Reversible structural transition in nanoconfined ice

The report on square ice sandwiched between two graphene layers by Algara-Siller et al. [Nature (London) 519, 443 (2015)] has generated a large interest in this system. By applying high lateral pressure on nanoconfined water, we found that monolayer ice is transformed to bilayer ice when the two graphene layers are separated by $\mathrm{H}=6,7$ Å. It was also found that three layers of a denser phase of ice with smaller lattice constant are formed if we start from bilayer ice and apply a lateral pressure of about 0.7 GPa with $\mathrm{H}=8,9$ Å. The lattice constant (2.5–2.6 Å) in both transitions is found to be smaller than those typical for the known phases of ice and water, i.e., 2.8 Å. We validate these results using ab initio calculations and find good agreement between ab initio O-O distance and those obtained from classical molecular dynamics simulations. The reversibility of the mentioned transitions is confirmed by decompressing the systems.

Figure 1

Schematic setup of confined water between two graphene layers held at a distance H from each other.

Figure 2

(a) The variation of calculated pressure with MD simulation time in nanoconfined water, which is initially monolayer (bilayer) ice and ended at bilayer (trilayer) ice. The legends refer to different channel sizes. (b) The different energy contributions (i.e., vdW, Coulomb, and total energy) of nanoconfined water as a function of lateral pressure for monolayer (bilayer) ice transformed to bilayer (trilayer) ice calculated using TIP4P. Here H = 6, 9 Å for monolayer (bilayer) to bilayer (trilayer) transitions, respectively.

Figure 3

(a) Side and (b) top view of a snapshot of our molecular dynamics simulation for bilayer ice at $T=298$ K. We obtain trilayer ice at about 0.7 GPa where we show a (c) side and (d) top view. We show only the oxygen atoms, where different colors refer to the different layers.

Figure 4

Reversibility of the response of confined ice to the applied lateral pressure. The vdW energy of ice and total vdW energy as a function of the lateral pressure. The inset shows the change in the number of H bonds during compression (until 1 ns) and expansion (beyond 1 ns). Here H = 6, 9 Å for monolayer (bilayer) to bilayer (trilayer) transitions, respectively.

Figure 5

The H-bonding energy per water molecule as a function of pressure using ReaxFF potentials. The inset indicates the H-bonding energy with time during compression.

Figure 6

The radial distribution function of the O-O distance, (a) in each of the layers in bilayer ice at $T=298$ K and (b) trilayer ice. In each panel, the insets indicate the $y$-axis density profile. Here H = 6, 9 Å for monolayer (bilayer) to bilayer (trilayer) transitions, respectively. The insets show the density profiles perpendicular to the layers.

Figure 7

(a) The optimized structure of 12 water molecules between two graphene layers and (b) the corresponding structure of single-layer water.

Figure 8

(a) The optimized structure of 32 water molecules between two graphene layers and (b),(c) the corresponding structure of each layer of formed bilayer water.

Figure 9

Four different initial configurations for monolayer (A1,A2,A3,A4) and bilayer (B1,B2,B3,B4) ice of 12 and 32 water molecules confined between graphene layers, and the top and side views of the corresponding final optimized structures Am and Bm (shown in Figs. 7 and 8).