High Resolution Electron Microscopy Study of Nanocubes and Polyhedral Nanocrystals of Cerium(IV) Oxide

In this research, high resolution transmission electron microscopy (HRTEM) and high angle annular dark field–scanning transmission electron microscopy (HAADF-STEM) studies of ceria­(IV) oxide CeO₂ nanocrystals (NCs) synthesized by a hydrothermal/two phase process were conducted. The synthesis route affords the possibility of controlling the shape of the CeO₂ NCs by changing the oleic acid/cerium ([OLA]/[Ce³⁺]) ratio. At a relatively low [OLA]/[Ce³⁺] ratio of 4, a polyhedral NC morphology was obtained with {111} and {200} termination facets. Increasing the [OLA]/[Ce³⁺] ratio to 8, while maintaining a constant reaction time and temperature during the synthesis, truncated cube-like CeO₂ NCs with {200}, {220}, and {111} termination facets was generated. These morphologies were identified by HRTEM and HAADF-STEM characterization. Fourier transform infrared (FT-IR) analysis and thermogravimetric analysis (TGA) confirm the presence of chemically bonded oleic acid (OLA) on the CeO₂ NC surface. It indicates that there is a relationship between the bonded OLA and the shape of the NC. Additionally, the identification of concave surfaces on {200} facets by HAADF-STEM characterization suggests that the formation of the cube-like CeO₂ morphology is a multiple step mechanism. On the basis of these observations new growth mechanisms for the CeO₂ morphology variants are proposed