International audienceThe O2 content and emission properties in silica nanoparticles after thermal treatments in oxygen rich atmosphere have been investigated by Raman and photoluminescence measurements. The nanoparticles have different sizes with average diameter ranging from 7 up to 40 nm. It is found that O2 concentration in nanoparticles monotonically increases with nanoparticles size. This finding is independent on the measurement technique and evidences that oxygen molecules are not present in all the nanoparticles volume. This dependence is interpreted on the basis of a structural model for nanoparticles consisting of a core region able to host the oxygen molecules and a surface shell of fixed size and free from O2
An experimental study of the O 2 diffusion process in nanoparticles of amorphous SiO 2 in the temperature range from 98 to 157 °C was carried out by Raman and photoluminescence techniques. We studied O 2 diffusion in high purity silica nanoparticles with a mean diameter of 14, 20, and 40 nm detecting the outgassing of molecules trapped during the manufacturing. The kinetics of diffusion is well described for all the investigated nanoparticles by the Fick's equation proving its applicability to nanoscale systems. The diffusion coefficient features an Arrhenius law temperature dependence in the explored temperature range, and the diffusion coefficient values are in good agreement with extrapolation of Arrhenius law from higher temperature studies.
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 800cm-1) ratio decreases with increasing the specific surface both for the as received and the loaded samples. By performing a thermal treatment (600 °C for 2h) 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 O2 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 non-radiative relaxation channels or because of the very low content of molecules trapped in this thin region
The O2 diffusion process in silica nanoparticles is experimentally\ud
studied in samples of average radius of primary particles ranging from 3.5 to 20 nm\ud
and specific surface ranging from 50 to 380 (m2/g). The investigation is done in the\ud
temperature range from 98 to 177 °C at O2 pressure ranging from 0.2 to 66 bar by\ud
measuring the interstitial O2 concentration by Raman and photoluminescence\ud
techniques. The kinetics of diffusion can be described by the Fick’s equation with an\ud
effective diffusion coefficient depending on the temperature, O2 pressure, and\ud
particles size. In particular, the dependence of the diffusion coefficient on the\ud
pressure and nanoparticles size is more pronounced at lower temperatures and is\ud
connected to morphological and physical factors
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