Hierarchical TiO2 Nanospheres with Dominant {001} Facets: Facile Synthesis, Growth Mechanism, and Photocatalytic Activity

Hierarchical TiO2 nanospheres with controlled surface morphologies and dominant {001} facets were directly synthesized from Ti powder by a facile, one-pot, hydrothermal method. The obtained hierarchical TiO2 nanospheres have a uniform size of 400500 nm and remarkable 78?% fraction of {001} facets. The influence of the reaction temperature, amount of HF, and reaction time on the morphology and the exposed facets was systematically studied. A possible growth mechanism speculates that Ti powder first dissolves in HF solution, and then flowerlike TiO2 nanostructures are formed by assembly of TiO2 nanocrystals. Because of the high concentration of HF in the early stage, these TiO2 nanostructures were etched, and hollow structures formed on the surface. After the F- ions were effectively absorbed on the crystal surfaces, {001} facets appear and grow steadily. At the same time, the {101} facets also grow and meet the {101} facets from adjacent truncated tetragonal pyramids, causing coalescence of these facets and formation of nanospheres with dominant {001} facets. With further extension of the reaction time, single-crystal {001} facets of hierarchical TiO2 nanospheres are dissolved and TiO2 nanospheres with dominant {101} facets are obtained. The photocatalytic activities of the hierarchical TiO2 nanospheres were evaluated and found to be closely related to the exposed {001} facets. Owing to the special hierarchical architecture and high percentage of exposed {001} facets, the TiO2 nanospheres exhibit much enhanced photocatalytic efficiency (almost fourfold) compared to P25 TiO2 as a benchmark material. This study provides new insight into crystal-facet engineering of anatase TiO2 nanostructures with high percentage of {001} facets as well as opportunities for controllable synthesis of 3D hierarchical nanostructures.