Optimizing MoS2 Edges by Alloying Isovalent W for Robust Hydrogen

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Optimizing MoS2 Edges by Alloying Isovalent W for Robust Hydrogen Evolution Activity Hao Wang, Liying Ouyang, Guifu Zou, Cheng Sun, Jun Hu, Xu Xiao, and Lijun Gao ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02162 • Publication Date (Web): 05 Sep 2018 Downloaded from http://pubs.acs.org on September 5, 2018

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Optimizing MoS2 Edges by Alloying Isovalent W for Robust Hydrogen Evolution Activity Hao Wang,1,2† Liying Ouyang,3† Guifu Zou,1* Cheng Sun,1 Jun Hu,3* Xu Xiao,2 and Lijun Gao1* 1

Soochow Institute for Energy and Materials InnovationS & Key Laboratory of

Advanced Carbon Materials and Wearable Energy Technologies of Jiangsu Province, School of Energy, Soochow University, Suzhou, 215006, China 2

A.J. Drexel Nanomaterials Institute and Department of Materials Science and

Engineering, Drexel University, Philadelphia, Pennsylvania 19104, United States 3

School of Physical Science and Technology, Soochow University, Suzhou, 215006,

China †

These authors contributed equally

*Corresponding authors E-mail: [email protected]; [email protected]; [email protected]

ABSTRACT: Previous studies revealed that HER activities on MoS2 could be boosted through exotic doping to introduce active centres. However, it is difficult to dope exotic elements into MoS2 with a high concentration due to their different chemical properties from host elements. Here, we theoretically and experimentally optimize the MoS2 edges through substituting Mo with different concentrations of isovalent W. The simulations predict that 50% substitution of Mo by W shows the best catalytic activity because of the reduction of H adsorption free energy and facilitation of charge transfer. Experiments demonstrate that the Mo0.5W0.5S2 significantly enhances HER performance of MoS2based compounds, having an onset potential of -37 mV and achieving 10 mA cm-2 with an overpotential of 138 mV. 1

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KEYWORDS: hydrogen evolution reaction, MoS2, edge activation, alloying, DFT calculations

1.

INTRODUCTION Hydrogen has be widely considered as an ideal energy carrier owing to the ultra-high

energy density and zero-pollution discharge.1 The sustainable H2 production by electrochemical water splitting requires efficient and stable electrocatalysts for hydrogen evolution reaction (HER).2 So far, platinum shows the highest HER activity, but high cost and scarcity restrict the scalable usage.3 Hence, developing efficient but cheap catalysts for HER is of great importance for scalable production of hydrogen through water splitting. Up to now, various earth-abundant transition metal compounds4-12 and carbonbased nanomaterials13-16 have been widely explored as HER catalysts. Among them, MoS2 shows great potential due to its high structural stability and catalytic activity.17 As has been well demonstrated, the HER activity of MoS2 mainly originates from edge sites rather than the tendentiously exposed basal plane.18 Consequently, many efforts have been devoted to engineering structures and defects of MoS2 to boost accessibility of edges for HER.19-25 Doping exotic elements is another possible approach to enhance HER activity by introducing active centers around the dopants. A series of heterogenous elements (such as Fe, Zn, N and O) were doped into MoS2 to respectively replace the Mo or S atoms, which indeed improved HER activity significantly.26-33 Notably, it is difficult to introduce these exotic elements into MoS2 with high concentration, because their chemical properties are different from the host elements Mo and S. However, W has the same valence electrons as Mo, so it has a closer chemical property with Mo than the other transition-metal elements. More importantly, WS2 has almost the same geometry and lattice constant as that of MoS2. Therefore, it is possible to drive a high concentration of W to substitute Mo in MoS2 so that desirable Mo1-xWxS2 alloys can be fabricated.34-36 In addition, W centers should be more active than the Mo centers because the W-5d electrons are more chemically active than the Mo-4d electrons for H adsorption. Herein, we report optimization of MoS2 edge by alloying isovalent W for HER. Theoretical 2


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simulations and experiments confirm the optimal HER performance in Mo0.5W0.5S2 among the compared Mo1-xWxS2 samples, delivering one of the best MoS2-based electrocatalytic HER performance. 2.

METHOD

2.1.

Theoretical calculations.

The Vienna ab-initio simulation package (VASP) was used to perform the firstprinciples calculations,37,38 at the level of the spin-polarized generalized-gradient approximation (GGA).39 The ionic cores were described according to the projector augmented wave (PAW).40,41 The energy cutoff for the plane wave expansion was 400 eV. To model one-dimensional nanoribbon, we inserted vacuum of 15 Å in the nonperiodic directions and sampled the Brillouin zone with 27×1×1 k-grid mesh. For the energy minimization procedure, the conjugated gradient method was used to completely relax the atomic positions. The force on every atom was smaller than 0.01 eV/Å. The Gibbs free energy (∆GH) for H adsorption can be estimated by18 ∆ = ∆ + ∆ − ∆ .

(1)

Here ∆EZPE and ∆SH respectively represent the zero point energy difference and entropy difference between the adsorbed state and the gas phase. In this work, ∆EZPE is 0.19±0.01 eV for all considered cases while T∆SH is -0.21 eV at room temperature.42 Therefore, we can rewrite Eq. (1) as ∆ = ∆ + 0.38. ∆EH is the adsorption energy of one more H on the edge of MoS2 with (n-1) H adsorbed, which can be calculated as

∆ = − − /

(2)

Here and are the total energies of MoS2 with and without H adsorbed on the edge, respectively. n is the number of H atoms at the edge and represents total energy of H2 molecule. 2.2.

Materials

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All chemicals of analytical reagents from Sinopharm Chemical Reagent Co., Ltd. were used

as

received.

Mo1-xWxS2 samples

were prepared

via anneal

of

(NH4)6Mo7O24·4H2O (AM), (NH4)10W12O41·xH2O (AT) and thiourea (TU) mixture. Typically, to synthesize pure Mo0.5W0.5S2 (x=0.5), aqueous solution containing 88 mg of AM (refer to 0.5 mmol of Mo), 127 mg of AT (refer to 0.5 mmol of W) and 1g of TU was prepared and then dried by rotate-evaporation at 70 oC for 2 h. The as-obtained white powder was then heated at 800 oC (2 oC min-1) for 2 h in Ar. To prepare more Mo1-xWxS2, different mole ratios of AM and AT with total mole of 1 mmol and 1 g of TU (unchanged) were used (detailed quantities of reactants see Table S1 in the Supporting Information). 2.3.

Characterizations

The morphologies of Mo1-xWxS2 were probed via scanning electron microscopy (SEM, HITACHI SU8010) and transmission electron microscopy (TEM, FEI Tecnai G20). The structures were investigated by X-ray diffraction (XRD, D/MAX-2000) and Raman microscope (HORIBA Jobin Yvon, HR800). The compositions were recorded by X-ray photoelectron spectroscopy (XPS, Thermal Fisher, mono Al Ka (1486 eV)). The element contents of Mo1-xWxS2 samples were tested using inductively coupled plasma spectroscopy (SHIMADZU, ICPS8100). 2.4.

Electrochemical tests.

Electrochemical measurements were carried out in a three-electrode cell with electrochemical workstation (CHI 660E) with a saturated Ag/AgCl electrode as reference electrode and a graphite rod as counter electrode. The working electrode was catalystcoated glassy carbon electrode (GCE) with mass loading of ~0.2 mg cm-2. The catalyst ink was prepared by dispersing 5 mg of Mo1-xWxS2 in mixture of 950 µL of ethanol and 50 µL of 5 wt% Nafion. N2-saturated 0.5 M H2SO4 was used as electrolyte. All the potentials without iR-correction were referenced to the reversible hydrogen electrode (RHE) according to the equation: = / ! + 0.059$% + 0.197

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Electrochemical impedance spectroscopy (EIS) was conducted in a frequency range of 100 kHz to 1 Hz with an amplitude of 5 mV, with the initial potential of -150 mV. 3.

RESULTS AND DISCUSSION A series of Mo1-xWxS2 alloys with x = 0, 0.25, 0.5, 0.75 and 1 are firstly studied

through the density functional theory (DFT) calculations. The HER activity of MoS2 is closely related to the Gibbs free energy (∆GH) of the reaction processes from H adsorption to H2 formation on MoS2.23 According to previous work,32 ∆GH can be defined as: ∆GH = ∆EH + 0.38 V, where ∆EH is the adsorption energy of H atoms (see Theoretical Calculations in Method). The ∆EH of the basal plane of MoS2 is as large as 1.48 eV (∆GH = 1.86 eV), indicating its inertness to HER. Previous experimental and theoretical studies have demonstrated that a Mo-terminated edge of MoS2 nanosheets is active for the HER.18 However, Mo atoms on the edge are not directly exposed to vacuum but covered by extra S atoms as it is shown in Figure 1a and Figure S1 in the Supporting Information.43 It is known that the presence of sulfur on metals can lead to strong interaction with adsorbed H and have impact on H coverage and kinetics.44 On a S-terminated edge, H atoms are actually adsorbed on S atoms as shown in Figure 1a and Figure S2 in Supporting Information. The ∆EH of H adsorption at S coverages of 25%, 50% and 75% are -0.61, -0.24 and -0.16 eV, respectively. Hence the edge S atoms are much more active than the basal S atoms, especially for low H coverage. The corresponding ∆GH are -0.23, 0.14 and 0.22 eV, respectively. Therefore, the HER performance reaches the maximum at H coverage of 50% (Figure 1b, x=0). On the other hand, our calculation indicates that the ∆EH is 1.85 eV (∆GH = 2.23 eV) for H adsorption on the basal plane of WS2, even larger than that of MoS2, so the basal plane of WS2 is also very inert to H adsorption. For H adsorption on the edge of WS2, the ∆EH at H coverages of 25%, 50% and 75% are -0.14, -0.32 and 0.14 eV, respectively. Obviously, the interaction between the edge S and H atoms is the strongest at the H coverage of 50%, which could arise from the different chemical activities of Mo-4d and W-5d orbits. Interestingly, ∆GH is only 0.06 eV for the H coverage of 50% (Figure 1b, x=1), implying that the HER activity of W edge is better than that of Mo edge.

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Figure 1. (a) Side view of atomic structures of the metal edge of MoS2, WS2 and W0.5Mo0.5S2 nanosheets. The green, blue, yellow and white spheres stand for Mo, W, S and H atoms, respectively. S1 and S2 indicate the two non-equivalent sites for H adsorption. (b) Optimal Gibbs free energy (∆GH) of Mo1-xWxS2 for HER at the metal edge with H coverage of 25% (red lines) and 50% (blue lines). (c)-(e) Projected density of states (PDOS) of Mo, W and S atoms at the edge of the MoS2 (c), WS2 (d) and W0.5Mo0.5S2 (e) nanosheets. The dashed and solid lines denote the PDOS before and after the H adsorption. The vertical dashed lines stand for the Fermi energy (EF).

To explore the effect of alloying W in MoS2, Mo0.5W0.5S2 which consists of equal quantity of Mo and W as a special prototype is investigated for its HER activity. After relaxation of atomic structures, we mainly considered the H adsorption on the edge of the first type of Mo0.5W0.5S2 where the Mo and W atoms are alternatively distributed on the edge with the lowest total energy (Figure S3 in the Supporting Information). As displayed in Figure 1a and Figure S2 in the Supporting Information, there are two types of nonequivalent S atoms, denoted as S1 and S2. The total energy of H adsorbed on S2 is lower by 0.07 eV than that on S1, which means that a H atom prefers S2 sites rather than S1 sites. However, the total energies are a little bit different, so the adsorption of H at S1

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sites cannot be excluded completely. At H coverage of 25%, the adsorption energies ∆EH for H adsorption on S1 and S2 sites are -0.34 and -0.41 eV, respectively, resulting in small amplitudes of ∆GH, 0.04 and -0.03 eV, respectively (Figure 1b). When the H coverage further increases to 50%, the adsorption energies ∆EH decrease to -0.46 and 0.51 eV for H adsorption on S1 and S2 sites, with the corresponding ∆GH of -0.08 eV and -0.13 eV. Therefore, the optimal H coverage is 25% for the Mo0.5W0.5S2. Interestingly, the amplitudes of ∆GH at the optimal H coverage (25%) for Mo0.5W0.5S2 are smaller than those for MoS2 and WS2 with the optimal H coverage (50%), which implies that HER activity of Mo0.5W0.5S2 is better than that of MoS2 and WS2. In fact, the lower optimal H coverage and smaller amplitude of ∆GH result in faster H adsorption and desorption processes, because it takes shorter time to accumulate H atoms at the active edge to the optimal coverage (i.e. 25%). We further calculated HER activities of Mo0.75W0.25S2 and Mo0.25W0.75S2 (Table S2 in the Supporting Information). The ∆GH are 0.04 and 0.08 eV for x = 0.25 and 0.75 at H coverage of 25%. Clearly, these values are smaller than that of MoS2 but larger than that of Mo0.5W0.5S2. Accordingly, it could be concluded that Mo0.5W0.5S2 has the optimal HER performance among Mo1-xWxS2 nanosheets. The other requirement for good HER performance is the good electric conductivity of catalysts, so the projected density of states (PDOS) of MoS2, WS2 and Mo0.5W0.5S2 nanosheets were addressed before and after H adsorption and the results are plotted in Figure 1c-e. Obviously, the edge Mo and W atoms have quite high PDOS near the Fermi energy (EF), which means that the nanosheets of these materials have conducting edges, even though their 2D counterparts are semiconductors with band gaps larger than 0.8 eV. As a result, the MoS2 and WS2 nanosheets have good HER activity as discovered in previous studies.18,45 After H adsorption (coverage of 25%), sharp peaks appear very close to the EF and a gap of about 0.4 eV is induced above the EF for all the cases. Consequently, electrons accumulate on the edge Mo and W atoms, which makes them more efficient to transport electrons to H+ from electrolyte and to those adsorbed H atoms for further desorption. To confirm the theoretical predictions, we fabricated a series of Mo1-xWxS2 nanosheet materials, including pure MoS2 (x = 0) and WS2 (x = 1). The Mo1-xWxS2 samples were 7


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prepared through a facile one-pot thermal treatment in Ar atmosphere of well-mixed ammonium molybdate, ammonium tungstate and thiourea. The SEM image (Figure 2a) displays the restacked nanosheet structure of Mo0.5W0.5S2. The TEM images (Figure 2b) reveal that Mo0.5W0.5S2 consists of ultrathin nanosheets, which are hierarchically assembled to fully expose the edges. The selected area electron diffraction (SAED) pattern (inset of Figure 2b) with the bright diffraction rings indicates the polycrystallinity owing to the nanoscale assembly of different-dimension crystallites. The TEM images of other Mo1-xWxS2 confirm the similar morphology to Mo0.5W0.5S2 samples (Figure S4 in the Supporting Information). As shown in Figure 2c and Figure S5, S6 in the Supporting Information, the energy dispersive spectroscopy (EDS) and element mapping results indicate that Mo, W, and S are uniformly distributed in Mo1-xWxS2 samples. As displayed in Table S3 in the Supporting Information, the elemental composition of Mo0.5W0.5S2 obtained by EDS corresponds to 16.7 at. % of Mo, 16.5 at. % of W, and 66.8 at. % of S, indicating that W to Mo ratio is close to 1:1. The detailed element contents of Mo1-xWxS2 samples were further determined by inductively coupled plasma spectroscopy (Table S4), which are in accordance with the EDS results. High-resolution TEM image of the standing edge structure is further observed (Figure 2d). The interlayer spacing of ~0.65 nm fits well to the interlayer distance between the (002) planes.45 Noted that the discontinuous crystal fringes along the curled edge are indicative of rich edges.46 The HRTEM characterization at the basal plane of Mo0.5W0.5S2 was also carried out (Figure 2e). The interplanar spacing of 0.27 nm is consistent with the d spacing of (100) planes. The defect-rich structure and disordered atomic arrangement could induce additional edges.46 To observe the distribution on Mo and W atoms in the nanosheet, HAADFSTEM was further investigated. As shown in Figure 2f, the bright and dim spots correspond to W and Mo atoms, respectively, suggesting the alternative distribution of Mo and W atoms in Mo0.5W0.5S2. It could refer to the first type of Mo0.5W0.5S2 with the lowest total energy (Figure S3a in the Supporting Information).

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Figure 2. (a) SEM and (b) TEM images of Mo0.5W0.5S2 samples. Inset of (b) gives the corresponding SAED pattern. (c) HAADF-STEM image and corresponding EDX mapping images Mo0.5W0.5S2 samples. (d) HRTEM image of the stand-up edges for Mo0.5W0.5S2 samples. (e) HRTEM image of basal plane of Mo0.5W0.5S2 nanosheet. (f) High-resolution HAADF-STEM image of Mo0.5W0.5S2 nanosheets.

To verify the microstructure and composition of the samples, XRD, Raman and XPS spectra of Mo1-xWxS2 samples were carefully examined. As shown in Figure 3a, the diffraction peaks of pristine MoS2 nanosheets match the 2H-MoS2 phase (JCPDS No. 371492) while those of WS2 nanosheets match the 2H-WS2 phase (JCPDS No. 08-0237). In addition, all the samples show the similar XRD patterns, implying that their crystal structures are rather close.47 It suggests MoS2 and WS2 have similar chemical environment, providing a solid base to design Mo1-xWxS2 alloys. According to the Scherrer equation, the thickness of Mo0.5W0.5S2 nanosheet was determined to be 6.7 nm, suggesting that the nanosheets consist of 10 S-Mo-S layers (Figure S7 in the Supporting Information).48 Raman spectra (Figure 3b) reveal that MoS2 exhibits two bands at 381 cm−1 and 408 cm−1 referring to the E2g and A1g modes, respectively, while WS2 shows the 9


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A1g and E2g modes at 418 cm−1 and 352 cm−1, respectively.49 With increasing W composition, the A1g mode shifts to higher frequency and the E2g mode related to MoS2 shifts to lower frequency, while the E2g mode of WS2 shows little shift but the intensity increases. The XPS full spectra (Figure S8 in the Supporting Information) reveal the presence of Mo, W, S for all the samples except the absence of W in pure MoS2 and Mo in pure WS2, respectively. The high-resolution XPS spectra for Mo3d, W4f and S2p of these samples are given in Figure 3c-e. The spectra of Mo3d region could be deconvoluted into three peaks referring to Mo3d3/2 (~232.5 eV), Mo3d5/2 (~229.3 eV) and S2s (~226.6 eV). In the case of W4f, three peaks around 32.7 eV, 34.8 eV and 38.1 eV are observed, which refer to W 4f7/2, W4f5/2 and W5p3/2, respectively. The S2p spectra could be deconvoluted into 2p3/2 at ~162.3 eV and 2p1/2 at ~163.5 eV. With increasing W composition, the Mo3d peaks shows a decrease in intensity while the W5p3/2 and W4f peaks show increases. Furthermore, the Mo3d and W4f peaks shift to lower binding energies while the S2p peaks shift to the higher side, as displayed in Figure S9 in the Supporting Information. This small shift should result from the reduced electron attraction strength of S and the enhanced electron attraction strength of Mo due to the bigger electronegativity of W (2.36) than that of Mo (2.16).49 The W atoms with relatively higher electronegativity could increase the positive charge density on the adjacent Mo atoms, serving as active sites for HER.

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Figure 3. (a) XRD patterns of the Mo1-xWxS2 samples. (b) Raman spectra of the Mo1samples, which display the in-plane (E2g) and out-of-plane (A1g) vibrations with regular shifts. High-resolution XPS spectra for (c) Mo 3d, (d) W 4f, (e) S 2p of the Mo1xWxS2 samples showing the shifting trends of characteristic peaks. All the measured XPS results are calibrated to the C1s peak at 284.8 eV and normalized by S2p3/2 peak intensity. xWxS2

The electrocatalytic HER activities of Mo1-xWxS2 nanosheets were measured on a three-electrode cell in N2-saturated 0.5 M H2SO4 electrolyte. Figure 4a shows the LSV curves for the Mo1-xWxS2 samples and commercial Pt/C catalyst (20 wt% Pt on Vulcan carbon black) with a scan rate of 2 mV s-1. The Pt/C exhibits the best catalytic activity with an onset potential (ηonset) of nearly 0 V. It is noted that Mo0.5W0.5S2 displays the lowest ηonset of -37 mV among all the control samples, while the MoS2 and WS2 nanosheets exhibit ηonset of -176 and -122 mV (Figure S10 in the Supporting Information), respectively. These results confirm the theoretically predicted order of HER activities for Mo1-xWxS2. The overpotential to achieve a current density of 10 mA cm-2 (η10) is determined to be 138 mV for Mo0.5W0.5S2, which is much lower than the other samples.

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Tafel plots derived from LSV curves are assessed to investigate the intrinsic activities of the catalysts (Figure 4b). The calculated Tafel slope (b) decreases from 83 mV dec-1 for MoS2 to 55 mV dec-1 for Mo0.5W0.5S2, indicating significantly boosted HER activity with Volmer-Heyrovsky mechanism.50 The exchange current densities (j0) for Mo1-xWxS2 were also obtained from the Tafel plots (Figure S11 in the Supporting Information). The j0 along with η10 and b for the Mo1-xWxS2 nanosheets are outlined in Figure 4c. Obviously, Mo0.5W0.5S2 possesses the highest j0 of 116 µA cm-2, the lowest η10 of 138 mV and b of 55 mV dec-1, indicating the best catalytic activity among the samples investigated in our experiments.51 Moreover, the HER performance in acidic solution for Mo0.5W0.5S2 sample and previous reported MoS2-based catalysts is outlined in Table S5 in the Supporting Information, further confirming that the Mo0.5W0.5S2 is one of the best MoS2based catalysts.16,29,30,34,35,52-56 To gain further insight into the origin of enhanced HER performance, the electrochemical surface area (ECSA) and electrochemical impedance spectroscopy (EIS) were investigated. The relative ECSA could be estimated from the electrochemical double-layer capacitance (Cdl) by employing electrochemical cyclic voltammograms with various scan rates in the potential region of 0.1-0.2 V vs. RHE (Figure S12 in the Supporting Information). Accordingly, the Cdl could be calculated from the plots of the ∆j (ja-jc) at 0.15 V against the scan rate, giving rise to the value of 3.07, 3.81, 4.16, 3.84, and 3.16 mF cm-2 for x=0, x=0.25, x=0.5, x=0.75, and x=1, respectively (Figure 4d). Assuming that the specific capacitance of a flat surface is ~ 40 µF cm-2 for 1 cm2 of real surface area, the ECSA is calculated to be 153.6, 190.6, 208, 192, and 158 cm2 for the MoS2, Mo0.75W0.25S2, Mo0.5W0.5S2, Mo0.25W0.75S2, and WS2, respectively. The highest electrochemical surface area (ECSA) of Mo0.5W0.5S2 among the Mo1-xWxS2 samples suggests the highest active surface area toward HER.26 Turnover frequency (TOF) could reveal the intrinsic per-site activity of a catalyst. Mo0.5W0.5S2 displays the highest TOF of 1.1 H2 s-1 at the overpotential of 200 mV (Figure S13 in the Supporting Information), addressing the superior HER activity of Mo0.5W0.5S2.52 Furthermore, EIS measurements were conducted to investigate the composition effects on the interfacial charge-transfer kinetics. As shown Figure 4e, Mo0.5W0.5S2 has demonstrated the smallest charge-transfer resistance (Rct) among all the samples, suggesting the fastest Faradaic process in HER 12


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kinetics. The smallest Rct for Mo0.5W0.5S2 is probably attributed to the modulation of electronic structure, which is in agreement with the above PDOS analyses.

Figure 4. (a) LSV polarization curves, and (b) corresponding Tafel plots of Mo1-xWxS2 samples and referenced Pt/C catalyst in 0.5 M H2SO4 at a scan rate of 2 mV s-1. (c) Overpotentials at the current density of 10 mA cm-2 (ƞ10), Tafel slopes (b), and exchange current densities (j0) for the Mo1-xWxS2 samples. (d) The difference of current densities between the anodic and cathodic sweeps versus scan rates for the Mo1-xWxS2 samples. (e) Nyquist plots of the Mo1-xWxS2 samples at the overpotential of 150 mV with the equivalent circuit given inset. (f) Polarization curves recorded for Mo1-xWxS2 before and after 2000 CV cycles in the range of 0.1 to -0.3 V vs RHE. Inset shows the current density as a function of time at a static overpotential of 150 mV.

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The cycling durability and long-term stability of Mo0.5W0.5S2 are also important to evaluate the performance of the electrocatalysts. As shown in Figure. 4f, the polarization curve shows negligible loss in current density after 2000 CV cycles. The unchanged morphology and structure of Mo0.5W0.5S2 after the cycling (Figure S14 in the Supporting Information) illustrate its excellent endurance towards HER process in the acid solution. Additionally, the current density shows little loss even after continuous operation for 12 h at a constant overpotential of 150 mV, demonstrating the outstanding long-term stability. CONCLUSIONS This work has realized high concentration of Mo substitution in MoS2 by W with similar valence electrons and chemical property for enhancing the HER performance. A series of Mo1-xWxS2 compounds were examined, and it was found through theoretical calculations and experiments that the Mo0.5W0.5S2 showed best HER performance. The first-principles calculations demonstrated that W substitution of MoS2 could significantly reduce the H adsorption free energy (∆GH) and facilitate charge transfer and consequently highly boost the HER performance. The value of ∆GH reaches minimum at a H coverage of 25% on Mo0.5W0.5S2, which is favorable for both of H adsorption and desorption processes on the surface of catalyst. Meanwhile, the electron conductivity of catalyst is also boosted as confirmed by PDOS calculations. Overall, this work both experimentally and theoretically demonstrated that high-concentration W substitution of Mo in MoS2 can deliver enhanced HER activity. AUTHOR INFORMATION Corresponding Authors [email protected] [email protected] [email protected] Author contributions

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†These authors contributed equally to this work. Notes The authors declare no competing financial interest. ASSOCIATED CONTENT Supporting Information. This Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Detailed modeling, SEM images, EDS mappings, enlarged XRD patterns, XPS full scans, shifting plots of XPS peaks, LSV curves, CV curves, ECSA calculations, TOF plots and summary of HER performance.

ACKNOWLEDGEMENTS We thank the financial support from National Natural Science Foundation of China (U1401248, 21671141, 11574223), “973 Program - the National Basic Research Program of China” Special Funds for the Chief Young Scientist (2015CB358600), and the National Science Foundation of Jiangsu Province (BK20150303). H. W. thanks the financial support from the China Scholarship Council (no. 201706920081).

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Optimizing MoS2 Edges by Alloying Isovalent W for Robust Hydrogen

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