Atomic-Scale Valence State Distribution inside Ultrafine CeO₂ Nanocubes and Its Size Dependence

Area of Science:

• Materials Science
• Nanotechnology
• Solid State Chemistry

Background:

• Understanding oxygen vacancies (V_O) in metal oxide nanocrystals is crucial for their properties.
• Atomic-scale analysis of cation valence states is key to elucidating V_O distribution.
• Challenges remain in characterizing V_O within nanocrystals.

Purpose of the Study:

• To investigate the layer-by-layer distribution of cerium valence states in ultrafine CeO₂ nanocubes (NCs).
• To explore the effect of nanocube size on Ce valence state distribution and V_O formation.
• To elucidate the mechanism behind the size effect on Ce valence states and lattice expansion.

Main Methods:

• Utilized state-of-the-art scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS).
• Performed layer-by-layer analysis of Ce valence states across CeO₂ NCs.
• Combined experimental data with theoretical calculations of V_O formation energy.

Main Results:

• Demonstrated a size-dependent distribution of Ce valence states in CeO₂ NCs (11.8 nm to 5.4 nm).
• Observed increased Ce³⁺ concentration in both surface and center layers of smaller NCs compared to larger ones.
• Linked nano-size effect to lattice expansion and reduced V_O formation energy in smaller NCs.

Conclusions:

• Nano-size effect induces lattice expansion in CeO₂ NCs below approximately 5 nm.
• Expanded lattice facilitates V_O formation due to lower energy requirements in smaller NCs.
• Provides fundamental insights into the formation and distribution of Ce³⁺ in ultrafine CeO₂ NCs.