Surface-Driven Electron Localization and Defect Heterogeneity in Ceria

Area of Science:

• Materials Science
• Surface Science
• Catalysis

Background:

• Ceria (CeO2) exhibits excellent catalytic and energy conversion properties.
• These properties are strongly linked to its defect chemistry, particularly oxygen vacancies (V_O^••).
• Traditionally, oxygen vacancies are assumed to be compensated by two Ce³⁺ ions (1 V_O^••: 2Ce³⁺).

Purpose of the Study:

• To investigate the local distribution of charge compensation in ceria nanoparticles.
• To understand how oxygen vacancies and electrons (Ce³⁺) interact at the surface versus the bulk.
• To determine the impact of nanoparticle size and reduction level on defect heterogeneity.

Main Methods:

• Hybrid quantum mechanical/molecular mechanical (QM/MM) defect calculations.
• Synchrotron X-ray photoelectron spectroscopy (XPS) measurements.
• Large-scale unbiased Monte Carlo simulations.

Main Results:

• The 1 V_O^••: 2Ce³⁺ ratio is a global approximation; local ratios vary significantly.
• Electrons preferentially segregate onto ceria surfaces, leading to a higher Ce³⁺ to V_O^•• ratio on surfaces compared to the bulk.
• Surface electron segregation is more pronounced in smaller nanoparticles and at lower reduction levels, though highly reduced conditions can alter this trend.

Conclusions:

• Local defect chemistry in ceria nanoparticles is complex and deviates from bulk assumptions.
• Surface segregation of electrons plays a critical role in ceria's defect behavior.
• Accurate modeling of defect heterogeneity is essential for optimizing ceria-based catalysts and energy devices.