Energy Level Engineering of MoS2 by Transition-Metal Doping for

Oct 15, 2017 - Water-splitting devices for hydrogen generation through electrolysis (hydrogen evolution reaction, HER) hold great promise for clean en...
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Article Cite This: J. Am. Chem. Soc. 2017, 139, 15479-15485

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Energy Level Engineering of MoS2 by Transition-Metal Doping for Accelerating Hydrogen Evolution Reaction Yi Shi, Yue Zhou, Dong-Rui Yang, Wei-Xuan Xu, Chen Wang, Feng-Bin Wang, Jing-Juan Xu, Xing-Hua Xia,* and Hong-Yuan Chen State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P. R. China

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S Supporting Information *

ABSTRACT: Water-splitting devices for hydrogen generation through electrolysis (hydrogen evolution reaction, HER) hold great promise for clean energy. However, their practical application relies on the development of inexpensive and efficient catalysts to replace precious platinum catalysts. We previously reported that HER can be largely enhanced through finely tuning the energy level of molybdenum sulfide (MoS2) by hot electron injection from plasmonic gold nanoparticles. Under this inspiration, herein, we propose a strategy to improve the HER performance of MoS2 by engineering its energy level via direct transition-metal doping. We find that zinc-doped MoS2 (Zn-MoS2) exhibits superior electrochemical activity toward HER as evidenced by the positively shifted onset potential to −0.13 V vs RHE. A turnover of 15.44 s−1 at 300 mV overpotential is achieved, which by far exceeds the activity of MoS2 catalysts reported. The large enhancement can be attributed to the synergistic effect of electronic effect (energy level matching) and morphological effect (rich active sites) via thermodynamic and kinetic acceleration, respectively. This design opens up further opportunities for improving electrocatalysts by incorporating promoters, which broadens the understanding toward the optimization of electrocatalytic activity of these unique materials.

1. INTRODUCTION In the modern industry of energy conversion, hydrogen holds great promise as the green energy carrier of the future due to its high energy density and zero effect of combustion product on environments.1,2 One of the most efficient pathway to produce hydrogen fuel is to electrolyze water in acidic media (hydrogen evolution reaction (HER), 2H+ + 2e− → H2).3,4 However, three fundamental limitations still exist during the process of electrocatalytic HER: low thermal efficiency, unstable catalytic performance, and extreme lack of cost-effective substitute catalysts for precise metals (i.e., Pt, Pd, and Rh). Thus, the need to decline precise metal usage in HER has sparked much passion for nonprecise metal catalysts with high efficiency and catalytic stability.5−9 Up to date, promising alternatives of precise metal electrocatalysts have been exploited, including various combinations of metals (core−shell metals, metal alloys, single-atom metal-support nanostructures), 3d transition-metal (3d-TM) sulfides, 3d-TM phosphides, 3d-TM nitrides, along with molecular catalysts.5−12 Among these alternatives, molybdenum sulfide (MoS2) has been recently considered as a promising and efficient nonprecise metal catalyst for HER,13−19 since it can catalyze HER with near zero free energy of H adsorption (ΔGoH ≅ 0 eV) analogous to Ptlike catalyst.20 Unfortunately, nanostructured MoS2 with large bandgap21 still suffers from sluggish kinetics of HER, which dramatically limits the overall electrocatalytic HER rate. To © 2017 American Chemical Society

overcome this obstacle, considerable attention has thus been devoted to improving the catalytic efficiency of MoS2 via minimizing the overpotential for driving HER.14−19 The intrinsic essence of decreasing the overpotential for HER is the modulation of energy level of the electrocatalyst comparable to this reaction. In this regard, electrons will be more favorable to exchange between electrocatalyst and proton in water, which activates this reaction thermodynamically. Enzyme in nature with a transition-metal ion as part of a metallomacrocyclic complex surrounded by a protein matrix usually shows high catalytic activity and selectivity in living systems. In enzyme catalysis, the electronic density of active sites will be manipulated by the surrounding peptides and residue chemical groups to the appropriate values matching the energy level of the substrates (reacting species), which realizes the feasibility of electron transfer from substrates to products. Lu et al. systematically studied the relationship between the energy levels of active iron centers in Heme and enzymatic activities.22,23 Learning the concept of “energy level matching” from nature, chemists endeavor to design functional artificial analogues of enzymes.20 In this case, similar phenomenon will also be observed on electrocatalyst using specific strategy to fine-tune its electronic density. The versatility for tailoring the Received: August 20, 2017 Published: October 15, 2017 15479

DOI: 10.1021/jacs.7b08881 J. Am. Chem. Soc. 2017, 139, 15479−15485

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Journal of the American Chemical Society

electrocatalytic activity toward HER, which is attributed to increased active sites and optimized 3d electron configuration. Turnovers of 15.44 s−1 at an overpotential of 300 mV are determined for Zn-MoS2, which is much better than the activities reported for MoS2 catalysts. We further propose that the activity and stability of Zn-MoS2 are synergistically optimized by electronic effect (energy level matching) and morphological effect (rich active sites), which dramatically enhances HER via thermodynamic and kinetic acceleration, respectively. The success of modulation HER activity via the element doping approach will offer a simple and promising pathway for designing efficient electrocatalysts for energy conversion.

electronic density of electrocatalyst will in turn influence the redox potential, nucleophilicity, or stability of the catalytically active site. On the basis of “energy level matching”, the engineering of electrochemical electronic structure will remove or at least diminish the barriers to the reaction process; thus, decreased overpotential in electrocatalysis can be achieved. Even though the active site on MoS2 has remained debatable since the first HER research about 40 years ago, a consensus has still been reached that the electrochemical activity of MoS2 arises from the unsaturated sulfur atoms along its edges,24 whose electronic structure totally differs from the catalytically inactive basal planes. To increase the number of active sites for HER, basal planes have been activated by chemical doping, strain control, integrating with localized surface plasmon resonance (LSPR), or phase conversion of MoS2 from 2H to 1T.18,25−29 In our previous research, we found that HER could be activated through modulating the energy level of MoS2 by hot electron injection from plasmonic gold nanoparticles to conduction band of the semiconductor.25 However, this procedure is restricted to the illuminating environment, which demands extra energy and complicated equipment. Therefore, we are seeking an alternative simple pathway for the same goal to adjust the electronic density of MoS2 for activating HER. In order to fully harness this material, chemical doping has been widely employed to promote the HER on MoS2 via increasing the conductivity and the number of active sites.14,15,27−29 The attractiveness of chemical doping for catalysts design includes facile synthetic procedure, high-yield production, and chemical stability. In the case of heteroatomdoped engineering, the electronic density of Mo and S atoms next to the doped heteroatoms will be significantly affected, therefore modulating the electrocatalytic activity of the in-plane atoms. In view of the classic “volcano” theory, near zero free energy of H adsorption (formation of bonds to atomic hydrogen is neither too strong nor too weak) on MoS2 with optimized energy level leads to maximum activity toward HER. On the other hand, the heteroatom-doped MoS2 (H−Mo−S) structures offer more active sites for HER, where synergistic effect among these atoms (H−Mo−S) will accelerate this reaction kinetically. With these key concepts of active sites and energy level modulation in mind, chemically doped MoS2 will be a useful model system for understanding the fundamental relationship between the MoS2 nanostructure and the thermodynamics and kinetics of HER. Herein, we propose an alternative means to engineer the electronic density of MoS2 for accelerating HER via doping the semiconductor with different transition metals (the resulting materials are denoted as M-MoS2, M = Fe, Co, Ni, Cu, and Zn). This doping process was accomplished by straightforward solvothermal methods that can be scaled readily (Scheme 1). Results show that Zn-doped MoS2 catalysts exhibit superior

2. EXPERIMENTAL SECTION Reagents. FeCl2 was purchased from Shanghai Richjoint Chemical Reagent Co., Ltd. Cu(NO3)2 and Co(NO3)2 were purchased from Xinbao Chemical Co., Ltd. (Shanghai Company). Zn(NO3)2 and Ni(NO3)2 were purchased from Xilong Chemical Co., Ltd. N,NDimethylformamide (DMF) was purchased from Sinopharm Chemical Reagent Co., Ltd. (NH4)2MoS4 was purchased from Alfa Aesar (Johnson Matthey Company). Molybdenum(IV) sulfide (MoS2) powder and n-butyllithium in hexane (1.6 M) were purchased from Sigma-Aldrich. All aqueous solutions were prepared with Millipore water (resistivity of 18.2 MΩ·cm). Material Synthesis. MoS2-based materials were prepared according to a modified method reported by Dai et al.19 For the preparation of different TM-doped MoS2, 22 mg of (NH4)2MoS4 was added to 10 mL of DMF solution of 4.25 × 10−5 mol corresponding metal ion (FeCl2, Co(NO3)2, Ni(NO3)2, Cu(NO3)2, and Zn(NO3)2). The mixture was sonicated at room temperature for ∼5 min to achieve a clear and homogeneous solution. The reaction solution was transferred to a 30 mL Teflon-lined autoclave in glovebox and then heated in an oven at 200 °C for 10 h. Product was collected by centrifugation at 6000 rpm for 20 min, washed with DI water, and recollected by centrifugation. The washing step was repeated at least three times to ensure that most DMF was removed. Finally, the product was redispersed in 10 mL of DI water, frozen by liquid nitrogen, and lyophilized overnight. Then, the black powder was calcined in tubular furnace at 350 °C for 1 h in Ar gas atmosphere (heating rate: 10 °C/min). Pure MoS2 was synthesized by the same procedure except for adding metal ion. Synthesis of Chemically Exfoliated MoS2 (ce-MoS2, 1T-MoS2) Nanosheets for Comparsion. Bulk MoS2 was intercalated by mixing MoS2 powder (0.6 g) with n-butyllithium in hexane (1.6 M, 6 mL) at 60 °C in Ar gas atmosphere. After 2 days, the suspensions were filtered and washed 3 times with hexane. Then, the resulting black powder was immediately suspended in 30 mL of Millipore water and sonicated for 1 h. Exfoliated material was then dialyzed for a week. The suspension was finally centrifuged at 5000 rpm to remove the unexfoliated materials. Characterization. The morphologies of samples were characterized by transmission electron microscopy (TEM, JEM-2100, Japan) by drying a droplet of sample solutions on Ni-grids with carbon film. Scanning electron microscopic (SEM) images were acquired on silicon wafers by S-4800 (Japan). Raman spectra were collected on a FTRaman Spectrometer (Bruker). XPS spectra were obtained on a PHI 5000 VersaProbe (Japan). The binding energy was calibrated by means of the C 1s peak energy of 284.6 eV. X-ray diffraction patterns (XRD) were carried out on a X’TRA (Switzerland). Electrochemical Characterization. A sample of 4 mg of catalyst and 80 μL of 5 wt % Nafion solution were dispersed in 2 mL of 4:1 v/v water/ethanol by sonication for 1 h to form a homogeneous ink. Then, 5 μL of the catalyst ink was loaded onto a freshly polished glassy carbon electrode of 3 mm in diameter. Linear sweep voltammetry (using CHI 440E instrument, Chenhua, China) with scan rate of 2 mV s−1 was conducted in 0.5 M H2SO4 using Ag/AgCl electrode (saturated KCl) as the reference electrode, a graphite rod as the

Scheme 1. Schematic Diagram of Doping MoS2 Material with Other Transition Metals

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DOI: 10.1021/jacs.7b08881 J. Am. Chem. Soc. 2017, 139, 15479−15485

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Journal of the American Chemical Society counter electrode, and a glassy carbon electrode as the working electrode. In all measurements, we used Ag/AgCl (saturated KCl) electrode as the reference. It was calibrated with respect to reversible hydrogen electrode (RHE). In 0.5 M H2SO4, E(RHE) = E(Ag/AgCl) + 0.238 V. All the potentials reported in our article are against RHE. Impedance measurements were performed at frequencies ranging from 0.1 Hz to 100 kHz with an amplitude of 10 mV at the onset potentials of each MoS2-based electrocatalyst where current density was 0.5 mA/ cm2.

1E) of Zn-MoS2 show the homogeneous distribution of Mo (indicated by green color), S (indicated by blue color), and Zn (indicated by red color) elements, demonstrating the uniform distribution of Zn atoms among the whole sheet. The X-ray photoelectron spectroscopy (XPS) spectrum of Zn 2p at ∼1020 and 1045 eV further confirms the presence of Zn in the structure of MoS 2 (Figure 2B). The amount of Zn incorporation in Zn-MoS2 is calculated as 4.33 atom %. It is clear that the binding energies of Mo 3d and S 2p are negatively shifted by ca. 0.47 and 0.40 eV, respectively, compared with the pure MoS2 (Figure 2C and D). This indicates the increase of electronic density in MoS2 after incorporation of Zn element. Raman shifts of pure MoS2 and Zn-MoS2 are shown in Figure 2E. The prominent Raman bands at 378 and 402 cm−1 are due to the in-plane E2g and out-of-plane A1g modes (Figure 2F), respectively. The A1g mode denotes the vibration for out-ofplane lattice with two S atoms directly bound to Mo atoms moving in opposite directions, while the E2g mode denotes the vibration for in-plane lattice with the two S atoms collectively moving in the same direction which is opposed to the movement of Mo atom (Figure 2F). The Raman spectrum of Zn-MoS2 shows similar characteristics like the pure one, which implies that the MoS2 layered feature in Zn-MoS2 is not changed. The physical characterizations demonstrate that Zn atoms have been uniformly doped in the MoS2 structure. Electrochemical measurements of various MoS2 samples were performed in a N2-saturated 0.5 M H2SO4 solution using a standard three-electrode configuration. As shown in Figure 3A (green line), due to large internal resistance, the bulk MoS2 exhibits little HER activity with a Tafel slope of 692 mV/dec as reported previously.30 It is well-known that the HER active centers of MoS2 originate from the edge deformed sites and leave predominant in-plane domains useless.27 Thus, the electrochemical activity of pure MoS2 toward HER is not yet satisfactory (black line). In order to achieve an enhanced HER catalytic activity of MoS2, we investigated the effect of Zn atom dopants on HER. Compared to pure MoS2, Zn-MoS2 exhibits superior electrocatalytic activity toward HER with the onset potential of ca. −0.13 V versus reversible hydrogen electrode (RHE) (red line), followed by a rapid increase in the cathodic current density with the increase of overpotentials, which exceeds that of the active ce-MoS2 (Figure S4). The results promise to narrow the gap with commercial Pt/C catalyst (Figure 3A, blue line). Raman spectra before and after Zn doping demonstrate that the activity modulation of HER is not caused by the variation of structure but can be ascribed to the regulation of the electronic density of Zn-MoS2. Results from Tafel plots can give more insights into the rate-limiting step of the HER. On the catalyst surface, the HER mechanism can usually be generalized for two pathways: (1) Volmer− Heyrovsky mechanism (H3O+ + e− = Hads + H2O and Hads + H3O+ + e− = H2 + H2O) and/or (2) Volmer−Tafel mechanism (H3O+ + e− = Hads + H2O and Hads + Hads = H2).19 These two pathways imply that the competitive processes of adsorption/ desorption of H atoms occurring at the catalyst should be matched to accelerate HER. A HER catalyzed by pure MoS2 may follow the Volmer−Heyrovsky mechanism, which has a lower H adsorption coverage (ΘH) and a larger Tafel slope (∼101 mV dec−1). Instead, the Tafel slope of Zn-MoS2 is calculated to be about 51 mV dec−1, which is more similar to Pt/C (32 mV dec−1), suggesting that HER on Zn-MoS2 resembles the reaction mechanism on Pt/C (Figure 3B). In a HER catalyzed by Zn-MoS2, the rate-limiting step at low

3. RESULTS AND DISCUSSION The Zn-MoS2 material was synthesized via one-pot solvothermal method by using (NH4)2MoS4 and Zn(NO3)2 as precursors (details in the Supporting Information). The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images (Figure 1B, C) show that Zn-MoS2

Figure 1. Morphologic characterizations of the catalysts. SEM images of pure MoS2 (A) and Zn-MoS2 (B). TEM (C) and HRTEM images (D) of Zn-MoS2. Down panel shows the element mapping images of Zn, Mo, and S of Zn-MoS2 (E).

is composed of roselike two-dimensional nanosheets with a layer spacing of 0.66 nm assigned to the (002) crystalline plane of MoS2. The high-resolution TEM (HRTEM) image (Figure 1D) indicates distinct ripples and corrugations, which implies the ultrathin structure of the sample. The appearance of small nanosheets with sharp edges indicates the high crystallization of the product. The morphology of MoS2 modified by Zn atoms will not be changed compared with the pure MoS2 (Figure 1A). There are no nanoparticles or large clusters appearing in the TEM image, which rules out the possibility of forming zinc sulfide compounds on the surface, consistent with the X-ray diffraction (XRD) patterns which show no other crystal phases (Figure 2A). Two peaks at high-angle region (33° and 56°) can be well indexed to (100) and (110) planes of the 2H crystal phase of Zn-MoS2 and pure MoS2 (JCPDS card no. 73-1508). Besides, the 2H crystal phase of Zn-MoS2 was further evidenced by UV−vis absorption, XPS, and Raman spectra (Figures S1−S3, detailed discussion shown in the Supporting Information). Corresponding EDX mapping images (Figure 15481

DOI: 10.1021/jacs.7b08881 J. Am. Chem. Soc. 2017, 139, 15479−15485

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Figure 2. Spectral characterizations of the catalysts. (A) XRD patterns of pure MoS2 and Zn-MoS2. XPS spectra of Zn-MoS2 as indicated in the Zn2p (B), Mo3d (C), and S2p (D) regions. (E) Raman spectra of pure MoS2 and Zn-MoS2. (F) Schematic diagram of different vibration modes of MoS2. The E2g and A1g modes represent the in-plane (two S atoms moving in same direction, opposite to Mo atom) and out-of-plane (two S atoms moving in opposite directions) vibrations for MoS2, respectively.

current density as compared to the initial curve, demonstrating the outstanding stability of Zn-MoS2 for HER. In view of the successful manipulation of electrocatalytic activity of MoS2 via Zn atoms doping, the HER activities of MoS2 doped with different tranistion metal atoms (Fe, Co, Ni, and Cu) were also investigated. As shown, various TM-MoS2 (Fe-, Co-, Ni-, and Cu-MoS2) also exhibit roselike nanostructure as Zn-MoS2 and the morphologies are not obviously changed after doping (Figure S6). The XPS surveys show the existence of Fe, Co, Ni, and Cu element (Figure S7A−D). It should be noted that compared to the ones for pure MoS2, the binding energies of Mo 3d and S 2p in different transition metals doped MoS2 are negatively shifted with different degrees (Figure S7E and F). We attribute the observed difference in negatively shifted binding energies to different electronic densities in MoS2 after transition metals incorporation. As mentioned above, the modulation of electronic density in catalysts leads to the changed catalytic activities. Compared to the pure MoS2, the electrocatalytic activities of M-MoS2 materials toward HER vary obviously with the doped transition metals (Figure 4A). As observed, Fe and Co doping significantly decrease the electrocatalytic activities of M-MoS2 toward HER, while the effect of Ni doping on electrocatalytic activity toward HER is negligible. Among these catalysts, Cuand Zn-doped MoS2 exhibits enhanced electrocatalytic activities toward HER, as evidenced by the lower onset potentials of ca.

Figure 3. Electrochemical activities of various catalysts toward HER. (A) Polarization curves of HER for various catalysts. (B) Tafel plots of the corresponding electrocatalysts derived from the early stages of HER polarization curves. (C) TOF values of the Zn-MoS2 catalyst (black line) and other MoS 2 -based catalysts reported recently.13−17,19,22,29,30 (D) Stability tests by measuring the polarization profiles for Zn-MoS2 catalyst before and after 1000 cyclic potential scans in 0.5 M H2SO4 at a scan rate of 20 mV s−1.

overpotentials is the recombination of two adsorbed H atoms, indicating that Zn doping promotes the HER following the Tafel mechanism, which dramatically contributes to HER. To provide more insights into the intrinsic activity, the turnover frequencies (TOFs) were measured to give the catalytic activities on a per-site basis (Figure S5). Based on the method of copper underpotential deposition (refers to the details in the Supporting Information), the number of active sites on various MoS2 samples is determined. The TOF of HER for pure MoS2 is 3.83 s−1. After Zn atoms incorporation, the TOF of Zn-MoS2 increases to 15.44 s−1, which is comparable to or even surpasses most state-of-the-art MoS2 reported (Figure 3C and Table S1).13−17,19,24,31,32 To value the stability of the Zn-MoS2 catalyst, long-term electrochemical measurement was carried out. As shown in Figure 3D, the profile for cathodic wave after 1000 cyclic potential sweeps displays an insignificant decline of

Figure 4. Electrochemical activities of various catalysts toward HER. (A) HER polarization curves of various catalysts as indicated. (B) Tafel plots of the corresponding electrocatalysts derived from the early stages of HER polarization curves. 15482

DOI: 10.1021/jacs.7b08881 J. Am. Chem. Soc. 2017, 139, 15479−15485

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Journal of the American Chemical Society −0.16 and −0.13 V, respectively. The negative effect of Fe, Co, Ni doping may be ascribed to the occupation of active sites on MoS2. This can be evidenced by the XRD patterns (Figure S8), in which new reflection peaks (for example, two sharp peaks around 20° in Co-MoS2 and Ni-MoS2) emerge. The emergence of new structure may occupy the active sites on MoS2, since the active site tends to be a more suitable nucleation site for heteroatoms doping. Compared with the turnover numbers (TONs) of Cu- and Zn-MoS2 (15.3 and 28.0), Fe-, Co-, and Ni-MoS2 show lower TONs with the values of 2.51, 3.49, and 5.01, respectively (Table S2), which in turn confirms the occupation of active sites. These facts indicate that the electrocatalytic activity of MoS2 can be optimized through different TM elements doping, as also evidenced by the Tafel plots (Figure 4B). The influence of doping amount on electrochemical activities of various TM-doped MoS2 toward HER was also investigated (Figure S9, specific atomic ratios of TMs in M-MoS2 shown in Table S3). The electrocatalytic activities of Fe-, Co-, and Ni-MoS2 are not closely related to the doping amount of TM since Fe-, Co-, and Ni-MoS2 with different doping amounts all show inert HER activities. With different amounts of Cu and Zn doping, Zn-MoS2 shows the optimal catalytic activity when the doping amount is onefold, while twofold for Cu-MoS2 (Figure S10). Over threefold amount doping (for example, fourfold) cannot be fully incorporated into MoS2 since the colors of the final reactants are similar to those of their corresponding precusors.Besides, Figure S11 shows the current densities of MoS2 and M-MoS2 (M = Fe, Co, Ni, Cu, and Zn) at η = 150 and 200 mV. It appears that overdoping (threefold) will inhibit the HER performance of all M-MoS2 samples. In small amount of doping, Cu and Zn dopings are all effective promoters. With onefold doping, the best promoter is Zn, which gives MoS2 a ∼13-fold increase in current density (49.5 vs 3.6 mA cm−2) at η = 200 mV. With 0.5- or 2-fold amount doping, the best promoter is Cu, which gives MoS2 a ∼3-fold increase of activity (9.3 vs 3.6 mA cm−2 or 12.1 vs 3.6 mA cm−2, respectively). On the other hand, Fe, Co, and Ni do not give any or only a small promotion to the activity of MoS2. In electrochemistry, extra energy, termed as overpotential, above the required thermodynamic potential is commonly applied to force the energy level of electrocatalysts comparable with the redox potential of specific reactions (Figure S12), where the electron will exchange between electrocatalyst and its substrate (reacting species). Our aim is concentrated on how to design an electrocatalyst with more proper matching of the energy level with the reduction potential of H2O, thus decreasing the overpotential (extra energy) applied for HER. It has been previously reported that HER can be accelerated via increasing the electronic density of MoS2.33−35 On the surface of electron-rich MoS2, the adsorption capacity of H atom will be weakened, indicating that the following H recombination and release process will become relatively easier.33,36,37 XPS results (vide supra) confirm the increased electronic density in MoS2 after TM-doping. As the energy level increases, the energy required to remove an electron from the catalyst surface will decrease, which activates HER thermodynamically. Ultraviolet photoelectron spectroscopy (UPS) was further used to determine the values of energy level (difference between the vacuum and Fermi level) of Zn-MoS2, Fe-MoS2, and pure MoS2 as representative examples, which are calculated to be 5.55, 6.58, and 6.86 eV, respectively, by subtracting the width of the He I UPS spectra (Figures S13−15) at excitation energy of

21.22 eV. UPS spectra of other transition metal doped MoS2 are also shown in Figures S16−19. Figure 5A depicts that

Figure 5. Energy level related HER mechanism. (A) Band structure diagram for Zn-MoS2, Fe-MoS2, and pure MoS2. (B) Schematic illustration of formation of undercoordinated Mo (δ+) and Zn (δ+) centers during the HER and the plausible reaction mechanism.

compared with the reduction potential of H2O, the energy level of Zn-MoS2 is properly positioned to permit electron transfer with much lower applied energy (lower overpotential), thus confirming Zn-MoS2 as a promised electrocatalyst for HER. However, energy level matching only reflects the feasibility of electron transfer and is an essential prerequisite for high electrocatalytic activity, which will also be codetermined by many other complicated factors (e.g., coordination environment, number of active sites, conductivity). Apart from the consideration of thermodynamics (energy level matching), kinetics (rich active sites) also plays a significant role in this reaction. The excellent HER catalytic activity of Zn-MoS2 might also be ascribed to the increased number of catalytic active sites (vide supra, evidenced by TON and TOF values) and the activation effect on H3O+ molecule through synergistic interaction between Mδ+···O (Moδ+···O or Znδ+···O) and Sδ‑··· H, as roughly represented in Figure 5B. In this bifunctional mechanism, the kinetics depend on a delicate balance between the rate of hydroxonium ion (H3O+) molecule dissociation step on Mδ+, recombination of Hads on Sδ−, and the rate for desorption of H2O that helps to accommodate the adsorption of the next H3O+ molecules.38,39 As mentioned above, the HER process relies on one primary discharge step (H3O+ + e− = Hads + H2O) and at least one kind of desorption steps (Hads + H3O+ + e− = H2 + H2O or Hads + Hads = H2). During the discharge step, H3O+ diffuses to the catalyst surface and then H3O+ picks up electrons and generates hydrogen adsorbed on the catalyst surface. In order to increase the kinetic rate and corresponding catalytic performance, we should focus on enriching the concentration of available H3O+. Due to the synergistic interaction between Mδ+···O and Sδ‑···H (Figure 5B), more H3O+ ions can be concentrated on the catalyst surface, which facilitates the discharge process. Tafel slope is related to the kinetic rate, the low value of which is indicative of facile kinetics as this typically reduces the overpotential demanded to achieve appreciable current density. After doping MoS2 with Zn, the Tafel slope decreases from 101 to 51 mV dec−1, suggesting the increased coverage of H and adsorption−desorption kinetics. In electrocatalysis, inherently excellent conductivity tends to afford rapid electron transport and extend electrons to more active sites, thus resulting in an increase of active sites utilization efficiency, which will offer markedly faster reaction kinetics.40 Electrochemical impedance spectroscopy (EIS) was performed to further characterize the charge transfer kinetics from active sites to the electrodes (Figure S20). As shown, Zn-MoS2 15483

DOI: 10.1021/jacs.7b08881 J. Am. Chem. Soc. 2017, 139, 15479−15485

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Journal of the American Chemical Society

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displays lower charge transfer resistance (Rct) than other MoS2based electrocatalysts (data shown in Table S2). The noticeable increased conductance of Zn-MoS2 will accelerate HER kinetics, which indeed improves the utilization efficiency of active sites (in accordance with TOFs). Only under the consideration of two aspects (thermodynamics and kinetics), the rational design of electrocatalyst will be realized.41,42

4. CONCLUSION In summary, we suggest that modulation of the energy level of electrocatalysts is an efficient means to improve the catalytic activity. Our results demonstrate that engineering the electronic density of MoS2 via different transition-metal doping can significantly influence the HER performance. Zinc-doped MoS2 (Zn-MoS2) has been designed as a model system to understand the fundamental relationship between the MoS2 nanostructure and the thermodynamics and kinetics of HER. Zn-MoS2 exhibits the superior electrochemical activity toward HER with the onset potential of −0.13 V vs RHE and the Tafel slope of 51 mV dec−1. The large enhancement can be attributed to the synergistic effect of electronic effect (energy level matching) and morphological effect (rich active sites) via thermodynamic and kinetic acceleration, respectively. The present work provides a facile and low-cost strategy for designing efficient electrocatalysts for HER, which promises to be a harbinger for broad applicability of this methodology for other catalytic systems.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b08881. Details of TONs and TOFs calculation; supporting data results (Figures S1−S19 and Tables S1−S3) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected]. ORCID

Chen Wang: 0000-0001-6544-4065 Jing-Juan Xu: 0000-0001-9579-9318 Xing-Hua Xia: 0000-0001-9831-4048 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from the National Key Research and Development Program of China (2017YFA0206500) and the National Natural Science Foundation of China (21327902, 21635004, and 21675079).

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REFERENCES

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DOI: 10.1021/jacs.7b08881 J. Am. Chem. Soc. 2017, 139, 15479−15485

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DOI: 10.1021/jacs.7b08881 J. Am. Chem. Soc. 2017, 139, 15479−15485