Recent progress on photocatalytic evolution of hydrogen gas over TiO2-x-based emerging nanostructures

In the recent decade, there has seen a significant advancement in TiO2-based photocatalytic production of hydrogen, as a powerful alternative to fossil fuels and in coping with the corresponding environmental pollution. Among the explored strategies for promoting the performance of pure TiO2, induction of structural defect in this semiconductor has been proven as robust approach for effective H2 evolution. This review brings together the recent information on emerging approaches to induce Ti3+ species and oxygen vacancies (OVs) in the structure of TiO2 and the related nanostructures/hybrids. Moreover, the employed characterization techniques and evidences of OVs and Ti3+ species presented in various TiO2-based structures were thoroughly discussed and depicted. Furthermore, the key determinants for enhanced H2 production over defective TiO2 were underlined for TiO2-x with homo/heterophase structures and/or doped with other materials, and in the presence/absence of cocatalyst. Besides, the plausible charge carriers transfer route in TiO2-x-based photocatalysts was indicated for H2 production under light illumination. The provided information in this review will light up the way toward economically viable and practical approaches for fabricating defective TiO2 nanomaterials, detecting defects, and producing H2 at higher rates and yields.