Recent advances inphotocatalytic hydrogen evolutionof AgIn5S8‐based photocatalysts

1 INTRODUCTION The continued consumption of conventional energy sources in the development of human society can lead to the serious issues of energy and environmental crises. [1-11] It is quite pressing to explore the clean and renewable energy to solve the current energy issues. Hydrogen is regarded as the most promising new energy source, which features high energy density and nonpollution. [12-20] Using the solar-to-energy (STE) technique, the photocatalytic hydrogen evolution (PHE) via water splitting is pivotal to realize the renewable hydrogen production.

[21-24] Fujishima and Honda explored that the photoelectrochemical catalysis can be realized on the TiO 2 electrode, which pioneered the PHE technique by TiO 2 photocatalyst. [25] Nevertheless, the excessively wide band gap of TiO 2 makes it unsuitable for the absorption of visible light during the corresponding PHE process. [21, 26] To this end, the development of narrow bandgap photocatalysts is of great importance to achieve efficient visible-light PHE. Compared with the pioneered photocatalyst of TiO 2 , metal sulfide (MS) photocatalysts generally have narrow bandgap, suitable band structure, and negative conduction band (CB) potential, which can not only realize the visible-light PHE but also have great potential to achieve high-efficiency hydrogen production rate.

[27-30] In the initial development period of MS photocatalysts, researchers have paid more attention to the fabrication and PHE application of binary MS (BMS) photocatalysts due to the simple phase and facile synthesis process. [31-33] Among the BMS photocatalysts, CdS has a suitable bandgap with relatively negative CB potential, making it feature a strong driving force for hydrogen evolution reaction (HER) under visible-light irradiation. [34-36] Therefore, the PHE rate on CdS-based photocatalysts has achieved higher than 10 mmol g −1 h −1 . [33] However, the CdS-based photocatalysts generally suffer from the photocorrosion phenomenon, leading to a serious decrease in photostability.

In this regard, researchers have attempted various efficient paths to overcome this issue. The most typical advanced CdS-based photocatalysts are dual-cocatalysts decorated rimous CdS spheres, [37] ternary heterojunction photocatalyst of CuWO 4 /CdS/carbon dots (CuWO 4 /CdS/CDs), [38] and S-scheme heterojunction photocatalyst of CdS/W 18 O 49 with oxygen vacancies, [39] which not only effectively hinder the photocorrosion phenomenon to a certain extent but also greatly enhance the charge separation. Apart from CdS, researchers have also explored other several BMS photocatalysts, like, MoS 2 , PbS, and ZnS, which have also been reviewed in detail by Zheng et al. [33] The authors highlighted the positive properties of them and also pointed out the issues to be addressed in the future researches.

Therefore, the exploration of new kinds of MS photocatalysts is quite meaningful for the realization of effective visible-light PHE. Ternary metal sulfide (TMS) photocatalyst generally possesses the semiconductor properties of tunable band structure, suitable bandgap, flexible element compositions, and high activity, which can effectively overcome the drawbacks of the traditional BMS photocatalysts to some extent. [40-49] Among the developed TMSs, the Ag-based photocatalysts own the unique advantages of extremely high photostability and environmental-friendly elements, which are the earliest developed TMS photocatalysts in PHE application. [50] Among them, the AgIn 5 S 8 possesses the suitable bandgap of 1.7–2.0 eV and high light absorption coefficient, which can effectively realize the visible-light PHE.

[51-57] Compared with the mature and intensively studied BMS of CdS, AgIn 5 S 8 not only displays high photostability but also composes of nontoxic elements. Therefore, it has a better potential to achieve the efficient PHE. Although AgIn 5 S 8 holds impressive semiconductor properties and better application prospects in PHE, the corresponding development is still in an early research stage. With the combination of theoretical and experimental methods, researchers have formulated corresponding optimization strategies, aiming to enhance the hydrogen production rate of AgIn 5 S 8 -based photocatalysts.

Nevertheless, the corresponding review of AgIn 5 S 8 -based photocatalysts in the application of PHE has been rarely reported. In this regard, we perform this review to provide a clear understanding and research direction of AgIn 5 S 8 -based photocatalysts. In this review, the crystal and electronic band structures of AgIn 5 S 8 are first introduced. Subsequently, the basic mechanism and charge transfer behaviors of the PHE are fully discussed and the properties of AgIn 5 S 8 to the PHE are highlighted.

Afterwards, we systematically and comprehensively reviewed the development of AgIn 5 S 8 -based photocatalysts, which contain the morphology control, Schottky junction formation through cocatalyst loading, and construction of different types of heterojunctions. Finally, some issues need to be further solved and the perspective of AgIn 5 S 8 -based photocatalysts is pointed out and some possible solutions are pointed out.