MIT engineers tiny graphene desalination membrane

Researchers at the Massachusetts Institute of Technology have published new research on the engineering of a very thin graphene membrane that could be used for tasks such as desalination without tearing the membrane apart.

The work appears in a paper, Selective Molecular Transport through Intrinsic Defects in a Single Layer of CVD Graphene, published in the American Chemical Society's journal ACS Nano on 2 October 2012.

Professor Rohit Karnik and a team from MIT, Oak Ridge National Laboratory, Indian Institute of Technology and King Fahd University of Petroleum & Minerals, Saudi Arabia, describe fabrication of a membrane by transferring a single layer of CVD graphene on a porous polycarbonate substrate.

The group settled on graphene, in part because of its extremely thin structure and its strength: a sheet of graphene is as thin as a single atom, but strong enough to let high volumes of fluids through without falling apart.

The team set out to engineer a membrane spanning 25 mm² -- a surface area that is large by graphene standards, holding about a quadrillion carbon atoms. They used graphene synthesized by chemical vapor deposition, borrowing expertise from another MIT research group.

Experiments showed that the membrane was not impermeable and that salts flowed through it. There was also a limit to the size of molecules that would pass through the membrane.

As a final experiment, Karnik and Sean O'Hern, also from MIT's Mechanical Engineering Department, observed the actual holes in the graphene membrane, looking at the material through a high-powered electron microscope at Oak Ridge in collaboration with Juan-Carlos Idrobo. They found that pores ranged in size from about 1 to 12 nm -- just wide enough to selectively let some small molecules through.

"No one has looked for holes in graphene before," says Karnik, associate professor of mechanical engineering at MIT. "There's a lot of chemical methods that can be used to modify these pores, so it's a platform technology for a new class of membranes."

When asked by D&WR how this research tied in with the MIT paper on graphene membranes reported in June 2012, Karnik replied that the previous study was a simulation showing the potential of graphene for high‑performance desalination, while his study was experimental. Experimentally, more progress was needed before desalination could be achieved, and this work was a first step in that direction, he said.

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