Oxygen Vacancy Defects and Cobalt Nanoparticle-Mediated Charge Separation in Black Ti³⁺ Self-Doped TiO₂ Mesoporous Nanotubes for Enhanced Solar-Driven Hydrogen Evolution and Tetracycline Degradation

The photocatalytic efficiency of TiO2 greatly suffers from the rapid recombination of photogenerated electron–hole (e–_h+) pairs, small specific surface area, and a large band gap. Self-doping can effectively improve the photocatalytic activity of TiO2. In this work, black Ti3+ self-doped TiO2 mesoporous nanotubes/N-doped carbon/Co nanoparticles (Ti3+-d-TiO2 MNTs/N-d-C/Co NPs) were successfully synthesized through the reduction of TiO2 MNTs by a Co2+–l-histidine (Co-l-his) complex at 800 °C under ordinary pressure and an inert atmosphere. The as-prepared ternary photocatalyst owned a narrower band gap of ∼2.6 eV, relatively high specific surface area of ∼91.0 m2 g–1, and average pore size of ∼12.4 nm. In comparison with the pristine TiO2 MNTs, the ternary photocatalyst exhibited a solar-driven photocatalytic hydrogen evolution rate of ∼3.9 mmol g–1 h–1, which was about ∼4 times than that of the pristine TiO2 MNTs of ∼0.9 mmol g–1 h–1, and enhanced capability of photocatalytic degradation of tetracycline. These improvements could be ascribed to the Ti3+ self-doping, formation of oxygen vacancy, and narrowed band gap of ∼2.6 eV, expanding the optical response to the visible light region, enhancing the separation efficiency of the photogenerated charge carriers. Meanwhile, the N-d-C/Co NPs could not only adsorb tetracycline and water molecules but also activate water molecules and promote H–OH cracking. As a result, enhanced solar-driven hydrogen evolution and tetracycline degradation were achieved.