A
team of University of Maryland nanotechnology researchers has solved
one of the most vexing challenges hindering the use of carbon
nanomaterials for better electrical energy storage and for enhancing the
fluorescence sensing capabilities of biosensors. The findings are
published in the July 12 issue of Nature Communications.

The
breakthrough research was led by Assistant Professor YuHuang Wang of
the Department of Chemistry and Biochemistry and conducted in the
university’s Nanostructures for Electrical Energy Storage center (an
Energy Frontier Research Center of the Department of Energy),
Northwestern University, and the Maryland NanoCenter.

Carbon
nanotubes (CNTs) are recognized has having enormous potential. They are
some of the most conductive structures ever made—highly efficient
electrodes with enormous surface area. To take full advantage of these
properties, however, CNTs must be soluble—that is, have the ability to
be dispersed in a liquid environment or to evenly coat a solid composite
material. Unfortunately, in their raw state CNTs are insoluble; they
clump together rather than disperse.

For
more than a decade, researchers have been developing new chemical
processes to address this challenge. One idea has been to create
permanent defects on the surfaces of CNTs and “functionalize” them so
they are soluble. Unfortunately, this also has the undesired side effect
of quickly destroying the CNTs’ electrical and optical properties.

Wang
and his team have developed a new functionalization process for CNTs
that delivers solubility and preserves electrical and optical
properties. They purposefully functionalize defects on the tubes in
useful—not random—places, creating strategic “functional groups.” These
carefully placed molecular groups allow CNTs to readily disperse while
retaining their optical properties and ability to conduct electric
current in large regions along the tube.

The
challenge has been to control the chemical reactions that produce the
functional groups on the CNTs. By using a chemical process called
Billups-Birch reductive alkylcarboxylation, Wang’s team found they could
progressively add new functional groups to the CNT wall in a controlled
way without introducing unintended new defects.

When
the CNTs are immersed in a chemical solution for a specific length of
time, the functionalized groups on the nanotubes lengthen by a
predictable amount. Each time the process is repeated, or as the time in
the solution increases, the sections grow longer. When the CNTs are
viewed under a special, high magnification electron microscope, it is
evident that the functionalization has progressed lengthwise along the
tube.

The
propagation can initiate from either naturally occurring or
intentionally introduced defects. Because the propagation mechanism
confines the reaction and strategically controls where the functional
groups grow, Wang’s team can produce clustered functional groups at a
controlled, constant propagation rate. It is the first clearly
established wet chemistry process that does so.

The
breakthrough makes it possible to create new functional structures such
as “banded” nanotubes with alternating segments of functionalized and
intact regions. The functionalized regions keep the CNTs from clumping,
making them among the most water-soluble CNTs known. At the same time,
the bands of intact, non-functionalized regions of the CNTs allow
electrical and optical properties to be retained.

“This
is important for the future use of these materials in batteries and
solar cells where efficient charge collection and transport are sought,”
Wang explains. “These CNTs also could be used as highly sensitive
biochemical sensors because of their sharp optical absorption and
long-lived fluorescence in the near infrared regions where tissues are
nearly optically transparent.”

“This
is a major step towards building the controlled nanostructures needed
to understand electrochemical science and its value for energy
solutions,” says University of Maryland NanoCenter Director, Professor
Gary Rubloff, a collaborator on the project.

The
research team also includes theoretical chemist Professor George Schatz
of Northwestern University, postdoctoral associates and graduate
students Shunliu Deng, Yin Zhang, Alexandra Brozena, who are equal
contribution first authors, as well as Maricris Mayes, Parag Banerjee
and Maryland NanoCenter staff member Wen-An Chiou.

“Confined propagation of covalent chemical reactions on single-walled carbon nanotubes”

Source: University of Maryland