NANO LETTERS
High-Yield Organic Dispersions of Unfunctionalized Graphene
2009 Vol. 9, No. 10 3460-3462
Christopher E. Hamilton, Jay R. Lomeda, Zhengzong Sun, James M. Tour,* and Andrew R. Barron* Richard E. Smalley Institute for Nanoscale Science and Technology and Department of Chemistry, Rice UniVersity, Houston, Texas 77005 Received May 26, 2009; Revised Manuscript Received July 13, 2009
ABSTRACT We report a simple, high-yield, method of producing homogeneous dispersions of unfunctionalized and nonoxidized graphene nanosheets in ortho-dichlorobenzene (ODCB). Sonication/centrifugation of various graphite materials results in stable homogeneous dispersions. ODCB dispersions of graphene avert the need for harsh oxidation chemistry and allow for chemical functionalization of graphene materials by a range of methods. Additionally, films produced from ODCB-graphene have high conductivity.
Graphene has enjoyed recent significant attention and is now as highly investigated as the related fullerenes and carbon nanotubes.1 Graphene is a single atomic layer of sp2 graphite carbon. Few- and single-layer graphene nanosheets were first achieved by mechanical exfoliation (“scotch-tape” method) of bulk graphite2 and later by epitaxial chemical vapor deposition.3 However, these methods are impractical for large-scale manufacturing. Chemical means are therefore the most realistic approach to graphene materials.4 The primary obstacle to achieving individual or few-layer graphene is overcoming the enormous interlayer van der Waals forces. To date, chemical efforts at graphite exfoliation have been focused primarily on intercalation, chemical derivatization, thermal expansion, oxidation-reduction, the use of surfactants, or some combination of these.5-9 The most common approach to graphite exfoliation is oxidation to graphite oxide (GO) by strong oxidizing agents.10 Although partial restoration of the graphitic structure is accomplished by subsequent chemical reduction6-8,11 to chemically converted graphene (CCG), the graphitic structure (with its desired properties) is not fully restored, and significant defects are introduced. Moreover, CCG is prone to aggregation unless stabilized.6,7,10 While, the dispersion of graphene has been reported using N,N-dimethylformamide (DMF)12 and N-methylpyrrolidone (NMP),13 the concentrations attainable are low (e0.01 mg.mL-1), and these highly polar solvents are not ideal in cases where reaction chemistry requires a nonpolar medium. Further, they are hygroscopic, making their use problematic when water must be excluded from reaction mixtures. Finally, DMF is prone to thermal and chemical decomposition. It is desirable, therefore, to * To whom correspondence should be addressed. E-mail: (A.R.B.) arb@ rice.edu; (J.M.T.)
[email protected]. 10.1021/nl9016623 CCC: $40.75 Published on Web 07/31/2009
2009 American Chemical Society
develop a method for the dispersion of unfunctionalized graphene in a solvent that allows for subsequent chemical functionalization. To date, no high-yield homogeneous graphene dispersion in a nonpolar solvent has been reported. We demonstrate herein a simple, high-yield method of producing homogeneous dispersions of graphene nanosheets in ortho-dichlorobenzene (ODCB) from various graphite materials. The advantage of our method lies in its high yield, its simplicity, and its avoidance of harsh oxidation chemistry. ODCB graphene dispersions also open up a wide range of possibilities for efficient chemical derivatization of graphene where nonpolar media are required. Finally, films cast from ODCB dispersions show excellent electrical conductivity. The choice of ODCB for graphite exfoliation was based on several criteria. First, ODCB is a common reaction solvent for fullerenes and is known to form stable SWNT dispersions presumably as a result of efficient via π-π interaction.14 Second, ODCB is a convenient high-boiling aromatic and is compatible with a variety of reaction chemistries. Third, Coleman and co-workers have suggested12 that good solvents for graphite exfoliation should have surface tension values of 40-50 mJ m-2. ODCB’s surface tension is 36.6 mJ m-2.15 Finally, ODCB, being aromatic, is able to interact with graphene via π-π stacking. Graphite is readily exfoliated in ODCB with homogenization and sonication.16 Three starting materials were successfully dispersed, microcrystalline synthetic graphite, thermally expanded graphite, and highly ordered pyrolytic graphite (HOPG). As a control, attempts to disperse graphite in several other aromatic solvents (benzene, toluene, xylenes, chlorobenzene, and pyridine) were not successful.17 Therefore, ODCB appears to be uniquely capable of exfoliating graphene. After centrifugation, homogeneous dispersions are
Figure 1. TEM images of single layer graphene from HOPG dispersion showing (a) monolayer and few layer of graphene stacked with smaller flakes; (b) selected edge region from (a); (c) selected area from (b) with FFT inset; (d) HRTEM of boxed region in (c) showing lattice fringes with FFT inset.
formed that are stable for long periods of time (months) with no aggregation. Dispersions of microcrystalline synthetic graphite have a concentration of 0.03 mg mL-1. Dispersions of expanded graphite and HOPG are less concentrated (0.02 mg mL-1).18 UV-visible spectra of graphene dispersions in ODCB are flat and featureless from 400-1000 nm. ODCB dispersions obey the Beer-Lambert law; at 660 nm microcrystalline, expanded graphite, and HOPG dispersions’ absorption coefficients are 4150, 2750, and 3110 L g-1 m-1, respectively.12 Sediment recycling is possible at least twice, yielding samples equivalent to the original dispersions (within 5% of original concentration by UV-vis). HRTEM (Figure 1) shows mostly few-layer graphene (n < 5) with single layers and small flakes stacked on top. Large graphitic domains are visible; this is further supported by selected area electron diffraction (SAED) and fast Fourier transform (FFT) in selected area. The exfoliation is confirmed by tapping mode AFM of dispersions sprayed onto silicon substrates, which show extremely thin flakes with nearly all below 10 nm (Figure 2) with an average height of 7-10 nm. The thinnest are less than 1 nm, that is, graphene monolayers. Lateral dimensions of nanosheets range from 100-500 nm. Raman spectroscopy (514 nm excitation) confirms the pristine graphitic structure of films produced from ODCB dispersions via vacuum filtration (see Supporting Information). Peaks are present at 1350 cm-1 (D-band), 1582 cm-1 (G-band), 2700 cm-1 (2D-band), and 3164 cm-1 (2G-band). The relatively low D-band intensities provide evidence of nearly defect-free graphene. Thus it appears that our method of high-shear mixing followed by brief (30 m) sonication efficiently exfoliates graphite without introducing defects. XPS of expanded graphite films shows composition C (88.5%), O (9.8%), and Cl (1.7%). The presence of oxygen is expected given that thermal expansion of graphite results Nano Lett., Vol. 9, No. 10, 2009
Figure 2. AFM image with height profile from a HOPG-ODCB dispersion. Uniform shape of flakes is attributed to exfoliation of a single graphitic crystallite. Scan area 2 × 2 µm.
Scheme 1. Reactions of ODCB-Graphene, Where R ) Dodecyl and Ar ) Phenyl
in some oxidation. As expected films deposited from microcrystalline graphite and HOPG contain less oxygen (
