Insights into TiO2 polymorphs: highly selective synthesis, phase transition, and their polymorph-dependent properties

Abstract

Despite the enormous efforts devoted to the research of titanium dioxide (TiO₂), controlling TiO₂ polymorphism, size and morphology still represents an interesting challenge since its stabilities, transitions and properties vary with them. Herein, we report a novel selective-synthesis of high purity anatase (A), rutile (R) and brookite (B) phase TiO₂ from solution under mild hydrothermal conditions. By adjusting the concentrations of chloroacetic acid, urea and sodium hydroxide, A-TiO₂ nanocrystals, R-TiO₂ nanorods and B-TiO₂ hollow spheres and nanorods can be easily produced as investigated by XRD and TEM characterisation. Based on time-varied experiments, two distinct phase transitions were observed under different hydrothermal conditions, namely the initially formed A-TiO₂ nanocrystals would transform into R-TiO₂ nanorods with augmented crystalline size under acidic conditions while they transformed into B-TiO₂ hollow spheres under basic conditions. Moreover, polymorph formation and transition mechanism were explored in detail based on systematic investigation into the effects of synthetic parameters on the products. Finally, the spectra and photocatalytic properties of the three polymorphs were investigated and discussed. The synthetic approach reported here allows for full control over polymorph selection in TiO₂ crystallization and provides new insights in this area.