Recent Progress in Photocatalytic Hydrogen Production Using 2D MoS₂ Based Materials

Abstract

Due to the increase in energy demand, photocatalytic hydrogen (H₂) production has received enormous interest from the scientific community due to its simplicity and cost-effectiveness. The photocatalyst (PC) plays a vital role in H₂ evolution, and it is well understood that an efficient PC should have a larger surface area and better charge separation and transport properties. Previously, extensive efforts were made to prepare the efficient PC for photocatalytic H₂ production. In some cases, pristine catalyst could not catalyze the catalytic reactions due to a fast recombination rate or poor catalytic behavior. Thus, cocatalysts can be explored to boost the photocatalytic H₂ production. In this regard, a promising cocatalyst should have a large surface area, more active sites, decent conductivity, and improved catalytic properties. Molybdenum disulfide (MoS₂) is one of the two-dimensional (2D) layered materials that have excellent optical, electrical, and physicochemical properties. MoS₂ has been widely utilized as a cocatalyst for the photocatalytic H₂ evolution under visible light. Herein, we have reviewed the progress in the fabrication of MoS₂ and its composites with metal oxides, perovskite, graphene, carbon nanotubes, graphitic carbon nitrides, polymers, MXenes, metal-organic frameworks, layered double hydroxides, metal sulfides, etc. for photocatalytic H₂ evolution. The reports showed that MoS₂ is one of the desirable cocatalysts for photocatalytic H₂ production applications. The challenges and future perspectives are also mentioned. This study may be beneficial for the researchers working on the design and fabrication of MoS₂-based PCs for photocatalytic H₂ evolution applications.

1. Introduction

In the previous years, energy consumption has significantly increased, but energy sources are still limited [1,2]. The increase in the energy consumption may be attributed to the rapid development of modern society and increased population [3]. The fossil fuels are limited, which motivated the researchers to find the alternative energy sources [4]. In this connection, hydrogen (H₂) production has attracted the scientific community due to its promising features such as simple working principle, low cost, and reasonable efficiency [5]. In 1972, Fujishima et al. [6] demonstrated the role of titanium dioxide (TiO₂)-based photoanodes for photoelectrochemical water splitting applications under ultraviolet irradiation. Various semiconductors, including graphitic carbon nitride (gCN) [7], TiO₂ [8,9], cadmium sulfide (CdS) [10], and zinc indium sulfide (ZnIn₂S₄) [11,12], and their hybrid composites were found to be promising catalysts for photocatalytic H₂ evolution reactions. The photocatalytic H₂ evolution depends on the physicochemical and optical properties of the photocatalyst (PC) [13]. Thus, it is of great significance to explore the promising PC for the photocatalytic H₂ evolution reaction. The electron-hole pairs are generated in the PC upon the illumination of light [14]. The electron-hole pairs play a crucial role in achieving a higher H₂ evolution rate [15]. An efficient photocatalytic system should separate the charge carriers efficiently, and rapid transfer of the electrons may enhance the H₂ evolution [16]. Unfortunately, most of the PC exhibits low stability and poor quantum efficiency. Thus, it is a challenge to enhance the performance of the photocatalytic H₂ evolution by tackling both quantum efficiency and stability issues [17]. The use of a cocatalyst strategy has been considered one of the effective approaches to improve the stability and photocatalytic H₂ evolution rate [18]. It is well understood that cocatalysts play a vital role in photocatalytic H₂ production. The presence of a cocatalyst may overcome the limitations (poor charge separation and poor quantum efficiency) of pristine PC materials. The cocatalyst may provide more active sites and enhance the charge separation/transport and reduce the recombination of photogenerated electron-hole pairs [18]. In reported studies, it was observed that incorporation of noble metals such as gold (Au) [19], silver (Ag) [20], palladium (Pd) [21], or platinum (Pt) [22] as cocatalysts can enhance the H₂ evolution rate. Unfortunately, their preciousness and high cost restrict their use as cocatalysts in photocatalytic H₂ evolution applications. Thus, precious metal-free cocatalysts need to be explored for photocatalytic H₂ evolution applications.

Molybdenum disulfide (MoS₂) is a two-dimensional (2D) layered structure that demonstrates excellent physicochemical properties [23,24]. MoS₂ has been used in a variety of applications, including photocatalysis [25], photovoltaics [26], sensors [27], energy storage [28], and H₂ evolution reaction [29]. MoS₂ also received extensive interest as a cocatalyst for H₂ evolution applications due to its decent electrical conductivity, tunable band structure, abundant edge sites, and strong chemical stability [25]. The presence of MoS₂ as a cocatalyst may also facilitate charge transfer and enhance H₂ evolution efficiency under visible light. MoS₂ has the capability to form the heterojunctions with various semiconductors and enhance the interfacial charge separation and surface reaction kinetics [18]. In addition, the use of MoS₂ as a cocatalyst is cost-effective for H₂ evolution studies.

Herein, we have compiled the last few year’s articles on MoS₂-based hybrid composites as cocatalysts/photocatalysts for photocatalytic H₂ evolution applications. This review article may benefit the materials scientists who are involved in the development of MoS₂-based PC materials for photocatalytic H₂ evolution applications.

2. Synthetic Methods for the Preparation of MoS₂

2.1. Exfoliation Approach

The liquid-phase exfoliation method is a well-known top-down approach for the preparation of 2D materials at large scale. Sousa et al. [30] reported the fabrication of Au NPs decorated MoS₂ using the liquid-phase exfoliation method. Authors added 150 mg MoS₂ bulk powder to 30 mL washed AuNPs and sonicated under pulse mode (40% amplitude; 7s on/5s off) under ice bath conditions (4 h). Furthermore, a bulk sample was collected from the exfoliated materials and washed two times with water. The detailed procedure for the fabrication of Au NPs decorated MoS₂ is illustrated in Figure 1a.

2.2. Physical Vapor Deposition (PVD) Approach

In the previous study, the PVD method was adopted for the preparation of ultra-thin MoS₂ films at low temperature [31]. The PVD method involves magnetron sputtering, which makes it scalable for the growth of thin films of MoS₂ over large areas of more than 1 m². The PVD method also has many advantages, which include cleanliness of the interfaces and control for precise atomic-scale thickness. It was also observed that the MoS₂-Ti composite was grown using the PVD method by using MoS₂ and Ti as targets, but the obtained MoS₂ was amorphous in nature [32,33]. The PVD method using sputtering technique is a promising approach for the preparation of thin films. In this process, molecules or atoms from the target materials are ejected and transferred to the substrate, and thin films are formed. The sputtering technique involves plasma for the generation of ions, which bombard the target materials. Anbalagan et al. [24] reported the fabrication of MoS₂ films using the sputtering method. It was observed that fabricated thin films of MoS₂ consist of few layers. It is suggesting that the sputtering method can be used to control the layers of the MoS₂.

2.3. Chemical Vapor Deposition (CVD) Approach

The CVD method was also utilized for the preparation of MoS₂. In a previous report, Kim et al. [34] adopted the CVD method for the fabrication of MoS₂. In the first step, the authors prepared a 0.8 nm thin Mo film. In this connection, c-plane (0001) sapphires were cleaned by using acetone/methanol/deionized water and buffer oxide etch (BOE). Furthermore, c-plane (0001) sapphires were rinsed with deionized water and dried with nitrogen gas. The 0.8 nm Mo thin film was deposited on a c-plane (0001) sapphire substrate using an e-beam evaporation approach. The synthetic process for the formation of CVD-method-based MoS₂ has been described in Figure 1b.

2.4. Hydrothermal/Solvothermal Approach

The hydrothermal and solvothermal methods are categorized on the basis of the type of solvents. Hydrothermal methods generally use water as solvents, whereas solvothermal methods involve the use of other solvents such as ethanol, N,N-dimethylformamide (DMF), or dimethyl sulfoxide (DMSO). Kim et al. [34] also prepared MoS₂ using the hydrothermal method by employing sodium molybdate and thiourea as Mo and S sources, respectively. The detailed procedure for the preparation of MoS₂ is shown in Figure 1c.

2.5. Calcination Method

Yan et al. [35] reported the synthesis of MoS₂-based composite using the calcination method. In another study, Yan et al. [36] also adopted a two-step calcination method for the formation of a 1T/2H MoS₂/gCN composite. Firstly, MoO₃ was obtained by the calcination of ammonium molybdate at 300 °C for 4 h under a muffle furnace. Furthermore, a certain weight of MoO₃ with different amounts of thiourea was carefully mixed, ground, and heated at 550 °C for 2 h under a nitrogen atmosphere, which yielded the desired product.

2.6. Sol-Gel Method

Taffelli et al. [37] reported facile sol-gel synthesis of MoS₂ film by dissolving ammonium molybdate in deionized water. Thioacetamide was also added and stirred until a complete dissolution took place. The color of the solution turned to light blue. Furthermore, diethylenetriaminepentaacetic acid was added to the solution, and the solution was stirred for another 1 h at room temperature, followed by 3 h at 60 °C. In this stage the color of the solution changes to dark brown viscous colloidal. The dilution step was performed using 2-butanol, 2-propanol, and ethanol. The obtained solutions with and without alcohols were used to fabricate the xerogel films. Authors stated that the use of alcohols was to obtain the uniform films.

2.7. Electrochemical Method

The electrochemical method is one of the promising approaches to deposit the materials with uniformity. In a previous report, MoS₂ was electrodeposited using an electrochemical method [38]. In this connection, sodium sulfide (Na₂S) and ammonium tetrathiomolybdate were mixed in ammonia buffer. The presence of excess Na₂S served as an S precursor as well as to prevent the precipitation of MoO₂ [39]. The prepared solution was deaerated with nitrogen gas and was stored under a nitrogen atmosphere. The PC-controlled automated system was used for the electrodeposition process. The working electrode was constructed from n-Si 100 (p-doped/resistivity = 1–5 Ω∙cm), and the exposure area was 1 cm in diameter. The electrode was cleaned as per the standards [40], and electrochemical depositions were performed at room temperature in the dark.

Figure 1. (a) Exfoliation method, (b) CVD method, and (c) hydrothermal method for the synthesis of MoS₂. Reproduced with permission [30,34].

Figure 1. (a) Exfoliation method, (b) CVD method, and (c) hydrothermal method for the synthesis of MoS₂. Reproduced with permission [30,34].

It can be understood that each method has its own limitations and promising features for a particular application. In this context, we have tried to provide a comparable analysis for all the above-mentioned methods in

Table 1. Advantages and disadvantages of various synthetic methods for the fabrication of MoS₂ as per our observations.

Method	Type	Advantages	Disadvantages	Expected Applications
Liquid phase exfoliation	Top-down	Simplicity, High yield	Poor efficiency and toxic nature of solvent	Suitable for large-scale production of few-layer MoS2 for basic research or coatings
PVD method	Bottom-up	Strong adhesion, low temperature deposition, scalability	High cost vacuum equipment, slo production speeds	Suitable for thin film based nano devices and microchips
CVD method	Bottom-up	Controlable layer numbers and size, large scale	High temperature	Suitable for high-performance electronic or sensor applications
Hydrothermal/solvothermal	Bottom-up	High yield and low temperature	Time consuming and limited control over phase purity	Photocatalysis, H2 evolution, and energy storage
Calcination method	Bottom-up	Thermal decomposition, volatile removal	High temperature, limited morphology control, Environemntal concerns	Photocatalysis, sensors
Sol-gel method	Bottom-up	Low processing temperature, cost-effective	Limited control over particle size and morphology, volume shrinkage	Thin film deposition for optoelectronic devices
Electrochemical deposition	Bottom-up	Uniform coating of films on substartes, precision, low temperature, control over reaction conditions, cost effective	Needs conductive substrate-Limited scalability-Requires controlled electrolyte chemistry	Electrochemical applications

.

3. Progress in H₂ Evolution

Recently, MoS₂-based composites are extensively used as PC materials for H₂ production. In this section, we have discussed recently published MoS₂-based PCs for H₂ production applications.

3.1. MoS₂/Metal Oxides

MoS₂ has been incorporated with metal oxides for various optoelectronic applications. In particular, MoS₂-incorporated metal oxides demonstrated excellent H₂ evolution with reasonably good stability and reusability. Liu et al. [41] reported the fabrication of MoS₂/titanium dioxide (TiO₂) with the existence of vacancy species to control the migration paths of the photo-generated charge carriers. It was proposed that the existence of the oxygen and sulfur vacancies (O_v and S_v) can induce defect energy levels in the prepared MoS₂/TiO₂ heterostructure, which can significantly capture the photogenerated holes and electrons, respectively. The presence of O_v and S_v may narrow the band gap by introducing defect energy levels and significantly extract the charge carriers to enhance their separation. Thus, the above proposed MoS₂/TiO₂ heterostructure (with two anionic defects, i.e., O_v and S_v) exhibits an H₂ evolution rate of 1.41 mmol·g⁻¹·h⁻¹ under the optimized conditions. Di et al. [42] stated that MoS₂ and cobalt oxide (CoO_x) dual cocatalysts with cadmium sulfide (CdS) nanorods (NRs) may be promising PCs for H₂ evolution under photocatalytic processes. It was observed that MoS₂ functions as a reduction cocatalyst for electron trapping, whereas CoO_x acts as a cocatalyst for hole trapping. The introduction of the dual cocatalyst enhances the lifetime of the photogenerated charge carriers and reduces the charge recombination rates and enhances the H₂ evolution performance. The CdS/MoS₂/CoO_x shows an H₂ production rate of 7.4 mmol·g⁻¹·h⁻¹ with a quantum efficiency (QE) of 7.6%. Zhang et al. [43] proposed an S-scheme heterojunction system for efficient H₂ production using MoS₂@MoO₃. The fabrication process for the formation of MoS₂@MoO₃ has been illustrated in Figure 2a. It was found that interface engineering provides the fast electron transmission path, and an S-rich surface improves surface reaction by generating active sites. Authors controlled the vulcanization by optimizing the amount of thioacetamide. The optimized conditions showed an excellent H₂ production rate of 12,416.8 µmol·g⁻¹·h⁻¹. Titanium dioxide (TiO₂) is one of the widely used PCs for H₂ production and photocatalytic applications. The combination of MoS₂ with TiO₂ nanosheets (NSs) heterojunction composite demonstrated an excellent H₂ production rate of 5423.77 µmol·g⁻¹·h⁻¹ [44]. The improved performance of the MoS₂/TiO₂ composite provides more active sites, and the presence of synergistic interactions improves the photocatalytic H₂ evolution under visible light. In another previous study [45], a ternary composite of zinc oxide (ZnO), zinc sulfide (ZnS), and MoS₂ was fabricated using a hydrothermally assisted method. The obtained ZnO/ZnS/MoS₂ composite was utilized as PC, which displayed an interesting H₂ evolution rate of 4.45 mmol·g⁻¹·h⁻¹. The H₂ evolution rate of 10.42 mmol·g⁻¹·h⁻¹ was also obtained by introducing piezo-potential by ultrasonication procedure during photocatalytic reaction. Z-scheme driven MoS₂/Co₃O₄-based studies exhibited interesting H₂ evolution of 3825 μmol·g⁻¹ [46]. It is understood that advanced functional materials with better electron transport paths could be prepared by the construction of heterojunction semiconducting materials with matched energy band structures. Thus, a Z-scheme heterojunction was developed using TiO₂ nanotubes (NTs), bismuth sulfide (Bi₂S₃), and MoS₂ [47], which delivered acceptable photocatalytic activity for pollutant removal and H₂ evolution. In another reported study for the degradation of ciprofloxacin, a reasonable H₂ evolution rate of 235 µmol·g⁻¹·h⁻¹ was obtained using a ZnO/MoS₂ composite, which may be attributed to the extended light absorption capacity, suitable band alignment, and effective charge transfer [48]. A novel composite of tungsten oxide, tungsten sulfide, and MoS₂ (WO₃/WS₂/MoS₂) also showed an H₂ production rate of 4907.2 µmol·g⁻¹·h⁻¹ [49]. It was also found that change in the W and Mo ratio does not exhibit a significant impact on H₂ evolution when W and Mo are close. TiO₂ NRs decorated MoS₂ nanospheres (NS) exhibited bi-functional properties for antibiotic degradation and H₂ production applications [50]. TiO₂ NRs/MoS₂ NS showed an excellent H₂ evolution rate of 7415 μmol·g⁻¹, which was approximately 24 times higher than that of pristine TiO₂ NRs. This revealed that the presence of synergism in the proposed PC enhanced the H₂ production rate. Nickel oxide (NiO) was also combined with graphitic carbon nitride and MoS₂ for the fabrication of novel PC for H₂ evolution studies [51]. The H₂ evolution activity of various constructed PCs was evaluated under similar conditions, and the authors observed that 1%MoS₂/Ni@NiO/gCN exhibited higher photocatalytic activity towards H₂ production, as shown in Figure 2b. The 1%MoS₂/Ni@NiO/gCN composite showed an interesting H₂ evolution rate of 7.98 mmol·g⁻¹·h⁻¹. The proposed MoS₂/Ni@NiO/gCN also demonstrated excellent stability for 5 cycles, as shown in Figure 2c. The presence of synergism I the proposed PC was responsible for the improved H₂ evolution activity, and a plausible mechanism for H₂ evolution is shown in Figure 2d. Phosphorous-doped MoS₂-modified TiO₂ was synthesized using a benign thermal annealing-assisted method [52]. It was found that TiO₂ has a hierarchical microsphere-shaped morphology. The proposed material demonstrated an H₂ production rate of 1550.30 µmol·g⁻¹·h⁻¹. Lead titanate (PbTiO₃) was also combined with MoS₂ for the degradation of tetracycline and H₂ evolution [53]. The proposed material MoS₂/PbTiO₃ involves an S-scheme heterojunction system for H₂ (217.09 µmol·g⁻¹·h⁻¹) evolution studies. The polarization properties improved the S-scheme heterojunction, and the bound charge S-scheme improved and may have enhanced charge separation efficiency. In another study [54], the MoS₂/TiO₂ nanocomposite was also prepared using the hydrothermal-assisted method. The morphology of the prepared composite was determined by scanning electron microscopy (SEM). The prepared MoS₂ has a microspherical-shaped hierarchical arrangement of interconnected NSs, which formed a petals-like structure (Figure 2e). The TiO2 NPs are agglomerated as shown in Figure 2f. The hybrid structure of the MoS₂/TiO₂ can be seen in Figure 2g, which exhibits the successful incorporation of TiO₂ NPs within the interstices of porous MoS₂. The H₂ evolution of 137.93 μmol·h⁻¹·g⁻¹ was observed for the MoS₂/TiO₂ based photocatalytic system (Figure 2h), which is higher compared to the pristine TiO₂ or MoS₂. The plausible mechanism for H₂ evolution is shown in Figure 2i. Authors also observed that the proposed PC is stable up to 5 cycles (Figure 2j).

Rutile TiO₂/MoS₂/CdS ternary heterojunction was also fabricated by Lin et al. [55] using a benign approach, and formation of the ternary heterojunction was confirmed by X-ray diffraction (XRD) technique. The XRD results indicated the presence of decent phase purity. It was found that radial rutile TiO₂ microspheres assembled from single-crystal NRs were prepared via nucleation and a seed-oriented growth approach. In a further step, authors prepared MoS₂ NPs that were combined with radial rutile TiO₂ using an in-situ photo-induced reduction reaction. In the last step, the authors deposited CdS NPs on the TiO₂/MoS₂ surface by using a wet chemistry method to fabricate the TiO₂/MoS₂/CdS ternary heterojunctions. This ternary composite-based photocatalytic study demonstrated H₂ production of 328 μmol·h⁻¹·g⁻¹. A novel honeycomb rod-like hierarchical MoO₃@MoS₂@ZnInS₄ composite was fabricated for photocatalytic H₂ evolution applications [56]. The prepared composite form p-n heterojunction demonstrated improved optical properties, and H₂ evolution of 11.74 mmol·g⁻¹·h⁻¹ was obtained, which is higher than pristine ZnIn₂S₄. The MoO₃ NRs acted as a scaffold for in situ growth of the MoS₂ interlayer. Authors also stated that MoS₂ facilitated the formation of an intimate heterojunction and efficiently promoted charge separation and transfer, which resulted in improved photocatalytic efficiency. Novel S-scheme α-Fe₂O₃/BiOBr (bismuth oxybromide)/MoS₂ ternary composite was also proposed as PC material for H₂ evolution studies [57]. Authors found that 0.5 wt % α-Fe₂O₃/BiOBr with 10 wt % MoS₂ demonstrated excellent properties and delivered an H₂ evolution rate of 57 mmol·g⁻¹·h⁻¹. Other reported work also showed the potential of MoS₂-modified strontium tungstate (SrWO₄) for H₂ evolution [58]. Unfortunately, this material shows poor H₂ evolution of 0.08 mmol·h⁻¹·g⁻¹. The chemical precipitation method was utilized for the preparation of MoS₂/zinc oxide (ZnO) composite [59]. The synthesized material involves a Z-scheme and displayed decent performance for photocatalytic degradation of water pollutants and H₂ evolution. The H₂ evolution of 858 μmol/g·h was observed for MoS₂/95 wt % ZnO. Platinum (Pt) is one of the widely used cocatalysts for H₂ evolution studies. Zhang et al. [60] reported the fabrication of novel Pt/TiO₂@MoS_2+x with polymethyl methacrylate (PMMA) for H₂ evolution studies in seawater. The PMMA-encapsulated Pt/TiO₂@MoS_2+x based investigations revealed that PMMA offers promising pathways for the photocatalytic H₂ evolution in seawater. Z-scheme-based investigations were also reported for effective H₂ production using MoS₂-based composites with bismuth tungstate and bismuth sulfide [61]. The MoS₂/Bi₂S₃/Bi₂WO₆ ternary composite was fabricated using hydrothermal and solvothermal methods. The proposed PC was explored for H₂ production in organic wastewater.

3.2. MoS₂/Carbon, Graphene, Graphitic Carbon Nitride, and CNTs

Three-dimensional (3D) porous boron nitride (BN)/reduced graphene oxide (rGO) skeleton embedded MoS₂ was prepared for the efficient H₂ evolution [62]. The schematic illustration of the preparation of the proposed PC is described in Figure 3. It was believed that the presence of the 3D porous structure, abundant active sites, fast interfacial charge mobility, and synergistic heterointerface may boost the photocatalytic activity of the proposed material. Thus, authors were able to achieve the interesting H₂ evolution of 1490.3 µmol·h⁻¹·g⁻¹ under simulated solar light.

Jiao et al. [63] constructed hollow carbon spheres coated with MoS₂/graphitic carbon nitride (gCN/CS@MoS₂) composite by a simple method. It was observed that layered MoS₂ was strongly attached to the CS surface and efficiently improved the contact area. The presence of synergism between MoS₂ and CS efficiently promoted separation of the electron-hole pairs of CN. Authors reported H₂ evolution of 732 μmol g⁻¹·h⁻¹ under the optimized conditions. Yuan et al. [64] fabricated an MoS₂/gCN composite using a one-step calcination route. The interesting H₂ evolution of 12 mmol·h⁻¹·g⁻¹ was achieved for the proposed PC under simulated solar light irradiation. The obtained results in terms of H₂ evolution rate and stability may be attributed to the few-layered MoS_2, which enhanced the separation of photogenerated electron-hole pairs. The room temperature sonication method was adopted for the preparation of CdS/MoS₂ composite [65]. It was observed that decoration of MoS₂ NSs with CdS reduces photo-corrosion and enhances the charge separation. The proposed material demonstrated H₂ evolution of 63.71 mmol·g⁻¹·h⁻¹ and 71.24 mmol g⁻¹·h⁻¹ under visible and simulated solar light irradiation, respectively. In other study [66], phase control and defect engineering were applied to prepare the metallic 1T-phase MoS₂ quantum dots (QDs) decorated 2D gCN NSS with the presence of N vacancies. The 1 T-MoS₂ QDs@g-C₃N_x was applied as PC for photocatalytic H₂ evolution, which exhibited excellent photocatalytic activity. The presence of N defects may facilitate the design of energy band structures of gCN, whereas MoS₂ QDs were found to be beneficial to enhance the charge carrier transport performance. The presence of defects and synergism were the key points for the improved photovoltaic performance of the 1 T-MoS₂ QDs@g-C₃N_x (15 wt %). Dual heterophase of MoS₂/MoC (molybdenum carbide)@rGO cocatalyst was prepared using the calcination method [67]. The observations revealed that the highest H₂ evolution rate for MoS₂-MoC@rGO/TiO₂ reached 575 μmol·h⁻¹·g⁻¹. The improved performance may be ascribed to the presence of rGO NSs, which accelerate the photoelectron transfer, whereas MoS₂ improves the number of interfacial H₂ evolution active sites. The 2D-2D stacked MoS₂/gCN composite was also explored as a piezophotocatalyst for effective H₂ evolution [68]. It was stated that MoS₂ NSs on the gCN surface may play a crucial role in H₂ evolution. Xu et al. [69] stated that 1T-rich MoS₂ with gCN hollow microspheres has excellent optical properties. Therefore, an efficient interface was established between 1T-rich MoS₂ and gCN, which improved H₂ evolution activity. 1T phase MoS₂/holey HCN NSs (2D-2D) heterostructure was prepared and characterized by X-ray diffraction (XRD), which authenticated the successful formation of the MoS₂/HCN composite [70]. The 1T MoS₂/HCN-4 has excellent stability after long-term testing and showed H₂ evolution of 2724.2 μmol⁻¹ h⁻¹·g⁻¹. Li et al. [71] also reported that 1T MoS₂-modified polymeric S-doped gCN (MASCN) is a promising PC material for photocatalytic H₂ evolution studies. The dispersity of the gold nanoparticles (Au NPs) may be enhanced by the presence of S-doped gCN. In addition, electrical conductivity may be enhanced by combining S-doped gCN and Au NPs. The 1T MoS₂ was also in situ formed within the induction of the Au NPs. Therefore, high-density active sites were observed with decreased interfacial charge transfer. The H₂ evolution activities show that MASCN has better photocatalytic properties compared to the other materials (Figure 4a). The H₂ evolution results for MA (2%) SCN with different MoS₂ loading amounts are shown in Figure 4b. The higher H₂ evolution of 4708.3 μmol/g/h was achieved under optimized conditions. The above proposal also shows decent stability for four cycles (Figure 4c). The mechanism for H₂ evolution is explained in Figure 4d. Liang et al. [72] reported the fabrication of one-pot synthesis-based cobalt sulfide (CoS₂)-modified MoS₂/gCN ternary composite. This type of ternary composite may accelerate the separation rates of photon-excited carriers. The reduced reaction energy barriers and increased active center were ascribed to the presence of MoS₂ in the prepared ternary composite. Thus, 0.02-CoS₂/MoS₂/g-C₃N₄ composite-based photocatalytic activity for H₂ evolution. gCN NSs loaded N-doped MoS₂ was fabricated by Wei et al. [73], which exhibits that the presence of N atoms may trigger the catalytic activity on the (002) plane of MoS₂. The MoN_1.2xS_2−1.2x reduced the charge transfer impedance at the interface. The presence of the catalytic sites and intrinsic charge separation features of MoN_1.2xS_2−1.2x may improve H₂ evolution. Hence, 5 wt % MoN_1.2xS_2−1.2x@g-CN based studies demonstrated an H₂ evolution rate of 360.4 µmol·g⁻¹·h⁻¹. Li et al. [74] achieved H₂ evolution of 1285 µmol·g⁻¹·h⁻¹ for gCN/MoS₂ composite under simulated solar light. The increased photogenerated carrier density and efficient charge separation were facilitated by the type-II heterojunction. The synergy in the prepared composite enhanced H₂ evolution under the photocatalytic process. Ning et al. [75] fabricated a novel 2D/2D/2D ternary heterojunction of ZnIn₂S₄/gCN/MoS₂ using solvothermal-assisted synthetic protocols. The proposed PC exhibited H₂ evolution of 13.6 mmol·g⁻¹·h⁻¹ under visible light. The improved performance may be attributed to the better interfacial contacts, shortened migration distance for photogenerated charges, and efficient pathways for electron separation/transport in type-II heterostructures. Another study [76] reported the fabrication of a double heterojunction (MoS₂/hybrid nanodiamond (HND)/gCN) using the hydrothermal method. The photocatalytic activity of the MoS₂/HND/gCN was evaluated for H₂ evolution, which exhibited decent H₂ evolution of 227 µmol·g⁻¹·h⁻¹. This may be attributed to the graphene shell of HND with the existence of higher carrier mobility, which can significantly shorten the charge migration time, whereas the diamond core may enhance the absorption capacity. The formation of the heterostructure at the interface of different components may also accelerate the charge transfer/separation, while MoS₂ QDs provide active sites and improve the photocatalytic H₂ evolution reaction. Li et al. [77] explored the use of a ternary composite of MoS₂/ZnIn₂S₄/GQDs heterojunction. MoS₂ NSs were obtained by the liquid-phase exfoliation method, and the ternary composite was fabricated via the hydrothermal-assisted method. The H₂ evolution of 21.63 mmol·h⁻¹·g⁻¹ was achieved using ternary composite. It may be concluded that the presence of synergism and improved charge separation/transfer enhanced the photocatalytic activity of the MoS₂/ZnIn₂S₄/GQDs heterojunction. It is well understood that optimization of the transfer channel for charge carriers and generating more active sites play crucial roles in photocatalytic systems. In this context, authors proposed novel strategies to enhance the charge carriers and active sites of the GO/CdS/MoS₂ ternary composite [78]. The proposed PC exhibited excellent stability for 20 h with insignificant loss of H₂ evolution rate. The highest H₂ production rate of 1.45 mmol·h⁻¹·g⁻¹ was obtained under optimized conditions. The enhanced photocatalytic activity was attributed to the holey structure of GO and the presence of active sites. According to Imam et al. [79], H₂ evolution of 15.6 mmol·g⁻¹·h⁻¹ can be obtained using a CdS/MoS₂/CNTs composite. The optimization of PC is of great significance to enhance the photocatalytic activity under visible light. Therefore, MoS₂/CNTs/CdS (15%) exhibited better catalytic activity towards H₂ evolution of 101.18 mmol·h⁻¹·g⁻¹, which is significantly 253 times higher compared to the pristine CdS [80]. gCN/CdZnS/MoS₂ heterojunction was proposed as PC for efficient H₂ evolution [81]. It was observed that a heterogeneous interface was formed between the gCN, CdZnS, and MoS₂. It can also be mentioned that a 2D/0D sandwich structure could be formed between the gCN, CdZnS, and MoS₂ that can exhibit improved optical properties for H₂ evolution. The H₂ evolution studies for CdZnS, gCN/CdZnS, and gCN/CdZnS/MoS₂ at different times are shown in Figure 4e. An excellent H₂ evolution rate of 57.02 mmol·g⁻¹·h⁻¹ was achieved under optimized conditions. The presence of the 2D structure of gCN and MoS₂ enhanced the stability of the proposed ternary composite for long-term applications.

In 2025, the MoS_2/gCN/ZnIn₂S₄ heterojunction was also fabricated, and its photocatalytic activities were evaluated for H₂ production applications [82]. The presence of heterojunction facilitated the charge carrier separation, and the introduction of MoS₂ promoted faster surface reactions. The optimized PC with 5 wt % g-CN and 2.0 wt % MoS₂ exhibited H₂ evolution of 5567 µmol·g⁻¹·h⁻¹.

3.3. MoS₂/Carbon/MOF/ZIF

Metal organic frameworks (MOFs) offer promising features such as high surface area and porosity, which makes them desirable candidates for catalytic and adsorption applications. MoS₂ was also combined with MOF to develop the hybrid composite material [83]. The constructed MOF/MoS₂ composite was explored for photocatalytic studies where MOF acted as a photosensitizer to enable the MoS₂ to absorb the visible light for more electron transfer. The presence of flower-like structures and decent optical properties of the MoS₂/MOF generated more active sites and improved the electron transfer rate. Therefore, it was observed that MOF/MoS₂ exhibits a low electron-hole recombination rate, which significantly improved the H₂ evolution rate (626.3 μmol·h⁻¹·g⁻¹). The multi-phase MoS₂ (1T/2H) was engineered via O-incorporation and combined with porous gCN using simple strategies [84]. The 7%-O-1T/2H-MoS₂/g-C₃N₄ exhibited the highest photocatalytic activity for an efficient H₂ evolution rate of 1487 μmol·h⁻¹·g⁻¹. Authors also proposed theoretical investigations that revealed that the presence of O distorted MoS₂ crystal and displaced some S atoms in the prepared 1T/2H-MoS₂ to tune the electronic structure. Thus, the proposed composite displayed improved conductivity with more active sites. Sima et al. [85] also proposed the fabrication of gCN/MoS₂ composite under benign conditions and explored it as a PC for efficient H₂ evolution under simulated light irradiation. The optimized conditions suggested that an interesting amount of H₂ of 913 μmol·g⁻¹·h⁻¹ can be obtained using gCN/MoS₂ composite. The solvothermal method was also adopted for the preparation of MOF-derived MoS₂/gCN heterojunctions PC, which displayed H₂ evolution of 1695.76 μmol·g⁻¹ at 4 h [86]. It was believed that MOF-derived MoS₂ increases light absorption capacity and enhances the separation of photogenerated charge carriers. The MOF-derived materials exhibit improved optical properties, and Liu et al. [87] also explored the potential of MOF-derived porous MoS₂/CdS heterostructures for H₂ evolution applications. In this context, the hydrothermal sulfurization method was adopted for the preparation of the MoS₂/CdS heterostructure. (Figure 5a) The presence of porous structure and large surface area enhance the photocatalytic activity of the prepared 5 wt % MoS₂/CdS heterostructure, which yielded H₂ evolution of 3318 μmol·h⁻¹·g⁻¹. The mechanism for H₂ evolution is shown in Figure 5b. Rehan et al. [88] obtained H₂ evolution of 58.2 mmol·g⁻¹·h⁻¹ using Au-anchored UiO-66-NH₂/ZnIn₂S₄/MoS₂ composite, whereas Tab et al. [89] reported H₂ evolution of 250.09 μmol using 1 T/2H-MoS₂@Zn-Ni MOF. The presence of a larger surface area, porosity, and synergism in the proposed PC enhanced the charge separation and improved H₂ evolution. Interfacial engineering-based strategies were also explored for the preparation of MoS₂/bimetallic MOF (MoS₂@Cu/Co-MOF) composite for piezo-photocatalytic H₂ production [90]. The fabrication of the MoS₂@Cu/Co-MOF composite has been presented in Figure 5c. The fabricated MoS₂@Cu/Co-MOF shows interesting H₂ evolution of 1308.028 µmol.

In another recent study [91], type II heterojunction was developed using binary 2D-MoS₂/ZIF-67 composite via the in-situ growth method. Authors found that the presence of the 2D structure of MoS₂ and the type II heterojunction improved the electron transfer, and an enhanced H₂ evolution of 8.13 mmol·g⁻¹·h⁻¹ was obtained for the MoS₂/ZIF-67 (MSZ-25) sample.

3.4. MoS₂/Polymers/MXenes

An S-scheme-based photocatalytic mechanism was proposed for photocatalytic H₂ production using protonated D-A typed polymer/MoS₂ as a promising PC [92]. It was observed that the proposed heterojunction promoted charge migration and increased active sites, which significantly contributed to boosting the H₂ evolution. Promoted charge separation was also observed for the 3D interconnected titanium carbide (Ti₃C₂) MXene/MoS₂/CdS composite [93]. The synthesis procedure is described in Figure 6a, whereas morphological features for different materials are shown in Figure 6b–e. The presence of tight interfacial contact shows improved catalytic activity, and the existence of synergism between the Ti₃C₂ and MoS₂ accelerated electron transfer. Therefore, optimized results indicated the H₂ evolution of 15.2 mmol·h⁻¹·g⁻¹ using Ti₃C₂/MoS₂/CdS. The H₂ evolution studies have been summarized in Figure 6f, while cyclic stability test results are shown in Figure 6g. The XRD studies suggested the presence of excellent stability of Ti₃C₂/MoS₂/CdS (Figure 6h).

The presence of defects, heterojunction, and synergy in the prepared MoS₂/Ti₃C₂/CdS PC boosted H₂ production to 4.1 mmol·h⁻¹·g⁻¹ [94]. Wu et al. [95] proposed the construction of a ternary composite of CdS, MoS₂, and Ti₃C₂ via in-situ growing. Authors observed that the presence of MoS₂ and Ti₃C₂ boosted photocorrosion resistance and photocatalytic activities of CdS. The electrons and holes of the CdS were significantly migrated to MoS₂ and Ti₃C₂, respectively, and the combined effects of Ti₃C₂ and MoS₂ inhibit the redox of Cd²⁺/S²⁻. Therefore, an excellent lifetime of 78 h and H₂ evolution of 14.88 mmol·h⁻¹·g⁻¹ were observed under the optimized conditions. Other recent studies demonstrated that charge carriers can be significantly separated using a Ti₃C₂-MoS₂-ZnIn₂S₄ composite [96]. Thus, the Ti₃C₂-MoS₂-ZnIn₂S₄ composite was prepared by sequentially growing the MoS₂/ZnIn₂S₄ layered structure on Ti₃C₂ MXene via a one-step hydrothermal method. The obtained Ti₃C₂-MoS₂-ZnIn₂S₄ composite shows improved visible light absorption and augmented active sites. The observations revealed that the Ti₃C₂-MoS₂-ZnIn₂S₄ composite formed a tight heterojunction interface, which is beneficial for the separation of charge carriers. The enhanced H₂ evolution of 4398 μmol·g⁻¹·h⁻¹ was obtained for Ti₃C₂-MoS₂-ZnIn₂S₄ composite. Mansoor et al. [97] reported the construction of MoS₂/TiO₂/Ti₃C₂ nanowires (NWs) composite via a hydrothermally assisted method. The obtained MoS₂/TiO₂/Ti₃C₂ composite shows improved photocatalytic H₂ evolution. The MoS₂/TiO₂/Ti₃C₂ composite shows H₂ evolution rates of 637.1 and 243.2 µmol·g⁻¹·h⁻¹ in triethanolamine (TEOA) and polylactic acid (PLA) sacrificial reagents, respectively.

3.5. MoS₂/Sulfides

Liu et al. [98] stated that interfacial charge dynamics is crucial for heterostructure photocatalysis. The typical chemical bath strategy was proposed for the formation of hexagonal CdS/exfoliated MoS₂ (MoS₂) for photocatalytic H₂ production applications. The photocatalytic H₂ evolution involves type II contact at the CdS/MoS₂ interface. The proposed photocatalytic system showed an H₂ evolution rate of 2.3 mmol·g⁻¹·h⁻¹. Peng et al. [99] designed and proposed the photocatalytic properties of the MoS₂/O-ZnIn₂S₄ composite towards H₂ evolution. It was observed that H₂ evolution of 4.002 mmol·g⁻¹·h⁻¹ can be obtained using the proposed MoS₂/O-ZnIn₂S₄ composite. Mo vacancy defective MoS₂ was combined with CdS NPs using a facile hydrothermal method [100]. This proposed composite demonstrated an H₂ production rate of 11,750 µmol·g⁻¹·h⁻¹, which is higher compared to the Mo vacancy-defective free MoS₂/CdS. It is suggested that the presence of defective (Mo) vacancies on the MoS₂ surface may enhance the photocatalytic properties of the defective MoS₂/CdS composite. The MoS₂/ZnIn₂S₄ composite was obtained using hydrothermal and chemical aqueous methods at low temperature [101]. It was found that pure ZnIn₂S₄ exhibits a low H₂ evolution rate of 30 μmol·g⁻¹·h⁻¹, which was further enhanced to 97.36 µmol·g⁻¹·h⁻¹ by employing a MoS₂/ZnIn₂S₄ composite. The larger surface area and efficient electron-hole pair separation catalytic active sites were responsible for the enhanced photocatalytic performance. Zhang et al. [102] found that interfacial contacts between MoS₂ and CdS play a crucial role in charge transfer. The hydrothermally obtained CdS/MoS₂ composite exhibited improved charge separation/transfer, which may be ascribed to the presence of MoS₂. Pure CdS exhibits H₂ evolution of 0.41 mmol·h⁻¹·g⁻¹, which was further enhanced to 35.24 mmol·h⁻¹·g⁻¹ using CdS/MoS₂ composite (Figure 7a). This proposed PC also demonstrated excellent stability for four cycles (Figure 7b).

Kumar et al. [103] developed an ultrathin layered Zn-doped MoS₂-modified CdS NRs composite using benign methods. It was believed that Zn doping may stimulate the charge transfer properties of Zn-MoS₂/CdS. The Zn-MoS₂/CdS demonstrated excellent improved photocatalytic activity for H₂ evolution (250 mmol·h⁻¹·g⁻¹). This improved photocatalytic performance may be ascribed to the high dispersion of few-layered Zn-MoS₂ and synergism. Kumar et al. [104] also reported the construction of Ni-doped MoS₂ NSs/CdS composite using simple approaches. This novel composite shows H₂ evolution of 249 mmol·h⁻¹·g⁻¹, which was found to be 70 times higher than pristine CdS. The decent conductivity, larger surface area, and improved active sites were responsible for the enhanced photocatalytic activity. The novel MoS₂/Zn_0.5Cd_0.5S with W/Z phase junctions was successfully prepared [105]. The authors observed that 3%MoS₂/Zn_0.5Cd_0.5S exhibited an enhanced H₂ production rate of 388.2 μmol/h. In another study [106], two cocatalysts were explored to enhance the photocatalytic H₂ evolution. In this connection, hollow carbon spheres (HCSs) and MoS₂ were loaded on ZnIn₂S₄ NSs. It was found that prepared ternary composite has strong interaction between MoS₂, HCSs, and ZnIn₂S₄. Therefore, the proposed ternary composite shows improved visible light absorption, low electron-hole recombination, and faster charge carrier transfer, which enhance the H₂ evolution rate to 620.9 μmol·g⁻¹·h⁻¹. Another research work obtained H₂ evolution of 32.94 mmol·g⁻¹·h⁻¹ using MoS₂/CdS composite [107]. MoS₂@silver sulfide (Ag₂S) composite was constructed by employing the hydrothermal method and ion exchange reactions [108]. The observations revealed that Ag₂S NPs are attached to flowers like MoS₂. Raman studies suggested that MoS₂ with a 1T/2H hybrid phase is present with Ag₂S and confirmed the formation of the composite. Furthermore, it was observed that the MoS₂@Ag₂S composite exhibits an S-scheme band structure, which can significantly facilitate the electron-hole separation for improved photocatalytic H₂ evolution (3516 μmol·g⁻¹·h⁻¹). A novel rGO-coated montmorillonite (rGO/Mt) was explored as a hydrophilic catalyst carrier, and it was combined with CdS and MoS₂ [109]. 100 mg of the proposed catalyst demonstrated excellent H₂ evolution of 1760 μmol·h⁻¹. It was stated that the presence of the hydrophilic Mt inhibited agglomeration of nanostructures. Thus, it improved the dispersibility in water, and the presence of conductive rGO facilitated electron transmission, and decent photocatalytic activity was observed for the above proposed PC. The photocatalytic activity (33.8 mmol‧g⁻¹‧h⁻¹) for the proposed 10% MoS₂/Zn_0.5Cd_0.5S was attributed to the presence of synergistic interactions and S-scheme band structure [110]. In addition, hierarchical structure enhanced the utilization of visible light and shortened the distance for the charge carriers. In another previous study [111], CdS/MoS₂/CNFs (CNFs = carbon nanofibers) was adopted as PC for photocatalytic H₂ evolution. It was observed that a compact heterojunction was formed between MoS₂ and CdS, which may significantly enhance the photogenerated carrier’s transfer. Flake-like structures of MoS₂ may provide abundant active sites that promote H₂ evolution reactions. Therefore, 3195.52 μmol·g⁻¹·h⁻¹ H₂ evolution was obtained under visible light irradiation. Wang et al. [112] reported Z-scheme-based photocatalytic H₂ evolution using MoS₂/Cd_0.6Zn_0.4S. Authors found that prepared material has an MoS₂/Cd_0.6Zn_0.4S plate-on-plate structured heterojunction. The optimized 0.8% MoS₂/Cd_0.6Zn_0.4S exhibits improved photocatalytic activity for H₂ evolution compared to the other samples. The major advantage of the proposed catalyst was the generation of interesting H₂ evolution of 47.68 µmol·g⁻¹ at 2.5 h in the absence of a sacrificial agent. However, authors also proposed that excellent H₂ evolution of 13,466.50 µmol·g⁻¹·h⁻¹ can be obtained using Na₂S/Na₂SO₃ sacrificial agents. Thus, sacrificial agents play a crucial role and enhance the photocatalytic activities of the PC. The CdZnS modified MoS₂ composite was also utilized for the evolution of H₂ under photocatalytic technique [113]. The CdZnS has a NRs structure, while MoS₂ consists of sheets-like surfaces, and a 6 wt % MoS₂-decorated CdZnS composite demonstrated higher H₂ evolution activity (75.89 mmol·g⁻¹·h⁻¹). The Cd_0.3Zn_0.7S NPs decorated 1 T-2 H mixed-phase MoS₂ NSs was also fabricated using the hydrothermal-assisted method [114]. The investigations revealed that 0.75% MoS₂/Cd_0.3Zn_0.7S has better photocatalytic activity due to the presence of synergistic interactions, and the interesting H₂ evolution rate reaches 11,172 μmol·g⁻¹·h⁻¹. A unique acorn-leaf-like CdS-modified layered MoS₂ NSs composite was developed using the hydrothermal method [115]. A very small amount of PC (10 mg) exhibited H₂ evolution of 70.05 mmol·g⁻¹·h⁻¹, which was higher compared to the pristine CdS. This is revealing that MoS₂ acted as a cocatalyst and improved the H₂ evolution activity of the fabricated composite. Sridevi et al. [116] prepared a 2D-2D ultrathin heterostructure of MoS₂/CdS using the solvothermal method. The presence of 2D-2D structure not only shortens distance for charge transport but also provides large contact areas, which enhance photocatalytic activity of the MoS₂/CdS composite. The optimized results indicated the presence of higher photocatalytic activity of 5 wt % 2D/2D MoS₂/CdS. P-doped Zn_0.3Cd_0.7S was also modified with MoS₂ for photocatalytic applications [117]. It was believed that the introduction of P atoms may elevate the concentration of S vacancies and tune the Fermi level near the S vacancies. The 1% MoS₂/P-Zn_0.3Cd_0.7S with porous coral-like surface morphological features demonstrated decent H₂ evolution of 30.65 mmol·g⁻¹·h⁻¹. In a previous effort, γ-ray radiation strategy was utilized to assemble dual vacancies for the preparation of MoS₂-CdS-γ [118]. It was observed that defect-rich CdS and flower-like 1T/2H MoS₂ exhibit synergistic interactions, and H₂ evolution reaches 37.80 mmol/h/g under visible light irradiation. The fluorinated CdS/MoS₂/ZnS composite was also adopted as a PC for photocatalytic H₂ production under visible light irradiation [119]. The presence of F- ions may accelerate the carrier interfacial transfer, and H₂ evolution may be improved under visible light. The interesting H₂ evolution of 11,902 μmol·g⁻¹·h⁻¹ was obtained under optimized conditions. The hollow MoS₂@ZnIn₂S₂ nanoboxes show p-n heterojunction, and observations suggested the generation of H₂ evolution of 22.25 mmol·h⁻¹·g⁻¹ [120]. The presence of the hollow structure, p-n heterojunction, active sites, and promoted separation of charge carriers were responsible for the improved photocatalytic activity of the 5% MoS₂@ZnIn₂S₄ nanobox PC. In another study [121], S vacancies induced 1T MoS₂ to be loaded on Zn₃In₂S₆ using the hydrothermal method. 1T MoS₂ significantly promoted separation of the charge carriers. The proposed composite has excellent photocatalytic properties, which can be attributed to the presence of abundant S defects and a larger contact interface between the Zn₃In₂S₆ and 1T MoS₂. Hu et al. [122] mentioned in his report that loading and stability of the PC are major concerns and therefore proposed the fabrication of a stable Zn_0.5Cd_0.5S/MoS₂ using two-step hydrothermal and physical methods. Authors used excess thiourea for the fabrication of MoS₂ and observed that excess use of thiourea produced Mo-O defects and improved layer spacing, which increases visible light utilization and enhances electron transfer. Authors used various sacrificial agents, but proposed materials exhibited excellent photocatalytic activity in the presence of lactic acid. This proposed Zn_0.5Cd_0.5S/MoS₂ composite shows excellent stability for H₂ evolution. Another study described the role of the O-doped MoS₂/CoS cocatalyst on the 1D Zn_0.1Cd_0.9S surface for the improved charge carrier separation and enhanced H₂ evolution [123]. The presence of the metallic nature of the O-MoS₂/CoS cocatalyst facilitated electron-hole separation and inhibited recombination. The 1D MoS₂/CoS/Zn_0.1Cd_0.9S showed H₂ production activity of 95.5 mmol·g⁻¹·h⁻¹. The CdIn₂S₄ NPs decorated MoS₂ microrods (MRs) (CdIn₂S₄@MoS₂) composite was fabricated using the solvothermal method [124]. The obtained CdIn₂S₄@MoS₂ composite exhibits the improved absorption of visible light and facilitated the separation/migration of the charge carriers, which further enhanced the photocatalytic activity of the CdIn₂S₄@MoS₂ composite. The ZnS@MoS₂ composite emerged as an efficient PC for photocatalytic H₂ production of 4663.5 µmol·g⁻¹·h⁻¹ [125]. The MoS₂ and NiS composite was developed using a benign hydrothermal method [126]. The prepared NiS@MoS₂ composite was utilized as a PC, which demonstrated H₂ production activity of 17,152.8 μmol·g⁻¹·h⁻¹ in the presence of the TEOA sacrificial agent under the Xe lamp source. The MoS₂/Cd_0.5Mn_0.5S Schottky junctions show promising features such as decent catalytic activity, synergy, and a larger surface area with improved active sites, which are beneficial for photocatalytic H₂ evolution [127]. Other reports show the potential of MoS₂ tipped Zn_0.1Cd_0.9S NRs composite for H₂ production applications [128]. This study shows photocatalytic H₂ production activity of 525 µmol·g⁻¹·h⁻¹ with 99% degradation of ofloxacin in wastewater. Men et al. [129] adopted a one-step hydrothermal method for the formation of the MoS₂/ZnIn₂S₄ composite. The obtained MoS₂/ZnIn₂S₄ composite H₂ evolution activity is 11.182 mmol·h⁻¹·g⁻¹. It is understood that modification of CdZnS is of great significance to improve its photocatalytic activities. In this context, Rh single atoms, i.e., Rh_1, were anchored on hollow microflowers shaped like MoS₂/S vacancy-rich CdZnS (CZS-S_Vs) using simple strategies [130]. The obtained Rh₁@MoS₂/CZS-S_Vs exhibited enhanced photocatalytic properties due to the generation of more active sites, accumulation of abundant electric charges, and presence of S atoms. The higher H₂ production activity of 39,827 μmol·h⁻¹·g⁻¹ was observed for the above-mentioned novel PC. It was also mentioned in another study that MoS₂/S_v-ZnIn₂S₄/ZnS (MS/S_v-ZIS/ZS) has decent photocatalytic properties and can be explored as a promising PC for photocatalytic H₂ production applications [131]. The MS/S_v-ZIS/ZS was found to be a promising PC material in terms of stability for 5 cycles and H₂ production rate. Dang et al. [132] utilized the hydrothermal method for the preparation of hollow CdS-modified MoS₂ composite and observed that MoS₂ has multi-phase (1T/2H MoS₂). The presence of 1T/2H MoS₂ reduced the direct contact between the solution and CdS, which retarded the corrosion. In addition, the presence of more active sites enhanced the photocatalytic activity of the H-CdS/MoS₂-3 to 1520.82 µmol·g⁻¹·h⁻¹. The Co-doped MoS₂/ZnIn₂S₄ composite demonstrated improved H₂ production activity, which may arise due to the presence of synergism between the Co-doped MoS₂ and ZnIn₂S₄ [133]. Deng et al. [134] reported the formation of a novel NRs cluster-like CdS@1 T/2H MoS₂ heterojunction with rich S-vacancies using the hydrothermal-assisted synthesis method. The optimized conditions revealed that CdS@1 T/2H MoS₂-4 possesses higher photocatalytic activity of 38.91 mmol·g⁻¹·h⁻¹ compared to the pristine CdS. These observations showed that MoS₂ is one of the promising cocatalysts that has the potential to boost the photocatalytic H₂ evolution.

3.6. Other MoS₂-Based PC Materials for H₂ Evolution

In this section, we have summarized some other recent examples of MoS₂-based PCs for photocatalytic H₂ production applications. In this connection, it would be worthy to mention that Zhang et al. [135] reported the fabrication of a novel PC by utilizing lead-free perovskite material/MoS₂. The MoS₂ was prepared by a modified hydrothermal method, whereas cesium silver bismuth bromide perovskite (Cs₂AgBiBr₆) decorated MoS₂ (MoS₂/CABB) composite was fabricated using the dissolution/recrystallization method, as shown in Figure 8a. The photocatalytic activity of the various proposed materials was evaluated in the presence of aqueous hydrobromide (HBr) solution. The H₂ evolution rate of various materials is displayed in Figure 8b. The optimized conditions demonstrated the generation of H₂ evolution of 87.5 μmol·h⁻¹·g⁻¹ under visible light. The plausible mechanism for H₂ evolution has been illustrated in Figure 8c. Authors also found that the proposed MoS₂-based PC has good stability for long-term H₂ evolution applications, as shown in Figure 8d.

It is suggesting that perovskite materials also have promising characteristics for photocatalytic H₂ evolution. Thus, another study also explored the photocatalytic activities of formamidinium lead bromide (FAPbBr₃)/MoS₂ composite [136]. Authors developed the FAPbBr₃/MoS₂ heterostructure by employing in-situ growth of FAPbBr₃ on the MoS₂ surface. The H₂ evolution of 1150 µmol·g⁻¹·h⁻¹ was observed for the FAPbBr₃/MoS₂-7 composite under visible light irradiation. Zhao et al. [137] also explored the construction of a novel 1T/2H MoS₂/tri(dimethylammonium) hexaiodobismuthate (DMA₃BiI₆) composite and examined its optical properties for photocatalytic H₂ production. The 1T/2H MoS₂/DMA₃BiI₆ composite shows excellent photocatalytic activity for hydroiodic acid splitting with H₂ evolution of 241.5 µmol·g⁻¹·h⁻¹ under visible light irradiation. Authors stated that improved performance of the 1T/2H MoS₂/DMA₃BiI₆ composite may be ascribed to the improved charge separation/transfer at heterojunction interfaces of the prepared composite material. In other previous work [138], a 3D-3D heterostructure of nickel selenide (NiSe₂)-MoS₂ was fabricated using the hydrothermal and in-situ thermal injunction approach. The obtained results confirmed the formation of a marigold-shaped structure of NiSe₂ that was attached to flower-shaped MoS₂. The 3D-3D nanojunction increased visible light absorption and enhanced the separation of the carriers. The improved H₂ production activity of 2473.7 μmol·h⁻¹·g⁻¹ was observed. In another approach, Cu₂(OH)₂CO₃-MoS₂@Ag₂S was explored as a PC, which shows H₂ production activity of 1.488 mmol·g⁻¹·h⁻¹ under visible light [139]. The high-yield exfoliation of MoS₂ monolayers was reported by Liu et al. [140] using novel strategies. It was indicated that the 2H phase of the MoS₂ was retained in monolayers of MoS₂. It was also observed that exfoliated MoS₂ has good cocatalytic properties towards H₂ evolution. Zhao et al. [141] proposed the role of a novel composite as PC for simulated solar light-induced H₂ evolution. In this connection, cobalt phosphide (CoP)-modified MoS₂ composite was prepared, which demonstrated reasonably good H₂ production activity of 76.45 μmol·g⁻¹·h⁻¹. Authors also stated that the proposed material has decent stability for photocatalytic applications. The highly active Co₂P/2H-1T MoS₂ cocatalyst was proposed as a cocatalyst, and the authors explored the potential of 2Co₂P/P-TiO₂/2H-1T MoS₂ for photocatalytic H₂ production reaction [142]. This above-mentioned PC demonstrated H₂ evolution of 4156 μmol/g under visible light, which is higher compared to the TiO₂ or 2H-1T MoS₂/TiO₂. The proposed PC also demonstrated reasonable stability for 8 cycles. The strongly promoted migration and separation of the electron-hole pairs were responsible for the enhanced photocatalytic activity of the proposed PC. The 2D O, P dual-doped MoS₂ NSs with 1T phase were prepared [143]. It was also found that prepared O, P-doped MoS₂ NSs have sufficient edge S active sites. Thus, the proposed 2D/2D O, P–MoS₂/NH₂-MIL-125(Ti) (where MIL-125 is a Ti based MOF) composite displayed enhanced photocatalytic activity of 339.3 μmol⋅g⁻¹⋅h⁻¹. Other study [144] reported the facile formation of Ni or Co doped black phosphorus (BP)/MoS₂ composite. The obtained BP/MoS₂-Ni and BP/MoS₂-Co heterojunction delivered H₂ production rates of 6.4139 mmol·h⁻¹·g⁻¹ and 7.4282 mmol·h⁻¹·g⁻¹, respectively, in the presence of Eosin-Y. Zhang et al. [145] proposed the facile construction of the Ag₃PO₄ @1T MoS₂ Z-scheme system for H₂ recovery from antibiotic wastewater. The distorted 1T MoS₂ Nss were stabilized by cubic Ag₃PO₄ (with O vacancies). The presence of interfacial O vacancies acted as an electron transporter and modulated the charge separation in the Z scheme system. Thus, H₂ production of 54.01 µmol·g⁻¹·h⁻¹ was obtained under the optimized conditions. The Co₉S₈/MoS₂/Ni₂P dual S-scheme based system exhibited H₂ evolution of 5.69 mmol·g⁻¹·h⁻¹ [146], whereas few-layered MoS₂ showed H₂ evolution of 246 mmol·g⁻¹·h⁻¹ [147]. The Ni complex modified flowers like MoS₂ was found to be a promising PC, which demonstrated an H₂ evolution rate of 3320 μmol·g⁻¹·h⁻¹ [148]. It was believed that improved photocatalytic activity may be ascribed to the efficient interfacial electron transport from MoS₂ to the Ni complex with octaaza-bis-α-diimine ligands, which surpassed the electron-hole recombination in the MoS₂. Therefore, H₂ was produced by the electron transfer-induced valence change of Ni ions. The presence of the flower-like structure of MoS₂ provides a larger surface area and high light absorption. In other reports, various MoS₂-based materials were also explored in photocatalytic H₂ evolution, which demonstrated acceptable efficiency and performance towards H₂ production [149,150,151,152,153,154,155,156,157,158]. The photocatalytic performance of various MoS₂-based hybrid materials for H₂ production is summarized in

Table 2. Photocatalytic H₂ evolution activities of various MoS₂ based materials.

PC/Cocatalyst	H2 Evolution	Light Source	Sacrificial Agent	References
MoS2/TiO2	1.41 mmol·g−1·h−1	300 W Xe lamp (320 < λ < 780 nm)	TEOA	[41]
MoS2@MoO3	12,416.8 µmol·h−1·g−1	5 W LED lamp (λ = 400–800 nm)	TEOA	[43]
ZnO/ZnS/MoS2	4.45 mmol·g−1 h−1	100 W Xe lamp (AM 1.5 G)	Na2S/Na2SO3	[45]
MoS2/ZnO	235 µmol·h−1·g−1	300 W Xe lamp	Na2S/Na2SO3	[48]
TiO2(Rod)/MoS2	7415 μmol·g−1	300 W Xe lamp	glycerol/water	[50]
α-Fe2O3/BiOBr/MoS2	57 mmol·g−1·h−1	300 W Xe lamp (λ > 400 nm)	Ethanol	[57]
MoS2 nanosheets/CdS nanorods	71.24 mmol·g−1·h−1	AM 1.5 G	10% Lactic acid	[65]
MoS2/SgCN	4708.3 µmol·h−1·g−1	300 W Xe lamp	Methanol	[71]
MoS2/ZIS/GQDs	21.63 mmol·g−1·h−1	300 W Xenon lamp	-	[77]
MOF/MoS2	626.3 μmol·h−1·g−1	350 W Xe lamp	Formic acid	[83]
MoS2/ZIF-67	8.13 mmol·g−1·h−1	-	Na2S/Na2SO3	[91]
MoS2/O-ZnIn2S4	4.002 mmol·g−1·h−1	300 W Xe lamp	Na2S/Na2SO3	[99]
Ni–MoS2/CdS	249 mmol·h−1·g−1	Simulated solar light	Lactic acid	[104]
CdS/MoS2/CNFs	3195.52 μmol·g−1·h−1	300 W Xe lamp (λ > 420 nm)	Na2S/Na2SO3	[111]
1% MoS2/P-Zn0.3Cd0.7S	30.65 mmol·g−1·h−1	300 W Xe lamp	Lactic acid	[117]
O-doped MoS2/CoS/Zn0.1Cd0.9S	95.5 mmol·g−1·h−1	Xe lamp AM 1.5 G	Lactic acid	[123]
Rh1@MoS2/CZS-SVs	39,827 μmol·h−1·g−1	300 W Xe lamp (λ > 420 nm)	Lactic acid	[130]
MoS2/Sv-ZnIn2S4/ZnS	9.5 mmol·g−1·h−1	300 W Xe lamp (λ > 420 nm)	TEOA	[131]
MoS2/CABB	87.5 μmol·h−1·g−1	300 W Xe lamp (λ > 420 nm)	-	[135]
Co9S8/MoS2/Ni2P	5.69 mmol·g−1·h−1	-	TEOA	[146]
MoS2/38AuNPs−2.75	35.08 mmol·g−1·h−1	300 W Xe lamp (λ > 400 nm)	15 vol % TEOA and 0.5 M Na2SO4	[149]
5% indene-C60 bisadduct (ICBA)/MoS2/CdS	978 μmol·h−1·g−1	300 W Xe lamp (λ > 400 nm)	-	[150]
MoS2 QDs/Cs3Bi2I9	6.09 mmol·h−1·g−1	300 W Xe lamp (λ > 400 nm)	-	[151]
O-MoS2/CdS	58.47 mmol·g−1·h−1	300 W Xe lamp (λ > 400 nm)	-	[152]
Ni0.08-MoS2/ZnIn2S4	7.13 mmol·h−1·g−1	300 W Xe lamp (λ > 400 nm)	TEOA	[153]
CoP@MoS2-2	4339.39 μmol·g−1·h−1	-	-	[154]
O-MoS2/CdS	532.8 μmol·h−1	LED light (420 nm)	lactic acid	[155]
N-Cd0.7Zn0.3S/1%MoS2	132 mmol·g−1·h−1	-	Na2S/Na2SO3	[156]
N-Cd0.7Zn0.3S/1%MoS2/1%MoC-Mo2C	168 mmol·g−1·h−1	-	Na2S/Na2SO3	[156]

.

As discussed above, numerous reports exhibited excellent H₂ evolution under visible light irradiation. As per our observations, MoS₂ nanosheets/CdS nanorods demonstrated H₂ evolution of 71.24 mmol·g⁻¹·h⁻¹ using lactic acid as a sacrificial agent [71]. In another study [104], the highest H₂ evolution of 249 mmol·h⁻¹·g⁻¹ was obtained using Ni–MoS₂/CdS. The O-doped MoS₂/CoS/Zn_0.1Cd_0.9S also exhibited excellent H₂ evolution of 95.5 mmol·g⁻¹·h^−1, which may be attributed to the better charge separation and synergistic interactions [123]. It was also found that Rh₁@MoS₂/CZS-S_Vs exhibited an excellent H₂ evolution rate of 39,827 μmol·h⁻¹·g⁻¹ owing to its synergistic interactions, improved charge mobility, abundant active sites, and formation of heterojunction [130]. The N-Cd_0.7Zn_0.3S/1%MoS₂/1%MoC-Mo₂C also showed excellent H₂ evolution of 168 mmol·g⁻¹·h⁻¹ [156]. It is indicated that optimization of atomic-scale engineering, interfacial contacts, and vacancy design are the promising key points for the fabrication of an efficient PC for H₂ production applications.

4. Conclusions, Limitations, and Future Perspectives

In summary, it is worthy to mention that MoS₂ is the promising 2D layered material for photocatalytic hydrogen evolution applications. MoS₂ has been widely used as a catalyst as well as a cocatalyst for photocatalytic hydrogen evolution reactions. MoS₂ has a layered structure and decent electrical conductivity, and the presence of flower-like surface morphological features of MoS₂ provides a larger surface area for the hydrogen evolution reaction. The MoS₂ has been incorporated with numerous materials such as metal oxides, polymers, MOFs, ZIFs, MXenes, rGO, gCN, and other materials using various synthetic methods. It was found that the presence of synergistic effects between the fabricated MoS₂-based hybrid materials and enhanced active sites reduced recombination reactions between photogenerated electron-hole pairs, and a larger surface area significantly improved hydrogen production under visible light. The formation of the heterojunctions and improved interfacial contacts also boosted the hydrogen production rate. Despite various advantages and enhanced hydrogen production, it is expected that a few more points need to be considered for future research directions, which are given below.

i.

The stability of MoS₂ for long-term application should be improved.

ii.

The mechanism for improved H₂ evolution is still not clear. A depth study and more clarifications are required to understand the role of cocatalysts in photocatalytic hydrogen production.

iii.

The design and development of cocatalysts at the molecular and atomic levels should be considered for future research.

iv.

Previous studies show that Ni, Co, or P doping to MoS₂ NSs enhances the hydrogen production. Thus, it is expected to explore single-atom-doped MoS₂ as a cocatalyst for hydrogen production.

v.

We believe that future research may also focus on density functional theory (DFT) and experimental investigations to analyze the electron transfer route for the enhanced hydrogen production.