Infrared reflectance spectroscopy of porous silicas

Fourier transform-infrared (FT-IR) specular reflectance spectroscopy was used to examine the fundamental phonon behavior of a series of porous silicas including porous Vycor, xerogels, aerogels, and colloidal solids. The spectra were evaluated using Kramers-Kronig analysis techniques, and the corresponding optical constants were determined via the Fresnel equations. The resulting spectra represent the first compilation of such data for low density silicas. The porous silicas revealed unique resonance modes for their respective imaginary dielectric functions and energy loss functions. A key distinction amongst the spectra was the change in the band shape of the antisymmetric SiOSi stretching modes. For instance, as the porosity level of the particulate systems increased, the maxima of the imaginary dielectric function shifted to higher frequencies while the maxima of the associated energy loss function shifted to lower frequencies. In essence, with increasing porosity, the maxima of the imaginary dielectric functions and the energy loss functions were converging toward frequencies intermediate between the transverse optical and longitudinal optical modes of fused silica. A similar trend was not observed for the semi-continuous silica matrices. Maxwell Garnett effective-medium models verified that these modes were a function of the porous microstructure and can be attributed to surface phonon modes. The effect of surface phonon modes was also evident in the absorption coefficient data. Contrary to the traditional view that changes in the absorption spectra of porous silicas are strictly due to molecular structure, this study has demonstrated that variations can be attributed, both qualitatively and quantitatively, to electrostatic screening effects of finite particles.