Laser photolysis of TiO₂ layers in the presence of aqueous iodide

Laser photolysis of TiO₂ porous layers has been studied using iodide as a hole-scavenger. The I₂^- transient product, which is produced inside the layer, decays predominantly by the second-order reaction with TiO₂ electrons and by disproportionation. The resulting I₃^- ion also reacts with the TiO₂ electron, although the rate of reaction is 2 orders of magnitude slower than I₂^-. Evaluation of the rate constants was achieved by fitting computed curves of absorbance vs time profiles to the experimental data. The rate constants k(e_TiO2^- + I₂^-) = (2.25 ± 0.6) × 10⁵, k(e_TiO2^- + I₃^-) = (1.65 ± 0.4) × 10³, and k(I₂^- + I₂^-) = (1.1 ± 0.15) × 10⁵ M^-1 s^-1 have been determined in the TiO₂ layer. The rate constants suggest relatively low diffusion rates for I₂^- radical ion and e_TiO2^-. These have been attributed to electron trapping at nanocrystallite boundaries as well as activation and steric barriers. The rate of reaction of I₃^- with the TiO₂ electron suggests that this reaction must be important in photoelectrochemical cells involving iodide and iodine. The high photocurrent yields observed in some of these cells, is attributed to dense coverage of the TiO₂ surface by the photosensitizer.