MECHANISM OF SURFACE-DIFFUSION OF GOLD ADATOMS IN CONTACT WITH AN ELECTROLYTIC SOLUTION

Temperature, 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 surfa ce diffusion process of gold adatoms in contact with a 0.5 M H2SO4 aqu eous solution. The measurements were carried out at five different pot ential values, negative and positive with respect to the zero charge p otential of polycrystalline gold (E(z) = -0.41 V(MSE)) 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 t o negative values for the activation entropies, instead of the expecte d positive value for the same magnitude derived from measurements made at the gold-vacuum interface. The activation enthalpy values measured in the potential range between 0 and 1000 mV(SHE) decrease with the p otential value, while the activation entropies increase with them, sho wing an example of the so-called compensation effect. These values are interpreted as the consequence of the formation of an activated compl ex for the surface diffusion process that is more ordered than the sys tem: electrode surface + adatom original. The enhancement of order mig ht be produced by the fact that before surface diffusion can take plac e, a partial charge transfer from the adatom to the electrode surface occurs which accumulates a positive charge on the adatom original, wit h the consequent orientating effect on the solvent dipoles forming par t of the double layer.