A team of researchers at MIT, Oak Ridge National Laboratory, and in Saudi Arabia succeeded in creating subnanoscale pores in a sheet of graphene, a development that could lead to ultrathin filters for improved desalination or water purification.
The new work, led by graduate student Sean O’Hern and associate professor of mechanical engineering Rohit Karnik, is the first step toward actual production of such a graphene filter.
Making these minuscule holes in graphene, a hexagonal array of carbon atoms, like atomic-scale chicken wire, occurs in a two-stage process. First, the graphene is bombarded with gallium ions, which disrupt the carbon bonds.
Then, the graphene is etched with an oxidizing solution that reacts strongly with the disrupted bonds, producing a hole at each spot where the gallium ions struck. By controlling how long the graphene sheet is left in the oxidizing solution, the MIT researchers can control the average size of the pores.
A big limitation in existing nanofiltration and reverse-osmosis desalination plants, which use filters to separate salt from seawater, is their low permeability: Water flows very slowly through them. The graphene filters, being much thinner, yet very strong, can sustain a much higher flow.
“We’ve developed the first membrane that consists of a high density of subnanometer-scale pores in an atomically thin, single sheet of graphene,” O’Hern says.