Graphene foam sensor built at RPI. Photo Credit: Nikhil Koratkar |
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 in the journal Scientific Reports, published by Nature Publishing Group.
“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.”
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.
Koartkar
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%. 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.
Graphene foam sensor built at RPI. Photo Credit: Nikhil Koratkar |
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.
High Sensitivity Gas Detection Using a Macroscopic Three-Dimensional Graphene Foam Network
Watch a short video of Koratkar talking about this research at: http://youtu.be/RHVW2kCr3Iw
