Effect of hydrogen on formation of nanoporous TiO2 by anodization with hydrogen-fluoride pretreatment

This study explores the effects of nano-(gamma-TiH and delta-TiH0.71) phases on the formation of nanoporous titanium oxide by anodization with HF pretreatment. The film characteristics were investigated by X-ray photoemission spectra, thin-film X-ray diffraction, transmission and electron scanning microscopy. Titanium hydride was a sacrificial barrier on titanium following HF pretreatment. The sacrificial barrier has nano-(gamma-TiH and delta-TiH0.71) phases. The titanium hydrides, gamma-TiH and delta-TiH0.71, with a tetragonal and an orthorhombic structure, respectively, were formed within titanium matrices during cathodization. The nanophases are directly dissolved after anodization. Nanoporosity and oxidation followed from phase transformations and dissolution reactions of the nanophases. The nano-(gamma-TiH and delta-TiH0.71) phases have important roles in forming nanoporous TiO2. Anodization with HF pretreatment not only induces a titanium hydride layer, but also forms nanostructural titanium oxide. Particle-like gamma-hydride and granular delta-hydride were observed on the Ti matrix and grain boundaries. The gamma-hydride has a tetragonal structure with lattice constant a = 0.421 nm, and the delta-hydride has an orthorhombic structure with lattice constant a = 0.434 nm. The (gamma + delta-hydride) microstructure was present on titanium grain boundaries. In the ( alpha + gamma) matrix, an alpha -> gamma -> delta transition occurred during cathodization. This result has never been previously reported. The surface that contains nanophases (gamma + delta-hydride) is readily dissolved in alkaline solution, so anodization for a short time at room temperature yields nanoporous TiO2. This fact is exploited to avoid having to prepare porous TiO2 by immersing Ti in an alkaline solution for a long time. As mentioned above, bioactive titanium with oxidation and nanostructures is believed to be capable of being prepared by anodization with HF pretreatment for a short time. (c) 2006 The Electrochemical Society.