Porous, n–p type ultra-long, ZnO@Bi2O3 heterojunction nanorods - based NO2 gas sensor: new insights towards charge transport characteristics

Herein, porous 1D n–p type ultra-long ZnO@Bi2O3 heterojunction nanorods have been synthesized by a solvothermal method and their complex charge transport characteristics pertaining to NO2 gas sensing properties have been investigated. The porous structure of the ZnO@Bi2O3 heterojunction nanorods assisted in achieving superior sensing properties compared to pristine ZnO nanorods. Temperature-dependent in situ electrical studies of the porous heterojunction nanorods explored the unique electron transport properties under different environments, which revealed the accumulation/depletion of electrons and charge carrier recombination leading to band bending at the metal oxide heterojunctions. The formation of electron depletion layers at n-ZnO/p-Bi2O3 interfaces is believed to increase the adsorption of oxidizing gas, resulting in a fast response time (10–12 s) and 10 times higher sensitivity than that of the ZnO nanorod-based sensor towards 500 ppb NO2. To study the structure–property correlation of the ultra-long ZnO@Bi2O3 heterojunction nanorods-based sensor, a crystallographic model supported by transmission electron microscopy analysis was adopted to understand the NO2 gas adsorption properties on the surface. The crystallographic model helps to visualize the dangling bonds and the ratio of metal to oxygen ions present at the exposed crystal planes. The results suggest that porous, ultralong n–p type ZnO@Bi2O3 heterojunction nanorods could be a promising candidate for a high performance NO2 sensor for real time applications.