Design and Epitaxial Growth of MoSe2âNiSe Vertical

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Design and Epitaxial Growth of MoSe2–NiSe Vertical Heteronanostructures with Electronic Modulation for Enhanced Hydrogen Evolution Reaction Xiaoli Zhou, Yun Liu, Huanxin Ju, Bicai Pan, Junfa Zhu, Tao Ding, Chunde Wang, and Qing Yang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b05006 • Publication Date (Web): 24 Feb 2016 Downloaded from http://pubs.acs.org on February 25, 2016

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Chemistry of Materials

Design and Epitaxial Growth of MoSe2–NiSe Vertical Heteronanostructures with Electronic Modulation for Enhanced Hydrogen Evolution Reaction Xiaoli Zhou,† Yun Liu,† Huanxin Ju,§ Bicai Pan,‡ Junfa Zhu,§ Tao Ding,† Chunde Wang,† Qing Yang*,† †

Hefei National Laboratory of Physical Sciences at the Microscale, Department of Chemistry, Laboratory of Nanomaterials for Energy Conversion and Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China (USTC), Hefei 230026, Anhui, P. R. China. § National Synchrotron Radiation Laboratory, USTC, Hefei 230029, Anhui, P. R. China. ‡ Department of Physics, USTC, Hefei 230026, Anhui, P. R. China. ABSTRACT: Transition-metal dichalcogenides (MX2, M = Mo or W, X = S or Se) have been regarded as one of the best alternatives for noble metal-free electrocatalysts of hydrogen evolution reaction (HER). Tremendous attempts have been made mainly focusing on the maximization of the number of active edge sites and conductivity of MX2-based electrocatalysts to enhance the HER performance. However, for MX2-based electrocatalysts, the acceleration of kinetic process to improve the HER performance still be neglected up to now. Here we report a colloidal epitaxial growth strategy to synthesize MoSe2–NiSe nanohybrids with welldefined heterointerfaces which are constructed by in situ growth of metallic NiSe nanocrystallites on the MoSe2 nanosheets. These high-quality vertical heteronanostructures with band alignment give rise to the electrons transferring from the metallic NiSe nanocrystallites to the MoSe2 matrix, achieving the electronic modulation of the MoSe2–NiSe nanohybrids for efficient electrocatalytic activity. The MoSe2–NiSe nanohybrids exhibit excellent HER catalytic properties with a low onset potential of -150 mV, a large cathodic current density (10 mA cm-2 at an overpotential of 210 mV) and a small Tafel slope of 56 mV per decade. The highly enhanced electrocatalytic properties were attributed to the electronic structure modulation from the synergetic interactions between NiSe nanocrystallites and MoSe2 nanosheets. We anticipate that constructing hybrid structures will be a powerful tool for achieving high performance electro-catalysts in solids.

INTRODUCTION Electrochemical reduction of molecular hydrogen from water holds enormous promise as a renewable and clean approach to address the global energy requirements and the environmental issues.1 A critical factor in this pursuit is the discovery of efficient and cost-effective catalysts for use in electrochemical energy conversion processes. However, up to date, Pt-based materials still represent the most efficient electrocatalysts for the hydrogen evolution reaction (HER),2-3 which have greatly hindered their practical applications on a large scale because of their scarcity and high cost. Therefore, numerous efforts have been carried out to search for noble metal-free HER catalysts. During the past few years, many potential alternatives for Pt-based electrocatalysts have been investigated, including transition metal sulfides,4-11 selenides,12-15 carbides,16-17 nitrides,18-20 phosphides,21-23 and metalfree catalysts.24-25 Among all these alternatives, transition metal dichalcogenides (MX2, M = Mo or W, X = S or Se) have attracted special attention due to their earth-abundant nature and high electrochemical activity.26-27 Theoretical and experimental studies have demonstrated that the active edge sites and the electric conductivity of the MX2 materials are crucial factors to determine the electrocatalytic HER performance.5,28 Thus, tremendous attempts have been made mainly focusing on the maximization of the number of active edge sites and conductivity of MX2-based electrocatalysts to enhance the HER performance.29-36 Notably, on the other hand, the reaction

kinetics also play a decisive role in the electrocatalytic performance. However, for MX2-based electrocatalysts, the improvement of the HER performance from the point of view of kinetic process has been neglected to date. As is known, the production of molecular hydrogen in the electrochemical process needs combination of multiple elementary steps, including the adsorption, electrochemical reduction and desorption of the reacting hydrogen species. Limitations in many single-component catalysts arise from the fact that they do not exhibit high catalytic activity for all these intermediate reaction processes. Pioneering works have demonstrated that rational fabrication of the hybrid materials could facilitate different parts of the overall multistep HER process and the synergetic interactions between the different catalytic active materials can greatly enhance the catalytic performance.37-39 Unfortunately, to the best of our knowledge, the optimization of the kinetic process for MX2-based electrocatalysts has not been reported up to now. Recently, our group demonstrated the colloidal synthesis of MoSe2 ultrathin nanosheets for HER,40 providing an ideal platform for us to investigate the optimization of the HER process by combining with other catalytic materials. Recently, some studies have been reported to fabricate the composite materials with the Ni-based compounds via integrating Ni(OH)2 into Pt electrodes,2 Ni/NiO into CoSe2,41 attaching NiO/Ni to carbon nanotube (NiO/Ni-CNT),42 and incorporating Fe, Co and Ni in amorphous MoSx films,43 leading

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to significantly improved HER performance by facilitating multistep HER process. Motivated by this strategy, an electrode of multi-component selenides is preferably chosen as the candidates to fabricate the MoSe2-based hybrid materials for HER. Among the transition metal selenides, the crystallographic structure of the hexagonal NiSe matches well with the hexagonal MoSe2 since the two selenides have the same space group P63/mmc and approximate d-spacings of (100) facet, providing a superior candidate to construct new hybrid system via vertical epitaxial growth of NiSe onto the as-obtained MoSe2 ultrathin nanosheets.40 In fact, high-quality heterointerfaces of the composite materials44 can also facilitate efficient and fast interfacial charge transfer, which is critical to the electrochemical reaction. On the other hand, due to the different band structure of the composite materials, the alignment of the energy levels at the interface usually relate to the electron transfer between the two composite components,45-46 leading to the electronic redistribution and modulation of catalytic performance. Therefore, constructing the MoSe2–NiSe hybrids is promising for achieving high performance HER electrocatalysts. In this work, we design and develop a colloidal epitaxial growth strategy to synthesize MoSe2–NiSe nanohybrids as one kind of vertical heteronanostructures for the first time, which provide an opportunity to investigate the synergetic interactions of the nanohybrids in the kinetic process and electronic modulations for HER. The small NiSe nanocrystallites in-situ grow and anchor on the MoSe2 nanosheets with fine dispersion to ensure the formation of high-quality heterointerfaces and prevent the agglomeration of NiSe nanocrystallites. As a proof of concept, the introduction of NiSe nanocrystallites on the MoSe2 nanosheets gives rise to highly enhanced HER performance compared with the pure MoSe2 catalyst. The MoSe2NiSe nanohybrids with vertical heteronanostructures exhibit high HER activity with a low onset potential of -150 mV, a large cathodic current density (10 mA cm-2 at an overpotential of ~ 210 mV) and small Tafel slope of 56 mV per decade. In addition, detailed characterizations and analyses reveal that the occurrence of electrons transfers from the NiSe nanocrystallites to the MoSe2 nanosheets. The injection of electrons to the MoSe2 nanosheets could increase their conductivity and thus improve their catalytic activity. Rational fabrication of the hybrid materials provides an efficient method to construct high performance electrocatalysts and investigate the underlying mechanism of electrochemical reaction.

EXPERIMENTAL SECTION Materials. Molybdenum(VI) dioxide bis(acetylacetonate) [MoO2(acac)2)] and oleylamine (OAm, 70%) were purchased from Sigma Aldrich. Dibenzyl diselenide [(PhCH2)2Se2, 95%] and nickel(II) acetylacetonate [Ni(acac)2, 98%] were purchased from Alfa Aesar. Absolute ethanol and toluene were obtained from Sinopharm Chemical Reagent Ltd., China. All chemicals were used in the experiments without further purification. Synthesis of MoSe2 nanoflowers. Synthesis of MoSe2 nanoflowers was performed according to our previous work40 with slight modification. In a typical procedure, 0.1 mmol MoO2(acac)2, 0.1 mmol (PhCH2)2Se2 and 6.0 mL OAm were all added into a three-neck flask at room temperature. Then the mixture was heated up to 240 °C. After 20 min, the resulting MoSe2 nanoflowers were obtained.

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Synthesis of MoSe2–NiSe nanohybrids. The stock solution was freshly prepared and preheated to 70 °C by mixing 0.1 mmol of Ni(II)(acac)2 and 0.05 mmol (PhCH2)2Se2 with 0.5 mL of OAm. Then, the stock solution was transferred into a syringe equipped with a needle. In a typical synthesis, 0.1 mmol (PhCH2)2Se2, 0.1 mmol MoO2(acac)2 and 6.0 mL OAm were all added into a three-neck 50 mL round-bottom flask at room temperature. The mixture was first heated to 130 °C for 30 min under an argon flow and magnetic stirring to remove water and other low boiling-point impurities. Then the mixture was heated up to 240 °C and kept at this temperature for 20 min. When the mixture was further heated up to 300 °C, 0.5 mL of OAm stock solution was rapidly injected into the above hot solution and allowed to be aged for 20 min. Finally, the solution was cooled down to room temperature naturally, and black products precipitated at the bottom of the flask were obtained. 5.0 mL toluene was then added into the crude solution and the products were isolated by centrifugation (8000 rpm for 5 min). To remove the excess surfactant, the samples were repeatedly washed with toluene and n-hexane. Other samples were synthesized under the same reaction conditions with slight modifications. Synthesis of pure NiSe nanoplates. In a typical procedure, 0.2 mmol Ni(II)(acac)2, 0.1 mmol (PhCH2)2Se2 and 6 mL OAm were all added into a three-neck flask at room temperature. Then the mixture was heated up to 300 °C. After 20 min, the resulting NiSe nanoplates were obtained. Characterization. The products were characterized using X-ray powder diffraction (XRD, performed on a Philips X’pert PRO X-ray diffractometer, Cu Kα, λ = 1.54182 Å), scanning electron microscopy (SEM, JSM-6700F) and Transmission electron microscopy (TEM, Hitachi H-7650). The high-resolution transmission electron microscopy (HR-TEM), electron energy loss spectra (EELS), high-angle annular darkfield scanning transmission electron microscopy (HAADFSTEM) images and corresponding energy-dispersive spectroscopic (EDS) mapping analyses were performed on a JEOL JEM-ARF200F TEM/STEM with a spherical aberration corrector. X-ray photoelectron spectra (XPS) were acquired on an ESCALAB MK II with Mg Kα as the excitation source. The nitrogen (N2) absorption/desorption isotherms were obtained using Micromeritics ASAP-2000 at 77 K. The electrical conductivity measurements were carried out on pressed pellets using a Keithley 4200-SCS Semiconductor characterization system by a two probe configuration. Ultraviolet Photoemission Spectroscopy. Ultraviolet photoemission spectroscopy (UPS) measurements were performed at the Catalysis and Surface Science Endstation of National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. The valence-band spectra were measured using synchrotron radiation light as the excitation source with a photon energy of 40 eV. The valence-band spectra were referenced to the Fermi level determined from the Au sample. A sample bias of -10 V was applied in order to observe the secondary electron cut-off. Calculations. The concerned NiSe crystal was computed by using the density functional theory (DFT). In our calculations, the local density approximation (LDA) in form of PerdewBurke-Ernzerhof (PBE)47 was adopted for exchangecorrelation potential. The norm-conserving pseudopotentials generated using the Troullier-Martins scheme,48 with atomic core and nonlocal components expressed in the fully separable form developed by Kleiman and Bylander,49-50 were used to represent the valence electrons. Sankey finite-range pseudo


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atomic or bitals (PAOs)51 were utilized as the split-valence double-ζ plus polarization basis set (DZP) for the valence electrons of all atoms involved. The Monkhorst-Pack scheme was used to sample the Brillouin zone,52 where the mesh of k space was sampled with 8×8×8. Electrochemical measurements. The as-synthesized samples (including MoSe2–NiSe nanohybrids, pure MoSe2, pure NiSe) were annealed at 400 °C for 30 min under flowing 5% H2/Ar to remove surfactants. The electrocatalytic properties were measured using a standard three-electrode system on an electrochemical workstation (CHI660E). A graphite rod was used as counter electrode, and a KCl-saturated Ag/AgCl electrode was used as reference electrodes, respectively. Typically, 4 mg of catalyst and 30 µL of Nafion solution (Sigma Aldrich, 5 wt%) were dispersed in 1.0 mL water-ethanol solution with volume ratio of 4:1 by sonicating for 1 h to form a homogeneous ink. Then 5 µL of the catalyst ink (containing 20 µg of catalyst) was loaded onto a glassy carbon electrode with 3 mm diameter (loading ~0.285 mg/cm2). The potentials reported in our work were referenced to the reversible hydrogen electrode (RHE): ERHE = EAg/AgCl + 0.059 × pH + 0.1988. The polarization curves were obtained by sweeping the potential from -0.3 to 0.2V vs RHE at room temperature with a sweep rate of 5 mV s−1. The electrochemical impedance spectroscopy (EIS) measurements were performed in the same configuration at overpotential η = 250 mV over a frequency range from 100 kHz to 0.01 Hz at the amplitude of 5 mV. Cyclic voltammetry (CV) was conducted between −0.3 and 0.2 V vs RHE at 50 mV s−1 to investigate the cycling stability. The current density (j) was based on geometric area of the electrode and all the polarization curves were corrected with iR-compensation that arose from the solution resistance. The resistance of 0.5M H2SO4 is about 10 Ω, which was determined by EIS. RESULTS AND DISCUSSION The representative approach of synthesizing MoSe2–NiSe nanohybrids is based on a colloidal epitaxial growth procedure as illustrated in Scheme 1, including preparation of MoSe2 flower-like nanosheets and the subsequent in situ growth of NiSe nanocrystallites onto the surface of MoSe2 nanosheets via a hot injection. Interestingly, our method is widely applicable for the growth of other metal selenides on MoSe2 nanoflowers at similar experimental conditions, such as MoSe2– Bi2Se3, MoSe2–CdSe and MoSe2–PbSe nanostructures (Figure S1 in supporting information). We believe that our facile and universal method could be a promising strategy towards the construction of novel 2D hybrid nanomaterials with well-

Scheme 1. Schematic illustration of the synthesis of MoSe2–NiSe nanohybrids.

Figure 1. (a) XRD patterns of the pure MoSe2 flower-like nanosheets, pure NiSe nanoplates and MoSe2–NiSe nanohybrids. (b) TEM image of the MoSe2–NiSe nanohybrids. (c) Bright-field and (d) dark-field TEM images of the MoSe2–NiSe nanohybrids.

defined structures and enhanced functionalities for various applications. The well-defined MoSe2–NiSe nanohybrids were successfully obtained through the colloidal epitaxial growth strategy, verified by systematic structural and morphological characterizations. As shown in Figure 1a, the XRD patterns reveal the coexistence of hexagonal MoSe2 phase and hexagonal NiSe phase in our obtained nanohybrids, which are in accordance with the standard JCPDS card, respectively. The morphology of the MoSe2–NiSe nanohybrids can be clearly observed by field-emission scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As depicted in Figure 1b and S2-3, it can be clearly seen that the MoSe2 flowerlike nanosheets are still retained and the NiSe nanocrystallites are well anchored on the surface of MoSe2 nanosheets with fine dispersion. No free NiSe nanocrystallites were found outside of the MoSe2 nanosheets. Even upon repeated washing and ultrasonication cycles the NiSe nanocrystallites could not be removed from the MoSe2 nanosheets, implying the strong conjunction between NiSe nanocrystallites and the supported MoSe2 nanosheets. Bright-field and dark-field TEM images (Figure 1c and d) further show that the NiSe nanocrystallites are embedded in the flower-like MoSe2 nanosheets. High-resolution TEM images further reveal the microscopic phase of the MoSe2-NiSe nanohybrids. As illustrated in Figure 2a-b and S4 a-b, two sets of parallel lattice fringes are distinctly observed, the interplanar distance of 0.31 nm agrees well with the (100) plane of hexagonal NiSe, while the interplanar distance of 0.28 nm can be indexed to the (100) plane of hexagonal MoSe2. The HRTEM images also suggest abundant defects in MoSe2 nanosheets. Meanwhile, the fast Fourier transform pattern (Figure 2a, inset) originated from HRTEM image (Figure 2a) shows two sets of well-aligned diffraction spots, which corresponded to MoSe2 and NiSe, respectively. Furthermore, cross-sectional HR-TEM images (Figure 2c and S4 c-d) show intimate contact between the MoSe2 nanosheets and the NiSe nanocrystallites. The observed lattice distance of about 0.65 nm corresponds to the (002) plane of hexagonal MoSe2. As for the in-situ formed NiSe nanocrystallites, the measured lattice distance is about 0.53 nm, in well consistence with the (001) planes of hexagonal NiSe. The crystallographic



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Figure 2. (a) HR-TEM image showing the MoSe2–NiSe heterostructural interface viewed parallel to the c axis of MoSe2 and fast Fourier transform electron diffraction (FFT-ED) patterns (inset). (b-c) Cross-sectional HR-TEM images of MoSe2-NiSe nanohybrids viewed perpendicularly to the c axis of MoSe2. (d) Schematic illustration of the epitaxial relationship between the NiSe nanocrystallites and MoSe2 nanosheets, projected along [001] zone axis/direction.

Figure 3. (a) HAADF-STEM image and corresponding EDS mapping images of the MoSe2–NiSe nanohybrids, showing the homogeneous distribution of Mo (yellow), Ni (red) and Se (cyan). (b) XPS spectra showing the binding energies of Mo 3d for MoSe2–NiSe nanohybrids and pure MoSe2. (c) XPS spectra showing the binding energies of Ni 2p for MoSe2–NiSe nanohybrids and pure NiSe. (d) Ni L3 and L2 EELS spectrum of MoSe2-NiSe nanohybrids and pure NiSe. The red arrow indicates the area where the EELS spectrum of MoSe2-NiSe nanohybrids was taken.


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Figure 4. Ultraviolet photoemission spectroscopy and energy band diagrams. (a) UPS spectra of the MoSe2 near the Fermi level energy and valence band maximum. (b-c) Onset level (Eonset) of the UPS spectra, for the MoSe2 and NiSe, respectively. (d) The calculated density of states (DOS) of NiSe. The Fermi level is set at 0 eV. (e) Schematic energy band diagrams of interfacial MoSe2/NiSe deduced by the UPS data in a-c. (f) The schematic illustration of the MoSe2–NiSe heterostructures and the electron transfer from the NiSe nanocrystallites to the MoSe2 substrate in the heterointerface.

orientation of NiSe nanocrystallites on the MoSe2 nanosheets is also visible in high-resolution HAADF-STEM images (Figure S5). The corresponding in-plan atomic illustration of MoSe2–NiSe heterointerface is schematically shown in Figure 2d. The epitaxial relationship in the MoSe2–NiSe system originates from the fact that the selenium layers in MoSe2 and NiSe exhibit the same hexagonal symmetry, with a lattice mismatch of about 10.17% (Table S1). All these results clearly demonstrate that the MoSe2–NiSe nanohybrids are successfully achieved with well-defined heterostructural interfaces (vertical heteronanostructures), which provides an ideal material model for further study of their intrinsic electrocatalytic properties. To further investigate the chemical composition and electronic structure of the MoSe2–NiSe nanohybrids, energy dispersive X-ray spectroscopy (EDS) mapping and X-ray photoelectron spectroscopy (XPS) analysis were performed. As shown in Figure 3a and S6, the HAADF-STEM image and corresponding EDS mapping analyses reveal uniform spatial distribution of Mo, Ni and Se in the MoSe2–NiSe nanohybrids. Both EDS and XPS results further confirm the existence of

Mo, Ni and Se in the as-synthesized nanohybrids (Figure S78). Figure 3b and 3c show the enlarged Mo 3d and Ni 2p XPS spectra of the MoSe2–NiSe nanohybrids, together with pure MoSe2 and pure NiSe samples, respectively. Notably, the binding energy of Mo 3d for the MoSe2–NiSe nanohybrids decreases by 0.3 eV compared with that for the pure MoSe2 (Figure S9), while the binding energy of Ni 2p for the MoSe2– NiSe nanohybrids increases by 1.0 eV compared with that for the pure NiSe (Figure S10). This change of the binding energy value for the MoSe2–NiSe nanohybrids means that there are electrons transferred from the NiSe nanocrystallites to theMoSe2 matrix. In order to further verify the electrons transfer from NiSe nanocrystallites to MoSe2 matrix, electron energy loss spectroscopy (EELS) was performed. As shown in Figure 3d, the EELS spectrum of Ni was performed on a pure NiSe and an individual NiSe nanocrystallite anchored on the MoSe2 nanosheets. The Ni L2,3 edge spectrum shows two peaks, L3 and L2, which associate with excited-electron transitions from 2p3/2 to 3d3/2, 3d5/2 and from 2p1/2 to 3d3/2, respectively. We can clearly see that the white-line intensity ratio L3/L2 of the MoSe2-NiSe nanohybrids (blue line) is slightly



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Figure 5. Electrocatalytic hydrogen evolution of different catalysts. (a) Polarization curves for HER on blank glassy carbon (GC) electrode and modified GC electrodes comprising MoSe2–NiSe nanohybrids, pure MoSe2, pure NiSe, physically mixed MoSe2 and NiSe (denoted as MoSe2+NiSe) and Pt. Catalyst loading is about 0.285 mg cm-2 for all samples. Sweep rate: 5 mV s-1. (b) Tafel plots of various catalysts derived from a. (c) Nyquist plots of various catalysts at an overpotential of 250 mV. Z' is real impedance and Z'' is imaginary impedance. The data were fitted using the modified Randles circuits shown in the insets. (d) Stability test of the MoSe2–NiSe nanohybrids catalyst.

higher than that of pure NiSe (orange line), implying the increased oxidation states of Ni in MoSe2-NiSe nanohybrids as compared to pure NiSe.53-54 Thus, the results of EELS are in well consistent with the Ni 2p XPS analysis, further confirming the electrons transfer from the NiSe nanocrystallites to the MoSe2 matrix. In order to further understand the changes in XPS and EELS spectra of the MoSe2–NiSe nanohybrids, ultraviolet photoelectron spectroscopy (UPS) experiments were carried out to determine the valence band (EV) and the work function (Φ) of MoSe2 and NiSe. As shown in Figure 4a, the valence band (EV) for MoSe2 is located at 1.06 eV below EF by linearly extrapolating the leading edge of the spectrum to the baseline. In addition, the Φ can be calculated using Φ = hν - Eonset, where hν is the incident photon energy (40.0 eV) and Eonset is the onset level related to the secondary electrons, as shown in Figure 4b-c. Hence, the Φ for MoSe2 and NiSe is 4.26 eV and 3.50 eV, respectively. The optical band gaps of the MoSe2 used in diagram are determined to be ~ 1.51 eV from literature.55 Meanwhile, we performed calculations at the level of density functional theory (DFT) for the NiSe crystal. Our calculations show that there exists density of states (DOS) near the Fermi level for NiSe, indicating that NiSe is intrinsically metallic (Figure 4d). Such a typical metallic behaviour of the pure NiSe is further reflected by the increased electrical resistivity as the temperature increases (Figure S11). Thus, we can schematically plot the energy band alignment diagram at the interface between MoSe2 and NiSe. As illustrated in Figure 4e, the Fermi level of NiSe is higher than that of MoSe2, which facilitates the electron injection from metallic NiSe into MoSe2 upon close contact to form heterostructures (Figure 4f). As a result, the injection of electrons to the MoSe2 nanosheets

can increase conductivity of the MoSe2–NiSe nanohybrids compared with the pure MoSe2 (Figure S11). The enhanced conductivity, therefore, facilitates fast electron transport between catalyst-electrolyte and catalyst-support electrode interfaces, which could further promote the electrochemical performance of HER. As expected, the MoSe2–NiSe nanohybrids exhibit high HER performance in 0.5M H2SO4. As shown in Figure 5a, the polarization curve of the MoSe2–NiSe nanohybrids exhibits a small onset potential of -150 mV versus the reversible hydrogen electrode (RHE) for the HER, beyond which the cathodic current increases rapidly. To achieve current densities of 1 and 10 mA cm-2, the MoSe2–NiSe nanohybrids require overpotentials of 160 and 210 mV, respectively. By contrast, pure MoSe2 exhibits inferior HER activity with a larger onset potential of -170 mV and lower catalytic current, while pure NiSe nanoplates only affected little HER activity. Also, the NiSe loading can be tuned by the injection of different amount of the precursors (Figure S12 and S13). To eliminate the influence of specific surface area variation on the HER activity, the MoSe2–NiSe nanohybrids, pure NiSe nanoplates and the MoSe2+NiSe mixture were further characterized by N2 adsorption−desorption isotherms. As illustrated in Figure S14 a-b, the specific surface area of the MoSe2–NiSe nanohybrids is calculated to be 67 m2 g-1, which is comparable to that of pure MoSe2 nanosheet with a BET surface area of 66.5 m2 g-1,40 while the specific surface area of pure NiSe nanoplates and the MoSe2+NiSe mixture is calculated to be 9.07 m2 g-1 and 47.83 m2 g-1, respectively (Figure S14 c-f). The HER polarization curves of all the samples have been normalized by the BET surface area of electrocatalysts (Figure S15).


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Obviously, the specific current density of MoSe2–NiSe nanohybrids is indeed higher than that of pure MoSe2, pure NiSe and MoSe2+NiSe mixture under the same overpotential, also presents a much earlier onset potential, directly confirming that the MoSe2–NiSe nanohybrids own a higher intrinsic electrocatalytic activity for HER than that of other comparisons. Furthermore, electrochemical double-layer capacitances (Cdl) are measured to evaluate the effective surface area of various catalysts (Figure S16). As shown in Table S2, MoSe2–NiSe nanohybrids exhibit much larger Cdl of 7.25 µF than the other counterparts, indicating the high exposure of effective active sites, which is responsible for the excellent HER activity. The HER kinetics of the above catalysts is probed by corresponding Tafel plots (η vs log j) (Fig. 5b). The Tafel slopes can be determined by fitting the linear portions of the Tafel plots to the Tafel equation (η = b log j + a, where j is the current density and b is the Tafel slope). A Tafel slope of about 56 mV decade-1 is observed for the MoSe2–NiSe nanohybrids catalyst, which is much smaller than the slope observed for the pure MoSe2 (95 mV decade-1), pure NiSe (118 mV decade-1), and MoSe2+NiSe mixture (74 mV decade-1). The value suggests that the Volmer–Heyrovsky mechanism takes effect in the HER of MoSe2–NiSe nanohybrids.8,11 In principle, a lower Tafel slope means that a catalyst requires a lower applied overpotential to generate a required current. Therefore, the MoSe2–NiSe nanohybrids with the smallest slope value display the best activity for HER. The high electrode kinetic metrics (including onset potential of -150 mV and the Tafel slope of 56 mV per decade) highlight the exceptional H2 evolving efficiency of the new MoSe2–NiSe nanohybrids catalyst (the detailed analysis of underlying mechanism as seen in SI). Furthermore, calculated turnover frequency (TOF)56 for each active site of MoSe2–NiSe nanohybrids reaches 5.6 s−1 at η = 250 mV and pH = 7, which is much higher than the TOF value of other samples (Figure S17 and Table S2), indicating the better intrinsic catalytic activity. To further prove better HER electrocatalytic efficiency of the MoSe2–NiSe nanohybrids, electrochemical impedance spectroscopy (EIS) was carried out to study the electrode kinetics under HER condition. The obtained Nyquist plot is shown in Figure 5c. The semicircle in the high-frequency range of the Nyquist plot attributes to the charge-transfer resistance (Rct). Generally speaking, Rct is related to the electrocatalytic kinetics and a lower value corresponds to a faster reaction rate. The Rct value of the MoSe2–NiSe nanohybrids electrode is the smallest one among all four catalysts (Table S2), indicating the fastest charge transfer process. To assess the durability of the MoSe2–NiSe nanohybrids electrocatalyst, accelerated linear potential sweeps were conducted repeatedly on the electrodes at a scan rate of 50 mV s-1. After a long-term cycling, the catalyst shows similar polarization curve as before with slight decay of current density (Figure 5d). The delamination of the catalyst from the electrode or the irreversible reaction of loaded NiSe in acidic medium presumably contributes to the slight loss in catalytic activity.16,27

CONCLUSIONS In summary, we have firstly demonstrated the construction of MoSe2–NiSe nanohybrids with well-defined heterointerfaces as an efficient electrocatalysts for HER, via in-situ colloidal epitaxial growth strategy. The colloidal pathway could also be extended as a universal strategy to prepare other MoSe2-based

heterostructures. Compared with the pure MoSe2 and pure NiSe, the MoSe2–NiSe nanohybrids catalyst exhibits excellent HER catalytic properties in acidic electrolyte with a low onset potential of -150 mV, a large cathodic current density (10 mA cm-2 at an overpotential of ~ 210 mV) and a small Tafel slope of 56 mV per decade. The enhanced HER performances have been attributed to the accelerated electrochemical reaction process resulting from the synergistic interaction between the MoSe2 substrate and anchored NiSe nanocrystallites. Furthermore, the increased conductivity of MoSe2–NiSe nanohybrids due to the electrons transfer from the loaded NiSe to the MoSe2 nanosheets also promotes the HER performance. This work provides new insights into the construction of high performance HER electrocatalysts from the view of kinetic process, opening the door for designing new efficient composite catalysts in the future.

ASSOCIATED CONTENT Supporting Information. TEM images and XRD patterns of other MoSe2-based heterostructures; calculation of lattice mismatch; SEM images, high-resolution HAADF-STEM images, EDS spectrum and XPS spectra of the MoSe2–NiSe nanohybrids; SEM image and TEM image of pure MoSe2 nanoflowers; TEM images of pure NiSe nanoplates; temperature dependent electrical resistivity; BET and additional electrochemical date; the analysis of HER mechanism. This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author *[email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Nature Science Foundation of China (51271173, 21571166, 21071136) and the National Basic Research Program of China (2012CB922001).

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