Low-temperature operating ZnO-based NO₂ sensors: a review

Owing to its excellent physical and chemical properties, ZnO has been considered to be a promising material for development of NO₂ sensors with high sensitivity, and fast response and recovery. However, due to the low activity of ZnO at low temperature, most of the current work is focused on detecting NO₂ at high operating temperatures (200-500 °C), which will inevitably increase energy consumption and shorten the lifetime of sensors. In order to overcome these problems and improve the practicality of ZnO-based NO₂ sensors, it is necessary to systematically understand the effective strategies and mechanisms of low-temperature NO₂ detection of ZnO sensors. This paper reviews the latest research progress of low-temperature ZnO nanomaterial-based NO₂ gas sensors. Several efficient strategies to achieve low-temperature NO₂ detection (such as morphology modification, noble metal decoration, additive doping, heterostructure sensitization, two-dimensional material composites, and light activation) and corresponding sensing mechanisms (such as depletion layer theory, grain boundary barrier theory, spill-over effects) are also introduced. Finally, the challenges and future development directions of low-temperature ZnO-based NO₂ sensors are outlined.