Environmentally persistent free radicals readily form on TiO2 but not on ZnO nanoparticles

Abstract

Environmentally persistent free radicals (EPFRs) are stable organic radicals and pollutants in atmospheric and soil particles. Knowledge of EPFR formation on pristine, unsupported engineered nanoparticles is limited. This study investigates the potential of TiO2 and ZnO nanoparticles in aiding aromatic precursors in forming EPFRs and is an experimental verification of our earlier prediction on nanoparticles’ potential to form EPFRs. TiO2 formed EPFRs regardless of size, while ZnO exhibited size-dependent behavior. Only smaller ZnO particles readily produced EPFRs. Larger ZnO particles only formed EPFRs when pretreated with ethanol. EPFRs formed on TiO2 and ZnO near room temperature, challenging the belief that relatively high temperatures are needed for EPFRs to form. The yields of EPFRs on pristine TiO2 and ZnO were comparable to those found in atmospheric particulate matter. We identified four types of EPFRs: carbon-centered phenoxyl, oxygen-centered phenoxyl, carbon-centered semiquinone, and oxygen-centered semiquinone radicals. Our study suggests the role of band bending in forming EPFRs on TiO2 and ZnO, in which the adsorption of aromatic precursor acts as an electron acceptor or donor. Our findings suggest that EPFRs may form in unexpected places and conditions where TiO2 and ZnO nanoparticles are present.