Water Confined in Nanocapillaries: Two-Dimensional Bilayer Squarelike Ice and Associated Solid-Liquid-Solid Transition

Despite recent experimental evidence of the two-dimensional (2D) square ice in graphene nanocapillaries, based on transmission electron microscopy (TEM) imaging, the AA-stacked bilayer square ice structure has not been observed in all previous classical molecular dynamics (MD) simulations nor found in recent unbiased first-principles structure searches. Herein, we report the MD simulations of 2D bilayer ice formation for water confined between two parallel hydrophobic walls (nanoslit). We find a bilayer ice whose simulated TEM imaging resembles that of bilayer squarelike ice. This bilayer ice also demonstrates dynamical stability in first-principles phonon computations. The realistic structure of this bilayer ice, however, consists of two hexagonal monolayers with the AB-stacking order, where the hexagonal rings are slightly elongated with two unequal inner angles, 107 and 146 degrees (rather than 120 degrees). The phase diagram of the nanoslit width versus temperature exhibits a solid-liquid-solid triple point, where the second solid phase is the well-known bilayer hexagonal ice (i.e., the bilayer ice I) with an AA-stacking order, which has been experimentally produced at ambient condition in a nanoslit of graphene and MoS2 sheet. Such a solidliquidsolid triple point exhibits some resemblance to that shown in the pressuretemperature phase diagram for bulk ice I-water-ice III phases.