Recent Progress and Approaches on Transition Metal Chalcogenides for Hydrogen Production

: Development of efﬁcient and affordable photocatalysts is of great signiﬁcance for energy production and environmental sustainability. Transition metal chalcogenides (TMCs) with particle sizes in the 1–100 nm have been used for various applications such as photocatalysis, photovoltaic, and energy storage due to their quantum conﬁnement effect, optoelectronic behavior, and their stability. In particular, TMCs and their heterostructures have great potential as an emerging inexpensive and sustainable alternative to metal-based catalysts for hydrogen evolution. Herein, the methods used for the fabrication of TMCs, characterization techniques employed, and the different methods of solar hydrogen production by using different TMCs as photocatalyst are reviewed. This review provides a summary of TMC photocatalysts for hydrogen production.


Introduction
Renewably produced hydrogen gas is used as an alternative clean fuel for transportation. It is regarded as a clean and efficient carrier of energy and is considered as an ideal alternative to fossil fuels to meet the growing energy demand and address environmental issues [1]. When compared with other combustible fuels (i.e., deposited fossil fuels such as coal, oil, and natural gas [2]) hydrogen possesses a high energy density (140 MJkg −1 ) and the end product of this combustion reaction is only water [3,4]. Therefore, the growing energy demand requires developments in the storage of hydrogen, their transport, and production techniques. Although great progress has been made in H 2 storage [5][6][7] and transport, the utilization of H 2 energy is stymied by the lack of inexpensive and efficient production methods [8]. Hydrogen can be produced in different ways ( Figure 1) including steam-methane reforming of natural gas, coal gasification, and biomass conversion [9], which are expensive and produce greenhouse gases such as carbon monoxide and carbon dioxide that are not favorable to the environment [10,11]. An attractive alternative technique, solar assisted water splitting, is considered as one of the most suitable and greener ways to produce hydrogen energy [12] and the technology of water splitting becomes mature day-by-day [13].
The simple method for the separation of water is electrolysis and various clean energy resources such as solar, geothermal, biomass, wind, ocean, or other renewables can be used for this process. Among them, using solar energy, water can be split by two methods: photoelectrochemical and photocatalytic water splitting. The simple method for the separation of water is electrolysis and various clea ergy resources such as solar, geothermal, biomass, wind, ocean, or other renewabl be used for this process. Among them, using solar energy, water can be split b methods: photoelectrochemical and photocatalytic water splitting.
Various types of photocatalytic materials have been utilized for photocatalyt plications. TiO2 [14][15][16][17], ZnO [18], mixed oxide materials [19], perovskite [20,21] [ 22], and metal-doped oxide materials [23,24] are some of them. Among these, tran metal chalcogenides (TMCs) have gained worldwide attention due to their electr lytic properties that include indirect band gaps, optoelectronic behavior, and the bility [25]. In addition, nanodots (quantum dots)/nano structures of metal chalcoge show enhanced catalytic activity at the edges. Furthermore, by limiting their partic in the 1-10 nm range, due to the quantum confinement effect, it is possible to utiliz der solar simulated irradiation [26,27]. Generation of exciton pairs and the eff charge separation in these semiconductor photocatalysts lead to an effective phot process. The efficiency of charge transport mainly depends on the crystallinity, pa and crystallite size of the materials among many other factors. Composites of TMC co-catalysts such as graphene [28], graphene oxides [29], carbon nitride [30], metal o [19,[31][32][33], CdLa2S4 [34], and metals [35] have been found to increase the conductiv electrons, provide active sites, and effectively separate the electron and hole pairs g ated by bandgap irradiation of the semiconductor photocatalysts. This review m focuses on the synthesis and characterization of various types of TMCs and their cation in hydrogen energy production through water splitting. This review may researchers by providing insights on the fabrication of highly efficient photocat with low cost and high efficiency.

Transition Metal Chalcogenides (TMCs)
TMCs have gained worldwide attention in recent decades. These catalysts have used in lithium-ion batteries, solar cells, and hydrogen evolution reactions. TMCs c excited by solar simulated irradiation and due to their indirect band gaps, optoelec behavior, small bandgap, and their stability are favorable candidates as photocata Nowadays, many researchers have focused on the production and photocatalytic ac of TMCs. These TMCs have been synthesized by different techniques that includ Various types of photocatalytic materials have been utilized for photocatalytic applications. TiO 2 [14][15][16][17], ZnO [18], mixed oxide materials [19], perovskite [20,21], dye [22], and metal-doped oxide materials [23,24] are some of them. Among these, transition metal chalcogenides (TMCs) have gained worldwide attention due to their electrocatalytic properties that include indirect band gaps, optoelectronic behavior, and their stability [25]. In addition, nanodots (quantum dots)/nano structures of metal chalcogenides show enhanced catalytic activity at the edges. Furthermore, by limiting their particle size in the 1-10 nm range, due to the quantum confinement effect, it is possible to utilize under solar simulated irradiation [26,27]. Generation of exciton pairs and the effective charge separation in these semiconductor photocatalysts lead to an effective photolysis process. The efficiency of charge transport mainly depends on the crystallinity, particle, and crystallite size of the materials among many other factors. Composites of TMCs with co-catalysts such as graphene [28], graphene oxides [29], carbon nitride [30], metal oxides [19,[31][32][33], CdLa 2 S 4 [34], and metals [35] have been found to increase the conductivity of electrons, provide active sites, and effectively separate the electron and hole pairs generated by bandgap irradiation of the semiconductor photocatalysts. This review mainly focuses on the synthesis and characterization of various types of TMCs and their application in hydrogen energy production through water splitting. This review may assist researchers by providing insights on the fabrication of highly efficient photocatalysts with low cost and high efficiency.

Transition Metal Chalcogenides (TMCs)
TMCs have gained worldwide attention in recent decades. These catalysts have been used in lithium-ion batteries, solar cells, and hydrogen evolution reactions. TMCs can be excited by solar simulated irradiation and due to their indirect band gaps, optoelectronic behavior, small bandgap, and their stability are favorable candidates as photocatalysts. Nowadays, many researchers have focused on the production and photocatalytic activity of TMCs. These TMCs have been synthesized by different techniques that include single-step hydrothermal synthesis [13,36,37], solvothermal method [38], sulfidation and selenization [39], free intercalation method [40], electrospinning technique, the successive ionic layer adsorption and reaction (SILAR) method [32], ultrasonic-exfoliation [23], electrodeposition method [27,41], photoreduction method [42], refluxing method [43], chemical vapor deposition (CVD) [44], template-free one-pot method [45], successive surface modification method [46], and microwave-assisted synthesis [47][48][49][50]. The following section will elaborate on some of the above-mentioned synthesis methods in detail. The hydrothermal technique is one of the frequently reported methods to produce nanomaterials with various morphologies and thicknesses [51]. This method involves a heterogeneous reaction in the presence of an aqueous solvent or mineralizers under high pressure and low temperature to dissolve and recrystallize the materials that are moderately insoluble under ordinary conditions in closed steel containers called autoclaves. If the water is used as the solvent, this process is called the hydrothermal method whereas in the solvothermal method, organic solvents are used [36,52]. This method can be used to synthesize nanomaterials with high crystallinities, favorable crystal facets, tunable morphologies, and controllable sizes by varying experimental factors such as temperature, solvent, time, etc. Hydrothermal synthesis consumes relatively low energy, and most importantly, it is an environmentally friendly method [53,54], and hence is a very common method. Figure 2 illustrates an example of the synthesis of MoS 2 .   Here, ammonium-hepta molybdate tetrahydrate and sulfur (S) powder were used as precursors. The black precipitates were collected and dried to obtain the required MoS 2 sample, which exhibited a layered structure [55].
In a different study, Jing Yu et al. synthesized SnS 2 nanosheets by the solvothermal method. The mechanism of their synthesis is illustrated in Figure 3, where Sn 2+ from SnCl 2 reacts with S 2− to form orthorhombic SnS micro flowers stacked with decentralized sheets, and further addition of a S 2− source results in SnS 2 with an increase in the reaction time [38].   In addition, surfactants such as SDS (sodium dodecyl sulfate), CTAB (cetyltrimethylammonium bromide), PVP (polyvinyl pyrrolidone) [56], etc. have been used to increase the crystal growth of the nanomaterials, and a hydrothermal synthesis of a WS 2 nanosphere by the addition of CTAB is discussed in Figure 4.   Figure 4a shows that the CTAB molecules were randomly dispersed in solution as single molecules (not aggregates) at low concentrations under the hydrothermal condition. After the amount of CTAB was adjusted to 1.5 mmol, a lamellar micelle was formed to minimize the free energy of the system. The external surface of the lamellar micelle contains a hydrophilic head, whereas the tail, which is hydrophobic, points to the micelle center [58]. In Figure 4b, WS 4 2− ions tend to form lamellar micelles due to complementary electrostatic interactions between the CTA + and WS 4 2− To keep the free energy of the system low, the lamellar micelle was formed as an ordered nano-microstructure, as indicated in Figure 4c. Figure 4d shows 3-D assemblies formed through van der Waals forces. Similar methods have been utilized to synthesize FeS 2 , NiS 2 , and CuS in a different work [37]. The WS 2 nanocrystal was formed from the reduction of WS 4 2− ions, as indicated in Figure 4e. The aggregation of WS 2 colloids reduced the surface free energy and formed nanosheets. The flower-like WS 2 nanospheres were obtained as indicated in Figure 4f, and it was concluded that hydrothermal synthesis in the presence of a surfactant such as CTAB and careful control of the precursor concentration can be utilized to obtain the desired flower-like structure [57].
Apart from this hydrothermal method, there is a clean, ultrafast (60 s) synthetic technique that can be conducted using microwave heating without any inert gas protection to achieve uniform TMCs [48,59].

Microwave-Assisted Synthesis
The microwave-assisted synthesis method has also been used to prepare TMCs with controllable sizes and shapes [60]. Figure 5 illustrates the microwave-assisted synthesis of a MoS 2 /graphene composite in the presence of ammonium tetrathiomolybdate (ATTM) and CS 2 on a graphene support. The reaction was carried out in a glass vial with equimolar amounts of ATTM and graphene. The reactants were mixed at a constant speed, and then the CS 2 solvent (200 µL) was added and mixed well. The solvent was evaporated after 10 min of air drying. Then, the mixture was kept under microwave irradiation. Here, the graphene served as a substrate to absorb the microwave energy and convert the incident radiation energy to heat. During the process, ATTM was reduced first to MoO 2 , and then converted to MoS 2 , as indicated in Figure 5    In different works, SnS 2 [61], CdS [62], CuS [63], NiS 2 [64], WS 2 [59], and CoS 2 [65] nanomaterials were synthesized using the microwave-assisted synthesis method.

Electrodeposition Method
Electrodeposition is a well-established technique for the preparation of nanomaterials for a wide range of materials (e.g., metal nanoparticles, nanosheet arrays [66], nanowires [67], nanofilms [68], laminated composites [69], multilayered composite coatings [70], and nanoparticle-reinforced composite coatings [71]) and this is a rapid and single-step process. It can be used to produce homogeneous and high-purity crystalline Energies 2021, 14, 8265 6 of 37 materials at the cathode of the electrochemical system during the coating process because of the external current (direct or pulsed) applied to the electrodeposition system [72][73][74]. Figure 6 illustrates the synthesis of MoS 2 quantum dots (QDs) on TiO 2 nano-tube arrays (NTAs) to form heterostructures using a facile electrodeposition method with a threeelectrode system. TiO 2 NTAs/Ti foil, Pt foil, and saturated Ag/AgCl electrodes were used as the working electrode, counter electrode, and reference electrode, respectively. MoS 2 QD solution and citric acid-sodium citrate buffer solution (0.1 M, pH 4.5) with a volume ratio of 1:1 was used as the electrolyte of this system. MoS 2 QDs were deposited onto TiO 2 NTA photoelectrodes by the electrodeposition process, and the obtained samples were rinsed with deionized water and dried at ambient temperature after different electrodeposition times of 5, 10, 20, and 30 min., respectively [27].

Photoreduction
Photoreduction, for example, the photodeposition of MoS 2 on rGO x /CdS was also utilized to prepare metal chalcogenide materials. An appropriate amount of precursor molecule (MoS 4 2− ) and surfactants were added and the pH of the solution was adjusted by adding an acid or base. Then, the suspension was bubbled with nitrogen to maintain an inert environment, then irradiated with a 350 W Xe lamp equipped with a cutoff filter (λ ≥ 420 nm) for a particular time interval. The incident intensity was 34 mWcm −1 . The MoS 4 2− ions were reduced to MoS 2 by the photoexcited electrons of rGO x /CdS. The final product, MoS 2 /rGO x /CdS, was obtained by filtration. The loaded amount of MoS 2 was determined by measuring the MoS 4 2− concentrations in both the reacted solution and the washed solution on a spectrophotometer at the wavelength of 468 nm [42].

Electron Beam Evaporation
There have been various methods used in the preparation of Au deposited on a NP/Zn x Cd 1−x S NW heterostructure. Ternary Zn x Cd 1−x S nanowires were synthesized by using silicon (100) wafers as substrates ( Figure 7). First, Au film was deposited on a Si substrate by the electron beam evaporation method. The substrates coated with 10 nm of Au film were placed into the downstream region of a three-zone horizontal quartz tube furnace and then annealed at 500 • C for 1 h.   The pressure was maintained at 0.01 torr throughout the annealing process. Ternary Zn x Cd 1−x S nanowires were synthesized by a simple one-step process. Before growth, commercial-grade ZnS powder (99.995%) CdS powder (99.999%), and a piece of Si (100) substrate coated with 10 nm of Au film as a catalyst were annealed. Argon flow was purged during the growth as a carrier gas and the pressure was maintained. After the heating process, the samples were cooled to room temperature in the furnace. The key factor in tuning the x ratio of the Zn x Cd 1−x S nanowires was found to be temperature [75].

Sulfidation and Selenization
Figure 8a schematically illustrates the experimental setup used for the growth of WO 3 nanowires (NWs) as well as WS 2 , WSe 2 , and their ternary WS 2(1−x) Se 2x nanotubes (NTs) on highly flexible carbon fibers (CFs) in a horizontal two-zone tube furnace [39]. High-density WO 3 NWs were used to cover the entire surfaces of the CFs vertically and uniformly, as shown in Figure 8c. The diameter and length of typical WO 3 NWs were found to be ca. 100 nm and the length was measured to be 5 µm. Then, the WS 2 and WSe 2 NTs were synthesized through sulfidization and selenization of WO 3 NWs at 800 • C, as demonstrated in Figure 8d,e.
For the synthesis of WS 2(1−x) Se 2x NTs, the ratio of S and Se precursors were successfully altered for the controllable element component of ternary NTs (Figure 8f). As WO 3 NWs were used as a conversion template, the diameter and length of the WS 2 and WSe 2 NTs were found to be comparable to those of WO 3 NWs. Synthesis of the WS 2 NTs was conducted by the sulfidization of the WO 3 NWs performed at 800 • C in an Ar atmosphere. The chemical reaction is described as

. Successive Ionic Layer Adsorption and Reaction (SILAR) Method
The SILAR method is a simple, cost effective, and rapid technique for the deposition of binary semiconducting thin films when compared with other methods. It can be applied in the deposition of large-area thin films. Since this technique was introduced by Nicolau in the mid-1980s [76], this method has been employed to grow thin films, especially CdS, ZnS, and CdZnS. In particular, this method is appropriate for depositing thin films of chalcogenide groups I-VI, II-VI, III-VI, V-VI, VIII-VI binary, and I-III-VI, II-II-VI, II-III-VI, II-VI-VI and II-V-VI ternary chalcogenides and composite films. The SILAR method consists of the adsorption of the cation precursor, water rinsing, adsorption of the anion precursor, followed by the appropriate reaction, and rinsing. These steps may be repeated. The schematic diagram illustrated in Figure 9 explains the mechanism of the SILAR growth.    In a typical process, the substrate is immersed separately in two precursor solutions and washed with water to remove the loosely bound species. By controlling the experimental conditions, the growth rates of thin films in the SILAR method can be varied between a quarter and a half of a monolayer [78][79][80]. Therefore, the aqua ligands remain at least partially intact during adsorption, thereby lowering the density of cations and anions in one layer. Furthermore, the growth of the thin film can be controlled at an accuracy of one SILAR cycle. Valkonen et al. [80] studied the effect of different types of substrates by depositing ZnS, CdS films on glass, ITO-coated glass, and single crystals of GaAs (100). In this paper, they mainly focused on promoting interest in the SILAR method as applied to tin disulfide thin films. The authors used the SILAR method to prepare large-area (∼10 cm 2 ) SnS 2 thin films on glass substrates under optimized deposition conditions. In addition, they reported SnS 2 thin films on a single crystal wafer of Si (111) to study the effect of substrate on the crystallinity of the films. In summary, these reports explain the feasibility of using the SILAR method for the preparation of TMC thin-films/coatings.

Refluxing Method
The refluxing method is a suitable method for the large-scale synthesis of TMCs such as SnS 2 . In their paper, Liu et al. reported a facile refluxing method without using any templates or surfactants to synthesize SnS 2 nanocrystals with controllable morphology. It was found that the as-obtained SnS 2 nanostructures exhibited good adsorption ability and enhanced visible light photocatalytic activity for the degradation of Methylene Blue.
In a typical procedure, SnS 2 was prepared by using SnCl 4 ·5H 2 O in the presence of ethylene glycol in a round bottom flask under stirring. The resultant mixture was heated to specific temperature followed by the addition of Na 2 S solution under vigorous stirring. The resulting solution was refluxed under different temperature (120 • C and 180 • C) to obtain different samples.
In another study, Aditha et al. synthesized Cu 2 ZnSnS 4 using the refluxing method. The experimental setup they used in their study is illustrated in Figure 10. A round bottom flask was fitted with a condenser connected to a water inlet and outlet to condense the vapor back into the reaction vessel to keep the total volume constant. In this method, the reaction temperature is the boiling point of the solvent in the reaction vessel [81].  CuCl 2 , ZnCl 2 , SnCl 2 ·2H 2 O, and thiourea were used as the precursor materials for producing Cu 2 ZnSnS 4 and the refluxing time was 8 h. The reaction mechanism is given below.
Further to this, several other studies have also been performed by using the refluxing method to synthesize metal chalcogenide materials such as CuS [82], MoS 2 [83], ZnS [84], and NiS 2 [85].

Chemical Vapor Deposition (CVD) Method
A typical bottom-up method, the chemical vapor deposition (CVD) method, is the process during which gaseous or vapor materials react at a certain temperature in the gas phase or gas-solid interfaces to form solid products with low structural defects [44]. In the CVD method, parameters such as choice of the precursor, temperature, pressure, flow rate, time, the distance between precursor and substrate, and the choice of substrates influences the morphology, layer number, lateral size, orientation, and degree of defects [86][87][88]. Kwon et al. [89] provided a deep understanding of the thermodynamic and kinetic factors influencing the CVD method. Most importantly, the CVD method exhibits prominent advantages in controlling 2-D materials at an atomic-level, which allow for clarity of the structure-activity relationships.
Tan et al. utilized WS 2 materials with a graphene electrode for ultrathin 2-D photo detectors [90]. In another study, a 2-D-layered tin disulfide was synthesized by Chang et al. using the CVD method for photo detection of transistors by the thickness-controlled interface dynamics (TCID) technique, which shows high performance and great potential for several other applications [91].
Cong et al. synthesized single crystalline WS 2 monolayers by the direct sulfurization of WO 3 powders with sulfur powder at 750 • C; 300 nm using SiO 2 /Si as the substrate. The substrate was covered with the WO 3 sample and another substrate was placed face down above it. After that, the setup ( Figure 11) was heated in a small end closed quartz tube, which in turn was placed inside a big quartz tube using a tubular furnace [92].  In addition, SnS 2 [93], NiS [94], NiS 2 [94], MoS 2 [95], and FeS 2 [96] nanomaterials were also synthesized by using this CVD method in different studies.

Liquid Cascade Centrifugation (LCC) Method
In addition to the above-mentioned methods, synthesis of TMC can also be performed by a liquid cascade centrifugation (LCC) method to achieve a highly efficient nanosheet with selected size and thickness [97]. LCC is an efficient, scalable, and versatile method that is based on a set of iterative centrifugation cascades designed to achieve highly efficient nanosheets and/or monolayer enriched dispersions. Mihai et al. synthesized liquidexfoliated 2D WS 2 and MoS 2 nanosheets by the liquid centrifugation cascade method and the prepared nanosheets were used for neuromorphic computing applications [98]. Figure 12 illustrates the LCC method used in their study to separate the nanosheets efficiently according to their size and thickness.  The following Table 1 discusses the different synthesis methods used in the preparation of TMC nanoparticles and the specific advantage(s)/disadvantage(s). Table 1. Summary of the different types of the synthesis method of TMC.

Synthesis Methods Advantages/Disadvantages
Hydrothermal/Solvothermal Method Produce nanomaterials with various morphologies and thicknesses at high pressure and low temperature; can synthesize high quality large crystals; cost of equipment is high Microwave-assisted Synthesis Required less time/rapid process; size can be controlled Electrodeposition Method Rapid and single-step process; used to produce homogeneous and high-purity crystalline materials at the cathode of the electrochemical system during the coating process

Characterization Techniques
There have been various techniques such as X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), UV-Visible spectroscopy/diffuse reflectance spec-Energies 2021, 14, 8265 12 of 37 troscopy (UV-DRS), and X-ray photoelectron spectroscopy (XPS) used to characterize the transition metal chalcogenides, which are discussed in the following section.

X-ray Diffraction Method (XRD)
X-ray diffraction method is a non-disruptive technique used to characterize the crystallite size, crystallinity, crystal structure, and phase composition of the material in a powder or film. Figure 13 shows the XRD patterns of SnS 2 nanosheets (NSs) that confirm the formation of 2T-type hexagonal SnS 2 NSs (space group P3m1) with cell parameters of a = b = 3.649 Å and c = 5.899 Å. A high crystalline and pure product was obtained when compared with the standard diffraction data of JCPDS no. 23-0677. The highest intensity peak was assigned to the (001) facet of the indicating hexagonal phase [38]. Figure 14 shows the XRD patterns for MoS 2 and WS 2 on carbon cloth at different thermolysis temperatures. The peaks at 26.8 • and 43.5 • (marked as "C") can be ascribed to the carbon cloth in all samples. As shown in Figure 14a, the presence of the (002) reflection peak at 2θ = 14.2 • indexed the periodicity along the direction perpendicular to the planar structure of MoS 2 . This was only observed for the MoS 2 samples heated at 1000 • C, but not at lower temperatures. For the WS 2 samples, peaks were observed at lower temperatures (i.e., 200 and 400 • C). As the thermolysis temperature increased, higher crystallinity of WS 2 was observed. For WS 2 samples prepared at 800 and 1000 • C, the reflection peak was found for WO 3 , which is marked with * in Figure 14b. X-ray diffraction method is a non-disruptive technique used to characterize the crystallite size, crystallinity, crystal structure, and phase composition of the material in a powder or film. Figure 13 shows the XRD patterns of SnS2 nanosheets (NSs) that confirm the formation of 2T-type hexagonal SnS2 NSs (space group P3 m1) with cell parameters of a = b = 3.649 Å and c = 5.899 Å. A high crystalline and pure product was obtained when compared with the standard diffraction data of JCPDS no. 23-0677. The highest intensity peak was assigned to the (001) facet of the indicating hexagonal phase [38]. Figure 14 shows the XRD patterns for MoS2 and WS2 on carbon cloth at different thermolysis temperatures. The peaks at 26.8° and 43.5° (marked as "C") can be ascribed to the carbon cloth in all samples. As shown in Figure 14a, the presence of the (002) reflection peak at 2θ = 14.2° indexed the periodicity along the direction perpendicular to the planar structure of MoS2. This was only observed for the MoS2 samples heated at 1000 °C, but not at lower temperatures. For the WS2 samples, peaks were observed at lower temperatures (i.e., 200 and 400 °C). As the thermolysis temperature increased, higher crystallinity of WS2 was observed. For WS2 samples prepared at 800 and 1000 °C, the reflection peak was found for WO3, which is marked with * in Figure 14b.  X-ray diffraction method is a non-disruptive technique used to characterize the crystallite size, crystallinity, crystal structure, and phase composition of the material in a powder or film. Figure 13 shows the XRD patterns of SnS2 nanosheets (NSs) that confirm the formation of 2T-type hexagonal SnS2 NSs (space group P3 m1) with cell parameters of a = b = 3.649 Å and c = 5.899 Å. A high crystalline and pure product was obtained when compared with the standard diffraction data of JCPDS no. 23-0677. The highest intensity peak was assigned to the (001) facet of the indicating hexagonal phase [38]. Figure 14 shows the XRD patterns for MoS2 and WS2 on carbon cloth at different thermolysis temperatures. The peaks at 26.8° and 43.5° (marked as "C") can be ascribed to the carbon cloth in all samples. As shown in Figure 14a, the presence of the (002) reflection peak at 2θ = 14.2° indexed the periodicity along the direction perpendicular to the planar structure of MoS2. This was only observed for the MoS2 samples heated at 1000 °C, but not at lower temperatures. For the WS2 samples, peaks were observed at lower temperatures (i.e., 200 and 400 °C). As the thermolysis temperature increased, higher crystallinity of WS2 was observed. For WS2 samples prepared at 800 and 1000 °C, the reflection peak was found for WO3, which is marked with * in Figure 14b.   Figure 15 shows the XRD patterns indexed to g-C 3 N 4 for different heating times and MoS 2 samples. The bottom (green) XRD pattern was indexed to a hexagonal phase of MoS 2 (JCPDS 37-1492) [100]. The peaks observed at 2θ = 32.6 • and 58.3 • corresponded to the (100) and (110) crystal planes of metal chalcogenide, respectively. In addition, the XRD patterns of CN2.5, CN10, and CN20 samples showed similar peaks, as indicated in Figure 15 (JCPDS 87-1526). The peak at 2θ = 27.4 • was indexed to the (002) stacking layered structure of g-C 3 N 4 and the 2θ = 13.0 • peak corresponded to the (100) in-plane of the repeating units [101]. The CM0.05 heterostructure pattern showed the sole g-C 3 N 4 peaks without the MoS 2 peaks due to the low content of MoS 2 [102][103][104].  In a different study, pure CoS 2 , pure TiO 2 , and 10 wt. % CoS 2 /TiO 2 were successfully synthesized using the hydrothermal method [105]. XRD patterns confirmed the formation of these nanoparticles and the Figure 16    In another study, SnS 2 /TiO 2 nanocomposites were synthesized by the hydrothermal method, and XRD analysis was carried out to characterize those materials. Figure 17  In addition, the particle size distribution can also be easily ease, and thus, many of the reported works have used this simple nique to study the TMC materials using SEM. Figure 18 exhibits the phase and morphology transformation ing the formation of monodispersed SnS2 nanosheets; consequentl action parameters including reaction time and the dosage of polyv on the morphology of SnS2 was also studied. PVP was used as a s ping agent to minimize aggregation of the colloidal particles. In addition, the particle size distribution can also be easily obtained with relative ease, and thus, many of the reported works have used this simple characterization technique to study the TMC materials using SEM. Figure 18 exhibits the phase and morphology transformation of intermediates during the formation of monodispersed SnS 2 nanosheets; consequently, the effects of the reaction parameters including reaction time and the dosage of polyvinylpyrrolidone (PVP) on the morphology of SnS 2 was also studied. PVP was used as a steric stabilizer or capping agent to minimize aggregation of the colloidal particles. Figure 18 exhibits the phase and morphology transformation of intermediate ing the formation of monodispersed SnS2 nanosheets; consequently, the effects of action parameters including reaction time and the dosage of polyvinylpyrrolidone on the morphology of SnS2 was also studied. PVP was used as a steric stabilizer o ping agent to minimize aggregation of the colloidal particles.    Figure 18a shows the structure of orthorhombic SnS microspheres stacked by sheets, which were first nucleated in the primary stage of a chemical reaction. When the reaction time increased to 2 h, small sheets expanded and grew onto larger sheets. At the same time, in Figure 18b, the spherical flower-like morphology remained and the regular nanosheets were deposited around micro-flowers. Figure 18c is the high-magnification SEM image with the reaction time of 2 h, which exhibits the deposited nanosheets. When the reaction time was prolonged to 5 h, uniform nanosheets were obtained with the single phase of hexagonal SnS 2 . Consequently, it was proven that the SnS phase would be oxidized into SnS 2 gradually as the reaction progresses. Furthermore, as the reaction time increases, SnS 2 nanosheets tend to grow and enhance their crystallinity [38], which was proven by SEM analysis.
In another study, a different TMC, NiS 2 particles with a hollow structure, were synthesized and characterized by FESEM. Figure 19a,b indicates the FESEM images of NiS 2 hollow microspheres prepared on a large scale. Figure 19b shows that the diameter of NiS 2 hollow microspheres was about 3 µm and it was found that the surface of the microspheres was rough. The thickness of the wall was found to be approximately 200 nm (Figure 19c), and the hollow structure can be clearly seen in this image. Figure 20d shows a high-resolution TEM image that displays the lattice fringes of NiS 2 . The lattice distance was 0.286 nm, which corresponds to the (200) crystal plane [13].
In a different study, TMC doped TiO 2 materials were synthesized and studied for hydrogen production. Sivagowri et al. synthesized a CoS 2 /TiO 2 nanocomposite using hydrothermal methods. As shown in the SEM images of pure CoS 2 (Figure 20a,b), aggregates stacked by flake-like structures were attained. The image for pure TiO 2 (Figure 20c,d) showed an irregular 3-D block-like structure covered with spongy like particles, whereas CoS 2 /TiO 2 indicates a hexagonal rod like structure decorated with sponge like materials, as illustrated in Figure 20e     The same group, in a different study, reported that a different morphology was attained with SnS 2 -TiO 2 nanocomposites when only the TMC was changed. In the SEM study, they observed that the pure TiO 2 nanoparticle showed an irregular 3-D block-like structure covered with sponge-like particles as indicated in Figure 21a. The micrograph obtained in the higher magnification (Figure 21b) image indicates that the aggregated particles had spongy-like structures. A flower-like structure composed with nanosheets was attained for pristine SnS 2 and is shown in Figure 21c Figure 22. The surface morphology of these SnS 2 /TiO 2 nanocomposites at different wt. % possessed nanosheets, and it can be seen that the sponge-like materials covered the structure and, therefore, exhibited significant aggregation [106].

X-ray Photoelectron Spectroscopy (XPS) Studies
X-ray photoelectron spectroscopy (XPS) is based on the photoelectric effect and was used to investigate the chemical state and surface elemental composition of the samples. Among the literature collected, it was found that only a few reports have presented the XPS analysis of TMC material, which may be due to the necessity of the analysis for the structure-activity correlation. In a study by Bo Gao et al., a MoS 2 /candle soot composite was synthesized and analyzed using XPS. Figure 23 shows the existence of the elements C, Mo, and S with a small amount of O. The corresponding peaks indicate the binding energies at 284.6 (C 1s), 161.5 (S 2p), 231.0 (Mo 3d), and 529.1 eV (O 1s), respectively. In addition, the atomic ratio between Mo and S was found to be 0.43. Figure 23b illustrates the presence of peaks with binding energies of 232.8 and 229.7 eV, which may be ascribed to Mo 3d 3/2 and 3d 5/2 of 2H-MoS 2 at the peaks at 228.9 eV for 3d 5/2 , and at 231.9 eV for 3d 3/2 , which was ascribed to 1T-MoS 2 [107]. The peak located at 226.9 eV corresponded to the binding energy of S 2s. In addition, the two weak peaks at 233.6 and 236.1 eV were due to the presence of the MoO 3 that formed during the sputtering process. From these results, they found that Mo 4+ dominates the product. Similarly, the two doublet peaks located at 162.6 eV and 164.1 eV corresponded to the 2p 3/2 and 2p 1/2 orbitals of 2H-MoS 2 . Furthermore, the binding energies at 161.6 eV and 163.2 eV were due to 1TMoS 2 . The high-resolution spectra of XPS further provides evidence for the existence of MoS 2 in the hybridized samples [108].
In a different study, Li and coworkers used XPS characterization to study the Pt doped CdS materials and Figure 24a illustrates the XPS spectrum of the Pt-CdS, indicating the co-existence of Cd, S, and Pt elements in the as-prepared Pt-CdS composite. The peaks at 405.6 and 412.3 eV corresponded to the Cd 3d 5/2 and Cd 3d 3/2 of CdS, respectively ( Figure 24b) [25]. In Figure 24c, the S 2p 1/2 and S 2p 3/2 peaks are located at 163.2 and 161.9 eV, respectively.

UV-Visible Spectroscopy/Diffuse Reflectance Spectroscopy (DRS)
Like all nanomaterials, to study the bandgap and the optical behavior of the TMCs, researchers have utilized UV-Visible diffuse reflectance spectroscopy. In this regard, in the study of the facile hydrothermal synthesis of a 3-D flower-like La-MoS 2 nanostructure for photocatalytic hydrogen energy production, Riaz et al. utilized this technique to study the bandgap of La doped MoS 2 materials, as illustrated in Figure 25.     The absorption spectra of pure MoS 2 and 3% La-MoS 2 indicate that the absorption of LaMoS 2 was significantly extended toward a longer wavelength than pure MoS 2 into the visible light range. Bandgap energy for pure MoS 2 calculated by using the equation (αhυ) 2 = A(hυ − Eg) allowed for direct transitions, resulting in a value of 1.84 eV, which is illustrated in the plot of (αhν) 2 vs. hν (eV) (Figure 25). This value is close to the bandgap energy of 1.8 eV for a monolayer of MoS 2 [110]. However, 3% La-MoS 2 showed a considerably small bandgap energy of 1.5 eV due to La 3+ doping, which significantly improved the light absorption properties of MoS 2 [109].
Gaur et al. used SnS 2 nanoparticles as photocatalysts for the reduction of Cr(VI) and their optical properties were investigated by DRS studies. Figure 26a,b shows DRS and Tauc plots of SnS 2 nanoparticles. Bandgap absorption in the range between 430 nm and 580 nm was observed. The bandgap calculated by using the Tauc plot indicates values between 2.31 eV and 2.94 eV [111]. In addition to these methods, Raman spectroscopic technique and photoluminescence studies (PL) were also utilized to analyze these TMC materials. However, there is a limited number of literature available, especially with the PL and AFM studies on the TMC materials toward hydrogen production application, and thus, were omitted in this review.

Raman Spectroscopy
L. Zhu et al. synthesized CoS 2 -based thin films as model catalysts for the oxygen reduction reaction, which were characterized by using Raman spectroscopy. Figure 27 shows characteristic peaks at 287 and 389 cm −1 as well as a shoulder around 410 cm −1 for the CoS 2 powder. This was in close agreement with the study of a CoS 2 single crystal [112]. The peak at 287 cm −1 was also associated with polysulfides. In contrast, the Raman spectrum obtained from the magnetron-sputtered thin film of CoS 2 showed a pronounced peak at 391 cm −1 , a broad shoulder at around 410 cm −1 , and another peak at 287 cm −1 , which exhibited a general similarity to the spectrum obtained for the standard CoS 2 . Furthermore, the Raman spectrum of commercial NiS 2 was also reported in their study and it showed distinctive peaks at 282 and 477 cm −1 , and a shoulder at 488 cm −1 (Figure 27). From this study, they confirmed the formation of CoS 2 with the expected characteristics.

Hydrogen Evolution Reactions
Production of hydrogen through water splitting is seen as an extremely impo renewable and clean energy [4]. Since water is abundant, and as novel materia synthesized and the technology of water splitting becomes mature, hydrogen c produced from water by different methods such as thermolysis (e.g., thermoche cycles), electrolysis (e.g., electrocatalysis), photolysis (e.g., photocatalysis), and bi (e.g., dark fermentation) [113].
Among them, electrolysis of water, which includes the cathodic hydrogen evol reaction (HER) [113] and anodic oxygen evolution reaction (OER) [113], remains li by the high cost of noble metals such as Pt [114] and Ru [115] based materials, whic typically used as electrocatalysts in different electrolyte materials that include acid base electrolytes [116]. In addition, different photocatalytic materials are being us utilize solar energy for splitting water [37,105,117]. Photoelectrochemical and p catalytic are the two main methods for water splitting. In the photoelectroche method, a semiconductor photoelectrode is typically used as the working electrode immersed in an aqueous electrolyte. A counter and reference electrode are used to plete the cell. An electrochemical potential is then applied to the photoelectrode. A source is used to excite the semiconductor electrode. If an n-type semiconductor is the photoexcited electrons generated in the photoanode travel to the counter elec (via an external wire) and reduce protons to form hydrogen. At the working elec

Hydrogen Evolution Reactions
Production of hydrogen through water splitting is seen as an extremely important renewable and clean energy [4]. Since water is abundant, and as novel materials are synthesized and the technology of water splitting becomes mature, hydrogen can be produced from water by different methods such as thermolysis (e.g., thermochemical cycles), electrolysis (e.g., electrocatalysis), photolysis (e.g., photocatalysis), and biolysis (e.g., dark fermentation) [113].
Among them, electrolysis of water, which includes the cathodic hydrogen evolution reaction (HER) [113] and anodic oxygen evolution reaction (OER) [113], remains limited by the high cost of noble metals such as Pt [114] and Ru [115] based materials, which are typically used as electrocatalysts in different electrolyte materials that include acid and base electrolytes [116]. In addition, different photocatalytic materials are being used to utilize solar energy for splitting water [37,105,117]. Photoelectrochemical and photocatalytic are the two main methods for water splitting. In the photoelectrochemical method, a semiconductor photoelectrode is typically used as the working electrode and is immersed in an aqueous electrolyte. A counter and reference electrode are used to complete the cell. An electrochemical potential is then applied to the photoelectrode. A light source is used to excite the semiconductor electrode. If an n-type semiconductor is used, the photoexcited electrons generated in the photoanode travel to the counter electrode (via an external wire) and reduce protons to form hydrogen. At the working electrode, the holes oxidize water to produce oxygen and an anodic photocurrent is produced. Thus, the products, hydrogen and oxygen, are produced at separate electrodes.
In the photocatalytic method, the semiconductor powder is suspended in water. The slurry is irradiated by an appropriate light source. Bandgap excitation leads to the generation of electrons and holes. These charge carriers migrate to the surface and dissipate their charges and produce hydrogen and oxygen, respectively. In photocatalysis, the product separation is required. Nowadays, researchers are putting great effort into maximizing the production of hydrogen by water splitting with the eventual aim to generate hydrogen through sustainable ways [13,[118][119][120].
The following table (Table 2) lists the different hydrogen production methods and their advantages and disadvantages.  [10,127,128] In recent years, the structure of the synthesized catalyst and the hydrogen evolution activities have been investigated theoretically using density functional theory (DFT) [129]. In this regard, theoretical studies of metal chalcogenide materials have been conducted. In a recent paper, Som et al. investigated the structural property of ZrS 2 , Janus ZrSSe, and ZrSe 2 and their catalytic activity from hydrogen evolution reactions [130]. From these calculations, they found that ZrS 2 possessed a larger bandgap energy of 1.58 eV, whereas ZrSSe and ZrSe 2 exhibited bandgap energies of 1.08 eV and 0.84 eV, respectively. In addition, Janus ZrSSe (DG 1 4 1:19 eV; DG: Gibbs free energy) was found to be the optimal catalyst [130] among the materials studied. Another study examined the electronic and optical properties and the potential applications of 2-D MoSe 2 /SnS 2 van der Waals heterojunction as a photocatalyst for overall water splitting. The DFT calculations showed that the MoSe 2 /SnS 2 heterostructure, as a direct Z-scheme photocatalyst, effectively absorbs visible light. The separation efficiency of photo-induced carriers was enhanced due to the induced built-in electric field. Moreover, when compared with the MoSe 2 and SnS 2 monolayers, the absorption intensity of the MoSe 2 /SnS 2 heterojunction was reinforced in the visible light region [131].
Park et al. worked on Nb-, Ta-, and Bi-based oxides/chalcogenides. They studied the crystal structure, band dispersion, effective masses of the charge carriers, band alignment, and pH dependence of these materials for hydrogen production and CO 2 reduction. They concluded that Bi-based chalcogenide materials, (KBiO 2 and KBiS 2 ) showed good performance over a wide range of pH conditions. In addition, the incorporation of chalcogenide narrows the bandgap and reduces the effective mass of the charge carriers when compared with their oxide counterparts [132]. Although these theoretical studies provide guidance toward the identification of materials required for efficient water splitting, it is necessary to test materials. The following sections summarize the reports of hydrogen evolution.

Photoelectrochemical Hydrogen Evolution
Electrochemical water splitting is a sustainable method for producing hydrogen on a large scale [133]. In photoelectrochemical hydrogen evolution, semiconductor material(s) are immersed in a water-based electrolyte and an external electric power/voltage is applied to dissociate water as hydrogen and oxygen molecules in the presence of a light source [126]. Electrochemical and photoelectrochemical hydrogen evolution have received much attention due to the fact that (i) the generation of O 2 and H 2 occurs at separate electrodes ( Figure 28), which eliminates the separation issue(s), and (ii) the potential for operation under ambient conditions [134]. Fujishima and Honda first described the photoelectrochemical cell (PEC) splitting of water using a crystalline titanium dioxide anode and a platinum cathode under ultraviolet (UV) irradiation and an external bias [24,135]. The anodic and cathodic reactions are as indicated below [136]. Different studies have been conducted in the field of electrochemical water splitting wherein Pt-and Ru-based materials [137] were used as electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. In recent Fujishima and Honda first described the photoelectrochemical cell (PEC) splitting of water using a crystalline titanium dioxide anode and a platinum cathode under ultraviolet (UV) irradiation and an external bias [24,135]. The anodic and cathodic reactions are as indicated below [136].
Anode Different studies have been conducted in the field of electrochemical water splitting wherein Pt-and Ru-based materials [137] were used as electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. In recent years, TMCs have been used. Ahn et al. used CoS 2 /rGO (2.3 mg/mL GO) hybrid materials that exhibited high electrocatalytic activity for hydrogen evolution reaction with an over potential of −150 mV versus RHE [138]. In another study, an impressive activity was attributed to the superior electrochemical performance of the Mo-CoS x skeleton, and the special configuration between MoS 2 and CoS x , which provide more active sites [139]. Apart from these studies, there have been several metal chalcogenide materials that have been utilized for electrochemical hydrogen evolution, which are summarized in the Table 3 below.  [156] 1T-MoS 2 10 mAcm −2 240 mV 68 mVdec −1 1 M KOH [157] CoSe/MoSe 2 10 mAcm −2 192 and 115 mV in acidic and alkaline NA Nafion ethanol solution [158] NA-Not Available.

Photocatalytic Hydrogen Evolution
Solar-assisted photocatalytic hydrogen evolution is an attractive technique to generate hydrogen and oxygen, because water and sunlight are abundant and renewable. Furthermore, the photocatalytic process can occur at ambient conditions [14]. Semiconductor materials are used as photocatalysts and their band structure consists of a filled valance band and an empty conduction band separated by a finite bandgap. Irradiation with incident energy greater than or equal to the bandgap of a material leads to excitation of electrons from the valence band to the conduction band edge [159][160][161]. This leaves holes in the valence band. These photoexcited electrons and holes are potent reducing and oxidizing agents and may react with adsorbed moieties on the surface of the photocatalytic material or may recombine to generate heat [9,136].
Photocatalytic reactions can be classified into two major categories that include uphill reaction, where the Gibbs free energy (∆G o ) is positive, or downhill reaction, where the (∆G o ) is negative (Figure 29). In this process, water molecules splitting into H 2 and O 2 , are accompanied by a large positive change in the Gibbs free energy (i.e., an uphill  [130]. During this process, light energy is transformed into chemical energy in a similar manner as the photosynthesis process. Therefore, this reaction is called artificial photosynthesis [136,162]. O2, are accompanied by a large positive change in the Gibbs free energy (i.e., an uphill reaction) [130]. During this process, light energy is transformed into chemical energy in a similar manner as the photosynthesis process. Therefore, this reaction is called artificial photosynthesis [136,162]. The photocatalytic hydrogen evolution through water splitting is a green route and has received significant attention toward simultaneously solving the current global environmental pollution and energy crisis [128]. Various photocatalysts have been well developed for the generation of hydrogen whereas TiO2 is still considered as a golden standard due to its exceptional stability under irradiation conditions, ease of synthesis, and cost [163]. However, a big disadvantage of TiO2 is that it requires UV light to be activated, thus limiting its utility since visible light cannot be used. Hence, other oxide and chalcogenide materials are constantly being explored and evaluated for photocatalytic hydrogen production. In addition, ZnO [33], mixed oxides [164], metal chalcogenides such as CdS [165], WS2 [166], g-C3N4 [30], and CdLa2S4 [34] have also been utilized for the hydrogen evolution reaction.

Principle and Mechanism of Photocatalytic Hydrogen Evolution
In general, photocatalytic hydrogen evolution reaction takes place through three major steps. The first step is the absorption of light by a catalyst to create exciton pair containing electrons and holes. In the second step, the excitons may separate and migrate to the surface of the photocatalyst, and finally, in the third step, the surface reactions take place. The electrons will reduce the protons to generate hydrogen molecules, and holes oxidize water molecules to produce oxygen, as illustrated in the Figure 30 below [167]. The photocatalytic hydrogen evolution through water splitting is a green route and has received significant attention toward simultaneously solving the current global environmental pollution and energy crisis [128]. Various photocatalysts have been well developed for the generation of hydrogen whereas TiO 2 is still considered as a golden standard due to its exceptional stability under irradiation conditions, ease of synthesis, and cost [163]. However, a big disadvantage of TiO 2 is that it requires UV light to be activated, thus limiting its utility since visible light cannot be used. Hence, other oxide and chalcogenide materials are constantly being explored and evaluated for photocatalytic hydrogen production. In addition, ZnO [33], mixed oxides [164], metal chalcogenides such as CdS [165], WS 2 [166], g-C 3 N 4 [30], and CdLa 2 S 4 [34] have also been utilized for the hydrogen evolution reaction.

Principle and Mechanism of Photocatalytic Hydrogen Evolution
In general, photocatalytic hydrogen evolution reaction takes place through three major steps. The first step is the absorption of light by a catalyst to create exciton pair containing electrons and holes. In the second step, the excitons may separate and migrate to the surface of the photocatalyst, and finally, in the third step, the surface reactions take place. The electrons will reduce the protons to generate hydrogen molecules, and holes oxidize water molecules to produce oxygen, as illustrated in the Figure 30 below [167]. The primary reactions (1-4) occurring during the process are listed below, and as indicated, the overall reaction involves four electrons. Table 4 lists the different metal chalcogenide materials used for hydrogen production and the amount of hydrogen evolved. The primary reactions (1)(2)(3)(4) occurring during the process are listed below, and as indicated, the overall reaction involves four electrons. Table 4 lists the different metal chalcogenide materials used for hydrogen production and the amount of hydrogen evolved.
Photocatalyst + 4hν → Photocatalyst + 4h + + 4e − (1) Generally, the rate of hydrogen evolution is denoted with the unit of µmolh −1 g −1 , which indicates the number of moles of hydrogen evolved in unit time using a unit amount of catalyst. This amount depends on several experimental conditions such as reaction vessel, light source, intensity of radiation, amount of catalyst, sacrificial agent, etc. [120]. However, it is noteworthy to mention here that there are inconsistencies among researchers in reporting the rate of hydrogen evolution and hence care must be exercised in making comparisons from one report to another. To provide a general quantification, quantum yield measurement is important in photolytic reactions to remove any variations in experimental conditions and for making meaningful comparisons among different photocatalysts and provide consistent benchmarking. Although the number of incident photons can be measured using a thermopile, it is difficult to determine the actual number of photons absorbed by a photocatalyst because of scattering and some reflection in a dispersed system. Photocatalytic/luminescence materials convert excitation energy into photons, and this conversion is usually not 100% efficient due to various losses through alternative deactivation paths. Therefore, two additional parameters such as the quantum yield (QY) and the quantum (energy) efficiency [191] have to be defined. In preparative photochemistry, the QY is defined as the product yield of a reaction calculated by the number of moles of reactant reacted per unit time. This type of QY has been preferred as it can be measured with small expenditure. However, it depends on the time section of the reaction in which it is determined. Therefore, the QY can be considered to be an apparent quantum yield (AQY), and this yield is mainly dependent on the time at which they are considered [192]. AQY is the ratio between the number of reacted electrons and the number of incident photons, and is used for the quantification of hydrogen evolution. The following Equation (5)  The turnover frequency (TOF) of photocatalysts based on the amount of a particular molecule can be calculated using Equation (6) [193]: TOF = moles of produced hydrogen moles of a particular molecule in photocatalyst × reaction time (h) Mangiri R. et al. used a dual co-catalyst of MoS 2 /CoSe 2 on CdS for the hydrogen evolution reaction (Figure 31), and the system evolved a higher rate of hydrogen with the yield of 191.5 mmol g −1 h −1 , which was 76.6 times higher than that of pristine CdS [182]. The turnover frequency (TOF) of photocatalysts based on the amount of a particular molecule can be calculated using Equation (6) [194]: Mangiri R. et al. used a dual co-catalyst of MoS2/CoSe2 on CdS for the hydrogen evolution reaction (Figure 31), and the system evolved a higher rate of hydrogen with the yield of 191.5 mmol g −1 h −1 , which was 76.6 times higher than that of pristine CdS [183]. In their study, CdS was used as a semiconductor, which can harvest the light and eject electrons from the conduction band of CdS to its valance band. Then, the photo-generated electrons in the conduction band of CdS can be easily transferred to MoS2 and CoSe2 due to the favorable band edge positions of the three materials. Thus, the rate of charge-carrier recombination is minimized and photocatalytic activity is enhanced in the composite photocatalyst in comparison to the pristine CdS materials. Here, the MoS2 and CoSe2 act as reactive sites to produce hydrogen. The holes produced can react with lactic acid and oxidize it to pyruvic acid [183].
In a different study, Riaz et al. used 3% La 3+ doped MoS2, and it exhibited an en- Figure 31. Schematic diagram of the mechanism of the hydrogen evolution reduction (HER) process. Reproduced with permission from [182].
In their study, CdS was used as a semiconductor, which can harvest the light and eject electrons from the conduction band of CdS to its valance band. Then, the photogenerated electrons in the conduction band of CdS can be easily transferred to MoS 2 and CoSe 2 due to the favorable band edge positions of the three materials. Thus, the rate of charge-carrier recombination is minimized and photocatalytic activity is enhanced in the composite photocatalyst in comparison to the pristine CdS materials. Here, the MoS 2 and CoSe 2 act as reactive sites to produce hydrogen. The holes produced can react with lactic acid and oxidize it to pyruvic acid [182].
In a different study, Riaz et al. used 3% La 3+ doped MoS 2 , and it exhibited an enhanced photocatalytic performance for hydrogen evolution with a 5.2 times higher hydrogen evolution rate compared with pure MoS 2 [109] (Figure 32). This has been attributed to the presence of trapping energy levels formed below the conduction band of MoS 2 due to the lanthanum ion-doping and minimizing recombination of electrons and holes. Surface adsorbed La 3+ atoms also ensured the rapid transfer of electrons from MoS 2 to capture protons for H 2 production [109]. Transition metal chalcogenides with and without incorporation of other materials such as metals [168], graphene [172], graphene oxide [42], carbon nitrides [169], dyes [195], and metal oxides [33] have been utilized for hydrogen production (Table 4). It can be seen that the use of composite photocatalysts, in general, leads to a higher rate of hydrogen production than the pristine transition metal chalcogenides. This has been attributed to the presence of heterojunctions that favor vectorial transfer of electrons that minimize electron-hole recombination.

Summary
This review details the recent approaches to the hydrogen evolution reaction using TMCs, which are being considered as potential catalysts for the production of an energy carrier, hydrogen, due to their favorable electronic and optical properties. The different synthesis methods of TMCs and their characteristic properties reported in the literature are discussed in detail, providing guidance for interested researchers to explore TMCs as exploratory materials. Furthermore, the catalytic activities of TMCs for hydrogen evolution reactions were also elaborated herein. Overall, this review provides an overview of the recent approaches to hydrogen production using TMCs.  Transition metal chalcogenides with and without incorporation of other materials such as metals [168], graphene [172], graphene oxide [42], carbon nitrides [169], dyes [194], and metal oxides [33] have been utilized for hydrogen production (Table 4). It can be seen that the use of composite photocatalysts, in general, leads to a higher rate of hydrogen production than the pristine transition metal chalcogenides. This has been attributed to the presence of heterojunctions that favor vectorial transfer of electrons that minimize electron-hole recombination.

Summary
This review details the recent approaches to the hydrogen evolution reaction using TMCs, which are being considered as potential catalysts for the production of an energy carrier, hydrogen, due to their favorable electronic and optical properties. The different synthesis methods of TMCs and their characteristic properties reported in the literature are discussed in detail, providing guidance for interested researchers to explore TMCs as exploratory materials. Furthermore, the catalytic activities of TMCs for hydrogen evolution reactions were also elaborated herein. Overall, this review provides an overview of the recent approaches to hydrogen production using TMCs.