Non-catalytic growth of metal oxide nanowires: Properties and growth mechanism investigations

This thesis is devoted to the non-catalytic syntheses of metal oxide nanowires (NWs), and investigations of their properties and growth mechanisms. Two different approaches were applied for the syntheses - metal resistive heating and vapor growth methods. The products were thoroughly characterized by electron microscopy, optical and X-ray characterization techniques. The synthesized NWs were examined for field emission (FE) and ultraviolet (UV) sensing applications. The resistive heating of various metals was demonstrated to be an efficient, simple and rapid method for the synthesis of CuO, Fe2O3, V2O5 and ZnO NWs under ambient air conditions. Fe2O3 NW formation was detected after just 2 s of heating; other metal oxide NWs were grown after 10 s. The NW growth mechanism during metal oxidation was explained based on observations of ZnO and Fe2O3 NW growth. The mechanism is based on the diffusion of metal ions to the surface through grain boundaries and to the tip of the growing NW through defect diffusion and by surface diffusion. FE from NWs grown by the resistive heating method exibited promising results for applications in vacuum electronic devices. Cold electron FE measurements showed that CuO NWs have a very low threshold electric field of 4 V/µm at a current density of 0.01 mA/cm². For the vapor growth of ZnO tetrapods (ZnO-Ts) a vertical flow reactor was designed and constructed. It was shown that the morphology of ZnO-Ts could be adjusted via the Zn vapor pressure in the reactor. The highest aspect ratio of ZnO-T legs was obtained at 700 °C, at a Zn partial pressure of 0.08 atm. ZnO-Ts demonstrated application possibilities for transparent and flexible UV sensors. Sensors based on ZnO-Ts showed a 45-fold current increase under UV irradiation with an intensity of 30 µW/cm² at a wavelength of 365 nm, and a response time of 0.9 s. The high performance of the device was explained by the multiple contact barriers.