GOLD ELECTROCRYSTALLIZATION ON CARBON AND HIGHLY ORIENTED PYROLYTIC-GRAPHITE FROM CONCENTRATED-SOLUTIONS OF LICL

The early stages of the electrolytic deposition of gold on carbon elec trodes from concentrated LiCl electrolytes have been investigated by v oltammetry, chronoamperometry, and microscopy. The analysis of current -time transients according to existing theories indicates that this pr ocess occurs by multiple three-dimensional nucleation, followed by dif fusion-controlled growth of nuclei. Diffusion coefficients calculated on the basis of nucleation theory were found to be higher than those f rom rotating disk experiments (RDE), suggesting that gold species are adsorbed on the electrode surface. Gold nucleation from 6 M LiCl canno t be classified as either instantaneous or progressive nucleation. The kinetic parameters for nucleation, aN(0) (nucleation rate) and N-0 (n umber density of active sites on the substrate surface), were estimate d by using a general Poisson nucleation law. Both quantities were foun d to vary with potential and with concentration of gold. The potential dependence of the nucleation rate, aN(0), was interpreted according t o classical and atomistic theory For low gold concentrations, where ad sorption of AuCl or AuCl3 might occur prior to nucleation, the number of atoms in the critical nucleus (N-c) was less than unity over the en tire potential range analyzed. For high gold concentrations the number of atoms forming the critical nucleus depends on overpotential. In al l solutions studied nucleation takes place on a limited number of acti ve sites.