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Non-oxidative intercalation and exfoliation of graphite by Brønsted acids

Nature Chemistry volume 6, pages 957–963 (2014)Cite this article

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Abstract

Graphite intercalation compounds are formed by inserting guest molecules or ions between sp2-bonded carbon layers. These compounds are interesting as synthetic metals and as precursors to graphene. For many decades it has been thought that graphite intercalation must involve host–guest charge transfer, resulting in partial oxidation, reduction or covalent modification of the graphene sheets. Here, we revisit this concept and show that graphite can be reversibly intercalated by non-oxidizing Brønsted acids (phosphoric, sulfuric, dichloroacetic and alkylsulfonic acids). The products are mixtures of graphite and first-stage intercalation compounds. X-ray photoelectron and vibrational spectra indicate that the graphene layers are not oxidized or reduced in the intercalation process. These observations are supported by density functional theory calculations, which indicate a dipolar interaction between the guest molecules and the polarizable graphene sheets. The intercalated graphites readily exfoliate in dimethylformamide to give suspensions of crystalline single- and few-layer graphene sheets.

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Figure 1: X-ray diffraction data from dried cryo-milled graphite (CMG)/acid films showing the progress of the intercalation reactions.
Figure 2: Edge-on electron microscopy images showing the expansion of graphite particles upon intercalation.
Figure 3: XPS spectra show only small changes in electron density in the sp2-bonded carbon sheets upon intercalation with acids.
Figure 4: Exfoliation of graphite is assisted by acid intercalation.
Figure 5: Most energetically favourable structures of phosphoric acid-intercalated graphite from DFT calculations, showing the direct interaction of acidic OH hydrogen atoms with the carbon sheets.
Figure 6: Differential charge density map of H3PO4-intercalated graphite and boron nitride.

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Acknowledgements

Electron and atomic force microscopy, FTIR and XPS were conducted at the Materials Characterization Laboratory of the Pennsylvania State University. The authors thank T. Clark and V. Bojan for help with TEM and XPS analysis and A. van Duin for help in developing the ReaxFF parameterization for modelling of the H3PO4/graphite system. This work is supported by US Army Research Office MURI grant W911NF-11-1-0362.

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Authors and Affiliations

  1. Department of Chemistry, The Pennsylvania State University, University Park, 16802, Pennsylvania, USA

    Nina I. Kovtyukhova & Thomas E. Mallouk

  2. Department of Physics, The Pennsylvania State University, University Park, 16802, Pennsylvania, USA

    Yuanxi Wang, Ayse Berkdemir, Mauricio Terrones, Vincent H. Crespi & Thomas E. Mallouk

  3. Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, 16802, Pennsylvania, USA

    Thomas E. Mallouk

  4. Research Center for Exotic Nanocarbons, Shinshu University, 4-17-1 Wakasato, Nagano, 380-8553, Japan

    Rodolfo Cruz-Silva

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  1. Nina I. Kovtyukhova
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Contributions

N.I.K. designed and performed the experiments. Y.W. performed computer simulations. A.B. collected Raman spectra. R.C. prepared CMG. T.E.M., V.H.C., M.T., Y.W. and N.I.K. supervised the research and contributed to the interpretation of data and writing of the paper.

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Correspondence to Thomas E. Mallouk.

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The authors declare no competing financial interests.

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Kovtyukhova, N., Wang, Y., Berkdemir, A. et al. Non-oxidative intercalation and exfoliation of graphite by Brønsted acids. Nature Chem 6, 957–963 (2014). https://doi.org/10.1038/nchem.2054

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