Nanoarchitectonics of La‐Doped Ni3S2/MoS2 Hetetostructural Electrocatalysts for Water Electrolysis

MoS2 with 2D structure shows efficient hydrogen evolution reaction (HER) performance because undercoordinated Mo–S edges have ideal hydrogen adsorption free energy. MoS2 usually does not satisfy the bifunctional catalysts because of the poor intrinsic oxygen evolution reaction (OER) catalytic activity. Herein, it is proposed to construct heterostructure with OER active components to induce efficient bifunctional catalytic activity along with heteroatom doping to modify the electronic structure to optimize the adsorption and desorption capabilities of reaction intermediates. La‐doped Ni3S2/MoS2 grown on nickel foam (La‐NMS@NF) is synthesized as bifunctional catalyst taking advantage of the excellent OER performance of Ni3S2. La‐NMS@NF evolves into nanoflower‐like structures with the addition of La dopant, which provides abundant pore channels to facilitate mass transfer and exposure of active sites. Density functional calculations reveal that the La‐doped Ni3S2/MoS2 heterointerface can optimize the water adsorption and H* adsorption/desorption, improving the HER performance. The La‐NMS@NF exhibits an overpotential of 154 and 300 mV for HER and OER at 100 mA cm−2 in 1.0 m KOH. Herein, a heteroatom‐driven heterostructure activation strategy for electron rearrangement and structural evolution in electrocatalysts to decrease energy consumption in overall water splitting is demonstrated.


Introduction
[22] Pt-based and Ru/Irbased catalysts have excellent catalytic activity for HER and OER, respectively.[25][26] Therefore, it is necessary to develop an inexpensive catalyst to facilitate the practical application of water splitting.
[29] The edge of MoS 2 exhibits absorption of hydrogen with Gibbs free energy (ΔG H* ) similar to that of Pt during HER due to the excellent H* adsorption ability of the unsaturated S site with unfilled p-electron orbitals, but German et al. demonstrated that higher binding energies are required for all sites of MoS 2 to adsorb oxygen-containing intermediates. [30,31]However, the inert basal plane, low electrical MoS 2 with 2D structure shows efficient hydrogen evolution reaction (HER) performance because undercoordinated Mo-S edges have ideal hydrogen adsorption free energy.MoS 2 usually does not satisfy the bifunctional catalysts because of the poor intrinsic oxygen evolution reaction (OER) catalytic activity.Herein, it is proposed to construct heterostructure with OER active components to induce efficient bifunctional catalytic activity along with heteroatom doping to modify the electronic structure to optimize the adsorption and desorption capabilities of reaction intermediates.La-doped Ni 3 S 2 /MoS 2 grown on nickel foam (La-NMS@NF) is synthesized as bifunctional catalyst taking advantage of the excellent OER performance of Ni 3 S 2 .La-NMS@NF evolves into nanoflowerlike structures with the addition of La dopant, which provides abundant pore channels to facilitate mass transfer and exposure of active sites.Density functional calculations reveal that the La-doped Ni 3 S 2 /MoS 2 heterointerface can optimize the water adsorption and H* adsorption/desorption, improving the HER performance.The La-NMS@NF exhibits an overpotential of 154 and 300 mV for HER and OER at 100 mA cm À2 in 1.0 M KOH.Herein, a heteroatomdriven heterostructure activation strategy for electron rearrangement and structural evolution in electrocatalysts to decrease energy consumption in overall water splitting is demonstrated.[34] Many approaches have been developed to address the existing issues, such as nanostructure design, doping heteroatom(s), and construction of heterostructures.[37][38][39] MoS 2 /Ni 3 S 2 heterostructure showed more excellent bifunctional catalytic performance compared to Ni 3 S 2 or MoS 2 . [40]Compared with MoS 2 , Ni 3 S 2 has excellent OER activity because Ni 3 S 2 possesses an electron configuration with spin-unpaired e g electrons, which exhibits strong interaction with *O through σ bonding. [41]Density functional theory (DFT) calculations illustrate that charge recombination at the Ni 3 S 2 /MoS 2 interface promotes the adsorption of hydrogen-containing intermediates at Mo-S edges and the adsorption of oxygen-containing intermediates at Ni sites. [42][45][46] Wang et al. demonstrated that Co doping leads to more filling of p orbitals of S at the edge of MoS 2 , which facilitates a stronger binding to H* and thus may lead to a lower energy barrier for H* adsorption. [47]Doping heteroatom(s) can also change the morphology of catalysts to regulate active site density and mass transfer.Tapas et al. found that La doping simultaneously changes the crystal phase and microstructure of MoS 2 /MoO 2 heterostructure, resulting in more exposed active sites, which enables an promotion in electrocatalytic water splitting activity comparable to that of noble metal catalysts. [48] similar phenomenon that heteroatom doping simultaneously changes the electronic structure and microscopic morphology is expected in Ni 3 S 2 /MoS 2 heterostructures.However, few studies have been conducted on La-doped MoS 2 -based heterostructures to tune simultaneously the electronic structure and morphology of bifunctional catalysts.As the first element of the lanthanide series, the large atomic radius of La can lead to the formation of lattice defect in the catalyst due to atomic radius difference, generating unsaturated bonds to tune the adsorption/desorption capacity of reaction intermediates.In addition, La with a unique orbital electronic structure (4f 0 5d 1 6s 2 ) has a stable þ3 valence state. [49]When a transition metal (Ni or Mo) is replaced by La, the d-electron transition of the transition metal will occur to achieve stable modulation of its electronic structure. [50]Therefore, La doping in MoS 2 -based heterostructures is desirable to achieve highly efficient bifunctional electrocatalysts to boost the HER and OER simultaneously.
In this work, inspired by the Ni 3 S 2 /MoS 2 heterostructures to introduce bifunctional catalytic sites and La doping to regulate heterostructures, La-doped Ni 3 S 2 /MoS 2 heterostructure nanoarrays anchored on Ni foam (NF) binder-free electrocatalysts (La-NMS@NF) are achieved by a hydrothermal process with adding La cations.In situ growth of Ni 3 S 2 /MoS 2 on NF with high conductivity and large pore channels can facilitate the release of gas products. [51]La-NMS@NF exhibits outstanding HER and OER performance, showing overpotentials of 154 and 300 mV to reach 100 mA cm À2 in alkaline solution.The excellent activities are attributed to the following aspects.(i) The La element is introduced to the hetero-interfaces of Ni 3 S 2 and MoS 2 , which can realize the rearrangement of outer electrons of Ni, Mo, and S to optimize intermediate adsorption and desorption.(ii) A flowerlike structure with a large surface area is achieved after doping with the appropriate amount of La, which is favorable for efficient contact between the active site and reaction intermediates and mass transport.This facile approach demonstrates that the synthesis strategy of La doped Ni 3 S 2 /MoS 2 heterostructure may facilitate the development of excellent electrocatalysts for energy conversion.

Microstructure and Composition
The La-NMS@NF catalyst is prepared by one-step hydrothermal method (Figure 1a).The decomposition of thioacetamide generates H 2 S to etch Ni to generate Ni ions.MoS 2 is easy to agglomerate to form aggregated nanosheets. [52]Ni 3 S 2 easily forms a nanosheet structure on the surface of nickel foam (Figure S1, Supporting Information).Sodium acetate can be used as a surfactant to form a uniformly mixed phase of Ni 3 S 2 and MoS 2 on the surface.The catalysts are obtained after a hydrothermal process, which can be directly used as anode/cathode electrodes for both HER and OER, respectively.The X-ray diffraction (XRD) pattern (Figure 1b) shows the crystal phase information of NMS@NF and La-NMS@NF with different La doping amounts.The five samples have similar XRD characteristic peaks and are well-matched with the standard cards of Ni 3 S 2 and Ni.The three strong peaks at 44.3°, 51.7°, and 76.1°correspond to the (1 1 1), (2 0 0), and (2 0 0) crystal planes of Ni (JCPDS 65-0380).The peaks at 21.8°, 31.2°,37.8°, 47.9°, 50.1°, and 55.2°correspond to (1 0 1), (1 1 0), (0 0 3), (1 1 3), (2 1 1), and (1 2 2) planes of Ni 3 S 2 (JCPDS 71-1687).Due to the less content of MoS 2 , the XRD peaks are not obvious. [53,54]No characteristic peaks of other phases are found in the XRD patterns of La-NMS@NF samples, indicating that La doping does not induce the generation of new phases.
As shown in Figure 1c, Ni 3 S 2 /MoS 2 grows on nickel foam with a network structure formed by assembling messy nanosheets.The morphology also varies with the doping amount of La.As shown in Figure S2a, Supporting Information, La1-NMS@NF is nano-spheres stacked by nanosheets when the amount of La is 1%.With the addition of La, the microstructure of La-NMS@NF is small nanosheets stacked into nanospheres.When the amount of La increases to 3%, the size of the nanospheres decreases (Figure 1d), forming a nanoflowerlike structure.This nanoflower-like structure is beneficial to expose more specific surface area.When the doping amount of La is increased to 5%, the morphology of La5-NMS@NF is a nano-spherical structure aggregated into an interconnected honeycomb structure (Figure S2f, Supporting Information).
The transmission electron microscopy (TEM) image of La-NMS@NF shown in Figure 2a

Analysis of the Coordination Environment
The X-ray photoelectron spectroscopy (XPS) spectra (Figure S4a, Supporting Information) indicates the presence of Ni, Mo, S, C, O, and La elements in La-NMS@NF and the presence of Ni, Mo, S, O, and C elements in NMS@NF.The element C (Figure S4b, Supporting Information) is derived from contaminated carbon.In Figure 3a, Mo 6þ peaks in NMS@NF and La-NMS@NF can be observed at 232.3 and 235.4 eV due to incomplete vulcanization or oxidation of air. [7]The binding energy at 228.8/231.6 eV of NMS@NF belong to the Mo 4þ 3d 5/2 /3d 3/2 , respectively. [40]However, the Mo 4þ of La-NMS@NF sample shift to the lower binding energy (ΔE = 0.3 eV) compared with the NMS@NF, indicating that the density of the outer electron cloud density of Mo increases.In Figure 3b, two pairs peaks in NMS@NF at 852.6/865.9eV, and 856.1/873.7 eV are ascribed to 2p 3/2 /2p 1/2 of Ni 0þ and Ni 2þ , respectively. [55]The peaks at 857.2 and 875.7 eV belong to Ni 3þ , which may be related to surface oxidation in air. [56]n addition, the binding energy at 861.4/879.9eV belongs to satellite peaks. [57]Compared with NMS@NF, the corresponding peaks of Ni 2þ shift by 0.3 eV to the high binding energy, indicating that the addition of La increases the outermost electron density and the relative loss of electrons of Ni.The other corresponding peaks of La-NMS@NF are the same as those of NMS@NF.In Figure 3c, the binding energy of NMS@NF at 161.8/163.7 eV corresponds to the 2p 3/2 /2p 1/2 of Mo-S, respectively.The peaks at 162.8 and 164.7 eV correspond to the characteristics of 2p 3/2 and 2p 1/2 of Ni-S, respectively. [58]urthermore, the binding energy at 168.7 eV may be related to SO 4 2À . [59]The 2p of Mo-S and Ni-S peak of La-NMS@NF shift 0.14 eV to high energy compared with that of the NMS@NF, resulting in a decrease in the outer electron density of S. Compared with the S of pure-phase MoS 2 [60] or Ni 3 S 2 , [61] the  3d, confirming the presence of La element in La-NMS@NF.The binding energy at 835.3 eV belongs to the satellite peak, which is related to the transition of La 3þ valence electrons to empty 4f orbitals [62] The charge difference between La and Ni, Mo drives the electron flow from S, Ni to Mo.The intensity of the Ni 2p peak is large enough to cover the La 3d 3/2 .In Figure S4c, Supporting Information, three binding energies at 529.8, 530.9, and 532.9 eV of O 1s belong to the metal-O peak, the adsorbed-OH peak, and the adsorbed HO peak, respectively. [7]After La doping, the integrated area of the H 2 O adsorption peak of O 1s increases.It shows that the La-NMS@NF catalyst has a better ability to adsorb water after introducing La dopants. [63]

Electrocatalytic Performance Toward HER
The IR-corrected liner sweep voltammetry (LSV) tests are performed in Figure 4a.NMS@NF without La doping exhibits high overpotential (230 mV) required for 100 mA cm À2 .La-NMS@NF (corresponding with La3-NMS@NF in Figure S7, Supporting Information) shows a low overpotential of 154 mV at 100 mA cm À2 .La-NMS@NF is also superior to other La-doped samples, La-NS@NF (374 mV) and La-NMO@NF (400 mV).The low overpotential of La-NMS@NF is due to the regulation of the electronic structure and microscopic morphology of the heterostructure by La doping.The HER activity of commercial Pt/C at a current density of 100 mA cm À2 is better than that of La-NMS@NF, showing a lower overpotential (114 mV).However, the required overpotential of Pt/C is equal to that of La-NMS@NF at about 200 mA cm À2 .La-NMS@NF has superior electrocatalytic performance than Pt/C at a large current density (>500 mA cm À2 ).In Figure 4b, the overpotential of La-NMS@NF (225 mV) is lower than that of Pt/C (392 mV) at 500 mA cm À2 .The normalized HER activities (Figure S14a, Supporting Information) show that the HER intrinsic activity of La-NMS@NF is superior to that of NMS@NF without La doping.In Figure 4c, La-NMS@NF (71 mV dec À1 ) exhibits a lower Tafel slope than La-NMO@NF (178 mV dec À1 ), La-NS@NF (196 mV dec À1 ), NMS@NF (80 mV dec À1 ), and Pt/C@NF (83 mV dec À1 ).According to the value of the Tafel slope, the prepared catalysts follow the Volmer-Heyrovsky mechanism. [64]The rate-determining step of NMS@NF and La-NMS@NF is the Heyrovsky process, indicating that the electrochemical desorption of its intermediates (H*) limits the reaction.However, the rate-determining step of La-NS@NF and La-NMO@NF is the Volmer process, indicating that they have poor adsorption capacity of intermediates. [65,66]The electrochemical impedance spectroscopy (EIS) curves and the corresponding equivalent circuit diagram of the prepared catalysts are shown in Figure 4d.Compared with the as-control catalysts, La-NMS@NF shows a smaller semicircle diameter of the curve, indicating that La-NMS@NF has the optimum charge transfer resistance (R ct ) for boosting HER activities.Compared with NMS@NF, R ct of La-NMS@NF decreases, which illustrates that La doping tunes the electronic structure of the heterostructure to achieve a fast reaction rate.The nanoflower-like morphology is conducive to the transport of substances and will also promote the improvement of reaction kinetics. [9,67]Double-layer capacitance (C dl ), which is proportional to ECSA, further evaluates the active area.In Figure 4e, the C dl of La-NMS@NF (103 mF cm À2 ) is superior to those of La-NMO@NF (6.7 mF cm À2 ), La-NS@NF (1.3 mF cm À2 ), and NMS@NF (65 mF cm À2 ), showing that La-NMS@NF exposes more effective catalytic active sites.In addition, La-NMS@NF is stable at 100 mA cm À2 for 50 h without a significant current decay (Figure 4f ).
The HER acticity of the samples with a range of La doping levels is systematically characterized in Figure S7, Supporting Information.As shown in Figure S7a, Supporting Information, La1-NMS@NF, La3-NMS@NF, La5-LNM@NF, and La7-NMS@NF catalysts have overpotentials of 159, 154, 194, and 180 mV at 100 mA cm À2 , outperforming the NMS@NF catalyst without La doping.Tafel slopes are calculated from LSV to characterize the reaction kinetics of Ni 3 S 2 /MoS 2 catalysts with different doping amounts.The Tafel slopes of La1-NMS@NF, La3-NMS@NF, La5-NMS@NF, and La7-NMS@ NF catalysts shown in Figure S7b, Supporting Information, are equal to 74, 71, 73, and 75 mV dec À1 , respectively.La3-NMS@NF shows outstanding HER activity and the fastest reaction kinetics.It confirms that the appropriate amount of La ions doped in NMS@NF causes the formation of nano-flowers with a 3D open structure and sufficient active sites, thus leading the enhanced HER activity.A nanoflower-like structure with open channels is conducive to transporting mass.Doping La atoms into Ni 3 S 2 /MoS 2 can introduce the formation of lattice distortion due to the different atom radii, which will induce more lattice defects.Lattice defects may lead to the generation of unsaturated S, which facilitates the acquisition of H 2 O by the catalyst. [68,69]La doping alters the electronic structure of Ni 3 S 2 /MoS 2 heterointerface, resulting in the balanced adsorption/desorption capacities of reaction intermediates for water splitting.In alkaline conditions, HER includes two processes: water adsorption/dissociation and H* absorption/desorption.In Figure 5a, the calculated ΔG H 2 O (0.37 eV) at the interface of La-NMS@NF is lower than that of Ni 3 S 2 surface (0.41 eV) and MoS 2 surface (0.4 eV), which indicates that H 2 O is more easily adsorbed to the S sites of the interface.Compared with NMS@NF without La doping, the ΔG H 2 O at the interface decreases after La doping, which is due to La doping induces a decrease in the electron cloud density of S sites, generating more empty orbitals for adsorbing H 2 O. [70] The adsorption/ desorption of the reaction intermediate H* on the active site should be properly balanced to facilitate the HER reaction.Due to the stronger electronegativity of S, H* is more easily adsorbed to S sites during the reaction. [42,71]As shown in Figure 5b, the ΔG H* at the interface of La-NMS@NF (0.01 eV) is lower than Ni 3 S 2 surface (0.52 eV), MoS 2 surface (1.08 eV), and the interface of NMS@NF (1.39 eV), indicating that the S site at the interface of La-NMS@NF has proper H* adsorption capacity.La doping can effectively reduce the ΔG H* of the interface, because La doping induces the electron cloud shift of the S site, which will lead to the desaturation of the valence orbital of the S atom, making it easier to adsorb the H* intermediate. [71]ccording to the earlier results, the HER mechanism is concluded in Figure 5c.In this model, the S sites at the interface provide more adsorption orbitals for H 2 O molecules.H 2 O molecules adsorbed on S are dissociated with the Vlomer step The dissociated H* and OH À are attracted by S and metal sites, respectively, due to the electrostatic attraction between metal sites and OH À .Afterward, the S site at the La-regulated interface has the advantage of adsorbing hydrogen, which accelerates the H* adsorption and H 2 desorption processes.

Electrocatalytic Performance Toward OER
As shown in Figure 6a, La-NMS@NF (corresponding with La3-NMS@NF in Figure S9, Supporting Information) displays a overpotential of 300 mV at 100 mA cm À2 , which is lower than that of the La-NMO@NF (390 mV), La-NS@NF (490 mV), NMS@NF (350 mV), and IrO 2 /C@NF (370 mV).The overpotential of La-NMS@NF is still low at about 200 mA cm À2 .In Figure 6b, the La-NMS@NF catalyst also exhibits a 350 mV overpotential at 200 mA cm À2 .The performance of La-NMS@NF is significantly improved by doping La.Its excellent performance can still be maintained at a high current density.The Tafel slope of La-NMS@NF is relatively low, which corresponds to its excellent OER activity.In Figure 6c, the Tafel slope of La-NMS@NF still has an advantage over other catalysts.As shown in Figure 6d, compared with NMS@NF, La-NS@NF, and La-NMO@NF, the Nyquist plot of La-NMS@NF possesses a smaller semicircle with a smaller charge transfer resistance (R ct ), indicating a faster charge transport and reaction kinetics.The larger active surface area is also a key factor for superior performance.In Figure 6e, the C dl of La-NMS@NF is larger than other catalysts.This indicates that the exposed effective electrochemical surface area (ECSA) is large, which is beneficial to the combination of reaction intermediates and the improvement of OER activity.The stability test is carried out at constant voltage (Figure 6f ).After 50 h of operation, the catalyst maintained %90% of the activity with a slight loss in current density, indicating its excellent stability.Shown in Figure S11, Supporting Information, is the scanning electron microscopy (SEM) after OER test, showing that the morphology after OER has not changed significantly.The crystal phase of the catalyst after the OER reaction was also well maintained (Figure S12, Supporting Information), indicating that the Ni 3 S 2 structure of the catalyst was stale in OER.The fluctuation of current may be caused by the accumulation of O 2 bubbles on the surface of the catalyst. [27]lectrochemical tests are carried out on a series of La1-NMS@NF, La-NMS@NF, La5-NMS@NF, and La7- NMS@NF with different amounts of doping in the same electrolyte.In Figure S9, Supporting Information, La-NMS@NF exhibits a relatively excellent activity (300 mV) at 100 mA cm À2 , which is superior to La1-NMS@NF (302 mV), La5-NMS@NF (303 mV), and La7-NMS@NF (310 mV).In particular, La-NMS@NF catalyst has a smaller Tafel slope and a larger ECSA.This shows that the doping amount of La also has a significant effect on OER performance.
According to previous studies, Ni sites are more likely to adsorb OH À during the OER process. [42]Ni 3 S 2 has a more significant effect on OER in La-NMS@NF and is generally considered the main active site of OER. [40,72,73]The OER process is shown in Scheme 1.When Ni 3 S 2 is used as the catalyst for OER, a high oxidation peak appears on the LSV curve during the reaction.76] As shown in the XPS pattern (Figure S13b, Supporting Information), the Ni 0þ peak of La-NMS@NF after OER disappeared.While the integral area of the high-valent Ni 3þ peak increased, which may be related to the surface chemical remodeling to form NiOOH. [77] As shown in Figure S13d, Supporting Information, all peaks of Mo 3d after OER are well maintained.The effect of La doping on the intrinsic activity of the catalyst can be judged by the normalized OER polarization curve.As shown in Figure S14b, Supporting Information, the overpotential of La-NMS@NF is slightly smaller than that of NMS@NF, which indicates La doping can improve the OER intrinsic activity of NMS@NF slightly.Therefore, the improvement of OER activity may come from two aspects.(i) The nanoflower-like morphology is more conducive to the exposure of active sites, thus increasing the density of OER active sites.(ii) La may also serve as a potential OER active site.La doping forms a La-S coordination structure, and the d electrons in the La valence shell allow for the polarization of the 6 s valence orbital, which may promote the chemical adsorption of oxygen-containing intermediates. [78]

Overall Water Splitting Performance
The performance of overall water splitting is carried out in alkaline electrolyzer with La-NMS@NF as illustrated in Figure 7a. 100 mA cm À2 can be achieved at a cell voltage of 1.72 V.In addition, the stability test was carried out at 100 mA cm À2 in 1.0 M Scheme 1.The OER mechanism diagrams of the La-NMS@NF catalyst in 1.0 M KOH.
KOH to evaluate the electrochemical durability of La-NMS@NF as the bifunctional catalyst.The performance of the catalyst is still well maintained after 40 h of operation as shown in Figure 7b.The current fluctuations seen in the stability tests may be related to the gas accumulation generated on the electrode surface during the catalytic process.In addition, the performance of the bifunctional catalyst is comparable to or even better than those of the previous electrocatalysts (Table S3, Supporting Information).

Conclusion
We reported an efficient La-NMS@NF bifunctional catalyst with excellent HER and OER activities by constructing a La-doped Ni 3 S 2 /MoS 2 interface that can achieve a voltage of 1.72 V at 100 mA cm À2 for overall water splitting.La-doped Ni 3 S 2 /MoS 2 with a nanoflower-like array structure grows on Ni foam, which is beneficial for the mass transfer and active sites exposure.The construction of the heterostructure is conducive to the formation of fast electron transfer channels for electrons at the Ni-S-Mo interface, which improves the conductivity of the catalyst as proved by EIS.The La dopant induces a decrease in the electron cloud density of Ni and S to optimize the adsorption and desorption, which is driven by the charge difference between the dopant La and Ni, Mo, and S at the interface.The increase of OER active sites is conducive to the improvement of OER activity.DFT proves that the enhanced HER activity is attributed to the favorable ΔG H* of S sites at Ni 3 S 2 /MoS 2 interface by La doping, which originates from the reduction of the outer electron cloud density of S sites for more empty orbitals to facilitate the H* adsorption.These findings confirm that the strategy of doping TMSs heterostructure with La elements can improve the catalytic activity of electrolyzed water and meet the requirements of overall water splitting.
Characterization: The phases and crystal structures of the synthesized catalysts were characterized by XRD (Rigaku Ultima IV) with Cu K α radiation.Scanning electron microscopy (SEM, ZEISS Gemini SEM 300) and TEM (JEOL JEM-F200) were used to characterize the microstructure.The chemical states of the elements in the catalyst were obtained using XPS (Thermo ESCALAB 250).
Electrochemical Measurements: The electrochemical performances were performed on Metrohm Autolab and CHI660E by a typical three-electrode cell in 1.0 M KOH solutions.All potentials of electrochemical tests were expressed as the potential of reversible hydrogen electrode in terms of 059 pH, where the pH was 13.8.LSV measurements were tested for both HER and OER.The Tafel slope can be calculated by η = a þ b log ( j), in which η was overpotential for HER or OER, j was current, and b was Tafel slope.EIS was performed in the frequency range from 0.1 to 100 000 Hz for HER and OER.The ECSA of electrocatalysts was tested by cyclic voltammograms.Overall water splitting tests were carried out in the two-electrode system in 1.0 M KOH.The electrochemical stability measurement of catalysts was characterized by chronoamperometry.
displays a sphere structure formed by stacking nanosheets of Ni 3 S 2 /MoS 2 .By observing the high-resolution TEM images in Figure 2b, the MoS 2 /Ni 3 S 2 hetero-interfaces exists in the hierarchical structure, in which MoS 2 is adjacent to Ni 3 S 2 .Figure 2c displays that the obvious lattice distance of 0.62 nm well matches the (0 0 2) lattice spacing of MoS 2 .The lattice fringes of 0.23 nm belong to (0 2 1) crystal planes of Ni 3 S 2 .The FFT images in Figure 2b,c also confirm the existence of Ni 3 S 2 and MoS 2 phases.The absence of a few atoms exists in the lattice of the TEM images (Figure S17, Supporting Information), indicating the presence of lattice defects.The lattice defects may be caused by La doping or the formation of Ni 3 S 2 /MoS 2 hetero-interface.The energy-dispersive X-ray spectrum (EDX) of the La-NMS@NF catalyst (Figure 2d) reveals the uniform distribution of elements including Ni, Mo, S, O, and La in the whole catalyst.The surface oxidation of the La-NMS@NF leads to the appearance of O element signal.

Figure 2 .
Figure 2. a-c) HR-TEM images of La-NMS@NF catalyst.d) HAADF-STEM picture of La-NMS@NF catalyst and EDX mapping images of Mo, Ni, S, O, and La, respectively.

The
ΔG H* and H 2 O adsorption energy (ΔG H 2 O ) at the Ni 3 S 2 surface, MoS 2 surface, and Ni 3 S 2 /MoS 2 interface is calculated in this model.

Figure 4 .
Figure 4. a) iR-Corrected polarization curves of catalysts.b) Overpotential (η 100 and η 500 ) of catalysts at 100 and 500 mA cm À2 , respectively.c) Tafel slopes of different catalysts.d) Electrochemical impedance spectra of catalysts.e) The C dl curves of different catalysts.f ) Electrochemical stability measurement of La-NMS@NF at 100 mA cm À2 (for 50 h) in 1.0 M KOH.

Figure 5 .
Figure 5. a) Water adsorption free energy ladder diagram.b) Hydrogen adsorption free energy ladder diagram.c) The HER mechanism diagrams of the La-NMS@NF catalyst in 1.0 M KOH.

Figure 6 .
Figure 6.a) iR-corrected polarization curves of different catalysts.b) Overpotential (η 100 and η 200 ) of different catalysts at 100 and 200 mA cm À2 , respectively.c) Tafel slopes of different catalysts.d) Electrochemical impedance spectra of different catalysts.e) The C dl curves different catalysts.f ) Electrochemical stability measurement of La-NMS@NF at 100 mA cm À2 (for 50 h) in 1.0 M KOH.

Figure 7 .
Figure 7. a) Polarization curve of the La-NMS@NF||La-NMS@NF electrode for overall water splitting in 1 M KOH at 5 mV s À1 .b) Electrochemical stability measurement of La-NMS@NF|| La-NMS@NF for overall water splitting at 1.72 V.