We report on the adsorption of phenol on ZnO(10-10) single crystal surface in an effort to understand how environmentally persistent free radicals (EPFRs) are formed. EPFRs persist for orders of magnitude longer than typical radicals and have significant public health consequences. Transition metal oxides are known to catalyze the formation of hazardous materials such as dioxins and furans, and a proposed model for this catalytic reaction involves the formation of EPFRs as an intermediate. To date, EPFR formation has been studied on powders. We are extending these studies to single crystal substrates because they are amenable to more unambiguous spectroscopic methods that cannot be applied to insulating powder samples. Ultraviolet photoelectron spectroscopy (UPS) and electron energy loss spectroscopy (EELS) were used to gain insights on the electron transfer process during chemisorption. UPS data of ZnO(10-10) exposed to phenol at room temperature, 220�C, and 350�C dosing showed clear indications of chemisorption. The indication of charge transfer is observed by a shift of 0.6 eV to lower binding energy (i.e., band bending). Work function measurements also show a relative shift of 1 eV to higher kinetic energy. This indicates a charge transfer from the molecule to the metal oxide surface upon chemisorption. EELS spectra were also obtained for the same temperatures. EELS spectra show features assignable to phenol. The EELS spectra for physisorbed phenol at -177�C shows energy loss features at 6.5 eV, which is attributed to the p?p* transition of phenol. These features shift to lower energy as the dosing temperature increases. Thus giving the indication that the HOMO-LUMO gap is decreasing as the dosing temperature increases. These data combined with the UPS spectra give a clear picture of the electronic structure changes that occur during chemisorption of phenol.

Interactions of phenol with ZnO(10-10) surface: UPS and EELS studies

247th National Meeting (2014)

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