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Bio-Inspired Crystallization Of Oxides In Inorganic Matrices

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Abstract

The energy crisis facing our planet requires solutions that take an interdisciplinary approach to the improvement of existing energy systems as well as the development of new energy sources. Moreover, the composition of the materials is important: thermally- and chemically-stable materials based on abundant, non-toxic elements are needed to support the sustainability of both the technology and our environment. Biological organisms present multiple examples of hierarchical structures that are optimized for a given function. In particular, biomineralized materials: (i) display crystallographic control across length scales; (ii) are often organic-inorganic composites due to the occlusion of components from the associated organic growth matrix; (iii) and exhibit tailored mechanical properties that are unique to their function. Of great importance to the development of advanced energy materials is the observation that biomineral architectures are built from crystallographically-defined structural elements with interfaces that span multiple length scales. Synthetically, the translation of biological mineralization strategies to oxide compounds is hindered by the low melting temperatures of biopolymer hydrogels that compose extracellular matrices. In order to successfully crystallize oxide compounds using a (bio-inspired) matrix-mediated approach, I had to identify and develop a hydrogel system with thermal stability and chemical compatibility to the growth conditions needed for the oxide. By moving to inorganic networks based on silica, I achieved a thermally-stable growth matrix. By forming these networks at low pH, I obtained a growth environment that was compatible with the crystallization of hematite. With these two features, hematite was crystallized under diffusion-limited conditions, which provided a means to to manipulate its structure and assembly from the atomic- to the microscale. By combining inductively coupled plasma atomic emission spectroscopy with Rietveld refinements to x-ray diffraction data, expansion of the hematite lattice along the c-axis was found to be correlated to increasing silicon in the crystals and the preferential growth of the coherent domains along [110] (perpendicular to the strained c-axis). Using single particle manipulation in a focused ion beam system, electron-transparent thin sections were prepared from precisely-defined geometric locations within the hematite crystals for analysis by transmission electron microscopy. Quantitative analysis on selected area electron diffraction patterns was used to unravel the net orientation of the hematite lattice with respect to the quasi-spherical form and to calculate the misorientation (mosaicity) between the coherent domains. The combined results of these analyses showed that silicon from the growth environment had consistently modified the architecture of hematite, from the atomic to the microscale, leading to microscale structures with surfaces composed of nanoscale, high catalytic activity {110} facets. With hydrogel growth as a demonstrated route to tune the hierarchical structure of a transition metal oxide to preferentially express desired planes, the bandgap and photocatalytic activity of the samples was studied, to reveal that these micro-scale hierarchical architectures outperform their nano-sized counterparts, presenting a new approach to the design of materials for advanced energy applications.

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2015-05-24

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Biomineralization; Crystallization; Transition Metal Oxides

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Estroff,Lara A.

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Disalvo,Francis J
Gruner,Sol Michael

Degree Discipline

Materials Science and Engineering

Degree Name

Ph. D., Materials Science and Engineering

Degree Level

Doctor of Philosophy

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Government Document

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dissertation or thesis

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