MIT Research News' Journal

Stamp-sized graphene sheets riddled 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 filtering certain molecules out of solutions vastly more efficient.

Such holes would typically be considered unwanted defects, but the MIT team has found that defects in graphene — which consists of a single layer of carbon atoms — can be an advantage in fields such as dialysis. Typically, much 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 tailored with pores small enough to let through certain molecules but not others, graphene could substantially improve dialysis membrane technology: The material is incredibly thin, meaning that it would take far less time for small molecules to pass through graphene than through much thicker polymer membranes.

The researchers also found that simply turning down the temperature during the normal process of growing graphene will produce pores in the exact size range as most molecules that dialysis membranes aim to filter. The new technique could thus be easily 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 an MIT postdoc and now an assistant professor at Vanderbilt University. “You don’t need anything else. Just reduce the temperature, and we have a fully 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

Kidambi and his colleagues previously developed a technique to generate nanometer-sized pores in graphene, by first fabricating pristine graphene using conventional methods, then using oxygen plasma to etch away at the fully formed material to create pores. Other groups have used focused beams of ions to methodically drill holes into 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 in 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 PhD student at Cambridge University, Kidambi spent much of his time looking for ways to make pristine, defect-free graphene, for use in electronics. In that context, he was trying to minimize the defects in graphene that occurred during chemical vapor deposition (CVD) — a process by which researchers flow gas across a copper substrate within a furnace. At high enough temperatures, of about 1,000 degrees Celsius, the gas eventually settles onto the substrate as high quality graphene.

“That was when the realization hit me: I just have to go back to my repository of processes and pick out those which give me 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 surprised us a little that it really works,” Kidambi says. “This [temperature] condition really gave us the sizes we 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 find use in biotechnological separations including in the preparation of drugs or molecular therapeutics, or perhaps in dialysis therapies.”

A Swiss cheese support

While the team is not entirely sure why a lower temperature creates nanoporous graphene, Kidambi suspects that it has something to do with how the gas in the reaction is deposited onto the substrate.

“The way graphene grows is, you inject a gas and the gas disassociates on the catalyst surface and forms carbon atom clusters which then form nuclei, or seeds,” Kidambi explains. “So you have many small seeds that graphene can start growing from to form a continuous film. If you reduce the temperature, your threshold for nucleation is lower so you get many nuclei. And if you have too many nuclei, they can’t grow big enough, and they are more prone to defects. We don’t know exactly what the formation mechansim of these defects, 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, and now pocked with holes, alone, it would likely come apart like paper-thin Swiss cheese if any solution of molecules were to flow across it. So the team adapted a method to cast a thicker supporting layer of polymer on top of the graphene.

The supported graphene was now tough enough to withstand normal dialysis procedures. But even if target molecules were to pass through the graphene, they would be blocked by the polymer support. The team needed a way to produce pores in the polymer that were significantly larger than those in graphene, to ensure that any small molecules passing through the ultrathin material would easily and quickly pass through the much thicker polymer, similar to a fish swimming through a port hole just its size, and then immediately passing through a much large tunnel.

The team ultimately found that by submersing the stack of copper, graphene, and polymer in a solution of water, and using conventional processes to etch away the copper layer, the same process naturally created large pores in the polymer support that were hundreds of times larger 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 this is the largest atomically thin nanoporous membrane made by direct formation of pores,” Kidambi says.

Currently, the team has produced pores in graphene measuring approximately 2 to 3 nanometers wide, which they found was small enough to quickly filter salts such as potassium chloride (0.66 nanometers), and small molecules such as the 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 did not filter out slightly larger molecules, such as the egg protein lysozyme (4 nanometers). The team is now working to tailor the size of graphene pores to precisely filter molecules of various sizes.

“We now have to control these size defects and make tunable sized pores,” Kidambi says. “Defects are not always bad, and if you can make the right defects, you can have many different applications for graphene.”

This research was supported, in part, by the U.S. Department of Energy.

A new path to solving a longstanding fusion challenge

A class exercise at MIT, aided by industry researchers, has led to an innovative solution to one of the longstanding challenges facing the development of practical fusion power plants: how to get rid of excess heat that would cause structural damage to the plant.

The new solution was made possible by an innovative approach to compact fusion reactors, using high-temperature superconducting magnets. This method formed the basis for a massive new research program launched this year at MIT and the creation of an independent startup company to develop the concept. The new design, unlike that of typical fusion plants, would make it possible to open the device’s internal chamber and replace critical comonents; this capability is essential for the newly proposed heat-draining mechanism.

The new approach is detailed in a paper in the journal Fusion Engineering and Design, authored by Adam Kuang, a graduate student from that class, along with 14 other MIT students, engineers from Mitsubishi Electric Research Laboratories and Commonwealth Fusion Systems, and Professor Dennis Whyte, director of MIT’s Plasma Science and Fusion Center, who taught the class.

In essence, Whyte explains, the shedding of heat from inside a fusion plant can be compared to the exhaust system in a car. In the new design, the “exhaust pipe” is much longer and wider than is possible in any of today’s fusion designs, making it much more effective at shedding the unwanted heat. But the engineering needed to make that possible required a great deal of complex analysis and the evaluation of many dozens of possible design alternatives.

Taming fusion plasma

Fusion harnesses the reaction that powers the sun itself, holding the promise of eventually producing clean, abundant electricity using a fuel derived from seawater — deuterium, a heavy form of hydrogen, and lithium — so the fuel supply is essentially limitless. But decades of research toward such power-producing plants have still not led to a device that produces as much power as it consumes, much less one that actually produces a net energy output.

Earlier this year, however, MIT’s proposal for a new kind of fusion plant — along with several other innovative designs being explored by others — finally made the goal of practical fusion power seem within reach. But several design challenges remain to be solved, including an effective way of shedding the internal heat from the super-hot, electrically charged material, called plasma, confined inside the device.

Most of the energy produced inside a fusion reactor is emitted in the form of neutrons, which heat a material surrounding the fusing plasma, called a blanket. In a power-producing plant, that heated blanket would in turn be used to drive a generating turbine. But about 20 percent of the energy is produced in the form of heat in the plasma itself, which somehow must be dissipated to prevent it from melting the materials that form the chamber.

No material is strong enough to withstand the heat of the plasma inside a fusion device, which reaches temperatures of millions of degrees, so the plasma is held in place by powerful magnets that prevent it from ever coming into direct contact with the interior walls of the donut-shaped fusion chamber. In typical fusion designs, a separate set of magnets is used to create a sort of side chamber to drain off excess heat, but these so-called divertors are insufficient for the high heat in the new, compact plant.

One of the desirable features of the ARC design is that it would produce power in a much smaller device than would be required from a conventional reactor of the same output. But that means more power confined in a smaller space, and thus more heat to get rid of.

“If we didn’t do anything about the heat exhaust, the mechanism would tear itself apart,” says Kuang, who is the lead author of the paper, describing the challenge the team addressed — and ultimately solved.

Inside job

In conventional fusion reactor designs, the secondary magnetic coils that create the divertor lie outside the primary ones, because there is simply no way to put these coils inside the solid primary coils. That means the secondary coils need to be large and powerful, to make their fields penetrate the chamber, and as a result they are not very precise in how they control the plasma shape.

But the new MIT-originated design, known as ARC (for advanced, robust, and compact) features magnets built in sections so they can be removed for service. This makes it possible to access the entire interior and place the secondary magnets inside the main coils instead of outside. With this new arrangement, “just by moving them closer [to the plasma] they can be significantly reduced in size,” says Kuang.

In the one-semester graduate class 22.63 (Principles of Fusion Engineering), students were divided into teams to address different aspects of the heat rejection challenge. Each team began by doing a thorough literature search to see what concepts had already been tried, then they brainstormed to come up with multiple concepts and gradually eliminated those that didn’t pan out. Those that had promise were subjected to detailed calculations and simulations, based, in part, on data from decades of research on research fusion devices such as MIT’s Alcator C-Mod, which was retired two years ago. C-Mod scientist Brian LaBombard also shared insights on new kinds of divertors, and two engineers from Mitsubishi worked with the team as well. Several of the students continued working on the project after the class ended, ultimately leading to the solution described in this new paper. The simulations demonstrated the effectiveness of the new design they settled on.

“It was really exciting, what we discovered,” Whyte says. The result is divertors that are longer and larger, and that keep the plasma more precisely controlled. As a result, they can handle the expected intense heat loads.

“You want to make the ‘exhaust pipe’ as large as possible,” Whyte says, explaining that the placement of the secondary magnets inside the primary ones makes that possible. “It’s really a revolution for a power plant design,” he says. Not only do the high-temperature superconductors used in the ARC design’s magnets enable a compact, high-powered power plant, he says, “but they also provide a lot of options” for optimizing the design in different ways — including, it turns out, this new divertor design.

Going forward, now that the basic concept has been developed, there is plenty of room for further development and optimization, including the exact shape and placement of these secondary magnets, the team says. The researchers are working on further developing the details of the design.

“This is opening up new paths in thinking about divertors and heat management in a fusion device,” Whyte says.

“All of the ARC work has been both eye-opening and stimulating of new ways of looking at tokamak fusion reactors,” says Bruce Lipschultz, a professor of physics at the University of York, in the U.K., who was not involved in this work. This latest paper, he says, “incorporates new ideas in the field with the many other significant improvements in the tokamak concept. … The ARC study of the extended leg divertor concept shows that the application to a reactor is not impossible, as others have contended.”

Lipschultz adds that this is “very high-quality research that shows a way forward for the tokamak reactor and stimulates new research elsewhere.”

The team included MIT students Norman Cao, Alexander Creely, Cody Dennett, Jake Hecla, Adam Kuang, Alex Tinguely, Elizabeth Tolman, Hannah Hoffman, Maximillian Major, Juan Ruiz Ruiz, and Brandon Sorbom, PSFC Research Scientists Daniel Brunner and Brian LaBombard, Professor Dennis Whyte, and Mitsubishi Electric Research Laboratories engineers Piyush Grover and Christopher Laughman. The work was supported by MIT’s Department of Nuclear Science and Engineering, the Department of Energy, the National Science Foundation, and Mitsubishi Electric Research Laboratories.