Top-on-top monolayer formation of foreign metals on gold single crystal surfaces

0013-46S6/92 $5.00 + 0.00

Eknochbnica Acta. Vol. 37, No. 12, pp. 2231-2244, 1992 Printed in Great Britain.

Q 1992. F’ergamonPress Ltd.

TOP-ON-TOP MONOLAYER FORMATION OF FOREIGN METALS ON GOLD SINGLE CRYSTAL SURFACES H. J. PAULING* and K. JijrrN@ *Institute of Physical Chemistry and Electrochemistry III, University of Karlsruhe, 7500 Karlsruhe 1, Germany TDECHEMA-Institute, Theodor-Heuss-Allee 25, 6G90 Frankfurt 15, Germany (Received 30 January 1992) Ah&met-The sequential deposition of two metals Me’ (Me’ = Ag) and Me” (Me” = Pb or Tl) on polyand single-crystal gold substrates was studied using the dipping technique and potential sweep voltammetry. In a first step, silver was deposited in submono-, mono-, or multilayer amounts under extreme diffusion contolled conditions at tixed under- and overpotentials. Using ex situ STM measurements, these thin silver films were found to be very regular even on an atomic scale, with a structure identical to that of the underlying gold substrate. In a second step, the deposition of Me” was performed cychc-voltammetrically in the corresponding underpotential region. From the peak structure of the Me” desorption voltammogram it can be concluded that the Ag fdm is epitaxially deposited. After the deposition of 2-3 silver monolayers the gold single crystal substrate is totally transformed into a silver singlecrystal substrate. At lower degrees of silver coverage the Me” voltammograms exhibit contributions which are typical of both gold and silver substrates. If the sequential order of deposition of both metals is reversed, which means the silver is plated subsequent to the formation of a monolayer of Me” adatoms, the desorption spectra exhibit a structure which is typical of the upd of Me” on a Ag substrate, which means that the Ag atoms must have penetrated the Me” layer. Key ivorcis:upd, thin films, monolayer, single crystal surfaces.

1. INTRODUCTION

During recent years, the phenomenon of underpotential deposition (upd) of metals Me’ on foreign metal substrates Me has been studied intensively on almost any possible substrate/adsorbate combination (Me/Me’) in aqueous and non-aqueous solutions. The thermodynamics and kinetic of the upd process as well as the structural and electrocatalytic properties of the metal depositions have been elucidated using poly- or monocrystalline substrates. There are several review papers where many of the results have been summarized[ l-51. In the last few years, a renewed interest in upd has been observed, stimulated by the invention of modem surface analytical in situ techniques, eg GIX, SEXAFS, STM, which in situ gain direct information on the structure, morphology and composition of electrode surfaces in contact with an electrolyte on an atomic scale[6-141. In general the results obtained are not only of theoretical interest for surface physics and chemistry, but also of practical importance for microgalvanic techniques or electrocatalysis. Most of the upd studies are concerned with the deposition of a single metal component on defined substrates. There are only a few investigations which consider the possibility of the deposition of multi-component systems on polycrystalline substrates[l5-17. Investigations of that kind may be interesting from different viewpoints for microsystem technology[l8], new electromagnetic storage devices and electrocatalysis. Ultrathin composition modulated structures and alloys have gained increasing importance in

materials science as functional coatings due to their unique and unconventional mechanical, electromagnetic and catalytic properties. Most of these materials are manufactured today by UHV sputtering techniqttes. However, “composition modulation alloys” can also be obtained by electrochemical deposition[l9,20]. Depending on the strength of specific interactions between substrate Me and adsorbate Me’ and the lateral interaction between the deposited metals Me’, Me”, . . , , different types of composite layers may possibly be realized on the basis of upd: (i) monolayer formation with statistically distributed metal atoms Me’ and Me” (Fig. la); (ii) monolayer formation of well separated 2-D domains of Me’ and Me” (Fig. 1b); (iii) formation of a sandwich layer structure of metal Me” on top of metal Me’ (Fig. lc); and (iv) formation of a 3-D alloy of either ideal mixture or defined stoichiometric composition (Fig. Id). An early paper of Schmidt and Gygax[ 151 reports on simultaneous upd in binary systems using a chamber type cell. From the overall charge amounts and the characteristic structures of the upd voltammograms they concluded that partial coverage of the surface by the more electropositive metal component leads to a blocking of surface sites for the deposition of the more electronegative component. This is consistent with the idea that the total adsorption capacity of the substrate is a 6nite quantity. Simultaneous upd of Pb and Tl on polycrystalline Ag substrate has also been studied by Bharathi et a1.[16]. They used a mixture of citrate and sulphuric acid to shift the upd region of both metals, due to

2237

H. J. PAULINGand K. J~&INZR

2238

a)

b) 4

with the more electropositive component Me’ = Ag and the more electronegative component Me” = Pb or Tl. Applying the dipping technique[21,22] the metal Me’ was deposited under semi-infinite linear diffusion controlled conditions prior to the upd of Me” in the same solution. The experimental technique used is much closer to future technical application. The use of single-crystal substrates provides additional information on structural correlations between the substrate and the deposited layers.

d) Fig. 1. Possible structure of layers which may be formed by adsorption of two different metals Me’ (0) and Me” (0) on the substrate Me (B).

complexation, into a potential range where both metals are codeposited simultaneously and bulk deposition of either does not occur. They found that upd of Pb and Tl lead to the formation of one monolayer composed of Pb and Tl atoms adjacent to each other. This corresponds to the finding of Schmidt and Gygax[lS] of a finite adsorption capacity. Using a more sophisticated twin-electrode thinlayer technique, Stucki[l7] used upd of Me” as a microprobe to study the effect of submono- up to multilayer amounts of Me’ deposited on polycrystalline Me substrates. With increasing amounts of Me’ the properties of the pure Me substrates were continuously tranformed to that of pure Me’ substrates. At Me‘ coverage below &, x 1.6, upd of Me” revealed the characteristics of a substrate surface composed of Me’ and Me. The deposit of the more electropositive metal Me’, even at submonolayer coverage, does not block the adsorption of the more electronegative metal Me” on top leading to the formation of sandwich metal monolayers. The aim of the present paper was to study the formation of two component metal films on Au single crystal surfaces in the system: Au(hkl)/Me’/Me”;(hkl)

= (11 l),(lOO)

2. EXPERIMENTAL The electrochemical measurements were performed in a three-compartment DUKAN glass cell held at a constant temperature of 298 K using a HAAKE L D8 thermostat. The experiments were carried out in two systems of the following compositions: I: 0.5 M NaClO, + lo-’ M HClO, + X M AgClO, + Y M Pb(ClO,), II: OSM

HClO.,+XM

AgClO,+

Y M TlClO,

with X = 1O-3-1O-5 or 0 and Y = 5 x 10m3or 0. The solution was prepared from suprapure reagents (Merck) and four-fold distilled water and was purged with purified nitrogen. As the metal perchlorates are not commercially available in suprapure quality, they were prepared from the corresponding suprapure carbonate salts and HCIO,. The working electrodes (we) were made from polyand single-crystalline gold (Au 99.999 wt%) cylinders with a diameter of 5 mm. For the dipping technique[21,22] they were mounted in glass tubes which could be moved vertically into the cell. Surface pretreatment was performed by means of mechanical and electrochemical polishing. Mechanical treatment consisted of wet felt polishing with 600 grit Sic and red felt cloth with 7, 1 and 0.3 pm diamond paste. Finally the gold electrodes were anodically polarized

Fig. 2. Experimental arrangement used for measuring sorption spectra in mixed upd and opd systems.

Top-on-top monolayer formation

2239

-E/mV A r&.&“Z+ -------------

o Me’ EWIMe.+ .---l

Me”

time t Fig. 3. Schematic polarization routines with the three possible waiting potentials (E,, E,, and En,) and the corresponding waiting times (I,. t,, and t,,,) at which the deposition of silver takes place.

for 10 min in 1 M Na, SO, solution at 100 mA cm-* to form a thin film of Au oxide which was removed by chemical dissolution in 5 M HCl. A platinum sheet (2cm*) separated from the main cell by glass diaphragms served as the counter electrode (ce). For the metal deposition experiments, the reversible Me/Me’+ electrode was used as reference electrode (re) in the corresponding solution. In particular, in system I a Ag wire was used at X # 0 and Y # 0, whereas a platinum sphere covered with a Pb deposit was used at X = 0 and Y # 0. In system II a conHg/Hg, SO.,, Na2S0, (sat.) electrode ventional with a Luggin capillary was used as re because

of the instability of the Tl/Tl+ electrode in acid solution. The electrochemical experiments were controlled by a newly developed digital potentiostat based on an IBM compatible computer with 286 CPU and an A/D-D/A combi-card for current and voltage measurements. The potentiostatic regulation of the cell was controlled by a software feedback routine. The complicated cycle voltammetric polarization routines and data acquisition, numerical integration, data presentation and storage were provided by software specifically developed by the author. A scheme of the experimental acquisition set-up is

Fig. 4. Monatomic steps on a Au( 111) single-crystal electrode visible after electrochemical polishing.

H. J. PAULINGand K. JiWruaa

2240

Fig. 5. Atomic structure corresponding to a Au(ll1) single crystal surface visible at a higher magnification on the monatomic steps. shown in Fig. 2. The general polarization routines employed for the investigation of the sequential deposition of two different metals Me‘ and Me” are shown in Fig. 3. Depending on the choice of the waiting potentials EW, and Ew,u and the waiting times t, and f,,, a defined coverage of Ag on the gold substrates can be achieved either at underpotentials (E > EABIAg+) with Ag coverages eae < 1 or at overpotentials (E < EAti,,*+)with eAg> 1. For a Ag+ concentration of X = 10e5, the deposition of Ag at under- and overpotentials takes place under extremely diffusion controlled conditions. Therefore, the degree of coverage can be easily controlled even at overpotentials by variation of t,,. The characteristic time for the formation of one Ag monolayer is about 20 min. The subsequent deposition of the metal Me” takes place at potentials Eies,Mc.r+< E < ENA*+ with a negligible additional denosition of AQ at these notentials. Due to the “strongly retarded deposition of Ag, it is also possible initially to deposit the metal Me” and to study subsequently the slow deposition of Ag under defined waiting conditions at and Ew,rn till. STM images of the electrode surfaces were made with a NanoScope II (Digital Instruments Corp., Santa Barbara, CA).

smooth, exhibiting monatomic steps (Fig. 4). At a higher magnification the atomic structure corresponding to the gold single-crystal surfaces became visible (Fig. 5). The STM images in Figs 4 and 5 are EH ImV 9650

950

1050

1250

200

100

600

l1 -90

E-EAgIAg+Imv E,lmV 19650

650

1050

AullllllAg' 9

2

1250 ol

L

0

t ._

3. RESULTS

AND DISCUSSION

-9 s

3.1. Ex situ STiU surface examination of the gold

electrodes Before the single-crystal electrodes were used for the measurements, the surfaces of the electrodes were examined by ex situ STM. After the electrochemical polishing the electrodes were found to be very

_K

R

-w.

200

*I b) 600 100 E-E nslAs*Imv

Fig. 6. Voltammograms of the system Au(hkl)/S x lo-’ M AgClO,+0.5MHClO,.(a)(hk/)=(1OO);(b)(hk/)=(111). dE/df = 10mV s-l; T = 298 K.

Top-on-top monolayer formation

Aulpoly)/Ag*

-161

0

600

600

1000

1200

I

2241

The sorption spectra on the polycrystalline substrate appear as superpositions of the sorption spectra on the (111) and (100) faces. There was no indication of the formation of a silver-gold alloy at tmderpotentials. Neither the shape of the peak structure nor the current integral was changed after long time polarization at E > E, + 10 mV. From integration of the potential current-density curves, it is possible to calculate the Ag coverage of the gold substrate:

1100

EH /mV

@Ag = :,

YS

Fig. 7. Voltammograms of the system Au(poly)/X M AgClO,+O.S M HCIO,. (-) X = lo-‘; (---) X = IO-$ c..) X = 5 x lo--‘. dE/dt = 10 mV s-‘; T = 298 K.

representative of the whole surface and are not only singular spots. Similar results were also found on the Au( 100) substrate. This proves that the electrochemical polishing technique used was very effective for the preparation of the gold single-crystal electrodes. 3.2. The single-component system Au(hkl)/Ag+ Knowledge of the sorption spectra of the corresponding single-component systems is a prerequisite for the interpretation of the sorption spectra obtained in the two-component systems. The upd systems Au/Pb2+ [23-251; Au/Tl+ [25]; Ag/Pb2+ [25-301 and Ag/Tl+[25,31] are well known in the literature for poly- and monocrystalline substrates, but have been reinvestigated in the present work. The single-component system Au/Ag+ was first studied by Lorenz et af.[32] and later by Takamura and Sato[33] using conventional electrochemical cells and by Schmidt and Stucki[34] using the twinelectrode thin-layer technique. In all these investigations only polycrystalline substrates were used. It was therefore necessary to study upd of Ag on Au(hkl) surfaces in the present work. Characteristic voltammograms of the system Au(hkl)/Ag+ are shown in Fig. 6a for (hkl) = (100) andinFig.6bfor(hkl)=(lll).FortheAu(lll)face the sorption spectrum exhibits a single adsorption (A,) and desorption (D,) peak at an underpotential AE = (E - i&As+ ) = 600 mV. In the voltammogram of the Au(100) face two adsorption (A2, A,) and desorption (Dr, D,) peaks are present at AE = 300 and 100 mV, respectively. Figure 7 shows the sorption spectrum of Ag on a polycrystalline gold substrate at three different silver ion concentrations. The three adsorption, desorption peaks are shifted towards negative potentials corresponding to the concentration dependence of the Nemst potential E, of the Ag/Ag+ electrode with decreasing silver ion concentration without affecting the peak structure of the voltammograms. At a silver ion concentration as low as 10m5M the adsorption peaks vanish due to the strongly diffusion limited deposition process at the corresponding sweep rate of u = 10 mV s-‘. However, even under these conditions adsorption equilibrium is attained .and the full silver monolayer is formed after about 20 min polarization at E = E,. The characteristic desorption peaks appear in the subsequent anodic stripping voltammogram, CJ Fig. 7.

where q is the experimental charge from integration and qS is the theoretical charge of the hexagonal Ag monolayer, equal to 220 PC crn2. Integration of the experimental curves, however, gave saturation charge values which are too low for a complete Ag monolayer. The charge corresponds to only one half of a silver monolayer. The reason is that part of the desorption charge is masked by the anodic current peak corresponding to the gold oxide formation at potentials E,, > 700 mV. Schmidt and Stucki[34], using the twin-electrode thin-layer technique, indeed found at polycrystalline electrodes an additional upd peak of Ag at AE z 800 mV. From independent charge q and coverage I measurements they could show that at the reversible potential E, = EAIIAI+the gold substrate is fully covered with one silver monolayer. 3.3. The two-component system Au(hkl)/Ag+/Pb2+ Figure 8 shows a typical cyclic voltammogram obtained in the two-component system Au(l1 l)/ Ag+/Pb2+ with both metal ions Ag+ (X = lo-’ M) and Pb2+ (Y = 5 x 10m3M) in solutions. The potential sweep starting at EH = 1200mV and initially covering the upd range of silver with the adsorption peak A, was interrupted at E,,, = EAglAB+ for t, = 20 min to permit formation of the Ag saturation on the Au substrate surface. On continuing the potential sweep, the adsorption/desorption peaks

-200 60.

0

200

DI -

LOO

E,, ImV 600

600

lOO0

1200

Aumlvb.gwb~’

h

E-Epb,pb”/mV

E -EAg,Ag4mV

Fig. 8. Voltammogram of the system Au(lll)/lO-5 M AgClO,+S x 10-l M Pb(C10,),+0.5 M NaClO,+ lo-’ M HClO,, with 0,, = I; E,,, = EAIIAl+;t, = 20 min. dE/dt = 10 mV s-l; T = 298 K.

H. J. PAULIS and K. Jthruaa

2242 E&mV

(al

-3

AullllWg*/Pbz'

-2

60-

-1

k

A, /D, can be attributed to the upd of Pb on Ag(ll1) and the peaks AZ/D2 to the upd of Pb on Au(ll1). The overall charge of both peaks corresponds to the formation of a Pb monolayer. Its separation into the contributions of gold and silver substrates indicates that formation of a Ag monolayer is obviously insufficient to completely screen the properties of the Au substrate. Another explanation of this effect could be an electronic interaction between the Pb adatoms and the Au substrate through a monolayer of silver; but this is improbable because of the relatively large distance between the Pb and Au atoms of about 300 pm. Interactions of this kind should only cause a slight shift of the adsorption peaks rather than a peak separation of about 100 mV. A possible explanation may be that deposited Ag atoms change places with Au substrate atoms during polarization in the overpotential region of silver, presumably initiated or catalysed by the upd of Pb. This process leaves half of the Au atoms in their original position so that the upd of Pb is split into two parts on Au and Ag atoms.

420.

-20

I.

Au~100~/Pb2~ I . I

-160- A

(cl

E-E,s,,+lmV

-66

Au1100~IA~%b2'

Fig. 9. (a) Voltammogram of the system Au(lll)/ 5 x lo-” M Ph(ClO,), + 0.5 M NaClO, + lo-’ M HClO,. (b) Voltammogram of the system Au(lll)/lO-s M NaClO, + Ph(ClO,), + 0.5 M AgClO, + 5 x lo-” M IO-)M HClO, with three different degrees of coverage: (1) 8,,,= 1; (2) 0*s= 1.5; (3) eAs=2.5, formed at E,,, = EWb+ = 500 mV and E,,,, = 350mV. (c) Voltammogram of the system Ag(lll)/5 x 10m3M Pb(ClO,),+ 0.5 M HClO.,. dE/dt = 10mV s-l; T = 298 K.

AZ/D, and A, /D, appear in the upd region of Pb and before termination of the sweep the characteristic desorption peak D3 arises in the upd region of Ag. It should be noted that bulk silver formation obviously does not occur during polarization at E < ENa+, otherwise an anodic dissolution current should have been observed on passing the reversible potential of the Ag/Ag+ electrode during the anodic half cycle of the sweep. Comparing the voltammograms of the single component systems Ag(l1 l)/Pbz+ and Au( 11l)/Pb2+ in Fig. 9a and c, respectively, with the voltammogram in Fig. 8, it is obvious that the peaks

E,/mV Fig. 10. (a) Voltannnogram of the system Au(lOO)/ 5 x 10-s M Pb(ClO,h + 0.5 M NaClO, + IO-) M HClp,. (b) Voltammogram of the system Au(l(KIfI; 7 AgClO, + 5 x lo-) M Ph(C10,)2 + 0.5 M lo-’ M HClO, with a silver coverage of 9, = 2.5 form! at Ew,,‘4slA + = 500 mV and &,, = 350 mV. (c) Voltammogram ots the system Ag(lOO)/5x 10m2M Pb(ClO,), + 0.5 M HClO,. dE/dt = 10mV s-l; T = 298 K.

Top-on-top monolayer formation AE= 50mV 17

&/mV

2243

predeposited at a potential EH= ENlre+ (Fig. lla) and at EH = - 150 mV after formation of the Pb monolayer (Fig. 1lb). The upd desorption spectra of Pb exhibit nearly identical shapes, indicating that the silver atoms deposited must have penetrated the lead monolayer and built a composite gold silver substrate identical to that formed in the absence of Pb on the gold surface. However, the entire upd peak structure is shifted by about 50mV to more positive potentials, indicating stronger interaction of the lead atoms with the composite surface. 3.4. The two-component system Au(hkl)/Ag+/Tl+

Fig. 11. Voltammograms of the system Au(lll)/lO-5 M AgClO,+5 x IO-‘M Pb(ClO,), + 0.5 M NaCIOa + lo-’ M HClO, with Ba = 1. (a) ‘The silver layer was beuosited at E, = 350 mV!nhe ..I v 1.At that ootential the cold electrode was not covered with lead adatoms. (b) The &er layer was deposited at E,,,= - 150mV/nhe [ v]. At that potential the gold electrode was covered with a monolayer of lead adatoms. dE/dr = 10 mV s-‘; T = 298 K.

It is possible to continuously increase the amount of Ag deposited on the Au substrate by additionally polarizing the electrode at overpotentials &,t = - 150 mV for a defined waiting time l,, . The E ABIAg+ results of such an experiment are shown in Fig. 9b. With increasing amounts of Ag the peaks AZ/D2 characteristic of the upd of Pb on Au(ll1) continuously decrease, while the peaks A, /D, , corresponding to the upd of Pb on Ag(1 1 1), increase. At a silver coverage of eAgx 2.5 the peaks A,/D, disappear almost completely (Fig. 9b) and the Au( 111) substrate appears to be totally transformed into a Ag( 111) substrate. This behaviour is in accordance with the results of Stucki[l7] on polycrystalline substrates and also with electroreflectance measurements on Ag deposits on Cu(ll1) substrates by Kdtx and Kolb[35]. The same behaviour as described for the Au(ll1) substrate is also found in the case of the Au( 100) substrate, as documented in Fig. 1Oa-c. For comparison, the voltammograms of the single component Pb upd system on the Ag-free substrates are shown in Fig. 10a and c. The voltammogram of the two-component system in Fig. lob was obtained after deposition of silver corresponding to a total coverage of 0,, = 2.5. The upd voltammogram of the two-component system in Fig. lob has nearly identical structure and peak positions compared with the single-component system Ag(lOO)/Pb*+ in Fig. 10~. This again clearly indicates the formation of a sandwich-type structure consisting of an epitaxial silver layer and a monolayer of lead on top. The experimental technique used also permits the deposition of the more electronegative metal Pb in the initial step by switching the potential immediately to Ew,,,, = EPblPW+ . The amount of Ag deposited at . overpotentials can be controlled varying the . . waiting time t,,,. Figure 11 shows two voltammograms with the same amount of Ag (eA8= 1) w

37,12--K

The same experiments as described for system I in Section 2.3 were also carried out in system II with Me” = TIC instead of Pb*+. The results are very similar to those obtained with Pb. At silver precoverages OAg= 2.4 the upd voltammograms of Tl are composed of both contributions of the single component systems Au(hkl)/Tl+ and Ag(hkl)/Tl+, except that the peaks D*, A*, which represent the formation of a second Tl monolayer on Ag substrates at low underpotentials, are not prominent when the silver coverage is below eAs= 1. At silver coverages OAg2 2.4 the properties of the gold substrate disappear almost completely and the upd diagrams of Tl+ become comparable with those of the Ag(hkl)/Tl+ system. This is shown, for example, in Fig. 12 for the Au( 111) single-crystal electrode. The two-component system (Fig. 12a) not only exhibits the sorption peaks A,-A,, D,-D, of the first Tl monolayer, but also the

EH/mV -350

50-550

-150

50

150

350

550

0’

LO-

-LO-

a) _

\A.

100 t

I01

ID’

200

Agllll)/TI’

LOO

600 E-ET,,,,.

80

1000

I

1200

I

I:V

Fig. 12. (a) Voltammogram of the systems Au(1 11)/10-5 M AgClO, + 5 x 10-s M TIClO, + 0.5 M HClO, with a silver coverage of 0_,s= 2.4 formed at E,,, = Ewc+ = 500 mV and E ,,,I1 = 350mV. (b) Voltammogram of the system Au(l11)/5 x IO-’ M TICIO, + 0.5 M HClO,. dE/dt = 10 mV s-l; T = 298 K.

H. J. PAULINGand K. Jtirr~~~

2244

peaks A*/D* of the second Tl monolayer are prominent at lower underpotentials. Indications of alloy formation between the metals Me’ and Me” were not found either in system I (Me” = Pb) or in system II (Me” = Tl).

4. CONCLUSIONS

9. W. J. Lorenz, L. M. Gassa, U. Schmidt, W. Obretenov, G. Stailcov,V. Bostanov and E. Budevski, Electrochim. Actu 37, 2173 (1992).

10. R, J. Behtn, Structure effects in the initial stages of Cu deposition on single crystalline Au electrodes, 5th Znternational Fischer Symposium on Adsorbares, ates and Inhibitors, Karlsruhe (1991).

11. H. Siegenthaler, Investigation of substrate reconstruction and lilm formation nrocesses bv in situ STM. 5th International

From the experimental results obtained in the two-component systems Au(hkl)/Me’/Me” with Me‘ = AgC and Me” = Pb2+ or Tl+, it can be concluded that Ag is epitaxially deposited on the gold single-crystal surfaces forming a very regular and non-porous film of a thickness from 0.2 up to 5 monolayers. The crystallographic structure of the substrates was reproduced. This is clearly reflected by the characteristic peak structures of the corresponding upd voltammograms of Pb or Tl. At low degrees of Ag coverage the voltammograms exhibit contributions of both Au and Ag substrates. Formation of the saturation coverage of Ag at the reversible potendoes not fully screen the interactions of Pb tial EAgiAe+ or Tl with the Au substrate. At a thickness of the silver film of approximately 2-3 monolayers, the upd diagrams resemble the typical shapes of the corresponding Ag(hkl) substrates. authors are indebted to the Arbeitsgemeinschaft Industrieller Forschungsvereinigung (AIF) and the Bundesministerium fur Wirtschaft for financial support of this work. Acknowledgements-The

REFERENCES 1. W. J. Lorenz and H. D. Hermann, J. electrochem. Sot. 121, 1167 (1974).

2. D. M. Kolb, Physical and electrochemical properties of metal monolayers on metalic substrates, in Adounces in Electrochemistry and Electrochemical Engineering (Edited by H. Gerischer and C. W. Tobias),Vol. 11. John Wilev. New York (1978). 3. K. Jiittner-and W. J. Lorenz,‘Z. phys. Chem. NF 122, 163 (1980).

4. R. R. Adzic, Electrocatalytic properties of the surfaces modified by foreign metal adsorbates adatoms, in Advances ttt Electrochemistry and Electrochemical Enaeneerinn (Edited bv H. Gerischer),,. Vol. 11. John

Wiey, New York (1978). 5. K. Jtittner, Electrochim. Acta 31, 917 (1986). 6. H. D. Abruna, X-Rays as Probes of Electrochemical Interfaces, Modern Aspects of Electrochemistry (Edited by J. GM. Bockris, R. E. White and B. E. Conway), Vol. 20. Plenum Press, New York. 7. 0. R. Melroy, M. F. Toney, G. L. Borges, M. G. Samant. J. G. Kortriaht and P. N. Ross. J. electroanal. Chem. &, 403 (1989). 8. D. M. Kolb, Structure studies with atomic scale resolution, 5th International Fischer Symposium on Adsorbates, Intermediates and Inhibitors, Karlsruhe (1991).

Zntermedi-

Fischer Symposium

onAdsorbates,

Znter-

__ mediates and Inhibitors, Karlsruhe (1991). 12. H. D. Abruna, In situ structure and compositional studies of electrochemical interfaces with X-rays, 5th International Fischer Symposium on Adsorbares,. Zntermediates and Inhibitors. Karlsruhe (1991). 13. J. G. Gordon, In situ.determination of the structure of upd metal monolayers by X-ray diffraction, 5th International Fischer Symposium on Adsorbares, mediates and Inhibitors, Karlsruhe (1991).

Inter -

14. G. Staikov, E. Budevski, M. Hiipfner, W. Obretenov, K. Jtlttner and W. J. Lorenz, Surf. Sci. 248,234 (1991). 15. E. Schmidt and H. R. Gygax, Helv. chim. Acta 49, 70 (1966).

16. S. Bharathi, V. Yeguaraman and G. Prabhakara Rao, Electrochim.

Acta 56, 1291 (1991).

17. S. Stucki. J. electroanal. Chem. 78. 31 (1977). 18. W. Ehrfeld, The LIGA process for micros&terns, in Micro System Technologies 90 (Edited by M. Reichel). Springer, Berlin (1991). 19. E. Budevski, Dechema Monogr. 121, 183 (1990). 20. D. S. Lashmore and P. Davies, J. electrochem. Sot. 135, 1218 (1988).

21. K. J. Bachmann and J. K. Dohrmann, J. electroanal. Chem. 21, 311 (1964). 22. J. W. Schultxe and D. Dickertman, Surf. Sci. 54, 489 (1976). 23. K. Engelsmann, Thermodynamik und Kinetik der Unterpotentialabscheidung von Blei auf einkristallinen Goldunterlagen. Dissertation, Universitat Karlsruhe (1978). 24. K. Engelsmann, W. J. Lorenz and E. Schmidt, J. electroanul. Chem. 114, 1 (1980). 25. D. M. Kolb, M. Przasnyski and H. Gerischer, J. electroanal. Chem. 54, 25 (1974). 26. H. Bort, K. Jtittner, W. J. ‘Lore& and E. Schmidt, J. electroanal. Chem. 90, 413 (1978). 27. W. J. Lorenz, E. Schmidt, G. Staikov and H. Bert, Faraday Symp. them. Sot. 12, 14 (1977). 28. H. Bort, Elektrolytische Abscheidung von Cadmium und Blei an Silbereinkristallfliichen im Unter- und Uberspannungsbereich. Dissertation, Universitiit Karlsruhe (1982). 29. H. Bort, K. Jiittner, W. J. Lorenz, G. Staikov and E. Budevski, Electrochim. Acta 28, 985 (1983). 30. G. Herren, H. Kiing and E. Schmidt, 32. ISE-Meeting, Dubrovnik, Ext. Abstr. B 26 (1981). 31. H. Siegenthaler, K. Jiittner, E. Schmidt and W. J. Lorenz. Electrochim. Actu 23, 1009 (1978). 32. W. J. Lorenx, 1. Moumtxis and E.‘Schmidt, J. electrounul. Chem. 33, 121 (1971). 33. T. Takamura and Y. Sato, J. electroanal. Chem. 47,245 (1973). 34. E. Schmidt and S. Stucki, J. electroanal. Chem. 39, 63 (1972). 35. R. Kiitz and D. M. Kolb, Surf Sci. 97, 575 (1980).