In Situ Growth of Nano-MoS2 on Graphite Substrates as Catalysts for Hydrogen Evolution Reaction

In order to synthesize a high-efficiency catalytic electrode for hydrogen evolution reactions, nano-MoS2 was deposited in situ on the surface of graphite substrates via a one-step hydrothermal method. The effects of the reactant concentration on the microstructure and the electrocatalytic characteristics of the nano-MoS2 catalyst layers were investigated in detail. The study results showed that nano-MoS2 sheets with a thickness of about 10 nm were successfully deposited on the surface of the graphite substrates. The reactant concentration had an important effect on uniform distribution of the catalyst layers. A higher or lower reactant concentration was disadvantageous for the electrochemical performance of the nano-MoS2 catalyst layers. The prepared electrode had the best electrocatalytic activity when the thiourea concentration was 0.10 mol·L−1. The minimum hydrogen evolution reaction overpotential was 196 mV (j = 10 mV·cm−2) and the corresponding Tafel slope was calculated to be 54.1 mV·dec−1. Moreover, the prepared electrode had an excellent cycling stability, and the microstructure and the electrocatalytic properties of the electrode had almost no change after 2000 cycles. The results of the present study are helpful for developing low-cost and efficient electrode material for hydrogen evolution reactions.


Introduction
Renewable energy is seen as a powerful way to reduce global carbon emissions, and it will become an integral part of energy in the future [1]. Sustainable, cheap, safe, and clean energy sources are a research hotspot in the pursuit of a green economy and due to increasing demands for energy [2,3]. Solar, wind, geothermal, bioenergy, and hydrogen energies are among the types of renewable energy [4]. Of these, hydrogen energy is clean, green, and sustainable and has a high energy density. It is also easy to produce. The evolution of hydrogen from natural gas is the common approach in current hydrogenevolution methods, but reserves are insufficient [5]. When coal is used to produce hydrogen, CO and CO 2 are generated during the reaction process, polluting the environment [6]. In the biological hydrogen evolution process, many impurities are generated, and purification is difficult [7]. Photocatalytic hydrogen evolution is a promising method, but there are still some unresolved issues. The instability of photocatalysts and the low efficiency of energy conversion limit its application [8][9][10].
The reaction conditions for electrocatalytic hydrogen evolution are mild, and no impurities are produced during the preparation process. Electrocatalytic hydrogen evolution has received much attention because of its virtuous cycle and its ease of operation [11,12]. In this case, the process of hydrogen preparation using electrolysis of water is considered to be a feasible approach [13]. To improve the efficiency of electrocatalysis for water decomposition, there is an immediate requirement to develop effective and reliable catalysts to expedite hydrogen evolution reaction (HER) dynamics [14,15]. Platinum is the most important HER catalyst owing to its suitable free energy of hydrogen adsorption [16]. However, as a noble metal catalyst, platinum is expensive and scarce, which limit its wide application [17,18]. Therefore, the development of low-cost and high-activity electrocatalysts is urgently needed [19,20].
Transition metal sulfides have become important materials due to their high-potential HER performance and low cost [21,22]. The sulfur element in transition metal sulfides can modulate the electronic properties of metals and improve their catalytic activity. Metal sulfides such as CoS 2 , NiS 2 , and MoS 2 have been shown to have good HER activity. Among them, CoS 2 has been widely studied because of its rich composition, semimetallic conductivity, large surface area, and significant catalytic activity. Importantly, CoS 2 has low chemisorption energy for hydrogen, which facilitates hydrogen generation [23]. Zhang et al. [24] used a hydrothermal method to deposit CoS 2 on titanium foil and showed that an extended treatment time of the hydrothermal reaction induced pyramidal CoS 2 morphology formation. The catalyst of the hydrothermal reaction at 15 h showed a very small onset potential of about 81 mV. Pyrite-based NiS 2 was reported to have unlimited potential to be widely used in hydrogen precipitation reactions due to its low cost, good stability, and ease of preparation [25,26]. Ma et al. [27] synthesized two-dimensional nickel disulfide grown on nickel foam with an average thickness of 10 nm and an overpotential of 161 mV at a current density of 100 mA·cm −2 . WS 2 has significant electrochemical activity due to its rapid diffusion of ions, easily accessible edge site structure, and large surface area [28]. Duan et al. [29] delaminated bulk WS 2 to obtain 2D WS 2 nanolayers and then doped the WS 2 nanolayers into graphene films. The catalyst exhibited extraordinary HER performance, with a high current density and long-lasting stability, because the 2D WS 2 surface area and the exposed edges were significantly increased, which was very favorable for HER. MoS 2 is a typical two-dimensional material [30,31]. Like graphite, it features a lamellar structure, with layers bonded internally by strong covalent bonds and layers connected through weak van der Waals forces [32]. It can be easily peeled off between layers an thus, has excellent properties for catalysis and lubrication. For these reasons, this new material has attracted much attention in recent years [33,34]. Therefore, MoS 2 is often used as a catalyst for photocatalytic and electrocatalytic hydrogen evolution. Lin et al. [35] prepared a 10 nm thick MoS 2 monolayer via a stripping method. The nanoscale MoS 2 monolayer had a larger contact area, which improved the activity of the catalyst in the photocatalytic reaction. Fu et al. [36] prepared PCN/MoS 2 via an ultrasonic dispersion method. The band structure of PCN and MoS 2 was staggered, which improved the separation rate of the photocarrier an thus, improved photocatalytic efficiency. However, MoS 2 , in its pure form, lacks catalytic activity. The inherent low electrical conductivity inhibits effective electron transfer and the associated electrochemical kinetics, affecting the speed of HER [37]. To improve the electrocatalytic properties of MoS 2 , researchers have used various methods [38]. A novel nanostructure with an increased surface area is an effective way to increase the number of exposed edges. Nguyen et al. [39] developed a hybrid material with a mesoporous nanosheet heterostructure, and a unique hollow core-shell structure with strong electronic interactions between the core part and the core shell was obtained. The electronic structure of MoS 2 can be effectively tuned by introducing heteroatoms in the MoS 2 base plane. Tang et al. [40] prepared Fe-hybridized MoS 2 nanosheets and found that doping with Fe not only affected the synthesis processes of MoS 2 nanosheets but also optimized the edge sites and the electronic properties of MoS 2 .
To date, most of the prepared HER catalytic agents have been sold in powder form. When performing electrocatalytic reactions, powdered electrocatalysts require Nafion as a binder to drip-coat catalysts on a conductive carrier. Powdered samples can suffer from flaking and loss during electrocatalysis, leading to errors in experimental testing. In addition, adhesive addition can reduce the conductivity of an electrode. Directly depositing nanometer catalysts on a conducting material is an effective way to solve the problem.
When a graphite substrate is used as the reaction substrate, there is direct contact between catalyst and electrolyte, and their contact area is increased. Additionally, there is no need to use Nafion as binder, and the conductivity of electrodes is not affected. This method optimizes the electrode preparation process and reduces the experimental cost.
Therefore, in the current study, we worked on preparing an effective and economical HER electrocatalyst for MoS 2 deposition in situ on a graphite substrate. The graphite substrate was used as a reaction substrate material to avoid catalyst dropout during hydrogen evolution. Moreover, this method reduced MoS 2 nanosheet buildup and exposed more edges to accelerate electron transfer, hence greatly improving the overall conductivity of the HER electrode.

Materials
Sodium molybdate (Na 2 MoO 4 ·2H 2 O) and thiourea (CH 4 N 2 S) were purchased from Tianjin Kemeiou Chemical Reagent Co., Ltd. (Tianjin, China). Nitric acid was purchased from Tianjin Fuchen Chemical Reagent Co., Ltd. (Tianjin, China). Graphite plates with a thickness of 2 mm were obtained from Zibo Baofeng Graphite Co., Ltd. (Zibo, China). The materials used in the experiments did not require additional treatment. To reduce the risk of sample contamination, deionized water was used throughout the experimental procedure.

Sample Preparation Method
Graphite plates were cut into a size of 10 mm × 10 mm, leaving a 3 mm wide section on one side for electrode clamping. We sanded the cut graphite plates with sandpaper until the surfaces were shiny and free of scratches. Then graphite plates were ultrasonicated in concentrated nitric acid for 2 h. After washing and drying, the acid-corroded graphite plates could be used as a substrate for depositing nano-MoS 2 catalyst.
The working electrodes that nano-MoS 2 in situ deposited on graphite substrates (MoS 2 @Gr) were prepared via a one-step hydrothermal method. The mixed solution for the hydrothermal process was prepared by ultrasonically dispersing Na 2 MoO 4 ·2H 2 O and CH 4 N 2 S in 60 mL of deionized water until completely dissolved. One graphite substrate was vertically placed in a 100 mL reaction vessel and subjected to hydrothermal reaction at 200 • C for 24 h. When finished with the reactions, products were cleaned with deionized water and ethanol for 5 min and then dried at 60 • C in a vacuum drying oven. Five different reactant concentrations were used in this study. The concentration of thiourea was 0.05, 0.07, 0.10, 0.12, or 0.15 mol·L −1 ; the concentration of Na 2 MoO 4 was one-third of that of thiourea. For convenience, the products with different thiourea concentrations were named MoS 2 @Gr-0.05, MoS 2 @Gr-0.07, MoS 2 @Gr-0.10, MoS 2 @Gr-0.12, and MoS 2 @Gr-0.15, respectively. In addition, pure MoS 2 powders were prepared under the same synthesis conditions with a thiourea concentration of 0.10 mol·L −1 , and this product was labeled MoS 2 .

Structural Characterizations
The microscopic appearance of the MoS 2 @Gr samples was determined with a scanning electron microscope (SEM, JEM-7100F, JEOL Ltd., Akishima, Japan). Structural properties of the electrodes were characterized via X-ray diffraction (XRD) using a Bruker D8 Discovertype powder diffractometer (Bruker, Billerica, MA, USA) with Cu-K α radiation. The elemental composition and valence bonding state of electrode surface were determined with an ESCALAB 250Xi X-ray photoelectron spectrometer (XPS) (Thermo Fisher Scientific, Waltham, MA, USA). In addition, a confocal Raman spectrum analyzer with an excitation wavelength of 532 nm for Raman analysis was applied. Contact angle measurement of surface wettability was performed using a contact angle meter with a camera (Powereach, JC2000, Shanghai, China). Atomic force microscopy (AFM, NYSE: A; Agilent, Santa Clara, CA, USA) was used to measure the roughness and surface area of graphite.

Electrochemical Tests
An electrochemical workstation (CHI660E) was selected for the electrochemical tests. A saturated calomel electrode (SCE) was used as a standard reference electrode, a platinum mesh served as a counter electrode, the prepared MoS 2 @Gr samples were directly treated as the working electrode, and 0.5 M H 2 SO 4 solution was substituted as the electrolyte. Electrochemical tests of pure MoS 2 powders were performed with the aid of a glassy carbon electrode, and the test electrode was obtained by referring to the preparation method of Wang et al. [41]. All the tests were carried out at room temperature. During electrochemical testing, E SCE was calibrated with a reversible hydrogen electrode (RHE) and converted to E RHE , where E RHE = E SCE + 0.059 pH + 0.241 V. Linear sweep voltammetry (LSV) was tested over a range of −0.6~0 V (vs. RHE), and the sampling rate was 2 mV·s −1 . The electrochemical impedance spectra (EIS) were measured in a frequency range of 100 kHz to 0.01 Hz with an AC voltage of 5 mV. To measure the double-layer capacitance (C dl ) of each electrode, cyclic voltammograms (CV) with different sweep rates (10, 20, 40, 60, 80, 100, or 120 mV·s −1 ) were collected in a voltage range of 0.15~0.35 V (vs. RHE). Chronoamperometry measurements were also recorded at an overpotential of 200 mV, and changes in polarization curves were compared after 2000 cycles of CV tests to investigate the stability of the catalyst.

Morphology and Structure of MoS 2 @Gr Samples
Graphite is very suitable for electrodes due to its high conductivity and low cost. However, its disadvantages are its low surface tension, a wide area without defects, fewer oxygen-containing functional groups on the surface, and poor hydrophilicity. There is no force between the catalyst and graphite substrate, which is not conducive to the adsorption of transition metal ions. Therefore, graphite substrates were first modified using acid corrosion to increase the surface area and improve the hydrophilicity of the substrate surfaces. Figure 1 shows the SEM images and contact angle of the polished and acidcorroded graphite substrate. It can be seen from Figure 1a that the polished graphite substrate surface was relatively smooth, and some small mottling existed on the substrate surface. This was because the graphite layers were not firmly bonded together during the pressing and sintering process. The water contact angle of the polished substrate was evaluated to be 92.4 • . Acid corrosion significantly changed the surface morphology of a substrate. Many graphite layers were exposed on the surface under the effect of ultrasonic and acid corrosion treatment. Moreover, the water contact angle of the acid-corroded substrate reduced to 70.3 • . Clearly, acid corrosion improved the specific surface area and hydrophilicity of the graphite substrate. This favored the growth of MoS 2 on graphite and the formation of a relatively stable structure. More importantly, the graphite substrates could be activated by nitric acid to increase the content of the oxygen-containing functional groups on the graphite substrate surface. The oxygen-containing functional groups could improve the hydrophilicity of graphite substrates, which is consistent with the contact angle measurements [42]. Figure 2 shows XPS spectra of polished graphite and acid-corroded graphite. As can be seen from Figure 2a,b, the polished and acid-corroded graphite substrates contained C and O elements. The C1s spectra of the polished and acid-corroded graphite substrates are shown in Figure 2c,d. The obvious peaks at 284.8 eV are both the binding states of C-C bonds. The peak at 286.2 eV corresponds to C-O-C binding, and the peak at 288.9 eV is O-C=O binding [43]. As can be seen in Table 1, after modification with HNO 3 , the proportion of the C-C peak significantly decreased, while the proportions of C-O-C and O-C=O binding states increased. This indicated that acidification increased the content of oxygen-containing functional groups. Oxygen-containing functional groups could improve the hydrophilicity of the graphite substrates, which is consistent with the results in Figure 1.    Figure 3a,b show the 3D surface morphology of the polished graphite. As can be seen from the AFM results in Figure 3a, the surface of the polished graphite was relatively flat, with a roughness R a of 75.6 nm and a surface area of 2.647 × 10 −9 m 2 . Figure 3c,d show the 3D morphology and roughness of graphite surface after HNO 3 acidification. The roughness R a and surface area of graphite increased to 718.4 nm and 3.84 × 10 −9 m 2 , respectively, as shown in Figure 3c, due to the formation of a fold layer on the surface. The fold layer increased the surface area of the graphite and provided more sites for MoS 2 deposition on the surface. The microstructures of MoS 2 @Gr electrodes were characterized via SEM. As shown in Figure 4, nano-MoS 2 was successfully deposited on the graphite substrate surface, while the reactant concentration had a great impact on the distribution morphology of the MoS 2 catalyst layers. It can be seen from Figure 4a that spherical MoS 2 particles were distributed on the substrate surface, and these flower spherical structures were not fully formed. The graphite substrate surfaces were not completely covered by MoS 2 particles, and graphite substrates could still be observed in some locations. The main reason for this is that the low concentration of the reactant prevented the formation of enough MoS 2 particles to cover the surface of graphite substrate. In contrast, the MoS 2 layer was uniformly distributed on the electrode surface of MoS 2 @Gr-0.10, as shown in Figure 4b. At high magnification, it can be seen that the deposition layer was also composed of flower spherical MoS 2 particles. Each spherical MoS 2 particle consisted of lots of nano-MoS 2 sheets with a thickness of about 10 nm. These nanosheets interspersed with each other and continuously accumulated and thickened to form regular microspheres. A higher reactant concentration was disadvantageous for nano-MoS 2 catalyst layer deposition. Cracks were clearly observed on the MoS 2 @Gr-0.15 electrode surface because the higher reactant concentration les to an increase in the deposition layer thickness. A thicker deposition layer on the electrode surface can easily crack during cleaning and drying process. It can also be observed from Figure 4 that the thickness of the MoS 2 flakes in each sample was close to 10 nm, and most of these nanoflakes vertically grew on the graphite substrate. According to Jaramillo et al. [44], the edges of folded flakes are more easily exposed, promoting the exposure of unsaturated sulfur atoms and improving hydrogen evolution activity. The XRD patterns obtained from the graphite substrate, MoS 2 @Gr, and pure MoS 2 powders are shown in Figure 5. The diffraction peaks of graphite were the characteristic peaks of graphite substrates, and no other impurity peaks could be observed in the substrate, as shown in Figure 5a. In comparison, small diffraction peaks at 2θ ≈ 14 • and 2θ ≈ 33 • corresponding to MoS 2 were detected in the MoS 2 @Gr samples. The diffraction peak intensity of MoS 2 became stronger with increasing reactant concentration, indicating that MoS 2 was successfully deposited on the graphite substrate surface. These results are consistent with the SEM results. In addition, the SEM images and XRD pattern of the pure MoS 2 powders are compared in Figure 5b. The analysis of the results indicated that there was no trace of impurities in the as-synthesized MoS 2 powders, and the powder sample consisted of spheroidal particles with an average diameter of about 3 µm. The high-magnification SEM image illustrated that each spheroidal particle was also formed of nano-MoS 2 sheets. The morphological features were similar to those of the deposited MoS 2 layers shown in Figure 4.
Raman measurements of the graphite substrate after acidification, pure MoS 2 , and MoS 2 @Gr-0.10 were carried out to further characterize the structural features of the samples, and the results are shown in Figure 6a. Two sharp peaks at 1347 cm −1 and 1596 cm −1 of the graphite substrate after the acidification sample represent D and G peaks, respectively, which reflect the degree of graphite disorder and order, respectively [45]. The D peak indicates a defective peak caused by the vibrations of sp 3 -hybridized carbon atoms in the amorphous carbon in a sample, and the G peak represents a graphite peak generated by vibrating sp 2 -hybridized carbon atoms in graphitic carbon [46]. The peaks at 378 cm −1 and 403 cm −1 for the pure MoS 2 sample indicated the existence of 2H phases of MoS 2 [47,48]. The two peaks correspond to two different vibrational modes of in-plane and out-of-plane Mo-S, where the frequency differences between the vibrational modes mean that MoS 2 nanosheets have a multilayer structure [49]. Both graphite and MoS 2 peaks could be observed in the MoS 2 @Gr-0.10 sample, also indicating that the MoS 2 catalyst layer was successfully deposited on the graphite substrate.  XPS testing was used to characterize the surface elemental composition and valence bonding state of the MoS 2 @Gr-0.10 sample, and the results are shown in Figure 6b-f. As can be seen in Figure 6b that the MoS 2 @Gr-0.10 sample was mainly composed of C, O, Mo, and S elements. The carbon mainly originated from the graphite substrate. In Figure 6d, two overlapped peaks can be observed. The peak at 530.7 eV indicates C=O bonding, and the peak at 532.1 eV indicates C-O-Mo bonding. According to Lu et al. [50], the presence of C-O-Mo bonding indicates that the chemical interaction between MoS 2 and graphite substrate occurs through C-O-Mo. This makes the MoS 2 and graphite substrate tightly bonded to each other, which facilitates the structural stability of catalyst layers. The main peaks at 229.6 eV and 232.8 eV shown in Figure 6e belong to Mo 3d 5/2 and Mo 3d 3/2 orbitals, respectively, corresponding to Mo 4+ [51,52]. Moreover, there was a weak peak corresponding to Mo 6+ at 235.8 eV, indicating that MoS 2 caused mild oxidation in the air. The peaks at 161.2 eV and 163.6 eV in Figure 6f belong to S 2p 3/2 and S 2p 1/2 , respectively, which correspond to the −2 valence in the sulfur of MoS 2 , reflecting the formation of the 2H phase of MoS 2 [53,54].

Electrochemical HER Performance
The electrochemical HER property of the prepared electrodes was tested in a typical three-electrode system. For comparison, the electrocatalytic properties of Pt and pure MoS 2 @Gr samples were also measured under the same conditions. The LSV curves of the samples are shown in Figure 7a. As shown, these samples produced different electrocatalytic performances. The graphite substrate had almost no catalytic activity in the HER process, and the pure MoS 2 also exhibited poor catalytic activity. In contrast, the prepared MoS 2 @Gr electrodes showed good HER catalytic performance. The overpotential of the MoS 2 @Gr electrodes at a current density of 10 mA·cm −2 first increased and then decreased with the increase in the reactant concentration, and the MoS 2 @Gr-0.10 sample had the lowest initial HER overvoltage of 113 mV. The variation in the HER catalytic properties of the MoS 2 @Gr electrodes was mainly affected by the microstructural difference caused by the reactant concentration (see Figure 4). For the MoS 2 @Gr-0.05 and MoS 2 @Gr-0.07 samples, the exposed MoS 2 edges were not enough on the electrode surface, resulting in poor HER catalytic performance. With respect to the MoS 2 @Gr-0.12 and MoS 2 @Gr-0.15 samples, excessive MoS 2 nanosheets accumulated on the electrode surface, leading to the edge coverage of active sites and lower conductivity of catalyst layer. The Pt electrode had the best general performance. In this study, nano-MoS 2 was successfully deposited on a graphite substrate, and its catalytic activity for HER may be further improved via structural modification [55], heteroatoms doping [56], etc.
Under ideal conditions, Tafel slopes usually represent the intrinsic properties of electrocatalysts and reaction kinetic issues that can well explain the active sites of catalysts and enable analysis of HER mechanisms [57]. According to Lin et al. [58,59], a smaller Tafel slope represents a faster current density growth when the overpotential increases by an equal amount. As shown in Figure 7b, the Tafel slopes of the MoS 2 @Gr-0.05, MoS 2 @Gr-0.07, MoS 2 @Gr-0.10, MoS 2 @Gr-0.12, and MoS 2 @Gr-0.15 electrodes were calculated as 81.1, 68.6, 54.1, 58.1, and 65.1 mV·dec −1 , respectively. Clearly, the MoS 2 @Gr-0.10 electrode had the lowest Tafel slope value after that of the Pt electrode.
The electron exchange process in electrochemical reactions usually occurs at an electrode surface between the electrons in an electrode and the ions in the solution, and this process often occurs in redox reactions. Therefore, EIS tests were conducted to describe the kinetic process of electron transfer rate at the electrode surface. A lower charge transfer resistance means faster HER kinetic properties. As shown in Figure 7c, Nyquist plots are presented as semicircles in the high-frequency region and slanted lines in the low-frequency region. These electrochemical measurements are represented by a fitted circuit in Figure 7c, where R s , R p , and R ct represent the resistance, electrode porosity, and charge transfer resistance of the electrolyte solution, respectively; CPE is a constant phase angle element that represents a solid double-layer capacitance of an electrode in real situations; and Z w represents ion diffusion process in liquid solution.
To better understand the electrochemical catalytic activity of each electrode, Tafel slopes and R ct values are compared in Figure 7d. It can be observed that Pt has the lowest Tafel slope value. For the other samples, the Tafel slope values present a trend of first decreasing and then increasing. Additionally, the R ct values first decrease and then increase, which is consistent with the Tafel slope values. The much smaller R ct values of the MoS 2 @Gr samples compared with those of the graphite substrate suggested that the MoS 2 @Gr samples had superior ion diffusion behavior. Among these MoS 2 @Gr electrodes, the MoS 2 @Gr-0.10 sample exhibited the best HER catalytic performance. Clearly, the microstructure of deposition catalyst layer was the decisive factor for HER catalytic activity. The microstructure of catalyst layer could be conveniently controlled by changing the hydrothermal reaction conditions, such as reactant concentration, reaction temperature, reaction time, and so on. Generally, there is a linear relationship between C dl and electrochemically active surface area (ECSA) [60]. Electrochemical CV curves were obtained in the voltage range of 0.15-0.35 V (vs. RHE) at multiple scan rates (10-120 mV·s −1 ), and the results are shown in Figure 8a-f. According to Feng et al. [61], C dl can be estimated by plotting the differences in current density (∆j = j a − j c ) at 0.241 V as a function of the scan rate. Based on the slopes of the line shown in Figure 8g, the C dl values were calculated and are displayed in Figure 8h. By comparing the C dl values of MoS 2 @Gr samples, we concluded that the ECSA of MoS 2 @Gr-0.10 was larger than that of the other samples. The higher C dl value of the MoS 2 @Gr-0.10 electrode was closely related to its microstructure feature: the increase in MoS 2 edges on the MoS 2 @Gr-0.10 electrode remarkably promoted its HER process.

Cycling Stability
In addition to electrocatalytic performance, stability is an important criterion to evaluate the performance of electrocatalysts. The electrochemical stability of the MoS 2 @Gr-0.10 electrode was determined via long-term CV tests. As shown in Figure 9a, after 2000 CV cycles, the change in the polarization curve of the sample was almost negligible. There was no loss of any current density, and the curves before and after CV cycles overlapped relatively well. This indicates that the MoS 2 @Gr-0.10 catalyst had very good stability and excellent HER performance during a long electrochemical time in 0.5 M H 2 SO 4 solution. After that, the electrode was subjected to a chronoamperometry, and the results are shown in Figure 9b. After 660 min durability experiments, the current density of the acidic solution showed variances but with small fluctuations.
The good cycling performance of the MoS 2 @Gr-0.10 electrode was mainly due to its structural stability during electrochemical cycling process. As illustrated in Figure 9d, nano-MoS 2 was still clearly visible on the electrode surface after 2000 CV cycling tests. There was no obvious exfoliation of MoS 2 sheets, indicating that the catalyst layer had a good adhesion force with the graphite substrate. This mainly benefitted from the inherent stability of graphite and the in situ growth of MoS 2 on the graphite substrate. In summary, based on the microstructural characterization of the materials and the comparison of the results of HER performance, we found that the MoS 2 @Gr-0.10 electrocatalyst had a good morphological structure and excellent HER performance. Although the in situ growth of MoS 2 on graphite substrates is a simple and rapid process, its activity can be further improved as a catalyst for hydrogen evolution. In subsequent work, the surface area of graphite can be further increased, the size of MoS 2 can be decreased, and metal atoms can be doped to improve the hydrogen evolution activity of the catalysts. The HER performance of MoS 2 @Gr was compared with that of the reported electrocatalysts, and the results are tabulated in Table 2.

Conclusions
(1) Nano-MoS 2 was successfully deposited on the surface of a graphite substrate via a one-step hydrothermal method, and the microstructure of the MoS 2 layers could be controlled by changing concentration of reactant. (2) A dense and uniform MoS 2 layer was the key factor to improve the HER catalytic activity of the MoS 2 @Gr electrodes. However, a higher reactant concentration led to an increase in the deposited MoS 2 layer thickness, which resulted in edge coverage of active sites and a decrease in the conductivity of the catalyst. Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.
Data Availability Statement: Data sharing is not applicable to this article as no new data were created or analyzed in this study.

Conflicts of Interest:
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.