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Title: Photocatalytic nanofibers composed of oxides of titanium, zinc, and bismuth for the degradation of pollutant dye in water, nitric oxide and VOC
Authors: Pei, Chun
Degree: Ph.D.
Issue Date: 2015
Abstract: Photocatalysis is a photoreaction of light-producing charge carriers on semiconductor surface with the reactants. Titanium dioxide (TiO₂) is found to be the most efficient photocatalyst under ultraviolet (UV) irradiation and as such it has been most commonly used. However, given TiO₂ can only be excited by UV which takes up 3-4% of the solar spectrum, considerable efforts have been made to increase the light activity of TiO₂ by broadening the responsive solar spectrum. In this study, various novel means have been investigated to improve the physicochemical properties of TiO₂ to enhance the photocatalytic activity for environmental applications. Specifically, semiconductor heterojunctions of TiO₂ coupled with other semiconductors to form composite nanostructure and TiO₂ mixed crystal phases have been investigated. Besides, the composite semiconductors are fabricated into the polycrystalline nanofibers as their photocatalytic performances are far superior when compared to nanoparticles. Composite photocatalysts have been developed using the synergistic effect of specific semiconductors with the goals of harvesting visible light energy and achieving higher photocatalytic performance by reducing both the band-gap energy and recombination rate of the photo-generated electron/hole pairs. Two cases, TiO₂/ZnO composite nanofibers and TiO₂/ZnO/Bi₂O3 (TZB) composite nanofibers, have been investigated to demonstrate the approach and benefits. Pure TiO₂ nanofibers and TiO₂/ZnO composite nanofibers are synthesized by electrospinning followed by calcination. The diameter of the nanofibers ranges from 70 to 130 nm for different concentrations of the precursor solutions. The photocatalytic activities of nanofibers are studied systematically by the degradation of Rhodamine B (RhB) under the 420 nm visible-light irradiation and the conversion of nitrogen monoxide (NO) gas under solar irradiation. Photocatalytical activity can be optimized by doping with an appropriate amount of Zn to achieve the highest surface oxygen vacancy.
Compared to pure phase of TiO₂, the mixed-phase TiO₂ materials have unique charge transfer and recombination dynamics, and fast diffusion of charge carriers to the surface, all of which improve the photocatalytic activity. The photocatalytic characteristics are investigated using TiO₂/ZnO nanofibers synthesized by calcinating at different temperatures to modify the anatase-to-rutile ratio, thereby changing the TiO₂ crystal structure. The trade-off effect between the electron/hole recombination rate and the surface area of the crystals can be balanced by optimizing the anatase-rutile ratio. Indeed, the anatase/rutile ratio (48:52) is optimized at calcination temperature of 650 °C, whereby the TiO₂/ZnO composite nanofibers have the highest photocatalytic efficiency both in the degradation of RhB in liquid and conversion of NO gas. TZB composite nanofibers are synthesized to further improve the photocatalytic efficiency. The TZB nanofibers exhibit much higher photocatalytic activity for the oxidation of NO under simulated solar irradiation than commercial TiO₂ nanoparticles and TiO₂/ZnO composite nanofibers. The TZB composite nanofibers have increased absorption in both UV and visible range when compared with TiO₂ nanoparticles. The enhanced photocatalytic activity of TZB is attributed to the difference in the energy band positions of the semiconductors, resulting in both lower band-gap energy and recombination rate. Moreover, the photocatalytic performances are more stable for TZB nanofibers than the TiO2 nanoparticles. In addition, a new kinetic model, taken into account of flow retention and physiochemical kinetics, is used to interpret the kinetic behavior of the photocatalytic reaction. Faster kinetics (i.e. higher throughput in the reactor) and higher conversion of NO are realized by optimizing the bismuth concentration in the composite nanofibers. The degradation pathway of o-xylene by TZB and the intermediate byproducts have also been investigated. This study has demonstrated that modified nanofiber photocatalyst can improve surface oxygen vacancy, reduce band gap, and reduce recombination rate thereby improving the photocatalytic oxidation on both gas contaminants and organic dyes in aqueous phase. Tests on the degradation of dye in aqueous phase, conversion of NO and degradation of o-xylene have proven far superior performance for the composite nanofibers as compared to the TiO₂ nanoparticles benchmark despite their surface-area-to-volume ratio are similar.
Subjects: Nanofibers
Water -- Purification
Hong Kong Polytechnic University -- Dissertations
Pages: ix, 182 leaves : illustrations ; 30 cm
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