Sol-gel synthesis: effect of precursors on the microstructure of silica hydrogels

PERULLINI M.; JOBBAGY M.; ALBABE BILMES S.; TORRIANI I.; CANDAL ROBERTO

Campinas, SP

Encuentro; 21 Reunion Anual de Usuarios del LNLS; 2011

Laboratorio Nacional de Luz Sincrotron

Sol-gel synthesis is an attractive route for the synthesis of new inorganic and hy-brid materials, from ceramics to biocompatible soft inorganic gels. The properties of the final product can be tuned by controlling the solution parameters that rules the sol synthesis and the sol-gel transition. However a the relationship between syn-thesis and ageing parameters, microstructure and macroscopic properties is still not completely understood. Those relationships are fundamental for the synthesis of materials with controlled microstructure in the 1.0-1000 nm range. In this work we explore the microstructure-optical quality relationship of silica hydrogels pre-pared by hydrolysis of tetraethoxysilane in acid media, following the alcohol free procedure, with focus in the role of synthesis parameters: pH (2.5-7.1), total silica concentration (3.6 to 10.7 percentage) and additives. The obtained hydrogels have an amorphous nanoporous monolithic structure. The optical properties were eval-uated by attenuance at 400 and 500 nm for samples aged 24 h in phosphate buffer (pH 6.5, 0.1 M). The microstructural characterization was performed by SAXS, (beam 2 of LNLS, wavelength=0.1488 nm, wave vector range: 0.09 - 2.2 nm-1 and sample stage in vacuum with mica windows). The log-log SAXS intensity plots are typical of the scattering from a mass fractal system. The fractal dimension (D) is estimated from the power-law decrease of the SAXS intensity in a q-range between the characteristic lengths of the fractal structure (R) and the characteristic length of the primary particles composing the structure (a). The values of the parameter a depend on synthesis pH but are independent of the silica concentration, whereas the values of D and R change significantly with silica concentration for a fixed pH. The difference observed in optical properties can be explained in terms of the fractal dimension and structure as both R and D decrease with increasing silica concentra-tion indicating that smaller and less branched fractal structures produce lower light scattering in the visible range.