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 <i>r</i> = <i>k</i>[S⁺]¹[TiO₂(e^–)]¹ with <i>k</i> = 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