Si-29 nuclear-magnetic-resonance and vibrational spectroscopy studies Of SiO2-TiO2 powders prepared by the sol-gel process

The sol-gel synthesis and structural characterization of SiO2-TiO2 powders are reported. Samples with titania concentrations in the range of 0 to 30 m ol%, calcined at temperatures between 120 and 1300 degreesC, were character ized by Si-29 magic-ang,le-spinning nuclear-magnetic-resonance (MAS NMR) an d cross polarization MAS NMR, Raman and Fourier transform infrared spectros copies, and powder x-ray diffraction. Tetrahedrally coordinated Ti (IV) ion s were found in the titanosilicate matrix. When the titania content increas es beyond approximately 10 mol% and upon calcination at temperatures over a pproximately 700 degreesC, the segregation of an amorphous silica-rich phas e occurs, together with the precipitation of anatase nanocrystallites with average sizes between 3 and 17 nm, depending on the temperature. An increas e in the heat treatment temperature, or the titania content, leads to an in crease in the degree of homocondensation (Si-O-Si and Ti-O-Ti bonds) and a simultaneous reduction in the degree of heterocondensation (Si-O-Ti bonds).