Square ice in graphene nanocapillaries

TL;DR

This study reveals that water confined between graphene sheets forms a square ice structure at room temperature, differing from traditional tetrahedral ice, with implications for nanoconfined water behavior.

Contribution

The paper provides high-resolution electron microscopy evidence of square ice in graphene nanocapillaries and demonstrates its likely prevalence in hydrophobic nanochannels through simulations.

Findings

Square ice forms at room temperature in graphene nanocapillaries.

Confined square ice exhibits AA stacking with high packing density.

MD simulations suggest square ice is common in hydrophobic nanochannels.

Abstract

Adsorbed layers of water are ubiquitously present at surfaces and fill in microscopic pores, playing a central role in many phenomena in such diverse fields as materials science, geology, biology, tribology, nanotechnology. Despite such importance, the crystal structure of nanoconfined water remains largely unknown. Here we report high-resolution electron microscopy of mono- and few- layers of water confined between two graphene sheets, an archetypal example of hydrophobic confinement. Confined water is found to form square ice at room temperature - a phase with symmetry principally different from the conventional tetrahedral geometry of hydrogen bonding. The square ice has a high packing density with a lattice constant of 2.83 angstrom and during TEM observation assembles in bi- and tri- layer crystallites exhibiting AA stacking. Our findings are important for understanding of interfacial phenomena and, in particular, shed light on ultrafast transport of water through hydrophobic nanocapillaries. Our MD simulations show that square ice is likely to be common inside hydrophobic nanochannels, independent of their exact atomic makeup.