Hydrothermal Synthesis of CeO2 Nanocrystals: Ostwald Ripening or Oriented Attachment?

The growth mechanism of CeO2 nanocrystals prepared by the hydrothermal method has been studied in this article. The synthesis of CeO2 nanocrystals follows two stages, the initial nucleation of CeO2 nuclei and the subsequent ripening of nuclei in the hydrothermal process. The nucleation involves the precipitation of Ce3+ cations by OH– ions to form Ce(OH)3 nanoparticles and the transition from Ce(OH)3 to 2–3 nm CeO2 nuclei through an oxidation and rapid dehydration process. In the hydrothermal process, the oriented attachment of nuclei through a lattice matched surface and subsequent Ostwald ripening results in the growth of CeO2 nanocrystals. The dominant mechanism for the ripening of nuclei in hydrothermal reactions is the oriented attachment. The addition of polyvinylpyrrolidone surfactant and adjustment of solution acidity can promote the dispersion of the nuclei and enhance the effective collision among them in the hydrothermal stage, resulting in the oriented aggregation of particles and further growth into larger CeO2 nanocrystals (∼15 nm). Because of the low solubility of CeO2 crystals in water, the Ostwald ripening process (dissolution/reprecipitation) only plays the second important role in the hydrothermal reactions, which converts the assembly clusters into nanocrystals with/without well-defined edges or contributes to the further growth of nuclei from 2–3 nm to 3–5 nm.