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Activating MoS2 for pH-Universal Hydrogen Evolution Catalysis Guoqing Li, Du Zhang, Yifei Yu, Shengyang Huang, Weitao Yang, and Linyou Cao J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b07450 • Publication Date (Web): 25 Oct 2017 Downloaded from http://pubs.acs.org on October 25, 2017

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Journal of the American Chemical Society

Activating MoS2 for pH-Universal Hydrogen Evolution Catalysis Guoqing Li1, Du Zhang2, Yifei Yu1, Shengyang Huang3, Weitao Yang2*, and Linyou Cao1,4,5* 1

Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695; 2Department of Chemistry, Duke University, Durham, NC 27708; 3 Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, NC, 27695; 4 Department of Physics, North Carolina State University, Raleigh, NC 27695; 5Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, NC 27695 ABSTRACT: MoS2 presents a promising catalyst for the hydrogen evolution reaction (HER) in water splitting, but its worse catalytic performance in neutral and alkaline media than in acidic environment may be problematic for practical application. This is because the other half reaction of water splitting, i.e., oxygen evolution reaction, often needs to be implemented in alkaline environment. Here we demonstrate a universal strategy that may be used to significantly improve the HER catalysis of MoS2 in all kinds of environment from acidic to alkaline, proton intercalation. Protons may be enabled to intercalate between monolayer MoS2 and underlying substrates or in the interlayer space of thicker MoS2 by two processes: electrochemically polarizing MoS2 at negative potentials (vs. RHE) in acidic media or immersing MoS2 into certain acid solutions like TFSI. The improvement in catalytic performance is due to the activity enhancement of the active sites in MoS2 by the intercalated protons, which might be related with the effect of the intercalated protons on electrical conductance and the adsorption energy of hydrogen atoms.

The

enhancement in catalytic activity by the intercalated proton is very stable even in neutral and alkaline electrolytes.

* To whom correspondence should be addressed.

ACS Paragon Plus Environment

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Email: [email protected], [email protected] INTRODUCTION The hydrogen evolution from water splitting is critical for the utilization of hydrogen energy, but its implementation has been delayed by the lack of low-cost high-performance catalysts that may be functional in all kinds of pH environment from acidic to alkaline.1-2 Earth abundant transition metal chalcogenide materials like molybdenum disulfide (MoS2) may provide a promising catalyst for the hydrogen evolution reaction (HER).1-2 However, despite a considerable amount of research effort,2-15 the catalytic performance of MoS2 remains unsatisfactory and is even worse in neutral and alkaline environment than in acidic media.16-17 Therefore, it is necessary to improve the catalytic performance of MoS2, in particular, in neutral or alkaline environment. This is important for practical application because the other half reaction of water splitting, i.e. oxygen evolution reaction, often requires to be implemented in neutral or alkaline electrolytes. Here we demonstrate a universal strategy to substantially improve the HER catalytic performance of MoS2 in all kinds of environment from acidic to alkaline, proton intercalation. The proton intercalation may be enabled by electrochemically polarizing MoS2 at negative potential (vs. RHE) in acidic electrolytes or immersing MoS2 into certain acid solution like TFSI. It may happen between monolayer MoS2 and underlying substrates or in the interlayer space of thicker MoS2. The improvement in catalytic performance results from the activity enhancement of the active sites in MoS2, including sulfur vacancies or edge sties18, by the intercalated protons. And this might be related with the effect of the intercalated protons on electrical conductance and the adsorption energy of hydrogen atoms. The enhancement effect of the intercalated protons for the catalytic activity is stable even in neutral and alkaline environment.


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EXPERIMENTAL SECTION Synthesis and transfer of monolayer MoS2 films and flakes: MoS2 films were synthesized using a self-limiting chemical vapor deposition (CVD) process that we have recently developed19. Briefly molybdenum chloride (MoCl5) powder (99.99%, Sigma-Aldrich) was placed at the center of the furnace and sulfur powder (Sigma-Aldrich) were placed at the upstream entry of the furnace. Receiving substrates (sapphire) were placed in the downstream of the tube. Typical conditions include a temperature of 850 °C, a flow rate of 50 sccm, and a pressure around 2 Torr. Monolayer MoS2 flakes were grown using a different CVD20, in which MoO3 (99.99%, SigmaAldrich) instead of MoCl5 was used as the precursor. Typical growth was performed at 750 °C under a flow of Ar gas in rate of 100 sccm and ambient pressure. The transfer of the monolayers followed a surface-energy-assisted transfer approach that we have developed previously.21 Briefly, a layer of polystyrene (PS) was spin-coated on the as-grown monolayers. A water droplet was then dropped on the top of the polymer. Due to the different surface energies of the monolayer and the substrate, water molecules could penetrate under the monolayer, resulting the delamination of the PS-monolayer assembly. We could pick up the polymer/monolayer assembly with a tweezers and transferred it to glassy carbon substrates. Finally, PS was removed by rinsing with toluene several times. Characterizations: Raman and PL measurements were carried out by a Horiba xPlora system equipped with an excitation wavelength at 532 nm. XPS measurements were carried out at X-ray photoelectron spectroscope (SPECS System with PHOIBOS 150 analyzer using an Mg Kα X-ray source). SEM and XRD measurements were performed at scanning electron microscope (SEM, JEOL JSM-6400F) and Rigaku SmartLab X-ray diffractometer using Cu-Kα radiation, respectively. The electrochemical characterization was performed in 0.5 M H2SO4 using a CH


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Instrument electrochemical analyzer (Model CHI604D) with a saturated calomel reference electrode (SCE) using a one-compartment cell system. A Pt mesh (2.54cm x 2.54cm) or a graphite rod were used as counter electrode, and the results did not show any obvious difference (Fig. S1). Nitrogen gas was bubbled into the electrolyte throughout the experiment. The potential shift of the SCE is calibrated to be -0.262 V vs. RHE. Typical electrochemical characterizations of the monolayers were performed using linear sweeping from +0 V to -0.5V (vs. RHE) with a scan rate of 5 mV/s. The electrochemical characterization in neutral solution was performance in 1M PBS solution. The mild base solution (pH=12) was prepared by mixing 327.0 mL 0.05M Na2HPO4 (99%, Sigma-Aldrich) and 173.0 mL 0.1M sodium hydroxide (NaOH) (98.5%, SigmaAldrich). DFT Computation: All computations in this work were performed with the plane-wave software code VASP (Vienne ab initio simulation package)22using the Perdew-Burke-Ernzerhof (PBE) exchange-correlation energy functional23 with its corresponding pseudopotentials. The semiempirical D2 model 24 was applied to describe the Van der Waals interactions. Periodic boundary conditions were used in all three directions for our supercell simulation models. A vacuum of >10 Å was employed in the non-periodic directions. The H+-intercalated models were calculated with one electron removed from the corresponding neutral systems. An opposite background charge is always adopted which is uniformly distributed throughout the simulation box, which may make the absolute value of the total energy behave well under periodic boundary conditions. This background charge only shifts the Hamiltonian by a global constant, and whenever relative energies are calculated, this same constant always cancels out upon subtraction. A plane-wave cutoff of 500 eV was chosen. For the Mo edge model, the Gamma-centered 2×1×1 k-point grid was applied for the geometry optimizations, and the 6×1×1 k-point grid for the single point


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energy calculations. For the S defect models, the Gamma-centered 2×2×1 k-point grid was applied for the geometry optimizations, and the 4×4×1 k-point grid for the single point energy calculations. The geometry optimizations were converged to 0.01 eV Å-1. The zero-point-energy (ZPE) correction was performed with the finite difference method. RESULTS AND DISCUSSION

Fig.1. Improved catalytic activity of monolayer MoS2 with electrochemical cycling in acidic electrolyte. Polarization curves of (a) typical monolayer MoS2 films, (b) monolayer MoS2 flakes in lateral size of around 2 µm, and (c) monolayer MoS2 film with < 2% sulfur vacancies with different cycles. The electrolyte is 0.5 M H2SO4. The number of cycles is given as shown. Insets of (a) and (b) are optical images of typical monolayer MoS2 films (in size of around 1 cm) and flakes (in size of around 2 µm). Corresponding Tafel slopes of these polarization curves are given in Fig. S2. The density of sulfur vacancies may be derived from XPS measurement as illustrated in Fig. S3. Fig. 1 shows the polarization curves collected from typical monolayer MoS2 films (Fig. 1a) and monolayer MoS2 flakes (Fig. 1b). The monolayer films and flakes were first grown on sapphire substrates using well-established chemical vapor deposition (CVD) processes19-20 and then transferred onto glassy carbon substrates for electrochemical characterizations.18, 21 The polarization curves were obtained using linear sweep voltammetry (LSV) from 0 V (vs RHE) to a voltage that can enable a current density of around 20 mA/cm2 as shown in Fig.1. Cyclic voltammetry (CV) was performed over similar voltages between each of the linear sweeps. Very interestingly, the catalytic performance of the film and the flakes both substantially increases with the cycling and eventually approaches to be stable. Usually the films request more cycles to


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reach the stable performance than the flakes. We also find that more cycles are needed for the flakes with larger size than smaller flakes. Additionally, extending the cycling range down to more negative voltages may lower the number of the cycles needed to reach stable activity, but may not change the level of the stable activity (Fig. S4). Similar increase in the catalytic performance of MoS2 with electrochemical cycling has actually been reported previously,8, 15, 25 but no understanding for the underlying mechanism is available yet except vaguely calling it as an activation process.

Fig.2. p-doping to monolayer MoS2 by electrochemical cycling. (a) XPS, (b) Raman, and (c) photoluminescence (PL) spectra of monolayer MoS2 films before (blue curves) and after (red curves) the electrochemical cycling that may enable stable catalytic activities. (d) Schematic illustration for the major role of proton intercalation between the monolayer and underlying structures. The protons adsorbed on top of the monolayer may only have a minor role. The characterization results for monolayer MoS2 flakes before and after electrochemical cycling are given in Fig. S8.

We may conclude that no substantial amount of new active site is generated during the cycling process. Our previous studies have demonstrated that MoS2 have two kinds of active sites for the hydrogen evolution reaction, edge sites and sulfur vacancies. Sulfur vacancies are the major active sites of the film, while edge sites are the major active sites of the flakes.18 Should the observed dramatic improvement in catalytic activity be due to the generation of new active sites, we would expect a very substantial change in the number of edge sites or sulfur


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vacancies, which could be realized by inducing cracks or lose of sulfur atoms. However, we did not observe any cracks in the film or flakes after the cycling from optical, SEM, and AFM measurements (Fig. S5-S6). We also did not find any substantial change in the stoichiometric ratio of Mo and S from XPS measurements. (Fig. 2a). These results indicate no substantial change in the number of edge sites and sulfur vacancies during the cycling. This conclusion is also supported by the very poor catalytic activity of the MoS2 films grown with little sulfur vacancies (Fig. 1c) and by the reasonable linear dependence of the exchange current density of the flakes on the length of the edge (Fig. S7), both of which indicate that the catalytic activity is eventually dictated by the as-grown active sites. Additionally, the Raman spectra of MoS2 show negligible change after the cycling, and this excludes out the possibility of phase change from 2H to 1T in the MoS2, which could substantially improve the catalytic activity and induce obvious changes in the Raman spectra.10-12, 26 Therefore, we may conclude that the improved catalytic performance during the cycling is not due to the generation of new active sites but due to catalytic activity enhancement of the as-grown active sites, which we refer as activation. To better understand the activation mechanism, we examine the PL, Raman, and XPS of monolayer MoS2 films and flakes before and after the cycling. All of the measurement results point toward that the cycling process may induce p-doping to MoS2. The PL of the MoS2 shows obviously stronger and blueshifts after the cycling (Fig. 2c). This may be ascribed to p-doping, which has previously been demonstrated able to enhance PL intensity and induce PL blueshift for monolayer MoS2.27 Monolayer MoS2 is intrinsically n-doped and p-doping may neutralize the free electrons to promote the emission of neutral excitons over trions (negatively charged


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excitons), the former showing stronger PL efficiency and shorter emission wavelengths than the latter (Fig. S9).27-29 Additionally, the A1g Raman peak blueshifts by ~ 0.5 cm-1, but no visible change may be found at the E12g peak after the cycling. This supports the notion of p-doping, as it is well known that the A1g peak of MoS2 tends to stiffen (blueshift) with p-doping due to strong susceptibility to the influence of electron-phonon coupling but the E12g peak may not be affected by doping.30 The p-doping is also supported by the XPS measurement, which shows a shift of around -0.14 eV in the S 2p and Mo 3d peaks after the cycling. This shift in binding energy is consistent with what was previously observed at p-doped MoS2.31-33

Fig.3. Negligible effect of the electrochemical cycling in neutral electrolytes. (a) Polarization curves, (b) Raman spectra, and (c) PL spectra of monolayer MoS2 films before and after 5000 cycles in 0.5 M solution of Na2SO4. The other cycling condition is identical to what used for the cycling of MoS2 in 0.5 M solution of H2SO4. Note that the polarization curves were collected in 0.5 M solution of H2SO4 while the cycling performed in a separate solution of 0.5 M Na2SO4. We may correlate the p-doping and catalytic activity enhancement of MoS2 to protons, more specifically, to the protons intercalated between the monolayer and underlying substrates. We performed electrochemical cycling at monolayer MoS2 in the solution of 0.5M Na2SO4 and found negligible improvement in the catalytic performance and trivial p-doping effect (Fig. 3).


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This result indicates that protons play a dominant role in the p-doping effect and catalytic activity enhancement observed at the MoS2 cycled in the acidic electrolyte, in which the only difference from the 0.5M Na2SO4 solution lies in the cation. This is consistent with the result of our previous work. We previously demonstrated from both experimental and theoretical grounds that protons may p-dope monolayer MoS2.34 The previous study also indicates that the p-doping is mainly contributed by the protons intercalated between the monolayer and underlying substrates, while the protons adsorbed on the top of the monolayer may only have trivial doping effect.34 Therefore, the observed p-doping effect in the MoS2 strongly suggests that the intercalation of protons between the monolayer and underlying substrates as illustrated in Fig. 2d is enabled during the cycling. Our other experimental observation may provide more support or insight for the proton intercalation and its correlation with the improvement of catalytic activity. The catalytic activity and p-doping effect gradually increase with the cycling. This indicates that the protons adsorbed on top of the monolayer, which is expected to happen immediately once the monolayer is immersed into the acidic electrolyte, only have negligible contribution. Additionally, the micrometer-scale flakes request much less cycles to achieve stable performance than the centimeter-scale films (Fig.1a-b), and extending the cycling range down to more negative voltages may decrease the number of cycles needed to reach stable catalytic activity (Fig. S4). This indicates that the intercalation of protons, which is hydrated in the acidic electrolyte, between the monolayer and underlying substrates is not spontaneous and requests driving force like negative electrochemical potentials to overcome certain energy barrier. To further confirm the role of proton intercalation in the improvement of catalytic activity, we examine the HER catalysis of monolayer MoS2 (films and flakes) treated with TFSI solution (1,2-dichlorobenzene and 1,2-dichloroethane used as solvent). We immersed monolayer


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MoS2 into the TFSI solution for minutes34-35 and then took it out for linear sweep voltammetry in acidic electrolytes. No electrochemical cycling is involved between the different linear sweeps. According to our previous studies, the immersion into TFSI solution may lead to the intercalation of protons between monolayer MoS2 and underlying substrates.34 The spontaneous protonintercalation solution could be due to the smaller size of non-hydrated protons in the TFSI solution , which may make the intercalation easier. We confirm the successful intercalation of protons after the TFSI treatment with PL, Raman, and XPS measurements, which shows similar enhancement and/or shifts as observed at the cycling in acidic media ( Fig. 4a-c). We have also confirmed no obvious change in the composition, structure, and morphology at the monolayers during the TFSI treatment,34 excluding the possibility of generation of new active sites or phase change. The TFSI treatment may enable enhancement in the catalytic activity similar to what observed at the electrochemical cycling. The catalytic performance increases with the number of TSFI treatment and eventually turns to be stable (Fig. 4d). The stable catalytic activity caused by the TFSI treatment is very similar to what caused by the cycling (Fig. 4d). We may ascribe the activity improvement by the TFSI treatment to protons as well, because no major activity improvement may be observed at the MoS2 treated by Li-TFSI solutions (Fig. S10), which have the same species as the TFSI except the cations. The gradual increase of catalytic activity with the number of treatment also suggests that the protons adsorbed at the surface of the monolayer, which is expected to happen immediately, do not play any major role. All of these indicate similar activation mechanisms for both TFSI-treated and electrochemically cycled MoS2, i.e. intercalation of protons. It is worthwhile to point out that the PL of the TFSI-treated monolayer shows negligible change after the catalytic characterization. This indicates that the intercalated protons are not reduced to hydrogen atoms during the electrocatalytic reaction, which would


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otherwise lead to much weaker PL intensity as hydrogen atoms are known able to n-dope the monolayer.36-37

Fig. 4. Effect of TFSI treatment on monolayer MoS2. (a) Polarization curves of monolayer MoS2 films with different times of TFSI treatment. Each TFSI treatment is done at 100 ºC for 10mins. The dashed line is the polarization curve collected from a similar film experienced extensive electrochemical cycling. (b) PL spectra of monolayer MoS2 films before (black curve) and after (yellow curve) TFSI treatment. The red dotted curve is the PL spectrum collected from the TFSI-treated MoS2 film experienced multiple linear sweeps. (c) Raman and (d) XPS spectra of monolayer MoS2 films before (black curve) and after (yellow curve) TFSI treatment. The dashed lines in (c) indicate the positions of the Raman peaks before the TFSI treatment. Similar results for the TFSI-treated monolayer MoS2 flakes are given in Fig. S11. Except the space between monolayer and underlying substrates, protons may also be enabled to intercalate in the interlayer space of thicker MoS2 (Fig. 5a). To illustrate this notion, we examine the catalytic performance of bilayer MoS2 films. Similar to monolayer MoS2, the catalytic activity of bilayer MoS2 substantially increases with the electrochemical cycling in acidic electrolyte and eventually turns to be stable (Fig. 5b). We believe that this improvement in catalytic activity is collectively contributed by the protons intercalated underneath the bilayer and in the interlayer space. The proton intercalation in the interlayer space is evidenced by the decrease of the frequency difference between the A1g and E12g peaks after the cycling (Fig. 5c). The Raman frequency difference is related with the coupling of neighboring layers, and the decrease in the frequency difference indicates a decrease of interlayer coupling in the bilayer,38-39 which suggests the intercalation of protons into the interlayer space. The PL of the bilayer


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blueshifts and shows increase in efficiency, consistent with the expectation for reduced interlayer coupling (Fig. 5d).40-41 To further confirm the interlayer intercalation of protons, we examine the effect of TFSI treatment on the catalytic activity of bilayer films. Our previous studies have demonstrated that TFSI treatment may lead to proton intercalation between MoS2 and underlying substrates, but less efficient in enabling the proton intercalation into the interlayer space of multilayer 2D materials due to the stronger interaction between 2D layers.34 Therefore, the TFSI treatment is expected to mainly induce proton intercalation between bilayer and the substrates. Indeed, while both TFSI treatment and electrochemical cycling lead to similar catalytic activity improvement at monolayer MoS2 (Fig. 4a), the electrochemical cycling may enable greater activity improvement at bilayer MoS2 than the TFSI treatment (Fig. 5b). The TFSI treatment also gives rise to smaller changes in the Raman and PL of bilayer MoS2 than the electrochemical cycling (Fig. 5c-d). By electrochemically cycling the TFSI-treated bilayer in acidic electrolytes may further improve the catalytic activity. All these results indicate that the electrochemical cycling may enable proton intercalation into the interlayer space of bilayer MoS2 (Fig. 5a).

Fig. 5. Proton intercalation at bilayer MoS2. (a) Schematic illustration for the different proton intercalation configuration at the bilayer MoS2 experiencing electrochemically cycling and TFSI treatment. (b) Polarization curves of bilayer MoS2 as a function of electrochemical cycling (solid curves) and TFSI treatment (dashed curves). Typical initial polarization curve of bilayer MoS2 with neither electrochemical cycling nor TSFI treatment is also plotted (black curve). (c) Raman


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and (d) PL spectra of bilayer MoS2 with no treatment (black), electrochemically cycled 3000 times (blue), and treated with TFSI solution (brown). The interlayer intercalation of protons and its effect on the catalytic activity may also be observed at even thicker MoS2 (Fig. 6). We synthesized MoS2 sheets in thickness of tens of nanometers and lateral size of micrometers on glassy carbon substrates following a process we developed previously,8 and then examined its catalytic performance as a function of electrochemical cycling. Similar to monolayer and bilayer MoS2, the catalytic activity of the MoS2 nanosheets substantially increases with the electrochemical cycling in acidic electrolytes, and eventually tends to be stable (Fig. 6a). And smaller nanosheets usually request less cycles to reach stable performance. We have confirmed no observerable change in the crystalline structures and composition of the nanosheets (Fig. 6b). However, XRD measurement shows that the (002) peak slightly shift toward smaller angle and shows obvious decrease in the intensity (Fig. 6c). Quantitative analysis indicates that the interlayer spacing increases by around 0.01 nm on average. Based on what we have discussed, it is reasonable to ascribe this increase in interlayer spacing to the intercalation of protons.

Fig.6. Proton intercalation at thick MoS2 nanosheets. (a) Polarization curves of MoS2 nanosheets as a function of electrochemical cycling in acidic electrolyte. The number of cycles is given as shown. Inset, SEM image for a typical MoS2 nanosheets (b) Raman spectra, and (c) XRD spectra of the MoS2 nanosheets before and after the electrochemical cycling.


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Fig. 7. Activation of HER catalysis at MoS2 in neutral and alkaline environment. (a)Polarization curves of monolayer MoS2 films without activation (black), with activation (red), and cycled for 2000 times after the activation (blue) at neutral solution pH = 7. (b)Polarization curves of monolayer MoS2 films without activation (black), with activation (red), and cycled for 2000 times after the activation (blue) at alkaline solution pH = 12. The activation process is performing cyclic voltammetry at the MoS2 film in acidic electrolyte till stable catalytic activity.

We believe that the enhancement effect of the intercalated protons is an important reason for the worse catalytic performance of MoS2 in neutral and alkaline environment than in acidic media as reported previously.16-17 The electrochemical cycling in neutral or alkaline electrolyte may not give rise to any substantial increase in the catalytic performance of MoS2, as there is no enough hydrogen ions available for intercalation (Fig. S12). However, the catalytic activity of MoS2 in neutral and alkaline environment may also be substantially improved by the proton intercalation. We activate MoS2 by performing electrochemical cycling in acidic electrolytes first, and then transfer the monolayer in neutral or alkaline electrolytes for evaluation of catalytic activity. The activated MoS2 monolayer shows much higher catalytic performance than the counterpart without being activated (Fig. 7). The activated MoS2 also shows remarkable stability in neutral and alkaline solutions with no obvious decrease in catalytic activity over > 2000 cycles.


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The stable enhancement effect results from the persistent presence of intercalated protons even in alkaline electrolyte, as evidenced by the negligible change in the Raman and PL of the MoS2 during the HER (Fig. S13).

There are two reasons for the intercalated protons not being

neutralized. First, it may be difficult for hydroxyl ions to intercalate into the confined space underneath monolayers or between the layers of thicker MoS2 as the size of hydroxyl ions is relatively big. Our experimental result suggests that the efficiency of the intercalation is inversely proportional to the size of the ions. For instance, the application of negative potential is necessary to drive the intercalation of hydrated protons in acidic electrolytes, but not necessary for the non-hydrated protons in TFSI, because hydrated protons may have larger sizes than nonhydrated ones. Additionally, we previously demonstrated that the cations with larger sizes like Na+ or K+ cannot intercalate underneath monolayer MoS2.34 Second, the intercalated protons might be chemically adsorbed onto the active sites, such as sulfur vacancies and edge sites, and this could prevent the intercalated protons to come out from the confined space. The intercalated protons may affect the catalytic activity through multiple effects. One effect lies in the charge transfer between the proton and MoS2, as indicated by the p-doping effect. The charge transfer may affect the catalytic activity by changing the electronic structure of the active site.42-44 Previous studies have demonstrated that the effect of charge transfer may be examined from the adsorption energy of hydrogen atoms.42-44 We perform first principle calculation for the adsorption free energy of hydrogen atoms, which is widely used as a figure of


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merit for the HER catalytic activity,4, 45 at the sulfur vacancies and edge sites of MoS2 with the presence of protons. The calculation indicates that the adsorption energy turns to be closer to 0 with the presence of H+ nearby as shown in Table 1 (see Fig. S13 and S14 for the structure used in the calculations). This indicates better HER catalytic activities at both sulfur vacancies and edge sites with the presence of protons, consistent with what we observed in experiments. It is worthwhile to point that the charge transfer between proton and MoS2 may induce p-doping effect, but this does not mean that any cations with p-doping effect would certainly be able to enhance the catalytic activity. For instance, our previous studies have demonstrated that Li-TFSI treatment may provide p-doping to monolayer MoS2,34 but we do not observe any improvement in the catalytic activity of MoS2 after Li-TFSI treatment (Fig. S10). Besides from the charge transfer effect, the intercalated protons might also affect the catalytic activity by increasing the electric conductance of MoS2 in both the vertical direction and horizontal directions. This is evidenced by the significant increase in exchange current density after the proton intercalation (Fig. S2 and S7), as recent studies have demonstrated that the exchange current density of MoS2 is related with the electrical conductance.15 More studies are necessary to further confirm and understand the effect of intercalated protons on the electrical conductance of MoS2. Table 1. Adsorption free energies of H atom Site

Without H+

With H+

Mo edge

0.100

0.044

S vacancy

-0.090

-0.052

CONCLUSION We have demonstrated that the intercalation of protons presents a universal strategy to significantly improve the HER catalytic performance of MoS2 for all kinds of environment from


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acidic to alkaline. This strategy may be generally applied to MoS2 either with atomically thin dimension such as monolayer or bilayer or with nanostructured features such as nanosheets. Protons can be enabled to intercalate between MoS2 and substrates or into the interlayer space of MoS2 by two approaches, electrochemically polarizing MoS2 at negative potentials (vs. RHE) in acidic media or immersing MoS2 in certain acid solutions like TFSI. This improvement in catalytic performance result from the catalytic activity enhancement of the active sites in MoS2 by the intercalated protons. The intercalated protons may affect the catalytic activity by enabling more favorable adsorption energy of hydrogen atoms and enhancing the electrical conductance. More studies are necessary to obtain better understanding. This result represents significant advance in fundamental understanding for the HER catalysis, and points toward a new way for the development of cost-effective high performance catalysts for HER at all kinds of pH environments. While this study focuses on MoS2, protons, and hydrogen evolution, the notion of using intercalation of species to tune the catalytic performance may generally be applied to other ions or molecules, other layered materials, and other catalytic reactions.


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ACKNOWLEDGEMENTS This work was supported by CCDM, an EFRC funded by U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (BES), under award #DE-SC0012575. Y.Z. acknowledges the support by DOE, Office of Science, BES, Materials Sciences and Engineering Division, under contract #DE-SC0012704. The authors acknowledge the use of the Analytical Instrumentation Facility (AIF) at North Carolina State University, which is supported by the State of North Carolina and the National Science Foundation. COMPETING FINANCIAL INTERESTS The authors declare no competing financial interests. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxx. 1. 2. 3. 4. 5.

Effect of counter electrodes on the catalytic measurement Tafel plots of the catalytic characterization. Catalyic performance of MoS2 flakes with different sizes. Effect of electrochemical cycling at smaller voltage ranges. Structural, compositional, Raman, and PL characterization of MoS2 after electrochemical cycling or TFSI treatment. 6. Effect of Li-TFSI treatment. 7. Effect of electrochemical cycling in neutral and alkaline solutions. 8. Geometrical feature of the structures used in first principle calculation.


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