Growth Kinetics of the Anodic Oxide Film on Platinum under Potentiodynamic Polarization Conditions

Abstract

A theory for the anodic oxidation of a metal under linear potentiodynamic conditions is derived based upon the Point Defect Model (PDM), by considering two contributions to the current; that from the metal oxidation reaction and that from capacitive charging. The veracity of the theory is demonstrated by analyzing linear potentiodynamic polarization curves for platinum in the oxide formation potential region. By optimizing the derived potential sweep rate-dependent current density expression on the measured ivs.V polarization data for the electrochemical oxidation of platinum in 0.1 M KOH solution at ambient temperature (22 ± 2 ºC) as a function of potential sweep rate, kinetic parameters for the growth and dissolution of the anodic film on platinum are extracted. The growth and dissolution rates of the oxide film are of the order of × 10¹² m/s (0.001 nm/s). The structure and electronic properties of the anodic film on platinum are also discussed. The film is n-type in electronic character, and is postulated to be a nano-crystalline structure probably comprising columnar, tetragonal unit cells or half cells (monolayer of oxygen) oriented with the c-axis perpendicular to the metal surface for an optimal epitaxial relationship with the substrate metal. For the very thinnest films (<0.1 nm thick), the film is postulated to comprise “buried” oxygen atoms or ions in the platinum surface.