How Engineers Repaired One-Atom Thick Graphene Filters

Ultrathin graphene membranes can filter our water, but defects far too common.

Graphene seems to be the next big thing in water filtration as scientists look to create ultrathin membranes to filter out contaminants. Only problem is, defects in the making of one-atom thick membranes are a common occurrence, causing leaks.

Hope is not lost though as engineers have found a way to repair the cracks and holes, filling them with a combination of chemical deposition and polymerization techniques.

The first of the two techniques used, addresses the smaller intrinsic defects. Using a process called “atomic layer disposition,” the team placed the graphene membrane in a vacuum chamber, pulsing in a hafnium containing chemical that normal does not interact with graphene.

In this scenario the chemical sticks to openings in the graphene, attracted to the area’s higher surface energy.

After several rounds of applied atomic layer deposition, the hafnium oxide successfully filled in the graphene’s nanometer-scale intrinsic defects.

This solution quickly unveiled a new issue, as the team realized it would require too much time to fill in the membranes larger defects.

Step two in the repair process involved “interfacial polymerization,” often employed in membrane synthesis. Submerging the membrane at the interface of two solutions, a water bath and an organic solvent unable to mix with water.

In these two solutions, the team of engineers dissolved two different molecules which react to form nylon. The nylon plugs formed only in the tears and holes, regions where the two molecules could come in contact – effectively sealing the remaining defects.

With the defects sealed, the next order of business was to ensure the membrane could still function as a filter.

For this, the team adopted a technique they developed the year before, etching tiny, uniform holes in graphene — small enough to let water molecules through.

Experiments with the repaired membrane included water with several types of contaminants including salt. The membrane rejected up to 90 percent of contaminants, but unfortunately let salt through at a faster rate the water.

Preliminary tests suggest graphene may be a viable alternative to existing filtration membranes, but techniques to seal its defects and control its permeability require improvements, says Rohit Karnik, associate professor of mechanical engineering at MIT.

“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,” Karnik says.

“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.”

Working from MIT’s Oak Ridge National Laboratory with King Fahd University of Petroleum and Minerals (KFUPM), the group’s results were first published in the journal Nano Letters.

What are your thoughts on the matter? What other implications can you see for graphene layers? Let us know in the comments below.

Image courtesy MIT News

Source: Jennifer Chu, MIT News