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

Time-resolved transient absorption spectroscopy has been used to study electron dynamics in aqueous anatase nanoparticles (pH = 4, 4.6 nm diameter) in the presence of hole scavengers: chemisorbed polyols and carbohydrates. These polyhydroxy compounds are rapidly oxidized by the holes on the nanoparticles; 50-60% of these holes are scavenged within the duration of 3.3 ns fwhm, 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 -CH2(OH)-CH2(OH)- ligand. This mode of binding accounts for the depletion of undercoordinated Ti atoms observed in the XANES spectra of 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 C-H proton to the environment, yielding a metastable, titanium-bound, ketyl radical.