Multiband light emission and nanoscale chemical analyses of carbonized fumed silica

Abstract

Fumed silica with a specific area of 295 m 2 /g was carbonized by successive phenyltrimethoxysilane treatments followed by annealing in inert atmosphere up to 650 C. Emission, excitation, kinetics, and photo-induced bleaching effects were investigated by steady state and time-resolved photoluminescence spectroscopies. The local chemistry was also studied by infrared transmission spectroscopy. Strong ultraviolet and visible photoluminescence was observed in the samples after the chemical treatments/modifications and thermal annealing. It has been shown that ultraviolet photoluminescence in chemically modified fumed silica is associated with phenyl groups, while near ultraviolet and visible emission in annealed samples originated from inorganic pyrolytic carbon precipitates dispersed in the silica host matrix. Two types of emission bands were identified as a function of the annealing temperature: one is in the near UV and the other is in the visible range. Based on the emission/excitation analysis of these two bands, as well as on correlations with the synthesis conditions, a structural-energy concept of light-emitting centers has been proposed. According to this model, the light-emitting centers are associated with carbon clusters that can be bonded or adsorbed on the silica surface. This has been validated by a detailed (S)TEM-electron energy-loss spectroscopy study, confirming the inhomogeneous distribution of nanoscale carbon precipitates at the surface of the silica nanoparticles. These carbon precipitates are mostly amorphous although they possess some degree of graphitization and local order. Finally, the fraction of sp 2 carbon in these nanoclusters has been estimated to be close to 80%. Published by AIP Publishing. https://doi.

Effect of Hydration Procedure of Fumed Silica Precursor on the Formation of Luminescent Carbon Centers in SiO2:C Nanocomposites

“…Spectral properties of PL was suggested to be determined by the structure of the clusters, their size distribution and/or presence of chemical bonds (CSi, CO) between the clusters and silica surface. This concept has been verified in more recent work where PhTMS was used as a carbon precursor . In the present paper, we follow this preliminary hypothesis and discuss PL properties in terms of carbon clusterization.…”

“…We suggest the following identification of emission bands in SiO 2 :C series. In our previous work the PL band at 330–340 nm in low temperature annealed phenylsilica has been assigned to excimer states of closely located phenyl groups . Indeed, FTIR spectra indicate the presence of phenyl groups after the annealing at 500 °C (Figure ).…”

“…It can be suggested that the fast component is related to the emission from phenyl group with contribution of charge transfer mechanism, which is quite possible taking into account the presence of nearby oxygen and silicon heteroatoms. Indeed, the presence of CO bonds has been previously demonstrated by XPS in SiO 2 :C powders synthesized at 600 °C . Other options of fast emission mechanism can be related to possible steric effects and/or other effects related to spatial interaction of phenyl groups which are closely located on the surface but have a limited motion freedom in condensed state …”

“…It is seen that silica consists of fused nanoparticles with diameter 10–15 nm. Previously it was demonstrated that neither chemical treatment nor thermal annealing affect morphology in nanoscale …”

“…Nanostructured silica seems to be a proper candidate as a matrix for CDs dispersion due to high specific surface and transparency. Indeed, fumed silica with specific surface of 295 m 2 g −1 and phenyltrimethoxysilane (PhTMS) were successfully used as morphological template and carbon precursor, respectively, for synthesis of highly luminescent SiO 2 :C nanocomposite by successive chemical/thermal treatments . Phenyl groups were grafted to silica surface by the reaction of methoxy groups of PhTMS with surface silanols via proton substitution.…”