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° (rather than 120°). 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 solid-liquid-solid triple point exhibits some resemblance to that shown in the pressure-temperature phase diagram for bulk ice I-water-ice III phases.