The impact of carrier gas on room-temperature trace nitrogen dioxide sensing of ZnO nanowire-integrated film under UV illumination

Chemiresistive gas sensors have been extensively explored for hazardous gas detection. Currently, an over-wheming majority of previous attention was focused on the parameter improvement of sensor performance while the impact of carrier gas species on the performance was severely ignored. Aiming to a deep insight into this issue, in this work we prepared zinc oxide (ZnO) nanowire-network sensor and explored its UV-activated sensing performance toward trace nitrogen dioxide gas (NO2) at room temperature (25 degrees C) under two carrier gases, i.e., dry nitrogen (N-2) and air. Within N-2, the sensor exhibited a reversible response of 157 toward 50 ppb NO2 and a sensitivity of 7.8/ppb, which was not only among the best showcases of the existing work, but much larger than those within air (11 and 0.091/ppb, respectively). Moreover, decent selectivity and long-term stability were demonstrated. Far more UV irradiation-induced electrons which reacted with adsorbed NO2 molecules on ZnO surface as well as smaller baseline resistance under N-2 than those under air jointly led to the superior response and sensitivity. After long-time UV exposure prior to gas-sensing tests within both carrier gas cases, the remaining oxygen ions (O-2(-)) were weakly bonded on ZnO surface, contributing to the reversible behaviors at room temperature. The interconversion between physisorbed O-2 molecules and ionic O-2(-) on ZnO surface was proposed to rationalize the sensing phenomena especially when no continuous oxygen was supplied under N-2 atmosphere, which enriched the current transduction mechanisms.