Rice University graduate student Daniel Hashim holds samples of a carbon nanotube-based sponge that holds great potential for cleaning up oil spills. Credit: Jeff Fitlow/Rice University

Researchers
at Rice University and Penn State University have discovered that
adding a dash of boron to carbon while creating nanotubes turns them
into solid, spongy, reusable blocks that have an astounding ability to
absorb oil spilled in water.

That’s
one of a range of potential innovations for the material created in a
single step. 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.

The
researchers, who collaborated with peers in labs around the nation and
in Spain, Belgium and Japan, revealed their discovery in Nature’s online
open-access journal Scientific Reports.

Lead
author Daniel Hashim, a graduate student in the Rice lab of materials
scientist Pulickel Ajayan, said 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% 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 said a sample remained elastic after about 10,000
compressions in the lab. The sponge can also store the oil for later
retrieval, he said.

“These
samples can be made pretty large and can be easily scaled up,” said
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 said the sponges described in the
paper can absorb more than a hundred times their weight in oil.

Ajayan,
Rice’s Benjamin M. and Mary Greenwood Anderson Professor in Mechanical
Engineering and Materials Science and of chemistry, said 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 said.

“The
interactions happen as they grow, and the material comes out of the
furnace as a solid,” Ajayan said. “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 said. “The nanotubes have to connect either through
some clever way of creating topological defects, or they have to be
welded together.”

This carbon nanotube sponge created at Rice University can hold more than 100 times its weight in oil. Oil can be squeezed out or burned off, and the sponge reused. Credit: Jeff Fitlow/Rice University

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 University 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 said. “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 said.

“Oil-spill
remediation and environmental cleanup are just the beginning of how
useful these new nanotube materials could be,” Terrones added. “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 suggested his nanosponges may also work as membranes for filtration.

“I don’t think anybody has created anything like this before,” Ajayan said. “It’s a spectacular nanostructured sponge.”

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.

Covalently bonded three-dimensional carbon nanotube solids via boron induced nanojunctions

Source: Rice University