Ostwald Isolation to Determine the Reaction Order for TiO2(e-)|S+ → TiO2IS 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 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(-))1S(+) -> TiO2IS 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 x 10(-16) cm(3) s(-1) (similar to 3 x 10(5) M-1 s(-1)). Under forward bias, the data were more complex. A recombination mechanism that incorporates a preequilibrium 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.