Growth of 1D TiO2 nanostructures on Ti substrates incorporated with residual stress through humid oxidation and their characterizations

Abstract

Different Ti substrates, such as particles (as-received and ball milled), plate and TEM grid were oxidized for the growth of one dimensional (1D) TiO2 nanostructures. The Ti substrates were oxidized for 4 h at temperatures of 700 °C–750 °C in humid and dry Ar containing 5 ppm of O2. The effects of residual stress on the growth of 1D TiO2 nanostructures were investigated. The residual stress inside the Ti particles was measured by XRD-sin2 ψ technique. The oxidized Ti substrates were characterized using field emission scanning electron microscope equipped with energy dispersive x-ray spectroscope, transmission electron microscope, x-ray diffractometer and x-ray photoelectron spectroscope. Results revealed that humid environment enhances the growth of 1D TiO2 nanostructures. Four different types of 1D morphologies obtained during humid oxidation, e.g. stacked, ribbon, plateau and lamp-post shaped nanostructures. The presence of residual stress significantly enhances the density and coverage of 1D nanostructures. The as-grown TiO2 nanostructures possess tetragonal rutile structure having length up to 10 μm along the 〈1 0 1〉 directions. During initial stage of oxidation, a TiO2 layer is formed on Ti substrate. Lower valence oxides (Ti3O5, Ti2O3 and TiO) then form underneath the TiO2 layer and induce stress at the interface of oxide layers. The induced stress plays significant role on the growth of 1D TiO2 nanostructures. The induced stress is relaxed by creating new surfaces in the form of 1D TiO2 nanostructures. A diffusion based model is proposed to explain the mechanism of 1D TiO2 growth during humid oxidation of Ti. The 1D TiO2 nanostructures and TiO2 layer is formed by the interstitial diffusion of Ti4+ ions to the surface and reacts with the surface adsorbed hydroxide ions (OH−). Lower valence oxides are formed at the metal-oxide interface by the reaction between diffused oxygen ions and Ti ions.