Specular reflectivity studies of the adsorption of halide anions on gold electrodes in 0.2. M HClO4

Abstract

The specular reflectivity of gold has been measured in 0.2 M HClO4 with and without halide ions present. The reflectivity potential curves depend on the halide concentration at potentials where halide ions are adsorbed. Increasing halide ion concentration decreases the reflectivity with a limiting value approached at higher halide ion concentrations. The decrease in reflectivity produced by anion adsorption also is potential dependent and approaches a limiting value at more anodic potentials. Hysteresis is evident in the reflectivity-potential curves recorded with cyclic linear potential sweeps when the sweep rates and halide ion concentrations are such that diffusion of halide ions to and from the electrode is expected to be rate controlling. Potential-step techniques and simultaneous reflectivity measurements have been used to demonstrate diffusion control of the adsorption kinetics at low halide concentrations. The factors involved in the change of the reflectivity of the gold-electrolyte solution interface with potential and anion adsorption are discussed. The change in the surface electron charge density appears to be particularly important. The reflectivity and voltammetry data have been used to determine the “adsorption potential” for the I−, Br−, and Cl− anions, (corresponding to the potential of the voltammetry peak or the maximum in the slope of the reflectivity-potential curve). These adsorption potentials have been found to fit a linear dependence on the third power of the ionic radius reasonably well as is to be expected if the principal factor responsible for the difference in adsorptivity of these three ions is their polarizability.