Among the various technologies for the production of hydrogen fuel, heterogeneous photocatalysis is one of the most promising, especially with the use of semiconductors, notably TiO2. However, the use of TiO2 is limited by hindrances for the photolysis of water, such as wide bandgap, a less negative conduction band reduction potential as compared to that of hydrogen evolution and the high electron/hole recombination rate. Deactivation of the semiconductor can be avoided by the addition of electron-rich compounds (sacrificial reagents) which react irreversibly with the hole, leading to a higher quantum efficiency. Another strategy is the Z scheme. In this system, two different photocatalysts (or photosystems) are combined using a suitable redox mediator. Furthermore, the bandgap can be adjusted by doping with transition metal oxides, with control of metal oxide valence band using p-orbitals of an anion, or s-orbitals of p-block metal ions, or by spectral sensitization. In view of these questions, the purpose of this review article is to describe and discuss recent studies that use a variety of materials for the photocatalytic generation of hydrogen.
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