| Feb 29, 2024 |
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(Nanowerk News) Carbon nanotubes have shown promise for everything from microelectronics to aviation to energy storage.
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Researchers think this material might one day fulfill the science fiction dream of creating an elevator to space.
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So why aren’t they used more often?
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University of Cincinnati chemist Noe Alvarez said one obstacle has been the frustrating inability to link carbon nanotubes to metal surfaces in a robust connection for sensors, transistors and other uses. These hollow tubes have a diameter of just a billionth of a meter but can be many centimeters long.
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“We want our experiments to be reproducible and consistent, but that’s not easily possible with nanotubes because we can’t control how well they’re connected to metal surfaces,” he said.
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But he and his collaborators have demonstrated a new chemical process that grafts nanotubes to metal surfaces to create a strong, consistent, conductive link. The study was published in the journal Nanoscale Advances (“Creating covalent bonds between Cu and C at the interface of metal/open-ended carbon nanotubes”).
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In past iterations, carbon nanotubes were dispersed in a solution to make what Alvarez likens to “wet spaghetti” that sticks to a metal surface.
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“But there is no robust connection. Nothing is really holding the nanotubes to the surface,” he said.
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So measurements of properties such as electrical conductivity were imprecise and inconsistent.
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Alvarez and his research partners at Texas A&M University, led by chemical engineering Professor Jorge Seminario, demonstrated ways to bond nanotubes chemically to copper, aluminum, gold and other metal surfaces.
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Alvarez and his collaborators received a $720,000 grant from the National Science Foundation to elaborate on their chemical discovery in the next three years.
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“Why don’t we see carbon nanotubes in widespread commercial applications even though they have so much potential? We have a lot to figure out,” UC doctoral student and study lead author Chaminda Nawarathne said.
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Alvarez and his co-authors discovered through computational calculations that carbon atoms in the organic link actually bond with two copper atoms, creating an especially strong bond.
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“That explains why our nanotubes once they’re chemically connected stay connected,” Alvarez said.
240222aAlvarez013.CR2 UC chemistry Professor Noe Alvarez and doctoral student Chaminda Nawarathne have come with a chemical process to bond carbon nanotubes to metals, which opens up a huge window of possibilities in energy storage, communications and biomedical engineering.
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Carbon nanotubes are used to create the blackest synthetic material on Earth, absorbing more than 99% of all light. Nanotube fibers are strong and lightweight. Photo/Andrew Higley/UC Marketing + Brand
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Carbon nanotubes are notoriously strong molecules, Alvarez said. Their molecular structure creates an elegant hexagonal lattice.
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“Carbon bonds are the strongest bonds. They’re covalent bonds. That’s why diamond is the hardest material because they are carbon-carbon bonds,” Alvarez said.
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While carbon atoms in diamonds are single bonds, carbon nanotubes have conjugated double-bonded atoms, making them even stronger than diamonds, Alvarez said.
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Cables made from strong but lightweight carbon nanotubes have been envisioned for creating “space elevators” that could ferry equipment into orbit, Alvarez said.
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A space elevator was depicted in the opening scene of the Brad Pitt movie “Ad Astra.”
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But strength is just one of their unique properties.
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Carbon nanotubes are used to create the blackest synthetic material on Earth. Alvarez said their strong bonds with metal could lead to better paints and coatings.
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“Nanotubes are fairly inert. They’re very stable. You can conjugate them without breaking their bonds. Semiconducting nanotubes also have fluorescence properties — they can generate light,” Alvarez said. “So the list of applications goes on and on.”
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Nawarathne said he is pursuing potential applications in energy storage.
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“Now that we can bond the carbon nanotubes to a current collector or metal probe, we can make very stable electrodes for supercapacitors,” Nawarathne said.
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UC chemistry students “grow” nanotubes on silicon disks using a process called catalytic chemical vapor deposition in equipment that heats reagents and an iron catalyst to 1,450 degrees Fahrenheit.
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“It’s red-hot,” Alvarez said, pointing to an object visible through a glass window in the oven-sized machine. “That’s like a baking pan. The catalyst goes in here.”
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After 45 minutes, a thin layer of carbon nanotubes appears on the silicon. From there, researchers were able to electrograft the nanotubes onto a variety of metal surfaces. Initially, they used bundles of nanotubes but with refined processes learned they can connect vertically aligned nanotubes.
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“It’s like trying to connect wool back to a sheep. You have yarn that has been sheared from the sheep. We’re able to connect individual fibers back to the sheep chemically,” he said.
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