For the first time, researchers at Penn State University and Rice University have created solid, spongy blocks of carbon nanotubes that have an astounding ability to clean up oil spills in water. Separating oil from seawater is just one of a range of potential applications for the new material formed using carbon and a dash of boron. The international team, which includes Mauricio Terrones, a professor of physics and of materials science and engineering at Penn State; Pulickel Ajayan, the Benjamin M. and Mary Greenwood Anderson Professor in Engineering at Rice University; and other scientists from the United States, Spain, Belgium and Japan, has published the results of its research in Nature’s online journal Scientific Reports.
Terrones explained that carbon nanotubes are tiny tubes with diameters ranging from 1-50 nanometers — much narrower than the width of a human hair. They are also 100 times stronger than steel and about one sixth the weight. “Our goal was to find a way to make three-dimensional networks of these carbon nanotubes that would form a macroscale fabric — a spongy block of nanotubes that would be big and thick enough to be used to clean up oil spills and to perform other tasks,” Terrones said. “We realized that the trick was adding boron — which is a chemical element that is next to carbon on the periodic table — because boron helps to trigger the interconnections of the material. To add the boron, we used very high temperatures and we then ‘knitted’ the substance into the nanotube fabric.”
Ajayan explained that the boron puts kinks and elbows into the nanotubes and promotes the formation of covalent bonds, which give the sponges their robust qualities. “The boron helps to tangle the sponges into a complex network,” Ajayan said. “In the past, people have made nanotube solids via post-growth processing but without proper covalent connections. The advantage with our method is that the material is created directly and comes out as a cross-linked porous network.”
First author Daniel Hashim, a graduate student at Rice University, explained that the spongy carbon-nanotube blocks that he and his team created are special for two reasons. “First, they are superhydrophobic, which means that they hate water, so they float really well. Second, they are oleophilic, which means that they love — and thus absorb — oil. In fact, they can absorb 123 times their weight in oil,” Hashim said. To demonstrate, Hashim dropped a nanotube sponge into a dish of water with used motor oil floating on top. The sponge soaked it up. He then put a match to the material, burned off the oil, and returned the material to the water to absorb more. “This material can be used repeatedly and stands up to abuse,” Hashim said. He also explained that a carbon-nanotube sponge remained elastic even after 11,000 uses in the lab. “Another interesting feature of these nanotube sponges, which are 99 percent air, is that they also conduct electricity and can easily be manipulated with magnets,” Hashim said.
Ajayan said that he and other members of the research team are continuing to work on how to make even larger sheets of the carbon-nanotube blocks. “For oil spills, you would have to make large-enough sheets or find a way to weld smaller sheets together,” Ajayan said. Terrones added that the team members also are looking into ways to exploit the three-dimensional structure of the nanotube sponges for use in other applications. “Oil-spill remediation and environmental clean-up are just the beginning of how useful these new nanotube materials could be,” Terrones said. “For example, we could make use these materials to make more-efficient and lighter batteries. We could use them as scaffolds for bone-tissue regeneration. We even could impregnate the nanotube sponge with polymers in order to fabricate robust and light composites for the automobile and plane industries.”
In addition to Terrones, Ajayan, and Hashim, other researchers who contributed to this study include Narayanan Narayanan, Myung Gwan Hahm, Joseph Suttle, and Robert Vajtai from Rice University; José Romo-Herrera from the University of Vigo in Spain; David Cullen and Bobby Sumpter from Oak Ridge National Laboratory in Tennessee; Peter Lezzi and Vincent Meunier from Rensselaer Polytechnic Institute; Doug Kelkhoff from the University of Illinois at Urbana-Champaign; E. Muñoz-Sandoval from the Instituto de Microelectrónica de Madrid; Sabyasachi Ganguli and Ajit Roy from the Air Force Research Laboratory in Ohio; David Smith from Arizona State University; and Humberto Terrones from Oak Ridge National Laboratory and the Université Catholique de Louvain in Belgium.
Support for this research comes from the National Science Foundation, the Air Force Office of Scientific Research, and the Center for Nanophase Materials Science (CNMS) of Oak Ridge National Laboratory.
Barbara Kennedy, Penn State University
