An emphatic study on role of spill-over sensitization and surface defects on NO 2 gas sensor properties of ultralong ZnO@Au heterojunction NRs

Abstract Ultralong ZnO@Au heterojunction NRs having a diameter 60 ± 2 nm and length extending to several micro-meters with uniformly distributed gold (Au) nanoclusters (4 ± 0.5 nm) were synthesized using a facile wet-chemical approach by in-situ reduction process. The ultralong NRs were structurally characterized by UV–visible (UV–vis), Room temperature photo luminescence (RTPL) spectroscopies, Scanning Electron Microscopy (SEM) and High Resolution Transmission Electron Microscopy (HRTEM), X-ray diffraction (XRD), Raman spectroscopy and X-ray photo-electron spectroscopy (XPS). The interfacial charge transfer process at the heterojunctions trigger the spill-over sensitization during NO2 sensing, which is further insinuated by the presence of surface oxygen defects and zinc interstitials present at the interfaces. Owing to their ultralong 1D nanorod structure, directed electron transport through the heterojunctions was found to significantly enhance the NO2 gas sensor response (S = 400) rapidly at a lower operating temperature (150 °C). The gas-material interactions were further analyzed by in-situ electrical characteristics to elucidate the anomalous sensing process of ultralong ZnO@Au heterojunction NRs by favoured band bending phenomenon, which was greatly influenced by the spill-over sensitization effect, localised surface plasmon and the dominant surface oxygen species namely O−.