Underpotential Deposition

As indicated in several chapters in this volume, many groups have studied underpotential deposition (UPD). UPD is an electrochemical process in which monolayers or submonolayers of a foreign metal adatom are deposited on a surface at potentials positive of the reversible or Nernst potential. The UPD monolayer can be thought of as deriving from a relatively strong adatom-substrate bond formed using less energy than the subsequent adatom-adatom bonds formed during bulk deposition. The attraction of many groups to studying UPD stems not only from the relative experimental simplicity of the process and the tendency of UPD adlattice structures to exhibit considerable structural complexity, but also from the importance of the UPD process in deposition, catalysis, and fundamental studies of monolayers [1]. We have used STM and AFM to examine several UPD lattice structures and to correlate these structures with electrochemical reactivity.

Fig. 1. AFM images (5 nm x 5 nm) of Bi UPD on Au(l 11) in 0.1 M HC104. (a) Au(l 11) surface found positive of Bi UPD. Atom-atom distance is 0.29 nm. (b) (2 x 2) Bi adlattice found at 200 mV vs -Egi/Bj3* • Atom-atom distance is 0.75 ± 0.02 nm. (c) Uniaxially commensurate, rectangular Bi adlattice found at 100 mV. Atom-atom distance is 0.3 ± 0.02 nm. (d) Schematic of Bi structures: left, rectangular lattice where p and p\ are primitive and nonprimitive (designated with U in the figure) unit cell vectors, respectively; right, (2 x 2) Bi adlattice showing open Au and Bi sites. The Bi adatoms are larger than Au; They are shown smaller here for clarity. Other arrangements with the Bi in bridging ar atop sites are also possible.

Fig. 1. AFM images (5 nm x 5 nm) of Bi UPD on Au(l 11) in 0.1 M HC104. (a) Au(l 11) surface found positive of Bi UPD. Atom-atom distance is 0.29 nm. (b) (2 x 2) Bi adlattice found at 200 mV vs -Egi/Bj3* • Atom-atom distance is 0.75 ± 0.02 nm. (c) Uniaxially commensurate, rectangular Bi adlattice found at 100 mV. Atom-atom distance is 0.3 ± 0.02 nm. (d) Schematic of Bi structures: left, rectangular lattice where p and p\ are primitive and nonprimitive (designated with U in the figure) unit cell vectors, respectively; right, (2 x 2) Bi adlattice showing open Au and Bi sites. The Bi adatoms are larger than Au; They are shown smaller here for clarity. Other arrangements with the Bi in bridging ar atop sites are also possible.

A favorite example is the underpotential deposition of Bi onto Au(lll). Monolayers of Bi formed through UPD on Au(lll) act as catalysts for the two-electron electroreduction of H202 to H20 in acid solutions. However, this catalytic activity occurs only in a narrow potential region [2], There are three different lattices observed in the UPD range by AFM [3]. The first of these, obtained at the most positive potentials and shown in Fig. 1(a), exhibits a 0.29 mn atom-atom spacing and a hexagonal structure. This is the bare Au(l 11) lattice which should be observed at these potentials. Moving into the UPD region, a different structure is observed (Fig. 1(b)). This first monolayer of Bi on Au(l 11) has a 0.58 mn spacing, hexagonal structure, and exhibits no rotation relative to the underlying Au. The structure corresponds to a (2 x 2) Bi adlattice shown at the right of Fig. 1(d). Finally, the third structure seen prior to bulk deposition of Bi is shown in Fig. 1(c). The lattice has now changed to a "uniaxially commensurate" (p x V3) structure which exhibits a V3 spacing in one direction (p2 in Fig. 1(d) (left)) but is incommensurate in the pi direction. These structures have been confirmed by the completely independent Surface X-ray scattering technique [4]. Interestingly, peroxide electroreduction occurs only in the potential region where the (2 x 2) Bi adlattice is present. This suggests that both substrate and adatom sites are a requirement for this reactivity.

One of the most interesting features exhibited in the UPD process described above and found in several other adatom-substrate pairs is the existence of "open" adatom lattice structures. In metal-on-metal depositions performed in the ultrahigh-vacuum (UHV) environment, open structures are never observed in the absence of formation of a surface alloy. Yet such structures are common in the electrochemical environment. In the case of Cu UPD on Au(lll) in the presence of sulfate, considerable effort has shown that the (V3 x V3)R30° lattice exhibited by this system forms due to coadsorption of sulfate with the UPD Cu [5], The sulfate forms a (V3 x V3)R30° lattice with 1/3 monolayer coverage, while the Cu atoms are arranged in a honeycomb lattice exhibiting a 2/3 surface coverage [6]. UPD of Ag on Au(lll) exhibits packing densities which correlate inversely with the size of the anion, with smaller anions giving rise to higher packing densities [7]. In the Bi case described above the same (2 x 2) lattice structure forms, regardless of the anion present in solution. We first thought that this behavior implied that the open structure resulted not from anion coadsorption, but rather from electrostatic repulsion between putatively partially discharged Bi adatoms. However, recent chronocoulometric measurements performed in our laboratory clearly indicate that the Bi adatom is fully discharged [8]. These measurements also indicate that hydroxide is co-adsorbed with the Bi in a 1:2 ratio. Thus the open adlattice structure is again due to anion co-adsorption although in this case the anion is not deliberately added to the solution but rather originates from the solvent itself. The presence of hydroxide in the potential region exhibiting maximal catalytic activity has important consequences for the mechanism of this activity [9], More importantly, this example shows that the SPM techniques are most powerful when utilized in conjunction with other techniques providing more quantitative information about the number and type of species present on the electrode surface.

Although the focus of UPD work is often on single-crystal surfaces of Au, Ag, or Pt, much important electrochemistry occurs on more technically relevant materials and materials which are less easily prepared. Work in the Gewirth group has focused on Cu surfaces with the goals of understanding aspects of Cu surface chemistry and developing relationships between this surface chemistry and the subsequent Cu bulk deposition behavior. The focus on bulk deposition of Cu is important because of the use of this process in industry.

Fig. 2. (a) 3 nm x 3 nm AFM image of electropolished Cu(100) in 0.1 M HC104 at -0.45 V vs. Hg2S04. The structure is square with an interatomic spacing of 0.36 nm. (b) 9 nm x 9 nm AFM image of two-domain region with Cu(l 10) (upper right) and (2x1) rows (lower left) observed at after a potential of -550 mV was applied.

Initial work examined structures formed on Cu(h,k,l) surfaces in the acid electrochemical environment. Figure 2 shows adlayer structures formed on the Cu(100) [10] and Cu(110) [11] electrode surfaces as imaged by in- situ atomic force microscopy. On the Cu(100) surface a (V2 x V2)R45° adlattice structure was observed at all but the

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