Bilayer ice is a recently discovered, new type of ice that can only form in very special conditions, such as between stacked 2D materials with fixed interlayer distances. The limited separation between the stack layers prevents ordinary 3D ice (for which the molecular structure is much better known) from forming.

The atomistic model developed by Danil Boukhvalov, Mikhail Katsnelson and Young-Woo Son produced a very simple result: bilayer ice forms in between stacked graphene oxide layers thanks to the fact that the lattice parameters of graphene and hexagonal ice are almost the same. “This is a very interesting finding in itself because although the binding between graphene and water is extremely weak, the hydrogen bonds in water are about an order of magnitude stronger, so the graphene provides the matrix that guides the crystallization of water into bilayer ice,” explains Boukhvalov.

In reduced graphene oxide, where the interlayer distance is just 0.6 nm, only one ice layer can be formed. However, in unreduced graphene oxide, in which the distance between graphene stacks is larger at 0.9 nm, a second layer of ice can develop, he continues. In contrast to ordinary hexagonal ice (Ih), out-of-plane hydrogen atoms in the water molecules in the second ice bilayer appear to “point” themselves in the direction of the first layer – an unusual situation that produces a structure that is rather more supple than that of the original Ih ice.

Unusual water permeation through graphene

And that is not all: the second layer of ice can also slide over the first one in a direction that follows the zigzag pattern of the graphene substrate itself. This phenomenon means that water permeates in a special way through graphene oxide layers - something that had already been observed in previous research but never explained until now, says Boukhvalov. Indeed, a team led by Andre Geim at the University of Manchester in the UK (who first isolated graphene back in 2004) found that water passes through a film of graphene oxide extremely fast (almost as though as if it were passing through empty air) while all other gases and liquids are completely blocked by the film.

The calculations also allowed the team to calculate the number of ice layers that form for specific graphene interlayer distances. “Interlayer distance in functionalized graphene can be varied quite smoothly and we can easily tune the formation of different numbers of ice layers,” adds Boukhvalov.

The results could help in the development of graphene-based filters and selective membrane separators, he told nanotechweb.org. Nitrogen-doped graphene or graphene on metal substrates could also make a good catalyst for a number of chemical reactions and the formation of ice between the graphene layers could change the material’s catalytic properties for the better, he added. Graphene with ice layers could also act as an efficient anti-corrosion surface since the layers would more easily slide over other material surfaces.

The researchers, who are reporting their work in Nano Letters, are now busy trying to better understand the effect of pressure on ice formation in graphene oxide layers and are also looking into how other polar liquids mix with carbon sheets.

Further reading

Mica pairs up with graphene (Oct 2010)
Laser beam exfoliates graphite to form chemically tunable platform (Jan 2013)