Abstract

Ceria nanoparticles are synthesized from aqueous cerium nitrate by a facile, room temperature, vapor diffusion based process, resulting in an unusual evolution of particle morphology without exogenous surface capping agents. Transmission electron microscopy, powder X-ray diffraction, and UV−visible spectroscopy reveal that as the hydroxyl ion concentration of the reaction medium increases through progressive diffusion of ammonia vapor into the precursor solution, crystals grow from amorphous small seeds to 10 nm particles with unusual development of well-defined crystal morphologies. Quasi-spherical particles grow to form truncated octahedra that subsequently grow via face reconstruction to yield cubic nanocrystals with {100} surfaces. The slow progressive increase in hydroxyl concentration resulting from the controlled diffusion of ammonia into the reaction medium permits us to observe the sequence of evolutionary transformations and deduce a probable mechanism. We suggest that the increase in OH− concentration provides the driving force for the increased growth rate along the  111  direction, while the exposed and otherwise unstable, polar {100} surfaces are stabilized through OH− adsorption or deprotonation of bound water. The progressive increase in basicity also leads to internal cracking that results in lattice expansion in the final CeO2 particles. The tunable, size- and shape-selective formation of ceria nanoparticles offers advantages for catalytic applications.