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Efficient, rapid one-electron photooxidation of chemisorbed polyhydroxyl alcohols and carbohydrates by TiO2 nanoparticles in an aqueous solution

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.