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 Z(n)(i) excited levels to the valence band or to V-Zn 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(center dot) EPFR. Instead, for ZnO/SiO2_W, intense features associated with oxygen species in proximity of V-O(+), V-Zn(-), and (V-Zn(-))(2)(-) centers dominate the spectra, while a minor contribution of the PhenO(center dot) 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 V-O(+) and V-Zn(-) 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.

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

Polizzi S.;
2019

Abstract

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 Z(n)(i) excited levels to the valence band or to V-Zn 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(center dot) EPFR. Instead, for ZnO/SiO2_W, intense features associated with oxygen species in proximity of V-O(+), V-Zn(-), and (V-Zn(-))(2)(-) centers dominate the spectra, while a minor contribution of the PhenO(center dot) 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 V-O(+) and V-Zn(-) 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.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10278/3719804
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