2D materials have been assembled into devices with the smallest possible man-made holes for water desalination. (Credit: University of Manchester)A graphene-based membrane was used to develop devices with the smallest possible anthropogenically engineered holes for water desalination. The slits made from graphene, hexagonal boron nitride and molybdenum disulfide (MoS₂) are several angstroms (0.1 nm) in size and allow ions with diameters larger than the size of the slit to permeate through.

Researchers from the National Graphene Institute (NGI) at the University of Manchester fabricated the slit devices from two 100-nm thick crystal slabs of graphite measuring several microns across obtained by shaving off bulk graphite crystals. Rectangular-shaped pieces of 2D atomic crystals of bilayer graphene and monolayer MoS₂ were placed at each edge of one of the graphite crystal slabs before placing another slab on top of the first. This produces a gap between the slabs that has a height equal to the spacers’ thickness.

The assembly is held together by van der Waals forces and the slits are roughly the same size as the diameter of aquaporins, which form pores in the membrane of biological cells to facilitate transport of water between cells.

Ions flow through the slits with voltage application when immersed in an ionic solution, and this ion flow constitutes an electric current. Measurement of ionic conductivity as ions passed through chloride solutions via the slits confirmed that ions could move through them as expected under an applied electric field.

These mechanisms observed in the new 2D materials are expected to contribute to the realization of high-flux water desalination membranes.