The team of researchers at MIT, Oak Ridge National Laboratory, and in Saudi Arabia succeeded in creating sub-nanoscale pores in a sheet of the one-atom-thick material.
The concept of using graphene, perforated by nanoscale pores, as a filter in desalination has been proposed and analyzed by other MIT researchers. 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 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 MIT researchers were able to control the average size of the pores.
A big limitation in existing nano-filtration 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,” says O’Hern.
For efficient desalination, a membrane must demonstrate a high rejection rate of salt, yet a high flow rate of water. One way of doing that is to reduce the membrane’s thickness, but this quickly renders conventional polymer-based membranes too weak to sustain the water pressure, or too ineffective at rejecting salt.
With graphene membranes, 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.
The permeability of such graphene filters, according to computer simulations carried out by other MIT researchers, could be 50 times greater than that of conventional membranes. But producing such filters with controlled pore sizes has remained a challenge. The new work, O’Hern says, demonstrates a method for actually producing such material with dense concentrations of nanometer-scale holes over large areas.
O’Hern's team was able to produce a membrane with 5 trillion pores per square centimeter. “To better understand how small and dense these graphene pores are, if our graphene membrane were to be magnified about a million times, the pores would be less than 1 millimeter in size, spaced about 4 millimeters apart, and span over 38 square miles, an area roughly half the size of Boston,” O’Hern explains.
With this technique, the researchers were able to control the filtration properties of a single, centimeter-sized sheet of graphene. Without etching, no salt flowed through the defects formed by gallium ions. With just a little etching, the membranes 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.
However, scaling up the process to produce useful sheets of the permeable graphene, while maintaining control over the pore sizes, will require further research, O’Hern concedes.
Key to illustration:
The MIT researchers used a four-step process to create filters from graphene: (a) a one-atom-thick sheet of graphene is placed on a supporting structure; (b) the graphene is bombarded with gallium ions; (c) wherever the gallium ions strike the graphene, they create defects in its structure; and (d) when etched with an oxidising solution, each of those defects grows into a hole in the graphene sheet. The longer the material stays in the oxidising bath, the larger the holes get (image courtesy of the researchers)