  Time-resolved transient absorption spectroscopy has been used to study
nanosecond and sub-microsecond electron dynamics in aqueous anatase
nanoparticles in the presence of hole scavengers: chemisorbed polyols and
carbohydrates. These polyhydroxylated compounds are rapidly oxidized by the
holes; 50-60% of these holes are scavenged within the duration of 355 nm
excitation laser pulse. The scavenging efficiency rapidly increases with the
number of anchoring hydroxyl groups and varies considerably as a function of
the carbohydrate structure. A specific binding site for the polyols and
carbohydrates is suggested that involves an octahedral Ti atom chelated by the
poly-OH ligand. This mode of binding accounts for the depletion of
undercoordinated Ti atoms observed in the XANES spectra of polyol coated
nanoparticles. We suggest that these binding sites trap a substantial fraction
of holes before the latter descend to surface traps and/or recombine with free
electrons. The resulting oxygen hole center rapidly loses a CH proton to the
environment, yielding a metastable C-centered radical.
