Investigation of Stacking Effects in Bilayer MoSSe about Photocatalytic

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C: Physical Processes in Nanomaterials and Nanostructures

Investigation of Stacking Effects in Bilayer MoSSe about Photocatalytic Water Splitting Songrui Wei, Jianwei Li, Xiaoqi Liao, Hao Jin, and Yadong Wei J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b04784 • Publication Date (Web): 18 Aug 2019 Downloaded from pubs.acs.org on August 19, 2019

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The Journal of Physical Chemistry

Investigation of Stacking Effects in Bilayer MoSSe about Photocatalytic Water Splitting Songrui Wei,† Jianwei Li,† Xiaoqi Liao,‡ Hao Jin,†,* and Yadong Wei,†,*

†College

of Physics and Energy, Shenzhen University, Nanhai Ave 3688, Shenzhen 518060, People's Republic of China

‡MOE

Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed

Matter and State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, People's Republic of China

*E-mail:

[email protected]; [email protected]

ABSTRACT

Stacking two-dimensional (2D) materials into the heterostructure, which is also known as van der Waals (vdW) epitaxy, is an effective method to tune the properties of the

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pristine monolayer structures. Recently, Janus monolayer MoSSe has been successfully fabricated in experiment, which might be beneficial for photocatalytic water splitting due to the inner electric field caused by inversion symmetry breaking. Considering that the bilayer structure has natural advantages over the monolayer structure in photocatalytic water splitting, we systematically investigate six stacking configurations of bilayer MoSSe. All these stacking configurations are stable according to their calculated formation energies. The stacking effect about properties such as band gap and charge distribution in vertical direction are much stronger than those in inplane direction. Only the AB_SSe case has a direct band gap when spin-orbit coupling (SOC) is considered. What’s more, AB_SSe is excellent for photocatalytic water splitting also because of its high adsorption spectrum, suitable band edge position and type II heterojunction. The direct band gap and type II heterojunction may be originated from the strong inversion symmetry broken. Our work is helpful to understand the mechanism of stacking effect in bilayer MoSSe and can promote the application of bilayer TMDs in photocatalytic water splitting.


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1. Introduction

Since the successful exfoliation of graphene, two-dimensional (2D) materials have received tremendous attentions over the past decade for their unique properties and potential applications in many fields such as superconductor, photocatalyst and spintronics.

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Among the 2D family, transition metal dichalcogenides (TMDs) is an

important membership and MoS2 is one of the most widely studied materials in TMDs.813

The layered MoS2 has some advantages in the application of photocatalytic water

splitting such as suitable direct band-gaps and appropriate band edge positions that perfectly meet the requirements of water redox reactions.11 But it also suffers from some disadvantages such as the high recombination rate of electrons and holes.4

Recently, Lu et al. and Zhang et al. independently synthesized Janus monolayer MoSSe in which one side of S atoms are totally replaced by Se atoms and the other side is intact as shown in Figure 1a and 1b.14,15 The corresponding band structure and density of state are shown in Figure 1c and 1d. One advantage of Janus MoSSe over MoS2 is that as the symmetry in out-of-plane direction is broken, Janus MoSSe has an


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inner electric field which can separate the electrons and holes and thus lower the recombination rate of electron-hole.16-19 Considering that stacking is an efficient method to tune the properties of 2D materials,20-26 the recombination rate may be further decreased if two MoSSe monolayers are stacked in a proper configuration and the electrons and holes are separated in different layers. But the common problem of the bilayer TMDs is that they usually have an indirect band gap which is not suitable for the application of photocatalytic water splitting.11, 23, 27, 28 To our knowledge, all the reported bilayer MoSSe have an indirect band gap.23 Fortunately, there are various stacking configurations of bilayer MoSSe and it still lacks a systematical investigation on all the possible stacking configurations. In this work, we systematically investigated all the possible configurations and considered the effect of SOC and finally found a stacking configuration of bilayer MoSSe with a direct band gap. This direct band gap of bilayer MoSSe is intrinsic and does not need the help of external factors such as strain, electric field or atomic doping. The effect of SOC in forming the direct band gap is also very remarkable so we analyzed the mechanism by calculating the band splitting caused by SOC.29


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Apart from the type of band gap, there are many other factors need to be considered in the application for photocatalytic water splitting.30-33 These factors usually include the adsorption spectrum, band edge position, type of heterojunction and stability et al. In this work, based on the spin-polarized density functional theory (DFT), we calculated the band structure, charge distribution, formation energy, adsorption spectrum and band edge etc. properties of bilayer MoSSe under various stacking configurations. The relationship among inversion symmetry, charge distribution, band gap and band splitting are discussed based on the above results. We are glad to find that the stacking configuration with direct band gap also has good property about other factors which are commonly considered in photocatalytic water splitting. The structure of this paper is arranged as following: Firstly, we give the simulation details about both the parameters used in the ab initio calculation and the definitions of different stacking configurations. Secondly, the most important results of this work are shown which mainly include the stability and layer distance, band structure, charge distribution and other important parameters in photocatalytic water splitting. We also discuss the relationship between different properties and the application of the stacked


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bilayer MoSSe in photocatalytic water splitting. At last, we give a brief summary of the above work.

Figure 1. Top (a) and side (b) view of monolayer MoSSe. Corresponding band structure (c) and projected density of state (d). Green, purple and yellow balls represent Se, Mo and S atoms, respectively. Blue rhombus region represents a primitive cell.

2. SIMULATION DETAILS

Our density functional theory (DFT) calculations are carried out within Perdew-BurkeErnzerhof (PBE) exchange-correlation functional and projector augmented-wave (PAW) pseudopotentials as implemented in Vienna ab initio simulation package (VASP).34-36 As


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GGA usually underestimates the band gap, we also perform hybrid Heyd–Scuseria– Ernzerh calculations (HSE06) to obtain more accurate band edge positions of the bilayer MoSSe.37 The cutoff energy for the plane-wave basis is 450 eV and a Monkhorst-Pack k-point mesh of 15×15×1 is used in the hexagonal Brillouin zone. In each configuration, atoms are fully relaxed by employing conjugate-gradient (CG) method. The total energy and atomic forces are converged to 10-5 eV and 0.01 eV/Å and a large vacuum spacing of at least 20 Å along out-of-plane direction is used in all the calculations. Various forms of vdW functionals are used to incorporate the vdw interactions.38 Six stacking configurations are studied, which are shown in Figure 2a. In the vertical direction, there are three kinds of stacking configurations: SeMoS//SMoSe, SMoSe//SeMoS and SMoS//SeMoSe. In the in-plane direction, we consider two stacking configurations: AA’ (mirror symmetry) and AB (translation symmetry) because their energies are nearly degenerate and much lower than the other three in-plane stacking configurations (Figure 2b). For simplicity, we label three vertical stacking configurations as SS, SeSe and SSe. So, the six stacking configurations can be expressed as: AA’_SS, AA’_SeSe, AA’_SSe, AB_SS, AB_SeSe, AB_SSe.


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3. RESULTS AND DISCUSSION

This section is mainly divided into four parts: the stability and layer distance, band structure, charge distribution and other important parameters in photocatalytic water splitting. The stability is shown by calculating the formation energy. The type of band gap is obtained from the band structure and the effect of SOC in forming the direct band gap is also investigated by calculating the band splitting caused by SOC. From the charge distribution, we can judge the type of band


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Figure 2. (a). Side and top view of combined six stacking configurations. (b). Five possible stacking configurations in in-plane direction. In this work, we only consider the more stable cases AA’ and AB.

structure of the heterostructure. Other important parameters in photocatalytic water splitting mainly include the adsorption spectrum and band edge.


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3.1 Stability and layer distance. The formation energy and layer distance of bilayer MoSSe are shown in Table 1. The formation energy is calculated by: Eformation=Ebilayer2*Emonolayer. Generally speaking, negative formation energy means the structure is stable. To show the results more straightforwardly, we also give the relative value of the formation energy by setting the formation energy of the AA’_SS case as 0. The sequence of stability of the six cases is: AA’_SeSe > AB_SeSe > AB_SSe > AA’_SeS > AB_SS > AA’_SS. It can be seen that the formation energy of all the stacking configurations are negative which means that they are all stable. Among them, AB_SSe is important because of its direct band gap. Its stability is moderate among the six cases. As the formation energy of AB_SSe is not the lowest one, we suggest that external asymmetrical factors may be needed to synthesize the structure in experiment. The external asymmetrical factors may include external electric field and magnetic field, asymmetric strain and asymmetric atomic doping. The above results are obtained by using vdW-D2-Grimme vdW functionals (D2). As the formation energy may be affected by the form of vdW functional, we also calculated the formation energy with other forms of vdW functionals which include no-vdW, vdW-


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D3-ZeroDampingGrimme (D3ZDG), vdW-D3-Becke-JonsonDamping (D3BJD), vdWTkatchenko-SchefflerMethod (TS) and vdW-dDsC (dDsC). The corresponding results are shown in Table 2. It can be seen that the form of vdW has remarkable influence on the formation energy. Comparing with D2, the formation energies of D3ZDG and D3BJD are lower and all the stacking configurations are more stable. On the other hand, the effects of TS and dDsC are very similar with no-vdW. Another factor that may affect the formation energy is the edge, which is especially important in photocatalytic water splitting. To investigate the effect of edge on formation energy, we calculated the formation energy of nanoribbons in different stacking configurations. The corresponding results are shown in Table 3. Firstly, the formation energies of different stacking configurations are all positive and the values are similar with the formation energy of MoS2 nanoribbons.39 It means that the bilayer MoSSe nanoribbon is less stable comparing with 2D bilayer MoSSe. Secondly, the sequence of stabilility of nanoribbon is exactly the same with that of the 2D bilayer MoSSe. It means that the sequence of stability of different stacking configurations may be some kind of intrinsic property for bilayer MoSSe.


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The sequence of layer distance of the six cases is: AB_SeSe > AA’_SeSe > AA’_SSe > AB_SSe > AA’_SS > AB_SS. The layer distance is mainly determined by the kind of atom at interface. As the size of Se atom is larger than S atom, the layer distance of SeSe is larger than SSe and SS. It can be also found that the sequence of stability and layer distance in vertical direction are the same. Namely, SeSe > SSe > SS. It may be because the attracting forces between the two monolayers are mainly contributed by the accumulated electrons in the middle of the bilayers and the Se atom can provide more electrons than S atom. Table 1. Formation energy and layer distance of the six cases

AA’_SS AA’_SeSe AA’_SSe AB_SS AB_SeSe AB_SSe Formation

energy

(meV) Relative value (meV) Layer distance (Å)

-157

-210

-186

-160

-205

-188

0

-53

-29

-3

-48

-31

3.08

3.21

3.14

3.03

3.24

3.12

Table 2. Formation energy of different stacking configurations and vdW forms (meV)

'no-

'vdw-D2-

'vdw-D3zeroDamping

'vdw-D3-Becke-

vdwTkatchenko-

'vdw-


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vdw'

Grimme'

Grimme'

JonsonDamping'

SchefflerMethod

dDsC'

' AA’-SS

-2.7

-157.6

-242.8

-275.5

-3.2

-2.7

-3.2

-211.2

-266.1

-305.6

-3.2

-3.2

AA’-SeS

-6.4

-187.1

-259.0

-295.4

-6.4

-6.4

AB-SS

-2.7

-160.4

-246.6

-284.8

-2.7

-2.7

AB-SeSe

-3.4

-205.2

-260.4

-299.2

-3.4

-3.4

AB-SeS

-6.6

-188.3

-261.1

-302.3

-6.6

-6.6

AA’SeSe

Table 3. Formation energy of bilayer MoSSe nanoribbon in different stacking configurations (meV)

Nanoribbon

AA’_SS

AA’_SeSe

AA’_SSe

AB_SS

AB_SeSe

AB_SSe

787.1

768.3

773.8

783.8

768.8

770.6

3.2 Band structure. The band structures of six stacking configurations considering SOC are shown in Figure 3. Firstly, we will analyze the relationship between the stacking configuration and the type of band gap. It can be inferred from the band structures that the direct band gap should form at K point. So, we calculate the energy difference of conduction band and valence band at K point (EK) and band gap of the whole band structure (E) respectively. If the difference


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Figure 3. Band structures of six cases with SOC. There is band splitting in case (3), (4), (5) and (6). Only case (6) has a direct band gap.

between them (ΔE = EK - E) is small, the case is close to a direct gap. The quantitative results of ΔE for different stacking configurations considering SOC are shown in the second line of Table 4. Only AB_SSe has a direct band gap and the sequence about closing to a direct gap for the six cases is: AB_SSe > AA’_SSe > AB_SeSe > AA’_SeSe > AA’_SS > AB_SS. It can be concluded that the stacking configuration whose vertical inversion symmetry is broken (SSe case) tends to form a direct band gap.


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Table 4. The energy difference between EK and E (defined in the text) for the six cases

AA’_SS AA’_SeSe AA’_SSe AB_SS AB_SeSe AB_SSe ΔE without SOC (eV) 0.5921

0.2623

0.1779

0.6174

0.2318

0.21

ΔE with SOC (eV)

0.2147

0.0978

0.5415

0.1546

0

0.5369

Secondly, we will investigate the effect of SOC on band gap. Theoretically, the SOC in bilayer MoSSe is different from the SOC in bilayer MoS2 on two aspects. Firstly, the strength of SOC and the weight of the atoms are positively correlated. As the Se atom is heavier than S atom, the effect of SOC in bilayer MoSSe is larger than that in bilayer MoS2. Secondly, the inversion symmetry in Janus MoSSe and bilayer MoSSe of AB_SSe and AA’_SSe case is broken. This will lead to the splitting of band caused by SOC. About our results, comparing the same case with and without SOC as shown in Table 4, it is found that the SOC always favors a direct gap. It is well known that the SOC may induce band splitting at conduction band (CB) and valence band (VB) of K point in some cases. So, we try to reveal the relationship between SOC and type of band gap by calculating the band splitting caused by SOC. In this work, all the discussions about band splitting are at CB and VB of K point. The quantitative results of


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band splitting for different cases are shown in Table 5 and can be summarized as following: (1) There is no band splitting for case AA’_SS and AA’_SeSe for both the CB and VB. (2) The band splitting of SSe is much larger than SS and SeSe for the VB. (3) The band splitting of AB is a little larger than AA’ for all the cases. Based on the above results, it can be concluded that the band splitting is strongly dependent on the symmetry of the stacking configuration. Only the case with broken inversion symmetry will have band splitting under SOC. The more seriously the inversion symmetry is broken, the larger the band splitting is. Now it is straightforward to understand the relationship between SOC and the type of band structure. For the cases with broken inversion symmetry, the band splitting caused by SOC will lift the valence band at K point and make it closer to a direct band gap. The AB’_SSe case has a direct band gap because the band splitting caused by SOC is large enough. It can also be inferred that the conclusion “the stacking effect in vertical direction is stronger than in-plane direction” may be related with the fact that the inversion symmetry broken in vertical direction is stronger than that in in-plane direction. So, the effect of SOC is important in


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determining the type of band gap and should not be ignored especially in the heavy atoms and the structures with broken inversion symmetry. Table 5. The band splitting of the six cases with SOC

AA’_SS AA’_SeSe AA’_SSe AB_SS AB_SeSe AB_SSe Band splitting of VB 0

0

0.1667

0.0658

0.0415

0.138

0

0.0134

0.0135

0.0106

0.0135

(eV) Band splitting of CB 0 (eV)

3.3 Charge distribution and projected band structure. The results of charge distribution include partial charge density and charge density difference. The partial charge densities of conduction band minimum (CBM) and valence band maximum (VBM) for the six stacking configurations are shown in Figure 4. It mainly describes the shape of the distribution of electrons in real space. Taking CBM of the AA’_SS case as an example, the charge is mainly distributed around the Mo atom so the CBM is mainly contributed by the Mo atom. More accurate results about the contribution of orbits can


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be obtained from the projection of band structure as shown in Figure 5 (the AA’_SS case). A fatter

Figure 4. Partial charge densities of CBM and VBM of six cases.

band means the contribution from the atom and orbital to the band is larger. Comparing the partial charge densities of six cases, the relationship between charge distribution and inversion symmetry of stacking configurations can be found. For the vertical


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stacking of SS and SeSe and in-plane stacking of AA’, the inversion symmetry is kept and the charge distribution in generally averaged in the two layers in the AA’_SS case and AA’_SeSe case. For the vertical stacking of SSe, the symmetry is seriously broken and the electrons of AA’_SSe and AB_SSe cases are distributed only in one layer. For the in-plane stacking of AB, the symmetry broken is weak and the charges are distributed in both layers but the distributions are different between the two layers in AB_SS and AB_SeSe case. So, it can be concluded that the charge distribution is strongly dependent on the inversion symmetry of the stacking configuration. A stronger inversion symmetry broken will induce a larger difference of charge distribution between the two layers. These results are important in judging the performance of bilayer MoSSe in photocatalytic water splitting. For the AA’_SSe and AB_SSe cases, the electrons of VBM are mainly from the top layer and the electrons of CBM are mainly from the bottom layer. This type of band structure is named the type II heterojunction. The separation of electron and hole in real space in type II heterojunction will lower the recombination rate and improve the performance of bilayer MoSSe in photocatalytic water splitting.


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The charge density difference is shown in Figure 6, which is calculated by subtracting the charge distribution of two monolayers from that of bilayer. Yellow means accumulation of electrons and blue means losing electrons. In all the six cases, electrons accumulate in the middle of the bilayer and this has strong influence on the stability of the bilayer structure. The charge distribution of atoms near the interface is strongly changed no matter whether S atom or Se atom forms the interface. The case of AB_SSe is different from the other five cases. In case AB_SSe,

Figure 5. Projected band structure of AA’_SS case. Width of line represents contribution of the orbit to the band.


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the charge distribution of outer Mo atom and S atom is also strongly changed in forming the bilayer while in other cases the atoms far from interface is almost intact. This characteristic of AB_SSe case may be also originated from the strong inversion symmetry broken and can be used to further increase the inner electric field.

Figure 6. Differential charge density of six cases. Yellow means accumulation of electrons and blue means losing electrons.

3.4 Band edge and absorption spectrum. The band edge position and absorption spectrum are two important factors that need to be considered in photocatalytic water splitting. For band edge position, it is required that the CBM should be higher than the reduction potential H+/H2 (-4.440 eV) and the VBM should be lower than the oxidation potential OH-/O2 (-5.670 eV). The gap between VBM and CBM should be larger than


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1.23 eV and be around 2~3 eV which is the main part of sunlight. In this work, we checked the band edge position and absorption spectrum of AB_SSe who has a direct band gap. The band edge position, the referenced oxidation/reduction potential are all given with respect to the vacuum level. The electrostatic potential is calculated by the HSE method as shown in Figure 7a. The band edges are calculated by subtracting the electrostatic potential of the Se side and S side from CBM and VBM respectively, and the relative results are shown in Figure 7b. It can be seen that AB_SSe perfectly satisfies the above requirement. The calculated energy level of CBM is −3.659 eV, which is 0.781 eV above the reduction potential H+/H2. VBM is −6.391 eV, which is 0.721 eV below the oxidation potential OH-/O2. The gap between VBM and CBM is 2.732 eV which is in the visible-light spectrum. The above discussions are all for the solutions of PH=0. For the solutions of other PH values, on one hand, the energy of electrons and holes are assumed to be constant under different PH values of solutions. On the other hand, based on our simulation, the oxidation/reduction potential of H2O increases with the PH value at the rate of 0.059 eV per PH value in the range of PH=0 to 7. In this way, the oxidation/reduction potential of H2O will be -4.027/-5.257 eV for


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PH=7. The bilayer MoSSe of AB_SSe is still active in such condition. But if the rule of oxidation/reduction potential of H2O and PH value holds for higher PH values, the oxidation/reduction potential of H2O will be -3.614/-4.844 eV for PH=14 and the bilayer MoSSe of AB_SSe will be inactive. The adsorption spectrum of AB_SSe case is shown in Figure 8. It can be seen the first peak is near 3 eV which is similar to the band gap of 2.732 eV. For the visible light range (1.63-3.12 eV), there is an obvious absorption. These results indicate that bilayer MoSSe stacked in the AB_SSe configuration is a light harvesting material and suitable for photocatalytic water splitting.

Figure 7. Electrostatic potential (a) and band edge position (b) of AB_SSe.


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Figure 8. Absorption spectrum of AB_SSe.

3.5 Overall discussion. We are surprised to find that almost all these properties are best for the AB_SSe case. Furthermore, it seems that all these promising properties are originated from the strong inversion symmetry broken of the AB_SSe case. In details, the direct band gap is induced by large band splitting which is related with the symmetry broken. The separation of electrons and holes in different layers only appears in the cases with strong inversion symmetry broken. The stability, adsorption spectrum and band edge position are also pretty good for the AB_SSe case. The problem of weaker inner electric field comparing with monolayer MoSSe may be also solved in the AB_SSe case because the electron distribution is strongly changed in forming the bilayer structure. On the other hand, the effect of SOC is also important and necessary in


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obtaining a direct band gap in bilayer TMDs. It may be necessary to double check the type of band gap by considering the effect of SOC in other bilayer TMDs whose inversion symmetry is broken.

4. CONCLUSIONS

In this work, we systematically investigated six kinds of stable stacking configurations of bilayer Janus MoSSe. We calculated the band structure, charge distribution, formation energy and adsorption spectrum based on the spin polarized DFT. From the above results, the properties related with photocatalytic water splitting such as band gap, band edge position, type of heterojunction are discussed. It is found that the AB_SSe case has a direct band gap and type II heterojunction simultaneously. These desired properties may be originated from the strongest inversion symmetry broken in AB_SSe case. We have tested that other important properties in photocatalytic water splitting such as adsorption spectrum and band edge position are also promising in AB_SSe case. It is also found that the effect of SOC is important in inducing a direct band gap in


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bilayer TMDs. This may encourage further investigation on photocatalytic water splitting based on bilayer TMDs.

AUTHOR INFORMATION

Corresponding Author

E-mail: [email protected]; [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Financial supports from the Postdoctoral Science Foundation of China (Grant No. 2018M643144), National Natural Science Foundation of China (Grant No. 11574217), and Shenzhen Key Lab Fund (Grant No. ZDSYS 20170228105421966) are gratefully acknowledged.

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TOC Graphic


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Figure 1. Top (a) and side (b) view of monolayer MoSSe. Corresponding band structure (c) and projected density of state (d). Green, purple and yellow balls represent Se, Mo and S atoms, respectively. Blue rhombus region represents a primitive cell.



Figure 2. (a). Side and top view of combined six stacking configurations. (b). Five possible stacking configurations in in-plane direction. In this work, we only consider the more stable cases AA’ and AB.


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Figure 3. Band structures of six cases with SOC. There is band splitting in case (3), (4), (5) and (6). Only case (6) has a direct band gap.



Figure 4. Partial charge densities of CBM and VBM of six cases.


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Figure 5. Projected band structure of AA’_SS case. Width of line represents contribution of the orbit to the band.



Figure 6. Differential charge density of six cases. Yellow means accumulation of electrons and blue means losing electrons.


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Figure 7. Electrostatic potential (a) and band edge position (b) of AB_SSe.



Figure 8. Absorption spectrum of AB_SSe 154x114mm (300 x 300 DPI)


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Investigation of Stacking Effects in Bilayer MoSSe about Photocatalytic

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