Doctor of Philosophy (PhD)
We have investigated the electronic structure of environmentally persistent free radicals (EPFRs) formed on two different metal oxides: Cu(II)O and ZnO. Cu(II)O is known to be one of the most active metal oxides to form EPFRs. ZnO, however, forms the longest-lived EPFRs. First, we examined the adsorption of phenol on different crystal faces of ZnO. Ultraviolet photoelectron spectroscopy (UPS) shows direct evidence of charge transfer away from the ZnO to the phenol. This differs from the hypothesized mechanism of EPFR formation that says that an electron is transferred to the metal oxide. UPS difference spectra also shows that the phenol HOMO on the ZnO(1010), ZnO(0001)-Zn, and ZnO(0001)-O differs significantly between each crystal face. The ZnO(1010) matches well with a calculated spectrum of a single phenol molecule while the other two surfaces show states either being depopulated or degeneracy being added to the phenol. Electron energy loss spectroscopy shows significant HOMO-LUMO gap narrowing of phenol on ZnO(0001)-Zn compared to gas phase phenol or physisorbed phenol. Low energy electron diffraction shows that phenol forms a semi-ordered c(2x2) superstructure only on the (1010) surface. Second, we examined the adsorption of phenol and chlorinated EPFR precursors on Cu(II)O nanoparticles. X-ray absorption near edge structure revealed that by heating Cu(II)O nanoparticles without the presence of any EPFR precursors Cu(II)O is reduced to Cu(I)2O. When Cu(II)O nanoparticles were exposed to phenol at 220 °C (minimal reduction by heating) reduction to Cu(I)2O was observed. This is direct evidence of charge transfer to the metal oxide in agreement with the hypothesized EPFR formation mechanism. The amount of reduction suggests that a significant fraction of metal atoms are on the interior of the nanoparticles and are unavailable for reduction, but the amount of reductions are inconsistent which could also suggests experimental flaws. Chlorinated EPFR precursors were adsorbed to Cu(II)O nanoparticles to observe the reactivity of the surface and measure their 3D structure using x-ray fluorescence spectroscopy. While we show distinct Cu peaks at the appropriate intensities no Cl was observed under any conditions which suggest possible dechlorination from either the spectrometer source or the nanoparticles.
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Thibodeaux, Chad Allen, "The Electronic Structure of Environmentally Persistent Free Radicals Formed on Metal Oxide Surfaces" (2015). LSU Doctoral Dissertations. 2151.