Ni 3 Se 4 @MoSe 2 Composites for Hydrogen Evolution Reaction

Featured Application: The Ni 3 Se 4 @MoSe 2 composites could be used as electrocatalyst for hydrogen production. Abstract: Transition metal dichalcogenides (TMDs) have been considered as one of the most promising electrocatalysts for the hydrogen evolution reaction (HER). Many studies have demonstrated the feasibility of signiﬁcant HER performance improvement of TMDs by constructing composite materials with Ni-based compounds. In this work, we prepared Ni 3 Se 4 @MoSe 2 composites as electrocatalysts for the HER by growing in situ MoSe 2 on the surface of Ni 3 Se 4 nanosheets. Electrochemical measurements revealed that Ni 3 Se 4 @MoSe 2 nanohybrids are highly active and durable during the HER process, which exhibits a low onset overpotential (145 mV) and Tafel slope (65 mV / dec), resulting in enhanced HER performance compared to pristine MoSe 2 nanosheets. The enhanced HER catalytic activity is ascribed to the high surface area of Ni 3 Se 4 nanosheets, which can both e ﬃ ciently prevent the agglomeration issue of MoSe 2 nanosheets and create more catalytic edge sites, hence accelerate electron transfer between MoSe 2 and the working electrode in the HER. This approach provides an e ﬀ ective pathway for catalytic enhancement of MoSe2 electrocatalysts and can be applied for other TMD electrocatalysts.


Introduction
Energy-saving and environmental protection are of great importance to develop a sustainable society in the 21st century. Currently, 80% of global energy is produced by the consumption of fossil fuels. However, the unsustainable fossil fuels will ultimately come to depletion because of the continuously growing population and expanding industrialization in the world, and their consumption will also lead to serious environmental pollution. So, there is an urgent need to explore alternative energy resources to substitute fossil fuels and gradually switch to a society dominated by sustainable and renewable energy [1,2]. Hydrogen energy is considered as one of the promising clean and renewable energies [3][4][5][6][7] because it possesses high energy density and its only outcome by combustion

Synthesis of Ni 3 Se 4 Nanosheets
Ni 3 Se 4 nanoparticles were synthesized as in previous work with some modifications [42]. In a typical synthesis procedure, 0.1 M NaOH solutions were prepared by dissolving 0.1 mol NaOH in 100 mL of ethanol. Subsequently, 12 mL of the solution was taken in each spout of a three-neck flask. Then, 0.2 mM NiCl 2 •6H 2 O, 0.2 mM selenium powder, and 2 mL of N 2 H 4 •H 2 O were added to the bottles, which were sealed well and preserved at 90 • C for 16 h with magnetic stirring. Thereafter, the black powders were collected by centrifugation and washed with ethanol repeatedly, followed by final drying in a vacuum oven for further characterization and synthesis of composites. The final Ni 3 Se 4 product weighed 98.4 mg.

Synthesis of Ni 3 Se 4 -MoSe 2 Composites
In a typical procedure, 0.2 mmol Se powder dissolved in 10 mL of OAm and dodecanethiol (9:1, vol%) was placed in a three-neck flask at room temperature. The suspension was first maintained at 120 • C for around 10 min with moderate stirring. To obtain the highly active Se precursor, the mixture was then heated up to 200 • C and aged for an additional 0.5 h. After it cooled down to room temperature, as-obtained Ni 3 Se 4 nanosheets (0.05, 0.1, and 0.2 mmol) mixed with 0.1 mmol of Mo(CO) 6 were added into 5 mL OAm and 15 mL ODE and then sufficiently mixed before injecting into the flask (the ratios of Ni 3 Se 4 to MoSe 2 were fixed at 1:2, 1:1, and 2:1, respectively). The products were then held at 250 • C for 0.5 h before being cooled to room temperature. Subsequently, the black powders were thoroughly washed alternately by hexane and ethanol and separated from solution by centrifugation. Further, an acid-picking process was applied to remove the organic molecules and improve the hydrophilic property of the products by dissolving them in acetic acid and maintaining them at 85 • C with vigorous magnetic stirring for 12 h. The final products were washed with alcohol, centrifuged, and dried for further characterization. Pristine MoSe 2 nanosheets were synthesized under the same conditions except adding Ni 3 Se 4 nanosheets. The Ni 3 Se 4 -MoSe 2 composites with ratios of 1:2, 1:1, and 2:1 weighed 31.2, 52.4, and 96.8 mg, and are denoted as Sample 1, 2, and 3 in the following discussion, respectively

Characterization
X-ray powder diffraction (XRD, Bruker New D8-Advance) patterns were recorded on an X-ray powder diffractometer with CuKα radiation (λ = 0.154 nm). Field-emission scanning electron microscopy (FE-SEM, Zeiss 300 VP) images were captured at an acceleration voltage of 10 kV. Transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX) mapping were performed using a JEOL (Tokyo, Japan) instrument. X-ray photoelectron spectroscopy (XPS) was conducted using a K-alpha plus (Thermo Fisher) instrument under vacuum pressure of at least 1 × 10 −5 bar using MgKα radiation (1250 eV) and a constant pass energy of 50 eV.

Electrochemical
Characterization HER performance test was implemented using a three-electrode cell, which consisted of a glassy carbon as working electrode (GCE, 3 mm in diameter), a graphite rod as counter electrode, and a saturated calomel as reference electrode and in 0.5 M H 2 SO 4 at room temperature. Then, 4 mg catalyst and 30 µL Nafion solution (5 wt%) were dispersed in 1.0 mL N, N-Dimethylformamide (DMF) and then sonicated for 0.5 h to form a homogeneous ink. Subsequently, 5 µL catalyst ink was then dropped onto the GCE and dried naturally. Linear sweep voltammetry (LSV) was performed between 0.2 and −1.0 V vs. RHE at a sweep rate of 5 mV/s. Electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range of 10 5 to 10 −1 Hz at the voltage of 0.27 V vs. RHE. All the potentials were calibrated to RHE using the equation: E(RHE) = E(SCE) + 0.272 mV.

Synthesis and Structural Characterization
Ni 3 Se 4 powders were synthesized by a facile and low-temperature method in ethanol (solvent), and the composites were obtained by growing MoSe 2 on the as-synthesized Ni 3 Se 4 nanosheets as depicted in Figure 1. The structural properties of Ni 3 Se 4 and Ni 3 Se 4 -MoSe 2 powders were investigated by XRD. As displayed in Figure 2, three dominant peaks appeared at 33.1 • , 44.8 • , and 50.6 • in the spectra of Ni 3 Se 4 , which were assigned to the (312), (514), and (310) crystal faces of the Ni 3 Se 4 phase, respectively (PDF card number 18-0890) [43]. Further, the obtained diffraction peaks of MoSe 2 can be indexed to the hexagonal 2H-MoSe 2 (JCPDS 29-0914). The XRD patterns of Ni 3 Se 4 -MoSe 2 were like that of Ni 3 Se 4 , but (002) peak assigned to the basal plane of MoSe 2 could still be observed in the patterns.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 10 Ni3Se4 powders were synthesized by a facile and low-temperature method in ethanol (solvent), and the composites were obtained by growing MoSe2 on the as-synthesized Ni3Se4 nanosheets as depicted in Figure 1. The structural properties of Ni3Se4 and Ni3Se4-MoSe2 powders were investigated by XRD. As displayed in Figure 2, three dominant peaks appeared at 33.1°, 44.8°, and 50.6° in the spectra of Ni3Se4, which were assigned to the (312), (514), and (310) crystal faces of the Ni3Se4 phase, respectively (PDF card number 18-0890) [43]. Further, the obtained diffraction peaks of MoSe2 can be indexed to the hexagonal 2H-MoSe2 (JCPDS 29-0914). The XRD patterns of Ni3Se4-MoSe2 were like that of Ni3Se4, but (002) peak assigned to the basal plane of MoSe2 could still be observed in the patterns.  Further, XPS analysis was carried out to explore the composition and chemical state of the composites. The survey spectrum of the Ni3Se4-MoSe2 composites clearly shows the peaks of Ni 2p, Mo 3d, Se 3d, and O 1s (Figure 3a). The O 1s peak occurred owing to the unavoidable oxidation that takes place during the synthesis process. To confirm the oxidation states of these three elements in Ni3Se4 powders were synthesized by a facile and low-temperature method in ethanol (solvent), and the composites were obtained by growing MoSe2 on the as-synthesized Ni3Se4 nanosheets as depicted in Figure 1. The structural properties of Ni3Se4 and Ni3Se4-MoSe2 powders were investigated by XRD. As displayed in Figure 2, three dominant peaks appeared at 33.1°, 44.8°, and 50.6° in the spectra of Ni3Se4, which were assigned to the (312), (514), and (310) crystal faces of the Ni3Se4 phase, respectively (PDF card number 18-0890) [43]. Further, the obtained diffraction peaks of MoSe2 can be indexed to the hexagonal 2H-MoSe2 (JCPDS 29-0914). The XRD patterns of Ni3Se4-MoSe2 were like that of Ni3Se4, but (002) peak assigned to the basal plane of MoSe2 could still be observed in the patterns.  Further, XPS analysis was carried out to explore the composition and chemical state of the composites. The survey spectrum of the Ni3Se4-MoSe2 composites clearly shows the peaks of Ni 2p, Mo 3d, Se 3d, and O 1s (Figure 3a). The O 1s peak occurred owing to the unavoidable oxidation that takes place during the synthesis process. To confirm the oxidation states of these three elements in Further, XPS analysis was carried out to explore the composition and chemical state of the composites. The survey spectrum of the Ni 3 Se 4 -MoSe 2 composites clearly shows the peaks of Ni 2p, Mo 3d, Se 3d, and O 1s (Figure 3a). The O 1s peak occurred owing to the unavoidable oxidation that takes place during the synthesis process. To confirm the oxidation states of these three elements in the composites, the high-resolution spectra of Ni 2p, Mo 3d, and Se 3d were also obtained ( Figure 3b). The spectrum of Ni 2p in the Ni 3 Se 4 -MoSe 2 composites could be deconvoluted into two doublets (2p 3/2 and 2p 1/2 ) along with two shake-up satellites, which appeared due to the spin-orbit coupling effect [44]. Specifically, the 2p 3/2 peak could be further deconvoluted into two components, which were located at the band energies of 853.6 and 855.8 eV. The lower energy band could be attributed to the +2 valence state of nickel (Ni 2+ ), whereas the higher energy could be attributed to the +3 valence state of nickel (Ni 3+ ) [43,45,46]. Similarly, the 2p 1/2 peak could also be resolved into two components that were located at the band energies of 870.9 and 873.5 eV, which were assigned to Ni 2+ and Ni 3+ , respectively [43,45,46]. Further, the satellite peaks appeared at a binding energy slightly positive to the peaks of Ni 2p 3/2 and Ni 2p 1/2 [45]. Compared to the spectrum of Ni 2p in pure Ni 3 Se 4 , all components and satellite peaks shifted slightly (approximately 0.1-1.2 eV) towards lower binding energy, which indicated the chemical bonding between Ni 3 Se 4 and MoSe 2 . For the spectra of Mo 3d (Figure 3c), the two peaks at binding energies of 229.0 and 232.2 eV were assigned to Mo 3d 5/2 and Mo 3d 3/2 of Mo (IV), confirming the presence of Mo 4+ , while the peak at binding energies of 235.6 eV was probably due to the Mo oxide [47], which was formed by oxidation of the metal Mo during the synthesis process. In addition, the Se 3d spectra of the Ni 3 Se 4 -MoSe 2 composites is shown in Figure 3d. The peaks at binding energies of 54.3 and 55.2 eV were corresponded to Se 3d 5/2 and Se 3d 3/2 , respectively, confirming the presence of Se in -2 valence state, whereas the peak at binding energies of 59.9 eV oxidized Se species (SeO x ). Further, the peaks of both Se 3d 5/2 and 3d 3/2 in the composites shifted slightly towards a higher binding energy, whereas the position of the SeO x peak remained unchanged.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 10 the composites, the high-resolution spectra of Ni 2p, Mo 3d, and Se 3d were also obtained ( Figure  3b). The spectrum of Ni 2p in the Ni3Se4-MoSe2 composites could be deconvoluted into two doublets (2p3/2 and 2p1/2) along with two shake-up satellites, which appeared due to the spin-orbit coupling effect [44]. Specifically, the 2p3/2 peak could be further deconvoluted into two components, which were located at the band energies of 853.6 and 855.8 eV. The lower energy band could be attributed to the +2 valence state of nickel (Ni 2+ ), whereas the higher energy could be attributed to the +3 valence state of nickel (Ni 3+ ) [43,45,46]. Similarly, the 2p1/2 peak could also be resolved into two components that were located at the band energies of 870.9 and 873.5 eV, which were assigned to Ni 2+ and Ni 3+ , respectively [43,45,46]. Further, the satellite peaks appeared at a binding energy slightly positive to the peaks of Ni 2p3/2 and Ni 2p1/2 [45]. Compared to the spectrum of Ni 2p in pure Ni3Se4, all components and satellite peaks shifted slightly (approximately 0.1-1.2 eV) towards lower binding energy, which indicated the chemical bonding between Ni3Se4 and MoSe2. For the spectra of Mo 3d (Figure 3c), the two peaks at binding energies of 229.0 and 232.2 eV were assigned to Mo 3d5/2 and Mo 3d3/2 of Mo (IV), confirming the presence of Mo 4+ , while the peak at binding energies of 235.6 eV was probably due to the Mo oxide [47], which was formed by oxidation of the metal Mo during the synthesis process. In addition, the Se 3d spectra of the Ni3Se4-MoSe2 composites is shown in Figure  3d. The peaks at binding energies of 54.3 and 55.2 eV were corresponded to Se 3d5/2 and Se 3d3/2, respectively, confirming the presence of Se in -2 valence state, whereas the peak at binding energies of 59.9 eV oxidized Se species (SeOx). Further, the peaks of both Se 3d5/2 and 3d3/2 in the composites shifted slightly towards a higher binding energy, whereas the position of the SeOx peak remained unchanged.  The morphologies and structures of the Ni 3 Se 4 and Ni 3 Se 4 -MoSe 2 composites with different amounts of Ni 3 Se 4 were characterized by FE-SEM and TEM. Figure 4 shows the FE-SEM and TEM images of pure Ni 3 Se 4 and Ni 3 Se 4 -MoSe 2 composites with a ratio of 1:2. The pure Ni 3 Se 4 nanosheets were observed to be aggregated and formed clusters with large surface areas, as shown in Figure 4a. Figure 4b shows the high-resolution transmission electron microscopy (HRTEM) image of Ni 3 Se 4 nanosheets with a lattice fringe of 0.27 nm, which was assigned to the (112) plane [48]. By in situ synthesis, uniform flower-like MoSe 2 nanosheets could be grown on Ni 3 Se 4 nanosheets (Figure 4c). The growth of vertical MoSe 2 nanoflowers was further confirmed by HRTEM (Figure 4d). Moreover, the EDX elemental mapping results shown (Supporting Information, Figure S1) revealed that MoSe 2 nanoflowers were dispersed uniformly on the surface of Ni 3 Se 4 . Further increasing the amound of Ni 3 Se 4 in the Ni 3 Se 4 -MoSe 2 composites resulted in agglomeration of the composites (Supporting Information, Figure S2) and could hinder the exposure of active edges of MoSe 2 .
Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 10 The morphologies and structures of the Ni3Se4 and Ni3Se4-MoSe2 composites with different amounts of Ni3Se4 were characterized by FE-SEM and TEM. Figures 4 shows the FE-SEM and TEM images of pure Ni3Se4 and Ni3Se4-MoSe2 composites with a ratio of 1:2. The pure Ni3Se4 nanosheets were observed to be aggregated and formed clusters with large surface areas, as shown in Figure 4a. Figure 4b shows the high-resolution transmission electron microscopy (HRTEM) image of Ni3Se4 nanosheets with a lattice fringe of 0.27 nm, which was assigned to the (112) plane [48]. By in situ synthesis, uniform flower-like MoSe2 nanosheets could be grown on Ni3Se4 nanosheets (Figure 4c). The growth of vertical MoSe2 nanoflowers was further confirmed by HRTEM (Figure 4d). Moreover, the EDX elemental mapping results shown (Supporting Information, Figure S1) revealed that MoSe2 nanoflowers were dispersed uniformly on the surface of Ni3Se4. Further increasing the amound of Ni3Se4 in the Ni3Se4-MoSe2 composites resulted in agglomeration of the composites (Supporting Information, Figure S2) and could hinder the exposure of active edges of MoSe2.

Electrocatalytic Properties
The electrocatalytic HER activities of Ni3Se4, MoSe2, and Ni3Se4-MoSe2 with different ratios were investigated in the 0.5 M H2SO4 solution in a three-electrode cell. For reference, commercial Pt/C (10 wt%) was also tested for comparison. As shown in Figure 5a, all Ni3Se4-MoSe2 composites showed low onset overpotentials at a cathode current density of 1 mA/cm 2 (η1). Specifically, Sample 1 showed the lowest onset overpotential of ~145 mV, while the overpotential for Samples 2 and 3 were ~160 and ~214 mV, respectively. Further, increasing the negative potential gave rise to a rapid increase in cathode current density. The overpotentials at the cathode current density of 10 mA/cm 2 (η10), which are usually regarded as indicators of HER performance [49], were 206, 242, and 310 mV for Samples 1, 2, and 3, respectively. All of them showed a decrease in η1 and η10 compared with pure MoSe2 nanosheets. The Tafel plots of catalyst samples and Pt/C are displayed in Figure 5b. The linear regions of Tafel plots derived from the polarization curve can be analyzed using the Tafel equation: η = b log j + a, where η is overpotential, b is Tafel slope, and j is current density [50,51]. Compared to pristine MoSe2 nanosheets (Tafel slope of 80 mV/dec), Sample 1 showed the lowest Tafel slope (65 mV/dec),

Electrocatalytic Properties
The electrocatalytic HER activities of Ni 3 Se 4 , MoSe 2 , and Ni 3 Se 4 -MoSe 2 with different ratios were investigated in the 0.5 M H 2 SO 4 solution in a three-electrode cell. For reference, commercial Pt/C (10 wt%) was also tested for comparison. As shown in Figure 5a, all Ni 3 Se 4 -MoSe 2 composites showed low onset overpotentials at a cathode current density of 1 mA/cm 2 (η 1 ). Specifically, Sample 1 showed the lowest onset overpotential of~145 mV, while the overpotential for Samples 2 and 3 were~160 and~214 mV, respectively. Further, increasing the negative potential gave rise to a rapid increase in cathode current density. The overpotentials at the cathode current density of 10 mA/cm 2 (η 10 ), which are usually regarded as indicators of HER performance [49], were 206, 242, and 310 mV for Samples 1, 2, and 3, respectively. All of them showed a decrease in η 1 and η 10 compared with pure MoSe 2 nanosheets. The Tafel plots of catalyst samples and Pt/C are displayed in Figure 5b. The linear regions of Tafel plots derived from the polarization curve can be analyzed using the Tafel equation: η = b log j + a, where η is overpotential, b is Tafel slope, and j is current density [50,51]. Compared to pristine MoSe 2 nanosheets (Tafel slope of 80 mV/dec), Sample 1 showed the lowest Tafel slope (65 mV/dec), while a slightly higher Tafel slope value was observed for Sample 2 (76 mV/dec) and Sample 3 (96 mV/dec).The HER performance of pure Ni 3 Se 4 nanoparticles was also investigated for comparison. They exhibited inferior performance towards HER. These results indicate that HER catalytic activity originates from MoSe 2 rather than from the inactive Ni 3 Se 4 . This observation is also confirmed by EIS, which was performed to investigate the impedance properties and the electron transfer kinetics during the HER [51]. The charge transfer resistance (R ct ) obtained from the impedance spectra (Figure 5c) showed that Ni 3 Se 4 nanoparticles were conductive and had lower R ct than pure MoSe 2 nanosheets, although they were not good for HER. We attribute the enhanced HER activity of MoSe 2 to the incorporation of Ni 3 Se 4 nanoparticles, which have large surface areas and can improve the conductivity of MoSe 2 nanosheets and hence promote electron transfer between MoSe 2 and the electrolyte. The η 1 , η 10 , Tafel slope, and R ct values of the three samples, along with those of Ni 3 Se 4 , MoSe 2 are summarized in Table 1.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 10 while a slightly higher Tafel slope value was observed for Sample 2 (76 mV/dec) and Sample 3 (96 mV/dec).The HER performance of pure Ni3Se4 nanoparticles was also investigated for comparison. They exhibited inferior performance towards HER. These results indicate that HER catalytic activity originates from MoSe2 rather than from the inactive Ni3Se4. This observation is also confirmed by EIS, which was performed to investigate the impedance properties and the electron transfer kinetics during the HER [51]. The charge transfer resistance (Rct) obtained from the impedance spectra ( Figure  5c) showed that Ni3Se4 nanoparticles were conductive and had lower Rct than pure MoSe2 nanosheets, although they were not good for HER. We attribute the enhanced HER activity of MoSe2 to the incorporation of Ni3Se4 nanoparticles, which have large surface areas and can improve the conductivity of MoSe2 nanosheets and hence promote electron transfer between MoSe2 and the electrolyte. The η1, η10, Tafel slope, and Rct values of the three samples, along with those of Ni3Se4, MoSe2 are summarized in Table 1.  The stability of the Ni3Se4-MoSe2 composites was evaluated by cyclic voltammetry tests from −0.4 to 0.2 V vs. RHE at 50 mV/s for 1000 cycles. The polarization curves of the Ni3Se4-MoSe2 composites showed negligible activity change after 1000 cycles, indicating their durability towards  The stability of the Ni 3 Se 4 -MoSe 2 composites was evaluated by cyclic voltammetry tests from −0.4 to 0.2 V vs. RHE at 50 mV/s for 1000 cycles. The polarization curves of the Ni 3 Se 4 -MoSe 2 composites showed negligible activity change after 1000 cycles, indicating their durability towards HER. Notably, the composites with ratios of 1:2 and 1:1 exhibited better stability than that with a ratio of 2:1, which is consistent with the HER performance results.

Conclusions
In summary, we successfully synthesized Ni 3 Se 4 -MoSe 2 composites by directly growing MoSe 2 on Ni 3 Se 4 nanosheets for the first time. The Ni 3 Se 4 nanosheets served as templates to support MoSe 2 and were expected to prevent MoSe 2 from aggregation and improve its conductivity. XRD, XPS, FE-SEM, and TEM were used to characterize the morphology and structure of the samples. The results revealed that MoSe 2 was chemically bonded on the surface of Ni 3 Se 4 . Further, electrochemical measurements of the composites verified that the HER performance was improved compared to pristine MoSe 2 nanosheets, whereas the Ni3Se4 nanosheets were not catalytically active to the HER but could reduce the charge transfer resistance and facilitate electron transfer between MoSe 2 and the electrolyte. The Ni 3 Se 4 -MoSe 2 composites with a ratio of 1:2 performed the best, with a small overpotential of 145 mV and a low Tafel slope of 65 mV/dec. Continued increase in the amount of Ni 3 Se 4 led to inferior HER performance. These results suggest that the HER activity of MoSe 2 nanosheets can be enhanced by constructing composites with Ni 3 Se 4 in appropriate ratios.

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