Stamp-sized graphene riddled sheets with holes could be boon for molecular separation

MIT engineers have found a way to directly "pinprick" microscopic holes into graphene as the material is grown in the lab. With this technique, they have fabricated relatively large sheets of graphene ("large," meaning roughly the size of a postage stamp), with pores that could make certain molecules filter out of solutions vastly more efficient.

Such loopholes would typically have been considered, but the MIT team has found that defects in graphene-which consists of a single layer of carbon atoms-may be an advantage in such fields. Typically, many thicker polymer membranes are used in laboratories to filter out specific molecules from solution, such as proteins, amino acids, chemicals, and salts.

If it could be done with a lot of small molecules, it would be possible to improve the membrane technology. The material is incredibly thin, it would be much more thicker polymer membranes.

The researchers also found that it is quite easy to achieve the right number of molecules in the production process. The new technique could easily be integrated into any large-scale manufacturing of graphene, such as a roll-to-roll process that the team has previously developed.

"If you take this to a roll-to-roll manufacturing process, it's a game changer," says lead author Piran Kidambi, formerly MIT's postdoc and now an assistant professor at Vanderbilt University. "You do not need anything else." "Just reduce the temperature, and we have a complete integrated manufacturing setup for graphene membranes."

Kidambi's MIT co-authors are Rohit Karnik, associate professor of mechanical engineering, and Jing Kong, professor of electrical engineering and computer science, along with researchers from Oxford University, the National University of Singapore, and Oak Ridge National Laboratory. Their paper appears today in Advanced Materials.

Pristine defects

This paper is a method for generating nanometer-sized pores in graphene. Other groups have used focused methods of graphically drilling holes in graphene, but Kidambi says these techniques are difficult to integrate into any large-scale manufacturing process.

"Scalability of these processes are extremely limited," Kidambi says. "They would take way too much time, and an industrially quick process, such pore-generating techniques would be challenging to do."

So he looked for ways to make nanoporous graphene in a more direct fashion. As a Ph.D. student at Cambridge University, Kidambi spent a lot of time doing pristine, defect-free graphene, for use in electronics. In that context, it has been tried to minimize the chemical vapor deposition (CVD) -a process by which researchers flow through a copper substrate within a furnace. At high enough temperatures, of about 1,000 degrees Celsius, the gas eventually settles on the substrate.

"That was when the realization hit me: I just got to go back to my repository of processes and pick out those defects, and try them in our CVD furnace," Kidambi says.

As it turns out, the team found that by simply lowering the temperature of the furnace to between 850 and 900 degrees Celsius, they were able to directly produce nanometer-sized pores as the graphene was grown.

"When we tried this, it's a little that it really works," Kidambi says. "This [temperature] we really need to make graphene dialysis membranes. "

"This is one of several advances that will ultimately make graphene membranes practical for a range of applications," Karnik adds. "They may be used in biotechnological separations in the preparation of drugs or molecular therapeutics, or perhaps in dialysis therapies."

A Swiss cheese support

While the team is not sure about the temperature, it is important to understand that Kidambi suspects that it has something to do with it.

"The way is graphene grows is, you have a gas and gas disassociates on the catalyst surface and forms carbon atom clusters which then form nuclei, or seeds," Kidambi explains. "So you have a lot of seeds that you can reduce the temperature, your threshold for nucleation is lower so you get many nuclei, and they can not grow big enough, and they are more prone to defects, we do not know exactly what the mechanics of these defects are, or pores, is, but we see it every single time. "

The researchers were able to fabricate nanoporous sheets of graphene. But as the material is incredibly thin, it is likely to be broken down. So the team adapted a method of casting a layer of polymer on the graphene.

The supported graphene was now tough enough withstand normal dialysis procedures. But even if target molecules were passed through the graphene, they would be blocked by the polymer support. The team needed a way to produce pores in the polymer that would have been much larger than those in the past hole just its size, and then immediately passing through a much wider tunnel.

The process is very simple, and the method of the process is much simpler, and the process is much easier. than the pores in graphene. Combining their techniques, they were able to create sheets of nanoporous graphene, each measuring about 5 square centimeters.

"To the best of our knowledge, so far is the largest atomically thin nanoporous membrane made by direct training of pores," Kidambi says.

Currently, the team has produced pores in graphene measuring approximately 2 to 3 nanometers wide, which they found to be small enough to filter as much salts as potassium chloride (0.66 nanometers), and small molecules such as amino acid L-Tryptophan (about 0.7 nanometers), food coloring Allura Red Dye (1 nanometer), and vitamin B-12 (1.5 nanometers) to varying degrees. The material is not so great (4 nanometers). The team is now working on the size of graphene pores to precisely filter molecules of various sizes.

"Kidambi says." "Defects are not always bad, and if you can make the right defects, you can have many different applications for graphene."

Explore further:
Scientists produce dialysis membrane made from graphene

More information:
Easy Manufacturing of Large-Area Atomically Thin Membranes by Direct Synthesis of Graphene with Nanoscale Porosity. Advanced Materials. doi.org/10.1002/adma.201804977