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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Triggering Catalytic Active Sites for Hydrogen Evolution Reaction by Intrinsic Defects in Janus Monolayer MoSSe Wenwu Shi, Guoqing Li, and Zhiguo Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01485 • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 25, 2019

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The Journal of Physical Chemistry

Triggering Catalytic Active Sites for Hydrogen Evolution Reaction by Intrinsic Defects in Janus Monolayer MoSSe Wenwu Shi*ab, Guoqing Lib and Zhiguo Wang*a a School of Electronics Science and Engineering, Center for Public Security Technology, University of Electronic Science and Technology of China, Chengdu, 610054, P.R. China b Department of Materials Science and Engineering, North Carolina State University, Raleigh, 27606, United States *Correspondence authors. E-mail: [email protected] (W. Shi); [email protected] (Z. Wang)

Abstract: Janus transition metal dichalcogenides have been predicted to be promising candidates for hydrogen evolution reaction (HER) due to their inherent structural asymmetry. While the effect of intrinsic defects, including vacancies, antisites, and grain boundaries, on their catalytic activity is still unknown. MoSSe provides an ideal platform for studying such defects, since theoretical calculation indicated that the formation energies of point defects and grain boundaries on MoSSe were lower than pristine MoS2 monolayer. In this work, density functional theory is utilized to study all the possible intrinsic defects on MoSSe monolayer for HER. MoSSe monolayer with 4|4, 4|8a, 5|7b, 8|10a GBs, vacancies (VS, VSe, VSSe, VMo, VMoS3) and anti-site defects (MoSSe, SeMo, SMo) shows enhanced HER performance. The adsorption behavior of hydrogen on defects were explained by using a “states-filling” model. The adsorption energy of hydrogen on the catalysis changes linearly with the work required to fill unoccupied electronic states within the catalysts. This work could provide a more comprehensive understanding of all the possible active sites of Janus transition metal dichalcogenides for HER.

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Introduction

Hydrogen, as a clean, sustainable and efficient energy resource, has been attracting huge interests. Water splitting is considered as a promising strategy for the hydrogen production since almost no impact on environmental would be caused during the process. While the key challenge of water splitting process is the development of low cost and efficient catalysts

1-3.

So far, platinum (Pt) and other Pt-group metals are known to the best catalyst for the hydrogen evolution reaction (HER), yet the commercialization of Pt-based catalyst was limited by its scarcity and the high cost 4. Two dimensional layered materials could be ideal alternative catalysts to replace Pt for HER, especially molybdenum transition metal dichalcogenides (TMDC), based on theoretical calculation

5-7.

However, the efficiency of

TMDC materials for HER is still inferior to Pt possibly due to the limited density of active sites sites

3, 8-9.

Although, extensive research work has been done to improve the density of active

10-12,

developing new strategies to increase catalytic activity of molybdenum

disulfide-based materials is still beneficial and necessary. Recently, Janus TMDC was predicted to show better catalytic activity comparing with pure TMDC materials due to their inherent structural asymmetry

13-17.

Density functional

theory (DFT) study showed the basal plane of Janus TMDC materials can be activated without applying large tensile strains and in the absence of significant density of vacancies 13. For pure TMDC materials, defects, including point defects (PDs) and line defect (grain boundaries, GBs), are known to be able to boost the catalytic activity

3, 18-22.

While the

question that if defects will have the similar effect on the Janus TMDC materials still needed be answered. The study of the electrochemical catalytic activity of all possible defects on

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Janus TMDC thin films is highly desired. MoSSe is a good represent for Janus TMDC materials and it can provide an ideal platform for studying such defects, since theoretical calculation indicated that the formation energies of PDs and GBs on MoSSe were lower than pristine MoS2 monolayer

16, 23-24.

The

GBs are often composed of 4-, 5-, 7-, 8- and 10-membered rings, which have been predicted by theoretical calculation

25

and identified experimentally

26-28.

Twelve kinds of PDs were

possible in Janus monolayer MoSSe, which are mono-sulfur vacancy (VS), mono-selenium vacancy (VSe), sulfur-selenium vacancy (VSSe), mono-molybdenum vacancy (VMo), vacancy complex of Mo and nearby three S atoms (VMoS3), vacancy complex of Mo and nearby three Se atoms (VMoSe3), vacancy complex of Mo with three nearby S-Se pairs (VMoS3Se3); antisite defects including one Mo atom substituting a S atom (MoS), one Mo atom substituting Se atom (MoSe), one Mo atom substituting one S-Se pair (MoSSe), a S atom substituting Mo atom (SMo) and a Se atom substituting Mo atom (SeMo). The excavation of the catalytic activity of all these possible defects can provide a comprehensive understanding of all the active sites for Janus TMDC materials. In this paper, the catalytic behavior the intrinsic defects in the Janus MoSSe monolayer were investigated by using DFT calculations. MoSSe monolayer with 4|4, 4|8a, 5|7b, 8|10a GBs, vacancies (VS, VSe, VSSe, VMo, VMoS3) and antisite defects (MoSSe, SeMo, SMo), which indicates that these defects can trigger basal plane catalytic activity for HER in Janus MoSSe monolayer. This study can serve as a guide for experimental research to designing better Janus TMDC catalysts for HER.

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Computational details All the DFT calculations were performed using norm-conserving pseudopotentials and the

Ceperley-Alder (CA) exchange-correlation functional 30.

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as implemented in SIESTA package

Spin polarization was considered. The valence electron wave functions were expanded

using a double-ζ basis set plus polarization functions

30.

All atomic positions and lattice

constants were freely relaxed using the conjugate gradient (CG) approximation until the force on each atom was less than 0.02 eV/Å. The Monkhorst-Pack k-point mesh scheme was used for the integration of Brillouin zone. To avoid the periodical interaction, a thickness of 30 Å vacuum space was added in the direction perpendicular to the monolayer. The formation energy of the GBs (𝐸f_GBs) and PDs (𝐸f_PDs) in Janus MoSSe were calculated using equations (1) and (2), respectively: 𝐸f_GBs = (𝐸GBs ― 𝐸perf)/2𝐿

(1)

𝐸f_PDs = (𝐸PDs ― 𝐸perf) + ∑𝑖𝑛𝑖𝜇𝑖

(2)

where 𝐸GBs(𝐸PDs) and 𝐸perf are the total energies of Janus MoSSe with and without GBs (PDs), respectively, L is the periodic length of the GBs. 𝜇𝑖 is the chemical potential of species i and ni is the number of exchanged species i that have been added to (ni0) the supercell. The adsorption energy ( Eads ) of a H atom on Janus MoSSe was calculated using equations (3): 1

∆𝐸ads = 𝐸H ― sub ― 𝐸sub ― 2𝐸H2 ― 𝐸BSSE

(3)

where 𝐸H ― sub and 𝐸sub are the total energy of Janus MoSSe monolayer with and without H atom adsorption, 𝐸H2 is the total energy of molecular hydrogen in the gas phase, and 𝐸BSSE is basis set superposition error (BSSE) 31, which is an overestimation of adsorption energy due 4


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to the unequal basis sets between the interacting bonded system and non-interacting separated systems. 𝐸BSSE was calculated using equation (4) by using the atomic counterpoise method with “ghost” atoms 32-33: 𝐸BSSE = (𝐸ghost ― H + 𝐸ghost ― sub) ― (𝐸noghost ― H + 𝐸noghost ― sub)

(4)

where 𝐸ghost ― H and 𝐸ghost ― sub are the energy of the H and MoSSe monolayer with replacement by ghost atoms, and 𝐸noghost ― H and 𝐸noghost ― sub are the energy of the system with only H and M atoms present. Then adsorption Gibbs free energy (∆𝐺H) for H on Janus MoSSe can be calculated by using equation (5) 10, 18, 34-35: ∆𝐺H = ∆𝐸ads +∆𝐸ZPE ―𝑇∆𝑆H

(5)

where ∆𝐸ZPE is the difference in zero-point energy of hydrogen between the adsorbed state and the gas phase. The zero-point energy can obtain from the phonon calculation. ∆𝑆H is the difference in entropy of hydrogen between the adsorbed state and the gas phase, which can be 1

written as ∆𝑆H = 2𝑆H2, where 𝑆H2 is the entropy of molecule hydrogen in the gas phase at a temperature of 300 K and pressure of 1 bar with 0.41 eV 36-38.

3.

Results and discussions In this work, 9 kinds of GBs including 4-4 rings (4|4), 4-8 rings (4|8a, 4|8b, 4|8c), 5-7

rings (5|7a, 5|7b, 5|7c), 8-10 rings (8|10a, 8|10b) in the Janus monolayer MoSSe were investigated, the optimized atomic structures are shown in Fig. 1a for 8|10a GBs and Fig. S1 for other GBs. The rings are marked with different colours. The calculated formation energies of the GBs (𝐸f_GBs) in Janus monolayer MoSSe are listed in Table 1. As periodic conditions were used in the simulation, so a pair of GBs are included in the supercells. The pair of GBs were separated with a distance (d). The distances between the GBs pairs are about 22.53, 5



25.79, 20.92, 23.67, 15.77, 17.68 and 20.98 Å for GBs composed of 4|8a, 4|8b, 4|8c, 4|4, 5|7a, 5|7b, 5|7c, 8|10a and 8|10b -membered ring, respectively. The calculations show that 𝐸f_GBs values are in the range between 0.37 and 1.34 eV/ Å. Whereas 𝐸f_GBs for these kind of GBs in MoS2 monolayer is in the rang between 0.55 and 2.32 eV 23. The formation energies for the GBs composed of 8|10a and 8|10b-membered rings are much larger than that for other ones, which are 1.34 and 1.10 eV/Å, respectively. However, they are smaller than that for the same type of GBs in MoS2 monolayers calculated with DFT calculation (2.32 eV/Å) and empirical potential method (2.45 to 2.55 eV/Å)

39.

The large formation energies of 8|10 GBs maybe

caused by the large bond rotation (60°) of metal-chalcogen bonds along with the formation S vacancies 40. The formation energies of 5|7 GBs in MoS2 monolayer are in the range between 0.483 to 1.004 eV/Å 41, the largest one is close to that of 8|10 GBs in the MoSSe monolayer. Given the 8|10 GBs and 5|7 GBs has been observed in MoS2 monolayer experimentally 40, the 8|10a and 8|10b GBs could exists in Janus monolayer MoSSe with reasonable stability. After comparing the values of 𝐸f_GBs, it was found that GBs in Janus MoSSe have lower formation energies than in MoS2 monolayer, such as the 𝐸f_GBs for 5|7b GBs are 0.73 and 1.004 eV in MoS2 monolayer 24, 41, which are 0.12 and 0.39 eV larger than 5|7b GBs in Janus MoSSe. This indicates GBs could be presents in the basal plane of Janus MoSSe monolayer like those GBs exist in MoS2 monolayer. 12 kinds of PDs were investigated in Janus MoSSe monolayer, the optimized atomic structure of VS is shown in Fig. 1b and others are shown in Fig. S2. The PDs were calculated by using 5×5×1 supercells with 5×5×1 k points. The calculated formation energies are listed in table 1, with the values of 𝐸f_PDs are in the range between 0.04 and 8.72 eV under the

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Mo-rich conditions and between 2.57 to 17.44 eV under the S-rich conditions. According to previous reported results of PDs in MoS2 monolayer, the formation energy of VS is 5.5 eV 42, VS, VMo and VMoS3 are 1.6, 4.6 and 6.2 eV 43, respectively. All of these formation energies are larger than the formation energies of 0.86, 4.02 and 4.25 eV for VS, VMo and VMoS3 in Janus MoSSe, respectively, which indicates that these PDs defects could be easily introduced into the Janus MoSSe. Some point defects have large formation energies, such as the formation energies are 8.50 (3.02), 17.44 (5.25), 7.83 (2.35), 4.02 (7.02), 9.74 (4.25) and 6.95 (1.68) eV for MoS, VMoS3Se3, MoSe, VMo, VMoS3 and VMoSe3 under the S (Mo) rich condition. To evaluated the dynamic stability of these PDs, the phonon dispersion of Janus MoSSe monolayers with these PDs were calculated using VIBRA package in the SIESTA

30

with frozen phonon method. Fig. 2a and 2b show the phonon dispersion of the Janus MoSSe monolayer with MoS and VMoS3Se3 PDs, which have large defect formation energies with 8.50 (3.02) eV and 17.44 (5.25) eV under the S (Mo) rich conditions, respectively. Fig. 2b and 2d indicate the enlarged part of phonon dispersions at the low frequencies region. There is no imaginary frequency presents in the phonon dispersion spectrum, indicating that the MoS and VMoS3Se3 PDs are dynamically stable in Janus MoSSe monolayer. The phonon dispersions of other PDs (MoSe, VMo, VMoS3 and VMoSe3) are showed in the Fig. S3, except for VMo and VMoSe3 PDs with the negligible imaginary frequency (~10 cm-1), the phonon dispersions of the MoSe and VMoS3 PDs show no imaginary frequency, which indicates that these PDs with large formation energies are dynamically stable in the Janus MoSSe monolayer. The catalytic performance for HER of these intrinsic defects in the Janus MoSSe

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monolayer is further studied. The Volmer reaction is the key step of HER, in which H atom is adsorbed on the catalytic site, the binding strength determinate the kinetics of H atom to form hydrogen molecule and release 44-45. All the possible adsorption positions for a single H atom on the Janus MoSSe with defects have been considered. The most stable adsorption configurations of H on Janus MoSSe with GBs and PDs are shown in Fig. 3 and Fig. 4, respectively. As shown in Fig. 3, the H atom prefers to be adsorbed on the S atom of MoSSe with 4|4, 4|8b, 8|10b GBs, whereas the H atom prefers to be adsorbed on the bridge site of Mo-Mo in MoSSe with 4|8a, 4|8c, 8|10a, 5|7a, 5|7b, 5|7c GBs. As shown in Fig. 4, the H atom prefers to locate at the vacancy site for MoSSe with VS, VSe and VMo PDs. The H atom are bonded to the S atom for MoSSe with SMo and SeMo PDs, and to the bridge site of Mo-Mo in MoSSe with VMoS3, VMoSe3, VSSe, VMoS3Se3 and MoSSe PDs, and to the Mo atoms for MoSSe with MoS and MoSe PDs. The energy preferable adsorption configurations and the corresponding adsorption Gibbs energies for H absorbed at the S-side and Se-side on the Janus MoSSe are shown in Figs. S4 and S5, respectively. The results show that H atom prefers to be adsorbed at the S-side whether the MoSSe with or without defects. The free energy diagrams for HER of Janus monolayer MoSSe with various GBs and PDs are shown in Fig. 5. The large positive value of adsorption Gibbs free energy with 1.99 eV for the pristine MoSSe indicate that the basal plane of pristine Janus MoSSe is catalytic inert for HER, which agrees with the previous report that the basal plane of MoSSe monolayer is catalytic inactive for HER

46.

The presence of GBs and PDs in monolayer

MoSSe can affect the adsorption behaviour of H on MoSSe, so these defects can be used to tune the catalytic behaviour of the basal plane of Janus MoSSe, Fig. 5a and 5b show the free

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energy of a H atom adsorption at Mo and S/Se atoms, respectively. ∆𝐺H are positive values with 1.06, 0.23 and 1.02 eV for H atom adsorbed on monolayer MoSSe with 8|10b, 4|8b GBs and VMoSe3 PDs, respectively, which indicates that the interactions between the H and these defects are still too weak to form intermediate state (H*). ∆𝐺H are large negative values for H atom adsorbed on MoSSe with 5|7a, 5|7c, 4|8c GBs and MoSSe, VMoS3Se3, SMo, MoS PDs, which indicates that the interaction between H and these defects is too strong to ensure a facile bond breakage to form H2 and release. ∆𝐺H is close to zero for H atom adsorbed on monolayer MoSSe with 4|4, 4|8a, 5|7b, 8|10a GBs and VS, VSe, VSSe, VMo, SeMo, SMo, VMoS3, MoSSe PDs, the corresponding values are -0.02, -0.15, -0.26, -0.25, -0.19, 0.10, -0.13, -0.27, -0.02, -0.25, -0.17 and 0.09 eV, respectively, which indicates that these sites are catalytic active for HER. The H adsorption process normally accompanies electron transfer between H atom and the catalyst. According to the equation (5), the ∆𝐺H largely depends on ∆𝐸ads, therefore, the strength of chemical bond energy of H to the catalyst plays an important role in the catalytic performance for HER. The strength of H adsorption on the MoSSe surface can be explained by using the “states-filling” model

47

where the ∆𝐸ads depends on the work required to fill

previously unoccupied electronic states of the Janus MoSSe. The easier the electrons can be filled, the stronger the binding is 48. The work required to fill the unoccupied electronic states of MoSSe monolayer can be described by equation (6) 47, and all the energies are referenced to the vacuum energy level: 𝜀′

𝑊filling = ∫𝜀

𝜀𝐷(𝜀)

LUS

𝑁H

𝑑𝜀

(6)

where ε is the Kohn-Sham (KS) energy, D(ε) is the density of states (DOS) of the Janus

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MoSSe, and ε' satisfies the charge-conservation criterion: 𝜀′

∫𝜀

𝐷(𝜀)

LUS

𝑁H

𝑑𝜀 = 1

(7)

where LUS is the lowest unoccupied state, 𝜀LUS is defined as the Fermi level and 𝜀CBM for a metal and nonmetal, respectively. The equations (6) and (7) resembles an integral formulation of the DOS of catalyst. So the electronic structure will significantly affect the value of 𝑊filling, the strength of H atom adsorption on the MoSSe will be enhanced as the 𝑊filling is minimized. Due to the adsorption of H on the defective structure, atomic reconstruction occurs, the reconstruction energy (ER) of these defect models cannot be ignored, in order to apply the “states-filling” model in the calculations; the chemical adsorption energy is calculated as: 1

∆𝐸1 = 𝐸H ― sub ― 𝐸sub ― 2𝐸H2 ― 𝐸BSSE ― 𝐸R

(8)

where the 𝐸R is the difference of total energy for monolayer MoSSe before and after H adsorption, which is listed in Table 1. Since the H prefers to be bonded to Mo or S/Se atoms, which depends on the defective type. The evolutions of ∆𝐸1 as a function of 𝑊filling are shown in Figs. 6a and 6b for H bonded to Mo or S/Se atoms, respectively. It can be seen from Fig. 6, ∆𝐸1 shows a linear dependence on the 𝑊filling. Large values of 𝑊filling indicates that electrons must overcome high barrier energy to fill the unoccupied states of MoSSe monolayer. The present results show that the site is catalytic active for HER as H bonded to Mo atom with 𝑊filling in the range between -3.62 and -3.24 eV, and H bonded to S/Se atoms with 𝑊filling in the range between -3.96 and -3.63 eV. The states-filling model can also be used to describe the adsorption behavior of atom on substrates, such as lithium 47 and sodium 49

on the graphene with PDs, potassium atom on the 2D phosphorene 50, and hydrogen atom

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on MoS2 4. This model described the physics of charge-transfer dominated binding since the state occupation depends on the charge transfer and determinates the bonding strength, which provides general guideline for designing effective catalysis for HER. The electronic structures of Janus MoSSe were further analysis to reveal the deep mechanism of enhanced catalytic activity for HER by introducing defects. The projected density of states (PDOS) of defective MoSSe with and without H adsorption are shown in Figs. 7, S6 and S7. The pristine Janus MoSSe shows a direct band gap with 1.55 eV with both the conduction band minimum (CBM) and valence band maximum (VBM) locate at the K points 16. For the MoSSe with Vs and 8|10a GBs, defective states appear below and above the Fermi energy. The new states mainly consist of Mo 4d orbital. The Fermi energy level shifts up after the H adsorption, which is due to the charge transferred from H atom to MoSSe. In pristine Janus MoSSe, the Fermi energy was shifted up by 0.75 eV after H adsorption, as shown in Fig. 7a. The H 1s orbital overlaps with Mo 4d orbital below the CBM, it indicates an unstable adsorption states. Because the available unoccupied state for H electron occupation is the CBM, which is 1.55 eV above the VBM. It is difficult for H donating its electron to Janus MoSSe by overcoming large barrier of 1.55 eV. Meanwhile, the gap states introduced by formation VS PDs and 8|10a GBs would provide an unoccupied state near the Fermi energy. When the H is adsorbed on the VS and 8|10a, the Fermi energy is shifted up by 0.44 and 0.10 eV in Fig. 7b and Fig. 7c, respectively. For the VS in MoSSe, the gap states are partially filled at the Fermi energy; and for the 8|10a GB in MoSSe, the gap states are fully filled near the Fermi energy. These result an increasing number of states near the Fermi energy. Most importantly, the hybridizations between the H 1s and Mo 4d orbitals are formed

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both below and above Fermi energy, which leads a stable bonding-antibonding adsorption states. Therefore, the MoSSe with defects including PDs and GBs have a better catalytic performance than pristine one. In order to further explore the nature of the good catalytic activity at the vicinity of the defects, we calculated the density of states alignment of Mo 4d orbital. Fig. 8 shows PDOS of pristine MoSSe, MoSSe with VS, and H atom adsorbed MoSSe with and without VS. When the S vacancy is introduced, the gap states near the fermi energy mainly consist of dz2, dxy and dyz states. Those gap states are freed up during the Mo-S bond breaking and lie above the Fermi energy, these empty states can more easily accept the electron from the adsorbed H atom. The dz2 state lies slightly above the VBM, while the two degenerate dxz and dyz both locate above the Fermi energy. When the H adsorbed, the dz2 state shifts up to the Fermi energy and the H 1s electron partially fills this state, thus leading to a strong H binding. Due to the charge distribution difference between the S and Se atoms, the internal electric field will be existed

16.

The effect of internal electric field on MoSSe and strain on the MoS2

possibly are the same for the improving catalytic activity. And this assumption was confirmed in a recent studied

13.

It indicates that strain less Janus MoSSe can reach a highly catalytic

activity for HER by introduced defects into basal plane. It would provide a simply method to achieve highly catalyze performance for HER via defecting the Janus transition metal dichalcogenides.

4.

Conclusion

In conclusion, the intrinsic defects including GBs and PDs in Janus MoSSe monolayer were

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systematically studied using DFT. The formation energies of PDs and GBs in Janus MoSSe were lower than that in the pristine MoS2 monolayer, indicating that these defects could be easily introduced into the Janus monolayer MoSSe. The catalytic performance of these intrinsic defects in Janus MoSSe monolayer for HER was further investigated. The calculated Gibbs free energies were close to zero for hydrogen adsorbed on Janus MoSSe monolayer with 4|4, 4|8a, 5|7b, 8|10a GBs, vacancies (VS, VSe, VSSe VMo, VMoS3) and antisite defects (MoSSe, SeMo, SMo), which indicates that these defects can triggering basal plane catalytic activity for HER in Janus MoSSe monolayer. The adsorption behaviour of hydrogen on the Janus MoSSe monolayer with defects was explained by using a “states-filling” model. The adsorption energy of hydrogen on the catalysis changes linearly with the work required to fill unoccupied electronic states within the catalysts. This work could provide a more comprehensive understanding of all the possible active sites of Janus transition metal dichalcogenides for HER.

Conflicts of interest There are no conflicts to declare.

ASSOCIATED CONTENT: Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: The optimized atomic structures of GBs and PDs, atomic structure of H adsorption on

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MoSSe with GBs and PDs, phonon dispersion of PDs with large formation energy, DOS of the GBs and PDs with and without H adsorption.

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

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3735-3739. 50. Liu, Y.; Xu, F.; Zhang, Z.; Penev, E. S.; Yakobson, B. I., Two-Dimensional Mono-Elemental Semiconductor with Electronically Inactive Defects: The Case of Phosphorus. Nano Lett. 2014, 14, 6782-6.

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Table 1 Distance (d) between the GBs pairs, formation energies of GBs (Ef_GBs) and PDs (Ef_PDs) in S-rich and Mo-rich conditions structure reconstruction energies (ΔER) caused by H atom adsorption for defects in Janus MoSSe monolayer. PDs

Ef_PDs (eV) S-rich Mo-rich

GBs

d (Å) Ef_GBs (eV/Å) ΔER (eV)

4|8a

22.53

0.38

0.01

VS

3.59

0.86

-0.002

4|8b

25.79

0.37

0.01

VSe

2.57

0.04

0.005

4|8c

20.92

0.57

0.02

VSSe

6.08

0.60

0.003

4|4

23.67

0.66

-0.24

VMo

4.02

7.02

0.001

5|7a

15.77

0.51

0.02

VMoS3

9.74

4.25

0.003

5|7b

17.68

0.61

0.02

VMoSe3

6.95

1.68

0.001

5|7c

20.98

0.50

0.12

VMoS3Se3

17.44

5.25

-0.300

8|10a

-

1.34

0.20

MoS

8.50

3.02

-0.645

8|10b

-

1.10

0.25

MoSe

7.83

2.35

0.001

SMo

3.24

8.54

-0.817

SeMo

3.72

8.72

-0.377

MoSSe

13.16

5.75

-0.569

18


ΔER (eV)

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Lists of figures captions: Figure 1 Atomic configurations of Janus MoSSe monolayer with (a) 4|8c GBs and (b) VS. The green, yellow and purple balls represent Se, S and Mo atoms, respectively. The 8- and 10-rings are marked with brown and green colors, respectively. VS is enclosed by a red circle. Figure 2 Phonon dispersions of Janus MoSSe monolayer with (a) MoS and (c) VMoS3Se3 defects along the high symmetry directions of the Brillouin zone. Right volume (b) and (d) indicate the enlarged part of phonon dispersions at the low frequencies region. Figure 3 The energetically preferable adsorption configurations for an H atom absorbed on Janus MoSSe monolayer with 4|4, 4|8a, 4|8b, 4|8c, 5|7a, 5|7b, 5|7c, 8|10a, and 8|10b GBs. The green, blue, yellow and purple balls represent Se, H, S and Mo atoms, respectively. Figure 4 The energetically preferable adsorption configurations for an H atom absorbed on Janus MoSSe monolayer with VS, VSe, VSSe, VMo, VMoS3, VMoSe3, VMoS3Se3, MoSe, MoS, MoSSe, SeMo and SMo point defects. The green, blue, yellow and purple balls represent Se, H, S and Mo atoms, respectively. Figure 5 Gibbs free energies of a hydrogen atom adsorbed at (a) Mo atoms and (b) S/Se atoms. Figure 6 Evolution of chemical adsorption energy (ΔE1) with Wfilling parameters for PDs and GBs. The H atom was absorbed at (a) Mo atoms and (b) S/Se atoms. Figure 7 Density of states of Janus MoSSe monolayer: (a) pristine MoSSe with and without 19



H atom adsorption, (b) MoSSe with VS PDs and H atom adsorbed, (c) MoSSe with 8|10a GBs and H atom adsorbed. The Fermi energy was indicated by dashed red line. Figure 8 Density of states of Mo 4d orbital: (a) pristine MoSSe, (b) MoSSe with VS PDs, (c) after H atom adsorption. The Fermi energy was indicated by dashed red line.

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(a)

(b)

W.W. Shi et al. Figure 1

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Triggering Catalytic Active Sites for Hydrogen ... - ACS Publications

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