Dimensional Janus Group-III Monochalcogenides - ACS Publications

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Optical Properties and Photocatalytic Applications of Two-Dimensional Janus Group-III Monochalcogenides Huang Aijian, Wenwu Shi, and Zhiguo Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12450 • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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The Journal of Physical Chemistry

Optical Properties and Photocatalytic Applications of TwoDimensional Janus Group-III Monochalcogenides Aijian Huang,1 Wenwu Shi,1,2* and Zhiguo Wang1* 1. School of Electronic Science and Engineering, Center for Public Security Technology, University of Electronic Science and Technology of China, Chengdu, 610054, P.R. China 2. Department of Materials Science and Engineering, North Carolina State University, Raleigh, 27606, United States *Corresponding author. E-mail: [email protected] (W. Shi); [email protected] (Z. Wang)

Abstract: Photocatalytic water splitting has received much attention for the production of renewable hydrogen from water, and two-dimensional (2D) materials show great potential for use as efficient photocatalysts. In this paper, the stabilities and electronic and optical properties of Janus group-III monochalcogenide M2XY (M=Ga and In and X/Y=S, Se and Te) monolayers were investigated using first-principles calculations. The band gaps of the Janus M2XY monolayers are in the range of 1.54 to 2.98 eV, which satisfies the minimum band gap requirement of photocatalysts for overall water splitting. Indirect-to-direct band gap transitions occur in the M2XTe (M=Ga and In and X =S and Se) monolayers. These transitions were induced by the valence band maximum at the Γ point being composed of the px and py orbitals of the M and Y atoms in M2XTe instead of the pz orbitals of the M and X atoms in the MX and other M2XY monolayers. The Janus M2XY monolayers have a considerable optical absorption coefficient (~3×104/cm) in the visible light region and an even larger absorption coefficient (~105/cm) in the near ultraviolet region. This study not only highlights the efficient photocatalytic performance of the 2D MX and M2XY monolayers but also provides an approach for tuning the band structures of 2D photocatalysts by forming Janus structures.

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1. Introduction Energy shortages and environmental pollution are two major problems in modern society. The splitting of water using semiconductors as photocatalysts for the conversion of solar light into hydrogen is a promising solution for producing clean renewable energy.1,

2

Numerous

semiconductors have been proposed as potential photocatalysts for water splitting. Unfortunately, several of these photocatalysts are inactive toward overall water splitting. Developing a desirable photocatalyst that can efficiently utilize solar energy to dissociate water and produce hydrogen is a major challenge.3-14 The water splitting reaction is driven by photoexcited electron-hole pairs; water is reduced to produce H2 (2H++ e−→H2) and oxidized to produce O2 (H2O+2h+→1/2O2+2H+) by the photoexcited electrons and holes at the surface of the photocatalyst, respectively. To be a good photocatalyst, a semiconductor should have a suitable band gap between 1.55 and 3.0 eV to maximize its efficiency toward solar energy adsorption. Apart from the band gap, the photocatalyst should also have suitable band alignment, that is, the conduction band minimum (CBM) should be higher than the hydrogen reduction potential (−4.44 eV vs. the vacuum level at pH = 0), and the valence band maximum (VBM) should be lower than the oxidation potential of H2O (−5.67 eV vs. the vacuum level at pH = 0), or the CBM should be more negative than the H+/H2 redox potential (0 V vs. normal hydrogen electrode (NHE) at pH = 0) and the VBM should be more positive than the oxidation potential of O2/H2O (1.23 V vs. NHE); therefore, the conventional minimum band gap needed for overall water splitting is 1.23 eV. Yang’s group15 proposed a mechanism that broke the band gap limitations of photocatalysts for water splitting by introducing a vertical intrinsic electric field (EF) into a 2D material,16 such as 2D 2


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In2Te3 (band gap of 1.14 eV), which can absorb infrared light to drive overall water splitting. At the same time, a good photocatalyst should be chemically and photochemically stable and economically affordable.17-30 Two-dimensional (2D) materials show great potential for use in photocatalytic water splitting due to their having a high surface area available for the photocatalytic reactions and a reduced migration path length for the photo-generated carriers to reach the reactive sites.17, 18, 20-22, 27 Transition-metal dichalcogenides, such as MoS and WS , g-C N and phosphorene have 2 2 3 4

been investigated as photocatalysts for water splitting.31-33 Other semiconductors, such as ZnO, CdS, WO3 and SrTiO3, have also been studied and have shown good photocatalytic performance.34 Although the application of 2D materials in photocatalysis field has been well studied, novel 2D photocatalytic materials with better photocatalytic behaviors are still highly desirable. Group III-VI monochalcogenides (MX, M = Ga and In, X = S, Se, and Te) are an emerging type of 2D material that has drawn considerable attention for their use in future optoelectronic devices.35-41 Both 2D GaS and GaSe have been successfully synthesized from their bulk counterparts by mechanical cleavage and solvent exfoliation.42, 43 These 2D GaS and GaSe materials exhibited higher optical responses than those of graphene or other 2D materials. Monolayer and few-layered InSe44 has also been successfully fabricated experimentally, and it had high electron mobility, good metal contacts, and a wide band gap. However, several of the 2D MX materials have poor absorption in the visible light spectrum due to their wide indirect band gap, such as GaS monolayers, which can only absorb a small portion of the solar spectrum in the ultraviolet region due to their relatively large band gap (>3.0 eV).17, 26 Zhuang et al.29 3



theoretically studied the detailed band structures of 2D MX and found that the band gap of 2D MX lies within 2.20-3.98 eV, while all 2D MX materials display indirect band gaps. Further tuning the electronic structure of 2D MX materials through band structure engineering is highly important for their practical application in the field of optoelectronics. Structural symmetry plays a crucial role in determining the electronic properties of materials. The Janus structures of other 2D materials have shown distinct physical properties compared to those of pristine materials. The band gaps of 2D MXenes can be tuned by forming Janus structures through different elemental functionalizations of the upper and lower sides of the material.45 The Janus Mo2SSe monolayer exhibited large Rashba spin splitting46 and strong piezoelectric effects.47 The 2D Janus MoSSe has a band gap of 1.48 eV,48 which is larger than that of MoSe2 and smaller than that of MoS2. Janus monolayers have also been investigated as photocatalysts for water splitting. Hu and Wei49 theoretically predicted that Janus Ga2SeTe, Ga2STe, and Ga2SSe monolayers would have suitable band edges for water splitting. Strain can be used to tune the band edges of a type-II heterojunction of Janus Ga2SeTe, Ga2STe, and Ga2SSe monolayers on α-Ga2S3 to achieve suitable band gaps for water splitting. The Janus transition metal oxide monolayers (TiSO, ZrSO, and HfSO) are expected to be promising photocatalysts for overall water splitting due to their suitable band gaps and optimal redox potentials.50 Moreover, the 2D Janus TiSP monolayer is catalytically active toward the hydrogen evolution reaction51. The phase transformation from the a stable 2H-phase to the metastable 1T'-phase in Janus MoSSe and MoSTe monolayers has also been investigated, and the phase transformation conditions depend on the type of elements in the materials (S, Se and Te)52. MX (M =Ga and 4


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In, X = S, Se and Te) monolayers are composed of four sublayer structures, which is different from the composition of the tri-layered MoS2 monolayer. No metastable phase for the MX monolayer has been reported; thus, stable MX monolayers were used for this study. In this paper, we investigated the electronic and optical properties of Janus 2D M2XY (M =Ga and In, X/Y= S, Se and Te where X≠Y) monolayers, the layer-dependent photocatalytic properties of mono-, bi, and tri-layered GaS and Janus Ga2SSe based on density functional theory (DFT), and their potential application in the field of photocatalytic water splitting. The results show that the band structure can be tuned by forming a Janus structure, which induced an indirect-todirect band gap transition. This work provides a general approach for designing a wide range of 2D-material photocatalysts for water splitting.

2. Simulation methods All the calculations were performed using the DFT as implemented in the Vienna ab initio Simulation Package (VASP).53 Potentials were generated with the projected augmented wave (PAW)54 method and used to describe the valence electrons. The exchange-correlation energy was calculated by using a generalized gradient approximation (GGA)55 as parametrized by Perdew-Burke-Ernzerhof (PBE). The band gaps and optical properties were calculated with the Heyd-Scuseria-Ernzerhof hybrid functional56 with a standard screening parameter of 0.2 (HSE06). The convergence of the total energy was tested by choosing a plane-wave cutoff energy of 520 eV. A 9×9×1 k-point mesh, generated by the Monkhorst-Pack scheme,57 was used for the Brillouin-zone integration. All the structures, including the atom positions and cell parameters, were fully optimized with the conjugant gradient method until the total energy was 5



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less than 10−5 eV and the force on each atom was less than 0.01 eV/Å. To avoid any artificial interactions between the layers and their images, a vacuum space of 20 Å was set along the direction perpendicular to the 2D surface. The atomic structures were analyzed by using the VESTA code.58

3. Results and discussion Bulk MXs are layered structures with either D6h or D3h symmetry depending on their interlayer stacking sequence. The layers of MXs are covalently bonded in an X-M-M-X sequence, while the interlayer is coupled by weak van der Waals (vdW) interactions.26, 29 The top and side views of the atomic structure of an MX monolayer are shown in Fig. 1a. An MX monolayer is composed of two vertically bonded metal-atom layers that are sandwiched by two chalcogen-atom layers. The top view shows that the MX monolayer is composed of a honeycombed structure of the M and X atoms, each occupying a triangular sublattice. A Janus M2XY monolayer is constructed by replacing the bottom layer of the chalcogen atoms with a different chalcogen atom, as shown in Fig. 1b. The Janus 2D M2XY (M =Ga and In, X/Y= S, Se, and Te where X≠Y) monolayers have broken mirror symmetry.59,

60

The calculated

structural parameters are summarized in Table 1. For all the MX monolayers, the Ga-Ga and In-In bond lengths are 2.47 and 2.83 Å, respectively. The M-M bond length (dM-M) increases when the metal atom is changed from Ga to In. The M-X bond length (dM-X) increases as the chalcogen atom changes from S to Te. The lattice constant (a) follows a similar trend since it is proportional to dM-X, which is due to the increase in the atomic radii of the M and X elements. The bond angle θ (∠XMM) also increases as the chalcogen atom changes from S to Te. The 6


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calculated structural parameters agree well with those previously calculated by Zhuang et al.29 The dM-M of the Janus M2XY monolayer is the same as that of the pristine MX monolayer, while the lattice constants of the M2XY monolayer lie between the values of pristine MX and MY monolayers. To characterize the thermodynamic stabilities of these Janus M2XY monolayers, the formation energy (∆E) was calculated as follows: ∆E = (Etot - 𝑛MEM - 𝑛XEX - 𝑛YEY)/𝑛tot, where Etot is the total energy of the Janus monolayer M2XY; EM, EX, and EY are the energies of individual M, X, and Y atoms in their stable solid phases, respectively; 𝑛M, 𝑛X, and 𝑛Y are the numbers of atoms of each element in the supercell; and 𝑛tot is the total number of atoms in the supercell. The calculated formation energies of the Janus M2XY and pristine MX monolayers are listed in Table 1. The calculated formation energies agree well with those of a previous report.21 The formation energies of the Janus M2XY monolayers are in the range of 0.63 eV to -0.41 eV, which is close to the -0.67 eV to -0.35 eV range of pristine MX monolayers, indicating that the Janus M2XY monolayers are energetically favorable. The phonon dispersions of the 2D MX and M2XY monolayers shown in Fig. 2 were calculated using a 5×5×1 supercell. It can be seen from the figure that the optical phonon energies for all the monolayers decrease as X/Y changes from S to Te. There is a negligible imaginary frequency for all the MX and M2XY monolayers, which indicates that they are dynamically stable. Since there are heavy elements, such as In and Te, present in the monolayers, the effect of spin orbital coupling (SOC) on the band structures of the InTe and In2SeTe monolayers was examined using the PBE functional. As shown in Fig. S1, the band structures calculated with the PBE functional and the SOC effect show no large differences from those calculated with 7



only the PBE functional, and the SOC effect is negligible; thus, the SOC effect was not considered in the following calculations. The electronic band structures of the pristine MX and Janus M2XY monolayers are shown in Fig. 3a and 3b, respectively. All the MX monolayers are indirect band gap semiconductors with their VBMs located between the Γ and K points, while the CBMs of the GaS and GaTe monolayers are located at the M points and the CBMs of the GaSe and InX monolayers are located at the Γ points. The band gap decreases as the chalcogen atom changes from S to Te for both the GaX and InX monolayers, as listed in Table 2. For the MX monolayers, the energy at the Γ point is only approximately 0.1 eV lower than the VBM, so it is expected that the VBM shifts to the Γ point through band edge engineering, thus enhancing the optical absorption in the visible light region. The band structures of the Janus M2XY monolayers are shown in Fig. 3b. It can be seen from Fig. 3b that the CBMs of all the Janus structures are located at the Γ point. Except for the Ga2SSe and In2SSe monolayers, the VBMs of the Ga2STe, Ga2SeTe, In2STe and In2SeTe monolayers shift to the Γ point, which leads to an indirect-to-direct band gap transition in the monolayers. Thus, the Janus Ga2STe, Ga2SeTe, In2STe and In2SeTe monolayers could be better than the MX monolayers for use in optoelectronic devices. The Janus M2XY band gaps have a narrow bandgap compared to those of the pristine MX and MY monolayers. Interestingly, the Janus M2XTe (M =Ga, In, X= S, Se) monolayers are direct band gap semiconductors. To investigate the origin of the transition from an indirect to a direct band gap, the orbital-decomposed band structures of the pristine MX and Janus M2XY monolayers were analyzed. The valence electrons of group-III and chalcogen atoms are located in s and p shells. As shown in Fig. 4a, the CBM of the InS monolayer located at the Γ point is mainly composed 8


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of the In 5s and S 3pz orbitals, while the VBM of the InS monolayer located between the Γ and K points is mainly composed of the In 5pz and S 3pz orbitals. The Janus In2SSe monolayer is formed by substitution of one layer of S by Se atoms. Additionally, the CBM, which sill is located at the Γ point, of the Janus In2SSe monolayer is mainly composed of In 5s, S 3pz, and Se 4pz orbitals (as shown in Fig. 4b). The VBM of the Janus In2SSe monolayer is composed of the In 5pz, S 3pz, and Se 4pz orbitals, which still have the same pz orbital characteristics as those of the InS monolayer; thus, the position of the VBM does not change. The CBM of the Janus In2STe monolayer is mainly composed of the In 5s, S 3pz, and Te 5pz orbitals (as shown in Fig. 4c), which are located at the Γ point. Instead of having pz orbital characteristics, the VBM of Janus In2STe is mainly composed of the In 5px, In 5py, Te 5px and Te 5py orbitals, which results in the VBM shifting to the Γ point, thus leading to the indirect-to-direct band gap transition. After analysis of the orbital-decomposed band structures of the other pristine MX and Janus M2XY monolayers (as shown in Fig. S2), the px and py orbital characteristics of the VBMs in the Janus M2XTe (M =Ga, In, X= S, Se) monolayers are the origin of the indirect-todirect band gap transition. The optical properties of a material can be deduced from the frequency-dependent complex dielectric function ε(ω) = 𝜀1(𝜔) +𝑖𝜀2(𝜔), as described in the literature.61 The real part 𝜀1(𝜔) of the dielectric function can be obtained by using the Kramers-Kronig relation,62 and the imaginary part 𝜀2(𝜔) can be calculated by summing all the empty states in the Brillouin zone.61 Thus, the absorption coefficient I(ω) was calculated using the following equation. 𝐼(𝜔) = ( 2)𝜔[ 𝜀1(𝜔) + 𝜀2(𝜔) ―𝜀1(𝜔)] 2

2

1 2

(1)

The real part 𝜀1(𝜔) and imaginary part 𝜀2(𝜔) of the complex dielectric function for the 9



pristine MX and Janus M2XY monolayers are shown in Fig. 5a and 5b, respectively. Several peaks with a main peak at 3-5 eV caused by the interband transition can be clearly seen in the imaginary part 𝜀2(𝜔). The first main peak of the imaginary part ε2(ω) shifts to lower energies as X changes from S to Te for both the GaX and InX monolayers, which agrees with the changing trends of the band gaps. Based on the dielectric function, the calculated absorption coefficients 𝐼(𝜔) of the pristine MX and Janus M2XY monolayers are shown in Fig. 6a and 6b, respectively. The first absorption peak shifts to lower energies as X changes from S to Te for both the GaX and InX monolayers, which agrees with the changing trends of the band gap dependence on the composition. The Janus structure does not affect the absorption coefficients of the M2XY and MX monolayers. Both types of monolayers have considerable optical absorption coefficients in the visible light region (1~3×104/cm) and in the near ultraviolet region (~105/cm). These absorption coefficients are better than or at least comparable to those of previously reported 2D materials, such as CdS63, ZnS64 and ZnV2O6.65 The InX monolayers have higher absorption coefficients than the GaX monolayers. For example, the GaS monolayer has nearly no optical absorption in the visible light region owing to its wide band gap. The Janus In2STe and In2SeTe monolayers have large absorption coefficients close to that of InTe, thus guaranteeing their ability to capture sunlight in the visible region; along with the direct band gap characteristics of the Janus M2XTe (M =Ga, In, X= S, Se) monolayers, this guarantees the possible application of Janus M2XY monolayers in the fields of photocatalytic water splitting. In addition to a large absorption coefficient in the visible light region and near the ultraviolet region, to efficiently utilize solar energy to dissociate water into hydrogen and oxygen, a 10


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photocatalyst should have the proper valence and conduction band edges for overall water splitting. The redox potential of water splitting depends on the pH value,66 and the standard reduction potential for H+/H2 can be calculated by EH + /H2 = -4.44eV + pH × 0.059eV, while the oxidation potential for O2/H2O can be calculated by EO2/H2O = -5.67eV + pH × 0.059eV. Thus, the standard redox potentials for H+/H2 and O2/H2O are -4.44 eV and -5.67 eV at pH=0, respectively, and the standard redox potentials for H+/H2 and O2/H2O are -4.027 eV and -5.257 eV at pH=7, respectively. An electrostatic potential difference has been observed at the surface of 2D Janus structures,16 which results in different values for the reduction potential of H+/H2 and the oxidation potential of O2/H2O. It should be noticed that the dipole correction 67 have an important influence on the electrostatic potential distribution perpendicular to the 2D M2XY monolayers. It was found that there is no vacuum level difference between the two sides for both 2D M2XY and MX monolayers without the dipole correction, as shown in Fig. S3. However, a distinct vacuum level difference (ΔΦ) exists on the two sides of 2D M2XY monolayer (as shown in Fig. S4), but not exists on those of 2D MX monolayers. The Janus structures possess an intrinsic built-in electric field due to the mirror asymmetry,16 thus the dipole correction should be considered for asymmetry structures. So in this work, all the following discussion are based on the simulation results considering the dipole correction. The Y atomic layer has smaller electronegativity than the X atomic layer in Janus M2XY monolayer as the atomic number of Y is larger than the X atom, thus an intrinsic electric field exist in the Janus layer. The larger the difference in atomic number between X and Y atoms, the larger the potential difference (ΔΦ) is. For example, the ΔΦ is 0.20 and 0.58 eV in Janus Ga2SSe and Ga2STe monolayers, respectively. The value of ΔΦ is in the range between 0.16 eV and 0.58 11



eV depending on the atomic type, which are able to cause the electronic band bending, thus affecting the photocatalytic performance. A comparison of the band edge positions of the pristine MX and Janus M2XY monolayers reference to the vacuum levels of both sides of Janus M2XY monolayer with the redox potentials for hydrogen evolution (H+/H2) and oxygen evolution (O2/H2O) at pH=0 and pH=7 is shown in Fig. 7. At pH=0, the band edges of the pristine MX monolayers straddle the water redox potentials, indicating that all the MX monolayers are potential photocatalysts for water splitting. All the Janus M2XY monolayers have suitable band alignments for overall water splitting. The VBM of Janus Ga2STe monolayer is -5.32 eV referenced to the vacuum of Te side, which is higher than the oxidation potential of water splitting (-5.67 eV), however it shifts to -5.90 eV as it referenced to the vacuum of S side. The Janus Ga2STe monolayer is photocatalytic active for water splitting due to the intrinsic electric field. Similar effects also exist in other Janus monolayers, such as Ga2SeTe. When the pH value is changed to 7.0, the InS, InSe, In2SSe, In2STe, and In2SeTe monolayers are not suitable for water splitting due to their CBMs being lower than the hydrogen reduction potential, whereas the Janus Ga2SSe, Ga2STe, Ga2SeTe monolayers are suitable for water splitting. As the Janus Ga2STe, Ga2SeTe monolayers are direct band gap semiconductor, which could be better than the MX monolayer regarding its use as a photocatalyst for water splitting. Based on the results of the present study and those reported in the literatures,15, 16, 49, 50 it can be concluded that this Janus structure is a potential photocatalyst for water splitting. The electronic properties of 2D materials show a dependence on their number of layers 68; as the layer number of 2H-MoS2 increases, its CBM and VBM shift to lower and higher energies, respectively.69 Thus, the optical and photocatalytic properties of bi- and trilayered GaS and 12


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Janus Ga2SSe were investigated. As shown in Fig. S5, there are five possible high-symmetry stacking configurations for bilayered GaS and Janus Ga2SSe70. Since the interlayer is coupled by vdW interactions,26, 29 the binding energy (Eb) was calculated using the following equation to characterize the energetically preferable stacking configuration: 𝐸b = 𝐸𝑛 ―𝑛𝐸0

(2)

where n is the number of layers, and En and E0 are the total energies of n-layered and monolayered GaS/Ga2SSe, respectively. The calculated binding energies are shown below the stacking configurations in Fig. S5. Both the GaS and Ga2SSe bilayers prefer the AB stacking configuration. Trilayered GaS and Ga2SSe prefer the ABC stacking configuration. The electronic, optical and photocatalytic properties of bi- and trilayered GaS and Janus Ga2SSe were calculated for their energy-favorable stacking configurations. The electronic band structures of mono-, bi-, and trilayered GaS and Janus Ga2SSe are shown in Fig. 8. The CBMs and VBMs of GaS shift to higher energies with increasing number of layers, and the band gaps decrease with increasing number of layers, as listed in Table 2. Thus, the electronic properties of GaS can be tuned by varying the number of layers. The lower band gap can improve the optical absorption in the visible light and near ultraviolet regions. The CBM of Janus Ga2SSe shifts to a lower energy with the increase of layer numbers, while the VBM shifts to a higher energy since the VBMs are mainly composed of the Ga 4pz, Se 4pz and S 3pz orbitals, which induce a decrease in the band gap with increasing number of layers, as shown in Fig. S6. The layer-dependent real part 𝜀1(𝜔) and imaginary part 𝜀2(𝜔) of the complex dielectric function for GaS and Janus Ga2SSe are shown in Fig. S7, and their absorption coefficients 𝐼 13



(𝜔) are shown in Fig. 9. The first absorption peak shifts to lower energies as the number of layer increases, which agrees with the change in the band gap energy. The optical absorption coefficient increases with increasing number of layers; for example, the optical absorption coefficient of trilayered Ga2SSe is 105/cm at ~5 eV. Thus, the optical absorption in both the visible light and near ultraviolet regions can be enhanced by increasing the number of layers. The valence and conduction band edges for mono-, bi-, and trilayered GaS and Janus Ga2SSe are shown in Fig. 10. All the structures have suitable band alignments for overall water splitting at pH=7. However, trilayered GaS is not suitable for oxygen evolution at pH=0 due to its EVBM being higher than the oxidation potential for O2/H2O. All the n-layered Janus Ga2SSe samples straddle the water redox potentials at both pH=0 and pH=7, which indicates Ga2SSe could potentially be a better photocatalyst than GaS for water splitting. 4. Conclusion In summary, the stabilities, electronic band structures, and optical absorption properties of 2D MX and Janus M2XY monolayers were investigated by using DFT calculations. Both the Janus M2XY and pristine MX monolayers have negative formation energies, indicating that both of them can be synthesized. Both the MX and M2XY monolayers show suitable band gaps, which were larger than the minimum energy required for the water splitting reaction. However, all the MX monolayers are indirect band gap materials, while the Ga2STe, Ga2SeTe, In2STe and In2SeTe monolayers have direct band gaps. The mechanism for this transition was attributed to the valence band maximum being composed of the px and py orbitals of the M and Y atoms in M2XY instead of the pz orbitals of the M and X atoms in the MX monolayer. Both the pristine MX and Janus M2XY monolayers have considerable absorption coefficients in the 14


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visible light region (~3×104/cm). This study not only highlights the efficient photocatalytic performance of the 2D MX and M2XY monolayers but also provides an approach for tuning the band structure of 2D photocatalysts by forming Janus structures.

ASSOCIATED CONTENT: Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: The band structures of InTe and In2SeTe monolayers with and without SOC, orbital decompositions of the band structures, electrostatic potential with or without dipole correction for MX and Janus M2XY monolayers, the stacking configurations, orbital decompositions of the band structures, dielectric function for bi- and tri-layered GaS and Janus Ga2SSe structures (PDF).

Acknowledgement: This work was financially supported by the National Natural Science Foundation of China (11474047) and the Fundamental Research Funds for the Central Universities (ZYGX2016J202). This work was carried out at the National Supercomputer Center in Tianjin, and the calculations were performed on TianHe-1(A). References: 1.

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Table 1. The structural parameters of the 2D MX and Janus M2XY monolayers. The lattice constant (a), the M-M bond length (dM-M), M-X bond length (dM-X), and M-Y bond length (dMY) are in units of Å. The X-M-M bond angle θ∠XMM and M-M-Y bond angle θ∠MMY are in units of degree. All parameters are denoted in Fig. 1. The calculated formation energies ΔE (eV/atom) are compared with those from Ref. 21. a

dM-Y

dM-X

GaS

3.62 2.47

2.36

117.4

-0.67

-0.66

GaSe

3.82 2.47

2.49

118.0

-0.60

-0.60

GaTe

4.13 2.47

2.70

118.2

-0.36

-0.37

Ga2SSe

3.72 2.47

2.39 2.47

115.9

119.5

-0.63

-0.62

Ga2STe

3.89 2.47

2.45 2.64

113.2

121.5

-0.44

-0.44

Ga2SeTe

3.98 2.47

2.55 2.66

115.7

120.3

-0.46

-0.46

InS

3.92 2.83

2.55

117.5

-0.55

-0.52

InSe

4.08 2.83

2.68

118.6

-0.54

-0.52

InTe

4.38 2.83

2.89

118.7

-0.35

-0.34

In2SSe

4.00 2.83

2.57 2.66

116.1

119.8

-0.54

-0.52

In2STe

4.16 2.83

2.62 2.83

113.4

122.0

-0.41

-0.39

In2SeTe

4.24 2.82

2.72 2.85

116.1

120.9

-0.43

-0.42

θ∠XMM

θ∠MMY

20


ΔE

Ref 21

dM-M

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Table 2. Band gap (Eg, eV) and band edge positions (EVBM and ECBM, eV) of the 2D MX and Janus M2XY monolayers. The band edges are referenced to the vacuum level at the X atomic side.

Monolayer GaS GaSe GaTe Ga2SSe Ga2STe Ga2SeTe InS InSe InTe In2SSe In2STe In2SeTe Bilayered GaS Ga2SSe Trilayered GaS Ga2SSe

EVBM

Ref 29

ECBM

−6.80 −6.34 −5.72 −6.69 −5.90 −6.03 −6.91 −6.46 −5.81 −6.79 −6.15 −6.23 −6.31 −6.44 −5.38 −6.32

−6.77 −6.36 −5.75

−3.51 −3.65 −3.59 −3.72 −4.29 −4.08 −4.35 −4.27 −3.80 −4.45 −4.61 −4.43 −3.40 −3.82 −2.77 −4.00

−6.96 −6.51 −5.88

Ref 29 −3.58 −3.38 −3.53

−4.25 −4.14 −3.68

21


Eg

Ref 29

Direct bandgap

3.29 2.69 2.13 2.97 1.61 1.95 2.56 2.19 2.01 2.34 1.54 1.80 2.91 2.62 2.61 2.32

3.19 2.98 2.22

No No No No Yes Yes No No No No Yes Yes No No No No

2.71 2.37 2.20


Figure 1 Top and side views of the 2D (a) MX and (b) Janus M2XY monolayers.

22


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Figure 2 Phonon dispersions of the (a) MX and (b) Janus M2XY monolayers.

23



Figure 3 The electronic band structures of the 2D (a) MX and (b) Janus M2XY monolayers. The vacuum level of X atomic side is set to be zero.

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Figure 4 The orbital decompositions of the band structures for the (a) InS (b) In2SSe and (c) In2STe monolayers. The vacuum level of X atomic side is set to be zero. 25



Figure 5 The real (ε1) parts of the dielectric function for the 2D (a) MX and (b) Janus M2XY monolayers. And the imaginary (ε2) parts of the dielectric function for the 2D (c) MX and (d) Janus M2XY monolayers.

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Figure 6 The optical absorption coefficients I(ω) for the 2D (a) MX and (b) Janus M2XY monolayers. The optical absorption coefficient in the visible light region for the (c) MX and (d) M2XY monolayers.

27



Figure 7 Band edge positions of the 2D MX and Janus M2XY monolayers relative to the vacuum level. The standard redox potentials for water splitting at pH= 0 (black dotted lines) and pH=7 (green dotted lines) are shown for comparison.

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Figure 8 The electronic band structures of (a) GaS and (b) Janus Ga2SSe for the corresponding number of layers. The vacuum level of X atomic side is set to be zero.

29



Figure 9 The optical absorption coefficient I(ω) for few-layered (a) GaS and (b) Janus Ga2SSe. The optical absorption coefficient in the visible light region for (c) GaS and (d) Ga2SSe.

30


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Figure 10 Band edge positions of few-layered GaS and Janus Ga2SSe relative to the vacuum level. The standard redox potentials for water splitting at pH= 0 (black dotted lines) and pH=7 (green dotted lines) are shown for comparison.

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Dimensional Janus Group-III Monochalcogenides - ACS Publications

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