Nanocrystalline anatase TiO 2 films were prepared from colloidal suspensions using particles with a nominal 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 ~125 nm between 470 K and 1220 K, with the crystallites remaining in the anatase phase. A phase transition to the rutile phase of TiO 2 occurs gradually in the temperature range 1220 -1320 K and the average crystallite size increases to ~160 nm. The E g vibration in anatase films reveals a blue shift and an asymmetric peak broadening towards higher frequencies for smaller particle size/lower annealing temperatures. A power-law dependence between the Raman peak width and the inverse of the crystallite size is found.
Nanocrystalline TiO 2 films were prepared from colloidal suspensions using a mixture of anatase and rutile TiO 2 particles. Their structure was examined by X-ray diffraction (XRD). The as-prepared specimens exhibit the presence of both anatase and rutile crystallites with average sizes of $30 and $50 nm, respectively, as determined from their characteristic diffraction peaks in XRD. Isochronal annealing of the films for 90 min in air at temperatures of up to 1270 K effected clear structural changes: between 470 and 870 K, the crystallite sizes do not change, with the crystallites remaining in their original phase. For annealing temperatures T a ! 970 K the anatase and rutile grain sizes increase, while above $1170 K, the anatase phase disappears and only crystallites in the rutile phase of TiO 2 are observed. Their size increases further with increasing annealing temperature and reaches $150 nm at 1270 K. Concurrently, the weight fraction of rutile changes from an initial value of $0.09 (for 470À870 K) to unity (above 1170 K). At T a ¼ 970 K, the increase of the duration of annealing from 90 to 360 min leads to a moderate increase of the crystallite size. The results indicate that the transformation of anatase particles is governed by the concurrent occurrence of interface and surface nucleation processes.
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