Heterostructure Ni3S4–MoS2 with interfacial electron redistribution used for enhancing hydrogen evolution

Developing highly effective and inexpensive electrocatalysts for hydrogen evolution reaction (HER), particularly in a water-alkaline electrolyzer, are crucial to large-scale industrialization. The earth-abundant molybdenum disulfide (MoS2) is an ideal electrocatalyst in acidic media but suffers from a high overpotential in alkaline solution. Herein, nanospherical heterostructure Ni3S4–MoS2 was obtained via a one-pot synthesis method, in which Ni3S4 was uniformly integrated with MoS2 ultrathin nanosheets. There were abundant heterojunctions in the as-synthesized catalyst, which were verified by X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). The structure features with interfacial electron redistribution was proved by XPS and density functional theory (DFT) calculations, which offered several advantages to promote the HER activity of MoS2, including increased specific surface area, exposed abundant active edge sites and improved electron transfer. Ni3S4–MoS2 exhibited a low overpotential of 116 mV at 10 mA cm−2 in an alkaline solution with a corresponding Tafel slope of 81 mV dec−1 and long-term stability of over 20 h. DFT simulations indicated that the synergistic effects in the system with the chemisorption of H on the (002) plane of MoS2 and OH on the (311) plane of Ni3S4 accelerated the rate-determining water dissociation steps of HER. This study provides a valuable route for the design and synthesis of inexpensive and efficient HER electrocatalyst, heterostructure Ni3S4–MoS2.


Introduction
With the increasing environmental protection demands, developing sustainable and fossil-free renewable energy plays a major role. 1 Due to its environmentally friendly, zeroemission, high-energy capacity and sustainable merits, hydrogen (H 2 ) has received extensive attention. 24][5] Currently, some noble metals, such as Pt, Rh and Ir, have been proved to possess excellent catalytic performance as HER electrocatalysts, including low overpotential, low Tafel slope and low impedance. 6,7However, the exorbitant cost and limited earth abundance of these noble metal materials have hindered their industrialization and commercialization. 8herefore, developing electrocatalysts that are low cost, highly active and stable from nonprecious and earth-abundant metal materials is urgent.
0][11] DFT calculations indicated that MoS 2 exhibits excellent HER performance in acidic solutions since its edge sites permit a near-optimal hydrogen adsorption free energy (DG H* ¼ 0.08 eV). 12Moreover, the tremendous amount of S sites in the basal plane of pure MoS 2 are quite inert and not sufficiently utilized. 13Unfortunately, MoS 2 has also been found to have poor activity in alkaline media, 14 even though alkaline catalysis is a more widespread application.Numerous strategies have been employed to improve the catalytic activity of 2D MoS 2 , such as generating the sulfur vacancies, 15 introducing heteroatoms, 16 changing conductive supports. 17It has been both experimentally and theoretically identied that the fabrication of heterogeneous nanostructures with abundant and accessible exposed active sites is a very effective way for improving the catalytic activity. 18oS 2 decorated with transition metals, such as Fe, Co and Ni, by heterostructure engineering has shown excellent electrocatalytic performance. 19Because of the versatile electronic structure of these metals and the ability to ll d orbitals with electrons from another transition metal, they provide distinctive catalytic properties.For example, nickel-based catalysts have been shown to have impressive potential for HER electrocatalysts due to their conductivity and low cost. 20,21Some reports have indicated that constructing heterostructure for MoS 2 via introducing nickel suldes (e.g., Ni 3 S 2 , 22 NiS 2 (ref.23)  and NiS 24 ) could provide superior HER activity.However, the intrinsic motivation of the enhanced catalytic performance is not clear.Moreover, the impact of constructing a heterogeneous structure via introducing Ni 3 S 4 into MoS 2 on HER performance remains to be studied.Moreover, an in-depth understanding of the interfacial electron redistribution for the improved HER performance is important for its wide application.
In this study, we fabricated the heterostructure Ni 3 S 4 -MoS 2 with a nanospherical morphology via a one-step hydrothermal strategy and applied it as an HER catalyst.To improve the electronic conductivity and expose abundant active edge sites of MoS 2 , we constructed the heterostructure via introducing Ni 3 S 4 into MoS 2 ultra-thin nanosheets, which signicantly enhanced the HER activity.The heterostructure of Ni 3 S 4 -MoS 2 was investigated via XPS, SEM and HRTEM techniques.For researching the electron redistribution on the interface of Ni 3 S 4 -MoS 2 and the mechanism of electro-catalysis during HER, we also applied the DFT simulation.It is indicated that the heterostructure Ni 3 S 4 -MoS 2 optimized water dissociation energies and H* absorption free energy.O and 15 mmol (1.14 g) of thiourea were dissolved in 30 mL of deionized water and dispersed by ultrasonication for 10 min to form a uniform solution.The mixture was transferred into a 50 mL Teon-lined stainless-steel autoclave and maintained at 180 C for 24 h.The as-prepared MoS 2 was washed with deionized water and ethanol several times and then dried in a vacuum at 60 C. Ni 3 S 4 -MoS 2 was synthesized by a simple one-pot step method that was the same as the above process for MoS 2 .However, in the Na 2 MoO 4 -$2H 2 O and thiourea mixed solution, different amounts of NiCl 2 $6H 2 O were added.Then, the Ni 3 S 4 -MoS 2 nanosheets were obtained aer the same hydrothermal treatment, washing and drying.

Experimental section
Ni 3 S 4 was prepared according to the method in the reported literature. 25Briey, 1.5 mmol (0.357 g) of NiCl 2 $6H 2 O and 10 mmol (0.60 g) of urea were dissolved in 30 mL deionized water, and the mixture was dispersed by ultrasonication for 10 min to form a uniform solution.The solution was then transferred into a 50 mL Teon-lined stainless-steel autoclave and maintained at 130 C for 2 h.The as-prepared Ni(OH) 2 was washed with deionized water and ethanol several times and then dried in a vacuum at 60 C.Then, the as-prepared Ni(OH) 2 precursor together with 30 mL of 15 mmol (3.6 g) sodium sulde (Na 2 -S$9H 2 O) aqueous solution was placed in the Teon-lined stainless-steel autoclave and maintained at 90 C for 9 h.The as-prepared Ni 3 S 4 was washed with deionized water and ethanol several times and then dried in a vacuum at 60 C for 12 h.Characterization X-ray diffraction (XRD) patterns were collected on a Rigaku XRD-6000 diffractometer using Cu Ka radiation from 3 to 80 at the scan rate of 10 min À1 .The morphologies were investigated via SEM (Zeiss SUPRA 55) at an accelerating voltage of 20 kV.A Brunauer-Emmett-Teller (BET, ASAP 2460) apparatus was used to measure the surface area.HRTEM images were recorded using a JEOL JEM-2010 eld-emission transmission electron microscope at an accelerating voltage of 200 kV, combined with energy-dispersive X-ray spectroscopy (EDS).XPS measurements were performed on a Thermo VG ESCALAB 250 X-ray photoelectron spectrometer with Al Ka radiation at a pressure of about 2 Â 10 À9 Pa.Inductively coupled plasma-optical emission spectrometry (ICP-OES) was adopted to analyze the chemical components of the catalysts.

Electrochemical measurements
Electrochemical measurements were performed on an electrochemical workstation (CHI 660E, CH Instruments Inc., Chenhua, Shanghai) using a three-electrode mode in an Ar-saturated 1 mol L À1 KOH aqueous solution.A platinum electrode was used as the counter electrode, a sliver/sliver chloride (Ag/AgCl) electrode was used as the reference electrode, and the asfabricated materials were used as the working electrodes.The potentials were converted to the RHE scale using the following Nernst equation: (E(RHE) ¼ E(Ag/AgCl) + 0.059 pH + 0.197).To accelerate the electrochemical performance tests, 5 mg of the as-prepared catalysts, 2 mg conductive carbon, 35 mL of the 5 wt% Naon solution and 1 mL anhydrous ethanol were mixed and ultrasonicated for 10 min to form homogeneous catalyst inks.The catalyst inks were dripped respectively onto the asprepared NF to obtain the working electrodes with a loading of $5 mg cm À2 , which were dried at 60 C for 1 h.The electrochemical impedance spectroscopy (EIS) tests were performed in the frequency range from 100 kHz to 0.1 Hz at an overpotential of 180 mV.The cyclic voltammograms (CV) were obtained between 0.1 and À0.3 V vs. RHE at 100 mV s À1 to investigate the cycling stability.The long-term stability tests were recorded by taking a chronoamperometric curve current density that reached 10 mA cm À2 .All data were presented were introduced to minimize interactions between adjacent layers in all supercells. 15All the atom positions in the model were optimized by the conjugate-gradient optimization procedure.
Gibbs free-energy of the adsorption atomic hydrogen was calculated as the following formula: where DE is the adsorption energy of adsorbed species on the given unit cell.DZPE and TDS are the zero-point energy and entropy difference of hydrogen in the adsorbed state and the gas phase, respectively.The value of ZPE and TS for the adsorbed species were calculated from the vibration frequencies, as shown in the previous literature. 15he SEM image of Ni 3 S 4 -MoS 2 identies that the morphology of the sample is a hierarchical nanosphere composed of nanosheets with a size of about 1 mm and a thickness of about 30 nm (Fig. 1b).By comparison Fig. 1b with c, it is found that the nanosheets of Ni 3 S 4 -MoS 2 are bigger and thinner than that of MoS 2 (average size is about 0.6 mm and thickness is 40 nm, as shown in Fig. S1 †).Besides, the heterostructure Ni 3 S 4 -MoS 2 has a larger specic surface area (2.7 m 2 g À1 ) than that of the pure MoS 2 (0.6 m 2 g À1 ).The information illustrates that the construction of the heterojunction increased the surface area of samples.The HRTEM images provide further details of the microstructure for the heterostructure Ni 3 S 4 -MoS 2 .The lattice spacing of 0.625 nm corresponds to the (002) plane of MoS 2 , 28,29 and 0.28 nm corresponds to the (311) plane of Ni 3 S 4 , 30 indicating that the sample consisted of Ni 3 S 4 and MoS 2 .The HRTEM image suggests that the (002) plane of MoS 2 and (311) plane of Ni 3 S 4 constitute important heterointerfaces in the composite (Fig. 1d).The crystal structure of the Ni 3 S 4 -MoS 2 composite was further veried by the select area electron diffraction (SAED) (Fig. 1e).It showed that the inner ring was strong and the outer ring pattern correspond to the (002) plane of the MoS 2 crystal and the (311) respectively, and the S 2p shis to the lower binding energy (about 0.2 eV, S 2p 3/2 and S 2p 3/2 ) compared with the pristine MoS 2 (Fig. 2c), indicating the lower valence state of S in Ni 3 S 4 -MoS 2 .The result was consistent with the previous reports for other transition metals decorated with MoS 2 . 33,34The lower valence state of S in Ni 3 S 4 -MoS 2 than that in MoS 2 indicated that S in Ni 3 S 4 -MoS 2 had a higher electric charge density that was contributed to H adsorption, which helped to improve the HER properties.The Ni 2p spectrum of Ni 3 S 4 -MoS 2 , as shown in Fig. 2d, exhibits the apparent Ni 2p 3/2 and 2p 1/2 peaks at 854.8 and 872.1 eV, respectively, that are attributed to Ni 2+ , and the other peaks at 856.9 and 874.3 eV, attributed to Ni 3+ . 21Two satellite peaks at 862.1 and 879.6 eV attributed to Ni 2p 3/2 and 2p 1/2 , respectively, were observed. 35It was further conrmed that the material contained compound Ni 3 S 4 , 36 which is in agreement with the XRD and HRTEM measurements.

Results and discussion
To investigate the electron redistribution on the heterostructure of Ni 3 S 4 -MoS 2 , the electron density different diagram and the Bader charge analysis was performed by DFT calculations (Fig. 2e and Table S3 † with that on the base Ni-2 in Ni 3 S 4 -MoS 2 , was decreased, indicating that the electrons were transferred from Ni to Mo or S on the interfaces.These data conrmed the electron redistribution in the heterostructure Ni 3 S 4 -MoS 2 .

HER catalytic behavior
The HER activities of MoS 2 , Ni 3 S 4 -MoS 2 and Ni 3 S 4 on NF were measured in a 1 M KOH solution.There was a signicant enhancement of the HER activity for Ni 3 S 4 -MoS 2 , as shown in Fig. 3a.The pure MoS 2 exhibited an overpotential of 10 mA cm À2 (h 10 ) at 235 mV, which is in agreement with the reported literature. 37The h 10 of Ni 3 S 4 -MoS 2 is 116 mV, which is much lower than that of the pure MoS 2 (235 mV) and pristine Ni 3 S 4 (318 mV).9][40] The HER catalytic performance of the electrocatalysts with different Ni contents in Ni 3 S 4 -MoS 2 was investigated, as shown in Fig. S3, † indicating that Ni 3 S 4 -MoS 2 with 7.7 wt% Ni has the lowest overpotential (116 mV) at 10 mA cm À2 in the alkaline solution.
The Tafel curves of MoS 2 , Ni 3 S 4 -MoS 2 and Ni 3 S 4 on NF are shown in Fig. 3b.The Tafel slope of Ni 3 S 4 -MoS 2 (81 mV dec À1 ) was much lower than that of MoS 2 (156 mV dec À1 ), indicating that Ni 3 S 4 played a key role in promoting the kinetics of HER.The EIS diagrams exhibited similar impedance characteristics, which implied similar electrochemical processes of these samples (Fig. 3c).Ni 3 S 4 -MoS 2 showed a much lower chargetransfer-resistance (R ct ) value when compared with the other catalysts, suggesting that Ni 3 S 4 -MoS 2 had better charge-transfer property and HER kinetics.In addition, not only the electric conductivity but also the wettability of   3d and S5 †).The C dl value of Ni 3 S 4 -MoS 2 was 282 mF cm À2 , which is much higher than that of MoS 2 (80 mF cm À2 ) and Ni 3 S 4 (54 mF cm À2 ) (Fig. 3e), indicating that the additional electrochemical active sites were generated aer Ni 3 S 4 was introduced.The C dl value of Ni 3 S 4 -MoS 2 was much more than that of MoS 2 and Ni 3 S 4 , indicating there are more active sites exposed for HER.
To evaluate the long-term stability of the heterostructure Ni 3 S 4 -MoS 2 , it was subjected to 3000 continuous CV cycles in an alkaline environment from 0 to À0.3 V vs. RHE.The LSV curves had no clear changes before and aer 3000 CV cycles (Fig. 3f), indicating that Ni 3 S 4 -MoS 2 has excellent catalytic stability during the electrochemical process.Besides, Ni 3 S 4 -MoS 2 has a stable HER current at a constant current of 10 mA versus time over a 20 h period in 1 M KOH (Fig. 3f).Simultaneously, the morphology of Ni 3 S 4 -MoS 2 was well preserved (Fig. S6 †), demonstrating excellent catalytic stability during the alkaline HER process.

Mechanism of Ni 3 S 4 -MoS 2 for HER
According to literature, 41,42 DFT calculations were also carried out to gain insight into the underlying mechanism of Ni 3 S 4 -MoS 2 towards the HER activity.In an alkaline medium, the HER reaction mainly includes three steps: water adsorbed on the catalyst, water dissociation, H* formation and H 2 generation. 43,44The water dissociation step is considered as the important step for the HER catalytic property in an alkaline solution.The chemisorption free energies of OH (DE OH ) and H (DE H ) on the different sites of Ni 3 S 4 -MoS 2 were calculated, respectively.To study the optimal chemisorption free energies (DE), several feasible positions were chosen for the adsorption of OH and H (Fig. S7 †).The chemisorption free energy of H adsorbed on the (002) plane of MoS 2 (DE H ¼ À0.81 eV) was lower than that absorbed on the (311) plane of Ni 3 S 4 (DE H ¼ À0.027 eV), indicating that H was inclined to be adsorbed on the (002) plane of MoS 2 .Compared with the (002) plane of MoS 2 (DE OH ¼ À3.0 eV), the (311) plane of Ni 3 S 4 showed a predominant binding energy towards OH (DE OH ¼ À4.8 eV), which is attributed to the bonding ability between OH and Ni (Fig. 4a).Therefore, OH is intensely adsorbed on the (311) plane of Ni Moreover, the heterostructure improved the electrical transport efficiency of Ni 3 S 4 -MoS 2 .It is found that the total density of state (DOS) curve of MoS 2 shows a clear band gap at the region around 0 eV, conrming the typical semiconductor characteristic.The peak of the valence band of the heterostructure Ni 3 S 4 -MoS 2 is close to 0 eV (Fig. 4d), leading to the enhanced excitation of charge carriers to the conduction band and showing better electric conductivity, which is consistent with the EIS tests.

Conclusions
In summary, we fabricated the sphere-shaped heterostructure Ni 3 S 4 -MoS 2 by a one-pot hydrothermal method.The assynthesized catalyst with the activated interfaces generated abundant active sites and improved the electrical transport efficiency.Beneting from engineering the heterostructure, the PaperRSC Advances without IR compensation, and all the electrochemical tests were tested at room temperature.Computational methodsAllrst principles calculations were performed via DFT in the Cambridge Sequential Total Energy Package (CASTEP) module in the Materials Studio.The MoS 2 (002) plane consisting of six layers of Mo and S, and the Ni 3 S 4 (311) slab composed of six layers of atoms were constructed as our models, because the (002) plane of MoS 2 and the (311) plane of Ni 3 S 4 were dominant crystal faces from the HRTEM images.The exchange-correlation interactions were treated within the generalized gradient approximation of the Perdew-Burke-Ernzerhof (PBE) type.The plane-wave cutoff energy was 400 eV, and a k-mesh of 3 Â 3 Â 1 was adopted to sample the Brillouin zone.The convergence threshold for energy and Hellmann-Feynman forces on each atom were set to 10 À5 eV and 0.01 eV ÅÀ1 .Vacuum layers of 15 Å
Fig. 3 The HER behavior of Ni 3 S 4 -MoS 2 , Ni 3 S 4 and MoS 2 in 1 M KOH.(a) The LSV curves and overpotential (h 10 ) without IR correction.(b) The Tafel slopes.(c) Nyquist plots collected at the overpotential of 180 mV.(d) CV curves of Ni 3 S 4 -MoS 2 at the scan rates of 5, 10, 15, 20, 25 and 30 mV s À1 , respectively.(e) Differences in current density variation (DJ ¼ J a À J c ) at 0.05 V vs. RHE plotted against scan rate fitted to linear regression for estimation of C dl values of Ni 3 S 4 -MoS 2 , Ni 3 S 4 and MoS 2 .(f) The initial and 3000th polarization curves of Ni 3 S 4 -MoS 2 .The inset is the chronoamperometric curve recorded at 10 mA for a continuous 20 h.
MoS 2 was inuenced by Ni 3 S 4 .The contact angle tests of the materials in 1 mol L À1 KOH electrolyte were carried out to explain such inuence.The contact angle decreased from 21.36 for MoS 2 to 15 for Ni 3 S 4 -MoS 2 (Fig. S3 †).It is shown that Ni 3 S 4 -MoS 2 had better wettability in the KOH electrolyte than that of initial MoS 2 .Electrochemical active surface area (ECSA) is a standard parameter applied in the evaluation of electrochemical catalysts.To investigate the exposed active sites, the ECSA of Ni 3 S 4 -MoS 2 , MoS 2 and Ni 3 S 4 were calculated by the double-layer capacitance (C dl ) through plotting CV curves.The CV curves of the samples were tested in the potential range of 0.0-0.1 V at the

Fig. 4
Fig. 4 (a) DFT-calculated adsorption energies of H and OH at different sites on the surfaces of Ni 3 S 4 -MoS 2 , respectively.(b) The illustration of a mechanism for the electrocatalytic HER under alkaline conditions.(c) Free energy diagrams on the surface of MoS 2 , Ni 3 S 4 and Ni 3 S 4 -MoS 2 in alkaline solution.(d) DOSs of pristine MoS 2 and Ni 3 S 4 -MoS 2 .
3 S 4 and H on the (002) plane of MoS 2 .The appropriate oxidation of Ni in Ni 3 S 4 -MoS 2 contributes to the adsorption energy of OH.Simultaneously, the partial reduction of S in Ni 3 S 4 -MoS 2 is benecial for the adsorption of H.It demonstrated a synergistic effect of Ni 3 S 4 -MoS 2 with chemisorption of H (on the (002) plane of MoS 2 ) and OH (on the (311) plane of Ni 3 S 4 ) accelerated the rate-determining water dissociation steps of HER.The free energy diagrams on the surfaces of MoS 2 , Ni 3 S 4 and Ni 3 S 4 -MoS 2 are shown in Fig. S8.† The DG H* of Ni 3 S 4 -MoS 2 was À0.36 eV, which was much lower than that of pure MoS 2 (1.92 eV), indicating the superior capacity of Ni 3 S 4 -MoS 2 for H* adsorption (Table S4 †), which beneted from the electron redistribution between Ni 3 S 4 and MoS 2 .For pure MoS 2 , the free energy barrier of water dissociation DG H 2 O is as high as 4.2 eV, which distinctly hindered the dissociation of H 2 O to H*.Moreover, the DG H2O of Ni 3 S 4 -MoS 2 was only À0.10 eV, which was much lower than that of MoS 2 (4.2 eV) and Ni 3 S 4 (4.3 eV).It is indicated that the DG H 2 O of Ni 3 S 4 -MoS 2 efficiently decreased because of the existence of a heterostructure.Hence, the HER process on Ni 3 S 4 -MoS 2 is highly accelerated and in accordance with our experimental results.

Ni 3 S 4 -
MoS 2 demonstrated the low overpotential of 116 mV with the corresponding Tafel slope of 81 mV dec À1 and long-term stability of over 20 h.DFT calculations proved that the heterostructure Ni 3 S 4 -MoS 2 resulted in electron redistribution, which indicated the presence of a synergistic effect with MoS 2 as the hydrogen acceptor and Ni 3 S 4 as the hydroxyl acceptor, and effectively reduced the intermediate energy barrier of the water dissociation.Hence showing outstanding HER performance in alkaline solution.This work opens the door to develop low-cost and high-activity HER electrocatalyst Ni 3 S 4 -MoS 2 via heterostructure engineering. 2