Construction of an Amethyst-like MoS2@Ni9S8/Co3S4 Rod Electrocatalyst for Overall Water Splitting

Transition metal sulphide electrocatalytic materials possess the bright overall water-splitting performance of practical electrocatalytic technologies. In this study, an amethyst-like MoS2@Ni9S8/Co3S4 rod electrocatalyst was constructed via a one-step hydrothermal method with in-situ-grown ZIF-67 nanoparticles on nickel foam (NF) as a precursor. The rational design and synthesis of MoS2@Ni9S8/Co3S4 endow the catalyst with neat nanorods morphology and high conductivity. The MoS2@Ni9S8/Co3S4/NF with the amethyst-like rod structure exposes abundant active sites and displays fast electron-transfer capability. The resultant MoS2@Ni9S8/Co3S4/NF exhibits outstanding hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrocatalytic activities, with low overpotentials of 81.24 mV (HER) at 10 mA cm−2 and 159.67 mV (OER) at 50 mA cm−2 in 1.0 M KOH solution. The full-cell voltage of overall water splitting only achieves 1.45 V at 10 mA cm−2. The successful preparation of the amethyst-like MoS2@Ni9S8/Co3S4 rod electrocatalyst provides a reliable reference for obtaining efficient electrocatalysts for overall water splitting.


Introduction
Studies have long focused on developing environmentally friendly and recyclable clean energy materials to replace depleted fossil fuels [1][2][3]. In this respect, hydrogen has become one of the most important material candidates for future energy technologies, owing to its cleanliness, renewability, and high calorific value [4][5][6]. Compared with numerous other hydrogen-production methods (e.g., coal gasification, steam methane reforming, biomass conversion, and photocatalytic hydrogen [7]), water-splitting technology has more potential since it only needs a certain voltage to produce high-purity hydrogen simply and without any pollution [8]. Typically, water splitting produces hydrogen and oxygen via the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) [9]. In the alkaline solution, the HER usually occurs through Volmer-Heyrovsky or Volmer-Tafel mechanism as follows [10,11]: H 2 O + e − + * → H * + OH − (Volmer step) H 2 O + e − + H * → H 2 + OH − + * (Heyrovsky Step) H * + H * → H 2 + 2 * (Tafel step) wherein * is the active adsorption site of the catalyst and (H * ) is the adsorbed hydrogen. The OER mechanism can be described as follows [12]: Ge et al. reported a non-agglomerated MoS 2 /CoP electrocatalyst exhibiting enhanced HER activity prepared by hydrothermal reaction of ZIF-67 precursor grown on titanium foil (TF) [47].
Herein, we demonstrate the preparation of an amethyst-like MoS 2 @Ni 9 S 8 /Co 3 S 4 rod electrocatalyst with high conductivity and rich active sites via a convenient hydrothermal method. Benefiting from the synergistic effects and improved electronic environment, MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF shows low overpotentials for the HER at 10 mA cm −2 and the OER at 50 mA cm −2 in alkaline solution and possesses a low full-cell voltage of overall water splitting at 10 mA cm −2 and excellent electrocatalytic stability.

Preparation of the ZIF-67/NF Precursor
The ZIF-67 precursor was directly grown on NF substrate via a simple process according to a previously reported method with some modifications [42]. First, NF (2 × 3 cm 2 ) was cleaned with 3 M HCl solution, deionized water, and ethanol for 10 min under ultrasonication in successive order and dried at 70 • C for 12 h. Next, 40 mg poly(sodium-p-styrene sulfonate) was added to 10 mL deionized water to form solution I. To achieve surface modification, the as-cleaned NF was soaked in solution I for 30 min under ultrasonication and then washed thrice with deionized water. Next, 40 mmol 2-methylimidazole (2-MeIM) was distributed in 50 mL methanol to form solution II, and 5 mmol Co(NO 3 ) 2 ·6H 2 O was added to 50 mL methanol to form solution III. The surface-modified NF was fully immersed in solution II for 30 min. Subsequently, solution III was poured into the resulting solution II with the surface-modified NF solution mixture and agitated for 30 min. The obtained blend was then allowed to stand at room temperature (25 • C) for 24 h. The desired product was obtained and washed thrice with deionized water. Finally, the product was dried at 70 • C in a vacuum oven for 12 h and named ZIF-67/NF.

Synthesis of Amethyst-like MoS 2 @Ni 9 S 8 /Co 3 S 4 Rods
First, 1 mmol Na 2 MoO 4 ·2H 2 O and 3 mmol thioacetamide (C 2 H 5 NS) were dissolved in 40 mL deionized water. The resultant solution was agitated for 30 min, after which one piece of the as-prepared ZIF-67/NF and the above solution was transferred into a 50 mL autoclave, where it was maintained at 180 • C for 6 h. Subsequently, the resultant sample was collected and washed several times with ethanol and deionized water. Finally, the amethyst-like MoS 2 @Ni 9 S 8 /Co 3 S 4 rod electrocatalyst was obtained after drying at 70 • C in a vacuum oven for 12 h. For comparison, MoS 2 /Ni 9 S 8 /NF without a ZIF-67 precursor, Mo-doped ZIF-67/NF without an S source, and S-doped ZIF-67/NF without a Mo source were synthesized using a preparation process similar to that of the amethyst-like MoS 2 @Ni 9 S 8 /Co 3 S 4 rods.

Material Characterization
The crystalline structures of all materials were characterized using a PANalytical X'Pert X-ray diffractometer (bruker D8 ADVACNCE, Bruker, Mannheim, Germany) with a Cu Kα radiation source at 30 kV. The morphologies and structures of the samples were observed using a Gemini SEM 300 scanning electron microscope (SEM) (Gemini, Friedrichshafen, Germany) and a JEOL JEM-2100F transmission electron microscope (JEOL Co., Ltd., Tokyo, Japan). X-ray photoelectron spectroscopy was conducted using a Thermo Scientific K-Alpha instrument (Waltham, MA, USA).

Electrochemical Measurements
All electrochemical tests were performed using a three-electrode system in a 1 M KOH electrolyte on a CHI 660E electrochemical workstation (CHI 660E, Shanghai, China). The as-prepared catalyst (1 × 1 cm 2 ) was used as the working electrode, and a graphite rod and Hg/HgO were used as the counter and reference electrodes, respectively. All potentials were converted to the reversible hydrogen electrode (RHE) potential according to the following equation: E RHE = E Hg/HgO + 0.0591pH + 0.098 [18]. Linear sweep voltammetry (LSV) was performed from −1.85 to −0.70 V (−0.20 to 1.40 V) vs. Hg/HgO for the hydrogen evolution reaction (HER) (oxygen evolution reaction (OER)) at a scan rate of 2 mV/s. Electrochemical impedance spectroscopy (EIS) was performed in the frequency range of 100 KHz to 0.01 Hz at −1.03 V vs. Hg/HgO (0.56 V) for the HER (OER). The double-layer capacitance (C dl ) and electrochemical surface area (ECSA) were obtained using cyclic voltammetry (CV) at scan rates of 20-100 mV s −1 in the range of −0.30 to −0.20 V vs. Hg/HgO. All polarization curves were corrected using iR compensation.

Results
An amethyst-like MoS 2 @Ni 9 S 8 /Co 3 S 4 rod electrocatalyst was synthesized via a onepot hydrothermal reaction ( Figure 1). A precursor comprising ZIF-67 deposited on NF was first obtained by the hydrothermal reaction at room temperature. As shown in Figure 2e, the prepared ZIF-67/NF precursor presented a classical uniform polyhedral morphology with a size range of 100-300 nm [48]. Subsequently, in the hydrothermal process, the ZIF-67/NF immersed in the solution first reacted with sulphides to release Co ions and generate Co 3 S 4 , resulting in the structural evolution of ZIF-67/NF [45]. Compared with S-ZIF-67/NF (Figure 2g), it can be seen that the structure of ZIF-67/NF became dense with sharp protrusions after reacting with sulphides, which may have a good structureorienting effect for the further formation of nanorods on it. Then, the Ni from the NF reacted with sulphides to generate Ni 9 S 8 nanorods, and Mo reacted with sulphides to form MoS 2 [46,47]. As the reaction proceeded, eventually, an amethyst-like MoS 2 @Ni 9 S 8 /Co 3 S 4 rod electrocatalyst was successfully obtained via this one-step hydrothermal method. Co., Ltd., Tokyo, Japan). X-ray photoelectron spectroscopy was conducted using a Thermo Scientific K-Alpha instrument (Waltham, MA, USA).

Electrochemical Measurements
All electrochemical tests were performed using a three-electrode system in a 1 M KOH electrolyte on a CHI 660E electrochemical workstation (CHI 660E, Shanghai, China). The as-prepared catalyst (1 × 1 cm 2 ) was used as the working electrode, and a graphite rod and Hg/HgO were used as the counter and reference electrodes, respectively. All potentials were converted to the reversible hydrogen electrode (RHE) potential according to the following equation: Hg/HgO. All polarization curves were corrected using iR compensation.

Results
An amethyst-like MoS2@Ni9S8/Co3S4 rod electrocatalyst was synthesized via a onepot hydrothermal reaction ( Figure 1). A precursor comprising ZIF-67 deposited on NF was first obtained by the hydrothermal reaction at room temperature. As shown in Figure  2e, the prepared ZIF-67/NF precursor presented a classical uniform polyhedral morphology with a size range of 100-300 nm [48]. Subsequently, in the hydrothermal process, the ZIF-67/NF immersed in the solution first reacted with sulphides to release Co ions and generate Co3S4, resulting in the structural evolution of ZIF-67/NF [45]. Compared with S-ZIF-67/NF (Figure 2g), it can be seen that the structure of ZIF-67/NF became dense with sharp protrusions after reacting with sulphides, which may have a good structure-orienting effect for the further formation of nanorods on it. Then, the Ni from the NF reacted with sulphides to generate Ni9S8 nanorods, and Mo reacted with sulphides to form MoS2 [46,47]. As the reaction proceeded, eventually, an amethyst-like MoS2@Ni9S8/Co3S4 rod electrocatalyst was successfully obtained via this one-step hydrothermal method.     (Figure 2c). ZIF-67 particles disappear compared with the precursor before sulphuration, indicating that ZIF-67 completely reacted with sulphides. The non-agglomerated rod-shaped structure of MoS2@Ni9S8/Co3S4/NF can expose rich active sites and facilitate the detachment of bubbles from the catalyst during water splitting [27,49]. The morphologies of the other comparative samples are significantly different from that of MoS2@Ni9S8/Co3S4/NF. Except for the ZIF-67/NF ( Figure 2e (Figure 2c). ZIF-67 particles disappear compared with the precursor before sulphuration, indicating that ZIF-67 completely reacted with sulphides. The non-agglomerated rod-shaped structure of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF can expose rich active sites and facilitate the detachment of bubbles from the catalyst during water splitting [27,49]. The morphologies of the other comparative samples are significantly different from that of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF. Except for the ZIF-67/NF ( Figure 2e) and S-ZIF-67/NF ( Figure 2g) discussed above, MoS 2 /Ni 9 S 8 /NF without ZIF-67 exhibits an open structure composed of nonuniform-sized blocks (Figure 2d). When the Mo source is introduced to the sample, Mo-ZIF-67/NF exhibits a network structure composed of nanosheets ( Figure 2f). The above results fully demonstrate that the Mo, S sources, and ZIF-67/NF precursor are essential for generating an amethyst-like rod structure of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF. The EDS mapping images of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF indicate a uniform distribution of elemental Mo, Ni, and S ( Figure 2h). The structure and composition of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF were further observed using TEM (Figure 2i). The results reveal that the MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF nanorod comprises a rough surface with a non-hollow structure. An HR-TEM analysis of MoS 2 @Ni 9 S 8 /Co 3  XRD tests were used to further determine the composition of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF. As shown in Figure  . Thus, to reduce the influence of NF-derived Ni, MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF powder was scraped from the NF for XRD analysis. As shown in Figure S1, the peaks at 14.   (Figure 4b), the peaks at 229.10 and 232.90 eV correspond to Mo 3d5/2 and 3d3/2, respectively, which may be assigned to Mo 4+ , whereas those at 232.10 and 235.90 eV can be ascribed to Mo 6+ [27,51]. The peak at 226.00 eV is assigned to the Mo-S bond [24]. The Ni 2p peaks of MoS2@Ni9S8/Co3S4/NF at 856.40 and 874.20 eV correspond to Ni 2p3/2 and 2p1/2, respectively, which are derived from Ni 3+ [30,52]. Moreover, two satellite peaks (Sat) (Figure 4c) at 862.70 and 880.70 eV are observed. Compared with those observed in the MoS2/Ni9S8/NF without a ZIF-67 spectrum, the Mo 3d and Ni 2p binding XPS analysis was performed to characterize the surface element compositions of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF. The full XPS spectrum of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF demonstrates the presence of elemental Mo, Ni, and S (Figure 4a). In the Mo 3d spectra of MoS 2 @Ni 9 S 8 / Co 3 S 4 /NF (Figure 4b), the peaks at 229.10 and 232.90 eV correspond to Mo 3d 5/2 and 3d 3/2 , respectively, which may be assigned to Mo 4+ , whereas those at 232.10 and 235.90 eV can be ascribed to Mo 6+ [27,51]. The peak at 226.00 eV is assigned to the Mo-S bond [24]. The Ni 2p peaks of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF at 856.40 and 874.20 eV correspond to Ni 2p 3/2 and 2p 1/2 , respectively, which are derived from Ni 3+ [30,52]. Moreover, two satellite peaks (Sat) (Figure 4c) at 862.70 and 880.70 eV are observed. Compared with those observed in the MoS 2 /Ni 9 S 8 /NF without a ZIF-67 spectrum, the Mo 3d and Ni 2p binding energies of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF exhibit positive shifts of 0.32 and 0.20 eV, respectively, which indicates the process of losing electrons in Mo and Ni elements of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF [53]. According to previous reports, transition metal sulphides that lose electrons can generate more positive charges, which is conducive to the adsorption of OH − and thus promotes the OER [33,54]. As shown in Figure S2, the peaks at 779.77 and 797.56 eV could correspond to Co 2p 3/2 and 2p 1/2 , respectively, which may be assigned to Co 3+ and Co 2+ [55], and other peaks are ascribed to satellite peaks. The peaks at 161.80 and 163.10 eV are attributed to S 2p 3/2 and S 2p 1/2 (Figure 4d), corresponding to the Ni-S bond [54], whereas those at 164.10 and 168.50 eV are ascribed to Mo-S and S-O bonds, respectively [27].  [53]. According to previous reports, transition metal sulphides that lose electrons can generate more positive charges, which is conducive to the adsorption of OH − and thus promotes the OER [33,54]. As shown in Figure S2, the peaks at 779.77 and 797.56 eV could correspond to Co 2p3/2 and 2p1/2, respectively, which may be assigned to Co 3+ and Co 2+ [55], and other peaks are ascribed to satellite peaks. The peaks at 161.80 and 163.10 eV are attributed to S 2p3/2 and S 2p1/2 (Figure 4d), corresponding to the Ni-S bond [54], whereas those at 164.10 and 168.50 eV are ascribed to Mo-S and S-O bonds, respectively [27]. The HER performances of the electrocatalysts were tested using a three-electrode system in 1.0 M KOH. As shown in Figure 5a, the linear sweep voltammetry (LSV) curves reveal that MoS2@Ni9S8/Co3S4/NF has the best HER performance. Figure 5b shows the overpotential values of all the samples at 10 and 200 mA cm −2 . At a low current density of 10 mA cm −2 , MoS2@Ni9S8/Co3S4/NF exhibits the lowest overpotential of 81.24 mV, which is superior to MoS2/Ni9S8/NF (96.08 mV), Mo-ZIF-67/NF (187.80 mV), ZIF-67/NF (250.41 mV), and S-ZIF-67/NF (256.63 mV). The optimal HER performance of MoS2@Ni9S8/Co3S4/NF originates from the high conductivity and the unique nanorods structure-exposed rich active sites [23,33]. Meanwhile, the improved electronic environment of MoS2@Ni9S8/Co3S4/NF enhances the adsorption of hydrogen-containing species and accelerates the HER [21,53]. At a high current density of 200 mA cm −2 , MoS2@Ni9S8/Co3S4/NF also exhibits superior HER catalytic activity, with an overpotential of 161.35 mV. Comparing the previously reported Mo/Co/Ni-S electrocatalysts for the HER shown in Table S1, the amethyst-like MoS2@Ni9S8/Co3S4/NF rod electrocatalyst presents the outstanding HER catalytic performance with a lower potential. The Tafel slope was used to study the electrocatalytic kinetics of electrocatalysts. As shown in Figure 5c  The HER performances of the electrocatalysts were tested using a three-electrode system in 1.0 M KOH. As shown in Figure 5a, the linear sweep voltammetry (LSV) curves reveal that MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF has the best HER performance. Figure 5b shows the overpotential values of all the samples at 10 and 200 mA cm −2 . At a low current density of 10 mA cm −2 , MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF exhibits the lowest overpotential of 81.24 mV, which is superior to MoS 2 /Ni 9 S 8 /NF (96.08 mV), Mo-ZIF-67/NF (187.80 mV), ZIF-67/NF (250.41 mV), and S-ZIF-67/NF (256.63 mV). The optimal HER performance of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF originates from the high conductivity and the unique nanorods structure-exposed rich active sites [23,33]. Meanwhile, the improved electronic environment of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF enhances the adsorption of hydrogen-containing species and accelerates the HER [21,53]. At a high current density of 200 mA cm −2 , MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF also exhibits superior HER catalytic activity, with an overpotential of 161.35 mV. Comparing the previously reported Mo/Co/Ni-S electrocatalysts for the HER shown in Table S1, the amethyst-like MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF rod electrocatalyst presents the outstanding HER catalytic performance with a lower potential. The Tafel slope was used to study the electrocatalytic kinetics of electrocatalysts. As shown in Figure 5c, The Tafel slopes of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF, MoS 2 /Ni 9 S 8 /NF, ZIF-67/NF, Mo-ZIF-67/NF, and S-ZIF-67/NF are 50.69, 56.60, 96.48, 116.83, and 118.48 mV dec −1 , respectively. The lower Tafel slope of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF indicates more-efficient HER electrocatalytic kinetics [9,23,27]. In addition, the electrocatalytic kinetics were investigated using electrochemical impedance (EIS) analysis. The charge transfer resistance (R ct ) is related to the electrocatalytic kinetics at the electrolyte-electrode interface. Generally, a smaller R ct represents a higher electron-transfer velocity [30]. Typically, the semicircle diameter in Nyquist plots is positively correlated with the value of R ct , and the specific R ct value can be obtained through equivalent circuit fitting [56]. Nyquist plots for all the synthesized samples are shown in Figure 5d, wherein MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF shows the smallest semicircle diameter. The R ct values of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF, MoS 2 /Ni 9 S 8 /NF, Mo-ZIF-67/NF, and S-ZIF-67/NF are determined as 2.30, 3.27, 8.72, and 23.93 Ω, respectively. ZIF-67/NF presents the highest R ct value (39.49 Ω), which is consistent with the previously reported conclusion that ZIF-67 materials have poor conductivity [8,13,33]. The smallest R ct of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF reflects the higher electron-transfer velocity and improved conductivity of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF [21]. Tafel slope of MoS2@Ni9S8/Co3S4/NF indicates more-efficient HER electrocatalytic kinetics [9,23,27]. In addition, the electrocatalytic kinetics were investigated using electrochemical impedance (EIS) analysis. The charge transfer resistance (Rct) is related to the electrocatalytic kinetics at the electrolyte-electrode interface. Generally, a smaller Rct represents a higher electron-transfer velocity [30]. Typically, the semicircle diameter in Nyquist plots is positively correlated with the value of Rct, and the specific Rct value can be obtained through equivalent circuit fitting [56]. Nyquist plots for all the synthesized samples are shown in Figure 5d, Figure 6a shows the LSV curves of the as-prepared electrocatalysts for the OER. Similar to the HER test results, MoS2@Ni9S8/Co3S4/NF exhibits superior OER activity. As shown in Figure 6b, the overpotential of MoS2@Ni9S8/Co3S4/NF is 159.67 mV at 50 mA cm −2 , which is lower than those of MoS2/Ni9S8/NF (194.97 mV), ZIF-67/NF (417.20 mV), Mo-ZIF-67/NF (423.97 mV), and S-ZIF-67/NF (439.77 mV). Moreover, MoS2@Ni9S8/Co3S4/NF only requires an overpotential of 230.05 mV to reach a current density of 100 mA cm −2 . The outstanding OER performance of MoS2@Ni9S8/Co3S4/NF is related to two factors: on the one hand, the combination of Ni9S8 and Co3S4 with OER catalytic activity can produce an effective synergistic effect of components [50,57]. on the other hand, according to the XPS test results, the slight shift of Ni binding energy indicates an improved electronic environment around Ni9S8, which makes it easier to adsorb OH − and thus promote the OER rate [22,52,53]. Compared with some previously reported electrocatalysts for the OER (Table  S2), MoS2@Ni9S8/Co3S4/NF exhibits competitive OER performance. Figure 6c shows the Tafel plots of the electrocatalysts used for the OER. The Tafel slope of  Figure 6a shows the LSV curves of the as-prepared electrocatalysts for the OER. Similar to the HER test results, MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF exhibits superior OER activity. As shown in Figure 6b, the overpotential of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF is 159.67 mV at 50 mA cm −2 , which is lower than those of MoS 2 /Ni 9 S 8 /NF (194.97 mV), ZIF-67/NF (417.20 mV), Mo-ZIF-67/NF (423.97 mV), and S-ZIF-67/NF (439.77 mV). Moreover, MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF only requires an overpotential of 230.05 mV to reach a current density of 100 mA cm −2 . The outstanding OER performance of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF is related to two factors: on the one hand, the combination of Ni 9 S 8 and Co 3 S 4 with OER catalytic activity can produce an effective synergistic effect of components [50,57]. on the other hand, according to the XPS test results, the slight shift of Ni binding energy indicates an improved electronic environment around Ni 9 S 8 , which makes it easier to adsorb OH − and thus promote the OER rate [22,52,53]. Compared with some previously reported electrocatalysts for the OER (Table S2), MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF exhibits competitive OER performance. Figure 6c shows the Tafel plots of the electrocatalysts used for the OER. The Tafel slope of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF (48.75 mV dec −1 ) is lower than those of MoS 2 /Ni 9 S 8 /NF (71.06 mV dec −1 ), ZIF-67/NF (92.36 mV dec −1 ), Mo-ZIF-67/NF (101.54 mV dec −1 ), and S-ZIF-67/NF (104.27 mV dec −1 ), thus reflecting the faster OER catalytic kinetics of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF. EIS analysis is performed to further study the electrocatalytic kinetics of the OER (Figure 6d). It can be seen that MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF shows the smallest semicircle diameter. The R ct values of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF, MoS 2 / Ni 9 S 8 /NF, ZIF-67, Mo-ZIF-67/NF, and S-ZIF-67/NF are determined to be 2.33, 2.80, 248.50, 123.30, and 241.90 Ω, respectively. Notably, the R ct value of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF is significantly lower than those of the other samples, indicating that MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF possesses the best electron-transfer ability in the OER process [53].  Electrochemical active surface area (ECSA) is a vital parameter for evaluating the performance of electrocatalysts [58]. Typically, the ECSA can be determined using the formula: ECSA /C , where ( ) is the double-layer capacitance and is the specific capacitance and is generally calculated by using 40.0 µF cm −2 [59]. The cyclic voltammetry (CV) curves of the as-prepared samples were measured across the potential range from −0.30 to −0.20 V (vs. Hg/HgO) at scanning speeds of 20-100 mV s −1 . The CV curves of MoS2@Ni9S8/Co3S4/NF are shown in Figure 7a, whereas those of MoS2/Ni9S8/NF, ZIF-67/NF, Mo-ZIF-67/NF, and S-ZIF-67/NF are shown in Figure S3a  Electrochemical active surface area (ECSA) is a vital parameter for evaluating the performance of electrocatalysts [58]. Typically, the ECSA can be determined using the formula: ECSA = C dl /C S , where (C dl ) is the double-layer capacitance and C s is the specific capacitance and is generally calculated by using 40.0 µF cm −2 [59]. The cyclic voltammetry (CV) curves of the as-prepared samples were measured across the potential range from −0.30 to −0.20 V (vs. Hg/HgO) at scanning speeds of 20-100 mV s −1 . The CV curves of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF are shown in Figure 7a, whereas those of MoS 2 /Ni 9 S 8 /NF, ZIF-67/NF, Mo-ZIF-67/NF, and S-ZIF-67/NF are shown in Figure S3a-d, respectively. The C dl can be obtained by fitting the relationship between half the current density at −0.25 V and the scanning speeds. The C dl values of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF, MoS 2 /Ni 9 S 8 /NF, ZIF-67/NF, Mo-ZIF-67/NF, and S-ZIF-67/NF are determined to be 45.32, 17.85, 0.66, 1.54, and 1.68 mF cm −2 (Figure 7b), and the corresponding ECSAs are 1133.0, 446. 3, 16.4, 38.5, and 42.0 cm 2 , respectively. Notably, the ECSA of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF is 2.5 and 69 times larger than those of MoS 2 /Ni 9 S 8 /NF and ZIF-67/NF, respectively. These results demonstrate that the MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF comprising an amethyst-like rod structure can provide a large ECSA and expose abundant active sites. Typically, ideal electrocatalysts must not only exhibit high HER and OER activities but also outstanding electrochemical stability. As shown in Figure 7c, during the chronopotentiometry (CP) test for the HER, the potentials of MoS2@Ni9S8/Co3S4/NF maintain a linear shape after 10, 50, and 100 mA cm −2 for 24 h, displaying excellent HER stability. Similarly, Figure 7d reveals the positive OER durability of MoS2@Ni9S8/Co3S4/NF. The 1000 CV cycles test further evaluated the stability of MoS2@Ni9S8/Co3S4/NF. As shown in Figure S4a, the LSV curves completely coincide before and after 1000 CV cycles for the HER, with almost no change in the overpotential. However, the overpotential of MoS2@Ni9S8/Co3S4/NF increases at the same current density for the OER after 1000 CV cycles ( Figure S4b). Compared to that of the HER (Figure S5a), the SEM images reveal a more-pronounced degradation of MoS2@Ni9S8/Co3S4/NF morphology after 1000 CV cycles for the OER ( Figure S5b), but the catalyst still attaches to the NF. The relatively weakened OER durability may be derived from the apparent degradation of MoS2@Ni9S8/Co3S4/NF morphology after 1000 CV cycles, which is due to the violent generation of bubbles during the OER process. Moreover, the samples after 1000 CV cycles were used for the XRD test. It can be seen that no other new diffraction peaks of samples appear after 1000 CV cycles ( Figure S6). In the sample after 1000 CV cycles for the HER, the peaks at 31. Owing to the superior electrochemical activities of MoS2@Ni9S8/Co3S4/NF for the HER and OER, MoS2@Ni9S8/Co3S4/NF was applied as a bifunctional catalyst for an overall Typically, ideal electrocatalysts must not only exhibit high HER and OER activities but also outstanding electrochemical stability. As shown in Figure 7c, during the chronopotentiometry (CP) test for the HER, the potentials of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF maintain a linear shape after 10, 50, and 100 mA cm −2 for 24 h, displaying excellent HER stability. Similarly, Figure 7d reveals the positive OER durability of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF. The 1000 CV cycles test further evaluated the stability of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF. As shown in Figure S4a, the LSV curves completely coincide before and after 1000 CV cycles for the HER, with almost no change in the overpotential. However, the overpotential of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF increases at the same current density for the OER after 1000 CV cycles ( Figure S4b). Compared to that of the HER (Figure S5a), the SEM images reveal a more-pronounced degradation of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF morphology after 1000 CV cycles for the OER (Figure S5b), but the catalyst still attaches to the NF. The relatively weakened OER durability may be derived from the apparent degradation of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF morphology after 1000 CV cycles, which is due to the violent generation of bubbles during the OER process. Moreover, the samples after 1000 CV cycles were used for the XRD test. It can be seen that no other new diffraction peaks of samples appear after 1000 CV cycles ( Figure S6). In the sample after 1000 CV cycles for the HER, the peaks at 31. Owing to the superior electrochemical activities of MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF for the HER and OER, MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF was applied as a bifunctional catalyst for an overall water-splitting test in a two-electrode system. The LSV curve (Figure 8a) shows that the MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF has a low full-cell voltage of 1.45 V at 10 mA cm −2 , and the cell voltage increases by only 0.03 V after holding at 10 mA cm −2 for 19 h (Figure 8b). These results demonstrate that MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF has excellent bifunctional electrocatalytic activity and stability for overall water splitting. water-splitting test in a two-electrode system. The LSV curve (Figure 8a) shows that the MoS2@Ni9S8/Co3S4/NF has a low full-cell voltage of 1.45 V at 10 mA cm −2 , and the cell voltage increases by only 0.03 V after holding at 10 mA cm −2 for 19 h (Figure 8b). These results demonstrate that MoS2@Ni9S8/Co3S4/NF has excellent bifunctional electrocatalytic activity and stability for overall water splitting.

Conclusions
In summary, we successfully constructed an amethyst-like MoS2@Ni9S8/Co3S4 rod electrocatalyst using a ZIF-67/NF precursor and a one-step hydrothermal method. By adopting a simple synthesis strategy, MoS2@Ni9S8/Co3S4/NF possesses high conductivity and numerous active edge sites. Meanwhile, the synergistic effect produced by the composite of MoS2, Ni9S8, and Co3S4 further improves the catalytic activity of the electrocatalyst. Therefore, MoS2@Ni9S8/Co3S4/NF exhibits lower overpotentials and outstanding electrochemical stability in a 1.0 M KOH solution. These results confirm that the as-prepared amethyst-like MoS2@Ni9S8/Co3S4 rod electrocatalyst possesses excellent bifunctional activity for overall water splitting.

Data Availability Statement:
The data presented in this study are available in this article.

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

Conclusions
In summary, we successfully constructed an amethyst-like MoS 2 @Ni 9 S 8 /Co 3 S 4 rod electrocatalyst using a ZIF-67/NF precursor and a one-step hydrothermal method. By adopting a simple synthesis strategy, MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF possesses high conductivity and numerous active edge sites. Meanwhile, the synergistic effect produced by the composite of MoS 2 , Ni 9 S 8, and Co 3 S 4 further improves the catalytic activity of the electrocatalyst. Therefore, MoS 2 @Ni 9 S 8 /Co 3 S 4 /NF exhibits lower overpotentials and outstanding electrochemical stability in a 1.0 M KOH solution. These results confirm that the as-prepared amethyst-like MoS 2 @Ni 9 S 8 /Co 3 S 4 rod electrocatalyst possesses excellent bifunctional activity for overall water splitting.

Data Availability Statement:
The data presented in this study are available in this article.

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