Abnormal n-p-n type conductivity transition of hollow ZnO/ZnFe2O4 nanostructures during gas sensing process: The role of ZnO-ZnFe2O4 hetero-interface

The hollow ZnO/ZnFe2O4 microspheres with heterogeneous structure are synthesized by direct pyrolysis of metal-organic frameworks. The as-prepared ZnO/ZnFe2O4 microspheres have well-defined spherical morphology with 1.5 pin in diameter and multiple porous shells constructed by interpenetrated ZnO and ZnFe2O4 heterogeneous nanoparticles. Interestingly, the hollow ZnO/ZnFe2O4 microspheres based gas sensors show interestingly temperature-dependent n-p-n type conductivity transition in detecting low-concentration VOC gases including ethanol, acetone, toluene and benzene. This interestingly n-p-n transition phenomenon is mainly ascribed to the trade-off of highly separated electron-hole pairs originated from the staggered type-II band alignment of in-shell ZnO-ZnFe2O4 hetero-interfaces, which is modulated by thermally-dependent ionization reaction of surface-absorbed oxygen molecules and extra electron injection due to surface reaction of reductive VOCs during gas-sensing process. This work presents a facile route to construct hollow nanostructures with heterogeneous feature and provides a new insight into sensing mechanism, exhibiting the potential application of ZnO/ZnFe2O4 microspheres in developing highly sensitive and selective gas-sensing materials in detecting low-concentration VOCs. (C) 2017 Elsevier B.V. All rights reserved.