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Reginald M. Penner

Jay A. Switzer

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NEWS OF THE WEEK
NANOTECHNOLOGY
September 24, 2001
Volume 79, Number 39
CENEAR 79 39 p. 14
ISSN 0009-2347
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NANOWIRE-BASED HYDROGEN SENSOR
Device responds faster than standard H2 sensors via unique mechanism

RON DAGANI

Reginald M. Penner had a nifty idea: Improve the speed with which a sensor can detect hydrogen gas by using thinner palladium wires than in a conventional H2 sensor. The response time should be faster, he reasoned, "because hydrogen has to diffuse a much shorter distance in a small wire."

NANOWIRE FABRICATION Palladium nanowires are electrochemically grown on the steps of a graphite surface and then embedded in an adhesive film. The film is then peeled off the graphite, flipped over, and placed on a glass slide.
ADAPTED FROM SCIENCE
The idea panned out, allowing Penner, a chemistry professor at the University of California, Irvine, and coworkers to fabricate a miniaturized sensor that Penner says works much better than conventional H2 sensors [Science, 293, 2227 (2001)].

Here's the twist: Compared with traditional H2 sensors, the new sensor operates via a totally different--and unexpected--mechanism. In conventional sensors, H2 is detected when it adsorbs to a macroscopic palladium wire, boosting the wire's electrical resistance. By contrast, in Penner's sensor, which is based on an array of up to 100 palladium wires having a diameter between 100 and 300 nm, H2 absorption lowers the wire's resistance by changing the wire's structure.

When palladium absorbs H2, a new phase (a palladium hydride) is produced, causing the metal lattice to expand by 3.5%. As a result, the palladium grains swell, closing nanoscopic gaps present in the wires. This, in turn, allows more current to pass through the wire.

"As far as I can tell, it's a unique sensing mechanism," comments Jay A. Switzer, an inorganic materials chemist at the University of Missouri, Rolla.

The sensor's performance is impressive, he adds. Compared with conventional H2 sensors, the new sensor responds faster (in less than 75 milliseconds), is less sensitive to other gases such as carbon monoxide and methane, and requires much less power (less than 100 nanowatts).

Because the detector is so small and can run on a watch battery, it could be made into a lapel pin to warn workers of a hydrogen leak. The device can detect H2 concentrations in air from 1 to 10%; concentrations of 4% and above are explosive.

Penner is patenting the new sensor and hopes to interest a manufacturer to license the technology and take part in its further development.

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