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.