Square ice in graphene nanocapillaries

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, etc. 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 Å and during TEM observation assembles in bi-and trilayer 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 suggest that square ice is likely to be common inside hydrophobic nanochannels, independent of their exact atomic makeup.Three-dimensional (3D) water exists in many forms, as liquid, vapor and as many as 15 crystalline and some amorphous phases of ice, with the commonly found hexagonal ice alone being responsible for the fascinating variety of snowflakes [1,2]. Less noticeable but equally ubiquitous is water present at interfaces and in microscopic pores where nanometer-scale confinement makes the structure of water and its dynamics radically different from bulk water [3,4]. Confined and interfacial water has attracted dedicated interest in fields ranging from life to earth to materials sciences, playing a crucial role in such diverse phenomena as protein assembly, nanofriction, filtration, dissolving, frost heaving, detergent cleaning, heterogeneous catalysis and so on [5][6][7][8]. It is now well established that near a solid surface, whether hydrophilic or hydrophobic, water forms a layered structure made up of distinct monolayers [3][4][5][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. However, the structure within these layers remains largely unknown. Molecular dynamics (MD) simulations [9-17] predicted a great variety of phases, although the results are sensitive to modelled conditions and some seem conflicting. For example, a buckled monolayer ice was found inside hydrophilic nanochannels [11] and a flat hexagonal ice inside hydrophobic ones below room temperature [12,15,22]. On the other hand, no in-plane order was observed inside mica (hydrophilic) and graphite (hydrophobic) nanochannels at and above room temperature [11,14]. A close analogue of planar square ice was reported in MD simulations of water inside carbon nanotubes [9,10,17]. In this case, water molecules

Multilayer Two-Dimensional Water Structure Confined in MoS2

“…In the current simulations also, after the start of Np y T equilibration at 298 K and 1 atm, the confined region between the two MoS 2 sheets becomes completely filled with water molecules in less than 200 ps if the S-to-S interlayer distance of the Interestingly, the density profiles of water for d = 6 and 8 Å are similar to those observed in graphene confined water 30 , suggesting the similar H-bond configurations of water regardless of material difference. However, the key disparity is that, for a given interlayer distance of MoS 2 sheets, clearly separated single, double, triple, and quadruple layers are seen at ambient 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 10 conditions; whereas for graphene, a large lateral pressure is needed not just to bring water to the graphitic confinement but also to obtain a flat configuration.…”

“…[16][17][18][19][20][21][22][23][24][25][26][27][28][29] Although the structures of confined water have been predicted for a variety of dimensions and materials using molecular dynamics (MD) simulations, the first experimental observation of the 2D water in between the two graphene sheets was obtained very recently using high-resolution transmission electron microscopy measurements (TEM). 30 This observation revealed the formation of a monolayer of planar "square" ice with a high packing density and, depending on the inter-graphene distance, the formation of bi-and trilayer crystallites of water. 30 The same authors have also reported the MD simulations of graphene-confined water that agreed with the experimental structure of the "square ice".…”

“…30 This observation revealed the formation of a monolayer of planar "square" ice with a high packing density and, depending on the inter-graphene distance, the formation of bi-and trilayer crystallites of water. 30 The same authors have also reported the MD simulations of graphene-confined water that agreed with the experimental structure of the "square ice". However this involved an enormous lateral van der Waals pressure of about 1 GPa (10,000 atm) to obtain a flat structure of water for the bilayer and trilayer distances.…”

Confined catalysis under two-dimensional materials

“…Thus, the interaction between Gr and CO is in the repulsion regime, and CO adsorption is destabilized in the confined space. It has been recently demonstrated that vdW forces between 2D materials can be so strong on a nanometer scale that huge pressure up to 1 GPa is built therein (40,41). Therefore, we conclude that the vdW interaction between the opposite walls of the 2D microenvironment has induced strong geometric constraint on trapped interlayer molecules, which contributes to the confinement effect.…”

Long-Range Repulsion Between Spatially Confined van der Waals Dimers

“…The repulsion presented in this Letter, which is missing from classical force fields and density functional approximations, would introduce an intermolecular repulsion, which may rationalize a higher flow rate with increasing confinement. The 2D oscillator dimer example is related to a recent experiment [52] where a previously unknown ice structure has been discovered when water molecules are encapsulated between two graphene sheets, thereby suggesting peculiar intermolecular interaction between water molecules under quasi-2D confinement. In fact, a recent quantum Monte Carlo study [53] of stable square ice between graphene sheets shows that most dispersion-corrected DFT functionals overestimate the binding in the water layer.…”