Georg Riegel

Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Titanium dioxide particles in the anatase crystal form are more efficient in typical photocatalytic applications than are particles in the rutile crystal form. Furthermore, substantial variations in photocatalytic efficiency are generally found among different samples of anatase, as well as rutile particles. The aim of the present study was to identify the dominant cause of these variations. Since electrons at the conduction-band edge of anatase have a larger "driving force" in the reduction of dissolved {dollar}\rm O\sb2{dollar}, it was of particular interest to investigate if the dissimilar efficiencies found for particles of rutile and anatase are primarily due to differences in the rate constant for reduction.;The use of electron paramagnetic resonance (EPR) spectroscopy, with spin-trapping for detection of the photoproduced hydroxyl radical ({dollar}\cdot{dollar}OH), allowed us to study a pre-steady-state component in the {dollar}\cdot{dollar}OH production. This component does not require the presence of a dissolved electron acceptor, and thus depends only on the rate constants for oxidation and recombination. Since the efficiency variations that characterize the steady state are found in this pre-steady-state component, we propose that changes, not in the rate constant for reduction, but in those for recombination and/or oxidation cause the observed variability in steady-state photocatalytic efficiency. An analysis of the pre-steady-state and steady-state efficiency variations, based on a simple kinetic model, indicates that these variations are specifically due to variations in the rate constant for recombination.;Size-quantized rutile particles in the form of quantum-wires were obtained from {dollar}\rm TiCl\sb4{dollar} hydrolysis. It is expected that almost the entire bandgap increase of {dollar}\sim{dollar}0.14 eV results in an increase of the oxidative power of holes at the valence-band edge. Thus, the role of the hole oxidative power could be investigated with the pre-steady-state {dollar}\cdot{dollar}OH production described above. This role appears to be of minor importance.;A detailed structural investigation of a fumed titanium dioxide photocatalyst (Degussa, P25) was carried out. It had been suggested earlier that this particularly efficient form of {dollar}\rm TiO\sb2{dollar} might be characterized by rutile overlayers on anatase particles, producing an enhanced efficiency of electron/hole separation. However, our results indicate that these overlayers are not present, and that the interfacial area between the two phases is thus limited to instances that arise from particle sintering during the manufacturing process.;We also propose a method for the investigation of {dollar}\rm TiO\sb2{dollar} particles with highly local surface probes such as scanning tunnelling and atomic force microscopes. Preliminary results have been obtained and are discussed.



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