A two-stage citric acid-sol/gel synthesis of ZnO/SiO2 nanocomposites:: study of precursors and final products

ZnO nanoparticles, of average size of 10-15 nm, homogeneously dispersed in a silica matrix were prepared by a two stage citric acid/sol-gel process and thermal treatments up to 700 degrees C. The precursors formed at the early stages of the synthesis and their thermal evolution were investigated by FTIR, Si-29 NMR MAS and CPMAS, C-13 CPMAS and T-1 rho(H-1). A unidentate complex was revealed in the gel, together with other complexes in which citrate carboxylate groups are bound to one Zn2+ ion or act as a bridge between two Zn2+ ions. A comparison of the results from nanocomposite and silica samples prepared by the same method showed that chemical interactions between amorphous silica and zinc ions are not present either in the precursors or in the final materials. As a consequence, ZnO particles do not react with silica matrix when they are heated up to 700 degrees C. This result is ascribed to a nanophase segregation of zinc citrate complexes from the host matrix.