Unveiling the Early Species of Silica-Based Materials: Raman Characterization of Silica Oligomers

Silica oligomers formed in solutions are difficult to characterize due to their size and chemical dynamics. Raman spectroscopy is well suited to track such structures, provided that a clear and unambiguous identification of the compounds is established. We report a comprehensive theoretical and experimental study of the spectra of key polysilicic acids species. We compute ab initio spectra at both the harmonic static and molecular dynamics levels that we compare with experimental Raman spectra. Silica-containing solutions with different polymerization ratios were obtained by tuning the pH in an aqueous media. Finally, we unambiguously assign vibrational modes to linear, cyclic, and cage-like silica oligomers. Our findings reveal trends in the ν_sym(Si-O-Si) band that correlate with the backbone structure supporting its use for the unique determination of these oligomers enabling reliable, quick and easy in situ identification of silicic acid oligomers in soluble silica mixtures.