Reduction of I2/I3- by Titanium Dioxide

A photoelectrochemical cell was designed that allowed the reactivity of oxidized iodide species with mesoporous nanocrystalline (anatase) TiO2 thin films to be quantified spectroscopically on nanosecond and longer time scales in half molar iodide acetonitrile solutions. Under forward bias conditions, TiO2 did not react with photogenerated iodine radical anions, I-2(-center dot), 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 x 10(9) M-1 s(-1). The absence of reactivity with I-2(-center dot) was unexpected. It appears that the reduction of I-2(-center dot) by TiO2(e(-)) does not complete kinetically with rapid I-2(-center dot) disproportionation. In contrast, TiO2(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.