Stabilization of two-dimensional penta-silicene for flexible lithium-ion battery anodes via surface chemistry reconfiguration

Donghai Wu a, Shuaiwei Wang a, Shouren Zhang a, Yibiao Liu a, Yingchun Ding b, Baocheng Yang a and Houyang Chen *c
aHenan Provincial Key Laboratory of Nanocomposites and Applications, Institute of Nanostructured Functional Materials, Huanghe Science and Technology College, Zhengzhou 450006, China
bCollege of Optoelectronics Technology, Chengdu University of Information Technology, Chengdu, 610225, China
cDepartment of Chemical and Biological Engineering, State University of New York at Buffalo, Buffalo, New York 14260-4200, USA. E-mail: hchen23@buffalo.edu

Received 6th August 2018 , Accepted 24th September 2018

First published on 24th September 2018


Silicon-based two-dimensional (2D) materials have unique properties and extraordinary engineering applications. However, penta-silicene is unstable. Herein, by employing first-principles calculations, we provide a facile surface chemistry method, i.e. functionalization, to acquire and reconfigure stable penta-silicene for use in flexible lithium-ion batteries. Our results of density functional theory calculations showed that the reconfigured penta-silicene nanosheets possess a broad range of properties, including semiconductors with an indirect bandgap, semiconductors with a direct bandgap, semimetals and metals. For fluorinated penta-silicene, a fluorine-concentration-induced transition from a semiconductor to a metal is found. For fully fluorinated penta-silicene, a mechanically induced transition from a semiconductor with an indirect bandgap to a semiconductor with a direct bandgap is obtained. Our calculation results showed the reconfigured penta-silicene is a high-performance anode for use in flexible lithium (Li)-ion batteries. A transition from a semiconductor to a metal with adsorption of Li atoms indicates a high electrical conductivity. It possesses low Li diffusion barriers (0.08–0.28 eV), demonstrating a high mobility of Li ions. The metallic feature and low Li diffusion barriers reveal that it has an ultrafast charge/discharge rate. This work suggests that surface chemistry reconfiguration provides new stable materials with excellent mechanical properties and tunable electronic properties for their promising applications in flexible metal-ion batteries and solar batteries as well as nanoelectronics devices.


1. Introduction

Two dimensional (2D) materials have attracted enormous interest since the classic 2D material graphene was reported in 2004.1 Beyond graphene, the pioneering 2D materials include group-IV graphene analogues (e.g. silicene and germanene),2,3 binary BN,4 ZnO,5 g-C3N4 nanosheets,6 transition-metal dichalcogenides (e.g. MoS2 and WS2),7 transition-metal carbides and nitrides (MXenes),8 black-phosphorene sheet9 and its As and Sb cousins.10 Most of these 2D nanosheets consist of rings containing an even number of atoms, especially hexagons.11 Nevertheless, 2D materials with rings containing an odd number of atoms are extremely rare. Recently, a new 2D carbon allotrope, penta-graphene, which is constructed by pentagonal carbon rings and exhibits extraordinary mechanical and thermal properties, has been proposed through density functional theory.12 A few other pentagonal 2D systems such as CN2,13 BP5,14 B2C,15 As2C16 and boron nitride monolayers17 have been successively predicted using first principles calculations. The pentagonal 2D materials exhibit unique properties such as unusual negative Poisson's ratios,12,18 ultrahigh ideal strength,12 ferromagnetism,19etc.

As the nearest neighbor of carbon in the same main group of the periodic table, silicon (Si) has numerous allotropes,3,11,20–22 such as the honeycomb silicene sheet with a regular stacking structure20 and the penta-SiC2 hetero-nanosheet with an analogous pentagonal structure.23 However, the pristine penta-silicene is dynamically unstable.24 Because the heteromorphic Si-based 2D monolayers with pure pentagonal structures are earth-abundant, hard, nontoxic, and compatible, and possess wide applications in electronic devices,11,21,25 several attempts have been adopted to stabilize this new allotrope.11,24,26 Surface modification, which can effectively improve the stability and tune the mechanical and electronic properties of 2D materials,22,24,27–32 is one of the useful strategies. For example, Li et al.29 have tuned the electronic and mechanical properties of penta-graphene via hydrogenation and fluorination. Ding and Wang24 found that the dynamic stability of penta-silicene can be restored by decorating with surface hydrogen. The Zeng group31 reported that the chemical functionalization (both hydrogenation and fluorination) can lead to the stabilization of penta-germanene. Due to the ultrahigh electronegativity, halogen, especially the fluorine (F) atom, is always the adherent functional group on the surface of various 2D materials in the experimental preparation process, which may have impact on many properties of materials.30 The fluorographene has been prepared successfully by the chemical and mechanical exfoliation of graphite fluoride in 2010.33 Ti3C2 synthesized by selectively etching the Al atoms with aqueous hydrofluoric acid is usually accompanied by surface F atoms, and the surface fluorination could significantly improve its intrinsic properties.8,34 It should be emphasized that the strategy of chemical functionalization to stabilize the structural stability of materials has been employed. However, previous studies mainly focused on the relationship between the properties of materials and their chemical functionalization.

In this paper, we adopt this strategy to reconfigure the properties for applications. 2D materials have also been regarded as promising electrode materials due to their large specific surface area and attractive electronic structures, which may achieve high capacity and fast ion diffusivity for metal-ion batteries.35–37 In addition, the steric hindrance effect in the surface of 2D materials, such as penta-silicene,35 Mo2C,38 phosphorene,39 and SiS,40 may accelerate the Li-ion diffusivity rate. Due to the potential unique properties12,18,19,41,42 of pentagonal 2D materials and the potential applications11,22,23,27,42–44 of silicon-based 2D materials, we adopted penta-silicene as an example of pentagonal 2D materials in this paper. By employing density functional theory calculations, which are carried out in VASP45 (theoretical methods are provided in Section S1 of the ESI), we acquire stable functionalized penta-silicene from unstable penta-silicene by introducing functional groups. We considered fluorination as functionalization and took penta-silicene as an example. We found that the fully fluorinated penta-silicene is stable and possesses extraordinary mechanical properties with high limiting strains. Dependence of electronic properties on the external strain and concentration of fluorine atoms was examined. We also investigated its applications in metal-ion batteries. The good electronic conductivity together with the low diffusion barrier signifies the high Li ion mobility in the fully fluorinated penta-silicene sheet, which could certainly accelerate the charge/discharge rate. Our results would provide fundamental information to stabilize unstable 2D materials via a surface chemistry approach and bring new significant 2D materials for promising engineering applications such as Li-ion batteries and solar batteries.

2. Results and discussion

2.1. Structure and stability

The pristine penta-silicene sheet is puckered as shown in Fig. S1a (ESI). The tetragonal unit cell, which constitutes four Si pentagons, contains two tetracoordinated silicon atoms (labeled Si1) and four tricoordinated silicon atoms (labeled Si2). Previous studies have reported that penta-silicene is unstable in dynamics,11,24,26 and our phonon spectrum (Fig. S2a, ESI) with noticeable imaginary frequencies also suggests its dynamical instability. The further eigenvector analysis indicates that the soft modes are related to the tricoordinated Si atoms, which tend to distort the Si pentagons to form more bonds with other atoms via the favourite sp3 hybridization.24 Motivated by this, we speculate the introduction of surface functional groups to stabilize the pristine penta-silicene. Hydrogenation or fluorination has been extensively used in two-dimensional nanomaterials for the improvement of physical and chemical performances.24,28,29,31,46–49 We introduce fluorine atoms on the three-fold coordinated Si atoms, forming a Si6F4 sheet. All Si atoms become four-fold coordinated atoms and the structure remains the integrated Si pentagons (Fig. S1b, ESI). The fully fluorinated penta-silicene sheet, Si6F4, looks like a chain of baskets lined up side by side. The formation energy Ef is defined by Ef = (EF-pSiEpSi − 2EF2)/4,28 and EF-pSi, EpSi and EF2 represent the total energies of the fully fluorinated penta-silicene, the pristine penta-silicene, and the free F2 molecule, respectively. The calculated formation energy Ef of Si6F4 is −4.458 eV. The negative Ef means that the surface fluorination of penta-silicene is an exothermic process, and the extremely negative value suggests its strong thermodynamic stability. No imaginary frequency was found in the entire Brillouin zone in our phonon spectrum calculations (Fig. S2b, ESI), indicating that it is dynamically stable. Meanwhile, its thermal stability was examined using the ab initio molecular dynamics (AIMD) simulations (with a 3 × 3 × 1 supercell containing 90 atoms) for 5 ps with a time step of 1 fs. The temperature and total energy converge to constants and its structure nearly retains the integrated nanostructure during the AIMD simulations at 800 K (Fig. S3, ESI), indicating that it possesses robust thermal stability at high temperatures. Taking the aforesaid results into account, one can conclude that fluorine functionalization on a 2D penta-silicene sheet could make the material stable in terms of thermodynamics and dynamics.

In such a fully fluorinated penta-silicene sheet, the buckling height between the top and bottom Si1 atoms increases from 2 Å (pristine penta-silicene) to 2.528 Å, the distance between the two adjacent Si2 atoms lengthened from 2.24 Å to 2.399 Å, while the distance between the Si1 and Si2 atoms elongated slightly from 2.36 Å to 2.367 Å, which is close to that of the hydrogenated penta-silicene.24 The bond length of Si2–F is approximately 1.626 Å, which is larger than that of C–F in fluorinated penta-graphene29 and that of the Si2–H bond in hydrogenated penta-silicene.24 The very close bond angles ∠Si1–Si2–Si1 (=105.3°) and ∠Si1–Si2–Si2 (=106.7°) show an evident sp3 hybridization character of these Si atoms, which are smaller than those in the pristine penta-silicene (112° and 111°).24

2.2. Mechanical properties

Since ultrathin 2D materials are susceptible to external influences, especially the mechanical deformation,50 we first calculate independent elastic constants of the fully fluorinated penta-silicene sheet from the energy–strain curve.12 The obtained elastic constants C11, C12 and C66 are 59.5 N m−1, 42.1 N m−1 and 43.9 N m−1, respectively, which are larger than those of hydrogenated penta-silicene24 and fluorinated penta-graphene.29 Moreover, they meet the mechanical stability criteria (i.e. C112C122 > 0 and C66 > 0) for 2D sheets,12 showing their mechanical stability. The in-plane Young's modulus (E) and Poisson's ratio (ν) are derived from the elastic constants using the formulae E = (C112C122)/C11 and ν = C12/C11. The moderate E of 29.7 N m−1 and ν of 0.71 suggest that the fully fluorinated penta-silicene sheet is a soft material.

To gain more insight into the mechanical performance, stress–strain relations under both biaxial and uniaxial loadings are further calculated. The tensile strain ε is defined as (ll0)/l0, where l0 and l are the strain-free and strained lattice constants, respectively. Because of the tetragonal symmetric crystal configuration, the uniaxial strains along the x and y directions are equivalent. When we stretch along the x axis, the lattice constants and internal atomic positions along the y axis are fully relaxed. The initial section of the stress–strain curves (Fig. 1a) presents linear trends. By fitting the initial slopes of this section, the Young's modulus of 29.6 N m−1 under the uniaxial strain is obtained. Besides, in the small uniaxial strain scope of the stress–strain curve, strains along the x and y directions have a relationship of εy = −νεx. From Fig. 1b, εy presents a linear variation with the equation of εy = −0.72εx when εx < 0.14, thus the Poisson's ratio ν is 0.72. The fitted E and ν values are in good agreement with the results obtained from elastic constants. As the applied strain further increases, the stress increases monotonically, until it reaches a critical point, and then the stress decreases. The largest tensile strains for biaxial and uniaxial tensile testing are 0.23 and 0.3, respectively, which are almost equal to those of the hydrogenated penta-silicene24 and are much larger than other 2D materials.14,51,52 Large critical strains indicate that the fully fluorinated penta-silicene sheet exhibits superior flexibility.24 Besides, it endures the stress as high as 6.07 N m−1 for the biaxial stretching and 10.8 N m−1 for the uniaxial tensile at the corresponding ultimate strains, indicating its outstanding mechanical properties.


image file: c8cp05008b-f1.tif
Fig. 1 (a) Stress–strain curves of a fully fluorinated penta-silicene sheet under biaxial and uniaxial stretching. (b) The variations of εyversus the uniaxial strain of εx.

We also calculated the phonon spectra of the fully fluorinated penta-silicene sheet under the critical points of biaxial and uniaxial tensions. No phonon softening occurs at the limiting strains (Fig. S2c and d, ESI), demonstrating that the sole mechanism for the failure of the fully fluorinated penta-silicene sheet is the elastic instability. Compared with other nanosheets suffering from the phonon instability before the elastic limit,52,53 the fully fluorinated penta-silicene sheet is more superior. Such excellent mechanical performance may be ascribed to its peculiar atomic configuration. Decorating F atoms on the tricoordinated Si2 atoms would form strong σ bonds with neighbours via sp3 hybridization. Thanks to the buckled basal plane with Si2 atoms up and down alternately, the change of the bond angle can effectively respond to the tensions rather than that of the bond length. Hence, the great strain could not break the fully fluorinated penta-silicene sheet, leading it to a superior flexible material with promising applications in nanodevices.

2.3. Electronic properties

We then examine its electronic properties. By using both PBE and HSE06 functionals, the band structures together with their corresponding densities of states (DOSs) of the pristine and fully fluorinated penta-silicene were calculated (Fig. 2). The pristine penta-silicene is a semiconductor with a smaller indirect band gap of 0.57 eV from the HSE06 functional (Fig. 2a). After full fluorination, the band gap is still indirect with the valence band maximum (VBM) locating at the M point and the conduction band minimum (CBM) lying at the Γ point (Fig. 2b). Obviously, the band structures of the fully fluorinated penta-silicene sheet calculated with the PBE and HSE06 functionals are essentially identical, except for the distinct gap values of 1.15 eV from the PBE functional and 1.82 eV from the HSE06 functional. It should be mentioned that, according to previous studies,54,55 the HSE06 functional provides more accurate band structures than the PBE functional. This appropriate band gap makes the fluorinated sheet a promising candidate for photoelectric materials. The DOS near the Fermi level (EF) is mainly contributed by the Si-p states, whereas deeper energy levels are mainly governed by the F-p states. The partial densities of states (PDOSs) were further calculated to identify the electronic orbital occupancy around EF. As seen from Fig. 2c, the VBM is dominated by the abundant p orbitals of Si1 atoms and moderate Si2-p orbitals, while the CBM is mainly composed of the p orbitals of Si2 atoms. For the bonding state, the position of orbital overlapping between Si2 and F atoms is located at a relatively low energy range from −0.6 to −1.2 eV, while that between the Si1 and Si2 atoms is from 0 to −0.6 eV. Such a behavior is also found in the antibonding states, i.e., from 0.9 to 2.4 eV for Si2–F orbitals and from 2.4 to 4.0 eV for Si1–Si2 orbitals. Based on this consequence, we plotted the corresponding energy level scheme, as shown in Fig. 2d. Evidently, between the Si1–Si2 bonding states in the plane and the Si2–F antibonding states out of the plane, a medium band gap is spontaneously opened for the fully fluorinated penta-silicene sheet, which possesses promising potential for applications in the photocatalysis process.
image file: c8cp05008b-f2.tif
Fig. 2 Band structures of pristine penta-silicene (a) and fully fluorinated penta-silicene (b) with PBE and HSE functionals. Partial density of states (PDOS) (c) and the orbital energy level schematic diagram (d) of a fully fluorinated penta-silicene sheet.

The external force plays a key role in the electronic structures of materials.14,26,27,31,56,57 We calculated the band structures of the fully fluorinated penta-silicene sheet under a series of biaxial and uniaxial strains (Fig. S4 and S5, ESI). For the biaxial strain, direct bandgaps were obtained at the Γ point with ε ≥ 0.15 (Fig. S4c and d, ESI). The indirect–direct band gap transition is primarily attributed to the movement of the VBM along the path of MXΓ upon increasing strains. Due to the high electronegativity of F atoms, the electrons are transferred from Si2 to F atoms, resulting in the extremely robust CBM (mainly controlled by the Si2-p state) locating at the Γ point. Although the position of the CBM is less influenced by the biaxial strain, the VBM, which is principally induced by the energy level of Si1 atoms, moves toward the Fermi level gradually with increasing strains. This is because that increasing strains would increase the charge encompassing around Si1 atoms.31 This phenomenon can also be explained from the corresponding PDOSs (Fig. S6, ESI). In the strained structures, the contribution of the Si1-p states below the Fermi level significantly increased compared to the strain-free state. Under a small biaxial strain that is lower than 0.15, the VBM shifts from the M point to the X point for the first time. As the strain further increases to 0.15, it shifts to the Γ point and forms a direct band gap, which is retained up to a limiting strain of 0.23. It should be mentioned that the strain level simulating in this study could be generally realized in experiments.31,58 For the uniaxial strain, no indirect–direct band gap transition occurs in the fully fluorinated penta-silicene sheet (Fig. S5, ESI). This is due to the relatively simultaneous movement of the VBM and CBM. When the strain increases to 0.15, the VBM shifts from the M point to the middle of the MΓ path. With further increasing the strain to 0.20, the VBM remains at the M point, while the CBM shifts from the Γ point to the M point, which is closely related to the significant increase of the Si1-p states above the Fermi level (Fig. S5c, ESI). For ε ≥ 0.20, the sheet retains such an indirect band gap up to a limiting strain of 0.3. Thus, it can be speculated that the variation of band gaps under strain has an intimate correlation with the Si1–Si2 electronic coupling interaction.31

Simultaneously, the strain-dependence band gaps of the fully fluorinated penta-silicene sheet are also depicted in Fig. 3. For the biaxial strain, by increasing the strain, the band gap (from the HSE functional) increases from 1.82 eV (ε = 0) to 2.05 eV (ε = 0.05), after which it decreases monotonously until it reaches a limiting strain of 0.23. Similar behavior is also found in the uniaxial strain, however, the maxima bandgap approaches to 2.18 eV (ε = 0.10). Such interesting curves in the band gap evolution are mainly attributed to the movements of the entire conduction band. Concretely, the VBM slightly lifts upward in a monotonic way, while the CBM first shifts upward and then shifts downward greatly. The CBM is always located at the Γ point, but the VBM moves its position far from the M point. Overall, the tensile strain can effectively modulate the electronic structure of the 2D sheet. More importantly, the indirect–direct semiconductor transition can be achieved by applying a biaxial strain exceeding 0.15, which is favorable for potential applications in the nanoelectronics field.


image file: c8cp05008b-f3.tif
Fig. 3 Band gaps from PBE and HSE functionals of the fully fluorinated penta-silicene sheet under biaxial strain (a) and uniaxial strain (b). Indirect and direct are short forms for the indirect bandgap and the direct bandgap, respectively.

Our previous study has proved that partial functionalization on 2D materials could bring attractive variation in band structures.28 Thus, we also explore the effect of concentration of fluorination (partial fluorination) on the electronic structures. As mentioned before, the four Si2 atoms are equivalent in the unit cell and the F atoms could be exclusively loaded on top of tricoordinated Si2 atoms forming the sp3 hybridization. Considering the symmetrical atomic arrangement, there are only four configurations for the partially fluorinated penta-silicene, i.e. Si6F3, Si6F2-I, Si6F2-II, and Si6F. Si6F2-I and Si6F2-II indicate the structures of the two fluorine atoms locating on one and two sides, respectively. The optimized configurations of these partially sheets, which are distorted to some extent, are displayed in Fig. S7 (ESI). The band structures and the corresponding PDOSs of these configurations are shown in Fig. S8 (ESI). There are two sub-bandgaps located below and above the sharp peaks at EF in the partially fluorinated sheet Si6F3 (Fig. S8a, ESI). From Fig. S8b (ESI), one can see that the energy bands around the Fermi level of Si6F2-I disperse greatly, even to a single band. PDOS analysis shows that such single bands are mainly originated from the Si2 atoms. In an overall view, as the concentration of the F atoms reduces, the band structures of the partially fluorinated sheets retain the indirect band gap while the gap value decrease greatly. Thus, one can conclude that fluorination could tune the band structures of penta-silicene, leading to transitions from a semiconductor (Si6F4 or Si6F3) to a semimetal (Si6F2-I) and from a semiconductor to a metal (Si6F2-II or Si6F).

2.4. Potential applications as flexible Li-ion battery anodes with fast charge/discharge rates

2D materials become popular electrode materials for rechargeable metal ion batteries in recent years.35,36,59–61 The 2D fully fluorinated penta-silicene nanosheets with furrow paths on their surfaces might be a suitable candidate for an anode material in metal-ion batteries. Thus, we examine the possible application of fully fluorinated penta-silicene sheets in metal ion batteries.

We first search the energetically favorable adsorption sites of a single Li atom on a fully fluorinated penta-silicene sheet. All possible adsorption sites, which are shown in Fig. S9 (ESI), are taken into account. After structural optimization, we found that A, B, C and D sites (Fig. 4) are favorable adsorption sites of a single Li atom, and the rest are relaxed to one of the above four sites during the optimization. The adsorption energy Eads was defined by Eads = EF-pSi + ELiEF-pSi+Li to examine the Li adsorption strength at different sites. EF-pSi+Li and EF-pSi are the total energies of the lithiated and the pristine fully fluorinated penta-silicene sheets, respectively, and ELi is the energy of an isolated Li atom.36 Under this definition, a positive value of Eads means an exothermic adsorption process and a higher Eads implies a more stable adsorption of Li atoms. The calculated results indicated that the A site is the most stable adsorption site with an Eads of 2.88 eV. The second most stable B site is 0.08 eV lower in energy. The fairly minor energy difference between the A and B sites suggests a low Li diffusion barrier along the A ↔ B path. The C site is 0.14 eV lower than the most stable A site, and the D site has an Eads of 2.60 eV. It should be mentioned that, as required,62 the Li adsorption energy of an excellent anode material should be larger than 1.63 eV of bulk lithium to avoid clustering and smaller than ∼3 eV to enable desorption. Moreover, the lithium insertion does not cause conspicuous volume change and the material retains its structural integrity.


image file: c8cp05008b-f4.tif
Fig. 4 Top (upper) and side views (below) of the four stable configurations (A site (a), B site (b), C site (c), and D site (d)) for a single Li atom adsorption on a fully fluorinated penta-silicene sheet after structural optimization.

We further calculate the possible maximum loading of lithium atoms on a fully fluorinated penta-silicene sheet. We found that four Li atoms could be adsorbed steadily on one side of the sheet (Fig. S10d, ESI). Taking both sides into account, the 2 × 2 × 1 supercell could hold eight Li atoms, corresponding to the chemical stoichiometry of Li2(Si6F4) (Fig. S10e, ESI).

The diffusion behavior of Li atoms in materials determines the charge/discharge capability, which is one of the most important indicators to assess the electrode materials. Based on the high symmetry of the fully fluorinated penta-silicene sheet, three possible diffusion paths between the energetically favorable Li adsorption sites (A sites) are identified (Fig. 5a). Specifically, in path 1, the Li atoms directly diffuse between two neighbouring A sites via the A ↔ B ↔ A direction, in path 2, the Li atoms diffuse from the unilateral A site to another A site crossing the C site (i.e. A ↔ C ↔ A direction), and in path 3, Li atoms’ diffusion is along the A ↔ D ↔ A direction. Integrating the three paths, it is concluded that Li atoms could diffuse along the entire surface of the fully fluorinated penta-silicene sheet. To gain theoretical insight into the Li mobility on the sheet, the energy barriers of these diffusion paths are calculated (Fig. 5b). Path 1 has the minimum Li diffusion energy barrier of only 0.08 eV, whereas paths 2 and 3 have Li diffusion energy barriers of 0.14 and 0.28 eV, respectively. Undoubtedly, path 3 is the rate-limiting step. The low Li diffusion barriers mean the rapid diffusion processes on the sheet, i.e., the Li atom is extremely mobile on the surface, which suggests the fast charge/discharge capability of the sheet.63 The Arrhenius equation reveals that the diffusion rate has an exponential dependence on the activation energy barrier,36 and slightly increasing the barrier could result in a large decrease in the diffusion rate.59 Significantly, the lowest energy barrier in path 1 is much lower than those of previously reported anode materials (e.g. 0.33 eV for graphene,64 0.23 eV for silicene,65 0.22 eV for MoS2,7 and 0.19 eV for the GeS sheet36). This implies that at room temperature, the Li diffusion capability of the fully fluorinated penta-silicene sheet is much faster than the above 2D materials.


image file: c8cp05008b-f5.tif
Fig. 5 Schematic illustration of diffusion paths of a single Li (a) and their corresponding diffusion barriers (b) on a fully fluorinated penta-silicene sheet.

It should be noted that the calculated diffusion barriers of the fully fluorinated penta-silicene are very close to those of the unadorned penta-graphene, indicating the steady performance of the fully fluorinated penta-silicene for Li ion batteries. Furthermore, the Bader charge of the Li atom is 0.08, indicating the Li+ cation existence in the adsorption state. In this regard, an external electric field could further enhance the mobility of charged Li ions on the fully fluorinated penta-silicene sheet during the operating processes.

The electrical conductivity, which directly concerns the electrochemical performance of electrode materials, was considered to understand the origin of the fast discharge/charge rates of Li ions on the fully fluorinated penta-silicene sheet. As mentioned in Section 3.3, the fully fluorinated penta-silicene sheet is a semiconductor exhibiting poor electrical conductivity. After adsorbing a Li atom, the system show its metallic feature (Fig. 6a). We speculate that the electron transferring from Li atoms to the sheet results in a semiconductor–metal transition, and exhibits good electrical conductivity. The transition can be explained as follows. The contribution of Li atoms to DOSs is tiny; however, the tiny contribution plays a key role in the total DOS and results in the entire band structures shifting to a lower energy region and passing through the Fermi level (Fig. 6a). Plenty of electronic states locating at EF would promote the electron mobility in the fully fluorinated penta-silicene sheet, and play a key role in the quick charge/discharge processes in Li-ion batteries. The charge density difference Δρ of the most stable adsorption structure (A site) is also taken into account (Fig. S11, ESI) and calculated by Δρ = ρF-pSi+LiρF-pSiρLi. ρF-pSi+Li, ρF-pSi, and ρLi are charge densities of the lithiated fully fluorinated penta-silicene sheet, of the pristine fully fluorinated penta-silicene sheet, and of the isolated Li atom, respectively. The charge density difference is mainly gathered around F and Si atoms, and the Li atom is an electron donor. Thus, the electrons are transferred from Li atoms to the fully fluorinated penta-silicene sheet, which conforms to the electronegativity sequence of F > Si > Li. The Bader charge analysis further manifests that a single Li atom donates 0.92 electrons to the sheet. To further reveal the bonding feature, the electron localization function (ELF) of a Li atom adsorption on the A site is calculated. As shown in Fig. 6b, the abundant shared-electrons between adjacent Si atoms indicate the covalent bonding characteristics of Si–Si bonds, the abundant electrons are tightly localized around F atoms while the electrons around the Li atoms almost approach to zero, revealing the ionic bonding characteristic of Li–F bonds, and the Si–F bonds exhibit both covalent and ionic bonding features.66 Noteworthily, the electron cloud even extending to above the top Li layers (cyan region) shows free electron gas behavior, which is expected to exhibit excellent electronic conductivity.35


image file: c8cp05008b-f6.tif
Fig. 6 The total DOS (a) and ELF plot (b) of a single Li adsorption on a fully fluorinated penta-silicene sheet.

Integrating the above-mentioned results and the flexibility of the functionalised penta-silicene sheets determined in Section 2.2, one can conclude that the reconfigured penta-silicene could be flexible LIB anodes with fast charge/discharge rates.

3. Conclusions

We described a simple surface chemistry reconfiguration approach, i.e. functionalization, to obtain multiple electronic properties, e.g. semiconductors with an indirect bandgap, semiconductors with a direct bandgap, semimetals and metals, from the same parent material. In this paper, we considered fluorination as functionalization, and took penta-silicene as an example. We carried out first principles simulations to examine the properties and applications of functionalized penta-silicene. Although the pristine penta-silicene sheet is dynamically unstable, the fully fluorine functionalized penta-silicene is thermodynamically, dynamically, thermally, and mechanically stable. It possesses superior flexibility and bears high biaxial and uniaxial tensile strains up to 0.23 and 0.30, respectively, which are much higher than those of other 2D materials. The fully fluorinated penta-silicene is a semiconductor with an indirect band gap of 1.82 eV (HSE06 functional), which is suitable for potential application in solar battery materials. Dependence of tunable electronic structures on the concentration of fluorination and external strain was determined. Our calculation results also showed that the fluorination-concentration-induced semiconductor-to-metal transition for fluorinated penta-silicene and the mechanically-induced indirect–direct bandgap transition for fully fluorinated penta-silicene occur. Our calculation results of adsorption and diffusion behaviors also show that the fully fluorinated penta-silicene sheet has low diffusion barriers (0.08–0.28 eV), indicating that it possesses fast diffusion of Li ions. Our results also show that the electrons are transferred from Li atoms to the fully fluorinated penta-silicene sheet, resulting in a transition from a semiconductor to a metal, which would be beneficial to electrical conductivity. The results of the metallic feature and high electrical conductivity as well as its mechanical properties indicate that the functionalized penta-silicene could be a flexible anode material possessing fast charge/discharge rates. The high stability, excellent mechanical properties, tunable electronic structures, remarkable adsorption and diffusion performances endow the fluorinated penta-silicene nanosheet with potential applications in high-performance flexible metal ion batteries, solar batteries and other nanodevices.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NSFC) (Grant No. 21206049 and 51702114), the Innovation Scientists and Technicians Team Construction Projects of Henan Province (CXTD2017002), the Natural Science Foundation of Henan Province (Grant No. 172102210381), the Zhejiang Top Academic Discipline of Applied Chemistry and Eco-Dyeing & Finishing (Grant No. YR2011014), and the Natural Science Foundation of Education Department of Henan Province (Grant No. 18A150011). We thank the High performance Computing Center of Huanghe Science and Technology College for the computational time provided.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c8cp05008b

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