Operando Characterization of an Amorphous Molybdenum Sulfide

Article pubs.acs.org/JPCC

Operando Characterization of an Amorphous Molybdenum Sulfide Nanoparticle Catalyst during the Hydrogen Evolution Reaction Hernan G. Sanchez Casalongue,† Jesse D. Benck,‡ Charlie Tsai,‡,∥ Rasmus K. B. Karlsson,⊥ Sarp Kaya,†,§ May Ling Ng,∥ Lars G. M. Pettersson,@ Frank Abild-Pedersen,∥ J. K. Nørskov,‡,∥ Hirohito Ogasawara,# Thomas F. Jaramillo,†,‡ and Anders Nilsson*,†,∥,# †

Joint Center for Artificial Photosynthesis (JCAP) Energy Innovation Hub, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, MS 976-JCAP, Berkeley, California 94720, United States ‡ Department of Chemical Engineering, Stanford University, 381 North-South Mall, Stanford, CA 94305, United States § Department of Chemistry, Koc University, Rumelifeneri Yolu, Sariyer, 34450 Istanbul, Turkey ∥ SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025, United States ⊥ Applied Electrochemistry, School of Chemical Science and Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden @ Department of Physics, AlbaNova University Center, Stockholm University, SE-106 91 Stockholm, Sweden # Stanford Synchrotron Radiation Lightsource (SSRL), SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025, United States S Supporting Information *

ABSTRACT: Molybdenum sulfide structures, particularly amorphous MoS3 nanoparticles, are promising materials in the search for cost-effective and scalable watersplitting catalysts. Ex situ observations show that the nanoparticles exhibit a composition change from MoS3 to defective MoS2 when subjected to hydrogen evolution reaction (HER) conditions, raising questions regarding the active surface sites taking part in the reaction. We tracked the in situ transformation of amorphous MoS3 nanoparticles under HER conditions through ambient pressure X-ray photoelectron spectroscopy and performed density functional theory studies of model MoSx systems. We demonstrate that, under operating conditions, surface sites are converted from MoS3 to MoS2 in a gradual manner and that the electrolytic current densities are proportional to the extent of the transformation. We also posit that it is the MoS2 edge-like sites that are active during HER, with the high activity of the catalyst being attributed to the increase in surface MoS2 edge-like sites after the reduction of MoS3 sites.

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INTRODUCTION A key hurdle for photoelectrocatalysis in a solar-to-fuel generator or for the electrolysis of water to avoid the use of expensive noble metal catalysts and still achieve high current densities during the electrolysis process.1−4 It is desirable to move away from rare elements and catalyze the water-splitting reaction through cheap, earth-abundant materials. This challenge has led to the exploration of molybdenum sulfides as catalysts for the hydrogen evolution reaction (HER)5−7 in a vast assortment of structures, such as nanoparticles,8 nanowires,9 mesoporous materials,10 and vertically aligned thin films.11 A recent improvement in this material has been the synthesis of amorphous molybdenum sulfide nanoparticles,12,13 whose high performance and usage of earth-abundant materials make them a promising candidate to use for a water-splitting © XXXX American Chemical Society

device. But this new material brings along additional questions, for ex situ measurements show that the nanoparticles undergo a composition change, starting as MoS3 and becoming more MoS2-like at the surface as they are used for HER.6,12,13 In the context of these measurements, it is not clear how this transformation takes place, if it happens solely during operation conditions or if there is a correlation between the extent of the transformation and the catalytic currents. Furthermore, it is also unclear which catalyst sites, that is which combination of Special Issue: John C. Hemminger Festschrift Received: June 1, 2014 Revised: October 27, 2014

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dx.doi.org/10.1021/jp505394e | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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molybdenum and sulfur atoms14,15, actively participate in the HER, which is crucial information for understanding the elementary steps of the reaction and the source of the high catalytic activity. It is important that these questions are answered, since they address both the suitability of the material for long-term use and the operating conditions of a potential device. Shedding light on these unknowns requires understanding the catalytic process as it takes place and that requires the use of characterization techniques with the ability to probe the surface under operating conditions. Ambient pressure X-ray photoelectron spectroscopy (APXPS)16,17 is a technique ideally suited for this task, with the chemical sensitivity of XPS enabling the differentiation of distinct chemical compositions while the near-ambient pressure capabilities afford access to relevant reaction conditions for electrochemical processes.18−21 In the present work, we use an electrochemical cell designed to be compatible with an APXPS system20,21 to investigate the changes that take place at the catalytic surface of amorphous MoS3 nanoparticles under operating conditions. We conclude that the catalyst changes from MoS3 to MoS2 in a gradual manner as a function of the catalytic currents. This assignment is supported by accompanying density functional theory (DFT) calculations. We also posit that it is the MoS2 sites that are active during HER, with the high activity of the catalyst being attributed to the increase in surface MoS2 edge-like sites after the reduction of MoS3 sites.

Figure 1. In situ APXPS electrochemistry. (a) Schematic drawing of a PEM electrochemical cell setup for APXPS investigations: the Nafion membrane is coated on the cathode/working electrode (WE) side with carbon-supported molybdenum sulfide nanoparticles. The anode/ counter electrode (CE) consists of carbon-supported platinum nanoparticles. Water vapor is introduced into the system through the APXPS gas cell and the counter electrode side. White and red circles represent hydrogen and oxygen (single blue circles represent protons); the black layer represents the porous carbon-Nafion support; gray circles represent the metallic platinum nanoparticles; pink circles represent the amorphous molybdenum sulfide nanoparticles. (b) Cyclic voltammetry of the molybdenum sulfide electrode collected with a 40 mV/s scan rate under 5 Torr of water. Due to the lack of a reference electrode, the voltage is expressed in terms of total applied cell voltage.

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EXPERIMENTAL SECTION Building on previous operando electrochemical APXPS studies,20,21 the experimental setup consisted of a PEEK polymer framework with two chambers separated by a PEM (Figure 1a). The membrane is made of Nafion 115, coated on one side with Pt nanoparticles supported on a Nafion/carbonblack mixture with catalyst loading of 4 mg/cm2 (particle diameter 10−20 nm) from Fuel Cell Store, Inc., Boulder, CO, which serves as the counter electrode. The working electrode was coated with amorphous molybdenum sulfide nanoparticles, synthesized by the procedure described by Benck et al.,12 using a two-step drop casting method: 0.2 g Vulcan XC-72 Carbon black and 0.44 mL 5% Nafion 117 solution (Aldrich) were dissolved in approximately 5 mL of isopropanol and mixed by means of 30 min of sonication. 50 μL of this mixture was added to a 1 cm2 area of the blank side of the membrane, resulting in a uniform carbon-Nafion coating. With the conducting support in place, 1 mg of amorphous molybdenum sulfide nanoparticles (particle diameter ∼50 nm) suspended in isopropanol were drop cast onto the surface. A control experiment was prepared by coating a second membrane through the same protocol but replacing the amorphous nanoparticles with 0.1 mg of crystalline MoS2 particles. These crystallites were obtained from extensive (30 min) high-frequency sonication of a commercially obtained MoS2 crystal. The assembled cells were introduced into the APXPS system on beamline 13-2 at the Stanford Synchrotron Radiation Lightsource (SSRL), where electrode and membrane humidification were ensured by the introduction of both saturated water vapor to the counter electrode through small tubes and water vapor gas to the working electrode via variable leak valves in the gas cell. Both electrodes were connected to an external Pinewave potentiostat, allowing for the characterization of the electrochemical properties of the system as a function of applied potential biases. Figure 1b shows a two-electrode cyclic voltammogram of the amorphous molybdenum sulfide

electrode under a 5 Torr water vapor atmosphere. The current corresponding to hydrogen reduction12 is observed at potentials below −1.0 V with respect to the counter electrode. All XPS and electrochemical measurements were performed at room temperature. All XPS spectra were collected from the molybdenum sulfide cathode, and their binding energies (BEs) are referenced to the Fermi level through calibrating the C 1s signal of the graphitic carbon to 284.5 eV under corresponding experimental conditions. The average collection time for each experimental condition was about 3 h. For the S 2p spectra, the signal intensity is normalized to the background intensity. Spectral deconvolutions and background subtractions (linear backgrounds) were performed using the Igor-Pro software. Each S 2p spectrum recorded in HER conditions was deconvoluted using asymmetric Gaussian−Lorentzian functions (60% Gaussian, 40% Lorentzian, 0.5 asymmetry), with BE for each component chosen from experimental or literature references. The inelastic mean-free path was calculated using the predictive G-1 equation in the NIST Electron InelasticMean-Free-Path database, version 1.2. B



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COMPUTATIONAL DETAILS

Apart from the XPS spectral shift calculations, all calculations were performed using plane-wave DFT employing ultrasoft pseudopotentials as implemented in the Quantum ESPRESSO code.22 An infinite striped model described previously was used to model the Mo-edge of MoS2.23−28 Both Mo edges and S edges are exposed, and the structure of the S edge was held fixed. A plane-wave cutoff and density cutoff of 500 and 5000 eV, respectively, were used. The bulk lattice constants were determined to be a = 3.19 Å and c = 13.05 Å, in reasonable agreement with experimental values of a = 3.16 Å and c = 12.29 Å.29 As in the subsequent modeling, only single layers were considered in the present study, the small discrepancy in the c value should not affect the conclusions. A unit cell containing four Mo atoms by four Mo atoms was used, with at least 9 Å of vacuum in the y direction (perpendicular to the edges) and 11 Å of vacuum in the z direction (perpendicular to the basal plane). This was the same setup used in previous studies.27,28 The Brillouin zone was sampled by a Monkhorst−Pack 2 × 1 × 1 k-point grid,30 and the criterion for structural relaxation was when the total forces were less than 0.05 eV/Å. The edge free energies were calculated following the same method as in previous studies.23,28,31 The Mo-edge is generally more stable than the S-edge, 23,32 so the amorphous surface was approximated by MoS2 Mo-edges. Adsorption free energies for H were also calculated in the same way as in the previous studies. S 2p XPS chemical shift calculations were carried out using the numerical-grid-based, projector-augmented wave electronic structure code GPAW (version 0.10) with the PAW setups of version 0.9.11271.33−35 The first principles DFT calculations were performed using the RPBE36 functional. The functional dependence of calculated XPS shifts has previously been shown to be insignificant on the GGA level.37 Gamma-point-only calculations using a grid spacing of 0.2 Å were carried out, using supercells which allowed a separation of at least 19 Å between core holes. XPS shifts were calculated for the Mo-edge of MoS2, with increasing coverages of S ranging from 50 to 100%. The S 2p XPS binding energies were calculated as the difference in total energies between the energy of the supercell with a 2p core hole on one S atom and that of the same supercell but without the core hole. The shift was then calculated as the difference between this XPS binding energy (BE) and a reference S 2p XPS BE of an S atom on the Mo-edge with 50% S coverage. All unique S atoms on the various modeled Moedges from the structure optimization studies above were probed in this way.

Figure 2. S 2p spectra of molybdenum sulfides. (a) S 2p signal of amorphous molybdenum sulfide nanoparticles under various experimental conditions collected with an incident photon energy of 900 eV. The water vapor pressure, applied cell voltage, and resulting cell current density are indicated for each spectrum. The 0−VIII labels next to each spectrum indicate the order in which they were taken. (b) S 2p signal of crystalline MoS2 under various experimental conditions collected with an incident photon energy of 900 eV. The water vapor pressure, applied cell voltage, and resulting cell currents are indicated for each spectrum. The IX−XII labels next to each spectrum indicate the order in which they were taken.

the system is taken to more extreme experimental conditions (V−VIII). Due to the two-electrode setup, the reported voltage values correspond to the total cell voltage (i.e., the potential required for the reaction at both electrodes and the potential drops due to series resistance). Nonetheless, with enough applied voltage, the cell reaches current density regimes on the order of mA/cm2, which remained constant during the XPS acquisition. The electrochemical data also show that humidification plays an essential role for the cell currents, with the transition from 1 to 5 Torr of water (III to IV), resulting in more than doubling the cell current, even with a lower applied voltage. This is attributed to the condensation of a water layer entirely covering the surface as the relative humidity reaches 30%,38 which underscores the need of using techniques, such as APXPS, that enable composition and chemical state measurements under the relevant operating conditions. Figure 2b shows the S 2p spectra for the crystalline MoS2 electrode probed with the same photon energy and similar reaction conditions as the amorphous MoS3 electrode. The water vapor pressure, applied voltage, and cell current density are shown for each spectrum, with the low experimental currents being attributed to low catalyst loading. When comparing the amorphous and the crystalline samples, it is

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RESULTS AND DISCUSSION Figure 2a shows the S 2p XPS spectra of an amorphous molybdenum sulfide nanoparticle electrode surface probed with incident photon energy 900 eV under vacuum (0) and under water vapor pressures of 1−5 Torr at open circuit (I) and different potential biases (II−VIII). The water vapor pressure, applied voltage, and cell current density are shown for each spectrum. The XPS spectra reveal that, as the system is humidified and taken to operating HER conditions, there is an irreversible change in the catalyst composition, with the growth of a lower BE feature at 162.0 eV. This change, which is independent of X-ray beam exposure (see the Supporting Information), is slow under humidification and μA/cm2 cell current densities (I−IV), but it becomes more prominent once C



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possible to observe that the growing feature at low BE in the amorphous nanoparticles at 162.0 eV coincides with that of the S 2p3/2 component in crystalline MoS2, in agreement with the ex situ observations. In contrast to the amorphous MoS3 sample, the crystalline MoS2 sample shows no changes in the XPS spectra after being subjected to similar humidification and potential biases. Taking into consideration that at 900 eV excitation energy, the inelastic mean free path for S 2p electrons is about 2.2 nm, we can conclude that the transformation occurs at the surface sites of the nanoparticles. Combining the MoS2 crystalline results with reference spectra from ex situ experiments,6,12,13 it is possible to deconvolute the spectra of the nanoparticles into two components. First, the sulfur atoms in MoS2 sites with S 2p3/2 and S 2p1/2 signatures at 162.0 and 163.2 eV, respectively; second, the higher BE features at 163.5 and 164.7 eV, assignable to S 2p3/2 and S 2p1/2 of the MoS3 sites (see DFT calculations below). The broadening of the peaks with respect to the crystalline sample suggests the existence of a continuum of compositions between MoS3 and MoS2, as supported by the DFT calculations; this would imply a highly defective structure. Figure 3 (panels a and b) shows two of the spectra in Figure 2a,

Table 1. Composition of MoS2 and MoS3 Components of the Correspondingly Labeled Spectra Shown in Figure 2a Based on the Curve Fitting Parameters of Figure 3 (panels a and b) (Individual Fits are shown in Figure S3 in the Supporting Information) spectrum

% MoS2

% MoS3

0 I II III IV V VI VII VIII

35.0 43.5 50.5 53.8 54.2 68.4 70.6 74.7 79.0

65.0 56.5 49.5 46.2 45.8 31.6 29.4 25.3 21.0

constant. This correspondence between performance and composition leads us to conclude that catalytic activity improves as MoS3 sites are transformed to MoS2, which we interpret to indicate that HER takes place mostly at the MoS2 sites. This correlation helps explain the ex situ observation of improved catalytic performance after the first reductive cycle:12 As made, the surface is covered with lower-activity MoS3 sites, but when taken to operating conditions, the nanoparticles are transformed into a new surface with an atomic coordination resembling that of highly active crystalline MoS2 edges, on which HER takes place,14,15 thus increasing the catalytic activity of the material. In a similar manner to the S 2p spectra, information on the catalyst can be obtained from collecting Mo 3d spectra. Figure 4a shows the Mo 3d and S 2s regions of XPS probed with an incident photon energy of 900 eV for experimental conditions (I), (III), (V), and (VII) in Figure 2a. Although similar trends in the MoS2 to MoS3 ratio can be observed, most of the information is obscured by the growth of high BE features at 232.3 and 235.5 eV, corresponding to the Mo 3d5/2 and Mo 3d3/2 components of molybdenum oxide (VI).39 We explain this change as due to the reaction of the outermost layers of the catalyst with air, creating a thin layer of oxide with similar processes seen ex situ.12 We attribute the presence of oxygen at the cathode to a small gas leak from the counter electrode chamber. This is supported by the Mo 3d to S 2s ratios, with the relative sulfur signal becoming less prominent under operating conditions but remaining measurable in both the S 2s and S 2p regions, suggesting that the oxide growth is indeed on the surface but constrained to only a few atomic layers. The decrease in S to Mo ratio also supports the stoichiometric change of the MoS3 to MoS2 transformation. Analogously to the S 2p spectrum analysis, Figure 4b shows the Mo 3d and S 2s BE range for the control crystalline sample for experimental conditions (I), (III), (V), and (VII) in Figure 2b. As with the S 2p data, the crystalline sample does not indicate any changes after being subjected to similar humidification and potential biases as the amorphous nanoparticles, suggesting that the growth of the oxide phase is also exclusive to the amorphous nature of the nanoparticles. A possible explanation for the oxidation process could be that, as the MoS3 sites are transformed into MoS2, some of the molybdenum atoms replace a sulfur bond with an oxygen bond, leading to the growth of a surface oxide layer. It is well-known that the local coverage of S on the MoS2 edges is strongly dependent on the operating environment.40,41

Figure 3. S 2p spectral deconvolution to determine the composition of the catalyst. (a) Curve-fitted S 2p spectrum of amorphous molybdenum sulfide nanoparticles under open circuit conditions (spectrum I in Figure 2a), where the green and red components correspond to MoS3 and MoS2, respectively. (b) Curve-fitted S 2p spectrum of amorphous molybdenum sulfide nanoparticles under hydrogen evolution conditions (spectrum VII in Figure 2a), where the green and red components correspond to MoS3 and MoS2, respectively.

corresponding to the humidified surface before (I) and after (VII) hydrogen evolution, curve-fitted with these two components. As observed in Figure 2a, there is a significant increase of the MoS2 component between the two conditions. Through a similar procedure (see the Supporting Information), it is possible to assess the relative composition in terms of MoS3 and MoS2 for each electrochemical condition, as shown in Table 1. We observe that the as-made nanoparticles consist mostly of MoS3 sites (65%), but when used for hydrogen evolution, the MoS3 sites are reduced to MoS2 sites during the electrolysis (proton diffusion to the bulk or sulfur diffusion to the reduced surface being two possible explanations for the transformation), with the MoS2 sites becoming 79% of the final composition of the portions of the catalyst probed in the experiment. Further analysis of the roles of MoS2 and MoS3 sites during HER reveals differences between their catalytic activities, where the electrochemical data in Figure 2a show increasing catalytic current density over time between spectra V and VI with increasing MoS2 composition, even with all other experimental variables (i.e., humidification and applied voltage) held D



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Figure 5. Plot of the edge free energy of the MoS2 Mo-edge as a function of the H2S pressure at URHE = 0 V. The legend indicates the S coverage θS (ML) and H coverage θH (ML) of each edge (written as θS*S θH*H, rounded to two decimals). Starting from an H2S pressure of 10−4 bar and lower, the structure with θS = 0.5 ML and θH = 0.5 ML becomes lowest in energy (edge structure shown in insert). Taking P(H2S) = 10−6 bar according to standard corrosion resistance42 (indicated by the vertical red line), the θS = 0.5 ML structures are clearly lowest in energy.

Figure 4. Mo 3d spectra of molybdenum sulfides. (a) Mo 3d signal of amorphous molybdenum sulfide nanoparticles collected with an incident photon energy of 900 eV. The labels next to each spectrum refer to the corresponding experimental conditions for S 2p spectra from Figure 2a. (b) Mo 3d signal of crystalline MoS2 collected with an incident photon energy of 900 eV. The labels next to each spectrum refer to the corresponding experimental conditions for S 2p spectra from Figure 2b.

hydrogen has been defined in terms of the electrode potential, following the computational hydrogen electrode model.45,46 We find the θS = 0.5 ML and θH = 0.25 ML structure to be lowest in energy under HER conditions, which corresponds to edges terminated with S monomers (Figure 5 inset). As shown with the triangular single-layered MoS2 nanocrystals,47 if Smonomers terminate the edges, the stoichiometry is close to MoS2 (HER conditions). At higher S coverages, the excess sulfur would result in S-dimers at the edge and stoichiometry that is close to MoS3. This is in agreement with the APXPS data (Figure 3), where the characteristic MoS3 peaks correspond to a bridging S22− ligand (such as an S-dimer termination), whereas the characteristic MoS2 peaks correspond to S2− (such as an S-monomer termination). To further verify whether the observed APXPS chemical shifts correspond to the conversion of an MoS3 structure to an active MoS2 phase, the S 2p XPS chemical shifts for S atoms on the Mo-edge of some of the considered MoSx systems were calculated using DFT. The calculations show that the formation of S dimers on the Mo-edge is associated with a positive shift versus a Mo-edge occupied by only S monomers. The shift is about 2 eV. The XPS energy shift for S atoms that are part of S dimers is thus positive, whereas the XPS shift for atoms that are S monomers is negative. The total XPS shift for S atoms that are surrounded by both S monomers and S dimers seems to be affected by both the positive contribution from being a part of an S dimer, and the negative contribution from being the neighbor of an S monomer. However, the calculated XPS shifts for increasing coverages from 50% to 100% indicate that a transformation from MoS2 to MoS3 should be associated with a net positive XPS chemical shift of 1.3−1.9 eV (see Table 2). This positive shift agrees with the APXPS data seen in Figure 3.

Regardless of the stoichiometry of the as-prepared catalyst, the local coverage of S on the under-coordinated sites will be further determined by the operating conditions for HER. Since the amorphous surface is expected to have a large percentage of under-coordinated sites, the local coverage of S at these sites could greatly impact the overall stoichiometry. One interpretation of the observed MoS3 → MoS2 transformation is thus in terms of changes in S coverage from synthesis to operation, with characteristic MoS3 sites viewed as MoS2-like sites with a higher S coverage that can change under HER conditions. DFT calculations were performed to investigate this hypothesis; as a first approximation, the surface of amorphous MoS3 is assumed to be similar to the Mo-edge sites of MoS2. Our approach to determining the edge structure under steadystate HER conditions is to start with the most thermodynamically stable Mo-edge structure at U = 0 V versus RHE and an H2S pressure of 10−6 bar (following standard corrosion resistance42) and take the structure where HER is more exergonic than either H2S desorption or further H adsorption. The free energies of the Mo-edge on MoS2, γ, at S coverages of θS = 0, 0.125, 0.25, 0.5, 0.625, 0.75, 0.875, and 1.0 ML and H coverages of θH = 0, 0.25, 0.5, 0.75, and 1.0 ML are shown as a function of the H2S pressure at U = 0 V versus RHE (Figure 5). The coverages were defined as fraction of a monolayer with respect to the total possible adsorption sites43 and the edge free energies were calculated as in previous studies.41,43,44 In determining the edge free energy, the chemical potential for E



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Table 2. Calculated S 2p XPS Shifts for Different S Coverages on the Mo Edgea S coverage 50% 62.5% 62.5% 62.5% 75% 75% 87.5% 87.5% 87.5% 100%

coordination of ionized S atom S monomer S monomer (surrounded by two S monomers) S monomer (surrounded by one S dimer and one S monomer) S atom part of an S dimer (surrounded by S monomers) S monomer (surrounded by S dimers) S atom part of an S dimer (surrounded by S monomers) S monomer (surrounded by S dimers) S atom part of an S dimer (surrounded by S dimers) S atom part of an S dimer (surrounded by one S monomer and one S dimer) S dimer

ASSOCIATED CONTENT

S Supporting Information *

Experimental controls on the effects of beam exposure, the full set of S 2p spectral deconvolutions for composition analysis, and structures used for the computational determination of S 2p chemical shifts. This material is available free of charge via the Internet at http://pubs.acs.org.

XPS shift vs 50% coverage 0.0 −0.05

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

−1.17 1.87

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, as follows: the experimental work was supported by the Joint Center for Artificial Photosynthesis Award DESC0004993. H.O. gratefully acknowledges the support from Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST). Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource (SSRL), a division of SLAC National Accelerator Laboratory and an Office of Science user facility operated by Stanford University for the U.S. Department of Energy. For work on molybdenum sulfide catalyst synthesis and development (J.D.B. and T.F.J.) and the DFT free-energy calculations (C.T, F.A-P., J.K.N), we acknowledge support by the Center on Nanostructuring for Efficient Energy Conversion (CNEEC) at Stanford University, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award DE-SC0001060. C.T, M.L.N., F.A-P., J.K.N., and A.N. acknowledge financial support from the U.S. Department of Energy (DOE), Office of Basic Energy Sciences to the SUNCAT Center for Interface Science and Catalysis. C.T. acknowledges support from the National Science Foundation Graduate Research Fellowship Program (GRFP) Grant DGE114747. R.K.B.K. acknowledges financial support from the Swedish Energy Agency and Permascand AB. XPS chemical shift calculations were performed using the resources of the High Performance Computing Center North (HPC2N), as provided by the Swedish National Infrastructure for Computing (SNIC).

1.04 1.34

a

The different atoms that were ionized are indicated in the structures shown in the Supporting Information.

We also find agreement between calculated hydrogen adsorption free energies for the Mo-edge of MoS2 with differing S coverages and the experimental observations of catalytic activity. The steady-state Mo-edge structure under HER, corresponding to the inset in Figure 5, has hydrogen adsorption free energy ΔGH = 0.06 eV, indicating that it should be highly active (thermo-neutral sites are expected to be active, with ΔG H = 0 eV corresponding to optimal HER activity45,48,49). This is in agreement with ΔGH on the active Mo-edge structures investigated in previous DFT studies.50,51 For θS > 0.5 ML, adsorption onto the S-dimers has hydrogen adsorption free energies ΔGH > 1.0 eV, indicating that the Sdimers are inactive for HER. Taken together with the calculated XPS chemical shifts, one plausible explanation for the origin of the observed enhancement in activity with increasing MoS2 sites involves a reduction of catalytically inactive S-dimers (corresponding to MoS3) to catalytically active S-monomers (corresponding to MoS2). Our theoretical results thus provide additional evidence that the active sites of MoS2 under reaction conditions are terminated by S monomers, affirming the crucial role of S coordination in determining catalytic activity.

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CONCLUSION

In conclusion, through the use of in situ APXPS electrochemistry combined with DFT calculations, we have been able to show that amorphous molybdenum sulfides can undergo a gradual chemical composition change of surface sites from MoS3 to MoS2 under hydrogen evolution conditions. In addition, there is a strong correlation observed between the current density drawn from the cell under HER conditions and the chemical composition observed by APXPS; the current increases with the emergence of MoS2-like states. DFT calculations corroborate these experimental findings. We therefore conclude that the MoS2 sites are the active sites in the HER for this material, explaining how MoS3 becomes catalytically active upon exposure to electrochemically reducing conditions.

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