Facile synthesis of nitrogen-doped TiO2 microspheres containing oxygen vacancies with excellent photocatalytic H2 evolution activity

Nitrogen dopant and oxygen vacancies in the TiO2 featured the promoted photocatalytic performance. However, it is still lack of simple and effective methods to introduce oxygen-deficient vacancies into TiO2. In this work, nitrogen-doped TiO2 microspheres assisted by formamide with high crystallinity and rich oxygen vacancies are prepared by a one-step solvothermal method without further sintering process and applied in photocatalytic under sunlight illumination. The anatase formamide-assisted TiO2 microspheres of 1.5 μm with oxygen vacancies and the doped nitrogen content of 1.8% were confirmed by the characterizations of Transmission Electron Microscope, Electron Paramagnetic Resonance, and X-ray Photoelectron Spectroscopy. Formamide-assisted nitrogen-doped TiO2 microspheres present higher absorption in the visible light area, slightly narrow band-gap, and slower charge recombination than the non-doped TiO2. The highest hydrogen evolution rate for formamide-assisted TiO2 under solar light irradiation was 8821.88 μmol h−1 g−1 with good cycling stability, which is 1.6 times of un-doped TiO2 and 1.2 times of TiO2 nanosheet spheres prepared with the assistance of dimethyl formamide. Furthermore, formamide-assisted TiO2 exhibits a good photocatalytic hydrogen evolution rate of 579.49 μmol h−1 g−1 under visible light irradiation, and its highest apparent quantum yield of 5.4% is acquired under 365 nm irradiation. The band gap structures of as-prepared TiO2, the charge transfer and separation, and the photocatalytic mechanism are also discussed in detail. This work provides a new strategy to effectively introduce oxygen vacancies and nitrogen dopants of TiO2 nanocrystals with significantly improved photocatalytic properties.