${\hbox {NO}}_{2}$ Gas Sensing Properties of ZnO/Single-Wall Carbon Nanotube Composites

Networks of ZnO/single-wall carbon nanotube (ZnO/ SWNT) composites were investigated as resistive gas sensors for NO₂ detection. Sensor films were spin coated on sapphire substrates. Sensors´ response, in dry air and in NO₂ atmosphere, was measured by resistance measurements in the temperature range from 25 to 300° C and gas concentration from 1 to 1000 ppm. At room temperature, ZnO/SWNT composites with equal weight ratio have shown quite high sensitivity to NO₂ concentrations as low as 1 ppm. Significant increase in the sensitivity of the composite at higher temperatures up to 300° C compared to that of ZnO free SWNTs. All tested sensors exhibited high stability and relatively low response and recovery times. The highest response values of the composite were obtained at temperatures ranging between 100 °C and 300 °C and gas concentrations higher that 50 ppm. For all gas concentrations, the ZnO/SWNT sensor shows much better performance at temperatures higher than 100° C compared to the SWNT sensor. Our results revealed that the measured sensitivity upon exposure to NO₂ strongly depends on the microstructure and the preparation conditions of the composite material. The gas detection mechanisms along with the optimal gas sensing parameters were discussed.