Entrapping of O₂ molecules in nanostructured silica probed by photoluminescence

We studied the emission of the O₂ molecules embedded in fumed silica (amorphous silicon dioxide) nanoparticles differing for diameters and specific surface. By using a 1064 nm laser as a source we recorded both the O₂ emission and the Raman signal of silica. Our experimental data show that the O₂ emission/Raman signal (at 800 cm^-1) ratio decreases with increasing the specific surface for both the as-received and the loaded samples. By performing a thermal treatment (600 °C for 2 h) we modified the structure and the water content of the smallest nanoparticles without observing any significant change in the O₂ emission/Raman signal ratio. Our data are explained by a shell model showing that the O ₂ emission is essentially due to the molecules entrapped in the core of the nanoparticles, whereas the contribution due to the surface shell, having a thickness of about 1 nm, is negligible because of its high content of Si-OH groups that introduce nonradiative relaxation channels or because of the very low content of molecules trapped in this thin region.