Manipulating the Formation of NH4TiOF3 Mesocrystals: Effects of Temperature, Surfactant, and pH

There are two main competitive processes involved in the formation of NH4TiOF3 mesocrystals. One leads to the assembly of individual NH4TiOF3 nanocrystals and the subsequent formation of NH4TiOF3 mesocrystals. The other converts the NH4TiOF3 nanocrystals into TiO2 (anatase) nanoparticles and results in the subsequent formation of TiO2 aggregates. Surfactant (Brij 58) molecules appear to facilitate the mesoscale assembly of NH4TiOF3 nanocrystals into NH4TiOF3 mesocrystals via the interaction of their hydrophobic tails. Elevated temperature and higher solution pH (adjusted by ammonia solution) increase the conversion rate of NH4TiOF3 nanocrystals into TiO2 (anatase) nanoparticles. This conversion process reduces the amount of NH4TiOF3 nanocrystals available for the mesoscale assembly and consequently increases the pore volume of the resultant NH4TiOF3 mesocrystals. Fluorination and {001} facets affect the photocatalytic performance of the calcined products (anatase TiO2) of the NH4TiOF3 mesocrystals. The present study demonstrates the key parameters that control the formation and stability of NH4TiOF3 mesocrystals and the photocatalytic behavior of nanocrystalline TiO2 derived from the mesocrystals.