Tungsten oxide is shown to be a very promising material for the fabrication
of highly selective ammonia sensors. Films of WO3 were deposited onto a si
licon substrate by means of the drop-coating method. Then, the films were a
nnealed in dry air at two different temperatures (300 and 400 degrees C). X
-ray photoelectron spectroscopy was used to investigate the composition of
the films. Tungsten appeared both in WO2 and WO3 oxidation states, but the
second state was clearly dominant. Scanning electron microscopy results sho
wed that the oxide was amorphous or nanocrystalline. The WO3-based devices
were sensitive to ammonia vapors when operated between 250 and 350 degrees
C. The optimal operating temperature for the highest sensitivity to ammonia
was 300 degrees C. Furthermore, when the devices were operated at 300 degr
ees C, their sensitivity to other reducing species such as ethanol, methane
, toluene, and water vapor was significantly lower, and this resulted in a
high selectivity to ammonia. A model for the sensing mechanisms of the fabr
icated sensors is proposed. (C) 2000 The Electrochemical Society. S0013-465
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