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 H2, NH3 and CO2 at high operating temperatures (≥400 °C) in N2 carrier gas. When exposed individually to 25 ppm NOx, 100 ppm NH3, 5000 ppm H2 and 1000 ppm CO2, the sensor is selective to NOx with higher sensor response (S ∼ 23) to NOx species compared to other gases (S ∼ 1). The sensor response (S) drastically decreases when either NH3 or H2 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 H2 at high temperatures (>400 °C). At >500 °C, NH3 and H2 reduce NOx to H2O and N2 which may explain the drastic reduction in NOx response at high temperatures (>500 °C). Exposure to H2 also etched the In2O3 film which comprises the stability when the sensor is operated in a H2 rich environment (>2500 ppm H2). In2O3 thin films deposited in an oxygen environment (25% O2) which presented the highest and quickest response were repeatable to 25 ppm NOx exposures with overall variation in sensor response <10%.