Formation of a two-dimensional helical square tube ice in hydrophobic nanoslit using the TIP5P water model

In extreme and nanoconfinement conditions, the tetrahedral arrangement of water molecules is challenged, resulting in a rich and new phase behavior unseen in bulk phases. The unique phase behavior of water confined in hydrophobic nanoslits has been previously observed, such as the formation of a variety of two-dimensional (2D) ices below the freezing temperature. The primary identified 2D ice phase, termed square tube ice (STI), represents a unique arrangement of water molecules in 2D ice, which can be viewed as an array of 1D ice nanotubes stacked in the direction parallel to the confinement plane. In this study, we report the molecular dynamics (MD) simulations evidence of a novel 2D ice phase, namely, helical square tube ice (H-STI). H-STI is characterized by the stacking of helical ice nanotubes in the direction parallel to the confinement plane. Its structural specificity is evident in the presence of helical square ice nanotubes, a configuration unseen in both STI and single-walled ice nanotubes. A detailed analysis of the hydrogen bonding strength showed that H-STI is a 2D ice phase diverging from the Bernal-Fowler-Pauling ice rules by forming only two strong hydrogen bonds between adjacent molecules along its helical ice chain. This arrangement of strong hydrogen bonds along ice nanotube and weak bonds between the ice nanotube shows a similarity to quasi-one-dimensional van der Waals materials. Ab initio molecular dynamics simulations (over a 30 ps) were employed to further verify H-STI's stability at 1 GPa and temperature up to 200 K.

Puckered Zigzag Monolayer Ice: Does a Confined Flat Four-Coordinated Monolayer Ice Always Have a Corresponding Puckered Phase?

We note that a flat, four-coordinated monolayer ice under confinement always has a corresponding puckered phase. Recently, a monolayer ice consisting of an array of zigzag water chains (ZZMI) predicted by first-principles calculations of water under confinement is a flat four-coordinated monolayer ice. Herein, to investigate whether puckered ZZMI exists stably, we perform molecular dynamics simulations of two-dimensional (2D) ice formation for water constrained in graphene nanocapillaries. We find a novel monolayer ice structure that can be viewed as the ZZMI puckered along the direction perpendicular to the zigzag chain (pZZMI). Unlike ZZMI that does not satisfy the ice rule, each water molecule in pZZMI can form four hydrogen bonds (HBs) via forming two stable intersublayer HBs and two intrasublayer HBs. This work provides a fresh perspective on 2D confined ice, highlighting the intrinsic connections between 2D confined ices.

Temperature-pressure phase diagram of confined monolayer water/ice at first-principles accuracy with a machine-learning force field

Understanding the phase behaviour of nanoconfined water films is of fundamental importance in broad fields of science and engineering. However, the phase behaviour of the thinnest water film - monolayer water - is still incompletely known. Here, we developed a machine-learning force field (MLFF) at first-principles accuracy to determine the phase diagram of monolayer water/ice in nanoconfinement with hydrophobic walls. We observed the spontaneous formation of two previously unreported high-density ices, namely, zigzag quasi-bilayer ice (ZZ-qBI) and branched-zigzag quasi-bilayer ice (bZZ-qBI). Unlike conventional bilayer ices, few inter-layer hydrogen bonds were observed in both quasi-bilayer ices. Notably, the bZZ-qBI entails a unique hydrogen-bonding network that consists of two distinctive types of hydrogen bonds. Moreover, we identified, for the first time, the stable region for the lowest-density [Formula: see text] monolayer ice (LD-48MI) at negative pressures (<-0.3 GPa). Overall, the MLFF enables large-scale first-principle-level molecular dynamics (MD) simulations of the spontaneous transition from the liquid water to a plethora of monolayer ices, including hexagonal, pentagonal, square, zigzag (ZZMI), and hexatic monolayer ices. These findings will enrich our understanding of the phase behaviour of the nanoconfined water/ices and provide a guide for future experimental realization of the 2D ices.

Does twist angle affect the properties of water confined inside twisted bilayer graphene?

Graphene nanoslit pores are used for nanofluidic devices, such as, in water desalination, ion-selective channels, ionic transistors, sensing, molecular sieving, blue energy harvesting, and protein sequencing. It is a strenuous task to prepare nanofluidic devices, because a small misalignment leads to a significant alteration in various properties of the devices. Here, we focus on the rotational misalignment between two parallel graphene sheets. Using molecular dynamics simulation, we probe the structure and dynamics of monolayer water confined inside graphene nanochannels for a range of commensurate twist angles. With SPC/E and TIP4P/2005 water models, our simulations reveal the independence of the equilibrium number density- n ∼ 13 nm^-2 for SPC/E and n ∼ 11.5 nm^-2 for TIP4P/2005- across twists. Based on the respective densities of the water models, the structure and dielectric constant are invariant of twist angles. The confined water structure at this density shows square ice ordering for SPC/E water only. TIP4P/2005 shows ordering at the vicinity of a critical density (n ∼ 12.5 nm^-2). The average perpendicular dielectric constant of the confined water remains anomalously low (∼2 for SPC/E and ∼6 for TIP4P/2005) for the studied twist angles. We find that the friction coefficient of confined water molecules varies for small twist angles, while becoming independent for twists greater than 5.1°. Our results indicate that a small, angular misalignment will not impair the dielectric properties of monolayer water within a graphene slit-pore, but can significantly influence its dynamics.

Two-dimensional partitioned square ice confined in graphene/graphite nanocapillaries

As one of the most fascinating confined water/ice phenomena, two-dimensional square ice has been extensively studied and experimentally confirmed in recent years. Apart from the unidirectional homogeneous square icing patterns considered in previous studies, the multidirectional partitioned square icing patterns are discovered in this study and characterized by molecular dynamics (MD) simulations. Square icing parameters are proposed to quantitatively distinguish the partitioned patterns from the homogeneous patterns and the liquid water. The number of graphene monolayers n is varied in this study, and the results show that it is more energetically favorable to form partitioned square icing patterns when the water molecules are confined between graphite sheets (n ≥ 2) compared to graphene (n = 1). This phenomenon is insensitive to n as long as n ≥ 2 because of the short-range nature of the interaction between water molecules and the carbon substrate. Moreover, it is energetically unfavorable to form partitioned square icing patterns for a single layer of water molecules even for n ≥ 2, verifying that the interaction between layers of water molecules is another dominant factor in the formation of partitioned structures. The conversion from partitioned structure to homogeneous square patterns is investigated by changing the pressure and the temperature. Based on the comprehensive MD simulations, this study unveils the formation mechanism of the partitioned square icing patterns.

First-Principles Molecular Dynamics Simulations of the Spontaneous Freezing Transition of 2D Water in a Nanoslit

As with bulk ices, two-dimensional (2D) ices exhibit diverse crystalline structures, and the majority of these 2D structures have been predicted based on classical molecular dynamics (MD) simulations. Here, the spontaneous freezing transition of 2D liquid water within hydrophobic nanoslits is demonstrated for the first time using first-principles MD simulations. Various 2D ices are observed under different lateral pressure and temperature conditions. Notably, the liquid water confined to a 6.0 Å-wide nanoslit can spontaneously freeze into a monolayer ice consisting of an array of zigzag water chains at 2.5 GPa and 250 K. Moreover, within an 8.0 Å-wide nanoslit and at 4.0 GPa and 300 K, a previously unreported bilayer ice forms spontaneously that has a structure resembling that of the double surface layers of bulk ice-VII. Both 2D crystalline ices do not obey the ice rule, suggesting first-principles simulation can access a certain phase space that is not easily approached using classical simulations.

Dynamics of water confined in a graphene nanochannel: dependence of friction on graphene chirality

Numerous recent studies reveal that water transport through graphene-based nanochannels exhibits unconventional behavior. Theoretical and experimental works have reported that the dynamic behavior of the constrained water could be affected by the atomic structure, surface curvature, confinement height, pressure, and mechanical strain of the confining channel. However, few studies concern the role of graphene chirality in the dynamics of confined water. In the present study, using equilibrium molecular dynamics (EMD) simulations, we simulated the water flow through different graphene-based channels to investigate the influence of graphene chirality on the dynamic behavior of the confined water. The friction coefficient at the water/graphene interface was found to be dependent on the crystallographic orientation of the graphene wall. The results also indicated that the chirality-dependent friction behavior was affected by the confinement height of the channel. Detailed analyses on the physical origin of such a chirality effect suggested that the chirality-dependent friction was ascribed to the water-graphene interaction energy barrier variation when the water flow was driven along the armchair edge and zigzag edge. These findings are beneficial to the design of graphene-based nanofluidic devices.

Interfacial water intercalation-induced metal-insulator transition in NbS2/BN heterostructure

Interfacial engineering, such as molecule intercalation, can modify properties and optimize performance of van der Waals heterostructures and their devices. Here, we investigated the pristine and water molecule intercalated heterointerface of niobium disulphide (NbS₂) on hexagonal boron nitride (h-BN) (NbS₂/BN) using advanced atomic force microscopy (AFM), and observed the metal-insulator transition (MIT) of first layer (1L-) of NbS₂ induced by water molecule intercalation. In pristine sample, interfacial charge transfers were confirmed by the direct detection of trapped static charges at the post-exposed h-BN surface, produced by mechanically peeling off the 1L-NbS₂ from the substrate. The interfacial charge transfers facilitate the intercalation of water molecules at the heterointerface. The intercalated water layers make a MIT of 1L-NbS₂, while the pristine metallic state of the following NbS₂ layers remains preserved. This work is of great significance to help understand the interfacial properties of 2D metal/insulator heterostructures and can pave the way for further preparation of an ultrathin transistor.

Comparative Study of the Effects of Temperature and Pressure on the Water-Mediated Interactions between Apolar Nanoscale Solutes

We perform molecular dynamics simulations to study the effects of temperature and pressure on the water-mediated interaction (WMI) between two nanoscale (apolar) graphene plates at 240 ≤ T ≤ 400 K and -100 ≤ P ≤ 1200 MPa. These are thermodynamic conditions relevant to, for example, cooling-, heating-, compression-, and decompression-induced protein denaturation. We find that at all ( T, P) studied, the potential of mean force between the graphene plates, as a function of plate separation r, exhibits local minima at specific plate separations r = r _n that can accommodate n water layers ( n = 0,1,2,3). In particular, our results show that isobaric cooling and isothermal compression have a similar effect on WMI between the plates; both processes tend to suppress the attraction and ultimate collapse of the graphene plates by kinetically trapping the plates at the metastable states with r = r _n ( n > 0). In addition, isobaric heating and isothermal decompression also have a similar effect; both processes tend to reduce the range and strength of the interactions between the graphene plates. Interestingly, at low temperatures, the WMI between the plates is affected by crystallization. However, crystallization depends deeply on the water model considered, SPC/E and TIP4P/2005 water models, with the crystallization occurring at different ( T, P) conditions, into different forms of ice.

Why different water models predict different structures under 2D confinement

Experiments of nanoconfined water between graphene sheets at high pressure suggest that it forms a square ice structure (Algara-Siller et al., Nature, 2015, 519, 443). Molecular dynamics (MD) simulations have been used to attempt to recreate this structure, but there have been discrepancies in the structure formed by the confined water depending on the simulation set-up that was employed and particularly on the choice of water model. Here, using classical molecular dynamics simulations, we have systematically investigated the effect that three different water models (SPC/E, TIP4P/2005 and TIP5P) have on the structure of water confined between two rigid graphene sheets with a 0.9 nm separation. We show that the TIP4P/2005 and the TIP5P water models form a hexagonal AA-stacked structure, whereas the SPC/E model forms a rhombic AB-stacked structure. Our work demonstrates that the formation of these structures is driven by differences in the strength of hydrogen bonds predicted by the three water models, and that the nature of the graphene/water interaction only mildly affects the phase diagram. Considering the available experimental data and first-principle simulations we conclude that, among the models tested, the TIP4P/2005 and TIP5P force fields are for now the most reliable when simulating water under confinement. © 2018 Wiley Periodicals, Inc.

Phase Diagram of Water Confined by Graphene

The behavior of water confined at the nanoscale plays a fundamental role in biological processes and technological applications, including protein folding, translocation of water across membranes, and filtration and desalination. Remarkably, nanoscale confinement drastically alters the properties of water. Using molecular dynamics simulations, we determine the phase diagram of water confined by graphene sheets in slab geometry, at T = 300 K and for a wide range of pressures. We find that, depending on the confining dimension D and density σ, water can exist in liquid and vapor phases, or crystallize into monolayer and bilayer square ices, as observed in experiments. Interestingly, depending on D and σ, the crystal-liquid transformation can be a first-order phase transition, or smooth, reminiscent of a supercritical liquid-gas transformation. We also focus on the limit of stability of the liquid relative to the vapor and obtain the cavitation pressure perpendicular to the graphene sheets. Perpendicular cavitation pressure varies non-monotonically with increasing D and exhibits a maximum at D ≈ 0.90 nm (equivalent to three water layers). The effect of nanoconfinement on the cavitation pressure can have an impact on water transport in technological and biological systems. Our study emphasizes the rich and apparently unpredictable behavior of nanoconfined water, which is complex even for graphene.