New research indicates that a novel method of fabricating graphene stacks could greatly benefit water filtration.
The spaces that form when sheets of nanomaterials like graphene — layers of single atoms — are stacked atop each other have been utilized by scientists in a wide range of applications. One of the potential uses for such gaps — more formally known as nanochannels — is in the filtration of water and other liquids, removing nanoscale contaminants.
In the last decade, a whole field has sprung up to study these spaces that form between 2-D nanomaterials. 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.
Robert Hurt, Professor, School of Engineering, Brown University
Stacked graphene-oxide (GO) films have been suggested in the filtration of water due to their highly selective nature and molecular ‘sieving’ that arises from their interlayer spacings. Another benefit of such materials is their relativity cheapness, which arises from the abundance of graphene and the fact that they can easily be mass-manufactured.
Hurt was part of a team from Brown University that set out to discover if they could orientate nanochannels to make these film structures more efficient in the filtration process. The team’s findings are discussed in a new paper published in the journal Nature Communications.
One of the most important aspects of the researchers’ work was tackling a long-standing problem that has impeded the use of nanomaterials in water filtration.
Pointing Nanochannels in the Right Direction
The issue with the use of nanochannels for water filtration concerns the direction in which those gaps are orientated. To consider this, imagine stacks of graphene sheets taking shape just like sheets of paper in a notebook.
Like a notebook, the stacks are wide horizontally but thin vertically. Consequentially, the channels that run between the sheets are also orientated horizontally. That presents a problem for filtration.
This horizontal orientation means that the liquid that is being filtered has to traverse quite a distance to travel from one end of the channels to the other. This wouldn’t be the case if the channel could be turned perpendicular to the orientation of the graphene sheets.
With a vertical orientation, the liquid would only have to travel the meager thickness of the stack to move through the whole channel.
Whilst this may sound like a relatively simple alteration to make, as Hurt points out, thus far materials scientists have failed to come up with an efficient way to make stacks with vertically orientated nanochannels. That was, until a postdoctoral researcher in his lab, Munchun Liu — now a researcher at the Massachusetts Institute of Technology (MIT) — hit on a new way to do just that.
A New Tilt on Nanochannels
The solution devised by Liu to the nanochannel orientation issue involves introducing a new element to the fabrication of nanomaterial stacks. Liu realized that the graphene sheets could be stacked on an elastic substrate.
The team applied tension to this substrate, stretching it and then stacked the graphene sheets atop it. When the tension is released, it causes the substrate to relax and return to its original shape. This causes the graphene sheets to wrinkle, forming sharp peaks and valleys.
“When you start wrinkling the graphene, you’re tilting the sheets and the channels out of the plane,” says Liu. “If you wrinkle it a lot, the channels end up being aligned almost vertically.”
This process creates channels that are vertical, but an extra step is required to make them useable. The whole structure is captured by epoxy embedding resulting in mechanically robust microscale membranes, where molecular transport is only allowed in the vertical direction. The ends are trimmed away and the structure is thinly sectioned to ensure that the nanochannels are uniform and run through the entire structure.
The team tested the structure they created — which they have dubbed vertically aligned graphene membranes or VAGMEs — by passing water vapor through it. They then tested the VAGME with a larger organic molecule — hexane, which comprises six carbon atoms — finding this molecule blocked by the structure, meaning that in a real-world application, it would have been filtered out.
“What we end up with is a membrane with these short and very narrow channels through which only very small molecules can pass,” says Hurt. “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.”
The team will now further develop VAGMEs with the aim of tailoring them for industrial and household applications.
¹ Liu. M., Weston. P. J., Hurt. R. H., , ‘Controlling nanochannel orientation and dimensions in graphene-based nanofluidic membranes,’ Nature Communications, [https://doi.org/10.1038/s41467-020-20837-2]
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