Waste catkins-mediated synthesis of in situ graphitic carbon-modified ZnO tubes rich in oxygen vacancy for boosting sensitive NO2 gas at near room temperature.

Boosting the low-temperature response of ZnO-based sensors still remains a key factor in precise detection of harmful NO₂ gas in complex environments. Herein, two in situ graphitic carbon (GC)-modified ZnO biomorphic tubes were controllably synthesized by separately annealing zinc salt-immersed catkins in air and H₂/N₂ atmospheres. Among them, 3.7 wt% GC/ZnO tubes obtained from calcination at 425 °C possess a broad mesoporous structure, large specific surface area, and rich oxygen vacancies. The synergetic effect of above advantageous characteristics can promote the transport of gas molecules and exposure of more active sites to accelerate the surface chemical reaction rate, thereby efficiently enhancing its sensitive performance to detect trace NO₂ at low-power consumption. At near room operating temperature of 50 °C, the 3.7 wt% GC/ZnO sensor achieves a high response (378 for 10 ppm NO₂ gas), along with other good comprehensive properties involving reversible dynamic response-recovery, low detection concentration, good selectivity, and moisture resistance. Furthermore, the related mechanism for enhanced low-temperature sensing response has been carefully explored.