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Catalysts 2017, 7(10), 285; doi:10.3390/catal7100285

Review
Two-Dimensional Material Molybdenum Disulfides as Electrocatalysts for Hydrogen Evolution
Lei Yang 1,*, Ping Liu 2, Jing Li 2 and Bin Xiang 2,*
1
Key Laboratory of Biomimetic Sensor and Detecting Technology of Anhui Province, School of Materials and Chemical Engineering, West Anhui University, Lu’an 237012, China
2
Department of Materials Science and Engineering, CAS Key Laboratory of Materials for Energy Conversion, Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei 230026, China
*
Correspondence: Tel.: +86-564-330-5690 (L.Y.); +86-551-6360-7833 (B.X.)
Received: 25 August 2017 / Accepted: 19 September 2017 / Published: 25 September 2017

Abstract

:
Recently, transition metal dichalcogenides (TMDs), represented by MoS2, have been proven to be a fascinating new class of electrocatalysts in hydrogen evolution reaction (HER). The rich chemical activities, combined with several strategies to regulate its morphologies and electronic properties, make MoS2 very attractive for understanding the fundamentals of electrocatalysis. In this review, recent developments in using MoS2 as electrocatalysts for the HER with high activity are presented. The effects of edges on HER activities of MoS2 are briefly discussed. Then we demonstrate strategies to further enhance the catalytic performance of MoS2 by improving its conductivity or engineering its structure. Finally, the key challenges to the industrial application of MoS2 in electrocatalytic hydrogen evolution are also pointed out.
Keywords:
MoS2; active edge sites; electrical conductivity; electronic structure; hydrogen evolution reaction

1. Introduction

In recent decades, the development of new energy sources has become a hot topic in academia [1,2,3,4]. Hydrogen, with its high energy density (143 kJ/g), has been proposed as a promising candidate to replace fossil fuels in the future due to the fact that the only combustion product is water, which is environmentally friendly [5,6,7]. The hydrogen evolution reaction (HER) has been considered one of the most effective ways to produce clean hydrogen energy [8,9,10,11]. Platinum- and noble metal-containing materials have been favored as electrocatalysts due to their high activity and chemical inertness [12,13,14]. However, the low abundance and high price restrict their large-scale application [10,15]. Therefore, exploring new catalysts that are abundant and of low cost has become important.
Recently, two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have been widely reported as promising non-noble metal electrocatalysts due to their abundance, low cost, and highly efficient catalytic activity. Several review papers have summarized the structure, synthesis, and composites of 2D TMDs, as well as their application in HER [16,17,18,19,20,21]. It is commonly accepted that unique 2D plenary structures provide ultrahigh specific surface area, atomic thickness, and an atomically flat facet [22]. Thus for the 2D TMDs, it is not only easy to achieve high catalytic activity, but also to modify the chemical and physical properties so as to further improve their catalytic performance [23]. Among the many layered TMDs electrocatalysts, MoS2 is the first to emerge as an active HER catalyst [24], and it continues to be explored as a prototypical model. Theoretical calculations and experimental results have revealed that the basal plane of MoS2 is semiconducting and catalytically inert, whereas the surface edges are metallic and chemically active [24,25,26]. It has been proposed that the key factor determining the hydrogen evolution efficiency of a catalyst is the Gibbs free energy for hydrogen adsorption on the active site (ΔGH) [24,27,28]. If the chemical bonds between the catalyst and the hydrogen are too strong, it would lead to permanent blocking of the catalytic surface. If the temporary chemical bonds are too weak, it would cause the adsorbate residence time to be too short for bond breaking [29,30]. Therefore, an optimal catalyst should have the Gibbs free energy for hydrogen adsorption close to zero (ΔGH ≈ 0) [29,30,31,32]. Figure 1a shows the “volcano plot” of exchange current density as a function of hydrogen adsorption Gibbs free energy for various HER catalyst materials [25]. The Pt and some other noble metal materials are at the top of the HER “volcano” and the hydrogen absorption energy of Pt is just slightly less than zero. The calculated Gibbs free energy for MoS2 edges is just +0.08 V when the H coverage is 25%, very close to the optimum value of 0 eV [24]. Therefore, MoS2 is a potential alternative to expensive noble metals.
Recently, great efforts have been dedicated to enhancing the electrocatalytic activities of TMDs in HER. Generally, there are three directions: the first is improving the catalytic activities of the edge sites; the second is exposing or increasing the number of active edge sites; the third is optimizing the electronic structure or improving the electrical conductivity of the catalyst [35,36,37,38]. This review attempts to summarize the recent progress in nanostructured MoS2 as representative TMD electrocatalysts toward the HER. Firstly, the role of the edges of MoS2 in a HER will be introduced. Then we will summarize the concrete approaches to further enhancing the catalytic performance of MoS2, like activating the inert S edges via doping, forming an amorphous structure, growing MoS2 on conductive carbon-based substrates, exerting strain, etc. In the end, we will briefly discuss the challenges pertaining to the rapid development of MoS2 as well as its industrial applications.

2. Edge Structures of MoS2

Bulk MoS2 is a layered material composed of a two-dimensional S-Mo-S “sandwich-like” structure [31,39]. For each individual layer, MoS2 prefers to expose two types of low index edge terminations: the ( 1 ¯ 010) S edge and the (10 1 ¯ 0) Mo edge, as shown in Figure 1b [33,40,41]. When the size becomes smaller, like the MoS2 cluster synthesized on Au (111), a triangular shape feature can be observed (Figure 1c) [33]. The triangular sharp domain only exposes one of the two types of low index edges [33,34]. According to the simple Wulff-type construction argument, the energetically favored one has priority to survive, so that the (10 1 ¯ 0) Mo edge terminates the MoS2 triangle domains [33,34]. Inside MoS2 “sandwich” layers, the Mo atoms symmetrically bond with six S atoms, which means that the Mo atoms are saturated with S atoms [33,34]. Density functional theory (DFT) calculations have shown that the Mo edges become unstable when there are dangling Mo bonds on the edge [33,34]. As the edges have no perfect trigonal prismatic coordination, the energetic favorable form of the edge can be the structure with one S (50% coverage) or two S atoms (100% coverage) per Mo edge atom (called S dimers), as shown in Figure 1d [33,34]. In both cases, similar to the situation with Mo atoms inside of MoS2, the Mo atoms are saturated by bonding with six S atoms. These results are consistent with the scanning tunneling microscope (STM) characterizations of MoS2 clusters, which show that the S atoms at the edge are shifted by half a lattice constant relative to the S atoms in the basal plane (Figure 1e) [33].
When the size of the MoS2 triangles cluster becomes even smaller (when the number of Mo atoms on the side of a triangle is less than 21), the edge only exposes the ( 1 ¯ 010) S edge with varying S coverage (Figure 1f) [34]. The 50% and 75% S coverage of the S edges is slightly less stable than that of the 100% saturated S edge [34].
As a HER catalyst, the active edges of nanostructured MoS2 are mainly from the Mo edges, whereas the S edges are inert [36,42,43]. The S edges constitute almost half of the exposed edge sites of the MoS2 catalyst [43]. Therefore, activating the inert edge sites is an efficient way to enhance its electrocatalytic activity.

3. Strategies for Improving the HER Activity of MoS2

3.1. Activating the Inert S Edges via Doping

To enhance the catalytic performance of MoS2 via activating the inert S edges, Bonde et al. have synthesized cobalt-doped MoS2 nanoparticles on Toray carbon paper using (NH4)6Mo7O24·4H2O and C4H4CoO4·4H2O as reactants [36]. Electrochemical characterizations showed that the cobalt exerted a promoted effect on MoS2 catalytic activities. The introduction of Co into MoS2 edges decreased the Tafel slope from 120 mV/dec to 101 mV/dec (Figure 2a). DFT calculations showed that the incorporation of cobalt into the MoS2 edges reduced the hydrogen adsorption ΔGH of S edge from 0.18 eV to 0.1 eV (Figure 2b), while the ΔGH of Mo edge (0.08 eV) was not affected due to its more stable structure.
In the form of Co-doped MoS2, the Co dopants might be distributed on the entire lattice structure of the MoS2 instead of being limited on the edges, which might also promote the terrace site activities [43]. The incorporation of Co metal atoms can change the morphology of MoS2. As a result, it is hard to distinguish whether the activity promotion effect was from doping or from the effect of an increased surface area caused by the morphology change of MoS2. To minimize this doubt, Wang et al. have synthesized a transition metal (Fe, Co, Ni, and Cu)-doped MoS2 nanofilm with fixed morphology via a chemical vapor deposition (CVD) method [43]. The doped MoS2 nanofilm was composed of vertically aligned molecular layers. The surface of the nanofilm was totally covered by the edge sites, confirming that the enhanced catalytic performance came from the transition-metal-doped edge sites. Figure 2c shows the polarization curves of the pristine and transition-metal-doped MoS2 nanofilm, respectively. For the Fe-, Co-, Ni-, and Cu-doped MoS2, the current densities at the overpotential of −300 mV were 2.3, 3.5, 2.4, and 2.6 mA/cm2, respectively, which were three times higher than that of pristine one. The Tafel slope (Figure 2d) was in the range of 117 to 103 mV/dec for those doped MoS2, lower than the 118 mV/dec in a pure MoS2 nanofilm. At the same time, the exchange current densities of the doped MoS2 were also larger than those of the pristine MoS2. To illustrate the enhanced behavior, they also performed DFT calculations. The calculated results suggested that the doped S edges had an energy value closer to thermo-neutral ΔGH and became similarly active to the pristine Mo edge, consistent with the results reported by Bonde’s group. Therefore, activating the inert S edges via a doping method is an efficient way to enhance the HER performance of MoS2.

3.2. Increasing the Number of Active Sites

3.2.1. Forming an Amorphous Structure

Previous studies have shown that the high HER activity of the edges is derived from unsaturated atoms at the edges [24,25,35]. Amorphous MoS2 has many coordinately unsaturated atoms that may serve as active sites and eventually lead to the evolution of hydrogen [44]. Shin and co-workers have successfully prepared an amorphous MoS2 catalyst on Au by the atomic layer deposition method (Figure 3a,b) [45]. Electrochemical characterizations showed that the amorphous MoS2 thin film exhibited excellent HER activity. Compared with the reported amorphous MoSx, the MoS2 thin film exhibited a much higher turnover frequency (TOF) of 3 H2/s at 0.215 V Vs Reversible Hydrogen Electrode (RHE). The Tafel slope was 47 mV/dec, close to the value in Pt. The amorphous MoS2 thin film also exhibited high electrical conductivity (0.22 Ω−1 cm−1 at room temperature) and low activation energy (0.027 eV).

3.2.2. Creating Defective MoS2 Nanosheets

Another efficient way to increase the number of active sites of MoS2 is defect engineering. Xie et al. have put forward a way to engineer a defect of MoS2 by forming cracks on the surface of MoS2 [46]. They achieved this by designing a reaction between the precursors and different amounts of thiourea. This indicated that the excess thiourea was necessary for the formation of the crack structure because the excess thiourea can be adsorbed on primary nanocrystallites, which partially hinder the oriented crystal growth. The defect-rich MoS2 ultrathin nanosheets with rich active sites exhibited excellent HER activity. The onset overpotential of the defect-rich MoS2 was −120 mV, much smaller than that of the defect-free one (Figure 3d). Compared with the defect-free MoS2, the defect-rich MoS2 also exhibited a smaller Tafel slope and larger cathodic current density. Besides the cracks, the disorder feature in MoS2 is also beneficial for its catalytic performance. By controlling the temperature during synthesis, Xie et al. have also realized the controllable disorder in the oxygen-incorporated MoS2 ultrathin nanosheets [47]. By reducing the reaction temperature from 200 °C to 140 °C in the Teflon-lined autoclave, atom construction on the basal surface was modified from regular to disorder. The high-resolution transmission electron microscope (HRTEM) image of MoS2 with certain disorder features is shown in Figure 3e. The disorder structure can offer a large number of unsaturated sulfur atoms, which are the active sites for HER. To investigate the HER activity, electrochemical measurements of the oxygen-incorporated MoS2 ultrathin nanosheets with different degrees of disorder have been undertaken. The electrochemical results suggested that the disorder feature had a promoting effect on the HER. When the degree of disorder increases, more unsaturated sulfur atoms can be exposed as active sites for HER, thus further enhancing its catalytic performance.
Plasma treatment is another efficient way to generate defects in MoS2 [48,49]. Wang’s group reported a simple plasma (O2 and Ar plasma) engineering method to modify the surface properties of MoS2 thin films [48]. For the O2 plasma treated MoS2, the current density at −350 mV under different treatment time of 120 s, 480 s, and 720 s was 6.11 mA/cm2, 15.17 mA/cm2, and 10.97 mA/cm2, respectively, much higher than that of pristine MoS2 thin films (1.39 mA cm−2). The Tafel slope for the 120 s, 480 s, and 720 s treatment time was 133 mV/dec, 105 mV/dec, and 120 mV/dec, respectively. The HER performance could be promoted by increasing the plasma irradiation time from 120 s to 480 s because it induced more defects (Figure 3f,g). However, as the treatment time was increased to 720 s, the current density decreased. It could be due to the removal of the MoS2 film under the long-term plasma etching. The HER performance results from Ar plasma-treated MoS2 were similar to that of O2 plasma treated MoS2. After the treatment of O2 or Ar plasma on MoS2, the generated defects enhanced the MoS2 electrocatalytic activity for HER.

3.2.3. Nanostructuring MoS2 with Various Morphologies

Increasing the number of active sites by maximally exposing the edges is another efficient way to enhance HER performance. Cui’s group successfully synthesized MoS2 thin films with vertically aligned layers, so that the edges can be maximally exposed on the surface [50]. During the synthesis, they deposited the Mo thin films on the substrate, then developed a rapid sulfurization process to convert Mo films into MoS2 films (Figure 4a). Benefiting from the edges on the surface, these edge-terminated films (Figure 4b) were expected to maximize the catalytic activity for a HER. The TOF of those vertically aligned MoS2 films at 0 V was 0.013 s−1. The exchange current density of the MoS2 films was 2.2 × 10−6 A/cm2, 10 times higher than that reported for the MoS2 nanoparticle. However, the Tafel slope was in the range of 105–120 mV/dec. Kibsgaard and co-workers have also synthesized MoS2 films to preferentially expose the edge sites [51]. The MoS2 films exhibited double-gyroid bicontinuous network morphology with nanoscale pores. The schematic illustration for the synthesis procedure is shown in Figure 4c. The high surface curvature combined with the nanoscale pores made the MoS2 films expose a high density of edge sites, leading to excellent electrocatalytic activity for HER (a low onset overpotential of −150 mV and a Tafel slope of 50 mV/dec).

3.2.4. Lowering the Dimension of MoS2

Lowering the dimension of MoS2 is also an efficient way to increase the number of active sites. Shi et al. have reported the scalable synthesis of monolayer MoS2 on Au foils by a CVD method [52]. Upon varying the growth temperature or the precursor substrate distance, a monolayer triangular shape MoS2 with different coverage was obtained. For the monolayer MoS2 with a coverage of 10%, the cathodic current density at η = −300 mV was 3.9 mA/cm2, and the Tafel slope was 74 mV/dec. It is indicated that the HER performance was much better than that of bulk MoS2 (Figure 5a–c). In addition, when the coverage was increased to 80%, the current density at η = −300 mV reached 50.5 mA/cm2, and the Tafel slope decreased to 61 mV/dec. Therefore, the increased coverage with the increased edge density can enhance the electrocatalytic performance of MoS2. Recently, we have successfully synthesized one-dimensional MoS2 nanobelts (Figure 5d) based on a CVD method with modified growth conditions [53]. High-resolution scanning transmission electron microscope (HRSTEM) characterization indicated that the (001) basal planes of the MoS2 nanobelt were vertically standing on the substrate (Figure 5e,f) and the edges of the base planes formed the top surfaces of the nanobelt. The onset overpotential of the nanobelts was −170 mV, much lower than that of monolayer MoS2. A smaller Tafel slope was observed in the nanobelts (70 mV/dec) compared with monolayer MoS2 (90 mV/dec). The edges on the top surface provide a high density of edge sites, so that the nanobelts can exhibit superior catalytic activity to the monolayer MoS2 (Figure 5g,h). Further decreasing the size of MoS2 by forming quasi-zero-dimensional nanoparticles could further improve the HER performance. Li’s group prepared mono-dispersed molybdenum sulfide nanoparticles (Figure 5i) from bulk MoS2 by a combination of ultrasonication and centrifugation [54]. X-ray photoelectron spectroscopic characterizations revealed that the as-synthesized sample was MoS2 nanoparticles with abundant S edges. These active edges made MoS2 exhibit outstanding HER electrocatalytic activity: an onset potential of −90 mV and a Tafel slope of 69 mV/dec.

3.3. Improving the Electrical Conductivity

3.3.1. Growing MoS2 on Conductive Carbon-Based Substrates

Another factor that obstructs the catalytic performance of MoS2 is its poor electrical transport [32,55]. As we know, both bulk 2H- and 3R-MoS2 are semiconductors with a band gap of 1.3 eV [56]. In general, the high conductivity of the catalyst would increase the electron transport from the active sites to the electrodes. Thus, increasing the electrical transport behavior of MoS2 is an efficient way to enhance its electrocatalytic activity [57]. In 2011, Dai’s group first reported the synthesis of MoS2 on reduced graphene oxide (rGO) sheets [58]. The resultant MoS2/rGO hybrid material has an abundance of small MoS2 nanoparticles dispersed on the surface of the conductive rGO nanosheets. The schematic structure and SEM images of the hybrid structure are shown in Figure 6a,b. Compared with pure MoS2 nanoparticles, the MoS2/rGO hybrid structure exhibited excellent HER activity: the overpotential was ~−100 mV and the Tafel slope was ~41 mV/dec (Figure 6c). This was attributed to the excellent electrical coupling effect between the underlying graphene sheets and the rich active edge sites. Similarly, Liu’s group synthesized a MoS2/carbon nanotube (CNT) nanocomposite network by a facile solvothermal method. The SEM image of the MoS2/ CNT composite is shown in Figure 6d [59]. Similar to the graphene sheets, the CNT could also provide a rapid electron transport channel from the less conducting MoS2 to the electrodes. As a result, the MoS2/CNT network exhibited excellent electrocatalytic activity for the HER (a low onset potential of −90 mV and a low Tafel slope of 44.6 mV/dec).

3.3.2. Fabricating MoS2-Based Heterostructures

Besides the carbon-based substrates, a hybrid structure can also enhance the MoS2 HER performance [60,61,62,63,64]. Jaramillo’s group designed vertically oriented core-shell MoO3/MoS2 heterostructure nanowires [60]. Figure 6e shows the scheme illustration and TEM image of the as-synthesized core/shell structure of MoO3/MoS2 nanowires. Experimental investigations indicated that the MoO3/MoS2 heterostructures exhibited enhanced HER activity: the overpotential was approximately −200–−150 mV, and the Tafel slope was 50–60 mV/dec. The MoO3 core with ~20–50 nm enabled facile charge transport events, and the MoS2 shell ~2–5 nm thick provided excellent catalytic activity. In addition, the stability of the catalyst is another important requirement for the HER application. There was no current degradation after 10,000 cycles in the stability test on MoO3/MoS2 heterostructures. The MoS2 shell could also serve as a protective layer during the HER process to keep the MoO3 away from the acidic environments. Huang et al. also tried the SnO2 as charge transport core by constructing a heterostructure of SnO2/MoS2 (Figure 6f) [61]. Similar to the MoO3/MoS2 heterostructures, the SnO2/MoS2 composite also exhibited excellent HER activity: a relatively low overpotential of −150 mV, a small Tafel slope of 59 mV/dec, and a large current density of 2.3 mA/cm2 at η = −150 mV.
Furthermore, Yu’s group have synthesized a heterostructure of CoSe2/MoS2 (Figure 7a) by growing MoS2 on a CoSe2/DETA nanobelt substrate [65]. In an acidic electrolyte, the CoSe2/MoS2 hybrid structure exhibited the best HER activity among the non-noble metal hydrogen evolution catalysts with an onset potential of −11 mV and a Tafel slope of 36 mV/dec (Figure 7b,c). The appreciation of the exceptional HER catalytic properties can be divided into four aspects. Firstly, the quasi-amorphous structure of MoS2 can increase the number of active edge sites in the MoS2/CoSe2 hybrid composite; secondly, the conductive CoSe2 can facilitate fast charge transport during the HER process; third, CoSe2 chemically interacted with MoS2 by forming a bond of S–Co that can further improve the HER activity of the heterostructure structure. Finally, the anchored MoS2 can also exert a positive effect on the reaction sites of CoSe2 and enhance the catalytic activity.

3.3.3. HER in Conductive MoS2 Prepared via Lithium Intercalation

It is well known that the hexagonal structure of MoS2 shows semiconducting behavior, while the trigonal structure of MoS2 shows metallic behavior [20]. Transferring MoS2 from the semiconductor phase to the metal phase may also decrease the charge transfer resistance, thus improving its catalytic performance. Jin’s group reported the synthesis of metallic 1T-MoS2 nanosheets via lithium intercalation [66]. X-ray diffraction (XRD), Raman scattering, current-sensing atomic force microscopy, and HRTEM characterizations confirmed the formation of 1T-MoS2 after lithium intercalation. As expected, dramatically improved HER activities were achieved in the as-synthesized 1T-MoS2 nanosheets. The overpotential at a current density of 10 mA/cm2 was −187 mV vs. RHE, and the Tafel slope was 43 mV/dec (Figure 7e). Similar results were also observed in the 1T-WS2 [3]. Cui’s group also advanced this structure-tuning investigation by discharging MoS2 based on a Li-ion battery [67]. Firstly, they synthesized the MoS2 films with molecular layers perpendicular to the substrate, as mentioned in Section 3.2.3. Then the MoS2 nanofilms, utilized as a negative electrode, were assembled into a half-cell to continuously tune the amount of intercalated Li ions. Figure 7f shows the discharge curve, which represents the lithiation process. The electrochemical characterizations indicated that as the amount of Li increased from x = 0 to x = 0.02 and x = 0.07, the Tafel slope decreased from 123 mV/dec to 84 and 60 mV/dec. The improved HER performance was mainly due to the lower oxidation states of Mo, as confirmed by the X-ray photoelectron spectroscopy (XPS) spectra. The oxidation states of Mo are related to the electron filling of bonding and antibonding between the active sites and atomic hydrogen. As the oxidation states of Mo are lowered, it would change the hydrogen bonding energy and activation barrier, thus improve the HER activity. When increasing the amount of Li to x = 0.28 and x = 0.85, the HER catalytic activities were further improved. This was mainly due to a 2H-1T phase transition in MoS2. The Tafel slopes in x = 0.28 and x = 0.85 were both saturated around 44 mV/dec, similar to the results observed in the 1T-MoS2 sheets exfoliated by n-Butyl lithium.

3.4. Optimizing the Electronic Structure of MoS2

3.4.1. Strain Effect

As discussed in Section 3.3.2, the introduction of defects on the basal plane of MoS2 could enhance the HER catalytic performance. To further promote the activity, Zheng’s group tried to exert strains on the MoS2 with S-vacancy [68]. According to the DFT calculations, some new bands appeared in the gap near the Fermi level in the MoS2 with S-vacancy. As a tensile strain was applied on the MoS2 with S-vacancy, these new bands moved closer to the Fermi level and the number of gap states increased. The increased gap states around the Fermi level favored the hydrogen adsorption on the S-vacancy sites, thus enhancing the HER activity. To confirm the calculations from the DFT, they first synthesized monolayer 2H-MoS2 via CVD method by using MoO3 and S powders as the precursors. Then they applied a tensile strain via a patterned Au nanocone through a capillary force. They also created S-vacancies on the basal plane by treating the MoS2 with Ar plasma, where the density of the S-vacancies could be controlled by varying the exposing time. Figure 8a shows the schematic illustration of the HER catalytic activity in a strain-affected MoS2 with S-vacancy. By optimizing the combinations of strain effect and S-vacancies (such as 3.12% S-vacancy with 8% strain and 12.5% S-vacancy with 1% strain), the MoS2 exhibited an excellent HER performance. The potential corresponding to 10 mA/cm2 for the strained MoS2 with S-vacancy was −170 mV, lower than that of unstrained MoS2 with S-vacancy (−250 mV), the strained MoS2 without S-vacancy, as well as the transferred MoS2 (Figure 8b). The Tafel slope and the TOFs-vacancy (at 0 V versus RHE) for the strained MoS2 with S-vacancy were 60 mV/dec and 0.08–0.31 s−1, respectively, better than that that of strained MoS2 without S-vacancy, the unstrained MoS2 with S-vacancy, as well as the transferred MoS2 (Figure 8c).

3.4.2. Catalytic Performance in Alloy Structures

Another efficient way to enhance the HER performance of MoS2 is to form MoS2-based alloys [69,70,71,72,73]. Our group synthesized monolayer MoS2(1−x)Se2x alloys and MoS2(1−x)Se2x nanobelt alloys via a CVD method by using MoO3, S powder, and Se powder as reactants [69,73]. By adjusting the weight ratio of S and Se powders, the monolayer MoS2(1−x)Se2x and MoS2(1−x)Se2x nanobelt alloys with different Se contents can be achieved. The HRSTEM image of monolayer MoS2(1−x)Se2x alloys with x = 0.39 (Figure 8d) indicated that some of the S atoms were successfully replaced by Se atoms in the MoS2 lattice structure. Raman scattering and photoluminescence characterizations confirmed that the electronic structures could be tuned in the MoS2(1−x)Se2x alloys. The electrochemical catalytic activity characterizations indicated that in the monolayer MoS2(1−x)Se2x alloys, the overpotential at the current density of 10 mA/cm2 was −300 mV (x = 0.39), −273 mV (x = 0.51) and −279 mV (x = 0.61), respectively. It showed smaller overpotential than that of pure monolayer MoS2 (−335 mV), as demonstrated in Figure 8e. The Tafel slope of monolayer MoS2(1−x)Se2x (x = 0.39, 0.51, 0.61) was 100 mV/dec, 119 mV/dec, 106 mV/dec, respectively (Figure 8f). Similar enhanced catalytic performance was also observed in the MoS2(1−x)Se2x nanobelt alloys [73]. The Gibbs free energy of hydrogen adsorption for a catalyst is strongly related to the density of states near the Fermi level. As the density of states near Fermi level increased, reduced ΔGH can be achieved. Compared with pure MoS2, the Mo in MoS2(1−x)Se2x alloys possesses a lower oxidation state. As a result, a more negative hydrogen adsorption energy can be achieved. Therefore, enhanced HER activities are achieved in the MoS2(1−x)Se2x alloys compared with pure MoS2. A similar trend was also observed in the WS2(1−x)Se2x alloys system [74]. We summarize the HER performance of a variety of MoS2 catalysts mentioned above in Table 1.

3.5. Some New Results

More interestingly, contrary to the traditional view that the number of edge sites is important, some studies also point out that the hopping efficiency of electrons in the vertical direction or the resistance plays a key role in the development of high-efficiency two-dimensional material catalysts [75,76,77]. Cao et al. have investigated layer-dependent MoS2 electrocatalytic activity by controlling the number of layers during the synthesis [76]. The polarization curves of monolayer, bilayer, and trilayer MoS2 films are shown in Figure 9a. Different from the previous study’s results suggesting that more active sites means higher electrocatalytic activity, the electrochemical characterizations indicated that the exchange current density decreased by a factor of ~4.47 with the addition of every layer. During the HER process, the electrochemical reaction only occurs at the outmost layer of the MoS2 film, and the electrons have to transfer from the glassy carbon electrode to the outermost layer [76]. As the potential barriers exit in the interlayer gap, the electron transfer in the perpendicular direction is through hopping (Figure 9b), which makes the thicker sample shows poor electrocatalytic performance. Recently, Shin’s group also investigated the electrocatalytic performance of MoS2 with a different number of edge sites [77]. Their experimental results indicated that the thicker zone with the maximum number of edge sites did not exhibit the best HER performance, while the thin zone consisting of basal planes resulted in the best performance. They attributed the low HER performance to the higher material resistance in the thicker layers, thereby limiting electron transfer for the HER. Therefore, strategies that can increase the electrons’ hopping efficiency in the vertical direction or lower the resistance are expected to be able to enhance the electrocatalytic performance of MoS2 materials.

4. The Photoelectrocatalytic HER

Besides the electrocatalysis, the application of photoelectrocatalytic HER of TMDs has also been investigated. The semiconducting TMDs have an indirect band gap of 1.0 eV to 1.5 eV, and a direct band gap of 1.4 eV to 2.3 eV [78,79,80,81,82]. When the photon energies are above the indirect and direct band gap, the TMDs will absorb the photon and show large absorption coefficients (≈105 cm−1 and ≈106 cm−1, respectively) [78,79,80,81,82]. For example, the WSe2 material shows high absorption with near-unity absorption peak occurring between 500 and 650 nm [79]. Fabricating TMDs/metal heterostructures is an effective way to further enhance their light absorption due to an increase in the local density of states (LDOS) near the semiconductor/metal interface [79,80,81]. The high light absorption combined with the active electrocatalytic performance make the semiconducting TMDs an excellent photoelectrocatalyst [78,79,80,81]. Lewis’s group investigated the photoelectrochemical performance of Pt-decorated p-type WSe2 photocathodes by using scanning photocurrent microscopy [78]. The linear sweep voltammograms indicated that the photocurrent increased markedly when the laser beam was irradiated on the Pt-decorated p-type WSe2 photocathodes (Figure 9c). For the MoS2, the metallic 1T phase possesses higher photoelectrocatalytic activity because it can serve as a good electron acceptor and transporter material [79]. Faster electron transport can effectively suppress the recombination processes of photogenerated charges and thus enhance photocatalytic activity under visible light. Chen’s group has proposed a strategy to efficiently increase the concentration of the 1T phase in exfoliated two-dimensional MoS2 nanosheets by using SC CO2-induced phase engineering [79]. As expected, the photocurrent of 1T@2H MoS2-based photoelectrochemical cells (Figure 9d) at −0.6 V was −1400 μA/cm2, much higher than that of pure 2H MoS2 (−400 μA/cm2).

5. Summary and Outlook

This review has summarized the recent developments of nanostructured MoS2 as electrocatalysts for hydrogen evolution. Specifically, activating the inert edge sites, improving the electrical conductivity, and optimizing the electronic structure are the main strategies to effectively enhance the electrocatalytic efficiency of MoS2. The excellent HER performance makes MoS2 a highly promising candidate to replace conventional noble-metal-based catalysts. However, there are still some issues to be solved before using these materials in industry. The first is the stability of MoS2 catalysts. The industrial applications of a catalyst should have long-term stability, not limited to several tens of hours, as demonstrated in the lab. The second challenge is the production cost and scalability. Even though MoS2 is an abundant material and can be synthesized from relatively abundant materials, reducing the cost is still important for MoS2 large-scale applications. In the end, further improving the electrocatalytic performance of MoS2 remains challenging. Although MoS2-based catalysts have exhibited excellent activities, their HER electrochemical catalytic activity is still unable to surpass that of the noble metals.

Acknowledgments

This work was supported by the Research Foundation for Young Talents of West Anhui University, the joint fund of the National Natural Science Foundation Committee of China Academy of Engineering Physics (NSAF) (U1630108) and the National Natural Science Foundation of China (21373196, 11434009). This work was partially carried out at the USTC Center for Micro and Nanoscale Research and Fabrication.

Author Contributions

Lei Yang and Bin Xiang contributed to the conception and the writing of this paper. Ping Liu and Jing Li contributed to the revision of this paper.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) “Volcano plot” of experimentally measured exchange current density as a function of the DFT-calculated Gibbs free energy of adsorbed atomic hydrogen for vary catalysts [25]; (b) Molecular model of a bulk truncated MoS2 hexagon with Mo and S edges being exposed [33]; (c) Triangular shape of MoS2 exposing Mo edges (Mo atoms terminate with S dimers) [33]; (d) Side view of Mo edges with 0%, 50% and 100% coverage, respectively [33]; (e) An atom-resolved STM images of MoS2 clusters [33]; (f) ( 1 ¯ 010) S edge with varying sulfur coverage [34]. Reproduced from [25] with permission from copyright 2007 American Association for the Advancement of Science, [33] with permission from copyright 2000 The American Physical Society, and [34] with permission from copyright 2007 Nature Publishing Group.
Figure 1. (a) “Volcano plot” of experimentally measured exchange current density as a function of the DFT-calculated Gibbs free energy of adsorbed atomic hydrogen for vary catalysts [25]; (b) Molecular model of a bulk truncated MoS2 hexagon with Mo and S edges being exposed [33]; (c) Triangular shape of MoS2 exposing Mo edges (Mo atoms terminate with S dimers) [33]; (d) Side view of Mo edges with 0%, 50% and 100% coverage, respectively [33]; (e) An atom-resolved STM images of MoS2 clusters [33]; (f) ( 1 ¯ 010) S edge with varying sulfur coverage [34]. Reproduced from [25] with permission from copyright 2007 American Association for the Advancement of Science, [33] with permission from copyright 2000 The American Physical Society, and [34] with permission from copyright 2007 Nature Publishing Group.
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Figure 2. (a) Tafel plots and polarization curves (inset) of the HER on MoS2 and cobalt-doped MoS2 [36]; (b) Left: molecular model of a hexagon MoS2 exposing both the S edge and the Mo edge. Right: differential free energies of hydrogen adsorption [36]; (c) Polarization curves and (d) Tafel plots of the pristine and transition metal doped MoS2 nanofilms [43]. Reproduced from [36] with permission from copyright 2008 The Royal Society of Chemistry and [43] with permission from Springer.
Figure 2. (a) Tafel plots and polarization curves (inset) of the HER on MoS2 and cobalt-doped MoS2 [36]; (b) Left: molecular model of a hexagon MoS2 exposing both the S edge and the Mo edge. Right: differential free energies of hydrogen adsorption [36]; (c) Polarization curves and (d) Tafel plots of the pristine and transition metal doped MoS2 nanofilms [43]. Reproduced from [36] with permission from copyright 2008 The Royal Society of Chemistry and [43] with permission from Springer.
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Figure 3. (a,b) SEM images of amorphous MoS2 thin film on Au [45]; (c) Schematic illustrations of additional active edge sites were designated by gray shading [46]; (d) Polarization curves of various samples as indicated [46]; (e) HRTEM image and corresponding FFT patterns of MoS2 with certain degree of disorder synthesized at 140 °C. The scale bar represents 5 nm [47]; (f) The polarization curves and (g) Tafel plots of O2 plasma-treated MoS2 [48]. Reproduced from [45,47] with permission from copyright American Chemical Society [46] with permission from copyright 2013 Wiley, and [48] with permission from copyright 2015 The Royal Society of Chemistry.
Figure 3. (a,b) SEM images of amorphous MoS2 thin film on Au [45]; (c) Schematic illustrations of additional active edge sites were designated by gray shading [46]; (d) Polarization curves of various samples as indicated [46]; (e) HRTEM image and corresponding FFT patterns of MoS2 with certain degree of disorder synthesized at 140 °C. The scale bar represents 5 nm [47]; (f) The polarization curves and (g) Tafel plots of O2 plasma-treated MoS2 [48]. Reproduced from [45,47] with permission from copyright American Chemical Society [46] with permission from copyright 2013 Wiley, and [48] with permission from copyright 2015 The Royal Society of Chemistry.
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Figure 4. (a) Schematic illustration of the synthesis process of vertically aligned MoS2 film [50]; (b) TEM image of vertically aligned MoS2 that clearly shows the exposed edges. The inset figure was the HRTEM image, which revealed individual layers consisting of three atomic planes in the sequence of S–Mo–S [50]; (c) Synthesis procedure and structural model for mesoporous MoS2 with a double-gyroid morphology [51]. Reproduced from [50] with permission from copyright 2013 American Chemical Society and [51] with permission from copyright 2012 Nature Publishing Group.
Figure 4. (a) Schematic illustration of the synthesis process of vertically aligned MoS2 film [50]; (b) TEM image of vertically aligned MoS2 that clearly shows the exposed edges. The inset figure was the HRTEM image, which revealed individual layers consisting of three atomic planes in the sequence of S–Mo–S [50]; (c) Synthesis procedure and structural model for mesoporous MoS2 with a double-gyroid morphology [51]. Reproduced from [50] with permission from copyright 2013 American Chemical Society and [51] with permission from copyright 2012 Nature Publishing Group.
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Figure 5. (a) Schematic illustration of the HER catalytic activity in the monolayer MoS2 [52]. (b) Polarization curves and (c) Tafel plots of monolayer MoS2 on Au foils with different coverage [52]. (d) Optical image of MoS2 nanobelts [53]. (e) Schematic illustration and (f) HRSTEM image of MoS2 nanobelts [53]. (g) Polarization curves and (h) Tafel plots of monolayer MoS2 and MoS2 nanobelts [53]. (i) TEM image of the MoS2 nanoparticles [54]. Reproduced from [52,53] with permission from American Chemical Society and [54] with permission from copyright 2013 The Royal Society of Chemistry.
Figure 5. (a) Schematic illustration of the HER catalytic activity in the monolayer MoS2 [52]. (b) Polarization curves and (c) Tafel plots of monolayer MoS2 on Au foils with different coverage [52]. (d) Optical image of MoS2 nanobelts [53]. (e) Schematic illustration and (f) HRSTEM image of MoS2 nanobelts [53]. (g) Polarization curves and (h) Tafel plots of monolayer MoS2 and MoS2 nanobelts [53]. (i) TEM image of the MoS2 nanoparticles [54]. Reproduced from [52,53] with permission from American Chemical Society and [54] with permission from copyright 2013 The Royal Society of Chemistry.
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Figure 6. (a) Schematic illustration and (b) SEM image of the MoS2/rGO hybrid [58]. The inset in (b) is the corresponding TEM image. (c) Tafel plots of several catalysts with a loading of 0.28 mg/cm2 as indicated [58]. (d) SEM image of the MoS2/CNT composite [59]. (e) Left: HRTEM image of MoO3/MoS2 heterostructure. Right: schematic illustration of the HER catalytic activity in the MoO3/MoS2 heterostructure [60]. (f) SEM image of MoS2/SnO2 nanotube heterostructure [61]. Reproduced from [58,60] with permission from copyright 2011. American Chemical Society and [59,61] with permission from copyright The Royal Society of Chemistry.
Figure 6. (a) Schematic illustration and (b) SEM image of the MoS2/rGO hybrid [58]. The inset in (b) is the corresponding TEM image. (c) Tafel plots of several catalysts with a loading of 0.28 mg/cm2 as indicated [58]. (d) SEM image of the MoS2/CNT composite [59]. (e) Left: HRTEM image of MoO3/MoS2 heterostructure. Right: schematic illustration of the HER catalytic activity in the MoO3/MoS2 heterostructure [60]. (f) SEM image of MoS2/SnO2 nanotube heterostructure [61]. Reproduced from [58,60] with permission from copyright 2011. American Chemical Society and [59,61] with permission from copyright The Royal Society of Chemistry.
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Figure 7. (a) Schematic illustration of MoS2/CoSe2 heterostructure [65]. (b) Polarization curves and (c) Tafel plots of MoS2/CoSe2 hybrid, pure MoS2, pure CoSe2, and Pt/C. Catalyst loading is about 0.28 mg/cm−2 [65]. (d) Top: the atomic models and SEM image of 2H-MoS2. Bottom: the atomic models and HRTEM image of chemically exfoliated 1T-MoS2 nanosheets [66]. (e) Polarization curves and Tafel plots (insert figure) of chemically exfoliated 1T-MoS2 and as-grown 2H-MoS2 nanosheets [66]. (f) Galvanostatic discharge curve representing the lithiation process in MoS2 [67]. Reproduced from [65] with permission from copyright 2015 Nature Publishing Group, [66] with permission from copyright 2013 American Chemical Society, and [67] with permission from copyright 2013 Proceedings of the National Academy of Sciences of the United States of America.
Figure 7. (a) Schematic illustration of MoS2/CoSe2 heterostructure [65]. (b) Polarization curves and (c) Tafel plots of MoS2/CoSe2 hybrid, pure MoS2, pure CoSe2, and Pt/C. Catalyst loading is about 0.28 mg/cm−2 [65]. (d) Top: the atomic models and SEM image of 2H-MoS2. Bottom: the atomic models and HRTEM image of chemically exfoliated 1T-MoS2 nanosheets [66]. (e) Polarization curves and Tafel plots (insert figure) of chemically exfoliated 1T-MoS2 and as-grown 2H-MoS2 nanosheets [66]. (f) Galvanostatic discharge curve representing the lithiation process in MoS2 [67]. Reproduced from [65] with permission from copyright 2015 Nature Publishing Group, [66] with permission from copyright 2013 American Chemical Society, and [67] with permission from copyright 2013 Proceedings of the National Academy of Sciences of the United States of America.
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Figure 8. (a) Schematic of the top and side views of MoS2 with strained S-vacancies on the basal plane [68]. (b) Polarization curves and (c) Tafel plots of various MoS2 catalysts as indicated [68]. (d) HRSTEM image of monolayer MoS2(1−x)Se2x alloy with x = 0.39 [69]. (e) Polarization curves and (f) Tafel plots of monolayer MoS2(1−x)Se2x with different Se contents [69]. Reproduced from [68] with permission from copyright 2016 Nature Publishing Group and [69] with permission from copyright 2015 The Royal Society of Chemistry.
Figure 8. (a) Schematic of the top and side views of MoS2 with strained S-vacancies on the basal plane [68]. (b) Polarization curves and (c) Tafel plots of various MoS2 catalysts as indicated [68]. (d) HRSTEM image of monolayer MoS2(1−x)Se2x alloy with x = 0.39 [69]. (e) Polarization curves and (f) Tafel plots of monolayer MoS2(1−x)Se2x with different Se contents [69]. Reproduced from [68] with permission from copyright 2016 Nature Publishing Group and [69] with permission from copyright 2015 The Royal Society of Chemistry.
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Figure 9. (a) Polarization curves of the monolayer (red, 1 L), bilayer (blue, 2 L), and trilayer (orange, 3 L) MoS2 films [76]; (b) Hopping of electrons in the vertical direction of MoS2 layers [76]; (c) Linear sweep voltammograms of p-MoS2/Pt obtained with and without laser beam irradiation, respectively [78]; (d) Schematic representation of photoelectrochemical cells with 1T@2H-MoS2 on FTO as photoanode [79]. Reproduced from [76,79] with permission from copyright American Chemical Society and [78] with permission from copyright 2016 The Royal Society of Chemistry.
Figure 9. (a) Polarization curves of the monolayer (red, 1 L), bilayer (blue, 2 L), and trilayer (orange, 3 L) MoS2 films [76]; (b) Hopping of electrons in the vertical direction of MoS2 layers [76]; (c) Linear sweep voltammograms of p-MoS2/Pt obtained with and without laser beam irradiation, respectively [78]; (d) Schematic representation of photoelectrochemical cells with 1T@2H-MoS2 on FTO as photoanode [79]. Reproduced from [76,79] with permission from copyright American Chemical Society and [78] with permission from copyright 2016 The Royal Society of Chemistry.
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Table 1. Onset overpotentials, overpotentials @ 10 mA/cm2, Tafel slopes, and exchange current densities of different MoS2 catalysts.
Table 1. Onset overpotentials, overpotentials @ 10 mA/cm2, Tafel slopes, and exchange current densities of different MoS2 catalysts.
CatalystOnset Overpotential (mV)Overpotential @ 10 mA/cm2 (mV)Tafel Slope (mV/dec)Exchange Current Densities j0 (μA/cm2)
Co-doped MoS2 nanoparticle-−400101-
Transition metal-doped MoS2 nanofilms--107–1182.2–4.9
Amorphous MoS2−165-470.027
Defect-rich ultrathin MoS2 nanosheets−120~−190508.91
Disordered oxygen-incorporated MoS2 nanosheets (140 °C)-~−210675.6
Plasma-engineered MoS2 thin films-~−300105-
MoS2 thin films with vertically aligned layers--105–1202.2
MoS2 films with double-gyroid morphology−200–−150~−220500.69
Monolayer MoS2−100~−2106138.1
MoS2 nanobelts−170−14070-
MoS2 nanoparticles−90~−220699.3
MoS2/rGO−100~−15541-
MoS2/CNT−90~−18044.6-
MoO3/MoS2 heterostructure−200–−150~−25050-60-
SnO2/MoS2 heterostructure−150~−22059-
CoSe2/MoS2 heterostructure−11~−703673
1T-MoS2 nanosheets-−18743-
Strained MoS2 with S-vacancies-−17060-
MoS2(1−x)Se2x nanobelts alloys−139-65-
Catalysts EISSN 2073-4344 Published by MDPI AG, Basel, Switzerland RSS E-Mail Table of Contents Alert
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