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Few-Layered Mo(1-x)WxS2 Hollow Nanospheres on Ni3S2 Nanorods Heterostructure as Robust Electrocatalysts for Overall Water Splitting Meiyong Zheng, Jing Du, Baopu Hou, and Cai-Ling Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07465 • Publication Date (Web): 21 Jul 2017 Downloaded from http://pubs.acs.org on July 22, 2017

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ACS Applied Materials & Interfaces

Few-Layered Mo(1-x)WxS2 Hollow Nanospheres on Ni3S2 Nanorods Heterostructure as Robust Electrocatalysts for Overall Water Splitting

Meiyong Zheng†, Jing Du†, Baopu Hou‡, and Cai-Ling Xu*†

†

State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous

Metal Chemistry and Resources Utilization of Gansu Province, Laboratory of Special Function Materials and Structure Design of the Ministry of Education, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, P. R. China ‡

Lanzhou No.33 High School, Lanzhou 730000, P. R. China Email: [email protected], [email protected]

ACS Paragon Plus Environment


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Abstract Owing to the unique optical, electronic and catalytic properties, MoS2 have received increasing interests in electrochemical water splitting. Herein, few-layered Mo(1-x)WxS2 hollow nanospheres modified Ni3S2 heterostructures are prepared through a facile hydrothermal method to further enhance the electro-catalytic performance of MoS2. The doping of W element optimizes the electronic structure of MoS2@Ni3S2, thus improves the conductivity and charge transfer ability of MoS2@Ni3S2. In addition, benefiting from the few-layered hollow structure of Mo(1-x)WxS2, the strong electronic interactions between Mo(1-x)WxS2 and Ni3S2, and the hierarchical structure of 1D nanorods and 3D Ni foam, massive active sites and fast ion and charge transportion are obtained. As a result, the optimized Mo(1-x)WxS2@Ni3S2 heterostructure (Mo-W-S-2@Ni3S2) achieves an extremely low overpotential of 98 mV for HER and 285 mV for OER at 10 mA cm-2 in alkaline electrolyte. Particularly, using Mo-W-S-2@Ni3S2 heterostructure as a bifunctional electrocatalyst, a cell voltage of 1.62 V is required to deliver a 10 mA cm-2 water splitting current density. In addition, the electrode can maintain at 10 mA cm-2 for at least 50 h, indicating the excellent stability of Mo-W-S-2@Ni3S2 heterostructure. Therefore, this development demonstrates an effective and feasible strategy to prepare high efficient bifunctional electrocatalysts for overall water splitting. Keywords: Few-layered, MoS2@Ni3S2, W-Doping, bifunctional electrocatalysts, water splitting


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1. Introduction Hydrogen has been extensively demonstrated as a secure, sustainable and clean energy resource to satisfy the ever-increasing global energy demands.1-2 Electrochemical water splitting is a promising strategy to obtain sustainable hydrogen energy, benefiting from the abundant water resources and the effective resolution of storage and conversion of energy.3 However, the electrolytic efficiency of water splitting is severely restricted by the high cell voltage (1.8-2.0 V instead of 1.23 V) because of the sluggish dynamic for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).4 Hence, electrocatalysts with efficiently catalytic capability are crucial to simultaneously accelerate the rate of HER and OER. Currently, Pt-based catalysts and Ru/Ir-based compounds have been regarded as the state-of-the-art electrocatalysts toward HER and OER,5 respectively, due to the near-zero free energy of adsorbed atomic hydrogen for Pt-based catalysts and the high-efficient and fast response for Ru/Ir-based compounds.6-7 Nevertheless, the large-scale practical application of these catalysts is significantly hampered in virtue of their scarcity and high cost. As a result, tremendous efforts have been devoted to searching for cost-effective, efficient and stable alternative catalytic materials (transition metal oxides,8 hydroxides,9 sulfides,10-11 phosphides,12 carbides13 and nitrides,14 etc.) for water splitting and great progress has been made during the past years. MoS2, as a widely used industrial catalyst, has been considered as a robust electrocatayst for HER because of the much lower cost than Pt and the top location of the volcano curve certified by theoretical calculation of the catalyst-hydrogen binding



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energy based on density functional theory (DFT).15 So far, most researches focused on the electrocatalytic capability of MoS2 for HER in acidic solution.16-18 However, water splitting performed in alkaline solution is a desired candidate for commercialization toward the production of mass hydrogen. Therefore, it is highly attractive to study the HER and OER performance of MoS2 in basic media. For example, Chen et al. synthesized ultrathin MoS2 nanosheets decorated carbon nanopapers, which showed an overpotential (at 10 mA cm-2) of 186 mV in basic media.18 The MoS2/Ni3S2 heterostructures were constructed by Feng’s group. At 10 mA cm-2, the applied overpotential of HER in basic media is approximately 110 mV.19 MoS2-Ni3S2 heteronanorods were also prepared by Gao’s group, which had been proved to show superior catalytic HER and OER performance in basic solutions.20 Nevertheless, bulk MoS2 still exhibits relatively inferior HER and OER activities in alkaline solutions because of the slow water dissociation dynamics. Accordingly, there is an urgent desire to develop MoS2-based active materials with high HER and OER electrocatalytic properties in alkaline solution simultaneously.21 Theoretical study shows that the introduction of foreign metal atoms with analogous atomic radius and electron configuration will inevitably lead to the unbalance of local Coulomb force and the derivate disturbance from the rearrangement of atoms will generate additional exposed edge sites.22 Moreover, ternary metal sulfides can afford richer redox active sites rather than the binary metal sulfides.23 Thus, the doping with heteroatom has been regarded as one feasible strategy to promote the catalytic performance of electrode materials by modifying the


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intrinsic electronic structure.24-25 Feng’s group engineered the water splitting sites by doping Ni atoms into MoS2 nanosheets to enhance the sluggish HER kinetics of MoS2 electrocatalysts in basic media.26 Furthermore, the morphology of electrocatalysts plays a significant role in their electrocatalytic performance. So far, a variety of MoS2 nanostructures have been prepared, such as nanoparticles,27 nanoplates,28 nanowires,29 nanoflower,30-31 vertically oriented sheets,32 double gyroid structures33 and hollow nanospheres.34-36 Among these diverse nanostructures, the hollow nanospheres can provide more active sites for the electrochemical reactions due to the large specific surface

area.37-38

Especially, the few-layered

MoS2

shows

the

improved

electrochemical performance, compared to the bulk MoS2, due to the much more active edges and fast charge transfer.39-40 Under these circumstances, we synthesized few-layered W-doped MoS2 hollow nanospheres on Ni3S2 nanorods by a facile one-step hydrothermal approach owing to the similar electronic structure between Mo and W. The substitution of W significantly changes the electronic structure of MoS2 rather than the crystal structure, which will finally improve the conductivity and reactivity of MoS2. The obtained Mo(1-x)WxS2@Ni3S2 heterostructure can be directly used as a 3D self-supporting HER and OER electrode without any binders. The optimized Mo(1-x)WxS2@Ni3S2 heterostructure (Mo-W-S-2@Ni3S2) presents a low overpotential of 98 mV and 168 mV at 10 and 100 mA cm-2 for HER, respectively, which is superior to those of the reported MoS2 electrocatalysts (> 100 mV at 10 mA cm-2). The OER overpotential of Mo-W-S-2@Ni3S2 heterostructure in alkaline solutions measured at 10 mA cm-2 is 285 mV, which is smaller than that of other



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transition metal chalcogenide compounds.41-42 More importantly, when the Mo-W-S-2@Ni3S2 heterostructures are used as bifunctional electrocatalysts (both the cathode and anode), a low cell voltage of 1.62 V is achieved at the current density of 10 mA cm-2. The enhanced catalytic properties of Mo-W-S-2@Ni3S2 heterostructure can be attributed to the massive active sites from W-doped MoS2 hollow nanospheres and the hierarchical structure of the abundant Ni3S2 nanorods grown on the 3D Ni foam. Thanks to the easy of synthesis, unique structure, high conductivity and enhanced activity, the Mo(1-x)WxS2@Ni3S2 heterostructures will exhibit an new avenue for high-efficient water splitting. 2. Experimental section 2.1 Reagents and Solutions. Sodium molybdate dehydrate (Na2MoO4•2H2O), sodium tungstate dihydrate (Na2WO4•2H2O), thioacetamide (CH3CSNH2) and other chemical reagents were all of analytical grade (AR) and were used without any additional purification. Iridium dioxide (IrO2) and platinum (nominally 20% on carbon black) were purchased from Sigma-Aldrich and Alfa Aesar, respectively. Nickel foam (thickness: 1.6 mm, pore density: 110 ppi) was purchased from Changsha Keliyuan. Deionized water (18.2 MΩ cm-1) generated by the Aike water system was used throughout the work. 2.2 Synthesis of Mo(1-x)WxS2@Ni3S2 heterostructure nanorods. Few-layered Mo(1-x)WxS2@Ni3S2 heterostructure were synthesized through a facile one-step hydrothermal method. In a typical process, 45 mg Na2MoO4•2H2O, 90mg thioacetamide and some amount of Na2WO4•2H2O (30 mg, 60 mg, 120 mg) were


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dissolved in 30 mL deionized water to obtain the clear solution under vigorous stirring for 30min at room temperature. Then the obtained homogeneous solution and a piece of Ni foam cleaned with acetone, ethanol and water were transferred to a 50 mL Teflon-lined stainless steel autoclave. The autoclave was then sealed and heated at 200 ºC for 24 h. After cooling down to room temperature, the samples were washed with deionized water three times and dried at 50 ºC. The as-prepared samples were demoted as Mo-W-S-1@Ni3S2, Mo-W-S-2@Ni3S2, Mo-W-S-3@Ni3S2, respectively. For the comparison, MoS2@Ni3S2 or WS2@Ni3S2 sample was synthesized by the same procedures when Na2WO4•2H2O or Na2MoO4•2H2O was absent, respectively. In addition, Ni3S2 modified Ni foam (denoted as Ni3S2@NF) was achieved by directly sulfuration of Ni foam in thioacetamide solution at 200ºC for 24 h.

2.3 Materials characterization. X-Ray diffraction (XRD) pattern of each sample was recorded on a Rigaku D/M ax-2400 at a scanning speed of 10° min-1 over a 2θ degree of 10-80°. The surface morphology of the sample was observed on a Hitachi S-4800 field-emission scanning electron microscope (FESEM). The chemical composition was analyzed by using energy dispersive X-ray spectroscopy (EDX). Transmission electron microscope (TEM), selected-area electron diffraction (SAED) and elemental mapping investigations were carried out on a TecnaiTM G2 F30 to characterize the microstructures of the samples. X-ray photoelectron spectroscope (XPS) measurements were performed on a VGESCALAB MKII X-ray photoelectron spectrometer with an Mg K α excitation source (1253.6 eV). The contents of Mo and W in the as-prepared samples were measured by Inductively Coupled Plasma Optical



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Emission Spectrometer (ICP-OES). The Raman spectra of as-prepared samples were conducted with a LabRAM HR 800 system at 532 nm laser.

2.4 Electrochemical Measurements. All the electrochemical performance was tested on the CHI760E electrochemical workstation at room temperature. The HER and OER electrochemical performances were executed in a three-electrode setup, and the overall water splitting was investigated in a two-electrode system. A standard Hg/HgO electrode and a Pt plate were used as the reference and counter electrodes, respectively. All polarization and CV curves were acquired at a scanning rate of 5 mV s-1 in 1 M KOH. Pt/C (20%) and IrO2 electrodes (loaded on nickel foam with 2 mg cm-2) were prepared as comparative experiments. The impedance spectra of the as-prepared samples in a three-electrode configuration were recorded at different applied voltages in 1 M KOH. All the plots displayed were calibrated to a Reversible Hydrogen Electrode (RHE) and corrected against the iR compensation. 3. Results and discussion 3.1. Material Characterizations A battery of Mo(1-x)WxS2@Ni3S2 composites were synthesized via one-step hydrothermal process. In order to determine the Mo/W molar ratio of Mo(1-x)WxS2@Ni3S2 samples, the Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) was performed (Table S1 in the Supporting Information). According to the results of ICP, the as-prepared samples are abbreviated to Ni3S2@NF, MoS2@Ni3S2,

Mo-W-S-1@Ni3S2,

Mo-W-S-2@Ni3S2,

WS2@Ni3S2, respectively.


Mo-W-S-3@Ni3S2 and

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The X-ray diffraction patterns of these samples are shown in Figure 1A and S1. As confirmed by the XRD results, all the samples demonstrate the strong diffraction peaks at 21.8°, 31.1°, 38.3°, 49.8°, 55.4°, 69.2°, 73.0° and 77.9°, which can be assigned to (101), (110), (021), (113), (300), (131), (214) and (401) planes of hexagonal Ni3S2 (JCPDS no. 44-1418),43 respectively, and the peaks of Ni foam at 44.9°, 52.2°, 76.7°, indicating the partial sulfuration of Ni foam after the hydrothermal treatment. Furthermore, no obvious characteristic diffraction peaks of MoS2 or WS2 in these samples are observed, owing to their low mass loading or amorphous structure.44 The Raman spectra of MoS2@Ni3S2, Mo-W-S-1@Ni3S2, Mo-W-S-2@Ni3S2, Mo-W-S-3@Ni3S2 samples are shown in Figure 1B. In terms of the MoS2@Ni3S2, the Raman shift at 406 cm-1 is ascribed to A1g mode of MoS2,45 which is representative of the out-of-plane Mo-S phonon mode vibration of MoS2. A very weak peak at 385 cm-1 can be assigned the in-plane Mo-S phonon mode vibration (E12g) of MoS2.46 The above result clearly proves the formation of MoS2. Additionally, It is already demonstrated that the MoS2 crystal termination can be inferred from the relative integrated intensities of A1g and E12g.47 A relatively larger ratio of A1g and E12g indicates the formation of edge-terminated structure, which will increase the number of exposed edge sites. From the Raman spectrum of the MoS2@Ni3S2, the intensity of A1g is much greater than that of E12g mode, which indicates the edge-terminated structure of as-prepared MoS2, and the enhanced catalytic performance. Furthermore, the Raman spectrum of Mo-W-S-1@Ni3S2 presents three main phonon modes, denoted to WS2-like E’ at 349 cm-1, MoS2-like E’ at 384 cm-1



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and A1g at 406 cm-1. Here, A1g phonon mode is considered to be the corresponding reference to distinguish MoS2 and WS2.48 For MoS2/WS2 heterostrutures, it splits into two frequencies corresponding to bulk MoS2 and WS2.49 The observed single A1g mode without peak-splitting for the Mo-W-S-1@Ni3S2 sample demonstrates the Mo1-xWxS2 alloys, not MoS2/WS2 heterostructure, are synthesized by one-step hydrothermal method. Moreover, with the increase of W content, the WS2-like E’ and MoS2-like E’ modes have an obvious blue-shift as compared to pure MoS2, indicating the enhanced electronic interaction between Mo and W. Additionally, the A1g of MoS2 is softened with respect to the bulk MoS2, which indicated the formation of the single or few layer structure.50

Figure 1. XRD patterns (A) and Raman spectra (B) of MoS2@Ni3S2, Mo-W-S-1@Ni3S2, Mo-W-S-2@Ni3S2 and Mo-W-S-3@Ni3S2 samples.

The FESEM images of as-prepared samples are shown in Figure 2 and S2. As shown in Figure S2A, Ni3S2@NF sample presents the block structure of polyhedron. However, with the addition of Mo source, MoS2@Ni3S2 sample shows a typical nanorod structure (Figure 2A), which are vertically oriented and uniformly distributed on the Ni substrate. The EDX result clearly confirms the existence of Mo, Ni and S


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elements (Figure S3A). Furthermore, Figure 2B, 2C and 2D reveal the similar nanorod morphology for Mo(1-x)WxS2@Ni3S2 heterostructures with different W contents. Additionally, the existence of Mo, Ni, S and W elements are confirmed by the EDX (Figure S3B, S3C and S3D). Finally, the WS2@Ni3S2 sample exhibits an analogous morphology with that of Ni3S2@NF (Figure S2B) while the Mo source is absent. All the results suggest that Mo source plays a crucial role for the formation of nanorod structure.51

Figure 2. FESEM images of (A) MoS2@Ni3S2, (B) Mo-W-S-1@Ni3S2, (C) Mo-W-S-2@Ni3S2 and (D) Mo-W-S-3@Ni3S2.

In order to reveal the detailed structure of the heterostructure nanorods, further insights of the morphology of MoS2@Ni3S2, Mo-W-S-1@Ni3S2, Mo-W-S-2@Ni3S2 and Mo-W-S-3@Ni3S2 samples were carried out by the transmission electron



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microscopy (TEM), as shown in Figure 3. Figure 3A shows a typical low-magnification TEM image of the synthesized MoS2@Ni3S2, in which some hollow spheres are developed from a nanorod core, forming a core-shell structure with average diameter of roughly 50 nm. Furthermore, HRTEM of MoS2@Ni3S2 sample (Figure 3B) shows the characteristic spacings of 0.287 nm and 0.273 nm for the (110) lattice planes of Ni3S2 core and (100) lattice planes of MoS2 shell, respectively, which further confirms the presence of MoS2.52 Figure 3C clearly shows the hierarchical structures of MoS2 shell, which displays the few layer structure with the 0.65 nm interlayer spacing corresponding to the (002) facet of MoS2. Compared with MoS2@Ni3S2, Mo(1-x)WxS2@Ni3S2 heterostructures present the similar core-shell structure (shown in Figure 3D, 3G and 3K). However, Mo(1-x)WxS2@Ni3S2 heterostructures (Figure 3E, 3H and 3L) exhibit the slightly larger crystal plane spacing of MoS2 than the pure MoS2, which indicates W element has been successfully doped into the crystal lattice of MoS2. The few layer structure of Mo(1-x)WxS2@Ni3S2 heterostructures are extremely visible in Figure 3F and 3M. To further investigate the constituent of inner core, a strong ultrasonic treatment was employed to remove the outer hollow spheres. The selected-area electron diffraction (SAED) pattern of the nanorod core (Figure 3I) clearly demonstrates the monocrystal structure of Ni3S2 core. Figure 3J is the dark field TEM image and EDX elemental mapping images of Mo-W-S-2@Ni3S2 sample. Obviously, Ni signals are mainly detected in the backbone region of nanorod, whereas Mo, W and S signals are uniformly distributed on the entire region of the heterostructure, once again validating


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the core-shell hierarchical structure of Mo-W-S-2@Ni3S2 samples.

Figure 3. Low-magnification and high-magnification TEM images of MoS2@Ni3S2(A, B, C), Mo-W-S-1@Ni3S2 (D, E, F), Mo-W-S-2@Ni3S2 (G, H) and Mo-W-S-3@Ni3S2 (K, L,M), SEAD pattern of Mo-W-S-2@Ni3S2 (I) after ultrasonic treatment; The dark-field image (J) and EDX elemental mapping of Mo, Ni, S, and W for Mo-W-S-2@Ni3S2 sample.



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The X-ray photoelectron spectroscopy (XPS) survey spectra (Figure S4) of Mo-W-S-1@Ni3S2, Mo-W-S-2@Ni3S2 and Mo-W-S-3@Ni3S2 samples distinctly show the presence of W element as compared to MoS2@Ni3S2 samples except for Mo, Ni and S element. Figure 4 show the high-resolution XPS spectra of Ni 2p, Mo 3d, W 4f and S 2p for the four samples. The Ni 2p spectrum of MoS2@Ni3S2 (Figure 4A) clearly affirms the Ni 2p3/2 and Ni 2p1/2 peaks at 855.8 and 873.4 eV, which corresponds to Ni+ and Ni2+ of Ni3S2,53 respectively. Otherwise, the peaks at 861.5 and 880.5 eV are satellite shakeup type peaks of Ni 2p3/2 and Ni 2p1/2. The small peaks at 852.2 eV and 869.7 eV can be attributed to metallic nickel originated from Ni foam. The signals of Ni in the Mo(1-x)WxS2@Ni3S2 samples display negative shifts of about 0.5 eV relative to the peaks of MoS2@Ni3S2, indicating strong electronic interactions between Ni3S2 and Mo-W-S heterostructure, which also suggests the establishment of coupling interfaces.51 The high-resolution Mo 3d spectrum of MoS2@Ni3S2 presents the spin-orbit splitting main peaks of Mo 3d5/2 and Mo 3d3/2 at 228.4 and 232.9 eV, respectively, which can be assigned to Mo4+.54 The peak at 235.1 eV is corresponded to Mo6+, attributed to partially oxidized of the surface. While the shoulder peak at 226.2 eV is the S 2s orbit of MoS2. Contrast with MoS2@Ni3S2, Mo 3d peaks of Mo(1-x)WxS2@Ni3S2 heterostructures shift to the lower binding energy, in which the more W-doping leads to the lower binding energy, moreover, the lower binding energy means the lower valence state of Mo, which is favorable for the enhanced HER properties in alkaline conditions.55 Figure 4C shows the W 4f spectra of these samples. No characteristic peaks of W are observed for MoS2@Ni3S2. Two


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main peaks located at 34.6 eV and 36.8 eV for Mo-W-S-1@Ni3S2 can be attributed to the W 4f 7/2 and W 4f 5/2, respectively, indicating the presence of W4+.56 In addition, a broad peak at ~ 39.5 eV is corresponding to the W6+. For Mo-W-S-2@Ni3S2 and Mo-W-S-3@Ni3S2, the binding energy is shifted to the negative direction, indicating the electronic structure alteration of W influenced by Mo element.57 In Figure 4D, the S 2p3/2 and S 2p1/2 of MoS2@Ni3S2 located at 161.8 and 163.0 eV, respectively, which can be assigned to the S2- states of MoS2 and Ni3S2.58 A broad peak at 168.8 eV is characteristic of S-O vibration.

Figure 4. High-resolution XPS spectra of (A) Ni 2p, (B) Mo 3d, (C) W 4f and (D) S 2p for MoS2@Ni3S2, Mo-W-S-1@Ni3S2, Mo-W-S-2@Ni3S2 and Mo-W-S-3@Ni3S2.

3.2. Electrochemical Measurements



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The HER performance of as-prepared samples was studied in 1M KOH solution with a scan rate of 5 mV s-1 (Figure 5 and S5). As can be seen from Figure S5A, the bare Ni foam, Ni3S2@NF and WS2@Ni3S2 samples show the large overpotential of 254 mV, 272 mV and 303 mV at 10 mA cm-2, respectively. Compared to them, an obvious cathode current occurred at MoS2@Ni3S2 catalyst with a low overpotential of 134 mV (Figure 5A). By contrast, the overpotential decreased stepwise when Mo-W-S-1@Ni3S2 (η10=109 mV) and Mo-W-S-2@Ni3S2 (η10=98 mV) were used as electrocatalysts. However, the overpotential of Mo-W-S-3@Ni3S2 rises to 134 mV in virtue of the excessive W content. Although the overpotential of Mo-W-S-2@Ni3S2 heterostructure is inferior to Pt/C electrode (η10=29 mV), but still outperform those of many preciously reported MoS2-based nanostructure catalysts, such as MoS2/Ni3S2 heterostructure (η10=110 mV),19 MoOx/MoS2 core–shell structure (η10=259 mV),59 Co9S8@MoS2/CNFs (η10=190 mV),60 respectively. Furthermore, Mo-W-S-2@Ni3S2 heterostructure merely need an impressively low overpotential of 168 mV to drive HER at the current density of 100 mA cm-2, illustrating the superior HER activity. This obviously suggests that the incorporation of W into MoS2@Ni3S2 plays a pivotal role in the enhancement of HER property. The Tafel slope is a key parameter to estimate the dominant HER mechanism at the interface of electrode and electrolyte. Detailed analysis of Tafel slope is executed to identify the rate-limiting step. Typically, three primary reactions are involved in HER of alkaline electrolyte. Essentially, the first procedure is the discharge reaction (known as the Volmer step), followed by either (a) the electrochemical desorption


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reaction (Heyrovsky step) or (b) the combination of two adsorbed protons (Tafel step), or both.61 As shown in following equations: M + H2O + e- → M-H+ OH- (Volmer)

(1)

M-H + H2O + e- →M + H2 + OH- (Heyrovsky)

(2)

2 M-H→2M + H2 (Tafel)

(3)

where M is representative of the catalyst active site. In the corresponding Tafel plots (Figure 5B), the commercial Pt/C catalyst still displays the smallest Tafel slope of 37 mV dec-1, similar to the previous reported value.62 The Tafel slope of Mo-W-S-2@Ni3S2 is 92 mV dec-1, lower than that of MoS2@Ni3S2 (117 mV dec-1), Mo-W-S-1@Ni3S2 (103 mV dec-1) and Mo-W-S-3@Ni3S2 (114 mV dec-1), implying a more rapid charge transfer kinetic. Benefiting from the lower Tafel slope, smaller overpotential is required to achieve the same cathodic current density, suggesting a faster HER response for Mo-W-S-2@Ni3S2 electrode. Additionally, according to the previous report,63 such a Tafel slope of the Mo-W-S-2@Ni3S2 heterostructure demonstrates a combined Volmer-Heyrovsky mechanism for HER. Figure 5C and S6A show the Electrochemical impedance spectroscopy (EIS) of the as-prepared samples, which were tested at the same applied voltage (-0.15V vs. RHE). The small series resistance of about 1.6 Ω for all materials elaborates the essentiality of the direct synthesis on a conductive substrate, such as Ni foam, which minimizes the parasitic ohmic losses. Compared to other samples, Mo-W-S-2@Ni3S2 heterostructure presents the smallest semicircle in the Nyquist plot, indicating the lowest charge-transfer resistance at the interface between catalysts and electrolyte.64 It



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suggests that a certain amount of W element can improve the electrical conductivity of MoS2, which is responsible to the enhanced electrochemical performance. The EIS of Mo-W-S-2@Ni3S2 heterostructure checked at different applied voltage are shown in Figure S6B. The more negative applied voltage leads to the lower charge-transfer resistance, indicating a positive correlation between the applied voltage and the charge-transfer.65 In addition, the cyclic voltammetry (CV) measurement was employed to acquire the electrochemical double-layer capacitance (Cdl), which is assumed to be linearly proportional to the electrochemical active surface area.66 The CV curves were swept from 1.174 - 1.274 V vs. RHE at various scan rates (20, 40, 60, 80, 100 mV s-1), as shown in Figure S7. Mo-W-S-2@Ni3S2 heterostructure exhibits the highest current density at the same scan rate, indicating the largest Cdl. Figure 5D shows that the calculated Cdl of Mo-W-S-2@Ni3S2 is 79.3 mF cm-2, significantly higher than that of MoS2@Ni3S2 (50.8 mF cm-2), Mo-W-S-1@Ni3S2 (64.8 mF cm-2) and

Mo-W-S-3@Ni3S2

(69.1

mF

cm-2),

indicating

the

Mo-W-S-2@Ni3S2

heterostructure possesses much more exposure of efficient active sites. Therefore, the excellent electrochemical properties of Mo-W-S-2@Ni3S2 can be attributed to the prominent synergistic effect between Mo(1-x)WxS2 and Ni3S2 induced by the abundant interface effect of core-shell nanorods and the hierarchical reaction platform constructed by the 3D framework of Ni foam, Ni3S2 nanorods and few-layer Mo(1-x)WxS2 hollow nanospheres, which can provided the large specific surface area and well conductive frameworks for the transportation of ion and electron. The stability of Mo-W-S-2@Ni3S2 heterostructure was executed by the


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continuous long-term cycling test. The comparison of the original curve and polarization curve after 3000 cycles is displayed in Figure 5E. A slight decay in the polarization curves are observed, indicating the robustness of the Mo-W-S-2@Ni3S2 catalyst in alkaline solution. Furthermore, Figure 5F provides a fifty-hour-long durable test for the Mo-W-S-2@Ni3S2 heterostructure conducted at 10 mA cm-2 in 1M KOH. The result also shows no obvious degradation of the curve during 50 h, which indicates the enormous potentials for implementing this catalyst into practical hydrogen evolution electrode.



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Figure 5. Polarization curves (A) and the corresponding Tafel plots (B) of MoS2@Ni3S2, Mo-W-S-1@Ni3S2, Mo-W-S-2@Ni3S2, Mo-W-S-3@Ni3S2 and Pt/C. Electrochemical impedance spectroscopy (C) and Plots of the current density vs. scan rate (D) for MoS2@Ni3S2, Mo-W-S-1@Ni3S2, Mo-W-S-2@Ni3S2 and Mo-W-S-3@Ni3S2. Polarization curves (E) of the Mo-W-S-2@Ni3S2 electrode recorded before and after 3000 continuous CV cycles. Chronopotentiometry measurement (F) for long-term stability tests of Mo-W-S-2@Ni3S2


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heterostructure.

Likewise, the OER activities of as-prepared samples were evaluated in 1 M KOH solution (Figure 6 and S8). Obviously, the OER properties of Ni foam, Ni3S2@NF and WS2@Ni3S2 can be almost ignored. Additionally, the overpotential of Mo-W-S-2@Ni3S2 heterostructure at 10 mA cm-2 is 285 mV, which is lower than that of MoS2@Ni3S2 (η10=310 mV), Mo-W-S-1@Ni3S2 (η10=296 mV), Mo-W-S-3@Ni3S2 (η10=298 mV) and comparable with that of IrO2 modified electrode (η10=280mV). Furthermore, the corresponding Tafel slopes are found to be 114, 105, 98 and 103 mV dec-1 for MoS2@Ni3S2, Mo-W-S-1@Ni3S2, Mo-W-S-2@Ni3S2 and Mo-W-S-3@Ni3S2, respectively, which suggests that Mo-W-S-2@Ni3S2 heterostructure can act as an ideal anode catalyst for water oxidation. It is worth noting that the excellent durability of Mo-W-S-2@Ni3S2 heterostructure for OER was also obtained. After continuous CV scanning, the slight change at the current density of 10 mA cm-2 (Figure 6C) are observed between the initial cycle curve and those going through 3000 CV cycles. Meanwhile, the chronopotentiometry response (Figure 6D) presents no appreciable increase in potential in the 50h interval for the oxygen evolution.



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Figure 6. Cyclic voltammetry curves (A) and the corresponding Tafel plots (B) of MoS2@ Ni3S2, Mo-W-S-1@Ni3S2, Mo-W-S-2@Ni3S2, Mo-W-S-3@Ni3S2 and IrO2. (C) Cyclic voltammetry curves of the Mo-W-S-2@Ni3S2 electrode recorded before and after 3000 continuous CV cycles. (D) Chronopotentiometry measurement of Mo-W-S-2@Ni3S2 heterostructure for long-term stability tests.

According to the above explanations, we conclude that Mo-W-S-2@Ni3S2 heterostructure can be used as a highly efficient bifunctional electrocatalyst for overall water splitting. As shown in Figure 7A, it exhibits high performance with a cell voltage of 1.62 V to afford 10 mA cm-2 water splitting current density in 1 M KOH solution with vigorous gas evolution on both electrodes. This potential is favorably comparable to the behavior of other nonprecious bifunctional catalysts reported in the recent literatures (Table S2). The cell’s capability of overall water


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splitting was examined at the current densities of 10 mA cm-2 for a long-term galvanostatic electrolysis (Figure 7B). Evidently, the cell voltage remains quite stable for 50 h, implying the outstanding ability of Mo-W-S-2@Ni3S2 heterostructure as bifunctional catalysts.

Figure 7. (A) Cyclic voltammetry and (B) Chronopotentiometry measurement of Mo-W-S-2@Ni3S2 heterostructure tested in a two-electrode setup as bifunctional electrocatalyst.

The FESEM images of Mo-W-S-2@Ni3S2 heterostructure after 50 h continuous test at 10 mA cm-2 in 1 M KOH solution for OER, HER and overall water splitting show no obvious change of the surface morphology, indicating the outstanding structure stability of Mo-W-S-2@Ni3S2 heterostructure (Figure S9-S11). XPS measurements of these samples are shown in Figure S12 and S13. According to the Ni 2p XPS spectra (Figure S13A), the peaks of metal Ni are almost disappeared, deriving from the surface oxidation of metal Ni in alkaline electrolyte.64 While the Ni 2p peaks of Ni3S2 still remain unchange before and after 50 h HER or OER. The Mo and W species in Mo-W-S-2@Ni3S2 heterostructure can remain nearly constant during the HER process. However, the XPS spectrum of Mo-W-S-2@Ni3S2 heterostructure after long-term OER shows that the peaks of Mo and W species turn weaker, due to the Mo4+ and W4+ are partially oxidized Mo6+ and W6+, which are consistent with the



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reported literatures.67-68 While the reasons for the change of S are still not very clear. All the results demonstrate that Mo-W-S-2@Ni3S2 heterostructure can maintain the excellent stability at least for 50 hours, whether for HER or OER. 4. Conclusion In summary, Mo(1-x)WxS2@Ni3S2 heterostructures were successfully synthesized via a facile hydrothermal procedure. In the composite catalysts, the few-layered Mo(1-x)WxS2 hollow nanospheres are conducive to exposure more active sites and facilitate the charge transport and the electrolyte diffusion. Moreover, the W dopant efficiently optimizes the electronic structure of MoS2 hollow nanospheres. The strong electronic interactions between Mo(1-x)WxS2 and Ni3S2 are originated from the establishment of the coupling interfaces, which will remarkably enhance the HER and OER performances. In particular, the Mo-W-S-2@Ni3S2 heterostructure exhibits an extremely low overpotential of 98 mV for HER and 285 mV for OER at 10 mA cm-2 in alkaline electrolyte, respectively. Furthermore, a Mo-W-S-2@Ni3S2 couple-based alkaline water electrolyzer approaches 10 mA cm-2 at cell voltages of 1.62 V. Excitingly, the Mo-W-S-2@Ni3S2 heterostructure shows robust stability in alkaline electrolyte, which can be maintained for at least 50 h. Supporting Information XRD, SEM characterization of Ni3S2/NF and WS2@Ni3S2; EDX and XPS spectra of MoS2@Ni3S2 and Mo(1-x)WxS2@Ni3S2; LSV curves, Nyquist plots, and additional CV curves. FESEM images and XPS survey spectra after catalysis, Table S1 and S2.


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Acknowledgments This work was supported by grants from Natural Science Foundation of China (NNSFC no. 21673105, 21503102), the Fundamental Research Funds for the Central University (lzujbky-2015-274, lzujbky-2016-K09) and Foundation of Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education) in Nankai University.

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