Ultrasensitive WO 3 gas sensors for NO 2 detection in air and low oxygen environment

We report here on the results of a study into the response of a tungsten oxide based low power MEMS gas sensor to ppb of nitrogen dioxide at low levels of ambient oxygen. It was found that the resistive gas sensors not only had a high sensitivity to NO 2 (3.4%/ppb vs. 0.2%/ppb obtained for commercial MOX) but can still operate reliably at lower oxygen levels (down to 0.5%) - albeit with slightly longer response and recovery times. The optimal operating temperature was determined to be ca. 350 °C and so easily within the range of a MEMS based SOI CMOS substrate. The response was sensitive to significant changes in ambient humidity, but was found to have low cross-sensitivity to CO, hydrogen, methane, and acetone even at much higher ppm levels. We believe that these tungsten oxide gas sensors could be exploited in harsh applications, i.e. with a low oxygen (lean) environment often associated in the exhaust gases from combustion systems.