Sensing Mechanism and Characterization of NO₂ Gas Sensors Using Gold-Black NP-Decorated Ga₂O₃ Nanorod Sensing Membranes

In this work, a vapor cooling condensation system was utilized to deposit various amounts of p-type gold-black nanoparticles (NPs) onto the surface of n-type gallium oxide (Ga₂O₃) nanorods forming p-n heterojunction-structured sensing membranes of nitrogen dioxide (NO₂) gas sensors. The role and the sensing mechanism of the various gold-black NP-decorated Ga₂O₃ nanorods in NO₂ gas sensors were investigated. The coverage and atomic percentage of the sensing membranes were observed using high-resolution transmission electron microscopy (HRTEM) measurements and energy-dispersive spectroscopy (EDS), respectively. For the NO₂ gas sensor using the sensing membrane of 60 s-deposited gold-black NP-decorated Ga₂O₃ nanorods under a NO₂ concentration of 10 ppm, the highest responsivity of 5221.1% was obtained. This result was attributed to the spillover effect and the formation of the p-n heterojunction, which increased more ionized-oxygen adsorption sites and promoted the reaction between NO₂ gas and Ga₂O₃ nanorods. Furthermore, the NO₂ gas sensor could detect the low NO₂ concentration of 100 ppb and achieved a responsivity of 56.9%. The resulting NO₂ gas sensor also exhibited excellent selectivity for detecting NO₂ gas, with higher responsivity at a NO₂ concentration of 10 ppm compared with that of the C₂H₅OH and NH₃ concentrations of 100 ppm.