Researchers from the University of Manchester have successfully created sub-nanoscale filtration devices using graphene
Graphene is transforming the way we approach a number of key materials problems, but one of its lesser-known qualities is permeability. Back in 2012, researchers at the University of Manchester discovered that graphene oxide –a sheet made from a single layer of carbon atoms – was impermeable to gases and liquids it tested, with the exception of water.
Studies at the University’s National Graphene Institute (NGI) proved that these properties could be used in filtration to remove small nanoparticles, organic molecules and even large salts from water. Further research published in April 2017 expanded on this, and managed to control the material’s tendency to swell in water, making it suitable for sieving out the common salts used in desalination.
In October, that was taken a step further, as NGI staff succeeded in fabricating a device with the smallest possible man-made holes. At several angstroms (0.1 nanometres) in size, it has allowed closer study of how ions pass through the material.
The slits are made from graphene, hexagonal boron nitride (hBN) and molybdenum disulphide (MoS2) and, in fact, allow ions with diameters larger than the size of the slit to permeate. The team says this will increase understanding of how similar scale biological filters such as aquaporins (or water channels) work and could help the development of better filters for desalination.
Changing the channel
Scientists have tried for years to create water channels capable of filtering small ions or individual water molecules. Yet those fabricated with traditional materials remain limited as a result of the intrinsic roughness of a material’s surface, whose spaces are typically at least ten times bigger than the diameter of smaller ions.
In this case the NGI team, led by Sir Andre Geim, made atomically flat, chemically inert slits at the sub-nanometre scale. These were constructed from two 100-nm slabs of graphite, in between which were placed 2D atomic crystals of bilayer graphene and monolayer MoS2. These crystals were placed at each edge of the graphite slabs, with another slab placed on top, producing a gap between with a height equal to the thickness of the central layer.
Geim explained: “It’s like taking a book, placing two matchsticks on each of its edges and then putting another book on top. This creates a gap between the books’ surfaces with the height of the gap being equal to the matches’ thickness. In our case, the books are the atomically flat graphite crystals and the matchsticks the graphene or MoS2 monolayers.”
The device is held together by van der Waals forces – the attraction of intermolecular forces between molecules – and each slit is roughly the same size as an aquaporin. The team says this is as small as the slits can get, given that any with thinner space would be unstable and collapse.
When a voltage is run across the device while immersed in an ionic solution, ions can flow through the slits, essentially creating an electric current. The team measured ionic conductivity as they passed through chloride solutions via the slits and found that ions could move through them as expected under an applied electric field. “When we looked more carefully, we found that bigger ions moved through more slowly than smaller ones like potassium chloride,” added post-doctoral researcher Dr Gopi Kalon, who headed the experiments.
The first author of the paper, which was published last month in Science, Dr Ali Esfandiar, continued: “The classical viewpoint is that ions with a diameter larger than the slit size cannot permeate, but our results show that this explanation is too simplistic. Ions in fact behave like soft tennis balls rather than hard billiard ones, and large ions can still pass – either by distorting their water shells or maybe shedding them altogether.”
The phenomenon reported in their paper is expected to enable new advances in desalination using molecular size exclusion, and the team believes their work could be a key step in the creation of high-flux water desalination membranes.