Microwave-assisted ionic liquid synthesis of Ti3+ self-doped TiO2 hollow nanocrystals with enhanced visible-light photoactivity

The realization of Ti3+ self-doping and exposure of a hollow structure in TiO2 photocatalysts has been proven to be an effective approach for significantly improving their photocatalytic performance. Herein, a facile and green strategy that consists of microwave-assisted ionic liquid synthesis followed by a vacuum-activation process is developed for synthesizing highly active Ti3+ self-doping TiO2 with a hollow structure. The results of X-ray diffraction, N-2 sorption, Raman, X-ray photoelectron spectroscopy, fluorescence spectrophotometry, scanning electron microscopy transmission electron microscopy and scanning transmission electron microscopy analyses revealed that the presence of the ionic liquid [Bmim][BF4] acted not only as a microwave absorbent for the solvothermal process but also as a morphology-controlling agent via a dissolution-recrystallization process, leading to a mesoporous structure. Moreover, the subsequent vacuum calcination process modified the TiO2 to provide highly air-stable Ti3+ and oxygen vacancies. Compared with the benchmark material P25 and other TiO2 nanostructures synthesized using [Bmim]Cl and different molar ratios of [Bmim][BF4], the Ti3+ self-doped TiO2 with a hollow structure was used as an efficient photocatalyst, and it exhibited a 4.6-fold enhancement in photocatalytic activity for the selective oxidation of benzyl alcohol due to its enhanced visible-light absorption, efficient molecular oxygen adsorption ability and improved charge-separation efficiency under visible-light irradiation. Furthermore, the unique disordered core/ordered shell protects the TiO2-x. nanoparticle core from further oxidation and effectively blocks oxidation between the Ti3+ and dissolved oxygen in the solvent, thereby allowing long-term recycling stability. The possible reaction mechanism for the photocatalytic selective oxidation of benzyl alcohol over Ti3+ self-doped TiO2 hollow nanocrystals has also been investigated. The results obtained in this study may shed light on improving the photoactivity in fabricating defective TiO2 and defect-based nanostructures and their applications in solar energy conversion. (C) 2016 Elsevier B.V. All rights reserved.