Underpotential Deposition

The underpotential deposition (UPD) of hydrogen and metal adlayers on single-crystal electrodes has long been investigated because it is well known that UPD processes are extremely sensitive to the atomic structure of the surface as well as to co-adsorbed anions [1-3]. The UPD of Cu on Au(l 11) in sulfuric acid is one of the most intensively investigated reactions using STM [26, 27], atomic force microscopy (AFM) [28], and more recently an X-ray scattering technique [29]. The structure of the first adlayer of Cu on Au(l 11) has recently been confirmed as a (a/3 x V3)R30° lattice with Cu atoms forming a honeycomb structure with a surface coverage of Cu of 2/3 by crystallographic measurement using x-rays, [29], while in-situ STM and AFM

Fig. 3. Cyclic voltammograms of Ag UPD on Pt(l 11) in sulfuric acid (a) [37] and on I/Pt(lll)(b) [8],

consistently showed the image corresponding to a simple (V3 x V3)R30° structure [2628]. According to the recent X-ray scattering determination of the first adlayer of Cu, the Cu adatoms form a honeycomb lattice, while a sulfate anion is co-adsorbed at the center of each honeycomb unit cell [29]. Therefore, the bright spots observed by STM and AFM should be considered as being due to adsorbed sulfate ions. Note that the adsorbed sulfate and bisulfate ions form the same structure with a (V3 x V7) symmetry on Au(lll), Pt(ll 1), and Rh(l 11) [30-33]. It is also noteworthy that in-situ STM also yielded an image of the simple ( S x V3 )R30° for the first UPD of Cu on Pt(l 11) in sulfuric acid, as shown in our previous paper [34]. It is also possible to assume in this case that the honeycomb lattice might be formed on Pt(l 11) as suggested previously by an indirect method [35].

The UPD of Ag on Au and Pt is also an interesting reaction to investigate with surface structure-sensitive techniques. It has clearly been demonstrated that the iodine adlayers on Pt(lll) and Au(lll) strongly affect the UPD of Ag [1, 8, 36]. For example, Fig. 3 illustrates a clear difference in the electrochemical response of the UPD of Ag on a well-defined Pt(l 11) in sulfuric acid (a) and on a Pt(l 11) with the (V7 x V7)R19.1° iodine adlayer (b), respectively. Two sets of well-defined UPD peaks in the cyclic voltammogram were observed on a well-ordered Pt(lll) in sulfuric acid [37]. On the other hand, the UPD of Ag occurred in three steps on the 1/Pt(l 1 1) [1, 8]. The structure of the adlayer formed during the UPD of Ag on 1/Pt(l 11) has previously been investigated with electrochemical UHV [1, 38]. Our recent preliminary in-situ STM study confirmed that the iodine adlayer structure was converted from (a/7 x V7)R19.1° to (3 x 3) by the formation of the first adlayer of Ag attached directly on the Pt surface [8]. Our result for the effect of the iodine adlayer on the first UPD is in good agreement with that reported by Hubbard et al. [38], However, STM images acquired after the formations of the second, third, and bulk deposited Ag layers seemed to correspond to a (V3 x V3)R30° structure. The STM images presented in our previous paper [8] could not explain the LEED patterns with three split spots reported by Hubbard et al., which seem to be similar to the pattern shown in Fig. 3(a), suggesting that the iodine adlayers with Ag layers on Pt(lll) might have incommensurate structures similar to those found on Au(l 11). Nevertheless, further experiments should be carrried out using both LEED and in-situ STM techniques to determine more accurately the structures obtained during the UPD of Ag.

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