Engineers at MIT, Oak Ridge National Laboratory, and King Fahd University of Petroleum and Minerals (KFUPM) have successfully sealed leaks in graphene in a two-step process. This new defect sealing technique may enable faster, more durable water filters.
Graphene’s unique properties make it a potentially ideal membrane for water filtration or desalination. But the main drawback in using Graphene is: Making membranes in one-atom-thick layers of graphene is a meticulous process that can tear the thin material — creating defects through which contaminants can leak.
So they have devised a process to repair these leaks, filling cracks and plugging holes using a combination of chemical deposition and polymerization techniques. Combining these two techniques, the researchers were able to engineer a relatively large defect-free graphene membrane.
Plugging graphene’s leaks
To plug graphene’s leaks, the team came up with a technique to first tackle the smaller intrinsic defects, then the larger transfer-induced defects. For the intrinsic defects, the researchers used a process called “atomic layer deposition,” placing the graphene membrane in a vacuum chamber, then pulsing in a hafnium-containing chemical that does not normally interact with graphene. However, if the chemical comes in contact with a small opening in graphene, it will tend to stick to that opening, attracted by the area’s higher surface energy.
The team applied several rounds of atomic layer deposition, finding that the deposited hafnium oxide successfully filled in graphene’s nanometer-scale intrinsic defects. However, O’Hern, a former graduate research assistant at MIT, realized that using the same process to fill in much larger holes and tears — on the order of hundreds of nanometers — would require too much time. So they came up with the second technique to fill in larger defects, using a process called “interfacial polymerization” that is often employed in membrane synthesis. After they filled in graphene’s intrinsic defects, the researchers submerged the membrane at the interface of two solutions: a water bath and an organic solvent that, like oil, does not mix with water.
In the two solutions, the researchers dissolved two different molecules that can react to form nylon. Once they placed the graphene membrane at the interface of the two solutions, they observed that nylon plugs formed only in tears and holes — regions where the two molecules could come in contact because of tears in the otherwise impermeable graphene — effectively sealing the remaining defects. Using a technique they developed last year, the researchers then etched tiny, uniform holes in graphene — small enough to let water molecules through, but not larger contaminants.
In experiments, the group tested the membrane with water containing several different molecules, including salt, and found that the membrane rejected up to 90 percent of larger molecules. The graphene was able to filter out most large-molecule contaminants, such as magnesium sulfate and dextran. However, it let salt through at a faster rate than water.
The preliminary tests suggest that graphene may be a viable alternative to existing filtration membranes, although techniques to seal its defects and control its permeability will need further improvements.
The research is published in the journal Nano Letters, represent the first success in plugging graphene’s leaks.
