Graphene sandwich squares away ice
Sandwiching water between two sheets of the wonder-material graphene causes it to freeze at room temperature in the form of 2D square crystals of ice, rather than its normal hexagonal crystal lattice, according to a team of researchers from the UK, Germany and China. This phase of ice could exist within some other nanostructures, such as carbon nanotubes, and could help explain why water moves unusually in these materials – a result that could have implications for developing more efficient filtration, desalination and distillation technologies.
Water exists in many forms, including liquid, vapour and as many as 15 crystal structures of ice. The hexagonal structure is responsible for the shape of snowflakes and the intricate patterns that form on surfaces when they freeze. "Less noticeable, but equally ubiquitous, is the water found at interfaces and confined in microscopic pores," says Irina Grigorieva of the University of Manchester in the UK. "In fact, monolayers of water cover every surface around us, even in the driest deserts, and fill in every single microscopic crack on Earth – for example those present in rocks. However, we know very little about the structure and behaviour of such microscopic water – especially when it is hidden from view in capillaries deep inside a material."
The newly discovered ice films, which are less than 1 nm thick, have a completely different symmetry to that of normal ice – which has a hexagonal structure. "The new phase of ice forms at room temperature, well above the 'normal' freezing temperature of water," explain team leaders Grigorieva and Andre Geim, who is also based at Manchester and is one half of the duo that won the Nobel prize for the discovery of graphene in 2010. "Apart from finding this new phase – not something that happens every day – our result will allow us to better understand the counterintuitive behaviour of water inside nanochannels, such as ultrafast permeation though graphene-oxide membranes."
The researchers employed high-magnification electron microscopy to look at the atomic structure of water trapped inside a transparent nanoscale capillary made from two sheets of graphene. Graphene is a layer of carbon atoms just one atom thick, and so does not obscure electron-microscopy imaging. "Thanks to strong adhesion between the graphene sheets that form the nanocapillary, pressures inside it can reach as high as 1 GPa, which appears to be an important factor in making water crystallize into ice," explains Grigorieva.
"To our surprise, we found small square crystals of ice at room temperature, provided the graphene capillaries were narrow enough – that is, those that allow for no more than three molecular layers of water," she says. "The water molecules form a square lattice, sitting along evenly spaced neat rows running perpendicular to each other." Such a flat square arrangement is completely uncharacteristic for bulk ice, in which the molecules always form small pyramidal structures, she adds.
Alan Soper of the UK's Rutherford Appleton Laboratory in Harwell, Oxford, agrees. "Water trapped between graphene sheets under these conditions is likely to crystallize, even at room temperature," he writes in a Nature News & Views article. "But the fact that it forms a square structure is unexpected."
The team says that it also attempted to find out how common square ice might actually be in nature using computer simulations. "Our results shows that if the layer of water is thin enough, it should form square ice independently of the exact chemical make-up of the confining nanopore walls," says Grigorieva. "It is thus likely that square ice is very common on the molecular scale and present at the tip of every microscopic crack or pore in all materials."
The new observations might be important for understanding how water moves through natural and man-made nanoscale channels. These include aquaporin (a widely occurring channel protein that regulates water flow across cell membranes) and carbon nanotubes.