Janus Structures of Transition Metal Dichalcogenides as the

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Janus Structures of Transition Metal Dichalcogenides as the Heterojunction Photocatalysts for Water Splitting Yujin Ji, Mingye Yang, Haiping Lin, Tingjun Hou, Lu Wang, Youyong Li, and Shuit-Tong Lee J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11584 • Publication Date (Web): 15 Jan 2018 Downloaded from http://pubs.acs.org on January 15, 2018

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The Journal of Physical Chemistry

Janus Structures of Transition Metal Dichalcogenides as the Heterojunction Photocatalysts for Water Splitting Yujin Ji, Mingye Yang, Haiping Lin, Tingjun Hou, Lu Wang*, Youyong Li*, Shuit-Tong Lee Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu 215123, China *

Email: [email protected], [email protected]

Abstract The Janus structures of transition metal dichalcogenides with an intrinsic dipole have been proposed as efficient photocatalysts for water splitting, and successfully synthesized recently. However, the mechanism for their superior photocatalytic activities are not understood. Here, we systematically investigate the photocatalytic activities of Janus molybdenum dichalcogenides (MoXY, X/Y=O, S, Se, and Te), by studying their band gaps, redox energy levels and electrons and holes separation, by first-principles calculations. The intrinsic dipoles in the Janus structures cause notable band bending to achieve favorable band edge positions relative to water redox potentials, which makes the Janus structures as efficient heterojunction photocatalysts. Electrons and holes are spatially separated on different surfaces of the Janus structure due to the internal electric field, which effectively inhibits the recombination of excitons and ensures photocatalytic activity with high efficiency.

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1. Introduction Photocatalytic water splitting into hydrogen and oxygen attracts intense interest over the past years, because it is potentially a green and renewable method to address the shortage of fuel resources and the global greenhouse effect without producing pollution. Compared to commercialization of silicon-based solar cells, photochemical cells experienced a tardy process due to the low conversion efficiency of water splitting, despite their early proposal for TiO2 in 1972.1 Besides TiO2, other metal oxides,2-3 metal sulfides,4-6 and metal nitrides7-8 have also been reported as the heterogeneous photocatalysts for water splitting. However, the low conversion efficiency is still a challenge for photochemical cells. In recent years, a diversity of two-dimensional (2D) nanomaterials with distinctive physical and chemical properties is coming to the forefront after the successful synthesis of one-atom-thick graphene in 2004.9 In particular, the 2D materials with a high specific surface area and a high ratio of exposed surface atoms are potentially outstanding candidates for surface catalysis.10-13 Commonly, a high-performance photocatalyst for water splitting should possess the following characteristics: appropriate band edge energy positions sandwiching water redox potentials (

and

), a

suitable band gap to harvest usable visible photons (larger than 1.23 eV), and good chemical stability. However, almost half of solar energy in ultraviolet and infrared area is not utilized. Recently, Yang's group have designed a surface-functionalized boron-nitride (BN) bilayer (F-BNBN-H) with an intrinsic dipole to produce hydrogen from dissociated water under near-infrared light, which relieves the restriction on the band gap requirement of photocatalysts.14 Afterwards, our group have theoretically proposed that graphene-like IV-VI group compounds with an internal electric field can serve as efficient photocatalysts for water splitting under a broad range of solar light from ultraviolet, visible to near-infrared area dependent on the thickness.15 Nevertheless, the above proposed materials have not been successfully synthesized yet. 2


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Recently, Lu et al. have reported a synthetic strategy to grow Janus MoSSe monolayer by fully replacing the top-layer S with Se atoms in MoS216 and the same structure was also theoretically proposed in our previous study17. The breaking of structural symmetry in the out-of-plane direction brings forth an intrinsic dipole moment, and induces a vertical piezoelectric response absent in the pristine monolayer MoS2. Further, the vertical piezoelectric polarization in the Janus MoSSe can be enhanced under the uniaxial or biaxial strains.18 Inspired by these unique properties, we propose that the Janus structures with an intrinsic dipole can be efficient photocatalysts for splitting water into hydrogen and oxygen. Previous theoretical studies have shown that most of the pristine monolayer transition metal dichalcogenides (TMD) is hard to concurrently fulfill the fundamental requirement of proper band edge positions

and a

reasonable band gap for photocatalysts.19 In contrast, the Janus TMD structures with an intrinsic dipole may nevertheless overcome this restriction. In this work, we take Mo as an example to explore the photocatalytic activities of 2D Janus molybdenum dichalcogenides (MoXY, X/Y=O, S, Se, and Te) by first-principles calculations. As expected, all the Janus MoXY structures have an intrinsic dipole moment due to the electronegativity difference of elements on both sides of the layer structure. This unique property of the Janus structures leads to efficient separation of holes and electrons into different surfaces and subsequent enhanced photocatalytic activity. Our research further highlights the promising applications of 2D TMD materials in the photocatalytic fields.

2. Computational details All spin-unrestricted calculations are implemented in theVienna Ab-initio Simulation Package (VASP) based on the projected augmented wave (PAW) method.20-22 The generalized gradient-corrected approximation with an exchange-correction formula presented by Perdew-Burke-Ernzerhof (GGA-PBE) is adopted for structural optimization.23 The cut-off energy is set to 500 eV and the convergence criteria of energy and force are set to 10-4 eV/atom and 0.02 eV/Å during self-consistent calculations. A Gamma centered grid of 3



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11×11×1 mesh for K-points is sampled in the reciprocal hexagonal lattices.24 The length of lattice along c-axis is 20 Å, which is large enough to avoid the coupling effect of two adjacent image layers. Meanwhile, a dipole correction along the c-axis is needed to obtain the converged electronic properties. The hybrid Heyd-Scuseria-Ernzerhof (HSE06) functional is adopted to get accurate electronic structures.25 The energy level of valence band maximum (VBM) is assumed to equal to the work function numerically and is calculated by the formula of where

is the electrostatic potential in vacuum, and

, is the Fermi energy

level. The energy level of conduction band minimum (CBM) is given by: , where

is the band gap calculated by the HSE06 functional.

When the material absorbs light of energy larger than its optical gap, electrons are excited from VBM to CBM and excitons, excited electron-hole pairs, are formed. Such excitons will subsequently dissociate into free electrons and holes through the supply of additional energy. We define this energy as the exciton binding energy. When studying the photocatalysis, the exciton binding energy ( to evaluate the photocatalytic performance. A low

) is an important descriptor

indicates a fast dissociation of

excitons into free electrons and holes, which can facilitate the photocatalytic process. Dvorak et al. have demonstrated that the product of localized state number around VBM (

) and CBM (

) has a proportional relation with the strength of

electron-hole interaction26: .

The calculation of

and

is defined as

4


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Here, the

is set to 2.5 eV.

3. Results and Discussions

Fig. 1. (a) Top view and side view of 2D Janus MoXY (X/Y= O, S, Se, and Te, X≠Y) structures; (b) the local density of states (LDOS) for Janus MoSSe. Molybdenum dichalcogenides (MoX2, X=O, S, Se, and Te) possess the structures composing of a Mo layer sandwiched by the same chalcogen atoms (O, S, Se, and Te) on the both sides. Our calculated lattice parameters for MoO2, MoS2, MoSe2, and MoTe2 are 2.83 Å, 3.19 Å, 3.32 Å, and 3.55 Å, respectively, which increase with the increasing atomic radius of chalcogen atoms. The corresponding Mo-X bond lengths are 2.05 Å, 2.42 Å, 2.54 Å, and 2.73 Å, respectively. Based on a MoX2 monolayer, the Janus MoXY monolayer is obtained by fully replacing the top-layer X with Y atoms (X/Y=O, S, Se, and Te, and

), as shown in Fig. 1 (a). The difference in atomic

size and electronegativity of X and Y breaks the pristine TMD structural symmetry and gives rise to inequivalent M-X and M-Y bond lengths. After replacing one X layer by a Y layer, six types of Janus MoXY structures are formed, and the optimized lattice parameters are shown in Table 1. It is found that the optimized lattice parameters of Janus MoXY structures are very close to one half of the lattice parameters of MoX2 plus MoY2. Meanwhile, the bond lengths of Mo-X and Mo-Y in Janus MoXY structures are almost the same as those in pristine MoX2 and MoY2 structures. Taking 5



MoSSe as an example, the bond lengths of Mo-S and Mo-Se are 2.42 Å and 2.54 Å, respectively, which are exactly the same as those in MoS2 and MoSe2. Fig. 1 (b) shows the local density of states (LDOS) for Janus MoSSe. Both S and Se atoms hybrid almost equally with Mo atom, which is consistent with the experimental results.16

Fig. 2. Band structures of (a) MoX2 (X=O, S, Se, and Te) and (b) Janus MoXY (X/Y=O, S, Se, and Te, and

) calculated by HSE06 functional.

Based on the optimal Janus MoXY structures, we further analyzed the electronic properties and compared them to the pristine MX2 structures. Fig. 2(a) shows the band structures for MoX2 (X=O, S, Se and Te), revealing that MoO2 is an indirect semiconductor with a band gap of 1.48 eV, while MoS2, MoSe2, and MoTe2 are all 6


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direct semiconductors with a band gap of 2.09 eV, 1.86 eV and 1.49 eV, respectively, under the HSE06 calculations. The Janus MoXY structures retain the semiconductive properties, with band gaps close to the average value of those in MoX2 and MoY2 except for MoOSe and MoOTe, which are consistent with the experimental results.16 For example, the band gap of MoSSe is 1.98 eV between the values of MoS2 (2.09 eV) and MoSe2 (1.86 eV), while the band gap of MoSeTe is 1.70 eV between the values of MoSe2 (1.86 eV) and MoTe2 (1.49 eV). The band gaps of MoOSe and MoOTe are 1.32 eV and 0.66 eV, respectively, which are smaller than that of the corresponding MoX2. The spin-orbital coupling effect was also considered and compared in Table S1 of Supporting Information, which has little effects on the band gaps of pristine MoX2 and Janus MoXY structures. Among all the Janus MoXY structures in Fig. 2, only MoSSe and MoSeTe are direct-band gap semiconductors. Importantly, due to asymmetric out-of-plane structural configurations, more electrons are transferred from Mo atoms to the X atoms with a larger electronegativity than to the Y atoms with a smaller electronegativity. This charge redistribution induces an intrinsic dipole in the out-of-plane direction leading to an internal electric field pointing from the Y atomic layer to X atomic layer.

Fig. 3. The electrostatic potential difference of monolayer (a) MoS2 and (b) MoSSe and (c) the relationship between electrostatic potential difference and electronegativity difference on the both sides in 2D Janus TMD materials.

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For the pristine MoX2 structures, there is no electrostatic potential difference between two X atomic layers because of the same element on both sides. However, for the Janus MoXY structures, a surface potential difference is observed. We have calculated the electrostatic potential differences and the intrinsic dipole moments for all the Janus MoXY structures, which are summarized in Table 1 and illustrated in Fig. 3. Due to the different elements on both sides of MoXY structures, the Y atomic layer with a smaller electronegativity value is positively charged and the X atomic layer with a larger electronegativity is negatively charged, generating an intrinsic dipole in MoXY structures. For example, MoSSe possesses an intrinsic dipole of 0.19 Debye, and an internal electric field in the direction pointing from Se surface to S surface and induces an electrostatic potential difference (∆Φ) of 0.77 eV between the X and Y surface. Interestingly, the potential difference

has a nearly linear relationship with

the electronegativity difference between X and Y atomic layers (Fig. 3c). MoOTe possesses the largest surface potential differences of 3.26 eV with a dipole moment of 0.77 Debye, due to the largest electronegativity difference between the O (3.50) and Te atom (2.01). The potential differences in the Janus MoXY structures are large enough (0.77-3.26 eV) to cause the electronic band bending. Under the absorption of water molecules, the electrostatic potential differences between both sides of the Janus MoXY structures will be further enhanced, for example, from 0.77 eV to 2.98 eV for MoSSe. To verify the photocatalytic activity and favorable band edge positions of MoX2 and Janus MoXY materials for water splitting, the energy alignments of VBM and CBM are calculated and shown in Fig. 4 and Table 1. Previous theoretical studies have demonstrated that MoS2 and MoSe2 are potential photocatalysts with suitable band edge positions for dissociating water,19 which is consistent with our results. However, for MoO2, the energy level of CBM is relatively low for water reduction, thus unable to drive electrons for water reduction, while for MoTe2, the energy level of VBM is too high compared with the oxidation potential of water, thus cannot provide enough power for holes to drive water oxidation. Therefore, the Janus MoXY structures with 8


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different surface potentials are predicted to have improved photocatalytic activity.

Table 1. The optimized lattice constant (a), band gaps (Eg-HSE06), intrinsic dipole moments (µ), electrostatic potential difference (

), energy differenceΔE1 (CBM

minus water reduction potential) andΔE2 (water oxidation potential minus VBM), and exciton binding energy (Eb) for pristine MoX2 and Janus MoXY. ∆E1 (eV)

∆E2 (eV)

Eb (eV)

0

-1.46

1.71

454.10

0

0

0.34

0.52

872.09

1.86

0

0

0.54

0.09

1022.80

3.55

1.49

0

0

0.79

-0.53

1129.30

MoOS

3.01

1.55

0.48

2.30

1.01

1.61

644.41

MoOSe

3.07

1.32

0.63

2.93

1.49

1.53

594.94

MoOTe

3.20

0.66

0.77

3.26

1.85

0.84

590.24

MoSSe

3.25

1.98

0.19

0.77

0.90

0.62

925.82

MoSTe

3.36

1.53

0.43

1.63

1.54

0.39

892.47

MoSeTe

3.43

1.70

0.26

0.97

1.29

0.15

1095.70

a (Å)

Eg-HSE06 (eV)

µ (Debye)

MoO2

2.83

1.48

0

MoS2

3.19

2.09

MoSe2

3.32

MoTe2

(eV)

First, we discuss the Janus MoOY structures. Due to electrostatic potential differences between O and Y (X=S, Se and Te) layers, the O layer possesses a more negative CBM position than the Y layer, while the Y layer possesses a more positive VBM position than the O layer, leading to electrons transfer from O to Y surface and holes from Y to O surface. So, the electrons and holes diffuse in the opposite directions and result in an efficient separation. This kind of band alignments is commonly found in semiconductor junctions. Here, we define the Janus MoXY 9



structures as the heterojunction photocatalyst. The CBM of Y layer and VBM of O layer are all located outside the redox potentials of water. Fig. 4a shows that compared to MoO2, the CBM levels in the Y layer for MoOS, MoOSe and MoOTe shift to -3.43, -2.95 and -2.59 eV, respectively, which lie above the reduction potential of H2O (-4.44 eV), so that photogenerated electrons can readily transfer from the conduction band to H2O to produce H2. On the other hand, the VBM levels of the O layer (-7.28 eV for MoOS, -7.20 eV for MoOSe and -6.57 eV for MoOTe) lie below the oxidation potential of H2O (-5.67 eV), so that the photoexcited holes in the valance band can readily flow to H2O to oxidize it into O2. Significantly, although the band gap of MoOTe of 0.66 eV is in the range of near-infrared light, the band edge positions of MoOTe are favorable relative to the redox potentials for water splitting due to a large surface potential differences between the O and Te layers. This condition breaks the rules of band gap requirement for traditional photocatalysts and broadens the spectral range of light absorption from visible to near-infrared. Similar results are also found in the Janus structures of MoSTe and MoSeTe. Invariably, the performances of MoSe2 and MoTe2 for water oxidation are relatively poor, because their VBM locations are close to (MoSe2) or higher than (MoTe2) the oxidation potential of water (Fig. 4b). For Janus MoSTe and MoSeTe structures, the VBM levels are improved and lowered to -6.06 eV and -5.82 eV, respectively, which are all lower than the oxidation potential of water. Therefore, the intrinsic dipole of the Janus structures may indeed influence the energy band alignments and simultaneously improve the photocatalytic performance for water splitting.

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Fig. 4. Band edge positions of pristine MoX2 (X=O, S, Se, and Te) and Janus MoXY (X/Y=O, S, Se, and Te, and

) structures compared with the redox potentials of

water based on HSE06 functional. The redox potentials of water are denoted as the black dashed line.

Besides proper band edge positions, exciton dissociation is also crucial for the photocatalytic process. Efficient separation of electrons and holes is an important factor for a good photocatalyst. The exciton binding energy, Eb, or the energy by which an exciton (electron-hole pair) is stabilized relative to free charge carriers due to their mutual electrostatic attraction, governs how easily and fast an exciton pair is dissociated into free electron and hole. Eb has to be as low as possible to facilitate the separation of electrons and holes. The Eb for the pristine MoX2 structures increases from 454 meV for MoO2 to 1129 meV for MoTe2. The exciton binding energy of MoS2 11



is 872 meV, which is consistent with the previous report of 897 meV.27 This value is obviously larger than that of the other catalytic crystals, like BiOBr (~280 meV).28 The exciton binding energies for Jansu MoXY structures are also calculated and summarized in Table 1. Our calculations show that the larger electronegativity difference between X and Y in the Janus MoXY structures, the smaller is the exciton binding energy. The Eb for MoOY (Y=S, Se and Te) are slightly larger than that of MoO2, but greatly smaller than that for MoY2 (Y=S, Se and Te). The MoOTe possesses the smallest Eb of 590 meV, which is almost one half of that in MoTe2. This can be attributed by the internal electric field induced by the intrinsic dipole in the Janus structures. The larger the internal electric field, the easier the exciton pairs dissociate. The internal electric field can serve as an auxiliary booster to break up an exciton pair, and cause the free electron and hole to diffuse in the opposite directions to ensure the photocatalytic reaction with high efficiency. Next, we investigate the photocatalytic properties of the multilayer MoXY, taking MoSSe as a representative. Because the energetic and electronic properties are not sensitive to the different stacking configurations, here we only consider the AB stacking to study the multilayer MoSSe from bilayer to five-layer. Similar to pristine MoS2, the inter-layer interactions in multilayer MoSSe are via Van der Waals forces, and the intra-layer chemical bonding is the same as that in the monolayer. Table 2 shows that the band gap of the multilayer MoSSe decreases from 1.53 eV for bilayer to 0.75 eV for trilayer, 0.37 eV for four-layer, and 0.16 eV for five-layer, respectively. However, the reducing band gaps do not influence the photocatalytic activities for water dissociation, as the layer increases, the intrinsic dipoles are increased to 0.73 Debye for four-layer and 1.10 Debye for five-layer MoSSe. Accordingly, there is a significant increase in the surface potential difference from 0.77 eV for monolayer MoSSe to 1.45 eV for bilayer and to 8.68 eV for five-layer MoSSe. From bilayer to five-layer MoSSe, all of them possess a surface potential difference larger than 1.23 eV (see Table 2), so that the fundamental limit of band gaps large than the energy of 1.23 eV for water splitting is relieved. The band energy alignments of VBM and CBM for 12


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the multilayer MoSSe with respect to the redox potentials of water are plotted in Fig. 5. It shows that the CBM of Se surface in the top layer of bilayer MoSSe lies at 1.26 eV above the water reduction potential, and the VBM of S surface in the bottom layer locates at 0.49 eV below the water oxidation potential. Further, the band edge positions for all multilayer MoSSe locate outside the water redox potentials with suitable energy differences for water splitting Therefore, our results demonstrate that multilayer MoSSe with smaller band gaps yet favorable redox potentials can serve as the photocatalyst, expanding the spectral range of solar light absorption from visible-light to infrared light for water splitting.

Table 2. The band gaps (Eg), intrinsic dipole moments (µ), electrostatic potential difference (

), energy difference ∆E1 (∆E2) between CBM (VBM) and water

reduction (oxidation) potential for multilayer Janus MoSSe structures. Eg (eV)

µ (Debye)

∆E1 (eV)

∆E2 (eV)

AB

1.53

0.18

1.45

1.26

0.49

ABA

0.75

0.31

2.44

1.58

0.38

ABAB

0.37

0.73

5.75

2.57

2.32

ABABA

0.16

1.10

8.68

4.02

3.59

(eV)

Due to the internal electric field, electrons will diffuse to the layer with a high surface potential and holes will migrate along the direction of the electric field to the layer with a small surface potential. We have examined the spatial distributions of VBM and CBM for multilayer MoSSe, which are shown in Fig. 5. We found that the charge density of CBM locates on the top layer, while the charge density of VBM distributes on the bottom layer, which generates a good separation of electrons and holes. Therefore, the internal electric field can serve as an auxiliary booster to break up an exciton pair and inhibit the probability of their recombination, which can ensure the 13



photocatalytic reaction with high efficiency. This phenomenon allows the hydrogen and oxygen evolution reactions to occur on different layers, thus ensuing an efficient photocatalytic reaction.

Fig. 5. (Above) The band edge positions for multilayer MoSSe based on HSE06 functional and (Below) the corresponding charge distributions of VBM (blue color) and CBM (red color) based on PBE functional. The redox potentials of water are denoted as the black dashed lines. The isosurface value for the charge density is set to 0.01.

4. Conclusion In summary, we have theoretically predicted the Janus transition metal dichalcogenides as the efficient photocatalysts for water splitting, and particularly they can overcome the fundamental restriction of band gap energy larger than water oxidation potential of 1.23 eV. Compared to the pristine MoX2 (X=O, S, Se and Te) materials, Janus MoXY (X/Y=O, S, Se and Te,

) structures exhibit superior

photocatalytic properties with favorable band edge positions for water splitting due to the surface potential differences caused by the intrinsic dipoles. The surface potential 14 ACS Paragon Plus Environment

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differences in the Janus MoXY structures are large enough (0.77-3.26 eV) to cause sufficient electronic band bending across the junction surface to meet the requirement of water redox potentials. Furthermore, the internal electric field in the Janus structures can serve as an auxiliary booster to break up excitons into free electrons and holes and separate themon different layer surfaces. The suitable band edge positions relative to water oxidation potential together with efficient separation of electrons and holes promote monolayer and multilayer Janus MoXY structures as superior photocatalysts with expanding spectral range of solar light absorption from visible-light to infrared-light. Our studies provide valuable insight into the enhanced photocatalytic activity of Janus MoXY structures and pave the viable way for developments of Janus TMD materials as efficient photocatalysts for water splitting.

Associated Content The Supporting Information is available free of charge on the ACS Publications website. The band gaps of the pristine MoX2 and Janus MoXY monolayer structures calculated by HSE06 with and without considering the spin-orbital coupling effect.

Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by the National Key Research and Development Program of China (Grants 2017YFA0204800 and 2017YFB0701600), the National Natural Science Foundation of China (Grants 21403146, 51761145013, 21673149). This is also a project supported by the Fund for Collaborative Innovation Center of Suzhou Nano Science & Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Janus Structures of Transition Metal Dichalcogenides as the

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