Shape preserving conversion reaction of siliceous structures using metal halides: Properties, kinetics, and potential applications

Diatoms (Bacillariophyta), provide examples of living organisms or bio-machines that have the capability of producing very complex shapes through self-assembly, may be utilized in potential microdevices, and may play an important role in nanotechnology. The recently invented BaSIC process has opened a route for such utilization, that is the chemistry of the diatom frustule may be modified from silica. A number of BaSIC reactions have been developed in order to convert diatom frustules, or generally speaking any promising 3-D microscale silica structures, into numerous different compositions. This dissertation reports the BaSIC reaction of halide gases (i.e., TiF 4, ZrF4, and ZrCl4) with 3-D silica structures, (i.e., diatom frustules, silicified direct-write assembly scaffolds, and Stober silica spheres). Additionally, potential applications of the converted titania diatom frustules (i.e., as a fast micron-sized ethanol sensor, and as a pesticide hydrolyzing agent) are also demonstrated.;A two-step gas-solid displacement process has been successfully shown to generate zirconia diatom frustule replicas without a loss of the original frustule shape (i.e., the cylinder shape of Aulacoseira diatom frustules). The successful conversion of silica structures into anatase replicas with feature preservation at the scale of ∼100 nm has been demonstrated through the work on the Corethron criophilum diatom spines. Reaction temperature played a critical role for the shape preservation. Certain atmospheric conditions were also required during the oxygenation (TiOF 2 to TiO2) process to avoid structural distortion. An ethanol sensor, based on a single TiO2-converted diatom frustule, has been successfully fabricated. This sensor exhibited a more rapid response to the ethanol concentration change compared to other sensors reported in the literature, and was able to detect at least 100 ppm of ethanol in air. Significant enhancements of the rate of hydrolysis of pesticide-like compounds were observed in the presence of F-doped anatase TiO2-converted diatom frustules.;X-ray powder diffraction data and Rietveld analyses have been successfully used to characterize the crystal structure, and the temperature-induced phase transformation (from the room temperature hexagonal R3¯ c structure to the higher temperature cubic Pm3m structure) of polycrystalline TiOF2 that was synthesized through metathetic reaction of silica with TiF4(g). To analyze the kinetics of reaction of silica with titanium tetrafluoride gas, a novel HTXRD reaction chamber was developed. The combination of the HTXRD conversion data, the analyses of microstructural evolution, and the analyses of nitrogen physisorption data have suggested that the kinetics of the reaction of Stober silica spheres with titanium tetrafluoride gas could be modeled by assuming the existence of 2 kinetic regimes. The initial kinetic regime followed chemical reaction control, while the second kinetic regime followed mass transport control through a TiOF2 layer of constant thickness. The total reaction time was influenced by the initial size of the silica spheres and the TiF4 partial pressure.