Ce3+ concentration control on the surface of ceria nanoparticles and the stability of surface Ce3+ in aqueous, silica, and PVA media

Abstract

Nanoceria (CeO2-x) with controllable surface Ce3+/Ce4+ ratios were directly synthesized without the use of surfactant organic molecules or complexing agents. All processes were carried out in aqueous solutions at room temperature, which is very advantageous for cost effective and large-scale productions. Importantly, higher Ce3+ concentrations on the surface of nanoceria have been achieved by increasing the pH of the aqueous solution for the synthesis. When nanoceria prepared at higher pH values were dispersed in deionized water to form aqueous colloidal solutions, the oxidation of excess surface Ce3+ to Ce4+ proceeded within ~6 h. This behavior of nanoceria with high surface Ce3+ concentration in aqueous media should be recognized for practical applications in biomedical systems because therapeutic potentials are based on the surface Ce3+/Ce4+ redox switch. Fortunately, the excess surface Ce3+ stability could be enhanced by embedding ceria nanoparticles in the mesopores of silica. The resulting ceria@silica composite revealed significantly smaller extents of oxidation as well as much slower rates of oxidation in aqueous media so that the oxidation continued for ~5 days after dispersing in water. In contrast, excess Ce3+ ions on the surface of nanoceria were relatively stable in atmospheric condition or PVA media. Therefore, the nanoceria with excess surface Ce3+ concentrations prepared in this work could be exploited for outdoor antioxidant applications of transparent UV-screening films.