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graphene composites: A density functional theory study
Accepted Manuscript Influence of interface structures on the properties of Molybdenum Disulfide/Graphene composites: a density functional theory study Wenyan Zan, Wei Geng, Huanxiang Liu, Xiaojun Yao PII:
S0925-8388(15)01459-0
DOI:
10.1016/j.jallcom.2015.05.149
Reference:
JALCOM 34271
To appear in:
Journal of Alloys and Compounds
Received Date: 2 February 2015 Revised Date:
16 May 2015
Accepted Date: 18 May 2015
Please cite this article as: W. Zan, W. Geng, H. Liu, X. Yao, Influence of interface structures on the properties of Molybdenum Disulfide/Graphene composites: a density functional theory study, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.05.149. 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.
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Influence of interface structures on the properties of Molybdenum Disulfide/Graphene composites: a density functional theory study
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Wenyan Zan1, Wei Geng1, Huanxiang Liu2, Xiaojun Yao1*
State Key Laboratory of Applied Organic Chemistry, Department of Chemistry, Lanzhou
School of Pharmacy, Lanzhou University, Lanzhou 730000, China
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University, Lanzhou 730000, China
Abstract
Density functional theory calculations were performed to study the photocatalytic
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properties of molybdenum disulfide/graphene composites by analyzing the structure, electronic properties and optical properties of molybdenum disulfide/graphene composites. Three typical structures of molybdenum disulfide considered in our work
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include pristine molybdenum disulfide and molybdenum disulfide with mononiobium
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doping. They were then composited with graphene, N-doped graphene and graphene with epoxy, respectively. The characteristics of these composites (MoS2/Graphene, MoS2/N-G, MoS2/O-G and Nb-MoS2/N-G) including binding energies, charge transfer, projected density of states, electron density and optical properties were calculated and analyzed. The binding energies of between MoS2 and
*
Corresponding author. Tel.: +86-931-891-2578; Fax: +86-931-891-2582. E-mail address: [email protected] 1
ACCEPTED MANUSCRIPT graphene were related to the extent of charge transfer. The data of projected density of states, band structures and optical properties gave an explanation of the mechanism for significant photocatalytic activity of MoS2/ N-doped graphene and Nb-doped
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MoS2/N-doped graphene composites.
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Key words: Molybdenum disulfide; Graphene; DFT; Photocatalytic properties
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Introduction
The p–n junctions have been widely used as the fundamental building blocks for the creation of semiconductor, such as photocatalysts[1-3].
P-type ultrathin MoS2 [4,
5] has received much attention because of its narrow band gap and electrostatic
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integrity. Graphene sheet with nitrogen atom doping can form an n-type semiconductor [6-8]. Because MoS2 and graphene layers shared the same hexagonal crystalline structure with the small lattice mismatch, MoS2/ graphene composites were
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of high quality and stable. At present, the growth of MoS2 on graphene has been
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reported [9-11]. Meng et al. have demonstrated that MoS2 /N-doped graphene showed significant photocatalytic activity toward the hydrogen evolution reaction in the wavelength range from the ultraviolet light through the near-infrared light. Maitra et al. also showed that MoS2 /N-doped graphene had outstanding performance in the production of H2 under visible-light illumination.[2, 3] Although the composite of molybdenum disulfide and N-doped graphene layers has been proved to be a kind of efficient photocatalyst in the experiment, some 2
ACCEPTED MANUSCRIPT key factors for the excellent performance including the interface and electronic properties of the composite remained poorly understood at atomic details. The interface features including stacking order, the positions of point defects and dopant
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atoms and whether chemical bonds were formed would greatly affect the optical and electronic properties of the composite. The study of the behavior of the electrons in the composite can essentially reveal the efficient photocatalystic
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mechanism of the composite. It’s worth mentioning that DFT calculations are
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powerful tools to explain and improve the experimentally observed photocatalytic activity of some composites [12-14].
Herein, we performed DFT calculations to characterize the interface between molybdenum disulfide and graphene surfaces. The aim of the present study was to
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establish a basic physical picture of these composites by interconnecting molybdenum disulfide with graphene layers and to study the interface structure and electronic properties. The material defects may emerge due to synthetic methods and conditions
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during the synthesis process of nanomaterials. The experimental and theoretical
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results have shown that doped TiO2 exhibit higher photocatalytic activity [13, 14]. We expect that by introducing Nb, N and O atoms into MoS2 and graphene layers separately, photocatalytic activity of molybdenum disulfide/ graphene composites may be enhanced. Hence, pristine molybdenum disulfide (MoS2) and molybdenum disulfide with mono niobium doping (Nb-MoS2) were composited with graphene, N-doped graphene (N-G) and graphene with epoxy (O-G). We calculated geometric structures, binding energies, charge density, charge transfer, the projected density of 3
ACCEPTED MANUSCRIPT states, band structures and optical properties of these composites (MoS2/Graphene, MoS2/O-G, MoS2/N-G and Nb-MoS2/N-G).
2. Computational details
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A 4×4 single graphene layer containing 32 carbon atoms was used to match 3×3 MoS2 monolayer. The lattice mismatch of the MoS2 and graphene layers was about 2.8 %. Vacuum space at Z axis was more than 15 Å to avoid the interaction between
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the periodic images. For the MoS2 and graphene layer, atoms and cell were fully
expression:
Eb = Etotal - EG - EMoS2
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relaxed. The binding energies of the composites were calculated by using the
(3)
Where the Etotal, EG and EMoS2 denote the total energy of MoS2/ graphene system,
adsorption structure.
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isolated graphene and MoS2, respectively. A negative Eb corresponds to a stable
The DFT calculations with the generalized gradient approximation (GGA) were
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performed in Dmol3 program[15, 16]. The Perdew, Burke, Ernzerhof (PBE)[17]
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exchange-correlation functional and Double-numeric quality basis set with the polarization functions (DNP) were employed. DFT semicore pseudopotentials (DSPPs) were used to treat the core electrons. Although the spin-polarized calculations were performed, the results indicated that all the superlattice models did not exhibit magnetism at their equilibrium lattice constants. The convergence criteria for the geometric optimization and energy calculation were set as follows: (a) an energy tolerance of 1.0×105Ha, (b) a maximum force tolerance of 0.002 Ha/Å, and 4
ACCEPTED MANUSCRIPT (c) a maximum displacement tolerance of 0.005Å. Because the weak interactions were not well described by the standard PBE functional, an empirical dispersion-corrected density functional theory (DFT-D) approach proposed by
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Grimme[18, 19] was adopted. The optical property calculations were performed with CASTEP code [20] of plane wave and ultrasoft pseudopotentials[21]. The interaction of a photon with an electron in the system is described according to time-dependent
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perturbations of the ground electronic state. The photon absorption or emission gives
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rise to transitions between occupied and unoccupied states. The spectra resulting from excitations can be thought of as a joint DOS between the conduction band and the valence band [22].
3. Results and discussion
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3.1. Geometric structures and binding energies
In this work, pristine molybdenum disulfide (MoS2) and molybdenum disulfide with mono niobium doping (Nb-MoS2) were used to study the nuances of
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molybdenum disulfide. For MoS2, the calculated bulk cell lattice constant were
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a=3.16 Å, c=12.29, which agreed well with the experimental value [23, 24]. The Mo-S bond length in the pure bulk system was 2.42 Å, which was close to the experimental value (2.41Å). For Nb-MoS2, a systematic study on Nb substitutional doping of MoS2 has been reported by DFT calculations. The calculated Nb-S bond length (2.47 Å) was in agreement with the reported value of 2.45 Å [25]. For MoS2/Graphene, MoS2/N-G and MoS2 / O-G, the positions of C and O atoms were located at different initial states including top of Mo and S sites and hollow sites. 5
ACCEPTED MANUSCRIPT As to Nb-MoS2 / N-G, the initial positions of Nb atom were fixed atop C sites or at hollow sites. After full relaxation, the optimized configurations of molybdenum disulfide/ graphene composites were shown in Fig. 1. For MoS2/Graphene and
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MoS2/N-G, the interlayer spacing was 3.36 Å and 3.33 Å respectively, which was slightly shorter than interlayer distance of graphite (4.01 Å). The interaction between MoS2 and Graphene layer was relatively weak and was belong to vdW interaction.
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There was no structural distortion observed on MoS2 and Graphene layers compared
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to the isolated components. For MoS2/O-G, after full relaxation, this configuration with O atom locating at top of Mo site was the most stable. There was no obvious structural distortion observed in graphene layer. The shortest distance between O and S atom was 2.81 Å. In the case of Nb-MoS2/N-G, the structure with Nb atom lying at
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hollow site of N-G was the most favorable. The interlayer distance was the same with MoS2/N-G (3.33 Å) and there is no structural distortion. The calculated binding energies of these composites (MoS2/Graphene,
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MoS2/N-G, MoS2/O-G and Nb-MoS2/N-G) were in an order of MoS2/O-G (-2.06eV),
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Nb-MoS2/N-G (-1.65 eV), MoS2/N-G (-0.89 eV) and MoS2/Graphene (-0.60eV). For MoS2/O-G, introducing O atom disturbed the stability of graphene. O-G preferred to composited with MoS2
For Nb-MoS2/N-G, MoS2/N-G and MoS2/Graphene, the
binding energies of these composites had an opposite tendency for the order of interlayer distances. Coulomb interaction was responsible for the mainly electrostatic interaction.
For Nb-MoS2, the Nb atom acted as an acceptor impurity for the
molybdenum disulphide. The interaction between Nb-MoS2 and N-G was stronger 6
ACCEPTED MANUSCRIPT than that of MoS2/N-G. 3.2. Charge density and charge transfer In order to further investigate the electronic properties and quantitative charge
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transfer at the composite interface, we studied the electronic total charge density and Mulliken charge population. The electronic total charge density slices for the composites (MoS2/Graphene, MoS2/O-G, MoS2/N-G and Nb-MoS2/N-G) were
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displayed in Fig. 2. As shown in Fig. 2, no electron orbital overlap between
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molybdenum disulphide (Nb-MoS2 and MoS2) and graphene (Graphene, O-G and N-G) layers was observed, indicating the weak vdW interactions between molybdenum disulphide and graphene systems.
The charge transfer of molybdenum disulphide layer, corresponding to the total
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sum of the Mulliken charge populations of the Mo and S atoms (for Nb-MoS2, Nb atom was included) was calculated. A positive value meant the charge transfer was
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from molybdenum disulphide to graphene systems, in contrast, charge transfer from graphene to molybdenum disulphide systems showed a negative value.
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For all composites, molybdenum disulphide showed negative charge. In other
words, charge tended to transfer from graphene to molybdenum disulphide layers. The degree of charge transfer was in the order of Nb-MoS2/N-G, MoS2/N-G, MoS2/Graphene and MoS2/O-G. The charge transfer values of MoS2/O-G, MoS2/ Graphene, MoS2/N-G and Nb-MoS2 /N-G were -0.083, -0.13, -0.30 and -0.54e, respectively. For Nb-MoS2, the Nb doping gave rise to an acceptor impurity for molybdenum disulphide, which was apt to get more charges from N-doped graphene; 7
ACCEPTED MANUSCRIPT therefore, the Mulliken charge population was more negative. For MoS2/ Graphene and MoS2/N-G, n-doping in graphene (N-G) preferred to provide charges, and graphene with epoxy had the relatively low ability to give charges.
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3.3 Projected density of states, band structures and optical properties We calculated the projected density of states (PDOSs) and band structures for the composites to understand electronic coupling and orbital contributions, which
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may reveal the mechanism of the photocatalytic process. The PDOSs of graphene,
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O-G, N-G, MoS2, Nb-MoS2 and their composites were plotted in Fig. 3 and Fig. 4. The Fermi level (Ef) was referenced at 0 eV.
The band structures of
MoS2/Graphene, MoS2/O-G, MoS2/N-G and Nb-MoS2/N-G were displayed in Fig. 5. We can see that introducing epoxy to graphene opened the band gap of
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graphene. As was shown in Fig. 3b, N atom doping for graphene made graphene become n-type semiconductor. Fig. 3d showed the band gap of molybdenum disulfide was 1.69eV, which agreed with the reported data [26]. It is well known that
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the DFT method usually underestimates the band gap of semiconductors due to the The trends in the relative change are pretty valuable[28].
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self-interaction error[27].
In Fig. 3d, the valence band was dominated by the molybdenum 4d orbital with some contributions from the sulfur 3p orbital. The conduction band was in a similar situation. In Fig. 3e, the substitution of one Mo atom by one Nb atom introduced an electron hole within the system and thus moved the Fermi level down. The valence and conduction band were mainly contributed to by the molybdenum 4d orbital. MoS2/Graphene was a semiconductor with no gap between its valence and 8
ACCEPTED MANUSCRIPT conduction bands. The linear dispersion bands of graphene were filled into the band gap of molybdenum disulfide layers. The characteristics of graphene were preserved. MoS2/Graphene did not show any photocatalytic activities, which was also proved in
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the experiment[29]. Compared with pure single-layer molybdenum disulfide, the PDOSs of molybdenum disulfide layer from MoS2/Graphene moved left. The C 2p orbitals of graphene occupied the top of valence band and the bottom of conduction
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band of MoS2/Graphene (Fig. 4a and Fig. 5a). MoS2/O-G had a band gap of 0.57 eV.
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The top of valence band and the bottom of conduction band were comprised of C 2p orbitals. Compared with individual MoS2, the PDOSs of MoS2 in MoS2/O-G moved to low energy level due to getting charges, as shown in Fig. 4b and Fig. 5b. Fig. 4c and Fig. 5c showed that MoS2/N-G was an n-type semiconductor, and Fermi level
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moved into the conduction band. The top of valence band and the bottom of conduction band of MoS2/N-G were dominated by the C 2p orbitals of N-doped graphene. It was easy to transport the electrons of MoS2 to graphene sheet under
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visible light irradiation. The N-doped graphene sheets were electron-deficient. Then
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e− and h+ were respectively fixed on N-doped graphene and molybdenum disulfide layers. This can improve the photoelectric efficiency. The PDOSs for MoS2 had the similar situation with that of MoS2/Graphene. The typical intensity and shape of the PDOSs for MoS2 had minor change. For Nb-MoS2/N-G (Fig. 4d and Fig. 5d), there was no band gap. Electrons from Nb-MoS2 can be easily transmitted to N-doped graphene sheet. The electron transmission was very favorable because the N-doped graphene sheets were in electron-deficient conditions. Then e− and h+ were 9
ACCEPTED MANUSCRIPT respectively located on N-doped graphene and Nb-doped molybdenum disulfide layers. The recombination of e− and h+ was greatly reduced. The photoelectric efficiency was enhanced. These special electronic properties of MoS2/N-G and
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Nb-MoS2/N-G explained why these composites can be effectively used for photocatalyst. Our calculation results about MoS2/N-G agreed well with the experimental results [2, 3]. Nb-MoS2/N-G can be a potential photocatalyst candidate
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owing to the high photocatalytic efficiency and relatively high stability.
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We further calculated the optical properties of these composites. The calculated real and imaginary parts of dielectric function for both E vector perpendicular and parallel to c axis(E c and E c) for these composites were displayed in Fig. 6. The values of real part of dielectric function at zero energy of monolayer MoS2 for both
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E vector perpendicular and parallel to c axis were 5.10 and 2.89. This is agreement with the reported data [30]. Fig. 6 showed that structure peaks of these composites appeared around 2.60 and 5.10eV correspond to E vector perpendicular and parallel
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to c axis. There was an additional peak at 0.74 eV corresponding to E vector
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perpendicular to c axis in MoS2/O-G. We focused on E vector perpendicular to c axis due to the layered structures of these composites. It was found that light absorption peaked at incident photon energy of around 2.60 eV for these composites. It was also observed that MoS2/N-G and Nb-MoS2/N-G systems had stronger light absorption by analyzing the peak intensities of these composites.
4. Conclusions We have investigated the structure and electronic properties of molybdenum disulfide 10
ACCEPTED MANUSCRIPT and graphene layers by employing density functional theory calculations. Several different composites including MoS2/Graphene, MoS2/N-G, MoS2/O-G and Nb-MoS2/N-G were studied. The structures, electronic properties and optical
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properties were calculated to reveal the interface interaction and photocatalytic mechanism of these composites. The interactions of these composites were relatively weak and were belong to vdW interactions. The binding energy of Nb-MoS2/N-G was
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stronger than that of MoS2/Graphene and MoS2/N-G. Projected density of states and
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band structures of composites clearly showed that because of these advantages including the favorable electron transmission. The reduced recombination of e− and h+ and relatively strong light absorption within the visible light region made MoS2/N-G and Nb-MoS2/N-G composites become efficient photocatalysts.
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Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No. 21475054) and the Fundamental Research Funds for the Central
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Figure captions Fig. 1
The most stable configurations of (a) MoS2/Graphene, (b) MoS2/O-G, (c)
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MoS2/N-G, (d) Nb-MoS2/N-G. Left are top views and right are front views. Gray, blue, pink, cyan, yellow and red spheres stand for the C, N, O, Mo, S and Nb atoms,
Electronic total charge density slices of (a) MoS2/Graphene, (b) MoS2/O-G,
(c) MoS2/N-G, (d) Nb-MoS2 /N-G.
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Fig. 2
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respectively.
Fig. 3 The projected density of states (PDOSs) of (a)Graphene, (b) N-G, (c) O-G, (d) MoS2, (e) Nb-MoS2. Fig. 4
The projected density of states (PDOSs) of (a) MoS2/Graphene, (b)
Fig. 5
The band structures of (a) MoS2/Graphene, (b) MoS2/O-G, (c) MoS2/N-G,
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(d) Nb-MoS2 /N-G. Fig. 6
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MoS2/O-G, (c) MoS2/N-G, (d) Nb-MoS2 /N-G.
The dielectric function for (a, b) MoS2/Graphene, (c, d) MoS2/O-G, (e, f)
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MoS2/N-G, (g, h) Nb-MoS2 /N-G. Black line means real part of dielectric function, and the red line means the imaginary part.
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Highlights MoS2/Graphene, MoS2/N-doped Graphene, MoS2/ Graphene with epoxy and Nb-doped MoS2/ N-doped Graphene were studied. electronic
and
optical
properties
disulfide/graphene composites were calculated.
of
molybdenum
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The
MoS2/N-doped Graphene and Nb-doped MoS2/ N-doped Graphene
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composites had better photocatalytic performance.
S0925-8388(15)01459-0
DOI:
10.1016/j.jallcom.2015.05.149
Reference:
JALCOM 34271
To appear in:
Journal of Alloys and Compounds
Received Date: 2 February 2015 Revised Date:
16 May 2015
Accepted Date: 18 May 2015
Please cite this article as: W. Zan, W. Geng, H. Liu, X. Yao, Influence of interface structures on the properties of Molybdenum Disulfide/Graphene composites: a density functional theory study, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.05.149. 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.
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Influence of interface structures on the properties of Molybdenum Disulfide/Graphene composites: a density functional theory study
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Wenyan Zan1, Wei Geng1, Huanxiang Liu2, Xiaojun Yao1*
State Key Laboratory of Applied Organic Chemistry, Department of Chemistry, Lanzhou
School of Pharmacy, Lanzhou University, Lanzhou 730000, China
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2
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University, Lanzhou 730000, China
Abstract
Density functional theory calculations were performed to study the photocatalytic
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properties of molybdenum disulfide/graphene composites by analyzing the structure, electronic properties and optical properties of molybdenum disulfide/graphene composites. Three typical structures of molybdenum disulfide considered in our work
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include pristine molybdenum disulfide and molybdenum disulfide with mononiobium
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doping. They were then composited with graphene, N-doped graphene and graphene with epoxy, respectively. The characteristics of these composites (MoS2/Graphene, MoS2/N-G, MoS2/O-G and Nb-MoS2/N-G) including binding energies, charge transfer, projected density of states, electron density and optical properties were calculated and analyzed. The binding energies of between MoS2 and
*
Corresponding author. Tel.: +86-931-891-2578; Fax: +86-931-891-2582. E-mail address: [email protected] 1
ACCEPTED MANUSCRIPT graphene were related to the extent of charge transfer. The data of projected density of states, band structures and optical properties gave an explanation of the mechanism for significant photocatalytic activity of MoS2/ N-doped graphene and Nb-doped
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MoS2/N-doped graphene composites.
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Key words: Molybdenum disulfide; Graphene; DFT; Photocatalytic properties
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Introduction
The p–n junctions have been widely used as the fundamental building blocks for the creation of semiconductor, such as photocatalysts[1-3].
P-type ultrathin MoS2 [4,
5] has received much attention because of its narrow band gap and electrostatic
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integrity. Graphene sheet with nitrogen atom doping can form an n-type semiconductor [6-8]. Because MoS2 and graphene layers shared the same hexagonal crystalline structure with the small lattice mismatch, MoS2/ graphene composites were
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of high quality and stable. At present, the growth of MoS2 on graphene has been
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reported [9-11]. Meng et al. have demonstrated that MoS2 /N-doped graphene showed significant photocatalytic activity toward the hydrogen evolution reaction in the wavelength range from the ultraviolet light through the near-infrared light. Maitra et al. also showed that MoS2 /N-doped graphene had outstanding performance in the production of H2 under visible-light illumination.[2, 3] Although the composite of molybdenum disulfide and N-doped graphene layers has been proved to be a kind of efficient photocatalyst in the experiment, some 2
ACCEPTED MANUSCRIPT key factors for the excellent performance including the interface and electronic properties of the composite remained poorly understood at atomic details. The interface features including stacking order, the positions of point defects and dopant
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atoms and whether chemical bonds were formed would greatly affect the optical and electronic properties of the composite. The study of the behavior of the electrons in the composite can essentially reveal the efficient photocatalystic
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mechanism of the composite. It’s worth mentioning that DFT calculations are
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powerful tools to explain and improve the experimentally observed photocatalytic activity of some composites [12-14].
Herein, we performed DFT calculations to characterize the interface between molybdenum disulfide and graphene surfaces. The aim of the present study was to
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establish a basic physical picture of these composites by interconnecting molybdenum disulfide with graphene layers and to study the interface structure and electronic properties. The material defects may emerge due to synthetic methods and conditions
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during the synthesis process of nanomaterials. The experimental and theoretical
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results have shown that doped TiO2 exhibit higher photocatalytic activity [13, 14]. We expect that by introducing Nb, N and O atoms into MoS2 and graphene layers separately, photocatalytic activity of molybdenum disulfide/ graphene composites may be enhanced. Hence, pristine molybdenum disulfide (MoS2) and molybdenum disulfide with mono niobium doping (Nb-MoS2) were composited with graphene, N-doped graphene (N-G) and graphene with epoxy (O-G). We calculated geometric structures, binding energies, charge density, charge transfer, the projected density of 3
ACCEPTED MANUSCRIPT states, band structures and optical properties of these composites (MoS2/Graphene, MoS2/O-G, MoS2/N-G and Nb-MoS2/N-G).
2. Computational details
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A 4×4 single graphene layer containing 32 carbon atoms was used to match 3×3 MoS2 monolayer. The lattice mismatch of the MoS2 and graphene layers was about 2.8 %. Vacuum space at Z axis was more than 15 Å to avoid the interaction between
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the periodic images. For the MoS2 and graphene layer, atoms and cell were fully
expression:
Eb = Etotal - EG - EMoS2
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relaxed. The binding energies of the composites were calculated by using the
(3)
Where the Etotal, EG and EMoS2 denote the total energy of MoS2/ graphene system,
adsorption structure.
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isolated graphene and MoS2, respectively. A negative Eb corresponds to a stable
The DFT calculations with the generalized gradient approximation (GGA) were
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performed in Dmol3 program[15, 16]. The Perdew, Burke, Ernzerhof (PBE)[17]
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exchange-correlation functional and Double-numeric quality basis set with the polarization functions (DNP) were employed. DFT semicore pseudopotentials (DSPPs) were used to treat the core electrons. Although the spin-polarized calculations were performed, the results indicated that all the superlattice models did not exhibit magnetism at their equilibrium lattice constants. The convergence criteria for the geometric optimization and energy calculation were set as follows: (a) an energy tolerance of 1.0×105Ha, (b) a maximum force tolerance of 0.002 Ha/Å, and 4
ACCEPTED MANUSCRIPT (c) a maximum displacement tolerance of 0.005Å. Because the weak interactions were not well described by the standard PBE functional, an empirical dispersion-corrected density functional theory (DFT-D) approach proposed by
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Grimme[18, 19] was adopted. The optical property calculations were performed with CASTEP code [20] of plane wave and ultrasoft pseudopotentials[21]. The interaction of a photon with an electron in the system is described according to time-dependent
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perturbations of the ground electronic state. The photon absorption or emission gives
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rise to transitions between occupied and unoccupied states. The spectra resulting from excitations can be thought of as a joint DOS between the conduction band and the valence band [22].
3. Results and discussion
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3.1. Geometric structures and binding energies
In this work, pristine molybdenum disulfide (MoS2) and molybdenum disulfide with mono niobium doping (Nb-MoS2) were used to study the nuances of
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molybdenum disulfide. For MoS2, the calculated bulk cell lattice constant were
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a=3.16 Å, c=12.29, which agreed well with the experimental value [23, 24]. The Mo-S bond length in the pure bulk system was 2.42 Å, which was close to the experimental value (2.41Å). For Nb-MoS2, a systematic study on Nb substitutional doping of MoS2 has been reported by DFT calculations. The calculated Nb-S bond length (2.47 Å) was in agreement with the reported value of 2.45 Å [25]. For MoS2/Graphene, MoS2/N-G and MoS2 / O-G, the positions of C and O atoms were located at different initial states including top of Mo and S sites and hollow sites. 5
ACCEPTED MANUSCRIPT As to Nb-MoS2 / N-G, the initial positions of Nb atom were fixed atop C sites or at hollow sites. After full relaxation, the optimized configurations of molybdenum disulfide/ graphene composites were shown in Fig. 1. For MoS2/Graphene and
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MoS2/N-G, the interlayer spacing was 3.36 Å and 3.33 Å respectively, which was slightly shorter than interlayer distance of graphite (4.01 Å). The interaction between MoS2 and Graphene layer was relatively weak and was belong to vdW interaction.
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There was no structural distortion observed on MoS2 and Graphene layers compared
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to the isolated components. For MoS2/O-G, after full relaxation, this configuration with O atom locating at top of Mo site was the most stable. There was no obvious structural distortion observed in graphene layer. The shortest distance between O and S atom was 2.81 Å. In the case of Nb-MoS2/N-G, the structure with Nb atom lying at
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hollow site of N-G was the most favorable. The interlayer distance was the same with MoS2/N-G (3.33 Å) and there is no structural distortion. The calculated binding energies of these composites (MoS2/Graphene,
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MoS2/N-G, MoS2/O-G and Nb-MoS2/N-G) were in an order of MoS2/O-G (-2.06eV),
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Nb-MoS2/N-G (-1.65 eV), MoS2/N-G (-0.89 eV) and MoS2/Graphene (-0.60eV). For MoS2/O-G, introducing O atom disturbed the stability of graphene. O-G preferred to composited with MoS2
For Nb-MoS2/N-G, MoS2/N-G and MoS2/Graphene, the
binding energies of these composites had an opposite tendency for the order of interlayer distances. Coulomb interaction was responsible for the mainly electrostatic interaction.
For Nb-MoS2, the Nb atom acted as an acceptor impurity for the
molybdenum disulphide. The interaction between Nb-MoS2 and N-G was stronger 6
ACCEPTED MANUSCRIPT than that of MoS2/N-G. 3.2. Charge density and charge transfer In order to further investigate the electronic properties and quantitative charge
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transfer at the composite interface, we studied the electronic total charge density and Mulliken charge population. The electronic total charge density slices for the composites (MoS2/Graphene, MoS2/O-G, MoS2/N-G and Nb-MoS2/N-G) were
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displayed in Fig. 2. As shown in Fig. 2, no electron orbital overlap between
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molybdenum disulphide (Nb-MoS2 and MoS2) and graphene (Graphene, O-G and N-G) layers was observed, indicating the weak vdW interactions between molybdenum disulphide and graphene systems.
The charge transfer of molybdenum disulphide layer, corresponding to the total
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sum of the Mulliken charge populations of the Mo and S atoms (for Nb-MoS2, Nb atom was included) was calculated. A positive value meant the charge transfer was
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from molybdenum disulphide to graphene systems, in contrast, charge transfer from graphene to molybdenum disulphide systems showed a negative value.
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For all composites, molybdenum disulphide showed negative charge. In other
words, charge tended to transfer from graphene to molybdenum disulphide layers. The degree of charge transfer was in the order of Nb-MoS2/N-G, MoS2/N-G, MoS2/Graphene and MoS2/O-G. The charge transfer values of MoS2/O-G, MoS2/ Graphene, MoS2/N-G and Nb-MoS2 /N-G were -0.083, -0.13, -0.30 and -0.54e, respectively. For Nb-MoS2, the Nb doping gave rise to an acceptor impurity for molybdenum disulphide, which was apt to get more charges from N-doped graphene; 7
ACCEPTED MANUSCRIPT therefore, the Mulliken charge population was more negative. For MoS2/ Graphene and MoS2/N-G, n-doping in graphene (N-G) preferred to provide charges, and graphene with epoxy had the relatively low ability to give charges.
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3.3 Projected density of states, band structures and optical properties We calculated the projected density of states (PDOSs) and band structures for the composites to understand electronic coupling and orbital contributions, which
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may reveal the mechanism of the photocatalytic process. The PDOSs of graphene,
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O-G, N-G, MoS2, Nb-MoS2 and their composites were plotted in Fig. 3 and Fig. 4. The Fermi level (Ef) was referenced at 0 eV.
The band structures of
MoS2/Graphene, MoS2/O-G, MoS2/N-G and Nb-MoS2/N-G were displayed in Fig. 5. We can see that introducing epoxy to graphene opened the band gap of
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graphene. As was shown in Fig. 3b, N atom doping for graphene made graphene become n-type semiconductor. Fig. 3d showed the band gap of molybdenum disulfide was 1.69eV, which agreed with the reported data [26]. It is well known that
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the DFT method usually underestimates the band gap of semiconductors due to the The trends in the relative change are pretty valuable[28].
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self-interaction error[27].
In Fig. 3d, the valence band was dominated by the molybdenum 4d orbital with some contributions from the sulfur 3p orbital. The conduction band was in a similar situation. In Fig. 3e, the substitution of one Mo atom by one Nb atom introduced an electron hole within the system and thus moved the Fermi level down. The valence and conduction band were mainly contributed to by the molybdenum 4d orbital. MoS2/Graphene was a semiconductor with no gap between its valence and 8
ACCEPTED MANUSCRIPT conduction bands. The linear dispersion bands of graphene were filled into the band gap of molybdenum disulfide layers. The characteristics of graphene were preserved. MoS2/Graphene did not show any photocatalytic activities, which was also proved in
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the experiment[29]. Compared with pure single-layer molybdenum disulfide, the PDOSs of molybdenum disulfide layer from MoS2/Graphene moved left. The C 2p orbitals of graphene occupied the top of valence band and the bottom of conduction
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band of MoS2/Graphene (Fig. 4a and Fig. 5a). MoS2/O-G had a band gap of 0.57 eV.
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The top of valence band and the bottom of conduction band were comprised of C 2p orbitals. Compared with individual MoS2, the PDOSs of MoS2 in MoS2/O-G moved to low energy level due to getting charges, as shown in Fig. 4b and Fig. 5b. Fig. 4c and Fig. 5c showed that MoS2/N-G was an n-type semiconductor, and Fermi level
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moved into the conduction band. The top of valence band and the bottom of conduction band of MoS2/N-G were dominated by the C 2p orbitals of N-doped graphene. It was easy to transport the electrons of MoS2 to graphene sheet under
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visible light irradiation. The N-doped graphene sheets were electron-deficient. Then
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e− and h+ were respectively fixed on N-doped graphene and molybdenum disulfide layers. This can improve the photoelectric efficiency. The PDOSs for MoS2 had the similar situation with that of MoS2/Graphene. The typical intensity and shape of the PDOSs for MoS2 had minor change. For Nb-MoS2/N-G (Fig. 4d and Fig. 5d), there was no band gap. Electrons from Nb-MoS2 can be easily transmitted to N-doped graphene sheet. The electron transmission was very favorable because the N-doped graphene sheets were in electron-deficient conditions. Then e− and h+ were 9
ACCEPTED MANUSCRIPT respectively located on N-doped graphene and Nb-doped molybdenum disulfide layers. The recombination of e− and h+ was greatly reduced. The photoelectric efficiency was enhanced. These special electronic properties of MoS2/N-G and
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Nb-MoS2/N-G explained why these composites can be effectively used for photocatalyst. Our calculation results about MoS2/N-G agreed well with the experimental results [2, 3]. Nb-MoS2/N-G can be a potential photocatalyst candidate
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owing to the high photocatalytic efficiency and relatively high stability.
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We further calculated the optical properties of these composites. The calculated real and imaginary parts of dielectric function for both E vector perpendicular and parallel to c axis(E c and E c) for these composites were displayed in Fig. 6. The values of real part of dielectric function at zero energy of monolayer MoS2 for both
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E vector perpendicular and parallel to c axis were 5.10 and 2.89. This is agreement with the reported data [30]. Fig. 6 showed that structure peaks of these composites appeared around 2.60 and 5.10eV correspond to E vector perpendicular and parallel
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to c axis. There was an additional peak at 0.74 eV corresponding to E vector
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perpendicular to c axis in MoS2/O-G. We focused on E vector perpendicular to c axis due to the layered structures of these composites. It was found that light absorption peaked at incident photon energy of around 2.60 eV for these composites. It was also observed that MoS2/N-G and Nb-MoS2/N-G systems had stronger light absorption by analyzing the peak intensities of these composites.
4. Conclusions We have investigated the structure and electronic properties of molybdenum disulfide 10
ACCEPTED MANUSCRIPT and graphene layers by employing density functional theory calculations. Several different composites including MoS2/Graphene, MoS2/N-G, MoS2/O-G and Nb-MoS2/N-G were studied. The structures, electronic properties and optical
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properties were calculated to reveal the interface interaction and photocatalytic mechanism of these composites. The interactions of these composites were relatively weak and were belong to vdW interactions. The binding energy of Nb-MoS2/N-G was
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stronger than that of MoS2/Graphene and MoS2/N-G. Projected density of states and
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band structures of composites clearly showed that because of these advantages including the favorable electron transmission. The reduced recombination of e− and h+ and relatively strong light absorption within the visible light region made MoS2/N-G and Nb-MoS2/N-G composites become efficient photocatalysts.
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Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No. 21475054) and the Fundamental Research Funds for the Central
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Figure captions Fig. 1
The most stable configurations of (a) MoS2/Graphene, (b) MoS2/O-G, (c)
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MoS2/N-G, (d) Nb-MoS2/N-G. Left are top views and right are front views. Gray, blue, pink, cyan, yellow and red spheres stand for the C, N, O, Mo, S and Nb atoms,
Electronic total charge density slices of (a) MoS2/Graphene, (b) MoS2/O-G,
(c) MoS2/N-G, (d) Nb-MoS2 /N-G.
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Fig. 2
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respectively.
Fig. 3 The projected density of states (PDOSs) of (a)Graphene, (b) N-G, (c) O-G, (d) MoS2, (e) Nb-MoS2. Fig. 4
The projected density of states (PDOSs) of (a) MoS2/Graphene, (b)
Fig. 5
The band structures of (a) MoS2/Graphene, (b) MoS2/O-G, (c) MoS2/N-G,
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(d) Nb-MoS2 /N-G. Fig. 6
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MoS2/O-G, (c) MoS2/N-G, (d) Nb-MoS2 /N-G.
The dielectric function for (a, b) MoS2/Graphene, (c, d) MoS2/O-G, (e, f)
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MoS2/N-G, (g, h) Nb-MoS2 /N-G. Black line means real part of dielectric function, and the red line means the imaginary part.
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Highlights MoS2/Graphene, MoS2/N-doped Graphene, MoS2/ Graphene with epoxy and Nb-doped MoS2/ N-doped Graphene were studied. electronic
and
optical
properties
disulfide/graphene composites were calculated.
of
molybdenum
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The
MoS2/N-doped Graphene and Nb-doped MoS2/ N-doped Graphene
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composites had better photocatalytic performance.