CONTROLLED GROWTH AND PROPERTY STUDY OF ONE-DIMENSIONAL AND TWO-DIMENSIONAL LAYERED NANOSTRUCTURES OF MOLYBDENUM OXIDE
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Abstract
Molybdenum trioxide (MoO3) is a versatile semiconductor material with a wide range of potential applications in photocatalysis, electrochromism/photochromism, sensing, and energy storage. Its highly anisotropic layered structure with weak van der Waals interactions makes it easy to form one-dimensional (1D) and two-dimensional (2D) layered nanostructures. This dissertation focuses on realizing the controlled growth of MoO3 nanostructures with tunable structures, exploring the corresponding growth mechanisms, and studying the resulting properties. It is composed of four major research themes, including (1) study of catalyst effects of alkali metal-based catalysts on the morphologies of MoO3 nanostructures, (2) low temperature growth of MoO3 nanoribbons with uniform shapes, (3) synthesis of 1D ITO (indium tin oxide)-MoO3 core-shell nanostructures with different morphologies, and (4) photocatalytic investigation of 2D MoO3 nanoflakes with enhanced low-temperature protonation performance and tunable plasmonic properties. As the core part of this dissertation, the last theme (chapter 5) covers material synthesis and processing, structure characterization and optical property measurement, and photothermal study of reaction mechanism. Formation of MoO3 nanoflakes was realized using a facile liquid exfoliation of MoO3 whiskers from chemical vapor deposition (CVD). A low-temperature protonation reaction of MoO3 nanoflakes with pure alcohol under visible light irradiation was discovered. The reaction is initiated by the visible light absorption from sub-bandgap defects and expedited by photothermal heating. The low reaction temperature provides a new low-cost method to produce substoichiometric semiconductors with tunable plasmonic behaviors. The reaction mechanism can be extended to other photocatalytic processes of MoO3 nanostructures to improve their efficiencies in utilizing solar energy.For the first theme (chapter 2), the growth resulted in different morphologies (e.g., rectangular fork-end nanoplates, long nano/micro-belts, and microtowers) and highly uniform millimeter long nanobelts were successfully synthesized across the entire substrate.During the second theme (chapter 3), high density and uniform growth of α-MoO3 rectangular nanoribbons (sharp edges) with the length ranging from 7 to 10 µm, typical thickness of 60-130 nm, and width around 350-800 nm was successfully established at the relatively low temperature of 350-550◦C with a minimum amount of source materials (as low as 6 mg). For the third theme (chapter 4), growth mechanisms of ITO nanowires (NWs) was discussed for specific temperature zones (high to low) inside the (CVD) reaction chamber. After successful growth of ITO NWs, shell layer materials (MoO3) were deposited on the ITO core by altering different growth parameters. Deposition of shell materials on ITO NWs led to three distinguished morphologies of longitudinal, transverse, and needle shaped structures. It was concluded that ultra-thin (50 nm) 1D core-shell nanostructures (ITO/MoO3) were successfully synthesized. Under this condition, as-synthesized ITO NWs were fully covered with a thin layer of shell materials.