MoS2âNi3S2 Heteronanorods as Efficient and Stable Bifunctional

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Heteronanorods of MoS2-Ni3S2 as Efficient and Stable BiFunctional Electrocatalysts for Overall Water Splitting Yaqing Yang, Kai Zhang, Huanlei Lin, Xiang Li, Hang Cheong Chan, Lichun Yang, and Qingsheng Gao ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03192 • Publication Date (Web): 16 Feb 2017 Downloaded from http://pubs.acs.org on February 17, 2017

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Heteronanorods of MoS2-Ni3S2 as Efficient and Stable Bi-Functional Electrocatalysts for Overall Water Splitting Yaqing Yang†, Kai Zhang‡, Huanlei Lin†, Xiang Li‡, Hang Cheong Chan†, Lichun Yang*,‡, and Qingsheng Gao*,† † Department of Chemistry, College of Chemistry and Materials Science, Jinan University, Guangzhou 510632, P. R. China ‡ School of Materials Science and Engineering, Guangdong Provincial Key Laboratory of Advanced Energy Storage Materials, South China University of Technology, Guangzhou, 510641, P. R. China.

ABSTRACT: Exploring noble-metal-free electrocatalysts with high efficiency for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is promising to advance the production of H2 fuel through water splitting. Herein, one-pot synthesis was introduced to MoS2Ni3S2 heteronanorods supported by Ni foam (MoS2-Ni3S2 HNRs/NF), in which the Ni3S2 nanorods were hierarchically integrated with MoS2 nanosheets. The hierarchical MoS2-Ni3S2 heteronanorods enable not only the good exposure of highly active hetero-interfaces, but also the facilitated charge transport along Ni3S2 nanorods anchored on conducting nickel foam, accomplishing the promoted kinetics and activity for HER, OER and overall water splitting. The

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optimal MoS2-Ni3S2 HNRs/NF presents a low overpotential (η10) of 98 and 249 mV to reach a current density of 10 mA cm-2 in 1.0 M KOH for HER and OER, respectively. Assembled as a electrolyzer for overall water splitting, such heteronanorods show a quite low cell voltage of 1.50 V at j = 10 mA cm-2 and remarkable stability for more than 48 hours, which are among the best of current noble-metal-free electrocatalysts. This work elucidates a rational design of heterostructures as efficient electrocatalysts, shedding some light on the development of functional materials in energy chemistry.

KEYWORDS: hetero-interfaces, metal sulfides, electrocatalysis, hydrogen evolution reaction, oxygen evolution reaction, overall water splitting

INTRODUCTION To address the rapid growth of energy consumption and the associated environment issues, renewable and clean energy sources have attracted immense research interest.1-2 Hydrogen is viewed as a promising energy carrier because of its cleanliness, high efficiency and renewability.3 For sustainable hydrogen production, water electrolysis using renewable energy sources (e.g., sunlight and wind) is a promising way,4 which requires a thermodynamic potential of 1.23 V for the splitting into hydrogen and oxygen.5-6 However, because of the inevitable dynamic overpotentials in hydrogen evolution reaction (HER) and oxygen evolution reaction (OER),5,7 the electrolysis efficiency is severely limited. Electrocatalysts are significant to lower the overpotentials. So far, platinum-based materials are the most effective HER electrocatalysts, while iridium and ruthenium oxides hold the benchmark for OER.5 Unfortunately, the application of these noble-metal-based catalysts is impeded by their high cost and low Earth-abundance.8-9


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Recent advances have been made by developing the alloys, oxides, sulfides, phosphides, carbides and nitrides of Earth-abundant transition metals.7,10-15 However, most studies focus on how to improve the electrocatalytic activity for HER or OER, but ignore the development of a bifunctional electrocatalyst with both high HER and OER activity. The good integration of HER and OER electrocatalysts into a single nanostructure has become one of the hottest issues in this field, holding the promise for efficient overall water splitting.16-17 Heterogeneous nanostructures, showing the synergistically promoted kinetics on varied active-sites and electron-reconfigured interfaces, emerge as the superior electrocatalysts to their single-component counterparts for HER or OER.18-22 Compatibly integrating HER and OER activity materials in a designed manner, they can serve as bi-functional electrocatalysts for overall water splitting. For example, nickel sulfides (e.g., Ni3S2, NiS2, NiS, etc.) being highly active for OER,23-24 unfortunately bear the restricted performance for overall water splitting due to unsatisfied HER activity.24-27 Their HER and the consequent overall water splitting activity can be promoted by integrating with nanosized MoS2,18,28 which shows efficient HER on undercoordinated Mo-S edges.29-30 To fulfill the synergy in heterostructures, rational engineering is necessary, in which the hetero-interfaces of every component should be fully manifested, and the heterostructures can guarantee the fast ion/electron transportation throughout the whole electrocatalysts.31-32 As highlighted in very recent reports, the heterostructures with onedimensional (1D) morphology deliver highly efficient electrocatalysis, because they can provide abundant active sites in radial direction, and facilitate charge transfer along axial dimension in interconnecting networks.22,33-37 In this regard, constructing 1D heterostructures of Mo-Ni-S is a key innovation toward the boosted HER, OER and overall water splitting. However, their fabrication and engineering are still prohibited by the harsh and multiple-step synthesis, which


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usually employ either chemical vapor deposition or surfactant-mediated solvothermal procedure.31-32 Herein, we propose a one-pot and facile fabrication of MoS2-Ni3S2 heteronanorods supported by Ni foam (MoS2-Ni3S2 HNRs/NF) via environment-friendly hydrothermal processes at mild temperature, accomplishing the synergistically enhanced activity for HER, OER, and overall water splitting. As shown in Scheme 1, 1D NiMoO4 firstly generates on Ni foam (NF) owing to the anisotropic growth of molybdates,38-39 and further evolves to MoS2-Ni3S2 as reacting with thioacetamide (TAA) at prolonged time, resulting in the inner Ni3S2 nanorods surface-decorated by MoS2 nanosheets. Such hierarchical nanostructures evenly composed of 1D Ni3S2 and twodimensional (2D) MoS2 would enable the well-exposed active-sites of Ni3S2 and MoS2, and more importantly their hetero-interfaces. In addition, the 1D heteronanorods with coarse surface and its direct attachment to the conducting NF provide pathways for efficient charge transport during the electrolysis, and promote gas bubbles evolving and releasing from catalyst surface. As expected, the optimal MoS2-Ni3S2 HNRs/NF presents a low overpotential (η10) of 98 and 249 mV to reach a |j| of 10 mA cm-2 in 1.0 M KOH for HER and OER, respectively. The superior activity toward overall water splitting is featured by a quite low cell voltage of 1.50 V at j = 10 mA cm-2 and the remarkable stability for more than 48 h, which outperforms that of commercial IrO2/C-Pt/C coupled electrocatalysts, and the most of current noble-metal-free electrocatalysts.


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Scheme 1. Schematic illustration for the fabrication of heterostructured MoS2-Ni3S2 HNRs/NF composites. AHM denotes for ammonium heptamolybdate tetrahydrate, and NF for nickel foam. RESULTS AND DISCUSSION To fabricate Ni3S2-based nanostructures, commercial nickel foam (NF) after removing surface oxides by 3 M HCl was employed as the substrate (Figure S1 in Supporting Information), which shows smooth surface in scanning electronic microscopy (SEM) investigation (Figure 1a). Bulky products in micron is received after NF reacts with TAA (Figure 1b), being confirmed as Ni3S2 (JCPDS No.: 08-0126) by X-ray diffraction (XRD, Figure S2 in Supporting Information). By contrast, as ammonium heptamolybdate tetrahydrate (AHM) is introduced, hierarchical nanostructures are obtained. With the increasing AHM feeding, the product evolves from nanorods to nanoparticles (Figure S3 in Supporting Information), and thus an optimal feeding AHM/NF ratio of 0.07 (in weight) was adopted for the fabrication. As the preparation is conducted for a shortened time (e.g., 8 hours), coarse nanorods are received (Figure 1c). In the corresponding transmission electron microscopy (TEM) investigation (Figure 1d), such nanorod clearly displays the (220) and (311) lattice fringes of α-NiMoO4 (JCPDS No. 32-0692), between which the interplanar angle of 68.5o is in accordance with theoretical value (68.8o). This α-NiMoO4 1D nanostructure should originate from the anisotropic arrange of molybdate anions.38-39 Meanwhile, the typical lattice fringe of MoS2(002) is also detected on the surface of NiMoO4 (Figure 1d), indicating the formation of MoS2 nanolayers in these hierarchical nanorods. With a prolonged reaction time to 24 hours, the above MoS2-NiMoO4 further evolves to MoS2-Ni3S2 due to the sufficient reactions with TAA, as confirmed by its XRD pattern (Figure S2 in Supporting Information). And, the core nanorods decorated with ultrathin nanosheets are clearly observed in SEM and TEM images (Figures 1e and 1f). The corresponding SAED pattern detected on a single


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nanorod is well indexed by Ni3S2 (JCPDS No. 08-0126), in which the interplanar angle between Ni3S2(012) and Ni3S2(011) is 89.3o, close to the theoretical value of 89.2o. Obviously, the 1D nanostructure is well retained during NiMoO4 evolves to Ni3S2. On Ni3S2 nanorod surface, MoS2 nanosheets displaying the typical (002) lattice (0.62 nm) are clearly observed. Interestingly, the MoS2-Ni3S2 interfaces are visible in such hierarchical structures, which are constituted by MoS2(002) with neighboring Ni3S2(0 12) and Ni3S2(011).

Figure 1. SEM images of (a) bare NF and (b) Ni3S2/NF. (c, e) SEM and (d, f) TEM images of the products obtained in the presence of AHM for (c, d) 8 and (e, f) 24 hours. Insets of d and f are the corresponding SAED patterns obtained on a single nanorod. Furthermore, the energy dispersive spectra (EDS) and corresponding elemental mapping are conducted to analyze the composition of the MoS2-Ni3S2 HNRs peeled off from NF by ultrasonic. As displayed in Figure 2a, the EDS identify the main composition of Ni, Mo and S in MoS2-Ni3S2 HNRs. The molar ratio of Ni and S is determined as 1.46 (inset of Figure 2a), in accordance with


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the molecular formula of Ni3S2. Regarding the high Ni/Mo ratio of 34.48, Ni3S2 is believed dominant in the HNRs, and MoS2 serves as modification on surface. This is consistent with the low MoS2 content (3.09 wt.%) determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Furthermore, the corresponding elemental mapping shows the uniform distribution of Ni, S and Mo in the HNRs (Figure 2b), indicating the even integration of Ni3S2 nanorods with MoS2 nanosheets.

Figure 2. (a) EDS and (b) corresponding elemental mapping of MoS2-Ni3S2 HNRs/NF. Inset of a is the atom (Ni, S and Mo) contents in MoS2-Ni3S2 HNRs. XPS analysis was employed to investigate the chemical states of elements in MoS2-Ni3S2 HNRs, along with that of bare Ni3S2 free from MoS2 decoration. The deconvoluted Ni 2p profiles clearly show the strong peaks of Ni 2p3/2 and 2p1/2 at respective 855.2 and 873.2 eV,40 as well as their satellite peaks, on both MoS2-Ni3S2 and bare Ni3S2 (Figure 3a). This is consistent with the previously reported Ni3S2 growing on NF substrate.24,28 The MoS2-Ni3S2 also presents the peaks attributed to Ni3S2 species at 852.9 and 871.4 eV,41 which is respectively blue-shifted in comparison with those of bare Ni3S2 (852.6 and 871.0 eV). It indicates that the electronic interactions between Ni3S2 and MoS2 lead to the charge redistribution on their interfaces.28 Meanwhile, the coincident binding energy of 162.5 eV (2p3/2) and 163.9 eV (2p1/2) for bridging S22- is shown by MoS2-Ni3S2 (Figure 3b), implying unsaturated S atoms on Ni-S and Mo-S sites.4243

The peak at 168.7 eV is associated with residual SO42- species,24 and its high intensity in MoS2-


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Ni3S2 suggests the oxidation of surface S by oxidative molybdates. In addition, MoS2-Ni3S2 exhibits the visible peaks for Mo4+ and Mo6+,42 which are absent in bare Ni3S2 sample (Figure 3c). The existence of Mo6+ is owing to the surface oxidation of MoS2 as exposed to air.

Figure 3. High-resolution XPS profiles of (a) Ni 2p, (b) S 2p and (c) Mo 3d in MoS2-Ni3S2 HNRs. To evaluate the electrocatalytic activity for HER, the MoS2-Ni3S2 HNRs/NF was tested in 1.0 M KOH solution using a typical three-electrode configuration, in which the foam with a size of 0.5 cm × 0.5 cm was directly used as working electrode. For comparison, bare NF, Ni3S2/NF, MoS2/NF, and commercial Pt/C supported by NF with the same mass loading were also tested. Figure 4a displays their polarization curves with iR correction. Undoubtedly, Pt/C on NF reveals the highest performance. The as-prepared MoS2-Ni3S2 HNRs/NF affords a remarkable HER activity, in which the cathodic current goes up sharply with the applied potential. The overpotential


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required for j = -10 and -100 mA cm-2 is as low as 98 and 191 mV, respectively, superior to those of NF (η10 = 308 mV), MoS2/NF (η10 = 235 mV), and Ni3S2/NF (η10 = 183 mV). The obvious improvement in HER activity indicates the synergy between MoS2 nanosheets and Ni3S2 nanorods. As further compared with the current noble-metal-free catalysts (Table S1 in Supporting Information), our MoS2-Ni3S2 HNRs/NF performs among the best. In the case of nickel-based electrocatalysts, the η10 of 98 mV presented on MoS2-Ni3S2 HNRs/NF is lower than that of the reported MoS2-Ni3S2 nanoparticles/NF (110 mV),28 NiCo-P nanocubes (150 mV),44 NiSe2 nanobelts (184 mV),45 Ni-Fe/C (219 mV)46 and Ni3S2 nanosheets/NF (223 mV),24 and comparable to that of NiSe nanowires/NF (96 mV),33 and MoS2@Ni/carbon cloth (91 mV).47 Typically, the alloys combining Mo with other transition metals (e.g., Ni, Co, and Fe) hold the benchmark of HER on noble-metal-free electrocatalysts.48-49 Showing the comparable or slightly lower activity, our MoS2-Ni3S2 HNRs/NF can open up new opportunities for Ni-based catalysts. In further comparison with other noble-metal-free electrocatalysts, the activity on MoS2-Ni3S2 HNRs/NF is better than that of MoC-Mo2C heteronanowires (120 mV),22 CoSe/Ti (121 mV),50 Zn0.76Co0.24S/CoS2 nanowires array (175 mV),51 1D Ni/Mo2C/C (179 mV),52 and Co-S/carbon paper (190 mV).53


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Figure 4. (a) Polarization curves and (b) Tafel plots of NF, Ni3S2/NF, MoS2-Ni3S2 HNRs/NF, Pt/C on NF and MoS2/NF. (c) Estimation of Cdl by plotting the current density variation (Δj = (ja - jc)/2, at 150 mV vs. RHE; data obtained from the CV in Figure S4 in Supporting Information) against scan rate to fit a linear regression. (d) Nyquist plots (at η = 200 mV). (e) η10 and Cdl of various MoS2-Ni3S2/NF derived from different feeding ratio. (f) Stability of the MoS2-Ni3S2 HNRs/NF and Pt/C on NF with an initial polarization curve and after 10,000 cycles in 1.0 M KOH, and (inset of f) the long-term durability tests at η = 150 mV (MoS2-Ni3S2 HNRs/NF) and 120 mV (Pt/C on NF) for HER in 1.0 M KOH electrolyte.


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Accordingly, their Tafel plots present the consistent enhancement in HER kinetics on MoS2Ni3S2 (Figure 4b). The MoS2-Ni3S2 HNRs/NF delivers a low onset overpotential (ηonset) of 31 mV, and a low Tafel slope (b) of 61 mV dec-1, superior to NF (ηonset = 250 mV, b = 149 mV dec-1), MoS2/NF (ηonset = 176 mV, b = 121 mV dec-1), and Ni3S2/NF (ηonset = 140 mV, b = 108 mV dec1

). The small Tafel slope of MoS2-Ni3S2 HNRs/NF indicates a fast increase of hydrogen generation

rate with applied overpotential, corresponding to the high activity presented in the polarization curves. According to the classic theory,54 HER in alkaline aqueous media proceeds in two steps (Equations 1 ~ 3), where the * indicates the active site for HER, and H* a hydrogen atom bound to an active site. The first one is a discharge step (Volmer-reaction) with a Tafel slope of 118 mV dec-1 (Equation 1), and the second one is either the ion and atom reaction (Heyrovsky-reaction) with a slope of 40 mV dec-1 (Equation 2) or the atom combination reaction (Tafel-reaction) with a slope of 30 mV dec-1 (Equation 3).54 Although the Tafel slope alone is insufficient to determine the specific mechanism, the evidently reduced slope on MoS2-Ni3S2 HNRs/NF (61 mV dec-1), as compared with that on Ni3S2/NF (108 mV dec-1), can confirms the promoted Volmer-step in HER kinetics. Feng et al have demonstrated the enhanced H binding on MoS2-Ni3S2 interfaces by density functional theory calculation.28 Our XPS analysis shows an obvious blue-shift of Ni 2p binding energy in heterostructured MoS2-Ni3S2 (Figure 3a), in comparison with that in Ni3S2. The reduced electron density around Ni is reasonably suggested, which can provide sufficient empty d-orbitals to enhance the binding with H atom.55 Such strengthened H binding will facilitate the kinetics of Volmer step, contributing to the obviously decreasing Tafel slope and the improved HER activity on MoS2-Ni3S2. H2O (l) + e- + * ⇌ H* +OH- (aq.)

H* + H2O (l) + e- ⇌ H2 (g) + OH- (aq.) + *

Vomer reaction Heyrovsky reaction

(1) (2)


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H* + H* ⇌ H2 (g) + 2*

Tafel reaction

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(3)

We further analyzed the surface area of the above composite materials, as well as their

contribution to electrocatalysis. At first, the BET surface is taken into account. As displayed in the Table S2 of Supporting Information, the MoS2-Ni3S2 HNRs/NF presents a lower BET surface area (~ 4.0 m2 g-1) than that of Ni3S2/NF (10.1 m2 g-1). By contrast, the former delivers the much higher HER activity than the latter. Because the BET surface just represents the total surface area of materials, including which are inactive for electrocatalysis, the enhanced HER activity on the MoS2-Ni3S2 HNRs/NF is reasonably independent to the BET surface. Therefore, another rational measurement of double-layer capacitances (Cdl) is herein employed, which is proportional to electrochemical surface area (ECSA) and can provide a relative comparison.56 Derived from the cyclic voltammograms (CV) vs. scan rate in 1.0 M KOH (Figure S4 in Supporting Information), the Cdl of 121.3 mF cm-2 on MoS2-Ni3S2/NF is greatly higher than that on Ni3S2/NF (30.3 mF cm2

), as shown in Figure 4c. The high Cdl value implies the enriched active-sites on sulfides for HER.

To further access intrinsic activity, the electrocatalytic currents are normalized by Cdl, regarding that Cdl represents the amount of active sites. As displayed by Figure S5 in Supporting Information, the obviously high value of MoS2-Ni3S2/NF confirms the intrinsically optimization of active sites on hetero-interfaces, which is contributed by the enhanced empty d-orbitals on Ni3S2 and the consequently promoted chemisorption of H* after MoS2 decoration. Meanwhile, the electrochemical impedance spectroscopy (EIS, Figure 4d) displays the varied charge-transfer resistant in the above MoS2-Ni3S2 HNRs/NF, Ni3S2/NF and bare NF. Correspondingly with the order in HER activity, the MoS2-Ni3S2 HNRs/NF delivers a obviously lower Rct than that of Ni3S2/NF and bare NF. This suggests the rapid electron transport for hydrogen evolution on the 1D nanostructures directly anchored on conducting NF. In addition, the


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impact of pH on the HER performance was study in the range of pH 14.0 ~ 7.4 (Figure S6 in Supporting Information). It’s clear that the higher pH level leads to the better electrocatalytic activity, which is possibly due to the solution conductivity improved at high electrolyte concentrations. The hierarchical nanostructures integrating MoS2 nanosheets on Ni3S2 nanorods would benefit the exposure of active MoS2-Ni3S2 interfaces, as indicated by the evolving HER activity associated with varied morphology. By varying feeding AHM/NF ratio in weight (RAHM/NF), the MoS2-Ni3S2 heterostructures evolve from nanorods to nanoparticles (Figure S3 in Suppporting Information), which show different HER activity (Figure S7 in Supporting Information). As displayed in Figure 4e, the η10 of MoS2-Ni3S2 is 109 and 98 mV as a low RAHM/NF of 0.02 and 0.07 is adopted, respectively, in which the HNRs are dominant. With an increasing RAHM/NF to 0.12 and 0.19, irrgular particles emerge accompanying with MoS2-Ni3S2 HNRs, and the η10 increases to 118 and 122 mV, respectively. As excessive AHM is employed in synthesis (RAHM/NF = 0.28), only the product of nanoparicles are reveived, resulting in the high η10 of 165 mV for HER. Accordingly, the Cdl measurement for these MoS2-Ni3S2/NF shows the consistent trends (Figure 4e), i.e., the maximum Cdl of 121.3 mF cm-2 is acchieved on MoS2-Ni3S2 HNRs/NF with a optimal feeding RAHM/NF of 0.07. This well indicates the promoted exposure of MoS2-Ni3S2 active interfaces on hierarchical nanorods . Operational stability is another important criterion for a HER catalyst. The long-term stability of our MoS2-Ni3S2 HNRs/NF was examined using cycling continuously for 10,000 cycles and chronoamperometry in 1.0 M KOH, with a reference of the commerical Pt/C supported by NF (Figure 4f). Although Pt/C delivers a high initial HER activity, it suffers an obvious loss of HER activity in tests. By contrast, the MoS2-Ni3S2 HNRs/NF affords similar j–V curves to the initial


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cycle with negligible loss of the cathodic current after 10,000 cycles, confirming the satisfied durability. As been further evaluated by prolonged electrolysis at η = 150 mV (inset of Figure 4f), MoS2-Ni3S2 HNRs/NF exhibits a high j about 40 mA cm-2 over 48 hours.

Figure 5. (a) CV curves, (b) Tafel plots of NF, Ni3S2/NF, MoS2-Ni3S2 HNRs/NF, MoS2/NF and IrO2/C supported by NF, and the corresponding (c) Nyquist plots (η = 300 mV). (d) Long-term durability of MoS2-Ni3S2 HNRs/NF for OER at 1.57 V vs. RHE in 1.0 M KOH. The MoS2-Ni3S2 HNRs/NF was further explored for electrocatalytic OER in 1.0 M KOH. Figure 5a displays the CV curves for MoS2-Ni3S2 HNRs/NF, Ni3S2/NF, MoS2/NF and bare NF at a scan rate of 1 mV s-1, along with that of the benchmark IrO2/C supported by NF with the same mass loading. In comparison with bare NF, the slightly enhanced activity is observed on MoS2/NF, indicates the low OER activity of MoS2, in accordance with previous reports.57 And the Ni3S2/NF presents a η10 314 mV. By contrast, MoS2-Ni3S2 HNRs/NF shows the remarkably improved


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activity featured by the low η10 of 249 mV, and the η100 of 341 mV, which outperforms that of conventional IrO2/C (IrO2: 20 wt%) on NF (η10 = 308 mV). Similar with the analysis for HER, the OER currents normalized by Cdl can reflect the intrinsic activity of the above composite electrocatalysts. As shown in Figure S8 in Supporting Information, the obviously higher value on MoS2-Ni3S2/NF, in comparison with Ni3S2/NF, well confirms the intrinsically enhanced activity associated with MoS2-Ni3S2 synergy. Such synergistic effects have been indicated by the blueshife of Ni 2p in the XPS profiles of MoS2-Ni3S2 HNRs (Figure 3a). The consequently enriched empty d-orbitals on Ni after MoS2 modification will contribute to the strengthened binding with OH* intermediate and thus the improved OER activity. Furthermore, the anodic (1.39 V) and cathodic (1.29 V) peaks on MoS2-Ni3S2 HNRs/NF correspond to the redox reaction of surface Ni species,28,58 in-situ generating efficient OER active-sites. Accordingly, the Raman and TEM investigations clearly identify the presence of oxidized species on the surface of MoS2-Ni3S2 after OER cycles (Figure S9 in Supporting Information). These peaks show the higher intensity in comparison with Ni3S2/NF, suggesting the promoted exposure of active-sites on hierarchical MoS2-Ni3S2. Remarkably, the low overpotential (η10 = 249 mV) presented by MoS2-Ni3S2 HNRs/NF confirms the superior OER activity to current noble-metal-free electrocatalysts (Table S3 in Supporting Information), e.g., Ni3S2 nanorods/NF (~ 420 mV),59 NiMo nanorods/Ti mesh (310 mV),60 CoP nanowires/Ti mesh (310 mV),61 exfoliated NiFe LDHs (305 mV),62 FeOOH/Co/FeOOH nanotubes (245 mV),63 1D Ni/Mo2C-porous carbon (368 mV),52 Au@Co3O4 (390 mV),64 etc. Such OER activity is comparable to that of benchmarking (NiCo)0.85Se/carbon cloth (255 mV),65 CoNi-LDH/Fe-porphyrin film (264 mV),66 and Co-phytate nanoplates/Cu (265 mV).67


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Figure 5b illustrates that the Tafel slope of the MoS2-Ni3S2 HNRs/NF is approximately 57 mV dec-1, lower than that of Ni3S2/NF (82 mV dec-1), MoS2/NF (80 mV dec-1), NF (91 mV dec-1), and even the benchmarking IrO2/C (59 mV dec-1). Its ηonset is 195 mV, outperforming Ni3S2/NF (281 mV), MoS2/NF (301 mV), NF (308 mV), and IrO2/C (280 mV). This result strongly suggests the kinetic merits of the MoS2-Ni3S2 HNRs for water oxidation. Coincident with the OER activity, MoS2-Ni3S2 HNRs/NF exhibits a smaller Rct as compared with Ni3S2/NF and NF (Figure 5c), indicating the fast Faradaic process on MoS2-Ni3S2 heterostructures. To access the stability of MoS2-Ni3S2 HNRs/NF for OER, a long term water-oxidation is conducted at η = 340 mV (Figure 5d), in which the steady OER activity is observed for 48 hours.

Figure 6. (a) Polarization curves and (b) long-term durability tests at 1.53 V of MoS2-Ni3S2 HNRs/NF for overall water splitting in 1.0 M KOH solution. (Inset of b) SEM image of MoS2Ni3S2 HNRs/NF after water electrolysis for 48 hours. The MoS2-Ni3S2 HNRs/NF is further utilized as a bi-functional electrocatalysts for overall water splitting in a two-electrode system (Figure 6). Remarkably, it affords a current density of 10 mA cm-2 in 1.0 M KOH at a cell voltage of 1.50 V, i.e., a combined overpotential of about 270 mV. Such activity is superior to those of NF, MoS2/NF and Ni3S2/NF, confirming that the synergy between MoS2 and Ni3S2 is efficient for overall water splitting. The MoS2-Ni3S2 HNRs/NF


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outperforms the couple of commercial 20 wt% IrO2/C and 40 wt% Pt/C (1.52 V at j = 10 mA cm2

), and the recently reported bi-functional electrocatalysts (Table S4 in Supporting Information),

e.g., MoS2-Ni3S2 nanoparticles/NF (1.56 V),28 Ni1-xFex/C (1.58 V),46 Fe-doped CoP nanoarray/Ti mesh (1.60 V),68 Ni3Se2/NF (1.61 V),69 Co-doped NiSe2/Ti mesh (1.62 V),70 NiS/NF (1.64 V),71 Co0.85Se/NiFe-LDH (1.67 V),72 NiCo2S4 nanowires/carbon cloth (1.68 V),73 and V/NiF (1.74 V),74 etc. Particularly, our MoS2-Ni3S2 HNRs/NF shows a lower OER activity (η10 = 249 mV) but a higher HER activity (η10 = 98 mV) than the recently reported MoS2-Ni3S2 nanoparticles/NF (η10 = 218 mV and 110 mV for OER and HER, respectively). For overall water splitting at j = 10 mA cm-2, MoS2-Ni3S2 HNRs/NF requires a lower cell voltage (1.50 V) than MoS2-Ni3S2 nanoparticles/NF (1.56 V),28 indicating the compatible integration of HER and OER associated with the hierarchical and heterogeneous structures. In addition, the Faradaic efficiencies for H2 and O2 evolutions were calculated by comparing the amount of experimentally quantified gas with theoretically calculated gas.68 The good agreement of both values (Figure S10 in Supporting Information) identifies the efficiencies approximate to 100% for both HER and OER, which can eliminate the O2 reduction at cathode and the H2 oxidation at anode. Moreover, the electrolyzer gives a smooth line with a stable current density of 17 mA cm-2 approximately for 48 h, when the applied voltage is set as 1.53 V (Figure 6b). The hierarchical nanostructures of MoS2-Ni3S2 are well remained even after overall water splitting for 48 hours (inset of Figure 6b), confirming the satisfied long-term durability of MoS2-Ni3S2 HNRs under the strong alkaline electrolyte. CONCLUSION In summary, the MoS2-Ni3S2 HNRs/NF was successfully achieved via facile hydrothermal processes, utilizing anisotropic molybdate intermediates to direct the growth of hierarchical MoS2-


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Ni3S2 heterostructures. In the composite electrocatalysts, Ni3S2 nanorods are evenly integrated with ultrathin MoS2 nanosheets, enabling the well-exposed hetero-interfaces. Such MoS2-Ni3S2 electrocatalysts display a superior performance for HER and OER, featured by the low η10 of 98 mV, 249 mV, respectively. As for overall water splitting, only a cell voltage of 1.50 V is required for a j of 10 mA cm-2, outperforming most of current noble-metal-free electrocatalysts, and even the coupled IrO2/C-Pt/C. Such superior catalytic performance should be ascribed to the MoS2Ni3S2 interfaces with favored chemisorption of H and O-contained intermediates, the effective exposure of active interfaces in hierarchical nanostructures, and the facilitated electron transport along 1D Ni3S2 that is anchored on NF substrate. Exploring the efficient water splitting over wellorganized metal-sulfide hetero-interfaces, this work will open up new opportunities to develop promising electrocatalysts via rational engineering on interfaces and nanostructures. EXPERIMENTAL SECTION Materials All reagents were purchased from commercial sources and in analytical or reagent grade when possible. Ammonium heptamolybdate tetrahydrate (AHM), urea, ethanol, thioacetamide, potassium hydroxide, sodium hydroxide, trisodium citrate dehydrate, and chloroiridic acid were purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). Hydrochloric acid was purchased from Guangzhou Chemical Reagent Factory. Ni foam was purchased from Shenzhen Green and Creative Environmental Science and Technology Co. Ltd (Shenzhen, China). Commercial Pt/C (40 wt%) and titanium wire were provided by Alfa Aesar. Phosphate Buffer Saline (PBS) was purchased from Invitrogen. All aqueous solutions were prepared using ultrapure water (> 18 MΩ). Catalyst preparation


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Synthesis of MoS2-Ni3S2 HNRs/NF: Commercial NF (1×1 cm2, ~ 42 g) was firstly immersed in acetone for 1 hour, and then HCl aqueous solution (3 M) for 2 hours to remove the surface oxides. Afterwards, the NF was placed into a Teflon-lined stainless steel autoclave, in which a varied amount of ammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24·4H2O, AHM), and fixed TAA (27 mg), urea (25 mg), ethanol (1.7 mL) and deionized water (1.3 mL), have been already loaded. After reactions at 240 oC for 24 hours, the MoS2-Ni3S2 HNRs/NF was received. The feeding ratio of AHM/NF in weight was varied in the range of 0.02 ~ 0.28 to tailor the MoS2Ni3S2 heterostrutures. And, the Ni3S2/NF was fabricated via the similar procedures without adding AHM. The loading mass on such NF was kept at 13 mg cm-2, which was calculated from the weight increment after catalyst preparation. Synthesis of IrO2/C: According to the previous reports,75-76 11.4 g of H2IrCl6 solution (0.35 wt%) and 0.19 g of trisodium citrate dehydrate were added to a 100 mL beaker with 50 mL of distilled water, which was then adjusted to pH of 7.5 by NaOH solution (1 M) and heated to 95 oC for 30 minutes with stirring. The mixture solution was transferred to a flask, and 0.184 g of carbon black (Vulcan XC72R) was added with stirring. The mixture was refluxed at 95 oC for 2 hours under O2 flow. After dried at 70 oC in vacuum, the solid was heated at 300 oC for 30 min under air flow to remove organic ligands, and the 20% IrO2/C was finally harvested. Physical characterization SEM was taken on a ZEISS ULTRA55. TEM, HRTEM, EDS and elemental mapping investigations were carried out with a JEOL JEM 2100F. XRD analysis was performed on Bruker D8 diffractometer using Cu Ka radiation (λ = 1.54056 Å). XPS was processed on a Perkine-Elmer PHI X-tool XPS, using C 1s (B. E. = 284.6 eV) as a reference. The Mo elemental analysis was determined by ICP-AES. N2 sorption isotherms were collected on a Quantachrome Autosorb-iQ-


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MP adsorption analyser at -196 oC (77 K). The Brunauer–Emmett–Teller (BET) specific surface area was calculated from adsorption data. Raman investigation was taken on a laser confocal Raman microspectrometer (XploRA, Horiba Jobin Yvon, Ltd.). The evolving H2 and O2 were analyzed by gas chromatography (FuLi instrument GC9790II) with TCD detector. Electrochemical measurements The electrochemical measurements of HER and OER were performed using an electrochemical workstation (CHI 760, CH Instruments, Inc., Shanghai) in a three-electrode system. The MoS2-Ni3S2 HNRs/NF can be used as working electrode directly, with a size of 0.5 cm × 0.5 cm. As for the MoS2, Pt/C and IrO2/C, the catalysts (4 mg) were firstly dispersed in 1 mL of 4:1 v/v water/ethanol and 10 μL of 5% PVDF solution by at least 30 min sonication, to form a homogeneous ink, and then loaded onto a preprocessed 0.25 cm2 NF by drop-coating. Graphite rod and saturated calomel electrode was used as the counter and reference electrode, respectively. Linear sweep voltammetry (LSV) for HER and cyclic voltammograms (CV) for OER were conducted with the scan rate of 1 mV s-1 in 1.0 M KOH (pH = 14.0). Electrochemical impedance spectra (EIS) were measured at corresponding potentials from 0.01 to 1 000 000 Hz for HER and OER, with an amplitude of 5 mV. For overall water splitting, the MoS2-Ni3S2 HNRs/NF was integrated as both anode and cathode in a two-electrode cell with a one-chamber setup (Figure S11 in Supporting Information). Polarization curves were recorded using LSV with a scan rate of 1 mV s-1. All the potentials reported in our manuscript were referenced to a reversible hydrogen electrode (RHE) by adding a value of (0.241 + 0.059 pH) V. The iR-correction was manually performed for every electrode, using potentio-electrochemical impedance spectroscopy. The compensated potential was corrected by Ecompensated = Emeasured – i×Rs, in which the Rs is the series resistance determined by electrochemical impedance spectroscopy.


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AUTHOR INFORMATION Corresponding Author *E-mail for Q. S. G.: [email protected] *E-mail for L. C. Y.: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS We appreciate the financial support from National Natural Science Foundation of China (21373102, 21433002, 51671089 and 51402110), Guangdong Natural Science Funds for Distinguished Young Scholar (2015A030306014) and Guangdong Program for Support of Topnotch Young Professionals (2014TQ01N036). SUPPORTING INFORMATION Photographs of the MoS2-Ni3S2 HNRs/NF and the setup for overall water splitting, XRD patterns, SEM images, CV curves, j normalized by Cdl (HER and OER), pH dependent HER activity of the MoS2-Ni3S2 HNRs/NF, the Raman and TEM investigation of after OER test, and the comparison of activity for HER, OER and overall water splitting. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES (1) Gray, H. B., Nat. Chem. 2009, 1, 112.


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Table of content

The MoS2-Ni3S2 heteronanorods supported by nickel foam were achieved via one-pot hydrothermal synthesis, exposing abundant hetero-interfaces for efficient electrocatalysis. The high activity for HER, OER and overall water splitting, outperforming the current noble-metal free counterparts, was contributed by the effectively exposed MoS2-Ni3S2 interfaces and the facilitated electron transport in hierarchical nanostructures.


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MoS2âNi3S2 Heteronanorods as Efficient and Stable Bifunctional

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