Letter pubs.acs.org/NanoLett
Tuning the MoS2 Edge-Site Activity for Hydrogen Evolution via Support Interactions Charlie Tsai,†,‡ Frank Abild-Pedersen,‡ and Jens K. Nørskov*,†,‡ †
Department of Chemical Engineering, Stanford University, 450 Serra Mall, Stanford, California 94305, United States SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States
‡
S Supporting Information *
ABSTRACT: The hydrogen evolution reaction (HER) on supported MoS2 catalysts is investigated using periodic density functional theory, employing the new BEEF-vdW functional that explicitly takes long-range van der Waals (vdW) forces into account. We find that the support interactions involving vdW forces leads to significant changes in the hydrogen binding energy, resulting in several orders of magnitude difference in HER activity. It is generally seen for the Mo-edge that strong adhesion of the catalyst onto the support leads to weakening in the hydrogen binding. This presents a way to optimally tune the hydrogen binding on MoS2 and explains the lower than expected exchange current densities of supported MoS2 in electrochemical H2 evolution studies. KEYWORDS: Molybdenum disulfide, hydrogen evolution, support interactions, van der Waals, density functional theory
I
exposed active sites could be greatly affected by the long-range vdW interactions between the catalyst and the support since the active catalyst is often only a single trilayer thick and all catalytic activity takes place close to the catalyst−support interface. An exchange-correlation functional explicitly including vdW correlations (BEEF-vdW)25 has recently been developed, which is able to describe both chemical bond formation26 and vdW interactions27,28 to a reasonable accuracy. Using this functional, we analyze the effect of the support on HER at the MoS2 edge sites for several different systems: unsupported MoS2, supported MoS2, unsupported stacked MoS2, and supported stacked MoS2. We conclude that the support has a substantial effect and that the support interaction can be used directly to tailor the catalytic activity of MoS2 for HER. The structures and electronic energies were calculated using plane-wave DFT employing ultrasoft-pseudopotentials. The QUANTUM ESPRESSO code29 and the BEEF-vdW exchangecorrelation functional were used for all the calculations. The plane-wave cutoff and density cutoff were 500 and 5000 eV, respectively. The basal plane of MoS2 has been found experimentally and theoretically to be chemically inert,8,9,30−32 and we concentrate on the chemistry of the edges in this study. Edge sites of single layer MoS2 particles were investigated using a semi-infinite MoS2 stripe model as described in detail previously.8,16,18 In the semi-infinite stripe, we can study both of the two possible edges, the (1010̅ ) Mo-edge and the (10̅ 10) S-edge. The Mo-
n order for H2 to be a real alternative to hydrocarbon fuels, a sustainable form of production is needed. One such route is to electrochemically evolve hydrogen by splitting water using electricity generated from solar or wind power.1 Although Pt is known to catalyze the hydrogen evolution reaction (HER) very efficiently, it suffers from being scarce and very expensive. The challenge is to find HER catalysts that are both effective and based on earth-abundant elements. Electrochemical hydrogen evolution involves hydrogen binding to the catalyst surface in the first step, and the hydrogen adsorption free energy has been shown to be a good descriptor for the rate of HER with an optimal binding energy of ΔG°H ≅ 0 eV.2−6 An optimal catalyst for HER will thus be one that binds hydrogen neither too strongly nor too weakly. Single trilayer MoS2 was predicted by theory to be a promising candidate for HER7 and MoS2 nanoparticles and clusters have subsequently been shown in a number of studies to be better than most nonprecious metals.7−15 The increased activity has been shown to arise from the metallic states that are located at the edges of the trilayer nanoparticles.16−21 It has been found that the Mo-edge is the one that is primarily exposed in single layers of MoS2 on Au(111),9,19 highly ordered pryolytic graphite (HOPG),17 or graphitic carbon.22 In the present paper,we take the theoretical analysis of MoS2 catalysts for HER one step further by explicitly including the effect of the catalyst support. This has not yet been possible since most common semilocal forms of density functional theory (DFT) calculations are completely inadequate for describing long-range van der Waals (vdW) interactions responsible for the adhesion of the MoS2 catalyst to the support.9,12,19,23,24 For MoS2 in particular, the activity of the © XXXX American Chemical Society
Received: November 30, 2013 Revised: January 30, 2014
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Table 1. Hydrogen Adsorption Free Energies and MoS2 Adhesion Energies (Structures Shown at Maximum Coverage)
a
The MoS2 adhesion energy was calculated for the top layer of the stacked MoS2 stripes
computational cost, one unit cell contained a MoS2 stripe with two Mo atoms in the x-direction and four Mo atoms in the y-direction. This gives two binding sites for hydrogen and was used to describe coverages of 0 and 0.5 ML. A second unit cell containing a MoS2 stripe that was four Mo atoms in both x- and y-directions was used to describe the 0.25 ML coverage. The MoS2 stripes were separated from each other by at least 9 Å in the y-direction. Periodic boundary conditions were used and 11 Å of vacuum in the z-direction was used to separate neighboring catalyst−support slabs. The Brillouin zone was sampled by a Monkhorst-Pack 4 × 1 × 1 k-point grid for the first unit cell and 2 × 1 × 1 k-point grid for the second unit cell.34 Structures were relaxed until the total forces were less than 0.05 eV/Å. We used the differential binding energy to describe the stability of hydrogen, defined by
edge is terminated with S monomers while the S-edge is terminated with a combination of S monomers and dimers as described previously.33 The lattice constant of MoS2 was determined to be 3.21 Å, which agrees reasonably with the experimental lattice constant of 3.16 Å.33 The semi-infinite stripes were placed on either three layers of Au(111) or one layer of graphene. In order to match the MoS2 stripe, the Au(111) lattice was strained by 4%, whereas the graphene was compressed by 1% and rotated. We expect that such small strains will not affect the interaction with the MoS 2 significantly. We define the coverage of hydrogen as the fraction of a monolayer with respect to the number of available S atoms on the edge θH(ML) =
nH (edge S atoms)
(1)
ΔE H = E(MoS2 + nH + support)
Previously it was determined that a hydrogen adsorption coverage of θH = 0.25 or 0.5 ML had the highest activity for hydrogen evolution, hence two unit cell sizes were used to describe these coverages.7 In order to save time and
− E(MoS2 + (n − 1)H + support) −
1 E(H 2) 2 (2)
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where E(MoS2 + nH + support) is the total energy for the MoS2 system with the catalyst support and n hydrogen atoms adsorbed on the edge, E(MoS2 + (n−1)H + support) is the total energy for (n − 1) adsorbed hydrogen atoms and E(H2) is the energy of a gas phase hydrogen molecule. The Gibbs free energy for hydrogen adsorption was then calculated as ΔG H◦ = ΔE H + ΔEZPE − T ΔSH
Au(111) and weaker binding between MoS2 and graphene or the MoS2 slab. Although the bottom layer of stacked MoS2 is strongly bound to Au(111), the top layer is very weakly bound (ΔEadh/Mo = 0.01 eV). The stronger bond to Au(111) than to MoS2 indicates that MoS2 will tend not to stack on Au(111). On graphene however, the adhesion energy of the top layer of stacked MoS2 is negative (ΔEadh/Mo = −0.12 eV), indicating a strong tendency for stacking. This is in complete agreement with experimental observations.9,37 We find a general trend on the Mo-edge where MoS2 that is bound more strongly to the support will lead to weaker hydrogen binding. As shown in Figure 1, the hydrogen
(3)
where ΔEZPE is the zero-point energy difference between the adsorbed state of the system and the gas phase state and ΔSH is the entropy difference between the adsorbed state of the system and the gas phase standard state (300 K, 1 bar). Here we have approximated the entropy of hydrogen adsorption as ΔSH ≈ 1/ 2(SH° 2) where SH° 2 is the entropy of gas phase H2 at standard conditions. The adhesion energy of the MoS2 on the support was defined by ΔEadhesion = [E(MoS2 + support) − E(MoS2 ) − E(support)] NMo atoms
(4)
where the energy of adhesion has been normalized to the number of Mo atoms in the catalyst trilayer. To calculate the energetic change due to reconstruction of S dimers and monomers on the MoS2 S-edge upon hydrogen adsorption we perform a single point energy calculation on the reconstructed structure with the hydrogen removed and subtract that by the energy of the corresponding clean edge structure. The single point energy calculation on the reconstructed structure with the hydrogen removed also provides the S−H bond formation energy on the reconstructed structure. Adhesion between layers of pure MoS2 is well described when vdW interactions are included via the BEEF-vdW functional. The interlayer distance was found to be 6.57 Å, in reasonable agreement with measured experimental values of bulk MoS2, which have been reported to be 6.2 Å.35,36 We have found that this discrepancy in vdW distance does not change the conclusions about hydrogen binding at the edge (see Supporting Information). The calculated hydrogen adsorption free energies and the MoS2 adhesion energies on different supports for the Mo-edge and S-edge are summarized in Table 1. The unsupported MoS2 results agree well with previous studies.7,12,18 The maximum hydrogen coverage for each system is highlighted in boldface; this represents the coverage where the final hydrogen added onto the catalyst is active for HER (i.e., having an adsorption free energy closest to 0.0 eV). For the Mo-edge, MoS2 supported on Au(111) and graphene showed hydrogen adsorption free energies that were markedly weakened compared to the unsupported MoS2. For MoS2 on Au(111) at 0.25 ML H coverage, the binding energy was increased by 0.56 eV whereas for MoS2 on graphene at 0.25 ML H coverage the binding energy was increased by 0.18 eV. The hydrogen bond weakening was much less on MoS2 supported on a MoS2 slab. For stacked MoS2 stripes with no support, the hydrogen binding is slightly weaker. However, when the stacked MoS2 stripes are supported on Au(111) or graphene, the top layer MoS2 has hydrogen binding that is similar to unsupported MoS2, suggesting that the support effect is shortrange. The adhesion energy of MoS2 onto the support also varies with the support. There is strong binding between MoS2 and
Figure 1. Change in hydrogen adsorption free energy on the Mo-edge at each coverage with MoS2 adsorption onto the support. For both 0.25 and 0.5 ML coverage, stronger MoS2 adsorption onto the support leads to weaker hydrogen adsorption. The dashed line indicates ΔGH = 0 eV, which intersects the 0.25 ML coverage line at ΔEadh/Mo = −0.3 eV.
adsorption free energy clearly rises with stronger MoS2 adhesion. Because hydrogen adsorbed at 0.25 ML coverage is bound too strongly (ΔGH = −0.36 eV), our results suggest that a support with ΔEadh/Mo between that of Au(111) and graphene with a ΔEadh/Mo ≈ − 0.3 eV could be found to achieve an optimal hydrogen adsorption free energy close to 0 eV. The hydrogen adsorbed at 0.50 ML coverage is already bound too weakly in unsupported MoS 2 , so support interactions of the van der Waals type will never improve the binding. Comparing the theoretical MoS2 hydrogen binding energy and the experimental HER exchange current density5,9,12 with that of various pure metals (Figure 2), we see that supported MoS2 shares comparable HER activity with the nonprecious metals. (The binding energies for pure metals in Figure 2 were calculated using the BEEF-vdW functional rather than the RPBE functional that has been used in previous studies.5,38 This represents a slight change in ΔGH of less than 0.03 eV.) Furthermore, the support-induced change in the hydrogen adsorption free energy leads to substantial difference in turnover frequency. The theoretical exchange current density of unsupported MoS2 (indicated by the open circle in Figure 2) is considerably larger than that of MoS2 supported on either Au(111) or graphene. The hydrogen adsorption free energy on the Mo-edge could therefore be improved either by choosing the best catalyst support (to tune the hydrogen in 0.25 ML) or by designing catalysts where the active edge sites are far enough from the support as to not be affected (to maintain the activity C
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0.14 eV and ΔGH = 0.12 eV, respectively). To understand why hydrogen binding at the S-edge is insensitive to the choice of support, we look at the structural change of the S atoms at the edge due to hydrogen adsorption (Table 2). For all cases, the S dimer opens. We find that for unsupported MoS2 and MoS2 on graphene, the opening of the S−S bond in the dimer is energetically costly (ΔErecon ≈ 0.8 eV). This is balanced by the energetically favorable S−H bond formation (ΔES−H ≈ −0.9 eV). For MoS2 on Au(111) the S−S dimer opening is already stable prior to hydrogen adsorption. Upon hydrogen adsorption, the additional energetic cost associated with minor shifting in the S dimers and monomers is about 0.6 eV more stable. We find the S−H bond formation energy to be weaker by approximately 0.6 eV as well and the competing effects of S−H bond formation and the S−S dimer opening ultimately lead to no change in the hydrogen adsorption free energies. This could be fortuitous, so we cannot conclude that support interactions are not advantageous for tuning the hydrogen adsorption free energy for HER at the S-edge. However, there could be a general compensation effect between the S−S bond destabilization and the S−H bond stabilization at work, so further studies on other supports would be needed to understand that. The strong interaction between the S-edge of MoS2 with Au(111) also suggests the possibility of the stable edge configuration changing. We investigated various S-edge configurations on the Au(111) support and determined that a larger coverage of S is possible (details in the Supporting Information). The S-edge covered with dimers becomes stable, with a ΔGH = −0.02 eV, comparable to the unsupported Moedge. If this configuration is present in significant amounts, it could serve as a way for improving hydrogen adsorption on the S-edge. Experimental STM studies have shown that the MoS2/ Au(111) overwhelmingly consists of exposed Mo-edges;9 however, this result opens up the possibility that a strongly interacting support can be chosen to modify the edge configurations. Because the underlying electronic structure ultimately defines the chemical properties of the edge sites, an analysis of the edge state electronic structure provides a deeper understanding of the trends seen in Figure 1. To elucidate how the support affects the electronic structure of the edge states and how that in turn affects the chemical properties of the edge sites, the atom-projected p-orbital density of states of the S atoms were calculated (Figures 3 and 4). The S atoms bind directly to the H and hence provide a direct means of analyzing the binding strength. Starting with the Mo-edge (Figure 3), the sharp peak from the projected states on the S atom at the edge corresponds to
Figure 2. Activity map for the hydrogen evolution reaction (HER) showing the exchange current density as a function of the calculated free energy of hydrogen adsorption, ΔGH. Surfaces with negative ΔGH have high coverage (θH = 1 ML) and surfaces with positive ΔGH have been assumed to have low coverage (θH = 0.25 ML), where θH is defined as the fraction of a monolayer with respect to the number of edge metal atoms. The full line is based on a detailed kinetic model for HER in which all input parameters are taken from DFT calculations.40 The dashed line indicates that metals with ΔGH < 0.2 eV/H tend to form oxides at U = 0 V. The blue points are experimental data as referenced in ref 5. The red solid points represent measured rates on MoS2 on either Au(111)9 or graphene12 (measured per active site and then transformed to the same number of active sites as Pt(111) in order to compare intrinsic rates to the metals) plotted at the calculated ΔGH from the present study. Using the kinetic model represented by the solid line, our calculated ΔGH for an unsupported MoS2 edge site leads to an estimate of the intrinsic exchange current for such a site (open red point).
of the hydrogen in 0.50 ML). Some recent examples of the latter strategy include thin films of MoS2 double-gyroids11 and vertically aligned MoS2 thin films.39 The S-edge consists of a combination of S monomers and S dimers and the relevant structure for HER involves hydrogen bound to the S dimer (Table 1).12 The hydrogen adsorption free energy for the 0.25 ML hydrogen on the S-edge is positive for all the structures considered so this is the only relevant hydrogen coverage. Once the theoretical overpotential for driving HER is achieved, hydrogen adsorption at a higher coverage will not be possible. From the hydrogen adsorption free energy, we confirm that the Mo-edge is the more active edge in HER. The ΔGH of the S-edge on unsupported MoS2 is 0.14 eV compared to the Mo-edge (ΔGH = 0.06 eV). For both the Au(111) and graphene support, the S-edge hydrogen adsorption free energy remained relatively unchanged (ΔGH =
Table 2. Energetic Change Due to Dimer Opening upon Hydrogen Adsorption in MoS2 S-edge Structures
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Figure 3. (Left) Projected p-orbital density of states of S for the Mo-edge. The edge S atoms are shown in blue and the terrace S atoms are shown in red. The shaded area corresponds to the filled states up to the Fermi level εF. (Right) Projected s-orbital density of states of adsorbed H showing the bonding and antibonding states relative to the Fermi level εF.
Figure 4. (Left) Projected p-orbital density of states for the S dimer on the S-edge. The edge S atoms are shown in blue and the terrace S atoms are shown in red. The shaded area corresponds to the filled states up to the Fermi level εF. (Right) Projected s-orbital density of states of adsorbed H showing the bonding and antibonding states relative to the Fermi level εF.
the active edge states, which is noticeably absent from the projected states for the terrace S atoms. For single-layered MoS2 supported on Au(111) and graphene, there is a visible downward shift of the p-states at the edge, whereas there is no noticeable shift for all other systems. The downward shift leads to more filled p-states, which typically leads to weaker binding.41 When the hydrogen adsorbate state interacts with the p-states, it will gives rise to bonding and antibonding states.42 This is seen in the 1s states of adsorbed hydrogen shown in the right part of Figure 3. Systems with p-states well below the Fermi level will result in more filled antibonding states in the 1s states and hence weaker hydrogen binding. We see a correspondence between the amount of shift in the pstates and s-states observed in Figure 3 and the trend shown in
Figure 1. MoS2/Au(111) has the greatest downward shift, resulting from the strongest catalyst adhesion onto the support and the weakest hydrogen binding. In comparison, MoS2/ graphene has a smaller shift in p-states, resulting from weaker catalyst adhesion onto the support and stronger hydrogen binding. Because the shape of the p-states remains relatively unchanged across the Mo-edges for all systems, the role of the support is in providing charge transfer that changes the filling of the p-states. Generally, our results show that the interaction between MoS2 and support is reflected in the shift of S p-states at the edge, which in turn indicates the relative hydrogen binding strength. The atom projected p-orbital density of states of the S-edge S atom is shown in Figure 4. Both the unsupported MoS2 and E
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ACKNOWLEDGMENTS C.T. acknowledges financial support from the NSF GRFP Grant DGE-114747. F.A-P. and J.K.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. We also thank Dr. Karen Chan for helpful discussions.
MoS2 supported on graphene have p-band centers at almost identical locations, which suggests similar hydrogen binding. Indeed, adsorbed H 1s states for both systems are similar and the hydrogen adsorption free energies are within 0.02 eV of each other (Table 1). For MoS2 supported on Au(111), the reconstruction of the S-edge geometry leads to changes in the electronic structure for both the p-states on the edge S atom and the 1s states on the adsorbed hydrogen (Figure 3), which appear to be more smeared out and downshifted. This agrees with our analysis that the S−H bond is weakened significantly for the MoS2 S-edge supported on Au(111) despite the ΔGH of the system remaining unchanged. The net affect of this electronic structure change and the charge transfer from the support is a hydrogen adsorption free energy that is the same as that of unsupported MoS2. In this study, we have used DFT to study the support effects of MoS2 and different catalyst supports on the hydrogen adsorption free energy at the MoS2 Mo-edge and S-edge sites. We see a general trend for the Mo-edge that strong adhesion of the MoS2 catalyst onto the support leads to weaker hydrogen binding. For the S-edge, we find a competing effect where strong support interactions simultaneously strengthen the edge reconstruction while weakening the S−H bond, leading to hydrogen adsorption free energies that are relatively unchanged. However, we also showed that when the support interactions are strong, as in MoS2 supported on Au(111), the stable S-edge configuration could possibly change. Our results suggest that the choice of support is crucial in determining the HER activity of supported MoS2 and that the catalytic activity at the Mo-edge can be optimally tuned by the choice of support. We show that it is possible to achieve a ΔGH close to thermoneutral with a support that binds MoS2 with ΔEadh/Mo ≈ − 0.30 eV. One way to achieve this is to modify the electronic structure of the graphene support to strengthen its binding to MoS2. Possibilities include controlling the defect density of the graphene support,43,44 doping the graphene support,45,46 and using graphene supported on metals.47−50 Ongoing work is being done to computationally screen a variety of catalyst supports in order to find the optimal one. This study also provides a way forward for studying the catalyst−support interactions in other layered transition metal dichalcogenide catalysts where vdW interactions are important. These will be examined in a further study.
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ASSOCIATED CONTENT
S Supporting Information *
Additional details on the determination of the S-edge configuration of MoS2/Au(111). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest. F
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