O-17 solution NMR characterization of the preparation of sol-gel derived SiO2/TiO2 and SiO2/ZrO2 glasses

O-17 nuclear magnetic resonance was used to investigate the classical two-step procedure for the preparation of sol-gel derived SiO2-TiO2 or SiO2-ZrO2 glasses. The formation of significant amount of Si-O-M bonds (M = Ti, Zr), resulting from the reaction of silanol groups with M-OR groups, was clearly demonstrated and seems to be kinetically favored when titanium (or zirconium) alkoxide is added to a prehydrolyzed solution of tetraethoxysilane. However, under the chosen experimental conditions, the as-formed oligomeric species are unstable, leading to a partial cleavage of the Si-O-Ti bonds; this phenomenon was not observed for the SiO2-ZrO2 system. Two approaches have been investigated to prevent such evolution: postaddition of water to promote condensation of the highly reactive Ti-OR groups or use of chemical modifiers to slow the reactivity of the titanium (or zirconium) alkoxide.