Sol-Gel Synthesis and Pyrolysis Study of Oxyfluoride Silica Gels

Abstract

Silicon oxyfluoride materials are synthesized by the sol-gel method using triethoxyfluorosilane as precursor, bearing the Si—F bond. SiO_(2−0.5x) F _x gel preparation requires peculiar experimental control of hydrolysis and condensation reactions. Maintenance of the Si—F bond during gelling, heating and aging was studied in the case of processes carried out under an argon atmosphere or in air. Fluorine contents in resulting samples were quantified by FT-IR and X-ray photoelectron spectroscopy (XPS); specific surface area and porosity of powdered samples were determined by N₂ adsorption. The thermal stability of oxyfluoride gels was studied by thermogravimetric-mass spectrometric (TG-MS) coupled analyses during heat treatment, under He flow. Mass spectra recorded during principal weight losses indicate the release of variously fluorinated silicon species resulting from Si—F/Si—O exchange reactions. The evolution of these species was observed at different temperatures, depending on gelling conditions. In particular, degradation of ≡Si—F moieties was prominent for gels aged in air, whereas samples processed under an argon atmosphere preserve the ≡Si—F bond up to 300°C.