Investigation on the generation process of HO2 radicals by γ-ray irradiation in O2-loaded fumed silica

We report an experimental investigation on the effects of γ-ray irradiation in three types of fumed silica previously loaded with O2 molecules. Our data indicate that the main effect of irradiation in these systems is to generate a very large concentration of HO2 interstitial radicals (about 1018 molecules/cm3). Furthermore, the number of generated HO2 was found to be larger in the samples with higher O2 contents before irradiation. This correlation suggests that HO2 radicals are induced by reaction of interstitial O2 molecules with radiolytic H atoms, as previously suggested for O2-loaded bulk amorphous silicon dioxide (a-SiO2 or silica) samples. However, at variance with respect to bulk materials, in fumed silica the radiolytic H does not arise from SiOH or SiOOH groups, as no EPR signal due to non-bridging oxygen hole centers (NBOHC) or to peroxy radicals (POR) is detected in the spectra of irradiated samples. As a reasonable alternative we propose that radiolytic hydrogen atoms could arise from a radiation induced breaking of interstitial H2O molecules, indicating that fumed silica in its pristine form could possess a very large concentration of interstitial water molecules.