From NH4TiOF3nanoparticles to NH4TiOF3 mesocrystals: steric hindrance versus hydrophobic attraction of F127 molecules

Abstract

The formation of NH₄TiOF₃ mesocrystals can be largely manipulated by controlling the colloidal and chemical stabilities of NH₄TiOF₃ nanoparticles as the building blocks. The factors studied in the present work are temperature and the concentration of F127 (a triblock copolymer). At 23 °C and 35 °C, the presence of 10% F127 gives rise to the formation of submicron particles, whereas, a lower concentration of F127 results in submicron particles and aggregates, and a higher concentration produces a combination of NH₄TiOF₃ mesocrystals and submicron particles. The process is dominated by the formation of NH₄TiOF₃ mesocrystals at 4 °C, where the chemical conversion from NH₄TiOF₃ nanoparticles to TiO₂ nanoparticles is significantly slowed down. A pancake-like conformation of F127 molecules on the hydrophilic particle surfaces is adopted to explain the concentration dependent steric effect and hydrophobic attraction imparted by their PPO chains. The TiO₂ nanocrystals derived from calcination show superior photocatalytic performance, in comparison to TiO₂ mesocrystals, due to the much higher specific surface area.