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.