g-C3N4 Heterostructure: A

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The Photocatalytic Properties of G-CN/gCN Heterostructure#a Theoretical Study 3

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Dongmei Liang, Tao Jing, Yuchen Ma, Jinxin Hao, Guangyu Sun, and Mingsen Deng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08699 • Publication Date (Web): 11 Oct 2016 Downloaded from http://pubs.acs.org on October 12, 2016

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The Journal of Physical Chemistry

The Photocatalytic Properties of g-C6N6/g-C3N4 Heterostructure：a Theoretical Study Dongmei Liang,†,‡ Tao Jing,*, ‡ Yuchen Ma,† Jinxin Hao,§ Guangyu Sun,§ and Mingsen Deng§ † School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, People’s Republic of China ‡ College of Physics and Electronic Engineering, Kaili University, Kaili Guizhou 556011, People’s Republic of China § Guizhou Provincial Key Laboratory of Computational Nano-Material Science, Appied Physics Institute, Guizhou Education University, Guiyang 550018, People’s Republic of China *E-mail: [email protected] Telephone number:+86-0855-8558686 ABSTRACT: As a promising photocatalytic material in water splitting and organic degradation, the polymeric graphitic g-C3N4 has attracted intensive research interest during the past decade due to the visible light response, non-toxicity, abundance, easily preparation, as well as high thermal and chemical stability. However, the low efficiency owing to the fast charge recombination limits its practical applications. In the present work, we systematically investigated the electronic structure and photocatalytic properties of layered g-C6N6/g-C3N4 heterostructure on the basis of first-principles calculations. The results show that the type-II heterojunction can be established between g-C6N6 and g-C3N4 monolayers due to a perfect lattice match and aligned band edges, facilitating the separation of photo-generated carriers. In addition, it is worthwhile to note that hole effective masses of g-C6N6/g-C3N4 heterostructure can be significantly reduced compared to pristine g-C3N4 due to orbital hybridization between the two monolayers, which is extremely favorable for the migration of photo-generated holes. The g-C6N6/g-C3N4 heterostructure has reduced band gap compared to pristine g-C3N4, which can further be tuned by biaxial strain. This work not only provides new insights into the physical and chemical properties of the g-C3N4-based heterostructures, but also suggests viable ways to prepare the highly efficient photocatalytic materials. 1

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1 INTRODUCTION The photocatalytic water splitting, as a promising technology for the direct conversion of solar energy into chemical energy, has attracted extensive attention during the past decades.1-5 Generally, for an ideal photocatalytic material possessing the ability to split water, the wide range of light response is essential for achieving high utilization efficiency of solar energy. Additionally, the conduction band minimum (CBM) and valence band maximum (VBM) should straddle the water redox potentials.6,7 However, the conventional photocatalytic materials, such as TiO2 and SrTiO3, suffer from low conversion efficiency of solar energy because of the narrow light response range and high carrier recombination rate. Thus, to develop stable, highly efficient, inexpensive, and non-toxic photocatalytic materials, various strategies were employed to improve their photocatalytic activity.8-13 To date, however, the low utilization efficiency of solar energy for current photocatalytic materials still restrains their large-scale applications. As recently developed photocatalytic materials, the polymeric graphitic carbon nitride (g-C3N4) has drawn tremendous research interest because of its fascinating properties, such as visible light harvest, non-toxicity, abundance, easy preparation, as well as high thermal and chemical stability.14-18 In addition, the two-dimensional (2D) structure provides a larger area, more activity sites, and shorter carrier migration distance compared to bulk materials, leading to the high photocatalytic performance. However, g-C3N4 suffers from the low quantum efficiency, as a consequence of fast recombination of charge carriers. To enhance the quantum efficiency of g-C3N4, the construction of heterostructure by coupling it with other semiconductors were generally adopted due to the improved separation of photo-generated carriers. Therefore, a number of g-C3N4-based heterojunction composites, such as Graphene/g-C3N4,19 g-CN/g-CNS,20 g-C3N4/MoS2,21 g-C3N4/CdS,22 g-C3N4/Bi2WO623 and g-C3N4/Ag3PO424 were investigated and exhibited the significantly enhanced photocatalytic performance compared to pristine g-C3N4. In these systems, the build-in electric field induced by charge rearrangement at the interface drives the rapidly separation and migration for photo-generated carriers, which can further 2


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enhance the photocatalytic activity. Generally, for a heterostructure photocatalyst, the lattice match between two semiconductors is advantageous for the formation of high quality interface, which can suppress the introduction of defects or dangling bonds. Additionally, the type-II band alignment is extremely important for the flow of electrons and holes toward opposite directions, leading to spatial separation between the two semiconductors. Recently, another new graphitic carbon nitride g-C6N6 was theoretically proposed and has potential application in photocatalytic water splitting due to suitable band edges that straddle the water redox levels.25 In g-C6N6, s-triazines are joined together by C–C bonds without the need for additional nitrogen atoms, which is different from g-C3N4. The photocatalytic water splitting properties of g-C6N6 have been investigated by recently theoretical study, implying that the photo-generated holes in g-C6N6 have the ability to oxide water even the absence of co-catalyst, while the hydrogen evolution needs the aid of a co-catalyst.26 Very recently, it has been suggested that g-C6N6 has high thermodynamic and kinetically stability and is likely to be synthesized by the reaction of cyanuric chloride with sodium metal. In addition, due to the stronger spin-orbital coupling (SOC) than both graphene and silicone, g-C6N6 possesses topologically nontrivial electronic states.27 Considering that both the VBM and CBM of g-C6N6 are higher than that of g-C3N4,25 the type-II band alignment can be expected to realize between the two monolayer carbon nitride materials. Furthermore, the lattice matching between g-C3N4 and g-C6N6 means the possible formation of high quality interface, which facilities the fast migration of charge carriers. In present work, we will discuss the electronic structure and photocatalytic properties of g-C6N6/g-C3N4 heterostructure. The purpose of this work is to explore the g-C3N4-based heterostructure with high photocatalytic performance. Thus, the electronic structure and photocatalytic properties of g-C6N6/g-C3N4 heterostructure are investigated on the basis of first-principles calculations. The results show that the formation of type-II heterojunction between the two carbon nitride monolayers promotes the separation of charge carriers at the interfaces. More notably, the light absorption redshift and 3



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improved hole mobility can be realized, which may significantly enhance the photocatalytic activity of g-C3N4. We further propose that strain can be used to modulate the band gap and extend the light response range of g-C3N4.

2 COMPUTATIONAL DETAILS The density function theory (DFT) calculations were performed using the Vienna ab initio simulation package (VASP) with projector-augmented-wave (PAW) pseudopotentials.28 The generalized gradient approximation (GGA) in the parametrization of Perdew–Burke–Ernzerhof (PBE) was adopted to simulate the electronic exchange-correction effect.29 For g-C6N6/g-C3N4 heterostructure, the weak van der Waals interactions are expected to be important because of the absence of strong bonding interactions between g-C6N6 and g-C3N4 monolayers. Because the standard PBE functional cannot give a good description for the weak interactions, an empirical atom-pairwise corrections proposed by Grimme30 in terms of the DFT+D2 scheme were utilized for descripting the long-range vdW interaction. A cutoff energy of 450 eV was used throughout all calculations. The energy convergence criterion was 10−5 eV, and the Hellmann−Feynman forces were relaxed to less than 0.02 eVÅ−1. Integrations over the Brillouin zone were performed using a 9×9×1 mesh for the DFT calculations.31 Test calculations indicate that the cutoff energy and k-points are sufficient to reach convergence for the total energies, as shown in supporting information (S1). On the other hand, to cover the well-known shortage of band gap underestimation for the PBE functional, the Heyd, Scuseria, and Ernzerhof (HSE06) screened hybrid density functional method was commonly employed to obtain accurate electronic structures for semiconductor materials.32 Generally, the HSE06 method cannot give accurate and physical bandgap values for 2D materials because of the neglect of exciton effect. The effect of neglecting excitons can somewhat cancel the underestimation of HSE06 bandgaps compared with the quasi-particle GW bandgaps, thus, the optical bandgap values obtained by HSE06 are close to the real optical bandgaps.33 Since the GGA functional can correctly describe the ground state 4


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properties, the HSE06 was not applied for total energy calculations, which can reduce the computational cost.34,35

3 RESULTS AND DISCUSSION After structural relaxation, as shown in Table 1, the calculated lattice parameter for g-C3N4 is a = 7.139 Å, while for g-C6N6 is a=7.126 Å, consistent with previous theoretical and experimental reports.14,18,25 This implies that a heterojunction nanocomposite can be constructed by the two monolayer carbon nitride materials due to the extremely low lattice mismatch of 0.18%. To investigate the electronic properties of pristine g-C3N4 and g-C6N6, the band structures of which were calculated and given in Fig 1. For g-C3N4, we can clearly see that the VBM locates at G point while the CBM is located at one point within the G-K line, corresponding to an indirect band gap semiconductor. This is different from the previous DFT study, indicating that the VBM of g-C3N4 occurs at G point but the CBM occurs at M point.19 More interesting, a large dispersion near the CBM can be found, indicating the high mobility of photo-generated electrons. However, the band structure near the VBM is much localized and suggests the low mobility of photo-generated holes, which is responsible for the low quantum yield of g-C3N4 in photocatalytic process. In the case of g-C6N6, both the VBM and CBM locate at K point, thus resulting in a direct band gap semiconductor with a value of 3.22 eV, which is larger than that of g-C3N4 and can lead to UV light absorption. In order to explore the properties of photocatalytic water splitting, we calculated the work function of g-C6N6 and g-C3N4 with respect to the vacuum level by HSE functional. It is known that the values for the reduction level of hydrogen (H+/H2) and oxidation level of H2O (O2-/O2) are -4.44eV and -5.67eV with respect to the vacuum level, respectively. Thus, the conduction and valance band edges with respect to H+/H2 and O2-/O2 levels can also be determined, respectively, as shown in Table 1. Discussing first g-C3N4, the CBM is found to be 1.13 eV higher than H+/H2 level, while the VBM is 0.45 eV lower than O2-/O2 level. This indicates that g-C3N4 possesses the ability in water splitting, consistent with the previous experimental and theoretical reports.25, 36-38 For g-C6N6, the CBM is 0.27 eV 5



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higher than H+/H2 level, while the VBM is 1.72 eV lower than O2-/O2 level. This means that g-C6N6 also has the ability to splitting water, despite a larger band gap compared with g-C3N4. In addition, the formation of type-II heterojunction and improvement of charge separation can be expected between g-C6N6 and g-C3N4 due to the suitable band edges. In order to explore the transfer properties of charge carriers, the effective masses of the two monolayer carbon nitride materials were calculated according to

 d 2 Ek  m = ±h  2   dk  *

−1

2

The region for parabolic fitting was within an energy difference of 26 meV around the CBM or VBM in the reciprocal space, which corresponds to the thermal excitation energy of charge carriers at room temperature.7,39 The calculated electron ( me∗ ) and hole ( mh∗ ) effective masses along different directions for g-C3N4 and g-C6N6 were summarized in Table 2. For g-C3N4, we can see that me∗ along G-K direction is 0.27, reflecting the high mobility of photo-generated electrons. However, the obtained values of mh∗ along G-K and G-M directions are 3.27 and 5.22, respectively. The large values of mh∗ mean the poor mobility of photo-generated holes and low photocatalytic activity of g-C3N4, consistent with previous band structure analysis. Contrary to the case of g-C3N4, me∗ of g-C6N6 along G-K direction is 0.89, which is slightly larger than that along K-M direction. This indicates that the photo-generated electrons of g-C6N6 have modest transfer ability. It is interesting to note that mh∗ along G-K and K-M directions are 1.84 and 1.68, respectively, which are significantly smaller than that of g-C3N4. Thus, the quick motion of holes might prohibit the carrier recombination with photo-generated electrons. The activity of photocatalytic materials can be expected to improve by the formation of composites, thus, the g-C6N6/g-C3N4 heterostructure that contains g-C6N6 and g-C3N4 monolayers is constructed. We first consider the thermodynamic 6


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stability of g-C6N6/g-C3N4 heterostructure. When g-C6N6 is placed on g-C3N4 to form heterojunction, it is difficult to obtain the lowest energy globally because the number of atomic arrangements is extremely large. Here, four representative arrangements of g-C6N6 on the g-C3N4 are considered to obtain a relatively stable structure, and the side views are given in Fig 2. The N atom from g-C6N6 that is marked in Fig 2b is placed directly above the hollow site and three different C atoms of g-C3N4 (N-C configurations), which are also marked in Fig 2a. As shown in Fig 2c (H1), both g-C6N6 and g-C3N4 maintain an almost planar atomic structure, indicating the weak interactions between the two monolayers. On the contrary, g-C3N4 shown in Fig 2d (H2), 2e (H3) and 2f (H4) have significant surface wrinkling, suggesting a stronger interface interaction compared to H1. To quantitatively compare the relative stability of these absorption structures, the binding energies were calculated by formula EB = Eg-C6 N6 /g-C3 N 4 − Eg-C6 N 6 − Eg-C3 N 4

where Eg-C6 N6 /g-C3N4 is the total energy of g-C6N6/g-C3N4 heterostructure, while Eg-C6 N 6 and Eg-C3 N 4 are the total energies of pristine g-C6N6 and g-C3N4 monolayers, respectively. For the four different structures, the calculated binding energies of per atom (referred as g-C3N4) are -108, -118, -119 and -134 meV, respectively. The corresponding equilibrium interlayer distances, defined as the nearest atomic distance perpendicular to the layer direction, are 3.02, 2.66, 2.88 and 2.63, respectively. It is clearly seen that H1 has the highest binding energy and largest interlayer distance, which can be ascribed to the weak interaction. While the other three structures have lower binding energies and shorter interlayer distances, indicating the strong interaction between g-C6N6 and g-C3N4 monolayers, which is supported by the surface wrinkling of g-C3N4. Among these structures, H4 has the lowest binding energy and shortest interlayer distance, indicating the large thermodynamic stability. This may promote the charge transfer due to large wave function overlap and strong built-in electric field. Additional four arrangements that the N atom from g-C6N6 is placed above N atoms of g-C3N4 (N-N configurations) are also been investigated, as 7



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shown in S2. It is noted that g-C6N6 and g-C3N4 in N-N configurations maintain an almost plane structure and the binding energies are significantly higher compared with N-C configurations, which can be attributed to the strong repulsive interaction. Therefore, N-N configurations will be not discussed in further investigation due to the worse thermodynamic stability. To further explore the interfacial mechanical properties, the binding energies per atom for the four different configurations as a function of the interface distance are shown in Figure 3. From which it can be found that the binding energies of four different structures drastically increase as the interfacial distant decrease, while a stable increase can be observed when the interfacial distant is increased. It is worth to note that, for all four heterostructures, the binding energies become positive value when the interlayer distance is reduced to 1.8 Å. In addition, H1 has the lowest binding energy in the whole range, suggesting that the surface wrinkling induced by absorption can reduce the binding energy and increase the thermodynamic stability. Since H4 has the lowest binding energy in equilibrium distance, we use this configuration for further investigation. To investigate the electronic structure of g-C6N6/g-C3N4 heterostructure, the band structure and density of states (DOS) were calculated and shown in Fig 4. It is found that g-C6N6/g-C3N4 heterostructure has indirect band gap with the value of 2.60 eV, as the CBM locates at one point within G-K line and the VBM locates at K point. Compared to pristine g-C3N4, the band near the CBM is more localized, suggesting the reduced electronic mobility. However, it is interesting to find that the VBM of g-C6N6/g-C3N4 heterostructure is more delocalized, which means that the mobility of photo-generated holes can be significantly improved. To firmly confirm it, we calculated the effective masses of charge carriers for this heterostructure. Indeed, the effective masses of photo-generated holes are reduced to 0.94 and 0.69 along G-K and M-K directions, respectively. This indicates the greatly enhanced carrier mobility that may lead to improved quantum efficiency. To deeply understand this, we further calculated the partial density of states (PDOS) of this heterostructure, as shown in Fig 4b and 4c. We can clearly see that the CBM is mainly composed of the 2p states of N and C atoms from g-C6N6, and slightly by the 2p states of N atoms and C atoms from 8


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g-C3N4. By contrast, the VBM is mainly composed of the 2p states of N atoms from g-C3N4, and the N 2s and C 2p states make the slight contribution. In addition, it is noted that the 2p states of N atoms from g-C6N6 also make some contribution to the VBM. Therefore, the significantly reduced effective masses of photo-generated holes can be attributed to the orbital hybridization between g-C6N6 and g-C3N4 monolayers, which results in more delocalized band structure and reduced hole effective masses. To explore more information about the electronic structure of g-C6N6/g-C3N4 heterostructure, we calculated the partial charge density, as shown in Fig 5. It can be found that the CBM mainly is contributed by the C and N atoms from g-C6N6 and the VBM is mainly contributed by the N atoms from g-C3N4. This indicates that a type-II heterojunction can be realized between the two monolayer carbon nitride materials. We can speculate that, under the visible light irradiation, the photo-generated electrons from the CB of g-C3N4 can transfer to the CB g-C6N6 to participate in the reduction reaction, while the photo-generated holes from the VB of g-C6N6 can be trapped by the N atoms of g-C3N4 to participate in the oxidation reaction. Therefore, the spatial separation of charge carriers between the two monolayers can be realized, which suppresses their recombination and enhances the photocatalytic activity. In order to unravel more information about the bonding mechanism, it is worthwhile to investigate charge density difference, ∆ρ = ρ g-C6 N 6 /g-C3 N 4 − ρ g-C6 N6 − ρ g-C3 N 4

which is defined by subtracting the electronic charge of g-C6N6/g-C3N4 heterostructure from that of the corresponding isolated g-C6N6 and g-C3N4 monolayers. Here, ρg-C6 N6 /g-C3 N4 , ρ g-C6 N6 and ρ g-C3 N 4 are charge densities of the g-C6N6/g-C3N4 heterostructure as well as g-C6N6 and g-C3N4 monolayers. From the Fig 6a, it can be found that the charge density rearrangement in both carbon nitride monolayers and the electronic charge transfer between them can be realized. In addition, other than vertical interactions between g-C6N6 and g-C3N4, interactions along lateral orientations are also crucial to promote charge transfer in same monolayer. To further explore the properties of charge transfer and rearrangement, the 9



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plane averaged charge density difference is calculated and plotted in Fig 6b. The electronic charge transfer from g-C3N4 to g-C6N6 can be found since g-C6N6 has lower VBM position compared with g-C3N4. Thus, the charge transfer between g-C6N6 and g-C3N4 introduces the build-in electric field, which produces a driving force to separate the photo-generated carriers to different monolayers and is also beneficial for the photocatalytic performance. A further Bader charge analysis shows that the charge rearrangement in planes can be realized, and the total charge transferring from g-C3N4 to g-C6N6 is about 0.032 |e|, as shown in S3. It is known that the strain effect plays an important role in modulating of electronic properties for two-dimensional materials.40,41 This would be very feasible because strain can be readily exerted on two-dimensional materials by various strategies, such as lattice mismatch, external load, bending, and by applying stress on materials.42 Previous studies indicated that strain can not only be used to tune the band gap of two-dimensional photocatalytic materials, but also decrease the energy of hydrogen adsorption and stabilize the adsorbed hydrogen at the surface.43 Here, we only focus on the band gap modification of g-C6N6/g-C3N4 heterostructure through strain engineering. Due to the limited size, strain exerted along the direction perpendicular to the plane of layers would be hard to control in experiments. Here, we focus on the effective band gap engineering by controlling biaxial strain, which is imposed on the planes of these systems by varying their planar lattice parameters. The magnitude of strain is defined as ε = ∆c/c0, here, the lattice constants of the unstrained and strained supercell are equal to c0 and c = ∆c+c0, respectively. Firstly, we focus on the strain effect on the band gap of pristine g-C6N6 and g-C3N4 monolayers and the calculated results are shown in Fig 7a. From which it is noted that, for g-C6N6 and g-C3N4 monolayers, the band gaps increase steadily with increasing isotropic strain from -7.5% to 7.5%. The total increments are only 0.08 and 0.10 eV for g-C6N6 and g-C3N4 monolayers, respectively, indicating that strain has slight influence on their light responsible range. We also investigate the strain effect on the band gap of heterostructure, as shown in Fig 7b. It is found that the band gap increases monotonically with increasing isotropic strain from 0% to 7.5%, while a 10


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oscillation character can be found when the isotropic strain varies from -7.5% to 0%. In addition, the band gap of this heterostructure can be modulated by imposing strain and the band gap reduction is nearly 0.18 eV at 5% compression. To further understand the origin of the band gap variation of g-C6N6/g-C3N4 heterostructure, we also calculated the strain dependence of the band gap for isolated g-C6N6 and g-C3N4 monolayers that retain pristine geometric structures in this heterostructure. Interestingly, the band gap variation of g-C3N4 with strain exhibits similar trend as g-C6N6/g-C3N4 heterostructure, as shown in Fig 7b. Different from g-C3N4, the band gap of g-C6N6 can be increased both under the tensile and compression strain. The reasons for band gap variation of g-C6N6/g-C3N4 heterostructure with the strain are complex. Beside the band gap variation of two monolayers, other factors may also be included, such as interfacial charge transfer and the band edge positions of two monolayers. Therefore, our results consistently indicate that the strain can be used to tune the band gap and light absorption properties of g-C6N6/g-C3N4 heterostructure, and the light absorption redshift can be expected by using compression strain.

4 CONCLUSIONS In

summary,

the

electronic

structure

and

photocatalytic

properties

of

g-C6N6/g-C3N4 heterostructure are investigated on the basis of density functional theory. It is found that g-C3N4 is an indirect band gap semiconductor, while g-C6N6 has a direct band gap with larger band gap value. For g-C6N6, the mobility of photo-generated holes is significantly higher than that of g-C3N4. Since the lattice constants of g-C6N6 and g-C3N4 are very similar, it is likely that the two monolayers can form the g-C6N6/g-C3N4 heterostructure. Due to suitable band edges, the type-II heterojunction can be realized between g-C6N6 and g-C3N4, which facilities the separation and migration of charge carriers. In addition, the reduced band gap and enhanced mobility of photo-generated holes can also improve the photocatalytic performance of g-C6N6/g-C3N4 heterostructure. Furthermore, we find that strain can be used to module the band gap of g-C6N6/g-C3N4 heterostructure. 11



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ACKNOWLEDGMENTS This work was supported by the Joint Fund of Guizhou Province of Science and Technology Bureau (Grant No. HL20147226 and Grant No. LKK201327); The Science Foundation of Guizhou Province of Education Bureau (Grant No. 2007036); The High-level Talent Project of Guizhou Province (No. TZJF[2006]38).

REFERENCES (1) Chen, X. B.; Shen, S. H.; Guo, L. J.; Mao, S.

S. Semiconductor Based

Photocatalytic Hydrogen Generation. Chem. Rev. 2010, 110, 6503–6570. (2) Hisatomi, T.; Kubota, J.; Domen, K. Recent Advances in Semiconductors for Photocatalytic

and

Photoelectrochemical

Water

Splitting.

Chem.

Soc.

Rev. 2014, 43, 7520–7535. (3) Zou, Z.; Ye, J.; Sayama, K.; Arakawa, H. Direct Splitting of Water under Visible Light Irradiation with an Oxide Semiconductor Photocatalyst. Nature 2001, 414, 625– 627. (4) Xiang, Q.; Cheng, B.; Yu, J. Graphene-Based Photocatalysts for Solar-Fuel Generation. Angew. Chem., Int. Ed. 2015, 54, 11350–11366. (5) Li, Q.; Meng, H.; Zhou, P.; Zheng, Y. Q.; Wang, J.; Yu, J. G.; Gong, J. R. Zn1– xCdxS

Solid Solutions with Controlled Bandgap and Enhanced Visible-Light

Photocatalytic H2-Production Activity. ACS Catal. 2013, 3, 882–889. (6)Tong, H.; Ouyang, S.; Bi, Y.; Umezawa, N.; Oshikiri, M.; Ye, J. Nano-photocatalytic Materials: Possibilities and Challenges. Adv. Mater. 2012, 24, 229–251. (7) Jing, T., Dai, Y., Ma, X., Wei, W., Huang, B. Electronic Structure and Photocatalytic Water-Splitting Properties of Ag2ZnSn(S1–xSex)4. J. Phys. Chem. C 2015, 119, 27900–27908. (8) Zhao, Z.; Li, Z.; Zou, Z. Water Adsorption and Decomposition on N/V-Doped Anatase TiO2 (101) Surfaces. J. Phys. Chem. C 2013, 117, 6172–6184. (9)

Ran, J.

J.; Zhang, J.; Yu, J.

G.; Jaroniec, M.; Qiao, S.

Z.

Earth-Abundant

Cocatalysts for Semiconductor-Based Photocatalytic Water Splitting. Chem. Soc. 12


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Rev. 2014, 43, 7787–7812. (10) Xu, T.; Zhang, L.; Cheng, H.; Zhu, Y. Significantly Enhanced Photocatalytic Performance of ZnO via Graphene Hybridization and the Mechanism Study. Appl. Catal., B 2011, 101, 382–387. (11)Awazu, K.; Fujimaki, M.; Rockstuhl, C.; Tominaga, J.; Murakami, H.; Ohki, Y.; Y oshida, N.; Watanabe, T. A plasmonic photocatalyst consisting of silver nanoparticles embedded in titanium dioxide. J. Am. Chem. Soc. 2008, 130, 1676–1680. (12) Chen, X.; Liu, L.; Yu, P. Y.; Mao, S. S. Increasing Solar Absorption for Photocatalysis

with

Black

Hydrogenated

Titanium

Dioxide

Nanocrystals.

Science 2011, 331, 746–750. (13) Li, H.; Bian, Z.; Zhu, J.; Zhang, D.; Li, G.; Huo, Y.; Li, H.; Lu, Y. Mesoporous Titania Spheres with Tunable Chamber Stucture and Enhanced Photocatalytic Activity. J. Am. Chem. Soc. 2007, 129, 8406–8407. (14) Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M. A Metal-free polymeric Photocatalyst for Hydrogen Production from Water under Visible Light. Nat. Mater. 2008, 8, 76–80. (15)

Zhang, G. G.; Huang, C. J.; Wang, X. C. Dispersing Molecular Cobalt in

Graphitic

Carbon

Nitride

Frame

works

for

Photocatalytic

Water

Oxidation. Small 2015, 11, 1215–1221 (16) Zhang, J.; Chen, X.; Takanabe, K.; Maeda, K.; Domen, K.; Epping, J. D.; Fu, X.; Antonietti, M.; Wang, X. Synthesis of a Carbon Nitride Structure for Visible-Light Catalysis by Copolymerization. Angew. Chem., Int. Ed. 2010, 49, 441–444. (17) Zhang, G.; Lan, Z.; Lin, L.; Lin, S.; Wang, X. Overall water splitting by Pt/g-C3N4 photocatalysts without using sacrificial agents. Chem. Sci. 2016, 7, 3062– 3066. (18) Liu, G.; Niu, P.; Sun, C.; Smith, S. C.; Chen, Z.; Lu, G. Q.; Cheng, H. M. Unique electronic structure induced high photoreactivity of sulfur-doped graphitic C3N4. J. Am. Chem. Soc. 2010,132, 11642–11648. (19) Li, X.; Dai, Y.; Ma, Y.; Han, S.; Huang, B. Graphene/gC3N4 bilayer: considerable band gap opening and effective band structure engineering. Phys. Chem. Chem. 13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 26

Phys. 2014, 16, 4230–4235. (20) Dong, F.; Zhao, Z. W.; Xiong, T.; Ni, Z. L.; Zhang, W. D.; Sun, Y. J.; Ho, W. K. In Situ Construction of g-C3N4/g-C3N4 Metal-Free Heterojunction for Enhanced Visible-Light Photocatalysis. ACS Appl. Mater. Interfaces 2013, 5, 11392–1140. (21) Hou, Y.; Laursen, A. B.; Zhang, J.; Zhang, G.; Zhu, Y.; Wang, X.; Dahl, S.; Chorkendorff, I. Layered Nanojunctions for Hydrogen Evolution Catalysis. Angew. Chem., Int. Ed. 2013, 52, 3621–3625. (22) Fu, J.; Chang, B. B.; Tian, Y. L.; Xi, F. N.; Dong, X. P. J. Novel C3N4–CdS composite photocatalysts with organic–inorganic heterojunctions: in situ synthesis, exceptional activity, high stability and photocatalytic mechanism. J. Mater. Chem. A 2013, 1, 3083–3090. (23) Li, M.; Zhang, L.; Fan, X.; Zhou, Y.; Wu, M.; Shi, J. Highly Selective CO2 Photoreduction to CO over g-C3N4/Bi2WO6 Composites under Visible Light. J. Mater. Chem. A 2015, 3, 5189–5196. (24) He, Y.; Zhang, L.; Teng, B.; Fan, M. New Application of Z Scheme Ag3PO4/g-C3N4 Composite in Converting CO2 to Fuel. Environ. Sci. Technol. 2015, 49, 649–656. (25) Srinivasu, K.; Modak, B.; Ghosh, S. K. Porous Graphitic Carbon Nitride: A Possible

Metal-Free

Photocatalyst

for

Water

Splitting. J.

Phys.

Chem.

C 2014, 118, 26479–26484. (26) Srinivasu, K.; Ghosh, S. K. Photocatalytic splitting of water on s-triazine based graphitic carbon nitride: an ab initio investigation. J. Mater. Chem. A 2015, 3, 23011– 23016. (27) Wang, A.; Zhang, X.; Zhao, M. Topological Insulator States in a Honeycomb Lattice of S-Triazines. Nanoscale 2014, 6, 11157–11162. (28) Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15–50. (29) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Atoms, Molecules, Solids, and Surfaces: Applications of the 14


Page 15 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Generalized Gradient Approximation for Exchange and Correlation. Phys.Rev. B 1992, 46, 6671–6687. (30) Grimme, S. Semiempirical Gga-Type Density Functional Constructed with a Long-Range Dispersion Correction. J. Comput. Chem. 2006, 27, 1787–1799. (31) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188–5192. (32) Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Erratum: ̀ ̀Hybrid Functionals Based on a Screened Coulomb Potential. J. Chem. Phys. 2006, 124, 219906. (33) Yang, J. H., Zhang, Y., Yin, W. J., Gong, X. G., Yakobson, B. I., Wei, S. H.. Two-Dimensional SiS Layers with Promising Electronic and Optoelectronic Properties: Theoretical Prediction. Nano Lett. 2016, 162, 1110–1117. (34) Cai, Y., Chuu, C. P., Wei, C. M., Chou, M. Y. Stability and electronic properties of two-dimensional silicene and germanene on graphene. Phys. Rev. B 2013, 88, 245408. (35) Guo, G. C., Wang, D., Wei, X. L., Zhang, Q., Liu, H., Lau, W. M., Liu, L. M.. First-principles study of phosphorene and graphene heterostructure as anode materials for rechargeable Li batteries. J. Phys. Chem. C 2015, 6, 5002–5008. (36) Yang, S.; Gong, Y.; Zhang, J.; Zhan, L.; Ma, L.; Fang, Z.; Vajtai, R.; Wang, X.; Ajayan, P. M. Exfoliated Graphitic Carbon Nitride Nanosheets as Efficient Catalysts for Hydrogen Evolution under Visible Light. Adv. Mater. 2013, 25, 2452–2456. (37) Gu, Q.; Liao, Y.; Yin, L.; Long, J.; Wang, X.; Xue, C. Template-Free Synthesis of Porous Graphitic Carbon Nitride Microspheres for Enhanced Photocatalytic Hydrogen Generation with High Stability. Appl. Catal., B 2015, 165, 503–510. (38) Han, Q.; Wang, B.; Gao, J.; Cheng, Z.; Zhao, Y.; Zhang, Z.; Qu, L. Atomically Thin Mesoporous Nanomesh of Graphitic-C3N4 for High-Efficiency Photocatalytic Hydrogen Evolution. ACS Nano 2016, 10, 2745–2751. (39) Ma, X.; Dai, Y.; Guo, M.; Huang, B. The Role of Effective Mass of Carrier in the Photocatalytic Behavior of Silver Halide-Based Ag@AgX (X = Cl, Br, I): A Theoretical Study. ChemPhysChem 2012, 13, 2304–2309. (40) Chen, Y.; Sun, Q.; Jena, P. SiTe Monolayers: Si-Based Analogues of 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Phosphorene. J. Mater. Chem. C 2016, 00, 1–3. (41) Qian, X.; Fu, L.; Li, J. Topological crystalline insulator nanomembrane with strain-tunable band gap. Nano Research 2015, 8, 967–979. (42) Jiao, Y.; Zhou, L.; Ma, F.; Gao, G.; Kou, L.; Bell, J.; Sanvito, S.; Du, A. Predicting Single-Layer Technetium Dichalcogenides (TcX2, X=S, Se) with Promising Applications in Photovoltaics and Photocatalysis. ACS Appl. Mater. Interfaces 2016, 8, 5385–5392. (43) Voiry, D.; Yang, J.; Chhowalla, M. Recent Strategies for Improving the Catalytic Activity of 2D TMD Nanosheets Toward the Hydrogen Evolution Reaction. Adv. Mater. 2016, DOI: 10.1002/adma.201505597.

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Table 1. The calculated lattice parameters, band gaps and band edges for g-C3N4 and g-C6N6 in comparison with previous theoretical and experimental results. EC and EV are relative to the H+/H2 and O2-/O2 levels for water splitting, respectively. g-C3N4

a=b (Å)

E g ( eV )

E V ( eV )

E C ( eV )

7.139

2.81

0.45

-1.13

a

b

2.73

Theoretical data

c

7.06

2.82d

This work

7.126

3.22

1.72

-0.27

d

d

This work Experimental date

g-C6N6

7.13

Theoretical data a

b

7.127

c

3.18

d

See ref 14. See ref 18. See ref 19. See ref 25.

Table 2. The calculated effective masses of charge carriers for g-C3N4 and g-C6N6. me∗ (G- K) g-C3N4

0.27

g-C6N6

0.89

me∗ (K- M) 0.93

mh∗ (G- K)

mh∗ (G- M)

3.27

5.22

1.84

17


mh∗ (K- M) 1.68


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Figure Captions Figure 1. The band structures for g-C3N4 (a) and g-C6N6 (b). Figure 2.The crystal structure of g-C3N4 (a) and g-C6N6 (b); the side views of the relaxed atomic structures of the four arrangements of g-C6N6/g-C3N4 heterostructure, referred as H1 (c), H2 (d), H3(e), and H4 (f). Figure 3. Binding energies of g-C6N6 of per atom on the g-C3N4 monolayer for the four configurations as a function of the distance between g-C6N6 and g-C3N4. Figure4. The band structures of g-C6N6/g-C3N4 heterostructure (a), and the PDOS of g-C3N4 (b) and g-C6N6 in this structure (c). Figure 5. The partial charge density for VBM (a) and CBM (b) of g-C6N6/g-C3N4 heterostructure with a 0.0004 eÅ-3 isosurface value. Figure 6. The charge density difference (a) and the corresponding planar averaged charge density difference (b) for g-C6N6/g-C3N4 heterostructure. Figure 7. The effect of strain on the band gap of pristine g-C6N6 and g-C3N4 (a) as well as on g-C6N6/g-C3N4 heterostructure (b).

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Figure 1

19



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Figure 2

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Figure 3

21



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Figure 4

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Figure 5

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Figure 6

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Figure 7

25



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Table of Contents (TOC) Graphic

The type-II heterojunction can promote the separation of photo-generated carriers between g-C6N6 and g-C3N4.

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g-C3N4 Heterostructure: A

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