Hexagonal Monolayer Ice without Shared Edges

Observation of Ice-Like Two-Dimensional Flakes on Self-Assembled Protein Monolayer without Nanoconfinement under Ambient Conditions

Directly correlating the morphology and composition of interfacial water is vital not only for studying water icing under critical conditions but also for understanding the role of protein-water interactions in bio-relevant systems. In this study, we present a model system to study two-dimensional (2D) water layers under ambient conditions by using self-assembled monolayers (SAMs) supporting the physisorption of the Cytochrome C (Cyt C) protein layer. We observed that the 2D island-like water layers were uniformly distributed on the SAMs as characterized by atomic force microscopy, and their composition was confirmed by nano-atomic force microscopy-infrared spectroscopy and Raman spectroscopy. In addition, these 2D flakes could grow under high-humidity conditions or melt upon the introduction of a heat source. The formation of these flakes is attributed to the activation energy for water desorption from the Cyt C being nearly twofold high than that from the SAMs. Our results provide a new and effective method for further understanding the water-protein interactions.

Rich Phase Behaviors of One-Dimensional Interfacial Water via Tuning Interfacial Water-Solid Interaction

Despite numerous studies of water structures at the two-dimensional water-solid interfaces, much less is known about the phase behaviors of water at the one-dimensional (1D) liquid-solid interface. In this work, the 1D interfacial water phase behavior on the outer surface of carbon nanotube-like (CNT-like) models is studied by tuning the Lennard-Jones potential parameter ε of the surface atoms at various temperatures. Extensive molecular dynamics simulations show that ice nanotubes (INTs) can be spontaneously formed on CNT-like model surfaces without nanoconfinement. INTs with flat-square walls (INTs-FSW) are formed on the CNT-like model surface when the ε value is beyond a critical value εc. The value of εc exponentially increases as temperature rises. Contrary to the prevailing formation of INTs-FSW at a relatively strong water-surface interaction, INTs with bilayer hexagonal walls are formed at a weak interfacial interaction with the ε value being in a modest range. An ε-T phase diagram is constructed for the 1D interfacial water on the CNT(100, 0) model surface. Rich phases of H2O are given in different regions of the phase diagram, depending on the water-surface interaction. This comprehensive study not only provides new insight into the phase behavior of 1D interfacial water but also can guide future experiments to produce INTs without nanoconfinement.

Deposition of Water Vapor on Au(001) Substrates: Effect of Temperature and Deposition Frequency

Ice formation from water vapor is a common phenomenon with significant implications for both natural ice formation and industrial processes. However, there remains controversy over how deposition frequency and substrate temperature affect the structural forms of deposition products and their formation processes. In this study, we employed molecular dynamics simulations to investigate the deposition process of water vapor onto a cold Au(001) substrate at different temperatures and deposition frequencies. We analyzed the effects of temperature and deposition frequency on the forms of deposition products including bilayer hexagonal ice, amorphous water, and their mixtures. Additionally, we identified and explained the unique formation of square ice as an unstable intermediate within specific temperature and deposition frequency ranges. We also discuss the crystallization processes of pancake- and droplet-like amorphous waters. This research contributes to a better understanding of ice formation, with implications for more accurate forecasting of natural ice formation and improved control of artificial ice processes.

Two rhombic ice phases from aqueous salt solutions under graphene confinement

Water exhibits rich ice phases depending upon its respective formation conditions, and in particular, the two-dimensional ice with nonhexagonal symmetry adsorbed on solids relates to the exceptional arrangement of water molecules. Despite extensive reporting of two-dimensional ice on various solid surfaces, the geometry and thermodynamics of ice formation from an aqueous salt solution are still unknown. In this Letter, we show the formation of single- and two-phase mixed two-dimensional rhombic ice from aqueous salt solutions with different concentrations under strong compressed confinement of graphene at ambient temperature by using classical molecular dynamics simulations and first-principles calculations. The two rhombic ice phases exhibit identical geometry and thermodynamic properties, but different projections of the oxygen atoms against solid surface symmetry, where they relate to the stable and metastable arrangements of water molecules confined between two graphene layers. A single-phase rhombic ice would grow from the confined saturated aqueous solutions since the previously stable rhombic molecular arrangement becomes an unstable high-energy state by introducing salt ions nearby. Our result reveals different rhombic ice phases growing from pure water and aqueous solutions, highlighting the deciding role of salt ions in the ice formation process due to their common presence in liquids.

Ab Initio Molecular Dynamics Investigation of the Solvation States of Hydrated Ions in Confined Water

Ionic transport in nanoscale channels with a critical size comparable to that of ions and solutes exhibits exceptional performance in water desalination, ion separation, electrocatalysts, and supercapacitors. However, the solvation states (SSs), i.e., the hydration structures and probability distribution, of hydrated ions in nanochannels differ from those in the bulk and the perspective of continuum theory. In this work, we conduct ab initio enhanced-sampling atomistic simulations to investigate the ion-specific SSs of monovalent ions (including Li+, Na+, K+, F-, Cl-, and I-) in the graphene channel with a width of 1 nm. Our findings highlight that the SSs of those ions are primarily determined by ion-water hydration, where ion-wall interactions play a minor role. The distribution of ions in layered confined water is a result of ion-specific hydration, which arises from the synergy of entropy and enthalpy. The free energy barriers for transitions between SSs are on the order of 1kBT, allowing for modulation through applying external fields or modifying surface properties. As the ion-wall interaction strengthens, as observed in vermiculite and carbides and nitrides of transition metal channels, the probability of near-wall SSs increases. These results help to improve the performance of nanofluidic devices and provide crucial insights for developing accurate force fields of molecular simulations or advanced theoretical approaches for ion dynamics in confined channels.

Water-Induced Bandgap Engineering in Nanoribbons of Hexagonal Boron Nitride

Different from hexagonal boron nitride (hBN) sheets, the bandgap of hBN nanoribbons (BNNRs) can be changed by spatial/electrostatic confinement. It is predicted that a transverse electric field can narrow the bandgap and even cause an insulator-metal transition in BNNRs. However, experimentally introducing an overhigh electric field across the BNNR remains challenging. Here, it is theoretically and experimentally demonstrated that water adsorption greatly reduces the bandgap of zigzag-oriented BNNRs (zBNNRs). Ab initio calculations show that water molecules can be favorably assembled within the trench between two adjacent BNNRs to form a polar ice layer, which induces a transverse equivalent electric field of over 2 V nm-1 accounting for the bandgap reduction. Field-effect transistors are successfully fabricated from zBNNRs with different widths. The conductance of water-adsorbed zBNNRs can be tuned over 3 orders in magnitude via modulation of the equivalent electrical field at room temperature. Furthermore, photocurrent response measurements are taken to determine the optical bandgaps of zBNNRs with water adsorption. The zBNNR with increased width can exhibit a bandgap down to 1.17 eV. This study offers fundamental insights into new routes toward realizing electronic/optoelectronic devices and circuits based on hexagonal boron nitride.

Water adsorption and dynamics on graphene and other 2D materials: Computational and experimental advances

The interaction of water and surfaces, at molecular level, is of critical importance for understanding processes such as corrosion, friction, catalysis and mass transport. The significant literature on interactions with single crystal metal surfaces should not obscure unknowns in the unique behaviour of ice and the complex relationships between adsorption, diffusion and long-range inter-molecular interactions. Even less is known about the atomic-scale behaviour of water on novel, non-metallic interfaces, in particular on graphene and other 2D materials. In this manuscript, we review recent progress in the characterisation of water adsorption on 2D materials, with a focus on the nano-material graphene and graphitic nanostructures; materials which are of paramount importance for separation technologies, electrochemistry and catalysis, to name a few. The adsorption of water on graphene has also become one of the benchmark systems for modern computational methods, in particular dispersion-corrected density functional theory (DFT). We then review recent experimental and theoretical advances in studying the single-molecular motion of water at surfaces, with a special emphasis on scattering approaches as they allow an unparalleled window of observation to water surface motion, including diffusion, vibration and self-assembly.

A folded ice monolayer

A highly stable ice monolayer with folded structural motifs is predicted by means of a novel tiling method augmented with ab initio calculations. This ice monolayer has every two neighboring water hexamers connected by a water square yet folded into two distinct planes, and is thus coined as a folded ice model. It is in the ground state in a range of water densities from 0.08 to 0.12 Å-2, with a stronger energy preference at a lower water density. Its stability shown by ab initio molecular dynamics simulations can sustain up to a temperature of 100 K. The tiling method also enables the prediction of a family of considerably stable ice monolayers with a variety of puckered structures. These results enrich our knowledge of low-dimensional water structures and pave a way to explore more exotic ice nanostructures under confinements.

A Helical Monolayer Ice

Understanding water wetting layers on solid surfaces is essential for many natural and industrial processes. Here we find a helical ice monolayer with every six water molecules helically arranged along the normal of the basal plane by performing an intensive structural search based on ab initio calculations. The helical ice is more stable than all previous models of monolayer and bilayer ices in a wide range of water densities both in vacuum and on weakly interacting substrates due to a stronger network of hydrogen bonds enabled by the helical geometry. More compelling is the fact that this model adequately explains a recent experimental ice monolayer grown on graphite in terms of the lattice parameter, water density, and Moiré pattern. The helical character in the new ice model echoes previously reported helical motifs in one-dimensional ice structures and suggests an unexpected capability of hydrogen bonds in driving the surface reconstruction of ice structures.

Rich topologies of monolayer ices via unconventional electrowetting

Accurate manipulation of a substance on the nanoscale and ultimately down to the level of a single atom or molecule is an ongoing subject of frontier research. Herein, we show that topologies of water monolayers on substrates, in the complete wetting condition, can be manipulated into rich forms of ordered structures via electrowetting. Notably, two new topologies of monolayer ices were identified from our molecular dynamics simulations: one stable below room temperature and the other one having the ability to be stable at room temperature. Moreover, the wettability of the substrate can be tuned from superhydrophobic to superhydrophilic by uniformly changing the charge of each atomic site of the dipole or quadrupole distributed in an orderly manner on the model substrate. At a certain threshold value of the atomic charge, water droplets on the substrate can spread out spontaneously, achieving a complete electrowetting. Importantly, unlike the conventional electrowetting, which involves application of a uniform external electric field, we proposed non-conventional electrowetting, for the first time, by invoking the electric field of dipoles and quadrupoles embedded in the substrate. Moreover, different topologies of water monolayers can be achieved by using the non-conventional electrowetting. A major advantage of the non-conventional electrowetting is that the contact-angle saturation, a long-standing and known limitation in the field of electrowetting, can be overcome by tuning uniformly the lattice atomic charge at the surface, thereby offering a new way to mitigate the contact-angle saturation for various electrowetting applications.

Transparent proton transport through a two-dimensional nanomesh material

Molecular sieving is of great importance to proton exchange in fuel cells, water desalination, and gas separation. Two-dimensional crystals emerge as superior materials showing desirable molecular permeability and selectivity. Here we demonstrate that a graphdiyne membrane, an experimentally fabricated member in the graphyne family, shows superior proton conductivity and perfect selectivity thanks to its intrinsic nanomesh structure. The trans-membrane hydrogen bonds across graphdiyne serve as ideal channels for proton transport in Grotthuss mechanism. The free energy barrier for proton transfer across graphdiyne is ~2.4 kJ mol-1, nearly identical to that in bulk water (2.1 kJ mol-1), enabling "transparent" proton transport at room temperature. This results in a proton conductivity of 0.6 S cm-1 for graphdiyne, four orders of magnitude greater than graphene. Considering its ultimate pore size of 0.55 nm, graphdiyne membrane blocks soluble fuel molecules and exhibits superior proton selectivity. These advantages endow graphdiyne a great potential as proton exchange material.