Meta-Hybrid Density Functional Theory Study of Adsorption of

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Meta-hybrid Density Function Theory Study of Adsorption of Imidazolium and Ammonium-Based Ionic Liquids on Graphene Sheet Mehdi Shakourian-Fard, Zahra Jamshidi, Ahmad Bayat, and Ganesh Kamath J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp512020q • Publication Date (Web): 16 Mar 2015 Downloaded from http://pubs.acs.org on March 16, 2015

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The Journal of Physical Chemistry

Meta-hybrid Density Function Theory Study of Adsorption of Imidazolium and Ammonium-Based Ionic Liquids on Graphene Sheet Mehdi Shakourian-Fard†, Zahra Jamshidi‡*, Ahmad Bayat†, and Ganesh Kamathǂ* †

Department of Chemistry, Sharif University of Technology, P.O. Box: 11365-9516, Tehran, Iran ‡ Chemistry and Chemical Engineering Research Center of Iran, Tehran, P.O. Box 14335-186, Iran ǂ Department of Chemistry, University of Missouri-Columbia, Columbia, Missouri 65211-7600, United States *Corresponding author: [email protected], [email protected]

Abstract In this study, two types of ionic liquids (ILs) based on 1-butyl-3-methylimidazolium [Bmim]+ and

butyltrimethylammonium

[Btma]+

cations,

paired

to

tetrafluoroborate

[BF4]-,

hexafluorophosphate [PF6]-, dicyanamide [DCA]-, and bis-(trifluoromethylsilfonyl)imide [Tf2N]anions were chosen as adsorbates to investigate the influence of cation and anion type on the adsorption of ILs on the graphene surface. The adsorption process on the graphene surface (circumcoronene) was studied using M06-2X/cc-pVDZ level of theory. Empirical dispersion correction (D3) was also added to the M06-2X functional in order to investigate the effects of dispersion on the binding energy values. The graphene…IL configurations, binding energies, and thermochemistry of IL adsorption on the graphene surface were investigated. Orbital energies,

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charge transfer behavior, the influence of adsorption on the hydrogen bond strength between cation and anion of ionic liquids, and the significance of non-covalent interactions on the adsorption of ILs on the graphene surface were also considered. ChelpG analysis indicated that upon adsorption of ILs on the graphene surface, the overall charge on the cation, anion, and graphene surface changes enabled by the charge transfer (CT) that occurs between ILs and graphene surface. Orbital energy and density of states (DOSs) calculations also show that the HOMO-LUMO energy gap of ILs decreases upon adsorption on the graphene surface. Quantum theory of atoms in molecules (QTAIM) analysis indicates that the hydrogen bond strength between cation and anion in ILs decreases upon adsorption on the graphene surface. Plotting the non-covalent interactions between ILs and graphene surface show the role and significance of cooperative π…π, C-H…π, and X…π (X = N, O, F atoms from anions) interactions in the adsorption of ILs on the graphene surface. The thermochemical analysis also indicates that the free energy of adsorption (∆Gads) of ILs on the graphene surface is negative, and thus the adsorption occurs spontaneously.

1. Introduction Graphite is one of the allotropes of carbon. It has found a variety of industrial applications in the manufacture of crucibles and molds, lubricants, heat shields, furnace linings, composite materials, and electrical contacts due to its weak interlayer interaction and conductivity. Because the interlayer interaction is rather weak, the properties of graphite are usually represented using a one-layer plate known as graphene. Graphite is also one of the most useful materials for adsorption of a variety of compounds such as proteins,1 gases such as oxygen,2 carbon dioxide,3 and methane,4 water,5 palladium,6 alkali metals,7-9 and titania,10 due to its large effective surface area. 2 ACS Paragon Plus Environment

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Another area that has shown a large growing demand is the semiconductor electronics industry. However, due to the absence of an energy band-gap in graphene, graphene devices cannot be switched off and, thus, show an intrinsic leakage current in the off state, which limits its application in logical circuit devices.11 A large number of research groups are focusing to solve this problem and develop methods to control the electronic structure of graphene. There are several methods for increasing the energy band-gap in graphene such as synthesis of graphene nanomesh,12 graphene nanoribbon,13-14 functionalization of graphene surface,15 molecular-doped bilayer graphene, and the use of external electric field.16-18 However, these methods also have several disadvantages associated with performance, accessibility, cost, and controllability. Therefore, there is a need to develop a simple and less expensive method for synthesizing the semiconducting graphene. As a process, the liquid-phase exfoliation of various layered solids (e.g., graphite, h-BN, WS2, MoS2) 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.19 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.19 In recent times, ionic liquids have been used to help in the exfoliation process and improving the electronic structure of graphene surfaces.20-21 Ionic liquids (ILs) are a class of novel compounds composed of organic cations and inorganic anions with high ionic conductivity and low melting point (below 100 °C). Further, Room Temperature Ionic Liquids (RTILs)22 have



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attracted significant attention as electrolytes in lithium ion batteries,23 proton exchange membranes in fuel cells, high energy density supercapacitors and solar cells24-25 due to salient properties such as wide electrochemical window, ionic conductivity, hydrophobicity, high ion density, nonvolatility, nonflammability, and good thermal stability.26 Several experimental27 studies and spectroscopic techniques have also been devoted to better understand graphite structural changes at the graphite/electrolyte interface.28-29 On the other hand, molecular dynamics simulations of graphite/ionic liquid interface30-35 have been performed by various groups for considering the arrangement of ions at a graphite/ionic liquid interface. Yan et al.31 have studied the interface structure for room temperature ionic liquids, 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and 1-octyl-3methylimidazolium hexafluorophosphate ([omim][PF6]), and the graphite surface by classical molecular dynamic simulations. Their results show that the [Bmim]+ cation tends to be arranged in parallel to the graphite surface at a distance of 3.6-3.7 Å. Kislenko et al.34-35 and Yan et al.31 have also shown that the [Bmim]+ cations are closer to the graphite surface than the [PF6]- anions so that the phosphorus atom stands at a distance of 4.1 Å from the surface. Electronic structure methods are accurate methods for determining the arrangement and adsorptive behavior of a wide range of nonpolar, polar, and ionic species on the solid surfaces. These methods are capable of handling aspects of interactions that can be difficult to capture using an empirical derived non-polarizable force field–properties such as polarizability,36 electron transfer, and hydrogen bonding. Recently, there has been a significant interest in using electronic structure methods for understanding the properties and interactions of chemicals and graphene surface as a proposed model for graphite. For example, adsorption of electron donor and acceptor molecules37 in order to find the influence of these molecules on the electronic


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property of graphene, adsorption of DNA nucleobases (guanine (G), adenine (A), thymine (T), and cytosine (C)) on graphene38 for exploring the binding energy mechanism and the relative binding strength of nucleobases. Ghatee et al.39 have used density functional theory (DFT) and the hybrid functional expand (B3LYP) with Pople’s medium 6-311G basis set in order to investigation of IL adsorption on the graphene surface. Wagle et al.40 have also used the same level of theory for determining the interactions between ILs and polycyclic atomic hydrocarbons such as naphthalene, anthracene, phenanthrene, pyrene, perylene, and coronene. In the interaction of ionic liquids with the garphene surface, it is generally believed that the process of adsorption is dominated by the noncovalent weak interactions which are usually not well described by the non long-range corrected DFT method used by Ghatee et al. and Wagle et al. Thus, it seems that these interactions should have a significant influence on adsorption energy of ILs, charge transfer process, and the configuration of ILs on the surfaces. The importance of these interactions on the adsorption of ionic liquids has been highlighted elsewhere in literature.38,41-42 In this study, a comprehensive investigation using quantum chemical calculations is performed to investigate the role of cation and anion on binding energy, ordering, and charge transfer between ILs and circumcoronene - proposed model for graphene. The ILs considered in this study are composed of 1-butyl-3-methylimidazolium [Bmim]+ and butyltrimethylammonium [Btma]+ cations, paired to tetrafluoroborate [BF4]-, hexafluorophosphate [PF6]-, dicyanamide [DCA]-, and bis-(trifluoromethylsulfonyl)imide [Tf2N]- anions (see Scheme 1).



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Scheme1. Structures of circumcoronene (graphene sheet), cations and anions for various IL combinations studied in this work. 2. Computational Details Full optimization and property calculations for the adsorption of ILs on graphene surface (circumcoronene model) were done at the M06-2X43 method using Dunning cc-pVDZ basis set. On the other hand, empirical dispersion correction (D3) developed by Grimme44 was also added to the M06-2X functional in order to investigate the effects of dispersion on the binding energy values. All of the calculations were carried out without symmetry restrictions. The harmonic vibrational frequencies and the corresponding zero-point vibrational energies (ZPVEs) were calculated for all of the optimized geometries, and real frequencies were obtained in all the cases. The binding energy was determined as the difference between the energy of G…IL complexes and the sum of the energies of the corresponding graphene surface and IL (∆Eb = E(G…IL) – (E(IL)


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+ E(G))). In addition to binding energy, enthalpy (H) and free energy (G) for the adsorption of ILs on graphene surface at 298.15 K were calculated using eq 1

∆X = X (G...IL) - (X(IL) + X(G) )

(1)

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

∆S =

(∆H - ∆G) 298.15

(2)

The counterpoise procedure (CP) by Boys and Bernardi was used to calculate the basis set superposition errors (BSSEs)45 and then, the binding energies for adsorption of various ILs on the graphene surface were corrected by the calculated BSSEs. The ChelpG charge analysis and density of state (DOS) calculations on the optimized structures were performed by Gaussian 09 program.46 NCIPLOT program47-48 was used for plotting non-covalent interaction regions between ILs and graphene surface which are responsible for adsorption of ILs on the graphene surface. In order to quantify the changes in the strength of hydrogen bond interactions between cation and anion of ionic liquids upon adsorption on the graphene surface, the quantum theory of atoms in molecules (QTAIM)49 analysis was also done using AIM2000 package50 on the wave functions generated at the M06-2X/cc-pVDZ level of theory. 3. Results and Discussions 3.1. Structures and Energetics of Ionic Liquid Ionic liquids based on two different cation types of 1-butyl-3-methylimidazolium [Bmim]+ and butyltrimethylammonium [Btma]+, paired to tetrafluoroborate [BF4]- (a tetrahedral anion), hexafluorophosphate [PF6]- (an octahedral anion), dicyanamide [DCA]- (an anion with π bonds), and bis-(trifluoromethylsulfonyl)imide [Tf2N]- anions (see Scheme 1) are chosen for adsorption on the graphene surface. [Bmim]+ cation can be adsorbed on the graphene surface via its positive 7 ACS Paragon Plus Environment


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five-membered aromatic ring, methyl and butyl alkyl groups, while there is an absence of aromaticity in [Btma]+ cation and therefore can be adsorbed only by methyl and butyl alkyl groups on the graphene surface. To find the most stable geometries of ILs considered in this study, we used the method described in literature.51 In this method, the region around the most stable geometry of the cations is divided into several regions and the most stable geometry of anions is located in these regions. There are also some sub-configurations in each region which are related to different orientations of anion with respect to the cation. For example, [BF4]- anion could interact with the cation through one, two, and three of its fluorine atoms in each region. Finally, all designed structures for interaction of [Bmim]+ and [Btma]+ cations with [BF4]-, [PF6]-, [DCA]- and [Tf2N]- anions were fully optimized at M06-2X/cc-pVDZ level of theory. The most stable geometries of ILs along with the distances of electronegative N, O, and F atoms of anions to C-H bond of imidazolium ring, methyl, and butyl groups of cations are shown in Figure 1. As seen from Figure 1, in ILs based on [Bmim]+ cation, anions have a good tendency to interact through the front of imidazolium ring with C-H bond of imidazolium ring and C-H bonds of methyl and butyl groups. Moreover, preliminary investigations have also suggested that these hydrogen bonds are the most characteristic non-covalent interactions in ILs systems.39-40,51-52 In ILs based on [Btma]+, anions lies in close to C-H bonds of methyl and butyl groups of cation. Recently, Castner et al.53 and Damodaran54 have also shown that anions in ILs based on tetraalkylammonium cation can be found close to the positive nitrogen atom by interaction with C-H bonds of alkyl groups. The nature of these hydrogen bond interactions in ILs can be analyzed by Bader’s topological QTAIM analysis49 in terms of topological parameters such as electron density ρ(r), the Laplacian


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of electron density (∇2ρ(r)), and the electronic energy density (H(r)) at the critical points (BCPs) of hydrogen bonds. H(r) is associated with the kinetic energy density (G(r)), which is always positive, and potential energy density (V(r)), which is always negative, by H(r) = G(r) + V(r) equation. Based on the sign of ∇2ρ(r) and H(r) at the bond critical points, Koch and Popelier55 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) bonds. ∇2ρ(r) and H(r) are also used to understand the covalent and electrostatic character of weak non-bonded interactions. The QTAIM analysis was performed using the calculation results at the M06-2X/cc-pVDZ level of theory. The computed electron density (ρ(r)), 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 ionic liquids are summarized in Table S1 (see 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 covalent and electrostatic character of the corresponding H-bonds, respectively. Thus, the Hbonds in the ILs are classified as weak and medium (∇2ρ(r), and H(r) > 0)) and strong (∇2ρ(r) > 0, and H(r) < 0) bonds in the nature. The electron density (ρ(r)) is used to describe the strength of a bond, a stronger bond associated with a larger ρ(r) value. In general, ρ(r) is greater than 0.20 au in shared (covalent) bonding and less than 0.10 au in a closed-shell interaction.56 From the values of electron density listed in Table S1, it can be concluded that the H-bonds between the [Bmim]+ and [Btma]+ cations and [BF4]-, [PF6]-, [DCA]-, and [Tf2N]- anions which are marked by the dotted line, are all closed-shell interaction.



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[Bmim][BF4]

[Bmim][PF6]

[Bmim][DCA]

[Bmim][TF2N]

[Btma][BF4]

[Btma][PF6]

[Btma][DCA]

[Btma][Tf2N]

Figure 1. The most stable geometries of ionic liquids studied at the M06-2X/cc-pVDZ level of theory. 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 bitter lime for phosphorus.


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1 2 3 Binding energy (∆Eb) of ILs is defined as the difference between the energy of IL and the sum 4 5 of energies of the cation and anion species (∆Eb = E(IL) – (E(Cation) + E(Anion))).51 Binding energies 6 7 8 and basis set superposition errors (BSSEs) for the most stable geometries of ILs are listed in 9 10 Table 1. 11 12 Table 1. Binding energy (∆Eb in kcal/mol) and basis set superposition error (BSSE in kcal/mol) 13 14 15 for the ILs and G…IL complexes. 16 17 M06-2X M06-2X-D3 Structure M06-2X M06-2X-D3 18 Structure ∆E BSSE ∆E +BSSE ∆E +BSSE ∆E BSSE ∆E + BSSE ∆Eb+BSSE 19 b b b b b 20 [Bmim][BF4] -98.74 11.08 -87.66 G[Bmim][BF4] -20.54 7.50 -13.04 -90.45 -29.14 21 [Bmim][PF6] -93.21 12.16 -81.05 G[Bmim][PF6] -22.35 7.34 -15.01 -84.15 -32.44 22 [Bmim][DCA] -91.48 5.86 -85.62 G[Bmim][DCA] -18.25 3.87 -14.38 -87.58 -25.59 23 N] -91.89 11.74 -80.15 G[Bmim][Tf N] -16.64 6.11 -10.53 [Bmim][Tf -83.82 -28.41 2 2 24 [Btma][BF4] -97.16 10.80 -86.36 G[Btma][BF4] -16.51 6.90 -9.61 -89.06 -25.95 25 -89.57 9.86 -79.71 G[Btma][PF6] -19.82 6.94 -12.88 [Btma][PF6] 26 -82.75 -27.50 27 [Btma][DCA] -88.52 4.58 -83.94 G[Btma][DCA] -17.30 3.77 -13.53 -85.95 -23.24 28 -82.35 -28.26 [Btma][Tf2N] -89.09 10.35 -78.74 G[Btma][Tf2N] -17.98 6.29 -11.69 29 30 31 For the ILs based on [Bmim]+ and [Btma]+ cations, the binding energies change from -80.15 32 33 34 kcal/mol to -87.66 kcal/mol and from -78.74 kcal/mol to -86.36 kcal/mol at the M06-2X/cc35 36 pVDZ level of theory, respectively. On the other hand, with the addition of empirical dispersion 37 38 39 correction (D3) to the M06-2X method the binding energy values for ILs based on [Bmim]+ and 40 41 [Btma]+ cations change from -83.82 kcal/mol to -90.45 kcal/mol and from -82.35 kcal/mol to 42 43 89.06 kcal/mol, respectively. As seen from Table 1, empirical dispersion correction improves the 44 45 46 binding energies about 2-4 kcal/mol. 47 48 The magnitude of the binding energy values for ILs containing the same cations of [Bmim]+ and 49 50 [Btma]+ follows the order [Bmim][BF4] (-90.45 kcal/mol)> [Bmim][DCA] (-87.58 kcal/mol)> 51 52 53 [Bmim][PF6] (-84.15 kcal/mol)> [Bmim][Tf2N] (-83.82 kcal/mol) and [Btma][BF4] (-89.06 54 55 kcal/mol)> [Btma][DCA] (-85.95 kcal/mol)> [Btma][PF6] (-82.75 kcal/mol)> [Btma][Tf2N] (56 57 58 59 11 60 ACS Paragon Plus Environment


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82.35 kcal/mol), respectively. As shown in Table 1, comparing the binding energy values for ILs containing the same anions shows that these values for ILs based on the [Bmim]+ cation are more favorable than ILs based on the [Btma]+ cation. 3.2. Ionic Liquid Adsorption on the Graphene Surface To understand the behavior of graphene in adsorption of ionic liquids, we used circumcoronene (C54H18) model composed of 19 aromatic rings.57-58 In this model, all the boundary carbon atoms of the circumcoronene have been saturated with hydrogen atoms (see Scheme 1) and thus, the ionic liquid adsorption can occur just on the carbon atoms of the circumcoronene. This model has shown good results for adsorption of chemicals on the graphene surface.37,39 The adsorption of ILs on the graphene surface can take place from various regions of ILs. These regions are five-membered aromatic ring, hydrogens of aromatic ring, alkyl (methyl and butyl) groups, and anion. In order to find the most stable geometry of graphene-IL complexes, we placed the IL structures in all possible states on the graphene surface. For better understanding the possible adsorption states, the initial structures for adsorption of [Bmim][BF4] ionic liquid on the graphene surface are shown in Figure S1 (see Supporting Information). After specifying the initial structures, they were optimized at the M06-2X/cc-pVDZ level of theory. The most stable geometries for adsorption of various ILs on the graphene surface are displayed in Figure 2. 3.2.1. [Bmim][Y] (Y = BF4-, PF6-, DCA-, and Tf2N-) adsorption on the graphene surface As seen from Figure 2, it can be seen that the [Bmim]+ cation in the ILs except [Bmim][Tf2N] tends to be arranged in parallel orientation with respect to the graphene surface with the distance of 3.01-3.21 Å and hydrogen atoms of methyl and methylene groups connected to the nitrogen


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atoms take the nearest distance with respect to the graphene surface. These hydrogen atoms are at a distance between 2.66 Å and 3.14 Å from graphene surface. It can also be found that other carbon atoms of butyl chain are bent slightly upward with respect to imidazolium ring with a weaker interaction relative to the graphene surface in such a way that the distance of the nearest hydrogen atoms is in the range of 3.47-4.13 Å. Our results are in good agreement with the MD simulation performed by Kislenko et al.34-35 The most stable geometry and orientation of the [Bmim][BF4] and [Bmim][PF6] ILs on the graphene surface are displayed in Figure 2. With the adsorption of [Bmim][BF4] and [Bmim][PF6] ionic liquids on graphene surface, the boron and phosphorous atoms of [BF4]- and [PF6]- anions lie at a distance of 3.20 and 3.72 Å, respectively from the surface and the triplets of fluoride atoms form planes parallel to the imidazolium ring surface and other fluoride atoms in [BF4]- and [PF6]- anions lie above the imidazolium ring surface. Our results indicate that the [Bmim]+ cation tends to interact with the surface at a closer distance than the [BF4]- and [PF6]- anions, although Ghatee et al.39 indicated that the [PF6]- anion tends to interact with the graphene surface at a closer distance than the [Bmim]+ cation. On the other hand, the most stable geometry of [Bmim][PF6] reported by Ghatee et al. is about 1.51 kcal/mol less stable than the most stable geometry reported in our work. Our results are also supported by MD simulation of the electrochemical interface between a graphite surface and [Bmim][PF6] performed by Kislenko et al.34-35 They indicated that in the first adsorption layer of [Bmim][PF6], the imidazolium ring in the [Bmim]+ cation tends to be arranged parallel to the surface at a distance of 3.5 Å, although the phosphorous atom in the [PF6]- anion is at a further distance from the graphene surface (4.1 Å). The agreement of our results with MD simulation results of Kislenko et al. indicates the importance of non-covalent interactions in adsorption of



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ionic liquids on the graphene surface, while Ghatee et al. used the density functional theory (DFT) and the hybrid B3LYP functional with Pople’s medium 6-311g basis set in order to optimize their structures which does not account for the dispersion energies.44 Recently, Cuong et al.59 have also indicated the importance of non-covalent interactions in the possibility of controlling the band-gap and carrier type of bilayer graphene using ionic liquids. In the next sections, the significance of non-covalent interactions is shown by plotting the contribution of these interactions in the adsorption of ionic liquids on the graphene surface. The geometry of [Bmim][DCA] ionic liquid on the graphene surface can be verified by the distance values shown in Figure 2. [DCA]- anion interacts effectively with the C2-H bond of imidazolium ring, methyl and butyl groups. One of the nitrogen atoms of [DCA]- anion lies in a plane parallel to imidazolium ring and interacts with C2-H bond and butyl group and takes a distance of 2.99 Å from graphene surface, although another nitrogen atom lies above the imidazolum ring with a distance of 5.89 Å from the graphene surface. The most stable geometry of [Bmim][Tf2N] is shown in Figure 2. As seen from Figure 2, it can be found that the important point in adsorption of [Bmim][Tf2N] is the orientation of [Bmim]+ cation with respect to the graphene surface. The [Bmim]+ cation tends to bend with respect to the graphene surface in such a way that the [Bmim]+ cation interacts with the graphene surface via hydrogen atoms of C2-H bond, methyl, and butyl groups. The nearest atom of cation from the graphene surface is the hydrogen atom of C2-H bond at a distance of 2.5 Å. As seen from Figure 2, the butyl chain of adsorbed [Bmim]+ cation shows a tendency to lie in the plane of the graphene surface. On the other hand, one oxygen atom of -SO2 group and two fluoride atoms of -CF3 group in the [Tf2N]- anion form a plane parallel to the graphene surface. The particular orientation of [Bmim]+ cation in [Bmim][Tf2N] with respect to graphene surface seems


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to be due to the big size of [Tf2N]- anion and the competition between cation and anion for interacting with the graphene surface. A comparison between the orientation of [Bmim][Y] (Y = BF4-, PF6-, and Tf2N-) adsorbed on the graphene and h-BN surface in our previous work60 indicates that the difference in the polarity of graphene and h-BN surface has a significant effect on the orientation of ILs adsorbed on the surfaces. For example, [Bmim][Tf2N] is adsorbed on the graphene surface by the interaction of graphene surface with the hydrogen atoms of C2-H bond, methyl, and butyl groups of [Bmim]+ cation and the oxygen and fluorine atoms of [Tf2N]- anion, although [Bmim][Tf2N] is primarily adsorbed by the interaction of oxygen and fluoride atoms of [Tf2N]- anion with the h-BN surface. In addition, the [Bmim]+ cation, while lies above the [Tf2N]- anion, interacts with the h-BN surface by its methyl group in the end of butyl chain. 3.2.2. [Btma][Y] (Y = BF4-, PF6-, DCA-, and Tf2N-) adsorption on the graphene surface The most stable geometry and orientation of [Btma][BF4], [Btma][PF6], [Btma][DCA], and [Btma][Tf2N] ILs on the graphene surface are displayed in Figure 2. The [Btma]+ cation in the ionic liquids assumes two orientations with respect to the graphene surface. The [Btma]+ cation in the [Btma][BF4] and [Btma][DCA] tends to be arranged in parallel orientation with respect to the graphene surface by interaction of butyl and one methyl group, while it takes a parallel orientation by interaction of butyl and two methyl groups in the case of [Btma][PF6] and [Btma][Tf2N]. It can be seen that the hydrogen atom of the methyl group has the nearest distance of 2.60 and 2.57 Å to the graphene surface in the [Btma][BF4], and [Btma][DCA], respectively, although it has the nearest distance of 2.66 and 2.70 Å in the [Btma][PF6] and [Btma][Tf2N], correspondingly. It is worth mentioning that there are no important changes in the orientation of anions relative to [Btma]+ cation upon adsorption on the graphene surface. As seen from Figure



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2, it can be found that the anions interact directly with the graphene surface through their fluorine, nitrogen, and oxygen atoms, while interacting closely with the positive part of the [Btma]+ cation. It is also interesting to note that [DCA]- anion can interact with the graphene surface by its π bonds as well as nitrogen atoms. The geometry of [Btma][Y] (Y = BF4-, PF6-, and Tf2N-) adsorbed on the graphene surface is different from that on the h-BN surface.60 For example, the [Btma]+ cation in [Btma][Tf2N] tends to be arranged in a parallel orientation with respect to the graphene surface by the interaction of butyl and two methyl groups, while it interacts with the h-BN surface60 by only two methyl groups. The [Tf2N]- anion is bent upward and interacts with the graphene surface by oxygen and fluorine atoms in –SO2 and –CF3 groups, while it tends to be parallel with respect to the h-BN surface and interacts via –SO2 and –CF3 groups.60

G[Bmim][BF4]

G[Bmim][PF6]


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G[Bmim][DCA]

G[Btma][Tf2N]

G[Btma][BF4]

G[Btma][PF6]

G[Btma][DCA]

G[Btma][Tf2N]

Figure 2. The most stable geometries for adsorption of ILs on the graphene surface optimized at the M06-2X/cc-pVDZ level of theory. One of the most important changes in ILs upon adsorption on the graphene surface is the change in the hydrogen bond strength between cation and anion. Bond critical point data for the 17 ACS Paragon Plus Environment


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H-bonds of ionic liquids before and after adsorption on the graphene surface are shown in Table S1 and Table S2, respectively. Based on the Bader’s theory, the hydrogen bond energies (EH…X) are also calculated using the equation of EH…X = 1/2V(r).61-62 As Tables S1 and S2 display, the changes in the sum of electron density (Σρ(r)) and hydrogen bond energy (ΣEH…X) values for the H-bonds of ILs before and after adsorption on the graphene surface are considerable. Although individual interactions were observed between ILs and the graphene surface, the hydrogen bonds within the ILs still have an important function in the adsorption system. Upon adsorption of ILs, the sum of electron density (Σρ(r)) and hydrogen bond energy (ΣEH…X) values in ILs decreases. The trend of electron density changes in BCPs of hydrogen bonds upon adsorption of ILs on the graphene surface is as follows: [Bmim][BF4]> [Bmim][PF6]> [Btma][BF4]> [Btma][PF6]> [Btma][DCA]> [Btma][Tf2N]> [Bmim][DCA]> [Bmim][Tf2N]. It is worth mentioning that the same results are also seen for the adsorption of [Bmim][Y] and [Btma][Y] (Y = BF4-, PF6-, and Tf2N-) on the h-BN surface, decreasing the sum of electron density (Σρ(r)) and hydrogen bond energy (ΣEH…X) values between cation and anion in the ionic liquids.60 In order to find the binding strength of cation and anion with the graphene surface, the data extracted from bond critical points (BCPs) formed between cation and graphene surface and also anion and graphene surface in G…IL complexes are summarized in Tables S3 and S4, respectively. The BCPs formed between [Bmim]+, [Btma]+ cations and graphene surface were produced through interaction between carbon atoms of graphene surface and hydrogen and carbon atoms of the cations. The [BF4]-, [PF6]-, [DCA]-, and [Tf2N]- anions also interact with the carbon atoms of graphene surface through their N, O, and F atoms. The presence of these weak interactions between cation, anion and graphene surface is shown by the green regions in noncovalent interaction plots. From sum of electron density (∑ρ(r)) and sum of binding energy


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(∑EX…Y, (X and Y = interacting atoms of cation, anion, and graphene surface)) values listed in Tables S3 and S4, we found that the [Bmim]+ and [Btma]+ cations in all G...IL complexes (with the exception of G[Bmim][BF4] and G[Btma][BF4] complexes) have a stronger interaction with the graphene surface than [PF6]-, [DCA]-, and [Tf2N]- anions. A comparison between our results for the adsorption of [Bmim][Y] and [Btma][Y] (Y = BF4-, PF6-, and Tf2N-) on the graphene and h-BN surface indicates that the [BF4]-, [PF6]-, and [Tf2N]- anions in the ILs have a stronger interaction with the h-BN surface than the [Bmim]+ and [Btma]+ cations,60 although [Bmim]+ and [Btma]+ cations in the ILs (with the exception of [Bmim][BF4] and [Btma][BF4]) have a stronger interaction with the graphene surface than the [BF4]-, [PF6]-, and [Tf2N]- anions. This result points out the significance of surface polarity and orientation of ionic liquids adsorbed on the surface in the binding strength between cation, anion and surface. 3.3. Graphene-Ionic Liquid Binding Energy The stability of graphene-ionic liquid complexes was evaluated by binding energy (∆Eb) values. The effect of basis set superposition error (BSSE) on the geometry of G…IL complexes was investigated and shown that the BSSE have no significant effect on the geometry of ILs on the graphene surface. On the other hand, the effect of BSSE on the ∆Eb values in the most stable geometry of G…IL complexes was evaluated, and then the ∆Eb values were corrected by BSSE and summarized in Table 1. In order to consider the effect of long-range correction, M06-2X-D3 functional which employs empirical dispersion corrections (D3) developed by Grimme44 was considered. Then, binding energies were calculated again and compared with those resulted from M06-2X/cc-pVDZ level of theory. As can be found in Table 1, for G…IL complexes which have ionic liquids based on the [Bmim]+ and [Btma]+ cations, the binding energies change from -10.53 to -15.01 kcal/mol and



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from -9.61 to -13.53 kcal/mol, respectively. The binding energy values for these complexes follow the order: G[Bmim][PF6] (-15.01 kcal/mol) > G[Bmim][DCA] (-14.38 kcal/mol) > G[Btma][DCA] (-13.53 kcal/mol) > G[Bmim][BF4] (-13.04 kcal/mol) > G[Btma][PF6] (-12.88 kcal/mol) > G[Btma][Tf2N] (-11.69 kcal/mol) > G[Bmim][Tf2N] (-10.53 kcal/mol) > G[Btma][BF4] (-9.61 kcal/mol). Since the adsorption of ILs on the graphene surface is influenced by van der Waals interactions, the inclusion of empirical dispersion correction (D3) to the M06-2X method improves the binding energy values about 10-18 kcal/mol. In addition, the order of binding energy values for these complexes changes as follows: G[Bmim][PF6] (-32.44 kcal/mol) > G[Bmim][BF4] (-29.14 kcal/mol) > G[Bmim][Tf2N] (-28.41 kcal/mol) > G[Btma][Tf2N] (-28.26 kcal/mol) > G[Btma][PF6] (-27.50 kcal/mol) > G[Btma][BF4] (-25.95 kcal/mol) > G[Bmim][DCA] (-25.59 kcal/mol) > G[Btma][DCA] (-23.24 kcal/mol). As shown in Table 1, comparing binding energy of G…IL complexes containing the same anions at the M06-2X-D3/cc-pVDZ level of theory shows that these values for complexes based on [Bmim]+ cation are more than those for complexes based on [Btma]+ cation. Generally, it can be concluded that the van der Waals interactions, cation and anion type, and their orientation on the graphene surface are factors which affect the binding energy values. A similar trend is also observed when the [Bmim][Y] and [Btma][Y] (Y = BF4-, PF6-, and Tf2N-) are adsorbed on the h-BN surface.60 In this trend, ILs based on the [Bmim]+ cation have more binding energies than ILs based on the [Btma]+ cation with the exception of h-BN[Bmim][Tf2N] and h-BN[Btma][Tf2N]. Comparing the ∆Eb values for adsorption of [Bmim][Y] and [Btma][Y] (Y = BF4-, PF6-, and Tf2N-) on the graphene and h-BN surface60 generally reveals that the binding energy values for adsorption on the graphene surface are more than those for adsorption on the h-


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BN surface. Also, the highest binding energy for IL adsorption on the graphene and h-BN surface is observed for [Bmim][PF6] and [Bmim][BF4], respectively. It is worth mentioning that the presence of defects such as monovacancies, multivacancies, pentagon-heptagon pairs, and adatoms [63] on the graphene structure due to carbon vacancies influences the chemical and physical characteristics of graphene [64] since it generates nonequivalent carbon atoms on the surface. Defective graphene can be obtained, for example, during generation of functionalized single graphene sheets from the thermal expansion of graphite oxide (GO) [65]. Defect sites present higher reactivity for adsorption, which makes chemical functionalization an easy method to detect imperfections on graphene [66]. On the other hand, defects can affect on the geometry of chemicals on the surface, binding energy values, the magnitude and direction of charge transfer, HOMO-LUMO energy gap, and thermodynamic properties. The results of this work lay the groundwork for extending this study to investigate ionic liquid interactions with defective graphene and hexagonal bonon-nitride sheets. 3.4. Orbital Energy and Density of States (DOSs) Calculations The energy difference between the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) is termed as the HOMO-LUMO energy gap. The magnitude of HOMO-LUMO energy gap depends on the relative magnitude of respective orbital energies. The HOMO-LUMO energy gaps of the graphene surface, ILs, and G…IL complexes are shown in Table 2. As seen from Table 2, the energy gap of ILs and G…IL complexes changes from 6.74 eV to 11.76 eV and from 4.23 eV to 4.28 eV, respectively. Our results show that the HOMO-LUMO energy gap of ILs decreases upon adsorption on the graphene surface. Similar results were also found upon adsorption of [Bmim][Y] and [Btma][Y] (Y = BF4-, PF6-,



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and Tf2N-) ILs on the h-BN surface and the HOMO-LUMO energy gap of these ILs decreases upon adsorption on the h-BN surface.60 The order of the HOMO-LUMO energy gap changes of ILs upon adsorption on the graphene surface is as follows: [Btma][PF6] (7.50 eV) > [Btma][BF4] (7.08 eV) > [Btma][Tf2N] (5.61 eV) > [Bmim][BF4] (5.45 eV) > [Bmim][PF6] (5.42 eV) > [Bmim][Tf2N] (4.8 eV) > [Btma][DCA] (3.38 eV) > [Bmim][DCA] (2.46 eV).

Table 2. The HOMO (in a.u), LUMO (in a.u), HOMO-LUMO energy gap (∆E in eV) of the graphene surface, ILs, and G…IL complexes, and the reduction of HOMO-LUMO energy gap of ILs, after adsorption on the graphene surface (∆∆E in eV). a

HOMO

LUMO

∆E

4.32

G[Bmim][BF4]

-0.2252

-0.0698

4.23

5.45

0.0139

9.68

G[Bmim][PF6]

-0.2285

-0.0727

4.24

5.42

-0.3480

0.0069

9.66

G[Bmim][DCA] -0.2311

-0.0738

4.28

2.46

[Bmim][DCA] -0.2436

0.0040

6.74

G[Bmim][Tf2N]

-0.2285

-0.0712

4.28

4.80

[Bmim][Tf2N]

-0.3263

0.0073

9.08

G[Btma][BF4]

-0.2168

-0.0599

4.27

7.08

[Btma][BF4]

-0.3788

0.0382

11.35

G[Btma][PF6]

-0.2230

-0.0665

4.26

7.50

[Btma][PF6]

-0.3983

0.0338

11.76

G[Btma][DCA]

-0.2249

-0.0683

4.26

3.38

[Btma][DCA]

-0.2414

0.0393

7.64

G[Btma][Tf2N]

-0.2271

-0.0701

4.27

5.61

[Btma][Tf2N]

-0.3303

0.0327

9.88

HOMO

LUMO

Graphene

-0.2238

-0.0650

[Bmim][BF4]

-0.3417

[Bmim][PF6]

a

∆E

b

Structure

Structure

∆∆E

∆E = E(LUMO) – E(HOMO), b∆∆E = ∆E(IL) – ∆E(G…IL)

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 energy gap due to molecular interactions.67 The DOS spectra for the graphene surface, ILs and ILs adsorbed on the graphene surface are shown in Figure 3. Upon adsorption of ILs on the graphene surface, DOS featuring ILs changes noticeably and their energies shift partly. The HOMO states in ILs based on the [Bmim]+ cation 22 ACS Paragon Plus Environment

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([Bmim][Y] (Y = BF4-, PF6-, DCA-, and Tf2N-)) move from negative energy values to more positive energy values, although the LUMO states move from positive energy values to more negative energy values upon adsorption on the graphene surface. The graphene surface involves states which cover partly the band gap of [Bmim][Y] (Y = BF4-, PF6-, DCA- and Tf2N-) ILs from the occupied side by about 3.169, 3.250, 0.340, and 2.660 eV, respectively to higher energies and from the unoccupied side by about 2.279, 2.169, 2.120, and 2.140 eV to lower energies, respectively. These changes lead to a reduction in the band gap of [Bmim][Y] (Y = BF4-, PF6-, DCA- and Tf2N-) ILs by 5.45, 5.42, 2.46, and 4.80 eV, respectively. Upon adsorption of [Btma][Y] (Y = BF4-, PF6-, DCA- and Tf2N-) ILs on the graphene surface, the shift of HOMO and LUMO energy levels of these ILs to the new energy levels are similar to that of HOMO and LUMO energy levels of [Bmim][Y] (Y = BF4-, PF6-, DCA- and Tf2N-) ILs. The graphene surface involves states which cover partly the band gap of [Btma][Y] (Y = BF4-, PF6-, DCA- and Tf2N-) ILs from the occupied side by about 4.410, 4.770, 0.450, and 2.810 eV, respectively to higher energies and from the unoccupied side by about 2.670, 2.730, 2.930, and 2.800 eV to lower energies, respectively. These changes lead to a reduction in the band gap of [Btma][Y] (Y = BF4-, PF6-, DCA- and Tf2N-) ILs by 7.08, 7.50, 3.38, and 5.61 eV, respectively. Our results for the ILs containing the same anions show that a more reduction is seen in the HOMO-LUMO energy gap (∆∆E) of ILs based on the [Btma]+ cation than ILs based on the [Bmim]+ cation upon adsorption on the graphene surface. Also, a comparison of the amount of reduction in HOMO-LUMO energy gap (∆∆E) of ILs containing the same anions ([Bmim][Y] and [Btma][Y] (Y = BF4-, PF6-, and Tf2N-)) upon adsorption on the graphene and h-BN surface60 indicates that a larger reduction is seen in the HOMO-LUMO energy gap of ILs adsorbed on the graphene surface than h-BN surface. These results are consistent with the magnitudes of



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interactions of various ILs with graphene and h-BN surfaces, respectively. A larger change in the HOMO-LUMO band gap is commensurate with higher magnitudes of interactions of ILs with the surface.

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


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3.5. Charge Transfer between Graphene Surface and Ionic Liquids For evaluating the atomic charge distributions of ILs, graphene surface and also their modification upon adsorption, CHELPG68 analysis was performed at the M06-2X/cc-pVDZ level of theory. Recent works suggest the importance and accuracy of CHELPG analysis to explore the charge distribution in adsorbate/adsorbent systems.37,40 The results of CHELPG analysis for estimation of the variation of cation, anion, and the graphene surface charges due to the adsorption process are listed in Table 3. Table 3. The charge difference of cation, anion, and graphene surface after adsorption (obtained by ChelpG approximation for M06-2X/cc-pVDZ wavefunctions), (values in e)a. Structure

b

∆q (cation)

c

∆q (anion)

∆q (graphene sheet)

[Bmim][BF4]

0.145

-0.154

-0.009

[Bmim][PF6]

0.061

-0.047

0.014

[Bmim][DCA]

0.051

-0.020

0.031

[Bmim][Tf2N]

0.057

-0.039

0.018

[Btma][BF4]

0.031

-0.066

-0.035

[Btma][PF6]

0.125

-0.128

-0.003

[Btma][DCA]

0.059

-0.010

0.049

[Btma][Tf2N]

0.089

-0.054

0.035

a

The charge of graphene surface before adsorption is zero. b∆q (cation) = qcation in IL (before adsorption) – qcation in IL (after adsorption), ∆q for cation is positive and the positive value means gaining charge. c∆q (anion) = qanion in IL (before adsorption) – qanion in IL (after adsorption), ∆q for anion is negative and the negative value means losing charge. According to Table 3, upon adsorption of [Bmim][Y] (Y = BF4-, PF6-, DCA-, and Tf2N-) ILs on the graphene surface, the overall charge on the graphene surface becomes positive except [Bmim][BF4], indicating charge transfer (CT) from the graphene surface to ILs. From charge analysis, it is obvious that the charge transfer between cation and anion is a considerable factor



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in the adsorption of ILs. Upon adsorption of these ILs, a significant change in point charges on the graphene surface are seen for [Bmim][DCA]. The adsorption of [Bmim][DCA] causes the [Bmim]+ cation to become less positive by 0.051e and the anion to be less negative by 0.020e. This results in a charge of 0.031e on the graphene surface. The adsorption process of [Bmim][BF4], [Bmim][PF6], and [Bmim][Tf2N] ILs causes the cation to become less positive by 0.145, 0.061, and 0.057e and the anion to be less negative by 0.154, 0.047, and 0.039e, respectively. This results in a charge of -0.009, 0.014, and 0.018e on the graphene surface, respectively. The adsorption of [Btma][Y] (Y = BF4-, PF6-, DCA- and Tf2N-) ILs on the graphene surface causes the cation to become less positive by 0.031, 0.125, 0.059, and 0.089e and the anions to be less negative by 0.066, 0.128, 0.010, and 0.054e, respectively. The sign of induced charges on the graphene surface indicates that charge transfer (CT) occurs from the [Btma][BF4] and [Btma][PF6] to graphene surface, although it is occurs

from the graphene surface to

[Btma][DCA] and [Btma][Tf2N] ILs. The order for the magnitude of charge transfer between different ILs and the graphene surface is as follows: [Btma][DCA] (0.049e) > [Btma][Tf2N] (0.035e), [Btma][BF4] (-0.035) > [Bmim][DCA] (0.031e)> [Bmim][Tf2N] (0.018e) > [Bmim][PF6] (0.014e) > [Bmim][BF4] (-0.009e) > [Btma][PF6] (-0.003e). A comparison between the charge of cation and anion in the [Bmim][Y] and [Btma][Y] (Y = BF4-, PF6-, and Tf2N-) ILs before and after adsorption on the graphene and h-BN surfaces60 indicates that the cation tends to become less positive (gaining charge) and the anion to be less negative (losing charge) after adsorption on the graphene and h-BN surfaces. The graphene and h-BN surfaces have variable negative or positive charges due to charge transfer between IL and the surfaces. On the other hand, the amount and direction of charge transfer between ILs and the


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surfaces are mostly different. For example, upon adsorption of [Bmim][Tf2N] IL on the graphene surface, the charge transfer occurs from graphene surface to IL (0.018e), although it occurs from IL to h-BN surface (-0.059e). 3.6. Non-Covalent Interaction (NCI) Plots For revealing the role and significance of non-covalent interactions in the adsorption of ionic liquids on the graphene surface, we used NCIPLOT program.47-48 This program reveals noncovalent interactions based on the peaks that appear in the reduced density gradient (RDG) at low densities. RDG isosurfaces for these peaks enable the visualization of weak interactions. The isosurfaces correspond to 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)favourable nature of the interactions and defines the isosurface coloring. The isosurfaces of sign (λ2) × ρ defined by Yang et al.47-48 are displayed in Figure 4, where the red and green regions represent the strong repulsion and attractive vdW interactions, respectively. The green region between ionic liquids and graphene surface indicates that vdW interaction is the key driving force to the adsorption of ILs on the graphene surface. The vdW interaction between graphene 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 seen from Figure 4, the type of cation and anion has an important influence on the configuration and orientation of ionic liquids on the graphene surface via vdW interactions. For example, [Bmim]+ and [Btma]+ cations interact with the graphene surface by imidazolium aromatic ring, methyl and butyl alkyl groups and [BF4]-, [PF6]-, [DCA]-, and [Tf2N]- anions interact by their fluorine, nitrogen, and oxygen



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atoms. As seen from Figure 4, [DCA]- anion in [Btma][DCA] IL can also interact with the graphene surface by its π bonds in addition to its nitrogen atoms.

G[Bmim][BF4]-top view

G[Bmim][PF6]-top view

G[Bmim][DCA]-top view

G[Bmim][BF4]-side view

G[Bmim][PF6]-side view

G[Bmim][DCA]-side view

G[Bmim][Tf2N]-top view

G[Btma][BF4]-top view

G[Btma][PF6]-top view


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G[Bmim][Tf2N]-side view

G[Btma][BF4]-side view

G[Btma][DCA]-top view

G[Btma][Tf2N]-top view

G[Btma][DCA]-side view

G[Btma][Tf2N]-side view


G[Btma][PF6]-side view


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Figure 4. The side and top view of 3-D graphic of reduced density gradient (RDG) calculation of G…IL complexes. Green color indicates vdW interaction and red strong non-bonded overlap (for interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 3.7. Thermochemistry of IL Adsorption on the Graphene 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 8 ILs present alone and when adsorbed on the graphene 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 graphene surface. Figure 5 shows the binding energy (∆Eb), enthalpy (∆Hads), free energy (∆Gads), entropy (∆Sads) of adsorption for the 8 different ILs on the graphene surface. The order of ∆Eb, ∆Hads, and ∆Gads values for adsorption of [Bmim][Y] (Y = BF4-, PF6-, DCA-, and Tf2N-) ILs on the graphene surface is as follows: [Bmim][PF6] > [Bmim][BF4] > [Bmim][DCA] > [Bmim][Tf2N], although a similar order is not seen for ∆Sads values ([Bmim][BF4] > [Bmim][PF6] > [Bmim][DCA] > [Bmim][Tf2N]). The ∆Eb and ∆Hads values for ILs based on the [Btma]+ cation ([Btma][Y] (Y = BF4-, PF6-, DCA-, and Tf2N-)) follow the order [Btma][PF6] > [Btma][Tf2N] > [Btma][DCA] > [Btma][BF4], although there is not a similar order for the ∆Gads and ∆Sads values. From the negative values of free energy (∆Gads), it could be concluded that the adsorption process of ILs on the graphene surface proceeds spontaneously. A similar trend was also seen when the


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[Bmim][Y] and [Btma][Y] (Y = BF4-, PF6-, and Tf2N-) ILs were adsorbed on the h-BN surface.60 Therefore, it could be concluded that the adsorption of ILs on both graphene and h-BN surfaces proceeds spontaneously. A comparison between the entropy of adsorption of ILs (∆Sads) on the graphene (Table S5 and Figure 5) and h-BN surface60 shows that ∆Sads values decrease upon adsorption of ILs on both surfaces. It could be due to the decrease of the translation degree of freedom. A further decomposition of entropy of adsorption into translational (∆St), rotational (∆Sr), and vibrational (∆Sv) components is also possible. As seen from Table S5, the contribution of these components in the total entropy (∆Sads) for adsorption of ILs based on the [Bmim]+ cation (except [Bmim][BF4] IL) follows the order ∆St > ∆Sv > ∆Sr, while the contribution of these components for adsorption of ILs based on the [Btma]+ cation follows the order ∆St > ∆Sr > ∆Sv. Finally, the most favorable adsorption enthalpies and free energies for ILs based on the [Bmim]+ and [Btma]+ cations were typically witnessed for adsorption of [Bmim][PF6] and [Btma][PF6] ILs on the graphene surface, respectively.



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Figure 5. The changes of ∆Eb without BSSE correction (a), ∆Hads (b), ∆Gads (c), and ∆Sads (d) for IL adsorption on the graphene surface. 4. Conclusion The adsorption of two types of ionic liquids based on 1-butyl-3-methylimidazolium [Bmim]+ and

butyltrimethylammonium

[Btma]+

cations,

paired

to

tetrafluoroborate

[BF4]-,

hexafluorophosphate [PF6]-, dicyanamide [DCA]-, and bis-(trifluoromethylsulfonyl)imide [Tf2N]anions on graphene surface have been investigated theoretically using M06-2X/cc-pVDZ level of theory. Our results indicate that the binding energy values for the adsorption of ILs based on [Bmim]+ cation on the graphene surface are more than those of ILs based on [Btma]+ cation with the exception of [Bmim][Tf2N] and [Btma][Tf2N] ILs. The magnitude of binding energy values for the adsorption of ILs on the graphene surface follows the order: [Bmim][PF6] (-15.01 kcal/mol) > [Bmim][DCA] (-14.38 kcal/mol) > [Btma][DCA] (-13.53 kcal/mol) > [Bmim][BF4] (-13.04) > [Btma][PF6] (-12.88 kcal/mol) > [Btma][Tf2N] (-11.69 kcal/mol) > [Bmim][Tf2N] (10.53 kcal/mol) > [Btma][BF4] (-9.61 kcal/mol). 32 ACS Paragon Plus Environment

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The [Bmim]+ cation in the ILs except [Bmim][Tf2N] tends to be arranged in parallel orientation with respect to the graphene surface at a distance of 3.01-3.21 Å. [Btma]+ cation interacts with the graphene surface by hydrogen atoms of methyl and butyl alkyl groups. [BF4]-, [PF6]-, [DCA]-, and [Tf2N]- anions interact by their fluorine, nitrogen, oxygen atoms and π bonds, particularly in [DCA]-, with the graphene surface. The non-covalent interactions responsible for the adsorption of ILs on the graphene surface are also shown by the green regions between ionic liquids and graphene surface in the non-covalent interaction plots. 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 graphite model involved in the interaction appreciably. The type of ionic liquid distinctively determines the charge modification of graphite model atoms. Upon adsorption of ionic liquids based on [Bmim]+ cation, the overall charge on the graphene surface becomes positive except for [Bmim][BF4], indicating the charge transfer from the graphene surface to ILs. In the ionic liquids based on [Btma]+ cation, the charge transfer from the graphene surface to the ILs are found in the adsorption of [Btma][DCA] and [Btma][Tf2N], although the charge transfer from IL to graphene surface are only seen in the adsorption of [Btma][BF4] and [Btma][PF6] ILs. Orbital energy and density of states (DOSs) calculations also show that the HOMO-LUMO energy gap of ILs decreases upon adsorption on the graphene surface. On the other hand, QTAIM analysis indicates significant reduction in the hydrogen bond strength between cation and anion after adsorption on the graphene surface. The results of QTAIM analysis also revealed that the [Bmim]+ and [Btma]+ cations in all G...IL complexes with the exception of G[Bmim][BF4] and G[Btma][BF4] complexes have a stronger interaction with the graphene surface than [PF6]-, [DCA]-, and [Tf2N]- anions.



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Finally, the results of thermochemical analysis (the negative values ∆Gads) indicate that the adsorption of ILs on the graphene surface proceeds spontaneously. The negative values of entropy of adsorption also indicate that the entropy decreases upon adsorption of ILs due to the decrease of the translation degree of freedom.

Supporting Information. This material contains Bond Critical Point (BCP) data resulted from QTAIM analysis for ILs and ILs adsorbed on the graphene surface, thermochemical parameters involved in the process of IL adsorption on the graphene surface, initial structures for adsorption of [Bmim][BF4] IL on the graphene surface, and geometries and SCF energies for all optimized structures. This material is also available free of charge via the Internet at http://pubs.acs.org.

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