Structural evolution upon thermal heating of nanostructured inorganic-organic hybrid materials to binary oxides MO2-SiO2 (M=Hf, Zr): a solid-state NMR study

The structural and compositional evolution of inorganic-organic silica-based hybrid materials and their conversion to mixed oxides, consisting of host silica with variable amounts of zirconia or hafnia (1:1 Si:M, 40:1 Si:M, M ) Zr, Hf), was studied by multinuclear solid-state NMR spectroscopy, FTIR spectroscopy, and thermogravimetry. It is shown that the as-prepared materials, composed of M4O2- (OMc)12 oxoclusters (M ) Zr, Hf; OMc ) methacrylate) and methacryloxymethyltriethoxysilane (MAPTMS), are not completely polymerized. Rather, a finite number of double bonds remain independent of the actual sample composition. Thermolysis between 300 and 500 °C is accompanied by drastic changes in the structural composition, as reflected by the complete loss of all organic components. The incorporation of oxoclusters is found to catalyze the cleavage of Si-C bonds. At the same time, continuous alterations in the inorganic/oxide part are registered. The degree of condensation, as derived from the 29Si NMR data, increases steadily with calcination temperature and is found to depend on the oxocluster amount. Thus, for the samples with the highest oxocluster content, a pronounced temperature dependence for the degree of condensation is observed that is absent for the samples with a small oxocluster content or for pure MAPTMS. 91Zr NMR measurements on the sample with the highest zirconium content indicate that upon calcination at 1000 °C, only a small amount of crystalline zirconia with tetragonal coordination exists. The major zirconia fraction distributed in the silica matrix is therefore in an amorphous state.