Physico-Chemical Characterization of Anodic Oxides on Hf as a Function of the Anodizing Conditions

Anodic films were grown to 5 V (Ag/AgCl) on mechanically polished Hf in 0.1 M ammonium biborate and 0.1 M NaOH. Independent of the anodizing conditions, the photoelectrochemical characterization allowed the observation of optical transitions at 3.25 eV, i.e. at photon energy lower than the bandgap of HfO2. They are attributed to localized states inside the gap of the oxide induced by the presence of oxygen vacancies. From the cathodic photocurrent spectra, it was possible to estimate an energy threshold of similar to 2.15 eV for internal electron photoemission phenomena. The impedance measurements proved the formation of insulating oxides with epsilon = 19. The anodizing occurs under a high field regime with an activation energy of 1.1 eV and an activation distance of 3.8 angstrom. (C) 2016 The Electrochemical Society. All rights reserved.