Metal Chalcogenides Janus Monolayers for Efficient Hydrogen

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Metal-Chalcogenides Janus Monolayers for Efficient Hydrogen Generation by Photocatalytic Water Splitting Rafael da Silva, Rafael Barbosa, Rosana Rabelo Mançano, Nathália Durães, Renato Borges Pontes, Roberto H. Miwa, Adalberto Fazzio, and Jose Eduardo Padilha ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02135 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019

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ACS Applied Nano Materials

Metal-Chalcogenides Janus Monolayers for Efficient Hydrogen Generation by Photocatalytic Water Splitting Rafael da Silva,† Rafael Barbosa,† Rosana Rabelo Mançano,‡ Nathália Durães,‡ Renato Borges Pontes,¶ R. H. Miwa,§ A. Fazzio,k and José Eduardo Padilha∗,‡ †Universidade Estadual de Maringá, Maringá 87020-900, PR, Brazil ‡Campus Avançado de Jandaia do Sul, Universidade Federal do Paraná, Jandaia do Sul 86900-000, PR, Brazil. ¶Instituto de Física, Universidade Federal de Goiás, Campus Samambaia, Goiânia 74690-900, GO, Brasil. §Instituto de Física, Universidade Federal de Uberlândia, C. P. 593, 8400-902, Uberlândia, MG, Brazil kBrazilian Nanotechnology National Laboratory (LNNano)/CNPEM, , Campinas C. P. 6192, 13083-970, SP, Brazil. E-mail: [email protected]

Abstract We investigated the structural, electronic and photocatalytic properties of the Janus monolayers (MLs) composed by the metal chalcogenides Ga2 XY and In2 XY (X= S, Se, Te; Y= S, Se, Te). The calculated phonon dispersion curves show that all Janus materials are mechanically stable, presenting the same structure as their pristine counterparts. Janus MLs are characterized by the presence of different chalcogen atoms lying on the

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opposite sides, giving rise to an electric dipole embedded perpendicularly to the ML, and thus the electronic band alignment becomes side dependent - a property which can be exploited for different technological applications. Excepting the Ga2 SSe, the other Janus crystals exhibit direct band gaps. Based on the GW Bethe-Salpeter approach, we found lower exciton binding energies (0.3–0.7 eV) in comparison with the pristine systems. Once the Janus monolayers combine small values of the exciton binding energies and electron-hole separation, our results reveal that they fulfill all the required conditions to be used in the hydrogen generation by photocatalytic water splitting.

Keywords Janus monolayers, photocatalysis, electronic properties, metal chalcogenides, water splitting, hydrogen generation, band alignment, first-principles simulations

1

Introduction

One important field of investigation that had started with the isolation of graphene, 1–3 was the area of two-dimensional (2D) materials. 4–6 Dozens of 2D materials, similar to graphene are now available and have been intensively studied. 4–6 They can exhibit several electronic characteristics, ranging from zero gap semiconductors, like graphene; 1–3 semiconductors, such as the transition metal dichalchogenides (TMDs), 7,8 and also insulators like the hexagonal boron nitride (h-BN). 9,10 2D crystals have been considering as the next-generation of advanced functional materials and, it is expected that they will be capable to overcome the limitations of the current electronic devices. For instance, some 2D materials could be used in the development of ultra-thin and flexible electronic and optoelectronic devices, with a drastic reduction in the characteristic lengths. 5,6,11–14 Van der Waals (vdW) crystals composed by metal chalcogenides GaSe, 15,16 In2 Se3 17,18 and InSe 19–21 have been recently used as key elements for

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optoelectronic and nanoelectronic applications. 15,22–25 They present a high on-off current ratio (∼ 103 ), high electron mobility (103 cm2 /(V.s), 15,23,24 and large broad-band spectral response. 26 In addition, they are mechanically flexible, 26 and similarly to the transition metal dichalcogenides, their band gaps can be tuned by the number of layers, as well as through the application of an external electrical field. 20,23,27,28 The atomically thin 2D materials can gain novel and exciting properties if the asymmetry is introduced across different planes. A possible way to break the mirror symmetry of 2D materials is to create a Janus structure. The Janus materials based on the group-VI chalcogenides were first theoretically proposed by Cheng et al. 29 and later experimentally investigated. 30–32 Aiming the fabrication of such materials, a new method of synthesis of 2D transition metal dichalcogenides was demonstrated by Lu et. al., 31 where by a controllable surface modification of MoS2 , they have obtained a Janus like material, by removing all sulfur atoms in one side, and replacing it by selenium atoms. Another synthesis route, proposed by Zhang et. al., 32 obtained the same material, a monolayer MoSSe, by sulfurization of MoSe2 . In the case of MoSSe, the symmetry break between the top/down faces, added to the difference in the electronegativity of S and Se, creates an intrinsic polarization, and as a result, this system presents an intrinsic piezoelectric response. 31,33,34 The Janus MoSSe is a direct band gap semiconductor, with the conduction band minimum and the valence band maximum both located at the K point in the reciprocal space, with a value of 1.478 eV 32 that is in between MoS2 and MoSe2 . These new synthesis routes, in principle, can be used in other layered materials with similar structures. Thus, they can pave the way for the obtention of completely new 2D Janus materials. Moving in this direction, theoretical studies have already pointed out for Janus conformation in graphene, 35 metal chalcogenides 36,37 and transition metal oxides. 38,39 These works have shown the potential applications of those Janus structures, such as photocatalysis 38–41 and piezoelectric applications. 36 For example, Chen et al., 38,39 have proposed a new class of Janus materials based on transition metal oxides and chalcogenides, that presents

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multifunctional properties, that could be used in photocatalysis and energy conversion. In another work, Guo et al., 36 have shown that the Janus materials of metal chalcogenides shows piezoelectric proprieties. However, a lot of theoretical and experimental investigations still need be done in order to provide a deep understanding of the new physics and phenomena that can emerge from the Janus materials, such the optical and excitonic properties of those systems. In this work we have studied the structural, electronic and photocatalytic properties of the Janus single layers of the metal chalcogenides Ga2 XY and In2 XY (X= S, Se,Te; Y= S, Se, Te). All systems, Ga2 SSe, Ga2 STe, Ga2 SeTe, In2 SSe, In2 STe and In2 SeTe are mechanically stable, presenting the same structure as its common counterparts (GaS, GaSe, GaTe, InS, InSe and InTe). Even though presenting the same structure, their electronic properties are completely distinct, mainly due to the difference in electronegativity of the chalcogens in the different sides of the materials. Except for the Ga2 SSe system, all others materials exhibit direct band gap. The small values of the exciton binding energies presented by the Janus monolayers, together with the electron-hole separation, due to the internal field of the material, show that those materials are promising materials for photocatalysis.

2 2.1

Results and Discussion Structural Stability

In Figs. 1(a) and 1(b) we present the top and side views in a ball-and-stick representation of a single layer Janus M2 XY, where M is a metal (M = Ga, In) and X and Y is a chalcogen (X, Y = S, Se, Te). The unit cell is represented by the shaded region, defined by the vectors a1 and a2 . All systems exhibit hexagonal lattices. The others structural parameters of the materials are also depicted in Fig. 1(b), i. e., dMM , dMX , dMY , t. In Table 1, we present all the structural parameters of the relaxed structures. As we can verify, the lattice constants of the Janus materials are always in between their pristine counterparts, for example, the 4


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(a)

(c)

a1

b1

K G

M

a2

y

b2 x

(b) z

M (Ga, In) X (S, Se, Te)

dM-X

X M

t

dM-M

M Y

Y (S, Se, Te)

dM-Y

Figure 1: (a) Top view of a ball-and-stick illustration of the structure of a single layer Janus M2 XY (M= Ga, In; X,Y= S, Se, Te). The unit cell is represented by the shaded region, defined by the vectors a1,2 . (b) Side view of a single layer Janus M2 XY highlighting the structural parameters: dM−M , dM−X , dM−Y and t. (c) Schematic representation of the first Brillouin zone of the hexagonal lattice. The reciprocal lattice vectors and the high-symmetry points are also presented. Janus Ga2 SSe has a lattice parameter of 3.64 Å, that is approximately the average value of the pristine GaS (a= 3.59 Å) and GaSe(a= 3.78 Å). The other compositions follow the same behavior. This shows that the structural parameters of the material are mainly governed by the chalcogen composition, and they follow the increasing of the atomic radius. Table 1: Structural parameters of the Janus monolayers of M2 XY (M= Ga, In; X,Y= S, Se, Te): Lattice parameter (a), bond lengths (dMM , dMX , dMY ) and the thickness of the layer (t), are defined in figure 1. All values are given in Å. Crystals GaS GaSe GaTe Ga2 SSe Ga2 STe Ga2 SeTe InS InSe InTe In2 SSe In2 STe In2 SeTe

a dMM 3.59 2.41 3.78 2.44 4.03 2.40 3.64 2.41 3.81 2.40 3.89 2.40 3.78 2.74 3.91 2.73 4.21 2.72 3.84 2.74 3.99 2.74 4.07 2.74

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dMX 2.33 2.46 2.64 2.34 2.40 2.49 2.50 2.61 2.81 2.52 2.55 2.65

dMY 2.33 2.46 2.64 2.42 2.58 2.60 2.50 2.61 2.81 2.60 2.77 2.78


t 4.52 4.72 4.89 4.64 4.71 4.79 5.19 5.37 5.56 5.29 5.36 5.45


2

2

2

2

2

2

Figure 2: Phonon dispersion (Left panels) and Energy Fluctuation (right panel) curves for: (a) Ga2 SSe; (b) Ga2 STe; (c) Ga2 SeTe; (d) In2 SSe; (e) In2 STe; (f) In2 SeTe. The temperature of the AIMD was set to 300K. The insets, in the right panels, show snapshots of the crystal structures for a time of 5ps (final configurations).

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To investigate the mechanical stability of the Janus materials we have calculated the phonon dispersion curves for all structures. We have considered in this work the finitedisplacement approach with supercell sizes of 4×4 primitive unit cells. The results are presented in Fig. 2(left panels). Although there is a small imaginary pocket near Γ, we find no imaginary frequencies in the Brillouin zone. This small instability in the flexural phonon branch, is very sensitive to details in the calculation, and is a common issue in first-principles calculations for 2D materials, as already pointed out by Falko et. al., 42 for the pristine cases of InS, InSe and InTe. In this way, our results allow us to infer that those Janus monolayer materials are dynamically stable. Also, our results about the In2 SSe phonon spectra agree very well with the one presented by Kandemir et. al. 37 and Guo et. al. 36 To further check the thermal stability of dynamically stable Janus materials, we have performed ab initio molecular dynamics simulations (AIMD) at 300K and 600K (not shown here), with a time step of 1 f s using Nosé heat bath scheme. 43 We have constructed a 4×4 supercell containing 16 formula units to minimize the constraint induced by periodicity. The atomic configuration of the monolayers remains nearly intact after more than 5 picoseconds. Plots of the variations of the total potential energy with respect to the simulation time and snapshots of the last configurations are shown in Fig. 2(right panels). These results demonstrate that the Janus monolayers considered in this work, once synthesized, are stable and can maintain their structural integrity at room temperature. Besides that, if one looks to the elastic constants of the Janus monolayers, calculated in the work of Guo et al., 36 2 > 0, it is possible to verify that all C11 = C22 > 0, C66 = 21 (C11 − C12 ) > 0 and C11 C22 − C12

of these constants satisfy the Born criteria, 44 which proves the mechanical stability of the Janus monolayers.

2.2

Electronic Properties

The HSE06 band structures for all Janus monolayers considered in this work are presented in Fig. 3. The top of the valence band was set to zero energy. For the Ga2 XY composition, 7



we can observe that the Ga2 SSe (Fig. 3(a)), is an indirect band gap semiconductor, where the top of the valence band is located in a k-point between M and Γ and the bottom of the conduction band is located at the M point. This behavior is the same for the pristine compositions of GaX. Meanwhile, Ga2 STe and Ga2 SeTe are characterized by direct band gap at the Γpoint, Figs. 3(b) and (c). We found energy gaps of 3.2 eV for Ga2 SSe, 1.71 eV for Ga2 STe and 2.16 eV for Ga2 SeTe, suggesting that those materials are suitable for optoelectronic applications. The In2 XY compositions present somewhat similar energy bands, Figs. 3(d), (e) and (f) for In2 SSe, In2 STe and In2 SeTe, characterized by direct energy gap at the Γ point. With respect to the band gap values, they are all in between the values of Ga2 XY, where the values are: 2.82 eV for In2 SSe; 1.59 eV for In2 STe and 1.97 for In2 SeTe. With this feature, if one constructs 2D van der Walls heterostructures with those materials, in principle, would be possible to cover almost all visible light spectra, making those materials good for opto and photovoltaic applications. We have also calculated the electronic band structures considering the spin-orbit interaction (SOC). Such results are shown in the Supporting Information5. In Fig. 4 we present the projected band structure, resolved in the atoms and also in the orbital composition, for two systems: (a) and (b) Ga2 SSe, that has an indirect band gap; (c) and (d) Ga2 STe, that has a direct band gap. These systems are representative for all other configurations that are not present here, and the behavior of the band structures are similar. If we look to atomic contribution, Figs. 4(a) and (c), we observe that the conduction band (CB) for all systems are mainly located in the metal (Ga/In), and comes majority from s orbital, Figs. 4(b) and (d). If one looks at the valence band (VB), for the Ga2 SSe system in Fig. 4(a) and (b), the top of the VB (VBM), that is located between M-Γ, its main contribution comes from the pz orbitals from Ga, and at Γ it is a hybrid state with pz orbitals from Ga and pxy orbitals from the Se atom. One important feature that we can observe in the atom projected band structure for the Ga2 SSe materials, is that the chalcogen contribution to the band structure are separated in energy [see Γ-point in

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(a)

2

(d)

2

(b)

2

(e)

2

(c)

2

(f)

2

Figure 3: HSE06-DFT calculated band structures for: (a) Ga2 SSe; (b) Ga2 STe; (c) Ga2 SeTe; (d) In2 SSe; (e) In2 STe and (f) In2 SeTe. The blue(red) color indicates the conduction(valence) band edge. The top of the valence bands are set to be zero. Fig. 4(a)], where the VB is located always in the atom that has smaller electronegativity. This behavior occurs in all Janus materials considered in this work. Finally, if we look to the VB for the Ga2 STe system, Figs. 4(c) and (d), this system is now a direct band gap semiconductor. This occurs, because due to the increase in the electronegativity difference between the two chalcogens, there are fewer electrons in the Te side, and the interaction between the pxy orbitals of the chalcogens becomes weaker, making this orbital contribution increase in energy and consequently being up from the pz contribution from the metal. This feature occurs in all systems, and it could benefit if one think in optical applications, because, for the pristine metal chalcogenides [GaX and InX (X= S, Se, Te)], the VB comes always from pz 16,42 orbitals and the CB is always s orbital, but this transition is forbidden from selection rules. However, in the Janus materials, the CB continues to be a s orbital, but the VB are now from pxy , and the optical transition s→pxy is allowed, and the optical behavior

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(a) Ga

S

Se

s

pxy

pz

Ga

S

Te

s

pxy

pz

(b)

(c)

(d)

Figure 4: (a) Atom projected band structure for the Ga2 SSe in the: Ga atom - left panel; S atom - middle panel; Se atom - right panel. (b) Orbital projected band structure for the Ga2 SSe in the: s orbital - left panel; pxy orbital - middle panel; pz orbital - right panel. (c) Atom projected band structure for the Ga2 STe in the: Ga atom - left panel; S atom - middle panel; Te atom - right panel. (d) Orbital projected band structure for the Ga2 STe in the: s orbital - left panel; pxy orbital - middle panel; pz orbital - right panel. The color scales indicate the (normalized) magnitudes of the atomic and orbital contributions to the energy bands. The zero energies were set at the valence band maximum.

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of the materials will be completely different. (d) Evac

(b)

Df=0.2eV

Vz (eV)

(a)

Evac

Eint

3.48 eV

Vz (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


S

Se

6.68 eV

(c)

6.48 eV

Eint S

3.28 eV

Eint

Eint Se S

Te Se

CBM

CBM

Gap 3.2 eV

Te VBM

VBM

Figure 5: (a) Average potential energy for the Ga2 SSe along the perpendicular direction. (b) Detail of the difference between the two different levels of the vacuum energy. (c) The direction of the driven electric field present due to the potential difference between different sides of the material. (d) Schematic illustration of the band offsets of the Janus Ga2 SSe, presenting the values of the CBM and VBM aligned with the different vacuum levels of the system. One key feature presented by those Janus systems, is that the mirror symmetry is broken, due to the different composition of the chalcogens on both sides of the layer, and together with different electronegativity of the chalcogens, there is a potential gradient normal to the basal plane of the system, that leads to a perpendicular intrinsic electric field Eint , which its direction follows the difference in the electronegativity, from the smaller to the higher, as shown in Fig. 5(c). In Fig. 5(a) we show the planar average of the electrostatic potential, along with the perpendicular plane direction, zˆ. The platform on the left and right sides of the material refers to the vacuum energy level. By comparing the vacuum energy level in both sides, we can note an asymmetry, presented in more details in Fig.5(b), where for the Ga2 SSe material, the left and right vacuum regions are different by about ∆φ = 0.2 eV in energy. If we know the vacuum energy and the corresponding energy of the valence band maximum (VBM) and conduction band minimum (CBM), we can obtain the ionization energy (IE) and electron affinity (EA), accordingly. In these materials, due to the existence of an intrinsic internal electric field (dipole), we have side dependent (by ∆φ) values of IE

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and EA. In Table 2 we present our results of ∆φ. Indeed, the largest ∆φ values are for the compositions with S and Te. This occurs due to the higher electronegativity difference involving these atoms, independently of the metallic atom. This difference leads to different behavior in the electronic structure. Table 2: Difference between the vacuum potentials for the Janus monolayers M2 XY (M= Ga,In; X,Y= S,Se,Te). System ∆φ(eV)

Ga2 SSe 0.20

Ga2 STe Ga2 SeTe 0.53 0.35

In2 SSe 0.20

In2 STe 0.47

In2 SeTe 0.3

As a reference, here we will consider the Ga2 SSe single layer. It is a semiconductor with an indirect band gap of 3.2 eV [see Fig.5(d)] As we have previously seen, for the Ga2 SSe, the difference between the vacuum potentials is roughly 0.2 eV. If one thinks in the construction of the 2D van der Waals heterostructure with those Janus materials, the energy bands of the two components will be approximately aligned with respect to the vacuum level, due to their weak van der Waals interaction, and as those materials present two different vacuum levels, different band alignments will be present. To provide a general guideline for the design of desirable heterostructures, a schematic diagram of the band alignments of a single layer Ga2 SSe is presented in Fig. 5(d).

2.3

Photocatalytic Properties

The pristine single layer materials (GaS, GaSe, GaTe, InS, InSe and InTe) have been considered as key materials for hydrogen generation by photocatalytic water splitting. 45 The water splitting occurs when the photons are absorbed by the semiconductor, generating a hole-electron pair. The excited electrons act in the hydrogen reduction reaction generating H2 : 2H+ + 2e− → H2 ; and the holes acts in the oxidation reaction, generating O2 : H2 O + 2h+ →1/2O2 + 2H+ . A good semiconductor for water splitting should present a band gap that exceeds the free energy of water splitting, around 1.23 eV and most importantly the CBM energy should be higher than the reduction potential of H+ /H2 and its VBM should 12


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be lower than the oxidation potential of O2 /H2 O. The usual values for the reduction and H + /H2

oxidation potential for water at pH= 0.0 are: Ered

O /H2 O

= -4.44 eV; Eox2

= -5.67 eV. For pH

dependent reactions, the redox potentials vary according to the Nernst equation, and if one considers different values of pH, the relation of the reduction and oxidation potentials are: H + /H2

Ered

O /H2 O

=(-4.44+pH× 0.059) eV; Eox2

=(-5.67+pH× 0.059) eV. 46 Therefore, for the water

splitting reactions, using values in reference to vacuum, the VBM energy must be lower than the O2 /H2 O reduction potential of -4.44 eV for pH=0 and -3.61 eV for pH=14, while the CBM energy must be higher than the H+/H2 oxidation potential of -5.67 eV for pH=0 and -4.84 eV for pH=14.

In2

In2

In2

In2

In2

In2

Ga2

Ga2

Ga2

Ga2

Ga2

Ga2

Ga2

pH 14 Ga2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 pH=14 pH=7

H+/H2

pH=0 pH=14 pH=7

O2/H2O

pH=0

Figure 6: Band edges for all Janus materials considered in this work. They are presented aligned with respect to both sides of the materials, as indicated. We also present the reduction/oxidation potential of water with respect to the pH value (green to yellow region). Aiming to verify the photocatalytic activity and the band edge positions of the Janus Ga2 XY and In2 XY structures for water splitting, in Fig. 6 we show the band edges alignments relative to the vacuum level, for all studied materials, compared to redox potentials of the water over a pH range of 0 to 14. All systems, except the Ga2 SSe, are direct band gap materials. It is worth noting that all materials fulfil the conditions to be a good photocatalyst material. Another important point is that those materials also work for the whole range of pH, as indicated by the green-yellow regions for the reduction and oxidation potentials. Finally, the intrinsic internal electric field of the Janus material could improve the dissociation of the exciton. As pointed out by Ji et al., 40 the intrinsic electric field of the 13



Janus materials can serve as an booster to break up an exciton pair, which promotes an electron-hole separation, ensuring a high efficiency in the photocatalytic reaction. Besides its electronic properties (sufficient wide band gap, the positions of CBM and VBM should be lower and higher than the reduction level of hydrogen and oxidation level of oxygen), a two-dimensional material to be a good photocatalytic material also has to present some important optical properties, as pointed by Singh et. al. 47 The system has to be a good sun light absorber, mainly in the visible light range, where around 40% of the solar energy is concentrated. This requirement (ability to use visible light) is very important since the non-equilibriium photophysical and photochemical processes initiate when a photon (with energy greater or equal to the band gap of the 2D Janus crystals) interacts with the material. The photon excites an electron from the VBM to CBM. In sequence, occurs a relaxation of the generated holes and electrons to the top and bottom of VB and CB. Depending on the band positioning and band bending configuration, these charge carriers can migrate to the surface of the such material and initiate REDOX reactions. Moreover, the exciton binding energy has to be small to facilitate the separation of charge carriers (electrons and holes). The optical absorption function is given by the imaginary part of the dielectric function. In Fig. 7 we present the complex dielectric function for all materials considered in this work. The GW plus Bethe-Salpeter equation approach is employed to predict the accurate optical absorption and excitonic nature of the M2 XY monolayers considered in this work. The exciton binding energy, that is defined as the difference between the GW band gap and the first absorption peak of the GW+BSE absorption spectrum, are presented figure 7. Due to the e-h interactions, the optical absorption of all the systems are improved, and bound exciton states below the single-particle band edge are observed. It can be seen that the values of exciton binding energies range from 0.30 eV to 0.68 eV. The values for the exciton binding energies, Eb , for all compositions are summarized in Table 3. One important point to observe, is that the exciton binding energy follows an opposite behavior than the intrinsic electric field, where if we compare the values of the exciton binding energy in Table 3 to

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the values of the potential difference presented in Table 2, we could conclude that for larger potential differences the exciton binding energies are smaller. This behavior is also present in Janus monolayers of transition metal dichalgogenides. 41 This effect is ascribed to the internal electric field in the 2D Janus material, that is not present in the common material. For example, the GaSe exciton binding energy is 0.66 eV, 48 which is higher than all Janus Ga2 XY. This small values of the exciton binding energies, together with the electron-hole separation due to the internal field of the material, shows that those materials are ideal for photocatalysis. 2

2

2

2

2

2

Figure 7: Imaginary part of the dielectric function calculated with GW and GW+BSE for: (a) Ga2 SSe; (b) Ga2 STe; (c) Ga2 SeTe; (d) In2 SSe; (e) In2 STe; (f) In2 SeTe. The black dashed lines indicate the band edges, that corresponds to the QP band gap calculated with GW. The blue dotted lines represent the first bright exciton.

Table 3: Exciton binding energies for the Janus monolayers M2 XY (M= Ga, In; X, Y= S, Se,Te). System Eb (eV)

Ga2 SSe 0.43

Ga2 STe 0.30

Ga2 SeTe 0.35

In2 SSe 0.68

In2 STe 0.47

In2 SeTe 0.56

In summary, we investigated the structural, electronic and photocatalytic properties of the Janus monolayers of the metal chalcogenides Ga2 XY and In2 XY (X= S, Se, Te; Y= S, Se, 15



Te). All materials, Ga2 SSe, Ga2 STe, Ga2 SeTe, In2 SSe, In2 STe and In2 SeTe are mechanically stable, presenting the same structure as their common counterparts (GaS, GaSe, GaTe, InS, InSe and InTe). Even though presenting the same structure, their electronic properties are distinct, mainly due to the difference in electronegativity of the chalcogens in different sides of the materials. Excepting the Ga2 SSe, all others materials exhibit direct band gaps. The small values of the exciton binding energies presented by those Janus monolayers, combined with the electron-hole separation due to the internal field of the material, show that those materials are promising materials for photocatalysis.

3

Methods

Our calculations were based on density functional theory, as implemented in the VASP code. 49,50 As conventional exchange-correlation functionals underestimate the band gap of semiconductors, we used the hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE06), 51,52 which gives more accurate band gaps and at the same time also accurate structural properties. The projector augmented wave potential (PAW) 53 was used to treat the ions-electrons interactions, and a plane wave cutoff energy of 500 eV are used to expand the valance electron wavefunctions (Ga: 4s2 4p1 ; In: 5s2 5p1 ; S: 3s2 3p4 ; Se: 4s2 4p4 ; Te: 5s2 5p4 ). We used 20 Å of vacuum to avoid spurious interactions with the periodic images, and also we used dipole corrections, due to the Janus materials presents an intrinsic internal electric dipole. The lattice parameters and atomic positions for all structures are relaxed until residual forces on the atoms are smaller than 0.001 eV/Å. The Brillouin zone is sampled using a 9×9×1 Monkhorst-Pack grid. 54 After the structural optimization, we confirm the dynamic stability of the Janus monolayers by calculating the phonon dispersion using PHONOPY code. 55 The thermal stability was verified by ab initio molecular dynamics simulations (AIMD) at 300K and 600K, with a time step of 1 f s using Nosé heat bath scheme. 43 The position of the valence- (VB) and conduction-band (CB) edges, with respect to the vacuum level, are

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determined by aligning the planar-averaged electrostatic potential within the layer with the vacuum region. Aiming to obtain the optical properties and the excitonic effects in the Janus materials, first we have carried out DFT calculations using the fully converged crystalline structure of the Janus materials. Using those results, we performed G0 W0 calculations to obtain the quasiparticle energies. The GW self-energies were calculated with the Plasmon-Pole Approximation. 56 To avoid the problem of the screening in low dimensional systems, we have used a truncated screened Coulomb interaction. The excitonic effect was determined using ab initio many-body perturbation theory, by solving the Bethe-Salpeter Equation (BSE). 57–59 The G0 W0 and BSE calculations were performed with the YAMBO code. 56 The convergence of quasi-particle band-gap with respect to the number of empty bands and the size of dielectric matrix and the Monkhorst-Pack grid were carefully checked. The TammDancoff approximation together with 5 valence bands and 5 conduction bands are included to solve the BSE.

4

Author Information

Corresponding Author *E-mail: [email protected]; [email protected]; The authors declare no competing financial interest.

5

Acknowledgment

This work was supported by the Brazilian agencies FAPEG, CNPq and FAPESP Project 17/02317-2. We would like to acknowledge the computing time provided by CENAPAD-SP.

17



Supporting Information Available The Supporting Information is available free of charge on the ACS Publications website at DOI: XX.XXXX/YYY Spin-orbit dependent band structure and projected band structure.

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In2

In2

In2

In2

In2

In2

Ga2

Ga2

Ga2

Ga2

Ga2

Ga2

Ga2

pH 14 Ga2

0 pH=7

H+/H2

pH=0 pH=14

H2O

pH=7

O2/H2O

pH=0

H2

Figure 8: TOC Graphics

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pH=14

+++++++

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H 2O

O2

Metal Chalcogenides Janus Monolayers for Efficient Hydrogen

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