Development of a continuous microwave reactor for the production of zinc oxide

Abstract

This study involves experimental design and development of continuous microwave reactor for the production of zinc oxide (ZnO) nanoparticles. The reactor consisted of a modified household microwave oven which was operated with a frequency of 2.45 GHz and a maximum output power of 800 W. The modified microwave reactor was equipped with two peristaltic pumps and connected to glass columns via T-junction to allow for mixing of the reactants, zinc nitrate hexahydrate (Zn(NO3)2.6H2O) and sodium hydroxide (NaOH), in the microwave system. The effect of process parameters such as retention time, microwave power and reactant concentration on the phase composition, particle morphology and optical properties of ZnO was investigated. At a fixed concentration of the reactants, crystalline phase of ZnO was formed as observed from the X-ray Diffraction (XRD) patterns. Also, the phase crystallinity of ZnO was found to improve when the retention time of the reactants in the reactor was increased up to 20 min and increasing of the microwave power until 600 W. Besides, increasing the concentration of reactants has successfully produced crystalline phase pure ZnO nanoparticles which were obtained using the mole ratio of Zn(NO3)2.6H2O:NaOH of 1:2. Transmission Electron Microscopy (TEM) image revealed spherical-shape ZnO nanoparticles with sizes ranging from 6 to 12 nm and the result matched well with the crystallite sizes determined from the XRD data using the Scherrer equation. The ZnO nanoparticles exhibited a strong absorption in the 390 nm region of the Ultraviolet-Visible (UV-Vis) spectra which is red-shifted from bulk ZnO (370 nm) with the band-gap value of 3.15 eV. The peak intensity was increasingly decreased along with increasing of retention time, microwave power and reactant concentration while the band-gap energies were found to decrease upon increasing of the retention time, microwave power and reactant concentration. The green band emission observed in the region of 350-470 nm in the Photoluminescence (PL) spectra suggests the presence of high oxygen vacancies in the ZnO lattice which then further reduced with increasing of retention time, microwave power and reactant concentration.