Tuning Growth Modes of Ceria-Based Nanocubes by a Hydrothermal Method

We control two growth modes, Ostwald ripening (OR) and oriented attachment (OA), to yield ceria nanocubes by a simple hydrothermal method, using an oleate modified precursor. Using a Ce(III) salt as an initial material, the OA mode is highly activated to form ceria nanocubes with well-developed (002) facets (ca. 8 nm), while OA growth is not allowed so that smaller nanocubes (ca. 5 nm) were formed if Ce(IV) salt is used for the synthesis. A combination analysis by X-ray photoelectron spectroscopy and Fourier transform IR indicates that the surface of ceria nanocubes synthesized with Ce(III) precursor have surface valence (Ce 3+ ) sites coordinating with oleate through bidentate bonding. We propose that the difference in Ce 3+ from the grain interior (Ce 4+ ) provides the high surface energy needed to activate the OA growth mode. The luminescence spectra of ceria nanocubes grown by OA rather resemble those of a microcrystalline sample, suggesting the presence of a nonstoichiometric ceria phase, such as Ce 2 O 3, at the domain boundary in an identical nanocube, while OR growth favors the production of point defects randomly distributed in the fluorite lattice. The OA mode was still sufficiently active to form well-defined nanocubes (ca. 8 nm) when Zr 4+ was doped to 20 mol % in ceria, while increasing the Zr 4+ content to 50 mol % drastically suppressed both OA and OR to form tiny spherical nanoparticles (ca. 2.0 nm).