Metal oxynitrides are promising photocatalysts due to their narrow bandgap, but their lower stability compared to metal oxides is a drawback. The introduction of high-entropy alloys with entropy-stabilization features has shown high potential for various functional applications in recent years. By considering these two types of materials, we developed a high-entropy oxynitride for photocatalytic water splitting. The material, with a general composition of TiZrHfNbTaO₆N₃ and a d⁰ electronic configuration, showed a narrow bandgap of 1.6 eV, which is much lower than the bandgaps of relevant binary and high-entropy oxides. The material exhibited photocurrent generation and photocatalytic hydrogen production with high chemical stability, suggesting the high potential of high-entropy oxynitrides as advanced low-bandgap and stable photocatalysts.

Renewable hydrogen production is a sustainable method for the development of next-generation energy technologies. Utilising solar energy and photocatalysts to split water is an ideal method to produce hydrogen. In this review, the fundamental principles and recent progress of hydrogen production by artificial photosynthesis are reviewed, focusing on hydrogen production from photocatalytic water splitting using organic–inorganic composite-based photocatalysts.

Photocatalytic hydrogen generation on low-bandgap black ZrO ₂ produced by high-pressure torsion.

Metal-free organic dyes containing benzo[b]phenothiazine were synthesized and effectively used for dye-sensitized visible-light-driven photocatalytic hydrogen production. The materials exhibited high stability and hydrogen production when numerous π-conjugated bridges were inserted as spacers between the donor and the anchor moiety. Photocatalytic hydrogen production was investigated in a TiO₂/dye/Pt structure using triethanolamine as the sacrificial reagent. Compound dye 3, which had the longest spacer between the donor and the acceptor, exhibited the best hydrogen production performance of the series examined in this study. It displayed a turnover number (TON) of 4460, a turnover frequency of 278 after 16 h, and a photo-quantum efficiency of 1.65% at 420 nm. Furthermore, it showed the longest electron injection lifetime because its coordination structure was considered to be vertically standing on the TiO₂ surface by theoretical calculations. On the other hand, dye 1 showed the lowest hydrogen production performance with a TON of 483 and very short electron injection lifetime. This observation is confirmed by the computation results, which showed the lying geometry of 1 with monodentate coordination of the dye with respect to the TiO₂ surface. This spacer effectproperty relationship study may provide a good strategy for the development of metal-free organic dyes for dye-sensitized photocatalytic water splitting.