Coverage-dependent adsorption and dissociation of H₂O on Al surfaces

The adsorption and dissociation of H₂O on Al surfaces including crystal planes and nanoparticles (ANPs) are systematically investigated by using density functional theory (DFT) calculations. H₂O adsorption strength follows the order ANPs > Al(110) > Al(111) > Al(100). Due to the smaller cluster deformation caused by the moderate H₂O adsorption, the relative magnitude of H₂O 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 H₂O 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 H₂O first increases and then decreases with the increase of water coverage. Moreover, each H₂O molecule can efficiently form up to two H bonds with two H₂O molecules. As a result, H₂O molecules tend to aggregate into cyclic structures rather than chains on Al surfaces. Furthermore, the dissociation energy barrier of H₂O 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.