UCLA

This dissertation presents the investigation of unique ZnO nanostructures grown in thermal oxidation, low-temperature oxygen injection CVD, and a new, low-cost and robust method of hybridizing ZnO nanostructures on carbon fiber through combustion synthesis. This new method will lead to novel multifunctional composites because ZnO nanostructures have unique piezoelectric and photoelectrochemical properties.

This work first investigated ZnO nanostructure growth mechanisms through vacuum thermal oxidation and CVD experiments. Through thermal oxidation, unique hollow structures were developed under substrate heating, demonstrating ZnO nanostructures growth through diffusion mechanisms. In the low-temperature oxygen fed CVD, many ZnO nanostructures including nanosheets, tetrapods could be consistently grown by stabilizing Zn vapor pressure and controlling the oxygen flow. This would prevent premature oxidation at lower temperatures before Zn and oxygen could deposit into ZnO nanostructures.

The second part of this work, investigated zinc combustion for ZnO nanostructure synthesis. It was found that the combustion reaction is affected by the amount of zinc precursor concentration and the heat transfer rate. The greater amount of zinc precursor requires a lower heat transfer rate to initiate the combustion process. Upon a sufficient heat transfer, combustion will occur. If the heat transfer rate is too low, no combustion occurs. Near the minimum heat transfer rate, the reaction is considered metastable. The results of the reaction were characterized through SEM, TEM, and XRD. Various ZnO nanostructures such as tetrapods, sheets, nanowires, and hollow sea urchins were observed for the combustion reaction, metastable reaction, and the non-combusted reaction. The nanostructures found in this system were match to similar ZnO nanostructures found in the thermal oxidation and CVD experiments. Based on the similarity, ZnO nanostructure growth mechanisms were proposed for the combustion process. ZnO nanostructures hybridized on the carbon fiber grow through deposition and diffusion mechanisms. The nanostructures' growths were affected by the heat transfer rate and oxygen availability.

Finally, a composite with piezoelectric response has been constructed and demonstrated. Upon mechanical deflection, the composite produced a voltage response of 0.1V. The carbon fiber hybridized with ZnO nanostructures preliminary results enable further development of multifunctional composites having sensing, damping, and energy harvesting capabilities.