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

A photoelectrochemical cell designed to characterize interfacial electron transfer at potentiostatically controlled mesoporous nanocrystalline (anatase) TiO 2 thin film electrodes was employed to characterize the reactivity of TiO 2 with di- and tri-iodide in acetonitrile solution. Tri- and di-iodide, I 3 − and I 2 •−, were generated by direct excitation of iodide (266 nm), band gap excitation of TiO 2 (355 nm), or both. The first iodide oxidation product observed spectroscopically after pulsed laser excitation was di-iodide, I 2 •− . The yield of I 2 •− measured 0.1 μs 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 TiO 2 and I 2 •−, even when the concentration of trapped electrons, TiO 2 (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 × 10 9 M −1 s −1 . Spectroscopic evidence for a reaction between TiO 2 (e − ) and I 3 − was observed. The kinetics for this reaction were complex and highly dependent on the TiO 2 (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.