Jin Wang

Date of Graduation

2009

Document Type

Thesis

Abstract

TiO2 are extensively used in the photocatalysts and photoelectrochemical devices for solar energy harvesting, hydrogen generation and environment remediation. The photocatalysis efficiency of TiO2 depends on the electron-hole separation and the electron-hole recombination processes. It remains a challenge to improve the generation efficiency of electron and holes and to reduce the electron-hole recombination rate. Furthermore, the large bandgap of pristine TiO2 limits its photocatalytic activity to the ultraviolet regime that accounts for about 5% of the spectral output of sunlight. This research aims to facilitate the photocatalytic applications of TiO2 through accomplishing three specific tasks: (i) synthesize one-dimensional (1-D) TiO 2 nanostructures by hydrothermal processing and tailor their structures; (ii) develop TiO2 (101) nanobelt as an efficient photocatalyst and gain fundamental understanding of its enhanced surface reactivity and reduced electron-hole recombination rate; (iii) extend the photocatalytic activity of TiO2 to the visible light range by doping nitrogen atoms into the TiO2 lattice. In the present work, hydrothermal processing has been developed as a cost-efficient, scale-up, reproducible synthetic route to fabricate 1-D single-crystalline titanate and titania nanostructures. The synthesis factors have been systematically investigated to ensure the repeatability and controllability of the method. The growth mechanism of 1-D nanostructures formation is also proposed. It is demonstrated how to tailor the titania nanomaterials in terms of the shape, crystal structure and phase composition. The photocatalytic activities of the TiO2 nanospheres, nanotubes and nanowires have been compared. It is found that the nanobelts exhibit the highest photocatalytic activity. In particular, single-crystalline anatase TiO2 nanobelts with two dominant surfaces terminated by the (101) facet, which are 60–400 nm wide, ∼10 nm thick, and up to 30 µm long, are superior in the photocatalytic activity to the nanosphere counterparts. The exposed (101) facet endows the nanobelts the enhanced reactivity with molecular O2 and thus facilitate the generation of superoxide radical ([special characters omitted]). Moreover, the nanobelts exhibit a lower electron-hole recombination rate than the nanospheres, which is due to three reasons: (i) higher charge mobility in the nanobelts where the charge carriers are able to move throughout the length dimension of single crystal; (ii) less localized states near the band edges and in the bandgap of the nanobelts; and (iii) enhanced charge separation due to trapping of photogenerated electrons by chemisorbed molecular O2 on the (101) facet. Our results indicate that the photocatalytic activity of TiO2 can be tailored by its shape and surface structure. Nitrogen-doped anatase titania nanobelts have been prepared via heat treatment in NH3. Higher treatment temperature results in higher nitrogen content and higher oxygen vacancies in the anatase TiO2 lattice. Nitrogen doping leads to an add-on shoulder on the edge of valence band, the localized N 2p levels above the valence band maximum and the 3d states of Ti3+ below the conduction band, which is confirmed by DFT calculation and X-ray photoelectron spectroscopy (XPS) measurement. Extension of the light adsorption from the ultraviolet (UV) region to the visible-light region arises from the N 2p levels near the valence band and the color centers induced by the oxygen vacancies and the Ti3+ species. Nitrogen doping allows a visible-light-responsive photocatalytic activity but lowers the UV-light-responsive photocatalytic activity. The visible-light photocatalytic activity originates from the N 2p levels near the valence band. The amount of oxygen vacancies and the associated Ti3+ species, whose content increases depending on the heat treatment temperature under NH3. These oxygen vacancies and the associated Ti3+ species act as recombination centers for the photo-induced electrons and holes, which is responsible for the reduced the UV-responsive photocatalytic activity. Owing their unique structures, single-crystalline nanobelts can be used as interesting building blocks or precursors in material fabrication. In this study, surface chemistry is found to induce in-situ phase transformation of hydrogen titanate nanobelts to titania nanomaterials in an aqueous solution. It is deduced that the interaction of proton or hydroxide ions with H 2Ti3O7 skeleton is the key to the phase transformation. Hydroxide ions could neutralize the hydrogen ions in H2Ti 3O7 skeleton, while hydrogen ions could adsorb onto under-coordinated oxygen sites. The adsorption of dissociate hydrogen/hydroxide ions modify the nanobelt surface, results in the unstable non-equilibrium energy state, which induces the overall phase transformation. The transition shows a tendency from titanate→anatase→rutile with increase in the acidity of the solution.

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