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.

Investigation of High-Sensitivity NO2 Gas Sensors with Ga2O3 Nanorod Sensing Membrane Grown by Hydrothermal Synthesis Method.

In this work, Ga2O3 nanorods were converted from GaOOH nanorods grown using the hydrothermal synthesis method as the sensing membranes of NO2 gas sensors. Since a sensing membrane with a high surface-to-volume ratio is a very important issue for gas sensors, the thickness of the seed layer and the concentrations of the hydrothermal precursor gallium nitrate nonahydrate (Ga(NO3)3·9H2O) and hexamethylenetetramine (HMT) were optimized to achieve a high surface-to-volume ratio in the GaOOH nanorods. The results showed that the largest surface-to-volume ratio of the GaOOH nanorods could be obtained using the 50-nm-thick SnO2 seed layer and the Ga(NO3)3·9H2O/HMT concentration of 12 mM/10 mM. In addition, the GaOOH nanorods were converted to Ga2O3 nanorods by thermal annealing in a pure N2 ambient atmosphere for 2 h at various temperatures of 300 °C, 400 °C, and 500 °C, respectively. Compared with the Ga2O3 nanorod sensing membranes annealed at 300 °C and 500 °C, the NO2 gas sensors using the 400 °C-annealed Ga2O3 nanorod sensing membrane exhibited optimal responsivity of 1184.6%, a response time of 63.6 s, and a recovery time of 135.7 s at a NO2 concentration of 10 ppm. The low NO2 concentration of 100 ppb could be detected by the Ga2O3 nanorod-structured NO2 gas sensors and the achieved responsivity was 34.2%.