Radiation Chemistry Provides Nanoscopic Insights into the Role of Intermediate Phases in CeO₂ Mesocrystal Formation

Abstract

The role of intermediate phases in CeO2 mesocrystal formation from aqueous CeIII solutions subjected to γ‐radiation was studied. Radiolytically formed hydroxyl radicals convert soluble CeIII into less soluble CeIV. Transmission electron microscopy (TEM) and X‐ray diffraction studies of samples from different stages of the process allowed the identification of several stages in CeO2 mesocrystal evolution following the oxidation to CeIV: (1) formation of hydrated CeIV hydroxides, serving as intermediates in the liquid‐to‐solid phase transformation; (2) CeO2 primary particle growth inside the intermediate phase; (3) alignment of the primary particles into “pre‐mesocrystals” and subsequently to mesocrystals, guided by confinement of the amorphous intermediate phase and accompanied by the formation of “mineral bridges”. Further alignment of the obtained mesocrystals into supracrystals occurs upon slow drying, making it possible to form complex hierarchical architectures.

Non-classical crystallization of CeO2 by means of in situ electron microscopy

“…This mutual alignment of the primary particles is generally in agreement with previously presented ex situ interpretations. 12 In the current study, the relative orientations of primary particle in the aggregates (i), (ii) and (iii) in Fig. 2 (or in the video “V02_LP-TEM_CeO 2 -nanoparticles”) change, even though those particles are already connected to each other.…”

“…3b ). 12 Moreover, the primary particles are found to mutually align in highly ordered structures (see Fourier transformation in Fig. S2 † ).…”

“…3b in ref. 12 ). This is quite surprising given the fact that for radiation synthesis of metal nanoparticles, the particle size displays strong dose rate dependence.…”

“…This is very well in line with the mechanism proposed for gamma-radiation induced synthesis of CeO 2 from Ce 3+ in aqueous solution. 12 In this mechanism, nucleation takes place in a gel-like intermediate phase consisting of hydrated Ce( iv )-hydroxides formed after the radiolytic oxidation of Ce 3+ . This subsequent nucleation, followed by growth of CeO 2 inside the intermediates, is not driven by radicals and, therefore, depend to a lesser extent on the irradiation dose rate.…”

“…In this work we explore a non-classical crystallization route using in situ LP-EM. We choose the formation of CeO 2 mesocrystals as a model process because of several reasons: (1) it allows us to use a water solution of a Ce 3+ salt without any organic surfactants/additives; (2) radiolytic oxidation has been relatively little studied both in a LP-EM and upon gamma-radiation, and the formation mechanism in LP-EM conditions is not clear; (3) we compare a radiolytic process in a LP-EM confined cell and in a large volume of a Gamma-Cell 12 thus enabling a unique study of the influence of the irradiation dose rate on nucleation and crystal growth by combining bulk and nanoscale approaches. Furthermore, we take advantage of the versatile irradiation conditions provided in LP-EM to study the influence of induced concentration gradients on the crystal growth during the radiolytic oxidation in solution.…”