Frontiers of Electrochemistry
Series Editors Jacek Lipkowski Depanment of Chemistry and Biochemistry University of Guelph Guelph, Ontario N1G 2W I Canada
Philip N. Ross I Cyclotron Road Lawrence Berkeley Laboratory University of California Berkeley. CA 94720, USA
IMAGING OF SURFACES AND INTERFACES Editors
JACEK L1PKOWSKI and PHILIP N. ROSS
Advisory Board D. M. Kolb, Ulm, Germany M. Van Hove, Berkeley, CA W. SchmickJer, Vltn, Germany R. Guidelli, Florence, Italy A. Wieckowski, Urbana, IL
W. O'Grady, Washington, DC M. J. Weaver, Lafayette, IN W. R. Fawcett. Davis, CA R. Parsons, Southhampton. United Kingdom S. Trasalli, Milan, Italy
Series Listing Adsorptioll of Molecules al Metal Eleclrodes. heek Lipkowski and Philip N. Ross. Eels. Structure of Electrified /1/lerjaces. Jaeek Lipkowski and Philip N. Ross. Eds. EleClrochemislry of Novel Materials, Jacek Lipkowski and Philip N. Ross, Eds. Electrocatalys;s, Jacek Lipkowski and Philip N. Ross, Eds.
WILEY·VCH New York • Chichester • Weinheim • Brisbane • Singapore • Toronto
CONTENTS
Preface This book is printed on acid·free p:lper.e CQpyright CI 1999 by
Wil~y-VCH,
Inc. All rights
Contributors r~sef\'ed.
I.
Published simultaneously in Canada. No part or this publication may be reproduc~d, stored in a retri~val system or transmil1~d in any form or by any means, electronk mechanical, ptx,llocopying, recording. scanning or othcrwis~. ~lcepl as permi1ted under Sections 107 or 108 of lhe 1976 United States COpyrighl Ac!. withoul either lh~ prior wrinen permission of the Publish~r, or authoriVltion through p:lym~nt of lhe appropriate per-copy fee 10 the Copyright Clearnnce Center, 222 Ros~wood Drive, Danvers. MA 01923. (978) 750-8400, fax (978) 75()·4744, RequeSts 10 the Publisher for permission should be addressed to lhe Permissions Department, John Wiley &. Sons. Inc.. 605 Third Av~nue, New York, NY 10158-0012, l212) 850·60 I I, fal (212) 850-6008. E-Mail: PER,\
[email protected],
Library oj Congress ClItaloging-in·Publicolion lkuo: Imaging of surfaces and interfaces / editors, bcek Lipkowski, Philip N. Ross. p. cm. _ (Frontiers of elttttOCllemistry : v. 5) Includes index. ISBN 0·471-24672-7 (cloth) I. Electrodes-Surfaces. 2. Inlerfaces (Physical sciences) 3. Spectroscopic imaging. 4. Surface chemistry, 5. ElCClrochemisU)'. I. Lipkowski, Jacck. n. Ross. P. N. (Philip N.) llJ. Series. QD57 1.143 1999 54 1.3'724-R.INQ
In both cases. (he process of Me deposition occurs in the so-called supersaturation range and is denoted as Me overpotential deposilion (OPO). I In Me electrocryslalliutioo on real substrates with a relalivel)' high density of cryslal imperfections. the surface inhomogeneities play lite most significant role in the process of deposition. In the case of Me deposition on a foreign substrate, Ihe fomtation of the new Me phase generally needs a preceding 3D nucleation. Therefore. the bulk deposition process of Me proceeds in the OPO range. Me phases, usually with lower dimensionality, can also be fomled. however, in the undersaturation range if the binding energy of Me adaloms on a foreign Subsu-3le is higher than that of Me adatoms on the native subSlrate. 1 This process is well known as Me underpotenlial deposition (UPO) and occurs at electrode pOlentials. E. more positi"e than the Nemst equilibrium potential of the 3D Me phase. E"k/M~.: •
where E~~"'1~- denotes the standard potential of the 30 Me bulk phase. aM":is the activity of Met· within the electrolyte. and a3DM~ represents the activity of the pure. condensed 30 Me phase. which is a constant equal to unity by convention. The potential difference AE = E - £Mc/Mr- > 0 represents the undcrpotential range. which is directly related to the undersaturation range AIJ. _ -lF6£ < 0 with respect to the Me phase. Me UPO is an electrosorption process with a charge-covering stoichiometry given by the so-called electrosorption valency:l
1.2
FORMATION OF LOW·DlMENSIONAL METAL PHASES
J
In the presence of substrate surface inhomogeneities. I,D (i '" 0.1.2). a stepwise formation of different low-dimensional Me phases becomes possible in the UPO range. However. cosorption or competitive sorption phenomena of electrolyte constituents differem from Met.. (e.g.. anions. may significantly influence the Me UPO process). The local foonation of low~dimensional Me phases on foreign substrates. S. plays an important role in modem nanotechnology. because future aspects of science and technology in many fields such as physics. chemistry. malerials science. electronics. sensors. biology. medicine, etc., are characterized by a miniaturization down to an atomic level. I Nanotechnology dealing with single atoms. molecules. or clusters will take the position of the micrometer technology. which has dominated the last 150 years. In surface nanotechnology. solid surfaces such as electron-conducting materials (metals. graphite. semiconductors. superconductors. electron-cooducting polymers. etc.). ion~conduct ing materials (solid electrolytes. ion-condueting polymers. membranes. etc.). and insulators have to be analyzed (analytical aspect) in the nanometer range. On the other hand. well-defined nanoslructuring and nanomodification of solid surfaces will play an increasing role in the future nanotechnology (preparalive aspects). Both aspects can only be studied using local probe microscopy (SPM) techniques such as STM. AFM. and related methods that have been developed during the past fifteen years.6-14 The first topic of this paper deals wilh theoretical and experimenlal results of the formation of low-dimensional Me phases on foreign substrates. The second topic contains the current state on nanOStOlcturing and nanomodification of electron-conducting solid surfaces under defined electrochemical conditions at solid/liquid interfaces using different in situ local probe methods. The role of surface inhomogeneilies in Me phase formation and local slOlcluring processes will be discussed.
(2)
where q is the relative specific ionic charge (charge density). r is the relative surface excess concentration of the adsorbed Me species. and IJ. denotes the chemical potential of Me:· in the electrolyte. In absence of cosorption or competitive sorption phenomena or at a constant relative surface excess concentration of anions in the interphase within a selected UPO range. the electrosorption valency is equal to the ionic charge of Me:"(-y = 0 denotes a supersaturation and O~.D < 0 an undersaluralion with respect 10 the corresponding condensed ;0 Me phase. 'Therefore. lhe potenlial differences E - £.DM~ can be defined as: £ - £.DM~ ~
t2
i
(b)
1.2.3
(7)
representing Ihe underpotential and the overpotemial with respect to the equilibrium potential of the corresponding condensed iO Me phase. As seen. the underpolenlial o£m and the overpotemial "liD are conditional and depend on the reference equilibrium potential £iOM•. Low-dimensional Me phases (i "" 2.1) are expected to exist on an ideally polarizable and fortign substrate in the undersalUration range E > E30M~. if the binding energy of Me adatoms on S is stronger Ihan Ihal of Me on the native substrate:
'I'Mo... _
(e)
(8) In this case. assuming an atomically flat and homogeneous substrate sur· face withoul any surface inhomogeneities. S becomes modified by an expanded and/or condensed 20 Me phase in the UPO range .6.£30 with respect 10 the condensed 3D Me phase. A 3D Me phase is formed on lOp of the 20 Me phase of the UPO modified substrate surface in the OPO range following either a Frank-van der Merwe or a Stranski·K.rastanov growth mode as schematically illuSlrated in Figures l.Ia and I.2b. 1 The growth mode depends on the crystallographic misfit between Me and S. which is defined by
Figure 1.2 Schemalic representation of different growlh modes in melal (Me) deposilion on foreign subStTale (5) depending on lhe binding energy of Me3d~ on S. "'Me.... _ s. compared 10 lhat of Meads on nalive substrale Me. ~M.No Me. and on the cryslallographic misfitf - (dO.M.-do.s)/dOS. characterized by the inlcralomic distances dO.M~ and dos 000 Me and 5 phases. respectively. (a) "Fl1lnk-van der Mcrwc" growlh mode (Me layer.by-layer formation) for '¥M~.. -S» '¥M'.. -M~ andf. 0; (b) "StranskiKraSlanov" growth mode (3D Me island (ormalion on lop of prcdcposiled 2D Me overlayers on 5) for +M¢.. -S» +M•• _M~ andf~ 0; (c) "Volmer-Weber" growlh mode (3D Mc island (onnation) for +101..... -s « i'M~.. _ M~ independent off. The fonnalion of the firsl Me UPD monolayer in (a) and (b) is indicated by a darker color.
8
LOW-DIMENSiONAL METAL PHASES AND NANOSTRUCTURING
f=
dO,Me - do. s
do. s
1.2 roRMATION or: Low·DIMENSIONAL MI::JAL PHASES
,
(9)
Bare stepped~,:""~j;~L.. _ _~::;:--::;;;'1L.. substrate ..
""'::tI
where dO,Me and do.s denote the atomic-nearest neighbor distances (alomic diameters) of 3D Me and S phases, respectively.
In the opposite case, ( 10)
3D Me clusters are fonned on UPD unmodified substrate surfaces at supersaturation l'/30 with respect to the 3D Me phase according to a Volmer-Weber growth mode independent off as demonstrated in Figure 1.2c. However, real substrates with surface inhomogeneities of different dimensionality (1;0 with i = 0, I,2) can induce a stepwise [annalian of low-dimensional Me phases under the condition
UJ
-~ .~ C
0where "'MeadS-I,D (i = 0,1.2) denotes the binding energy of Me adatorns on an
iD surface inhomogeneity of the foreign substrate S. "'Me>'1e phase on a stepped foreign substrate S. (3) 2D nucleation on atomically Ital terraces of S: (b) 20 nucleation al monalOmic steps of S: (c) 2D nucleation at monatomic step comers of S.
•
htlnd, i' Me~ _ SI¢P "" 'i'MCod> _MCOlb yields dG eo " S,ep ... O. and (he fonnation of a condensed 2D Me 3dl phase occurs without activation energy for 20 nucleation
along Ihc sleps. If '¥MCadI-Slcps »'I'Me....
Meads'
a formation of aID Me phase
at monatomic steps becomes thermodynamically possible in Ihe undersaturation range £ > E2DMe (with respect 10 Ihc condensed 2D Me phase). The width of such an 10 Me deposit fOnJ1Cd al a monatomic step is not necessarily restricted to montllomic dimensions, bUI rather depends on the step activity as previously mentioncd. These effects play an important role in step decoration processes in Me UPD syslCms at relatively high underpotetllials.
J.2. J.2 S/Ibstrate Structure alld Low-Dimellsiollal Me Phases The assumption of atomically flat ideal substrate surfaces is rather unrealistic. Real crystals are always imperfect in some respect and exhibit various structural imperfections of different dimensionality:
• OD or point imperfeclions (atomic disorder. chemical impurities. elC.) • 10 or line imperfections (edge and screw dislocations. etc.) • 20 or planar imperfections (grain or subgrain boundaries. stacking faults, ctc.) • 3D or volume imperfections (crystal domains with different chemical and/or physical properties). The crystal surface, which can be considered as a 20 crystal imperfection, plays an important role in the process of phase formation and crystal growth.
The heterogeneity of a real crystal surface can be welt characterized on a nanometer scale by in si!U local probe techniques. Emergence poinls of screw dislocations, monatomic steps separated by atomically flat terraces, 20 islands and pilS, and so on. can be directly observed under defined electrochemical condilions as illustrated in Figure 1.6. The atomic struc!Ure of flat terraces and monatornic steps can be imaged by in situ local probe melhods with lateral alomic resolution as shown in Figure 1.7. The struc!Ure of low-dimensional Me phases depends on the following parameters: the vertical Meod$ - liD interaction energy, .... Me.... -l;o, the lateral Meads - MeiliAl METAL PHASES
23
lOW-DIMEf'SIONAl METAL PHASES A"O NANOSTRUCTURL'IG
low-dimensional Me phases exist tentiat ranges.
Weak Me-S Interaction
1.2.2 Stability Range of 3D Me Phase
No low-Dimensional Me Phases
Underpotential
Overpotential
Strong Me-S Interaction
~
l'l 550 mY; (c) condensed commensurate Ag overlayer with an AU(IOO)-(1 x I) Ag quadratic Structure at AEjD S 200 mV,l.3),)s
LOW.DIMENSIONAL METAL PIIASES A.."":O f',lANOSTRUcrURJNG
28
1.2
FOR.... ATION OF LOW·DIMENSIONAL MI;.'AL PIIASES
images show only the Au(lIl)-(.,J3x..J3)R 300 HSO; sublauice as iIIusmucd in Figure 1.15b. 26 The (onnalion of diffcrenl condensed 20 Me phases lakes place 31 '110 via a first order phase transition and can be indircClly detected by electrochemical 1l1clhods and directly observed by in situ SPM. As expected. condensed
i.b) :
1.00
2.00
• • • •
•• •
•• •• • • • • • • • •• • • •• • • • • • • •• • • • •• • • • •• • •• • • • • • • •• • • • • • • • •• "• • •• •• • • • • ••• •••• • •
.'
.' :
0 0
•
3.00
7~
~
(a) 0.0
'.00
.
.,
·
0.3
AE.M
'1
(
3.00
3.00
1.00
•
o 1.00
2.00
7~
Figure 1.15 Cyclic voltammogram Ind
(b) i"igurf 1.14 In situ STM images or20 Pb domains wilh cxpandt'd :-8(100)«2 x 2) Pb and Au(IOO)-c(2 x 2) Pb 5lJUCnrn:s. (a) system A&(100)/5 x 10- M Pb(CIO"h .... 10-2 M HeIO.. al T,.. 298 K and 4£30 2 175 mV; (b) system Ag(100)/5 x 10- 3 M Pb(CIO"h + 10- 2 M HeIO" II T = 298 K and ~£3D '" 550 mV.l.J6
7~ In
situ STM images in (he system
Au(lIl)jIO-J M CUS04 .. 5 x 10- 2 M H2S04 at T:: 298 K. (a) atomic scroclUre of the bare Au( 111 J substrate at 300 mY :S 6t'}D :S 500 mY; (b) AU(IIIHv3 x v3)R 30" HSO; overlayer structure at 100 mY:S 6EJo:S 220 mV scabilizing the Au(III)2(,f) x v3)R 30"' Pb honeycomb slnKturc. "'hich is IlOl detectable by in SIlU STM; (c) Condensed commensUI'111C Cu ovcrlayer with an Au(JIIHI x I) Cu suucture at 6E}o :S 20 mY. By courtesy of D. M. Kolb.26
3.
1.2
LOW·DlMF.NSIONAL METAL PHASES AND NANOSTRUCTURING
20 Me phases with commensurate S(hkf)/-(Ix I) Me structures are formed in systems with either negligible or negative crystallographic misfit. As examples. the Au(IOO)-(lx I) Ag and Au( I J I)-(Ix 1) Cu structures are shown in Figures 1.I3c and 1.15c. respectively.'.26.JDS According 10 theory. condensed and iSOlropically and anisOlropically compressed incommensurate 20 Me phases with closed packed 20 hcp $ICUClUres are formed in systems with a significant positive crystallographic misfit.' As examples. the jSQ(ropically compressed Ag(III)-hcp Ph and the anisotropically compressed Ag(lOO)·hcp Ph structures are shown in Figures 1.16 and 1.17. respectively.l·21.36.J7 The linear dependence of the interatomic distance of Me atoms in condensed and compressed overlay. ers on '120 according to eq. (15) is demonstrated in Figure 1.18 for the Ag(III)hcp R 4.5 0 Pb Structurc.lJ1ln this model system. E20Pb was determined from cyclic voltanunetry and in situ STM measuremenls yielding a value of about E lOPb '" E)OPb + 150 mY as noted in the following discussion.
FORMATION OF LOW-DIMENSIONAL METAL PHASES
31
•••
..
, (.)
,.. L"""01
.•
AJ(1t0l
,
.• ,..
,
•
••• •
••
$11101
15
(b)
.F'igurf' 1.17 Anisolropically compressed 20 hcp Ph overlayer on Ag( 100) observed the system Ag(loo)/S x 10 ) M Pb(CI04n + 10 2 M HCI04 :It T 298 K and ll.£~D '" 80. mY. (~), in situ STM image of the 2D hcp Ph overillyer showing a superSlr~c~ure with mOIre p
(a3)
C
-20
200
V
Coverage of lerraces islands excluded
~
~
E - CJO Pb [mV] Figure 1.20
strained in the potential range -150 mY $1120 $: -50 mY as demonstrated in Figure 1.18. The peak 0) is related to the depletion of the lOp parts of step edge regions only. The dissolution of Ihe condensed 20 Pb phase on top of isolated islands and on flat lerraces occurs in peak 02. The peak 0 1 reflects lhe dissolution of lhe step decoration. This interprelation was possible only recently on the basis of in situ SPM resUlts. 4o.4l The fonnation of the ID Pb phase (decoration of monatomic sleps) and the initial Slage of the formation of the condensed 2D Pb phase starting at monatomic steps is shown by the in situ STM line scan in Figure 1.22 corresponding to the stages (a1) or (d3) in Figure l.2l. The nucleation and growlh of the condensed 20 Pb phase strongly depends not only on the supersaturation. but also on the substrate surface morphology as illuslrated by in situ STM line scans combined with slep polarization experimenls in Figure 1.23. Theslep polari7.ation routine used in these experiments stans from an adsorbate-free initial Slale at 6£10 10 low and high supersaturations wilh respecl to lhe 20 Pb phase al '11~6 and at '11~6, respectively, and vice versa.
c:
~
300
Cyclic vohammogram measured with a scan rUle of IdE/drl = I mV S-l in lhe system Ag(IJl)/5 x 10- 3 M Pb(CIO~n + 5 x 10- 1 M NaCIO~ + 5 x 10- 3 M IICI04 3t T " 298 K. An and On with n '" J .2.3 denote cathodic adsorption and anodic desorption pea),;s. respectively. Equilibrium potentials. £iOPb (i = 1.2,3), of the corresponding iD Pb phases as well as the overpotential. 1'/20, and the underpotcntiaJ, 6£20. with respect to the condensed 20 Pb phase. are indicated in the upper abscissa.
Dissolution of step decoration
1
regions of step edges and on islands
VI VI
.-0 (d2)
~
Depletion of top pan regions of slep edges~
0,
11
~
....::;, 0
-~~--...,r0~
A3 Deposition on lop part
0
-
Dissolution from terraces islands Included
. l:----------c-:;:,
-30
100
(d4)
(d3)
~
rJI
-10
-40
Bare surface
(a2)
0
~,
Stage
~
-"
~S"P d,,,,,,,tion 0
35
__ ~~_);:~~~-'~'~-~-'-'~"c"c"~'-'--!!"","--' P;l TltfTae8 " "........ , TemIC.
n
c:
0
•
!-i- AE
20
~
FORMATION OF LOW·DIMENSIONAL METAL PHASES
O2 :
30
"E
1.2
F
Completed monoIyer
tJ
4
Figure J.21 Schemmic representation of Ihc slepwise deposition and dissolution or low-dimensional Pb phases in the system Ag(11 J)/Pb 2... ClO;.
I I
-
I Xl...
"/"
E20 ... E ID ...
0","
20nm
Potential
Figu~ ~.~~ In shu STM line SC3n plot showing the StCp decoration by a 10 Pb phase and the Imllal stage or 2D phase rOrmation on a stepped Ag( 1I I) substrate in Ihe system 3 Ag(lll)/4 x 10- M Pb(CI04h + 10- 2 M HCl04 at T = 298 K. B)' courtesy or H
SicgeOlh3Icr.40·4l
.
36
LOW.DIMENSIONAL Ml:"TAL PHASES AND NANOSTRUCTURING
-.-.oi!l- ----~~ -
Is. o~
"'_.-=-:,:",--~
------
25nm
",m
•
~
--
FORMATION OF LOW·Dl.\1ENSIONAL METAL PHASES
37
The covering of atomically smooth lerraces of a stepped surface al '1~~ and 1j~b is shown in the lower pan of Figure 1.233 corresponding to the stages (33) and (d