Abstract: The effect of calcining temperatures on the particle size coarsening, the specific surface area lowering and band gap blue shifting of nanocrystalline TiO₂ in
the anatase, mixed-phases, and rutile phase was investigated. The rate of the particle size coarsening and the surface area lowering of TiO₂ in the rutile
phase are much higher than that in the anatase phase. This result means that it is hard to prepare nanocrystalline TiO₂ in the rutile by anatase-to-rutile
transformation or reactions at high temperature. When nanocrystalline TiO₂ in the rutile phase is heated under vacuum, it has higher surface area as well
as in finer size than that of one calcined in the air, however, heating under vacuum always results in nonstoichiometry and crystal defect, which is detrimental to its applications.

Key words: titania, calcining, specific surface area, particle size