Factors Affecting the Exfoliation of Graphite Intercalation Compounds

Feb 23, 2015 - (38) A plane-wave basis with an energy cutoff of 400 eV was used .... investigations were carried out for electronegative intercalants,...
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Factors Affecting the Exfoliation of Graphite Intercalation Compounds for Graphene Synthesis Gabin Yoon,†,‡ Dong-Hwa Seo,†,∥ Kyojin Ku,†,‡ Jungmo Kim,§ Seokwoo Jeon,§ and Kisuk Kang*,†,‡ †

Department of Materials Science and Engineering, Research Institute of Advanced Materials (RIAM), Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-742, Republic of Korea ‡ Center for Nanoparticles Research, Institute for Basic Science (IBS), Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-742, Republic of Korea § Department of Materials Science and Engineering and Graphene Research Center of KI for the NanoCentury, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea S Supporting Information *

ABSTRACT: We investigate the mechanism of the intercalation-aided exfoliation of graphite using van der Waals force-corrected density functional theory (DFT) calculations. From a comparative study on various intercalation systems, we find that, depending on the intercalant species, the exfoliation energies vary significantly, not only due to the size of intercalants but also due to interactions with the host graphite. While it is generally perceived that an expanded interlayer distance with intercalants weakens the binding between graphene layers, as the van der Waals forces decrease, the calculations reveal that the intercalation of electronegative or electropositive intercalants (e.g., Li, K, F, Cl, and Br) result in a 1.5−5-fold higher exfoliation energy than pristine graphite due to additional binding forces from charge transfer between intercalants and graphene layers. Furthermore, we demonstrate that this additional binding force could be manipulated with cointercalation or neutral intercalants, which hints at effective exfoliation strategies with graphite intercalation compounds. This theoretical study broadens our understanding of the mechanism underlying graphite exfoliation and will facilitate development of more effective exfoliation strategies for other related layered materials.

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van der Waals forces; the binding force scales roughly to r−6, where r is the distance between graphene layers. Thus, expanding the interlayer distance can reduce the binding energy significantly and is expected to facilitate the exfoliation of graphene layers. Many studies have shown that a variety of species can be intercalated into the graphite and are capable of expanding the graphene interlayers, such as K,19,20 Li,21 S,22 FeCl3,23,24 ICl, IBr,25 hydrate salt,26 Brønsted acids,27 and KCl− NaCl−ZnCl2.28 Subsequent exfoliation with these graphite intercalation compounds (GIC) could successfully produce a large quantity of graphene, although the flake size would be relatively small and contain inherent defects.29−33 In this work, we investigate a series of graphite intercalation compounds used for liquid phase exfoliation by van der Waals force-corrected first-principles calculations. While the exfoliation process of GICs is not clearly understood, nor is the role of intercalants, systematic calculations were performed to unveil the underlying mechanism of the exfoliation. From a comparative study on various intercalation systems, we found that, depending on the intercalant species, the exfoliation

INTRODUCTION Graphene holds great promise in a variety of potential applications, such as in the fields of electronics, energy storage/conversion, optics, and photovoltaics.1−5 Its remarkable physical properties, due to its unique electronic structure, provide exceptionally high electronic and thermal conductivity, chemical stability, and mechanical flexibility, which make graphene a potentially “breakthrough” material for these applications.6−9 Since isolation of single layer graphene was first clearly demonstrated using the well-known “Scotch tape method” in 200410 after a lot of other approaches,11−14 several graphene synthesis methods have been developed to facilitate its use as a feasible industrial material. Among them are mechanical exfoliation, chemical vapor deposition (CVD), and molecular assembly.10,15−17 While these methods produce generally highquality graphene, they lack the capability to yield large quantities of graphene and have extremely high synthesis costs, a critical barrier to large-scale industrial applications.18 Liquid phase exfoliation, on the other hand, is a comparatively low-cost synthesis method and can produce large amounts of graphene. The main concept of liquid phase exfoliation is based on cleaving the graphite flake in solution, because graphite is composed of graphene layers bonded with © 2015 American Chemical Society

Received: December 9, 2014 Revised: January 19, 2015 Published: February 23, 2015 2067

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Figure 1. (a) Top-viewed structure of LiC6. (b) Side-view of LiC6. (c) Formation energies of LiC6 and graphite as a function of the interlayer distance. Exfoliation energy increases from 55 to 285 meV. (d) Isosurface of charge distribution in LiC6 upon Li intercalation. Yellow indicates the charge gained, and blue the charge lost. Isosurface is set to 0.03 e−/Å3. In-plane charge distributions are also shown in graphene layer, and 0.4 Å above graphene layer. used projector-augmented wave (PAW) pseudopotentials36,37 as implemented in the Vienna ab initio simulation package (VASP).38 A plane-wave basis with an energy cutoff of 400 eV was used and appropriate numbers of k-points were used, depending on the size of the unit cells. All geometric relaxations were performed until all forces of the system converged within 0.05 eV/Å. Quantification of charge transfer was performed with bader charge analysis code.39 To describe the exfoliation of graphite or GICs, we constructed slab models adopting vacuum/slab/vacuum layered geometry. The thickness of a vacuum slab was set to >11 Å, which is sufficiently larger than the widely accepted thickness in a number of slab/vacuum geometry calculations.40−42 The overall thickness of the unit cell is ∼35−45 Å depending on the size of the system. In our model systems, exfoliation was defined as the separation of a single surface layer from the thick slab. On the other hand, the split of slabs yielding more than two surface layers on each side was defined as cleavage. Interlayer cohesive energies (Ecoh) of each system were calculated, where Ecoh is

energies vary significantly, not only due to the size of the intercalants but also due to interactions with the host graphite. While the expanded interlayer distance generally weakens the van der Waals forces between graphene layers upon intercalation, additional binding forces are generated between intercalants and graphene layers due to charge transfer occurring between them. Furthermore, we demonstrated that this additional binding force could be manipulated with cointercalation or neutral intercalants, which hints at effective exfoliation strategies using GICs. Our fundamental study is believed to aid in understanding the mechanism of the liquid phase exfoliation of graphite and provides further insights into the exfoliation of other related layered materials.

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COMPUTATIONAL DETAILS

All calculations were performed based on a density functional theory (DFT) platform using Perdew−Burke−Ernzerhof (PBE) parametrized with a spin-polarized generalized gradient approximation (GGA) for exchange-correlation energy.34 Because the conventional DFT method fails to describe long-range electron correlations, such as van der Waals (vdW) interactions, the semiempirical dispersion potential proposed by Grimme (DFT-D2)35 was implemented in the DFT results. Parameters for the DFT-D2 method, such as dispersion coefficients and vdW radii, were adopted as proposed by Grimme. We checked the validity of the DFT-D2 vdW functional by implementing it in a graphite system and comparing the result with that from GGA alone. An interlayer distance of 3.24 Å, in good agreement with experimental results, was obtained using the DFT-D2 method, whereas negligible dispersion forces were seen without the DFT-D2 vdW functional. We

Ecoh = Esys(d = ∞) − Esys(d = de) and de is the equilibrium distance between graphene layers in GICs. Note that Ecoh is normalized by the number of surface carbon atoms. The exfoliation and cleavage were compared to determine the preferable graphene forming mechanism. However, in all calculations, Ecoh of the exfoliation process was always less than those of the cleavage processes, indicating that exfoliation of a single surface layer of graphite is most likely to occur before cleavage. This also agrees with the previous report by Girifalco and Lad,43 which used lattice summations with Lennard−Jones potentials between carbon atoms. Thus, the entire model systems in this work was constructed assuming that only the exfoliation process occurred, and interlayer cohesive energies Ecoh were denoted as exfoliation energies Eex. We also 2068

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Chemistry of Materials investigated slab number dependency on exfoliation energy. Almost no variation on the exfoliation energy of graphite was observed in slabs thicker than five graphene layers; thus, we adopted similar slab sizes in all model systems.

Figure S1 in the Supporting Information). The result was that the empty graphene layers open up before the Li-filled layers, indicating that less energy was required to cleave the Li-free layer. This is consistent with the result of LiC6, which showed much stronger binding with Li intercalants present in the layers. This observation also indicates that the empty graphene layers in partially filled GICs are inclined to be separated in advance and contribute mainly to the exfoliation process, even in the presence of intercalants. Moreover, we found that the exfoliation energy of LiC12 was almost comparable to that of the pristine graphite.The difference was less than ∼1%, meaning that the weakest link in the staging type of GICs lies in separating empty graphene layers. The lesson from the calculation of LixC6 is that not only the weakening force between graphene layers with expanded interlayer distance but also the formation of new ionic bonding upon intercalation should be taken into account; they compete with each other. This led to our second GIC under investigation: K-intercalated graphite. Because the ionic radius of K is far larger than that of Li (1.38 Å vs 0.76 Å in the sixcoordinated case), it would be expected that K ions would expand the interlayer distance more effectively than Li ions.52 Figure S2a and b shows the structure of fully intercalated phase, KC8. Its structure varied slightly from LiC6 because of the size difference between K and Li ions. Due to the large size of K ions, they occupy a smaller number of sites in the interlayer space than Li ions, with in-plane ordering of 2 × 2 in KC8. While carbon layers stack as AAAA··· in both compounds, the stacking sequence of K ions in KC8 is αβγδ···, different from the αα··· stacking of Li ions in LiC6. K intercalation significantly enlarged the interlayer distance, from 3.24 Å in the pristine graphite to 5.23 Å, also far larger than 3.69 Å in LiC6.48 Accordingly, the exfoliation energy of KC8 became 85.1 meV, which is much smaller than with LiC6, as shown in Figure S2c. However, the exfoliation energy of K-intercalated graphite was still higher than that of the pristine graphite. This was because charge redistribution in KC8 occurred around the electropositive K ions and graphene layer, strengthening the bond, as revealed from the electronic structure calculation (Figure S2d). This result indicates that an attractive force between cation intercalants and the graphene layer is inevitable, despite the substantially enlarged interlayer space, and is generally stronger than the van der Waals interaction. It would be challenging to reduce the exfoliation energy by intercalating electropositive intercalants into graphite. To assess the role of intercalants of GICs in general, similar investigations were carried out for electronegative intercalants, such as F, Cl, and Br in hypothetical structures of XC6 (X = F, Cl, Br). Note that there has been no experimental report on GICs comprising F, Cl, or Br. The hypothetical XC6 (X = F, Cl, Br) structures were modeled with √3 × √3 in-plane ordering of intercalants and a stacking sequence of graphene layers with AA···. Upon intercalation, interlayer distances between graphene layers increased gradually as larger ions were introduced. According to our calculations, they increased from 3.24 Å in pristine graphite to 4.61, 5.06, and 5.61 Å in FC6, ClC6, and BrC6, respectively (RF− (1.33 Å) < RCl− (1.81 Å) < RBr− (1.96 Å) in the six-coordinated case). Figure 2a shows that exfoliation energies of each compound decreased markedly as the equilibrium interlayer distance increased with the intercalation of larger anions. In FC6, the exfoliation of a layer of graphene required 193 meV, but only 114 or 87 meV is required in the cases of ClC6 and BrC6, respectively.

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RESULTS AND DISCUSSION Among a variety of graphite intercalation compounds, we first focused on Li-intercalated graphite, which was reported recently to be an effective medium for graphite exfoliation44,45 and has been studied widely in the field of lithium rechargeable batteries as an electrode material.46,47 Figure 1a and b shows the structure of Li-intercalated graphite, LiC6, which represents the phase with a maximum amount of Li insertion into the graphite.48 Li occupies the interlayer space with √3 × √3 inplane ordering, and the stacking sequence of the graphene layers adopts AAAA···, in contrast to ABAB··· in the pristine graphite. Our calculation shows that the interlayer distance between graphene layers increased from 3.24 to 3.69 Å upon Li intercalation, in agreement with previous experimental results.49 However, to our surprise, the exfoliation energy increased significantly, from 55 to 285 meV despite the expanded graphene interlayer distance, which was expected to, consequently, reduce the van der Waals forces between graphene layers (Figure 1c). This result contradicted to our expectations as well as some previous explanations on the role of GICs in the exfoliation process, which simply correlated the expanded graphene layers with the exfoliation capability.33,44,50 To comprehend this phenomenon, we investigated how the Li intercalants interacted with graphene layers by calculating the charge redistribution upon Li intercalation in the graphite. Figure 1d illustrates that additional charge (described as “yellow cloud”) populates between Li intercalants and graphene layers, strongly suggesting that a significant charge transfer occurs from the electropositive Li to the carbons. Additionally, a small amount of depletion of electrons in the C−C bonds was also observed in the ab plane of Figure 1d (right, bottom), which causes elongation of the C−C bond length, from 1.423 to 1.438 Å. This increase in the C−C bond length is attributable to charge transfer from σ bonds to π bonds of carbon, due to the formation of the ionic bond between Li and graphene.51 Note that the change of C−C bond length in this model system is due to the redistribution of charges; the modification of the C− C bond length may presumably affect the fracture strength of the framework, thus the final lateral size and thickness of the exfoliated graphene flakes in the practical experiments. This indicates that the intercalated Li produces a binding force between graphene layers by sharing ionic bonds with adjacent graphene layers. While a slight reduction in van der Waals forces is also expected due to the enlarged interlayer distance, the former dominates in LiC6, resulting in the increase in the exfoliation energy. In some of the GICs, such as LixC6 (x < 1), intercalants often preferably occupy certain interlayer spaces of graphite, that is, staging of graphite. For example, in stage n, intercalants occupy every nth interlayer of graphite, while other layers remain empty. Thus, during exfoliation of such staging GICs, there are two possible cleavage planes: one with empty interlayers and the other filled with intercalants. These two distinguishable planes can potentially exhibit different exfoliation energies. Stage II Li-intercalated graphite LiC12 was chosen to examine this, where both Li-filled and Li-free layers are present alternately in the structure. We gradually separated all of the graphene layers in LiC12, monitoring each interlayer space (see 2069

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Figure 2. (a) Formation energies of XC6 (X = F, Cl, Br) and graphite as a function of the interlayer distance. Exfoliation energy increases from 55 to 193, 114, and 87 meV, respectively. (b) Isosurface of charge distribution in FC6 upon F intercalation. Yellow indicates the charge gained, and blue the charge lost. Isosurface is set to 0.03 e−/Å3. In-plane charge distributions are also shown in the graphene layer, and 0.4 Å above graphene layer.

Figure 3. (a) Structure of (C6H6)C32. (b) Isosurface of charge distribution in (C6H6)C32 upon benzene intercalation. Isosurface is set to 0.6 e−/Å3. Note that no significant charge transfer was observed even at 20 times larger isosurface than LiC6, FC6. (c) Formation energy of (C6H6)C32 and graphite as a function of the interlayer distance. Exfoliation energy decreases from 55 to 16.6 meV.

Nevertheless, the exfoliations of such GICs are still less favorable than the pristine graphite, the exfoliation energy of which is as low as 55 meV. Charge analyses of those compounds also showed same cause as in cation intercalation; that is, the formation of ionic bonds. As shown in Figure 2b, electron clouds transfer from graphene layers to electronegative F intercalants, and subsequently the C−C bond length decreases slightly, from 1.423 to 1.421 Å, in agreement with previous work.51 It should be noted that the ambipolar nature of the graphite enables both the negative and positive polarization of graphite. This suggests that ionic bonds are also formed upon intercalation of electronegative intercalants, resulting in the increased exfoliation energy. Because both positive and negative intercalants robustly bind graphene layers due to the ambipolar nature of the graphite, intercalants that have similar electronegativity to carbon atoms were investigated in an attempt to minimize any possible ionic bond formation. Among such possible intercalants, we chose benzene, which consists of only C and H atoms, in a hypothetical GIC with a composition of (C6H6)C32. Figure 3a shows the most plausible structure calculated in the given composition, where C atoms in the benzene molecule reside at hollow sites in the C6 ring of AA··· stacked graphite layers. As a result of benzene intercalation, the interlayer distance of graphite increased to 6.21 Å, which is far larger than those of GICs previously considered with cations or anions. Moreover, no significant charge transfer was observed, as shown in Figure 3b. It is believed that the relatively large interlayer distance of 6.21 Å in (C6H6)C32 is attributable in part to the size of the benzene molecule itself, but mostly to negligible charge transfer and the resulting lack of attractive force between graphene layers. It was found that the exfoliation energy of (C6H6)C32 decreased markedly, from 55 to 16.6 meV, upon benzene

intercalation as a consequence. Considering that the room temperature lattice vibration energy is of the order of 25 meV, it would seem that this level of exfoliation energy would be readily surmounted in practical experimental conditions. In contrast to most GICs with electropositive or electronegative intercalants, which formed new ionic bonds with a net increase in the exfoliation energy, the intercalation of neutral intercalants such as benzene molecules could decrease van der Waals forces between graphene layers without producing any additional attraction force. This indicates that the key to effective graphite exfoliation through intercalation is to identify a proper neutral intercalants with an appropriate size. Aside from the intercalation of neutral molecules, such as benzene, we examined the cointercalation of both electronegative and electropositive ions as a possible means of enlarging the interlayer space without a charge transfer to graphene layers. By cointercalation, some ions or molecules that are not capable of being solely intercalated into graphite can be inserted, forming ternary compounds, as in the cases of a Li-solvent system53−55 and a metal−chloride system.23,28 Among them, we first investigated the K and Cl cointercalated system as an example. The most stable configuration of the K and Cl cointercalated system was selected after calculations of more than 400 possible arrangements of K and Cl (Figures 4a and S3) within a framework of the KC8 structure. Although it should be noted that the ClC8 structure is purely hypothetical, configurations of (KCl)0.5C8 showed negative formation energies, meaning that the intercalation of KCl into graphite may be preferable to the segregation into KC8 and ClC8. In fact, a recent intercalation experiment with graphite and ternary KCl−NaCl−ZnCl2 salts showed evidence of Cl cointercala2070

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Figure 4. (a) In-plane arrangement of K and Cl in (KCl)0.5C8 before and after the relaxation. (b) Side view of the relaxed structure of (KCl)0.5C8. Graphene layers are bent due to the agglomeration of KCl. (c) Charge distribution upon Cl intercalation into KC8. Isosurface is set to 0.03 e−/Å3. (d) Formation energy of (KCl)0.5C8 and graphite as a function of interlayer distance. Exfoliation energy decreases from 55 to 24.2 meV. (e) Exfoliation energies of K1−x(H2O)xC8 at various compositions. Dashed line indicates the exfoliation energy of graphite. (f) Plot of exfoliation energy versus interlayer distance in all the systems investigated.

tion.28 Figure 4a describes in-plane arrangement of K and Cl in (KCl)0.5C8. It can be seen that a strong attraction exists between positive K ions and negative Cl ions in the plane, which breaks the 2 × 2 in-plane ordering of intercalants. This agglomeration of KCl also has an effect on the interlayer ordering, as shown in Figure 4b. Graphene layers are notably bent and modulated due to the arrangement of agglomerates. Nevertheless, the average interlayer distance between graphene layers increased to 5.99 Å, which is significantly larger than with KC8 (5.23 Å) and ClC6 (5.06 Å). This marked increase in interlayer space indicates that the binding nature of the material has been altered in the cointercalated system. Comparative electronic structure analyses on KC8 and (KCl)0.5C8 suggested that the absence of charge transfer from KCl intercalants may have contributed to the increased interlayer distance. As shown in Figure 4c, no interlayer charge transfer was observed in (KCl)0.5C8, in contrast to KC8 and ClC6. Instead, significant charge transfer occurs from K ion to Cl ions, not to the graphene layers. The negligible charge transfer to graphene layers and the correspondingly weakened binding with graphene layers results in a large interlayer distance and small exfoliation energy of 24.2 meV in the (KCl)0.5C8 system in Figure 4d.

Another possible cointercalation scenario is the ion−solvent molecule intercalation into graphite. In practical experimental conditions, subsequent intercalation of solvent molecules into GICs is plausible. In fact, there have been several reports on exfoliation of layered materials using solvent intercalation.56−58 To describe this ternary system, including the solvent, we constructed a simple model cointercalation structure of K and H2O. Structure modeling followed the previous KCl system; that is, a KC8 framework, where K and H2O are arranged randomly. In this case, different ratios of K and H2O intercalation were investigated to mimic the experimental conditions where sequential water intercalation takes place in the host system in Figure 4e. When all the intercalants are K ions, the exfoliation energy is 85.1 meV, as shown before. However, as the relative amount of H2O molecules increases, the exfoliation energy decreases gradually, down to 26.3 meV at the entire H2O intercalation. We found that, in pure H2Ointercalated graphite (H2O)C8, no charge transfer was observed, similar to the cases of (C6H6)C32 and (KCl)0.5C8. However, the maximum amount of charge transfer was found with KC8. This observation confirms that the charge transfer to graphene layers play an important role in determining the exfoliation energy, and subsequent intercalation of solvents lowers the amount of charge transfer, facilitating the exfoliation 2071

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calants are included. This material is available free of charge via the Internet at http://pubs.acs.org.

process. Even though we investigated only one kind of solvent (H2O) in this study, we believe that other solvents would also play an important role in the exfoliation process as reported in several experimental studies.56−58 These calculations suggest that exfoliation energy depends strongly on charge transfer between intercalants and graphene layers, which compete with the change in van der Waals forces. This phenomenon is further clarified by the intercalation of divalent Ca into graphite. As shown in Figure 4f, where we plotted all the GICs examined and their corresponding exfoliation energy as a function of interlayer spacing, the exfoliation energy of Ca-intercalated graphite is much higher than F-intercalated graphite in spite of their similar interlayer distances. It is because graphene layers have a larger amount of charge transfer with divalent Ca than monovalent F, resulting in stronger binding forces. Degree of charge transfer not only depends on the valence charges, but also presumably on the difference of electronegativity between carbon and intercalants. In Figure S4, it is clearly observed that the number of charges transferred is proportional to the difference of electronegativity between carbon and intercalants in monovalent halogen intercalated graphite systems. In Figure 4f, it is shown that the exfoliation energy decreases when (i) large intercalants are introduced and (ii) more electroneutral molecules are intercalated. This tendency was also demonstrated in a series of Kx(H2O)1−xC8 systems, indicating that solvent cointercalation is necessary in ion-intercalated graphite exfoliation.

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Corresponding Author

*E-mail: [email protected]. Web page: energylab.snu.ac.kr. Present Address

∥ D.-H.S.: Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by (1) Supercomputing Center/ Korea Institute of Science and Technology Information with supercomputing resources including technical support (KSC2013-C3-038), (2) the Energy Efficiency & Resources (Project no. 20112010100140), and (3) Human Resources Development program (20124010203320) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy.

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REFERENCES

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CONCLUSIONS In this work, various graphite intercalation compounds were investigated to determine the mechanism of graphite exfoliation through the intercalation method. In contrast to conventional expectations, we found that the intercalation of alkali metals and halogens did not facilitate graphite exfoliation, but rather increased exfoliation energies, even though the interlayer distance of the graphene was expanded, to ∼5.2 Å, ∼2 Å greater than that of graphite. By observing charge transfers with graphite intercalation compounds, it was shown that the main reason for this was the formation of ionic bonds between electronegative or electropositive intercalants and graphene layers upon intercalation. In the case of most GICs with electropositive or electronegative intercalants, the formation of new ionic bonds resulted in a net increase in the exfoliation energy. In contrast, the intercalation of neutral intercalants, such as benzene molecules or KCl, decreased the van der Waals forces between graphene layers without producing additional attraction forces. Also, the aid of sequential solvent intercalation markedly suppressed the charge transfer from intercalants to graphene layers, reducing the barrier to exfoliation. Although our results do not perfectly describe the exfoliation system, because of incomplete consideration of the dynamic exfoliation process of GICs in solvents which happens far away from equilibrium, they do provide guidelines for selecting intercalants for graphite exfoliation. We believe that our calculation results provide insight into the exfoliation of not only graphite but also other related layered materials.

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AUTHOR INFORMATION

ASSOCIATED CONTENT

S Supporting Information *

Additional information regarding the structure, exfoliation energies, formation energies of GICs and the degree of charge transfer dependency on elemental electronegativity of inter2072

DOI: 10.1021/cm504511b Chem. Mater. 2015, 27, 2067−2073

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DOI: 10.1021/cm504511b Chem. Mater. 2015, 27, 2067−2073