Mono- and Bilayer ZnSnN2 Sheets for Visible-Light Photocatalysis

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Mono- and Bi-Layer ZnSnN Sheets for Visible Light Photocatalysis: First-Principles Predictions Dangqi Fang, Xi Chen, Pengfei Gao, Yang Zhang, and Shengli Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07115 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 10, 2017

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The Journal of Physical Chemistry

Mono- and Bi-Layer ZnSnN2 Sheets for Visible Light Photocatalysis: First-Principles Predictions

D. Q. Fang,1, * X. Chen,1 P. F. Gao,1 Y. Zhang,1 and S. L. Zhang1 1

MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, School of Science, Xi’an Jiaotong University, Xi’an 710049, China

*corresponding author: [email protected]

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ABSTRACT: The search for two-dimensional semiconductor materials suitable for visible light photocatalysis is an active research field. In this work, using first-principles calculations, we explore the stability, electronic structure, and optical property of monolayer and bilayer ZnSnN2 sheets. The phonon spectra confirm their dynamical stability. The PBE0 hybrid functional calculations predict that monolayer and bilayer ZnSnN2 are direct-band-gap semiconductors with band gaps of 2.39 eV and 2.62 eV, respectively. Based on band alignment and optical property calculations, we find that monolayer and bilayer ZnSnN2 possess not only sufficient band gaps but also appropriate band edge positions for photocatalytic water splitting in the visible light region. Moreover, the band gaps and band edge positions of monolayer and bilayer ZnSnN2 can be effectively tuned by applying biaxial strain, which may enhance photocatalytic performance. Our results provide guidance for experimental synthesis efforts and future application of two-dimensional ZnSnN2 sheets.

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I. INTRODUCTION

Hydrogen produced from photocatalytic water splitting provides an environmentally friendly and renewable energy source, which plays an important role in solving the energy crisis and environment problem. One of the crucial issues for solar hydrogen production is to discover and explore highly efficient semiconductor photocatalysts. Since the discovery of graphene, great efforts have been devoted to the synthesis of new two-dimensional (2D) materials because they exhibit novel electronic, optical, and mechanical properties different from their bulk counterparts. Recently, the use of 2D materials in the field of photocatalytic water splitting has attracted much interest.1-9 2D materials not only have a high specific surface area available for photocatalytic reactions, but also minimize the migration distance of photogenerated electrons and holes, thus reducing possibility of electron-hole recombination.7 These intrinsic advantages of 2D materials potentially enhance the photocatalytic performance. Sun et al. synthesized freestanding SnS2 single-layers with three atom thickness,1 which yielded a photocurrent density of 2.75 mAcm-2 at 1.0 V, roughly 72 times larger than that of bulk SnS2 and an incident photo-to-current conversion efficiency (IPCE) of 38.7% at 420 nm in contrast to only 2.33% for bulk SnS2. Computational studies can help accelerate the discovery, characterization, and design of 2D materials. Some 2D materials such as GaX (X=S, Se, Te),10 InX (X=S, Se, Te),10 MoS2,11 WS2,11 PtS2,11 PtSe2,11 α-ZrNX (X=Cl, Br, I),12 MnPSe3,13 Phosphorene14 have been proposed theoretically for photocatalyst application. The search for other possible 2D materials will likely lead to new materials with 3



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unexpected properties that are suitable for various applications, including photocatalysis. ZnSnN2, a new earth-abundant element semiconductor, has recently been synthesized15-20 and exhibits a band gap of around 1.7 eV,16 making it potentially valuable material for solar cell and other optoelectronics.21 The dimensionality reduction is one of the effective avenues to bring novel properties for the layered or non-layered materials.3, 4, 22, 23 For example, large-area freestanding single layers of non-layered ZnSe with four-atomic thickness have been fabricated using a strategy involving a lamellar hybrid intermediate, which exhibit a photocurrent density of 2.14 mAcm-2, 195 times higher than that of bulk ZnSe and an IPCE of 42.5% compared to 0.25% of the bulk counterpart.3 Al Balushi et al. reported the synthesis of 2D GaN via a migration-enhanced encapsulated growth (MEEG) technique utilizing epitaxial graphene,22 which provides a foundation to realize many other classes of materials that are not traditionally 2D. For the ternary ZnSnN2 semiconductor, however, the properties in the 2D case are unclear. Are the ZnSnN2 sheets with few atomic layers stable? How are the electronic structures of ZnSnN2 altered due to reduced dimensionality? In this paper, we investigate the stability, electronic structure, and optical property of monolayer and bilayer ZnSnN2 sheets using first-principles calculations. We find that 2D ZnSnN2 sheets studied are dynamically stable. Surprisingly, they show sufficient band gaps and appropriate band edge positions for water splitting driven by visible light.

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II. METHODS

Our calculations were performed using the density functional theory (DFT) as implemented in the Vienna ab initio simulation package (VASP).24 The exchange-correlation functional was described within the generalized gradient approximation (GGA) parameterized by the Perdew-Burke-Ernzerhof (PBE).25 The electron-ion interaction was described by the projector augmented-wave (PAW) method.26, 27 A vacuum region of 12 Å perpendicular to the plane of 2D sheets was used to avoid the interactions between neighboring sheets. For the geometry optimization, the cutoff energy for the plane-wave basis was set to 500 eV and Brillouin-zone sampling was carried out using Γ-centered k-point meshes of 5×5 ×5 and 5×5×1 for the bulk and 2D ZnSnN2 unit cells, respectively. All atoms were relaxed until the Hellmann-Feynman forces acting on them were less than 0.01 eV/Å. For the electronic band structures and the optical property we used the PBE0 hybrid functional28 to compensate for the underestimation of the band gap by the PBE functional. The PBE0 band structure calculations used the geometry optimized using the PBE functional. For the optical property calculations, Brillouin-zone sampling was performed using a Γ-centered 19×19×1 k-point mesh. The imaginary part of the dielectric function was determined using the summation of transitions over the conduction band states.29 The phonon spectra of ZnSnN2 sheets were calculated using the finite displacement method and the Phonopy code.30 A cutoff energy of 600 eV for the plane-wave basis was used in the phonon calculations.

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III. RESULTS AND DISCUSSION

A. Atomic Structure and Stability Analysis. We first optimize the lattice parameters of the bulk ZnSnN2 in the orthorhombic structure (space group Pna21)20 and obtain a=6.82 Å, b=5.91 Å and c=5.54 Å, which are in good agreement with experimental result.16 The initial structures of monolayer and bilayer ZnSnN2 sheets are cleaved from the (001) plane of the bulk structure, as shown in Figure 1. The optimized structure of monolayer ZnSnN2 is nearly planar with a very small buckling parameter of 0.02 Å. The lattice parameters are found to be a=6.82 Å and b=5.91 Å and the average lengths of the Zn-N and Sn-N bonds are 1.94 Å and 2.00 Å, respectively. For the optimized bilayer ZnSnN2, the lattice parameters differ slightly with the bulk values, as shown in Table 1, whereas the atomic structure is dramatically reconstructed. The Zn-N bonds at the interface in the initial structure are broken and the Sn and N atoms form new bonds with length of 2.22 Å along the c direction. We also examine 2D ZnSnN2 sheets cleaved from the (010) plane of the bulk orthorhombic phase. After full optimizations of the lattice parameters and atomic coordinates, the monolayer ZnSnN2 cleaved from the (010) plane changes into the same planar structure as that cleaved from the (001) plane. However, the optimized bilayer ZnSnN2 cleaved from the (010) plane has a higher energy by 2.06 eV per supercell than that cut from the (001) plane. Therefore, in this work we investigate the properties of mono- and bi-layer ZnSnN2 sheets cleaved from the (001) plane. The energetic stability of 2D ZnSnN2 sheets with respect to the bulk phase is determined by the formation energy ∆E = E2D/N2D－E3D/N3D, where E2D and E3D are 6


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the energies of the 2D sheet and 3D bulk unit cells, respectively. N2D and N3D denote the number of atoms in the respective unit cells.31 Table 2 shows the formation energy ∆E of some 2D materials from bulk materials. The calculated formation energies for monolayer and bilayer ZnSnN2 are 0.41 eV/atom and 0.20 eV/atom, respectively, which are comparable to those of 2D GaN and ZnO sheets. It is worth noting that few-layer GaN and ZnO have indeed been synthesized successfully on substrates.22, 23 This indicates that it is feasible to grow 2D ZnSnN2 sheets on appropriate substrate using usual synthesis techniques, such as chemical vapor deposition, molecular beam epitaxy, or migration-enhanced encapsulated growth technique.22 To examine the dynamical stability of 2D ZnSnN2 sheets, the phonon dispersion curves of monolayer and bilayer ZnSnN2 are calculated, as shown in Figure 2. Except a small pocket near Γ, there is no trace of imaginary frequencies in the phonon spectra calculated using 2×2 supercell. The pocket of instability is not a real physical effect, but reflects the difficulty in achieving numerical convergence for the flexural phonon branch for 2D materials.32 We use a larger supercell, i.e. 3×3, for the phonon spectra calculation of monolayer ZnSnN2, and find that the imaginary frequencies disappear, as shown in Figure 2a. Therefore, the phonon spectra suggest that monolayer and bilayer ZnSnN2 sheets are dynamically stable. B. Electronic Structure. Figure 3a and Figure 3b present the electronic band structures of monolayer and bilayer ZnSnN2 sheets calculated using the PBE0 functional, respectively. Both monolayer and bilayer ZnSnN2 sheets show direct band gap at the Γ point and there are no dangling bond states in the band gap region. The 7



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values of direct band gaps calculated using the PBE and PBE0 functionals for the bulk phase and 2D sheets are listed in Table 1. The PBE functional severely underestimates the band gap of bulk ZnSnN2 while the PBE0 functional gives a reasonable value of 1.79 eV comparable with the experimental data.16 The PBE0 functional predicts that the band gaps of monolayer and bilayer ZnSnN2 sheets are 2.39 eV and 2.62 eV, respectively, which are larger than that of the bulk phase due to the quantum-confinement effect. These results imply that monolayer and bilayer ZnSnN2 can efficiently harvest the visible light. To further understand the band characteristic of 2D ZnSnN2 sheets, we plot the total and projected density of states in Figure 4. It can be seen that the valence band maximum (VBM) are mainly contributed by N-2p and Zn-3d orbitals, while the conduction band minimum (CBM) are ruled by Sn-5s and Zn-4s orbitals. C. Band Alignments and Optical Properties. To check the possibility of 2D ZnSnN2 sheets as visible-light driven photocatalyst for water splitting, we analyze their band edge positions and optical properties. For a 2D semiconductor to be usable for photocatalytic water-splitting, the band gap must exceed the free energy of water splitting of 1.23 eV and be smaller than about 3 eV to allow the absorption of a large fraction of the solar spectrum.7 Besides the magnitude of the band gap, the band edges must straddle water redox potentials. The standard reduction and oxidation potentials relative to vacuum for H+/H2 and O2/H2O at pH=0 are -4.44 eV and -5.67 eV, respectively, which will increase with pH as pH×0.059 eV.10 To verify these properties, the energy positions of VBM and CBM relative to the vacuum level for 8


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monolayer, bilayer, and bulk ZnSnN2 are calculated using the PBE0 functional, as shown in Figure 5. Though the CBM of bulk ZnSnN2 is slightly lower than the reduction potential of H+/H2, the band edges of monolayer and bilayer ZnSnN2 are situated in energetically favorable positions for water splitting. The change of the band edges for monolayer and bilayer ZnSnN2 can be ascribed to the atomic reconstruction and quantum confinement effect in the 2D case. The optical properties of these 2D ZnSnN2 sheets are further examined with the PBE0 functional. Figure 6 shows the calculated imaginary part of dielectric function. The optical adsorption is fairly strong over the visible-light energy range, indicated that these 2D ZnSnN2 sheets have promising potential to be used as visible-light driven photocatalyst for water splitting. D. Strain Effect. Strain is one of the effective methods to tune the band structures of materials. Here, we examine the strain effect on the band gaps and band edge positions of 2D ZnSnN2 sheets by applying biaxial strain. Figure 7 shows the evolution of the total energy of primitive unit cell and the band edge positions for monolayer and bilayer ZnSnN2 as a function of biaxial strains in the range from -2% to +6%. The negative strain indicates compression, while the positive strain denotes tension. For the monolayer ZnSnN2, both the band gap and the band edge position decrease when the strain varies from -2% to +6%. However, the band gap becomes indirect with the VBM at the X point and the CBM at the Γ point when the strain is larger than 1%. For the bilayer ZnSnN2, the energy position of CBM decreases whereas that of VBM increases when the strain changes from -2% to +6% and the 9



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band gap is direct in the whole range of strain. We observe for bilayer ZnSnN2 that the distance between the Zn atom at one layer and the N atom at the other layer changes from 3.28 Å to 2.72 Å when the biaxial strain varies from 0% to +6%. The repulsion between the N-2p and Zn-3d orbitals leads to the lift of the VBM of bilayer ZnSnN2. It is noteworthy that monolayer and bilayer ZnSnN2 have an indirect band gap of 1.70 eV and a direct band gap of 2.01 eV, respectively, under the tensive strain of 6%, and their band edge positions still straddle the redox potentials of water.

IV. CONCLUSIONS In this work, we investigate the geometric, electronic, and optical properties of monolayer and bilayer ZnSnN2 through first-principles calculations. Monolayer ZnSnN2 has a nearly planar structure while bilayer ZnSnN2 prefers a reconstructed structure. The phonon spectra suggest that monolayer and bilayer ZnSnN2 are dynamically stable. Both monolayer and bilayer ZnSnN2 are direct-band-gap semiconductors with band gaps of 2.39 eV and 2.62 eV, respectively, predicted by the PBE0 hybrid functional. Different from bulk ZnSnN2, monolayer and bilayer ZnSnN2 have appropriate band edge positions for water splitting. The optical property calculations confirm their strong light adsorption in the visible light region. In addition, we show that the band gaps and band edge positions of monolayer and bilayer ZnSnN2 can be effectively tuned by applying biaxial strain. The results indicate that monolayer and bilayer ZnSnN2 are promising candidates for photocatalytic water splitting in the visible light region. 10


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ACKNOWLEDGMENTS The work described in this paper is supported by the National Natural Science Foundation of China (Grant Nos. 11604254 and 11374237), by the Fundamental Research Funds for the Central Universities (Grant No. xjj2015060), and by the HPCC Platform of Xi’an Jiaotong University.

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Table 1. Calculated Lattice Parameters Monolayer, and Bilayer Znsnn2 a Bulk 6.82 Monolayer 6.82 Bilayer 6.78 16 Experimental values of bulk 6.753

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(in Å) and Band Gap Eg (in eV) of Bulk, b 5.91 5.91 5.93 5.842

c 5.54

Eg(PBE) 0.11 0.65 0.93

5.462

Table 2. Formation Energy ∆E (eV/atom) of 2D Materials from Bulk Materials Material Monolayer Bilayer Ref. ZnSnN2 0.41 0.20 Our work GaN 0.36 0.31 Our work InN 0.43 0.30 Our work ZnO 0.23 0.17 Our work Silicene 0.76 9 Germanene 0.99 9 Graphene 0.063 9 GaX (X=S, Se, Te) 0.058-0.068 9 MoS2 0.076 9

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Eg(PBE0) 1.79 2.39 2.62

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Figure 1. The atomic structures of monolayer and bilayer ZnSnN2 sheets before and after optimization. N, Zn, and Sn atoms are denoted by blue, gray, and yellow balls, respectively. The primitive unit cells used in calculations are shown by the black rectangles.

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Figure 2. Phonon dispersion curves of (a) monolayer ZnSnN2 calculated using both 2×2 and 3×3 supercells and (b) bilayer ZnSnN2 calculated using 2×2 supercell.

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(a)

(b)

Figure 3. Electronic band structures of (a) monolayer and (b) bilayer ZnSnN2 calculated using the PBE0 functional.

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Figure 4. Total and projected density of states (DOS) of (a) monolayer and (b) bilayer ZnSnN2.

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Figure 5. Band alignment of monolayer, bilayer, and bulk ZnSnN2 calculated using the PBE0 functional. The vacuum level is set to 0 eV. The chemical reaction potentials for H+/H2 and O2/H2O are plotted with the dashed lines.

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Figure 6. The imaginary part of the dielectric function of (a) monolayer and (b) bilayer ZnSnN2 along the a(xx), b(yy) and c(zz) directions.

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Figure 7. Evolution of the total energy of primitive unit cell and the band edge positions as a function of biaxial strains for monolayer [(a) and (b)] and bilayer [(c) and (d)] ZnSnN2.

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Mono- and Bilayer ZnSnN2 Sheets for Visible-Light Photocatalysis

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