pH-dependent structure and energetics of H2O/MgO(100)

The structure and energetics of water on MgO(100) surfaces are studied by atomic force microscopy (AFM) and density-functional theory (DFT). Computationally, the adsorption of water monomers, small water clusters and water monolayers on MgO(100) surfaces is considered. The calculations predict the non-dissociative adsorption for water monomers. The potential energy surface for single monomers is characterized by very low diffusion barriers. Increasing water coverage leads to the formation of structures containing alternatively dissociated and molecularly adsorbed water molecules. The magnitude of the calculated adsorption energy per water molecule increases from 0.57 eV for the water monomer to 0.79 eV for the water monolayer. The present experimental and theoretical results show furthermore that the stability of MgO(100) surfaces in the presence of water depends on its pH value. The etching of MgO(100) surfaces in aqueous medium is studied with the AFM in situ with pH value changing from basic to acidic. While the atomically flat MgO(100) surface remains stable in basic and neutral pH ranges, it is easily etched when the pH turns below a value of 6. This agrees qualitatively with the present DFT calculations showing that square pits resulting from the etching reduce the MgO(100) surface energy in acidic environments.