ZnO/SiO 2 nanocomposites obtained by impregnation of mesoporous silica C. Cannas, M. Mainas, A. Musinu*, G. Piccaluga Dipartimento Scienze Chimiche, Cittadella Universitaria, S.S 554 Km 4.5, 09042, Monserrato (CA), Italy Abstract The synthesis of ZnO–SiO 2 nanocomposites through impregnation of a commercial mesoporous silica with zinc nitrate aqueous or ethanolic solutions is reported. During thermal treatment the samples evolve toward the formation of nanocrystalline ZnO particles (zincite phase) dispersed onto amorphous silica stable up to the temperature of 700  C. The nanoparticle size distribution is affected by the process parameters used in the impregnation. A narrower and more homogeneous distribution is obtained using ethanolic instead of aqueous solutions. At higher temperatures the samples evolve towards the formation of a zinc silicate phase. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: 61.43.G; 61.16.B; 61.46 1. Introduction ZnO-based nanocomposites are of considerable inter- est in ceramics technology for applications as varistors, sensor elements and photoluminescent materials. In particular, varistors used in high voltage applications require a small grain size in order to keep the varistor volume low, while the overall properties of varistors are greatly improved by the reduction of the zinc oxide particles to nanometer range. A potential application in the optoelectronic industry stems from the size-depen- dent optical properties, associated with the quantum size effect and with the existence of a high percentage of atoms at the nanoparticle surface. Several preparation methods, like sol-gel [1–3], impregnation [4,5] and molecular capping [6,7], have been proposed for the dispersion of ZnO nanoparticles in silica [1–5] or polymeric [6,7] matrices in order to avoid the tendency of nanoparticles to aggregate. This study is focused on the synthesis and character- ization of nanocomposites in which ZnO nanocrystals are incorporated into commercial mesoporous silica through impregnation with zinc nitrate aqueous or ethanolic solutions, and submitted to the appropriate thermal treatments. The effect of the solvent, time and temperature of impregnation is investigated. Structural evolution of the samples towards thermal treatments and chemical modifications of the precursor deposited onto silica are studied by BET, XRD and TEM techniques. 2. Experimental Four samples were prepared by impregnating 1 g of silica (Aldrich, surface area=300 m 2 /g, average poros- ity=150 A ˚ ) with 50 ml of aqueous or ethanolic solu- tions of zinc nitrate 1.6 M. Impregnation was performed in three steps in which two aliquots of 15 ml and one of 20 ml of the solution were taken into contact with silica under continuous stirring. At the end of each step a cycle of centrifugation was carried out to separate solid by the impregnation liquid. In the final cycle, solid was washed with a zinc free solution having the same pH of the final suspension that varies in dependence of the different experimental conditions. After drying at 60  C for 48 h, TG measurements were carried out on a Met- tler Toledo 851 under oxygen flow (heating rate 10 K min 1 ). The data indicate that the weight loss is almost complete at T > 450  C, therefore the solids were sub- mitted to five consecutive steps of calcination at the temperature of 500, 600, 700, 800 and 900  C, leaving the samples at each temperature for 1 h up to 700  C and30minupto900  C. Chemical analyses, carried out with a plasma ICP Perkin Elmer 2000, are reported in Table 1, together with the experimental preparation conditions and the acronyms for the samples which will 0266-3538/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0266-3538(03)00040-X Composites Science and Technology 63 (2003) 1187–1191 www.elsevier.com/locate/compscitech * Corresponding author. E-mail address: musinu@vaxca1.unica.it (A. Musinu).