MoS2-CdS heterojunction with enhanced photocatalytic activity: A first principles study

Accepted Manuscript MoS2-CdS heterojunction with enhanced photocatalytic activity: A first principles study Xiaojun Lian, Mang Niu, Yan Huang, Daojian Cheng PII:

S0022-3697(17)31200-3

DOI:

10.1016/j.jpcs.2018.04.020

Reference:

PCS 8536

To appear in:

Journal of Physics and Chemistry of Solids

Received Date: 3 July 2017 Revised Date:

13 April 2018

Accepted Date: 16 April 2018

Please cite this article as: X. Lian, M. Niu, Y. Huang, D. Cheng, MoS2-CdS heterojunction with enhanced photocatalytic activity: A first principles study, Journal of Physics and Chemistry of Solids (2018), doi: 10.1016/j.jpcs.2018.04.020. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

MoS2-CdS Heterojunction with Enhanced Photocatalytic Activity: A First Principles Study Xiaojun Liana, Mang Niub, Yan Huang a*, and Daojian Chenga* Beijing Key Laboratory of Energy Environmental Catalysis, State Key Laboratory of

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a

Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China b

College of Science, China University of Petroleum (East China), Qingdao 266580, Shandong

Province, People’s Republic of China

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Abstract

investigated

based

on

density

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In this study, the electronic properties of the MoS2-CdS heterojunction were functional

theory

calculations

within

the

Perdew–Burke–Ernzerhof and Heyd–Scuseria–Ernzerhof approximations. MoS2 serves as the photosensitizer in the MoS2-CdS heterojunction. Due to the proper band gap (2.11 eV) of the MoS2-CdS heterojunction even under irradiation by visible light, the electron in the MoS2-CdS heterojunction can readily transfer from the valence

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band maximum of CdS to the conduction band minimum of MoS2 and generate electron–hole pairs. A built-in potential of 0.45 eV can be generated in the heterojunction, so the electron–hole pairs may be separated efficiently in the heterojunction. In addition, the intensity of the optical absorption by the MoS2-CdS

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heterojunction is stronger than that by the monolayer CdS, thereby indicating that the

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MoS2-CdS heterojunction could be used as a hybrid photocatalyst in the future.

Keywords: density functional theory, heterojunction, Heyd–Scuseria–Ernzerhof, MoS2-CdS, photocatalytic activity

*

Authors

to

whom

correspondence

should

be

[email protected] and [email protected]

1

addressed.

Electronic

addresses:

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1. Introduction Efficient, earth-abundant, and sustainable photocatalysts are essential for the utilization of solar power [1]. Hybrid photocatalysts usually include two components, such as TiO2-CdS [2-5], graphene-MoS2 [6], and MoS2-CdS [7] heterojunctions.

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These heterojunctions can enhance the separation of photogenerated holes and electrons [8], so the absorption edge of the light absorption range can be extended toward the visible light region [9]. In addition, heterojunctions can effectively inhibit the recombination of photogenerated electrons and holes in photocatalytic reactions

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[10] because electron–hole pairs are readily generated and separated under irradiation by visible light. Thus, heterojunctions have been widely applied in the fields of solar

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cells, bipolar transistors, and strain sensors [11], [12].

Molybdenum disulfide (MoS2) is a semiconductor and a potential photocatalyst because it has a two-dimensional planar structure and a narrow band gap energy of 1.9 eV, which is aligned well with the solar spectrum [4], [13]. Thus, it can convert visible light into photogenerated charges and transfer these photogenerated charges freely within its microscopic monolayer. However, the photocatalytic activity of pure

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MoS2 is inert because its band gap is excessively narrow. Alternatively, MoS2 could be combined with other wide band gap semiconductor to form heterojunctions as hybrid photocatalysts. For example, when MoS2 is combined with some other catalysts, such

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as CdS, TiO2, and C3N4, the composites formed may exhibit substantial photocatalytic activities in the hydrogen evolution reaction [14-20]. Recently, it was reported that the

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MoS2-CdS heterojunction is an efficient hybrid photocatalyst in the photocatalytic H2 production reaction [8]. The MoS2-CdS heterojunction exhibits photochemical stability during the photocatalytic hydrogen evolution reaction [21]. Studies have shown that electron–hole pairs in the MoS2-CdS heterojunction can be excited by visible light [22]. This charge separation reduces the possibility of recombination and increases the quantum efficiency of the MoS2-CdS heterojunction. However, there have been no theoretical studies of the MoS2-CdS heterojunction and the mechanism of photocatalytic enhancement in the MoS2-CdS heterojunction is still not fully understood. Therefore, it is necessary to study the electronic properties and the mechanism of photocatalysis enhancement for the MoS2-CdS heterojunction. 2

ACCEPTED MANUSCRIPT In this study, the electronic properties of monolayer CdS, MoS2, and MoS2-CdS heterojunctions were investigated based on density functional theory (DFT) calculations

within

the

Perdew–Burke–Ernzerhof

(PBE)

and

Heyd–Scuseria–Ernzerhof (HSE) approximations. The electronic and optical absorption properties of these systems were determined. The effects of CdS on the

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band gap and optical absorption in MoS2 were investigated in detail to explain the high photocatalytic activity of the MoS2-CdS heterojunction. Finally, we considered the mechanism responsible for photocatalytic enhancement by the MoS2-CdS

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heterojunction.

2. Computational details

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We conducted DFT calculations within the PBE and HSE approximations using the Vienna Ab-initio Simulation Package (VASP) software package, which is based on the projected augmented plane wave method. The PBE [23] approximation of the generalized gradient approximation was used to describe the exchange and correlation potential. The plane wave cut-off was set as 500 eV. Numerous computations were performed to guarantee the convergence of the k-point mesh [24] with a suitable

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number for this system. Structural optimization was conducted until the force on each ion in the system was reduced to less than 0.01 eV/Å and the optimized structures were used to calculate the electronic structures. The convergences of the k-point mesh

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and the cut-off energy were set to within 0.001 eV. In order to obtain accurate electronic structures, we performed hybrid density

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function calculations within the HSE06 approximation [25]. The contribution of exchange included the short-range part and long-range part in the HSE06 hybrid function. The short-range part of PBE exchange was weighted by 25 percent Hartree–Fock exchange. The equation for the exchange relationship in HSE06 is as follows:

HSE EXC =

1 HF,SR 3 EX （µ）+ EXPBE,SR（µ）+EXPBE,LR（µ）+ECPBE , 4 4

(1)

where LR represents the long-range part and SR indicates the short-range part of the exchange interaction, and µ is a parameter that describes the range of the Coulomb kernel, i.e., μ = 0.2 Å–1 in this study. Before exploring the properties of the MoS2-CdS 3

ACCEPTED MANUSCRIPT heterojunction, we studied the structural properties of the MoS2 monolayer. The MoS2 monolayer was selected as a photosensitizer because it is suitable for solar energy conversion due to its optimum band gap [26]. The crystal phase of CdS includes cubic sphalerite and hexagonal wurtzite phases. In this study, we considered the cubic sphalerite phase [19, 20]. The optimized lattice

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parameters for the MoS2 monolayer, CdS monolayer, and MoS2-CdS heterojunction are summarized in Table 1. In the present study, the heterojunction was built by placing the CdS monolayer on the MoS2 monolayer, as shown in Figure 1(a–c), with

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36 atoms (S: 22; Cd: 6; Mo: 8). The MoS2 monolayer and CdS monolayer were chosen from the (0 1 0) face and (1 0 0) face, respectively. In addition, we set a vacuum region of 20 Å to guarantee the decoupling with neighboring systems. The

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Monkhorst–Pack k-point mesh [24] was set as 1 × 3 × 1 for the original structure of MoS2 (0 1 0). Both the atomic positions and structural parameters were optimized until the strength on each ion was within 0.01 eV/Å. The monolayer MoS2 sheet selected as the substrate and the unit cell of 1 × 3 MoS2 (5.832 Å × 12.372 Å) matched well with that of a 4 × 1 monolayer CdS.

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3. Results and discussion

According to the calculations, the most stable structures and surface lattice parameters for the heterojunction were obtained by optimizing the initial structures

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based on the PBE approximation. We then calculated the band gap of the MoS2-CdS heterojunction based on DFT calculations within both the PBE and HSE

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approximations. The results indicated that we could obtain a more accurate band gap that was closer to the experimental value [4, 22, 27] by using the HSE approximation rather than the PBE approximation. The binding energy Eb was calculated using the following equation:

E = (E

where E

, E

+E

, and E

−E

),

(2)

are the energies of MoS2, CdS, and MoS2-CdS,

respectively. Eb was positive for MoS2-CdS, i.e., 0.134 eV, and thus the structure of MoS2-CdS was stable. As shown in Figure 1, the distance between the MoS2 slab and CdS slab was 3.180 Å after relaxation. In addition, we studied the photocatalytic properties of the MoS2-CdS [28] 4

ACCEPTED MANUSCRIPT heterojunction in the range of visible light (1.7–3.1 eV). The band gap of the MoS2-CdS heterojunction was about 2.11 eV, which was narrower than that of the MoS2 monolayer, as shown in Figure 2. In addition, the valence band was located on the M point for the MoS2-CdS heterojunction. The calculated results showed that the MoS2-CdS heterojunction is an indirect band gap semiconductor material with a band

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gap of 2.11 eV. In addition, Table 2 shows the different band gaps obtained with various methods. The built-in potential is defined as the conduction band minimum (CBM) offset between the CdS and MoS2 surfaces, and we used the CBM of CdS

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subtracted from the CBM of MoS2, which was around 0.45 eV. Therefore, the photogenerated electron could be removed from the valence band of CdS slab to the conduction band of MoS2, and the conduction band energy of CdS and MoS2 was

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completely hybridized. The photogenerated electrons can be transferred after the separation of electrons and holes. Therefore, the CdS monolayer could significantly change the photocatalytic activity of the MoS2 material in the visible region. To understand the indirect-to-direct band gap transition, we studied the influence of strain on the band structure of the heterojunction according to the band gap change

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in the MoS2-CdS heterojunction. Figure 3 indicates that as the lattice parameter of the heterojunction increased slightly from 5.15 Å to 6.04 Å, the band gap changed from an indirect transition to a direct transition. The indirect-to-direct band gap transition of the heterojunction appeared when the lattice value was larger than 5.54 Å (tensile

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stress). In addition, the band gap of the heterojunction decreased in a linear manner when the lattice parameter was in the range of 5.54 Å to 6.04 Å. In the case of tensile

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stress, the tensile stress of the lattice increased with the band gap of the heterojunction to 2.11 eV (direct gap at the C point), as shown in Figure 3. Therefore, the indirect-to-direct band gap transition and the increased band gap (2.11 eV) of the heterojunction can be attributed to the tensile stress of the heterojunction (8.02% and 7.72% along the x and y directions, respectively) in the MoS2-CdS. We analyzed the partial density of states (PDOS) and total density of states (TDOS) [29] for the MoS2-CdS heterojunction by using the HSE approximation, as shown in Figure 4. Figures 4(a) and 4(b) indicate that the valence band maximum (VBM) of the MoS2 monolayer mainly comprised the S 2p state mixed with some of 5

ACCEPTED MANUSCRIPT the Mo 3d state, whereas the CBM mainly comprised the Mo 3d state mixed with a small amount of the S 2p state. Thus, we may conclude that the electron could be diverted from the S 2p orbital to the Mo 3d orbital. The TDOS and PDOS for CdS are shown in Figures 4(c) and 4(d), respectively. The VBM of CdS comprised the S 2p state, with the Cd 3d state in the CBM. Figures 4(e) and 4(f) show the TDOS and

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PDOS for the MoS2-CdS heterojunction, respectively. We observed that the electronic properties of the MoS2 monolayer were effectively regulated by the CdS. The VBM of the MoS2-CdS heterojunction comprised the S 2p state, with the Mo 3d state in the

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CBM. The electrons could be diverted from the MoS2 S 2p state and the CdS S 2p state into the Mo 3d state. The band gap of the MoS2-CdS heterojunction was about 2.11 eV, which was narrower than that of the CdS monolayer (around 2.42 eV).

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Therefore, for the semiconducting MoS2-CdS heterojunction, the electron could be diverted from the valence band to the conduction band under visible light irradiation, thereby leaving a hole in the valence band. The photogenerated electron could be infused into the conduction band and an electron could be removed from the valence band of the CdS slab to the conduction band of MoS2. Thus, the oxidation reaction

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and redox reaction could occur in the MoS2 and CdS sheets, respectively. In conclusion, the MoS2-CdS heterojunction could greatly improve the photocatalytic activity due to the proper band gap of the MoS2-CdS heterojunction and because the electrons and holes may be separated and transferred.

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Moreover, we calculated the three-dimensional differential charge density of the MoS2-CdS heterojunction. We subtracted the electronic charge of the monolayer

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MoS2 from the MoS2-CdS heterojunction to understand the charge separation and transfer process clearly, as shown in Figure 5, where the cyan and yellow regions represent the charge depletion and accumulation in the space, respectively. The charge redistribution occurred mainly at the interface of the MoS2-CdS heterojunction. The charged interface of the MoS2-CdS heterojunction resembled a p-n junction. The electron and hole could be separated at the p-n junction. Figure 5 indicates that numerous charge transfers could occur. Thus, the charged interface region of the MoS2-CdS heterojunction could lead to the valid separation of photogenerated electrons and holes. In addition, the analysis indicated that the photogenerated 6

ACCEPTED MANUSCRIPT electron–hole pairs could be separated via the great charge transfer on the interface, thereby improving the photocatalytic activity of the MoS2-CdS heterojunction under visible light. We also calculated the optical absorption of the MoS2-CdS heterojunction in order to study its optical absorption properties. In particular, we calculated the complex

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dielectric function: ε = ε1 + iε2, after optimizing the electronic structure. The real part (ε1) and imaginary part (ε2) were calculated based on the PBE approximation. The equation for the corresponding absorption spectrum is as follows [30]: [

(

(

]

(

,

(3)

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I(ω = 2ω(

where I(ω is the optical absorption coefficient and ω is the angular frequency. The

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ultraviolet–visible absorption spectra for the MoS2-CdS heterojunction are shown in Figure 6. In general, the optical absorption property can reflect the photocatalytic activity of a photocatalyst. The optical absorption spectra shown in Fig. 6 demonstrate that the MoS2-CdS heterojunction exhibited enhanced visible light absorption compared with the MoS2 and CdS monolayers. Thus, the MoS2-CdS heterojunction could absorb more photons compared with the MoS2 monolayer under irradiation by

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visible light. We conclude that the MoS2-CdS heterojunction is associated with the effective absorption of visible light, photo-excitation, electron injection, and the charge separation process. Therefore, our results suggest that the MoS2-CdS

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heterojunction is a promising hybrid photocatalyst under visible light.

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4. Conclusion

In this study, we have conducted DFT calculations within the PBE and HSE

approximations to explore the electronic properties of the MoS2-CdS heterojunction. We found that the MoS2-CdS heterojunction had a proper band gap of 2.11 eV. The electron–hole pair could be separated efficiently in the heterojunction because of the built-in potential of 0.45 eV. The heterojunction could facilitate the electron transfer from the VBM to the CBM under visible light. The MoS2-CdS heterojunction exhibited stronger optical absorption than the CdS monolayer. We consider that the MoS2-CdS heterojunction could be used as a hybrid photocatalyst in the future.

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Acknowledgments This study was supported by the National Natural Science Foundation of China (21576008, 21603275, 91634116, 91334203), the Fundamental Research Funds for the Central Universities (PYCC1705), and PetroChina Innovation Foundation

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(2016D-5007-0505).

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Table 1 Lattice parameters of heterojunctions. Structure

Lattice parameters (Å)

Dz (Å)

b

MoS2 (1 × 1)

5.182

3.127

-

CdS (1 × 1) CdS (1 × 4) MoS2-CdS

5.832 5.830 5.830

3.093 12.37 12.37

3.180

Eb (eV)

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a

0.134

HSE06

1.25 2.17 1.02

1.78 2.41 2.11

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MoS2 CdS MoS2-CdS

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Ref.

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Band gap（eV） ）

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Table 2 Different band gaps obtained using various methods.

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1.8[27] 2.42[22] -

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Figure captions Figure 1 Structures of the MoS2 monolayer (a), CdS monolayer (b), and MoS2-CdS (c) heterojunction.

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Figure 2 Band structure of the MoS2-CdS heterojunction along the high symmetry path of the Brillouin zone. The blue, pink, and green lines represent the bands for MoS2, CdS, and

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the MoS2-CdS heterojunction, respectively. The gray line represents the Fermi level.

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Figure 3 Strain dependence of the band gap structure of the MoS2-CdS heterojunction.

Figure 4 TDOS and corresponding PDOS for MoS2, CdS, and MoS2-CdS heterojunctions obtained using the HSE approximation.

Figure 5 Three-dimensional charge density difference in the MoS2-CdS heterojunction (cyan

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region: charge depletion; yellow regions: charge accumulation).

Figure 6 Optical absorption curves obtained for the MoS2-CdS heterojunction, MoS2

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monolayer, and CdS monolayer.

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Figure 1 Structures of the MoS2 monolayer (a), CdS monolayer (b), and MoS2-CdS (c) heterojunction.

Figure 2 Band structure of the MoS2-CdS heterojunction along the high symmetry path of the Brillouin zone. The blue, pink, and green lines represent the bands for MoS2, CdS, and the MoS2-CdS heterojunction, respectively. The gray line represents the Fermi level. 12

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Figure 3 Strain dependence of the band gap structure of the MoS2-CdS heterojunction.

Figure 4 TDOS and corresponding PDOS for MoS2, CdS, and MoS2-CdS heterojunctions obtained using the HSE approximation.

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Figure 5 Three-dimensional charge density difference in the MoS2-CdS heterojunction (cyan

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region: charge depletion; yellow regions: charge accumulation).

Figure 6 Optical absorption curves obtained for the MoS2-CdS heterojunction, MoS2 monolayer, and CdS monolayer.

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Research highlights

• Properties of MoS2-CdS heterojunction investigated from first

•

MoS2-CdS

heterojunction

exhibits

enhanced

visible

light

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photocatalytic activity.

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principles.

• Proper energy gap of the MoS2-CdS heterojunction is 2.11 eV.

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• Built-in potential of 0.45 eV could be generated in the composite.

• Intensity of optical absorption is stronger than that in the CdS

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monolayer.