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

Kinetic isolation conditions were identified that enabled determination of the reaction order for interfacial charge recombination at a sensitized mesoporous TiO 2 thin film. An external bias was used to maintain a fixed and known number of oxidized sensitizers, S +, or TiO 2 electrons, TiO 2 (e – )s. Pulsed laser excitation resulted in excited state injection and the subsequent TiO 2 (e – )|S + → TiO 2 |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 [TiO 2 (e – )] 1 with k = 5.0 × 10 –16 cm 3 s –1 (∼3 × 10 5 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 TiO 2 (e – )s is small and electron transfer becomes more dominant when the number is large.