Strategies to improve electrocatalytic performance of MoS2-based catalysts for hydrogen evolution reactions

Electrocatalytic hydrogen evolution reactions (HERs) are a key process for hydrogen production for clean energy applications. HERs have unique advantages in terms of energy efficiency and product separation compared to other methods. Molybdenum disulfide (MoS2) has attracted extensive attention as a potential HER catalyst because of its high electrocatalytic activity. However, the HER performance of MoS2 needs to be improved to make it competitive with conventional Pt-based catalysts. Herein, we summarize three typical strategies for promoting the HER performance, i.e., defect engineering, heterostructure formation, and heteroatom doping. We also summarize the computational density functional theory (DFT) methods used to obtain insight that can guide the construction of MoS2-based materials. Additionally, the challenges and prospects of MoS2-based catalysts for the HER have also been discussed.


Introduction
With the rapid growth of the economy, the traditional, polluting sources of energy are being consumed heavily, resulting in environmental degradation; 1-3 thus, developing sustainable and clean energy has become urgent. Among all the alternative energies, hydrogen has signicant advantages due to its high energy density and environmental friendliness. 4 Currently, mass H 2 production mainly comes from steam reforming of hydrocarbons, which is highly energy-consuming and relies on the polluting petroleum industry. [5][6][7] To promote the adoption of the H 2 economy, in the past several decades, new H 2 production technologies, like photocatalytic and electrocatalytic water splitting, have been developed. [8][9][10][11][12] Compared with photocatalytic water splitting, the electrochemical approach has more advantages in terms of energy efficiency and achieving effective product separation. Electrochemical water splitting consists of two half-reactions, which are named as the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). 13 The OER is hard to occur, because it involves four-electron transfer. 14 The reaction mechanism is as follows: Total reaction: In acidic solution: Cathode: 2H + (aq) + 2e À / H 2 (g) Anode : H 2 OðlÞ/2H þ ðaqÞ þ 1 2 O 2 ðgÞ þ 2e À In neutral and alkaline solution: Cathode: 2H 2 O(l) + 2e À / H 2 (g) + 2OH À (aq) Anode : 2OH À ðaqÞ/H 2 OðlÞ þ 1 2 O 2 ðgÞ þ 2e À Generally, the HER consists of the three following steps: Volmer step: H + + e À / H* Heyrovsky step: H* + H + + e À / H 2 (7) Tafel step: 2H* / H 2 + 2* In acidic electrolytes, the HER occurs when protons are adsorbed at the surface of the MoS 2 and consequently, step (6) takes place, which is then followed by step (7) or (8). The overall reaction rate relies on the Gibbs free energy of H 2 adsorption (DG H* ). In theory, the closer this value is to zero, the greater is the HER activity. The reactions (7) or (8) are inhibited when the binding of H to the catalyst surface is weak. Therefore, catalysts with excellent HER performance tend to exhibit good adsorption of H atoms, which is conducive to charge transfer and bond breakage, resulting in the formation of H 2 . 9, [15][16][17][18] Under standard conditions, initiating HER requires a dynamic overpotential, which essentially involves additional energy consumption; thus, electrocatalysts are integral to abate the overpotential. As of today, Pt-based materials are the most widely used electrocatalysts; however, Pt is scarce and costly, making it essential to develop cost-effective electrocatalysts. Currently, a wide range of materials, including transition metal alloys, [19][20][21][22][23][24][25][26][27] transition metal carbides, [28][29][30][31][32][33][34][35][36] transition metal dichalcogenides, [37][38][39][40][41][42][43][44][45] transition metal phosphides, [46][47][48][49][50] and so on, are being studied as possible alternative HER electrocatalysts. Among them, MoS 2 has attracted wide attention due to its unique two-dimensional layered structure and adjustable band gap width, which makes MoS 2 has a property of ease incorporation into the lattice (heteroatoms and vacancies) and adjustability of electron transport. However, experimental and theoretical studies reveal that the HER activity of MoS 2 remains unsatisfactory due to three main issues. First, the HER activity of MoS 2 correlates with the active edge sites rather than basal planes, but the edge sites just account for a small part of the surface area of MoS 2 . Second, the semiconducting basal plane of MoS 2 has a low intrinsic electronic conductivity, which leads to poor charge transfer. Third, the aggregation and restacking of MoS 2 hinders the availability of edge sites. Herein, we discuss three important approaches that have been demonstrated to improve the intrinsic catalytic activity and HER performance of MoS 2 effectively, i.e., defect engineering, formation of the heterostructure, and heteroatom doping (Fig. 1).

Properties of MoS 2
MoS 2 is a typical layered transition metal dichalcogenides composed of stacking S-Mo-S layers by van der Waals interactions, leading to a sandwich-like layered structure. 51,52 The basal plane of MoS 2 is generally believed to be inert toward HER, while the edge sites of MoS 2 layers possess much higher chemical reactivity than the basal plane. 13 Based on the arrangement of the S atoms, three phases of MoS 2 can be obtained, i.e., 1T, 2H and 3R. 53 The 2H phase MoS 2 is thermodynamically stable, but the conductivity of 2H phase MoS 2 is poor. 54,55 Compared to 2H MoS 2 , 1T MoS 2 is metallic and is 10 7 times more conductive than 2H MoS 2 . 56 Nevertheless, 1T MoS 2 is thermodynamically unstable and does not exist in nature 57 (Fig. 2). Under some specic conditions,1T MoS 2 can also exist and phase transformation is one of the most effective methods to improve the catalytic performance of MoS 2 . 58, 59 Zhao et al. 60 investigated the HER dynamics in the MoS 2 basal plane of the 1T and 2H phases. They calculated the absorbed H free energies on the 2H/1T structural interfaces and their work offered a new way to increase the number of MoS 2 active sites on the basal plane. DFT calculations demonstrated that MoS 2 had a similar H binding energy as that of Pt. 61 In theory, MoS 2 should exhibit outstanding HER performance; however, bulk MoS 2 displays poor performance due to the following reasons. (i) The active sites of bulk MoS 2 cannot be fully exposed. 62 (ii) The superimposition of the van der Waals force in the interior of the bulk MoS 2 leads to an incongruous effect of the active sites. 63 (iii) The covalently bonded atoms in the layers lead to poor HER performance. 64 (iv) The electronic conductivity of bulk MoS 2 is low. 65 Thus, many efforts have been devoted to designing a reasonable MoS 2 nanostructure and modulating the MoS 2 electronic structure. 66 nanosheets. [75][76][77][78][79] These nanostructures can maximally expose active edge sites of MoS 2 and shorten the electron transfer distance. 80

Synthetic methods for MoS 2
In order to synthesize MoS 2 , many strategies have been developed, including mechanical exfoliation, 81-83 solution exfoliation, 84-86 chemical vapor deposition [87][88][89] (CVD), hydrothermal synthesis, 90-92 electrodeposition methods, 93-95 metal-organic chemical vapor deposition (MOCVD), [96][97][98] and molecular beam epitaxy (MBE). [99][100][101] These methods can be classied as topdown methods (including mechanical exfoliation and solution exfoliation) and bottom-up methods (including hydrothermal synthesis, CVD, MOCVD and MBE). In the top-down methods, the morphology of MoS 2 can be well controlled since the structure of the nal product is inherited from the original materials. 102 However, top-down methods have intrinsic shortcomings; for instance, the nanoscale morphology of the nal product is hard to adjust since it is difficult to change the distance between ions and atoms in top-down methods. 103 In bottom-up methods, a large and complicated system are formed by atoms, molecules and nanoparticles through self-assembly or weak interactions, and the usage of materials is more efficient. 104 The specic surface area and thickness of MoS 2 can be better controlled than in top-down methods. For example, in hydrothermal method, the thickness and size of MoS 2 can be modulated by simply tuning the hydrothermal time, temperature and growth parameters of solution concentration. 105 The main disadvantage of bottom-up methods is the difficulty of large-scale synthesis. 106,107 In the rest of this section, we try to list out the general methods of synthesizing MoS 2 for the electrochemical HER (Table 1).

Hydrothermal synthesis
Hydrothermal method is to seal the reactants in the reactor, and the reaction process is completed through chemical transmission. Liquid or gaseous water is the pressure transmission medium under high temperature and high pressure, and most of the reactants can be dissolved in water, so that the reaction can be carried out in the critical state of gas phase and liquid phase solvent. Compared with other powder preparation methods, the powder prepared by hydrothermal methods has the advantages of complete grain development, small particle size, uniform distribution, light particle agglomeration, cheaper raw materials, and appropriate stoichiometry and crystal shape are easily obtained. [108][109][110][111] Typically, the ceramic powder prepared by hydrothermal methods does not need hightemperature calcination treatment, which prevents grain growth, defect formation and introduction of impurities in the calcination process, so the prepared powder has high sintering activity. [112][113][114][115] Liu et al. 116 prepared ultrathin MoS 2 nanosheets grown on an in situ-formed polysaccharide matrix derived from glucose condensation via a hydrothermal method. 0.80 g of glucose with a varied amount of ammonium heptamolybdate tetrahydrate ((NH 4 ) 6 Mo 7 O 24 $4H 2 O) and thiourea (CS(NH 2 ) 2 ) was dissolved in 15.0 mL of distilled water, and then the solution was turned to a Teon-lined stainless-steel autoclave, with a hydrothermal treatment at 240 C for 48 h. Aer centrifugation, washing and annealing, the MoS 2 /C nanospheres was successfully synthesized. Wang et al. 117 prepared 1T/2H MoS 2 via a facile hydrothermal method with the addition of ammonium bicarbonate. During the hydrothermal process, ammonium bicarbonate was decomposed into small molecules and ions as guests, including NH 4 + , H 2 O and CO 2 , which were inserted into the lamellar structures of MoS 2 , inducing the formation of multiphasic 1T/

Chemical vapor deposition
Chemical vapor deposition (CVD) is a method of synthesizing coatings and nanomaterials deposited on a substrate surface by chemical gas or vapor reaction. It is the most widely used technology for depositing a variety of materials in the semiconductor industry, including a wide range of insulating materials, most metal materials and metal alloy materials. [121][122][123] Theoretically, the CVD process is very simple: two or more gaseous raw materials are introduced into a reaction chamber, which then react with each other to form a new material that is deposited on the wafer surface. However, experimentally, the reaction that takes place in the reaction chamber is very complex, and there are many factors that must be considered. The deposition parameters, which can vary over a wide range, include the pressure in the reaction chamber, the temperature of the wafer, the ow rate of the gas, the distance of the gas through the wafer, the chemical composition of the gas, the ratio of one gas to another, the role of the intermediate product of the reaction, and whether other reactions are needed. [124][125][126][127][128] Pumera et al. 129 synthesized MoS 2 lms via powderless gas deposition. The CVD fabrication method they used is suitable for industry because this powderless and one-step process eliminates the deviations of MoS 2 growth which are likely to arise from the usage of a powder precursor. Yu et al. 130 hybridized MoS 2 micro-owers on VS 2 by a one-pot CVD method and their catalyst exhibited excellent HER performance. Atomic layer deposition (ALD) is a method in which substances can be deposited on the surface of the substrate, layer by layer, in the form of a monoatomic lm. 131 ALD has similarities with ordinary chemical deposition, except that the chemical reaction of the new layer of atomic lm is directly dependent on the previous layer. In this way, only one layer of atoms is deposited per reaction. Due to its highly controllable deposition parameters (thickness, composition, and structure), atomic layer deposition technology, excellent deposition uniformity and consistency, it has wide application potential in the eld of micro-nanoelectronics and nanomaterials. Tan et al. 132 successfully synthesized layered MoS 2 via the ALD method. Typically, MoS 2 lms were prepared by alternating exposure to molybdenum chloride (MoCl 5 ) and hydrogen disulde (H 2 S) vapors. Due to the self-limiting reactions of the vapors, the number of MoS 2 lm layers can be precisely controlled by the number of deposition cycles.

Exfoliation method
At present, the most effective preparation methods of single-or few-layer MoS 2 are exfoliation methods, which can be divided into mechanical exfoliation, lithium-ion intercalation exfoliation and liquid phase ultrasonic exfoliation. In the mechanical exfoliation method, MoS 2 powder is directly stripped to form a thin layer of MoS 2 nanoakes with a special gummed tape. This is the most original and traditional preparation method. The thickness of MoS 2 that was rst peeled off in 1965 ranged from a few layers to dozens of layers. 133 Aer the improvement of this stripping process, monolayer MoS 2 was successfully obtained. 134 The most obvious advantage of the mechanical exfoliation method is that the process is simple, can be obtained manually, and the stripping efficiency is high. Since this is direct exfoliation, the monolayer MoS 2 can maintain a good single crystal structure and offer high carrier mobility. However, this method has obvious disadvantages, such as low yield and poor repeatability. The lithium-ion intercalation-based exfoliation method can be used, for instance the Li x MoS 2 (x $ 1) intercalation compound is obtained by the reaction of the lithium-ion intercalation agent (such as n-butyl lithium) and MoS 2 powder (intercalation process). Then the intercalation compound reacts violently in water and other protonic solvents, and the stripping process occurs. Thus, multilayer and monolayer MoS 2 can be obtained by ultrasonic extraction. In 1986, Morrison et al. 135 prepared monolayer MoS 2 by this method. The efficiency of the Li ion intercalation stripping was found to be very high, with a yield close to 100%. The quality of the prepared nanosheets was very good, making it suitable for massproduction, and the method could be used to peel many other inorganic layered compounds. However, this preparation method is complex, the intercalation agent is sensitive to the environment, leading to a high production cost, and long preparation time. Moreover, it is difficult to control the degree of intercalation, which makes the post-treatment difficult. For this reason, Zeng et al. 136 used an electrochemical lithium battery device to control the lithium ion insertion and stripping process, so as to prepare a variety of monolayer materials such as MoS 2 . This electrochemical lithium-ion intercalation exfoliation method can effectively avoid many of the abovementioned problems and obtain high yield monolayer MoS 2 . The basic principle of the liquid-phase ultrasonic exfoliation method is to put MoS 2 and other layered materials into a suitable solvent, use the ultrasonic wave to vibrate and peel off the lower layer of materials from the block, and then centrifuge to obtain thin MoS 2 nanosheets. In 2011, Coleman et al. 137 prepared monolayer and multilayer nanosheets such as MoS 2 , BN and WS 2 . The solvent selected by the liquid phase ultrasonic exfoliation method is generally a polar solvent with good dispersity and solubility for layered materials and similar surface energy. Coleman et al. 138 prepared thin MoS 2 material by adding a sodium cholate surfactant and stripping it in an aqueous phase. However, Zhou et al. 139 used ethanol/water mixed lowboiling solvents to successfully peel off nanosheets such as MoS 2 , making the peeling method cheaper and environmentally friendly, and peeling MoS 2 under water phase conditions has become a new choice for people. The liquid-phase ultrasonic peeling method is simple and fast, and can be suitable for largescale production, but generally the peeling degree is not high, the concentration of the obtained nanosheet solution is small, and the dependence on the ultrasonic conditions is high, and the ultrasonic power is too large or too small, which is not conducive to the formation of nanosheets.

Electrodeposition method
Electrodeposition method is an attractive way to synthesize MoS 2 thin lm in large scale because it uses cheap equipment, enables the deposition in large area and easy control of growth parameters through applied potential, current, pH and temperature of the bath. 140 Many researchers have studied the mechanism of an electrochemical deposition of MoS 2 from aqueous and little from non-aqueous solutions. A variety of electrodeposition techniques have been reported for the preparation of MoS 2 lms using anodic oxidation or cathodic reduction.
Ping et al. 141 developed an edge-guided electrodeposition approach for the construction of high quality edge contacted metal-2D MoS 2 and further demonstrated the potential of this approach in the fabrication of high performance electrocatalyst. Hwang et al. 142 developed a novel MoS 2 electrode with a nanocarbon (NC) coating as a catalytic cathode for hydrogen production in a microbial electrolysis cells (MEC), while treating simulated urine. The MEC performance of HER using the MoS 2 -NC cathode was characterized and evaluated under different applied voltages and various dilution factors for adjusting conductivity. Lyndi et al. 143 prepared MoS 2 lms via an electrodeposition method. Electrodeposition of molybdenum disulde lms was carried out with a 10 mM (NH 4 ) 2 MoS 4 (ammonium tetrathiomolybdate) with 0.1 M LiClO 4 adjusted to pH 10 with NH 4 OH using chronoamperometric deposition. Jiang et al. 144 prepared the MoS 2 -Co 3 S 4 /NF heterostructure layer by Co-MOF vulcanization and cyclic voltammetry (CV) method, which showed better electrochemical performance than the single-layer Co 3 S 4 /NF. The as-prepared MoS 2 -Co 3 S 4 heterostructure layer was decorated by ZnCo-LDH, the hydrogen evolution performance was further improved.
Though the above synthetic methods have been demonstrated to be an effective way to synthesize bulk MoS 2 , the HER performance of bulk MoS 2 is still not satisfactory. In the following part, we summarize three main strategies to promote the HER performance of MoS 2 (Fig. 3).

Defect engineering
Many physical and chemical properties of materials are closely related to the existence of defects. [145][146][147] Generally, defects can be classied as vacancies, grain boundaries and disorders. [148][149][150] Furthermore, vacancies can consist of, for instance, Mo vacancies, S vacancies, O vacancies, N vacancies and C vacancies. [151][152][153][154][155] Thermodynamically, it's unavoidable to introduce vacancies in the process of synthesizing MoS 2 . 156 Many studies have examined the impact of vacancies on the HER performance of MoS 2 . [157][158][159][160][161][162][163][164][165][166][167] Theoretically, the rational design of defects on the basal plane of MoS 2 can enhance the HER performance of MoS 2 because the basal plane of MoS 2 is inert while the edge sites are signicantly more active.
Li et al. 168 designed vertically aligned MoS 2 with a selectively cleaved Mo-S bond. Their work showed a new way to synthesize MoS 2 with a vacancy and their synthesis route is shown in Fig. 4(a). Fig. 4(b) shows that the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images conrm the existence of micro-cubic c-MoS 2 -C morphology. Aberration-corrected high-angle annular dark-eld scanning transmission electron microscopy (AC HAADF-STEM) images show the existence of the vacancy. This unique defect is bene-cial to a thermoneutral H-adsorption energy, which leads to accelerate HER kinetics. The energy-dispersive X-ray spectroscopy (EDS) elemental mapping results further conrm the formation of MoS 2 nanosheets with Mo and S defects. The electrocatalytic performance of the defect-rich MoS 2 is shown in Fig. 5. Further DFT calculations reveal that the newly generated interior edge sites break the periodic electronic structure of the MoS 2 basal plane, leading to an optimized surface charge conguration for H adsorption and desorption.
Li et al. 74 conducted a study of the catalytic activities of all potential reaction sites, such as edge sites, S vacancies and grain boundaries. For the rst time, they demonstrated that S vacancies provide another active site for HER in addition to the well-known active edge sites ( Fig. 6(a)). In order to identify the inuence of S vacancy states and concentration on catalysis, Wang et al. 169 synthesized MoS 2 with homo-vacancies, with an H 2 O 2 chemical etching strategy (Fig. 6(b)). They performed DFT calculations of DG H* to explore the effect of S vacancy concentration and distribution on the HER performance. The results showed that 12.50% of the single S-vacancies yielded the optimal HER performance. They synthesized a vertically aligned MoS 2 nanosheet guided by these theoretical DFT calculations. The X-ray diffraction (XRD) results in Fig. 7(a) demonstrate that the sample phase was rm and did not change aer H 2 O 2 etching. To determine the S-vacancy concentration as a function of the etching process, three X-ray photoelectron spectroscopy (XPS) spectra of P-MoS 2 and MoS 2Àx are shown in Fig. 7(a). The S : Mo ratio in each spectrum was calculated and it was converted to the corresponding S-vacancy concentration. The electrochemical results are shown in Fig. 7(b) indicate that MoS 2 with 12.11% S-vacancy exhibited the best HER performance and the ratio of S-vacancy was very close to the theoretically predicted optimal value of 12.50%. Furthermore, the superiority of single S-vacancies over agglomerate S-vacancies was found to originate from more effective engineering of the surface electronic structure. The synergistic modulation of catalytic performance from both vacancy concentration and distribution broadens the vacancy design eld and can be further extended to other types of TMDs and for other catalytic reactions. Forming pits on MoS 2 monolayers has been found to be desirable for promoting HER performance. Park et al. 170 investigated the effect of adventitious C on the pit formation in the MoS 2 layers. They used an aperture type in situ ETEM to study the reaction between MoS 2 monolayers and O 2 . The HER activity of MoS 2 was enhanced in the presence of pits, which can be ascribed to the increase of active edge sites (Fig. 8(b)). They also conducted DFT calculations and found that the presence of C nanoparticles on monolayer MoS 2 was favorable for progressive oxidation (Fig. 8(a)). Although the existence of defects can signicantly improve the HER performance of MoS 2 , the key is the ability to detect the existence of defects. Thus, we need to study the relationship between defect types and electronic modes. Up till now, the existence of defects was observed indirectly because the concentration of defects is typically very low. One of the detecting methods is Raman spectroscopy, which can effectively detect edge defects and show distinct electronic structure. The vital task of defect engineering is to enhance the electrochemical activity of the MoS 2 basal plane.

Formation of heterostructure
The construction of heterostructured composites is an effective way to enhance the activity of MoS 2 . 171-181 Wei et al. 182 synthesized core-shell structured CoS 2 @MoS 2 by a hydrothermal method. As shown in Fig. 9(a), the CoS 2 @MoS 2 nanoparticles had a uniform size distribution, ranging from 20-30 nm. The HER performance of CoS 2 @MoS 2 in Fig. 9(b) shows that the overpotential of HER was only 276 mV and the Tafel slope was 60 mV dec À1 . Mei et al. 183 constructed a PCN@MoS 2 @C sandwich-like heterostructure by a facile hydrothermal method.
The PCN@MoS 2 @C exhibited excellent HER performance within a wide pH range (Fig. 9(c)).
Constructing heterostructures with multiple electrocatalytically active components simultaneously to improve HER performance is regarded as an effective strategy. He et al. 184 synthesized CoS 2 /SnO 2 @MoS 2 (denoted as CSM) nanotubes by a fast template-engaged hydrothermal treatment. The SEM and TEM images of the obtained CSM are shown in Fig. 10(a) and  (b). These images revealed a porous structure, which enlarged the surface area of the material and boosted the mass transport process, leading to excellent HER performance (Fig. 10(c)). Huang et al. 185 designed an exfoliated few-layer FePS 3 decorated with vertical MoS 2 nanosheets through a facile hydrothermal reaction. The synergistic effect between the FePS 3 and MoS 2 led to excellent HER performance. This effect can regulate the electronic structure and improve the electron conductivity. As shown in Fig. 10(d). The overpotentials of FeS 3 @MoS 2 were 175 mV and 168 mV in alkaline and acidic media, respectively. Furthermore, the heterostructure could prevent agglomeration during electrocatalytic cycling, which led to high stability.
Tahira et al. 186 reported a stable electrocatalyst composed of TiO 2 nanorods decorated with MoS 2 nanosheets by a two-step hydrothermal method. This electrocatalyst showed an enhanced HER performance compared with bulk MoS 2 ( Fig. 11(a)). Cao et al. 187 prepared cobalt molybdenum nitride with a nanoscale morphology using a two-step solid-state reaction. This compound did not belong to any layered structural family prepared for HER; however, it exhibited HER activity comparable to MoS 2 ( Fig. 11(b)).
Theoretically, heterostructure engineering of MoS 2 with graphene is an effective and facile method and the enhanced catalytic performance can be attributed to two factors. First, the induced graphene substrate prevents the agglomeration of MoS 2 nanosheets; thus, creating sufficient edge sites for HER. Second, the excellent electronic conductivity of graphene encourages rapid electron transfer, which is benecial for the absorption of the H atom and reduces the Gibbs free energy. Furthermore, the heterojunction of the MoS 2 @graphene hybrid provides new chemical properties and may be benecial for an enhanced HER performance. Wang et al. 189 constructed a MoS 2 @rGO hybrid catalyst by growing MoS 2 arrays on the rGO surface via a one-step hydrothermal method. The MoS 2 @rGO presented a three-dimensional ower-like morphology, with plentiful active edge sites exposed, which contributed to good HER performance. Yan et al. 190 reported a bottom-up method for the large-scale synthesis of 3D hybrid architectures constructed from graphene, MoS 2 and graphitic carbon nitride nanosheets by self-assembly. Due to the distinct architectural features such as 3D interconnected porous networks and low charge-transfer resistance, the MoS 2 -CN/G exhibited superior HER performance with low overpotential, small Tafel slope and long-term durability. Wu et al. 191 designed MoS 2 /MoN heterostructures with tuned components and the catalyst showed excellent HER performance in acid and alkaline electrocatalyst. DFT calculations suggested that the MoS 2 /MoN interface could optimize the H atom adsorption kinetic energy, accelerating the HER process. Liu et al. 192 reported a hierarchical ZnS@C@MoS 2 core-shell nanostructure in a bottom-up strategy combined with a selective etching method. The co-existence of ZnS and porous carbon shell not only improved the electrical conductivity but also separated and loaded the MoS 2 nanosheets, which effectively exposed active edge sites. This catalyst showed an excellent HER performance with an overpotential of 118 mV and a Tafel slope of 55.4 mV dec À1 . Li et al. 193 reported a 1T-MoS 2 /CoS 2 heterostructure via a self-sacricing template method. In this work, they compared the cohesive energy of in- plane heterostructure and out-plane heterostructure by DFT calculation and revealed that the cohesive energy of in-plane heterostructure (À5.1 eV) is much lower than that of outplane (À4.5 eV), which demonstrated that the in-plane heterostructure is thermodynamically more stable. Xiong et al. 194 reported Co-doped MoS 2 nanosheets via a facile one-step hydrothermal method. As shown in Fig. 12(a), the obtained Co-MoS 2 exhibited superior HER activity with an overpotential of 60 mV/90 mV at a current density of 10mA cm À2 for HER in 0.5 M H 2 SO 4 and 1.0 M KOH, respectively. They conducted DFT calculations ( Fig. 12(b)) and revealed that the DG H* value of pristine MoS 2 was about $2.06 eV while the DG H* value decreased to À0.20 eV aer Co doping, demonstrating that Co doping is an effective method for enhancing HER activity of MoS 2 . They also studied the OER activity origin of Codoped MoS 2 by recording the ex situ Co and Mo K-edge EXAFS spectra of Co-MoS 2 .
Iron is a typical nonprecious and earth-abundant element, and Fe-containing materials are more effective in adsorbing hydroxyl, which leads to its unique conductivity to water splitting. 13 Previous studies 195,196 have veried these observations through XPS characterizations. Xue et al. 197 synthesized a series of Fe-doped MoS 2 nanomaterials via a facile one-pot solvothermal method. The Fe-MoS 2 displayed high cycling stability and high HER activity with a small overpotential of 173 mV at a current density of 10 mA cm À2 (Fig. 12(c)). As shown in Fig. 12(d), the XPS full spectrum conrmed the existence of Fe, S, Mo and O.
Ni doping has a signicant impact on surface topography and valence state of Mo in MoS 2 , which can accelerate the HER reaction kinetics. Wang et al. 198 prepared Ni-doped MoS 2 via a hydrothermal method. The layer number of MoS 2 can be estimated by the difference in frequency between two characteristic Raman peaks. The results indicate that Ni doping decreases the vibration frequency of MoS 2 and increases the number of layers of MoS 2 . Fig. 12(e) shows that the Ni-MoS 2 displayed enhanced HER performance with an overpotential of 164 mV to reach a current density of 10 mA cm À2 in acidic media and a small charge transfer resistance (R ct ) of 89 U.
In addition to Co, Fe, Ni doping, 198-201 some other transition metals can also promote the HER activity of MoS 2 . Huang et al. 202 prepared a self-supporting electrode composed of Cu mesh-supported graphene and MoS 2 by electroplating zinc, followed by a hydrothermal reaction. The self-supporting catalytic electrode has the advantages of avoiding complex pretreatment of the active substance and the usage of binders. As shown in Fig. 13(b), the MoS 2 -rGO-Cu exhibited excellent HER performance with a low Tafel slope of 54 mV dec À1 and a small overpotential of À300 mV at a current density of 400 mA cm À2 . The excellent HER performance can be ascribed to the good electron transport capability of the Cu mesh, and specically, the self-supporting electrode can be fabricated into any desired shape, which is well maintained while the HER takes place. Luo et al. 203 designed Pd-doped MoS 2 via a spontaneous interfacial redox technique. The Pd doping converted 2H phase MoS 2 into a stabilized 1T structure. Furthermore, Pd atoms occurred at the Mo site, which simultaneously introduced an S vacancy. As shown in Fig. 13(a), the Pd-doped MoS 2 exhibited good cycling stability and excellent HER activity with an overpotential of 78 mV at a current density of 10 mA cm À2 . The DFT calculations demonstrated that the S atom next to the Pd sites had a low H atom adsorption energy of À0.02 eV. Sun et al. 204 designed a novel intralayer V-doping method to synthesize semi-metallic V-doped MoS 2 nanosheets. The introduction of V atoms regulated the intrinsic electrical properties of MoS 2 , leading to an enhanced in-plane conductivity and shortened electron transfer paths. The V-doped MoS 2 showed excellent catalytic activity with an overpotential of 130 mV and a small Tafel slope (Fig. 13(c)). Zhang et al. 205 reported a single Ru atom-doped MoS 2 electrocatalyst for HER. As shown in Fig. 14(b) and (c), the DFT calculations revealed that the synergistic effects of the single Ru atom doping, S vacancies and phase transition of MoS 2 modulated the electronic structure of MoS 2 and signicantly reduced the energy barrier of the Volmer step, leading to enhanced HER performance of Ru-MoS 2 (Fig. 14(a)). Multidoping methods are also a promising way to enhance the HER activity of MoS 2 . Yu et al. 206 reported a typical MOFengaged synthetic method to prepare Ni and Co incorporated MoS 2 nano-boxes. The ultrathin nanosheets obtained exhibited enhanced electrochemical activity for HER ( Fig. 13(d)). In recent years, materials with single-atom structure have been recognized as promising HER catalysts due to the maximized atom efficiency and tunable electronic properties. Ni et al. 207 reported that Ru-based single-atom catalysts are promising HER catalysts due to its excellent HER performance, which is comparable to commercial Pt/C catalysts. However, Ru-based single atom catalysts have problems such as large amount of Ru and unsatisfactory stability. Ge et al. 208 synthesized dual-metallic Ru and Ni atoms decorated MoS 2 , which resolve the above problems well. The introduction of Ni has two key roles. On the one hand, the introduction of highly electronegative heteroatom Ni could regulate the electronic structure of Ru and optimize the H* adsorption during the water splitting process, thus vastly enhancing the HER property. On the other hand, the incorporation of highly electronegative heteroatom Ni in the substrate exposes abundant active sites and improves electron transfer of the catalyst, and further anchors the single atom Ru and reduces the consumption of precious metal Ru atoms.
4.3.2. Nonmetal-doping method. Nonmetal doping can enhance the electrocatalytic activity of MoS 2 . Wang et al. 209 synthesized amorphous P-doped MoS 2 nanoparticles hydrothermally. The P-doping promoted the amorphization of MoS 2 ; thus, affecting the HER performance of MoS 2 . Deng et al. 210 prepared O-doped MoS 2 via an electrochemical anodic activation method. The HER activity of O-doped MoS 2 signicantly improved at an overpotential of 50 mV, at a current density of 1 mA cm À2 (Fig. 15(a)). As shown in Fig. 15(b), the XPS images conrmed the strong electron interactions aer O doping, which indicated the existence of Mo-O bonds and demonstrated that the O doping was successful. Liu et al. 211 reported O and P dual-doped MoS 2 nanosheets, which have a porous structure and conductive network via a one-pot hydrothermal approach. As shown in Fig. 15(c), the O, P multi-doped MoS 2 showed superior electrocatalytic performances compared with O-doped MoS 2 nanosheets. Wang et al. 212 prepared MoS 2 decorated Ni-Fe-N-doped carbon nanotubes via a hydrothermal method. According to the SEM results shown in Fig. 15(d), the catalysts were hierarchically structured. Due to the synergistic effect of the NiFe alloy and N-doped carbon nanotubes, the catalysts exhibited excellent HER performance with a cell voltage of 1.6 V at a current density of 10 mA cm À2 . Tian et al. 213 synthesized N, P dual-doped MoS 2 on hollow carbon spheres by a template and hydrothermal process. The synergistic effect of N, P dual-doped carbon and MoS 2 led to the enhanced electrocatalytic performance of MoS 2 with an overpotential of 147 mV, at a current density of 10 mA cm À2 and Tafel slope of 72 mV dec À1 . As shown in Fig. 15(e), the XPS spectra conrmed the co-existence of N and P atoms.
Metal-doping and nonmental-doping can be synergistically used to improve the HER performance of MoS 2 . Meng et al. 214 reported a 3D mesoporous hybrid structure of Co-doped MoS 2 and graphene. The Co-doping enhanced the intrinsic electrocatalytic activity of the in-plane S sites. The graphene network was found to be electronically conductive and robust, which improved the conductivity of electrons and enhanced the stability of the hybrid structure. The catalysts showed good cycling stability and an excellent HER performance, with an overpotential of 143 mV at a current density of 10 mA cm À2 . Wu et al. 215 designed Co, Se dual-doped electrocatalysts via a solvothermal and post-low-temperature selenylation process. Due to the abundance of interfaces, the electrocatalysts exhibited excellent HER performance, with a low overpotential of 58 mV at a current density of 10 mA cm À2 and a small Tafel slope of 84 mV dec À1 . Sun et al. 216 reported plasma-induced N, Pt-doping and phase modulation of MoS 2 nanosheets. The synergetic doping of N and Pt heteroatoms regulates the electronic and coordinated structures of MoS 2 and further facilitates water adsorption and dissociation during the HER. Notably, this plasma bombardment method is highly feasible for the largearea fabrication of MoS 2 -derived HER active electrodes.

Theoretical study of the HER
The studies of electronic structures of MoS 2 offer theoretical guidance for designing efficient catalysts. For MoS 2 , a typical 2D material, the theoretical calculation of the basal plane positions, Mo edge and S edge is quite signicant. Furthermore, it is also benecial to investigate the adsorption energy of intermediates. Jacob et al. 217 proposed an H-binding mechanism for S atoms in the basal plane of MoS 2 . This model was tested and veried by DFT calculations; herein, the energy barrier can be tuned by introducing metal dopants and defects. Their study revealed the electronic structure mechanism which tends to inuence the basal plane S atoms activation ( Fig. 16(a) and (b)). Xian et al. 218 synthesized the monodispersed single-crystal Pt nanoparticles decorated on 2H phase MoS 2 nanosheets via a wet chemical method. They construct the atomic model of Pt truncated octahedron loaded on MoS 2 , and choose four Pt sites (Fig. 16(c)). The charge-density difference is showed in Fig. 16(d), indicating the variations of the charge distributions for Pt and S at the interface of Pt-MoS 2 atomic model from those for pure Pt and S. The results reveal that the Pt-S bonds are formed. Then, they calculate the free energy of adsorbed atomic hydrogen (DG H* ), including MoS 2 (001), Pt (100), Pt (100) interface , Pt (1-11) and Pt (1-11) interface (Fig. 16(e)). The results indicate that the Pt (100) interface owns the smallest DG H* as well as the best HER activity. Further, they found that with the studied Pt atoms changing to the interface, the D-band center move away from the Fermi level (Fig. 14(f)), indicating the decrease of DG H* , which conrms the superiority for Pt (100) interface in HER performance. In published DFT studies, the electronic structures of MoS 2 -based materials are usually computed by VASP and the electron-ion interactions are described by the projector-augmented wave method. All calculations are typically implemented with PBE exchange-correlation functional on periodically repeated slabs. The details of the molecular model-building, HER mechanism and free-energy calculations are provided in the relevant ref. 219-222.

Conclusions and outlook
With the depletion of traditional fossil energy, the development of green energy such as hydrogen energy has become urgent. Electrocatalytic water splitting to produce hydrogen offers a means to develop new energy sources. Up to now, signicant progress has been made in achieving effecting hydrogen evolution reactions (HERs) using promising electrocatalysts such as transition metal suldes (TMDs), transition metal carbides (TMCs), transition metal phosphides (TMPs) and transition metal nitrides (TMNs). As a typical electrocatalyst, MoS 2 has been widely studied and much progress has been made. In this review, we summarize the synthetic methods of MoS 2 and the effective strategies adopted to optimize the HER performance of MoS 2 . The introduction of defects signicantly enhanced the HER activity of the MoS 2 -based materials, which was also veried theoretically. The formation of heterostructures is another effective way to enhance the HER activity of MoS 2, which can modulate the electronic structure. Furthermore, doping metal atoms (Fe, Co, Ni, Cu, Mn, V, Ru etc.) or nonmetal atoms (C, N, F, P, S, Se etc.) into MoS 2 can also improve its electrocatalytic HER performance.
However, there are still some challenges for the commercial application of MoS 2 -based materials. Firstly, even though ultrathin MoS 2 -based materials can be synthesized by CVD and hydrothermal methods, the large-scale production of MoS 2 is still a challenge. Secondly, the standards for evaluating electrocatalytic stability have not yet been developed. Thirdly, while DFT calculations can provide a detailed theoretical insight into the reaction mechanism of a single active center, research involving the existence of multiple active centers is still lacking. Fourthly, the catalytic inuence of H 2 O on the MoS 2 surface and interlayers is still not completely understood because of the difficulty to develop bifunctional electrocatalysts for all electrolytes. Lastly, vacancies, doping atoms, and heterojunctions usually have a positive effect on the electrocatalytic process. However, the reaction mechanism needs to be further explored.
While there is still a long way to go for the development of MoS 2 -based electrocatalytic materials, the research interest in this area is increasing. Moreover, with the rapid development of computational tools, it is possible to make advances in many experimental areas, including MoS 2 -based materials. From this point of view, we can foresee that MoS 2 -based materials will be very promising for hydrogen production applications.

Conflicts of interest
The authors declare no competing nancial interest.