Polymorphism of water in two dimensions

The structure of confined water is governed by a delicate interplay between water-substrate and water-water interactions. Motivated by recent experimental studies on water in graphene nanocapillaries, two-dimensional water layers have been addressed by first-principles electronic structure calculations. In order to identify the structure-determining factors, first calculations for free-standing water layers have been performed. We demonstrate that two-dimensional water structures consisting of square bilayers, rhombic bilayers, truncated-square bilayers or secondary-prism bilayers are energetically more favorable than the traditionally considered hexagonal bilayer. These 2D water structures are stabilized by a compromise between high coordination and optimum tetrahedral bonding geometry. The identified structure determining factors responsible for the polymorphism of water in two dimensions will be operative in any confined water structure. The graphene-water interaction associated with the confinement of water within graphene sheets has an additional, but rather weak influence on the structural stability of water layers.