Interstitial O2 distribution in amorphous SiO2 nanoparticles determined by Raman and photoluminescence spectroscopy

Abstract

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

Properties of HO 2 • radicals induced by γ-ray irradiation in silica nanoparticles

“…This estimate provides a new conversion factor that allows to obtain the interstitial O 2 concentration from the PL intensity in the Raman spectra. This latter value is independent from those obtained from data previously reported in the literature [8,24,36] because the methodology used is different. The previous values were obtained in three separate experimental works [8,24], so it is of considerable importance that the conversion factor obtained by us (about 1.5 · 10 17 cm -3 ) is compatible with the values that can be obtained basing on previous investigation (K AEOX50~1 .8 · 10 17 cm -3 obtained from the work done by Kajihara et al [8]; K AEOX50~3 .5 · 10 17 cm -3 from the work done by Skuja et al [36]; K AEOX50~0 .9 · 10 17 cm -3 from the work done by Agnello et al [24]) using the amplitude ratio of the SiO 2 Raman bands at 1000-1200 cm -1 and the O 2 Raman/PL spectra.…”

“…We have to point out that this estimation was limited to a-SiO 2 bulk samples. Here we can extend the use of this procedure to silica nanoparticles considering some multiplying factors to take into account quantum yield variations, as recently reported [24]. They are due to the different life times of the various nanostructured materials studied here with respect to bulk silica [17].…”

“…Table 2 Average diameters and life times taken from ref. [24] and conversion factors used to obtain the interstitial O 2 concentration in each material studied here. Life times not experimentally obtained are marked with an asterisk and pertain to the materials SIG200-12 and SIG395.…”

“…This objective was reached by considering the life times of the materials reported in ref. [17,24]. We hypothesized the same life time for all the types of nanoparticles with the same average diameter.…”

“…We have taken advantage from the work done by Agnello et al [17,24] where interstitial O 2 concentrations were estimated in several a-SiO 2 nanostructured samples following the method suggested by Kajihara et al [8] but considering the differences, reported above or in the following, between nanoparticles and bulk silica. In this context it is worth noting that previous studies have shown that the estimations of O 2 absolute concentration by optical measurements are in agreement within 50% [8,24]. We have obtained the conversion factor K i , specific for every nanostructured material, by using the following equation:…”