Monolayer Group IVâVI Monochalcogenides: Low-Dimensional

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Monolayer Group IV-VI Monochalcogenides: LowDimensional Materials for Photocatalytic Water Splitting Chandra Chowdhury, Sharmistha Karmakar, and Ayan Datta J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12080 • Publication Date (Web): 17 Mar 2017 Downloaded from http://pubs.acs.org on March 18, 2017

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The Journal of Physical Chemistry

Monolayer Group IV-VI Monochalcogenides: LowDimensional Materials for Photocatalytic Water Splitting Chandra Chowdhury, Sharmistha Karmakar, Ayan Datta* Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur- 700032, West Bengal (India). Email: [email protected]

Abstract Harvesting solar energy for artificial photosynthesis is an emerging area in alternative energy research. In the present letter, we have investigated the photocatalytic properties of single-layer group IV-VI monochalcogenides, MXs (M = Ge, Si, Sn and X = S, Se) based on first-principles electronic structure calculations. Our dispersion corrected DFT calculations show that these materials have moderate cohesive energies (< 120 meV/atom) which are indicative of favorable isolation of MX monolayers by mechanical, sonicated or liquid-phase exfoliation. The calculated band gaps using hybrid density functional method (HSE06) reveal that all of the MXs show larger band gaps than the minimum energy required for the water splitting reaction (1.23 eV). Considering band edge alignments, all the MXs other than SiS have an acceptable alignment of conduction band minima but not the valence band maxima. We have evaluated the overpotentials for both oxygen and hydrogen evolution reactions. Interestingly, considering contribution from overpotentials, we have tuned the band alignments by varying the pH of the medium. At a basic pH, GeS and SiSe exhibit excellent photocatalytic properties whereas for SiS, an acidic pH is required. Additionally, the optical absorption spectrum shows excellent absorption in the visible 1

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region indicating efficient harvesting of solar radiation. They are substantially stable even in aqueous environment indicating their robust stability at ambient electrochemical conditions.

Introduction Hydrogen, the most abundant element exists in both water and biomass. Due to its high energy yield (122 kJ/g) and no environmentally harmful products at its end, hydrogen is considered as an ideal energy storage medium.1,2To replace the usage of fossil fuels, several alternative energies are being actively developed. Photocatalytic water splitting using semiconductors as hosts for the conversion of solar light into hydrogen is an attractive technology for producing clean and renewable energy without pollution or byproducts.3-5The main challenge for this process is to find a suitable photocatalyst which can efficiently utilize solar energy to dissociate water and produce hydrogen.6,7There are several stringent requirements for a material to behave as a good photocatalyst. The semiconductor should have both chemical and photochemical stability, efficient visible light absorption capability, appropriate electronic band structure for overall water splitting with proper valence and conduction band edge alignment and of course, the material should economically affordable. A schematic demonstration of the photocatalytic water splitting is shown in Figure 1 (a).

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Figure 1. (a) Schematic diagram of photocatalytic water splitting on a semiconductor surface and (b) Side and top views of single-layer group IV-VI (MX) structures. When light is irradiated on a semiconductor with an energy equivalent to or greater than the band gap of semiconductor (photocatalyst), electrons in the valence band are excited to conduction band whereby holes are produced in the valence band. The holes then take part in oxygen evolution reaction (OER) to produce O2:H2O + 2h+→½ O2 + 2H+ and electrons cause hydrogen evolution reaction (HER) which leads to H2 : 2H+ + 2e-→H2. The minimum band gap required for these two processes to occur simultaneously is 1.23 eV which is simply the free energy (∆𝐺) for water splitting.6As almost half of the solar energy spectrum falls in the visible region, the band gap of photocatalyst should lie in the window 1.5 -3.0 eV so as to utilize solar light efficiently.8Apart from band gap, the photocatalyst should also have a suitable band alignment namely, the bottom of the conduction band must be more positive than the reduction 3



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potential of H+/H2 (-4.44 eV at pH = 0) and the top of the valence band must be more negative than the oxidation potential of O2/H2O (-5.67 eV at pH = 0).9In some cases, a co-catalyst is required to overcome large overpotentials of the individual processes (OER and HER).9,10To understand the mechanism of these reactions on semiconductor surfaces and to estimate the overpotentials required for half-cell reactions, computational studies have been found to be highly valuable.11,12 Two dimensional (2D) materials for photocatalytic water splitting has been extremely promising because of their large surface to volume ratio. This not only reduces the possibility of electronhole recombination but also enhances their catalytic properties.13,14Several experimental and theoretical studies have explored graphene and related 2D materials for such photocatalytic water splitting.15-19Besides graphene, various other 2D materials such as MoS2, functionalized silicane and

germanane,

various

MXenes

(like

Zr2CO2&

Hf2CO2),

single

layer

group–III

monochalcogenides and group-IVB nitride halides have also shown potential applications for excellent photocatalyst.20-24Interestingly, SnS2 can be tuned to behave as an outstanding photocatalyst under an acidic pH (pH = 6.6) and through the application of an external bias potential.25,26More recently, a new elemental 2D material namely black phosphorene (Pn) which because of Pseudo Jahn-Teller (PJT) effects is also buckled in a stapler-clip symmetry (unlike silicene/germanene which are buckled along D3d axis) have gained significant attraction to materials science community.27-33It has been shown recently that good photocatalytic behavior can be induced in black Pn under strain.34,35 The 2D analogues of group IV monochalcogenides, MX (M=Si, Ge, Sn and X=S or Se) form buckled honeycomb lattice similar to that of phosphorene.36,37Amongst the six structures, the bulk phases of the four monochalcogenides (GeS, SnS, GeSe and SnSe) is known to exist in 4


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orthorhombic crystal structure similar with black-Pn.38Synthesis of these monolayers can be achieved through exfoliation either by physical or by chemical processes.39,40Computational and experimental groups have explored the different forms of these monochalcogenides.41-44Hanakata et al showed that among Group-IV monochalcogenides, SnS and GeSe monolayers have potentials to act as functional materials for information storage.41Their large piezoelectric coefficients make them suitable for large scale applications in sensors and energy harvesting.42In our previous study, we have shown that the monolayers of these monochalcogenides, maily SiS and SiSe have potential to act as anode materials in Li ion battery and we have also identified the role of quantum mechanical tunneling in the migration of Li ion.43By using density functional theory and semiclassical Boltzmann theory Tyagi et al showed the stability, electronic and transport properties of SnSe nanoribbons.44Recently, group IV-VI monochalcogenides like silicon monosulfide and monoselenide have been predicted to form stable free standing monolayers and therefore, syntheses of α-SiS and α-SiSe monolayers can be readily attained through different efficient deposition technique.37,45,46Structural resemblance and similar electronic properties like phosphorene make them attractive for applications in electronic and opto-electronic devices. In the present article, in order to examine their potential as photocatalysts, we have systematically investigated the stabilities of MXs and studied their electronic structures using state-of-the art hybrid density functional calculations. Next we determined the band edge alignment and optical absorption spectra and finally we unraveled the stability of the monochalcogenides in aqueous solution. The relatively low exfoliation energies, appropriate band gaps, desirable band edge positions and high optical absorption make GeS, SiS and SiSe excellent materials for photocatalytic water splitting. 5



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Computational Details

All calculations are performed within the framework of density functional theory (DFT) calculations using the projector augmented wave method as implemented in Vienna Ab Initio Simulation Package (VASP).47The exchange-correlation term has been described within the generalized gradient approximation (GGA) parameterized by the Perdew-Burke-Ernzerhof (PBE) functional and to account for van der Waals interactions we used the empirical correction method proposed by Grimme (DFT-D2).48,49In all computations the kinetic energy cut-off is set to be 500 eV for the plane wave expansion. All the structures are fully relaxed in conjugant gradient method and the convergence threshold was set as 10-4 eV for energy and 0.01 eV/Å for force. The Brillouin zone has been sampled by 7 × 7 × 1 Monkhorst-Pack grid for the monolayer and 7 × 7 × 7 for the bulk.50To avoid the spurious interactions between periodic images a 15Å vacuum distance along the c-direction was considered. In order to determine band edge positions accurately, we have used hybrid HSE06 functional.51For calculations using the HSE06 functional, we have used a k mesh of 5 × 5 × 1 to describe the Brillouin zone. The positions of VBM and CBM were defined as ECBM/VBM = EBGC ± ½ EgHSE06 where EBGC is the band gap center energy calibrated with respect to the vacuum level. As BGC is quite insensitive to the choice of exchange correlation functional, we have used the less expensive PBE functional for calculating this.52

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To obtain the optical absorption spectra of MXs, we have calculated the frequency dependent dielectric function where the Brillouin-zone is sampled with a 21×21×1 MonkhorstPack grid and the imaginary part of the dielectric function which arises due to the interband transition and is calculated using the following equation:53

∈!

2𝑒 ! 𝜋 ћ𝜔 = Ω ∈!

| 𝛹!! 𝑢𝑟|𝛹!! |! 𝛿(𝐸!! − 𝐸!! − 𝐸) !,!,!

where Ω, ω, u, 𝜈 and c denote the unit-cell volume, photon frequency, the vector implying the polarization of the incident electric field, the valence band and the conduction band respectively. To calculate the free energy change in the oxidation/reduction reactions, we have utilized the method developed by Norskov and co-workers according to which the change in free energy of an electrochemical reaction is computed as54-56: ∆𝐺 = ∆𝐸 + ∆𝑍𝑃𝐸 − 𝑇∆𝑆 + ∆𝐺! + ∆𝐺!" where ∆𝐸 is the DFT computed reaction (electronic) energy, ∆𝑍𝑃𝐸 is the difference in zero point energy, ∆𝑆is the change in entropy and ∆𝐺! = −𝑒𝑈, where U is the electrode potential. ∆𝐺!" corresponds to the free energy correction due to the concentration of H+ in solution and is calculated as ∆𝐺!" = 2.303𝑘! 𝑇× 𝑝𝐻, where 𝑘! was the Boltzmann constant and T is the temperature of the medium. It is important to note that though for metal surfaces, the water molecules on the surfaces stabilize the intermediates (OH*, O*, OOH*)

11

for semiconducting

surfaces like those considered here, we neglect the effect of the aqueous environment. This is because of the smaller change in Gibbs free energy due to interaction with water and this approach has been utilized for other two dimensional materials as well.18,55,57-59To analyze the free energy dependence of various intermediates at varying potential and pH conditions, we

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computed the surface Pourbaix diagram of SiS surface as representative example using previously utilized computational schemes.60-63To obtain zero point energy corrections, the vibrational frequencies were calculated through density functional perturbation theory (DFTP). Entropy of the molecules in gas phase were obtained from NIST database. The free energy of (H+ + e-) was considered as that of 1/2 (H2). Overpotential is defined as the difference of equilibrium potential and the potential where all reaction steps become downhill. To evaluate stability of the monolayers in water, solvation energies were computed as the sum of the cohesive and the hydration energies. To this regard, cohesive energy was calculated using the PBE functional in VASP followed by hydration energy using the GAUSSIAN 09 package using the B3LYP/aug-cc-pVQZ level of theory.64-65We have also considered ion association where solvated cation-anion pair is formed and we compute almost identical solvation energy also for such cases as well.

Result and Discussions

Figure. 1 (b) shows the structure of MX layers. The structural parameters of MXs (calculated using the DFT-PBE functional) are reported in Table 1. Our computations show excellent agreement with the previous reports.36,66 Table 1. Structural parameters calculated using PBE functional, band gaps of single layer MXs calculated using both PBE and HSE06 functionals. dM-X(I) and dM-X(II) represent the two unique M-X distances.

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System

a (Å)

b (Å)

SiS SiSe

3.39 3.63

4.65 4.80

dM-X (I) (Å) 2.34 2.46

dM-X (II) (Å) 2.31 2.51

θM-X-M (°) 96.2 100.9

θX-M-X (°) 94.6 92.6

Eg(PBE) Eg(HSE06) (eV) (eV) 1.37 2.04 1.04 1.61

GeS

3.66

4.50

2.42

2.48

104.2

95.2

1.58

2.42

GeSe

3.92

4.38

2.53

2.61

98.5

97.3

1.28

1.68

SnS

4.05

4.36

2.61

2.71

102.7

96.8

1.52

2.04

SnSe

4.20

4.60

2.74

2.83

96.9

95.5

1.10

1.69

For a feasible photocatalyst, it should be thermodynamically stable. Relative to the bulk phase, the formation energy (Ef) of the single layer MX is defined as:

𝐸! =

𝐸!! 𝐸!! − 𝑁!! 𝑁!!

where E3D and E2D are the energies and N3D and N2D are the number of atoms of bulk and single layer MXs, respectively. As the bulk forms of SiS and SiSe are unknown, for the sake of comparison, we have considered the bulk phases of these two as like others and calculated the formation energies. The calculated formation energies are plotted in Figure 2. The Ef are moderate and comparable to that of other successfully synthesized 2D materials.23,67So it is expected that these MXs can be synthesized by typical experimental techniques like chemical vapor deposition or molecular beam epitaxy. Among MXs, ultrathin single crystal GeS and GeSe nanosheets have already been synthesized which suggests the possibility of exfoliating monolayers from the bulk structure.68Brent and co-workers synthesized few layer SnS nanosheets successfully by liquid phase exfoliation and ultrathin single crystalline SnSe nanosheets have also been synthesized recently.39,69Clearly, our computations present an optimistic picture for the exfoliation of IV-VI two-dimensional structures. 9



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Figure 2.Formation energies of single-layer MXs from their corresponding bulk counterparts.

Encouraged by the excellent stability of IV-VI monolayers, we explored their electronic properties for photocatalytic water splitting. Figure 3 shows the band structures of single-layer MX obtained from the HSE06 functional. The corresponding band structures obtained from PBE functional are reported in the Supporting Information (Figure S1) file. The band gaps of MXs are summarized in Table 1 for both PBE and HSE06 functional.

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Figure 3.The electronic structures of single-layer (a) SiS, (b) SiSe, (c) GeS, (d) GeSe, (e) SnS and (f) SnSe. The band structures are calculated using HSE06 functional.

It is observed that for all MXs, the band gap exceeds 1.23 eV, the minimum gap required for photocatalytic water splitting. Interestingly, GeSe and SnSe have a direct band gap while the rest are indirect band gap semiconductors. The band gaps range from 1.61 eV-2.42 eV and can therefore in principle, efficiently harvest the visible light of solar radiation. Interestingly, as reported by Zhang and coworkers, indirect band gap materials are more suitable for photocatalytic activity.70For such cases, as the excited electrons emit photons to eventually relax back to the valence band, the presence of different k-vectors for the CBM and VBM reduces the possibility of recombination of photo-generated electrons and holes.

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Along with a sufficient band gap, the semiconductor also needs to possess suitable band edge alignment for the oxidation and reduction potentials. Figure 4 represents the band alignments of MXs with respect to the normal hydrogen electrode (NHE).

Figure 4.(a)Band edge positions of single layer MXs related to the vacuum level calculated with the HSE06 functional and the standard redox potentials for water splitting at pH=0. Band alignments of (b) SiSe, (c) GeS, (d) GeSe at various pH. With an exception of SiS for which both VBM and CBM are favorably positioned for OER and HER, the other MXs only have the correct position for CBM but not VBM. So other than SiS, all other MX require an external bias potential for OER. Such bias potentials can be tuned by

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changing the pH of the medium as the standard oxidation and reduction potential of water can be written as71 EoxO2/H2O = - 5.67 + 0.059 × pH and EredH+/H2O = - 4.44 + 0.059 × pH So, we can tune the band edge alignment of MXs by controlling the pH to match the redox potential of water. Here, one important point is that as our system resembles phosphorene and SnS2 for which the band alignments are insensitive to the change of pH and hence, we have assumed here that for our studied MXs, the band edge positons do not change with change in pH.26,34,72-74As the position of VBM for SnS and SnSe exceed the O2/H2O level in water by 0.7 eV, the standard magnitude for external bias voltage, one might safely discard these two MXs for possible applications in photocatalytic water splitting.75Nevertheless, there is an opportunity for utilizing them as catalysts for hydrogen evolution reaction. For SiSe, at a mildly basic pH (pH = 8.0), one attains suitable band alignment for water splitting similar as that of phosphorene.34 For GeS, a strongly acidic condition (pH= 5.0) and for GeSe a strongly basic condition (pH = 12.0) make them suitable for water splitting. Figure 4 (b), 4 (c) and 4 (d) show the band alignments of SiSe, GeS and GeSe at their suitable pH. Besides this, for some cases though the catalysts satisfy the basic conditions, it might not act as an efficient photocatalyst as the oxygen and hydrogen evolution may suffer from high overpotentials.55So it is of our prime interest to evaluate the over-potentials for individual half-cell reactions for the rest MXs (SiS, SiSe, GeS and GeSe). We have used the model proposed by Norskov and co-workers to estimate the overall water splitting reaction.54

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The oxygen evolution reaction proceeds in four steps as follows: * + H2 O

→

*OH + (H+ + e-)

(1)

*OH

→

*O + (H+ + e-)

(2)

*O + H2O

→

*OOH + (H+ + e-)

(3)

*OOH

→

* + O2 + (H+ + e-)

(4)

where *A refers to species adsorbed on MX surface. For all elemental steps, the free energy changes were calculated as specified in the computational details section. We have also studied the HER on MXs surfaces and the mechanism consists of following steps: * + (H+ + e-) →*H

(5)

*H + (H+ + e-) →H2 + *

(6)

The HER may also be mediated by water molecules and the mechanism can also be alternatively proposed as: *H2O + (H+ + e-) →*H3O

(7)

*H3O + (H+ + e- ) → *H2O + H2

(8)

Here we considered both the cases. For all MXs, we have considered a supercell of 16 atoms. In the following section, we have considered MXs individually and calculated the corresponding over-potentials for OER and HER and tuned the band edge positions accordingly. For OER, the optimized geometries of all intermediates adsorbed on MXs surfaces are shown in Figure S2. In the first reaction step (Eq. 1), the water molecule adsorbed on MXs surface is transferred to *OH by releasing one protonated electron (H+ + e-). We initially considered the situation without any external potential simulating the condition for absence of any irradiation on the sample. For the case of GeS, the free energy change of this reaction is found to be + 1.26 eV

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but for GeSe it is found to be + 0.93 eV. In the case of SiS and SiSe, the energies required are + 0.75 eV and + 0.60 eV, respectively. In the next step, the OH which was adsorbed on MXs surface releases one (H+ + e-) pair and the O-atom gets adsorbed (*O) on the surfaces. For this reaction, all the MXs require a relatively smaller energy with respect to the first step. For GeS, GeSe, SiS and SiSe these are + 0.29 eV,+0.56 eV,+ 0.53 eV and + 0.32 eV respectively. In the third step of the reaction, there is an interaction of the second water molecule with that of the surface which adsorbed O (*O) and for this reaction one electron-proton pair is released and an intermediate with adsorbed OOH (*OOH) is formed. The free energy change for this reaction step is found to be +1.41eV, +1.78 eV, +1.43 eV and +1.32eV for GeS, GeSe, SiS and SiSe, respectively. In the final step, the *OOH releases one electron-proton pair along with the release of di-oxygen (O2) from the surface. The free energies for this step are +0.12 eV, +0.42 eV, +0.23 eV and +0.88 eV for GeS, GeSe, SiS and SiSe, respectively for this process. For each case, the zero point energy (ZPE) corrections to the free energies are reported in Table S2. Figure 5 describes the free energy plots at different potentials for all MXs.

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Figure 5.Calculated free energy changes during oxygen evolution reaction at different potentials for (a) SiS, (b) SiSe, (c) GeS, (d) GeSe.

As observed from Figure 5, for all MXs each of the elementary reaction steps are uphill in the absence of any external potential (U = 0.0 V) for the OER. The rate-limiting step is the third step (interaction of H2O with O-atom adsorbed on MX) as it has the highest free energy for all MXs. At an intermediate potential of U = 0.72 V, the first and third reaction steps still remain uphill for GeS, GeSe and SiS though the second and fourth steps become downhill. For SiSe, the third and 16


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fourth steps are uphill at this potential. Further optimal increase in U can indeed make all the four step favorable (exergonic). For example, for GeS at U = 1.42 V, all the reaction steps are downhill and the corresponding over-potential for OER is 0.19 V. The over-potentials required for both SiS and SiSe are small enough and the potentials are 0.22 V and 0.10 V respectively. But, for GeSe the over-potential for OER is relatively large (0.57 V). For HER to occur on the MXs surfaces, there are two possible pathways as represented by Eq. 58. In the first path, H-atom gets adsorbed on the surface and one hydrogen molecule is released. For the other pathway, the adsorbed H2O (*H2O) forms adsorbed H3O (*H3O) which eventually releases a hydrogen molecule. The optimized geometries of *H and *H3O are reported in S3. For both OER and HER, it is observed that the intermediates mainly adsorb on the metal atom (M) rather than chalcogenides atom (X). The adsorption geometries and energies of other sites are shown it Figure S4. It is seen that in all cases, the second pathway requires larger free energy than the first one. Table 2 reports the change in Gibbs free energy for HER on MX surfaces.

Table 2. Calculated free energy changes during hydrogen evolution reaction in different pathways. System

GeS

GeSe

Free energy change (eV)

Isolated H atom

In the presence of H2O

∆G1

+ 0.23

+0.42

∆G2

- 0.23

-0.42

∆G1

+ 0.48

+1.02

∆G2

- 0.48

-1.02

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SiS

SiSe

∆G1

+ 0.32

+ 0.52

∆G2

- 0.32

- 0.52

∆G1

+ 0.50

+ 0.57

∆G2

- 0.50

- 0.57

From Table 2 it is clear that the over-potentials of MXs are quite reasonable18,55 and they are particularly small enough for GeS, SiS and SiSe systems. Now a good catalyst for HER should exhibit a small adsorption energy for H (|∆GH|~0). For that reason the change in free energies at different coverages are computed which are are shown in Figure 6. The adsorption energies with different number of adsorbents are calculated as ∆EH = EnH* - E(n-1)H* - ½ EH2 where EnH*, E(n-1)H* and EH2 denotes energies of surface with n hydrogen atoms, (n-1) hydrogen atoms and energy of H2 molecule, respectively. As in the 2×2×1 supercell, proton prefers to adsorb on the metal atoms (Si/Ge) rather than S/Se atoms, we have considered the coverages in such a configuration. From Figure 6, it is seen that for SiS and GeS, the maximum HER activity (free energy change is close to zero) reach at a coverage of θ=2/4 and θ=3/4 respectively.

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Figure 6: Free energy diagram of HER under different coverages of (a) SiS, (b) SiSe, and (c) GeS. Considering the over-potentials of MXs, the band alignments can be modulated by tuning the pH in GeS, SiS and SiSe for photocatalytic water splitting. For GeS, a small basic pH = 8.5 suffices for suitable alignment and this is in excellent agreement with recent experimental report.76To overcome the small over-potential of SiS, an acidic pH = 5.0 is required to modulate the VBM and CBM positions suitably while for SiSe an alkaline pH~12.0 is necessary.

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To construct the surface Pourbaix diagram, we have considered Eq. (1) and Eq. (2) following the previous reports.60 The free energy change of *OH and *O is calculated as described in computational details section namely, ∆G1 = ∆G(*OH) = ∆G0(*OH) – eUSHE – kbT ln10 × pH ∆G2 = ∆G(*O) = ∆G0(*O) – eUSHE – kbT ln10 × pH which finally comes as, ∆G(*OH) = ∆G1 and ∆G(*O) = ∆G1 + ∆G2 To determine the most stable state of SiS surface, we considered different coverages of *OH and *O on surface and calculated the free energy changes per surface atom by following the equation, ∆G(U,pH)per surface atom = ∆G0(U=0, pH=0)per adsorbate – neU × !

!!"#$%&!'( !"#$%&' !"#$

. The lowest

free energy change determines the most stable surface state. Here instead of plotting a three dimensional graph, we considered first the dependence with potential fixing pH=0 which is shown in a phase diagram in Figure 7a and then the pH dependence of the formation potential of the most stable surface state is governed by only a change of slope of -0.059eV/pH (Nernst behavior) in the Pourbaix diagram.

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Figure 7: (a) Phase diagram showing the stability of SiS surface with different surface coverages at varying potentials, (b) surface Pourbaix diagram for SiS surface. ML represents the monolayer of SiS.

From Figure 7a, it is clear that the lowest line corresponds to the surface state of ¼ ML OH and the bare surface starts to oxidize after a potential of 0.75 V. Figure 7b depicts the change in electrode potential with that of pH. It is clearly seen that at pH=0, the bare surface is stable upto USHE = 0.75 V without any adsorbate and the change of potential with change of pH is shown by the olive shaded region in Figure 7b. After that water starts to oxidize on the surface firstly with coverage of *OH and then gradually oxidizes to *O with increasing the value of potential. Besides the above criteria, a photocatalyst must have excellent optical absorption mainly in the visible region to harvest solar energy efficiently. The optical absorptions of GeS, SiS and SiSe are shown in Figure 8. Since the PBE functional is generally known to underestimate the band

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gaps, a constant shift of the absorption curves by the difference of PBE and HSE06 functionals (∆𝐸!!!"# = 𝐸!!"#!" − 𝐸!!"# ) is implemented to overcome this shortcoming.

Figure 8.Imaginary part of dielectric function ∈! (ћ𝜔) (z-component), calculated using the PBE functional followed by a constant energy shift of ∆𝐸!!!"# = |𝐸!!"#!" − 𝐸!!"# | to correct for the band gap underestimation at the PBE functional.

For GeS, the absorption begins in the visible region and gets maximized at ultraviolet region similar to that of phosphorene.34Encouragingly, for SiS and SiSe, the computed UV-Vis spectra show significant absorption intensity in the visible region suggesting highly efficient utilization of solar radiation. Besides this, exciton binding energy plays an important role for predicting a material to be efficiently harvest solar energy. Tuttle et al calculated the exciton binding energies 22


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for SiS and GeS monolayers and for SiS it is similar to those of other two-dimensional materials like MoS2 which are predicted to be a good photocatalyst.20,77,78 For GeS, it is quite higher compared to other analogous materials. But GeS has high carrier mobility and in particular the electron mobility is much higher than those of hole mobility which confirms the lower recombination rate of electron-hole pair which can be beneficial for being a good photocatalyst.79We have calculated the average static dielectric constants of SiS, SiSe and GeS as 4.06, 5.78 and 3.72 respectively. For SiS and GeS this values agree well with previous report.77 Based on this, we may safely assume that the largest dielectric constant of SiSe would be indicative of a lower exciton binding energy. This demonstrates that single layer GeS, SiS and SiSe are potential materials for solar water splitting. We have also estimated the stability of single layer GeS, SiS and SiSe by calculating the solvation energy when these materials are decomposed into Ge/Si and S/Se ions in aqueous solution. The solvation energy is the sum of cohesive energy obtained from periodic calculations and hydration energy obtained from molecular quantum chemistry calculations. Such computation scheme has been applied previously.23,26The resulting solvation energies of SiS, SiSe and GeS are 696, 619, and 478 kJ/mol, respectively which clearly indicate their poor solvation behavior. Also, the endothermic solvation enthalpies indicate that the monolayers would be stable even in aqueous media. We have also taken into account of associations of ions in aqueous medium for which again, the solvation enthalpies remain endothermic (See Supp. Info. File, Fig S5).

Conclusion To summarize, we have explored the possibility of using MXs as an effective photocatalyst. The exfoliation energies of MXs are quite moderate indicating successful synthesis of these materials. All these materials show an effective band gap > 1.23 eV which is suitable for a 23



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photocatalyst. We have studied each intermediate reaction steps involved in OER and HER and estimate the over-potentials for all MXs. The calculated over-potentials of OER for GeSe, SnS and SnSe are quite large which limits their application as a potential material for photocatalyst though they are suitable for half-cell HER. Therefore, we predict an exciting possibility for employing them these as a Z-scheme photocatalytic systems using a co-catalyst. Besides, it is also possible to tie up these materials with other materials which show good OER activity (like striazine based graphitic carbon nitride)18 and investigate the photocatalytic activity of such new heterostructures. On the other hand, SiS, SiSe and GeS exhibit good photocatalytic activity at NHE condition (pH=0), basic (pH=8.0) and acidic (pH=5.0) respectively. For further consideration of the over-potentials we have tuned the band alignments with the pH of the solution and show that GeS, SiS and SiSe have excellent activities at pH values of 8.5, 5 and 12 respectively. SiS and SiSe have the significant absorption in the visible region which coupled with their endothermic solvation enthalpies further support their candidature as materials for photocatalytic water splitting.

ASSOCIATED CONTENT Supporting Information. Cartesian Coordinates of unit cell used, Band structures using PBE functional, Structures of intermediate geometries of species formed in OER and HER, Complete Gaussian 09 references. AUTHOR INFORMATION Corresponding Author:

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[email protected] ACKNOWLEDGMENT C.C. and S.K. thank CSIR India for SRF. AD thanks INSA, DST and BRNS for partial funding.

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