Reduction of I2/I3− by Titanium Dioxide

A photoelectrochemical cell was designed that allowed the reactivity of oxidized iodide species with mesoporous nanocrystalline (anatase) TiO 2 thin films to be quantified spectroscopically on nanosecond and longer time scales in half molar iodide acetonitrile solutions. Under forward bias conditions, TiO 2 did not react with photogenerated iodine radical anions, I 2 −•, that were found instead to disproportionate with a rate constant that was within experimental error the same as that measured in fluid acetonitrile solution, k = 3 × 10 9 M −1 s −1 . The absence of reactivity with I 2 −• was unexpected. It appears that the reduction of I 2 −• by TiO 2 (e − ) does not complete kinetically with rapid I 2 −• disproportionation. In contrast, TiO 2 (e − ) was found to decrease the concentration of tri-iodide, I 3 −, and presumably molecular iodine, I 2, that was expected to be present in low equilibrium concentrations. The findings have relevance to unwanted charge recombination processes in dye sensitized solar cells.