MoS2 Quantum Dots: Effect of Hydrogenation on Surface Stability and

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

MoS2 Quantum Dots: Effect of Hydrogenation on Surface Stability and H2S Release Prashant P. Shinde, Shashishekar P Adiga, Shanthi Pandian, Sanoop Ramachandran, Krishnan S Hariharan, and Subramanya Mayya Kolake J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04198 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 8, 2018

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The Journal of Physical Chemistry

MoS2 Quantum Dots: Effect of Hydrogenation on Surface Stability and H2S Release Prashant P. Shinde,∗ Shashishekar P. Adiga, Shanthi Pandian, Sanoop Ramachandran, Krishnan S. Hariharan, and Subramanya M. Kolake Computational Simulations Group (SAIT-India), Samsung R&D Institute India-Bangalore 560037, India E-mail: [email protected]

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Abstract We employ density functional theory to investigate effects of hydrogenation on the energetic stability and electronic properties of triangular MoS2 nanoclusters with Sedges. Excess edge sulfur atoms relative to the bulk stoichiometry along the edges are passivated by hydrogen atoms. We find that, the hydrogen coverage for maximum stability can be calculated by (n − 2)/2(n − 1), where n is the number of S atoms along an edge. The energetics reveal a preference for zigzag arrangement of adsorbed hydrogens on the edges. Our calculations show vanishing HOMO-LUMO gaps mainly due to the presence of dangling bonds at the edges and can be considered metal-like. We find that the activation energy required to release H2 S lies in between 0.47 and 0.62 eV and this value is in good agreement with the recently reported experimental value.

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Introduction

Monolayer molybdenum disulfide (MoS2 ), a two-dimensional (2D) atomic crystal, has been receiving scientific as well as technological interests because of its distinctive electronic 1–3 and catalytic properties. 4–6 In contrast to graphene, the electronic structure of MoS2 monolayer shows a direct energy gap of 1.80 eV, 1 which is a defining property for digital logic applications. The tunability of the gap to 1.20 eV for the bulk MoS2 and the high mechanical flexibility facilitates its use in flexible electronic devices. 7–9 Moreover, a room-temperature carrier mobility of at least 200 cm2 V−1 s−1 and current on/off ratios of 108 have been demonstrated 10 for the monolayer. In order to fulfil the demands for device applications, controllable large-scale synthesis of MoS2 thin films of uniform thickness and composition is of great importance. 7,11–13 Recent years have witnessed tremendously increased activity in the synthesis of large-area, high-quality MoS2 thin films on insulating substrates. These studies reported presence of triangular nanoclusters of MoS2 stabilized by excess sulfur atoms 8,14–19 on edges during the growth. Till today, limited number of studies 8,20–22 have been published detailing the controlled 2


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fabrication of large-scale MoS2 monolayers by chemical vapor deposition technique. A recent study by Kang et al. 23 focuses on a wafer-scale synthesis of MoS2 thin films using molybdenum hexacarbonyl (Mo(CO)6 ) and diethyl sulfide ((C2 H5 )2 S) precursors. The metalcomplex, Mo(CO)6 , was used as the source for the metal atom whereas the thiol molecule, (C2 H5 )2 S, was used as the source for the chalcogen atom. In addition to these precursor molecules, H2 and Ar were introduced with a controlled flow rate. The carrier gas (Ar) transports carbonaceous and other reactive species to outlet. In experiments, the presence of H2 gas was shown to play a crucial role in the growth of MoS2 monolayers. The authors in Ref. 23 reported large-scale growth of MoS2 grains of size more than 10 µm with decreasing H2 flow. Here H2 acts as an additional reducing agent to the chalcogen precursor. Similar study 24 revealed no transition metal dichalcogenide (TMDC) nanosheet growth without H2 in the carrier gas. Moreover, the role of hydrogen under industrial hydrotreating/hydrodesulfurization (HDS) conditions and to the HDS process dissociative adsorption of hydrogen is paramount. 25 By combining atomically resolved scanning tunnelling microscopy (STM) images and density functional theory (DFT) calculations Lauritsen et al. 26 studied the interaction of atomic hydrogen with triangular MoS2 clusters. The authors conclude that the adsorbed hydrogen changes the chemical properties of the nanoclusters substantially. The hydrogen molecules present in the reaction chamber attack the catalytically active sites on the edges and lead to the formation of coordinatively unsaturated sites through the release of H2 S molecules. The H2 S release or HDS activity occurs preferentially at the MoS2 edges. Thus, there are sufficient evidences supporting the fact that hydrogen is crucial for achieving the large scale growth of TMDC layers. Inspired by these observations, we perform DFT based calculations to investigate the effect of hydrogenation on the energetic stability and electronic properties of S-edge MoS2 nanoclusters. The MoS2 nanoclusters achieve maximum stability when all the excess S atoms are passivated by a hydrogen atoms. We find that the activation energy barrier for H2 S release lies in between 0.47 and 0.62 eV. Our investigation of energies associated with H-adsorption and H2 S release have implications

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for optimal H2 partial pressure during the growth of MoS2 monolayer. Additionally, our DFT results suggest a possible way to engineer the electronic properties of the nanoparticles from semiconducting to metallic through hydrogenation.

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Computational Methods

Our DFT calculations are performed using the Vienna Ab initio Simulation Package (VASP) code. 27 We have used the Perdew-Burke-Ernzerhof form of exchange-correlation functional 28 implemented within the projector augmented wave method. 29 All calculations have been carried out with an energy cut-off of 450 eV for the plane wave expansion. With the energy cut-off the calculated in-plane lattice parameter, 3.16 Å for MoS2 monolayer is in good agreement with the experimental value of 3.15 Å. 30 For geometry optimization, the cluster was placed in a large enough orthorhombic periodic cell such that there was a vacuum space of at least 10 Å in all directions to avoid the interaction between the cluster and its periodic images. Also, we considered a larger periodic cell with vacuum space of 20 Å between periodic images. However, we did not find significant difference in the total energy. The total energy is converged with a tolerance of 10−5 while the force on each ion is considered to be converged if its magnitude becomes less than 0.005 eV/Å. The van der Waals (vdW) dispersion interactions are taken into account by involving the DFT-D3 method by Grimme et al. 31 The influence of spin-polarization is included in the self-consistent field calculation following a geometry optimization calculation. To determine the minimum energy path, transition states and intermediate states between H2 adsorption and H2 S release we used nudged elastic band (NEB) method. 32–35 The structure of 2H-MoS2 monolayer consists an atomic layer of Mo atoms sandwiched between two atomic layers of S atoms. 36 The metal atoms are arranged in a hexagonal lattice and are positioned in a trigonal prismatic coordination with six S atoms as their nearest neighbours. 37 For MoS2 monolayer, the calculated lattice parameter (a0 ) and the

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Mo−S bond length are 3.16 Å and 2.41 Å respectively. To model an MoS2 nanocluster we consider a triangular morphology exposing the under-coordinated S atoms along the edges. This edge is called (¯1010) S-edge as shown in Figure 1. The other edge type is called (10¯10) Mo-edge. In the present work, we considered intermediate size clusters i.e., MoS2 nanocluster with n = 6 and 7 edge S atoms.

n = 7: S-edge MoS2 nanocluster

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6

5

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1 S-edge Mo-edge

MoS2 monolayer

Figure 1: Construction of triangular-shaped MoS2 nanoclusters from MoS2 monolayer. (Left): S-edge MoS2 cluster with n = 7 edge S atoms, (Right): Mo-edge MoS2 cluster with the blue atoms forming the edge. Large gray balls, Mo atoms; small gray balls, S atoms.

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Results and Discussion

Upon geometry optimization of the S-edge MoS2 nanocluster with n = 6 edge S atoms, the Mo−S bond length in the center of the cluster (Figure 2b) remains unchanged compared to 2.41 Å in the monolayer. We do not expect a significant effect of the edges on the geometry parameters for large clusters. The most significant changes in the bond length, ±5%, have been observed on the edges of the nanoclusters due to under-coordinated edge S atoms. In the later text, we refer the total number of edge S atoms as NSedge and number of adsorbed hydrogens as NH . We observe structural reconstructions of the corner two sulfur atoms coordinated by a single Mo atom (see red dotted ovals in Figure 2b). The large chemical potential of under-coordinated S atoms favors the formation of bulk sulfur. Indeed, the two sulfur atoms form a dimer and the solid state of MoS2 becomes unstable. 25 A

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very recent STM studies 37,38 have revealed energetically favorable configuration with sulfur pairing at the edges. The sulfur atoms on the edges are catalytically active sites for hydrogen adsorption and HDS reactions. It should be noted that to understand and control the reactions pathways, details of hydrogen interaction need to be explored. Therefore, in the following section we investigate the effects of hydrogenation on the energetic stability of Sedge MoS2 nanoclusters.

Dimerization

Mo

Top view

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re ctu Stru xation a Rel

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+H

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e) I

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III II


H

Side view

Figure 2: Optimized structures of hydrogenated triangular MoS2 nanoclusters with S atoms forming the edges. a) Isolated MoS2 nanocluster, b) relaxed MoS2 nanocluster, c) hydrogenated nanocluster with NH = 2 (θH ≈ 7%), and d-e) hydrogenated cluster with NH = 12 (θH = 40%). The cluster size is indicated by n (= 6).

3.1

Excess Sulfur and Hydrogenation

As mentioned in Introduction, thermodynamically most stable MoS2 nanoparticles appear as triangular-shaped clusters. 16,37 The edges of these clusters show excess sulfur atoms that are under-coordinated. We first calculate the number of excess S atoms in a given S-edge cluster. Let n be the number of S atoms on an edge of the cluster shown in Figure 2a. The total number of S atoms in the cluster is given by, n(n + 1). Correspondingly, the total number of Mo atoms is, (n − 1)n/2. Therefore, the number of excess S atoms is given by subtracting 2(n − 1)n/2 from n(n + 1), i.e., 2n. For a given S-edge cluster with the excess sulfur atoms, the ratio of number of chalcogen atoms (NS ) to the number of metal atoms (NMo ) is always higher than 2. In the present 6


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work, the NS /NMo ratio is ∼2.75. It was reported 16 that the clusters with the ratio above 3.5 are energetically disfavored. It is evident that the under-coordinated or excess S atoms on the edges predominantly contribute to the states near the Fermi level and they determine the energetic stability and the electronic properties of the cluster. 39 To stabilize a cluster, the dangling bonds on the edges must be saturated. We now investigate the energetic stability of the cluster with n = 6 S atoms on an edge (see Figure 2a) by using hydrogen as a probe particle. The stabilization energy for a cluster is defined as,

∆E = E(MoS2 + NH ) − E(MoS2 ) − NH E(H2 )/2

(1)

where E(MoS2 + NH ), E(MoS2 ), and E(H2 ) indicate the total energies for a hydrogenated cluster, a non-hydrogenated cluster and a hydrogen molecule, respectively. The total number of hydrogen molecules are indicated by NH /2. We also define the differential stabilization energy to describe the stability of hydrogens as

∆EH2 = E(MoS2 + NH ) − E(MoS2 + (NH − 2)) − E(H2 )

(2)

Here E(MoS2 + (NH - 2)) is the total energy for a hydrogenated cluster with (NH - 2) adsorbed hydrogens. We now define the hydrogen coverage as the ratio of number of hydrogen atoms adsorbed to the total number of edge S atoms as

θH =

NH NSedge

(3)

For n = 6 case, NSedge = 6(n − 1) = 30. Here, the number 6 represents the total number of S-edges in an MoS2 cluster. To understand the effects of dangling bonds on the properties of the nanoclusters, we passivate the under-coordinated edge S atoms with a single H atom. The Mo atoms in a given cluster are coordinated by six S atoms and are bulk-like. When an H2 molecule interacts

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Ebare 0

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өH%

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Frontier Orbital Energy [eV]

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ΔE [eV]

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-4.6

100

-4.8

Figure 3: Calculated stabilization energy for n = 6 S-edge MoS2 nanocluster. The reference energy for the stabilization energy is taken as the total energy of the non-hydrogenated cluster (Ebare ). For the frontier orbitals the scale is shown on the right side. The green region between pink and black horizontal line segments represents the size of HOMO-LUMO gap. with an edge S atom it dissociates into two H atoms and forms two S-H groups with bond length of 1.35 Å. Here the hydrogen coverage, θH is ≈7%. The two S-H groups are nearest neighbors to each other (site I in Figure 2e). We find that the energy of adsorption is lowered by 0.40 eV if the second H was adsorbed on to the second nearest S atom (site II). The gain in energy is due to the reduction in steric hindrance between the two H atoms. We then followed this procedure to find the energetically most favorable second adsorption site by placing it on different possible edge S atoms. The adsorption at the third nearest neighbor site marked by III in Figure 2e further lowered the energy of the system by 0.15 eV. In the end, we found that the energy was lowered again (∼0.20 eV) by placing the second H atom in the middle of a second edge. The energetics for the relaxation of the hydrogens reveal that H atoms prefer to occupy a site in the middle of edges. At a hydrogen coverage of ∼14% (i.e., two H2 molecules), a third H atom preferred to occupy a place in the middle of the third edge. The remaining H atom from the second H2 molecule preferred to be adsorbed on the third nearest neighbor site on an edge. By increasing the hydrogen coverage to 40%, 8


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N6-edge cluster N7-edge cluster

0.5

ΔE[eV]/H2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0 -0.5 -1 -1.5 -2

0

10

20

30

40 θH%

50

60

70

80

Figure 4: Calculated differential stabilization energy for n = 6 and 7 S-edge MoS2 nanoclusters. we observe that H atoms arrange themselves in a zigzag fashion as shown in Figure 2e. With this information we now develop a formula for the percentage of hydrogen coverage (θH ) that gives maximum stability to an S-edge cluster. Let us consider a cluster with “n” S atoms along an edge as shown in Figure 2. For a stable cluster with the zigzag arrangement of H atoms one can adsorb maximum 3(n − 2) hydrogen atoms along the edges. The total number of edge S atoms in a plane is, n + n − 1 + n − 2 = 3(n − 1). Therefore, the hydrogen coverage for maximum stability can be calculated by taking the ratio of the two terms and is given by n−2 −→ ∆Emax 2(n − 1)

(4)

The factor 2 in the denominator indicates the number of chalcogen planes. Thus, for a given stable S-edge cluster the hydrogen coverage is a function of n. When n → ∞, the ratio goes to 0.5 which implies there are no excess S atoms in a monolayer. In Table 1 we report the calculated stabilization energy for different hydrogen coverages on the n = 6 cluster. We consider only partial passivation of dangling bonds at the edges. For the first dissociative adsorption of a hydrogen molecule the calculated stabilization energy 9



Table 1: Relative stabilization energy (∆E) for different hydrogen coverages (θH ) and HOMO-LUMO gap System (a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (l) (m) (n)

NH 0 2 4 6 8 10 12 14 16 18 20 22 24 30

θH (%) 0.00 6.67 13.33 20.00 26.67 33.33 40.00 46.67 53.33 60 66.67 73.33 80 100

∆E (eV) Gap (meV) 0.00 250 -1.42 14 -2.34 12 -3.18 62 -4.39 109 -5.59 95 -6.19 71 -6.03 219 -5.30 47 -4.72 82 -4.70 39 -4.67 121 -5.11 436 -4.85 47

is -1.42 eV. We used the total energy of a bare (non-hydrogenated) cluster as the reference energy to calculate the stabilization energy. The negative sign indicates that hydrogen adsorption on the S edges brings stability to the cluster. A recent study by Prodhomme et al. 40 showed that hydrogens are most stable when bonded to the edge sulfur atoms. Another computational study 41 showed that hydrogen passivation of S atoms modifies the structural and electronic properties of a monolayer quantum dot, although it remains metallic. Indeed, the passivation leads to stabilization of clusters. Figure 3 shows the variation of stabilization energy as a function of hydrogen coverage (θH ). Upon further hydrogenation, the stabilization energy of the cluster becomes more and more negative and it reaches minimum (-6.20 eV) when the hydrogen coverage, θH , reaches 40%. At this hydrogen coverage, all the adsorbed H atoms arrange themselves in a zigzag fashion. The zigzag arragement is favored because all the adsorbed hydrogens experience minimum steric hinderance. The formula developed above (Eq. 4) for maximum stability gives the hydrogen coverage to be 40%. For this coverage, the cluster is most

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stable. We observe similar trend for the larger clusters with n = 7 and n = 8. For n = 7 (n = 8) S-edge cluster, the formula predicts that the maximum stability can be achieved by setting the hydrogen coverage θH to 42 (43)% (Figure S1 for n = 7 case only, Supporting Information) and our DFT calculations support this. The calculated stabilization energy for n = 7 at θH ≈ 42% (n = 8 at θH ≈ 43%) S-edge cluster is -8.06 eV (-8.40 eV). With further increase in the nanocluster size we can expect the stabilization energy to become more negative and negligible effects of the corners. Moreover, for the larger clusters, n = 7 and 8, the sulfur excess on the edges drives a rearragement of the edges and stabilize the clusters. By increasing the size of the cluster to ∞ the hydrogen coverage calculated from our formula approaches to 50%. Above the hydrogen coverage of 40% (for n = 6 case), the stabilization energy becomes positive (∼1.0 eV) compared with that for the most stable configuration and weakens the stability for more hydrogen adsorption. We can expect the hydrogen evolution reaction to be preferable above the hydrogen coverage of θH = 40%. The dissociative adsorption of hydrogen atoms on the edges could obstruct the growth of MoS2 monolayer. The concentration of H atoms plays a crucial role in the S extraction from the edges. 42–44 A recent experimental study by Lauritsen et al. 16 reported that MoS2 nanoclusters prefer triangular shape under different sulphiding conditions. The clusters with the number of Mo atoms more than 21 prefer to expose Mo-edges. A comprehensive analysis of the nanoclusters revealed that a rearrangement of the sulfur atoms on the edges stabilize the clusters. Interestingly, the sulfur excess relative to bulk stoichiometry drives the reconstruction of the edges. Larger the sulfur excess lesser is the stability. In the present work, we only consider MoS2 clusters with the number of Mo atoms close to 21 (S-edges with NMo =15 (n = 6 case), 21 (n = 7 case), and 28 (n = 8 case)). As discussed above, the cluster with n = 6 S-edge (NMo = 15) achieves stability through hydrogenation as the NS /NMo ratio (2.80) for it is higher than the clusters with n = 7 (2.66) and n = 8 (2.57). We consider the effect of this stabilization on the H2 S release in the following section. In Fig. 4 we plot the differential stabilization energy to investigate the stability of ad-

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sorbed hydrogens. The differential stabilization energy captures the effect of adsorption of additional H atoms to a hydrogenated cluster. By taking the total energy difference between successive hydrogenated clusters we can investigate the favourability or non-favourability for more hydrogen adsorption. As shown in Fig. 4, the calculated differential stabilization energy, ∆EH2 , indicates that at a low hydrogen coverage the hydrogen atoms bind strongly to the cluster edges. The ∆EH2 is nearly -1.40 eV when the clusters are adsorbed by two hydrogen atoms. The dissociative adsorption of hydrogens suggest that the stability of MoS2 clusters is improved. With increase in hydrogen coverage (θH < 40%), the average stabilization energy is -1.08 eV. Thus, our DFT results show that the dissociative adsorption of hydrogens on the S-edges is highly exothermic. Here we emphasize that longer distance between S-H groups arranged in a zigzag fashion, as a result of repulsive inetraction, stabilize the hydrogenated edges. For more hydrogen adsorption (θH ∼ 50%), the stabilization is less exothermic. At a hydrogen coverage of 45% (n = 7 case), the stabilization energy is ∼ -0.15 eV. Above the hydrogen coverage θH > 50% we observe that ∆EH2 is in general positive. This indicates that further hydrogenation is energetically not favourable. The reason for this is an increased repulsive interaction between adjacent S-H groups. The zigzag arrangement for hydrogen atoms is possible if hydrogen diffusion is kinetically favoured. Indeed, Prodhomme et al. 40 showed that the diffusion of hydrogen ad-atoms on MoS2 edges is kinetically favoured compared with H2 and H2 S associative desorption. The authors in Ref. 40 reported that a hydrogenated S-edge in which half of the terminal sulfur atoms are bonded with one hydrogen atom, S-edge 100%S+50%H model, is the most stable state at lower H2 pressure. In their model the hydrogen coverage θH is 50%. In addition, the theoretical calculation on the normal modes for the S-edge 100%S+50%H model is in good agreement with the vibrational spectroscopy data. This concludes that the clusters are thermodynamically stable at the lower values of H2 pressure with decrease in temperature. Our formula predicts that for an infinitely long S-edge, the hydrogen coverage of θH = 50% stabilizes the cluster. However, at different industrial working conditions, high temperatures and high pressures,

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formation of different surface chemical species e.g. hydride, coordinatively unsaturated sites have been reported. 45–48 These species are mainly suspected for HDS mechanism creating sulfur vacancies at high hydrogen pressure. Therefore, in the next section, we consider the effect of hydrogen induced stabilization on the extraction of an S atom from a hydrogenated cluster. It is well known that nanoclusters behave differently than their bulk counterparts and that their properties can be tailored by functionalization. Here, in the present study we suggest a way to engineer the electronic properties of the nanoparticles upon hydrogenation. For the calculated ground state nanoclusters the variation of HOMO-LUMO gap as a function of hydrogen coverage θH is shown in Figure 3. The green region between the frontier energy levels, marked by pink (EHOMO) and black (ELUMO) horizontal line-segments, indicates the size of the gap. The gap for the non-hydrogenated cluster (θH = 0) is 250 meV. This small value compared to the monolayer band gap value indicates metallic nature of the cluster. Recently, Bollinger et al. 25 showed that different MoS2 nanoclusters may be either metallic or semiconducting. The metallic behavior is caused by the presence of dangling bonds at the edges of nanoclusters. Also, it is confirmed by photoelectron spectroscopy experiments 49 that the platelets of MoS2 and WS2 of similar size have a metallic character. The study revealed no detectable HOMO-LUMO gap i.e., the gaps are smaller than their calculated value of 300 meV for the clusters. Our calculated value of the gap, 250 meV, is in very good agreement with the study. With increase in hydrogen coverage, the HOMO-LUMO gap decreases to 12 meV for θH ≈ 14% and then increases to 219 meV at θH = 47%. This means the nanocluster is still metallic in nature due to unsaturated dangling bonds on the edges. The energetically most stable configuration with 40% hydrogen coverage shows only 71 meV HOMO-LUMO gap. This is because the corresponding electronic states from the dangling bonds lie very close to the Fermi level. At 80% hydrogen coverage we observe a huge jump in the gap value. The HOMO-LUMO gap for the cluster with θH = 80% is 436 meV. In addition, we observe slight increase in the stabilization energy at this coverage. For

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the fully coordinated cluster in which all the edge S atoms are passivated with a hydrogen atoms, θH = 100%, the gap is 47 meV. We observe similar trend for the next large cluster with n = 7 (Figure S1 and Table T1, Supporting Information). All the hydrogenated clusters show a very small HOMO-LUMO gap. The HOMO-LUMO gap value for n = 7 cluster with θH ≈ 42% is 110 meV which is close to the value 71 meV for n = 6 cluster with θH ≈ 40%. At hydrogen coverage of θH ≈ 47% the gap increases to 174 meV. Both sizes, n = 6 and n = 7, show a very similar gap values at around θH ≈ 45%. It is well known that nanoclusters show a large surface to volume ratio that strongly determines their properties. In the triangular nanoclusters considered in the present study the edges are responsible for the deviation from bulk properties and for vanishing the gap in the electronic structure. Indeed metallic-like states were observed 25 for MoS2 clusters with triangular morphology. Moreover, magic clusters of MoS2 49 and the size-dependence of the gap of rhombohedral MoS2 nanoclusters 50 hold promise for optoelectronic applications. In order to get further insight into the metallic nature of the hydrogenated clusters, we plot total (TDOS) and projected density of states (PDOS) as shown in Figure 5. The top panel (a, b, and c) shows the TDOS for spin-up (green curve) and spin-down (red curve) channels whereas the bottom panel (d, e, and f ) shows the PDOS for hydrogen coverages θH = 6.67 (≈7)%, 40%, and 100%. For θH ≈ 7%, a large density of states cross the Fermi level indicating metallic behavior of the cluster. This metallic behavior is caused by dominating character of p-states from unpaired electronic states on the edges (see black curves in Figures 5d, 5e, and 5f ). The energy of the unpaired electrons on the edge S atoms lie close to the Fermi level, and they determine the fundamental properties of the system. The contribution of d-states from the metal atom lie below -2 eV. We observe similar metallic trend for the hydrogen coverages θH = 40% and 100%. In all the plots presented, no clear energy gap is observed because the electronic states are Gaussian broadened by 0.10 eV. Similar metallic-like behavior was reported by Li et al. 39 for Mo-edge MoS2 clusters passivated by S atoms. The dominating behavior of the states on the edge atoms determines

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the electronic properties of MoS2 nanoclusters. a)

Density of states

40

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θH ≈ 7%

c)

θH = 40%

θH = 100%

20 0 20 40 6

Projected density of states

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d)

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S [p]

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Mo [d]

S [p]

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Figure 5: The total and projected density of states for n = 6 S-edge MoS2 nanocluster upon hydrogenation obtained by using Gaussian broadening of 0.10 eV. a) and d) for θH ≈ 7%, b) and e) for θH = 40%, and c) and f) for θH = 100%. Vertical dashed lines represent the Fermi energy. The spin-up and spin-down states are represented by ↑ and ↓ arrows respectively.

3.2

H2 S Release

To understand the effect of hydrogen adsorption on the two-fold coordinated edge S atoms we investigate the energetics of formation and release of H2 S molecules. Figure 6 shows the potential energy surface for the extraction of one S atom from the edge. From a kinetic point of view, knowledge about the rate determining step of vacancy formation is very important. In this section, we concentrate on the extraction of an edge sulfur atom by a hydrogen molecule and release of the formed H2 S molecule. We used the NEB method to detremine the transition state (TS) and intermediate states (IM). As shown in Figure 6a, an H2 molecule of the gas phase undergoes a homolytic dissociative adsorption on the edge and forms an HSH group (Figure 6c). The physisorption of H2 is exothermic in energy by 0.10 eV. The weak vdW interaction between a bare cluster and an H2 molcule activates the H−H bond 15



by increasing length of the bond to 0.87 Å. In an isolated H2 molecule the bond length is 0.75 Å. The calculated activation energy, 0.10 eV, for the elementary step matches well with the value reported by Paul et al. 47 Then the molecule undergoes a dissociative adsorption reaction. We find the dissociation energy on the edge to be 0.75 eV. This calculated value is close to the value 0.97 eV reported by Dumeignil et al. 48 for the metallic edge. After dissociation, one of the H atoms forms a chemical bond with an S atom with a bond length ≈ 1.40 Å whereas the free radical (H∗ ) is at a distance of 2 Å from the S atom. The bond between the radical and the S atom is indicated by S---H∗ in Figure 6b. We call this state as the first transition state, TS1. It has been shown 47,51 that the hydrogen atoms form stable bonds with catalytically active S2− anions on the edges. Once the H atoms form bonds with an edge S atom, the HSH group undergoes a rotation of about 15◦ (see Figure 6d as TS2) and departs as an H2 S molecule shown in Figure 6e. This rotation mechanism could be a pathway to the activation of an H2 S molecule release from the edge. Local atomic relaxations at the defect site lower the total energy of the system by 0.11 eV. Our calculated value 0.47 eV for the activation of H2 S release is in good agreement with the value 0.52 ± 0.1 eV estimated from sulfur exchange experiments. 48 We continue investigating similar mechanism of H2 S release from a stable MoS2 cluster. Here, we consider the lowest energy configuration at hydrogen coverage, θH ≈7% (a single H2 adsorption case) as the stable cluster. The new model system as shown in Figure 6f consists of the stable cluster with two previously formed SH groups on the edge and one more H2 molecule interacting with it. The dissociative adsorption of the second H2 molecule leads to the formation of two more SH groups on the surface. Here a new SH group is a part of an HSH group (see Figure 6g). The remaining fourth H atom is adsorbed on an S atom next to the newly formed HSH group. The corresponding S−H bonds are of length 1.36 Å. The geometry of the transition state (TS) with ∼15◦ rotation to the HSH group is shown in Figure 6g. The activation energy for this endothermic reaction is 0.60 eV. By comparing the activation energy values for H2 S release in the two cases discussed above, we conclude

16


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b)

S-H

d)

S---H*

g)

~15° Rotation

~15° Rotation

H2 S

h) TS2

TS1

TS

H2 S

-0.11 eV 0.75 eV

0.60 eV

0.45 eV

IS

e)

IS

c)

a)

0.33 eV

FS

IM

-0.02 eV

FS

f)

Figure 6: Potential energy surface for H2 physisorption, dissociation, and H2 S release from the S-edge MoS2 nanocluster. (a-e): H2 molecule interaction and H2 S release, (f-h): Another H2 interaction with the lowest energy configuration for θH ≈7% and H2 S release. The horizontal dotted line marks energy of the initial state (IS) as the reference energy. that hydrogen induced stabilization increases the energy barrier for H2 S release. We further perform kinetic calculations for the cluster with n = 7 S atoms on the edges. The calculated energy barrier for the non-hydrogenated cluster is 0.62 eV (Figure S2, Supporting Information). Lauritsen et al. 16 reported that under different sulfiding conditions a rearragement of the sulfur atoms on the edges stabilize the clusters. The excess sulfur atoms relative to bulk stoichiometry drives the reconstruction at the edges. Larger the sulfur excess lesser is the stability. For the cluster with n = 7 S-edge atoms the NS /NMo ratio (2.66) is smaller than that for n = 6 S-edge cluster (2.80). The increase in the barrier value for the non-hydrogenated cases can be attributed to the large size (less sulfur excess) and negligible effects of the corners. We expect similar energy barrier for hydrogenated clusters because an increase in the hydrogen concentration will only increase the rate of the H2 S release and not the activation energy.

4

Conclusions

Using density functional theory we systematically explore energetic stability and the electronic properties of triangular MoS2 quantum dots with S-edges upon hydrogenation. A 17



zigzag arrangement for hydrogen adsorption on the edges maximize the stability of the cluster. The hydrogen coverage that maximize the stabilization energy can be calculated from (n − 2)/2(n − 1), where n is the number of S atoms along an edge. Our calculations indicate that the hydrogenated clusters show vanishing HOMO-LUMO gaps mainly due to the presence of dangling bonds at the edges and can be considered metal-like. The activation of the gas phase hydrogen molecule is a crucial step for hydrogenation or hydrodesulfurization of MoS2 nanoclusters. We find that the activation energy barrier for H2 S release from an MoS2 nanocluster lies in between 0.47 and 0.62 eV.

Acknowledgement We acknowledge useful discussion with Dr. Hyeon-Jin Shin from Device Lab, SAIT Korea. Financial support from SAIT-Korea is gratefully acknowledged.

Supporting Information Available The variation of stabilization energy (∆E) as a function of hydrogen coverage (θH ) for n = 7 S-edge MoS2 cluster is shown in Figure S1. The HOMO-LUMO gap for the calculated clusters is also shown. In Table T1, we report the calculated stabilization energy for different hydrogen coverages on the n = 7 S-edge cluster. The Figure S2 shows the potential energy surface for the extraction of one S atom from the edge of n = 7. This material is available free of charge via the Internet at http://pubs.acs.org/.

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E a = 0.47 eV


S-edge

MoS2 Quantum Dots: Effect of Hydrogenation on Surface Stability and

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