N-doped TiO₂ with stable surface Ti³⁺ for visible light photocatalytic hydrogen production

Pure TiO₂ exhibits limited responsiveness to visible light, while the doping of Ti³⁺ significantly enhances its absorption efficiency in the visible spectrum, improving the efficiency of photogenerated electron utilization. However, during the direct reduction of TiO₂ using NaBH₄, the instability of surface Ti³⁺ ions and their susceptibility to oxidation in air limit their performance in photocatalytic applications. Nitrogen doping into TiO₂ effectively addresses this issue, improving the stability of Ti³⁺ ions and significantly enhancing visible-light-driven photocatalytic activity. Experimental results indicate that reducing 1 g of TiO₂ with 0.2 g of NaBH₄, followed by calcination in an ammonia environment at 300℃, yields a catalyst with optimal visible-light photocatalytic hydrogen evolution activity, achieving a hydrogen production rate of 847.8 μmol h⁻¹ g⁻¹, representing a 177-fold enhancement over pristine anatase TiO₂. After modification, the band gap of the catalyst decreased from 3.2 eV to 2.83 eV. This study elucidates the synergistic effects among Ti³⁺ species, nitrogen dopants, and oxygen vacancies in TiO₂-based photocatalysts, demonstrating their crucial roles in enhancing visible-light-driven hydrogen evolution.