Date of Award

8-2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Gerd Duscher

Committee Members

Kurt Sickafus, Kalyanaraman Ramakrishnan, Gong Gu

Abstract

TiO2 [titanium dioxide] nanoparticles (NPs) exhibit a variety of properties substantially departed from the optical and catalytic properties of the known bulk phases. A special interest in the engineering of functionality is how structures and bandgaps interact to enhance the photo-activity of TiO2. However, fundamental understanding of bandgap-related changes is difficult at these length scales because of scarcity of methods capable of probing the bandgap and the structure of isolated small particles. In this research, we adopt a series of highly spatially resolved transmission electron microscopy (TEM) techniques to characterize the structures of amorphous TiO2 nanoparticles (NPs) used as precursors and the TiO2 NPs with narrowed band gap derived from these amorphous precursors. The results show: (1) The amorphous TiO2 precursors consist of TiO6 [titanium atom surrounded by six oxygen atoms] octahedra randomly connected with each other; (2) An unconventional phase transformation occur wherein anatase and TiO2(B) coexist when annealing the amorphous TiO2 NPs in a temperature range from 400°C to 900°C in oxygengas; (3) Chromium and nitrogen co-doped TiO2 NPs have significantly narrowed band gaps originating from extra states on the top of valence band; (4) Black rutile NPs are produced by annealing amorphous TiO2 NPs at 700 °C in argon gas, which have a (crystalline TiO2) core-(amorphous Ti2O3 [titanium sesquioxide]) shell structure. Formation of Ti2O3 originates from diffusion of oxygen vacancies towards vacuum in the amorphous precursors; (5) The black anatase NPs are produced by annealing amorphous TiO2 NPs at 400 °C in argon gas, whose structures are (i) core-shell, or (ii) randomly distributed phases. These two structures originate from different degrees of crystallization and diffusion rate of oxygen vacancies; (6) Cubic-TiO2 is induced by the electron beam, which originates from the migration of interstitial Ti atoms grown on a rutile template; (7) We also discuss the origins of deviations from known phases’ EELS spectra which is the most important tool of electronic structure characterizations in this research; (8) Finally, we found a group of intense peaks in the interface between silicon and TiO2. These peaks may originate from interface plasmon excitations.

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