We synthesized TiO2 mesocrystals using a hydrothermal method and investigated the effect of calcination temperature (100–800 °C) on their morphology, crystallinity, and photocatalytic activity. While no appreciable changes in the shape, dimension, and crystal structure of the TiO2 nanoparticles (NPs) were observed as the calcination temperature increased to 300 °C, the crystallinity improved with increasing temperature. The mesocrystal form of the NPs began to disappear at 400 °C, and the specific surface area significantly decreased with increasing temperature owing to the reduced boundaries between the subunits and surface roughness of the NPs. The photocatalytic activity of the TiO2 NPs improved when the temperature increased to 300 °C because of the enhanced crystallinity and elimination of byproducts; on the other hand, it degraded above 400 °C due to the decreased surface area. These results suggest that controlling the calcination temperature is an effective way to tailor the morphology, crystallinity, and photocatalytic activity of TiO2 NPs.

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.