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    In nanotube science, is boron nitride the new carbon?

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    Engineers at MIT and the University of Tokyo have produced centimeter-scale structures, large enough for the eye to see, that are packed with hundreds of billions of hollow aligned fibers, or nanotubes, made from hexagonal boron nitride.

    Hexagonal boron nitride, or hBN, is a single-atom-thin material that has been coined “white graphene” for its transparent appearance and its similarity to carbon-based graphene in molecular structure and strength. It can also withstand higher temperatures than graphene, and is electrically insulating, rather than conductive. When hBN is rolled into nanometer-scale tubes, or nanotubes, its exceptional properties are significantly enhanced.

    The team’s results, published today in the journal ACS Nano, provide a route toward fabricating aligned boron nitride nanotubes (A-BNNTs) in bulk. The researchers plan to harness the technique to fabricate bulk-scale arrays of these nanotubes, which can then be combined with other materials to make stronger, more heat-resistant composites, for instance to shield space structures and hypersonic aircraft.

    As hBN is transparent and electrically insulating, the team also envisions incorporating the BNNTs into transparent windows and using them to electrically insulate sensors within electronic devices. The team is also investigating ways to weave the nanofibers into membranes for water filtration and for “blue energy” — a concept for renewable energy in which electricity is produced from the ionic filtering of salt water into fresh water.

    Brian Wardle, professor of aeronautics and astronautics at MIT, likens the team’s results to scientists’ decades-long, ongoing pursuit of manufacturing bulk-scale carbon nanotubes.

    “In 1991, a single carbon nanotube was identified as an interesting thing, but it’s been 30 years getting to bulk aligned carbon nanotubes, and the world’s not even fully there yet,” Wardle says. “With the work we’re doing, we’ve just short-circuited about 20 years in getting to bulk-scale versions of aligned boron nitride nanotubes.”

    Wardle is the senior author of the new study, which includes lead author and MIT research scientist Luiz Acauan, former MIT postdoc Haozhe Wang, and collaborators at the University of Tokyo.

    A vision, aligned

    Like graphene, hexagonal boron nitride has a molecular structure resembling chicken wire. In graphene, this chicken wire configuration is made entirely of carbon atoms, arranged in a repeating pattern of hexagons. For hBN, the hexagons are composed of alternating atoms of boron and nitrogen. In recent years, researchers have found that two-dimensional sheets of hBN exhibit exceptional properties of strength, stiffness, and resilience at high temperatures. When sheets of hBN are rolled into nanotube form, these properties are further enhanced, particularly when the nanotubes are aligned, like tiny trees in a densely packed forest.

    But finding ways to synthesize stable, high quality BNNTs has proven challenging. A handful of efforts to do so have produced low-quality, nonaligned fibers.

    “If you can align them, you have much better chance of harnessing BNNTs properties at the bulk scale to make actual physical devices, composites, and membranes,” Wardle says.

    In 2020, Rong Xiang and colleagues at the University of Tokyo found they could produce high-quality boron nitride nanotubes by first using a conventional approach of chemical vapor deposition to grow a forest of short, few micron-long carbon nanotubes. They then coated the carbon-based forest with “precursors” of boron and nitrogen gas, which when baked in an oven at high temperatures crystallized onto the carbon nanotubes to form high-quality nanotubes of hexagonal boron nitride with carbon nanotubes inside.

    Burning scaffolds

    In the new study, Wardle and Acauan have extend and scale Xiang’s approach, essentially removing the underlying carbon nanotubes and leaving the long boron nitride nanotubes to stand on their own. The team drew on the expertise of Wardle’s group, which has focused for years on fabricating high-quality aligned arrays of carbon nanotubes. With their current work, the researchers looked for ways to tweak the temperatures and pressures of the chemical vapor deposition process in order to remove the carbon nanotubes while leaving the boron nitride nanotubes intact.

    “The first few times we did it, it was completely ugly garbage,” Wardle recalls. “The tubes curled up into a ball, and they didn’t work.”

    Eventually, the team hit on a combination of temperatures, pressures, and precursors that did the trick. With this combination of processes, the researchers first reproduced the steps that Xiang took to synthesize the boron-nitride-coated carbon nanotubes. As hBN is resistant to higher temperatures than graphene, the team then cranked up the heat to burn away the underlying black carbon nanotube scaffold, while leaving the transparent, freestanding boron nitride nanotubes intact.
    By using carbon nanotubes as a scaffold, MIT engineers grow forests of “white graphene” that emerge (in MIT pattern) after burning away the black carbon scaffold. Courtesy of the researchersIn microscopic images, the team observed clear crystalline structures — evidence that the boron nitride nanotubes have a high quality. The structures were also dense: Within a square centimeter, the researchers were able to synthesize a forest of more than 100 billion aligned boron nitride nanotubes, that measured about a millimeter in height — large enough to be visible by eye. By nanotube engineering standards, these dimensions are considered to be “bulk” in scale.

    “We are now able to make these nanoscale fibers at bulk scale, which has never been shown before,” Acauan says.

    To demonstrate the flexibility of their technique, the team synthesized larger carbon-based structures, including a weave of carbon fibers, a mat of “fuzzy” carbon nanotubes, and sheets of randomly oriented carbon nanotubes known as “buckypaper.” They coated each carbon-based sample with boron and nitrogen precursors, then went through their process to burn away the underlying carbon. In each demonstration, they were left with a boron-nitride replica of the original black carbon scaffold.

    They also were able to “knock down” the forests of BNNTs, producing horizontally aligned fiber films that are a preferred configuration for incorporating into composite materials.

    “We are now working toward fibers to reinforce ceramic matrix composites, for hypersonic and space applications where there are very high temperatures, and for windows for devices that need to be optically transparent,” Wardle says. “You could make transparent materials that are reinforced with these very strong nanotubes.”

    This research was supported, in part, by Airbus, ANSYS, Boeing, Embraer, Lockheed Martin, Saab AB, and Teijin Carbon America through MIT’s Nano-Engineered Composite aerospace STructures (NECST) Consortium. More

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      in Environment

      Using graphene foam to filter toxins from drinking water

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      Some kinds of water pollution, such as algal blooms and plastics that foul rivers, lakes, and marine environments, lie in plain sight. But other contaminants are not so readily apparent, which makes their impact potentially more dangerous. Among these invisible substances is uranium. Leaching into water resources from mining operations, nuclear waste sites, or from natural subterranean deposits, the element can now be found flowing out of taps worldwide.

      In the United States alone, “many areas are affected by uranium contamination, including the High Plains and Central Valley aquifers, which supply drinking water to 6 million people,” says Ahmed Sami Helal, a postdoc in the Department of Nuclear Science and Engineering. This contamination poses a near and present danger. “Even small concentrations are bad for human health,” says Ju Li, the Battelle Energy Alliance Professor of Nuclear Science and Engineering and professor of materials science and engineering.

      Now, a team led by Li has devised a highly efficient method for removing uranium from drinking water. Applying an electric charge to graphene oxide foam, the researchers can capture uranium in solution, which precipitates out as a condensed solid crystal. The foam may be reused up to seven times without losing its electrochemical properties. “Within hours, our process can purify a large quantity of drinking water below the EPA limit for uranium,” says Li.

      A paper describing this work was published in this week Advanced Materials. The two first co-authors are Helal and Chao Wang, a postdoc at MIT during the study, who is now with the School of Materials Science and Engineering at Tongji University, Shanghai. Researchers from Argonne National Laboratory, Taiwan’s National Chiao Tung University, and the University of Tokyo also participated in the research. The Defense Threat Reduction Agency (U.S. Department of Defense) funded later stages of this work.

      Targeting the contaminant

      The project, launched three years ago, began as an effort to find better approaches to environmental cleanup of heavy metals from mining sites. To date, remediation methods for such metals as chromium, cadmium, arsenic, lead, mercury, radium, and uranium have proven limited and expensive. “These techniques are highly sensitive to organics in water, and are poor at separating out the heavy metal contaminants,” explains Helal. “So they involve long operation times, high capital costs, and at the end of extraction, generate more toxic sludge.”

      To the team, uranium seemed a particularly attractive target. Field testing from the U.S. Geological Service and the Environmental Protection Agency (EPA) has revealed unhealthy levels of uranium moving into reservoirs and aquifers from natural rock sources in the northeastern United States, from ponds and pits storing old nuclear weapons and fuel in places like Hanford, Washington, and from mining activities located in many western states. This kind of contamination is prevalent in many other nations as well. An alarming number of these sites show uranium concentrations close to or above the EPA’s recommended ceiling of 30 parts per billion (ppb) — a level linked to kidney damage, cancer risk, and neurobehavioral changes in humans.

      The critical challenge lay in finding a practical remediation process exclusively sensitive to uranium, capable of extracting it from solution without producing toxic residues. And while earlier research showed that electrically charged carbon fiber could filter uranium from water, the results were partial and imprecise.

      Wang managed to crack these problems — based on her investigation of the behavior of graphene foam used for lithium-sulfur batteries. “The physical performance of this foam was unique because of its ability to attract certain chemical species to its surface,” she says. “I thought the ligands in graphene foam would work well with uranium.”

      Simple, efficient, and clean

      The team set to work transforming graphene foam into the equivalent of a uranium magnet. They learned that by sending an electric charge through the foam, splitting water and releasing hydrogen, they could increase the local pH and induce a chemical change that pulled uranium ions out of solution. The researchers found that the uranium would graft itself onto the foam’s surface, where it formed a never-before-seen crystalline uranium hydroxide. On reversal of the electric charge, the mineral, which resembles fish scales, slipped easily off the foam.

      It took hundreds of tries to get the chemical composition and electrolysis just right. “We kept changing the functional chemical groups to get them to work correctly,” says Helal. “And the foam was initially quite fragile, tending to break into pieces, so we needed to make it stronger and more durable,” says Wang.

      This uranium filtration process is simple, efficient, and clean, according to Li: “Each time it’s used, our foam can capture four times its own weight of uranium, and we can achieve an extraction capacity of 4,000 mg per gram, which is a major improvement over other methods,” he says. “We’ve also made a major breakthrough in reusability, because the foam can go through seven cycles without losing its extraction efficiency.” The graphene foam functions as well in seawater, where it reduces uranium concentrations from 3 parts per million to 19.9 ppb, showing that other ions in the brine do not interfere with filtration.

      The team believes its low-cost, effective device could become a new kind of home water filter, fitting on faucets like those of commercial brands. “Some of these filters already have activated carbon, so maybe we could modify these, add low-voltage electricity to filter uranium,” says Li.

      “The uranium extraction this device achieves is very impressive when compared to existing methods,” says Ho Jin Ryu, associate professor of nuclear and quantum engineering at the Korea Advanced Institute of Science and Technology. Ryu, who was not involved in the research, believes that the demonstration of graphene foam reusability is a “significant advance,” and that “the technology of local pH control to enhance uranium deposition will be impactful because the scientific principle can be applied more generally to heavy metal extraction from polluted water.”

      The researchers have already begun investigating broader applications of their method. “There is a science to this, so we can modify our filters to be selective for other heavy metals such as lead, mercury, and cadmium,” says Li. He notes that radium is another significant danger for locales in the United States and elsewhere that lack resources for reliable drinking water infrastructure.

      “In the future, instead of a passive water filter, we could be using a smart filter powered by clean electricity that turns on electrolytic action, which could extract multiple toxic metals, tell you when to regenerate the filter, and give you quality assurance about the water you’re drinking.” More

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      in Energy

      Cleaning up industrial filtration

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      If you wanted to get pasta out of a pot of water, would you boil off the water, or use a strainer? While home cooks would choose the strainer, many industries continue to use energy-intensive thermal methods of separating out liquids. In some cases, that’s because it’s difficult to make a filtration system for chemical separation, which requires pores small enough to separate atoms.

      In other cases, membranes exist to separate liquids, but they are made of fragile polymers, which can break down or gum up in industrial use.

      Via Separations, a startup that emerged from MIT in 2017, has set out to address these challenges with a membrane that is cost-effective and robust. Made of graphene oxide (a “cousin” of pencil lead), the membrane can reduce the amount of energy used in industrial separations by 90 percent, according to Shreya Dave PhD ’16, company co-founder and CEO.

      This is valuable because separation processes account for about 22 percent of all in-plant energy use in the United States, according to Oak Ridge National Laboratory. By making such processes significantly more efficient, Via Separations plans to both save energy and address the significant emissions produced by thermal processes. “Our goal is eliminating 500 megatons of carbon dioxide emissions by 2050,” Dave says.

      Play video

      What do our passions for pasta and decarbonizing the Earth have in common? MIT alumna Shreya Dave PhD ’16 explains how she and her team at Via Separations are building the equivalent of a pasta strainer to separate chemical compounds for industry.

      Via Separations began piloting its technology this year at a U.S. paper company and expects to deploy a full commercial system there in the spring of 2022. “Our vision is to help manufacturers slow carbon dioxide emissions next year,” Dave says.

      MITEI Seed Grant

      The story of Via Separations begins in 2012, when the MIT Energy Initiative (MITEI) awarded a Seed Fund grant to Professor Jeffrey Grossman, who is now the Morton and Claire Goulder and Family Professor in Environmental Systems and head of MIT’s Department of Materials Science and Engineering. Grossman was pursuing research into nanoporous membranes for water desalination. “We thought we could bring down the cost of desalination and improve access to clean water,” says Dave, who worked on the project as a graduate student in Grossman’s lab.

      There, she teamed up with Brent Keller PhD ’16, another Grossman graduate student and a 2016-17 ExxonMobil-MIT Energy Fellow, who was developing lab experiments to fabricate and test new materials. “We were early comrades in figuring out how to debug experiments or fix equipment,” says Keller, Via Separations’ co-founder and chief technology officer. “We were fast friends who spent a lot of time talking about science over burritos.”

      Dave went on to write her doctoral thesis on using graphene oxide for water desalination, but that turned out to be the wrong application of the technology from a business perspective, she says. “The cost of desalination doesn’t lie in the membrane materials,” she explains.

      So, after Dave and Keller graduated from MIT in 2016, they spent a lot of time talking to customers to learn more about the needs and opportunities for their new separation technology. This research led them to target the paper industry, because the environmental benefits of improving paper processing are enormous, Dave says. “The paper industry is particularly exciting because separation processes just in that industry account for more than 2 percent of U.S. energy consumption,” she says. “It’s a very concentrated, high-energy-use industry.”

      Most paper today is made by breaking down the chemical bonds in wood to create wood pulp, the primary ingredient of paper. This process generates a byproduct called black liquor, a toxic solution that was once simply dumped into waterways. To clean up this process, paper mills turned to boiling off the water from black liquor and recovering both water and chemicals for reuse in the pulping process. (Today, the most valuable way to use the liquor is as biomass feedstock to generate energy.) Via Separations plans to accomplish this same separation work by filtering black liquor through its graphene oxide membrane.

      “The advantage of graphene oxide is that it’s very robust,” Dave says. “It’s got carbon double bonds that hold together in a lot of environments, including at different pH levels and temperatures that are typically unfriendly to materials.”

      Such properties should also make the company’s membranes attractive to other industries that use membrane separation, Keller says, because today’s polymer membranes have drawbacks. “For most of the things we make — from plastics to paper and gasoline — those polymers will swell or react or degrade,” he says.

      Graphene oxide is significantly more durable, and Via Separations can customize the pores in the material to suit each industry’s application. “That’s our secret sauce,” Dave says, “modulating pore size while retaining robustness to operate in challenging environments.”

      “We’re building a catalog of products to serve different applications,” Keller says, noting that the next target market could be the food and beverage industry. “In that industry, instead of separating different corrosive paper chemicals from water, we’re trying to separate particular sugars and food ingredients from other things.”

      Future target customers include pharmaceutical companies, oil refineries, and semiconductor manufacturers, or even carbon capture businesses.

      Scaling up

      Dave, Keller, and Grossman launched Via Separations in 2017 — with a lot of help from MIT. After the seed grant, in 2015, the founders received a year of funding and support from the J-WAFS Solutions program to explore markets and to develop their business plans. The company’s first capital investment came from The Engine, a venture firm founded by MIT to support “tough tech” companies (tech businesses with transformative potential but long and challenging paths to success). They also received advice and support from MIT’s Deshpande Center for Technological Innovation, Venture Mentoring Service, and Technology Licensing Office. In addition, Grossman continues to serve the company as chief scientist.

      “We were incredibly fortunate to be starting a company in the MIT entrepreneurial ecosystem,” Keller says, noting that The Engine support alone “probably shaved years off our progress.”

      Already, Via Separations has grown to employ 17 people, while significantly scaling up its product. “Our customers are producing thousands of gallons per minute,” Keller explains. “To process that much liquid, we need huge areas of membrane.”

      Via Separations’ manufacturing process, which is now capable of making more than 10,000 square feet of membrane in one production run, is a key competitive advantage, Dave says. The company rolls 300-400 square feet of membrane into a module, and modules can be combined as needed to increase filtration capacity.

      The goal, Dave says, is to contribute to a more sustainable world by making an environmentally beneficial product that makes good business sense. “What we do is make manufacturing things more energy-efficient,” she says. “We allow a paper mill or chemical facility to make more product using less energy and with lower costs. So, there is a bottom-line benefit that’s significant on an industrial scale.”

      Keller says he shares Dave’s goal of building a more sustainable future. “Climate change and energy are central challenges of our time,” he says. “Working on something that has a chance to make a meaningful impact on something so important to everyone is really fulfilling.”

      This article appears in the Spring 2021 issue of Energy Futures, the magazine of the MIT Energy Initiative.  More