We fabricated nitrogen dioxide (NO2) gas sensors with p-type carbon nanotubes (CNTs) / n-type tin dioxide (SnO2) nanoparticle heterojunctions using one-step dielectrophoretic assembly and investigated the effect of CNT/SnO2 ratio on their NO2 gas detection properties. CNTs and SnO2 nanoparticles were mixed in various ratios, suspended in deionized water, and assembled by dielectrophoresis. The normalized response of fabricated CNT/SnO2 heterojunction gas sensors against 1 ppm NO2 was ∼80 in an N2 atmosphere and ∼20 in artificial air, where UV irradiation was used only for initialization. To reduce the effect of oxygen (O2), we also conducted continuous UV irradiation with various intensities during the initialization and gas detection. The CNT/SnO2 pn heterojunction gas sensor had a maximum normalized response of 19 for 1 ppm NO2 in artificial air, while that of the SnO2 sensor was 3. Furthermore, plotting the gas sensor response as a function of NO2 concentration reveals that the sensor detected an NO2 gas concentration as low as 20 ppb in artificial air.
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