A single flake of graphene oxide roughly 40 microns wide, seen under an electron microscope, sits atop a copper support. Such “giant” flakes form into a gel-like liquid crystal in solution. Image: Rice University/University of Colorado at Boulder |
Giant flakes of graphene oxide in water aggregate like a
stack of pancakes, but infinitely thinner, and in the process gain
characteristics that materials scientists may find delicious.
A new paper by scientists at Rice
University and the University of Colorado
details how slices of graphene in a solution arrange themselves to form a
nematic liquid crystal in which particles are free-floating but aligned.
That much was already known. The new twist is that if the
flakes—in this case, graphene oxide—are big enough and concentrated enough,
they retain their alignment as they form a gel. That gel is a handy precursor
for manufacturing metamaterials or fibers with unique mechanical and electronic
properties.
The team reported its discovery online in Soft Matter.
Rice authors include Matteo Pasquali, a professor of chemical and biomolecular
engineering and of chemistry; James Tour, the T.T. and W.F. Chao Chair in
Chemistry as well as a professor of mechanical engineering and materials
science and of computer science; postdoctoral research associate Dmitry Kosynkin;
and graduate students Budhadipta Dan and Natnael Behabtu. Ivan Smalyukh, an
assistant professor of physics at the University
of Colorado at Boulder, led research for his group, in which
Dan served as a visiting scientist.
“Graphene materials and fluid phases are a great
research area,” Pasquali says. “From the fundamental point of view,
fluid phases comprising flakes are relatively unexplored, and certainly so when
the flakes have important electronic properties.
“From the application standpoint, graphene and graphene
oxide can be important building blocks in such areas as flexible electronics
and conductive and high-strength materials, and can serve as templates for
ordering plasmonic structures,” he says.
By “giant,” the researchers referred to irregular
flakes of graphene oxide up to 10,000 times as wide as they are high. (That’s
still impossibly small: on average, roughly 12 microns wide and less than a
nanometer high.) Previous studies showed smaller bits of pristine graphene
suspended in acid would form a liquid crystal and that graphene oxide would do
likewise in other solutions, including water.
This time the team discovered that if the flakes are big
enough and concentrated enough, the solution becomes semisolid. When they
constrained the gel to a thin pipette and evaporated some of the water, the
graphene oxide flakes got closer to each other and stacked up spontaneously,
although imperfectly.
“The exciting part for me is the spontaneous ordering
of graphene oxide into a liquid crystal, which nobody had observed
before,” says Behabtu, a member of Pasquali’s laboratory. “It’s still
a liquid, but it’s ordered. That’s useful to make fibers, but it could also
induce order on other particles like nanorods.”
He says it would be a simple matter to heat the concentrated
gel and extrude it into something like carbon fiber, with enhanced properties
provided by “mix-ins.”
Testing the possibilities, the researchers mixed gold
microtriangles and glass microrods into the solution, and found both were
effectively forced to line up with the pancaking flakes. Their inclusion also
helped the team get visual confirmation of the flakes’ orientation.
The process offers the possibility of the large-scale
ordering and alignment of such plasmonic particles as gold, silver, and
palladium nanorods, important components in optoelectronic devices and
metamaterials, they report.
Behabtu adds that heating the gel “crosslinks the
flakes, and that’s good for mechanical strength. You can even heat graphene oxide
enough to reduce it, stripping out the oxygen and turning it back into
graphite.”
