Abstract:

Converting solar light into chemical energy is currently a hot topic for addressing the worldwide energy and environmental crises. However, the utilization of solar energy greatly suffers from its low energy flow density and discontinuous space-time distribution, which are essential for a reasonable energy conversion strategy toward effective storage and utilization. To this end, photocatalytic water splitting is a promising method for utilizing solar light to produce environmentally friendly hydrogen energy; yet, the efficiency needs to be improved. Generally, such processes can be divided into three elementary steps: light absorption, charge separation and migration, and surface redox reaction. The overall performance is determined by the cumulative efficiencies of the above three steps. The construction of cocatalysts is among the extensive efforts taken to improve the solar conversion efficiency. First, the cocatalysts possess higher work function than the semiconductors, and the photogenerated electrons migrate from semiconductor to cocatalysts, thereby promoting the charge separation. Second, cocatalysts usually lower the activation energy and provide abundant surface reactive sites. Particularly, the addition of cocatalysts can remarkably accelerate the four-electron transfer O₂ evolution kinetics, which usually requires much higher overpotential and is often considered as the bottleneck for water splitting. Third, cocatalysts can timely remove the photogenerated charges from the surface of the semiconductor and subsequently inhibit the photocorrosion and improve the stability of the photocatalysts. Moreover, the cocatalysts also retard the backward recombination of H₂ and O₂. In general, cocatalysts for water splitting can be classified into three categories: H₂ evolution cocatalysts, O₂ evolution cocatalysts, and dual cocatalysts. The H₂ evolution cocatalysts mainly contain noble metals such as Pt, Au, and other transition metals such as Co, Ni, and Cu and their phosphides or sulfides, which are capable of trapping electrons and promoting proton reduction. The O₂ evolution cocatalysts are often noble metal oxides and transition metal (hydro)oxides and corresponding phosphates, which are always efficient in adsorbing and dissociating water molecules. To realize the overall water splitting, H₂ evolution cocatalysts and O₂ evolution cocatalysts are often integrated on one photocatalyst, which results in the so-called dual cocatalyst system. Furthermore, the performance of cocatalysts can be improved by modulating the loading amount, morphology, particle size, etc. In addition, composites such as Pt/Ni(OH)₂ cocatalyst can not only provide both H₂ and O₂ evolution sites but also accelerate the intrinsic surface redox kinetics by promoting H₂O activation, thus being much more active than the conventional dual cocatalyst system. This review summarizes the important role and design principle of cocatalysts in photocatalytic systems. The construction and functional mechanism of H₂ evolution cocatalyst, O₂ evolution cocatalyst, and dual cocatalysts in overall water splitting photocatalysts are discussed in detail, and the design strategy of new cocatalysts toward water activation is proposed.

Key words: Photocatalytic water splitting, Cocatalyst, Charge separation, Water molecular activation