¹⁷O Solution NMR Characterization of the Preparation of Sol−Gel Derived SiO₂/TiO₂ and SiO₂/ZrO₂ Glasses

17 O nuclear magnetic resonance was used to investigate the classical two-step procedure for the preparation of sol−gel derived SiO 2 −TiO 2 or SiO 2 −ZrO 2 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 SiO 2 −ZrO 2 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.