Different Kinetic Reactivity of Electrons in Distinct TiO2 Nanoparticle Trap States.

Electrons added to TiO2 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 TiO2 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 TiO2/e - with sub-stoichiometric oxidants showed a surprising pattern: an initial fast (seconds) decrease in TiO2/e - absorbance followed by a secondary, slow (minutes) increase in the broad TiO2/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 TiO2/e - by O2, which like 4-MeO-TEMPO is a proton-coupled electron transfer (PCET) oxidant, and by the pure electron transfer (ET) oxidant KI3. 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 TiO2 reactivity. This differential trap state reactivity is likely to influence the performance of TiO2 in photochemical/electrochemical devices, and it suggests an opportunity for tuning catalysis.