Control of Exposed Facet and Morphology of Anatase Crystals through TiO_xF_y Precursor Synthesis and Impact of the Facet on Crystal Phase Transition

Anatase crystals with 30% and 80% of {001} facet exposures are prepared by decomposition of two fluorine-contained precursors (TiO x F y ). The two precursors were obtained via solvothermal process by controlling the F – and Ti 4+ concentrations and had a flower-like and hexagonal pencil-like structure, respectively. The two anatase products by calcination of the corresponding precursors at 700 °C have unique morphologies: sheet-like single crystals with square pores and oriented assembly of truncated-octahedron anatase crystals, respectively. Both of them are inherited from the morphologies of the precursor particles. We reveal for the first time that the phase transition temperature of the crystal with larger {001} facet percentage is lower, and the in situ SEM experiments indicate that numerous defect sites first form on {001} facet during the phase transformation of anatase to rutile, while the {101} facet on the same crystal remains smooth. The formation of defect sites is attributed to the preferential (relative to {101} facet) diffusion of bulk vacancy defect toward the {001} facet and agglomeration on this facet during the annealing, which is an important process to start the anatase to rutile phase transformation.