Edge-Driven Phase Transitions in 2D Ice

Abstract

Two-dimensional (2D) water, confined by atomically flat layered materials, may transit into various crystalline phases even at room temperature. However, to gain full control over the crystalline state, we should not only confine water in the out-of-plane direction but also restrict its in-plane motion, forming 2D water clusters or ribbons. One way to do this is by using an electric field, in particular the intrinsic electric field of an adjacent polar material. We have found that the crystalline phases of 2D water clusters placed between two hexagonal boron nitride (h-BN) nanoribbons are crucially determined by the nanoribbons' edges, the resulting polarity of the nanoribbons, and their interlayer distance. We make use of the density functional theory with further assistance of molecular dynamics simulations to establish the comprehensive phase diagrams, demonstrating transitions between liquid and solid phases and between the states of different crystalline orders. We also show that the crystalline orders are maintained when water flows between h-BN channels under external pressure. Our results open a promising pathway toward the control of the water structure and its flow by the use of the microscopic electric field of polar materials.