Derivitization of Pristine Graphene withWell-Defined Chemical FunctionalitiesLi-Hong Liu,† Michael M. Lerner,‡ and Mingdi Yan*,†

†Department of Chemistry, Portland State University, Portland, Oregon 97207-0751 and ‡Department of Chemistry,Oregon State University, Corvallis, Oregon 97331-4003

ABSTRACT Covalent functionalization of pristine graphene poses considerable challenges due to the lack of reactive functional groups.Herein, we report a simple and general method to covalently functionalize pristine graphene with well-defined chemical functionalities.It is a solution-based process where solvent-exfoliated graphene was treated with perfluorophenylazide (PFPA) by photochemical orthermal activation. Graphene with well-defined functionalities was synthesized, and the resulting materials were soluble in organicsolvents or water depending on the nature of the functional group on PFPA.

KEYWORDS Graphene, azides, covalent functionalization

Graphene, a material having a two-dimensional atomiclayer of sp2 carbon, has emerged as a nanoscalematerial with a wide range of unique properties.1-3

In order to realize the many potential applications thatgraphene can offer, the availability of graphene with a well-defined and controllable surface and interface properties isof critical importance. Despite numerous studies on theproperties and potentials of graphene, robust methods forproducing chemically functionalized graphene are stilllacking.4,5 The most common method for the covalentfunctionalization of graphene employs graphene oxide(GO),6 which is prepared by treating graphite particles withstrong acids.7 The oxidation process produces various oxygen-containing species, the nature and density of which aredifficult to control.

Covalent functionalization of pristine graphene posesconsiderable challenges due to its lack of reactive functionalgroups. Herein, we report a simple and general method forthe covalent functionalization of pristine graphene. Theapproach is based on perfluorophenylazide (PFPA),8,9 which,upon photochemical or thermal activation, is converted tothe highly reactive singlet perfluorophenylnitrene that cansubsequently undergo CdC addition reactions with the sp2

C network in graphene to form the aziridine adduct. We haveconfirmed the covalent bond formation between PFPA andgraphene using X-ray photoelectron spectroscopy.10,11 Bycontrolling the functional group on the PFPA (Scheme 1),graphene with well-defined chemical functionalities can beprepared in a single step using a simple solution-basedprocess.

PFPAs bearing alkyl (1), ethylene oxide (2), and perfluo-roalkyl groups (3) (Scheme 1) were synthesized and used inthis study (see Supporting Information for detailed synthesis

and characterization of the compounds). These functionalgroups were chosen to impact the solubility and surfaceenergy of the resulting graphene. Pristine graphene wasprepared by exfoliating graphite in o-dichlorobenzene (DCB),a procedure that has been shown to produce graphene flakesin high yield.12 Sonication of graphite in DCB followed bycentrifugation gave a well-dispersed graphene solution,which was collected and used in the subsequent reactions.These graphene flakes consisted primarily of four to fivelayers of graphene and thin graphite, as indicated by Ramanspectroscopy and atomic force microscopy.10 Covalent func-tionalization of graphene was accomplished by either ther-mal or photochemical activation. The thermal reactions werecarried out by heating the graphene flakes with PFPA 1, 2or 3 in DCB at 90 °C for 72 h. For the photochemicalreactions, the graphene solution was mixed with PFPA 1, 2,

* To whom correspondence should be addressed, yanm@pdx.edu.Received for review: 07/15/2010Published on Web: 08/06/2010

SCHEME 1. Functionalization of Pristine Graphene with PFPA

pubs.acs.org/NanoLett

© 2010 American Chemical Society 3754 DOI: 10.1021/nl1024744 | Nano Lett. 2010, 10, 3754–3756

or 3 and sonicated for 10 min, and the resulting solution wasirradiated under ambient conditions with a 450 W medium-pressure Hg lamp for 60 min. In both cases, a large excessof PFPA compound was used to ensure complete function-alization of graphene. The product was then centrifuged andwashed extensively with DCB and acetone to remove excessreagents and was dried in vacuum.

After graphene was functionalized with PFPA 1 and PFPA3, the products were soluble in DCB, and homogeneoussolutions were obtained (panels a and c of Figure 1). In thereaction of PFPA 2 with graphene, however, the mixture wasno longer homogeneous in DCB after the reaction. Theproduct precipitated from the solution and was insoluble inDCB (Figure 1b). In fact, the product, after isolation from thereaction mixture and purification, was soluble in waterinstead (II, Figure 1d). On the other hand, the productsfunctionalized with PFPA 1 and PFPA 3 were soluble in DCBand not in water (I and III, Figure 2c). These observationswere strong evidence that graphene was indeed functional-ized with the corresponding PFPA derivative. The solubilityof graphene was thus drastically altered as a result of thechemical functionalization, and the functionalized grapheneproducts dispersed well in the corresponding solvents. The

solutions were stable, and no precipitates were observedafter the solutions were set at ambient conditions for over24 h.

The successful functionalization of graphene was furtherconfirmed by FT-IR spectroscopy. Figure 2b shows the IRspectrum of the graphene functionalized with PFPA 3. Theintense absorption bands at 1340 and 1140-1200 cm-1 inPFPA 3 (marked with *, Figure 2a) are the axial CF2 stretch-ing and asymmetric CF2 stretching vibrations, respectively.13

These fingerprints, as well as the ester absorption at 1730cm-1, were observed in the graphene functionalized withPFPA 3 (Figure 2b). These peaks were absent in the pristinegraphene that had not been functionalized (Figure 2c). Takentogether, the results demonstrate that the graphene wasindeed functionalized with PFPA 3.

In conclusion, we have developed a simple, general, andpowerful method to derivatize pristine graphene using PF-PAs. The functionalization was a solution-based process,carried out by thermal or photochemical activation. Thefunctional group on the PFPA introduced well-defined chemi-cal functionalities and rendered the resulting graphenesoluble in organic solvents or in water. The method devel-oped is readily applicable to different forms of grapheneregardless of its size, shape, or configuration. The ability tocontrol the chemical functionalities, to fine-tune the solubilityand surface properties of graphene greatly enhances theprocessability of graphene-based materials. The functionalgroups can be furthermore derivatized with additional mol-ecules and materials, opening up a myriad of opportunitiesin grapheme-based materials synthesis and nanodevicefabrication.

Acknowledgment. This work was supported by OregonNanoscience and Microtechnologies Institute (ONAMI) and ONRunder Contract N00014-08-1-1237 and NIH (2R15GM066279,R01GM080295).

Supporting Information Available. General instrumenta-tion, Experimental Section, and synthesis of PFPAs. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

REFERENCES AND NOTES(1) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183–191.(2) Geim, A. K. Science 2009, 324, 1530–1534.

FIGURE 1. Reaction mixture after graphene was treated with PFPA (a) 1, (b) 2, and (c) 3 by thermal reaction. (d) Graphene functionalized withPFPA 1 (I), 2 (II), and 3 (III) in the mixed solvents of water (top layer) and DCB (bottom layer).

FIGURE 2. FT-IR spectra of (a) PFPA 3, (b) PFPA 3-functionalizedgraphene, and (c) pristine graphene.

© 2010 American Chemical Society 3755 DOI: 10.1021/nl1024744 | Nano Lett. 2010, 10, 3754-–3756

(3) Allen, M. J.; Tung, V. C.; Kaner, R. B. Chem. Rev. 2010, 110, 132–145.(4) Park, S.; Ruoff, R. S. Nat. Nanotechnol. 2009, 4, 217–224.(5) Loh, K. P.; Bao, Q. L.; Ang, P. K.; Yang, J. X. J. Mater. Chem. 2010,

20, 2277–2289.(6) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. Chem. Soc.

Rev. 2010, 39, 228–240.(7) Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339.(8) Leyva, E.; Platz, M. S.; Persy, G.; Wirz, J. J. Am. Chem. Soc. 1986,

108, 3783–3790.

(9) Yan, M. Chem.sEur. J. 2007, 13, 4138–4144.(10) Liu, L.-H.; Zorn, G.; Castner, D. G.; Solanki, R.; Lerner, M. M.; Yan,

M. J. Mater. Chem. 2010, 20, 5041–5046.(11) Liu, L.-H.; Yan, M. Nano Lett. 2009, 9, 3375–3378.(12) Hamilton, C. E.; Lomeda, J. R.; Sun, Z.; Tour, J. M.; Barron, A. R.

Nano Lett. 2009, 9, 3460–3462.(13) Frey, S.; Heister, K.; Zharnikov, M.; Grunze, M.; Tamada, K.;

Colorado, R.; Graupe, M.; Shmakova, O. E.; Lee, T. R. Isr. J. Chem.2000, 40, 81–97.

© 2010 American Chemical Society 3756 DOI: 10.1021/nl1024744 | Nano Lett. 2010, 10, 3754-–3756