Writing in a research paper for the journal Nature Communications, researchers from Brown University have demonstratde a proof-of-concept method for re-orienting the tiny gaps – called nano-channels – that form between sheets of two-dimensional nano-materials like graphene to make water filters.
“In the last decade, a whole field has sprung up to study these spaces that form between 2-D nano-materials,” said co-author Professor Robert Hurt. “You can grow things in there, you can store things in there, and there’s this emerging field of nano-fluidics where you’re using those channels to filter out some molecules while letting others go through.”
Up until now, the problem with using these nano-channels for filtration was their horizontal orientation with respect to the sheets. This makes filtration much less efficient as water, instead of traversing the relatively thin height of the stack, has to travel across its much greater length and width.
So how does one flip the orientation of the nano-channels without affecting their desirable qualities? This is where Muchun Liu, a former postdoctoral research in Hurt’s lab, comes in. Liu designed a method whereby graphene sheets are stacked on a stretched-out elastic substrate, which is then released of tension, making the stack contract. This results in nearly vertical wrinkles, or channels.
After the desired orientation of the nano-channels is achieved, the assemblage – which the researchers dubbed VAGMEs (vertically aligned graphene membranes) – is encased in epoxy, and the tops and bottoms are then trimmed away, thereby opening the channels all the way through the material.
“What we end up with is a membrane with these short and very narrow channels through which only very small molecules can pass,” Hurt said. “So, for example, water can pass through, but organic contaminants or some metal ions would be too large to go through. So you could filter those out.”
Testing performed to ascertain the VAGME’s filtration capabilities showed that water vapour can easily travel across the nano-channels, while hexane, which is a larger organic molecule, gets stuck and is thereby filtered out.
Hurt and his team plan to continue working on the new material, which they claim might be eventually used for developing advanced household filters and larger-scale industrial filtration systems.