Be careful, nano-TiO2 will undergo phase transitions when annealed at temperatures above ca. 250°C, check the literature for more details (e.g. by Zhang and Banfield in Chem. Mater. 2005, Phys. Rev. B 2008, Chem. Rev. 2014, etc.). Basically, anatase is the stable modification for smaller particles, while heating leads to larger particles, where rutile TiO2 is thermodynamically favourable. Intermediate, brookite may also be present... Hope this helps, Dirk