1D aligned, n–p and n–n type ZnO heterojunction nanofibers for NO2 sensors: exploration of conduction mechanism using in situ impedance spectroscopy

Abstract

Highly aligned 1D n–p type ZnO/Bi₂O₃ and n–n type ZnO/In₂O₃ heterojunction nanofibers (HNFs) have been developed using coaxial electrospinning approach, aiming at exploring the effect of 1D aligned heterojunction nanofibrous structure on their NO₂ sensing properties. The aligned HNFs have been structurally and morphologically characterized using various spectroscopic and microscopic analyses. The n–p heterojunctions of ZnO/Bi₂O₃ NFs and n–n heterojunctions of ZnO/In₂O₃ NFs exhibited significantly boosted sensitivity towards NO₂ compared to pristine ZnO NFs. The superior responses of ZnO/Bi₂O₃ and ZnO/In₂O₃ HNFs have been attributed to their unique electron transport properties originated from work function differences, leading to the formation of interfacial accumulation and depletion of electrons resulting in surface band bending. The charge depletion of Bi₂O₃/ZnO HNFs was estimated to be higher due to the interaction of NO₂ gas at an operating temperature of 300 °C, resulting in quick response time (8–10 s) and 10 times superior sensitivity than ZnO NFs. Whereas, n-In₂O₃/n-ZnO HNFs-based sensor exhibited a reduced operating temperature (200 °C) with superior sensitivity (S ∼ 340%), rapid response (5–7 s) with minimal interference towards exposure to trace-level NO₂ (500 ppb). The complex sensing mechanisms associated with n–p/n–n type HNFs have been deduced using in situ AC impedance spectroscopic studies, which endorsed the contributions from the modulation of grain and grain boundary resistance and charge transfer between n–p and n–n type materials. The results suggest that the boosted gas sensing properties of aligned 1D n–p and n–n type HNFs under atmospheric pressure conditions can pave the way for development of inexpensive and highly sensitive NO₂ gas sensors.