Letter | Published:

Square ice in graphene nanocapillaries

Nature volume 519, pages 443445 (26 March 2015) | Download Citation

Abstract

Bulk water exists in many forms, including liquid, vapour and numerous crystalline and amorphous phases of ice, with hexagonal ice being responsible for the fascinating variety of snowflakes1,2. Much less noticeable but equally ubiquitous is water adsorbed at interfaces and confined in microscopic pores. Such low-dimensional water determines aspects of various phenomena in materials science, geology, biology, tribology and nanotechnology3,4,5,6,7,8. Theory suggests many possible phases for adsorbed and confined water9,10,11,12,13,14,15,16,17, but it has proved challenging to assess its crystal structure experimentally17,18,19,20,21,22,23. Here we report high-resolution electron microscopy imaging of water locked between two graphene sheets, an archetypal example of hydrophobic confinement. The observations show that the nanoconfined water at room temperature forms ‘square ice’—a phase having symmetry qualitatively different from the conventional tetrahedral geometry of hydrogen bonding between water molecules. Square ice has a high packing density with a lattice constant of 2.83 Å and can assemble in bilayer and trilayer crystallites. Molecular dynamics simulations indicate that square ice should be present inside hydrophobic nanochannels independently of their exact atomic nature.

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Acknowledgements

This work was supported by the DFG (Germany), the European Research Council, the EU Graphene Flagship, the National Natural Science Foundation of China, the Ministry of Science, Research and Arts of Baden-Wuerttemberg (Germany), the Office of Naval Research, the Air Force Office of Scientific Research, the Anhui Provincial Natural Science Foundation (China), the Finnish Cultural Foundation and the Fundamental Research Funds for the Central Universities of China. MD simulations were carried out at Supercomputing Center of the University of Science and Technology of China.

Author information

Affiliations

  1. Central Facility for Electron Microscopy, Group of Electron Microscopy of Materials Science, University of Ulm, 89081 Ulm, Germany

    • G. Algara-Siller
    • , O. Lehtinen
    •  & U. Kaiser
  2. Chinese Academy of Sciences Key Laboratory of Mechanical Behavior and Design of Materials, Department of Modern Mechanics, University of Science and Technology of China, Hefei, Anhui 230027, China

    • F. C. Wang
    •  & H. A. Wu
  3. School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK

    • R. R. Nair
    • , A. K. Geim
    •  & I. V. Grigorieva

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Contributions

U.K., I.V.G and A.K.G. proposed and directed the project. G.A.-S. and O.L. designed the experiments and samples, performed TEM measurements and analysed them. H.A.W. and F.C.W. carried out MD simulations (with feedback from A.K.G.). I.V.G. and A.K.G. wrote the manuscript with help from all the authors.

Competing interests

The authors declare no competing financial interests.

Corresponding authors

Correspondence to U. Kaiser or H. A. Wu or I. V. Grigorieva.

Extended data

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Videos

  1. 1.

    Dynamics of square ice

    An accelerated time sequence of 101 frames recorded over approximately 4 minutes, showing dynamics of the 2D ice. The sequence was recorded with an exposure time of 1 second per frame and acquisition rate of 1 frame per 2 seconds. The dose rate in the sequence was maintained at 1.7∙107 e s-1 nm-2. To compose the sequence, raw images have been aligned and corrected for slightly non-uniform illumination.

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DOI

https://doi.org/10.1038/nature14295

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