Morphology-preserving chemical conversion of bioorganic and inorganic templates
Vernon, Jonathan P.
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The generation of nanostructured assemblies with complex (three-dimensional, 3D) self-assembled morphologies and with complex (multicomponent) tailorable inorganic compositions is of considerable technological and scientific interest. This research demonstrates self-assembled 3D organic templates of biogenic origin can be converted into replicas comprised of numerous other functional nanocrystalline inorganic materials. Nature provides a spectacular variety of biologically-assembled 3D organic structures with intricate, hierarchical (macro-to-micro-to-nanoscale) morphologies. Morphology-preserving chemical conversion of such readily available, structurally complex templates will provide a framework for chemical conversion of synthetic organic templates and, potentially, production of organic/inorganic composites. Four research thrusts are detailed in this dissertation. First, chemical conversion of a nanostructured bioorganic template into a multicomponent oxide compound (tetragonal BaTiO₃ via layer-by-layer surface sol-gel coating and subsequent morphology-preserving microwave hydrothermal processing was demonstrated. Second, photoluminescence was imparted to bioorganic template structures through morphology-preserving chemical conversion to exhibit both the dramatic change in properties such processing can provide, and the potential utility of chemically transformed templates in anti-counterfeiting / authentication applications. Third, the reaction mechanism(s) for morphology-preserving microwave hydrothermal conversion of TiO₂ to BaTiO₃, were studied with the aid of Au inert markers on single crystal rutile TiO₂. Finally, constructive coating techniques (SSG) and moderate temperature (< 500C) heat treatments were utilized to modify and replicate structural color and were coupled with deconstructive focused ion beam microsurgery to prepare samples for microscale structure/property interrogation. Specifically, the effects of coating thickness and coating composition on reflection spectra of structurally colored templates were examined. Also, the effects of the replacement of natural material with higher index of refraction inorganic materials on optical properties were studied. The three processing research thrusts constituting chapters 1, 2 and 4 take advantage of moderate temperature processing to ensure nanocrystalline materials, either for shape preservation or to prevent scattering in optical applications. The research thrust presented in chapter 3 examines hydrothermal conversion of TiO₂ to BaTiO₃, not only to identify the reaction mechanism(s) involved in hydrothermal conversion under morphology-preserving conditions, but also to introduce inert marker experiments to the field of microwave hydrothermal processing.