PALLADIUM-CHLORIDE TO PALLADIUM METAL 2-DIMENSIONAL NUCLEATION AND GROWTH PHENOMENA

The reduction of a monolayer of surface-bound Pd(II) to Pd(0) on a palladium substrate reveals two-dimensional nucleation and growth phenomena. Using well-known 2D nucleation-growth theories, this reduction is shown to proceed by an instantaneous nucleation and growth mechanism. However, when a submonolayer of Pd(II) is present, this mechanism fails to account for the experimentally observed high cathodic currents seen at zero time. A model incorporating preexisting Pd(0) cylindrical sites on the partially oxidized Pd(0) surface has been successfully applied to account for the discrepancy between the experimental results and current 2D theories. Using this modified 2D model, values for the mathematical product of cylindrical growth rate and the square root of the nucleation site densities have been determined, and the overpotential dependence of the growth rate has been confirmed and quantified. These 2D nucleation-growth phenomena have practical consequences on the performance of the Pd(II)/Pd(0) system as a faradaic supercapacitor, and probably on the performance of other 2D faradaic supercapacitor systems. In addition, because many electrodes undergo monolayer surface oxidation-reduction reactions in other solvent systems, especially aqueous electrolytes, these 2D nucleation-growth phenomena may play a role in these important surface-modifying redox systems as well.