Mechanism of Hydrogen Evolution Reaction on 1T-MoS2 from First

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Mechanism of Hydrogen Evolution Reaction on 1T-MoS2 from First Principles Qing Tang, and De-en Jiang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01211 • Publication Date (Web): 20 Jun 2016 Downloaded from http://pubs.acs.org on June 26, 2016

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Mechanism of Hydrogen Evolution Reaction on 1T-MoS2 from First Principles Qing Tang and De-en Jiang* Department of Chemistry, University of California, Riverside, CA 92521, USA *To whom correspondence should be addressed. E-mail: [email protected].

Abstract: The 1T phase of transition metal dichalcogenides (TMDs) has been demonstrated in recent experiments to display catalytic activity for hydrogen evolution reaction (HER), but the catalytic mechanism has not been elucidated so far. Herein, using 1T MoS2 as the prototypical TMD material, we studied the HER activity on its basal plane from periodic density functional theory (DFT) calculations. Compared to the non-reactive basal plane of 2H phase MoS2, the catalytic activity of the basal plane of 1T phase MoS2 mainly arises from its affinity for binding H at the surface S sites. Using the binding free energy (∆GH) of H as the descriptor, we found that the optimum evolution of H2 will proceed at surface H coverage of 12.5% ~ 25%. Within this coverage, we examined the reaction energy and kinetic activation barrier for the three elementary steps of the HER process. The Volmer step was found to be facile, while the subsequent Heyrovsky reaction is kinetically more favorable than the Tafel reaction. Our results suggest that at low overpotential, HER can take place readily on the basal plane of 1T MoS2 via the Volmer–Heyrovsky mechanism. We further screened the dopants for the HER activity and found that substitutional doping of the Mo atom by metals such as Mn, Cr, Cu, Ni, and Fe can make 1T MoS2 a better HER catalyst. Keywords: 1T MoS2, hydrogen evolution reaction, catalytic activity, basal plane, VolmerHeyrovsky mechanism, substitutional doping

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1. Introduction Molecular hydrogen is the cleanest energy carrier. Water splitting from either light or renewable energy1,2 provides the most sustainable supply of H2. Electrochemical hydrogen evolution reaction (HER) in acidic media is most effectively catalyzed by Pt and its alloys. However, the scarcity and high cost of the Pt-group metals prevent wider adoption of Pt.3 For years, the scientific community has been actively exploring earth-abundant materials to replace Pt. Many types of promising HER electrocatalysts have been developed,4,5 including non-noble transition metals (Fe, Co, Ni),6,7 transition metal phosphides (Ni2P,8 CoP9), carbides (Mo2C),10 nitrides,11 metal-free catalysts (for example, fullerenol,12 carbon nanotubes,13 boron14 or nitrogen doped graphene,15 C3N416), and pyrite-type cobalt phosphosulphide (CoPS) - currently the best non-Pt catalyst.17 Recently, molybdenum disulfide (MoS2) has been identified as a promising active and acidstable catalyst for HER.18-20 MoS2 belongs to a family of layered transition-metal dichalcogenides (TMDs) where the individual S-metal-S sandwich layers weakly interact with each other. MoS2 naturally occurs as the semiconducting and thermodynamically favorable 2H phase. The bulk layered MoS2 itself, however, has poor HER activity, and its (0001) basal planes are electrocatalytically inert due to the poor electrical conductivity which hinders charge transfer kinetics. In 2005, Nørskov et al. suggested from theoretical studies that the electrocatalytic hydrogen evolution on 2H-MoS2 nanoparticles is mainly driven by the metallic edges.21 In 2007, Chorkendroff et al. experimentally proved that the exposed edge sites (a sulfided Mo edge) of 2H-MoS2 are indeed the active sites for HER.22 Moreover, a direct experimental comparison between the basal and edge planes of MoS2 verified that the edge plane exhibits much higher electrochemical activity and faster heterogeneous electron transfer (HET) rate over the inactive


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basal plane.23 This understanding has ignited significant interest in the investigations and developments of 2H-MoS2 nanostructures with high percentage of active edge sites as electrocatalysts for achieving better HER performance.24-33 The discovery of MoS2-based HER catalyst opens up an exciting field for research activity. However, the overall catalytic activity of 2H-MoS2 is still limited by poor electrical transport and inefficient charge transfer at the interface, which can be tuned by electrochemical pretreatment such as electrochemical reduction.34,35 Excitingly, recent advances have showed that the HER catalytic activity can be significantly enhanced when the semiconducting 2H phase MoS2 is converted into metallic 1T phase MoS2 nanosheets.36-39 The Tafel slope was greatly reduced from 110 mV/decade for 2H-MoS2 to 43 mV/decade for 1T-MoS2, approaching that of Pt at 30 mV/decade.38 However, the detailed mechanism is unclear. The meta-stable 1T phase is not found in nature, but can be made via intercalation of the 2H-MoS2 lattice with lithium or organolithium compounds.36,40 In 2H phase, each Mo center is prismatically coordinated by six surrounding S atoms, whereas in the 1T phase the Mo atom is octahedrally coordinated to six neighboring S. Recent experimental41 and computational42 results revealed that 1T MoS2 has high reactivity towards surface covalent functionalization. By contrast, the 2H phase cannot be functionalized directly. Particularly for the catalytic properties, the surface reactivity of the conducting 1T MoS2 nanosheets towards binding H as well as the improved charge transfer kinetics might be responsible for their substantially enhanced HER catalytic activity compared to few-layer 2H-MoS2 nanosheets. Unlike 2H MoS2 where the catalytic activity arises from the edges, expriments by Chhowalla et al. sugggested that the active sites of chemically exfoliated 1T MoS2 nanosheets are mainly located on the basal plane and the contribution of the metallic edges to the overall HER efficiency is relatively small.36 The much


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greater active surface area of 1T nanosheets with respect to the edge portion thus guarantees the higher HER activity. In addition, the catalytic activity of exfoliated MoS2 depends strongly on the organolithium compounds used: exfoliations with n-butyllithium and t-butyllithium produced 1T MoS2 sheets that were catalytically more active than those obtained with methyllithium.43 It is worth noting that the polymorph control from 2H phase to 1T phase has demonstrated improved catalytic and electrochemical activity not only for MoS2 but also for other analogous TMD materials such as MoSe2, WS2, and WSe2.44-46 This implies that the phase control of MX2 structures may play an important role on their HER properties. Despite the exciting experimental advances, the reaction mechanism and reaction pathways of the HER process with 1T MX2 catalysts remain unknown. As a representative MX2 material, understanding the HER process on 1T MoS2 will shed light on the understanding of other MX2 structures that might be relevant for the given application. Several recent theoretical studies examined the HER activity of 1T MoS2 and related MX2 materials47-54 and mainly focused on the H adsorption free energy. But a complete HER pathway with the explicit consideration of the water layer has not been reported. More important, the metastatble 1T MoS2 can be stabilized by chemical functionalization, so it would be highly desirable to find out whether the functionalized 1T is active for HER. Since hydrogen atom can functionalize the 1T-MoS2 phase and adsorbed H is also important for HER in acidic conditions, here we use density functional theory (DFT) calculations to examine the HER activity and mechanism on the basal plane of the 1T MoS2 monolayer at different hydrogen coverages. We further consider tuning the HER activity by doping.


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2. Method The DFT simulations of the adsorption and reaction process involved in HER on 1T MoS2 were performed by using the Vienna ab initio simulation package (VASP).55 The ion-electron interaction is described with the projector augmented wave (PAW) method.56 Electron exchangecorrelation is represented by the functional of Perdew, Burke and Ernzerhof (PBE) of generalized gradient approximation (GGA).57 A cutoff energy of 400 eV was used for the planewave basis set. The MoS2/electrolyte interface is modeled using 1T MoS2 monolayer with one layer of water film on top and excess H atoms in the water layer. In this model, we used the recently developed DFT-D3 method to include dispersion correction for the hydrogen bonding interactions between water molecules.58 The advantage of including the water layer is that we can correlate the potential with the calculated work function and study the proton-electron transfer reaction (Volmer & Heyrovsky), which would be impossible for previous DFT calculations without considering the water layer. The calculations were carried out in periodically repeated 6×4 3 and 9×4 3 rectangular supercells of 1T MoS2 monolayer with 4×4×1 and 2×3×1 Monkhorst Pack k-point sampling, respectively. The climbing-image nudged elastic band (CI-NEB) method59 implemented in VASP was used to determine the diffusion energy barrier and the minimum energy pathways for H2 evolution on 1T MoS2 surface. The convergence threshold for structural optimization was set to be 0.02 eV/Å in force. The transition states were obtained by relaxing the force below 0.05 eV/Å. Note that with adsorption of H, the 1T MoS2 lattice will become structurally distorted to the 1T´phase, as denoted in other theoretical papers.47,50,51 In this paper, we do not explicitly distinguish the two phases but use the 1T phase for both cases.


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The free-energy of the adsorption atomic hydrogen (∆GH) is obtained by ∆GH = ∆EH + ∆EZPE - T∆SH. ∆EH describes the energy needed to increase the coverage by one hydrogen atom, which is calculated as follows: ∆EH = E[MoS2+nH] – E[MoS2+(n-1)H] – ½ E[H2], where E[MoS2+nH] and E[MoS2+(n-1)H] represent the total energy of the 1T-MoS2 system with n and n-1 adsorbed hydrogen atoms on the surface, respectively, while E(H2) represents the total

energy of a gas phase H2 molecule. ∆EZPE is calculated by: ∆

,

where denotes the zero-point energy of n adsorbed hydrogens on 1T-MoS2 (without the

contribution of 1T-MoS2 catalyst), and denotes the zero-point energy of gas phase H2. The

calculated vibrational frequencies of H adsorbed on 1T-MoS2 are 2280 cm-1, 690 cm-1 and 570 cm-1, which do not differ notably for the different H coverages. The calculated vibrational frequencies for H2 gas are 2730 cm-1, 210 cm-1 and 150 cm-1. Thus ∆EZPE is simplified as 0.06

eV. Further, ∆SH is obtained by: ∆ ≅ , where is the entropy of H2 in the gas phase at standard condition, and the gas phase value can be taken from reference ( ~130 J·mol-1·K-1).60 Therefore, ∆GH can be rewritten as ∆GH = ∆EH + 0.26, where 0.26 eV is a correction constant.

Figure 1. Calculated differential free energy of hydrogen adsorption (∆GH) as a function of hydrogen coverage on 1T-MoS2 surface in vacuum.


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3. Results and discussion 3.1 Gibbs free energy (∆GH) of atomic hydrogen on the surface and the optimal coverage. ∆GH of atomic hydrogen adsorbed on a catalyst is considered as a good descriptor of HER activity.61 An optimum HER activity can be achieved at a value of ∆GH close to zero (∆GH~0), indicating that the free energy of adsorbed H is close to that of the reactant or product. Lower ∆GH will lead to strong bonding to adsorbed hydrogen, while higher ∆GH will make the protons bonded too weakly to the catalyst surface, both leading to slow HER kinetics. The calculated free energy ∆GH as a function of surface H coverage in the absence of water and a bias potential is shown in Figure 1. The adsorption geometries at different H coverage were provided in Supporting Information (Figure S1). Once H is adsorbed, the 1T phase structurally transforms to the 1T' phase (Figure S2). This structure change helps adsorption of H on the 1T phase, but another important factor is the 1T phase’s metallicity and partially filled Mo 4d states.42 From Figure 1, one can see that the free energy close to zero occurs around 12.5% ~ 25% H coverage (the ∆GH value is between -0.28 eV and 0.13 eV). Therefore, it is most likely that the hydrogen evolution reaction is mainly driven by the hydrogen adsorption at 12.5% ~ 25% coverage. Particularly, the appropriate H adsorption energy suggests that the basal plane of 1T MoS2 is catalytically active, in good agreement with other theoretical results.47,48,50 The adsorbed H prefers to bind to the surface S atoms, regardless of the coverage. Our previous work showed that the metastable 1T MoS2 can be stabilized by hydrogenation and after a crossover coverage of 25%, the 1T phase becomes more stable than the 2H phase.42 Therefore, considering ∆GH and the relative stability together, one can see that 25% coverage of hydrogen is optimal.


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3.2 The water-solid interface. Now that we have found out the optimal coverage of H on 1TMoS2, next we examine its HER activity. To simulate the water-solid interface under an electrochemical potential, here we adopted an approach previously used by Nørskov et al. in elucidating the reaction mechanism of HER on Pt(111) electrode.62 The interface region (or the Helmholtz layer) is often approximated by about 3 Å thick electrical double layer, which can be represented by one water layer. Each proton concentration in the water layer corresponds to a certain electrode potential versus the normal hydrogen electrode (NHE), where the electrode potential is deduced from the calculated work function. By varying the number of hydrogen atoms added, the electrostatic potential of the water–solid interface can be varied. In our system, the water–solid interface in an acid solution was modeled with one layer of proton-containing water molecules on the surface of H-adsorbed 1T-MoS2 monolayer. We considered intermediate-sized 6×4 3 (Figure 2a) and 9×4 3 (Figure 2b) lateral cells of 1TMoS2 monolayer with 25% H coverage. To determine a stable concentration of protons in the surface water layer, we performed 10 ps of DFT molecular dynamics simulation at different numbers of protons in the water layer and found that the 1T-MoS2 monolayer having two protons (or excess hydrogen atoms) in the water layer containing 16 H2O molecules is stable. Moreover, to determine the stable water structure, we performed parallel DFT-MD simulations with three different initial water structures: (1) the molecular plane of H2O molecule parallel to the surface; (2) the H atom of each H2O molecule pointing down towards the surface; (3) the H atom of each H2O molecule pointing up away from the surface. The structure with the lowest energy after MD runs was chosen as the model for the subsequent mechanistic studies. The annealed water model is different from the initial water structure. Specifically, in the annealed structure, the H2O molecules atop surface S atoms will adopt the H-down configuration, due to the hydrogen bond


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between H and the surface S, while those H2O molecules with adsorbed H underneath will adopt the flat-lying configuration. Figure 2a presents the optimized structure of 1T-MoS2 containing 2 solvated protons at 25% H coverage, where the two hydronium ions are indicated by arrows. The water layer in our model is disordered, probalby caused by the structural distortion of the 1T MoS2 lattice, the adsorbed H atoms on the surface, and the hydronium ions in the water layer. To our knowledge, there has been no experimental study of the exact structure of the water overlayer on 1T MoS2. The calculated electrode potential (relative to NHE) corresponding to the model system in Figure 2a is -0.22 V. This is in good agreement with the experimentally measured overpotential, where the exfoliated 1T-MoS2 nanosheets exhibit an onset of HER activity at a low overpotential of approximately -0.20 V vs reversible hydrogen electrode. In our following study on the elementary reaction steps of HER process, we will keep the proton concentration in the water layer fixed to be 1/8, i.e., 2 protons out of 16 water molecules for the 6×4 3 superstructure (Figure 2a). To examine the effect of cell size, we also considered a larger unit cell, the 9×4 3 superstructure, with its water layer (Figure 2b) that contains 3 protons out of 24 water molecules, thereby maintaining the same H+/H2O ratio of 1/8.


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Figure 2. Model for the solvated 1T-MoS2 system: (a) the 6×4

3 superstructure and (b) the 9×4 3

superstructure), both at 25% surface H coverage with 1/8 proton concentration in the water layer. Top panel: top view; bottom panel, side view. Color code: yellow, S; cyan, Mo; pink, adsorbed surface H; white and red balls, H3O+, indicated by arrows; white and red sticks, water molecules.

3.3 The first step of the HER process: the Volmer reaction. For HER in acid media, two types of possible pathways have been proposed for reducing protons to hydrogen, namely, the Volmer–Heyrovsky

or

the

Volmer–Tafel

mechanism (Scheme 1). The Volmer reaction refers to the initial adsorption of protons from the acid solution to form adsorbed H (H+ + e- → Had), which is usually considered to be fast. In the Volmer–

Scheme 1. Mechanisms of hydrogen evolution reaction. Hads means H atom adsorbed on the catalyst surface.

Heyrovsky mechanism, a solvated proton from the water layer reacts with one adsorbed surface hydrogen to form H2 (Had + H+ + e- → H2), while in the Volmer–Tafel mechanism, two adsorbed surface hydrogens next to each other react to form an H2 molecule (Had + Had → H2). Note that for the Volmer or Heyrovsky reaction, performing the proton-electron transfer reaction in a limited size supercell with periodic boundary conditions will result in changes of the electrode potential along the reaction path. Here we take the “extrapolation scheme” as proposed by Nørskov et al. to circumvent this problem.63,64 Specifically, we calculate the reaction energies and activation energies for a charge transfer reaction (Volmer or Heyrovsky reaction) for different supercell sizes, and then we extrapolate the results (such as activation energy) to the limit where the change in potential during the reaction, ∆U, approaches to zero. ∆U is the work-function difference between the initial state and the final state.


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First we consider the initial step of HER process: the Volmer reaction. Figure 3a presents the minimum-energy path of transferring one of the solvated protons to the 1T-MoS2 surface for the 6×4 3 supercell; one can see that the activation energy is small (at 0.004 eV). At the transition state, the proton H is bonded between O and S atoms, and the O-H and S-H bond lengths are about 1.25 Å and 1.64 Å, respectively. We also considered a larger unit cell, the 9×4

3 supercell, and the minimum-energy path shows an activation energy of 0.05 eV and a similar transition state (Figure 3b). To extrapolate the reaction and activation energies to the limit where the potential does not change (∆U close to 0 V), we also included the data for another system with a different hydrogen coverage in the final state and the results are shown in Figure 4. We obtained a reaction energy (∆E between final state and initial state) of -0.07 eV (Figure 4a) and an activation energy of 0.16 eV (Figure 4b). This indicates that the Volmer process (Scheme 1) is thermally neutral (∆E ~ 0) and has a small energy barrier at the electrode potential around -0.2~0.3 V vs. NHE and hydrogen coverage around 25% on the 1T-MoS2 monolayer.

Figure 3. The minimum-energy pathways of the Volmer reaction on two models of the 1T-MoS2 surface: (a) the 6×4

3 superstructure, with a final H coverage of 4/16 and a final proton concentration of 2/16; (b) the 9×4 3 superstructure, with a final H coverage of 6/24 and a final proton concentration of 3/24; inserts show side views of


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the initial, transition, and final state structures, where the transferred H to the surface at the final state is shown as a white ball.

Figure 4. (a) Reaction energy (∆E) and (b) activation energy (Ea) for the Volmer reaction as a function of the change in electrode potential ∆U. 4/16, 5/24 and 6/24 indicate the surface H coverages in the final states; for all the three cases, the proton concentration of the final state is 1/8.

3.4 The second step of the HER process via the Heyrovsky reaction. In the second step of the HER process, two elementary steps to evolve H2 are possible (Scheme 1): the Tafel reaction or the Heyrovsky reaction. We start with the Heyrovsky reaction whereby a proton in the water layer reacts with an adsorbed hydrogen to form H2, and the calculated minimum-energy paths at two different supercell sizes were presented in Figure 5. One can see that an adsorbed hydrogen atom on a sulfur atom approaches a hydrogen atom of a hydronium ion in the water layer, then the adsorbed H breaks away from the surface, and H2 molecule forms inside the water layer. In the case of 6×4 3 supercell (Figure 5a), we obtained an activation energy of 0.95 eV. The activation energy decreases to 0.81 eV for the 9×4 3 supercell (Figure 5b). At the transition state, the adsorbed hydrogen (pink) is partially broken away from the surface and close to the accepting hydrogen (white); the distance between the two interacting H atoms is 1.12 Å in Figure


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5a and 1.19 Å in Figure 5b. After crossing the transition state, the final state with a weakly adsorbed H2 molecule above the surface was formed, where the H-H bond length is 0.76 Å. When both the reaction energy (∆E) and the activation energy (Ea) are extrapolated to ∆U = 0 V, we get ∆E of -0.80 eV (Figure 6a) and Ea of 0.62 eV (Figure 6b) from the intercepts. So the energy barrier for the Heyrovsky reaction is much higher than that for the Volmer reaction (Ea ~ 0.16 eV), indicating that the H desorption process should be the rate-determining step of the Volmer-Heyrovsky pathway.

Figure 5. The minimum-energy pathways of the Heyrovsky reaction on two models of the 1T-MoS2 surface: (a) the 6×4

3 superstructure, with an initial H coverage of 4/16 and an initial proton concentration of 3/16; (b) the 9×4

3 superstructure, with an initial H coverage of 6/24 and an initial proton concentration of 4/24; inserts show side views of the initial, transition, and final state structures.


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Figure 6. (a) Reaction energy (∆E) and (b) activation energy (Ea) for the Heyrovsky reaction as a function of the change in electrode potential ∆U. 3/16, 4/24, and 5/24 indicate the surface H coverages in the final states.

3.5 The second step of the HER process via the Tafel reaction. We next turn to the Tafel reaction, where two surface H atoms chemically desorb to form H2. Since there is no charge transfer involved in the Tafel process (the work functions of the initial and final states remain nearly unchanged, so the potential of the reaction system will stay the same), it is not necessary to use the extrapolation method as used for the Volmer and Heyrovsky reactions to determine ∆E and Ea. In this case, we calculated the energetics of Tafel reaction on the 6×4 3 superstructure. Figure 7a presents the minimum-energy path of the Tafel reaction with an initial hydrogen coverage of 5/16. The initial state has an H-H distance (two adjacent H atoms bonded to the surface S atoms) of 3.56 Å. The two H atoms then approached and formed a transition state with H-H distance of 1.24. In the final state, the evolved H2 molecule desorbs from the surface with H-H bond length of 0.76 Å. The activation energy for this Tafel-step reaction is 1.07 eV, with a favorable reaction energy of -0.79 eV.

Figure 7. (a) Reaction paths from NEB calculations showing the energy barrier for Tafel reaction on 6×4

3 1T-

MoS2. Here the initial state corresponds to proton concentration of 2/16 and surface hydrogen coverage of 5/16, where two adsorbed H atoms react to form H2 that desorbs from the surface, and the final state corresponds to


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hydrogen coverage of 3/16. (b) The calculated activation energy for the Tafel reaction as a function of initial hydrogen coverage and the corresponding electrode overpotential.

Since the Tafel reaction is a surface-dominated process and less dependent on the water layer, it will be more sensitive to the surface hydrogen coverage. As shown in Figure 7b, as the hydrogen coverage increases, the activation barrier of the Tafel reaction decreases from 1.19 eV for hydrogen coverage of 4/16 to 0.89 eV for hydrogen coverage of 7/16. We also computed the corresponding electrochemical potential (from work function) at different hydrogen coverages (Figure 7b). One can see that the potential becomes more negative with coverage and the Tafel step of homolytic hydrogen desorption from surface becomes easier under more negative overpotential. For the electrode potential between -0.2 ~ -0.3V where the HER happens on 1TMoS2, the corresponding hydrogen coverage lies between 4/16 and 5/16, and the activation barrier for the Tafel reaction is still higher than 1 eV. In addition, Figure 1 shows that 25% (4/16) is an optimal coverage, in terms of binding of hydrogen with 1T-MoS2.

Figure 8. Free energy diagram for the Volmer-Heyrovsky route (a) and Volmer-Tafel route (b) on 1T MoS2 at electrode potential of -0.22 V (~25% surface H coverage).


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3.6 The overall mechanism. In Figure 8, we plotted a free energetic diagram of the VolmerHeyrovsky and Volmer-Tafel reaction pathways on 1T-MoS2 at electrode potential of -0.22 V and ~25% surface H coverage (here the relative energy for Volmer, Heyrovsky and Tafel reactions is calculated based on the same 6×4 3 supercell). By comparing the energy barriers for the Tafel and Heyrovsky reactions on 1T-MoS2, one can find a considerably lower energy barrier for the Heyrovsky reaction than that for the Tafel reaction. This suggests that the Heyrovsky reaction is much faster and the Volmer–Heyrovsky mechanism is the main pathway of HER on 1T-MoS2 at an electrode potential of -0.22 V and a moderate hydrogen coverage of ~25%. Figure 9 illustrates the overall HER process on 1T MoS2 surface, starting from 1T MoS2 to adsorbed sulfur-bound hydrogen with a small barrier of 0.16 eV, followed by the Heyrovsky reaction of the adsorbed H with attacked (H3O)+ proton to generate H2 gas with a barrier of 0.62 eV (the right bottom illustrates the transition state structure of the Heyrovsky process).

Figure 9. Overall reaction mechanism for HER on the surface of 1T MoS2.


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Experimentally the Tafel slope is viewed as an intrinsic property of electrocatalysts, which is determined by the rate-limiting step and allows for the distinction of possible mechanisms for HER. Normally, the HER kinetic models suggest that the Volmer, Heyrovsky or Tafel reaction will be the rate-determining step for a Tafel slope of about 120, 40 or 30 mV per decade, respectively.65 In fact, the experimentally measured Tafel slope from exfoliated metallic nanosheets of 1T-MoS2 at a low overpotential of ~0.2 eV is about 40 mV/decade.36,38 This low Tafel slope suggests that the Heyrovsky-step-limited Volmer–Heyrovsky mechanism is responsible for HER catalyzed by 1T-MoS2 nanosheets, in agreement with our DFT results. Here we note that in the microkinetic analyses of some experimental HER data, the Heyrovsky mechanism was found to be the rate-limiting step in some extreme cases.66 3.7 Comparison with previous DFT studies. Recently, Goddard et al. reported DFT calculations to determine the HER pathways on the Mo-edge of 2H MoS2 monolayer using both a Mo10S21 cluster model and a periodic slab.67 Based on free-energy barriers, they found that the HER process at 2H MoS2 edges takes place mainly through the Volmer−Heyrovsky mechanism and the complete pathway involves adsorption of hydrogen on S, diffusion of hydrogen from S to Mo to form molybdenum hydride (Mo-H), and formation of H2 from (Mo-H) and a hydronium ion (H3O+). Although here we also found that HER happens via the Volmer−Heyrovsky process on the basal plane of 1T MoS2, the detailed mechanism is very different: the formation of molybdenum hydride on 1T MoS2 is highly unfavorable; instead, we found that the Heyrovsky process can happen directly between hydrogen adsorbed on sulfur and a hydrogen from the hydronium ion. It is interesting to note that our estimated activation energy from the Heyrovsky step on 1T MoS2 is about 0.62 eV, which is comparable to the energy barrier of 0.78 eV on 2H MoS2 from Goddard et al. This implies that the basal plane of 1T MoS2 could be more


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electrocatalytically active than the edges of 2H MoS2. Considering the high reactivity and important catalytic role of the unsaturated edges, it would also be worthwhile in the future to extend the current study to investigate the HER activity at the edge sites of 1T MoS2. More important, the high activity of the basal plane of 1T MoS2 suggests that the 1T phase can provide much more active areas for HER and that one could further engineer the basal plane such as by doping or creating defects to enhance the HER activity. Several recent theoretical reports examined the HER activity of 1T MoS2, 47-54 most of them using the ∆GH descriptor. Lau et al.,47 Kuo et al.,50 Du et al.51 and Abild-Pedersen et al.48 reported that both the basal surfaces and the edges sites of 1T-MoS2 are active for HER, with ∆GH close to thermoneutral. Our findings on the reactivity of the basal plane are in agreement with their results. Abild-Pedersen et al.48 also calculated the possibility of H2 desorption via the Tafel mechanism on the 1T-MoS2 basal plane, and found an insurmountably high barrier of about 1.5 eV, which suggests that the Tafel mechanism is less unlikely the relevant route for HER. However, their calculations did not consider the interfacial solvent or the Heyrovsky mechanism. Our complete HER pathway further confirmed that the Tafel route is less favorable than the Heyrovsky one. 3.8 Doping of 1T-MoS2. Doping has been widely used to tune the physical and chemical properties of materials. It is well-known that the hydrodesulfurization (HDS) performance of MoS2 can be significantly increased by addition of metal promoters such as Co or Ni.68-70 The introduction of metal dopants such as Pt, Fe, Co and Ni has been recently demonstrated to promote the catalytic activity of 2H MoS2 for hydrogen generation.71-75 Moreover, there have also been experimental reports of anion doping in 2H MoS2 lattice by O76 and Cl,77 leading to dramatically enhanced HER activity. Here we tested the doping effect on the HER activity of 1T


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ACS Catalysis

MoS2 by substitutional doping of the Mo site with other transition metals. As shown in Figure 10a, the S atom highlighted by the dashed circle denotes the H binding site, the neighboring purple atom denotes the substitutional doping site for the Mo atom. We found that doping of the Mo site by V, Cr, Mn, Fe, Co, Ni, Nb, Ta, W and Re induces negligible structural changes to the 1T-MoS2 structure, while Cu doping induces slightly larger lattice distortion (about 0.6% tensile strain). So the impact of doping here is more on the electronic strucuture than on the strain, unlike the case of graphitic carbon nitride where strain induced by substitutional doping can tune the HER activity.78 The calculated ∆GH of the doped systems is shown in Figur 10b, as compared to the pristine 1T MoS2. One can see that doping with Mn, Cr, Cu, Ni and Fe for Mo reduces ∆GH, and among them, doping with Cr leads to a ∆GH value more close to zero. Hence, we predict that the HER catalytic activity of 1T MoS2 nanosheets can be improved through doping with Mn, Cr, Cu, Ni, or Fe.

Figure 10. (a) Top view of hydrogenated 1T-MoS2 with Mo substitutional doping. The circled atom indicates the H binding site, the purple atom indicates the Mo doping site. (b) The calculated free energy diagram for hydrogen evolution of doped and pristine 1T MoS2.


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4. Summary and conclusions In conclusion, we carried out DFT calculations in this work to understand the electrocatalytic HER mechanism on the basal plane of conducting 1T MoS2 nanosheets. Calculated ∆GH indicates that the HER process is most likely to occur at the surface hydrogen coverage between 12.5%~25%, where the adsorbed H prefers to be bonded to surface S atoms. We computed reaction pathways and kinetic barriers for the three elementary steps involved in HER process. We found that at ~25% hydrogen coverage (electrochemical potential close to 0.22 V vs hydrogen electrode), the Volmer reaction is very fast (activation barrier ~ 0.16 eV); while for the hydrogen desorption process, the Heyrovsky reaction (activation barrier ~ 0.62 eV) has relatively much lower barrier than the Tafel reaction (activation barrier ~ 1.07 eV). Our finding revealed that the HER happens mainly via the Volmer–Heyrovsky mechanism on the basal plane of 1T MoS2 electrode at a low overpotential. The previously measured Tafel slope (~ 40 mv/decade at overpotential of -0.20 V) supports this mechanism. In addition, we discovered that the HER activity of 1T MoS2 can be further increased by doping of the MoS2 lattice with other metal atoms (e.g., Mn, Cr, Cu, Ni, Fe). These results offer useful insights for understanding and improving the HER activity of the 1T phase of MoS2 and other 2D TMDs nanosheets.

Acknowledgments This work was supported by the University of California, Riverside. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract DE-AC02-05CH11231.


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ACS Catalysis

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. •

Hydrogen adsorption geometries on 1T MoS2

•

Structure change to 1T MoS2 caused by H adsorption

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Mechanism of Hydrogen Evolution Reaction on 1T-MoS2 from First

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