Strain Effect on the Dissociation of Water Molecules on Silicene

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Strain Effect on the Dissociation of Water Molecules on Silicene: Density Functional Theory Study Quanguo Jiang, Jianfeng Zhang, Huajie Huang, Yuping Wu, and Zhimin Ao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00426 • Publication Date (Web): 18 Apr 2019 Downloaded from http://pubs.acs.org on April 18, 2019

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The Journal of Physical Chemistry

Strain Effect on the Dissociation of Water Molecules on Silicene: Density Functional Theory Study Quanguo Jiang,1 Jianfeng Zhang,1* Huajie Huang,1 Yuping Wu,1 Zhimin Ao2 1 College of Mechanics and Materials, Hohai University, Nanjing 210098, China 2 Guangzhou Key Laboratory Environmental Catalysis and Pollution Control, Guangdong Key Laboratory of Environmental Catalysis and Health Risk Control, Institute of Environmental Health and Pollution Control, School of Environmental Science and Engineering, Guangdong University of Technology, Guangzhou, 510006, China Abstract Dissociative adsorption of water molecules on silicene is an efficient way to open the band gap of silicene for electronic applications. However, the dissociation of H2O molecules on silicene is difficult due to the Pauli exclusion effect between H2O molecules and silicene. By using density functional theory calculations, we investigated the effect of strain on the dissociative barrier of H2O molecules on silicene. Our results demonstrate that the tensile strain can significantly reduce the dissociative energy barrier of H2O molecules on silicene, while the compressive strain has slight effect on the dissociation barrier. In addition, the dissociation barrier reduces from 0.85 to 0.27 eV with the tensile strain of 9%, where the reaction time for the dissociation of H2O on silicene can be reduced significantly from 2.23 × 102 to 3.60 × 10-8 s. The mechanism for the effect of strains on the depressed dissociation barrier of H2O molecule on silicene can be understood through analysing the density

*

Corresponding authors: [email protected]; [email protected]

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of states and orbital charge of the water/silicene system. The band gap of silicene is 0.37 eV after the dissociation of H2O molecule and the carrier mobility is relatively high, which is important for its applications in logic circuits. Further study indicates that the dissociation of H2O is favorite at atomic vacancy and grain boundary of silicene with and without strain.


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1. Introduction

As an analogy of graphene1, silicene with two dimensional honeycomb structures has attracted academic and industrial interests since it was experimentally fabricaited,2-11 due to its high mobility of Dirac fermions. Unlike the flat honeycomb lattice of graphene, silicene has low buckling of about 0.44 Å,12 which allows more flexibility to functionalize their electronic properties.13–17 Moreover, silicene is more compatible with modern semiconductor technology based on silicon than graphene.18 Therefore, silicene is believed to have a bright future for electronic applications,19 and silicene based field-effect transistor (FET) has been fabricated through a growthtransfer-fabrication method,20,21 which shows a carrier mobility of ≈100 cm2 V−1 s−1 at room temperature. However, the zero band gap of silicene22 has restricted its application in electronics, such as FET, where an on-off current ratio between 104 and 107 is required for obtaining well switching capabilities, and the semiconducting channel with a bandgap close to 0.4 eV or more is necessary.23,24 As a result, opening a sizeable band gap close to 0.4 eV for silicene is desired for its application in logic circuits to obtain well on-off current ratio. The band gap of silicene can be tuned by inert substrates, such as MoS2,25,26 BN,27,28 SiC,28 and silicane,29,30 where the high carrier mobility is kept but it only opens a limited band gap smaller than 0.1 eV, which is too weak for the roomtemperature operation of FETs. In addition, adsorption of small molecules31,32 and organic molecules33 was also investigated to open the band gap of silicene, which opens a band gap smaller than 0.2 eV. The dissociative adsorption of small molecules is also an important method to open the band gap of silicene. It has been reported that hydrogenation34-36,41 of silicene is also effective methods to tune the band gap of silicene. However, the fully hydrogenated silicenes (silicane) are wide-gap insulators, 3 ACS Paragon Plus Environment


where an indirect band gap of 3.51 eV is found based on the more accurate hybrid HSE06 functional calculation.36 Furthermore, it is found that the properties of silicene can be tuned by the hydrogenation ratio,36 where half hydrogenated freestanding silicene generates a direct band gap of 1.79 eV and becomes a ferromagnetic semiconductor. In our previous study, it is found that the dissociative adsorption of H2O molecules makes Al-doped graphene keep conductive.37 Motivated by this fact, the band gap and conductivity of silicene is also expected to be tuned by dissociative adsorption of H2O molecules. However, H2O molecules are inert38-41 to the silicene due to the Pauli exclusion effect,38 and the dissociation of water molecule on silicene is difficult. The possibility for the dissociation of molecules can be evaluated by the energy barrier, where reducing the energy barrier would facilitate the dissociative reactions. Therefore, external stimulations should be applied to depress the dissociation barrier for H2O molecules on silicene. Strain field is effective to change the surface activity of low dimensional materials.42-49 It is reported that strain can enhance the binding energy and charge transfer between water and the substrate on an in-plane graphene/silicene heterostructure.48 Our recent work also reported that the dissociative barrier of H2 molecules on silicene can be significantly depressed in the presence of strains.49 Therefore, strain is expected to be an effective way to manipulate the activity of silicene toward H2O molecules dissociation. In this work, the effect of both tensile and compressive strains on the dissociative energy barrier of water molecule on pristine silicene and defective silicene are systemically studied by using DFT method. The mechanism of reducing the dissociative energy barrier can be understood through analyzing the partial density of states (PDOS), orbital overlaps, and charge transfer between H2O molecules and


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silicene. Finally, the band structure of silicene with the dissociative adsorption of H2O molecule is studied and its potential application in logical electronic devices is discussed. 2. Simulation methods

All the DFT results in this work were calculated by using the DMol3 module.50 The exchange-correlation functional has adopted generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE)51 functional. The core electrons were replaced by a single effective potential, i.e. core treatment is implemented with DFT semicore pseudopotentials (DSPPs) for relativistic effects. The basis set has used a double numerical plus polarization (DNP) method. The convergence tolerance of energy is taken as 10-5 Ha (1 Ha = 27.21 eV), with a maximum allowed force and displacement of 0.002 Ha/Å and 0.005 Å respectively. To test the accuracy of the maximum allowed force, we have calculated the adsorption energy of a H2O molecule on the top site of a Si atom in silicene and the minimum energy barrier for the H2O molecule dissociation in the absence of strain field with the maximum allowed force of 0.007 Ha/Å in the following, which shows that the results are the same to that with the maximum allowed force of 0.002 Ha/Å. Therefore, the maximum allowed force (0.002 Ha/Å) is low enough and adopted in this work. It was reported that the exchangecorrelation functional has much smaller effect on the energy barriers of calculated reaction than that of adsorption energies.52 In order to investigate the minimum energy pathway for the dissociative adsorption of water molecule on silicene, linear synchronous transition/quadratic synchronous transit (LST/QST)53 and nudged elastic band (NEB)54 tools in DMol3 were employed. To take into consideration of the effect of van der Waals forces, Grimme scheme55 for DFT-D correction is used. Spin-polarized calculations are added for band structure



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observation. The simulation has used three-dimensional periodic boundary conditions with 4 × 4 × 1 supercell for all studied structures, where a vacuum value of 20 Å is employed above the silicene surface to avoid the interaction between silicene and its periodic images. All atoms have been relaxed to reach their most stable positions, where the kpoint is set to 6 × 6 × 1 for structure relaxations, and the density of states (DOS) are calculated with a finer k-point grid of 15 × 15 × 1 based on previous report.56 For the silicene nanoribbon with 585 boundary, 1 × 6 × 1 k-point meshes are used. To better describe the band gap of the silicene system, the more accurate hybrid functional HSE0657 is used in CASTEP code58, where the Norm-conserving pseudopotentials and an energy cutoff of 750 eV are used. The adsorption energy Ead of molecules on silicene is determined by, Ead = EH2O/silicene - (Esilicene+EH2O)

(1)

where EH2O/silicene, Esilicene and EH2O are total energies of the H2O/silicene system, the isolate silicene and molecules, respectively. Based on the relaxed configurations of the silicene system, we apply a biaxial tensile or compressive strain by scaling the axial lattice constant of pristine silicene or silicene with single atom vacancy and double atoms vacancy. The positive strain (ε > 0) refers to biaxial expansion, while the negative one (ε < 0) corresponds to biaxial compression. For silicene nanoribbon with 585 boundary, we applied a uniaxial strain along the periodic direction. Then the silicene is further optimized at a given strain. 3. Results and discussion

Previous literature41 has reported that the dissociation of water molecule on silicene is hindered by high energy barriers, where the detailed reaction progress and energy barrier are absent. To comprehensively understand this dissociation progress, we study the adsorption and dissociation of H2O molecule on silicene carefully in the 6 ACS Paragon Plus Environment

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following. The adsorption of H2O molecule on silicene in the absence of strain is first studied as the initial structure (IS) for the dissociative reaction. There are four possible adsorption sites as shown in Fig. 1a: the top, bridge, down and hollow sites, labelled as IST, ISB, ISD and ISHol, respectively. Based on the DFT-D calculations with consideration of van der Waals force through the Grimme scheme55 and eq. 1, the obtained Ead values are listed in Table 1. Based on our calculations, the favourable adsorption site of the H2O is at the hollow site with an adsorption energy Ead = -0.19 eV, as shown in Fig. 1a, which is similar to that in ref. 38. In order to study the effect of the simulation cell size on the results, we also performed calculations with H2O adsorbed on the hollow site using a 5 × 5 supercell and found adsorption energy Ead = -0.19 eV, which is the same with the result obtained with the 4 × 4 supercell, indicating that the 4 × 4 supercell is large enough for this simulation. For the final structure (FS) after the dissociation of H2O molecules, H and OH are chemically adsorbed on silicene. Due to the different dissociative species (H and OH), there are many relative positions for H and OH co-adsorption. Therefore, the OH group is first fixed, and relative position of the H atom is investigated. There are two possible adsorption sites for OH groups, i.e., sites a and 1 as shown in Fig. 1b. When OH group is located at site a, there are three possible adsorption sites for H atom from 1 to 3, where the configurations are labelled as FS1, FS2 and FS3, respectively. When OH is located at site 1, there are also three possible adsorption sites for H atom from a to c, where the configurations are labelled as FSa, FSb and FSc, respectively. According to the above DFT-D calculations, the relative total energies compared to ISHol are 0.01, 0.02 and 0.02 eV for IST, ISB and ISD, respectively. The adsorption energies from IST to ISHol have the same energetic level. Thus, all reaction paths for the dissociative adsorption of H2O on silicene from the initial structure (IST-



ISHol) to final state (FS1-FSc) may occur and all are considered. After LST/QST and NEB calculations, the energy barriers along all the possible pathways are shown in Table 2, where the minimum energy pathway is from IST to FS1. The detailed reaction pathway is shown in Fig. 2, where IS, TS, FS, Ebar = ETS – EIS, and reaction energy Er = EFS – EIS for the H2O dissociative adsorption on silicene are given. We can see that the reaction needs two steps: the H2O molecule first dissociates into free H atom and adsorbed OH group at the TS state after overcoming an energy barrier of 0.85 eV, then the H atom and OH group find their most stable positions at FS state with releasing an energy of 1.33 eV. A total energy of 0.48 eV is released for the dissociation of H2O on silicene in the absence of strain field. Although this reaction releases an energy of about 0.48 eV in total (EFS - EIS), there is a high potential energy barrier of 0.85 eV for the first step. Consequently, the first step for the dissociation of H2O molecule with larger Ebar value is the rate-limiting step. It has been reported that surface reactions at ambient temperature may occur when Ebar < 0.75 eV.59 Therefore, the dissociative adsorption of water on pristine silicene is difficult at room temperature, which are consistent with the previous report that the H2O molecule is inert to the silicene surface.41 High reaction temperature or other external energy source is required to facilitate the dissociation. It is known that applying a tensile or compressive strain is an alternative way to alter the electronic distribution and chemical potential of silicene.42-48, 60 In addition, the barrier of the dissociation of molecular hydrogen on silicene can be greatly reduced by applying strain on the substrate as discussed above.49 Therefore, strain field is considered here to investigate the possibility for lowering the dissociation energy barrier of water molecule on silicene. To investigate the effect of strain, we apply axial strain on the H2O/silicene system, where the direction of positive strain


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field is shown in Fig. 1a. The pathways for this reaction in the presence of strain field with different intensities are shown in Fig. 3. Before studying the dissociation progress of water molecules, the effect of strain field on the adsorption of water molecule on silicene is first studied, the corresponding results are shown in Table 1. As discussed above, the favourable adsorption site for the water molecule is at the hollow site of silicene in the absence of strain field as shown in Fig. 1a. In the presence of tensile strain, the adsorption energy of water molecule at the hollow site keeps almost unchanged with increasing tensile strain as shown in Table 1. However, the adsorption energy at site D increases with increasing tensile strain, and it is larger than that at hollow site when the tensile strain is larger than 9% as shown in Table 1. Therefore, the water molecule prefers to adsorb at hollow site when the tensile strain is smaller than 9%, while the water molecule prefers to adsorb at the site D of silicene with the tensile strain larger than 9% (see ISD in Fig. 3c). With a strain field of 8%, although the most stable adsorption site for the adsorption of water molecule on silicene is still ISHol, the water molecule chemically adsorbs at the site D with a Si-O bond length of 2.100 Å. Further increasing the tensile strain to 12%, the adsorption energy for H2O in ISD configuration is -0.36 eV, which is much larger than that in IST, ISB and ISHol configurations as shown in Table 1, which indicates that the H2O molecule prefers to adsorb at site D in the presence of tensile strain field 12%. The water molecule keeps physical adsorption on silicene in IST, ISB and ISHol under different tensile strain field as indicated by the distance between the O atom of H2O and the nearest Si atom (lO-Si) and the distance between the O atom of H2O and silicene surface (lO-Silicene) in Table 1. In the presence of compressive strain, the structure of silicene will be distorted when the compressive strain is larger than -2%. Therefore, only the adsorption of water molecule on silicene



with compressive strain of -2% is studied, where the water molecule prefers to adsorb at the top site of silicene. Because the adsorption energies from IST to ISHol in the presence of different strain field have the same energetic level when the strain field is smaller than 9%, all reaction paths for the dissociative adsorption of H2O on silicene from the initial structure (IST-ISHol) to final state (FS1-FSc) with strain field from -2% to 9% are considered and the energy barriers are shown in Table 2. The detailed reaction pathway in the presence of different strain field is shown in Fig. 3. With the strain field of 4% (see Fig. 3a), although the preferred configuration of the reactant is still ISHol, the dissociative barriers from IST-ISHol to FS1-FSa are all calculated and shown in Table 2, where the minimum reaction pathway is from ISD to FSa with an energy barrier of 0.57 eV. Note that in the absence of strain field, the minimum reaction pathway is from IST to FS1 with energy barrier of 0.85 eV (see Fig. 2), which decreases to 0.57 eV with strain field of 4%. This is reasonable due to the fact that the distance between Si1 and Sia becomes larger and it is easier for the Si1 atom to move out and bind with OH group, which depresses the dissociative barrier of H2O molecule from ISD state to FSa state. Since the surface reactions at ambient temperature occurs when Ebar < 0.75 eV,59 the H2O molecule can spontaneously dissociate on pristine silicene to obtain FSa product in the presence of 4% tensile strain field. In addition, the energy barriers from IST to FS1 and that from ISB to FS1 are also reduced to 0.68 and 0.66 eV, respectively. However, the reaction pathway from ISD to FSa has the lowest energy barrier of 0.57 eV and the dissociation of H2O on silicene most likely occurs along this path. Further increasing the tensile strain field to 8%, the dissociative barriers from ISHol-ISD to FS1-FSa are also calculated and shown in Table 2, where the minimum reaction pathway is from ISD to FSa with an energy barrier of 0.29 eV (see Fig. 3b).


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Since the energy barrier is smaller than the critical barrier of 0.75 eV,59 the water molecule can spontaneously dissociate on pristine silicene to obtain FSa. Note that the energy barriers from IST to FS1 and that from ISB to FS1 are significantly reduced to 0.46 and 0.43 eV, respectively. While the energy barriers from ISHol to FS2 is still very large being 0.96 eV. Further increasing the tensile strain field to 9%, the possible dissociation pathways from ISHol-ISD to FS1-FSa are all calculated and shown in Table 2, where the minimum dissociation energy barrier of H2O on silicene is 0.27 eV from ISD to FSa and the detailed reaction pathway is shown in Fig. 3c. Since the energy barrier is smaller than the critical barrier of 0.75 eV,59 the water molecule can spontaneously dissociate on pristine silicene to obtain FSa. Note that the energy barriers from IST to FS1 and that from ISB to FS1 are 0.41 and 0.36 eV, respectively, which are both smaller than the critical barrier of 0.75 eV. However, the most stable geometry for the adsorption of water molecule on silicene is ISD (see Fig. 3c). Therefore, the dissociation of H2O on silicene prefers the path from ISD to FSa. Further increasing the tensile strain to 12%, the adsorption energy for H2O in ISD configuration is -0.36 eV, which is much larger than that in IST, ISB and ISHol configurations as shown in Table 1, which indicates that the H2O molecule prefers to adsorb at site D in the presence of tensile strain field 12%. The dissociation barrier for H2O molecule on silicene is decreased to 0.23 eV and the reaction process is more exothermic with reaction energy of -0.92 eV, indicating that the reaction can go through smoothly at low temperature and obtain the FSa product. Thus, the tensile strain can promote the dissociative adsorption of H2O molecule on silicene and makes the reaction process more exothermic. The effect of compressive strain on the dissociation of H2O molecule on silicene



is also studied in Fig. 3d. Because the structure of silicene is distorted when the compressive strain is larger than -2%, further compressive strain is not applied. With the strain field of -2%, although the H2O molecule prefers to adsorb at top Si atom, the dissociative barrier from ISHol-IST to FS1-FSa is shown in Table 2, where the minimum reaction path is from ISH to FS2 with an energy barrier of 0.79 eV. In the compressive strain, moving up for the Si1 atom at the down site is difficult, therefore, the barrier from IST to FS1 and that from IST to FS1 are both larger than that without strain field. However, the compressive strain makes the distance between Sia atom and O atom smaller, which lowers the energy barrier from ISHol to FS2. Since surface reactions at ambient temperature may occur when Ebar < 0.75 eV,59 the dissociative reaction here is still difficult to occur at room temperature. Overall, the tensile strain can significantly reduce the energy barrier for the IST, ISB and ISD configurations, while it has little effect on the barrier for the ISHol configuration. For the ISHol configuration, when the strain value increases from 0% to 4%, the O-Sia distance lO-Si increases from 3.612 to 3.810 Å, and then decreases to 3.618 Å with 8% strain field. Correspondingly, the energy barrier from ISHol to FS2 increases from 0.94 to 1.01 eV with strain increasing from 0% to 4%, and then decreases to 0.99 eV with 8% strain field as shown in Table 1. Therefore, the energy barrier from ISHol is insensitive to the tensile strain field and slightly affected by the O-Sia distance. In contrast, the compressive strain field can decrease the energy barrier slightly to 0.79 eV for ISHol configuration due to the reduced O-Sia distance lOSi of

3.132 Å. Therefore, the reduced energy barrier is sensitive to both the initial state

and strain field. To better understand the effect of energy barrier reduction on the reaction time, the reaction time for each step in Fig. 2 and Fig. 3 is estimated by the following


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equation,61 1

τ=

e

  Ebar   K BT

  

(2)

where ν is in order of 1012 Hz, KB is the Boltzmann constant and T = 298.15 K. The reaction time for the reaction in Figs. 2 and 3 are: τ1 = 2.23  102 s without strain, τ2 = 4.19  10-3 s with 4% tensile strain, τ3 = 7.86  10-8 s with 8% tensile strain, τ4 = 3.60  108 s with 9% tensile strain, and τ5 = 2.17  101 s with 2% compressive strain, respectively. Thus, the dissociation of H2O on silicene possesses very fast reaction kinetics when the tensile strain is larger than 4%. The effect of strain field on Ebar can be understood by considering the interaction between the bands of H2O and silicene through analyzing their partial density of states (PDOS) in Figs. 4a-4e. The PDOS and orbitals of a free H2O molecule are also shown in Fig. 4f, in order to completely understand how H2O interacts with the silicene substrate. All orbitals are labelled in Fig. 4f, which indicates that the interaction between H2O molecule and silicene surface is mainly determined by the highest occupied molecule orbital (HOMO, 1b1) and lowest unoccupied molecule orbital (LUMO, 4a1), which are most close to the Fermi level among all orbitals. For the physical adsorption of H2O, the relative positions of HOMO (1b1) and LUMO (4a1) of the adsorbed H2O with respect to the Dirac point determine the charge transfer between substrate and H2O, because HOMO is completely located on O atom, while LUMO is fully located on H atoms.62 Therefore, the H2O losses electron when the O atom points to the silicene surface, while the H2O obtains electron when the H atom points to the silicene surface. For example, in the absence of strain field, the H2O at top site (IST in Fig. 1) loses 0.088 e, while the one at hollow site (ISHol in Fig. 2) loses 0.009 e. This is reasonable, because H2O at top site lies in parallel to silicene 13 ACS Paragon Plus Environment


surface, the HOMO orbital interacts with the Si atom as shown in Fig. 4a, which causes electron depletion; when H2O lies at hollow site, the H atoms point to the near Si atom, the LUMO orbital attracts electrons from Si atoms while the HOMO orbital loses electrons, which causes the H2O molecule at hollow site to lose less electrons to silicene compared with the case at top site. The interaction of HOMO and LUMO orbitals of H2O at hollow site of silicene makes larger adsorption energy than that at the top site in the absence of strain field. The O-H bond points to the silicene surface in the ISB and ISD configurations, thus the LUMO orbital interacts with the silicene and H2O molecule obtains 0.009 and 0.008 e in ISB and ISD, respectively. The distance between O atom of H2O and the nearest Si atom (lO-Si), as well as the distance between O atom of H2O and silicene surface (lO-Silicene) in Table 1 indicates that the H2O molecule physically adsorbs on silicene without strain field. The O-H bond length lO-H is the largest and the O-Si distance lO-Si is the smallest in IST, which indicates the strongest interaction between H2O and silicene among the four adsorption configurations, which is confirmed by the minimum energy barrier from IST to FS1 among all the reaction paths. When applying 4% tensile strain, the reaction path with minimum energy barrier is from ISD to FSa (see ISD in Fig. 3a), thus the PDOS for the atoms in ISD is shown in Fig. 4b. The OH bonds point to the silicene surface for ISD, where the LUMO orbital interacts with the Si atom weakly (see Fig. 4b) and attracts electrons from Si atoms, which is confirmed by the fact that the H2O molecule obtains 0.007 e as shown in Table 1. The O-Si distance lO-Si is 3.746 for the IST as shown in Table 1, indicating a physical adsorption for the H2O molecule. The O-H bond length lO-H is 0.973 Å, which is similar to that of 0.972 Å in ISD without strain. It indicates that the reduced energy barrier is not caused by the weakness of OH bond in H2O molecule. The


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reduced barrier should be caused by the fact that the tensile strain makes the SiD atom have more space to move up and bind with the OH group to reduce the energy barrier. Increasing the tensile strain field to 8%, the H2O adsorbs on SiD atom with forming O-Si bond (see ISD in Fig. 3b) due to the hybridization between 1b1 orbital of water and p orbital of Si atom. About 0.166 e is transferred from the adsorbed H2O molecule to silicene. In addition, the overlap between H and O atoms for 3a1 orbital are significantly depressed compared with the H band of free molecule in Fig. 4c, which implies that the O-H interaction is depressed and the dissociation of H2O becomes easier. This is confirmed by the small dissociative energy barrier of 0.29 eV as shown in Fig. 3b. Further increasing the tensile strain field to 9%, there is also chemical bond between the H2O and SiD atom (see ISD in Fig. 3c) and about 0.167 e is transferred from the adsorbed H2O molecule to silicene. Therefore, the overlap between H and O atoms for 3a1 orbital in Fig. 4d is similar to that of ISD in tensile strain field of 8% in Fig. 4c, which is confirmed by the slightly decreased energy barrier of 0.27 eV as shown in Fig. 3c. Overall, the tensile strain field can enhance the reactivity of the silicene and lower Ebar for the dissociative adsorption of H2O on silicene. Because the atomic structure of silicene is distorted when the compressive strain is larger than -2%, only the electronic structure of initial structure in the presence of compressive strain of 2% is studied. The reaction path with minimum energy barrier is from ISH to FS2 (see ISH in Fig. 3D), thus the PDOS for the atoms in ISH is shown in Fig. 4e. Here, about 0.040 e is transferred from the H2O molecule to silicene in Table 1, indicating enhanced interaction between HOMO orbital of H2O and silicene in Fig. 4e, compared with that of ISH without strain field, where 0.009 e is transferred from the H2O molecule to silicene. Thus, the dissociative energy barrier is slightly



decreased to 0.79 eV in Fig. 3d. Since surface reactions at ambient temperature occur when Ebar < 0.75 eV,59 the dissociative reaction of H2O on silicene cannot go through in the presence of the compressive strain field at room temperature. The band structures of silicene with different adsorbates in the absence of strain field are then studied in Fig. 5. The band structure of pristine silicene is shown in Fig. 5a, where linear dispersion is shown near the Fermi energy with zero band gap. The band structure of silicene with adsorption of H2O molecule at hollow site (ISHol) is also calculated, where a direct band gap of 0.03 eV is obtained, indicates that the physical adsorption of H2O molecule has little effect for opening the band gap of silicene. The dissociative adsorption of H2O molecule has significant effect on the band structure of silicene. The band structure of silicene with OH at site 1 and H at site a (FSa) is shown in Fig. 5b, where a direct band gap of 0.37 eV is obtained, which is close to the band gap of 0.4 eV to obtain perfect on-off current ratio for its application in logic circuits as discussed in Refs. 23 and 24. The nearly linear dispersion near Fermi energy is found in Fig. 5b, indicating that the carrier mobility of FSa structure should be high. In addition, the electron orbitals of the highest valence band (HVB) and lowest conduction band (LCB) are connective in the FSa system, which is similar to that of pristine silicene in Fig. 5a. The carrier mobility of electron and hole for FSa structure is further estimated based on the band structures in Fig. 5b. The effective mass of electrons (me) and holes (mh) for FSa are first calculated by m = ħ2[∂2E(k)/∂k2]-1, where ħ is the reduced Planck constant, k is the wave vector, and E(k) is the dispersion relation. The calculated me and mh values of silicene (FSa) are 6.75 and 2.68 times larger than those of pristine silicene, respectively. Note that in pristine silicene, μe = 2.57 × 105 cm2 V-1 s-1, and μh = 2.22 × 105 cm2 V-1 s-1, which is calculated by μ = et/m (t is the scattering time).63 As


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a result, the estimated me and mh for FSa are 0.39 × 105 cm2 V-1 s-1 and 0.83 × 105 cm2 V-1 s-1, respectively, which is still much larger than that of bulk Si (1400 cm2 V-1 s-1). Therefore, the obtained FSa structure is suitable for application in high speed electronic devices. Typically, adsorption of small molecules starts from defect sites,41 such as vacancies and grain boundaries, since defects are generally chemically more active. Single silicon vacancy (SV), double silicon vacancy (DV) and 585 grain boundary are common defects in 2D materials.64 Therefore, the dissociation for H2O molecule on silicene with SV, DV and 585 grain boundary are studied as shown in Fig. 6. For the defective silicenes, there are mainly three possible adsorption sites as shown in Figs. 6a, 6c and 6e: the top, hollow1 and hollow2 sites, labelled as IST, ISHol and ISHol_2, respectively. Based on the DFT-D calculations, it is found that the H2O molecule chemically adsorbs at the top site of Si atom with adsorption energies of -0.42, -0.33 and -0.28 eV for SV, DV and 585 boundary, respectively, where about 0.199, 0.181 and 0.187 e are transferred from the adsorbed H2O molecule to silicene, and the O-H bond length elongates to 0.986, 0.986 and 0.989 Å for SV, DV and 585 configurations. While the H2O molecule is physically adsorbed at hollow1 and hollow2 sites for all kind of defects with small adsorption energy as shown in Table 3, where small charge transfer is happened and the O-H bond length is slightly changed compared with that of free H2O molecule. Therefore, the IST structure is adopted as the initial structure in the subsequent transition state search calculations for all the defective silicene. For the final structure (FS) after the dissociation of H2O molecules, there are two possible configurations are labelled as FS1 and FS2 for silicene with SV and DV defects as shown in Figs. 6a and 6c. For silicene with 585 boundary, there are three possible configurations labelled as FS1, FS2 and FS3 as shown in Fig. 6e. Thus, all reaction



paths for the dissociative adsorption of H2O on silicene from the initial structure IST to final state FS1, FS2 and FS3 are considered. After LST/QST and NEB calculations, the energy barriers along all the possible pathways are shown in Table 4, where the minimum energy pathway is from IST to FS1 for SV and DV defects, while that is from IST to FS2 for 585 boundary defect. The detailed reaction pathway is shown in Fig. 6. The energy barrier is 0.03 and 0.25 eV from IST to FS1 for SV and DV defects as shown in Figs. 6b and 6d, respectively, while it is 0.37 eV from IST to FS2 for 585 boundary in Fig. 6f. Since the surface reactions at ambient temperature may occur when Ebar < 0.75 eV,53 the water molecule could spontaneously dissociate on silicene with SV, DV and 585 boundary defects at room temperature. The effect of strain on the adsorption and dissociation of H2O molecule on defective silicene is also studied. Because the atomic structure of defective silicene is distorted when the compressive strain 2% is applied, only the effect of tensile strain with a value of 4% is studied. For silicene with SV defect, the H2O molecule spontaneously dissociates into H and OH for IST configuration after geometric optimization in the presence of a tensile strain field 4%, while the H2O molecule keeps physical adsorption for ISHol and ISHol_2 configurations, where the QH2O, lO-H, lOS,

and lO-Silicene are similar to those for ISHol and ISHol_2 configurations without strain.

For silicene with DV defect, the H2O molecule spontaneously dissociates into H and OH for IST and ISHol_2 configurations after geometric optimization in the presence of a tensile strain field 4%, while the H2O molecule keeps physical adsorption for ISHol configurations, where the QH2O, lO-H, lO-S, and lO-Silicene are also similar to those for ISHol configuration without strain. Therefore, the tensile strain can promote the dissociation of H2O molecule on silicene with SV and DV defects.


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For silicene with 585 boundary, in the presence of tensile strain field 4% the H2O molecule keeps physical adsorption for ISHol and ISHol2 configurations, while the adsorption energy of H2O for IST configuration is -0.27 eV and charge transfer is 0.986 e, both slightly smaller than those for IST without strain, which corresponds to a slightly larger barrier of 0.44 eV from IST to FS2, but it is still much smaller than the critical barrier of 0.75 eV. Therefore, the dissociation of water on silicene with defects can occur spontaneously at room temperature. 4. Conclusion The dissociative adsorption of water molecules on silicene in the presence of different strains has been studied by using DFT calculations. We found that the tensile strain can significantly reduce the dissociative energy barrier of H2O molecule on silicene when the tensile strain is large than 9%, while the compressive strain has slight effect due to the fact that large compressive strain would destroy the atomic structure of silicene. Further study indicates that the H2O molecule can spontaneously dissociate into H and OH at the defective positions with and without strain. Through analysing PDOS and orbitals, it is found that the 1b1 orbital of H2O molecule plays a significant role for the enhanced interaction between H2O and silicene, which weakens the bond strength of 3a1 orbital of H2O. As a result, the OH bond strength is weakened and the dissociation barrier of H2O on silicene is depressed. The band gap is 0.37 eV for silicene after the dissociation of H2O, which is suitable for its applications in logic circuits. Acknowledgements We acknowledge supports by the Fundamental Research Funds for National Natural Science Foundation of China (Grant No. 21703052, 21607029, 21777033), the Central Universities (Grant Nos. 2017B12914 and 2015B01914), China Postdoctoral 19 ACS Paragon Plus Environment


Science Foundation (2015M571652), Natural Science Foundation of Jiangsu Province (BK20161506), National 973 Plan Project (2015CB057803), Science and Technology Program of Guangdong Province (2017B020216003), Science and Technology Program of Guangzhou City (201707010359), and “1000 Plan” for Young Professionals Program of China.


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Table 1. The adsorption energy Ead, Mulliken charge QH2O, and bond length lO-H of H2O molecule on silicene in the presence of different strains. The distance between O atom of H2O and the nearest Si atom (lO-Si), as well as the distance between O atom of H2O and silicene surface (lO-Silicene) are also shown. Strain IST ISB ISD ISHol IST QH2O (e) ISB ISD ISHol IST lO-H (Å) ISB ISD ISHol IST lO-Si (Å) ISB ISD ISHol lO-Silicene(Å) IST ISB ISD ISHol Ead (eV)

-2% -0.21 -0.19 -0.18 -0.20 0.105 0.056 -0.004 0.040 0.975 0.974 0.972 0.974 2.549 2.942 4.105 3.132 2.548 2.726 3.359 2.808

0 -0.18 -0.17 -0.17 -0.19 0.088 -0.009 -0.008 0.009 0.975 0.972 0.972 0.973 2.657 3.809 4.043 3.612 2.630 3.590 3.542 3.159

4% -0.17 -0.15 -0.16 -0.19 0.048 -0.004 -0.007 0.001 0.973 0.972 0.973 0.972 3.000 3.647 3.746 3.810 2.972 3.466 3.286 3.207

8% -0.16 -0.15 -0.17 -0.19 0.076 0.029 0.166 0.006 0.975 0.973 0.993 0.973 2.733 3.192 2.100 3.618 2.710 2.983 2.050 3.006


9% -0.17 -0.15 -0.21 -0.20 0.074 0.026 0.167 0.004 0.975 0.974 0.995 0.973 2.752 3.197 2.064 3.628 2.736 2.993 1.996 2.999

12% -0.18 -0.17 -0.36 -0.19 0.085 0.012 0.162 0.021 0.975 0.973 0.996 0.974 2.649 3.477 2.035 3.266 2.627 3.294 2.027 2.596

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Table 2. The energy barriers (Ebar) and reaction energy (Er) for the dissociation of water molecule on pristine silicene along different reaction pathways in the presence of different strain are also shown. The dash indicates that the corresponding reaction path cannot occur.

Strain

IST

ISB

ISD

ISHol

IST-FS1 IST -FS2 IST-FS3 IST-FSa IST-FSb IST-FSc ISB-FS1 ISB -FS2 ISB-FS3 ISB-FSa ISB-FSb ISB-FSc ISD-FS1 ISD-FS2 ISD-FS3 ISD-FSa ISD-FSb ISD-FSc ISHolFS1 ISHolFS2 ISHolFS3 ISHol-FSa ISHolFSb ISHol-FSc

-2%

0

4%

8%

9%

Ebar Er Ebar Er Ebar Er Ebar Er Ebar Er 0.87 -0.58 0.85 -0.48 0.68 -0.61 0.46 -0.83 0.41 -0.91                                                   0.86 -0.60 0.87 -0.49 0.66 -0.62 0.43 -0.85 0.36 -0.96                                                   0.85 -0.61                                 0.57 -0.59 0.29 -0.84 0.27 -0.86   1.14 0.07                           0.79

0.28

0.94 -0.19 1.01 -0.25 0.99 -0.43 0.96 -0.49









 

 

 

 













1.50 -0.76 1.46 -0.81

      1.20 -0.03 1.15 -0.29 1.15 -0.39 




1.59 -0.73






Table 3. The adsorption energy Ead, Mulliken charge QH2O, and bond length lO-H of H2O molecule on silicene in the presence of different strains. The dash indicates that the H2O molecule automatically dissociates to H and OH on the silicene with SV and SV defects in the presence of tensile strain field 4% after geometric optimization and the corresponding parameters are not measured. The distance between O atom of H2O and the nearest Si atom (lO-Si), as well as the distance between O atom of H2O and silicene surface (lO-Silicene) are also shown.

IST ISHol ISHol_2 IST QH2O (e) ISHol ISHol_2 IST lO-H (Å) ISHol ISHol_2 IST lO-Si (Å) ISHol ISHol_2 lO-Silicene(Å) IST ISHol ISHol_2 Ead (eV)

SV DV 585 Boundary 0 4% 0 4% 0 4% -0.42 -2.15 -0.33 -2.01 -0.28 -0.27 -0.20 -0.20 -0.19 -0.20 -0.19 -0.19 -0.19 -0.19 -0.23 -2.01 -0.20 -0.21 0.199 0.181 0.187 0.184   -0.002 -0.002 -0.005 -0.014 -0.010 -0.011 0.034 -0.004 0.015 0.014 0.014  0.986 0.986 0.989 0.986   0.972 0.973 0.973 0.973 0.972 0.972 0.973 0.973 0.974 0.973 0.973  2.028 2.042 2.077 2.097   3.836 3.693 3.947 3.997 4.127 4.149 3.192 3.858 3.245 3.558 3.570  1.981 1.770 1.863 1.927   3.158 3.226 3.540 3.515 3.441 3.460 2.953 3.362 2.023 2.053 2.283 


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Table 4. The energy barriers (Ebar) and reaction energy (Er) for the dissociation of water molecule on the defective silicene along different reaction pathways with different strain field are shown. The dash indicates that the corresponding reaction path cannot occur. Strain SV

IST → FS1 IST → FS2 DV IST → FS1 IST → FS2 585 Boundary IST → FS1 IST→ FS2 IST → FS3

0% 4% Ebar (eV) Er (eV) Ebar (eV) Er (eV) 0.03 -0.97 0 -2.15     0.25 -0.84 0 -2.01     0.79 -0.77 0.71 -0.81 0.37 -0.66 0.44 -0.64    



Figure captions: Fig. 1. (Color online) (a) The most stable configuration of silicene adsorbed with H2O molecule in the absence of strain field, where the letters indicate the possible H2O adsorption sites and the arrows indicate the direction of the applied tensile strain. (b) The most stable configuration for the dissociative H2O molecule on silicene with fixing OH group on site a in the absence of strain field, where the numbers indicate the possible adsorption sites for the H atom. The yellow, brown, red and white balls are respectively Si of top layer, Si of down layer, O and H atoms in this and following figures.


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Fig. 2. (Color online) The minimum reaction pathway for the dissociative adsorption of H2O molecule on pristine silicene from IST to FS1 without strain field. IS, TS, and FS represent initial structure, transition structure and final structure, respectively. Their atomic structures are given by the insets. The energy of ISHol is taken to be 0. The unit of Ebar and Er is eV, where Ebar is the energy barrier and Er is the reaction energy.



Fig. 3. (Color online) The minimum reaction pathway for the dissociative adsorption of water molecule on pristine silicene from IS to FS in the presence of different strain fields.


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Fig. 4. (Color online) Panels (a)-(e) show the PDOS of the hydrogen, oxygen and the silicon atoms of IS in the presence of different strains, where the Si atom is the nearest one to the oxygen atom. The black, blue and red curves are the PDOS of the Si atom, O atom, and H atoms of the reactant, respectively. Insects are the charge arrangement of 1b1 or 4a1 orbital for the adsorbed H2O molecule. The vertical lines indicate the Fermi level. The PDOS and molecular orbitals of free H2O are also labelled in (f).



Fig. 5. (Color online) The band structures of the pristine silicene (a), and the silicene after the dissociative adsorption of H2O with OH at site 1 and H at site a (FSa) (b). Inset is the electron orbital of the highest valence band (HVB) and lowest conduction band (LCB), which is shown as a black line. The vertical lines denote the Fermi level.


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Fig. 6 (Color online) The most stable configuration for the molecular and dissociative adsorption of H2O on silicene with SV (a), DV (c) and 585 boundary (e), where the letters indicate the possible adsorption position for the H2O molecule and the numbers indicate the possible adsorption sites for the H atom after dissociation. The minimum reaction pathways for the dissociative adsorption of H2O molecule on silicene with SV (b), DV (d) and 585 boundary (f) without strain field are also shown.



Table of Contents (For Table of Contents Only)

The tensile strain will promote the dissociation of H2O molecule on silicene, which can modulate the band gap of silicene for applications in logic circuits.


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Strain Effect on the Dissociation of Water Molecules on Silicene

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