Growth of brookite TiO₂ nanorods by thermal oxidation of Ti metal in air

The multiphase TiO₂ containing brookite and rutile were grown by thermal oxidation of Ti metal in the range of 650–800 °C. The surface seeds formed by HCl acid solution etching at 80 ℃ promoted the growth of brookite nanorods. The co-growth of two nanorods with a diameter of about 150 nm was observed at 800 °C. The growth mechanism was discussed in consideration of the diffusion behaviour as a function of oxidation temperature and time. The thermal oxidation is a diffusion-controlled process as described by D_s = 3.4exp[−165.5/RT]. The diffusion coefficient at 800 °C is 40 times that at 700 °C, which is the transformation temperature for the diffusion of oxygen in Ti and self-diffusion of Ti. The cathodoluminescent property was evaluated and a strong broad emission band from 400 to 700 nm was observed. The luminescence mechanism is related to the self-trapped excitons on the TiO₆ octahedral, oxygen vacancies, the formation of Ti³⁺ ions and associated defects.