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

Abstract

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.