Activating Inert Basal Planes of MoS2 for Hydrogen Evolution

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Activating inert basal planes of MoS2 for hydrogen evolution reaction through the formation of different intrinsic defects Yixin Ouyang, Chongyi Ling, Qian Chen, Zilu Wang, Li Shi, and Jinlan Wang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01395 • Publication Date (Web): 27 May 2016 Downloaded from http://pubs.acs.org on May 31, 2016

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Chemistry of Materials

Activating inert basal planes of MoS2 for hydrogen evolution reaction through the formation of different intrinsic defects Yixin Ouyang1, Chongyi Ling1, Qian Chen1, Zilu Wang1, Li Shi1 , and Jinlan Wang*1,2 1

Department of Physics, Southeast University, Nanjing 211189, China

2

Synergetic Innovation Center for Quantum Effects and Applications (SICQEA), Hunan Normal University, Changsha 410081, China

Abstract: Nanoscale molybdenum disulphide (MoS2) has attracted ever-growing interest as one of the most promising non-precious catalysts for hydrogen evolution reaction (HER). However, the active sites of pristine MoS 2 are located at the edges, leaving large area of basal planes useless. Here, we systematically evaluate the capabilities of 16 kinds of structural defects including point defects (PDs) and grain boundaries (GBs) to activate the basal plane of MoS2 monolayer. Our first-principle calculations show that six types of defects (i.e., Vs, VMoS3, MoS2 PDs, 4|8a, S bridge and Mo-Mo bond GBs) can greatly improve the HER performance of the in-plane domains of MoS2 . More importantly, Vs and MoS2 PDs, S bridge and 4|8a GBs exhibit outstanding activity in both Heyrovsky and Tafel reactions as well. Moreover, the different HER activities of defects are well understood by an amendatory band-center model, which is applicable to a broad class of systems with localized defect states. Our study provides a comprehensive picture on the defect-engineered HER activities of MoS2 monolayer and opens a new window for optimizing the HER activity of two-dimensional dichalcogenides for future hydrogen utilization.

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Introduction Electrochemical water splitting is a promising route to the sustainable production of hydrogen.1 The corresponding devices always need catalyst to facilitate low overpotential and fast kinetics for the efficient hydrogen evolution reaction (HER).2 Currently, platinum (Pt) and other Pt-group metals are the most popular catalysts for HER,3,4 while the high cost and rare reserves limit their widespread utilization. The development of low-cost, earth-abundant, and high-performance alternatives to Pt for electrochemical hydrogen production is thus highly demanding. 5 Recently, molybdenum disulphide (MoS2) nanosheet as promising alternative to Pt for HER has drawn persistent interest.6-9 However, a number of studies have revealed that the electrocatalytic HER performance of nanoscale MoS 2 mainly arises from its edge states, while the basal plane is inert, which leaves large area of basal planes useless.10-12 So if the inert basal plane sites can be optimized, meanwhile the edge activity can be maintained, the catalytic activity of MoS2 monolayer will be enormously improved. Very recently, it was found that inert basal plane of MoS2 could be activated by introducing sulphur (S) vacancies,13 cracks and triangular holes14 into the monolayer surface. In fact, abundant structural defects, including point defects, grain boundaries, and edges, have been observed in MoS 2 monolayer experimentally,15-21 and they can tailor the electronic,22 magnetic,23 and optical properties,24 and creating new functionalities25 in a wide range. Whether will the structural defects bring additional active sites into MoS2 monolayer and activate the inert basal planes for HER?


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In this work, we construct 16 types of structural defects varying from point defects (PDs) to grain boundaries (GBs) into the basal plane of MoS2 monolayer (Figure 2(a)), which have been widely observed in experiments, and explore their HER activity by employing first-principle method. Our calculations show that 6 kinds of defects, i.e., Vs, VMoS3, MoS2 PDs, 4|8a, S bridge and Mo-Mo bond GBs, can greatly improve the HER performance of MoS2 , leading to the inert in-plane domain activated. More importantly, defects, such as Vs, MoS2 PDs, S bridge and 4|8a GBs, show excellent activity in both Heyrovsky and Tafel reactions. We also develop an amendatory band-center model to interpret the defect-dependent HER activity in MoS2.

Method and model Compared to graphene, which only contains one atomic layer of carbon, structural defects in MoS2 are more complicated due to the three atomic layer structure and binary element system involved. Here, we study 9 kinds of point defects and 7 kinds of grain boundaries of MoS2 monolayer, including monosulfur vacancy (VS), disulfur vacancy (VS2), vacancy complex of Mo and nearby three sulfur (V MoS3), vacancy complex of Mo nearby three disulfur pairs (V MoS6), antisite defects where a Mo atom substituting a S atom (MoS), a Mo atom substituting a S2 atom (MoS2 ), a Mo2 column substituting a S2 column (Mo2S2), a S atom substituting a Mo atom (SMo ), or a S2 column substituting a Mo atom (S2 Mo), grain boundaries with 21.8°-tilt composed 5-7 rings (5|7a, 5|7b), 60°-tilt composed 4-4 rings (4|4), 4-8 rings (4|8a, 4|8b), Mo-Mo bond, and S bridge. The periodic structure model of point defects is easy to build, but



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it is complicated for grain boundaries. In Figure 1, we show the way to build periodic structure model of grain boundaries taking 5|7a and 5|7b GBs as examples. We first construct two MoS2 nanosheets which have same structure and same orientation tropism, then we inset another MoS2 nanosheet which has the same structure but different orientation tropism between above two MoS2 nanosheets. Tailoring the boundaries and reassembling the three nanosheets into a coherent whole, the periodic structure with grain boundary will emerge.

Figure 1. The construction of periodic structure models of 5|7a and 5|7 b GBs. The 5|7b and 5|7a GBs regions are marked by invert colors. The periodic-unit is framed by the red line. The yellow and purple balls refer to the S and Mo atoms, respectively.

All the calculations were performed by employing spin-polarized density functional theory (DFT) within a general gradient approximation parameterized by Perdew, Burke, and Ernzerhof,26 as implemented in Vienna ab initio simulation package.27,28 The electron-ion potential was described by the projected augmented wave method,29 and a kinetic energy cutoff of 400eV was used for the plane wave expansion. The vacuum spaces in all supercells were larger than 15Å above the MoS 2 plane to avoid any artificial interaction. The effect of van der Waals (vdW) interactions was included


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for weak interaction cases using the semiempirical correction scheme of Grimme, DFT-D2.30 The climbing-image nudged elastic band (cNEB) method31 was employed to locate the minimum energy paths and the transition states of H adsorption on MoS 2 monolayer.

Result and discussion The HER is a multi-step electrochemical process taking place on the surface of an electrode. Generally accepted reaction mechanisms in acid solution are: 32 i) Electrochemical hydrogen adsorption (Volmer reaction), H++M+e-⇌M-H*

(1)

ii) Electrochemical desorption (Heyrovsky reaction), M-H*+H ++e-⇌M+H2

(2)

or chemical desorption (Tafel reaction), 2M-H*⇌2M+H2

(3)

where H* designates a hydrogen atom chemically adsorbed on an active site of the electrode surface (M). Chemical adsorption and desorption of H atoms on an electrode surface are competitive processes. According to the Sabatier principle,33 to achieve the maximum reaction rate, the Gibbs free energy for H* adsorption on a catalyst surface (G H) should be close to zero. So in this work, the Gibbs free energy is used to evaluate the HER activity of MoS2. The Gibbs free energy of the three steps are calculated by using the standard hydrogen electrode as the reference through 1 GH (1)  EH1  TS  eU  kT ln CH   EZPE 2


(4)


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1 GH (2)   EH1  TS  eU  kT ln( PH2 / CH  )  EZPE 2

(5)

GH (3)  EH 1  EH2  TS  kT ln PH2  EZPE

(6)

G H(1), G H(2) and G H(3) are the Gibbs free energy of Volmer reaction, Heyrovsky reaction and Tafel reaction, respectively. E H n is the adsorption energy of the nth H atom and is defined as EH  EMoS  nH  EMoS ( n1) H  n 2 2

1 EH . U is the electronic 2 2

voltage of an excited electron versus the standard hydrogen electrode, while PH and 2

CH  are the relative partial pressure of H 2 and the relative concentration of H + in the solution, respectively. TS is the gas-phase entropy contribution of a hydrogen molecule at 298 K (It is a constant, 0.40 eV). EZPE is the zero-point energy difference between the adsorbed state of the system and the gas phase state. Considering that the phonon contribution to the free energy is quite small, it is reasonable to ignore the zero-point energy. We first study the Volmer reaction occurring at the different point defects and grain boundary regions. All the possible adsorption positions for a single H atom have been considered and the most stable adsorption configurations are displayed in Figure 2(a). For VS and VS2 PDs, the adsorbed H atom locates at the S vacancy. For the cases of VMoS3 and VMoS6 PDs, 5|7a, 5|7b, 4|8a, 4|8b and Mo-Mo bond GBs, the H atom prefers to absorb on the bridge site of two Mo atoms. Nevertheless, the H atom favors to bond with the Mo atom instead of the S atom in MoS2, MoS and Mo2S2 PDs regions, while it tends to bond with the S atom with dangling bonds for S2 Mo, SMo PDs and S bridge GBs. Regarding of 4|4 GBs, the H atom absorbs at the top site of the S atom. The free energy for H* adsorption (G H(1)) at different PDs and GBs of MoS2 are presented in


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Table 1 and Figure 2(b), (c). It can be vividly seen from the figure that the Gibbs free energy of all the H adsorption sites in defect regions are much smaller than that of the perfect MoS2 basal plane (0001) plane (1.92eV),34 indicating that these structure defects can indeed break the inertia of basal plane and enhance the interaction between H atom and adsorption site. Moreover, different defects have different impacts on the H binding strength. Defects, such as 4|4 GBs, S2 Mo and SMo, present G H(1) of very positive values, indicating that the interactions between H and MoS2 basal plane are still too weak to form sufficiently strong bonding for facilitating the proton-electron transfer process. For the case of VMoS6 and 4|8b GBs, the calculated G H(1) is very negative, demonstrating that the interaction is too strong to assure a facile bond breaking and the release of gaseous H 2 . It is worth noting that the calculated G H(1) of VS PDs, VMoS3 PDs, MoS2 PDs, 4|8a GBs, S bridge GBs and Mo-Mo bond GBs is only -0.06, -0.13, 0.09, 0.15, -0.13 and 0.04eV, respectively, indicative of highly catalytic activity for the Volmer reaction. In comparison with Pt (G H=-0.09eV) or Mo-edge (G H=0.08eV), these 6 kinds of defects have comparable or even better catalytic activity for HER Therefore, the presence of PDs and GBs do bring additional active sites into MoS2 basal planes as expected.



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Table 1. Gibbs free energy (GH(1)) of Volmer reaction and structure reconstruction energies (ER) of MoS2 caused by H atom adsorption. The ER is defined as the difference between the free energy of defects before and after reconstruction. Structure

ΔGH(1) (eV)

ΔER (eV)

Structure

ΔGH(1) (eV)

ΔER (eV)

VS

-0.06

0.08

Mo2S2

-0.19

0.05

VS2

-0.18

0.18

5|7a

-0.26

0.23

VMoS3

-0.13

0.32

5|7b

0.27

0.13

VMoS6

-0.97

0.02

4|8a

0.15

0.10

S2Mo

1.33

0.15

4|8b

-0.56

0.24

SMo

1.22

0.17

S bridge

-0.13

0.10

MoS2

0.09

0.34

Mo-Mo bond

0.04

0.18

MoS

-0.17

0.15

4|4

1.38

0.26

Figure 2. (a) The most stable adsorption positions for singe H atom absorbing at VS, VS2, VMoS3, VMoS6, MoS2, MoS, Mo2S2, S2Mo, SMo PDs and 5|7a, 5|7b, 4|8a, 4|8b, Mo-Mo bond, S bridge, 4|4 GBs. The yellow, purple and cyan balls refer to the S, Mo, and H atoms in defect regions, respectively. Defect regions are colored and framed by the dash line. (b) Energetics of the HER process on different catalysts: 5|7a, 5|7b, 4|8a, 4|8b, Mo-Mo bond, S bridge , 4|4 GBs and Pt. (c) Energetics of HER process on different catalysts: VS, VS2, VMoS3, VMoS6, MoS2, MoS, Mo2S2, S2Mo, SMo PDs and Pt.


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The H adsorption process normally accompanies with electron cloud overlap between H atom and the catalyst and structure reconstruction of the catalyst. Therefore, the adsorption energy of the H atom (EH), of which the free energy (G H(1)) largely depends on, are decided by two factors, the chemical bond energy of H to the catalyst (EI) and the structure reconstruction energy of the catalyst (ER). Generally, the structure reconstruction energy of pure crystal electrode materials is very small and not significantly contributive to G H(1). However, the structure reconstruction energy in defective systems cannot be neglected due to the relatively poor structural stability of defects. The ER of different PDs and GBs are listed in Table 1. We only focus on the six defect structures (VS, VMoS3, MoS2, 4|8a, S bridge, Mo-Mo bond) which have active sites. Except the VMoS3 and MoS2 PDs, the ER of other four defects is close to zero. The small ER indicates the better structure stability and less structure relaxation of these four defect structures, which is conducive to the H atom fast adsorption and release. It is known that chemical properties of materials are actually decided by the underlying electronic structure, an analysis of the density of states (DOS) can help us understand the interaction between H atom and MoS 2 defects, and thus give us insight into the different HER activity of defective structures. We plot the DOS of defects in Figure 3(a) which the H atom interacts directly with Mo atoms, such as VS, VS2, VMoS3, VMoS6 , MoS2 , MoS, Mo2S2 PDs and 5|7a, 5|7b, 4|8a, 4|8b and Mo-Mo bond GBs. Only the d-projected DOS of Mo atoms is presented since Mo d orbitals play the decisive role during the formation of the H-Mo bonds. To analyze the variation of H binding



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strength of different defects, we combine these results with the d-band center theory of transition-metal,35-37 where the chemical bonding energy of H to the catalyst EI can be described as: 38,39 E I  

V2 | d s |

(7)

In this equation, V is the coupling matrix element and can be assumed as a constant value; εs is the energy level of adsorbate states and can be set to be 0 eV; εd is the energy level of d states and its value is d band center. The value of εd is calculated by: 

 xρ( x)dx εd    ρ( x)dx

(8)

The x and ρ(x) correspond to abscissa and ordinate of Figure 3(a), respectively. The integral domain is from minimum energy to maximal energy of d electrons. According to Eq. 7, the variation in chemical bonding energy is then determined by the -|εd | and the chemical bond will be the strongest when -|εd | is maximized. Considering that the structure reconstruction energy in these defective systems cannot be ignored, to apply this theory to our systems, we define the H bond energy as EI=EH-ER, where EH is the H* adsorption energy, ER is the structure reconstruction energy. As shown in Fig. 3(b), the EI values do not display a declining trend with the increase of -|εd |. However, when we change the integral domain of εd from the whole energy range to [-1eV, 0eV], which is under and near the Fermi level, the EI values of all investigated models show an approximate linear trend in relation to -|ε'd |. This means that only the d-states under and near the Fermi level play an important role in the H adsorption process, and the closer the -|ε'd | to the Fermi level, the stronger the H-Mo


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bond is. As we mention above, too strong or too weak chemical bonds are neither suitable to HER, and an appropriate -|ε'd | can ensure an appropriate strength of H-Mo bond and catalytic activity for the HER. For the case of S2 Mo PDs, SMo PDs, S bridge GBs and 4|4 GBs, H interacts directly with the S atoms, so the p-projected DOS of the S atoms should be considered in the Eq. 7. Similar to the situation of H atom interacting directly with Mo atom, the E I values show an approximate linear trend in relation to -|ε’p | of S atoms. That is, the bond energy between hydrogen atom and the active site of defective systems is actually decided by the localized defect states under and near the Fermi level rather than states of all energy ranges. This amendatory band-center model developed here should be of generality and be applicable to a broad class of defective systems, even to the doped system with localized states near Fermi level.



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Figure 3. (a) Projected d-orbital density of states of Mo atoms for H adsorption sites. The shaded area corresponds to the filled states up to the Fermi level εF. The red and blue dashed lines indicate -|εd| and -|ε’d |, respectively. These were calculated from the structures given in Fig. 1(a) with one H removed. (b) Relationship between ΔEI and -|εd| for various defects. (c) Relationship between ΔEI and -|ε’d| for various defects. (d) Relationship between ΔEI and -|ε’p| for various defects.

Above Gibbs free energy calculations show that 6 kinds of structural defects including VS, VMoS3 , MoS2 DP and 4|8a, S bridge, Mo-Mo bond GBs have active sites for the Volmer reaction. Whether they are also active sites for the following reactions


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in HER? To answer this question, we further study the catalytic activity of these 6 kinds of structural defects for the second step in the HER, i.e., Heyrovsky or Tafel reaction. According to the recent literature, various morphologies of MoS 2 nanostructure showed different reaction mechanisms at different experimental environments.7, 40-42 Therefore, both of above two mechanisms should be considered. The activation barrier following Heyrovsky mechanism cannot be computed by traditional density functional theory as the electrons in initial and final states are not balanced. However, according to Eqs. (4) and (5), when the overpotential is zero, G H(2) approaches to -G H(1). Thus, the active sites for Volmer reaction will keep catalytic activity for Heyrovsky reaction. That is, all these 6 kinds of defects which have small G H(1) should be active sites for Heyrovsky reaction as well. Comparing with Heyrovsky reaction, Tafel reaction is a more complicated process, and it demands large number and appropriate distribution of active sites. However, if the rate-limiting step of the HER is Tafel reaction, the experimental Tafel slope will correspond to the theoretical minimum limit,43 and the smaller the Tafel slope, the better the catalytic performance. So it is vital to investigate the activity of defect regions for Tafel reaction. Obviously, the Tafel reaction, i.e., the recombination reaction of two H atoms only happens when the two H atoms are adjacent enough (Otherwise, it will be Heyrovsky reaction). Thereby, we first locate the most stable absorption sites of the second H atom near the first absorbed H atom, and then calculate the corresponding H absorption free energy (G H(3)), as shown in Figure 4(b). For Vs and MoS2 PDs, S bridge and 4|8a GBs, the free energy (G H(3)) is still



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close to 0 eV, indicating the good HER activity of these sites for Tafel reaction. The two absorbed H atoms from recombination to release as H2 is a process of the free energy increase. The big value of the free energy rise usually corresponds to high reaction barrier which will cause low turnover frequency and reduce the hydrogen production rate. Therefore, Vs and MoS2 PDs, S bridge and 4|8a GBs with low free energy rise are excellent active centers not only for Heyrovsky reaction but also for Tafel reaction. To gain profound understanding on the Tafel mechanism and the kinetics of H atom recombination and release as H2, we take 4|8a GBs and Vs PDs as two representatives to locate the minimum energy paths and the transition states via cNEB method. The corresponding initial, intermediate and final states are displayed in Figure 4(c) and (d). The reaction barrier to break the Mo-H bonds and to form the H-H bond is 1.58eV for 4|8a GBs and 1.32eV for Vs PDs, respectively. Moreover, comparing with the Tafel reaction happening at MoS2 edge sites,44 the H recombination and release paths of Vs PDs and 4|8a GBs are much simpler, due to the intensive and appropriate distribution of the active sites. This is beneficial to the Tafel reaction which will make it occur easily and quickly and thereby improve the hydrogen yield.


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Figure 4. (a) Catalytic reaction pathways and energies of the HER on MoS2 4|8a GBs with two different reaction mechanisms (Volmer–Heyrovsky or Volmer–Tafel reaction). (b) Free energy diagrams for the second H atom adsorption and the H2 evolution. Calculated energy profile involved in the recombination of 2H* on 4|8a GBs (c) and Vs PDs (d).

Conclusion In summary, we have proposed a possible approach to activate the inert basal planes of MoS2 through the formation of different intrinsic defects. We have systematically assessed 16 kinds of structural defects including point defects and grain boundaries of MoS 2 monolayer as catalytic centers for HER. Our calculations demonstrate that in addition to the single S vacancies, other defects such as VMoS3 and MoS2 PDs, 4|8a, S bridge and Mo-Mo bond GBs in the basal plane of MoS2 monolayer all have catalytic sites for HER. Moreover, Vs and MoS2 PDs, S bridge and 4|8a GBs exhibit outstanding activity in both Heyrovsky and Tafel mechanisms. The catalytic activity of different defects is well understood in light of an amendatory



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band-center model, which can be applied to a broad class of defective systems with localized states. Our findings suggest that increasing the designated point defects and grain boundaries can optimize MoS2 basal planes and increase the number of catalytic sites. In fact, it has been reported that there are several experimental routes to control the emergence of designated defects. For example, in mechanical exfoliation and chemical vapour deposition grown MoS2, most of the defects are VS PDs, while in physical vapour deposition grown MoS2, MoS2 PDs are dominant.17 In contrast, different grain boundaries of vapour deposition grown MoS2 can be obtained by modulating the morphology of substrate.21 Therefore, it is a feasible strategy to greatly improve the catalytic efficiency of MoS 2 for the HER by introducing the designated point defects and grain boundaries into in-plane of domains of MoS2. The strategy proposed here is of generality and can be applied to other 2D dichalcogenides materials.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: H adsorption configurations on different defects and their corresponding G H for HER, and PDOS of S atoms for H adsorption sites (PDF)

AUTHOR INFORMATION Corresponding Author *Email: [email protected]


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ACKNOWLEDGMENTS This work is supported by the NSFC (21525311, 21173040, 21373045, 11404056) and NSF of Jiangsu (BK20130016) and SRFDP (20130092110029, 20130092120042) in China. The authors thank the computational resources at the SEU and National Supercomputing Center in Tianjin.

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Activating Inert Basal Planes of MoS2 for Hydrogen Evolution

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