The non-classical crystallization of CeO 2 nanoparticles

Abstract

Over the past couple of decades, new in situ characterization techniques such as liquid-cell TEM have revitalized efforts to understand the mechanisms of crystal formation. The spontaneous, room-temperature crystallization of CeO 2 from mM concentrations of cerium(IV) ammonium nitrate (CAN) in water was studied using UV-Vis absorption spectroscopy, transient absorption spectroscopy, x-ray diffraction, and high-resolution transmission electron microscopy. Characterization of the final nanoparticles revealed polycrystalline CeO 2 nanoparticles that are stable from aggregation over a period of months. Most of the nanoparticles are between 3 and 9 nm, although a small proportion of larger particles between 10 and 150 nm were also detected. Crystallization is accompanied by a large change in absorption which can be modeled by the presence of just two species. These species are argued to be an amorphous, hydrated intermediate that is converted to nanocrystalline CeO 2 over a period of minutes to hours. The rate-limiting step of the amorphous to crystalline transition involves a proton transfer reaction, as evidenced by a solvent kinetic isotope effect of ~10. Ultrafast transient absorption measurements show a drastic difference between the optical properties of the crystalline nanoparticles and the amorphous precursors. This system is an excellent model system for studying non-classical crystallization because the minutes-to-hours time scale and the small sizes of the nanoparticles and precursors allow for in situ observation of crystallization using steady-state absorption spectroscopy. This system would also lend itself well to characterization by other techniques such as liquid-cell TEM or x-ray absorption spectroscopy.