Sub-nanoscale research: If magnified a million times, the graphene pores would be less than 1mm in size
Researchers have devised a way of making tiny holes of controllable size in sheets of the material graphene, a development that could lead to ultra-thin filters for improved desalination or water purification.
The team of academics at Massachusetts Institute of Technology (MIT) and Oak Ridge National Laboratory in the US, along with colleagues in Saudi Arabia, succeeded in creating sub-nanoscale pores in a sheet of the one-atom-thick material, which is one of the strongest known.
The concept of using graphene perforated by nanoscale pores is the first step towards production of improved filters.
Making these minuscule holes in graphene – a hexagonal array of carbon atoms – 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 oxidising 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 oxidising solution, the researchers can control the average size of the pores.
A big limitation in existing nanofiltration and reverse-osmosis desalination plants is the low permeability of the filters used to separate salt from seawater: water flows through them very slowly. 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 sub-nanometre-scale pores in an atomically thin, single sheet of graphene,” said graduate student Sean O’Hern of MIT, who is working on the research along with Rohit Karnik, associate professor of mechanical engineering.
For efficient desalination, a membrane must demonstrate “a high rejection rate of salt, yet a high flow rate of water,” added O’Hern. One way of doing that is to decrease the membrane’s thickness, but this renders conventional polymer-based membranes too weak to sustain the water pressure, or too ineffective at rejecting salt.
With graphene membranes, said O’Hern, it becomes simply a matter of controlling the size of the pores, making them “larger than water molecules, but smaller than everything else” – whether salt, impurities or particular kinds of biochemical molecules.
According to computer simulations, the permeability of such graphene filters could be 50 times greater than that of conventional membranes, as demonstrated earlier by a team of MIT researchers led by graduate student David Cohen-Tanugi of the department of materials science and engineering. But producing such filters with controlled pore sizes has remained a challenge.
O’Hern said the new work demonstrates a method for actually producing such material with dense concentrations of nanometre-scale holes over large areas.
“We bombard the graphene with gallium ions at high energy,” he said. “That creates defects in the graphene structure, and these defects are more chemically reactive.” When the material is bathed in a reactive oxidant solution, the oxidant “preferentially attacks the defects” and etches away many holes of roughly similar size.
O’Hern and his co-authors were able to produce a membrane with
5 trillion pores per cm2, well suited to use for filtration.
“To better understand how small and dense these pores are, if our graphene membrane were to be magnified about a million times, the pores would be less than 1mm in size, spaced about 4mm apart, and span over 38 square miles,” said O’Hern.
With this technique, the researchers were able to control the filtration properties of a single, centimetre-sized sheet of graphene. Without etching, no salt flowed through the defects formed by gallium ions. With just a little etching, the membrane started allowing positive salt ions to flow through. With further etching, the membranes allowed both positive and negative salt ions to flow through, but blocked the flow of larger organic molecules.
With even more etching, the pores were large enough to allow everything to go through.
Scaling up the process to produce useful sheets of the permeable graphene, while maintaining control over the pore sizes, will require further research.
Karnik said that such membranes, depending on their pore size, could find various applications. Desalination and nanofiltration may be the most demanding, since the membranes required for these plants would be very large. But for other purposes, such as selective filtration of molecules – for example, for removal of unreacted reagents from DNA – even the very small filters produced so far might be useful.
“For biofiltration, size or cost are not as critical,” Karnik says. “For those applications, the current scale is suitable.”