Graphene stacked for filters
Researchers have developed new graphene-based water filters.
When sheets of two-dimensional nanomaterials like graphene are stacked on top of each other, tiny gaps known as ‘nanochannels’ form between the sheets. These channels have a wide variety of potential uses.
A team of engineers in the US has found a way to orient nanochannels in a way that makes them extremely useful for filtering contaminants from water and other liquids.
“In the last decade, a whole field has sprung up to study these spaces that form between 2-D nanomaterials,” says Robert Hurt, a professor in Brown University’s School of Engineering and coauthor of the research.
“You can grow things in there, you can store things in there, and there’s this emerging field of nanofluidics where you’re using those channels to filter out some molecules while letting others go through.”
One problem has been that graphene stacks are thin in the vertical direction compared to their horizontal length and width.
This means the channels between the sheets are oriented horizontally too, which is not ideal for filtration, because liquid has to travel a relatively long way to get from one end of a channel to the other.
But researchers have now come up with a good way to make vertically oriented graphene nanochannels.
The method involves stacking graphene sheets on an elastic substrate, which is placed under tension to stretch it out. After the sheets are deposited, the tension on the substrate is released, which allows it to contract. When that happens, the graphene assemblage on top wrinkles into sharp peaks and valleys.
“When you start wrinkling the graphene, you’re tilting the sheets and the channels out of plane,” says Muchun Liu, who came up with the idea as part of her postdoctoral research.
“If you wrinkle it a lot, the channels end up being aligned almost vertically.”
Once the channels are nearly vertical, the assemblage is encased in epoxy, and the tops and bottoms are then trimmed away, which opens the channels all the way through the material. The researchers have dubbed the assemblages VAGMEs (vertically aligned graphene membranes).
“What we end up with is a membrane with these short and very narrow channels through which only very small molecules can pass,” Prof 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.”
Proof-of-concept testing demonstrated that water vapor could pass easily through a VAGME, while hexane — a larger organic molecule — was filtered out.
The researchers plan to continue developing the technology, with an eye toward potential industrial or household filtering applications.