Researchers at Manchester University’s National Graphene Institute (NGI) have assembled 2D materials with sub-nanometre slits that hold potential for water desalination.
The materials are made from graphene, hexagonal boron nitride (hBN) and molybdenum disulphide (MoS2). According to the researchers, they were able to manufacture slits in these materials just several angstroms (0.1nm) in diameter. At this scale, it was possible to study how individual ions behaved while passing through the slits. The work, which is published in the journal Science, also sheds light on how similar scale biological filters function in nature.
The researchers made their slit devices from two 100nm thick crystal slabs of graphite measuring several microns across. They then placed rectangular-shaped pieces of 2D atomic crystals of bilayer graphene and monolayer MoS2 at each edge of one of the slabs, placing another slab on top of the first. This produced a gap between the slabs that had a height equal to the spacers’ thickness.
“It’s like taking a book, placing two matchsticks on each of its edges and then putting another book on top,” explained Sir Andre Geim, one of the Manchester University physicists awarded a Nobel Prize in 2010 for their work on graphene.
“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 are the graphene, or MoS2 monolayers.”
The assembly is held together by van der Waals forces and the slits are roughly the same size as the diameter of aquaporins, integral membrane proteins that serve as water transfer channels in living organisms. According to the Manchester team, slits smaller than this are not possible, as the attraction between opposing walls would cause thinner slits to collapse. To their surprise, the researchers also found that ions slightly bigger than the slits were able to squeeze through, and the research could lead to the development of high-flux water desalination membranes.
“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,” said Dr Ali Esfandiar, the paper’s first author.
“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.”