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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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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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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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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