Thermally induced structural modifications and O2 trapping in highly porous silica nanoparticles

In this work we investigate by Raman spectroscopy the effect of isochronal (2 h) thermal treatments in air in the temperature range 200-1000 °C of amorphous silicon dioxide porous nanoparticles with diameters ranging from 5 up to 15 nm and specific surface 590-690 m2/g. Our results indicate that the amorphous structure changes similarly to other porous systems previously investigated, in fact superficial SiOH groups are removed, Si-O-Si linkages are created and the ring statistic is modified, furthermore these data evidence that the three membered rings do not contribute significantly to the Raman signal detected at about 495 cm−1. In addition, after annealing at 900 and 1000 °C we noted the appearance of the O2 emission at 1272 nm, absent in the not treated samples. The measure of the O2 emission has been combined with electron paramagnetic resonance measurements of the γ irradiation induced HO2 radicals to investigate the O2 content per mass unit of thin layers of silica. Our data reveal that the porous nanoparticles have a much lower ability to trap O2 molecules per mass units than nonporous silica supporting a model by which O2 trapping inside a surface layer of about 1 nm of silica is always limited.