Intrinsic defect engineered Janus MoSSe sheet as a promising photocatalyst for water splitting

Abstract

The Janus MoSSe sheet has aroused significant attention due to its band edge position and intrinsic dipole moment, making it a strong candidate for water splitting photocatalysis. However, weak water adsorption seriously prevents its further application. Here, first-principles calculations are used to explore the effect of intrinsic defects on water adsorption and conversion at the Janus MoSSe sheet. First-principles calculation results clearly show that intrinsic defects (S_vac, Mo_anti, and Mo_int) can effectively alter the interaction between water and the MoSSe sheet. Except for S_vac defects, the adsorption energy of water at Mo_anti or Mo_int defects can be significantly increased by −1.0 to −1.5 eV with respect to the weak water adsorption on a pristine MoSSe sheet of about −0.24 eV. More importantly, the energy barrier for water conversion can be dramatically lowered by 48% to 0.7 eV at Mo_anti or Mo_int defects, together with a more stable final state. Such significant enhancement of the adsorption energy is attributed to the red shift of water energy levels, resulting from the strong interaction between O2p orbitals and Mo3d orbitals. It is shown that the intrinsic defects have the potential to change the photocatalytic reactivity of the surface, and thus this may serve as an important way to design photocatalysts for water splitting.

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Introduction

Photocatalytic water splitting is gaining increased interest, since it is a promising “green chemistry” approach for the direct conversion of water into clean energy H₂ driven by solar light.^1–3 However, low-efficiency production of H₂ seriously prevents its further applications. Thus, it is urgent to search for a photocatalyst with high stability and efficiency. As a potential photocatalyst, several key criteria should be satisfied as follows. Firstly, a suitable band gap larger than 1.23 eV is required for the photocatalyst to harvest the solar light.⁴ Then, the photocatalyst must have decent band edge positions for overall water splitting, with the conduction band minimum (CBM) higher than the reduction potential (E_H⁺/E_H2 = −4.44 eV) and valence band maximum (VBM) lower than the oxidation potential (E_O2/E_H2O = −5.67 eV). In addition, separation of the photogenerated electron–hole pair is also a crucial factor to reduce the possibility of carrier recombination.⁵

Based on the above criteria, various semiconductors including TiO₂, ZnO, CdS, Fe₂O₃, Bi₂WO₆, Cu₂O, etc. have been proposed as photocatalysts to increase the efficiency for water splitting.^1,6 However, some of these photocatalysts such as TiO₂ are inactive for overall water splitting into O₂ and H₂ in the absence of sacrificial reagents because of a large band gap and low quantum efficiency.⁷ Recently, low dimensional materials have also aroused extensive attention due to the fundamental properties such as large surface area, tunable electronic structure, and abundant reactive sites.^8–12 For example, researchers have found that two dimensional g-C₃N₄, and transition metal dichalcogenides (TMDs) have suitable band gaps and robust absorption in the solar spectrum, which is suitable for water splitting.^13–15 Especially, it has reported that TMDs such as MoS₂ exhibits a direct band gap of about 1.9 eV as it is tailored into single layer, and the band edge can across the reduction and oxidation potential of water splitting.^16,17 However, the carrier mobility of electron and hole in the pristine MoS₂ is relatively low, leading to the low efficiency in water splitting activity and hindering its further applications.^18–20

Except for the traditional TMDs, forming a Janus structure is also an efficient strategy to enhance the physical and chemical properties of the materials.^21,22 For example, Guo et al. found that the Janus group III chalcogenide structure could greatly enhance the piezoelectric effect associated with the intrinsic dipole perpendicular to the plane structure.²³ Furthermore, the enhanced dipole of the Janus structure can generate an intrinsic electric field, which facilitates the separation of the photogenerated carriers in the photocatalytic reactions.²⁴ On the other hand, some researches have proposed some new Janus structures with excellent photocatalytic property. For example, Yang et al. have proposed a novel family of Janus single-layer group-III monochalcogenides (J–MX, J = Ga, M = In, X = S and Se) using first-principles calculations, and found their band gaps are suitable for water photocatalytic splitting.²⁵ And Gao et al. also proposed eight IV–V compound materials showing excellent photocatalysts for water splitting reactions with high efficiency of visible light.²⁶

Recently, Lu et al. have grown a Janus monolayer of transition metal dichalcogenides named MoSSe through breaking the out-of-plane structural symmetry of the single layer MoS₂.²⁷ Janus MoSSe is structurally viewed as MoS₂ with all of S atoms on one side replaced by Se atoms, and exhibits an electronic structure similar to that of MoS₂.²⁸ Unlike MoS₂, an intrinsic dipole exists along the vertical direction of the MoSSe structure, and Yin et al. have found the carrier mobility of Janus MoSSe structure could be greatly affected by the dipole and layer thickness by DFT calculation.²⁹ Further HSE06 result proposed by Ma et al. shows that the Janus MoSSe sheet can be a potential water splitting photocatalyst with wide solar-absorption range and retard carrier recombination.³⁰ Based on the above results, other works on photocatalytic water splitting have been performed. For example, using HSE06+SOC method, Din et al. found that the appropriate band alignments for visible light water splitting can be achieved through forming MoSSe/GeC heterostructure.³¹ Further study showed that despite the band gaps of MoSSe/XN (X = Al, Ga) heterostructures being smaller than 1.23 eV, the band edge positions are always suitable for visible-infrared water splitting.³² On the other hand, Guan et al. found that few layer Janus MoSSe are suitable for visible light water splitting through thickness administrating.³³ Therefore, it can be deduced that Janus MoSSe related materials can be excellent photocatalysis for water splitting. Unfortunately, the adsorption energy of molecular water at pristine MoSSe sheet is about 0.16 eV, suggesting an extremely weak van der Waal (vdW) interaction similar to CO₂ adsorption at TiO₂ surface,^34,35 which will seriously hinder the process of water conversion.

So far, to enhance the adsorption of molecular species at the surface, there are several approaches such as surface modification, transition metal or nonmetal doping, and defect introduction.^36–39 Among these methods, introducing intrinsic defect may be superior to other approaches.⁴⁰ First, intrinsic defects are inevitable during the material growth and are also difficult to eliminate. Furthermore, defect introduction can not only vary the reactivity of the local surface structure, but also change the electronic structure of the material. For example, Zhang et al. reported that the band gap and work function of monolayer Janus MoSSe can be modified by simple hydrogenation method.⁴¹ On the other hand, during the photon related process, the presence of defects can change the quantum yield of photonic excitation by tuning the energy band structure, modifying electron–hole separation, and carrier transport property.⁴² For example, it has been reported that chalcogenide vacancy exists at TMDs.⁴³ Chaurasiya et al. have found that intrinsic defects can effectively alter the adsorption behaviors of NO₂, NO, and NH₃ on Janus MoSSe sheet.⁴⁴ However, the detailed role of the intrinsic defects on introducing the adsorption state and enhancing the photoreaction of water is still unknown.

The purpose of this work is to unveil the effects of intrinsic defects on water splitting at Janus MoSSe sheet. Particular attention will be focused on the following questions: (i) What is the most stable configuration of the Janus MoSSe sheet with intrinsic defects like? (ii) What effects do the intrinsic defects have on water adsorption and conversion? (iii) What factors are crucial to govern the interaction between water and substrate? Our results clearly show that Mo_anti defect favors forming two Mo–S bonds and two Mo–Mo bonds, while Mo_int favors be located over lattice Mo site. The adsorption and conversion of water can be strongly enhanced by intrinsic defects. Thus, intrinsic defect introduction exhibits great potential for the design of photocatalysis for water splitting.

Computational method

The first-principles calculations were conducted by the spin-polarized density functional theory (DFT) within periodic boundary conditions, as implemented in the CP2K/Quickstep package.⁴⁵ The CP2K implementation of DFT employs the hybrid Gaussian and plane wave (GPW) basis sets and the norm conserving Goedecker–Teter–Hutter pseudopotentials to describe the ion–electron interactions.⁴⁶ The Gaussian functions consisting of a double-ζ plus polarization (DZVP) basis set was used for all calculations. Perdew–Burke–Ernzerhof (PBE) functional was employed for the exchange-correlation term based on the generalized gradient approximation (GGA).⁴⁷ The energy cutoff for the real space grid was set at 500 Ry, which yields total energies converged to at least 0.001 eV per atom. Reciprocal space sampling was restricted to the Γ-point, due to the rather large size of the used simulation cells. To consider the weak interaction between water and Janus MoSSe sheet, vdW contribution was included with the Grimme's scheme (DFT-D3).⁴⁸ A vacuum spacing of 15 Å was placed between each adjacent images. Transition states along the reaction pathways were searched by Climbing Image Nudged Elastic Band (CI-NEB) approach.⁴⁹ The interaction between molecule and the substrate was described by the adsorption energy, which is expressed as,

E_ad = E_sub+mol − E_sub − E_mol (1)

where E_sub+mol, E_sub, and E_mol are the total energy of the molecule adsorbed at substrate, isolated substrate, and isolated molecule in the same simulation box, respectively.

Results and discussions

The adsorption of water molecule at defective Janus MoSSe sheet

The primitive cell of Janus MoSSe sheet is a hexagonal structure as denoted by the green line see Fig. 1(a), which consists of S–Mo–Se atomic tri-layers with the upper Se, middle Mo, and lower S atoms as shown in Fig. 1(b). The relaxed cell of Janus MoSSe is in P3m1 space group, and the Mo atom is in six-fold, and the S (Se) atom is in three-fold coordinated. Here, a (4 × 4) supercell is used to model the Janus MoSSe sheet for water adsorption as shown in Fig. 1. For comparison, the molecular water (M-H₂O) adsorbed at pristine Janus MoSSe and MoS₂ sheet are firstly considered. The S side of the MoSSe is firstly examined, and there are two possible adsorption configurations existing after relaxation, denoted as P-M₁ and P-M₂ respectively, as shown in Fig. 1. In P-M₁, the H₂O is adsorbed at hollow site surrounded by three Mo atoms with H atoms pointing to S atoms (Fig. 1(a)). In P-M₂, the H₂O is adsorbed vertically over a single lattice Mo (Fig. 1(c)). The calculated adsorption energy for P-M₁ is −0.243 eV, which is 0.04 eV more negative than that of P-M₂, suggesting P-M₁ is slightly more stable. This result agrees well with the previous PBE prediction of 0.16 eV.³⁰ Similar to S side, the water adsorption on Se side of MoSSe is about −0.240 eV, which is a little smaller than that of S side. As for MoS₂ sheet, the adsorption of water is smaller than the above two with the value at about −0.219 eV. In all, we can find the adsorption energy of water on these three cases are quite close, where S side of MoSSe sheet has the largest water adsorption energy still a weak vdW interaction. The larger adsorption of water consistent with the shorter H–S bond length. Therefore, in the text, we choose S side of MoSSe as the typical one to investigate the water behavior.

Fig. 1 Optimized structures for single water molecule adsorbed at a (4 × 4) Janus MoSSe sheet. (a) and (c) top views of possible water adsorption configurations, and (b) the corresponding side view of (a) at pristine case. (d)–(f) the top views of possible water adsorption structures at S_vac Janus MoSSe sheet. The primitive cell is denoted in green line. The Mo atom is light violet ball, S atom is yellow ball, Se is light green ball, O atom is red ball, and H atom is light pink ball for water adsorbed Janus MoSSe sheet, hereinafter.

It has been reported that defect introduction is an effectively approach to alter the surface reactivity. Here, intrinsic defects including S vacancy (S_vac), Mo antidefect (Mo_anti), and Mo interstitial (Mo_int) are considered for the Janus MoSSe sheet. It should be noted that only S side is considered, since the property of Se side is similar to that of S side. Firstly, one S_vac defect is introduced in the (4 × 4) supercell. Three typical water adsorption configurations are obtained from the geometry optimization calculations, denoted as S_vac-M_n (n = 1, 2, 3) as shown in Fig. 1. In S_vac-M₁, one H atom of M-H₂O is located at the S_vac, and the other H atom is pointing to the adjacent S atom in Fig. 1(d). In S_vac-M₂, the O atom of M-H₂O resides at the S_vac with the two H atoms pointing outwards. In S_vac-M₃, the M-H₂O is absorbed at a hexagonal hollow site that is away from S_vac. The corresponding calculated adsorption energies for S_vac-M_n are provided in Table 1. It is shown that the M-H₂O at S_vac-M₁ has the most negative adsorption energy of −0.321 eV, which is about 0.08 eV more negative than the M-H₂O adsorption energy for the pristine sheet. This more negative adsorption energy of S_vac-M₁, as compared to P-M₁, is consistent with substantially shorter Mo–H distance of 3.19 Å for S_vac-M₁ (vs. 4.29 Å for P-M₁). To know the difference between MoSSe and MoS₂ sheet, the water adsorbed at S_vac defect MoS₂ sheet is also examined. The result shows that the adsorption energy is about −0.297 eV at S_vac MoS₂, which is a little lower than that at S_vac_MoSSe sheet. Thus, introducing S_vac defect will slightly enhance the water adsorption on the Janus MoSSe sheet.

Table 1 The adsorption energy (E_ad), and the corresponding relaxed geometry parameters of water adsorbed at pristine and S_vac Janus MoSSe sheet

System	M-H2O	Ead	H–S (Å)	H–Mo (Å)	O–H (Å)	∠HOH (degree)
Pristine	P-M1	−0.243	2.661	4.290	0.972	103.414
	P-M2	−0.239	2.645		0.972	103.35
	M-H2O				0.97	104.131
Svac	Svac-M1	−0.321	2.985	3.190	0.971	103.187
	Svac-M2	−0.307	2.879	3.397	0.972	103.262
	Svac-M3	−0.230	2.658	4.290	0.972	103.492

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Other than S_vac defect, anti-defect is also considered by replacing one S atom by a Mo atom in the (4 × 4) supercell of the Janus MoSSe sheet. Three kinds of Mo_anti defects are allowed for by the (4 × 4) supercell, and the corresponding relaxed structures are denoted as Mo_anti-n (n = 1, 2, 3) as shown in Fig. 2. In Mo_anti-1, the Mo substitute resides at the exact position of the replaced S and forms three equal Mo–Mo bonds (Fig. 2(a)). In Mo_anti-2, the Mo substitute is slightly displaced towards the hollow center, forming two Mo–S and two Mo–Mo bonds (Fig. 2(b)). In Mo_anti-3 the Mo substitute is slightly displaced towards the adjacent lattice Mo, forming two Mo–S bonds and a Mo–Mo bond in Fig. 2(c). Among these three configurations, Mo_anti-2 defect is found to be more stable, as suggested by the predicted relative energies (Table 2).

Fig. 2 (a)–(c) The possible Mo_anti defect at Janus MoSSe sheet. (d)–(f) The water molecule adsorption at Mo_anti-2 structure. The Mo_anti atom is in green ball.

Table 2 The relative energy of Mo_anti or Mo_int defect at possible site in Janus MoSSe sheet. The corresponding relaxed geometry parameters are also provided. It should be mentioned that the relative lower total energy is set to zero point

Defect type	Localized sites	Relative energy	Mo–Mo (Å)	Mo–S (Å)
Moanti	Moanti-1	0.450	2.522	3.115
	Moanti-2	0	2.430	2.443
	Moanti-3	0.366	2.439	2.457
Moint	Moint-1	0.127	2.881	2.194
	Moint-2	0	2.443	2.430

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The adsorption of molecular water at the Mo_anti-2 site is investigated. Three adsorption configurations are resulting from the geometry optimization calculations, denoted as Mo_anti-2-M_n (n = 1, 2, 3), as shown in Fig. 2. In Mo_anti-2-M₁, the M-H₂O is tiltedly adsorbed at Mo_anti atom site with H atoms pointing to the two nearest S atom. In Mo_anti-2-M₂, the M-H₂O resides right atop the Mo_anti atom with H atoms pointing to two nearest S atoms. Mo_anti-2-M₃ is structurally close to Mo_anti-2-M₁ except for the pointing directions of the H atoms. The corresponding adsorption energy and relaxed geometry parameters for the three configurations are shown in Table 3. It can be found that the adsorption energies of M-H₂O at all three modes are more negative than −1.0 eV. Among the three configurations, Mo_anti-2-M₁ has the most negative M-H₂O adsorption energy (−1.42 eV), together with the shortest Mo–O distance of ∼2.2 Å. Relative to M-H₂O at pristine sheet, it is unexpected to find that the adsorption energy is dramatically increased by −1.18 eV. Furthermore, the difference between MoSSe and MoS₂ sheet are checked. Compared with Mo_anti MoSSe, the water adsorption at Mo_anti MoS₂ is about 0.036 eV larger than that at MoSSe sheet, suggesting Mo_anti defect at MoSSe has larger effect on water adsorption. Therefore, Mo_anti defect can strongly alter the adsorption behavior of water at Janus MoSSe sheet, which would be a potential way to promote the photocatalytic water splitting.

Table 3 The calculated adsorption energy (E_ad), and the corresponding relaxed geometry parameters of water molecule adsorbed at Mo_anti or Mo_int defective Janus MoSSe sheet

System	M-H2O	Ead	H–S (Å)	O–Mo (Å)	O–H (Å)	∠HOH (°)
Moanti-2	Moanti-M1	−1.415	2.324	2.199	0.937	102.650
	Moanti-M2	−1.061	2.905	2.228	0.978	106.704
	Moanti-M3	−1.203	2.610	2.228	0.981	104.163
Moint-2	Moint-M	−1.202	2.418	2.295	0.975	103.501

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Interstitial defect is another common intrinsic defect type for transition metal dichalcogenides. The models for Janus MoSSe with Mo_int defect are constructed by adding one Mo atom to the (4 × 4) supercell. Two types of Mo_int defects are obtained from the geometry optimization calculations, denoted as Mo_int-1 and Mo_int-2, as shown in Fig. 3. For Mo_int-1, the Mo_int atom is located at hollow site forming three Mo–S bonds. In Mo_int-2, the Mo_int atom is located right atop the lattice Mo atom forming three Mo–S bonds. Although the Mo_int atom at Mo_int-2 seems repulsively with lattice Mo atom, the calculated relative energy shows Mo_int-2 is about 0.13 eV lower than that of Mo_int-1, suggesting Mo_int-2 is more stable.

Fig. 3 (a) and (b) The top view of possible Mo_int defect at Janus MoSSe sheet. (c) The adsorption of molecular water at Mo_int MoSSe sheet. The Mo_int atom is light orange ball.

Based on the comparatively more stable Mo_int-2 structure, the behavior of M-H₂O adsorption at Mo_int defective MoSSe sheet is investigated, and the corresponding calculated result is displayed in Table 3. Only one type of M-H₂O adsorption configuration is found, denoted as Mo_int-M (Fig. 3). The corresponding M-H₂O adsorption energy for Mo_int defective sheet is about −1.2 eV, comparable to that for the Mo_anti defective sheet. Furthermore, the difference between MoSSe and MoS₂ sheet are checked. Compared with Mo_int MoSSe, the water adsorption at Mo_int MoS₂ is close to that at MoSSe sheet, suggesting Mo_anti defect at both MoSSe and MoS₂ sheet has identical effect on water adsorption. Thus, Mo_int defect can also effectively enhance water adsorption at defective MoSSe sheet. In all, the S_vac defect can slightly increase the water adsorption, while Mo_anti and Mo_int can significantly enhance the interaction between water molecule and substrate.

The adsorption behavior of dissociated water at defective Janus MoSSe sheet

Above result shows the intrinsic defects have a prominent effect on the adsorption of water molecule at Janus MoSSe sheet. In order to know whether these defects could affect the water splitting, the adsorption behavior of dissociated water (D-H₂O) is also explored. According to the possible atomic site, two types of D-H₂O modes are existing for each defective MoSSe sheet as shown in Fig. 4. As for S_vac defective sheet, the relaxed configuration of D-H₂O can be denoted as S_vac-D_n (n = 1, 2) for short. For example, the H atom of D-H₂O in S_vac-D₁ is forming H–S next to S_vac defect, while the OH is located at the site of S_vac forming three Mo–O bonds. In S_vac-D₁, it forms two Mo–O bonds for S_vac-D₂. The case for Mo_anti defect is quite different because of the unsaturated bonds of lattice Mo atom near Mo_anti defect. Thus, it can form two different configurations of D-H₂O with the H linked to lattice S atom in Mo_anti-D₁ or to lattice Mo atom in Mo_anti-D₂. As for D-H₂O at Mo_int defect sheet, the H of D-H₂O is linked to S atom next to Mo_int atom in Mo_int-D₁, while both the OH and H atoms are directly absorbed to Mo_int atom in Mo_int-D₂.

Fig. 4 The relaxed structures of dissociated water at defective Janus MoSSe sheet. The top view of dissociated water at S_vac (a) and (b), Mo_anti (c), (b), and Mo_int (e), (f) defective Janus MoSSe sheet.

The calculated adsorption energy, energy difference, and the corresponding relaxed geometry parameters are displayed in Table 4. Surprisingly, it can be found that the adsorption energy of D-H₂O at S_vac Janus MoSSe sheet is positive, where S_vac-D₃ has a relative lower adsorption energy at about 0.658 eV. In order to check whether the M-H₂O favors to form D-H₂O, the energy difference between M-H₂O and D-H₂O is provided. The result shows that the energy difference is 0.985 eV for water at S_vac defect sheet, showing M-H₂O is more stable. Thus, the water molecule may be difficult to split at S_vac Janus MoSSe sheet. As for water on Mo_anti defect sheet, the adsorption energies for D-H₂O are negative, with Mo_anti-D₂ lower adsorption energy at about −1.625 eV. Moreover, it is interesting to find that the energy difference is a negative value of −0.21 eV, indicating D-H₂O is more stable. Thus, the molecular water may be favored to dissociate at Mo_anti defect sheet. Similar to the case at Mo_anti defect sheet, the adsorption energy for D-H₂O at Mo_int defective Janus MoSSe sheet are all negative. And the energy difference is about −0.993 eV, which is larger than the case at Mo_anti sheet at about −0.21 eV. The lower adsorption energy of D-H₂O is related with shorter Mo–O bond length. For example, the bond length of Mo–O at S_vac-D₁ sheet is changing from 2.117 Å to 1.91 Å for Mo_anti-D₂, even shorten to 1.892 Å for Mo_int-D₂. Therefore, the Mo_anti and Mo_int defects can greatly enhance the adsorption energy of D-H₂O, which may promote the process of photocatalytic water splitting.

Table 4 The calculated adsorption energy (E_ad), and the corresponding relaxed geometry parameters of D-H₂O at defective Janus MoSSe sheet. The energy difference (E_dif) is the total energy difference between M-H₂O and D-H₂O, and the negative value means D-H₂O is more stable

System	D-H2O	Ead	Edif	H–S	O–Mo	O–H
Svac	Svac-D1	0.664	0.985	1.359	2.177	0.983
	Svac-D2	0.896	1.217	1.36	2.201	1.003
Moanti-2	Moanti-D1	−0.800	0.615	1.362	1.914	0.978
	Moanti-D2	−1.625	−0.209	1.778 (H–Mo)	1.916	0.977
Moint-2	Moint-D1	−1.414	−0.211	1.389	1.901	0.971
	Moint-D2	−2.195	−0.993	1.679 (H–Mo)	1.892	0.972

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Discussion

The above result shows that the intrinsic defects can dramatically enhance the interaction between water and Janus MoSSe sheet. To check the role of these defects, the structural and electronic property of intrinsic defects are discussed. First of all, the structural parameter of the defective structure is examined, since introducing defect will induce local structure distortion. It can be found that the bond length of Mo–S around the defect will be strongly shrunk, where it is shorten by 0.02 Å for S_vac and 0.17 Å for Mo_int defect sheet. Furthermore, these defects will introduce unsaturated Mo atom at the Janus MoSSe sheet, which will enhance the reactivity of sheet.

Except for the local structure distortion, the electronic structures of the defective structure are also examined since defects provide unique electronic property, which are crucial to the reactivity of surface. Here, the projected partial electronic density of state (PDOS) for defective Janus MoSSe sheet adsorbed with molecular water are calculated together with pristine case for comparison, and the corresponding calculated result is shown in Fig. 5. It should be noted that only the electronic density of states for Mo, S, O, and H atoms are provided, since the water is adsorbed at S side. For pristine sheet, we can find that the PDOS of adsorbed water remains in three intact peaks similar to single water molecule. This strongly suggests that the adsorbed water is hardly interact with pristine Janus MoSSe sheet, corresponding to weak adsorption energy of −0.16 eV.³⁰ As a S_vac defect exists in the structure, the three peaks are almost preserved, but the location of the peaks have a little red shift toward lower energy level, suggesting water adsorption is more favored corresponding larger adsorption energy of −0.23 eV. The case for Mo_anti and Mo_int defects are much different. First of all, it is unexpected to find that the three peaks disappear, and the PDOS of adsorbed water disperses into a large area, suggesting a strong interaction between O2p orbital of water with Mo3d orbitals of Mo_anti or Mo_int defect. Furthermore, the PODS of the adsorbed water is dispersed in the arrange from −6.6 eV to −5.0 eV for Mo_anti defect, and −6.0 eV to −5.2 eV for Mo_int defect sheet. Thus, a dramatic red shift of the PDOS for the adsorbed water is relative to the water adsorbed at pristine Janus MoSSe sheet. This results in larger adsorption energy of water at Mo_anti or Mo_int defect Janus MoSSe sheet with the value of −1.4 eV or 1.2 eV. Therefore, the origin of enhanced adsorption energy of water at defective S side of sheet should be the orbitals interaction between O2p of water and Mo3d of defects, together with the red shift of water energy level.

Fig. 5 The projected partial electronic density of states (PDOS) of the M-H₂O adsorbed at defective MoSSe sheet. The PDOS of H₂O adsorbed at pristine Janus MoSSe sheet is also provided in (a) for comparison. (b)–(d) represent the PDOS of M-H₂ adsorbed at S_vac, Mo_anti, and Mo_int defective MoSSe sheet, respectively. The PDOS of Mo, S, O, and H atoms are in red (short dash), green (short dash), blue, and orange lines. The black dot line denotes the VBM, which is setting at zero eV.

Conversion of water at defective Janus MoSSe sheet

As mentioned in introduction, Janus MoSSe sheet is a promising photocatalyst for water splitting, however it is limited by the weak water adsorption. Fortunately, the above result shows that S_vac, Mo_anti and Mo_int intrinsic defects can greatly enhance the adsorption of water at Janus MoSSe sheet. Thus, it is of interest to know the effect of intrinsic defect on water conversion. Generally, it involves both the most stable M-H₂O state and D-H₂O state during the water splitting process, which can be expressed as:

H₂O → H⁺ + OH⁻ (2)

This reaction process involves two steps with the water molecule adsorbed at the Janus MoSSe sheet; consecutively, one H of M-H₂O transfers to lattice S or Mo atom to form H–S or Mo–H bond, leaving OH bond at the sheet.

The calculated energy barrier of water conversion to H and OH at defective Janus MoSSe sheet are shown in Fig. 6. It should be mentioned that D-H₂O is unable to form at pristine Janus MoSSe sheet. Thus, we only consider the water conversion at the defective sheet. The result shows that it requires 1.31 eV energy to overcome the transition state for water splitting at S_vac defect sheet. During this process, the H atom of water towards to the adjacent lattice S atom through a transition state to form an H–S bond and an O–H bond as shown in Fig. 7(a). Additionally, the final state of D-H₂O is a little lower than that of transition state, but 1.21 eV larger than initial state of M-H₂O. Thus, water may be difficult to split at S_vac defect Janus MoSSe sheet.

Fig. 6 Illustration of reaction pathway via M-H₂O to form H and OH bond. The sum of energies of the water and substrate is the zero reference for energy. The transition state is denoted as TS for short. The relative energy for water splitting at S_vac, Mo_anti, and Mo_int defect sheet are in red, blue, and green lines, respectively.

Fig. 7 Selected configurations along the conversion pathway of water at defective Janus MoSSe sheet. The detailed defects are displayed in (a) S_vac, (b) Mo_anti, and (c) Mo_int defective Janus MoSSe sheet.

Compared to S_vac defect, the behavior of other defects such as Mo_anti and Mo_int defects appears completely different. For example, for Mo_anti defect, one H atom of water links to unsaturated lattice Mo atom through a transition state as shown in Fig. 7(b). The corresponding energy barrier for the water splitting is about 0.71 eV, which is dramatically lower than that at S_vac sheet. Furthermore, it is interesting to find that the relative energy of the final state is about 0.21 eV lower than the initial state. As for Mo_int defect, it has larger impact than S_vac and Mo_anti defects on water splitting. The H atom of water towards to Mo_int atom forming Mo–H bond as shown in Fig. 7(c). The calculated result shows that it needs 0.7 eV to overcome the transition state, close to the case at Mo_anti sheet. Interestingly, the final state of D-H₂O is about −0.99 eV, extremely lower than the initial state. Therefore, the Mo_anti and Mo_int defects have greatly impact on decreasing the energy barrier of water splitting, indicating introducing defect should be a promising approach to modify the reactivity of low dimensional materials.

Conclusion

In summary, the first-principles calculations are carried out to explore the effect of intrinsic defects on water adsorption and conversion at Janus MoSSe sheet. The result shows that relative to pristine case, the adsorption energy of M-H₂O increases to −0.32 eV under S_vac defect, while D-H₂O is difficult to form. Unlike S_vac defect, the effect of Mo_anti and Mo_int defect is much stronger than S_vac defect on water behavior at Janus MoSSe sheet. The water adsorption energy at both Mo_anti and Mo_int sheet can be increased by −1.0 eV. More importantly, the energy barrier for water splitting can be lowered by 40% to 0.7 eV at Mo_anti or Mo_int defects sheet, and the final state of D-H₂O is stable than the initial state. This significant enhancement of the adsorption energy originates from the strong interaction between O2p orbital and Mo3d orbitals. These results presented here showed that the intrinsic defect has the potential to change the property of materials, thus it is an important way to design photocatalyst for water splitting.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Science Challenge Project (TZ2018004) and the National Natural Science Foundation of China (No. 51572016, U1930402, 11804090, 11847213, 11774298, and 21503012). It is supported by the Scientific Research Fund of Hunan provincial Education Department, China (Grant No. 17C0626, and 2019JJ50148), and Scientific Research Fund of University (No. E517558). This research is supported by a Tianhe-2JK computing time award at the Beijing Computational Science Research Center (CSRC) and the Special Program for Applied Research on Super Computation of the NSFC Guangdong Joint Fund (the second phase) under Grant No. U1501501.

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