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

Abstract

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, t p, 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 E p, 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 10 8-109 V m-1. © 2004 Elsevier B.V. All rights reserved.