Thesis (M.Eng.)--Chulalongkorn University, 2007

ZnO nanopowders with various crystallite sizes (8.8-47.0 nm) and nanocrystalline Au-ZnO and Pt-ZnO catalysts have been prepared by flame spray pyrolysis (FSP) method and characterized by X-ray diffraction (XRD), nitrogen adsorption, UV–vis spectroscopy, photoluminescence (PL), and transmission electron microscopy (TEM). Photodegradation of methylene blue (MB) was used to evaluate the photocatalytic performance of these powders. Commercial photocatalysts P25 and JRC were used for comparison purposes. The FSP-ZnO 47 nm exhibited the highest photocatalytic activity. Further increase of the ZnO crystallite size by annealing at high temperature, however, gradually decreased the photocatalytic activity of ZnO. The results suggest that there were competing effects of crystal quality, surface area, surface oxygen vacancies and surface defects on the photocatalytic activity of ZnO. However, ZnO with crystallite size ≥ 30 nm showed better performance than the commercial P25 and JRC TiO2 photocatalysts. In the case of Pt/ZnO and Au/ZnO prepared by FSP, the experimental results suggest for an optimum content of 3 wt.% Au/ZnO that gave the best photocatalytic performance for MB degradation. Pt/ZnO, on the other hand, showed poor photocatalytic activity compared to the undoped ZnO due to the different type of Fermi level equilibration with diameter and the different metal cluster sizes/density deposited on ZnO.