MIT team makes progress in graphene filtration control

A new method for controlling the creation of subnanometer-scale pores in graphene sheets could make all the difference in the use of the technology in desalination, according to a research team at the Massachusetts Institute of Technology.

Following on from previous research, graduate student Sean O’Hern and associate professor of mechanical engineering Rohit Karnik say that their new work demonstrates a method for actually producing such material with dense concentrations of nanometer-scale holes over large areas.

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.

The permeability of such graphene filters, according to computer simulations, 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 & Engineering. But producing such filters with controlled pore sizes has remained a challenge.

With this technique, the researchers were able to control the filtration properties of a single, centimeter-sized sheet of grapheme. Without etching, no salt flowed through the defects. 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.

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 says.

The team’s paper, Selective Ionic Transport through Tunable Subnanometer Pores in Single-Layer Graphene Membranes, was published in Nano Letters on 3 February 2014.