Single-Layer MoS2 with Sulfur Vacancies: Structure and Catalytic

Feb 15, 2014 - Radisavljevic , B.; Whitwick , M. B.; Kis , A. Integrated Circuits and Logic Operations Based on Single-Layer MoS2 ACS Nano 2011, 5, 99...
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Single-Layer MoS2 with Sulfur Vacancies: Structure and Catalytic Application Duy Le, Takat B. Rawal, and Talat S. Rahman*

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Department of Physics, University of Central Florida, Orlando, Florida 32816, United States ABSTRACT: Single-layer MoS2 is proving to be a versatile material for a wide variety of electronic, optical, and chemical applications. Sulfur depletion, without destabilization of the single layer, is considered a prudent way for making the basal plane of the layer catalytically active. Based on the results of our density-functional-theory examination of vacancy structures on one side of an MoS2 layer, we show that the formation energy per sulfur vacancy is the lowest (energetically favorable) when the vacancies form a row and that the longer the row, the lower the formation energy. In addition, we find that the lowest energy barrier for the diffusion of sulfur vacancy at the row structures through the exchange of a vacancy with a nearby sulfur atom is 0.79 eV and that this barrier increases as the row elongates. We also evaluate the propensity for catalytic activity of an MoS2 layer with two types of sulfur-vacancy structures (row and patch) and find the energetics for alcohol synthesis from syngas to be more favorable for the layer with a sulfur-vacancy patch.

1. INTRODUCTION As a prototype of two-dimensional transition-metal dichalcogenides (MX2, where M = Mo, W, Nb, Ta, ... and X = S, Se, Te), single-layer molybdenum disulfide (MoS2) is proving to be a versatile material for a wide variety of applications owing to its low dimensionality and its optical direct gap of about 1.8−1.9 eV.1,2 Researchers have achieved the growth of large, high-quality MoS2 sheets3−6 and have proposed and investigated possible applications such as low-power transistors,7,8 phototransistors,9,10 complex electronic circuits,11,12 hydrogen evolution reaction, hydrosulfurization, optoelectronics, and energy storage applications.13 An MoS2 layer consists actually of three atomic layers: Mo layer sandwiched between two S layers. Since there is no dangling bond at the basal planes terminated by sulfur atoms, the planes are inert and these sulfur atoms are not expected to participate in catalytic activity of MoS2-based materials. Only the edges of MoS2, containing undercoordinated S or Mo atoms are expected to interact strongly with adsorbates, the particular reactive site being specific to the reaction in question. Nevertheless, MoS2 is used in petrochemical hydrodesulfurization and hydrodenitrogenation processes.14 With the aid of alkali, MoS2 is also used (e.g., by Dow/Union Carbide company15) for converting synthesis gases (syngas)CO, CO2, and H2to alcohol. The particular optical direct band gap of the single-layer MoS2 also makes it a potential candidate for an efficient and viable material for electroand photoelectrocatalytic hydrogen evolution.16 Efforts are also underway to make MoS2 reactive through the creation of more active sites via etching16,17 or by growing it vertically.18−20 Another possibility is to grow small nanoparticles of MoS221 which would expose a number of reactive sites. Alternatively, sulfur depletion without destabilization of the single layer could provide a facile route for making the basal plane of the layer catalytically active. It is worth mentioning that if © 2014 American Chemical Society

sulfur depletion can be performed in a controlled manner, it would be possible to create a large number of active sites in the basal plane, far more than the number to be found at the edges. Controlled creation of vacancy structures ready to be further tested for chemical activity was thus the motivation of the recent work which employed low-energy argon sputtering to create sulfur vacancy on MoS2.22 Creation of sulfur vacancy on MoS2 is also possible under electron irradiation.23 Note that at sulfurvacancy sites, it is the undercoordinated Mo atoms that are exposed to the surface allowing their d-states to form bonds with adsorbing species and thereby offer opportunities for facilitating chemical reactions. A theoretical understanding of the energetics of formation of sulfur-vacancy structures and of their properties is essential as it will encourage experimentalists to develop techniques for growing or creating such films with controlled concentrations and distributions of sulfur vacancies for optimal catalytic activity in industrially important reactions. It will also be a first step in the predictive modeling of MoS2-based catalysts for broader applications. Herein, based on our density-functional-theory (DFT) calculations, we first report that sulfur vacancies on MoS2 prefer to form rows. Second, we show that the diffusion barriers of vacancies increase as the vacancy row elongates, adding to the preference of the formation of the longer rows. Finally, by investigating the potential energy profiles for alcohol synthesis from syngas over MoS2 with sulfur-vacancy structures, we suggest the possibility of using such MoS2 structures as catalysts for this technologically relevant chemical reaction. Received: November 15, 2013 Revised: February 12, 2014 Published: February 15, 2014 5346

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2. COMPUTATIONAL DETAILS We use the Vienna Ab initio Simulation Package (VASP)24,25 to perform DFT simulations employing the projector-augmented wave (PAW)26,27 and plane-wave basis set methods. We use the Perdew−Burke−Ernzerhof functional (PBE)28 to describe exchange-correlation interactions. If not otherwise noted, we use an (8 × 8) MoS2 structure at the center of a supercell whose height is 15 Å as our model system. We create sulfur vacancies by removing a certain number of sulfur atoms on one side of the MoS2 layer. We set energy cutoff for plane-wave expansion at 500 eV to achieve total energy convergence to within 1 meV. We relax atomic positions and lateral dimensions of the (8 × 8) MoS2 layer with and without vacancy structures until all components of forces acting on each atom reach 0.01 eV/Å and the structures are stress-free. Given the large size of the supercell (containing 184−192 atoms), we sample the Brillouin zone with one k-point at the zone-center, for computational feasibilities. With the above setting, the (relaxed) lattice parameter of MoS2 is 3.182 Å, which is within 1% of the experimental value (3.16 Å).29 The formation energy per vacancy of a cluster containing n sulfur vacancies is defined as E Form =

Figure 1. Formation energy of sulfur vacancies on MoS2. Each data point (circle) represents the formation energy of a vacancy structure. The magenta line connects the data of sulfur-vacancy row.

For the case of three sulfur vacancies, we generate all possible vacancy cluster geometries (five total) and calculate the formation energy of these structures. It turns out that the structure in which three vacancies are in a row is the one with the lowest formation energy (5.76 eV). Similarly, for four sulfur vacancies, we generate all possible arrangements of the vacancies satisfying the condition that the distance between two neighboring vacancies must not exceed the lattice parameter of MoS2. From the generated structures, we calculate the average distance d between two arbitrary sulfur vacancies and bin structures with the same d into one group. We next identify distinct structures from each group. We usually find two such structures per group, depending on their symmetry (all structures in the same group have the same shape; if the shape does not have mirror symmetry we will find two distinct structures per group because the basal plane of MoS2 does not have mirror symmetry). Finally, we calculate the formation energies of each structure (12 total). Interestingly, once again we find that the structure in which all four sulfur vacancies are in a row has the lowest formation energy (5.72 eV). We follow exactly the same procedure for five sulfur vacancies for which we obtain 43 different cluster geometries. Once again we find the same result: the lowest formation energy (5.68 eV) occurs when five sulfur vacancies are in a row. For the record we provide in Figure 1 the geometries of few other five-vacancy structures considered here. Calculated formation energies of all structures mentioned above and that of an infinite row of sulfur vacancies are plotted in Figure 1. In addition to displaying those structures in which sulfur vacancies form rows having the lowest formation energy, it shows that for small number of vacancies the formation energy varies linearly with the length of the row (or number of vacancies in the row). Also, the formation energy is the lowest (5.46 eV) when vacancies form an infinite row. The results obtained above are compelling enough that we have not carried out calculations for structures containing more than five sulfur vacancies which would have required much larger supercells and hence computational resources. We have, in addition, performed calculations for well-ordered distribution of single sulfur vacancies using (6 × 6) MoS2 supercells in which the sulfur vacancies are distributed to form 6 × 6, 3 × 3, and 2 × 2 superstructures. We found that the formation energies are 5.84, 5.84, and 5.93 eV, respectively, which are higher than that in row

1 (E Mo64S128−n + nES − E Mo64S128) n

where EMo64S128, EMo64S128−n, and ES are the total energy of an (8 × 8) MoS2, of an (8 × 8) MoS2 with a vacancy cluster containing n sulfur vacancies, and of an isolated sulfur atom. In calculating energy barriers for the diffusion of vacancies, we use the Nudged Elastic Band (NEB) method30 for preliminary determination of a minimum energy path. We then apply the more accurate Climbing Image Nudged Elastic Band (CI-NEB) method31 to refine the calculation of the transition pathway. This way of combining NEB and CI-NEB methods is found to be more efficient for searching transition states than performing CINEB alone. Note that in this work we have used the PBE functional, which does not explicitly include van der Waals (vdW) interaction, as it provides reasonably accurate descriptions of covalently bonded systems (as is recognized in the DFT community) and is computationally economical (it is less expensive than ab initio functionals that account for vdW interactions such as vdWDF,32−34 vdW-DF2,35 optB88-vdW36,37). Note also that in a single layer of MoS2 with sulfur vacancies vdW interactions do not dominate. The interactions of the different molecules/ adsorbates and the MoS2 layer with sulfur vacancies are also predominantly covalent. Nevertheless, we did perform tests calculations with three functionals that include vdW interactions. While vdW-DF and vdW-DF2 strongly overestimate the lattice parameter of MoS2 (3.232 and 3.282 Å, respectively), optB88vdW yields a very good value (3.188 Å, similar to that obtained by PBE). We also find little difference in the binding energy of CO on vacancy-laden MoS2 calculated with optB88-vdW and PBE (2.40 eV as compared to 2.22 eV).

3. RESULTS AND DISCUSSION 3.1. Tendency for Forming Sulfur-Vacancy Row. The formation energy of a single sulfur vacancy in a (8 × 8) MoS2 supercell is found to be 5.85 eV, while that for two sulfur vacancies next to each other is 5.81 eV per vacancy. These initial results indicate that sulfur vacancies have a slight preference to be next to each other. We thus consider in the next part only the case in which sulfur vacancies are adjacent to each other. 5347

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structures indicating once again that the row structures of S vacancies are energetically more favorable. 3.2. Stability of Sulfur-Vacancy Row. The question that we address next is the stability of the sulfur-vacancy row once it has been created. In particular: what is the energy barrier for a nearby sulfur atom to diffuse into and subsequently spoil the row vacancy structure? To answer this question, we calculate the diffusion barrier of a single sulfur vacancy and of sulfur vacancy in various row structures. Essentially it is the energy cost of the exchange between a sulfur vacancy and a neighboring sulfur atom. Following the procedure of combining NEB and CI-NEB methods outlined above for such calculations, we first find that the barrier is 2.32 eV for the diffusion of a single sulfur vacancy. This high barrier suggests the immobility of single sulfur vacancy at low or room temperature conditions. For the case of two sulfur vacancies, we find that the barriers for spoiling row structure are about 2.42, 2.32, and 2.34 eV as the vacancy marked as ① exchanges its position with S atoms numbered 1, 2, and 4, respectively (see Figure 2). Herein, circled (①, ②, ...) and boldface (1, 2, 3, ...) numbers represent, respectively, sulfur-vacancy sites and sulfur atoms next to vacancy row. The lowest energetic barrier for a sulfur atom to get into the two-sulfur-vacancy structure occurs when sulfur atom 3 diffuses to the vacancy ①. The barrier for this process is found to be 0.79 eV. In this case, the row structure of two-sulfur vacancies is not spoiled, but rather it rotates 60°. Note that, since the diffusion barriers of other sulfur atoms to the two-sulfur-vacancy row are large (>2 eV), the two-sulfur-vacancy row is not expected to diffuse from one to another location. Interestingly, as the number of sulfur vacancies in the row increases to three, the energy barrier for separating a vacancy from the row is very high, similar to the two-sulfur-vacancy case. In this case, the lowest energetic barrier for spoiling row structure occurs when sulfur atom 3 diffuses to vacancy ① (see in Figure 3). The barrier for this process is 0.88 eV, higher than the lowest barrier in the case of two-sulfur-vacancy row. More interestingly, the diffusion of the vacancy at the center of the row (i.e., when sulfur atom 3 diffuses to vacancy ②) needs to overcome a higher barrier at 0.92 eV, indicating that the vacancy near the edge of the row is more mobile that the one in the middle of the row. As the length of the row continues to increase to four, the corresponding energy barriers are 0.96 and 1.04 eV for the diffusion of sulfur atom from 3 to vacancy ① and from 4 to vacancy ② (see Figure 4). This again attests that as the length of vacancy row increases (1) the lowest energy barrier for spoiling the row structure increases, and (2) the diffusion of the vacancies at the center of the row is less probable than that of the vacancies at the ends. In the limit of an infinite row, the lowest barrier for sulfur-vacancy diffusion increases to 1.51 eV (0.36 eV for reverse process). From all calculated barriers above, it is clear that the barrier for the diffusion of sulfur vacancies is the lowest in the case of twosulfur-vacancy structure. This structure is the most mobile: it can rotate but hardly diffuse to another location. The diffusion of single sulfur vacancy needs to overcome a huge barrier of 2.32 eV. Thus, the formation of vacancy row is not because of coalescence via the diffusion of sulfur vacancies from one location to another but rather through the diffusion of a vacancy within a vacancy patch or by direct creation of vacancy row (it costs lower energy to elongate the row than to enlarge the vacancy structures or to create a single vacancy). It is important to emphasize that the longer the row structure, the more stable it is. It is also worth

Figure 2. Ball-and-stick model of two-sulfur-vacancy row. Sulfur vacancies and sulfur atoms next to the vacancies are marked with circled and boldface numbers (identical ones are marked with the same numbers). Diffusion barriers of sulfur atom near vacancies are listed on the left. Numbers in parentheses are the barriers for reverse processes. Cyan, dark blue, and yellow balls represent Mo, Mo at vacancy row, and S atoms, respectively.

Figure 3. Ball-stick model of three-sulfur-vacancy row. Notation as in Figure 2.

Figure 4. Ball-stick model of four-sulfur-vacancy row. Notation as in Figure 2.

mentioning that in the event that a sulfur vacancy diffuses away from a row structure, it tends to diffuse back as the barriers of reverse processes are always lower (see Figure 2, Figure 3, and Figure 4), except for the case of two-sulfur-vacancy row for which the two processes are equivalent. During the writing of this manuscript, we became aware of the very recent experimental and theoretical work of Komsa et al.38 who have provided a recipe for controlling the orientation of the vacancy rows, for example, using strain. For single-vacancy row, our results are in excellent agreement with both theoretical simulations and experimental images reported in this work. 3.3. MoS2 with Sulfur Vacancies for Catalytic Application. The existence of the sulfur-vacancy rows and the possibility to control their directions38 suggest a possibility of using them for catalytic applications since, as discussed in the Introduction section, sulfur vacancies will provide great conditions for catalytic activity: Mo atoms and their d-electrons are exposed to adsorbates. It is important to note that the variation of formation energy of various vacancy structures is not large (see Figure 1), especially for structures with small number of sulfur vacancies; thus, the existence of structures other than vacancy row with higher formation energy is also possible. To postulate our ideas of utilizing MoS2 with S vacancies as catalysts, we perform ab initio simulations based on DFT to investigate the energetic pathway of an industrially important reaction not only on sulfurvacancy row but also on seven-sulfur-vacancy patch. Doing so allows us to verify the idea of using MoS2 with sulfur vacancies for catalytic applications and to be able to emphasize the ability to 5348

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We next determine whether or not the adsorption of CO on MoS2 with sulfur-vacancy row happens spontaneously. We use (CI)-NEB, as outlined above, for seven images interpolated from final state (FS) that is the adsorption configuration of CO on MoS2 with sulfur-vacancy row determined above and initial state (IS) that is the optimized configuration of a CO molecule initially placed about 3.0 Å above the determined adsorption site. We find that the CO molecule needs to overcome an adsorption barrier of about 0.55 eVthe energy difference between transition state (TS) and ISas seen in Figure 5a. The same procedure is applied to study the adsorption of H2 molecule onto the MoS2 with sulfur-vacancy row. We first place H2 molecule at various adsorption sites on the MoS2 sheet with sulfur-vacancy row and subsequently perform structural relaxations. We find that the H2 molecule does not chemically adsorb onto the row meaning that the molecule is moving away from the MoS2 after structural relaxation no matter where H2 is initially placed. We then search for the adsorption sites for two H atoms. We find that the two atoms prefer to adsorb at the sulfurvacancy sites as shown in Figure 5b, labeled as FS. This result suggests that the H2 molecule dissociatively adsorbs onto MoS2 with sulfur-vacancy row. The binding energy of dissociative adsorption of H2 molecule is about 0.42 eV. Similar to the adsorption of CO, the dissociative adsorption of H2 molecule is not spontaneous. The barrier of this adsorption is found to be 0.82 eV. After the adsorptions of CO and H2, the reaction of CO* and H* results in the formation of CHO*. We find IS of this reaction by placing an H atom adsorbing at the sulfur vacancy next to the adsorption site of CO* and relaxing the obtained structure. The FS of this reaction is the configuration in which CHO* has been formed and adsorbed on the MoS2 with sulfur-vacancy row. We find such configuration by placing the CHO at various possible adsorption sites on the vacancy row and relax them to find the one with the lowest energy. We then perform (CI)-NEB calculation for seven images interpolated from the determined IS and FS. We find that the barrier for this reaction is 1.09 eV (see Figure 5c). We repeat the process for calculating the reaction barrier in subsequent steps. The results are summarized in the potentialenergy profile of the CO hydrogenation process as displayed in Figure 6. This profile describes the potential energy of each state with respect to the initial state (which is set to be zero corresponding to the state in which CO and two H2 molecules

control and improve catalytic activities of MoS2 with sulfur vacancies. We are particularly interested in designing structures and concentrations of sulfur vacancies on MoS2 basal planes for (higher) alcohol synthesis: converting syngas (CO, H2) to (higher) alcohol, the formation of which is an important part of an economy based on renewable fuels. At the first step, we focus on the synthesis of methanolthe simplest form of alcohol. In principle, the methanol synthesis from syngas is a CO hydrogenation process that consists of the following main steps: (i) CO(g) adsorption to form CO*, (ii) H2 dissociative adsorption to form atomic hydrogen H*, (iii) CO* + H* → CHO*, (iv) CHO* + H* → CH2O*, (v) CH2O* + H* → CH3O* (superscript * denotes that the species adsorbed onto the MoS2 while subscript (g) indicates gas-phase species). 3.3.1. On MoS2 with Sulfur-Vacancy Rows. We first search for the adsorption site of CO molecule on the sulfur-vacancy row by placing the molecule at several possible sites and performing structural relaxation to find the minimized energy structures. The lowest energy configuration is shown in Figure 5a, labeled as FS

Figure 5. (CI)-NEB results of the adsorption of CO (a), dissociative adsorption of H2 (b), and the formation of CHO* (c) on MoS2 with sulfur-vacancy row. IS, FS, and TS are, respectively, abbreviations of initial state, final state, and transition state. Cyan, yellow, black, red, and magenta (smallest) balls represent Mo, S, C, O, and H atoms, respectively.

(standing for f inal state). We find that the molecule prefers to chemisorb at the hollow site with a sulfur atom underneath (sulfur-vacancy site) with a binding energy of 0.12 eV. Herein, the binding energy is defined as the absolute value of the difference between the total energy of the whole system (molecule absorbs onto the MoS2 sheet) and the sum of total energy of the MoS2 sheet and that of the isolated molecule (which is in a 15 × 15 × 15 Å3 box). Note that for reducing computational cost, we use a (7 × 6) MoS2 supercell for calculations reported in this section.

Figure 6. Potential energies along the reaction pathway of the formation of CH3OH via the CO hydrogenation on MoS2 with a row of sulfur vacancies (solid connections) and with a patch of seven sulfur vacancies (dotted connections). Thicker-longer bars represent the intermediate states while thinner-shorter bars represent transition states. Numbers (in eV) are energetic barriers. Superscript * indicates adsorbed specie. Subscript (g) indicates gas phase. 5349

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much larger than that in the case of sulfur-vacancy row. Not only does the adsorption energy of CO increase dramatically, but its adsorption onto the seven-sulfur-vacancy patch is also found to be barrierless. For the adsorption of H2, we found that the molecule dissociatively adsorbs and that the resulting H atoms adsorb preferably at the hollow sites with a sulfur atom underneath. The barrier for this process is found to be 0.15 eV, significantly lower than that on the sulfur-vacancy row. We also constructed potential-energy profile of various states in the CO hydrogenation process on MoS2 with sulfur-vacancy patch (as displayed in Figure 6 with dotted connections), similarly to what was done in the case of sulfur-vacancy row. As shown, reaction barriers reduce to 0.5 eV or less for most processes. More importantly, the potential energies of the intermediate states are much lower than that of CO(g) + 2H2(g), CH3OH, and CH2O + H2(g). This suggests that if one supplies CO and H2 to MoS2 with seven-sulfur-vacancy patches, CO and H atoms will react to form CHO*, CH2O*, and CH3O*. The desorption of formaldehyde in this case is not favorable because this reaction is endothermic as it requires about 2.26 eV per molecule. However, the formation of methanol from methoxy is an endothermic reaction for which the barrier is 1.19 eV. This endothermic reaction with high barrier is the rate-limiting step of the hydrogenation process.

are in gas phase and well separated from the MoS2 layer) and transition states (which are found by (CI)-NEB calculations). As one can see, CO and H2 adsorb onto the MoS2 layer with sulfurvacancy row with a barrier of 0.55 and 0.82 eV, respectively. Note that since there is no particular preference for order of the adsorption of CO and H2, we display two possible processes from CO + H2 to CO* + 2H* in the profile. The adsorbed species, CO* and two H*, are the ingredients for the next reactionthe formation of CHO* as described above. Next CHO* will react with H* to form CH2O*. This particular reaction needs to overcome a barrier of 0.80 eV and is shown next in the profile. From CH2O*, there are two possible scenarios, as seen in the profile: (1) CH2O* will desorb to form gas-phase formaldehyde or (2) CH2O reacts with H* (coming from another dissociatively adsorbed H2 with a barrier of 0.82 eV) to form methoxy which then adsorbs on the vacancy row (CH3O*). The desorption in the first scenario is found to be spontaneous, while the reaction in the second occurs with a barrier of 0.72 eV. The next reaction shown in the profile is the formation of methanol (CH3OH) from CH3O* and H*. The barrier for this reaction is 1.40 eV. CH3OH is not found to chemisorb on the sulfur-vacancy row but to physisorb with a binding energy of about 60 meV. Since methanol does not chemically bind onto the vacancy row, it will desorb at moderate temperatures and should be the final product of the CO hydrogenation process. The calculated barriers for the reactions of CO hydrogenation on the sulfur-vacancy row discussed above are higher than 0.7 eV, except for that for the adsorption of CO. These barriers are higher than what we need for achieving high reaction rates under normal synthesis conditions. It is because most of the reactions in this case (such as CO* + H* → CHO*, CHO* + H* → CH2O*) involve the diffusion of H atoms. The barrier of such diffusion is itself 0.8 eV and may be attributed to the narrow size of the vacancy row. Apart from the high reaction barrier, a few other factors may inhibit the synthesis of CH3OH from CO and H2 over sulfur-vacancy row. For example, there are large potential energies of the intermediate states (CHO*, CH2O*) which suggests hindrance in these states to form methanol as the final product. Instead, the poisoning of active sites from CO and atomic H adsorption may be dominant. Even if the reaction could pass through the high potential energy states, the third problem preventing the synthesis of methanol is the formation of methoxy (CH3O) on the surface as the final step. This is possible as the adsorption of methoxy on the surface is quite strong (with potential > −2 eV) and the barrier for converting methoxy to methanol is found to be 1.4 eV, which is the highest barrier in the whole process. The last obstacle is the formation of formaldehyde (CH2O). As shown in Figure 6, once CH2O* forms, it spontaneously desorbs from the surface to form formaldehyde because the process is found to be barrierless. Thus, the formation of formaldehyde is more favorable than that of methanol. 3.3.2. On MoS2 with Sulfur-Vacancy Patches. The limited performance of the sulfur-vacancy row could be attributed to the narrow size of the vacancy row which does not allow room for facile reactions. One way around this is to create patches of sulfur vacancy. For purposes here, we have a seven-sulfur-vacancy patch whose formation energy is found to be 6.0 eV. We have checked the stability of the structure by performing ab initio molecular dynamics simulations for 12 ps.22 While the steps in our investigation of this case are identical to that used for the sulfurvacancy row, the results are interestingly different. The adsorption energy of CO is found to be 2.22 eV, which is

4. CONCLUSIONS In summary, using DFT simulations we have first calculated the formation energies of sulfur vacancies in various structures and shown the tendency for the elongation of sulfur-vacancy row on the MoS2 basal plane. Our calculations of the energy barriers for events that would change the vacancy cluster geometry from the row show that the lowest barrier to spoil the row structure increases as the row elongates and that the barrier for the reverse process is always smaller (except for the case of two-vacancy row in which the two barriers are identical). Moreover, the calculations show high barrier for sulfur-vacancy diffusion suggesting that the formation of row vacancy is not via the coalescence of sulfur vacancies from one location to another but rather via direct creation of the row or via the diffusion of sulfur vacancy within a vacancy cluster. Our calculations also indicate that once the row-vacancy structure is formed, it tends to elongate and that the longer sulfur-vacancy row, the more stable it is. As for catalytic applications, even though we find the sulfurvacancy rows on MoS2 to be not facile for alcohol synthesis from syngas, analysis of the rate-limiting step gave us ideas about alternative ways to enhance the catalytic activity of the MoS2 basal plane. One such possibility is via enlarging the size of the sulfur-vacancy region (we use seven-sulfur-vacancy patch), which we find to provide lower potential energies of intermediate states, resulting in an increase of exothermic processes, which in turn support the hydrogenation of CO. In addition, the barriers for such processes reduce to much smaller values as compared to those on the sulfur-vacancy row. Methanol synthesis on the sulfur-vacancy patch, however, faces a rate-limiting step, whose barrier is about 1.19 eV. In short, the above analysis has allowed us to establish that alcohol synthesis from syngas is possible through the manipulation of the sulfur-vacancy geometry. Even though our present analysis is far from complete (missing other intermediate states, final products, etc.), our results shed some light into the usage of sulfur vacancy on MoS2 for alcohol synthesis. Such predictive modeling could serve as the first step toward a rational design of alcohol synthesis catalyst based on sulfur vacancies on basal plane of single-layer MoS2, where we 5350

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The Journal of Physical Chemistry C

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can take advantage of large surface/bulk ratio to maximize catalytic efficiency.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +1-407-823-1480. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported in part by the U.S. Department of Energy under grant DE-FG02-07ER15842. We thank the Extreme Science and Engineering Discovery Environment (XSEDE), under grant DMR13009, and STOKES Advanced Research Computing Center at the University of Central Florida for providing computational resources for this work.

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REFERENCES

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dx.doi.org/10.1021/jp411256g | J. Phys. Chem. C 2014, 118, 5346−5351