Diffusive equilibrium properties of O2 in amorphous SiO2 nanoparticles probed via dependence of concentration on size and pressure

An experimental study on the diffusive equilibrium value of interstitial O2 in silica nanoparticles was carried out on samples with average particles diameter 40, 14, and 7 nm. The investigation was performed by measuring the concentration of interstitial O2 by Raman and photoluminescence techniques. The dependence of diffusive equilibrium concentration on pressure and temperature was investigated in the pressure range from 0.2 to 76 bar and in the temperature range from 98 to 244 °C. The equilibrium concentration of interstitial O2 follows Henry’s law at pressures below 13 bar whereas a departure from this model is observed at higher pressures. In particular, O2 concentration saturates above about 60 bar, reaching a temperature-independent value for fixed particles size. The concentration of dissolved O2, under fixed thermodynamical conditions, is size dependent, being lower in smaller particles. These results evidence an effect of interaction among diffusing species in the dynamics of transport and give support to a shell-like model for the distribution of O2 in nanoparticles.