2D materials with ‘smallest possible’ holes sieve salt from seawater

RESEARCHERS at the University of Manchester’s National Graphene Institute (NGI) have assembled membranes with the smallest possible manmade holes which can separate salts from seawater.

The membranes have slits just 0.1 nm wide, and are made from the 2D materials graphene, hexagonal boron nitride (hBN) and molybdenum disulphide (MoS₂). Up to now, membrane pores have been limited in how small they can be made, due to the intrinsic roughness of the materials used. The channels through them are usually at least ten times the size of the hydrated diameter of small ions. The tiny slits in the materials made by physicist Sir Andre Geim and the team approach the size of the molecules themselves and are smooth and chemically inert.

The researchers were surprised to find that ions with a larger diameter than the pore were still able to pass through. The study will allow researchers to gain a better understanding of fundamental ion transport mechanisms, like those found in biological systems, called aquaporins and lead to the development of new membranes.

The researchers first shaved 100 nm thick crystal slabs of graphite off bulk graphite crystals. They then placed 2D atomic crystals of bilayer graphene and monolayer MoS₂ at each edge of a graphite slab, and placed another slab on top. It is the gap created between the slab that becomes the channel. The structure is held together by van der Waals forces between the components.

“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,” said Geim.

The gaps, or channels, are about the same size as aquaporins. Smaller sized slits are not possible as the attractive van der Waals forces between the two layers are then small enough to close the slit. When the researchers applied a voltage across the slits in a salt solution, the ions moved through the slits. Larger ions were found to move more slowly than smaller ones.

“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,” said researcher Ali Esfandiar.

The researchers believe that learning about such mechanisms will enable the creation of high-flux membranes for water desalination.

Science doi.org/cfng