The nanofibers lean in different directions depending on where they are located in relation to the chromium grid, because the ions are being drawn to the grid and strike the catalysts at various angles.
Researchers from North
Carolina State University, the Oak Ridge National
Laboratory, and CFD Research Corporation have found a new way to develop
straight carbon nanofibers on a transparent substrate. Growing such nanofiber
coatings is important for use in novel biomedical research tools, solar cells,
water repellent coatings, and others. The technique utilizes a charged chromium
grid, and relies on ions to ensure the nanofibers are straight, rather than
curling—which limits their utility.
“This is the first time, that I know of, where someone has been able to grow
straight carbon nanofibers on a clear substrate,” says Anatoli Melechko, PhD, an
associate professor of materials science and engineering at NC State and coauthor
of a paper describing the research. “Such nanofibers can be used as
gene-delivery tools. And a transparent substrate allows researchers to see how
the nanofibers interact with cells, and to manipulate this interaction.”
Specifically, the nanofibers can be coated with genetic material and then
inserted into the nucleus of a cell—for example, to facilitate gene therapy
research. The transparent substrate improves visibility because researchers can
shine light through it, creating better contrast and making it easier to see
what’s going on.
The researchers also learned that ions play a key role in ensuring that the
carbon nanofibers are straight. To understand that role, you need to know how
the technique works.
The nanofibers are made by distributing nickel nanoparticles evenly on a
substrate made of fused silicon (which is pure silicon dioxide). The substrate
is then overlaid with a fine grid made of chromium, which serves as an
electrode. The substrate and grid are then placed in a chamber at 700 C, which
is then filled with acetylene and ammonia gas. The chrome grid is a negatively
charged electrode, and the top of the chamber contains a positively charged
Electric voltage is then applied to the two electrodes, creating an electric
field in the chamber that excites the atoms in the acetylene and ammonia gas.
Some of the electrons in these atoms break away, creating free electrons and
positively charged atoms called ions. The free electrons accelerate around the
chamber, knocking loose even more electrons. The positively charged ions are
drawn to the negatively charged grid on the floor of the chamber.
Meanwhile, the nickel nanoparticles are serving as catalysts, reacting with
the carbon in the acetylene gas (C2H2) to create graphitic carbon nanofibers. The
catalyst rides on the tip of the nanofiber that forms beneath it, like a
rapidly growing pillar. The term graphitic means that the nanofibers have
carbon atoms arranged in a hexagonal structure—like graphite.
One problem with growing carbon nanofibers is that the surface of the
catalyst can become obstructed by a carbon film that blocks catalytic action,
preventing further nanofibers growth. Here’s where those ions come in.
The ions being drawn to the chromium grid are moving very quickly, and they
choose the shortest possible route to reach the negatively-charged metal. In
their rush to reach the grid, the ions often collide with the nickel catalysts,
knocking off the excess carbon—and allowing further nanofibers growth.
Because the ions are being drawn to the chromium grid, the angle at which
they strike the catalysts depends on where the catalyst is located relative to
the grid. For example, if you are looking down at the grid, a catalyst just to
the right of the grid will appear to be leaning right—because ions would have
been striking the right side of the catalyst in an attempt to reach the grid.
These nanofibers are still straight—they don’t curl up—they simply lean in one
direction. The bulk of the nanofibers, however, are both straight and
“This finding gives us an opportunity to create new reactors for creating
nanofibers, building in the chromium grid,” Melechko says.