First-principles molecular dynamics study of water dissociation on the γ-U(100) surface

Based on first-principles molecular dynamics simulations at finite temperatures, we systematically study the adsorption and dissociation of water molecules on the gamma-U(1 0 0) surface. We predict that water molecules spontaneously dissociate upon approaching the native gamma-U(1 0 0) surface. The dissociation results from electronic interactions between surface uranium 6d states and 1b(2), 3a(1), and 1b(1) molecular orbitals of water. With segregated Nb atoms existing on the surface, adsorbing water molecules also dissociate spontaneously because Nb 3d electronic states can also interact with the molecular orbitals similarly. After dissociation, the isolated hydrogen atoms are found to diffuse fast on both the gamma-U surface and that with a surface substitutional Nb atom, which is very similar to the 'Hot-Atom' dissociation of oxygen molecules on the Al(1 1 1) surface. From a series of consecutive molecular dynamics simulations, we further reveal that on both the gamma-U surface and that with a surface substitutional Nb atom, one surface U atom will be pulled out to form the U-O-U structure after dissociative adsorption of 0.44 ML water molecules. This result indicates that oxide nucleus can form at low coverage of water adsorption on the two surfaces.