2.1.1. Cadmium Sulfide

CdS is a widely-investigated metal sulfide-based photocatalyst that has a narrow band gap and high hydrogen production activity. The synergetic effects of this coupled composite photocatalyst on hydrogen evolution can be studied by investigating the decay behavior of photoexcited carriers. The synergetic effect is important for the enhancement of hydrogen production by sulfides of transition metals. Wang et al. [17] reported that (ZnO)1/(CdS)0.2 composite showed noticeably slower decay kinetics as compared to bare ZnO and CdS when observed with time-resolved fluorescence emission decay spectra. This finding indicates that the photocatalyst (ZnO)1/(CdS)0.2 is able to produce the highest H2 evolution among different ZnO/CdS heterostructures due to the transfer of photoexcited carriers between CdS and ZnO, which may hinder recombination. A similar trend of this synergetic effect was also observed in other studies, such as those incorporating g-C3N4 on the outer shell of a CdS core [18], g-C3N4/Ni(OH)2-CdS [19], g-C3N4Ni/CdS [20], co-loading of MoS2 and graphene to CdS nanorods [21], Ni3N/CdS [22], and MoS2/CdS [23]. Transition metal chalcogenides are appropriate candidates for composite photocatalysts due to their conduction band positions, which are appropriate for the reduction reaction of water to form hydrogen [24], and their excellent luminescence and photochemical properties. Table 1 summarizes the photocatalytic performances of CdS-based photocatalysts.


**Table 1.** Photocatalytic performances of CdS-based photocatalysts.

#### 2.1.2. Copper Sulfide

CuS/ZnS composites are efficient photocatalysts for hydrogen evolution. Wang et al. reported that CuS/ZnS nanomaterials exhibit high visible light-induced H2 generation activity. The H2 generation rate increases with increasing Cu2<sup>+</sup> ions. However, as with other cocatalysts, when the maximum amount of Cu2<sup>+</sup> is reached (above 7 mol %), the hydrogen evolution rate decreases significantly. This is due to light shielding by excess CuS, which reduces the number of active sites on the surface [25]. In addition to the solvothermal method for the fabrication of CuS cocatalysts, the growth of CuS/g-C3N4 by an in-situ method has been investigated by other authors. That study found that CuS nanoparticles were uniformly distributed on g-C3N4 nanosheets [26].

Chang et al. found that CuS-ZnS1−xOx/g-C3N4 heterostructured photocatalyst had a high photocatalytic H2 generation property. That study demonstrated that the decoration of CuS on the surface helps to enhance the absorption of the heterostructured photocatalyst [27]. In addition, CuS is used to decorate free-standing ZnS-carbon nanotube films because CuS can form heterojunctions with ZnS to improve the separation of photoexcited charge carriers, resulting in higher rates of hydrogen production [28]. In another work, Markovskaya et al. suggested the important role of CuS in enhancing the hydrogen evolution rate of the photocatalyst Cd0.3Zn0.7S. The optimized performance (3520 μmol h<sup>−</sup>1g−1) was obtained with 1 mol % CuS/Cd0.3Zn0.7S [29]. Similar contributions were also found for CuS/TiO2 nanocomposites [30,31]. Table 2 lists the photocatalytic activity of CuS-based composite photocatalysts.

**Table 2.** Photocatalytic activity of copper sulfide as the cocatalyst.

