Highly Sensitive Detection of Dilute Nitrogen Dioxide Using Tungsten Oxide Thick Film Sensor

Abstract

WO₃ thin and thick film sensors have been investigated for highly sensitive detection of dilute NO₂. WO₃ powders were prepared by pyrolysis and precipitation method, and calcined at various temperatures. By using these WO₃ powders, the WO₃ thin and thick film sensors were fabricated on Au microelectrodes by suspension dropping. The entirely different thickness effect on NO₂ sensitivity was observed depending on the preparation method of powder. For the powder prepared by pyrolysis, the sensitivity decreased with increasing film thickness. On the other hand, the unique thickness effect was observed for powder prepared by precipitation, i.e., the NO₂ sensitivity had a maximum at optimal film thickness. The WO₃ thick film sensor using 400°C calcined powder showed the highest sensitivity at thickness of 7 μm. The sensitivity was as high as 45 to 0.05 ppm NO₂ at 200°C, and hardly influenced by coexisting water vapor, suggesting the possibility of environmental monitoring using WO₃ sensor. The surface morphology and the effect of oxygen partial pressure were investigated in order to clarify the unique thickness effect.