Ice Nanotubes Research Articles

Water transport in double-walled carbon nanotubes

Water confined in nanoscale environments exhibits rich phase behaviors. Recently, double-walled carbon nanotubes (DWCNTs) have been found to be able to generate ice nanotubes with large sizes, but their dynamics are still not explored. In this work, upon using molecular dynamics simulations we systematically investigate the water transport in DWCNTs, focusing on the effect of CNT length and temperature. At low temperatures, with the increase in CNT length, the water flow decreases linearly first and then has a sudden reduction to zero because of the liquid-ice phase transition; while at high temperatures the water flow decreases linearly without freezing. Obviously, the critical CNT length for the formation of ice increases at higher temperatures, because longer CNTs facilitate the ordered water structures. Furthermore, for a short CNT the water flow exhibits a perfect linear increase with the temperature; while for long CNTs, the water flow is zero at low temperatures because of the ice phase, and then increases suddenly. Apparently, the critical melting temperature shifts to higher values for longer CNTs. The translocation time has an overall opposite behavior compared to the water flow, and the water occupancy and dipole orientation are also sensitive to the CNT length and temperature. Our results reveal the relationship between water structures and dynamics within DWCNTs, and should have great implications for the design of novel nanofluidic devices.

Controllable structure phases and tunable freezing zones of water confined in multi-walled carbon nanotube capillaries

One-dimensional nanoconfined water exhibits rich structure phases. The freezing behavior of water confined in single-walled carbon nanotubes has been studied by both experimental measurements and theoretical simulations, however, it lacks of studying the freezing behavior of water confined in multi-walled carbon nanotubes (MWCNTs). Herein, we investigated the freezing behavior of water confined in MWCNTs using molecular dynamics simulations. Flat-square-walled ice nanotubes (FSW-INTs) were formed in the gaps with the inter-wall spacing of 6.66 Å, while, puckered-rhombic-walled ice nanotubes (PRW-INTs) were observed in the gaps with the inter-wall spacing of 8.61 Å. Then, the INT structures formed among the inter-wall gaps of MWCNTs can be controlled by tuning the inter-wall spacings of MWCNTs. Specifically, various groups of FSW-INTs and/or PRW-INTs were designed to form by using MWCNTs with the inter-wall spacing of 6.66 and/or 8.61 Å. In addition, it was found that water could be frozen in a simultaneous way or be frozen in a zone-by-zone form by tuning the inter-wall spacings of the MWCNTs due to the freezing temperatures of INTs are related with their diameters. The study will provide deep insights into the freezing behavior of water in MWCNTs.

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 30ps) were employed to further verify H-STI's stability at 1GPa and temperature up to 200K.

Edge-sharing water prisms.

Regular-shaped water clusters and nanostructures can be of particular interest for the self-assembly of complex structures and functional materials involving water molecules. Polyhedral water clusters are the most well studied. The low-energy structures of water hexamer, octamer and decamer are shaped like right prisms. The cavities of gas hydrates also have a polyhedral shape. Ice nanotubes are of no less interest both as configurations of global energy minima and in terms of possible applications. This article presents the results of the first systematic study of the structure and properties of a special class of water clusters. It reveals different combinations of three right prisms sharing an edge. According to modern calculations, the configurations of global energy minima of water clusters with the number of molecules being eighteen and twenty have this structure precisely. The topological characteristics of the edge-sharing water prisms are studied and the formulas for estimating the residual entropy are obtained. For these clusters (3-prisms), the usefulness of using a discrete model of intermolecular interaction [strong-weak-effective-bond model], previously developed for polyhedral water clusters, is shown. Calculations of the binding energy were performed using the non-additive Amoeba potential. To understand the relative stability of 3-prisms among other cluster forms, we use the structures recently reported in a published database containing over 3 × 106 unique water cluster networks (H2O)N of size N = 3-25 [Rakshit et al., J. Chem. Phys., 2019, 151(21), 214307, DOI: 10.1063/1.5128378].

Ferroelectricity of ice nanotube forests grown in three-dimensional graphene: the electric field effect.

Generating diverse ferroelectric ice nanotubes (NTs) efficiently has always been challenging, but matters in nanomaterial synthesis and processing technology. In the present work, we propose a method of growing ice NT forests in a single cooling process. A three-dimensional (3D) graphene structure was selected to behave as a representative container in which a batch of (5, 0) ice NTs was formed simultaneously under the cooling process from molecular dynamics simulation. Other similar 3D graphene structures but with different hole configurations, like uniform triangle or both triangle and pentagon, were also tested, revealing that ice NTs with different tube indices, i.e. both (3, 0) and (5, 0), could also be formed at the same time. Intriguingly, the orientations of the dipole moments of the water molecules of an ice NT formed were independent of each other, making the net ferroelectricity of the whole system weakened or even cancelled. An electric field could help change the orientation of the water molecules of the already obtained ice NTs and even twist the tube to be a spiral (5, 1) one if it was applied during the cooling process, such that the net ferroelectricity was greatly improved. The underlying physical mechanism of all phase transition phenomena, including the improvement of the ferroelectricity under an electric field, were explored in depth from the phase transition curves and structural point of view. The obtained results are of significant application value for improving the preparation efficiency of nano-ferroelectric materials, which are prosperous in nano-devices.

Possible formation of H2 hydrates in different nanotubes and surfaces using molecular dynamics simulation.

In this work, we simulated water molecules confined in carbon, boron nitride (BN), and silicon carbide (SiC) nanotubes with similar sizes. We also simulated water molecules confined between parallel graphene, BN, and SiC surfaces in two cases: (a) a similar geometric surface density of water of 0.177/Å2, in which the number of gas molecules was 18% of the total water molecules, and (b) a similar density profile of water of 0.04-0.05 dalton per Å3. To examine H2 hydrate formation, we added guest H2 molecules to the confined water molecules in the nanotube and surface systems. We analyzed the formed shapes, adsorption energies, radial distribution functions (RDFs), and self-diffusion coefficients of the confined molecules in gas hydrate formation. Our results showed that a more ordered heptagonal ice nanotube was formed in the BN nanotube than that in the other systems. After the addition of H2 molecules in the different nanotubes, some of the H2 molecules occupied the wall of the ice nanotube and some of them positioned in the hollow space. Although gas hydrates were created in all surface systems, ordered gas hydrate shapes were formed only in the graphene system. The adsorption energy for guest H2 molecules between the different surfaces was negative, which means that the formation of H2 hydrates between these surfaces is a spontaneous process (unlike that in the nanotube systems). According to RDF results, the BN nanotube and graphene surfaces are proper systems to form more ordered H2 hydrate structures. The confined water molecules have much higher diffusion coefficients in the BN nanotube and graphene surfaces than in the other systems. The F 4 parameter also substantiated hydrate formation in the different nanostructures. In a new configuration of BN and SiC systems with density profiles similar to that of the graphene system, the H2 hydrate was not formed completely as in the case of the graphene system. H2 hydrates formed in the new BN and SiC surfaces were less than those formed in the primary structures (with a geometrical density similar to that of the graphene system) and the graphene system.

Cubic water clusters as building blocks for self-assembly

The processes of self-organization in a system of cubic water clusters of D2d and S4 symmetry are studied. In the course of modeling, using the AMOEBA force field, a tendency to the formation of ice nanotubes and spatial nanostructures formed by tubular fragments is found. It was established that the quadrupole moments of the cubic isomers D2d and S4 are linear. However, the quadrupole–quadrupole interaction becomes decisive only at a great distance. It is very small in magnitude and therefore of little use. The structure of ice nanotubes obtained by combining D2d and S4 isomers has been studied. It is shown that these nanotubes significantly exceed all other configurations of square ice nanotubes in the stabilization energy. An atlas of the most probable structures with the number of cubes from 2 to 4 is presented. The methods of joining nanotubes and some peculiarities of their growth are discussed.

Structure and stability of nitrogen hydrate in a single-walled carbon nanotube under external electric fields

The effects of an external electric field on the structure and stability of the nitrogen hydrate confined in a single-walled carbon nanotube (CNT) were studied by using molecular dynamics (MD) simulations. It was found that the structure of the nitrogen hydrate, the occupancy and distribution of the nitrogen molecules inside the nanotube depend sensitively on the direction of the external electric field. A parallel electric field can destabilize the nitrogen hydrate and cause the release of nitrogen molecules from the ice nanotube of the hydrate. While a vertical electric field can redistribute the nitrogen molecules from the core to the shell of the hydrate. The occupancy of the nitrogen molecules of the hydrate follows a sigmoid-like function as the direction of the electric field changes. Our findings may aid in the development of methods to control gas release and encapsulation by using electric fields.

Evidence of Formation of 1–10 nm Diameter Ice Nanotubes in Double-Walled Carbon Nanotube Capillaries

Water exhibits rich phase behaviors in nanoscale confinement. Since the simulation evidence of the formation of single-walled ice nanotubes (INTs) in single-walled carbon nanotubes was confirmed experimentally, INTs have been recognized as a form of low-dimensional hydrogen-bonding network. However, the single-walled INTs reported in the literature all possess subnanometer diameters (<1 nm). Herein, based on systematic and large-scale molecular dynamics simulations, we demonstrate the spontaneous freezing transition of liquid water to single-walled INTs with diameters reaching ∼10 nm when confined to capillaries of double-walled carbon nanotubes (DW-CNTs). Three distinct classes of INTs are observed, namely, INTs with flat square walls (INTs-FSW), INTs with puckered rhombic walls (INTs-PRW), and INTs with bilayer hexagonal walls (INTs-BHW). Surprisingly, when water is confined in DW-CNT (3, 3)@(13, 13), an INT-FSW freezing temperature of 380 K can be reached, which is even higher than the boiling temperature of bulk water at atmospheric pressure. The freezing temperatures of INTs-FSW decrease as their caliber increases, approaching to the freezing temperature of two-dimensional flat square ice at the large-diameter limit. In contrast, the freezing temperature of INTs-PRW is insensitive to their diameter. Ab initio molecular dynamics simulations are performed to examine the stability of the INT-FSW and INT-PRW. The highly stable INTs with diameters beyond subnanometer scale can be exploited for potential applications in nanofluidic technologies and for mass transport as bioinspired nanochannels.

A hybrid topological and shape-matching approach for structure analysis.

Properties of crystalline and amorphous materials are characterized by the underlying long-range and local crystalline order. Deformations and defects are structural hallmarks of plasticity, ice formation, and crystal growth mechanisms. Partitioning topological networks into constituent crystal building blocks, which is the basis of topological identification criteria, is an intuitive approach for classification in both bulk and confinement. However, techniques reliant on the convex hull for assigning orientations of component units fail for non-convex blocks. Here, we propose a new framework, called Topological Unit Matching (TUM), which exploits information from topological criteria for an efficient shape-matching procedure. TUM is a general family of algorithms, capable of quantifying deformations and unambiguously determining grains of bulk and confined ice polymorphs. We show that TUM significantly improves the identification of quasi-one-dimensional ice by including deformed prism blocks. We demonstrate the efficacy of TUM by analyzing supercooled water nanoparticles, amorphous ice, and phase transitions in an ice nanotube. We also illustrate the superiority of TUM in resolving topological defect structures with minimal parameterization.

Residual entropy of twisted and helical ice nanotubes

To study the properties of ice nanotubes, the exact statistics of proton disorder are of interest. In this paper, a new version of the transfer-matrix method is applied to compute the number of defect-free configurations in twisted and helical ice nanotubes. For the calculation of the transfer matrices themselves, this new version of the transfer-matrix method uses small conditional matrices of size 2 x 2 or 4 x 4. For different types of nanotubes, the number of proton configurations in the unit cells of different lengths and the asymptotic values of the residual entropy are presented. For wide tubes, the convergence of the residual entropy to the entropy of the well-known two-dimensional ice model is discussed.

Absorption of mechanical energy via formation of ice nanotubes in zeolites.

Molecular dynamics simulations are carried out for a heterogeneous system composed of bulk water and pure-silica zeolites of the AFI type. My simulations show, for the first time, the spontaneous crystallization of water in hydrophobic zeolite channels by compression, while the water outside remains liquid. The formation of ice nanotubes results in a molecular bumper behavior in the absence of chemical reactions, although the mechanism has been explained by the appearance of silanol defects. In contrast, the same zeolite-water system exhibits a weak shock-absorber behavior at higher temperatures. My study shows that the phase transitions of confined water dramatically change its intrusion/extrusion behavior and alter the energetic performance by varying the temperature alone. The results offer a new perspective for a better design of hydrophobic nanoporous materials utilized with water.

D-SEAMS: Deferred Structural Elucidation Analysis for Molecular Simulations.

Structural analyses are an integral part of computational research on nucleation and supercooled water, whose accuracy and efficiency can impact the validity and feasibility of such studies. The underlying molecular mechanisms of these often elusive and computationally expensive processes can be inferred from the evolution of ice-like structures, determined using appropriate structural analysis techniques. We present d-SEAMS, a free and open-source postprocessing engine for the analysis of molecular dynamics trajectories, which is specifically able to qualitatively classify ice structures in both strong-confinement and bulk systems. For the first time, recent algorithms for confined ice structure determination have been implemented, along with topological network criteria for bulk ice structure determination. We also propose and validate a new order parameter for identifying the building blocks of quasi-one-dimensional ice. Recognizing the need for customization in structural analysis, d-SEAMS has a unique code architecture built with nix and employing a YAML-Lua scripting pipeline. The software has been designed to be user-friendly and extensible. The engine outputs are compatible with popular graphics software suites, allowing for immediate visual insights into the systems studied. We demonstrate the features of d-SEAMS by using it to analyze nucleation in the bulk regime and for quasi-one- and quasi-two-dimensional systems. Structural time evolution and quantitative metrics are determined for heterogeneous ice nucleation on a silver-exposed β-AgI surface, homogeneous ice nucleation, flat monolayer square ice formation, and freezing of an ice nanotube.

Confined water-mediated high proton conduction in hydrophobic channel of a synthetic nanotube

Water confined within one-dimensional (1D) hydrophobic nanochannels has attracted significant interest due to its unusual structure and dynamic properties. As a representative system, water-filled carbon nanotubes (CNTs) are generally studied, but direct observation of the crystal structure and proton transport is difficult for CNTs due to their poor crystallinity and high electron conduction. Here, we report the direct observation of a unique water-cluster structure and high proton conduction realized in a metal-organic nanotube, [Pt(dach)(bpy)Br]4(SO4)4·32H2O (dach: (1R, 2R)-(–)-1,2-diaminocyclohexane; bpy: 4,4’-bipyridine). In the crystalline state, a hydrogen-bonded ice nanotube composed of water tetramers and octamers is found within the hydrophobic nanochannel. Single-crystal impedance measurements along the channel direction reveal a high proton conduction of 10−2 Scm−1. Moreover, fast proton diffusion and continuous liquid-to-solid transition are confirmed using solid-state 1H-NMR measurements. Our study provides valuable insight into the structural and dynamical properties of confined water within 1D hydrophobic nanochannels.

Structure and phase transition behaviors of water in carbon nanotube under the electric field and high pressure

The structure and phase transition behaviors of the confined water in a (3, 15) carbon nanotube (CNT) (diameter = 1.26 nm) under the different axial electric fields and the ultra-high pressure P = 1 and 10 GPa were studied by perform molecular dynamics simulations. Depending on the pressure and the strength of the applied fields, the confined water presents four ice structures. Under zero of fields, the hollow 〈7, 0〉 ice nanotube (ice-NT) is the unique ice phase to the water confined in (3, 15) CNT. Applying an electric field triggers a helical water chain forming on the axis of the ice-NT. The structural order of water competes with its degree of polarization in its first–order solid-liquid phase transition. Therefore, high-intensity fields are not conductive to the formation of the ice. However, continuous phase transition tends to occur in the case of ultra-high pressure.

On the mechanical stability and buckling analysis of carbon nanotubes filled with ice nanotubes in the aqueous environment: A molecular dynamics simulation approach

In this article, molecular dynamics (MD) simulations are utilized to investigate the buckling behavior of carbon nanotubes (CNTs) containing ice nanotubes in the vacuum and aqueous environment. The obtained results show that unlike the critical strain, the critical buckling load of CNT containing ice nanotube is higher than that of pure CNT in the vacuum. It is also indicated that the sensitivity of critical buckling load and the critical strain of CNT containing ice nanotube to the variation of length decreases when the nanostructure is subjected to the aqueous environment. Additionally, it is observed that the calculated critical buckling load and the critical strain of CNTs filled with ice nanotubes in the aqueous environment are respectively larger and smaller than those obtained in the vacuum. It is further observed that CNTs lose their symmetric buckling mode shape as they are filled with ice nanotubes in the vacuum. The results of these simulations can be used as a benchmark for further studies in designing novel potential applications such as proton electronic-based nanoelectromechanical systems (NEMS).

Analysis of Carbon Nanotube Arrays for Their Potential Use as Adhesives Under Harsh Conditions as in Space Technology

For their potential use as dry adhesives under harsh conditions as in space, we investigated the temperature dependence of the adhesion of carbon nanotube arrays by atomic force microscopy (AFM) from $$-\,20\,^\circ \hbox {C}$$ to $$+\,240\,^\circ \hbox {C}$$ . In order to mimic the specific conditions for space application as close as possible, we glued tiny meteoritic particles to AFM cantilevers as probes. The measurements revealed that the adhesion forces of about $$2.5\,\hbox {N/cm}^2$$ are practically constant in the investigated temperature range. Long-term measurements with 1000 attachment–detachment cycles demonstrated the long-term stability of carbon nanotube arrays. The overall properties did not change after exposing the sample to simulated space condition. Additionally, we measured the adhesion between ice and carbon nanotubes at $$-\,20\,^\circ \hbox {C}$$ and obtained similar results. For these measurements, we used a bundle of CNTs as probe and grow a closed ice layer as sample surface.

Formation of CO2 Hydrates within Single-Walled Carbon Nanotubes at Ambient Pressure: CO2 Capture and Selective Separation of a CO2/H2 Mixture in Water

Carbon dioxide (CO2) capture and separation are two currently accepted strategies to mitigate increasing CO2 emissions into the atmosphere due to the burning of fossil fuels. Here, we show the simulation results of hydrate-based CO2 capture and selective separation from the CO2/H2 mixture dissolved in water, both using single-walled carbon nanotubes (SW-CNTs). The spontaneous formation of quasi-one-dimensional (Q1D) polygonal CO2 hydrates under ambient pressure was observed within SW-CNTs immersed in CO2 aqueous solution. Moreover, highly selective adsorption of a CO2 over a H2 molecule is observed in the Q1D polygonal ice nanotube due to a much lower value of the potential mean force (PMF) difference for a CO2 molecule than for a H2 molecule enclosed in the corresponding hydrate. The simulation results indicate that the formation of Q1D hydrates can be an effective approach for CO2 capture or for the separation of CO2 from H2 in the mixture.

Freezing Temperatures, Ice Nanotubes Structures, and Proton Ordering of TIP4P/ICE Water inside Single Wall Carbon Nanotubes

A very recent experimental paper importantly and unexpectedly showed that water in carbon nanotubes is already in the solid ordered phase at the temperature where bulk water boils. The water models used so far in literature for molecular dynamics simulations in carbon nanotubes show freezing temperatures lower than the experiments. We present here results from molecular dynamics simulations of water inside single walled carbon nanotubes using an extremely realistic model for both liquid and icy water, the TIP4P/ICE. The water behavior inside nanotubes of different diameters has been studied upon cooling along the isobars at ambient pressure starting from temperatures where water is in a liquid state. We studied the liquid/solid transition, and we observed freezing temperatures higher than in bulk water and that depend on the diameter of the nanotube. The maximum freezing temperature found is 390 K, which is in remarkable agreement with the recent experimental measurements. We have also analyzed the ice structure called "ice nanotube" that water forms inside the single walled carbon nanotubes when it freezes. The ice forms observed are in agreement with previous results obtained with different water models. A novel finding, a partial proton ordering, is evidenced in our ice nanotubes at finite temperature.

Evidence of low-density and high-density liquid phases and isochore end point for water confined to carbon nanotube

Possible transition between two phases of supercooled liquid water, namely the low- and high-density liquid water, has been only predicted to occur below 230 K from molecular dynamics (MD) simulation. However, such a phase transition cannot be detected in the laboratory because of the so-called "no-man's land" under deeply supercooled condition, where only crystalline ices have been observed. Here, we show MD simulation evidence that, inside an isolated carbon nanotube (CNT) with a diameter of 1.25 nm, both low- and high-density liquid water states can be detected near ambient temperature and above ambient pressure. In the temperature-pressure phase diagram, the low- and high-density liquid water phases are separated by the hexagonal ice nanotube (hINT) phase, and the melting line terminates at the isochore end point near 292 K because of the retracting melting line from 292 to 278 K. Beyond the isochore end point (292 K), low- and high-density liquid becomes indistinguishable. When the pressure is increased from 10 to 600 MPa along the 280-K isotherm, we observe that water inside the 1.25-nm-diameter CNT can undergo low-density liquid to hINT to high-density liquid reentrant first-order transitions.