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 degrees C under ordinary pressure and an inert atmosphere. The as-prepared ternary photocatalyst owned a narrower band gap of similar to 2.6 eV, relatively high specific surface area of similar to 91.0 m(2) g(-1), and average pore size of similar to 12.4 nm. In comparison with the pristine TiO2 MNTs, the ternary photocatalyst exhibited a solar-driven photocatalytic hydrogen evolution rate of similar to 3.9 mmol g(-1) h(-1) which was about similar to 4 times than that of the pristine TiO2 MNTs of similar to 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 similar to 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.