Mechanism of surface diffusion of gold adatoms in contact with an electrolytic solution

Tempera ture, potential, and time-dependent measurements of the change of the roughness factor of electrodispersed gold electrodes allowed a derivation of the activation enthalpy and entropy values for the surface diffusion process of gold adatoms in contact with a 0.5 M H2SO4 aqueous solution. The measurements were carried out at five different potential values, negative and positive with respect to the zero charge potential of polycrystalline gold (Ez = -0.41 VMsE) and a potential value within the potential range at which gold oxides are formed. The low preexponential factors deduced from In D vs T-1 plots give rise to negative values for the activation entropies, instead of the expected positive value for the same magnitude derived from measurements made at the goldvacuum interface. The activation enthalpy values measured in the potential range between 0 and 1000 mV SHE decrease with the potential value, while the activation entropies increase with them, showing an example of the so-called compensation effect. These values are interpreted as the consequence of the formation of an activated complex for the surface diffusion process that is more ordered than the system: electrode surface + adatom original. The enhancement of order might be produced by the fact that before surface diffusion can take place, a partial charge transfer from the adatom to the electrode surface occurs which accumulates a positive charge on the adatom original, with the consequent orientating effect on the solvent dipoles forming part of the double !ayer.