**4. Conclusions and Perspective**

In this review, we have attempted to summarize the efforts performed in the field of metal sulfide-based photocatalysts. Metal sulfide-based heterogeneous photocatalysts are promising candidates for photocatalytic hydrogen generation. Recent developments in the material design, process parameters, and performance of metal sulfide-based photocatalysts have been systematically discussed. The major problem limiting the photocatalytic H2 generation rates of photocatalysts is the fast recombination of photoexcited electron–hole pairs. This problem can be solved by decorating with cocatalysts or incorporating noble metal nanoparticles, conductive polymers, or porous conductive substrate. The formation of heterogeneous junctions helps to promote the transport of photogenerated carriers. In-situ C *K*-edge NEXAFS spectra provide a new method to investigate the electronic density of the photocatalyst. It can pave the way for the rational design of photocatalysts for efficient H2 generation. In addition, tuning the surface texture can increase the contact surface area with reactants, and the light absorption can be increased by the light trapping effect. Changing the crystal structure can also enhance the activity of a photocatalyst because of the facet effect. The fabrication of immobilized photocatalysts and magnetically separable photocatalysts makes the recycling and repeated use of photocatalysts easier to handle than photocatalyst dispersion does. The influences of doping and pH have also been discussed. The photocatalytic activity increases as the particle size of the photocatalyst decreases. This review provides a systematic overview of recent progress on the performances of various metal sulfide photocatalysts, together with some important concepts or methods to improve

and characterize their performances. Key experimental parameters and an in-situ characterization method can be applied to the research of other photocatalytic materials.

In our opinion, future research efforts should be focused on the following issues. First, we should develop in-situ spectroscopy and microscopy techniques for investigating the surface active sites, electronic states, and chemical/physical changes of the photocatalysts, together with intermediates and the mechanism of the photocatalytic reactions. Second, efforts must be focused on developing outstanding materials that can achieve both high activity and excellent reusability. Possible directions for future research are developing new heterojunction structures, increasing charge transfer, enhancing light harvesting efficiency, and achieving high activity and excellent stability of recycled photocatalysts after repeated photocatalytic H2 production processes. Finally, to implement the use of these photocatalysts for photocatalytic hydrogen generation in industry, future works should also focus on the optimized design of reactor systems for scaled-up photocatalytic processes.

**Funding:** This research was supported and funded by the Ministry of Science and Technology, under the contract of MOST 105-2221-E-035-087-MY3.

**Acknowledgments:** The authors thank the financial support from the Ministry of Science and Technology under the contract of MOST 105-2221-E-035-087-MY3.

**Conflicts of Interest:** The authors declare no conflict of interest.
