Zinc oxide (ZnO) is a wide band gap (3.4eV at 300K) II-VI semiconductor with an exciton binding energy up to 60meV and is promising in the realization of excitonic or polaritonic lasing effect. Photoluminescence is widely used in studying the band gap and defect levels of ZnO. However, understanding in defects of ZnO is still far from satisfaction and remains controversial. Different authors suggest different explanations and mechanisms. 　In the present study we investigate in the photoluminescence spectra of four kinds of ZnO single crystal, namely as-grown (not implanted) Zn-face polished, Zn-implanted, O-implanted and He-implanted. The samples are annealed both in air and argon gas at a temperature of 350, 650, 750, 900 and 1200oC. The results show that O-implanted sample is weaker in excitonic emission and has an annealing effect tendency not consistent with that of Zn-implanted and He-implanted. Ion implantation would introduce defects in favor of yellow luminescence and the defects would anneal out gradually as the annealing temperature is rising. / published_or_final_version / Physics / Master / Master of Philosophy

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

In this study, the properties of nickel and antimony doped tin oxide (NATO) electrode materials were investigated in relation to the electrochemical generation of ozone. The performance of NATO materials was correlated to ·OH radical generation and oxygen adsorption properties. Long-time ozone generation results suggested that surface property changes, including surface morphology, chemical composition and electro-catalyst thickness, could lead to ozone production rate decreased from 137 to 0 mg·h-1 and the current efficiency declined from 18% to 0. The loss of Ni in the electrode was suggested for the decrease in ozone generation. Moreover, material characterization results indicated the presence of NiOOH and multiple oxidation states of Sb (+3 and +5), which were proposed as the critical sites for the electrochemical generation of ozone. In addition, NATO nanocrystals of 3.5 ~ 7.5 nm in size prepared by the hydrothermal method were used as an alternative route to fabricate electrodes. The highest current efficiency of 41% was achieved on NATO material of 6% Sb in the precursor, which led to the lowest resistivity of 2.38 ± 0.03 Ω·cm in the product NATO material. This further demonstrated the applicability of NATO materials used as electro-catalysts for the electrochemical generation of ozone. Hydroxyl free radicals (·OH) can be regarded as one of the most important intermediates for ozone generation. The presence of ·OH radicals was quantified by fluorescence spectroscopy with terephthalic acid as probes. Quantitative analysis results showed that Ni dopant could significantly enhance ·OH generation, while over-doping of Sb and Ni can decrease the generation of ·OH radicals. An oxygen chemisorption study on NATO materials showed that more active sites available for oxygen chemisorption lead to higher catalytic activity for ozone generation. The highest oxygen chemisorption capacity of 49.76 μmol·g-1 was achieved on NATO-5 (Sn:Sb:Ni=1000:16:2), which showed the highest current efficiency of 43%. In addition, temperature programmed oxygen adsorption and desorption showed different patterns on different NATO materials. This suggested that oxygen adsorption on NATO materials has a correlation to the electrochemical generation of ozone. In addition, oxygen adsorption was further investigated with near ambient oxygen adsorption. Oxygen adsorption isotherm results indicated that both physisorption and chemisorption can occur on the surface of SnO2 based material (NATO-5) with or without hydrogen pretreatment. When NATO-5 was treated with hydrogen, adsorption was mainly in the form of chemisorption. However, it was mainly in the form of physisorption without hydrogen pretreatment. By comparing NATO-6 (Sn:Sb:Ni=1000:16:0) with NATO-7 (Sn:Sb:Ni=1000:0:2), it was found that Sb was more important in the oxygen adsorption ability of NATO materials compared to Ni doping. Based on the findings in this study, two active sites (Sb and Ni sites) were proposed for ·OH generation and oxygen adsorption in order to explain the mechanism of ozone generation on NATO materials. Also, electrochemical generation of ozone was correlated with oxygen adsorption and ·OH generation. / published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

published_or_final_version / Dental Materials Science / Master / Master of Science in Dental Materials Science

Solid oxide fuel cells (SOFCs) are power generation devices that offer many great advantages compared to lower temperature fuel cells; for example, they are able to operate at high efficiencies without the use of expensive precious metal catalysts, and are also able to directly utilize hydrocarbon fuels without the need of an external reformer. Unfortunately, the conventional high operating temperature of these devices (T ≈ 1000 °C) requires the use of expensive, specialized materials that can withstand these high temperatures. This issue has generated considerable interest in reducing the operating temperature of these devices to the intermediate-temperature (600 – 800 °C) to allow for the use of less-expensive materials, such as stainless steel. However, the conventionally utilized SOFC cathode materials exhibit poor electrochemical performance at these reduced temperatures. Currently considered alternative intermediate temperature cathodes, such as Ba₀.₅Sr₀.₅Co₀.₈Fe₀.₂O₃₋δ (BSCF), offer improved performance, but have a large thermal expansion coefficient (TEC), leading to cell failure. In light of these issues, this dissertation focuses on the development of low TEC cathodes for intermediate temperature SOFCS (IT-SOFCs). The primary focus of this dissertation is on the swedenborgite-type RBaCo₃MO₇₊δ (R = Y, In, and Ca; M = Zn and Fe) series of cathodes. Due to their tetrahedrally-coordinated M site, the cobalt ions in these materials do not undergo a spin-state transition, and have TECs similar to conventional SOFC electrolyte materials. The long-term phase stability of these materials was addressed, and it was discovered that a slight In substitution significantly promoted phase stability. In the Y₁₋[subscript x] In [subscript x] BaCo₃ZnO₇₊δ series, it was observed that x = 0.1 successfully stabilized the phase without observable degradation of performance. Similarly, a high-Ca content material (Y₀.₅In₀.₁Ca₀.₄BaCo₃ZnO₇₊δ) was successfully stabilized, though Ca is known to destabilize the phase; furthermore, this compound showed improved performance compared to YBaCo₃ZnO₇₊δ. Lastly, the replacement of the performance-inhibiting Zn with Fe was investigated, and the Y₀.₉In₀.₁BaCo₃Zn₀.₆Fe₀.₄O₇₊δ sample showed low temperature performance rivaling BSCF. Other work in this dissertation focuses on the application of functional silver materials for use in SOFCs, with good performance; these materials were easily manufactured, and they showed performance drastically greater than the conventionally utilized platinum. / text

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published_or_final_version / Physics / Master / Master of Philosophy