New type of 'ice' discovered in graphene interlayers

Scientists have discovered a new type of ice that forms between the layers of graphene oxide.
Researchers at the Korea Institute for Advanced Study and Radboud University Nijmegen in the Netherlands used atomistic models to obtain a novel type of 'bilayer' ice.
The findings could help in the development of graphene-based filters and separators, better graphene catalysts and even anti-corrosion surfaces based on the carbon material.
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, website 'nanotechweb.Org' reported.

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 owing 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 crystallisation of water into bilayer ice," said 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 said.

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.
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, said Boukhvalov.

The study was published in journal Nano Letters.