Temperature-dependent grain growth and phase transformation in mixed anatase-rutile nanocrystalline TiO2 films

Nanocrystalline TiO2 films were prepared from colloidal suspensions using a mixture of anatase and nitile TiO2 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 similar to 30 and similar to 50 mn, 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 nitile grain sizes increase, while above similar to 1170 K, the anatase phase disappears and only crystallites in the rutile phase of TiO2 are observed. Their size increases further with increasing annealing temperature and similar to 150 nm at 1270K. Concurrently, the weight fraction of rutile changes from an initial value of similar to 0.09 (for 470-870 K) to unity (above 1170K). At T-a = 970 K the increase of the duration of annealing from 90 to 360 min leads to a moderate in-crease of the crystallite size. The results indicate that the transformation of anatase particles is governed by the concurrent occurrence of interface and surface nucleation (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim