New graphene nanochannel water filters

Date:
January 21, 2021
Source:
Brown University
Summary:
Researchers have shown that tiny channels between graphene sheets
can be aligned in a way that makes them ideal for water filtration.



FULL STORY ==========================================================================
When sheets of two-dimensional nanomaterials like graphene are stacked
on top of each other, tiny gaps form between the sheets that have a wide variety of potential uses. 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).


========================================================================== Story Source: Materials provided by Brown_University. Note: Content may
be edited for style and length.


========================================================================== Journal Reference:
1. Muchun Liu, Paula J. Weston, Robert H. Hurt. Controlling nanochannel
orientation and dimensions in graphene-based nanofluidic membranes.

Nature Communications, 2021; 12 (1) DOI: 10.1038/s41467-020-20837-2 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/01/210121132409.htm

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