²⁹Si 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 mol%, calcined at temperatures between 120 and 1300 °C, were characterized by ²⁹Si magic-angle-spinning nuclear-magnetic-resonance (MAS NMR) and cross polarization MAS NMR, Raman and Fourier transform infrared spectroscopies, and powder x-ray diffraction. Tetrahedrally coordinated Ti (IV) ions were found in the titanosilicate matrix. When the titania content increases beyond approximately 10 mol% and upon calcination at temperatures over approximately 700 °C, the segregation of an amorphous silica-rich phase occurs, together with the precipitation of anatase nanocrystallites with average sizes between 3 and 17 nm, depending on the temperature. An increase in the heat treatment temperature, or the titania content, leads to an increase 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).