Download files
Abstract
In this thesis, a range of visible-light-active WO3 electrodes are fabricated by the electrochemical anodization method and studied for its solar energy conversion ability, predominantly as a photoanode for photoelectrochemical (PEC) reactions. The work starts with the synthesis of relatively thick flower-like nanostructured WO3 films by anodizing tungsten foil in fluoride-containing electrolyte under non-stirring static condition. The as-anodized samples comprise of monoclinic hydrated tungstite (WO3 2H2O), while films that were subsequently calcined contain predominantly monoclinic WO3. It was found that the supersaturation condition established during the anodization process favors the formation of the flower-structured WO3 2H2O through an anodization/precipitation-recrystallization process. The flower-structured films also exhibit higher photoresponse as compared to the typical mesoporous structure formed under similar anodization condition with non-static condition.
Following the synthesis of these flower-structured WO3 films, a self-photorechargeability phenomenon was discovered with these WO3 films demonstrating simultaneous generation and storage of photo-excited electrons during PEC reactions. By introducing alkali cations such as Na+ and K+ in the electrolyte, the WO3 photoelectrode possessed the ability of storing light energy in the form of trapped electrons. Subsequently, the influence of crystallinity of the flower-structured WO3 films directed towards the optimization of both PEC water splitting and self-photorechargeability performance is also conducted. The effect of crystallinity was found to be crucial in the self-photorecharge property of these films.
The last part of the thesis focuses on the development of bismuth tungstate (Bi2WO6) by transforming anodized flower-structured WO3 2H2O films into orthorhombic Bi2WO6 by hydrothermal treatment for PEC water splitting under visible light irradiation. The H2O molecules in the layered anodized WO3 2H2O film are proposed to be substituted by the [Bi2O2]2+ layers during the hydrothermal treatment process. The importance of an initial layered structure is also highlighted by the absence of Bi2WO6 formation in non-layered WO3 films.