Doctor of Philosophy (PhD)
Department of Physics and Astronomy
We have studied the morphology and electronic structure of transition metal oxides (TMOs) nanoparticles and single-crystal surfaces which are known to be active for the formation of environmentally persistent free radicals (EPFRs) from organic precursors. We have also investigated the effect of simulated solar irradiation on the formed EPFRs.
First, we examined the change in the vibrational and structural properties of TiO2, ZnO, CuO, and Fe2O3 nanoparticles due to phenol adsorption at high temperature and thus EPFR formation on the surface of these TMOs. The paramagnetic signal observed by electron paramagnetic resonance (EPR) indicates the formation of phenoxyl-type radical. Vibrational spectroscopy has further confirmed the formation of EPFRs by the disappearance of –OH groups which indicates the chemisorption of the organic precursor on the metal oxide surface, as well as observing both ring torsion mode and C-H in-plane bend characteristic of phenol adsorption on the studied systems. In this study, we have also investigated the change of the oxidation of the metal cations upon phenol adsorption at elevated temperature. We have found that the direction of charge transfer (redox) during phenol chemisorption is strongly dependent on surface properties as well as surface defects of the metal oxide surface.
Second, we studied the adsorption behavior of phenol on the surface of alpha-Fe2O3 (0001) single-crystal surface at both high temperature and ambient to probe the changes in the electronic structure due to adsorption. At room temperature dosing, phenolic bands have been clearly observed in the UPS spectrum indicating the physisorption of phenol on the metal oxide surface. Upon dosing at high temperature, both techniques have shown clear features that strongly suggest a charge transfer from the organic precursor to the metal oxide. This evidence agrees well with the EPFRs proposed formation mechanism, and can guide future experimental and computational studies.
The spin density of EPFRs formed by phenol adsorbed on TiO2 nanoparticles at high temperature is found to increase upon exposure to simulated solar radiation as well as a mild shift in the g-value. Furthermore, solar irradiation induces the transformation of phenoxyl radical to a combination of catechol and some quinone product.
Sakr, Nadra Ibrahim, "Vibrational and Electronic Structure of Environmentally Persistent Free Radicals Formed on Metal Oxide Nanoclusters and Single-Crystal Surfaces" (2020). LSU Doctoral Dissertations. 5132.