Researchers develop new graphene nanochannel water filters
Brown University
In research published in the journal Nature Communications, a team of Brown University researchers has found a way to orient those gaps, called nanochannels, in a way that makes them more useful for filtering water and other liquids of nanoscale contaminants.
“In the last decade, a whole field has sprung up to study these spaces that form between 2-D nanomaterials,” said Robert Hurt, a professor in Brown’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.”
There’s a problem, however, with using these nanochannels for filtration, and it has to do with the way those channels are oriented.
Like a notebook made from stacked sheets of paper, graphene stacks are thin in the vertical direction compared to their horizontal length and width. That means that the channels between the sheets are likewise oriented horizontally.
That’s not ideal for filtration,
because liquid has to travel a relatively long way to get from one end of a
channel to the other. It would be better if the channels were perpendicular to
the orientation of the sheets. In that case, liquid would only need to traverse
the relatively thin vertical height of the stack rather than the much longer
length and width.
But
until now, Hurt says, no one had come up with a good way to make vertically
oriented graphene nanochannels. That is until Muchun Liu, a former postdoctoral
researcher in Hurt’s lab, figured out a novel way to do it.
Liu’s
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,” said Liu, who is now a researcher at Massachusetts Institute of
Technology. “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,” 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.
The
research was supported by the National Institute of Environmental Health
Sciences Superfund Research Program (P42 ES013660).
