Chemical Vapor Deposition of One Dimensional Tin Oxide Nanostructures: Structural Studies, Surface Modifications and Device Applications (2010)

One-dimensional (1D) metal oxide nanostructures such as wires, rods, belts and tubes have become the focus of intensive research for investigating structure-property relationship under diminishing dimensions and probing their possible scientific and technological applications. Chemical vapor deposition (CVD), based on catalyzed vapor-liquid-solid (VLS) growth mechanism, is an efficient way to synthesize 1D metal oxide nanostructures, which can be implored by combining molecular precursors with CVD-VLS growth. This thesis contains results obtained on a molecule-based CVD approach to grow metal oxide nanowires, elaboration of experimental parameters enabling control over random and orientated growth. (1) Controlled synthesis, growth mechanism and plasma-treatment of SnO2 nanowires. Uniform and high-density single crystalline SnO2 NWs were fabricated by optimization of deposition temperature, precursor temperature, size of catalyst and angle of graphite holder, and the electrical, photoluminescence, gas sensing and field emission properties were also systematically investigated, it enabled us to have a better understanding of SnO2 nanowires. The technical highlights of this work include the successful demonstration of oriented growth of SnO2 nanowires arrays on TiO2(001) substrates by MB-CVD method for the first time. A growth model for the nanowire morphology based upon crystallographic relation between the substrate and NW material is proposed. Electrical and gas sensing properties of SnO2 [101] single nanowire showed that oriented nanowire arrays can be potentially used towards diameter- and orientation-dependent sensing unit for detection of gas molecules. Surface modification of SnO2 nanowires in an argon-oxygen (Ar/O2) plasma treatment caused preferential etching of the oxygen atoms from surface and the inner volume (lattice) producing a non-stoichiometric overlayer, resulting in the higher sensitivity for ethanol gas at lower operating temperature and exhibited improved transducing response towards changing gas atmospheres. (2) New architectures of SnO2 nanowire based 1D heterostructure: Synthesis and properties. New morphological SnO2 nanowire based heterostructures (such as SnO2@TiO2, SnO2@SnO2, SnO2@VOx and SnO2@CdS) were fabricated by chemical surface modification via a two-step process. Structural characterization of SnO2/TiO2 core-shell structures revealed the formation of mixed-cation phases of composition SnxTi1-xO2 (x = 0.857 ~ 1.0) depended on the annealing temperatures, the excellent electrical property and gas sensing performance of SnO2/TiO2 core-shell structures are attributed to nanowire based sensor applications. The SnO2@SnO2 heterostrucutres with contact angle (CA) of 133° exhibited a superhydrophobic property in comparison with the superhydrophilic SnO2 nanowires (CA = 3°). Switchable surface wettability of SiOx coated SnO2@SnO2 heterostructure (CA = 155.8°) was observed by alternation of UV irradiation, dark storage and O2 annealing. Geometric microstructure was the major determinant in the switchable wettability from superhydrophilic to superhydrophobic. The SnO2@CdS QDs heterostructures were fabricated by a chemical bath deposition (CBD) method via hydroxide cluster growth mechanism, and had a remarkably enhancement in photoconductivity than non-coated SnO2 nanowires when the wavelength was below 450 nm. The work carried out in this thesis is supported by Federal Ministry of Education and Research (BMBF) in the frame of the priority program "BMBF-NanoFutur" (FKZ 03X5512).