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

This study explores the effects of nano-(γ-TiH and δ-TiH _0.71) 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-(γ-TiH and δ-TiH_0.71) phases. The titanium hydrides, γ-TiH and δ-TiH_0.71 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-(γy-TiH and δ-TiH _0.71) phases have important roles in forming nanoporous TiO ₂. Anodization with HF pretreatment not only induces a titanium hydride layer, but also forms nanostructural titanium oxide. Particle-like γ-hydride and granular δ-hydride were observed on the Ti matrix and grain boundaries. The γ-hydride has a tetragonal structure with lattice constant a = 0.421 nm, and the δ-hydride has an orthorhombic structure with lattice constant a = 0.434 nm. The (γ + δ-hydride) microstructure was present on titanium grain boundaries. In the (α + γ) matrix, an α → γ → δ transition occurred during cathodization. This result has never been previously reported. The surface that contains nanophases (γ + δ-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.