Investigation of titanium dioxide polymorph selectivity as influenced by the amorphous precursor state

Abstract

Polymorphism in the titanium dioxide system provides an opportunity for enhancing performance and functionality through targeted synthesis of phases higher in energy than the most thermodynamically favorable phase. Both the anatase and brookite polymorphs are metastable relative to the most thermodynamically stable structure, rutile. Along with their metastability, these two polymorphs also exhibit photocatalytic activity. The ability to selectively synthesize each of these titanium dioxide polymorphs would prove invaluable in the pursuit of optimizing material properties for photocatalysis. A fundamental understanding of the mechanisms that guide polymorph formation in this system is critical for proficiently directing the formation of metastable structures from amorphous precursors. Elucidating these mechanisms is the goal of the work contained in this dissertation. A variety of characterization techniques including transmission electron microscopy, electron energy-loss spectroscopy, and electron diffraction-based pair distribution function calculations were utilized to study the atomic structure and chemistry of the crystalline polymorphs and their respective amorphous precursors. Selective synthesis of the rutile, anatase, and brookite polymorphs is presented through manipulation of the amorphous phase that precedes them. Manipulating oxygen content in the amorphous precursor through controlling the oxygen pressure during synthesis is shown to crystallize anatase from amorphous precursors grown under high oxygen pressures (> 5 mTorr), rutile from low oxygen pressures (< 0.1 mTorr), and brookite from intermediate oxygen pressures (0.5 - 1 mTorr). Pair distribution functions measured from the amorphous precursors show that these changes in Ti–O stoichiometry reconfigure the relative arrangements of TiO6 octahedra. Similarities are found between the octahedral structure in the amorphous precursor and the unit cell structure of the crystalline polymorph it forms. These results suggest structural templating in the amorphous phase as a mechanism for preferentially nucleating and growing both thermodynamically stable and metastable polymorphs. A concept of amorphous precursor engineering for directing the crystallization of titanium dioxide polymorphs is postulated and discussed based on the research contained in this dissertation. This concept demonstrates a wide-ranging potential for synthesis of metastable phases in polymorphic systems where the stabilization of higher energy phases provides a route to superior functionality over their stable counterparts.