Shan Ni,Hongnan Qu,Huifang Xing,Zihao Xu,Xiangyang Zhu,Menglei Yuan,Meng Rong,Li Wang,Jiemiao Yu,Yanqing Li,Liangrong Yang,3,,Huizhou Liu,3,

1 CAS Key Laboratory of Green Process and Engineering,State Key Laboratory of Biochemical Engineering,Institute of Process Engineering,Chinese Academy of Sciences,Beijing 100190,China

2 School of Chemical Engineering,University of Chinese Academy of Sciences,Beijing 100049,China

3 Qingdao Institute of Bioenergy and Bioprocess Technology,Chinese Academy of Sciences,Qingdao 266061,China

Keywords:Oxygen evolution reaction Transition-metal sulfides heterostructures Heterointerface Built-in electric field

ABSTRACT Developing highly efficient,durable,and non-noble electrocatalysts for the sluggish anodic oxygen evolution reaction(OER)is the pivotal for meeting the practical demand in water splitting.However,the current transition-metal electrocatalysts still suffer from low activity and durability on account of poor interfacial reaction kinetics.In this work,a facile solid-state synthesis strategy is developed to construct transition-metal sulfides heterostructures(denoted as MS2/NiS2,M=Mo or W)for boosting OER electrocatalysis.As a result,MoS2/NiS2 and WS2/NiS2 show lower overpotentials of 300 mV and 320 mV to achieve the current density of 10 mA·cm-2,and smaller Tafel slopes of 60 mV·dec-1 and 83 mV·dec-1 in 1 mol·L-1 KOH,respectively,in comparison with the single MoS2,WS2,NiS2,as well as even the benchmark RuO2.The experiments reveal that the designed heterostructures have strong electronic interactions and spontaneously develop a built-in electric field at the heterointerface with uneven charge distribution based on the difference of band structures,which promote interfacial charge transfer,improve absorptivity of OH-,and modulate the energy level more comparable to the OER.Thus,the designed transition-metal sulfides heterostructures exhibit a remarkably high electrocatalytic activity for OER.This study provides a simple strategy to manipulate the heterostructure interface via an energy level engineering method for OER and can be extended to fabricate other heterostructures for various energy-related applications.

1.Introduction

Electrocatalytic water splitting for hydrogen production has been deemed a promising and sustainable approach to solve society’s energy shortage and environmental problems[1,2].However,as a half-reaction of water splitting,the sluggish oxygen evolution reaction (OER) hinders the practical application of water splitting because it involves complex four-electron-transfer steps [3].Therefore,it is critical to develop high-performance OER electrocatalysts for promoting the reaction kinetics and reducing the overpotential.Although some precious metals and their oxides(RuO2and IrO2) have been considered as the state-of-the-art electrocatalysts for the OER,their large-scale applications are seriously restricted by the high cost,scarcity,and undesirable stability[4,5].Until now,tremendous efforts have been devoted to exploring efficient alternatives with great activity and durability for OER electrocatalysis,including transition metal oxides [6,7],phosphides[8,9],sulfides [10–12],selenides [13,14],and carbides [15–17].

Among these non-noble metals based OER electrocatalysts,nanostructured transition sulfides have attracted the researcher’s extensive attention due to their low cost and intrinsic electrochemical activity in alkaline media [18,19].Although the fact that these candidates were proven effectively with commendable performance,the OER performances need to be further improved in comparison with Ir/Ru-based catalysts and are still far from satisfactory to meet the commercialization in reality.Since OER is a multistep process containing the adsorption and desorption of reactants and products as well as the electron transfer that takes place on the surface of catalysts,so the electrocatalytic activities of catalysts are closely related to the surface electronic structure of materials [20–23].Many approaches have been exploited to promote the OER activity by tailoring the surface properties,such as heteroatom incorporation [24],heterostructure design [25,26],and defect engineering [27].Specially,the heterojunctions by interfacial engineering not only possess the intrinsic properties of each component but also obtain some prominently improved activities,which are mainly ascribed to the synergetic effects related to the heterointerfaces[28].Additionally,inspired by semiconductor physics,when two semiconductors with different energy levels are coupled,a built-in electric field and two opposite space charge regions will be formed as soon as their Fermi levels reach thermodynamic equilibrium state through spontaneous migration of electrons across the heterointerface [29,30].Obviously,the built-in electric field inside a heterostructure will boost the charge transport and the charged regions would have great potential to modulate the absorption process of reactants [31,32].Therefore,the rational design of semiconductor heterostructures with suitable energy band structures would be an efficient strategy for manipulating the surface charge density of catalysts for improving the performance of specific catalytic reactions.For example,Ma et al.constructed CoS2/CoS n-n heterojunction with uneven charge distribution,showing boosted electrocatalytic activity for aqueous polysulfide/iodide redox flow batteries [33].Unfortunately,to the best of our knowledge,although engineering interface of electrocatalysts by constructing heterojunctions provides a promising strategy for accelerating OER kinetics,they often suffer from laborious multistep processes and unsatisfactory yields,ultimately impeding its exploitation in industrial production.Given this,the simple and scalable fabrication of transitionmetal sulfides heterostructures electrocatalysts with suitable energy band structures remains challenging.

On the basis of the above considerations,herein,a facile solidstate synthesis method has been developed to construct transition-metal sulfides heterostructures electrocatalysts (denoted as MS2/NiS2,M=Mo or W).The morphology and structure of the resulting MS2/NiS2heterostructures were systematically employed along with their electrocatalytic activity and stability.Moreover,the mechanistic insights into the activity enhancement are provided in terms of the theory of energy band.Benefiting from a built-in electric field and the regulation of charge distribution in the heterojunction interface,the MS2/NiS2exhibit distinctly reduced OER overpotential and enhanced activity than corresponding single MS2and NiS2in alkaline KOH solution.This work suggests that highly efficient OER electrocatalysts can be designed through heterostructure construction and interface engineering.

2.Experimental

2.1.Materials

Potassium hydroxide (KOH),and thiourea (CH4N2S) were purchased from Xilong Chemical Co.,Ltd.Nickel acetate tetrahydrate(NiC4H6O4·4H2O),tungsten trioxide (WO3),molybdenum trioxide(MoO3),as well as ruthenium dioxide(RuO2)were purchased from Aladdin Industrial Corporation (Shanghai,China).Nafion solution(5%(mass)) were purchased from Sigma-Aldrich.All these materials were of analytical grade.A Millipore system was employed for preparing the water with a resistance of 18.2 MΩ·cm.

2.2.Preparation of MS2/NiS2

Preparation of MoS2/NiS2:0.41 g NiC4H6O4·4H2O,1.20 g MoO3,as well as 3.04 g CH4N2S were added into the agate mortar.After being grinded with 1 h,the acquired homogeneous powder was transferred into a ceramic crucible.Then,it was annealed in a tube furnace under Ar atmosphere at 550 oC for 2 h with a heating rate of 10 oC·min-1.When the tube furnace was cooled to environmental temperature,the targeted catalyst (MoS2/NiS2) was acquired.Similarly,the NiS2and MoS2were synthesized via the identical method as MoS2/NiS2,except without the using of MoO3or NiC4H6-O4·4H2O,respectively.

Preparation of WS2/NiS2:the synthesis method was similar with the MoS2/NiS2via employing 1.93 g WO3instead of MoO3.Moreover,the WS2was prepared via the identical method as WS2/NiS2,except for not adding NiC4H6O4·4H2O.

2.3.Characterization

Field emission scanning electron microscopy(FE-SEM,Zeiss Sigma300) and transmission electron microscopy (TEM,JEOL JEMF200) were employed for investigating the morphology and structure information of the synthesized materials.Powder X-ray diffraction (XRD) patterns of these materials were obtained via a diffractometer (X’PERT PRO MPD,scanned range of 5o-90o) with Cu Kα radiation.All the N2adsorption-desorption tests of obtained samples were performed on an Autosorb-IQ analyzer at 77 K,after being degassed at 120 oC with 12 h under the vacuum environment.The AutoChem II 2920 with a TCD detector was employed for performing the NH3temperature-programmed desorption(NH3-TPD) tests from room temperature to 500 °C with a heating rate of 15 oC·min-1.Fourier transform infrared spectroscopy (FTIR) spectrums were acquired on a spectrometer (BRUKER TENSOR 27) with attenuated total reflection (ATR) accessory.Raman spectra were collected on the Raman spectrometer(Jobin Yvon LabRAM Aramis) at the excitation laser of 633 nm.The electron paramagnetic resonance (EPR) measurements were performed on the Bruker A300.X-ray photoelectron spectroscopy (XPS) tests were carried out on a Kratos spectrometer(AXIS Ultra DLD,Al Kα radiation).And the acquired XPS spectrums were further calibrated according to the peak of C 1 s (284.8 eV).With barium sulfate as the reference,the ultraviolet–visible (UV–vis) diffuse-reflectance spectra were obtained via the Shimadzu UV2450 UV–vis spectrophotometer.The corresponding metal contents in prepared materials were investigated via the inductively coupled plasma optical emission spectrometry (ICP-OES,PerkinElmer).

2.4.Catalytic-activity evaluation of the synthesized electrocatalysts

A CHI 660E electrochemical workstation was used to conduct the electrochemical measurements in KOH solution (1.0 mol·L-1).And the saturated Ag/AgCl,platinum plate,as well as catalystmodified carbon cloth were served as reference electrode,counter electrode,and working electrode,respectively.According to the equation:ERHE=0.197 V+EAg/AgCl+0.059×pH,the acquired potential (EAg/AgCl) was converted to the corresponding reversible hydrogen electrode potential (ERHE).Meanwhile,the prepared catalyst (4 mg) was dispersed into a homogeneous solution (470 μl water,30 μl Nafion (5%(mass)),and 500 μl ethanol) through sonicating (1 h).After that,for obtaining the working electrode(0.3 mg·cm-2),a certain amount of the acquired ink (22.5 μl) was drop-coated on a carbon cloth (1 cm×3 cm).The KOH solution was purged via high purity O2gas (30 min) before the electrochemical measurement.Afterwards,the prepared electrode was swept via the CV procedure for stabilizing the current of electrode(from 1 to 1.8 V (vs.RHE),20 cycles).Linear sweep voltammetry(LSV)tests were performed via ohmic potential drop(iR)correction of 95% with the scan rate of 5 mV·s-1.The overpotential (η) was acquired through the following formula:η=ERHE–1.23.And according to the equation:η=a+b×lg|j|(a,b,and j denote intercept,Tafel slope,and current density,respectively),the Tafel slopes were also determined.CV tests were conducted from 1.2 to 1.3 V(vs.RHE) with different scan rates (from 20 to 140 mV·s-1) for determining the Cdl.And for the current density at 1.25 V (vs.RHE) with different scan rates,the half the slopes of these differences were these corresponding values of Cdl.Then,in the light of the formula:ECSA=A×Cdl/Cs(A is 0.3 cm2(the geometric area of working electrode),and Csis 0.004 mF·cm2(the specific capacitance for electrode with the smooth planar surface)),the ECSA was obtained.The equation:(jECSA=j/ECSA) was further used to calculate the ECSA-normalized current density.Electrochemical impedance spectroscopy (EIS) was performed from 100 kHz to 1 Hz.Moreover,the Mott-Schottky measurements were conducted from 0.4 to 1.2 V (vs.Ag/AgCl) with an AC frequency of 1 kHz.The chronoamperometry (i-t) tests were employed under a fixed bias of 10 mA·cm-2for evaluating the durability of synthesized materials.The corresponding LSV curves were collected after the i-t measurements.

Fig.1.Mott-Schottky plots of (a) NiS2,(b) MoS2,and (c) WS2 electrocatalysts at the frequency of 1 kHz;schematic diagrams of the band structure of NiS2,MoS2,and WS2 before (d) and after contact (e).

3.Results and Discussion

3.1.Synthesis and characterization of catalysts

In order to validate the formation of strong interaction and charge redistribution at the interface of heterostructures,UV–Vis diffusive reflectance spectroscopy and Mott-Schottky plots were measured to construct the corresponding energy level diagrams.As shown in Fig.1a-c,the Mott-Schottky plots of MoS2,WS2,and NiS2exhibit negative slopes,which demonstrate that they are ptype semiconductors with the flat band potential (Efb) of 1.11,1.07,and 0.83 V (vs.RHE,-5.61,-5.57 and -5.33 eV vs.vacuum level,respectively),respectively,derived from the x-intercept in the Mott-Schottky plots at 1 kHz frequency.Additionally,it is usually believed that the valence band potential (EVB) for the p-type semiconductor is about 0.1–0.3 V more negative than their Efband it is generally defined as 0.2 eV [33,34].Thereby,the EVBfor MoS2,WS2,and NiS2can be estimated as -5.81,-5.77,and-5.53 eV,respectively.Moreover,the band-gap energies of NiS2,MoS2,and WS2are further calculated to be about 1.78,1.84,and 2.29 eV(Fig.S1,see Supplementary Material),respectively,according to the Kubelka-Munk remission function [33].Consequently,the energy band diagrams of NiS2,MoS2,WS2,and the corresponding heterostructures can be constructed (Fig.1d).Upon forming heterostructures,a built-in electric field is spontaneously generated at the interface of heterojunctions due to the obvious difference in Fermi levels,and thus reinforce electronic interactions between NiS2and MS2(Fig.1e).Importantly,such a built-in electric field will remarkably boost interfacial carrier transport and hence strengthen the conductivity [34].In addition,according to the Lewis acid-base concept[35],the self-driven electron donation from the NiS2to MS2around the heterojunction interfaces endows the NiS2with enriched positive charges and more acid,which is beneficial to enhance the oxidative ability and promote the adsorption of OH-,thus leading to the improved OER activity.Besides,after being soaked in 1 mol·L-1KOH,the bonding form of these absorbed water molecules and hydroxide ions over the surfaces of electrocatalysts was investigated via the Fourier transform infrared spectrum with an attenuated total reflection technique (FT-IR ATR) (Fig.S2).For the MoS2/NiS2,the peak located at 1606 cm-1should be assigned to the bending vibration of O-H bonds,which is originating from the absorbed water and hydroxide ions.In comparison with those of MoS2and NiS2,the distinct red-shift of 9 cm-1and 22 cm-1for the O-H peak over MoS2/NiS2,respectively,implies that the O-H bond on MoS2/NiS2turns into longer.This result demonstrates that the adsorption of hydroxide ions can be boosted when MoS2/NiS2as electrocatalysts [36,37].Analogously,for the WS2/NiS2,the O-H bond at 1609 cm-1also shows a red-shift of 8 cm-1and 19 cm-1relative to those of WS2and NiS2,respectively.NH3-TPD tests were also employed for evaluating the absorptivity ability of the designed heterojunctions for the OH-.As shown in Fig.S3,MoS2/NiS2and WS2/NiS2display the new and stronger NH3-TPD peaks in comparison with the NiS2,implying their enhanced acidity and affinity of heterojunctions for the OH-[38,39].In the meantime,the redistribution of electron structure will significantly result in band bending at the interface and pulling down the valence band of NiS2.While the downshift of valence band can increase the gap between EOERand EVBof NiS2,which makes the NiS2with better match energy levels for the OER,and hence accelerate the electron transfer from OH-to the VB of NiS2[40,41].Moreover,the valence electrons are tightly in connection with the d states,hence the movement of a valence band indicates the shift of corresponding d-band center in comparison with the Fermi level.Since the d-band center is intervenient between antibonding and bonding,the d-band center is closely related to the absorbate-metal interplay in the light of the d-band theory[42,43].It indicates that the energy state of antibonding orbitals is reduced and can be easily occupied as the d-band center downshifts,that lessens the interplay between the surface of these electrocatalysts and adsorbed oxygenated species,result in low reaction barrier of the rate-limiting step and thus boost OER performance [44,45].In consequence,the constructed MS2/NiS2heterostructures are supposed to improve the OER activity due to the strong built-in electric field at interfaces.

Fig.2.Schematic illustration of the synthetic steps for MS2/NiS2 heterostructures.

In this study,MS2/NiS2heterostructures and unitary MS2or NiS2were synthesized through a facile solid-state process (Fig.2),which is practicable for a large-scale fabrication without complicated treatments.As demonstrated on the powder X-ray diffraction(XRD) patterns,diffraction peaks located at 27.2°,31.4°,35.2°,38.9°,45.2°,53.3°,55.8°,58.7°,and 61.1° in NiS2(Fig.S4a) can be assigned to be a pyrite crystal structure (JCPDF #11-0099) [46],whereas MoS2(Fig.S5a) can be confirmed with peaks at 14.1°,33.2°,and 58.8°(JCPDF#73-1508)[47],WS2(Fig.S5b)can be identified with peaks at 14.1°,32.9°,and 59.6° (JCPDF #08-0237) [48].Characteristic diffraction peaks for MoS2and WS2clear appear in MoS2/NiS2and WS2/NiS2,respectively,but with very weak peaks for NiS2being detectable due to relatively low loadings,indicating the successful preparation of MS2/NiS2samples without any peaks for impurities.

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed for investigating the microstructure and the morphology of these obtained materials.SEM images clearly show that all samples have analogous clumpy morphology(Fig.S4b,S6).The HRTEM images of pure NiS2(Fig.S3-d-e) exhibit lattice fringes with an interplanar spacing of 0.28 nm corresponding to (2 0 0) plane [49].In addition,these highresolution TEM (HRTEM) images show the distinguished layer structure of MoS2and WS2(Fig.3a-b,e-f).The clear lattice fringes in Fig.3b,f with an interplanar spacing of 0.62 nm are assigned to the (0 0 2) planes of MoS2and WS2[50,51].Fig.3c-d,g-h present the representative TEM images of MS2/NiS2heterostructures,indicating the typical layer structure retained.The HRTEM images from the selected area (Fig.3d,h) of MS2/NiS2show lattice fringe spacings of 0.62 nm and 0.28 nm,corresponding to the interplanar distances of (0 0 2) and (2 0 0) planes of MS2and NiS2,respectively,further confirming the formation of MS2/NiS2heterostructures.All these results are consistent with the above XRD patterns.Moreover,the intersecting lattice fringes of MS2and NiS2manifest that the intimate contact between MS2and NiS2might form at the interface,which is considered playing a crucial role in enhancing the catalytic kinetics for OER.

The compositions of the MS2/NiS2heterostructures were further studied by Raman analysis as shown in Fig.4a,e.The observed Raman bands for MoS2are found at 382 and 405 cm-1,which corresponds to the in-planeand out-of-plane A1g[50],respectively.For WS2,the characteristic Raman bands at 357 and 421 cm-1are relative toand A1g[51],respectively.More importantly,compared with those of MoS2and WS2,the Raman peaks of MoS2/NiS2and WS2/NiS2shift toward lower wavenumbers,further implying the strong electronic interactions between MS2and NiS2domains.Furthermore,the related surface area and pore structure of these prepared catalysts were also investigated through N2adsorption/desorption measurements(Fig.S7a).The specific surface areas calculated via Brunauer-Emmett-Teller (BET) of the NiS2,MoS2,WS2,MoS2/NiS2,and WS2/NiS2are 69.39,13.53,14.79,25.21,and 30.04 m2·g-1,respectively.And these corresponding pore size distribution curves acquired based on the Barrett-Joyner-Halenda imply that these synthesized materials are predominantly mesoporous(Fig.S7b).Besides,the specific weight ratios of corresponding metals in NiS2,MoS2,WS2,MoS2/NiS2,and WS2/NiS2were analyzed via ICPOES (Table S1).

Fig.3.TEM and the corresponding HRTEM images of the catalysts.(a,b) MoS2;(c,d) MoS2/NiS2;(e,f) WS2;(g,h) WS2/NiS2.

The surface chemical compositions and electronic interactions of MS2/NiS2were then investigated by X-ray photoelectron spectroscopy (XPS).The resulted survey spectra indicate the existence of Mo,Ni,and S in MoS2/NiS2,and W,Ni,and S in WS2/NiS2evidently.As shown in the Mo 3d high-resolution spectrum of MoS2(Fig.4b),these main peaks located at 229.6 and 232.7 eV should be indexed into Mo 3d5/2and Mo 3d3/2,respectively,indicating the existence of Mo4+valence state of MoS2.In the meantime,a couple of peaks at 230.2 and 233.8 eV should be assigned to the MoSxOyspecies.And the peak at 236.7 eV reveals the emergence of Mo6+,suggesting the slight oxidation of MoS2surface exposed to air[52].Similarly,in the W 4f region of WS2(Fig.4f),two peaks center at 32.6 and 34.8 eV correspond to W 4f7/2and W 4f5/2orbitals,respectively,confirming the W4+in WS2.In addition,peaks with higher energies for W6+4f7/2at 35.8 eV and W6+4f5/2at 38.1 eV are also shown due to the inevitable oxidation [53].One should note that these peaks on MS2/NiS2exhibit a negative shift to lower binding energies as compared to MS2,implying the strong coupling interaction between MS2and NiS2.Besides,in the Ni 2p spectrum of NiS2(Fig.S8a),two pairs of peaks can be observed:these peaks situated at Ni 2p3/2(856.1 eV) and Ni 2p1/2(874.4 eV)should be corresponding to the Ni2+species;while these peaks situated at Ni 2p3/2(859.6 eV)and Ni 2p1/2(878.3 eV)should be attributed to Ni3+species [54].Notably,the binding energies of Ni 2p(Fig.4c,g)present a positive shift for MS2/NiS2in comparison with NiS2,further evidencing the strong electronic interaction between MS2and NiS2.Moreover,in the S 2p region of NiS2(Fig.S8b),two peaks center at 162.1 and 163.7 eV should be attributed to S 2p3/2and S 2p1/2orbitals[55],respectively.The peaks of S 2p of MS2/NiS2show negative shifts compared with those of MS2as shown in Fig.4d,h.To sum up,these phenomena indicate that interfacial charge transfer exactly exists from NiS2to MS2induced by the strong electronic interaction,which results in the hole accumulation on NiS2and may significantly improve the electrocatalytic OER activity because the hole is often regarded as the active site for the oxidation reaction and enhances affinity for the OH-[56],as verified in the following part.These XPS results are in good agreement with the previous energy diagrams analysis.Moreover,the EPR was further employed for characterizing the sulfur vacancies in these synthesized samples.As displayed in Fig.S9,the MoS2/NiS2and WS2/NiS2electrocatalysts exhibit stronger EPR signals in comparison with the NiS2,MoS2and WS2,implying the increased sulfur vacancies after forming the heterojunction due to the energy levels mismatch between the corresponding sulfides [57].The sulfur vacancies may optimize the adsorption free energy of these oxygen-containing intermediates and thus promote the OER activity of the sulfide heterostructures [58].

Fig.4.Spectral characterizations of the catalysts.(a)Raman spectra of MoS2 and MoS2/NiS2;high-resolution XPS spectra of(b)Mo 3d in the MoS2 and MoS2/NiS2,(c)Ni 2p in the NiS2 and MoS2/NiS2,and(d)S 2p in the MoS2 and MoS2/NiS2;(e)Raman spectra of WS2 and WS2/NiS2;high-resolution XPS spectra of(f)W 4f in the WS2 and WS2/NiS2,(g)Ni 2p in the NiS2 and WS2/NiS2,and (h) S 2p in the WS2 and WS2/NiS2.

Fig.5.Catalytic activities of various catalysts toward the OER in 1.0 mol·L-1 KOH electrolyte.(a) LSV curves in the presence of MoS2,WS2,NiS2,MoS2/NiS2,WS2/NiS2 and RuO2;(b) Tafel slopes corresponding to (a);(c) comparison of overpotentials at 10 mA·cm-2 and Tafel slopes of MoS2,WS2,NiS2,MoS2/NiS2,WS2/NiS2 and RuO2;(d) LSV curves normalized by ECSA of MoS2,WS2,NiS2,MoS2/NiS2,WS2/NiS2 and RuO2;(e)LSV curve for MoS2/NiS2 before and after i-t test;(f)LSV curve for WS2/NiS2 before and after i-t test.

3.2.Electrochemical activity of catalysts

The electrocatalytic performance toward OER of these MS2/NiS2heterostructures was evaluated in an O2-saturated 1.0 mol·L-1KOH with a standard three-electrode system.Firstly,the electrocatalytic performance of the MS2/NiS2electrocatalysts with different Ni contents was evaluated for obtaining the targeted sample with optimized electrocatalytic activity.Obviously,the MoS2/NiS2heterojunction with a molar ratio of 1:5 (denoted as MoS2/NiS2)has the optimal catalytic activity for OER(Fig.S10a).Fig.5a shows the linear sweep voltammetry (LSV) curves of MoS2/NiS2,MoS2,NiS2,and RuO2with 95% iR-compensation at a scan rate of 5 mV·s-1.To obtain the current density of 10 mA·cm-2,which is a common criterion to evaluate OER activity [59],the MoS2/NiS2heterojunction just needs an overpotential of 309 mV,much lower than that of MoS2(407 mV),NiS2(355 mV),RuO2(381 mV) and other reported non-noble OER catalysts (Table S2).Moreover,the current density of the MoS2/NiS2heterojunction could increase to 50 mA·cm-2at the overpotential of 349 mV,which is 221,107,and 91 mV lower than those of the MoS2,NiS2,and RuO2,respectively,suggesting that the as-fabricated MoS2/NiS2heterostructure can prominently expedite the OER process.The Tafel plots have also been investigated to reveal catalytic kinetics of the OER process.As shown in Fig.5b,MoS2/NiS2heterojunction has a Tafel slope of 60 mV·dec-1,which is significantly lower than MoS2(159 mV·dec-1),NiS2(96 mV·dec-1),and RuO2(82 mV·dec-1),highlighting the cooperative interactions between MoS2and NiS2in MoS2/NiS2heterostructure.Particularly,the variation of the Tafel slop also indicates a change in corresponding ratedetermining step for OER [60,61].The Tafel slope of 60 mV·dec-1for MoS2/NiS2heterojunction implies that the rate-limiting step is the chemisorption of OH-[62].Thus,to compare the deviations of catalytic activity more visually,overpotentials at 10 mA·cm-2and Tafel slopes of these aforementioned samples are listed in Fig.5c,that further evidencing the excellent electrocatalytic activity of MoS2/NiS2heterostructure for OER due to the strong interfacial electronic interaction.The electrochemical impedance spectroscopy (EIS) was also utilized to evaluate the chargetransfer capabilities of these electrocatalysts (Fig.S11).Remarkably,the MoS2/NiS2heterojunction displays a much smaller charge-transfer resistance (Rct) in comparison with those of MoS2and NiS2,implying the most favorable charge transfer and reaction kinetics for electrocatalytic OER upon heterojunction.The EIS results further verify that the built-in electric field at the interface can accelerate charge transfer,which match well with the above analysis of energy band diagrams.Also,to clarify the inherent origin of the OER performance enhancement,the electrochemically active surface area(ECSA)was determined via measuring the electric double-layer capacitance (Cdl) in the non-Faradaic potential regions (Fig.S12).It demonstrates that NiS2possesses a bigger Cdlcompared with MoS2,while MoS2/NiS2heterojunction exhibits a middle Cdlvalue.The ECSA-standardized LSV curves are also shown in Fig.5c,which illustrate an OER activity trend of MoS2/NiS2＞NiS2＞MoS2,indicating that the remarkably boosted activity of MoS2/NiS2heterojunction is most probably associated with the high intrinsic OER activity originating from the indispensable contribution of the interfacial electronic coupling [63,64].Moreover,MoS2/NiS2also displays much bigger TOF (Fig.S13) and standardized activity via BET surface area(Fig.S14),implying the excellent OER performance of MoS2/NiS2.In addition to the catalytic activity,the stability is also an important parameter for evaluating superior electrocatalysts in practical application.In accordance with the chronoamperometry (i-t) measurement result (Fig.S15a),MoS2/NiS2heterostructure displays excellent stability,persistently running for 15 h at 10 mA·cm-2.Besides,these LSV curves of MoS2/NiS2acquired before and after the durability measurement show negligible variation (Fig.5e),again verifying the superior stability of MoS2/NiS2heterojunction.Furthermore,post-electrolysis structural analyses(Fig.S16a-c)indicate that the morphology and structure of MoS2/NiS2heterojunction have no distinct changes after the durability measurement.Nevertheless,the XPS spectra of MoS2/NiS2exhibits noticeable variations after the durability test(Fig.S17a-c),implying the surface oxidation of sulfides due to the extremely oxidizing OER environment in line with former reports [65,66].Practically,the cooperative impact between these sulfides and these oxidized species,that the residual sulfides core boosts the movement of electron and the surface layer of oxyhydroxides or oxides offers active sites toward OER,can expedite the kinetics for OER [67].

To illustrate the universality of synthetic strategy,the OER performance of these prepared WS2/NiS2and WS2was also studied.Analogously,the WS2/NiS2heterojunction with a molar ratio of 1:5 (denoted as WS2/NiS2) shows the optimal catalytic activity for OER(Fig.S10b).In line with the tendency observed for preceding molybdenum-based samples,the WS2/NiS2heterojunction reveals the best OER electrocatalytic performance with an overpotential of 354 and 421 mV to reach a current density of 10 and 50 mA·cm-2,respectively (Fig.5a).Tafel plots further prove the kinetic superiority for OER of WS2/NiS2with a comparatively low value of 83 mV·dec-1(Fig.5b).Compared with the individual WS2and NiS2,WS2/NiS2heterojunction displays significantly enhanced OER performances in terms of the Tafel slopes and overpotentials at 10 mA cm-2(Fig.5c).The results of EIS further suggest that the WS2/NiS2heterojunction has more rapid chargetransfer capacity (Fig.S11),implying that the conductivity has been strengthened arising from the formation of heterostructure.Meanwhile,these ECSA and ECSA-standardized LSV results also demonstrate that WS2/NiS2heterojunction possesses high intrinsic activity (Fig.S12,5d),which can be attributed to the synergistic effect of WS2and NiS2.WS2/NiS2also shows much bigger TOF(Fig.S13) and standardized activity via BET surface area(Fig.S14),suggesting the superior OER performance of WS2/NiS2.Moreover,the synthesized WS2/NiS2heterojunction exhibits superior durability for OER activity as well as robustness for structure(Fig.S15b,5f).Analogous OER-triggered oxidation of a surface of the electrocatalyst can also be detected in WS2/NiS2(Fig.S16d-f,S17d-f).

4.Conclusions

In summary,an interfacial engineering strategy has been presented to develop MS2/NiS2hybrids as OER electrocatalysts via a simple solid-state method.The intimate coupling between MS2and NiS2enables charge redistribution at the heterointerface due to the different band structures,which can significantly boost the electrocatalytic activity of the catalysts by promoting charge transfer,improving absorptivity of OH-,and modulating the energy level more comparable to the OER.As expected,the MS2/NiS2hybrids show excellent electrocatalytic performance for OER with low overpotentials,small Tafel slopes,and remarkable stability in alkaline medium.The present work will make a significant impact on the designment of strongly coupled hybrid materials as promising electrocatalysts for OER and other advanced energy technologies.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21922814,22138012,21961160745,21921005,22178349,22078333,22108281 and 31961133019),Excellent Member in Youth Innovation Promotion Association,Chinese Academy of Sciences (Y202014),and Shandong Energy Institute(Grant Number SEI 1202133).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.09.026.