Graphene is a sheet of carbon just one atom thick in which the atoms are arranged in a honeycomb lattice. Graphene oxide is like ordinary graphene but is covered with molecules such as hydroxyl groups. Graphene-oxide sheets can easily be stacked on top of each other to form extremely thin, but mechanically strong, membranes. These membranes consist of millions of small flakes of graphene oxide with nanosized empty channels (or capillaries) between the flakes.

Two years ago, a team of researchers led by Andre Geim – who discovered graphene in 2004 – found that graphene oxide membranes were impermeable to all gases and vapours except for water. In fact, Geim and colleagues found that water passes through a film of graphene oxide extremely fast while all other gases and liquids are completely blocked by the film. Even helium, which is extremely difficult to block out, cannot pass through the membranes but water vapour goes through so quickly, it is as if the membranes were not there at all.

Capillaries shrink when dry

The graphene oxide sheets are arranged in such a way that there is room only for one layer of water molecules. In the absence of water, however, the capillaries shrink and do not let anything through this way. This is why the material is impermeable to everything but water.

Now, the same team has found that when the membranes are immersed in water, as opposed to just being exposed to water vapour or ambient humidity, they appear to swell slightly and are able to block all molecules or ions with a hydrated size larger than 9 angstroms. (A hydrated sugar molecule, for example, has a diameter of 10 angstroms). What is more, the membranes are able to distinguish between atomic species that differ in size by only a few percent. And, importantly, ions that are smaller than 9 angstroms across pass through the membranes 1000 times faster than expected from simple diffusion processes alone.

"Ion sponging"

“We believe that this last phenomenon comes thanks to another exceptional property of graphene oxide membranes that we have called ‘ion sponging’”, co-team leader Rahul Nair told nanotechweb.org. “The capillaries between the individual graphene oxide flakes appear to act rather like powerful little hoovers that ‘suck up’ small ions.”

The Manchester researchers produced their membranes by stacking thousands of individual layers of graphene oxide on top of each other using simple techniques such as vacuum filtration and spray coating. The graphene oxide sheets are separated from each other by around 6–7 angstroms when dry, but when they are immersed in water, this separation increases to around 11–12 angstroms because not one, but two layers of water are lodged between the sheets. “Water layers formed inside this ultra-narrow graphene capillary can move very freely, something that helps ions and molecules that are smaller than the size of the capillary itself to pass through,” explained Nair.

Decontamination and desalination applications on the horizon

According to the team, the membranes could be ideal for removing valuable salts and molecules from contaminated larger molecules – for example, during oil spills. “More importantly, our work shows that if we were able to further control the capillary size below 9 angstroms, we should be able to use these membranes to filter and desalinate water.”

Indeed, the team says that it is now busy looking at ways to control the graphene oxide mesh size and reduce it to around 6 angstroms so that the membranes can filter out even the smallest salts in seawater. “We might achieve this by preventing the graphene oxide laminates from swelling when they are placed in water,” said Nair.

“Our ultimate goal would be to make a filter device from the carbon-based material that allows you to obtain a glass of drinkable water from seawater using a handheld mechanical pump,” added team member Irina Grigorieva.

The current work is detailed in Science DOI: 10.1126/science.1245711.

Further reading

Graphene oxide goes large scale (Feb 2010)
Graphene oxide framework packs in hydrogen (Mar 2010)
New form of ice seen in graphene interlayers (Aug 2013)
Carbon membranes excel at separating molecules (Jan 2012)