Semiconductive Behavior of Anodic Titanium Oxide Films with Different Thicknesses

Abstract

The high corrosion resistance of metal in aqueous environment depends on a thin oxide film formed on its surface. Oxide films on many metals and alloys exhibit the semiconductive property that is considered to be related to corrosion behavior. Therefore elucidating the semiconductive property of oxide film is necessary to understand corrosion resistance of metals and alloys. In order to discuss the semiconductive property of oxide film, in-situ analytical techniques are desired as the denature of the oxide film during evaluation is avoided. Photoelectrochemical response and electrochemical impedance spectroscopy have been applied to study the semiconductive property of oxide film in-situ. In the present work, the semiconductive property of anodic Ti oxide films with different thickness was analyzed using the techniques. Materials were pure Ti discs embedded into epoxy resin. Prior to experiments, its surface was mirror-finished, then ultrasonically cleaned in methanol, ethanol and deionized water for 5 min, successively. The mirror-finished Ti surface was anodized at various potentials ranged from 3 V to 7 V for 60 min in a 0.01 M Na2SO4 solution and phosphate buffer solution, then examined with electrochemical impedance spectroscopy and photoelectrochemical response. For the electrochemical impedance spectroscopy, the capacitance of space charge region was measured and analyzed to estimate donor density and flat band potential. The donor density decreased and the flat band potential increased with increasing film formation potential for all samples. This result indicates that the electronic property of the oxide films is correlated with the film formation potential. In the photoelectrochemical response, a photocurrent exhibited similar transients under all conditions. A spectrum of the photocurrent was separated into two components; a spike photocurrent and a subsequent steady photocurrent. At high measuring potentials, spike charge estimated from the spike photocurrent and the steady photocurrent decrease with increasing the film formation potential. At lower potentials, on the other hand, the spike charge and the steady photocurrent increase with increasing the film formation potential. This result suggests that not only film formation conditions but also measurement conditions influence the photoelectrochemical response. In the presentation, the results will be discussed based on results for electrochemical impedance spectroscopy.