Electronic Signatures of a Model Pollutant–Particle
System: Chemisorbed Phenol on TiO<sub>2</sub>(110)
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Abstract
Environmentally persistent free radicals
(EPFRs) are a class of
composite organic/metal oxide pollutants that have recently been discovered
to form from a wide variety of substituted benzenes chemisorbed to
commonly encountered oxides. Although a qualitative understanding
of EPFR formation on particulate metal oxides has been achieved, a
detailed understanding of the charge transfer mechanism that must
accompany the creation of an unpaired radical electron is lacking.
In this study, we perform photoelectron spectroscopy and electron
energy loss spectroscopy on a well-defined model system–phenol
chemisorbed on TiO<sub>2</sub>(110) to directly observe changes in
the electronic structure of the oxide and chemisorbed phenol as a
function of adsorption temperature. We show strong evidence that,
upon exposure at high temperature, empty states in the TiO<sub>2</sub> are filled and the phenol HOMO is depopulated, as has been proposed
in a conceptual model of EPFR formation. This experimental evidence
of charge transfer provides a deeper understanding of the EPFR formation
mechanism to guide future experimental and computational studies as
well as potential environmental remediation strategies