Magnetic, luminescence and gas sensing properties of various zinc oxide nanostructures: the influence of surface modification by gold on the gas sensing properties
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Various morphologies of zinc oxide (ZnO) including particles, spheres, flowers and sheets achieved by varying the pH from 7 to 13 were successfully synthesized using the microwave-assisted hydrothermal method. Effect of pH and annealing on morphological, optical and magnetic properties was investigated. Annealing altered the morphology of the ZnO structures obtained at pH levels of 9 and 13 whereby spheres and sheets were transformed into particles and platelets, respectively. The decrease in surface area and porosity of the ZnO structures was also observed with post-annealing. Green emissions assigned to oxygen vacancies (VO) dominated the PL spectra of the as prepared ZnO structures. Whereas for annealed ZnO structures, green emissions only dominated the PL spectra of the ZnO structures produced at lower pH levels (pH 7 and 9) while those of the structures obtained at higher pHs were dominated by blue emissions assigned to zinc interstitials (Zni). The sensing performance of the ZnO nanostructures to CO, CH4, NO2, H2 and NH3 at temperatures ranging from room temperature (RT) to 450°C was investigated. The study conducted on the influence of irradiation time to structural, luminescence, magnetic and sensing properties of ZnO nanorods revealed an increase in the surface area of the rods which correlated with the decrease of the lengths and widths with increasing irradiation time. High sensing response to CO at 350 °C was achieved. Surface defects on the ZnO nanorods were attributed for the high response to CO through the confirmation from PL and EPR analyses. ZnO and Au loaded ZnO nanorods were also synthesized through the microwave-assisted hydrothermal method to study the effect of Au loading on sensing properties. The distribution of the Au nanoparticles on the surface of the ZnO nanorods was controlled by varying the Au concentration as 0.5, 1, 1.5, 2, 2.5 wt%. XRD, SEM, TEM and X-ray photoelectron spectroscopy (XPS) studies confirmed the presence of the Au nanoparticles on the ZnO nanorods surface. It was found that the sensors were selective to NH3 and the 0.5 wt% sensor showed the highest response to NH3 as compared to the other sensors. The mechanisms involved in the improved sensing response of the Au modified ZnO sensors were explained in detail.