Understanding Size-Dependent Morphology Transition of Triangular

Subscriber access provided by UB + Fachbibliothek Chemie | (FU-Bibliothekssystem)

Article 2

Understanding Size-Dependent Morphology Transition of Triangular MoS Nanoclusters: The Role of Metal Substrate and Sulfur Chemical Potential Peng Zhang, and Yong-Hyun Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12643 • Publication Date (Web): 30 Dec 2016 Downloaded from http://pubs.acs.org on January 3, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28

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

The Journal of Physical Chemistry

Understanding Size-Dependent Morphology Transition of Triangular MoS2 Nanoclusters: The Role of Metal Substrate and Sulfur Chemical Potential Peng Zhang1,* and Yong-Hyun Kim2,* 1

2

Beijing Computational Science Research Center, Beijing 100094, China

Graduate School of Nanoscience and Technology, KAIST, Daejeon 305-701, Republic of Korea Corresponding Author: [email protected] Corresponding Author: [email protected]

S1

ACS Paragon Plus Environment


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

Abstract Molybdenum disulfide (MoS2) nanoclusters have recently attracted enormous interest, due to their promising applications as catalysts in hydrodesulfurization of fossil fuels. It has been demonstrated that the catalytic activity of MoS2 nanoclusters closely relates to their equilibrium morphology, which is in turn quite sensitive to various factors, such as the synthesis environments, the cluster size, and the substrates. Here, we carry out the density functional theory (DFT) calculations to study the size-dependent morphology change of triangular MoS2 nanoclusters with all these factors systematically considered. Our results indicate that the stability of triangular MoS2 nanoclusters is mainly determined by their edge and corner energies, and the variation of the ratio of the edge to corner energies with respect to the cluster size, chemical potential of sulfur, and substrates could induce a structural transition for their equilibrium morphology. By setting the chemical potential to fit experimental conditions, our calculations reveal a size-dependent morphology transition of triangular MoS2 nanoclusters on Au(111) substrate, which is quantitatively consistent with experiments. In addition, the electronic structures of triangular MoS2 nanoclusters are carefully studied. The results indicate that the metallic edge states, which is important for the hydrodesulfurization catalysis, are very sensitive to the substrates and only the clusters with Mo edge on Au(111) is found to have the one-dimensional metallic edge states. This result implies that in addition to the Mo edge, the metallic substrates may also play an important role in understanding the experimentally observed catalytic activity of MoS2 nanoclusters, which has never been considered before.

S2


Page 2 of 28

Page 3 of 28

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


1. Introduction The study of supported MoS2 nanoclusters is of particular interest, due to their important applications as hydrotreating catalysts used to upgrade and clean up sulfur-containing compounds from crude oil feedstock in the hydrodesulfurization (HDS)1. In its bulk form, MoS2 is known to be chemically rather inactive, but when dispersed as nanoclusters on a support, the material shows a significant catalytic activity in the hydrogenation of aromatic S-containing compounds, which occur naturally in oil fractions. This activity is normally ascribed to the edge sites of MoS2 nanoclusters, which have a different local stoichiometry and structure from the inert MoS2 basal planes. For single-layer MoS2 nanoclusters that constitute the basic catalytically active phase in all MoS2-based catalysts, their edge structures can be classified into two different categories, the (1010) Mo edge and the (1010) S edge, with the former typically saturated by sulfur. Recent experimental2-3 and theoretical4-5 investigations have indicated that both of the Mo and S edges could be catalytically active, however, the detailed catalytic processes over these two edges are quite different from each other. For example, it was found that both the hydrogenation and direct desulfurization reactions could occur at the fully S-saturated Mo edge, whereas the S vacancies must be created for the reactions to proceed at the S edge5. The activity of fully S-saturated Mo edge was demonstrated to be mainly ascribed to their one-dimensional metallic edge states, which are absent for the S edge2, 4, 6. These studies elucidate that the catalytic activity, as well as the catalytic reaction pathway, may vary significantly with different edge structures, and thus a

S3



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

key parameter of improving the catalytic performance of MoS2 nanoclusters is to carefully control their equilibrium structure, so that the most active edges could be exposed. It has long been realized that the equilibrium structure of MoS2 nanoclusters may be very inconsistent, and different morphologies could exist depending on the synthesis conditions and the supporting substrates7-11. Based on the DFT calculations, Schweiger et al.9 calculated the formation energies of various edge structures and predicted that the equilibrium morphology of MoS2 nanoclusters would evolve from a triangular to hexagonal shape with different edge structures as the chemical potential of sulfur changes from sulfur-rich to sulfur-poor conditions. Moreover, Bollinger et al.11 calculated the formation energies of the Mo edge with either the S-monomer or S-dimer configuration for both the free-standing and Au(111) supported MoS2 nanoclusters, which implied that the substrate may significantly influence the relative stability of different edge structures and thus the equilibrium morphology of MoS2 nanoclusters. More recently, a size-dependent structural progression has been observed for triangularly shaped MoS2 nanoclusters by scanning tunneling microscopy (STM), which implies that the equilibrium structure of MoS2 nanoclusters may also relate to their size12-13. According to the experiments, MoS2 nanoclusters with a size larger than 1.5 nm prefer to be terminated by the Mo edge, whereas a striking edge reconstruction from the Mo edge to the S edge occurs for smaller clusters12. Further investigations revealed that the adsorption property of HDS refractory dibenzothiophnene (DBT) is enhanced for smaller MoS2 nanoclusters (with size < 1.5 nm), compared to the larger ones, which should correlate closely with their structural change13. These studies reveal an interesting relationship of the equilibrium structure and catalytic activity of MoS2 nanoclusters with their size; however, the S4


Page 4 of 28

Page 5 of 28

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


underlying mechanism for this size-related structural progression, as well as the possible influences from the synthesis conditions and substrates, is still unclear. In this work, we carry out the DFT calculations to study the size-dependent morphology transition of triangular MoS2 nanoclusters and analyze how the equilibrium morphology can be affected by the cluster size, the chemical potential of sulfur, and the substrates. The Au(111) surface and hexagonal BN (h-BN) monolayer are considered, in order to capture the general effects of metallic and insulating substrates, respectively. Our calculations reveal that the stability of triangular MoS2 nanoclusters is mainly determined by their edge and corner energies, and the variation of the ratio of the edge to corner energies with respect to the size can induce a structural transition for their equilibrium morphology. Moreover, the chemical potential of sulfur and metallic substrates are revealed to significantly change both of the edge and corner energies, and thus alter the equilibrium morphology of triangular MoS2 nanoclusters. In addition, our detailed analysis of electronic structures indicates that the metallic edge states are very sensitive to the morphology of MoS2 nanoclusters, as well as the substrates. Actually, only the clusters with Mo edge on Au(111) is found to have the one-dimensional metallic edge states, which is quite different from the results obtained by using MoS2 nanoribbon. A significant deduction from our results is that the catalytic activity found for MoS2 nanoclusters may be not only determined by their edge morphology, i.e., the Mo edge, but also by metal substrate.

2. Computational details

S5



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

Page 6 of 28

DFT calculations. The present calculations are carried out by using the projector augmented wave (PAW) method14 within the DFT as implemented in the Vienna ab initio simulation package (VASP)15. The exchange and correlation energy is treated within the spin-polarized

generalized

gradient

approximation

(GGA)

parameterized

by

Perdew-Burke-Ernzerhof (PBE)16. The energy cutoff for the plane-wave expansion is set to 500 eV. The Monkhorst-Pack k-point meshes are set as 1×1×1 for MoS2 nanoclusters and 15×1×1 for MoS2 nanoribbons. For the geometry optimization, all structures are fully relaxed until the Hellman-Feynman forces acting on each atom are less than 0.01 eV/ Å. To treat the interaction between MoS2 nanoclusters and Au(111) (or h-BN) substrate, we employ van der Waals correlated DFT-D3 method implemented in VASP17. To identify the utility of DFT-D3 method, we also conducted the DFT-D2 calculations of n = 6 MoS2 nanoclusters with Mo edge on h-BN substrate and compared with the DFT-D3 result. The calculated interlayer distance within the DFT-D2 method is 3.32 Å which is very similar to the value obtained by DFT-D3 method (3.26 Å). Thermodynamics. The thermodynamic stability of MoS2 nanoclusters can be estimated by calculating their surface formation energies (Ω) by Ωn = En − n E − n μ , 1 where En is the total energy of a cluster with size n, n is the amount of Mo atoms in the cluster and n is the excess S atoms in the cluster compared to the bulk stoichiometry. Here, E is the total energy of MoS2 monolayer per unit formula and μ is the chemical potential of sulfur with its range determined by

μ = E + 2E + ΔH

S6


− 2μ , 2

Page 7 of 28

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


and

E +

ΔH 2

≤ μ ≤ E , 3

(E ) is the total energy of elementary solid Mo (S) and ΔH where E

is the

formation enthalpy of monolayer MoS2 per unit formula. Here, we choose the maximum value of μ , corresponding to the strong sulfiding condition, to be zero, and thus the minimum value of μ , corresponding to the sulfo-reductive condition, is calculated to be −1.3 eV. In the experimental synthesis process, μ is determined by the ratio of partial pressure of H2S (PH S)) to that of H2 (PH ) by PH S μ = μ − μ = μ T, P − μ T, P + k " Tln 4 PH and *+, μ%& = h%& T, P − h%& T = 0K, P + E%& + E%& − TS%& T, P 5 *+, is where T is the temperature, P is the standard pressure, h%& is the enthalpy of gas, E%&

the zero point vibrational energy, E%& is the total energy of gas molecule and S%& is the entropy of gas. As illustrated in Fig. 1, the surface formation energy of MoS2 nanoclusters can be re-written as the sum of the edge and corner energies, Ωn = 3n − 3λ + χ, 6 where λ is the edge energy per zigzag unit (zz.) and χ is the total corner energy that equals to Ω3 by definition. Because χ can be easily calculated by eq. (1), this transfers the problem of calculating the formation energy of MoS2 nanoclusters to a problem of calculating the edge energy, which can be done by taking the energy difference between similar structures of different cluster sizes, assuming that χ does not change with the cluster size18, S7



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

λ=

1 1 1Ωn − χ2 = 1En − E3 − ∆n E − ∆n μ 2, 7 3n − 3 3n − 3

where ∆n = n n − n 3, 8 ∆n = n n − n 3. 9 Combing the obtained edge and corner energies, the relative stabilities of MoS2 nanoclusters with different structures can be compared, ∆Ωn = Ω n − Ω n = 1χ − χ 2 + 13n − 3λ − λ 2, 10 where the subscript denotes clusters with the Mo and S edges, respectively. Here, a negative (positive) value of ∆Ω implies that MoS2 nanoclusters with the Mo edge (S edge) are stable.

3. Results and discussion Edge and corner energies of triangular MoS2 nanoclusters. Fig. 1 illustrates the optimized structures of triangular MoS2 nanoclusters (to be simple, we will only use MoS2 nanoclusters below, however it corresponds to the triangular ones) terminated by either the (1010) Mo edge or the (1010) S edge, with their size represented by n that equals to the number of Mo atoms per triangular side. For the MoS2 nanoclusters with Mo edge, the outmost Mo atoms are fully saturated by sulfur to form the six-fold coordination. The saturated S atoms are found to contract in the direction perpendicular to the cluster basal plane and form the so-called S2 dimers19. For the MoS2 nanoclusters with S edge, on the other hand, the outmost S atoms occupy the same sublattice as the S atoms in the cluster basal plane, however, a structural reconstruction is found to occur at the corners. For convenience, we define the smallest clusters (n = 3) studied in this work as composed of only bulk MoS2 and S8


Page 8 of 28

Page 9 of 28

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


corners (Mo or S corners), while the larger ones composed of bulk MoS2, corners and additional edges, as shown in Fig. 1. Although this definition of corner is artificial, it doesn’t change the conclusion of this work. As the size of MoS2 nanoclusters increases, e.g. from n = 3 to n = 6, we can see from Fig. 1 that both the Mo and S corners keep almost unchanged, while the Mo and S edges grow along the triangular sides of clusters. The thermodynamic stability of MoS2 nanoclusters can be estimated by calculating their surface formation energy which can be further decomposed into the edge and corner energies (see Method). To estimate the influence of different types of substrates, the free-standing, h-BN supported and Au(111) supported MoS2 nanoclusters are studied in this work, with the latter two given in Fig. 2 (the results for free-standing case are given in Table S1 in supporting information (SI)). In this figure, we set the energies of Mo edge and corner to be zero and use them as the reference to estimate the relative energies of their S counterparts. The chemical potential of sulfur (μ ) is determined to vary from 0 to −1.3 eV, corresponding to the variation from sulfiding to sulfo-reductive conditions. For the h-BN supported MoS2 nanoclusters as shown in Fig. 2(a), the relative stability of S edge, compared to the Mo edge, is linearly enhanced as μ decreases from 0 to −1.3 eV. The energy differences between λ (BN) and λ (BN) are calculated to be −0.51 and 0.36 eV at μ = 0 and μ = −1.3 eV, respectively, with a crossover occurring at μ = −0.76 eV. Analogously, the relative stability between the Mo and S corners also has a linear dependence on μ , and the energy differences between χ (BN) and χ (BN) are found to be −1.63 and 3.57 eV at μ = 0 and −1.3 eV, respectively, yielding a crossover at μ = −0.41 eV, as shown in Fig. 2(b). These results indicate that the Mo edge and Mo corner are generally more stable at higher μ , whereas S9



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

their S counterparts are more stable at lower μ , which is consistent with the previous DFT calculations9. Compared to the free-standing ones, our results show that the h-BN supported MoS2 nanoclusters generally yield similar edge and corner energies, implying that h-BN has little impact on the stability of MoS2 nanoclusters. This may be mainly because h-BN is chemically inert and electronically insulating, and thus only weak van der Waals interaction could exit between the clusters and substrate, which would not essentially alter the stability of the clusters. For the Au(111) supported MoS2 nanoclusters, the relative stability between the Mo and S edges (corners) has a similar trend to that of h-BN supported ones, however, downward shifts can be clearly seen for the edge and corner energies, as shown in Fig. 2. From Fig. 2(a), we can see that the surface energies of Mo and S edges are lowered by 0.03 and 0.37 eV, respectively. As a result, the energy differences between λ (Au) and λ (Au) are calculated to be −0.17 and 0.70 eV at μ = 0 and μ = −1.3 eV, respectively, which yields a crossover at μ = −0.24 eV that is much higher than that between λ (BN) and λ (BN). Moreover, as shown in Fig. 2(b), the formation energies of Mo and S corners are also reduced by 1.50 and 3.72 eV, respectively. As a consequence, the S corner becomes entirely more stable than the Mo corner within the whole range of μ , and no crossover between χ (Au) and χ (Au) is observed. This indicates that Au(111) can significantly stabilize the MoS2 nanoclusters, compare to the free-standing and/or h-BN supported ones. We expect that this stabilization effect may be ascribed to the possible charge transfer between the MoS2 nanoclusters and Au(111), which could not occur for insulating h-BN. It would be easier to understand this stabilization effect by considering the edge (or corner) states as defect states. S10


Page 10 of 28

Page 11 of 28

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


If Au(111) transfers electrons to these defects states, it can stabilize the MoS2 nanoclusters. The band alignment analysis, as shown in Fig. S1, indicates that the Fermi level of Au(111) is energetically higher than the edge states, and thus the charge transfer from Au(111) to MoS2 nanoclusters is likely to occur. Moreover, the charge transfer between the MoS2 nanoclusters and Au has also been observed in experiments, which was reported to be responsible for the visible bulk in their STM images7.

Size-dependent equilibrium morphology of triangular MoS2 nanoclusters. To study the morphology progression, the surface formation energy difference (∆Ω) between the MoS2 nanoclusters with different morphologies are calculated as a function of the chemical potential of sulfur and cluster size, as shown in Fig. 3. Here, both the h-BN and Au(111) supported MoS2 nanoclusters are given. In Fig. 3, the variation of ∆Ω is represented by the color change, varying from blue (-10 eV) to red (10 eV), with the boundary (where ∆Ω = 0) marked by the black solid line. The blue (red) color depicts the region where the clusters with Mo edge (S edge) are stable, and consequently a structural transition would occur when ∆Ω go across the boundary. As μ decreases, it is clear to see that the color changes horizontally from blue to red, implying that the clusters with Mo edge are more stable at higher μ , while those with S edge are more stable at lower μ , in agreement with our previous results that the Mo edge (corner) is generally more stable than the S edge (corner) at high μ , and vice versa. On the other hand, as the cluster size increases, the color (no matter blue and red) gets deepened vertically, indicating enhanced energy difference between the clusters with different morphologies. We already see in Fig. 1 that as the size increases, only the edges of clusters S11



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

grow, while the corners keep unchanged. Thus, the deepening color can be ascribed to the growing edges and the energy difference between the Mo and S edges λ − λ , i.e., the blue gets deepened when λ − λ > 0, while the red gets deepened when λ − λ < 0 . Consequently, it can be seen in Fig. 3 that the boundary varies as a function of the cluster size, implying a possible size-dependent morphology of MoS2 nanoclusters. For the h-BN supported MoS2 nanoclusters as shown in Fig. 3(a), the boundary monotonously shifts from −0.41 eV to −0.67 eV, as the cluster size goes from n = 3 to n = 10. If the clusters continue to grow, further decrease of the boundary should be expected, however, its value could never be smaller than −0.76 eV even for larger clusters, as will be discussed below. We can therefore define a structural transition region (STR) within which the structural transition could occur. As shown in Fig. 3(a), the STR for the h-BN supported MoS2 nanoclusters with the size larger than n = 3 is within a chemical potential range of −0.76 < μ < −0.41 eV. Without the STR, it is clearly seen that either the Mo edge or the S edge would be universally found for the clusters, with no structural transition could occur. Within the STR, however, the structural transition can happen as long as the cluster size crosses the boundary. Since it has been demonstrated that h-BN exerts little influence on the stability of MoS2 nanoclusters, it is natural to expect that the MoS2 nanoclusters supported on other inert and insulating substrates would have a similar STR and structural transition behavior. For the Au(111) supported MoS2 nanoclusters as shown in Fig 3.(b), the boundary also decreases with the increasing size, similar to the h-BN supported case. However, the blue and red regions are shown to entirely shift to the right-hand side (the end of μ = 0) and the STR S12


Page 12 of 28

Page 13 of 28

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


relocates to between 0 and −0.24 eV. As a result, the small clusters with a size of n < 5 are found to always favor the Mo edge for the whole range of μ and no structural transition could occur for them. This behavior is quite different from the h-BN supported clusters, for which the structural transition is allowed to occur for any given n. On the other hand, for the clusters with a size of n ≥ 5 , the structural transition can always occur, but at a much higher μ than the corresponding h-BN supported ones. This shift of phase space is because of the different magnitudes of stabilization effect that Au(111) exerts on the clusters with different morphologies. As shown in Fig. 2, a more significant reduction of formation energy by Au(111) can be seen for the S edge and corner, compare to their Mo counterparts. As a result, the relative stability of Au(111) supported clusters with S edge would be more enhanced at any given μ and n, compared to their counterpart with Mo edge,. In the STM experiments12, the MoS2 nanoclusters were supported on Au substrate and synthesized under a nearly pure H2S atmosphere at a temperature around 400 K. By using JANAF thermochemical Tables20, we estimate μ for these synthesis conditions to be around −0.10 eV, which is consistent with the previous DFT calculations9. It is shown in Fig. 3(b) that, for μ around −0.10 eV, our results predict that a structural transition for the equilibrium morphology of Au(111) supported clusters would occur at n = 6, which is quantitatively consistent with the experimental observations12. This consistency with experiments implies that the substrates play a key role in determining the equilibrium morphology of MoS2 nanoclusters. It should be noted that the results obtained for Au(111) might be translatable to other metallic substrates, because the charge transfer between the metallic surface and MoS2 nanoclusters could always occur. S13



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

The above results imply that the size-dependent morphology of MoS2 nanoclusters may be observed, only if the synthesis conditions and substrates are carefully chosen, so that μ can fall into the STR. However, one issue still left to clarify is how to determine the important STR. To answer this question, we can re-examine the calculated edge and corner energies. As shown in Fig. 2, for the h-BN supported MoS2 nanoclusters, the crossovers between λ (BN) and λ (BN) and between χ (BN) and χ (BN) are calculated to be at μ = −0.76 and μ = −0.41 eV, respectively, which give the two boundaries of the SRT given in Fig. 3(a). For μ > −0.41 eV (or μ < −0.76 eV), it is clearly seen that both the Mo edge and Mo corner are more (or less) stable than their S counterparts, resulting in the MoS2 nanoclusters universally possessing the Mo (or S) edge, regardless of their size. However, for −0.76 < μ < −0.41 eV, λ (BN) is lower than λ (BN), but χ (BN) is higher than χ (BN), which together causes an energetic competition between the structure with the stable edge and that with the stable corner. The equilibrium morphology of MoS2 nanoclusters is then determined by the most stable one of these two competitive structures. For the small MoS2 nanoclusters, the corner energy is more important than the edge energy and thus the structure with the stable corner (S corner) would be adopted. However, as the cluster size increases, the edge energy is gradually increased and will finally dominate the corner energy, leading to a structural transition from the one with the S edge to that with the Mo edge. The STR of the Au(111) supported MoS2 nanoclusters is determined in a similar way, however, the charge transfer between the clusters and Au(111) changes the crossover between λ (Au) and λ (Au) to μ = −0.24 eV and that between χ (Au) and χ (Au) to beyond the available range of μ , and thus relocates the STR to −0.24 < μ < 0 eV. S14


Page 14 of 28

Page 15 of 28

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


Electronic structures of triangular MoS2 nanoclusters. An infinite sheet of MoS2 monolayer is a well-known semiconductor with a band gap of ca. 1.7 eV. However, terminating the MoS2 nanoclusters with edge and corner would remarkably change their band structure. Fig. 4 presents the spin-polarized density of states (DOS) for the h-BN and Au(111) supported MoS2 nanoclusters, together with the partial charge density of localized states at the Fermi level. For comparison, the electronic structures of free-standing MoS2 nanoclusters and nanoribbon are also calculated and given in SI. It is clearly seen that the free-standing and h-BN supported clusters have essentially same band structures, implying that h-BN has almost no impact on the electronic properties of MoS2 nanoclusters. For MoS2 nanoribbon that was always used as the computational model to study the edge states of MoS2 nanoclusters, our results indicate that both the Mo and S edges exhibit a metallic character and the metallic states come mainly from either Mo atoms or S2 dimers at the Mo edge or S atoms at the S edge, which is consistent with the previous DFT studies4, 11. However, it is interesting to see that the direct calculation of MoS2 nanoclusters actually gives essentially different results. As shown in Fig. 4(a), the h-BN supported (or free-standing) clusters with Mo edge generally have a semiconducting character, with a distinguishable band gap. Increasing the size from n = 5 to n = 7 does not close the band gap, implying that the semiconducting character is not size-dependent. This discrepancy between the cluster and ribbon model should be very crucial, because the one-dimensional metallic states of Mo edge are usually viewed to be responsible for the catalytic activity of MoS2 nanoclusters2, 4-6. We notice that the most important difference between these two models is that the nanoribbon employs infinite-long edges and periodic boundary conditions, while the clusters have S15



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

finite-long edges terminated by corners. Detailed structural analysis show that the structural reconstruction occurs for the Mo edge in MoS2 nanoclusters (see Fig. S4), rather than that in MoS2 nanoribbon. This edge reconstruction then tends to stabilize the edge structure by splitting the edge states at the Fermi surface and thus lowering the energy of electrons below the energy gap, which yields a semiconducting character of MoS2 nanoclusters. This metal-to-semiconductor transition is usually found for one-dimensional metallic chains, known as the Peierls transition. It should be noted that the edge reconstruction is found to be energetically unfavorable in MoS2 nanoribbon, since it causes lattice distortions in its bulk and thus increases the elastic energy of the whole material, which offsets the energy gain from the band splitting. While for the clusters, the stress from the bulk lattice distortion can be released by the corner reconstruction, and thus the edge reconstruction prefers to occur. The band splitting at the Fermi level can be also seen in Fig. 4(b) that visualizes the partial charge density at the top valence band. From the figure, we can clearly see either the p orbital of S atoms (n = 6) or the d orbital of Mo atoms (n = 5 and n = 7), but not both of them. When supported on Au(111), an essential band broadening and band shift can be seen for the MoS2 nanoclusters with Mo edge, as shown in Fig. 4(a), implying that the charge transfer between the clusters and Au(111) occurs. As a result, a partially occupied band across the Fermi level is found for the clusters, which reveals that they have a metallic character. Detailed inspection of the corresponding partial charge density indicates that both the S2 dimers and Mo atoms at the edge contribute to the states near the Fermi level, which yield the one-dimensional metallic edge states as found in MoS2 nanoribbon. Moreover, a detectable contribution from the bulk and corner Mo atoms can also be seen in Fig. 4(b). It is interesting to see that the S16


Page 16 of 28

Page 17 of 28

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


partial charge density obtained for Au(111) supported clusters shows a good agreement with their STM images observed in experiments12-13. The spin-polarized DOS for the MoS2 nanoclusters with S edge is presented in Fig. 5(a). The results show that the h-BN supported clusters are metallic and have a spin-polarized ground state, with a spin-down conducting channel. The corresponding partial charge density in Fig. 5(b) indicates that the metallic states mostly come from the edge S atoms which have a lower coordination than their bulk counterpart. Because of the lower coordination, significant amounts of unpaired electrons (dangling bonds) should exit on the edge S atoms, which causes a local magnetism and induces a half-metallic character for the S edge. Compared to the partial charge density found for MoS2 nanoribbon, we find that the S edge of MoS2 nanoclusters yield an almost same charge distribution. This is because that the S atoms at S edge are well confined in the hexagonal lattice and thus no edge reconstruction prefers to occur. As the cluster size increases, no significant size-dependent character can be observed, expect that the edge states grow along the triangular side. When supported on Au(111), the charge transfer between the clusters and substrate is found to occur, which tends to saturates the dangling bonds on edge S atoms and thus diminishes their local magnetism, as shown in Fig. 5(a). Compared to the h-BN supported case, the states near the Fermi level are shown to come primarily from the Mo atoms that are nearest to the S edge, rather than the S edge atoms, as shown in Fig. 5(c). This edge passivation by the charge transfer can be used to explain our previous results that Au(111) tends to significantly stabilize the S edge.

4. Conclusions S17



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

In conclusion, we have performed the DFT calculations to investigate the size-dependent morphology change and the metallic edge states of triangular MoS2 nanoclusters. To be more practical, h-BN and Au(111) are considered as the substrates in this work, in order to capture the general influences of the metallic and insulating substrates used in experiments and industrial. The first-principles total energy calculations indicate that the stability of MoS2 nanoclusters is mainly determined by their edge and corner energies, both of which vary depending on the chemical potential of sulfur. As the chemical potential changes from sulfiding to sulfo-reductive conditions, our results indicate that the relative stabilities of both the Mo edge and corner are gradually reduced, compared to their S counterparts, which is consistent with the previous DFT investigations9. More interestingly, it is found that both of the edge and corner energies are also quite sensitive to the substrates that are always omitted in the previous work. It is shown that the insulating h-BN substrate exert little effect on the stability of MoS2 nanoclusters, while the metallic Au(111) surface can significantly stabilize the supported clusters with S edge. Our results indicate that the stabilization effect of Au(111) should be ascribed to the charge transfer between the clusters and substrate, which could saturate the dangling bonds at the S edge. Since the influences exerted by the substrates are demonstrated to depend mainly on their ability to transfer charge, our results should not be limited to the special h-BN and Au(111), but applicable to all insulating and metallic substrates, such as the widely used γ-alumina, amorphous SiO2 and graphene (or graphite). In addition, our results reveal that the morphology progression of MoS2 nanoclusters can be understood by the variation of the ratio from the edge to corner energies with respect to the cluster size. As the size increases, the most important factor that determines the equilibrium S18


Page 18 of 28

Page 19 of 28

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


morphology of MoS2 nanoclusters is shown to change from the corner energy to the edge energy, which induces a structural transition from the one with S edge to that with Mo edge. Moreover, the Au(111) substrate is shown to have a significant impact on this morphology progression, since it stabilizes the Mo and S edges in different magnitudes. By setting the chemical potential of sulfur to fit experimental conditions, we predict a size-dependent structural progression of MoS2 nanoclusters on Au(111), which is quantitatively consistent with the experimental observations12-13. More interestingly, it is also revealed by our calculations that the morphology progression of MoS2 nanoclusters can only occur within a special region of chemical potential of sulfur, i.e. the STR, which depends closely on the substrates. Based on these results, we suggest that precise control for the morphology of MoS2 nanoclusters may be achieved by carefully co-adjusting the synthesis conditions, cluster sizes and substrates. In the end, the electronic structure of MoS2 nanoclusters are discussed. Compared to the results obtained in MoS2 nanoribbon, our direct studies of MoS2 nanoclusters reveal quite different results. For the h-BN supported clusters, our results indicate that the Mo edge generally have a semiconducting character, which is mistaken to be metallic by using MoS2 nanoribbon. We find that this essential difference comes from the edge reconstruction which is found to occur for MoS2 nanoclusters, rather than MoS2 nanoribbon. On the other hand, the S edge of MoS2 nanoclusters are found to reproduce the results found in MoS2 nanoribbon, because the edge construction could not occur. When supported on Au(111) clusters, both the Mo and S edges are shown to have a metallic character, mainly because of the charge transfer between the clusters and substrate. However, the one-dimensional metallic states are only S19



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

Page 20 of 28

found for the clusters with Mo edge, but not for those with S edge, which is qualitatively consistent with their STM images observed in experiments12-13. Since the one-dimensional metallic states have been demonstrated to be very important for their catalytic activity, it can be expected from our results that the MoS2 nanoclusters with Mo edge on metallic substrates could be good candidates as hydrotreating catalysts.

Supporting information Calculated edge and corner energies for free-standing, BN and Au(111) supported MoS2 nanoclusters, band alignment between the Au(111) and supported MoS2 nanoclusters, spin-polarized density of states and partial charge density of free-standing MoS2 nanoribbon, and the spin-polarized density of states and partial charge density of n = 6 MoS2 nanoclusters supported on h-BN.

Acknowledgment This

work

was

supported

by

the

National

Research

Foundation

of

Korea

(2015R1A2A2A05027766) and the Global Frontier R&D (2011-0031566: Centre for Multiscale Energy Systems) programs.

S20


Page 21 of 28

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


References 1. Prins, R., Catalytic Hydrodenitrogenation. Adv. Catal. 2001, 46, 399-464. 2. Lauritsen, J. V.; Nyberg, M.; Vang, R. T.; Bollinger, M.; Clausen, B.; Topsøe, H.; Jacobsen, K. W.; Lægsgaard, E.; Nørskov, J.; Besenbacher, F., Chemistry of One-Dimensional Metallic Edge States in MoS2 Nanoclusters. Nanotechnology 2003, 14, 385-389. 3. Lauritsen, J. V.; Bollinger, M. V.; Lægsgaard, E.; Jacobsen, K. W.; Nørskov, J. K.; Clausen, B. S.; Topsøe, H.; Besenbacher, F., Atomic-Scale Insight into Structure and Morphology Changes of MoS2 Nanoclusters in Hydrotreating Catalysts. J. Catal. 2004, 221, 510-522. 4. Bollinger, M. V.; Lauritsen, J. V.; Jacobsen, K. W.; Norskov, J. K.; Helveg, S.; Besenbacher, F., One-Dimensional Metallic Edge States in MoS2. Phys. Rev. Lett. 2001, 87, 196803-196806. 5. Moses, P. G.; Hinnemann, B.; Topsøe, H.; Nørskov, J. K., The Hydrogenation and Direct Desulfurization Reaction Pathway in Thiophene Hydrodesulfurization over MoS2 Catalysts at Realistic Conditions: A Density Functional Study. J. Catal. 2007, 248, 188-203. 6. Lauritsen, J. V.; Nyberg, M.; Nørskov, J. K.; Clausen, B. S.; Topsøe, H.; Lægsgaard, E.; Besenbacher, F., Hydrodesulfurization Reaction Pathways on MoS2 Nanoclusters Revealed by Scanning Tunneling Microscopy. J. Catal. 2004, 224, 94-106. 7. Helveg, S.; Lauritsen, J. V.; Lægsgaard, E.; Stensgaard, I.; Nørskov, J. K.; Clausen, B.; Topsøe, H.; Besenbacher, F., Atomic-Scale Structure of Single-Layer MoS2 Nanoclusters. Phys. Rev. Lett. 2000, 84, 951-954. 8. Kibsgaard, J.; Lauritsen, J. V.; Lægsgaard, E.; Clausen, B. S.; Topsøe, H.; Besenbacher, F., Cluster-Support Interactions and Morphology of MoS2 Nanoclusters in a Graphite-Supported Hydrotreating Model Catalyst. J. Am. Chem. Soc. 2006, 128, 13950-13958. 9. Schweiger, H.; Raybaud, P.; Kresse, G.; Toulhoat, H., Shape and Edge Sites Modifications of MoS2 Catalytic Nanoparticles Induced by Working Conditions: A Theoretical Study. J. Catal. 2002, 207, 76-87. 10. Cao, D.; Shen, T.; Liang, P.; Chen, X.; Shu, H., Role of Chemical Potential in Flake Shape and Edge Properties of Monolayer MoS2. J. Phys. Chem. C 2015, 119, 4294-4301. 11. Bollinger, M. V.; Jacobsen, K. W.; Nørskov, J. K., Atomic and Electronic Structure of MoS2 Nanoparticles. Phys. Rev. B 2003, 67, 085410-085426. 12. Lauritsen, J. V.; Kibsgaard, J.; Helveg, S.; Topsoe, H.; Clausen, B. S.; Laegsgaard, E.; Besenbacher, F., Size-Dependent Structure of MoS2 Nanocrystals. Nat Nanotechnol 2007, 2, 53-58. 13. Tuxen, A.; Kibsgaard, J.; Gøbel, H.; Lægsgaard, E.; Topsøe, H.; Lauritsen, J. V.; Besenbacher, F., Size Threshold in the Dibenzothiophene Adsorption on MoS2 Nanoclusters. ACS Nano 2010, 4, 4677-4682. 14. Kresse, G.; Joubert, D., From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758-1764. 15. Kresse, G.; Furthmüller, J., Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169-11183. S21



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

16. Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. 17. Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H., A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (Dft-D) for the 94 Elements H-Pu. J. Chem. Phys. 2010, 132, 154104-154122. 18. Zhang, S. B.; Wei, S. H., Surface Energy and the Common Dangling Bond Rule for Semiconductors. Phys. Rev. Lett. 2004, 92, 086102-086105. 19. Lucking, M. C.; Bang, J.; Terrones, H.; Sun, Y.-Y.; Zhang, S., Multivalency-Induced Band Gap Opening at MoS2 Edges. Chem. Mater. 2015, 27, 3326-3331. 20. Chase Jr, M.; Curnutt, J.; Downey Jr, J.; McDonald, R.; Syverud, A.; Valenzuela, E., Janaf Thermochemical Tables, 1982 Supplement. J. Phys. Chem. Ref. Data 1982, 11, 695-940.

S22


Page 22 of 28

Page 23 of 28

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


Fig. 1 Optimized structures of triangular MoS2 nanoclusters with (a) Mo edge, and (b) S edge. The Mo and S atoms are indicated by the purple and yellow balls, respectively, and the cluster size is represented by n.

S23



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

Fig. 2 (a) Edge energy and (b) corner energy of triangular MoS2 nanoclusters. The solid and dashed lines represent h-BN and Au(111) supported MoS2 nanoclusters, respectively.

S24


Page 24 of 28

Page 25 of 28

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


Fig. 3 Surface formation energy differences between triangular MoS2 nanoclusters with the Mo and S edges on (a) h-BN and (b) Au(111). The blue and red represent the regions where ∆Ω < 0 and ∆Ω > 0, respectively, with the solid line marks their boundary. The dashed lines indicate the boundaries of structural transition region.

S25



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

Fig. 4 (a) Spin-polarized density of states of MoS2 nanoclusters with Mo edge and (b) and (c) corresponding partial charge density. The partial charge density is the sum of spin-up and spin-down densities and plotted for the energy states (eV) within [−0.1, 0] with respect to the Fermi level, with the isosurface of 0.01 e/Å3.

S26


Page 26 of 28

Page 27 of 28

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


Fig. 5 (a) Spin-polarized density of states of MoS2 nanoclusters with S edge and (b) and (c) corresponding partial charge density. The partial charge density is the sum of spin-up and spin-down densities and plotted for the energy states (eV) within [−0.1, 0.1] with respect to the Fermi level, with the isosurface of 0.01 e/Å3.

S27



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

TOC Graphic

S28


Page 28 of 28

Understanding Size-Dependent Morphology Transition of Triangular

Recommend Documents