Origin of Coverage Dependence in Photoreactivity of
Carboxylate on TiO<sub>2</sub>(110): Hindering by Charged Coadsorbed
Hydroxyls
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Abstract
The influence of reactant coverage
on photochemical activity was
explored using scanning tunneling microscopy (STM) and ultraviolet
photoelectron spectroscopy (UPS). We observed diminished reactivity
of carboxylate species (trimethyl acetate, TMA) on TiO<sub>2</sub>(110) as a function of increasing coverage. This effect was not linked
to intermolecular interactions of TMA but to the accumulation of the
coadsorbed bridging hydroxyls (HO<sub>b</sub>) deposited during (thermal)
dissociative adsorption of the parent, trimethylacetic acid (TMAA).
Confirmation of the hindering influence of HO<sub>b</sub> groups was
obtained by the observation that HO<sub>b</sub> species originated
from H<sub>2</sub>O dissociation at O-vacancy sites have a similar
hindering effect on TMA photochemistry. Though HO<sub>b</sub>’s
are photoinactive on TiO<sub>2</sub>(110) under ultrahigh vacuum conditions,
UPS results show that these sites trap photoexcited electrons, which
in turn likely (electrostatically) attract and neutralize photoexcited
holes, thus suppressing the hole-mediated photoreactivity of TMA.
This negative influence of surface hydroxyls on hole-mediated photochemistry
is likely a major factor in other anaerobic photochemical processes
on reducible oxide surfaces
