Written by AZoNanoJan 22 2021
Stacking multiple sheets of two-dimensional nanomaterials such as graphene on top of each other leads to the formation of small gaps between the sheets that could find an extensive range of potential uses.
The channels between graphene sheets are horizontal, which is not great for applications like water filtration. But researchers from Brown University have shown a way to flip those channels to make them vertical in relation to the sheets, which is an ideal filtration orientation. Image Credit: Hurt lab/Brown University.In a study published in the Nature Communications journal, a research team from Brown University describes the discovery of a new method to direct those gaps, known as nanochannels, such that they are more beneficial for filtering nanoscale 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. 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, Study Co-Author and Professor, School of Engineering, Brown University
Related StoriesUsing Graphene Based Solar Cells for Solar ApplicationsThe Current and Future Production of GrapheneA Guide to GrapheneHowever, the use of these nanochannels for filtration poses a problem, which is related to the way those channels are directed. Similar to a notebook created from piled sheets of paper, graphene stacks are thin in the vertical direction than their horizontal width and length.
This implies that the channels present between the sheets are similarly oriented horizontally. That is not optimal for filtration, since the liquid must travel comparatively too far to get from one end of a channel to another.
It would be ideal if the channels were in the direction perpendicular to the orientation of the sheets. If that is the case, then liquid would only require to traverse the relatively thin vertical height of the stack instead of the much longer width and length.
According to Hurt, to date, an ideal method to make vertically oriented graphene nanochannels has not been developed yet. That was until Muchun Liu, who is a former postdoctoral researcher in Hurt’s laboratory, discovered a novel method to achieve it.
In Liu’s technique, graphene sheets are stacked on an elastic substrate, which is positioned under tension to drag it out. As soon as the sheets are deposited, the tension on the substrate is liberated, which enables it to shrink. When that process takes place, the graphene assemblage on top wrinkles into sharp valleys and peaks.
When you start wrinkling the graphene, you’re tilting the sheets and the channels out of plane. If you wrinkle it a lot, the channels end up being aligned almost vertically.
Muchun Liu, Researcher, Massachusetts Institute of Technology
As soon as the channels become almost vertical, the assemblage is wrapped in epoxy, and the tops and bottoms are further trimmed away, which unlocks the channels via the material. The team has named the assemblages VAGMEs—vertically lined up 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. 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.
Robert Hurt, Study Co-Author and Professor, School of Engineering, Brown University
A proof-of-principle investigation showed that water vapor could pass easily via VAGME, while hexane, which is a huge organic molecule, was filtered out. The team intends to continue developing the technology, with the aim of possible industrial or household filtering applications.
The study was financially supported by the National Institute of Environmental Health Sciences Superfund Research Program (P42 ES013660).
Journal Reference
Liu, M., et al. (2021) Controlling nanochannel orientation and dimensions in graphene-based nanofluidic membranes. Nature Communications. doi.org/10.1038/s41467-020-20837-2.
Source: https://www.brown.edu/
