Heterojunction gas sensors for environmental NO2 and CO2 monitoring

The NO2 and CO2 sensing properties of a heterojunction gas sensor formed between n-type ZnO and a p-type composite based on a mixture of BaTiO3/CuO/La2O3 have been evaluated and compared with the performance of its component p- and n-type materials. It was found that the individual ZnO and BaTiO3/CuO/La2O3 sensors showed resistance increases when exposed to NO2. When exposed to CO2, the ZnO based sensor showed no response, while the BaTiO3/CuO/La2O3 sensor showed a small resistance increase. The heterojunction sensor was found to be insensitive to <3000 ppm CO2 although at higher concentrations it showed a small decrease in resistance. The resistance also decreased when exposed to low levels of NO2, indicating that a different detection mechanism was operative at the heterocontact compared with the single-phase materials.