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Scientists may have found the keystone use for graphene oxide

by Jessica Hall

This looks like a graphene story at first glance, but it isn’t.

Graphene has two major problems: it’s tough to make, and we don’t really know what its keystone use is. High-quality graphene is made by vapor deposition, except that that’s extremely expensive, which is one reason why graphene doesn’t see wider use. But we’re inching toward better techniques for studying and using graphene. Recently scientists discovered a new way of using one derivative form of graphene called graphene oxide, and beyond its use in research it could see a lot of use in commercial applications. It turns out graphene oxide makes a pretty great filter for desalinating water. And it’s all because of physical chemistry.

Graphene oxide comes in sheets, like sheets of graphene. That’s about where the similarities end. Where graphene has (ideally) a perfectly regular, one-atom-thick structure of adjoining benzene rings, graphene oxide is several layers thick. Graphene oxide is also loaded with oxygen — hence the oxide — and that gives it some very different chemical properties than regular graphene, which is pure carbon.

Graphene is a great conductor, if you can make it behave. But that isn’t what graphene oxide does best. In somewhat the same way as oxidation rusts metal, making it a less effective conductor, graphene oxide isn’t necessarily what we’re after in terms of applications for the semiconductor industry. It just isn’t quite as compelling as graphene, or even reduced graphite oxide. No, graphene oxide has a different set of talents.

The chemical properties of graphene oxide make it swell up when it gets water on it. The formal term is hydration. Hydrated graphene oxide forms a molecular mesh of very regular size, which does not permit anything larger than 9 angstroms through it. It’s like a tiny, unforgiving sieve. That’s why it’s a great water filter. The tiny, regular holes in the hydrated oxide mesh are of a size that’s small enough to permit H2O molecules, but not larger compounds. Researchers call these tiny pores graphene capillaries.

The problem is that salts are smaller than 9 angstroms. So the researchers thought a while, and then mashed a layer of graphene oxide between two layers of epoxy, which left the graphene oxide nowhere to expand into but its own empty space. Envision yourself squashing a bunch of water balloons between two sheets of Plexiglas, and that’s not too far off what happens when you hydrate graphene oxide while it’s constrained. The spaces between units get smaller. And in that smushed form factor, the holes in the mesh are small enough that most things no longer fit through. Researchers from the University of Manchester achieved a graphene oxide pore size of less than 7 Å, and at that size, even hydrated ions don’t fit through. Just water. The researchers achieved 97 percent filtration of NaCl ions, and it’s safe to expect that further development would yield good results with many different solutes.

Thoroughly oxygenated graphite oxide molecule, including epoxide, hydroxyl and carboxyl groups. These substituents give graphite oxide very different properties to “regular” graphene. Image in public domain, via Wikipedia

Predictably, this is some shiny news for water desalination. Osmotic movement across a semipermeable membrane happens without human energy input, which makes this development compelling. And it’s even better because of the microfluidics aspect of the story. Between the graphene capillaries and the wicking action of water between two narrowly separated plates, this is absolutely ripe for passive exploitation and low-power applications, which could make it really great for deployment in the roughest regions of interior, water-challenged countries. There may be semiconductor applications, but none with the same potential for ubiquitous worldwide deployment.

If I haven’t lost you yet, here’s one last crazy idea: Scrub excess carbon from factory vents, cars, and/or the atmosphere, transform the CO2 into graphite through the appropriate series of reactions, and use that retrieved carbon mass to provide clean water for the developing world at the cost (“cost”) of some reagents plus a cleaner atmosphere for everyone. Then when they’re used up, sink them in a vessel somewhere deep, sequestering the carbon. Disposable graphene-oxide water filters that are safe for making formula with, practically free, and good for the environment. Crazy? Crazy.

Now read: What is graphene?

Originally published at www.extremetech.com on April 12, 2017.

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