Reduction of titanium dioxide photoactivity
Titanium dioxide, "titania" is used for a wide range of everyday applications, such as in sunblocks, paints, paper, and some plastics. For these uses it is necessary to reduce the photoactivity of the titania for either product quality or consumer safety. Titanium dioxide also finds application in the field of photocatalysis for pollution abatement and in this instance a high photoactivity is desirable. This thesis describes the development and characterisation of two main strategies intended to reduce titania photoactivity: the addition of iron dopants to suppress the number of surface radicals (Chapters 3 and 4), and the addition of an organic coating to the titania to scavenge the radicals (Chapters 5 and 6). The titania was iron doped in two ways, either only on the surface, or in the lattice and on the surface (shown by Electron Paramagnetic Resonance "EPW' spectra). The effect of several novel organic coatings on the photoactivity was tested: a phosphate, an organophosphate based coating and an organosilane. Two types of titanium dioxide were modified, a rutile sample used in commercial sunblocks and Degussa P25, a titania commonly used in research. The efficiencies of these strategies were tested by several methods to determine the effect of solution medium on the photoactivity. A new technique of 2-nitrophenol degradation was developed to measured photoactivity in aqueous solutions and compared with the well characterised technique of 2-propanol oxidation. In addition, a test based on salicylic acid degradation was developed and shown to measure the photoactivity by direct hole oxidation. Infra red spectroscopy and surface area determination were used to analyse how the coatings formed on the titania surface. IR spectra show that the headgroups bond to the surface and so the coatings are hydrophobic. Careful analysis showed that organosilanes form on the surface as crosslinked polymers. Lastly, work on 4-nitrophenol degradation carried out under my guidance complements earlier work on 2-nitrophenol degradation to show that UV absorption by the solution affects the measured kinetics of photocatalytic degradation.