Phase and morphological transitions of titania/titanate nanostructures from an acid to an alkali hydrothermal environment

A systematical investigation was established to uncover the phase transition of titania/titanate nanostructures in acidic and alkaline media. X-ray diffraction was used to identify the phase composition and phase transition behavior, and field emission scanning electron microscopy was employed to elucidate the morphological transitions of the titania/titanate products, which resulted in the determination of a phase and morphological transition diagram of titania/titanate nanostructures in connection with the acid/alkali concentrations. The crystal growth of rutile nanorods and anatase nanoparticles was proposed following Ostwald's ripening mechanism, but crystal growth of brookite and titanate products should follow another formation mechanism according to Ostwald's step rule, which reveals the phase transitions from layered structure to brookite, dititanate or trititanate under hydrothermal treatment. The anatase samples and anatase–rutile composite obtained in 0.1 M HCl exhibited excellent photocatalytic activities in the photodegradation experiments of methyl orange, salicylic acid and phenol solution under UV irradiation.