Article pubs.acs.org/JPCC
Charge Mediated Semiconducting-to-Metallic Phase Transition in Molybdenum Disulfide Monolayer and Hydrogen Evolution Reaction in New 1T′ Phase Guoping Gao,† Yan Jiao,‡ Fengxian Ma,† Yalong Jiao,† Eric Waclawik,† and Aijun Du*,† †
School of Chemistry, Physics and Mechanical Engineering Faculty, Queensland University of Technology, Garden Point Campus, Brisbane, QLD 4001, Australia ‡ School of Chemical Engineering, University of Adelaide, Adelaide, SA 5005, Australia S Supporting Information *
ABSTRACT: The phase transition of single layer molybdenum disulfide (MoS2) from semiconducting 2H to metallic 1T and then to 1T′ phases, and the effect of the phase transition on hydrogen evolution reaction (HER) are investigated within this work by density functional theory. Experimentally, 2H-MoS2 has been widely used as an excellent electrode for HER and can get charged easily. Here we find that the negative charge has a significant impact on the structural phase transition in a MoS2 monolayer. The thermodynamic stability of 1T-MoS2 increases with the negative charge state, comparing with the 2H-MoS2 structure before phase transition and the kinetic energy barrier for a phase transition from 2H to 1T decreases from 1.59 to 0.27 eV when 4e− are injected per MoS2 unit. Additionally, 1T phase is found to transform into the distorted structure (1T′ phase) spontaneously. On their activity toward hydrogen evolution reaction, 1T′-MoS2 structure shows comparable hydrogen evolution reaction activity to the 2H-MoS2 structure. If the charge transfer kinetics is taken into account, the catalytic activity of 1T′-MoS2 is superior to that of 2H-MoS2. Our finding provides a possible novel method for phase transition of MoS2 and enriches understanding of the catalytic properties of MoS2 for HER.
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identified as the active catalytic center for HER,30,34,35 single layer 2H-MoS2 with largely exposed sulfur edge sites is considered to be a promising candidate for HER.13,30,36,37 However, one of the limitations of 2H-MoS2 as fuel cell cathode material is its poor charge transfer properties.29,37,38 Two methods have been developed to increase the cathodic current: growing 2H-MoS2 on conductive supports, and transforming 2H-MoS2 to conductive 1T-MoS2. Experiments have demonstrated MoS2 grown on conductive supports,38−40 such as graphene,40 have showed markedly faster HER kinetics. However, conducting 1T-MoS2 nanosheets have also exhibited superior electrocatalytic performance for HER.18,41,42 This has been attributed to its metallic properties, where the elimination of Schottky barriers improved the charge transfer.21,41 Phase transition from 2H-MoS2 to 1T-MoS2, or stabilization of 1T-MoS2, has been realized by both chemical method6,43 and physical method.44 Li+ intercalation is the most studied chemical method to promote the phase transition.18,45−47 However, a previous study on the Li-intercalated 1T-MoS2 showed that only a quarter of the edge sites are active for HER.16 Hence, to achieve a higher HER efficiency, a new
INTRODUCTION MoS 2 is one of the most studied transition metal dichalcogenides (TMD)1,2 and is considered to be a promising electrocatalyst for the hydrogen evolution reaction (HER). Several prototypes of MoS2 have been reported, including 2H, 1T, and 1T′.3−8 Among them, 2H9,10 and 1T3,11,12 have attracted the most extensive attention from both theory calculations and experiments in recent years.13−15 Due to the localization behavior of the d-band of transition metal,16,17 the electronic properties of the two phases are dramatically different:8 the 2H-MoS2 phase is semiconductive, while the 1T-MoS2 phase is metallic.18 Generally, the 1T phase is unstable under ambient condition and transforms back to the thermodynamically stable 2H phase17 or 1T′ (distorted 1T) phase.8 Previous studies showed that 1T phase could be stabilized by electron donation via ion intercalation,14,19,20 and the 1T and 2H phases can coexist in the same TMD nanosheets.11,21−23 However, the mechanism for the phase transition is not elucidated yet. Due to the weak van der Waals interaction between layers, MoS2 can be exfoliated into monolayer structure, which exhibits unique and fascinating electronic properties2,24−29 that could be used in a variety of potential applications, including HER,30 energy storage,31 high-performance electronics,32 and sensitive photodetectors.33 Since sulfur edge sites of MoS2 were © 2015 American Chemical Society
Received: May 15, 2015 Revised: May 21, 2015 Published: May 21, 2015 13124
DOI: 10.1021/acs.jpcc.5b04658 J. Phys. Chem. C 2015, 119, 13124−13128
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The Journal of Physical Chemistry C
the 1T-MoS2 by 0.80 eV per three-atom-MoS2 unit. If the Mo atoms of 1T-MoS2 distort along the y direction slightly, it is 1T′-MoS2 form (see Figure 1c,f), and the band gap of 1T′MoS2 is 0.006 eV (see Figure 1i). The energy of 1T′ phase is 0.15 eV lower than 1T phase per MoS2 unit. In the following, we investigate the phase transition between 2H and 1T MoS2 and then to 1T′ phase. The phase transition involves one of the S atoms moving from one pyramidal position to the other pyramidal position of the unit cell (see Figure S1). Figure 2a shows the pathway of the transition of 2H to 1T under charge states from 0e− to 4e− per MoS2. The relative energy of 1T-MoS2 gradually decreases from positive (0.80 and 0.31 eV in neutral state and 1e−, respectively) to negative (−0.05, −0.10, and −0.30 eV in charge state of 2e−, 3e−, and 4e−, respectively). This indicates that charge injection can stabilize the 1T-MoS2; in the case where more than 1e− is injected, 1T-MoS2 is more stable than 2H-MoS2. The phase transition energy barrier also decreases from 1.59 eV in neutral to 1.11, 0.71, 0.54, and 0.27 eV in charge states of 1e−, 2e−, 3e−, and 4e−, respectively. The general trend of MoS2 phase transition is that negative charge inducing 2H-MoS2 transform into 1T-MoS2 easily. In charge state of 4e−, not only is the 1TMoS2 form more stable than 2H-MoS2 by 0.30 eV, but also the barrier of 0.27 eV is easily overcome. Intercalation lithium (Li) or sodium (Na) on MoS2, which are studied extensively in recent experiments14,31,42,45 and theoretical calculations,56−58 introduce excessive electrons into MoS2 and promotes the phase transition. We intercalate a layer of lithium (Li) and sodium (Na) atoms on the top of twodimensional MoS2 surface to compare the ion’s effect on the transition of 2H-MoS2 to 1T-MoS2 with charge effect discussed above. The relative energies of Li and Na intercalated 1T-MoS2 are 0.16 and 0.53 eV, respectively. Meanwhile, the barrier for the Li and Na intercalated MoS2 phase transition from 2H to 1T is only 1.00 and 1.25 eV, respectively. By comparing these results with that of phase transition in the different states, ion intercalation could enhance the stability of 1T-MoS2 and reduce the barrier of the transition for 2H-MoS2 to 1T-MoS2 to some extent. However, charge injection is a superior method to promote the phase transition. To explore the mechanism for the phase transition, we calculate the different charge densities of negatively charged, Li intercalated, and Na intercalated MoS2 (see Figure S2). In all three cases, the charge transfer direction is similar, which indicates that the mechanism of ion intercalation and negative charge injection are the same. Charge mainly accumulates on the S atom induced in the transition, while it is depleted in the area among S and Mo. These all indicate that extra electrons weaken the Mo−S bonds. Meanwhile, there is a small amount of electrons to be transferred into the nonbonding d-orbitals of Mo. According to ligand field theory and refs 16 and 59, the nonbonding d-orbitals of 2H-MoS2 (group 6 TMD) split into three d-orbitals: dz2 (filled), dx2−y2,xy (empty), and dxz,yz (empty). The extra electron entrancing into nonbonding d-orbitals of MoS2 makes it an isoelectronic of group 7 TMD, whose nonbonding d-orbitals prefer to split into two d-orbitals: dyz, xz, xy (partially filled) and dz2,x2−y2 (empty) and exhibit a metallic character. The increase of d-electron count destabilizes the 2H-MoS2. Therefore, the extra electron, from charge injection or ion intercalation, promotes the phase transition by both weakening the Mo−S bond and decreasing the relative energy of 1T-MoS2.
approach, which can not only transform 2H-MoS2 to 1T-MoS2 easily but also maintain the number of active sites per unit MoS2, should be explored. Converting single layer MoS2 from 2H to 1T phase by physical methods, including strain effect and charge injection, are possible ways to obtain metallic 1T-MoS2 without sacrificing the number of active sites. For example, Voiry et al. demonstrated that 2H-WS2 could transform to 1TWS2 with improved catalytic activity by strain effect.48 Another physical factor, charge injection, which could be found in the phase transition of MoS2 in Li ion battery,49 remains unexplored. Therefore, in-depth theoretical understandings of the HER activity on 1T-MoS2 or 1T′-MoS2 are highly desired, in addition to the phase transition process. Experimentally, 2H-MoS2 has been widely used as an excellent electrode for HER and can get charged easily. Within this study, we investigate the effect of charge injection on the conversion of 2H-MoS2 to 1T-MoS2 by using the nudged elastic band (NEB) method50,51 as implemented in the Vienna ab initio simulation package.52,53 Then HER properties on distorted 1T′-MoS2 are studied by density functional theory (DFT) calculations based on Dmol3 software.54,55 More details on computational methods can be seen in the Supporting Information. Our investigations predict that an easy physical method, i.e., charge injection, can transform 2H-MoS2 to 1T′MoS2 with high density of active edges toward HER. These active sites in 1T′-MoS2 engender superior electrocatalytic HER activity compared to the well-developed 2H-MoS2.
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RESULTS AND DISCUSSION MoS2 possesses a variety of polytypic structures, including 2H, 1T, and 1T′. The geometry structures and their electronic band structures are shown in Figure 1. The space-group of 2H-MoS2 is P6̅m2, where the Mo site is in a trigonal prism coordination (Figure 1a,d). The space-group of 1T-MoS2 is P3̅m1, with the Mo in octahedral coordination (Figure 1b,e). Figure 1g,h shows that the electronic properties of 2H-MoS2 and 1T-MoS2 are quite different: the band gap of 2H-MoS2 is 1.74 eV, while 1TMoS2 is metallic. The optimized 2H-MoS2 is more stable than
Figure 1. Top view and side view of 2H, 1T, and 1T′ 4 × 4 × 1 MoS2 and their band structures. Cyan, Mo; yellow, S. 13125
DOI: 10.1021/acs.jpcc.5b04658 J. Phys. Chem. C 2015, 119, 13124−13128
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Figure 2. (a) Minimum energy pathways of the phase transition from 2H to 1T in different charge states or with ion intercalated. All the energies are relative to the total energy of 2H-MoS2 in corresponding charge states or with corresponding ion intercalated. (b) Spontaneous atom relaxation from 1T to new 1T′ phase.
Furthermore, if the angle of 1T-MoS2’s lattice vectors a and b are changed from 60° to 90° and supercell is enlarged from 1 × 1 × 1 cell to 1 × 2 × 1, the Mo atoms of 1T-MoS2 relaxes along the y direction, and the lattice transforms into the distorted 1T′-MoS2 form after cell relaxation with no barrier (see Figure 2b). The relaxation energy from 1T to 1T′ phase is −0.15 eV per MoS2 unit. Therefore, 1T phase would transform into the distorted structure (1T′ phase) spontaneously. Finally, we calculate the catalytic properties of 1T′-MoS2 on HER by Dmol3. The overall HER pathway includes two steps: Volmer step (bonding of hydrogen on the catalyst (H*) from initial state (H+ + e− + *)) and Heyrovsky60 or Tafel61 step (releasing of molecular hydrogen (1/2H2 + *), where * donates the catalyst). Experiments62 have proven that Volmer step is the rate-determining step of HER on MoS2. Therefore, the Gibbs free-energy of the adsorption atomic hydrogen (ΔG0H) is a key quantity to describe the HER activity of the catalyst. The ΔG0H of an ideal catalyst for HER should be close to 0 (ΔG0H → 0).63 In addition, previous DFT studies show that the activities sites of MoS2 mainly are Mo-edge with 50% S coverage30,37,64,65 rather than the basal plane.66,67 As the Mo of 1T′-MoS2 forms a 1D zigzag chain, there are two different Mo-edges determined by the directions. To simulate the two different Mo-edges, we build a model of 7Mo × 4Mo slab, and both the (101̅0) and (1̅010) Mo-edges are covered by 50% S (see Figure S3). The ΔG0H of the two 1T′-MoS2 Mo-edges was calculated under two coverages, 0.25 and 0.50 ML, as shown in Figure 3. The binding energy of hydrogen adsorbed on (10̅ 10) Mo-edge at 0.25 ML is too strong with a free energy of −0.421 eV, while the binding energy of hydrogen adsorbed on (1̅010) Mo-edge at 0.50 ML has a free energy of 0.187 eV. On the (101̅0) Moedge, the free energy of hydrogen is close to zero at both 0.25 ML (0.062 eV) and 0.50 ML (0.059 eV). The free energy of hydrogen on (1010̅ ) Mo-edge of 2H-MoS2 at 0.50 ML is 0.015 eV. These results indicate that the catalytic activity on (101̅0) Mo-edge of 1T′-MoS2 is comparable to the catalytic activity of 2H-MoS2. However, the poor charge transfer in semiconducting 2H-MoS2 significantly limited the kinetics of HER as suggested by experiments.39 If taking the charge transfer into
Figure 3. Free energy diagram of HER processing on (101̅0) (left panel) and (1̅010) (right panel) of 1T′-MoS2 under different coverage at equilibrium potential. Cyan, Mo; yellow, S; white, H.
account, the catalytic performance of 1T′-MoS2 exhibits superior catalytic activity, largely attributed to its conductivity.
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CONCLUSIONS
In conclusion, we report that injected negative charge can significantly enhance the stability of 1T-MoS2 and promote the phase transition from semiconducting 2H-MoS2 to metallic 1TMoS2, which then further distorts into the 1T′-MoS2 phase. Remarkably, 1T′-MoS2 demonstrates comparable catalytic activity to the 2H-MoS2 phase, based on free energy calculations. Taking the charge transfer into account, the metallic 1T′-MoS2 would show superior catalytic activity compared to semiconducting 2H-MoS2. Our results suggest a practical method for the transition of 2H-MoS2 to 1T-MoS2 and enrich the information on catalytic properties of 1T′-MoS2 for HER application. 13126
DOI: 10.1021/acs.jpcc.5b04658 J. Phys. Chem. C 2015, 119, 13124−13128
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ASSOCIATED CONTENT
S Supporting Information *
Computational details, schematic diagram of 2H-MoS2 to 1TMoS2 phase transition, different charge densities of negatively charged, Li intercalated, and Na intercalated MoS2, relaxation energy, geometric structures of 1T′-MoS2 stripes for HER, and differential hydrogen adsorption energy on 1T′-MoS2. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b04658.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge generous grants of high-performance computer time from computing facility at Queensland University of Technology and Australian National Facility. A.D. greatly appreciates the Australian Research Council QEII Fellowship (DP110101239) and financial support of the Australian Research Council under Discovery Project (DP130102420).
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DOI: 10.1021/acs.jpcc.5b04658 J. Phys. Chem. C 2015, 119, 13124−13128
