Catalyst-free SiO₂ sonogels

Abstract

The sol-gel process has been used extensively to produce a great variety of glasses and ceramics using tetraetoxisilane (TEOS) as precursor of the SiO₂ matrix. The majority of synthetic approaches have been focused on the alkoxide-alcohol-water system. In all cases the alkoxide reactions occur through an acid and/or basic catalyzed hydrolysis condensation process. The sonication of acidified water/TEOS has been used as an alternative method to stimulate the hydrolysis of the alkoxide and avoid the use of ethanol as common solvent. These so-called sonogels have been extensively studied, and interesting new properties have been well established. In this work the occurrence of a reaction between neutral water and TEOS activated by high intensity ultrasonic irradiation is evidenced. As a result of this reaction high purity SiO₂ gels are obtained. An ad-hoc experimental setup was used which allows control of the reaction atmosphere during the sonication step, as well as recording of the pH and temperature of the system. The sonication scheme consisted of successive irradiation and short silent periods. A reaction scheme is proposed based on the formation and recombination of radicals during the ultrasonic irradiation step, which results mainly in the formation of a poly-hydroxylated Si species, while the poly-condensation reactions occur after the irradiation step. The high purity SiO₂ sonogels obtained are of interest due to their potential applications as supports for non-linear optical materials, catalytic active phases or in drug delivery.