Electrodeposition of ultrathin Pd, Co and Bi films on well-defined noble-metal electrodes: studies by ultrahigh vacuum-electrochemistry (UHV-EC)

Three illustrative cases involving the electrodeposition of ultrathin metal films of varying reactivities onto noble-metal substrates were investigated: (i) Pd on Pt(111), a noble admetal on a noble-metal surface; (ii) Bi on Pd(111), a less noble admetal on a noble-metal surface; and (iii) Co on polycrystalline Pd and Pd(111), a reactive metal on a noble-metal surface. The interfacial electrochemistry of these prototypical systems was characterized using a combination of electrochemical methods (voltammetry and coulometry) and ultrahigh vacuum electron spectroscopies (Auger electron spectroscopy, AES; low energy electron diffraction, LEED; and X-ray photoelectron spectroscopy, XPS). Potential-controlled adsorption-desorption cycles of aqueous bromide exerted surface smoothening effects on ultrathin Pd films with defect sites (steps). This procedure, dubbed as electrochemical (EC) annealing, constituted a nonthermal analogue to conventional annealing. EC-annealed ultrathin Pd films exhibited long-range surface order and remained free of oxygen adspecies. Pdadatoms occupying step-sites were selectively dissolved and/or rearranged to assume equilibrium positions in a well-ordered (1x1) film. Electrodeposition of Co was found to be highly surface-structuresensitive. While virtually no Co electrodeposition transpired on a clean Pd(111) surface, Co was voltammetrically deposited on (i) a Pd(111) electrode roughened by oxidation-reduction cycles; and (ii) thermally annealed polycrystalline Pd, which is a composite of the (111) and (100) facets. Electrodeposition of Co was also observed to be kinetically hindered and slow potential scan rates (0.1 mV/s) were required. Well-defined ultrathin Bi films were potentiostatically electrodeposited onto Pd(111); a Stranski-Krastanov growth mode was indicated. The electrochemical reactivity of ultrathin Bi films was characterized using two surface probes: aqueous iodide and D-glucose. (i) Exposure of the prepared Bi adlayers (????Bi 0.33) to aqueous iodide gave rise to (??????3x??????7) I-on-Bi superlattice. The same superlattice was obtained if Bi was electrodeposited onto Pd(111)(??????3x??????3)R30o-I. (ii) With respect to electrooxidation of D-glucose on Pd(111), the presence of Bi adlayers inhibited the by-product-induced "surface poisoning" of Pd(111) but reduced its electrocatalytic efficiency.