2D Materials in Devices Help Separate Salts in Seawater

Two-dimensional (2D) materials have been successfully assembled into devices with the tiniest possible manmade holes for water desalination.

A team of Researchers at the National Graphene Institute (NGI) at The University of Manchester have been successful in fabricating minute slits in a new membrane that is merely several angstroms (0.1 nm) in size. This has allowed for the analysis of how various ions pass through these minute holes.

The slits are composed of graphene, molybdenum disulfide (MoS₂), and hexagonal boron nitride (hBN), and surprisingly, permit ions with diameters larger than the size of the slit to permeate through. The size-exclusion studies allow for an improved understanding of how similar scale biological filters such as aquaporins work and so will help in the development of high-flux filters for water desalination and associated technologies.

For Researchers keen to study the behavior of fluids and their filtration, it has been an ultimate but apparently distant goal to controllably fabricate capillaries with dimensions nearing the size of small ions and individual water molecules.

Researchers have been attempting to replicate naturally-occurring ion transport systems, but this has proved to be no easy mission. Channels fabricated with basic methods and conventional materials have unluckily been limited in size by the inherent roughness of a material’s surface, which is typically at least ten times larger than the hydrated diameter of small ions.

Earlier this year graphene-oxide based membranes created at the NGI attracted substantial attention as potential candidates for new filtration technologies. This research using the new toolkit of 2D materials reveals the real-world potential of providing clean drinking water from salt water.

To better comprehend the central mechanisms behind ion transport, a team headed by Sir Andre Geim of The University of Manchester made atomically flat slits measuring only several angstroms in size. These channels are chemically inert possessing smooth walls on the angstrom scale.

The Researchers formed their slit devices from two 100 nm thick crystal slabs of graphite measuring several microns across that they acquired by shaving off bulk graphite crystals. They then positioned rectangular-shaped pieces of 2D atomic crystals of bilayer graphene and monolayer MoS₂ at each edge of one of the graphite crystal slabs before positioning another slab on top of the first. This creates a gap between the slabs that has a height matching the spacers’ thickness.

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 are the graphene, or MoS₂ monolayers.

Sir Andre Geim, The University of Manchester

The assembly is kept together by van der Waals forces and the slits are approximately the same size as the diameter of aquaporins, which are essential for living organisms. The slits are the smallest size possible since slits with thinner spacers are unbalanced and collapse because of attraction between opposite walls.

Ions flow through the slits if a voltage is applied across them when they are soaked in an ionic solution, and this ion flow constitutes an electric current. The team measured the ionic conductivity as they traveled through chloride solutions via the slits and discovered that ions could travel through them as anticipated 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.

Dr Gopi Kalon, Postdoctoral Researcher who headed the experimental effort

Dr Ali Esfandiar, who is the First Author of the paper, adds, “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.”

This new research published in Science reveals that these freshly observed mechanisms play a vital role for desalination using the size exclusion and is a crucial step to forming high-flux water desalination membranes.