Highly dispersed metal oxide nanoparticle sols are of great interest in science and engineering because of their broad potential applications in optics, electronics, sensors, catalysis, biomedicines, and even in cosmetics. Here, we report the size-controlled synthesis of cerium oxide nanoparticle sols by a reflux method with poly(vinylpyrrolidone) as a coating agent to afford stable monodispersions of spherical particles with size of 50-120 nm. The size of nanoparticles can be easily controlled by appropriate adjustment of the molecular weight of the added polymer. Colloidal crystals of cerium oxide are also obtained by evaporation of the cerium oxide sol. As cerium oxide has a larger refractive index than silica and polymers typically used for the preparation of colloidal crystals, the present sols are expected to be useful for the fabrication of high-performance photonic crystals.
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