Spontaneous high-concentration dispersions and liquid crystals of graphene

Journal name:
Nature Nanotechnology
Volume:
5,
Pages:
406–411
Year published:
DOI:
doi:10.1038/nnano.2010.86
Received
Accepted
Published online

Abstract

Graphene combines unique electronic properties and surprising quantum effects with outstanding thermal and mechanical properties1, 2, 3, 4. Many potential applications, including electronics and nanocomposites, require that graphene be dispersed and processed in a fluid phase5. Here, we show that graphite spontaneously exfoliates into single-layer graphene in chlorosulphonic acid, and dissolves at isotropic concentrations as high as ~2 mg ml−1, which is an order of magnitude higher than previously reported values. This occurs without the need for covalent functionalization, surfactant stabilization, or sonication, which can compromise the properties of graphene6 or reduce flake size. We also report spontaneous formation of liquid-crystalline phases at high concentrations (~20–30 mg ml−1). Transparent, conducting films are produced from these dispersions at 1,000 −1 and ~80% transparency. High-concentration solutions, both isotropic and liquid crystalline, could be particularly useful for making flexible electronics as well as multifunctional fibres.

At a glance

Figures

  1. Solubility and solvent quality of graphite dispersions.
    Figure 1: Solubility and solvent quality of graphite dispersions.

    a, Comparison of chlorosulphonic acid dispersion of graphite (25 mg ml−1 initial concentration) obtained from different sources as indicated below the vials. A dark upper portion (top) is obtained for all the sources after 12 h of centrifugation (5,000 r.p.m.), with a grey-coloured lower portion (bottom). The yellow line on the vials indicate the interface between the top and bottom phases in the three vials. The soluble portion was removed and isolated for solubility determinations. b, Comparison of acid-induced shifts in the liquid-phase Raman G-peak for graphite dispersed in the same mixtures of chlorosulphonic acid in sulphuric acid. The G-peak shift, denoted as dG, is a quantitative measure of the degree of protonation. The image in the insert shows a qualitative comparison between graphite dissolution into different solvents, showing graphite in the vials with a Teflon-coated stir bar to promote dissolution. Starting from the left, graphite is dissolved in NMP, 50, 65 and 80 vol% chlorosulphonic acid (HSO3Cl) in sulphuric acid (H2SO4) and pure chlorosulphonic acid. The dispersions were prepared at 10 mg ml−1. The acid dispersions were then centrifuged at 5,000 r.p.m. for 12 h and the NMP dispersion was centrifuged for 3 h. The amount of centrifugation time is different for the two cases because the settling time is linearly dependent on the density differential between the particles and solvent. Thus, the centrifugation time is scaled by this density difference. c, Solid-state Raman spectra of the initial graphite dry powder and the graphite quenched from the acid dispersion. The two spectra are virtually identical, indicating that protonation is reversible. Both the liquid- and solid-phase Raman spectra were taken with excitation wavelengths of 514 nm and long working distances on a ×50 lens.

  2. G[prime] band Raman spectra performed using an excitation laser wavelength of 514 nm.
    Figure 2: G′ band Raman spectra performed using an excitation laser wavelength of 514 nm.

    ac, Spectra obtained using graphoil as a starting material. The top (a) and bottom (b) phase spectra were obtained on solid samples by quenching the acid-dispersed material. The experimental curves were fitted with three Lorentzians centred at ~2,690 (G′3DA), 2,710 (G′2D) and 2,730 (G′3DA), respectively. The G′ peak of the top phase is mostly composed of the G′2D peak, and the undispersed bottom phase is mostly composed of G′3DA and G′3DB peaks, although it has a higher G′2D peak compared to the initial graphite powder (c).

  3. Evidence for single-layer dissolution.
    Figure 3: Evidence for single-layer dissolution.

    a, Low-magnification TEM showing a small flake of graphene. Note how the CNT network beneath the graphene flake is clearly visible. b, HRTEM of a single-layer edge with fast Fourier transform (FFT) insert. c, HRTEM of a few-layer edge with its FFT. Adsorbates of unknown origin are clearly visible at high magnification, similar to flakes obtained by micromechanical cleavage22. d, Electron diffraction showing the typical intensity profile along the line delimited by the two yellow arrows (e) (see also the diffraction intensity as a function of tilting angle in S-07). f, AFM image and its height profile showing a flake with a height of 0.5 nm.

  4. Graphene as a rigid platelet.
    Figure 4: Graphene as a rigid platelet.

    a,b, Cryogenic-TEM images of graphene flakes dispersed in chlorosulphonic acid. A graphene flake (a) is shown close to the TEM lacey carbon edge at very low imaging conditions (<10 electrons Å−2). After some irradiation from the electron beam of 50–80 electrons Å−2 (b), the contrast between the graphene and acid is heightened close to the graphene edges as acid is preferentially etched at these sites. No dark lines (which indicate folding points) can be observed.

  5. Evidence for the graphene liquid-crystalline phase.
    Figure 5: Evidence for the graphene liquid-crystalline phase.

    a–c, Light micrographs of a high-concentration (~2 wt%) graphene dispersion in chlorosulphonic acid (scale bar, 50 µm): transmitted light (a); transmitted polarized light with analyser and polarizer crossed at 90° (b); crossed analyser and polarizer rotated by 45° with respect to image b (c). d–f, Light micrographs of high-concentration (~2 wt%) oxidized nanoribbons dispersion in chlorosulphonic acid (scale bar, 200 µm): transmitted light (d); transmitted polarized light with analyser and polarizer crossed at 90° (e); crossed analyser and polarizer rotated by 45° with respect to image e (f).

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Author information

  1. These authors contributed equally to this work

    • Natnael Behabtu &
    • Jay R. Lomeda

Affiliations

  1. Department of Chemical and Biomolecular Engineering, Rice University, Houston, Texas 77005, USA

    • Natnael Behabtu,
    • Micah J. Green,
    • Dmitri Tsentalovich,
    • A. Nicholas G. Parra-Vasquez &
    • Matteo Pasquali
  2. Richard E. Smalley Institute for Nanoscale Science and Technology, Rice University, Houston, Texas 77005, USA

    • Jay R. Lomeda,
    • Amanda L. Higginbotham,
    • Alexander Sinitskii,
    • Dmitry V. Kosynkin,
    • James M. Tour &
    • Matteo Pasquali
  3. Department of Chemistry, Rice University, Houston, Texas 77005, USA

    • Natnael Behabtu,
    • Jay R. Lomeda,
    • Micah J. Green,
    • Amanda L. Higginbotham,
    • Alexander Sinitskii,
    • Dmitry V. Kosynkin,
    • Dmitri Tsentalovich,
    • A. Nicholas G. Parra-Vasquez,
    • James M. Tour &
    • Matteo Pasquali
  4. Department of Chemical Engineering, Technion—Israel Institute of Technology, Haifa 32000, Israel

    • Judith Schmidt,
    • Ellina Kesselman,
    • Yachin Cohen &
    • Yeshayahu Talmon
  5. Department of Mechanical Engineering and Materials Science, Rice University, Houston, Texas 77005, USA

    • James M. Tour
  6. Present address: Department of Chemical Engineering, Texas Tech University, Lubbock, Texas 79409, USA

    • Micah J. Green

Contributions

J.L. and N.B. conceived, designed and performed the experiments including dispersion and film fabrication. J.L. and A.S. performed AFM. N.B. and D.T. performed and interpreted the Raman measurements. N.B. characterized the liquid crystallinity. N.B. and A.N.G.P.V. designed the HRTEM experiments. A.S. fabricated the electronic devices. N.B., J.L. and A.S. performed electrical measurements. N.B. and A.S. performed SEM. N.B. performed STEM and electron diffraction. N.B. and A.L.H. prepared HRTEM samples, performed HRTEM experiments and interpreted the images. D.K. provided nanoribbons and graphite oxides. Y.T., Y.C., J.S., M.J.G. and E.K. performed HRTEM and cryo-TEM experiments and interpreted the images. N.B., M.J.G., A.L.H., A.S., J.L., Y.T., J.M.T. and M.P. co-wrote the paper. M.P., Y.T., Y.C. and J.M.T. supervised the project.

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The authors declare no competing financial interests.

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