The effect of stabilized ZnO nanostructures green luminescence towards LPG sensing capabilities

Abstract

ZnO nano-rods and flower-like structures have been successfully synthesized by the hydrothermal process. These nanostructures were annealed at various temperatures (210, 310, 510, and 710 °C) both in air and vacuum (~10−4 mbar) for 1h and subsequently allowed to cool down slowly or rapidly. These structures were subsequently characterised by various techniques for structure and morphology. Laser photoluminescence spectroscopy performed with a continuous He–Cd laser chopped at 120 Hz and spectra were collected three times to study the emission stability over time. The un-annealed ZnO exhibited a very broad-band of defect related emission due to oxygen vacancies with the exciton recombination completely suppressed. These defect emissions were unstable and degraded under ultraviolet laser excitation, reducing in intensity after each emission measurement. Interestingly, annealing these ZnO samples in air together with thermal quenching significantly improved both the exciton emission and these defect-related emissions, thereby improving the overall emission stability including the green luminescence. The liquefied petroleum gas (LPG) sensing performance has been examined. The ZnO nanostructure with stable green luminescence has been found to exhibit the highest sensing response of about 80 at 200 °C low operating temperature compared to the other nanostructures. This showed a clear link between the green luminescence stability and LPG sensing properties. Therefore, the main innovation of this work is to link the stability of oxygen vacancies to the LPG detection and response and mechanical properties of these ZnO nanostructures, contrary to previous studies which this part is sought. The ZnO sensor further showed an excellent reproducibility for long-term stability, linear dependence, low detection limit, and higher selectivity towards LPG as compared to other interference gases (i.e. NH3, C2H2, H2, and CO2). The response and recovery times were as 400s and 100s, respectively. The sensitivity and stability of the samples towards LPG were also tested. In addition, the mechanical-stress improved sensor proved to be resilient and reliable even after subjected to harsh humid environment of ultra-violet light irradiation of 8 h in air.