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A carbon nanotube sponge that can soak up oil in water with incredible efficiency is now in the works.

Carbon nanotubes, which consist of atom-thick sheets of carbon rolled into cylinders, have captured scientific attention because of their high strength, potential high conductivity and light weight. However, producing nanotubes in bulk for specialized applications often faced limitations because of difficulties in controlling the growth process as well as dispersing and sorting the produced nanotubes. Not anymore.

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That is because Bobby Sumpter at the Department of Energy’s (DOE’s) Oak Ridge National Laboratory (ORNL) was part of a multi-institutional research team that set out to grow large clumps of nanotubes by selectively substituting boron atoms into the otherwise pure carbon lattice.

Sumpter and Vincent Meunier, now of Rensselaer Polytechnic Institute (RPI), conducted simulations on supercomputers, including Jaguar at ORNL’s Leadership Computing Facility, to understand how the addition of boron would affect the carbon nanotube structure.

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“Any time you put a different atom inside the hexagonal carbon lattice, which is a chicken wire-like network, you disrupt that network because those atoms don’t necessarily want to be part of the chicken wire structure,” Sumpter said. “Boron has a different number of valence electrons, which results in curvature changes that trigger a different type of growth.”

Simulations and lab experiments showed the addition of boron atoms encouraged the formation of “elbow” junctions that help the nanotubes grow into a 3-D network.

“Instead of a forest of straight tubes, you create an interconnected, woven sponge-like material,” Sumpter said. “Because it is interconnected, it becomes three-dimensionally strong, instead of only one-dimensionally strong along the tube axis.”

Further experiments showed the team’s material, which is visible to the human eye, is extremely efficient at absorbing oil in contaminated seawater because it attracts oil and repels water.

“It loves carbon because it is primarily carbon,” Sumpter said. “Depending on the density of oil to water content and the density of the sponge network, it will absorb up to 100 times its weight in oil.”

The material’s mechanical flexibility, magnetic properties, and strength lend it additional appeal as a potential technology to aid in oil spill cleanup, Sumpter said.

“You can reuse the material over and over again because it’s so robust,” he said. “Burning it does not substantially decrease its ability to absorb oil, and squeezing it like a sponge doesn’t damage it either.”

The material’s magnetic properties, a result of the team’s use of an iron catalyst during the nanotube growth process, means a magnet can easily control it or remove it in an oil cleanup scenario. This ability is an improvement over existing substances used in oil removal, which usually stay behind after cleanup and can degrade the environment.

The experimental team submitted a patent application on the technology through Rice University. A research paper entitled “Covalently bonded three-dimensional carbon nanotube solids via boron induced nanojunctions,” is also available.

The research team included researchers from ORNL, Rice University; Universidade de Vigo, Spain; RPI; University of Illinois at Urbana-Champaign; Instituto de Microelectronica de Madrid, Spain; Air Force Office of Scientific Research Laboratory; Arizona State University; Universite Catholique de Louvain, Belgium; The Pennsylvania State University; and Shinshu University, Japan.

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