Recently, there has been a strong demand for reliable
and selective solid-state NOx sensors for environmental
monitoring and automotive applications [1]. Several ntype
semiconducting oxides, such as SnO2, TiO2, ZnO,
WO3, and α-Fe2O3, have been studied for applications in
NOx sensors [2–6]. WO3 among these is the single oxide
showing the highest sensitivity to NOx with characteristics
good enough to consider its practical application for
air-quality monitoring [7,8].
If the responses of gas sensitive elements for reactive
gases are to be improved, surface reactions are much
more relevant than bulk changes, so the specific surface
area of the films must be as large as possible. The
screen-printing method is adequate for satisfying this requirement.
Also, a nanocrystalline material for which
the surface-to-bulk ratio is much greater than that of
a material with large grains may have a porosity which
yields a large interface between the solid and the gaseous
medium. In these respects, nanocrystalline WO3 thick
films are very desirable for highly sensitive NO2 gas sensors.
We report here a method for preparing nanosized
WO3 powders by using a sol-precipitation method with a tungsten-ethoxide solution synthesized directly by promoting
a reaction of WCl6 and ethanol without a catalyst.
Also, we discuss the fabrication of nanocrystalline
WO3 thick films and the effect of the particle sizes of the
powders on the electrical properties of those thick films
in air and NO2.