Science & Technology - Posted by Mike Williams-Rice on Monday, April 16, 2012 12:08 - 7 Comments
Carbon nanotube sponge devours oil
RICE (US) — By adding boron to carbon nanotubes, scientists have created spongy, reusable blocks that absorb oil spilled in water.
Oil absorbance is one of a range of potential innovations for the material created in a single step by researchers at Rice University and Penn State.
The team found for the first time that boron puts kinks and elbows into the nanotubes as they grow and promotes the formation of covalent bonds, which give the sponges their robust qualities.
Straight from the Source
The researchers, who collaborated with peers in labs around the nation and in Spain, Belgium, and Japan, revealed their discovery in the journal Scientific Reports.
Lead author Daniel Hashim, a graduate student in the lab of Rice materials scientist Pulickel Ajayan, says the blocks are both superhydrophobic—they hate water, so they float really well—and oleophilic—they love oil.
The nanosponges, which are more than 99 percent air, also conduct electricity and can easily be manipulated with magnets.
To demonstrate, Hashim dropped the 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 sponge to the water to absorb more.
The robust sponge can be used repeatedly and stands up to abuse; he says a sample remained elastic after about 10,000 compressions in the lab. The sponge can also store the oil for later retrieval, he says.
“These samples can be made pretty large and can be easily scaled up,” says Hashim, holding a half-inch square block of billions of nanotubes. “They’re super-low density, so the available volume is large. That’s why the uptake of oil can be so high.” He says the sponges described in the paper can absorb more than a hundred times their weight in oil.
Ajayan, professor in mechanical engineering and materials science and of chemistry, says multiwalled carbon nanotubes grown on a substrate via chemical vapor deposition usually stand up straight without any real connections to their neighbors. But the boron-introduced defects induced the nanotubes to bond at the atomic level, which tangled them into a complex network.
Nanotube sponges with oil-absorbing potential have been made before, but this is the first time the covalent junctions between nanotubes in such solids have been convincingly demonstrated, he says.
“The interactions happen as they grow, and the material comes out of the furnace as a solid,” Ajayan says. “People have made nanotube solids via post-growth processing but without proper covalent connections. The advantage here is that the material is directly created during growth and comes out as a cross-linked porous network.
“It’s easy for us to make nano building blocks, but getting to the macroscale has been tough,” he says. “The nanotubes have to connect either through some clever way of creating topological defects, or they have to be welded together.”
When he was an undergraduate student of Ajayan’s at Rensselaer Polytechnic Institute, Hashim and his classmates discovered hints of a topological solution to the problem while participating in a National Science Foundation exchange program at the Institute of Scientific Research and Technology (IPICYT) in San Luis Potosí, Mexico.
The paper’s co-author, Mauricio Terrones, a professor of physics, materials science and engineering at Penn State with an appointment at Shinshu University, Japan, led a nanotechnology lab there.
“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 says.
“We realized that the trick was adding boron—a chemical element 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.”
The researchers have high hopes for the material’s environmental applications. “For oil spills, you would have to make large sheets of these or find a way to weld sheets together (a process Hashim continues to work on),” Ajayan says.
“Oil-spill remediation and environmental cleanup are just the beginning of how useful these new nanotube materials could be,” Terrones adds.
“For example, we could 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 to fabricate robust and light composites for the automobile and plane industries.”
Hashim suggests his nanosponges may also work as membranes for filtration.
“I don’t think anybody has created anything like this before,” Ajayan says. “It’s a spectacular nanostructured sponge.”
The paper’s additional authors contributed to the research from Rice, the University of Vigo, Spain, Oak Ridge National Laboratory, Rensselaer Polytechnic Institute, University of Illinois at Urbana-Champaign, Instituto de Microelectrónica de Madrid, Air Force Research Laboratory, Arizona State University, and the Université Catholique de Louvain, Belgium.
The National Science Foundation and the Air Force Office of Scientific Research Project MURI program for the synthesis and characterization of 3-D carbon nanotube solid networks supported the research.
More news from Rice University: www.media.rice.edu/media/