Surface Modification and Decoration of Nano-Honeycombed ZnO Structure With Gold-Black Nanoparticles for High Responsivity Detection of NO₂ Gas Sensors

Nitrogen dioxide (NO2) gas sensors were fabricated using nano-honeycombed zinc oxide (ZnO) sensing membranes grown by a hydrothermal synthesis method with various trisodium citrate dihydrate concentrations. The trisodium citrate dihydrate could inhibit the c-axis direction growth of ZnO nanorods to modify the surface morphology. Consequently, the modified sensing membranes with larger specific surface area could be obtained. Under 10-ppm NO2 concentration and the optimal operating temperature of 200 °C, the responsivity of the NO2 gas sensors using the nano-honeycombed ZnO sensing membranes grown with a trisodium citrate dihydrate concentration of 1.00 mM for 2.5 h was 55.6%. To enhance the performances by inducing surface defect states to perform NO2 molecules absorption sites, the diluted HCl solution was utilized to etch the nano-honeycombed ZnO structures for various times. The responsivity of the NO2 gas sensors using the nano-honeycombed ZnO sensing membranes etched by diluted HCl solution for 7 s was 338.5% under 10-ppm NO2 concentration and the optimal operating temperature of 180 °C. To further improve the responsivity of the NO2 gas sensors, p-type gold-black nanoparticles decorated on the nano-honeycombed ZnO sensing membranes were deposited using a vapor cooling condensation system for various times. The responsivity of the NO2 gas sensors using nano-honeycombed ZnO sensing membranes decorated with Au content of 0.51 at% was improved to 540.1% under 10-ppm NO2 concentration and the optimal operating temperature of 170 °C. Even under the very low 0.1-ppm NO2 concentration, the resulting NO2 gas sensors could detect it.