Since the first nanoparticle syntheses, research has focused on producing various nano-catalysts of different compositions and uses. Over the years, the importance of their shape, hence their exposed facets, grew in interest. The differences between facets, may sometimes remain unclear and controversial, but more and more effects in terms of reactivity and/or selectivity are shown in recent works. In this framework, rod and cubic-shaped cerium oxide nanoparticles were synthesized using a hydrothermal microwave-assisted process. Systematic TEM imaging and analysis reveal that rods are progressively replaced by cubes with increasing synthesis time (with fixed temperature and pH). Besides, a more careful look at these cubic-shaped particles highlights a slight anisotropy – depending on the zone axis they are imaged along – leading us to reconsider their nucleation and growth mechanisms.

These processes were first studied using radiation damages induced in ceria rods’ complex microstructures under focussed electron beam. Their periodical packing composed of single crystals aligned along a common low-index axis were firstly reduced and new nanometer-sized domains were formed with different (micro)structures. These zones tend to split from the initial rod in case of severe irradiation conditions and may, in-fine, lead to cuboidal particles via a solid-state transformation process.

As-synthesized cubes were then characterized by means of high resolution TEM coupled with molecular dynamics simulations. As expected, they exhibit single crystalline structure and {001} lateral facets are enclosed by {111} sharp corners and {011} average flat edges made of alternating {111} steps. More surprisingly, whatever the particle size might be, projected edges and corners width remains constant and corresponds to the diagonal of a four by four square formed by {002} planes. By keeping constant this truncation, surface plane ratios can be tuned only by adjusting ceria particles synthesis conditions.

Further on, when submitted to reducing conditions, both simulations and experiments tend to show a partially reversible flattening of the {011} edges combined with the observation of superlattice reflections and Moiré fringes. This indicates that oxygen anions have been removed from the fluorine CeO₂ structure inducing an ordered oxygen vacancies lattice. Finally, oxygen vacancies movement into the ceria network will be discussed.

Figures:

HRTEM images and digital diffractograms (insets) extracted from in-situ video sequence recorded during electron beam exposure of a CeO2 <112> oriented rod. Superstructure marked diffraction spot on figure b is induced by the reduction of the sample

MD simulation of a cuboidal CeO2 nanoparticle showing the stepped surfaces of average {110} edges. a) global view. b) closer profile view.

HRTEM micrographs of the {111}-stepped {110} edges. a) before reducing by the electron beam. b) during reduction. A flattening of the surface becomes visible, as well as Moiré fringes and superstructure spots on the digital diffractogram. c) After relaxing time, surface shows faceting again, and Moiré fringes and extra spots disappear.

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EMC Abstracts - https://emc-proceedings.com/abstract/ceria-road-to-cubes-a-combined-experiment-and-md-simulation-study/