MoS₂-Based Nanocomposites for Photocatalytic Hydrogen Evolution and Carbon Dioxide Reduction

Abstract

Photocatalysis is a facile and sustainable approach for energy conversion and environmental remediation by generating solar fuels from water splitting. Due to their two-dimensional (2D) layered structure and excellent physicochemical properties, molybdenum disulfide (MoS₂) has been effectively utilized in photocatalytic H₂ evolution reaction (HER) and CO₂ reduction. The photocatalytic efficiency of MoS₂ greatly depends on the active edge sites present in their layered structure. Modifications like reducing the layer numbers, creating defective structures, and adopting different morphologies produce more unsaturated S atoms as active edge sites. Hence, MoS₂ acts as a cocatalyst in nanocomposites/heterojunctions to facilitate the photogenerated electron transfer. This review highlights the role of MoS₂ as a cocatalyst for nanocomposites in H₂ evolution reaction and CO₂ reduction. The H₂ evolution activity has been described comprehensively as binary (with metal oxide, carbonaceous materials, metal sulfides, and metal–organic frameworks) and ternary composites of MoS₂. Photocatalytic CO₂ reduction is a more complex and challenging process that demands an efficient light-responsive semiconductor catalyst to tackle the thermodynamic and kinetic factors. Photocatalytic reduction of CO₂ using MoS₂ is an emerging topic and would be a cost-effective substitute for noble catalysts. Herein, we also exclusively envisioned the possibility of layered MoS₂ and its composites in this area. This review is expected to furnish an understanding of the diverse roles of MoS₂ in solar fuel generation, thus endorsing an interest in utilizing this unique layered structure to create nanostructures for future energy applications.

1. Introduction

Overusing fossil fuels, growing environmental pollution, and global climate change are the most significant challenges in this century. This crisis has emphasized developing renewable and clean energy sources for future purposes. One of the potential alternatives for this dilemma is solar energy. Semiconductor photocatalysis is a process that effectively converts solar energy to chemical energy. It is considered one of the most widely explored solar-fuel strategies for the photocatalytic splitting of water into H₂ and O₂ and the photocatalytic reduction of CO₂ into hydrocarbon fuels.¹⁻⁴ So far, noble metals, viz., Au, Pd, and Pt, are the most efficient catalysts for either H₂ evolution or CO₂ reduction reactions. However, over the past two decades, significant progress has been achieved in creating an efficient catalyst for H₂ evolution and CO₂ reduction utilizing non-noble metals, including metal oxides,⁵⁻¹⁰ metal sulfides,^11,12 chalcogenides,^13,14 carbonaceous materials,^15,16 and carbides.¹⁷⁻¹⁹ Among these, the discovery of materials having two-dimensional (2D) structures has been a significant triumph in developing and expanding the energy industry. Primarily, such a journey started with the invention of graphene in 2004 by Andre Geim and Kostya Novoselov.^20,21 Their outstanding properties significantly influence energy-related fields, such as solar cells, photo and electrocatalysis, light-emitting diodes (LEDs), transistors, and photodetectors.²²⁻²⁷ Like graphene, layered transition metal dichalcogenides (TMDCs) were introduced as another class of 2D materials with intriguing properties that could potentially replace noble metals in catalytic applications. Molybdenum disulfide (MoS₂) mainly shows wide applications in various energy-related fields.²⁸⁻³⁰ Most importantly, MoS₂-based nanocomposites have developed significantly in photocatalysis, such as pollutant degradation,^31,32 H₂ production,^33,34 and CO₂ reduction.^35,36 Therefore, we explore the advancement of nanocomposites based on MoS₂ in photocatalytic H₂ evolution and CO₂ reduction.

2. Photocatalytic Hydrogen Evolution Reaction (HER)

2.1. Basic Principles of HER

The overall photocatalytic splitting of H₂O to produce H₂ follows three basic steps. The first step is the electron–hole pair generation through photon absorption, which occurs inside the photocatalyst. Second-step involves the separation of charge carriers and their transport to the surface of the photocatalyst, and the third step constitutes the subsequent surface reduction of photoexcited electrons to produce hydrogen (Figure 1). Water splitting to produce H₂ is a thermodynamically uphill reaction, and the free energy associated with this process is 237.2 kJ mol^–1, corresponding to 1.23 eV. Therefore, the photocatalyst has a minimum band gap energy of 1.23 eV for water splitting, and the band gap should be lower than 3 eV for maximum solar energy utilization. Another criterion is related to the positions of the valence band (VB) and conduction band (CB) of the photocatalyst; the conduction band minimum (CBM) of the photocatalyst should be above the water reduction potential, and the valence band minimum (VBM) should be below the water oxidation potential.³⁷ Thus, this limitation of band gap is a significant problem associated with the low yield of H₂ production in most photocatalysts.

Figure 1.

Schematic illustration of the sequences of photocatalytic H₂O splitting in a photocatalyst: (a) irradiation of light and photon absorption, (b) generation and separation of charge carriers, (c) migration of electron and hole to the surface of the photocatalyst, and (d) oxidation–reduction reactions on the surface to produce H₂ and O₂.

2.2. Structure and Properties of MoS₂

Hydrogen (H₂), having an energy density of 140 MJ kg^–1, is an ideal solution for future energy and environmental crises.³⁸⁻⁴⁰ Since the pioneering discovery by Fujishima and Honda in 1972 through the photoelectrochemical splitting of H₂O, H₂ production via solar energy using semiconductor photocatalysts has received greater attention from researchers.^41,42 Further research is ongoing to produce highly active photocatalysts for photocatalytic H₂ evolution with high catalytic activity and lower overpotential. The development of nanocomposites of MoS₂ for photocatalytic H₂ generation is of great interest due to its excellent chemical stability, low cost, nontoxicity, and 2D-layered structure.⁴³⁻⁴⁵

It is well-known that bulk MoS₂ has a bandgap of 1.2 eV and has interestingly been explored as a lubricant, 2D transistor, and hydrodesulfurization catalyst. As shown in Figure 2a, it is composed of vertically stacked, weakly interacting layers held together by van der Waals interactions. In monolayer MoS₂, the Mo atoms are sandwiched between two layers of sulfur atoms with a thickness of 0.65 nm. On the basis of the atomic stacking order, MoS₂ has three structural polymorphs: 2H (two layers per repeat unit, hexagonal symmetry), 3R (three layers per repeat unit, rhombohedral symmetry), and 1T (one layer per repeat unit, tetragonal symmetry)^46,47 (Figure 2b). MoS₂ has numerous intriguing properties due to its unique layered structure. Because of the antibonding state created by the interaction between molybdenum dz² and sulfur p_z orbital at the top of the valence band, MoS₂ exhibits excellent stability versus photocorrosion in solution. Previous research on bulk MoS₂ revealed that it would not be an effective HER catalyst during light illumination.^48,49 However, studies have demonstrated that the fabrication of nanostructured MoS₂ materials can considerably enhance HER activity, which is driven by several factors. For bulk MoS₂, the CB edge potential is positioned at −0.16 V, more than the H⁺/H₂ redox couple (−0.41 V vs SHE, pH 7). As a result, it fails to satisfy the condition of water splitting. Alternatively, the CB edge potential tends to be −0.53 eV in monolayers, indicating the ability to reduce protons to generate H₂.^50,51 This kind of indirect-to-direct bandgap transition of MoS₂ was accomplished by decreasing the layer numbers, which resulted in an increase in bandgap from 1.2 eV (bulk MoS₂) to 1.9 eV (monolayer MoS₂), eliminating the potential barrier, and MoS₂ could be used as appropriate material for the water-splitting reaction.^47,52 Hinnemann et al. employed DFT calculations to determine the free energy of hydrogen adsorption (GH* on the MoS₂ edge) to find out the active edge sites of MoS₂ for the H₂ evolution reaction. The results demonstrated that GH* is significantly close to thermoneutrality, suggesting that MoS₂ could be an effective HER catalyst. This result was also confirmed experimentally by the electrochemical measurements of MoS₂ particles grown on graphite.⁵³ Bonde et al. made similar calculations and determined the free energy of adsorption on MoS₂ edges.⁵⁴ No̷rskov’s group has recently modified these values. On the basis of their findings, the suitable edge configurations correspond to Mo edges covered with 50% S having GH* = 0.06 eV instead of S being covered with 100% S having GH* = −0.45 eV.⁵⁵ Meanwhile, Jaramillo et al. provided experimental evidence for the role of MoS₂ edges in the HER reaction. They systematically produced different sizes of MoS₂ nanoclusters and correlated the electrochemical behavior and edge length of MoS₂ nanoclusters by STM analysis. The results show that the HER activity was associated linearly with many edge sites on the MoS₂ catalyst. Thus, the edge sites of MoS₂ were identified as active sites for the HER reaction.⁵⁶ Hence, the first part of this review focuses on 2D layered MoS₂-based nanocomposites as photocatalysts for H₂ evolution reactions. It envisages the unique role of MoS₂ in increasing the photocatalytic activity of different composites for future energy applications.

Figure 2.

(a) Three-dimensional representation of the structure of a MoS₂ monolayer. Reproduced from ref (46). Copyright 2014 American Chemical Society (b) Schematic structural polytypes: including 2H, 3R, and 1T MoS₂ polytypes. Reproduced with permission from ref (44). Copyright 2016 Elsevier.

2.3. MoS₂-Based Composites for Photocatalytic Hydrogen Evolution Reaction

2.3.1. MoS₂/Metal Oxide Composites

The fundamental issues associated with metal oxide as photocatalysts are the ultrafast charge recombination and photocorrosion within the photocatalysts. Various heterostructures have been developed to overcome these issues by combining metal oxides with MoS₂ and providing promising, low-cost, efficient catalysts.^57,58 The edges of 2D MoS₂ nanosheets have more catalytically active sites that show superior photocatalytic activity. This edge site activity can be further enhanced efficiently by suppressing the stacking of MoS₂ flakes. This modified edge-enriched ultrathin MoS₂ nanosheets uniformly embedded into yolk–shell structured TiO₂ exhibited excellent photocatalytic H₂ evolution. The composite shows an H₂-evolution rate of 2443 μmol g^–1 h^–1 than pure TiO₂ (247 μmol g^–1 h^–1) with a PL lifetime of 5.17 ns. Since the energy differences between the CB of TiO₂ and MoS₂ are comparable, the electrons are more easily transferred from the excited TiO₂ to MoS₂ nanosheets. Because of their higher active sites and enriched edges, ultrathin MoS₂ nanoflakes potentially act as an electron acceptor rather than bulk MoS₂.⁵⁹ Using a plasma sputtering method, a thin layer (0.5 to 10 nm) of MoS₂ is selectively deposited on the top of anodic anatase TiO₂ nanotubes (TiNTs). It has been tested in an open-circuit for photocatalytic H₂ evolution reaction. Using solar simulator (100 mW/cm²), a significant increase in H₂-evolution can be observed using a nominal 1 nm thick MoS₂ on the top of a 6 μm thick TiNTs layer. The band gap positions of the TiNTs/MoS₂ structure were tuned from 1.2, 1.3, and 1.8 eV for the deposition of 10, 3, and 1 nm MoS₂, respectively. Here, MoS₂ serves as a photosensitizer and as an H₂ evolution cocatalyst.⁶⁰

Several studies have shown that engineered defects at the 2D MoS₂ nanosheet edges can enhance the photocatalytic H₂ evolution efficiency. Metal oxides can be combined with this defect-rich ultrathin form of MoS₂ nanosheets to improve light-harvesting efficiency and increase the photocatalytic H₂ evolution rate.⁶¹ Zhu et al., for example, developed a series of MoS₂-TiO₂ catalysts for photocatalytic H₂-evolution via a simple ball-milling process using MoS₂ as a precursor. The recombination of photogenerated charge carriers decreased the photocatalytic performance of pure TiO₂. The H₂ evolution activity increases considerably with MoS₂ loading, reaching a maximum photocatalytic activity of 150.7 μmol h^–1 for the optimum 4.0 wt % MoS₂-TiO₂ catalyst. The too-quick ball-milling process produces significantly more defective MoS₂ structures, which is essential to the practical application of this simple, cost-effective, and sustainable method.⁶² In addition to TiO₂, the coupling of MoS₂ with other metal oxide photocatalysts has also been investigated. Defect-rich MoS₂-nanosheets combined with nitrogen-doped ZnO nanorod composites has recently been developed.⁶³ Under solar light irradiation, an optimized heterostructure consisting of 15 wt % defect-rich MoS₂ nanosheets coated on N-ZnO (N-ZnO-MoS₂) exhibited the maximum H₂ evolution, 17.3 mmol h^–1 g^–1 (Figure 3a). The effective encapsulation of defect-rich MoS₂ nanosheets over N-ZnO nanorods resulted in a defect-induced interfacial contact region, contributing to increased photocatalytic activity. Figure 3b illustrates the pathway of enhanced photocatalytic H₂ evolution reaction. Under sunlight irradiation, the electron transfer occurs from the in situ generated ZnS to the conduction band of N-ZnO, followed by the defect-rich MoS₂ nanosheets. The defect-rich structure has several active edge sites with many exposed unsaturated “S” atoms that show a significant affinity for H⁺ ions to produce H₂ during this reaction and boost visible light absorption. Using a simple hydrothermal approach, Yuan et al. investigated the effect of the interaction of MoS₂-nanosheet with ZnO nanoparticles.⁶⁴ Noble metals have replaced MoS₂ as a cocatalyst for many photocatalytic H₂ evolution reactions. For comparison, composites of ZnO with noble metals were also made.

Figure 3.

(a) Volume of H₂ generated using the N-ZnO-MoS₂ photocatalysts. (b) Schematic illustration of photocatalytic H₂ generation on the N-ZnO-MoS₂ heterostructure under natural sun light. Reproduced from ref (63). Copyright 2019 American Chemical Society.

Among these, Rh exhibited the maximum photocatalytic H₂ evolution rate of 153 μmol h^–1 g^–1, around one-fifth of the 1 wt % MoS₂-ZnO composite photocatalyst (768 μmol g^–1 h^–1). Hence, layered MoS₂ could be used as a substitute for noble metals in photocatalytic H₂ evolution reactions. Similarly, a noble metal-free visible light H₂ evolution was achieved through Zn-5, 10, 15, 20-tetrakis (4-carboxyphenyl)-porphyrin dye (Zn TCPP) sensitized MoS₂/ZnO heterostructures as a photocatalyst. The effect of MoS₂ loading on the photocatalytic activities of the Zn TCPP-MoS₂/ZnO hybrid system was investigated using triethanolamine (TEOA) aqueous solution as a sacrificial agent. The results suggested that, even without any noble metals, the as-prepared composite loaded with 0.5 wt % MoS₂ delivered an H₂ evolution rate of 75 μmol h^–1 g^–1. The amount of MoS₂ loaded on the ZnO surface plays an essential role in trapping the conduction band electrons injected by the zinc porphyrin dye (ZnTCPP*), thereby suppressing the recombination of photogenerated charge carriers. This study provides insight into developing noble metal-free visible-light responsive ZnO-based photocatalysts using zinc porphyrin dyes as a light-harvesting material and MoS₂ as a cocatalyst.⁶⁵

A photocatalytic H₂ evolution of ß-Tm³⁺, Yb³⁺: NaYF₄/MoS₂-Ta₂O₅ photocatalyst was reported by Shu et al. MoS₂ was deposited onto the surface of ß-Tm³⁺, Yb³⁺: NaYF₄/Ta₂O₅ nanocomposite as a cocatalyst. The presence of an up-conversion luminescence agent and MoS₂ cocatalyst could significantly increase the photocatalytic H₂ evolution rate of the Ta₂O₅ catalyst. Because of its outstanding electronic properties, MoS₂ provides more active sites to accept photogenerated electrons from the conduction band (CB) of Ta₂O₅ for H₂ evolution, reducing electron–hole recombination and increasing photocatalytic H₂ evolution activity of ß-Tm³⁺, Yb³⁺: NaYF₄/MoS₂-Ta₂O₅ catalysts.⁶⁶ A MoS₂/Bi₂O₃ Z-scheme heterojunction catalyst with rich oxygen vacancies was prepared in another study. The superior photocatalytic activity stemmed from oxygen vacancies that enhance the interfacial interaction between MoS₂ and Bi₂O₃.⁶⁷ Monolayer metal sulfides are promising for photocatalysis due to their excellent activity and high surface area. Especially their excellent charge carrier mobility shows high promise in photocatalysis.⁶⁸

For the first time, catalytic H₂ production and pollutant degradation catalyzed by MoS₂ monolayers loaded on the m-BiVO₄ surface were studied using hybrid density functional calculations and long-range dispersion correction methods. Interfacial adhesion energy, binding energy, and equilibrium distance indicate that the association of MoS₂ monolayer with m-BiVO₄ (010) is a van der Waals interaction, resulting in a high reduction capacity toward H₂ generation. With a high optical absorption coefficient, the monolayer of MoS₂ serves as a suitable electron acceptor, and more active sites can accept reactive species during the photocatalytic process.⁶⁹ Semiconductor photocatalysis through p-n heterojunctions is essential for improving future hydrogen storage. Swain et al. discovered a p-n heterojunction between p-type MoS₂ and n-type CeO₂ in a p-n heterojunction. The suggested p-n heterojunction method provides photoinduced charge transfer and separation. Following optimization of the overall experiment, it was discovered that the 2 wt % MoS₂/CeO₂ heterojunction nanocomposite has the most remarkable rate of H₂ evolution of 508.44 μmol h^–1.⁷⁰Table 1 summarizes the different MoS₂/metal oxide composites, the synthesis method, and photocatalytic parameters to produce H₂ from the photocatalytic H₂O splitting.

Table 1. Summarized List of Photocatalytic Hydrogen Evolution of MoS₂ with Metal Oxides and Carbonaceous Materials.

photocatalyst	synthesis method	light source	amount of catalyst	sacrificial reagent	H2 evolution activity	ref
yolk shell TiO2/ultrathin MoS2	hydrothermal	300 W Xe lamp	50 mg	80 mL aqueous solution contains 20 mL methanol	2443 μmol g–1 h–1	(59)
TiO2/MoS2	ball-milling	300 W Xe lamp	0.2 g	100 mL of 15% mixed methanol-H2O solution	150.7 μmol h–1	(62)
N-ZnO/defect rich MoS2	hydrothermal	natural sunlight	10 mg	50 mL aqueous solution contains 0.3 M Na2S and Na2SO4	17.3 mmol g–1 h–1	(63)
ZnO/MoS2	hydrothermal	300 W Xe lamp	100 mg	100 mL aqueous solution contains 0.1 M Na2S and 0.1 M Na2SO3	768 μmol g–1 h–1	(64)
Zn-porphyrin MoS2/ZnO	hydrothermal	300 W LED lamp	100 mg	100 mL aqueous solution of 0.02 M TEOA	75 μmol g–1 h–1	(65)
Bi2O3/MoS2	hydrothermal	300 W Xe lamp	25 mg	50 mL aqueous solution contains 0.02 mol L–1 TEOA	3075.21 μmol g–1 h–1	(67)
CeO2/MoS2	hydrothermal	150 W Xe lamp	20 mg	20 mL aqueous solution of methanol	508.44 μmol h–1	(70)
activated carbon/MoS2	liquid phase reaction	30 W white light LED lamp	0.1 g	200 mL 10 vol % TEOA aqueous solution	872.3 μmol h–1	(72)
g-C3N4/MoS2	hydrothermal	300 W Xe lamp	20 mg	20 mL of H2O-methanol (4:1 v/v) solution	1497 μmol g–1 h–1	(73)
hollow g-C3N4/MoS2	impregnation and sulfidation	300 W Xe lamp	20 mg	100 mL aqueous solution contains 10 vol % lactic acid	26.8 μmol h–1	(75)
g-C3N4/MoS2	impregnation method	300 W Xe lamp	0.1 g	120 mL of aqueous solution contains 25% methanol	23.10 μmol h–1	(76)
g-C3N4/MoS2	hydrothermal	300 W Xe lamp	50 mg	120 mL of aqueous solution contains 25% methanol	867.6 μmol g–1 h–1	(77)
g-C3N4/MoS2	hydrothermal	300 W Xe lamp	50 mg	250 mL 0.1 M TEOA aqueous solution	1155 μmol g–1 h–1	(78)

2.3.2. MoS₂/Carbonaceous Materials

To enhance the catalytic efficiency of MoS₂ nanosheets, different layered carbon-based materials, including graphene, activated carbon, carbon nanofibers, and carbon nanotubes, have been incorporated. The resulting 2D/2D heterostructures with large contact areas will significantly benefit catalysis. Interfacial engineering between the layered heterojunctions as interface electron channels increases the HER performance.⁷¹ Recently, Yin et al. reported an activated carbon/MoS₂ composite via a facile one-pot liquid phase reaction. The activity was mainly attributed to the in situ incorporation of activated carbon, resulting in an increased electrical conductivity due to the formation of thinner and smaller MoS₂ nanosheets.⁷² A hydrothermal sulfurization and liquid phase exfoliation method was used to synthesize the modified gC₃N₄-MoS₂ heterojunction for the first time. An excellent system for exposing active edge sites to photochemical processes was obtained by adjusting the lateral diameters of MoS₂ nanosheets from 18 to 52 nm. The optimized structure with 20 wt % MoS₂ loading and a lateral size of 39 nm had the maximum photocatalytic H₂ activity of 1497 μmol h^–1 g^–1 and a quantum efficiency of 3.3% at 410 nm.

The density of MoS₂ edge sites created at the interface of the MoS₂/gC₃N₄ heterojunction is responsible for this increased HER activity. The MoS₂ nanosheets with a large fraction of surface edge sites enhance the electron transfer.⁷³ Inorganic molten salts offer a suitable platform for high 2D/2D heterostructures. A 2D/2D gC₃N₄-MoS₂ visible-light-driven photocatalyst is developed via simple pyrolysis of melamine and (NH₄)₂MoS₄ in a ternary molten LiCl-NaCl-KCl solution at 550 °C. The surface area of the composite was about 57 cm² g^–1, more than that of pure gC₃N₄. MoS₂ assembly suppresses the overgrowth of gC₃N₄-nuclei and enhances the synthesis of a hybrid structure with a greater porosity and H₂ evolution efficiency.⁷⁴

A highly stable hollow carbon nitride nanosheet (HCNS) integrated MoS₂ was recently constructed by Zheng et al. for an H₂ evolution reaction. Applying lactic acid as a hole acceptor, the photocatalytic performance of MoS₂/HCNS nanocomposites for H₂ evolution was investigated. Upon 0.5 wt % MoS₂ loading, HCNS indicated an increase in the H₂ evolution rate of 26.8 μmol h^–1. The improved photocatalytic efficiency was ascribed to developing surface heterostructures across MoS₂ and HCNS, facilitating charge carrier separation and migration.⁷⁵ Ge et al. used a simple impregnation technique to produce MoS₂-gC₃N₄ composite photocatalysts. The maximum H₂ evolution rate of 23.10 μmol h^–1 was obtained for the 0.5 wt % MoS₂-gC₃N₄ composite.⁷⁶ Using a simple hydrothermal process, a unique 3D flowerlike hexagonal 2H-MoS₂ was produced as thin stacked nanosheets, which were then combined with gC₃N₄ nanosheets of the same layered structure. The synthesized samples exhibited excellent photocatalytic and electrocatalytic activity with lower overpotential and substantial current densities. The SEM micrographs of pristine MoS₂ demonstrated 3D flowerlike nanostructures having a diameter of 400–600 nm and thicknesses of 10 nm. Remarkably, the nano platelike subunits are linked with one another to develop 3D flowerlike networks. It overcomes disordered stacking of MoS₂ layers by disclosing many active edge sites and increasing the specific surface area to decrease the diffusion pathway for both reactants and surface charges. The MoS₂/gC₃N₄ composites have a thinner and fluffier microstructure than pure gC₃N₄, showing the introduction and deposition of MoS₂ nanoflowers within the framework of gC₃N₄ nanosheets (Figure 4a,b). The 0.5% composition has the maximum photocatalytic activity of 867.6 μmol h^–1 g^–1.

Figure 4.

(a) SEM images of MoS₂. (b) 0.5% of MoS₂/gC₃N₄ photocatalyst. Reproduced with permission from ref (77). Copyright 2018 Elsevier.

The photocatalytic activity was elevated to 2.8 times higher than pure gC₃N_4. Introducing a unique flowerlike MoS₂ structure boosted more increased and stable H₂ evolution activity. Several H⁺ ions in the solution effectively bond to unsaturated active “S” atoms along the edges, converting them into H₂.⁷⁷ Yuan et al. recently reported a 2D-2D MoS₂/gC₃N₄ photocatalyst having an H₂ evolution rate of 1155 μmol g^–1 h^–1. The photoluminescence lifetime (PL) decreased as the concentration of MoS₂ increased, confirming that MoS₂ plays a crucial role in the effective separation of charge carriers.⁷⁸ The chemical vapor deposition (CVD) process was used to synthesize 3D edge-oriented graphene with an edge-rich MoS₂ nanoarray corresponding to gC₃N₄. The high conductivity and transparency of the 3D graphene layer provide a large exposed surface area for incorporating many photocatalysts, and developed 3D graphene/E-MoS₂ structures exhibit superior optical and electrical characteristics. The maximum H₂ evolution activity of 2232.7 μmol g^–1 h^–1 was achieved under white light irradiation.⁷⁹ Binary composites of MoS₂-carbon nitride (MoS₂-CN) have recently been found as a substrate for piezo-photocatalytic hydrogen evolution reactions. The structure of the MoS₂ on CN significantly impacts the piezoelectric activity of the binary composites. The MoS₂ nanosheet acts as a piezo potential generator to induce an electric field on CN; thus, the recombination of the charge carriers is inhibited under low-frequency vortex vibration.⁸⁰Table 1 outlines the various MoS₂/carbonaceous composites and the synthesis methods and photocatalytic parameters used to produce H₂ by photocatalytic water splitting.

2.3.3. MoS₂/Metal Sulfides

Many metal chalcogenides, such as CdS, In₂S₃, Bi₂S₃, and CuS, have narrow band gaps and excellent absorption in the visible light range, making them suitable candidates for photocatalytic H₂O splitting to release hydrogen.⁸¹⁻⁸³ However, metal sulfides also had significant drawbacks, such as fast electron–hole pair recombination and serious photocorrosion in photocatalysis. MoS₂, as a catalyst in association with typical metal chalcogenides, can increase solar energy absorption in the visible range. For example, Li et al. demonstrated cost-effective and efficient photocatalysis based on MoS₂ and CdS nanospheres.⁸⁴ MoS₂ is intimately formed on CdS and has an unusually high H₂-evolution activity of 37.31 mmol g^–1 h^–1.

The agglomeration-resistant structure, low overpotential, and the large number of active sites of MoS₂ contribute to the exceptional photocatalytic efficiency of these heterostructures.⁸⁴ The HER activity of MoS₂ nanosheets is linked to the active “S” atoms on the exposed edges rather than the S atoms presented on the basal planes, and this can be increased further by reducing the layer numbers of MoS₂.⁸⁵ In particular, CdS-MoS₂ hybrids have attracted more attention, and several heterogeneous photocatalysts have been generated from CdS nanocrystals by integrating with MoS₂ cocatalysts. For example, few-layered MoS₂ nanosheets loaded with CdS nanorods produce a high H₂ evolution rate (1009 mmol h^–1 g^–1) with an apparent quantum efficiency (AQE) of 71% at 420 nm. The few-layer MoS₂ edges attached to CdS contain the more active sites, minimizing the distance and resistance of the transfer of electrons from the basal plane for the H₂ evolution process.⁸⁶

Ma et al. also developed a 2D-2D CdS/MoS₂ heterojunction for photocatalytic H₂ evolution reaction. Under visible light, the heterojunctions could attain hydrogen evolution activity of 1.75 mmol g^–1 h^–1, using an aqueous solution of sulfide and sulfate, and this became 2.03 times greater than pure CdS NSs. The increased activity can be due to the loading of MoS₂ NSs, which effectively promotes the separation of charge carriers.⁸⁷ A possibility of photocatalytic H₂ evolution using biomass-derived sacrificial reagents such as glucose has been applied to a series of flowerlike CdS/MoS₂ binary heterostructure composites. After 900 min of continuous testing, the 40% CdS/MoS₂ catalyst achieved the maximum H₂ evolution rate (55.0 mmol g^–1 h^–1) and maintained high stability. The synergistic interaction of CdS and MoS₂ facilitates effective charge carrier separation. Furthermore, glucose performed as a better sacrificial agent, creating more protons during photocatalysis.⁸⁸ Doping of MoS₂ with heteroatoms is a simple and effective approach for producing an excellent catalyst for H₂ production from water splitting. A Zn-doped few-layered MoS₂ on the surface of CdS was reported. The doped MoS₂ material was demonstrated to be a good cocatalyst for enhancing the photocatalytic activity of CdS. It also facilitates photoinduced charge transfer by providing many active edge sites for catalytic reactions and increasing long-term stability for useful applications. The optimized structure (6 wt % Zn-MoS₂) shows the maximum catalytic activity of 241 mmol h^–1 g^–1, which is ∼75 times more than pure CdS.⁸⁹

Similarly, Guo et al. synthesized the nickel subsulfide Ni₃S₂/MoS₂ heterostructure. The Ni₃S₂ nanostructures were developed on Ni foam via a sulfuration approach by the CVD method, and MoS₂ was formed along with the Ni₃S₂ nanostructures via a hydrothermal process. After synthesizing, the MoS₂ achieved direct interaction with the Ni₃S₂ structure, resulting in effective interfacial interaction. The hybrid enhances broadband absorption covering 300 to 800 nm, yielding a significant increase in hydrogen evolution (540.75 μmol g^–1 h^–1).⁹⁰ Zhao et al. synthesized CdS nanorods with MoS₂ nanosheets, and the composite achieved an H₂ production rate of 63.71 and 71.24 mmol g^–1 h^–1 in visible light and stimulated sunlight irradiation, respectively. The resulting heterojunction effectively separates photogenerated holes and electrons. Decorating CdS and MoS₂ helped to reduce the photocorrosion of CdS nanorods by stabilizing the sulphions.⁹¹ Similar results were obtained in another CdS/MoS₂ heterojunction.⁹²

Multicomponent sulfides form heterostructures with MoS₂ received considerable attention for applications in photocatalytic hydrogen production.^93,94 By adding a transition metal into the CdS solid solution, photocorrosion of CdS can be eliminated and transition metal doped CdS can be formed. Among these, Cd_xZn_1–xS takes more attention due to its excellent photocatalytic activity.

By combining with MoS₂, a new Cd_xZn_1–xS/MoS₂ heterojunction was synthesized by an electrostatic self-assembly method.⁹⁵ Similar results were also obtained for a 2D/2D ZnIn₂S₄/MoS₂ composite, achieving a H₂ evolution rate of 221.71 μmol h^–1 was achieved under visible light irradiation. The close contact of ZnIn₂S₄ with MoS₂ offers several channels for carrier migration. Furthermore, the active S atoms at exposed edges of MoS₂ give more active sites for proton reduction.⁹⁶ Whereas Chand et al. adopted a two-step process including hydrothermal and simple chemical methods to fabricate MoS₂-ZnIn₂S₄ composites. The H₂ production rate was also increased by adding MoS₂ as a cocatalyst. MoS₂ is used as an oxidation catalyst to strengthen the oxidation half-cell reaction and synchronously strengthen the reduction half-cell reaction; thus, the overall photocatalytic H₂ evolution is enhanced.⁹⁷

Many other reports have been published incorporating layered MoS₂ with ternary chalcogenides. Hexagonal ZnIn₂S₄ has received more attention due to its structural similarity with MoS₂. It is facile to develop an intimate 2D/2D ZnIn₂S₄/MoS₂ heterojunction by growing a layer of MoS₂ on the hexagonal ZnInS₄ surface. The CB position of MoS₂ was less negative than that of ZnIn₂S₄, resulting from the direct migration of photogenerated electrons possible through ZnIn₂S₄ to MoS₂.

The optimized 5% MoS₂/ZnIn₂S₄ composition exhibits a hydrogen production rate of 3891.6 μmol g^–1 h^–1.⁹⁸ Huang et al. adopted a two-step hydrothermal method to construct the ZnIn₂S₄/MoS₂ composite. The interaction of MoS₂ can effectively promote electron–hole separation and extends the photocatalytic lifetime.⁹⁹ For the first time, a solvothermal process was adopted for the controlled growth of ZnIn₂S₄ nanosheets on few-layered MoS₂ slices to construct the MoS₂/ZnIn₂S₄ hybrid. The exfoliated 2D platform framework of MoS₂ slices can be an excellent supporting medium for the in situ growth of ZnIn₂S₄ nanosheets, enhancing interfacial charge transfer and minimizing self-agglomeration.¹⁰⁰ Other reports also showed the highly enhanced photocatalytic performance for the H₂-evolution of ZnIn₂S₄/MoS₂ composites.^101,102

The ZnIn₂S₄/MoS₂ composite was synthesized using an environmentally friendly biomolecule-assisted microwave heating method. Using visible light irradiation, a ZnIn₂S₄/MoS₂ composite having an optimized weight percentage of MoS₂ was used to reduce harmful Cr (VI) ion to Cr (III) ion.

Furthermore, even following three cyclic tests, this catalyst demonstrated high stability. The ZnIn₂S₄ and MoS₂ combine to produce a type II heterostructure responsible for the increased activity.¹⁰³ Similarly, flowerlike p-MoS₂/n-MgIn₂S₄ hybrids were synthesized and studied for photocatalytic efficiency in H₂-evolution and N₂-reduction. By inducing an internal electric current at the interface of two photocatalysts, the p-n heterojunction provides an excellent potential for separating photogenerated charge carriers. MoS₂ nanosheets provide many exposed active sites through S–S bonding on the edges and offer efficient excitons separation.¹⁰⁴ Swain et al. developed another type II heterostructure with excellent charge separation efficiency. A set of roselike p-MoS₂ nanoflowers are decorated with n-type cubic CaIn₂S₄ (c-CIS) micro flowers incorporating different amounts of MoS₂. Highly dispersed MoS₂ nanoflowers with more active edge sites provided the platform for growing CaIn₂S₄, resulting in a hierarchical network (MoS₂/c-CIS) with intimate contact. The p-n heterojunction character of the composite is revealed by electrochemical studies such as Mott–Schottky analysis and LSV curves. The lower slope of 0.5 wt % p-MoS₂/n-CaIn₂S₄ compared to pure CaIn₂S₄ indicated the increased donor density capacity, which was the primary source of the photocatalytic performance (Figure 5a). When the potential was applied, pure c-CIS showed a very low photocurrent, i.e., 0.09 mA, although all composites showed a significant photocurrent. The plot is asymmetrical in forward and reversed biasing (Figure 5b). The modified photocatalysts comprising 0.5 wt % p-MoS₂/n-CaIn₂S₄ had a higher H₂ generation rate of 602.35 μmol h^–1, having photocurrent densities of 0.743 mA cm^–2. The type II heterojunction photocatalyst was developed to improve charge separation effectiveness by shifting electrons from the upper Fermi level of n-type c-CIS to the lower Fermi level p-type MoS₂ (Figure 5c).¹⁰⁵Table 2 outlines the various MoS₂/metal sulfides composites and the synthesis methods and photocatalytic parameters used to produce H₂ by photocatalytic water splitting.

Figure 5.

(a) Mott–Schottky plots of pure c-CIS and 0.5% MoS₂/c-CIS. (b) LSV curves of pure c-CIS and different MoS₂/c-CIS composites. (c) Possible charge transfer pathway for H₂ evolution by p–n junction mechanism. Reproduced from ref (105). Copyright 2018 American Chemical Society.

Table 2. Summarized List of Photocatalytic Hydrogen Evolution of MoS₂/Metal Sulfide Composites and MoS₂/MOFs Composites.

photocatalyst	synthesis method	light source	amount of catalyst	sacrificial reagent	H2 evolution activity	ref
CdS/MoS2	hydrothermal	300 W Xe lamp	20 mg	80 mL aqueous solution contains 8 mL lactic acid	37.31 mmol g–1 h–1	(84)
CdS/MoS2	hydrothermal	300 W Xe lamp	20 mg	100 mL aqueous solution contains lactic acid	1009 mmol g–1 h–1	(86)
CdS/MoS2	hydrothermal	300 W Xe lamp	50 mg	80 mL aqueous solution contains 0.5 mol L–1 Na2S and 0.5 mol L–1 Na2SO3	1.75 mmol g–1 h–1	(87)
CdS/MoS2	hydrothermal	300 W Xe lamp	0.1 g	100 mL aqueous solution contains 0.1 M glucose	55 mmol g–1 h–1	(88)
Ni3S2/MoS2	CVD and hydrothermal	150 W Xe lamp	0.05 g	40 mL distilled H2O contains 0.1 mol L–1 Na2S and 0.1 mol L–1 Na2SO3	540.75 μmol g–1 h–1	(90)
ZnIn2S4/MoS2	electrostatic self-assembly	visible light	–	36 mL deionized H2O + 4 mL lactic acid	4.97 mmol g–1 h–1	(93)
ZnIn2S4/MoS2	wet chemical method	300 W Xe lamp	25 mg	90 mL DIW H2O + 10 mL TEA	8898 μmol g–1 h–1	(94)
ZnIn2S4/MoS2	solvothermal	300 W Xe lamp	80 mg	100 mL aqueous solution contains 0.35 M Na2S and 0.25 M Na2SO3	3891.6 μmol g–1 h–1	(98)
p-MoS2/n-ZnIn2S4	hydrothermal	150 W Xe lamp	20 mg	100 mL aqueous solution contains 0.35 M Na2S and 0.25 M Na2SO3	320.2 μmol h–1	(102)
ZnIn2S4/MoS2	hydrothermal	150 W Xe lamp	0.1 g	100 mL aqueous solution contains 0.35 M Na2S and 0.25 M Na2SO3	200.1 μmol g–1 h–1	(103)
MgIn2S4/MoS2	hydrothermal	150 W Xe lamp	20 mg	20 mL aqueous solution contains 0.35 M Na2S and 0.25 M Na2SO3	570.8 μmol h–1	(104)
CaIn2S4/MoS2	hydrothermal	150 W Xe lamp	20 mg	20 mL aqueous solution contains 0.025 M Na2S and 0.025 M Na2SO3	602.35 μmol h–1	(105)
MoS2-MOF/Co3O4	hydrothermal	300 W Xe lamp	10 mg	15% (v/v) TEOA (30 mL) aqueous solution	629.75 μmol	(106)
MoS2@Cd-MOF	solvothermal	300 W Xe lamp	30 mg	150 mL aqueous solution contains 35 mM Na2S and 18 M Na2SO3	5587 μmol g–1 h–1	(108)
Cu0.9Co2.1S4@MoS2	hydrothermal	300 W Xe lamp	–	aqueous solution contains TEOA	40156 μmol g–1 h–1	(110)

2.3.4. MoS₂/Metal–Organic Frameworks (MOFs)

Metal–organic frameworks (MOFs), a new class of porous and crystalline materials exhibiting massive coordination centers, have found significant applications in photocatalysis.^106,107 The photocatalytic activity of MoS₂ in water splitting for H₂ evolution can be enhanced by constructing heterogeneous interfaces with MOFs or MOF-derived metal oxides or sulfides. The sulfurization of the core–shell MoS₂@Cd-MOF constructed a new CdS/MoS₂ heterojunction. The CdS/MoS₂ presents uniform loading of CdS nanoparticles on the MoS₂ flowers. The composite exhibited a higher specific surface area of 78 m² g^–1, and the optimized CdS/MoS₂ heterojunction showed an average H₂ evolution rate of 5587 μmol g^–1 h^–1. In the heterojunction, the conduction band minimum of MoS₂ is lower than that of CdS, so the photoexcited electrons transferring from CdS to MoS₂ become possible. The MoS₂ was a cocatalyst to trap photoexcited electrons and holes and improve photocatalytic activity.¹⁰⁸

MoS₂ combined with terephthalic acid to form MoS₂-MOF and Co₃O₄ was deposited on these catalytic surfaces in methanol solution under light irradiation to form a MoS₂-MOF/Co₃O₄ photocatalyst. MoS₂-MOF constitutes a new type of catalyst that could provide a large specific surface area, pore size, and active sites to improve electron transport efficiency. The deposition of Co₃O₄ further accelerates the charge separation. The MoS₂-MOF structure appears as a leaf, and Co₃O₄ is structured into this system as a venation. This catalytic system has an excellent fluorescence quenching rate and lifetime, and photocatalytic hydrogen evolution reached 629.75 μmol using an eosin Y-sensitized system; this is a successful example of preparing an organic framework composed of metal sulfides as an H₂ evolution photocatalyst.¹⁰⁶ A simple hydrothermal approach developed a unique 3D nanofibrous aerogel-based MoS₂@Co₃S₄ composites. The heterojunction showed an exceptional H₂-evolution activity of 228.2 μmol g^–1 h^–1 and delivered excellent photocatalytic dye degradation toward Cr (VI), sulfamethoxazole, and bacteria.¹⁰⁹ A bimetallic metal–organic frameworks catalyst was coupled with MoS₂ forming a heterojunction of the composition Cu_xCo_3-xS₄@MoS₂. After optimization, the flowerlike Cu_0.9Co_2.1S₄@MoS₂ catalyst delivered 40156 μmol g^–1 h^–1 of visible light H₂ production. These structures of Cu_0.9Co_2.1S₄@MoS₂ hybrid can fully expose the catalytic sites and maintain structural stability over a long period. The photocatalytic mechanism sensitized by eosin Y (EY) proceeds via a reductive electron transfer process. TEOA reductively quenches the excited EY molecules to produce EY*. Subsequently, electrons were transferred from the EY* molecule to the conduction band of Cu_0.9Co_2.1S₄@MoS₂, where the protons are reduced to form molecular H₂ (Figure 6a). The stability of the photocatalyst was retained steadily after three consecutive cycles with an apparent quantum efficiency of about 3.2% (Figure 6b)¹¹⁰ (Table 2). A MOF-199 modified MoS₂ heterojunction was reported by Qiao et al. to achieve high H₂ evolution via water splitting (626.3 μmol g^–1 h^–1). The synthesized MOF/MoS₂ composites show the combined properties of MoS₂ and MOF; thus, the active sites can be significantly enhanced. Here the MOF-199 acts as a photosensitizer to assist MoS₂ in harvesting visible light by transferring photoexcited electrons to MoS₂.¹¹¹

Figure 6.

(a) Photocatalytic process for H₂ production on Cu_0.9Co_2.1S₄@MoS₂ photocatalyst using visible-light irradiation. (b) Stability tests of Cu_0.9Co_2.1S₄@MoS₂ in hydrogen evolution for three cycles under visible irradiation. Reproduced from ref (110). Copyright 2019 American Chemical Society.

2.3.5. Ternary Composites of MoS₂

For photocatalytic H₂ evolution processes, MoS₂-based ternary materials have received considerable interest. Because of its numerous active sites and efficient separation of charge carriers, the MoS₂ ternary systems produce more H₂ than binary composites.^112,113 Zhu et al. has synthesized a ternary CeO₂@MoS₂/gC₃N₄ catalyst with excellent photocatalytic performance for H₂ production. The optimized 0D/2D composite produced a short and efficient multielectron transfer mechanism. The H₂ evolution rate achieved 65.4 μmol/h, roughly 8.3 and 17.5-fold higher than pure gC₃N₄ and CeO₂. The composite also exhibited an extended quantum efficiency of 10.35% at a wavelength of 420 nm.¹¹⁴ Transition metal doping (Co) of MoS₂ effectively tunes the geometry and electronic structure for enhancing HER activity. A ternary CoMoS₂/rGO/gC₃N₄ was constructed by a solvothermal method. The synergistic effect between the nanosheets of gC₃N₄, rGO, and Co-doped MoS₂ enhances the HER activity rate to 684 μmol g^–1 h^–1. The enriched electrons on CoMoS₂ with abundant active sites and lower atomic hydrogen absorption energy demonstrated high charge separation efficiency for reducing the protons to H₂.¹¹⁵ In another study, a multifunctional ternary photocatalyst of MoS₂ was introduced by Kumar et al. They developed a photocatalytic and electrocatalytic H₂ production reaction for the ternary ZnO/MoS₂/RGO heterojunction. As shown in Figure 7a, H₂ evolution increases with increasing the MoS₂-RGO content. The optimized composition achieved the highest H₂ evolution (28.616 mmol g^–1 h^–1). Because it comprises a layered structure containing exposed S atoms at its edge sites with a high affinity toward H⁺ ions, the nanosized MoS₂ facilitates H₂ evolution (Figure 7b). During HER reaction, ZnO dissolves in an alkaline sulfide solution and the in situ-generated ZnS enhances interfacial charge transport to MoS₂ and RGO. The photocatalytic process also indicates that the synergistic effect of RGO-MoS₂ nanosheets and in situ formed ZnS efficiently reduce charge recombination in ZnO and significantly accelerates the H₂ evolution reaction (Figure 7c).¹¹⁶ In another system, a noble metal-free heterostructured WS₂-MoS₂ integrated on CdS nanorods served as active sites for H₂ production. By accelerating the photoinduced charge transfer and reducing electron–hole recombination, the WS₂-MoS₂ heterostructure ultrathin nanosheets became an efficient cocatalyst for enhancing the photocatalytic hydrogen evolution rate.¹¹⁷ CdS nanospheres with MoS₂ and NiP ternary nanohybrids were synthesized for the first time by using a hydrothermal process and a MOF-templated approach. The composite has a high photocatalytic H₂ evolution capacity of 72.76 mmol h^–1 g^–1 under optimum conditions.¹¹⁸

Figure 7.

(a) H₂ volume evolved in various ternary ZnO/MoS₂/RGO photocatalysts. (b) Layered structure of nanosized MoS₂ showing exposed edges of sulfur atoms for H⁺ ions in aqueous solution for HER performance. (c) A plausible mechanism for enhanced electron transfer across the ZnO-MoS₂-RGO heterojunction under solar irradiation using Na₂S-Na₂SO₃ as a sacrificial reagent. Reproduced with permission from ref (116). Copyright 2017 Wiley.

Yu et al. described a biomolecule-assisted synthesis of CdS/MoS₂/graphene hollow spheres. l-Cysteine worked as a sulfur source and was important for the self-assembly of the graphene structure to produce the ternary heterostructure. The CdS/MoS₂ heterojunction developed by in situ MoS₂ growth offers better contact. It allows more photogenerated electrons in CdS to move to the MoS₂ layer and combine with H⁺ to produce H₂ at considerably lower overpotentials.¹¹⁹ Similar results were obtained when MoS₂ was modified with CdS and PANI.¹²⁰ A ternary structure recently comprised of MoS₂/SiO₂/GO was reported as a photocatalytic degradation and H₂ evolution catalyst. The as-synthesized lotuslike ternary photocatalyst has a suitable energy band structure for removing cotton pulp black liquor (CPBL) and simultaneously produces H₂ (233.4 μmol). The loading of MoS₂ onto this SiO₂/GO structure dramatically improves the dispersion and specific surface area of MoS₂, endowing its superior photocatalytic activity.¹²¹ As an excellent noble metal-free photocatalyst for H₂ generation, ternary MoS₂/ZnCdS/ZnS dual nanocomposites were fabricated. The material produces H₂ at a 79.3 mmol g^–1 h^–1 rate and an apparent quantum yield of 47.9 at 420 nm. MoS₂ has a higher specific perimeter and many active sites and easily separates and transports electron–hole pairs with enhanced photocatalytic performance.¹²² A chemical vapor deposition (CVD) method was adopted to construct a novel MoO₂/MoS₂/TiO₂ ternary structure. Here few-layered MoS₂ nanoflakes were deposited on TiO₂ nanotubes, enabling the directional transfer of photogenerated charge carriers and showing nearly full spectrum absorption with excellent photocatalytic activity for H₂ evolution.¹²³ Feng et al. described a binary Mn_0.2Cd_0.8S/Co₃O₄ transformed to ternary Mn_0.2Cd_0.8S/MoS₂/Co₃O₄. By producing a type-II heterojunction, the synergistic interaction of the Mn_0.2Cd_0.8S/Co₃O₄ p–n junction and MoS₂ enhance photocatalytic H₂ production. The few-layered MoS₂ served as a cocatalyst with a low CB position, providing electron transport from the CB of Mn_0.2Cd_0.8S to the CB of MoS₂ and Co₃O₄, respectively.¹²⁴

Systematic incorporation of the different constituents in a ternary composite was essential for increasing solar absorption and H₂ evolution reactions. Liu et al. constructed an Au-Cu nanoalloy/TiO₂/MoS₂ ternary composite by combining the Au-Cu nanoalloy and TiO₂/MoS₂ nanosheets. The high surface area of the TiO₂/MoS₂ hybrid improves sunlight absorption, and the Au-Cu nanoparticles serve as a bridge to connect the MoS₂ and TiO₂ via the surface plasmon resonance phenomenon.¹²⁵ Chen et al. synthesized a CdS/MoS₂/Mxenes ternary hybrid for the first time. When irradiated to visible light, the compound exhibits remarkable photocatalytic activity, with an H₂ production rate of 9679 μmol g^–1 h^–1.¹²⁶ Another MoS₂/CQDs/ZnIn₂S₄ nanocomposites exhibit enhanced H₂ evolution activity with an apparent quantum efficiency of 25.6%.¹²⁷ The separation of photogenerated charge carriers and the resulting photocatalytic activity was considerably enhanced by the fabrication of 0D/2D heterostructures. Guan et al. developed a ZnIn₂S₄/MoS₂-RGO nanosheets 0D/2D nanocomposite through a fast, low-temperature hydrothermal process. The MoS₂/RGO nanosheets support inhibiting 0D ZnIn₂S₄ nanoparticles from aggregation in the ternary heterostructure, resulting in a significantly higher H₂ generation than pure ZnIn₂S₄. The ZnIn₂S₄/MoS₂-RGO composite benefited from the superior visible light absorption characteristics of MoS₂ and RGO.¹²⁸ The sheetlike MoS₂/CdS/TiO₂ ternary structure exhibited exceptional stability and a high H₂ evolution rate of 4146 μmol g^–1 h^–1 under visible light. In this case, the conduction band of MoS₂ enhances the transfer of photoexcited electrons from sunlight on CdS to porous TiO₂ for photocatalytic H₂ generation. Conducting MoS₂ can increase the conductivity of the photocatalyst and reduce the photocorrosion of CdS as a hole collector.¹²⁹ Another ternary composite with similar results includes CuInS₂QDs/TiO₂/MoS₂ and gC₃N₄/red phosphorus/MoS₂. The H₂ evolution activity in these structures was 1034 μmol g^–1 h^–1 and 257.9 μmol g^–1 h^–1, respectively.^130,131 The effective electrospinning and photodeposition process developed a unique ternary structure, MoS₂/CdS-TiO₂ nanofiber. The electro-spun TiO₂ nanofibers have the advantages of improved CdS and MoS₂ dispersion. The influence of MoS₂ deposition on H₂ production was investigated. When the concentration of MoS₂ increases from 0.5 to 1%, the H₂ evolution rate also increases. The photocatalytic performance for H₂ evolution in the 1% MoS₂/CdS-TiO₂ sample is significantly higher (280.0 μmol h^–1) than others (Figure 8a). The active hydrogen sites of MoS₂ can minimize the overpotential and enhance photocatalytic H₂ evolution. On the basis of the process (Figure 8b), photoexcited electrons can instantly be transported to the H₂ evolution active sites of MoS₂ and indirectly migrated to MoS₂ through TiO₂ as a bridge, enhancing the H₂ evolution rate. The unique structure of the heterojunction accomplished this and close intrafacial contact.¹³² Ternary composites of CdS/MoS₂/MWCNTs composite photocatalyst were reported by Jo et al. The few-layered MoS₂ nanosheets exhibit high absorption and photoluminescence. A transient photocurrent density of 11.12 mA cm² illustrates the charge separation ability of the hybrid, confirming the transport of electrons from photoexcited CdS to MoS₂ edges along MWCNTs.¹³³ Similar results were obtained for the bifunctional ternary heterostructure In₂S₃/MoS₂/CdS composite.¹³⁴ The defect-rich MoS₂ in Cd_1–xZn_xS/MoS₂/graphene hollow spheres performs as a low-cost cocatalyst to accelerate the photocatalytic H₂ evolution reaction. Few-layered MoS₂ having exposed active edges can take electrons directly from CdS or as an electron transfer medium via graphene. This heterostructure obtained an optimal H₂ production activity of 2.97 mmol h^–1 g^–1 (Table 2).¹³⁵ Furthermore, photocatalysts for HER over ZnIn₂S₄ were synthesized using MoS₂ nanosheets on the surface of hollow carbon spheres (C/MoS₂) and Co₃O₄ nanoparticles. The optimized C/MoS₂@ZnIn₂S₄/Co₃O₄ composite shows a photocatalytic H₂ evolution rate of 6.7 mmol g^–1 h^–1. The enhanced photocatalytic activity is due to a dual photo generated process in which C/MoS₂ produces a Schottky heterojunction involving ZnIn₂S₄ and ZnIn₂S₄ which makes an S-scheme heterojunction with Co₂O₄. Additionally, the core–shell structure of the composite encourages intimate contact between constituents, enabling the separation of charge carriers.¹³⁶ Similar observations were reported in another ZnIn₂S₄ ternary structure containing hollow carbon spheres (HCSs) with MoS₂. The synergistic effect of MoS₂ and HCSs effectively facilitates H₂ production.¹³⁷ Ran et al. fabricated a microporous “S” doped NH₂-UiO-66 bridged ZnIn₂S₄/MoS₂ sheet structure (MS/ZIS) photocatalyst. The synthesized MS/ZIS photocatalyst exhibits a relatively high H₂ production rate of 5.69 mmol h^–1 g^–1. The “S” doped NU66 creates a new electron transport path, which benefits electron transport to the surface and active edge sites of MoS₂.¹³⁸

Figure 8.

(Left) Photocatalytic H₂ evolution rate on MoS₂/CdS-TiO₂ nanofibers. (Right) Schematic representation of MoS₂/CdS-TiO₂ nanofibers for photocatalytic H₂ evolution in the presence of visible light irradiation (λ ≥ 420 nm). Reproduced with permission from ref (132). Copyright 2017 Elsevier.

A two-step solvothermal approach followed by an in situ deposition technique was used to generate the oxygen-incorporated MoS₂ (O-MoS₂) coated on 1D Cd_1–xZn_xS with NiO_x ternary nanostructures. NiO_x nanoparticles were effectively grown on Cd_1–xZn_xS@O-MoS₂ to construct unique Cd_1–xZn_xS@O-MoS₂/NiO_x composites. By exchanging the S-sites, oxygen is integrated into the MoS₂ lattice. The longitudinal dimension of 1D CdZnS nanocrystals may serve as an effective channel. If the produced O-MoS₂ contains many structural defects, the photoexcited electrons in CdZnS and O-MoS₂ may also transfer to NiO_x, extending the recombination procedure considerably (Figure 9).¹³⁹ For photocatalytic H₂ production, a ternary heterostructure of Ti₃C₂/MoS₂/CdS was produced. In this report, MoS₂ was formed in situ on Ti₃C₂ via electrostatic self-assembly, and hydrothermal techniques exhibited a vertically aligned framework suitable for more active edge sites for H₂ production. This binary structure served as a cocatalyst for H₂ evolution. It increases electron transport and prevents the recombination of electron–hole pairs formed from CdS by availing of the synergistic interactions of the MoS₂/Ti₃C₂ structures.¹⁴⁰

Figure 9.

Schematic showing the photocatalytic HER over CZ_0.15S@O-MoS₂/NiOx. Reproduced with permission from ref (139). Copyright 2019 Wiley.

Noble metals are generally recognized as excellent cocatalysts for photocatalytic H₂O splitting reactions. A substantial surface plasmon resonance (SPR) phenomenon can contribute to a greater absorption cross-section with higher energy density of trapped electrons, resulting in rapid H₂ production in photocatalytic splitting of water.^141,142 Ternary composites of MoS₂ containing noble metals have attracted much attention due to its high H₂ evolution activity. An Au nanoparticle deposited ternary Au-MoS₂/Ti₃C₂ photocatalyst was prepared using a hydrothermal method. The synthesized composite can modify electron transport paths. Due to the surface plasmon resonance effect of Au nanoparticles, the deposited Au nanoparticles on the surface of MoS₂ were excited, thereby improving the photocatalytic hydrogen evolution rate.¹⁴³ Similarly, the Pt-decorated ternary system, MoS₂/Pt-TiO₂, was efficiently developed by depositing MoS₂ nanosheets to Pt-TiO₂ frameworks. Appropriate ratios of MoS₂ and Pt inhibited the recombination of photogenerated charge carriers. The rate of H₂ evolution reaches 4.18 mmol h^–1 g^–1, which is around 41.8 times higher than pure TiO₂. This unique method provides a path for transferring excited electrons from TiO₂ particles using MoS₂ and Pt nanoparticles as a link to facilitate proton reduction into H₂.¹⁴⁴ The sequential attachment of Au nanocrystals and CdS onto two-dimensional exfoliated MoS₂ nanosheets (e-MoS₂) was successfully fabricated by forming internally coupled plasmonic metal semiconductor hybrids. Due to their distinct band structure, the CdS and exfoliated MoS₂ will have a wide optical absorption range. Moreover, its 2D morphology allows e-MoS₂ to construct hybrids effectively with exposed surfaces.¹⁴⁵ Similar results were observed for the compound AgInZnS/MoS₂-GO, which exhibited the maximum hydrogen evolution activity of 1302 μmol h^–1. The higher H₂ evolution activity of the composite arises due to the synergetic effect of graphene and MoS₂, where graphene facilitates the charge transfer, and MoS₂ provides more active sites for H₂ evolution.¹⁴⁶ The Au nanoparticle-incorporated ternary CdS-Au/MoS₂ core–shell hollow structure demonstrated the strongest visible-light photocatalytic H₂ evolution. Electron and hole recombination was efficiently reduced after the formation of heterostructures by CdS, MoS₂ nanosheets, and Au NPs, resulting in increased H₂ generation. Figure 10 illustrates the reaction pathway for H₂ evolution. The SPR effects of Au NPs stimulate photon absorption and accelerate electron transport, acting as an electron mediator from CdS to MoS₂. During visible light excitation, the electrons produced by CdS are transported into the CB of MoS₂ nanosheets via the Au NPs. MoS₂ nanosheets are electronic sinks and provide active sites for the photocatalytic H₂ evolution reaction.¹⁴⁷

Figure 10.

Schematic illustration for the charge separation and enhanced H₂ evolution activity in CdS-Au/MoS₂ composites. Reproduced from ref (147). Copyright 2018 American Chemical Society

In another study, AuNPs@MoS₂/ZnO nanorod hybrid structure has been successfully synthesized and shows excellent stability for H₂ production. The MoS₂ seemed like a monodispersed sphere with a diameter of ∼400 nm and composed of tangled MoS₂ nanosheets. Introducing Au NPs into MoS₂ microspheres results in a core–shell hybrid with improved plasmonic absorption. Adding ZnO nanorods into this system further enhances optical absorption. Mutual interactions between Au NPs (or ZnO nanorods) and MoS₂ spheres efficiently prevent MoS₂ nanosheets from peeling off from the spheres. Thus, after a 32 h test, the hybrid exhibited promising H₂ production activity of 13.56 μmol g^–1 min^–1 with increased stability of 91.9%. This is one of the highest photocatalytic activities among the MoS₂-based photocatalysts.¹⁴⁸Table 3 summarizes the different MoS₂/noble metal composites, the synthesis method, and the photocatalytic parameters for producing H₂ from the photocatalytic water splitting.

Table 3. Summarized List of Photocatalytic Hydrogen Evolution of Ternary Composites of MoS₂.

photocatalyst	synthesis method	light source	amount of catalyst	sacrificial reagent	H2 evolution activity	ref
CeO2/MoS2/g-C3N4	hydrothermal	3 W UV LEDs	50 mg	80 mL aqueous solution containing 0.5 M Na2SO3 and 0.5 M Na2S	65.4 μmol h–1	(114)
CoMoS2/rGO/C3N4	solvothermal	300 W Xe lamp	100 mg	mixed solution of TEOA and H2O (volume ratio equal to 1/5)	684 μmol g–1 h–1	(115)
ZnO/MoS2/RGO	hydrothermal	natural sunlight	5.0 mg	45 mL aqueous solution contains 0.1 M Na2S and 0.1 M Na2SO3	28.61 mmol g–1 h–1	(116)
CdS/WS2/MoS2	hydrothermal	150 W Xe lamp	1.0 mg	20 vol % lactic acid	209.79 mmol g–1 h–1	(117)
CdS/Ni2P/MoS2	hydrothermal	150 W Xe lamp	1.0 mg	15 mL aqueous solution containing 3 mL lactic acid	72.76 mmol g–1 h–1	(118)
CdS/MoS2/graphene	hydrothermal	300 W Xe lamp	30 mg	45 mL aqueous solution + 5 mL lactic acid	1913 μmol g–1 h–1	(119)
MoO2/MoS2/TiO2	CVD	300 W Xe lamp	–	100 mL aqueous solution contains 0.5 M ethylene glycol and 0.5 M Na2SO4	110 μmol h–1 cm–2	(123)
Mn0.2Cd0.8S/MoS2/CoO4	solvothermal	300 W Xe lamp	0.05 g	100 mL aqueous solution contains 0.5 mol L–1 Na2S and 0.5 mol L–1 Na2SO3	16.45 mmol g–1 h–1	(124)
Au-Cu nanoalloy-TiO2/MoS2	hydrothermal and annealing	300 W Xe lamp	5 mg	aqueous solution contains 0.35 M Na2S and 0.25 M Na2SO3	0.75 mmol g–1 h–1	(125)
MXene/CdS/MoS2	hydrothermal	300 W Xe lamp	5 mg	50 mL aqueous solution contains 0.25 M Na2S and 0.35 M Na2SO3	9679 μmol g–1 h–1	(126)
ZnIn2S4/CQDs/MoS2	hydrothermal	300 W Xe lamp	50 mg	100 mL aqueous solution contains 10 mL TEOA	133 μmol h–1	(127)
ZnIn2S4/MoS2/RGO	hydrothermal	300 W Xe lamp	50 mg	100 mL 20% v/v lactic acid solution	425.1 μmol g–1 h–1	(128)
CuInS2 QDs/TiO2/MoS2	ultrasonic dispersion and annealing	300 W Xe lamp	50 mg	250 mL aqueous solution contains 0.1 M Na2S and 0.1 M Na2SO3	1034 μmol g–1 h–1	(130)
g-C3N4/red phosphorus/MoS2	in situ photodeposition	300 W Xe lamp	10 mg	9 mL aqueous solution contains 1 mL TEOA	257.9 μmol g–1 h–1	(131)
CdS/TiO2/MoS2	electrospinning andphotodeposition	300 W Xe lamp	20 mg	80 mL aqueous solution contains 10 vol % lactic acid	280 μmol h–1	(132)
CdS/MoS2/MWCNTs	hydrothermal	300 W Xe lamp	20 mg	45 mL H2O + 5 mL lactic acid	676.0 μmol g–1 h–1	(133)
Cd0.8Zn0.2S/MoS2/graphene	hydrothermal	300 W Xe lamp	50 mg	96 mL H2O + 4 mL lactic acid	2.97 mmol g–1 h–1	(135)
NH2-UiO-66 bridged ZnIn2S4/MoS2	hydrothermal	300 W Xe lamp	40 mg	80 mL aqueous solution contains 10% TEOA	5.69 mmol g–1 h–1	(138)
Cd1–xZnxS@OMoS2/NiOx	hydrothermal	300 W Xe lamp	10 mg	100 mL aqueous solution contains 0.35 M Na2S and 0.25 M Na2SO3	66.08 mmol g–1 h–1	(139)
Au-MoS2/Ti3C2	hydrothermal	300 W Xe lamp	30 mg	50 mL methanol solution (deionized H2O: 70%, methanol: 30%)	12000 μmol g–1 h–1	(143)
Pt-TiO2/MoS2	hydrothermal	300 W Xe lamp	20 mg	100 mL solution contains 0.25 M Na2S and 0.5 M Na2SO3	4.18 mmol g–1 h–1	(144)
CdS-Au/MoS2	hydrothermal	300 W Xe lamp	10 mg	50 mL aqueous solution contains 10% lactic acid	237.90 μmol g–1 h–1	(147)
Au-ZnO/MoS2	hydrothermal	300 W Xe lamp	0.05 g	50 mL deionized H2O contains 0.3 M Na2S and 0.3 M Na2SO3	13.56 μmol g–1 min–1	(148)

3. Photocatalytic Carbon Dioxide Reduction Reaction (PCO₂RR)

Carbon dioxide (CO₂), the most abundant greenhouse gas, contributed significantly to global warming. Different techniques have been developed to decrease the amount of CO₂ in the atmosphere, including absorption, bioconversion, and photocatalytic reduction.^149−151 Compared to photocatalytic H₂ evolution, CO₂ reduction by photocatalysis is a more complicated and challenging photochemical process. The products of photocatalytic CO₂ reduction are selective and depend on several conditions. To address the significant issues of the preliminary modification processes, it must tackle both the challenges of thermodynamic and kinetic factors. Being a stable linear molecule, CO₂ has a high bond energy of 750 kJ mol^–1. The thermodynamic aspect of photocatalytic CO₂ reduction requires an efficient light-responsive semiconductor that can capture enough external energy to break the C=O bond, essential for producing the C–H bond and synthesizing hydrocarbons.^152,153 The interactions of intermediates with the surface of the catalyst determine product selectivity in CO₂ reduction. Therefore, to attain maximum output during photocatalytic CO₂ reduction, the photocatalyst should first satisfy the fundamental criteria: interact with the substrate molecules to start the transition process on the catalyst’s surface (CO₂ or H₂O). Another governing factor is photon absorption for exciting electrons from the catalyst’s valence band (VB) to the conduction band (CB). After excitation, the higher energy electrons in the CB showed higher potentials for CO₂ reduction. As a result, catalysts with suitable band gaps (1.4–3 eV) are necessary to attain the maximum CO₂ reduction.^154,155 Since the first report by Inoue and co-workers on the photocatalytic reduction of CO₂ to formaldehyde and methanol,¹⁵⁶ various photocatalytic materials have been developed for photocatalytic CO₂ reduction, including metal oxides,^157,158 sulfides,¹⁵⁹ perovskite-like materials,¹⁶⁰ double-layered hydroxides,¹⁶¹ and layered materials.^162,163 Among them, the transition metal dichalcogenides (TMD) find their application for CO₂ transformation through electrocatalytic, thermochemical, and photocatalytic ways. Even though photocatalytic H₂ evolution using TMD is well-known, their use in photocatalytic CO₂ reduction has been recently reported. Mainly, photocatalytic CO₂ reduction using MoS₂ is an emerging area that is considered a future photocatalyst for CO₂ reduction and a favorable, cost-effective alternative for noble metal catalysts, exhibiting remarkable CO₂ electrochemical reduction activity with a high current density and low overpotential. The 2D layered nanostructures enable the modification and tunability of other TMDs. The atomic thickness allows for excellent optical transparency and mechanical flexibility, possibly allowing more surface-active sites over MoS₂. The small diffusion path length and presence of terminated edges of MoS₂ act as an active center for the CO₂ reduction reaction, facilitating multiple electron transfers.^164−166

Interestingly, MoS₂-based catalysts exhibit superior electrochemical selectivity over alcohols than other catalysts. In particular, alcohol synthesis from syngas has also been evaluated.^167,168 Asadi et al. in 2014 reported that a layer-stacked bulk MoS₂ containing molybdenum (Mo)-terminated edges showed superior CO₂ reduction activity. The metallic nature of MoS₂, the large d-electron density, and the availability of molybdenum-terminated edges are the primary reasons for the catalytic activity, which facilitates multiple electron transfer processes in the CO₂ reduction pathway.¹⁶⁹ Recently, researchers have focused on photocatalytic CO₂ reduction using MoS₂-based photocatalysts. Xie et al., for example, showed that the catalytic process over MoS₂ proceeds via a carboxylate pathway, viz., CO₂ → COOH* → CO* → CO to produce CO as the final product.

Due to the low energy barrier of CO, the method suggests more excellent selectivity toward CO than other products like HCOOH and CH₃OH. Theoretical calculations indicate that the center of the d band at Mo exposed atoms at the edges is much closer than the metallic catalysts (e.g., Pt); thus, close interactions occurred with the adsorbed species. Meanwhile, the partial density of states (PDOS) of the activated CO₂* and the exposed Mo atoms shows the strong interactions of Mo and O compared to the Mo–C interactions, which initiates the cleavage of the C=O bond to activate CO₂*. The exposed edges are responsible for adsorbing the CO₂ molecule and activating the two neighboring Mo atoms. The first transfer of e^– or h⁺ across the MoS₂ edges is thought to be a hydrogenation step that controls the reduction pathway. It forms the unstable intermediate HO-CO*, which quickly decomposes to OH* and CO*. The hydrogenation of COOH* controls the product selectivity for CO and other hydrocarbons.¹⁷⁰ Due to the low band gap potential, the charge recombination occurring over the surface of MoS₂ is becoming a significant challenge. Forming products like CH₃OH and CH₄ is also difficult because this involves 6 e^– and 8 e^– processes, respectively, compared to products like CO, which require only two e^– that can be directly used as fuels and transformed into other powers through energy-intensive processes.¹⁷¹ Therefore, effective doping and heterojunction formations are required to get a more significant number of electrons available for reducing CO₂, thereby increasing the efficiency of the MoS₂-based photocatalysts for CO₂ reduction. The remaining sections will focus on the roles of MoS₂ and the various transformations that occurred as a transpire photocatalyst in CO₂ reduction reactions.

3.1. MoS₂ Nanostructures for Photocatalytic CO₂ Reduction

The MoS₂ nanoflower-like morphology with dangling bonds at the stacked sheet edge makes it ideal for photocatalytic CO₂ reduction. Meier et al. reported a controllable edge-rich nanoflower morphology with abundant edge sites through a chemical vapor deposition (CVD) method. Photocatalytic activity increases with increasing edge plane density of nanoflower due to unsaturated and dangling bonds. During CVD synthesis, the energy gap (Eg) was separately varied from 1.38 to 1.83 eV. Carbon monoxide (CO) was the most prominent product of CO₂ photoreduction in the presence of H₂O. The production rates of CO nearly doubled after a post-treatment reduction step. This reliable CVD technique contributes to developing efficient and stable 2D-MoS₂ nanoflower materials for CO₂ reduction.¹⁷² MoS₂ nanosheets combined with metal oxides were identified as a cost-effective substitute for noble metal cocatalyst for photocatalytic reduction of CO₂. Tu et al. reported the formation of a 2D-MoS₂ nanosheet on TiO₂ nanosheets. To reduce CO₂ by UV–vis light irradiation, pure TiO₂ nanosheets and MoS₂/TiO₂ hybrid nanosheets were placed in a CO₂-saturated 1 M NaHCO₃ aqueous solution. The CH₃OH production of 0.5 wt % MoS₂/TiO₂ reached to 10.6 μmol g^–1 h^–1, which was 2.9 times that of pure TiO₂ (3.7 μmol g^–1 h^–1). The fast electron transport from TiO₂ to MoS₂ is indicated by the PL decay lifetime of 0.51 ns. The weak van der Waals interactions in the S–Mo–S layers result in Mo-terminated edges containing high d-electron density, which contributes to excellent and selective photocatalytic CO₂ reduction.¹⁷³ Similarly, few-layered MoS₂ with uniformly formed mesoporous TiO₂ on the surface of 3D graphene showed higher photocatalytic CO₂ reduction. The MoS₂ layers in the 3D aerogel perform as a cocatalyst to minimize the backward charge transfer through graphene to TiO₂. The few-layered MoS₂ with unsaturated S atoms on exposed edges improved its activity and generated a high electron concentration in the composite between the MoS₂ and graphene layers. The yield of CO formation was up to 92.33 μmol g^–1 h^–1.¹⁷⁴

Jia et al. recently synthesized a MoS₂/TiO₂ heterojunction photocatalyst for CO₂ reduction. The ultrathin MoS₂ nanosheets were equally combined with the TiO₂ nanoparticles. The conduction band (CB) edge potential of MoS₂ (−0.93 V) was more negative than that of TiO₂ (−0.55 V), so the photogenerated electrons on the surface of MoS₂ sheets could easily migrate to TiO₂. CO and CH₄ were the major products in the experimental process, and the 10% MoS₂/TiO₂ composite gave the maximum yield. It is about 268.97 μmol g^–1 and 49.93 μmol g^–1, respectively.¹⁷⁵ Xu et al. constructed a 1D/2D TiO₂/MoS₂ nanostructure composite. Here, the MoS₂ sheet arrays with lateral dimensions of 80 nm with thicknesses of up to 2 nm were firmly connected to the surface of TiO₂ nanofiber. The CO₂ adsorption and surface area of the produced samples increase as the MoS₂ loading increases. With increasing CO₂ loading, the CH₄ and CH₃OH synthesis rates increase, reaching a maximum of 2.86 and 2.55 μmol g^–1 h^–1, respectively, with a quantum yield of 0.16% for the best TiO₂/MoS₂ sample (TM10). The DFT calculations further demonstrate that MoS₂ has a higher work function than TiO₂, resulting in electron transfer from MoS₂ to TiO₂ upon photoexcitation.¹⁷⁶

A type II p-n heterojunction charge transfer model was formed by coupling the α-Fe₂O₃ and MoS₂ composite. The loading of α-Fe₂O₃ promoted 2H to 1T MoS₂ phase conversion and increased charge carrier migration. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis revealed a deoxygenation pathway for CO₂ conversion. Several intermediate species are formed upon the adsorption of CO₂, which ultimately results from excellent CO₂ reduction performance to CH₄ and CH₃OH.¹⁷⁷ In another system, a few-layered MoS₂-loaded flowerlike Bi₂WO₆ nanocomposite was reported by Dai et al.¹⁷⁸ The as-prepared composites showed an excellent photoreduction of CO₂ into hydrocarbons compared to pure Bi₂WO₆. The intimate interfacial contact between MoS₂ and Bi₂WO₆ significantly influenced the transfer of photogenerated charge carriers. The composite also showed much stronger absorption within the visible region than pure Bi₂WO₆. The photocurrent response of the composites (Figure 11a) provided information about the higher separation efficiency of photogenerated charge carriers. The methanol and ethanol obtained from the optimized sample (0.4 wt % MoS₂) after 4 h of visible light irradiation were 36.7 and 36.6 μmol g^–1, respectively, 1.94 times higher than pure Bi₂WO₆.

Figure 11.

(a) Transient photocurrent response of Bi₂WO₆ and MoS₂/Bi₂WO₆ nanocomposites under visible light irradiation. (b) Proposed CO₂ photoreduction and charge transfer mechanism in the MoS₂/Bi₂WO₆ nanocomposite. Reproduced with permission from ref (178). Copyright 2017 Elsevier.

A possible photocatalytic mechanism (Figure 11b) was also proposed. It indicated that the CO₃^2–, HCO₃^–, and H₂CO₃ formed in the aqueous solutions might serve as reactive carbon sources during the photoreduction process. The 2D-π conjugated structure could serve as an electron-accepting material and contribute to separating e^–-h⁺ pairs for CO₂ reduction.¹⁷⁸ A flowerlike MoS₂/Ag/SnO₂ ternary composite that serves as a bifunctional catalyst for photocatalytic CO₂ reduction and degradation of pollutants was recently produced using a simple hydrothermal process. Because of the high specific surface area, several folded edges, and ultrathin nature, Ag nanoparticles and SnO₂ NSs were distributed uniformly on the MoS₂ surface, enhancing photoexcited charge transport. The modified MoS₂/Ag/SnO₂ nanocomposites exhibit outstanding visible-light photocatalytic activity for converting CO₂ to methane (CH₄). The quantum efficiency calculated at λ = 420 nm was found to be 2.38% which substantiates the high CO₂ conversion efficiency of the catalyst. SnO₂ accepts the photogenerated electron of MoS₂ through Ag in the heterojunction, which directly reduces CO₂ and produces CH₄ as the final product. Besides CO₂ reduction, the composite also shows high activities for pollutant degradation.¹⁷⁹ Another binary composite using 2D MoS₂ nanosheets with a metal sulfide has been reported. The p-n heterostructure Bi₂S₃-MoS₂ nanocomposite revealed improved sunlight absorbance and photocatalytic reduction of CO₂. The high catalytic performance was due to the increased S-defects on the MoS₂ surface, hence increasing interfacial interaction between MoS₂ and Bi₂S₃.¹⁸⁰ As mentioned in the first section, graphitic carbon nitride (gC₃N₄) can be an efficient photocatalyst to construct the heterojunction with layered MoS₂. Studies have been reported regarding the applications of MoS₂/g-C₃N₄ nanocomposites for photocatalytic reactions to convert CO₂ to hydrocarbons. Recently, Qin et al. mentioned the synthesis of MoS₂/gC₃N₄ nanocomposites to convert CO₂ into fuels by a simple hydrothermal followed by a calcination process. The synthesized 10% MoS₂/g-C₃N₄ heterojunction exhibited the highest conversion efficiency in reducing CO₂ to CO up to 58.59 μmol g^–1 and showed significant stability after a seven-hour reaction. Z-Scheme charge transfer was constructed by correctly coupling the two semiconductor photocatalysts for CO₂ reduction.¹⁸¹

Combining semiconductors with noble metal NPs (Ag, Au, or Pt) caused localized surface plasmon resonance (LSPR) to show great potential in photocatalytic CO₂ reactions. Interestingly, studies have confirmed that the generation of LSPR can optimize the structure of MoS₂. Sun et al. demonstrated the Au nanoparticle decorated ultrathin MoS₂ nanosheet acts as an excellent heterogeneous catalyst for photocatalytic CO₂ reduction. The DFT calculations also endorse the experimental data. DRIFTS spectrum proposed a reaction pathway for the formation of CH₄, CO₂ → COO– → −COOH → −HCO→ −HCOH → −CH₂OH→ −CH₃ → CH₄ via the hydrogenation process. For the Au-MoS₂ sample, the reaction barrier for forming a COOH intermediate decreases from 2.17 to 1.82 eV, suggesting faster CO₂ reduction.¹⁸² Similarly, a DFT study on Ag-loaded 2H-MoS₂ was stimulated by Ouyang et al. for the photocatalytic reduction of CO₂. From the series of Ag loading on the 2H-MoS₂ nanosheet, the 20 wt % Ag/MoS₂ is most suitable for converting CO₂ to CH₄. The Gibbs free energy was reduced from 2.830 to 0.925 eV for the 20 wt % Ag/2H-MoS₂ nanocomposite.¹⁸³

For adjusting the band gap of MoS₂, phase conversion via chemical exfoliation was used. In a recent study, Zheng et al. reported the synthesis of Ag/2H-MoS₂ composites and their photocatalytic activity in reducing CO₂ to methanol. A wet chemical process was invented to exfoliate MoS₂ nanosheets and transform them from the 1T phase to the 2H phase. This composite obtained a larger specific surface area through Li-ion intercalation and exfoliation than the hydrothermally synthesized MoS₂. The Ag nanoparticles can act as a pool for collecting the photogenerated electrons of MoS₂ nanosheets and suppress the recombination of charge carriers. The maximum yield of methanol obtained after 10 h irradiation at 20 wt % Ag/2H-MoS₂ was about 365.08 μmol g^–1 h^–1.¹⁸⁴ A study by Lu et al. demonstrated the use of a noble metal-free catalyst (Mn_0.2Cd_0.8S) wrapped in a series of MoS₂ ultrathin nanosheets for photocatalytic energy conversion. The optimal loading of MoS₂ content was 3%, giving an H₂ production rate of 335.02 μmol h^–1 and a CH₃OH production rate of 2.13 μmol h^–1. The production of methanol increases with increasing the MoS₂ content. The improved photocatalytic efficiency benefits from the intimate interfacial contact between Mn_0.2Cd_0.8S and MoS₂, which effectively promotes the separation and transportation of photogenerated charge carriers.¹⁸⁵

Recently, a composite comprising MoS₂ and a halide perovskite material, CsPbBr₃, formed a stable heterojunction. The heterojunction contacts between the two components increased the lifetime confirmed by the PL, surface, and transient photovoltage (SPV, TPV) and DFT studies. The temperature-programmed CO₂ desorption (TPD) analyses showed that introducing MoS₂ increased the ability of the CO₂ uptake of the composite.¹⁸⁶ CO₂ photoreduction and simultaneous H₂ production were achieved by a ternary nanocomposite of MoS₂ and N-containing polymer polypyrrole (PPy) on reduced graphene oxide (rGO). The resulting rGO-MoS₂/PPy nanocomposites reveal that rGO-MoS₂ acts as a rigid template, ensuring consistent growth and heterogeneous nucleation of PPY, thereby increasing their photocatalytic reduction of CO₂ into CH₄, CO, and H₂. Composites containing optimal amounts of PPy (rGO-MoS₂/PPy-150) exhibited the highest photocatalytic activity to CO (3.95 μmol g^–1 h^–1), CH₄ (1.50 μmol g^–1 h^–1), and H₂ (4.19 μmol g^–1 h^–1), with a quantum yield of 0.30%.¹⁸⁷ Similarly, a simple solvothermal method constructed a novel S–C–S heterojunction by Yin et al. The MoS₂ was assembled into 3D-uniform nanoflower balls after a simultaneous coupling connection with rGO and SnS₂. The valence band (VB) positions of pure MoS₂ (1.61 eV) and SnS₂ (1.50 eV) indicate that they can form an effective Z-type heterojunction so that the electrons can quickly be excited from the MoS₂ CB to VB in SnS₂ and thereby to rGO to reduce CO₂ to CO and CH₄.

The multilevel electron transport produces CO and CH₄ up to 68.53 and 50.55 μmol g^–1 h^–1, respectively.¹⁸⁸ A binary photocatalyst composed of the SiC@MoS₂ nanoflower was reported by Wang et al. for simultaneous photocatalytic CO₂ reduction and H₂O oxidation under visible light. All the SiC@MoS₂ samples show much higher CH₄ evolution than the individual components. The optimized samples show the CH₄ evolution up to 323 μL g^–1 h^–1 and O₂ evolution of about 620 μL g^–1 h^–1 with high photocatalyst stability. The photocatalytic reduction of CO₂ follows the hydrogenation pathway (Figure 12). One CO₂ molecule is converted into one CH₄ by four sequential hydrogenation half-cell reactions. This pathway takes advantage of the photoreduction potential of SiC and the powerful photoreduction ability of MoS₂ nanosheets.¹⁸⁹

Figure 12.

Reaction pathway for photocatalytic CO₂ reduction by H₂O on SiC@MoS₂. Reproduced from ref (189). Copyright 2018 American Chemical Society.

Layered double hydroxides (LDH) are a new materials class that has recently attracted much interest. A study by Zhao et al. demonstrated that the heterojunction of MoS₂ with Co-Al-LDH through an electrostatic interaction facilitates the charge transfer and syngas production by controlling the heterojunction concentration. Photogenerated electrons spontaneously transfer from LDH CB to MoS₂, and the syngas ratio (H₂:CO) was precisely turned from 1.3:1 to 15:1 by altering only the catalyst concentration in the photocatalytic CO₂ reduction under visible light.¹⁹⁰

A multidimensional ternary heterojunction of Bi₂S₃/TiO₂/MoS₂ was reported by Alkanad et al. for the photocatalytic CO₂ reduction reaction where Bi₂S₃ and TiO₂ were embedded and wrapped within the MoS₂ nanosheet. The in situ irradiated photoelectron spectroscopy (ISI-XPS) reveals the switchable feature of the heterostructure. Electron flow follows the S-scheme approach in UV–vis-NIR (CH₃OH and C₂H₅OH as the major products), whereas, in vis-NIR, it forms a type II heterojunction (CH₄ and CO), thus exhibiting a selective photocatalytic CO₂ reduction reaction. Moreover, the quantum efficiency of the system reaches 4.23% at 600 nm wavelength.¹⁹¹ Another higher CO₂ reduction capacity was observed in a ZnO/MoS₂ nanosheet composite where the hollow structure of MoS₂ encapsulates the ZnO nanoparticles and enables multiple refractions of photons in the cell. MoS₂ as a 2D sheet would enhance electron mobility, so the photogenerated electrons and holes are accumulated on the surface of MoS₂. Therefore, CB and VB of MoS₂ simultaneously reduce CO₂ and oxidize H₂O.¹⁹² Metallic or nonmetallic doping creates defects in the MoS₂ lattice and facilitates charge transfer at the interface to facilitate CO₂ reduction. Such a ternary system in which In₂S₃ was decorated with MoO₃@MoS₂ was fabricated for visible light photoreduction of CO₂ to CH₄ and CO. The presence of In₂S₄ induces S-rich sites and O vacancies in the composite for CO₂ reduction, and a maximum yield of CH₄ (29.4 μmol g^–1 h^–1) and CO (35.6 μmol g^–1 h^–1) was obtained.¹⁹³ Similarly, duel In₂S₄-NiS decorated MoO₃@MoS₂ was reported by Zheng et al. through an in situ sulfurized approach of nonthermal plasma. Increasing the In₂S₄-NiS content into the MoO₃@MoS₂ system increased the CH₄ yield of 49.11 μmol g^–1 h^–1.¹⁹⁴ Metal–organic frameworks (MOFs) were an excellent platform for developing heterogeneous catalysts for CO₂ reduction. By integrating MoS₂ nanosheets into hierarchically porous defective UiO-66 (d-UiO-66), a composite of d-UiO-66/MoS₂ was prepared for the selective photocatalytic reduction of CO₂ to CH₃COOH. The unsaturated Mo atoms on the MoS₂ nanosheet would contact d-UiO-66 to form Mo–O–Zr bimetallic sites for selective CO₂ reduction. By loading the composite on a nylon 66 microfiltration membrane, CH₃COOH was formed with a rate of 39.0 μmol g^–1 h^–1 without any sacrificial reagent.¹⁹⁵ Similarly, a new In₂O₃/MoS₂ comodified ZnO nanorods also showed CO₂ photoreduction to CO and CH₄. Surface modification of the In₂O₃ and MoS₂ species would create transfer channels for holes and electrons on the photocatalyst’s surface and improve the photocatalytic activity.¹⁹⁶

3.2. MoS_2-Based Photoelectrocatalytic CO₂ Reduction

Cu₂O is a promising low-cost photocatalyst for CO₂ reduction, which showed higher selectivity for methanol formation. But their poor conductivity and charge recombination render their application. To overcome these drawbacks of Cu₂O, heterojunctions with suitable components have been formed. Among these structures, a new MoS₂-Cu₂O heterostructure was fabricated by Singh et al. Here two types of MoS₂, p-MoS₂ and n-MoS₂, were used to form heterostructures with CuO: Cu₂O-c (cubic morphology) and Cu₂O-co (cuboctahedron morphology). This new system exhibited both CO₂ reduction and water oxidation capacity. Among these structures, the p-MoS₂/Cu₂O-c type system showed higher activity, following a Z-scheme charge transfer to yield CH₃OH, and DFT calculations further confirm it.¹⁹⁷ Recently, MoS₂-rods/TiO₂ nanotubes (NTs) heterojunction was introduced for exploiting photoenhanced electrocatalytic reduction (PEEC) of CO₂ to methanol.

The resulting composite exhibited excellent optical performance but had an unmatched conduction band minimum (CBM) of −0.15 eV with low Faradaic efficiency. Therefore, developed a new PEEC to reduce CO₂ to methanol on MoS₂-rods/TiO₂ NTs. Adding illumination to the electrocatalytic system improved the Faradaic efficiency to 111.58% more than the same electrocatalytic (EC) system. They introduced a unique mechanism for the PEEC reduction of CO₂ to methanol. The reduction of CO₂ to CH₃OH consists of six electronic reactions, as shown in Figure 13a. On one side, when applying −1.3 V potential on the cathode, the anode potential is 1.82 V versus SCE and can oxidize H₂O to O₂ and protons. Then the protons will reach the cathode for CO₂ EC reduction. Moreover, the catalyst has a valence band (VB) suitable for in situ PC oxidation of H₂O to generate protons. So, it provides more protons for EC reduction of CO₂ to methanol with high efficiency (Figure 13b).¹⁹⁸Table 4 summarizes the recent progress in photocatalytic CO₂ reduction using MoS₂ nanocomposites.

Figure 13.

(a) Probable speculation reaction pathway for the reduction of CO₂ to CH₃OH. (b) Mechanism of the PEEC reducing CO₂ process at MoS₂-rods/TiO₂NTs electrode. Reproduced with permission from ref (198). Copyright 2014 Elsevier.

Table 4. Summarized List of Phototocatalytic Carbon Dioxide Reduction Using MoS₂ Compounds.

photocatalyst	synthesis method	light source	amount of catalyst	major products	ref
MoS2 nanoflower	CVD	solar simulator	150 mg	CO	(172)
MoS2/TiO2	hydrothermal	300 W Xe lamp	0.1 g	CH4 (10.6 μmol g–1 h–1)	(173)
TiO2/MoS2/graphene	hydrothermal	300 W Xe lamp	–	CO (92.33 μmol g–1 h–1)	(174)
MoS2/TiO2	hydrothermal	300 W Xe lamp	50 mg	CO (268.97 μmol g–1)	(175)
				CH4 (49.93 μmol g–1)
TiO2/MoS2	hydrothermal	350 W Xe lamp	50 mg	CH4 (2.86 μmol g–1 h–1)	(176)
				CH3OH (2.55 μmol g–1 h–1)
α-Fe2O3/MoS2	hydrothermal	300 W Xe lamp	50 mg	CH4 (121 μmol g–1 h–1)	(177)
				CH3OH (41 μmol g–1 h–1)
MoS2/Bi2WO6	hydrothermal	300 W Xe lamp	50 mg	CH3OH (36.7 μmol g–1)	(178)
				C2H5OH (36.6 μmol g–1)
SnO2/Ag/MoS2	hydrothermal	300 W Xe lamp	–	CH4 (20 μmol)	(179)
				CO (9 μmol)
Bi2S3/MoS2	hydrothermal	150 mW cm–2 Xe lamp	0.2 g	CO (40 μmol g–1)	(180)
				CH4 (2.5 μmol g–1)
MoS2/g-C3N4	hydrothermal	300 W Xe lamp	50 mg	CO (58.59 μmol g–1)	(181)
Au/MoS2	hydrothermal and ultrasonic dispersion	300 W Xe lamp	0.025 g	CH4 (19.38 μmol g–1 h–1)	(182)
Ag/MoS2	hydrothermal, calcination and reduction	200 W Hg lamp	20 mg	CH3OH (365.08 μmol g–1 h–1)	(184)
MoS2/Mn0.2Cd0.8S	hydrothermal	300 W Xe lamp	50 mg	CH3OH (2.13 μmol h–1)	(185)
polypyrrole/rGO/MoS2	hydrothermal and ultrasonication	300 W Xe lamp	50 mg	CO (3.95 μmol g–1 h–1)	(187)
				CH4 (1.50 μmol g–1 h–1)
MoS2/SnS2/rGO	solvothermal	8 W Hg lamp	20 mg	CO (68.53 μmol g–1 h–1)	(188)
				CH4 (50.55 μmol g–1 h–1)
SiC/MoS2	self-assembly	300 W Xe lamp	10 mg	CH4 (323 μL g–1 h–1)	(189)
Bi2S3/TiO2/MoS2	microwave synthesis	300 W Xe lamp	50 mg	CH3OH (119 μmol g–1)	(191)
				C2H5OH (102.7 μmol g–1)
MoS2@ZnO	calcination	150 W Hg lamp	0.1 g	CH3OH (170.5 μmol g–1)	(192)
In2S3/MoO3@MoS2	in situ sulfurization	200 mW cm–2 Xe lamp	10 mg	CH4 (29.4 μmol g–1 h–1)	(193)
				CO (35.6 μmol g–1 h–1)
In2S3/NiS/MoO3@MoS2	in situ sulfurization	300 W Xe lamp	10 mg	CH4 (49.11 μmol g–1 h–1)	(194)
				CO (6.19 μmol g–1 h–1)
ZnO/In2O3/MoS2	hydrothermal and ultrasonication	300 W Hg lamp	100 mg	CO (5.628 μmol g–1 h–1)	(196)
				CH4 (3.048 μmol g–1 h–1)

4. Conclusions and Perspectives

This review focused on 2D layered MoS₂-based nanostructures for photocatalytic H₂ evolution and CO₂ reduction to hydrocarbon fuels, which have been discussed in the literature over the past several years.

The layered structure of MoS₂ with more active edge sites mainly contributed to its photocatalytic activity. The first part of this review illustrates the H₂ evolution reactions of binary and ternary composites of MoS₂. Binary composites cover the composites of MoS₂ with metal oxides, carbonaceous materials, metal sulfides, and metal–organic frameworks, and ternary heterosystems comprise the combinations of different binary structures with MoS₂. Many studies have shown that combining MoS₂ as a cocatalyst can overcome the barriers to H₂ evolution in many systems. The recombination of charge carriers in several metal oxides is reduced by forming binary structures with MoS₂, where the tuning of the band gap of metal oxide was achieved.⁶⁰ The 2D/2D binary heterostructures formed with MoS₂ and carbonaceous materials provide a high surface area that significantly benefits higher H₂ evolution activity. Especially the H₂ production was increased to a large extent by introducing thinner and fluffier flowerlike MoS₂ structures in the framework of gC₃N₄ nanosheets.^76,77 The significant drawbacks like photocorrosion and fast electron–hole recombination of metal sulfides can be effectively eliminated by forming binary structures with MoS₂. Binary heterostructures formed with multimetal sulfides like ZnIn₂S₄, CuInS₂, and MgIn₂S₄ with MoS₂ are also widely used as a photocatalyst for H₂ production. The flowerlike morphology of MoS₂ provides an excellent platform for catalytic activity.^104,105 The peculiar porous structure of MOFs exhibits many coordination centers for H₂ evolution activity, and the association of MoS₂ further increases its activity. Apart from these binary combinations, MoS₂-based ternary composites have much interest due to their efficient multielectron transport mechanism for H₂ production. In these systems, MoS₂ acts as a channel to accept more photogenerated electrons for combining H⁺ to produce H₂ at lower overpotentials.^131,132 Ternary composites of MoS₂ containing noble metals always show a high H₂ evolution rate with excellent stability. Such plasmonic metal hybrid systems with MoS₂ contributed much better photocatalytic activity than others. For example, an Au-deposited MoS₂/ZnO system exhibited the highest H₂ evolution rate with good stability.¹⁴⁸

Though encouraging progress has been achieved toward photocatalytic H₂ evolution using MoS₂-based nanocomposites, some problems and challenges still exist. One of the crucial difficulties is that MoS₂ alone has negligible photocatalytic activity, and its bulk form is an indirect-band gap semiconductor. The band structure of MoS₂ varies with the number of layers. When it became a monolayer, it gave a high optical absorption coefficient and showed better HER performance. Therefore, more research is needed to explore single-layer MoS₂ as a hydrogen evolution catalyst. The combinations of MoS₂ with other transition metal dichalcogenides (TMDs) such as WS₂, MoSe₂, and WSe₂ would give much better activity. For example, the WS₂-MoS₂ binary structure with CdS was an efficient noble-metal-free catalyst with high durability. So, the development of such type of hybrid metal sulfide combinations would be more acceptable.¹⁹⁹

Furthermore, a few reports are available using MoS₂ with inorganic compounds like Mxenes, giving the highest H₂ evolution rate.¹²⁶ Similar results were obtained for the 3D structure of graphene containing MoS₂. In this context, efforts for efficient photocatalysts are also to be developed with different compositions for enhanced H₂ evolution performance.⁷⁹ In most cases, H₂ evolution occurs in aqueous solutions using diverse sacrificial reagents like triethanolamine, lactic acid, Na₂S-Na₂SO₃ pair, and methanol as electron donors. The use of such sacrificial reagents in MoS₂-based nanocomposites for H₂ evolution implied a higher cost for future applications on a large scale. So, it is essential to develop MoS₂-based photocatalysts for H₂ production without using additional sacrificial reagents.

Photocatalytic CO₂ reduction to hydrocarbon fuels mainly involves multielectron transfer mechanisms and includes a variety of redox potentials. As a result, several products can be generated during the photocatalytic CO₂ reduction process, and the product selectivity is related to the chemical pathways of the reaction. Thus, the role of MoS₂ in photocatalytic CO₂ reduction must be better understood to improve selectivity for a particular fuel. Layered MoS₂ has been widely employed to improve photocatalytic H₂-production performance. However, research on MoS₂-based photocatalysts for CO₂ reduction is still in the early stages. Efforts in developing layered MoS₂-based photocatalysts capable of simultaneous H₂ evolution and CO₂ conversion are an area of current interest.^185,187 Besides the photochemical conversion of CO₂, MoS₂-based systems for CO₂ reduction using photoelectrochemical splitting are highly required due to their high Faradaic efficiency. Theoretical studies stimulate the electronic properties of MoS₂-nanocomposites, which will help one to understand the efficiency of the catalyst in photoelectrochemical studies.^197,198 In short, developing 2D layered MoS₂-based nanocomposites is a substantial breakthrough in photocatalytic H₂ evolution and CO₂ reduction.

Author Present Address

^§ Assistant Professor, Department of Chemistry, Morning Star Home Science College, Angamali South, Kerala 683573, India

The authors declare no competing financial interest.