Titanium Dioxide Photocatalyst Thin Films With Composite Nanostructures

Titanium dioxide is a promising photocatalyst because of its stable chemical property, nontoxic and high oxidation ability of the photogenerated holes. The practical application of the titania dioxide photocatalyst is limited to data due mainly to the easy combination of photogenerated electron and hole pairs as well as poor utilization efficiency of solar energy. This thesis studied effects of composite nanostructures on the titania photocatalyst. Firstly, a porous titania substrate was fabricated through reacting metallic Ti with 30 wt% H₂O₂ at 80 °C for 1 h. The obtained substrate was sol-gel spin-coated with titania with various layers. Secondly,, titania nanorods substrate was prepared by direct oxidation of metallic Ti with 30 wt% H₂O₂ at 80℃ for 72 h. Titania nanoparticles were then filled into the gaps among the nanorods through an infiltration sol-gel procedure. The microstructure and photocatalytic property of the achieved titania film with composite nanostructures were studied, using rhodamine B in water as a molecular probe. The possible mechanism of the improved activity due to the novel nanostructure were discussed.The photocatalytic activity (PA) of the layered composite film fabricated using the porous titania substrate is higher than that achieved using the as-pickled titanium substrate. The ambient photoluminescence (PL) spectra suggested that charge separation of the layered composite film fabricated using the porous titania substrate is promoted. The difference in band structure between the porous titania substrate and the sol-gel spin-coated titania top layer, which was caused by different fabrication history and different crystal size, as estimated by Scherrer formula from XRD patterns, is the possible reason for the easy charge separation, which resulted in higher PA for the layered composite titania film using the porous titania substrate when compared to that obtained using the as-pickled Ti substrate. With increasing film thickness of the spin-coated titania top layer, the absorption of UV photons increased. The PA of the layered composite titania film increased firstly for up to 3 coating layers, which then decreased with further increasing coating layer. The charge separation was firstly meliorated then deteriorated with increasing thickness of the spin-coated titania layer, as revealed by ambient PL spectra. The layered composite titania film with 3 sol-gel titania layers had the best charge separation effect among all layered composite titania films, which agrees well with the PA evaluation result. The PA of the layered composite titania film was found to increase in general with increasing contents of the commercial Degussa P25 titaniananoparticles dispersed previously into the titania sol. Field-emission scanning electron microscopy (FE-SEM) observation illustrated that, a better surface quality of the layered composite titania film could be obtained through using PEG 2000 as additive in the titania sol, but the PA become lower because of the increasing grains caused by higher heat treatment temperatures utilized to ensure totally removal of the additive.Ttitania nanoparticles were filled into the gaps among the nanorods through an infiltration sol-gel procedure to form a composite titania film with ordered nanostructure. X-ray diffraction spectra indicated that the titania nanorod film was a mixture of anatase and rutile while the so-gel derived titania film was pure anatase. FE-SEM observations show that titania nanoparticles were embedded into the titania nanorod film. PL spectra suggested also the enhanced separation of electron and hole pairs for the obtained composite titania film over the corresponding titania nanorod film. The composite titania film exhibited improved ability to photodegrade rhodamine B in water compared with the titania nanorod film. Its apparent reaction rate constant, fitted using a pseudo-first order reaction, was 3 times of that obtained by the sol-gel derived titania film with identical weight. The improved PA for the composite titania film could be contributed to the enhanced separation of electron and hole pairs due to the embedding of the titania nanoparticles within the titania nanorods.