Uranyl interaction with the hydrated (1Â 1Â 1) nickel face: A periodic density functional theory investigation

Periodic DFT calculations using plane waves basis sets with the GGA formalism were performed in order to study the behavior of the UO22+ uranyl ion at the water-Ni(1 1 1) interface. First, the adsorption of water molecules interacting with an optimized surface model has been studied. At low coverage, isolated water molecules adsorb preferentially on top of the surface nickel atoms, the plane defined by the H2O molecule being almost parallel to the surface. When the water coverage increases from 1/9 ML to 2/3 ML (ML = monolayer coverage), hydrogen bonds are created leading to the formation of water hexamers, as suggested experimentally on Ni(1 1 1) and other metallic surfaces [A. Michaelides, A. Alavi, D.A. King, Physical Review B 69 (2004) 113404. M. Nakamura, M. Ito, Chemical Physics Letters 325 (2000) 293-298]. Higher water coverage induces the formation of an additional water layer physisorbed over the cyclic hexamers. In a second step, the behavior of UO22+ with respect to the hydrated Ni(1 1 1) face has been investigated. Evidence that two adsorption modes can take place was put forward: a first one with an outer sphere adsorption mechanism, where two water molecules of the uranyl ion first hydration shell are shared with four water hexamers, and a second one through a strong Ni-O−yle bond formation. Even though the second surface complex is energetically the most stable, the necessary activation energy to reach it makes it improbable.