SnS2

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High Photocatalytic Activity of Heptazine-Based G-CN/SnS Heterojunction and Its Origin: Insights From Hybrid DFT Jianjun Liu, and Enda Hua

J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07914 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 8, 2017

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The Journal of Physical Chemistry

High photocatalytic activity of heptazine-based g-C3N4/SnS2 heterojunction and its origin: Insights from hybrid DFT Jianjun Liu*†‡ ,Enda Hua§ †

School of Physics and Electronic Information, Huaibei Normal University, Huaibei, Anhui, 235000, P. R. China

‡

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, P. R. China

§

School of Physical Sciences, University of Science and Technology of China, Hefei 230026, P. R. China

E-mail: [email protected]

1

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Abstract g-C3N4-based composite structure exhibit excellent photocatalytic performance. However, their photongenerated carrier transfer and photocatalytic reaction mechanism was yet unclear. In this study, a 2D/2D g-C3N4/SnS2 heterojunction was systematically investigated by a hybrid density functional approach. Results indicated that the g-C3N4/SnS2 heterojunction was a staggered band alignment structure, and band bending occurred at the interface. A built-in electric field from the g-C3N4 surface to the SnS2 surface was formed by Interfacial interaction. When visible light irradiation, excited electrons in the CBM of SnS2 easily recombined with the holes in the VBM of g-C3N4 under the electric field force. As a result, photogenerated electrons and holes naturally accumulate at the CBM of g-C3N4 and the VBM of SnS2, respectively. The effective separation of holes and electrons in space was advantageous them to participate in catalytic reactions on a different surface. Consequently, a direct Z-scheme photocatalytic reaction mechanism was established to enhance the photocatalytic activity of g-C3N4/SnS2 heterojunction. Our results not only reveal photocatalytic reaction mechanism of g-C3N4/SnS2 heterojunction but also provide a theoretical guidance for the design and preparation of novel g-C3N4 based composite structures.

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1. Introduction Two-dimensional materials, such as graphene, graphite carbon nitride (g-C3N4), transition metal dichalcogenides (e.g., MoS2, SnS2, and WS2), boron nitride, and phosphorene, have been considered for potential applications in catalysis, optoelectronics, energy storage and sensing because of their unique geometric and electronic structures.1-5 Among these two-dimensional materials, g-C3N4, as a graphene analogous structure, exhibits excellent photocatalytic properties in the decomposition of organic pollutant, splitting water to produce hydrogen and photoreduction of CO2.6-7 Although g-C3N4 possesses geometric characteristics similar to those of graphene, the band structures of g-C3N4 were different from those of graphene. The band gap of graphene is zero, As such, graphene is unsuitable for photocatalysis. By contrast, the band gap of g-C3N4 is approximately 2.7 eV, and its band edge potentials of conduction and valence bands are -1.1 and 1.6 V(vs. NHE). With these properties, g-C3N4 can decompose organic pollutants and split water.8 In addition, g-C3N4 is characterized by excellent thermodynamic and chemical stabilities. Therefore, g-C3N4 is regarded as a promising visible light responsive photocatalytic material. Similar to other typical photocatalytic materials, such as TiO2 or CdS, g-C3N4 also shows some disadvantages, including small specific surface area, high recombination rate of photoexcited electron-hole pairs, and low visible light absorption rate.9 These drawbacks impede further g-C3N4 applications in photocatalysis. Therefore, appropriate measures should be implemented to restrain these disadvantages and improve the photocatalytic performance of g-C3N4. 3



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Bulk g-C3N4 has also been chemically exfoliated into few layer or monolayer g-C3N4 nanosheets. The specific surface area of g-C3N4 can be improved through chemical exfoliation. More photocatalytic reaction sites can also be formed through this process. Consequently, its photocatalytic efficiency can be improved.10-11 A low recombination rate of the photogenerated carriers of g-C3N4 can also be effectively obtained by constructing g-C3N4 based heterostructures12 or modificating g-C3N4 with cocatalysts13 to promote the effective separation of photoexcited electron-hole pairs, and thus reduce their recombination rate.14 For example, g-C3N4/TiO2,15 g-C3N4/WO3,16 g-C3N4/CdS17 and g-C3N4/BiOBr18 heterostructures possess excellent photocatalytic properties. And NiS,19 NixPy,20 Ni12P5,21 Ni2P nanoparticles22 and carbon nanodots23 as cocatalysts with g-C3N4 showed significantly increased performance in water splitting or reduction of CO2. Considering these aspects, we can use monolayer g-C3N4 nanosheets with graphene oxide,24 Bi2WO6,25 transition metal dichalcogenides, such as SnS2,26 to construct 2D/2D heterojunction. In this manner, specific surface areas and active sites can be increased, and electrons and holes can be effectively

separated

and

migrated.27

Recently,

g-C3N4/SnS226,

28-31

nano

heterostructures show an excellent photocatalytic performance in splitting water to produce hydrogen and degradation of organic dyes. Although the morphological characteristics and photocatalytic performance of g-C3N4/SnS2 nano heterostructures have been described in detail, the nature of their enhanced photocatalytic activity remains controversial. For example, Zhang et al.26 reported that g-C3N4/SnS2 is a traditional type II heterostructure, Deng et al.31 hypothesized that g-C3N4/SnS2 is a 4


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type I heterostructure. Liu et al.30 and Di et al.32 indicated that g-C3N4/SnS2 is a direct Z-scheme photocatalytic heterostructure. As such, the photocatalytic mechanism of g-C3N4/SnS2 nanoheterojunction should be investigated through first-principle calculation

method

to clarify

the

experimental

findings

on

g-C3N4/SnS2

heterostructures and enhance our understanding of the photocatalytic reaction of g-C3N4-based heterostructures. Chen et al.33 theoretically investigated the electronic structure and optical properties of triazine-based g-C3N4/SnS2 composite and indicated that it was type II heterostructure. However, theoretical calculations revealed that heptazine-based g-C3N4 is more stable than triazine-based g-C3N4.34 Differences in g-C3N4 structure can cause variations in energy bands and electronic structure.35 Experimentally prepared g-C3N4/SnS2 nanocomposites are composed of heptazine-based

g-C3N426,28-31.

Therefore,

heptazine-based

g-C3N4/SnS2

heterostructure should be theoretically explored to verify experimental findings accurately. Up to date, There was no theoretical report about heptazine-based g-C3N4/SnS2 heterostructure. In addition, the band bending attributed to the interaction of interfaces is an important factor in the photocatalytic reaction of heterostructure , and should thus be considered in theoretical research. In this work, heptazine-based g-C3N4/SnS2 nano heterojunction was constructed. The interfacial binding energy, energy band composition, electron density, electrostatic potential, and optical absorption coefficient of g-C3N4/SnS2 nano heterojunction were systematically investigated using a hybrid density functional method. The interfacial interactions, effective mass of electrons and holes, charge 5



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density difference, and work function of g-C3N4/SnS2 heterostructure were also explored. The built-in electric field between interfaces are unfavorable to the formation of the II type heterostructure. By contrast, they promotes a direct Z-scheme photocatalytic reaction. Therefore, monolayer g-C3N4/SnS2 nano heterojunction is a typical direct Z-scheme photocatalytic heterostructure. The oxidation and reduction abilities of direct Z-scheme heterostructure are stronger than those of traditional type I or II heterostructure.

2. Computational Methods In this work, all of our calculations based on denstiy functional theory were adopted the projected augmented wave (PAW) method using the Vienna ab initio simulation package (VASP).36-37 The generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) form was adopted for the exchange and correlation energy. Taking into account the long range effect of van der Waals (vdW) force, The vdW interactions were added with the DFT-D2 method of Grimme.38 The plane-wave cutoff energy was set as 520 eV. The first Brillouin zone (BZ) was sampled with a 11 × 11 × 1 Monkhorst-Pack mesh. All the structural optimizations were finished until the energy on each atom were less than 1.0 × 10−5 eV and the forces on each atom were less than 0.01 eVÅ-1 . In order to obtain more accurate band gaps，the calculations of band structures and density of states (DOS) were used the screened hybrid HSE06 functional. The HSE06 functional contained 24% short-range Hartree-Fock exchange. The screening parameter µ was set 0.2 Å−1. 6


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3. Results and discussion The heptazine structure of g-C3N4 belongs to the orthorhombic system, and its space group is Cmc21. The optimized lattice constants of monolayer g-C3N4 were a = b = 7.14 Å, which were consistent with the experimental data and other calculated results.39-40 SnS2 belongs to the trigonal system, and its space group is P3m1 . The optimized lattice constants of monolayer SnS2 were a = b = 3.68 Å, and the error was 1.0% compared with the experimental value (3.64 Å). The 2D/2D g-C3N4/SnS2 heterostructure consisted of a 1×1 monolayer of g-C3N4 supercell and a monolayer of 2 × 2 SnS2 supercell. The lattice mismatch was less than 3.0%. A lattice constant of 7.21 Å of g-C3N4/SnS2 heterostructure was obtained by calculating the minimum total energy of monolayer g-C3N4 and SnS2 (Figure 1). The thickness of the vacuum region of g-C3N4/SnS2 heterostructure was 15 Å, which could be used to eliminate the mirror effect of its adjacent periodic structure. The constructed g-C3N4/SnS2 heterostructure is shown in Figures 2a–2c. The calculated minimum distance between g-C3N4 and SnS2 was 3.01 Å, which was close to the interface distance of other g-C3N4 based heterostructures.41-43 At the g-C3N4/SnS2 interface, the C–N bond lengths were approximately 1.33, 1.37, and 1.49 Å and the Sn–S bond lengths were 2.56 and 2.57 Å. These values slightly differed from the C–N bond lengths (1.33, 1.39, and 1.48 Å) of monolayer g-C3N4 nanosheet and the Sn–S bond length (2.58 Å) of monolayer SnS2 nanosheet. Thus it can be concluded that the interaction between the g-C3N4 surface and the SnS2 surface is relatively weak. 7



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Figure 1: Optimized Lattice parameter of the g-C3N4/SnS2 nano heterostructure

The thermal stability and interfacial interaction of g-C3N4/SnS2 heterostructure can be described by the formation energy of the interface expressed as follows: EF = Eg-C3 N 4 /SnS2 − Eg −C3 N4 − ESnS2

where

Eg-C3 N 4 /SnS2 ,

Eg − C3 N 4 and

(1)

ESnS2 are the total energies of g-C3N4/SnS2

heterojunction, monolayer g-C3N4, and SnS2 nanosheet, respectively. In general, a negative formation energy indicates that the heterojunction formation is exothermic. Thus, a stable interface structure is easily formed. The calculated interfacial formation energy of g-C3N4/SnS2 heterostructure was -0.57 eV. Therefore, the g-C3N4/SnS2 heterostructure is a thermodynamically stable. Furthermore, the calculated interfacial formation energy per unit area was 13.0 meV/Å2,which characterizes a typical van der Waals interaction.44 These results and the calculated interface distance indicate that g-C3N4/SnS2 nanostructure is a van der Waals heterojunction. 8


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Figure 2: (a) top view,(b) side view, and (c) three dimensional structure of g-C3N4/SnS2 heterostructure after geometry optimization.

The band structures and DOS of g-C3N4/SnS2 heterostructure were calculated through HSE06 hybrid functional method. To investigate g-C3N4/SnS2 heterostructure comprehensively, we determined the band structure of the monolayer g-C3N4 and SnS2 nanosheet (Figure 3a and 3b) and found that monolayer g-C3N4 nanosheet was an indirect band gap structure. Its valence band maximum (VBM) was located at the Г point and the conduction band minimum (CBM) was located at the M point. A calculated band gap of 2.76 eV was obtained by using HSE06 hybrid functional method. The experimental values (2.7 eV) were closer to the results of the HSE06 hybrid functional method than to the band gap (1.36 eV) calculated with the PBE functional method. Monolayer SnS2 nanosheet was also an indirect band gap structure. 9



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Its VBM was located between Г and M points but not at a high symmetry point. Moreover, the CBM is at the M point. The calculated band gaps of monolayer SnS2 nanosheet were 2.38 and 1.59 eV through HSE06 hybrid functional and PBE functional methods, respectively. The experimental band gap of the monolayer SnS2 nanosheet was approximately 2.29 eV.

45

Therefore, the band gap calculated by the

HSE06 hybrid functional method for the monolayer SnS2 nanosheet is more accurate than that obtained by the PBE functional method.

Figure 3: Energy band structures of (a) monolayer g-C3N4 nanosheet and (b) monolayer SnS2 nanosheet calculated by HSE06 hybrid functional method

The effective mass of electrons and holes in photocatalytic materials significantly affects Photocatalytic activity because the photocatalytic activity of semiconductor materials depends on the effective separation and migration of carriers and the effective mass of carriers determines the migration of photogenerated 10


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carriers.46 Therefore, the effective mass of carriers can be used to explain the photocatalytic activity of semiconductors. The effective mass of photoexcited electron and hole is calculated as follows:

m* = h 2 (

d 2 E −1 ) dk 2

(2)

where E is the band edge energy, k is the wave vector, and ћ is the reduced Planck constant. We can fit the functional relationship between E and k by the calculated energy band to obtain the effective mass of the photoexcited electrons at VBM and CBM. Table 1 lists the calculated effective mass of the photogenerated carriers of the monolayer g-C3N4 and SnS2 nanosheets. The effective mass of the photoexcited electrons of monolayer g-C3N4 and SnS2 nanosheet was less than that of the photogenerated holes. The effective mass of the photogenerated electrons and holes of g-C3N4 and SnS2 nanosheets varied with different paths. The mobility of carriers ( µ ) is inversely proportional to the effective mass and mathematically expressed as follows:

µ=

qτ m∗

(3)

Where q is the charge number, and τ is the scattering time. This equation indicates that a low effective mass corresponds to a high mobility, and this phenomenon promotes the migration of carriers and reduces the recombination rate of photoexcited electron and hole. Therfore, numerous photogenerated electrons and holes moved to the surface and participated in a redox reaction and thus increased the photocatalytic reaction rate. In Table 1, the effective mass of the photogenerated electrons of 11



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monolayer SnS2 and g-C3N4 nanosheet was smaller than that of photogenerated holes, and this finding suggested that photogenerated electrons migrated more easily than photogenerated holes did. In addition, the effective mass of the photoexcited electron of the monolayer SnS2 nanosheet was lower than that of the monolayer g-C3N4 nanosheet. Thus, the photoexcited electron of monlayer SnS2 nanosheets exhibit higher mobility. For the monolayer g-C3N4 nanosheet, the effective mass of photogenerated holes along different directions slightly varied, and this slight variation implied that the migration of the photogenerated holes of monolayer g-C3N4 nanosheet along different directions was almost similar. Conversely, the effective mass of the hole and electron of SnS2 nanosheet in different directions remarkably varied. Therefore, the mobility of the SnS2 nanosheet in different directions varies and exhibits anisotropy.

Table 1: Calculated effective mass of electron on the CBM and hole on the VBM of monolayer g-C3N4 and SnS2 (point P is the valence band maximum of SnS2)

g-C3N4

SnS2

M→Γ

M→K

Γ→M

Γ→K

P→Γ

P→M

me*/m0

0.42

0.80

—

—

—

—

mh*/m0

—

—

1.98

1.96

—

—

me*/m0

0.24

0.70

—

—

—

—

mh*/m0

—

—

—

—

13.8

1.18

12


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The band structure of g-C3N4/SnS2 nano heterostructure was calculated using the HSE06 hybrid functional method (Figure 4). In Figure 4a, the g-C3N4/SnS2 heterostructure was a typical indirect band gap structure with a band gap of 1.66 eV. The VBM of g-C3N4/SnS2 heterostructure was found at K point, and the CBM was located at Г point. Compared with the band structures of monolayer g-C3N4 and SnS2 nanosheet, the band structure of g-C3N4/SnS2 heterostructures was not a simple superposition of the band structure of monolayer g-C3N4 and SnS2 nanosheet. Their band structure was influenced by van der Waals interaction between g-C3N4/SnS2 interfaces. Figure 4a shows that the band distribution of g-C3N4/SnS2 heterostructure was staggered near the forbidden band. In the conduction band, the electron orbit of g-C3N4 was found in the higher level of the conduction band, and the electron orbit of SnS2 was located in the lower energy level of the conduction band. Figure 4b illustrates that the CBM of the g-C3N4/SnS2 heterostructure was occupied by the electron orbitals of SnS2. In the valence band, the electron orbit of g-C3N4 was detected in the higher level of the valence band, and the electron orbit of SnS2 was obsevered in the lower energy level of the valence band. Figure 4c reveals that the VBM of the g-C3N4/SnS2 heterostructure was occupied by the electron orbitals of g-C3N4.

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Figure 4: (a) Energy band structure of g-C3N4/SnS2 heterostructure calculated by HSE06 hybrid functional method. The red circle represents electron orbits of g-C3N4, the green circle represents the electron orbits of SnS2, and the size of the circle represents the weight of the orbit

The total density of states (TDOS) and the partial density of states (PDOS) of g-C3N4/SnS2 nano heterostructure were calculated using HSE06 hybrid functional method to analyze the electronic structure of g-C3N4/SnS2 nanoheterostructure (Figure 5). In particular, the VBM of g-C3N4/SnS2 nano heterostructure was mainly composed of N 2p, N 2s, and C 2p orbitals, whereas the CBM of the g-C3N4/SnS2 nano heterostructure was dominated by Sn 5s and S 3p orbitals. Constructing g-C3N4/SnS2 nano heterostructure did not introduce localized states in the forbidden band mainly 14


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because the C–N bond in the g-C3N4 surface and the Sn–S bond in SnS2 surface were in saturated coordination, and the van der Waals force between g-C3N4 and SnS2 surfaces was weak and therefore could not produce a localized state. Therefore, the energy band levels of g-C3N4 and SnS2 were mutually staggered after they interact with each other, and this result is consistent with the calculated results of the band structure. The staggered energy band level of g-C3N4/SnS2 nano heterostructure could promote the separation of photogenerated carriers, and thus effectively improve the photocatalytic activity.

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Figure 5: The calculated TDOS and the PDOS of g-C3N4/SnS2 heterostructure using HSE06 hybrid functional method.

The work function is an important physical parameter in studying on the energy band alignment and charge transfer of semiconductor heterostructure. The work function is numerically equal to the difference between vacuum and Fermi levels and is expressed as follows:

Φ = Evac − EF

(4)

where Ф，Evac, and EF represent work function, vacuum level, and Fermi level, respectively.43 The work function of monolayer g-C3N4 and SnS2 nanosheets could be obtained by calculating the electrostatic potential of their slab models. Figure 6 shows the electrostatic potential of monolayer g-C3N4 and SnS2 nanosheets calculated with HSE06 hybrid functional. In figure 6a and 6b, the work functions of the monolayer g-C3N4 and SnS2 nanosheets were 5.85 and 7.08 eV, respectively. The work function of monolayer g-C3N4 nanosheets was less than that of the monolayer SnS2 nanosheet. The result indicated that, when monolayer g-C3N4 nanosheet were interacted with the monolayer SnS2 nanosheet to form the g-C3N4/SnS2 heterojunction, the electrons would flow from the g-C3N4 surface to the SnS2 surface until they reached the same Fermi levels. g-C3N4 and SnS2 surface became charged because of the electron transfer and a bulit-in electric field was formed at their interface. The direction of the electric field is from the g-C3N4 surface to the SnS2 surface.

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Figure 6: The calculated Electrostatic potential of monolayer g-C3N4 and SnS2 nanosheets using HSE06 hybrid functional method.

We further investigated the charge density difference of g-C3N4/SnS2 nano heterostructure in detail. This charge density difference can be calculated as follows: ∆ρ = ρ g-C3 N 4 /SnS2 − ρ g − C3 N 4 − ρSnS2

(5)

where ρ g-C3 N 4 /SnS2 , ρ g −C3 N4 , and ρSnS2 represent the charge densities of g-C3N4/SnS2 heterostructure, monolayer g-C3N4 and SnS2 nanosheet, respectively. The calculated charge density difference of g-C3N4/SnS2 heterostructure is shown in Figure 7a and 7b.The yellow area represents electron accumulation, and the cyan area shows the electron depletion. At the interface of g-C3N4/SnS2 heterostructure, the g-C3N4 surface was mainly dominated by the cyan area but was slightly covered bt the yellow area. The SnS2 surface was mainly filled by the yellow area but was partially occupied by the cyan area. The planar-averaged charge density difference along the Z axis of the g-C3N4/SnS2 heterostructure was calculated and shown in figure 7c. The charge distribution of the g-C3N4/SnS2 heterostructure at the interface can be obtained directly from the figure 7c. We can found that the surface of SnS2 at the interface is 17



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gathering electrons, which has negative charge, while the g-C3N4 surface gathers holes, which has positive charge. It is consistent with the above analysis based on work function. A built-in electric field at their interface was formed, and the electric field direction was from the g-C3N4 surface to the SnS2 surface.

Figure 7: (a) The side view and (b) the top view of the charge density difference for g-C3N4/SnS2. (c) Planar-averaged electron density difference along with Z direction for g-C3N4/ SnS2 heterostructure.

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We further calculated the planar-averaged electrostatic potential along the Z axis for the g-C3N4/SnS2 nano heterostructure (Figure 8a). As mentioned above, electron transfer occurred in the interface region until two Fermi energies reached the same level when monolayer g-C3N4 nanosheet interacts with monolayer SnS2 nanosheet to form g-C3N4/SnS2 heterostructure. The Fermi energy of g-C3N4/SnS2 heterostructure was −6.13 eV with respect to the vacuum level. In this process, the VBM and CBM potentials of g-C3N4 and SnS2 in the g-C3N4/SnS2 heterostructure were also changed. After contact of g-C3N4 and SnS2, the energy bands bent at the interface, and the energy barrier was formed (Figure 8b). Combining the DOS of the g-C3N4/SnS2 with the Fermi energy of g-C3N4/SnS2 heterostructure, we can calculate the potentials of the VBM and the CBM of g-C3N4 and SnS2 in the g-C3N4/SnS2 heterostructure (Figure 8b). For SnS2, the VBM potential was 2.57 V (vs. NHE), and the CBM potential was 0.19 V (vs. NHE). For g-C3N4, the VBM potential was 1.97 V (vs. NHE), and the CBM potential was -0.79 V (vs. NHE). In Figure 8b, the g-C3N4/SnS2 heterostructure was a staggered band alignment structure. Moreover, the CBM and VBM of g-C3N4 at the interface were bent up and the CBM and VBM of SnS2 were bent down. Under visible light irradiation of g-C3N4/SnS2 heterostructure, the built-in electric field at their interface formed by band bending were harmful to the photogenerated electrons at the CBM of g-C3N4 move to the CBM of SnS2. These conditions were also not conducive to the photogenerated hole at the VBM of SnS2 move to the VBM of g-C3N4. However, the built-in electric field at their interface were beneficial to the transition of the photoexcited electrons at the CBM of SnS2 to 19



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the VBM of g-C3N4. Consequently, photoexcited electrons easily recombine with photoexcited holes at the VBM of g-C3N4. In this way, more photogenerated electrons were accumulated at the CBM of g-C3N4, and more photogenerated holes became accumulated at the VBM of SnS2. The photogenerated electrons and holes were located on the surface of g-C3N4 and SnS2, respectively. Thus, photoexcited electrons and holes became effective separation in space. These phenomena induced more photoexcited electrons and holes to participate in the photocatalytic reaction of the surface and thus improve photocatalytic efficiency.

Therefore, the 2D/2D

g-C3N4/SnS2 nano heterostructure was a direct Z type photocatalytic heterostructure. Compared with the traditional type II heterostructure, the direct Z type heterostructure yielded a more negative potential of CBM and more postive potential of VBM. As a result, the Z type heterostructure exhibited strong reduction and oxidation abilities. For the direct Z type g-C3N4/SnS2 heterostructure, the potential of CBM was -0.79 V, and the potential of VBM was 2.57 V. These findings demonstrated that Z type g-C3N4/SnS2 heterostructure has sufficient ability to produce hydrogen and photodegrade organic pollutants.

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Figure 8: (a) Electrostatic potential distribution and (b) direct Z type photocatalytic reaction mechanism of 2D/2D g-C3N4/SnS2 heterostructures Furthemore, water adsorption on the surface of g-C3N4/SnS2 heterostructure and monolayer g-C3N4 nanosheet were investigated. The adsorption energy can be calculated as follows: 21



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Eads = Eslab + EH 2O − Eslab+H 2O

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（6）

where Eslab+H2O was the total energy of the system , Eslab was the total energy of the slab, and EH 2 O was the total energy of an isolated water molecule. A postive value of the adsorption energy indicates that the adsorption is exothermic with respect to a free water molecule. The calculated adsorption energy of g-C3N4/SnS2 heterostructure and monolayer g-C3N4 nanosheet were 3.07 and 2.61 eV, respectively. The larger adsorption energy means that the adsorption ability of g-C3N4/SnS2 heterostructure to the water molecule is stronger than that of g-C3N4 nanosheet, which is beneficial to the catalytic reaction of water molecule on the surface. When the water molecule adsorbed on the surface of g-C3N4/SnS2 heterostructure, the O-H bond lengths were 0.982 and 0.983 Å, respectively. As a comparison, the O-H bond lengths were 0.976 and 0.977 Å, respectively as water molecule adsorbed on the surface of the monolayer g-C3N4 nanosheet (Figure 9a and 9b). g-C3N4/SnS2 heterostructure made O-H bond of water molecule adsorbed on the surface longer, and the interaction between O and H in water molecule decrease, which in favor of breaking covalent bond of water molecule to generation of hydrogen ion and hydroxyl ion. These calculations further demonstrated that g-C3N4/SnS2 heterostructure have stronger ability to split water than g-C3N4 nanosheet.

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Figure 9: Water molecule adsorbed on the surface of (a) monolayer g-C3N4 nanosheet and (b) g-C3N4/SnS2 heterostructure

The optical absorption coefficient is the fundamental physical quantity to describe the optical properties of materials. This parameter can be calculated with the following equation:

α (ω ) = 2ω  ε1 (ω ) 2 + ε 2 (ω ) 2 − ε 1 (ω )  



1/2

(7)

where ε1 (ω ) is the real part of the dielectric function, ε 2 (ω ) is the imaginary part of the dielectric function, and ω is the light frequency.47 The optical absorption coefficients of monolayer g-C3N4 nanosheet, monolayer SnS2 nanosheet and 2D/2D g-C3N4/SnS2 nano heterostructure were calculated (Figure 10). In the visible light range (1.55-3.1 eV), the optical absorption coefficient of monolayer SnS2 nanosheet was larger than that of monolayer g-C3N4 nanosheet. The optical absorption coefficient of 2D/2D g-C3N4/SnS2 heterostructure was significantly greater than that of the monolayer g-C3N4 and SnS2 nanosheet. This finding indicated that visible light utilization by g-C3N4/SnS2 heterostructure was higher than by monolayer g-C3N4 and SnS2 nanosheet. Hence, the construction of 2D/2D g-C3N4/SnS2 heterostructure 23



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helped enhance the conversion efficiency of solar energy, and expand the scope of optical responses.

Figure 10: Optical absorption coefficients of monolayer g-C3N4, monolayer SnS2 nanosheets and 2D/2D g-C3N4/SnS2 heterostructure

4. Conclusions In summary, a 2D/2D g-C3N4/SnS2 nano heterostructure was constructed. Its energy band structure, DOS, electron density difference, and electrostatic potential were calculated through HSE06 hybrid density functional method. Our results showed that g-C3N4/SnS2 nano heterostructure was an indirect band gap structure with a band gap of 1.66 eV. Th projected DOS and band structure indicated that the g-C3N4/SnS2 heterostructure was a staggered band alignment structure. The work functions and the charge density difference revealed that g-C3N4 interacted with SnS2 to form a heterojunction, and its two surfaces were positively and negatively charged. Band 24


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bending at the interface occurred as the Fermi energy changed. The built-in electric field was formed at interface, and the direction was from g-C3N4 surface to the SnS2 surface. Under visible light irradiation, the built-in electric field accelerated the photogenerated electrons at the CBM of SnS2 to recombine with the photogenerated holes at the VBM of g-C3N4. As a result, the VBM of SnS2 and CBM of g-C3N4 accumulated more holes and electrons, respectively. The separation of photoexcited hole and electron in space was advantageous to more holes and electrons to participate in the photocatalytic reaction on the surfaces. Therefore, 2D/2D g-C3N4/SnS2 nano heterostructure was a direct Z scheme photocatalytic reaction mechanism. This result suggested that g-C3N4/SnS2 nanoheterostructure exhibited strong redox ability because of the more positive potential of VBM and the more negative potential of CBM. The calculated optical absorption coefficient also indicated that the optical absorption ability of g-C3N4/SnS2 nano heterostructure was stronger than that of monolayer g-C3N4 and SnS2 nanosheet. The first principle calculations revealed the mechanism of the photocatalytic reaction of g-C3N4/SnS2 heterostructures from an atomic level and provided a theoretical guidance for the design and preparation of novel g-C3N4 based heterostructures.

Acknowledgements This work was supported by the Natural Science Foundation of Education Department of Anhui Province (KJ2017A385) and Natural Science Foundation of Anhui Province

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(1208085MA04). Thanks for the guidance of Prof. J.G. Yu of Wuhan University of Technology.

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SnS2

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