Noncovalent Functionalization of Monolayer MoS2 with Carbon

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Non-Covalent Functionalization of Monolayer MoS2 with Carbon Nanotubes: Tuning Electronic Structure and Photocatalytic Activity Zhaogang Zhang, Wei-Qing Huang, Zhong Xie, Wangyu Hu, Ping Peng, and Gui-Fang Huang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06793 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 20, 2017

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The Journal of Physical Chemistry

Non-Covalent Functionalization of Monolayer MoS2 with Carbon Nanotubes: Tuning Electronic Structure and Photocatalytic Activity Zhaogang Zhang1,2, Wei-Qing Huang1∗, Zhong Xie1, Wangyu Hu3, Ping Peng3, Gui-Fang Huang1# 1 2

School of Physics and Electronics, Hunan University, Changsha 410082, China

College of Physics Science and Engineering Technology, Yichun University, Yichun Jiangxi 336000, China 3

School of Materials Science and Engineering, Hunan University, Changsha 410082, China

Abstract: Carbon nanotubes (CNTs)/two-dimensional transition metal dichalcogenides (MX2) hybrids have shown unique physical properties, making them promising materials for various applications ranging from photocatalysis to solar energy conversion. Understanding the interfacial interactions is highly desirable for designing these hybrids having excellent performance. Here we systematically study the interfacial interaction in single-wall CNT/monolayer MoS2 hybrids and its effects on electronic and optical properties by first-principles calculations. It is found that CNT is interacted non-covalently with monolayer MoS2 forming van der Waals heterostructures, and their interfacial interaction is closely related to tube diameter. In these hybrids, interestingly, MoS2 gaining or losing electrons depends also on tube diameter: (3, 0)CNT would gain some electrons from MoS2, whereas other CNTs lose some electrons. The small band gap makes these hybrids having a strong optical absorption in visible light region. The type-II, staggered, band alignment in CNT/MoS2 hybrids can facilitate the separation of photoexcited electrons and holes, improving the photocatalytic activity. Moreover, the CNTs are not only an effective sensitizer, but also a highly active co-catalyst in hybrids. These results have revealed the mechanism of enhanced photocatalytic performance of CNT/MoS2 hybrids observed in experiments, and help for developing highly efficient carbon nanomaterial-based nanophotocatalysts.

∗

. Corresponding author. E-mail address: [email protected] #. Corresponding author. E-mail address: [email protected] 1

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1. Introduction Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) with the chemical formula MX2 (M=Mo, W; X=S, Se) have recently attracted considerable attention due to their attractive physical, chemical and optical properties for the potential applications in catalysis, energy storage, photodetectors, and electronic devices1-2. Among TMDs, molybdenum disulfide (MoS2) is of particular interest in high-performance catalysis owing to relatively high efficiency, low cost, and high stability3-5. However, the low conductivity and insufficient number of active sites restrict its practical applications. To address these problems, several strategies have been developed to optimize their catalytic performance: (i) improving the electrical conductivity of MoS2 by incorporation conductive materials such as low-dimensional allotropes of carbon, (ii) increasing the number of active sites of MoS2. It is verified that only the S–Mo–S edges in MoS2 sheets are active for hydrogen atom adsorption, and the [1010] Mo edges are mainly responsible for bringing about a favorable free energy change for hydrogen atom adsorption6. Increasing the number of active edge sites in MoS2 can be tuned via reducing dimensions to produce single-layer or several-layer MoS2, introducing defects on the basal planes7-8, controlling of aligned morphology 9 and formation of heterostructures10. Particularly, coupling MoS2 sheet with other materials has been shown to be one of the most effective strategies

to

improve

its

photocatalytic

performance11.

Various

nanocomposites have been designed and demonstrated high activity10,

12-13

MoS2-based

. For examples,

MoS2/TiO2 exhibits a higher rate of hydrogen generation by two orders of magnitude than pure TiO2, as well as improved photostability14. The rate of H2 evolution on MoS2/CdS is increased by up to 36 times than that on CdS under visible light, and its activity is even higher than those of the CdS loaded with different noble metals15. In general, the enhanced photocatalytic activity is usually ascribed to the reduced recombination rates of photo-excited charge carriers owing to the formation of type-II band alignment at the interface, and provided abundant reactive sites in the MoS2 nanocomposite16-17. Low-dimensional carbon nanomaterials such as fullerene, carbon nanotubes (CNTs) and graphene (GR), have unique electronic properties and large specific surface area, and can be used as an important component to enhance the catalytic performance of nanocomposites18-23. Various 2


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CNT-based composites, such as MoS2/CNT composites have been synthesized by hydrothermal method24-26, direct mixing method27, and thermal decomposition method28, and showed enhanced the catalytic activity and stability compared to isolated MoS226, 29-35. In particular, a paradigm is CdS/MoS2/CNTs nanocomposites36, which shows a strong visible-light photocatalytic H2-evolution and exhibits a maximum photocurrent density of 11.12 mA/cm2, which is 12 times higher than that of pristine CdS. The enhancement is assumed to be the presence of the CNTs as a co-catalyst/support, increasing electron transfer from photoexcited CdS to MoS2 edges through CNTs. For the MoS2/CNT composites, Li et al. ascribed the excellent catalytic activity to the synergistic effect from the dense catalytic sites at amorphous MoSx surface and fluent charge transport along N-doped CNT forest29. Similarly, Ye et al. suggested that the CNT network not only promotes the charge transfer in corresponding catalytic process but also enhances the stability of the active sites in MoSx35. Nevertheless, Yuan et al. found that the enhanced catalytic activity with increased CNT content may not correlate with the number of active sites, but is very likely due to increased conductivity, suggesting that conductivity could be the main limiting factor26. Obviously, there is so far no consensus on the enhancement mechanisms. More complicated, in fact, is the interaction of MoS2 with CNTs, and less is clear about its effects on the photocatalytic activity. This is, essentially, due to the fact that The MoS2/CNT samples synthesized at various conditions usually involve metallic and semiconducting tubes since the real CNTs are generally their mixture. Moreover, a wide range of CNT diameter (0.3~20.0 nm) has considerable impact on the performance of composites because the band structure of CNTs varies with the diameter37, thus affecting the band alignment at the CNT-MoS2 interface, in turn, photocatalytic activity. These issues, which are extremely difficult to disclose by experiments, call for a scientific understanding from the electronic level for designing CNT-based composites with desirable properties. Here, the interfacial interaction of CNTs with various diameters adsorbed on 2D TMDs is systematically explored via a representative monolayer MoS2, to reveal the origin of the enhanced photocatalytic performance by using large-scale density functional theory (DFT) computations. We explicitly show that the interfacial interaction in CNT/MoS2 hybrid is related to tube diameter of CNTs. Monolayer MoS2 in the hybrids would gain some electrons from CNT with smallest 3



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diameter, while lose some electrons to CNT with large diameter. The band gap of CNT/MoS2 hybrid is decreased compared to that of MoS2. The type-II, staggered, band alignment in the interface can facilitate the separation of photoexcited electrons and holes, improving the photocatalytic activity of CNT/MoS2 hybrids. The results can be rationalized by available experiments, thereby partly resolving a debate on the role of CNT in hybrids, and pave the way for developing highly efficient carbon-based nanophotocatalysts.

2. Computational method To construct the hybrid of monolayer MoS2 and CNTs, the (1 × 1 × 3) CNTs are respectively used to represent typical ∼1.3 nm CNTs. The calculated supercells are composed of a (4 × 3) monolayer MoS2 (containing 24 Mo and 48 S atoms) and different carbon tubes with length of 12.6 Å in its axial direction. This only causes minor axial strain, leading to a 1.3% lattice mismatch. The vacuum depth is as large as 15 Å for all the hybrids to avoid artificial interaction in a supercell (16.2 ×12.6 × 25 Å3). The local density approximation (LDA) with inclusion of the vdWs interaction is chosen because long-range vdW interactions are expected to be significant in such these hybrids. All of the theoretical calculations are performed using the DFT/LDA method implemented in the plane wave basis CASTEP code38. A Morkhost-Pack mesh of k points, 2 × 2 × 1 and 4 × 4 × 1 points, is used, respectively, to sample the two-dimensional Brillouin zone for geometry optimization and for calculating the density of states (DOSs). The cutoff energy for plane waves is chosen to be 400 eV, total energy and all forces on atoms converge to less than 10−6 eV and 0.01 eV/Å, respectively.

3. Results and discussion 3.1 Geometric Structure and Formation Energy To elucidate the effect of tube diameter (D) on the interaction between CNT and monolayer MoS2, seven nanotubes with different diameters ranging from 2.35 to 7.83 Å (See Table 1) are selected. Fig. 1 presents four optimized geometric structures of the typical representative CNT/MoS2 hybrids. Parts a1 and a2 illustrate the side views of the metallic (3,0)CNT- and (9,0)CNT-MoS2 hybrids; Parts b1 and b2 are for the semiconducting (5,0)CNT- and (10,0)CNT-MoS2 hybrids, respectively. For the CNT-MoS2 hybrids, the interlayer spacing 4


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between the coaxial of CNTs and the top S atom of monolayer MoS2 after optimization are calculated to be 4.0~6.9 Å (See Table 1), which is in accordance with those experimental results of 6.2~7.1 Å

24-25, 35

. The equilibrium distances between the nanotube wall and the top S atom of

monolayer MoS2 after optimization are calculated to be 2.78~3.02 Å (See Table 1), which is consistent with those between monolayer MoS2 and other materials (2.62 Å for MoS2/fullerene39, 3.11-3.19 Å for MoS2/GR40-41 , 2.84 Å for MoS2/CeO242, 2.60 Å for MoS2/SnO217). Such large equilibrium distance shows that the CNTs interact with monolayer MoS2 through a weak vdWs force. After optimization, the CNTs and monolayer MoS2 in the hybrids are nearly unchanged, indicating that the CNT-MoS2 interaction is indeed vdW rather than covalent.

Fig. 1. The optimized geometry (side view) for different CNTs on monolayer MoS2: (a1-b2) are for (3, 0), (5, 0), (9, 0), and (10, 0) CNTs, respectively. The equilibrium spacing between the nanotube wall and the top S atom layer is denoted by d. Gray, red and cyan spheres represent C, S and Mo atoms, respectively.

In the (3, 0)CNT/MoS2, the equilibrium distance after optimization is 2.78 Å, smaller than those (2.9-3.0 Å) in other CNT/MoS2 hybrids. Generally, the smaller the interfacial spacing, the 5



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stronger the interaction at the interface. The smallest distance in the (3, 0)CNT/MoS2 shows that the interfacial interaction between the metallic (3, 0)CNT and monolayer MoS2 is strongest among these hybrids we studied here. Table 1 clearly shows that the interfacial spacing (or interaction) between CNT and MoS2 is a weak function of diameter. This is due to the fact that the interfacial interaction in CNT/MoS2 hybrid is mainly related to two competing effects: the smaller diameter CNT has smaller contact area with monolayer MoS2, whereas the number of contacts per unit volume will be smaller for larger nanotubes due to small aspect ratio.

Table 1. The diameter, band gap Eg of pure CNTs and the formation energy (Ef), band gap Eg*, interfacial spacing (d) between the nanotube wall and the top S atom of monolayer MoS2, interfacial spacing (L) between the coaxial of CNTs and the top S atom of monolayer MoS2 of optimized CNT/MoS2 composites. Diameter

Eg

Ef

Eg*

d

L

(Å)

(eV)

(eV)

(eV)

(Å)

(Å)

MoS2

CNT

CNT(10,0)/MoS2

7.83

0.834

-2.708

0.779

2.97

6.89

-0.048

0.048

*CNT(9,0)/ MoS2

7.05

0

-0.959

0.582

2.86

6.39

-0.082

0.082

CNT(8,0)/ MoS2

6.27

0.555

-0.279

0.602

3.02

6.16

-0.058

0.058

CNT(7,0)/ MoS2

5.48

0.409

-0.897

0.317

2.89

5.63

-0.042

0.042

*CNT(6,0)/ MoS2

4.70

0

-0.662

0

3.03

5.38

-0.058

0.058

CNT(5,0)/ MoS2

3.92

0.136

-0.755

0.147

2.99

4.95

-0.012

0.012

*CNT(3,0)/ MoS2

2.35

0

-0.438

0

2.78

3.96

0.122

-0.122

Hybrid

Bader Charge (e)

Note: CNT marked * indicates that the individual CNT is metallic.

The stability of the CNT/MoS2 hybrids can be assessed by their formation energy, which is defined as:

= − −

(1)

where E , E , and E represent the total energy of the relaxed CNT/MoS2, pure CNT, and monolayer MoS2, respectively. By this definition, negative Ef suggests that the interface is stable. As can be seen from Table 1, the formation energy of all CNT/MoS2 hybrids is negative, 6


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ranging from -0.2 and -2.7 eV, indicating a rather strong interaction between CNT and monolayer MoS2, and the high thermodynamically stability of these hybrids. Among them, the CNT(9,0)/MoS2 and CNT(10,0)/MoS2 hybrids with lower formation energy would be more easily formed, in line with their larger contact area of the CNT with MoS2.

3.2 Density of states The electronic structure variation of heterostructures is closely related to the interfacial interaction. Fig. 2 presents the density of states (DOSs) of CNTs and monolayer MoS2 before and after formation of the hybrid, respectively. Bulk MoS2 is an indirect semiconductor with a band gap (Eg) of 1.2 eV and undergoes a transition to a direct band gap semiconductor when thinned to a monolayer43, which has been confirmed experimentally44. Our calculated monolayer MoS2 is a direct semiconductor with a band gap (Eg) of 1.89 eV, agreeing well with previous studies43, 45.

Fig. 2. DOSs for the hybrids (a1) CNT(3,0)/MoS2, (a2) CNT(9,0)/MoS2, (b1) CNT(5,0)/MoS2, (b2) CNT(10,0)/MoS2, (a1*) metallic (3,0) CNT, (a2*) metallic (9,0) CNT, (b1*) semiconducting (5,0) CNT, and (b2*) semiconducting (10,0) CNT, respectively. The Fermi level is set to zero.

7



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The calculated DOSs of the CNTs and the CNT/MoS2 hybrids depict the electronic properties and energy level alignment in detail (Fig. 2 and Table 1). The DOSs in parts a2*and b2* of Fig. 2 show that (9, 0) CNT is metallic and (10, 0) CNT is semiconducting with a band gap of 0.92 eV, which is comparable to the previous studies46. Fig. 2 clearly illustrates that each component of the combined DOSs in the CNT/MoS2 hybrids changes very little with respect to those of individuals and are mutually staggered near the Fermi level, basically maintaining the nature of their respective DOSs as isolated individuals, which can further determine the fact that the interfacial interaction is indeed weak vdW rather than covalent. Compared with pure semiconducting (7, 0) and (10, 0) CNTs, their hybrids with monolayer MoS2 have smaller ban gap; whereas for (5, 0), (8, 0) CNTs and their hybrids, it is just the opposite (Table 1). For the metallic CNT/MoS2 hybrids, only CNT(9,0)/MoS2 hybrid opens a band gap of 0.582 eV due to the stress effect in CNT(9,0), in accordance with the previous work47. Although the band gap variation of CNT/MoS2 hybrids is non-monotonic with tube diameter, all band gaps calculated are small (< 0.8 eV, as listed in Table 1), which is considered ideal to absorb most of the sunlight, indicating that these hybrids are the candidate photocatalyst materials in visible light region. Therefore, the small band gap, compared to that of monolayer MoS2, is one of major factors for the observed enhanced photocatalytic performance of CNT/MoS236. The decreased band gap of CNT/MoS2 hybrid can be attributed to the C 2p states of CNT appearing in the gap of monolayer MoS2, which is clearly displayed in Fig. 2. More detailed analysis shows that the near-gap electronic structure of CNT/MoS2 hybrid varies with tube diameter. As the small CNTs (such as (3, 0) and (5, 0) tubes) are coupled to monolayer MoS2, their energy levels are embedded in the band gap of monolayer MoS2 (Figs. 2 (a1, b1)). This is very clearly displayed in Fig. 3, in which one can see that the highest-occupied level (HOL) is formed by C 2p states mixing a small S states in CNT(5, 0)/MoS2, even only consisted of C 2p states in CNT(3, 0)/MoS2, where their lowest-unoccupied levels (LUL) are only composed of the C 2p orbits. Therefore, the CNT(3, 0)/MoS2 and CNT(5, 0)/MoS2 hybrids are type-I heterojunctions. For practical purpose (for example, photocatalysis), this is unfavorable for the separation of photogenerated electron−hole pairs. In contrast, a type-II, staggered, band alignment exists between monolayer MoS2 and CNT with large diameter (> 7.0 Å) (Figs. 2 (a2, b2)), i.e., 8


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CNT(9, 0)/MoS2 and CNT(10, 0)/MoS2 hybrids. The two right-most columns in Fig. 3 make this very clear: HOL is C 2p states, and LUL is 4d states. In photocatalysis, such band alignment is conducive to the separation of electron-hole pairs, i.e., the electrons in CNTs can be directly excited to monolayer MoS2 under light irradiation, leading to the robust separation of photoexcited charge carriers between the two constituents. This indicates that the CNT with large diameter would be a sensitizer for monolayer MoS2. Therefore, CNT with large diameter is the preferable choice to achieve high photoactivity when CNT is used to decorated monolayer MoS2. These results indicate that choosing appropriate diameter of CNT is especially critical to obtain high efficiency of electron-hole separation in the CNT/MoS2 hybrid.

Fig. 3. Maps of the electron and hole density distributions for LUL (a1–b2) and HOL(a1*–b2*) for the hybrid (a1) CNT(3,0)/MoS2, (a2)

CNT(9,0)/MoS2, (b1) CNT(5,0)/MoS2, (b2)

CNT(10,0)/MoS2. The blue and yellow represent the electron and hole density distributions for LUL and HOL, respectively; the isovalue is 0.007 e/Å3. Herein, HOL and LUL are determined by the highest-occupied and lowest-unoccupied levels, respectively.

3.3 Charge Density Difference and Mechanism Analysis To investigate the effect of tube diameter on charge transfer and redistribution at the interface in these CNT/MoS2 hybrids, 3D charge density difference are calculated, ∆ = / − − , where / , and are respectively the charge densities of the 9



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hybrids, monolayer MoS2 and CNTs in the same configuration, as shown in Fig. 4. The blue and yellow represent charge accumulation and depletion, respectively. Obviously, the charge redistribution mostly occurs at the CNT- MoS2 interface, and depends strongly on tube diameter. In the CNT(3,0)/MoS2 hybrid, the charge distribution is the most violent, involving all C atoms in CNT, the top layer S atom and Mo atom in MoS2 (Fig. 4 (a1)). This further demonstrates that the interfacial interaction is strong in CNT(3,0)-MoS2 interface, which is consistent with the shorter interface distance.

Fig. 4. 3D Charge density differences for (a1) CNT(3,0)/MoS2, (b1) CNT(5,0)/MoS2 and (a2) CNT(9,0)/MoS2, (b2) CNT(10,0)/MoS2, respectively. The blue and yellow represent charge accumulation and depletion, respectively; the isovalue is 0.0015 e/Å3. (c1), (c2) Profile of the planar averaged self-consistent electrostatic potential for the (a1) CNT(3,0)/MoS2, (b1) CNT(5,0)/MoS2 and (a2) CNT(9,0)/MoS2, (b2) CNT(10,0)/MoS2 as a function of position in the z-direction. (d1), (d2) Profile of the planar averaged charge density difference for the (a1) CNT(3,0)/MoS2, (b1) CNT(5,0)/MoS2 and (a2) CNT(9,0)/MoS2, (b2) CNT(10,0)/MoS2 as a function of position in the z-direction. The horizontal dashed line is the position of both the bottom layer of the CNT surface and the top layer of the MoS2 surface. 10


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The charge redistribution of all C atoms is largely because that even the top C atoms is near the S atom due to its smallest diameter (~2.35 Å) of (3, 0)CNT. As the tube diameter is increased, only those C atoms at the bottom of CNT could influenced by the interaction from the top S atoms. Therefore, the charge redistribution at the CNT-MoS2 interface is weaken with increase of tube diameter (Figs. 4 (a1-b2)). Such diversity enables us to visualize the different features caused by interfacial interactions resulted from varying diameter of CNT. Most importantly, closer inspection to Fig. 4 reveals that the interfacial interaction results into the negatively charged Mo atoms in the monolayer MoS2. This indicates that some Mo atoms at basal planes, initially catalytically inert, would turn out to be active sites, which are beneficial to the improved photocatalytic performance of the CNT/MoS2 Hybrid. The planar averaged charge density difference along the direction perpendicular to monolayer MoS2 is plotted in Figs. 4 (d1) and (d2), which presents quantitative result of charge transfer and redistribution. The horizontal dashed lines are the positions of both the bottom layer of the CNT and the top S layer of monolayer MoS2. The positive (negative) values indicate electron accumulation (depletion). The largest efficient electron accumulation localized above the S atoms in the top layer is about 4.35 × 10-4 e/Å3 in the CNT/MoS2 hybrids; while the largest local efficient electron depletion at the lowest layer C atoms is about -8.76 × 10-4 e/Å3 in the CNT(3,0)/MoS2, bigger than those (2.70~2.96× 10-4 e/Å3) in the CNT(9,0)/MoS2 and CNT(10,0)/MoS2 hybrids. This provides further evidence that the charge redistribution at the (3, 0)CNT-MoS2 interface the most significant, and shows different directions of charge transfer in these hybrids, which will be discussed below. To quantitatively investigate the charge variation at the interface, a Mulliken population analysis of the plane-wave pseudopotential calculations has been operated on the CNT, monolayer MoS2, and CNT/MoS2 hybrids. In the CNT/MoS2 hybrids, the interfacial interaction would result in a change of the Mulliken charge of some atoms and the Mulliken charges on different atoms are described in Fig. 5, in which several typical values are denoted. The Mo atoms have -0.02, -0.02, -0.01, -0.01 in the CNT(3,0)/MoS2, CNT(5,0)/MoS2, CNT(9,0)/MoS2 and CNT(10,0)/MoS2 hybrids (-0.02 in pure monolayer MoS2), indicating that the electrons of Mo atoms are changed due to the coupling of CNT with large diameter. The top-most S atoms have a Mulliken charge of 11



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+0.01, and +0.04, +0.03, +0.03, +0.03 in pure monolayer MoS2 and CNT/MoS2 hybrids, respectively. The charge variation declares that the top-most S atoms of the CNT/MoS2 hybrids would lose more electrons than those in the isolated monolayer MoS2. It would be the physical mechanism that enhance stability of CNT/MoS2 photocatalyst, just as in the case of MoS2/fullerene 39 and MoS2/GR 48.

Fig. 5. The Mulliken charge distribution maps of (a1) CNT(3,0)/MoS2, (b1) CNT(5,0)/MoS2, (a2) CNT(9,0)/MoS2, (b2) CNT(10,0)/MoS2, respectively, with an isovalue of 0.5 e/Å3. Gray, red and cyan spheres represent C, S and Mo atoms, respectively.

Although the C atoms in the CNT have a Mulliken charge of approaching zero, those C atoms in the CNT/MoS2 hybrids have different Mulliken charges because the interfacial interaction is varied. Fig. 5 shows that in the CNT/MoS2 hybrids, the C atom near monolayer MoS2 has a Mulliken charge of -0.02 and -0.02 in the CNT(9,0)/MoS2 and CNT(10,0)/MoS2 12


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hybrids, whereas those corresponding C atoms in the CNT(3,0)/MoS2 and CNT(5,0)/MoS2 hybrids have a much larger Mulliken charge of −0.05 and -0.03, responsible for a change in nanotube diameter in the CNT/MoS2 hybrids. The effective net charge from one constituent to another in these composites can be studied by the Bader method. Interestingly, the direction of net charge transfer in these hybrids depends upon the tube diameter. When the tube diameter is bigger than 3.9 Å, the calculated Bader charge reveals that some charge is transferred from CNTs to MoS2, thus leading to hole doping for the CNT. For examples, 0.012, 0.082 and 0.048 electrons transfer from CNT to MoS2 in the CNT(5,0)-, CNT(9,0)-, and CNT(10,0)-MoS2 hybrids, respectively (Table 1). On the contrary, the amount of charge transferred at the interface is 0.122 from MoS2 to CNT in the CNT(3,0)/MoS2 hybrid, which is larger than those (0.012 ~ 0.082) in other hybrids, in agreement with the fact that the former has the smallest interfacial distance (see Table 1). The origin of the interfacial electron transfer could be understood by comparing their work functions. The calculated work function of monolayer MoS2 is 5.36 eV

42

, those of CNTs with large tube

diameter (>3.9 Å) are smaller than 5.3 eV, whereas that of (3, 0) CNT is bigger than 6.0 eV 49. The spontaneous interfacial charge transfer in these hybrids can therefore be simply rationalized in terms of the differences between their work functions. The interfacial charge transfer would surely alter the electrostatic potential distribution in these hybrids. Figs. 4 (c1) and (c2) display the profile of the planar averaged self-consistent electrostatic potential for the CNT/MoS2 hybrids as a function of position in the z-direction. Monolayer MoS2 is a typical S–Mo–S sandwich structure, where the potential at the Mo atomic plane is higher than that at the S atomic plane. The potential along the CNT radial direction is like-sinusoidal. At the interface, a large potential difference (~41 eV) between CNT and MoS2 can be observed, while the average electrostatic potential distributions in monolayer MoS2 and CNT are almost unchanged. Under light irradiation, the large built-in potential at the CNT-MoS2 interface can promote the separation and migration of photogenerated carriers in the hybrids. As a consequence, the photocatalytic activity and stability of the MoS2 photocatalyst would be greatly enhanced due to the coupling of CNTs.

3.5. Optical Properties Recent experiments have shown that the CNT/MoS2 composites have high photocatalytic H2 13



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production activity under visible light irradiation. To explore its underlying mechanism, the UV-vis absorption spectra of monolayer MoS2 and CNT/MoS2 hybrids are respectively calculated over the electronic structure obtained with the LDA including vdWs interaction, as shown in Fig. 6. The absorption edge of monolayer MoS2 occurs at about 1.89 eV owing to its intrinsic transition from the Mo 4d to S 3p orbitals. One can see that the optical absorption edge of the CNT/MoS2 hybrid shifts towards the longer wavelength due to their decreased band gaps, compared to those of pure MoS2 (see Fig. 6). The red shift is usually attributed to the electron transition from the C 2p to Mo 4d states, or C 2p to C 2p states. Especially, the absorption intensity of the CNT/MoS2 hybrid is obviously increased in the visible-light region (> 450 nm). Among these hybrids, CNT(8, 0)/MoS2 and CNT(10, 0)/MoS2 hybrids display the largest absorption intensity in the long wavelength region (500~800 nm).

Fig. 6. Calculated absorption spectra of the CNT/MoS2 hybrids and pure monolayer MoS2

The change of absorption intensity in the CNT/MoS2 hybrid can be explained using the following basic relation: =

ħ

∑-,-.|〈#|$|#′〉| '(#)(1 − '(#′))+(,- − ,-. − ħ/) 14


(2)

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where ɛ2 is the imaginary part of the dielectric function, ħɷ is the energy of the incident photon, p is the momentum operator 2(ħ⁄3 )(5⁄56), (|#〉) is a crystal wave function and '(#) is Fermi function. The real part ε1(ω) of the dielectric function can be obtained from imaginary part according to the Kramer–Kronig relationship. The optical absorption coefficient I(ω) can be evaluated as follows: =⁄

7(ɷ) = √2ɷ :;ɛ= (ɷ) + ɛ (ɷ) − ɛ= (ɷ)?

(3)

In the vis-light region, the weak optical absorption of pure MoS2 is due to the very low Mo 4d states in the CB bottom. For the CNT/MoS2 hybrid, C 2p states at VB top are predominant components, C 2p and Mo 4d hybridized orbitals are at the lower part of CB (Fig. 2). The large states near band-gap of these hybrids correspond to the big values of s−p and p−p matrix elements in Eq. 2, thus resulting into the strong optical absorption of the CNT/MoS2 hybrids in the vis-light region. We now speculate on the origin of the enhanced photocatalytic activity and stability of the CNT/MoS2 hybrids obtained in experiments. For the CNT/MoS2 hybrids, firstly, the C 2p states of CNTs are embedded into the band gap of MoS2, resulting into much smaller band gap compared to their individuals. The very small band gap (< 0.8 eV) enables the CNT/MoS2 hybrids to absorb more light in not only vis-light region but also near-infrared spectral regions (Fig. 6). Secondly, the photoexcited charge carriers can be effectively separated in these hybrids in photocatalysis. For the CNT/MoS2 hybrids, metallic CNT can greatly improve the electrical conductivity of the MoS2 catalyst and photoexcited charge can be shuttled freely along the conducting network of the CNT bundle, thus improving its photostability. As for the CNT(9,0)/MoS2 and CNT(10,0)/MoS2 hybrids, the LUL and HOL of MoS2 is both more negative than those of CNTs, forming a type-II heterojunction (Fig. 3). Under vis-light irradiation, the photogenerated electron is easily transferred into MoS2, leading to a large amount of electrons accumulated at the MoS2. Simultaneously, this continuous process can promote the effective separation of photoexcited electron–hole pairs, and also effectively improve the photocatalytic activity. Finally, some charged C atoms in CNTs, initially catalytically inert, will turn out to be active sites due to charge transfer, making the CNTs to be a highly active co-catalyst in these hybrids. In addition, the Mo atom 15



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becomes active species in the photocatalytic reaction due to gaining electrons from CNTs. It is generally accepted that the catalytic activity of MoS2 plates is derived from the under coordinated sulfur edge sites, while their basal planes remain catalytically inert. During the photocatalytic process, the charge transfer from C 2p to Mo 4d orbitals makes the monolayer MoS2 to be a highly active co-catalyst in the hybrids. The combined effects of the above factors can result in enhanced vis-light photocatalytic performance of the CNT/ MoS2 hybrids.

4. Summary In summary, we have investigated the mechanism of enhanced photocatalytic activity of the CNT/MoS2 hybrids by analyzing the electronic and optical properties under the framework of DFT. The results show that the interfacial interaction in CNT/MoS2 hybrid is closely related to tube diameter. In these hybrids, interestingly, monolayer MoS2 gaining or losing electrons also depends on tube diameter: (3,0)CNT would gain some electrons from MoS2, whereas other CNTs lose some electrons. C 2p states greatly broadens the upper VB of CNT/MoS2 hybrids, resulting into their small band gap (< 0.8 eV). As a consequence, the CNT/MoS2 hybrids have a strong absorption from ultraviolet to infrared region. The type-II, staggered, band alignment in CNT/MoS2 hybrids can facilitate the separation of photoexcited electrons and holes, enabling the high photocatalytic efficiency. It is demonstrated that the CNTs are not only an effective sensitizer, but also a highly active co-catalyst in CNT/MoS2 hybrids. These results have revealed the mechanism of enhanced photocatalytic performance of CNT/MoS2 hybrids observed in experiments,

and

help

for

developing

highly

efficient

carbon

nanomaterial-based

nanophotocatalysts.

5. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51772085).

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Properties of Molybdenum Disulfide/Graphene Composites: A Density Functional Theory Study. J. Alloy. Compd. 2015, 649, 961-967. 49. Shan, B.; Cho, K. J. First Principles Study of Work Functions of Single Wall Carbon Nanotubes. Phys. Rev. Lett. 2005, 94, 4.

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Noncovalent Functionalization of Monolayer MoS2 with Carbon

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