Gas sensors are ubiquitous. They have enabled realization of clean and efficient automobile engines as well as widespread use for detection of poisonous gases. Most of these sensors are designed for large volumes, but nanowire sensors have the advantage that they could be easily integrated into chips and, in principle, tailored to detect even single molecules. Now, scientists from Wuhan University and Peking University in China and Nanyang Technological University in Singapore1 show that CeO2 (ceria oxide) nanowires coated with Pt nanocyrstals can be effective gas sensors of carbon monoxide.

Jinchai Li and his colleagues used CeO2, a catalyst widely used for removing pollutants like carbon monoxide, nitrous oxide and hydrocarbons. These catalytic properties actually improve in nanostructure form.

Fig. 1: The ceria oxide nanowires grow in a sort of ‘nest’. To make the actual sensors, a single nanowire is pulled from the bundle and deposited on a chip.

The CeO2 nanowires were several microns long and 50 nm in diameter (Fig.1) and were coated with Pt nanocrystals ~ 3.5 nm in diameter.

How do the nanowires detect a change in gas composition? As prepared, a certain amount of oxygen is adsorbed on the nanowire surface. Since oxygen tends to take conducting electrons from the wire surface, the resistance of the wires increases with increasing adsorbed oxygen. Gasses like carbon monoxide, though, tend to remove oxygen, effectively releasing conduction electrons back into the wire and decreasing the resistance of the wire. For example, at 50 ppm of carbon monoxide in air, the resistance of the wires dropped to half what it is in air. The role of the nanocrystalline Pt was to increase the concentration of oxygen species on the surface of the wire in air.

The researchers tested their nanowires in carbon monoxide, gasoline, hydrogen sulfide, hydrogen and ethanol. The detection ability of the nanowire for these other gasses is quite weak – an advantage for using the wires as selective carbon monoxide sensors.

“The nanowire sensors have an excellent long life performance with stable sensing characteristics,” says Li. “They could be useful for detecting carbon monoxide in industrial or mining applications or dangerous gas leaks in the kitchen or near the gas heater.”