Efficient Carrier Separation and Band Structure Tuning of Two

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C: Energy Conversion and Storage; Energy and Charge Transport

Efficient Carrier Separation and Band Structure Tuning of Two-Dimensional CN/GaTe Van Der Waals Heterostructure 2

Yujie Bai, Qinfang Zhang, Ning Xu, Kaiming Deng, and Erjun Kan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04440 • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on June 27, 2018

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The Journal of Physical Chemistry

Efficient Carrier Separation and Band Structure Tuning of Two-Dimensional C2N/GaTe van der Waals Heterostructure

Yujie Bai†, Qinfang Zhang†,* Ning Xu†,* Kaiming Deng‡, Erjun Kan‡

†

Physics Department, Yancheng Institute of Technology, Yancheng 224051, Jiangsu, P. R. China

‡

Department of Applied Physics and Institution of Energy and Microstructure, Nanjing University

of Science and Technology, Nanjing 210094, Jiangsu, P. R. China.

*Corresponding author:

e-mail: [email protected]; [email protected];

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The Journal of Physical Chemistry 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

Abstract Efficient carrier separation and suitable band structure are critical for developing better nanoscale optoelectronic devices. However, so far, researchers have not developed a single material system that can satisfy these requirements. Here we design a novel C2N/GaTe van der Waals heterostructure based on the density functional theory. Our results suggest that this heterostructure is an indirect band gap semiconductor (1.39 eV) with intrinsic type-II band alignment, facilitating the separation of photogenerated carriers. Meanwhile, this heterostructure exhibits improved visible optical absorption compared with that of the isolate C2N and GaTe monolayers. More fascinatingly, we find that an intriguing indirect-to-direct band gap semiconductor transition can be induced at the compressive strain of 3%. Simultaneously, the band gaps and carrier effective masses can also be significantly reduced by the biaxial strain. Furthermore, the band edge positions of C2N/GaTe heterostructure can be effectively tuned to straddle the redox potentials of water splitting by isoelectronic anion S and Se substitution at the Te site, and the enhanced optical absorptions are also observed in the doped heterostructures, indicating that S (Se)-doped C2N/GaTe heterostructures are potential photocatalysts for water splitting. In addition, effective spatial separation of photogenerated carriers is expected to occur for all of the above cases. These findings suggest that C2N/GaTe heterostructure is a promising candidate for application in future nanoelectronics and optoelectronics device and also provide some valuable information for future experimental research.


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1. INTRODUCTION Since the first isolation of graphene in the experiment,1 two-dimensional (2D) materials have been taken wide attention because of their tremendous opportunities for applications in nanoelectronics and optoelectronic devices. So far, a great variety of 2D materials have been subjected to extensive investigations by theory or experiment, including graphitic carbon nitrides C2N, C3N4, C6N6,2-4 and transition metal dichalcogenides, black phosphorus, silicene, MXenes,5-8 and so on. However, in order to improve the performance in their large-scale practical applications, the electronic structure of 2D materials usually should be tailored. For example, the band gap of C2N can be effectively modulated by increasing the concentrations of P and As substitution to enhance the visible light adsorption.9 Through the structural distortion engineering, photogenerated electron-hole pairs can be effectively separated for g-C3N4, resulting in achieving high photocatalytic activity.10 It is well recognized that the suitable band structure and efficient charge separation are critical to efficiently utilize solar energy, including solar cells, photocatalytic water splitting, and photodegradation of contaminant.11-13 However, photogenerated electron-hole pairs in the most of single semiconductor materials usually occupy the same regions spatially, forming the wave function large overlap between electrons and holes and resulting in a high rate of recombination.14 Additionally, a large number of semiconductor materials only absorb a small portion of the solar spectrum in the ultraviolet region due to their large band gaps, such as TiO2 and SrTiO3.15,16 Until now, researchers still have not developed a single material system that can satisfy these requirements, despite the significant efforts have been made. In recent years, van der Waals (vdW) heterostructure which is formed by putting a monolayer on top of another monolayer has been demonstrated as an efficient way to modify the property of 2D materials, such as charge spatial separation and band structure tuning.17,18 For example, MoS2/AlN(GaN) vdW heterostructures are highly efficient visible-light photocatalysts for water splitting, where hydrogen and oxygen gas will be produced at the opposite surfaces.19 ZrS2/BN(C3N4) vdW heterostructures ACS Paragon Plus Environment


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have good charge separation and remarkable optical properties in the visible-light region.20 More interestingly, recently, a new C2N monolayer material with uniform pore and nitrogen-atom distributions has been successfully synthesized via a bottom-up wet-chemical reaction.3 Because of visible-light absorption, low-cost fabrication, eco-friendly and earth-abundant components, the new C2N monolayer material stimulates researchers to extensively investigate.21,22 A large number of studies suggest that pristine or modified C2N monolayer could be a new promising 2D semiconductor

material

for

potential

applications

in

nanoelectronics

and

optoelectronics device.23-26 However, there is still a great challenge for large-scale practical applications: the valence band maximum (VBM) and conduction band minimum (CBM) are not well-separated spatially, resulting in significantly inhibiting electron-hole pairs generation due to the fast recombination of photogenerated carriers. In order to realize the effective generation and separation of electron-hole pairs, a promising approach is to form C2N-based vdW heterostructure with other monolayer materials. In fact, previous studies have revealed that spatial separation of photongenerated

carriers

can

be

effectively

realized

in

C2N/CdS(MoS2)

heterostructures, where electrons and holes are localized in C2N and CdS(MoS2) monolayers, respectively.27,28 The group-III monochalcogenide monolayers MX (M = Ga, In, and X = S, Se, Te), an emerging type of 2D materials, have been extensively investigated for future optoelectronic devices.29,30 Among these 2D materials, GaTe monolayer could form a better photoelectric heterostructure with C2N monolayer, because the lattice mismatch between C2N unit cell and GaTe (2×2×1) supercell is only 0.7% and the VBM and CBM of C2N are lower than those of GaTe monolayer, indicating that type-II heterostructure could be realized between these two monolayers. Therefore, in this work, we investigate the possibility of C2N/GaTe heterostructure as an effective carrier separation and band structure tuning material based on the first-principles calculations. Our calculated results show that C2N/GaTe heterostructure is an indirect band gap semiconductor with intrinsic type-II band alignment, facilitating the ACS Paragon Plus Environment

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separation of photogenerated electron-hole pairs, and the optical absorption spectrum further shifts toward the visible light region. More interestingly, an intriguing indirect-direct band gap semiconductor transition for C2N/GaTe heterostructure can be induced by compressive strain, which enhances the high absorption coefficient and the high efficient electron-hole pair generation. Furthermore, the band edge positions of the C2N/GaTe heterostructure can further be tuned by isovalent anion S and Se doping to straddle the redox potentials levels of water and realize overall photocatalytic water splitting reaction. 2. COMPUTATIONAL METHODS All calculations have been performed based on the density functional theory with the projector-augmented-wave (PAW) method which is implemented in the Vienna ab initio simulation package (VASP).31,32 The generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) parametrization is used to describe the exchange and correlation potential.33,34 The DFT-D2 method of Grimme is taken into account the long-range effect of vdW force.35 The vacuum space of 20 Å along the z-direction is employed to avoid the interaction between the periodic images. The plane-wave cutoff energy is set to be 500 eV and both the convergence criteria for energy and force are set to 1×10-5 eV, and 0.01 eV Å-1, respectively. The Monkhorst-Pack k-point mesh of 9×9×1 is used for all the monolayers and heterostructures.36 In order to avoid the disadvantages of DFT-PBE calculations, Heyd–Scuseria–Ernzerhof (HSE06) hybrid functional is adopted to accurately calculate the electronic structures and optical absorption properties of the GGA-PBE optimized phases.37,38 Here the mixing exchange parameter of 0.25 and screening parameter of 0.2 Å-1 are adopted in all calculations. Moreover, the more reliable tetrahedron method with the Blöchl correction has been adopted to calculate the density of states (DOS) and projected density of states (PDOS).39 In order to study the optical absorption property of pristine C2N and GaTe monolayers and C2N/GaTe heterostructure, the frequency dependent complex dielectric functions ε (ω ) = ε1 (ω ) + iε 2 (ω ) is calculated. The imaginary term ε 2 can be



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derived from a summation over a sufficiently large number of empty states, and the real part ε1 is deduced from imaginary part ε 2 by the well-known Kramer-Kronig relation, respectively. The absorption coefficient α (ω ) can be related to these two parts of the dielectric functions, using the following formula40

α (ω ) = 2ω ( ε1 (ω ) 2 + ε 2 (ω ) 2 − ε1 (ω )) 3.

1

2

(1)

RESULTS AND DISCUSSION

3.1 The pristine C2N and GaTe monolayers Firstly, the geometric structures and electronic properties of pristine C2N and GaTe monolayers have been investigated in our work. The pristine C2N monolayer is a holey structure, which is constructed by benzene rings and pyrazine rings, forming a big hole surrounded by six N atoms, as shown in Figure1(a). While the pristine GaTe monolayer has a repeating unit of Te-Ga-Ga-Te sheet built by six-membered Ga3Te3 rings, as demonstrated in Figure 1(b). In order to test the reliability of our methods, the calculated lattice constant a (b), band gap (Eg) and band edge positions with respect to vacuum are listed in Table 1. In our calculations, the optimized lattice constants for C2N and GaTe monolayers are 8.328 and 4.135 Å, respectively. In the case of C2N monolayer, the bond lengths are found to be 1.336, 1.469, and 1.429 Å for the C-N, C-C1, and C-C2 bonds, while the bond lengths of Ga-Ga and Te-Ga in the GaTe monolayer are 2.473 and 2.708 Å, respectively. Table 1. The calculated lattice constants a (b), band gaps and the band edge positions with respect to vacuum as obtained by HSE06. system

a=b(Å)

Eg(eV)

ECBM(eV)

EVBM(eV)

This work

8.328

2.42

-4.11

-6.53

Other work

8.325

This work

4.135

Other work

4.140

C2N

GaTe

9

42

9,41

2.46

-4.09

2.04

-3.69

42

2.10

41

29

-3.53

41

-6.55 -5.73 -5.75

29

The band structure and density of states (DOS) calculated with the HSE06 hybrid functional, as shown Figure 1(c, d), indicate that C2N monolayer is a direct band gap


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semiconductor (2.42 eV) with the CBM and VBM both locating at the Г point. And the valence bands are primarily attributed to 2p states of C atoms with a small contribution of 2p states of N, while the conduction bands predominantly derived from C 2p and N 2p states, implying the strong hybridization between them. For the GaTe monolayer, as shown Figure 1(e, f), it is an indirect band gap semiconductor (2.04 eV) with the CBM locating at the Г point, while VBM locating at the K→Г path in the two-dimensional hexagonal Brillouin zone. The valence band mainly originates from the Ga 4p and Te 5p states, and the conduction band mainly stem from Ga 4s and Te 5p states. As shown in Table 1, the calculated lattice constants and electronic properties are in good agreement with previous theoretical results,9,29,41,42 which implies that the calculation methods in this paper are reasonable and the calculated results are reliable.

Figure 1. (a, b) Schematic views of C2N and GaTe monolayers, the unit cells are presented with the violet line. (c-d, e-f) are the band structures and DOS for C2N and GaTe monolayers, respectively. The dashed lines represent the Fermi levels.

3.2 Geometric and electronic structure of the C2N/GaTe heterostructure The optimized lattice constants for C2N and GaTe monolayers are 8.328 and



4.135 Å, respectively. Therefore, the C2N/GaTe heterostructure has been proposed based on a model, in which a 1×1×1 unit cell of C2N monolayer is used to match a 2×2×1 supercell of GaTe monolayer with the tiny mismatch of only 0.7%. Compared with the other heterostructures investigated previously,19,20 the present lattice mismatch value is very small and C2N/GaTe heterostructure can be constructed readily with little interfacial strain. Here, three different structures for C2N/GaTe heterostructures in terms of their stacking conformation, as shown in Figure 2(a-c), are considered in this work. After structure optimization, it is obviously that both C2N and GaTe monolayers maintain their original structures without obvious distortion, indicating the interaction between these two monolayers is weak. In order to obtain a relatively stable structure, the binding energy ( Eb ) is calculated according to the following equation

Eb = EC2 N/GaTe − EC2 N − EGaTe (2) where EC2 N/GaTe is the total energy of C2N/GaTe heterostructure, while EC2 N and EGaTe are the total energies of isolate C2N and GaTe monolayers, respectively. As listed in Table S1, the calculated binding energies for three different stacking conformations (H1, H2, H3) are -0.71, -0.69, -0.73 eV, and the corresponding equilibrium interlayer distances are 3.45, 3.42 and 3.29 Å, respectively. The calculated results indicate that C2N and GaTe monolayers could form thermodynamically stable van der Waals C2N/GaTe heterostructures. Among them, conformation H3 is the ground state structure due to the lowest binding energy.


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Figure 2. The top and side views of the relaxed atomic structures of the three conformations of C2N/GaTe heterostructure, referred as H1 (a), H2 (b), H3 (c).

The band structure and DOS of C2N/GaTe heterostructure are calculated based on the HSE06 hybrid functional method. As shown in Figure 3(a), the band structure indicates that C2N/GaTe heterostructure is a semiconductor with an indirect band gap of 1.39 eV, which is smaller than those of isolated C2N (2.42 eV) and GaTe (2.04 eV) monolayers, implying that forming C2N/GaTe heterostructure can effectively narrow the band gap and promote the photogenerated electron transferring from VBM to CBM easily. Furthermore, the valence band mainly originates from GaTe monolayer; while the conduction band is primarily attributed to C2N monolayer, which is consistent with our detailed TDOS results (Figure 3(b)), suggesting that photogenerated electron-hole pairs should realize efficient spatially separation. To gain further insight, the projected density of state (PDOS) is plotted in Figure 3(c), it is clearly seen that the valence bands primarily originate from the p states of Te and Ga atoms, while the conduction bands mainly stem from p states of N and C atoms.



Figure 3. (a) Band structure for C2N/GaTe heterostructure; the bands plotted in red and blue indicate the bands dominated by C2N and GaTe monolayers, respectively. (b, c)Total and projected density of states for C2N/GaTe heterostructure (Fermi level is indicated by a dashed line).

To understand the carrier separation in more detail, the band alignment is illustrated in Figure 4(a). The conduction bands (CB) and valence bands (VB) of the GaTe monolayer are higher in energy than the corresponding bands of the C2N monolayer, implying the formation of type-II heterostructures. The valence band offset (VBO) and conduction band offset (CBO) between the C2N and GaTe monolayers are calculated to be about 0.80 and 0.42 eV, respectively. When C2N/GaTe heterostructure is irradiated under sunlight, the photogenerated electrons in GaTe monolayer can easily transfer to the conduction bands of C2N monolayer due to the observed CBO. On the contrary, the VBO promotes the transfer of photogenerated holes in C2N monolayer to the valence bands of GaTe monolayer. Therefore, both VBO and CBO promote effective separation of photogenerated carriers, which is consistent with the partial charge density of conduction bands and valence bands for C2N/GaTe heterostructure, as shown in Figure 4(b). The charge density difference of C2N/GaTe heterostructure is shown in Figure 4(c), indicating that a large amount of charge redistribution occurs at the interface region. Through Bader charge analysis,43 the quantity of the charge transfer from the GaTe to C2N monolayer is 0.066e. Thus, spatial separation of photongenerated carrier can be effectively achieved in C2N/GaTe


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heterostructure, where electrons and holes accumulate in C2N and GaTe monolayers, respectively, inhibiting their recombination rate and improving photoelectric conversion efficiency. The band structures of metastable state configuration H1 and H2 are also investigated, as shown in Figure S1. Clearly, these two configurations are still indirect band gap semiconductors with the conduction bands and valence bands originating mainly from C2N and GaTe monolayers, respectively, indicating they also have good spatial separation for photogenerated carriers.

Figure 4. (a) Schematic representation of the band alignment, (b) the partial charge density of the valence band (VB) and conduction band (CB) (the isovalue is 0.002 e/Å3), and (c) the charge density difference (isosurface value is 0.0002 e/Å3) for C2N/GaTe heterostructure. Blue and red refer to regions of electron depletion and accumulation, respectively.

In order to achieve high-efficiency application in photoelectric heterostructure materials, wide and strong optical absorption in the optimal visible light regions is naturally expected. Thus, we further investigate the optical absorption spectra for the isolated C2N and GaTe monolayers as well as C2N/GaTe heterostructure. As shown in Figure 5, it is noted that both C2N and GaTe monolayers can adsorb visible light. Moreover, the optical absorption spectrum of C2N/GaTe heterostructure is found to be further extended to the visible light region, indicating that visible light utilization of C2N/GaTe heterostructure is more efficient than that of the isolated C2N and GaTe monolayers. The enhancement of the optical absorption can be understood by the fact that the interlayer coupling between two monolayers induces the band gap narrowing and the charge transfer. ACS Paragon Plus Environment


Figure 5. Calculated optical absorption spectra for isolate C2N and GaTe monolayers as well as the C2N/GaTe heterostructure, respectively.

3.3 Strain effects of the C2N/GaTe heterostructure It is well known that applying external strain is an effective approach to tune electronic property of 2D materials. For example, the band gaps and effective masses for GaX (X = S, Se, Te) monolayers can be modulated by applying both compressive and tensile strain.44 Upon the application of strain, an indirect-direct and insulator-metal transition can be observed in GeSe/phosphorene heterostructure.45 Here, we mainly focus on the in-plane biaxial strain to tune the electronic property of C2N/GaTe heterostructure. The in-plane biaxial strain on C2N/GaTe heterostructure is simulated by changing the crystal lattice parameter and calculated according to the following equation

ε = [( a − a0 ) a0 ] × 100% (3) where a0 and a represent the lattice parameters under the strained and unstrained conditions, respectively. The band gap of C2N/GaTe heterostructure as a function of the strain is depicted in Figure 6(a). It is obviously that the band gap increases monotonically when the compressive strain changes from -5% to -2%, while the band gap decreases monotonically with changing strain from -2% to 4%. Furthermore, an indirect-to-direct gap semiconductor transition can be induced at the compressive strain of more than 3%. These results indicate that electronic property of the C2N/GaTe heterostructure is expected to be well tuned by biaxial strain. To check


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whether the strains considered here are all within the elastic limit, we calculate the strain energy per atom according to the following equation Es = ( Estrained − Eunstrained ) n (4) Where n is the number of atoms in the system. As shown in Figure 6(a), the strain energy curve is smooth as a quadratic function under the biaxial strain, indicating that the strain considered in this work is within the elastic limit and the strain is reversible.

Figure 6. (a) Calculated band gap and strain energy as functions of the in-layer biaxial strain for the C2N/GaTe heterostructure. (b) Effect of strain on the band gap of isolate C2N and GaTe monolayers as well as on the C2N/GaTe heterostructure. Here, the positive and negative strains represent tension and compression, respectively.

In order to further understand the band gap evolution of C2N/GaTe heterostructure under biaxial strain, we also investigate the strain effect on the band gap of isolate C2N and GaTe monolayers. Interestingly, the band gap variation of GaTe monolayer with strain exhibits a similar trend to the C2N/GaTe heterostructure, as shown in Figure 6(b). While it is different from GaTe monolayer, the band gap of C2N monolayer increases steadily with increasing the strain from -5% to 4%. Therefore, the band gap variation of C2N/GaTe heterostructure with the biaxial strain may be complex. Besides the band gap variation of two isolate monolayers, interfacial coupling may also play a role in it. The band structures of the C2N/GaTe heterostructure as a function of the biaxial strain are shown in Figure 7. One can see that C2N/GaTe heterostructures maintain indirect gap semiconductors with the CBMs locating at the Г point, while the VBMs locating at the Г→K path when the strain changes from -2% to 4%, while direct gap ACS Paragon Plus Environment


semiconductors transition occur with changing strain from -5% to -3% with the CBMs and VBMs both locating at the Г point. At the same time, C2N/GaTe heterostructures still maintain the property of efficient separation of photogenerated electron-hole pairs where electrons and holes are localized in C2N and GaTe monolayers, respectively, during the whole range of strain, as shown in Figure S2.

Figure 7. (a-i) Band structures of C2N/GaTe heterostructure as functions of the in-layer biaxial strain: (a) -5%, (b) -4%, (c) -3%, (d) -2%, (e) -1%, (f) 1%, (g) 2%, (h) 3%, (i) 4%.

It is well known that direct band gap semiconductors will have high absorption coefficient and high efficient electron-hole pair generation,46 which is beneficial for application in optoelectronic devices. To investigate the transition from an indirect to a direct band gap of the C2N/GaTe heterostructure when applying the compressive strain larger than 3%, we analyze the band structure in more detail. The above discussion indicates that indirect-to-direct band gap transition is due to the VBMs shifting to the Г point. Therefore, we mainly focus on the strain effect on the valence band (Г point) of the C2N/GaTe heterostructure. As shown in Figure 8, with increase the compressive strain, the valence band (Г point) of C2N/GaTe heterostructures are


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primarily attributed to p orbital of Te atoms with a small amount of p orbital of Ga atoms. Further analysis of the Te p orbital, as shown in Figure 9, indicates that Te px, py orbitals shift upward gradually at the Г points, while Te pz orbital shifts downward when the compressive strain changing from 0% to -5%. It is worth noting that Te px, py orbitals at the Г point are raised and become the VBM when applying the compressive strain is larger than 3%, resulting in forming direct band gap semiconductors. When applying biaxial compressive strain, the three p orbitals of Te atom would be affected differently by the structure deformation. The interactions of Te px, py orbitals may become stronger under the in-plane shrinking condition, leading to the elevation of Te px, py states at the Г point. While the strength of the Te pz orbitals interaction become weaker under out-of-plane expansion condition, resulting in Te pz states shifting in the downward direction. This may explain the transition from an indirect to a direct band gap semiconductor.



Figure 8. Orbital decomposition of the band structure for Ga and Te p orbitals in the C2N/GaTe heterostructure as functions of the in-layer biaxial strain: (a) 0%, (b) -1%, (c) -2%, (d) -3%, (e) -4%, (f) -5%. The size of the circles in each band denotes the contributions from different atomic orbitals. The horizontal dashed lines represent the Fermi levels.


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Figure 9. Orbital decomposition of the band structure for Te px, py and pz orbitals in the C2N/GaTe heterostructure as functions of the in-layer biaxial strain: (a) 0%, (b) -1%, (c) -2%, (d) -3%, (e) -4%, (f) -5%. The size of the circles in each band denotes the contributions from different atomic orbitals. The horizontal dashed lines represent the Fermi levels.

In order to investigate the properties of carrier under biaxial strain, the effective masses of the C2N/GaTe heterostructure are calculated according to the following equation m ∗ = h 2 (∂ 2 E ∂K 2 ) (5)

The calculated electron (me*) and hole (mh*) effective masses under the different biaxial strain are summarized in Table 2. We can see that the effective masses of me* and mh* are larger with the strain changing from -1% to 1%. The large values mean poor mobility of photogenerated electrons and holes. On the contrary, when the applying strain is larger than 1%, the effective masses of me* and mh* become more and more small, reflecting the high mobility of photogenerated carriers. Therefore, carrier mobility of C2N/GaTe heterostructure can be effectively improved by biaxial strain, which is very important for photoelectric device application.



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Table 2 The effective masses of C2N/GaTe heterostructure along the Г→K direction under the different biaxial strain according to the HSE06 functional (in units of electron mass) Biaxial strain

-5%

-3%

-1%

0%

1%

3%

Electrons(Г→K)

0.67

0.87

1.32

1.46

1.27

0.69

Holes(Г→K)

0.72

0.76

2.86

1.98

1.12

0.63

3.4 Doping effects of the C2N/GaTe heterostructure The above discussion indicates that the C2N/GaTe heterostructure has effective charge separation and a decent band gap (1.39 eV), suggesting it can be regarded as a potential photocatalyst for water splitting. As shown in Figure 10, we align the band edge positions of isolate C2N and GaTe monolayers as well as the C2N/GaTe heterostructure with respect to the water redox potentials. It is observed that the band edge positions of isolate C2N and GaTe monolayers both straddle the redox potentials of water, implying that they have potential application to photocatalytic water splitting. Nevertheless, the VBM of C2N/GaTe heterostructure is higher in energy than the oxidation potential of O2/H2O, suggesting it cannot produce O2 by water splitting.

Figure 10. Band edge alignment of the isolate C2N, GaTe monolayers and C2N/GaTe heterostructure as well as O, S, Se doped C2N/GaTe heterostructures with respect to the water redox potentials, respectively.

In order to improve the performance of materials in practical applications, multiple strategies are often used together in the field of material research. For example, ultrathin P-doped ZnO nanosheet decorated with atomic MoS2 layer has


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been prepared, which indicates a superior photocatalytic activity due to the synergetic effects, including the enhanced light adsorption efficiency, inhibition of charge recombination, and so on.47 Herein, the isoelectronic anion O, S, Se substitution at the Te site in the C2N/GaTe heterostructure are considered to make the valence band maximum shifting in the downward direction, because the p orbital energies of O, S, Se are lower than that of Te p orbital.48 As shown in Figure S3, four different doped configurations are considered, and the lowest-energy structures for doped systems all adopt configuration D, in which the dopants prefer to occupy the outside rather than inside of the heterostructure. Then, we calculate the formation energy according to the following equation Eform = Edoped − EC2 N/GaTe + ETe − EX (6)

where Edoped and EC2 N/GaTe are the total energies of doped and undoped C2N/GaTe heterostructure, and ETe and EX are the total energies of an isolated Te and X (X=O, S, Se) atoms, respectively. As listed in Table S2, the formation energies are all negative values, implying that doped systems are stable and easy to achieve in the experiments. Through doping, the interlayer distances are smaller than that of perfect C2N/GaTe heterostructures, implying that the interlayer coupling becomes stronger.



Figure 11. The band structures and the partial charge density of the valence band (VB) and conduction band (CB) for O, S, Se doped C2N/GaTe heterostructures are in (a-c) and (d-e), respectively. (The isovalue is 0.002 e/Å3).

The band structures of doped heterostructures are calculated, as depicted in Figure 11(a-c). The band structures indicate that doped heterostructures are all still indirect band gap semiconductors with the CBM locating at the Г point, while VBM locating at the Г→K path, which is similar to the perfect C2N/GaTe heterostructure. In addition, the valence bands are predominantly derived from GaTe monolayer; while the conduction bands mainly originate C2N monolayer (as shown in Figure S4), implying that the photogenerated carriers still maintain efficient spatial separation, which is consistent with the partial charge density of the conduction bands and valence bands for doped heterostructures, as shown in Figure 11(d-e). Based on the Bader charge analysis,43 we find that the quantity of the charge transfer from the GaTe to C2N monolayer for O, S, Se doped C2N/GaTe heterostructures are 0.072e, 0.069e and 0.067e, respectively, which is larger than that of pristine C2N/GaTe heterostructure. The larger amount of transferred charge can further enhance the


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photocatalytic performance of these doped heterostructures. Through isoelectronic anion O, S, Se substitution, whether or not the band edge positions could be tuned to match with the redox potentials of water splitting? We illustrate the alignment of the band edge positions of doped heterostructures with respect to the redox potential of water (see in Figure 10). Compared with the undoped heterostructure, it is obviously that band edge positions of doped heterostructures shift downward to straddle the redox potentials of water, except the case of O-doped case whose CBM is lower than the reduction potential of H+/H2. It is clear see that the CBMs and VBMs move upward monotonously for doped heterostructures when the dopants move down from O to Se, which is because the p orbital energy decreases gradually from O to Se in the same group.48 The optical absorption spectra of undoped and doped C2N/GaTe heterostructures are also demonstrated in Figure 12 based on the HSE06 functional. It is clear that the optical absorptions of the doped C2N/GaTe heterostructures extend more into the visible region compared with the undoped case, perhaps due to the doping effect which causes stronger interlayer coupling and the larger amount of charge transfer between two monolayers. Therefore, decent band edge positions, enhanced visible optical absorption, and effective separation of photogenerated carriers indicate that S, Se doped C2N/GaTe heterostructures are promising visible-light photocatalysts for overall water splitting.

Figure 12. Calculated optical absorption spectra for C2N/GaTe heterostructure as well as the O, S, Se doped C2N/GaTe heterostructures , respectively.



4.

CONCLUSIONS In summary, we design a new two-dimensional C2N/GaTe van der Waals

heterostructure based on the density functional theory. Because of little lattice mismatch as well as the negative formation energy, C2N/GaTe heterostructure should not be difficult to synthesize experimentally. The calculated results indicate that C2N/GaTe heterostructure is an indirect band gap semiconductor with type-II band alignment, ensuring the spatial separation for photogenerated electron-hole pairs, where electrons and holes are localized in C2N and GaTe monolayers, respectively. Simultaneously, the C2N/GaTe heterostructure exhibits enhanced optical absorption in the visible light region compared with the isolate C2N and GaTe monolayers. More fascinatingly, the indirect-to-direct band gap semiconductor transition is observed at the compressive strain of 3%. At the same time, the band gaps and carrier effective masses can also be effectively tuned by biaxial strain. In addition, through S, Se substitutions at the Te sites, the band edge positions of C2N/GaTe heterostructure shift downward to match with the redox potentials of water, and the corresponding optical absorption spectra are enhanced obviously in the visible region, indicating that S(Se)-doped C2N/GaTe heterostructures are potential photocatalysts for water splitting. These theoretical predictions indicate that the C2N/GaTe heterostructure is a promising candidate for application in optoelectronics device.

Supporting Information Table S1 and S2: The interlayer distance, binding energy, band edge positions and band gaps of three pristine and doped configuration C2N/GaTe heterostructures, respectively; Figure S1: Band structures for metastable state configuration C2N/GaTe heterostructure; Figure S2: The partial charge density of C2N/GaTe heterostructure as functions of the in-layer biaxial strain; Figure S3: Computational model for the doped C2N/GaTe heterostructure; Figure S4: The DOS for O, S, Se doped C2N/GaTe heterostructures are in (a-c), respectively.

Corresponding Authors ACS Paragon Plus Environment

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*E-mail: [email protected]. *E-mail: [email protected].

Notes The authors declare no competing financial interest

ACKNOWLEDGMENTS This work is supported by the NSFC (11704324, 11474246, 11774178 and 11750110415), by the College Natural Science Research Project of Jiangsu Province (17KJB140026), and by the Natural Science Foundation of Jiangsu Province (BK20160061).

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Efficient Carrier Separation and Band Structure Tuning of Two

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