TIME-RESOLVED RISE OF I-2(-) UPON OXIDATION OF IODIDE AT AQUEOUS TIO2 COLLOID

The rise of the one-electron oxidation intermediate I2- formed upon band-gap excitation of bare TiO2 colloidal particles in aqueous iodide solution has been resolved for the first time. These measurements are based on a very sensitive transient absorption technique that allows monitoring of I2- at iodide concentrations as low as 10(-4) M. The linear dependence of the pseudo-first-order rise constant on iodide concentration (slope = (7.9 +/- 1.2) X 10(9) L mol-1 s-1) suggests that the observed step reflects the reaction of surface-adsorbed I atoms with iodide from the surrounding solution phase. The adsorbed I atoms are formed by the reaction of surface-adsorbed iodide with valence band holes with in the 5-ns duration of the 355 band-gap excitation pulse. Analysis of the decay kinetics of I2- and measurement of the concurrent I3- yield revealed that interparticle reduction by trapped conduction band electrons is the main decay path of I2-. These results are compared with a previous time-resolved study of iodide photooxidation at dye-sensitized TiO2 colloids.