The Growth Mechanism Of Oxide Nanowires

One-dimensional oxide nanomaterials have been widely used in a variety of applications including sensors,energy-related devices,and chemical catalysis owing to their unique optical,thermal,electrical,and magnetic properties.However,vapor growth methods particularly VLS have achieved limited success in fabricating oxide nanowires with an emphasis on simple oxides.In addition,the growth mechanism of oxide nanowires is also far from fully understood.In previous work of our group,TiO₂nanowires with a length of a few microns were observed on the surface of TiO₂ substrate after heat treatment of Au-coated single-crystal(110)-TiO₂substrate at high temperature.A mechanism was proposed emphasizing the role of Au surface in adsorbing and transporting the vapor products of vaporization.In this thesis,we examined the growth mechanism of oxide nanowires by modifying the composition of metallic particles(e.g.Au/Ag-TiO₂ system)which would subsequently change the surface adsorption and transport behavior of the particles.Besides,composition tuning was used to promote the growth of complicated oxides(e.g.Au/Mn-MgAl₂O₄).Our results help to understand the growth theory of oxide nanostructures from new aspects,and provide a possible method to synthesize oxide nanowires especially for multi-component oxides.The main results are summarized as follows:(1)With Au/Ag particles,the growth direction of TiO₂ nanowires was changed resulted from the aggregation of Ag at growth front.This was consistent with the different surface adsorption and transport properties of Au-rich domains and Ag-rich domains,thus solidifying the proposed growth mechanism.(2)The growth of oxide nanostructures on MgAl₂O₄ substrate could be promoted by Au/Mn droplets.Two types of nanostructures were obtained including Al-rich spinel nanopillars and Al-deficient orthogonal enstatite nanowires under the same growth mechanism with Au/Ag-TiO₂ system,indicating the generality of this vaporization method for synthesizing oxide nanowires.(3)The influence of heat treatment conditions on the growth of oxide nanowires was investigated.The results showed that high temperature and sufficient time are required for successful fabrication of oxide nanowires.(4)According to the analysis of equilibrium pressure,producing rate,and consuming rate of vaporization products,vaporization of source materials is the key limiting factor for oxide growth.