Abstract
Photocatalytic degradation and water splitting have received considerable attention as green methods to address problems related to contamination and greenhouse effects. Dyes and other chemical contaminations can be degraded by photocatalysts under sunlight irradiation. Charge carriers created in the photocatalyst migrate to the surface and are trapped by absorbed H2O, HO−, or O2, thereby forming highly reactive radicals. These radicals can decompose organic pollutants into smaller molecules, such as CO2 and H2O. Photocatalytic water splitting is similar to photocatalytic degradation: charge carriers are first created in the photocatalysts under light irradiation and then migrate to the surface. The subsequent step, however, is different: charge carriers on the catalyst surface react with H2O molecules to create H2 and O2.This thesis focuses on the synthesis of titania and titanate nanomaterials using the hydrothermal method, and their applications to photocatalytic degradation and water splitting.
First, TiO2 core-shell nanostructures decorated with Ag2O were synthesized. The ratio between Ag and Ti was controlled to 5%, 10%, 15%, 20%, or 25% during precursor preparation. The final ratios of Ag and Ti in the production were detected by energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). The photocatalytic degradation abilities of the resultant materials were characterized by methylene blue (MB) degradation, and a mechanism of the photocatalytic phenomena observed was proposed based on the band structure of the catalyst heterojunctions. Our results showed that interface states were created at the interface between Ag2O and TiO2. Due to the existence of interface states, an energy barrier was generated between the conduction band of Ag2O and TiO2. This energy barrier determined the transportation of the photogenerated electrons between Ag2O and TiO2, and then influenced the photocatalytic ability of the Ag2O/TiO2 heterojuction.
The second part of this paper reports on the photocatalytic abilities of different facets of anatase TiO2 nanocrystals. We found that different surface treatment methods could change the photocatalytic order of different facets. Anatase TiO2 nanocrystals with {001} and {101} facets were synthesized by the hydrothermal method using hydrofluoric acid to control the ratio of the different facets. F– ions were removed using two methods: washing with NaOH aqueous solution and calcination. The photocatalytic activities of the facets were characterized by dye degradation, and the defects found on the facets were investigated by X-ray photoelectron spectroscopy. Calcination enhanced the density of oxygen vacancies on the {001} facet and improved the photocatalytic activity of the facets. In this dissertation, we evaluated the influence of surface treatment on the defects and photocatalytic activities of the different facets of anatase TiO2. It was found that both alkaline solution washing and calcination can remove the F– ions absorbed on the surface of TiO2 efficiently. When the TiO2 nanosheets were treated by alkaline solution washing, TiO2-101 is more efficient than TiO2-001. It is because the {101} facets have more oxygen vacancies (Ti2O3) than those of the {001} facets. But when the samples were treated by calcination, the photocatalytic efficiency order was reversed, TiO2-001 became more efficient than TiO2-101. It is because that the oxygen vacancies (Ti2O3) on the surface of {101} facets diffused into inner layer when the sample was calcined. The oxygen vacancies in the inner layer acted as recombination centers which leaded the annihilation of carriers. At the same time, the oxygen vacancies on the surface acted as reaction center for the dye degradation. So more oxygen vacancies on the surface means the sample is more efficient.
The third section of this paper demonstrates the synthesis of SrTiO3 nanocubes via the hydrothermal method. The photocatalytic water splitting abilities of these catalysts were investigated using offline water splitting system. Water splitting results were evaluated. It was found that the mixture of SrTiO3 and PbTiO3 (4:1 mole ratio) doubled the hydrogen generation rate than that in the pure SrTiO3 nanocubes. Decorating SrTiO3 with Pt (1 w%) can treble the hydrogen generation rate than pure SrTiO3. In order to make use of the visible light, we doped Pb into SrTiO3 to modify the energy band gap of SrTiO3. The results showed that when doped with Pb, the absorption wavelength shifted to the visible region. The photocatalytic hydrogen generation rate was increased after doping with Pb, but decreased when doping was excessive.
| Date of Award | 15 Mar 2016 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Lawrence WU (Supervisor) |