Charge Redistribution of Co9S8/MoS2 Heterojunction Microsphere Enhances Electrocatalytic Hydrogen Evolution

The electrocatalytic hydrogen evolution activity of transition metal sulfide heterojunctions are significantly increased when compared with that of a single component, but the mechanism behind the performance enhancement and the preparation of catalysts with specific morphologies still need to be explored. Here, we prepared a Co9S8/MoS2 heterojunction with microsphere morphology consisting of thin nanosheets using a facile two-step method. There is electron transfer between the Co9S8 and MoS2 of the heterojunction, thus realizing the redistribution of charge. After the formation of the heterojunction, the density of states near the Fermi surface increases, the d-band center of the transition metal moves downward, and the adsorption of both water molecules and hydrogen by the catalyst are optimized. As a result, the overpotential of Co9S8/MoS2 is superior to that of most relevant electrocatalysts reported in the literature. This work provides insight into the synergistic mechanisms of heterojunctions and their morphological regulation.


Introduction
With the increase in global population and rapid economic development, environmental degradation and the energy issue are starting to gain attention, and this created a demand for new clean and efficient energy sources. Considering its renewable attributes and high energy density, hydrogen is one of the most promising green energy sources [1][2][3]. There are four main ways to produce hydrogen: from fossil fuels, from industrial by-products, from biomass, and from the electrolysis of water [4]. The first three of these require unsustainable sources of raw materials. Hydrogen production from electrolytic water splitting has gained worldwide attention because of its environmental friendliness, production flexibility, and product purity [5][6][7]. Efficient electrolytic hydrogen production from water requires effective electrocatalysts to reduce the energy consumption of the hydrogen evolution reaction (HER). Pt-based materials are benchmark catalysts for the HER, but their high cost and scarcity hinder large-scale commercial applications [8][9][10]. Therefore, it is crucial to design electrocatalysts that are both affordable and high-performing in order to enable the development of hydrogen energy.
In recent years, materials based on transition metal sulfides have been considered as potential alternatives to noble metal catalysts due to their low cost, high activity, and stability [11][12][13][14]. Density functional theory calculations indicate that MoS 2 has a ∆G H* value close to that of Pt, demonstrating that it may be a promising HER electrocatalyst [15]. Subsequent electrochemical characterization has confirmed that MoS 2 does indeed have excellent HER activity [16]. In fact, MoS 2 has many different phases [17]. Among them, the H and T phases, in which Mo atoms are located at the centers of triangular prisms and octahedra, respectively, are common. Because 2H-MoS 2 is semiconducting and 1T-MoS 2 is metallic, 1T-MoS 2 is usually considered suitable as an electrocatalyst. Co 9 S 8
The hydrothermal approach was used to create a CoMo-LDH precursor. Firstly, 60 mL deionized water was used to dissolve 2 mmol Co(NO 3 ) 2 ·6H 2 O, 3 mmol Na 2 MoO 4 ·2H 2 O, and 6 mmol HMTA. The prepared solution was then placed into a 100 mL reactor after thorough stirring, and the hydrothermal process was kept at 90 • C for 6 h. Secondly, the precipitates were cleaned with deionized water and alcohol after cooling to room temperature and then dried and centrifuged to obtain the CoMo-LDH precursor.
The prepared CoMo-LDH precursor was transferred to the tubular furnace. S powder was selected as the sulfur source, and 500 mg S powder was placed in an aluminum oxide crucible and put in upstream of the airflow. Next, 50 mg CoMo-LDH was placed in another aluminum oxide crucible and put in downstream of the airflow. The whole reaction process was conducted under the protection of an argon hydrogen (Ar/H 2 ) atmosphere. The incoming gas flow rate was set to 40 standard cubic centimeters per minute (sccm). The Co 9 S 8 /MoS 2 samples were prepared at the following temperatures for 2 h: 350 • C, 450 • C, 550 • C, and 650 • C (the heating rate was 5 • C/min). After the tube furnace had cooled to room temperature, the product in the aluminum oxide crucible located downstream of the gas was removed to obtain the desired sample.
First, 5 mmol cobalt acetate ((CH 3 COO) 2 Co·4H 2 O) and 5 mmol thiourea were dissolved in 30 mL ethylene glycol. The solution was then transferred to a 50 mL high-pressure reactor after 30 min stirring and sealed and held at 200 • C for 48 h. The precipitate was centrifuged once the autoclave had reached room temperature. It was washed three times with deionized water and alcohol and then dried in an oven at 60 • C for eight hours. Co 9 S 8 was eventually obtained.

Characterizations
X-ray diffractions were characterized with Cu K radiation using D/MAX 2500 equipment. The morphology and energy dispersive spectrometer (EDS) characterizations were carried out using scanning electron microscopy (Hitachi S4800). A high-resolution transmission electron microscope (JEM-F200) was used to determine the microstructure. A Thermo Scientific K-Alpha apparatus was used to collect the X-ray photoelectron spectra with an Al K source.

Electrochemical Measurements
The electrochemical properties were determined using a three-electrode electrochemical workstation (CHI 660). A platinum electrode was employed as the counter electrode. A glassy carbon electrode with the sample coated according to the following process was used as the working electrode. First, 5 mg electrocatalyst sample and 80 mL Nafion solution were uniformly dispersed in 1 mL ethanol. The surface of the 3 mm diameter glassy carbon electrode was then splattered with 5 mL electrocatalyst ink. Finally, it was dried at 50 • C to produce the necessary working electrode. An Ag/AgCl (in saturated KCl solution) was chosen for the reference electrode and 1 M KOH (PH = 13.8) was used as the electrolyte. The linear sweep voltammetric (LSV) curves were tested (iR-corrected) at 5 mV/s. Electrochemical impedance spectra Nyquist plots were tested at −400 mV (vs. Ag/AgCl) from 10 5 Hz to 0.01 Hz. Cyclic voltammetry curves were measured at different scan rates (20 mV/s; 40 mV/s; 80 mV/s; 120 mV/s; 160 mV/s; 200 mV/s). As for the electrochemical stability test, the initial LSV curve (iR-corrected) was tested first, then 1000 cyclic voltammetry cycles were scanned at a sweep rate of 5 mV/s. Another LSV curve (iR-corrected) was tested to compare with the initial LSV curve. The chronoamperometric curve was tested with an overpotential that maintained an initial current density of 10 mA/cm 2 . The test time was set to 50 h. The potential vs. Ag/AgCl electrode was converted to potential vs. RHE according to the following formula: E (RHE) = E (Ag/AgCl) + 0.197 V + 0.059 × pH. The electrochemically active surface area (ECSA) of the catalyst on the glassy carbon electrode was estimated according to the following equation: ECSA = C dl /C s × S, where C dl is the electrochemical double-layer capacitance, which can be obtained by testing cyclic voltammetric curves at different sweep rates, C s is the specific capacitance of the sample, which can be estimated as 0.04 mF/cm 2 in the present work, and S is the surface area of the glassy carbon electrode, which is 0.071 cm 2 in the present work.

DFT Calculations
The mechanism was revealed by first principle calculations using the Vienna ab initio simulation software [26,27]. In order to optimize the structure and determine the free energy of all structures, the program uses the projected enhancement wave pseudopotential [28] and the generalized gradient approximation of the Perdew-Burke-Ernzerhof (PBE) exchange correlation functional [29]. In the adsorption energy calculation, plane waves basis cutoff energy sets of 500 eV and a Monkhorst-Pack mesh of 2 × 2 × 1 was used in K-sampling. All atom locations were totally relaxed until the residual force on each atom was less than 0.02 eV/−1.20 eV/ of vacuum layer along the z-direction, with the electronic self-consistent iteration set to 10 −5 eV.

Structural and Morphological Characterization
The synthesis of the Co 9 S 8 /MoS 2 sample was based on the sulfuration of the precursor CoMo-LDH. The precursor was obtained from a hydrothermal reaction in accordance with the literature [30]. Its composition was confirmed by X-ray diffraction (XRD) (Figure 1a). In addition, energy dispersive spectrometer (EDS) analysis showed that it was composed of three elements-Co, Mo, and O ( Figure 1b)-indicating its high purity. The scanning electron microscope (SEM) characterization showed that it presented a morphology of micrometer spheres made of nanosheets (Figure 1c,d). After the CoMo-LDH was vulcanized at 450 • C in a reducing atmosphere, the product was composed of Co 9 S 8 (PDF-#75-2023) and 2H-MoS 2 (PDF-#37-1492) (Figure 2a). The product still showed a microsphere morphology consisting of nanosheets (Figure 2b,c), indicating that the sulfidation reaction at a high temperature did not change the overall morphology of the precursor. The presence of many pores between the nanosheets helped to increase the surface area as well as facilitate the migration of electrolytes and bubbles for subsequent electrochemical reactions [31].
To determine how the temperature of the vulcanization reaction affects its products, the precursors were also vulcanized at 350 • C, 550 • C, and 650 • C. XRD characterization showed that the products at these different temperatures were all Co 9 S 8 /MoS 2 ( Figure 3). The diffraction peaks for the product at 350 • C were weaker than those of the other products, indicating its poor crystallinity. SEM characterization showed that the nanosheets became thicker as the reaction temperature increased ( Figure 4). From the results of the transmission electron microscopy (TEM) characterization, a distinct microsphere can be observed ( Figure 2d). The thin nanosheets can be observed after zooming in on the images ( Figure 2e). From the high-resolution TEM image (Figure 2f), clear lattice stripes can be seen. The 0.173 nm stripe corresponds to the (440) crystal plane of the Co 9 S 8 , while the 0.624 nm stripe corresponds to the (002) crystal plane of the MoS 2 . The presence of clear interfaces between them indicates that they form heterojunctions. The EDS elemental mapping revealed that the distribution of Co, Mo, and S in the sample was uniform ( Figure 2g). In addition, separate Co 9 S 8 and MoS 2 were prepared for a comparative analysis of the properties of the Co 9 S 8 /MoS 2 . Their components as well as the morphology of the nanosheets were confirmed using XRD and SEM characterization ( Figure 5).

Structural and Morphological Characterization
The synthesis of the Co9S8/MoS2 sample was based on the sulfuration of the precursor CoMo-LDH. The precursor was obtained from a hydrothermal reaction in accordance with the literature [30]. Its composition was confirmed by X-ray diffraction (XRD) ( Figure  1a). In addition, energy dispersive spectrometer (EDS) analysis showed that it was composed of three elements-Co, Mo, and O ( Figure 1b)-indicating its high purity. The scanning electron microscope (SEM) characterization showed that it presented a morphology of micrometer spheres made of nanosheets (Figure 1c,d). After the CoMo-LDH was vulcanized at 450 °C in a reducing atmosphere, the product was composed of Co9S8 (PDF-#75-2023) and 2H-MoS2 (PDF-#37-1492) (Figure 2a). The product still showed a microsphere morphology consisting of nanosheets (Figure 2b,c), indicating that the sulfidation reaction at a high temperature did not change the overall morphology of the precursor. The presence of many pores between the nanosheets helped to increase the surface area as well as facilitate the migration of electrolytes and bubbles for subsequent electrochemical reactions [31]. To determine how the temperature of the vulcanization reaction affects its products, the precursors were also vulcanized at 350 °C, 550 °C, and 650 °C. XRD characterization showed that the products at these different temperatures were all Co9S8/MoS2 ( Figure 3). The diffraction peaks for the product at 350 °C were weaker than those of the other products, indicating its poor crystallinity. SEM characterization showed that the nanosheets became thicker as the reaction temperature increased (Figure 4). From the results of the transmission electron microscopy (TEM) characterization, a distinct microsphere can be observed (Figure 2d). The thin nanosheets can be observed after zooming in on the images (Figure 2e). From the high-resolution TEM image (Figure 2f), clear lattice stripes can be seen. The 0.173 nm stripe corresponds to the (440) crystal plane of the Co9S8, while the 0.624 nm stripe corresponds to the (002) crystal plane of the MoS2. The presence of clear interfaces between them indicates that they form heterojunctions. The EDS elemental mapping revealed that the distribution of Co, Mo, and S in the sample was uniform (Figure 2g). In addition, separate Co9S8 and MoS2 were prepared for a comparative analysis of the properties of the Co9S8/MoS2. Their components as well as the morphology of the nanosheets were confirmed using XRD and SEM characterization ( Figure 5).         To determine the chemical make-up and elemental states of the samples, X-ray photoelectron spectroscopy (XPS) was employed. The survey spectra of the Co 9 S 8 , MoS 2 , and Co 9 S 8 /MoS 2 are shown in Figure 6a. The Co 9 S 8 /MoS 2 sample consists of Co, Mo, S, and O elements, the Co 9 S 8 sample consists of Co, S, and O elements, and the MoS 2 sample consists of Mo, S, and O elements. The O elements in the samples are a result of the oxidation of the sample surfaces. The C elements are artificially added to correct the data. For the high-resolution XPS spectra of the Co 2p electrons (Figure 6b), the peaks located near 778.4 eV and 793.6 eV are associated with Co 3+ and correspond to the Co 2p 3/2 and Co 2p 1/2 electrons of the Co 3+ , respectively [32]. The peaks located near 781.5 eV and 797.7 eV are associated with Co 2+ and they correspond to the Co 2p 3/2 and Co 2p 1/2 electrons of the Co 2+ , respectively. In addition, the peaks near 787.7 eV and 803.0 eV are the satellite peaks of the Co 2p electrons. The area of the Co 3+ peak is significantly larger in the Co 9 S 8 /MoS 2 compared with the Co 9 S 8 , indicating that a portion of the electrons is transferred from the Co in the Co 9 S 8 /MoS 2 . For the high-resolution XPS spectrum of the Mo 3d electrons (Figure 6c), four peaks were exhibited at 226.0 eV, 228.9 eV, 232.1 eV, and 235.5 eV. They are attributed to the S 2s, Mo 4+ 3d 5/2 , Mo 4+ 3d 3/2 , and Mo 6+ electrons, respectively [33]. Compared with that of the MoS 2 , the Mo 4+ 3d peak of the Co 9 S 8 /MoS 2 changes toward lower binding energy, showing that the Mo in the Co 9 S 8 /MoS 2 has gained some electrons. The appearance of the Mo 6+ is due to the oxidation of some Mo elements on the surface of the sample. For the high-resolution XPS spectrum of the S 2p electrons (Figure 6d), the peaks between 161 eV and 164 eV can be fitted by two peaks. They correspond to the S 2− 2p 3/2 and S 2− 2p 1/2 electrons, respectively [34]. In the Co 9 S 8 , the peak at 168.5 eV is attributed to the S-O, which results from oxidizing superficial S. According to the aforementioned findings, there is charge transfer between the Co 9 S 8 and MoS 2 in the Co 9 S 8 /MoS 2 , which leads to the redistribution of electrons.

Electrocatalytic Performance
To investigate the performance of these samples for the electrocatalytic hydrogen evolution reaction (HER), they were characterized electrochemically. The electrolyte used was 1 M KOH solution. Figure 7a shows the linear scanning voltammetry (LSV) test curves of the samples (which have been 95% iR compensated). The figure shows that the Co 9 S 8 /MoS 2 exhibits greater HER activity than the Co 9 S 8 or the MoS 2 alone at the same current density. The overpotential is only 118 mV at a current density of 10 mA cm −2 , which is much lower than that of the Co 9 S 8 at 234 mV or the MoS 2 at 257 mV. This indicates that there is a synergistic effect between the Co 9 S 8 and the MoS 2 which facilitates the hydrogen evolution reaction. In addition, the activity of the Co 9 S 8 /MoS 2 catalysts fabricated in this work is better than the overpotential values of most relevant heterojunction materials reported in the literature (Table 1) [35][36][37][38][39][40][41][42][43][44]. The Tafel slope of the Co 9 S 8 /MoS 2 is 92.6 mV dec −1 , which is lower than that of the Co 9 S 8 at 106.1 mV dec −1 or the MoS 2 at 127.8 mV dec −1 (Figure 7b). The above Tafel slope values also indicate that their HER processes follow the Volmer-Heyrovsky reaction mechanism [45].

Electrocatalytic Performance
To investigate the performance of these samples for the electrocatalytic hydrogen evolution reaction (HER), they were characterized electrochemically. The electrolyte used was 1 M KOH solution. Figure 7a shows the linear scanning voltammetry (LSV) test curves of the samples (which have been 95% iR compensated). The figure shows that the Co9S8/MoS2 exhibits greater HER activity than the Co9S8 or the MoS2 alone at the same current density. The overpotential is only 118 mV at a current density of 10 mA cm −2 , which is much lower than that of the Co9S8 at 234 mV or the MoS2 at 257 mV. This indicates that there is a synergistic effect between the Co9S8 and the MoS2 which facilitates the hydrogen evolution reaction. In addition, the activity of the Co9S8/MoS2 catalysts fabricated in this work is better than the overpotential values of most relevant heterojunction materials reported in the literature (Table 1) [35][36][37][38][39][40][41][42][43][44]. The Tafel slope of the Co9S8/MoS2 is 92.6 mV dec −1 , which is lower than that of the Co9S8 at 106.1 mV dec −1 or the MoS2 at 127.8 mV dec −1 (Figure 7b). The above Tafel slope values also indicate that their HER processes follow the Volmer-Heyrovsky reaction mechanism [45].   [35] MoS2@CoS2/G 118 53 [36]   To compare the actual electrochemically active surface area (ECSA) of the Co 9 S 8 , MoS 2 , and Co 9 S 8 /MoS 2 electrodes, a CV test was used in the non-Faraday region to calculate the double-layer capacitance value (C dl ) of the electrode, since ECSA is proportional to C dl . The CV curves of the samples at different sweep rates are shown in Figure 8a-c. The fitted C dl values are shown in Figure 8d. The Co 9 S 8 /MoS 2 electrode has the largest C dl value (29.5 mF cm −2 ), which is higher than the double-layer capacitance values of both the Co 9 S 8 (22.2 mF cm −2 ) and the MoS 2 (7.47 mF cm −2 ). This indicates that the prepared Co 9 S 8 /MoS 2 electrode has the largest electrochemically active surface area and the most active sites, which is favorable for the HER process. To reveal the effect on the intrinsic activity after the formation of the Co 9 S 8 /MoS 2 heterojunction, the current density was normalized by the electrochemically active surface area (ECSA). As is shown in Figure 9a, the overpotential of the Co 9 S 8 /MoS 2 is the smallest among the three at the same current density, which indicates that the formation of the Co 9 S 8 /MoS 2 heterojunction does improve the intrinsic electrocatalytic hydrogen evolution activity of the single components Co 9 S 8 and MoS 2 . To further investigate the electrode kinetics of the HER process for the Co 9 S 8 /MoS 2 heterojunction materials, the electrochemical impedance spectra (EIS) of the Co 9 S 8 , MoS 2 , and Co 9 S 8 /MoS 2 materials were tested. Typically, the impedance decreases with a decreasing Nyquist curve arc radius. It can be clearly observed from Figure 9b that the Co 9 S 8 /MoS 2 has a much lower impedance value than the comparative sample. This result indicates that the construction of the Co 9 S 8 /MoS 2 heterojunction can effectively improve the reaction kinetics of the catalyst. Stability is also an important index for evaluating hydrogen precipitation catalysts. There was no significant increase in current density after 50 h of chronoamperometry tests (Figure 9c). In addition, Figure 9d shows that after 1000 CV tests the overpotential increased by just 2 mV at a current density of 10 mA cm −2 . These results indicate that the Co 9 S 8 /MoS 2 heterojunction catalyst has excellent electrochemical stability. In addition, the hydrogen evolution activity was compared for the Co 9 S 8 /MoS 2 samples obtained at different sulfidation temperatures. As is shown in Figure 9e, the best HER activity was obtained from the sample vulcanized at 450 • C. This may be the result of the combined effect of the crystallinity and morphology of the samples. On the one hand, the sample sulfided at 350 • C is less crystalline, and on the other hand the nanosheets of the products become rougher at higher temperatures (550 • C and 650 • C), which leads to a smaller electrochemically active area. Therefore, 450 • C is the optimal temperature for sulfidation treatment.
Co9S8/MoS2 samples obtained at different sulfidation temperatures. As is shown in Figure  9e, the best HER activity was obtained from the sample vulcanized at 450 °C. This may be the result of the combined effect of the crystallinity and morphology of the samples. On the one hand, the sample sulfided at 350 °C is less crystalline, and on the other hand the nanosheets of the products become rougher at higher temperatures (550 °C and 650 °C), which leads to a smaller electrochemically active area. Therefore, 450 °C is the optimal temperature for sulfidation treatment.

DFT Calculations
First-principles calculations were carried out to shed more light on the cause of the enhanced activity following the creation of the Co9S8/MoS2 heterostructure. The optimized structural model is shown in Figure 10.

DFT Calculations
First-principles calculations were carried out to shed more light on the cause of the enhanced activity following the creation of the Co 9 S 8 /MoS 2 heterostructure. The optimized structural model is shown in Figure 10. By calculating the charge density difference of the material, the charge transfer between the different atoms can be visualized. As is shown in Figure 11a, at the interface between the Co9S8 and MoS2, the electron cloud density decreases on the Co9S8 side and increases on the MoS2 side. This indicates that electrons flow from the Co9S8 to the MoS2, and this result is consistent with the results of the XPS characterization. The density of states (DOS) curve (Figure 11b) shows that the MoS2 is semiconducting with a forbidden band width of 1.50 eV. The Co9S8 and Co9S8/MoS2 both exhibit metallicity. The Co9S8/MoS2 has a higher density of states at the Fermi surface than the Co9S8, indicating that it is more conductive, which would favor the HER process. Hydrogen adsorption free energy (ΔGH*) is often used to indirectly reflect the activity of the catalyst [46]. By calculating the charge density difference of the material, the charge transfer between the different atoms can be visualized. As is shown in Figure 11a, at the interface between the Co 9 S 8 and MoS 2 , the electron cloud density decreases on the Co 9 S 8 side and increases on the MoS 2 side. This indicates that electrons flow from the Co 9 S 8 to the MoS 2 , and this result is consistent with the results of the XPS characterization. The density of states (DOS) curve (Figure 11b) shows that the MoS 2 is semiconducting with a forbidden band width of 1.50 eV. The Co 9 S 8 and Co 9 S 8 /MoS 2 both exhibit metallicity. The Co 9 S 8 /MoS 2 has a higher density of states at the Fermi surface than the Co 9 S 8 , indicating that it is more conductive, which would favor the HER process. Hydrogen adsorption free energy (∆G H* ) is often used to indirectly reflect the activity of the catalyst [46]. A positive ΔGH* value indicates that it is difficult for the catalyst to adsorb hydrogen, whereas a negative ΔGH* value suggests that hydrogen is adsorbing onto the catalyst too tightly. The ideal ΔGH* value is zero. As can be seen in Figure 11c, both the Co9S8 and the MoS2 have positive free energy values for hydrogen adsorption. The ΔGH* value of the Co9S8 (0.39 eV) is much smaller than that of the MoS2 (2.5 eV), indicating that the Co9S8 has better HER activity than the MoS2. The ΔGH* value of the Co9S8/MoS2 is further optimized to only −0.24 eV, which is even closer to zero, indicating that the Co9S8/MoS2 has the best HER activity among the three. The adsorption energy of the catalyst on water molecules is another parameter that can reflect the HER activity [47]. This is due to the fact that the occurrence of the hydrogen evolution reaction in alkaline media first involves the adsorption of water molecules. As is shown in Figure 11d, the adsorption energy of the Co9S8/MoS2 for water molecules is −0.30 eV. It is −0.22 eV for the Co9S8 and 0.05 eV for the MoS2. This indicates that the adsorption of water molecules by the Co9S8/MoS2 is the best A positive ∆G H* value indicates that it is difficult for the catalyst to adsorb hydrogen, whereas a negative ∆G H* value suggests that hydrogen is adsorbing onto the catalyst too tightly. The ideal ∆G H* value is zero. As can be seen in Figure 11c, both the Co 9 S 8 and the MoS 2 have positive free energy values for hydrogen adsorption. The ∆G H* value of the Co 9 S 8 (0.39 eV) is much smaller than that of the MoS 2 (2.5 eV), indicating that the Co 9 S 8 has better HER activity than the MoS 2 . The ∆G H* value of the Co 9 S 8 /MoS 2 is further optimized to only −0.24 eV, which is even closer to zero, indicating that the Co 9 S 8 /MoS 2 has the best HER activity among the three. The adsorption energy of the catalyst on water molecules is another parameter that can reflect the HER activity [47]. This is due to the fact that the occurrence of the hydrogen evolution reaction in alkaline media first involves the adsorption of water molecules. As is shown in Figure 11d, the adsorption energy of the Co 9 S 8 /MoS 2 for water molecules is −0.30 eV. It is −0.22 eV for the Co 9 S 8 and 0.05 eV for the MoS 2 . This indicates that the adsorption of water molecules by the Co 9 S 8 /MoS 2 is the best among these three. For compounds composed of 3d metals, the position of the d-band center is related to the adsorption of hydrogen [48]. Lower adsorption of hydrogen by the catalyst is typically implied by a downward shift of the d-band center. The d-band centers of the Co 9 S 8 and MoS 2 are situated at −1.32 eV and −0.35 eV, respectively, as is shown in Figure 11e,f. They are pushed downward to −1.39 eV and −1.07 eV for the Co 9 S 8 /MoS 2 , respectively. Combined with the previous results concerning the positive ∆G H* values of the Co 9 S 8 and MoS 2 , the lesser hydrogen adsorption by the catalyst in this case, which is more advantageous to the HER process, is indicated by the d-band centers having moved down. The above results indicate that the charge transfer between the Co 9 S 8 and the MoS 2 after the formation of the Co 9 S 8 /MoS 2 heterojunction achieves charge redistribution, thus optimizing the adsorption characteristics of the active sites of the electrocatalyst for hydrogen and water molecules, and thereby realizing the improvement of the electrocatalytic hydrogen evolution performance.

Conclusions
In summary, we prepared a Co 9 S 8 /MoS 2 heterojunction by vulcanizing a CoMo-LDH precursor. The vulcanization temperature of 450 • C was suitable to obtain a product with good crystallinity as well as nanosheet thickness. The Co 9 S 8 /MoS 2 manifests the morphology of a microsphere made of thin nanosheets. Some electrons of the Co 9 S 8 of the heterojunction were transferred to the MoS 2 , which led to the optimization of the charge distribution. After the formation of the heterojunction, the density of states near the Fermi surface increase, the d-band centers of the transition metals shifted downward, and the adsorption of both water molecules and hydrogen by the catalyst were optimized. As a result, the overpotential of the Co 9 S 8 /MoS 2 at the current density of 10 mA cm −2 was only 118 mV, which is better than those of most Co 9 S 8 /MoS 2 -related HER electrocatalysts reported in the literature. This work provides clarification concerning the mechanisms of performance enhancement and morphology regulation of heterojunction electrocatalysts.
Author Contributions: Formal analysis and methodology, L.Z.and J.Z.; conceptualization and investigation, A.X., Z.L. and Z.W.; writing-original draft; writing-review and editing, L.Z. and S.S.; software and project administration, W.Z. and G.W.; data curation and funding acquisition, S.S. and G.W. All authors have read and agreed to the published version of the manuscript.
Funding: This work was financially supported by the National Natural Science Foundation of China (52271184, 52072255) and the Natural Science Foundation of Zhejiang Province, China (LY23E020001, LTY20E020001).

Data Availability Statement:
The data presented in this study are available upon request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.