The use of electrochemical scanning tunnelling microscopy (EC-STM) in corrosion analysis: Reference material and procedural guidelines (EFC 44) (European Federation of Corrosion Publications)

The use of electrochemical scanning tunnelling microscopy (EC–STM) in corrosion analysis

European Federation of Corrosion Publications NUMBER 44

The use of electrochemical scanning tunnelling microscopy (EC–STM) in corrosion analysis Reference material and procedural guidelines R. Lindström, V. Maurice, L. H. Klein and P. Marcus

Published for the European Federation of Corrosion by Woodhead Publishing and Maney Publishing on behalf of The Institute of Materials, Minerals & Mining CRC Press Boca Raton Boston New York Washington, DC

Cambridge England

Woodhead Publishing Limited and Maney Publishing Limited on behalf of The Institute of Materials, Minerals & Mining Published by Woodhead Publishing Limited, Abington Hall, Abington Cambridge CB21 6AH, England www.woodheadpublishing.com Published in North America by CRC Press LLC, 6000 Broken Sound Parkway, NW, Suite 300, Boca Raton, FL 33487, USA First published 2007 by Woodhead Publishing Limited and CRC Press LLC © 2007, Institute of Materials, Minerals and Mining The authors have asserted their moral rights. This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. Reasonable efforts have been made to publish reliable data and information, but the authors and the publishers cannot assume responsibility for the validity of all materials. Neither the authors nor the publishers, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming and recording, or by any information storage or retrieval system, without permission in writing from Woodhead Publishing Limited. The consent of Woodhead Publishing Limited does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from Woodhead Publishing Limited for such copying. Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. Library of Congress Cataloging in Publication Data A catalog record for this book is available from the Library of Congress. Woodhead Publishing ISBN-13: 978-1-84569-235-3 (book) Woodhead Publishing ISBN-10: 1-84569-235-7 (book) Woodhead Publishing ISBN-13: 978-1-84569-256-8 (e-book) Woodhead Publishing ISBN-10: 1-84569-256-X (e-book) CRC Press ISBN-13: 978-1-4200-5119-3 CRC Press ISBN-10: 1-4200-5119-9 CRC Press order number: WP5119 ISSN 1354-5116 The publishers’ policy is to use permanent paper from mills that operate a sustainable forestry policy, and which has been manufactured from pulp which is processed using acid-free and elementary chlorine-free practices. Furthermore, the publishers ensure that the text paper and cover board used have met acceptable environmental accreditation standards. Typeset by SNP Best-set Typesetter Ltd., Hong Kong Printed by TJ International Ltd, Padstow, Cornwall, England

Contents

Series introduction

vii

Volumes in the EFC series

ix

Preface

xiii

1

Introduction

1

2

Background to the copper system

3

3 3.1

Experimental description Surface preparation 3.1.1 Mechanical polishing 3.1.2 Electropolishing 3.1.3 Annealing Experiment preparation 3.2.1 Preparation of solutions 3.2.2 Preparation of tips 3.2.3 Cleaning the cell Setting the microscope 3.3.1 Mounting the head and adjusting the height 3.3.2 Setting the cell and adding electrolyte Running the microscope 3.4.1 Tip approach 3.4.2 Imaging 3.4.3 Electrochemistry 3.4.4 Tip withdrawal

7 7 7 7 8 9 9 9 11 11 12 13 14 14 15 16 17

Moiré pattern formation on Cu(111) Cyclic voltammetry Imaging at fixed potential Sweeping the potential during imaging

19 19 19 25

3.2

3.3

3.4

4 4.1 4.2 4.3

v

vi

Contents

5 5.1 5.2

Problems with imaging Instability and drift Poorly resolved images

27 27 27

6

References

31

European Federation of Corrosion Publications: Series introduction

The EFC, incorporated in Belgium, was founded in 1955 with the purpose of promoting European co-operation in the fields of research into corrosion and corrosion prevention. Membership of the EFC is based upon participation by corrosion societies and committees in technical Working Parties. Member societies appoint delegates to Working Parties, whose membership is expanded by personal corresponding membership. The activities of the Working Parties cover corrosion topics associated with inhibition, education, reinforcement in concrete, microbial effects, hot gases and combustion products, environment sensitive fracture, marine environments, refineries, surface science, physico-chemical methods of measurement, the nuclear industry, the automotive industry, computer based information systems, coatings, tribo-corrosion and the oil and gas industry. Working Parties and Task Forces on other topics are established as required. The Working Parties function in various ways, e.g. by preparing reports, organising symposia, conducting intensive courses and producing instructional material, including films. The activities of the Working Parties are coordinated, through a Science and Technology Advisory Committee, by the Scientific Secretary. The administration of the EFC is handled by three Secretariats: DECHEMA e.V. in Germany, the Société de Chimie Industrielle in France, and The Institute of Materials, Minerals and Mining in the United Kingdom. These three Secretariats meet at the Board of Administrators of the EFC. There is an annual General Assembly at which delegates from all member societies meet to determine and approve EFC policy. News of EFC activities, forthcoming conferences, courses, etc. is published in a range of accredited corrosion and certain other journals throughout Europe. More detailed descriptions of activities are given in a Newsletter prepared by the Scientific Secretary. The output of the EFC takes various forms. Papers on particular topics, for example, reviews or results of experimental work, may be published in vii

viii

Series introduction

scientific and technical journals in one or more countries in Europe. Conference proceedings are often published by the organisation responsible for the conference. In 1987 the, then, Institute of Metals was appointed as the official EFC publisher. Although the arrangement is non-exclusive and other routes for publication are still available, it is expected that the Working Parties of the EFC will use The Institute of Materials, Minerals and Mining for publication of reports, proceedings, etc. wherever possible. The name of The Institute of Metals was changed to The Institute of Materials on 1 January 1992 and to The Institute of Materials, Minerals and Mining with effect from 26 June 2002. The series is now published by Woodhead Publishing and Maney Publishing on behalf of The Institute of Materials, Minerals and Mining. P. McIntyre EFC Series Editor The Institute of Materials, Minerals and Mining London, UK EFC Secretariats are located at: Dr B. A. Rickinson European Federation of Corrosion, The Institute of Materials, Minerals and Mining, 1 Carlton House Terrace, London SW1Y 5DB, UK Dr J. P. Berge Fédération Européenne de la Corrosion, Société de Chimie Industrielle, 28 rue Saint-Dominique, F-75007 Paris, FRANCE Professor Dr G. Kreysa Europäische Föderation Korrosion, DECHEMA e.V., Theodor-HeussAllee 25, D-60486 Fankfurt, GERMANY

Volumes in the EFC series

1

Corrosion in the nuclear industry Prepared by the Working Party on Nuclear Corrosion

2

Practical corrosion principles Prepared by the Working Party on Corrosion Education (Out of print)

3

General guidelines for corrosion testing of materials for marine applications Prepared by the Working Party on Marine Corrosion

4

Guidelines on electrochemical corrosion measurements Prepared by the Working Party on Physico-Chemical Methods of Corrosion Testing

5

Illustrated case histories of marine corrosion Prepared by the Working Party on Marine Corrosion

6

Corrosion education manual Prepared by the Working Party on Corrosion Education

7

Corrosion problems related to nuclear waste disposal Prepared by the Working Party on nuclear Corrosion

8

Microbial corrosion Prepared by the Working Party on Microbial Corrosion

9

Microbiological degradation of materials – and methods of protection Prepared by the Working Party on Microbial Corrosion

10

Marine corrosion of stainless steels: chlorination and microbial effects Prepared by the Working Party on Marine Corrosion

11

Corrosion inhibitors Prepared by the Working Party on Inhibitors (Out of print) ix

x

Volumes in the EFC series

12

Modifications of passive films Prepared by the Working Party on Surface Science and Mechanisms of Corrosion and Protection

13

Predicting CO2 corrosion in the oil and gas industry Prepared by the Working Party on Corrosion in Oil and Gas Production (Out of print)

14

Guidelines for methods of testing and research in high temperature corrosion Prepared by the Working Party on Corrosion by Hot Gases and Combustion Products

15

Microbial corrosion (Proc. 3rd int. EFC workshop) Prepared by the Working Party on Microbial Corrosion

16

Guidelines on materials requirements for carbon and low alloy steels for H2S-containing environments in oil and gas production Prepared by the Working Party on Corrosion in Oil and Gas Production

17

Corrosion resistant alloys for oil and gas production: guidance on general requirements and test methods for H2S service Prepared by the Working Party on Corrosion in Oil and Gas Production

18

Stainless steel in concrete: state of the art report Prepared by the Working Party on Corrosion of Reinforcement in Concrete

19

Sea water corrosion of stainless steels – mechanisms and experiences Prepared by the Working Parties on Marine Corrosion and Microbial Corrosion

20

Organic and inorganic coatings for corrosion prevention – research and experiences Papers from EUROCORR ’96

21

Corrosion–deformation interactions CDI ’96 in conjunction with EUROCORR ’96

22

Aspects of microbially induced corrosion Papers from EUROCORR ’96 and the EFC Working Party on Microbial Corrosion

23

CO2 corrosion control in oil and gas production – design considerations Prepared by the Working Party on Corrosion in Oil and Gas Production

Volumes in the EFC series 24

Electrochemical rehabilitation methods for reinforced concrete structures – a state of the art report Prepared by the Working Party on Corrosion of Reinforcement in Concrete

25

Corrosion of reinforcement in concrete – monitoring, prevention and rehabilitation Papers from EUROCORR ’97

26

Advances in corrosion control and materials in oil and gas production Papers from EUROCORR ’97 and EUROCORR ’98

27

Cyclic oxidation of high temperature materials Proceedings of an EFC workshop, Frankfurt/Main, 1999

28

Electrochemical approach to selected corrosion and corrosion control studies Papers from 50th ISE Meeting, Pavia, 1999

29

Microbial corrosion (Proceedings of the 4th international EFC workshop) Prepared by the Working Party on Microbial Corrosion

30

Survey of literature on crevice corrosion (1979–1998): mechanisms, test methods and results, practical experience, protective measures and monitoring Prepared by F. P. IJsseling and the Working Party on Marine Corrosion

31

Corrosion of reinforcement in concrete: corrosion mechanisms and corrosion protection Papers from EUROCORR ’99 and the Working Party on Corrosion of Reinforcement in Concrete

32

Guidelines for the compilation of corrosion cost data and for the calculation of the life cycle cost of corrosion – a working party report Prepared by the Working Party on Corrosion in Oil and Gas Production

33

Marine corrosion of stainless steels: testing, selection, experience, protection and monitoring Edited by D. Féron

34

Lifetime modelling of high temperature corrosion processes Proceedings of an EFC Workshop 2001. Edited by M. Schütze, W. J. Quadakkers and J. R. Nicholls

xi

xii

Volumes in the EFC series

35

Corrosion inhibitors for steel in concrete Prepared by B. Elsener with support from a Task Group of Working Party 11 on Corrosion of Reinforcement in Concrete

36

Prediction of long term corrosion behaviour in nuclear waste systems Edited by D. Féron of Working Party 4 on Nuclear Corrosion

37

Test methods for assessing the susceptibility of prestressing steels to hydrogen induced stress corrosion cracking by B. Isecke of EFC WP11 on Corrosion of Reinforcement in Concrete

38

Corrosion of reinforcement in concrete: mechanisms, monitoring, inhibitors and rehabilitation techniques Edited by M. Raupach, B. Elsener, R. Polder and J. Mietz on behalf of Working Party 11 on Corrosion of Steel in Concrete

39

The use of corrosion inhibitors in oil and gas production Edited by J. W. Palmer, W. Hedges and J. L. Dawson

40

Control of corrosion in cooling waters Edited by J. D. Harston and F. Ropital

41

Corrosion by carbon and nitrogen: metal dusting, carburisation and nitridation M. Schutze and H. Grabke

42

Corrosion in refineries J. Harston

43

The electrochemistry and characteristics of embeddable reference electrodes for concrete Prepared by R. Myrdal on behalf of Working Party 11 on Corrosion of Steel in Concrete

44

The use of electrochemical scanning tunnelling microscopy (EC–STM) in corrosion analysis: reference material and procedural guidelines Prepared by R. Lindström, V. Maurice, L. H. Klein and P. Marcus on behalf of Working Party 6 on Surface Science

Preface

The European Federation of Corrosion’s Working Party on Surface Science and the Mechanisms of Corrosion and Protection (EFC WP6) has defined, as one of its objectives, to develop a reference material and reference guidelines for the application of electrochemical scanning tunnelling microscopy (EC–STM) in corrosion science. The great opportunity given by EC–STM, to be able to study the relationship between surface structure and surface reactivity in situ on electrodes in contact with an electrolyte, cannot be overemphasised in corrosion research. This report is aimed at describing the reference material and providing procedural guidelines. STM-users are instructed how to obtain high resolution data (i.e. atomic resolution) on a carefully prepared copper single-crystal surface. Philippe Marcus Chairman of Working Party 6 ‘Surface Science and Mechanisms of Corrosion and Protection’

xiii

1 Introduction

Electrochemical scanning tunnelling microscopy, EC–STM (sometimes termed in situ STM), is a powerful technique for the study of structural effects of electrochemical oxidation–reduction reactions that take place at metal–electrolyte interfaces. EC–STM allows characterisation of the topography of an electrode surface under potentiostatic control with atomic resolution. Thus, EC–STM enables the in situ localisation of electrochemical reactivity if it is accompanied by changes in sample topography, e.g. for deposition, adsorption or dissolution reactions. The initial stages of corrosion can be followed, such as passivation phenomena or initiation of pitting. The structure of adlayers and oxides can be correlated to the substrate lattice structure and phenomena such as reconstruction, growth and misfit can be investigated. The great opportunity to use in situ STM in corrosion research in order to be able to understand the relationship between surface structure and surface reactivity of electrodes in the presence of adsorbed chemical species and solvent molecules cannot be overemphasised. Scanning tunnelling microscopy (STM) was invented by Binning and Rohrer in 1981 [1] and was immediately found to be an invaluable surface analysis technique with atomic resolution in ultra high vacuum (UHV). Later (1986), the STM technique was further developed by Sonnenfield and Hansla [2] to operate in electrolyte solution. Today the EC–STM may operate at ambient conditions with a four-electrode configuration where the electrochemical potential of the tip and the substrate can be independently controlled with respect to a common reference electrode. In the last few decades, many successful experiments with STM have been made under electrochemical conditions, including studies of structures of adsorbed species, underpotential deposition and electrochemical dissolution and anodic oxidation of metals and semiconductors. The first system investigated was the underpotential deposition of Cu on Au(111), which demonstrated that STM not only resolves individual atoms but also the adlayer structure at the solid–liquid interface [3]. This first study has been 1

2

Use of EC–STM in corrosion analysis

followed by investigations of the structure of adlayers of anions, and both organic and biological molecules. Due to the fact that STM with atomic resolution requires a well-defined, atomically flat electrode surface, the choice of substrate and its preparation is an important concern. Electrodes investigated so far include noble metals, base metals, semiconductors and some metal oxides. Single-crystalline surfaces of controlled orientation must be used in order to understand the surface structure–reactivity relationships on the atomic scale. Single crystals of Pt, Au, Ir, Rh, Pb, and perhaps Ag in various orientations can be prepared by the flame-annealing and quenching method. However, base metals such as Ni, Co, Fe and Cu cannot be treated by the same method because they are oxidised in the flame as well as in the air. Nevertheless, atomically flat terrace-step structures on these metals can be obtained by etching, chemically or electrochemically, prior to high temperature annealing under a hydrogen atmosphere. The need for conducting substrates for STM imaging, precludes experiments on metals forming thick, electronically insulating surface oxide films. For these materials in situ AFM is more appropriate, but it is much more difficult to produce atomically resolved images by this method. Several volumes and review articles have been published on in situ STM and related techniques such as in situ atomic force microscopy (AFM), including results obtained on the various substrates of the metals mentioned above and semiconductors [3–10]. This report is aimed at describing a reference material and at providing procedural guidelines for the application of STM in corrosion science, as part of the objectives defined by The European Federation of Corrosion’s Working Party on Surface Science and the Mechanisms of Corrosion and Protection (EFC WP6). The description is intended to help STM-users to learn how to obtain high resolution data (i.e. atomic resolution) on a carefully prepared copper single-crystal surface. As a reference system, sulphate adsorption on Cu(111) in 5 mM H2SO4(aq.) has been selected. This system was chosen because, at potentials below that for oxidation, sulphate adsorbs on Cu(111) and forms an adlayer with a well-defined superstructure [11–19]. The formation of a so-called moiré pattern, characterising the superstructure at the nanometric scale, can nicely and relatively easily be followed by EC–STM. This report does not review results on other systems nor describe the microscope set-up, as this information can be found in other publications and manuals of commercially available instruments.

2 Background to the copper system

Copper is relatively stable in aqueous solutions, but oxidises in air. At slightly cathodic potentials, Cu is immune in the whole pH range (see the Pourbaix diagram in Fig. 2.1). At anodic potentials, copper is amphoteric and dissolves to Cu2+ at low pH, forms crystalline Cu2O and CuO at neutral to weakly alkaline pH and dissolves to form soluble hydroxide complexes at high pH. At slightly acidic pH, Cu2+ forms soluble complexes with sulphate and closer to neutrality, these precipitate as copper hydroxy sulphates. The growth, stability and thickness of the anodic oxides are dependent on potential and anodising time [20–34]. Their structure has been studied with EC–STM [35–41]. At a potential below that for the oxidation of copper in sulphuric acidic solutions, sulphate adsorbs on the copper surfaces. A representative steadystate cyclic voltammogram of sulphate adsorption–desorption on Cu(111) in 5 mM H2SO4(aq.) (pH = 2) is shown in Fig. 2.2 [18]. Sulphate adsorption below the oxidation potential on Cu(111) is a suitable system for learning EC–STM, because sulphate adsorbs, forming an adlayer with a well-defined superstructure [11–19], which can be relatively easily imaged by EC–STM. The superstructure forms a moiré pattern, characterised by a regular distribution of local minima and maxima extending to the edges of the atomically flat terraces of the surface. The separation between the minima and maxima greatly exceeds the atomic distance of the adsorbed species and can, therefore, be seen at relatively low resolution as shown in Fig. 2.3a. The formation of the moiré pattern on Cu(111) was first reported by Kruft et al. [11] in 1997. At higher resolution, it is evident that the moiré pattern is built up by rows of intensity maxima corresponding to adsorbed sulphate ions (Fig. 2.3b). In between these rows, it is proposed, depending on the authors [11–19], that hydrogen or hydronium ions or water molecules are coadsorbed on the surface, forming zigzag chains of lower intensity spots as marked in the upper right corner of the image in Fig. 2.3b [14]. A model of 3

[Cu2+]TOT = 10.00 µM

[SO42–]TOT = 5.00 mM

1.5

C u4SO4(O H )6(s )

1.0

ESHE / V

C u 2+

CuSO4

CuO(s)

Cu(O H ) 4 2–

0.5

C u2 O ( s )

0.0

Cu(O H ) 2 –

–0.5

Cu(s)

–1.0 0

2

4

6

8

10

12

14

pH

T= 25 °C

Fig. 2.1 Pourbaix diagram calculated at [Cu ] = 10 µM and [SO42−] = 5 mM at 25°C and I = 0. The stability diagrams have been calculated using the input-sed-predom2 program [42] based on the Solgaswater algorithm [43]. 2+

Anodic corrosion 0.05

j[mA/cm2]

Sulphate adsorption regime

0.00

Hydrogen evolution

Sulphate desorption –0.05

Copper redeposition

–0.6

–0.4

–0.2

0.0

0.2

0.4

E [V] vs RHE

Fig. 2.2 A representative steady-state cyclic voltammogram of Cu(111) in 5 mM H2SO4(aq.) (pH = 2), sweep rate = 10 mV s−1 as reported by Broekmann et al. [18].

Background to the copper system

a

b

c

Cu

5 nm

5

SO4(2-x)-

H3O+ or H+

1 nm

Fig. 2.3 Moiré pattern formed on Cu(111) in 5 mM H2SO4 as reported by Maurice et al. [19]. The STM images are captured at E = −0.1 V/SHE, Etip = 0.15 V, tunnelling current It = 1 nA. (a) The moiré pattern at nanometric resolution, 50 × 50 nm, Z range = 1.13 nm; (b) atomic resolution showing the adsorbed sulphate ions and co-adsorbed cations in a zigzag arrangement in between the sulphate rows, 7.9 × 7.9 nm, Z range = 0.12 nm; (c) model of the structure of the sulphate adlayer forming a ( 3 × 7 ) structure on the Cu(111) surface.

the co-adsorbed layer structure formed below the oxidation potential on Cu(111) is shown in Figure 2.3c. The moiré pattern formed on Cu(111) in 5 mM H2SO4 has 0.4 nm deep depressions and cell parameters of 2.8 ± 0.2 nm, 3.2 ± 0.2 nm, and 57 ± 5° (the unit cell is marked in Fig. 2.3a and 2.3b), whereas the corresponding parameters for the adsorbed sulphates are 0.46 ± 0.02 nm, 0.65 ± 0.02 nm, and 75 ± 3° (see unit cell marked in Fig. 2.3b and 2.3c) [15]. The bare copper is found to be 0.25 ± 0.02 nm and 62 ± 3° [16]. Thus, the adsorbed sulphate species orders in ( 3 × 7 ) superstructures corresponding to a coverage of about 0.2 SO4(2−x)− (x = 1). The charge transfer (calculated from the charge of the cathodic peak of sulphate desorption) corresponds to about 0.2 electrons per metal atom [14], confirming the surface coverage deduced from the structural data. The ( 3 × 7 ) structure is also found on Au(111) [44], Pt(111) [45], and Rh(111) [46] imaged in the same electrolyte, but the moiré pattern appears only on the Cu(111) surface. This is explained by the smaller lattice parameters of Cu(111) in comparison to the other metal surfaces listed above [14]. Evidently, the moiré pattern is formed for a specific ratio between the sulphate distances in the ( 3 × 7 ) adlayer structure and the copper atom distances in the Cu(111) lattice and thus does not appear on copper surfaces with other orientations [16]. It is generally argued that the origin of the moiré pattern on Cu(111) in acidic sulphate solution is due to a misfit between the Cu(111) lattice and the sulphate adlayer. The misfit causes a reconstruction of the uppermost metallic layer, beneath the adsorbed layer, giving rise to the regularly-

6

Use of EC–STM in corrosion analysis

arranged depressions in the adsorbed film [14]. By using a higher tunnelling current and a small bias voltage, it has been shown that the uppermost Cu layer can be observed through the sulphate layer and has larger lattice parameters compared to those of the bulk metal [18]. However, these results have not been confirmed by others [19]. Maurice et al. [19] have studied the influence of pH on the adsorption of sulphate on Cu(111). It was shown that at higher pH, water/hydroxide adsorption is of increased importance. In alkaline sulphate-containing solutions (5 mM SO42−, pH 10.5), only hydroxide is adsorbed on the Cu(111) surface. In neutral sulphate-containing solutions (5 mM SO42−, pH 7), sulphate is co-adsorbed with water, forming partially ordered structures, but no moiré structure was observed. Additionally, in neutral to alkaline sulphate solutions, it was shown that copper oxide is formed on the copper surface at more anodic potentials.

3 Experimental description

3.1

Surface preparation

Copper single-crystals can be purchased from various suppliers. The Cu(111) orientation has to be accurate within ±1°. The dimensions of the sample have to match the STM stage of the instrument used. The procedure below is typical for disc-shaped samples with a diameter of 10 mm and a thickness of 2 mm. Good surface preparation is essential to obtain atomically resolved EC–STM images. Extended surface terraces, flat at an atomic scale, are achieved on Cu(111) by surface preparation based on mechanical polishing followed by electropolishing in phosphoric acid and by annealing in a hydrogen atmosphere.

3.1.1 Mechanical polishing The surface is mechanically polished on a cloth with 6, 3, 1 and 1/4 micron diamond spray. Polishing takes time and several hours are required for the finer grades. The finished mirror surface must not have any holes or scratches on the entire surface. Any damage on the surface will introduce pitting in the electropolishing step. Copper is a soft material and can readily be damaged if impacted, dropped or touched by hard tools. If this happens, the damaged surface has to be removed in order not to form polycrystalline regions in the annealing process.

3.1.2 Electropolishing The perturbed layer formed by the mechanical polishing is removed before annealing by electropolishing. In the case of copper, viscous electropolishing at 1.8 V in 60% H3PO4(aq.) for four minutes has been shown to give successful results. Before the procedure, the sample is washed in acetone, ethanol and finally Millipore water (see Section 3.2.1). The sample is 7

8

Use of EC–STM in corrosion analysis + Cu foil used as cathode –

Cu wire used as anode

60% H3PO4(aq.)

Cu crystal

Fig. 3.1

Set-up for electropolishing.

immersed in the acid, placed on a copper wire with the metal surface pointing upwards, being parallel to the solution surface (see Fig. 3.1). The solution should not be stirred. The counter electrode is a copper plate. There should be no air bubbles on the surface of the sample before switching on the current. After the four minutes, the sample is removed from the solution without switching off the current. The surface is promptly rinsed with 10% H3PO4 and then Millipore water, after which the surface is dried in a nitrogen gas stream. The sample is now ready to be annealed.

3.1.3 Annealing Annealing in a hydrogen atmosphere reduces air-formed oxides and removes all adsorbed species on the metal surface. In addition, the roughness of the surface smooths out during the high-temperature treatment. The copper samples are annealed at 750°C in a hydrogen atmosphere for about 15 hours. Normal precautions should be taken when handling hydrogen, which is a flammable and explosive gas. The crystals are placed in a quartz tube, which is connected to the set-up, as shown in Fig. 3.2. The annealing set-up is initially pumped to about 10−5 mbar vacuum. Thereafter, a moderate hydrogen flow (one bubble per second) slightly above atmospheric pressure is established. The oven, set at the desired temperature, is then positioned in order to surround the tube containing the crystals. After annealing, the samples are kept in the closed tube under a hydrogen atmosphere until it is time to start the STM experiment.

Experimental description

9

Pressure controller 7 Pump valve

Samples 3

4 oven Oven 8

5 6

Manometer H2 tube

2 Hydrogen valve

1

H2 out 9 Pump 1 A

10 Pump 2 B

Fig. 3.2 Set-up of annealing system. The entire system is made of silica glass. The hydrogen line is marked with dark grey valves (1–6) and the pump valves (7–9) are in light grey. The wash bottle flow indicators are filled with glycerine. The sample tube is attached to the set-up with ball connections, sealed by O-rings and clips. The primary pump (A) is a rotary pump and the secondary (B) is an oil diffusion pump.

3.2

Experiment preparation

In addition to the preparation of the metal surface described previously, tips and solutions have to be made, and finally, just before an experiment, the electrochemical cell, tools and glassware must be rigorously cleaned.

3.2.1 Preparation of solutions The electrolytes used for EC–STM have to be of the highest quality. The 5 mM H2SO4 solution in this description was prepared from Suprapur 96% H2SO4 (Merck®) and deionised water (Millipore®, resistivity > 18 MΩ · cm, Millipore, France).

3.2.2 Preparation of tips Tungsten tips are commonly used for EC–STM. Due to the hardness of tungsten, these tips remain undamaged when lightly touching a softer sample surface. Tungsten is also stable in a broad pH range.

10

Use of EC–STM in corrosion analysis + a

b

– –

+

Fig. 3.3 Tip making: (a) Fixed etching by dipping the tungsten wire tip in 3 M NaOH(aq.) at around 10 V. The counter electrode is a shielded platinum wire; (b) moving the wire through a droplet of the same solution held by a platinum ring at around 2 V.

a

b

c

Heater +

Tip Z

_

Y X

Fig. 3.4 Wax covering: (a) Heating device made of platinum wire connected to a power supply. The tip holder can be moved in the x, y, and z directions; (b) wax is melted on the heated wire so that a droplet fills the crotch. 5 mm of the tip is passed through the droplet to be completely covered; (c) the heating is increased and the top of the covered tip is moved towards the heater, which melts the wax until a tiny point is visible.

The tips are etched electrochemically in 3 M NaOH or KOH at 2–10 V versus Pt. About 2 cm of tungsten wire (3 mm in diameter) is required to form each tip and it is useful to prepare a few before an experiment. There are different techniques for the etching procedure. In the easiest, the tip is dipped (about 5 mm) vertically into the alkaline solution (see Fig. 3.3a). Another method is to pass the tungsten wire through a droplet of alkali solution in a platinum ring charged at a potential of about 2 V until the wire divides, leaving a very sharp tip (Fig. 3.3b). Either or a combination of the two methods may be used. Sometimes the other end of the wire also has to be gently etched in order to fit into the microscope head. After sharpening, the tip, except for its very end, must be covered by wax or lacquer for insulation. A U-shaped heating device made of platinum wire is used (see Fig. 3.4a). The procedure is in two steps. First, the device is

Experimental description

11

heated to the temperature needed for the wax (e.g. Apiezon wax) to melt and the heating wire crotch is filled by a droplet of the wax. The tip is covered by passing 4–5 mm of the tip through the droplet, as shown in Fig. 3.4b. Second, the very end of the tip is uncovered by gently moving the covered tip towards the heater, now set red-hot, so that the wax on the tip slowly glides down. When a tiny point is visible in a travelling microscope, it is withdrawn quickly from the heat (see Fig. 3.4c). To test if the insulation is satisfactory, the tip is mounted in the STM and dipped into Millipore water. The tunnelling current should be less than