Structural properties of core and surface of silica nanoparticles investigated by Raman spectroscopy

We studied the experimental Raman spectra of various commercial silica nanoparticles of average diameter from 7 to 40 nm and specific surface from 50 to 380 m²/g. We found that the peculiarities of the particles Raman spectra systematically depend on their specific surface. In detail, the peak position of the R band at about 440 cm^-1 shifts towards high wavenumbers following an almost linear dependence on the specific surface. Similarly, the amplitudes of the D1 and D2 bands, at about 495 and 605 cm ^-1, respectively, increase linearly with the same quantity. Our results are interpreted in the frame of the shell model for the nanoparticles clarifying that the network of the core of the nanoparticles is comparable to the bulk silica materials, whereas the surface shell has a ring statistic shifted to the low member rings and features an higher density.