Recent Progress on Metal Sulﬁde Composite Nanomaterials for Photocatalytic Hydrogen Production

: Metal sulﬁde-based photocatalysts have gained much attention due to their outstanding photocatalytic properties. This review paper discusses recent developments on metal sulﬁde-based nanomaterials for H 2 production, acting as either photocatalysts or cocatalysts, especially in the last decade. Recent progress on key experimental parameters, in-situ characterization methods, and the performance of the metal sulﬁde photocatalysts are systematically discussed, including the forms of heterogeneous composite photocatalysts, immobilized photocatalysts, and magnetically separable photocatalysts. Some methods have been studied to solve the problem of rapid recombination of photoinduced carriers. The electronic density of photocatalysts can be investigated by in-situ C K -edge near edge X-ray absorption ﬁne structure (NEXAFS) spectra to study the mechanism of the photocatalytic process. The e ﬀ ects of crystal properties, nanostructure, cocatalyst, sacriﬁcial agent, electrically conductive materials, doping, calcination, crystal size, and pH on the performance of composite photocatalysts are presented. Moreover, the facet e ﬀ ect and light trapping (or light harvesting) e ﬀ ect, which can improve the photocatalytic activity, are also discussed.


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][2][3][4][5][6][7][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][10][11].In comparison with their metal oxide analogs, sulfide semiconductors with narrower band gaps hold more promise as photocatalysts for H 2 production [12].However, the H 2 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 H 2 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.

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 H 2 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-C 3 N 4 on the outer shell of a CdS core [18], g-C 3 N 4 /Ni(OH) 2 -CdS [19], g-C 3 N 4 Ni/CdS [20], co-loading of MoS 2 and graphene to CdS nanorods [21], Ni 3 N/CdS [22], and MoS 2 /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.

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 Ag 2 S-coupled ZnO@ZnS core-shell nanorods to achieve efficient H 2 production.The maximum hydrogen production rate is reached when the AgNO 3 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 Ag 2 S on ZnO@ZnS nanorods, photoexcited electrons can effectively transfer from ZnS to Ag 2 S or migrate to the conductive wire mesh substrate.The reaction of electrons and H + can produce H 2 .Similarly, Yue et al. fabricated a novel Ag 2 S/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 Ag 2 S and CNF is also important for inhibiting the recombination of charge carriers [33].Moreover, nanosheets of ZnS:Ag 2 S also exhibit a trend of enhancement similar to that of the above mentioned nanostructure.As reported by Yang et al., porous ZnS:Ag 2 S 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 H 2 generation performances of photocatalysts loaded with silver sulfide.
Catalysts 2019, 9, x FOR PEER REVIEW 4 of 25 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 + 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.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].

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 H 2 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-C 3 N 4 has been studied by Hao et al.In that study, the close contact between ZnS and g-C 3 N 4 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].
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.

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][40][41][42][43][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].In addition to nanostructures, the chloroplast-like structure of Bi 2 S 3 /ZnS also possesses high photocatalytic activity due to its band gap structure.Based on the reported reaction mechanism, photogenerated electrons migrate from Bi 2 S 3 to ZnS to generate H 2 , while holes transfer from ZnS to Bi 2 S 3 .This efficient charge separation improves the H 2 generation rate in this chloroplast-like structure [37].
Efficient H 2 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 H 2 generation performances of ZnS-based photocatalysts.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][40][41][42][43][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 H 2 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 (∆A) between the irradiated and dark states of the Ag L 3 -edge, C K-edge, and Bi lL 3 -edge of BiOI/graphene photocatalyst (BG10), and A10B/G10 photocatalyst.In comparison with BG10, A10B/G10 presents more positive ∆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 ∆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.
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 (∆A) between the irradiated and dark states of the Ag L3-edge, C K-edge, and Bi lL3-edge of BiOI/graphene photocatalyst (BG10), and A10B/G10 photocatalyst.In comparison with BG10, A10B/G10 presents more positive ∆A values of NEXAFS for the Bi L3-edge and the Ag L3-edge under in situ light exposure, revealing the increased amounts of the unoccupied density of states (DOS) of the Ag Ledge 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 ∆A values of NEXAFS for the C Kedge 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.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.

Reduced Graphene Oxide and Graphene Oxide
Since the discovery of graphene (G) and/or graphene oxide (GO) [50], they have been widely studied [51][52][53].GO consists of graphene nanosheets with epoxy or hydroxyl group-modified basal planes and carbonyl/carboxylic acid-modified edges [54][55][56].In contrast to graphite, GO can be exfoliated easily and dispersed in aqueous solution because of these hydrophilic groups on the surface [57][58][59].Hence, a few studies have used graphene oxide as the supporting material.For instance, in a recent study done by Peng et al., GO-cadmium sulfide was immobilized and well dispersed on a graphene oxide sheet.Increasing the GO-loading up to 5 wt % promoted the hydrogen generation rate to a maximum of 314 µmol h −1 [60].Hou et al. found that the introduction of GO can extend the lifetime of the photoexcited carriers because they can act as both electron transporter and electron acceptor [61].
A number of authors have also driven the further development of reduced graphene oxide.For instance, Zhang et al. enhanced solar photocatalytic hydrogen production by introducing reduced graphene oxide nanosheets and Zn x Cd 1−x S. That research provided a green method for using reduced graphene oxide (RGO) as a support material to enhance the performance of Zn x Cd 1−x S photocatalyst, for the hydrogen production of RGO-Zn 0.8 Cd 0.2 S was 450% higher than that of pristine Zn 0.8 Cd 0.2 S [62].Similar effects after the incorporation of reduced graphene oxide have been evidenced in other works, such as studies of ZnO-CdS/RGO [63] and ternary NiS/Zn x Cd 1−x S/RGO nanocomposites [64].Table 6 presents the photocatalytic H 2 production properties of graphene oxide loaded with composite photocatalysts.

Conductive Polymer
Many papers have reported on the improvement of photocatalytic activity due to the loading of conductive polymers, such as polythiophene, polypyrrole, PEDOT, and PSS.Conductive polymers are capable of inducing charge separation in the composite photocatalysts [65].A recent study showed that polyaniline (PANI)/ZnS synthesized by solvothermal method increases the hydrogen evolution rate up to 6750 µmol h −1 g −1 because PANI has unique electron and hole transporting properties [66].PANI-ZnS composite photocatalysts exhibit improved dispersibility, light harvesting, and photocurrent response.Wang et al. [67] also reported on conducting polymers (such as polypyrrole, poly-3,4-ethylenedioxythiophene (PEDOT), and PANI) on the surface of CdS nanorods.It was found that the rate of hydrogen production of polyaniline@CdS was almost 5 times that of PEDOT@CdS and 3 times that of polypyrrole@CdS.That study showed that polyaniline is an efficient conducting material for modifying CdS nanomaterials, for it enables better light penetration than does a polypyrrole shell.Zieli ńska et al. [68] reported that the hydrogen production of PANI/NaTaO 3 photocatalyst under UV light irradiation was about twice that of pristine NaTaO 3 photocatalyst.PANI/NaTaO 3 exhibits a lower PL spectrum than that of NaTaO 3 , indicating a slower recombination of photoexcited charge carriers.The photoluminescence (PL) spectrum confirmed that the enhancement resulted from the efficient charge separation process.Sasikala et al. studied the photocatalytic hydrogen production performance of MoS 2 -PANI-CdS photocatalysts [69].The incorporation of MoS 2 and PANI improved the visible light absorption ability and improved the lifetime of photoexcited electron-hole pairs of the composite photocatalysts.To achieve high photocatalytic activity, 4% MoS 2 and 5% PANI are the optimum amounts for the composite photocatalysts.The enhanced light absorption and lifetime of the photoexcited charge carriers result in improved activity of the photocatalysts.Figure 4

Conductive Polymer
Many papers have reported on the improvement of photocatalytic activity due to the loading of conductive polymers, such as polythiophene, polypyrrole, PEDOT, and PSS.Conductive polymers are capable of inducing charge separation in the composite photocatalysts [65].A recent study showed that polyaniline (PANI)/ZnS synthesized by solvothermal method increases the hydrogen evolution rate up to 6750 μmol h −1 g −1 because PANI has unique electron and hole transporting properties [66].PANI-ZnS composite photocatalysts exhibit improved dispersibility, light harvesting, and photocurrent response.Wang et al. [67] also reported on conducting polymers (such as polypyrrole, poly-3,4-ethylenedioxythiophene (PEDOT), and PANI) on the surface of CdS nanorods.It was found that the rate of hydrogen production of polyaniline@CdS was almost 5 times that of PEDOT@CdS and 3 times that of polypyrrole@CdS.That study showed that polyaniline is an efficient conducting material for modifying CdS nanomaterials, for it enables better light penetration than does a polypyrrole shell.Zielińska et al. [68] reported that the hydrogen production of PANI/NaTaO3 photocatalyst under UV light irradiation was about twice that of pristine NaTaO3 photocatalyst.PANI/NaTaO3 exhibits a lower PL spectrum than that of NaTaO3, indicating a slower recombination of photoexcited charge carriers.The photoluminescence (PL) spectrum confirmed that the enhancement resulted from the efficient charge separation process.Sasikala et al. studied the photocatalytic hydrogen production performance of MoS2-PANI-CdS photocatalysts [69].The incorporation of MoS2 and PANI improved the visible light absorption ability and improved the lifetime of photoexcited electron-hole pairs of the composite photocatalysts.To achieve high photocatalytic activity, 4% MoS2 and 5% PANI are the optimum amounts for the composite photocatalysts.The enhanced light absorption and lifetime of the photoexcited charge carriers result in improved activity of the photocatalysts.Figure 4 presents a possible mechanism for the electron transfer of MoS2-PANI-CdS photocatalysts.Incorporating polyaniline helps to separate photoinduced charges across the ZnS-polyaniline interfaces.The photocatalysts had an optimized activity of 2500 µmol/g −1 h −1 , resulting from their matched band structure, porous microstructure, and conductive Ni foam as substrate.The surface turned from hydrophobic to superhydrophilic after Ni-doped ZnS was grown on the surface of the Ni foam.In addition, the porous microstructure facilitated the transport of reactant and generated a large amount of surface active sites, and the conductive Ni foam aided in the separation of photoexcited carriers.Figure 5 shows a proposed mechanism, illustrating the band structure of the photocatalysts and the transport of photoexcited electrons.When the composite photocatalysts were irradiated with light, the photoexcited electrons were effectively separated by their transportation from ZnS to NiO.In addition, stainless-steel wire mesh is also a good candidate for the preparation of porous immobilized photocatalysts [71]. .Ni foam was decorated with doped ZnS to prepare the porous immobilized photocatalysts.The photocatalysts had an optimized activity of 2500 μmol/g −1 h −1 , resulting from their matched band structure, porous microstructure, and conductive Ni foam as substrate.The surface turned from hydrophobic to superhydrophilic after Ni-doped ZnS was grown on the surface of the Ni foam.In addition, the porous microstructure facilitated the transport of reactant and generated a large amount of surface active sites, and the conductive Ni foam aided in the separation of photoexcited carriers.Figure 5 shows a proposed mechanism, illustrating the band structure of the photocatalysts and the transport of photoexcited electrons.When the composite photocatalysts were irradiated with light, the photoexcited electrons were effectively separated by their transportation from ZnS to NiO.In addition, stainless-steel wire mesh is also a good candidate for the preparation of porous immobilized photocatalysts [71].

Magnetic Materials-Based Composite Photocatalysts
Magnetic nanomaterials have been used as the core for the preparation of magnetically separable photocatalysts.To improve the activity, photocatalyst nanomaterials should be highly dispersible in solution.Nonetheless, it will become more difficult to separate and reuse the nanoparticles of photocatalysts by centrifugation for repeated operation if such good dispersal stability is achieved.Hence, the introduction of magnetic nanomaterials in photocatalysts will enable efficient separation for the repeated use of photocatalysts.For instance, in studies conducted on NiCo2O4@ZnS and Fe3O4@ZnS core-shell nanoparticles, the recycled photocatalyst exhibited a good hydrogen evolution rate even after being recycled three times [72].The ZnS shell deposited on a magnetic core decreases the magnetic saturation of core-shell microspheres.CoFe2O4@ZnS nanoparticles exhibit superparamagnetic properties where no residual magnetism is left after repeated use of the photocatalyst [73].Figure 6 shows the dispersion and magnetic separation of calcinated CoFe2O4@ZnS-0.5h photocatalyst.The superparamagnetic property is quite important for photocatalytic H2 generation in practical operations.After the magnet is removed, there should be no residual magnetism so as to prevent the aggregation of recycled photocatalysts.

Magnetic Materials-Based Composite Photocatalysts
Magnetic nanomaterials have been used as the core for the preparation of magnetically separable photocatalysts.To improve the activity, photocatalyst nanomaterials should be highly dispersible in solution.Nonetheless, it will become more difficult to separate and reuse the nanoparticles of photocatalysts by centrifugation for repeated operation if such good dispersal stability is achieved.Hence, the introduction of magnetic nanomaterials in photocatalysts will enable efficient separation for the repeated use of photocatalysts.For instance, in studies conducted on NiCo 2 O 4 @ZnS and Fe 3 O 4 @ZnS core-shell nanoparticles, the recycled photocatalyst exhibited a good hydrogen evolution rate even after being recycled three times [72].The ZnS shell deposited on a magnetic core decreases the magnetic saturation of core-shell microspheres.CoFe 2 O 4 @ZnS nanoparticles exhibit superparamagnetic properties where no residual magnetism is left after repeated use of the photocatalyst [73].Figure 6 shows the dispersion and magnetic separation of calcinated CoFe 2 O 4 @ZnS-0.5hphotocatalyst.The superparamagnetic property is quite important for photocatalytic H 2 generation in practical operations.After the magnet is removed, there should be no residual magnetism so as to prevent the aggregation of recycled photocatalysts.

Loading with Metal
The majority of the prior research to improve the activity of photocatalysts involved the loading of noble and non-noble metals and the coupling of semiconductors.Then the photogenerated electrons could be transported to the noble metals or delocalized and transported between photocatalysts.

Noble Metal Loading
The photocatalytic activity for the production of hydrogen is affected by the noble metal loaded on the photocatalyst.Noble metal co-catalysts can accelerate the transport of photoexcited charge and thus create H2 desorption sites.This will eventually lead to higher H2 generation activity [74].Although the rate of H2 generation will increase with increasing noble metal loads, the loading amount will reach a maximum, above which further increases of a noble metal will not decrease the photocatalytic activity.The reduction is probably due to two factors: (i) excess noble metal will lead to shielding of the incident light; and (ii) a higher amount of noble metal decorated on the photocatalyst surface will causes light scattering of the samples, thus reducing the effective irradiation absorbed by the reaction suspension [75,76].
Zhou et al. studied the ternary heterojunction photocatalyst CdS/M/TiO2 (M = Ag, Au, Pd, Pt).Some photocatalysts enriched with the noble metals are better than pristine TiO2 and binary heterojunction photocatalysts.For example, the hydrogen evolution rate of photocatalyst loaded with Pd (CdS/Pd/TiO2) is 6.7 times higher than that of CdS/TiO2 [77].
The photocatalytic H2 generation activity also increases as greater amounts of Au are loaded onto ZnS flowers.When the Au load is less than 4%, the photocatalytic activity is able to reach 3306 μmol h −1 g − 1.Further increasing the Au load to 6% reduces the activity.Such improvement of photoactivity can be ascribed to the following reasons.First, in comparison with the conduction band minimum of ZnS, Au has a lower Fermi level.The photoexcited electrons can be transported to Au.Second, the Au(I) loaded on the ZnS lattice will extend the light absorption wavelength and enhance the light harvesting efficiency [78].Moreover, when Au nanoparticles are incorporated on S,Nmodified TiO2 (SNT), the Au particles promote the visible light-driven hydrogen production activity.Because of surface plasmon resonance, 3.5 nm Au particles deposited on TiO2 can increase light absorption.The amount of hydrogen generated by the 3Au-SNT is 9 times that of pure SNT [79].
In addition to Au, Pt is frequently incorporated in semiconductor photocatalysts.In a previous study by Yu et al., Pt was loaded onto a Cu2ZnSnS4 (CZTS) semiconductor with a maximum content of 1%, and the hydrogen production rate increased.However, the performance decreased when the Pt load was further increased, due to the optical shielding effect.The production of hydrogen by 1% Cu2ZnSnS4-Pt was 8 times higher than that of bare Cu2ZnSnS4.Intimate contact between CZTS and Pt increases the production of hydrogen [80].Similarly, in CdS photocatalyst, the hydrogen evolution

Loading with Metal
The majority of the prior research to improve the activity of photocatalysts involved the loading of noble and non-noble metals and the coupling of semiconductors.Then the photogenerated electrons could be transported to the noble metals or delocalized and transported between photocatalysts.

Noble Metal Loading
The photocatalytic activity for the production of hydrogen is affected by the noble metal loaded on the photocatalyst.Noble metal co-catalysts can accelerate the transport of photoexcited charge and thus create H 2 desorption sites.This will eventually lead to higher H 2 generation activity [74].Although the rate of H 2 generation will increase with increasing noble metal loads, the loading amount will reach a maximum, above which further increases of a noble metal will not decrease the photocatalytic activity.The reduction is probably due to two factors: (i) excess noble metal will lead to shielding of the incident light; and (ii) a higher amount of noble metal decorated on the photocatalyst surface will causes light scattering of the samples, thus reducing the effective irradiation absorbed by the reaction suspension [75,76].
Zhou et al. studied the ternary heterojunction photocatalyst CdS/M/TiO 2 (M = Ag, Au, Pd, Pt).Some photocatalysts enriched with the noble metals are better than pristine TiO 2 and binary heterojunction photocatalysts.For example, the hydrogen evolution rate of photocatalyst loaded with Pd (CdS/Pd/TiO 2 ) is 6.7 times higher than that of CdS/TiO 2 [77].
The photocatalytic H 2 generation activity also increases as greater amounts of Au are loaded onto ZnS flowers.When the Au load is less than 4%, the photocatalytic activity is able to reach 3306 µmol h −1 g −1 .Further increasing the Au load to 6% reduces the activity.Such improvement of photoactivity can be ascribed to the following reasons.First, in comparison with the conduction band minimum of ZnS, Au has a lower Fermi level.The photoexcited electrons can be transported to Au.Second, the Au(I) loaded on the ZnS lattice will extend the light absorption wavelength and enhance the light harvesting efficiency [78].Moreover, when Au nanoparticles are incorporated on S,N-modified TiO 2 (SNT), the Au particles promote the visible light-driven hydrogen production activity.Because of surface plasmon resonance, 3.5 nm Au particles deposited on TiO 2 can increase light absorption.The amount of hydrogen generated by the 3Au-SNT is 9 times that of pure SNT [79].
In addition to Au, Pt is frequently incorporated in semiconductor photocatalysts.In a previous study by Yu et al., Pt was loaded onto a Cu 2 ZnSnS 4 (CZTS) semiconductor with a maximum content of 1%, and the hydrogen production rate increased.However, the performance decreased when the Pt load was further increased, due to the optical shielding effect.The production of hydrogen by 1% Cu 2 ZnSnS 4 -Pt was 8 times higher than that of bare Cu 2 ZnSnS 4 .Intimate contact between CZTS and Pt increases the production of hydrogen [80].Similarly, in CdS photocatalyst, the hydrogen evolution reaches a quantum efficiency of 51% and a maximum of 4800 µmol h −1 when 0.65 wt % of Pt is loaded on CdS.However, the activity can be further increased by loadings of 0.3 wt % Pt and 0.13 wt % PdS on CdS.The co-loading of noble and non-noble metals onto pristine CdS promotes the splitting of H 2 S into H 2 and S [81].Likewise, the addition of Pt to CuS-TiO 2 enhances the evolution of hydrogen because excited electrons from the CB of CuS or through CB of TiO 2 can be transferred directly to Pt sites, resulting in the reduction of protons to hydrogen [82].Moreover, loading Pt on novel Cd x Cu y Zn 1−x−y S also enhances the photocatalytic performance.The presence of 0.5 wt % Pt increases the H 2 production rate to 557 µmol h −1 , as compared to 350 µmol h −1 produced by Cd 0.1 Cu 0.01 Zn 0.89 S alone [83].
The presence of Cu in a photocatalyst facilitates carrier separation and also increases light absorption.As reported in a study of In and Cu co-doped ZnS photocatalysts by Kimi et al. [84], photocatalytic performance is strongly related to the amount of doped Cu.With the suitable amount of doped Cu (0.03), hydrogen evolution reaches a maximum that is 8 times higher than that of hydrogen produced by In(0.1)-ZnS photocatalyst.However, with the incorporation of more Cu, the photocatalytic activity becomes lower than that of single doped In(0.1)-ZnS.An excessive amount of Cu causes light scattering, and the excess Cu also acts as recombination sites to halt the photocatalytic reaction.
Although the incorporation of noble metals in photocatalysts will increase the rate of hydrogen production, different noble metals have various effects on the enhancement.When four different noble metals, Pt, Rh, Pd, and Ru, are decorated on CdS/TiO 2 photocatalyst, the amount of photogenerated H 2 by Pt loaded (640 µmol h −1 ) photocatalyst is the highest [85].Table 7 shows the photocatalytic H 2 generation for noble metal-loaded photocatalysts.

Transition Metal Doping
As observed from a few recent studies, transition metals (TM) have begun attracting attention because doping with TM can significantly enhance the photocatalytic performance by efficiently promoting the separation process of photoexcited holes and electrons.For instance, Chen et al. developed an in-situ photodeposition method to load Co on CdS nanorods.That work reported a highest photocatalytic activity of 1299 µmol h −1 with the optimum loading of 1.0 wt % [87].In comparison with nickel and iron, cobalt has the ability to improve the rate of H 2 generation.From the work done by Zhou et al., the enhancement of the activity of MoS x was found to be in the order of Co > Ni > Fe [88].This improved performance results from the higher amount of doped Co and the capability of Co to activate the S-edge sites [89].A quick screening technology has been reported to find out the optimized composition of the photocatalyst.M-ZnS based photocatalysts (M = Cr, Cu, Ni, Mo, and Ag) for photoelectrochemical water oxidation applications can be screened rapidly using scanning electrochemical microscopy (SECM) with an optical fiber by finding, the spot with the highest photocurrent among the photocatalyst arrays [90].
Ni doping can enhance the photocatalytic activity of H 2 generation by increasing the absorption of light of the doped photocatalyst.In contrast, when the amount of Ni loaded on ZnS-graphene composites is increased, it will degenerate the crystalline property of the photocatalyst [91].In addition, the incorporation of Ni on Cd 1−x Zn x S microsphere photocatalyst increases the rate of hydrogen production to 191 µmol h −1 g −1 when an optimum amount of 0.1 wt % Ni is loaded.The increment results from the decreased particle size and increased surface area of the photocatalyst [92].Incorporating a metal such as Ni onto the photocatalyst can accelerate the process of transferring electrons to the surface and decrease the band gap, leading to increased photocatalytic activity.A similar trend of enhancement by Ni doping was also observed with stainless steel wire mesh@doped ZnS photocatalyst [93].Pristine stainless steel wire mesh C60 has a hydrophobic surface (water contact angle = 103 • ).In contrast, the water contact angle of ZnS decorated wire mesh photocatalyst C60S0.5 is 0 • .Effective contact between the sacrificial solution and the photocatalyst surface is very important because the photocatalyst is used for photocatalytic H 2 generation in aqueous sacrificial solution.Improving the hydrophilicity of the photocatalyst will lead to increased activity.Table 8 lists the photocatalytic H 2 production performances for transition metal doped photocatalysts.

Non-Metal Doping
Gopinath et al. [94] reported that the band gap of semiconductor oxide was reduced by doping with anions because of the broadening or the upward shifting of VB.Asahi et al. [95] studied the effects of C, N, F, P, or S doping on TiO 2 -based photocatalysts.Their results showed that N doping can decrease the band gap of the photocatalyst due to the energy state overlapping between the N 2p states and O 2p states.Such band gap narrowing was also found for S doped photocatalysts.Up to the present, only a few studies have demonstrated the use of anion doping to improve the visible light-induced hydrogen generation performance.As non-metal doping is more difficult to prepare by conventional chemical methods, it has received little attention.Tsuji et al. demonstrated the effectiveness of halogen codoped Pb-ZnS photocatalyst in photocatalytic activity.Although Pb-doped ZnS already has a high hydrogen evolution rate, codoping of halogen is still useful, for it facilitates the relaxation of the distortion produced by doping with large Pb ions.The photocatalytic activity of halogen and Pb codoped ZnS is three times higher than that of the Pb-doped ZnS photocatalyst [96].

Calcination
By changing the treatment temperature and ambient gas condition, the post thermal treatment of metal sulfide photocatalysts at elevated temperature in air or oxygen usually results in effective electron-hole separation and enhanced photocatalytic activity, due to the formation of metal sulfide-metal oxide heterojunction.Hong et al. [97] reported that ZnS-ZnO composite prepared by thermal treatments from preformed ZnS particles showed improved charge separation and photocatalytic activity.Optimizing oxide content in ZnS-ZnO photocatalyst by controlling O 2 partial pressure (16.9 kPa) and temperature (500 o C) can help to achieve a H 2 production rate of 494.8 µmol g −1 h −1 .
The crystallinity and surface area of a photocatalyst can be modified by calcination.Based on previous studies, the H 2 production rate increases as the calcination temperature increases.However, there is an optimum temperature for the maximum H 2 evolution rate.Further increment of temperature will cause negative effects on the H 2 production rate.This negative effect results from the decrease in the surface area after heating at high temperature.These findings have been evidenced in Ce-doped ZnO/ZnS [98], ZnS 1−x−0.5yO x (OH) y (1:1) [99], and CdS/TiO 2 [85].Table 9 presents the influence of calcination treatment on the activity of photocatalysts.

Effects of pH Level
The photocatalytic evolution of hydrogen is affected by the pH level of the sacrificial agent solution.However, the effect of pH on photocatalytic performance depends on the mechanism of the reaction.According to previous research by Markovskaya et al., the pH level of the sacrificial agent can be manipulated by adding NaOH or acetic acid.The addition of acetic acid initially increases the rate of hydrogen evolution, up to a sharp peak at pH 7.5.Further increasing the acetic acid causes a drop in the evolution rate.The H 2 production in Na 2 S/Na 2 SO 3 is described as [29] Na 2 S + Na According to reaction (1) of the reaction mechanism, the addition of hydroxyl ions thermodynamically impedes the H 2 production reaction, while an increase in the concentration of acetic acid promotes the reaction.The dependence of the H 2 generation rate on the pH level has been reported for Ni-doped CdS nanorods [100].Both samples under pH 14.7 conditions performed better than the other two.These results suggested that the concentration of OH -is an important factor that will affect the efficiency of hydrogen generation.

Sacrificial Agent
The sacrificial agent plays the important role of electron donor, efficiently consuming holes to prevent recombination of charge carriers on the surface of the photocatalyst [100].Charge recombination is one of the factors that may hinder the performance of photocatalytic reactions.Adding suitable sacrificial agents helps to solve the problem.For different reactions, different sacrificial agents should be used.For photocatalytic hydrogen production reactions, the sacrificial agent acts as the hole scavenger to reduce the charge recombination of the photoexcited electron-hole pairs.Then electrons can react with H + and enhance the photocatalytic performance for hydrogen production.Metal sulfide-based photocatalysts exhibit excellent activities in aqueous solution containing sacrificial reagents Na 2 S and Na 2 SO 3 .In addition to Na 2 S and Na 2 SO 3 , some other candidates-such as methanol [31], triethanolamine [20,101], lactic acid [23,44], glycerol [46], and 2-propanol [102]-can act as the sacrificial agents for metal oxysulfide or metal oxide/metal sulfide composite photocatalysts.It will be more constructive from the viewpoints of energy production and environmental protection if the sacrificial agents are sourced from the chemical waste or byproducts of industrial processes.One example is glycerol, which is a byproduct of biodiesel manufacturing.In the chemical industry, sulfur-containing side products and waste are common.In addition to the type of sacrificial agent, its concentration also affects the photocatalytic activity.As an example, the effect of glycerol concentration on the performance of photocatalysts has been studied [46].Glycerol can react with photogenerated holes to hinder the recombination of electron-hole pairs.Optimized graphene and glycerol contents can achieve the maximum H 2 generation activity (1070 µmol h −1 g −1 ).Table 10 lists the photocatalytic activity of photocatalysts using different sacrificial agents.

Morphology
The morphology of a photocatalyst also affects the performance, for the surface area and the surface active sites are influenced by the structure.Photocatalysts have different morphologies, including 3D and porous morphologies, nanosheets, nanorods, nanoflowers, and nanowires [14,19,25,32,34,47,77,103].Recently, Amirav et al. studied tunable nanorod heterostructures.They demonstrated that a longer CdSe seeded rod was more active than a shorter rod of the same diameter, as the surface active sites were located further apart.However, nanorods with comparable rod lengths but smaller diameters will provide higher activity [102].Panmand et al. reported that more structural defects and surface states are created on CdS decorated Bi 2 S 3 nanowires.Figure 7 presents the morphology of a composite photocatalyst.The photogenerated charge carriers of photocatalysts with such defect energy levels can be effectively separated, leading to enhanced photocatalytic activity [104].
Catalysts 2019, 9, x FOR PEER REVIEW 14 of 25 of glycerol concentration on the performance of photocatalysts has been studied [46].Glycerol can react with photogenerated holes to hinder the recombination of electron-hole pairs.Optimized graphene and glycerol contents can achieve the maximum H2 generation activity (1070 μmol h −1 g −1 ).Table 10 lists the photocatalytic activity of photocatalysts using different sacrificial agents.

Morphology
The morphology of a photocatalyst also affects the performance, for the surface area and the surface active sites are influenced by the structure.Photocatalysts have different morphologies, including 3D and porous morphologies, nanosheets, nanorods, nanoflowers, and nanowires [14,19,25,32,34,47,77,103].Recently, Amirav et al. studied tunable nanorod heterostructures.They demonstrated that a longer CdSe seeded rod was more active than a shorter rod of the same diameter, as the surface active sites were located further apart.However, nanorods with comparable rod lengths but smaller diameters will provide higher activity [102].Panmand et al. reported that more structural defects and surface states are created on CdS decorated Bi2S3 nanowires.Figure 7 presents the morphology of a composite photocatalyst.The photogenerated charge carriers of photocatalysts with such defect energy levels can be effectively separated, leading to enhanced photocatalytic activity [104].In addition, a 2D morphology (such as a nanosheet) can help to improve the photocatalytic activity.Zhang et al. reported that ZnIn 2 S 4 /g-C 3 N 4 heterojunction nanosheets demonstrated higher H 2 production rates compared to single heterojunction components [101].The contact of the components to form heterojunctions is very important [105].A nanorod array structure with a small inter-rod distance will not easily form heterojunctions with close contact between different components.In contrast, a 2D heterostructure has enhanced photocatalytic performance because the van der Waals interaction of the 2D heterostructure junction between 2D metals (1T-MoS 2 ) and the 2D semiconductor (O-g-C 3 N 4 ) minimizes the Schottky barrier, thus improving the efficiency of charge transfer [106].
Moreover, the notable high hydrogen production rate and good stability achieved by mesoporous monoclinic CaIn 2 S 4 with surface nanostructures implies the importance of structure in enhancing the H 2 evolution rate.Ding et al. reported that monoclinic CaIn 2 S 4 (m-CaIn 2 S 4 ) exhibits a lower ability than cubic CaIn 2 S 4 (c-CaIn 2 S 4 ) to absorb visible light, but it also has better photocatalytic performance.The better performance results from the larger surface area, higher pore volume, more negative conduction band potential, and efficient separation of photoexcited carriers of m-CaIn 2 S 4 [107].
Furthermore, a core-shell structure will also benefit the photocatalyst, as it may have an increased surface area and change the surface properties.For example, Chang et al. reported that the growth of Ag 2 S-ZnO@ZnS core-shell nanorods on metal wire mesh had modified the surface from hydrophobic to superhydrophilic.In addition, the H 2 production activity also increased with the increasing thickness of the ZnS shell [32].A similar trend was also evidenced in studies of NiCo 2 O 4 @ZnS and Fe 3 O 4 @ZnS core shell photocatalysts [72].Table 11 presents the performances of photocatalysts with different structures.3.6.1.Facet Effect It has been reported that the photocatalysis reaction mainly occurs at the surface of the photocatalyst.The exposure of certain facets leads to greatly improved activity of the photocatalysts, known as the facet effect.Therefore, the preparation of photocatalysts with specific morphologies and structures is an important topic in the photocatalysis field.The facet effect has been observed for some oxide-based photocatalysts.Li et al. [109] reported that efficient separation of photoexcited electron-hole pairs can occur between different facets of photocatalytic nanomaterials.In comparison with their analogs with randomly distributed cocatalysts, selective deposition of a reduction cocatalyst and an oxidation cocatalyst on the {010} and {110} facets of BiVO 4 leads to higher photocatalytic activity.Ohno et al. [110] found the effect of facets on the photocatalytic activity of TiO 2 photocatalysts.For the rutile TiO 2 nanomaterials, the {110} facet can act as an effective reduction site, and the {011} facet can offer a site for effective oxidation.TiO 2 photocatalysts show high activity because of the synergistic effect between the {110} and {011} facets.− anions, which may help the oxidation process and speed up the consumption of photogenerated holes (Figure 8).This impedes the electron-hole recombination process, leading to improved photocatalytic activity.
electron-hole pairs can occur between different facets of photocatalytic nanomaterials.In comparison with their analogs with randomly distributed cocatalysts, selective deposition of a reduction cocatalyst and an oxidation cocatalyst on the {010} and {110} facets of BiVO4 leads to higher photocatalytic activity.Ohno et al. [110] found the effect of facets on the photocatalytic activity of TiO2 photocatalysts.For the rutile TiO2 nanomaterials, the {110} facet can act as an effective reduction site, and the {011} facet can offer a site for effective oxidation.TiO2 photocatalysts show high activity because of the synergistic effect between the {110} and {011} facets.Similar results were also found for the sulfide-based photocatalysts.Song et al. found that, in comparison with 2-D Cu2MoS4 nanosheet with the exposed {001} facet, Cu2MoS4 nanotube with the exposed {010} facet exhibited effectively improved the performance for photocatalytic degradation and water splitting [111].Shen et al. reported that the crystal facets of ZnIn2S4 with a 3D-hierarchical persimmon-like structure will influence the photocatalytic activity of ZnIn2S4.Extending the reaction time did not reveal any significant influences on the band gap or surface area of ZnIn2S4.Hence, the increase in the percentage of the {006} facet enhances hydrogen production [108].The atomic structure of the {006} facet mainly consists of unsaturated metal cations.During the H2 generation reaction, the exposed unsaturated Zn and In cations of ZnIn2S4 will attract S 2− and SO3 − anions, which may help the oxidation process and speed up the consumption of photogenerated holes (Figure 8).This impedes the electron-hole recombination process, leading to improved photocatalytic activity.

Light Trapping (Light Harvesting)
It is reported that properly patterned surface textures can lead to dramatically enhanced light absorption by the photocatalyst because of the light trapping effect [32].Some textured structuresincluding nanowire arrays [112], ordered mesoporous structures [113], micro-hole arrays [114], and hemisphere-array films [115,116]-are able to increase the light harvesting and photocatalytic performance of photocatalysts.Zhang et al. [112] reported that a 3D ZnO nanowire array-CdS sample exhibited substantial light-trapping enhancement in the visible light region.A schematic illustration of interface scattering when the light was irradiated on the surface of porous photocatalyst is provided in Figure 9 [116].To enhance the light absorption efficiency, these surface textures allow multiple reflections and light scattering within the nanostructures.The incident light can travel

Light Trapping (Light Harvesting)
It is reported that properly patterned surface textures can lead to dramatically enhanced light absorption by the photocatalyst because of the light trapping effect [32].Some textured structuresincluding nanowire arrays [112], ordered mesoporous structures [113], micro-hole arrays [114], and hemisphere-array films [115,116]-are able to increase the light harvesting and photocatalytic performance of photocatalysts.Zhang et al. [112] reported that a 3D ZnO nanowire array-CdS sample exhibited substantial light-trapping enhancement in the visible light region.A schematic illustration of interface scattering when the light was irradiated on the surface of porous photocatalyst is provided in Figure 9 [116].To enhance the light absorption efficiency, these surface textures allow multiple reflections and light scattering within the nanostructures.The incident light can travel through the cavities and decrease the optical loss.Efficient light trapping can be achieved by tuning the shape and roughness of the textured surface [117].
through the cavities and decrease the optical loss.Efficient light trapping can be achieved by tuning the shape and roughness of the textured surface [117].

Fabrication Method
Photocatalysts can be prepared by numerous methods, including the coprecipitation, cation exchange, chemical bath deposition, hydrothermal, and solvothermal methods.The co-precipitation method may lead to differences between the obtained final element composition in solid solution and the stoichiometric ratio [118].The hydrothermal method requires a great amount of time to prepare a well-crystallized solid solution [119].In the thermolysis method, the products are treated under high temperature to achieve fast fabrication, high crystallinity, and photocatalytic performance of the solid solution [120].
Li et al. found that Zn1−xCdxS photocatalyst fabricated using the simple Zn-Cd-Tu complex thermolysis method showed better performance than did those synthesized by the coprecipitation and hydrothermal methods [118,121].Such a thermolysis method is preferred for the following reasons: (i) the precursors can be well mixed and reacted to prepare Zn-Cd-Tu complex by the ultrasonication process; (ii) the loss of precursors can be prevented during the fabrication process; and (iii) Zn1−xCdxS with a tiny crystallite size can be prepared because thiourea releases S 2− ions slowly and offers some N and C atoms as the pinning points in the Zn-Cd-Tu complex [122].
Zhang et al. reported that a particular fabrication method will enable a photocatalyst to perform better in photocatalytic activity.The photocatalysts Cd1−xZnxS are prepared by three different methods: thermal sulfuration, co-precipitation without thermal treatment, and co-precipitation with thermal treatment.The photocatalyst which is synthesized by the thermal sulfuration method has better performance because the fabrication method allows uneven distribution of S 2− ions, thus leading to a charge gradient

Crystal Size
Hydrogen production activity is also influenced by the crystal size of the photocatalyst.Li et al. [125] studied the photocatalytic hydrogen production performances of size-selected CdS nanoparticles decorated with co-catalyst Pt nanoparticles.When the size of CdS nanoparticles decreases from 4.6 to 2.8 nm, the H2 generation quantum yield can increase from 11% to 17%.Such a dependence was observed because the driving force of photoinduced carrier transfer from CdS to vacant states of Pt nanoparticles is size-dependent.Baldovi et al. [126] prepared MoS2 quantum dots by laser ablation of MoS2 particles in suspension and investigated their photocatalytic hydrogen production performance.Two types of MoS2 nanoparticles exhibited higher activity than that of bulk

Fabrication Method
Photocatalysts can be prepared by numerous methods, including the coprecipitation, cation exchange, chemical bath deposition, hydrothermal, and solvothermal methods.The co-precipitation method may lead to differences between the obtained final element composition in solid solution and the stoichiometric ratio [118].The hydrothermal method requires a great amount of time to prepare a well-crystallized solid solution [119].In the thermolysis method, the products are treated under high temperature to achieve fast fabrication, high crystallinity, and photocatalytic performance of the solid solution [120].
Li et al. found that Zn 1−x Cd x S photocatalyst fabricated using the simple Zn-Cd-Tu complex thermolysis method showed better performance than did those synthesized by the coprecipitation and hydrothermal methods [118,121].Such a thermolysis method is preferred for the following reasons: (i) the precursors can be well mixed and reacted to prepare Zn-Cd-Tu complex by the ultrasonication process; (ii) the loss of precursors can be prevented during the fabrication process; and (iii) Zn 1−x Cd x S with a tiny crystallite size can be prepared because thiourea releases S 2− ions slowly and offers some N and C atoms as the pinning points in the Zn-Cd-Tu complex [122].
Zhang et al. reported that a particular fabrication method will enable a photocatalyst to perform better in photocatalytic activity.The photocatalysts Cd 1−x Zn x S are prepared by three different methods: thermal sulfuration, co-precipitation without thermal treatment, and co-precipitation with thermal treatment.The photocatalyst which is synthesized by the thermal sulfuration method has better performance because the fabrication method allows uneven distribution of S 2− ions, thus leading to a charge gradient [123].A similar trend is also evidenced in the work done by Park et al. on a ternary CdS/TiO 2 /Pt hybrid.CdS/(Pt-sgTiO 2 ) has the highest hydrogen production because of the electron transfer from CdS to Pt through TiO 2 [124].

Crystal Size
Hydrogen production activity is also influenced by the crystal size of the photocatalyst.Li et al. [125] studied the photocatalytic hydrogen production performances of size-selected CdS nanoparticles decorated with co-catalyst Pt nanoparticles.When the size of CdS nanoparticles decreases from 4.6 to 2.8 nm, the H 2 generation quantum yield can increase from 11% to 17%.Such a dependence was observed because the driving force of photoinduced carrier transfer from CdS to vacant states of Pt nanoparticles is size-dependent.Baldovi et al. [126] prepared MoS 2 quantum dots by laser ablation of MoS 2 particles in suspension and investigated their photocatalytic hydrogen production performance.Two types of MoS 2 nanoparticles exhibited higher activity than that of bulk MoS 2 .When the size of MoS 2 nanoparticles was decreased from 15-25 nm to 5 nm, the photocatalytic hydrogen generation performance was almost doubled.Holmes et al. [127] and Grigioni et al. [128] reported the dependence of the photocatalytic water splitting activity on the size of CdSe nanoparticles.The photocatalytic activity increases as the size of CdSe nanoparticles decreases.Figure 10 presents the UV-vis absorption spectra and quantum yields of CdSe quantum dots photocatalysts with various size.They also reported that the light harvesting capability and the conduction band energy should compromise to achieve maximal photocatalytic H 2 generation activity.There is an optimal size of 2.8 nm to achieve maximal photocatalytic H 2 generation activity considering the compromise among light harvesting, band structure, and charge separation.MoS2.When the size of MoS2 nanoparticles was decreased from 15-25 nm to 5 nm, the photocatalytic hydrogen generation performance was almost doubled.Holmes et al. [127] and Grigioni et al. [128] reported the dependence of the photocatalytic water splitting activity on the size of CdSe nanoparticles.The photocatalytic activity increases as the size of CdSe nanoparticles decreases.Figure 10 presents the UV-vis absorption spectra and quantum yields of CdSe quantum dots photocatalysts with various size.They also reported that the light harvesting capability and the conduction band energy should compromise to achieve maximal photocatalytic H2 generation activity.There is an optimal size of 2.8 nm to achieve maximal photocatalytic H2 generation activity considering the compromise among light harvesting, band structure, and charge separation.

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

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 H 2 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 H 2 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 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 H 2 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.

Figure 1 .
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].

Figure 1 .
Figure 1.Morphology and a proposed mechanism of the photocatalytic H 2 production by metal wire mesh based immobilized photocatalysts with Ag 2 S-coupled ZnO@ZnS core-shell nanorods.Figure adapted from [32].
presents a possible mechanism for the electron transfer of MoS 2 -PANI-CdS photocatalysts.Incorporating polyaniline helps to separate photoinduced charges across the ZnS-polyaniline interfaces.Catalysts 2019, 9, x FOR PEER REVIEW 8 of 25

Figure 4 .
Figure 4.A proposed mechanism for the electron transfer of MoS2-PANI-CdS photocatalysts when illuminated.Figure adapted from [69].

Figure 4 .
Figure 4.A proposed mechanism for the electron transfer of MoS 2 -PANI-CdS photocatalysts when illuminated.Figure adapted from [69].

Figure 5 .
Figure 5.A possible mechanism for the photoexcitation and carrier transporting process of Ni-doped ZnS/NiO/Ni foam photocatalysts.Figure adapted from [70].

Figure 5 .
Figure 5.A possible mechanism for the photoexcitation and carrier transporting process of Ni-doped ZnS/NiO/Ni foam photocatalysts.Figure adapted from [70].
Similar results were also found for the sulfide-based photocatalysts.Song et al. found that, in comparison with 2-D Cu 2 MoS 4 nanosheet with the exposed {001} facet, Cu 2 MoS 4 nanotube with the exposed {010} facet exhibited effectively improved the performance for photocatalytic degradation and water splitting [111].Shen et al. reported that the crystal facets of ZnIn 2 S 4 with a 3D-hierarchical persimmon-like structure will influence the photocatalytic activity of ZnIn 2 S 4 .Extending the reaction time did not reveal any significant influences on the band gap or surface area of ZnIn 2 S 4 .Hence, the increase in the percentage of the {006} facet enhances hydrogen production [108].The atomic structure of the {006} facet mainly consists of unsaturated metal cations.During the H 2 generation reaction, the exposed unsaturated Zn and In cations of ZnIn 2 S 4 will attract S 2− and SO 3

Figure 8 .
Figure 8. Schematic illustration of the hydrogen generation reaction on the {006} facets of the Pt loaded ZnIn2S4 photocatalysts.Figure adapted from reference [108].

Figure 8 .
Figure 8. Schematic illustration of the hydrogen generation reaction on the {006} facets of the Pt loaded ZnIn 2 S 4 photocatalysts.Figure adapted from reference [108].
Photocatalysts can be prepared by numerous methods, including the coprecipitation, cation exchange, chemical bath deposition, hydrothermal, and solvothermal methods.The co-precipitation method may lead to differences between the obtained final element composition in solid solution and the stoichiometric ratio[118].The hydrothermal method requires a great amount of time to prepare a well-crystallized solid solution[119].In the thermolysis method, the products are treated under high temperature to achieve fast fabrication, high crystallinity, and photocatalytic performance of the solid solution[120].Li et al. found that Zn1−xCdxS photocatalyst fabricated using the simple Zn-Cd-Tu complex thermolysis method showed better performance than did those synthesized by the coprecipitation and hydrothermal methods[118,121].Such a thermolysis method is preferred for the following reasons: (i) the precursors can be well mixed and reacted to prepare Zn-Cd-Tu complex by the ultrasonication process; (ii) the loss of precursors can be prevented during the fabrication process; and (iii) Zn1−xCdxS with a tiny crystallite size can be prepared because thiourea releases S 2− ions slowly and offers some N and C atoms as the pinning points in the Zn-Cd-Tu complex[122].Zhang et al. reported that a particular fabrication method will enable a photocatalyst to perform better in photocatalytic activity.The photocatalysts Cd1−xZnxS are prepared by three different methods: thermal sulfuration, co-precipitation without thermal treatment, and co-precipitation with thermal treatment.The photocatalyst which is synthesized by the thermal sulfuration method has better performance because the fabrication method allows uneven distribution of S 2− ions, thus leading to a charge gradient[123].A similar trend is also evidenced in the work done by Park et al. on a ternary CdS/TiO2/Pt hybrid.CdS/(Pt-sgTiO2) has the highest hydrogen production because of the electron transfer from CdS to Pt through TiO2[124].

Figure 10 .
Figure 10.UV-vis absorption spectra and quantum yields of CdSe quantum dot photocatalysts with various size.Figure adapted from [128].

Figure 10 .
Figure 10.UV-vis absorption spectra and quantum yields of CdSe quantum dot photocatalysts with various size.Figure adapted from [128].

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.

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

Table 3 .
Photocatalytic H 2 generation property of photocatalysts loaded with silver sulfide.

Table 6 .
Photocatalytic H 2 production for graphene oxide loaded with composite photocatalysts.

Table 6 .
Photocatalytic H2 production for graphene oxide loaded with composite photocatalysts

Table 8 .
Photocatalytic hydrogen production performances for transition metal doped photocatalysts.

Table 9 .
Effects of calcination treatment on photocatalytic performance.

Table 10 .
Photocatalytic activity of photocatalysts using different sacrificial agents.

Table 10 .
Photocatalytic activity of photocatalysts using different sacrificial agents

Table 11 .
Photocatalytic activity of photocatalysts with different structures.