Facet-controlled anatase TiO2 nanoparticles through various fluorine sources for superior photocatalytic activity.

Reactive surface-exposed anatase TiO2 (a-TiO2) is highly desirable for applications requiring superior photocatalytic activity. In order to obtain a favorable surface, morphology control of the a-TiO2 using capping agents has been widely investigated. Herein, we systematically study the effects of different F sources (HF, TiF4, and NH4F) as the capping agent on the morphology control and photocatalytic activities of a-TiO2 in a hydrothermal process. When either HF or TiF4 was added, large truncated bipyramids formed with the photocatalytically active {001} facet, whereas the NH4F was not effective for facet control, yielding nanospheres similar to the pure a-TiO2. The morphology changes were related to the decomposition behaviors of the F sources in the solvent material: HF and TiF4 decomposed and supplied F(-) ions before a-TiO2 nucleation, which changed the nucleation rate and growth direction, leading to the resultant a-TiO2 morphology. On the other hand, NH4F supplied F(-) ions after a-TiO2 nucleation and could not change the growth behavior. In terms of the photocatalytic effect, the HF- and TiF4-treated a-TiO2 effectively decomposed ∼90% and ∼80% of methylene blue, respectively, in 1 h, while ∼60% was decomposed for the NH4F-treated a-TiO2. Note that pure a-TiO2 photocatalytically decomposed only ∼10% of methylene blue over the same time. These results pave the way to precise control of the facet of TiO2 through using different capping agents.