NO2 sensitivity of a heterojunction sensor based on WO3 and doped SnO2

The NO2 sensing properties of a heterojunction gas sensor formed between WO3 and 3 wt.% Nd2O3 doped SnO2 were evaluated using DC and AC measurements at 300 °C and compared with the responses obtained from single component tungsten oxide and doped tin oxide sensors. The heterojunction sensor showed rectifying behaviour with a forward bias direction defined as SnO2−/WO3+. The heterojunction sensor and the component materials were all found to be sensitive to NO2, showing almost linear resistance increases with concentration when exposed to 0–5 ppm NO2. However, the sensitivity of the heterojunction sensor was almost an order of magnitude greater than either of the component sensors. Under reverse bias the effective sensor resistance did not change with NO2 concentration. AC impedance spectroscopy demonstrated that the improved NO2 response of the heterojunction sensor occurred primarily as a result of increases in the forward bias resistance of the heterojunction interface. The long-term stability of the heterojunction sensor was characterised over 53 days using continuous DC resistance measurements, under an applied forward bias voltage of 0.4 V. The sensor showed good stability, with low drift and a reproducible response to NO2 in the concentration range 0–10 ppm throughout the test period.