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Electrochemical molecular intercalation and exfoliation of solution-processable two-dimensional crystals

Nature Protocols volume 18, pages 2814–2837 (2023)Cite this article

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Abstract

Electrochemical molecular intercalation of layered semiconducting crystals with organic cations followed by ultrasonic exfoliation has proven to be an effective approach to producing a rich family of organic/inorganic hybrid superlattices and high-quality, solution-processable 2D semiconductors. A traditional method for exfoliating 2D crystals relies on the intercalation of inorganic alkali metal cations. The organic cations (e.g., alkyl chain–substituted quaternary ammonium cations) are much larger than their inorganic counterparts, and the bulky molecular structure endows distinct intercalation and exfoliation chemistry, as well as molecular tunability. By using this protocol, many layered 2D crystals (including graphene, black phosphorus and versatile metal chalcogenides) can be electrochemically intercalated with organic quaternary alkylammonium cations. Subsequent solution-phase exfoliation of the intercalated compounds is realized by regular bath sonication for a short period (5–30 min) to produce free-standing, thin 2D nanosheets. It is also possible to graft additional ligands on the nanosheet surface. The thickness of the exfoliated nanosheets can be measured by using atomic force microscopy and Raman spectroscopy. Modifying the chemical structure and geometrical configuration of alkylammonium cations results in different exfoliation behavior and a family of versatile organic/inorganic hybrid superlattices with tunable physical/chemical properties. The whole protocol takes ~6 h for the successful production of stable, ultrathin 2D nanosheet dispersion in solution and another 11 h for depositing thin films and transferring them onto an arbitrary surface. This protocol does not require expertise beyond basic electrochemistry knowledge and conventional colloidal nanocrystal synthesis and processing.

Key points

  • A layered semiconductor crystal that undergoes electrochemical intercalation with alkyl chain–substituted quaternary ammonium cations can be exfoliated into free-standing 2D nanosheets by sonication. The resulting 2D nanosheets in solution are spin-coated to form thin films on versatile substrates for electronic applications.

  • The main advantage is the preservation of semiconducting characteristics (e.g., MoS2) in the exfoliated nanosheets and assembled thin films.

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Fig. 1: Solution-based chemical methods for the synthesis of 2D crystal in liquid.
Fig. 2: Schematic illustration of the electrochemical molecular intercalation and exfoliation process.
Fig. 3: Step-by-step illustration of the electrochemical molecular intercalation and exfoliation process.
Fig. 4: Examples of current profile recorded during electrochemical molecular intercalation.
Fig. 5: Photograph of the bulk crystals before and after electrochemical molecular intercalation.
Fig. 6: The nanosheet dispersion in solution.
Fig. 7: AFM analysis of the exfoliated 2D nanosheets.
Fig. 8: TEM images of various 2D nanosheets.
Fig. 9: Raman and photoluminescence spectra of the exfoliated MoS2 nanosheets.
Fig. 10: XPS spectra of MoS2 nanosheets after various post-treatments.
Fig. 11: XRD patterns of a MoS2 nanosheet film after various post-treatments.
Fig. 12: XRD patterns of various exfoliated 2D nanosheets.

Data availability

The main data supporting the findings of this study were previously published27,44. Additional data are available from the corresponding authors upon reasonable request.

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Acknowledgements

Z.L. acknowledges the start-up grant from Tsinghua University and the National Natural Science Foundation of China (NSFC, grant no. 22275113). Q.H. acknowledges support from Early Career Scheme Project 21302821 and General Research Fund Project 11314322 from the University Grants Committee of Hong Kong.

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Authors and Affiliations

  1. Department of Chemistry, Engineering Research Center of Advanced Rare Earth Materials (Ministry of Education), Tsinghua University, Beijing, China

    Shengqi Wang, Junying Xue, Jing He, Yongping Dai, Tingyi Xia & Zhaoyang Lin

  2. Department of Materials Science and Engineering, University of California Los Angeles, Los Angeles, CA, USA

    Dong Xu & Yu Huang

  3. Department of Materials Science and Engineering, City University of Hong Kong, Kowloon, Hong Kong, China

    Qiyuan He

  4. Department of Chemistry and Biochemistry, University of California Los Angeles, Los Angeles, CA, USA

    Xiangfeng Duan

  5. California NanoSystems Institute, University of California Los Angeles, Los Angeles, CA, USA

    Xiangfeng Duan

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Contributions

Z.L. and X.D. designed the experiments. Z.L. developed the protocol and performed the experiments. S.W., J.X., J.H., Y.D., T.X. and Q.H. performed the material synthesis and film deposition. D.X. and Y.H. carried out the free-standing film transfer experiment. All authors wrote and reviewed the manuscript.

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Correspondence to Qiyuan He, Xiangfeng Duan or Zhaoyang Lin.

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Nature Protocols thanks Martin Pumera, Guoxiu Wang and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Key references using this protocol

Li, L. et al. Adv. Mater. 34, e2207392 (2022): https://doi.org/10.1002/adma.202207392

Yan, Z. et al. Science 375, 852–859 (2022): https://doi.org/10.1126/science.abl8941

Piatti, E. et al. Nat. Electron. 4, 893–905 (2021): https://doi.org/10.1038/s41928-021-00684-9

Key data used in this protocol

Lin, Z. et al. Chem 7, 1887–1902 (2021): https://doi.org/10.1016/j.chempr.2021.03.022

Lin, Z. et al. Nature 562, 254–258 (2018): https://doi.org/10.1038/s41586-018-0574-4

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Wang, S., Xue, J., Xu, D. et al. Electrochemical molecular intercalation and exfoliation of solution-processable two-dimensional crystals. Nat Protoc 18, 2814–2837 (2023). https://doi.org/10.1038/s41596-023-00865-0

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