Initial stages of palladium deposition on Au()

Initial stages of palladium deposition on Au(hk l)Part III: Pd on Au(110)

L.A. Kibler, M. Kleinert, V. Lazarescu 1, D.M. Kolb *

Department of Electrochemistry, University of Ulm, 89069 Ulm, Germany

Received 10 May 2001; accepted for publication 16 October 2001

Abstract

The deposition of palladium onto the unreconstructed Au(1 1 0) surface was studied by cyclic voltammetry and in situ

scanning tunnelling microscopy. An ordered adlayer of [PdCl4]2� was imaged with atomic resolution on the bare

Au(1 1 0) surface. Pd deposition starts at monoatomic high steps by forming a layer that grows onto the lower terrace.

Coulometric data point towards the deposition of approximately three monolayer equivalents in the Pd underpotential

region. This high coverage and the presence of holes in the Au(1 1 0) surface after the complete anodic dissolution of the

Pd deposit are explained by surface alloy formation. Furthermore, the palladium overlayers on Au(1 1 0) appear to be

rather rough, because there is no strict layer-by-layer growth. Important aspects of the initial stages of palladium

deposition on the three low-index Au surfaces are summarised and the influence of the crystallographic orientation of

the substrate as well as the effect of different Pd film thicknesses on the electrochemical properties are briefly dis-

cussed. � 2001 Published by Elsevier Science B.V.

Keywords: Alloys; Gold; Metal–electrolyte interfaces; Palladium; Scanning tunneling microscopy

1. Introduction

Studies of the electrochemical deposition ofmetals on well-ordered single crystalline substratesallow for a better understanding of the funda-mental aspects of metal deposition [1,2]. Suchknowledge is of great interest for, e.g. derivingstructure–reactivity relationships in electrocataly-

sis [3], or producing thin overlayers of well-definedstructure.

The deposition of palladium has been consid-ered to be a promising case for basic investiga-tions, because of the high catalytic activity [4].Furthermore, the difficulties in preparing andhandling massive palladium single crystals weretried to be circumvented by the use of ultrathinpalladium overlayers. Indeed, palladium deposi-tion was found to be structure-sensitive: epitaxiallygrown overlayers were obtained on gold and plat-inum single crystal substrates [5,6]. In addition,the electrochemical characterisation of palladiumoverlayers on single crystal substrates was ex-tremely useful for developing simple methods

Surface Science 498 (2002) 175–185

www.elsevier.com/locate/susc

*Corresponding author. Tel.: +49-731-50-25400; fax: +49-

731-50-25409.

E-mail address: dieter.kolb@chemie.uni-ulm.de (D.M.

Kolb).1 Present address: Institute of Physical Chemistry, I.G.

Murgulescu, 77208 Bucharest 6, Romania.

0039-6028/01/$ - see front matter � 2001 Published by Elsevier Science B.V.

PII: S0039-6028 (01 )01684-3

to prepare high-quality palladium single crystals[7].

The deposition of Pd on Au(1 1 1) and Au(1 0 0)has recently been studied by our group, especiallywith regard to the initial stages [5,8,9]. The oxi-dation of small organic molecules on the electro-chemically grown ultrathin palladium films onAu(hk l) was found to depend significantly on thecrystallographic orientation of the substrate andon the film thickness [10,11]. In order to under-stand such relations it is essential to have availabledetailed information about the palladium surfacemorphology. In this respect, recent progress hasbeen made by the use of in situ scanning tunnellingmicroscopy (STM) [5,8,9,12,13] and surface X-rayscattering [14].

In general, Pd is deposited onto Au fromaqueous solutions of its chlorine compounds, e.g.PdCl2 or K2PdCl4. From such electrolytes, priorto palladium deposition, [PdCl4]

2� adsorbs on thebare Au(1 1 1) and Au(1 0 0) surfaces and formsordered adlayers, the structures of which havebeen studied by in situ STM [8,9,12,13]. Thiscomplex anion has also been found to adsorb onthe first Pd monolayer on Au(1 1 1) [8], whereasit is displaced by chloride in the case of Pd onAu(1 0 0) [9]. On these unreconstructed Au(1 1 1)and Au(1 0 0) surfaces, palladium nucleates first atsurface defects like monoatomic high steps, andat higher overpotentials also on terraces [8,9].In the underpotential deposition (upd) region,a pseudomorphic Pd monolayer is formed onAu(1 1 1) [8,14]. No alloy formation was observedfor this system [8], while a Pd/Au surface alloy isformed when Pd is deposited onto Au(1 0 0) [9].Deviations in the electrochemical properties ofthin Pd films on Au(1 0 0) from those of well-ordered Pd(1 0 0) surfaces [10] are most probablycaused by this alloying process.

In this communication, the initial stages of pal-ladium deposition onto unreconstructed Au(1 1 0)are investigated and general trends of palladiumdeposition onto the three low-index Au surfacesare presented. These surfaces after flame-annealingare known to be reconstructed, even in an elec-trochemical environment under certain conditions,e.g. when specific adsorption of anions is avoided[15]. For the present three-part study, Pd was

always deposited at potentials, where gold surfacesare unreconstructed. Structural changes upon thetransition from freshly prepared reconstructed toð1� 1Þ unreconstructed phases have an influenceon the initial stages of metal deposition, since de-fects that arise from such lifting of the recon-struction can act as nucleation centres. However,compared to Au(1 1 1) or Au(1 0 0), there is littleinformation in the literature about the electro-chemical behaviour of Au(1 1 0), probably becauseit is still difficult to prepare large and well-orderedterraces for this relatively open surface. The exis-tence of an order–disorder transition at about 650K [16] restricts the annealing to lower tempera-tures. With the conventional flame-annealing andquenching technique, Au(1 1 0) surfaces are ob-tained with rather small domains and an inho-mogeneous structure [17]. For this reason, we havedeveloped a method [18], which allows to prepareAu(1 1 0) surfaces of high quality. Terraces of upto 100 nm width were obtained and it was possibleto use in situ STM as a tool to study the electro-chemical Pd deposition on Au(1 1 0) at an atomiclevel. The characterisation of a well-orderedAu(1 1 0) electrode in aqueous sulfuric acid solu-tion by cyclic voltammetry (CV) and in situ STMwith regard to reconstruction phenomena will bedescribed elsewhere [18].

2. Experimental

The experimental procedure is similar to thatdescribed in the previous publications [8,9]. TheAu(1 1 0) single crystals were purchased by Ma-TecK (J€uulich, Germany), where they had beenpolished down to 0.03 lm and oriented to betterthan 1�. The electrode used for CV was 3 mm indiameter, with a gold wire attached at its rear forbetter handling. Before each experiment, it wasannealed for 1 min in a propane flame at dim redheat, cooled down for 1 min in air just above asmall beaker filled with Milli-Q water, into whichit was subsequently dipped and finally, protectedby a drop of water, transferred to the electro-chemical cell. For in situ STM, a larger singlecrystal was used (12 mm in diameter). It was an-nealed at about 200 �C for several hours (usually

176 L.A. Kibler et al. / Surface Science 498 (2002) 175–185

over night) in a drying locker. The surface wasthen cleaned by a short high-temperature anneal-ing of a few seconds in a hydrogen flame, cooleddown for several minutes under a nitrogen streamand mounted in the STM cell.

The STM images were recorded with a To-pometrix TMX Discoverer 2010 and are all takenwith the scan direction from top to bottom. Anelectrochemically etched Pt–Ir wire was used asSTM tip. It was coated with an electrophoreticpaint to reduce the Faradaic current at the tip–electrolyte interface below 50 pA.

Pt wires served as counter electrodes in bothcases, but for STM measurements a platinum wireserved also as a quasi-reference electrode. Other-wise a saturated calomel electrode (SCE) was used,and in the following, all potentials are quotedagainst SCE. As mentioned in previous work [8],the Pt reference electrode allows for highly cleanconditions, but suffers somewhat from instabilitiesand hence, the potential values for STM mea-surements are less precise than those obtained withan SCE. In addition, shielding effects due to thethin-layer configuration of tip and sample mayalso cause small potential shifts. The solutionswere prepared from H2SO4 (Merck, suprapur),PdCl2 (Merck, zur Synthese), PdSO4 (Aldrich,98%), HCl (Merck, suprapur) and Milli-Q water(18.2 MX cm, 3 ppb total organic carbon).

3. Results

3.1. Cyclic voltammetry

The bulk deposition of palladium on the un-reconstructed Au(1 1 0) surface from 0.1 MH2SO4 þ 1 mM PdCl2, which starts at 0.52 V, ispreceded by two distinct deposition steps (upd) asindicated by both the cyclic voltammogram andthe charge isotherm (Fig. 1). Each data point forthe charge isotherm (Fig. 1b) represents the totalcharge, recorded during the potential scan with 1mV/s from 0.75 V to selected final potentials be-tween 0.62 and 0.52 V, and holding the electrodepotential there for 20 min (Fig. 2). Obviously, thecurrent drops to zero as long as the potentials areconfined to the upd region. The formation of bulkpalladium starts around 0.52 V, where the depo-sition current is seen to increase after the initialdecline brought about the potential stop (Fig. 2,broken line).

The two upd peaks in Fig. 1a overlap and thusare not well resolved. Nevertheless, there is a clearchange in the deposition mechanism around 0.56V as indicated by the two cathodic peaks inthe cyclic voltammogram (Fig. 1a) and the de-pendence of total cathodic charge on electrodepotential (Fig. 1b). The charges consumed inthe first step and in the whole upd region were

Fig. 1. Pd deposition on Au(1 1 0) from 0.1 M H2SO4 þ 1 mM PdCl2: (a) cyclic voltammograms with different negative potential limits

for Pd upd on Au(1 1 0) (scan rate: 1 mV/s) and (b) maximum charge densities as derived from the current–time curves in Fig. 2.

L.A. Kibler et al. / Surface Science 498 (2002) 175–185 177

found to be about 350 and 800 lC/cm2, respec-tively (Fig. 1b).

Although the first cathodic peak is relativelysharp, its anodic counterpart is far from lookingalike (Fig. 1a, dotted curve). The stripping of thepalladium overlayer, which has been formed in thefirst deposition process, is spread out over a ratherwide potential range. The anodic branch revealstwo very small humps located around 0.65 and0.75 V. However, when the deposition time waslonger, like in the potential-arrest experiments(Fig. 2), only a single dissolution peak is observed,which is shifting continuously towards more neg-ative potentials with lower deposition potential(Fig. 3a and b). On the other hand, the strippingof the palladium deposited in the second step(Edeposition < 0:55 V) takes place in a much nar-rower potential region yielding a pronounced peakclosely located to the bulk phase dissolution (Figs.1a and 3a).

Decreasing the PdCl2 concentration from 1 to0.1 mM is seen to shift the potential of Pd depo-sition and dissolution (Fig. 3b) in positive direc-tion. This is obviously due to the lower freechloride concentration in 0.1 mM PdCl2 solution(for similar considerations, see Ref. [8]). How-ever, it is worth mentioning that variation of the

chloride concentration up to 0.1 M did not changesignificantly the shape of the cyclic voltammo-grams. Contrary to the highly irreversible Pddeposition on Au(1 1 1) and Au(1 0 0) from chlo-ride-free solution, palladium deposited fromPdSO4—at least for low coverage—on Au(1 1 0)can be stripped in the positive-going potentialsweep (see Fig. 4). At higher coverage, Pd disso-lution in sulfate solution is much slower.

3.2. Scanning tunnelling microscopy experiments

Fig. 5 shows a high-resolution STM image of awell-prepared Au(1 1 0) electrode in 0.1 M H2SO4

at 0.55 V. At such a positive potential, the (1� 2)reconstruction is no longer stable and the surfaceis in its unreconstructed (1� 1) state. Atomicallyflat terraces are seen, separated by monoatomichigh steps, which show a pronounced frizziness[18]. Close inspection of Fig. 5 shows even theindividual surface atoms. When PdCl2 is added tothe supporting electrolyte, an ordered adlayer isobserved at positive potentials, where Pd deposi-tion does not yet start (Fig. 6a). The STM imageshows square and rectangular spots arranged inrows, which run parallel to the [1 �11 0] direction.

Fig. 2. Current–time curves for Pd deposition on Au(1 1 0). The potential was scanned with 1 mV/s from 0.75 V to various potentials in

the Pd upd regime and held for 20 min (the potential program is drawn schematically as inset). The broken line indicates the onset of

Pd bulk deposition at 0.52 V.

178 L.A. Kibler et al. / Surface Science 498 (2002) 175–185

The distance of the spots along the row is 8:7� 0:9�AA. Details of this structure will be given in a forth-coming publication [19].

In Fig. 6, a sequence of STM images for theinitial stages of Pd deposition on Au(1 1 0) isshown. Fig. 6a represents the gold surface, coveredby the ordered adlayer before Pd deposition andFig. 6j shows the electrode covered by about a 7ML equivalent of Pd bulk deposit. In between(Fig. 6b–i), the potential of the Au(1 1 0) electrodeis confined to the upd regime. We reemphasise thatthe potential values obtained by the Pt quasi-ref-erence electrode in the STM cell differ slightly buterratically from those measured with the SCE.This is reflected by the observation that 0.49 V still

refers to the upd range (Fig. 6e–i). As expected,surface defects were found to play an importantrole in the nucleation of the Pd on Au(1 1 0). Thepalladium deposition process starts by a decora-tion of the monoatomic high steps, the deposit

Fig. 3. (a) Series of current–potential curves for Pd dissolution

from Au(1 1 0) in 0.1 M H2SO4 þ 1 mM PdCl2 after holding the

potential for 30 min at different values Edeposition in the upd re-

gion (the dotted line corresponds to bulk deposition region,

already, scan rate: 10 mV/s). (b) Plot of dissolution peak po-

tentials Edissolution from (a) and respective data for 0.1 mM PdCl2against the deposition potential Edeposition.

Fig. 4. Cyclic voltammograms for Au(1 1 0) in 0.1 M H2SO4 þ0:1 mM PdCl2 (dotted line) and 0.1 M H2SO4 þ 0:1 mM PdSO4

(solid and broken line). In the latter case, the negative poten-

tial limit was shifted by �0.1 V in the second cycle (scan rate:

1 mV/s).

Fig. 5. STM image (32 nm� 32 nm) for an unreconstructed

Au(1 1 0) surface in 0.1 M H2SO4 at 0.55 V (IT ¼ 2 nA).

L.A. Kibler et al. / Surface Science 498 (2002) 175–185 179

growing onto the lower terrace (Fig. 6b). The de-position process then continues by formation of

two-dimensional islands on the flat terraces (Fig.6c–e). The second layer starts to grow before the

Fig. 6. Sequence of 10 STM images, (a–i) 30 nm� 30 nm, (j) 500 nm� 500 nm, showing the initial stages of Pd deposition onto

Au(1 1 0) from 0.1 M H2SO4 þ 0:1 mM PdCl2 þ 0:6 mM HCl. Deposition potentials as indicated in the figures. The location of one

monoatomic high step in (a) is represented by the white line in (b) and (c) as guide for the eye (IT ¼ 2 nA).

180 L.A. Kibler et al. / Surface Science 498 (2002) 175–185

first layer is completed (Fig. 6f and g) as the ob-served overlapping of the peaks in the cyclic vol-tammogram (Fig. 1a) also suggests. An adlayersimilar to, although significantly less well-orderedthan the one in Fig. 6a is observed on top ofthe grown islands. After about 15 min, depositionat 0.49 V ceased and high-resolution imagesat more negative potentials did not yield any fur-ther information. Large scale images for bulkPd reveal a rather rough and scale-like surface(Fig. 6j).

Complete dissolution of such a rather thick Pdoverlayer at 0.75 V led to the very same adlayerstructure on gold as before Pd deposition (Fig. 7a).However, the Au(1 1 0) surface is now full ofmonoatomic deep holes of different size, whichsuggests that surface alloying had taken placeduring Pd deposition.

3.3. Electrochemical behaviour

The electrochemical properties of thin palla-dium films on Au(1 1 0), which were determined in0.1 M H2SO4 after transfer to another electro-chemical cell, were found to be strongly dependenton the Pd coverage. The latter was estimated fromthe charge flown during the deposition reaction,and proved to be decisive for the voltammetricprofile. This is demonstrated in Fig. 8. The cyclicvoltammograms show two characteristic potentialregions: The region of surface oxidation and re-duction for E > 0:4 V and the hydrogen adsorp-tion and desorption region between 0.1 V and �0:2V. The latter process can be ideally studied withthin Pd films, because hydrogen absorption isshifted towards more negative potentials as com-pared to massive Pd single crystals [5].

Fig. 6 (continued)

L.A. Kibler et al. / Surface Science 498 (2002) 175–185 181

For the case (c)—3 ML Pd on Au(1 1 0)—thevoltammetric shape in the oxidation region is verysimilar to that reported for a massive Pd(1 1 0)electrode [20], but the peak around 0.5 V is not assharp. For 0.6 V as positive potential limit (brokenline in Fig. 8c), there is a perfect balance betweenanodic and cathodic charge (about 150 lC/cm2)for this peak, indicating that the current is due tooxide formation and reduction (or OH� adsorp-tion and desorption) rather than Pd dissolution.The latter process becomes important for more

positive potentials, since the anodic charge exceedsthe cathodic charge by far. (Redeposition from asolution containing no Pd salt would be practicallynil.) The oxidation peak at 0.47 V, which resem-bles the one of massive Pd(1 1 0), appears to bepresent only for average overlayer thickness of 2ML and more (see case b), while it is completelyabsent at lower coverage. For monolayer and sub-monolayer deposits, Pd dissolution at positivepotentials is predominant, as inferred from thecharge imbalance seen in case (a) and from chan-

Fig. 7. STM images of Au(1 1 0) in 0.1 M H2SO4 þ 0:1 mM PdCl2 þ 0:6 mM HCl after dissolution of several layers of palladium at

0.75 V, showing the [PdCl4]2� adlayer on a surface full of holes: (a) 30 nm� 30 nm, (b) 105 nm� 105 nm (IT ¼ 2 nA).

Fig. 8. Cyclic voltammograms for Pd overlayers of four different coverages on Au(1 1 0) in 0.1 M H2SO4 (scan rate: 10 mV/s).

182 L.A. Kibler et al. / Surface Science 498 (2002) 175–185

ges in the voltammogram in the following cycles. Itappears that the anodic peak at 0.7 V is related topartial dissolution of Pd in the first or/and secondlayer, because it is absent for higher coverage.

Hydrogen adsorption is seen to occur in twooverlapping steps, resulting in current peaks atabout �0:06 and �0:18 V (Fig. 8c). The totalcharge after correction for double layer chargingamounts to about 280 lC/cm2, a value similar tothat obtained for Pt(1 1 0) [21]. Because hydrogenadsorption on platinum is well-known to be aconvenient tool for titration of surface atoms(e.g. for surface roughness determination), similarconsiderations may be applied to palladium. If thecharge for hydrogen adsorption in Fig. 8c relatesto a Pd(1 1 0) surface, the clearly lower chargevalues in Fig. 8a and b (80 and 190 lC/cm2 for 1and 2 ML equivalents of Pd, respectively) signalincomplete coverage of the gold substrate, despiteample Pd on the surface. This again supports thehypothesis of massive alloying between Pd depositand Au(1 1 0) substrate.

4. Discussion

4.1. Adsorption of [PdCl4]2�

The adsorption of metal chloro complexes wasfound to play a key role in electrochemical depo-sition of Pt [22,23], Rh [24] and Pd [8,9,12,13,25]on Au(1 1 1) and Au(1 0 0) surfaces. The square-type maxima in the STM images in Fig. 6 arevery similar to those observed for [PdCl4]

2� onAu(1 0 0) in the same electrolyte [9], showing that[PdCl4]

2� is adsorbed on Au(1 1 0) too. So far, theplanar [PdCl4]

2� molecule was observed to liedown on the Au(1 1 1) and Au(1 0 0) surfaces whenbeing adsorbed. However, for the more open(1 1 0) surface, one could imagine, that the mole-cule may also be aligned perpendicular to thesurface in the substrate rows. Such a geometrycould be the origin of the thinner rows in the ad-sorbate image (Fig. 6), although we cannot com-pletely rule out that the latter may representcoadsorbed chloride, which is assumed to be pre-sent in the [PdCl4]

2� adlayer on Au(1 0 0) [9]. Inany case, the anisotropy of the Au(1 1 0) surface is

reflected in the ordered structure of the adsorbate,forming parallel chains along the [1 �11 0] directionof the substrate. Details of the adlayer structureand its dependence on the electrode potential willbe described elsewhere [19].

Thus, well-ordered adlayers of [PdCl4]2� were

found for all three low-index Au single crystalfaces. From inspection of STM images like in Fig.6i, [PdCl4]

2� appears to be also adsorbed on the Pdoverlayers on Au(1 1 0), as it has been observed forPd on Au(1 1 1) [8]. In the latter case, [PdCl4]

2�

seems to play a crucial role for the growth be-haviour, since the formation of three-dimensionalclusters was observed for Pd deposition in thehydrogen adsorption region, i.e. with no [PdCl4]

2�

on the surface [25], in contrast to two-dimensionalgrowth for deposition potentials, where [PdCl4]

2�

is adsorbed. However, there might also be an effectof the potential proper, i.e. different overvoltage.

4.2. Pd deposition on Au(110)

The cyclic voltammograms and the charge iso-therms in Fig. 1 indicate that the Pd upd onAu(1 1 0) proceeds in two steps. This is often re-lated to the formation of superstructures, as foundfor many upd systems [1,2]. However, the resultspresented in Section 3, especially the STM images,do not point in this direction. Discharge of oneMe2þ ion per unit cell on the unreconstructedAu(1 1 0) surface corresponds to 272 lC/cm2.However, the (1 1 0) surface is only covered com-pletely, when 2 ML of metal are deposited. Con-sequently, 2 ML (corresponding to 544 lC/cm2)deposited in two steps would be expected to beinvolved in the upd of palladium on the Au(1 1 0)surface as previously found for Cu upd on theunreconstructed (1 1 0) surfaces of Pt and Pd[26,27].

For Pd upd on Au(1 1 0), there is also a clearchange in mechanism after deposition of just morethan 1 ML (Fig. 1b), that gives rise to two updpeaks in the voltammogram (Fig. 1a). However,the measured total charge was in fact found tobe around 800 lC/cm2, which corresponds roughlyto the deposition of 3 (!) ML of Pd, assumingthat reduction of Pd2þ is the main contributionto this cathodic charge. Anion effects should be

L.A. Kibler et al. / Surface Science 498 (2002) 175–185 183

negligible, since [PdCl4]2� is adsorbed both on the

Au(1 1 0) substrate and on the Pd adlayers asmentioned above (see Section 4.1). The differencein the current–potential characteristics for ad-sorption and desorption (Fig. 1a) points towardsslow dissolution kinetics or towards some side re-action, such as surface alloy formation as in thecase of Pd deposition on Au(1 0 0) [9]. There isan obvious time effect on the desorption curves(compare Fig. 1a with Fig. 3a) and, in addition,the holes in the Au(1 1 0) surface, which are ob-served in STM after dissolution of palladium (Fig.7), are a strong indication for an alloying process.It might be possible, that in the course of thissurface alloy formation additional Pd is depositedin the underpotential region besides the 2 ML,which can be expected for a (1 1 0) surface. If asurface Au atom and a Pd atom change their po-sitions, one may understand that more Pd can bedeposited in the upd region due to the strong in-teraction of the two metals. In this context it isinteresting to recall, that for a Au3Pd(1 1 0) alloy, asegregation of Au with a topmost layer con-centration of 100 at.% Au was reported [28]. Thepalladium deposition process on Au(1 1 0) wasseen in the STM images to start by a decoration ofmonoatomic high steps (Fig. 6b and c), however,in contrast to Pd deposition on Au(1 1 1) and onAu(1 0 0), deposition on the upper terrace quicklycommences. Since the growth of palladium doesnot proceed in a perfect layer-by-layer mode, exactPd coverage data, which could support the depo-sition of 3 ML of Pd in the upd region, cannot beobtained from the STM images.

So far, we have observed, that the tendency ofalloy formation during Pd deposition is stronglyinfluenced by the crystallographic orientation ofthe Au substrate and is increasing in the orderAuð111Þ < Auð100Þ < Au(1 1 0) [8,9]. Intuitively,this order is reasonable, since the surface atomsare packed less and less densely. However, we arestill far from a mechanistic model for the alloyingprocess. Adatom diffusion that involves exchangeof substrate atoms could be related to surface al-loying. If such exchange diffusion is absent forclose-packed surfaces, one might have an expla-nation that Pd deposition on Au(1 1 1) does notlead to surface alloy formation.

4.3. Electrochemical behaviour of Pd overlayers onAu(110)

Unlike for Au(1 1 1), a pseudomorphic Pdmonolayer is not obtained on Au(1 1 0) by elec-trochemical deposition. However, in the sub-monolayer region and up to a coverage of about 2ML, where both gold and palladium atoms arepresent on the surface due to alloying, interestingelectrochemical and electrocatalytic properties areexpected. The thicker Pd films, which are ratherrough, are known to behave similar to a massivePd(1 1 0) single crystal surface [10]. This meansthat Pd adlayers on Au(1 1 0) like the one shownin Fig. 6j are indeed epitaxially grown. Thus, theelectrochemical and electrocatalytic properties ofelectrochemically deposited Pd films on Au(1 1 0)approach with increasing coverage the behaviourof a massive Pd(1 1 0) electrode, as indicated byliterature data [10] and by our preliminary results.However, large deviations from bulk behaviourare observed for the thinner palladium films (up to2 ML), where both gold and palladium atoms arepresent on the surface. The potentials for palla-dium electrodissolution, oxide formation (and COoxidation, to be complete) are strongly influencednot only by the crystallographic orientation ofthe Au substrate, but also by the surface mor-phology and composition, which is determinedby the amount of Pd deposited. This is also truefor Pd on Au(1 0 0), but not in that extent forPd on Au(1 1 1). In the latter case, the electro-chemical properties are determined by the presenceof pseudomorphic Pd overlayers. In addition,monoatomic high steps on the Au(1 1 1) substratewere found to play a decisive role for the oxidationof the Pd films. This means that different poten-tials were observed for oxidation of well-ordered‘‘Pd(1 1 1)’’ terraces (�0.8 V) and of Pd defect sites(�0.6 V). Similar effects were found for steppedPd(1 1 1) electrodes [29].

The situation changes when Pd is deposited onAu(1 0 0) or Au(1 1 0). For these systems, there areclear indications of surface alloy formation. Thismeans that for low coverages, surface structuresare formed, the properties of which do not re-semble those of massive Pd(1 0 0) or Pd(1 1 0)electrodes. Only for thicker Pd films, where Au

184 L.A. Kibler et al. / Surface Science 498 (2002) 175–185

atoms are no longer on the surface, the electro-chemical properties are similar to the massiveelectrodes.

5. Conclusions

(1) [PdCl4]2� forms ordered adlayers on all three

low-index faces of gold. On Au(1 0 0) and onAu(1 1 0), chloride might be coadsorbed.

(2) [PdCl4]2� is also adsorbed on electrochemi-

cally deposited Pd overlayers on Au(1 1 1) and onAu(1 1 0), whereas it is replaced by chloride for Pdon Au(1 0 0).

(3) The deposition of Pd on Au(hk l) starts inthe upd region by the decoration of surface defectsand is followed by the formation of two-dimen-sional islands.

(4) In the case of Pd deposition on Au(1 0 0) andAu(1 1 0), there are strong indications of alloyformation, which may be explained by adatomexchange diffusion.

(5) Pseudomorphic Pd layers are only formedon Au(1 1 1). Pd growth on Au(1 1 0) does notfollow a perfect layer-by-layer mechanism and nopseudomorphic Pd overlayers were seen in theSTM images. Pd films thicker than 3 ML exhibit apotentiodynamic behaviour typical for Pd(1 1 0)surface, proving an epitaxial film growth.

Acknowledgements

One of us (V.L.) acknowledges gratefully anAlexander von Humboldt stipend as well as fi-nancial assistance of ANSTI.

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