Understanding the Mechanism of Photocatalysis Enhancements in the

Article pubs.acs.org/JPCC

Understanding the Mechanism of Photocatalysis Enhancements in the Graphene-like Semiconductor Sheet/TiO2 Composites Mang Niu, Daojian Cheng,* and Dapeng Cao* Division of Molecular and Materials Simulation, State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China S Supporting Information *

ABSTRACT: The electronic properties of monolayer transition-metal dichalcogenide MX2 (M = Mo and W; X = S and Se) interfaced TiO2(110) composites were investigated by hybrid density functional theory. In the MX2/TiO2(110) composites, MX2 serves as an efficient photosensitizer, and the electron−hole pair can, therefore, be easily generated by visible-light irradiation and be effectively separated by the electron injection from MX2 to TiO2. This mechanism is quite different from the one of the foreign elements doped TiO2, in which the electron is directly excited from the midgap impurity states into the CB of TiO2, leading to an optical absorption edge extending to the visible-light region. Moreover, we reveal that the prerequisite of designing the highly efficient semiconductor−TiO2 photocatalytic composites is to select the proper semiconductor, which holds the band gap of ∼2.0 eV and generates a built-in potential of 0.3−0.5 eV in the composite, as a photosensitizer, which can be also considered as a fundamental criteria to screen the suitable semiconductor and further to design the TiO2-based heterojunction composites for improving visible-light photocatalysis.

1. INTRODUCTION

Recently, the semiconductor−TiO2 composites have been receiving much attention to improve the visible-light photoactivity of TiO2.27−31 For example, the MoS2/TiO2 composites show a great enhancement of photocatalytic hydrogen production28,29 and dye degradation30,31 under visible illumination. The nanosized MoS2 in the MoS2/TiO2 composites is considered to be a photosensitizer, which can absorb visible light and transfer photoexcited electrons from MoS2 to TiO2. However, the mechanism of photocatalysis enhancement in MoS2/TiO2 composites is not understood completely yet, because the electronic properties of MoS2/TiO2 composites and its heterojunction interfaces have not been investigated fully. Definitely, in order to understand the mechanism of photocatalysis enhancements and to design new TiO2-based heterojunction composites for visible-light photocatalysis, it is necessary to further investigate the electronic properties of such semiconductor−TiO2 heterojunctions. In this work, we investigate the electronic properties of monolayer transition-metal dichalcogenide MX2 (M = Mo and W; X = S and Se) interfaced TiO2(110) composites by using hybrid density functional calculations and find that the MoS2/ TiO2(110) heterojunction exhibits the best performance in visible-light photocatalysis among these systems due to the effective visible-light absorption, photoexcited electron injection, and charge separation process in the composite.

TiO2, as a wide-band-gap semiconductor, has been considered as one of the most promising photocatalysis and photovoltaic materials for solar-energy conversion and environmental purification.1−8 For example, TiO2-based photocatalysts3,4 and dye-sensitized solar cells (DSSCs)6−8 allow the use of sunlight for the degeneration of toxic pollutants and for the conversion of solar energy to electrical power, respectively. However, the wide band gap of TiO2 (3.0 eV for rutile and 3.2 eV for anatase) only absorbs ultraviolet (UV) light (only 5% of the sunlight energy) for photocatalytic activation, and its photocatalytic activity is very low under visible-light irradiation (about 45% of the sunlight energy). To improve the photocatalytic performance of TiO2, it is crucial to narrow its band gap for increasing visible-light absorption. It is well-known that the incorporation of foreign elements is the commonly used method to modify the electronic structures of TiO2.9−21 Although the doping method can improve the visible-light photocatalytic performance of TiO2 to some extent, the photocatalytic efficiency of these doped systems is still limited by the relatively high recombination rate and the low carries mobility of photoexcited electron−hole pairs.22−24 Moreover, it is very difficult to synthesize the doped TiO2 with high concentration of dopants by experiments, and the doped TiO2 may possess low stability against photocorrosion.25,26 Clearly, an alternative doping-free approach, which can improve the photocatalytic performance of TiO2, would be more favorable. © 2014 American Chemical Society

Received: December 22, 2013 Revised: February 16, 2014 Published: February 25, 2014 5954

dx.doi.org/10.1021/jp412556r | J. Phys. Chem. C 2014, 118, 5954−5960

The Journal of Physical Chemistry C

Article

Moreover, we also propose a fundamental criterion on efficient design of the semiconductor−TiO2 composites for visible-light photocatalysis.

Table 1. Lattice Parameters and the Band Gaps within the Exchange and Correlation Functional of PBE and HSE06 for Bulk Rutile TiO2 and Monolayer MX2 (M = Mo and W; X = S and Se) Sheet

2. CALCULATION METHODS The DFT calculations were carried out using the Vienna Abinitio Simulation Package (VASP)32,33 with the frozen-core allelectron projector-augment-wave (PAW)34,35 method. The Perdew−Burke−Ernzerhof (PBE)36 of the generalized gradient approximation (GGA) was adopted to describe the exchange and correlation potential. The cutoff energy for the plane-wave basis set was set to 500 eV. Extensive tests were carried out to ensure the convergence with respect to the number of k-point mesh37 for all systems. The geometry optimizations were performed until the forces on each ion were reduced below 0.01 eV/Å, and the resulting structures were then used to calculate the electronic structures. To obtain the correct electronic structures (especially the band gap energies) of MX2 and TiO2 systems, we used the Heyd−Scuseria−Ernzerhof (HSE06)38,39 hybrid functional. In the HSE06 hybrid functional, the exchange contribution is divided into short- and long-range parts and the short-range part of the PBE exchange is weighted by 25% Hartree−Fock (HF) exchange. Before exploring the properties of the MX2/TiO2 heterojunctions, we first investigate the structural properties of monolayer MX2 and TiO2. Here, we choose monolayer MX2 sheets as photosensitizers mainly because these monolayer MX2 sheets have optimum band gaps for solar energy conversion or visible-light photocatalysis,40 which is more suitable than the bulk phase of MX2. The structure of the monolayer MX2 sheet is shown in Figure 1a. Similar to BN, SiC, ZnO, and other

Eg (eV)

lattice parameters (Å) TiO2 MoS2 MoSe2 WS2 WSe2

a

c or h

PBE

HSE06

4.660 3.182 3.318 3.182 3.316

2.969 3.127 3.338 3.140 3.355

1.663 1.673 1.44 1.812 1.542

3.03 2.142 1.889 2.303 2.004

sheets (MS2) and MoSe2 and WSe2 sheets (MSe2) have quite similar lattice parameters. This similarity indicates that the difference of metal species only shows a slight effect on the lattice parameters when the MX2 sheets have the same chalcogen (S and Se). In general, there are two common crystallographic TiO2:rutile (tetragonal, P42/mnm) and anatase (tetragonal, I41/amd).42 The rutile TiO2(110) and anatase TiO2(101) surfaces are the most stable among the low index surfaces of the two species. In the present work, we choose a four-layer rutile TiO2(110) slab as the substrate mainly because a 2 × 1 unit cell of rutile TiO2(110) has a 2D rectangular cell of 5.938 Å × 6.591 Å, which can match well with a √3 × 2 rectangular unit cell of monolayer MX2 sheets. The unit cell and four-layer(110) slab of rutile TiO2 are displayed in Figure 1b, and the rectangular unit cell of 2 × 1 rutile TiO2(110) and the √3 × 2 MX2 sheet are shown in Figure 1c, respectively. The MX2− TiO2 hybrid systems were modeled by placing a monolayer MX2 sheet on the top of four-layer TiO2(110) slabs. A vacuum region of 20 Å above TiO2(110) slabs was used to ensure the decoupling between neighboring systems.

3. RESULTS AND DISCUSSION In our calculations, the most stable contact geometries of MX2/ TiO2(110) heterojunctions were obtained by optimizing the different initial configurations, and the surface lattices of these systems were also optimized during the structural relaxation. The van der Waals (vdW) interactions in the DFT-D2 method of Grimme,43 which accounts for long-range dispersion correction, have been checked for the MX2/TiO2(110) hybrid systems. The results indicate that it has only negligible influence on the total energies and the stable geometries. The optimized geometries of MX2/TiO2(110) heterojunctions are illustrated in Figure 2. For all systems, the MX2 sheets and the TiO2(110) slabs have almost identical geometries after relaxation; i.e., the center bottom-layer X atoms of MX2 sheets absorbed on the bridge sites of the O atom surface of TiO2(110) slabs (see Figure 2e). The monolayer MX2 sheet remains the flat graphene-like structure without large distortions. The optimized surface lattices, MX2−TiO2(110) binding energies, and MX2−TiO2(110) distances of MX2/ TiO2(110) heterojunctions are listed in Table 2. The MX2− TiO2(110) binding energies per rectangular unit cell, calculated by Eb = (EMX2 + ETiO2(110) − EMX2/TiO2(110)), were found to be positive values, indicating that the MX2/TiO2(110) hybrid systems form a stable interface. In addition, the interactions between the MX2 sheets and TiO2(110) slabs are relatively weak with the binding energies range from 0.024 to 0.033 eV and the MX2−TiO2(110) distances (Dz) range from 3.231 to

Figure 1. (a) The top and side views of monolayer the MX2 sheet (M = Mo and W; X = S and Se. a and b are the lattice constants, and h represents the height of the MX2 sheet. (b) The unit cell of rutile TiO2 and the side view of the four-layer rutile TiO2(110) slab. The rectangular region with a black dashed line edge is the monolayer TiO2(110) surface. (c) The top views of the 2 × 1 unit cell and √3 × 2 rectangular unit cell of rutile TiO2(110) and monolayer MX2 sheet, respectively.

heterogeneous graphene-like sheets, the monolayer transitionmetal dichalcogenide MX2 is a two-dimensional (2D) sheet where M atoms occupy one sublattice of the hexagonal sheet and X atoms occupy the other. The obtained thickness and lattice constant of the MoS2 sheet are 3.127 and 3.182 Å, respectively. These calculated values agree well with the previous calculation results (3.13 and 3.19 Å, respectively).41 The optimized lattice parameters of monolayer MX2 are summarized in Table 1. It can be seen that the MoS2 and WS2 5955



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monolayer MX2 sheets and bulk rutile TiO2 were also calculated for comparison, as displayed in Figure 3. It is found that the monolayer MX2 sheet is a direct band gap semiconductor, and the calculated band gaps are 2.142, 1.889, 2.303, and 2.004 eV for MoS2, MoSe2, WS2, and WSe2, respectively. These results are also in good agreement with the previous theoretical results.41 On the basis of HSE06 hybrid functional calculations, the bulk rutile TiO2 is a direct band gap semiconductor with the band gap of 3.03 eV, which is precisely consistent with the experimental value of 3.0 eV. Because the band gaps of monolayer MX2 sheets are around 2.0 eV, the semiconducting MX2 sheets are suitable for visible-light absorption. The band gap energies of the monolayer MX2 sheet and bulk rutile TiO2 within the exchange and correlation functional of PBE and HSE06 are listed in Table 1. Compared with the PBE results, the HSE06 calculations show greater band gaps with increments of ∼0.5 and 1.4 eV for MX2 sheets and bulk rutile TiO2, respectively. As listed in Table 1, the HSE06 hybrid functional results are more precise than standard PBE functional results compared to the experimental data. The HSE06 calculated density of states (DOSs) for MX2/ TiO2(110) hybrid systems are displayed in Figure 4, where the MX2/TiO2(110) heterojunctions show a typical type-II band alignment where the CB of MX2 locates at higher energy states than that of the TiO2(110) surface. The band gaps of MX2 sheets in the MX2/TiO2(110) heterojunctions are around 1.5 eV (see Figure 5), so the electron can be excited from the valence band (VB) to the CB of the semiconducting MX2 sheets under visible-light irradiation, yielding a hole in its VB. Then the photogenerated electron is injected into the CB of TiO2, and the positively charged MX2 sheets can remove an electron from the VB of TiO2, transferring the photogenerated hole to the VB of TiO2. Therefore, the oxidation and redox reactions can take place in TiO2 and MX2 sheets, respectively. Actually, the intrinsic band gap of TiO2 is almost not changed after the formation of the hybrid interface with the MX2 sheets, indicating that MX2/TiO2(110) heterojunctions still keep the photoactivity under UV irradiation. Therefore, such a band alignment in the MX2/TiO2(110) hybrid systems is of great advantage for the photocatalytic activity enhancement. These findings rationalize the enhanced visible-light photocatalytic activity of the MX2/TiO2(110) hybrid systems reported by experiments.28−31 Since the MX2 sheets act as a photosensitizer in the MX2/TiO2(110) heterojunctions, the electron−hole pair

Figure 2. Optimized geometries of (a) MoS2/TiO2(110), (b) MoSe2/ TiO2(110), (c) WS2/TiO2(110), and (d) WSe2/TiO2(110) heterojunctions. Dz are the distances between MX2 (M = Mo and W; X = S and Se) sheets and TiO2(110) slabs. (e) The structure of center bottom-layer X atoms of the MX2 sheet absorbed on the bridge sites of surface O atoms of the TiO2(110) slab.

Table 2. Optimized Lattice Parameters of Rutile TiO2(110) Surface and MX2/TiO2(110) (M = Mo and W; X = S and Se) Heterojunctions; and the Absorption Properties of Monolayer MX2 Sheets on Rutile TiO2(110) Slabs lattice parameters (Å) TiO2(110) MoS2/TiO2(110) MoSe2/TiO2(110) WS2/TiO2(110) WSe2/TiO2(110)

a

b

Eb (eV)

Dz (Å)

5.938 5.855 5.892 5.848 5.889

6.591 6.506 6.562 6.505 6.563

0.024 0.033 0.025 0.033

3.239 3.246 3.231 3.257

3.257 Å. Although the binding energies of MoSe2/TiO2(110) and WSe2/TiO2(110) heterojunctions are stronger than those of MoS2/TiO2(110) and WS2/TiO2(110) ones, the distances of MoSe2−TiO2(110) and WSe2−TiO2(110) are larger than those of MoS2−TiO2(110) and WS2−TiO2(110), which can be attributed to the larger atom radius of Se (Se, 1.22 Å; S, 1.09 Å). To understand the photocatalytic performance of the MX2/ TiO2(110) heterojunctions, we calculated the electronic structures of MX2/TiO2(110) hybrid systems by using the HSE06 hybrid functional. The band structures of individual

Figure 3. HSE06 calculated band structures of monolayer MX2 sheets (M = Mo and W; X = S and Se) and bulk rutile TiO2 along high symmetry lines of the Brillouin zone. The energy zero is taken as the Fermi level and is displayed with a blue dashed line. 5956



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MX2 and TiO2 bands can still be identified by referencing the corresponding energy ranges in the DOSs. Owing to the formation of an interface, the band gaps of monolayer MX2 sheets are reduced to 1.225, 1.720, 1.323, and 1.562 eV for MoS2, MoSe2, WS2, and WSe2, respectively. Furthermore, the direct band gaps of monolayer MoS2, WS2, and WSe2 become indirect band gaps in the MX2/TiO2(110) heterojunctions (see Figure 6). These band gap reduction and transition can be attributed to the interface strains induced by hybridization with TiO2, which is in good agreement with the previous theoretical study by Yun et al.40 In their work, the direct gap would become an indirect gap, and in particular, the tensile strain reduces the gap energy when the strain is applied in the monolayer of MX2 (M = Mo and W; X = S, Se, and Te). It is also found in Figure 6 that the CB energy bands of the MX2 sheets and TiO2(110) slabs are hybridized significantly, indicating a strong wave function overlap, which drives an efficient adiabatic photogenerated electron injection from MX2 sheets to the TiO2(110) surface.27 In addition to the charge injection, the efficient charge separation also plays a key role in improving photocatalytic performance. The built-in potential is defined as the energy difference between the effective TiO2 CBM and MX2 CBM (CBM offset), and they are 0.526, 1.577, 0.948, and 1.581 eV for MoS2/TiO2(110), MoSe2/TiO2(110), WS2/TiO2(110), and WSe2/TiO2(110) systems, respectively. On one hand, a large built-in potential can drive charge carriers, resulting in effective separation. On the other hand, due to the fixed band gap of the photosensitizer in the hybrid system, the larger built-in potential would reduce the oxidation potential of the hole in the valence band maximum (VBM) of a photosensitizer, leading to a lower photocatalytic activity. For example, the built-in potential is 1.581 eV in the MoSe2/ TiO2(110) heterojunction, and the CB of TiO2 almost hybridizes with the VB of MoSe2 nearby the Fermi level (see Figure 5). As a result, the injected electrons in the CBM of TiO2 would recombine the holes that remained in the VBM of MoSe2 after electron injection and carriers separation. Therefore, the MoS2/TiO2(110) and WS2/TiO2(110) heterojunctions are more suitable for photocatalysis than the MoSe2/ TiO2(110) and WSe2/TiO2(110) ones. To achieve high photocatalytic performance, we suggest that the desirable semiconductor−TiO2 heterojunctions should have a photosensitizer with the band gap of ∼2.0 eV and the built-in potential of 0.3−0.5 eV, which can be considered as a efficient

Figure 4. HSE06 calculated DOSs of MoS2/TiO2(110), MoSe2/ TiO2(110), WS2/TiO2(110), and WSe2/TiO2(110) heterojunctions. The Fermi level of all of these systems is displayed with a black dashed line.

can be easily generated by visible-light irradiation and then be effectively separated by the electron injection from MX2 to TiO2. This mechanism is quite different from the one of the foreign elements doped TiO2,16,19,21 in which the electron is directly excited from the midgap impurity states into the CB of TiO2, leading to an optical absorption edge extending to the visible-light region. Considering the fact that monolayer MX2 sheet-decorated TiO2 can lower the electron−hole recombination rate and would not induce large lattice distortion, it can be concluded that this decoration method is better than the doping one. The HSE06 calculated band structures of the MX2/ TiO2(110) hybrid systems are illustrated in Figure 5. In the hybrid systems, although the energy bands of MX2 sheets hybridize with those of TiO2 to some extent, the majority of the

Figure 5. HSE06 calculated band structures of MX2/TiO2(110) (M = Mo and W; X = S and Se) composites along high symmetry lines of the Brillouin zone. The red, blue, and green dots represent the energy bands of TiO2, MX2, and MX2−TiO2 hybrids, respectively. The band gaps of MX2 sheets in MX2/TiO2(110) heterojunctions are illustrated with a black line with an arrow, and the values are also displayed in black numbers. The energy zero is taken as the Fermi level. 5957



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Figure 6. Three-dimensional charge density difference of MoS2/TiO2(110), MoSe2/TiO2(110), WS2/TiO2(110), and WSe2/TiO2(110) composites. The green region represents charge accumulation, and the blue region indicates charge depletion. The isosurface value is 8.0 × 10−5 e/Bohr.3

strategy to design the TiO2-based heterojunction composites for enhancing visible-light photocatalysis. To clarify the charge transfer and separation process, we calculated the three-dimensional charge density difference by subtracting the electronic charges of the individual MX2 sheet and TiO2(110) slab from that of a hybrid MX2/TiO2(110) composite, as shown in Figure 6. It is found that the charge redistribution mainly occurs at the interface region of the MoS2/TiO2(110) heterojunction, while there is almost no charge transfer from the TiO2(110) slab matrix to the interface. The charged interface region of MoS2/TiO2(110) is very similar to the space charge region of the p−n junction, in which the electron−hole pair can be effectively separated under a built-in potential of 0.526 eV. In contrast, the charge redistribution can be seen in the whole TiO2(110) slabs beside the interfaces for MoSe2/TiO2(110), WS2/TiO2(110), and WSe2/TiO2(110) heterojunctions, indicating the large amount of charge transfer between the MX2 (MoSe2, WS2, and WSe2) sheet and TiO2(110) slab. To quantify the charge transfer between the individual MX2 sheet and TiO2(110) slab, we performed the Bader analysis44 of the charge densities of MX2/ TiO2(110) heterojunctions. It is found that there are 0.021, 0.085, 0.039, and 0.073 electrons transferred from the MX2 sheet to the TiO2(110) slab in MoS2/TiO2(110), MoSe2/ TiO2(110), WS2/TiO2(110), and WSe2/TiO2(110) systems, respectively. The large amount of charge transfer between the MX2 (MoSe2, WS2, and WSe2) sheet and TiO2(110) slab indicates a strong donor−acceptor interaction. It suggests that the MoS2/TiO2(110) composite is the best candidate for photocatalysts among the four heterojunctions, owing to the proper built-in potential of the MoS2/TiO2(110) composite and ideal charge redistribution in the interface region.

alignment. Such a band alignment and the CB energy band hybridization of MX2 sheets with TiO2(110) slabs facilitate the injection of photoexcited electrons from MX2 to TiO2(110) slabs. Moreover, the charge transfer in the MX2/TiO2(110) interface leads to large built-in potentials, which is advantageous for the electron−hole separation in the interface region. On the basis of the fact that the monolayer MX2 sheetdecorated TiO2 can lower the electron−hole recombination rate and would not induce large lattice distortion, it can be concluded that this decoration method is much better than the commonly used doping one by the incorporation of foreign elements into TiO2 for enhanced visible-light photocatalysis. By analyzing the electronic properties of MX2 /TiO 2(110) heterojunctions, we suggest that the desirable semiconductor−TiO2 hybrid systems should have a photosensitizer with the band gap of ∼2.0 eV and have a proper built-in potential of 0.3−0.5 eV. It is expected that this proposed fundamental strategy can be used for the design of TiO 2 -based heterojunction composites for enhanced visible-light photocatalysis in the future.

4. CONCLUSIONS In summary, we have performed hybrid density functional calculations to explore electronic properties of monolayer transition-metal dichalcogenide MX2 (M = Mo and W; X = S and Se) interfaced TiO2(110) composites. It is found that the semiconducting MX2-decorated TiO2(110) composites display a significant photocatalysis enhancement. On one hand, the monolayer MX2 in the composites benefits for visible-light absorption, indicating that the monolayer MX2 acts as an effective photosensitizer. On the other hand, the MX2/ TiO2(110) heterojunctions show a typical type-II band

Notes

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ASSOCIATED CONTENT

* Supporting Information S

The atomic coordinates of optimized configurations for MX2/ TiO2(110) heterojunctions (M = Mo and W; X = S and Se). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D. Cheng). Fax: +861064427616. *E-mail: [email protected] (D. Cao). The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National NSF of China (91334203, 21106003, 21274011, 21121064), the National 863 Program (2013AA031901), Beijing Novel Program (Z12111000250000), University Scientific Research Funding (ZZ1304), Outstanding Talents Plans (RC1301) and “Chemical Grid Project” of BUCT, and the Beijing Computing Center (BCC). 5958



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