Layers of water adsorbed on a mica surface have been trapped under a graphene sheet and their structure determined by atomic force microscopy.
When surfaces come in contact with water, their wetting process is often determined by the initial steps of water adsorption. But although the formation of ice at surfaces has been observed at low temperatures, the weak, dynamic nature of water–surface interactions has prevented similar studies under ambient conditions. James Heath and co-workers from the California Institute of Technology have now circumvented1 this issue by using graphene as a coating material, and determining the structure of water on a mica surface with an atomic force microscope (AFM).
The researchers deposited a graphene monolayer on mica under a humid atmosphere. The sheet uniformly coats the wet surface and protects it from direct interactions with the probe tip of the AFM, thus enabling the elucidation of the water patterns formed on mica. At a relative humidity of 40%, large, flat, faceted islands are observed. These correspond to the formation of the first adsorbed layer of water, trapped under the graphene sheet.
Increasing the relative humidity revealed that the adsorption occurs one layer at a time. The first two layers resembled ice, and formed through islands that appeared preferentially near defects of the mica surface — suggesting that these defects play the role of nucleation centres. Subsequent layers adopted a liquid-like structure. In addition to the insight gained on water structure, the work shows that graphene may serve as a coating material for applications in surface science.
References
Xu, K., Cao, P. & Heath, J. R. Graphene visualizes the first water adlayers on mica at ambient conditions. Science 329, 1188–1191 (2010).
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Pichon, A. Under a graphene cover. Nature Chem (2010). https://doi.org/10.1038/nchem.869
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DOI: https://doi.org/10.1038/nchem.869