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

Different Ti substrates, such as particles (as-received and ball milled), plate and TEM grid were oxidized for the growth of 1D TiO 2 nanostructures.The Ti substrates were oxidized for 4 h at temperatures of 700-750 °C in humid and dry Ar containing 5 ppm of O 2 .The effects of residual stress on the growth of 1D TiO 2 nanostructures were investigated.The residual stress inside the Ti particles was measured by XRD-sin 2 ψ technique.The oxidized Ti substrates were characterized using field emission scanning electron microscope (FESEM) equipped with energy dispersive X-ray (EDX) spectroscope, transmission electron microscope (TEM), X-ray diffractometer (XRD) and Xray photoelectron spectroscope (XPS).Results revealed that humid environment enhances the growth of 1D TiO 2 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 TiO 2 nanostructures possess tetragonal rutile structure having length up to 10 µm along the <1 0 1> directions.During initial stage of oxidation, a TiO 2 layer is first formed on Ti substrate.Lower valence oxides (Ti 3 O 5 , Ti 2 O 3 and TiO) then form underneath the TiO 2 layer and induce stress at the interface of oxide layers.The induced stress plays significant role on the growth of 1D TiO 2 nanostructures.The induced stress is relaxed by creating new surfaces in the form of 1D TiO 2 nanostructures.A diffusion based model is proposed to explain the mechanism of 1D TiO 2 growth during humid oxidation of Ti.The 1D TiO 2 nanostructures and TiO 2 layer is formed by the interstitial diffusion of Ti 4+ 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.