Enhanced photocatalytic hydrogen evolution under visible-light using C, N co-doped mesoporous TiO2 nanocrystals templated by ionic liquids

Introducing heteroatoms into TiO2 photocatalysts is an effective way to improve its visible-light driven photocatalytic performance. However, the strategies to realize it always require hazardous reagents and/or high temperature. Herein, we put forward a facile and environmentally benign strategy for synthesizing C, N co-doped mesoporous TiO2 nanocrystals. The preparation consists of microwave heating of a simple mixture of H2O, imidazolium-based ionic liquid (IL) and tetrabutyl titanate and subsequent heat-treatment. During the synthesis, the IL acts as reaction solvent, microwave absorbent, C and N sources, and structure-directing agent. Such a strategy not only simplifies the synthesis by avoiding the employment of additional reactants and structure-directing agents towards constructing TiO2 with small sizes, well-developed mesoporous structure and abundant C, N dopants in the lattices, but also lows down the post-processing temperature, which can relieve the secondary growth of TiO2 grains. Benefiting from the mesoporous structure and heteroatom co-doping, the synthesized material shows high specific-surface area, excellent hydrophilicity, elevated valence-band edges, abundant defect levels and narrowed band-gap, which contribute to the improved visible-light absorption, reduced photogenerated electron-hole recombination, and rapid charge transfer and surface-catalyzed reactions. As a result, the material exhibits higher photocatalytic H2 generation performance under the visible-light irradiation than many conventional types of doped TiO2. Besides, the formation mechanism of this TiO2 and the relationships of IL with the morphology and co-doping process have been revealed. This work highlights great potential of IL-assisted microwave synthesis in developing heteroatoms co-doped TiO2 with enhanced photoactivity and application in the solar energy conversion.