Insight into the Influence of ZnO Defectivity on the Catalytic Generation of Environmentally Persistent Free Radicals (EPFRs) in ZnO/SiO2 Systems

The present study aims at supplying a more in-depth picture of the generation of environmentally persistent free radicals (EPFRs) from phenol (PhOH) on ZnO/SiO2 systems, by exploring the properties of ZnO nanoparticles (NPs) with different intrinsic defectivity grown on highly porous silica with a spherical (ZnO/SiO2-S) and wormlike morphology (ZnO/SiO2-W). In detail, besides an extensive structural, morphological, and surface investigation, the occurrence of inequivalent defect centers in the samples was tracked by photoluminescence (PL) experiments, which unveiled, for ZnO/SiO2-W, intense blue emissions possibly involving radiative recombination from Zni excited levels to the valence band or to VZn levels. Electron spin resonance (ESR) spectra corroborated these results and revealed a remarkably different behavior of the samples in the EPFR formation model reaction. In fact, upon PhOH contact, the ESR spectrum of ZnO/SiO2-S showed the exclusive presence of a weak isotropic signal ascribable to a PhenO• EPFR. Instead, for ZnO/SiO2-W, intense features associated with oxygen species in proximity of VO +, VZn -, and (VZn -)2 - centers dominate the spectra, while a minor contribution of the PhenO• radical can be discovered only by signal simulation. These outcomes definitively envisage a role of the intrinsic defectivity of ZnO NPs on the final yield and stability of EPFR generation, with VO + and VZn - defects possibly involved in dissociative adsorption or oxidation processes at the oxide surface. Although this work focuses on ZnO, it is expected to foster a critical re-examination and integration of important results on other metal oxide/silica systems already reported in the literature, offering the chance to better evaluate the dependence of EPFR generation on the oxide defects chemistry.