The frequency at which droplets emerge is controlled by an acoustic trigger, which can be tuned so that each droplet containing a protein or virus meets an incoming pulse of x-rays.
graphite oxide (GO) into full-fledged supercapacitors turns out to be
simple. But until a laboratory at Rice University figured out how, it
was anything but obvious.
Professor Pulickel Ajayan and his team discovered they could transform a
sheet of GO into a functional supercapacitor by writing patterns into
it with a laser. Scientists already knew that the heat of a laser could
convert GO?the oxidized form of graphite, or carbon-based pencil
lead?into electrically conducting reduced graphite oxide (RGO). By
writing patterns of RGO into thin sheets of GO, the Rice researchers
effectively turned them into free-standing supercapacitors with the
ability to store and release energy over thousands of cycles.
discovery was reported this week in the online edition of Nature
Nanotechnology. The surprising find was that GO, when hydrated, can hold
ions and serve as a solid electrolyte and an electrically insulating
is quite easy, as GO soaks up water like a sponge and can hold up to 16% of its weight,” said Wei Gao, lead author of the paper and a
graduate student in the Ajayan Lab.
fundamental breakthrough here is that GO, when it contains water, acts
as an ionic conductor,” said Ajayan, Rice’s Benjamin M. and Mary
Greenwood Anderson Professor in Mechanical Engineering and Materials
Science and of chemistry. “So we’re able to convert a sheet of GO into a
supercapacitor without adding anything. All you need are a pattern and
the electrodes, and you have a device. Of course the devices also
perform in the presence of external electrolytes, which is even better.
think you’re going to see a lot of tiny devices that need smaller power
sources. Intermediate-sized devices might also be powered by this
material; it’s very scalable.”
a control experiment, the team sucked all the water out of an
RGO-GO-RGO device in a vacuum to kill its ionic conductivity. Exposing
it to air for three hours completely restored its supercapacitor
function, another potentially handy characteristic.
build a fully functional supercapacitor, conducting electrode materials
need to be separated by an insulator that contains the electrolyte.
When laser-written patterns of conducting RGO are separated by GO, the
material becomes an energy storage device, Gao said. The patterns can be
layered top and bottom or on the same plane.
their experiments, heat from a laser at Rice’s Oshman Engineering
Design Kitchen sucked oxygen out of the surface to create the dark,
porous RGO, which provided a level of resistance and restrained the
GO-contained ions until their controlled release. Patterns were written
in the GO with nearly one-micron accuracy.
Essentially, the devices exhibited good electrochemical performance — without the chemicals.
of the devices at Rice and by colleagues at the University of Delaware
showed their performance compares favorably with existing thin-film
micro-supercapacitors. They exhibit proton transport characteristics
similar to that of Nafion, a commercial electrolyte membrane discovered
in the 1960s, Ajayan said.
the lab won’t make flat supercapacitors in bulk anytime soon, Ajayan
said the research opens the way to interesting possibilities, including
devices for use in fuel cells and lithium batteries.
said the discovery is surprising “because a lot of people have been
looking at graphite oxide for five or 10 years now, and nobody has seen
what we see here. We’ve discovered a fundamental mechanism of graphite
oxide? an ionic conducting membrane?that is useful for applications.”
of the paper are graduate student Neelam Singh, former postdoctoral
researcher Li Song and Lijie Ci, postdoctoral researcher Zheng Liu,
research scientist Arava Leela Mohana Reddy and Robert Vajtai, a faculty
fellow in mechanical engineering and materials science, all of Rice;
and graduate student Qing Zhang and Binngqing Wei, an associate
professor of mechanical engineering, both at the University of Delaware.
Nanoholdings LLC funded the research.
Direct laser writing of micro-supercapacitors on hydrated graphite oxide films