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

摘要

Three illustrative cases involving the electrodeposition of ultrathin metalfilms of varying reactivities onto noble-metal substrates were investigated: (i) Pdon Pt(111), a noble admetal on a noble-metal surface; (ii) Bi on Pd(111), a lessnoble admetal on a noble-metal surface; and (iii) Co on polycrystalline Pd andPd(111), a reactive metal on a noble-metal surface. The interfacialelectrochemistry of these prototypical systems was characterized using acombination of electrochemical methods (voltammetry and coulometry) andultrahigh 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 bromideexerted surface smoothening effects on ultrathin Pd films with defect sites(steps). This procedure, dubbed as electrochemical (EC) annealing, constituted anonthermal analogue to conventional annealing. EC-annealed ultrathin Pd filmsexhibited long-range surface order and remained free of oxygen adspecies. Pdadatoms occupying step-sites were selectively dissolved and/or rearranged toassume 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) electroderoughened by oxidation-reduction cycles; and (ii) thermally annealedpolycrystalline Pd, which is a composite of the (111) and (100) facets.Electrodeposition of Co was also observed to be kinetically hindered and slowpotential scan rates (0.1 mV/s) were required.Well-defined ultrathin Bi films were potentiostatically electrodepositedonto Pd(111); a Stranski-Krastanov growth mode was indicated. Theelectrochemical reactivity of ultrathin Bi films was characterized using twosurface probes: aqueous iodide and D-glucose. (i) Exposure of the prepared Biadlayers (????Bi 0.33) to aqueous iodide gave rise to (??????3x??????7) I-on-Bi superlattice.The same superlattice was obtained if Bi was electrodeposited ontoPd(111)(??????3x??????3)R30o-I. (ii) With respect to electrooxidation of D-glucose onPd(111), the presence of Bi adlayers inhibited the by-product-induced "surfacepoisoning" of Pd(111) but reduced its electrocatalytic efficiency.