Combined use of XAFS, XRD and TEM to unravel the microstructural evolution of nanostructured ZrO2–SiO2 binary oxides: from nanometres down to the molecular domain

In this paper, the detailed study of the microstructural evolution under annealing of zirconium-based inorganic–organic hybrid materials to give silica–zirconia mixed oxides was addressed by X-ray absorption fine structure (XAFS) spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). The silica materials embedding different amounts of ZrO2 nanoparticles were prepared by copolymerisation of the organically modified oxozirconium cluster (Zr4O2(OMc)12 (OMc = methacrylate)) with methacryloxypropyltrimethoxysilane (MAPTMS). By the free radical copolymerisation of the oxoclusters bearing 12 methacrylate groups with the methacrylate-functionalised siloxanes, a stable anchoring of the clusters to the silica network was achieved. The thermal treatment of these hybrids at high (≥500 °C) temperatures yielded the SiO2–ZrO2 mixed oxides. The microstructural evolution upon heating was studied at increasing temperatures, namely 500, 600, 700, 900, 1000 and 1300 °C. Furthermore, different samples characterised by different Zr : Si atomic ratios and annealed at 1000 °C were comparatively analysed to study the effect of the composition on the evolution of the hybrids to give the mixed oxides. In a third experiment, samples characterised by the same composition were annealed at the same temperature by using either a conventional muffle or a microwave oven in order to evidence whether the different processing could also affect the microstructural features of the final oxide materials. Through XRD and XAFS it was demonstrated that at temperatures above 800 °C, crystallisation of tetragonal zirconia occurs in the samples of high zirconium concentration treated in muffle, whereas amorphous oxide materials form upon annealing in microwave oven. The presence of zirconia nanoclusters having an average size of 5–10 nm was evidenced by TEM.