Size effect on phase transition sequence of TiO2 nanocrystal

Titanium dioxide nanocrystal with a mixture of anatase and brookite was prepared by sol-gel method. The effects of the grain size and the phase content on the transition were studied for TiO2 nanocrystal annealed in air for 1 h at the temperature range 200-650 deg C. The experimental results indicate that the phase transition sequence depends on relative grain size between anatase and brookite. Under the same grain sizes anatase, brookite, and rutile is the most stable at the size smaller than 4.9 nm, between 4.9 and 30 nm, and larger than 30 nm, respectively. Thermodynamic analysis gives an empirical expression on a critical grain size of brookite Dc, which dominates the transition sequence between anatase and brookite. As the size of brookite Db reaches Dc, brookite and anatase phases transform directly to rutile. When Db > Dc, anatase phase transforms to brookite and than brookite to rutile and/or directly to rutile; when Db < Dc, brookite phase transforms to anatase and then anatase to rutile and/or directly to rutile. The origin of the transition sequence was discussed from the view of free energy.