Nanogravimetry study of the initial stages of anodic surface oxide film growth at Au in aqueous HClO4 and H2SO4 by means of EQCN

The dependence of the initial stages of anodic oxide film growth at Au on time or log time, tau(h), was investigated by means of nanogravimetry using a electrochemical quartz crystal nanobalance (EQCN) which provides a new dimension for such studies, complementary to voltammetry, chronoamperometry and ellipsometry. Films were formed potentiostatically in sulphuric or perchloric acids. The mass to charge ratio determined over the potential range corresponding to Au oxide formation is found to be ca. 6-7 g per mole of electrons, which is close to the value expected for formation of Au/O-ads on the electrode surface. The relations between charge and 109 tau(h), or mass and log tau(h), were evaluated. The charge for prior formation of 0 species, determined from reduction voltammetry, increases linearly with the log of the holding time. However, it is of special interest that the mass change in HClO4 solutions does not increase linearly with the log of the holding time. This is interpreted in terms of co-adsorption of anions and displacement of water molecules. The results enable mechanisms of oxide film formation at Au to be critically appraised. (C) 2003 Elsevier Science Ltd. All rights reserved.