Humidity-Enhanced Effects of Sputtered Au Nanoparticles on the Ethanol Sensing Performance of Thermally Evaporated ZnO Nanowires

Abstract

Mitigating the adverse effects of humidity on the sensing performance of one-dimensional (1D) metal oxide semiconductors has been a key challenge in gas sensor development. This study investigated the humidity-enhanced effect of gold (Au) nanoparticles on the ethanol response of 1D ZnO nanostructures. ZnO nanowires were grown using a thermal evaporation–condensation method with Au-catalyzed growth kinetics for the fabrication of conductometric chemical sensors. Au nanoparticles were then additionally decorated on ZnO nanostructures by sputtering with varying times from 2 to 10 s. Structural examinations demonstrated Au nanoparticles with diverse diameters (10–70 nm) widely distributed on ZnO nanowires exhibiting lengths of 0.1–1 μm and diameters of 10–30 nm. All fabricated sensors were systematically evaluated toward NO2, NH3, C3H6O, CO, and C2H5OH at 250–400 °C under dry and humid conditions. ZnO nanowires functionalized with Au for 5 s were found to exhibit the highest response of 93.75 to 70 ppm of C2H5OH, which was significantly greater than that of the pristine ZnO nanowire sensor (1.86) at 50% relative humidity (RH) and the optimum sensing temperature of 350 °C. Interestingly, the optimally Au-decorated ZnO nanowire sensor showed humidity-enhanced behaviors with significantly boosted ethanol responses at 50% RH. In addition, the optimal sensor displayed a low theoretical limit of detections (3.7 ppb) and high C2H5OH selectivity against interfering gases, including NO2, NH3, C3H6O, and CO. The observed ethanol response improvement and humidity-enhanced effect might be ascribed to the attributes of ohmic metal–semiconductor junctions formed at Au–ZnO interfaces and the capabilities of very fine sputtered Au nanoparticles to enhance oxygen chemisorption and suppress the formation of surface hydroxyl and hydronium species on ZnO nanowires. According to the results, Au-decorated ZnO nanowires fabricated by evaporation–condensation and sputtering could be potential candidates for use as practical C2H5OH sensors.