Significantly enhanced cocatalyst-free H₂ evolution from defect-engineered Brown TiO₂

TiO₂ is the extensively investigated materials for various photocatalytic reforming and water splitting. Superior stability towards photo-corrosion, appropriate band energy levels driving most photocatalytic reactions, and low-cost production are promising features of TiO₂. However, a primary limitation with TiO₂ is that it only absorbs ultraviolet light constituting less than 5% of the solar spectrum. In this work, we use a facile, low temperature, vacuum-free, and solution-route synthesis approach to rationally induce oxygen vacancy/Ti³⁺ defects to reduce the bandgap of TiO₂ to 2.0 eV (3.2 eV for pristine white TiO₂) to form brown TiO₂ with enhanced visible-light absorption. The mechanism of defect formation is systematically deduced from the detailed investigation through Raman spectroscopy, spin-sensitive technique, high-resolution microscopy, and surface analysis. The brown TiO₂ yielded 8.1 mmol h⁻¹g⁻¹_cat H₂ evolution without any cocatalyst under natural sunlight, which is a factor two higher than pristine (white) TiO₂. To the best of our knowledge, the observed H₂ evolution rate is the highest reported value under natural sunlight for any TiO₂-based photocatalyst. This work demonstrates the applicability of brown TiO₂ to fabricate large-area photocatalyst panels for the cost-effective production of solar H₂.