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 (Ga 2 O 3 ) nanorods forming p–n heterojunction-structured sensing membranes of nitrogen dioxide (NO 2 ) gas sensors. The role and the sensing mechanism of the various gold-black NP-decorated Ga 2 O 3 nanorods in NO 2 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 NO 2 gas sensor using the sensing membrane of 60 s-deposited gold-black NP-decorated Ga 2 O 3 nanorods under a NO 2 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 NO 2 gas and Ga 2 O 3 nanorods. Furthermore, the NO 2 gas sensor could detect the low NO 2 concentration of 100 ppb and achieved a responsivity of 56.9%. The resulting NO 2 gas sensor also exhibited excellent selectivity for detecting NO 2 gas, with higher responsivity at a NO 2 concentration of 10 ppm compared with that of the C 2 H 5 OH and NH 3 concentrations of 100 ppm.