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Article Cite This: ACS Omega 2019, 4, 4023−4028
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Surface Morphology and Sulfur Reduction Pathways of MoS2 Mo Edges of the Monolayer and (100) and (103) Surfaces by Molecular Hydrogen: A DFT Study Sergei Posysaev and Matti Alatalo* Nano and Molecular Systems Research Unit, University of Oulu, PO Box 3000, Oulu FI-90014, Finland
ACS Omega 2019.4:4023-4028. Downloaded from pubs.acs.org by 191.96.170.102 on 02/24/19. For personal use only.
S Supporting Information *
ABSTRACT: We have performed a density functional theory study of the MoS2 monolayer and the MoS2 (100) and (103) surfaces in relation to the early stages of the hydrodesulfurization reaction. In many X-ray diffraction (XRD) results, the (103) surface exhibits a higher peak than the (100) surface, yet one of the most frequently occurring surface has not been studied extensively. By analyzing experimental studies, we conclude that the (103) surface of MoS2 is the most frequently occurring edge surface when the sample size is thicker than ∼10−15 nm. Herein, we report the first comparison of reaction paths for the formation of a sulfur vacancy on the (103) surface of MoS2, monolayer, and (100) surface of MoS2. The reason for the occurence of the (103) surface in the XRD patterns has been established. We point out the similarity in the reaction barriers for the monolayer and (100) and (103) surfaces and discuss the reason for it. Moreover, we found a more energetically favorable step in the reaction pathway for the formation of a sulfur vacancy, which allowed us to refine the previously established pathway.
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purification process. A promising study by Hu et al.20 showed experimentally and theoretically that a stepped edge surface, which consists of (100), (103), (110), and (200) planes of the MoS2 crystal, has significantly enhanced the electrocatalytic activity, particularly in the hydrogen evolution reaction. The band gap of MoS2 varies from 1.23 eV in bulk21 up to 1.9 eV in a monolayer,22 which indicates a different chemical environment between the two. It allows us to assume that catalytic efficiency can depend on the structure of MoS2 and on the surface termination. In applications, MoS2 is mostly used in the form of nanoparticles,1,23−25 suggesting that several surface orientations may be of interest. The most studied MoS2 surface is (100) (Figure 1). It is often encountered in experiments, but several experimental studies showed also the presence of the (103) edge surface of MoS2 in samples,20,24−30 and in some of them, the (103) surface exhibits the highest peak in the X-ray diffraction (XRD) patterns.24,26−28 In the case of (103), the surface area is ∼20% larger than the area of the (100) surface consisting of the same number of atoms, which can be useful for some HDS reactions, for example, with large molecules. As per our knowledge, no one has investigated the reaction pathway for the formation of a sulfur vacancy on the (103) surface theoretically so far, even though it may have catalytic efficiency distinct from those of the (100) surface and the monolayer.
INTRODUCTION Layered transition metal dichalcogenides (TMDs) have been intensively studied for decades and have an opportunity to be applied in nanoelectronics and optoelectronics,1 photocatalysis,2,3 and as a transistor.4 Fuel purification is among the most important applications of TMDs due to the demand for cleaner fuel that rises each year within stricter environmental regulations.5 Several approaches have been tried to improve the processes of fuel purification such as changing the morphology of the catalyst,6,7 doping,8 and novel supports.9,10 Nevertheless, the need for developing highly effective catalysts still persists. Sulfur dioxide is an air pollutant; it affects human health11 and can provoke acid rains. One of the sources of sulfur dioxide emission is fuel combustion. To reduce the emission of sulfur dioxide, hydrodesulfurization (HDS) reaction is used, during which sulfur is removed from refined petroleum products and natural gas. The edge planes of MoS2 are used as a catalyst for such a reaction. Morphologies of MoS2 nanoparticles in hydrodesulfurization catalysis have been studied by scanning tunneling microscopy, and the structures of catalytically active sites were investigated.12 Several theoretical studies have examined the elementary reaction steps, the catalytic activity of the (100) surface of MoS2,13−16 cobalt promotion of MoS2 that increases the hydrogenation activity,17 and the effect of the hydrogen and sulfur coverage on MoS2 edges18,19 by means of density functional theory (DFT). The latter studies have shown that sulfur vacancies can affect the adsorption of sulfur compounds and improve the fuel © 2019 American Chemical Society
Received: October 29, 2018 Accepted: February 13, 2019 Published: February 22, 2019 4023
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retained a “bulklike” structure, which led to a small decrease in surface energies for the relaxed surfaces. Effect of a fixed bottom layer on the surface energies of both models was estimated by relaxing models without atomic constraints. The energy decrease caused by freeing all atoms is almost the same for both surfaces (−0.037 J/m2 for (100) and −0.031 J/m2 for the (103) surface). Relaxing the surfaces without atomic constraints also yielded only minor changes in the atomic positions. To examine the surface energy of the most stable sulfur coverage under HDS reactions by eq 1, half of the sulfur atoms have been transferred from the S edge to the Mo edge. The obtained Mo50-S50 surfaces have significantly lower surface energies, which is in agreement with previous results.19,31 On the basis of the computed surfaces, the (103) surface is more stable than the (100) surface both in the case of ideal surfaces and in the case of the most stable sulfur coverage under HDS reactions. Our result agrees with theoretical studies33 and experimental results, where the (103) XRD peak of MoS2 is the highest24,26−28 but contradicts with refs 20, 29, 30, where the (100) peak is higher. Apparently, the XRD peak intensity depends on the thickness of MoS2 samples. The (103) peak is higher for the crystal size larger than ∼10−15 nm and smaller for thinner samples. It was verified by increasing the ball-milling time in the work by Cheng et al.27 Formation of Sulfur Vacancy. The key step in fuel purification is the formation of a sulfur vacancy preceded by dissociative adsorption of H2, diffusion of the hydrogen atoms on the MoS2 surface, creation of a H2S molecule, and subsequent associative desorption.13,14 The lowest activation barrier for H2 dissociative adsorption as well as the lowest activation energies for H2S associative desorption was found on the Mo edge with 50% sulfur coverage.14 Moreover, for the (103) surface, the Mo edge with 50% sulfur coverage can be created by a planar cut from the ideal structure33 along with other terminations (Figure S1). According to Paul and Payen,13 the energy of the departure of the first H2S molecule from the Mo edge of the Mo50-S50 surface is lower (ΔE = 0.49 eV) than that from the S edge of Mo50-S50. A more recent study by Prodhomme et al.14 showed the same trend (ΔE = 0.92 eV). It allows us to assume that most sulfur vacancies appear at the Mo edge and give a reason to explore the formation of sulfur vacancies only on this edge of the Mo50-S50 surface. In this section, we present energy pathways for the elimination of the H2S molecule from the Mo edge of the (100) monolayer, (100) surface, and (103) surface (Figure 2) and provide a slightly altered pathway that is more energetically favorable. The reaction pathway for the formation of sulfur vacancy is presented in Figure 3. The pathway consists of the same steps as proposed by Prodhomme et al.14 To obtain consistent results for different surfaces and simulate the case where the molecules and the slab are far from each other, particular care should be taken. Energies for the first reaction step (S1) and the last reaction step (S6) are taken as sums of separate calculations of the molecules (H2 and H2S consequently) and the slab. Otherwise, the energy of the systems slightly varies with the position of the H2 and H2S molecules above the surfaces, which leads to disagreement between results for different systems. We describe the pathway only for the (100) monolayer in detail because the local structures of the Mo edge are the same for the (100) monolayer, (100) surface, and (103) surface
Figure 1. (100) and (103) surfaces of MoS2. Mo atoms are in turquoise and S atoms in yellow.
In this article, our aim is to demonstrate the reason for the occurrence of the (103) surface of MoS2 in XRD patterns by DFT calculations. We shall also shed light on the possible applications of the (103) surface in the HDS reaction and on the difference in reaction barriers between the monolayer and edge surfaces of MoS2 and propose new reaction pathways. Moreover, we provide experimentalists with information about similarities in the local structure of the examined surfaces for sulfur coverage on an ideal MoS2 surface and on the most stable sulfur coverage under HDS reactions.
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RESULTS AND DISCUSSION Surface Energies and Structures. Both the (100) and (103) surfaces of MoS2 have been experimentally observed20,24−30 and have some of the highest peaks in XRD patterns among edge surfaces. Other surfaces have also been observed, but due to the complexity of calculation, we limit this study to two above-mentioned surfaces. Previous theoretical investigations showed that during the HDS reaction, sulfur coverage on the edge surface of MoS2 changes.18,19 The most stable sulfur coverage under HDS reactions is the so-called Mo50-S50,19,31 where 50% of sulfur transfers from the S edge to the Mo edge, or in other words, there is one sulfur atom per Mo atom on the surface. The ideal MoS2 surface has two sulfur atoms per Mo atom on the S edge surface and is called Mo0-S100. In this study, we compare only the stability of Mo50-S50 and Mo0-S100 sulfur coverages for (100) and (103) surfaces by assuming that the difference in surface energy between both surfaces is similar for different sulfur coverages. A more recent study by Rosen et al.32 showed that the most stable coverage under HDS was found to be Mo50-S100. We keep 50% of sulfur coverage on the S edge because this work concentrates on the activity of the Mo edge, and we expect that a change in the sulfur coverage on the S edge does not have a big impact on the catalytic activity of the Mo edge due to the large distance between the Mo and the S edges. Cleavage energy values showed a higher possibility of the formation of the (103) surface (Table 1). The most energetically favorable surface was defined by surface energy, which reflected the energy loss when creating the surface, compared to the bulk. Relaxing the surfaces yielded only minor changes in the atomic positions. The surfaces therefore mostly Table 1. Cleavage and Surface Energies (J/m2)
(100) surface (103) surface
cleavage energy
surface energy (Mo0-S100)
surface energy (Mo50-S50)
2.117 1.853
2.084 1.827
1.793 1.561 4024
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Figure 2. Slab models of Mo50-S50 sulfur coverages: (a) monolayer, (b) (100) surface, and (c) (103) surface. The local structures of Mo edges are indicated by dashed ellipses.
Figure 3. Reaction pathway for the formation of a sulfur vacancy. Black, green, and blue paths relate to the (100) monolayer, (100) surface, and (103) surface, respectively. Paths are shifted vertically for a better readability. The energies of the S1 state are taken as references for each reaction path. Mo atoms are in turquoise, S atoms are in yellow, and H atoms are in green and red (the two hydrogen atoms are depicted with different colors for clarity). Geometrical structures are depicted for the (100) monolayer surface.
To further show that the chemical properties of S atoms are not affected by the overall structure of the surfaces and have the same electronic structure on the Mo edges of the examined surfaces the atom-projected p-orbital density of state of S atoms was calculated (Figure 4). The peak position near the Fermi level reflects the active state, whereas the shift of the peak usually leads to a change in the binding strength.34,35 All sulfur atoms on the Mo edges have no noticeable difference in the peak positions for all of the structures considered (the peak positions are −0.74, −0.79, and −0.64 eV for (103) and (100) surfaces and monolayer, respectively). The shape of the pstates remains relatively unchanged across the Mo edges for all systems. From this, we confirm that the overall structure of the surfaces does not affect the chemical properties of the S atoms on the Mo edges. Moreover, it was shown that the
(Figure 2), and according to our results, the barriers among the three models are similar (Figure 3). First, the energy barrier for dissociative adsorption of H2 is 0.41 eV. The adsorption is found to be an exothermic process (−0.16 eV). After the first step, hydrogen atoms start to diffuse on the surface, beginning with transition state 2 (TS2) where the hydrogen atom depicted as green goes to the other side of the edge of the MoS2 layer (barrier is 0.58 eV). Further, the other hydrogen depicted as red diffuses to another sulfur atom and stabilizes the system in comparison to S1 by −0.53 eV. The highest barrier of 0.86 eV should be overcome to break the H−S bond of the red hydrogen atom and form a H2S molecule adsorbed on the surface. The last step of H2S desorption is an endothermic process and proceeds with almost no significant barrier. 4025
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hydrogen goes to another side of the edge of the MoS2 layer (TS3a) and the rest of the pathway resembles the pathway in Figure 3. Another one skips the S4 state and goes from S3a to S5 (purple dashed line). The barrier of TS3b is only 0.04 eV higher than that of TS4, which makes this pathway possible under the working conditions of HDS but likely rarely occurring. The last steps are the same as in Figure 3: the formation of the H2S molecule adsorbed on the surface with the consequent desorption.
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CONCLUSIONS In this work, we demonstrated that the reason for the occurrence of the (103) surface of MoS2 in XRD patterns is its low surface energy and the possibility to construct the most energetically favorable sulfur coverage by a planar cut. Special attention was paid to energy barriers on different MoS2 surfaces. Our results for the Mo edge of the (100) monolayer are very close to the results obtained in the work of Prodhomme et al.14 Barriers among the (100) monolayer, (100) surface, and (103) surface differ by less than 0.07 eV. By concluding that the monolayer and (100) and (103) surface have similar catalytic activities, it allows manufactures/ experimentalists to use MoS2 catalysts equally well in the form of monolayers, samples with the highest (100) peak in XRD patterns, as well as samples with the highest (103) peak. The probable reason for such similarities can be the weak van der Waals interaction between layers of MoS2, which can change the energy of the band gap but apparently has a vanishingly small impact on the catalytic activity of the edge surfaces of MoS2. Furthermore, we conclude that the barriers for the reaction pathway for the formation of a sulfur vacancy on (10X) surfaces (where, X = 0, 1, 2, etc.) should be the same because the local structure of the monolayer on the surface is the same for all of these surfaces, only the empty space between the layers of MoS2 as well as the surface area grows
Figure 4. Projected p-orbital density of states of S for the Mo edge of the monolayer, (100) surface, and (103) surface relative to the Fermi level.
concentration of H2 affects the formation of a sulfur vacancy on the edge surface of MoS2.14,36 We expect the same effect of the concentration of H2 for the (103) surface by taking into account the similar chemical activity of both surfaces. We provide a slightly altered reaction pathway for the formation of a sulfur vacancy, which is energetically more favorable (Figure 5). Here, the TS2 state was changed from the green hydrogen twisting from one side of the layer to another different transition state labeled as TS2a where the red hydrogen moves from a Mo to a S atom. The barrier for TS2a is 0.28 eV lower than that for TS2. Likewise, the new S3a state is 0.27 eV more energetically favorable than S3. There are at least two possible steps after S3a. In the first one, the green
Figure 5. Reaction pathway for the formation of a sulfur vacancy. The steps for the proposed new pathway are depicted in red. The possible step that skips S4 is in purple. 4026
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with increasing X. If the bulk structure of MoS2 is not needed for the HDS reaction, full exfoliation of MoS2 can be beneficial because a monolayer standing alone is able to adsorb bigger molecules than the bulk. In addition, a new reaction pathway for the formation of a sulfur vacancy was found. It does not change the activation energy of the reaction neither reduce the highest barrier of the reaction. The refined pathway is more energetically favorable and provides a deeper understanding of the reaction.
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COMPUTATIONAL METHODS DFT Calculations. Spin-polarized ab initio calculations were performed with DFT, using the projector augmented wave method as implemented in the Vienna ab initio simulation package (VASP).37,38 The Perdew−Burke−Ernzerhof functional was used to treat the electron exchange and correlations, together with the van der Waals (DFT-D3) correction.39,40 The plane wave cut-off energy was set to 350 eV. The convergence criterion was chosen so that the maximum force on one atom is smaller than 0.02 eV/Å. The calculations were performed with periodic boundary conditions. The Brillouin zone was sampled using a Monkhorst− Pack k-point set containing 2 × 2 × 1 k-points during the structural optimization.41 Surface energy (Es) is calculated as Es(n) =
1 (En − nE b) 2A
Figure 6. Slab models for the calculation of the reaction paths: (a) (100) surface and (b) (103) surface.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b02990. Planar terminations of the MoS2 (103) surface (PDF)
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(1)
AUTHOR INFORMATION
Corresponding Author
*E-mail: matti.alatalo@oulu.fi.
where n is the number of unit cells, A is the surface area, En is the total energy of the relaxed model, and Eb is the total energy of crystalline MoS2 (per unit cell). For cleavage energy calculation, En is the total energy of the unrelaxed model. During the search for transition states (TS), two methods were applied in the following order: first, climbing image nudged elastic band42 and then the dimer method.43 For each TS, the dynamical matrix was calculated and verified that it contains exactly one imaginary frequency. Construction of MoS2 Edge Surfaces. The (100) and (103) surfaces of MoS2 were tested in depth and in width until surface energy convergence of less than 0.01 J/m2 was reached. Final surfaces contain four unit cells in the x direction, five in the z direction for surface energy calculations, and four in x and z directions for the calculation of the reaction paths (Figure 6), resulting in 40 Mo and 80 S atoms and 32 Mo and 64 S atoms, respectively. The vacuum region in the z direction was set to 15 Å to avoid mirror interactions between nearby images. During relaxation, all atoms were allowed to relax except for the two bottom layers of MoS2, which represent the bulk of MoS2. The bulk unit cell of MoS2 consists of two layers: Mo atoms of one layer are exactly on top of S atoms of the second layer and vice versa. Such structure leads to two types of edges: a Mo edge with Mo atoms on top and a S edge wherein S atoms are on top (Figure 6). The (100) monolayer is constructed from the Mo edge in a similar fashion as used in previous models13−15 consisting of a single MoS2 layer separated by 12 Å in the y direction. Two Mo layers in the y direction are used to model the “as-cleaved from the bulk” structure for the (100) and (103) surfaces of MoS2. All slabs are carved in such a way that the upper and lower termination will construct a bulk crystal if properly stacked upon each other.
ORCID
Sergei Posysaev: 0000-0001-6161-870X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We would like to thank Olga Miroshnichenko for useful discussions. We acknowledge CSC-Scientific Computing Ltd., Espoo, Finland for providing computational resources.
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