Images of carbon nanofibers grown from nickel nanoparticle catalysts: (left) without removing the ligands and (right) after removing the ligands from the nanoparticles before nanofiber growth. Note how the nanofibers grown from nanoparticles with ligands are more uniform in diameter and distribution. Credit: North Carolina State Univ.
Carbon nanofibers hold promise for technologies ranging from medical imaging
devices to precise scientific measurement tools, but the time and expense
associated with uniformly creating nanofibers of the correct size has been an obstacle.
Now, a study from North Carolina State Univ. demonstrates an improved method
for creating carbon nanofibers of specific sizes, as well as explaining the
science behind the method.
“Carbon nanofibers have a host of potential applications, but their utility is
affected by their diameter—and controlling the diameter of nanofibers has
historically been costly and time-consuming,” says Dr. Anatoli Melechko, an
associate professor of materials science and engineering at NC State and
co-author of a paper describing the study.
Specifically, the researchers have shown that nickel nanoparticles coated with
a ligand shell can be used to grow carbon nanofibers that are uniform in
diameter. Ligands are small organic molecules that have functional groups that
bond directly to metals. Nickel nanoparticles are of particular interest because—at
high temperatures—they can serve as catalysts for growing carbon nanofibers.
“What we learned is that the ligand shell, which is composed of
trioctylphosphine, undergoes chemical changes at high temperatures—gradually
transforming into a graphite-like shell,” says Dr. Joe Tracy, a co-author of
the paper and assistant professor of materials science and engineering at NC
State. “These ‘graphitic’ shells prevent the nickel nanoparticles from lumping
together at elevated temperatures, which is a problem for high-temperature
applications involving nanoparticles.”
Using nanoparticles to grow nanofibers is useful, because the fibers tend to
have the same diameter as the nanoparticles they are growing from. If you need nanofibers
that are 20 nm in diameter, you would simply use nanoparticles that are 20 nm
in diameter as your catalyst.
“This is why controlling the diameter of the nanoparticles is important. If
they begin to lump together at high temperatures, you end up growing nanofibers
of many different, larger sizes,” Melechko says. “This research gives us a
better fundamental understanding of the relationship between nickel
nanoparticles, ligands and carbon nanofiber synthesis.”
Using nanoparticles to grow nanofibers has another benefit—it allows you to
define where the nanofibers grow and how they are arranged. If you need the
nanofibers to grow in a specific pattern, you would arrange the nanoparticles
in that pattern before growing the fibers.
The paper, “Effects of Ligand Monolayers on Catalytic Nickel Nanoparticles
for Synthesizing Vertically Aligned Carbon Nanofibers,” was published in ACS
Applied Materials & Interfaces.