The preparation and characterisation of robust, transparent nanocrystalline thin films of anatase TiO<SUB>2</SUB> and their use in photochemical systems for the photodegradation of organics (4-chlorophenol, methylene blue and stearic acid), and the evolution of hydrogen from aqueous solutions of sacrificial electron donors (SEDs) is described. Quantum yields for the photooxidation of 4-chlorophenol in TiO<SUB>2</SUB> are 0.5-1% and these depend upon the total number of photons absorbed and are independent of both the absorbed photon flux and the distribution of charge carriers in the film. In contrast the rate of photoreduction of methylene blue appears to be surface limited as quantum yields for methylene blue reduction fall from 1.7% to 0.5% as film thickness increases. Quantum yields for photooxidation of solid films of stearic acid are 1-4%. In studies of hydrogen evolution from aqueous solution in the presence of SEDs, the quantum yields are low and surface limited unless a Pt or Au cathode is present. The geometry and size (over a ten-fold range) of the Pt cathode is not influential in determining hydrogen evolution quantum yields, nor is the spatial separation of the electrodes. At constant total ion concentration (0.1M), quantum yields are 0.112 (0.007) for pH 2-6 and 0.029 (.003) for pH 8-12. In acidic solution hydrogen evolution occurs at both the TiO<SUB>2</SUB> surface (30%) and the Pt surface (70%) whereas the alkaline solution hydrogen is evolved exclusively from the TiO<SUB>2</SUB> surface. Hydrogen evolution yields are highly dependent upon the nature and concentration of the SED. The maximum quantum yield obtained was 0.27 for 1 M HCl in 50/50 v/v methanol/water. The attempted preparation of visible light absorbing WO<SUB>3</SUB> and dye-sensitised TiO<SUB>2</SUB> films in these photosystems is also described.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:637009 |
| Date | January 2002 |
| Creators | Fretwell, R. |
| Publisher | Swansea University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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