Highly Sensitive Ethanol Gas Sensors of Au Nanoparticle-Adsorbed ZnO Nanorod Arrays via a Photochemical Deposition Treatment

Abstract

Vertically aligned zinc oxide (ZnO) nanorod (NR) arrays were successfully grown through a cheap hydrothermal route synthesis method (HTM) (95 °C, 3 h) prepared by 25 mM zinc nitrate hexahydrate [Zn(NO₃)₂·6H₂O] and 25 mM hexamethylenetetramine (C₆H₁₂N₄, HMTA). After simple photochemical deposition treatment [0.5 mM chloroauric acid (HAuCl₄·4H₂O)] under ultraviolet (UV) light irradiation, the noble gold nanoparticles (Au NPs) obviously decorated onto the ZnO NR surface. One-dimensional (1D) ZnO NR arrays without and with decorated Au NPs were called ZAuO-0 and ZAuO-1 nanostructures, respectively. Field-emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HR-TEM) were performed to explore the surface morphologies and elemental compositions of 1D NR arrays. All nanostructures synthesized easily at a low temperature were structurally uniform with good crystal performance and perpendicular to the substrate surface. The morphological images explored that the average length and diameter of the 1D ZAuO-0 nanostructures were 1.78 μm and 76.9 nm, whereas the average length and diameter of the 1D ZAuO-1 nanostructures were 1.81 μm and 78.2 nm, respectively. 1D ZnO nanostructures were analyzed with an X-ray diffraction (XRD) instrument, and results exhibited that all samples had a hexagonal wurtzite phase with the (002) plane as the most preferred c-axis orientation. The optical characteristics of all devices were investigated by photoluminescence (PL) analysis and UV–visible absorbance, and the results revealed the excitonic- (UV) and defect-related (green) emission regions. Energy-dispersive X-ray spectroscopy (EDX) was further performed to study the composition of the 1D ZAuO-1 NR arrays, which were found to comprise Zn (53.85 atom %), O (44.36 atom %), and Au (1.79 atom %) contents. As a result, all sensors performed best at 270 °C and had the optimal detection ethanol (C₂H₅OH) gas concentration of 100 ppm. Moreover, the 1D ZAuO-1 sensor exhibits a superior gas response performance (86.9%) over the 1D ZAuO-0 sensor (68.9%) and has good selectivity over ethanol gas. These remarkable ethanol-sensing features revealed the potential of the prepared 1D ZAuO-1 NR arrays as promising gas sensors.