Elementary steps of electrochemical oxidation of single-crystal planes of Au—I. Chemical basis of processes involving geometry of anions and the electrode surfaces

Abstract

Single-crystal planes of Au electrodes in the (111), (100) and (110) orientation were studied by the potentiodynamic sweep method in solutions of anions which adsorb weakly (HClO 4) and strongly (H 2SO 4) on Au. It is shown how the specific adsorption of anions which appear to be fully discharged on the (111) and (110) planes, but very little or not at all on the (100) planes, depends on the symmetry of the arrangement of surface atoms in relation to the three-fold geometry of the tetrahedral anions. This in turn determines the type and extent of a coordinative deposition (UPD) of OH in between adsorbed anions, which seems to occur with only partial charge transfer (γ = 0.5). The resolution of the processes of OH deposition in sub-lattices amongst the adsorbed anions is found to be specific for the three low-index planes examined and is interpreted in terms of the states of adsorption of the anions in relation to lattice geometry and charge transfer. The adsorbed anions, by having a stabilizing effect on the MOH (1−γ)-species on the surface, influence also the kinetics of the process of replacement of adsorbed anions by deposited OH on the surface and the concerted M/OH turn-over process which constitutes the beginning stage of formation of bulk-phase oxide material on the electrode.