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

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.