Advanced 2D molybdenum disulfide for green hydrogen production: Recent progress and future perspectives

Abstract

The development of renewable and affordable energy is crucial for building a sustainable society. In this context, establishing a sustainable infrastructure for renewable energy requires the integration of energy storage, specifically use of renewable hydrogen. The hydrogen evolution reaction (HER) of electrochemical water splitting is a promising method for producing green hydrogen. Recently, two-dimensional nanomaterials have shown great promise in promoting the HER in terms of both fundamental research and practical applications due to their high specific surface areas and tunable electronic properties. Among them, molybdenum disulfide (MoS₂), a non-noble metal catalyst, has emerged as a promising alternative to replace expensive platinum-based catalysts for the HER because MoS₂ has a high inherent activity, low cost, and abundant reserves. At present, greatly improved activity and stability are urgently needed for MoS₂ to enable wide deployment of water electrolysis devices. In this regard, efficient strategies for precisely modifying MoS₂ are of interest. Herein, the progress made with MoS₂ as an HER catalyst is reviewed, with a focus on modification strategies, including phase engineering, morphology design, defect engineering, heteroatom doping, and heterostructure construction. It is believed that these strategies will be helpful in designing and developing high-performance and low-cost MoS₂-based catalysts by lowering the charge transfer barrier, increasing the active site density, and optimizing the surface hydrophilicity. In addition, the challenges of MoS₂ electrocatalysts and perspectives for future research and development of these catalysts are discussed.

Graphical abstract

Keywords

molybdenum disulfide (MoS₂) / hydrogen evolution reaction (HER) / active site / electrocatalyst / modification strategie

1 Introduction

Conventional fossil fuels can no longer meet the needs of the human society, which has prompted us to develop renewable and green energy sources. Hydrogen, a carbon-free energy source, has attracted tremendous attention as a potential replacement for fossil fuels in the future. Electrocatalysis and photocatalysis are currently two of the most promising hydrogen production technologies. Photocatalytic hydrogen production has the advantages of low cost and simple hydrogen production. Since the introduction of photocatalytic hydrogen production, researchers have developed a variety of photocatalysts for hydrogen formation, such as graphite carbon nitride (g-C₃N₄) [1], linear conjugated polymers [2], conjugated microporous polymers [3], and covalent organic frameworks (COFs) [4]. However, the current efficiencies for photocatalytic hydrogen production are inadequate for commercial application. Water electrolysis for hydrogen production is a well-established industry with advantages arising from low hydrogen impurity levels and decreased carbon emissions. In particular, water electrolysis comprises the anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER). Therefore, the performance is affected by both the anode and cathode processes. In previous papers, the progress made with the OER [5,6] are carefully reviewed, and in this paper, focus is laid on HER.

To date, water splitting and the HER often necessitate the use of precious metals, such as Pt. However, these metals are both costly and scarce [7]. In this regard, the development of nonnoble metal catalysts with high activity and stability is being pursued [8]. Promising substitutes for the noble metal catalysts used for the HER include transition metal oxides (TMOs), transition metal carbides (TMCs), transition metal phosphides (TMPs), transition metal nitrides (TMNs), and transition metal dichalcogenides (TMDs) [9]. TMDs have become popular and promising non-noble metal electrocatalysts for HER because of their atomic-level thicknesses, complete exposure of active sites, adjustable electronic structures, diverse modulation strategies, and low hydrogen adsorption free energy [10]. In particular, molybdenum disulfide (MoS₂) has been intensively investigated. MoS₂ is one of two-dimensional (2D) TMDs nanomaterials. MoS₂ is a widely-used material for electrocatalytic hydrogen evolution due to its high catalytic activity at edge active site, easy accessibility, and low cost.

Conventionally, it is believed that the unsaturated sulfur atoms located at the edges of MoS₂ are the catalytic active centers, while the basal planes make no contribution to HER. The edge structure of MoS₂ is very similar to that of the active site of nitrogenase and exhibits ΔG_H*\cong 0 [11]. In HER, nitrogenase has a hydrogen formation activity similar to that of Pt. As a result, MoS₂ is said to have a Pt-like activity. Jaramillo et al. [12] conducted a point-to-point comparison of the edge active sites of MoS₂ with the sites on the Pt (111) face. They found that the exchange current density at the MoS₂ edge was significantly higher than that of common metals and only lower than that of Pt group metals. MoS₂ has almost no active sites in the basal planes and is prone to stacking, which causes some of the active sites at the edges to disappear. Modifications have been proposed to enhance the activity of MoS₂, including phase engineering, morphology design, defect engineering, heteroatom doping, and heterostructure construction (Fig.1). Herein, the recent progress made in modifying MoS₂ for use in the HER is reviewed and perspectives on future R&D and beyond are provided.

2 Structural features of MoS₂

MoS₂ has a layered structure comprising individual S−Mo−S units, the adjacent layers are bonded through van der Waals (VDW) forces, and the S and Mo atoms are bonded via strong ionic-covalent bonds [13]. The bonding mechanism is similar to that of graphite [14]. MoS₂ is commonly a black-gray semiconductor in the form of solid powder, which exhibits a metallic luster and a good chemical/thermal stability [15].

MoS₂ has four crystal structures, including a distorted tetragonal phase (1T), hexagonal phases (1H and 2H), and a rhombohedral phase (3R), whose structures are shown in Fig.2(a)–2(d) [16]. Among them, 1T-MoS₂ and 3R-MoS₂ are metastable states, while 2H-MoS₂ widely exists under natural conditions. The arrangement of atomic layers (S−Mo−S) in the 1T phase of MoS₂ is significantly different from those in the 2H and 3R phases [17]. Particularly, the 1T phase has a regular octahedra with Mo atoms at the centers and coordination by S atoms, and there is only one layer for the repeat units in the crystal unit cell. However, the 2H phase has triangular prisms with Mo atoms at the center, and there are two layers of repeat units in the crystal unit cell. The repeat unit has hexagonal symmetry. Similarly, the 3R phase also has triangular prisms but contains three layers of repeat units in the crystal unit cell. The repeat unit has rhombohedral symmetry.

In MoS₂ nanosheets, the crystal phases, sizes, and chemical compositions determine the electrocatalytic properties [18]. Martis et al. [19] used four-dimensional scanning transmission electron microscopy (4D-STEM) (Fig.2(e)) to generate annular dark field (ADF)-STEM (Fig.2(g)) and center of mass images (Fig.2(h) and 2(i)). This reveals that 2H-MoS₂ is a 2D semiconductor composed of Mo atoms sandwiched between two S atoms, and it has a direct band gap. The semiconducting properties and direct bandgap make it highly suitable for catalytic applications.

3 Modification of MoS₂ for HER

Although the active sites of MoS₂ have a hydrogen binding energy comparable to that of Pt, they are mostly located at the edges and are hardly present at the basal planes of MoS₂. The blocky structure of MoS₂ results in a prevalent concentration of VDW forces within the layers, leading to a lower coordination of the active sites at the edges. Furthermore, the atoms in the central layer of the blocky MoS₂ form covalent bonds, which results in fewer exposed active sites. Consequently, the HER performance is diminished [21]. To address this problem, researchers have invested tremendous effort in modifications.

3.1 Phase engineering

Unlike semiconductor-phase MoS₂, 1T-MoS₂ demonstrates a metal-like electrical conductivity, which is the reason that it is often referred to as metal-phase MoS₂. 1T-MoS₂ achieves this electrical conductivity due to the presence of unoccupied empty orbitals located near the Fermi energy level [22]. Reasonably, after the transformation from the 2H phase to the 1T phase, the electronic conductivity of MoS₂ is increased by approximately 10⁷ times. The phase transition of monolayer MoS₂ is accomplished through lateral displacement of S atoms within the S planes (as shown in Fig.3(a)), which is facilitated by intralayer atomic plane gliding [23]. In 2H-MoS₂, a layer of S atoms in the S−Mo−S structure undergoes displacement, resulting in the transformation from the original triangular symmetry to an octahedral coordination structure. The electrocatalytic activities of the different MoS₂ crystalline phases are completely different. This is attributed to the fact that active sites in 2H-MoS₂ are only generated by the edge uncoordinated S atoms, while the basal surface of 1T-MoS₂ exhibits a catalytic activity akin to the edge atoms. Moreover, the refined electronic conductivity and hydrophilicity of the 1T phase synergistically improve the electrolyte accessibility during electrolysis. Compared to semiconducting 2H-MoS₂, metallic 1T-MoS₂ exhibits a larger band gap, which promotes faster electron transfer. Manipulating the proportions of different phases in MoS₂ can alter the reaction rates for electrocatalytic reactions [24].

1T-MoS₂ is typically obtained via the “top-down” and the “bottom-up” methods: The “top-down” approach employs 2H-MoS₂ as the raw material for the production of 1T-MoS₂ by chemical, electrochemical, physical or a combination of physical and chemical techniques [17]. For instance, heavy electron doping, such as with Li⁺, Na⁺, and K⁺ intercalation, plasmonic hot electron doping, electron irradiation, and substitutional doping, is employed to introduce foreign species into 2H-MoS₂, thereby achieving the 2H → 1T transformation [25]. The “bottom-up” approach is primarily used to synthesize 1T-MoS₂ by using Mo sources and S sources and methods such as low-temperature chemical vapor deposition (CVD) and hydrothermal and solvothermal methods.

In “top-down” synthetic methods, the main approach is chemical/electrochemical intercalation with alkali metal ions. In MoS₂, the interlayer spacing between the atoms is relatively large, and the VDW forces between them are weak. Therefore, alkali metals are easily intercalated into the atomic layers. In general, researchers chemically exfoliate 2H-MoS₂ by inserting ions such as Li⁺, Na⁺, and K⁺ to obtain 1T-MoS₂. The resulting 1T-MoS₂ material is usually hydrophilic. Liu et al. [26] found that the 2H to 1T phase transition of MoS₂ in the lithium intercalation method is induced by insertion of the lithium atoms. The insertion of lithium reduces the energy barrier for movement of the S atoms. In 2H-MoS₂, the phase transition is induced by individual transitions of the S atoms rather than collective behavior. Tan et al. [27] proposed an approach that combined ball milling and chemical Li intercalation to achieve a high-yield syntheses of small monolayer MoS₂ nanoparticles, a high percentage of the 1T phase, and an exceptionally large specific surface area. Chen et al. [28] demonstrated a simple and direct strategy for intercalating zero-valent metals into bulk MoS₂ (Fig.3(b)). Since block-like 2H-MoS₂ itself does not have a strong reduction capacity, lithium was first intercalated into bulk MoS₂ to obtain 1T'-Li_xMoS₂. It was found that 1T'-Li_xMoS₂ had an increased electron density and a larger reduction potential, and this characteristic was used for in-situ reduction of the metal ions on the surface of Li_xMoS₂ to achieve intercalation of the zero-valent metal atoms. As shown by time-of-flight secondary ion mass spectroscopy (ToF-SIMS) (Fig.3(c)), the zero-valent metal atoms in Pt nanoparticles were successfully intercalated into MoS₂. From the stable 2H phase to the unstable 1T' phase, the insertion of Li also increases the interlayer distance of MoS₂, which allows for diffusion exchange between MoS₂ and the target metal salt ions, and the Li ions inserted into MoS₂ can be removed. Inserted metals can improve the stability of the MoS₂ 1T' phase for use in the HER.

Although the intercalation method is widely used in industrial production, it is limited by the use of hazardous organic solvents and explosive Li⁺ for embedding the ions, which requires inert atmospheres, as well as the slow kinetics of the intercalation/exfoliation processes. Electron irradiation can be used to optimize the properties and defects of materials by controlling the irradiation conditions, and the irradiation parameters can be used to adjust the structures of materials, thereby changing the chemical and physical properties and obtaining the best catalytic performance. Compared with the intercalation method, electron irradiation achieves a more precise modulation of materials without chemical residues affecting the HER performance. Dong et al. [29] performed electron irradiation on MoS₂ in an atmospheric environment with a high-voltage electron accelerator. After electron beam irradiation, the MoS₂ nanostructures exhibited a strong desulfurization and formation of the 1T phase in the prepared MoS₂, which was controlled by adjusting the energy density of electron irradiation. As the irradiation energy was increased, the content of the 1T phase increased, which generated more active sites and significantly improved the electrocatalytic activity of the inert MoS₂ base. Phase transition-induced atomic migration alters the charge distribution on the atomic surfaces, facilitating rapid transfer of H₂ at the catalytic interface. However, irradiation of MoS₂ with excessive energy can cause severe aggregation, resulting in decreases in the specific surface area and conductivity. It was necessary to control the energy density within a suitable range to obtain a good HER performance.

The “top-down” method of MoS₂ phase transitions can also be induced by plasma treatment and atomic doping. Zhu et al. [30] demonstrated that a weak Ar plasma treatment to activate the 2H → 1T phase transition of MoS₂ provided a 2H to 1T phase conversion ratio of about 40%. The proportion of the MoS₂ 2H → 1T phase transition realized by elemental doping has been limited from 40% to 60%, mainly because elemental doping occurs on the surface and cannot adjust the internal lattice. Sun et al. [31] discovered a way to promote the 2H → 1T phase transition of MoS₂ nanosheets by utilizing the synergistic effect of Pt atom doping and an N₂ plasma, and the specific steps are shown in Fig.4(a). They first directly grew 2H MoS₂ on carbon cloth (CC) by the hydrothermal method and then immersed the CC coated with MoS₂ nanosheets in H₂PtCl₆·6H₂O (0.5 mol/L, 10 mL) for Pt atom doping. The impregnated material was dried and then bombarded with an N₂ plasma for 2 h. The interlayer spacing of a material that was not soaked in the H₂PtCl₆·6H₂O solution increased from 0.64 to 0.67 nm, while the interlayer spacing of the material soaked in the H₂PtCl₆·6H₂O solution increased to 0.69 nm (Fig.4(b)). The larger interlayer spacing indicated that more of the 2H phase in the MoS₂ was converted to the 1T phase, suggesting that Pt doping improved the conversion rate of the 1T phase. The proportions of the 1T phase in N-MoS₂ and N, Pt-MoS₂ nanosheets are 62% and 87%, respectively. Synergistic doping of N and Pt atoms altered the atomic arrangement and electronic structure of MoS₂ (Fig.4(e) and Fig.4(f)), which subsequently activated the S vacancies and formed empty 2p_z orbitals that facilitated water adsorption and dissociation. N, Pt-MoS₂ nanosheets exhibited a lower overpotential (Fig.4(d)) similar to that of 20 wt% of Pt/C, suggesting that N, Pt-MoS₂ has a lower charge transfer resistance.

“Bottom-up” synthesis methods can address the complex synthesis limitations in “top-down” methods. Therefore, “bottom-up” one-pot methods such as hydrothermal and solvothermal syntheses are promising and direct strategies for preparing 1T-MoS₂. Solvothermal synthesis offers a simple approach to 1T-MoS₂. However, the effectiveness of this process in the 2H → 1T phase conversion is restricted, and it may generate unforeseen byproducts such as MoO₂ and MoO_3. Mai et al. [32] used a direct and effective solvothermal synthesis that optimized and quantified the 1T and 2H phases in MoS₂. During the solvothermal synthesis, the S²⁻ ions in thiourea (NH₂CSNH₂) reduced Mo⁶⁺ in the ammonium molybdate tetrahydrate ((NH₄)₆Mo₇O₂₄·4H₂O) to Mo⁴⁺, and then S²⁻ combined with Mo⁴⁺ to form MoS₂. Thiourea also produced OH⁻ during hydrolysis, which combined with Mo⁶⁺ and Mo⁴⁺ to form Mo_xO_y. They found that S²⁻ was more reactive at high temperatures, which inhibited the formation of Mo_xO_y. However, an overly high temperature converts the 1T phase to the 2H phase. Mai et al. [32] found that 220 °C was the optimal synthetic temperature, since it inhibited byproduct generation and provided a high proportion of the 1T phase, thus solving the main problem of the solvothermal method in the synthesis of 1T-MoS₂. Zhu et al. [33] synthesized 1T-MoS₂ with a magneto-hydrothermal method, which provided MoS₂ with a pure 1T phase at a magnetic field of 9T. The ΔG_H* of 1T-MoS₂ was 0.7 eV lower than that of 2H-MoS₂ (1.96 → 1.26 eV), and the modification with trace amounts of Ru reduced the ΔG_H* of 1T-MoS₂ from 1.26 to 0.02 eV. Therefore, it adsorbed hydrogen ions more easily.

Zhang et al. [34] developed a one-step solvothermal method to obtain 1T-MoS₂ with extended interlayer spacing derived from Mo-based organic frameworks. Metal-organic frameworks (MOFs) have been used for HER due to their exceptional properties, including high specific surface areas, tunable pore sizes, easily modifiable compositions, and diverse morphological structures. However, most MOFs have low conductivities and stabilities, which significantly limit their application. They used N, N-dimethylformamide (DMF) oxide to increase the surface spacing of MoS₂ and successfully synthesized 1T-MoS₂ with a 10.87 Å layer spacing. The specific experimental steps for the preparation of 1T-MoS₂ are shown in Fig.5(a). The uniform solution yielded Mo-based MOFs (Mo-MOFs) at 120 °C, which were totally transformed into 1T-MoS₂ after heating to 200 °C (Fig.5(b)). Scanning electron microscopy (SEM), transmission electron microscopy (TEM), HRTEM, and water contact angle images for 2H-MoS₂, 180-1T-MoS₂, and 200-1T-MoS₂ (Fig.5(d)) showed that their structures became progressively looser and more dispersed. As the samples progressed from 2H-MoS₂ to 180-1T-MoS₂ and then to 200-1T-MoS₂, their interlayer spacings increased, and they became more hydrophilic. This strategy avoided structural collapse of the MOFs during carbonization and coverage of the active sites on the materials. Furthermore, the HER catalytic performance of the MoS₂ was improved by increasing the crystal face spacing and reducing the adsorption–desorption potential for protons.

In industrial production, the “top-down” method is the main method used to obtain the 1T-phase of MoS₂. Generally, the “top-down” method is used to obtain monolayer 1T-MoS₂, but the structural stability of monolayer 1T-MoS₂ is poor. Due to the interlayer S-S VDW forces, 1T-MoS₂ is gradually converted to 2H-MoS₂, and the MoS₂ monolayers are stacked on top of each other, which masks the catalytic active sites. The “bottom-up” method is an inexpensive and simple route to 1T-MoS₂, but the hydrothermal/solvothermal reaction is not easily observed and difficult to regulate. Phase transformation is an effective strategy for modulating electronic properties and increasing the number of active sites, but there are still many challenges in phase transformation modification. The monolayer dispersion of 1T-MoS₂ impedes modification of the microstructure and modulation of the electronic structure, and the 1T-MoS₂ basal atoms, although active, are still less reactive than the edge sites, which restricts the catalytic activity. Researchers have introduced S vacancies, doped elements and conductive carbon substrates into MoS₂ to obtain highly efficient hydrogen generation with 1T-MoS₂. Phase-change engineering still has very broad prospects, and it is worth exploring in depth.

3.2 Morphology designs

Bulk MoS₂ usually has a low electronic conductivity. One way to improve the catalytic performance of MoS₂ is to adjust the electronic structure by creating an appropriate microstructure. Excellent work was performed to unveil the relationships between the morphology and electrochemical activity of MoS₂. In 2005, Hinnemann et al. [11] used density functional theory (DFT) calculations to show that the catalytically active sites of MoS₂ were located at its edges, while the basal surface was inert and usually not involved in construction of the catalytic active centers. Subsequently, researchers used various studies to maximize the density of edge active sites and expose more edge areas per unit area by morphological design engineering to modify the MoS₂ catalyst. For example, nanospheres, nanowires, nanosheets, vertically aligned nanosheet arrays, and mesoporous structures were constructed (Fig.6(a)).

Jiang et al. [35] synthesized MoS₂ nanoparticles via an electrochemical method and uniformly dispersed them on a reduced graphene oxide-modified carbon nanotube/polyimide (PI/CNT-RGO) film. The MoS₂ nanoparticles had highly exposed edge sites and exhibited a low overpotential and electrocatalytic activity for the HER. Yi et al. [36] used a solid-state synthesis to prepare MoS₂ nanorods in a one-step pyrolysis process with Mo-MOFs as the molybdenum source in the presence of thiourea. Behranginia et al. [37] synthesized MoS₂ with a 3D crystal structure and bare Mo-edge atoms by CVD. In this method, the temperature was precisely controlled to maintain the same vaporization rate for S and MoO₃ and prevent the generation of undesirable byproducts. Accordingly, the MoS₂ grew directly on the defects of glassy carbon (GC) and graphene/GC, forming well-defined 3D-MoS₂. The obtained MoS₂ had massive active edge sites, which provided a strong synergistic effect for HER by improving the electron transfer rate. Li et al. [38] employed a hydrothermal process with the surfactant polyethylene glycol (PEG) to synthesize MoS₂ samples with monodisperse spherical morphologies. This MoS₂-PEG sample had a minimum particle diameter of 250 nm with a larger interlayer spacing, shorter plates, and fewer stacked layers, resulting in a higher catalytic activity. Bhimanapati et al. [39] used CVD to deposit MoS₂ powder onto high-quality graphite paper and silicon substrates to obtain flower-like MoS₂ structures.

Recently, Van Nguyen et al. [40] used (NH₄)₆Mo₇O₂₄·4H₂O and thioacetamide (TAA, CH₃CSNH₂) with heating and annealing in the presence of ammonia and hydrochloric acid, respectively, to obtain MoS₂ nanoflowers (MoS₂ NFs) and ultrasmall MoS₂ nanoflowers (MoS₂ SNFs). The step of generating intermediates was controlled by varying the temperature and the concentration of H to obtain nanoflowers of different sizes. Specifically, the sizes of the obtained MoS₂ SNFs were only 50–90 nm, which were significantly smaller than conventional MoS₂ nanoflowers (900–1500 nm). The decreased sizes exposed more active edge sites. In addition, N-doping enhanced the HER kinetics.

The structural modifications above were designed to increase the densities of edge sites by reducing the thicknesses and sizes of the particles, while the mesoporous structure was mainly intended to enhance the catalytic activity by increasing the ratio of the edges to the basal planes and expose more active sites. Kibsgaard et al. [41] used the previously reported double-gyroid (DG) mesostructure of Pt and SiO₂ and synthesized the first fully contiguous large-area mesoporous MoS₂ thin film (Fig.6(b)). Compared to traditional 2D mesoporous structures, three-dimensional (3D) mesoporous structures have higher structural stabilities and diffusion rates that are an order of magnitude faster than those of other mesoporous structures. Deng et al. [42] reported a well-defined mesoporous MoS₂ foam (mPF-MoS₂), as shown in Fig.6(c). The mPF-MoS₂ was obtained by attaching Mo precursor to SiO₂ nanospheres and reacting with CS₂ on the surface to form MoS₂, which were self-assembled into vertically aligned structures, followed by SiO₂ template etching using hydrofluoric acid solution. The MoS₂ nanosheets oriented and grown vertically around the mesopores, providing many edges and active sites. A distinct mesopore can be observed in the SEM image of mPF-MoS₂ (Fig.6(d)), while the images of Fig.6(f)–Fig.6(i) reveal the presence of abundant spherical voids resulting from the removal of SiO₂ nanospheres, during the formation of a uniform porous framework. Many uniform mesopores in mPF-MoS₂ facilitated mass transport and delivery of H₃O⁺ and H₂. Moreover, the curved surfaces of MoS₂ with abundant mesopores in a 2D plane induced strain, which enhanced the electrocatalytic activity. Notably, mPF-MoS₂ with uniform mesopores exhibited a significantly improved HER performance when compared to randomly oriented MoS₂ nanosheets.

Among the many nanostructures studied, quantum dots (QDs) have numerous advantages. Ren et al. [44] synthesized MoS₂ QDs from sodium molybdate and dibenzyl disulfides raw materials by a hydrothermal approach. These QDs exhibited zero-dimensional structures with high specific surface areas and unique defect-rich configurations, which provided abundant active edge sites to enhance the efficiency of the HER. Additionally, the monolayer structure of the MoS₂ QDs enabled smoother electron transfer on the basal plane by eliminating interlayer barriers in the vertical direction. As a result, these MoS₂ QDs demonstrated a remarkable electrocatalytic activity, making them highly promising electrocatalysts for hydrogen production. Vikraman et al. [45] synthesized MoS₂ directly via chemical bath deposition with (NH₄)₆Mo₇O₂₄·4H₂O and CH₄N₂S as the Mo and S sources, respectively (with the experimental setup shown in Fig.7(a)). This approach provided control of the thicknesses of the MoS₂ crystal layers while maintaining large-area uniformity on different substrates. Along with nucleation and growth of the MoS₂ crystals, the resulting MoS₂ layers exhibited the properties of QDs, including edge effects and improved catalytic performance. SEM images (Fig.7(b)–Fig.7(m)) revealed that the MoS₂ layers were composed of crystalline QDs. After 2 min, the deposited MoS₂ exhibited a relatively smooth surface, and with increasing deposition time, larger grains uniformly grew on the surface of the MoS₂. The growth of MoS₂ crystal QDs required uniform nucleation, with larger agglomerates appearing as the layer thickness increased. It has been demonstrated that the thicknesses of MoS₂ crystal QD films can be adjusted, and uniformity can be maintained.

Morphological design exposes more edge activity, which is important for improving the hydrogen evolution activity of MoS₂ by expanding the exposure of surface atoms while increasing atom utilization (see Tab.1 for electrochemical properties of typical catalysts). A controlled morphology enables fast kinetics for the hydrogen adsorption/desorption process. However, constructing edge-enriched MoS₂ is challenging due to the thermodynamic disadvantages of edge sites relative to the basal plane. Enhanced active site exposure originating from morphological design can provide a higher catalytic HER activity. However, morphology design and optimization of MoS₂ does little to enhance the intrinsic activity and accelerate electron transfer from the substrate surface. The controlled morphology of MoS₂ should be used in conjunction with other modification methods to improve the overall catalytic HER activity and conductivity.

3.3 Defect engineering

Defect engineering is highly effective in creating active sites in 2D MoS₂. Defect engineering involves creating defects on MoS₂ to manipulate the microstructure. The generation of defects increases the number of active sites, modifies the coordination environments around the active centers, and optimizes the electronic structure. Defect engineering strategies can be divided into edge defects, atomic vacancy defects, and doping defects; the first two are intrinsic defects and the latter describes extrinsic defects.

Intrinsic defects are those arising from the incomplete structure of a crystal itself, without any foreign atoms doped (SEM images of common intrinsic defects are shown in Fig.8(a)‒Fig.8(e) [46]). These defects are usually classified into atomic vacancies and edge defects. Atomic vacancies are usually S atom vacancies in the MoS₂ substrate, while edge vacancies are mainly caused by the rich boundaries and pore structure in 2H-MoS₂. The process involves the introduction of vacancies on the substrate, flexible adjustment of the local catalytic environments surrounding the adjacent atoms, formation of unsaturated coordination states, and optimization of hydrogen adsorption/desorption, all of which fundamentally optimize the HER activity [47]. While both metallic and nonmetallic sites positioned at the edges of MoS₂ exhibit catalytic activity, higher proportions of edge defects enhance the exposure of these active sites to obtain a better catalytic activity.

The catalytic activities of the metal sites in the basal plane of MoS₂ are often hindered by the presence of nonmetallic sulfur atoms covering these sites, resulting in a limited catalytic activity. DFT calculations revealed that the S vacancies (S_v) present in the basal plane of MoS₂ migrated along the empty conduction band previously occupied by S_v-related S 3p and Mo 4d orbitals. The primary factor contributing to the HER activity of MoS₂ defects is the alignment of energy levels between the empty conduction band and the reduction of H⁺ ions [48]. The presence of S_v in MoS₂ has been shown to enhance the kinetics of HER by increasing the exposure of unsaturated S−Mo bonds. This exposure optimizes the ΔG_H* for the surface of MoS₂. At an optimal ΔG_H* value of 0 eV, hydrogen exhibits equilibrium binding that is neither too strong nor too weak. Li et al. [49] used DFT calculations (Fig.8(f) depicts the activation of the basal plane) to show that the introduction of 3.12% S_v (% represents the percentage of S atoms removed) provided stability, and the exposed Mo atoms provide sites for hydrogen bonding, resulting in a decrease in ΔG_H* to 0.18 eV (Fig.8(g)). The DFT calculations also showed that the valence band of MoS₂ shifted downward when different 3d transition metal (3d-TM) dopants replaced the unsaturated Mo atoms surrounding S_v sites (Fig.8(h)–8(i)) [50]. This indicated that coupling between the inert plane S defects and the surrounding 3d-TM dopants activated the MoS₂ basal plane, thereby enhancing the electrical conductivity and electron mobility.

Li et al. [49] obtained MoS₂ with abundant S vacancies and an excellent catalytic activity via treatment with an Ar plasma. Xu et al. [51] designed and synthesized a Frenkel-defect monolayer MoS₂ (FD-MoS₂) catalyst by annealing monolayers of MoS₂ in an Ar plasma and generating intrinsic defects, as described in Fig.9(a). FD-MoS₂ exhibited a phenomenon in which some of the Mo atoms spontaneously left their lattice sites, which created vacancies, and the atoms lodged into adjacent interstitial sites. This resulted in a distinct surface charge distribution for the MoS₂, which enhanced the adsorption of H atoms at the vacancies. An increased proportion of Frenkel defects does not always lead to continuous improvement in HER performance. Monolayer MoS₂ is easily damaged and fragmented into small islands during bombardment with the Ar plasma. Furthermore, a strong Ar plasma can disrupt the bonding of MoS₂, resulting in small fragments and a loss of structural integrity. This indicates that strong ions may not be ideal for generating S_v sites on MoS₂. Cheng et al. [52] found that by treating monolayer MoS₂ with an H₂ plasma, the atomic ratio of Mo and S was changed from 1:1.97 to 1:1.20. This provided a wider range of atomic ratios than defect generation with an Ar plasma or O₂ plasma. They successfully induced the formation of S_v sites on the basal planes of MoS₂ monolayers with a remote H₂ plasma (Fig.9(b)) and investigated the catalytic performance with different vacancy densities. Fig.9(c) demonstrates that the structures of the monolayer MoS₂ nanosheets remained intact and unaffected by treatment with the H₂ plasma, i.e., there was no fragmentation. Compared with the initial MoS₂, the MoS₂ treated with the H₂ plasma for 15 min showed a 544 mV decrease in the overpotential at 10 mA/cm² (Fig.9(d)), as well as a significantly decreased Tafel slope of 77.6 mV/dec (Fig.9(e)). In other performance tests, the MoS₂ monolayers treated with the H₂ plasma also showed a good performance and a high stability for HER in acidic environments (the electrochemical performance being illustrated in Fig.9(f) and Fig.9(g)).

The introduction of S vacancies on MoS₂ substrates with plasma or annealing treatments usually requires high temperature/pressure synthetic conditions, which is not energy efficient. The use of mild electrochemical desulfurization has been effective in generating S vacancies on MoS₂ substrates. Tsai et al. [53] showed with DFT calculations that the S vacancies in the basal plane were thermodynamically favored at a strong reducing potential. S vacancies were formed in the basal plane by hydrogenation of S atoms and subsequent elimination as H₂S gas. Therefore, the defect concentration could be controlled by changing the desulfurization potential. The active sites remained stable over extended periods of desulfurization. Laser ablation in liquid (LAL) is another environmentally friendly technique for generating very high pressures and temperatures at the moment of laser ablation and rapid quenching between the laser pulses for defect formation. Meng et al. [54] synthesized pristine MoS₂ (P-MoS₂) through a hydrothermal method, dispersed the P-MoS₂ in ethanol under ultrasonic treatment, and then obtained laser-treated MoS₂ (L-MoS₂) through pulsed laser irradiation. S vacancies were generated on the basal plane of L-MoS₂ after the laser treatment, and there were only slight deformations.

Air oxidation etching is commonly used for introducing active defects, which results in the formation of well-oriented triangular pits, also known as antidots, on the surfaces of MoS₂ samples. These pits improve the catalytic performance of bulk MoS₂. Lv et al. [55] oxidized and etched MoS₂ samples by heating them with air in a mini-CVD furnace. To investigate the microscopic oxidation etching process of 2D MoS₂ at the atomic level, the researchers used the TEM etching technology, heated the furnace to 300 °C within 10 min and maintained this temperature for 5 min for etching. With ADF-STEM images, they analyzed the types and distributions of the residual edge structures at different positions to understand the mechanism for oxidation etching. Their results revealed a kinetic pathway for oxidation etching of 2D MoS₂, in which the process was initiated from the edges of the material, as well as from the atomic defects. The edge sites of MoS₂ were accompanied by intrinsic structural defects. By increasing the exposure of these edge active sites, both the density of edge defects and the electrocatalytic performance for the HER were improved. Xie et al. [56] achieved this by inducing controlled cracks on the nanosheet surface to expose active edge sites.

The introduction of atomic vacancy defects optimizes the local electronic configuration of the MoS₂ planes, and the introduction of edge defects generates more active sites, both of which modulate the intrinsic properties of MoS₂ to produce more active centers. This results in synergistic modulation of the vacancy concentration and distribution and thus provides significant improvement in the catalytic performance. However, intrinsic defects, due to the presence of atomic vacancies and edge defects, make it difficult to distinguish their individual roles in catalytic behavior. Moreover, the defect structures are less stable, as they are subject to remodeling in the presence of strong acid and base. The introduction of defects into bulk MoS₂ by dopant atoms is discussed in the next section.

3.4 Heteroatom doping

Heteroatom doping is a simple method used to enhance the catalytic activity of MoS₂. It involves embedding dopant atoms into the MoS₂ structure to create a uniform and stable composite catalyst. The primary purpose is to modulate and optimize the catalytic performance by combining the 2D structure of the basal plane with the electronic structures of the doped atoms. The doped atoms are firmly incorporated into the MoS₂ nanosheets through crystal alignment or strong covalent bonds. Because they remain coordinatively unsaturated, they enhance the electronic structure of the MoS₂ surface, increase the intrinsic activities of individual catalytic sites, provide more active sites for catalytic reactions, and reduce the ΔG_H*. The changes occurring in chemical bonding during the doping process are essential for optimizing the performance of MoS₂. Atomic doping can be divided into two main types: metal (cation) doping and nonmetal (anion) doping [57].

MoS₂ has a very high specific surface area and an excellent chemical stability and can be used as a support for Pt-based catalysts. According to both DFT and experimental results [58], strong bonds are formed between S and Pt atoms. This strong electronic coupling between the S and Pt atoms lowers the d-band center of Pt, which facilitates electron transport during HER [59]. Therefore, MoS₂ with a large specific surface area and an excellent stability can be used as a support for Pt-based catalysts. The contact interface between the Pt and MoS₂ is thus a highly active catalytic site for HER. Shan et al. [60] employed a wet chemical method with sodium borohydride (NaBH₄) as the reducing agent to synthesize a series of MoS₂ catalysts with different Pt particle coverages and investigated their catalytic properties. In addition, it was reported that modifications of Pt nanoparticles led to lattice strain and defects in MoS₂ so that the Pt−S bonds increased the specific surface area, prevented the agglomeration of Pt, and improved the activity and stability.

By introducing single atoms into the catalyst, the electronic structures of the active sites can be strained. Inspired by this approach, Jiang et al. [61] used CVD and chemical etching to fabricate nanoporous MoS₂ with a bicontinuous structure, which was termed np-MoS₂. Considering the remarkable performance of Ru in HER, they anchored single Ru atoms on MoS₂, which was denoted as Ru/np-MoS₂, and investigated the synergistic effect between the Ru sites and Sv. Based on a previous report [62] that explored doping with isolated metal atoms to activate the MoS₂ substrate, the researchers used theoretical calculations and proposed that the introduction of individual Ru atoms into MoS₂ could lead to displacement of the surrounding S atoms. This process would be accompanied by a phase transformation, resulting in the formation of Ru/1T-MoS₂. Theoretical results indicated that the incorporation of Ru atoms replaced the Mo atoms, demonstrating the potential of Ru doping in generating S_v sites. DFT has been used to study the synergistic effects of Ru atom doping and the tensile strain on MoS₂ (Fig.10(a)). It was observed that the external strain increased the accumulation of OH⁻ and H₂O within the S_v sites, thereby enhancing the synergistic effect between the sulfur vacancies and Ru sites (Fig.10(b)). Fig.10(c) shows a greater degree of orbital overlap between the Mo 3d orbitals and O_ads 2p orbitals below the Fermi level, while the bonding energy between *OH₂ and \text{M}{\text{o}}_{\text{Sv}} was stronger, and ΔG_H* (Fig.10(d)) was also closer to 0 for Ru/1T-MoS₂. Consequently, HER was accelerated at the Ru sites. The resulting catalyst exhibited an exceptional electrochemical performance in alkaline environments for HER.

The two dopants mentioned above were noble metals. In addition to noble metal doping, Shi et al. [63] used non-noble metals (Fe, Co, Ni, Cu, and Zn) and doped MoS₂ via a one-pot hydrothermal method. The heteroatoms reduced the hydrogen adsorption barrier and enhanced the HER catalytic activity of MoS₂. In their study of atom-doped catalysts, they found that doping of Fe or Co into MoS₂ did not improve the catalytic performance. The improvement in HER performance by Ni-doping was found to be negligible. Interestingly, Cu and Zn doping of MoS₂ provided improved catalytic activity for HER. The different doping amounts of the Cu and Zn atoms impacted the electrocatalytic performance in HER. For example, 1-fold was the optimal doping amount for Zn−MoS₂, while Cu-MoS₂ exhibited a better activity when the doping amount was 2-fold. Among them, Zn exhibited the best activity with 1-fold doping; η = 200 mV, and the current density of MoS₂ increased by 13 times, indicating a fast electron transfer. The starting potential was −0.13 V vs. RHE, and the Tafel slope was reduced from 101 to 51 mV/dec. Li et al. [64] used a simple one-step hydrothermal method to dope vanadium (V) into MoS₂ nanosheets grown vertically on carbon paper. The MoS₂ samples doped with 5% vanadium showed the ΔG_H* values closest to 0. Vanadium doping increased the number of active sites while causing the MoS₂ 2H → 1T phase transition. Both doping and the phase transition sharply reduced the resistance, significantly increased the conductivity of the samples, promoted electron transfer, and improved the intrinsic activity of MoS₂ while enhancing the HER performance.

In addition to doping MoS₂ with various metals, some researchers also reported nonmetallic doping (such as O, C, P, N, F, S, and Cl) to optimize ΔG_H* and form noncrystalline structures or lattice distortions. This approach exposed more active centers. Nonmetallic doping changed the electronegativity and produced distorted bond lengths and angles, resulting in a redistribution of charge density at the doped sites. Oxygen incorporation into MoS₂ nanosheets provided more unsaturated S atoms as active sites, which facilitated charge transfer during electrocatalysis [65]. Yang et al. [66] used vertical growth and fabricated a nanosheet array of etched O-MoS₂ on CC with NH₄F etching. The resulting nanosheets exhibited sharp edges and high crystallinities due to their growth on CC. The NH₄F etching process introduced disordered structures into the O-MoS₂ nanosheets, resulting in an abundance of unsaturated S atoms that served as active sites for electrocatalysis and reduced the charge transfer resistance. The number of unsaturated S atoms obtained in the resulting samples was closely related to the etching temperature.

In conclusion, the introduction of heteroatoms regulates the electronic structure of MoS₂ to obtain ΔG_H*\cong0. However, agglomeration often occurs during introduction of the metal atoms, which reduces atomic utilization, and the S vacancies provide doping sites for the atoms. Therefore, some researchers created S vacancies in MoS₂ prior to, or at the same time as, the introduction of metal atoms to improve atomic utilization. Although there have been many reports of doping MoS₂ with heteroatoms, the synergistic effects of diatoms or even triatoms should be further investigated.

3.5 Heterostructure construction

Due to the synergistic effect of the interface, heterostructures exhibit an enhanced electrochemical performance compared to single-component structures. Abundant interfaces between different components can be introduced into mixed catalysts, thereby optimizing the electronic configuration and improving the electrochemical activity [67].

Through synergistic interactions in multiscale electronic structures, the intrinsic activity of each active site and the density of active sites are enhanced, leading to an improved electrochemical accessibility. Theoretical findings suggested that Ru doping activated the inert basal planes of MoS₂, enhancing processes such as water adsorption/dissociation and hydrogen adsorption/desorption.

Zhang et al. [68] designed a catalyst with tunable compositions and novel core-shell structures for Ru-doped MoS₂ nanosheets, in which the outer shell surrounded multiwalled carbon nanotubes (CNTs) called Ru MoS₂/CNTs. The catalyst was characterized by small layers of Ru-doped MoS₂ nanosheets tightly wrapped around the CNTs. Theoretical calculations suggested that stable doping of Ru atoms into 2H-MoS₂ could be achieved by substituting Mo atoms and coordinating with 6 adjacent S atoms in the substrate. However, due to the low intrinsic reactivities of the Mo edge sites, the effect of Ru edge doping on the HER was minimal [69]. The experimental results suggested that the core-shell structure of Ru-MoS₂/CNTs reduced the energy barriers and enhanced mass transfer. The optimal HER activity was achieved with Ru MoS₂/CNT and an Ru percentage of 5% (mass fraction).

Nguyen et al. [70] discovered a non-noble metal-doped heterostructure. This structure consisted of Co and Nb incorporated into hierarchical MoS₂ ultrathin nanosheets that were then hydrothermal (HT) grown directly on micro-TiO₂ hollow spheres (referred to as Co, Nb-MoS₂/TiO₂ HSs). The specific steps are shown in Fig.11. The surface of the spherical hollow structure was coated with ultrathin and mesoporous MoS₂ nanosheets, which increased the contact area of the electrolyte, enhanced diffusion of the electrolyte, and improved the charge transfer capability. The dual doping with Co and Nb changed the electronic structure of the MoS₂ host, which enhanced the catalytic performance and increased the electrochemically active surface area (ECSA). Polystyrene microspheres (PS) and titanium dioxide were used to create core-shell microspheres (PS@TiO₂) as substrates for deposition. The surfaces of the spheres were rough and provided many nanovoids that served as chambers for loading additional MoS₂ nanosheets. This configuration exposed a greater number of active sites at the periphery [71]. Additionally, electrons were easily transferred from TiO₂ to the outside Co, Nb-MoS₂ NSs, thus providing a good catalytic performance in HER.

Shah et al. [72] combined metal- and nonmetal-containing MoS₂ nanosheets with extended interlayer spacings, which were grown vertically on N-doped nickel-carbon (Ni@NC) substrates. This resulted in the formation of Ni@NC@MoS₂ submicrospheres. This unique heterostructure exhibited an excellent catalytic performance for HER and a low Tafel slope of 47.5 mV/dec. Furthermore, the Ni@NC@MoS₂ catalysts showed an improved HER catalytic activity even after 3000 cycles in acidic conditions.

In addition to metal-doped heterostructures, nonmetal-doped heterostructures are also available. Wang et al. [73] synthesized a porous carbon network confining ultrasmall nanocrystals of N-doped MoS₂ (N-MoS₂/CN) via a self-templating strategy (Fig.12(a) and 12(j)). The SEM images (Fig.12(b)) show the distinct porous nature of the MoS₂/g-C₃N₄ composite, in contrast to the comparatively bulky structure observed for the g-C₃N₄ bulk material [74]. The type-II isotherm of N-MoS₂/CN with a hysteresis loop is shown in Fig.12(c), indicating a mesoporous structure [75]. The X-ray diffraction (XRD) pattern (Fig.12(d)) for N-MoS₂/CN exhibited broad peaks with low intensities, indicating the presence of ultrasmall crystallites of MoS₂ and defects induced by the N dopants in the MoS₂ structure. As shown in Fig.12(e), the hierarchical interconnected structure of N-MoS₂/CN was similar to that of a loofah sponge and exhibited a mesoporous morphology. The construction of N-doped edge sites and heterostructures resulted in an overpotential of 114 mV at 10 mA/cm² for N-MoS₂/CN (Fig.12(f)) and a low Tafel slope of 46.8 mV/dec (Fig.12(g)), suggesting a fast Heyrovsky-dominated Volmer-Heyrovsky mechanism [76]. The strong electronic coupling between the N-MoS₂ nanocrystals and the carbon matrix made a great contribution to this improved activity. By extrapolation from the Tafel diagram, the exchange current density for N-MoS₂/CN was 140 μA/cm², which was markedly higher than those of MoS₂ (11 μA/cm²), MoS₂/CS (1.6 μA) and MoS₂/CS (1.6 μA). This catalyst also presents a promising stability (Fig.12(h)).

In heterostructure construction, the bonding interactions at the interfaces between different components often increase the rate of electron transfer. The conductivity, hydrophilicity, chemical stability, and active site density of the heterostructure can be modulated by bonding with different materials. The different energy band structures of different phases enable charge transfer at the interface, which facilitates electron modulation of the heterostructure surfaces. Efficient construction of heterostructures, especially those that combine high specific surface areas with fast mass transfer rates, remains a significant challenge.

4 Summary and outlook

4.1 Summary

The application of MoS₂ as an old but promising electrocatalyst toward HER has been widely explored. Even the most recent research works are still focusing on MoS₂ [77–81]. The continuous interest on this material is due to its tailorability, which is highly important to replace the state-of-the-art Pt/C HER catalyst.

To obtain well-defined MoS₂ catalyst, researchers have proposed a series of modification strategies to improve the intrinsic HER activity of MoS₂, including phase engineering, morphology design, defect engineering, heteroatom doping, and heterostructure construction. These strategies are efficient to control the active sites on MoS₂. For example, the 1T phase of MoS₂ is preferred due to its high active site density and fast charge transfer. Conventional synthesis methods lead to low yield of exfoliated 1T-MoS₂. Precise phase engineering, e.g., the “top–down” and “bottom–up” strategies as discussed in this paper, is thus important to adjust the electronic structure and obtain a stable 1T phase. Besides, the morphology control is another nonnegligible factor, such as high ratio of edge to plane surface to obtain more available active sites, size reduce to facilitate mass transfer. In addition, atomic vacancy defects can enhance the catalytic performance by modulating the electronic structure and thus altering the local catalytic environment of the surrounding neighboring atoms. Edge defects can enhance catalytic performance by promoting the exposure of edge active sites. Among the various methods of defect engineering, the type of defects is hard to be controlled, so that the effect of specific types of defects on catalytic performance is still not clear. Therefore, methods for preparing a single type of defect need to be explored. The introduction of heteroatoms induced a Fermi energy level shift of MoS₂, which introduced excess electrons and holes into MoS₂ and increased more active sites. Moreover, the dopant atoms interact with the electrons on the surface of MoS₂, changing the MoS₂ electron transfer process and thus altering the distribution of the local charges at active sites. Although various heteroatom doping in MoS₂ has been investigated, it is still a great challenge to synthesize homogeneously heteroatom doped MoS₂ catalysts. Heterostructures improve the activity and durability of electrocatalysts by compositing different components and achieve the redistribution of electrons at the interface to play a cooperativity role, and by modulating the composition and crystalline phases of the materials so that the different interfacial components form a so-called coupling effect. However, the coupling effects between different components are still poorly understood. More work is still needed to investigate the coupling effect of different interfacial components in heterostructures.

4.2 Outlook

Although MoS₂ has been intensively and extensively explored since it was discovered, it still attracts wide interest due to its unique properties, particularly toward HER [82]. To promote the application of MoS₂ catalyst in HER, there are some perspectives to be noted.

Among the various preparation methods for MoS₂-based catalysts, one of the major difficulties is that the existing preparation methods are unable to precisely regulate the structure of active sites of MoS₂-based catalysts. Therefore, some improved manufacturing techniques are expected to be used to achieve precise control of the material structure, for example, atomic layer deposition, metalorganic CVD and wet chemical methods of colloidal synthesis.

Significant progress has been made in the modification of MoS₂ in recent years, but the recognition of active sites during the electrochemical process as well as their evolution are still unclear, which poses barriers for further activity and stability optimization. The in-situ characterization technique is a good solution to overcome these difficulties. It can not only understand the surface morphology and structure of initial catalysts, but also observe the catalytic active sites under the working conditions and monitor the dynamic change. For example, in-situ X-ray photoelectron spectroscopy (XPS) and in-situ X-ray absorption spectroscopy (XAS) can be used to monitor the electronic state, determine the number of active sites, and judge the initial and intermediate states of a reaction in time. These in-situ characterization techniques can guide the synthesis of highly active and stable MoS₂ catalysts by exploring the relationship between the structure and properties of different modified MoS₂.

Theoretical calculations are generally used to study the electronic properties of catalysts, electron transfer energy barriers, catalyst activity and stability by calculating the energy band structure of catalysts, adsorption energy, differential charge density and energy profile for the HER process. The minimum energy path during hydrogen evolution can be modeled by calculations. The results obtained experimentally can be interpreted theoretically by calculations. By combining theoretical calculations with practical experimental results, it is possible to gain insights into the underlying reaction mechanisms and degradation mechanisms, which can guide future research and development of MoS₂ with excellent properties.

In summary, the modification of MoS₂ has been greatly developed, showing that it indeed has a great potential for replacing noble metal toward HER. Although MoS₂ is still in its early stages of industrialization, it has the potential to be implemented in membrane-electrode assemblies as an alternative to noble metal catalysts.

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