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Graphene Foam Detects Explosives, Emissions Better Than Today’s Gas Sensors
Rensselaer Polytechnic Institute Engineering
Researchers Overcome Decade-Old Hurdle Hampering Development of
Nanostructure-based Gas Sensors
Photo Credit: Nikhil Koratkar
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A new study from Rensselaer Polytechnic Institute
demonstrates how graphene foam can outperform leading
commercial gas sensors in detecting potentially dangerous and
explosive chemicals. The discovery opens the door for a new
generation of gas sensors to be used by bomb squads, law
enforcement officials, defense organizations, and in various
industrial settings.
The new sensor successfully and repeatedly measured ammonia
(NH3) and nitrogen dioxide (NO2) at
concentrations as small as 20 parts-per-million. Made from
continuous graphene nanosheets that grow into a foam-like
structure about the size of a postage stamp and thickness of
felt, the sensor is flexible, rugged, and finally overcomes the
shortcomings that have prevented nanostructure-based gas
detectors from reaching the marketplace.
Results of the study were published today in the journal
Scientific Reports, published by Nature Publishing
Group. See the paper, titled “High Sensitivity Gas Detection
Using a Macroscopic Three-Dimensional Graphene Foam Network,”
at:
http://www.nature.com/srep/2011/111123/
srep00166/full/srep00166.html
“We are very excited about this new discovery, which we
think could lead to new commercial gas sensors,” said
Rensselaer Engineering Professor Nikhil Koratkar,
who co-led the study along with Professor Hui-Ming Cheng at the
Shenyang National Laboratory for Materials Science at the
Chinese Academy of Sciences. “So far, the sensors have shown to
be significantly more sensitive at detecting ammonia and
nitrogen dioxide at room temperature than the commercial gas
detectors on the market today.”
Watch a short video of Koratkar talking about this research
at: http://youtu.be/RHVW2kCr3Iw
Over the past decade researchers have shown that individual
nanostructures are extremely sensitive to chemicals and
different gases. To build and operate a device using an
individual nanostructure for gas detection, however, has proven
to be far too complex, expensive, and unreliable to be
commercially viable, Koratkar said. Such an endeavor would
involve creating and manipulating the position of the
individual nanostructure, locating it using microscopy, using
lithography to apply gold contacts, followed by other slow,
costly steps. Embedded within a handheld device, such a single
nanostructure can be easily damaged and rendered inoperable.
Additionally, it can be challenging to “clean” the detected gas
from the single nanostructure.
The new postage stamp-sized structure developed by Koratkar
has all of the same attractive properties as an individual
nanostructure, but is much easier to work with because of its
large, macroscale size. Koratkar’s collaborators at the Chinese
Academy of Sciences grew graphene on a structure of nickel
foam. After removing the nickel foam, what’s left is a large,
free-standing network of foam-like graphene. Essentially a
single layer of the graphite found commonly in our pencils or
the charcoal we burn on our barbeques, graphene is an
atom-thick sheet of carbon atoms arranged like a nanoscale
chicken-wire fence. The walls of the foam-like graphene sensor
are comprised of continuous graphene sheets without any
physical breaks or interfaces between the sheets.
Koratkar and his students developed the idea to use this
graphene foam structure as a gas detector. As a result of
exposing the graphene foam to air contaminated with trace
amounts of ammonia or nitrogen dioxide, the researchers found
that the gas particles stuck, or adsorbed, to the foam’s
surface. This change in surface chemistry has a distinct impact
upon the electrical resistance of the graphene. Measuring this
change in resistance is the mechanism by which the sensor can
detect different gases.
Additionally, the graphene foam gas detector is very
convenient to clean. By applying a ~100 milliampere current
through the graphene structure, Koratkar’s team was able to
heat the graphene foam enough to unattach, or desorb, all of
the adsorbed gas particles. This cleaning mechanism has no
impact on the graphene foam’s ability to detect gases, which
means the detection process is fully reversible and a device
based on this new technology would be low power—no need for
external heaters to clean the foam—and reusable.
Koratkar chose ammonia as a test gas to demonstrate the
proof-of-concept for this new detector. Ammonium nitrate is
present in many explosives and is known to gradually decompose
and release trace amounts of ammonia. As a result, ammonia
detectors are often used to test for the presence of an
explosive. A toxic gas, ammonia also is used in a variety of
industrial and medical processes, for which detectors are
necessary to monitor for leaks.
Results of the study show the new graphene foam structure
detected ammonia at 1,000 parts-per-million in 5 to 10 minutes
at room temperature and atmospheric pressure. The accompanying
change in the graphene’s electrical resistance was about 30
percent. This compared favorably to commercially available
conducting polymer sensors, which undergo a 30 percent
resistance change in 5 to 10 minutes when exposed to 10,000
parts-per-million of ammonia. In the same time frame and with
the same change in resistance, the graphene foam detector was
10 times as sensitive. The graphene foam detector’s sensitivity
is effective down to 20 parts-per-million, much lower than the
commercially available devices. Additionally, many of the
commercially available devices require high power consumption
since they provide adequate sensitivity only at high
temperatures, whereas the graphene foam detector operates at
room temperature.
Koratkar’s team used nitrogen dioxide as the second test
gas. Different explosives including nitrocellulose gradually
degrade, and are known to produce nitrogen dioxide gas as a
byproduct. As a result, nitrogen dioxide also is used as a
marker when testing for explosives. Additionally, nitrogen
dioxide is a common pollutant found in combustion and auto
emissions. Many different environmental monitoring systems
feature real-time nitrogen dioxide detection.
The new graphene foam sensor detected nitrogen dioxide at
100 parts-per-million by a 10 percent resistance change in 5 to
10 minutes at room temperature and atmospheric pressure. It
showed to be 10 times more sensitive than commercial conducting
polymer sensors, which typically detect nitrogen dioxide at
1,000 part-per-million in the same time and with the same
resistance chance at room temperature. Other nitrogen dioxide
detectors available today require high power consumption and
high temperatures to provide adequate sensitivity. The graphene
foam sensor can detect nitrogen dioxide down to 20
parts-per-million at room temperature.
“We see this as the first practical
nanostructure-based gas detector that’s viable for
commercialization,” said Koratkar, a professor in the Department of Mechanical,
Aerospace, and Nuclear Engineering at Rensselaer. “Our
results show the graphene foam is able to detect ammonia and
nitrogen dioxide at a concentration that is an order of
magnitude lower than commercial gas detectors on the market
today.”
The graphene foam can be engineered to detect many different
gases beyond ammonia and nitrogen dioxide, he said.
Studies have shown the electrical conductivity of an
individual nanotube, nanowire, or graphene sheet is acutely
sensitive to gas adsorbtion. But the small size of individual
nanostructures made it costly and challenging to develop into a
device, plus the structures are delicate and often don’t yield
consistent results.
The new graphene foam gas sensor overcomes these challenges.
It is easy to handle and manipulate because of its large,
macroscale size. The sensor also is flexible, rugged, and
robust enough to handle wear and tear inside of a device. Plus
it is fully reversible, and the results it provides are
consistent and repeatable. Most important, the graphene foam is
highly sensitive, thanks to its 3-D, porous structure that
allows gases to easily adsorb to its huge surface area. Despite
its large size, the graphene foam structure essentially
functions as a single nanostructure. There are no breaks in the
graphene network, which means there are no interfaces to
overcome, and electrons flow freely with little resistance.
This adds to the foam’s sensitivity to gases.
“In a sense we have overcome the Achilles’ heel of
nanotechnology for chemical sensing,” Koratkar said. “A single
nanostructure works great, but doesn’t mean much when applied
in a real device in the real world. When you try to scale it up
to macroscale proportions, the interfaces defeats what you’re
trying to accomplish, as the nanostructure’s properties are
dominated by interfaces. Now we’re able to scale up graphene in
a way that the interfaces are not present. This allows us to
take advantage of the intrinsic properties of the
nanostructure, yet work with a macroscopic structure that gives
us repeatability, reliability, and robustness, but shows
similar sensitivity to gas adsorbtion as a single
nanostructure.”
Along with Koratkar, co-authors of the paper are: Rensselaer
graduate students Fazel Yavari and Abhay Varghese Thomas; along
with professors W.C. Ren, H.M. Cheng and graduate student Z.P.
Chen of the Shenyang National Laboratory for Materials Science
at the Chinese Academy of Sciences.
This research was supported in part by the Advanced Energy
Consortium (AEC), the National Science Foundation of China, and
the Chinese Academy of Sciences.
For more information on Koratkar’s graphene research at
Rensselaer, visit:
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Published
November 23,
2011 |
Contact: Michael Mullaney
Phone: (518) 276-6161
E-mail: mullam@rpi.edu |
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