Thesis--Ph. D., University of Hong Kong. / Typewritten.

Thesis (M.S.)--West Virginia University, 2003. / Title from document title page. Document formatted into pages; contains xiii, 85 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 83-85).

Photocatalysis has been attracting much research interest because of its wide applications in renewable energy and environmental remediation. Among the photocatalysts, metal oxide semiconductors are the best choice due to distinctive properties, durability and cost effectiveness. However, the mechanisms of photocatalysis are still not entirely clear and the photocatalytic activity of the metal oxide materials needs to be improved. Therefore, this thesis concentrates on the study of the photocatalytic mechanism and the factors affecting the photocatalytic activity. The photocatalysis of different commercial metal oxide nanoparticles has been studied. Different photocatalytic experiments have been designed to find out the mechanisms of photocatalysis. It is found that the dominant mechanism of photocatalysis is direct charge transfer between metal oxide and organic compounds. Reactive oxygen species, such as hydroxyl radicals also play a minor role in the photocatalysis. ZnO tetrapods were synthesized by evaporating Zn powder in the flow of Ar gas at high temperature in this study. They exhibit excellent photocatalytic activity. By the comparison between ZnO tetrapods and different ZnO nanoparticles, it is found that the photocatalytic activity of the metal oxide is strongly affected by the native defects, especially by the nonradiative defects. 1D TiO2 nanotubes and ZnO nanorods were synthesized by anodization and hydrothermal growth respectively. ZnO nanorods with alumina and titania shell were prepared by solution-based methods. These structures are interesting as supported catalysts, which is important for practical applications, since it enables simple removal of photocatalyst from treated water. The core shell structures are expected to improve the stability of ZnO. The results also imply that the photocatalytic avtivity of materials is affected by the native defects. Two different solution based methods hydrothermal growth and electrodeposition to grow CuxO layer were used. The hydrothermal grown CuxO exhibit excellent photocatalytic activity and good photocorrosion resistance. It was also found that the photocatalytic activity of the CuxO prepared by hydrothermal methods can be recovered by simple immersion of the sample into the precursor solution. / published_or_final_version / Physics / Master / Master of Philosophy

The structure and physical properties of layered oxides and carbon materials were studied. Two layered carbon materials were studied: carbon nanotubes (CNTs) synthesized by electron irradiation from amorphous carbon in situ in a transmission electron microscope (TEM) and a carbon and silver nanocomposite consisting of graphitic carbon nanospheres encapsulating Ag nanoparticles. In the CNT experiments, the effect of electron irradiation in the TEM is shown to alter drastically their structure and properties, even being able to transform amorphous carbon into a CNT. This suggests a possible alternative synthesis technique for the production of CNTs, in addition to providing a method for tailoring their properties. The structure and magnetic properties of the carbon and silver nanocomposite was characterized with x-ray diffraction, scanning and transmission electron microscopy techniques, and magnetic susceptibility measurements with a superconducting quantum interference device (SQUID) magnetometer. While the sp² bonding gives a grapheme sheet its mechanical properties, the p[subscript pi] electrons are responsible for its electronic and magnetic properties. In a flat graphene sheet the p[subscript pi] electrons are itinerant, but in a narrow p[subscript pi] band. The introduction of curvature to the graphene sheets that encapsulate the Ag nanoparticles is demonstrated to narrow the p[subscript pi] band sufficiently to result in "ferromagnetic" behavior. A model that is able to explain spin localization and ferrimagnetic spin-spin interactions in graphitic materials with positive curvature is presented. Layered oxides from the family of the P2 Na[subscript x]CoO₂ structure were synthesized and their properties studied. Na[subscript x]CoO₂ has a rich phase diagram ranging form a promising Na-rich thermoelectric composition to the hydrated Na-poor composition Na[subscript 0.33]CoO₂· 1.3H₂O that is superconductive. Intermediate to these two Na compositions exists an insulating phase with x [approximately equal to] 0.5 that presents a variety of interesting structural, magnetic, thermoelectric, and electronic behavior. Investigations of Na[subscript x]CoO₂ that probe the role of H₂O in the superconductive Na[subscript 0.33]CoO₂· 1.3H₂O are presented and conclude that H₂O plays a more active role than a passive lattice spacer. The relationship between Na ordering and an interesting magnetic behavior observed with [chi](T) measurements of annealed NaxCoO₂ and Sr[subscript x/2]CoO₂ samples is determined and found to correspond to a (2a x 2a) superstructure. The properties of NaxCoO₂ (x [approximately equal to] 0.5) are reviewed and thermoelectric S(T) measurements are made in order to develop a model that is able to explain the salient features of the NaxCoO₂ (x [approximately equal to] 0.5) phase. / text

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