Synthesis of multi-dimensional ZnO nanostructures in aqueous medium for the application of gas sensor

Novel cacti-like structure and nanoneedles of zinc oxide (ZnO) were grown onto glass substrate using chemical route at comparatively low temperature (90 °C), and employed for the application of gas sensor. The grown nanostructures were characterized by X-ray diffraction (XRD), Field Emission Scanning Electron Microscope (FE-SEM) and Transmission Electron Microscope (TEM) and photoluminescence (PL) spectroscopy. FE-SEM and TEM images showed that vertically aligned ZnO nanoneedles were formed on substrate and secondary branches emanated from primary aligned nanoneedles. PL spectra showed distinctively different peaks for nanoneedles and cacti-like structure where the peak intensities for cacti structures are as high as the one compared for aligned nanoneedles. Also, the intense visible peak detected for cacti structure confirmed the presence of defects due to oxygen vacancy in the grown nanostructures. Further, gas sensing behaviors were studied for these two different nanostructures against nitrogen dioxide (NO2) gas, and their selectivities toward reducing gases such as hydrogen disulfide, ethanol and liquefied petroleum gas were compared. It was found that cacti-like structure exhibited high gas response at 200 °C and is selective for NO2 gas as compared with that for nanoneedles. The improved gas response is due to high surface area of cacti structure and presence of oxygen vacancies. Moreover, the contact between two adjacent cacti branches creates barrier potential at junction, which controls its resistance of sensors resulting in high gas sensing. Therefore, novel cacti-like nanostructure demonstrated to be the best candidate as NO2 gas sensor at low cost and temperature.