Different Kinetic Reactivities of Electrons in Distinct TiO2 Nanoparticle Trap States

Electrons added to TiO 2 and other semiconductors often occupy trap states, whose reactivity can determine the catalytic and stoichiometric chemistry of the material. We previously showed that reduced aqueous colloidal TiO 2 nanoparticles have two distinct classes of thermally-equilibrated trapped electrons, termed Red/ e - and Blue/ e - . Presented here are parallel optical and electron paramagnetic resonance (EPR) kinetic studies of the reactivity of these electrons with solution-based oxidants. Optical stopped-flow measurements monitoring reactions of TiO 2 / e - with sub-stoichiometric oxidants showed a surprising pattern: an initial fast (seconds) decrease in TiO 2 / e - absorbance followed by a secondary, slow (minutes) increase in the broad TiO 2 / e - optical feature. Analysis revealed that the fast decrease is due to the preferential oxidation of the Red/ e - trap states, and the slow increase results from re-equilibration of electrons from Blue to Red states. This kinetic model was confirmed by freeze-quench EPR measurements. Quantitative analysis of the kinetic data demonstrated that Red/ e - react ~5 times faster than Blue/ e - with the nitroxyl radical oxidant, 4-MeO-TEMPO. Similar reactivity patterns were also observed in oxidations of TiO 2 / e - by O 2 , which like 4-MeO-TEMPO is a proton-coupled electron transfer (PCET) oxidant, and by the pure electron transfer (ET) oxidant KI 3 . This suggests that the faster intrinsic reactivity of one trap state over another on the seconds-minutes timescale is likely a general feature of reduced TiO 2 reactivity. This differential trap state reactivity is likely to influence the performance of TiO 2 in photochemical/electrochemical devices, and it suggests an opportunity for tuning catalysis.