PHOTOCHEMISTRY AND PICOSECOND ABSORPTION-SPECTRA OF AQUEOUS SUSPENSIONS OF A POLYCRYSTALLINE TITANIUM-DIOXIDE OPTICALLY TRANSPARENT IN THE VISIBLE SPECTRUM

Optically transparent suspensions of (Degussa P25) titanium dioxide wh ich mimic well the photochemical and photophysical properties of the b ulk material were prepared by sequential centrifugation. Quantum yield s of photocatalyzed oxidation of I- and of propanal were measured unde r monochromatic 313 nm irradiation and found to be very similar to tho se estimated for concentrated P25 slurries but higher than those measu red in colloidal TiO2 solutions. A unit limiting quantum yield for pro panal photooxidation suggests very efficient utilization of photogener ated holes. Time-resolved absorption spectra were measured in the 50 p s-10 ns time interval following excitation by a 30 ps, 355 nm laser pu lse and found to be definitely different from picosecond spectra obser ved earlier for TiO2 colloids. It is proposed that the observed transi ent signal may arise from a photogenerated hole that undergoes thermal ization and surface-trapping with a lifetime of approximately 1 ns.