Trends in Physisorption of Ionic Liquids on Boron-Nitride Sheets

Article pubs.acs.org/JPCC

Trends in Physisorption of Ionic Liquids on Boron-Nitride Sheets Mehdi Shakourian-Fard,† Ganesh Kamath,*,‡ and Zahra Jamshidi*,§ †

Department of Chemistry, Sharif University of Technology, P.O. Box 11365-9516, Tehran, Iran Department of Chemistry, University of MissouriColumbia, Columbia, Missouri 65211, United States § Chemistry and Chemical Engineering Research Center of Iran, Tehran, P.O. Box 14335-186, Iran ‡

S Supporting Information *

ABSTRACT: The adsorption of ionic liquids (ILs) on the hexagonal boron-nitride (h-BN) surface was studied at the M06-2X/ cc-pVDZ level of theory. Three types of ionic liquids based on 1-butyl-3-methylimidazolium [Bmim]+, 1-butylpyridinium [Bpy]+, and butyltrimethylammonium [Btma]+ cations, paired with tetrafluoroborate [BF4]−, hexafluorophosphate [PF6]−, and bis(trifluoromethylsilfonyl)imide [Tf2N]− anions were chosen as the adsorbates to better understand the trends in adsorption behavior of ILs on the h-BN surface. We have identified the various stable configurations of the h-BN-ionic liquid (h-BN···IL) complexes based on their binding energies and investigated the effect of charge transfer behavior and noncovalent interactions on the adsorption of ILs. ChelpG analysis indicated that, upon adsorption of ionic liquids on the h-BN surface, the overall charge on the cation, anion, and h-BN surface changes and the transfer (CT) between ILs and h-BN surface occurs. The order for the magnitude of charge transfer between different ILs and the h-BN surface is as follows: [Bmim][Tf2N] (−0.059e) > [Btma][PF6] (0.036e) > [Bpy][Tf2N] (0.028e) > [Btma][Tf2N] (0.021e) > [Bmim][PF6] (0.009e) > [Bpy][BF4] (0.007e) > [Bpy][PF6] (−0.006e) > [Btma][BF4] (−0.003e) > [Bmim][BF4] (−0.001e), respectively. Orbital energy and density of states (DOSs) calculations also show that the HOMO−LUMO energy gap of ILs decreases upon adsorption on the h-BN surface. The order of the HOMO−LUMO gap energy changes of ILs upon adsorption on the h-BN surface is as follows: [Btma][PF6] (3.25 eV) > [Btma][BF4] (2.84 eV) > [Bpy][PF6] (2.41 eV) > [Bpy][BF4] (2.29 eV) > [Bmim][BF4] (1.76 eV) > [Bmim][PF6] (1.54 eV) > [Btma][Tf2N] (1.26 eV) > [Bmim][Tf2N] (1.19 eV) > [Bpy][Tf2N] (0.86 eV), respectively. The binding energies based on QTAIM analysis indicate that the [BF4]−, [PF6]−, and [Tf2N]− anions in the ILs have a stronger interaction with the h-BN surface than [Bmim]+, [Bpy]+, and [Btma]+ cations. The role of cooperative π···π, C−H···π, and X···π (X = N, O, F atoms from anions) interactions on the adsorption of ILs on the h-BN surface was elucidated by analyzing the noncovalent interactions between ILs and the h-BN surface. Energy decomposition analysis (EDA) carried out for the h-BN···IL complexes indicates that the contribution of the ΔEdisp component in each complex is also more than electrostatic (ΔEelect) and orbital (ΔEorb) components (ΔEdisp > ΔEelect > ΔEorb), with the exception of the h-BN[Btma][BF4] complex whose ΔEdisp and ΔEelect components are almost equal. For the complexes with the same cations, dispersion interaction increases by increasing size of anion from [BF4]− to [PF6]− and [Tf2N]−. This is confirmed by more favorable enthalpy of adsorption for ILs on the h-BN surface. The thermochemical analysis also indicates that the free energy of adsorption (ΔGads) of ILs on the h-BN surface is negative, and thus, the adsorption occurs spontaneously. Our first-principles study offers fundamental insights into the nature of the physisorption and solvation behavior of ionic liquids on h-BN. received particular attention,2−4 not surprisingly, since they have several physical features in common (e.g., both are soft and

1. INTRODUCTION Isolation of single sheets of graphene, a truly two-dimensional (2D) material, and the discovery of its unique properties have given rise to vigorous research of other 2D systems.1 A structural analog of graphene, a single sheet of hexagonal boron nitride (h-BN) comprised of alternating boron and nitrogen atoms, has © XXXX American Chemical Society

Received: June 24, 2014 Revised: September 30, 2014

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dx.doi.org/10.1021/jp506277n | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Scheme 1. Structures of Hexagonal Boron Nitride (h-BN) Model and Cations and Anions of ILs Studied in This Work

lubricious), which has resulted in h-BN being termed as “white graphene”. The relatively large difference in electronegativity between nitrogen (3.0) and boron (2.0) imparts a greater localization of π (p) electrons in BN than in graphite. Therefore, in contrast to graphene, BN is a wide band gap semiconductor, is a good electrical insulator, and is highly resistant to oxidation;5 therefore, it is more suitable for very high temperature (up to 1000 K) applications in which graphene would burn.6,7 As a result, h-BN nanosheet building blocks offer intriguingly complementary application perspectives to graphene in nanocomposite materials, in nanoscale electrical devices, and in optical and biomedical systems working under extreme conditions. However, some theoretical studies scrutinized the ability of h-BN sheets to adsorb molecules such as CO2,8 CO,9 O3,10 phenol,11 and chitosan,12 which implies that a h-BN sheet can be used as a gas sensor. In addition, some studies have also focused on the adsorption behavior of nucleobase molecules on the h-BN surface to explore their self-assembly mechanism and the performance of this material as a biosensor.13,14 Notably, the spacing between the layer planes in h-BN is relatively large (0.3615 nm), or more than twice the spacing between the atoms within the plane, meaning that h-BN (like graphite) should be able to accommodate larger intercalating elements. Indeed, the bonds between the planes of h-BN are even weaker than those found in graphite, suggesting promise for use in “soft” prospective exfoliation processes.5 As a process, exfoliation of layered solids has had a transformative effect on materials science and technology by opening up properties found in the two-dimensional (2D) exfoliated forms, not necessarily seen in their bulk counterparts.15 Such exfoliation leads to materials with extraordinary values of crystal surface area. This can result in dramatically enhanced surface activity, leading to important applications, such as electrodes in supercapacitors or batteries. Another result of exfoliation is quantum confinement of electrons in two dimensions, transforming the electron band structure to yield new types of electronic and magnetic materials.15 The liquid-phase exfoliation of various 2D layered materials (e.g., graphite, h-BN, WS2, MoS2) in the presence of various surface energy-matched organic solvents, aqueous surfactant solutions, or ionic liquids (ILs)5,16,17 appears to be a particularly attractive approach to prepare 2D sheets that should be compatible with industrial production. Based on our knowledge, only Kamath et al.5 have reported the exfoliation of a h-BN bilayer with ionic liquids using molecular dynamics (MD) simulation. They calculated the free energies of

exfoliation and dispersion for hexagonal boron nitride (h-BN) monolayers from a model h-BN bilayer using adaptive bias force−molecular dynamics (ABF−MD) simulations to advocate the use of ionic liquids (ILs) in assisting the liquid exfoliation of h-BN to yield individually stabilized (isolated) nanosheets. In this study, we present the first quantum chemical calculations to probe the adsorption properties of ionic liquids on the h-BN surface. Ionic liquids based on three different cation types of 1-butyl-3-methylimidazolium [Bmim]+, 1-butylpyridinium [Bpy]+, and butyltrimethylammonium [Btma]+, paired to tetrafluoroborate [BF4]− (a tetrahedral anion), hexafluorophosphate [PF6]− (an octahedral anion), and bis(trifluoromethylsulfonyl)imide [Tf2N]− (a big anion) anions (see Scheme 1), were chosen and their interactions, orderings, and adsorption features on the h-BN surface are modeled using a h-BN plate composed of 19 rings which is isoelectronic with circumcoronene.18

2. COMPUTATIONAL DETAILS All calculations were carried out by using the M06-2X “Minnesota 2006 functional with double Hartree-Fock exchange” developed by Zhao and Truhlar et al. and Dunning’s cc-pVDZ basis set.19,20 The geometry optimization of ILs, onelayer of hexagonal boron nitride (h-BN), and h-BN···IL complexes was carried out without symmetry restrictions in the singlet ground state. The absence of imaginary frequencies in the calculated vibrational frequencies of the optimized structures ensured that structures are stable. Zero-point energy (ZPE) corrections were calculated and taken into account in the calculation of binding energy (ΔEb). The binding energy was determined as the difference between the energy of h-BN···IL complexes and the sum of the energies of the corresponding h-BN surface and IL (ΔEb = E(h‑BN···IL) − (E(IL) + E(h‑BN))). In addition to binding energy, enthalpy (H) and free energy (G) for the adsorption of ILs on the h-BN surface at 298.15 K were calculated using eq 1 ΔX = X(h‐BN···IL) − (X(IL) + X(h‐BN))

(1)

where X = H and G. The entropy (S) of adsorption was calculated at 298.15 K from eq 2.

ΔS =

(ΔH − ΔG) 298.15

(2)

Boys−Bernardi’s counterpoise procedure (CP) was used to calculate the basis set superposition errors (BSSEs).21 Atomic charges were calculated to fit the electrostatic potential according B



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one, two, and three of its fluorine atoms in each region. Finally, all of these structures for interaction of [Bmim]+, [Bpy]+, and [Btma]+ cations with [BF4]−, [PF6]−, and [Tf2N]− anions were designed and fully optimized at the M06-2X/cc-pVDZ level of theory. Figure 1 shows the most stable geometries of ILs along with the distances of electronegative N, O, and F atoms of anions from the C−H bonds of the imidazolium ring and methyl and butyl groups of cations. As can be found in Figure 1, in ILs based on [Bmim]+ cation, [BF4]− and [PF6]− anions have a significant interaction with the imidazolium ring through the C−H bond of the imidazolium ring and the C−H bonds of the methyl and butyl groups by their fluoride atoms. The [Tf2N]− anion prefers to be located below the imidazolium ring and in close contact with the C−H bonds of the methyl and butyl groups and the imidazolium ring. Interaction of [BF4]−, [PF6]−, and [Tf2N]− anions with the [Bmim]+ cation leads to an increase of the C2−H (C−H bond between two nitrogen atoms in the imidazolium ring) bond length of about 0.01, 0.009, and 0.008 Å, respectively. In [Bpy][BF4] and [Bpy][PF6] ionic liquids, [BF4]− and [PF6]− anions interact with the C−H bonds of the butyl group and the pyridinium ring. Similar to the case of [Bmim][Tf2N], in the [Bpy][Tf2N] ionic liquid we find that the [Tf2N]− anion prefers to lie below the pyridinium ring and in close contact with the C−H bonds of the butyl group and the pyridinium ring. Our results indicate that in comparing ILs based on [Bmim]+ and [Bpy]+ cations, changes in the C−H bond lengths of pyridinium ring are less than those of the imidazolium ring. The structural properties of the ILs determined based on the [Bmim]+ and [Bpy]+ cations are in good agreement with the literature studied.32,38 In the ILs based on the [Btma]+ cation, the anions lie close to the C−H bonds of the methyl and butyl groups of the cation. Recently, Castner and co-workers39,40 and Damodaran and co-workers41 have also shown that anions in ILs based on tetraalkylammonium cation can be found close to the positive nitrogen atom by interaction with the C−H bonds of the alkyl groups. The nature of these hydrogen bond interactions in ILs can be analyzed by Bader’s topological QTAIM analysis23 using topological parameters such as electron density ρ(r), the Laplacian of electron density (∇2ρ(r)), and the electronic energy density (H(r)) at the bond critical points (BCPs) of hydrogen bonds. H(r) is associated with the kinetic energy density (G(r)), which is always positive, and the potential energy density (V(r)), which is always negative, using the equation H(r) = G(r) + V(r). Based on the sign of ∇2ρ(r) and H(r) at the bond critical points of hydrogen bonds, Koch and Popelier42 proposed that H-bonds are classified as weak and medium (∇2ρ(r), and H(r) > 0), strong (∇2ρ(r) > 0, and H(r) < 0), and very strong (∇2ρ(r), and H(r) < 0). ∇2ρ(r) and H(r) are also used to understand the covalent and electrostatic nature of weak interactions. The QTAIM analysis was performed for wave functions obtained at the M06-2X/cc-pVDZ level of theory. The computed electron density (ρ(r)), the Laplacian of electron density (∇2ρ(r)), kinetic energy density (G(r)), potential energy density (V(r)), and the electronic energy density (H(r)) at the H-bond’s BCPs between cation and anion of the ionic liquids are summarized in Table S1 in the Supporting Information. A positive value of ∇2ρ(r) at the BCPs of H-bonds (listed in Table S1) indicates that these interactions should be classified as a closed-shell (electrostatic) type of bonding. On the other hand, negative and positive values of H(r) for H-bonds imply the

to the ChelpG22 method. To check the sensitivity of the h-BN sheet to the adsorption of ILs, the electronic properties of the considered h-BN···IL complexes were analyzed from their density of state (DOS) spectra. Noncovalent interaction plots (NCIPLOTS) were produced for visualization of noncovalent interaction regions between ILs and the h-BN surface. To realize changes created in the strength of hydrogen bond interactions between cations and anions of ionic liquids upon adsorption on the h-BN surface and compare the interaction strengths of cations and anions with the h-BN surface, the quantum theory of atoms in molecules (QTAIM)23 analysis was also performed. Finally, energy decomposition analysis (EDA) was performed to understand the nature of interaction between ILs and the h-BN surface at the PBE-D3/TZP level of theory. Optimization of the ILs, the h-BN surface, and the h-BN···IL complexes, and the calculation of ChelpG charge analysis were performed by the Gaussian 09 program.24 The density of state (DOS) spectra and the HOMO−LUMO band gap of ILs and h-BN···IL complexes were calculated using the GaussSum 2.2 program.25 The NCIPLOT program26,27 was used for visualization of noncovalent interaction regions between ILs and the h-BN surface. The AIM2000 package28 was used to analyze bond critical point (BCP) data in ionic liquids and h-BN···IL complexes from the wave functions generated at the M06-2X/ cc-pVDZ level of theory. Finally, EDA was performed by ADF (2010.01) software29−31 to understand the nature of interactions in h-BN···IL complexes.

3. RESULTS AND DISCUSSIONS 3.1. Structures and Energetics of Ionic Liquids. Ionic liquids based on three different cation types of 1-butyl-3methylimidazolium [Bmim]+, 1-butylpyridinium[Bpy]+, and butyltrimethylammonium [Btma]+, paired to tetrafluoroborate [BF4]− (a tetrahedral anion), hexafluorophosphate [PF6]− (an octahedral anion), and bis(trifluoromethylsulfonyl)imide [Tf2N]− (a big anion) anions (see Scheme 1), are chosen for adsorption on a boron-nitride (h-BN) sheet. [Bmim]+ and [Bpy]+ cations can be adsorbed on the h-BN surface via their positive five- and six-membered aromatic rings, and also methyl and butyl alkyl groups. [Btma]+ cation, which does not have an aromatic part, is adsorbed only by methyl and butyl alkyl groups. Although cation−anion interactions in ionic liquids are mainly dominated by Coulombic forces, the hydrogen bonding also plays an important role. For example, for imidazolium based ionic liquids, the hydrogen bonding between cation and anion is mainly determined by the most acidic hydrogen at the cation in position C2 (carbon atom between two nitrogen atoms in the imidazolium ring). This effect is evident when the hydrogen atom at the C2 position is replaced by a methyl group. For example, 1-butyl-2-methyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C4C1mim][Tf2N] has a lower binding energy than its structural isomer (1-pentyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C5mim][Tf2N] ionic liquid), emphasizing the importance of the hydrogen bonding between the ions.32 To find the most stable geometries of ILs considered in this study, we used the method described previously in the literature.33−37 In this method, the regions around the most stable geometry of the cations are divided into several regions and the most stable geometry of anions is located in these regions. There are also some subconfigurations in each region which are related to different orientations of anion with respect to the cation. For example, the [BF4]− anion could interact with the cation through C



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Figure 1. Most stable geometries of ionic liquids studied at the M06-2X/cc-pVDZ level of theory (all bond lengths are in Å).

(−82.95 kcal/mol) > [Bpy][Y] (−79.87 kcal/mol) (Y = BF4−, PF6−, and Tf2N−). Our results are in good agreement with the results of Fernandes et al. in exploring the influence of cation type on the binding energy.32 They have indicated that the reduction of the electrostatic strength due to the charge delocalization in cations with aromatic rings (such as imidazolium and pyridinium), as compared to a localized charge on the nitrogen atom of saturated rings (such as pyrrolidinium and piperidinium), is reflected in the relative values of binding energies. Aromatic-based ILs (pyridinium- and imidazolium-based) present lower relative binding strength values when compared with their saturated counterparts (piperidinium- and pyrrolidinium-based). A comparison between ILs based on [Btma]+ cation and cations with aromatic rings ([Bmim]+ and [Bpy]+) (for the same anion type) indicates that the binding energy values of ILs based on [Btma]+ cation lie between the binding energy values of ILs based on [Bmim]+ and [Bpy]+ cations. Therefore, the magnitude

covalent and electrostatic nature of the corresponding H-bonds, respectively. Thus, the H-bonds in the ILs are classified as weak and medium (∇2ρ(r), and H(r) > 0)) and strong (∇2ρ(r) > 0, and H(r) < 0) in nature. Binding energy is one of the most powerful measurements for estimating the strength of the noncovalent interactions. Binding energy (ΔEb) of ILs is defined as the difference between the energy of the IL and the sum of the energies of the cation and anion species (ΔEb = E(IL) − (E(Cation) + E(Anion))) and is listed in Table 1. The influence of cation type on the binding energy was investigated with the fixed [BF4]−, [PF6]−, and [Tf2N]−anions. As Table 1 shows, the increase in cation size from a five (imidazolium) to a six (pyridinium)-membered ring (in ILs with the same anion type) increases the distance between the charges while delocalizing them as well and, therefore, resulting in a decrease in the binding energy. The magnitude of the average binding energy values for these ILs follows the order [Bmim][Y] D



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Table 1. Binding Energies (ΔEb in kcal/mol), Zero-Point Energy Corrections (ΔEZPE in kcal/mol), and Basis Set Superposition Errors (BSSEs in kcal/mol) for the ILs and h-BN···IL Complexes Structure

ΔEba

ΔEZPE

ΔEBSSE

Structure

ΔEbb

ΔEZPE

ΔEBSSE

[Bmim][BF4] [Bmim][PF6] [Bmim][Tf2N] [Bpy][BF4] [Bpy][PF6] [Bpy][Tf2N] [Btma][BF4] [Btma][PF6] [Btma][Tf2N]

−87.66 −81.05 −80.15 −84.41 −77.99 −77.21 −86.36 −79.71 −78.74

2.23 2.30 2.09 2.11 2.17 1.84 2.14 2.31 2.08

11.08 12.16 11.74 10.20 11.62 10.51 10.80 9.86 10.35

h-BN[Bmim][BF4] h-BN[Bmim][PF6] h-BN[Bmim][Tf2N] h-BN[Bpy][BF4] h-BN[Bpy][PF6] h-BN[Bpy][Tf2N] h-BN[Btma][BF4] h-BN[Btma][PF6] h-BN[Btma][Tf2N]

−10.16 −9.87 −6.35 −11.84 −10.60 −7.36 −9.99 −9.33 −7.21

1.682352 1.864957 1.936493 2.121608 1.869349 2.120353 1.817894 1.312749 2.294173

11.04 11.39 11.49 11.19 11.12 11.49 10.16 10.48 11.24

ΔEb corrected by ΔEZPE and ΔEBSSE.

a,b

the adsorption of [Bmim][BF4] on the h-BN surface, [Bmim]+ cation tends to arrange in a parallel orientation with respect to the h-BN surface at a distance of 3.17 Å. The boron atom in the [BF4]− anion lies at a distance of 3.18 Å from the surface, three of the fluoride atoms form a plane parallel to the imidazolium ring surface, and the fourth one lies above the imidazolium ring surface. In this structure the butyl chain of the adsorbed [Bmim]+ cation is oriented toward the [BF4]− anion and also interacts with the h-BN surface through its C−H bonds in such a way that the distance of the nearest carbon atom of the butyl chain from the surface is around 3.34 Å. The optimized h-BN···[Bmim][PF6] structure is shown in Figure 2. In this structure, three of the fluoride atoms are present at a distance of 2.66−2.74 Å with the surface. In this case, the [PF6]− anion tends to interact with the surface at a closer distance than the imidazolium ring. As seen from Figure 2, the [Bmim]+ cation interacts with the surface through the hydrogens present on the methyl and butyl groups and those present at the bottom of imidazolium ring. The butyl chain of the imidazolium ring orients itself parallel with respect to the h-BN surface. Under these circumstances, the middle carbon atom and methyl group at the end of the butyl chain lie 3.68 and 3.35 Å above the h-BN surface, respectively, indicating a physisorption type process. The orientation of [Bmim][PF6] on the h-BN surface upon adsorption is different from the case of [Bmim][PF6] adsorption on the graphene surface reported in the literature.18 Ghatee et al. indicated that the [PF6]− anion tends to interact with the graphene surface at a closer distance than the imidazolium ring and the ring tends to be arranged in parallel orientation to the surface but tilts itself slightly in such a way that the nitrogen atom connected to the methyl group is nearer to the surface than the nitrogen atom connected to the butyl group. The alkyl chain of the imidazolium cation is placed almost parallel to the surface such that the distance of the nearest H atoms is in the range of 3.26−3.99 Å.18 This comparison shows that the adsorption behavior of ionic liquids on the h-BN (a hydrophilic surface) sheet is different from that of the graphene (a hydrophobic surface) sheet. This difference indicates that the hydrophobicity/ hydrophilicity characteristic of the surface has an important influence on the adsorption of ionic liquids on such surfaces. The geometry of the h-BN···[Bmim][Tf2N] complex is shown in Figure 2. In this structure, two oxygen atoms of −SO2 groups and four fluoride atoms of −CF3 groups in the [Tf2N]− anion form a plane parallel to the h-BN surface. The [Tf2N]− anion tends to interact with the boron and especially nitrogen atoms of the surface via its oxygen and fluoride atoms. Our results are supported by the recent MD simulation performed by Kamath et al.5 They indicated that the [Tf2N]− interactions with the

of the average binding energy values for these ILs follows the order: [Bmim][Y] (−82.95 kcal/mol) > [Btma][Y] (−81.60 kcal/mol) > [Bpy][Y] (−79.87 kcal/mol) (Y = BF4−, PF6−, and Tf2N−). As shown in Table 1, comparing binding energies for ILs containing the same anions shows that the magnitude of ΔEb follows the order [X][BF4] (−86.14 kcal/mol) > [X][PF6] (−79.58 kcal/mol) > [X][Tf2N] (−78.70 kcal/mol) (X = Bmim+, Bpy+, and Btma+). The decrease of binding energies can be due to the increase in anion size and increase in charge delocalization by going from [BF4]− to [PF6]− and [Tf2N]− anions. A similar effect has been reported in the correlation between proton affinity and charge delocalization of ionic liquid anions, whereby a greater degree of charge delocalization lowers the proton affinity.43 3.2. Adsorption of Ionic Liquids on the h-BN Surface. For understanding the effect of the h-BN surface on the adsorption of ionic liquids (ILs), we used a h-BN sheet composed of 19 rings containing boron and nitrogen atoms and passivated with hydrogens. This model is isoelectronic with circumcoronene, which has been shown to be a good representation model analog for graphene.18,44,45 In this model, all the boundary boron and nitrogen atoms have been saturated with hydrogen atoms (see Scheme 1). The adsorption of ILs on the h-BN surface could occur due to favorable interactions of various regions of ILs with the h-BN surface. These regions are the five- and six-membered aromatic rings, hydrogen atoms of the aromatic rings, alkyl (methyl and butyl) groups of the cation, and N, O, and F atoms of anions. Upon adsorption of ILs on the h-BN surface via these regions, the h-BN surface exhibits an out-of-plane mode, which is due to noncovalent interactions between the h-BN surface and ILs. In order to find the most stable geometries of the h-BN···IL complexes, we placed the IL structures in all possible conformations with respect to the h-BN surface. For better understanding the possible adsorption configurations, the initial structures for adsorption of an [Bmim][BF4] ionic liquid on the h-BN surface are shown in Figure S1. After specifying the initial structures, these h-BN···IL complexes were optimized at the M06-2X/ccpVDZ level of theory. The most stable geometries for adsorption of various ILs on the h-BN surface are displayed in Figure 2. The nearest distances of anion and cation from the h-BN surface and the position of anion relative to cation (Figure 2) reveal the orientation of the ILs with respect to the h-BN surface. a. [Bmim][Y] (Y = BF4−, PF6−, and Tf2N−) Adsorption on the h-BN Surface. As shown in Figure 2, [Bmim]+ cation in [Bmim][Y] (Y = BF4−, PF6−, and Tf2N−) ionic liquids tend to take different orientations with respect to the h-BN surface. In E



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Figure 2. Most stable geometries for adsorption of ILs on the h-BN surface optimized at the M06-2X/cc-pVDZ level of theory (bond lengths are in Å). (a) h-BN[Bmim][BF4], (b) h-BN[Bmim][PF6], (c) h-BN[Bmim][Tf2N], (d) h-BN[Bpy][BF4], (e) h-BN[Bpy][PF6], (f) h-BN[Bpy][Tf2N], (g) h-BN[Btma][BF4], (h) h-BN[Btma][PF6], and (i) h-BN[Btma][Tf2N]. Dark gray is used to color carbon, light gray for hydrogen, blue for nitrogen, red for oxygen, beige for fluorine, olive for sulfur, pink for boron, and lime for phosphorus.

distance from the h-BN surface. The imidazolium ring in the [Bmim]+ cation tends to be arranged in parallel orientation to the surface above the [Tf2N]− anion, and the butyl chain is bent

h-BN surface arise primarily from weak interactions between the heteroatoms of the anion and the N atom from the h-BN surface. The [Bmim]+ cation tends to be oriented and placed at a further F



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to the h-BN surface seems to be due to the surface dipolarity of h-BN. As seen from Figure 2, the heteroatoms of the [Tf2N]− anion have less distance from the h-BN surface than the carbon atoms of the [Bpy]+ cation, indicating a higher interaction with the surface. The [Tf2N]− anion also interacts directly with the h-BN surface, while interacting closely with the positive part of the pyridinium ring. c. [Btma][Y] (Y = BF4−, PF6−, and Tf2N−) Adsorption on the h-BN Surface. The most stable geometries and orientation of [Btma][BF4], [Btma][PF6], and [Btma][Tf2N] on the h-BN surface are displayed in Figure 2. We find that the [Btma]+ cation in the ionic liquids takes two orientations with respect to the h-BN surface. The [Btma]+ cation in the [Btma][BF4] and [Btma][PF6] tends to be arranged in a parallel orientation with respect to the h-BN surface by interaction of one butyl and one methyl group, while it interacts with the h-BN surface via its two methyl groups in [Btma][Tf2N]. On the other hand, three of the fluoride atoms in the [BF4]− and [PF6]− anions, two oxygen atoms of the −SO2 group, and two fluoride atoms of the −CF3 group in the [Tf2N]− anion form a plane parallel to the h-BN surface. As seen from Figure 2, the anions interact directly with the h-BN surface through the fluoride and oxygen atoms, while interacting closely with the positive part of the [Btma]+ cation. One of the most important changes in IL structures upon adsorption on the h-BN surface is in the magnitude of the hydrogen bond strength between the cation and the anion. Bond critical points (BCPs) data for the H-bonds between the cation and the anion of ionic liquids and adsorbed ILs on the h-BN surface are summarized in Table S1 and Table S2, respectively. Based on the QTAIM analysis, the hydrogen bond energies (E(H···X)) are also calculated using the equation of E(H···X) = 1/2V(r).46,47 As seen from Tables S1 and S2, the changes in the sum of electron density (∑ρ(r)) and hydrogen bond energy (∑E(H···X)) values for the H-bonds of ILs before and after adsorption of ILs on the h-BN surface are significant. Although the individual interactions were observed between ILs and the h-BN surface, the hydrogen bonds inside the ILs still have an important effect in the adsorption system. Upon adsorption of ILs, the sum of electron density (∑ρ(r)) (along the BCPs) and hydrogen bond energy (∑E(H···X)) values decreases. The order of electron density changes in BCPs of hydrogen bonds upon adsorption of ILs on the h-BN surface (based on the [Bmim]+, [Bpy]+, and [Btma]+ cations) is as follows: [Bmim][BF4] > [Bmim][PF6] > [Bmim][Tf2N], [Bpy][BF4] > [Bpy][PF6] > [Bpy][Tf2N], and [Btma][PF6] > [Btma][Tf2N] > [Btma][BF4], respectively. The order for electron density changes in BCPs of hydrogen bonds upon adsorption of ILs on the h-BN surface is as follows: [Bmim][BF4] > [Btma][PF6] > [Bmim][PF6] > [Btma][Tf2N]> [Bpy][BF4] > [Btma][BF4] > [Bpy][PF6] > [Bpy][Tf2N] > [Bmim][Tf2N], respectively. In order to find the binding strength of cation and anion with the h-BN surface, the data extracted from bond critical points (BCPs) formed between cation, anion, and h-BN surface in h-BN···IL complexes is summarized in Tables S3 and S4, respectively. As seen from binding energy values, E(Y···X), (X and Y = interacting atoms of cation, anion, and h-BN surface) listed in Tables S3 and S4, we find that the [BF4]−, [PF6]−, and [Tf2N]− anions in the ILs have a stronger interaction with the h-BN surface than the [Bmim]+, [Bpy]+, and [Btma]+ cations. The [BF4]−, [PF6]−, and [Tf2N]− interactions with the h-BN surface arise primarily from interactions between the O, N, and F heteroatoms of the anions and the N atoms from the h-BN surface. The [Bmim]+, [Bpy]+, and [Btma]+ interactions occur

downward with respect to the imidazolium ring such that the methyl group at the end of the butyl chain could interact with the surface at a distance of 3.27 Å. These results are in good agreement with the results of the MD simulation.5 The MD simulation results have indicated that in the interaction of the [Bmim]+ cation with the h-BN surface the butyl carbon interactions with the h-BN surface tend to dominate over the ring carbon or methyl interactions with the h-BN surface. The comparison of our results for adsorption of [Bmim][Tf2N] on the h-BN surface and the results of the MD simulation performed by Kamath et al. for adsorption of [Bmim][Tf2N] on a graphene surface17 show that the crucial stabilizing interactions in a graphene···[Bmim][Tf2N] complex should arise from π−π interactions between the imidazolium ring and the basal plane of a graphene sheet, although the stabilizing interactions in a h-BN···[Bmim][Tf2N] complex arise primarily from weak interactions between the heteroatoms of the [Tf2N]− anion and the N atom from the h-BN surface, indicating the significance of anion−π interactions. The surface dipolarity of h-BN may account for the weak interactions of the [Tf2N]− anion with the h-BN surface. b. [Bpy][Y] (Y = BF4−, PF6−, and Tf2N−) Adsorption on the h-BN Surface. The most stable geometry and orientation of the [Bpy][BF4] and [Bpy][PF6] on the h-BN surface are displayed in Figure 2. With the adsorption of [Bpy][BF4] and [Bpy][PF6] on the h-BN surface, the boron and phosphorus atoms of the [BF4]− and [PF6]− anions lie at distances of 3.20 and 3.73 Å, respectively, from the surface and three of the fluoride atoms form planes parallel to the imidazolium ring surface and the other fluoride atoms in the [BF4]− and [PF6]− anions lie above the imidazolium ring surface. In the [Bpy][BF4] and [Bpy][PF6] structures, the [Bpy]+ cation tends to be arranged in a parallel orientation with respect to the h-BN surface at a distance of 3.20 and 3.15 Å, respectively. The butyl chain of the adsorbed [Bpy]+ cation is oriented slightly toward the [BF4]−and [PF6]− anions and also interacts with the h-BN surface through the C−H bonds in such a way that the distance of the nearest carbon atoms of the butyl chain from the surface is 3.36 and 3.33 Å, respectively. In these structures, the [BF4]− and [PF6]− anions tend to interact with the nitrogen atoms of the h-BN surface via their fluoride atoms. The h-BN surface also interacts with the [Bpy]+ cation via weak interactions between its nitrogen atoms and the carbon and nitrogen atoms of the pyridinium ring and the C−H bonds of the butyl chain. To further confirm the presence and role of noncovalent interactions in adsorption of ILs on the h-BN surface, the contribution of noncovalent interactions was evaluated as shown in the next section. The geometry of the h-BN···[Bpy][Tf2N] complex is shown in Figure 2. Note that the important point in adsorption of [Bpy][Tf2N] is the orientation of the [Bpy]+ cation with respect to the h-BN surface. The pyridinium ring tends to be arranged in a perpendicular orientation with respect to the h-BN surface and interacts via its C−H bonds with the h-BN surface, although the butyl chain tends to be parallel with respect to the h-BN surface atoms, leading to a larger surface interaction of the IL with the surface. A recent MD simulation performed by Kamath et al.5 also indicates that there is a higher probability of populations of butyl interactions with the h-BN surface than the π−π interactions between the pyridinium ring and h-BN surface. It is worth mentioning that this is in stark contrast to the [Bpy][Tf2N] interaction with the graphene surface, where π−π interactions were found to be prevalent.17 The particular orientation of the [Bpy]+ cation in [Bpy][Tf2N] with respect G



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[Bmim][Y] (Y = BF4−, PF6−, and Tf2N−) ILs display a DOS featuring roughly a Maxwellian distribution in HOMO states. The LUMO states are nearly extended from zero to the positive energies. Upon adsorption on the h-BN, DOS featuring ILs change noticeably and their energies shift partly. The h-BN surface involves states which cover partly the band gap of [Bmim][Y] (Y = BF4−, PF6−, and Tf2N−) from the occupied side by about 1.413, 1.637, and 1.302 eV, respectively, to higher energies and from the unoccupied side by about 0.350 and 0.116 eV to lower energies for [Bmim][BF4] and [Bmim][Tf2N], respectively, although the LUMO states have a small shift to higher energies in [Bmim][PF6] upon adsorption on the h-BN surface. These changes lead to a reduction in the band gap of [Bmim][Y] (Y = BF4−, PF6−, and Tf2N−), by 1.76, 1.54, and 1.19 eV, respectively. It is worth mentioning that the DOS for the case of adsorption of [Bmim][PF6] on the h-BN surface is in contrast to the DOS for the adsorption of [Bmim][PF6] on the graphene surface.18 The reductions in band gap of [Bmim][PF6] upon adsorption on the h-BN and circumcoronene surfaces are 1.54 and 4.28 eV,18 respectively. The difference in the DOS and band gap of [Bmim][PF6] adsorbed on the h-BN and graphene may be due to the difference in polarity between h-BN (hydrophilic surface) and graphene (hydrophobic surface) surfaces and the difference in the orientation of [Bmim][PF6] on the surfaces. As seen from Figure 3, upon adsorption of [Bpy][Y] (Y = BF4−, PF6−, and Tf2N−) on the h-BN surface, the HOMO states move to higher energies such that the states of the h-BN surface cover partly the band gap of ILs from the occupied side of about 2.15, 2.28, and 0.96 eV, respectively, to higher energies. The LUMO states of [Bpy][BF4] and [Bpy][PF6] move slightly to lower energies, although the LUMO states of [Bpy][Tf2N] move with little change to higher energies. These changes lead to a reduction of 2.29, 2.41, and 0.86 eV in the band gap of [Bpy][Y] (Y = BF4−, PF6−, and Tf2N−), respectively. A comparison of our results for the DOS of ILs adsorption on the h-BN surface and the coronene (chosen as graphite model) surface reported by Baker et al.22 indicates that the HOMO−LUMO band gap for the ILs composed of [Bpy]+cation and [BF4]−, [PF6]−, and [Tf2N]− anions decreases upon adsorption on both h-BN and coronene surfaces, and this decrease in the band gap primarily results from a rise in the HOMO states energy and little change in the LUMO states energy. As seen from Figure 3, the HOMO states in [Btma][Y] (Y = BF4−, PF6−, and Tf2N−) are extended from −10.31, −10.84, and −8.99 eV to the more negative energies, although the LUMO states are extended from 1.04, 0.92, and 0.89 eV to the more positive energies, respectively. Upon adsorption of [Btma][Y] (Y = BF4−, PF6−, and Tf2N−) on the h-BN surface, the HOMO states move to higher energies such that the states of the h-BN surface cover partly the band gap of ILs from the occupied side of about 2.57, 3.00, and 0.98 eV, respectively, to higher energies. The LUMO states of ILs move slightly to lower energies of about 0.27, 0.24, and 0.27 eV, respectively. These changes lead to a reduction of 2.84, 3.25, and 1.26 eV in the band gap of [Btma][Y] (Y = BF4−, PF6−, and Tf2N−), respectively. In general, it seems that the cation and anion type, IL configuration, and orientation of ILs on the h-BN surface influence the DOSs of these ILs. 3.5. Charge Transfer between Ionic Liquids and h-BN Surface. To investigate the atomic charge distributions of ILs, the h-BN surface, and also their modification by the adsorption, ChelpG49 analysis has been carried out at the M06-2X/cc-pVDZ level of theory. Recent works confirm the accuracy of ChelpG analysis to explore charge distribution in adsorbate/adsorbent

between B and especially N atoms of h-BN and H, C, N atoms from cations. The presence of these weak interactions between cation, anion, and h-BN surface is revealed by the green regions in noncovalent interaction plots shown in the next sections. 3.3. Binding Energy of h-BN···IL Complexes. The stability of h-BN···IL complexes was further evaluated by computing the binding energies (ΔEb) between the h-BN surface and ionic liquids. As seen from Table 1, the low value of binding energy (−7 to −12 kcal/mol) proves that the van der Waals type of interaction can be significant. The binding energy values for adsorption of ILs based on [Bmim]+, [Bpy]+, and [Btma]+ cations on the h-BN surface follow the order h-BN[Bmim][BF4] (−10.16 kcal/mol) > h-BN[Bmim][PF6] (−9.87 kcal/mol) > h-BN[Bmim][Tf 2N] (−6.35 kcal/mol), h-BN[Bpy][BF4] (−11.84 kcal/mol) > h-BN[Bpy][PF6] (−10.60 kcal/mol) > h-BN[Bpy][Tf2N] (−7.36 kcal/mol), and h-BN[Btma][BF4] (−9.99 kcal/mol) > h-BN[Btma][PF6] (−9.33 kcal/mol) > h-BN[Btma][Tf2N] (−7.21 kcal/mol), respectively. The order for all 9 ILs is h-BN[Bpy][BF4] > h-BN[Bpy][PF6] > h-BN[Bmim][BF4] > h-BN[Btma][BF4] > h-N[Bmim][PF6] > h-BN[Btma][PF6] > h-BN[Bpy][Tf2N] > h-N[Btma][Tf2N] > h-BN[Bmim][Tf2N], respectively. This order indicates that the cation and anion type, the arrangement of the cation and anion in ILs, and the adsorption behavior of ILs on the h-BN surface could be the most important factors influencing the binding energy values between ILs and the h-BN surface. 3.4. Orbital Energy and Density of States (DOS) Calculations. The HOMO, LUMO, and HOMO−LUMO energy gaps of the ILs, h-BN, and h-BN···IL complexes are shown in Table 2. This table shows that the HOMO−LUMO Table 2. HOMO−LUMO Gap Energies of ILs, h-BN, and h-BN···IL Complexes

a

Structure

ΔEa (eV)

Structure

ΔE (eV)

h-BN sheet [Bmim][BF4] [Bmim][PF6] [Bmim][Tf2N] [Bpy][BF4] [Bpy][PF6] [Bpy][Tf2N] [Btma][BF4] [Btma][PF6] [Btma][Tf2N]

8.98 9.68 9.66 8.86 8.24 8.30 7.12 11.35 11.76 9.88


7.92 8.12 7.67 5.95 5.89 6.26 8.51 8.51 8.62

ΔE = E(LUMO) − E(HOMO).

energy gap of ILs falls between 7.12 and 11.76 eV and the band gap of ILs decreases upon adsorption on the h-BN surface. The density of state (DOS) of a system describes the number of states per interval of energy at each energy level that are available to be occupied by electrons. DOS is a useful technique to investigate the changes in the HOMO−LUMO gap due to molecular interactions.48 The DOS spectra of h-BN, ILs, and ILs adsorbed on the h-BN surface are presented in Figure 3. The order of the HOMO−LUMO energy gap changes of ILs upon adsorption on the h-BN surface is as follows: [Btma][PF6] (3.25 eV) > [Btma][BF4] (2.84 eV) > [Bpy][PF6] (2.41 eV) > [Bpy][BF4] (2.29 eV) > [Bmim][BF4] (1.76 eV) > [Bmim][PF6] (1.54 eV) > [Btma][Tf2N] (1.26 eV) > [Bmim][Tf2N] (1.19 eV) > [Bpy][Tf2N] (0.86 eV), respectively. The H



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Figure 3. Density of states (DOSs) calculated at the M06-2X/cc-pVDZ level of theory before and after adsorption of ionic liquids on the h-BN surface.

Table 3. Charge Difference of Cation, Anion, and h-BN Surface after Adsorption (Obtained by ChelpG Approximation for M06-2X/cc-pVDZ Wavefunctions) (values in e)a Structure

Δqb (cation)

Δqc (anion)

Δq (h-BN sheet)

[Bmim][BF4] [Bmim][PF6] [Bmim][Tf2N] [Bpy][BF4] [Bpy][PF6] [Bpy][Tf2N] [Btma][BF4] [Btma][PF6] [Btma][Tf2N]

0.125 0.087 0.013 0.119 0.064 0.094 0.060 0.118 0.055

−0.126 −0.077 −0.072 −0.112 −0.070 −0.066 −0.063 −0.082 −0.034

−0.001 0.009 −0.059 0.007 −0.006 0.028 −0.003 0.036 0.021

It seems that the difference in the polarity of surfaces51 (coronene with nonpolar homonuclear C−C intralayer bonds and h-BN with highly polar B−N bonds) and the difference in the orientation of ILs on surfaces are important factors influencing the amount and direction of charge transfer. According to Table 3, upon adsorption of [Bpy][Y] (Y = BF4−, PF6−, and Tf2N−), the overall charge on the h-BN surface becomes positive except for [Bpy][PF6], indicating charge transfer from the h-BN surface to IL. The charge analysis also shows that adsorption of [Bpy][Y] (Y = BF4−, PF6−, and Tf2N−) causes the cation to turn less positive by 0.119, 0.064, and 0.094e, respectively, and the anions to be less negative by −0.112, −0.070, and −0.066e, respectively, and accordingly, the net charges induced on the h-BN surface are 0.007, −0.006, and 0.028e, respectively. The charge variations on cation, anion, and h-BN surfaces for these complexes are also shown in Figure S2. The charge transfer process for adsorption of [Bpy][Y] (Y = BF4−, PF6−, and Tf2N−) on the h-BN surface is different from the coronene surface such that the charge is transferred from [Bpy][PF6] to the coronene surface, although for [Bpy][BF4] and [Bpy][Tf2N] charge is transferred from the coronene surface to ILs.22 The adsorption of [Btma][Y] (Y = BF4−, PF6−, and Tf2N−) on the h-BN causes the cation to become less positive by 0.060, 0.118, and 0.055e and the anions less negative by −0.063, −0.082, and −0.034e, respectively. The sign of induced charges on the h-BN surface indicates that charge transfer (CT) occurs from the [Btma][BF4] to the h-BN surface, although it is done from the h-BN surface to [Btma][PF6] and [Btma][Tf2N]. The order for the magnitude of charge transfer between different ILs and the h-BN surface is as follows: [Bmim][Tf2N] (−0.059e) > [Btma][PF6] (0.036e) > [Bpy][Tf2N] (0.028e) > [Btma][Tf2N] (0.021e) > [Bmim][PF6] (0.009e) > [Bpy][BF4] (0.007e) > [Bpy][PF6] (−0.006e) > [Btma][BF4] (−0.003e) > [Bmim][BF4] (−0.001e), respectively. 3.6. Noncovalent Interaction (NCI) Plots. It is generally believed that the adsorptions of ILs on the surfaces are dominated by the noncovalent weak interactions,50 which are usually well described by the novel hybrid meta-GGA functionals such as M06-2X, developed by Truhlar et al.19,20 For revealing

a

The charge of the h-BN surface before adsorption is zero. Δq (cation) = qcation in IL (before adsorption) − qcation in IL (after adsorption); Δq for cation is positive, and the positive value means gaining charge. c Δq (anion) = qanion in IL (before adsorption) − qanion in IL (after adsorption); Δq for anion is negative, and the negative value means losing charge. b

systems.22,50 The results of ChelpG analysis are summarized in Table 3 and also shown in Figure S2. The adsorption of [Bmim][Y] (Y = BF4−, PF6−, and Tf2N−) on the h-BN causes the cation to become less positive (gaining charge) by 0.125, 0.087, and 0.013e and the anions to be less negative (losing charge) by −0.126, −0.077, and −0.072e, respectively. This has induced a charge of −0.001, 0.009, and −0.059e on the h-BN surface. The sign of induced charges on the h-BN surface indicates that charge transfer (CT) occurs from [Bmim][BF4] and [Bmim][Tf2N] to the h-BN surface, while it is done from the h-BN surface to [Bmim][PF6]. The amount and direction of charge transfer for adsorption of [Bmim][Y] (Y = BF4−, PF6−, and Tf2N−) ILs on the h-BN surface are different from those for adsorption on the coronene surface reported by Wagle et al.22 They have indicated that the charge transfer occurs from the coronene surface to the [Bmim][Y] (Y = BF4−, PF6−, and Tf2N−) ILs upon adsorption on the surface. I



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Figure 4. Side and top view of 3-D graphic of reduced density gradient (RDG) calculation of h-BN···IL complexes. Green color indicates vdW interaction between cation, anion, and h-BN surface.

both favorable and unfavorable interactions, as differentiated by the sign of the second density Hessian eigenvalue (λ2). The sign of this eigenvalue is able to characterize both the strength and (un)favorable nature of the interactions based on the iso-surface coloring. The red and green regions represent the strong repulsion interaction and attractive vdW interactions, respectively.

the role and effect of noncovalent interactions in the adsorption of ionic liquids on the h-BN surface, we used the NCIPLOT program.26,27 This program reveals noncovalent interactions based on the peaks that appear in the reduced density gradient (RDG) at low densities. RDG iso-surfaces for these peaks enable the visualization of weak interactions. The iso-surfaces correspond to J



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Table 4. Energy Decomposition Analysis (EDA, in kcal/mol) for h-BN···IL Complexes at the PBE-D3/TZP Level of Theory

a

Structure

ΔEPauli

ΔEelect

ΔEorb

ΔEdisp

ΔEinta


35.35 34.12 27.36 35.03 33.00 33.60 31.99 30.88 29.22

−19.21(34.4%) −19.24(34.6%) −14.72(35.7%) −19.14(33.9%) −17.56(32.6%) −17.86(31.6%) −18.72(36.7%) −16.53(32.5%) −14.77(31.4%)

−14.77(26.5%) −14.50(26.1%) −7.50(18.2%) −15.54(27.5%) −14.14(26.3%) −14.28(25.3%) −13.70 (26.8%) −14.42(28.3%) −12.56(26.7%)

−21.79(39.1%) −21.78(39.2%) −19.02(46.1%) −21.82(38.6%) −22.09(41.1%) −24.34(43.1%) −18.59(36.4%) −19.96(39.2%) −19.70(41.9%)

−20.42 −21.40 −13.88 −21.47 −20.78 −22.89 −19.03 −20.03 −17.81

ΔEint values calculated by ADF are without basis set superposition errors (BSSEs).

from Table 4, the contribution of the ΔEdisp component in each complex is more than that of the ΔEelect and ΔEorb, components (ΔEdisp > ΔEelect > ΔEorb), with the exception of the h-BN[Btma][BF4] complex whose ΔEdisp and ΔEelect components are almost equal. It is worth mentioning that the percent contribution of the ΔEdisp component in the complexes containing the same cations increases with the change of anion type from [BF4]− to [PF6]− and [Tf2N]−. For example, the percent contribution of the ΔEdisp component in h-BN[Bmim][Y] (Y = BF4−, PF6−, and Tf2N−) complexes is about 39.1%, 39.2%, and 46.1%, respectively. Table 4 also indicates that the electrostatic interaction ΔEelect and also orbital interaction, ΔEorb, which accounts for charge transfer, polarization, and electronpair bonding, has contributions of about 33% and 25%, respectively, to the attractive energy. Therefore, the electronic structure can also easily be affected by the adsorption of IL. 3.8. Thermochemistry of IL Adsorption on the h-BN Surface. Thermochemistry involves quantitative measures made in order to understand the energetics associated with a system, particularly energy and heat associated with chemical reactions and/or physical transformations. In this study, we have calculated the binding energy, enthalpy, free energy, and entropy of 9 ILs present alone and when adsorbed on the h-BN surface (Table S5). An estimation of these quantities aids in understanding the feasibility of such an interaction process. We have placed a special emphasis on the effects of different cation−anion combinations of the IL in order to determine the stability of adsorption of ILs onto the h-BN surface. Figure 5 shows the binding energy (ΔEb), enthalpy (ΔHads), free energy (ΔGads), and entropy (ΔSads) of adsorption for the 9 different ILs on the h-BN surface, and also the correlations between enthalpy, free energy of adsorption, and HOMO−LUMO gap of h-BN···IL complexes. On the whole, the binding energy and enthalpy of adsorption of ILs gradually decrease with an increase in the anion size (from [BF4]− to [Tf2N]−) of ILs containing the same cations, although there is no clear trend in ΔGads and ΔSads as the anion size increases (from [BF4]− to [PF6]− and [Tf2N]−). The negative values of entropy in Figure 5 indicate that the entropy decreases upon adsorption of ILs due to a decrease of the translation degree of freedom. A further decomposition of entropy into translational (ΔSt), rotational (ΔSr), and vibrational (ΔSv) components is also possible. As seen from Table S5, the contribution of these components to the total entropy (ΔSads) follows the order ΔSt > ΔSr > ΔSv. Obviously, ΔSt and ΔSr components in ILs containing the same cations decrease (toward more negative values) as a function of anion size from [BF4]− to [PF6]− and [Tf2N]−, while ΔSv increases without a clear trend in the anion size.

The iso-surfaces of sign (λ2) × ρ defined by Yang et al.26,27 are displayed for h-BN···IL complexes in Figure 4. The green region between ionic liquids and the h-BN surface indicates that vdW interaction is the key driving force for the adsorption of ILs on the h-BN surface. As seen from Figure 4, the cation and anion type has an important influence on the configuration and orientation of ionic liquids on the h-BN surface via vdW interactions. The vdW interaction between the h-BN surface and ionic liquids is very obvious via cooperative π···π, C−H···π, and X···π (X = N, O, F atoms from anions) interactions. As shown in Figure 4, the [BF4]−, [PF6]−, and [Tf2N]− interactions with the h-BN surface arise primarily between the O, N, and F heteroatoms of the anions with the B and N atoms from the h-BN surface. The [Bmim]+, [Bpy]+, and [Btma]+ interactions occur between imidazolium, pyridinium aromatic rings, methyl and butyl alkyl groups and the h-BN surface. The green region (vdW interaction) between cation, anion, and h-BN surface can support the weak interactions that are identified also in Tables S3 and S4 based on QTAIM analysis. 3.7. Energy Decomposition Analysis (EDA). The nature of interaction between the ionic liquids (ILs) and the h-BN surface can be determined by the energy decomposition analysis (EDA) method.52 In this method, the interaction energy between the two fragments, ΔEint, is split up into four physically meaningful components: ΔE int = ΔE Pauli + ΔEelect + ΔEorb + ΔEdisp

(3)

ΔEPauli gives the repulsive four-electron interactions between occupied orbitals. ΔEelect gives the electrostatic interaction energy between the fragments, which is calculated with a frozen electron density distribution in the geometry of the complex. It can be considered as an estimation of the electrostatic contribution to the interaction energy. The orbital interaction, ΔEorb, in any MO model, and also in Kohn−Sham theory, accounts for charge transfer (i.e., donor−acceptor interactions between occupied orbitals on one moiety with unoccupied orbitals of the other, including the HOMO−LUMO interactions) and polarization (empty/occupied orbital mixing on one fragment due to the presence of another fragment). In addition, ΔEdisp has been calculated when the dispersion corrected density functional has been used; this term basically is the difference between total energy based on dispersion corrected DFT (DFT-D or DFT-D3) and nondispersion corrected DFT methods.43 Therefore, by going from the DFT to DFT-D (or DFT-D3) functional the ΔEPauli, ΔEelect, and ΔEorb values remain unchanged and the dispersion correction appears as an extra term. Table 4 lists the results of the EDA calculations at the PBE-D3/TZP level of theory for h-BN···IL complexes. As seen K



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Figure 5. Changes of ΔEb without BSSE correction (a), and of ΔHads (b), ΔGads (c), and ΔSads (d) for IL adsorption on the h-BN surface; (e and f) the changes of ΔHads and ΔGads with respect to the HOMO−LUMO energy gap of h-BN···IL complexes.

BN[Bmim][BF4] (−10.16 kcal/mol) > BN[Bmim][PF6] (−9.87 kcal/mol) > BN[Bmim][Tf2N] (−6.35 kcal/mol), BN[Bpy][BF4] (−11.84 kcal/mol) > BN[Bpy][PF6] (−10.60 kcal/mol) > BN[Bpy][Tf2N] (−7.36 kcal/mol), and BN[Btma][BF4] (−9.99 kcal/mol) > BN[Btma][PF6] (−9.33 kcal/mol) > BN[Btma][Tf2N] (−7.21 kcal/mol), respectively. This order indicates that anion type, the arrangement of anion in IL, and the adsorption behavior of ILs on the h-BN surface could be the most important factors influencing the binding energy values between ILs and the h-BN surface. A comparison of our results for adsorption of ILs on the h-BN surface and the previous results based on density functional theory (DFT) and the hybrid B3LYP functional with Pople’s medium 6-311 g basis set and also molecular dynamics (MD) simulations for adsorption of ILs on a graphene surface indicates that the adsorption behavior of ILs on the h-BN (a hydrophilic surface) sheet is different from that of the graphene (a hydrophobic surface) sheet. This difference indicates that the hydrophobicity/hydrophilicity characteristic of a surface has an important influence on the adsorption of ionic liquids on the surfaces. The noncovalent interaction plots also show the role and significance of cooperative π···π, C−H···π, and X···π (X = N, O, F atoms from anions) interactions on the adsorption of ILs on the h-BN surface. QTAIM analysis indicates a significant reduction in the sum of hydrogen bond energy (∑E(H···X)) and electron density (∑ρ(r)) values of H-bonds in ILs upon adsorption on the h-BN surface. The results of QTAIM analysis on the h-BN···IL complexes show that the [BF4]−, [PF6]−, and [Tf2N]− anions in the ILs have a stronger interaction with the h-BN surface than the [Bmim]+, [Bpy]+, and [Btma]+ cations. The anion interactions with the

As shown in Figure 5 and Table S5, the free energy (ΔGads) values for adsorption of ILs are negative. Therefore, it is expected that the adsorption of ILs on the h-BN surface proceeds spontaneously. The most favorable adsorption enthalpies were typically witnessed for adsorption of [Bpy][BF4] on the h-BN surface. We propose that the suitable orientation of [Bpy][BF4] on the h-BN surface provides an opportunity for strong interaction of [Bpy][BF4] with the h-BN surface. This strong interaction of [Bpy][BF4] with the h-BN surface is in excellent agreement with the smaller HOMO−LUMO gap observed for the h-BN···[Bpy][BF4] complex. Inspection of the thermodynamic quantities for these ILs interacting with the h-BN surface exposes the fact that reduction in the HOMO−LUMO band gap of ILs containing the same cations is correlated with the enthalpy (ΔHads) and free energy (ΔGads) of adsorption. It is noteworthy that, overall, ionic liquids containing [Tf2N]− anion ([Bmim][Tf2N], [Bpy][Tf2N], and [Btma][Tf2N]) display the least favorable interactions across all ionic liquids adsorbed on the h-BN surface (in terms of ΔHads).

4. CONCLUSION To conclude, we have performed a comprehensive firstprinciples study on the adsorption of three types of ionic liquids based on 1-butyl-3-methylimidazolium [Bmim]+, 1-butylpyridinium [Bpy]+, and butyltrimethylammonium [Btma]+ cations, paired to tetrafluoroborate [BF4]−, hexafluorophosphate [PF6]−, and bis(trifluoromethylsulfonyl)imide [Tf2N]− anions on a hexagonal boron-nitride (h-BN) surface at the M06-2X/ccpVDZ level of theory. Our results indicate that the binding energy values for adsorption of ILs based on [Bmim]+, [Bpy]+, and [Btma]+ cations on the h-BN surface follow the order L



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h-BN surface also occur mostly from interactions between the O, N, and F heteroatoms of the anions with the N atoms from the h-BN surface, and the cation interactions arise from interactions between B and especially N atoms of the h-BN surface and H, C, N atoms from cations. The ChelpG analysis confirms that the adsorption process changes the anion and cation charges. Importantly, as demonstrated by ChelpG analysis, the adsorption process also modifies the charge of the h-BN model involved in the interaction appreciably. The type of ionic liquid distinctively determines the charge modification of h-BN model atoms. Our results also show that the charge transfer process between ILs and the h-BN surface is different from the graphene surface reported in the literature. Thus, this difference indicates the importance of the hydrophobicity/hydrophilicity characteristic of the surface on the charge transfer process again. A comparison between HOMO−LUMO energy gaps and density of states (DOSs) for adsorption of ILs on the h-BN surface and graphene surface reported in the literature indicates that the HOMO−LUMO energy gap of ILs decreases upon adsorption of ILs on the h-BN and graphene surfaces. In general, it seems that, for the cation and anion type, the IL configuration and orientation of ILs on the h-BN surface can be the most significant factors influencing the amount and direction of charge transfer between the IL and h-BN surface, the HOMO−LUMO energy gap, and the density of states of ILs. Energy decomposition analysis (EDA) shows that the contribution of the ΔEdisp component in each complex is generally more than the ΔEelect and ΔEorb components and the contribution of the ΔEdisp component in the complexes including the same cations increases with the change of anion type from [BF4]− to [PF6]− and [Tf2N]−. The thermochemical analysis indicates that in general the binding energy and enthalpy of adsorption gradually decrease with an increase in the anion size from [BF4]− to [PF6]−and [Tf2N]− for ILs containing the same cations, although there is no clear trend in ΔGads and ΔSads as the anion size increases from [BF4]− to [PF6]− and [Tf2N]− anions. The thermochemistry of adsorption also shows that the adsorption of ILs on the h-BN surface occurs spontaneously. Our study offers fundamental insights into the structure and energetics of adsorption of ionic liquids in 2-D materials such as h-BN. The results of this work lay the groundwork for extending this study to investigate ionic liquid interactions with other 2-D materials such as MoS2 and phosphorene.

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ASSOCIATED CONTENT

S Supporting Information *

Bond critical point (BCP) data resulted from QTAIM analysis for ILs and ILs adsorbed on the h-BN surface, thermochemical parameters involved in the process of IL adsorption on the h-BN surface, initial structures for adsorption of [Bmim][BF4] IL on the h-BN surface, and the results of ChelpG analysis. This material is available free of charge via the Internet at http://pubs. acs.org.

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Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. M



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Trends in Physisorption of Ionic Liquids on Boron-Nitride Sheets

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