**1. Introduction**

Hydrogen, which has high energy yield and less greenhouse gas emissions after combustion, is an environmentally friendly and attractive fuel. Hydrogen production and hydrogen economy, together with potential applications in fuel cell hydrogen electric cars and hydrogen-diesel fuel co-combustion, are hot topics in research related to hydrogen energy [1–8]. Hydrogen is an alternative energy source for fossil fuels. Research on generating electricity from solar energy has also gained increasing attention. Environmentally friendly photocatalytic water-splitting by semiconductor nanomaterials is very important for the development of a hydrogen economy. The photoexcited electron–hole pairs play an important role in the water splitting process. Photocatalysts should be able to absorb radiation and efficiently reduce protons to hydrogen molecules with photogenerated electrons.

Metal sulfide-based photocatalysts are widely investigated because of their unique physical and chemical properties [9–11]. In comparison with their metal oxide analogs, sulfide semiconductors with narrower band gaps hold more promise as photocatalysts for H2 production [12]. However, the H2 production performance of metal sulfide-based composite photocatalysts is still limited because of the fast recombination of photoexcited carriers [13]. In addition, photocorrosion is also a major problem for metal sulfide-based photocatalysts. Therefore, many researchers have focused on solving the problems of photocorrosion and carrier recombination, examining the formation of a heterojunction by decorating cocatalysts, introducing noble metal nanoparticles, or using porous conductive substrate. It has been reported that the formation of a heterojunction interface with other semiconductors

or coupling with other cocatalysts can overcome the carrier recombination and photocorrosion problems [14]. The formation of a junction helps to promote the transport of photoexcited electrons. In addition, it is reported that the performance of photocatalysts can also be enhanced further by doping, modifying the surface texture, and tuning the crystal structure [15,16]. Furthermore, current research on metal sulfide-based photocatalysts is focused on two hot topics: the search for novel metal-sulfide nanomaterials that do not contain acutely toxic metals such as cadmium and lead, and the search for cocatalyst nanomaterials that do not contain noble metals.

In this review, we discuss recent developments in the use of metal sulfide-based nanomaterials as either photocatalysts or cocatalysts for H2 production, especially those in the last 10 years. Recent progress on the fabrication and performance of metal sulfide-based composite photocatalysts is reviewed, including the form of heterogeneous composite photocatalysts, immobilized photocatalysts, and magnetically separable photocatalysts. In addition, the effects of experimental parameters—including noble metal loading, transition metal doping, anion doping, calcination, the pH level of solution, sacrificial agent, structure of photocatalysts, facet effect, light trapping effect, crystal size of photocatalysts, and fabrication methods—on photocatalytic performance are also systematically reviewed.
