MEMS-compatible, gold nanoisland anchored 1D aligned ZnO heterojunction nanofibers: unveiling the NO2 sensing mechanism with operando photoluminescence studies

Abstract

Production and alignment of heterojunction metal oxide semiconductor nanomaterial-based sensing elements for microsensor devices have always posed fabrication challenges since they involve multi-step synthesis processes. Herein, we demonstrate a coaxial electrospinning with an in situ photoreduction process for the fabrication of MEMS-compatible, 1D aligned gold nanoisland (GNI) anchored ZnO-based Heterojunction Nanofibers (HNFs) and their complex plasmon-mediated NO₂ gas sensing mechanism has been investigated using operando photoluminescence studies. Evaluation of the gas sensing properties of aligned ZnO-GNI HNFs has exhibited excellent sensor response (196%) with rapid response time towards 500 ppb NO₂ gas at reduced operating temperature (200 °C). Operando studies using photoluminescence and electrical measurements ascertained the existence of supplementary active sites in the multicrystalline nanofibers owing to the existence of Au nanograins over aligned ZnO nanofibers (NFs), thereby inducing a spill-over zone and aiding the charge transfer phenomenon. The operando PL studies also revealed the plasmonic effect of metallic Au and the role of zinc interstitial (Zn_i) defects and oxygen vacancies (V⁺_o), which influence the charge transfer between ZnO and surface anchored Au nanoislands. This study revealed atomistic insights into the structural defects and charge transport properties of metal oxide semiconductor heterojunction materials, and the coaxial integration method of 1D aligned ZnO-GNI HNFs paves the way for unique strategies and the development of suitable MEMS based gas sensors for real-time applications.