Experimental and First-Principles Investigation of MoWS2 with High

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Experimental and First-Principles Investigation of MoWS2 with High Hydrogen Evolution Performance Honglin Li, Ke Yu,* 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, China ABSTRACT: Electrochemically splitting water for hydrogen evolution has attracted a lot attention and developed into a promising approach to produce hydrogen energy. Searching for high-activity and economical electrocatalysts to replace Pt-based catalysts remains a great challenge. In this paper, we reported a concise and effective strategy to fabricate the novel MoWS2 composite for use as the electrocatalyst through a hydrothermal method for the first time. The final obtained MoWS2 composite demonstrated a well-defined hierarchical structure and established that its densely stacked nanopetals act as the active sites in the corresponding hydrogen evolution reaction (HER) processes. Experimental results show that the composites can deliver a clearly promoted HER activity and are superior to the pure structure. In order to give a more in-depth explanation, we also performed a first-principles calculation to further survey the electronic properties, compound form, and HER mechanism of different structures. The charge distribution of MoWS2 composite indicates that electrons can directionally transfer from WS2 to the neighboring MoS2 and form an “electron-rich” configuration, which is beneficial to increase the HER rate and promote the overall performance. This thorough research will not only provide new thought to the analyses and elucidation of the inner mechanism of the HER process for this kind of two-dimensional composite but also guide further work on the basis of experimental and calculation results. KEYWORDS: MoS2, heterojunction, hydrogen, electrocatalysis, electronic structures

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INTRODUCTION

it still remains a great challenge to develop highly effective and acidity-stable catalysts. Recently, two-dimension-based materials have been explored as competitive catalysts in HER due to their high stability and remarkable catalytic performances. Enlightened by the recent chemical synthesis studies, metal non-oxides with certain nanostructures, for example, WSe 2 , WC, Mo 2 C, and MoS 2 , 15−17 have been extensively explored for HER applications. In these studies, MoS2- or WS2-based materials are very promising alternatives because of their wide distribution, acidic stability, and favorable electrocatalytic properties after some kind of modification.18,19 WS2/MoS2 consisted of two S layers sandwiching a W/Mo layer by van der Waals interactions.20 Both theoretical21 and experimental22 research demonstrated that their HER abilities are mainly derived from the edge sites of the 2D structures. In principle, two factors could influence the extensive use of these materials.23−25 On the one hand, the involvement of van der Waals forces will lower the surface area and reduce the active sites.26,27 On the other hand, the low overall conductivity will also restrict the electron transfer and restrain the related HER processes.28−30 In order to deal with these issues, designing electrocatalysts with increased edge sites as well as good conductivity would be a feasible way to enhance the HER

Hydrogen, as a clean and efficient fuel source, can be currently produced by steam reformation or partial oxidation of hydrocarbon, which involves carbon dioxide release and a high temperature precondition.1,2 The increase of environmental concerns has triggered an aggressive investigation of low-pollution alternative techniques to produce hydrogen.3,4 Considered from this aspect, H2 can be directly obtained from the abundant and nontoxic water resources; thus, hydrogen evolution through light or electricity are promising and appealing pathways.5 It is worthwhile to explore new techniques to produce hydrogen in an economical and renewable manner to control the consumption of fossil fuel and reduce CO2 emission. Utilization of efficient cathode electrocatalysts for the hydrogen evolution reaction (HER) is the key to achieve an optimal performance for water splitting. Until now, people have devoted great efforts to producing hydrogen by resorting to electrochemical or photocatalytic routines of water splitting.6−8 Various traditional HER catalysts, such as nickel alloy, polymeric carbon nitride, or carbides, have been widely used in many cases.9−11 Generally, nickel alloy can provide a nice HER ability when working in alkaline electrolytes, but the corresponding degradation of activity is significant in acidic solutions.12 Platinum (Pt) is certainly an ideal option due to its excellent electrocatalytic performance in acidic conditions; however, it has been limited in widespread application because of its scarcity and high cost.13,14 Therefore, © 2016 American Chemical Society

Received: August 2, 2016 Accepted: October 18, 2016 Published: October 18, 2016 29442

DOI: 10.1021/acsami.6b09620 ACS Appl. Mater. Interfaces 2016, 8, 29442−29451

Research Article

ACS Applied Materials & Interfaces

cell lattices were optimized until the forces on each atom are less than 0.01 eV/Å. A k-point grid of 5 × 5 × 1 was used for the geometry relaxation and a 500 eV cutoff energy was adopted in all calculations, which was sufficient enough to reach convergence for the corresponding calculations.36

activity. The most common methods to overcome the above issues are increasing the active sites and trying to improve the systematic conductivity through physical vapor deposition or hydrothermal methods for HER utilization. Although considerable research has been conducted for the WS2/MoS2-based materials, there is still no report on the use of MoWS2 composite material as electrocatalyst of the HER application. In this case, we synthesize the novel MoWS2 composites via a convenient and high-producing hydrothermal route. The corresponding processes are facile and can avoid special processing approaches. Owing to the specific structure and electronic configuration of MoWS2, the fabricated MoWS2 composite demonstrated a well-separated architecture with massive nanopetals that could deliver a high HER activity. The different samples were systematically characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Xray photoelectron spectroscopy (XPS), as well as Raman spectroscopy. To better explore the inner mechanism theoretically, the first-principles calculation was further conducted to analyze the synergistic effect within WS2/MoS2 and to try to give a clear physical picture. In brief, the calculated results could reasonably clarify the reason why the composite structure showed a better HER activity, and also these analytical methods could provide meaningful references to future research of the other HER catalysts.

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RESULTS AND DISCUSSION Parts a and b of Figure 1 demonstrate the morphology of the hydrothermal fabricated pristine MoS2 and S6 composite,

EXPERIMENTAL METHODS AND CALCULATION

Synthesis of Nanopetal-Nanostructured MoWS2. The general steps for the fabrication of pristine MoS2 and MoWS2 are as follows: different qualities of Na2MoO4·2H2O and Na2WO4·2H2O based on 4 mmol of Mo(1‑x)WxS2 (x = 0, 0.03, 0.06, 0.09, 0.12, 0.15, 0.18, and 0.21), abbreviated as S1−S8, were dissolved in eight 80-mL portions of pure water. Then, CN2H4S, H2C2O4, and HONH3Cl were dispersed in the above aqueous solution with continuous stirring for 30 min. The solution was then transferred to a 100 mL autoclave and heated in 200 °C for 24 h. After hydrothermal reaction, the obtained products should be repeatedly cleaned by ethanol and distilled water to remove impurities. Sample Characterization. In this section, X-ray diffraction was used to characterize the crystal structure of different samples with Cu Kα radiation (λ = 0.1541 nm). The 2θ scanning range is from 10° to 70° with 0.02° intervals to collect the diffraction data. Raman spectra analyses were carried out by a Jobin-Yvon LabRAM HR 800 microRaman spectrometer. X-ray photoelectron spectrometry (XPS) analyses were performed by an ESCALAB 250Xi.31 The FESEM characterization and TEM measurements were accomplished by using JEOL-JSM-6700F and JEOL-JEM-2100 instruments, respectively. All the above measurements were performed at room temperature.32,33 Electrochemical Measurements. All electrochemical studies were experimented on a glassy carbon electrode (GCE) in oxygenremoved H2SO4 solution (0.5 M). Five milligrams of various synthesized catalysts was dissolved in 1 mL of H2O−2-propanol (4:1 v/v) and then mixed with 30 μL of Nafion (5 wt %). A homogeneous mixture formed after 30 min ultrasonic treatment, and it was further coated on the GCE for the following measurements. Linear sweep voltammetry (LSV) with a 5 mV s−1 scan rate was used to quantitatively determine the HER activities of various catalysts.34 In the electrochemical reaction, catalyst-coated GCE acted as the working electrode. Pt and saturated calomel electrode (SCE) served as the counter electrode and the reference electrode, respectively. Before the above experiment, the H2SO4 solutions must be purged with pure N2 for 1 h to completely remove the dissolved oxygen. Computational Details. The density functional theory (DFT) based Vienna Ab Initio Simulation Package (VASP) was employed with the projector-augmented wave (PAW) method to describe the electron−ion interaction.35 To begin, the pristine and composite unit

Figure 1. (a, b) SEM images of the pristine MoS2 and S6 composite, respectively. (c) The TEM image of the S6 composite. (d) HRTEM image of individual S6 nanopetals. (e) The EDX mapping of Mo, S, and W.

respectively. As shown in Figure 1a, each individual MoS2 sphere demonstrates a 0.3 μm averaged diameter. The surface of this spherical morphology is primarily constituted by densely stacked 2D curved nanopetals. These secondary structures aggregate in a crosswise and random way. It also shows that these heavily staggered 2D nanopetals can grow in a direction perpendicular to the surface, and thus the spherical architecture can be generated finally. Figure 1b shows SEM images of the S6 composite. On the whole, it is roughly coincident with the pristine structure, which reveals that each composite structure shows a 0.5 μm averaged diameter. The inset in Figure 1b shows the detailed morphology of the S6 composite and it possesses massive smaller nanopetals attached on the surface that are also similar to the surface morphology of the pristine structure, as exhibited in the inset of Figure 1a. Figure 1c shows the TEM image of the S6 composite. The surrounding parts of S6 are covered by semitransparent substances attributed to nanopetals. Figure 1d is the HRTEM image of individual nanopetals of S6, which can be used to survey its nanostructure further. The overall HRTEM image of its nanopetals reveals the layered growth nature of S6, and the fabricated nanopetals structure is inclined to stack with a 0.608 nm spacing (3.04 nm for five layers) in a highly dense way. This sharp-edged HRTEM image strongly suggests the crystallographic characteristic of S6 and convincingly indexes to the typical hexagonal structure. The elemental distribution of Figure 1e for the composite presents the coexistence of Mo, S, and W elements. In conclusion, the above comprehensive SEM and TEM 29443


Research Article

ACS Applied Materials & Interfaces surveys of pristine MoS2 and S6 composite convincingly prove the formation of the corresponding nanopetal-structured 3D hierarchical architecture. In regard to the formation mechanism of this nanopetaled structure, we consider that MoS2/MoWS2 nanopetals are greatly influenced by the hydrothermal environment. The amorphous MoS2/WS2 would first evolve under 200 °C in the course of the initial hydrothermal reaction period. In the following reduction process from Na2MoO4·2H2O/ Na2WO4·2H2O to MoS2/WS2, these amorphous primary nanoparticles may freely roll up to form spherical structure with dense curls on the surface to eliminate the dangling bonds and lower the total energy. The layered 2D nature of MoS2/ WS2 induces these primary structures to aggregate into spheres spontaneously afterward. Since MoS2 and WS2 have perfectly matched lattice constants, the composite structures would also grow into curls in the hydrothermal environment and finally encourage the confined growth of such hierarchical structure without significant change. Figure 2a shows the XRD patterns of the synthesized pristine MoS2 and different MoWS2 composite structures. The pristine

Figure 3. (a) Raman spectra of the different vibrational modes for pristine MoS2 and S6 composite. (b) Overall XPS pattern of S6. (c) C 1s spectrum. (d) Mo 3d and S 2s spectra. (e) W 4f spectrum. (f) S 2p spectrum.

the basal plane.38 After W-doping, blue shifts of the E12g (375.7 cm−1) and A1g (404.8 cm−1) peaks appear for S6, which is attributed to the van der Waals force in MoWS2 sheets resulting in a higher force constant for atomic vibration. In Figure 3b−f, the S6 sample is further characterized by XPS survey. Figure 3b presents the typical overall S6 XPS spectrum. The characteristic binding energies at 232.5 and 229.3 eV attributed to Mo 3d3/2 and 3d5/2 are observed in Figure 3d, respectively, which proves the existence of Mo4+.39 In Figure 3e, the characteristic binding energies at 32.9 and 34.9 eV are ascribed to W 4f7/2 and W 4f5/2. The W 5p5/2 peak is also observed, indicating the existence of W4+. For the binding energies of S 2p1/2 and 2p3/2 for divalent sulfide ion (S2−), the 2p3/2 peak was observed at 162.2 eV. The stoichiometric ratio estimated from the XPS survey on the surface is around 2.21 for S/(Mo + W), implying that the film is close to Mo0.85W0.15S2 (S6 sample). The slightly higher stoichiometric ratio of this hydrothermally synthesized sample has relevant with the presence of MoS3. It was reported that the amorphous MoS3 thin film has a formal charge state of Mo4+.40 The observation of S 2p3/2/2p1/2 at 162.5/163.8 eV originates from bridging S22− and apical S2− of the unsaturated S atoms, respectively.41 To survey the catalytic activities of the composites toward HER in comparison with pristine MoS2, different samples were deposited on GCEs in N2-saturated 0.5 M H2SO4 solution with a three-electrode configuration. Figure 4a shows the polarization curves within a 0 to −0.35 V vs RHE cathodic potential window. Clearly, the Pt electrode demonstrates an overpotential close to zero and thus can deliver the highest HER activity in comparison with the other catalysts. Figure 4b indicates that the HER process on the Pt surface has a Tafel

Figure 2. (a) XRD patterns for S1−S8. (b) Detailed XRD spectra of different samples ranging from 10° to 18°.

MoS2 presents a high purity. The observed diffraction peaks at 13.94°, 33.3°, and 58.9° correspond respectively to (002), (101), and (110) planes of the hexagonal MoS2. No other peak is observed, and the overall spectrum can be well-assigned to the standard hexagonal MoS2 (JCPDS Card No. 37-1492).37 As for S2−S8 composites, the diffraction peaks are also clear and sharp, implying a high degree of crystallinity for these hydrothermally synthesized samples. Figure 2b shows the detailed XRD spectra of the different samples. The standard diffraction peak along (002) of MoS2 and WS2 are at 14.38° and 14.32°, respectively. The Bragg’s equation states that 2d sin θ = nλ, where d, θ, n, and λ are interplanar spacing, diffraction angle, diffraction series, and X-ray wavelengths. The measured diffraction peaks of (002) for these samples are all slightly less than the standard values, implying a larger interlayer spacing and the involvement of strain between the layers for the curve hierarchical structure of the synthesized samples. Figure 3a shows the Raman spectra of pristine MoS2 and S6. The two dominant peaks at 374.6 and 403.9 cm−1 for S1 correspond to the E12g and A1g modes of hexagonal MoS2 crystal, respectively. The vibration direction of the A1g mode is along the c-axis and corresponds to the in-layer displacements of Mo and S atoms. The E12g mode vibration direction is within 29444


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Figure 4. (a) Polarization curves for the eight electrocatalysts. (b) The corresponding Tafel plots of different structures. The straight lines are fitted by the Tafel equation, η = b log(j) + a, in which b denotes the Tafel slope. (c) Durability test for S6 catalyst by CVs after 1000 cycles. Negligible HER current lost after circulation. (d) Nyquist plots of various electrocatalyst-modified GCEs.

Figure 5. (a−h) Voltammograms plots for S1−S8 at various scan rates (10−100 mV s−1). (i) Plots of the capacitive currents as a function of scan rate.

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corresponding HER activities. Different from the conventional wisdom that a larger catalytically active surface area will necessarily generate a higher catalytic activity, our results elucidate that this area makes no difference in HER performance. Even though the Cdl is a useful criterion of HER performance for a certain catalyst, as reported previously, sometimes it is not reasonable to characterize the intrinsic activities of different catalysts only using the Cdl values, since the hydrogen evolution ability of active sites may differ widely. With that in mind, turnover frequency (TOF/s−1, the number of H2 molecules evolved per second per active site) calculated by an electrochemical approach seems much more reasonable to appraise the catalytic activity.44 Assuming that the HER process induces the entire cathodic current, the TOF can be obtained by using the equation TOF = j/nFN, where n, F, and N are the stoichiometric number of electrons consumed in the electrode reaction (n = 2 for the HER), the Faraday constant, and the number of active sites in a unit area, respectively.45 The calculated TOFs at an overpotential of 250 mV from S1 to S8 are 0.146, 0.166, 0.219, 0.247, 0.284, 0.345, 0.282, and 0.168 s−1, respectively. Then, an interesting volcano-shaped trend is displayed in Figure 6, in which S6 has the highest TOF value,

slope of 32 mV/decade. The resulting Tafel slope of the pristine MoS2 is about 172 mV/decade. It can be seen that much higher electrocatalytic activities are obtained for seven MoWS2 composite catalysts compared with the pristine structure. They present lower onset Tafel slopes of about 168, 141, 122, 116, 89, 131, and 139 mV/decade for S2−S8, respectively, implying superior HER activities, and the cathodic current densities will rise rapidly under a more negative potential. Generally, a smaller Tafel slope denotes a faster HER process and is advantageous for practical applications. The optimal catalytic performance of the S6 composite in this case is preliminarily attributed to the massive active edge sites, which can make a positive contribution to the overall HER process, and the synergistic effect between WS2 and MoS2. The stability of S6 toward HER is assessed by the continuous long-term cycling test (1000 cycles) in an acidic environment to test the HER durability. In this course, catalyst poisoning or delaminating from GCE will inevitably weaken the reaction activity. As shown in Figure 4c, a negligible decay of the cathodic current for S6 is obtained at the end of cycling. The engineering of this nanopetals morphology ensures the structural stability. This substantial long-term stability of S6 shows great promise in the fabrication of an economic and efficient hydrogen evolution electrode in water electrolysis systems. Electrochemical impedance spectroscopy (EIS) measurements are further performed to survey the interface reactions and electrode kinetics of different catalysts in the HER process. Generally, the EIS Nyquist can be fitted to an equivalent circuit (inset of Figure 4d), in which Rct is the charge-transfer resistance. Sample S6 shows the lowest charge transfer resistance (Rct = 888.4 Ω) among these synthesized catalysts, which suggests the fastest charge transfer for HER.42 Also, the obtained semicircles are in accord with the HER activities shown in Figure 5a. Overall, the charge transfer resistance of the MoWS2 composites are much smaller than that of pristine MoS2, verifying that the incorporated W can indeed improve the electrical conductivity. Thus, electrons can be delivered directly and efficiently between electrocatalyst and electrolyte during the corresponding electrocatalytic reaction. A faster charge transfer during the HER reaction contributes to a better HER activity of the MoWS2 composites for the achievement of both structural and electronic synergistic effects in HER process. In the following, we estimated the differences in electrochemically active surface area for the eight fabricated electrocatalysts. The catalytically active area is measured by a cyclic voltammetry (CV) method at various scan rates (10−100 mV· s−1) in the 0.02−0.12 V vs RHE region (Figure 5a−h), which is associated with electrochemical double-layer charging after repeated potential cycling tests of eight samples. In this case, the electrochemically active surface area can be computed by electrochemical double-layer capacitance (Cdl). From the CV curves, the Cdl is calculated by plotting the ΔJ at 0.07 V vs RHE in the CV against the scan rate.43 The corresponding plots are exhibited in Figure 5i, in which the slopes are twice as the Cdl value. The dependence of the current on the scan rate in the full region is linear for all eight samples, which is in keeping with capacitive charging behavior. The calculated Cdl values of the eight samples are 0.49, 0.63, 0.53, 0.54, 0.56, 0.65, 0.45, and 0.70 mF cm−2 for S1−S8, respectively. These obtained results clearly denote that the electrochemically active surface areas of these eight samples are not the decisive factor for the

Figure 6. Calculated turnover frequencies for S1−S8.

being ∼2.36 times higher than that of pristine MoS2. This denotes the remarkable promoted activities of MoWS 2 composites for HER, and these values can roughly accord with the HER activities analyzed above. In order to further make it clear why the composite structures performed much better than that of pristine MoS2 theoretically, we conducted a first-principles calculation to survey the inner mechanism. First, the geometric construction of the composite structure should be identified. Generally, the defects formation energy of an intentionally doped structure is an important physical quantity to characterize the solubility of a dopant. The defects formation energy in the neutral charge state can be computed as Ef(D) = Etot(D) − Etot(perfect) − ∑niμi, where Etot(D) and Etot(perfect) are the total energies of a defective supercell and a perfect supercell, respectively.46 ni is the incorporated (ni > 0) or removed (ni < 0) atoms when the defect sites are formed. μi represents the corresponding chemical potential. In principle, the chemical potential of a certain element strongly depends on the reaction environment, and it should be S-rich or W/Mo-rich (the actual situation is between the two). In this case, the real hydrothermal reaction condition is S-rich; therefore, the considered μMo/W is μMo/W = 29446


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ACS Applied Materials & Interfaces μ(MoS2/WS2) − 2μ(S), and the above defects formation energy expression can be rewritten as E f MoS2 − W/WS2 − Mo(D) = EMoS2 − W/WS2 − Mo(D) − Etot(perf ect MoS2 /WS2) − μ(W/Mo) + μ(Mo/W)

When a W/Mo atom doped the perfect MoS2/WS2 structure, the defects formation energies of MoS2−W/WS2−Mo under Srich reaction conditions are calculated to be −0.016 and 4.862 eV, respectively. When calculating the doped structures, only neutral charge states are taken into account. The +6 states of W/Mo are used as the reference of the doping sources, by which the chemical potentials μi of W and Mo atoms are determined, since Na2WO4·2H2O and Na2MoO4·2H2O were used as raw materials in the experiments. Theoretically speaking, a lower defects formation energy signifies a more stable doped structure and a higher solubility of the dopant in a certain lattice. Therefore, the positive defects formation energy of the Mo-doped WS2 (WS2−Mo) structure indicates that it is nearly nonexistent. In additon, the negative value of −0.016 eV suggests that the W-doped MoS2 (MoS2−W) structure is a possible doped configuration; however, the formation energy of pristine MoS2 is calculated to be −0.712 eV, and this value is far lower than the defects formation energy of the MoS2−W structure, implying that the pristine structure is much more stable than the MoS2−W doped structure and thus it can steadily form under the hydrothermal conditions. Given these concerns, we consider that a reasonable and prudent composite structure should be a sandwiched MoS2/WS2/MoS2 configuration. This is because the Mo source is much more stable than the W source in reality and the doped structures are very difficult to form. Also, the XRD results show no other peak except for standard hexagonal MoS2/WS2 peaks. It is generally considered that the different lattice constants of two structures will inevitably induce some lattice mismatch and strain effect in the composite structure, which will influence the construction of the computational model. However, in this case, the experimental lattice constants for the MoS2 and WS2 are a = b = 3.161 Å, c = 12.295, α = β = 90°, γ = 120° and a = b = 3.196 Å, c = 12.503 Å, α = β = 90°, γ = 120°, respectively, and both possess the same hexagonal structure and P63/mmc (194) crystallographic space group. That is, MoS2 and WS2 can be perfectly matched and no further modification is needed. To model the MoWS2 hybrid, we consider that the composite is composed by a WS2 layer inserted into the MoS2 interlayer and a three-layer structure is formed. On account of the perfect matched lattice between MoS2 and WS2, as discussed above, there is no need to create a bigger superlattice structure to release the strain effect. This is thought to be in accordance with the experimental data. To begin, the pristine and hybrid structures are fully relaxed until the forces on each atom are less than 0.01 eV/Å. These relaxed constructions will be used in the following computational surveys. Figure 7a,b shows the band structures of pristine MoS2 and sandwiched MoWS2. The band gap of the pristine structure is calculated to be 1.14 eV, which accords well with the reported experimental values,47,48 and this ensures the credibility of our calculations. As for the MoWS2 composite, the band gap was evidently reduced to 0.88 eV, which is clearly smaller than that of pristine MoS2, and thus the conductivity gets promoted. Figure 7c,d shows the partial charge density of the valence band maximum (VBM) and

Figure 7. (a, b) Band structures of pristine MoS2 and MoWS2 composite, respectively. (c, d) The partial charge density of the valence band maximum and the conduction band minimum for the MoWS2 composite, respectively.

conduction band minimum (CBM) for MoWS2 and clearly illustrates that the VBM and CBM of this composite are respectively delocalized in MoS2 and WS2, implying that the incorporation of WS2 can significantly influence the corresponding electronic structure. To better understand the electron carrier properties of the two structures, the electron effective masses are computed. On the basis of the semiconductor theory, the electron carriers’ effective mass at the CBM (orange and cyan bands shown in parts a and b of Figure 7 for pristine MoS2 and MoWS2, respectively) can be calculated as49 1 1 ⎛ ∂ 2E ⎞ = 2⎜ 2⎟ mi ℏ ⎝ ∂ki ⎠

where E and k are the energy and the wave vector, respectively. Cross terms of k-dependence of energy have been neglected. The electron effective masses of pristine MoS2 and WS2 are denoted as m1 and m2, respectively. The direction corresponds to GM of CBM in the first Brillouin zone. Forty k-points are used along the GM direction in the reciprocal space within the calculation of effective masses and can be confirmed fine enough. The final calculated effective masses of m1 and m2 are 0.022 m0 and 0.016 m0, respectively, where m0 represents the free electron mass. This means that the electron effective mass of MoWS2 is much less than that of the pristine structure. 29447


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corresponding reaction course and that electrons can directionally transfer from the WS2 layer to MoS2 and form an “electron-rich” condition. This further inspired us to seek the inner HER mechanism. The considered active adsorption sites include the basal plane and Mo and S edges. It should be noted that the Mo edge configuration is not a simple truncation of the bulk structured MoS2; it should be terminated with extra S atoms.52 The Mo edge with 50% sulfur-atom-covered termination matches the previously observed and predicted results.53 It is generally considered that the criteria standard for an ideal HER catalyst, for example, Pt, is that the adsorbed H atom’s Gibbs free energy should be close to the thermoneutral state (i.e., ΔG ≈ 0). According to thermodynamics, the generation of surface H* would be suppressed if the adsorption is endothermic (ΔG > 0), while the recombination of H* to form H2↑ would also be restrained when it is too exothermic on the contrary (ΔG < 0).54 The ΔG of H atom at the S edge and basal plane are calculated to be 0.73 and 2.46 eV, respectively, while the ΔG of the Mo edge equals −0.352 eV; thus, these sites should mainly undertake the overall HER process. Allowing for the above analyses that the ideal ΔG should be around 0 eV and maintain the binding of the H atom to an appropriate degree, the Mo edge shall perform much better than the other sites. When the extra electrons transfer from the WS2 layer to MoS2, it is speculated that the edge sites of MoS2 will accelerate the hydrogen evolution process. Figure 9a demonstrates the variation trend when extra electrons are added to the MoS2. It is obvious that the ΔG values have a consistent variation trend and get an increase with an increase of extra electrons. It is remarkable that the ΔG of the Mo edge approaches the thermoneutral states. Consequently, it is safe to conclude that the HER activity should be improved when many more electrons are involved in the corresponding electrochemical process because the main active site of HER on MoS2 is Mo edge sites. The MoWS2 can provide electronic regulation by engineering electron transfer to lower the ΔG at the active sites, which then offer faster proton adsorption/desorption kinetics. Previously, Yu et al.55 investigated the HER catalytic performance of atomically thin films of MoS2 under controlled growth. Compared with previous reports, there was something different behind the experiment. They considered that the key to obtain a high-efficiency electrocatalyst was to increase the electrons’ hopping efficiency in the vertical direction based on their systematic experiments rather than simply increase the active edge sites. This is because the electrons’ transfer toward the z direction is through hopping and the potential barriers exist in the interlayer gap of MoS2.56 Figure 9b shows the computed average electrostatic potential of the pristine MoS2 and MoWS2 composite, in which the oscillation patterns for the two structures signify the periodicity of the supercell. The averaged potential for the MoWS2 is lower than that of pristine MoS2 by −0.309 eV, implying an easier transfer of electrons from the electrode to the MoWS2 composite in the direction perpendicular to the basal plane and an increase of hopping efficiency in the corresponding HER routine, which also accords well with the above experimental results. It is generally accepted that the band gap is closely associated with the electrical conductivity. In this work, the smaller band gap of the MoWS2 composite results in a better conductivity, and this result corresponds well with the EIS measurements. Besides, electrons directionally transfering from the WS2 layer to MoS2 will further facilitate the forward reaction of the above

Generally speaking, a smaller effective mass signifies a better transport behavior. MoWS2 composite has a narrower band gap and a smaller effective mass in this case, so one can draw a conclusion that the composite possesses a better electron transport behavior and can facilitate the overrall HER process.50 To qualitatively analyze the charge transfer between MoS2 and WS2 layers, charge density difference maps are plotted in Figure 8. The charge difference reveals the charge redistribution

Figure 8. (a, b) The 3D charge density distribution of the MoWS2 composite along the (001) direction. (c) Plane-averaged charge difference.

for the composite system of isolated portions. In Figure 8a,b, the yellow/blue region represent charge accumulation/ depletion, respectively. The difference pattern of the MoWS2 composite indicates that the major charge transfer appears around the connected interface. A larger yellow/blue region between the two parts suggests a certain amount of electron transfer from the WS2 layer to MoS2. Figure 8c shows the 2D charge density difference along the (001) direction. According to this figure, positive/negative regions represent a gain/loss of charge for the formation of an integral structure. It is evident that the charge redistribution mainly occurs around the interface region, and the electrons can transfer from the WS2 layer to MoS2 directionally. A quantitative calculation based on Bader charge analysis should be performed. Bader charge analysis is usually employed to calculate the electrons transfer between different parts of a composite system quantitatively.51 As presented above, a significant charge difference exists between the WS2 layer and MoS2, and a distinct difference is also reflected in the change of the Bader charges. A charge transfer of 0.102 e from the WS2 layer to MoS2 is found, suggesting the directional transfer of electrons. This directional transfer of electron will inevitably facilitate the overall HER processes. A detailed explanation will be given below. The main feature of the MoWS2 composite, as indicated above, is that massive active edge sites play an active role in the 29448


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the successful preparation of the dense nanopetal-structured MoWS2 that demonstrated the improved electrocatalysis activities but also described full DFT calculation details involving the inner mechanisms of how the composite delivered an improved performance. The related analytical methods and the achieved conclusions adequately integrate experimental results with calculation conclusions. We believe that this work will provide a significant reference for the research on hybrid systems in the future.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 21 54345198. Fax: +86 21 54345119. 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. 61274014, 61474043, 61425004, 61574055), Innovation Research Project of Shanghai Education Commission (Grant No. 13zz033), the Open Project Program of Key Laboratory of Polar Materials and Devices, MOE (Grant No. KFKT20140003), and Outstanding Doctoral Dissertation Cultivation Plan of Action (Grant No. PY2015045).

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REFERENCES

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Figure 9. (a) The variation of ΔG profiles versus the amount of extra electrons for the Mo edges of MoS2. (b) Averaged electrostatic potential of the pristine MoS2 and MoWS2, in which the vacuum levels are set to zero.

steps and achieve a better HER activity. In brief, the fabricated MoWS2 composite can provide massive MoS2 active sites and a smaller band gap, and the composite also has a lower ΔG, which can effectively improve the systematic conductivity and promote the overall HER activity.

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CONCLUSION In summary, the synergistic regulation of both structure and electronics ensures superior HER performance in the MoWS2 catalysts. In this work, a convenient and high productivity hydrothermal approach has been proposed to synthesize the novel MoWS2 composites. Considering their unique structural feature of dense nanopetals that act as exposed active edges, the synthesized composites exhibited high HER activities, and the S6 sample delivered the optimal electrocatalysis activity. The formation of the MoWS2 composite significantly increases the HER performance, with low Tafel slope as well as excellent stability after 1000 cycles. We also conducted an electronic properties survey of the pristine MoS2 and MoWS2 composite using the DFT method. It was concluded that the better performance behavior of the composite is rooted in its intrinsic features. A smaller band gap and the synergistic effect between WS2/MoS2 make electrons directionally transfer from the WS2 layer to MoS2, contributing a quick and efficient hydrogen production process and facilitating the improvement of the electrocatalysis activity. This original work not only involved 29449


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Experimental and First-Principles Investigation of MoWS2 with High

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