2.1.3. Silver Sulfide

Recently, silver sulfide was used as a cocatalyst of ZnS photocatalyst to enhance the hydrogen evolution rate. For instance, Hsu et al. reported the use of Ag2S-coupled ZnO@ZnS core–shell nanorods to achieve efficient H2 production. The maximum hydrogen production rate is reached when the AgNO3 concentration is 2 mM; further increasing the concentration only decreases the hydrogen production rate [32]. Figure 1 demonstrates the morphology of and a possible mechanism for the photocatalytic reaction. Because of the growth of Ag2S on ZnO@ZnS nanorods, photoexcited electrons can effectively transfer from ZnS to Ag2S or migrate to the conductive wire mesh substrate. The reaction of electrons and H<sup>+</sup> can produce H2. Similarly, Yue et al. fabricated a novel Ag2S/ZnS/carbon nanofiber ternary composite to increase the hydrogen production rate well above that of the reported ZnS composite photocatalyst. In addition, the synergetic effect of Ag2S and CNF is also important for inhibiting the recombination of charge carriers [33]. Moreover, nanosheets of ZnS:Ag2S also exhibit a trend of enhancement similar to that of the above mentioned nanostructure. As reported by Yang et al., porous ZnS:Ag2S nanosheets were synthesized such that the porous nanostructure provided a large surface area for intimate contact with the sacrificial solution [34]. Table 3 shows the H2 generation performances of photocatalysts loaded with silver sulfide.

**Figure 1.** Morphology and a proposed mechanism of the photocatalytic H2 production by metal wire mesh based immobilized photocatalysts with Ag2S-coupled ZnO@ZnS core–shell nanorods. Figure adapted from [32].


**Table 3.** Photocatalytic H2 generation property of photocatalysts loaded with silver sulfide.

#### 2.1.4. Zinc Sulfide

ZnS is an excellent photocatalyst for photocatalytic water splitting, for it can produce high negative potentials of photoexcited electrons. Xin et al. reported that ZnS@CdS-Te photocatalysts with a p–n heterostructure exhibited improved H2 generation rates. Based on the possible mechanism, after the loading of ZnS on the CdS-Te nanostructure, more surface active sites can be produced, leading to increased hydrogen generation activity [35]. Figure 2 presents a possible mechanism of ZnS@CdS-Te photocatalysts. Moreover, a similar heterojunction between ZnS/g-C3N4 has been studied by Hao et al. In that study, the close contact between ZnS and g-C3N4 increased the capacity of light harvesting and efficiency of charge separation. The key factor that enhanced the photocatalytic hydrogen was the two-photo excitation of ZnS [36].

**Figure 2.** Possible mechanism of ZnS@CdS-Te composite photocatalysts. Figure adapted from reference [35].

In addition to nanostructures, the chloroplast-like structure of Bi2S3/ZnS also possesses high photocatalytic activity due to its band gap structure. Based on the reported reaction mechanism, photogenerated electrons migrate from Bi2S3 to ZnS to generate H2, while holes transfer from ZnS to Bi2S3. This efficient charge separation improves the H2 generation rate in this chloroplast-like structure [37].

Efficient H2 generation has been achieved by CdS-ZnS photocatalyst without facilitation by a cocatalyst. This includes the study done by Jiang et al. on a CdS nanorod/ZnS nanoparticle photocatalyst. The highly efficient hydrogen production likely resulted from the rapid transport of carriers in the core–shell nanorod structure [38]. Table 4 presents the H2 generation performances of ZnS-based photocatalysts.


**Table 4.** Photocatalytic H2 generation performances of ZnS-based photocatalysts.

*2.2. Electrically Conductive Materials (Non-Noble Metal)-Based Composite Photocatalysts*

#### 2.2.1. Graphene

In addition to decoration with noble and non-noble metals, combining a photocatalyst with graphene or carbon dots is also a useful way to improve activity [39–44]. Graphene can be incorporated solely or co-loaded with other compounds. Loading graphene provides several benefits, such as a large surface area and enhanced separation of photoexcited electron–hole pairs. In other words, the

exceptional electron transfer capability of graphene and intimate contact between the photocatalyst and graphene can help to transport photoexcited electrons efficiently, thus improving the activity for photocatalytic hydrogen generation. As shown in work done by Azarang et al. on nitrogen-doped graphene-supported ZnS nanorods, the photocatalytic activity of ZnS was multiplied by as much as 6 times when solely graphene was loaded and by 14 times when loaded with NG-ZnS [45].

Chang et al. [46] found that the incorporation of graphene with ZnO-ZnS nanoparticles improved the rate of H2 generation from glycerol. The irradiated and dark states of in-situ C *K*-edge NEXAFS spectra were monitored to investigate the electronic properties of the photocatalyst. In-situ NEXAFS spectra revealed that photoexcited electrons can be transported from ZnO-ZnS nanomaterials to graphene. NEXAFS spectra were used to investigate the interfacial electronic states of AgI/BiOI/graphene (A10B/G10) samples [47]. Figure 3a–c presents the intensity change (Δ*A*) between the irradiated and dark states of the Ag *L*3-edge, C *K*-edge, and Bi l*L*3-edge of BiOI/graphene photocatalyst (BG10), and A10B/G10 photocatalyst. In comparison with BG10, A10B/G10 presents more positive Δ*A* values of NEXAFS for the Bi *L*3-edge and the Ag *L*3-edge under in situ light exposure, revealing the increased amounts of the unoccupied density of states (DOS) of the Ag L-edge and Bi L-edge after light exposure due to donating photoexcited electrons for BiOI and AgI. In contrast, compared with A10B/G10, B/G10 exhibits more negative Δ*A* values of NEXAFS for the C *K*-edge under light irradiation. The reduced C *K*-edge indicates the decrease of unoccupied DOS of graphene after light irradiation due to receiving the photoexcited electrons. These results imply the migration of a charge from AgI to BiOI and then to graphene under light exposure. (Figure 3d). The activity can be improved by the formation of graphene/BiOI and BiOI/AgI heterostructures.

**Figure 3.** Intensity change between the irradiated and dark states of (**a**) Bi l*L*3-edge, (**b**) C *K*-edge, and (**c**) Ag *L*3-edge. (ΔA = Alight − Adark) of B/G10 and A10B/G10 photocatalysts, (**d**) a proposed mechanism for the photocatalytic H2 generation reaction by A10B/G10 [47].

The incorporation of graphene provides not only the above-mentioned advantages but also good water dispersity of the composite photocatalyst. This advantage was confirmed by Zhang et al. based on the results of CdSe/CdS-Au (QD-Au) core–satellite heteronanocrystal assembled on graphene nanosheets [48]. In addition to the common graphene nanosheets, Chang et al. also reported using flower-like graphene with a 3D porous structure for application in photocatalytic activity. The application of flower-like graphene enables the photocatalyst to have a higher BET surface area, which leads to enhanced photocatalytic hydrogen production [49]. Table 5 shows the hydrogen generation properties of graphene-loaded composite photocatalysts.


**Table 5.** Photocatalytic H2 production properties of graphene-loaded composite photocatalysts.
