STUDY OF THICK ANODIC OXIDE-FILMS ON PT BY SPECTROSCOPIC ELLIPSOMETRY

We describe a study of the oxide films grown on a platinum (Pt) electrode by anodization at 2.1 V in a 0.5 M H2SO4 solution. These oxide films were studied before by single lambda ellipsometry (1) and by cyclic voltammetry (2). Our present study employed a spectroscopic ellipsometer, and thus allowed us to measure in situ the spectrum of the anodic oxide film in the wavelength range 300 to 800 nm, at various stages of film growth. We show that both film thickness and optical spectrum can be solved at each stage in oxide growth by assuming a highly absorbing thin film (congruent-to 30 angstrom) adjacent to the metal surface, and, on top of it, a second film, which is most probably a hydrous form of PtO2. The latter film reaches thicknesses of several hundred angstrom after more than two days of anodization. The thick film seems to grow at a constant rate, unaffected by its thickness. The optical spectrum of the latter film shows a nonzero extinction coefficient, k(f), in the visible, which tends to increase toward the near-UV end of the lambda-domain probed, suggesting a bandgap > 3.5 eV. A typical range for the value of triple overdot n(film) of the thicker external oxide layer is triple overdot n = (1.70 +/- 0.03) - (0.15 +/- 0.05)i.