Synthesis and Characterization of One Dimensional (1d) Nanostructures for Energy Conversion.

Due to the global warming and faster depletion of fossil fuels, alternate cleaner energy such as thermoelectric and solar energy conversion is getting increasing attention to the researchers around the world. One-dimensional (1D) nanostructured materials show immense potential to enhance the efficiency of the existing energy conversion devices. However, reliable and scalable synthetic approaches to produce these functional 1D nanostructures are still lacking. The main objective of this thesis work is to synthesize boron-based and TiO2 1D nanostructures and understand their growth mechanisms, so that theoretical and practical foundations for future large-scale production of these 1D nanostructures can be laid. Boron-based 1D nanostructures are promising thermoelectric materials for power generation due to their unique "naturally-formed" superlattice feature. TiO2 1D nanostructures are promising materials to improve the efficiency of dye sensitized solar cells (DSSC) due to their enhancement of surface area and abundant electron transport. Vapor phase growth synthetic strategy was adopted in this work. Catalytic materials-assisted growth of alkaline-earth metal hexaboride (MB6, M=Sr, Ba) 1D nanostructures was achieved by pyrolysis of diborane (B2H6) over alkaline-earth metal oxide (MO) or alkaline-earth metal carbonate (MCO3) powders at elevated temperatures (∼890--960C) and low pressure (∼165 mTorr) in a homebuilt hot-walled low pressure chemical vapor deposition (LPCVD) system. Nickel (Ni), gold (Au) and palladium (Pd) metals were the effective catalytic materials. The as-synthesized MB6 1D nanostructures were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy and X-ray Diffraction (XRD). Results show that the MB6 nanostructures are single crystalline with the preferred growth direction along [001]. The SrB6 nanowires are ∼20--100 nm in diameter and ∼2--5 microm in length. The BaB6 nanostructures have a rectangular cross section (width ∼50--200 nm) and are ∼5--20 microm in length. The growth of these MB6 nanostructures can be attributed to a non-traditional vapor-solid-liquid (VLS)-like basal growth mechanism. Some preliminary work has been done on the study of thermal stability of these nanostructures using a homebuilt cold-walled LPCVD system. These MB6 1D nanostructures will find potential applications in thermoelectric energy conversion devices as n-type materials. TiO2 1D nanostructures such as nanowires and nanoribbons were synthesized by direct heating of Ni-coated TiO powders in argon at 850--920C and ∼755 torr in a tube furnace. The nanostructures were characterized by SEM, TEM and Raman spectroscopy. The 1D nanostructures were found to be single crystalline rutile TiO2. The preferred growth direction is along the [110] direction. The nanowires are roughly ∼10--50 nm in diameter, and 5--20 microm in length. The nanoribbons are 0.4--2 m in width and 5--20 microm in length. The growth of nanostructures under different conditions of substrate, catalytic material, reaction environment and chemical composition of solid TixOy precursor was studied. The possible growth mechanisms were also explored. In order to simplify the process even further, TiO2 nanowires were grown from Ti grid and Ti powders by the same mechanism. Tensile test was performed on individual TiO2 nanowires to determine the elastic modulus (48 to 170 GPa) and tensile strength (1.7 GPa). This growth technique provides a cost effective approach to fabricate TiO2 1D nanostructures for device integration, for example DSSC anode.