Structural transformations in nanocrystalline anatase TiO 2 films upon annealing in air

Nanocrystalline anatase TiO ₂ films were prepared from colloidal suspensions using particles with a size of 12 nm. Their structure was examined by Raman spectroscopy and X-ray diffraction (XRD). The as-prepared specimens exhibit exclusively features due to the anatase phase of TiO ₂ (e.g., the E _g, B _1g and A _1g vibration modes in Raman spectroscopy and the characteristic diffraction peaks in XRD). Isochronal annealing of the films in air at temperatures of up to 1320 K effected clear structural changes, observed both in Raman and XRD: the crystallite size increases from ∼13 nm to ∼90 nm between 720 K and 1170 K, with the crystallites remaining in the anatase phase. A phase transition to the rutile phase of TiO ₂ occurs in the temperature range 1170-1320 K and the average crystallite size increases to ∼160 nm.