Coverage-dependent adsorption and dissociation of H 2 O on Al surfaces.

The adsorption and dissociation of H 2 O on Al surfaces including crystal planes and nanoparticles (ANPs) are systematically investigated by using density functional theory (DFT) calculations. H 2 O adsorption strength follows the order ANPs > Al(110) > Al(111) > Al(100). Due to the smaller cluster deformation caused by the moderate H 2 O adsorption, the relative magnitude of H 2 O adsorption strength on ANPs and crystal planes is opposite to the trend of adatoms like O* and/or N*. The energy barrier for the decomposition of H 2 O into H* and OH* is larger on ANPs than on crystal planes, and it decreases with the increasing cluster size. Due to the competition between the hydrogen (H) bonding among water molecules and the interaction between the water molecules and the substrate, the adsorption strength of H 2 O first increases and then decreases with the increase of water coverage. Moreover, each H 2 O molecule can efficiently form up to two H bonds with two H 2 O molecules. As a result, H 2 O molecules tend to aggregate into cyclic structures rather than chains on Al surfaces. Furthermore, the dissociation energy barrier of H 2 O drops with the increasing water coverage due to the presence of H bonds. Our results provide insight into interactions between water and Al, which can be extended to understand the interaction between water and other metal surfaces.