Janus Group-III Chalcogenide Monolayers and Derivative Type-II

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C: Energy Conversion and Storage; Energy and Charge Transport

Janus Group-# Chalcogenide Monolayers and Derivative Type-# Heterojunctions as Water Splitting Photocatalysts with Strong Visible Light Absorbance Lei Hu, and Dongshan Wei J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06575 • Publication Date (Web): 16 Nov 2018 Downloaded from http://pubs.acs.org on November 17, 2018

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The Journal of Physical Chemistry

Janus

Group-Ⅲ

Chalcogenide

Monolayers

and

Derivative

Type-Ⅱ

Heterojunctions as Water Splitting Photocatalysts with Strong Visible Light Absorbance

Lei Hu1,2, Dongshan Wei1* 1

School of Electronic Engineering, Dongguan University of Technology, Dongguan, Guangdong

523808, China 2

School of Environmental and Chemical Engineering, Chongqing Three Gorges University,

Chongqing 404100, China *To whom correspondence should be addressed. E-mail: [email protected]

1 ACS Paragon Plus Environment

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Abstract Search for two-dimensional (2D) water splitting photocatalysts is crucial to solve energy crises and environmental problems. In this research, we study the electronic and photocatalytic properties of single-layer Ga2X1X2 (Ga2SeTe, Ga2STe and Ga2SSe) and newly proposed α-Ga2S3/Ga2SSe-A, αGa2S3/Ga2SSe-B and α-Ga2S3/Ga2SSe-C van der Walls heterojunctions using first-principles calculations. Theoretical results indicate Ga2X1X2 monolayers present suitable band edges. 2D αGa2S3/Ga2SSe-B and α-Ga2S3/Ga2SSe-C belong to type-Ⅱ heterojunctions, and under biaxial strains embody suitable band edges. Comparisons of the valence band maximum (VBM) charge and electric dipole of α-Ga2S3/Ga2SSe-A and α-Ga2S3/Ga2SSe-B demonstrate it is possible to achieve suitable band edges for water splitting by switching electric dipoles. Especially, the three Ga2X1X2 monolayers, αGa2S3/Ga2SSe-B and α-Ga2S3/Ga2SSe-C heterojunctions absorb a large amount of visible light, promising they are photocatalysts for water splitting. More importantly, we find the optical absorption coefficients of 2D monolayers and heterojunctions in previous calculations are several times underestimated because the effective volume is not taken into consideration. To obtain reliable absorption coefficients, the real and imaginary parts of dielectric function must be renormalized.

1. Introduction Hydrogen from photocatalytic water splitting plays an important role in solving energy crises and 2 ACS Paragon Plus Environment

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environmental problems.1, 2 One of the central issues for solar hydrogen production is to explore highly efficient photocatalysts. Recently, utilization of two-dimensional (2D) materials as photocatalysts is attracting increasing attention.3-8 2D materials not only possess extremely large surface areas beneficial for photocatalytic reactions but also minimize the migration distance of photogenerated electrons and holes, thereby reducing possibilities of electron-hole recombination.9 On the other hand, group Ⅲ-Ⅵ monolayers (GaX, X=S, Se,Te) embody novel electronic,10, 11 optical12-14 and mechanic properties.15, 16

GaX monolayers are potential photocatalysts for water splitting because they present suitable band

edges and absorb a significant amount of visible light.17 Group Ⅳ-Ⅴ (Ⅳ=Si, Ge; Ⅴ=N, P) monolayers with similar structures as GaX monolayers have absorption from visible-light to far-ultraviolet wavelengths, resulting in excellent potential for solar energy conversion.18 These inspire us to explore the electronic and photocatalytic properties of Janus group-Ⅲ chalcogenide monolayers (Ga2X1X2 , X1, X2=S, Se, Te), which are derivatives of GaX monolayers and experimentally feasible as they are mechanically and dynamically stable.19 Another class of Ⅲ-Ⅵ monolayers (α-M2X3, M=Ga, In; X=S, Se, Te) consisting of five sublayers stacked in the sequence of X-M-X-M-X were theoretically predicted by Debbichi et al.,20

and were

demonstrated to exhibit significant out-of-plane polarization.21 The small mismatch of in-plane lattice constants of Ga2X1X219 and α-M2X321 monolayers promises stable van der Waals heterojunctions. 2D heterojunctions are classified into type-Ⅰ, type-Ⅱ and type-Ⅲ heterojunctions.22,

23

The type-Ⅰ

heterojunctions have conduction band minimum (CBM) and valence band maximum (VBM) states in the same layer. The VBM and CBM of type-Ⅱ heterojunctions are located at different layers, and thereby holes and electrons are high efficiently separated.6 The type-Ⅲ heterojunctions have no band gap, and thus cannot be applied to produce hydrogen via water splitting, because the bandgap of water splitting catalysts is wider than 1.23 eV. So far, various type-Ⅱ heterojunctions such as g-C3N4/C2N24, AN/BP25 and C2N/MoS226 have been reported to enhance charge separation. However, most of previously reported type-Ⅱ heterojunctions such as AN/BP and g-C3N4/C2N only have limited absorption of visible light, which results in low efficiency of solar energy conversion. Here, by using first-principles calculations we demonstrate some 2D α-M2X3/Ga2X1X2 heterojunctions belong to type-Ⅱ heterojunctions and have strong absorption in a wide visible-light range. Especially, we find the optical absorption coefficients of all the low-dimensional materials including monolayers27, 28 and 3 ACS Paragon Plus Environment


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2D heterojunctions6, 24-26 are several times underestimated in previous theoretical works. To solve such a problem, the effective unit cell volume must be taken into consideration when calculating absorption coefficients. 2. Calculation details In the present work, the electronic and photocatalytic properties of 2D Ga2X1X2 (Ga2SeTe, Ga2STe and Ga2SSe) monolayers and α-M2X3/Ga2SSe heterojunctions are studied using firstprinciples calculations. All the calculations are on the basis of density functional theory (DFT) using the projector-augmented wave method (PAW)29 as implemented in the Vienna Ab intio Simulation Package (VASP).30-32 The generalized gradient approximation (GGA) parametrized by Perdew, Burke, and Ernzerhof (PBE)33 with van der Waals correction proposed by Grimme (DFT-D2)34 is applied. The plane-wave cutoff energy is set to 450 eV. To avoid interactions between adjacent 2D monolayers or heterojunctions, a large vacuum space of more than 14 Å is added in the direction perpendicular to atomic planes. A dense k-point grid of 11 × 11 × 1 is used to optimize geometric structures, and equilibrium structures are obtained when the forces acting on all atoms are less than 0.001 eV/Å. To overcome problems of bandgap underestimation in GGA-PBE calculations, the HSE06 hybrid functional is applied to calculate electronic band structures.35-37 Furthermore, the energy difference between HSE06 and PBE bandgaps is used for scissors corrections in optical calculations. The imaginary part of dielectric function due to direct interband transitions is given using the Fermi golden rule

ε 2 (ω)' 

4π 2 V2

 w

iVB , jCB

k

k j pa k i

2

 ( kj   ki   ) ,

(1)

k

where V, 𝜔, 𝑉𝐵 and 𝐶𝐵 are the unit cell volume, photon frequency, valence and conduction bands, respectively. The real part of dielectric function is extracted from a Kramers-Kronig transformation

ω'ε (ω') 4  ε1 (ω)'=1+ P  dω' 2 2 2 , 0 π ω'  ω

(2)

where P is the principle value of the integral. Based on the complex dielectric function, the optical absorption coefficient is obtained using the following formula

α(ω)'= 2ω[(ε1 (ω) '2 +ε 2 (ω) '2 )1/2 -ε1 (ω)']1/2 . 3. Results and discussions 3.1 Single-layer Ga2X1X2 4 ACS Paragon Plus Environment

(3)

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Figure 1(a) displays top and side views of single-layer Ga2SSe, which can also be considered as schematics of Ga2STe and Ga2SeTe monolayers. As can be seen, Ga2X1X2 monolayers consist of four sublayers stacked in the sequence of X1-Ga-Ga-X2. Their structural parameters and bandgaps calculated using the PBE functional are shown in Table 1, which are very close to previously reported values.19 The PBE functional usually underestimates bandgaps of semiconductors because of quasiparticle self-energy corrections, and thereby the HSE06 hybrid functional is applied to calculate accurate band structures. The calculated band structures are shown in Figure 2. Single-layer Ga2SSe displays an indirect bandgap of 3.20 eV with the VBM located between the Γ (0.0, 0.0, 0.0) and K (1/3, 2/3, 0) points and the CBM located at the M (0.0, 1/2, 0.0) point. The indirect bandgap is slightly narrower than the energy gap (3.39 eV) at the Γ point. Ga2SSe and Ga2STe monolayers exhibit direct bandgaps with the VBM and CBM both located at the Γ point. Interestingly, the direct bandgap (2.02 eV) of single-layer Ga2SeTe is in the visible-light range (1.64-3.19eV), implying the Ga2STe monolayer absorbs a significant fraction of visible light. The bandgap (1.61eV) of monolayer Ga2STe is in the infrared range, and thus Ga2STe absorbs all visible light. Efficient water splitting photocatalysts should be insoluble in water. As shown in the supporting information (SI), we find gallium (Ga)-X1(X2) bonds of single-layer Ga2X1X2 are inherently covalent by performing electron localization function (ELF)6 and Bader charge analyses. Considering the poor insolubility of covalent compounds such as WS2 nanosheets in water,6, 38 single-layer Ga2X1X2 are stable in water. 3.2 α-Ga2S3/Ga2SSe heterojunctions The optimized in-plane lattice constants of single-layer Ga2X1X2 are very close to that of monolayer α-Ga2X3.39 For instance, the in-plane lattice constant (3.653Å) of monolayer Ga2SSe gets very close to that (3.59 Å) of single-layer α-Ga2S3. Moreover, Ga2X1X2 and α-Ga2X3 monolayers both belong to C3V crystal symmetry. These promise stable 2D heterojunctions by stacking Ga2X1X2 and α-Ga2X3 layers. Hence, we devise three types of 2D heterojunctions based on representative Ga2SSe and α-Ga2S3 monolayers. As shown in Figure 1(c), the first α-Ga2S3/Ga2SSe heterojunction is constructed by placing α-Ga2S3 on the top of Ga2SSe and named as α-Ga2S3/Ga2SSe-A, where the sulphur (S) atom of α-Ga2S3 is on the very top of the S atom from Ga2SSe. The α-Ga2S3/Ga2SSe-B heterojunction is constructed by switching the electric dipole of α-Ga2S3 in α-Ga2S3/Ga2SSe-A. The 5 ACS Paragon Plus Environment


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α-Ga2S3/Ga2SSe-C heterojunction is achieved via a translation of the α-Ga2S3 layer in αGa2S3/Ga2SSe-B, as shown in Figure 1(e), where the Ga atom from the Ga2SSe layer is on the very top of the Ga atom in the α-Ga2S3 layer. As summarized in Table 2, the optimized in-plane lattice constants of α-Ga2S3/Ga2SSe heterojunctions are very close to that of Ga2SSe and α-Ga2S3 monolayers, suggesting the lattice mismatch is very small. The largest lattice mismatch occurs in Ga2SSe of α-Ga2S3/Ga2SSe-C, still being smaller than 1%. The formation energies for the proposed heterojunctions are defined as Ef = ( E ― E1 ― E2)/S1, where E, E1 and E2 respectively denote total energies of the heterojunctions, Ga2SSe and α-Ga2S3 monolayers. S1 is the interface area of the heterojunctions. As shown in Table 2, the calculated formation energies calculated using DFT-D2 are negative. The formation energies of 2D heterojunctions are usually connected with the vdW functional. As suggested by Table S2 of SI, the formation energies calculated using DFT-D3, TS and dDsC are also negative. The small lattice mismatch and negative formation energy imply these heterojunctions are experimentally feasible. As shown in Figure 1, the interlayer distances d are defined as the distances between the two neighboring sulfur sublayers in α-Ga2S3/Ga2SSe heterojunctions, namely, the z-coordinate differences of sulfur atoms from the two 2D constituent layers. The theoretical interlayer distances are summarized in Table 1. To further unravel binding characteristics, the three-dimensional charge density differences are calculated by subtracting the electronic charge of α-Ga2S3/Ga2SSe heterojunctions from that of independent Ga2SSe and α-Ga2S3 monolayers. Figure 3 suggests that the charge redistribution at the interface of α-Ga2S3/Ga2SSe-C is more pronounced than that of α-Ga2S3/Ga2SSe-A and αGa2S3/Ga2SSe-B, indicating α-Ga2S3/Ga2SSe-C forms a more stable interface. Figure 4 shows the band structures of the heterojunctions calculated using the HSE06 functional. The α-Ga2S3/Ga2SSe-A heterojunction displays an indirect bandgap, with the CBM located at the M point and VBM located between the M and Γ points. The α-Ga2S3/Ga2SSe-B and α-Ga2S3/Ga2SSe-C heterojunctions also present indirect bandgaps, with the CBM located at the M point and the VBM located at the Γ point. Interestingly, the bandgaps of α-Ga2S3/Ga2SSe-B and α-Ga2S3/Ga2SSe-C are about 2.0 eV, and their energy gaps at the Γ point get very close to respective bandgaps, which are greatly favorable for absorbance of visible light. The total and partial densities of states of the heterojunctions are calculated using HSE06 and shown in Figure 4. The VBMs of α-Ga2S3/Ga2SSe-B 6 ACS Paragon Plus Environment

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and α-Ga2S3/Ga2SSe-C are composed of Ga 4p, S 3p and Se 4p states. By contrast, Ga 4p and Se 4p states near the VBM of α-Ga2S3/Ga2SSe-A are somewhat shifted to the low energy region. Resultantly, a sharp peak arises near the VBM for α-Ga2S3/Ga2SSe-B and α-Ga2S3/Ga2SSe-C, which will result in large possibilities of electrons jumping to the vacuum energy level. Figure 5 displays the decomposed charge density of the VBM and CBM of α-Ga2S3/Ga2SSe heterojunctions. The VBM of α-Ga2S3/Ga2SSe-A is localized in α-Ga2S3 and Ga2SSe, and their CBM is localized in the Ga2SSe layer. The VBM is localized in the Ga2SSe layer for α-Ga2S3/Ga2SSe-B and α-Ga2S3/Ga2SSe-C heterojunctions, while their CBM is localized in the α-Ga2S3 layer. This suggests α-Ga2S3/Ga2SSe-B and α-Ga2S3/Ga2SSe-C heterojunctions belong to type-Ⅱ band alignments, which is very helpful to enhance charge separation and water splitting efficiencies. 3.3 Photocatalytic ability The oxidation/reduction ability can be evaluated by comparing the CBM and VBM energy levels with the redox potential of water. The VBM and CBM energy levels related to vacuum (EVBM and ECBM) is determined by EVBM = EBGC ― 1/2Eg and ECBM = EBGC + 1/2Eg according to the method proposed by Toroker et al. 17, 40, 41 EBGC is the energy level of bandgap centers relative to vacuum and expressed as EBGC = (E′VBM + E′CBM) 2 ― Evaccum. Here, Evaccum is the electrostatic potential in the vacuum region. E′CBM and E′VBM are CBM and VBM energy levels exacted from band structures where the fermi energy level is not reset, in contrast to usual cases where fermi energy levels are set to zero. Considering Eg = E′CBM ― E′VBM, we further obtain EVBM = E′VBM ― Evaccum and ECBM = E′CBM ― Evaccum. Briefly, the positions of VBM (CBM) relative to vacuum can be straightly obtained by taking the energy difference of the electrostatic potential in vacuum and the energy level of VBM (CBM) from band structure calculations. Through performing test calculations, we find Evaccum of Ga2X1X2 monolayers and αGa2S3/Ga2SSe heterojunctions is insensitive to the choice of the PBE or HSE06 functional. Thus, the less computationally expensive PBE functional is applied to calculate Evaccum. The theoretical Evaccum of Ga2X1X2 and α-Ga2S3/Ga2SSe are summarized in Table 1 and 2, respectively. As mentioned above, E′CBM and E′VBM are extracted from HSE06 band structures where the fermi level is not reset. The CBM and VBM energy levels (ECBM and EVBM) relative to vacuum are shown in Figure 6. The ECBM (-3.62 eV) of single-layer GaS is consistent with the previous value calculated with HSE06 (-3.58 7 ACS Paragon Plus Environment


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eV),17 which confirms the reliability of our calculation. For water splitting reactions at PH=0, the reduction potential for H+/H2 and the oxidation potential for O2/H2O are -4.44 and -5.67 eV, respectively. The band edges of Ga2SSe and Ga2SeTe monolayers locate in energetically favorable positions for water oxidation and reduction reactions. The ECBM of single-layer Ga2STe is higher than the reduction potential (-4.44 eV) of H+/H2; nevertheless, the VBM (-5.69 eV) is only slightly lower than the oxidation potential (-5.67 eV). Moreover, the reduction and oxidation potentials of water splitting are shifted upward with PH by PH × 0.059 eV. Therefore, a little change in PH of solutions results in Ga2STe being unfavorable for water splitting. As for α-Ga2S3/Ga2SSe heterojunctions, their VBMs are all lower than -5.67 eV. However, the ECBM (-4.43 eV) of α-Ga2S3/Ga2SSe-B is slightly higher than -4.44 eV. The ECBM (-4.46 eV) of αGa2S3/Ga2SSe-C is very close to the reduction potential of H+/H2. It is worthy of notice that the ECBM of α-Ga2S3/Ga2SSe-A is shifted down to 5.00 eV and in an unfavorably position for hydrogen production, which is mainly caused by larger Evaccum shown in Table 2. The larger Evaccum indicates more energy is needed to move electrons of α-Ga2S3/Ga2SSe-A into vacuum, in agreement with the above analysis of the density of states. As shown in Figure 5, the α-Ga2S3 layer can be considered as an electric dipole, where the minus sign ( ― ) represents the center of negative charges (S2 ― ) and the positive sign ( + ) denotes the center of positive charges (Ga3 + ).39 Obviously, the distance between the VBM charge and positive charge center of α-Ga2S3/Ga2SSe-A is shorter than that of αGa2S3/Ga2SSe-B and α-Ga2S3/Ga2SSe-C, again demonstrating it needs more work to move electrons of α-Ga2S3/Ga2SSe-A into vacuum. Strain engineering is efficient in tuning the positions of CBMs and VBMs.5, 42 As shown in SI, single-layer Ga2X1X2 can sustain 20% tensile strain and at least 16% compressive strain in the biaxial directions. The examined three α-Ga2S3/Ga2SSe heterojunctions sustain 20% biaxial tensile strain and 16% biaxial compressive strain. Here, we explore strain effects on the band edge positions of Ga2X1X2 monolayers and α-Ga2S3/Ga2SSe heterojunctions by applying biaxial strains ranging from -4% to +4%. At the given biaxial strain, the atomic positions are relaxed with PBE and then the band structures are calculated using HSE06. The ECBM and EVBM of strained Ga2X1X2 and α-Ga2S3/Ga2SSe are respectively shown in Figures 7(a) and 7(b). For single-layer Ga2SSe, the CBM and VBM straddle water redox potentials under strains from -4% to +4%. As mentioned above, at zero strain the EVBM 8 ACS Paragon Plus Environment

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(-5.69 eV) of single-layer Ga2STe is too close to the oxidation potential (-5.67 eV) of O2/H2O. Surprisingly, the VBM (-5.90 eV) of Ga2STe at +1% strain becomes more energetically favorable. The water redox potentials lie within the bandgap of single-layer Ga2SeTe under strains from 0% to 3%. Nevertheless, at both compressive and tensile strains the CBM of α-Ga2S3/Ga2SSe-A is far below 4.44 eV because of larger Evaccum shown in Figure 7(c), which hampers hydrogen production via splitting water. By contrast, the CBMs of α-Ga2S3/Ga2SSe-B and α-Ga2S3/Ga2SSe-C under compressive strains become favorable to release H2. Considering the electric dipole directions of the α-Ga2S3 layer in α-Ga2S3/Ga2SSe-A and α-Ga2S3/Ga2SSe-B are opposite, we propose a new method to achieve suitable band edges for water splitting by switching electric dipoles of van der Waals heterojunctions. Another vital requirement for efficient photocatalytic materials is to absorb enough visible. Optical absorption coefficients directly reflect the absorption range and intensity. However, the absorption coefficients of 2D monolayers27, 28 and heterojunctions18, 43 are underestimated by several times in previous calculations. Most of the space in the unit cell of 2D materials is not occupied because a large vacuum space is added perpendicular to 2D atomic planes when simulating physical properties. Thus, the unit cell volume V in eqn. (1) should be replaced by the effective unit cell volume Veff, as shown in recent calculations of nonlinear optical (NLO) coefficients.13,

44, 45

Obviously, when the

effective unit cell volume Veff is included, NLO coefficients of 2D materials are several times enhanced. To obtain reliable linear optical properties such as absorption coefficients, we renormalize the imaginary and real parts of dielectric function. As shown in eqs. (1) and (2), ε1(ω)′ and ε2(ω)′ are respectively the real and imaginary parts of dielectric function of 2D materials in a unit cell volume V with a vacuum space, and the direct output of some ab initio software. For 2D materials with an V

effective unit cell volume Veff, the imaginary part of dielectric function is expressed as ε2(ω) = Veffε2 (ω)′. Considering

V Veff

z

z

= h, ε2(ω) can be expressed as ε2(ω) = h ε2(ω)′, where z and h are the z-

lattice parameter of unit cell and effective thickness of 2D materials respectively. The real part of z

dielectric function is further obtained using the formula ε1(ω) = 1 + h[ε1(ω)′ ―1]. Clearly, the optical absorption coefficient obtained from eqn. (3) will be significantly modified as the effective unit 9 ACS Paragon Plus Environment


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cell volume is taken into account. The effective unit cell volume of 2D materials is obtained by multiplying the in-plane area of unit cell and effective thickness. The effective thickness h of single-layer Ga2X1X2 is simply taken as the average of the effective thickness of monolayer GaX1 and GaX2 (X1, X2=S, Se, Te)13 and summarized in Table 1. As mentioned above, the S atom from the Ga2SSe layer is on the very top of the S atom from the α-Ga2S3 layer for α-Ga2S3/Ga2SSe-A and α-Ga2S3/Ga2SSe-B heterojunctions. Moreover, as suggested by Figures 3(a) and 3(b), their charge redistribution at the interface is not pronounced. Therefore, the effective thickness of α-Ga2S3/Ga2SSe-A (α-Ga2S3/Ga2SSe-B), denoted as hA(hB), can be considered as the sum of that of single-layer Ga2SSe and α-Ga2S3.39 The α-Ga2S3/Ga2SSe-C heterojunction embodies charge redistributions at the interface; additionally, its interlayer distance is shortened in comparison with that of α-Ga2S3/Ga2SSe-B. The effective thickness of α-Ga2S3/Ga2SSeC, expressed as ℎ𝐶, is thereby taken as ℎ𝐶 = ℎ𝐵 ―(𝑑𝐵 ― 𝑑𝑐). 𝑑𝐵 and 𝑑𝑐 are the interlayer distances of α-Ga2S3/Ga2SSe-B and α-Ga2S3/Ga2SSe-C, respectively. The calculated effective thicknesses of αGa2S3/Ga2SSe heterojunctions are summarized in Table 2. The calculated absorption coefficients of potential water splitting photocatalysts including Ga2X1X2 monolayers, α-Ga2S3/Ga2SSe-B and α-Ga2S3/Ga2SSe-C heterojunctions are displayed in Figure 8. The examined monolayers and heterojunctions possess two independent absorption coefficients, i.e. αzz and αxx dictated by their C3V point group. For single-layer Ga2SeTe, the absorption edge is at 2.02 eV, corresponding to the direct bandgap at the Γ point. αxx of Ga2SeTe achieves the maximum value of ~ 30000 cm-1 at 2.17 eV; compared with αxx, αzz is greatly enhanced in the energy range from 2.54 to 3.19 eV. The absorption coefficients for single-layer Ga2STe with +1% strain exhibit similar characteristic with that of Ga2SeTe with zero strain. In comparison with Ga2SeTe, the absorption edge of Ga2STe with 1% strain is redshifted, which enables Ga2STe with 1% strain absorbs more visible light. The indirect bandgap (3.20eV) of single-layer Ga2SSe is beyond the visible range, resulting in no absorbance of visible light. As shown in Figure 7(a), the bandgap of Ga2SSe is decreased under both biaxial tension and compression. Even though the bandgap of GaSSe with -4% strain is decreased to 2.48 eV, the absorption of visible light is nearly negligible. In contrast, GaSSe with +4% strain shows large absorption from 2.66 to 3.19 eV. Single-layer Ga2SSe with -4% strain displays the minimum direct energy gap of 3.52 eV at the Γ point, being much larger than the indirect 10 ACS Paragon Plus Environment

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bandgap of 2.48 eV, as shown in Figure 2(e). On the other hand, as shown in Figure 2(f), the energy gap of 2.59 eV at the Γ point for singe-layer Ga2SSe with +4% strain nearly approaches the indirect bandgap of 2.54 eV. The direct interband transition process at the Γ point improves the visible light absorbance of singe-layer Ga2SSe with +4% strain since in such a process the electron does not need a large momentum. On the other hand, even though the visible light absorbance of α-Ga2S3/Ga2SSe-B and α-Ga2S3/Ga2SSe-C with -4% strain is much decreased in comparison with that of single-layer Ga2X1X2, the absorbance of these two heterojunctions in the visible range is at least twice larger than that of the type-Ⅱ heterojunctions including AN/BP,25 g-C3N4/C2N,24 MoS2/AlN and MoS2/GaN.46 In brief, single-layer Ga2X1X2, α-Ga2S3/Ga2SSe-B and α-Ga2S3/Ga2SSe-C heterojunctions exhibit large absorbance in the visible range because of moderate bandgaps and the effective unit cell volume being taken into account. 4 Conclusions We have carried out first-principles calculations of the electronic and photocatalytic properties of Ga2X1X2 monolayers and 2D α-Ga2S3/Ga2SSe heterojunctions. Single-layer Ga2STe and Ga2SeTe are direct bandgap semiconductors with bandgaps of 1.61 and 2.02 eV, respectively. The bandgap of single-layer Ga2SSe is decreased to ~ 2.5 eV under biaxial strains. The α-Ga2S3/Ga2SSe-B and αGa2S3/Ga2SSe-C heterojunctions belong to type-Ⅱ heterojunctions, and their indirect bandgaps are in the visible range. The band edges of single-layer Ga2X1X2 are in favorable positions for water splitting, and under a wide range of biaxial compressive strain those of α-Ga2S3/Ga2SSe-B and α-Ga2S3/Ga2SSeC are also favorable for water splitting. Comparisons of the VBM charge and electric dipole of αGa2S3/Ga2SSe-A and α-Ga2S3/Ga2SSe-B demonstrate it is possible to achieve suitable band edges for water splitting by switching electric dipoles. Especially, the visible light absorbance of examined monolayers and heterojunctions are quite large, and therefore they are promising photocatalysts for water splitting. As for the calculation method, we simplify the commonly used equation of band edge positions, which will accelerate the exploration process of water splitting catalysts; we renormalize the imaginary and real parts of dielectric function, which will help to obtain reliable optical absorption coefficients. Summarily, our work will stimulate researches in photocatalytic properties of 2D monolayers and heterojunctions.



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Supporting Information Stability of Ga2X1X2 monolayers in water, stability of Ga2X1X2 monolayers and α-Ga2S3/Ga2SSe heterojunctions at different strains, formation energies and bandgaps calculated using various vdW functtionals.

Acknowledgments This work has been partially supported by National Natural Science Foundation of China (61605206, 61771138), the National Key Research and Development Program of China (2017YFF0106303)

and

Dongguan

Industry

University

Research

Cooperation

Project

(2015509102211)

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Table 1. In-plane constant a, effective thickness h (Å), bandgap Eg (eV), electrostatic potential in the vacuum region Evaccum (eV) of single-layer Ga2X1X2 Material

a

h

EPBE g

EHSE06 g

Evaccum

Ga2SSe

3.653

7.871

2.34

3.20

3.30

Ga2STe

3.821

8.040

0.87

1.61

3.76

Ga2SeTe

3.899

8.058

1.25

2.02

3.51

Table 2. In-plane constant a , effective thickness h, interlayer distance d (Å), bandgap Eg(eV), electrostatic potential in the vacuum region Evaccum (eV), and formation energy Ef (meV/Å2) of proposed heterojunctions calculated with the D2 correction. Evaccum Ef Material EPBE EHSE06 a h d g g α-Ga2S3/Ga2SSe-A 3.617 16.721

3.806

1.73

2.76

5.97

-7.06

α-Ga2S3/Ga2SSe-B

3.617 16.721

3.792

1.24

2.19

5.32

-8.34

α-Ga2S3/Ga2SSe-C

3.620 15.995

3.066

1.24

2.08

5.32

-8.82

Figures and Captions


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Figure 1 Top and side views of (a) Ga2SSe and (b) α-Ga2S3. Side view of 2D heterojunctions including (c) Ga2SSe/α-Ga2S3-A, (d) Ga2SSe/α-Ga2S3-B and (e) Ga2SSe/α-Ga2S3-C.

Figure 2 Band structure of single-layer (a) Ga2SeTe, (b) Ga2STe, (d) Ga2SSe with 0% strain, (e) Ga2SSe with -4% strain and (f) Ga2SSe with 4% strain calculated using HSE06. The fermi energy level is reset to zero. The high symmetry k-point path in the Brillouin Zone, as shown in (c), is chosen as Γ (0, 0, 0) → K (-1/3, 2/3, 0) → M (0, 1/2, 0) → Γ (0, 0, 0).



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Figure 3 Charge redistribution of 2D heterojunctions including (a) α-Ga2S3/Ga2SSe-A, (b) αGa2S3/Ga2SSe-B and (c) α-Ga2S3/Ga2SSe-C calculated using PBE. The blue and red regions respectively indicate the accumulation and depletion of electrons. The isosurface is 0.00015 eBohr-3.

Figure 4 Band structures ， total and partial densities of states of (a) α-Ga2S3/Ga2SSe-A, (b) αGa2S3/Ga2SSe-B and (c) α-Ga2S3/Ga2SSe-C calculated using HSE06 where the fermi energy level is reset to zero. The high symmetry k-point path is the same as that of monolayer Ga2X1X2.


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Figure 5 The charge density of the VBM and CBM for (a) α-Ga2S3/Ga2SSe-A, (b) α-Ga2S3/Ga2SSeB and (c) α-Ga2S3/Ga2SSe-C. The blue and red regions respectively indicate the CBM and VBM. The isosurface value is 0.004 eBohr-3

Figure 6 Band alignments of (a) GaS, (b) Ga2SSe, (c) Ga2STe, (d) Ga2SeTe monolayers, (e) αGa2S3/Ga2SSe-A, (f) α-Ga2S3/Ga2SSe-B and (g) α-Ga2S3/Ga2SSe-C heterojunctions. The two dotted lines are the H+/H2 reduction potential (-4.44 eV) and the O2/H2O oxidation potential (-5.67 eV), respectively.



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Figure 7 Strain effects on the band edge positions of (a) single-layer Ga2X1X2 and (b) derivative heterojunctions. (c) Strain effect on the electrostatic potential in the vacuum region Evaccum (eV). Positive and negative strains respectively denote tension and compression.

Figure 8 Absorption coefficients αxx and αzz of (a) Ga2SeTe with 0% strain and Ga2STe with +1% strain, (b) Ga2SSe with +4% and -4% strain, (c) α-Ga2S3/Ga2SSe-B with -4% strain and αGa2S3/Ga2SSe-C with -4% strain. The solid and dotted curves respectively denote αzz and αxx.


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TOC Graphic


Janus Group-III Chalcogenide Monolayers and Derivative Type-II

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