Cause of slow phase transformation of TiO₂ nanorods

The phase transformation from anatase to rutile in nanocrystalline TiO ₂ nanorods comprised of length and diameter of approximately 35 and 4 nm, respectively, has been investigated. TiO₂ nanorods were synthesized at 100 °C with titanium isopropoxide as a presusor, oleic acid (OLEA) as a surfactent, and trimethylamine as a catalyst. Characteristic absorption peaks of OLEA were observed from the OLEA-caped TiO₂ nanorods after synthesis and disappeared after annealing process. No phase transformation (from anatase to rutile) were observed after annealing at 750 °C for 2 h. Only small portion of phase transformation occurred after annealing at 850 °C for 2 and 3 h. A relatively slow phase transformation from anatase to rutile might be due to the small diameter of TiO₂ nanorods resulting favored surface free energy of the anatase phase.