Zr and Hf oxoclusters as building blocks for the preparation of nanostructured hybrid materials and binary oxides MO2-SiO2 (M = Hf, Zr)

Silica materials embedding ZrO2 or HfO2 were prepared by copolymerisation of organically modified oxozirconium or oxohafnium clusters M4O2(OMc)(12) (M = Zr, Hf and OMc = methacrylate) with (methacryloxymethyl) triethoxysilane or (methacryloxypropyl)trimethoxysilane. Free radical copolymerisation of the oxoclusters bearing 12 methacrylate groups with the methacrylate-functionalized siloxanes allows stable anchoring of the clusters to the silica network formed by the hydrolysis and condensation of the alkoxy groups. This route represents a valuable strategy to yield a very homogeneous dispersion of the MO2 precursors inside the silica matrix. The composition and the microstructural features of the starting hybrid gels were studied by solid state C-13 and Si-29 NMR spectroscopy and FT IR transmission spectroscopy. Their evolution upon mild heating (up to 180 degrees C) was followed by FTIR Attenuated Total Reflectance spectroscopy (ATR), while their thermal behaviour was studied by thermogravimetric analysis (TGA). The covalent incorporation of the clusters into the silica hybrid matrix was studied at several temperatures. Through X-Ray Diffraction (XRD) and Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS) it is demonstrated that temperatures above 800 degrees C yield binary oxides MO2-SiO2 (M = Zr, Hf). Delayed crystallisation of HfO2 and tetragonal ZrO2 from 450 degrees C to at least 800 degrees C was detected, which is ascribed to the presence of a homogeneous dispersion of the guest oxide in the silica matrix.