Di- and Tri-iodide Reactivity at Illuminated Titanium Dioxide Interfaces

A photoclectrochemical cell designed to characterize interfacial electron transfer at potentiostatically controlled mesoporous nanocrystalline (anatase) TiO2 thin film electrodes was employed to characterize the reactivity of TiO2 with di- and tri-iodide in acetonitrile solution. Tri- and di-iodide, I-3(-) and I-2(center dot-), were generated by direct excitation of iodide (266 nm), band gap excitation of TiO2 (355 nm), or both. The first iodide oxidation product observed spectroscopically after pulsed laser excitation was di-iodide, I-2(center dot-). The yield of I-2(center dot-) measured 0.1 mu s after pulsed 355 nm laser excitation decreased with the application of a forward bias. Under all conditions studied, there was no direct evidence for a reaction between TiO2 and I-2(center dot-), even when the concentration of trapped electrons, TiO2-(e(-))s, was increased with a forward bias. Di-iodide was found instead to disproportionate to yield tri-iodide, I-3(-), and iodide with a disproportionation 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). Spectroscopic evidence for a reaction between TiO2(e(-)) and I-3(-) was observed. The kinetics for this reaction were complex and highly dependent on the TiO2(e(-)) concentration, behavior qualitatively consistent with a multiple trapping model. These findings may have relevance to unwanted charge recombination processes in dye-sensitized solar cells.