Comprehensive Chemical Kinetics, Volume 19: Simple Processes at the Gas-Solid Interface

COMPREHENSIVE CHEMICAL KINETICS

COMPREHENSIVE Section 1 .

THE PRACTICE AND THEORY OF KINETICS ( 3 volumes)

Section 2.

HOMOGENEOUS DECOMPOSITION AND ISOMERISATION REACTIONS ( 2 volumes)

Section 3.

INORGANIC REACTIONS ( 2 volumes)

Section 4.

ORGANIC REACTIONS ( 6 volumes)

Section 5.

POLYMERISATION REACTIONS ( 3 volumes)

Section 6.

OXIDATION AND COMBUSTION REACTIONS ( 2 volumes)

Section 7 .

SELECTED ELEMENTARY REACTIONS ( 1 volume)

Section 8.

HETEROGENEOUS REACTIONS ( 4 volumes)

Section 9.

KINETICS AND CHEMICAL TECHNOLOGY (1 volume)

Section 10. MODERN METHODS, THEORY, AND DATA

CHEMICAL KINETICS EDITED BY

C.H. BAMFORD M.A.,Ph.D.,Sc.D. (Cantab.),F.R.I.C.,F.R.S. Formerly Campbell-Brown Professor of Industrial Chemisiry, University of Liverpool

The late C.F.H. TIPPER Ph.D. (Bristol), D.Sc. (Edinburgh) Senior Lecturer in Physical Chemistry, University of Liverpool AND

R.G. COMPTON M.A., D.Phil. (Oxon.) Lecturer in Physical Chemistry; University o f Liverpool

VOLUME 19

SIMPLE PROCESSES AT THE GAS-SOLID INTERFACE

ELSEVIER AMSTERDAM-OXFORD-NEW 1984

YORK-TOKY 0

ELSEVIER SCIENCE PUBLISHERS B.V.

Molenwerf 1, P.O. Box 211,1000 AE Amsterdam, The Netherlands

Distributors for the United States and Canada

ELSEVIER SCIENCE PUBLISHING COMPANY INC.

52 Vanderbilt Avenue New York, N.Y. 10017

ISBN 0-444-41631-5 (Series) ISBN 0-444-42287-0 (Vol. 19) with 117 illustrations and 8 tables

@ Elsevier Science Publishers B.V., 1984 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system o r transmitted in any form o r by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science Publishers B.V., P.O. Box 330, 1000 AH Amsterdam, The Netherlands Printed in The Netherlands

COMPREHENSIVE CHEMICAL KINETICS

ADVISORY BOARD Professor S.W. BENSON Professor SIR FREDERICK DAINTON Professor G. GEE Professor G.S. HAMMOND Professor W. JOST Professor G.B. KISTIAKOWSKY Professor K.J. LAIDLER Professor M. MAGAT Professor SIR HARRY MELVILLE Professor S. OKAMURA Professor N.N. SEMENOV Professor Z.G. SZABO Professor 0. WICHTERLE

Volumes in the Series Section 1. Volume 1 Volume 2 Volume 3

The Practice of Kinetics The Theory of Kinetics The Formation and Decay of Excited Species Section 2.

Volume 4 Volume 5

SELECTED ELEMENTARY REACTIONS (1 volume

Selected Elementary Reactions Section 8.

Volume 19 Volume 20 Volume 21 Volume 2 2

OXIDATION AND COMBUSTION REACTIONS ( 2 volumes)

Liquid-phase Oxidation Gas-phase Combustion Section 7.

Volume 18

POLYMERISATION REACTIONS ( 3 volumes)

Degradation of Polymers Free-radical Polymerisation Non-radical Polymerisation Section 6.

Volume 16 Volume 17

ORGANIC REACTIONS (6 volumes)

Proton Transfer Addition and Elimination Reactions of Aliphatic Compounds Ester Formation and Hydrolysis and Related Reactions Electrophilic Substitution at a Saturated Carbon Atom Reactions of Aromatic Compounds Section 5.

Volume 14 Volume 14A Volume 15

INORGANIC REACTIONS

Reactions of Non-metallic Inorganic Compounds Reactions of Metallic Salts and Complexes, and Organometallic Compounds Section 4 .

Volume 8 Volume 9 Volume 10 Volume 12 Volume 1 3

HOMOGENEOUS DECOMPOSITION AND ISOMERISATION REACTIONS

Decomposition of Inorganic and Organometallic Compounds Decomposition and Isomerisation of Organic Compounds Section 3.

Volume 6 Volume 7

THE PRACTICE AND THEORY OF KINETICS

HETEROGENEOUS REACTIONS ( 4 volumes)

Simple Processes at the Gas-Solid Interface Complex Catalytic Processes Reactions of Solids with Gases Reactions in the Solid State

Section 9. Volume 23

KINETICS AND CHEMICAL TECHNOLOGY (1 volume)

Kinetics and Chemical Technology Section 10. MODERN METHODS, THEORY, AND DATA (1 volume)

Volume 24

Modern Methods in Kinetics

Contributors to Volume 19 M. BOWKER

New Sciences Group, Imperial Chemical Industries, Runcorn, Cheshire. Gt. Britain

J. CUNNINGHAM

Department of Chemistry, University College, Cork, Ireland

C.T. FOXON

Philips Research Laboratories, Solid State Electronics Division, Redhill, Surrey, Gt. Britain

B.A. JOYCE

Philips Research Laboratories, Solid State Electronics Division, Red hill, Surrey, Gt. Britain

D.A. KING

Donnan Laboratories, The University, Liverpool, Gt. Britain

M.A. MORRIS

Department of Chemistry, Imperial College of Science and Technology, London, Gt. Britain

Section 8, which comprises the four volumes 19-22, deals with reactions which occur at gas-solid and solidsolid interfaces other than the degradation of solid polymers which has already been reviewed in Volume 14A. Reactions at the liquidsolid interface are not considered, but those involving electrochemical processes will be covered in subsequent volumes. With respect to chemical processes at gassolid interfaces, it has been necessary to discuss surface structure and adsorption as a lead-in t o the consideration of the kinetics and mechanisms of catalytic reactions. Volume 1 9 is devoted to considering simple processes occurring at the gas solid interface. Chapter 1 serves as an introduction and deals with the methodology of experimental surface science. Experimental results for metal surfaces on both adsorption and desorption kinetics and surface diffusion are discussed in terms of the current theories of these processes. Chapter 2 deals in the same way with these processes on semi-conductor surfaces. Finally, Chapter 3 is concerned with radiation and photoeffects at gassolid interfaces. The editors thank Professor D.A. King for invaluable advice.

C.H. Bamford The late C.F.H. Tipper R.G. Compton

Liverpool January 1984

This Page Intentionally Left Blank

Preface

.................................................

ix

Chapter I (M.A. Morris. M . Bowker and D.A. King) Kinetics of adsorption. desorption and diffusion at metal surfaces . . . . . . . . . . 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 The development of the science of solid surfaces . . . . . . . . . . . . . . . 1.2 The gassolid surface interaction potential . . . . . . . . . . . . . . . . . . . 1 . 3 Order4isorder phenomena in adsorbed layers . . . . . . . . . . . . . . . . 1.3.1 Dipole-dipole interactions . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.2 Indirect coupling interaction . . . . . . . . . . . . . . . . . . . . . . . . 1.3.3 Direct coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.4 Substrate atom sharing interactions . . . . . . . . . . . . . . . . . . . . 1.3.5 Importance of lateral interactions . . . . . . . . . . . . . . . . . . . . . 1.4 Adsorbate-induced static distAacements of substrate atoms . . . . . . . . 2. Experimental techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Surface crystallography. chemical composition and electronic structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1 Low energy electron diffraction (LEED) . . . . . . . . . . . . . . . . 2.1.2 Auger electron spectroscopy (AES). . . . . . . . . . . . . . . . . . . . 2.1.3 Photoemission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.4 Vibrational spectroscopies . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Adsorption kinetics and absolute coverages . . . . . . . . . . . . . . . . . . 2.2.1 Uptake techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Temperature-programmed desorption . . . . . . . . . . . . . . . . . . 2.2.3 Radiotracer techniques. . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4 Nuclear reaction method . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.5 Microbalance techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.6 Relative coverage measurement techniques . . . . . . . . . . . . . . . 2.2.7 Reflection detector techniques . . . . . . . . . . . . . . . . . . . . . . . 2.2.8 Absolute random flux technique. . . . . . . . . . . . . . . . . . . . . . 2.3 Desorption kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 Temperature-programmed desorption . . . . . . . . . . . . . . . . . . 2.3.2 Isothermal desorption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3 Electron.. photon-, ion- and field-stimulated desorption . . . . . . 2.4 Surface diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.1 Scanning methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2 Field emission and field ion microscopies . . . . . . . . . . . . . . . . 2.4.3 Other methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 . Adsorption kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 The data base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1 Zero coverage sticking probabilities . . . . . . . . . . . . . . . . . . . . 3.1.2 Variations of s with surface coverage . . . . . . . . . . . . . . . . . . . 3.2 Mechanisms and rate laws in adsorption . . . . . . . . . . . . . . . . . . . . . 3.2.1 Energy accommodation and trapping. . . . . . . . . . . . . . . . . . . 3.2.2 Precursor states in reactive gas-olid interactions. . . . . . . . . . .

1 1 1

3 6 7 7 7 8 8 9 10 10 10 13 15 16 17 17 20 21 21 22 23 24 26 27 27 29 29 31 31 34 39 41 41 41 55 57 58 62

3.2.3Models for adsorption kinetics . . . . . . . . . . . . . . . . . . . . . . . 3.2.4Activated adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Desorption kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Theory and analysis of desorption spectra . . . . . . . . . . . . . . . . . . . 4.1.1Theoretical aspects of thermal desorption. . . . . . . . . . . . . . . . 4.1.2 Integral order desorption with coverage-independent parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.3Systems with variable desorption energies. . . . . . . . . . . . . . . . 4.1.4Systems with variable pre-exponentials . . . . . . . . . . . . . . . . . 4.1.5 Desorption order. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.6The effect of precursor states . . . . . . . . . . . . . . . . . . . . . . . . 4.1.7The influence of lateral interactions . . . . . . . . . . . . . . . . . . . 4.2 The data base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1Crystal plane orientation . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2Surface cleanliness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3 Temperature-programmed reaction spectroscopy . . . . . . . . . . . 4.3 The desorption data bank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Interstate conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Surface diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

64 82 84 86 86 90 94 97 98 101 104 108 108 108 122 125 141 142 142 149 163 163

Chapter 2 (B.A. Joyce and C.T. Foxon) Adsorption. desorption and migration on semiconductor surfaces . . . . . . . . . . . 1 . Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Justification for the subject matter . . . . . . . . . . . . . . . . . . . . . . . . 1.2 The “charge-transfer’’ model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Experimental approach. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Surface crystallography . Diffraction techniques. . . . . . . . . . . . . . . . 2.1.1 Low energy electron diffraction (LEED) . . . . . . . . . . . . . . . . 2.1.2 Reflection high energy electron diffraction (RHEED). . . . . . . . 2.2 Surface compositional analysis. Auger electron spectroscopy (AES) . . 2.3 Surface electronic structure. Photoelectron spectroscopies . . . . . . . . 2.4 Surface kinetic measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.1 Modulated molecular beam methods . . . . . . . . . . . . . . . . . . . 2.4.2Thermal desorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Atomically clean semiconductor surfaces . . . . . . . . . . . . . . . . . . . . . . . 3.1 Electronic structure of semiconductor surfaces . . . . . . . . . . . . . . . . 3.1.1Self-consistent pseudo-potential calculations . . . . . . . . . . . . . . 3.1.2 Realistic tight-binding calculations . . . . . . . . . . . . . . . . . . . . 3.2 Crystallography of semiconductor surfaces . Relaxation and reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Preparation of clean surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2 Gallium arsenide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Determination of the crystallographic and electronic structure of clean semiconductor surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . .

......................................

......................................

3.4.3 GaAB{ 110) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Gas- emiconductor surface interactions . . . . . . . . . . . . . . . . . . . . . . .

181 181 181 182 183 183 183 187 189 190 192 193 195 197 197 199 200 200 201 202 204

206 206 210 215 221

4.1 Adsorption of hydrogen on silicon . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1 Introduction and survey of early work . . . . . . . . . . . . . . . . . . 4.1.2 Hydrogen atom adsorption on Si 111 . . . . . . . . . . . . . . . . . 4.1.3 Hydrogen atom adsorption on Si 110 . . . . . . . . . . . . . . . . . 4.1.4 Hydrogen atom adsorption on Si 100 . . . . . . . . . . . . . . . . . 4.1.5 Direct observation of hydride surface phases . . . . . . . . . . . . . . 4.1.6 Theoretical considerations . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Oxygen adsorption on silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Chlorine adsorption on silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Oxygen adsorption on GaAs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1 Oxygen adsorption on clean { 110) GaAs surfaces, . . . . . . . . . . 4.4.2 Oxygen adsorption on other orientations . . . . . . . . . . . . . . . . 5 . Metal interactions with semiconductor surfaces . . . . . . . . . . . . . . . . . . . 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Metalsemiconductor interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1 Goldsilicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2 Silver-ilicon .................................. 5.2.3 Group I11 metals (Al, Ga, In)--silicon . . . . . . . . . . . . . . . . . . . 5.2.4 Caesium-silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.5 Metals+roup 111-V compounds . . . . . . . . . . . . . . . . . . . . . 5.2.6 Mechanisms of metabsemiconductor interface interactions . . . . 5.2.7 “Classical” models of metal desorption from semiconductor surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Semiconductorsemiconductor interfaces . . . . . . . . . . . . . . . . . . . 5.4 Interaction of Group V elements with GaAs surfaces . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11

221 221 223 227 229 230 231 232 233 242 246 247 252 253 253 254 255 258 259 260 260 269 270 275 277 280

Chapter 3 (J. Cunningham) 291 Radiation and photoeffects at gaslsolid interfaces . . . . . . . . . . . . . . . . . . . . . 1. General introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291 1.1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291 293 1.2 Origins of radiation sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . 293 1.2.1 Collective-electron models . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2 Active-site models and their sensitivity to radiation . . . . . . . . . 2 9 6 1.2.3 Combinations of collective-electron and active-site models . . . . . 301 303 1.2.4 Surface-state models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Spectroscopic aspects of irradiated gas/solid interfaces . . . . . . . . . . . 310 1.3.1 Electron spectroscopy of surfaces . . . . . . . . . . . . . . . . . . . . . 311 325 2 . Photoeffects at gas/solid interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . 327 2.1 Photophysical effects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327 2.1.1 Experimental aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 2.1.2 Results and interpretations . . . . . . . . . . . . . . . . . . . . . . . . . 354 2.2 Photochemical effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354 2.2.1 Experimental aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359 2.2.2 Results and interpretations . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3 Modifying effects of surface dopants. . . . . . . . . . . . . . . . . . . 394 3 Effects induced by irradiation with high-energy photons or particles . . . . . 397 3.1 Energy deposition and localisation at the gas/solid interface . . . . . . . 398 399 3.2 Experimental aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401 3.3 Results and interpretations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Effects on adsorption-desorption processes during irradiation . . 401 405 3.3.2 Chemical effects during irradiation . . . . . . . . . . . . . . . . . . . .

.

3.3.3 Effects persisting at the interface after irradiation . . . . . . . . 4 . Perspectives and prospectus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Index

..................................................

..

413 417 419 429

Chapter 1

Kinetics of Adsorption ,Desorption and Diffusion at Metal Surfaces M.A. MORRIS, MICHAEL BOWKER and DAVID A. KING

1. Introduction 1.1THE DEVELOPMENT O F THE SCIENCE OF SOLID SURFACES

torr, approximately one monolayer of gas molAt a pressure of ecules collides with a smooth surface every second. For most gas-metal combinations, adsorption is a remarkably efficient process; the probability that a normal background constituent of a vacuum system, such as O,, CO, N, or H,, will become adsorbed during a single collision with a clean metal surface (the sticking probability) is high, often between 0.1 and 1, and the contamination rate is therefore prohibitively high at these ambient pressures. Studies of the gas-solid interface under reproducible and controlled conditions were therefore contingent on the developments in ultrahigh vacuum technology which occurred several decades ago, providing methods for routine access to experimental environments with background pressures in the range lo-'' t o lo-" torr. Subsequently, a powerful array of surface-sensitive, surface-specific analytical, crystallographic, dynamical and spectroscopic techniques has been developed, providing no less of a revolution in the study of solid surfaces than occurred in most other areas of physical chemistry in the 1930s and 1940s. The literature which this revolution has spawned, and the consequential developments in the science of solid surfaces, is both formidable and daunting (particularly to the reviewer!). In this chapter, we attempt a comprehensive survey of the impact of these developments on the study of kinetic processes at metal surfaces. These studies have highlighted both the important relationship between kinetics and structure in the surface layer and the role of weakly held, short-lived adsorption states (usually described as precursor states) in the formation of stable adsorbed layers. To place the kinetic studies in context, we therefore include in this introduction a brief survey of adsorption potential energy curves and of structural phenomena at surfaces. In the period prior to the development of ultrahigh vacuum (UHV) technology, it was widely believed that the process of forming a strongly held, chemisorbed layer was activated [ 11, although even then there was a suspicion that this might be due to the difficulty of attaining and maintaining clean surfaces. Several research groups during this period did, however, manage t o work under conditions where clean surfaces could be References p p . 163-1 79

2

produced, noteworthy among these being Langmuir in the U.S.A. and Roberts in the U.K. Thus, the early work of Taylor and Langmuir [2] on the adsorption of Cs on polycrystalline tungsten provided accurate, absolute measurements of adsorption rates and surface coverages which still form the basis of modern developments in kinetic schemes for adsorption. The work of Roberts [3, 41 on accommdoation coefficients and the adsorption of H 2 , N2 and O2 on tungsten filaments was equally noteworthy in establishing the high efficiency of adsorption on clean metal surfaces, the non-activated nature of the process, and the role of lateral interactions and adsorbate configuration on the coverage dependence of the rate of adsorption. The development of the inverted Bayard-Alpert gauge in the 1950s (the first device capable of measuring low pressures and still the most commonly used) was followed closely by the flash filament technique for thermal desorption and sticking probability measurements, pioneered by Ehrlich [5] and Redhead [ 6 ] . At the same ,time, the field emission microscope, developed by Muller [7] in the later 1930s, began t o be used extensively in adsorption studies, particularly by Becker [ 81, Gomer [91 and Ehrlich and Hudda [ 101 , This work demonstrated in dramatic fashion the crystal plane specificity of the adsorption process. From that time on, research workers could not be content with preparing clean metal surfaces for study; the crystal plane exposed at the surface had to be specified as well. This led to a resurgence of interest in a technique discovered by Davison and Germer [ 111 in 1927 and practised in splendid isolation over the intervening period by Farnsworth [ 121 : low energy electron diffraction (LEED). Germer himself returned to this method in the later 1950s and his adaptation of Ehrenberg’s technique [14] of displaying the backscattered electron diffraction pattern on a fluorescent screen [ 131 is now the sine qua non of every surface science laboratory. Even then, however, the state of cleanliness of the surface was largely a matter of (sometimes misguided) faith; only tungsten surfaces, for which simple cleaning procedures had been established from the flash filament and field emission microscopy studies, could be examined with any confidence; results from the early single crystal studies of Delchar and Ehrlich [15] and of Estrup and Anderson [16] have stood up well to the test of time. In the 1960s, a spectroscopic method, based on a suggestion by Lander [17] was developed [ 181 for determining the chemical composition, and hence the state of cleanliness, of solid surfaces: Auger electron spectroscopy (AES). Only then could the surface science community proceed with the confidence to study the interaction of gases with crystallographically and chemically well-defined solid surfaces. From this period on, numerous powerful spectroscopic and crystallographic techniques, often confusing the literature with their acronyms, have been developed for surface studies: photoelectron spectroscopy; ion scattering and ion channeling; electron energy loss and reflectionadsorption infrared vibrational spectroscopies;

3

atomic and molecular beam scattering; etc. Rather late in the day, the science of solid surfaces had come of age, The state of the field is summarised in a recent series of review articles [ 191 . 1 . 2 THE GAS-SOLID SURFACE INTERACTION POTENTIAL

A major breakthrough in the study of gassolid interactions was the development by Lennard-Jones [20] in 1924 of a potential curve for the interaction. For a gas-metal system where the interaction is strong enough to form a chemisorbed species, it was shown that an incoming molecule passes through two minima, the first a broad, shallow well (the physisorption well attributed to van der Waals forces) and the second a deeper well corresponding to the formation of a chemical bond. The physisorption well was originally calculated by summing the “6 : 12” potentials between the incoming species and each surface atom

f = c(?+$) i

where r is the interatomic distance. Potential wells have been simulated using an estimation of the London constant [21] K , and choosing a range of values of K 2 to fit the data; good approximations to K, are available for many systems [22] and have been applied to adsorption data with apparent success [23]. However, it is now recognised that the physisoprtion well on metals is not produced solely by dispersion forces. Ehrlich [24] found that the heats of adsorption of the rare gases Ar, Kr and Xe on tungsten were 8, 18 and 35 kJ mole-’, respectively, compared with condensation heats (i.e. rare gas atom on rare gas solid) of 6.7, 9.0 and 12.7 kJ mole-’, respectively. The disparity is larger the larger the polarisability of the adatom. Engel and Gomer [25] found that the adsorption heats of rare gases on W single crystal planes were not as predicted by simple pairwise addition of dispersive forces. And it was also discovered that the rare gases induced surprisingly large changes in work function, of the order of 1 eV for Ar, Kr and Xe [25-271. The large electric field gradient at the surface must be expected to cause a polarisation of the adatom, giving an additional energy term %Fa2,where F is the field strength at the equilibrium distance from the surface and a the volume polarisability of the adsorbed species. This theory has been supported [28-301 and has been used to explain the surface roughness effect on adsorption heat q , the rougher the surface, the larger is F and hence q. Engel and Gomer [25], however, pointed out that the surface field falls off very rapidly with distance. It should be noted that, due to electron overspill at a clean metal surface, a surface dipole exists at the surface with a negutiue charge outwards from the surface. This is well established, both theoretically [ 311 and experimentally [32]. Yet physisorbed species on metal surfaces invariably produce a decrease in the work References p p . 163-1 79

4

A

Fermi level ( p )

/

Fig. 1. Electron potential against distance from surface. (From ref. 33.)

function, suggesting that a dipole is induced in the physisorbed species with the positive charge away from the surface, in the opposite sense to that anticipated from the surface electrostatic field. The solution is hinted at in Fig. 1, which shows the electron potential at a metal surface versus distance from the surface as computed by Lang [33], corresponding to a Fermi wavelength of 8 . 6 6 a . It is seen that the electrostatic surface potential barrier $J is only a small part of the total effective barrier Veff, at the surface. Although $J is positive, and electrons are repelled and nuclei attracted by the electrostatic field, Veff is negative and IV,,, I > /$I. The electrons of an adatom physisorbed t o the surface thus tend to move into the region between the adatom nucleus and the surface. Clearly, this effect contributes to both the depth of the physisorption well and the size and sign of the induced dipole. (Theoretical treatments of physisorption potential energy wells are reviewed by Kasperma [34], Steele [35] and Gerahsimenko [36], but do not take account of this last point .) Two further experimentally determined properties of physisorbed species should be mentioned. (i) LEED studies on single crystal planes have shown that physisorbed layers may be ordered, as first noted by Palmberg [37] for Xe on P d ( l l 0 ) and by Chesters and Pritchard [38] for Xe on Cu(100). These ordered layers appear t o be dominated by adatomadatom interactions. Photoemission also provides evidence for such interactions [ 391 . (ii) Activation energies for diffusion of physisorbed species have been determined by field emission methods, as described in detail later, and it is concluded that, in general, the diffusion barrier is about 40% of the desorption barrier. There are, therefore, preferred site configurations for physisorbed species at a metal surface, the diffusion barrier referring t o the saddle point between sites. Although such barriers have been elegantly calculated for rare gas atoms on rare gas solids [40], all theoretical treatments t o date of the interaction

between gases and metal surfaces have treated the surface as a continuum. Figure 1 does, however, provide an intuitive basis for understanding this site preference and also the larger observed heats of adsorption on rough surfaces. A t an open site, the adsorbed species can approach closer to the surface, where Veff is large; between such sites, or on a smooth surface, the adatom sits proud of the surface, where V,,, is small. The potential energy well for chemisorption is associated with the more familiar chemical bond, although the valence band of the solid provides many unique features, and is reviewed by Grimley [41].The experimental distinction between physisorbed and chemisorbed states is now readily made by photoemission studies of the combined adsorbateadsorbent system, thus (thankfully) committing the otherwise rather theological discussions of borderline cases to past history. Chemisorption heats (see ref. 42) usually lie within the range 30 < q < 600 kJ mole-' and measured adatomadsorbent atom equilibrium distances are usually very close to those observed in solid state of molecular analogues. (Such measurements are obtained by LEED or, more accurately, by surface EXAFS.) Two schematic combined potential energy wells for the interaction of a gaseous species with a surface are shown in Fig. 2, illustrating the importance of the crossover point of the chemisorption and physisorption wells for adsorption and desorption kinetics. In the first case, adsorption is activated; in the second, it is non-activated. (There are, in fact, only a few well-documented cases of activated chemisorption.) Recently, Lundqvist et al. [43] have made detailed calculations of the potential interaction between H, and a magnesium surface which substantiate the presence of two minima. Their work is reviewed elsewhere [44]. It must be borne in mind that diagrams such as Fig. 2 grossly oversimplify the

I

I

Distance from surface

___)

Fig. 2. Crossed potential energy curves for physisorption and chemisorption. (a) Nonactivated adsorption; (b) activated adsorption.

References p p . 163-1 79

6

interaction. For example, a distinction must be made between the physisorbed species at a site filled by a chemisorbed species and the species at an empty site. And more than one chemisorption well should be envisaged at a given site configuration: for example, a molecular, upright bridged diatomic; a molecular, “lying-down” diatomic; and the dissociated adatoms of the original diatomic molecule, As discussed later, the physisorption well and (to a lesser extent) other intermediate wells, almost invariably play a decisive role in adsorption and desorption kinetics. 1.3 ORDER-DISORDER PHENOMENA IN ADSORBED LAYERS

If there were no lateral interactions between adsorbed species, equilibrium adsorption would occur with random occupation of preferred site configurations. However, since the advent of LEED as a tool for determining the long-range structural order of a surface it has been known that disorder in thermally equilibrated chemisorbed overlayers is the exception rather than the rule, even at relatively low surface coverages. In general, therefore, the lateral interaction energy between adsorbed species, w , is large compared with thermal energy, kT. The importance for kinetic processes, originally suggested by Roberts [4] and substantiated in kinetic studies of adsorption, desorption and diffusion kinetics on metal single crystal planes by King and co-workers [45-481, lies in both the effect that w has on the activation energy for these processes and its effect on the configuration of the adlayer and of empty sites, and hence on the probability of formation of the required activated complex for the kinetic process. The long-range order in an equilibrated adlayer at submonolayer coverages at low temperatures, in a system where no static displacement of adsorbent atoms is induced by the adsorbate (see Sect. 1.4), is entirely determined by the range, strength and sign of the lateral interactions between adsorbed species, provided that each species occupies an identical site. Once the preferred sites are filled, further adsorption can take place into less preferred sites, or the site configuration of the entire overlayer may be switched. In addition t o a coverage dependence of the overlayer configuration, there is a strong temperature dependence. For example, in the simplest cases an ordered overlayer can be disordered by random occupation of a range of sites. Order-disorder transitions in chemisorbed layers have been documented in several instances and in some cases the surface phase diagrams have been determined [49-53] . Phase transitions in physisorbed layers on exfoliated graphite have recently excited much interest [ 541. The origin of lateral interactions between adsorbed species has been widely discussed and it seems unlikely that an adsorbate/adsorbent system exists where these interactions are not important. A number of contributory effects have been identified in recent years, the dominant effect for individual systems being determined by the range over which

7

the interaction occurs and by the nature of the adsorbateadsorbent bond.

1.3.1 Dipole-dipole interactions [ 4 , 55, 561 If the surface dipole is strong and the distance between dipoles small, the repulsive force between two dipoles aligned parallel to each other may be dominant. This is, however, only likely to be the case for ionic adsorption, as in alkali metals adsorbed on transition metals. Despite the relatively weak nature of this interaction for most systems, the importance of dipole-dipole coupling in vibrational spectroscopy is now well established E571.

1.3.2 Indirect coupling interaction The importance of this effect was first recognised by Koutecky [ 581 for adsorption on semiconductor surfaces, and the work of Grimley [ 591 and of Einstein and Schrieffer [60] has firmly established its influence in chemisorption on metal surfaces. It arises from the interaction between the adsorbed species and the itinerant valence electrons of the adsorbent. The effect is illustrated by considering two H atoms separated in vacuum by a distance such that the wave functions of the 1s electrons are zero between them; there is no direct interaction, On the surface, however, the valence band electrons of the solid provide a path which allows coupling between the H atoms, at the same separation distance. This interaction is relatively long range, is oscillatory (it may be repulsive for nearestneighbours, attractive for next nearest-neighbours, etc.) and is dependent on crystallographic direction in the surface. It is widely regarded as a dominant effect in the formation of ordered structures at low fractional surface coverages.

1.3.3 Direct coupling This effect arises from the interaction between orbitals on adjacent adsorbed species and is clearly only important over very short distances. Energy dispersion, observed by angle-resolved photoemission, in adsorbate energy levels for Xe on Pd(100) [39] has been attributed t o this effect and these effects in high-coverage chemisorbed layers have been theoretically examined by Batra and Ciraci [61]. The contribution to the total lateral interaction energy, even in crowded overlayers, is small. The direct interaction between adsorbed species is dominated by the repulsive interaction between neighbouring molecular orbitals: thus, the saturation coverage of CO on various metal single crystal planes suggests a close-packing intermolecular separation which is close to that found in solid CO. At favourable separation distances between adsorbed species, the van der Waals forces will provide an attractive interaction. References p p . 163-1 79

8

1.3.4 Substrate atom sharing interactions If an adsorbed species, such as a H atom, is adsorbed in a bridged site between two substrate atoms, a second adatom adsorbed in a bridge with one substrate atom common to the first will produce a direct bonding interaction. This effect has been theoretically examined by Grimley and Torrini [62] and may be energetically dominant. It is clearly a specific, short-range interaction.

1.3.5 Importance of lateral interactions Le Bosse et al. [63] have made a theoretical comparison of the importance of the first three of these interactions in chemisorption, with particular relation to the adsorption of sodium on copper. Experimentally, lateral interactions have been determined by observing orderdisorder transitions using LEED [ 50-531 and from desorption [45,641 and diffusion kinetics [48] as documented in Sects. 3.2.3 and 4.1.6 of this review. LEED is a useful structural tool in this respect, because the width and intensity of fractional-order diffracted beams provide a direct measure of the degree of order in the overlayer. To obtain lateral interaction energies from the data requires the use of a two-dimensional lattice gas formalism, which enables the Hamiltonian for the system to be reduced to one equivalent to a magnetic spin system of the Ising type, as solved in one dimension by Onsager [65].(The equivalence was shown by Lee and Young [66].) These analyses are therefore restricted to systems where (i) the substrate is undisturbed by the adsorbate (no static displacements of substrate atoms) and (ii) all adsorbed species occupy identical surface sites and are in the same configuration. For a double-spaced, i.e. c(2 x 2) structure, on a square twodimensional lattice (surface net), where only nearest-neighbour repulsive interactions, wnn, exist, the solution is

where T, is the order4isorder transition temperature. This simple theory, based on only pairwise nearest neighbour (n.n.) repulsive interactions, predicts a phase diagram which is symmetrical about a fractional site coverage of 0.5;this has never been observed. Thus, for one of the simplest systems examined to date, oxygen adatoms on a W { l l O } surface, it has been found necessary to take into account n.n., next nearest neighbour (n.n.n.) and three-body (trio) interactions to explain the results [67, 681. The existence of trio interactions removes the equivalence between filled and empty sites, thus also removing the symmetry about the half-monolayer coverage. The consequential theoretical complexities have to some extent been overcome by the use of Monte Car10 techniques [ 69-73 ] . Pairwise interaction energies, repulsive and attractive, for

9

various chemisorption systems have been determined with values up t o 10-20 kJ mole-’. As an example, the order- disorder transition temperature for O/W{llO}at half monolayer coverage occurs at 700 K. 1.4 ADSORBATE-INDUCEDSTATIC DISPLACEMENTS OF SUBSTRATE ATOMS

The two-dimensional lattice statistics approach described in the previous section is restricted to a “checkerboard”mode1 of the adsorption process, in which a rigid surface provides fixed sites for the adsorbate and is itself undisturbed by the adsorbate. However, there are a number of established examples of systems where the substrate atoms are structurally rearranged in the process of adsorption. The uptake of oxygen by a clean metal surface often results in the formation of a thin oxide film at the surface, with a structure replicating that of a bulk oxide. Here, clearly, a substantial rearrangement of substrate metal atoms is involved. Even at low oxygen adatom coverages, however, substrate reconstruction may occur as, for example, when oxygen is chemisorbed on W(100) [74] or Cu [75] single crystal surfaces. In several instances, oxygen chemisorption on to a unreconstructed surface at low coverages is followed by oxide nucleation as the coverage is increased; for oxygen on Ni films and single crystal surfaces, this occurs at around 0.25 of a monolayer [ 76-78] . Reconstruction is not limited to oxygen adsorption: for example, the annealed state of CO on W{OOl} appears to result from surface reconstruction [ 791. A further complication occurs for several systems where the adsorbate is taken into an underlayer and does not occupy surface sites. This occurs for N atoms on Ti{0001} [80] and on W{llO) [81]; N adatoms form an overlayer at coverages up to half a monolayer on W { 001) and at temperatures below 900 K, but at higher temperatures or coverages they are taken into the bulk [82]. In several cases, adsorption can result in the lifting of a surface reconstruction present on a clean surface. For example, the stable structures of the {loo} and (110) surfaces of Pt and Ir are not the truncated bulk structures. Chemisorption of CO, NO, oxygen or hydrogen, under certain conditions, causes a removal of non-integral order diffraction beams from the clean surface LEED patterns [83, 841. This is widely interpreted to indicate the stabilisation of the truncated bulk structure by the adsorbate and clearly involves a static displacement of substrate atoms from their stable positions in the clean surface. In the case of the (100)surfaces of these metals, which are believed to form a hexagonal close-packed surface layer structure when clean [ 851, this involves a substantial rearrangement of substrate surface atoms. The surface layer density is lowered during chemisorption and this can only result from a significant transport of metal atoms across the surface, with the formation of defects such as steps and kinks. The kinetics of these processes themselves form a significant challenge to the experimentalist, which have hardly been References p p . 163-1 79

10

touched on. Their existence provides an added dimension of complexity t o the study of adsorption, desorption and diffusion in these systems. A further category of adsorbate-induced substrate atom displacements has been revealed in recent studies by the groups at Liverpool University in the U.K. and at Brown University in the U.S.A. on the W{lOO} and Mo{100} surfaces. On these surfaces, the surface atoms are inherently unstable to small lateral displacements from their bulk lattice positions, as first revealed by the observation of reversible surface phase changes which occur on cooling below -370 K [83,841. On the W(1OOj surface, the low temperature phase is formed by alternate static displacements of surface atoms, by -0.2& in the ( n o ) and (110) directions, forming a zig-zag chain-like structure [83].Hydrogen adsorption, at adatom coverages between 0.1 and 0.2 of a monolayer (defined in terms of the number of W atoms in the surface), causes a symmetry switch in the W surface layer, with atoms alternately displaced in the (TOO) and (100) directions, producing a dimer structure [85].Further adsorption to saturation with two monolayers of hydrogen adatoms results in a return of surface tungsten atoms to bulk lattice positions. Adsorption of nitrogen adatoms at very low coverages (- 2% of a monolayer) completely inhibits the low temperature zig-zag structure [ 861. At a N adatom coverage of 0.4monolayer two-dimensional compressed islands are formed in which the interatomic spacing between surface W atoms is reduced compared with that in the bulk [87].And at exactly 0.5 monolayer, the surface W atoms are returned to bulk lattice positions: only at this coverage is the checkerboard model operative. Clearly, lattice statistical models are an oversimplification in the treatment of these systems; the work at Liverpool was originally instigated as a check on the applicability of these models.

2. Experimental techniques 2.1 SURFACE CRYSTALLOGRAPHY, CHEMICAL COMPOSITION A N D ELECTRONIC STRUCTURE

This section is intended as a brief introduction to some of the more widely used techniques for surface characterisation. More detail is available in a number of books [88-911. 2.1.1 Low energy electron diffraction (LEED)

A collimated, primary beam of electrons with a diameter of 0.5-1 mm at energies typically within the range 15-350 eV is impinged on a surface and the elastically back-scattered electrons, after travelling through a field-free region, are spatially analysed. In the commonly used display LEED system, this is achieved by passing the scattered electrons through

11

two (or more) hemispherical grids, the second at a retarding voltage t o eliminate inelastic electrons, and the diffracted beams are then accelerated into a hemispherical phosphor screen. The LEED pattern displayed on the screen is the reciprocal lattice (or net, in two dimensions) of the real space surface net. Figure 3(a) is a schematic representation of the LEED process, demonstrating the relationship between the real set, the reciprocal set and the diffracted beams. The resultant LEED photograph is shown as a typical example in Fig. 3(b). In the energy range 15-250 eV, the escape depth of the back-scattered electrons is 5-10 A and the observed pattern thus represents the combined symmetry parallel to the surface of the top few atomic layers. In the majority of cases, clean single crystal planes of metals produce LEED patterns with the symmetry of the bulk, although there are notable exceptions, as mentioned in Sect. 1.4. Superstructures formed by adsorbed layers, with a real space unit cell (or mesh, in two dimensions) larger than that of the substrate, are readily observed through the appearance of the corresponding additional fractional order beams in the LEED pattern. The information from the LEED pattern itself is limited by the fact that it is the reciprocal net of the surface diffraction grating: the nature of diffraction centres (e.g. adsorbate atoms or molecules) and the registry between overlayer and substrate (e.g. whether adsorbed species are in on-top sites, bridged sites, or four-fold hollow sites on a {loo} surface) are not provided by the pattern. The periodicity normal to the surface is, in fact, weakly felt by the combined incoming and outgoing wavefield of the electron beam and this forms the basis of the so-called intensity-voltage, or I-V, spectral analysis. The intensity of a given diffracted beam is related, albeit in a complex way (due t o multiple scattering or dynamical effects), to the energy of the primary electron beam and analyses of these spectra can provide the spacings normal to the surface required for a complete structural analysis [ 911 . Two sets of notation are commonly used to describe overlayer structures observed in diffraction experiments, the Wood notation [92] and a matrix notation. Although the latter is more flexible, the former is more widely used and we shall restrict ourselves to it in this review. The nomenclature is based on a comparison between the unit mesh of the topmost layer, the overlayer, and that of the second, unreconstructed, substrate layer. If a and b are the unit mesh vectors of the substrate layer and a, and b, the unit mesh vectors of the overlayer, then Wood’s notation for an overlayer of adsorbed species A on the {hkl} plane of a crystal M is

M{hkl}-

i‘::‘Y) -x-

-Rr#~-[[8]

(3)

where $I is the angle of rotation of the overlayer mesh relative t o the substrate mesh. For example, the structures shown in Fig. 4, representing a half-monolayer of adsorbate on a {loo} surface, are described as References p p . 163-1 79

12

bea rn

Fig. 3. (a) The relation between the surface net and the reciprocal surface net as observed by LEED. (b) The LEED photograph.

13

I

Fig. 4. Adsorbate structures on a b.c.c.{100} surface which would produce a c ( 2 X 2) pattern in LEED.

0

0

0

0

0

0

0

0

0

0

0

0

0000000 0000000

0000000 0000000

(a)

(b)

0

0

OO0O 0 ooooooooooooo 0 0 0 0 0000000 0 0 0 0 0 0 0 (C)

(d)

mmmo ooooo~o 0000000 (el

0000000 ( f )

Fig. 5. Six possible substrate-adsorbate structures that result in the same LEED pattern. (From ref. 714.)

M{100}- ( f l x f l ) R 4 5 ' ; M(100) - ~ ( x 22).

alternatively, they are also often given as

All three structures in Fig. 4 produce the same LEED pattern. This is more explicilty shown in Fig. 5; all these structures are equivalent in LEED display. Note, however, that they would all produce different I-V spectra.

2.1.2 Auger electron spectroscopy ( A E S ) An electron or photon incident at a surface with sufficient energy may result in the ejection of an electron from a core level (X) of an atom in References p p . 163-1 79

14

the surface region. Auger electron ejection is an auto-ionisation process of the ion so created: an electron in a lower binding energy level (core or valence) rapidly s) falls into the core level vacancy, in the process ejecting a second electron, the Auger electron, from a lower binding energy core or valence level. Provided the atom is close to the surface, this electron may be ejected into the vacuum, with a kinetic energy characteristic of the energy level structure of the atom (and independent of the primary electron or photon beam energy), hence providing a means of chemically identifying the atom. The alternative decay process, the emission of a photon, is inefficient when the binding energy of the core level X is 1015

1.78 2 2.1

1

5 x 10j4 5 x 1014 8x

lOI4

2 x 1014 2.5 x 1 0 ' ~ 8x lOI4 6 X lOI4 2 x 1014

C C C

5 x loJ4 7.5 x 1 0 ' ~ High High High 6 x lOI4

0.1(123 K) 0.08 0.72 0.735 0.4

A

C C C C C C C

E C A C C C

A

636 695 591 636 6 36 503 503 697 698 629 636 636 636 205 629 205 700 701 234 596 702 504 504 505 505 505

50

0.5-

0

0

Y

01

1

TI Zr Hf

I

V Nb Ta

I

Cr Mo W

l

fin Tc Re

C

Fe Ru

0s

I

CO Rh Ir

!

NI

Pd Pt

Fig. 16. The variation of SO across the periodic table for CO ( 0 ) and N2

m

(0).

I

v

I

'

V Nb Ta

Cr MO

W

Mn TC Re

Fe RU 05

Co Rh

Ir

Ni

Cu

Pd Pt

%

Fig. 17. Variation of so across the periodic table. Shown are points for 0 2

(v)

( 0 )and H2

51

metals are plotted as a function of group across the periodic table. The behaviour of the isoelectronic CO and N2 molecules is remarkably similar: hlgh reactivity (with so approaching unity) t o the left, a minimum in the middle (around Mn, Ru and Fe) and higher values to the right. The accumulation of evidence from a variety of studies indicates a further trend; both CO and N2 are dissociated on metals to the left of Fe, and non-dissociatively adsorbed on metals t o the right, with heats of adsorption decreasing from left t o right. A full discussion of these trends has been made by Broden et al. [351] and recently Nieuwenhuys [352] has extended the discussion t o correlate the change in work function across the periodic table with the amount of electron back-donation between CO and N2 and the metal. The magnitude of this back-donation is, of course, a measure of the dissociation tendency since the back-donation is the cause of the C-0 or N-N bond weakening. For oxygen, high so values, close t o unity, are reported for metals in Groups 2-6, only decreasing significantly with the filled d-band metals Cu, Ag and Au. For H,, the reactivity trend appears t o be the reverse of that for CO and N,, with a maximum around the centre of the periodic table. Adsorption heats and heats of formation of the metal hydrides, on the other hand, again fall from left to right. Dissociative adsorption does not occur on Cu, Ag and Au. Studies on single crystals have revealed that the sticking probability may be a sensitive function of crystal plane; this appears t o be more important for H, and N2 than for CO and 02, although similar trends are observed for all these gases on the same metal. Of the body centred cubic (b.c.c.) metals, tungsten is the most extensively studied and the most accurate available values for so are plotted for the principal zones of tungsten in Fig. 18. The range of so values for N2 is illustrated by the extremes, from 0.70 on W(320) to 3 x on W(llO}. It would appear that the closest-packed plane, { l l o ) , is the least reactive for N2, H,, CO and 0 2 ,although the trend is clearly less important for CO and 0,. We have chosen Pt t o illustrate the trends for an face centred cubic (f.c.c.) metal, (Fig. 19), although here there is some difficulty arising from the fact that the {110} and (100) surfaces of these metals are reconstructed in the stable state. Both surfaces can, however, be prepared in a metastable (1x l) state and, where available, so values are plotted for both states of these surfaces. For H,, O2 and COY maximum reactivity is observed for the relatively open (110) face, even in the (1x 2) reconstructed state. Very marked differences in the reactivity between the {loo} (5 x 20) and (1x 1) surfaces have been reported. Interestingly, it is widely accepted that the (5 x 20) reconstructed form of this surface, which has a low sticking probability for H2 and 0,, possesses a hexagonal, close-packed surface structure. For both f.c.c. and b.c.c. metals, therefore, it appears that the more open faces are more reactive. However, this conclusion must be treated with caution; for example, the very open References p p . 163-1 79

52

c 8

8

8

8

0 0

0 0

0 0

0

0

0

(211)

poly.

0

0 0 (100)

(110)

(111)

Fig. 18. Variation of sticking probability with Pt crystal plane for

HZ.

.,

C O ; 0 , O 2 and 0,

{Ill} plane of tungsten is one of the least reactive for nitrogen chemisorption [ 461 . Much has been written about the high reactivity of steps at metal surfaces and there are several examples in chemisorption. For example, the {llO} plane of W has a low sticking probability (- 3 x for nitrogen chemisorption at 300 K, but on the stepped {320) plane, with (110) terraces, so = 0.70. On the other hand, a singular, step-free W(lOO} surface is also very reactive (so = 0.59) and it has been suggested that the reactivity of stepped planes such as {320} is related t o the structural similarity of the steps t o the {loo}plane itself [47]. A further dramatic illustration of the dependence of so on the presence of steps is demonstrated by the work of Salmeron et al. [353] on the stepped Pt surface. Using a moJecular beam source, they demonstrated that the reactivity for dissociative chemisorption of H2 (as indicated by the formation of HD from a mixture of H2 and D 2 )was 7 times higher when the beam was directed “at” t h e steps than when it was directed over the steps, as shown in Fig. 20. Several studies have been made of the dependence of so on crystal and gas temperature. Since these results are critically important in establishing the adsorption mechanism, the detailed presentation of results is left until

53

l.op1.0-

0 0

0

0

0

0 0

0

ul

0

-

0.5

0 0 0

0 o

n

0 (111)

.,

(110)

(211)

Fig. 19. Sticking probability versus W crystal plane for the adsorption of CO.

0 , H2 ; 0,N2 ;and

0,

0,;

the following section of this chapter. Here, we simply note several categories of adsorption systems. (i) For several well-studied systems, such as N 2 / W [46, 3541 and 0 2 / P t [355], so decreases with increasing crystal temperature, although in both systems a low temperature limiting value, less than unity, is reached (0.62 for N2 on W { 100)) and recent results also show that there is a high temperature asymptotic value greater than zero (-0.05 for N2 on W{lOO}) [46]. For these systems, it has also been found that so decreases with increasing gas temperature [46, 3551. (ii) In some cases, so is insensitive t o crystal temperature. For example, King et al. [170] found that the reactive sticking probability for 02,i.e. the probability that an incident O 2 molecule is desorbed as atoms or oxide molecules, was the same a t 2200 K on a polycrystalline W filament as so a t 300K (-0.7). For H2/W{100},so has been reported [356] as independent of crystal temperature over the range 150- 300 K. For these systems, comparable studies of the gas temperature dependences have not been reported. (iii) In a minority of cases, so is found t o increase with increasing crystal temperature; this is true for H2 on Cu [358] and for N2 on Fe [357]. In these cases, an increase in so with gas temperature can be anticipated and the process is properly described as activated adsorption. References p p . 163-1 79

64

'1

:I z

0.0

-90 Pt {332} f = 1 0 H ~

05

O 0 1:

0

Pt{lll}

-80 -60 -40 - 2 0

0

20

40

60

80

Angle of incidence. B / D E G

Fig. 20. H D production as a function at angle of incidence (6)of the molecular beam normalised t o the incident Dz intensity. (a) Pt(332) with steps perpendicular t o the beam. ( b) Pt(332) with step edges parallel t o the beam. (c) P t { l l l } surface. (From ref. 353.)

A few studies have been made of the dependence of so on incidence angle using molecular beam techniques. Apart from the study on a stepped surface illustrated in Fig. 20, relatively small effects have been noted. King and Wells [46]found no dependence (within ? 0.01) for N2 on polycrystalline W and on W(100). Steinbriichel and Schmidt [ 359 J found significant variations only for H2 on W{lOO} (Fig. 21) and, in this case, it is probably again reasonable to assume that the results are dominated by step effects. Recently, Bickley et al. [360] have compared accurate

55

W(100)

08

O L 60

0

30

90

lLLLuAL 0

Angle of incidence, 8

30

60

90

Angle of Incidence. 8

Fig. 21. The variation of sticking probability with angle of incidence 8. s ( 0 ) is the sticking probability at 8 = 0 . (From ref. 359.)

values obtained for angle-integrated measurements with molecular beam values for normal incidence and conclude that the differences imply a significant variation with incidence angle for N,, H, and D, on W{OOl}, but only at angles close to grazing incidence. 3.1.2 Variations of s with surface coverage Provided that the adsorbate is not absorbed into the bulk, as with H, on Ti, and is not reversibly desorbed, all the surface sites will eventually become blocked and s therefore falls to zero. In considerations of the coverage dependence of s, the precise definition must be borne in mind: it is the probability that an incident particle is eventually adsorbed into a state with a lifetime 7 greatly in excess of the experimental time period. For example, even when s is zero at saturation coverage in a strongly bound state, the trapping probability into weakly bound adsorbed states may still be high. In a number of cases, such as CO on Ni [361, 3621 or W { 110) [ 3631 at 300 K, the heat of adsorption falls strongly with coverage and the desorption rate from the chemisorbed layer at high coverages becomes hgh. In these cases, what is experimentally measured is the net sticking probability, given by the ratio (adsorption - desorption rate)/bombardment rate, which will become zero even when the absolute s for adsorption into the chemisorbed state is finite. The shape of plots for s against coverage is a further important indicator of adsorption mechanism. The coverage may either be signified in absolute terms by N , the number of adsorbed species cm-’, or by 8,the fraction of sites filled. The use of the latter must be treated with caution; it is only unambiguously defined when the number of sites is equal t o the number References p p . 163-1 79

56

of substrate atoms cm-*, although even in that case surface reconstruction is an obvious complication, and we have confined our usage t o this definition. Since specific site orientations or symmetries are often required by the adsorbate (which may vary with coverage), the coverage often reaches a saturation value well below 8 = 1;we refer here t o this value as N , or 8,. In a consideration of the very wide range of results published t o date (Table l), we have proposed six different categories of adsorption in terms of the sticking probability-coverage profiles. The six categories are illustrated in Fig. 22, where relative sticking probabilities are schematically plotted against relative coverages. In category A, s falls linearly with coverage; this is anticipated for non-dissociative Langmuir adsorption, i.e. a simple site-blocking mechanism, but typical examples are the dissociative chemisorption of H2 [ 364, 3651 and 0, [ 366, 3671 on W{lOO}. In category B, the curve is convex with respect t o the origin; this is anticipated for dissociative Langmuir adsorption and has been observed for O 2 on P t { l l l } [368] and NO on Ag [369, 3701. Category C is the most common; here the curve is concave to the origin, with s initially almost independent of coverage; examples are N2 on various W single crystal planes and CO on a wide range of transition metals. This is usually taken t o indicate a mechanism in which trapping can take place into a precursor state at a filled chemisorbed site, with subsequent diffusion t o an empty site. In category D, s falls to a minimum at relatively

A

D

B

N

N

E

N

N

C

F

N

N

Fig. 22. Experimentally observed variations of s with N . The letters refer to the listing in Table 1.

57

low coverages, subsequently rising to a plateau before falling again at saturation. First observed by Horgan and King [371] for O 2 on Ni films, this characteristic shape is taken as an indication of chemisorption followed by an adsorbate-induced reconstruction, involving incorporation of the adatoms, at the coverage corresponding t o the minimum. In category E, s rises from an initial relatively low value to a smooth maximum and then falls again. This has only been observed for adsorption into relatively weakly bound states, e.g. for the formation of a y-N2 layer on W { l l O } at low temperatures [372]. In category F, the sticking probability profile is characterised by one or more distinct steps at characteristic coverages, observed, for example, with O 2 on polycrystalline W wires at very high exposures [170, 3731. The steps indicate several distinct stages in the formation of the overlayer, each with a characteristic initial sticking probability. Where coverage dependences have been reported in the literature, the shape is indicated in Table 1 by the lettering A-F, referring to Fig. 22. Clearly, a wide range of different chemisorption rate laws is indicated. 3.2 MECHANISMS AND RATE LAWS IN ADSORPTION

The data base discussed in the previous section provides an indication of the complexity and variety of the steps involved in the formation of a chemisorbed layer when an initially clean surface is exposed t o a gas. The questions posed by these results include the following. What are the important mechanistic steps or processes involved when a molecule strikes a clean surface? How is the excess kinetic energy of the incident particle dissipated? Why is one metal, or one crystal plane of a given metal, more reactive than another? What time-dependent electronic processes occur during the formation of a chemisorbed species? How can the presence of steps be accounted for? And what are the factors governing the variation of sticking probability with surface coverage, surface temperature, gas temperature and incidence angle? In this chapter, we attempt t o survey some of the answers that have been given in the literature, although it should be stressed at the outset that considerable experimental and theoretical work is still required before definitive answers can be provided, even in relation to a single adsorption system. In Sect. 1.2, we discussed briefly the potential energy surfaces for the electronic ground state of a system in which a molecule approaches a surface and is chemisorbed. An attractive potential energy well always exists for a physisorbed state and we therefore begin by examining the question of trapping into this state and energy accommodation in general. We then move on to a discussion of the evidence for, and the properties of, precursor states before considering specific rate laws in adsorption kinetics. The application of these rate laws is exemplified by the adsorption of N2 on W{lOO}, which has been most extensively studied. During Referencesp p . 163-1 79

58

the discussion, we consider also the electronic processes involved in chemisorption and include the relevance of such phenomena as exoelectron emission and chemiluminescence. 3.2.1 Energy accommodation and trapping Where only a physisorption potential energy well exists at a surface for an incident particle, such as helium, three processes can be distinguished: (i) The particle may be elastically scattered back into the gas phase, i.e. without energy loss. Such a collision may lead to diffraction. (ii) During a single collision, the particle may lose kinetic energy and be inelastically scattered. (iii) The particle may lose sufficient kinetic energy to be trapped in the potential energy well with a lifetime T determined by the well depth 4 , i.e. 7 = T~ exp(q/RT) where T~ = 1/v and v is a frequency factor. It is subsequently inelastically scattered into the gas phase. Transfer of kinetic energy during an inelastic collision must take place t o excitations of the combined system, solid plus adsorbate. These excitations are vibrational modes, of which phonons are the low lying ones, electronic modes, two-dimensional translation, and, for polyatomic incident molecules, rotational modes. Each of these energy-loss processes is considered in turn. The loss of kinetic energy by excitation of lattice phonons has received considerable theoretical attention. In the model of McCarroll and Ehrlich [ 3741 , the lattice is simulated as a semi-infinite chain of particles connected via a characteristic force constant and the gassolid interaction is described using a modified harmonic potential. In all these models, energy transfer increases with an increase in (a) the mass ratio p , (incident particle mass)/(substrate atoms mass), and ( b ) the depth of the adsorption potential energy well. Electronic excitations have only received theoretical treatment in recent years [ 375-3781. Kinetic energy is lost through the production of “electron+ole pairs” in the substrate. For metal substrates, the electron-hole pair excitation energies are continuous from zero upwards and, even for rare gases, this may be an important loss mechanism. For insulators, on the other hand, there is a threshold for electron-hole pairs, with excitation modes below it. (Interestingly, for insulators and semiconductors, sticking probabilities are generally considerably lower than for metals, as discussed in Chap. 2 of this volume.) Brako and Newns [377] have used an electron-hole pair model t o determine the trapping probability as a function of incoming particle velocity, reporting a limiting value for low velocity particles. The implication is that, at low gas temperatures, the process may be dominated by losses other than electron-hole pairs. Boato et al. [3791 have discussed evidence, based on molecular beam studies of H2 on LiF surfaces, for the transfer of

59

kinetic energy to rotational and translational modes of the combined system. Thorman and Bernasek [ 3821 have recently developed a method to measure the vibrational and rotational temperatures of a desorbing gas, in this instance N, from a polycrystalline iron surface. Their method basically consists of exciting the desorbing gas molecules with an electron beam and monitoring the subsequent fluorescence to yield the internal energy distribution. Their results indicated that there was a large steric factor controlling nitrogen atom recombination and that when sulphur was present on the surface it caused the formation of a large energy barrier perpendicular to the surface. There are no gas-metal systems for which the dominant loss mechanism has been determined. However, it can be anticipated that developments in angle-resolved inelastic atom beam scattering experiments, exemplified by the recent work of Feuerbacher and Allison [380] with scattering from LiF(100}, will make good this deficiency. In cases where single surface phonons are responsible for the inelasticity in He scattering, timeof-flight measurements with the detector scanned away from the molecular beam enable the dispersion curves for surface phonons to be constructed. Two experimental parameters relating to energy transfer have been quite widely measured, the accommodation coefficient, ac, and the trapping probability a. In the former, all loss processes are integrated; it is defined by E-EE, ac = ES - 4 where E is the average energy of molecules re-emitted from the surface at an effective temperature TL, E g is the average energy of molecules in the incident gas (temperature T g ) ,and E, is the average energy of molecules emitted with the temperature of the solid, T,. For a monatomic gas, the accommodation coefficient is then simply given by

Experimentally, ac may be determined using a metal filament by measuring the incremental electrical energy that has to be applied to the filament to maintain a constant temperature when gas is introduced to the experimental chamber [381]. The trapping probability, a, is the probability of adsorption into a weakly held state on the surface and is measured under conditions where the adsorbate lifetime is small compared with the experimental time period. It is most readily measured by molecular beam techniques; a typical experimental arrangement is depicted in Fig. 23.Depending on the sophistication of the equipment, the parameters which may be measured include the velocity distribution of incident and scattered particles and References p p . 163-1 79

60

zr$z I -to

mect-anical pump

port

15cm

Fig. 23. Typical molecular beam system as used by Somorjai and co-workers [ 3531.

the intensity distribution of scattered particles at variable incidence angles. High fluxes can be achieved using bundles of fine capillaries possessing a large length-to-radius ratio [383, 3841. Wharton and coworkers [ 3851 have measured velocity and angular distributions for Ar atoms scattered from polycrystalline tungsten for supersonic incident beam energies over the range 300-2000K and surface temperatures between 350 and 1900K. They find that direct inelastic scattering involving a single encounter of the gas atom with the surface is the most important process; no elastic or quasi-elastic scattering was observed. At the lowest surface temperatures, trapping-desorption scattering was also observed. Direct inelastic scattering for an incident angle of 45' could be characterised, for all energies and surface temperatures, by the simple expression (KE)e = 0.83(KE)i + 0.20(KETs) (23) where (KE)d, (KE)i and (KE,,) are the kinetic energy of the scattered Ar, the incident Ar, and Ar in equilibrium at the surface temperature, respectively. This work has clearly established the importance of the direct inelastic scattering channel, involving the loss of insufficient incident particle kinetic energy for trapping to occur. However, it is probably difficult to distinguish experimentally between trappingdesorption and inelastic scattering at high surface temperatures, when the lifetime of the trapped species approaches the time of a single collision.

61

Merrill and Weinberg [ 3861 measured the intensity distribution of scattered particles in molecular beam experiments and distinguished between lobular and diffuse (cosine law) scattering. They estimated the fraction of trapped species to be given by the fraction of diffusely scattered particles. For room temperature Ar atoms incident on W ( 1 0 0 ) at 370 K, they report a = 0.45 using this criterion. Wharton and co-workers [ 3851 , analysing velocity distributions of scattered particles, find a 0.43 for low incident Ar atom kinetic energies (analogous to an effusive source temperature of 150 K) on a polycrystalline tungsten ribbon at 393 K and suggest that Merrill and Weinberg overestimate a ( a decreases with increasing incident energy) since inelastic scattering and surface roughness also contribute t o diffuse scattering. Menzel [ 3871 interpreted isothermal energy accommodation experiments in terms of a model which assumes that trapping leads to full equilibration while inelastic scattering leads t o a fixed fractional energy loss t o the surface; for Ar atoms incident on polycrystalline tungsten, both at room temperature, a value for a of 0.20 is derived by this method. In Fig. 24 experimental values of ac and a values for rare gases on tungsten are plotted as a function of the adsorption heat. Both ac and a increase in proportion to the well depth and we also note that ac and a have similar values, at least for the range of experimental conditions used by Merrill and Weinberg [ 3861 . Impurity adatoms, such as H, N and 0, have a dramatic influence on measured accommodation coefficients; for example, a monolayer of oxygen adatoms on polycrystalline tungsten raises the ac for helium from 0.02 for a clean surface to 0.6. Roberts [388] and co-workers made use of this sensitivity to the presence of adsorbates t o determine sticking probabilities for reactive gases on tungsten. West and Somorjai [ 3891 have used the extent of He elastic scattering as a sensitive measure of surface cleanliness. From these experiments, therefore, we can conclude that the trapping probability is a sensitive function of (a) potential energy well depth, q , in the adsorbed state; (b) the mass ratio p of the incident particle t o the substrate atom; and (c) the kinetic energy of the incident particle (slower atoms having a higher trapping probability than fast atoms). A number of theoretical models have been developed to encompass these results, which we only briefly summarise here. They include (a) continuum theories [390] , in which the solid is treated as a continuum and perturbation theory is used to simulate the interaction; (b) classical lattice models [391]; and (c) quantum models, in which an attractive potential well is associated with the gassolid interaction [ 386, 3891 . Hard cube theories, developed by Logan and Keck [392] and by Stickney [ 3921 , have had some success in reproducing experimentally observed dependencies. A detailed discussion of these theories is given in a recent review by Goodman and Wachman [420]. References p p . 163-1 79

62 He Ne

0.e

.

0

Kr

Ar

Xe

1

.

I

I

I

10

20

30

Heat of physisorption

/ kJ

mole-'

Fig. 24. Variations of accommodation coefficients and trapping probabilities for rare gases on polycrystalline W (open data points) and W(110) (filled data points). Target temperatures: A, 375 K; 0 , 575 K;0 , 7 7 5 K;and 0,1300 K. (From ref. 386.)

3.2.2 Precursor states in reactive g a s s o l i d interactions Even when the result of a gassolid collision is the formation of a stable chemisorbed species, weakly bound precursor states can play a major role in the kinetic process. Evidence for such precursor states has recently been reviewed by Cassuto and King, [21]who draw a distinction between intrinsic precursor states, which exist at empty surface sites, and extrinsic precursor states, which exist over sites filled with chemisorbed species. The ability of colliding species to be trapped in these states and to be efficiently transported across the surface is an important mechanistic feature in adsorption. A confusion in nomenclature can arise when a metastable, or "virgin", chemisorbed state can be formed on the surface as an intermediate between physisorbed and stable chemisorbed states: for example, at low temperatures, a virgin, nondissociatively chemisorbed state of CO is formed on tungsten which can be converted t o a dissociatively bound state on heating [ 1021.In the few cases that have been investigated,

63

however, it has been found that these states are virtually immobile over their temperature range of stability [396]. In kinetic formulations involving precursor states, an essential feature is that they are transitory, since the assumption is made that the steady state population is negligibly small. Clearly, at temperatures where a virgin state is frozen in, this does not apply; it is then the stable chemisorbed state. At higher temperatures, however, these states may qualify as precursors. Extrinsic precursor states, as defined above, have been observed and characterised in low temperature shadowed field emitter tip experiments [9]. Diffusion from a low temperature (20K) multilayer deposit of adsorbates such as H,, N2, 0, and CO on tungsten proceeds, on raising the temperature, with a sharp boundary resulting from efficient transfer to the chemisorbed state as second layer, or precursor, species reach empty sites. At higher temperatures, the boundary advances a fixed distance ( Y ~ ) ~and / ~then moves no further, independent of the amount of adsorbate in the original deposit. Following Gomer [9], this distance is approximately related to the desorption energy E i from the precursor and its diffusion energy E L by the expression

E i - E A = RTln ( ( ~ ~ ) ' / ~ / a ) (24) where a is the root mean square jump distance. The average number of hops made before desorption is simply ( x 2 > / a 2 For . example, for O2 on W, E A 4 kJmole-' and E i N 12 kJ mole-' [396] ; thus, at 300 K, where the precursor lifetime is only -lO-'Os, the extrinsic precursor is capable of making 600 hops over filled sites. Recent low temperature (-30K) UPS studies of 0,on A l { l l l } [297] show clear evidence for an extrinsic precursor. The striking kinetic consequence of the mobile extrinsic precursor adsorption rates which may be effectively coverage-independent over a wide range of coverage - in fact constituted the first experimental evidence for its existence [2, 101. It is not, however, the only evidence, as has recently been suggested [ 2971 . Experimental evidence for the existence of intrinsic precursor states is rather more difficult to come by. The common observation that the initial sticking probability, so, often decreases with increaing substrate temperature is consistent with the existence of such a state, as discussed here. Indirect evidence is also provided by molecular beam studies, for example, Hayward and Walters [401] (for H2 on W{OOl}) and Engel [402] (for 0, on Pd{lll}) have observed scattered particle intensity distributions which, even at a fractional coverage in the chemisorbed layer close to zero, exhibit a strong directional lobe in the specular direction superimposed on a cosine law distribution. The specular lobe clearly contains molecules scattered at the first collision, while the cosine law component is most readily attributed to the particles which are trapped in the precursor state and then scattered back into the gas phase. Of

-

Reference8 p p . 163-1 79

64

course, a further fraction of the incident particles are transferred to the chemisorbed state; this may occur by direct transfer into the chemisorbed state, without trapping, or after trapping. For H, on W{lOO} at 300 K, dayward and Walters [401] estimate that, at zero coverage, 3% of the incident particles are reflected at the first collision, 16%are trapped and then re-emitted with a cosine distribution, and 80% pass into the chemisorbed state. The existence of a mobile intrinsic precursor state can also be inferred from the variation of the zero-coverage sticking probability, so, with stereographic angle across the [ 1001 zone for nitrogen on tungsten single crystal planes reported by King and co-workers [ 471. The (110) plane is relatively inactive (so < lo-,), but planes with {110}terraces and “(lOO}” steps were found to have very high sticking probabilities (e.g. on W{320}, about 80% of incident particles will strike these terraces) are efficiently trapped and transported to the step sites. Finally, in systems where trapping into the precursor state is efficient, relative to the process of direct transfer on impact into the chemisorbed state, it would be possible to freeze the oncoming molecule into the precursor state by cooling to very low temperatures ( 5 30 K); spectroscopic techniques could then be employed to examine the state of the precursor and its conversion on heating. In early kinetic models, it was assumed that all molecules incident at the surface are trapped into a precursor state. However, the loss of sufficient excess kinetic energy of the incident particle to result in trapping at the first collision is not necessarily efficient and values lower than unity may be anticipated. Since we wish to reserve the term sticking probability for the probability that an incident particle is finally chemisorbed, the probability of capture into precursor states is described as the trapping probability, a. Values of a are difficult to estimate from experiments with reactive gases, although the velocity distribution has been measured for unreactive gases [32] and, more recently, for nitrogen on polycrystalline tungsten [ 4031 . In molecular beam experiments with rare gases on tungsten, the total scattering intensity could be represented as a superimposition of directional scattering (the quasi-specular portion) and diffuse scattering (the trapped portion), allowing for an estimate of a , As discussed in Sect. 3.2.1, both a and the thermal accommodation coefficient rise monotonically with increasing heat of physisorption, from -0.02 for He to -0.04 for Kr and -0.7 for Xe. By inference, it might be expected that values for reactive gases would cover the same range, as indicated in ref. 429. It is difficult at this time t o obtain reliable theoretical estimates for trapping probabilities. 3.2.3 Models for adsorption kinetics The simplest kinetic model for adsorption is that proposed by Langmuir [422], in which it is assumed that (i) the surface is homogeneous; (ii) every

65

adsorbed species occupies identical sites to every other species at all coverages; (iii) there are no interactions between adsorbed species, other than preclusion of occupation of a site by more than one adsorbed species; and (iv) there is no mechanism for the transport of impinging gas molecules across the surface. The adsorption rate is then simply the product of the impingement rate at the surface, an activation energy term, and a term representing the probability that a site or array of sites is empty, i.e

r,

=

ZP exp (- E , / R T ) f ( 0 )

(25)

where Z is the Knudsen collision factor ( 2 1 ~ r n k T ) - ”For ~ . non-dissociative adsorption, f ( 6 ) = 1 - 8 ; for dissociative adsorption of a diatomic mol-e) ecule f ( 0 ) is (1- f3)2 if the adsorbed layerismobileandZ(1- e)2/(2 if the layer is immobile, where Z is the number of nearest neighbour sites on the surface. Many other cases have been statistically solved; for example, for nondissociative adsorption of a molecule which prevents adsorption at nearest neighbour sites

f(e) = 1 - 3 6 =

+:e2

3 [I - 6 )

+ $ e 3

-q1- el3

+ 5(1 - e ) 4 1

o < e < 0.5 0.5 < e < 1

(26) (27)

Expressed in terms of the sticking probability, the Langmuir expression is simply s = s,f(e)

(28)

and we note that very few adsorption systems follow Langmuir adsorption kinetics (Sect. 3.1.2). The most serious deficiency of this model is its failure to describe the initial virtual independence of s on 8,observed for a large number of systems, particularly at low temperatures. Taylor and Langmuir [2] recognised the importance of introducing a second weakly held adsorbed layer, with a short lifetime, in order to account for their observation that the sticking probability for Cs on W is close to unity, even at coverages approaching saturation in the chemisorbed layer. Kinetic models including a mobile precursor state were derived by Morrison and Roberts [423], Becker and Hartman [424] Ehrlich [425] and Kisliuk [426]. Two distinct approaches have been used to model precursor state kinetics. (1) A successive site statistical model, introduced by Kisliuk [426] for adsorption and adapted by King [298] for desorption. (2) The chemical reaction kinetics approach, involving rate coefficients and the stationary state approximation, followed by Becker and Hartman [ 4241 , Ehrlich [425] and recently developed by Gorte and Schmidt [297] and Cassuto and King [421]. It has recently been shown by Schonhammer [427] and Cassuto and King [421] that the two approaches produce the same kinetic expressions. Variants of these models have References p p . 163-1 79

66

been produced by Kohrt and Gomer [214], Kieffer and Bootsma [428], Lopez-Sancho and de Segovia [373] and King and co-workers [45-481, the latter introducing the influence of lateral interactions between adsorbed species through order-disorder theory. The most general expressions are those recently derived by Cassuto and King [421] for both adsorption and desorption kinetics, incorporating the influence of lateral interactions in the adlayer and the precursor state. We follow their derivation here. Representing chemisorbed species as A,, the intrinsic precursor as A*(A;), the extrinsic precursor as A’(A;) and the gas phase species as A,(A2,) we have (a) for nondissociative adsorption adsorption

desorption

(b) for dissociative adsorption adsorption

67

desorption

Here, k,, kd and k , are the rate coefficients for adsorption, desorption, and migration from the intrinsic precursor state, k k and k i are the rate coefficients for migration and desorption from the extrinsic precursor state, kD is the rate coefficient for transfer from the chemisorbed state to the intrinsic precursor state, and a* and a' are the trapping probabilities for molecules incident at intrinsic and extrinsic precursor sites, respectively. Direct transfer from gas phase to chemisorbed state or vice versa is included through the probability, s, for adsorption and the rate coefficient, k c , for desorption [427]. In order to generalise the rate expressions, we now introduce a group of terms F(8) which are only functions of the surface coverage 8. For a particular case, such as nondissociative adsorption, these terms may be evaluated and inserted into the appropriate rate expression. F, is the occupation probability that a site (or sites) exists in a configuration which can lead to desorption (= 8 for nondissociative adsorption). F, is the probability that an intrinsic precursor, in hopping, moves to a site configuration where an extrinsic precursor state can exist (= 8 for nondissociative adsorption). FL is the probability that an extrinsic precursor, in hopping, moves to a site configuration where an intrinsic precursor state may exist [ = (1- 8) for non-dissociative adsorption] . Fa is the probability that an intrinsic precursor state is at a site configuration where chemisorption can occur (= 1 for non-dissociative adsorption). F* is the probability that a collision takes place at an intrinsic precursor site [= (1- 8 ) for nondissociative adsorption]. F' is the probability that a collision takes place at an extrinsic precursor site (= 8 for nondissociative adsorption). It follows from these definitions that F, + F d = 1 ; F * +F' = 1;FL = F*;F,

= F'

(29)

These last two relations arise from the fact that the probability of finding References p p . 163-1 79

68

an intrinsic or extrinsic precursor position does not depend on the origin of the atom or molecule (gas phase or surface). Applying the stationary state approximation for both desorption and adsorption to the kinetic schemes (Rl-R4) with the assumption that A* and A’ (or A: and A;) coverages are negligibly small yields

+ kaFa + k,F,)[A* or A*,] + k k F ’ [ A ’ o r A;] [A* or A*,] - (k& + k k F k ) [ A ’ o r A;] = 0

k D F D- (kd k,F,

= 0

(30)

and (kd (k&

+ kaFa + k,F,)[A* + k k F k ) [ A ‘ o r A;]

or A*,] - k k F ’ [ A ’ o r A;] = a * Z F * -k,F, [A* or A*,] = a’ZF’

(31) where Z [ = p/(21rrnkT)1’2] is the collision rate per unit area. Since the rate of desorption, rd, and the rate of adsorption, ra, can be written as rd = kd[A* or A*,]

+ k&[A’or A;] + k,FD

(32)

and ra = kaFa[A* or A*,]

+ scZF*Fa

(33)

the general rate expressions are obtained as

ra =

g k a F a [a*F*

haFa

+ kd

+ {a’F’Fkk k /(k’ + k k F k ) } ] + scgF*Fa {k, F, k&/(hh 4-k k FL )}

Using the relationships (29), these are further simplified to

and

Under equilibrium conditions, the kinetic schemes become k,

A,, ( o r 2 A , )

A’ ( o r

- kd

*.

.. (or ,

(34) (35)

69

Application of detailed balancing gives = kkF;[A’orA’,]

k,[A*orA*,,]F,

kd [A* or A*,] = a*gF*

kh [A’or A;] = a’gF’ from which, together with relations (29), we have

The general form of the equilibrium isotherm is readily obtained from the equalities FDkD= kaFa[A* or A;], kd [A* or A*,] = a*gF* as

In the simplest cases, where there is no interaction between adsorbed species, eqn. (40) reduces to the Langmuir isotherm form, for dissociative or non-dissociative adsorption; with pairwise interactions between adsorbed species, eqn. (40) is equivalent to the Fowler-Guggenheim [320] isotherm. From the equilibrium kinetic scheme, it can also readily be shown that

The relationship (39) allows us to further simplify the general rate expression, far from equilibrium, as

and 0, =

+ +

+ +

kd}] gkaFaF*a*[1 {kmFm/((l- F,)a*k,/a’) t s,gF*Fa (43) kaFa kd [ 1 {k, F,/((l - F,)a*km/a’> kd}]

+

Thus, apart from the term a‘,the rate expressions can be reduced to a form which only contains rate coefficients referred to the intrinsic precursor state. If transfer from the gas phase to the precursor state is non-activated the energy diagram in Fig. 2 is applicable. Writing rate coefficients k as the product v exp (- E/RT) and noting from Fig. 2 that Em-Ed

= EL-8;

we have, from eq. (39) References p p . 163-1 79

(44)

70

Thus, only if the frequency factor ratio was unity would we have a* = a'. In the models of Kisliuk [426] and King and Wells [46], this assumption is implicitly made (Kisliuk originally assuming that both trapping probabilities were unity). The physical significance of this assumption is that the trapping probability, and hence energy transfer, are the same for molecules incident on bare sites and on sites occupied by chemisorbed species. While this may appear to be unrealistic on the basis of a phonon energy transfer mechanism, since energy transfer would be substantially increased if the mass ratio of the incident particle to adsorbent atom were increased, there are several examples in the literature (see Table 1) where the sticking probability is less than unity but accurately independent of coverage over a wide range of coverage at low substrate temperatures; this result requires that a* = a'. (On the other hand, there are several recent examples in the literature where s has been found to increase with coverage at sufficiently low coverages and temperatures [ 3221 .) Making the approximation and also assuming, following Kisliuk [426] and King [46, 2981, that ac = 0 (i.e. ignoring direct transfer), eqn. (39) is transformed to

Further simplications can be achieved by using normalised rate coefficients, which are similar to the probability terms used in Kisliuk's statistical method of kinetic analysis, viz.

P, = km/Zk Pa = k$Zk

PL = k k / Z k '

(47)

+ +

+

where ZP = ZP' = 1, Zk = k, kd k, and Zk' = k & k k . The advantage of such a formulation is that the rates of adsorption and desorption can now be expressed in terms of only one of the precursor states, since we can write

P& = Pm/(pm+ Pd) and Pi = The result is rd

=

pd/(pd

k D ( 1 -pa)PdFd papmFa(1 - Fm) +pd [1 - p a ( l

+ P,) - Fa)]

(48)

(49)

71

These expressions give an exact identity with rate laws derived using the statistical approach by Kisliuk [426] and King [46, 2981. Lateral pairwise interactions between nearest neightbour (n.n.) occupied sites can be treated in the quasichemical approximation [45, 46, 641 by considering the equilibrium

2OA+00+AA (51) for which the energy change is o,the pairwise interaction energy. OA, 00 and AA represent n.n. pairs of sites, where 0 is an empty site and A is a filled site. Using material and energy balance at equilibrium in the overlayer for a fractional surface coverage, 0 , the approximation yields

eAA

=

eoo = eOA =

e - 2e (1 - e)/(o+ 1) i-e-2e(i-e)/(~+i) 4 e ( i - e ) / ( ~+ 1)

(52)

where D = [ 1 - 40(1 - O ) ( l - exp w/RT)] 1’2. for nondissociative adsorption is

The desorption energy

and for dissociative adsorption

where z is the number of n.n.6 to a given site in the surface mesh and

E: is the desorption energy when the n.n. occupancy approaches zero. ( a )Non-dissociative adsorption with single site occupancy If the gaseous species is adsorbed intact and occupies only a single site at the surface and if the adsorption rate into an empty site is independent of n.n. occupancy, the only term affected by lateral interactions is the desorption energy Ed [eqn. (53)] in the rate coefficient kD. We have Fa = 1 ; F * = l--B;andF, = which yields, from eqns. (13) and (14)

and

References p p . 163-1 79

FD

= 0,

(55)

72

(b ) D issoc ia tive adso rp t io n

For dissociative adsorption t o occur, it is assumed that a n.n. empty pair site configuration (00)is required, while the prerequisite for desorption is assumed to be a filled n.n. pair site configuration (AA). Although not explicitly stated, two different models have been used in the literature to describe the nature of the intrinsic precursor state; not surprisingly, these model assumptions lead to different rate expressions. Kisliuk [ 4261 and Gorte and Schmidt [297 ] assume that the intrinsic precursor occupies a single empty chemisorption site, while King and Wells [46] assume that is occupies an n.n. pair of empty chemisorption sites. (i) If the intrinsic precursor occupies a single site, the functions F, defined in Sect. 4, become FD =

I

eAA

I

F* = 1 - 8

(We note that F, is the probability than an intrinsic precursor, on a single empty site, is in a n.n. configuration to a second empty site.) Thus, from eqns. (49), (50) and (52), we have

(ii) If the intrinsic precursor occupies a pair of n.n. empty sites, the coverage-dependent functions, F, become

FD = ~ A Fa = 1 F,

=

F* =

A

i-eoo

coo-

(63)

73

These expressions transform the desorption and adsorption rate expressions to

and

In this model, the rates of adsorption, migration and desorption from the intrinsic precursor are ka[A*,], h , [A*,],and k,[A*,], respectively, and it follows that the normalised rate coefficients defined earlier are identical with the probabilities f,, f, and f d defined by King and Wells [46] and equation (65) is readily transformed into their rate expression for dissociative adsorption derived by the statistical method. The influence of the precursor and of lateral interactions between adatoms is demonstrated for dissociative adsorption on a symmetric surface in the theoretical curves of sticking probability as a function of fractional coverage shown in Fig. 25, taken from King and Wells [46]. Increasing the influence of the precursor state leads to a curve which tends to be initially coverage-invariant, while increasing the ratio of the repulsive n.n. interaction energy w t o T tends t o result in a termination of adsorption at half a monlayer. The latter results from the tendency of the overlayer to form a double-spaced structure at half a monolayer coverage; if completely ordered, i.e. if w / T is large, at 0 = 0.5 there would be no n.n. empty pair sites in the surface where dissociative adsorption could occur. King and co-workers [45-48] have examined the applicability of these kinetic models t o the adsorption of nitrogen on tungsten single crystal planes, The nitrogen-tungsten system is a good example of crystallographic anisotropy in adsorption kinetics at T = 300 K. Thus, so on the {110} plane is < and on the (320) plane is -0.74. Adams and Germer [430] proposed a simple model t o account for this. They suggested that the N adatom can only be adsorbed into the fourfold symmetric, fivefold coordinate site characteristic of the (100) plane, with occupation of alternate sites. Thus, the reactivity of any W crystal plane for N adsorption is expected to be proportional t o the number of (100) sites on that plane. All planes on the crystallographic triangle defined by the zones containing the {Ill}, (110) and {211} planes, being devoid of {loo} sites, should, therefore, be inactive. This model was particularly consistent with LEED and work function data for the planes on the (001) zone studied. Singh-Boparai et al. [47] studied the adsorption kinetics and surface coverages for N, on a variety of tungsten single-crystal planes. A References p p . 163-1 79

74

I

1

1

I

I

I

1.0.

I

1

I

-

-

0.8

4 o.60.40.2I

Fig. 25. Upper curves: computed sticking probability profiles for a fixed degree of short-range order in the overlayer (B = 0.99) and various values of the precursor state parameter K. Lower curves: computed sticking probability profiles for a fived value of K (= 0.05) and variable B, illustrating the effect of short-range order in the chemisorbed overlayer. (From King and Wells [ 461.)

collection of sticking probability data is shown in Fig. 26 and a comparison with the number density of [loo] sites on each plane indicated that the prediction of the Adams and Germer model is not fully confirmed. The {110}, {111} and {411} planes are clearly less reactive than the remaining planes studied; although the (411j plane contains [loo] sites, they are not in nearest-neighbour relationship to each other and the (110) and {Ill} are devoid of [loo]sites. Thus, the {loo},(310)and

75

\

\ \ \ I

8 Surface coverage/atom c m - '

Fig. 26. The variation of s with N for various tungsten single crystal planes under N 2 exposure. T,= Tg= 300 K. (From ref. 47.)

(320) planes, all containing [ 1001 sites in nearest-neighbour positions, are the most reactive, but the order of reactivity and saturation coverages for these planes is not as predicted. It was concluded [ 471 that the dissociation of the nitrogen molecule into the state occurs only at the [ l o o ] site pairs, but the chemisorbed atoms thus formed may subsequently migrate out on t o the [110] sites and terraces, thus populating other areas of the crystal with 0 adatoms. King and Wells [46] developed the Kisliuk model for dissociative adsorption t o take account of ordering in the chemisorbed overlayer resulting from the existence of lateral interactions between chemisorbed species in nearest-neighbour positions. s could then be expressed as

the terms being defined above. To extend the model to stepped planes, Singh-Boparai et al. [47] introduced a parameter {, which is the probability that a physisorbed species is in a trap in the vicinity of a [ l o o ] site pair, i.e. the ratio of [ 1001 sites t o total sites on the surface. This yields the general expression appropriate to N, adsorption on all W planes [47] References p p . 163-1 79

76

(

s = a 1+-

l+k--

(ce:o

I)]-’

Best fit parameters for the planes studied were obtained: with these parameters, eqn. (67) is a quantitative description of all the data over a wide range of substrate temperatures. The only significant variation is in the experimentally determined parameter a. It was concluded that the trapping probability is highest for planes with the highest surface density of W atoms. This parameter thus increases across the (100) zone, from [ 1001 to [ 1101, while the [ 1001 nearest-neighbour site pair density decreases in the same direction. These two factors together produce a maximum in so across the (001) zone. King and co-workers derived their models on the assumption that a is independent of substrate temperature but sensitive t o gas temperature. With this assumption, the fit t o the data for the {loo} [46] and stepped planes [47] was found to be good. These kinetic formulations have recently been brought into question by two independent sets of investigations. (i) In a recent elegant study of the velocity distribution of nitrogen back-scattered from a polycrystalline tungsten foil, Auerbach and co-workers [ 4031 were able t o distinguish direct inelastic scattering from trapping-desorption scattering and concluded that their data were consistent with a significant decrease in the precursor state trapping probability with increasing substrate temperature and inconsistent with both previous kinetic formulations [46, 3541. (ii) Top layer tungsten atoms on W(100) have been shown t o be unstable t o arbitrary lateral displacements, the displacement vector being determined by the surface condition [ 83, 841 . During nitrogen adsorption, for example, contracted domains are formed at a fraction coverage 0 = 0.4, with surface W atoms returned t o their bulk lattice positions; at higher coverages, Winters et al. [431] have produced evidence that N adatoms are incorporated into a reconstructed surface layer. Thus, the “checkerboard” surface model on which the order-disorder model of King and Wells [ 461 was based can no longer be considered to give an accurate description of the site availability on the surface. Alnot and King [432] have returned to the measurement of sticking probabilities. for N, on W {loo} in order to extend the earlier data base t o higher temperatures, as a test of the kinetic models in the light of the recent work described above, and to check for anomalous effects at 0 = 0.4 which could be attritmhd to the presence of contracted domains. In particular, whereas the model of King and Wells [46] predicts a high temperature limiting value of s at low surface coverages, the analysis of Auerbach and co-workers [403] predicts that s should approach zero at high temperatures. The molecular beam technique was extended in this study to the measurement of “reactive” sticking coefficients, s,, by In observing the rate of isotopic mixing from mixtures of 30N2and 28N2.

77

contrast t o the measurements of s, these measurements were made at high temperatures where the steady state coverage in the adsorbate is low, thus allowing an extension of the range of substrate temperatures which can be investigated. (In principle the method is similar t o that used for polycrystalline wires by Yates and Madey [433].) In the beam experiment, it is readily shown that for an equimolar mixture of iostopes

where PZ9 and P,, are the steady state pressures of 29N2and ’ON, in the adsorption cell during beaming on the crystal. As with the measurement of s, the absolute accuracy of this method is simply determined by the accuracy in measuring a pressure ratio; no calibration factors are involved. From the work by Alnot and King, it is apparent that there is no kinetic effect associated with the phenomena of surface reconstruction. It seems likely that dissociative chemisorption proceeds in the channels between islands, and N atoms then “sew” the islands together, finally removing the surface layer contraction at 8 = 0.5. Thus, despite contracted domains, the number of adsorption sites effectively remains constant as adsorption proceeds, which explains the success of the “checkerboard” model. The results are illustrated in Figs. 27 and 28. The authors then apply their results t o the King and Wells model [46] with considerable success. The data provide support for the suggestion that CY is virtually independent of T,. The Alnot and King study contradicts some of the conclusions reached by Auerbach and co-workers et al. [403] from their time-of-flight study of N, scattered from polycrystalline W. The following facts are conclusively demonstrated in the Auerbach investigation. (i) Unreacted N, is scattered into only two channels, a direct channel which is entirely inelastic and a trapping4esorption channel: no elastic peak is observed. (ii) At high beam and surface temperatures (> l o o O K ) , the average accommodation coefficient is 0.46, in agreement with an earlier study of Cassuto et al. [429]. The distinction between the direct inelastic and trapping-desorption channels is most clearly demonstrated for a beam temperature of 3400 K at T , = 400 K, where a lobular distribution is found for the former and a cosine distribution for the latter. However, Alnot and King [432] note several unresolved contradictions in the analysis of their data; in particular, their assumption that the trapping-desorption velocity distribution is Boltzmann cannot be justified. It can be seen that, for the N2-W system, the “trapping-precursor” model for adsorption is all-important. It should not be expected that this is so all the time. This is clearly shown by the work of Hayward and Walters [ 4011 who measured the angular distribution of hydrogen scattered from W ( 1 0 0 ) at 300K as a function of the amount of hydrogen References p p . 163-1 79

78

I"--7 10'' N Coverage /atoms cm-'

Fig. 27. The variation of s with surface coverage, N , when N2 is adsorned on W{lOO), for various substrate temperatures, T,. A comparison with the work of King and Wells [46] isalsoincluded.., T s = 3 0 0 K [ 4 6 ] ; . , T s = 4 3 3 K [ 4 6 ] ; A , T s = 3 0 0 K [ 4 3 2 ] ; 0 , T,= 320K [432];0, T s = 4 0 5 K [432].(FromAlnotandKing [432].)

chemisorbed on the surface. Of the molecules incident on the clean surface, 3% were elastically reflected, 16%were inelastically scattered and the remainder were chemisorbed. A polar plot of H, scattered from the clean surface is shown in Fig. 29, which illustrates the clear distinction between lobular and cosine law scattering. The elastic component was found to be a sensitive function of the hydrogen coverage, rising to 17% in the specular beam alone (i.e. excluding diffraction peaks) a t saturation. Two states of hydrogen on W{lOO}, denoted 0, and PI, have long been identified, the former being observed alone at low coverages. It was found that the elastic component only rose above 3% at coverages where the peak was first observed in the desorption spectrum, i.e. when portions of the overlayer are saturated with adatoms. The implication is that, either the trapping probability, 01, is smaller (- 0.82) on hydrogen-saturated areas of the surface than on clean areas (-0.97), or a proportion of the incident molecules which would have been elastically reflected in the absence of the chemisorbed potential energy will pass directly into the chemisorbed state on the clean or pzcovered surface. The latter is probably the correct explanation, particularly as the physisorption well depth for H, is small, so that one would anticipate

79

0

4 GO

800

1200

1600

C r y s t a l temperature, T, / K

Fig. 28. Variation of S O with the crystal temperature T, for nitrogen on W{lOO]. The beam temperature, T B , is also varied. 0, T B= 191 K;n,o, T B= 400 K;*, T B= 533 K; o,isotopic experiments; -, theory. (From Alnot and King [432 I.)

a very small value for a. In general, it is concluded that where stickmg probabilities greater than about 0.05 are observed for hydrogen adsorption, both so > a and so > ac and the process is dominated by a direct passage through the physisorption well, without trapping. [It may be argued that, even at saturation coverage, the proportion of molecules diffusely scattered in the experiment of Hayward and Walters [401] (-0.82) is much higher than anticipated for H,; however, it is well known that adatoms cause an appreciable increase in the accommodation coefficient.] The usual precursor state mechanisms are therefore not a good basis for the description of hydrogen adsorption on metals. The systems discussed above are, in many ways, “ideal” in that adsorption is very site-specific or limited to the surface layer. Many systems are known t o absorb as well as adsorb, This effect is sometimes reflected in sticking probability versus coverage profiles. These may show an increase in s because of a sudden freeing of surface sites due t o the absorption process. One example is O2 on Al(111) [434]. However, the adsorbed species may form at all coverages and the s versus N profiles look like those of a typical mobile precursor-trapping model. Fromm [435] has proposed a model t o fit this absorption-adsorption mechanism. His References p p . 163-1 79

80

O0

I

\

'

1

Surface normal

\

30"

/

/ I 0.05

0.00

u.u3 Molecular flux,

U.IU

u.13

1 dN

N dn

Fig. 22. A polar plot of hydrogen scattered from clean W{lOO} at an incidence angle of 34 . (From ref. 401.)

model is based on the assumption that, if there are large gradients of the chemical potential from the surface t o the bulk, then gas atoms should be able t o migrate freely until a certain point below the surface. This produces an s versus N profile which shows an initial plateau. After this, there will be an exponential decay of s correlated with an activation energy for the initial chemisorption; the position has been reached where the energy released by chemisorption can no longer compensate for the energy needed t o push atoms into relatively high-energy interstial sites. Equating free energy expressions for the processes, Fromm obtained

where A G$(eff) is the effective Gibbs activation energy for adsorption from the gas phase t o the chemisorbed state and AG: is the activation energy for diffusion. 8, as usual, represents relative coverage, and Oi the is equal t o AG$efo (the coverage value where s begins to fall. If AG!!(,,,,

81

effective activation energy between different sites in the metal), then it was found that S _ -

I."--

kT

SO

e -8,

8, being the coverage at the surface. This model was used t o fit the adsorption of 0, and N, on various metal films [435, 4361 and good agreement is claimed between the experimmental and theoretical work. The authors also note that the length of the plateau is a measure of the ratio of heat of solution to activation energy for diffusion. Thus, for a given metal, the length of plateaus for 0 and N adsorption/adsorption should be rationally related and the ratio of the two plateau lengths calculable. However, the model is not as broadly applicable as implied by Fromm, if it is applicable at all. In adsorption on metal films, the porous structure and the rough outer surface of the film play an important role in determining the shape of the sticking probability versus coverage profile [175]. In many instances, there is no possibility of absorption into the bulk, such as molecular N, adsorption on Ni films [ 5861, but the profiles still have the shape discussed by Fromm. For example, King and Tompkins [ 5841, in treating data for N, adsorption on molybdenum and titanium films, distinguish clearly between adsorption on the outer film surface, with a sticking pmbability s,, and adsorption by gaseous diffusion into the film pores, where surface adsorption occurs. The sticking probability on the outer surface is related to the value for an ideally smooth surface, sa, by the expression s,

=

Sa

(1- (1- S a ) > { l - (1/R)J where R is the outer surface roughness factor, The measured sticking probability, s, is a sum of s,, s, and a term relating t o adsorption into the porous microstructure, sp. For example, it is estimated that, for N, adsorption on molybdenum films at 3 0 0 K , the initial measured sticking probability of 0.75 is composed of s, = 0.70, sp = 0.15 (averaged over all collisions) and, with R = 3, s, = 0.44. For N, on titanium films, the corresponding values are s, = 0.46, sa = 0.22 a t 300 K. In a comparison of N, adsorption on nickel and palladium films, King [586] found that the zero coverage sticking probabilities were dependent on sintering temperature and arrived at smooth surface values of 0.56 and 0.67, respectively. Moreover, with nickel, the characteristic distinction between outer surface and porous structure adsorption was clearly noted in the sticking probability profiles, with the extent of porous structure adsorption decreasing dramatically with increasing sintering temperture. With palladium, however, this distinction could not be made, and this was References p p . 163-1 79

82

Fig. 30. Potential energy wells for activated and interstate conversion in adsorbed layers. (a) Shows that the stable state is reached via an intermediate “virgin state” but passage to it is non-activated. (b) Shows the case where passage from one well to the next is activated.

attributed t o rapid surface diffusion of nitrogen on palladium at 78K, providing an efficient means of adsorption into the pores.

3.2.4 Activated adsorption Here, it is necessary t o distinguish between true activated adsorption

83

(where the PE cross-over point lies above the PE zero, as defined by gas molecule and surface being an infinite distance apart) and inter-stake conversion where a metastable chemisorbed state (the “virgin” state) may be formed a t low temperatures and conversion t o the stable chemisorbed state occurs on warming. The latter may o r may not be true activated adsorption, depending, as illustrated in Fig. 30, on the energies at the two PE cross-over points. Contrary to the view expressed elsewhere [ 4371 , activated adsorption can be distinguished by the dependence of sticking probability on surface or gas temperature, depending on the mechanistic circumstances. Both types of information are useful. If adsorption is trapping-dominated, the T, dependence is given by the King and Wells [46] model where eXp ( - E d / R T ) / N , eXp (-Ea/RT)]-’

(70) and so increases with T, for activated adsorption (E, > E d ) . However, if adsorption proceeds by direct transfer through the precursor state, as concluded above for the case of H, on W{lOO}, a very small T, dependence might be anticipated. For trapping-dominated activated adsorption, higher gas temperatures lead t o lower trapping probabilities and hence lower so; for direct transfer, so will increase with T,. Early studies of H, on polycrystalline Cu [438] show that it is an activated process with an activation energy of roughly 20-40 kJ mole-’. Balooch et al. [358] have reported a molecular beam study of hydrogen adsorption on Cu{lOO), (110) and (310) in which the sticking probability is inferred from the reaction rate between H, and atomic D t o form HD at high crystal temperatures. The data show that the HD production rate increases significantly as the energy of the molecules increases and as the angle of incidence decreases towards the surface normal; it is concluded that there are substantial activation energies (12-20 kJ mole-’ ) for adsorption, the barriers depending on crystallographic orientation and acting essentially perpendicular t o the surfaces. Interestingly though, the H, sticking probability was found t o approach a non-unity limiting value (0.14 for Cu(llO), 0.10 for Cu{lOO) and (310)) at high impinging beam energy. A systematic study of the T, dependence for this system would be useful, but this work has established the H,/Cu system as a case of true activated adsorption which proceeds by direct transfer through the precursor state. If proving whether an adsorption process is activated or non-activated is difficult having only the temperature dependence of s, these difficulties have now been removed. There has been a recurrent interest in the angular dependence of the flux and energy distribution of molecules desorbing or scattered from surfaces. Methods for measuring the 8 dependence (8 is the polar angle measured to the surface normal) of the desorption flux, Nd(d), the average kinetic energy and the normalised speed ratio have been established. These methods are usually molecular beam scattering So

a[l

+vd

References p p . 163-1 79

84

experiments [358, 439, 4401 in which Nd(@ is measured at steady state, or permeation experiments [447-4491 in which Nd(@is again measured at steady state after permeation of the gas through the crystal. A method which, unlike the methods above, measures only truly desorbing particles has been developed [ 4501 . The measurements of N d ( 6 ) performed for heavier gases (N,, 0,) show a distribution which normally adheres t o a cosine law [451]. However, in permeation experiments with hydrogen [ 447-4491, strong deviations have been noted. The results are usually expressed as cosxO, where x is > 1. This enhancement of desorption flux along the surface normal is due to the presence of an activation energy for the adsorption process. Recently, Cosser [ 4521 has developed a theoretical model for the desorption from a surface through a Lennard-Jones potential field for both the activated and non-activated cases. The work by Cosser et al. [450] has clearly shown that the low sticking probability of N, on a perfect W {110} plane is an activated process and that chemisorption occurs at step and defect sites followed by diffusion and occupation of [ 1101 sites. They arrive at a value of 17.4 kJ mole-' for the activation energy for adsorption on the (110) terraces. If the s versus N profile for a mobile precursor is typically initially independent of coverage, then a typical profile for the case of activated adsorption follows an exponential decay and s can be written [273, 4531

s

= uf(0) exp

(-E/RT)

(71) where u is the condensation coefficient, E is the activation energy and f (0) as usual introduces the site-dependent coverage term. Activated adsorption processes are often investigated using transition state theory and rate laws and examples of this are common in the literature [4544561. Analyses of this kind lead to expressions which are qualitatively useful but which often contain a number of terms which are difficult to evaluate. 4. Desorption kinetics In this section, the details of thermal desorption from surfaces will be considered. The rate of the process can be represented in an ideal form by the Polanyi-Wigner equation

dN

-- =

dt

vNmexp( - E d / R T )

(72)

where N is the surface coverage, v is the frequency factor and m is the order of the desorption process. The usual routine of desorption experiments is t o adsorb gas on to the sample at a temperature a t which the

85 I 100

1

I

I

I

I

I

I

I SE/

v

I 5-’

48

96

14 4

-

n 200

I

I

I

220

240

260

I

1

280 300 t /m5

I

I

I

320

340

360

Fig. 31. The effect of pumping speed ( S E / V on ) the shape of a desorption trace. (After Ehrlich [ 51 ).

desorption rate is low and then ramp the temperature while monitoring the gas phase for desorbing products. Clearly, from eqn. (72), in such an experiment there are two time-dependent variables affecting the desorption rate; as the exponent increases, the surface coverage decreases and the resulting convolution gives a peaked form to the desorption trace (see Fig. 31). Section 4.1.1 describes the desorption characteristics of systems obeying eqn. (72). I t must be stressed that eqn. (72) represents an ideal desorption process, where both v and Ed are coverage-independent parameters. Unfortunately, very few systems behave in this ideal fashion: desorption is the reverse process of adsorption and, as has been described above for adsorption, several properties of the adlayer severely affect the kinetics of the basic desorption process. Thus, in the following sections, the effects on desorption kinetics of surface inhomogeneity, changes in desorption mechanism, precursor states and lateral interactions between adspecies, will be considered. The effects which these parameters have are considerable References p p . 163-1 79

86

and may result in the broadening or narrowing of the widths of desorption peaks, unexpected shifts in the peak temperature for different coverages and may even produce multiple peaks in the desorption spectrum. Finally in this section, a comprehensive compilation of desorption data which has appeared in the surface science literature over the last 10 years or so, is presented in tabular form; sample spectra for a variety of adsorption systems are also given as an overview of the desorption process. By the way of an introduction to desorption kinetics, a fuller, more theoretical discussion of the type of rate equation for thermal desorption presented above is given in the following subsection. 4.1 THEORY AND ANALYSIS OF DESORPTION SPECTRA

4 . 1 . 1 Theoretical aspects of thermal desorption

Theory has concentrated on trying to predict the more difficult parameter in the basic kinetic equation, i.e. the pre-exponential factor. Two major approaches have been used. ( i ) Collision theory. The older of the two approaches attempts to describe the collisional event between two reacting particles. The likelihood of collision is determined by a parameter known as the reaction cross-section. This quantity is invariant at all but the lowest collision energies where the time of a collision may be long enough t o allow some electronic interaction between particles. There will also be a threshold energy below which a reaction will not occur. Analytically, this is modelled by assuming the reactants are hard spheres, but an extra factor (a steric factor) has often to be introduced to account for pre-exponentials lower than the theories can predict; i.e. the direction of collision is important. This method has now largely been replaced, its major weaknesses being that it cannot predict values for the steric factor and that internal changes (other than rotational) cannot be included in the model. (ii) Transition state theory. This theory is based on the model of an activated complex being the intermediate state between reactants and products. For surface processes the following can be envisaged. PB Molecular adsorption

TB

TB

PB

i B

TB

AB

AB AE

M M M M M M

M M M M M M

M M 4!t

M M M

Molecular desorpt ion Transition sites

Dissociotive adsorption

A

B

B A

B

I I I I I I

A---B

M M M M M M

M M M M M M

A

A---0

A---E

I I I I I I

AB

AB

+B M

M

h

M M

Recombination

M

87

The simple form of the Polanyi-Wigner equation is based on the assumption that any particle possessing the requisite activation energy desorbs during the period of a single vibration. If the recombination mechanism is written as

A(a) + B(a)

2

(AB)C) -P A%

where the rate-determining step in desorption is the formation of the activated complex and, furthermore, assuming that this is in equilibrium with the adsorbed species, then

where the f values are molecular partition functions. Separating out zero point energy differences

If the total number of sites available for adsorption is N,, then [A] [B] = N,f (0) where f(0) is some function involving coverage. Thus

assuming that R d = K#[AB#], that is that the transmission coefficient to gas phase products from the activated state is unity. For the molecular desorption case

and so, in comparison with the Polanyi-Wigner equation pre-exponential

kT q# v(l) = - h qAB (77) kT q * 1 u ( z ) = - ___ h qAqBNs Such relationships indicate two types of behaviour. First, considering the non-dissociative adsorption: (a) if the activated complex and adsorbed species have the same degrees of freedom, then v kT/h, i.e. around 6 x 10l2 s-' at 300 K; (b) if the activated complex is less strongly bound than the adsorbate, then it may possess more degrees of freedom and so q#/qAB > 1. If the activated complex gains translational freedom, i.e. becomes delocalised, then this ratio could be as high as lo4 and so pre-exponentials as high as l O I 7 s-l could be expected.

i

-

-

References p p . 163-1 79

88

For the dissociatively adsorbed case, several possibilities exist: (a) if all particles are localised, then the partition function ratio is close to unity and pre-exponentials would be approximately kT/N, h or 6 x 1 O - j cm' s-' at 300 K; (b) if the activated complex is mobile, then pre-exponentials may be as high as 60; (c) if the adsorbates are mobile but the transition state is less so (due t o its strained configuration), then qP/qAqB< 1. For both cases, we note that lateral interactions in the adlayer will affect the mobility of the adsorbed species and so, particularly at high coverages, anomalous pre-exponential values may be anticipated. Also, and this factor would transmission coefficients may be as low as need to be included in eqn. (75), resulting in desorption at a slower rate than anticipated from the activation energy value. Alternatively, the rate equation can be written as [338, 3391

Rd = N,f(0)

kT - exp (- AG4/RT) h

(78)

where AG' is the Gibbs free energy change for the process, given by Ed - TA@. In this formulation, for first-order desorption, we now have

v

=

kT -exp h

(A@/R)

(79)

and the entropy term is implicit in the pre-exponential factor. The upper limit of AS' would correspond with desorption from a completely localised layer to a completely non-localised transition state and this is the translational entropy of a 2D gas. As;#,,, for such a system is given by Kemball [340] as

As#,,,

= Rln(AHTA)

+ 275

-

(80)

where A is the surface area per adsorbate, giving a figure of 130 J K-' mole-' at room temperature. Many desorption experiments have now shown v values which are well away from the kT/h value. The desorption of CO from Ru(00l) [341] gave v 1 O l s s-' . This prompted Ibach et al. [342] to investigate the CO/Ni{ 111) system and they pointed out that a value of l O I 3 s - l can only be expected if the adsorbate is mobile and has free rotation or has states of low excitation energy. The model they used to elucidate the kinetics was a modification of an earlier theoretical treatment derived by Landau and Lifshitz [343]. It is to be expected that the chemical potentials of gas phase and adsorbed species will be equal (pa = p g ) . From Fermi statistics, the probability of site occupation on the surface is

-

+

W = a0 = {exp [(ea - pa)/kT] I}-' where E , is the energy of the site. Thus

89

pa = E ,

+ kTln [ a 0 / ( 1- a d ) ]

and from Landau and Lifshitz pg = E ,

+ k T { ( h 2 / 2 n m k T ) 3 / 2 ( R T f rInf vP)}

(82)

where P is pressure, f, and f, are the complete rotational and vibrational partition functions and E , is the ground-state energy of the gas phase. For the atomic case, f, = fv = 1and, equating pa with p, P =

ae

(1- a0)[FzT(2nmkT)3/2h-3 1 exP [- (Eg

- Ea)/kTl

where E , - E , is simply the heat of adsorption; P/(2nmhT)'/2is the bombardment rate and if S(0, T ) are the fraction adsorbed into chemisorbed states, then at equilibrium

Rd = R , = S ( d , T ) P ( 2 ~ r n k T ) - ' / ~

(83)

Thus Rd

= (Ye(l-&!0)-'

kT 2nmkT h h2

s ( 6 , T)exP (-

E d / RT )

(84)

and so the pre-exponential factor is given by kT 2nmkT S(0, T ) h h2 Ns Thus, since the sticking probability term is present, precursor state kinetics can be introduced into the desorption. Furthermore, high values of v could be predicted and it is both coverage and temperature dependent. If f, is not unity, as for the molecular adsorption case, then provided hW > k T fr = (U/h2)kT

v =

a(1-aq-1 -

~

~

where I is the reduced mass and so

Using isothermal desorption of CO from Ni{ 111)Ibach et al. [ 3 4 2 ] found that v = l O I 7 s-l, and eqn. (86) indicated a value of 6 x 10l6s-' at low coverages, in good agreement with the experiment. Bauer et al. [ 3 4 4 ] have used a similar formalism, but included a transmission coefficient. Petermann [ 3451 purported to show that the temperature dependence of the transmission coefficient for the system H,/Ni{ 100) decreases from 0.1 to 0.02 between 690 and 8 3 0 K . Such temperature dependence is not completely surprising since it relates to the efficiency of energy transfer and Suhl et al. [ 3461 have shown that there is a change in the transmission coefficient near a paramagnetic to ferromagnetic transition. References p p . 163-1 79

90

Such discussions then, allow deviations of a few orders of magnitude s-’, but for even greater from the normal pre-exponential value of l o L 3 deviations, the basic postulate of the above argument appears t o break down, i.e. equilibrium between the reactants and transition state is not achieved. A rate coefficient defining energy transfer from adsorbent to adsorbate must then be introduced and Kramers [ 3471 has treated this case; if this process is rate-limiting, it causes the pre-exponential factor t o be drastically reduced. The model can be modified by the inclusion of an additional rate coefficient t o account for the relaxation of the surface population from a non-equilibrium t o an equilibrium state and so it is to be expected that v is strongly temperature-dependent [ 348, 3491 . The model has been successfully applied t o the desorption of neon from xenon a t very low temperatures for which v was found t o be lo5 s-’ [ 3501 .

-

4.1.2 Integral order desorption with coverage-independent parameters Even in the simplest desorption systems (those for which there is n o influence of lateral interactions or precursor states, for instance), multiple desorption states are generally observed. These can arise from states of different bonding geometry, and hence binding energy, on the surface. Unfortunately, the literature in this field has become littered with a variety of different symbolisms for different states which make comparisons between spectra confusing. The Greek letters y, a and p are commonly used, referring t o states with increasing heat of adsorption in that order; metastable states, which can be converted t o more strongly bound states by heating, are designated as “virgin” or v states. Even within these categories, there can be sub-states and these are differentiated by numerical subscripts, 0, and pz for instance, the higher number indicating the more strongly bound state. Such complications in desorption spectra clearly complicate their analysis; in order t o simulate a spectrum, each separate state thought t o be present has t o be assigned a theoretical shape; these can be then summed and adjusted for a good “fit”. I t has been shown [263] that the fitting peaks is relatively insensitive to the model chosen for the surface layer. It is assumed that the peaks in such spectra reflect exactly the equilibrium population in the adsorbed layer and that on heating there is n o mixing of population between states; this latter phenomenon can be termed “interstate conversion” and will be discussed later. Once again, in the simplest cases, v and Ed are assumed to be constant over the whole coverage and temperature regime. For a number of states, eqn. (72) can be written in a summed form, for i desorption states, as

The aim in the early 1960s was t o produce a method of treating desorption

91

spectra quickly and easily t o produce accurate determinations of these parameters and such work was pioneered by Ehrlich [5] and Redhead [6] whose methods are still commonly used. Redhead presents a number of methods of analysis of desorption peaks and these are given below. ( a ) Peak temperature analysis

This is the simplest and probably the least accurate method of determining the kinetic parameters involved in thermal desorption processes. T o minimise inaccuracies, the desorption peak should be measured using a linear heating ramp given by T = To Pt (88) where T is the temperature a t time t , T o is the starting temperature and 0 the heating rate (aTlat). Furthermore, the system should have a high and constant pumping speed t o avoid peak distortion. ( A means of experimentally evading this restriction has been presented by Kneringer and Netzer [ 2641 who perform two separate experiments, one where desorption is carried o u t with direct line of sight to a mass spectrometer and one without, the difference in pressures then being representative of the undistorted desorption from the front face of the sample.) In general, if the pressure at saturation of the surface is Pes,then L = KFP,,

(89)

The terms having been defined earlier. If re-adsorption during desorption is negligible, then ARd

+L

=

k S P + hV(dP/dt)

(90)

where A is the sample area. If we write P*

>P

-

Peq,then

(dP*/dt) -k P*/Y = aRd

(91) where a = A/kV and Y = V/S; Y is known as the characteristic pumping time of the system. Thus two extremes of experimental conditions can prevail. Firstly, if S is small, i.e. if Y > desorption time, then Rd a dP*ldt, but secondly, if the pumping speed is high, then Rd P* and so the desorption curve of pressure versus time does represent the desorption rate. For a linear heating rate, eqn. (72) can be represented in the form Q:

which can be differentiatied t o give

References p p . 163-1 79

[ p = aT/at; eqn. (SS)]

92

Since at T = T,, (dZN/dt2)= 0, then in general

In particular, for the first-order case

and so clearly, if rn = 1, the peak temperature is independent of initial coverage; for the second-order case, there is an inverse relationship with coverage, i.e. higher coverages give lower peak temperatures. The values of Ed can be calculated from the peak temperature above using eqn. (94) and assuming v = 1013s-l, of the order of molecular vibrational frequencies. However, much better determinations can be made by varying the heating rate, 0, and plotting ln(p/T;) versus 1/T, (for the first-order case) for the various peak temperatures obtained; as the heating rate increases, so also does the peak temperature. Such a method was originally proposed by Booth [ 2651, but it has been pointed out by Lord and Kittelburger [266] that, for accurate determination, P must be varied by a t least an order of magnitude and preferably by two orders of magnitude (though this is often not experimentally possible due t o sensitivity problems a t low heating rates and distortion and peakmerging effects at very high rates as discussed in detail by Chan et al. [ 2671 ). For other orders, the peak temperature is coverage-dependent and so Ed can be determined from the variation in T , with N o from a plot In (N," T,' ) versus 1/T,. ( b ) Line shape analysis

This method is useful for well-defined, non-overlapping peaks. Equation (72) can be integrated in the form

(96) N1

TI

The left-hand side = In (Nl/N,) for rn = 1

(97)

and = (l/N2)- ( l / N l )f o r m = 2

(98)

The right-hand side may be integrated by parts using a substitution u = Ed/RT and the mathematical identity

I U

-m

(- t , dt =

t

U

x H(u) for ( u ) % 1

(99)

93

+

where H(u) = 1 ( l / u ) the derivation yields

+ ( 2 / u Z ) t. . . Using only two terms in this series,

- T:

exP (-Ed/RTi

)I

For the first-order case and utilising eqns. (72), (97) and

(100)

(loo), then

and the desorption rate curve is asymmetric about T,. For the secondorder case

(102) and so, when T/T,

+

1,then

and the peak is symmetrical about T,. Despite its wide use, the Redhead model has inherent difficulties such as the heating rate problems mentioned above. An alternative analysis method has been proposed and is outlined below.

( c ) Line width analysis. Edwards [ 2681 performed an analysis based on desorption peak widths similar to a method developed by Schmidt [269]. The analysis showed that the type of heating program had very little effect on peak width. The first-order case has been treated similarly. At the half-peak maximum values

dN

1dN tP

Under these conditions, a solution of eqn. (72) is given by In [N(t)/No] =

(105)

where u = - &/RT and A ( T o )is a small constant close to zero. Thus N(tp) = No exp {-j(a)) References p p . 163-1 79

94

+

--

where Q = E d / R T p and j ( a )= 1 - ( 2 / a ) ( 6 / a 2 )- ( 2 4 / a 3 ) .For a linear heating rate [ eqn. (SS)] j ( Q ) = exp

(-3) -(YTp (1

2T'

+

6(T')2 ~ u T ' ) ~ )

1 n 2 + ~ - E-d 7 + j ( ~ ) . .. RT Using a power series technique where s = 01 s = so

- ( E / R T )and E =

1/a

+ SE + S 2 E 2 + . . .

(108)

If AW is the peak width, inserting eqn. (108) into eqn. ( 1 0 7 ) and requiring that eqn. ( 1 0 7 ) is true for arbitrary values of E , the solution yields a value for so, sl, s 2 . . . and these values can then be used to write AW TP

-

2.464RTp

il-

1.40RTp Ed

~

3.53RT, *

Ed

Ed

. .)

or, on inversion

This type of analysis is simple to carry out but suffers from the assumption that there is no static overlap and so, in overlapping curves, deconvolution has t o be used. 4.1.3 Systems with variable desorption energies

In many desorption experiments, the desorption activation energy depends on coverage and examples of the kinds of variation observed are shown in Fig. 32. Desorption of the type shown in curves D and E can also be produced by the presence of two states on the surface. There are several sources of desorption energy variation but the two main causes are (i) lateral interactions and (ii) surface inhomogeneity. The latter has been treated for the case of small uniform patches [270] ; if they are of equal surface area dA, then En

=

E:

(111)

- C,

where En is the value of Ed for the nth patch and C is a constant equal to dE,/dA. In terms of the general rate equation, for the first-order case N=Nt

N , (exp E- (Ed0 - c,)/RTI dA

-~Wt/dt = v N=O

=

[ ( v R T / C )exp (- Eg/RT)][exp ( C N , / R T )- 13

(112)

95

N

A

B

N

C

N

D

c

c N

E

N

F

N

G

N

H

N

Fig. 32. Experimentally observed variations of Ed with coverage. The letters below the examples are used in Table 2 to indicate the shape, if investigated, in the experiments.

and since exp ( C N , / R T )% 1,eqn. (112) simplifies to

- cWt/dt = b exp ( C N t / R T )

(113)

where b is the contents of the first bracket on the right-hand side of eqn. (112). Integration yields

(1 - N t )

=

RT -In

C

(t + to/bo)

where bo = R T / C b , and so values for E: and C can be evaluated from a plot of In (dN,/dt) versus reciprocal temperature giving (E: - C N , ) and, if contant temperature measurements are made, then a plot of (1- N , ) versus In ( t t o ) will give a value for C which can be back-substituted t o give E!. This type of treatment has been used to explain the applicability of the Elovich equation to many metal-gas systems [271] ; the equation can be written as

+

dlv, dt

=

a exp ( b N t )

where a and b are constants. Brunauer et al. [270] and Winter [272] have used this relationship for a linear activation energy dependence on coverage and for patches of varying Ed values. Winter showed that only three types of patches are enough to produce the right form for a uniform surface provided that the adsorption is activated [272]. It must be noted that lateral interactions are unlikely to produce variations as large as References p p . 163-1 79

96

those considered in the above treatments, being usually only of the order of 1 0 kJ mole-' or so. The effect of lateral interactions will be dealt with in Seot. 4.1.6, but here the modifications of the basic equations used for fitting variable desorption energies will be introduced. A linear variation of the energy is given by Ed

= E:

+

C8

(115)

where 8 is the fractional surface coverage and c is usually negative. Such variations in desorption energy can yield ambiguous results since Redhead [ 6 ] has shown that a good fit using the simple second-order analysis could be due to a first-order mechanism with a falling E d . Thus, for the firstorder desorption

Rd = v exp [- (E: - cO)/RT]' 8 (116) The slope between 8 and 8 2 (in an order plot of In R d versus In 8 ) can be written as 1 + [ C ( d , - e , ) / R T I ln(82/81) (117) The slope should be 1 for the first-order case, but if a value for c of 20 kJ mole-' is considered, then between 8 = 0.1 and 0.2, the value of the coverage-dependent term in eqn. (117) is about 0.1 and the order is apparently high. The order approaches an apparent value of 2 for a straight line between two points at 8 = 0.1 and 0.5 at a temperature of 1500 K. Variable Ed values and their coverage dependence can be determined graphically by making the usual isothermal order plot for a set of desorption spectra at different initial coverages and then taking rate values from that plot at constant 8 values for the various isotherms. Such treatments have been used extensively by Falconer and Madix [ 2741 . McCabe and Schmidt [275] have used a revised form of the Redhead equation, eqn. (116), including coverage variations of E d , viz.

If the coverage variations are significant, cO/RT % 1but c8 usually is small compared with E:, then, at least approximately

and so a plot of In (BT,,) versus reciprocal temperature gives a line of slope E:; in the second-order case, the ordinate plotted would be ln(8Ti). It was pointed out that, since changes in T , are usually fairly small, changes in In (T p )and In (Ti) are even smaller and so such plots are very insensitive and will invariably give straight lines even if the wrong order is assumed. A line width analysis by Chan et al. [ 2 6 7 ] revealed that, for the system CO/R {Ilo}, there was an apparent decrease in Ed with coverage.

97

Like the Edwards line shape analysis, the mathematics involved is fairly complex and for brevity is not repeated in detail here. They began by making the general desorption equation dimensionless, viz. d8 R ( Z ) = - = VPexp ( - E / Z )

dz where Z = T/T* and T* = l K , N* is the saturation coverage and E = E d / R T * . This equation then allowed the authors to calculate the relative coverage 8 and the desorption rate R at the peak maximum. These could be written

RM = drn)8; exp ( - E M )

The desorption rate could then be expanded using a Taylor series about 2, (peak temperature) and eventually the half-width could be written as

+

A U 112 = 22, [ EM (EM ZM)] 1’2 (122) The authors make the point after reaching this stage that expansion in terms of a Taylor series is only valid when 2 - ZM is small, something that is only true when desorption occurs over very small temperature ranges. This inherent weakness is emphasised by the authors using a numerical analysis to calculate parameters for CO/Ni (111) and CO/Pt (110). For the CO/Pt{llO) system, the authors found a 10%variation of Ed with coverage and this magnitude of variation has been shown to cause significant deviations from linearity in simple Arrhenius plots. The analysis was fairly successful in that it produced good agreement with previous data for a number of wide-ranging systems. But it emphasised that line width analyses, while yielding parameters from small amounts of experimental data not available via normal methods, are particularly prone t o errors arising from successive approximations. 4.1.4 Systems with variable pre-exponentials

It has been shown that variations in Ed with coverage can be used t o explain the non-linearity of kinetic plots. However, such deviations can also be explained by pre-exponential variations and, in most cases, a shift of Ed or v values, or more rarely, a change in both together, can be used t o fit experimental data successfully. A further problem with a simple analysis is that compensation effects between parameters can make the system appear simple, while give misleading values for kinetic terms. Most experiments assume a constant v, because of the difficulties of making accurate measurements of the pre-exponential. Experiments which have assumed a dependence of Ed on 8 can also be explained by applying a Referencesp p . 163-1 79

98

variation of the v term as shown by Pisani et al. [263]. Tamm and Schmidt [ 2761 have used a coverage-dependent pre-exponential fit t o their second-order desorption spectra for CO on W single crystals. The desorption was considered to occur by molecular desorption, rate-limited by recombination of diffusing atomic species. This model was derived formally in an earlier paper [ 2771 . The pre-exponential v2 is given by

where v is the vibrational frequency, a is the lattice unit diffusing distance and a, is the minimum separation of an atom pair during collision. Using a and a, as variable parameters, the desorption spectra could be fitted successfully. It is believed that v and Ed can be linked together by a compensation effect and Cassuto et al. [ 2781 have shown this intrinsically, They used the relationship V = Vo

exp [-Ed/RT]

to fit their data.

4.1.5 Desorption order In the discussion so far, orders of one or two have been considered. Surface processes with other orders (and fractional orders) have been found and changes in Arrhenius plot slopes can be caused by changes in desorption mechanism. Equation (94) shows the general form of the dependence of the kinetic parameters on the desorption order in terms of initial coverages. Another important relationship can be derived from the work of Falconer [ 2801 who showed that N i m - 1 ) = m-lNbm-l) (125) where N , is the coverage of material on the surface at the desorption peak maximum, and so, by substitution in eqn. (94), we have

As pointed out above, the desorption order markedly effects the shape of the desorption curve and the behaviour of the peak temperature with variation of initial coverage. Zero-order kinetics are shown by an increase in peak temperature with coverage and zero-order surface processes have now been observed for many systems [281-2861. Schwartz et al. [287] performed isothermal desorption measurements on the H2/Ti system and determined an order of 1.5, explaining this finding in terms of surface compound formation, with a stoichiometry of TiH1.,. A very good example of the confusion which can reign in this field is exhibited by the

99

system O2(Ag{llO}. Orders between zero and three have been postulated [ 2881 for the desorption process, although it is known that the adsorbate is in the atomic form. Bowker [289] has shown that the observed desorption effects can be explained in terms of the expected second-order process, but that the order term in the kinetic relationship is obscured by the effects of attractive lateral interactions between adatoms. (Such effects will be discussed in detail below.) The balance between two competing reactions of different order has been discussed by Klein and Yates [ 2901 in relation t o results which were obtained for the NO/W (110) combination. They consider two possible states of dissociated N atoms, N, being more strongly bound than Nx, there being an activation energy barrier between the two states. Thus kl

N X + O ' k z NYkand therefore

3N2W

where y is the concentration of the intermediate state. If the adsorption (N, + f N,) proceeds over a very low activation energy barrier 2dY-

dt

- k , x = 2ky2 - k 2 ( l - [O]

-X)Y

(128)

where [O] is the concentration of adsorbed oxygen. Assuming that y has a steady state coverage during most of the desorption process, i.e. (dyldt) C= 0, then at low coverages ( x >> y 2 and 1 % [ O ] x ) and

+

k 1%

Y = _k2

Thus

Hence, at low coverages, the kinetics will be essentially second order. A t higher coverages and ignoring the presence of oxygen

In general, for NO d"21 = dt 2k2(1- [O] - ~ ) ' [ 1 - { 1

+ 8klkx/k$(l- [O] - x ) ~ } ~ " ]+ 8klk2x 8k (132)

References p p . 163-1 79

100

It has been shown, in general, that zero-order desorption is likely from desorbing layers of surface compounds [ 2911, as has been seen for Xe on C(OOO1) [ 2 9 2 ] and for oxide films on W [ 2 9 3 ] ( W 0 2 oxide states are observed in the desorption spectra). Following the Venables and Bienfait analysis [ 2911, the coverage, N , is made up of atoms in a solid phase, S, in the first layer which covers a fraction, A , of the substrate and an adsorbed gas phase, 1, is in the first layer while 2 denotes second layer adsorbed gas species. If the atoms arrive from the vapour phase at a rate R (per unit area) and are accommodated and the layer evaporates at a rate R e , then

RdN = l - R e (R-R,)(l-A)+(R-R,)A (133) where R l and R 2 are the evaporation rates of phases 1 and 2 and N is the absolute coverage. The phase S only evaporates via phase 1 and/or phase 2 . Now R = P(2~rnkT)”~ (134) If the deposit is in equilibrium with the vapour, R = R e and the four phases are in equilibrium; thus, using chemical potentials

+ RTlnP

(135) where po is the standard chemical potential of the vapour. At equilibrium also 11s

= P1 = p2 = pv = po

Re = R1 = R2 = R (136) If R is reduced from its equilibrium value, the latter two relationships do not apply, but if interchange with the vapour is the slow step, then Ps = P l =

(137)

112

and

(138) Re = R , = R 2 Under these circumstances, the desorption order is zero and from eqns. ( 1 3 3 ) and ( 1 3 4 ) D -

D

where Ps is the equilibrium vapour pressure of solid phase S at temperature T . The zero-order form of eqn. ( 1 3 9 ) seems reasonable since compound desorption leaves an identical material behind and so the rate is independent of the surface density of material. Venables and Bienfait [ 2911 also showed that, under such circumstances, experiment should yield high values of v due to the presence of an extra entropy term (see Sect. 4.1.1) in the pre-exponential factor.

101

4.1.6 The effect o f precursor states

In Sect. 3.2.2, the effects of precursor states on adsorption kinetics have been discussed. Since even the earliest adsorption experiments show evidence for the influence of weakly bound intermediate states, following the principle of microscopic reversibility it might be expected that desorption kinetics would show the influence of such species. However, desorption experiments are usually carried out at considerably higher temperatures and average lifetimes in such states will be much lower. Shanabarger [294, 2951 was the first to use a model based on such effects, studying the systems H,/Fe (films) and H,/Ni (films). He proposed that hydrogen is dissociated on these surfaces and that the formation of a molecular precursor bottle-necks the desorption process. This results in isothermal desorption techniques measuring only the desorption energy of this precursor. This is obviously a misconception as the rate-limiting step in the process has to be desorption from the chemisorbed state and thus kinetics are measured principally from this state, although values may be affected by the precursor to a small extent. The first work to examine precursor effects in a quantitative manner was performed only recently by King [298]. It is based on the reversal of the kinetic model described in Sect. 3.2.2. The rate of transfer from the chemisorbed state to the precursor is given by

ri = Vee-EIRT (140) Because of the reasons discussed earlier, it is necessary to write that

rd = Fri

(141) F being a fraction since not all the molecules reaching the precursor desorb. By describing the possible events for a molecule in terms of probabilities, F was found to be given by

and so the effect of F on desorption spectra could be examined. This was done by computer simulation and showed that, for many chemisorption systems, precursor states have little effect on desorption kinetics; an example is shown in Fig. 33 for illustration. These systems are those for which initial sticking probabilities are low near the desorption temperature or fall off rapidly with coverage. Many other systems, however, exhibit significant mobility of the precursor state when desorption from the chemisorbed state is occurring. These systems are revealed in adsorption experiments by So being independent of coverage at low coverages. These systems were shown to produce broad desorption spectra shifted to higher temperatures than might be expected. Prompted by King’s work, Gorte and Schmidt [297] prepared a model, References p p . 163-1 79

102

o.oe

A"

c

vl

k

-2

0.06

0 C

?i

b

% 0.04 D I

0.02

0 1

Temp ./K

Fig. 33. A series of spectra, computer simulated from eqn. (143), illustratin the influence of the precursor state on spectra. All curves correspond to v = 1 0 ' s - I , f d = 38.3 kJmole-' and einitial = 7 , with heating rate 1 0 Ks-I. ( 1 ) F = 1; (2) K = 0 * 5 , F,=10/12, F d = 1 / 2 , S o / a = 1 ; ( 3 ) K = F d = 0 . 1 , F , = l , so/CU=l; (4) K = 0.01, Fd = l o + , F, = 1, so/& = 1. (After King [298] .)

f

also invoking a precursor as an important step in the desorption process. They adapted an approach based on rate laws. Writing

-

kd

;k

A*(a)-

A(€!)

(144)

and assuming a steady state in the precursor state A*(a) Rd

=

k*k,0 k* -t ka(l - 0 )

If k* 3 k,, then the kinetics are first order, but if k, % k*, then

(145)

103

and higher orders and more complicated desorption spectra are to be expected. The authors continued the development t o examine dissociative adsorption and differentiated between precursors existing over filled and over empty sites. These two precursors have now been assigned the names of extrinsic (filled) and intrinsic (empty) [421]. A new rate law mechanism can now be drawn up, viz.

A2

It is then possible to show that the rate of desorption is given by

so that the “apparent order” observed in experiments is dependent on the relative magnitudes of the rate coefficients. This is exactly the same conclusion that had been reached by King earlier. In many ways, the recent paper by Cassuto and King [421] is the definitive paper of its type. The model presented there includes all effects that may complicate “ideal” desorption spectra. From the reaction scheme (R4) drawn up earlier (p. 67) and applying detailed balancing, it is possible to write

Thus, apart from the term a’,this is now in a form which contains rate coefficient referred t o the intrinsic precursor. As discussed earlier in Sect. 3.2.3, a* = a’ and a, = 0 (i.e. ignoring direct transfer) so that

References p p . 163-1 79

104

kkk&

(149)

and, using normalised rate coefficients, rd can now be written

The Ps represent normalised rate coefficients. If the intrinsic precursor occupies a single site, for dissociative adsorption and no lateral interactions, the function F simply become Fd = 0 2 , F, = 6 , and Fa = (1- 6)z as shown previously (Sect. 3.2.3).Thus

which is a simplified version of the expression derived by Gorte and Schmidt [eqn. (147)] [297], The work by Cassuto and King is readily adapted to any adsorption system, but it must be noted that in order to obtain reasonable expressions, it is always necessary t o make a number of approximations using a detailed knowledge of the adsorption system under consideration. 4.1.7 The influence of lateral interactions

It has been shown that lateral interactions, like precursor states, play a fundamental role in determining adsorption kinetics (see Sect. 3.2.3) and they are of similar importance in thermal desorption measurements. This has been studied theoretically [ 300-3021. The existence of multiple peaks in desorption spectra may, in some cases, be explained by the presence of lateral interactions between adspecies, as first described by Toya [315] in discussing the two peaks seen for hydrogen desorption from W(100) (previously considered to be due t o two distinct binding sites). Goymour and King [ 451 used the quasi-chemical approximation t o obtain a good quantitative description of the effect of lateral interactions on second-order thermal desorption and applied their model t o the desorption of CO from W. Typical desorption spectra for this system are shown in Fig. 34. (A different model has been applied to the problem by Adams [ 3141 .) Strong nearest-neighbour repulsive interactions dominate the spectra and result in the appearance of the low-temperature peak, well separated from the higher-temperature, low-coverage desorption peak. The presence of repulsive interactions between adatoms lowers the heat of adsorption as the coverage increases and for recombinative desorption, the pre-exponential factor takes on a complicated coverage dependence, obscuring the meaning of desorption order. Goymour and King [ 451 have given the desorption rate in that case as

105 I

22 -

I

lA

20-

Y

-3 -0

>E s a

18-

161412-

L

0

2

10-

U r

0

8-

0,

c

6? .r

yip

4-

2-

Fig. 34. 0-CO desorption spectra from polycrystalline tungsten. The spectra shown are best fits t o the experimental data using computer simulation of eqn. (152). Because of the presence of two states, the coverage term, 8, was replaced by (8, OB). For the spectra shown, the values of (8, 8,) are A, 0.1; B, 0.2;C,0.3; D,0.48; E,0.68; F, 0.75;G, 0.88. (From ref. 45.)

+

+

where D = (1- 48(1 - 8 ) [ l - exp ( w / R T ) }1'2 ] and contains the interaction energy w . E is also dependent on the coverage and magnitude of lateral interactions, viz .

E , being the zero coverage desorption energy. The peak seen at 950K for CO adsorbed on tungsten is observed at coverages above 8 = 0.5 when (provided the interaction energy is high) the pre-exponential term becomes (28 - l), i.e. first-order dependence (and so the peak temperature is coverage independent). The desorption energy appropriate to the peak is E , Zw and so the peak appears well separated from the low-coverage peak by eight times the interaction energy (in the case of a four-fold symmetric surface). For the CO/W case (Fig. 34), the repulsive interaction energy was found to be 20 kJ mole-'. At low coverages, the interactions have little effect since the adatoms locate in high-energy sites and so the desorption peak is more nearly second order in character and at very low coverages the desorption energy is Eo.

+

References p p . 163-1 79

106 I

'

I

I

I

1

I

I

Fig, 35. Desorption of O2 from A g ( l l 0 ) . The data points are the experimental values at various coverages. The solid lines are predicted from eqn. (152). (From ref. 289).

An example of recombinative desorption with attractive interactions has recently been reported for the O,/Ag(llO) adsorption system and, again, the desorption order is obscured by the interactions [ 2891 . Figure 35 shows desorption spectra for this system which look more like those for first-order desorption (asymmetric with the desorption integral to the left of the peak being much greater than that to the right). The interaction energy is attractive and has a value of 14 kJ mole-' and so has a large value over most of the coverage range, leaving a near first-order dependence of the desorption rate, while at the maximum possible coverage in this case, 8 = 0.5, E = E , 2 0 (since the (110) is a two-fold symmetric surface). The situation is simpler for the first-order desorption systems since, in that case, only the desorption energy is affected by lateral interactions; the criterion of occupied nearest sites being essential for desorption is not needed. An example of the effect of attractive lateral interactions on desorption can be seen in Fig. 36, taken from the work of Jones and Perry [ 4571 on the Hg/W (100) system. These workers initially concluded that the desorption was zero order [457] since, as Fig. 36 shows, the peak temperature shifts to higher temperature with increasing coverage. However, this conclusion was in stark contrast to the adsorption heat with increasing coverage. Subsequently, Jones and Perry [ 4581 intepreted their

+

107

Fig. 36. Desorption of Hg from W{lOO}. (After Jones and Perry [457].)

desorption data in terms of attractive lateral interactions between the mercury adatoms amounting t o 6 kJ mole-' between pairs of atoms. From these examples, it is clear that much care needs to be taken with thermal desorption data and that ambiguous results may be obtained. When lateral interactions are a possibility in an desorption system, parallel measurements on adsorption or LEED determinations can help eliminate such ambiguities. In the previous discussions on adsorption, with respect t o eqns. (152) and (153), and in a consideration of surface diffusion mechanisms which is t o follow, the role of lateral interactions in the kinetics of these processes has been elucidated using the quasi-chemical approximation of Fowler and Guggenheim [ 3201 . Adams and Germer [ 3191 have also considered the effect of lateral interactions on thermal desorption measurements, but quantify it in terms of the Bethe-Peierls approximation [318]. Using this method, Adams and Germer [ 3191 compute desorption spectra which are qualitatively similar to those shown in Fig. 34 for repulsive lateral ,interactions. This is not surprising since, as Fowler and Guggenheim have discussed [ 3201 , the quasi-chemical and Bethe-Peierls approximations, although derived from different bases, give identical formulations [ 3201 . However, the quantitative fit t o the desorption spectra for CO desorption from W(210) was poor, probably due to the fact that, in contrast to Goymour and King [45], Adams and Germer used a first-order model for the CO desorption; it is now generally accepted that the &states of CO desorption are from dissociated states. References p p . 163-1 79

108

Other systems which have been simulated with these types of model are now manifold. King [ 3211 has extended his work to hydrogen desorption from various tungsten planes. On the (110) plane, o was found to be repulsive to the extent of 6 kJ mole-', but for the (100) and (111) planes the results could not be fitted exactly, possibly due to the presence of different binding energy sites. Lateral interaction effects have been inferred to explain the presence of two /3 states in the desorption of H, from P t { l l l ) [303]. Other examples of systems exhibiting these effects are CO/Ru{lOiO) [307] and Ru(001) [308], O,/W(lOO) [309], H,/Ni{100) [310] and N i { l l l ) [306] and CO/Mo{100) [311]. 4.2 THE DATA BASE

In Fig. 37, a large number of representative desorption spectra are presented for simple adsorption systems in which the adsorbate desorbs from the surface without decomposition or reaction. Even a simple molecule such as NO can undergo disproportionation reactions on surfaces to yield N, into the gas phase. The influences of surface structure and surface cleanliness on desorption spectra are discussed below. Section 4.2.3 deals with a specialised branch of thermal desorption known as temperature programmed reaction spectroscopy. 4.2.1 Crystal plane orientation In general, desorption spectra are very dependent on the types of site exposed at a surface and, in principle, it should be possible to determine the area ratios of different types of crystallography on inhomogeneous surfaces in this way. Investigations of this kind, but in terms of differing amounts of step sites on a well-defined surface, have been conducted by Collins and Spicer [324]. Figure 38 shows the desorption of CO from Pt{ l l l } , Pt [6{111} x {lOO}] and Pt [6{111} x {111}] and clearly shows a high-temperature state for the stepped surfaces over the whole coverage range; Collins and Spicer separated out these contributions from steps and terraces. Figure 39 shows the results for hydrogen desorption which exhibit similar features. These results indicate that the step density on a surface can be determined, if unknown, by desorption from the sample. 4.2.2 Surface cleanliness Once again, Pt will be used as a sample system to illustrate the effect of one adspecies on the desorption of another. Single crystals of Pt often contain large surface impurities of S, P, Ca, C, C1 and 0 and these adatoms are often very difficult to remove. The influence of S at the surface on the desorption of CO from Pt is shown in Fig. 40 from the work of Bonze1 and Ku [325]. Clearly, S substantially affects the desorption, decreasing both the desorption energy of the adsorbate and the amount adsorbed. Oxygen adsorbs on platinum t o form an oxide and McCabe and

109

Fig. 37. (a)

References p p . 163-1 79

110

Fig. 37. (b)

111

100

500 100

Fig. 37. (c)

References p p . 163-1 79

600

100

500-

112

Fig. 37. (d)

113

114

1100

1500

2500

Fig. 37. (g) and (h)

573

115

T/K

623 400

323

1800

300

900

1200

Fig. 37. ( i )

Referencesp p . 163-1 79

700 400

700

500

600

1300

1000

900

293

1300

573

c

116

Fig. 37. (j)

117

373

300

673

600

273

400

Fig. 37. (k)

References p p . 163-1 79

673 300

573 343

1200

300

600

118

200

Fig. 37. (1)

1550

119

N2/MOi100)

154

400

100

T/K 100

200

400

c

800

T/K

373

a1

I

673

c

180

I

200

Fig. 37. (m)

References p p . 163-1 79

1100

120 (n) N

2/

Re(1iloment)

I/K C

900

1500

300

120

1100

1500

Fig. 37. Some desorption spectra from the simpler adsorbate/substrate combinations. References are given with the spectra. (See Table 2.)

300

I“’

bl

(a1

500 T/K

700

300

500

T/K

700

300

n

500

700

T/ K

Fig, 38. TDS of CO as a function of CO exposure (1 L = 1 Langmuir = 10-6Torr s). (a) Pt (111). Heating rate % 1 5 Ks-’. The small amount of desorption for T 500 K is attributed to desorption from a small density of defects present on the {lll}surface, as well as to desorption from the edges of the crystal. (b) Pt6\111) X (100). Heating rate 1 2 K s - I . (c) Pt6(111) X (111). Heating rate ‘v 1 4 K s - . The desorption for T 500 K in (b) and (c) is attributed to adsorption at steps, while that at T 500 K is attributed to adsorption on the terraces. (After Collins and Spicer [324]).

>

>


50% for repulsive and < 50%for attractive lateral interactions. In diffusion studies, determinations are usually made s f D as a measure References p p . 163-1 79

146

of the diffusion rate and to determine the activation energy for surface migration. In order to measure D from the curves above, the simplifying assumption of a coverage-independent D must be abandoned. Thus, in terms of the one-dimensional form of Fick’s law at =

2ax (D(N)!y)

where N is the coverage, t is the diffusion time, x is the distance moved at coverage N and D ( N ) is the concentration-dependent diffusion coefficient. D ( N ) cannot be taken outside the brackets as is usually done. Boltzmann [ 405, 4061 derived a general solution for D ( N ) in this situation, with the boundary conditions pertaining in the cases above, as

where 17 is a reduced variable and equals ~ t - ” Matano ~ . [408] used this equation t o analyse diffusion profiles from the bulk interdiffusion of nickel and copper. He used profiles produced after a particular time of diffusion t and used eqn. (161) in the form

1 d x D ( N ) = -- 2t div

1 N

XdN

N1

where rxdN = 0 N1

and when t = 0 and x > 0, N = N o and when t = 0 and x < 0, N = N I . The latter condition is the “conservation of matter” rule during diffusion; the process involved must be solely surface diffusion and the equation cannot be used accurately in the above form when competing processes such as desorption or bulk diffusion take place within the diffusion zone. Diffusion coefficients obtained in this way are shown in Fig. 48 for the results of diffusion in two dimensions with and without lateral interactions. D is much reduced at high coverages for attractive interactions and is increased for repulsive interactions. Dilute layers tend t o the value of D for no lateral interactions. In the random walk situation for the fourfold symmetric surface, D is related t o r by [409]

D

=

ix2rz

(163)

Since z = 4 and X = 1 (the jump distance A is one lattice unit) then D = I‘ at low coverage. Thus, when z’ = 3 (75% coverage) r = 0.34 (the value assigned) and D is approximately the same value (Fig. 48). Similar results have been obtained using an analytical expression t o include the effects of lateral interactions. Making use of the work of

147

a

i

0.3t

i

100

Coverage

(%I

Fig. 48. The variation of D with coverage. Three examples are shown: (a) attractive, (b) repulsive and (c) no interactions existing between adspecies. (From Ref. 234.)

Fowler and Guggenheim and the quasi-chemical approximation [ 3201

i

I

(1-26) [I - 4 e ( i - e ) ~ 1 1 / 2 where E m ( 6 ) is the diffusional activation energy at coverage 8, Em is that a t zero coverage and B = 1-exp ( w / R T )(known as the short-range order parameter). Figure 49 shows the variation in Em(6)with 6 for various interaction energy values. Now eqn. (155) becomes E m ( 6 ) = Em

+-zw2

1-

o(e) = D~ exp [ - E , ( ~ ) / R T ]

(165)

and thus, in Fick’s law

Irdroducing thereducedvariablesx’ = x / l and t’ = [Do exp ( - E m / R T ) ] t / 1 2 where x is the unit surface distance and 1 is the total distance across the surface considered ( t is the time unit) and differentiating the right-hand side of the reduced equation

References p p . 163-1 79

148

1401

----

2

n L7 .:.

loo'

(C)

._. -. _. ._._. .. ...- -. . ....... ........... ... ....... . .......- -....._.

0:1

d.2

013

d.4

d.5

d.6

017

0!8

d.9

0

Coverage (inonoloyer)

Fig. 49. The variation of Em@) with coverage for different values of the lateral interaction energy. w = (a) 4- 4 kJ mole-' ; (b) 4- 2 kJ mole-' ; (c) 0; ( d ) - 2 kJ mole-' ; (e) - 4 kJ mole-'. (From ref. 234.)

This expression was numerically solved in terms of 8 and x using the Crank-Nicolson finite difference method. With w = 0, symmetric profiles (as in Fig. 44) were obtained, yielding concentration-independent D values on analysis using the Boltzmann-Matano method. Profiles similar to those from the simulation were obtained for finite values of w and diffusion coefficients showed the same trends (see Fig. 48). From the analytical results, a plot of B C (the boundary cross-over value) was made as a function of the interaction parameter ( w / R T )as shown in Fig. 50. It is possible that this curve could be used to estimate lateral interaction energies directly from diffusion profiles providing next nearest neighbour (n.n.n.) lateral interactions are negligible. When n.n.n. interactions do occur, if they are of the same sign as the n.n. interactions, then the effect will be to push the OC value to a greater extreme, indicating a nearest neighbour w value which would be somewhat high. The extension of the simulation programme to include n.n.n. lateral interactions is described in the following section in connection with experimental work on O/W (110). These methods have been applied on the assumption of a perfectly flat surface. However, surfaces may have steps at specific intervals across them

149

,

1

* 1.5 ’

O

r

I

T

1

\

i t

4 I

-0.5

-1.0

-2.0

1 1 d

Fig. 50. The position of the boundary cross-over value with the interaction parameter. (From ref. 234.)

and the step-sites may have “deeper” potential wells associated with them; thus, once the adspecies is trapped in such a sites, Em will be higher, reducing the rate of diffusion from them. Such features could be simulated in the difffusion programme, though as yet this has not been attempted. The determination of diffusion properties can also be carried out by static observation at a particular point on the surface and by following the change in concentration with time, rather than by the fuller method of profile analysis. Such methods have been used by Renard and Deloche [ 2611 (equation given earlier) and Abramenkov et al. [ 2601 ;these methods of analysis are detailed elsewhere [406, 4101 and will not be discussed here. 5.2 RESULTS

This survey is not meant to include all the measurements in this field of study; instead, those works which the authors consider to be the most significant to -date are presented together with results which illustrate a particular investigative method. In section 5.3,a table is presented which References p p . 163-1 79

150

contains most of the results obtained as regards the surface diffusion of adsorbates on non-supported metal samples. Some of the most elegant work in this area was that that done by Langmuir and Taylor [258] in the very early days of surface science and the method adopted was described in Sect. 2.4.3. They deposited low coverages of caesium on a polycrystalline wire and found a strong dependence of the diffusion coefficient on the initial Cs coverage. At a fixed temperature of 812 K, they found D = 3.45 x lo-’ cm2 s-’ with an initial coverage (on the high value side of the original boundary) of 2.73 x l o ” atom m-’, whereas with a coverage of 1.73 x 1017 atom m-’, D was lower a t 1.4 x cm2 s-l. These workers considered this variation to be due to repulsive mutual interactions between the Cs adatoms on the surface. Shortly after this, Brattain and Becker [259] investigated the diffusion of thorium on a polycrystalline tungsten ribbon using the method described in Sect. 2.4.3. They, too, found that the diffusion could not be fitted at all coverages with one diffusion coefficient, but D decreased with decreasing coverage by a t least a factor of two. In 1935, Bosworth [231] examined the surface diffusion of sodium and potassium on tungsten using the method given earlier in Sect. 2.4.l(a). He found that, with only one “dose” of sodium on the surface, although the band diminished in size, no matter was seen to leave the patch area (see Fig. 51). However, after carrying o u t this procedure many times without actually cleaning the surface (just allowing the patch signal t o decay), eventually he could deposit a patch which would spread with some semblance of a surface diffusion mechanism. Using various maximum coverages (at the centre of the patch) from 2 x l o L 9t o 6 x l O I 9 atom m-’, he found little variation in D with coverage. From an Arrhenius plot of the temperature variation of the diffusion coefficient, he found an activation energy for diffusion of only 24 kJ mole-’. He postulated that the behaviour of the sodium in the initial doses was due t o its diffusion into a “micro-structure” in the polycrystalline surface, which after many doses eventually became saturated. With potassium, no diffusion was apparent at room temperature but on heating, there was less tendency to diffuse into this “micro-structure” and he measured diffusion over a large coverage range. The profiles were more like those which would be expected for a true diffusion process, the coverage decreasing continuously with distance from the peak (see Fig. 52). He found a strong coverage dependence of E m , being 69 kJ mole-’ a t the lowest coverage measured (0.06 x 10l8atom m-’) and 28 kJ mole-’ a t 4.8 x 10l8atom m-’. He attributed this variation to repulsive dipole-dipole interactions between adatoms, making the diffusion easier at higher coverages. By far the major proportion of the surface diffusion data available at present has been obtained using the field-emission microscope. Not only can this technique yield valuable quantitative results concerning this process, but a layer of atoms can actually be seen to move over

151

Distance along strlp/cm

Fig. 51. Diffusion profiles for sodium adsorbed on tungsten. (From Bosworth [ 2 3 1 ] . ) A, 5 rnin at 295 K ; B, 1 0 min at 295 K ; C, 20 rnin at 295 K ; D, 40 min at 295 K; E, 60 min at 295 K ; F, 1 rnin at 415 K ; G , 3 rnin at 415 K ; H , 5 rnin at 415 K ; I , 1 1 rnin at 415 K.

the surface in a most dramatic fashion. Some of the most significant contributions to this field of study were made by Gomer and co-workers in the later 1950s [246-2481. In 1957, Gomer and Hulm [248] investigated the diffusion of oxygen over a field emission tip by “shadowing” it with oxygen and they discovered several modes of migration. First of all, at high coverages and very low temperatures (27-70K) there was diffusion with a “moving boundary” due to migration of the physisorbed oxygen species over the chemisorbed layer, with precipitation and chemisorption on to the surface where the physisorbed species encountered a clean region : this mechanism did not operate above 8 0 K because of the short lifetime of the physisorbed species at that temperature. Diffusion took place in the chemisorbed layer above 400K, again in a moving boundary fashion: the activation energy for diffusion of the sharp boundary was 95 (f 4)kJ mole-’ . The third References p p . 163-1 79

152 A

2 1.0

D i s t a n c e in c m

Fig. 52. The diffusion of potassium adsorbed o n tungsten. The curves show the increase in patch width as a function of heating time at 480 K. (From Bosworth [ 231.)

fi

[Ill1 n

4

.

, ,

,.

e

(C)

,7

~

S u r face c h onnels

)

Fig. 53. Hard sphere model of three interchangeable configurations for the atom pairs of Mo on the W(211) plane. Small open circles on each surface channel indicate the adsorption sites for adatoms, separated, respectively, by 2.7 A. (From ref. 414.)

153

mode was observed after this boundary reached the { l l O } plane; a new boundary was then seen to radiate outwards from the {llO}, but this stopped after a short distance unless the dose was initially directed on t o the (110) surface: Em for this process was found to be 104 (+ 4) kJ mole-’. Finally, if the temperature was further raised to 600K at this stage, a boundary-free migration process occurred over an activation energy barrier of 1 2 6 (+ 6) kJ mole-’ (this being the value a t low surface coverages). They explained this variation in mechanism as due t o the presence of different proportions of 2, 3, 4 and 5 coordinate sites on the different crystallographic regions of the tip. On the {lOOj-type regions, the sites they proposed are mainly 2 and 5 coordinate sites whereas on (110)types, they are 3 and 4. Thus the lowest energy diffusion is from 2 coordinate sites, which are filled at the highest coverages (least binding energy, less bonds to be broken in diffusion); low coverages exhibit diffusion from the tightrbinding 5-coordinate sites and the middle range of diffusion is in the (110} region. Theoretical calculations using the density functional approximation have recently been performed by Huntington and co-workers [411] and lend support to these usually accepted ideas of plane dependence of surface diffusion. They investigated the variation of diffusional activation energies with surface crystallography and the adatom binding site on the surface. They found a general decrease in activation energy for migration with increasing close packing (smoothness) of the surface. For an atom sitting in a four-fold or three-fold hollow in the surface, this change was considerably more dramatic than for an atom binding directly above a surface atom. Chen and Gomer [412] have recently advanced their earlier work by investigating the diffusion of oxygen on the {110} plane of a tungsten field emission tip using the “flicker-noise” technique and a probe-hole attachment as described earlier in Sect. 2.4.2(a). They studied the diffusion between 300 and 800K and coverages from 0.15 t o 0.56 of a monolayer. At intermediate temperatures, they found predominantly a mechanism of single particle diffusion and found Ed to be 60 kJ mole-’ a t 8 < 0.2, increasing t o around 94 kJ mole-’ at 8 = 0.56, while Do ranged cm2 s-’. In the high temperature range, there was to from evidence f o r - t h e diffusion of multiples of atoms in clusters while a t the lowest temperatures, diffusion was by the mechanism of adatoms “flipping” from one adsorbed state of a certain work function to another state of different work function, rather than by density fluctuations inducing local differences in 4 values. An idea of the presence of lateral interactions between adspecies can be gained from such studies since, in a situation with attractive interactions, the current fluctuations should decrease with increasing temperature, while the reverse is true for repulsive interactions [413]. In fact, Chen and Gomer observed the latter, which is in apparent contradiction References p p . 163-1 79

154

t o the increasing values of the diffusion activation energy which they also found. Field ion microscopy provides us with a facility for observing the diffusion of individual atoms on an FEM tip. The results with heavy metal atoms show clearly that lateral interactions have an important effect on the diffusion of individual species on the surface: indeed, this technique reveals some rather surprising aspects of sub-microscopic diffusion. On highly channelled planes such as (211jW and {321jW, diffusion of M o adatoms is observed to take place along the [ l l l ] channels [414, 4151 and not perpendicular to them; furthermore, when two atoms approach each other in adjacent channels within a couple of atomic spacings, they “lock” into each other’s motion and then diffuse as a dimer (though with single atomic “hops”) as shown in Fig. 53. It is intuitively rather surprising, however, that the activation energy barrier t o diffusion for this entity was only 26 kJ mole-’ on the (211) plane, half that for the single atoms. Reed and Ehrlich [416] found that, for Ir adatom dimers on the same plane, diffusion proceeded with E m = 64 kJmole-’, whereas for the single adatoms it was 54 kJ mole-’. They conclude that the interactions, as might be expected, are very dependent on adatom size. They observed chains of atoms as long as 4 units all moving in correlated motions in adjacent channels. It is important for a more detailed understanding of the surface diffusion process that experiments are conducted with single crystals which have a well-defined surface structure and stoichiometry. On polycrystalline surfaces, there appears to be a surface substructure and in FEM microscopic samples, the diffusion effects are dominated by the close proximity

-1

0.1

0.2

0.3

0.4

I

I

I

I

I

0.5

0.6

0.7

0.0

0.9

Coverage,

e

Fig. 54. The variation of D with surface coverage, for O2 on W ( l l O } . 76OoC. (From Butz and Wagner [ 2 4 0 ] .)

0,

880°C; X I

155

of planes with differing activation energies for diffusion. Experiments with single crystal planes should reveal the effect of any lateral interactions between adspecies on the diffusion parameters. Very little work in this respect has been carried out to date. However, Love and Wiederick [417] have recently compared the diffusion of caesium on polycrystalline and (110) single crystal samples of tungsten. They used a similar method t o that of Bosworth, but had good UHV conditions. For the polycrystalline samples, they found a similar effect t o that of Bosworth (the adsorption of photoactivity by the surface) except that they separated two contributing processes, one with activation energy of 170 kJ mole-’ and the other of 17 kJ mole-’. The former they attributed t o diffusion into the bulk, down grain boundaries, and the latter t o migration into some “surface structure”. Again, after several doses, this structure could be saturated and surface diffusion observed. For the (110) single crystal, this effect was also seen, but was much less extensive, the adsorption effect being saturated after the adsorption of 0.1 monolayer of Cs: subsequently, they found Em was 57 kJ mole-’ for small coverages between lo-’ and 5 x monolayers. Butz and Wagner [ 2401 investigated the diffusion of oxygen on (110) W in the monolayer region using the technique described in Sect. 2.4.l(c). The diffusion of the atomic 0 species was examined in the range 10401150 K and they found Em t o be 113 (k 8) kJ mole-’ and D t o be approximately cmz s-’ at a coverage of half a monolayer and a temperature of 11OOK. D showed a strong dependence on the oxygen coverage and this is illustrated in Fig. 55. The coverage profile results were analysed using the Boltzmann-Matano method described earlier, but it must be noted that matter was lost during their diffusion sequences as shown in

Distance (pm)

Fig. 55. The coverage versus distance profiles obtained by Butz and Wagner [ 2 4 0 ] for 02 diffusion on W { l l O ) at 88OoC.

References p p . 163-1 79

156

t 80-

,2.70 B

m

m-

U

$

0

'

50-

L

40-

10 0

40

Distance ( l a t t i c e units)

Fig. 56. The diffusion profiles observed after 1000 time units when the interaction between molecules extend t o allow both nearest neighbour and next nearest neighbour interactions. 0 , Diffusion in one dimension; 0 , diffusion in two dimensions. (From Bowker and King [ 2341 .)

Fig, 56: the area under the left-hand side of the profile (the matter diffused out from the original boundary) is considerably less than the area above the curve on the right-hand side (matter lost from the original covered area during diffusion). It appears that another process may be taking place a t the same time as the surface diffusion, at least on the low concentration side of the profile. However, prompted by this work and some results obtained recently by Lagally and co-workers [ 501, Bowker and King [407] have extended their simulation model of surface diffusion t o include next-nearest neighbour lateral interactions. Lagally and co-workers [ 501 investigated the p ( 2 x 1) low-energy electron diffraction (LEED) pattern for oxygen adsorbed on {llO}W, measuring the decrease in intensity of the adsorbateinduced spots with increasing substrate temperature. Using a model they devised t o fit this variation a t different coverages, they deduced the presence of repulsive n.n. interactions of 14.4 kJ mole-' and attractive n.n.n. interactions of 6.7 kJmole-' (this oscillating nature of the sign of the interaction energy is a result of its indirect nature, effected via the conduction band electrons [ 591 ). Using these lateral interaction values, Bowker and King [407] obtained the profiles shown in Fig. 56 for diffusion in two dimensions and D values found using the Boltzmann-Matano analysis are presented as a function of coverage in Fig. 57. The qualitative agreement with Butz and Wagner's results produced using the independent data of Lagally and co-workers is encouraging and

157

01

0

I 10

I

I

I

I

20

30

40

50

I

60

I

I

70

80

I

90

I

100

Coverage (%)

Fig. 57. The variation o f D with coverage for diffusion in two dimensions as found by the computational studies of Bowker and King [234].

indicates that the simple ideas of the surface diffusion process encompassed in this model bear a reasonable semblance t o reality. These experimental data have also been qualitatively fitted by Asada and Masuda [ 2551 using the kind of statistical-mechanical treatment outlined in Sect. 4.1.6, adapted to take account of more distant lateral interactions than simply nearest neighbours, and using the Bethe-Peierls approximation [ 3181. Continuing their diffusion studies using scanning electron microscopy, Butz and Wagner have reported elegant work on the surface diffusion of gold and palladium on tungsten planes [418, 4191. Many fascinating details of surface diffusion were observed in this study and, once again, interatomic lateral interactions seemed to affect the results in a marked way. Thus on the {110}plane, kinks in the diffusion profile between zero and one monolayer coverage are produced, corresponding t o ordered phases on the surface (although these kinks are diminished at high temperature and long times due t o the great surplus of material, in excess of one monolayer, deposited in the original adsorbate patch). Such profiles are very similar to those shown in Fig. 57, obtained under the influence of repulsive and attractive interactions. The dominating influence on the diffusion rate was, however, the presence of second layer adatoms since, as found in the FEM studies cited earlier, diffusion is then in the “boundary” regime. This is because the Pd-W or Au-W bond strength is relatively stronger than that for Pd-Pd or Au-Au and thus, on reaching the limit of the second layer boundary, the adatoms drop into more immobile trap sites on the tungsten surface. On stepped W( llO} surfaces, it was found that diffusion was very anisotropic [ 4191 ; palladium could not scramble over steps (which were {loo}type sites and so act as “traps”) as well as over the close-packed { l l O } terraces. As a result, a fascinating phenomenon References p p . 163-1 79

TABLE 3 Surface diffusion parameters Adsorbent (PI-)

Adsorbate

Temp. range

Em

(K)

(kJ mole-’ )

DT (cm s-’

DO )K

Coverage

(cm2s-’)

Coverage dependence

Method

Ref. ~

W(po1ycrystalline wire)

CS

650-812

58.5

(3.4 x 10-5)812

0.2

3 x 10”atom m-’

Strong increase in D with increase in coverage

Desorption sampling

258

550-850

55

( 6 x 10-5)m

0.23

Very low to 5 x monolayer

Dilute layer

Photoemission profile

232

x l0”atom m-’

E m is an average over coverage range

Photoemission profile

231

-40

0.12 x 1 0 ’ ~ a t o mm-’ 1 . 2 x 10i8atom m-’ 4.8 x 10’”atorn m-’

Photoemission profile

231

1.6 x 1.3 x -32

0.5 x 10’8atom m - 2 1.5 x 10”atom m-’ 2.7 x 1O”atom m-’

Strong increase in D with increase in coverage Increase in E m (decrease in D ) with increase in coverage

FEM

715

Average Em for coverage range and for diffusion over different planes of tip

FEM

716

W(oxidised polYcrYddine ribbon)

Na

300-800

24

K

295-750

63 50 28

W (over several planes of tip sufrace)

K

-600

21 40

470-540

40 80

2-6

( 2 x 10-9Mx, (4 x 10-8)Mx, (8 x 10-6),,

3x 1.5 x

-

1 monolayer

-0

?

LI Q,

W((Oll}--t { E l } )

Ti

YI

500-750

106 79 64 24

800-960

180 180

-

700

500-600 530-600 310-340

0.64 0.18 0.03 0.002

-

-(3 X 10-

46 96

(7 x (10- 11)-

< 31

(1O'3)ml5 )TOO

-

-10-l 107 x lo-' 1.4 x

Au

W(across one side of tip)

Hg

2OHOO

W({110}-*{loo}) W(around {211} region)

Yb

600-1 100

43 68

W({110}+ {loo}) W(amund (111) region)

Nd

600-1100

73 62

w(P0lYclYstdine ribbon)

Th

1535-1655

450

Scanning AES

7 35

Low coverage

E m is an

FEM

717

Em has maximum at

FEM

718

E m is an

FEM

719

Decrease in Em with in crease in 0

FEM

720

Decrease in Em with in crease in 0

FEM

721

FEM

250

0.3 monolayer 0.6 monolayer 1.O monolayer

-

1 monolayer

6

lo-'

e * 0.5 average

Probably about 1 monolayer

6(t150

W({101}+{100))

Strong increase in D with increase in coverage average

-

-(2 x

96 120 86

0.25 monolayer 0.50 monolayer 1-7 monolayers

0.2 monolayer to 10-30.5-1 monolayer

-

1 monolayer

Average Em over coverage range of moving bound-

-

1 monolayer

Average E m as above

FEM

250

Increase in D with increase in tl

Thermionic emission

259

X Y

I-

cn W

TABLE 3 (continued)

Adsorbent (plane)

Adsorbate

Mo

Ir

Ge

Temp. range (K)

Em

DT

Do

(kJmole-’)

( C ~ S - ’ ) ~

(em2 s-’ )

235-290

55 25 53

( 2 x 10-:;)270

50 65

(10-16)%

-

300

350-750 -700

W({llO}-+ {loo})

Si

Si

1000-1200

-750

-

1100

Coverage

Coverage dependence

Method

Ref.

9 x lo-’ 2 x lo-’ 1x

Single adatom Dimer Single adatom

Very low coverage only

FIM

414, 415

5 x lo-’ 9x

Single adatom Dimer

Very low coverage only (onedimensional diffusion)

FIM

416

22 24 165 125

High coverage

Migration in second Ge layer

FEM

722

150

“Small amounts”, < monolayer

Em dependent

FEM

723

Large coverage Lower coverage

Diffusion in 2nd Si layer?

FEM

724

Average value for Em

FEM

249

Coverage average E m

FEM

.725

Coverage average E m

Scanning AES

233

Average E , over coverage range boundary

FEM

248

FEM

248

(3 X 10-18)z70 (5 lo- )270 (6x 10-

1300

Low coverage

72 92 152 172

C

850-1100

230

N

-650

85 -150

N

80*900

88

(2 x

10-8)m

0.014

0

500-55 0 560-650

105 126

(3 x 10-l~ (8 x 10-10;::

-0.03 82

0

400-480

65

-

-

-

1 monolayer

Low coverage

-

1 monolayer

on amount Si deposited and region observed

(probe hole)

e=1 e = 0.5

Increase in D with decrease in 09 from 1 to 0.5

Scanning electron beam (secondary electron emission)

234

113

Average for Em given: maximum in D at 0 = 0.4

Scanning work function probe

240

1040-1450

220

High coverage value for Em

co

-

150

-

Scanning electron beam (secondary electron emission)

1 monolayer

FEM

726

co

-750

210

-

Average over all planes, over coverage range

1 monolayer

From dissociation of C 0 2: average over planes and coverage

FEM

727

- 1 . 8 ~ 1 0 --1monolayer ~

E , averaging over varying surface geometry and boundary coverage range

FEM

728

Multilayer

Diffusion in 2nd layer

FEM

729

Multilayer

Diffusion in 2nd layer

FEM

730

< 1 monolayer

First stage of investigated diffusion-average E m over low coverage range

SIMS

260

0

918-1303

106 99

0

1030-1150

0

-(I x~o-~),,,,

-0.04 -(i4 x 10-6)lz13 -0.25

k

0,

cu

W(across half tip)

W (across half tip)

coz

(physisorbed)

W( 110 + 100 ) WJllOI+ [Ill),

H

W(across tip)

Ar

650

60-70

-

-

180

25

18-21

-

-

< 3.8 > 4.5

(physisorbed)

W((310)’

(100))

Ar (physisorbed) KI (physisorbed) Xe (physhorbed)

Mo (polycrystalline) Cu

20 20

10

2.5

-80

16

800-1100

52

(4 x 10-6),070

- lo-)

~

Q,

U

TABLE 3 (continued) Temp. range

Em

(K)

(kJmole-')

Adsorbent @be)

Adsorbate

Mo(211) --* (100)

Si

Ni(across tip)

H

250-280

30

Ni((ll1))

Tritium (physisorbed)

13-20

0.84

H

-

Ni(cornpresed powder sample) Ta(around (110)) Ta((010}+ (111))

Hg

DT (cms-l)K

Do (cm' s-')

covemge dependence

Method

Ref.

1 monolayer

Coverage, average

FEM

731

.- 1 monolayer

Coverage, plane average

FEM

732

1 monolayer

Coverage average

Radio activity sampling

261

G S now rate

733

FEM

124

Coverage

215

(14 x 10-6),s

3x

5

300

88-120

i!,

5

1 monolayer

DecreaseinE, by half in going from

e=oto 8=1

163

could be observed in 2D Auger scans; originally deposited circular patches of palladium became ovals, spreading faster in the direction parallel with the steps; on the (110) plane itself, diffusion was isotropic. 5.3 SUMMARY

A summary of the results to date is given in Table 3. Where the data are available, column 5 lists diffusion coefficient values a t a particular temperature T. It is apparent from the table that by far the majority of work has been done with tungsten. This is because of the ease of forming FEM tips from this material, its activity to adsorption and its ease of cleaning (in polycrystalline and single crystal samples the major impurity is carbon, which is easily removed by heating the sample in oxygen). There is evidently a great need for data from other materials and particularly from singlecrystal samples. With the availability of many powerful new tools in surface science, there will certainly be an upsurge in this field of work and many more fascinating aspects of surface diffusion will be exposed to view. References 1

7 8

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

H.S. Taylor, J. Am, Chem. SOC.,53 (1931)578;Trans. Faraday SOC.,28 (1932) 181. J.B. Taylor and I. Langmuir, Phys. Rev., 44 (1933)423. J.K. Roberts, Proc. R. SOC.London Ser. A, 142 (1933)518. J.K. Roberts, Proc. R . SOC.London Ser. A, 152 (1935)445. G. Ehrlich, J . Appl. Phys., 32 (1961)4;Adv. Catal., 14 (1963)255. P.A. Redhead, Vacuum, 12 (1962) 203. P.A. Redhead, J.P. Hobson and E.V. Kornelsen, The Physical Basis of Ultra-high Vacuum, Chapman and Hall, London, 1968. E.W.Miiller, Z. Phys., 106 (1937)541. J.A. Becker, Solid State Phys., 7 (1958)379. R. Gomer, Discuss. Faraday SOC.,28 (1959)23. G. Ehrlich and F.G. Hudda, J. Chem. Phys., 35 (1961)1421. C.J. Davison and L.H. Germer, Phys. Rev., 30 (1927)705. H.E. Farnsworth, Phys. Rev., 34 (1929)679. R.E. Schlier and H.E. Farnsworth, J. Appl. Phys., 25 (1954)1333. L.H. Germer and C.D. Hartman, J. Phys. Chem. Solids, 14 (1960)75. W.Ehrenberg, Philos. Mag., 18 (1934)878. T.A. Delchar and G. Ehrlich, J. Chem. Phys., 42 (1956)2686. P.J. Estrup and J. Anderson, J. Chem. Phys., 45 (1966)2254. J.J. Lander,Phys. Rev., 91 (1953)1382. R.F. Weber and W.T. Peria, J. Appi. Phys., 38 (1967)4355. D.A. King and D.P. Woodruff (Eds.) The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vols. 1-4, Elsevier, Amsterdam, 1981-1983. J.F. Lennard-Jones, Proc. R. SOC.London Ser. A, 106 (1924)463. F. London, Trans. Faraday SOC.,33 (1937)8. A. Miiller, Proc. R . SOC.London Ser. A, 154 (1936)624. R.A. Pierotti and G.D. Halsey, J. Phys. Chem., 63 (1959)680. G. Ehrlich, Trans. 8 t h Vac. Symp., Pergamon Press, Oxford 1961. T. Engel and R. Gomer, J. Chem. Phys., 52 (1970)5572.

164 26 27 28 29 30 31

32 33 34 35 36 37 38 39 40 41

42 43 44

45 46 47 48 49 50

51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

G. Ehrlich and F.G. Hudda, J. Chem. Phys., 30 (1959) 493. M.A. Chesters, M. Hussain and J. Pritchard, Surf. Sci., 35 (1973) 161. P.G. Hall, Chem. Commun., (1950) 877. J. Muller, Chem. Commun., (1970) 1197. D.F. Klemperer and D. Snaith, Surf. Sci., 28 (1971) 209. J.E. Inglesfield and B. Holland, in D.A. King and D.P. Woodruff (Eds.), The Chemical Physics of Solid State Surfaces and Heterogeneous Catalysis, Vol. 1, Elsevier, Amsterdam, 1981, Chap. 3. K. Besocke and H. Wagner, Phys. Rev., 88 (1973) 4597. N.D. Lang, Solid State Phys., 28 (1973) 260. J.H. Kasperma, Ph.D. Thesis, Catholic University of Nijmegen, 1972. W.A. Steele, Interaction of Gases With Solid Surfaces, Pergamon Press, New York, 1974, Chap. 2. V.I. Gerahsimenko. Sov. Phys. Solid State, 1 9 (1977) 1677. P.W. Palmberg, Surf. Sci., 25 (1971) 598. M.A. Chesters and J. Pritchard, Surf. Sci., 28 (1971) 460. M. Scheffler, K. Horn, A.M. Bradshaw and K. Kambe, Surf. Sci., 80 (1979) 69. F. Ricca, C. Pisani and E. Garrowe, in F. Ricca (Ed.), Adsorption-Desorption Phenomena, Academic Press, London, 1972. T.B. Grimley, in D.A. King and D.P. Woodruff (Eds.), The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis Vol. 2 ., Elsevier, Amsterdam, 198 3, Chap. 2. G.A. Somorjai, Chemistry in Two Dimensions: Surfaces, Cornell University Press, London, 1981, p. 283. J.K. Norskov, A. Houmoller, P.K. Johannson and B. Lundquist, Phys. Rev. Lett., 46 (1981) 257. B. Lundqvist, B. Kasemo and D.A. King, in D.A. King and D.P. Woodruff (Eds.), The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 3B, Elsevier, Amsterdam, in press. C.G. Goymour and D.A. King, J. Chem. SOC.Faraday Trans. 1,69 (1973) 749. D.A. King and M.G. Wells, Proc. R. SOC.London Ser. A, 339 (1974) 245. S.P. Singh-Boparai, M. Bowker and D.A. King, Surf. Sci., 53 (1978) 55, M. Bowker and D.A. King, Surf. Sci., 7 1 (1978) 583. J.C. Tracey, J. Chem. Phys., 56 (1972) 2736. T.M. Lu, G.C. Wang and M.G. Lagally, Phys. Rev. Lett., 39 (1977) 411. G.C. Wang, T.M. Lu and M.G. Lagally, J. Chem. Phys. 69 (1978) 479. J.C. Bucholz and M.G. Lagally, Phys. Rev. Lett., 35 (1975) 422. P.A. Thiel, J.T. Yates, Jr. and W.H. Weinberg,Surf. Sci., 82 (1979) 22. J. Benn, K. Christmann and G. Ertl, Solid State Commun., 25 (1978) 763. A.R. Kortan, P.I. Cohen and R.L. Park, J. Vac. Sci. Technol., 69 (1979) 54. A. Thomy, X. Duval and J. Regnier, Surf. Sci. Rep., 1 (1981) 1. R. Gomer, Solid State Phys., 30 (1975) 93. J.P. Muscat and D.M. Newns, Solid State Commun., 11 (1972), 737; J. Phys. C, 7 (1974) 2630. A. Crossley and D.A. King, Surf. Sci., 95 (1980) 131. J. Koutecky, Trans. Faraday SOC.,54 (1958) 1038. T.B. Grimley, Proc. Phys. SOC., 40 (1967) 751; Proc. Phys. Soc., 92 (1967) 776; T.B. Grimley and S.M. Walker, Surf. Sci., 1 4 (1969) 395. T.L. Einstein and J.R. Schrieffer, Phys. Rev. B, 7 (1973) 3269. I.P. Batra and S. Ciraci, Proc. 7th Int. Vac. Congr. and 3rd Int. Conf. Solid Surf., Vienna, 1977, Vol. 2, p. 1141. T.B. Grimley and M. Torrini, J. Phys. Chem., 6 (1973) 868. J.C. Le Bosse, J. Lopez and J. Rousseau, Surf. Sci., 72 (1978) 125. D.L. Adams. Surf. Sci.. 42 11974) 12. L. Onsager, Phys. Rev.: 85 i 1 9 7 4 j 12.

165 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87

88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105

T.D. Lee and C.N. Young, Phys. Rev., 87 (1952) 410. W.Y. Ching, D.L. Huber,M.G. Lagally and G.C. Wang, Surf. Sci., 77 (1978) 550. T.L. Einstein, Surf. Sci., 84 (1979) L497. G. Ertl and D. Schillinger, J. Chem. Phys., 66 (1977) 2569. W.Y. Ching and D.L. Huber, Surf. Sci., 77 (1978) 550. E. Domany, M. Schick and J.S. Walker, Solid State Commun., 30 (1979) 331. E.D. Williams, S.G. Cunningham and W.H. Weinberg, J. Vac. Sci. Technol., 15 (1978) 417. G. Doyen, G. Ertl and M. Plancher, J. Chem. Phys., 62 (1975) 2957. E. Bauer and H.M. Dramer, Surf. Sci., 93 (1980) 407. M.G. Wells and D.A. King, J. Phys. C, 7 (1974) 4053. M.J. Braithwaite, R.W. Joyner and M.W. Roberts, J. Chem. SOC.Faraday Disc u s . , 60 (1975) 89. L. McDonnel and D.P. Woodruff, Surf. Sci., (1974) 505. A.M. Horgan and D.A. King, Surf. Sci., 23 (1970) 359. P.J. Holloway and J.B. Hudson, Surf. Sci., 43 (1974) 123; Surf. Sci., 43 (1974) 123. D.F. Mitchell, P.B. Sewell and H. Cohen, Surf. Sci., 61 (1976) 355. C. Steinbruchel and R. Gomer, Surf. Sci., 67 (1971) 21. H.D. Shin, F. Jona, D.W. Jepson and M.P. Marcus, Surf. Sci., 60 (1976) 445; Phys. Rev. Lett., 36 (1976) 798. C. Somerton and D.A. King, Surf. Sci., 89 (1979) 391. K. Griffiths and D.A. King, Vide, 201 (1980) 403. M.K. Debe and D.A. King, J. Phys. C, 10 (1977) L303; Surf. Sci. 8 1 (1979) 193. T.E. Felter, R.A. Barker and P.J. Estrup, Phys. Rev. Lett., 38 (1977) 1138. D.A. King and G. Thomas, Surf. Sci. 92 (1980) 201. K. Griffiths, G. Thomas and D.A. King, Vacuum, 31 (1981) 671. K. Griffiths and D.A. King, J. Phys. C, 12 (1979) L343. K. Griffiths, C. Kendon, D.A. King and J. Pendry, Phys. Rev. Lett., 46 (1981) 1584. G.A. Somorjai, Principles of Surface Chemistry, Prentice Hall, Englewood Cliffs, NH, 1972. G. Ertl and J. Kuppers, Low Energy Electrons and Surface Chemistry, Verlag Chemie, Weinheim, 1974. M.W. Roberts and C.S. McKee, Chemistry of the Metal-Gas Interface, Oxford University Press, Oxford, 1978. M.A. Van Hove and S.Y. Tong, Surface Crystallography by LEED, Springer Series in Chemical Physics, Vol. 2, Springer-Verlag, New York, 1979. E.A. Wood, J. Appl. Phys., 35 (1964) 1306. R.F. Weber and W.J. Peria, J. Appl. Phys., 38 (1967) 4355. R.L. Park, Surf. Sci., 48 (1975) 80. J.E. Houston and R.L. Park, Rev. Sci. Instrum., 43 (1972) 1437. See, for example, K.D. Sevier in K. Siegbayn (Ed.), Low Energy Electron Spectrometry, Wiley, New York, 1972; Alpha-, Beta- and Gamma-Ray Spectroscopy, North Holland, Amsterdam, 1965. N.C. MacDonald and J.R. Waldrop. Appl. Phys. Lett., 19 (1971) 315. D. Menzel, Surf. Sci., 14 (1969) 340. E.N. Sickafus, Surf. Sci., 19 (1970) 181. G.A. Somorjai, Surf. Sci., 34 (1973) 156. M. Housley and D.A. King, Surf. Sci., 62 (1977) 81. M.E. Bishop and J.C. Riviere, J. Appl. Phys., 40 (1969) 1740. D. Menzel and R. Gomer, J. Chem. Phys., 41 (1964) 311; 41 (1964) 3329. P.A. Redhead, Can. J. Phys., 42 (1964) 886. J.T. Yates and D.A. King, Surf. Sci., 32 (1972) 479;Surf. Sci., 38 (1973) 114. P.A. Redhead, Nuovo Cimento Suppl., 5 (1967) 586.

166 106 107 108 109 110 111 112 113 114 115

116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138

J.T. Yates, T.E. Madey and T.K. Payne, Nuovo Cimento Suppl., 5 (1967)558. R.D. Adams and E.E. Donaldson, J. Chem. Phys., 42 (1965)777. W.J. Lange, J. Vac. Sci. Technol., 2 (1965)74. P. Kronauer and D. Menzel, in F Ricca (Ed)., Adsorption-Desorption Phenomena, Academic Press, London, 1972. D. Menzel, P. Kronauer and W. Irland, Ber. Bunsenges. Phys. Chem., 75 (1971) 1074. R. Franchy and D. Menzel, Phys. Rev. Lett., 43 (1979)865. G.W. Fabel, S.M. Cox and D. Lichtmann, Surf. Sci., 40 (1973)571. T.E. Madey and J.T. Yates, Surf. Sci., 63 (1977)203. J.C. Campuzano and D.A. King, J. Phys. C, 13 (1980)4858. D.P. Woodruff, M.M. Traum,M.H. Farrel, N.V. Smith,P.D. Johnson, D.A. King, R.L. Benbow and Z. Hurych, Phys. Rev. B, 21 (1980).D.P. Woodruff, P.D. Johnson, M.M. Traum, M.H. Farrel, N.V. Smith, R.L. Benbow and Z. Hurych, Surf. Sci., 104 (1981)282. T.E. Felter and P.J. Estrup, Surf. Sci., 76 (1978)464. H. Niehus, Surf. Sci., 92 (1980)88. P.R. Davis, E.E. Donaldson and D.R. Sandstrom, Surf. Sci., 34 (1973)117. G.M. Bliznakoy and M.P. Kiskinova, J. Catal. 61 (1980)305. M.A. Chesters, B.J. Hopkins, A.R. Jones and R. Nathan, J. Phys. C, 7 (1974) 4486. M.L. Knotek and P.J. Fiebelman, Phys. Rev. Lett., 40 (1978)984. G.D. Watts, A.R. Jones and B.J. Hopkins, Surf. Sci., 45 (1974)705. D.E. Eastman, in E. Passaglia (Ed.), Techniques of Metals Research, Vol. 6, Wiley-Interscience, New York, 1971. E.O. Kane, Phys. Rev. Lett., 12 (1964)97. G.W. Gobeli, F.G. Allen and E.O. Kane, Phys. Rev. Lett., 12 (1964)96. W.Eberhardt and F.J. Mihajel, Phys. Rev. Lett., 42 (1979)1375. K. Jacobi, M. Schrieffer, K. Kambe and F. Forstman, Solid State Commun., 22 (1977)17. J.C. Campuzano, D.A. King, C. Somerton and J.E. Inglesfield, Phys. Rev. Lett., 45 (1980)1649. M.W. Holmes and D.A. King, Proc. R. SOC.London Ser. A, 376 (1981)565. D.A. Shirley, J. Stohr, P.S. Wehner, R.S. Williams and G. Apai, Phys. Scr., 16 (1977)398. K.Horn, M. Scheffler and A.M. Bradshaw,Phys. Rev. Lett., 41 (1978)822. P. Hofmann, C.V. Muschwitz, K. Horn, K. Jacobi, A.M. Bradshaw and K. Kambe, Surf. Sci., 89 (1979)327. G. Gewinner, A.C. Peruchetti, R.Rifanger and A. Jaegle, Solid State Commun., 36 (1980)785. J.C. Fuggle and D. Menzel, Surf. Sci., 79 (1979)1. G.B.Fisher and J.L. Gland, Surf. Sci., 94 (1980)445. F.P. Uptler, R.A. Wille and M. Grunze, Surf. Sci., 102 (1981)75. M. Bowker and R.J. Madix, Surf. Sci, 102 (1981)542. M. Grunze, F. BOZSO,G. Ertl and M. Weiss, Appl. Surf. Sci., 1 (1978)241. C.W. Seabury, T.N. Rhodin, R.J. Purtell and R.P. Merril, Surf. Sci., 93 (1980)

117. 139 H.Conrad, G. Ertl, J. Kuppers and E.E. Latta, Solid State Commun., 17 (1978) 613. 140 O.L. Allyn, T. Gustafsson and E.W. Plummer, Solid State Commun., 24 (1977) 631. 141 T. Gustafsson, E.W. Plummer, D.E. Eastman and J.L. Freeduk, Solid State Commun., 17 (1975)391. 142 S.I. Atkinson, C.R. Brundle and M.W. Roberts, Faraday Discuss. Chem. SOC.,58 (1974)62.

167

143 144 145 146 147 148 149 150 151 152 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180

181

182 183

J.T. Yates, T.E. Madey and N.E. Erickson, Chem. Phys. Lett., 39 (1976)113. C. Steinbruchel and R. Comer, Surf. Sci., 7 (1977)21. R.W. Joyner, M.W. Roberts and S.P. Singh-Boparai, Surf. Sci., 104 (1981)L199. K. Kishi and S. Ikeda, Appl. Surf. Sci., 5 (1980)7. J.T. Yates, T.E. Madey and N.E. Erickson, Surf. Sci., 43 (1974)257. T. Fleisch, G.L. Ott, W.N. Delgass and N. Winograd, Surf. Sci., 8 (1979)1. P.R. Norton, J.W. Goodale and E.B. Selkirk, Surf. Sci., 83 (1979)189. G. Borden, G. Pirug and H.P. Bonze], Surf. Sci., 72 (1978)45. T.E. Madey, J.T. Yates and N.E. Erickson, Chem. Phys. Lett., 19 (1973)487. D. Pfnur, D. Menzel, F.M. Hoffman, A. Ortega and A.M. Bradshaw, Surf. Sci., 93 (1980)431. R.A. Shigeishi and D.A. King, Surf. Sci., 75 (1978)L397. M.J. Krebs and H. Luth, Appl. Surf. Sci., 14 (1977)337. K. Horn and J. Pritchard, Surf. Sci., 55 (1976)701. J.T. Yates, R.G. Greenler, I. Ratajczykowa and D.A. King, Surf. Sci., 36 (1973) 739. J.T. Yates and D.A. King, Surf. Sci., 30 (1972)601. E. Evans and D.L. Mills, Phys. Rev. B, 5 (1972)4125.D.M. Newns, Phys. Lett. A, 60 (1977)461.A.M. Bar0 and H. Ibach, Surf. Sci., 103 (1981)248. S. Andersson, Vacuum, to be published. B.A. Sexton and R.J. Madix, Surf. Sci., 105 (1981)177. B.A. Sexton, Chem. Phys. Lett., 63 (1979)451. S. Andersson, Solid State Commun., 21 (1977)75. H. Froitzheim, H. Hopster, M. Ibach and S. Lehwald, Appl. Phys., 13 (1977) 197. H. Hopster and H. Ibach, Surf. Sci., 77 (1978)109. H. Froitzheim, H. Ibach and S. Lehwald, Surf. Sci., 63 (1977)56. R.J. Gorte and J.L. Gland, Surf. Sci., 102 (1981)348. J.C. Bertolini and B. Tardy, Surf. Sci., 102 (1981)131. L.B. Loeb, The Kinetic Theory of Gases, Dover Publications, London, 3rd edn., 1961. S. Dushman, Scientific Foundations of Vacuum Technique, Wiley, London, 1962. D.A. King, T.E. Madey and J.T. Yates, J. Chem. Phys., 55 (1971)3236. C.G. Goymour and D.A. King, J. Chem. SOC.Faraday Trans. 1.69 (1973) 736. , . B.B. Dayton, Trans. 3rd Vac. Symp., American Vacuum Society, New York, 1956. H. Winters, J. Chem. Phys., 62 (1975)2454. D.O. Hayward, D.A. King and F.C. Tompkins, Chem. Commun., (1965)178. D.O. Hayward, D.A. King and F.C. Tompkins, Proc. R . SOC.London, Ser. A, 297 (1967)305. C.K. Beck, Science, 110 (1949)371. J.A. Dillon and H.E. Farnsworth, J. Chem. Phys., 22 (1954)1601. A.D. Crowell, J. Chem. Phys., 32 (1960)1576. A.D. Crowell and L.D. Matthews, Surf. Sci., 7 (1967)79. K. Kleik, A.C. Zettlehoier and H. Leiderheiser, J. Chem. Phys., 53 (1970)9. E.E. Dohann, Surf. Sci., 54 (1976)529. R.P. Godwin and E. Luscher, Surf. Sci., 3 (1965)42. E. von Goeler and R.N. Peacock, J. Chem. Phys., 39 (1963) 169. A. Czanderna, Vacuum Microbalance Techniques, Plenum Press, New York, 1981. A. Czanderna, in S.P. Wolzkey and E.J. Zadannuk (Eds.), Ultra Micro Weight Determination in Controlled Environments, Wiley-Interscience, New York, 1969. M. Nagasaka, F. Uyeda and T. Yamashima, Vacuum, 23 (1973)51. M. Nagasaka and T. Yamashima, J. Vac. Sci. Technol., 8 (1971)605. L.L. Levenson, Nuovo Cimento Suppl, 5 (1967)321. C.F. Bryson, V. Cazcarra and L.L.Levenson, J. Vac Sci. Technol., 11 (1974)411.

168 184 R.J. MacDonald and D. Haneman, J. Appl. Phys., 37 (1966) 1609. 185 J.W.T. Ridgway and D. Haneman,Surf. Sci., 24 (1971) 451. 186 B. Kasemo, Proc. 7th Int. Vac. Congr. and 3rd Int. Conf. Solid Surf. Vienna, 1977, Vol. 2, p. 1197; B. Kasemo and E. Tornqvist, Appl. Surf. Sci., 3 (1979) 307. 187 R.A. Shigeishi and D.A. King, Surf, Sci., 58 (1976) 379. 188 G. Blyholder, J. Phys. Chem., 68 (1964) 2772. 189 M. Primte, J.M. Basset, M.V. Mathieu and M. Prettre, J. Vatal., 29 (1973) 213. 190 A. Spitzer and H. Luth, Surf. Sci., 102 (1981) 29. 191 J. Topping, Proc. R. SOC.London Ser. A, 114 (1927) 67. 192 R.R. Rye, B.D. Barford and P.G. Cartier, J. Chem. Phys., 59 (1973) 1693. 193 V.P. Ivanov, B.N. Goldo and V.N. Kolomiichuk, Sov. Phys. Solid State, 16 (1974) 240. 194 P.R. Norton and P.T. Richards, Surf. Sci., 44 (1974) 293. 195 G. Wedler, M. Papp and G. Schroll, Surf. Sci., 44 (1974) 463. 196 W.M.H. Sachtler and G.J.H. Dorgelo, Z. Phys. Chem. (Frankfurt a m Main) 25 (1980) 69. 197 H. Jones, in S. Flugge (Ed.), Handbuch der Physik, Vol. 19, Springer-Verlag, Berlin, 1956, p. 227. 198 K.L. Chopra and L.C. Bobb, in M. Francombe and H. Sat0 (Eds.), Single Crystal Films, Pergamon Press, Oxford, 1964, p. 373. 199 R.C. O’Mandley and D.K. Burge, Surf. Sci., 48 (1975) 214. E. Meyer and J.J. Vrakking, Surf. Sci., 33 (1972) 271. 200 (a) G.A. Bootsma and F. Meyner, Surf. Sci., 1 4 (1969) 52. ( b ) F.M. Hadraken, E.P. Kleffer and G.A. Bootsma, Surf. Sci., 83 (1979) 45. ( c ) M . Albers, W.J.J. van der Wol and G.A. Bootsma, Surf. Sci., 68 (1977) 47. 201 A.A. Bell and R. Gomer, J. Chem. Phys., 44 (1966) 1065. 202 C. Wang and R. Gomer, Surf. Sci., 74 (1978) 389. 203 D.O. Hayward and N. Taylor, J. Sci. Instrum., 44 (1967) 327. 204 A.M. Horgan and D.A. King, Nature (London), 2 1 (1968) 760. 205 D.A. King and M.G. Wells, Surf, Sci., 29 (1972) 454. 206 T.E. Madey, Surf. Sci., 33 (1972) 355. 207 M.A. Morris and D.A. King, Vacuum, 30 (1980) 23. 208 W. Steckelmacher, Vacuum, 30 (1980) 261. 209 G. Ehrlich, J. Chem. Phys., 34 (1961) 29. 210 T.A. Delchar and G. Ehrlich, J. Chem. Phys., 42 (1965) 2686. 211 G. Carter, Vacuum, 12 (1962) 245. 212 R.R. Rye and B.D. Barford, Surf. Sci., 27 (1971) 667. 213 M. Housley, R. Ducros, C. Piquard and A. Cassuto, Surf. Sci., 68 (1977) 277. 214 C. Kohrt and R. Gomer, J. Chem. Phys., 62 (1970) 3283. 215 L.A. Peterman, in F. Ricca (Ed.), Adsorption-Desorption Phenomena, Academic Press, London, 1972. W. Jecand and D. Menzel, Surf. Sci., 42 (1974) 485. 216 J.C. Tracy, J. Chem.Phys., 56 (1972) 2736, 2748. 217 F.L. Hughes and H. Levinstein, Phys. Rev., 113 (1959) 1029. 218 J.B. Hudson and J.S. Sandejas, J. Vac. Sci. Technol., 4 (1967) 230. 219 J. Perel, R.H. Vernon and H.L. Daley, J. Appl. Phys., 36 (1965) 2157. 220 M.D. Scheer and J. Fine, J. Chem. Phys., 37 (1962) 107; 38 (1963) 307. 221 A.Y. Cho and D.D. Hendricks, J. Appl. Phys., 40 (1969) 339. 222 J.B. Hudson and C.M. Lo, Surf. Sci., 36 (1973) 141. 223 C.R. Helms and R.J. Madix, Surf. Sci., 52 (1975) 677. 224 T. Engel, J. Chem. Phys., 69 (1978) 378. 225 M. Salmeron, R.J. Gale and G.A. Somorjai, J. Chem. Phys., 70 (1979) 2807. 226 J.J. Czyzewski, T.E. Madey and J.T. Yates, Phys. Rev. Lett., 32 (1974) 777. T.E. Madey, J.J. Czyzewski and J.T. Yates, Surf. Sci., 49 (1975) 465.

169 227 228 229

230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273

D.W. Bassett, in M.W. Roberts and J.M. Thomas (Eds.), and Defect Properties of Solids, Vol. 2, The Chemical Society, London 1973, p. 34. Ya. M. Fogel, Sov. Phys. Usp., 1 0 (1967) 17. M. Barber and J.C. Vickerman, in M.W. Roberts and JN. Thomas (Eds.), Specialist Periodical Reports: Surface and Defect Properties of Solids, Vol. 5 , The Chemical Society, London, 1976. J. Stensgaard, L.C. Feldman and P.J. Silverman, Phys. Rev. Lett., 42 (1979) 247. R.C.L. Bosworth, Proc. R. SOC. London Ser. A, 150 (1935) 58; 154 (1936) 112. H.M. Love and H.D. Wiederick, Can. J. Phys., 47 (1969) 657. A. Polak and G. Ehrlich, J. Vac. Sci. Technol., 1 4 (1977) 407. M. Bowker and D.A. King, Surf. Sci., 9 4 (1980) 564. A. Dekker, Solid State Physics, Macmillan, London, 1960, Chap. 17. D. Voreades, Surf. Sci., 60 (1978) 326. G. Ashwell, C.J. Todd and R. Heckingbottom, J. Phys. E, 6 (1973) 435. R.G. Muskett, J. LessCommon Met., 22 (1970) 175. D. Lichtmann and J.C. Campuzano, Jpn. J. Appl. Phys. Suppl, 2 ( 2 ) (1974) 189. R. Butz and H. Wagner, Surf. Sci., 6 3 (1977) 448. K. Besocke and S. Berger, Rev. Sci. Instrum., 47 (1976) 840. R.W. Wood, Phys. Rev., 5 (1897) 1. W. Schottky, 2. Phys., 1 4 (1923) 63. D.L. Adams and L.H. Germer, Surf. Sci., 40 (1968) 1361. S.P. Singh-Boparai and D.A. King, Surf. Sci., 5 3 (1975) 55. R. Gomer, R. Wortman and R. Lundy, J. Chem. Phys., 26 (1957) 1147. R. Wortman, R. Gomer and R . Lundy, J. Chem. Phys., 27 (1957) 1099. R. Gomer and J.K. Hulm, J. Chem. Phys., 27 (1957) 1363. R. Klein, J. Chem. Phys., 22 (1954) 1406; 31 (1959) 1306. I.S. Marinova and Y.V. Zubenko, Sov. Phys. Solid State, 1 2 (1976) 401. S. Normura and E. Sugata, Jpn. J. Appl. Phys., 9 (1970) 1229. L.D. Schmidt and R . Gomer, J. Chem. Phys., 42 (1965) 3573. G. Vladimirov, B. Medvedev and J. Sokol’skaya, Sov. Phys. Solid State, 1 2 (1970) 1118. E.W. Plummer and R.D. Young, Phys. Rev. B, 1 (1970) 2088. M. Asada a n d M. Masuda, Surf. Sci., 99 (1980) L429. A.B. Meador, T.M. Lin and H.B. Huntington, Surf. Sci., 97 (1980) 53. R. Butz and H. Wagner, Surf. Sci., 87 (1979) 69. I. Langmuir and J.B. Taylor, Phys. Rev., 40 (1932) 463. W.M. Brattain and J.A. Becker, Phys. Rev., 4 3 (1933) 428. A. Abramenkov, V. Slezov, L. Tanatarov and Y. Fogel, Sov. Phys. Solid State, 1 2 (1971) 411. M. Renard and D. Deloche, Surf. Sci., 35 (1973) 487. W.F. Roseneark, J. Am. Chem. Soc., 5 3 (1931) 1951. C. Pisani, G. Rabino and P. Ricca, Surf. Sci., 41 (1974) 277. G. Kneringer and F. Netzer, Surf. Sci., 49 (1979) 125. A.M. Booth, Can. J. Chem., 32 (1954) 214. F.M. Lord and J.S. Kittelberger, Surf. Sci., 4 3 (1974) 173. C.H. Chan, R. Aris and W.H. Weinberg, Appl. Surf. Sci., 4 3 (1974) 173. D. Edwards, Surf. Sci., 54 (1976) 1. L.D. Schmidt, Catal. Rev., 9 (1974) 115. S. Brunauer, K.S. Love and C.G. Keegan, J. Am. Chem. SOC.,64 (1972) 751. C. Aharoni, F.C. Tompkins, Adv. Catal., 21 (1970) 1. E. Winter, J. Catal., 4 (1965) 134. D.O. Hayward and B.M. Trapnell, Chemisorption, Butterworths, London, 1964.

170 274 275 276 277 278 279 280 28 1 28 2 28 3 28 4 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 31 2 31 3 314 315 316 317 318 319 320

J.L. Falconer and R.J. M a d k , J. Catal., 48 (1977) 262. R.W. McCabe and L.D. Schmidt, Surf. Sci., 65 (1977) 189. P.W. Tamm and L.D. Schmidt, J. Chem. Phys., 54 (1971) 4775. P.W. Tamm and L.D. Schmidt, J. Chem. Phys., 5 1 (1969) 5352; 52 (1970) 1150. A. Cassuto, J. Fusy and A. Pentenero, Jpn. J. Appl. Phys. Suppl; 2 ( 2 ) (1974) 79. C.H. Steinbruchel, Appl. Phys. Lett., 1 9 (1979) L13. J.L.Falconer, PhD Thesis, Stanford University, 1974. J.R. Arthur and A. Cho, Surf. Sci., 38 (1973) 394. M. Bienfait and J.A. Venables, Surf. Sci., 64 (1977) 425. G. le Gay, M. Mannerville and R. Kern, Surf. Sci., 65 (1977) 261. R. Kern and G. le Gay, J. Phys. (Paris), 38 (1977) 155. M. Bertucci, G. Le Gay, M. Mannerville and R. Kern, Surf. Sci., 85 (1979) 471. R. Kern, Surf. Sci., 9 3 (1979) 101. J.A. Schwartz, R.S. Polizzotti and J.J. Burton, Surf. Sci., 67 (1977) 10. H.A. Engelhardt and D. Menzel, Surf. Sci., 57 (1976) 591. M. Bowker, Surf. Sci., 100 (1980) L472. R. Klein and J.T. Yates, Jpn. J. Appl Phys. Suppl., 2 ( 2 ) (1974) 461. J.A. Venables and M. Bienfait, Surf. Sci., 61 (1970) 667. J. Suzanne, J.P. Coucomb and M. Bienfait, J . Cryst. Growth, 31 (1975) 87. D.A. King, T. Madey and J.T. Yates, J. Chem. Phys., 55 (1971) 3247. M. Shanabarger, Surf. Sci., 52 (1975) 689. M. Shanabarger, Solid State Common., 1 4 (1974) 1015. J. Lapujoulade and K. Neil, Surf. Sci., 35 (1973) 288. R. Gorte and L.D. Schmidt, Surf. Sci., 76 (1978) 559. D.A. King, Surf. Sci., 64 (1977) 43. R.A. Shigeishi and D.A. King, Surf. Sci., 58 (1976) 379. G. Ertl, J. Vac. Sci. Technol., 14 (1977) 435. J.M. McCreery and G . Wolken, J. Chem. Phys., 64 (1976) 2845. R.A. Van Santeen, Surf. Sci., 5 3 (1975) 35. K. Christmann, G. Ertl and T. Pignet, Surf. Sci., 54 (1976) 365. D.L. Adams and L.H. Germer, Surf. Sci. 32 (1972) 205. H. Weiss, G. Ertl and F. Nietsche, Appl. Surf. Sci., 2 (1979) 614. K. Christmann, R.J. Behm, G. Ertl, M.A. van Hove and W.H. Weinberg, J. Chem. Phys., 70 (1979) 4168. R. Ku, N.A. Gjostein and H.P. Bonzel, Surf. Sci., 64 (1977) 465. T.E. Madey and D. Menzel, Jpn. J. Appl. Phys. Suppl, 2 ( 2 ) (1974) 229. H. Bacal, J. Desplat and T. Alleau, J. Vac. Sci. Technol., 9 (1972) 89. J. Lapujoulade and K. Neil, Surf. Sci., 35 (1973) 288. T. Fuller and P.J. Estrup, Surf. Sci., 54 (1976) 179. P. Jewsbury and J.L. Beeby, J. Phys. C, 8 (1975) 353. S. Holloway, P. Jewsbury and J.L. Beeby, Surf. Sci., 6 3 (1977) 339. S.B. Whitehouse and P. Jewsbury, Appl. Phys., 1 9 (1979) 387. D.L. Adams,Surf. Sci., 42 (1974) 12. T. Toya, J. Vac. Sci. Technol., 9 (1972) 890. A.R. Miller, The adsorption of Gases o n Solids, Cambridge University Press, 1949. J.K. Roberts and A.R. Miller, Proc. Cambridge Philes. SOC.,35 (1939) 293. S. Glasstone, K.J. Laidler and H. Eyring, The Theory Of Rate Processes, McGraw-Hill, New York, 1941. R.E. Peierls, Proc. Cambridge Philos. SOC., 32 (1936) 471. D.L. Adams and L. Germer, Surf. Sci., 23 (1970) 419. R.H. Fowler and E.A. Guggenheim, Statistical Thermodynamics, Cambridge University Press, 1939.

171 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 34 3 344 345 346 347 348 349 350 35 1 352 353 354 355 356 35 7 358 359 360 361 362

D.A. King, Surf. Sci., 47 (1975) 384. M. Alnot, J. Fusy and A. Cassuto, Surf. Sci., 72 (1978) 467. M. Alnot, J. Weber, J. Fusy and A. Cassuto, Appl. Surf. Sci., 2 (1979) 578. D.M. Collins and W.E. Spicer, Surf. Sci., 69 (1977) 85. H.P. Bonze1 and R. Ku, J. Chem. Phys., 58 (1973) 4617. R.J. Madix, Adv. Catal., 29 (1981) 1. R.J. Madix, Surf. Sci., 89 (1979) 540. M. Bowker and R.J. Madix, Surf. Sci. 102 (1981) 542. D. Ying and R.J. Madix, J. Catal., 61 (1980) 48. L.A. Larsen and J.T. Dickenson, Surf. Sci., 46 (1974) 473. J.L. Falconer and R.J. Madix, Surf. Sci., 46 (1974) 473. M. Bowker and R.J. Madix, Surf. Sci., 95 (1980) 190. I.E. Wachs and R.J. Madix, J. Catal., 5 3 (1978) 208. B. Sexton, Surf. Sci., 88 (1979) 299. L.W. Swanson and R . Gomer, J. Chem. Phys., 39 (1963) 2813. L.J. Rigby, Can. J. Phys., 42 (1964) 1256. M. Bowker and D.A. King, J. Chem. Soc. Faraday Trans. 1, 76 (1980) 758. C. Leung, M. Vass and R . Gomer, Surf. Sci., 66 (1977) 67. W.F. Egelhoff, V.W. Linnett and D.L. Perry, Faraday Discuss. Chem. Soc., 58 (1974) 35. C.G. Goymour and D.A. King, J. Chem. Soc. Faraday. Trans. 1 , 6 8 (1972) 289. A.K. Mazumdar and H. Wassmuth, Surf. Sci., 34 (1973) 249. C. Kemball, Proc. R. Soc. London Ser. A, 187 (1946) 73. P. Feulner, H. Engelhard and D. Menzel, Chem. Phys. Lett., 5 9 (1978) 481. H. Ibach, W. Erley and H. Wagner, Surf. Sci., 92 (1980) 29. L.D. Landau and E.M. Lifshitz, Lehrbuch der Theoritsche Physik, Vol. V, Adademie-Verlag, Berlin, 1975, p. 141. E. Bauer, F. Bonczek, H. Poppa and G. Todd, Surf. Sci., 5 3 (1975) 87. L.A. Peterman, in F. Ricca (Ed.), Adsorption-Desorption Phenomena, Academic Press, New York, 1972, p. 277. H. Suhl, J.M. Smith and P. Fumar, Phys. Rev. Lett., 25 (1970) 1442. H.A. Kramers, Physica, 7 (1940) 824. P.J. Pagni and J.C. Keck, J. Chem. Phys., 58 (1973) 1162. P.J. Pagni, J. Chem. Phys., 58 (1973) 2940. B. Bendow and J.C. Ying, J. Vac. Sci. Technol., 9 (1972) 8041; Phys. Rev. B, 75 (1973) 622. G. Broden, T.N. Rhodin, C. Brucker, R. Benbow and Z. Hurych, Surf. Sci., 5 9 (1976) 593. B.E. Nieuwenhuys, Surf. Sci., 1 0 5 (1981) 505. M. Salmeron, R.J. Gale and G.A. Somorjai, J. Chem. Phys., 67 (1977) 5324. L.R. Clavenna and L.D. Schmidt, Surf. Sci., 22 (1970) 365. N. Pacia, B. Weber and A. Pentenero, Surf. Sci., 49 (1975) 330. P.W. Tamm and L.D. Schmidt, J. Chem. Phys., 55 (1971) 4253. I.D. Gay, M. Textor, R. Mason and Y. Iwasawa, Proc. R. Soc. London, 856 (1977) 25. M. Balooch, M.J. Cardillo, D.R. Miller and R.E. Stickney, Surf. Sci., 46 (1974) 358. C.S. Steincruchel and L.D. Schmidt, Phys. Rev. B, 1 0 (1974) 4209; Phys. Rev. Lett., 32 (1974) 11. H. Bickley, J.S. Arlow, M.A. Morris and D.A. King, Vacuum, 31 (1980) 507. J.G. McCarty and R.J. Madix, Surf. Sci., 74 (1978) 109. T.N. Taylor and P.J. Estrup, J. Vac. Sci. Technol., 1 0 (1973) 26. N.H. Madden, J. Kuppers and G. Ertl, J. Chem. Phys., 58 (1973) 3401. 0. Beeck, Adv. Catal., 2 (1950) 151. G. Wedler, H. Papp and G. Schroll, Surf. Sci., 44 (1974) 463.

172 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391

392 393 394 395 396 397 398 399

C. Kohrt and R. Gomer, Surf. Sci., 40 (1973) 71. G. Thomas and D.A. King, Surf. Sci., 92 (1980) 201. T.E. Madey, Surf. Sci., 36 (1973) 281. M.G. Wellsand D.A. King, J. Phys. C, 7 (1974) 4053. L.R. Clavenna and L.D. Schmidt, Surf. Sci., 33 (1972) 11. H.P. Bonze1 and R. Ku, Surf. Sci., 40 (1973) 85. R.A. Marbrow and R.M. Lambert, Surf. Sci., 61 (1976) 317. P.J. Goddard, J. West and R.M. Lambert, Surf. Sci., 7 1 (1978) 447. A.M. Horgan and D.A. King, in G.A. Somorjai (Ed.), The Structure and Chemistry of Solid Surfaces, Wiley, New York, 1969. M. Bowker and D.A. King, J. Chem. SOC.Faraday Trans. 1 , 7 5 (1979) 2100. S.M. LopezSancho and J.L. de Segovia, Surf. Sci., 30 (1972) 419. B. McCaroll and G. Ehrlich, J. Chem. Phys., 35 (1963) 523. J.K. NQrskov and B.I. Lundqvist, Surf. Sci., 89 (1979) 251. R. Sedlmeir and W. Brenig, Z. Phys., 1330 (1980) 245. J.W. Gadzuk and H. Metill, Phys. Rev. B, 22 (1980) 2603. R. Brako and D.M. Newns, Solid State Commun., 33 (1980) 713. E. Muller-Marthan, T.V. Ramafrishman and G. Toulouse, Solid State Commun., 9 (197 1)99. G. Boato, P. Cantini and L. Mattera, J. Chem. Phys., 65 (1976)544. B. Feuerbacher and W. Allison, Phys. Rev. Lett., 45 (1980) 2040. M. Knudsen, Ann. Phys., 34 (1911) 593. D.O. Goodman, Surf. Sci., 24 (1971) 667; Prog. Surf. Sci., 5 (1975) 261. J. Wachman, J. Am. Rocket SOC.,32 (1962) 2. R.P. Thorman and S.L. Bernasek, J. Chem. Phys., 74 (1981) 6448. G.A. Somorjai and S.B. Brumbach, Crit. Rev. Solid State Sci., (1974) 429. M. Salmeron, R.J. Gale and G.A. Somorjai, J. Chem. Phys., 67 (1977) 5324. K.C. Janda, J.E.Hurst, C.A. Becker, J.P. Cowin, D.J. Auerbach and L. Wharton, J. Chem. Phys., 72 (1980) 2403. R.P. Merrill and W.H. Weinberg, J. Vac. Sci. Technol., 8 (1979) 718; J. Chem. Phys., 56 (1972) 2881. D. Menzel, Proc. Int. Conf. Rarified Gas Dynamics, Vol. 2,1974, p.El4.1. J.K. Roberts, Some Problems in Adsorption, Cambridge University Press, Cambridge, 1939. L.A. West and G.A. Somorjai, J. Chem. Phys., 54 (1971) 2864. L. Landau, Phys. Z. Sowjetunion? 8 (1935) 489. The theories used are based o n work by N.B. Cabrera and R.W. Zwanzig, Discuss. Faraday Sco., 28 (1959) 16; R.W. Zwanzig, J. Chem. Phys., 32 (1960) 1173 and work has been continued extensively: F. Goodman, J. Phys. Chem. Solids, 23 (1962), 1491. F.C. Hurlbut, Univ. Calif. Eng. Proj. Rep. HE 150, 1962, p. 208. R.M. Logan and J.C. Keck, J. Chem. Phys., 44 (1966) 195; 49 (1968) 860. R.E. Stickney, in G.A. Somorjai (Ed.), The Structure and Chemistry of Solid Surfaces, Wiley, New York, 1961, p. 41. A.F. Devonshire, Proc. R. SOC.London Ser A, 158 (1937) 269. F.O. Goodman, J. Phys. Chem. Solids, 24 (1963) 1451;Surf. Sci., 92 (1980) 165. A. Cassuto and D.A. King, Surf. Sci., 102 (1981) 288. D. Menzel and R. Gomer, J. Chem. Phys., 41 (1964) 3329. C. Leung, M. Vass and R. Gomer, Surf. Sci., 67 (1977) 21. D.A. King, J. Vac. Sci. Technol., 1 7 (1980) 241. R. Gomer, Discuss. Faraday SOC.,28 (1959) 23. P. Hofmann, K. Horn, A.M. Bradshaw and K. Jacobi, Surf. Sci., 82 (1979) L610. Reviewed by M.W. Roberts and C.S. McKee, Chemistry of the Metal-Gas Interface, Oxford University Press, Oxford, 1978.

173 400 401 402 403 404 405 406 407 408 409 410

R. Gorte and L.D. Schmidt, Surf. Sci., 76 (1978)559. D.O. Hayward and M.R. Walters, Jpn. J. Appl. Phys. Suppl, 1 (2)(1974)587. T. Engel, J. Chem. Phys., 69 (1978)373. K.C. Janda, J.E. Hurst, C.A. Becker, J.P. Cowin, D.J. Auerbach and L. Wharton, Surf. Sci., 93 (1980)270. P.R. Norton and J.W. Goodale, personal communication, 1983. L. Boltzmann, Wien Ann., 53 (1934)53. J. Crank, The Mathematics of Diffusion, Clarendon Press, London, 1936,p. 36. M. Bowker and D.A. King, Surf. Sci., 72 (1978)208. C. Matano, Jpn. J. Phys., 8 (1933)109. See, for example, J.R. Manning, Diffusion Kinetics for Atoms in Crystals, Vol. 2, Van Nostrand, New York, 1968. W. Jost, Diffusion o n Solids, Liquids and Gases, Academic Press, New York,

1972. 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431

A.B. Meador, T.H. Lin and H.B. Huntington, Surf. Sci., 97 (1980)53. J.R. Chen and R. Gomer, Surf. Sci., 79 (1979)413. A. Bell, R. Gomer and H. Reiss, Surf. Sci., 55 (1976)494. T. Sakata and S.Nakamura, Surf. Sci., 51 (1975)313. T.Sakata and S. Nakamura, Jpn. J. Appl. Phys., 14 (1975)943. D.A. Reed and G. Ehrlich, Philos. Mag., 32 (1975)1095. H.M. Love and H.D. Wiederick, Can. J. Phys., 47 (1969)657. R. Butz and H. Wagner, Surf. Sci., 87 (1979)69. R. Butz and H. Wagner, Surf. Sci., 87 (1979)85. F.O. Goodman and H.Y. Wachman, Dynamics of Gas-Surface Scattering, Academic Press, New York, 1976. A. Cassuto and D.A. King, Surf. Sci., 102 (1981)388. I. Langmuir, J. Am. Chem. SOC.,40 (1918)1361. J.L. Morrison and J.K. Roberts, Proc. R. SOC.London Ser. A, 173 (1939)13. J.A. Becker and C.D. Hartman, J. Phys. Chem., 57 (1953)157. G. Ehrlich, J. Phys. Chem., 59 (1955)473. P. Kisliuk, J. Phys. Chem. Solids, 3 (1957)95;5 (1958)78. K. Schonhammer, Surf. Sci., 83 (1979)L633. K.P. Kieffer and G.A. Bootsma, Proc. 7th Int. Vac. Congr., Vol. 2, 1977,p. 877. A. Cassuto, S. Tang and J.B. Fenn, presented a t 7th Int. Symp. Rarefied Gas Dynamics, Pisa, 1970. D.L. Adams and L.H. Germer, Surf. Sci., 30 (1972)205. H.F. Winters, P. Morgan, S. Tougards and J. Onsgaard, Suppl. Rev. Vide, 2091

(1980)201. 432 P. Alnot and D.A. King, Surf. Sci,, in press, 433 J.T. Yates and T.E. Madey, J. Chem. Phys., 43 (1965)1055;J. Chem. Phys., 44 (1966)1675. 434 P.D. Garland, Surf. Sci., 62 (1977)373. 435 E. Fromm, Surf. Sci., 52 (1975)401. 436 E. Fromm and C. Mayer, Surf. Sci., 52 (1975)415. 437 L.D. Schmidt, in E. Drauglis and R.I. Jaffee (Eds.), The Physical Basis for Heterogeneous Catalysis, Plenum Press, New York, 1975,p. 451. 438 C.S. Alexander and J. Pritchard, J. Chem. SOC. Faraday Trans. 1, 68 (1972) 202. 439 R.L. Palmer, J.N. Smith, Jr., H. Saltsburg and D.R. O’Keefe, J. Chem. Phys., 53 (1970)1666. 440 M.J. Cardillo, B. Balooch and R.F. Stickney, Surf. Sci., 50 (1975)263. 447 W.Van Willigen, Phys. Letts. A, 28 (1968)80. 448 M. Ballooch and R.F. Stickney, Surf. Sci., 44 (1974)310. 449 G. Comsa, E. David and B.J. Schumacher, Surf. Sci. 85 (1979)45.

174 450 45 1 45 2 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 47 1 472 473 474 475 476 477 478 479 480 48 1 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497

R.C. Cosser, S.R. Bare, S.M. Francis and D.A. King, Vacuum, 3 1 (1980) 503. F.C. Hurlbut, J. Appl. Phys., 28 (1957) 844. R.C. Cosser, to be published. A Clarke, The Theory of Adsorption and Catalysis, Academic Press, New York, 1970, p. 220. R.H. Fowler, Proc. Cambridge Philos. SOC.,31 (1935) 260. K.J. Laidler, in P. Emmett and I. Emmett (Eds.), Catalysis, Vol. 1, Reinhold, New York, 1954, p. 195. G. Ehrlich, J. Chem. Phys., 31 (1959) 1111. R.G. Jones and D.L. Perry, Surf. Sci., 71 (1978) 59. R.G. Jones and D.L. Perry, Surf. Sci., 8 2 (1979) 540. C. Backx, C.P.M. De Groot and P. Biloen, Surf. Sci., 104 (1981) 300. M. Bowker, M. Barteau and R.J. Madix, Surf. Sci., 92 (1980) 528. G. Rovida, F. Pratesi, M. Maglietta and E. Ferroni, Surf. Sci., 43 (1974) 230. V.N. Ageev and N.I. Ionov, Sov. Phys. Tech. Phys., 16 (1972) 1742. J.L. Taylor, D.F. Ibbotson and W.H. Weinberg, Surf. Sci., 79 (1979) 349. V.P. Ivanov, G.K. Boneskov and V.J. Sauchenko, Surf. Sci., 61 (1976) 207. W. Greenes and R.E. Stickney, Surf. Sci., 11 (1968) 395. P.A. Redhead, Can. J. Phys., (1964) 886. N.P. Vasko, G. Yu. Ptushinski and B.A. Chuikov, Surf. Sci., 14 (1968) 448. G.M. Bliznakoy and M.P. Kiskinova, J. Catal., 61 (1980) 299. M. Alnot, A. Cassuto, J. Fusy and A. Pentenero, Jpn. J. Appl. Phys. Suppl., 2 ( 2 ) (1974) 79. D.M. Collins, W.F. Spicer and J.B. Lee, Surf. Sci., 55 (1976) 389. G. Kneringer and F.P. Netzer, Surf. Sci., 49 (1975) 125. M. Wilf and P.T. Dawson, Surf. Sci., 65 (1977 ) 399. C.T. Campbell, G. Ertl, M. Kuipper and J. Segner, Surf. Sci., 107 (1981) 220. J.L. Gland, Surf. Sci., 9 3 (1980) 487. K. Schwama and F. Bechtold, Surf. Sci., 65 (1977) 277. J.L. Gland and V.N. Korchok, Surf. Sci., 75 (1978) 733. P.A. Thiel, J.T. Yates and W.H. Weinberg, Surf. Sci., 82 (1979) 22. R. Klein and A. Shim, Surf. Sci., 69 (1977) 403. P.D. Reed, R.M. Lambert and C.M. Comrie, Surf. Sci., 64 (1977) 603. V.N. Ageev and N.I. Ionov, Sov. Phys. Solid State, 1 3 (1971) 1305. A.K. Mazumdar and M.W. Wassmuth, Surf. Sci., 30 (1972) 617. W. Engelmaier and R.E. Stickney, Surf. Sci., 11 (1968) 370. J.A. Becker, F.J. Becker and R.G. Brandes, J. Appl. Phys., 32 (1981) 411. M.W. Wassmuth, H. Werner and A.F. Mazumdar, J. Vac. Sci. Technol., 9 (1972) 835. N.P. Vasko, Yu.G. Ptushinski and R.A. Chuikov, Surf. Sci., 14 (1968) 448. E. Bauer, H. Popp and Y. Vishuanath, Surf. Sci., 58 (1978) 517. M. Bacal, L. Desplat and T. Alleau, J. Vac. Sci. Technol., 9 (1972) 851. T. Engel, M. Niehus and E. Bauer, Surf. Sci., 52 (1975) 237. N.P. Vasko, Yu. G. Ptushinski and A.A. Mitryaeu, Sov. Phys. Solid State, 1 5 (1974) 1942. T. Engel, T. von dem Hagen and E. Bauer, Surf. Sci., 62 (1977) 361. R. Dus and W. Lisowski, Surf. Sci., 61 (1976) 635. M.E. Bridge, C.M. Comrie and R.M. Lambert, J. Catal., 58 (1979) 28. H.D. Malev, Sov. Phys. Tech. Phys., 17 (1973) 2009. J. Yasumori, N . Momma and M. Kiyomi, Jpn. J. Appl. Phys. Suppl., 2 ( 2 ) (1974) 485. E. Choenet and R.W. Coughlin, J. Catal., 27 (1972) 243; 28 (1973) 44. J. Benziger and R.J. Madix, Surf. Sci., 9 4 (1980) 119. D.E. Ibbotson, T.S. Wittrig and W.H. Weinberg, Surf. Sci., 110 (1981) 294.

175 498 499 500 501 502 503 504

V.J. Mimeault and R.S. Hansen, J. Chem. Phys., 45 (1965) 2240. R.J. Bickley, M.W. Roberts and W.C. Story, J. Chem. SOC.A, (1971) 2774. G.E. Moore and F.C. Unterwald, J. Chem. Phys., 40 (1964) 2626. H.R. Han and L.D. Schmidt, J. Phys. Chem., 75 (1971) 227. M. Mahning and L.D. Schmidt, Z. Phys. Chem., 80 (1972) 71. S.M. KO and L.D. Schmidt, Surf. Sci., 42 (1974) 508. D.I. Hagen and F.F. Donaldson, Surf. Sci., 45 (1974) 61; J. Nucl. Mater., 53

(1974) 281. 505 S. Johnson and R.J. Madix, Surf. Sci., 108 (1981) 77. 506 K. Christmann, Z. Naturforsch. Teil A, 34 (1979) 22. 507 K. Christmann, 0. Schober, G. Ertl and N. Neumann, J. Chem. Phys, 60 (1974) 4528. 508 G. Ertl and D. Kueppers, Ber. Bunsenges. Phys. Chem., 75 (1971) 1017. 509 J. McCarty, J. Falconer and R.J. Madix, J. Catal., 30 (1973) 235. 510 J. Lapujoulade and K.S. Neil, J. Chem. Phys., 57 (1972) 3535. 511 A.W. Aldag and L.D. Schmidt, J. Catal., 22 (1971) 280. 512 M. Conrad, G. Ertl and E.E. Latta, Surf. Sci., 41 (1974) 435. 513 J.J. Stephan, V. Ponec and W.M.H. Sachtler, J. Catal., 37 (1975) 81. 514 H. Procop and J. Voelter, Surf. Sci., 33 (1972) 69. 515 H.U.P. Wiesendanger, J. Catal., 2 (1963) 538. 516 R.W. McCabe and L.D. Schmidt, Proc. 7th Int. Vac. Congr., Vol. 2, 1977, p. 120. 517 F.P. Netzer and G. Kneringer, Surf. Sci., 5 1 (1975) 526. 518 W.H. Weinberg, D.R. Monroe, V. Lampton and R.P. Merril, J . Vac. Sci. Technol. 14 (1977) 444. 519 M. Salmeron, R.J. Gale and G.A. Somorjai, J. Chem. Phys., 70 (1979) 2807. 520 K.E. Lu and R.R. Rye, Surf. Sci., 45 (1974) 677. 521 K. Christmann and G. Ertl, Surf. Sci., 60 (1976) 365. 522 R. Ducros, J.J. Ehrhardt, M. Alnot and A. Cassuto, Surf. Sci., 55 (1976) 509. 523 L.R. Dainellson, M.J. Dresser, E.E. Donaldson and J.J. Dickenson, Surf. Sci., 7 1 (1978) 599. 525 V.D. Bekov, Y.K. Ustinov and A.P. Komar, Sov. Phys. Tech. Phys., 2 1 (1976) 1415. 526 J.A. Schwarz, R.S. Polizzotti and J.J. Burton, Surf. Sci., 67 (1977) 10. 527 L.J. Rigby, Can. J. Phys., 43 (1962) 1965. 528 Yu. G. Ptushinskii and B.A. Chuikov, Kinet. Catal. USSR, 5 (1964) 444. 529 F. Roman and J.L. de Segovia, Proc. 7th Int. Vac. Congr., Vol. 2,1979, p. 971. 530 R.A. Barker and P.J. Estrup, Phys. Rev. Lett., 41 (1978) 1307. 531 T.E.Madey, Surf. Sci., 29 (1972) 571. 532 M.E. Bridge, C.M. Comrie and R.M. Lambert, Surf. Sci., 67 (1977) 393. 533 K. Yoshida, Jpn. J. Appl. Phys., 20 (1981) 823. 534 J. Keuppers and A. Plagge, J. Vac. Sci. Technol., 13 (1976) 259. 535 C.M. Comrie and W.H. Weinberg, J. Chem. Phys., 64 (1976) 250. 536 P.A. Zitdan, G.K. Bereskov and A.I. Baronin, Chem. Phys. Lett., 44 (1976) 528. 537 D.I. Hagen, B.E. Nieuwenhuys, G. Rovida and G.A. Somorjai, Surf. Sci., 57 (1976) 632. 538 D.A. Degras, J. Chim. Phys., 64 (1967) 405. 539 E. Gillet, J.C. Chiarena and M. Gillet, Surf. Sci., 54 (1976) 601. 540 S. Semancic and P.J. Estrup, Surf. Sci., 104 (1981) 26. 541 L. Lecante, R. Riwan and C. Guillot, Surf. Sci., 35 (1973) 271. 542 L.D. Matthews, Surf. Sci., 24 (1971) 248. 543 V. Viswanath and L.D. Schmidt, J. Chem. Phys., 59 (1973) 4184. 544 E. Gillet, J.C. Chiamena and M. Gillet, Surf. Sci., 66 (1977) 596.

176 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593

D.A. Degras, Nuovo Cimento Suppl., 5 (1967)420. J. Lapujoulade, Nuovo Cimento Suppl., 5 (1967)433. M. Kiskinova and D.W. Goodman, Surf. Sci., 108 (1981)64. C.R. Helms and R.J. Madix, Surf. Sci., 52 (1975)677. M.M. Madden, J. Kueppers and G. Ertl, J. Chem. Phys., 58 (1973)3401. K. Christmann, 0.Schober and G. Ertl, J. Chem. Phys., 60 (1974)4719. H. Conrad, G. Ertl, J. Kueppers and E.E. Latta, Surf. Sci., 57 (1976)475. G.M. Bliznakov and G.M. Kiskinova, J. Catal., 61 (1980)299. H. Conrad, G. Ertl, J. Kuch, and E.E. Latta, Surf. Sci., 43 (1974)462. K. Kawasaki, H.Miki, T. Kyoko and T. Kodama, Surf. Sci., 64 (1977)349. W.L. Winterbottom, Surf. Sci., 36 (1973)195. M.A. Barteau, E.I. KO and R.J. Madix, Surf. Sci., 102 (1981)99. R.W. McCabe and L.D. Schmidt, Surf. Sci., 66 (1977)101. H.P. Bonzel and R. Ku, J. Chem. Phys., 58 (1973)4617. H.P. Bonzel and J.J. Burton, Surf. Sci., 52 (1975)223. C.M. Comrie and R.M. Lambert, J. Chem. SOC. Faraday Trans. 1, 72 (1976) 1659. G. Ertl, H. Neumann and K.M. Streit, Surf. Sci., 64 (1977)393. P.R. Norton, J.W. Goodale and E.B. Selkirk,Surf. Sci., 83 (1979)189. S.R. Keleman, T.E. Fischer and J.A. Schwarz, Surf. Sci., 81 (1979)440. D.H. Winicur, J. Hurst, C.A. Becker and L. Wharton, Surf. Sci., 109 (1981) 263. C.T. Cambell, G. Ertl, H. Kuppers and J. Segner, Surf. Sci., 107 (1981)207. T.H. Lin and G.A. Somorjai, Surf. Sci., 107 (1981)573. D.H. Collins and W.E. Spicer, Surf. Sci., 69 (1977)85. R.P.H. Gasser, R. Thwaites and J. Wilkinson, Trans. Faraday SOC., 63 (1967) 195. M. Alnot, J.J. Ehrhardt, F. Fusy and A. Cassuto, Surf. Sci., 46 (1974)81. V.V. Gorodetskii and B.E. Niuwenhuys, Surf. Sci., 105 (1981)299. D.G. Chastner, B.A. Sexton and G.A. Somorjai, Surf. Sci., 71 (1978)59. T.E. Madey and D. Menzel, Jpn. J. Appl. Phys. Suppl., 2 (2)(1974)229. P.D. Reed, R.M. Lambert and C.M. Comrie, Surf. Sci., 59 (1970)33. K. Yu. Ustinov, Sov. Phys. Tech. Phys., 17 (1973)1701. D.A. Degras, Nuovo Cimento Suppl., 5 (1967)408. G. Ehrlich, J. Chem. Phys., 11 (1962)71. D.A. King, C.G. Goymour and J.T. Yates, Proc. R. Soc. London Ser. A, 331 (1972)361. W.L. Winterbottom, J. Vac. Sci. Technol., 9 (1972)936. C. Wang and R. Gomer, to be published. V.J. Mimeault and R.S. Hansen, J. Phys. Chem., 70 (1966)3001. T.Oguri, J. Phys. SOC.Jpn., 18 (1963)1280. A.A. Parry and J.A. Pryde, Br. J. Appl. Phys., 18 (1967)329. S.M. KO and L.D. Schmidt, Surf. Sci., 42 (1974)408. D.A. King and F.C. Tompkins, Trans. Faraday Soc., 64 (1968)496. J. Lapujoulade and K.S. Neil, C.R. Acad. Sci. Ser. C, 274 (1972)2125. D.A. King, Surf. Sci., 9 (1968)375. M. Grunze, R.K. Driscoll, G.N. Burland, J.C.L. Cornish and J. Pritchard, Surf. Sci., 89 (1979)381. M. Wilf and P.T.Dawson, Surf. Sci., 60 (1976)501. R.A. Shigeishi and D.A. King, Surf. Sci., 62 (1977)379. K.Schwaha and E. Bechtold, Surf. Sci., 66 (1977)383. J.T. Yates and T.E. Madey, J. Chem. Phys., 51 (1969)334. M.P. Hill, S.M.A. Lecchini and B.A. Pethica, Trans. Faraday Soc., 62 (1966) 229. T.E. Madey and J.T. Yates, J. Chem. Phys., 44 (1966)1675.

177 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 61 1 612 61 3 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 6 34 635 636 637 638 639 64 0 64 1

M.F. Winters and D.F. Horner, Surf. Sci., 24 (1971) 587. J.T. Yates. R. Klein and T.E. Madey, Surf. Sci., 58 (1976) 469. P.W. Tamm and L.D. Schmidt, Surf. Sci., 26 (1971) 286. P.J. Goddard, K. Schwama and R.M. Lambert, Surf. Sci., 7 1 (1976) 351. P.J. Goddard and R.M. Lambert, Surf. Sci., 67 (1977) 180. R.A. Marbow and R.M. Lambert, Surf. Sci., 71 (1978) 107. D. Spencer and R.M. Lambert, Surf. Sci., 107 (1981) 237. W. Erley, Surf. Sci., 94 (1980) 281. M.P. Cox and R.M. Lambert, Surf. Sci., 107 (1981) 547. H.M. Kramer and E. Bauer, Surf. Sci., 107 (1981) 1. E. Bechtold, Appl. Surf. Sci., 7 (1981) 231. J.L. Gland, Surf. Sci., 7 1 (1978) 327. J.L. Gland and E.B. Kollin, Surf. Sci., 104 (1981) 478. L.R. Danielson, M.J. Dresser, E.E. Donaldson and J.T. Dickinson, Surf. Sci., 7 1 (1978) 559. F. Solymosi and J. Kiss, Surf. Sci., 108 (1981) 641. F. Solymosi and J. Kiss, Surf. Sci., 108 (1981) 368. S. Golub and D.G. Kedek, Surf. Sci., 45 (1974) 213. G. Gdowski and R.J. Madix, Surf. Sci., 105 (1981) 307. T.S. Wittrig, D.E. Ibbotson and W.H. Weinberg, Surf. Sci., 102 (1981) 506. B. Fischer and J.L. Gland, Surf. Sci., 94 (1980) 446. Y. Akisika, M. Miyamorah, I. Tamari, H. Nishijima and M. Onch, Surf. Sci., 9 3 (1980) 327. G.L. Price and B.G. Baker, Surf. Sci., 9 1 (1980) 571. G.L. Price, B.A. Sexton and B.G. Baker, Surf. Sci., 60 (1976) 506. H. Conrad, G. Ertl, J. Kueppers and E.E. Latta, Surf. Sci., 50 (1975) 296. C.M. Comrie, R.M. Lambert and W.H. Weinberg, Surf. Sci., 57 (1976) 619. R.J. Gorte and J.L. Gland, Surf. Sci., 102 (1981) 348. R. Ducros, M. Alnot, J.J. Ehrhardt, M. Housley, G. Piquard and A. Cassuto, Surf. Sci., 94 (1980) 154. P.D. Reed, C.M. Comrie and R.M. Lambert, Surf. Sci., 72 (1978) 423. E. Cornet, R.W. Coughlin, J. Catal., 27 (1972) 246; 28 (1973) 414. J. Benzinger and R.J. Madix, Surf. Sci., 96 (1980) 114. L. Johnson, M.J. Dresser and E.E. Donaldson, J. Vac. Sci. Technol., 9 (1972) 852. A.M. Horgan and I. Dallins, Surf. Sci., 41 (1974) 624. R. Chappell and D.O. Hayward, J. Vac. Sci. Technol., 9 (1972) 1052. R.W. McCabe and L.D. Schmidt, Surf. Sci., 60 (1976) 85. P.R. Norton and P.T. Richards, Surf. Sci., 44 (1974) 129. S.M. KO and L.D. Schmidt, Surf. Sci., 47 (1975) 557. J.A. Schwarz, R.S. Polizzotti and J.J. Burton, Surf. Sci., 67 (1977) 10. Yu. G. Ptushinskii and B.A. Chuikhov, Kinet. Catal. USSR, 5 (1964) 444. B.D. Barford and R.R. Lye, J. Chem. Phys., 60 (1974) 1046. V.V. Gorodetskii, B.E. Nieuwenhuys, W.H.H. Sachtler and G.K. Boreskov, Surf. Sci., 108 (1981) 225. T.E. Madey, Surf. Sci., 36 (1973) 281. P.G. Hall and D.A. King, Surf. Sci., 42 (1974) 810. E. Fromm and C. Meyer, Surf. Sci., 74 (1978) 259. M. Albers, G.A. Bootsma, W.J. van der Waal and D.L. Gijzeman, Surf. Sci., 77 (1978) 1. R.A. Marbrow and R.M. Lambert, Surf. Sci., 7 1 (1978) 107. W.M. Krueger and S.R. Pollack, Surf. Sci., 20 (1972) 263. P.D. Gartland, Surf. Sci., 62 (1977) 183. A.M. Bradshaw, P. Hofmann and W. Wyrobisch, Surf. Sci., 68 (1977) 269.

178 64 2 A.M. Bradshaw, P. Hofmann, K. Horn and K. Jacobi, Surf. Sci., 8 2 (1979) L610. 643 G. Rovida and H. Maglietta, Proc. 7th Int. Vac. Congr., Vol 2,1977, p. 905. 644 T. Matsuyama and A. Ignatiev, Surf. Sci., 102 (1981) 18. 645 M.E. Bridge and R.M. Lambert, Surf. Sci., 8 2 (1979) 413. 646 P. Hofmann, R. Unwin, W. Wyrobisch and A.M. Bradshaw, Surf. Sci., 72 (1978) 635. 647 F.M. Habreken, F.P. Heffer and G.A. Bootsma, Proc. 7th Int. Vac. Congr., Vol. 2,1977, p. 877. 648 C.R. Brundle, Surf. Sci., 66 (1977) 581. 649 P.B. Sewell, M. Cohen and D.F. Mitchell, Surf. Sci., 33 (1972) 335. 650 I.T. Moriguch and S. Nakansh, Jpn. J. Appl. Phys. Suppl., 2 ( 2 ) (1974) 89. 65 1 G.W. Simmons and D.J. Dwyer, Surf. Sci., 48 (1975) 373. 652 M.A. Chesters and J.C. Riviere, Proc. 7th Int. Vac. Congr., Vol. 2, 1977, p. 873. 653 N.G. Dorfield, J.B. Hudson and R. Zuhr, Surf. Sci., 57 (1976) 460. 654 V.P. Ivanov, G.K. Boreskov and V.I. Sauchenko, Surf. Sci., 61 (1976) 207. 655 T. Miura and Y. Tuzi, Jpn. J. Appl. Phys. Suppl., 2 ( 2 ) (1974) 85. 657 P.R. Norton, R.L. Tapping and J.W. Goodale, Surf. Sci., 65 (1977) 13. 658 T. Matsushita and J.M. White, Surf. Sci., 67 (1977) 122. 659 H. Conrad, G. Ertl, J. Kueppers and E. Latta, Surf. Sci., 65 (1977) 245. 660 B. Weber, J. Fusy and A. Cassuto, J. Chim. Phys., 66 (1969) 708. 661 Y.K. Peng and P.T. Dawson, Can. J. Chem., 52 (1974) 3507. 662 R.J. Voelter, M. Procop and M. Bernd, Surf. Sci., 39 (1973) 453. 663 P.R. Norton, J. Catal., 38 (1975) 211. 664 N. Pacia, B. Weber and A. Pentenero, Surf. Sci., 49 (1975) 330. 664 G. Pirug, G. Broden and H.P. Bonzel, Proc. 7th Int. Vac. Congr., Vol. 2, 1977, p. 967. 666 R. Ducros and R.P. Merril, Surf. Sci., 55 (1976) 227. 667 F.P. Netzer and R.A. Wiley, Surf. Sci., 74 (1978) 547. 668 K. Schwaha and E. Bechtold, Surf. Sci., 65 (1977) 277. 669 J. Fusy, B. Begeard and A. Cassuto, Surf. Sci., 46 (1974) 177. 670 J.C. Fuggle, T.E. Madey, H. Steinkilberg and D. Menzel, Surf. Sci., 52 (1975) 521. 671 T.W. Orent and R.S. Hansen, Surf. Sci., 67 (1977) 325. 67 2 N. Pacia, H.G. Lintz and A. Pentenero, Surf. Sci., 36 (1973) 701. 673 A.F. Dabiri, R.F. Stickney and U.S. Aramati, Surf. Sci., 40 (1973) 205. 674 R.M. Prince and G.R. Floyd, Surf, Sci., 74 (1978) 342. 675 U.P. Zingerman, Sov. Phys. Solid State, 1 4 (1972) 419. 676 J.M. Lopez-Sancho and J.L. d e Segovia, Surf. Sci., 30 (1972) 419. 677 J.L. Desplat, Surf. Sci., 34 (1973) 588. 678 M.G. Wells and D.A. King, J. Phys. C, 7 (1974) 4053. 679 T.E. Madey, Surf. Sci., 3 3 (1972) 355. 680 E. Besocke and S. Berger, Proc. 7th Int. Vac. Congr., Vol. 2,1977, p. 893. 68 1 T.E. Madey, Surf. Sci., 94 (1980) 489. 68 2 B.E. Niewenhuys and G.A. Somorjai, Surf. Sci., 72 (1978) 8. 68 3 E.E. Domann, Surf. Sci., 54 (1976) 529. 684 T.E. Felter and P.J. Estrup, Surf. Sci., 7 6 (1978) 464. 685 E. Gillet, J.C. Chiarena and M. Gillet, Surf. Sci., 66 (1977) 596. 686 J.C. Campuzano, R. Dus and R.G. Greenler, Surf. Sci., 102 (1981) 172. 687 H. Conrad, G. Ertl and J. Kueppers, Surf. Sci., 76 (1978) 323. 688 J.M. Martinez and J.B. Hudson, J. Vac. Sci. Technol., 1 0 (1973) 35. 689 C. Wang and R. Gomer, Proc. 7th Int. Vac. Congr., Vol. 2,1977, p. 1155. 690 H. Ibach. W. Erley and H. Wagner. Surf. Sci.. 9 2 (1980) 29. 691 G. Ertl, M. Grunze and H. W e k , J. Vac. Sci.’Technol.,’l3 (1976) 134.

179 69 2 69 3 694 695 696 691 698 699 700 701 102 703 704 105 706 707

K. Kishi and M.W. Roberts, Surf. Sci., 62 (1977) 252. M. Textor, R. Mason and Y. Iwasawa, Proc. R. Soc. London, 356 (1977) 25. T. Oguri, J. Phys. Soc. Jpn., 19 (1962) 1977. A.H. Dickey, Surf. Sci., 50 (1975) 515. R.A. Shigeishi and D.A. King, Surf. Sci., 62 (1977) 379. U. Bauder and E. Fromm, Surf. Sci., 52 (1975) 415. T. Griffiths and J . Pryde, Trans. Faraday Soc., 63 (1967) 2522. P.T. Dawson and Y.T. Peng, Surf. Sci., 33 (1972) 565. D.L. Adams and L.H. Germer, Surf. Sci., 26 (1971) 109. T.E. Madey and J.T. Yates, Nuovo Cimento Suppl, 5 (1967) 463. J.T. Yates, R. Klein and T.E. Madey, Surf. Sci., 58 (1976) 409. M.E. Bridge and R.M. Lambert, Surf. Sci., 94 (1981) 469. F.G. Barker and R.P. Gasser, Surf. Sci., 39 (1973) 136. N.V. Richardson and J.K. Sass, Surf. Sci., 103 (1981) 496. G. Smith, Surf. Sci., 32 (1972) 527. D. Briggs, R.A. Marbrow and R.M. Lambert, Chem. Phys. Lett., 53 (1978)

708 709 710 711 712 713 7 14

W. Erley and H. Wagner, Surf. Sci., 66 (1977) 371. H.F. Winters, J . C h e m . Phys., 64 (1970) 3495. R.A. Shigeishi, Surf. Sci., 72 (1978) 61. B.J. Hopkins, A.R. Jones and R.J. Winton, Surf. Sci., 57 (1976) 266. J.M. Wilson, Surf. Sci., 53 (1975) 330. M. Weiss, G. Ertl and E. Nitschke, Appl. Surf. Sci., 2 (1979) 614. R.A. Armstrong, in R.B. Anderson and P.T. Dawson (Eds.), Experimental Methods in Catalytic Research, Academic Press, New York, 1976. L. Schmidt and R. Gomer, J. Chem. Phys., 42 (1965) 3573. H. Utsugi and R. Gomer, J. Chem. Phys., 37 (1962) 1706. C. Bhatia and M. Sinha, Surf. Sci., 43 (1974) 369. G. Vladimirov, B. Medvedev and I. Sokol’skaya, Sov. Phys. Solid State, 12

462.

715 716 7 17 718

(1970) 413,1118. 719 7 20 121 722 723 724 725 726 721 128 129 730 731 132 733 134 735 136

A. Melmed, J. Chem. Phys., 43 (1965) 3057. G. Vladimirov and T. Sokol’skaya, Sov. Phys. Solid State, 12 (1970) 1224. L.W. Swanson, R.W. Strayer and L.E. Davis, Surf. Sci., 9 (1968) 165. T. Sokol’skaya and N. Mileshkina, Sov. Phys. Solid State, 6 (1965) 1401. 0. Swenson and M. Sinha, J. Vac. Sci. Technol., 9 (1972) 942. K. Noimann, Sov. Phys. Solid State, 7 (1966) 1642. G. Ehrlich and F. Hudda, J. Chem. Phys. 35 (1961) 1421. R. Klein, J. Chem.Phys., 3 1 (1959) 1306. D. Hayward and R. Gomer, J. Chem. Phys., 30 (1959) 1617. R. Gomer, R. Wortman and R. Lundy, J. Chem. Phys., 26 (1957) 1147. R. Gomer, J. Phys. Chem., 63 (1959) 468. G. Ehrlich and F. Hudda, J. Chem. Phys., 30 (1959) 493. G. Venkatachalam and S. Sinha, Surf. Sci., 44 (1974) 157. R. Wortman, R . Gomer and R. Lundy, J. Chem. Phys., 27 (1957) 1099. C. Satterfield and H. Iino, Ind. Eng. Chem. Fundam., 7 (1968) 214. K. Bobev and Z. Mireva, Surf, Sci., 5 1 (1975) 513. M.A. Morris, C. Barnes and D.A. King, Surf. Sci., in press. G. Wedler, D. Borgmann and K.P. Geuss, Surf. Sci., 47 (1975) 592.

This Page Intentionally Left Blank

Chapter 2

Adsorption, Desorption and Migration on Semiconductor Surfaces B.A. JOYCE and C.T. FOXON

1. Introduction 1.1 JUSTIFICATION FOR THE SUBJECT MATTER

The general topic of chemisorption on semiconductors, which encompasses the processes described in the title, has generated a vast literature for which Peshev et al. [l] have provided a bibliography of almost 3000 papers covering the period 1946-1972. If we examine the nature of much of the work cited there, we find it to be predominantly chemically orientated, both in the description of the materials studied and in the mechanisms proposed. Discussion of the crystallography and electronic structure of the semiconductor surface tends to be in bulk terms and measurement techniques for electronic effects are often simple adaptations of those devised for the determination of bulk properties. In large part, this somewhat unsatisfactory approach stems from the use of oxide semiconductors as vehicles for study. From a chemical viewpoint, their choice is entirely logical since they are widely used as catalysts, but in every other respect they are unsuitable. Stone [ 2 ] has pointed out some of the limitations, which include the poor quality of oxide crystals in terms of structural perfection, stoichiometry and purity, the virtual absence of any theoretical or experimental evaluation of their surface electronic and crystallographic structure, except in the most elementary terms [ 3 ] , and a very limited technology for clean surface preparation, such that part of the reported work may not have related to semiconductor surfaces at all because of the level of contamination. Given this framework, it is perhaps not surprising that work on such systems has been largely empiric, providing a vast catalogue of specific information, not necessarily very reproducible, but little in the way of a general understanding. We will include a brief summary of the type of theory which has developed from such an approach, but the remainder of this review will follow a quite different pattern. We will deal only with elemental (Si and Ge) and III-V compound semiconductors whose crystal perfection and purity is extremely high, where atomically clean surfaces can be routinely prepared and where there is a good basis of understanding of surface electronic structure and crystallography. After a brief discussion of experimental techniques, we will discuss the fundamental concepts of surface electronic states and surface reconstruction in References p p . 280-289

182

semiconductors and the preparation of clean surfaces, and to complete this section we will give examples of measurements and calculations on specific clean surfaces. We will then deal with the adsorption of simple gases on well-characterized surfaces to indicate the extent t o which theory and experiments are beginning to converge. Finally, we will treat the adsorption and desorption of metals and use these processes to illustrate instability effects on clean, reconstructed surfaces. In one sense, therefore, this review will be much more limited in scope than most previous reviews on this subject, but we hope t o show how the application of the more recently developed experimental and theoretical methods of surface physics is leading to results of general significance. First, though, we will summarize very briefly the more conventional chemical approaches. 1.2 THE “CHARGE-TRANSFER” MODEL

The concepts of charge transfer adsorption and catalysis have been formulated and developed by Wagner and Hauffe [4],Garner et al. [ 51, Wolkenstein [ 3, 61, Hauffe [ 71 Aigrain and Dagas [ 81, Weisz [ 91, Garrett [ l o ] and several others referred to in those publications. The basic idea is simply that electrons are transferred to or from the semiconductor from or t o the reactants, so that some parts of the overall interacting system are donors and some acceptors. If we consider first the adsorption stage, there is electronic equilibrium between the semiconductor and the adsorbate, which is partially ionized and some fixed amount of charge will be transferred, forming a surface dipole. Where a complete surface reaction is involved, each separate charge-transfer equilibrium will be upset to some extent by the simultaneous presence of other adsorbates, but the net flow of charge to or from the semiconductor is zero. There is also a steady state flux of neutral atoms or molecules between the surface and the gas phase. It is, however, the process of charge transfer which is of particular interest in this discussion, since even though it may not be the rate-limiting reaction step, it is the one which depends on the electronic properties of the semiconductor. The original version of the model assumes a semiconductor adsorbent with no intrinsic surface states, so that before adsorption occurs the bands are flat to the surface. Wolkenstein [6] even refers to “weak” and “strong” chemisorption; with the former an adsorbed molecule is bound only by covalent forces, whilst, with the latter, charge exchange with the semiconductor takes place. The important point is that the model does not stipulate that the chemisorption bond must be completely ionic. The approach most in keeping with current ideas was developed by Krusemeyer and Thomas [ 111, who considered intrinsic surface states, which produced band bending, and showed that following adsorption they are replaced by states characteristic of the adsorbed material.

183

Whilst in a qualitative sense this theory has a certain basic validity, it does not provide very real physical insight into the electronic and structural effects now known to be associated with adsorption on semiconductors. In the following sections, we shall attempt to show how the subject has been developed from the much greater understanding of semiconductor surfaces which is now available.

2. Experimental approach The general topic of surface analysis has already generated several books and some hundreds of review articles, so it is clearly outside the scope of this chapter t o provide a comprehensive discussion of all of the techniques presently available for the study of clean surfaces and adsorption/desorption processes. Instead, we shall consider a few of the methods most relevant to semiconductors which have perhaps been treated less extensively elsewhere. We shall also concentrate on the type of information available rather than the details of its production. In Fig. 1, we have attempted t o illustrate the complete range of surface evaluation techniques, from which we see that a surface can be probed by electrons, ions, photons, neutral particles and phonons (temperature programming), and the products of the resulting interactions analysed in various ways. Fortunately, the bulk of information on semiconductor surfaces has been obtained from the application of a very limited selection from this range of techniques, which we summarize below. They include Auger electron spectroscopy (AES), low energy electron diffraction (LEED), reflection high energy electron diffraction (RHEED), ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS), molecular beam scattering and thermal desorption. For semiconductor device structures, the various modes of operation of the scanning electron microscope (SEM) have been used extensively, but this is outside the scope of this article. References 12-18 are to selected books and review articles which give general surveys of most methods of surface analysis. 2.1 SURFACE CRYSTALLOGRAPHY. DIFFRACTION TECHNIQUES

We shall be concerned principally with electron diffraction, but recently some important results have been obtained using neutral atom (He) beam diffraction, to which we make brief reference in Sect. 3. The applications of interest are t o the determination of the crystallography of clean surfaces and also of surface-adsorbate systems. 2.1.1 Low energy electron diffraction ( L E E D )

This is by far the most widely used method of obtaining surface crystallographic information and as such has been extensively reviewed [19261. Details of experimental techniques are included in most of these References p p . 280-289

184 R BS LEIS SlMS \

ellipsometry Ramon scattering

n

‘I

Moleculair beam scatter i rm reactive and non-teactive (including diffraction)

Fig. 1 . Experimental techniques available for surface studies. SEM = Scanning electron microscopy (all modes); AES = Auger electron spectroscopy; LEED = low energy electron diffraction; RHEED = reflection high energy electron diffraction; ESD = electron stimulated desorption; X ( U)PS = X-ray ( U V ) photoelectron spectroscopy; ELS = electron loss spectroscopy; RBS = Rutherford back scattering; LEIS = low energy ion scattering; SIMS = secondary ion mass spectrometry; INS = ion neutralization spectroscopy.

articles and Fig. 2 shows typical arrangement. The evaluation of surface structures has two aspects: the determination of the space group symmetry (Bravais lattice) and the determination of the unit cell which occupies the individual points of the Bravais lattice. In principle, the former can be obtained simply from the directions of the diffracted beams, whilst an intensity analysis is required t o obtain the unit cell structure. This is valid for X-ray diffraction from solids, but for surfaces the geometric information is complicated by the loss of translational symmetry normal t o the surface. The periodicity parallel to the surface, however, is reflected in the law of conservation of parallel momentum

k;

=

kj

+g

where k; is the surface parallel component of the propagation vector ko which has magnitude 277/X ( X = electron wavelength), k/; is the surface parallel component of the propagation vector of a diffracted beam and g is a reciprocal lattice vector of the Bravais net characterizing the translational symmetry parallel t o the surface. The spot pattern relating t o

185

Fi Qment

Fig. 2. Arrangement of a display-type LEED system. Primary electron energy range 20-500 eV ( hX 2.7-0.55 A). The incident beam is at near normal incidence and the diffracted beams are transmitted through spherical grids biassed to remove inelastically scattered electrons. The screen is biassed to 5 kV to enable the elastically scattered electrons to be accelerated sufficiently to produce spots on the fluorescent screen.

eqn. (1)is therefore a manifestation of the space group symmetry of the lattice parallel to the surface. For the usual case of a normally incident primary beam (k9 = 0), the directions of the diffracted beams correspond directly to the reciprocal lattice directions. If we combine the definition of elastic scattering

k; = ( 2 r n E / h 2 ) 1 ' 2sin 0

(2) where E is the primary beam energy and 0 the exit angle, with eqn. (l), we can derive an expression for the exit angles as a function of primary beam energy and unit cell spacing parallel to the surface. The cell dimensions parallel to the surface are therefore obtained directly from the diffraction pattern, but there may not be a unique third cell dimension, since not only may the structure not repeat normal to the surface, but also the surface normal plane spacings may vary into the bulk, i.e. there may be surface dilation or contraction. The determination of the packing sequence and layer spacing of the top few atomic layers (the distance over which LEED information is produced) does, in fact, require the measurement and analysis of the diffracted beam References pp. 280-289

186

intensities. In addition, if a valid intensity analysis is available, the position of atoms within the unit cell, and hence the complete surface structure, can be obtained. This analysis is, however, a very complex problem, because there is both a very strong inelastic interaction between the incident electrons and the valence electrons of the upper 2-5 layers of the solid, and very high elastic electron-ion core cross-sections, which make multiple elastic scattering phenomena important in analysing diffracted intensities. This dynamical effect makes the use of single scattering models (the Born approximation) inappropriate and means that the theoretical basis of X-ray crystallography [27] cannot be used in the evaluation of LEED intensities. Several reviews of the dynamical theory of LEED have been published 119, 28-30] and we will not pursue it here beyond stating the problem which has been tackled and the basis of its solution. An incident plane wave with wave vector k is incident on a flat, infinitely extended surface in which the atomic structure is perfectly two-dimensionally periodic. The extension normal t o the surface into the crystal is infinite. The problem is t o find the intensities of the back-scattered electron waves and the solution is obtained by first considering scattering by single atoms and then the multiple scattering between atoms. It has three steps ( i ) calculation of the multiple scattering inside a single atom, (ii) calculation of the multiple scattering inside an atomic layer, and (iii) calculation of the multiple scattering between atomic layers. Alternative approaches t o the intensity analysis problem have been proposed, all of which involve some degree of approximation. The quasidynamic method [31] includes all orders of multiple scattering between layers, but only single scatterings from atomic centres within a layer. The kinematic (single scattering) method [ 321 includes only one scattering from each atomic centre. These approximation techniques tend to be used, in the interest of computational economy, where highly complex surface structures are involved, but it must be emphasized that almost all surface structures so far determined have been evaluated on the basis of the dynamical method. However, for some semiconductor surfaces where there is a comparatively small e l e c t r o n a t o m scattering cross-section, some approximation methods may give reasonable results. Although in principle it is possible to analyse the intensity data using one of these methods t o find an unknown surface structure, in practice this cannot be achieved. Instead, the LEED data is used t o decide whether a particular proposed model of the surface structure is correct by comparison with theoretical intensity data derived for that model, i.e. any acceptable model must be able t o pass the LEED test. In addition to attempting to unravel basic surface crystallography, the other main application of LEED is the identification of surface defect structures [15, 331, including point defects, arrays of atomic steps, domain structures and facets.

187

For random distributions of point defects, such as vacancies or adsorbed atoms, the effect is for an increase in the overall background diffracted intensity at the expense of intensity in the pattern. Where the distribution is non-random, no uniform background is found, there is only additional intensity in the vicinity of the diffracted beams. Surface steps are revealed because LEED is only sensitive t o a few atomic layers, so that interference occurs between beams diffracted from neighbouring terraces due to horizontal and vertical shift. Depending on primary beam energy and direction, and the direction of the diffracted beam, the interference will be constructive or destructive. In the former case, there is no change of spot shape, but if the interference is destructive, spot splitting or broadening occurs. Consequently, it is important t o study the variation of spot shape with experimental conditions (especially primary beam energy) to derive the number and distribution of atomic steps. Additional diffraction spots can also be the manifestation of superstructures and domains where the periodicity of the surface atom arrangement differs from that of the substrate. A superstructure may be due t o regular, perhaps partial, occupation of available surface sites, or to periodic displacement of surface atoms with respect to bulk positions. An area containing a superstructure arrangement and having perfect periodicity within this area is called a domain. Finally, during certain processes such as film growth or thermal etching, new, essentially low index planes may be formed on part of the surface. They are referred t o as facets and make definite angles with flat portions of the surface. If they are large enough, their diffraction pattern is that of an independent, inclined face. The problem is more complex when they are small, but they can then be effectively treated as steps or domains. 2.1.2 Reflection high energy electron diffraction (RHEED) The alternative electron diffraction technique is referred to as reflection electron diffraction and uses a comparatively high energy primary beam (typically 10-50 keV) at a grazing incidence of = 1-3". There is, however, very little difference between LEED and RHEED in the depth of material probed, since, for example, a 50 keV electron at 3" incidence angle will have approximately the same momentum perpendicular t o the surface as a normally incident 150 eV electron. To obtain the complete space group symmetry with RHEED, however, it is necessary to use at least two primary beam directions. The geometric arrangement is illustrated in Fig. 3 and the information available from the angular distribution and intensity of the diffracted beams is similar in almost every respect to that obtained from LEED. The use of a grazing incidence beam does, however, reveal more details of surface topography, particularly if there are small asperities on the surface. References p p . 280-289

188

lncide beam Crystal

I

Fig. 3 . Geometric arrangement of a RHEED system showing zeroth- and first-order diffracted beams. Primary Zlectron energy range z 10-50 keV ( A z 0.12-0.055 8). Incident beam angle X 1-3 . In most cases, all of the electrons which are scattered (elastic and inelastic) into the angular range of observation are allowed t o reach the screen.

When such features exist, they are penetrated by the electron beam so the material is represented by a three-dimensional point lattice and diffraction only occurs when the Ewald sphere intersects a point. This produces a transmission-type spot pattern. For smooth surfaces, the diffraction pattern appears as a set of streaks normal t o the shadow edge on the fluorescent screen, due t o the interaction of the Ewald sphere with the rods projecting orthogonally t o the plane of the two-dimensional reciprocal lattice of the surface. The reciprocal lattice points are drawn out into rods because of the very small beam penetration into the crystal (2-5 atomic layers). We would emphasize, however, that despite contrary statements in the literature, the appearance of a streaked pattern is a necessary but not sufficient condition by which to define an atomically flat surface. Several other factors, such as the size of the crystal surface region over which the primary wave field is coherent and thermal diffuse scattering effects (electron-phonon interactions) can influence the intensity modulation along the streaks. The evaluation of crystal structure is complicated by precisely the same multiple scattering and inelastic scattering processes that occur in LEED. The experimental and theoretical aspects of RHEED have been reviewed elsewhere [23, 25, 341 and, in general, with a few notable exceptions [35--371, the technique has not been used very extensively for semiconductors. We have included it here, however, by virtue of its geometric compatibility with molecular beam scattering, which is being used increasingly t o obtain kinetic data from reactions on semiconductor surfaces (see Sects. 2.4 and 5). The combined arrangement makes it possible to

189

obtain crystallographic and kinetic information simultaneously during the course of a surface reaction. 2.2 SURFACE COMPOSITIONAL ANALYSIS. AUGER ELECTRON SPECTROSCOPY (AES)

This particular technique has been the subject of so many reviews and articles (see,,for example, refs. 12-17) that no details of its theory or practice need be repeated here. The important feature lies in its ability t o measure surface composition with a sensitivity of 1-0.176 for any particular surface atom (except H). In absolute terms, the accuracy is probably not better than ? 20% (unless very well-defined standards are available), but the relative accuracy can be better than k 576, with approximately the same reproducibility. AES has been applied in three areas of semiconductor surface studies. The simplest is the assessment of amounts of contamination present on a surface, and the absence of peaks other than those associated with the substrate is used effectively to define an atomically clean surface. I t should be realised, however, that there could still be up to 0.01 monolayer, or x 1013 atomscm-* of any element (hydrogen cannot be detected) on the surface, even though no additional Auger features are present. The second application is to the direct measurement of adsorptiondesorption processes using the Auger peak height of the particular element as a monitor. The principal limitation is the possible influence of the electron beam on the adsorbate, which can result in beam-induced desorption, adsorption or dissociation. The basis of electron-stimulated desorption (ESD) was established some time ago independently be Menzel and Gomer [38] and Redhead [ 3 9 ] . Electron impact causes Franck-Condon transitions of bound electrons in the adsorbed species into excited states. There is, therefore, a probability of dissociation with subsequent desorption of the particular species involved. As an example of these effects on semiconductor surfaces, Joyce and Neave [ 401 have reported results on silicon, while Ranke and Jacobi [ 411 have discussed the electron-stimulated oxidation of GaAs. AES has also been used to assess the surface composition (stoichiometry) of 111-V compounds and alloy semiconductors [ 421 . In principle, the relative peak heights from transitions involving the constituent elements can provide this information, but again some caution must be exercised in the interpretation. The information depth, particularly for the higher energy (2 1keV) electrons, is not restricted to the surface layer, and in materials such as GaAs the thermal history of the substrate can have a serious effect on the cation :anion ratio in the outermost layers [43]. Unless any treatment prior t o the AES measurement is precisely defined, the ratio has no significance and, in general, AES measurements of this type do not provide fundamental information and can only be used to monitor specific surface and sub-surface effects. References p p . 280-289

190

2.3 SURFACE ELECTRONIC STRUCTURE. PHOTOELECTRON SPECTROSCOPIES

The various forms of photoelectron spectroscopy presently available permit a straightforward determination of occupied and unoccupied surface states. The most comprehensive and authoritative collection of reviews is in the book edited by Feuerbacher et al. [44],while Ertl and Kiippers [ 151 also provide useful information. Here, we will only attempt to summarize how the principal versions of the technique can be used in the determination of surface electronic structure. In this context the crucial factor is that photoemission spectra represent a direct manifestation of the initial and final density of states of the emitting system. Because selection rules (matrix element effects) can be involved in the transition, the state densities may not always correspond to those derived from the band structure, but in practice there is frequently a rather close correspondence. If we consider first photoelectron energy distribution spectra, in which radiation of a fixed wavelength is used t o generate photoelectrons which are detected over wide (up to 2 n steradians) angular ranges (“angle integrated”) and energy analysed, it is simply occupied electron states which are being probed. The general geometric arrangement for photoemission experiments is shown in Fig. 4, where the direction of emission is defined by the polar angle 4, and the azimuthal angle, 8. If now the direction, or momentum, as well as the energy of the photoemitted electrons is measured (“angle resolved photoelectron spectroscopy”, ARPES), the wave vector of the free photoelectron is determined. To relate this photoelectron t o its initial and final state in the solid, the surface parallel momentum component, k , , is conserved during photoemission, and thus k , of the initial state is determined by

k , = (2rnEk/h2)1’2sin q5

(3)

hw

Fig. 4. Direction of photoemitted electrons, defined by the polar (@)and azimuthal

(8)angles, which provide information on the momentum distribution of electronic states.

191

where Ek, the measured kinetic energy of the electron is given by E k = hv Ei, in which hu is the incident photon energy, CP the work function of the solid and Ei the initial state energy ($I is the polar angle). For occupied surface states the two-dimensional band structure E (ki) can therefore be determined. Occupied surface states are detected with much greater sensitivity by angle resolved measurements than by angle integrated, since by measuring only those electrons having an energy with kl matching that of a surface state, there is strong emission from the surface state but bulk emission is suppressed. The alternative approach involves the so-called photoemission yield spectroscopies, which enable empty surface states t o be probed. In these, the incident photon energy is varied while the electron energies are or are not resolved. The technique can firstly be used t o investigate transitions between core levels and empty states, usually by using synchrotron radiation so that sufficient photon energy is available for the excitation. Core levels have negligible dispersion in k-space, so the measurement reveals the unoccupied conduction band and surface state transition state densities, since electrons are generated by optical transitions from a core level t o either empty surface states or conduction band states. By using much lower energy photons, Guichar e t al. [45] have developed a high sensitivity technique which probes transitions between occupied surface and bulk states and states at or above the vacuum level. The partial yield of electrons in an energy window A E at a fixed final state energy E , as a function of photon energy, where E is fixed at < 5 eV so that only secondary electrons are measured, is referred t o as partial yield spectroscopy. When E > 5 eV, although the technique is experimentally identical, it is used t o study initial state and excitonic effects and is known as constant final state spectroscopy. Constant initial state techniques require the photon energy and electron energy analyser t o be scanned synchronously so that h v - E is kept constant. In this way, the photon energy dependent partial yield of electrons in an energy window A E a t a fixed initial state energy E, = E - hv is measured. Core level t o empty surface state transitions are enhanced by selecting the appropriate Ei corresponding t o a minimum in the valence band emission. Considered from a very simple mechanistic viewpoint, the various photoemission yield techniques can be rationalized as follows. A core state (occupied state) is excited by photoadsorption and de-excited by one or more of three possible processes, viz. Auger transitions, direct recombination of surface excitations or direct recombination processes of excitons involving the valence bands. The electrons resulting from these processes are inelastically scattered and so generate secondary electrons. Therefore, by separating the optical excitations from Auger de-excitation - CP

+

References pp. 280-289

192

and recombination processes, yield spectroscopy measurements of either the secondary electrons (partial yield spectrum), the non-inelastically scattered electrons (constant initial state spectrum) or the total number of emitted electrons (total yield spectrum) show core level to surface state excitation features. Surface sensitivity is an intrinsic property of photoemission measurements. The incident light penetrates far into the solid, but the escape depth of excited electrons is very short (Fig. 5), although there are local variations related to direction-dependent band structure effects. Surface sensitivity can be further enhanced by appropriate choice of experimental parameters such as photon energy, angles of incidence and emission, etc., which take advantage of selection rules favouring surface processes. Finally, we can summarize general features of photon energy ranges and sources. Threshold (yield) spectroscopy uses energies around 5 eV to probe the region dominated by the emission threshold and to determine work functions and surface states. The UV photoelectron spectroscopy (UPS) range is effectively defined by resonance light sources with energies from 1 0 to X 4 2 e V , which spans the valence band of most materials. The upper energy range uses characteristic X-ray lines (0.1-5keV) to excite the electrons. The most important source is, however, synchrotron radiation [44,461, which provides a continuous spectrum over a wide energy range. The experimental distinction between UPS and XPS is therefore no longer necessary. In addition, the light is highly polarized, so it may be used to check wave function symmetry selection rules. 2.4 SURFACE KINETIC MEASUREMENTS

The two techniques which have provided most of the direct surface rate measurements of adsorption and desorption on semiconductors are

$ I , , , ,

=

I

1

10

lo2

o3

1

lo4

Kinetic energy (eV)

Fig. 5. Energy dependence of the escape depth of excited electrons showing the mean free path as a function of kinetic energy.

193 Substrate

Source

Detector (a)

(a)

Substrate

Source

Detector (b)

1

Identification of desorbed species Desorption r a t e Sticking coefficients j O r d e r s of chemical r e a c t)i o n s ) Thermal accommodation COeff IcientS Surface lifetimes Binding energies

Fig. 6 . Principle of modulated molecular beam measurements.

modulated molecular beam investigations and temperature-programmed thermal desorption. We will describe briefly the essential features of each.

2.4.1 Modulated molecular beam methods The basic principles are illustrated in Fig. 6 . A molecular beam produced from a Knudsen source is directed on t o a substrate and part of the desorbed flux is detected using a mass spectrometer. In surface scattering/ desorption studies, it is essential t o distinguish between signals in the mass spectrometer arising from background gases in the vacuum system and those produced by the desorbed species. This can be achieved by modulating either the adsorbed or the desorbed molecular beam and measuring the resulting time-dependent signal in the mass spectrometer. The simplest method of modulation consists of opening or closing an appropriately positioned beam shutter which produces a step function change in molecular beam intensity. This has been used by a number of workers [47491 but has severe limitations when used for volatile species because the resulting pressure fluctuations in the vacuum system can produce low frequency time-dependent signals in the mass spectrometer which d o not relate to the desorption process. This problem can be overcome by using periodic modulation of the molecular beams together with a synchronous detection system [ 50-521 . The technique can be further improved because of the fundamental connection between time and frequency domains so that signal averaging and Fourier transform technqiues can be combined in order t o extract the maximum possible information from the signal in the mass spectrometer [ 531. In analysing data from modulated molecular beam experiments, it is References p p . 280-289

194

assumed, often implicitly, that the behaviour can be treated as a timeinvariant linear system. In such cases, it is well established (see, for example, ref. 54) that the response, y ( t ) , to an arbitrary stimulus, x ( t ) , is given by

where h (7)is the response of the same system t o a unit impulse at time -7. An important consequence of this relationship is that only those frequency components present in the stimulus x ( t ) will be observed in the output y ( t ) .This can be seen directly by Fourier transforming eqn. (4) to give

(5) where Y(f), H(f) and X(f) are the Fourier transforms of y ( t ) , h ( 7 ) and x ( t ) ,respectively. The complex convolution integral is reduced t o a simple product and it is obvious that an excitation at a frequency f only results in a response at the same frequency. If, as in the case of gassurface studies, the overall response is determined by a number of processes, then the one of interest may be extracted by deconvolution techniques, which are particularly simple in the frequency domain [ 531 . In particular, attenuation and phase shift of the signal produced by the flight time of molecules from the modulator t o the detector and any non-ideal response of the detector may be taken into account [ 551 . Using these techniques, the following kinetic parameters can be determined. (i) In principle, the accommodation coefficient of an adsorbed species can be deduced by studying the effect of substrate temperature on the amplitude and phase of the signal in the detector. In practice, however, difficulties may be encountered if the detector used is non-ideal [ 561 . (ii) The surface lifetime of an adsorbed gas may be deduced since this will give rise t o a characteristic frequency-dependent attenuation and phase shift of the desorbed pulse. The shortest lifetime which can be measured is typically to s depending on the molecular flight times involved and the accuracy of the measurements. (iii) The sticking coefficient of incident molecules can also be measured from the frequency-independent attenuation of the signal in the mass spectrometer. The accuracy of these measurements is usually determined by signal-to-noise considerations and is typically within 1%.(iv) The order of chemical reactions can be studied either by using a small perturbation of the incident flux and changing the average surface concentration with a second unmodulated source, or by modulating the desorbed flux and measuring directly its intensity as a function of adsorption rate. Olander and Ullman [ 561 have shown that, even when a non-linear surface process is involved in the overall chemical reaction, an effective linearization is frequently imposed Y(f) = H ( f ) * X ( f )

*

195

by other reactions steps. The degree of non-linearity can be assessed directly when the various Fourier components of the signal are measured simultaneously [ 53 J . 2.4.2 Thermal desorption

Thermal desorption studies have the attraction of comparatively simple experimentation, but face severe problems in the evaluation of unambiguous, unique rate parameters from the measurements. The subject has been reviewed several times recently (see, for example, refs. 57-61), particularly in relation to gas-metal systems, so here we will concentrate on its specific applications to semiconductors, where it has been used almost exclusively to study metal absorbatesemiconductor surface interactions. Since this topic provides the subject matter for Sect. 5, we will limit the discussion in this section t o the basic experimental approach and available methods of data analysis. We will leave t o Sect. 5 the critical appraisal of the validity of these methods as applied to solid adsorbates, and the interaction models which have been postulated. The two primary objectives of thermal desorption measurements from semiconductors are the detailed physical understanding of surface kinetics and the derivation of practical mechanisms of adsorbatesurface interactions with a view to technological application (metalsemiconductor contacts, Schottky barriers, doping, etc.). Following adsorption at a particular temperature, thermal desorption may be performed either isothermally or by application of a programmed temperature rise, usually linear with time. Isothermal desorption involves a temperature jump, which is very difficult to achieve in practice, whereby the substrate temperature is rapidly raised from the adsorption temperature to some constant temperature at which desorption is monitored, the procedure being repeated for several different desorption temperatures. Alternatively, a programmed temperature rise is applied and desorption measured continuously with time (and hence temperature). This is experimentally simpler than the isothermal method, but the detailed analysis of desorption spectra is complex, often leading to ambiguities. In principle, adsorption energies, adsorption state populations and details of kinetic processes can be obtained. The basic problem, by whatever means the experiment is performed, is t o establish the correct form of the rate equation which describes the desorption. The usual procedure is to postulate the simple, empiric form described as the Polanyi-Wigner equation

-dn = n x v , exp dt

(-z) AE

where n is the surface coverage, x is the order of the desorption process, u, is a frequency factor and A E is the desorption activation energy. For a References pp. 280-289

196

first-order process, there is an analogous rate equation, deduced by Frenkel [62], in which the inverse specific rate is defined as a time constant, 7, and the inverse frequency factor is T~ ; eqn. (6) then becomes 7 = T~

exp ( A E I k T )

(7)

Given such postulated, but not general, rate equations, a variety of methods is available for evaluating the rate parameters. We will consider the uniqueness or otherwise of the rate parameters subsequently, but first we will describe briefly the analytical procedures with specific reference t o their application t o metal-semiconductor systems. The simplest, and in many cases most accurate, is the peak temperature method first described by Redhead [63]. A E can be directly obtained by measuring the temperature, T p , at which the desorption rate is a maximum, then by setting d 2 n / d t 2 = 0, an expression for AE can be derived in terms of v, Tp and the known heating rate 0;v is usually assumed t o be 1013 s-l . A second approach is an initial state analysis, in which the high coverage, low temperature part of the spectrum is fitted t o a large linear constant and an exponential raised to a constant. The object is t o obtain the desorption activation energy for the high coverage limit before the surface is changed by the rapid heating. Not only is this method less accurate than Redhead’s, but, because with most metalsemiconductor systems the adatom-adatom interaction dominates the adatomsurface interaction, both of the above analyses only determine the enthalpy of evaporation of the metal. An alternative approach, referred t o as a “complete” analysis, has been discussed by King [ 591 and Bauer et al. [ 6 4 ] . The desorption spectra are analysed by a quasi-isotherm method, where the isotherms are obtained by measuring desorption rates and coverages at selected temperatures from a family of desorption curves corresponding t o different initial coverages. Although this method can be used at the low coverage, high temperature end of the desorption spectra, it then only gives information on the final state reached by the adsorbate during the application of the programmed temperature rise. No assumptions are made concerning the coverage independence of v and A E , but the Polanyi-Wigner rate equation is assumed, and in addition there is the implicit assumption that the order is constant at any fixed coverage, irrespective of the initial coverage. It will be seen in Sect. 5 that this is frequently not the case for solid absorbates. If we return now t o the question of the uniqueness of the rate parameters determined from thermal desorption measurements, we see that all of the analytical methods depend on the assumption of a rate equation whose validity, in general, is not tested. In particular, when there are adsorbate-adsorbate (lateral) interactions, or where desorption occurs via a precursor state, the coverage dependence in the pre-exponential term is not a simple function and the concept of reaction order is not meaningful.

197

Other effects which can influence the form of the rate equation include a coverage dependence of the activation energy and an apparent order which changes with coverage as a function of initial coverage. It is clear that great caution must be exercised in the interpretation of desorption spectra if a unique, unambiguous solution is to be obtained. Unfortunately, this is all too rare an occurrence, but the reader is referred t o the article by Petermann [ 571 for an excellent appraisal of the overall situation.

3. Atomically clean semiconductor surfaces To perform any meaningful adsorption, desorption or surface diffusion studies, the nature of the surface on which these processes are occurring must be adequately characterized in terms of structure, composition and electronic state. Ideally, it should be free from extraneous impurities and exhibit no segregation of bulk dopant atoms. Crystallographically, it should be of a single orientation, although it may well not have the same structure as the equivalent plane in the bulk, i.e. the surface may undergo some reconstruction. In addition, it is an advantage if it is smooth on an atomic scale and essentially free from steps. A detailed understanding of chemical bonding mechanisms in adsorption requires a knowledge of the electronic energy levels of the surface, which will, of course, be modified with respect t o the bulk, so the existence of specific surface states must be recognized. Such a surface provides a basis for the study of its interactions with other species; the effect of particular surface perturbations, e.g. atomic steps, can be considered once the ideal surface behaviour is understood. This section will begin with a discussion of the fundamental concepts of the electronic and crystallographic structure of semiconductor surfaces, followed by a description of the methods used to prepare surfaces in as ideal a state as possible experimentally. The emphasis will be on Si and GaAs as typical examples of elemental and compound semiconductor, respectively, and with which the great majority of published work has been carried out. We will conclude with some examples of the determination, experimentally and theoretically, of the electronic and crystallographic structure of specific surfaces of elemental and compound semiconductors. 3.1 ELECTRONIC STRUCTURE OF SEMICONDUCTOR SURFACES

We shall only attempt to introduce some of the more important physical concepts involved, since several excellent and comprehensive reviews already exist [65-711. The electron states in an infinite periodic solid are described by Bloch functions, and are therefore non-localized, extending over all of real space. The introduction of a surface imposes a spatial restriction in one direction References p p . 280-289

198

and the electron wave functions are modified close to the solid-vacuum interface. New electron states may then be allowed at the surface having energies corresponding t o a band-gap in the bulk. Since periodicity in the directions parallel t o the surface is not affected by the one-dimensional restriction, the electron state in the presence of a surface can be characterized by a two dimensional Bloch wave vector k,, which is uniquely defined within a polygon representing the surface Brillouin zone. The surface states may thus be delocalized in the directions parallel to the surface, but they are localized at the vacuum-crystal interface, and have wave functions which decay into the bulk. All electron states at a surface, comprising those bulk bands which extend t o the surface plus localized states, are described by a local density of states (LDOS) which is given by

where $ E is an eigenstate of the system at energy E . If the real space co-ordinate, r , is limited to the surface, eqn. (8) defines the surface LDOS. For any particular k i , a surface state may only occur in a bulk band gap, and its dispersion relation ~ ( k y is) defined by following it as a function of k l . I t may well overlap with bulk states for other parallel wave vectors, since kl is a good quantum number. A state can therefore be a surface state for a limited range of kl and be degenerate with bulk bands for other values of k, . In relation t o the localization of charge in a surface state, or how its charge is distributed between the surface region and the tail in the bulk, if a surface state is near the middle of a wide gap, it will be highly surface localized, whilst if the gap is small or the state has its energy near the edge of the gap, a significant fraction of the charge will be in the decaying tail. Where an electron state exists as a surface state for a limited range of k,, being degenerate with bulk bands for other k, , the state can decay into the bulk, and is called a surface resonance, since kl is not a good quantum number. In calculations of surface states, self-consistency is an important consideration, i.e. whether the charge distribution arising from the calculated electron states is consistent with the potential put into the calculation. Results of self-consistent calculations give true local state densities, including bulk and surface states, and the distinction between surface resonances and surface states becomes irrelevant. The methods used to calculate surface states need not concern us here in any detail, but i t will be instructive to give a brief indication of the two approaches currently employed (self-consistent calculations of the electronic energy and surface potential and realistic tight binding models), since this will provide some insight into semiconductor surface bonds and hence into chemisorption.

199

3.1.1 Self-consistentpseudo-potential calculations In principle, surface atomic and electronic structures are both available from self-consistent calculations of the electronic energy and surface potential. Until recently, however, such calculations were rather unrealistic, being based on a one-dimensional model using a square well crystal potential, with a semi-infinite lattice of pseudo-ions added by first-order perturbation theory. This treatment could not adequately describe dangling bond surface bands. Fortunately, the situation has improved enormously as the result of an approach due t o Appelbaum and Hamann (see ref. 70 and references cited therein), which is based on the following concepts. (i) As a result of their diatomic, low co-ordination number structure, semiconductors tend t o behave like macromolecules, especially with regard to valence bands, which determine the energy position of occupied surface states and hence the self-consistent surface potential. The molecular character of semiconductors becomes especially significant at the surface, as Shockley [ 721 originally indicated. He pointed out that the occupied valence states of a semiconductor were separated from the unoccupied conduction band states by an energy gap and that in diamond structure materials the valence states corresponded to an s p 3 hybridization configuration different from the s 2 p* atomic valence configuration. A necessary consequence is that surface states would occur in the energy gap and the ratio of the number of surface state bands to the number of bulk valence bands is equivalent to the ratio of the number of dangling bonds per unit cell to the total number of bonds in the corresponding unit cell. These concepts have the additional benefit of linking chemical bonding and surface states. (ii) The energy bands of semiconductors are known in considerable detail over an energy range of several Rydbergs and for the technologically important materials they are better known than for all molecular systems except H:. Consequently, bulk energy bands can be used t o construct atomic pseudo-potentials which are very accurate in the bulk, so that at the surface it is only necessary to determine the self-consistent valence contribution to the crystal potential. Since both bulk and surface states are molecular in character, the wave functions of atoms in both types of position can be calculated by the same method. Appelbaum and Hamann [ 701 assume two-dimensional periodicity along the surface and make the same Fourier expansion of the pseudo-wave function as for the bulk, except that at each of a set of discrete surface normal co-ordinates a different set of expansion coefficients is used. These sets can be integrated from outside the surface into the bulk. Well inside the bulk, these wave functions are matched t o bulk states of similar lateral symmetry and the matching condition determines energies and wave functions. References p p . 280-289

200

The result of such calculations show that, on a clean semiconductor, surface atomic sites in equilibrium always differ substantially from those of a semi-infinite lattice and there is an inward force on these surface atoms, since the presence of a dangling bond on a surface atom strengthens the back bonds to atoms in the second layer. This means that the back bonds assume some double bond character (i.e. the bond order becomes greater than unity). The consequent change in bond length leads to so-called surface relaxation (see Sect. 3.3). 3.1.2 Realistic tight-binding calculations

Tight binding (TB) and linear combination of atomic orbitals (LCAO) methods represent the more chemical approach to the problem of surface state calculations. They are basically fitting techniques, but, given a reasonable choice of parameters, they can add considerable detail to the basis provided by self-consisten t calculations. The method, as applied to surfaces, was initiated by Hirabayashi [73] and developed into a useable form by Pandey and Phillips [ 74, 751. The major problem with tight-binding calculations is the change of parameters from bulk values when the surface relaxes, causing first and second nearest neigh b o w bond distances to change. This is resolved, however, by the availability of self-consistent calculations for relaxed surfaces. The method as developed by Pandey and Phillips assumes that the wave functions of a thin slab can be written as

where k, is the surface Bloch wave vector, Gj ( r - R m ) is an orthoganalized atomic orbital of either s- or p-symmetry about an atomic site R m . Seven parameters then characterize the matrix elements of the Hamiltonian and they are calculated from the mean squared error between TB and pseudopotential energy levels for the bulk at many points in the zone for the valence and lowest conduction bands, varying them to find the absolute minimum. To incorporate relaxation, they assume that the matrix elements M j k ( R m) between nearest neighbour orbitals $ j ( r ) and $ k ( r -+ R , ) depends on R m through a Hiickel relationship

Mjk(Rrn) = Mjk(R", exP [P(IRO,I - IRm I)] (10) where P is an empiric overlap parameter and RZ is the separation of orbitals j , k before relaxation. is fixed from self-consistent calculations. 3.2 CRYSTALLOGRAPHY OF SEMICONDUCTOR SURFACES. RELAXATION AND RECONSTRUCTION

A plethora of electron diffraction results amply testifies to the fact that clean surfaces of semiconductors undergo relaxation or reconstruction;

201

that is, the surface structure is characterized by a two-dimensional net with primitive unit meshes different from the bulk as the result of a phase transformation in which the surface atoms are displaced from the positions they would occupy if the bulk lattice were simply terminated. The references to the subject are far too numerous to quote individually, so we simply list a selection [76--811 in which the treatment is fairly general and where many references are given t o specific materials. In covalent materials, forming a surface by division along a lattice plane breaks directional bonds, so atoms close to the surface cannot recover the energy to form stable tetrahedral s p 3 orbitals by bonding to their neighbours. These atoms therefore rearrange themselves to obtain more covalent bonding energy. With ionic materials, the corresponding effect is the generation of unbalanced forces on ions close t o the surfaces, which also leads to surface reconstruction. Substantial reconstruction of semiconductor surfaces is therefore to be expected, a prediction well substantiated in practice. This contrasts with metals, where the spatially extended and much less directional bonding makes reconstruction an unusual occurrence. In elemental semiconductors and the polar faces of compound semiconductors, an odd number of electrons is formed per surface atom by the creation of a surface. The solid therefore undergoes a metal-insulator phase transition [ 821 t o produce an even number of electrons per surface unit cell, thus reducing its symmetry in the plane of the surface. For non-polar faces of compound semiconductors, the simple truncated bulk geometry is already insulating in character because anionic and cationic species are electronically inequivalent. No distortions which reduce the symmetry are therefore necessary to provide stability, but the unbalanced ionic forces and unsaturated covalencies can produce quite large (“ 0.5 8 ) atomic movements (“surface relaxation”). Thus to define the atomic geometry of a clean semiconductor surface, it is necessary to determine (1)the depth of the reconstructed layer, ( 2 ) its structure, and ( 3 ) its registry with respect to the underlying substrate. 3.3 PREPARATION O F CLEAN SURFACES

Three methods have been used to produce clean surfaces, with varying degrees of success. The simplest involves heating to some prescribed temperature under UHV conditions. An alternative, if heating alone is inadequate, is rare gas ion bombardment followed by annealing in UHV to remove the crystallographic damage introduced by the bombardment. Finally, to obtain a specific crystal plane, in situ cleaving in UHV, sometimes followed by annealing, can be used. The degree of cleaning achieved is most commonly measured by AES, although both LEED and SIMS have also been used. As a general rule, surfaces are defined as being atomically clean when the level of contamination is less than 0.01-0.001 monolayers, References p p . 280-289

202

i.e. the detection limit of the assessment methods. Each of the cleaning techniques has various limitations depending on which semiconductor is being considered and it will be instructive to consider silicon (germanium is closely similar) and GaAs as particular examples.

3.3.1 Silicon It is comparatively simple to obtain a clean silicon surface by heat treatment alone. The usual contaminants are carbon and oxygen; the latter is removed as SiO, at temperatures > 1100 K according to the reaction SiO,(,,

+ Si,,,

+

2 SiO,,,

although oxygen is not desorbed as such in either the atomic or molecular form [83]. T o remove carbon, it is necessary t o raise the temperature rapidly t o a value a t which the diffusivity and solid solubility of carbon in silicon are high enough t o enable it t o diffuse into the bulk and not precipitate as 0-Sic a t the surface. Typically, temperatures necessary t o remove carbon are 2 1450 K [ 84,851 . Although heat treatment can produce surfaces with < 0.001 monolayers of residual impurity, two effects can occur during the cleaning process which modify the surface significantly, one being topographic, the other electrical. Dealing first with topographic changes, usually referred t o as thermal etching, three apparently different effects have been reported, but they can probably be rationalized. Thus, either n o changes occur, or irregular pits are produced, or the pits assume a crystallographic form which depends on the particular face being cleaned [86-901. It has been found [ 911, however, that pits are only formed if carbon is present on the surface for fairly lengthy periods during heat treatment (i.e. the temperature is too low). They become crystallographic, usually with (311) facets when the partial pressure of oxygen and/or water vapour exceeds torr during heating. Heat treatment at temperatures 2 1450 K generally does not introduce topographic changes. Uncontrolled electrical changes occurring in the surface regions of silicon substrates during heat treatment in vacuum are rather more difficult t o avoid and are also less readily detected than topographic changes. It was first noted by Allen [92] that silicon surfaces became strongly p-type on heating in vacuum system constructed from borosilicate glass, irrespective of the initial characteristics of the silicon used. For temperatures above 1270 K, a surface layer could contain between lo', and 1015 acceptor atoms ern-,. The mechanism suggested by Law [93] and confirmed in greater detail by Allen e t al. [94] is that boron is transported from the glass to the silicon surface as volatile H3BO3 or HBO, by reaction between water vapour and B 2 0 3 in the glass. Acceptor concentrations as high as 1019 cm-3 have been measured after heat treatment.

-

203

However, the effect is not confined t o borosilicate glass vacuum systems, but has also been observed with stainless steel systems [ 95, 961 , although it can be avoided if the bakeout stage is omitted. This phenomenon is frequently ignored because, unless a direct electrical assessment of the surface region is made, it may not be apparent. In the work quoted above with stainless steel systems, the effect was evaluated by forming either Schottky diodes [96] or MOS capacitors [95] and performing electrical measurements in situ. Inert gas ion bombardment was widely used as a cleaning technique in much of the early work on silicon surfaces [86, 88, 93, 971. Ion beam current densities between 1 and lo3 pA c m - 2 , with energies from 200 eV t o sz 1.3 keV were used, combined with a wide range of outgassing, bombardment times and annealing treatments. Substantial crystallographic damage is introduced by ion bombardment, but an annealing temperature of 1000 K was fairly commonly believed t o be sufficient t o remove it. However, this is almost certainly too low a temperature, since it has been shown by transmission electron microscopy that many dislocation loops resulting from vacancy condensation are still present after this treatment [ 9 8 ] . Statements t o the effect that “sharp” LEED patterns are produced by annealing for 1 0 min at 1200 K have been made [ 9 9 ] , but the “sharpness” of a LEED pattern is not really an adequate criterion by which t o judge crystal perfection. In addition t o the direct crystal damage, inert gas ions become trapped in the surface region and, although many are desorbed on annealing, it is an activated process and those bound in deeper states may not be released. There is no question that ion bombardment can produce a clean surface; it is the crystallographic perfection and the extent t o which the defects may introduce electronic states that is open t o doubt. A final problem which has been reported [ l o o ] is that bombardment and annealing can cause surface roughening. Cleavage involves creating a fresh surface in the UHV chamber and, provided that the base pressure is low ( 700 K. The precise stoichiometry and orientation relationships of the silicides need not concern us further here, but it is crucially important to be aware of their presence and their influence on surface analytical investigations. Their formation has been carefully studied by AES combined with depth-composition profiling [ 277, 2801 and it was shown to be essential to follow the development of the Si peaks due to silicides at 88.5 and 95eV. It is not sufficient t o monitor only the main Au (69.5eV) and Si (92 eV) peaks to assess accurately the surface composition [ 278, 2791 , since this does not take account of compound formation. The most detailed information of Au-Si room temperature interactions has been obtained by Braicovich et al. [281] for S i { l l l } 2 x 1 cleaved surfaces. They used photoemission with synchrotron radiation covering the photon energy range from 10 to 200eV and studied Au coverages from 6 = 0.15 monolayer to 6 = 160 monolayers. We give a brief summary of their results, which not only indicate the degree of complexity of some References p p . 280-289

256

s

7t

0

1

I

I

1

I

I

1

2

3

4

5

6

1

7

c = 0/0.15 Fig. 31. Reduced intensity of Au 4 f emission as a function of reduced coverage on a Si substrate. The broken line is the expected result if there is no Au-Si admixing at the interface (after Braicovich et al. [281]).

metal-semiconductor surface interactions, but also provides a basis for the discussion of desorption behaviour. In the low coverage region (0 = 0.15-2.0 monolayers), the major effect is the rapid reduction, as a function of coverage, of emission from Si dangling bond states present on the clean surface. The mechanism for this can be seen if emission from the Au 4f levels is also followed as a function of coverage. If the Au film simply grows on the Si surface with no intermixing, this emission should be proportional t o coverage. In practice, it is not, but is indicative of a substantial gold deficiency for all coverages (Fig. 31), which can only be explained by intermixing to depths > 5 from the surface. 5 is the approximate escape depth of the photoelectrons, but the actual intermixed depth has been shown t o be = 1 5 8 . The disappearance of surface states can therefore be attributed to surface disruption by Au atoms and the formation of a wide interface. The Au 5 d emission is consistent with the Au being dispersed (i.e. atomic-like) and interacting electronically with Si, suggesting the possibility of alloy formation. With increasing coverage up to 15 monolayers, emission from Au becomes less atomic-like and more similar to that from bulk material, but at the same time the energy separation between the Au 4f and Si 2 p

a

a

257 84.1I

99.9;

'

'

'

I

I

I

I

10 I 20 30 40 Au coverage, 0 (monolayers)

I

I

50

Fig. 32. Au 4 f and Si 2 p binding energes (referred to the Fermi level) as a function of Au coverage (after Braicovich et al. [ 281 ] ).

-

_______ -_ _ _ _ _ -_____ Au silicide

Possible surface segregation of SI

Au + dissolved SI Au silicide

Si substrate

Fig. 3 3 . Schematic representation of the overall effects occurring during Au deposition on Si at room temperature.

levels increases (Fig. 32). A progressive change is therefore taking place in the Au-Si bonding and since it is the Si binding energy which is increasing, the reaction is probably occurring predominantly near the metal-vacuum interface. For coverages beyond 15 monolayers, there is still no pure Au present, but only a Au film containing dissolved Si between two regions of silicide. There is, additionally, a surface enrichment of silicon. The effect of heating is to diffuse more Au further into the Si substrate, leaving a surface more silicon-rich. Figure 33 is a schematic representation of the composition of a room temperature deposit. References p p . 280-289

258

5.2.2 Siluel--silicon

There seems t o be a general consensus [282--2851 that this is a nonreactive interface, i.e. while Ag might be chemically bonded to Si, the interface is essentially planar with no dissociation of the semiconductor and no anomalous diffusion. However, no definitive results have been reported to support this belief and there is evidence which suggests the formation of a silicide phase at room temperature [ 2831. Some difference in condensation behaviour apparently occurs above and below = 470K. Below this temperature, the growth follows a layerby-layer two-dimensional mode, while above it exhibits so-called StranskiKrastanov characteristics in which an epitaxial mono- or multi-layer is first formed on the surface and three-dimensional crystals of the deposit then grow on top of (or from within) this layer. Clearly, when the number density of these crystals becomes high, the difference between the two growth modes is semantic and some workers take the viewpoint that all growth in this system follows the Stranski-Krastanov mode. Four groups [ 282-284, 2861 have studied deposition on (111)7 x 7 Si surfaces prepared by thermal cleaning, which may not have been atomically clean, and although McKinley et al. [ 2851 used a (111)2 x 1 surface prepared by in situ cleavage, their evaporation systems (W spiral and Ta foil) were potentially sources of large amounts of contamination. Venables et al. [286] have made direct observations of the film morphology by UHV-SEM, but the composition and planarity of the interface have been deduced from AES, ELS and UPS. Using AES, Le Lay et al. [282] followed the decay of the Si 92eV peak and the growth of the Ag 353eV peak with increasing deposition and, from the shape of the curves as a function of coverage, deduced the growth mode above and below 470K substrate temperature. They did not, however, consider other peaks which could have been produced from a separate Ag- Si phase. Strictly, they should only have interpreted the results they presented in terms of elemental Ag and Si, but they assumed that the peaks they measured were direct indications of the total amounts of Ag and Si. Housley et al. [283] also used AES, but in addition t o observing the main peaks, they computed difference spectra and showed that a new peak appeared at 82eV. Although they were not able t o follow its height as a function of coverage, they suggest it derived from a bond having Si 3 p Ag 5s character which occurred at the interface. Alternatively, it could be produced by peak shifting/splitting due to silicide formation, as in the Au-Si system. From ELS measurements, Derrien et al. [284] observed a bulk Ag spectrum after three monolayers deposition at room temperature, but only after 30 monolayers at 470K. They attributed this t o the different

259

growth modes at the two temperatures, but did not consider the possibility of chemical interaction. In the only work reported on a (111)2 x 1surface prepared by in situ cleavage [ 2851 , there appears to be evidence for Stranski-Krastanov growth mode with pronounced three-dimensional crystallite formation even at room temperature. From UPS measurements on these films, however, the most noticeable features are the rapid attenuation of photoemission from the Si dangling bond state and the growth of Ag d state emission. This increased emission is not proportional to coverage, however, but becomes asymptotic after x 1 monolayer, an effect attributed t o the Stranski--Krastanov growth mode. However, in the Au-Si case, similar observations [ 2811 were interpreted in terms of intermixing, with the disappearance of the dangling bond states then being caused by surface disruption and the formation of a wide interface region. From the available evidence, it seems reasonable t o conclude that the ideality of the Ag Si interface is not proven, but that any interaction effects are significantly less than those occurring with Au- Si. 5.2.3 Group III metals (Al, Ga, In)--silicon Work on these combinations is in large part attributable t o Rowe et al. [ 287-2921 with some theoretical input from Chalikowsky [ 2931 . Again, the interface is not easy to define, but t o quote Rowe et al. [ 2911 “results clearly show that the electronic states at the metal semiconductor interfaces are different from those of the clean silicon surface, or from those one would expect at an abrupt junction”. The experimental results have been obtained almost entirely from LEED, UPS and LEELS measurements for coverages ranging from a fraction of a monolayer to x 20 monolayers, but an important point has been the use of molecular beam techniques for metal deposition to minimze contamination effects. (The metal was effused from clean Knudsen sources.) If we consider first the LEED observations, the initial state is a thermally produced (111] 7 x 7 structure. The 7 x 7 periodicity is retained up to x 1 monolayer of metal deposit, but as an extrinsic 7 x 7 pattern induced by the metal rather than a simple decrease in intensity of the Si 7 x 7 reflections. This was quoted as evidence of an essentially two-dimensional morphology for the metal deposit, since the formation of three-dimensional nuclei with clean silicon between them would have only reduced the intensity of the intrinsic 7 x 7 pattern. Beyond one monlayer, growth could follow a Stranski-Krastanov mode, however. There are two important effects of metal deposition on the electronic structure of the silicon surface. The first is the saturation of the dangling bonds and removal of the associated back bond states by a very small metal coverage so that the Fermi level is not pinned by dangling bond References pp. 280-289

260

states but by new metal-induced states. Secondly, the main one-electron transition related t o the bulk Si band structure is virtually unchanged, which means that the metal- silicon bonding a t the interface is similar t o bulk Si covalent bonding. This is consistent with Group I11 metals being substitutional impurities, but the interface cannot then be described by a sharp metal- silicon boundary. In the proposed model, initial metal atoms are chemisorbed either as substitutional impurities in the silicon lattice, with strongly localized bonds, or t o fill free surface vacancies. With further deposition, the bonds formed are still much more covalent than could occur for a pure metallic region, but nevertheless the interface region formed at this stage has considerable metallic character, as shown by the high electron density and the tailing of states into the gap. Its effect is t o introduce a high density of new states in the gap and pin the Fermi level in a new position. 5 . 2 . 4 Caesuim-silicon

This system has been studied by Goldstein [294] and Levine [295], and seems to be an example of very site-specific adsorption in which the Cs atoms occupy four-fold coordination sites above the uppermost Si atoms. There is some similarity with other systems in that the Si dangling bond states are removed by Cs deposition t o be replaced by Cs-induced gap states. There is, however, no evidence for interface instability effects.

5.2.5 Metals--Group III- -V compounds Many of the interface phenomena which occur with metal-Si systems occur in a more exaggerated way for metals-- Group III-V compounds and a very large literature has been generated in the past few years. Much of it is concerned with a detailed study of the electronic properties of interfaces in relation t o Schottky barrier formation, t o which we will only make passing reference. However, the chemical effects are very pronounced and we will concentrate on this aspect as providing a particularly good insight into the complexity of the interactions. A useful starting point is t o consider the room temperature deposition of a single metal, Au, on the in situ cleaved (110) surfaces of GaSb, GaAs and InP, studied in detail by the Spicer group [296-3001 and in specific aspects by Brillson e t al. [301, 3021. The experimental techniques used were principally UPS/SXPS (with synchrotron radiation) and AES in combination with sputter profiling. Gold coverages from a fraction of a monolayer t o over 100 monolayers were investigated. Since the interpretation of interface behaviour from such measurements is critically dependent on the morphology of the metal film, we will deal with this problem first. No electron micrographs are available, so structure must be deduced from spectroscopic data. For very thin films, the development of the Au 5 d bands provides the relevant information. The

261

spin-orbit splitting between the d S l 2 and d3,2 levels is X 1.5 eV in atomic gold and = 2.3 eV in bulk gold. For coverages < 0.2 monolayer, the measured splitting using UPS is very close to 1.5eV, indicating that the gold is dispersed and atomic-like; it has not clustered into three-dimensional islands. Nevertheless, this coverage of gold (0.2 monolayer) is sufficient t o pin the Fermi level at the surface and we will discuss this point in more detail subsequently. For Au coverages 2 4 monolayers, the splitting reaches the bulk metal value, but the valence bands still do not resemble bulk Au because the formation of alloys or compounds with the semiconductor components prevents the Au atoms from assuming the lattice structure of gold metal. We cannot, however, use these observations t o confirm that growth occurs by a two-dimensional layer-by-layer process. The same splitting would be observed for large islands separated by areas of clean semiconductor and we must turn to the SXPS results t o provide a model for thicker films. The spectra obtained by the Spicer group for GaSb and GaAs are shown in Figs. 34 and 35, respectively (InP is qualitatively similar to GaAs). They are outer core-level spectra of Au and the semiconductors taken for increasing Au coverages from 1 monolayer to > 100 monolayers, and since the photoelectron escape depths at

rAu-4f

Ga Sb + Au

Sb-4d

hw =120eV

I I I I I . . 85 30 20 10 VBV Binding energy

(eV)

Fig. 34. Photoemission spectra for different Au coverages on GaSb (after Chye et al. P O 0 1 ).

References p p . 280-289

262 ____-----

Ga As + A u

h w = 165eV

Binding energy

(eV)

Fig. 35. Photoemission spectra for differen [3001).

4u coverages on GaAs (after Chye et al.

a),

the energies concerned are very short (5-10 the information in each of the spectra relates only to the surface region. Before extracting morphological data from these spectra, we will consider the most important feature, the presence of peaks from the Group I11 and V elements, even after 100 monolayers of Au have been deposited, despite the x 5a photoelectron escape depth. With GaSb, there is a preferential segregation of Sb to the surface, but for GaAs and InP, the Group I11 and V elements are present in roughly equal amounts at the surface. In all cases, this must involve considerable interaction and possible dissociation at the interface followed by migration through a continuous Au film, since if growth occurred in the form of three-dimensional islands dispersed on a clean semiconductor surface, the spectra would all show equal amounts of Group I11 and V elements. More information on the compositional profiles through the Au films can be obtained from AES combined with sputtering and two examples from the work of Chye et al. [300]are shown in Figs. 36 and 37 for GaSb and GaAs, respectively. It is quite clear that substantial amounts of both Group I11 and Group V elements become distributed throughout the whole of gold films at least 200a thick deposited at room temperature and in the case of Sb, there is additionally considerable surface segregation. The nature and composition of any separate phases which form is not known, but it is abundantly clear that

263 1

Depth 100

(A) 200

I

I

f

0

5 10 15 Sputtering time (min)

Sb

20

Fig. 36. Compositional profile of an Au -GaSb interface obtained by AES and ionmilling (after Chye et al. [ 3001 ).

that these interfaces are very unstable and cannot be treated in a simple geometric fashion. A second materials combination which has attracted a large amount of interest and generated a substantial literature [ 303-3111 is GaAs-Group I11 metals (Ga, A1 and In). Again, the surface in question is an in situ cleaved {llO} and we will only be concerned with room temperature deposition. Starting from a clean surface with no band bending, the first effect of A1 deposition, at x 0.1 monolayer coverage, is an upwards band bending of ,> 0.5eV which pins the Fermi level at the surface near mid-gap. The chemical effects occurring as a function of coverage have been followed by observing photoemission from A1 2 p , Ga 3 d and As 3 d core states. The results of Skeath et al. [307]are shown in Fig. 38 and illustrate a number of important features: from the shifts in A1 References p p . 280-289

264

Depth ( A ) 0 I

100

200

I

I

, Ga A s t Au

3 Sputtering time (min1 Fig. 31. Compositional profile of an Au--GaAs interface obtained by AES and ionmilling (After Chye et al. [300]).

energy at different coverages, it can be seen that two sequential low coverage states occur, both of which are different from bulk Al, although by the time 8 has been deposited, the 2 p binding energy has assumed its bulk value. The Ga and As 3 d levels both shift towards lower binding energies, which simply corresponds to the upwards band bending, and although this is the only effect observed for As, difference curves show a substantial increase in emission on the low binding energy side for the Ga 3 d level. This is consistent with the emission expected from free metallic Ga and it occurs at A1 coverages 2 1 monolayer. For coverages < 1 monolayer, however, the emission is shifted t o higher binding energies, which is indicative of very small Ga clusters dispersed on the surface (i.e. more atomic-Ga-like). The proposed interaction model involves the formation of an A1 coverage to = 0.1 monolayer, with very little penetration of A1 into the lattice, but with increasing A1 deposition there is an increasing tendency for A1 to replace Ga. The thermodynamic driving force is the higher heat of formation of AlAs than GaAs (117 kJ mole-', cf. 71 kJ mole-' ). It is speculated that the initial surface accumulation arises because a Ga surface vacancy, created thermally by A1 adsorption, will more probably be refilled by a Ga atom until a comparatively large A1 population has formed. In

a

265

Initial s t a t e e n e r g y (eV below E, 1

Fig. 38. A1 2 p , As 3 d and Ga 3d core level spectra for several A1 overlayer thicknesses (in 8) on GaAs (after Skeath et al. [ 3 0 7 ] ) .

spite of this first stage stability, however, extensive diffusion of both Ga and As occurs and over 200 of A1 are required before the As 3d level is n o longer observable with 130 eV radiation [ 3081 . Mele and Joannopoulos [ 312, 3131 have performed tight-binding calculations for ordered half-monolayer coverages of A1 chemisorbed on GaAs based on a model in which A1 displaces Ga, which then bonds t o surface As atoms as though the lattice were continued as normal, i.e. a simple exchange reaction. Surface densities of states were computed for various chemisorption configurations and t o account for the observed upwards band bending it is necessary t o postulate a relaxation effect whereby the A1 atom rotates away from its “bulk” position, which lowers the surface electronic energy by 0.3eV per unit cell as the As-Al-As bond angle is reduced. In this new structure, the dangling bond orbitals of the threefold coordinated cation are slightly rehybridized, gaining in s character and shifting t o lower energies. The interaction of Ga with GaAs shows some differences from that with Al. There is a tendency at sub-monolayer coverage t o form two-. dimensional clusters, which do not greatly disturb the GaAs surface lattice electronic structure. When more than one monolayer is deposited, the coverage tends t o become uniform with some As diffusing into this overlayer. The bonding of Ga t o the surface raises an interesting point; the simplest arrangement puts the Ga atom in the position it would occupy if it were part of the next layer. Total energy calculations [309] show this t o correspond t o a local minimum and, somewhat surprisingly, t o a surface geometry which is ideal and unrelaxed. If Ga is adsorbed into a relaxed site, the total energy is appreciably higher. This model is consistent with surface Fermi-level pinning by Ga and with LEED observations.

a

References p p . 280-289

266

However, there is an alternative possibility, which additionally produces better agreement with UPS results. This is a two-fold coordination geometry in which the chemisorbed Ga is bonded to a surface Ga and a surface As atom with roughly equal bond lengths. The vertical distance of the adsorbed Ga atom from the substrate surface plane is then only about half that which it would be in the one-fold site. This configuration requires the repulsive Ga ion-Ga ion interaction energy t o be 4.3eV (i.e. the value of a Ga-Ga atom pair). The actual value is not known, but is estimated t o be % 4 eV, so the two models cannot be distinguished on the basis of total energy calculations. Finally, we may consider the GaAs-Cs system, which is of considerable technological importance for negative electron affinity photocathodes, i.e. where there is n o potential barrier between the conduction band minimum in the bulk of the solid and the vacuum level at the surface. This condition is achieved by downward band bending at the surface brought about by heavy p-type bulk doping and unfilled electronic states at the GaAs-Cs interface. We will not be concerned with device aspects here, but a useful review, emphasizing the physics, has been written by Spicer [314]. The device importance has, however, gven rise t o wide ranging investigations [315--3211 and we will summarize the salient features. We will again restrict our comments t o results with { l l O ) surfaces since, although other orientations have been used, it is not clear to what extent extrinsic effects (damage, contamination, etc.) may have had an influence. Only comparatively low coverages (5 4 monolayers) have been investigated, with surfaces prepared either by in situ cleavage or ion bombardment and annealing, and the critical factor is the nature of the adsorbed species. Since the ionization energy of a free Cs atom (Ei = 3.89 eV) is lower than the work function of clean, cleaved GaAs, caesium should be adsorbed as positive ions with compensating negative charge located in a space charge layer or in surface states, i.e. a dipole layer is formed. In spite of this apparent difference, the effect. of Cs on the surface electronic structure of {11O}GaAs is very similar t o that of Au in that the Fermi level is pinned at the surface ( z 0.45eV above the valence band edge) at coverages well below one monolayer (= 0.2 monolayer). Such pinning can only occur if a large number of surface states is present in the gap, but since there are no gap states with the clean surface, they must be created by the adsorbed Cs. This point was first made by Scheer and van Laar in 1969 [315]. Contrary t o the case with Au, however, with Cs adsorption there is n o suggestion of, or evidence for, interface instability. The adsorption mechanism has been studied by a combination of LEED, AES and work function changes [317, 319-3211. At room temperature, Cs adsorbs on (110) GaAs in an ordered c ( 4 x 4 ) structure for coverages around one monolayer [ 317, 3211 and although the symmetry

300°C overnight and an involvement of H,O, in this process was envisaged following protonation of photoadsorbed 0; to HO, according

-

to HO, HOZ

-

+ HOZ + OH

hu

H,Oz

H2O

+ 0,

(2W

+02

O H + OH HzO2 Pre-adsorption of H 2 0 2 was found to greatly enhance the slow 0, photodesorption which exhibited diffusion kinetics for low (or zero) preadsorbed H,O,, but tended towards a zero-order reaction, with dPOz/dt constant, for hydrated surfaces having higher H 2 0 2 coverage. The extent of retention and nature of the bonding of H, 0, were found t o be strongly dependent upon whether the TiO, surface was fully hydrated, or dehydrated at 200°C [Ti0,-(20O0)] o r also dehydroxylated in vacuo at 400°C [TiO, -( 400°)] . Inner-sphere peroxo complexes of H , 0 2 were envisaged for H 2 0 , on the latter surfaces, whereas on Ti0,-(200) it was suggested that at least part of the adsorbed H , 0 2 formed outer-sphere complexes rather than displacing hydroxyls. Excitons produced in TiO, by illumination were suggested to diffuse to surface locations (extrinsic surface states) involving Ti4+ with a basic OH group in its coordinationi sphere. These sites may thereupon be converted into a Ti3+ion and a free OH radical (by localisation of the electron o r the hole of the electronhole pair, respectively) and thereby become photoactivated for H 2 0 , adsorption and decomposition. Such a mechanism is strictly outside the scope of this subsection concerned with purely photophysical processes,

349

but merits presentation here because of strong similarities to mechanism (29) assumed by those workers t o be responsible for fast photoadsorption of 0, (in the absence of H,O,). H

H

Y

Whilst thus reaffirming that basic surface hydroxyl groups are essential for all photoprocesses, Munuera et al. [ 1431 express the view that the degree of hydration of the surface determines, through the ease of protonation, whether 0; becomes HO, or, instead, produces 0;- surface species on extensively dehydroxylated surfaces [cf. eqn. (29)]. This latter process may also account for the secondary uptake of I8O2 shown in Fig. 8 ( b ) (iii) after flash photolysis of "0, /Ti02 interfaces. The need to take surface states of various kinds into account has also developed in 0, /ZnO interfaces using either direct techniques for observing photosorption or indirect techniques based on photoelectronic effects. Thus IR studies of ZnO surfaces [ 1441 have shown that surface hydroxyl groups represent one abundant surface species on samples vacuumoutgassed at temperatures < 520 K. Outgassing at temperatures > 670 K, which can serve t o reduce the surface concentration of hydroxyl impurities, produces, instead, some metal-excess non-stoichiometry in surface layers [ 791. The resultant density of electron-donating surface sites, such as Zno or Zn', are capable of strongly influencing the surface reactivity. Zinc oxide is not alone among metal oxides in posing the problem of identifying outgassing conditions severe enough to remove residual surface impurity species without simultaneously producing a degree of surface reduction through thermolysis: for example, TiO, and Cu,O surfaces pose similar problems [58, 79, 1451. However, examination of the very extensive literature on direct and indirect studies of oxygen sorption on zinc oxide [ 146-1561 reveals a particularly marked influence of surface pretreatment upon observed photoeffects, which suggests that the 0, /ZnO system poses the problem in a particularly apparent form, presumably because of the relatively low temperatures (>320°C) at which thermolysis of the ZnO surface becomes appreciable [ 146,1471. Several workers [ 147-1491 have concluded that electron-donating surface states can exist on prereduced ZnO surfaces and can give rise to an inversion in the sign of the surface double layer and band-bending, since injection of electrons into the bulk from donor surface states can leave the surface positively charged relative to the bulk (i.e. an accumulation-type surface layer). Eger et al. [148] concluded that such accumulation layers were formed during oxygen photodesorption f;om ZnO and that the rate of photodesorption varied exponentially with the excess surface concentration of electrons. An important role of donor surface species in oxygen photosorption at References p p . 4 19-42 7

350

0 2 / Z n 0 interfaces was also envisaged by Arijs and Cardon [149] in their theoretical and experimental investigations of the effect of a surface accumulation Iayer. They concluded that “very satisfactory qualitative agreement” emerged between their indirect electrical measurements and a model for oxygen chemisorption and photosorption based on electron transfer between bulk and surface states [ 1491. Shapira et al. [ 1161 reported excellent agreement of their experimental results on photoconductivity and photodesorption at 0, /ZnO interfaces with predictions of a charge transfer-type model. A novel feature of their model was the role proposed for impurity carbon atoms as the sites near which physically adsorbed oxygen captured an electron from the conduction band t o form surface CO, species. Desorption was proposed to result from the capture of a photogenerated hole by CO,, but otherwise the treatment of charge localisation/delocalisation at the surface and its intercharge with the bulk were treated in similar fashion to that detailed for eqn. (25), above. It was predicted, and observed experimentally, that the rate of change of surface conductivity decreased reciprocally with the square root of the time of exposure to band-gap illumination, whilst parallel behaviour was observed for photosorption of carbon dioxide. The species CO, is, however, paramagnetic with a characteristic EPR signal, and postulates of surface CO, as an important intermediate lack the expected experimental support from results of many observations on O,/ZnO or O,/TiO, interfaces under UV illumination in situ in EPR spectrometers [61a, 791. However, the formation of diamagnetic carbon- or carboxylate-type surface species are extensively reported for “carbon contaminated” metal oxide surfaces. Their photodissociation could account for the observed release of CO, without need t o invoke the paramagnetic CO, intermediate (see scheme 57, p. 397). Other workers [ 5 2 , 6 4 , 101, 150-1531 have commented unfavourably, however, upon the limited and rather qualitative nature of the agreement attainable between observed photoelectronic effects at 0, /ZnO interfaces and the predictions of models based upon charge-transfer mediated solely by collective-electron factors. A variety of additional factors have been identified by these workers as also exerting important influences on observable photophysical effects. Thus Beekmans [ 1501 has elaborated a model in which the conductivity of a ZnO powder layer is controlled by the conductance at narrow “necks” connecting adjacent ZnO grains, and has reported a fair measure of agreement between requirements of this model and photoconductivity measurements at UV-illuminated 0, /ZnO interfaces [ 1501. The spectral dependence of the photoconductivity of (lOi0) faces of ZnO single crystals at 77 K has, on the one hand, been interpreted by Luth and Heiland [64] in terms of photoexcitation of electrons from two sets of surface states a t 170 and 450meV below the conduction band. Since 110 peak occurred in t h e spectral response curve corresponding to the 170 rneV surface states whenever a surface depletion

351

layer was present at the O2/ZnO interface, they concluded that occupancy of this surface state was strongly dependent upon band bending. On the other hand, observations on surface photovoltage on ZnO single crystals, and particularly those on the inversion of surface voltage by illumination, have been interpreted by Sproules and co-workers [118] in terms of photo-excitation of electrons from the valence band into surface states having a wide distribution of energies within the forbidden band gap. Recently, these latter workers have argued that the dominant surface states at O,/ZnO interfaces are associated primarily with adsorbed oxygen and that occupancy of these states does not follow simple electronic theories for electron equilibration [ 521. Rather, they argue that these surface states are quasi-isolated and that operation of both electronic and non-electronic factors must be taken into account in attempting to calculate the probability of electron localisation as chemisorbed oxygen. They propose eqn. (30) to allow for the dual requirement that electrons must penetrate the surface barrier, q V , / k T , in order to reach the surface and that oxygen species at the interface must overcome an activation energy E , in order to be bonded t o the surface.

Other symbols in this equation have the same significance as in eqns. (22) and (26). The first bracketed term in eqn. (30) corresponds to the operation of an electronic factor in charge localisation at the surface, viz. the penetration of electrons from the bulk through the Schottky surface barrier, V,, at a rate exponentially related to - V,. The second bracketed term allows for the operation of a thermally activated nonelectronic factor, viz. the transformation of physisorbed molecular oxygen into a metastable activated form capable of localising electrons with “normal” cross-section. Lagowski et al. attempted to “separate out” the nonelectronic factor by measurements at fixed surface potential and interpreted their results as indicative of an activation energy of 0.72eV and a cross-section of cm2 for oxygen chemisorption. It is worth noting here that eqn. (30) and the ideas underlying it are the converse of mechanism (27) discussed above in relation to flash-initiated ISO, desorption from ZnO. That mechanism saw the need for photogenerated holes to penetrate to the surface (rather than electrons) and envisaged activated desorption (rather than chemisorption) as the non-electronic factor controlling the rate. The complexities considered in the previous paragraph in relation t o surface states at non-illuminated 0 2 / Z n 0 interfaces can be further exacerbated under UV illumination through creation of new surface states by surface photolysis. Recent evidence in support of this additional complication has been provided by observations of order-of-magnitude References pp. 419-427

352

enhancements of the surface conductivity of lithium-doped ZnO single crystals upon prolonged illumination in vacuo by a xenon arc lamp [ 1531 . The single crystal surfaces had been outgassed and partially reduced in vacuo of N m - ? , prior to surface re-oxidation which produced very low initial surface conductivity. Subsequent enhancement of the surface conductivity by prolonged illumination was attributed to photolytic formation of excess zinc in the surface region with growth of an accompanying accumulation layer. The latter could gradually be removed through interaction with oxygen and a cubic equation was developed t o relate conductivity to the number densities of chemisorbed oxygen, ionised surface donors and total surface charge. Agreement with the cubic equation was qualitative rather than quantitative [ 1531 . One final related photophysical process at O,/ZnO interfaces deserves brief mention, viz. photoassisted “place exchange” between I8O2 in the gas phase and 1602previously chemisorbed on the surface during exposure at 673K and during cooling t o room temperature. When such O,/ZnO interfaces were briefly evacuated to < torr and an isotopically pre-equilibrated mixture of ( 2 5 % 1602 50% I6O1’O t 25% ’80,) was admitted at low pressures, UV illumination produced a rapid rise in the mole fraction of 1602in the gas phase, with a corresponding such that AX3,= - (AX3, AX3y)[1 5 4 ]. decline in 1 8 0 2 and 160’80 The surface conditions which emerge from the foregoing considerations as those most likely to minimise the effects of surface states arising from impurities or defects at the O,/metal oxide interface would appear to be: (i) outgassing at temperatures sufficiently high to remove chemisorbed impurities such as OH- or CO,, but not so high as t o cause extensive surface decomposition; (ii) repeated re-oxidation in pure oxygen at sufficiently high temperatures and for sufficiently long times to “burn o f f ” any residual carbon impurities; (iii) UHV sample-handling procedures t o minimise surface recontamination and (iv) use of such low photon fluxes that possibilities for surface photolysis are negligible during photosorption measurements. Unfortunately, these criteria have not been satisfied simultaneously for any of the studies of oxygen photodesorption made by direct mass spectrometric observations on the release of gases into the gas phase from illuminated 0, /metal oxide interfaces. Not surprisingly, therefore, a measure of disagreement has emerged between results obtained in such widely differing sample conditions that different surface states may be expected to dominate the effects observed in different studies. Results with 0, /ZnO illustrate this difficulty. Thus, surface carbon impurities predominated in the studies by Shapira et al. [ 1161 leading t o C 0 2 as the only photoadsorbed product at room temperature with 0.25 s flashes of photons at 365 nm with flux density 3.3 x 10l6 cm-, s-’ ; photodesorption of I8O2 was detected as the major product released from 180,/Zn0 interfaces flash-illuminated by a 5 0 p s pulse consisting of 10l8 photons with wavelengths between 360 and 640nm, but an influence of surface states

+

+

353

related to metal-excess surface non-stoichiometry appeared probable for those flash-illuminated lSO2/ZnO interfaces due to some surface reduction during prior outgassing [ 1361 ; on the other hand, direct observations by Steinbach and Harborth [154] and by Hirschwald and Thule [155] on the products released t o the gas phase from ZnO surfaces maintained at temperatures of about 7 7 0 K during exposure t o very high light fluxes showed oxygen atoms to be the dominant evolved species. Here again, a marked initial influence of the degree of prior surface reduction was noted but the volume of C 0 2 produced was very low. In view of claims that increasing temperature at the O2/ZnO interface causes the predominant chemisorbed oxygen species t o shift from 0; t o 0-,the apparent conflict between the release of monatomic oxygen at high temperatures but O2 at 300 K may be explicable, in part, because photogenerated holes neutralise mainly 0; at 300K but 0- or 02-at high temperatures, and in part because high temperatures are needed to desorb monatomic oxygen prior t o its dimerisation on the surface.

( c ) Photoeffects whose origins are unresolved Several interpretations of photophysical processes considered above envisaged individual photons being instrumental in causing localisation/ delocalisation of an electron between an adsorbate-related state and a collective-electron state arising from bands in the bulk of the adsorbent or at its surface. Some impetus towards consideration of alternative collective excitations, e.g. of small surface regions of the absorbent (as distinct from localised activation of an adsorbate or its bonding t o a surface ion within an incomplete coordination sphere) is developing from a variety of phenomena observed under laser illumination including the following: (i) photon-initiated field ion mass spectrometry (PIFIMS) on the C2 H4 /Ag system [ 1561 ; (ii) laser-induced desorption studies [ 1571 ; and (iii) long wavelength photoemission of electrons from 0 2 / M g and related systems [ 1071. The PIFIMS technique differs from other photodesorption methods described above by obtaining the photoassisted removal of an adsorbate as ions rather than as neutral species and by observing such removal from an extremely small and well-characterised area of adsorbate, viz. the tip of a metal field emitter as used in a field ion microscope. Mass analysis of the ions being emitted from selected regions of the tip and imaged on the display screen is accomplished by time-of-flight mass spectrometry when the ion beam is passed through an aperture in the screen. PIFIMS was observed when laser pulses of 2-5ns pulse length and wavelengths of about 400nm became incident on a silver emitter a t which the field gradient had been reduced t o such a level that ion emission was negligible in the dark with the emitter tip carrying ethylene adsorbate. Below the onset of normal field ionisation, hydrocarbon ions like C,H:, ( 2 < n < 6) were observed by Referencespp. 419-427

354

the PIFIMS technique and attributed to a direct electronic excitation mechanism whose exact nature is as yet unclear. Dynamic mass spectrometry has been used t o detect and identify species desorbed by incidence of an intense laser probe on selected small regions of various solid substrates [157]. Either positive or negative ions (but not neutral species) were detected with appropriate ion optics. A notable feature is the small amount of ion fragmentation seen in the mass spectra. Commercial instrumentation for examining surfaces by laser-assisted microscopy with mass analysis (LAMMA) has been developed. Incidence of photons at 410 and 520 nm but not at 610 nm has been reported to cause photoelectron emission from magnesium surfaces during the early stages of oxidation [107].The criteria applied to distinguish these long wavelength photoelectrons (LWPE) from processes of exoelectron or photoexoelectron emission were: (i) that LWPE should be detectable even when the base pressure was reduced to 5 x lo-'' torr; and (ii) that the LWPE component should reduce to zero when the incident light was turned off. A marked contrast emerged between aluminium and magnesium foil surfaces pretreated in similar manner on an UHV system, since aluminium produced no LWPE with incident light of wavelengths between 600 and 350 nm, whereas LWPE with 4 eV energy spread from Mg was readily observed under excitation at 500, 520, and 410 nm. Photoelectric work functions as low as 0.3 eV were observed and attributed to the development during oxidation of patches having low work functions (termed exopatches). The exact nature of energy levels on such surfaces and of the manner in which photons interact with them remain to be resolved. 2.2 PHOTOCHEMICAL EFFECTS

Photons of energies 2--8eV incident on gas/solid interfaces may produce, in addition to the photophysical processes considered above, the rupture and/or formation of chemical bonding within adsorbates or between them on the surface. These photochemical processes at the interface may be further subdivided into: (i) photoassisted surface reactions yielding products which remain at the interface and so irreversibly alter its chemical composition and reactivity in the selected reaction conditions; and (ii) photocatalytic processes wherein the products from photoinitiated reactions at the interface are continuously removed to the gas phase in the reaction conditions (e.g. by thermally assisted or photoassisted desorption) with the result that the active surface is continuously regenerated and can become responsible for high turnover accomplished per photoactivated surface site (t.a.p.s.*).

2.2.1 Experimental aspects Different experimental approaches and equipment are frequently appropriate depending on whether the photochemical effect under study

355

UV source

t =

-- 6

2 6 0 torr

m

- 5 111

2 4

z

- 3

2

6 0 torr

K

> 2 1 I

1

100

200

References pp. 4 1 9 4 2 7

P(torri

0

100

200

300

356

G.C.

carrier

Ka‘

I I V \ourcis

e

.

outlet

1

i

reactor

reactor

ty

G.c

I

Continuous reactant flow

Pulsed reactant flow

I

(C)

Fig. 10. (a) Photoreactor developed for the study of selective photo-odixations in conditions of continuous illumination with a continuous flow of (carrier gas hydrooxygen) through a thin layer of finely powdered photocatalyst. Reproduced carbon with permission and minor adaptation from ref. 158. (b) Results obtained with photoreactor showing pressure dependence of the photoassisted steady-state rate of acetone 0 2 He)/ formation under continuous UV-illumination of a dynamic (isobutane TiOz ) interface. (c) Comparison of flow diagrams and positions of sampling valve for utilisation of photoreactor in “pulsed-reactant” versus “continuous reactant flow” conditions.

+

+

+

+

at an illuminated gas/solid interface is truly a photocatalytic process or is, instead, a noncatalytic surface reaction. True photocatalytic processes are here understood t o be those in which the photoinitiating making and/or breaking of internal chemical bonds within the adsorbate(s) proceeds to t.a.p.s.* > 100 with the assistance of the surface, but does not lead to significant permanent chemical alteration of the catalyst surface. In such cases, the emphasis in experimental measurements has normally centred on the nature and kinetics of changes in the composition of the gas phase, allied occasionally t o studies of reactant and product species on the surface using spectroscopic techniques. Relatively straightforward kinetic behaviour is often assumed for pure photocatalysed reactions on the basis that the light intensity and the surface concentration of active sites remain constant throughout the photoprocess. Such simplifying assumptions can predict proportionality between the rate of the photocatalysed reaction and the percentage of the corresponding photoproduct present when a photodynamic steady state is reached, i.e. when reactant gas(es) passing at constant flow rate through a fixed and illuminated bed of catalyst emerge with a percentage conversion to products which no longer varies with duration of illumination. Such dynamic flow photoreactors employing gas chromatographic techniques to separate and quantitatively measure the photoproducts have been widely used in combination with continuous

351

illumination to study processes thought t o be true photocatalysed reactions [158--1601. An example of this type of photoreactor, as developed by Teichner and his co-workers [158] for studying the photo-oxidation of alkanes over very finely divided powdered samples of TiO,, specially prepared in a flame reactor which yielded non-porous particles of high surface area, is shown in Fig. 10. Hydrocarbon products indicative of selective partial photo-oxidation were measured at high sensitivity with a flame ionisation detector, whilst CO, product from oxidation was measured with a thermal conductivity detector. N o conversion to products was obtained except when the alkane reactant simultaneously encountered TiO, , UV illumination and appreciable pressures of oxygen gas. Similar prerequisites for the successful continuing operation of photocatalysed oxidation over TiO, and ZnO have been reported by other research groups using dynamic flow photocatalytic reactors [ 158-1601 with continuous UV illumination and oxygen pressures of 5 x lo3 to 5 x lo4 N m-2 . These techniques allow valuable information on the dependence of any steady state extent of conversion to various products on the pressure of reactant or of oxygen and on the flow rate and light intensity to be accumulated readily. They are less satisfactory for describing any build-up or decay of the activity of the solid catalyst under illumination [ 161,2561 or for providing data on the following points: the nature of adsorbed species present at the UV-illuminated interface; the identity of adsorption sites and their extent of coverage by reactant and/or product; the nature of the active sites on which photoassisted surface reaction takes place; details of the manner in which incident radiation activates the surface sites on the adsorbate. Further, the high sensitivity attainable with flame ionisation detection allows small conversions to be readily observed, and it is essential, as stressed recently by Childs and Ollis [ 1621 to check that activity persists and remains constant throughout turnover to product at the illuminated interface of numbers of reactant molecules orders of magnitude greater than the number of surface sites activated by illuminated (i.e. t.a.p.s.* > 100). The foregoing criterion of constant activity of the illuminated gaslsolid reaction throughout large turnover of the photoinduced chemical change at the interface does not apply to non-catalytic photochemical changes. Since such non-catalytic photochemical reactions are frequently accompanied by significant chemical alteration in the surface layer of the solid, marked variations in rate of reaction can occur as the extent of surface reaction increases, e.g. Elovich-type kinetics, with rate falling off exponentially with extent of reaction, can be expected if each non-reversible surface reaction inhibits subsequent reaction events at the illuminated interface. Work by Hemminger et al. [163] illustrated how UHV procedures and electron spectroscopic techniques could be combined with gas chromatographic techniques in order t o delineate (i) the growth of methane product to a limiting value from a photoassisted reaction involving References p p . 4 1 9 4 2 7

A'

C r o s s section A A '

,

I I

4

t

30

7 E

2

m Y

0 4

a 3

C

zoo % x X

0

300

0

2

4 6 Time (h)

8

Fig. 11. (a) Schematic of closed photocatalytic reactor with provision for tumbling powdered catalyst past a continuously operating source of UV illumination and for sampling an illuminated mixture of (alcohol vapour oxygen) to a gas chromatograph for analysis. (b) Results obtained with photoreactor showing photoinduced evolution of acetone (0, left scale), pressure fall for isopropanol (m, left scale) and oxygen uptake (0, m, right scale) during photo-oxidation of isopropanol at 300 K on TiO, outgaeed at 600 K. (1) From a monolayer of isopropanol but without isopropanol in the gao phase (circles); (2)as ( l ) , but also with an initial pressure of 56 N m-z of isopropanol in the gas phase (triangles); (3) as (I), but also with isopropanol initially in the gas phase (squares). Initial oxygen pressure in all experiments was 1.33 kN Reproduced with permission from ref. 161.

.,.,

+

.,

359

H,O and CO, over a single crystal of SrTi03 and (ii) what changes occurred at the surface of the SrTi03 crystal as a consequence of such reaction. Whilst such parallel observations upon the surface by kinetic and non-kinetic techniques carry particular force for non-catalytic radiation-induced surface reactions, they are also highly desirable in the early stages of photocatalysed reactions t o reveal possible variations in the rate of reaction with increasing t.a.p.s.*, such as would result from photoformation of active sites o r from their destruction by illumination [256]. Various types of “sealed” or recirculatory reactions have been devised which allow for periodic or continuous sampling of gases for analysis, following the introduction of gaseous reactants over the catalyst and initiation of illumination. One such sealed photoreactor, featuring continuous UV illumination and constant tumbling of the TiO, particles in the light flux, is illustrated later in Fig. 11 and was utilised by Stone and co-workers [ 1611 together with gas chromatographic analysis t o follow the photo-oxidation of propan-2-01 to acetone. Another common modification of sealed catalytic reactors involves the inclusion of some means (usually magnetically driven glass-enclosed pistons) for the recirculation of gaseous reactants through the sealed system and over the surface of the catalyst [ 164-166, 265, 2661 . Within this context of being able to follow the variation of surface activity with the extent of exposure to, and conversion of, reactant, it is also worth noting modifications of dynamic flow reactors into a pulsed-reactant mode as distinct from the continuousreactant flow normally employed. Such pulsed reactant systems can make it possible to follow the adsorption on the non-illuminated solid prior t o the study of conversions brought about by illumination. It is also possible in these systems to limit the duration of illumination t o the transit time of the reactant pulse and to follow for successive pulses any variation in the percentage conversion with pulse number. Relatively minor variation of the positioning of gas sampling values relative to dynamic flow photoreactors of the type shown in Fig. lO(c) allows their conversion for operation in this pulsed reactant mode [256] . Photoassisted reactions under much lower pressures of reactant can be made by utilising mass spectrometric detection allied to either dynamic flow or sealed photoreactors [ 541.

2.2.2 Results and interpretations Reactions involving molecular oxygen as one of the reactants which undergo photoassisted bond cleavage and rearrangement over illuminated gas/solid interfaces represent the category of photochemical reactants which has received most attention. Photochemical processes falling into this category are considered first whilst those not involving oxygen as a reactant are treated later in this section. References p p . 419-427

360

( a ) Oxygen isotope exchange ( O I E )

Photoassisted scrambling of oxygen isotopes between molecular oxygen species over an illuminated interface is conceptually the simplest representative of this category of reaction since, apart from adsorbate-adsorbent bonds, it involves rupture/rearrangement of only oxygen-oxygen bonds. Equation (31) represents a surface-assisted scrambling process involving redistribution of oxygen isotopes between molecular oxygen species which originate from and return to the gas phase, viz. '60,(g)i

+ l"io,(g),

-

2 '"o'80(g)i

(31)

where the use of the subscript i denotes that species become involved in this equilibration whilst present at the 0, /solid interface. The term homophase has been suggested for this OIE process since it causes variations in isotopic composition only of the gas phase and proceeds without significant exchange between lattice and gas [ 167-1701. Two features useful for distinguishing this scrambling process from other OIE events are ( i ) that it does not produce any change in the atom percentages of l 6 0 o r l 8 0 in the gas phase, since it does not involve loss of either oxygen isotope from the gas phase through incorporation into surface layers of solid catalyst, and (ii) that it may be possible on catalysts which do not have oxygen in their surface layers. The latter feature has recently been reported [ 167 1 as being satisfied over zinc sulphide catalysts which were shown to catalyse process (31) with an optimum rate at 180 K . Operation of OIE in accordance with eqn. (31) has likewise been reported for several metal oxides at temperatures < 300 K in the absence of illumination, but only when surfaces were prereduced. N o photoeffect was detected, however, on illumination of ZnS, despite a report [168] that oxygen does photoadsorb on ZnS. Additional possibilities for exchange of oxygen isotopes over metal oxide catalysts become possible since exchange may occur between oxygen from the gas phase and from the lattice. A classification of oxygen isotope exchange processes as R o , R' or R2 in character has been suggested according to the number of monatomic oxygen lattice species, 01, which transfer into the gas phase for each gaseous oxygen molecule experiencing isotopic scrambling at the surface [ 1691. Thus, eqn. (31) described Ro-type exchange, which has also been termed homomolecular or homophase exchange and which proceeds without oxygen exchange between lattice and gas-phase oxygen [ 1701. On the other hand, type R' and R2 exchange can be represented respectively by the equations

+

{'802(g)}y-

160;-

{'SO*(g)}y-t

' 6 0 nI -

= {'60180(g)},n-

+ 1 6 0 In -

+ 180;+ '80;- + 180;-

= {'6O2(g)}y-

(32)

(33)

which involve exchange between lattice oxygen, Or-, and molecular , The superscripts n- and oxygen present at the interface, ( ( 0 (g)}m-.

361

m - are used for generality here to indicate that lattice oxygen, 01,and molecular oxygen may carry different charges (usually from 0 to -2) while at the interface. The symbol (g) denotes species which originate from o r return to the gas phase. It is possible experimentally, by the choice of appropriate isotopic compositions of the mixture admitted into contact with the metal oxide surface, to favour Ro-type processes over R' and R2 or vice versa. Thus, if a starting mixture of (I6O2 l80,)which is far from equilibrium in relation t o process (31) is employed, illumination of the (I6O, 1802)/ metal oxide interface may provide sites and a reaction pathway on the catalyst surface which facilitates rapid approach of the system to the equilibrium situation defined by

+

+

Such isotopic exchange over non-illuminated TiO, samples with partially reduced surfaces had been studied by Russian workers [171] and a correlation established between the activity and the extent of reduction, which was controlled by pre-heating in H, or CO and monitored by the magnitude of the ESR signal of the Ti3+ species [ 1611. The activity did not correlate with the number of paramagnetic 0, species detected at the O,/TiO, interface by ESR and the suggestion was made that other oxygen anion radicals were involved in the sites active for oxygen exchange. Activity fell off with increasing oxygen exposure and preoxidised TiO, was inactive. These results are quoted since they define the Ro-type activity expected for prereduced TiO, towards isotopically enriched oxygen in the absence of illumination. Similar effects have been reported recently from the author's laboratories for prereduced samples of TiO,, ZnO and MgO, but with the additional observation that exposure t o low light intensities prevented the decay in surface activity noted by Russian workers or restored activity t o samples from which initial activity had decayed in the dark [172a]. The kinetics of the variation of isotopic composition after contacting the ( 1 6 0 2 ''0,) gas phase with a freshly reduced surface accurately obeyed reversible second-order kinetics over zinc oxide surfaces a t 300 and 77 K [172b, 2671. Data for TiO, surfaces exposed t o illumination indicated a small contribution from R' and/or RZ in addition t o an Ro contribution. Following the illumination-induced restoration of Ro -type exchange activity t o dark-decayed samples of TiO, , ZnO or MgO, pronounced "memory" effects took the form of another gradual decline in activity in the dark. These memory effects strongly resemble those reported by Russian workers for oxidised samples of ZnO, MgO and lithium-doped zinc oxide [127]. Each set of observations has been interpreted on the basis that incident light promotes electrons from relatively deep traps into very shallow or untrapped states at the interface. The Russian workers envisaged the deeper traps as lattice 0; type species

+

References p p . 4 1 9 4 2 7

362

and accounted for photoadsorption plus OIE in terms of

q-

+

1802

-

'8o;(ads)

(34b).

Promotion of electrons between surface electronic energy levels of the types depicted in Fig. 3 ( b ) forms the basis of an alternative model developed by Cunningham et al. [154] for photoassisted Ro-type processes. According t o this explanation, activity requires surface locations of the type e;I(O-Zn,,,), i.e. surface ion pairs, in which the zinc ions are at positions of high coordinative unsaturation and near which an electron has been localised. In terms of Fig. 3(b), this would correspond to localisation/ promotion of electrons into the band of surface states associated with zinc dangling bonds, although that band should be extended towards lower energies to allow for the lower Madelung potential of zinc ions with high coordinative unsaturation. The availability of e; I (0-Zncus ) allows activation of molecular 1802(or 1 6 Q 2 ) from the gas phase by the tendencies of these sites both to donate an electron towards the antibonding orbitals of O2 and to reduce the high coordinative unsaturation of Zn;,, through interaction with a lone pair on one of the oxygen atoms. 'Two limiting cases of activation may be distinguished: strong activation, whereby the molecular oxygen becomes dissociatively adsorbed into one bound and one non-bonded monatomic surface oxygen species [cf. step (a) of eqns. (35) ]; and weak activation, whereby bonding in the molecular oxygen is weakened and redistributed in a weak surface donor-acceptor [cf. eqn. (35'd)l. Either type of complex between Q2 and e- I(Q-Zn,,,) activation could be envisaged, as per schemes (35) and (35'), respectively, to lead t o Ro -type OIE exhibiting the reversible second-order kinetics observed experimentally when ( 1 6 0 2 "02)was contacted a t 295 or 77 K with prereduced zinc oxide surfaces in the absence of illumination.

+

363

In this scheme, the initial dissociative adsorption is followed by fast formation of a triatomic O3 intermediate (with equal probability from 1602 o r l8OZ of the isotopically non-equilibrated 1 6 0 2 "O2 gas phase) which serves as the propagator of step (c). The latter has the character of a chain reaction, scrambling the gas phase into isotopic equilibration with reversible second-order kinetics as observed experimentally. Another aspect of experimental observations at 295K readily accounted for by eqns. (35) was the diminution of activity as successive doses of the isotopically nonequilibrated (1602 1 8 0 2 ) were introduced, since step (a) shows destruction of Zncus.The alternative weak-activation case is

+

+

0 2 (g)

+ e-l (O-Zncus)

+

-

(35'd)

O2(O-Zncus)-

2

(g) [ 1 6 ~ 1 8 ~ ( ~ ~ n c7 ,,)]I6Ol8O(g) [1602(o-z~,,,)]-

1602

+

(35'f)

In this scheme, the activated but non-dissociated oxygen species formed in step (d), with equal probability from 1 6 0 2 o r 1 8 0 2 ,serve as the propagator of steps (e) and (e'), each of which is reversible second order and chain-reaction in character [cf. step (f)] . Since low temperatures should enhance surface concentrations of the weak complex envisaged in step (d), which furthermore avoids any significant energy of activation, this scheme offers advantages in accounting for the OIE observed at 77 K. Figure 3(b) made it clear that the requirement, inherent in both schemes (35) and (35'), for electron localisation as e-I(O-ZnC,,) was unlikely to be satisfied in the absence of illumination unless the surface itself featured an excess of donors, such as can arise from the zinc-excess non-stoichiometry of surface layers observed for zinc oxide surfaces outgassed in vacuo at high temperatures. The fact that such surface nonstoichiometry was known to be drastically reduced by oxidation in oxygen at 6 7 0 K , then provided an explanation for the lack of Ro-type activity of pre-oxidised zinc oxide surfaces in the dark, i.e. that oxidation strips electrons from e - ] (0-Zncus) locations by localising them on new surface 0'- species. Interfaces between ( r602 1 8 0 2 ) and these preoxidised ZnO surfaces were found, however, t o be remarkably lightsensitive, since even room light restored Ro-type activity with apparent quantum efficiencies having values up to 30 depending upon the light

+

References p p . 419-427

364

intensity and degree of surface pre-oxidation. Photoinduced reformation of e- I( O-Zncus) situations by promotion of electrons from deeper traps, which could be F: or M,-type centres involving electron localisation on surface anion vacancies V,, , could account for the restoration of Ro-type activity via schemes (35) or (35') on illumination. When the isotopic composition of the oxygen gas initially admitted into contact with a metal oxide surface is already in equilibrium in relation t o eqn. (31a), then no alteration in the isotopic composition of the gas phase can be expected under illumination unless illumination opens up a type R' o r R2 pathway for exchange between oxygen from the lattice with that from the gas phase. Recent French work [ 1731 on photoassisted OIE over TiO, , ZnO, S n 0 2 and ZrO, has provided convincing kinetic evidence for isotopic scrambling via an R'-type mechanism whenever UV light at high intensity became incident on the interface between each of these metal oxides and gaseous oxygen which was initially in isotopic equilibrium in relation t o eqn. (31a). A striking contrast thus emerges between the predominance of an R'-type mechanism (with a need for monatomic oxygen intermediates on the illuminated interface) under the conditions of those French experiments and the predominance of an Ro-type mechanism reported by many Russian workers. The contrast may derive in part from the use of isotopically equilibrated gaseous oxygen by the French workers and in part from the higher light intensities employed in their studies. The intensity data quoted by the French workers indicate a photon flux at least two orders of magnitude greater than employed by Russian workers [ 1271 . Consequently, the possibilities for the formation of a monatomic oxygen species at the interface by photolysis of the surface layer of the solids were greater in the French work. A possible mechanism by which photolysis may contribute to OIE is depicted schematically as

pnotoiysls

Photolysis of the surface of ZnO in vacuo during illumination by photons of energies greater than the bulk band gap of 3.2eV has been reported by several workers [153-1551. The relationship of such photolysis to OIE was re-examined recently in the author's laboratories using the full UV output from a xenon flash tube (ca. 4.7 x 10l8 photons at 200-340 nm per flash of duration 50 ps). The time profiles for rise, with t:,, 0.20s of transient pulses at m / e = 32 (corresponding t o an additional partial pressure of molecular oxygen product, A 1 6 0 2 , appearing in the gas phase as a consequence of surface photolysis), were measured

365

using the dynamic mass spectrometer system (cf. Fig. 8). With no gaseous oxygen admitted over the ZnO surface, which was instead maintained under the residual pressure of lo-' N m-' achieved by the ion pump, the magnitude of the pressure pulses at m / e = 32, initiated by single flashes incident on the vacuum/ZnO interface without the intervention of filters, diminished progressively for a series of equal pulses delivered at 2 0 s intervals. This progressive decline was taken as indicative of photolysis of surface layers of the ZnO, with each pulse leading to a pulse-related growth in metal-excess non-stoichiometry of the surface. In terms of a model advanced by Hirschwald and Stolze [146] for the photolysis of ZnO, increasing surface concentrations of excess zinc can cause decreasing efficiency of photolysis by acting as recombination centres and so competing against surface oxide ions for photogenerated holes as in the final steps of + h' ZnO t hv-

(e

h+ /

+of--00,-

+ h)/ZnO* -------(,

lO,(g)

(37) hi

Previous work illustrated the observation that non-reproducibility in magnitude of the photolytic I6O2 with flash number could be overcome N m-' over by maintaining oxygen at dynamic pressures of lo-' t o the metal oxide surface during the flash. The last column of Table 2 summarises the results of a comparison of the sensitivity of various metal oxides t o surface photolysis, which was made using the DMS system and by introducing I8O2 at a dynamic pressure of 3 x N m-* over flash UV-illuminated metal oxide surfaces as a means of minimising variations of 1602in successive pulses. The very fact that the presence of "0, at these low pressures transformed the system from non-reproducible pulse heights at rnle = 32 suggested a role for I8O2 in preventing or reversing surface photolysis. The manner of its action in this regard was indicated by the observation of a parallel flash-initiated OIE process. Data in the fourth column of Table 3 summarises the measurements made on this flash-initiated OIE, as detected with the DMS system for various 1802/metal oxide interfaces, through integration of the area under the l 6 0 l 8 O transient at m / e = 34. These data show that a flash-initiated transient at mle = 34 was readily detected for 180,/Cr20, and "02/ ZnO-In and was more than an order-of-magnitude larger than the trace (ca. 177 ) present in the isotopically equilibrated m o u n t of 160180 reactant gas (ca. 99% I8O2).N o significant flash-initiated enhancement of content occurred, however, with this highly enriched 1 8 0 2 gas the 160180 present over V,O,, TiO? or NiO. Neither did any enhancement occur when the enriched I8O2 gas but no metal oxide was present within the cylindrical quartz jacket of the photoreactor. These observations ruled out any possibility that the OIE process might have originated from isotopic References p p . 4 19-- 4 2 7

366

or place exchange processes involving residual I6O2 retained on walls of the photoreactor or samples (all of which had been outgassed at 670 K). A model such as that shown in eqn. (36) and based on the interaction between photoactivated l8O5 and I6O produced from the lattice by photolysis can provide a better explanation of this OIE process. Such a model would predict qualitative correlation between the extent of 160180 detected and the arithmetic product (amount of I6O2 released by photolysis x amount of I8O2 released by photodesorption) because the former term should reflect the rate of formation of I6O from photolysis i.e. as a precursor of I6O2) and the latter should reflect the rate of photoassisted activation of l x 0 it o "0;. Values of the product of these two terms, as evaluated from the flash-initiated transients at m / e = 32 and 36 listed in columns five and six of Table 3 would predict the sequence C r , 0 3 > Fe, O4 > V2 0, > Co, 0, > ZnO > CuO > T i 0 2 > NiO > ZnO-Li and the sequence observed experimentally by measurements o n m/e = 34 correlates well with that sequence (V, 0, being the single exception), when values of < 3 x l O I 3 are recognised as below the limit for accurate comparison. ( b) Pho to-oxida t ions in uo lv ing m olecu lar oxygen

The literature on photochemical oxidation of other species with molecular oxygen as the oxidant at illuminated interfaces is very much less extensive than reported for dye-sensitised photo-oxidations involving singlet molecular oxygen ( ' A ) as an important intermediate [ 1741 . Indeed, it is difficult as yet to identify any photoassisted reaction at the gaslsolid interface which can be unequivocally attributed to 0, ( ' A ) , although its possible involvement has been considered by some workers [ 175, 2681 . Published investigations of photoassisted surface reactions with groundstate oxygen species as reactant may be subdivided instead into three categories: (i) investigations of photo-oxidation of carbon monoxide by molecular oxygen, which has served as a model reaction for exploring photoactivation of oxygen at the illuminated surface into a state reactive towards CO; (ii) investigations of selective photo-oxidations of alkanes to partially oxidised products, which has been studied with a view t o identifying selective and efficient photoassisted pathways t o important partially oxidised chemicals, e.g. acetone, ethylene oxide o r phenol; (iii) tests of the contributions by photoassisted redox processes at surfaces towards selective photo-oxidation of oxidisable species (such as alcohols), which can act as hole-trapping species and so complement the electrontrapping action of 0, at the illuminated interfaces. Developments in these three areas will be separately outlined.

( i ) Carbon monoxide photo-oxidation. Investigations of carbon monoxide photo-oxidation prior t o the decade covered by this chapter had established that it was possible t o detect an increase in the rate of oxidation on the illumination of mixtures of CO and 0, in contact with metal oxide

361

catalysts which had been pretreated with oxygen [176, 1771. In the temperature range 470 k 100 K , the reaction under illumination had been variously reported as a true photochemical reaction with no temperature coefficient or activation energy [ 1 7 8 ] , as zero-order with respect t o carbon monoxide or dioxide pressure but first-order with respect t o oxygen pressure [ 1791, and as proceeding via photochemisorption of 0; followed by (C0)i

+ (0i)i

(C0i)i + ( 0 ) i

(38)

Other early investigations by Steinbach [ 180,1811 of the heterogeneously catalysed oxidation of carbon monoxide examined experimentally the then widely held view that electronic factors, in particular the position of the Fermi level at the gas/semiconductor interface, should exert a strong influence on the rate of reaction through modifications of Eact,the activation energy for the surface-catalysed reaction. Following observations consistent with the dependence of the activation energy of carbon monoxide oxidation upon the position of the Fermi level at the non-illuminated surfaces of the semiconducting oxides ZnO, NiO and Co,O, [ l 8 0 ] , Steinbach [ 1811 utilised similar techniques to determine the effects of UV illumination upon E,,, with NiO. Modifications in position of the Fermi level at the surfaces of lOOnm thick NiO grains were sought through their deposition as a thin layer on t o transparent evaporated films of the metals Ag, Au, Pd and Pt. Since the work functions, $, of these < GPd < @ p t < $NiO,it was metals vary in relation t o NiO as $Ag < argued that depletion of electrons from surface layers of particles of NiO in contact with these metals should be greatest on Ag layers and least on Pt, with corresponding shifts of the Fermi level upwards and away from the upper edge of the valence band of this p-type semiconductor. With the exception of Ag, the expected increase in Eactfor CO oxidation in the dark was observed, since Eact increased from 90 kJ mole-' over NiO t o 124.2, 133.7 and 163.1kJmole-' for NiO/Pt, NiO/Pd and NiO/Au, respectively. Illumination through the transparent metal support brought Eact for the latter systems back down again t o 81.9, 86.6 and 94.5kJ mole-', as would be consistent with photoinduced decreases in the net transfer of charge from NiO t o metal and with diminution of the upward shift of the Fermi level. Subsequent kinetic investigations of this supposedly ideal reaction in the temperature range 600-733 K showed that illumination influenced not only an apparent activation energy, but also the pre-exponential factor in a manner considered t o be indicative of parallel thermal and photocatalysed reactions [ 1821. With illumination at increasing light intensities but fixed wavelength, the activation energy decrease followed a parabolic law. Light of progressively shorter wavelengths produced progressively stronger decreases in apparent activation energy. However, a striking observation was made that the efficiency of any wavelength diminished upon simultaneous illumination at other References p p . 419-427

368

wavelengths. This latter effect, together with the non-linear dependence on intensity, was interpreted as evidence that the overall light reaction was not the sum of simultaneous, independent reactions on differently excited surface sites. Activation energy effects were, rather, considered to arise from the effects of illumination on the strength of the zinc-axygen bonds at the surface. This concept of surface bond strength being dependent upon the intensity and energy of the photons incident on the surface contrasts with the interpretations of other workers. Investigations of the spectral dependence of the photocatalytic oxidation of carbon monoxide by pure zinc oxide and its solid solutions with lithium oxide ( ZnO-Li) or aluminium oxide (ZnO-Al) led Zakharenko et al. [ 1271 more recently to the conclusion that the quantum yield of the photoassisted oxidation depended largely on the surface charge present on the surface of the zinc oxide. Differences [cf. Fig. 9(a)] in the spectral dependences of the photoadsorption of oxygen (with a maximum at 2.5eV) and of the photo-oxidation of CO (which showed the same spectral dependence as the band-edge absorption) for pure ZnO led those Russian workers to conclude, contrary t o eqn. (38), that 05, which would reach maximum surface coverage under illumination by photons at 2.5 eV, cannot be the form of oxygen active in the oxidation of CO on the illuminated surfaces. They concluded, rather, that lattice 0;species, produced by hole-trapping on surface 0 2 -ions, were the species active in photo-assisted oxidation. Measurements of photo- and thermoelectric work functions of these materials were interpreted as indicating the location of the Fermi level approximately at the mid-point of the band gap for pure and lithium-doped ZnO, but as being displaced into the top half of this gap for ZnO-Al. Migration of photogenerated holes from within the bulk of ZnO to surface 0'- ion sites was thus suggested as being favoured by a greater negative surface potential than would exist on ZnO-A1. The Russian workers found much larger quantum efficiencies (ca. 0.24-0.3) over ZnO than over ZnO-Al (ca. 0.018) under illumination by photons inside the band edge. The lithium-doped ZnO samples presented a more complex picture, since photocatalytic act,ivity for CO activation exhibited a well-defined peak at 2.5 eV (i.e. 0.7 eV outside the band edge). Quantum efficiencies were 0.032, 0.024 and 0.016 for illumination intensities of 2.2 x l o i 3 , 5.3 10i4 and 4.7 x photons s-' , and this process was attributed to photogeneration of surface 0- sites from surface defect centres involving lithium [cf. eqn. ( 1 9 ) ] . That Russian work, like many of the earlier studies. was carried out with oxidised ZnO surfaces and there are unresolved differences between the results, e.g. Russian claims to observe the effect at room temperatures, in contrast t o the higher temperatures reported elsewhere as necessary. Orders of reaction with respect to carbon monoxide and oxygen were different from those reported by earlier workers [177-1791, but their report of first-order dependence on P,, agreed with Steinhach and Barth [ 1821. 'I

369

100

700

400

Pco ( t o r r )

torr

Fig. 12. Dependence of t h e photoassisted rates of carbon monoxide oxidation over TiOz (upper plots) and of isopropanol oxidation over ZnO (lower plots) on reactant pressure. Observed rate photo-oxidation at each pressure is subdivided, in the manner of eqns. (39) in the text, into a rate V , displaying a Langmuir-Hinshelwood pressure dependence and another rate, V , , increasing linearly with reactant pressure and consistent with an Eley-Rideal process. Reproduced with permission from refs. 183 and 257.

Comparisons of the activity of TiOz and G a z 0 3 with that of ZnO for promoting photo-oxidations of carbon monoxide at room temperature have been made by Thevenet et al. [183] using illumination inside the band edge (210-390 nm) and a gas chromatographic method for monitoring the rather small extents of conversion (ca. 1.3%)attained in one pass through a dynamic photoreactor. The photocatalytic activities of TiO, and G a z 0 3 were stated to be greater than for ZnO, S b z 0 3 o r SnO, . Kinetic analysis of the dependence of the extent of photoconversion on oxygen pressure at constant Pco yielded linear plots in the format of eqn. (39a), which could correspond to the LangmuirHinshelwood-type dependence of rate on surface coverage, O 0 , controlled by eqn. (39b), viz. Referencespp. 4 1 9 4 2 7

370

Dependence on Pco for fixed Po, was not fitted by a single function but was separable empirically into two processes with photoassisted velocities, V1 and V , which obeyed eqns. (39c) and (39d) respectively. Agreement with this analysis is illustrated in Fig. 12(a). The component described by eqn. (39c) was attributed to a Langmuir-Hinshelwood dependence on Oc0 at low Pco . The Eley-Rideal type described by eqn. (39d) became more important a t higher Pco .

v,

=

k 1 Kco pco1 + KcoPco

Thenevet et al. considered that the relationships (39) might originate from any one of three mechanisms, two of which would involve chemisorbed oxygen as the oxidant whilst the third would involve lattice oxygen. Lattice oxygen was favoured on the basis of a single mass spectrometric experiment showing that the CO, photoproduct from CO I8O2 was mainly CI6O2 and not C 1 6 0 1 8 0Largely . on the basis of this experiment, these authors favoured a mechanism previously suggested by Mars and Kreveleri [184] as being responsible for their results and kinetics. That mechanism envisages reaction occurring via collision of CO from the gas phase with lattice oxygen, followed by restoration of lattice oxygen through surface re-oxidation by oxygen from the gas phase. The component of the photocatalysed kinetics expressed by eqn. (39d) was tentatively identified as a mechanism involving collision of CO from the gas phase with adsorbed and dissociated oxygen, i.e. Eley-Rideal in character. Photo-oxidation of carbon monoxide has been re-examined by Steinbach and Harborth [ 1541 using UHV techniques for the preparation of clean surfaces of ZnO single crystals and mass spectrometry for monitoring reaction thereon. Photoassisted oxidation t o CO, over ZnO single-crystal surfaces at temperatures between 670 and 720K was observed with a quadrupole mass spectrometer (QMS) in the presence of N m-’ of 0, and CO. It was argued that true photoassisted processes should be distinguishable from thermally catalysed oxidation on the basis that only the former should respond rapidly to changes in illumination intensity. This led t o the incident photolytic light being chopped and a lock-in amplifier was used to select only those components of the output from the QMS which rapidly followed the intermittent light

+

371

intensity. Steinbach and Harborth [154] concluded that only the partial pressures of C 0 2 and atomic oxygen, i.e. Pcoz and P o , responded sufficiently rapidly t o be classified as genuine photoproducts. Slower changes in other gas-phase components, e.g. O,, H 2 0 , CO, and zinc vapour, were attributed to thermally assisted processes, some of which were promoted by the very high flux of photons focussed on the ZnO crystal from a 1 O O O W lamp. [However, the features envisaged above in relation t o mechanism (27) for photodesorption at I8O2/Zn0 interfaces also merit consideration here, viz. that the photoassisted surface reaction may be fast, but that desorption of observable products may be thermally activated and be the slow rate-determining process.] In the absence of gas phase CO, atomic oxygen was the dominant photoproduct above the interface between loT6Nm-2 of O 2 and the ZnO single crystal surface, provided the latter was at > 670 K. Unfortunately, no direct evidence was obtained in this study as to the sites on the ZnO surface from which this atomic oxygen originated, e.g. whether it resulted from removal of surface 0- from normal lattice sites or from oxygen atom-like species at surface defects. However, Hirschwald [ 1551 , who also studied ZnO photolysis in conditions of UHV, has commented on the much greater ease of photolysis t o atomic oxygen from a nearly stoichiometric oxygen-rich surface layer prepared on polycrystalline ZnO by prior oxidation ( 1 5 h , l O O O K , 1 0 4 N m - 2 0,) than from zinc-rich surfaces produced by vacuum treatment similar t o that used by Steinbach. In Hirschwald's view, the absence of significant photolysis of ZnO below ca. 623K may be attributed t o a retention of zinc (which has an activation energy of 32 kcalmole-' for desorption). Consequently, zinc formed in limited initial photolysis or thermolysis remains on the surface and acts as efficient recombination centres for reconverting adjacent surface 0- (i.e. holes at the illuminated surface) into ZnO. Despite some unresolved points, which undoubtedly stem from differences in the pretreatment of the solid and from differing light intensities, the above interpretations tend to converge on a description of carbon monoxide photo-oxidation on metal oxide catalyst as involving (i) photogeneration from the lattice of a monatomic oxygen species, which Russian workers identify as O;, and (ii) reaction of this species with CO. Since transfer of charge between the bulk and surface regions of the illuminated adsorbent is thus envisaged in the rate-determining step, some sensitivity of this photo-oxidation towards the extent of bandbending would be expected and has been claimed recently by Van Damme and Hall [185] over various perovskites. Those workers report a photocatalytic (PC) enhancement over SrTiO, . Studies of the temperature dependence of this effect revealed that, for temperatures < 573 K, activation energy for the process under illumination was lower than for the reaction in the dark. At higher temperatures, the results obtained over SrTiO, in the light were not experimentally distinguishable from those in References p p . 419-427

372

the dark. However, as the temperature was lowered, irradiation produced a measurable enhancement and an Arrhenius plot of the dark reaction remained linear, whilst that for reaction under illumination levelled off. This latter behaviour was seen as “characteristic” of a photocatalytic process. Both TiOz and BaTi0, also responded t o illumination, whereas LaCo0, and Ba(Fe0,33Ti0,67)Oz.67, which were much more active for CO oxidation in the dark, did not respond t o illumination. The temperature dependence over BaTiO, was remarkable, since in the dark it evidenced those features quoted earlier as “characteristic” of a photocatalytic process and levelled off at temperatures below ca. 6 0 0 K . The title ferrocatalytic (FC) was suggested for this effect, since it was attributed to spontaneous polarisation in surface layers of ferroelectric BaTi0, and t o enhancing effects of associated charge fields on the transfer of charge from the bulk to the surface of BaTiO,. Destruction of the polarised surface layers, or compensation of their effects, by hole-electron pairs generated by illumination were advanced as explanations of the observation that FC effects were found to be quenched after exposure to bandgap illumination. The interrrelationships between PC and FC effects suggested by this work are interesting and deserving of detailed kinetic study t o check the validity of an underlying assumption that the same rate-determining step operated for CO oxidation in all their systems. Interestingly, parallel studies on the oxidation of H, by O2 over the perovskite catalysts (and ZnO) failed t o give evidence of any PC or FC effects on that reaction. Recently, Anpo et al. “61 have observed that the introduction of carbon monoxide at low pressures over the oxides V2OS,MOO, and CrO, dispersed on porous Vycor glass (PVG) diminished the phosphorescence attributable to a triplet state of a charge-transfer excited state e.g. (M o 6 + - 0 2

-

)

9 hv

(Mo”-O-)*

Quenching of the luminescence was accompanied by photoadsorption of CO and photoformation of CO, as detailed in Table 4. Anpo et al. consider: (i) that these results support the conclusion that photoreduction of the oxides with CO molecules proceeds via their charge transfer triplet excited states; and (ii) that this is the first report connecting the photoreactivity of oxide catalysts with the lifetime of (M-0) excited states (but see Sect. 3.3.2). On the basis of a further observation that none of the oxides NiO, Co304, Cr203, FeZO3 or CuO showed activity towards photoreduction with CO, they tentatively advance the idea that such activity may arise only in oxides (such as V z O s , MOO, or CrO,) in which metalboxygen double bonds arise, since they argue that only there would the oxygen of the charge-transfer excited state be similar to a “free 0- ion”. However, that argument ignores the possibilities for photoformation of 0-

373

TABLE 4 Relationship between the lifetimes of excited triplet states and the initial rates of photoreaction at 300 K

Lifetime of excited triplet states (ps) Initial rate of C02 photoformation (lo-'' moles-') Initial rate of CO photoadsorption (lo-'' moles-' ) Quantum yield of C02 photoformation Quantum yield of C02 photoadsorption a

V2 0,/PVGa

Moo3/PVGa

Cr03/PVGa

218

63

2.9

1.44

0.43

0.01

4.33

1.22

0.01

0.043

0.01

0.11

0.03

PVG = porous Vycor glass.

species similar to free 0- from oxide ions existing at surface sites of high coordinative unsaturation.

(ii) Alkane photo-oxidation. A gas chromatographic technique similar t o that employed for their study of carbon monoxide photo-oxidation over TiOz has been utilised by Teichner and co-workers [187, 1881 t o establish the general features of alkane photo-oxidation for a mixture of helium, oxygen and hydrocarbon passed over TiO, at ca. l a t m . total pressure. Studies were made with non-porous finely divided particles having the anatase crystal structure and particle sizes of 6-100 pm. Photoassisted reactions were initiated by the incidence of UV illumination from a 125W medium pressure Hg arc lamp on the interface between these anatase particles and the continuously flowing mixture of helium and reactants. Emergent gases from the dynamic photoreactor were sampled at intervals for separation on appropriately packed columns and were analysed by GLC using either a flame ionisation detector (for hydrocarbon photoproducts) or, at lower sensitivity, by a thermal conductivitytype detector (for permanent gas products, CO,, etc). N o significant amounts of photoproduct emerged continuously from the dynamic photoreactor except when TiO,, oxygen and UV illumination were present simultaneously. Under the latter conditions, small percentage conversions of reactant hydrocarbon (typically 1-5%) could be photoinduced continuously for reactants undergoing one pass over an illuminated layer of the anatase particles dispersed as a thin layer on a membrane permeable to the reactant gases and photoproducts. Despite rather low percentage conversions, the photoassisted processes were classified as photocatalytic, apparently on the basis that stationary activity could be maintained for several hours under illumination, following an initial rise of the References p p . 4 1 9 4 2 7

314

photoassisted activity during the initial 10-30 min of illumination. These features were illustrated in Fig. 10. A very rapid decline in activity occurred when illumination ceased. Quantum efficiencies of 0.1-1 for alkane photo-oxidation have been claimed for the process under optimum conditions. For all the alkanes investigated, except methane and ethane, partial photo-oxidation to aldehyde and ketone products represented a significant fraction of the total photoproducts detected. Results expressed as the ratio of the number of moles of particular product formed, to the number of moles of alkane consumed in the reaction (termed the selectivity, S) showed C 0 2 t o be the dominant product. Ketones and aldehydes were present as products of selective photooxidations. Based on an implicit assumption that similar factors will determine product distribution in photo as in thermally assisted processes, it was argued that distribution of photoproducts among the ketones, aldehydes, etc. indicated photoassisted attack on each carbon atom of the parent alkane. This latter deduction was made for all the alkanes investigated from the distribution of photoproducts. Teichner and co-workers [ 187, 188, 1901 considered that a modified selectivity criterion, S , , (representing the ratio of the number of moles of alkane required to form a particular product to the total number of moles of the alkane consumed in the reaction) formed a better basis for the comparison of various pathways of photooxidation. Such S , values should total loo%, e.g. the followingS, values were observed during isobutane photo-oxidation: C 0 2 23%; acetone 61%; 2-methylpropanol 7%; and t-butanol 9%. These values were considered t o favour a consecutive reaction scheme of the form isobutane + t-butanol + acetone. Indirect support for photo-oxidation via a route involving tbutanol as an intermediate was deduced from the equality in rates of the photoassisted conversion of isobutane to acetone or of t-butanol to acetone. Parallel reactions leading, respectively, t o acetone or t o t-butanol via a hydroperoxide (such as might be expected t o result by attack of 0; or singlet molecular oxygen) were considered less probable in view of the fact that feeding the photocatalytic reactor with the hydroperoxide of isobutane yielded only minor amounts of acetone and no trace of tbutanol. The tentative hypothesis of an alcohol intermediate in this and other selective photo-oxidations raised the interesting question as t o how a UVilluminated (alkane O,)/TiOz interfaces could achieve insertion of oxygen into C-H bonds. No definitive evidence as to the feasibility or mechanism of such photoassisted oxygen insertion was forthcoming from that study, although Teichner mentioned the possibility that the alkoxy radicals (CH,), CO may initially form at the interface in the photoassisted conversion of isobutane t o acetone and t-butanol. Later work by Courbon et al. [ 1731, which has already been discussed above in relation t o OIE at lSOz/Til60g interfaces, showed that. the photoassisted oxidation of

+

315

+

isobutane inhibited any OIE in {**O, (CH,), CH}/Ti160: systems, thereby indicating that the same monatomic oxygen species at the TiO, surface could participate in both photochemical processes. Since dissociated oxygen species were favoured as the sites of OIE under the conditions of the experiments of Courbon et al., the conclusion was drawn that monatomic oxygen also represents the photoactivated surface species which attacks isobutane in its photo-oxidation over TiO,. The nonclassical nature of the selective oxidation products formed over the UV illuminated (hydrocarbon 0,)/TiO, interfaces has also been confirmed through the observations of alkylbenzaldehydes as major photoproducts from alkyltoluenes [ 1891. Rate expressions (40a) and (40b) which are formally similar t o those derived by Mars and van Krevelen for hydrocarbon oxidation on nonilluminated oxide catalysts, were advanced by Formenti et al. [188] as a basis for linearising their experimental data for reactant pressure dependencies in isobutane conversion t o acetone over TiO, under continuous UV illumination at fixed intensity.

+

1 Rate

--

-

1 koPt

1 + K,KoP, -~

1

tkr

In these equations, K c represented an equilibrium constant for the physical adsorption of isobutane (or other hydrocarbon) according to a Langmuir isotherm; k o was a rate constant for oxygen adsorption (irreversible below 300°C) at a rate given by k o c (1- O o ) , where N was set equal t o 0.5 for dissociative adsorption; the rate of oxygen adsorption was assumed equal to the rate of its consumption, which in turn was set equal to acetone formation, koOoO,; and Pc or Po were the pressures of the hydrocarbon or oxygen reactants, respectively. The rate expression (40a) and the assumptions underlying it have recently been criticised by Childs and Ollis [ 1621 who put forward a re-interpretation of the experimental data of Formenti et al. [188] on isobutane and of the data of Walker et al. [ 1901 on photo-oxidation of 2-methyl-2-butyl alcohol over Ti0,. Teichner et al. had reported comparable photoassisted rates of acetone formation from the alkane and the alcohol, leading to the postulate of a common rate-determining process, viz. dehydration of an assumed alcohol intermediate, for the two photo-oxidations. Childs and Ollis adopted this idea t o arrive at the following parallel reaction schemes.

Scheme (41)for isobutane

(1) R2CHR’(g) ( 2) 2 s + o ,

+ S ,--”(R,CHR’.S)

+

References p p . 41 9-42 7

2(0*S)

376

(3a) (R,CHR’*S)

+ (0.S)

k3

- +S

(R2COHR S)

Scheme (42) for alcohol KROH

(1) R,COHR’(g)

+S 5(R2COHR‘.S)

KO

(2) 2 s + o 2 2 2 ( 0 . S ) Kr (3a) (R,COHR*S) S ( R 2 C = R ‘ * S )+ (H2O.S)

-

+ (3b) (R,COHR’-S) + S k ’ -

+ (H20*S)

[R=C(R)R‘*S]

Further rapid reactions applicable to ( 4 1 ) and ( 4 2 ) (4) (R,C=R’) (5a) O=R’ (5b) O=R’

+ 0,

-

A

R2C=0

+ O=R’

desorbed product (if R’ > CH,)

CO,

+ H,O (if R’ = CH,)

The kinetics of each process are determined by the slower processes (3a) and (3b). These lead to an olefin-type surface intermediate (or its precursor) via a dehydration step requiring two sites for its completion, one t o accommodate the olefin-related species and one a water molecule. The requirement for a second, vacant site is characteristic of the model, leading in the case of increasing alcohol pressure to autoinhibition due to decreasing availability of vacant sites. With additional assumptions of (i) Langmuir-type adsorption isotherms for both alcohol and oxygen adsorption, and (ii) the absence of inhibition by product, rate expression (42a) was deduced for acetone formation from alcohol.

If, as a first approximation, oxygen pressure dependence was assumed to be weak, this could be simplified t o

Satisfactory agreement was shown between this expression and the experimental results of Walker et a]. [190] on alcohol pressure dependence. No adequate test of oxygen pressure dependence was possible on the basis of that study. Re-adoption of the assumptions of zero inhibition by products and of Langmuir-type adsorption of the alkane reactant, together with a similar pseudo-equilibrium expression for surface coverage by oxygen, was shown t o lead to rate expressions (41a) and (41b) for the photoassisted production

of acetone from isobutane. Rate =

(

2J=P

k,C$P,NPC [1+(K0Po)N K,P,

+

=

I,(P{)

+P{Pc]2

+ S,P,NP,

+

In these expressions, a = k 3 / ( k - 3 k 4 ) , 0 = KrK,, I is the intercept and S the slope of plots according to eqn. (41b). The fact that linear plots resulted forN = 0.5 provided support for the involvement of dissociatively adsorbed oxygen in the alkane photo-oxidation. In general, the experimental data of Formenti et al. could be adequately accounted for.

( i i i ) Partial photo-oxidation of isopropanol. Partly as a follow-up to their observation that alcohols appeared as minor products and probable intermediates in the photocatalysed oxidation of hydrocarbons over TiO,, French workers have applied similar gas chromatographic techniques t o study photo-oxidation of various alcohols over TiO, and have observed selective photo-oxidations to ketone and aldehyde products [ 1901 . Investigations of alcohol vapour/metal oxide interfaces have also been reported utilising a range of other techniques including a recirculating rotary photoreactor [ 1611 , IR analysis [ 9 3 ] , mass spectrometry and thermal desorption [ 1911, dynamic mass spectrometry [ 192, 1931 and oxygen-labelling techniques [ 1941 . Isopropanol photo-oxidation has been particularly widely studied and was found by IR observations on the composition of the vapour phase to be sensitised by W03, SnO, and ZrO,, as well as by conventional ZnO and Ti02 photocatalysts [ 9 3 ] . IR observations on adsorbed species showed that surfaces of silica gel and y-Al,03 also acted t o promote photo-oxidation of isopropanol to acetone. In addition to the bands at ca. 1700cm-' attributable to carbonyl frequencies of adsorbed acetone, other IR bands appeared at ca. 1600 and 1400 em-' , indicative of surface compounds similar to acetate ions. Photo-oxidation of methanol produced a surface photoproduct with formate-like absorption. Whilst unreacted alcohol could be largely removed by heating and evacuation, these latter carboxylate-like bands were not completely removed even at 6 2 3 K [ 921 and would appear t o originate from a photoassisted surface reaction rather than true photocatalysed processes. IR results thus indicate a slow build-up of strongly held carboxylate species as products of a photoassisted surface reaction proceeding in parallel with photocatalysed oxidative dehydrogenation of alcohol. The composition of the gas phase over an isopropanol/TiOz system illuminated in a rotary photoreactor has been examined with a monolayer of isopropanol pre-adsorbed on the powdered TiO, [ 1611 . Acetone References p p . 419-427

378

photoproduct was not released in significant and continuously increasing amount unless oxygen gas was also present at the illuminated interface and this process is illustrated in Fig. 11, together with the converse photoassisted depletion of oxygen and of isopropanol from the gas phase. The figure also illustrates an effectively constant photoassisted rate of uptake of oxygen, which appears independent of the amount of isopropanol present in the gas phase. This contrasted with an enhancing effect of increased isopropanol partial pressure on the rate of release of acetone to the gas phase, and the contrast led to suggestions that isopropanol could dislodge acetone from the TiO, surface in a 1:l molar ratio [161]. Admission of water vapour to the TiO, surface, after outgassing at 600 K and after contact with isopropanol, did not significantly change the course of reaction from that shown in Fig. 11. However, when water was admitted t o the TiO, surface after outgassing at 6 0 0 K and prior t o admission of isopropanol, a “hydrated surface” resulted, displaying lower rates of oxygen depletion from the gas phase and of acetone production. The latter was not dependent on the amount of isopropanol in the gas phase over the hydrated surface. Bickley and Jayanty [191] considered that these differences could be accounted for in terms of a dependence of the rates of acetone and isopropanol evolution to the gas phase on complex equilibria involving adsorption/desorption and mutual displacement of these species and water a t the surface. The positions of these equilibria were envisaged t o vary as photo-oxidation proceeded with the stoichiometry 0 2 (g)

+ 2 C3H70H = 2 (CH,),CO + 2 H2O

The nature and reactivity of the photoactivated surface site or species could not readily be inferred from kinetic data such as those in Fig. l l ( b ) . A mechanism for the photo-oxidation was, nevertheless, advanced which represented the trapping of photogenerated holes by pre-existing surface hydroxyls as the initiation step, followed by electron capture on adsorbed molecular oxygen and by reaction of the resultant 0; with adsorbed alcohol via either proton or H atom transfer. The choice of surface OH; as the photoactivated site for the initiation of the reaction appeared to rest mainly upon the linkage of two qualitative observations on the effects of high temperature outgassing on the activity of TiO, surfaces: (i) that the activity for alcohol photo-oxidation was reduced by outgassing at 1073 K; (ii) that the activity for oxygen photoadsorption (in the absence of alcohol) declined progressively as the extent of dehydroxylation of the surface was reduced by outgassing at temperatures between 600 and 1073K. A mechanism based on hole trapping by surface hydroxyls was adopted by Bickley and Jayanty [191] in interpreting the results of a re-examination of isopropanol photo-oxidation over fully oxidised T i 0 2 . This employed programmed thermal desorption (PTD) to remove species from the T i 0 2 surface and mass spectrometry to identify them. The hydroxyl-based mechanism was complicated by observations indicating

379

that a strongly ,held form of adsorbed isopropanol was in competition with water for sites capable of yielding surface hydroxyls. Another difficulty which attached to the use of thermal desorption was the occurrence of extensive thermally assisted dehydration and dehydrogenation processes (see also ref. 54), which made it difficult to disentangle photoassisted from thermally assisted products. The photoassisted oxidation of isopropanol and another secondary alcohol, butan-2-01, over TiO, and ZnO has been re-examined recently in the author’s laboratory [ 2561 employing a Lyon-type photoreactor either with a continuous flow of reactants or with pulses of reactant admitted as desired to a surface not otherwise exposed to reactant gases. Results either with intermittent reactant pulses and illumination or with continuous dynamic flow and illumination showed that, with a fixed pressure of 0, but varying pressures of secondary alcohol admixed with N, carrier gas, photoassisted oxidative dehydrogenation of the secondary alcohols was the dominant photocatalysed process with a rate dependent on alcohol pressure in a manner similar to eqns. (39c) and (39d), i.e. the observed rate could be separated into one process displaying a LangmuirHinshelwood-type dependence on alcohol pressure at low PROHand another which increased linearly with alcohol pressure [cf. Fig. 12(b)] and so would be consistent with an Eley-Rideal-type photoassisted process. The latter process was attributed to a photoactivation of surface oxide ion sites by hole capture, producing 0- surface intermediates which reacted during their short lifetime with physisorbed alcohol or alcohol directly striking the photoactivated site from the gas phase. The Langmuir-Hinshelwood-type process was considered t o involve alcohols chemisorbed on a subset of particularly active O:& sites which, on photoactivation by hole capture, activated the chemisorbed alcohol for attack by adsorbed 0,. On ZnO surfaces, this latter process dominated at alcohol pressures < 500 N m-*, but levelled off at ca. 2.5 x lo3 N m-?. The EleyRideal type processes made increasingly important contributions at pressures >2.5 x lo3 Nm-’ and exceeded that from the LangmuirHinshelwood-type processes at alcohol pressures of 5 x lo3 N rn-, [cf. Fig. 12(b)]. Kinetic data did not suffice t o determine whether these increasing contributions involved reaction of physisorbed alcohol or alcohol colliding from the gas phases with photoactivated oxygencontaining locations such as that envisaged in eqn. (36). Pulsed-reactant procedures [ 192-1961 allowed pulsing of the alcohol and/or the photon flux in systems involving alcohol vapour flowing over a powdered sample of the metal oxide (cf. Fig. 10). Surfaces of the metal oxide were preconditioned by heating to 623K either in the N2 stream (referred t o as mildly reduced surfaces) or in a stream of oxygen (referred t o as pre-oxidised surfaces) and cooled prior t o contact with a pulse of reactant(s). Such reactant pulses then passed through the packed GLC column for separation and subsequent quantitative analysis by the flame ionisation detector. Adsorption of reactant alcohol on the preconditioned

-

References p p . 4 1 9 - 4 2 7

380

sample, even in the absence of UV illumination, showed up as a diminution of the amount reaching the detector [195]. With pulses of 0.5 ml total capacity and composition [N,: (CH,),CHOH: 0 , 0.45 atm; 0.05 atm: 0.5 atm] , the amount adsorbed on the metal oxide increased linearly with the number of reactant pulses for the first 1 2 pulses but then levelled off gradually t o a limiting value. Alcohol adsorption in the dark a t 350 K was not accompanied by the release of a detectable amount of products which would correspond t o surface-assisted oxidative dehydrogenation or dehydration of the alcohol. Illumination of the (alcohol 02)/metal oxide interfaces during passage of the reactant pulses did result in the detection of pulses of such products. The dominant product pulses from secondary alcohols corresponded to the prompt formation and release of ketone products requiring photoassisted oxidative dehydrogenation of the reactant alcohol, e.g. butan-2-one from butan-2-01 [ 195, 2561 . Product pulses corresponding t o photoassisted dehydration t o alkene were < 5 % of the total photoproduct yield. However, a lower aldehyde corresponding t o photo-initiated C,-C, bond cleavage in the alcohol did form at significant rates. Depending upon the identities of the secondary alcohol and of the catalyst, the rate of such - (C,-C, )* processes varied from 40 to 160% that of the (-H,) process, the higher yields being obtained for longer-chain alcohols. For all the (alcohol O2 )/metal oxide systems studied, photoassisted conversion t o any of the indicated products was greatest for the first reactant pulse and then declined on admission and analysis of subsequent reactant pulses, this decline being greatest and occurring over the smallest number of pulses for very finely divided samples of high surface area and high initial photoactivity (e.g. a decline t o 12%of initial activity occurred from pulse 1 t o pulse 6 for a TiO, sample of area 170 m2 g-'). Some decay in the level of photocatalytic activity of ZnO or TiOz from an initially high value to some intermediate but reproducible level after use for 0.5-3 h, is not unique t o operation in a pulsed-reactant mode nor t o one laboratory. It seems appropriate, therefore, t o recall that changes in the electronic properties of such systems can be brought about at such gas/semiconductor interfaces on this time scale by UV illumination (cf. Sect. 2.1.2 and Fig. 1) and that such electronic changes can affect rate coefficients and surface concentrations for reaction (cf. Sect. 1.2.3). Changes in photoelectronic factors thus appear likely t o make significant contributions to variations in the rate of photocatalysed reactions with time of illumination. Other likely contributory factors were the poisoning/ inactivation of a sub-set of particularly active surface sites by: (i) photoassisted surface reactions, such as formation of carboxylate or carbonatetype surface centres via a side reaction accompanying the predominant (- H2 )* or (-H,O)* process from the alcohols; or (ii) by photoassisted removal from the surface of some essential component of the specially active sites, such as removal of Of& or OH; as water vapour during

+

+

381

exposure t o alcohol and t o UV illumination. Evidence for the operation of the former of these processes at (CH3),CHOH/Zn0 interfaces came from observations that prior exposure of the ZnO surface t o acetic acid or C 0 2 , which were likely t o produce carboxylate and carbonate-type species, respectively, did cause the (- H, ) photoactivity t o decrease in proportion t o the extent of such prior exposure. Consequently, the decline in activity noted experimentally represents, in all probability, the combined influent? of site poisoning and photoelectronic factors. In such circumstances, quantitative analysis of kinetics during the initial decline in activity would require that the rates of change be independently monitored, both for the surface electronic properties and for the destruction or blockage of active sites. This problem appears not yet t o have been rigorously treated, particularly in dynamic flow photoreactions with gas chromatographic detection, where the working assumption usually adopted has been that rates of conversion attained after the initial decline in activity provide a good measure of stable photocatalytic activity. Despite widespread recognition of the necessity for some gas-phase pressure of 0, if the photocatalytic activity for partial oxidation of alcohols is t o continue t o high t.a.p.s.* over ZnO and TiO,, definitive evidence on the role or roles played by 0, is still awaited. The various suggestions made include: (i) 0, (ads) acting as an electron trap, thereby complementing the hole-trapping roles of surface OH- or surface alcohol species and so diminishing the rate of electron-hole recombination; (ii) entry of 0; into the sequence of reactions, e.g. by abstraction of H+ or H atom from an alcohol-related surface intermediate produced in the primary photoactivation step; (iii) involvement of O,, or of surface oxygen species produced by chemisorption/desorption equilibria, in the formation of charge-transfer-type exciplexes, e.g. as per eqns. (43), in which all square-bracketted or asterisked species exist only on the surface. ht

+ [ \/ CHOH . . .O,]--

[

\ C H O H . . . O,]* /

The possibilities of surface exciplex formation recognised by these equations are: hole capture by surface complexes between alcohol and 0; preformed at the dark interface [cf. eqn. ( 4 3 a ) l ; production of a lowlying electronically excited state of O;, e.g. by hole capture on 0; and subsequent encounter with adsorbed or gas-phase alcohol [cf. eqn. ( 4 3 b ) l ; and adsorbent-initiated excitation of adsorbed alcohol, plus subsequent encounter with O2 from the gas phase or a weakly adsorbed state. Available kinetic data for ZnO and TiO, point t o an approximately first-order Refe:ances p p . 419-427

382

dependence on Po, at pressures of 20-400 torr, but this does not provide definitive discrimination between the roles of molecular oxygen suggested under (i), (ii) and (iii) above, in cases where photo-oxidation proceeded to high t.a.p.s.* in those gas chromatographic experiments. Some insight into the probable roles of molecular 0, at low t.a.p.s.* has emerged from experiments employing low pressures (ca. 0.1 torr) allied to mass spectrometric monitoring of the of isopropanol and of 1802, gas phase composition, over various 3d transition metal oxides at room temperature [196]. Even in the absence of UV illumination or of any gas phase O,, conversions t o acetone product equivalent t o the (-H2) reaction of isopropanol with the first surface monolayer of metal oxide (or a significant fraction thereof) were observed over pre-oxidised surfaces of C r 2 0 3 , F e 2 0 3 , V 2 0 5 and TiO, . Support for the interpretation of these conversions as originating from surface reactions at t.a.p.s.* < 1 came from observations that the extent of conversion diminished sharply over Cr203, Fe203 and Ti02 after outgassing in vacuo a t 675 K, a pretreatment which yielded extensively dehydroxylated and slightly oxygendeficient surface layers. Whenever partial pressures of ca. 0.1 torr each of 1802and isopropanol were established over the indicated oxides or over ZnO and the systems were exposed to the output (at 3 0 0 < X < 800 nm) of a 500 W medium pressure Hg-arc lamp, photoassisted growth of acetone products was observed, except in the case of V 2 0 5 . Acetone photoproduct was enriched t o 28 ? 10% in "0 over the other oxides except high purity TiO,, the enrichment being influenced by the extent of surface pre-oxidation/outgassing. Since no comparable enrichment was found from the "02 (CH3)2CH'60H systems in the dark, nor from "02 (CH3)2 C l 6 0 systems under illumination, it could beqoncluded that oxygen isotope exchange between molecular "02 and ,CHOH or C 'O could not be responsible for the observed incorporation. Photo/ assisted exchange between H, l 8 0 and (CH,), CI6O was observed, however, over ZnO and Cr203, with rates more than adequate t o account for the incorporation of oxygen-18 into the acetone photoproduct from isopropanol over those oxides [196]. One two-step mechanism for l80 incorporation consistent with these observations would envisage: (i) reaction of oxygen-18 surface species derived from 1802with surface hydrogen fragments from the (- H,)* process; and (ii) subsequent rapid oxygen exchange between the resultant H, l80and the acetone fragment from the (-H,) process. Additional hypotheses would be necessary in this mechanism to account for limitation of the l80 incorporation t o 28 t 10% over most of the oxides [e.g. that the (-H2) fragments can react also with surface lattice '60;u,species as well as l80from and for the lack of any significant incorporation of l80into acetone over one high purity TiOz sample or over V 2 0 , [e.g. that reaction of (- H,) with surface oxygen species on those surfaces does not produce the particular H2 "0 surface species which can engage in OIE with acetone]. Unproven

+

+

383

hypotheses would likewise appear necessary t o reconcile experimental observations with an alternative two-step mechanism involving: (a) dehydration yielding surface H2 l6O and propene-like intermediates; (b) attack of l8O species derived from "02 on the newly formed or incipient double bond of the alkene. Since tests with the rate of oxidation of propene by I8O2 over illuminated ZnO and Ti02 indicated rates of "0 incorporation too low to account for those observed from [I8O2 i(CH3)2CH160H]/ZnO systems, one unproven hypothesis necessary t o reconcile observations with this latter mechanism would be an alcoholrelated surface intermediate having an incipient double bond more susceptible to attack by "02than surface propene. Neither (i) and (ii) nor (a) and (b) can be regarded at this stage as providing a fully satisfactory explanation of oxygen-18 incorporation into the acetone photoproduct from isopropanol under low pressure conditions at t.a.p.s.* < 1. When gaseous oxygen was present together with alcohol vapour at partial pressures 2 10 torr, mass spectroscopy studies confirmed the occurrence of continuing photoassisted dehydrogenation of secondary alcohols up to large t.a.p.s.* over ZnO or Ti02 (anatase, Degussa P25). Experiments with a gas chromatographic continuous reactant flow technique, which were weighted towards photocatalytic processes continuing to high t.a.p.s.* and were rather insensitive for photoassisted surface reactions proceeding only to t.a.p.s.* < 1 , again demonstrated photocatalytic activity over ZnO and Ti02 [ 1961. However, similar experiments with VzOs, CrzO3, Fe203, Co304, NiO and CuO or Cu20, failed to yield unequivocal evidence for photodehydrogenation (- H2)*, photodehydration (- H,O)* or photoinitiated C,-C, bond cleavage - (C,-C, )* continuing t o high t.a.p.s.* over these oxides featuring cations with partially filled d shells. Cunningham et al. [196] have suggested that this is a consequence of photogenerated holes losing their predominantly 0--like character in transition metal oxides having partially filled d levels (or d bands). Holes in these latter systems come, instead, to have the character of oxidised cation sites and consequently fail to show the selective reactivity of 0 - towards alcohols seen in ZnO and Ti02. [It is worth noting here that similar arguments would be expected to apply to carbon monoxide oxidation via photogenerated 0species and that SrTi03 and TiOz and BaTi03 did show the expected photocatalytic activity [185], whereas LaCo03 and Ba(Fe,.,, Ti2.0)02.6, featuring cations with partially filled d shells did not.] A recently published kinetic analysis of data on the partial photooxidation of oxygenated isopropanol vapour over a high-purity zinc oxide powder (NJZ-SP500) at 350 K, yielded

as the rate expression most consistent with photoassisted conversions to References p p . 419-427

384

acetone in both continuous and pulsed reactant conditions [256]. The possibilities for dependence on the square root of the intensity of the UV illumination (310-390 nm) were recognised as attaching to processes, such as the following, initiated by localisation of photogenerated holes at the surface.

..

h+

\

+ / CHOH(ads) \

h+ + [ CHOH . . . 0 2 ] -

/

\

CHOH+(ads)

’

(45c)

\

[ C H O H . . .Oz]*

/

(45d)

In high intensity conditions such that (i) the number of holes reaching the surface t o engage in such reactions is a small fraction of those produced within ZnO by Zabs and (ii) most photogenerated holes recombine with the higher concentration of photogenerated electrons rather than with [ e-] in the dark, then [ h+] becomes proportional t o Z . As required by this idea that the driving force of the photoassisted reaction is the small fraction of photogenerated holes which arrive a t the surface having avoided recombination, the quantum efficiencies were low, e.g. 3 x for a flux of UV photons of 7.6 x 10l8 s-’ on to the catalyst [256]. Although there is evidence in homogeneous systems for the selective abstraction of a-hydrogen by 0- (or OH) from secondary alcohols, and although eqn. (45a) gives rise t o 0--type species, operation of this abstraction process alone as the rate-determining step was not considered t o explain adequately the appearance in eqn. (44) of both OROH and Oo,. Rather, this second-order dependence was considered t o point t o the operation of one or more of the following bimolecular or termolecular surface reactions involving surface species activated by hole capture.

’”

--

+ O2 + \CHOH ‘CO + OH; + HOz / \ / \ CO + OH; + HO, OiUs+ [ CHOH . . . 02] / \’ 0: + ,CHOH ‘CO + H z 0 2 / \ \ ( CHOH)’ + 0; CO + H 2 0 2 / / OiUs

/

(45e) (45f) (45g)

Dependence on Z’’2 and on 8 R O H would arise naturally from the operation of these processes as the rate-determining steps. Dependence on Po, can also be readily accommodated, viz. directly in eqn. (45e), or via dark equilibria which determine the surface concentrations of alcohol-O2 complexes [cf. eqn. (45f)], or 0; species [cf. eqns. (45g) and ( 4 5 h ) l . The successful deconvolution of the alcohol pressure dependence into a Langmuir-Hinshelwood component predominating at low PRoHand an

385 TABLE 5 Extent of adsorption of alcohols on to pre-oxidised rutile layers and rate constants for subsequent photoreaction with "Oz Alcohol

Preadsorbed alcohola

ko (- ROH)b (molecules-' )

k l (- " 0 ~ )ko(R1 ~ Rz CO)d (s-l) (molecules-' )

Propan-2-01 Butan-2-01 2-Methylpropan-2-01

1 x 1019 2 x 1019 2 x lOI9

6.7 x 1013 1.0 x 1014 2.7 x 1014

1.8 x 1 0 - ~ 3.3 x 1 0 - ~ 3.2 x 1 0 - ~

2.3 x 1014 3.2 x 1014 5.2 x 1014

a Values expressed as molecules pre-adsorbed in

2000 s-l on to standard weight of pre-oxidised rutine on quartz support in static reactor. Pseudo equilibrium was reached with 66 N m-2 of alcohol vapour in the reactor. Pseudo zero-order rate constant, k o (- ROH), expressed as molecules lost to the surface of the rutile sample per second, corresponding to continuous slow adsorption of alcohols. Pseudo first-order rate constant for photo-initiated loss of I8O2 from the gas phase, as evaluated from first-order plots of log P ( I8O2)against time. Pseudo zero-order rate constant for photo-initiated appearance of ketone photoproduct in the gas phase, expressed as molecules released into reactor s-l under UV illumination.

Eley-Rideal or VCI component predominating at high PRO,may also be accommodated within this set of possible surface reactions [e.g. eqns. (45f) and (45h) are predominantly Langmuir-Hinshelwood in character, whereas eqns. (45c) and (45g) could introduce Eley-Rideal or VCI character]. It is worth noting that no comparably successful explanation of the dependence upon isopropanol pressure could be achieved using models of the Mars-van Krevelen type [256].

(iv) Photoassisted conversions of other alcohols. An approach frequently adopted in mechanistic studies of elimination reactions from alcohols in the absence of irradiation is the comparison of rates and/or selectivities of thermally assisted conversions for alcohols whose differing structures could be expected t o favour one or other of the classical E, , E, or EIcb pathways t o elimination reactions. For example, comparisons of isopropanol with t-butanol and neopentyl alcohol might be undertaken to test the relative importance and rate-determining character of Cp-H and C,-H bond rupture. However, comparison of photocatalysed conversions of isopropanol with the available scanty data for photoconversions and other alcohols suggests that such arguments and approaches may be inadequate and misleading if based solely on heterolytic bond ruptures, with loss of H', H- and OH- from alcohols in E l , E, or EIcb pathways. Some inadequacy of such conventional mechanisms was noted in relation t o data from ref. 194 reproduced in Table 5 summarising comparisons made in closely similar conditions between three different alcohols undergoing photoassisted conversions over a high-purity rutile sample ( TiOz MR128 References p p . 4 1 9 4 2 7

386

from New Jersey Zinc), Rates of an approximately first-order photoassisted depletion of lSO2 from the gas phase greatly exceeded the measured rates of appearance of ketone as the major photoproduct in each case. Acetone, whose formation as major photoproduct from the tertiary alcohol required photoinitiated cleavage of a C,--C, bond, had nevertheless a rate of photoassisted conversion comparable with the rates of simple (- H2)* from the secondary alcohols. The observed (- C,-C, )* rate was, furthermore, much greater than the photoassisted oxidation of butenes in similar conditions, thereby again posing difficulties for pathways based on dehydration and subsequent oxidative scission of a butene intermediate from t-butanol. The ketone product evolved during UV illumination of [alcohol " 0 2 ] reactants over Ti02 surfaces preoxidised by 1 6 0 2 showed very low (

E

20.0 I I

II

30

29

31

"/"

l------

30.0

20.0

rye, .\

:\ (ii) O

0

.\

Lo-o-o-.

I

I 4.0 Pressure "m-2

8.0

x 10-4)

32

393

0.36

9 m

0.31

.. 61

al

F

\ 0 m ,I

< F 0.2(

t 0.21

I

I

Fig. 1 3 (a) Evidence obtained from the DMS system for the release of nitric oxide as the major product observable in the gas phase 100 ms after exposure of an 1 4 N 1 5 N ' 6 0 / ZnO interface to an intense 50ps pulse of photons of wavelengths 340-640 nm. The solid line depicts a fast (10ms) scan across m / e = 28-38 at a dynamic pressure of 1 4 N 1 5 N ' 6 0of N m-2. The broken line depicts a similar scan made 1 0 0 ms after a flash illumination. ( b ) The effects of additions of gases known t o react with 0 - on the magnitude of the nitric oxide transients depicted in (a). ( i ) Additions of isopropanol; (ii) additions of carbon monoxide. (c) Time profiles on commencing continuous illumination of alcohol/ZnO interfaces and also showing temporary enhancing effect of NzO on photodehydrogenation. ( i ) Time profile of acetone photoproduct from (CH3)zCHOH over zinc oxide, showing rapid decay of photoactivity to zero; (ii) as for (i), but showing that a small dynamic pressure of NzO enhanced acetone photoproduct. Reproduced with permission from refs. 136b and 193.

any detectable enhancement of m / e = 29 (14N"N+ ) or m / e = 32 (l60:), all pointed to the presence of nitric oxide, rather than nitrogen or oxygen, as the major photoproduct in the gas phase at 100 ms after flash illumination. The data shown in Fig. 13(a) were taken with photons of X = 340-640 incident on the ''N'5N'60/Zn0 interface. Experiments at References p p . 4 1 9 4 2 7

394

different flash intensities established that nitric oxide formation varied with the square of the intensity and suggested that the nitric oxide product could stem from operation of a new photoactivation process requiring the interaction of preexisting surface species or sites on ZnO with two photogenerated intermediates. On the basis of evidence [indicated in Fig. 13(b)] that 0; intermediates took part in the flash-initiated production of both 14Ni60 and l S N L 6 Oproducts, the mechanism represented by eqns. (55) was proposed t o account for these results and for the observed second-order dependence on flash intensity, the latter coming from the requirement of the process for two photogenerated holes in steps (b) and (d), respectively. ZnO h,+ +

+ 2 hv

-

160;-

1 6 0 1 4 ~ 1 + 5 ~1 6~0 ;

2 h:/ Z n 0 2 -

(553)

160-

(55b)

1

-

__c

h: + (16014N15N160)-

(16014N15~160),1

16014N

+

(55c)

15N160

(55d) Figure 13(b) illustrates supporting evidence for the occurrence of m 1 6 0 1 intermediate a t the illuminated interface, as required by reaction (55c). This takes the form of a plot showing the progressive diminution of the mass spectral peaks of nitric oxide on the addition of species known t o compete for 0-, viz. carbon monoxide [202] or isopropanol [203], into the flow of 1 4 N ' 5 N ' 6 0 over ZnO a t low pressures. Those results under flash illumination not only confirmed the importance of 0; as a surface intermediate on UV-illuminated ZnO, but also pointed t o the important changes in product distribution and in intensity dependence which could accompany the high instantaneous flux of UV photons in the 5 0 p pulses attained with 2005 flash lamp, relative t o studies under continuous illumination. Studies of the influence of nitrous oxide on photodehydration of isopropanol over ZnO under continuous UV illumination showed N2 as the main product from N 2 0 at pressures of 1-102 N m T 2 .Figure 13(c) illustrates (i) a transient increase in the (-H2) product from isopropanol on commencing continuous illumination of the (CH3)2CHOH/Zn0interface and (ii) an enhancing effect of N,O on the size of this transient. i

2.2.3 Modifying effectso f surface dopants The neaessity for mabti&ung dioxygen, or another.gas-phase oxidant, at significant pressure emerged clearly in the work reported above, if continuing photocatalysed conversions of alkanes or alcohols were to be achieved over undoped T i 0 2 or other single-component solid catalysts. Recently, however, it has been shown that the presence of small particles of platinum dispersed on the surfaces of TiO, catalysts can make it possible t o dispense with the need for O2 or other gas-phase oxidant and

395

can promote photodehydrogenation of methanol vapour over the Pt/Ti02 catalysts at room temperature [207]. Mass spectral analysis of the gas phase in the static photoreactor demonstrated a photoassisted formation of H2 and HCHO as the major photoproducts forming with a quantum efficiency of ca. 0.45 from CH,CH vapour a t an initial pressure of 1-3 torr. Parallel and much more extensive studies on the production of H2 and other products from CI-C4 alcohols in the liquid state confirmed that the H2 originated predominantly via (-H2)* from primary or secondary alcohol and that such conversion proceeded to t.a.p.s.* > 100 only when T i 0 2 particles in the illuminated suspension had Pt (or Pd) dispersed on their surfaces. Pichat et al. [258] considered that Pt/TiO, fulfilled a dual role: (i) in transferring free electrons from the Ti02 support (presumably into collective-electron states of the Pt particle); and (ii) in assisting H2 production via migration of alcohol-related H-atom fragments between the support and the metal. Similar ideas have often been employed t o account for the electrocatalytic activity of Pt for H2evolution reactions. In a further extension of this analogy, Pichat et al. [ 2071 have remarked that the low oxidation-reduction potential of the HCHO/CH3OH, CH3CHO/C2H, OH couples, viz. approx. 0.19 V, make the dehydrogenation of alcohols a much less demanding reaction than dehydrogenation of water (see below). Large decreases in the limiting extents and rates attaching t o the photoassisted splitting of water, H 2 0 + H2 402,over UV-illuminated semiconductor surfaces have been noted for water vapour relative t o those attainable at liquid water/semiconductor interfaces [ 208-2141 . For example, Van Damme and Hall [214] estimate that the turnover number per surface OH group can be lower by four orders of magnitude at the gas/solid interface. They point t o evidence implicating direct photodecomposition of surface OH groups on TiO, as the process responsible for the production of H2 with low efficiency and low t.a.p.s.*. As yet, the origins of reportedly large enhancements in photoassisted water splitting when liquid water or aqueous electrolyte are in contact with the illuminated semiconductor surfaces appear unclear and, in any event, are outside the scope of this chapter. One aspect of such reports, viz. claims that surface-doped semiconductors, such as Pt/Ti02 or Pt/SrTi03 or R u 0 2 / T i 0 2 , are capable of yet greater efficiency, has, however, been tested for some gas/solid systems. Thus, Sato and White [215] have reported that the photoassisted water-gas shift reaction H 2 0 CO + H2 -I-C 0 2 , is genuinely photocatalytic over platinised T i 0 2 catalysts with a quantum efficiency of 5 x at 25°C. Comparisons of photoassisted water splitting over Pt/Ti02 with those over Ti02 showed that platinisation gave moderate enhancement ( x 3) in H2 production from water vapour. The mechanistic scheme (56) proposed for the water-gas shift reaction implicated such water splitting,

+

+

h+ -t H 2 0

-

References p p . 41 9-42 7

OH(ads)

+ H+

(56a)

396

h+

+ OH(ads)

CO(g)

-

-

O(ads) + H+

CO(ads)

-

+ OH(ads) CO(ads) + O(ads) H+ + eH(ads) CO(ads)

Pt

2H(ads)

Pt

(56b)

-

C02(g)

+ H(ads)

C02(g) (56f)

H2(g)

(56g)

It was noted that Pt/Ti02 prepared without H2 pretreatment showed much lower photocatalytic activity than Pt/H2-doped T i 0 2 . Recent studies by UPS and XPS of the effects of UV illumination on the chemisorption of 02,H2 and H 2 0 on reduced and stoichiometric SrTiO, (111) surfaces further highlight the crucial role of metal-excess non-stoichiometry within surface layers in determining photoactivity [ 2161. Thus, stoichiometric SrTiO, (111)surfaces were chemically inert towards H2, O2 or H 2 0 and were not photolysed t o surfaces with detectable Ti3+ after UV illumination for 17 h. This contrasted strongly with surfaces of black non-stoichiometric SrTi03 crystals having Ti3+ in the surface layers, which reacted very rapidly and extensively with O2 but less strongly with H, or H 2 0 . Photoregeneration of Ti3+ by subsequent exposure (at a flux of 1014-1010 photons ion-2) t o photons inside the band edge was observed after exposure t o O2 or H2 but, unfortunately, not after exposure to H 2 0 . The unfortunate aspect of this latter observation is the difficulty it introduces into envisaging continuing photocatalytic activity of H 2 0 ( g ) / SrTiO, interfaces through repeated photoregeneration of Ti3+, such as would be required for repeated photoassisted oxidation of H2O(g) over illuminated SrTiO, . The limited production of methane on UV illumination of an [H,O(g) C02(g)]/SrTi03 single crystal interface has been reported, but only in the rather special circumstances that a Pt foil was in contact with the rear non-illuminated face of the SrTi0, single crystal [163]. The attainment in such systems of continuing photoassisted production of methane, which is uphill in energy by 842 kJ mole-' relative t o H 2 0 ( g ) C02(g),would be of great importance as an artificial inorganic analogue of the endoenergic photosynthetic process in biological systems. However, the limited yield of methane so far reported from the [H20(g) C02(g)]/ (SrTi03-Pt) system was comparable in magnitude with the number of surface sites and so fell far short of the ideal of a photosynthetic process proceeding t o high t.a.p.s.*. The sensitising effect of platinisation for heterogeneous photocatalysis continues, however, to excite interest, e.g. photo-oxidation of hydrocarbons on platinised Ti02 has been reported [ 2171 . Consideration of strong metal support interactions (SMSI) in such cases is outside the scope of this chapter except t o note that metals other than Pt have been contacted with T i 0 2 .

+

+

+

397

Substances other than platinum have been contacted with TiO, surfaces during illumination in search of alternative promoters of efficient photocatalysed conversions or of endoenergic reactions at interfaces. Carbon has been advocated for this purpose on the grounds that, by acting as a scavenger for oxygen species from water splitting, wasteful recombination of oxygen with hydrogen may be decreased and more efficient photoassisted production of H2 achieved. On this basis, comparable photoassisted rates of production of H, and carbon oxides would be expected and have been claimed by Kawai and Sakata [218] from liquid water or water vapour over mixed catalysts containing RuO,, TiO, and amorphous carbon. Recent re-examination of photoassisted water vapour conversion in these systems utilised H, "0 and carbon-13 t o explore the origins of H, and CO, as the major products [219]. Analysis of the isotopic composition of the CO, product, which contained all possible isotopic species from 12C1602t o 13C1802, and of a minor yield of CO, led to scheme (57) as a representation of a sequence of events photoinitiated at a coordinatively unsaturated O2 - lattice sites and leading t o a carbonicacid like intermediate. f ?+\

13-

.

.

"nu-

.=

__

"OH-

/

Since mixtures of Cl6O, and H, ''0 admitted to the TiO, surface underwent insignificant isotopic exchange in the dark but were scrambled rapidly under illumination, this supported the idea that some such carbonic acid-like surface species could act as an intermediate for photoassisted scrambling of oxygen-18 between CO, and H 2 0 . If correct, mechanism (57) illustrates how photoinitiated interactions between a promoter and the primary photocatalyst can play a central role in photocatalysed reactions, just as thermally assisted interactions between promoters and primary catalysts can be important in normal catalysis. However, from their study of the reactions of water vapour with carbon and ethylene over illuminated Pt/TiO,, Sato and White [215] conclude that the mechanism and the sites involved in the photocatalysed oxidation of carbon observed in that system are not clear and that 0- may be attached t o carbon not titania. An interesting difference between the results from the carbon-promoted Pt/TiO, and Ru"+-TiO, systems was that the latter did not exhibit photoactivity for the water-gas shift reaction.

3. Effects induced by irradiation with high-energy photons or particles It is important t o recognise from the outset of this section that the majority of photoeffects considered above should also be capable of References p p . 41 9-42 7

398

operation under the influence of the higher-energy radiations now to be considered. This follows from the twin facts that (i) the photoeffects were initiated by excitation of the solid catalyst and/or the gaseous reactants into their respective electronically excited states and that (ii) such excitations account, in most cases, for about one half of the energy deposited into the systems by the higher-energy radiations. Generally speaking, any new radiation-induced phenomena will instead have their origins in’ different types of energy-depositing events or in differing spatial distribution of energy deposition. 3.1 ENERGY DEPOSITION AND LOCALISATION AT THE GAS/SOLID INTERFACE

Relative t o the photoassisted surface processes considered in Sect. 2, additional features can be expected t o arise under the action of highenergy radiations (such as y-rays, electrons, neutrons or a-particles at MeV energies) or other ionising radiations (such as X-rays, ions or electrons at keV energies). This expectation stems from the following new features in modes of energy deposition [220, 2211 t o gas/solid systems from these types of radiation: (i) all of these radiations produce ionisation as well as electronic excitation of atoms, molecules or ions located in regions of energy deposition. Consequently, additional features stemming from ionised states can be expected; (ii) the spatial distribution of energy deposition events along the path of an incident beam of highenergy radiation is much more inhomogeneous than from a beam of photons of energy < 8 eV, which typically generates one electronically excited state per photon absorbed and does not produce such states in close proximity, except in conditions of very intense illumination. With many high-energy radiations, on the other hand, each energy deposition event inherently produces many electronically excited and many ionised states in close proximity in localised regions of the irradiated medium which can be referred t o as “spurs”. Processes involving interactions between two or more radiation-activated states can occur within such spurs. Consequently, additional features stemming from such second- or higher-order processes can be expected if spurs occur a t the gas/solid interface; (iii) possibilities for selectively depositing energy either into the adsorbate or the adsorbent appear less favourable under high-energy radiations in view of the twin facts that the gas phase will not be transparent to high-energy radiations, whilst great depths of penetration into the solid are possible with such radiations. The extent of this difficulty can be expected to vary according t o the rate of linear energy transfer (LET) per unit length of path of the radiation. Radiations of high LET (such as a-particles or ion beams) are likely t o produce short columns of dense and overlapping spurs within the solid, and to a lesser extent in the gas phase, whereas radiations of low LET (such as 1MeV y-rays o r electron beams) penetrate deeply into the solid and produce spurs well

399

separated from one another; (iv) possibilities for displacement of atoms or ions from their original positions in surface regions of the solid lattice are greatly enhanced via momentum transfer from particle irradiations of high LET, threshold energies being low for such displacements, except for electrons where thresholds are ca. 1MeV. The occurrence of such displacements a t catalyst surfaces can be expected to alter the surface densities of active sites and so t o bring about changes in catalytic activity, which persist after exposure to particle irradiation. Energy-loss events of the types just enumerated initiate radiationinduced processes not only at the gaslsolid interface, but also in regions of the solid or gas phases distant from this interface [ 2591 . No general coverage of these latter processes is attempted in this section, which limits consideration to changes brought about by irradiation in the nature or kinetics of processes actually occurring at irradiated gas/solid interfaces. This limitation on the coverage to be attempted means that radiation-induced processses in regions of the gaseous or solid phases distant from the interface are of interest here only in as much as properties of the interface thereby experience modification, e.g. by migration thereto of energetic states or species produced by energy-deposition events distant from the interface. The extent of the contribution by processes of the latter type will, in turn, depend upon the efficiency, range and time span required for transporting t o the interface those excited/ionised states of the solid or gaseous phases which have the capability of altering surface processes. The relative weighting t o be given t o energy-deposition events distant from the interface will be strongly dependent upon assumptions concerning those factors and so will be difficult t o evaluate. An empirical approach often adopted t o estimate the extent of their contribution t o radiation-induced processes at the gaslsolid interface involves, as a first step, the calculation of the total energy deposited into the solid, ADsolid, e.g. expressed either as total electron volts (eV) deposited or as roentgen adsorbed dose (rad, where 1rad. s-' = 100 erg g-' s-' ). The total number of events of a particular kind brought about at the gas/solid interface by deposition of this total dose throughout the solid, e.g. AN(diss) for radiolysis of an adsorbed species, is used to calculate an apparent G value for the process, viz. 100 x AN(diss)/AD, expressed as eV deposited. Where such apparent G values exceed unity, significant contributions by distant energy-deposition events are indicated. Since the fraction of energy deposited at the interface is usually very small, it would be much more usual for the G value, which has the significance of the number of events of a gwen kind achieved per 100 eV deposited, t o have values 4 1 when calculated thus on the basis of energy deposited throughout the solid. 3.2 EXPERIMENTAL ASPECTS

Because of the elaborate precautions and attendent experimental difficulties arising from the need to protect personnel and equipment from Referencespp. 4 1 9 4 2 7

400

harmful effects of penetrating radiations such as neutrons, y-rays and high-energy (>1MeV) electron or ion beams, investigations of changes induced by such radiations have been evaluated much more frequently by measurements made after, rather than during, irradiation. Relevant techniques include: (i) ESR for the detection of paramagnetic centres produced by trapping of radiation-induced holes or electrons at the gas/ solid interface during irradiation; (ii) studies of the adsorptive capacity of the sample as determined by the amount taken up on the surface after irradiation; (iii) studies of the catalytic activity of the irradiated surface through measurements on the rate of a test reaction. Such techniques are favoured by powdered samples of high surface area and this experimental constraint resulted in many of the early studies on surface effects produced by high-energy radiations being made with powdered specimens [ 71 . Almost invariably, the surface conditions of such powdered samples were less well characterised than the single-crystal specimens examined by more recent surface spectroscopic techniques. Consequently, some uncertainties can arise as t o the possible roles of adsorbed impurities in modifying radiation-induced effects on powdered samples. In favourable cases, however, measurements made after irradiation can yield useful information on new post-irradiation interactions at gaslsolid interfaces, recent examples being provided by ESR observations reported on the formation of 0:- at the surface of powdered Ti02 samples on contact with low pressures of O2 after X-irradiation at 77 K [222] and on the formation of a variety of surface molecular ion radicals on contacting CO, oxygen or C 0 2 with H-Y type zeolites previously exposed to yirradiation [ 2231. The latter experiments yielded information on the location and reactivity of trapped-hole type centres present at the surface after y-irradiation and illustrate the application of conventional spectroscopic techniques to study post-irradiation effects of high-energy radiation. Conventional procedures of this type are not ideally suited t o the selective study of changes produced at gas/solid interfaces, since energy is deposited throughout the gaseous and solid phases. Furthermore, considerable complexities related t o the protection of personnel and equipment can arise when high radiation fluxes are employed and information is desired on effects during irradiation. By a fortunate coincidence, these difficulties can, in some cases, be overcome with the aid of equipment and procedures developed for modern ESCA techniques of surface analysis (Table 1). The primary objective of many of the incident radiations listed in Table 1 is ionisation and, in some cases, arrangements can be made for energy deposition t o be concentrated in regions close t o the surface. Small escape depths for emitted electrons mean, in any event, that these carry information mainly about species in the topmost few layers of the solids. The high sensitivities attainable in energy analysis of such electrons means that information can be obtained without the need t o

401

use very high fluxes of ionising radiations. Furthermore, the stainless steel containment vessels utilised as a vacuum envelope can, with appropriate precautions, reduce radiation levels to acceptable values during irradiation. Consequently, the UHV systems marketed commercially for surface spectroscopic examination of single-crystal specimens (cf. Chap. 2 of this volume) have resulted in marked increases in the ease and frequency of measurements detailing the changes produced in surface properties during exposure t o the radiations listed in Table 1. One typical application of these facilities is their use t o study the desorption of molecular species or ions from such surfaces as stimulated by an incident electron beam and detected simultaneously with a suitably placed mass spectrometer [ 2241 . The degree of surface coverage at galsolid interfaces on exposure t o small pressures of chemisorbing gases has been examined in this way [ 41, as also has the spatial distribution of adsorbed species over an inhomogeneous surface using a scanning electron beam [225]. It is also possible in iondesorption and UPS studies t o deduce the directionality of adsorbate bonding via analysis of the angular distribution of desorbed ions or photoelectrons, respectively. The desorption of adsorbed impurities through exposure t o argon-ion bombardment is another routine procedure in these UHV systems [ 2271 and can be monitored during irradiation by secondary ion mass spectrometry (SIMS) or, after irradiation, by AES examination of the surface. Changes in the degree of ordering of the surface after exposure t o ion bombardment can also routinely be monitored through LEED studies. Other radiation-induced changes in the physical properties of the surfaces of single-crystal specimens can be studied through the incorporation of special probes; for example, changes in work function can be detected after irradiation via contact potential difference measurements, whilst changes in surface conductance can be monitored with electrodes evaporated on t o the surface. A continuing expansion in the application of such techniques seems certain in view of their versatility in detecting and characterising radiation-induced changes in the physical properties of initially well-defined surfaces. 3.3 RESULTS AND INTERPRETATIONS

3.3.1 Effects o n adsorption-desorption processes during irradiation In a recent review of the extensive Russian literature on the effects of ionising and high-energy radiations on processes a t gaslsolid interfaces, Sokol’skii et al. [2] surveyed reports of enhanced chemisorption of hydrogen, oxygen and carbon monoxide on A1203 and other oxides during irradiation by X- or y-rays. They concluded that electrons and holes were the main agents for the enhancement of adsorptive capacity during irradiation and cited the large increase in conductivity which accompanied y-ray-induced chemisorption of hydrogen on ZnO at room References p p . 419-427

402

temperature as evidence for the capture of radiation-induced holes by the chemisorbing H2. Ionising and high-energy radiations can readily produce conduction-band electrons and valence-band holes, even for such wide band-gap oxides as Alz 0 3 .The view of these Russian workers was that migration of such non-equilibrium charge carriers to the surface could influence chemisorption. The initial rate of oxygen chemisorption was directly proportional t o the quantity of A1203 catalyst and radiation intensity but was independent of oxygen pressure. Chemisorption stopped when yirradiation ceased. Radiation-enhanced chemisorption of hydrogen obeyed the expression

which appeared consistent with activation of surface sites by radiation. Here, p is the pressure of hydrogen at time t, a is the surface density of hydrogen adsorption centres, Z is the dose rate of the y-rays, S is the total area of the catalyst and 8 is the fraction of adsorption centres occupied. 5.5 x 10” cm-’ was A maximum density of adsorption centres of attained under a dose rate of 350 rad. s-’. At similar dose rates, the quantity of ethylene taken up by the y-alumina surface increased during y-irradiation, whilst a volume of carbon monoxide was taken up almost completely. These differences demonstrate some degree of specificity in the gas-irradiated A1203 interactions. Effects reported on silica gels during irradiation by X-rays or y-rays offer evidence that factors other than radiation enhancement of the density of charge carriers may influence the adsorptive capacity in those systems. Thus Puncocharova et al. [ 2261 have considered that irradiation can cause vapour nucleation centres in capillary-condensed water and can cause a transition in the adsorbate from a metastable t o a stable state within pores of certain dimensions. Infrared studies by Ermatov and Koserov [ 921 indicated dissociative adsorption of water in H,O/silica gel systems under X-ray irradiation and, conversely, that irradiation with X-rays, y-rays and neutrons had dehydroxylating and dehydrating effects on the surface of the silica gel. The OH or H groups thus liberated from the surface may initiate chemical reactions (see Sect. 3.3.2) as well as leaving behind sites capable of enhanced adsorption. Results contrasting markedly with the foregoing and showing net radiation-induced desorption rather than adsorption, have been obtained not only in containment vessels subject t o intense beams of ionising radiations [ 1] , but also with well-characterised single-crystal surfaces exposed t o the radiations employed in modern surface spectroscopic techniques (cf. Table 1).The phenomenon of radiation-induced desorption from the walls of containment vessels has acquired new technological interest from the probability that plasma-induced desorption from the

-

403

walls of Torus or Tomahawk type accelerators for controlled fusion may present a serious source of contamination of the plasma [ l ] . This could include desorption through the action of electrons, X-rays and high-energy optical photons and those aspects of the topic have been treated in ref. 1. Desorption stimulated by electron beams a t high energy has also been reviewed recently [ 227-2291 showing its wide occurrence, drawing attention t o the very poor agreement on quantitative cross-section values, and indicating general agreement that the process involves direct interaction of the bombarding electron with the adsorbed species. In view of these recent reviews, coverage of those aspects of desorption from surfaces during exposure t o electron-beam irradiation is not again attempted here. Consideration is given, however, t o desorption effects during exposure t o other ionising radiations and particularly to important new results on the desorption of ions from surfaces under synchrotron radiation. A technique widely employed in the study of surfaces by UHV techniques is the use of a beam of rare gas ions at moderately low energies (300-1500 eV) t o remove adsorbed surface species, together with atoms in the top layer of the solid, by sputtering. The effects of such ion bombardment on the stay-time (i.e. mean residence time) of physisorbed xenon has recently been investigated [227] using a pulsed molecular beam of xenon incident on nickel surfaces simultaneously with 300 eV ions at a flux of - 3 n A . The stay-time of xenon from the pulsed molecular beam on the cooled nickel surface a t temperatures of 92-125K was reduced by ion bombardment, typically from 2 x t o 2.5 x s at 111K by exposure t o He' or Ar+ ions for 1h. The argument was made that, since this flux corresponded to only one in ten surface atoms being struck over the 1h period, the probability for direct knock-on displacement of xenon would be too small t o account for the significant reduction in stay-time. The result was considered to be more consistent with the removal of surface oxide by sputtering, since higher binding energies for xenon had been reported for oxide-covered tungsten than for the clean metal. Just as the greatly increased use of collimated electron beams for LEED and Auger studies of surfaces has allowed many observations on electronstimulated desorption, so t o o has the development of beams of low-energy X-rays for X-ray photoelectron spectroscopy (XPS) led to observations on resultant desorption processes. Recently, Franchy and Menzel [ 2281 reported the detection of ionised desorption products, including H+ and O', entering a quadrupole mass spectrometer as desorption products from stainless steel or tungsten surfaces during irradiation by AlK, X-rays of energies < 12 keV. Desorption probabilities were < ions photon-' for these ionic species and a preliminary observation was made on the desorption of H, as a neutral species released during the X-ray irradiation. Observations of neutral species desorbing under ionising radiations encounter greater difficulties and are less well characterised than for ions. References p p . 419-427

404

A mechanism for the desorption of positive ions from adsorbed layers on surfaces has been proposed by Knotek and Fiebelman [229] which predicts similar ion desorption from ionically bonded species at surfaces by either electrons or photons of sufficient energy t o create core holes. Interatomic Auger decay then produces a positive charge on the surface species, which experiences strong repulsion from substrate cations in their maximum valency state, e.g. 0' ions repelled from W6' ions of W 0 3 . Recent studies by Madey et al. [230, 2311 have demonstrated that ionyield plots of 0' from W ( 1 1 1 ) shows similar correlation with core-hole binding energies for tungsten atoms whenever stimulated either by photons of energies 20-120 eV from a synchrotron (photon-stimulated ion desorption, PSID) or by electrons of energies ca. 550eV (electronstimulated ion desorption, ESID). Furthermore, these studies show similar angular distribution of the 0' ions from PSID or ESID. Plots of 0' ion yield versus energy of the photons selected from the synchrotron radiation were dominated by peaks at - 4 5 and -55eV, irrespective of whether PSID of 0' originated from a W 0 3 oxide layer, from a monolayer of oxygen on W ( 1 1 1 ) or from 0.5 monolayer coverage. This led Madey et al. t o conclude that W6' species were present even in monolayers or fractional monolayers of oxygen, since the Auger decay model of ion desorption requires maximal valency for the cationic species. For the desorption of 0' from the oxide layer, ion yields were low both for PSID and ESID, viz. 3 x ions photon-' at hv = 55 eV in PSID and ions electron-' at 500 eV. A twenty-fold reduction in the 1x PSID yield of 0' (or possibly OH', which was not distinguishable from 0' by the time-of-flight method used t o identify ions) from an oxide layer resulted from exposure of the clean oxide t o H atoms. Photoexcitation of these surfaces gave PSID of H+ with a dominant threshold at lower values (220 eV) than for 0' but the angular distribution patterns were similar. This led to suggestions of a linear surface W-0-H species with H' desorption proceeding via excitation of a surface OH bond, possibly by an 0 (2s) core hole (- 22 eV) with excitation of the W substrate playing a lesser role than for 0' (or OH') [ 2301 . Madey [231] in an elegant set of experiments with a polyhedral tungsten crystal having a central W(110) facet and four flats with orientations close t o that close-packed surface, has also examined the role of steps and defects in the ESID of 0' at various oxygen coverages. He concluded from the study that: (i) there was little 0' ESID from a flat W ( 1 1 0 ) surface even at high coverages; (ii) the surface sites from which ESID originated were predominantly located at steps and defects; (iii) ESID of 0' was very sensitive t o local site geometry; (iv) the temperature of adsorption exerted a major influence on the details of the adsorbed structures and resultant ESID patterns. These conclusions are strongly supportive of those drawn by Somorjai [36] concerning the importance of several aspects of adsorbate-active site interactions in activating stepped crystals of Pt for various chemisorption and catalytic processes.

-

405

3.3.2 Chemical effects during irradiation Just as prompt adsorption or desorption effects during irradiation served to signal rapid conversions of radiation energy in the systems considered above, so should observations of prompt chemical reactions under irradiation serve as signals for rapid transformations of radiation energy into Eact. Prompt radiation-induced chemical effects should ideally be distinguished from slower effects by the use of short radiation pulses and fast detection techniques. However, the majority of reported observations on chemical effects at gas/solid interfaces during irradiation derive from procedures lacking any time resolution. As such, they represent the net effects, not only of primary and secondary radiation-induced chemical changes, but also of any subsequent changes capable of operation during the period for which the system is continuously exposed t o irradiation. Notwithstanding this deficiency, experimental observations on the overall chemical effects during irradiation have sometimes been made the basis for interpretations seeking t o establish the nature of very early events in the conversion of radiation energy into EaCt.An interesting case in point concerns the interpretations advanced for correlations claimed t o exist between the efficiencies of radiation-induced chemical changes at gas/solid interfaces and the width of the forbidden band gap of the solid [ 21. Thus, large differences have been reported in the relative extent t o which radiolysis of such adsorbates is promoted by various metal oxide surfaces, e.g. ZnO with a band gap of 3.2 eV significantly accelerated the y-radiolysis of adsorbed H 2 0 , whereas V 2 0 5 with a band gap of 0.5 eV produced little or no enhancement under similar conditions [ 2321 . Such observations have led t o claims of a useful empirical correlation between the band gap of the solid and the extent t o which it can promote radiolysis of adsorbates. Indeed, Sokol’skii et al. have alluded in their review [ 2 ] t o a “universal character” of this correlation. They have instanced the sequence SiO, : Al, O3 : ZrO, : MgO > ZnO: ( Al, 03,SiO, ) > NiO > Ni > Pt/SiO, , in which yield of cyclohexane dehydrogenation on the various solids under continuous y-irradiation (3.4 x 10l6 eV g-’ s - l ) was reported t o decrease as the band gap decreased. This sequence is inverted relative t o order-ofmagnitude activities of the catalysts in thermally assisted dehydrogenation. Despite the deficiency noted above, postulates advanced as reasons for existence of such correlations were: (i) that the activation of an adsorbed cyclohexane molecule proceeds from the recombination of a free electron with a radiation-generated hole, initially trapped by the adsorbate; and (ii) that the energy released in such a recombination should relate t o the width of the forbidden band gap minus the heat of adsorption. Relevant radiation-initiated processes may be summarised as MX

-

C6H,,(ads)

+ h+

Referencespp. 419-427

+ h)/MX

-

e-, h’, (e

C6H:,(ads)

(59b)

e; 4-C6H:2(adS)

-

C6H:,(adS)

-

C6H10

+ HZ/MX

(59d)

and are likely t o be completed within s. It has been argued that eqn. (59d) would result in the release of the largest recombination energies to the cyclohexane molecule on large band gap dielectrics (e.g. SiO,, A1203, MgO), the smallest recombination energy on metallic catalysts, and intermediate energies on semiconductors of medium band gap (e.g. ZnO). Whilst espousing the universal character of such mechanisms, Sokol’skii et al. [2] noted the possibility of serious competition between them and charge recombination a t other local surface centres, such as impurities or defects, Overall efficiencies would undoubtedly be affected by competition from other electron-hole recombination processes [ 1961 , but it is improbable that the empirical correlation between the efficiency and the band gap of the solid could arise in that way. A more probable origin can be suggested from a more detailed consideration of processes similar t o (59d) and in particular from considerations of how non-radiative decay of the excited state produced by electron-hole recombination may vary with band gap. A basis for these considerations is provided by the “energy-gap law” for radiationless transitions for which there is theoretical and experimental support in molecular systems [ 233, 2341. According t o this energy-gap law, the radiationless decay constants, k,,, for a series of excited states based on the same chromophore are determined by the vibrational overlap between ground and excited state and can increase exponentially with decreasing energy gap between these states if certain conditions are satisfied [233]. Good agreement with the law, with k,, varying over two orders of magnitude, has been demonstrated recently for a series of osmium(I1) complexes in which the relevant transitions had metal-to-ligand charge transfer character [234]. By analogy with such systems, if electron-hole recombination a t the surface of an irradiated metal oxide is envisaged to produce an excited state a t the surface as represented in the first step of (59e), it is reasonable t o suggest, that k,, for the radiationless decay represented in eqn. (59e) will increase exponentially with decreasing energy gap for charge transfer between an oxygen anion and the metal cation (i.e. the band gap of non-transition metal oxides). e-

+ (M:’

-0;)

--+

(M,Z+-o:-)*

k,_ (M:+. . . OZ-)

(59e) The lifetime of the excited state shown in eqn. (59e) should, in consequence, be much larger for large band-gap oxides such as A1203 or SiOz on MgO, than for oxides having smaller band gaps. The former will thus have greater possibilities for interaction of the excited state (prior to its non-radiative decay) with an adsorbate, e.g. with cyclohexane via an

407

exciplex as in

( M F + . . . Of-)*

+

Z +-

of-) *

(M:+L

0:-1

(59f)

Whilst the intervention of radiationless decay in the manner of eqns. (59e) and (59f) would be consistent with the correlations claimed by Russian workers, such ideas must be regarded as tentative until a more searching test can be made, preferably using techniques with built-in time resolution. However, it is worth noting, that if the radiationless decay process were itself t o be selected as the vehicle by which the energy needed t o drive dehydrogenation was transferred into an adsorbate such as cyclohexane (in analogous fashion t o the transfer into vibrational modes of the ligands in the series of osmium (11) complexes mentioned above [ 2341 ) the efficiency of that process would be predicted t o increase with decreasing band gap of the oxides. Various sequences of radiation-initiated processes leading t o the dissociation of adsorbed methanol by ionising radiations have been proposed by Russian workers [81, 2351. Zhabrova et al. [81] suggested that positively charged species created by the ionising radiation became localised on chemisorbed methanol and that annihilation of this centre through recombination with an electron resulted in the formation of hydroxymethyl (CHzOH) radicals which could either combine to yield ethylene glycol or dissociate t o yield formaldehyde. Their comparisons of the enhancing effect of various solids upon methanol radiolysis, including a conceptually interesting comparison of graphite and diamond, were interpreted in terms of this proposed mechanism. A one hundred-fold intensification of the radiolysis by diamond (band gap 7eV) and the lack of significant enhancement by graphite were interpreted in terms of the aforementioned correlation of radiation catalytic activity and band gap. An important point not fully resolved in that study was the identification of what exceptionally favourable conditions existed at the diamond surfaces t o make them ten times more effective than materials of comparable band gap (e.g. SiOz or A1203 on which the decomposition of methanol under condinuous y-irradiation exceeded the rate of homogeneous radiolysis by a factor of only ten). It is also puzzling in the context of the claimed correlation with band gap that the activity of the wide-band oxide MgO (band gap 8.7 eV) was rather similar t o ZnO or NiO, which have much smaller band gaps. Infrared studies of silica gels exposed t o y-irradiation in the presence of methanol or water vapour have been differently interpreted by Ermatov and Koserov [ 921 who proposed radiation-induced dehydroxylation and dehydration of the surfaces followed by the reaction of adsorbate with the surface sites so produced. This References p p . 419-427

408

concept of reaction between adsorbates and surface sites activated by ionising radiation also emerged from the work of Vedrine e t al. [225], who in their ESR studies of y-irradiated zeolites of the H-Y type, found that the presence of adsorbed H,O or NH3 on the zeolites during y-irradiation prevented the build-up of detectable concentrations of trapped-hole (V-type) centres, but that, instead, surface OH or NH, radicals appeared. The radicals were not observed except when H 2 0 or NH3, respectively, was present during irradiation. The intensity of the corresponding ESR signals was stated t o be a t a maximum for monolayer coverage. Adsorption of gaseous reactant on, or immediately adjacent to, the precursors of the V centres and subsequent reaction with the V centres during irradiation was envisaged as the mode of formation of surface OH and NH2 radicals. The absence of adsorbent-related paramagnetic species from H2 O/SiO2 systems after y-irradiation has likewise been interpreted in terms of the reactions of radiation-induced surface centres with adsorbed H 2 0 . This gains support from reports of the expected converse process, viz. increases in the yield of hydrogen product from these H20/Si02 systems relative to homogeneous systems similarly irradiated. In the terminology introduced in Sect, 2.1, initiation of these processes may be attributed t o active site charge-transfer (ASCT). Photoinitiated oxygen isotope exchange (OIE) in mixtures of (1602 I8O2) over an irradiated metal oxide was considered in Sect. 2.2.2.(a) as one of the simpler representative cases of the radiation-induced cleavage and rearrangement of bonds other than the adsorbate-adsorbent bonds. Recent work by Trokhimets e t al. [237] showed that exposure of (I6O2 1802)/A1203interfaces t o soft X-irradiation a t room temperature produced a rapid constant rate of OIE which was greatly enhanced relative t o non-irradiated interfaces. Those results were interpreted in terms of the creation of a radiation-induced, steady-state concentration of surface sites active for exchange. They would be consistent with a mechanism, such as eqn. (35), driven by ASCT following localisation of radiationinduced carriers on pre-existing defects. The activity of a pre-irradiated interface diminished rapidly after the cessation of irradiation or could immediately be eliminated by contact with H 2 , as would be consistent with spontaneous decay of the trapped charges through recombination or, alternatively, through reaction with H2. Dehydrogenation and dehydration processes at irradiated alcohol/ semiconductor interfaces has been envisaged by Vol’kenshtein and coworkers [8,15, 2361 as being dominated not by ASCT but by collectiveelectron properties of the solid adsorbent. The essential first step towards dehydrogenated product consists, in their view, in the cleavage of the RO-H bond of the alcohol, which they class as an acceptor-type reaction (i.e. one which will, in terms of a collectiveelectron theory, be accelerated by increasing availability of electrons at the surface). The essential first step towards dehydrated product is envisaged as cleavage of the R - O H

+

+

409

bond, which is classed as donor-type reaction (i.e. one accelerated by the availability of holes at the surface). Contact potential difference measurements were utilised by Spitsyn et al. [236] as a means of evaluating radiation-induced increases in the availability of electrons at the surface of an yttrium oxide semiconducting catalyst, as caused by the incorporation of radioactive 91 Y isotopes. Parallel measurements on the influence of the incorporated 9 1 Y (which emitted 0 rays at 1.55meV and X-rays at 1.2 meV) on dehydrogenation and dehydration indicated that the rate of the former was progressively enhanced by increasing the specific activity of t h e . sample, whereas the rate of the latter progressively declined. Qualitative agreement was claimed with the requirements of what would be classed here as a collectiveelectron charge-transfer mechanism for simultaneous alcohol dehydration and hydrogenation. However, the following should be noted: (i) that experiments were carried out at 640-690 K, since only in that region was the static charge formed during radioactive decay considered t o flow entirely from the sample, and that many metal oxides exhibit high catalytic activity for thermally assisted elimination reactions of alcohols in that temperature range via more conventional Lewis acid-base catalysed mechanisms [ 2371 ; (ii) that since severe difficulties can arise, as detailed elsewhere, in distinguishing definitively between the operation of ASCT and CECT, the data available on the yttrium oxide d o not exclude the possibility that ASCT predominates in charge transfer a t the irradiated oxide surface, whilst collective electron factors exert an important influence on the occupancy of active sites by radiation-induced electrons or holes and on the lifetimes of such occupied states. The enhancing effects of y-irradiation on the methanation of carbon oxides over supporting ruthenium catalysts have been reported by Gupta et al. [239]. Using a microcatalytic reactor with H2 carrier gas and carbon oxides introduced in either pulsed-reactant or continuous-flow modes [e.g. see Fig. 10(b)] they demonstrated that in situ y-irradiation enhanced the turnover of CO or C 0 2 t o methane for temperatures 400600K. Figure 14 illustrates their results in continuous flow mode for methanation of 2%COz in H2 over two different catalysts, viz. ruthenium supported on alumina (RA) or on molecular sieve (RM). Progressive slow growth during 30-90 min irradiation characterised the effect and resembled similar slow growths to steady-state photocatalytic activity in other systems. Enhancement by y-irradiation depended upon temperature and the ratio of steady-state activity with and without y-irradiation was greater at lower temperatures (e.g. the ratio equalled 18 over RM at 400K, but 3.5 at 425K) in a manner consistent with a reduction in activation energy of the overall process under irradiation (e.g. a reduction of Eact from 7.3 t o 4.2 kJ mole-' over RM). Gupta et al. concluded that radiolysis of COz was not important. They favoured the radiation-induced acceleration of the rate of reaction of COz with H,, possibly through References p p . 41 9-42 7

410

Time ( m h l

Fig. 14. Data illustrating effect of y-irradiation o n t h e methanation of carbon dioxide over supported Ru catalysts comprising ruthenium o n alumina (Ru/AI) o r ruthenium in molecular sieve (Ru/M). Effects of 7-irradiation a t the indicated temperature on the growth of methane product observed from a continuous flow of C 0 2 in a Hz carrier gas over (i) Ru/M and (ii) Ru/AI. Note the growth in methane yields from C02-H2 reaction at different temperatures as a function of y dose and its decay with time subsequent to removal of the catalyst from t h e y-source.

some weakening of the bonding of a surface ( R u - € 0 , ) complex via a mechanism not fully resolved, following the thermally induced transport of radiation energy from the support material t o the ruthenium. The question of energy transfer from deep within the adsorbent to the surface to effect chemical changes in the adsorbate, emerges yet more strongly from recent studies of the y-radiolysis at 7 7 K of methane mainly physisorbed on a range of y-alumina samples previously outgassed above 5 7 0 K . Norfolk has interpreted these observations in terms of the existence of exposed lattice ions on to which methane can become chemisorbed during y-irradiation, via a VCI or Eley--Rideal-type process, to produce surface precursors of methane or of C, and C3 hydrocarbons [ 2381 . Subsequent thermal desorption of hydrocarbon material from the irradiated y-Al,03 then yielded a mixture of C, t o C, alkanes and alkenes, which was expressed as total product carbon (TPC) given by

411

+

+

+

[chemisorbed CH4 2(C2H4 C2H6) 3(C3H6 iC3H8)].Typical plots of the yield of TPC desorbed t o the gas phase as a function of radiation dose and of coverage by physisorbed methane are illustrated in Fig. 15(a) and (b): respectively. Evidence that the radiation energy deposited within the bulk contributed to the conversion of physisorbed methane came from observations that plots of desorbed TPC as a function of the adsorbed dose had slopes which corresponded t o a yield of G(TPC) 2.0, i.e. to the radiation activation of 2 sites per lOOeV absorbed from the y-rays by the entire bulk of the A1203 sample. (If only energy absorbed directly be adsorbate had yielded TPC, that would have required one conversion per 0.02 eV.) An important feature of the proposed radiationinduced reaction sequence was the migration t o the surface of free charge carriers or excitons created within the bulk by irradiation. The formation of activated sites through hole capture on exposed oxygen sites and electron localisation on exposed cations was envisaged as in eqn. (60a) followed by dissociative chemisorption of methane as in eqn. (60b) during irradiation and by desorption of TPC during subsequent heating as in eqn. ( ~ O C ) viz. ,

-

A diminution in TPC yield when methane was added after, rather than during, irradiation [cf. Fig. 15(c)] was taken t o indicate that the first step and at least part of the second take place during irradiation, as would be consistent with a radiation-assisted Eley-Rideal or VCI-type process. Qualitative comparison of the yield G(TPC) = 2.0 with G(ion pair) X 3 led to the conclusion that, in favourable circumstances, up t o two-thirds of the excited charge carriers generated throughout the Al,03 sample by y-irradiation could be available for adsorbed phase radiolysis, thereby implying efficient energy transfer from y-A1203 t o adsorbed methane under y-irradiation at 77 K. Factors identified as affecting the yield from adsorbed methane radiolysis included the surface area, the surface density and distribution of exposed anion sites, the rate at which excited charge carriers generated in the solid reach the surface, and the availability of sufficient adsorbed methane t o react with such charge carriers. The operReferences p p . 4 1 9 4 2 7

412

1c

8 c

Y

-

u)

0

Y

0

x

(U

0

E -5 U

a

+

0

I

2

6

4

Radiation

dose

0

(Mrad)

I

300

700 500 Desorption temperature ( K )

413

ation of similar factors had been suggested by previous workers who also noted high efficiencies for the chemical changes brought about in adsorbates by high-energy irradiation of high-surface-area adsorbents. Whilst such results on radiation-induced processes imply efficient energy transfer t o the gas/solid interface from within the solid where much of it was deposited, definitive evidence is still lacking as to the nature of the energy transfer process(es), e.g. whether by processes (lo), (11)or (12) or others. The resolution of this question represents a challenging problem. 3.3.3 Effects persisting at the interface after irradiation The changes in physical properties of surfaces which persist after irradiation for times much longer than the expected decay times of the primary radiation-induced species have sometimes been referred t o as “memory” effects and are considered in this subsection for samples exposed t o ionising and high-energy radiations. It is instructive t o recall that memory effects were rare for photoinduced processes at gas/solid interfaces and generally were explained as a consequence of the trapping of photoinduced charge carriers by surface defects where they remain available t o promote charge-transfer processes with gases subsequently admitted (e.g. the promotion of oxygen isotope exchange by surface-trapped electrons). Sokol’skii et al. [ 21 concluded in their review article that similar processes are the main agents for post-radiation enhancements of the adsorptive capacity of oxides, including alumina and silica gel. Thus the appreciable ability of silica gel to adsorb hydrogen, which persists after y-irradiation, has been correlated with the persistence of hole-type colour centres which were removed on hydrogen adsorption. The adsorptive capacity of irradiated silica gel was reported t o increase with the extent of its contamination by aluminium. A subsequent study of y-irradiated aluminosilicates in the form of the H-Y zeolites confirmed the ready formation of V-type paramagnetic surface sites under irradiation and their neutralisation when H2 was later admitted [ 2231 . Enhanced post-irradiation adsorption of 0,’ CO and CO, could be interpreted in terms of their adsorption on the pre-

Fig. 15. Radiolysis of methane adsorbed on y-alumina expressed in terms of total product carbon (TPC, chemisorbed CH4 4- 2(C2H4 C2H6) + 3(C3H6 + C ~ H S ) ] . (a) Variation in TPC yield with radiation dose delivered to the CH3/A1203system for samples previously outgassed at V , 623 K ; 0,673 K ; A, 723 K ; or 0, 928 K. ( b ) Variation in TPC yield with coverage by physisorbed methane on similarly pretreated A1203 samples dosed with methane and then y-irradiated at two different dose rates of 0,1.1 and 0 , 0.29 M h-’ . (c) Comparison of TPC desorption curves for 3 equivalently pretreated A1203 samples to which methane was added: 0 , before y-irradiation at 77 K ; 0 , after y-irradiation at 77 K but before warm-up; after y-irradiation at 77 K and warm-up to 300 K for 1h.

+

*,

References p p . 419-427

414

cursors of these V-type centres, either a t the lattice oxygen or in its immediate proximity. The resultant ESR spectra have been assigned to 0; and GO+ radicals with the structures

Results of the type just described for previously irradiated powdered samples containing SiOz and/or A1203 thus appear consistent with the operation of mechanisms involving charge localisation a t surface sites during irradiation and with subsequent charge transfer t o chemisorbing gases. The rate of publication of papers dealing with the changes in catalytic activity of powdered metal oxide samples after their exposure t o ionising radiations had declined in the past decade relative t o that comprehensively detailed by Taylor [7] through July 1967. Two of the most striking and reproducible post-irradiation effects evident a t that time concerned a greatly enhanced rate of H2/D2 exchange on silica gels after irradiation and a concomitant decrease in activation energy from 37 mJ mole-' with unirradiated t o 8 k J mole-' with irradiated samples. Criticisms levelled at other systems, on the basis that surface poisons were inadequately removed prior t o irradiation, did not appear to apply t o the silica gel system, since the enhancement of activity for the H2/Dz exchange could be reproduced on samples outgassed at high temperatures. Taylor [7] set out the pros and cons of assigning the enhanced activity t o the trapping of radiation-generated holes at surface locations where they remained available for catalysing Hz /D2 exchange after irradiation. Clarification of the nature of trapped-hole-type centres involving surface 0- species and transition metal ions dispersed on the surfaces of silica gels has come from subsequent studies by Kazanskii et al. [74]. The formation of an active surface 0- species by hole capture during y-irradiation of Mo6+ so dispersed was represented as

which includes a second metal ion of unspecified coordination as the corresponding electron trap. The ESR signals of the 0- species thus produced from V205/SiOz,Mo03/Si02 and W03/SiOz has similar magnetic parameters and structures t o radicals produced by the decomposition of nitrous oxide on prereduced surface sites according t o 0 0 ,,oNM06+.' *M05' ... + N20 + N2 '0

'0

-

'0

' 0

415

The radical anion 0- when produced in the gas phase is characterised by high reactivity for H-atom abstraction reactions and the species formed by the above reactions react rapidly in this manner with H2, NH3, CH,OH, CH4. It has been proposed that the reaction with H2 produces H-atom-like species which are active for H2/D2 exchange. Some characteristics of adsorption chemiluminescence on y-irradiated silica gel, alumina gel, NaY zeolite and other silicates have been reported by Russian workers [ 240, 2411 who utilized samples activated in vacuo a t 670 or 770K. A high dose of y-irradiation (13Mrad) was delivered at room temperature to the vacuum-activated samples, after which gaseous 02,C 0 2 , H2 or NH3 was admitted rapidly to pressures typically in the range 660-8 x lo3 Nm-2. The total light emission from a particular material was greatest on admission of H2 and was usually a t least one order of magnitude less with 02.However, very low efficiency of the emission process was inferred from the fact that the number of photons collected were factors of t o lo-'' lower than the number of paramagnetic species detected in the irradiated samples by ESR. Factors other than the trapping of radiation-generated charge carriers at pre-existing surface defects have been emphasised in the interpretations of catalytic effects persisting for long times after irradiation by neutron or high-energy particles. Sokol'skii e t al. [242, 2431 compared the catalytic activity for the hydrogenation of ethynyldimethyl methanol and butynediol exhibited by palladium and platinum blacks prepared by H2 reduction of irradiated and non-irradiated samples of the corresponding metal oxides. Higher activity was found for blacks prepared from the neutron-irradiated samples even more than a month after irradiation. The differences later noted between the electrochemical charging curves of electrodes coated with blacks prepared from neutron-irradiated or non-irradiated oxides were interpreted in terms of the increased dispersion and increased heat of H2 adsorption for the pre-irradiated samples [ 2441. Associated increases in extent of surface coverage by hydrogen were postulated as the origin of enhanced catalytic activity. The manifestation of these effects, even after H2 reduction of the neutron-irradiated oxides, could, in part, be understood on the basis of the extensive production of vacancies and other structural effects in the oxides by neutron irradiation, with consequent influence of such defects on the dispersion and density of defects on the Pd-black or Pt-black formed from the neutron-irradiated oxides. The radiolytic oxidation of porous reactor moderator graphite in C 0 2 represents another effect induced by neutron irradiation in which the radiation-induced formation of structural defects is proposed t o play an important role [245]. It has, however, been demonstrated that neutron irradiation also affects the electronic properties of various carbons [ 2461 and one interpretation placed on such effects was that a dose of 1neutron per cm2 created four electronic holes in the valence band per cm3. The References p p . 419-427

416

resulting complex possibilities for the radiation-induced modification of the surface (including, on the one hand, energy deposited in the bulk but transferred t o the surface quickly by electronic transfer processes or slowly by defect migration and, on the other hand, energy deposition into surface regions) makes this a difficult area of investigation on which specialist texts should be consulted. Interpretations envisaging that highenergy radiations not only lead t o the ’trapping of charge carriers at preexisting defects, but also generate additional surface defects which can further enhance post-irradiation adsorption at the gas/solid interface, appear consistent with several correlations between defect sites and chemisorption : (i) sulphur ion vacancies on CdS surfaces have been identified as sites for radiation-enhanced oxygen chemisorption [ 2471 ; (ii) increased nucleation of “islands” of Au or Ag from the gas phase on t o previously irradiated alkali halide single crystals [248] ; (iii) initial sticking coefficiencies of oxygen on tungsten (110) faces have been shown t o be increased by a factor of 3.6 in the presence of monatomic steps [ 2491 ; (iv) defects or other extraneous sites contribute to the higher binding energies a t 6’ < 0.2 for argon on tungsten [ 7 1 ] . Reliable evidence concerning the types and surface densities of defects and how these are affected by irradiation will, in general, require the use of singlecrystal samples and more sophisticated procedures than utilised for the foregoing studies on powdered solids. In conclusion, the advantages of a coordinated application of modern surface spectroscopic techniques t o study changes in the physical properties of the surfaces of ZnO single crystals after irradiation may be illustrated by recent results. Thus, Margoninski and Eger [ 2501, by their comparisons of the surface conductance or the LEED patterns from the (000%)oxygen-rich or the (0001) zinc-rich surface before and after Ar-ion bombardment, showed parallel decreases in surface resistance and degree of order in the surface layer t o limiting values. Bombardment with He’ Or H+ ions extinguished the LEED pattern and further diminished surface resistance. This added efficiency of the lighter ions was interpreted in terms of their greater penetration into the ZnO lattice with the creation of donor centres and disruption of order. Characteristic low-energy loss spectra at electron energies 2-30eV were also taken on the (OOOi)/‘O’ face and (0001)/‘Zn’ face before and after argon-ion bombardment and/or exposure to hydrogen atoms. The strong similarities of the ELS spectra from the two different faces a t 2.8 eV were interpreted as indicative of a vibration characteristic of surface ZnO molecules rather than of a dangling bond state on a surface ion. Changes in the ELS spectra were difficult t o disentangle from a marked sensitivity of the surfaces towards very small exposures t o hydrogen in the initial study. A more recent study [251] sought correlations between the changes brought about in the surface

417

conductivity of polar ZnO faces by exposure t o H2 or O2 and the simultaneous changes in the corresponding ELS spectra in the range 3-20eV. Correlations were found for the ELS peaks at 5.3 and 14.3 eV, indicating that surface states at these energies contribute electrons t o the surface conduction band, but no correlation was found for the band at 3eV. In another study of the post-irradiation changes in the physical properties of ZnO single crystals, Holmstrom et al. [252] detected changes, after prolonged exposure t o 3 keV electrons, in the AES spectrum and also in the work function of the surface. Radiation-induced changes in the Auger peaks for carbon and zinc were interpreted as evidence for the ESD of carbon and for the production of excess zinc through radiolysis of the ZnO surface. The latter was attributed t o the production within O.1pm of the surface of ca. 100 electron-hole pairs per incident 2 keV electron and t o surface decomposition by the high density of radiation-generated holes. Post-irradiation decreases in the work function were likewise interpreted in terms of the accumulation of positive charge in the surface layers.

4. Perspectives and prospectus In the author’s view, the work surveyed allows identification of the following advances during the past decade. Firstly, from the point of view of experimental procedures brought t o bear on the problem, there has been a marked and welcome increase in the applications of UHV procedures and surface spectroscopic techniques. This has allowed better definition of the surface condition of the solid catalysts prior t o and after irradiation and has made possible, in favourable single-crystal cases, the identification of surface defects, impurities or topographical features which may act as active sites. The expanded use of such techniques is desirable in future work in the search for a definitive correlation between the radiation-induced activity of surfaces and the measured concentrations of defects on the surfaces. Secondly, from the point of view of the models utilised t o account for radiation-induced processes at the gaslsolid interface, early and somewhat antagonistic use of electronic or active-site concepts of surface activity has been supplanted by the recognition and simultaneous utilisation of both concepts as mutually complementary. Thus, changes in surface activity persisting after irradiation have generally been explained in terms of “active-site charge transfer”, which envisages electronic-type localisation of radiation-induced carriers at special surface traps which, however, also serve as active sites for charge-transfer to adsorbing gases. Expanded use of these “active site charge transfer” concepts should prove profitable for effects during irradiation. Thirdly, it References p p . 41 9-427

418

has been increasingly recognised that the usually high degree of coordinative unsaturation associated with non-regular surface sites carries with it strong implications for the modification of electronic energy levels and reactivity associated with such sites. For metal oxides, and some other binary non-metallic solids, irradiation can promote such sites into excited states with new and interesting surface reactivities. The emergence of surface-state models and theories of electronic configurations and energy bands at surfaces appears t o offer a new framework within which the enhanced reactivities of such sites in their ground and electronically excited states will increasingly be understood. Fourthly, from the kineticist’s point of view, attention has increasingly been drawn t o mechanisms in which the rate-determining process a t pressures approaching 1atm involves interaction between a radiation-activated site and a gaseous reactant encountering the activated site either directly from the gas phase or whilst in a weakly adsorbed molecular form. Such radiation-induced Eley-Rideal or VCI mechanisms contrast with the predominance of Langmuir-Hinshelwood-type photoassisted processes involving strongly chemisorbed species at low reactant pressures. Further kinetic study and exploitation of such photoassisted Eley-Rideal-type mechanisms at appreciable pressures can be expected in view of the indications already received that, relative to Langmuir-Hinshelwood processes, a greater fraction of surface sites can become activated for Eley-Rideal processes under irradiation and that these may be less susceptible t o poisoning. Fifthly, although progress in characterising and understanding the radiationinduced desorption of neutral species has been disappointing, a satisfactory mechanism for the desorption of positive ions has been developed and gives ground for optimism that radiationless processes involved in the desorption of neutrals may eventually become better understood. A lack of comparable advances in other important areas may also be discerned and it is to be hoped that these may receive increased attention along the following lines in future work: (i) efforts t o time-resolve, and t o display simultaneously, changes in electronic and in other surface processes through the use of pulsed irradiation procedures coupled t o fast detection techniques. Some particular difficulties which can attach t o such studies on heterogeneous systems were indicated above; (ii) more explicit consideration of the manner of initiation by radiation and of the relative importance, not only of mechanisms based on heterolytic bond rupture, but also of radical-type surface processes at the iiradiated interface; (iii) increased emphasis on systematic programmes for evaluating the lifetimes of singlet and triplet electronically excited states a t interfaces and for correlating lifetime changes with details of surface structure and composition of the interface region. Advances will be needed in these areas t o allow reliable correlations t o be established between the efficiencies of radiative and radiationless processes at the interface and early events in the complex sequence of steps which can be initiated by irradiation a t gaslsolid interfaces.

419

References 1 M. Kaminsky, Adv. Chem. Ser., 158 (1976) 1. 2 D.V. Sokol’skii, K.K. Kuzembaev and I.V. Kel’man, Usp. Khim. 46 (1977) 828. 3 R.I. Bickley, Chemical Physics of Solids and their Surfaces, Specialist Report, The Chemical Society, London, 1980, Chap. 5. 4 (a) T.E. Madey and J.T. Yates, J. Vac. Sci.Technol.,8 (1971) 525;(b) D.Menze1, Surf. Sci., 47 (1975) 370. 5 Faraday Discuss. Chem. SOC.,58 (1974) 6 A. Scharmann and W. Kriegsels, in A. Bohan and A. Scharmann (Eds.), Symp. Exoelectron Emission Dosimetry, Zvikovske Podrahi, Prague, 1976, p. 5. 7 E.H. Taylor, Adv. Catal., 18 (1968) 114. 8 Th. Wolkenstein, Adv. Catal., 23 (1973) 157. 9 Symp. Electron. Phenom. Chemisorp. Catal. Semiconductors, de Gruyter, Berlin, 1969, pp. 1-24. 1 0 R. Coekelbergs, A. Crucq and A. Frennet, Adv. Catal. 13 (1962) 55. 11 ( a ) M. Pope and H. Kallmann, Discuss. Faraday Soc., 5 1 (1971) 7; ( b ) R.L. Van Ewyk, A.V. Chadwick and J.D. Wright, J. Chem. SOC. Farad. Trans. 1, 76 (1980) 2194. 1 2 K.J. Laidler, in P.H. Emmett (Ed.), Catalysis, Vol. 1, Reinhold, New York, 1959, (a) p. 119, ( b ) p. 190. 1 3 A.P. Shuklov, B.N. Shelimov and V.B. Kazanskii, Kinet. Katal., 18 (1977) 780. 1 4 J.H. de Boer, Dynamical Character of Adsorption, Oxford University Press, Oxford, 1968, p. 35. 1 5 F.F. Volkenshtein, The Electronic Theory of Catalysis on Semiconductors, Pergamon Press, Oxford, 1963, p. 1. 1 6 R.J. Morrison and J.P. Bonnelle, J. Catal., 25 (1972) 416. 17 B. Claudel, in M. Kleitz and J. Dupuy (Eds.), Electrode Processes in Solid State Ionics, Reidel, Dordrecht, 1976, pp. 45-81. 18 Th. Wolkenstein, Prog. Surf. Sci., 6 (1975) 213. 1 9 E. Segal and M.Teodorescu, Rev. Roum. Chim., 1 2 (1967) 75. 20 S. Baidiyaroy and P. Mark, Surf. Sci., 30 (1972) 53. 21 D. Kohl, H. Moorman and G. Heiland, Surf. Sci., 7 3 (1978) 160. 22 W.H. Cropper, Science, 137 (1962) 955. 23 D.A. Dorsden, Ann. Rep. Chem. SOC.C, (1979) 3. 24 R.P. Messmer, in E. Drauglis and R.I. Jafee (Eds.), The Physical Basis for Heterogeneous Catalysis, Plenum Press, New York, 1975. 25 G. Blyholder, J. Chem. Phys., 62 (1975) 3193. 26 J.E. Demuth, Surf. Sci., 65 (1977) 369. 27 F.C. Tompkins, in G.C. Bond, P.B. Wells and F.C. Tompkins (Eds.), 6th Int. Congr. Catal., London, 1976, The Chemical Society, London, 1977, p. 32. 28 W.M.H. Sachtler and P. Van der Mark, Surf. Sci., 18 (1969) 62. 29 C.F. Melius, J.W. Moskowitz, A.P. Mortola, M.B. Baille and M.A. Ratner, Surf. Sci., 59 (1976) 279. 30 C.V. Pittman and R.C. Ryan, Chem. Technol., 8 (1978) 170. 31 H.A. Taylor and N. Thon, J. Am. Chem. Soc., 74 (1952) 4169. 32 N. Thon and H.A. Taylor, J. Am. Chem. SOC.,7 5 (1953) 147. 33 A.S. Portet and F.C. Tompkins, Proc. R. SOC.London Ser. A, 217 (1953) 529. 34 J.M. Tatibouet and J.E. Germain, J. Catal., 72 (1981) 375. 35 M. Boudart, in G.C. Bond, P.B. Wells and F.C. Tompkins (Eds.), Proc. 6th Int. Congr. Catal., London 1976, The Chemical Society, London, 1977, p. 1. 36 (a) G. Somorjai, Adv. Catal. 26 (1977) 1 ; ( b ) G. Somorjai, Surf. Sci., 8 9 (1979) 496. 37 S.L. Bernasek and G.A. Somorjai, J. Chem. Phys., 62 (1975) 3149.

420 38 M. Boudart, A. Delbouille, E.G. Derouane, V. Indovina and A.B. Walters, J. Am. Chem. SOC.,94 (1972) 6622. 39 (a) S. Coluccia, A.M. Deane and A.J. Tench, J. Chem. SOC. Faraday Trans. 1 , 74 (1978) 2913; ( b ) S. Coluccia, A.M. Deane and A.J. Tench, in G.C. Bond, P.B. Wells and F.C. Tompkins (Eds.), Proc. 6th Int. Congr. Catal., London, 1976, The Chemical Society, London, 1 9 7 7 , pp.171, 1 8 2 ; (c) S. Coluccia, A. Barton and A.J. Tench, J. Chem. SOC. Faraday Trans. 1 , 7 7 (1981) 2203. 40 M. Che, C. Naccache and B. Imelik, J. Catal., 24 (1972) 328. 41 D. Cordischi and V. Indovina, J. Chem. SOC.Faraday Trans. 1 , 7 0 (1974) 2189. 42 D. Cordischi, V. Indovina and M. Occhiuzzi, J. Chem. SOC.Faraday Trans. 1 , 7 4 (1978) 456. 4 3 A.J. Tench and G.T. Pott, Chem. Phys. Lett., 2 6 (1974) 590. 44 S.R. Morrison, Surf. Sci., 5 0 (1975) 329. 45 S. Collucia, A.J. Tench and R. Segall, J. Chem. SOC.Faraday Trans. 1 , 7 5 (1979) 1769. 46 F.S. Stone and A. Zecchina, in G.C. Bond, P.B. Wells and F.C. Tompkins (Eds.), Proc. 6th Int. Congr. Catal., London, 1976, The Chemical Society, London, 1977, p. 1 6 2 . 47 B.D. Flockhard, Surface and Defect Properties of Solids, Vol. 2, The Chemical Society, London, 1973. 48 G. Parravano, in K. Hauffe and Th. Wolkenstein (Eds.), Electronic Phenomena in Chemisorption and Catalysis on Semiconductors, De Gruyter, Berlin, 1969, p. 111. 49 A. Bielanski and J. Deren, in K. Hauffe and Th. Wolkenstein (Eds.), Electronic Phenomena in Chemisorption and Catalysis o n Semiconductors, De Gruyter, Berlin, 1969, p. 156. 5 0 P. Roussel and S.J. Teichner, Catal. Rev., 6 (1972) 133. 5 1 S.R. Morrison, Chem. Technol., 7 (1977) 570. 52 J. Lagowski, E.S. Sproules and H.C. Gatos, J. Appl. Phys., 48 (1977) 3566. 53 J. Cunningham and A.L. Penny, J. Phys. Chem., 78 (1974) 870. 54 J. Cunningham, D.J. Morrissey and E.L. Goold, J. Catal., 5 3 (1978) 6 8 . 55 G.A. Somorjai and R.R. Haering, J . Phys. Chem., 67 (1963) 1150. 5 6 R.F. Copeland, J. Phys. Chem., 75 (1971) 1967. 57 J.R. Schrieffer and P. Soven, Phys. Today, 2 9 (1975) 24. 58 (a) R.H. Tait and R.V. Kasowski, Phys. Rev., 2 0 (1979) 5178; ( b ) R.V. Kasowski and R.H. Tait, Phys. Rev., 20 (1979) 5168. 5 9 (a) S.J. Gutman and M.J. Kelly, Surface and Defect Properties of Solids, Vol. 5, The Chemical Society, London 1976, p . 1 ; ( b ) G. Chiarotti, S. Nannarone, R. Pastora and P. Chiaradia, Phys. Rev. Sect. B, 4 (1971) 3398; ( c ) G. Betteridge and V. Heine, J. Phys. C, 6 (1973). 6 0 W. Hirschwald, P. Bonaswicz, L. Ernst, M. Grade, D. Hoffman, S. Krebs, R. Littbarski, G. Neumann, M. Grunze, D. Kolb and H.J. Schulz, in E. Kaldis (Ed.), Current Topics in Materials Science, Vol. 7 , North-Holland, Amsterdam, 1981. 6 1 (a) W. Gopel, Bunsen Ges. Phys. Chem., 8 2 (1978) 7 1 6 ; ( b ) W.A. Harrison, Surf. Sci., 5 5 (1976) 1. 6 2 (a) I. Ivanov and J. Pollman, J. Vac. Sci. Technol., 19 (1981) 3 4 4 ; ( b ) I. Ivanov and J. Pollman, Solid State Commun., 36 (1980) 361. 6 3 ( a ) P.E. Gregory and W.E. Spicer, Appl. Phys. Lett., 25 (1974) 511;(b) W. Ranke and K. Jacobi, Prog. Surf. Sci., 1 0 (1981) 1. 64 (a) H. Luth and G. Heiland, Phys. Status Solidi, 1 4 (1972) 5 7 3 ; ( b ) G. Heiland and H. Luth, in D.A. King and D.P. Woodruff (Eds.), The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 3A, Elsevier, Amsterdam, in press.

421 65 66 67 68 69

70 71

72 73 74 75 76 77 78 79 80 81 82

83 84 85 86 87 88 89 90 91 92

H. Kakado, T. Nakayama and E. Inoue, J. Phys. Chem. Solids, 3 4 ( 1 9 7 3 ) 1 ; 35 (1974) 1169. J. Van Laar and J. Huijeer, J. Vac. Sci. Technol., 1 3 ( 1 9 7 6 ) 769. (a) P. Mark, E. So and M. Bonn, J. Vac. Sci. Technol., 1 4 (1977) 365; (b) P. Makk and W.F. Creighton, Appl. Phys. Lett., 27 (1975) 400. P. Pianetta, I. Lindau, C.M. Garner and W.E. Spicer, Phys. Rev. B, 1 8 , (1978) 2792. (a) A.B. Lidiard and M.J. Norgrett, in Computational Solid State Physics, Plenum Press, London, 1972 p . 3 8 5 ; ( b ) W.C. Mackrodt, R.F. Stewart, J.C. Campbell and I.H. Hillier, J. Phys. (Paris), C6 ( 1 9 8 0 ) 6 6 ; (c) C.R.A. Catlow, W.C. Mackrodt, M.J. Norgrett and A.M. Stoneham, Philos. Mag., 3 5 ( 1 9 7 7 ) 177. (a) E. Brockawik and J. Haber, J . Catal., 7 2 ( 1 9 8 1 ) 379; ( b ) M. Tsukada, N. Tsuda and F. Minami, J. Phys. SOC.Jpn., 4 9 ( 1 9 8 0 ) 1115; (c) M. Tsukada, E. Miyazaki and H. Adachi, J. Phys. SOC.Jpn., 50 ( 1 9 8 1 ) 3032. (a) R.L. Park, in R.B. Anderson and P.T. Dawson (Eds.), Experimental Methods in Catalytic Research, Academic Press, New York, 1 9 7 6 ; (b) J.M. Honig, Proc. Symp. Chem. Phys. Surf. Met. Oxides, Kalpakkam, 1976, p. 254; (c) V. Landman and G.G. Kleiman, Surface and Defect Properties of Solids, Vol. 6 , The Chemical Society, London, 1976. A. Terenin, Adv. Catal., 15 ( 1 9 6 4 ) 227. (a) G. Kortum, Reflectance Spectroscopy, Springer Verlag, Berlin, 1 9 6 9 ; ( b ) A. Bagchi, R.E. Barrera and B.B. Dagupta, Phys. Rev. Lett., 44 ( 1 9 8 0 ) 1475. (a) V.B. Kazanskii, in G.C. Bond, P.B. Wells and F.C. Tompkins (Eds.), Proc. 6th Int. Congr. Catal. The Chemical Society, London, 1977, p.50; ( b ) S.L. Kaliqguine, B.N. Shelimov and V.B. Kazansky, J. Catal., 55 (1978) 384. G.T. Pott and W.H. Stork, Catal. Rev., 1 2 ( 1 9 7 5 ) 163. (a) J.H. Lunsford, Adv. Catal., 22 ( 1 9 7 3 ) 265; ( b ) J.H. Lunsford, Catal. Rev., 8 (1974) 135. A.J. Tench and P. Holyroyd, Chem. Commun., ( 1 9 6 8 ) 471. N.B. Wong and J.H. Lunsford, J. Chem. Phys., 55 ( 1 9 7 1 ) 3007. R.D. Iyengar and M. Codell, Adv. Colloid Interface Sci., 3 ( 1 9 7 2 ) 365. I.M. Prudnikov and Yu. P. Solonitsyn, Kinet. Katal., 1 3 (1972) 380. G.M. Zhabrova, V.I. Vladimirova, A.A. Gesalov and B.M. Kadenatsi, in K. Hauffe and Th. Wolkenstein (Eds.), Electronic Phenomena in Chemisorption and Catalysis o n Semiconductors, De Gruyter, Berlin, 1969, p. 236. T. Kwan, in K. Hauffe and Th. Wolkenstein (Eds.), Electronic Phenomena in Chemisorption and Catalysis o n Semiconductors, De Gruyter, Berlin, 1969, p. 193. (a) J. Cunningham, J.J. Kelly and A.L. Penny, J. Phys. Chem., 7 4 ( 1 9 7 0 ) 1992; (b) J. Cunningham, J.J. Kelly and A.L. Penny, J. Phys. Chem., 75 ( 1 9 7 1 ) 617. W.B. Williamson, J.H. Lunsford and C. Naccache, Chem. Phys. Lett., 9 ( 1 9 7 1 ) 33. N.B. Wong, Ben-Taarit and J.H. Lunsford, J. Chem. Phys., 6 0 ( 1 9 7 4 ) 2149. K. Aika and J.H Lunsford, J. Phys. Chem., 8 1 ( 1 9 7 7 ) 1393; 8 2 (1978) 1794. A.J. Tench, Trans. Faraday SOC.,68 (1972) 1181. (a) V.V. Nikisha, B.N. Shelimov and V.B. Kazanskii, J. Catal., 28 ( 1 9 7 3 ) 230; (b) V.V. Nikisha, B.N. Shelimov and V.B. Kazanskii, 15 (1974) 678. S.A. Surin, A.D. Shuklov, B.N. Shelimov and V.B. Kazanskii, Kinet. Katal., 19 (1978) 435. A.J. Tench and R.L. Nelson, J. Colloid Sci., 26 ( 1 9 6 8 ) 364. H.L. Hair, Infrared Spectroscopy in Surface Chemistry, Arnold, London, 1967. (a) S.E. Ermatov and T.S. Koserov, Deponirovannye Rukopisi Viniti, Vsesajuznyj institut naucnoj i techniceskoj informacii, 1973, 7635-73; (b) S.E. Ermatov and T.S. Koserov Izv. Akad. Nauk Kaz. SSR, Ser. Fiz. Mat., 1 3 ( 1 9 7 5 ) 114.

422 9 3 (a) V.N. Filimonov, Dokl. Akad. Nauk SSSR., 158 (1964) 1 4 0 8 ; (b) A. Mansour, H. Balard, and E. Papirer, Bull. SOC.Chim. Fr., (1981) 1-236. 94 (a) H. Knozinger and B. Stubner, J.Phys.Chem.,82 (1978) 1526;(b)B.Stubner, H. Knozinger, J. Conrad and J.J. Fripiat, J. Phys. Chem., 8 2 (1978) 1811. 95 V.E. Heinrich, Prog. Surf. Sci., 9 (1979) 143. 96 (a) J.H. Sinfeld, Rev. Mod. Phys., 51 (1979) 569; (b) J.H. Sinfield, Proc. Symp. Photoelectron Spectrosc., Surf. Chem. Catal., Cardiff, 1980, Faraday Div. Chem. SOC;(c) J.H. Sinfeld and J.H. Viz, J. Catal., 5 6 (1979) 1. 97 J.A. Kirby, A.S. Robertson, J.P. Smith, A.C. Thompson, S.R. Cooper and M.P. Klein. J. Am. Chem. SOC.,103 (1981) 5537. 98 W. Bach and H.D. Brauer, Faraday Discuss. Chem. SOC.,58 (1974) 237. 99 H. Kuhn, D. Mobius and H. Bucher, in A. Weissberger and B. Rossiter (Eds.), Physical Methods in Chemistry, Vol. 1,Wiley, New York, 1972. 100 H. Gerischer, Faraday Discuss. Chem. SOC.,58 (1974) 220. 1 0 1 J. Lagowski, H.C. Gatos and C.L. Balestra, J. Appl. Phys., 4 9 (1978) 2821. 102 ( a ) P.B. Gilman, Pure Appl. Chem., 49 (1977). 357; ( b ) P.B. Gilman, in H. Gerischer and J.J. Katz (Eds.), Light Induced Separation in Biology and Chemistry, Dahlern Conference, Berlin, 1979, pp. 187-203. 1 0 3 K. Vacek, Czech. J. Phys., B21 (1971) 303. 104 V. Fidler, K. Vacek and J. Fiala, Czech. J. Phys., B23 (1973) 1102. 105 Th. Wolkenstein, G.P. Peka and V.V. Malak Hov, J. Lumin., 5 (1972) 251, 261. 106 E. Lendvay, J. Phys. Chem., 6 9 (1965) 740. 107 G.C. Allen, P.M. Tucker, B.E. Hayden and D. Klomperer, Surf. Sci., 1 0 2 (1981) 207. 108 K.J. Haas, D.C. Fox and M.J. Katz, J. Phys. Chem. Solids, 26 (1965) 1779. 109 A.A. Lisachenko and F.I. Vilesov, Kinet. Katal., 13 (1972) 420. 1 1 0 (a) D. Lichtman and Y. Shapira, OTIS Rep. COO-2425-4, 1975; ( b ) N. Van Thieu and D. Lichtman, Surf. Sci., 1 0 3 (1981) 535. 111 R. Schubert and K.W. Boer, J. Phys. Chem. Solids, 3 2 (1971) 77. 112 S. Baidyaroy, W.R. Bottoms and P. Mark, Surf. Sci., 28 (1971) 517. 113 P. Genequand, Surf. Sci., 25 (1971) 643. 114 J. Cunningham and N. Samman, in D. Price and J.F.J. Todd (Eds.), Heyden, London, 1975, p. 247. 115 J. McK. Nobbs, Vacuum, 23 (1973) 391. 1 1 6 ( a ) Y. Shapira, S.M. Cox and D. Lichtman, Surf. Sci., 50 (1975) 5 0 3 ; ( b ) Y. Shapira, S.M. Cox and D. Lichtman, Surf. Sci., 5 4 (1976) 4 3 ; ( c ) Y. Shapira and A. Friedenberg, J. Appl. Phys., 51 (1980) 710. 117 B. Kasemo and E. Tornqvist, Phys. Rev. Lett., 4 4 (1980) 1555. 118 J. Lagowski, E.S. Sproules and H. Gatos, Surf. Sci., 3 0 (1972) 653. 119 B. Kramer, J.T. Wallmark and P. Mark, J. Vac. Sci. Technol., 1 2 (1975) 713. 120 ( a ) A. Many, J. Shappir and V. Shakad, in E. Drauglis, R.D. Gretz and R.I. Jaffee (Eds.), Molecular Processes on Solid Surfaces, McGraw Hill, New York, 1969, pp. 199-224. ( b ) J. Shappir and A. Many, Surf. Sci., 1 4 (1969) 169. 1 2 1 S. Aziz and A. Mishriky, Egypt. J. Phys., 4 (1973) 103. 1 2 2 (a) K. Tanaka and K. Tamanu, Bull. Chem. SOC. Jpn., 37 (1965). 1 8 6 2 ; ( b ) H. Noller and K. Thomke, J. Mol. Catal., 6 (1979) 375. 1 2 3 T.B. Grimley, Faraday Discuss. Chem. SOC., 58 (1974) 7. 124 Yu. P. Solonitsyn, in B.S. Neporent (Ed.), Elementary Photo Processes in Molecules, Consultants Bureau, New York, 1968, pp. 327-339. 125 D.B. Kennedy, M. Ritchey and J. Mackenzie, Trans. Faraday SOC., 5 4 (1958) 119. 1 2 6 (a) G.A. Korsunovskii, Kinet. Katal., 3 (1962) 296; ( b ) Yu. P. Solonitsyn, Kinet. Katal., 7 (1966) 130.

423 127 V.S. Zakharenko, A.E. Cherkashin, N.P. Keier and S.V. Korcheev, Kinet. Katal., 1 6 (1975) 182. 128 H. Moesta and H.D. Breuer, Surf. Sci., 17 (1969) 439. 129 K. Weber, Doctoral Thesis, Bonn, 1973. 130 M.J. Drinkwine, Y. Shapira and D. Lichtman, in M. Kaminsky (Ed.), Radiation Effects on Solid Surfaces, American Chemical Society, Washington, 1976, p. 171. 131 P. Mark, J. Phys. Chem. Solids, 26 (1965) 965. 132 P. Mark, J. Phys. Chem. Solids, 26 (1965) 1767. 1 3 3 A. Many and A. Katzir, Surf. Sci., 6 (1967) 279. 134 M. Petrera, F. Trifiro and G. Benedek, Proc. 2nd Int. Conf. Solid Surf., 1974, Jpn. J. Appl. Phys. Suppl., 2 (1974) 31. 135 J. Cunningham, B. Doyle and N. Samman, J. Chem. SOC. Faraday Trans. 1 , 72 (1976) 1495. 136 (a) J. Cunningham, B. Doyle, D.J. Morrissey and N. Samman, in G.C. Bond, P.B. Wells and F.C. Tompkins (Eds.), Proc. 6th Int. Congr. Catal., London, 1976, The Chemical Society, London, 1977, p.1093; ( b ) J. Cunningham, B. Doyle and D.J. Morrissey, in D. Price and J.F.J. Todd (Eds.), Dynamic Mass Spectrometry, Vol. 5, Heyden, London, 1976, p. 195. 137 P.C. Cosby, P.A. Bennett, J.R. Peterson and J.T. Moseley, J. Chem. Phys., 63 (1975) 1612. 138 M. Formenti, H. Courbon, F. Juillet, A. Lissatchenko, J.R. Martin, P. Meriadeau and S.J. Teichner, J. Vac. Sci. Technol., 9 (1972) 947. 139 (a) J.M. Herrman, J. Disdier and P. Pichat, Proc. 7th Int. Vac. Congr. and 3rd Int. Conf. Solid Surf., Vienna, 1977, p. 951; ( b ) J.M. Herrman, J. Disdier and P. Pichat, J. Chem. Soc. Faraday Trans 1, 77 (1981) 2815. 140 R.I. Bickley and F.S. Stone, J. Catal., 31 (1973) 389. 141 (a) A.H. Boonstra and C.A. Mutsaers, J. Phys. Chem., 79 (1975) 1940; (b) A.H. Boonstra and C.A. Mutsaers, J. Phys. Chem., 79 (1975) 1694. 142 G. Munuera, A.R. Gonzalez-Elipe, J. Soria and J. Sanz, J . Chem. SOC.Faraday Trans. 1 , 7 6 (1980) 1535. 143 ( a ) G. Munuera, V. Rivis-Arnau and A. Saucedo, J. Chem. Soc. Faraday Trans. 1, 75 (1979) 736; (b) A.R. Gonzalez-Elipe, G. Munuera and J. Soria, J. Chem. SOC.Faraday Trans. 1 , 7 5 (1979) 748. 144 K. Atherton, G. Newbold and J.A. Hockey, Discuss. Faraday Soc., 52 (1971) 33. 145 G.G. Libowitz, Prog. Solid State Chem., 2 (1965) 216. 146 W. Hirschwald and F. Stolze, Z. Phys. Chem. (Frankfurt a m M a i n ) , 77 (1972) 21. 147 G. Heiland, Z. Phys., 138 (1954) 495; 142 (1955) 415. 148 D. Eger, Y. Goldstein and A Many, RCA Rev., 36 (1975) 508. 149 E. Arijs and F. Cardon, J. Solid State Chem., 6 (1973) 310, 319 150 N.M. Beekmans, J. Chem. SOC.Faraday Trans. 1 , 7 4 (1978) 31. 151 P. Morgen, J.H. Onsgaard and S. Tougaard, J. Appl. Phys., 47 (1976) 5094. 152 J. Attal, Colloq. Int. Appl. Technol. Vide Ind. Semicond. Component. Electron. Microelectron, (C.R.) 3rd Soc. Fr. Ing. Tech. Vide, Paris, 1971, p. 363. 153 E. Arijs, F. Cardon and W.M. Van Der Vorst, Z. Phys. Chem. (Leipzig) 94 (1975) 255. 154 F. Steinbach and R. Harborth, Faraday Discuss. Chem. Soc., 58 (1974) 143. 155 W. Hirschwald and E. Thall, Faraday Discuss. Chem. Soc., 58 (1974) 176. 156 S. Nishigaki, W. Drasksel and J.M. Block, Surf. Sci., 87 (1979) 389. 157 A.J.H. Boerboom, P.G. Kistemaker, M.A. Posthumus and H.L.C. Meuzelaar, in D. Price and J.F.J. Todd (Eds.), Dynamic Mass Spectrometry, Vol. 5, Heyden, London, 1978, p. 114.

424 158 M.Formenti, J. Juillet and S.J. Teichner, Bull. SOC.Chim. Fr., (1976) 1031. 159 R.B. Cundall, R. Rudham and M.S. Salim, J. Chem. SOC.Faraday Trans. 1 , 72 (1976) 1642. 160 J. Cunningham, B.K. Hodnett and A. Walker, Proc. R. Ir. Acad., R.I.C. Centenary Issue, (1977) 411. 161 R.I. Bickley, G. Munuera and F.S. Stone, J. Catal., 31 (1973) 398. 162 (a) L.P. Childs and D.F. Ollis, J. Catal., 66 (1980) 4891; ( b ) L.P. Childs and D.F. Ollis, J. Catal., 67 (1981) 35. 163 J.C. Hemminger, R. Carr and G.A. Somorjai, Chem. Phys. Lett., 57 (1978) 100. 164 M.I. Temkin, S.L. Kiperman and L.I. Luk'yanova, Dokl. Akad. Nauk SSSR, 74 (1950) 763. 165 I.P. Sidorov, V.V. Shishkova and M.I. Temkin, Tr. GIAP, 6 (1956) 323. 166 J. Cunningham and B.K. Hodnett, J. Chem. SOC.Faraday Trans. 1, 77 (1981) 2777. 167 K. Tanaka, A. Kazasaka, A. Yamazaki and K. Miychara, J. Phys. Chem., 8 1 (1977) 268. 168 T.A. Borodina and T.S. Minakova, Zh. Fiz. Khim., 52 (1978) 1203. 169 J. Novakova, Catal. Rev., 4 (1970) 77. 170 G.K. Boreskov, Adv. Catal., 1 5 (1964) 285. 1 7 1 A.P. Griva, V.V. Nikisha, B.N. Shelimov and V.B. Kazanskii, Kinet. Katal., 1 5 (1974) 104. 172 (a) J. Cunningham, E.L. Goold and E.M. Leahy, J. Chem. SOC.Faraday Trans. 1 , 75 (1979) 305; (b) J. Cunningham, E.L. Goold and J.L.G. Fierro, J. Chem. SOC.Faraday Trans. 1 , 7 8 (1982) 785. 173 ( a ) H. Courbon, M. Formenti and P. Pichat, J. Phys. Chem., 82 (1977) 500; ( b ) P. Pichat, J.M. Hermann, H. Courbon, J. Disdier and M.N. Mezzanega, Can. J. Chem. Eng., 60 (1982) 27. 174 (a) D.R. Kearns, Chem. Rev., 71 (1971) 395; ( b ) L.R. Manring and C.S. Foote, J. Phys. Chem., 8 6 (1982) 1257; (c) C. Munuera, A. Navio, and V. Rives-Arnaud, J. Chem SOC.Faraday Trans. 1 , 7 7 (1981) 2747. 175 (a) A.U. Khan, Chem. Phys. Lett., 4 (1970) 567; ( b ) B. McCarroll, J. Chem. Phys., 50 (1969) 4658. 176 P.C. Gravelle and S.J. Teichner, Adv. Catal., 20 (1969) 167. 177 F.S. Stone, Adv. Catal., 1 3 (1963) 1. 178 G.M. Schwab, H. Noller, F. Steinbach and M. Venuqoplan, Nature (London) 193 (1962) 774. 179 W. Doerffler and K. Hauffe, J. Catal., 3 (1964) 171,156. iao F. Steinbach, Nature (London), 215 (1967) 152; Angew. Chem., 79 (1967) 1019. 181 F. Steinbach, Nature (London), 221 (1969) 657. 182 F. Steinbach and R. Barth, Ber. Bunsenges. Phys. Chem., 7 3 (1969) 884. 183 A. Thevenet, F. Juillet and S.J. Teichner, Proc. 2nd Int. Conf. Solid Surf., 1974, Jpn. J. Appl. Phys. Suppl., 2 (1974) 529. 184 P. Mars and D.W. Van Krevelen, Chem. Eng. Sci. Spec., 9 (1954) Suppl. 41. 185 H. Van Damme and K. Hall, J . Catal., 6 9 (1981) 371. 186 M. Anpo, I. Tanabashi and Y. Kubokowa, J. Phys. Chem., 8 6 (1982) 1. 187 N. Djeghri, M. Formenti, F. Juillet and S.J. Teichner, Faraday Discuss. Chem. soc., 58 (1974) 185. 188 M. Formenti, F. Juillet, P. Meriaudeau and S.J. Teichner, Chem. Technol., 1 (1971) 680. 189 M.-N. Mozzanega, J.M. Herrmann and P. Pichat, Tetrahedron Lett., 34 (1977) 7965. 190 A. Walker, M. Formenti, P. Meriaudeau and S.J. Teichner, J. Catal., 50 (1977) 237

425 191 R.I. Bickley and R.K.M. Jayanty, Faraday Discuss. Chem. SOC.,58 (1974) 194. 192 (a) J. Cunningham and P. Meriaudeau, J. Chem. SOC. Faraday Trans. 1, 72 (1976) 1499; ( b ) J. Cunningham, E. Finn and N. Samman, Faraday Discuss. Chem. SOC.,58 (1974) 160. 193 J. Cunningham, D.J. Morrissey and E.L. Goold, J. Catal., 5 3 (1978) 68. 194 J. Cunningham and B. Doyle and E.M. Leahy, J. Chem. SOC.Faraday Trans. 1, 75 (1979) 2000. 195 J. Cunningham, B.K. Hodnett, M. Ilyas, J. Tobin and E.M. Leahy, Faraday Discuss. Chem. SOC.,72 (1981) 283. 196 J. Cunningham, B.K. Hodnett, M. Ilyas, E.M. Leahy and J.P. Tobin, J. Chem. SOC.Faraday Trans. 1,78 (1982) 3297. 197 J. Cunningham, G. A1 Sayed and M. Ilyas, Can. J . Chem., t o be published. 198 C.G. Harkins, W.W. Shang and T.E. Leland, J . Phys. Chem., 7 3 (1969) 130. 199 A.D. Shuklov, S.A. Surin, B.N. Shelimov and V.B. Kazanskii, Kinet. Katal., 1 6 (1975) 468. 200 A.D. Shuklov, B.N. Shelimov and V.B. Kazanskii, Kinet. Katal., 18 (1977) 413. 201 K. Tanaka and G. Blyholder, J. Phys. Chem., 75 (1971) 1037. 202 T.L. Moruzzi and A.V. Philips, J. Chem. Phys., 45 (1966) 4716. 203 J. Warman, J. Phys. Chem., 72 (1968) 52. 204 K. Tanaka and G. Blyholder, J. Phys. Chem., 76 (1972) 1807. 205 P. Pitchat, H. Courbon, J. Disdier, M.N. Mozzanega and J.-M. Herrmann, Proc. 7 t h Int. Congr. Catal., Tokyo, 1980. 206 D.R. Kennedy, M. Ritchie and J. Mackenzie, Trans. Faraday SOC.,(1956) 119. 207 P. Pichat., J.-M. Herrmann, J. Disdier, H. Courbon and M.-N. Mozzanega, Nouv. J. Chim., 5 (1981) 559. 208 A. Fujishima and K. Honda, Nature (London), 238 (1972) 37; Bull. Chem. SOC.Jpn., 44 (1971) 1148. 209 M.S. Wrighton, D.S. Ginley, P.T. Wolexanski, A.B. Ellis, D.L. Morse and A. Linz, Proc. Natl. Acad. Sci. U.S.A., 72 (1975) 1518. 210 V.A. Kotelnikov and A.N. Terenin, Dokl. Akad. Nauk SSSR, 174 (1967) 1366. 211 M.S. Wrighton, Chem. Eng. News, 57 (1979) 29. 212 L.L. Basov, Yu. P. Solonitsin and Yu. P. Efimov, Adv. Photonic Phenom., 3 ( 4 ) (1974). 213 T. Kawai and T. Sakata, Chem. Phys. Lett. 72 (1980) 87. 214 H. Van Damme and W.K. Hall, J. Am. Chem. SOC., 101 (1979) 4373. 215 (a) S. Sat0 and J.M. White, J. Am. Chem. SOC.,102 (1980) 7206; ( b ) S. Sat0 and J.M. White, Chem. Phys. Lett., 72 (1980) 83. 216 S. Ferrer and G.A. Somorjai, Surf. Sci., 94 (1980) 41; 97 (1980) 304. 217 I. Izami, W.W. Dunn, K.O. Wilbairn, Fu-Ren F. Fan and A.J. Bard, J. Phys. Chem., 8 4 (1980) 3207. 218 (a) T. Kawai and T. Sakata, J. Chem. SOC.Chem. Commun., (1979) 1047; (b) T. Kawai and T. Sakata, J. Chem. SOC.Chem. Commun., (1980) 694; (c) Nature (London), 286 (1980) 474. 219 J. Cunningham, J.P. Tobin and P. Meriaudeau, Surf. Sci., 108 (1981) 465. 220 A. Mozumder, Adv. Radiat. Chem., 1 (1969) 1. 221 I. Santar and J. Bednar, in “Radiation Chemistry, Vol. 1 , Adv. Chem., 1 (1968) 523. 222 T.E. Madey, J.J. Czyewski and T. Yates, Surf. Sci., 49 (1975) 465; 57 (1976) 580. 223 A. Joshi and L.E. Davis, J. Vac. Sci. Technol., 1 4 (1977) 1310. 224 A.R. Gonzalez-Elipe, J. Soria and G. Munnuera, Chem. Phys. Lett., 57 (1978) 265.

426 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 24 0 241 242 243 244 245 246 247 248 249 250 25 1 252 253 254 255 256

J.C. Vedrine, J. Massardier and A. Abou-Kais, Canad. J. Chem., 5 4 (1976) 1678. J. Puncocharova, A. Fojrik and 0. Kadler, Chem. Zvesti, 29 (1975) 319. R.G. Wilmeth and S.S. Fischer, Surf. Sci., 72 (1978) 693. R. Franchy and D. Menzel, Proc. 7th Int. Vac. Congr. and 3rd Int. Conf. Solid Surf., Vienna, 1977, p. 1209. M.L. Knotek and P.J. Fiebelman, Phys. Rev. Lett., 40 (1978) 9 6 4 ; Phys. Rev. B, 18 (1978) 6531. T.E. Madey, R. Stockbauer, J.F. van det Veen and D.E. Eastman, Phys. Rev. Lett., 4 5 (1980) 187. T.E. Madey, Surf. Sci., 94 (1980) 483. G.B. Pariishii, Yu. A. Mischenko and V.B. Kazanskii, Kinet. Katal., 6 (1965) 625. ( a ) R. Engleman and J. Jortner, Mol. Phys., 18 (1970) 1 4 5 ; ( b ) K.F. Freed and J. Jortner, J. Chem. Phys., 5 2 (1970) 6272. J.V. Caspar, E.M. Kother, B.P. Sullivan and T.J. Meyer, J. Am. Chem. Soc., 104 (1982) 630. G.M. Panchenkov, A.L. Plyushch, V.V. Erchenkov, D.A. Kuzovkin, N.K. Shirayaeva and M.D. Dancher, Zh. Fiz. Khim., 51 (1977) 1480. V.I. Spitsyn, F.F. Vol’kenshtein, G.I. Pirogova, S.F. Timashev, R.I. Korosteleva and A.A. Sopina, Izv. Akad. Nauk SSSR Ser. Khim., 4 (1972) 771. A.I. Trokhimets, T.I. Beschuertnaya and A.A. Ivko, Vesn. Akad. Nauk BSSR. Ser. Khim. Nauk, 6 (1977) 50. (a) D.J. Norfolk and T. Swan, J. Chem. Soc. Faraday Trans. 1 , 7 3 (1977) 1454 ( b ) D.J. Norfolk and T. Swan, J. Chem. SOC.Faraday Trans. 1 , 7 4 (1978) 1976. N.M. Gupta V.S. Kamble and R.M. Ivea, J. Catal., 6 6 (1980) 101. E.L. Krylova and D.I. D o h , Kinet. Katal., 7 (1966) 977. N.S. Bubyreva, P.I. Dolin, A.A. Kononovich and N.D. Rozenblyum, Kinet. Katal., 6 (1965) 936. D.V. Sokol’skii, B.T. Nadykto, A.M. Pak, E.I. Ten, Yu. D. Kuznetsov and L.D. Rozmanova, Khim. Vys. Energ., 5 (1971) 74. D.V. Sokol’skii, A.M. Pak, B.T. Nadykbo, L.D. Rozomanova, A.V. Korolev and G.P. Chursin, Khim. Vys. Energ., 5 (1971) 4. D.V. Sokol’skii, A.M. Pak and L.D. Rozmanova, Elecktrokhimiya, 11 (1975) 1685. B.T. Kelly, B.W. Ashton, R. Lind and V.Y. Labaton, Ext. Abstr. Bien. Conf. Carbon, 1 2 (1975) 319. J. Rappeneau, G. McCaud, A. Paccault, A. Marchand, J. Amiell, Ext. Abstr. Bien. Conf. Carbon, 1 2 (1975) 13. S. Baidyaroy, W.R. Bottoms and P. Mark, Surf. Sci., 29 (1972) 165. T.N. Rhodin, P.W. Palmberg and C.J. Todd, in E. Drauglis, R.D. Gretz and R.F. Jaffee (Eds.), Processes o n Solid Surfaces, McGraw-Hill, New York, 1969, pp. 499-530. K. Besocke and S. Berger, Vak. Tech., 27 (1978) 66. Y, Margoninski and D. Eger, Proc. 7 t h Int. Vac. Congr. and 3rd Int. Conf. Solid Surf., Vienna, 1977, p. 525. Y. Margoninski, Surf. Sci., 94 (1980) L167. R.P. Holmstrom, J. Labonski and H.C. Gatos, Surf. Sci., 75 (1978) L781. F.J. Arlinghaus, J.G. Gay and J.R. Smith, in J.R. Smith (Ed.), Theory of Chemisorption, Springer-Verlag, Berlin, 1980, p. 71. E. Garrone, A. Zecchina and F.S. Stone, Philos. Mag., 42 (1980) 683. E. Garrone, A. Zecchina and F.S. Stone, J. Catal., 6 2 (1980) 396. J. Cunningham and B.K. Hodnett, J. Chem. SOC. Faraday Trans. 1 , 77 (1981) 2777.

427 257 J. Cunningham, E.L. Gadd and J.L.G. Fields, J. Chem. Soc. Faraday Trans. 1, 7 8 (1982) 785. 258 P. Pichat, Nouv. J. Chim., 5 ( 1 9 8 1 ) 627. 259 J. Cunningham, in E.T. Kaiser and L. Kevan (Eds.), Radical Ions, Interscience, New York, 1 9 6 8 , p. 475. 260 N.H. Tolk, M.M. Traum, J.C. Tully and T.E. Hadley (Eds.), Proceedings of the 1st International Workshop o n Desorption Induced by Electron Transitions, Springer, Heidelberg, 1982. 2 6 1 M.J. Drinkwine and D. Lichtmann, Prog. Surf. Sci., 8 (1977) 123. 262 J.A. Applebaum and D.R. Hamann, in J.R. Smith (Ed.), Theory of Chemisorption, Springer-Verlag, Berlin, 1980, p. 43. 263 C.R.A. Catlow, W.C. Mackodt, M.J. Norgrett and A.M. Stoneham, Philos. Mag., A40 ( 1 9 7 9 ) 161. 264 ( a ) K. Aika and J.H. Lunsford, J. Phys. Chem., 81 (1977) 1393; (b) K. Aika and J.H. Lunsford, J . Phys. Chem., 8 2 ( 1 9 7 8 ) 1794. 265 E. Christoffel, Catal. Rev. Sci. Eng., 2 4 ( 1 982) 159. 266 F.V. Hanson and J.E. Bensen, J . Catol., 31 (1973) 471. 267 J. Cunningham and E.L. Godd, J. Chem. SOC. Faraday Trans. 1, 7 7 (1981) 837. 268 ( a ) A. Spitzer and H. Luth, Surf. Sci., 118 (1982) 121; ( b ) A. Spitzer and H. Luth, Surf. Sci., 118 ( 1 9 8 2 ) 136.

This Page Intentionally Left Blank

Index

A accommodation coefficient, 59, 61, 62, 64 active sites, 293, 296, 298-303, 340, 417,418 adsorption/desorption processes, 295, 296, 299-301, 304, 307, 320, 324, 325, 327, 329, 331, 340, 343, 376, 401--403, 405, 406-408, 414, 416, 418 AES, see Auger emission spectroscopy AI, 334,339 aluminium, as doping agent in zinc oxide, 335,368 -, on gallium arsenide, 263-265, 275, 276 -, on silicon, 259 aluminium oxide, as catalyst, 402 -, band gap, 406,407 -, chemisorption of gases on, 401 -, decomposition of methanol, 407 -, finely divided, 320 -, irradiated sample, 414 -, outgassed, 319 -, oxygen interface, 320, 409 -, photo-oxidation, 377 -, radiation-induced surface holes, 320 ammonia, adsorbed on aluminium oxide, 316 -, - iron, 139 -, - platinum, 140 -, - rubidium, 140 -, - silica, 316 -, hydrogen abstraction from, 415 -, radical formation from, 408 ANI, 340, 341, 343 antimony, photocatalytic activity of antimony(II1) oxide, 369 -, photoconductivity of antimony(V) oxide, 347 AREDC, 218 argon, adsorbed on tungsten, 3, 161 -, bombardment with Ar', 416, 417 -, weakly bonded to metals, 311

ARPES, 190, 203 arsenic, formation of gallium arsenide from As2, 278, 279 -, interaction of A& with gallium arsenide surfaces, 277, 279 ASCT, 300, 310, 3 3 2 , 4 0 8 , 4 0 9 Auger electron spectroscopy, 2, 13, 23, 31,183,189-192,201,236,242,255, 258, 260, 262, 266, 267, 312, 330, 401,403,404,417

B barium, on tungsten, 158 Bayard--Alpert gauge, 2 benzene, formation from cyclohexene, 399 beryllium oxide, with adsorbed methanol, 320 bismuth molybdate, catalysed oxidation of propene, 301 Bravais lattice, 184 Brillouin zones, 198, 227, 253 bromine, physisorbed on metals, 139, 31 1

C cadmium sulphide, CdS/gas interface, 340, 34 1 -, CdS/oxygen interface, 295, 332, 333, 339,340,342 -, photoadsorption of oxygen, 327 -, photosorption of oxygen, 331 -, sulphur in vacancies, 416 cadmium telluride, empty surface states, 307 canted ridges, 230 carbon, impurity in clean silicon surfaces, 202 -, on tungsten, 160 carbon dioxide, as impurity, 328, 352

430

-, -, -, -,

on nickel, 21 on tungsten, 1 6 1 photodesorption of, 331, 352 production in oxidation of alkanes, 357,374 -, reaction with hydrogen, 409 -, use as moderator, 416 carbon monoxide, adsorbed, 30, 41, 50-52 -, as impurity, 328 -, chemisorbed, 401 -, contamination of silicon surface, 233 -, on chromium, 4 7 , 1 1 5 , 1 3 1 -, on cobalt, 4 7 , 1 1 5 , 1 3 1 -, on copper, 23 -, on iron, 115, 1 3 1 -, on molybdenum, 4 7 , 1 0 8 , 1 1 5 , 1 3 1 -,on nickel, 47, 88, 89, 97, 115, 132, 155 -, on palladium, 47, 97, 108, 177 -,on platinum, 47, 97, 108, 117, 120, 121,133 -, on rhenium, 47,116, 134 -, on rhodium, 48, 117, 135 -, on ruthenium, 48, 88, 108, 117, 135 -, on titanium, 117, 135 -,on tungsten, 9, 48, 62, 63, 98, 104, 111,118,135,141,161 -, photo-assisted reaction, 390 -, photodesorption from metal surfaces, 336-339,347 -, photo-oxidation of, 366, 373 -, reaction with HzO, 395 catalysis, 292, 296, 298, 299, 301, 308, 310, 311, 320, 354, 356, 357, 367, 369, 371, 374, 377, 383, 390, 391, 394, 395, 397, 400, 402, 404, 406, 407,409,417 -, non-metallic, 300 CECT, 302, 331,332,409 CeOz, photoconductivity of, 347 cesium, adsorbed o n gallium arsenide, 266-269 -, - silicon, 260 -, --tungsten, 39, 155, 158 chemiluminescence, 41 5 chemisorption, 182, 198, 221, 223, 226228, 231, 232, 237, 242, 246, 251, 252, 260, 265, 266, 268, 278, 280, 293-296, 298-302, 311, 317, 319, 325, 329, 331, 335, 340, 342-344, 352, 360, 379, 401, ,402, 404, 407, 416,418

chlorine, absorbed on copper, 139 - gold, 139 - palladium, 139 - platinum, 139 -, - rhodium, 139 -, - silicon, 242-246 -, - tungsten, 139 chromium, metal ion dispersed o n aluminium oxide, 316 -, - silicon dioxide, 316 chromium(II1) oxide, interface with oxygen, 335 -, photoreduction of carbon monoxide, 372 -, pre-oxidised surface, 382 clean surfaces, 1 , 2, 108, 183, 201-206, 210, 214, 215, 222, 224, 229, 230, 232-234, 236, 246, 247, 251, 254, 262, 2 6 3 , 2 6 6 , 2 6 9 , 3 2 6 , 3 2 7 , 4 0 4 -, preparation of, 1 , 14, 258, 259, 329, 403 clusters, copper/ruthenium, 321 -, zinc/oxygen, 308 -, zirconium/oxygen, 308 co-axial filament flow reactor, 354 cobalt(1V) oxide, Fermi levels of, 367 -, photoreduction, 372 collective electron, 293, 296, 301-303, 340,398,408,409 condensation coefficient, 8 4 copper, absorbed on molybdenum, 161 -, - - tungsten, 159 copper(I) oxide, surface reduction through thermolysis, 349 copper/ruthenium clusters, 321 C.R.O., 332 crystallographic etch pits, 205, 243 CVD, 254

-, -, -,

D Dember effect, 332, 340 demetallisation, 23 density of states, 296, 304, 308, 342 depolarisation, 268 desorption mechanisms, 308-31 7 deuterium, exchange with hydrogen, 298, 299, 3 8 8 , 3 8 9 , 4 1 4 , 4 1 5 dipole-dipole interaction, 7 direct coupling, 7 direct hit, 296 disorder/order, 6, 226, 227, 307, 320

431 E

-, photoemission spectra, 261 -, semiconductorsemiconductor inter-

electron tunnelling, 302, 310 Eley-Rideal mechanism, 298, 370, 379, 385,412, 418 ellipsametry, 24, 233, 237, 247 Elovich equation, 247 Elovich kinetics, 333, 335, 341 ELS, 227, 234, 236, 237, 239, 247, 248, 250,251,253, 258,417 ESCA, 325, 400 ESD, 312,417 ESID, 404 ESR, 204, 306, 317-319, 325,361,400, 408,414,415 ethane, quenching of magnesium oxide luminescence, 316 Ewald sphere, 198 EXAFS, 5, 237,321, 322

-, topological defects of surface, 307

F F+ centres, 320, 364 Fick’s law, 146 field emission microscopy, 2, 34-38, 142, 154-163 field ion microscopy, 38, 39, 230 flash filament method, 17, 28 fluorescein, adsorbed on zinc oxide, 339 fluorine, adsorbed on platinum, 139 formic acid, adsorbed on copper, 1 2 3 -, - nickel, 124 Franck-Condon transitions, 189

face, 275, 276

-, TPD spectra, 273, 274 -, with adsorbed cesium, 266, 267 -, with adsorbed gold, 262 gallium arsenide { 100) face, interaction with Group V metals, 279 -, oxygen adsorbed on clean faces, 253 -, semiconductor-semiconductor interface, 276 gallium arsenide { 110) face, cesium adsorbed on, 266 -, cleaning of, 215-221 -, gold adsorbed on, 266 -, oxygen adsorbed on clean faces, 247.25 2 -, reaction with oxygen, 251 -, semiconductor interface interaction, 270 -, semiconductor-semiconductor interface, 276, 277 gallium( 111) oxide, photocatalysis, 369 gassemiconductor interface, 295, 296, 301, 307,310, 332,343 germanium, on tungsten, 160 gold, adsorbed on gallium antimonide, 260-262 -, - gallium arsenide, 260-262, 269 -, -silicon, 255-257, 259 -, - tungsten, 157 -, hydrogen chemisorbed on, 299 -, interface with nickel oxide, 367 -, islands of, 416

G

H gallium, adsorbed on gallium arsenide, 263-267 -, - silicon, 254 gallium antimonide, gold film on, 262 -, oxidation of, 251 -, oxygen adsorbed on, 301 -, spectra of, 251 -, spectra of gold+aSb, 263 gallium arsenide, cleaning of surface, 204-206 -, electron stimulated oxidation, 199 -, energy levels of, 306 -, interaction with aluminium, 265 -, interaction with Group V elements, 277,278

HREELS, 1 6 hydrogen, activating outgassed aluminium oxide, 319 -, - magnesium oxide, 319 -, adsorbed, 41, 5+52 -, adsorbed on chromium, 42,109, 126 -, - cobalt, 42, 109, 126 -, - copper, 53, 83, 109, 126 -, - iridium, 42, 109,126 -, -iron, 42, 101, 109, 126 -, - magnesium, 126 -, -molybdenum, 4 2 , 1 0 9 , 1 2 6 - ,_ nickel, 40, 42, 56, 63, 77, 83, 89, 104,108,128,162

432

-, -niobium, 42, 109, 126 -, -palladium, 42, 110, 127

krypton, adsorbed on tungsten, 3, 161

-, physisorbed on metals, 31 1

-, --platinum, 42, 110, 121, 124

-, -rhenium, 43,110, 128 -, -, -, -, -, -,

-rhodium, 43, 110, 128 -ruthenium, 43, 110, 128 -silica gel, 320, 414 -tantalum, 43, 110, 128 -titanium, 97, 109, 126 - tungsten, 2, 10, 43, 54, 56, 63, 77, 8 3 , 1 0 4 , 1 0 8 , 1 1 2 , 1 2 8 , 161 -, chemisorbed on transition metals, 296 -, desorption of, 403 -, deuterium exchange, 298, 299, 414, 415 -, - catalysed by magnesium oxide, 298, 300 -, formation of palladium black, 415 -, formation of platinum black, 415 -, reaction on platinum surfaces, 298 -, reaction with carbon dioxide, 408 -, - carbon monoxide, 408 -, - hydrogen atoms, 415 hydrogen peroxide, pre-adsorption of, 349,368

I indirect coupling interaction, 7 indium, adsorbed on gallium arsenide, 263 -, - silicon, 259 -, - tungsten, 159 indium phosphide, analysis of surface for gold, 262 -, spectra of gold-InP, 261 -, with adsorbed gold, 260 -, with adsorbed oxygen, 307 interstate conversion, 141 iodine, physisorbed on metals, 311 indium, adsorbed on tungsten, 1 6 0 -, - zinc oxide, 365 -, with chemisorbed hydrocarbons, 299 iron, pre-oxidised surface, 382 iron(111) oxide, photoreduction of carbon monoxide, 372 isothermal desorption, 29

K kinks, 230 Kisluik model, 70, 74

LaCo03, photocatalysis of, 383 Lagowski model, 301 LAMMA, 354 Langmuir adsorption, 2, 39, 56, 64, 65, 6 9 , 1 5 0 , 376 Langmuir evaporation, 205, 249 Langmuir-Hinshelwood recombination mechanism, 298, 369,370, 385,418 LDOS, 198, 208,220,221, 238, 241 Lead, adsorbed on gallium arsenide, 273, 274 LEED, 2, 4 - 6 , 8-11, 73, 107, 156, 183, 185-188, 201, 203, 204, 206-208, 210, 215, 219, 222, 233, 242, 244, 247, 255, 259, 265, 266, 268, 297, 298, 3 1 1 , 4 0 1 , 4 0 3 , 4 1 6 LEELS, 237, 259, 277 Lennard-Jones potential, 3, 8 4 LET, 398,399 lithium, zinc oxide doped with, 335, 352, 361, 368 lithium fluoride, with adsorbed formaldehyde, 322 LWPE, 354

M M+ centre, 364 magnesium oxide, band gap, 406,407 -, catalyst for deuterium/hydrogen exchange, 300, 388, 389 -, F+ type centres, 320 -, isotropic exchange over, 361 -, luminescence of, 316 -, outgassed, 319 -, reaction with N20,389 --,“smoke”, 315 magnesium-oxygen interface, 353 mass spectroscopy, 327, 330, 347, 353, 365,370,377,401 MBE, 206, 247,253, 272,277, 278 mercury, adsorbed on tantalum, 162 -, -tungsten, 106, 159 metal excess surface species, 295 metal oxide layers, 9, 233, 234, 242, 247, 251, 253, 331, 352, 367, 377, 379, 403,404-409,415, 416,418

433

methanol, adsorbed o n beryllium oxide, 320 -, - silicon dioxide, 320 -, hydrogen abstraction, 415 microbalance, 331 -, techniques, 22 molybdenum, adsorbed o n tungsten, 152, 154,160 molybdenum(II1) oxide, ESR signal, 415 --,photoreduction of carbon monoxide, 372 N N i Z + ,dispersed o n alumina, 316 - silica, 316 nickel, photodesorption of carbon monoxide from, 336 -, with physisorbed xenon, 403 nickel oxide, band gap of, 407 -, enriched "0 passed over, 365 -, Fermi level of, 367 -, NiO/metal interface, 367 Ni(C0)4, 338 nitrogen;adsorbed, 41, 50, 51 -, adsorbed o n copper, 48 -, - indium, 136 -, - iron, 48, 5 3 -, -molybdenum, 48,81,119, 136 -, -nickel, 48, 81, 119, 137 -, - niobium, 48, 131 -, -palladium, 119, 137 -, -platinum, 48, 119, 137 -, -rhenium, 48,119, 137 -, - ruthenium, 120 -, - silver, 48 -, -tantalum, 48, 137 -, - titanium, 48, 81 -, - tungsten, 9, 28, 31, 34, 49, 53, 54, 57, 63, 73, 120, 137, 160 -, - vanadium, 48 nitrogen monoxide, adsorbed o n nickel, 138 -, - platinum, 138 -, - rhenium, 138 -, - rhodium, 138 -, - ruthenium, 138 -, -silver, 56, 138 -, -tungsten, 56, 138 -, quenching luminescence of magnesium oxide, 316 nitrous oxide, adsorbed o n platinum, 140

-,

-, causing band bending, 294 -, interface with cadmium sulphide, 341

-, photoreduction, 389-394 -, quenching luminescence of magnesium oxide, 316 NMR, 320

0 osmium(I1) complexes, 407, 408 oxidation of alcohols, 366-373 oxygen, adsorbed, 41,50-52 -, adsorbed on aluminium, 44, 63, 79 -, - copper, 9 , 4 4 -, - gallium antimonide, 307 -, - gallium arsenide, 307 -, -indium, 44, 130 -, - indium phosphide, 3, 91 -, - iron, 44 -, -molybdenum, 44, 130 -, -nickel, 9 , 4 5 , 57 -, -niobium, 113 -, -palladium, 45,63,113,129 -, - platinum, 45, 53,56, 113,114, 130 -, - rhenium, 46, 130 -,-rhodium, 46, 1 1 4 , 130 -, -ruthenium, 46, 1 1 4 , 130 -, -silica gel, 320 -, -silver, 43, 99, 106, 113, 129 -, - tantalum, 46, 130 -, - tungsten, 2, 8 , 9, 33. 46, 63, 100, 108,114, 131, 160, 161 -, -zinc selenide, 307 -, -zirconium, 48 -, anion radical formation, 317 -, aluminium oxide /02 system, 319 -, causing band bending, 294 -, chemisorption on aluminium oxide, 401 -, -zinc oxide, 301 -, contamination on silicon surfaces, 234 -, interaction with magnesium, 353 -, interface with cadmium sulphide, 295, 332, 333, 336, 340, 342, 350-352, 371, 374 -, - tin(1V) oxide, 295, 332, 333, 336, 340,342,350-352,371,372 -, - titanium oxide, 295, 332, 333, 336, 340,342,350-352,371,372 -, -zinc oxide, 295, 332, 333, 336, 340, 342,350-352,371,372 -, irreversible photoadsorption o n titanium oxide, 347, 348

434

-, isotropic exchange, 360-366,

374, 375,380,409 -, photoadsorption on cadmium sulphide 331 -, -zinc oxide, 3 3 5 -, photodesorption from titanium oxide, 347,348 -, photo-oxidation of carbon monoxide, 370-373 -, reaction with gallium antimonide, 251 -, silicon d i o x i d e / 0 2 , 239-242 -, silicon-oxygen bond length, 257 -, sorption o n zinc oxide, 349 -, sticking coefficient o n silicon, 234 oxygen-oxygen interaction, 353 oxygen-xygen stretch, 2 3 9 oxygen pressure effect, 234 oxygen/zirconium(IV) oxide system, 319 P palladium, adsorbed o n tungsten, 1 5 7 palladium black, 416 -, preparation of, 415 Pd/nickel oxide interface, 367 PEDS, 306 photo-oxidation, of alkanes, 373-376 -, of carbon monoxide, 366-373 -, of isopropanol, 377--412 -, of other alcohols, 385-388 photoadsorption/desorption, 327-335, 339, 343, 348, 3 5 2 , 3 7 1 photoelectronic effect, 331, 3 3 2 photoemission, 15, 214, 217, 218, 223, 225, 243-246, 2 5 5 , 2 6 3 , 304 photogenerated holes, 332, 340, 343, 365,384 photoluminescence, 315, 324 PIFIMS, 3 5 3 platinum, { 111)surfaces, 298 -, activating crystals of, 404 -, chemisorption of hydrocarbons on, 299 -, desorption of chemisorbed species from, 300 -, reaction of active sites, 299 -, reaction with hydrogen, 298 -, single crystal of, 298, 300 platinum black, 416 -, preparation of, 415 platinumltitanium oxide, 395-397 Polanyi-wigner equation, 8 4 , 8 7 , 195, 196

potassium, adsorbed o n tungsten, 1 5 2 , 158 potential energy curves, 5 , 57, 8 2 pre-adsorption, 339, 348, 377 precursor states, 6 2 4 9 , 72, 77, 84, 8 5 , 1 0 1 , 236, 252, 271, 272, 2 7 1 propene, oxidation of using bismuth molybdate, 3 0 1 PSID, 404 PTD, 3 7 8 pulsed techniques, 332, 4 1 8

R radiation-induced modifications, 296, 299, 3 0 2 , 3 1 1 , 3 9 9 RAIRS, 1 6 Resistivity, 2 3 RHEED, 187-189 Rose Bengal dye, adsorbed o n glass, 322 -, - gold, 322 -, -zinc oxide, 339 -, spectra o f , 3 2 3 S scanning electron microscope, 1 8 3 , 258, 274 SCI, 3 3 4 , 3 3 5 , 3 3 9 sensitisation, 342 SEXAFS, 252 silica, see silicon dioxide silicon, adsorbed with cesium, 260 -, - gold, 255-257, 260 -, -indium, 259 -, -molybdenum, 1 6 2 -, - silver, 258-259 -, - tungsten, 1 6 0 -, cleaning of surface, 202-204 -, germanium silicate interface, 277 -, intrinsic surface state, 3 0 3 , 306 -, reaction with arsenic, 278 -, -chlorine, 242-246 -, - gallium, 259 -, -hydrogen, 223-232 -, - oxygen, 234-242 -,{loo} face, hydrogen adsorbed o n , 2 29-230 -, { l l O } face, hydrogen adsorbed on, 2 2 7-2 2 9 -, { 111) face, effect of substrate orientation and reconstruction, 235 -, -, hydrogen adsorbed o n , 223-227

435

-, -, reaction with aluminium, 259 -, -, -chlorine, 244 -, -, - gallium, 259 -, -, - gold, 255 -, -, - indium, 2 5 9 -, -, -silver, 258 silicon dioxide, band gap, 406 -, decomposition of methanol, 407 -, finely divided, 3 2 0 -, irradiated sample, 408, 409 -, methanol adsorbed o n , 320 -, molybdenum(V1) oxide/Si02 interface, 415 -, photo-oxidation on, 3 2 0 -, tungsten( VI) oxide/Si02 interface, 415 -, vanadium(V) oxide/SiOz interface, 415 -, water absorbed o n , 3 2 0 , 408 Silver, adsorbed o n silicon, 258-259 silver/ethane interface, 353 silver halides, adsorption of dyes o n silver bromide, 339 -, photoluminescence of, 324 silver islands, 416 silver/nickel oxide interface, 367 SIMS, 30 sodium, adsorbed o n tungsten, 1 5 8 spurs, 398 SSI, 340 steps, 298, 2 9 9 , 3 0 4 , 3 0 8 , 3 1 6 , 404 sticking coefficient, 1 9 4 , 222, 229, 233, 234, 236, 242, 247, 252, 253, 267,278,279 sticking probability, 1, 1 7 , 24. 41, 76, 81, 101 -, zero coverage, 41-55 strontium titanate, photocatalysis, 383 -, photocatalytic enhancement, 3 7 1 -, stoichiometric { 111)surface, 396 -, use in catalysis, 359 structure-sensitive reactions, 298, 299 sulphur arsenide, interaction with gallium arsenide surfaces, 275, 276 sulphur dioxide, adsorbed on tungsten, 140 surface diffusion, 31-41, 82, 143, 1 9 7 surface enhancement, 209 surface-insensitive reactions, 298 surface roughness, 8 1 , 205 surface state, 303-310, 340, 3 4 3 -, extrinsic, 307-308, 3 4 8 -, intrinsic, 303-307

surface steps, 187. 213, 234 symmetric model, 2 0 9

T TEM, 2 0 3 , 2 1 6 , 277 temperature-programmed desorption, 20, 27-29, 9 1 , 109-120, 253, 268, 272, 273, 275 terraces, 52, 6 4 , 213 thorium, adsorbed o n tungsten, 1 5 9 tin(1V) oxide, interface with oxygen, 3 4 3 -, photocatalysis of, 369, 377 -, photoconductivity of, 347 titanium, adsorbed o n tungsten, 1 5 9 titanium oxide, band bending of, 294 -, density of states, 304 -, ESR, 3 1 9 , 4 0 0 -, gases desorbed from, 347 -, interface with oxygen, 295, 340, 343, 344, 350 -, interpretation of surface properties by surface state model, 306 -, irreversible photoadsorption of oxygen on, 334 -, isotropic exchange over, 3 6 1 -, photocatalysis, 369, 372, 378, 380383 -, photoconductivity, 347 --,photo-oxidation of alkanes, 357, 359, 373 -, surface reduction through thermolysis, 349 -, undoped, 394 -, with metal excess surface species (Ti3+),3 9 5 titanium oxidelnitric oxide system, 3 9 1 titanium oxide/platinum, 395-397 Tomahawk accelerator, 4 0 3 Torus accelerator, 4 0 3 TPC, 4 1 1 , 4 1 2 TPRS, 1 2 2 trapping probability, 59, 6 4 , 7 8 tungsten oxide, ESR signal, 4 1 5 -, photoconductivity of, 347 -, sensitisation, 415

U UPS, 1 8 3 , 1 9 0 , 222,226-228, 2 3 0 , 2 3 6 , 238, 247, 248, 251, 255, 258, 259, 261, 2 6 6 , 2 7 6 , 3 1 2 , 3 2 1 , 3 9 6 , 4 0 1

436 V

Z

vanadium, dispersed on aluminium oxide, 316 -, -silicon dioxide, 316 vanadium(V) oxide, ESR signal, 4 1 5 -, photoconductivity, 347 -, pre-oxidised surface, 382 -, with carbon monoxide introduced over, 3 7 2 -, with enriched l8 O2 passed over, 3 6 5 van der Waals, 294 V centres, 3 2 0 , 4 0 8 , 414 VCI, 2 9 8 , 3 8 5 , 4 1 2 , 4 1 8 virgin states, 9 0 VPE. 254

zinc oxide, adsorption of dyes, 339 band bending of, 394 band gap, 4 0 5 4 0 7 chemisorption of hydrogen, 4 0 1 -oxygen, 3 0 1 , 3 4 4 doped with aluminium, 335, 352, 361, 368 -, -lithium, 335, 352, 361, 368 -, extrinsic surface state, 307 -, Fermi level of, 367 -, interface with nitrous oxide, 389, 390, 3 9 3 , 3 9 4 , 401 -, interface with oxygen 295, 336, 350, 352 -, interpretation of surface properties by surface state model, 306 W -, isotropic exchange over, 361 -, lattice, 417 water, o n iridium, 1 4 0 -, photoadsorption of oxygen, 335 -, on iron, 1 4 0 -, photocatalysed reaction, 357, 368, -, o n platinum, 1 4 0 377,380,381,383 Wood notation, 11 -, photoconductivity, 347, 350 work function, 3 , 223, 232, 266, 268, -, photodesorption, 3 7 1 367,368,401,418 -, photolysis of, 364, 365, 3 7 1 -, photo-oxidation, 320 -, photosorption of thin films on, 330 ” A -, pre-oxidised surface, 3 6 3 -, single crystal, 324, 351, 371, 416, 417 -, sorption of oxygen, 349 xenon, adsorbed on copper, 4 -, with metal excess surface species -, - palladium, 4 , 7 (Zn’/Zn+), 295 -, -tungsten, 3 , 1 1 , 6 2 zinc selenide, adsorbed oxygen, 307 -, discharge, 332 -, disordered surface, 307 -, physisorbed o n metals, 4 1 1 -, empty surface states, 307 - - nickel, 4 0 3 XPS, 1 8 3 , 1 9 0 , 222,226-228,230, 236, zirconium(1V) oxide, interface with oxygen, 347 238, 247, 248, 251, 255, 258, 259, -, photoconductivity, 347 261, 266.276, 331. 396. 403 X-ray crystallography,~l86 -, sensitisation of, 377 ’

-, -, -, -, -,