Researchers from MIT, King Fahd University of Petroleum and Minerals (KFUPM) and Oak Ridge National Laboratory have come up with a novel method to fill the defects or holes in graphene.
The unique properties of graphene make it ideal for use as water desalination or filtration membranes. However, graphene is only one-atom-thick and the detailed manufacturing process of membrane filters can tear it causing defective membranes that fail to trap all the contaminants.
In this work, the engineers employed a mix of ‘polymerization’ and ‘chemical deposition’ and with the help of an existing method created very small, but uniform pores in graphene, which are so tiny that only water molecules can pass through them.
Using this novel method, the engineers have created a large graphene membrane without any cracks or holes. The defect-free graphene membrane was about the size of a penny, which is significant as graphene needs to be produced at the centimeter or larger scale so as to meet the size requirements of filtration membranes.
Once the crack-sealing and pore creation processes were over, water was pumped into the graphene membrane, and it was able to filter water at the same rate as that of existing desalination or filtration membranes. Large impurities such as magnesium sulfate and dextran molecules were successfully removed by the graphene membrane.
“We’ve been able to seal defects, at least on the lab scale, to realize molecular filtration across a macroscopic area of graphene, which has not been possible before,” Karnik says.
“If we have better process control, maybe in the future we don’t even need defect sealing. But I think it’s very unlikely that we’ll ever have perfect graphene — there will always be some need to control leakages. These two [techniques] are examples which enable filtration.”
“The current types of membranes that can produce freshwater from saltwater are fairly thick, on the order of 200 nanometers,” O’Hern says.
“The benefit of a graphene membrane is, instead of being hundreds of nanometers thick, we’re on the order of three angstroms — 600 times thinner than existing membranes. This enables you to have a higher flow rate over the same area.”
The potential of graphene to act as a filtration membrane was being studied for a few years now by O’Hern and Karnik. Although the group produced membranes using graphene grown on copper substrate in 2009, copper being impermeable, the fabricated graphene needed to be transferred to a porous substrate. The team noted that cracks were created in graphene during this transfer and also that these defects were caused by impurities in the original material.
The research group used “atomic layer deposition” to fix intrinsic defects in graphene. The graphene membrane was pulsed in a chemical containing hafnium inside a vacuum chamber. Although the chemical does not interact with graphene under normal circumstances, it can stick to the tiny openings in graphene, where the surface energy is relatively high. After many cycles of atomic layer deposition, the hafnium oxide filled up all nanometer-scale intrinsic holes in graphene.
However, since O’Hern knew that it would be too time-consuming to fill in larger defects in graphene using the same method, the research team used “interfacial polymerization” to fill in larger holes in graphene. Once the intrinsic defects were fixed, the graphene membrane was immersed at the interface of two liquids - water and an immiscible organic solvent.
Two different molecules which can form nylon on reaction with each other were added to the two solutions. It was observed that nylon plugs were formed only at the tears and holes in graphene, which are the only areas where the two molecules can react and form nylon. These plugs sealed the larger defects in graphene.
The researchers then used a technique developed by them earlier to engrave minute but uniform holes in the graphene membrane. These holes were tiny enough to selectively let only water pass through and trap larger impurities.
Finally, the researchers tested the graphene membrane thus produced by pumping water having salt and several other molecules through it. They observed that up to 90% of larger molecules were retained by the membrane, though salt was allowed to pass through at a more rapid rate than water.
Although initial testing reveals that graphene can be a feasible alternative to currently used filtration membranes, Karnik feels that further advancement in the sealing and permeability controlling techniques is key.
“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.”
De-en Jiang, an assistant professor of chemistry at the University of California at Riverside, sees the defect-sealing technique as “a great advance toward making graphene filtration a reality.”
“The two-step technique is very smart: sealing the defects while preserving the desired pores for filtration,” says Jiang, who did not contribute to the research. “This would make the scale-up much easier. One can produce a large graphene membrane first, not worrying about the defects, which can be sealed later.”
According to associate professor of mechanical engineering at MIT, Rohit Karnik, this MIT study reports the first ever successful sealing of the holes in graphene. The first author of the paper is Sean O’Hern, who is a former graduate research assistant at MIT. MIT graduate student Doojoon Jang, former graduate student Suman Bose, and Professor Jing Kong are the other contributors.
This research work was partially supported by the Center for Clean Water and Clean Energy at MIT and KFUPM, the National Science Foundation, and the U.S. Department of Energy.