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.