Heterostructured NO₂ Gas Sensors Using Decorated p-Type Reduced Graphene Oxide Nanoparticles on Surface Modified n-Type ZnO Nanorods

This report studied the p-type reduced graphene oxide (rGO) nanoparticles/n-type zinc oxide (ZnO) nanorods heterostructured sensing membranes of nitrogen dioxide (NO2) gas sensors grown by the hydrothermal synthesis method with various graphene oxide contents. To enhance the effective sensing area, the roughened hill-like ZnO seed layer was formed to grow more amount of ZnO nanorods. The as-synthesized sensing membranes were characterized by scanning electron microscope (SEM), high-resolution transmission electron microscope (HR-TEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and energy dispersive spectroscopy (EDS). To improve the sensing performances of NO2 gas sensors, the oxygen functional group existed in the graphene oxide nanoparticles was reduced using an annealing process in a hydrogen ambient at $400~^{\circ }$ C for 4 min. The resulting rGO nanoparticles had less amount of oxygen functional group and provided more amount of molecular adsorption sites. By investigating the influence of the diameter of ZnO nanorods and the heterostructured area of rGO nanoparticles/ZnO nanorods, the response of 8.93 and the optimal operating temperature of $135~^{\circ }$ C were achieved for the NO2 gas sensors grown with the graphene oxide content of 10 mg/mL. Furthermore, a very low NO2 concentration of 500 ppb could be detected.