Water adsorption on a NaCl (001) surface: A density functional theory study

Water adsorption on a NaCl (001) surface has been studied using density functional theory (DFT) with a generalized gradient approximation (GGA). Water monomer is more likely to locate near the top site with its O atom adjacent to Na^+ and its H atoms attracted by the nearest Cl^-. For the most stable configuration, the dipole plane of the water monomer tilts downward to the surface. The adsorption energy ranges from 0. 20. 3em{0ex}to0. 3em{0ex}0. 40. 3em{0ex}eV∕molecule for most typical sites. In the case of small water cluster adsorption (dimers and tetramers), the hydrogen bond and the water-substrate interaction come into play. We found that the hydrogen bond plays an important role in determining water adsorption configuration besides the mechanical and chemical constraints of the substrate. A systematic study of the water overlayer adsorption at higher coverages ranging from 1 monolayer (ML), 1. 5 ML, and 1. 75 ML to 2 ML on NaCl (001) was also conducted, where we found hydrogen bond patterns with alternating square and pentagonal or hexagonal rings dominate at coverages 1 ML. The new structural characteristics are due to the compromise of maximizing hydrogen bonding between water molecules and the water-NaCl interaction. The water-substrate interaction is gradually reduced and water-water interaction is increased as the coverage increases. Our results of the energetics and structures have been compared with available experiments.