12-01 :: January 2012

nanotimes News in Brief

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 com- monly 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 chose ammonia and nitrogen dioxide as test gases to demonstrate the proof-of-concept for this new detector, because ammonium nitrate is present in many explosives. Different explosives including nitrocellulose gradually degrade, and are known to produce nitrogen dioxide gas as a byproduct.

As a 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 under- go a 30 percent resistance change in 5 to 10 minutes when exposed to 10,000 parts-per-million of ammo- nia.

In the same time frame and with the same change in resistance, the graphene foam detector was 10 times

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as sensitive. The graphene foam detector’s sensitivity is effective down to 20 parts-per-million, much lower than the commercially available devices. Additio- nally, many of the commercially available devices require high power consumption since they provide adequate sensitivity only at high temperatures, whe- reas the graphene foam detector operates at room temperature.

“We see this as the first practical nanostructure- based gas detector that’s viable for commercializa- tion,” 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 new graphene foam gas sensor 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 gra- phene 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 resi- stance. 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.