Source: TCE Today | Helen Tunnicliffe | May 8, 2015
Graphene can be made with tiny pores, and is 600 times thinner than existing water purification membranes, allowing water through easily at high flowrates. It should therefore be ideal for water filtration or desalination. However, the manufacturing process introduces defects and tears, allowing contaminants and salt to pass through. The researchers, from MIT, Oak Ridge National Laboratory and King Fahd University of Petroleum and Minerals, led by mechanical engineering professor Rohit Karnik, use chemical deposition and polymerisation in their repair process.
Previously, the researchers made graphene for water filtration on a copper substrate, which allows the production of large areas of graphene. Removing it from the non-porous copper onto a porous substrate is the stage that causes the problems, while there are also smaller intrinsic defects.
To fix the intrinsic defects, the researchers place the graphene membrane in a vacuum chamber and use atomic layer deposition to pulse in hafnium oxide. This would not usually interact with graphene, but is attracted to the gaps in the sheet due to the higher surface energy, where it sticks and blocks the holes.
This process, while effective for smaller defects, cannot fix larger holes in a sensible timescale. Instead, Karnik and the team used a process called interfacial polymerisation, that is commonly used in membrane production, adapted for graphene. After the graphene sheet has been coated with hafnium oxide, the researchers submerge the membrane at the interface of water and an immiscible organic solvent, each containing a molecule that when reacted together, form nylon. Where there are tears in the membrane, the two molecules meet, react, and form a nylon plug, sealing the defect.
The researchers then etch tiny holes into the membrane to allow water through. One thing they say is significant is that the graphene membranes created are about the size of penny, approaching the size that would be useful in a commercial membrane.
“Water desalination and nanofiltration are big applications where, if things work out and this technology withstands the different demands of real-world tests, it would have a large impact. But one could also imagine applications for fine chemical- or biological-sample processing, where these membranes could be useful. And this is the first report of a centimeter-scale graphene membrane that does any kind of molecular filtration. That’s exciting,” says Karnik.
Tests showed that some salt is allowed through the membrane, although 90% of other molecules were blocked, so further development work will be necessary.