Treatment with a
microwave oven and chlorine removes stubborn iron catalyst residues from
carbon nanotubes,
according to researchers at Rice University and Swansea University.
The two-step process
may make them more suitable for sensitive applications.
(Credit: Virginia
Goméz Jiménez/Swansea University)
(January 22, 2016) Rice,
Swansea scientists use household oven to help decontaminate carbon
nanotubes
Amid all the fancy equipment found in a typical
nanomaterials lab, one of the most useful may turn out to be the humble
microwave oven.
A standard kitchen microwave proved effective as part of a
two-step process invented at Rice and Swansea universities to clean carbon
nanotubes.
Basic nanotubes are good for many things, like forming into
microelectronic components or electrically conductive fibers and composites;
for more sensitive uses like drug delivery and solar panels, they need to be as
pristine as possible.
Multiwalled
nanotubes before treatment with a Rice University/Swansea University
process to remove
catalyst residue from their surfaces and from inside.
The process begins
with heating the nanotubes in a standard kitchen microwave oven.
(Credit: Virginia
Goméz Jiménez/Swansea University)
Nanotubes form from metal catalysts in the presence of
heated gas, but residues of those catalysts (usually iron) sometimes remain
stuck on and inside the tubes. The catalyst remnants can be difficult to remove
by physical or chemical means because the same carbon-laden gas used to make
the tubes lets carbon atoms form encapsulating layers around the remaining
iron, reducing the ability to remove it during purification.
In the new process, treating the tubes in open air in a
microwave burns off the amorphous carbon. The nanotubes can then be treated
with high-temperature chlorine to eliminate almost all of the extraneous
particles.
The process was revealed today in the Royal Society of
Chemistry journal RSC Advances.
A multiwalled
carbon nanotube cleaned with a process developed at Rice University and
Swansea University
shows iron catalyst residue has been removed from the surface,
while most
particles have been removed from inside the nanotube’s walls.
The process is
expected to make nanotubes more suitable for applications
like drug delivery
and solar panels.
(Credit: Virginia
Goméz Jiménez/Swansea University)
The labs of chemists Robert Hauge, Andrew Barron and Charles
Dunnill led the study. Barron is a professor at Rice in Houston and at Swansea
University in the United Kingdom. Rice’s Hauge is a pioneer in nanotube growth
techniques. Dunnill is a senior lecturer at the Energy Safety Research
Institute at Swansea.
There are many ways to purify nanotubes, but at a cost,
Barron said. “The chlorine method developed by Hauge has the advantage of not
damaging the nanotubes, unlike other methods,” he said. “Unfortunately, many of
the residual catalyst particles are surrounded by a carbon layer that stops the
chlorine from reacting, and this is a problem for making high-purity carbon
nanotubes.”
The researchers gathered microscope images and spectroscopy
data on batches of single-walled and multiwalled nanotubes before and after
microwaving them in a 1,000-watt oven, and again after bathing them in an
oxidizing bath of chlorine gas under high heat and pressure. They found that
once the iron particles were exposed to the microwave, it was much easier to
get them to react with chlorine. The resulting volatile iron chloride was then
removed.
Eliminating iron particles lodged inside large multiwalled
nanotubes proved to be harder, but transmission electron microscope images
showed their numbers, especially in single-walled tubes, to be greatly
diminished.