Nitrogen oxides-sensing characteristics of WO₃-based nanocrystalline thick film gas sensor

TiO₂ added tungsten trioxides with grain size of about 3 nm, were synthesized by sol-coprecipitation of WCl₆ and TiCl₄ solution with ammonium hydroxide and surfactant. Tungsten trioxide and titanium oxides were also synthesized by sol-precipitation from WCl₆ and TiCl₄, respectively. After calcining these materials, the thick film sensors were fabricated by the screen-printing technology. These materials were investigated about the characteristics of their structural properties by means of X-ray diffraction (XRD) measurements, the surface morphology by the SEM photography and specific surface area by the BET method. The grain size of precursor obtained by coprecipitation of TiCl₄ (4 wt.%) and WCl₆ was about 3 nm and that of WO₃ precursor by simple precipitation was about 8 nm. TiO₂ added WO₃ nanocrystalline thick films showed higher sensitivity to NO_x over wide temperature ranges, than those of pure WO₃ or the simple mixture of WO₃ and TiO₂ powder. It was shown that the particle size and surface area of sensing materials make an important role of the sensing characteristics of WO₃-based thick film sensor. The TiO₂ added WO₃ nanocrystalline sensor showed excellent sensitivity to low level NO_x concentrations (0.5-30 ppm), fast response, recovery reaction, and good selectivity at 350 °C.