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Unzipping of Graphene by Fluorination M. Wu,† J. S. Tse,*,†,‡ and J. Z. Jiang*,† †
International Center for New-Structured Materials (ICNSM), Zhejiang University, and Laboratory of New-Structured Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou, P.R. China, and ‡Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan, Canada, S7N 5E2
ABSTRACT Ab intio static and dynamic calculations show that successive synadditions of F2 to the double bonds of graphene resulted in the rehyrbrization of fluorinated carbons. This in combination with steric repulsions between F atoms led to a distortion of the local geometry and severe buckling of the graphene. The weakened C-C bonds can then be broken by pyrolysis. The results demonstrate that fluorination can be a promising chemical route for cutting of graphene into smaller fragments under controlled experimental conditions. SECTION Dynamics, Clusters, Excited States
S
unzipping mechanism, in this letter the results of the study of fluorination-induced unzipping of graphene with first-principle calculations are disccussed. It is demonstrated that sp2 to sp3 rehybrization of the carbon together with large steric repulsions between F atoms after the addition of F2 resulted in (spontaneous) breaking of carbon double bonds. The theoretical results suggested that fluorination requires milder conditions for unzipping of graphene and may be a more efficient alternative to the oxidation process. The starting model on the study of fluorination is a hypothetical graphene sheet modeled by a planar fused C54H18. This model has been used in many previous theoretical studies. Density functional theory (DFT) calculations with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional19 were performed with the SIESTA program20 employing localized double-ζ plus polarization (DZP) basis sets.21 The predicted calculated structures and energies on some configurations were checked against the results obtained from the molecular program GAMESS22 using similar quality basis sets and the PBE functional. Consistent results were always obtained with both approaches, and the absolute agreements are very good. For example, the calculated heat of reaction for F2 þ C2H4 f H2FC-CFH2 of 5.89 eV is comparable to the estimated value of 5.69 eV. Fluorine molecules (F2) were added successively to selected sites onto the surface of the model graphene. Additions to the top and bottom of the graphene sheet were also studied. It is found that the energetics of the latter reaction (i.e., F at both the top and bottom) is often higher energy than addition on a single side (syn-addtion), and these results will not be discussed further (see Supporting Information). The salient results from the study of F2 to the model graphene sheet are summarized as follows: (i) Initial fluorination was found to
ince its discovery in 2004, graphene, a new member of carbon based materials,1 has been the focus of numerous theoretical2 and experimental3 studies due to its novel electronic properties. There are challenging problems in the fabrication and manipulation of this material. For example, extensive efforts have been made4-8 to improve the fabrication of high quality film with large surface area. The ability to manipulate graphene into different physical forms is also critical since graphene in nanoribbons with different edge states,9-11 or as nanoflakes,12 or in the form of carbon atomic chains13 has been shown to exhibit very different electronic characteristics. The development of reliable and efficient techniques to produce various forms of graphene is essential to exploit practical applications of this versatile material. So far, the primary way to manipulate graphene is lithography. This method, however, is limited to small (micrometer) scale fabrication. To fully realize the unique properties of graphene, methods for manipulation at the macroscopic scale need to be developed. Oxidation has been shown to be a method for the unzipping of carbon nanotubes14,15 and graphene.16 Under well-controlled conditions, graphene or carbon nanotubes can be cut in the transverse direction,15 yielding shorter fragments, or unzipped in the longitudinal direction, forming nanoribbons.14 There has been comparatively little effort to explore other chemical approaches for the manipulation of the nanostructure in graphene. Another possibility is through fluorination. It has been shown that a large amount of fluorine can be covalently attached to carbon nanotubes.17 Subsequent heating in an argon atmosphere at 1000 °C led to the “cutting” of nanotubes.18 In view of the similarity of the surface morphology between single-wall carbon nanotubes and graphene, fluorination may also be an effective method for the unzipping of graphene. From thermochemical data, the exothermic heat of reaction for the fluorination of a CdC double bond is 5.69 eV. This favorable energetics will help to drive the kinetics of the reaction. To explore this possibility and to understand the
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Received Date: March 15, 2010 Accepted Date: April 7, 2010 Published on Web Date: April 13, 2010
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start from the edge sites. (ii) Addition of subsequent F2 always favors the CdC bond adjacent to the proceeded pair. (iii) The energetics of F2 addition to graphene is always exothermic. In the graphene sheet model, the successive addition of eight or more F2 molecules, depending on the choice of CdC sites, led to the formation of zigzag chains or parallel rows of F atoms across the surface (Figure 1). The formation of C-F bonds destroys the CdC π bond and changes the hybridization of the carbon from planar sp2 to tetrahedral sp3, thus elongating the C-C bond and distorting the local geometry. Furthermore, to reduce the repulsive interaction between the two adjacent F atoms, the eclipsed configuration of the two syn-C-F bonds causes them to rotated away from each other and results in the twisting of the C-C bond. The net effect is a severe
distortion of the local structure around the fluorinated carbon atom. This effect becomes more significant when more F2 moelcules are added. Consequently, the graphene sheet is buckled severely. For example, as shown in Figure 1e, when F atoms were added along the meridian of the graphene sheet in parallel rows, it bent from planarity to almost 88°. The twisting and buckling, however, did not lead to the breaking of C-C bonds. The longest C-C distance of 1.60 Å found among all the configurations studied here is not much longer than a normal F-substituted C-C single bond of 1.58 Å. No cracking of the graphene sheet was observed even after BornOppenheimer ab initio molecular dynamics (BOMD)23 calculations performed at 1500 K on optimized structures of all the models. The preliminary survey of the reactions of F2 on the model graphene sheet, however, offers a clue to a possible process of the breaking of CdC bonds of graphene. In the model graphene, the limited length scale provides the flexibility for the bonds to bend easily under the strain of fluorination. A relevant question is what would happen for a system if this degree of freedom is restricted? This scenario can be mimicked by an infinite graphene ribbon model imposed with the periodic boundary conditions (see Supporting Information). Graphene ribbons in the armchair (AGR) and zigzag (ZGR) conformation were studied. An armchair model with H-terminated edges and a length of 17.123 Å was constructed from a repetitive C72H16 model. Similarly, a zigzag model with a length of 14.829 Å was constructed from C72H12. It is known24 that the armchair-like edge is more energetic than the zigzag-like edge. This is confirmed from test calculations by putting a pair of F atoms on the edge of the ribbon models. It is found that the binding energy for F adsorbed on the edge of AGR is indeed lower than that on ZGR. In the ensuing discussion, only results on the AGR model are discussed. Two configurations, 2R and S2R shown in Figure 2a,b, for the addition F2 to the ribbon were considered. In the 2R model, F2 addition initiated at the CdC bond with both C's terminated by an H atom on the edge of the ribbon. The addition of F2 in the S2R model started from the
Figure 1. Optimized structures for F2 addition to a graphene sheet. The top and side views are shown for each configuration. The red balls represent the carbon atoms, which have longest bond lengths, while the blue balls stand for the special case in which the C-C bond is broken.
Figure 2. Two configurations (2R and S2R) with F2 addition to the AGR (see text). Plots of optimized structures at different perspectives. Top and side views of the optimized fully fluorinated structures for 2R and S2R. Directions of the views are shown in the figures. The red balls represent the carbon atoms of the longest C-C bonds in the configurations.
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Figure 3. (a) Plot of the longest C-C bond length (dmax) against the number of adsorbed F atoms onto the 2R and S2R models. The solid lines are guides for the eyes. (b) Heat of reaction for the successive addition of molecular fluorine to the 2R and S2R model.
carbon nanotubes into shorter fragments after fluorination at a temperature of 1000 °C.18 It is noteworthy that no F was found in the “cut” fragments after thermal annealing in argon.18 The energetics for fluorination is favorable to successive addition of F2 to the graphene. The heats of reaction for the addition reactions, F2 þ (n - 1)F2-ANR (2R or S2R) f (nF2)-ANR (2R or S2R), are summarized in Figure 3b. Several important points should be noticed. Regardless of the model structure, all successive F2 heats of adsorption are negative. This implies that the product is always energetically more stable than the reactants. The initial heat of the addition reaction of 4.0 eV is slightly smaller than the 5.89 eV calculated for the addition of F2 to C2H4. Moreover, depending on the addition sites, even for the heat of reaction change with the number of F2 molecules added, the averaged values of 3.88 and 3.09 eV for 2R and S2R, respectively, are not significantly different from the initial value of 4.0 eV. Therefore, even though there is increasing repulsion upon F2 addition to the CdC double bonds, the repulsive energy is not large enough to inhibit the reaction. The mechanism of oxygen-driven unzipping of graphitic materials has been studied with first-principles calculations.16 It is surmised that the necessary condition for the cracking is the insertion of oxygen atoms (radicals) into CdC double bonds on the opposite ends of the hexagonal ring, forming two epoxides. Cleaving of the C-C linkage is facilitated by the formation of C-O-C bonds. In comparison, the unzipping of graphene using fluorine follows a different mechanism. The breaking of CdC bonds arises from severe distortion of the local geometry as a result of the change in hybridization of the carbon and the avoidance of repulsion between F atoms. However, in both cases, the rate-determining steps are (i) attachment of O or F to the double bonds and (ii) the cleavage of CdC bonds. The initial reaction for oxidation requires the first generation of O radicals and subsequent insertion to the CdC bonds strategically located in the graphene hexagonal rings. In comparison, the syn-addition of F2 to a double bond (C2H4) is shown to be a two-step process.25,26 The lowest energy pathway first step is through the formation of an intermediate diradical followed by the dissociation of the F-F bond to produce CH2CH2F• and F•. The first reaction is exothermic, and the energy is more than sufficient to overcome the activation barrier required for the breaking of the F-F bond.26 Therefore, fluorination and the eventual unzipping of graphene is
elongated CdC bond at the edge with only one C terminated by an H atom. In essence, the predicted trend of the addition of F2 to the nanoribbon follows that of the graphene sheet. As shown above, the most important parameter leading to the cracking of graphene is twisting of the C-C bond. For this purpose, the longest C-C distance in the infinite graphene model as a function of the number of adsorbed F2 molecules is reported in Figure 3. It is shown that the C-C distance increases from the nominal value single bond of 1.54 Å to about 1.70-1.72 Å upon the addition of eight or more F2 molecules. The very long C-C distance indicates that the bond is significantly weakened and may have been broken. This assertion is confirmed by unrestricted (spin polarized) calculations that, at this C-C separation, the energy of the spin unpaired triplet state (spin unpaired electrons delocalized around the C atoms of the broken bond) is more stable by ca. 0.3 to 0.4 eV, and there is no more covalent bonding between the C atoms. In addition, ab initio molecular dynamics (MD) calculations were performed on the 2R and S2R configurations at 1500 K. It was found that the temporal separation between the long C 3 3 3 C distances became even larger, extending to a maximum of 1.92 Å. Examination of the atom trajectories reveals that the C-C bond has been broken. Results from static geometry optimization and dynamic MD calculations show that the fluorination starts from the edge of the graphene, and successive fluorination forms a zigzag row of distorted C sites. The cracking of graphene is initiated at the most strained part of the fluorinated chains at the middle of the sheet and then propagates to the edges. Eventually, upon heating, the graphene unzipped spontaneously. The breaking of the C-C bonds is the consequence of the highly strained environment at the carbons due to the rehybrization and steric repulsion of the F atoms. In a restricted periodic system, the graphene cannot buckle freely to relieve the strain, thus the only alternative is to break the C-C bonds. Figure 2 shows the distortion of the 2R and S2R graphene ribbons after full fluorination in different perspectives. It is evident that the ribbons buckle upward along the direction of the C-F bonds. When compared to Figure 1, the bending of the ribbon is not as severe as in the graphene sheet. The theoretical results show that a continuous band of F across the graphene can be formed when reacted with F2, and subsequent heating will lead to unzipping. This suggestion is consistent with experimental observation on the fluorination of carbon nanotubes17 and the “cutting” of single-walled
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expected to be a facile reaction. Reactions of graphene or carbon nanotubes with molecular fluorine to fabricate smaller fragments may be a competitive or even more efficient process than oxidation. 16
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SUPPORTING INFORMATION AVAILABLE Results that
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were mentioned in the text but not described in detail is presented. This material is available free of charge via the Internet at http://pubs. acs.org.
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AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: john.
[email protected];
[email protected].
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ACKNOWLEDGMENT Financial support from the National Natural
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Science Foundation of China (Grant Nos. 60776014, 60876002, and 10804096), Zhejiang University-Helmholtz cooperation fund, the Ministry of Education of China (Program for Changjiang Scholars), the Department of Science and Technology of Zhejiang Province and Zhejiang University is gratefully acknowledged.
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