b. Tianjin Key Laboratory of Structure and Performance for Functional Molecules, Key Laboratory of Inorganic-Organic Hybrid Functional Materials Chemistry, Ministry of Education; College of Chemistry, Tianjin Normal University, Tianjin 300387, China;
c. School of Materials and Chemical Engineering, Anhui Jianzhu University, Hefei 230601, China
Hydrogen, produced from water splitting by using renewable solar energy, is an ultimately clean energy and plays a vital role in solving the energy crisis caused by the depletion of fossil fuels and the serious environment problems [1-2]. Since the pioneering work carried out by Fujishima and Honda that overall water splitting can be achieved via the photaocatalyst of TiO2 , photocatalytic water splitting has been intensively investigated in the past years [4-8]. Till now, over 130 semiconductor materials, including SrTiO3 , Ag3PO4 , NaTaO3 , and CdS , have been widely studied as promising photocatalysts. However, the practical application of photocatalytic water splitting for hydrogen production is limited, since most of photocatalysts are active only under ultraviolet (UV) irradiation (i.e. SrTiO3 and NaTaO3), where others (i.e. Ag3PO4 and CdS) are not stable during the photocatalytic process although they are capable of harvesting visible light. Hence, developing high efficiency and stable photocatalysts for water splitting aspects are theoretically and experimentally needed.
Metal-free photocatalysts have the advantages of nontoxicity and good process ability, and gradually become important catalysts. In 2009, Wang et al. reported that graphite-like carbon nitride (g-C3N4) can produce hydrogen from water under visible light irradiation in the presence of a sacrificial donor . It has attracted intensive attention because of its high chemical and thermal stability, promising electronic properties, and low cost. However, the energy conversion efficiency of the incoming solar energy is low due to the marginal visible light absorption. The g-C3N4 can only absorb the photons with wavelength shorter than 450 nm. Moreover, g-C3N4 suffers from a high recombination rate of photo generated electrons and holes, resulting in low hydrogen production efficiency . Therefore, in order to enhance the photocatalytic activity of this material under visible light, the optical absorption and the separation efficiency of photogenerated electron-hole pairs need to be greatly improved. Many approaches have been proposed to overcome these two limitations, such as doping with heteroatoms and loading surface co-catalysts [15, 16]. A more effective strategy is to construct g-C3N4 based heterostucture, such as g-C3N4/TiO2 , g-C3N4/ZnO , g-C3N4/rGO , g-C3N4/GQD [14, 20], g-C3N4/WO3 , g-C3N4/BiVO4 [22, 23], g-C3N4/Zn2GeO4 , and g-C3N4/Ag3PO4 . These g-C3N4/semiconductor heterostructures greatly reduce the possibility of electron-hole recombination and potentially enhance the photocatalytic performance. Recently, the especially appealing and intriguing heterostructures are g-C3N4 coupled with transition-metal dichalcogenides (TMD), owing to their suitable band gap and high hydrogen evolution reaction activity . For example, Hou et al.  reported that g-C3N4/MoS2 heterostructures exhibited enhanced photocatalytic hydrogen evolution activity under visible-light irradiation, and their performance was comparable to that of Pt/g-C3N4 under their reaction conditions.
Recently, Sun et al.  synthesized free-standing SnS2 single-layer with three atom thickness through a convenient and scalable liquid exfoliation strategy. And it reached a visible light conversion efficiency of 38.7%, which is expected to offer an excellent platform to achieve efficient visible light water splitting. However, it is not suitable for the H2 production since its conduction band edge position does not match the reduction potential of water. Motivated by this experimental investigation, we try to examine the possibility of the SnS2 sheet hybridizing with g-C3N4. And it is also interesting to explore the interfacial effect on the structural, electronic and optical properties of the g-C3N4/SnS2 heterostructure, which are vitally important to characterize its photocatalytic activity. Here, by performing extensive density functional theory (DFT) calculations, we try to explore the interfacial interaction between monolayer g-C3N4 and SnS2 sheet and align the valence and conduction band edge positions of g-C3N4 and SnS2. Theoretical results clearly reveal that a nice type-II band alignment, which is desirable for visible light water splitting, appears in this proposed heterostructure. Moreover, the built-in interfacial electric field due to the charge transfer between g-C3N4 and SnS2 sheet can further enhance the photogenerated electron-hole separation. That is to say, SnS2 sheet is a promising candidate as a non-noble metal co-catalyst to improve the photocatalytic performance of g-C3N4 photocatalyst.II. COMPUTATIONAL DETAILS
Our calculations are performed using the Vienna ab initio simulation package (VASP) [29, 30]. We use the frozen-core projector augmented wave approach to describe the interaction between the core and valence electrons . To optimize the geometric structures, the generalized gradient approximation of Perdew-Burke-Ernzerhof (PBE)  form with van der Waals (vdW) correction proposed by Grimme  is chosen due to its good description of long-range vdW interactions. The energy cutoff is set to be 520 eV. A Monkhorst-Pack mesh of 10×10×1 and 2×2×1 k-points  is used to sample the two-dimensional Brilliouin zone for the geometry optimizations and electronic structure calculations, respectively. The vacuum space is set to be at least 20 Å to separate the neighboring slab interactions. All geometry structures are fully relaxed until the convergence criteria of energy and force are less than 10-5 eV and 0.01 eV/Å, respectively. The Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional [35, 36] is used to obtain the correct electronic structures of g-C3N4, SnS2 sheet, and g-C3N4/SnS2 heterostructure. The Hartree-Fock exchange mixing parameter (a) in the default HSE06 functional is set to be 0.25, and the band gaps of monolayer g-C3N4 and SnS2 sheet are predicted to be 3.19 and 2.52 eV, respectively, which are seriously overestimated [13-37]. In order to reproduce the experimental band gaps of free g-C3N4 and SnS2 sheets, the value of mixing parameter is reduced from 0.25 to 0.175 in this work.
To explore the optical properties of g-C3N4/ SnS2 heterostructure, the optical absorption spectra are simulated by converting the complex dielectric function to the absorption coefficient αabs according to the following relation ,
Here, the ε1 and ε2 are the real and imaginary absorption factors, respectively.III. RESULTS AND DISCUSSION}
To examine the reliability of the computational parameters, we perform benchmark calculations for the isolated single-layer g-C3N4 and SnS2 sheets. Figure 1(a) shows the optimized g-C3N4 monolayer, the lattice parameters a=b=4.79 Å. All C atoms have three nearest N atoms, which are divided into two kinds (labeled as N1 and N2, the N1 atoms are coordinated with two C atoms, leaving a non-bonding character, while the N2 atoms are fully connected with three C atoms). The C-N1 and C-N2 bonding lengths are about 1.33 and 1.47 Å, respectively. The calculated band structure, as shown in Fig. 1(c), shows that the g-C3N4 monolayer is a semiconductor with a direct energy gap of 2.71 eV. Figure 1(e) shows the calculated density of states (DOS) displays that the valence band maximum (VBM) is mainly dominated by the p orbitals of N1 atoms, and the conduction band minimum (CBM) is composed of p orbitals of N2 and C, which well reflects the different chemical bonding environments of monolayer g-C3N4. These results agree well with previous experimental measurements and theoretical results [13, 28, 37, 39]. As for SnS2 sheet, the top and side views of an optimized SnS2 sheet are illustrated in Fig. 1(b). Clearly, it has a CdI2-type hexagonal structure consisting of a S-Sn-S triple layer. The vertical S-S distance is 2.99 Å and the Sn-S bond length is 2.58 Å. The bandstructures and DOS results are shown in Fig. 1 (e) and (f), respectively. It is clear that SnS2 sheet displays an indirect band gap energy with a suitable size of 2.12 eV from the HSE06 functional, which is consistent with the experimental data . In the energy range of -1.0 eV to 0.0 eV, the valence bands mainly originate from S3p orbitals. As for the CBM, it is contributed by the hybridized states between S3p and Sn5s orbitals. This is a little different from TMD. For example, the conduction bands of single-layer MoS2 come from the hybridization between S3p and Mo3d orbitals . These calculations imply that the adopted computational method and parameters are appropriate.
To improve the optical absorption and the separation efficiency of photogenerated carriers of g-C3N4, a single-layer SnS2 can be loaded on it and act as a non-noble metal co-catalyst. The top and side views of this proposed heterostructure are illustrated in Fig. 2 (a) and (b), respectively. Here, a 4×4 supercell of SnS2 (including 16 Sn and 32 S atoms) is covered on a 3×3 supercell of g-C3N4 (27 C and 36 N atoms) with the small lattice mismatch of less than 3.0%. Clearly, the g-C3N4 monolayer displays an obvious geometric distortion due to the presence of SnS2 sheet, and the corresponding buckling distance (h1) is about 0.98 Å. The vertical distance (h2) between monolayer g-C3N4 and SnS2 sheet is predicted to be 3.09 Å. This is a typical vdW equilibrium space, which is consistent with other g-C3N4-based semiconductor heterostructures [19, 20, 23, 37, 41, 42].
To describe the thermodynamic stability of this proposed g-C3N4/SnS2 heterostructure, we calculate the interface formation energy (Ef), which is defined as
Now we turn to calculate the band structures, and the corresponding total DOS and projected DOS of the g-C3N4/SnS2 heterostructure at the HSE06 level, and plot them in Fig. 2(c). Clearly, the g-C3N4/SnS2 heterostructure is an indirect semiconductor. Due to the interaction between g-C3N4 monolayer and SnS2 sheet, the band gap of the g-C3N4/SnS2 heterostructure is reduced to about 1.79 eV, which is less than the band gaps of g-C3N4 monolayer and SnS2 sheet. This reduced band gap implies that the electron in the VB of the g-C3N4/SnS2 heterostructure can be more easily excited to the CB, consequently, it will result in the red shift of the optical absorption edge. The VBM of g-C3N4/SnS2 heterostructure is contributed by g-C3N4 monolayer, while the CBM mainly originates from SnS2 sheet. Due to the relative large separation and the weak interaction between the g-C3N4 and SnS2, the main features of the calculated partial DOS of Sn, S, N, and C atoms in the g-C3N4/SnS2 heterostructure, as shown in Fig. 3, are similar to the results for the isolated g-C3N4 monolayer and the SnS2 sheet, as shown in Fig. 1 (e) and (f), respectively. While the CBM mainly comes from the states of SnS2 sheet. Moreover, the VBM of g-C3N4 is 0.22 eV higher than that of g-C3N4, whereas the CBM of g-C3N4 is 1.01 eV lower than that of SnS2. That is to say, the proposed g-C3N4/SnS2 heterostructure is a typical type-II band alignment structure, in which the valence band offset (VBO) between g-C3N4 and SnS2 is about 0.22 eV, and the conduction band offset (CBO) is about 1.01 eV.
To explore the charge transfer between g-C3N4 monolayer and SnS2 sheet in the proposed heterostructure, we calculate the charge density difference by subtracting the charge density of g-C3N4/SnS2 heterostructure from that of the free-standing g-C3N4 monolayer and SnS2 sheet, and plot in Fig. 4(a). The cyan region indicates the charge accumulation while the purple region represents the charge depletion. Clearly, charge redistribution mainly occurs at the g-C3N4/SnS2 interface region, while there is almost no charge change in the SnS2 sheet. To illustrate the charge transfer more clearly, Fig. 4(b) shows the planar-averaged charge density difference along the z-direction, here, the positive values represent electron accumulation, while the negative values indicate the electron depletion. It is clear that the charge transfer from the g-C3N4 to the SnS2 sheet in the heterostructure occurs at the interface region. Based on the Bader analysis, the average charge transfer from g-C3N4 to SnS2 sheet is about 0.33 $|e|$. This can be easily understood, since the work functions of the g-C3N4 monolayer and SnS2 sheet are predicted to be 5.17 and 6.98 eV, respectively. That is to say, the spontaneous interfacial charge transfer from the g-C3N4 monolayer to SnS2 sheet can be simply rationalized in terms of the large difference between two calculated work functions. Note that this spontaneous interfacial charge transfer result in a built-in electric field at the interface region, which can promote photogenerated electrons from the CB of SnS2 to the CB of g-C3N4, and also improve photogenerated holes transfer from the VB of g-C3N4 to the VB of SnS2. It means that this built-in interface electric filed can effectively enhance the photogenerated electron-hole separation.
A key index to the photocatalyst performance is the optical absorption since it is strongly related to their photocatalytic activities. We calculate the optical absorption curves of the g-C3N4 and the g-C3N4/SnS2 heterostructure, and plot them in Fig. 5, respectively. The optical absorption for the free-standing g-C3N4 monolayer occurs at about 3.0 eV, which is attributed to the intrinsic transition from the N1 2p orbitals to the CB. Clearly, the g-C3N4/SnS2 heterostructure can effectively harvest the visible light since the absorption edge shifts from UV region to infrared light. Moreover, the predicted absorption spectra depend on the polarization direction. The optical absorption coefficients along the vertical direction are significantly larger than that along the parallel direction.
In general, the oxidation/reduction ability of a photocatalyst for water splitting can be evaluated by the band edge positions of VBM and CBM, compared with the reduction and oxidation potentials of water. We thus determine the band edge positions of the free-standing g-C3N4 monolayer and SnS2 sheet, according to the method of Toroker et al. . Figure 6 illustrates the band edge alignments versus the normal hydrogen electrode (NHE), where the band edge positions of CBM and VBM of g-C3N4 monolayer are -0.83 and 0.65 eV, while the CBM and VBM of SnS2 sheet locate at 0.58 and 1.47 eV, respectively. These results agree well with the previous experimental and theoretical investigations [24, 44]. Note that the CBM of SnS2 sheet is more positive than the reduction potential of H+/H2, thus a bias potential of at least 0.58 V is required to drive the hydrogen evolution.
When the g-C3N4 monolayer covers on SnS2 sheet, the relative positions of the CB and VB of g-C3N4 and SnS2 will change with the Fermi level due to the charge redistribution . According to the above calculated DOS, as shown in Fig. 2, the CBM and VBM positions of the proposed g-C3N4/SnS2 heterostructure are plotted in Fig. 6 (right side). As mentioned above, this heterostructure is a type-II heterojunction. When the g-C3N4/SnS2 heterostructure absorbs photon energy under solar light irradiation, electrons in the VB are excited to the CB, and simultaneously, photogenerated holes are generated in the VB. These photogenerated electrons in the CB of SnS2 sheet can be easily moved to the CB of the g-C3N4 due to the observed CBO. Conversely, the existence of VBO induced the photogenerated holes in the VB of g-C3N4 moving to the VB of SnS2. That is to say, the oxidation and redox reactions will take place in g-C3N4 and SnS2 sheets, respectively.
In short, due to the observed band offsets built-in interface polarized electric field, we believe that in the proposed g-C3N4/SnS2 heterostructure the energy-wasteful electron-hole recombination could be effectively reduced, and then, the photcatalytic quantum efficiency can be greatly improved. That is to say, the proposed g-C3N4/SnS2 heterostructure is a promising g-C3N4 based water splitting photocatalyst.IV. CONCLUSION
In summary, we perform extensive hybrid DFT calculations to examine the geometric, electronic, optical properties as well as the band edge alignment of the proposed g-C3N4/SnS2 heterostructure. Theoretical results clearly reveal that the g-C3N4 monolayer weakly couples to SnS2 sheet, and forms a typical vdW heterojunction. The predicted band gaps, band edge positions and optical absorptions suggest that the g-C3N4/SnS2 heterostructure can effectively harvest visible light, and both VBM and CBM locate in energetically favorable positions for both water oxidation and reduction reactions. Moreover, the built-in interface polarized electric field within the interface region, owing to the charge transfer from the g-C3N4 monolayer to SnS2 sheet, is desirable for the photogenerated carrier separation. The built-in interface polarized electric field and the nice band edge alignment imply that the g-C3N4/SnS2 heterostructure is a promising water splitting photocatalyst with good performance.V. Acknowledgments
This work is supported by the National Key Basic Research Program (No.2014CB921101), the National Natural Science Foundation of China (No.21503149, No.21273208, and No.21473168), the PhD foundation of Tianjin Normal University (No.52XB1408), and the Innovative Program of Development Foundation of Hefei Center for Physical Science and Technology. Jing Huang thanks the Natural Science Foundation of the Anhui Higher Education Institutions (No.KJ2016A144) and the Natural Science Foundation of Anhui Province (No.1408085QB26). Computational resources have been provided by CAS, Shanghai and USTC Supercomputer Centers.
|||Kudo and Y. Miseki A., Chem. Soc. Rev. 38 , 253 (2009). DOI:10.1039/B800489G|
|||T. Hisatomi, J. Kubota, and K. Domen, Chem. Soc. Rev. 43 , 7520 (2014). DOI:10.1039/C3CS60378D|
|||Fujishima and K. Honda A., Nature 238 , 37 (1972). DOI:10.1038/238037a0|
|||R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, and Y. Taga, Science 293 , 269 (2001). DOI:10.1126/science.1061051|
|||J. Wang, H. Sun, J. Huang, Q. Li, and J. Yang, J. Phys. Chem (2014).|
|||X. Chen, S. Shen, L. Guo, and S. S. Mao, Chem. Rev. 110 , 6503 (2010). DOI:10.1021/cr1001645|
|||Z. Wang, Y. Liu, B. Huang, Y. Dai, Z. Lou, G. Wang, X. Zhang, and X. Qin, Phys. Chem. Chem. Phys. 16 , 2758 (2014). DOI:10.1039/c3cp53817f|
|||J. Wang, Q. Meng, J. Huang, Q. Li, and J. Yang, J. Chem. Phys. 140 , 174705 (2014).|
|||Iwashina and A. Kudo K., J. Am. Chem. Soc. 133 , 13272 (2011). DOI:10.1021/ja2050315|
|||Z. Yi, J. Ye, N. Kikugawa, T. Kako, S. Ouyang, Stuart-Williams H., H. Yang, J. Cao, W. Luo, Z. Li, Y. Liu, and R. L. Withers, Nat. Mater. 9 , 559 (2010). DOI:10.1038/nmat2780|
|||H. Kato, K. Asakura, and A. Kudo, J. Am. Chem. Soc. 125 , 3082 (2003). DOI:10.1021/ja027751g|
|||X. Zong, H. Yan, G. Wu, G. Ma, F. Wen, L. Wang, and C. Li, J. Am. Chem. Soc. 130 , 7176 (2008). DOI:10.1021/ja8007825|
|||X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, and M. Antonietti, Nat. Mater. 8 , 76 (2008).|
|||J. Liu, Y. Liu, N. Liu, Y. Han, X. Zhang, H. Huang, Y. Lifshitz, S. T. Lee, J. Zhong, and Z. Kang, Science 347 , 970 (2015). DOI:10.1126/science.aaa3145|
|||Y. Zhang, T. Mori, J. Ye, and M. Antonietti, J. Am. Chem. Soc. 132 , 6294 (2010). DOI:10.1021/ja101749y|
|||Y. Di, X. Wang, A. Thomas, and M. Antonietti, ChemCatChem 2 , 834 (2010). DOI:10.1002/cctc.201000057|
|||J. Zhou, M. Zhang, and Y. Zhu, Phys. Chem. Chem. Phys. 17 , 3647 (2015). DOI:10.1039/C4CP05173D|
|||Y. Wang, R. Shi, J. Lin, and Y. Zhu, Energy Environ. Sci. 4 , 2922 (2011). DOI:10.1039/c0ee00825g|
|||L. Xu, W. Q. Huang, L. L. Wang, Z. A. Tian, W. Hu, Y. Ma, X. Wang, A. Pan, and G. F. Huang, Chem. Mater. 27 , 1612 (2015). DOI:10.1021/cm504265w|
|||G. Gao, Y. Jiao, F. Ma, Y. Jiao, E. Waclawik, and A. Du, Phys. Chem. Chem. Phys. 17 , 31140 (2015). DOI:10.1039/C5CP05512A|
|||F. Zhan, R. Xie, W. Li, J. Li, Y. Yang, Y. Li, and Q. Chen, RSC Adv. 5 , 69753 (2015). DOI:10.1039/C5RA11464K|
|||C. Li, S. Wang, T. Wang, Y. Wei, P. Zhang, and J. Gong, Small 10 , 2783 (2014). DOI:10.1002/smll.201400506|
|||J. Zhang, F. Ren, M. Deng, and Y. Wang, Phys. Chem. Chem. Phys. 17 , 10218 (2015). DOI:10.1039/C4CP06089J|
|||L. Sun, Y. Qi, C. J. Jia, Z. Jin, and W. Fan, Nanoscale 6 , 2649 (2014). DOI:10.1039/c3nr06104c|
|||S. Kumar, T. Surendar, A. Baruah, and V. Shanker, J. Mater. Chem (2013).|
|||X. Chia, Y. S. Eng A., A. Ambrosi, S. M. Tan, and M. Pumera, Chem. Rev. 115 , 11941 (2015). DOI:10.1021/acs.chemrev.5b00287|
|||Y. Hou, Z. Wen, S. Cui, X. Guo, and J. Chen, Adv. Mater. 25 , 6291 (2013). DOI:10.1002/adma.201303116|
|||Y. Sun, H. Cheng, S. Gao, S. Q. Liu Z., F. Lei, T. Yao, J. He, S. Wei, and Y. Xie, Angew. Chem. Int. Ed. 51 , 8727 (2012). DOI:10.1002/anie.v51.35|
|||Kresse and J. Furthmuller G., Phys. Rev (1996).|
|||Kresse and J. Furthmuller G., Comput. Mater. Sci. 6 , 15 (1996). DOI:10.1016/0927-0256(96)00008-0|
|||P. E. Blochl, Phys. Rev (1994).|
|||J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. 77 , 3865 (1996). DOI:10.1103/PhysRevLett.77.3865|
|||S. Grimme, J. Comput. Chem. 27 , 1787 (2006). DOI:10.1002/(ISSN)1096-987X|
|||J. Monkhorst and J. D. Pack H., Phys. Rev (1976).|
|||J. Heyd, G. E. Scuseria, and M. Ernzerhof, J. Chem. Phys. 118 , 8207 (2003).|
|||J. Heyd, G. E. Scuseria, and M. Ernzerhof, J. Chem. Phys. 124 , 219906 (2006).|
|||J. Wang, Z. Guan, J. Huang, and Li and J. Yang Q., J. Mater. Chem A2 , 7960 (2014).|
|||M. Gajdos, K. Hummer, G. Kresse, Furthmüller J., and F. Bechstedt, Phys. Rev B73 , 045112 (2006).|
|||F. Wu, Y. Liu, G. Yu, D. Shen, Y. Wang, and E. Kan, J. Phys. Chem. Lett. 3 , 3330 (2012). DOI:10.1021/jz301536k|
|||L. Zhuang and R. G. Hennig H., Chem. Mater. 25 , 3232 (2013). DOI:10.1021/cm401661x|
|||J. Liu, J. Phys. Chem C119 , 28417 (2015).|
|||J. Cui, S. Liang, X. Wang, and J. M. Zhang, Phys. Chem. Chem. Phys. 17 , 23613 (2015). DOI:10.1039/C5CP03173G|
|||M. C. Toroker, D. K. Kanan, N. Alidoust, L. Y. Isseroff, P. Liao, and E. A. Carte, Phys. Chem. Chem. Phys. 13 , 16644 (2011). DOI:10.1039/c1cp22128k|
|||L. Zhuang and R. G. Hennig H., Phys. Rev B88 , 115314 (2013).|
b. 天津师范大学化学学院, 天津 300387;
c. 安徽建工学院材料与化工学院, 合肥 230022