Growth of surface oxides on gold electrodes under well-defined potential, time and temperature conditions

Surface oxides on Au electrodes are formed by application of anodic polarization in 0.5 M aqueous H2SO4 at polarization potentials, Ep, from 1.45 to 2.00 V, for polarization times, tp, up to 104 s and at 278 ⩽ T ⩽ 328 K. Such polarization conditions result in the growth of thin oxides that reveal one feature in the linear-sweep voltammetry (LSV) oxide-reduction profiles, the OC1 peak, which corresponds to the AuO reduction. The oxide growth behavior is influenced by Ep, tp and T, and the higher the Ep, the longer is tp, and the higher the T, the thicker is the oxide layer. The OC1 peak shifts towards less positive potentials upon an increase of Ep or/and tp, but moves towards more positive potentials upon an increase of T. Theoretical data treatment indicates that the Au oxide growth follows two distinct kinetic laws, each arising from a different growth mechanism: (i) logarithmic growth for the oxide for which the thickness is up to 2 ML of AuO and (ii) inverse-logarithmic growth for oxides for which the thickness is greater than 3 ML of AuO. The transition from logarithmic to inverse-logarithmic kinetics occurs when the oxide thickness is in the 2-3 AuO ML range. The logarithmic growth originates from the interfacial place exchange between Ochem and the top-most Au surface atoms, whereas the inverse-logarithmic law arises from the growth being limited by the escape of the Au cation from the metal into the oxide at the inner metal/oxide interface. The surface dipole moment of the Auδ+-Ochemδ- species that drives the place exchange is consistently 1.5 ± 0.1 D. The electric field that assists the interfacial Au cation escape is of the order of 108-109 V m−1.