Manipulating the Formation of NH<sub>4</sub>TiOF<sub>3</sub> Mesocrystals: Effects of Temperature, Surfactant, and pH

There are two main competitive processes involved in the formation of NH<sub>4</sub>TiOF<sub>3</sub> mesocrystals. One leads to the assembly of individual NH<sub>4</sub>TiOF<sub>3</sub> nanocrystals and the subsequent formation of NH<sub>4</sub>TiOF<sub>3</sub> mesocrystals. The other converts the NH<sub>4</sub>TiOF<sub>3</sub> nanocrystals into TiO<sub>2</sub> (anatase) nanoparticles and results in the subsequent formation of TiO<sub>2</sub> aggregates. Surfactant (Brij 58) molecules appear to facilitate the mesoscale assembly of NH<sub>4</sub>TiOF<sub>3</sub> nanocrystals into NH<sub>4</sub>TiOF<sub>3</sub> mesocrystals via the interaction of their hydrophobic tails. Elevated temperature and higher solution pH (adjusted by ammonia solution) increase the conversion rate of NH<sub>4</sub>TiOF<sub>3</sub> nanocrystals into TiO<sub>2</sub> (anatase) nanoparticles. This conversion process reduces the amount of NH<sub>4</sub>TiOF<sub>3</sub> nanocrystals available for the mesoscale assembly and consequently increases the pore volume of the resultant NH<sub>4</sub>TiOF<sub>3</sub> mesocrystals. Fluorination and {001} facets affect the photocatalytic performance of the calcined products (anatase TiO<sub>2</sub>) of the NH<sub>4</sub>TiOF<sub>3</sub> mesocrystals. The present study demonstrates the key parameters that control the formation and stability of NH<sub>4</sub>TiOF<sub>3</sub> mesocrystals and the photocatalytic behavior of nanocrystalline TiO<sub>2</sub> derived from the mesocrystals.