| dc.description.abstract | The focus of this dissertation is to investigate the synthesis of crystalline transition metal oxide (TMO) structures in a counter-flow diffusion flame. This thesis examines the underlying mechanisms for the flame synthesis of 1D, 2D and 3D nano and micron scaled TMO structures on the solid support and in the flame gas-phase. The understanding of the growth mechanisms of the synthesis of single metal structures, along with hybrid and heterostructures with complex morphologies is essential to tailor the fabrication of TMO nanoscale structures. These new novel structures enable a multiplicity of applications in critical areas of today’s world economics such as clean energy and sustainability (i.e. solar energy, batteries, smart buildings, gas sensors, etc.). The source material, solid supports of high purity transition metals, in the form of wires are introduced in the flame medium to trigger the synthesis of crystalline nanostructures. The insertion of single and dual solid supports in the oxygen-enriched diffusion flame is investigated along with the rapid formation of structures in the gas-phase. The solid support serves as, both, the material source and the site for disposition, allowing for crystallization of the formed structures at different locations around the surface of the metal probes. The study presented herein commenced with the fabrication of 1D and 3D tungsten-oxide nanostructures on the surface of a single solid support (i.e., W probe) inserted in the opposed-flow oxy-fuel flame. The unique 1D nature of the counter-flow diffusion flame, along with is high thermal and chemical species gradients favors the transformation of bulk transition metals into 1D, 2D, and 3D structures with complex morphologies. The physico-chemical properties of the transition metal probe (i.e., diameter and elemental composition) and flame composition (i.e., oxygen content in the oxidizer stream) were varied to investigate their influence on the synthesized nanostructures. We hypothesize that the concentration rate of metal oxide vapors generated from the surface of the probe, once inserted in the oxygen-rich flame medium, along with flame residence time, flame oxidizer composition and precursor location in the flame volume are key parameter for controlling the morphology of the structures. All of the mentioned factors can influence flame temperature and composition of oxygen species, along with the rates of oxide generation, subsequent crystallization and morphologies of the structures formed. Moreover, the unique structure of the 1D flame geometry with one region rich in carbon species and the other in oxygen radicals can be leveraged for the synthesis of even more complex assemblies such as hybrid carbon-metal oxide structures.
The formation of triangular, rectangular, square, and cylindrical 3D channels with completely hollow or semi-hollow morphologies was achieved by varying the probe diameter from 1.0 to 0.5 mm. Whereas the increase of the O2 content to 100% and the employment of a 1 mm high purity W probe resulted in the growth of 2D ribbon-like micron-sized structures. The lattice spacing of ~0.38 nm measured for the 1D tungsten-oxides closely matches that of monoclinic WO3 structure. X-ray photoelectron spectroscopy analysis revealed that the larger 3D structures also consist of WO3, confirming that the chemical composition of the structures remains the same while tailoring both the probe and flame parameters. The flame geometry was used to synthesize hybrid C-metal oxide structures. This was achieved by inserting a high purity 1 mm diameter W probe into the oxygen rich flame environment (formed with an oxidizer composition of 50% O2 + 50% N2) to form 1D tungsten-oxide structures. The newly formed structures were exposed to the carbon rich environment of the flame, using a sleeve, to coat their surfaces with various layers of carbon shells, thus, forming 1D hybrid nanowires composed of tungsten-oxide cores covered with a uniform carbon sheath. The physical features of the grown structures on the solid support present lengths up to 50 µm and diameters ranging from 20 to 50 nm. This study reveals the existence of a common generic mechanism consisting of tungsten-oxides/hydroxides layers being formed on the probe surface (fuel side) exposed to the high-temperature oxidative environment. These oxides/hydroxides layers are then evaporated/sublimated as the metal probe continues being exposed to the flame volume. The formed oxide vapors are then transported by the gas flow towards the stagnation plane and crystalizes in the form of 1D nanomaterials on the upper surface of the probe where the temperatures are lower. The proposed growth mechanism for the 3D structures consists of coalescing of 1D tungsten-oxide nanorods. The growth of the 3D channel hollowed structures is favored by material deposition from the gas-phase along the edges of early formed large cubical structures (Berg effect). Thereby, resulting in deposits of hollowed structures since solely a limited amount of materials can diffuse to the heart of the formed 3D structure. Regarding the hybrid nanowires, a two-step synthesis mechanism is proposed: (i) tungsten-oxide nanorods are formed in the oxygen-rich flame region; (ii) the rapid formation of carbon shells from hydrocarbon species attained from the carbon-rich zone of the flame during the probe removal process.
The gas-phase structure/particle synthesis sets the foundation for increase production of tungsten-oxide nanostructures. The synthesis of well-defined faceted octagonal prisms (octahedron nanoplatelets) and elongated structures of high aspect ratio in the form of rod-like nanocrystals is performed in the flame gas-phase employing a solid-fed precursor flame technique using a 99.9% purity 1 mm diameter W probe as the precursor source. The growth mechanism of the formed nanoscale structures involves the oxidation of the surface of the metallic probe, evaporation of the newly formed oxide layer followed by transport of the metal oxide vapors toward the hydrocarbon-rich zone of the flame. As the transport takes place, the metal oxide vapors are crystallized into well-defined octahedron nanoplatelets and elongated rod-like nanocrystals. The lattice d-spacing of grown nanoplatelets and of the rod-like nanocrystals was measured to be ~0.38 nm closely corresponding to the (002) plane of monoclinic WO3 structures.
The last part of this research consisted of studying the impact of dual solid support synthesis to aid the formation of hybrid and complex structures with a multiplicity of morphologies. W and Mo probes of 1 mm in diameter are inserted in an oxygen-rich zone at flame positions of Z = 13 and Z = 11 mm, respectively. Experiments consisted of simultaneous and varied probe insertion in the flame medium. Evidently, controlling flame position (temperature/radicals’ concentration), residence time (rate of oxide vapors production), and probe insertion sequence results in the formation of highly complex structures with multiple morphologies (e.g., polyhedral-, tree-, flower-, forest-, and grass-like nanostructures). The growth of these unique structures is driven by the formation of 1D “backbone” structures that serves as precursors for the synthesis of the multiple structures with complex morphologies. HR-TEM/EDX/SAED analyses were used to characterize the formed structures. A proposed growth mechanism for the synthesis of these hybrid complex structures is also presented. It is interesting to note that tungsten-oxide structures with unique morphologies were grown not only on the solid support surface but also in the flame gas-phase providing insightful knowledge regarding the mass production of these materials with a wider range of applications. | en_US |
