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

The (1 1 1), (1 1 0), and (0 0 1) 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 (1 1 1) > (1 1 0) > defective (0 0 1) > polar (0 0 1). The effect of thickness is shown to modestly modulate the surface stability and chemical activity of the (1 1 0) surface. The high work function Φ of 6.19 eV indicates the chemical inertia of the most stable (1 1 1) surface, and the surface O-vacancy with concentration CV = 25% can efficiently lower Φ to 4.35 eV, which is a crucial indicator of the difference in the surface chemical activities between PuO2 and α-Pu2O3. For the polar (0 0 1) 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 〈1 0 0〉 direction. We also investigate the relative stability of PuO2 surfaces in an oxygen environment. Under oxygen-rich conditions, the stoichiometric O-terminated (1 1 1) is found to be the most stable surface. Whereas under O-reducing conditions, the on-surface O-vacancy of CV = 1/9 is stable, and for high reducing conditions, the (1 1 1) surface with nearly one monolayer subsurface oxygen removed (CV = 8/9) becomes most stable.