Two-dimensional Na–Cl crystals of unconventional stoichiometries on graphene surface from dilute solution at ambient conditions

Abstract

NaCl in a 1:1 stoichiometry is the only known stable form of the Na–Cl crystal under ambient conditions, and non-1:1 Na–Cl species can only form under extreme conditions, such as high pressures. Here we report the direct observation, under ambient conditions, of Na2Cl and Na3Cl as two-dimensional (2D) Na–Cl crystals, together with regular NaCl, on reduced graphene oxide membranes and on the surfaces of natural graphite powders from salt solutions far below the saturated concentration. Molecular dynamics and density functional theory calculations suggest that this unconventional crystallization process originates from the cation–π interaction between the ions and the π-conjugated system in the graphitic surface, which promotes the ion–surface adsorption. The strong Na+π interaction and charge transfer lead to stoichiometries with an excess of Na+. With unique electron and spin distributions and bonding, the resulting 2D crystals may have unusual electronic, magnetic, optical and mechanical properties.

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Fig. 1: 2D Na–Cl crystals from salt solutions far below the saturated concentration.
Fig. 2: XRD patterns of rGO membranes immersed in solutions at different concentrations.
Fig. 3: Na+ and Cl adsorbed on a graphene sheet and between two graphene sheets.
Fig. 4: XRD pattern of K–Cl crystals obtained from rGO membranes immersed in unsaturated (3.0 M) KCl solution together with four control systems.

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Acknowledgements

We thank P. Ball, Y. Tu, R. Wan and Y. Gao for their constructive suggestions. Support from the National Science Fund for Outstanding Young Scholars (no. 11722548), the National Natural Science Foundation of China (no. 11574339, 41430644, 11404361 and 21671131), the Scientific Research and Developed Fund of Zhejiang A & F University (no. 2017FR032), the Key Research Program of the Chinese Academy of Sciences (no. KJZD-EW-M03), the Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (no. QYZDJ-SSW-SLH053), the Deepcomp7000 and ScGrid of the Supercomputing Center, Computer Network Information Center of Chinese Academy of Sciences, the Special Program for Applied Research on SuperComputation of the NSFC-Guangdong Joint Fund (the second phase) and the Supercomputer Center of CAS is acknowledged.

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H.-P.F. had the idea to observe Na–Cl crystals in the rGO membranes in salt solutions of concentrations below the saturated concentration based on the ion–π interactions. H.-P.F., M.-H.W., G.-S.S. and L.Y. conceived and designed the experiments and simulations. L.C., G.-S.S., D.-Y.L. and Z.Q. performed the experiments. G.-S.S. and Y.-Z.Y. performed the simulations. G.-S.S., L.C., H.-P.F., S.-S.L. and L.-H.L. analysed the data, G.-S.S., L.C., H.-P.F. and M.-H.W. co-wrote the paper. All the authors discussed the results and commented on the manuscript.

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Correspondence to Minghong Wu or Haiping Fang.

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Shi, G., Chen, L., Yang, Y. et al. Two-dimensional Na–Cl crystals of unconventional stoichiometries on graphene surface from dilute solution at ambient conditions. Nature Chem 10, 776–779 (2018). https://doi.org/10.1038/s41557-018-0061-4

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