Hydrogen energy is the ultimate environment-friendly energy and the most promising form of energy to replace traditional energy sources such as coal, oil, and natural gas ^{[1][2][3][4][5]}[1,2,3,4,5]. At present, the production of hydrogen mainly relies on the cracking of traditional energy sources, which belongs to false decarbonization ^[6][7][8][6,7,8]. Hydrogen production by solar photovoltaic power generation is the most promising method of hydrogen production ^[9][10][9,10]. The key to electrocatalytic hydrogen production lies in the development and utilization of an electrocatalyst. Because the precious metal platinum has good catalytic hydrogen production performance, it is the best hydrogen production catalyst at present; however, its high cost and scarce resources seriously hinder its application in catalytic production.

As a non-precious metal catalyst with the most potential to replace precious metal platinum, MoS₂ has attracted increasing attention ^{[11][12][13][14][15][16]}[11,12,13,14,15,16]. The key factors that determine the catalytic hydrogen production performance of MoS₂ mainly relate to two aspects: one is the number of active sites, and the other is the true activity of the active site ^{[17][18][19][20][21][22]}[17,18,19,20,21,22]. It is generally believed that the base surface of MoS₂ is chemically inert and does not have the performance of catalytic hydrogen production; moreover, at the same time, the edge of MoS₂ has high catalytic hydrogen evolution activity ^{[23][24][25][26][27]}[23,24,25,26,27]. More and more people are using various methods to expose the edge of MoS₂ to improve its catalytic hydrogen production performance ^{[28][29][30][31][32]}[28,29,30,31,32]. The latest research shows that the conversion of sheet MoS₂ into bands can expose the edge sites and improve the catalytic hydrogen production performance ^{[33][34][35][36][37][38][39][40][41]}[33,34,35,36,37,38,39,40,41]. It is also possible to directly generate branchlike MoS₂ by controlling the proportion of precursors during the growth of MoS₂, thereby increasing the edge site of MoS₂ ^{[42][43][44][45][46][47][48]}[42,43,44,45,46,47,48]. Defect engineering and phase engineering are also strategies to regulate the catalytic hydrogen production performance of MoS₂ ^{[49][50][51][52][53][54][55][56]}[49,50,51,52,53,54,55,56]. Although the catalytic hydrogen production performance of MoS₂ can be adjusted through various regulatory strategies, the corresponding catalytic mechanism is still very controversial. It has been reported that the inert base plane can undergo a transient phase transition in the catalytic process to play the catalytic role, which is contrary to the common understanding that the catalytic activity only occurs at the edge [57].

2. Edge

It is well known that the edge site of MoS₂ has high catalytic HER activity, and a lot of research has focused on how to expose the edge of MoS₂. Recent studies have shown that the MoS₂ can be designed with a rich edge structure such as a paired edge nanoribbon, which can further enhance the catalytic hydrogen evolution reaction (HER) activity of MoS₂.

2.1. Nanoribbon

According to research on edge-dominated electrochemical reaction kinetics in ultra-narrow MoS₂ nanoribbons, ideal energetics for HER could be obtained. Large arrays of MoS₂ nanoribbons were acquired using a templated subtractive patterning process (TSPP), which significantly enhanced the turn-over frequency, exchanged the current density, and lowered the Tafel slope because of improved charge transfer efficiency. Utilizing the naturally occurring bilayer and multilayer regions in graphene, and taking advantage of the bottom-up approach of graphene, the pattern is transferred from the graphene mask to the surface of the MoS₂ material through a pattern transfer process, thus forming an aligned MoS₂ nanoribbon array with a controlled direction. Since the formation of nanoribbons is random to a certain extent. With a length-to-width ratio of more than 7000 and a high density, the strips are more efficient than other strategies for patterning MoS₂ nanoribbons. The observation of a single nanoribbon over a long distance shows that the fractures are solved and the structural stability of the nanoribbon is ensured. Using the electron diffraction technique, the crystal properties of MoS₂ nanoribbons can be determined and characterized. The six-fold symmetry diffraction pattern was observed using the SAED model. The orientation of the nanoribbon was determined not to affect the crystal structure. The atomic arrangement in the TEM shows an orderly structure with no obvious defects. The difference in brightness may be due to the atomic number of the atoms; the darker atoms are Mo and the brighter ones are of S. High basal plane quality, which makes their nanoribbon array an ideal model system for studying the source of HER enhancement. 

2.2. Fractal MoS

₂

Since the catalytic active site of 2H-MoS₂ is mainly at its edge, controlling the morphology and structure of MoS₂ to expose more edges can further improve the hydrogen evolution reaction (HER) of MoS₂. Then, if MoS₂ is grown in a multi-branched and multi-edge morphology structure in the chemical vapor deposition (CVD) growth process, the HER performance of MoS₂ can be improved. Therefore, Yu G. et al. synthesized MoS₂ with different morphologies by adjusting the proportion of precursor in the process of MoS₂ generation by CVD. Fractal MoS₂ and triangular MoS₂ were obtained by controlling the proportions of MoO₃ and S, respectively, and the coverage rate of fractal MoS₂ and triangular MoS₂ was determined to be 20.5% and 22.7% using image analysis software. When MoO₃ is sufficient, a triangular MoS₂ can be generated, while, at a low dose of MoO₃, a fractal MoS₂ will be generated. After the formation of MoS₂ with different morphologies, their catalytic properties were further evaluated.

3. Sulfur Vacancies

Vacancies are considered to be the limiting doping states that promote atomic rearrangements and modulate the electronic structure over a wide range. Many methods have been successfully implemented to introduce vacancies in 2D TMDs, such as hydrogen plasma exposure, H₂ annealing, Ar²⁺ beam irradiation, and helium ion beam irradiation, showing great potential for catalytic reactions. However, all of the above methods require additional intervention from external stimuli; therefore, it is difficult to generate controllable vacancies directly by growth. Defect engineering is an effective strategy to accelerate the catalytic hydrogen production performance of MoS₂. However, introducing defects such as sulfur vacancies on the MoS₂ basal plane is still a major challenge. Currently, sulfur vacancies are mainly introduced into MoS₂ by using post-treatment methods such as plasma treatment, ultrasonic, ball milling, and other methods. However, if sulfur vacancies can be introduced directly during the preparation of MoS₂, it would be an excellent strategy to prepare sulfur vacancy defects. Man et al. proposed that sulfur vacancies can be introduced into the MoS₂ basal plane by controlling the reaction conditions during the MoS₂ growth through a salt-assisted CVD method [55]. The density of sulfur vacancies could be controllable by controlling the added amount of KCl during the CVD growth process, and some kind of change has occurred during the process. In order to explore the relationship between the density of sulfur vacancies and the catalytic hydrogen production performance of MoS₂, a micro-nano HER test platform was built to precisely evaluate the catalytic performance of monolithic MoS₂ with sulfur vacancies. It was found that the samples with abundant sulfur vacancies had the best catalytic hydrogen production performance and the lowest Tafel slope. The method of thermochemical annealing sodium hypophosphite to produce MoS₂-active defects is proposed; meanwhile, it can spontaneously produce PH₃ to regulate the MoS₂ lattice. By controlling the reaction conditions, active defects are formed at the basal plane and edges, thereby exposing more metal active sites and improving the Hydrogen Evolution Reaction (HER) performance of MoS₂. The development of efficient and low-cost MoS₂ catalysts for practical applications is important. Sodium hypophosphate is set at around 200 °C to produce PH₃ gas, and MoS₂ is annealed using PH₃ gas at 500 °C. PH₃ reacts with MoS₂ to produce defects that replace the S atom in the MoS₂ lattice through defects, resulting in P doping. Due to the active chemical properties of doped P, the active P element can be oxidized to a phosphate layer coating on the surface of MoS₂ and form phosphate without phosphate compounds in the MoS₂ crystal. The phosphate could be eventually removed from the crystal lattice of the MoS₂ crystal if it is dissolved in water or acid solution, thus creating defects again.

4. Doping

Doping engineering has become an effective strategy to improve MoS₂ base activity [57]. The most common doping engineering practice is to improve its electronic structure through heteroatom doping, thereby enhancing its intrinsic catalytic activity. This is mainly because the electronic structure of the base atoms is modulated using defect engineering to improve its surface conductivity [58]. In addition, heteroatom doping has important effects on chemical bond formation, adsorption/desorption processes, and the Gibbs free energy of the reaction ^{[59][60][61][62]}[59,60,61,62]. Therefore, the electrocatalytic activity of MoS₂-based catalysts can be effectively improved according to the electronegativity difference and the type and number of heteroatoms. 1T-phase MoS₂ (1T-MoS₂) has been widely concerned in hydrogen evolution reaction (HER) because it exhibits better charge transport characteristics and can expose more active sites. Although 1T-MoS₂ is a good HER material under acidic conditions, it produces a higher overpotential under alkaline conditions. At the same time, the conditions of an alkaline solution are more suitable for HER. The design and modification of the catalytic site at the atomic level can deepen the understanding of the active site of the catalyst, which is essential to enhance the activity of the catalyst. In this regard, Jing Gu et al. used Anderson-type polyoxometalates as a precursor to doping the metal active site onto 1T-MoS₂ at the atomic level to improve the HER activity of 1T-MoS₂. Tthe precursor Anderson-type POM nanoclusters, [XH₆Mo₆O₂₄]ⁿ⁻ (denoted as XMo₆, where X represents the doped metal atoms such as Fe, Co, Ni, etc.) is characterized by different shapes.

5. Phase

It is generally believed that the base plane of MoS₂ is inert; however, it has been reported that the inert base plane can undergo a transient phase transition in the catalytic process to play the catalytic role, which is contrary to the common understanding that the catalytic activity is only at the edge. The HER catalytic mechanism of 1T-MoS₂ remains elusive and controversial. Therefore, it is necessary to further understand the mechanism of MoS₂ catalytic hydrogen production.

5.1. An Irreversible Phase Transition during Photocatalytic Hydrogen Evolution

It is widely believed the active sites of 2H-MoS₂ for catalytic hydrogen production are located at the edges, while its basal plane is inert. Moreover, it has been reported that the conversion of the 2H phase into the 1T phase by phase transformation is an ideal strategy to enhance the catalytic HER performance of MoS₂. However, the HER catalytic mechanism of 1T-MoS₂ remains elusive and controversial. It is difficult to explain the nature of the better catalytic performance, which is originally from the improved electrical conductivity, the increased intrinsic activity of the active site, or the number of active sites. In order to explore this problem, the Wang group made ultra-thin MoS₂ nanosheets that were vertically grown on TiO₂ nanofibers, and this vertical growth could introduce the strain. The 1T-MoS₂ with sulfur vacancies and strain could be obtained by further lithium intercalation. Using this sample as the catalyst for HER, it was found that its catalytic performance gradually increased during the process of catalytic hydrogen production. This self-optimization of the catalytic performance is most likely due to the structural transformation of the catalyst during the catalytic HER process. In order to investigate this transformation, the catalyst after the catalytic reaction was structurally traced. The HRTEM shows that the 1T phase has transformed into the 1T’ phase, with the super-lattice structure from the Mo atom clustering into Zigzag chains. This suggests that the 1T’ phase is the true active phase for catalytic HER. A molecular dynamics simulation was performed to research the transition from the 1T phase to the 1T’ phase.

5.2. Transient Phase Transition during the Hydrogen Evolution Reaction

2H-MoS₂ is one of the most promising noble metal-free electrocatalysts in the hydrogen evolution reaction (HER). With regard to its HER mechanism, the widely accepted view, so far, is that its marginal sites have high HER activity, while its basal plane is inert during the HER process. However, Zhai et al. found that this conclusion was incorrect and verified it using ATR-SEIRAS and XAFS. 

6. Conclusions

A number of strategies have been developed to improve the catalytic production performance of MoS₂, and the mechanism of MoS₂ catalytic hydrogen production has also been proposed. The catalyst with an ideal atomic structure should be prepared in view of the controversy over the mechanism of MoS₂ catalytic hydrogen production, and the catalyst should be used as a model to explore the mechanism of catalytic hydrogen production, combined with first-principles calculation and in situ characterization methods. The catalytic hydrogen production performance of MoS₂ should be standardized by constructing a micro-nano structure device, and the catalytic hydrogen production performance should be attributed to the catalytic active site with a specific atomic structure.