First-principles study of surface properties of PuO2: Effects of thickness and O-vacancy on surface stability and chemical activity

TL;DR

This study uses density-functional theory+U to analyze how surface orientation, thickness, and oxygen vacancies influence the stability and chemical activity of PuO2 surfaces, revealing key factors affecting their properties.

Contribution

It provides a comprehensive first-principles analysis of PuO2 surface stability and chemical activity considering surface orientation, vacancies, and environmental conditions, which was not previously detailed.

Findings

(111) surface is most stable under oxygen-rich conditions.

O-vacancies significantly reduce work function and alter chemical activity.

Surface stability varies with oxygen environment and vacancy concentration.

Abstract

The (111), (110), and (001) surfaces properties of PuO2 are studied by using density-functional theory+U method. The total-energy static calculations determine the relative order of stability for low-index PuO2 surfaces, namely, O-terminated (111) > (110) > defective (001) > polar (001). The effect of thickness is shown to modestly modulate the surface stability and chemical activity of the (110) surface. The high work function of 6.19 eV indicates the chemical inertia of the most stable (111) surface, and the surface O-vacancy with concentration C_V=25% can efficiently lower the work function to 4.35 eV, which is a crucial indicator of the difference in the surface chemical activities between PuO2 and \alpha-Pu2O3. For the polar (001) surface, 50% on-surface O-vacancy can effectively quench the dipole moment and stabilize the surface structure, where the residual surface oxygen atoms are arranged in a zigzag manner along the <100> direction. We also investigate the relative stability of PuO2 surfaces in an oxygen environment. Under oxygen-rich conditions, the stoichiometric O-terminated (111) is found to be the most stable surface. Whereas under O-reducing conditions, the on-surface O-vacancy of C_V = 1/9 is stable, and for high reducing conditions, the (111) surface with nearly one monolayer subsurface oxygen removed (C_V = 8/9) becomes most stable.