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Initial stages of palladium deposition on Au(hkl )
Surface Science 498 (2002) 175–185 www.elsevier.com/locate/susc
Initial stages of palladium deposition on Au(h k l) Part III: Pd on Au(1 1 0) 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 discussed. Ó 2001 Published by Elsevier Science B.V. Keywords: Alloys; Gold; Metal–electrolyte interfaces; Palladium; Scanning tunneling microscopy
1. Introduction Studies of the electrochemical deposition of metals on well-ordered single crystalline substrates allow for a better understanding of the fundamental aspects of metal deposition [1,2]. Such knowledge is of great interest for, e.g. deriving structure–reactivity relationships in electrocataly* Corresponding author. Tel.: +49-731-50-25400; fax: +49731-50-25409. E-mail address: [email protected] (D.M. Kolb). 1 Present address: Institute of Physical Chemistry, I.G. Murgulescu, 77208 Bucharest 6, Romania.
sis [3], or producing thin overlayers of well-defined structure. The deposition of palladium has been considered to be a promising case for basic investigations, because of the high catalytic activity [4]. Furthermore, the difficulties in preparing and handling massive palladium single crystals were tried to be circumvented by the use of ultrathin palladium overlayers. Indeed, palladium deposition was found to be structure-sensitive: epitaxially grown overlayers were obtained on gold and platinum single crystal substrates [5,6]. In addition, the electrochemical characterisation of palladium overlayers on single crystal substrates was extremely useful for developing simple methods
0039-6028/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 0 3 9 - 6 0 2 8 ( 0 1 ) 0 1 6 8 4 - 3
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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, especially with regard to the initial stages [5,8,9]. The oxidation of small organic molecules on the electrochemically grown ultrathin palladium films on Au(h k l) was found to depend significantly on the crystallographic orientation of the substrate and on the film thickness [10,11]. In order to understand such relations it is essential to have available detailed information about the palladium surface morphology. In this respect, recent progress has been made by the use of in situ scanning tunnelling microscopy (STM) [5,8,9,12,13] and surface X-ray scattering [14]. In general, Pd is deposited onto Au from aqueous solutions of its chlorine compounds, e.g. PdCl2 or K2 PdCl4 . From such electrolytes, prior to palladium deposition, [PdCl4 ]2 adsorbs on the bare Au(1 1 1) and Au(1 0 0) surfaces and forms ordered adlayers, the structures of which have been studied by in situ STM [8,9,12,13]. This complex anion has also been found to adsorb on the first Pd monolayer on Au(1 1 1) [8], whereas it is displaced by chloride in the case of Pd on Au(1 0 0) [9]. On these unreconstructed Au(1 1 1) and Au(1 0 0) surfaces, palladium nucleates first at surface defects like monoatomic high steps, and at higher overpotentials also on terraces [8,9]. In the underpotential deposition (upd) region, a pseudomorphic Pd monolayer is formed on Au(1 1 1) [8,14]. No alloy formation was observed for this system [8], while a Pd/Au surface alloy is formed when Pd is deposited onto Au(1 0 0) [9]. Deviations in the electrochemical properties of thin Pd films on Au(1 0 0) from those of wellordered Pd(1 0 0) surfaces [10] are most probably caused by this alloying process. In this communication, the initial stages of palladium deposition onto unreconstructed Au(1 1 0) are investigated and general trends of palladium deposition onto the three low-index Au surfaces are presented. These surfaces after flame-annealing are known to be reconstructed, even in an electrochemical 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 surfaces are unreconstructed. Structural changes upon the transition from freshly prepared reconstructed to ð1 1Þ unreconstructed phases have an influence on the initial stages of metal deposition, since defects that arise from such lifting of the reconstruction can act as nucleation centres. However, compared to Au(1 1 1) or Au(1 0 0), there is little information in the literature about the electrochemical behaviour of Au(1 1 0), probably because it is still difficult to prepare large and well-ordered terraces for this relatively open surface. The existence of an order–disorder transition at about 650 K [16] restricts the annealing to lower temperatures. With the conventional flame-annealing and quenching technique, Au(1 1 0) surfaces are obtained with rather small domains and an inhomogeneous structure [17]. For this reason, we have developed a method [18], which allows to prepare Au(1 1 0) surfaces of high quality. Terraces of up to 100 nm width were obtained and it was possible to use in situ STM as a tool to study the electrochemical Pd deposition on Au(1 1 0) at an atomic level. The characterisation of a well-ordered Au(1 1 0) electrode in aqueous sulfuric acid solution by cyclic voltammetry (CV) and in situ STM with regard to reconstruction phenomena will be described elsewhere [18].
2. Experimental The experimental procedure is similar to that described in the previous publications [8,9]. The Au(1 1 0) single crystals were purchased by MaTecK (J€ ulich, Germany), where they had been polished down to 0.03 lm and oriented to better than 1°. The electrode used for CV was 3 mm in diameter, with a gold wire attached at its rear for better handling. Before each experiment, it was annealed for 1 min in a propane flame at dim red heat, cooled down for 1 min in air just above a small beaker filled with Milli-Q water, into which it was subsequently dipped and finally, protected by a drop of water, transferred to the electrochemical cell. For in situ STM, a larger single crystal was used (12 mm in diameter). It was annealed at about 200 °C for several hours (usually
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over night) in a drying locker. The surface was then cleaned by a short high-temperature annealing of a few seconds in a hydrogen flame, cooled down for several minutes under a nitrogen stream and mounted in the STM cell. The STM images were recorded with a Topometrix TMX Discoverer 2010 and are all taken with the scan direction from top to bottom. An electrochemically etched Pt–Ir wire was used as STM tip. It was coated with an electrophoretic paint to reduce the Faradaic current at the tip– electrolyte interface below 50 pA. Pt wires served as counter electrodes in both cases, but for STM measurements a platinum wire served also as a quasi-reference electrode. Otherwise a saturated calomel electrode (SCE) was used, and in the following, all potentials are quoted against SCE. As mentioned in previous work [8], the Pt reference electrode allows for highly clean conditions, but suffers somewhat from instabilities and hence, the potential values for STM measurements are less precise than those obtained with an SCE. In addition, shielding effects due to the thin-layer configuration of tip and sample may also cause small potential shifts. The solutions were prepared from H2 SO4 (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).
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3. Results 3.1. Cyclic voltammetry The bulk deposition of palladium on the unreconstructed Au(1 1 0) surface from 0.1 M H2 SO4 þ 1 mM PdCl2 , which starts at 0.52 V, is preceded by two distinct deposition steps (upd) as indicated by both the cyclic voltammogram and the charge isotherm (Fig. 1). Each data point for the charge isotherm (Fig. 1b) represents the total charge, recorded during the potential scan with 1 mV/s from 0.75 V to selected final potentials between 0.62 and 0.52 V, and holding the electrode potential there for 20 min (Fig. 2). Obviously, the current drops to zero as long as the potentials are confined to the upd region. The formation of bulk palladium starts around 0.52 V, where the deposition current is seen to increase after the initial decline brought about the potential stop (Fig. 2, broken line). The two upd peaks in Fig. 1a overlap and thus are not well resolved. Nevertheless, there is a clear change in the deposition mechanism around 0.56 V as indicated by the two cathodic peaks in the cyclic voltammogram (Fig. 1a) and the dependence of total cathodic charge on electrode potential (Fig. 1b). The charges consumed in the first step and in the whole upd region were
Fig. 1. Pd deposition on Au(1 1 0) from 0.1 M H2 SO4 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.
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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.
found to be about 350 and 800 lC/cm2 , respectively (Fig. 1b). Although the first cathodic peak is relatively sharp, its anodic counterpart is far from looking alike (Fig. 1a, dotted curve). The stripping of the palladium overlayer, which has been formed in the first deposition process, is spread out over a rather wide potential range. The anodic branch reveals two very small humps located around 0.65 and 0.75 V. However, when the deposition time was longer, like in the potential-arrest experiments (Fig. 2), only a single dissolution peak is observed, which is shifting continuously towards more negative potentials with lower deposition potential (Fig. 3a and b). On the other hand, the stripping of the palladium deposited in the second step (Edeposition < 0:55 V) takes place in a much narrower potential region yielding a pronounced peak closely located to the bulk phase dissolution (Figs. 1a and 3a). Decreasing the PdCl2 concentration from 1 to 0.1 mM is seen to shift the potential of Pd deposition and dissolution (Fig. 3b) in positive direction. This is obviously due to the lower free chloride concentration in 0.1 mM PdCl2 solution (for similar considerations, see Ref. [8]). However, it is worth mentioning that variation of the
chloride concentration up to 0.1 M did not change significantly the shape of the cyclic voltammograms. Contrary to the highly irreversible Pd deposition on Au(1 1 1) and Au(1 0 0) from chloride-free solution, palladium deposited from PdSO4 —at least for low coverage—on Au(1 1 0) can be stripped in the positive-going potential sweep (see Fig. 4). At higher coverage, Pd dissolution in sulfate solution is much slower.
3.2. Scanning tunnelling microscopy experiments Fig. 5 shows a high-resolution STM image of a well-prepared Au(1 1 0) electrode in 0.1 M H2 SO4 at 0.55 V. At such a positive potential, the (1 2) reconstruction is no longer stable and the surface is in its unreconstructed (1 1) state. Atomically flat terraces are seen, separated by monoatomic high steps, which show a pronounced frizziness [18]. Close inspection of Fig. 5 shows even the individual surface atoms. When PdCl2 is added to the supporting electrolyte, an ordered adlayer is observed at positive potentials, where Pd deposition does not yet start (Fig. 6a). The STM image shows square and rectangular spots arranged in rows, which run parallel to the [1 1 0] direction.
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Fig. 4. Cyclic voltammograms for Au(1 1 0) in 0.1 M H2 SO4 þ 0:1 mM PdCl2 (dotted line) and 0.1 M H2 SO4 þ 0:1 mM PdSO4 (solid and broken line). In the latter case, the negative potential limit was shifted by 0.1 V in the second cycle (scan rate: 1 mV/s).
Fig. 3. (a) Series of current–potential curves for Pd dissolution from Au(1 1 0) in 0.1 M H2 SO4 þ 1 mM PdCl2 after holding the potential for 30 min at different values Edeposition in the upd region (the dotted line corresponds to bulk deposition region, already, scan rate: 10 mV/s). (b) Plot of dissolution peak potentials Edissolution from (a) and respective data for 0.1 mM PdCl2 against the deposition potential Edeposition .
The distance of the spots along the row is 8:7 0:9 . Details of this structure will be given in a forthA coming publication [19]. In Fig. 6, a sequence of STM images for the initial stages of Pd deposition on Au(1 1 0) is shown. Fig. 6a represents the gold surface, covered by the ordered adlayer before Pd deposition and Fig. 6j shows the electrode covered by about a 7 ML equivalent of Pd bulk deposit. In between (Fig. 6b–i), the potential of the Au(1 1 0) electrode is confined to the upd regime. We reemphasise that the potential values obtained by the Pt quasi-reference electrode in the STM cell differ slightly but erratically from those measured with the SCE. This is reflected by the observation that 0.49 V still
Fig. 5. STM image (32 nm 32 nm) for an unreconstructed Au(1 1 0) surface in 0.1 M H2 SO4 at 0.55 V (IT ¼ 2 nA).
refers to the upd range (Fig. 6e–i). As expected, surface defects were found to play an important role in the nucleation of the Pd on Au(1 1 0). The palladium deposition process starts by a decoration of the monoatomic high steps, the deposit
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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 H2 SO4 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).
growing onto the lower terrace (Fig. 6b). The deposition process then continues by formation of
two-dimensional islands on the flat terraces (Fig. 6c–e). The second layer starts to grow before the
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Fig. 6 (continued)
first layer is completed (Fig. 6f and g) as the observed overlapping of the peaks in the cyclic voltammogram (Fig. 1a) also suggests. An adlayer similar to, although significantly less well-ordered than the one in Fig. 6a is observed on top of the grown islands. After about 15 min, deposition at 0.49 V ceased and high-resolution images at more negative potentials did not yield any further information. Large scale images for bulk Pd reveal a rather rough and scale-like surface (Fig. 6j). Complete dissolution of such a rather thick Pd overlayer at 0.75 V led to the very same adlayer structure on gold as before Pd deposition (Fig. 7a). However, the Au(1 1 0) surface is now full of monoatomic deep holes of different size, which suggests that surface alloying had taken place during Pd deposition.
3.3. Electrochemical behaviour The electrochemical properties of thin palladium films on Au(1 1 0), which were determined in 0.1 M H2 SO4 after transfer to another electrochemical cell, were found to be strongly dependent on the Pd coverage. The latter was estimated from the charge flown during the deposition reaction, and proved to be decisive for the voltammetric profile. This is demonstrated in Fig. 8. The cyclic voltammograms show two characteristic potential regions: The region of surface oxidation and reduction for E > 0:4 V and the hydrogen adsorption and desorption region between 0.1 V and 0:2 V. The latter process can be ideally studied with thin Pd films, because hydrogen absorption is shifted towards more negative potentials as compared to massive Pd single crystals [5].
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Fig. 7. STM images of Au(1 1 0) in 0.1 M H2 SO4 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 H2 SO4 (scan rate: 10 mV/s).
For the case (c)—3 ML Pd on Au(1 1 0)—the voltammetric shape in the oxidation region is very similar to that reported for a massive Pd(1 1 0) electrode [20], but the peak around 0.5 V is not as sharp. For 0.6 V as positive potential limit (broken line in Fig. 8c), there is a perfect balance between anodic and cathodic charge (about 150 lC/cm2 ) for this peak, indicating that the current is due to oxide formation and reduction (or OH adsorption and desorption) rather than Pd dissolution. The latter process becomes important for more
positive potentials, since the anodic charge exceeds the cathodic charge by far. (Redeposition from a solution containing no Pd salt would be practically nil.) The oxidation peak at 0.47 V, which resembles the one of massive Pd(1 1 0), appears to be present only for average overlayer thickness of 2 ML and more (see case b), while it is completely absent at lower coverage. For monolayer and submonolayer deposits, Pd dissolution at positive potentials is predominant, as inferred from the charge imbalance seen in case (a) and from chan-
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ges in the voltammogram in the following cycles. It appears that the anodic peak at 0.7 V is related to partial dissolution of Pd in the first or/and second layer, because it is absent for higher coverage. Hydrogen adsorption is seen to occur in two overlapping steps, resulting in current peaks at about 0:06 and 0:18 V (Fig. 8c). The total charge after correction for double layer charging amounts to about 280 lC/cm2 , a value similar to that obtained for Pt(1 1 0) [21]. Because hydrogen adsorption on platinum is well-known to be a convenient tool for titration of surface atoms (e.g. for surface roughness determination), similar considerations may be applied to palladium. If the charge for hydrogen adsorption in Fig. 8c relates to a Pd(1 1 0) surface, the clearly lower charge values in Fig. 8a and b (80 and 190 lC/cm2 for 1 and 2 ML equivalents of Pd, respectively) signal incomplete coverage of the gold substrate, despite ample Pd on the surface. This again supports the hypothesis of massive alloying between Pd deposit and Au(1 1 0) substrate.
4. Discussion 4.1. Adsorption of [PdCl4 ]2 The adsorption of metal chloro complexes was found to play a key role in electrochemical deposition 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 squaretype maxima in the STM images in Fig. 6 are very similar to those observed for [PdCl4 ]2 on Au(1 0 0) in the same electrolyte [9], showing that [PdCl4 ]2 is adsorbed on Au(1 1 0) too. So far, the planar [PdCl4 ]2 molecule was observed to lie down on the Au(1 1 1) and Au(1 0 0) surfaces when being adsorbed. However, for the more open (1 1 0) surface, one could imagine, that the molecule may also be aligned perpendicular to the surface in the substrate rows. Such a geometry could be the origin of the thinner rows in the adsorbate image (Fig. 6), although we cannot completely rule out that the latter may represent coadsorbed chloride, which is assumed to be present in the [PdCl4 ]2 adlayer on Au(1 0 0) [9]. In any case, the anisotropy of the Au(1 1 0) surface is
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reflected in the ordered structure of the adsorbate, forming parallel chains along the [1 1 0] direction of the substrate. Details of the adlayer structure and its dependence on the electrode potential will be described elsewhere [19]. Thus, well-ordered adlayers of [PdCl4 ]2 were found for all three low-index Au single crystal faces. From inspection of STM images like in Fig. 6i, [PdCl4 ]2 appears to be also adsorbed on the Pd overlayers on Au(1 1 0), as it has been observed for Pd on Au(1 1 1) [8]. In the latter case, [PdCl4 ]2 seems to play a crucial role for the growth behaviour, since the formation of three-dimensional clusters was observed for Pd deposition in the hydrogen adsorption region, i.e. with no [PdCl4 ]2 on the surface [25], in contrast to two-dimensional growth for deposition potentials, where [PdCl4 ]2 is adsorbed. However, there might also be an effect of the potential proper, i.e. different overvoltage. 4.2. Pd deposition on Au(1 1 0) The cyclic voltammograms and the charge isotherms in Fig. 1 indicate that the Pd upd on Au(1 1 0) proceeds in two steps. This is often related to the formation of superstructures, as found for many upd systems [1,2]. However, the results presented in Section 3, especially the STM images, do not point in this direction. Discharge of one Me2þ ion per unit cell on the unreconstructed Au(1 1 0) surface corresponds to 272 lC/cm2 . However, the (1 1 0) surface is only covered completely, when 2 ML of metal are deposited. Consequently, 2 ML (corresponding to 544 lC/cm2 ) deposited in two steps would be expected to be involved in the upd of palladium on the Au(1 1 0) surface as previously found for Cu upd on the unreconstructed (1 1 0) surfaces of Pt and Pd [26,27]. For Pd upd on Au(1 1 0), there is also a clear change in mechanism after deposition of just more than 1 ML (Fig. 1b), that gives rise to two upd peaks in the voltammogram (Fig. 1a). However, the measured total charge was in fact found to be around 800 lC/cm2 , which corresponds roughly to the deposition of 3 (!) ML of Pd, assuming that reduction of Pd2þ is the main contribution to this cathodic charge. Anion effects should be
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negligible, since [PdCl4 ]2 is adsorbed both on the Au(1 1 0) substrate and on the Pd adlayers as mentioned above (see Section 4.1). The difference in the current–potential characteristics for adsorption and desorption (Fig. 1a) points towards slow dissolution kinetics or towards some side reaction, such as surface alloy formation as in the case of Pd deposition on Au(1 0 0) [9]. There is an 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 observed 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 this surface alloy formation additional Pd is deposited in the underpotential region besides the 2 ML, which can be expected for a (1 1 0) surface. If a surface Au atom and a Pd atom change their positions, one may understand that more Pd can be deposited in the upd region due to the strong interaction of the two metals. In this context it is interesting to recall, that for a Au3 Pd(1 1 0) alloy, a segregation of Au with a topmost layer concentration of 100 at.% Au was reported [28]. The palladium deposition process on Au(1 1 0) was seen in the STM images to start by a decoration of monoatomic high steps (Fig. 6b and c), however, in contrast to Pd deposition on Au(1 1 1) and on Au(1 0 0), deposition on the upper terrace quickly commences. Since the growth of palladium does not proceed in a perfect layer-by-layer mode, exact Pd coverage data, which could support the deposition of 3 ML of Pd in the upd region, cannot be obtained from the STM images. So far, we have observed, that the tendency of alloy formation during Pd deposition is strongly influenced by the crystallographic orientation of the Au substrate and is increasing in the order Auð1 1 1Þ < Auð1 0 0Þ < Au(1 1 0) [8,9]. Intuitively, this order is reasonable, since the surface atoms are packed less and less densely. However, we are still far from a mechanistic model for the alloying process. Adatom diffusion that involves exchange of substrate atoms could be related to surface alloying. If such exchange diffusion is absent for close-packed surfaces, one might have an explanation that Pd deposition on Au(1 1 1) does not lead to surface alloy formation.
4.3. Electrochemical behaviour of Pd overlayers on Au(1 1 0) Unlike for Au(1 1 1), a pseudomorphic Pd monolayer is not obtained on Au(1 1 0) by electrochemical deposition. However, in the submonolayer region and up to a coverage of about 2 ML, where both gold and palladium atoms are present on the surface due to alloying, interesting electrochemical and electrocatalytic properties are expected. The thicker Pd films, which are rather rough, are known to behave similar to a massive Pd(1 1 0) single crystal surface [10]. This means that Pd adlayers on Au(1 1 0) like the one shown in Fig. 6j are indeed epitaxially grown. Thus, the electrochemical and electrocatalytic properties of electrochemically deposited Pd films on Au(1 1 0) approach with increasing coverage the behaviour of a massive Pd(1 1 0) electrode, as indicated by literature data [10] and by our preliminary results. However, large deviations from bulk behaviour are observed for the thinner palladium films (up to 2 ML), where both gold and palladium atoms are present on the surface. The potentials for palladium electrodissolution, oxide formation (and CO oxidation, to be complete) are strongly influenced not only by the crystallographic orientation of the Au substrate, but also by the surface morphology and composition, which is determined by the amount of Pd deposited. This is also true for Pd on Au(1 0 0), but not in that extent for Pd on Au(1 1 1). In the latter case, the electrochemical properties are determined by the presence of pseudomorphic Pd overlayers. In addition, monoatomic high steps on the Au(1 1 1) substrate were found to play a decisive role for the oxidation of the Pd films. This means that different potentials 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 stepped Pd(1 1 1) electrodes [29]. The situation changes when Pd is deposited on Au(1 0 0) or Au(1 1 0). For these systems, there are clear indications of surface alloy formation. This means that for low coverages, surface structures are formed, the properties of which do not resemble those of massive Pd(1 0 0) or Pd(1 1 0) electrodes. Only for thicker Pd films, where Au
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atoms are no longer on the surface, the electrochemical properties are similar to the massive electrodes.
5. Conclusions (1) [PdCl4 ]2 forms ordered adlayers on all three low-index faces of gold. On Au(1 0 0) and on Au(1 1 0), chloride might be coadsorbed. (2) [PdCl4 ]2 is also adsorbed on electrochemically deposited Pd overlayers on Au(1 1 1) and on Au(1 1 0), whereas it is replaced by chloride for Pd on Au(1 0 0). (3) The deposition of Pd on Au(h k l) starts in the upd region by the decoration of surface defects and is followed by the formation of two-dimensional islands. (4) In the case of Pd deposition on Au(1 0 0) and Au(1 1 0), there are strong indications of alloy formation, which may be explained by adatom exchange diffusion. (5) Pseudomorphic Pd layers are only formed on Au(1 1 1). Pd growth on Au(1 1 0) does not follow a perfect layer-by-layer mechanism and no pseudomorphic Pd overlayers were seen in the STM images. Pd films thicker than 3 ML exhibit a potentiodynamic behaviour typical for Pd(1 1 0) surface, proving an epitaxial film growth.
Acknowledgements One of us (V.L.) acknowledges gratefully an Alexander von Humboldt stipend as well as financial assistance of ANSTI.
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Initial stages of palladium deposition on Au(h k l) Part III: Pd on Au(1 1 0) 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 discussed. Ó 2001 Published by Elsevier Science B.V. Keywords: Alloys; Gold; Metal–electrolyte interfaces; Palladium; Scanning tunneling microscopy
1. Introduction Studies of the electrochemical deposition of metals on well-ordered single crystalline substrates allow for a better understanding of the fundamental aspects of metal deposition [1,2]. Such knowledge is of great interest for, e.g. deriving structure–reactivity relationships in electrocataly* Corresponding author. Tel.: +49-731-50-25400; fax: +49731-50-25409. E-mail address: [email protected] (D.M. Kolb). 1 Present address: Institute of Physical Chemistry, I.G. Murgulescu, 77208 Bucharest 6, Romania.
sis [3], or producing thin overlayers of well-defined structure. The deposition of palladium has been considered to be a promising case for basic investigations, because of the high catalytic activity [4]. Furthermore, the difficulties in preparing and handling massive palladium single crystals were tried to be circumvented by the use of ultrathin palladium overlayers. Indeed, palladium deposition was found to be structure-sensitive: epitaxially grown overlayers were obtained on gold and platinum single crystal substrates [5,6]. In addition, the electrochemical characterisation of palladium overlayers on single crystal substrates was extremely useful for developing simple methods
0039-6028/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 0 3 9 - 6 0 2 8 ( 0 1 ) 0 1 6 8 4 - 3
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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, especially with regard to the initial stages [5,8,9]. The oxidation of small organic molecules on the electrochemically grown ultrathin palladium films on Au(h k l) was found to depend significantly on the crystallographic orientation of the substrate and on the film thickness [10,11]. In order to understand such relations it is essential to have available detailed information about the palladium surface morphology. In this respect, recent progress has been made by the use of in situ scanning tunnelling microscopy (STM) [5,8,9,12,13] and surface X-ray scattering [14]. In general, Pd is deposited onto Au from aqueous solutions of its chlorine compounds, e.g. PdCl2 or K2 PdCl4 . From such electrolytes, prior to palladium deposition, [PdCl4 ]2 adsorbs on the bare Au(1 1 1) and Au(1 0 0) surfaces and forms ordered adlayers, the structures of which have been studied by in situ STM [8,9,12,13]. This complex anion has also been found to adsorb on the first Pd monolayer on Au(1 1 1) [8], whereas it is displaced by chloride in the case of Pd on Au(1 0 0) [9]. On these unreconstructed Au(1 1 1) and Au(1 0 0) surfaces, palladium nucleates first at surface defects like monoatomic high steps, and at higher overpotentials also on terraces [8,9]. In the underpotential deposition (upd) region, a pseudomorphic Pd monolayer is formed on Au(1 1 1) [8,14]. No alloy formation was observed for this system [8], while a Pd/Au surface alloy is formed when Pd is deposited onto Au(1 0 0) [9]. Deviations in the electrochemical properties of thin Pd films on Au(1 0 0) from those of wellordered Pd(1 0 0) surfaces [10] are most probably caused by this alloying process. In this communication, the initial stages of palladium deposition onto unreconstructed Au(1 1 0) are investigated and general trends of palladium deposition onto the three low-index Au surfaces are presented. These surfaces after flame-annealing are known to be reconstructed, even in an electrochemical 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 surfaces are unreconstructed. Structural changes upon the transition from freshly prepared reconstructed to ð1 1Þ unreconstructed phases have an influence on the initial stages of metal deposition, since defects that arise from such lifting of the reconstruction can act as nucleation centres. However, compared to Au(1 1 1) or Au(1 0 0), there is little information in the literature about the electrochemical behaviour of Au(1 1 0), probably because it is still difficult to prepare large and well-ordered terraces for this relatively open surface. The existence of an order–disorder transition at about 650 K [16] restricts the annealing to lower temperatures. With the conventional flame-annealing and quenching technique, Au(1 1 0) surfaces are obtained with rather small domains and an inhomogeneous structure [17]. For this reason, we have developed a method [18], which allows to prepare Au(1 1 0) surfaces of high quality. Terraces of up to 100 nm width were obtained and it was possible to use in situ STM as a tool to study the electrochemical Pd deposition on Au(1 1 0) at an atomic level. The characterisation of a well-ordered Au(1 1 0) electrode in aqueous sulfuric acid solution by cyclic voltammetry (CV) and in situ STM with regard to reconstruction phenomena will be described elsewhere [18].
2. Experimental The experimental procedure is similar to that described in the previous publications [8,9]. The Au(1 1 0) single crystals were purchased by MaTecK (J€ ulich, Germany), where they had been polished down to 0.03 lm and oriented to better than 1°. The electrode used for CV was 3 mm in diameter, with a gold wire attached at its rear for better handling. Before each experiment, it was annealed for 1 min in a propane flame at dim red heat, cooled down for 1 min in air just above a small beaker filled with Milli-Q water, into which it was subsequently dipped and finally, protected by a drop of water, transferred to the electrochemical cell. For in situ STM, a larger single crystal was used (12 mm in diameter). It was annealed at about 200 °C for several hours (usually
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over night) in a drying locker. The surface was then cleaned by a short high-temperature annealing of a few seconds in a hydrogen flame, cooled down for several minutes under a nitrogen stream and mounted in the STM cell. The STM images were recorded with a Topometrix TMX Discoverer 2010 and are all taken with the scan direction from top to bottom. An electrochemically etched Pt–Ir wire was used as STM tip. It was coated with an electrophoretic paint to reduce the Faradaic current at the tip– electrolyte interface below 50 pA. Pt wires served as counter electrodes in both cases, but for STM measurements a platinum wire served also as a quasi-reference electrode. Otherwise a saturated calomel electrode (SCE) was used, and in the following, all potentials are quoted against SCE. As mentioned in previous work [8], the Pt reference electrode allows for highly clean conditions, but suffers somewhat from instabilities and hence, the potential values for STM measurements are less precise than those obtained with an SCE. In addition, shielding effects due to the thin-layer configuration of tip and sample may also cause small potential shifts. The solutions were prepared from H2 SO4 (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).
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3. Results 3.1. Cyclic voltammetry The bulk deposition of palladium on the unreconstructed Au(1 1 0) surface from 0.1 M H2 SO4 þ 1 mM PdCl2 , which starts at 0.52 V, is preceded by two distinct deposition steps (upd) as indicated by both the cyclic voltammogram and the charge isotherm (Fig. 1). Each data point for the charge isotherm (Fig. 1b) represents the total charge, recorded during the potential scan with 1 mV/s from 0.75 V to selected final potentials between 0.62 and 0.52 V, and holding the electrode potential there for 20 min (Fig. 2). Obviously, the current drops to zero as long as the potentials are confined to the upd region. The formation of bulk palladium starts around 0.52 V, where the deposition current is seen to increase after the initial decline brought about the potential stop (Fig. 2, broken line). The two upd peaks in Fig. 1a overlap and thus are not well resolved. Nevertheless, there is a clear change in the deposition mechanism around 0.56 V as indicated by the two cathodic peaks in the cyclic voltammogram (Fig. 1a) and the dependence of total cathodic charge on electrode potential (Fig. 1b). The charges consumed in the first step and in the whole upd region were
Fig. 1. Pd deposition on Au(1 1 0) from 0.1 M H2 SO4 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.
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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.
found to be about 350 and 800 lC/cm2 , respectively (Fig. 1b). Although the first cathodic peak is relatively sharp, its anodic counterpart is far from looking alike (Fig. 1a, dotted curve). The stripping of the palladium overlayer, which has been formed in the first deposition process, is spread out over a rather wide potential range. The anodic branch reveals two very small humps located around 0.65 and 0.75 V. However, when the deposition time was longer, like in the potential-arrest experiments (Fig. 2), only a single dissolution peak is observed, which is shifting continuously towards more negative potentials with lower deposition potential (Fig. 3a and b). On the other hand, the stripping of the palladium deposited in the second step (Edeposition < 0:55 V) takes place in a much narrower potential region yielding a pronounced peak closely located to the bulk phase dissolution (Figs. 1a and 3a). Decreasing the PdCl2 concentration from 1 to 0.1 mM is seen to shift the potential of Pd deposition and dissolution (Fig. 3b) in positive direction. This is obviously due to the lower free chloride concentration in 0.1 mM PdCl2 solution (for similar considerations, see Ref. [8]). However, it is worth mentioning that variation of the
chloride concentration up to 0.1 M did not change significantly the shape of the cyclic voltammograms. Contrary to the highly irreversible Pd deposition on Au(1 1 1) and Au(1 0 0) from chloride-free solution, palladium deposited from PdSO4 —at least for low coverage—on Au(1 1 0) can be stripped in the positive-going potential sweep (see Fig. 4). At higher coverage, Pd dissolution in sulfate solution is much slower.
3.2. Scanning tunnelling microscopy experiments Fig. 5 shows a high-resolution STM image of a well-prepared Au(1 1 0) electrode in 0.1 M H2 SO4 at 0.55 V. At such a positive potential, the (1 2) reconstruction is no longer stable and the surface is in its unreconstructed (1 1) state. Atomically flat terraces are seen, separated by monoatomic high steps, which show a pronounced frizziness [18]. Close inspection of Fig. 5 shows even the individual surface atoms. When PdCl2 is added to the supporting electrolyte, an ordered adlayer is observed at positive potentials, where Pd deposition does not yet start (Fig. 6a). The STM image shows square and rectangular spots arranged in rows, which run parallel to the [1 1 0] direction.
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Fig. 4. Cyclic voltammograms for Au(1 1 0) in 0.1 M H2 SO4 þ 0:1 mM PdCl2 (dotted line) and 0.1 M H2 SO4 þ 0:1 mM PdSO4 (solid and broken line). In the latter case, the negative potential limit was shifted by 0.1 V in the second cycle (scan rate: 1 mV/s).
Fig. 3. (a) Series of current–potential curves for Pd dissolution from Au(1 1 0) in 0.1 M H2 SO4 þ 1 mM PdCl2 after holding the potential for 30 min at different values Edeposition in the upd region (the dotted line corresponds to bulk deposition region, already, scan rate: 10 mV/s). (b) Plot of dissolution peak potentials Edissolution from (a) and respective data for 0.1 mM PdCl2 against the deposition potential Edeposition .
The distance of the spots along the row is 8:7 0:9 . Details of this structure will be given in a forthA coming publication [19]. In Fig. 6, a sequence of STM images for the initial stages of Pd deposition on Au(1 1 0) is shown. Fig. 6a represents the gold surface, covered by the ordered adlayer before Pd deposition and Fig. 6j shows the electrode covered by about a 7 ML equivalent of Pd bulk deposit. In between (Fig. 6b–i), the potential of the Au(1 1 0) electrode is confined to the upd regime. We reemphasise that the potential values obtained by the Pt quasi-reference electrode in the STM cell differ slightly but erratically from those measured with the SCE. This is reflected by the observation that 0.49 V still
Fig. 5. STM image (32 nm 32 nm) for an unreconstructed Au(1 1 0) surface in 0.1 M H2 SO4 at 0.55 V (IT ¼ 2 nA).
refers to the upd range (Fig. 6e–i). As expected, surface defects were found to play an important role in the nucleation of the Pd on Au(1 1 0). The palladium deposition process starts by a decoration of the monoatomic high steps, the deposit
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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 H2 SO4 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).
growing onto the lower terrace (Fig. 6b). The deposition process then continues by formation of
two-dimensional islands on the flat terraces (Fig. 6c–e). The second layer starts to grow before the
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Fig. 6 (continued)
first layer is completed (Fig. 6f and g) as the observed overlapping of the peaks in the cyclic voltammogram (Fig. 1a) also suggests. An adlayer similar to, although significantly less well-ordered than the one in Fig. 6a is observed on top of the grown islands. After about 15 min, deposition at 0.49 V ceased and high-resolution images at more negative potentials did not yield any further information. Large scale images for bulk Pd reveal a rather rough and scale-like surface (Fig. 6j). Complete dissolution of such a rather thick Pd overlayer at 0.75 V led to the very same adlayer structure on gold as before Pd deposition (Fig. 7a). However, the Au(1 1 0) surface is now full of monoatomic deep holes of different size, which suggests that surface alloying had taken place during Pd deposition.
3.3. Electrochemical behaviour The electrochemical properties of thin palladium films on Au(1 1 0), which were determined in 0.1 M H2 SO4 after transfer to another electrochemical cell, were found to be strongly dependent on the Pd coverage. The latter was estimated from the charge flown during the deposition reaction, and proved to be decisive for the voltammetric profile. This is demonstrated in Fig. 8. The cyclic voltammograms show two characteristic potential regions: The region of surface oxidation and reduction for E > 0:4 V and the hydrogen adsorption and desorption region between 0.1 V and 0:2 V. The latter process can be ideally studied with thin Pd films, because hydrogen absorption is shifted towards more negative potentials as compared to massive Pd single crystals [5].
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Fig. 7. STM images of Au(1 1 0) in 0.1 M H2 SO4 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 H2 SO4 (scan rate: 10 mV/s).
For the case (c)—3 ML Pd on Au(1 1 0)—the voltammetric shape in the oxidation region is very similar to that reported for a massive Pd(1 1 0) electrode [20], but the peak around 0.5 V is not as sharp. For 0.6 V as positive potential limit (broken line in Fig. 8c), there is a perfect balance between anodic and cathodic charge (about 150 lC/cm2 ) for this peak, indicating that the current is due to oxide formation and reduction (or OH adsorption and desorption) rather than Pd dissolution. The latter process becomes important for more
positive potentials, since the anodic charge exceeds the cathodic charge by far. (Redeposition from a solution containing no Pd salt would be practically nil.) The oxidation peak at 0.47 V, which resembles the one of massive Pd(1 1 0), appears to be present only for average overlayer thickness of 2 ML and more (see case b), while it is completely absent at lower coverage. For monolayer and submonolayer deposits, Pd dissolution at positive potentials is predominant, as inferred from the charge imbalance seen in case (a) and from chan-
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ges in the voltammogram in the following cycles. It appears that the anodic peak at 0.7 V is related to partial dissolution of Pd in the first or/and second layer, because it is absent for higher coverage. Hydrogen adsorption is seen to occur in two overlapping steps, resulting in current peaks at about 0:06 and 0:18 V (Fig. 8c). The total charge after correction for double layer charging amounts to about 280 lC/cm2 , a value similar to that obtained for Pt(1 1 0) [21]. Because hydrogen adsorption on platinum is well-known to be a convenient tool for titration of surface atoms (e.g. for surface roughness determination), similar considerations may be applied to palladium. If the charge for hydrogen adsorption in Fig. 8c relates to a Pd(1 1 0) surface, the clearly lower charge values in Fig. 8a and b (80 and 190 lC/cm2 for 1 and 2 ML equivalents of Pd, respectively) signal incomplete coverage of the gold substrate, despite ample Pd on the surface. This again supports the hypothesis of massive alloying between Pd deposit and Au(1 1 0) substrate.
4. Discussion 4.1. Adsorption of [PdCl4 ]2 The adsorption of metal chloro complexes was found to play a key role in electrochemical deposition 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 squaretype maxima in the STM images in Fig. 6 are very similar to those observed for [PdCl4 ]2 on Au(1 0 0) in the same electrolyte [9], showing that [PdCl4 ]2 is adsorbed on Au(1 1 0) too. So far, the planar [PdCl4 ]2 molecule was observed to lie down on the Au(1 1 1) and Au(1 0 0) surfaces when being adsorbed. However, for the more open (1 1 0) surface, one could imagine, that the molecule may also be aligned perpendicular to the surface in the substrate rows. Such a geometry could be the origin of the thinner rows in the adsorbate image (Fig. 6), although we cannot completely rule out that the latter may represent coadsorbed chloride, which is assumed to be present in the [PdCl4 ]2 adlayer on Au(1 0 0) [9]. In any case, the anisotropy of the Au(1 1 0) surface is
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reflected in the ordered structure of the adsorbate, forming parallel chains along the [1 1 0] direction of the substrate. Details of the adlayer structure and its dependence on the electrode potential will be described elsewhere [19]. Thus, well-ordered adlayers of [PdCl4 ]2 were found for all three low-index Au single crystal faces. From inspection of STM images like in Fig. 6i, [PdCl4 ]2 appears to be also adsorbed on the Pd overlayers on Au(1 1 0), as it has been observed for Pd on Au(1 1 1) [8]. In the latter case, [PdCl4 ]2 seems to play a crucial role for the growth behaviour, since the formation of three-dimensional clusters was observed for Pd deposition in the hydrogen adsorption region, i.e. with no [PdCl4 ]2 on the surface [25], in contrast to two-dimensional growth for deposition potentials, where [PdCl4 ]2 is adsorbed. However, there might also be an effect of the potential proper, i.e. different overvoltage. 4.2. Pd deposition on Au(1 1 0) The cyclic voltammograms and the charge isotherms in Fig. 1 indicate that the Pd upd on Au(1 1 0) proceeds in two steps. This is often related to the formation of superstructures, as found for many upd systems [1,2]. However, the results presented in Section 3, especially the STM images, do not point in this direction. Discharge of one Me2þ ion per unit cell on the unreconstructed Au(1 1 0) surface corresponds to 272 lC/cm2 . However, the (1 1 0) surface is only covered completely, when 2 ML of metal are deposited. Consequently, 2 ML (corresponding to 544 lC/cm2 ) deposited in two steps would be expected to be involved in the upd of palladium on the Au(1 1 0) surface as previously found for Cu upd on the unreconstructed (1 1 0) surfaces of Pt and Pd [26,27]. For Pd upd on Au(1 1 0), there is also a clear change in mechanism after deposition of just more than 1 ML (Fig. 1b), that gives rise to two upd peaks in the voltammogram (Fig. 1a). However, the measured total charge was in fact found to be around 800 lC/cm2 , which corresponds roughly to the deposition of 3 (!) ML of Pd, assuming that reduction of Pd2þ is the main contribution to this cathodic charge. Anion effects should be
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negligible, since [PdCl4 ]2 is adsorbed both on the Au(1 1 0) substrate and on the Pd adlayers as mentioned above (see Section 4.1). The difference in the current–potential characteristics for adsorption and desorption (Fig. 1a) points towards slow dissolution kinetics or towards some side reaction, such as surface alloy formation as in the case of Pd deposition on Au(1 0 0) [9]. There is an 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 observed 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 this surface alloy formation additional Pd is deposited in the underpotential region besides the 2 ML, which can be expected for a (1 1 0) surface. If a surface Au atom and a Pd atom change their positions, one may understand that more Pd can be deposited in the upd region due to the strong interaction of the two metals. In this context it is interesting to recall, that for a Au3 Pd(1 1 0) alloy, a segregation of Au with a topmost layer concentration of 100 at.% Au was reported [28]. The palladium deposition process on Au(1 1 0) was seen in the STM images to start by a decoration of monoatomic high steps (Fig. 6b and c), however, in contrast to Pd deposition on Au(1 1 1) and on Au(1 0 0), deposition on the upper terrace quickly commences. Since the growth of palladium does not proceed in a perfect layer-by-layer mode, exact Pd coverage data, which could support the deposition of 3 ML of Pd in the upd region, cannot be obtained from the STM images. So far, we have observed, that the tendency of alloy formation during Pd deposition is strongly influenced by the crystallographic orientation of the Au substrate and is increasing in the order Auð1 1 1Þ < Auð1 0 0Þ < Au(1 1 0) [8,9]. Intuitively, this order is reasonable, since the surface atoms are packed less and less densely. However, we are still far from a mechanistic model for the alloying process. Adatom diffusion that involves exchange of substrate atoms could be related to surface alloying. If such exchange diffusion is absent for close-packed surfaces, one might have an explanation that Pd deposition on Au(1 1 1) does not lead to surface alloy formation.
4.3. Electrochemical behaviour of Pd overlayers on Au(1 1 0) Unlike for Au(1 1 1), a pseudomorphic Pd monolayer is not obtained on Au(1 1 0) by electrochemical deposition. However, in the submonolayer region and up to a coverage of about 2 ML, where both gold and palladium atoms are present on the surface due to alloying, interesting electrochemical and electrocatalytic properties are expected. The thicker Pd films, which are rather rough, are known to behave similar to a massive Pd(1 1 0) single crystal surface [10]. This means that Pd adlayers on Au(1 1 0) like the one shown in Fig. 6j are indeed epitaxially grown. Thus, the electrochemical and electrocatalytic properties of electrochemically deposited Pd films on Au(1 1 0) approach with increasing coverage the behaviour of a massive Pd(1 1 0) electrode, as indicated by literature data [10] and by our preliminary results. However, large deviations from bulk behaviour are observed for the thinner palladium films (up to 2 ML), where both gold and palladium atoms are present on the surface. The potentials for palladium electrodissolution, oxide formation (and CO oxidation, to be complete) are strongly influenced not only by the crystallographic orientation of the Au substrate, but also by the surface morphology and composition, which is determined by the amount of Pd deposited. This is also true for Pd on Au(1 0 0), but not in that extent for Pd on Au(1 1 1). In the latter case, the electrochemical properties are determined by the presence of pseudomorphic Pd overlayers. In addition, monoatomic high steps on the Au(1 1 1) substrate were found to play a decisive role for the oxidation of the Pd films. This means that different potentials 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 stepped Pd(1 1 1) electrodes [29]. The situation changes when Pd is deposited on Au(1 0 0) or Au(1 1 0). For these systems, there are clear indications of surface alloy formation. This means that for low coverages, surface structures are formed, the properties of which do not resemble those of massive Pd(1 0 0) or Pd(1 1 0) electrodes. Only for thicker Pd films, where Au
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atoms are no longer on the surface, the electrochemical properties are similar to the massive electrodes.
5. Conclusions (1) [PdCl4 ]2 forms ordered adlayers on all three low-index faces of gold. On Au(1 0 0) and on Au(1 1 0), chloride might be coadsorbed. (2) [PdCl4 ]2 is also adsorbed on electrochemically deposited Pd overlayers on Au(1 1 1) and on Au(1 1 0), whereas it is replaced by chloride for Pd on Au(1 0 0). (3) The deposition of Pd on Au(h k l) starts in the upd region by the decoration of surface defects and is followed by the formation of two-dimensional islands. (4) In the case of Pd deposition on Au(1 0 0) and Au(1 1 0), there are strong indications of alloy formation, which may be explained by adatom exchange diffusion. (5) Pseudomorphic Pd layers are only formed on Au(1 1 1). Pd growth on Au(1 1 0) does not follow a perfect layer-by-layer mechanism and no pseudomorphic Pd overlayers were seen in the STM images. Pd films thicker than 3 ML exhibit a potentiodynamic behaviour typical for Pd(1 1 0) surface, proving an epitaxial film growth.
Acknowledgements One of us (V.L.) acknowledges gratefully an Alexander von Humboldt stipend as well as financial assistance of ANSTI.
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