Computational Screening of Defective Group IVA Monochalcogenides

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Computational Screening of Defective Group IVA Monochalcogenides as Efficient Catalysts for Hydrogen Evolution Reaction Qian Wu, Wei Wei, Xingshuai Lv, Baibiao Huang, and Ying Dai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02783 • Publication Date (Web): 20 Apr 2019 Downloaded from http://pubs.acs.org on April 20, 2019

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The Journal of Physical Chemistry

Computational Screening of Defective Group IVA Monochalcogenides as Efficient Catalysts for Hydrogen Evolution Reaction Qian Wu, Wei Wei,* Xingshuai Lv, Baibiao Huang, Ying Dai* School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China *

Corresponding authors:

[email protected] (W. Wei) [email protected] (Y. Dai)

ABSTRACT Electrocatalysis has the potential to become a more sustainable approach to generate hydrogen as a clean energy source. So exploring stable, eco-friendly and nonprecious catalysts for hydrogen evolution reaction (HER) is the key for the proposed hydrogen economy. In this work, by means of density functional theory (DFT) calculations, we systematically evaluate the stability, electrical conductivity and HER activity to screen the best catalysts among defective group IVA monochalcogenides MXs (M = Ge, Sn; X = S, Se). Our results reveal that M vacancy can trigger superior catalytic activities compared with the bare MXs basal plane. Especially, SnSe with Sn vacancies and GeSe with Ge vacancies with hydrogen adsorption free energy (∆GH*) ideally being near zero were screen out from the considered MXs. The defective SnSe can exhibit high HER activities at low defect concentrations and present excellent electrical conductivity. These performances are comparable to, or even better than the currently used Pt for the HER. Furthermore, the detailed analysis of strain engineering and binding strength

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schematically unravel the mechanism of boosted hydrogen evolution. Our work introduces defective group IVA monochalcogenides as the promising HER catalysts for future energy applications and hold great promise to be realized experimentally.

Introduction Electrochemical water splitting has been regarded as the most promising strategy to resolve the environmental and energy problems of using fossil fuels for hydrogen production.1−6 On the other hand, catalysts are necessary for water splitting to destroy the strong O–H bonds to speed up the hydrogen evolution reaction (HER). Currently, platinum (Pt) is widely accepted as the most efficient catalyst due to its outstanding performance (near-zero overpotentials, low Tafel slope and high stability) for the HER. 7−9 However, high cost and scarcity hamper the commercial applications as an industry-level electrocatalyst.10 In this context, developing inexpensive and earth-abundant catalysts that perform comparable to and even better than Pt becomes challenging. 2,11−14 Two-dimensional (2D) materials show superior physical and chemical properties, some of them have been demonstrated as the potential candidates to replace Pt for the HER, such as transition metal dichalcogenides (TMDs),15−22 MXenes23−28 and carbon-based nanomaterials.29,30 However, obstacles such as semiconducting feature and inert basal plane still exist in 2D catalysts, therefore, seeking for effective catalysts becomes the key for electrochemical water splitting.17,20,21 Group IVA monochalcogenides MXs (M = Ge, Sn; X = S, Se) monolayers have been successfully synthesized in experiments, 30,31 which have been intensively investigated for solar cells, catalysts and anchors for lithium–sulfur batteries in light of the unique structures and electronic properties. 32−35 Inspired by the

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fact that MXs have been predicted as the good catalysts for oxygen reduction reaction (ORR),36 we are wondering if MXs can be good catalysts for HER. If yes, how to further improve the ability of MXs for HER? To answer the questions mentioned above, in this paper, we investigate the properties of MXs for HER, aiming at searching for effective catalysts to complete the experimental investigations. By computationally screening defective MXs monolayers of different defect concentrations, hydrogen adsorption free energy (ΔGH*) and corresponding catalytic activities for HER are evaluated. Our results demonstrate that creating vacancy is an effective strategy to trigger the HER activities of MXs inert surface, and even with low defect concentration defective MXs could have great HER performance. In particular, SnSe with Sn vacancies and GeSe with Ge vacancies are identified as the most promising catalysts for HER among the considered MXs, with ΔGH* ideally being near zero. Our results indicate that defective SnSe and GeSe could be the new and ideal candidates of HER catalysts, with the activities even better than current Pt.

Computational details All calculations were performed on the basis of density functional theory (DFT) as implemented in the Vienna ab initio Simulation Package (VASP)37,38 with the projector augmented wave (PAW).39,40 In test calculations for GeS monolayer with M and X vacancies, we confirmed that defective GeS is spin non-polarization. In addition, the total energies with and without spin polarization are almost the same, thus we did not consider the spin-polarization in our calculations. The exchange–correlation interactions were described by generalized gradient approximation (GGA)41 with the Perdew–Burke–

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Ernzerhof (PBE) functional.42 To correctly optimize the chemisorption energies, van der Waals (vdW) interactions were taken into account by the empirical correction scheme of Grimme (DFT+D2).43 The cut off energy was set to 500 eV, the convergence criteria for residual force and energy on each atom during structure relaxation were set to 0.01 eV/Å and 10-5 eV, respectively. The vacuum space was set to larger than 20 Å to avoid interactions between periodic images. The Brillouin zone was sampled with the Monkhorst–Pack mesh with the k-point grid of 7×7×1 for 2×2×1 supercell, 3×3×1 for 3×3×1 and 4×4×1 supercells.44 In this study, solvation effect is not taken into account due to the slight influence on the properties of HER.45,46 The hydrogen adsorption Gibbs free energy (ΔGH*)4,47−49 was considered to characterize the HER catalytic activity for MXs, ∆𝐺

∗

= ∆𝐸

∗

+ ∆ZPE − 𝑇∆𝑆

where 𝑇∆𝑆 is the entropy difference between adsorbed H* and H 2 in gas phase at 298.15 K, and ∆𝐸

is the differential hydrogen adsorption energy

∗

∆𝐸 with 𝐸

∗

, 𝐸

and 𝐸

∗

=𝐸

∗

−𝐸

1 − 𝐸 2

representing the total energies of MXs monolayers with

one hydrogen atom adsorbed on the surface, MXs monolayers and H2 molecule, respectively. ∆ZPE is the difference in zero-point energy between adsorbed H* and molecular H2 in gas phase, which could be calculated as50 ∆ZPE = ZPE with ZPE 1

∗

∗

1 − ZPE 2

being the zero-point energy of one adsorbed H* on the catalyst obtained by

2 ℎ𝜈 without the contributions of the catalyst, and ZPE 4


is the zero-point energy of

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the molecular H2 in gas phase. In Table 2, zero-point energy (ZPE) and TS (T = 298.15 K) of defective MXs with M vacancies of different defect concentrations are summarized. In addition, for H2 in gas phase is also shown. In the cases of defective GeS and SnS, ZPE and TS of H* adsorption are not sensitive to catalysts and defect concentrations. Therefore, ∆𝐺

∗

for defective GeS and SnS can be rewritten as ∆𝐺

Similarly, ∆𝐺

∗

∗

= ∆𝐸

∗

+ 0.28 eV

for defective GeSe and SnSe can be rewritten as ∆𝐺

∗

= ∆𝐸

∗

+ 0.26 eV

Results and discussion In previous studies, defects and stain have been demonstrated to play important roles in boosting the HER activities of monolayer TMDs. 52,53 By creating chalcogen vacancies in 2H MoS2 and Janus TMDs, the ΔGH* could be effectively decreased. However, large strain and high defect concentration are indispensable, which are difficult to be realized experimentally. For MXs monolayers, the Gibbs free energy ΔGH* for bare GeS, GeSe, SnS and SnSe are calculated to be 1.831, 1.937, 1.702 and 1.888 eV, respectively, which are far away from the ideal value of zero. It therefore indicates that the basal planes of MXs are inert for HER. As discussed latter, however, point defects could significantly improve the HER activity of MXs. It has been characterized that, for instance, intrinsic Sn vacancies inevitably appear and p-type conductivity can be easily achieved in SnS monolayer. 53,54 In case of single-crystal GeSe nanosheets, experimental studies also indicate that Ge vacancies exist, with the smallest formation energy among all the possible point defects in GeSe. 55 As a function

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of Se chemical potential, the formation energy of Ge vacancy varies within a small energy range, but always positive. It thus means the formation of Ge vacancy is endothermic. In Figure 1a, MXs with metal or non-metal vacancy are shown, where one metal (Ge, Sn) or non-metal (S, Se) atom is removed to simulate the defects. In order to take the effects of defect concentration, MXs supercells of 2×2×1, 3×3×1 and 4×4×1 are adopted. In these cases, defect concentrations correspond to 12.5%, 5.55% and 3.12%, respectively. The defect formation energy 𝐸 is calculated as,56 𝐸 =𝐸 where 𝐸

− (𝑛 𝜇 + 𝑛 𝜇 )

is the total energy of the defective structures, 𝑛

,

and 𝜇

,

are the atom

number and chemical potential, respectively. In conditions of M-rich, chemical potential 𝜇

is calculated as the energy of one M atom in bulk diamond structure, while for X-rich

environments chemical potential 𝜇

is referred to the energy of one X atom from

molecular crystal (𝑅 3 phase) Se6 and S8.56,57 Figure 1b shows the formation energy of defective MXs with different defect concentrations, where we can be found that the formation of either M defects or X defects is endothermic. It was argued that defects with formation energy smaller than 1.5 eV will be experimentally created with large probability, even though the positive formation energy.53 In general, more energy is needed to create the M defects in M-rich conditions, with an exception of GeSe. As we can see from Figure 1b, for GeSe, of all defect concentrations, the defect formation energies of X vacancies are all more positive than M vacancies, so M vacancies are always favorable to form under both M- and X-rich conditions, which is in agreement with previous conclusion. 53,56 In addition, defect formation energy shows concentration dependence, especially for GeS and SnS, which prefer high defect concentrations.

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As shown in Figure 2a, hydrogen adsorption on defective MXs indicates that the hydrogen atom will fill the vacancy site, bonding with the atom underneath the vacancy. In Table 1, hydrogen adsorption energies ∆𝐸

∗

on defective MXs are summarized, the

coverage-dependent adsorption can be found. In general, hydrogen adsorption on MXs with X vacancies is endothermic, indicating a weak adsorption. At the same time, defective GeSe and SnSe with M vacancies of high defect concentration could not provide stable sites for H adsorption, thus we will not discuss the HER properties of them. In Figure 2b, ΔGH* for hydrogen adsorption on defective MXs with M vacancies are shown. In comparison with bare MXs, interestingly, M vacancies can significantly improve the HER activity. In general, M vacancies will lead to superior HER activity with the ΔGH* close to zero. In particular, ΔGH* decreases from 1.888 eV for pristine SnSe to –0.080 eV for SnSe with 5.55% Sn vacancies. As the Se/Sn vacancy concentration decreases to 3.12%, ΔGH* on SnSe turns out to be -0.050 eV. In case of GeSe with 5.55% and 3.12% Ge vacancies, ΔGH* are as low as -0.021 and 0.022 eV, respectively. It is known that ΔGH* for current Pt is –0.090 eV, therefore, by virtue of the ideal hydrogen adsorption free energies, GeSe and SnSe with metal vacancies could be promising candidates to replace Pt as highly efficient HER catalysts. It should be pointed out that, with respect to MoS2, 12.5% S vacancy as well as 1% tensile strain or 3.12% S vacancy and 8% tensile strain can also tune the ΔGH* to zero.51,52 Here, we emphasize that exclusive metal vacancies in strain-free SnSe and GeSe can realize near ideal zero ΔGH*. In the following, the discussion will be focused SnSe with 5.55% Sn vacancies. As shown in Figure 3a, band structure indicates the semiconducting feature of perfect SnSe with an indirect band gap of 0.75 eV, which is adverse to the electrical conductivity of the

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catalysts. Remarkably, as indicated in Figure 3b, by introducing Sn vacancy, the defective SnSe turns into metallic characteristic, indicative of a better conductivity than the original SnSe, thus guarantees the good catalytic properties of HER. Due to the close relationship between HER quantities and the binding strength, we calculate the crystal orbital Hamilton population (COHP) of Sn and Se atoms around the Sn vacancy before and after introducing Sn vacancy. As shown in Figure 3c, the COHP of perfect SnSe indicates states bellow the Fermi level (from –2 to 0 eV) are occupied antibonding states σ*. As a Sn atom is removed, the occupation of the antibonding states σ * decreases, which means increased bonding strength with hydrogen atoms. In turn, the bonding strength is strongly related to the HER activity.58 Because H atoms bond to the Se atoms in both perfect and defective SnSe, we also investigate the COHP of H and Se atom of perfect and defective SnSe with H adsorption. Figure 3d indicates that Sn vacancies could decrease the occupation of the antibonding states σ *, which further proves the results of Figure 3c. Figure 3e displays the density of states (DOS) of perfect, defective and hydrogen adsorbed SnSe. It clearly shows the electronic structure change after introducing defects and subsequent hydrogen adsorption. After introducing defects in SnSe, valence states bellow the Fermi level become unoccupied, which originates from the electron back donation from Sn to Se. In this respect, hydrogen bonding strength will be strengthened, therefore, ΔGH* will be decreased. In addition, defective SnSe with H adsorption remains the metallic character, guaranteeing the desirable conductivity for HER catalysts. In short, introducing Sn vacancies in SnSe can effectively tune the overpotentials to boost the hydrogen evolution. Once hydrogen atoms are absorbed, H 1s electrons would increase the occupation of the antibonding states σ *, as can be observed

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in the DOS. Figure 4a shows that the total density of states (TDOS) of the defective SnSe are mainly devoted by s and p orbitals of Sn atom and p orbital of Se atom. In Figure 4b, the mechanism of bonding strength is schematically explained. As demonstrated in previous works,51,52 strain can change the hydrogen adsorption, thus affect the HER activity. In Figure 4c, ΔGH* of MXs with metal vacancies as a function of strain is shown. In principle, strain alters the bonding strength between M and X atoms and, subsequently, changes the bonding strength with adsorbed hydrogen atoms. In particular, hydrogen adsorption will be strengthened as strain changes from compressive to tensile, and ΔGH* monotonically decreases as the strain increases. It is worth noting that for SnSe with Sn vacancy, ΔGH* can be tuned to be as low as 0.026 eV under a tensile strain of 1%, indicative the promoting effect of strain. It also indicates that, in comparison to 2H MoS2,51 MXs monolayers can achieve similar or even better catalytic activity without extreme conditions, thus is much easier to be implemented experimentally. In Figure 4d and 4e where COHP and DOS of defective SnSe varies as a function of strain, the results further confirms the assumption that decreased occupation of antibonding states σ * will strengthen the hydrogen adsorption. Figure 5 shows the volcano curve of MXs with M vacancies, and Pt is also shown for comparison. In the volcano curve, exchange current density i0 is calculated as a function of the ΔGH* of hydrogen adsorption according to Nørskov’s assumption, 23,59-61 If ΔGH*  0, i0 at pH = 0 is calculated as 𝑖 = −𝑒𝑘

1 1 + exp(−∆𝐺 ∗ /𝑘 𝑇)

If ΔGH* > 0, i0 is expressed as 9



𝑖 = −𝑒𝑘

1 1 + exp(∆𝐺 ∗ /𝑘 𝑇)

where 𝑘 is the rate constant and set to 1.23,59-61 The volcano peak is considered as the position of the best catalytic activities of the catalysts,59 then the HER performance of defective MXs can be quantitatively evaluated by the position of its i0 and ΔGH* with respect to the volcano peak (the closer to the peak, the better the catalyst). As indicated in Figure 5, according to the above equation, all the calculated values maintain a good linear relationship, which is consistent with previous results. 23,60,61 Especially for GeSe and SnSe with metal vacancies, the positions are much closer to the peak with larger exchange current density even than Pt. It is therefore indicative that defective GeSe and SnSe could be promising candidates for HER to replace currently used Pt.

Conclusion In summary, we theoretically predicted a new family of defective MXs to be promising HER catalysts, and screened out defective SnSe and GeSe as the best HER catalysts. Our results show that the inert MXs basal planes could be activated by introducing M vacancies. By comparing the hydrogen adsorption Gibbs free energy ΔGH* of defective MXs, M vacancies lead to better HER performance than the perfect MXs structures. Especially for defective GeSe with Ge vacancies and SnSe with Sn vacancies (strainfree), the HER catalytic activities are even better than Pt. It is conclusive that M vacancies decrease the occupation of antibonding states σ *, enabling strong bonding with hydrogen atoms. In addition, M vacancies make the hydrogen-adsorbed systems metallic, guaranteeing the desirable conductivity. Our results provide a new paradigm in searching for excellent HER catalysts without complex materials design. 10


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Acknowledgements This work is supported by the National Natural Science Foundation of China (No. 51872170 and 21333006), the Taishan Scholar Program of Shandong Province, the Young Scholars Program of Shandong University (YSPSDU), and the 111 project (B13029)

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2049−2055.

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Table 1 Hydrogen adsorption energy ∆𝐸

∗

Page 20 of 25

on MXs (GeS, GeSe, SnS, SnSe) with M and

X vacancies of different defect concentrations. The unit is eV. 12.5%

5.55%

3.12%

GeS-VGe

-0.121

-0.575

-0.608

GeSe-VGe

0.341

-0.271

-0.128

SnS-VSn

-0.012

-0.744

-0.750

SnSe-VSn

0.156

-0.331

-0.300

GeS-VS

0.173

0.441

0.317

GeSe-VSe

0.359

0.384

0.421

SnS-VS

0.571

0.489

0.493

SnSe-VSe

0.464

0.403

0.418

∆𝐸

∗

Table 2 Zero-point energies ZPE (in eV) and entropies multiplied by T (T = 298.15 K) (in eV) of defective MXs with M vacancies in different defect concentrations and H2 in gas phase. ZPE

TS

defect concentration

12.5%

5.55%

3.12%

12.5%

5.55%

3.12%

GeS-VGe

0.235

0.239

0.233

0.021

0.014

0.020

GeSe-VGe

0.206

0.208

0.206

0.021

0.028

0.023

SnS-VSn

0.236

0.236

0.237

0.028

0.027

0.026

SnSe-VSn

0.205

0.203

0.204

0.033

0.035

0.034

H2

0.272

0.408

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Figure Captions Figure 1 (a) Top view of MXs 3×3×1 supercell with metal and nonmetal vacancies, gray and yellow spheres represent M and X atoms, respectively. The purple circles denote the vacancies. (b) Defect formation energy of MXs with metal and nonmetal vacancies of different defect concentrations. In Figure 1b, the formation energy is shown as a function of M/X chemical potential. In particular, M-rich indicates that the vacancies are generated in the M-rich environment, while X-rich means that the vacancies are created in the Xrich atmosphere. M (metal) –rich indicate the chemical potential 𝜇

is calculated as the

energy of one M atom in bulk diamond structure, while for X (nonmetal) -rich environments chemical potential 𝜇

is referred to the energy of one X atom from

molecular crystal (𝑅 3 phase) Se6 and S8.

Figure 2 (a) Hydrogen atom (white spheres) adsorption on MXs with metal (top) and nonmetal (bottom) vacancies. (b) ΔGH* for hydrogen adsorption on MXs with metal vacancies of different defect concentrations.

Figure 3 (a) Band structure of pristine SnSe, the dotted line indicates the Fermi level. (b) Band structure of defective SnSe 3×3×1 supercell with Sn vacancies, the dotted line indicates the Fermi level. (c) –pCOHP of Sn and Se atoms around the Sn vacancy before and after introducing Sn vacancies, negative –pCOHP indicates antibonding states σ *. (d) –pCOHP of H and Se atoms before and after introducing Sn vacancies with H atom adsorbed, negative -pCOHP indicates antibonding states σ *. (e) Density of states (DOS) of perfect SnSe, SnSe with Sn vacancies and defective SnSe with hydrogen atom, the

21



dotted line indicates the Fermi level.

Figure 4 (a) Total density of states (TDOS) and projected density of states (PDOS) of the defective SnSe with Sn vacancies, the dotted line indicates the Fermi level. (b) Schematic of bonding and antibonding states of Sn s/p and Se s/p orbitals, the dotted line indicates the Fermi level (EF). (c) ΔGH* of MXs with metal vacancies versus applied strain. The blue region denotes the best catalytic region for HER. (d) –pCOHP of Sn–Se bonds around the Sn vacancies as a function of applied strain. (e) Density of states (DOS) of H atom adsorbed defective SnSe versus applied strain, the dotted line indicates the Fermi level.

Figure 5 Volcano curve of exchange current density i0 as a function of ΔGH* for hydrogen adsorption on MXs with metal vacancies, that of Pt is shown for comparison.

22


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Figure 1

Figure 2

23



Figure 3

24


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Figure 4

Figure 5

25


Computational Screening of Defective Group IVA Monochalcogenides

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