Hydroxylation-induced surface stability of AnO2 (An = U, Np, Pu) from first-principles

The relative stabilities of clean and hydroxylated surfaces of three actinide dioxides (AnO2, An = U, Np, Pu) have been investigated using first-principles methods within the DFT + U framework. In the case of the clean surfaces, the calculated surface energies are consistently the lowest for the (111) surface for all three AnO2 compositions, followed by the (110) and (100) surface energies. In the case of UO2, for instance, the calculated surface energies are 0.78, 1.05, and 1.47 J/m2 for the (111), (110), and (100) surfaces, respectively, reinforcing the well-established surface energy trend for metal-dioxides: (111) < (110) < (100). Dissociated water, at one monolayer coverage, is adsorbed preferentially onto the (100) surface for all three AnO2 systems. In the case of UO2 the water adsorption energy on the (100) surface (− 1.34 J/m2) is almost four times higher than the adsorption energy on the (111) surface (− 0.35 J/m2), and almost twice as large as the adsorption energy on the (110) surface (− 0.77 J/m2). Similar trend in the adsorption energies is observed for both NpO2 and PuO2. As a result, the relative stability of the hydroxylated AnO2 (An = U, Np, Pu) surfaces changes to (100) < (110) < (111). The effects of the geometric relaxations on the clean and hydroxylated surfaces are discussed.