In situ mass and ellipsometric study of hydrous oxide film growth on Pt in alkaline solutions

Abstract

The growth and reduction of compact (α-) and hydrous (β-) oxide films on polycrystalline Pt electrodes in aqueous 0.1 M NaOH solutions have been investigated using cyclic voltammetry, as well as in situ ellipsometry and the quartz crystal microbalance (QCMB) techniques. All α-oxide films, formed in base with time at constant potentials up to 1.9 V, or by multi-cycling of the potential, are non-hydrated in nature, even when covered by a thick β-oxide film. Two different forms of α-oxide film are generated, suggested to be PtO at lower potentials (≤1.6 V) and PtO 2 at higher potentials (>1.6 V), based on their different growth rates and optical properties. All β-oxide films are different from the α-oxide, being hydrated to varying extents. Based on the measured refractive index of 2.5, and their mass, thin β-oxide films are suggested to be PtO 2.H 2O, when formed using standard growth conditions (2 V/s between 0.5 and 2.8 V). As more β-oxide film is formed, the outer regions become more hydrated ( n as low as 1.8), and the suggested film composition is PtO 2.1.2H 2O. Pt β-oxide films are even more hydrated when formed using more negative E − and E + limits during growth (while maintaining the other limit constant at either 0.5 or 2.82 V), resulting in film masses and a refractive index consistent with PtO 2.3.5H 2O.