Abstract

Water in nano-confinement is ubiquitous in biology and geology, and highly relevant for nanotechnology. Unfortunately, the difficulty in interpreting experiments and contrasting results from simulations using a plethora of models has prevented a consensus on the phase behavior of nanoconfined water. Here we predict the full pressure-temperature phase diagram of monolayer water confined between graphene at first-principles accuracy by combining advances in state-of-the-art methods such as diffusion Monte Carlo, density functional theory, machine learning and advanced statistical sampling. We find that monolayer water exhibits surprisingly rich and diverse phase-behavior that is highly sensitive to the lateral pressure applied by graphene sheets. At low pressures, monolayer water exhibits rich polymorphsim with a non-monotonic dependence of the melting temperature with pressure. At typical experimental confinement pressures, monolayer water can melt via a two-step mechanism, through an intermediate ``hexatic like” phase, in agreement with the KTHNY theory of phase transitions in 2D materials. The hexatic phase exhibits quasi-rotations of water molecules similar to the free rotor phase of high-pressure hydrogen. High pressures increase autoprotolysis events, and eventually lead to a new superionic phase at mild conditions compared to bulk. Our work comprehensively describes the phase behavior of monolayer confined water and outlines clear experimental routes for observing the hexatic and superionic phases. Furthermore, it suggests that confinement could be a route towards probing superionic materials at mild conditions, opening prospects for the development of efficient battery materials.