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

The growth and reduction of compact (α-) and overlying hydrous (β-) oxide films on polycrystalline Au electrodes in aqueous 0.1 M H2SO4 solutions have been investigated using potentiostatic, cyclic voltammetry, ellipsometric and quartz crystal microbalance (QCMB) techniques. All α-oxide films, formed with time at constant potentials up to 2.6 V, or by multicycling of the potential, are non-hydrated in nature, even when covered by a thick β-oxide film. The α-oxide film composition is suggested to be AuO below 1.5 V, and a mixture of AuO+Au2O3 at potentials above this, becoming predominantly Au2O3 at very high potentials. Up to three monolayers of Au2O3 can be formed. When formed at constant potential, the β-oxide film becomes increasingly hydrated as it thickens with time of growth, with a mass to charge ratio and refractive index consistent with Au2O3·H2O and later with Au2O3·2H2O. In contrast, the β-oxide film formed by multicycling has a higher mass overall, and becomes less hydrated as it thickens with time, with a mass and refractive index consistent with Au2O3·10H2O at short times, ranging to Au2O3·2H2O as the film thickens.