Selectivity, stability and repeatability of In2O3 thin films towards NOx at high temperatures (≥500 °C)

Solid state sensors need to be selective to the target gas and stable in their performance in the sensing environment. We have investigated the selectivity, stability, and repeatability of indium oxide (In2O3) thin film sensor to detect NOx (25 ppm) in presence of other exhaust gas pollutants including H-2, NH3 and CO2 at high operating temperatures (>= 400 degrees C) in N-2 carrier gas. When exposed individually to 25 ppm NOx, 100 ppm NH3, 5000 ppm H-2 and 1000 ppm CO2, the sensor is selective to NOx with higher sensor response (S similar to 23) to NOx species compared to other gases (S similar to 1). The sensor response (S) drastically decreases when either NH3 or H-2 are added to the NOx test gas. In2O3 samples without promoter layers exhibited higher sensor response to NOx in presence of 100 ppm NH3 when compared to Au promoter samples. Fourier transform infrared spectroscopy (FT-IR) was employed in order to investigate the gas-phase reactions between NOx and NH3 or H-2 at high temperatures (>400 degrees C). At >500 degrees C, NH3 and H-2 reduce NOx to H2O and N-2 which may explain the drastic reduction in NOx response at high temperatures (>500 degrees C). Exposure to H-2 also etched the In2O3 film which comprises the stability when the sensor is operated in a H-2 rich environment (>2500 ppm H-2). In2O3 thin films deposited in an oxygen environment (25% O-2) which presented the highest and quickest response were repeatable to 25 ppm NOx exposures with overall variation in sensor response <10%. Published by Elsevier B.V.