Novel ZnO nanostructures: synthesis, growth mechanism, and applications
Molefe, Fokotsa Victor
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The ZnO nanostructures were successfully synthesized by chemical bath deposition method (CBD) to study the influence of parameters such as reaction temperature, time, precursor concentration and the annealing temperature respectively. The main motivation for this thesis is to successfully synthesise novel ZnO nanostructures and understand the growth mechanism. In this work, the thermal, structural, morphology, optical, and luminescence properties of ZnO were investigated in details by means of thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), x-ray photoelectron spectroscopy (XPS), ultraviolet visible (UV-vis) spectroscopy and photoluminescence (PL) spectroscopy techniques. From TGA results when increasing both reaction and annealing temperature we observed the increase in thermal stability of ZnO due to the removal of adsorbed species in the material. The melting temperatures (as determined through DSC) decreased due to crystallization of ZnO with the increase in both reaction and annealing temperature. X-ray diffraction (XRD) indicated that all the ZnO nanostructures prepared at 80 ℃ crystallizes in the wurtzite structure with the mean lattice parameters a = b = 3.25 Å and c = 5.18 Å and there is an increment in the particle size resulting into the improvement of crystallinity of the material. In materials prepared at lower reaction temperature, reaction time, and precursor concentration, traces of zinc hydroxide Zn(OH)2 were observed. When Zn(OH)2 decomposes into ZnO, the entire surface morphology through the study of ZnO consisted of agglomerated nanoflakes. The EDS results confirmed the presence of Zinc (Zn) and Oxygen (O) as the major product, and the ratio of Zn to O increased as ZnO becomes more crystalline. The UV-Vis reflectance spectra showed that the absorption band edges shift to the higher wavelength with an increase in reaction time, temperature, molar concentration precursors, and annealing temperature. As a result the band gap energy of ZnO nanostructures determined using Kubelka Munk’s equation was found to decrease due to quantum confinement effects and the increase in particle size. In general, the photoluminescence (PL) analysis showed that ZnO nanoflakes prepared at different parameters have almost the same characteristics. PL measurements revealed broad emission that extends from UV region to the visible region. The luminescence intensity of this emission was quenched when increasing parameters mentioned above, and these quenching is attributed to the decrease in concentration of defect related emissions. It is well known that when using chemical reaction methods such as CBD the emission intensity quenches as Zn(OH)2 dehydrates into ZnO. The slight red-shift in the emission band is also observed which is attributed to band gap narrowing.