First-Principle and Experiment Framework for Charge Distribution at

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First-Principle and Experiment Framework for Charge Distribution at the Interface of the Molybdenum Dichalcogenide Hybrid for Enhanced Electrochemical Hydrogen Generation Xiang Lei,† Ke Yu,*,†,‡ Honglin Li,† Zheng Tang,† and Ziqiang Zhu† †

Key Laboratory of Polar Materials and Devices (Ministry of Education of China), Department of Electronic Engineering, East China Normal University, Shanghai 200241, People’s Republic of China ‡ Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi 030006, People’s Republic of China ABSTRACT: In this work, we present a novel hybrid structure, which has been produced by compositing MoSe2 on MoS2. This promising composite originating from two types of two-dimensional (2D) transition-metal chalcogenides can be used as electrocatalyst for catalyzing protons to hydrogen via the so-called hydrogen evolution reaction (HER), in which MoSe2 can play a role as cocatalyst. The crystal structure and morphology have been systematically studied by SEM, TEM, XPS, and XRD technologies. Then, density functional theory (DFT) based first-principle investigation was employed to accurately describe the electronic redistributing behavior by analyzing charge density change at the interface of the hybrid, which could provide theoretical support for material structure design. The optimized Gibbs free energy change and contracted band structure of the hybrid predicted the enhancement on HER activity and have been proved by corresponding experiments. According to HER measurements, enhanced output current density, small Tafel slope, and long-term stability have been achieved on this hybrid owing to compositing MoSe2 cocatalyst on MoS2. Both theoretical and experimental investigation in this work elucidate the potential of this hybrid for electrochemical hydrogen generation and pave a promising pathway for electrocatalyst design solution. exist between chalcogen layers,7 illustrate a favorable electrochemical property and various electrochemical applications due to the different numbers of d-electrons of transition-metal elements.8,9 Focusing our attention on earth-abundant molybdenum disulfide (MoS2),10,11 it has been considered as an ideal HER catalyst according to computational and experimental studies.12 It is commonly believed that HER ability of MoS2 mainly derives from S−Mo−S edges, which are active for hydrogen atom adsorption.13,14 There are some existing difficulties such as few exposed active edges and low conductivity on MoS2, which hinders further development of this promising catalyst material.15 Composing MoS2 with other cocatalyst functional material is considered an efficient way to promote H2 production through the HER process. Previously, MoO3/MoS2,16 MoS2/CoSe2,17 and MoS2/metal18−20 compositing systems have been successfully fabricated for HER and achieved enhanced performance of increased current density and decreased Tafel slope. Recently, a semiconducting TMD of MoSe2 with the same atomic structure as MoS2 has been applied in electrochemical H2 production.21−23 The intrinsic narrow bandgap decides the MoSe2 can be more conductive

1. INTRODUCTION Recently, renewable resources are playing a more and more important role in our energy demand with the pressing pressure of global environmental issues.1 In order to ensure the continuously increasing demands of energy supply, it is such an urgent task to find an efficient means for renewable energy storage.2 Generally, an appealing way to solve this problem is the conversion of electricity to hydrogen with the hydrogen evolution reaction (HER), in which water is reduced to molecular hydrogen via an electrochemical process.3,4 Using catalysts is an available and economic method to avoid the problem of a large overpotential. Conventionally, noble metals, such as platinum and its alloys, are the most active catalysts for HER and can dramatically enhance the reaction rate at a very low overpotential for high efficiency energy conversion.5 However, the high material price and poor natural abundance of those noble metals restrict the widespread applications of the electrochemical water splitting technique.6 Therefore, it results in a hot issue of studies to explore low-cost alternatives based on earth-abundant elements such as HER catalysts that are highly active and chemically stable. The layered transition-metal dichalcogenides (TMDs), which consist of a “sandwich” structure of a transition-metal layer (e.g., Mo, W, Nb) between two chalcogen layers (e.g., S, Se, Te), while van der Waals (vdW) interactions along the z axis © XXXX American Chemical Society

Received: May 19, 2016 Revised: June 25, 2016

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DOI: 10.1021/acs.jpcc.6b05076 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 1. (a) and (b) SEM images of the pristine MoS2 under different magnifications. (c) TEM and (d) HRTEM images of the pristine MoS2.

than MoS2.24,25 These works point to the possibility to access new and efficient HER catalysts by combining the promising MoSe2 and MoS2. In this paper, we reported the synthesized catalyst of the MoS2-coated MoSe2 (defined as MoSe2@MoS2) hybrid with excellent HER performance and mechanism analysis, and several analysis methods are employed to characterize the morphology and structure of this hybrid material obtained by a facile hydrothermal process. In order to theoretically explain the HER activity and charge transfer at the interface between molybdenum dichalcogenides, precise analysis of the charge density variation at the hybrid interface has been innovatively applied to provide an accurate description of the electronic redistribution and band structure contraction in terms of hybrid functional calculations. The MoSe2@MoS2 hybrid shows a significantly boosted current density with flat Tafel slope and slight output loss after a long-term measurement. These results suggest the MoSe2@MoS2 hybrid could be considered as a nonnovel metal HER catalyst, and our work would extend the TMD-based electrocatalyst researching field.

of as-prepared MoS2 powders were dispersed in 75 mL of deionized water, and then 5 mL of 50% N2H2·4H2O was added to yield a dark solution with the S:Se ratio of 10:1 under constant stirring. This solution was transferred into a 100 mL Teflon-lined stainless-steel autoclave, which was sealed and heated at 180 °C for 20 h. Therefore, the MoSe2@MoS2 hybrid was obtained, and the dark results were collected, washed, and dried via the methods described above. Meanwhile, pristine MoS2 and pure MoSe2 were synthesized by the same method described above. All the as-prepared products were annealed in argon at 600 °C for 2 h to remove the organic residue. 2.2. Sample Characterization. The field emission scanning electron microscopy (FESEM, JEQL-JSM-6700F) at an accelerating voltage of 200 kV and transmission voltage electron microscopy (TEM, JEOL-JEM-2100) were applied to characterize the structures and morphologies of the as-prepared products at an accelerating voltage of 200 kV. Samples for the TEM analysis were prepared by drying a drop of cyclohexane solution containing the nanomaterials on the surface of a carbon-coated copper grid. The lattice structures of the different samples were characterized by X-ray diffraction (D8 Advance/BRUKER AXS GMBH) with Cu Kα radiation (λ = 0.1541 nm) in the 2θ scanning range from 10° to 70° to collect diffraction data. X-ray photoelectron spectroscopic (XPS) measurements were performed on a Thermo Scientific ESCALAB 250Xi spectrometer to give information on elements and structure, and the binding energies were referenced to the C 1s of the carbon contaminants at 284.7 eV during those tests. 2.3. Computational Methods. In this work, we carried out the first-principles density functional theory (DFT) calculations to give the theoretical basis of the MoSe2@MoS2 hybrid, which has been widely used for the calculation of TMDs.26 The DFT-based calculations were all performed within projector-augmented-wave (PAW) pseudopotentials,27 as implemented in the Vienna ab initio Simulation Package (VASP) code.28 We consider the Mo orbitals 4p6 4d5 5s1 and the chalcogen orbitals 3s2 3p4 for S and 4s2 4p4 for Se as valence states. Then, we used the generalized gradient approximation

2. EXPERIMENTAL SECTION 2.1. Synthesis of MoSe2@MoS2 Hybrid. In this work, the MoSe2@MoS2 hybrid was synthesized via hydrothermal routes. All the chemical reagents were of analytical grade and used without further purification. In brief, 0.50 g of Na2MoO4·2H2O, 0.79 g of CN2H4S, and 0.33 g of oxalic acid were added to a 100 mL beaker with 80 mL of deionized water to form a homogeneous colorless solution after magnetic stirring for 30 min. Then, the as-prepared transparent solution was transferred into a 100 mL Teflon-lined stainless-steel autoclave which was sealed and maintained at 200 °C for 24 h following. After aircooled to room temperature naturally, the resulting black precipitates were collected and washed with ethanol and deionized water several times alternately. The suspension was centrifuged and dried at 60 °C for 6 h to obtain black powders. For the composition of MoSe2 on MoS2, 1 mmol of (NH4)6Mo7O24·4H2O, 1 mmol of Se powders, and 10 mmol B

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Figure 2. (a) and (b) SEM images of the MoSe2@MoS2 hybrid under different magnifications. (c) Schematic diagram for the formation processes of MoS2 nanoflower and MoSe2@MoS2 hybrid, respectively. The hybrid assembled from the MoSe2 nanosheet anchored on a prepared MoS2 nanoflower. (d) and (e) TEM images of the pristine MoS2 under different magnifications. (f) HRTEM image of an area of the surface of MoSe2@ MoS2 hybrid. (g) The energy-dispersive spectrometer (EDS) mapping images of Mo, S, and Se around composite section. The white boxes denote the scan area of the element distribution.

to form a homogeneous ink. Then 5 μL of the dispersion (containing 20 μg of catalyst) was loaded onto the work electrode with polishing preprocessing. All measurements were carried out at the same optimized loading weight of 0.285 mg/ cm2. Linear sweep voltammetry beginning at +0.3 V and ending at −0.3 V vs RHE with a scan rate of 2 mV/s was conducted. Cyclic voltammetry (CV) was conducted between 0.4 and 0.5 V vs RHE at the scan rate of 20, 40, 60, mV/s etc. to investigate the cycling stability for 200 cycles at room temperature with the same standard three-electrode setup. Electrochemical impedance spectroscopy (EIS) was performed when the working electrode was biased at a constant value of 0.7 V vs RHE while sweeping a frequency from 100 000 to 0.01 Hz. The timedependent current density curve of MoS2, MoSe2, and MoSe2@ MoS2 hybrid was conducted under a static overpotential of −0.3 V vs RHE for the continuous operation for 12 h.

(GGA) with the parametrization of Perdew−Burke−Ernzerhof (PBE)29 to deal with the exchange and correlation potentials. As an influential issue to the structure of MoSe2 and MoS2 layers, the added vdW interaction was taken into account by the DFT+D/PBE method proposed by Grimme.30 A Monkhorst− Pack k-point grid of 5 × 5 × 1 was used for the geometry optimization as the Brillouin zone k-point mesh, while a kinetic energy cutoff of 500 eV for the plane wave expansion was used for the calculations. The convergence criterion for energy between two consecutive steps was chosen to be 10−5 eV. and atomic positions are relaxed until the maximum Hellmann− Feynman forces have converged to 0.02 eV/Å. A 15 Å vacuum layer between two-dimensional single layers was introduced to avoid interlayer interactions in the out-of-plane direction. 2.4. Electrocatalytic Measurements for the HER. All of the electrochemical measurements were performed on a CHI660E electrochemical workstation in a three-electrode system with a saturated calomel electrode (SCE, Hg/HgCl2 in saturated KCl) as the reference electrode, a Pt wire as the counter electrode, and a glassy carbon electrode (GCE) with 3 mm diameter loading different catalyzing materials of MoS2, MoSe2, MoSe2@MoS2, and Pt/C (15%) as the working electrode. All the potentials were converted to values with reference to a reversible hydrogen electrode (RHE) from SCE (V vs RHE = V vs SCE + 0.26 V). HER performances were conducted in 0.5 M H2SO4 electrolyte (purged with pure N2 for 30 min). Generally, 4 mg of as-prepared catalyst and 30 μL of Nafion solution (5 wt %) were dispersed in the mixed solution of 0.8 mL of water and 0.2 mL of isopropanol by sonicating for 0.5 h

3. RESULTS AND DISCUSSION 3.1. Characterization. Figure 1 presents the morphology and microscopic structures of the pristine MoS2 characterized by SEM and TEM images. The morphology of pristine MoS2 can be described as the flower-like sphere which consists of a large number of petals according to the sequence of SEM images at various magnifications in Figure 1a and b. As shown in Figure 1a, each MoS2 nanoflower has an average diameter of 1−2 μm. Figure 2b is a high magnification SEM image, which shows that abundant petals appear on the surface of nanoflowers. The TEM and HRTEM measurements are emerged in Figure 1 to further investigate the formation C

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Figure 3. (a) XRD patterns of the pristine MoS2, pure MoSe2, and MoSe2@MoS2 hybrid. Inset is an enlargement of an area in (a). (b) Typical overall XPS pattern. (c) C 1s spectrum of pristine MoS2 and MoSe2@MoS2 hybrid. XPS spectra of (d) Mo 3d, (e) S 2p, and (f) Se 3d signals recorded for the MoSe2@MoS2 hybrid. The insets illustrate corresponding regions of pristine MoS2 and MoSe2@MoS2 hybrid in (d−f).

consistent with the SEM plots (Figure 2e illustrated the enlargement marked area in Figure 2d). Besides, layeredstructure stacking by isolated TMD monolayers with different interlayer distance in the HRTEM image of the MoSe2@MoS2 hybrid is illustrated in Figure 2f. For the section we used the right bubble to indicate the presence of an interlayer distance of 0.648 nm (the average value for 5 layers), which could be certainly consistent with 2H-MoSe2. Meanwhile, the measured interlayer separation of 0.635 nm (the average value for 5 layers) outlined in the left bubble exhibits a broadened lattice spacing when comparing to pristine MoS2 (0.62 nm, from Figure 1c), which could be attributed to the compositing of MoSe2 with wide lattice spacing. By the analysis of TEM, it can be concluded that the lattice structure of MoS2 can be influenced via the synthesis of a hybrid. Next, a typical EDS mapping of the MoSe2@MoS2 hybrid was carried out to investigate the chemical composition of the products. The TEM image and corresponding EDS mapping analyses (Figure 2g) reveal the distribution of Mo, S, and Se in this hybrid structure. Figure 3a shows the X-ray diffraction (XRD) patterns of the MoSe2@MoS2 hybrid as well as pristine MoS2 and pure MoSe2. The diffraction peaks indicated in red and blue curves correspond to the typical hexagonal MoS2 (a = b = 0.316 nm, c = 1.229 nm, JCPDS: 37-1492) and hexagonal MoSe2 (a = b = 0.329 nm, c = 1.292 nm, JCPDS: 29-0914), respectively. Then, with the introduction of MoSe2 compositing layers, the peak around 14° index to the (002) plane for TMDs shifts left compared with the pristine one, illustrating the broadening of interlayer distance, in which it is corresponding to the TEM results. Meanwhile, the chemical compositions and binding states for the MoSe2@MoS2 hybrid were investigated using Xray photoelectron spectroscopy (XPS). XPS analysis of MoS2 and the MoSe2@MoS2 hybrid shows the presence of all the constituents in Figure 3b, and the two survey spectra shared the same peak of C 1s at 284.7 eV (as shown in Figure 3c). Comparing to the spectrum of MoS2, a distinct peak at 178.7 eV in the spectrum of the MoSe2@MoS2 hybrid refers to the Se Auger, indicating MoSe2 compositing. Then, the high-

features and crystallinity of the MoS2 nanostructure. In Figure 1c of TEM, it can be seen that each section of the nanoflowers presenting a shape of a petal is actually an individual stack of 2D MoS2 thin layers. A large amount of active sites can be attributed to widely distributed petals, which would offer much more active sites for HER. The insertion of a high-resolution TEM image in Figure 1c exhibits that the pristine MoS2 was grown in high density with an interlayer separation of 0.62 nm, which is consistent with the value of MoS2 interlayer spacing of the (002) plane, and the atomic spacing of MoS2 is obtained from the average values for five layers with 3.10 nm to reduce biases. The selected area electron diffraction (SAED) results reflect the (101), (103), and (110) planes of 2H-MoS2 clearly in the inset of Figure 1d. All the measurement results present the evidence to back the preparation of 2H-MoS2. SEM and TEM images of the as-prepared MoSe2@MoS2 hybrid are illustrated in Figure 2. As reflected in the SEM images with different magnifications in Figure 2a and b, the MoSe2 nanosheets grew on the surface of the spherical MoS2 to form a hybrid structure. It can be observed that obvious distinction exists in the surface of the sphere after the MoSe2 compositing process when comparing Figure 2b to the inset of Figure 1b. The MoSe2 nanosheets are in a more loose state, while MoS2-based petals are tightly aggregated and point toward a common inner center. Figure 2c indicates the formation processes of the MoS2 nanoflower and MoSe2@ MoS2 hybrid, respectively. In Step 1, Mo4+ bound with S2− to form single-layerd MoS2, and several MoS2 layers stack via vdW interaction and result in a MoS2 petal. A large amount of petals align together to fabricate a spherical structure. The MoSe2 nanosheets prepare via stacking monolayer MoSe2 originating from a binding interaction between Mo4+ and Se2− in the following Step 2. Adding obtained MoS2 as the extra raw material, formed MoSe2 nanosheets well anchor on the surface of the spherical product, and the end product of the MoSe2@ MoS2 hybrid finally fabricates. Figure 2d and e shows the low-magnification TEM image of the MoSe2@MoS2 hybrid. It can be observed that mass nanosheets anchored on the surface of spheres, which is D

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The Journal of Physical Chemistry C resolution spectra of the MoSe2@MoS2 hybrid as well as MoS2 are illustrated in Figure 3d−f. From Figure 3d, the binding energies of Mo 3d3/2 and Mo 3d5/2 orbitals located at 231.6 and 228.3 eV indicate that the elemental chemical state of Mo is mainly the Mo4+ oxidation state in the hexagonal 2H phase of MoSe2 and MoS2.31 Remarkably, as shown in the inset of Figure 3d, note that the peaks corresponding to the MoSe2@ MoS2 hybrid move to low binding energy when comparing to pure MoS2 (Mo 3d3/2 and Mo 3d5/2 orbits locate at 232.4 and 229.2 eV, respectively). It is believed that the displacement of Mo4+ 3d peaks can be caused by decreasing of the Mo oxidation state originating from compositing MoSe2 among MoS2.32 Generally, electrons on the d orbital in the Mo transition metal element play an influential role for electrocatalysis affecting the bond with adsorbates.33 Thus, we can modify the HER performance of MoS2 by forming a MoSe2@MoS2 hybrid, and the S 2p level at 225.7 eV is also observed in this plot. The 2p peak of the S element is split into well-defined 2p1/2 and 2p3/2 peaks at 163.0 and 162.3 eV, corresponding to the S2− oxidation state (Figure 3e). It is noteworthy that the Se 2p orbital appears in that area and has an obvious influence on the S 2p levels when compared to the results of pure MoS2 in the inset of Figure 3d. Two fitted peaks at 55.4 and 54.5 eV (Figure 3f) attributable to the core levels of Se 3d3/2 and Se 3d5/2, respectively, are characteristic of Se2− of the MoSe2@MoS2 hybrid.34 All of the above characterization results prove the success of the composition of MoSe2 on MoS2. 3.2. First-Principle Investigation. According to SEM, TEM, and XRD measurements, the MoSe2@MoS2 hybrid has been successfully synthesized via hydrothermal routes, and a lower binding energy of d orbital electrons in the Mo element results in the hybrid in attractive potential in electrocatalysis when considering the compositing of MoSe2 on MoS2, as reflected from XPS measurements. First-principle investigations are herein employed to further verify our hypothesis. Figure 4a to c shows the band structures of pristine MoS2, pure MoSe2, and MoSe2@MoS2 hybrid, respectively. The MoSe2@MoS2 hybrid exhibits a narrower bandgap (0.75 eV) in comparison with pristine MoS2 (1.34 eV) and pure MoSe2 (1.18 eV), and the bandgap value well matches with the calculating results for their bulk structure.35 All band structure calculations are based on the DFT+D/PBE approach. A greatly reduced bandgap can be obtained with the introduction of MoSe2 compositing layers, depending on the results of the comparison. It is noteworthy that the bandgap of the hybrid is smaller than that of both MoS2 and MoSe2, implying an easier electron transfer from the valence band maximum (VBM) to the conduction band minimum (CBM) and a better conductivity when comparing to MoS2 and MoSe2. The corresponding calculated total density of states (DOS) is carried out to further elucidate the variation of band structure among various catalysts. As shown in Figure 4d, the band gap of the MoSe2@MoS2 hybrid has thinned about 0.6 eV with respect to that of the pristine MoS2, indicating that the existence of MoSe2 on MoS2 could result in more charge carriers and higher conductivity of catalyst, and the difference of binding energy between d-orbit-Mo/p-orbit-S and d-orbit-Mo/d-orbit-Se would be responsible for the narrowed bandgap. Besides, we have further calculated the charge density distributions and charge density difference investigation of MoSe2@MoS2 to directly visualize the influence of the electronic structure induced by compositing MoSe2 as well as provide much in-depth information on the interaction between MoS2 and MoSe2. In Figure 5a, the isosurface of charge

Figure 4. Band structures for (a) pristine MoS2, (b) pure MoSe2, and (c) MoSe2@MoS2 hybrid and total density of states for (d) pristine MoS2, (e) pure MoSe2, and (f) MoSe2@MoS2 hybrid. The yellow shading clearly indicates the decrease of bandgap after MoSe2 composition. The Fermi energy EF is set to zero in all of the four panels and is indicated by the black dashed lines.

Figure 5. (a) Charge density plot for the MoSe2@MoS2 hybrid. Black lines represent the contour lines of the charge density. (b) The 3D different charge density distribution of the MoSe2@MoS2 hybrid along the (001) direction. The cyan and yellow regions represent the charge depletion and accumulation space, respectively.

E

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MoSe2@MoS2 hybrid as well as pristine MoS2 and pure MoSe2 were performed in 0.5 M H2SO4 electrolyte using the above three-electrode system to support our proposition obtained by theoretical calculating basis. Figure 6a represents the iR-

densities at the interface between MoS2 and MoSe2 is shown. The density is plotted in a plane formed by Mo, S, and Se atoms, and along a contour the electron density is constant. A positive value (red) indicates electron accumulation, and a negative value (blue) denotes electron depletion. These charge distributions clearly show the electron accumulation in the middle region of MoS2 and MoSe2. Figure 5b shows the charge density difference along the (001) direction, which can quantitatively estimate charge redistribution. The charge density difference is defined as Δn(r) = n MoSe2 @MoS2(r) − n MoS2(r) − n MoSe2(r)

(1)

where nMoSe2@MoS2(r), nMoS2(r), and nMoSe2(r) are the electron densities of MoSe2@MoS2, MoS2, and MoSe2, respectively.36 All atoms in this calculating structure were kept at the same positions as they were in the integral structure in computing of nMoS2(r) and nMoSe2(r). The charge depletion and accumulation spaces are revealed in cyan and yellow, respectively. First, we note that this redistribution of charge is limited to a localization contacting region between MoS2 and MoSe2, surrounding Se and S atoms. Furthermore, according to this three-dimensional (3D) figure, positive space represents a gain of charge for the formation of the integral part, while the negative one denotes a loss of charge, which objectively generates an efficient directional transfer of electrical charge from MoSe2 to MoS2; it is consistent with charge density distribution illustration. Then, a built-in electric field further appears on the basis of the directional movement of charge. This electrical coupling will facilitate the following rapid electron transfer from electrode to MoSe2 and then to MoS2, forming an excessive negative charge density. Generally, the Gibbs free energy change for hydrogen adsorption (ΔG) on the edges of the catalysts can be used to judge the activity of the catalysts.37,38 It is commonly believed that a thermo-neutral situation with ΔG = 0 eV can offer catalyst an ideal reaction environment for HER because an endothermic hydrogen adsorption could suppress the generation of the surface Hads, while an exothermic could make it difficult for Hads to form H2.39 Considering the charge transfer from MoSe2 to MoS2, the resulting excessive negative charge density can effectively prompt the HER process by adjusting ΔG shifting to the thermo-neutral situation and finally optimize the total HER process.40 The ΔG at Mo-edge, S-edge, and basal plane for individual MoS2 are equal to 0.04, 0.20, and 2.05 eV, respectively. As for the individual MoSe2, the ΔG of M-edge and Se-edge are 0.06 and 0.13 eV, while we calculated a 1.69 eV ΔG for the basal plane. The calculated ΔG of the two individual materials are in good accord with the previous theoretical values of MoS215,37 and can be compatible to other TMDs.41,42 This also ensures the credibility of our calculations. The electron transfers from MoSe2 to MoS2 can facilitate HER activity by optimizing ΔG to provide an accelerated proton transformation. According to the prediction of the theoretical calculation, we expect the MoSe2@MoS2 hybrid could represent enhanced HER performance by higher cathodic current density and lower charge-transfer resistance because of contracted bandgap and balanced Gibbs free energy owing to promoted charge transfer among catalysts with the introduction of MoSe2 cocatalyst. 3.3. HER Electrochemical Measurements. The theoretical calculation forecasts that the MoSe2@MoS2 hybrid has more activities as a HER catalyst with the cocatalyst of MoSe2. Then the HER electrochemical measurements of the as-obtained

Figure 6. (a) Polarization curves and (b) the corresponding Tafel plots of the pristine MoS2, pure MoSe2, the MoSe2@MoS2 hybrid, and Pt/C (15%). (c) Stability test of the MoSe2@MoS2 composite electrocatalyst. Time dependence of the current density under a constant overpotential of 0.30 V vs RHE. (d) An enlargement of the boxed area in (c) and (e) polarization data for MoSe2@MoS2 hybrid sample sweeps between −0.30 and 0 V vs RHE with a scan rate of 0.1 V/s, showing the current density changes after 1000 CV cycles. All the measurements were performed in N2-saturated 0.5 M H2SO4 electrolyte. (f) Schematic illustration of the atomic structure of MoSe2@MoS2 hybrid and the synergistic effect of MoSe2 cocatalyst in HER routine.

corrected linear sweep voltammograms (LSVs) of MoS2, MoSe2, MoSe2@MoS2 hybrid, and Pt/C. The cathodic current density (j), which is considered as an important evaluating criterion for HER activity, reaches 35 mA/cm2 at an overpotential (η) around −300 mV for the MoSe2@MoS2 hybrid, whereas the pristine MoS2 and pure MoSe2 show poor catalytic activity (15 and 21 mA/cm2 at an overpotential around −300 mV, respectively). The hybrid structure shows a litter higher onset value of −175 mV vs RHE than that of pristine MoS2 (−145 mV vs RHE), while it is much lower than pure MoSe2 (−220 mV vs RHE). Meanwhile, significant H2 evolution (j = 10 mA/cm2) was observed at −220 mV vs RHE, which was much smaller than MoS2 (−260 mV vs RHE), suggesting an excellent enhancement on catalytic activity of the MoSe2@MoS2 hybrid. Furthermore, we performed Tafel analysis of the HER polarization curves according to calculations, as shown in Figure 6b. The linear regions were fit by the Tafel equation (η = b·log j + a, where b is the Tafel slope43,44) with the slope information extracted in the plot. In our work, the pristine MoS2 exhibits a Tafel slope of 93 mV/ F

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investigate the behavior of electrodes under HER operating circumstances (Figure 7a). The Nyquist plots reveal a decrease

decade, close to many values for 2H-MoS2 in the previous work.45,46 The corresponding Tafel plots indicate that the MoSe2@MoS2 hybrid possesses a smaller Tafel slope (65 mV/ decade) than pristine MoS2. Generally, three principle steps for converting H+ to H2 have been proposed for HER in acidic solution.37 The first step is a discharge step (Volmer reaction) H3O+ + e− → Hads + H 2O

(2)

in which protons (H3O+) are adsorbed onto the active sites on the surface of the catalyst and combined with electrons (e−) to form adsorbed hydrogen atoms (Hads) and a slope of 120 mV/ decade should be observed. Then, the recombination step (Tafel reaction) or electrochemical desorption step (Heyrovsky reaction) are the following Hads + Hads → H 2 −

Hads + H3O + e → H 2 + H 2O

(3) (4)

where Tafel slopes of 30 and 40 mV/decade can be obtained, respectively. In the present work, the Tafel slope is 65 mV/ decade for the MoSe2@MoS2 hybrid, illustrating the Heyrovsky reaction plays a dominant role in determining the HER rate of this catalyst material, and the Tafel slope of 65 mV/decade in this work is indeed close to the values of 3D MoS2/MoSe2 nanosheet−graphene networks47 and MoS2/MoSe2 host lattice.48 Meanwhile, the chronoamperometry measurement was employed to check the long-term stability of the MoSe2@ MoS2 hybrid catalyst loaded on GCE. This process was conducted in 0.5 M H2SO4 for 12 h on the static overpotential of −300 mV vs RHE, resulting in a continuous HER process occurring to generate molecular H2. As shown in Figure 6c, the serrate current density of the MoSe2@MoS2 hybrid decreases gradually in the first 4 h, and then the reduction starts to slow and remains at a stable value of ∼28 mA/cm2 even after a long period of 12 h. It could be concluded that the current density presents only slight degradation even after a long period of 40 000 s because the reaction would be inhibited due to the consumption of H+ in electrolyte. H2 bubble release and accumulation are alternate during the generating process, and it causes a typical serrate shaped output.49 In detail, accumulating bubbles closes catalyst off the electrolyte to form a decreasing output, and then current density mutates to the initial value after bubble release (see Figure 6d).To further investigate the stability in acidic environment, CV of the MoSe2@MoS2 hybrid-modified GCE was conducted for 1000 cycles from −0.4 to 0.2 V vs RHE at 100 mV/s, and the LSV measurements were performed and illustrated in Figure 6e. The negligible difference between the curves measured before and after 1000 cycles can be seen, suggesting the good durability of asprepared hybrid. All in all, both the LSV measurements at different cycles and chronoamperometry measurement demonstrate the good chemical stability of the as-prepared hybrid catalyst. According to first-principle calculations and HER measurements, we introduce a schematic analysis to illustrate the effect of MoSe2 cocatalyst for this hybrid structure in HER routine. As shown Figure 6f, an excessive negative charge density originated from the electron transfer from MoSe2 to MoS2 to adjust the Gibbs free energy and optimize the HER process by promoting proton transformation into H2 on the active edges of MoS2. Besides the current density and Tafel slope, the electrochemical impedance spectroscopy (EIS) is performed to

Figure 7. (a) EIS Nyquist plots. The inset of (a) shows the highfrequency Nyquist plots of this three catalysts. Cyclic voltammetry curves of the (b) pristine MoS2 and (c) MoSe2@MoS2 hybrid electrocatalysts at various scan rates (20−200 mV/s) in the region of 0.15−0.25 V vs RHE. (d) The differences in current density variation (Δj = ja − jc) at an overpotential of 0.20 V plotted against scan rate fitted to a linear regression enable the estimation of Cdl and relative electrochemically active surface area for the pristine MoS2 and MoSe2@MoS2 hybrid electrocatalysts.

in the charge-transfer resistance (Rct) for the MoSe2@MoS2 hybrid (46 Ω) compared to the pristine MoS2 (360 Ω) and pure MoSe2 (160 Ω), indicating a smaller reaction resistance and higher catalytic activity for the HER in acid solution for the MoSe2@MoS2 hybrid electrode. This change in the behavior of electrochemistry also corresponds well with the above band structure and DOS calculating results. Next, cyclic voltammetry (CV) was performed at various scan rates (20, 40, and 60 mV/ s, etc.) in 0.15−0.25 V vs RHE region to estimate the effective active surface area of different samples by measuring the double-layer capacitance (Cdl), and the Cdl is proportional to the surface area. The typical CV plots of pristine MoS2 and MoSe2@MoS2 hybrid are presented in Figure 7b and c, respectively. The double-layer capacitance is estimated by plotting the current density differences of Δj = ja − jc at 0.20 V vs RHE against the scan rate, where the double-layer capacitance value of Cdl is equivalent to half of the linear slope in Figure 7d. The calculated values of Cdl are 0.7 and 10.9 μ C for pristine MoS2 and MoSe2@MoS2 hybrid, respectively. It is noteworthy that the much higher relative active surface could be obtained by compositing MoSe2 on MoS2 to form a hybrid structure because of a remarkable larger value of Cdl for the hybrid. It is suggested that the MoSe2@MoS2 hybrid possesses a much larger active surface area and more active sites for hydrogen production. On the basis of the above electrochemical measurement results, the MoSe2@MoS2 hybrid exhibits enhanced HER activity as well as distinguished stability. Four possible reasons could be responsible for the enhancement of HER performance: (1) extra active edges have been added to this hybrid structure along with processing of MoSe2 composition and offer large amounts of unsaturated sulfur and selenium atoms for HER reaction; (2) the bandgap has been markedly reduced G

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The Journal of Physical Chemistry C because of the existence of Mo4+ with lower binding energy from MoSe2, thus improving the intrinsic conductivity of the hybrid and promoting the combination between Hads and catalyst; (3) the rapid electron transfer from MoSe2 to MoS2 results in an excessive negative charge density which could satisfy the ΔG = 0 eV requirement and finally facilitate the overall HER routine; (4) the excellent mechanical properties of TMDs and high crystallinity of hybrid guarantee the stability during long-term H2 generating.

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4. CONCLUSIONS In conclusion, the MoSe2@MoS2 hybrid has been synthesized via a facile hydrothermal chemical method and applied as the catalyst electrode for HER. We herein employ the DFT-based first-principle calculations to describe the charge redistribution and changes of bank structure. According to the electron density analysis, the charge transfer from MoSe2 and MoS2 in this hybrid can construct. The excellent HER activities with high current density, small Tafel slope, and long-term stability have been achieved in the hybrid with distinct morphology, which can been proved by the corresponding characterizations. This enhancement can be attributed to the extra active sites, the reduced bandgap, and optimized Gibbs free energy change, and our work in this paper could explore new ideas in the research field of alternative for platinum-based HER electrocatalysts.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-21-54345198. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the NSF of China (Grant Nos. 61574055, 61274014, 61474043, 61425004) and the Open Project Program of Key Laboratory of Polar Materials and Devices, MOE (Grant No. KFKT20140003), East China Normal University.

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REFERENCES

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DOI: 10.1021/acs.jpcc.6b05076 J. Phys. Chem. C XXXX, XXX, XXX−XXX