Temperature-dependent grain growth and phase transformation in mixed anatase-rutile nanocrystalline TiO_2 films

摘要

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 ~50nm, respectively, as determined from their characteristic diffraction peaks in XRD. Isochronal annealing of the films for 90min 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 7_a>970K the anatase and rutile grain sizes increase, while above ~1170K, 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 470-870 K) to unity (above 1170K). At 7_aa = 970K, 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.