Experimental observation of liquid-solid transition of nanoconfined water at ambient temperature.

Nanoconfined water exhibits many abnormal properties compared with bulk water. However, the origin of those anomalies remains controversial due to the lack of experimental access to the molecular-level details of the hydrogen-bonding network of water within a nanocavity. Here we address this issue by combining scanning probe microscopy with nitrogen-vacancy-centre-based quantum sensing. Such a technique allows us to characterize both dynamics and structure of water confined between a hexagonal boron nitride flake and a hydrophilic diamond surface by nanoscale nuclear magnetic resonance. We observe a liquid-solid phase transition of nanoconfined water at ambient temperature with an onset confinement size of ~1.6 nm, below which the water diffusion is considerably suppressed and the hydrogen-bonding network of water becomes structurally ordered. The complete crystallization is observed below a confinement size of ~1 nm. The liquid-solid transition is further confirmed by molecular dynamics simulation. These findings shed new light on the phase transition of nanoconfined water and may form a unified picture for understanding water anomalies at the nanoscale.