Solar H generation from water has been intensively investigated as a clean method to convert solar energy into hydrogen fuel. During the past few decades, many studies have demonstrated that metal complexes can act as efficient photoactive materials for photocatalytic H production. Here, we review the recent progress in the application of metal-complex chromophores to solar-to-H conversion, including metal-complex photosensitizers and supramolecular photocatalysts. A brief overview of the fundamental principles of photocatalytic H production is given. Then, different metal-complex photosensitizers and supramolecular photocatalysts are introduced in detail, and the most important factors that strictly determine their photocatalytic performance are also discussed. Finally, we illustrate some challenges and opportunities for future research in this promising area.

Cocatalysts have been extensively used to accelerate the rate of hydrogen evolution in semiconductor-based photocatalytic systems, but the influence of interface state between semiconductor and cocatalyst has been rarely investigated. Here, we demonstrate a feasible strategy of two-dimensional (2D) nanojuctions to enhance solar hydrogen generation of the MoS2/TiO2 system. Loading of 2D MoS2 nanosheets on the surface of 2D anatase TiO2 nanosheets with exposed (001) facets greatly increases the interfacial contact. At an optimal ratio of 0.50 wt % MoS2, the 2D-2D MoS2/TiO2 photocatalyst shows the highest H2 evolution rate of 2145 μmol h–1 g–1, which is almost 36.4 times higher than that of pure TiO2 nanosheets. The apparent quantum yield of hydrogen evolution system reaches 6.4% at 360 nm. More importantly, the 2D-2D MoS2/TiO2 composite exhibits photocatalytic activity much higher than those of noble metal (such as Pt, Rh, Ru, Pd, and Au) loaded TiO2 photocatalysts. The decisive factor in improving the photocatalytic H2 production activity is an intimate and large contact interface between the light-harvesting semiconductor and cocatalyst. The effective charge transfer from TiO2 to MoS2 is demonstrated by the significant enhancement of photocurrent responses in 2D-2D MoS2/TiO2 composite electrodes. This work creates new opportunities for designing and constructing highly efficient photocatalysts by interface engineering.

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Two metal-organic coordination polymers [Ag(bipy)(UO(2))(bdc)(1.5)] (bipy=2,2'-bipyridyl, bdc=1,4-benzenedicarboxylate) and [Ag(2)(phen)(2)UO(2)(btec)] (phen=1,10-phenanthroline, btec=1,2,4,5-benzenetetracarboxylate) were obtained by hydrothermal assembly of the d(10) metal silver and the 5f metal uranium with mixed ligands. Both compounds form two-dimensional networks with pi-pi overlap interactions between the aromatic fragments in the neighboring layers. In aqueous suspension the two water-insoluble materials show photocatalytic degradation performance superior to that of commercial TiO(2) (Degussa P-25) when tested on nonbiodegradable rhodamine B (RhB) as model pollutant. The relationship between the structure of the photocatalysts and the photocatalytic activity was also elucidated. On the basis of the monitored intermediate species and the final mineralized products, it is proposed that the possible reaction mechanism for the photodegradation (oxidation) of RhB in aqueous solution catalyzed by the two assembly compounds involves photoexcitation of uranyl centers and molecular oxygen.

Atomically thin, single-crystalline InVO 4 sheets with the uniform thickness of ∼1.5 nm were convincingly synthesized, which was identified with strong, low-angle X-ray diffraction peaks. The InVO 4 atomic layer corresponding to 3 unit cells along [110] orientation exhibits highly selective and efficient photocatalytic conversion of CO 2 into CO in the presence of water vapor. Surface potential change measurement and liquid photoluminescence decay spectra confirm that the atomically ultrathin structure can shorten the transfer distance of charge carriers from the interior onto the surface and decrease the recombination in body. It thus allows more electrons to survive and accumulate on the surface, which is beneficial for activation and reduction of CO 2 . In addition, exclusively exposed {110} facet of the InVO 4 atomic layer was found to bind the generating CO weakly, facilitating quick desorption from the catalyst surface to form free CO molecules, which provides an ideal platform to catalytically selective CO product.

Recent developments in cadmium sulphide-based photocatalysts including heterojunctions, solid solutions and quantum dots for photocatalytic H2 production are reviewed.

Facilitated by TiO2 particles absorbing La3+ in hydrosol, La-doped TiO2 was prepared by a sol-hydrothermal method. Electron paramagnetic resonance and Brunauer−Emmett−Teller (BET) surface area analysis showed that the obtained La-doped anatase TiO2 surface provided a higher density of oxygen vacancies without a change in the BET surface area. A theoretical calculation was carried out to explain the generation mechanism of the increased oxygen vacancies. The results showed that the La-doped anatase TiO2 (101) surface tends to engender oxygen vacancies. The photoelectric conversion efficiency of dye-sensitized solar cells fabricated from 1 mol % La-doped TiO2 reached 6.72%, which gave an efficiency improved by 13.5% compared with that of the cells fabricated from pure TiO2. The improvement in the efficiency was ascribed to more dye absorbed on the surface of TiO2.

High photoinduced charge-carrier-separation efficiency plays a crucial factor in determining the photocatalytic activities of photocatalysts, and it remains challenging to steer the charge separation in an accurate manner. Herein, we address this important challenge by growing the Co2P cocatalyst onto the edges of black phosphorus (BP) nanosheets to craft Co–P bonds in the Co2P/BP nanosheets photocatalyst. As demonstrated by the photocurrent measurement and first-principle calculation, the Co–P bonds acting to faciliate atomic-level charge-flow steering can improve the photogenerated charge-carrier transfer between BP nanosheets and the Co2P cocatalyst, resulting in the improved photocatalytic performance of the Co2P/BP photocatalyst for H2 generation. As expected, the photocatalytic H2 generation rate of the Co2P/BP nanosheets photocatalyst is 39.7 times greater than that of bare BP nanosheets. Moreover, the Co2P grown on the edges of BP nanosheets inhibits the degradation of the BP nanosheets, resulting in its good stability for photocatalytic H2 production.