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SnO hetero-bilayer as a promising photocatalyst for solar water splitting
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 7 8 1 6 e2 7 8 2 4
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Theoretical design of a novel 2D tetragonal ZnS/ SnO hetero-bilayer as a promising photocatalyst for solar water splitting Jia Zhou a,b a b
School of Science, Harbin Institute of Technology, Shenzhen 518055, China School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China
highlights
graphical abstract
ZnS/SnO hetero-bilayer is a promising photocatalyst for solar water splitting. A significant quantum confinement effect has been observed. ZnS/SnO hetero-bilayer is a polarized semiconductor with a built-in electric field.
article info
abstract
Article history:
Van der Waals (vdW) hetero-bilayers are emerging as unique structures to enhance the
Received 27 June 2019
performance of two-dimensional (2D) layered nanomaterials for next-generation electronic
Received in revised form
and optoelectronic devices. In this work, we employ first-principles calculations to study
22 August 2019
the novel tetragonal ZnS/SnO hetero-bilayer (BL). The state-of-the-art computations based
Accepted 4 September 2019
upon quasiparticle GW and BetheSalpeter equation (BSE) are utilized to study the elec-
Available online 27 September 2019
tronic and optical properties of this novel vdW hetero-bilayer. We reveal that ZnS/SnO BL is a polarized semiconductor with a clear built-in electric field, and possesses a special band
Keywords:
characteristic favorable for reducing the carrier recombination. It is also demonstrated that
Solar water splitting
strain and external electric field are among the effective methods to modulate the elec-
Two-dimensional materials
tronic and optical properties of ZnS/SnO BL. Our work suggests that ZnS/SnO BL has an
vdW hetero-bilayers
excellent optical absorption in the solar spectrum, rendering the material a viable candi-
Carrier separation
date for optoelectronic applications, in particular for solar water splitting.
GW approximation
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
E-mail address: [email protected]. https://doi.org/10.1016/j.ijhydene.2019.09.047 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction During the past decades, a growing number of 2D nanomaterials have been drawing tremendous attention due to their novel nanoscale properties which differ from their bulk counterparts. These advantages open the possibility for using 2D nanomaterials in future generations of nanoelectronics and optoelectronic devices, such as topological insulators, thermal conductors, etc. [1e5] Particularly, 2D nanomaterials are emerging to be ideal candidates as photocatalysts owing to their tunable band gap by the layer thickness, large specific surface area providing plenty of reactive sites, and high carrier mobility decreasing the recombination rate of photo-induced electrons and holes [6]. Nevertheless, single-component 2D materials alone are rarely suitable for practical applications [7]. Therefore, it is attractive to tailor the electronic and transport properties of 2D nanomaterials by a variety of means, such as doping, strain, and external electric field. In addition, there are plenty of ongoing investigations on constructing hetero-structures by two or more 2D materials interacting via the van der Waals (vdW) forces by experiments and computations, aiming to combine the advantages of building layered materials together [8e11]. For instance, graphene and hexagonal boron nitride (h-BN) vdW heterostructures are among the first research targets because they have similar structural parameters but drastically different properties, and abundant novel physics and functionalities have then been demonstrated [12e16]. Other vdW heterobilayers, such as phosphorene/MoS2 hetero-bilayers and transition metal dichalcogenide hetero-bilayers have also been studied and are proposed for largescale 2D optoelectronics [8,10,11,17]. More recently, group II-VI compound layered materials, including ZnS and ZnSe, have stepped into the limelight owing to their outstanding properties [18e23]. In 2012, Xie and co-workers reported a fabrication of large-area freestanding 2D sheets of ZnSe with four-atom thickness and a honeycomb lattice by a strategy involving a lamellar hybrid intermediate [18]. In photoelectrochemical test for solar water splitting, the 2D layered sheet of ZnSe exhibited a photocurrent density of 2.14 mA cm2 at 0.72 V versus Ag/AgCl under Xe lamp irradiation, which is almost 200 times higher than its bulk
counterpart. It is remarkable that their observation triggers an intensive explorations on a series of other freestanding layered nanosheets of ZnX and CdX (X ¼ S, Se) as potential synthesis targets by theoretical investigations [19e25]. Among these theoretical studies, our previous work on various singlelayered (SL) and bi-layered BL sheets containing tetragonal ZnSe and ZnS predicted by using density functional theory (DFT) that they would potentially have an outstanding performance on solar energy harvesting [23,26,27]. Furthermore, we explored the possibility of the formation of ZnS/ZnSe and ZnS/PbO vdW hetero-bilayers [28,29]. Our calculations showed these vdW hetero-bilayers demonstrated a great electron-hole separation by allocation of photo-induced electrons and holes in separate two layers, making them potentially more suitable for applications in photocatalytic systems. It is also worth noting that in Xie's work, they used indium tin oxide, a typical n-type semiconductor, as substrate to support hexagonal monolayer ZnS and ZnSe to achieve the superior solar water splitting performance. On the other hand, tin monoxide (SnO) possesses an intrinsic p-type semiconductor character, and has a tetragonal litharge layered structure, similar to our predicted tetragonal ZnS [30]. Previous studies showed SnO SL has high carrier mobility [31], and it could be stacked to form vdW hetero-bilayers with graphene and h-BN [32]. Therefore, we in this work intend to study the electronic and optical properties of ZnS/SnO vdW heterobilayer by means of DFT, accompanied by quasiparticle GW and BSE methods. Meanwhile, we also investigate the impact from strain and external electric field on the electronic and
interlayer distance Table 1 e Lattice parameter (a, in A), electronic band gap (Eg, in eV), and optical band (d, in A), gap (Eopt, in eV) of ZnS SL, SnO SL, and ZnS/SnO BL, and interlayer stabilization energy (DE, in meV/f.u.) of BLs with respect to the individual SLs. a ZnS SL SnO SL ZnS/SnO BL
d DE
3.95 e e 3.81 e e 3.89 2.45 77
Eg (PBED3)
Eg (G0W0)
Eopt (G0W0BSE)
2.94 3.03 1.62
5.10 4.86 3.12
4.04 3.88 2.83
Fig. 1 e (a) Side view and (b) top view of ZnS/SnO BL with unit cell highlighted with read dashed line. (c) 2D Brillouin zone with high symmetry points.
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Fig. 2 e (a) Band structure of ZnS/SnO BL with partial charge density of the CBM and VBM, and (bef) total/partial DOSs of ZnS/ SnO BL.
optical properties of ZnS/SnO BL. Our calculations demonstrate that the proper combination of different layered nanomaterials could lead to a novel 2D material, not only possessing typical type-II band alignment, but also with superb optical absorption, which indicates that ZnS/SnO BL can become a promising candidate for applications in photovoltaic devices.
Computational details First-principles calculations were performed based on Vienna ab initio simulation package (VASP) [33e36], by using planewave basis sets and projector-augmented wave (PAW) pseudopotentials [37,38]. For the structural relaxations and energy
calculations, The generalized gradient approximation with the Perdew-Burke-Ernzerhof (PBE) parametrization was used [39,40]. In addition, the van der Waals interactions were taken into account using Grimme's DFT-D3 method [41]. A cutoff energy of 500 eV for the plane-wave basis set was utilized throughout all calculations. The 2D layered materials were simulated using a dense G-centered 24 24 1 mesh of Monkhorst-Pack points for integration over the Brillouin zone [42]. A vacuum space of 20 A was added to eliminate interactions between the neighboring systems in the structural relaxation while fully relaxing the other lattice vectors. All atoms were relaxed until the force on each atom was less than 0.01 eV/ A. The convergence threshold for the total energy was chosen as 105 eV/atom. For the study of property, many-body perturbation theory including quasiparticle G0W0 and BSE
Fig. 3 e Band alignment of ZnS SL, SnO SL, and ZnS/SnO BLs with different strains calculated with G0W0. Standard redox potentials for water splitting at pH ¼ 0 are shown for comparison.
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were used unless noted otherwise [43,44]. A reduced cutoff energy of 150 eV and a reduced k-mesh of 12 12 1 was used for the response function. A total of 192 and 96 bands were used in the G0W0 calculation with 96 frequency points for BLs and SLs, respectively, which was then followed by the BSE calculation using the G0W0 quasiparticle energies and the PBE wave functions. The excitonic state calculation for BLs (SLs) included the 30 (15) lowest conduction bands and 32 (16) highest valence bands.
Results and discussion Geometries and energetics Fig. 1a, b shows the side view and top view of ZnS/SnO BL. respectively. The tetragonal ZnS SL has been studied in our previous studies [23,27], and SnO SL has a similar structure. In ZnS/SnO BL, each layer is of three-atom thickness, with either Zn or O atoms coordinated tetragonally with S or Sn atoms. For vdW hetero-structures, it is very important for the composing parts to have close in-plane lattice constants. In the current study, the lattice constants of ZnS SL (3.95 A) and SnO SL (3.81 A) are quite close with a ca. 3% mismatch. As same as ZnS and SnO bulk and multi-layers, ZnS/SnO BL favors approximately AA stacking (taking Sn equivalent to S; Zn equivalent to O). In Table 1, the interlayer stabilization energy per formula unit of ZnS/SnO BL is 77 meV, which is a little smaller than that of ZnS/PbO BL (89 meV) in our previous study [28]. The smaller interlayer stabilization energy is in part due to the somewhat bigger mismatch between SnO SL and ZnS SL in the lattice constants than between PbO SL and ZnS SL. However, the layer-layer distance between ZnS and SnO is 2.45 A, a little shorter than that between ZnS and PbO (2.46 A) in ZnS/PbO BL, indicating a comparable stability of the proposed ZnS/SnO BL. The optimal lattice constant for ZnS/SnO BL (3.89 A) lies right in the middle of SnO SL (3.81 A) and ZnS SL (3.95 A).
Electronic structure
Fig. 4 e Optical absorbance spectra A(u) by G0W0-BSE for (a) ZnS/SnO BL, (b) ZnS SL, and (c) SnO SL.
For the properties of ZnS/SnO BL, we studied its band structure and density of states (DOS) in the first place. Fig. 2a shows the band structure of ZnS/SnO BL, calculated by the PBE-D3 method. The 2D Brillouin zone is depicted in Fig. 1c. ZnS/ SnO BL exhibits an indirect band gap, and the calculated band gap is 1.62 eV. In ZnS/SnO BL, the valence band maximum (VBM) is somewhere between the G point and the M point, while the conduction band minimum (CBM) locates at the G point. The band gap of ZnS/SnO BL is significantly smaller than those of its components ZnS SL (2.94 eV) and SnO SL (3.03 eV). The calculated band gap of SnO SL is in a good agreement with the result from other group (3.00 eV) [45], while the results of ZnS SL is from our own study [29]. The drastically decreased band gap when forming ZnS/SnO BL indicates there might be strong interaction between ZnS layer and SnO layer since their geometries change little. To obtain deeper insight into the significant downhill of the band gap, the partial density of states (DOS) of ZnS/SnO BL is analyzed and shown in Fig. 2bef. Apparently, the conduction bands (CB)
close to the Fermi level are almost dominated by the Zn's s state, while the valence bands (VB) between 0.0 and 1.0 eV are from the Sn's sp states and the O's p states. Bader charges calculated for Zn and O atoms amount to þ0.8 and 1.2, respectively. On the other hand, Bader charges for the Sn and S atoms on the interface are þ1.3 and 0.9, while for those not on the interface, Bader charges are þ1.2 and 0.8, respectively. For direct comparison, Bader charges calculated for Zn and S atoms in pure ZnS SL amount to ±0.8, and Bader charges calculated for Sn and O atoms in pure SnO SL amount to ±1.2. Therefore, there is a clear charge transfer between two layers with ZnS layer obtaining extra 0.1 charges from SnO layer, resulting in a built-in electric field pointed from SnO layer to ZnS layer. The charge transfer further reinforces the n-type character of ZnS and the p-type character of SnO, suggesting a possible photo-induced electron-hole separation between ZnS layer and SnO layer. The partial charge distribution corresponding to the CBM/VBM of ZnS/SnO BL is also shown in
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Fig. 2a. The CBM is mostly composed of Zn s atomic orbitals in the ZnS layer, while the VBM is of Sn sp atomic orbitals and O p atomic orbitals in the SnO layer, confirming charge carrier separation in the two layers upon exciton dissociation. The separation of electrons and holes in ZnS/SnO BL can significantly suppress charge recombination, thus enhancing its prowess in optoelectronic devices. In addition, we calculated the effective mass of electrons and holes for ZnS/SnO BL in the same way as we did in our previous work [27]. As usual, the effective mass is stated in units of the rest mass of an electron, me. The electron effective mass of ZnS/SnO BL is 0.25me, very close to that of the pure ZnS SL, validating again the CBM of ZnS/SnO BL mainly comes from ZnS part. On the other hand, the hole effective mass of ZnS/SnO BL is 2.02me, ca. eight times of the electron effective mass. The high mass ratio mh =me implies the new bi-layered material could more likely be utilized in tunable n-type devices [22]. Since the band gaps are usually underestimated by the PBE functional [46], we then applied more accurate quasiparticle G0W0 method to calculate the band gap for the abovementioned 2D materials. The band gap of SnO SL increases to 4.86 eV by the G0W0 calculations from 3.03 eV by DFT. From our previous study, the band gap of ZnS SL is 5.10 eV by G0W0, and 2.94 eV by DFT [29]. However, the G0W0 band gap of ZnS/ SnO BL narrows to 3.12 eV with respect to the individual SLs. The band gap of ZnS/SnO BL falls in the visible light region compared with those of the composing SLs, indicating an enhanced capacity to absorb sunlight. Photocatalytic water splitting attracts much attention at this point. To efficiently photocatalyze the splitting of water, it is a fundamental
requirement that the band edges must straddle the redox potentials of the water-splitting reaction [47,48]. That is to say, the CBM energy of a suitable photocatalyst should be higher than the reduction potential of Hþ/H2 and the VBM energy lower than the oxidation potential of O2/H2O. To this end, we calculated the band edge positions ECBM and EVBM with respect to the vacuum level with G0W0 method by aligning the energy levels of different 2D layered materials so that the vacuum levels of them are all set to be zero. The redox potentials of the water splitting reaction have something to do with the pH value. In this work, we adopted the commonly accepted value of Ered Hþ =H2 ¼ 4:44 eV þ pH 0:059 eV for the standard reduction potential for Hþ/H2, and the oxidation potential for O2/H2O is Eox O2 =H2 O ¼ 5:67 eV þ pH 0:059 eV [49]. Fig. 3 shows the CBM/ VBM positions of ZnS/SnO BL, as well as ZnS SL and SnO SL. Obviously, the band alignment demonstrates the type-II [7] nature of ZnS/SnO BL, with its CBM position close to that of ZnS SL and VBM position close to SnO SL. This observation is in good agreement with our previous DOS analysis. As stated previously, a suitable photocatalyst ought to have a broad absorption range, especially within the visible light region, as well as an efficient carrier separation. The accurate G0W0 calculations show the band gap of ZnS SL or SnO SL is ca. 5 eV, well over the upper limit of the visible light (ca. 3.2 eV), therefore impeding the application in photocatalysis. But, it endows ZnS SL and SnO SL to obtain an improved capacity to absorb the sunlight in the visible light region by stacking into vdW hetero-bilayers. Similar behaviors have been found in many vdW hetero-bilayers, however the suitable layered nanostructure duo must be screened carefully in order to possess such property.
Fig. 5 e Optical absorbance spectra A(u) by G0W0-BSE for ZnS/SnO BL with different biaxial strains: (a) ¡2%, (b) ¡1%, (c) þ1%, and (d) þ2%.
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Optical properties Besides a suitable band gap and band alignment, it is equally important for photocatalytic materials to absorb a great amount of the incoming sunlight. Here, the optical property of ZnS/SnO BL was addressed by using the optical absorbance A(u), which was defined as the fraction of photons with energy E ¼ ħu absorbed by the 2D layered nanosheets. For the inplane polarized light, A(u) was then calculated using the equation A(u) ¼ u L ε2/c, in which u is the frequency of photon, L is the interlayer spacing between the isolated layers, ε2 is the imaginary part of the dielectric function, and c is the light speed in vacuum [50]. We calculated the imaginary part of the dielectric function ε2 by solving BetheSalpeter equation. We have used this strategy on a number of layered materials in our previous studies [23,26,51]. The computed optical absorbance A(u), which is a function of the photon frequency u, for ZnS/SnO BL, ZnS SL, and SnO SL is shown in Fig. 4. The optical band gaps shown in Fig. 4 are also collected in Table 1: 3.88 eV, 4.04 eV, and 2.83 eV for SnO SL, ZnS SL, and ZnS/SnO BL, respectively. Hence, the exciton binding energies are 0.98 eV, 1.06 eV, and 0.29 eV for SnO SL, ZnS SL, and ZnS/SnO BL, showing a clear excitonic effect. It is
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interesting that the exciton binding energy for ZnS/SnO BL is less than one third of those of SLs, indicating a strong interlayer impact. It is another important factor for the potential application of the layered nanosheets to have a strong photoabsorption in the visible light range. Obviously, ZnS/SnO BL has a much stronger photoabsorption in the visible light range (1.6e3.2 eV) than the composing SLs. This superior behavior, accompanied by the clear electron-hole separation upon photoabsorption, renders ZnS/SnO BL an excellent candidate in optoelectronic applications.
Effect of strain Strain is among effective methods to modulate the properties of nanomaterials. It has been confirmed by many theoretical and experimental investigations that various properties of 2D materials could be tuned by the biaxial strain [10,22,52]. Thus, it is tempting to explore whether it could be another practical route to tune the properties of ZnS/SnO BL by applying biaxial strains, in particular electronic and optical properties. We in this work applied biaxial strains ranging from 2% to þ2%, which could be realized by varying the in-plane lattice constant (a0) gradually and optimizing the atomic coordinates at
Fig. 6 e Optical absorbance spectra A(u) by HSE06 functional for ZnS/SnO BL with different external electric field: (a) 0.0, (b) (c) þ0.2 V/A, (d) þ0.3 V/A, (e) þ0.4 V/A, and (f) þ0.5 V/A. þ0.1 V/A,
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Fig. 7 e Optical absorbance spectra A(u) by HSE06 functional for ZnS/SnO BL with different external electric field: (a) 0.0, (b) (c) ¡0.2 V/A, (d) ¡0.3 V/A, (e) ¡0.4 V/A, and (f) ¡0.5 V/A. ¡0.1 V/A, each strain accordingly. After biaxial strains, ZnS/SnO BL still maintains an indirect band gap. The band gap increases when applying compressive strain (3.30 eV by G0W0 at 2%), while decreases when applying tensile strain (2.96 eV by G0W0 at þ2%). Strain effect on band edge of ZnS/SnO BL was also studied, and the results are shown in Fig. 3. It is interesting VBM positions almost stay put, while CBM positions climb gradually when introducing extra compressive strain. Fig. 5 shows the calculated optical absorbance A(u) as a function of the photon frequency u for ZnS/SnO BL with different strains. The optical band gap has a similar trend to the electronic band gap: 3.00 eV by G0W0-BSE at 2% and 2.68 eV by G0W0-BSE at þ2%. It could be anticipated that 1% change of the strain results in ca. 0.1 eV change of the band edges. The effectiveness of the photoabsorption modulation by biaxial strain could be of significant help on the utilization of this novel 2D material in photovoltaic devices.
Effect of external electric field Many recent research studies have also shown that applying an external electric field is an effective way to tune the electronic properties of vdW BLs [10]. Our calculation has already
shown ZnS/SnO BL is a dipole system, with a clear electron transfer from SnO to ZnS layer resulting in a built-in electric field pointed from top layer (ZnS) to bottom layer (SnO), so hopefully the external vertical electric field could be even more effective for the property modulation. Figs. 6 and 7 show the optical absorbance A(u) for ZnS/SnO BL with different external electric fields calculated on the frequency-dependent dielectric constants at the HSE06 [53] level. The strength of the applied electric field varies from 0 to 0.5 V/ A, and the positive or negative sign indicates the direction of the external electric field is along the built-in electric field or opposite to the builtin electric field. It should be noted that the band gap computed by HSE06 is usually smaller than by G0W0. For instance, the band gap of ZnS/SnO BL is 3.12 eV by G0W0, but 2.68 eV by HSE06. The discrepancy between GW and HSE06 has been thoroughly discussed in many studies [22]. Our previous study demonstrated that the trends which were observed by PBE and HSE06 methods for t-ZnX SLs, DLs, and 3D bulks are quite close, suggesting that the basic physics discovered by these methods ought to be convincible [27]. Under the external electric field, ZnS/SnO BL still maintains an indirect band gap, and a similar band structure. In general, the band gap of ZnS/ SnO BL increases when applying positive external electric
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field, while decreases when applying negative external electric field. Consequently, the position of the first peak in the optical absorbance redshifts up to 3.95 eV when applying positive external electric field, while blueshifts up to 3.41 eV when applying negative external electric field.
Conclusions To conclude, we have studied the electronic and optical properties of a novel polarized ZnS/SnO hetero BL by using DFT method, as well as the state-of-the-art many-body perturbation theory including quasiparticle G0W0 and BSE. The band gap of ZnS/SnO BL is significantly smaller than the individual SnO SL and ZnS SL. Moreover, compared to the individual SLs, ZnS/SnO BL demonstrates an excellent electron-hole separation by allocation of the photo-induced electrons and holes to different layers. Additionally, the excitonic effects have been confirmed to be strong for all of the layered nanostructures based on the G0W0-BSE calculations. Our calculations show ZnS/SnO BL has a superior photoabsorption in the visible light range, rendering it more efficient for solar energy harvesting and utilization. Strain and external electric field effects on the electronic and optical properties have also been investigated. According to our study, the novel ZnS/SnO BL would be a good candidate as an excellent photocatalyst for solar water splitting, among a wealth of optoelectronic applications.
Acknowledgements We gratefully acknowledge the National Natural Science Foundation of China (No. 51602079) for financial support. Computer time made available by the National Supercomputing Center of China in Shenzhen (Shenzhen Cloud Computing Center) is gratefully acknowledged.
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Theoretical design of a novel 2D tetragonal ZnS/ SnO hetero-bilayer as a promising photocatalyst for solar water splitting Jia Zhou a,b a b
School of Science, Harbin Institute of Technology, Shenzhen 518055, China School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China
highlights
graphical abstract
ZnS/SnO hetero-bilayer is a promising photocatalyst for solar water splitting. A significant quantum confinement effect has been observed. ZnS/SnO hetero-bilayer is a polarized semiconductor with a built-in electric field.
article info
abstract
Article history:
Van der Waals (vdW) hetero-bilayers are emerging as unique structures to enhance the
Received 27 June 2019
performance of two-dimensional (2D) layered nanomaterials for next-generation electronic
Received in revised form
and optoelectronic devices. In this work, we employ first-principles calculations to study
22 August 2019
the novel tetragonal ZnS/SnO hetero-bilayer (BL). The state-of-the-art computations based
Accepted 4 September 2019
upon quasiparticle GW and BetheSalpeter equation (BSE) are utilized to study the elec-
Available online 27 September 2019
tronic and optical properties of this novel vdW hetero-bilayer. We reveal that ZnS/SnO BL is a polarized semiconductor with a clear built-in electric field, and possesses a special band
Keywords:
characteristic favorable for reducing the carrier recombination. It is also demonstrated that
Solar water splitting
strain and external electric field are among the effective methods to modulate the elec-
Two-dimensional materials
tronic and optical properties of ZnS/SnO BL. Our work suggests that ZnS/SnO BL has an
vdW hetero-bilayers
excellent optical absorption in the solar spectrum, rendering the material a viable candi-
Carrier separation
date for optoelectronic applications, in particular for solar water splitting.
GW approximation
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
E-mail address: [email protected]. https://doi.org/10.1016/j.ijhydene.2019.09.047 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction During the past decades, a growing number of 2D nanomaterials have been drawing tremendous attention due to their novel nanoscale properties which differ from their bulk counterparts. These advantages open the possibility for using 2D nanomaterials in future generations of nanoelectronics and optoelectronic devices, such as topological insulators, thermal conductors, etc. [1e5] Particularly, 2D nanomaterials are emerging to be ideal candidates as photocatalysts owing to their tunable band gap by the layer thickness, large specific surface area providing plenty of reactive sites, and high carrier mobility decreasing the recombination rate of photo-induced electrons and holes [6]. Nevertheless, single-component 2D materials alone are rarely suitable for practical applications [7]. Therefore, it is attractive to tailor the electronic and transport properties of 2D nanomaterials by a variety of means, such as doping, strain, and external electric field. In addition, there are plenty of ongoing investigations on constructing hetero-structures by two or more 2D materials interacting via the van der Waals (vdW) forces by experiments and computations, aiming to combine the advantages of building layered materials together [8e11]. For instance, graphene and hexagonal boron nitride (h-BN) vdW heterostructures are among the first research targets because they have similar structural parameters but drastically different properties, and abundant novel physics and functionalities have then been demonstrated [12e16]. Other vdW heterobilayers, such as phosphorene/MoS2 hetero-bilayers and transition metal dichalcogenide hetero-bilayers have also been studied and are proposed for largescale 2D optoelectronics [8,10,11,17]. More recently, group II-VI compound layered materials, including ZnS and ZnSe, have stepped into the limelight owing to their outstanding properties [18e23]. In 2012, Xie and co-workers reported a fabrication of large-area freestanding 2D sheets of ZnSe with four-atom thickness and a honeycomb lattice by a strategy involving a lamellar hybrid intermediate [18]. In photoelectrochemical test for solar water splitting, the 2D layered sheet of ZnSe exhibited a photocurrent density of 2.14 mA cm2 at 0.72 V versus Ag/AgCl under Xe lamp irradiation, which is almost 200 times higher than its bulk
counterpart. It is remarkable that their observation triggers an intensive explorations on a series of other freestanding layered nanosheets of ZnX and CdX (X ¼ S, Se) as potential synthesis targets by theoretical investigations [19e25]. Among these theoretical studies, our previous work on various singlelayered (SL) and bi-layered BL sheets containing tetragonal ZnSe and ZnS predicted by using density functional theory (DFT) that they would potentially have an outstanding performance on solar energy harvesting [23,26,27]. Furthermore, we explored the possibility of the formation of ZnS/ZnSe and ZnS/PbO vdW hetero-bilayers [28,29]. Our calculations showed these vdW hetero-bilayers demonstrated a great electron-hole separation by allocation of photo-induced electrons and holes in separate two layers, making them potentially more suitable for applications in photocatalytic systems. It is also worth noting that in Xie's work, they used indium tin oxide, a typical n-type semiconductor, as substrate to support hexagonal monolayer ZnS and ZnSe to achieve the superior solar water splitting performance. On the other hand, tin monoxide (SnO) possesses an intrinsic p-type semiconductor character, and has a tetragonal litharge layered structure, similar to our predicted tetragonal ZnS [30]. Previous studies showed SnO SL has high carrier mobility [31], and it could be stacked to form vdW hetero-bilayers with graphene and h-BN [32]. Therefore, we in this work intend to study the electronic and optical properties of ZnS/SnO vdW heterobilayer by means of DFT, accompanied by quasiparticle GW and BSE methods. Meanwhile, we also investigate the impact from strain and external electric field on the electronic and
interlayer distance Table 1 e Lattice parameter (a, in A), electronic band gap (Eg, in eV), and optical band (d, in A), gap (Eopt, in eV) of ZnS SL, SnO SL, and ZnS/SnO BL, and interlayer stabilization energy (DE, in meV/f.u.) of BLs with respect to the individual SLs. a ZnS SL SnO SL ZnS/SnO BL
d DE
3.95 e e 3.81 e e 3.89 2.45 77
Eg (PBED3)
Eg (G0W0)
Eopt (G0W0BSE)
2.94 3.03 1.62
5.10 4.86 3.12
4.04 3.88 2.83
Fig. 1 e (a) Side view and (b) top view of ZnS/SnO BL with unit cell highlighted with read dashed line. (c) 2D Brillouin zone with high symmetry points.
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Fig. 2 e (a) Band structure of ZnS/SnO BL with partial charge density of the CBM and VBM, and (bef) total/partial DOSs of ZnS/ SnO BL.
optical properties of ZnS/SnO BL. Our calculations demonstrate that the proper combination of different layered nanomaterials could lead to a novel 2D material, not only possessing typical type-II band alignment, but also with superb optical absorption, which indicates that ZnS/SnO BL can become a promising candidate for applications in photovoltaic devices.
Computational details First-principles calculations were performed based on Vienna ab initio simulation package (VASP) [33e36], by using planewave basis sets and projector-augmented wave (PAW) pseudopotentials [37,38]. For the structural relaxations and energy
calculations, The generalized gradient approximation with the Perdew-Burke-Ernzerhof (PBE) parametrization was used [39,40]. In addition, the van der Waals interactions were taken into account using Grimme's DFT-D3 method [41]. A cutoff energy of 500 eV for the plane-wave basis set was utilized throughout all calculations. The 2D layered materials were simulated using a dense G-centered 24 24 1 mesh of Monkhorst-Pack points for integration over the Brillouin zone [42]. A vacuum space of 20 A was added to eliminate interactions between the neighboring systems in the structural relaxation while fully relaxing the other lattice vectors. All atoms were relaxed until the force on each atom was less than 0.01 eV/ A. The convergence threshold for the total energy was chosen as 105 eV/atom. For the study of property, many-body perturbation theory including quasiparticle G0W0 and BSE
Fig. 3 e Band alignment of ZnS SL, SnO SL, and ZnS/SnO BLs with different strains calculated with G0W0. Standard redox potentials for water splitting at pH ¼ 0 are shown for comparison.
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were used unless noted otherwise [43,44]. A reduced cutoff energy of 150 eV and a reduced k-mesh of 12 12 1 was used for the response function. A total of 192 and 96 bands were used in the G0W0 calculation with 96 frequency points for BLs and SLs, respectively, which was then followed by the BSE calculation using the G0W0 quasiparticle energies and the PBE wave functions. The excitonic state calculation for BLs (SLs) included the 30 (15) lowest conduction bands and 32 (16) highest valence bands.
Results and discussion Geometries and energetics Fig. 1a, b shows the side view and top view of ZnS/SnO BL. respectively. The tetragonal ZnS SL has been studied in our previous studies [23,27], and SnO SL has a similar structure. In ZnS/SnO BL, each layer is of three-atom thickness, with either Zn or O atoms coordinated tetragonally with S or Sn atoms. For vdW hetero-structures, it is very important for the composing parts to have close in-plane lattice constants. In the current study, the lattice constants of ZnS SL (3.95 A) and SnO SL (3.81 A) are quite close with a ca. 3% mismatch. As same as ZnS and SnO bulk and multi-layers, ZnS/SnO BL favors approximately AA stacking (taking Sn equivalent to S; Zn equivalent to O). In Table 1, the interlayer stabilization energy per formula unit of ZnS/SnO BL is 77 meV, which is a little smaller than that of ZnS/PbO BL (89 meV) in our previous study [28]. The smaller interlayer stabilization energy is in part due to the somewhat bigger mismatch between SnO SL and ZnS SL in the lattice constants than between PbO SL and ZnS SL. However, the layer-layer distance between ZnS and SnO is 2.45 A, a little shorter than that between ZnS and PbO (2.46 A) in ZnS/PbO BL, indicating a comparable stability of the proposed ZnS/SnO BL. The optimal lattice constant for ZnS/SnO BL (3.89 A) lies right in the middle of SnO SL (3.81 A) and ZnS SL (3.95 A).
Electronic structure
Fig. 4 e Optical absorbance spectra A(u) by G0W0-BSE for (a) ZnS/SnO BL, (b) ZnS SL, and (c) SnO SL.
For the properties of ZnS/SnO BL, we studied its band structure and density of states (DOS) in the first place. Fig. 2a shows the band structure of ZnS/SnO BL, calculated by the PBE-D3 method. The 2D Brillouin zone is depicted in Fig. 1c. ZnS/ SnO BL exhibits an indirect band gap, and the calculated band gap is 1.62 eV. In ZnS/SnO BL, the valence band maximum (VBM) is somewhere between the G point and the M point, while the conduction band minimum (CBM) locates at the G point. The band gap of ZnS/SnO BL is significantly smaller than those of its components ZnS SL (2.94 eV) and SnO SL (3.03 eV). The calculated band gap of SnO SL is in a good agreement with the result from other group (3.00 eV) [45], while the results of ZnS SL is from our own study [29]. The drastically decreased band gap when forming ZnS/SnO BL indicates there might be strong interaction between ZnS layer and SnO layer since their geometries change little. To obtain deeper insight into the significant downhill of the band gap, the partial density of states (DOS) of ZnS/SnO BL is analyzed and shown in Fig. 2bef. Apparently, the conduction bands (CB)
close to the Fermi level are almost dominated by the Zn's s state, while the valence bands (VB) between 0.0 and 1.0 eV are from the Sn's sp states and the O's p states. Bader charges calculated for Zn and O atoms amount to þ0.8 and 1.2, respectively. On the other hand, Bader charges for the Sn and S atoms on the interface are þ1.3 and 0.9, while for those not on the interface, Bader charges are þ1.2 and 0.8, respectively. For direct comparison, Bader charges calculated for Zn and S atoms in pure ZnS SL amount to ±0.8, and Bader charges calculated for Sn and O atoms in pure SnO SL amount to ±1.2. Therefore, there is a clear charge transfer between two layers with ZnS layer obtaining extra 0.1 charges from SnO layer, resulting in a built-in electric field pointed from SnO layer to ZnS layer. The charge transfer further reinforces the n-type character of ZnS and the p-type character of SnO, suggesting a possible photo-induced electron-hole separation between ZnS layer and SnO layer. The partial charge distribution corresponding to the CBM/VBM of ZnS/SnO BL is also shown in
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Fig. 2a. The CBM is mostly composed of Zn s atomic orbitals in the ZnS layer, while the VBM is of Sn sp atomic orbitals and O p atomic orbitals in the SnO layer, confirming charge carrier separation in the two layers upon exciton dissociation. The separation of electrons and holes in ZnS/SnO BL can significantly suppress charge recombination, thus enhancing its prowess in optoelectronic devices. In addition, we calculated the effective mass of electrons and holes for ZnS/SnO BL in the same way as we did in our previous work [27]. As usual, the effective mass is stated in units of the rest mass of an electron, me. The electron effective mass of ZnS/SnO BL is 0.25me, very close to that of the pure ZnS SL, validating again the CBM of ZnS/SnO BL mainly comes from ZnS part. On the other hand, the hole effective mass of ZnS/SnO BL is 2.02me, ca. eight times of the electron effective mass. The high mass ratio mh =me implies the new bi-layered material could more likely be utilized in tunable n-type devices [22]. Since the band gaps are usually underestimated by the PBE functional [46], we then applied more accurate quasiparticle G0W0 method to calculate the band gap for the abovementioned 2D materials. The band gap of SnO SL increases to 4.86 eV by the G0W0 calculations from 3.03 eV by DFT. From our previous study, the band gap of ZnS SL is 5.10 eV by G0W0, and 2.94 eV by DFT [29]. However, the G0W0 band gap of ZnS/ SnO BL narrows to 3.12 eV with respect to the individual SLs. The band gap of ZnS/SnO BL falls in the visible light region compared with those of the composing SLs, indicating an enhanced capacity to absorb sunlight. Photocatalytic water splitting attracts much attention at this point. To efficiently photocatalyze the splitting of water, it is a fundamental
requirement that the band edges must straddle the redox potentials of the water-splitting reaction [47,48]. That is to say, the CBM energy of a suitable photocatalyst should be higher than the reduction potential of Hþ/H2 and the VBM energy lower than the oxidation potential of O2/H2O. To this end, we calculated the band edge positions ECBM and EVBM with respect to the vacuum level with G0W0 method by aligning the energy levels of different 2D layered materials so that the vacuum levels of them are all set to be zero. The redox potentials of the water splitting reaction have something to do with the pH value. In this work, we adopted the commonly accepted value of Ered Hþ =H2 ¼ 4:44 eV þ pH 0:059 eV for the standard reduction potential for Hþ/H2, and the oxidation potential for O2/H2O is Eox O2 =H2 O ¼ 5:67 eV þ pH 0:059 eV [49]. Fig. 3 shows the CBM/ VBM positions of ZnS/SnO BL, as well as ZnS SL and SnO SL. Obviously, the band alignment demonstrates the type-II [7] nature of ZnS/SnO BL, with its CBM position close to that of ZnS SL and VBM position close to SnO SL. This observation is in good agreement with our previous DOS analysis. As stated previously, a suitable photocatalyst ought to have a broad absorption range, especially within the visible light region, as well as an efficient carrier separation. The accurate G0W0 calculations show the band gap of ZnS SL or SnO SL is ca. 5 eV, well over the upper limit of the visible light (ca. 3.2 eV), therefore impeding the application in photocatalysis. But, it endows ZnS SL and SnO SL to obtain an improved capacity to absorb the sunlight in the visible light region by stacking into vdW hetero-bilayers. Similar behaviors have been found in many vdW hetero-bilayers, however the suitable layered nanostructure duo must be screened carefully in order to possess such property.
Fig. 5 e Optical absorbance spectra A(u) by G0W0-BSE for ZnS/SnO BL with different biaxial strains: (a) ¡2%, (b) ¡1%, (c) þ1%, and (d) þ2%.
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Optical properties Besides a suitable band gap and band alignment, it is equally important for photocatalytic materials to absorb a great amount of the incoming sunlight. Here, the optical property of ZnS/SnO BL was addressed by using the optical absorbance A(u), which was defined as the fraction of photons with energy E ¼ ħu absorbed by the 2D layered nanosheets. For the inplane polarized light, A(u) was then calculated using the equation A(u) ¼ u L ε2/c, in which u is the frequency of photon, L is the interlayer spacing between the isolated layers, ε2 is the imaginary part of the dielectric function, and c is the light speed in vacuum [50]. We calculated the imaginary part of the dielectric function ε2 by solving BetheSalpeter equation. We have used this strategy on a number of layered materials in our previous studies [23,26,51]. The computed optical absorbance A(u), which is a function of the photon frequency u, for ZnS/SnO BL, ZnS SL, and SnO SL is shown in Fig. 4. The optical band gaps shown in Fig. 4 are also collected in Table 1: 3.88 eV, 4.04 eV, and 2.83 eV for SnO SL, ZnS SL, and ZnS/SnO BL, respectively. Hence, the exciton binding energies are 0.98 eV, 1.06 eV, and 0.29 eV for SnO SL, ZnS SL, and ZnS/SnO BL, showing a clear excitonic effect. It is
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interesting that the exciton binding energy for ZnS/SnO BL is less than one third of those of SLs, indicating a strong interlayer impact. It is another important factor for the potential application of the layered nanosheets to have a strong photoabsorption in the visible light range. Obviously, ZnS/SnO BL has a much stronger photoabsorption in the visible light range (1.6e3.2 eV) than the composing SLs. This superior behavior, accompanied by the clear electron-hole separation upon photoabsorption, renders ZnS/SnO BL an excellent candidate in optoelectronic applications.
Effect of strain Strain is among effective methods to modulate the properties of nanomaterials. It has been confirmed by many theoretical and experimental investigations that various properties of 2D materials could be tuned by the biaxial strain [10,22,52]. Thus, it is tempting to explore whether it could be another practical route to tune the properties of ZnS/SnO BL by applying biaxial strains, in particular electronic and optical properties. We in this work applied biaxial strains ranging from 2% to þ2%, which could be realized by varying the in-plane lattice constant (a0) gradually and optimizing the atomic coordinates at
Fig. 6 e Optical absorbance spectra A(u) by HSE06 functional for ZnS/SnO BL with different external electric field: (a) 0.0, (b) (c) þ0.2 V/A, (d) þ0.3 V/A, (e) þ0.4 V/A, and (f) þ0.5 V/A. þ0.1 V/A,
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Fig. 7 e Optical absorbance spectra A(u) by HSE06 functional for ZnS/SnO BL with different external electric field: (a) 0.0, (b) (c) ¡0.2 V/A, (d) ¡0.3 V/A, (e) ¡0.4 V/A, and (f) ¡0.5 V/A. ¡0.1 V/A, each strain accordingly. After biaxial strains, ZnS/SnO BL still maintains an indirect band gap. The band gap increases when applying compressive strain (3.30 eV by G0W0 at 2%), while decreases when applying tensile strain (2.96 eV by G0W0 at þ2%). Strain effect on band edge of ZnS/SnO BL was also studied, and the results are shown in Fig. 3. It is interesting VBM positions almost stay put, while CBM positions climb gradually when introducing extra compressive strain. Fig. 5 shows the calculated optical absorbance A(u) as a function of the photon frequency u for ZnS/SnO BL with different strains. The optical band gap has a similar trend to the electronic band gap: 3.00 eV by G0W0-BSE at 2% and 2.68 eV by G0W0-BSE at þ2%. It could be anticipated that 1% change of the strain results in ca. 0.1 eV change of the band edges. The effectiveness of the photoabsorption modulation by biaxial strain could be of significant help on the utilization of this novel 2D material in photovoltaic devices.
Effect of external electric field Many recent research studies have also shown that applying an external electric field is an effective way to tune the electronic properties of vdW BLs [10]. Our calculation has already
shown ZnS/SnO BL is a dipole system, with a clear electron transfer from SnO to ZnS layer resulting in a built-in electric field pointed from top layer (ZnS) to bottom layer (SnO), so hopefully the external vertical electric field could be even more effective for the property modulation. Figs. 6 and 7 show the optical absorbance A(u) for ZnS/SnO BL with different external electric fields calculated on the frequency-dependent dielectric constants at the HSE06 [53] level. The strength of the applied electric field varies from 0 to 0.5 V/ A, and the positive or negative sign indicates the direction of the external electric field is along the built-in electric field or opposite to the builtin electric field. It should be noted that the band gap computed by HSE06 is usually smaller than by G0W0. For instance, the band gap of ZnS/SnO BL is 3.12 eV by G0W0, but 2.68 eV by HSE06. The discrepancy between GW and HSE06 has been thoroughly discussed in many studies [22]. Our previous study demonstrated that the trends which were observed by PBE and HSE06 methods for t-ZnX SLs, DLs, and 3D bulks are quite close, suggesting that the basic physics discovered by these methods ought to be convincible [27]. Under the external electric field, ZnS/SnO BL still maintains an indirect band gap, and a similar band structure. In general, the band gap of ZnS/ SnO BL increases when applying positive external electric
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field, while decreases when applying negative external electric field. Consequently, the position of the first peak in the optical absorbance redshifts up to 3.95 eV when applying positive external electric field, while blueshifts up to 3.41 eV when applying negative external electric field.
Conclusions To conclude, we have studied the electronic and optical properties of a novel polarized ZnS/SnO hetero BL by using DFT method, as well as the state-of-the-art many-body perturbation theory including quasiparticle G0W0 and BSE. The band gap of ZnS/SnO BL is significantly smaller than the individual SnO SL and ZnS SL. Moreover, compared to the individual SLs, ZnS/SnO BL demonstrates an excellent electron-hole separation by allocation of the photo-induced electrons and holes to different layers. Additionally, the excitonic effects have been confirmed to be strong for all of the layered nanostructures based on the G0W0-BSE calculations. Our calculations show ZnS/SnO BL has a superior photoabsorption in the visible light range, rendering it more efficient for solar energy harvesting and utilization. Strain and external electric field effects on the electronic and optical properties have also been investigated. According to our study, the novel ZnS/SnO BL would be a good candidate as an excellent photocatalyst for solar water splitting, among a wealth of optoelectronic applications.
Acknowledgements We gratefully acknowledge the National Natural Science Foundation of China (No. 51602079) for financial support. Computer time made available by the National Supercomputing Center of China in Shenzhen (Shenzhen Cloud Computing Center) is gratefully acknowledged.
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