A multi-technique study of compact and hydrous Au oxide growth in 0.1 M sulfuric acid solutions

The growth and reduction of compact (alpha-) and overlying hydrous (beta-) oxide films on polycrystalline Au electrodes in aqueous 0.1 M H2SO4 solutio ns have been investigated using potentiostatic, cyclic voltammetry, ellipso metric and quartz crystal microbalance (QCMB) techniques. All alpha -oxide films, formed with time at constant potentials up to 2.6 V, or by multicycl ing of the potential, are non-hydrated in nature, even when covered by a th ick beta -oxide film. The alpha -oxide him composition is suggested to be A uO below 1.5 V, and a mixture of AuO + Au2O3 at potentials above this, beco ming predominantly Au2O3 at very high potentials. Up to three monolayers of Au2O3 can be formed. When formed at constant potential, the beta -oxide fi lm becomes increasingly hydrated as it thickens with time of growth, with a mass to charge ratio and refractive index consistent with Au2O3.H2O and la ter with Au2O3. 2H(2)O. In contrast, the beta -oxide film formed by multicy cling has a higher mass overall, and becomes less hydrated as it thickens w ith time, with a mass and refractive index consistent with Au2O3. 10H(2)O a t short times, ranging to Au2O3. 2H(2)O as the film thickens. (C) 2001 Else vier Science B.V. All rights reserved.