Ostwald Isolation to Determine the Reaction Order for TiO2(e–)|S+→ TiO2|S Charge Recombination at Sensitized TiO2 Interfaces

Abstract

Kinetic isolation conditions were identified that enabled determination of the reaction order for interfacial charge recombination at a sensitized mesoporous TiO2 thin film. An external bias was used to maintain a fixed and known number of oxidized sensitizers, S+, or TiO2 electrons, TiO2(e–)s. Pulsed laser excitation resulted in excited state injection and the subsequent TiO2(e–)|S+ → TiO2|S reaction was quantified spectroscopically. The data provide compelling evidence that the rate law for charge recombination under reverse bias is r = k[S+]1[TiO2(e–)]1 with k = 5.0 × 10–16 cm3 s–1 (∼3 × 105 M–1 s–1). Under forward bias, the data were more complex. A recombination mechanism that incorporates a pre-equilibrium diffusional encounter between injected electrons and oxidized sensitizers is proposed. This and previously reported data indicate that diffusion limits recombination when the number of TiO2(e–)s is small and electron transfer becomes more dominant when the number is large.