Thermally Induced Structural Modification of Silica Nanoparticles Investigated by Raman and Infrared Absorption Spectroscopies

We report an experimental investigation by Raman and infrared (IR) absorption spectroscopies on the structural modifications induced by isochronal thermal treatments on amorphous SiO2 nanoparticles (fumed silica). In particular, three different commercial types of this material, characterized by particle mean diameters of 7, 14, and 40 nm, were subjected to thermal treatments from 100 up to 1000 \ub0C. We found that some properties of fumed silica, such as the SiOSi mean bond angle, ring size distribution, and surface adsorbed water content, are drastically different from those of common bulk silica materials and intimately related to the particles\u2019 dimension. The SiOSi mean bond angle, probed by the main Raman line peaked at about 440 cm-1, is modified by thermal treatments above 400 \ub0C and tends toward typical values of bulk silica materials, whereas the three-membered ring population, probed by the D2 line peaked at about 600 cm-1, changes but does not reach bulk silica features. The surface adsorbed water content, estimated by IR measurements, gradually decreases, starting from 100 \ub0C. The peculiar properties of fumed silica, which suggest a strained atomic network structure, together with the investigation of its modifications induced by thermal treatments, are interpreted in terms of a shell-like model of the constituting particles. In particular, the model assumes that each particle comprises a surface shell characterized by a network structure highly strained and a core shell with a less strained structure more similar to that of bulk silica. This model, discussed on the basis of our experimental results, suggests that the structural property modifications induced by thermal treatments into the surface shell are related to the dehydroxylation process and to the buildup of particle-to-particle linking (sintering effects), whereas the structural modifications into the core shell arise from the network relaxation activated by thermal treatments