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L. Armelao, C. Eisenmenger-Sittner, M. Groenewolt, S. Gross, C. Sada, U. Schubert, E. Tondello, A. Zattin:
"Zirconium and hafnium oxoclusters as molecular building blocks for highly dispersed ZrO₂ or HfO₂ nanoparticles in silica thin films";
Journal of Materials Chemistry, 15 (2005), 18; 1838 - 1848.

A novel synthetic route for the preparation of ZrO₂ or HfO₂ nanoparticles homogeneously dispersed in SiO₂ thin films was developed. This route is based on the copolymerisation of organically modified crystalline oxozirconium or oxohafnium clusters (M₄O₂(OMc)₁₂, M = Zr, Hf; OMc = OC(O)-C(CH₃)=CH₂) with (methacryloxymethyl)triethoxysilane (MAMTES, CH₂=C(CH₃)C(O)O-CH₂Si(OCH₂CH₃)₃). These crystalline clusters, which are the precursors for the corresponding metal oxides (MO₂), were prepared via the sol-gel route by reaction of zirconium or hafnium butoxide (M(OBu)₄) with methacrylic acid. The copolymerisation of the clusters with the methacrylate-functionalised siloxane was photoinitaited by Irgacure 184 and allowed the anchoring of the cluster to the forming silica network. The solution was cast into films by dip-coating and UV cured (10 min, 125 W) to promote the copolymerisation of the methacrylate groups of the cluster with those of the silane. Transparent and homogeneous films 200-450 nm thick were obtained after calcination at 800 °C in air. This route allowed the production of a very homogeneous dispersion of the MO₂ precursors inside the silica matrix. The surface and in-depth composition of the thin films was investigated through IR, X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). SIMS and XPS depth profiles evidenced a very homogenous distribution of both zirconium or hafnium throughout the silica films and sharp film-substrate interfaces. The surface morphology of the coatings was investigated through atomic force microscopy (AFM), which showed smooth, homogeneous and crack-free surfaces. Through X-ray diffraction (XRD) the crystallisation of hafnium and zirconium oxides was revealed, while the presence of isolated crystalline nanoparticles having a diameter of 5-10 nm was evidenced by transmission electron microscopy (TEM). A pull-off test indicated a very good adhesion of the films to the substrate.

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