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

ABSTRACT: 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.

SUBMITTER: Peper JL

PROVIDER: S-EPMC8232823 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:The influence of inorganic anions on the photoreactivity and aggregation of titanium dioxide nanoparticles (NPs) was assessed by dosing carbonate, chloride, nitrate, phosphate, and sulfate as potassium salts at multiple concentrations. NP stability was monitored in terms of aggregate morphology and electrophoretic mobility (EPM). Aggregate size and fractal dimension were measured over time by laser diffraction, and the isoelectric point (IEP) as a function of anion and concentration was obtained by measuring EPM versus pH. Phosphate, carbonate, and to a lesser extent, sulfate decreased the IEP of TiO2 and stabilized NP suspensions owing to specific surface interactions, whereas this was not observed for nitrate and chloride. TiO2 NPs were exposed to UV-A radiation, and the photoreactivity was assessed by monitoring the production of reactive species over time both at the NP surface (photogenerated holes) and in the bulk solution (hydroxyl radicals) by observing their reactions with the selective probe compounds iodide and terephthalic acid, respectively. The generation of photogenerated holes and hydroxyl radicals was influenced by each inorganic anion to varying degrees. Carbonate and phosphate inhibited the oxidation of iodide, and this interaction was successfully described by a Langmuir-Hinshelwood mechanism and related to the characteristics of TiO2 aggregates. Chloride and nitrate do not specifically interact with TiO2, and sulfate creates relatively weak interactions with the TiO2 surface such that no decrease in photogenerated hole reactivity was observed. A decrease in hydroxyl radical generation was observed for all inorganic anions. Quenching rate constants for the reaction of hydroxyl radicals with each inorganic anion do not provide a comprehensive explanation for the magnitude of this decrease, which arises from the interplay of several physicochemical phenomena. This work shows that the reactivity of NPs will be strongly influenced by the makeup of the waters they are released into. The impact of anion species on hydroxyl radical inhibition was as follows: carbonate > chloride > phosphate > nitrate > sulfate.

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

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

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