Narrow Nanotubes Research Articles

Ionic Coulomb blockade controls the current in a short narrow carbon nanotube.

We use all-atom molecular dynamics simulations to investigate ionic conduction in a short, charged, single-wall carbon nanotube. They reveal ionic Coulomb blockade (ICB) oscillations in the current as a function of the fixed charge on the wall, and an associated occupancy staircase. Current peaks related to fluctuations around the 2 → 1 and 1 → 0 steps in occupancy are clearly resolved, in agreement with ICB theory. Current peaks were also observed at constant occupancy. These unpredicted secondary peaks are attributed to edge effects involving a remote knock-on mechanism; they are attenuated, or absent, for certain choices of model parameters. The key parameters of the system that underlie the current oscillations are estimated using ICB theory and the potential of the mean force. Future perspectives opened up by these observations are discussed.

Structural model of oxidatively unzipped narrow single-walled carbon nanotubes

In this study, we present a synthetic method for the preparation of oxo-graphene nanoribbons (oxo-GNRs) by oxidative cleavage of C–C bonds of enriched (6,5)-single-walled carbon nanotubes (SWCNTs). We applied a mild oxidation protocol to oxidize SWCNTs involving sulfuric acid as dispersion medium and potassium permanganate as oxidant. The obtained oxo-GNRs exhibit stunning fluorescence properties with emission maxima of about 300 nm and 400 nm, respectively, depending on oxidation time. The analyses reveal that functional oxo-groups are predominantly located at the rims of oxo-GNRs. Next to lactones, 1,3-ketones are identified, which can be further reacted with hydrazine, most likely forming pyrazoline groups. Based on our data, we propose a chemical structure model for the oxo-GNRs. The model is based on an intact inner π-system, responsible for photoluminescence properties and oxo-functionalized rims of GNRs.

Ion Coupling, Bonding, and Transfer in Narrow Carbon Nanotubes.

Narrow carbon nanotubes (nCNT) are unique mimics of biological channels with water-ion selectivity attractive for applications such as water purification and osmotic energy harvesting, yet their understanding is still incomplete. Here, an ab initio computation is employed to develop the full picture of ion transfer in nCNT including specificity and coupling between ions. The thermodynamic costs of ion transfer are computed for single ions and ion pairs and used to evaluate different local coupling scenarios including strong (pairing) and weak (free-ion) coupling as well as "electroneutrality breakdown" (EB), possible for cations only due to their chemisorption-like interaction with nCNT. The results also indicate that nCNT behaves as a highly polarizable metal-like shell, which eliminates the dielectric energy when CNT accommodates coupled cation and anion. This allows facile computation and comparison of the full transfer costs, including translation entropy, for different ions in different coupling modes to identify the dominant regime. EB transfer appears most favorable for K+, while anions strongly favor transfer as pairs, except for chloride which favors weak coupling and, at neutral pH, transfers as a trace ion coupled to both cation and OH-. The results demonstrate that, in general, observed ion permeation and conduction in nCNT, especially for anions, reflect a complex ion-specific and composition-dependent interplay between different ions.

Enhanced fluidity of water in superhydrophobic nanotubes: estimating viscosity using jump-corrected confined Stokes-Einstein approach.

Accurately predicting the viscosity of water confined within nanotubes is vital for various technological applications. Traditional methods have failed in this regard, necessitating a novel approach. We introduced the jump-corrected confined Stokes-Einstein (JCSE) method and now employ the same to estimate the viscosity and diffusion in superhydrophobic nanotubes. Our study covers a temperature range of 230-300 K and considers three nanotube diameters. Results show that water inside superhydrophobic nanotubes exhibits a significantly lower viscosity and higher diffusion than those inside hydrophobic nanotubes. Narrower nanotubes and lower temperatures accentuate these effects. Furthermore, water inside superhydrophobic nanotubes display a lower viscosity than bulk water, with the difference increasing at lower temperatures. This reduction is attributed to weaker water-water interactions caused by a lower water density in the interfacial region. These findings highlight the importance of interfacial water density and its influence on nanotube viscosity, shedding light on nanoscale fluid dynamics and opening avenues for diverse applications.

Superfluidity Meets the Solid State: Frictionless Mass Transport through a (5,5) Carbon Nanotube.

Superfluidity is a well-characterized quantum phenomenon which entails frictionless motion of mesoscopic particles through a superfluid, such as ^{4}He or dilute atomic gases at very low temperatures. As shown by Landau, the incompatibility between energy and momentum conservation, which ultimately stems from the spectrum of the elementary excitations of the superfluid, forbids quantum scattering between the superfluid and the moving mesoscopic particle, below a critical speed threshold. Here, we predict that frictionless motion can also occur in the absence of a standard superfluid, i.e., when a He atom travels through a narrow (5,5) carbon nanotube (CNT). Because of the quasilinear dispersion of the plasmon and phonon modes that could interact with He, the (5,5) CNT embodies a solid-state analog of the superfluid, thereby enabling straightforward transfer of Landau's criterion of superfluidity. As a result, Landau's equations acquire broader generality and may be applicable to other nanoscale friction phenomena, whose description has been so far purely classical.

Observation and Isochoric Thermodynamic Analysis of Partially Water-Filled 1.32 and 1.45 nm Diameter Carbon Nanotubes

Recent measurements of fluids under extreme confinement, including water within narrow carbon nanotubes, exhibit marked deviations from continuum theoretical descriptions. In this work, we generate precise carbon nanotube replicates that are filled with water, closed from external mass transfer, and studied over a wide temperature range by Raman spectroscopy. We study segments that are empty, partially filled, and completely filled with condensed water from -80 to 120 °C. Partially filled, nanodroplet states contain submicron vapor-like and liquid-like domains and are analyzed using a Clausius-Clapeyron-type model, yielding heats of condensation of water inside closed 1.32 nm diameter carbon nanotubes (3.32 ± 0.10 kJ/mol and 3.72 ± 0.11 kJ/mol) and 1.45 nm diameter carbon nanotubes (3.50 ± 0.07 kJ/mol) that are lower than the bulk enthalpy of vaporization and closer to the bulk enthalpy of fusion. Favored partial filling fractions are calculated, highlighting the effect of subnanometer changes in confining diameter on fluid properties and suggesting the promise of molecular engineering of nanoconfined liquid/vapor interfaces for water treatment or membrane distillation.

From normal to anomalous diffusion of water molecules in narrow carbon nanotubes with defects, gases, and salts

In this letter, we study the diffusion of water molecules inside narrow carbon nanotubes in the presence of nanotube defects, gas molecules and salt ions by using molecular dynamics simulations. Mean square displacement (MSD) with a power-law fitting parameter , and the probability distribution function (PDF) with an distribution parameter a, of water molecules are calculated. It is found that within the diffusion time scale, water molecules exhibit a superdiffusion behavior with inside a perfect nanotube and the superdiffusion is weakened in the presence of gas molecules and salt ions. Meanwhile, a normal diffusion behavior with is observed inside a defective nanotube, and it is replaced by a subdiffusion behavior with when gas molecules and salt ions are involved. PDF of water molecules is found to be Gaussian-like with subject to superdiffusion and normal diffusion, while it becomes a center-peaked and long-tailed Lévy distribution with a < 2 subject to subdiffusion. Furthermore, it is found that the diffusion behavior of water molecules approaches normal diffusion as the radius size of the nanotube increases. Our result implies anomalous transport behavior of water in nanochannels due to the common presence of defects, gases and salts in nanochannels.

Electrical image potential and solvation energies for an ion in a pore in a metallic electrode or in a nanotube.

Electrical image potentials can be important in small spaces, such as nanoscale pores in porous electrodes, which are used in capacitive desalination and in supercapacitors, as argued by Bazant's group at Massachusetts Institute of Technology. It will be shown here that inside pores in porous metallic materials the image potentials can be considerably larger than near flat walls, as a result of the fact that the dielectric constant for an electric field perpendicular to a wall is much smaller than the bulk dielectric constant of water. Calculations will be presented for the image potential in spherical and cylindrically shaped pores. The calculations for cylindrical pores can also be applied to nanotubes. It was believed for a long time, on the basis of molecular dynamics simulations, that in order to push a salt solution through a small radius nanotube, work must be done against the solvation energy of the ions, which is larger inside a narrow nanotube than it is in the bulk. The relatively large image charge potential energy in narrow nanotubes, however, tends to oppose this increase in the solvation energy. The degree to which the image potential facilitates the flow of the salt ions into nanotubes will be discussed.

Ion Transport, Selectivity and Charge Formation Mechanisms in Carbon Nanotube Porins: An Ab Initio Study

Narrow carbon nanotubes (CNTs) desalinate water, mimicking water channels of biological membranes, yet the physics behind selectivity, especially, relative roles of water and ion interactions within CNT and with surrounding matrix, is still unclear. Here we report ab initio (DFT) inverstigation of the energies involved in transfer of water and ions from bulk solution into bare and water-filled (5,5) nanotubes of diameter 0.68 nm and (6,6) nanotubes of diameter 0.8 nm, more common in experimental studies.In the first part, we will clarify the effect of the medium surrounding CNT, defined by its dielectric constant ε. Since nanotubes need to be embedded in an insulating matrix, ε may have a direct impact on water and ion transfer, yet its effect has not been previously investigated. The transfer energies computed ab initio in the full range 1 < ε < ∞ permit a transparent breakdown to three main contributions: binding to CNT, intra-CNT hydration, and dielectric energy. The results for (5,5) and (6,6) tubes indicate that dielectric energy is small in the case of water transfer. It is however very significant in the case of ions and scales linearly with 1/ε, reminiscent of the Born solvation model, with the slope in the range 75-130 kJ/mol for all analyzed ions and CNTs. This suggests that the dielectric energy is of the order of several tens kJ/mol for CNTs embedded in lipid bilayers (e ~2) and may easily turn ion transfer form preferential uptake to strong exclusion, as observed for K+. In contrast, Cl- appear to be strongly excluded for all ε, thus dielectric energy only enhances the exclusion. Curiously, opposite of conclusions previously drawn from classical MD simulations, ab initio simulations demonstrate that water and ion arrange in a single file in (5,5) tubes, yet it is strongly distorted in (6,6) tubes. Figure below displays most favorable arrangement of water (A) and ions (B,C) in (6,6) tubes. As a result, some species are solvated by more than two water molecules, which significantly affects their overall ion transfer energies, but not as mush its dielectric part.In the second on-going study, we carry out full thermodynamic analysis (including both enthalpy and entropy) of the effect of OH- and H3O+ ions, inherently present in water, on transfer of salt (KCl) and ionic conductance within CNTs. Its main purpose is to rationalize data on ion conductance and its dependence on KCl concentration and pH. Published experimental results indicate that (6,6) channels strongly exclude chloride in neutral conditions thereby conductance predominantly occurs as simultaneous uptake and migration of K+ and OH- ions. This suggests that the energy associated with OH- transfer is several tens kJ/mol lower than for Cl-, however, simulations show no such difference, and cannot explain the abnormally high contribution of OH- to CNT conductance. Based on simulations results, we also reject ion pairing (that may explain recent anion transfer data) and recently proposed electroneutrality breakdown in narrow channels as plausible explanations of the unusually high OH- contribution. However, simulations, currently on-going, point to possibility of some unorthodox transport regimes that might reasonably explain these and other experimental findings. Figure 1

Unusual properties of the electric double layer in an extremely narrow nanotube. A grand canonical Monte Carlo and classical DFT study

A grand canonical Monte Carlo (GCMC) simulation technique at a constant electrode–electrolyte potential drop and classical density functional theory (CDFT) have been employed to study the ion distribution and the differential capacitance of an electric double layer (EDL) in extremely narrow nanotubes. In the applied constant-potential GCMC method, to preserve the electroneutrality of the system after a single ion exchange, the electrode charge is suitably modified. The resulting charge fluctuations are used to calculate the differential capacitance of the EDL. Results for the ion distributions and differential capacitance in the nanotubes are reported for symmetric +1: 1 ionic valences with a common ionic diameter of 0.4 nm at electrolyte concentrations of 0.1 m, 1.0 m, and 2.5 m, a cylinder radius of 0.6 nm, potential drop varying from −0.6 V to 0.6 V, a relative permittivity of 78.5, and a temperature of 298.15 K. These results are compared with corresponding data for size-asymmetric systems with cations of diameters of 0.32 nm and 0.48 nm and ion-valence asymmetric systems of +2: 1 and +3: 1. At higher electrode surface charges, the counter-ion density distributions show an unusual accumulation along the axis of the nanotube. Differential capacitance plots have four or fewer, often three, maxima. The central minimum surrounded by two maxima undergoes a transition from a minimum to a maximum as the electrolyte concentration is increased. The positions of the side maxima indicate the electrode potential at which the rate of the unusual counter-ion accumulation is at its highest. Comparison between the results of simulations and CDFT suggests the latter to be reliable for investigation of the electrical capacitance properties of extreme nanoscale supercapacitors, even if its degree of accuracy for particle distributions decreases with increasing surface charge.

Universal Zigzag Edge Reconstruction of an α-Phase Puckered Monolayer and Its Resulting Robust Spatial Charge Separation.

Edges are important, because they dictate the stability and properties of nanoribbons. Here, we reveal a universal reconstruction of the ZZ edge into a (2 × 1) tubed [ZZ(Tube)] edge, enabling an ultimate narrow nanotube to terminate nanoribbons for α-puckered group-V elemental and compound monolayers (GeS/Se and SnS/Se). The reconstructed edge formations are confirmed by CALYPSO. The ZZ(Tube) edge forms easily, is highly stable, and is semiconducting. Remarkably, the ZZ(Tube) edge always exhibits a type-II band structure and robust spatial charge separation. For a compound monolayer monochalcogenide, mild (2 × 1) ZZ(S-R) occurs at the chalcogenide-terminated edge. TDDFT simulations indicate that charge separation occurs only at 672 fs, while the lifetime is over 5 ns, thus facilitating robust spatial charge accumulation. These remarkable features of ZZ(Tube) edge-terminated α-puckered nanoribbons are ideal for optoelectronic and photocatalytic applications.

Competition of quantum effects in H 2/D 2 sieving in narrow single-wall carbon nanotubes

Nanoporous materials have the potential to be used as molecular sieves to separate chemical substances in a mixture via selective adsorption and kinetic sieving. The separation of isotopologues is also possible via the so-called quantum sieving effect: the different effective size of isotopologues due to their different Zero Point Energy (ZPE). Here we compare the diffusion coefficients of Hydrogen and Deuterium in (8,0) Single Walled Carbon Nanotubes obtained with quantum dynamics simulations. The diffusion channels obtained present important contributions from resonances connecting the potential wells. These resonances, which are more important for H than for D , increase the low-temperature diffusivity of both isotopologues, but prevent the inverse kinetic isotope effect reported for similar nanostructured systems.

Protonation of Cytosine-Rich Telomeric DNA Fragments by Carboxylated Carbon Nanotubes: Insights from Computational Studies.

In this work, we studied, using computational methods, the protonation reactions of telomeric DNA fragments being due to interaction with carboxylated carbon nanotubes. The applied computational methodology is divided into two stages. (i) Using classical molecular dynamics, we generated states in which carboxyl groups are brought to the vicinity of nitrogen atoms within the cytosine rings belonging to the DNA duplex. (ii) From these states, we selected two systems for systematic quantum chemical studies aimed at the analysis of proton-transfer reactions between the carboxyl groups and nitrogen atoms within the cytosine rings. Results of molecular dynamics calculations led to the conclusion that sidewall-functionalized carbon nanotubes deliver carboxyl groups slightly more effectively than the on-tip-functionalized ones. The latter can provide carboxyl groups in various arrangements and more diverse quality of approach of carboxyl groups to the cytosines; however, the differences between various arrangements of carboxyl groups are still not big. It was generally observed that narrow nanotubes can access the cytosine pocket easier than wider ones. Quantum chemical calculations led however to the conclusion that a direct proton transfer from the carboxyl group to the nitrogen atom within the cytosine ring is impossible under normal conditions. Precisely, we detected either very high activation barrier for the proton-transfer reaction or instability of the reaction product, i.e., its spontaneous decomposition toward reaction substrates.

Selectivity of ion transport in narrow carbon nanotubes depends on the driving force due to drag or drive nature of their active hydration shells.

Molecular dynamics simulations have revealed the important roles of hydration shells of ions transported through ultrathin carbon nanotubes (CNTs). In particular, ions driven by electric fields tend to drag their hydration shells behind them, while for ions transported by pressure, their hydration shells can actively drive them. Given the different binding strengths of hydration shells to ions of different sizes, these active roles of hydration shells affect the relative entry rates and driving speeds of ions in CNTs.

Ion Solvation and Transport in Narrow Carbon Nanotubes: Effects of Polarizability, Cation−π Interaction, and Confinement

Understanding ion solvation and transport under confinement is critical for a wide range of emerging technologies, including water desalination and energy storage. While molecular dynamics (MD) simulations have been widely used to study the behavior of confined ions, considerable deviations between simulation results depending on the specific treatment of intermolecular interactions remain. In the following, we present a systematic investigation of the structure and dynamics of two representative solutions, that is, KCl and LiCl, confined in narrow carbon nanotubes (CNTs) with a diameter of 1.1 and 1.5 nm, using a combination of first-principles and classical MD simulations. Our simulations show that the inclusion of both polarization and cation-π interactions is essential for the description of ion solvation under confinement, particularly for large ions with weak hydration energies. Beyond the variation in ion solvation, we find that cation-π interactions can significantly influence the transport properties of ions in CNTs, particularly for KCl, where our simulations point to a strong correlation between ion dehydration and diffusion. Our study highlights the complex interplay between nanoconfinement and specific intermolecular interactions that strongly control the solvation and transport properties of ions.

Effect of nanotube diameter on the transport of water molecules in electric fields

Electric fields control the water dipole orientation and result in a rotation-translation coupling transport of water through narrow carbon nanotubes (CNTs); however, it is not clear if this phenomenon still holds when the confined water is close to bulk structures for increasing the CNT diameter. Herein, we confirm that this transport phenomenon still exists for larger CNTs with bulk-like water structures. Using molecular dynamics simulations, we investigate the transport of water molecules through CNTs with different diameters in electric fields. The water flow decreases as a whole with the increase of field strength, while the water flux changes oppositely with a two-stage behavior, leading to the increase of unidirectional transport efficiency ƞ. Similar to the experimental wetting phenomenon of CNTs, our simulated water occupancy displays a linear increase with the field strength. Moreover, we find a simple equation of τ = σ*N/flow that can accurately predict the water translocation time τ according to the water occupancy N and flow, where σ is a fixed coefficient for each CNT diameter and numerically close to the ambient temperature of 300 K. The water dipole angels can be highly regulated by the electric field, as the values change from nearly 900 with random orientation to below 300 that along the field direction, which can be further elucidated by the unique dipole distributions. The water dipole orientation exists a two-stage change that accounts for the two-stage water flux behavior. These results enrich our knowledge of the transport behavior of water molecules under external fields, and are helpful for the design of stable and high-efficient nanofluidic devices.

Electrochemistry of redox-active molecules confined within narrow carbon nanotubes

Confinement of molecules within nanocontainers can be a powerful tool for controlling the states of guest-molecules, tuning properties of host-nanocontainers and triggering the emergence of synergistic properties within the host-guest systems. Among nanocontainers, single-walled carbon nanotubes - atomically thin cylinders of carbon, with typical diameters below 2 nm and lengths reaching macroscopic dimensions - are ideal hosts for a variety of materials, including inorganic crystals, and organic, inorganic and organometallic molecules. The extremely high aspect ratio of carbon nanotubes is complemented by their functional properties, such as exceptionally high electrical conductivity and thermal, chemical and electrochemical stability, making carbon nanotubes ideal connectors between guest-molecules and macroscopic electrodes. The idea of harnessing nanotubes both as nanocontainers and nanoelectrodes has led to the incorporation of redox-active species entrapped within nanotube cavities where the host-nanotubes may serve as conduits of electrons to/from the guest-molecules, whilst restricting the molecular positions, orientations, and local environment around the redox centres. This review gives a contemporary overview of the status of molecular redox chemistry within ultra-narrow carbon nanotubes (nanotubes with diameters approaching molecular dimensions) highlighting the opportunities, pitfalls, and gaps in understanding of electrochemistry in confinement, including the role of nanotube diameter, size and shape of guest-molecules, type of electrolyte, solvent and other experimental conditions.

Water diffusion in carbon nanotubes: Interplay between confinement, surface deformation, and temperature.

In this article, we investigate, through molecular dynamics simulations, the diffusion behavior of the TIP4P/2005 water confined in pristine and deformed carbon nanotubes (armchair and zigzag). To analyze different diffusive mechanisms, the water temperature was varied as 210 ≤ T ≤ 380 K. The results of our simulations reveal that water presents a non-Arrhenius to Arrhenius diffusion crossover. The confinement shifts the diffusion transition to higher temperatures when compared with the bulk system. In addition, for narrower nanotubes, water diffuses in a single line, which leads to its mobility independent of the activation energy.

Water and Ion Transfer to Narrow Carbon Nanotubes: Roles of Exterior and Interior.

Narrow carbon nanotubes (CNTs) desalinate water, mimicking water channels of biological membranes, yet the physics behind selectivity, especially the effect of the membrane embedding CNTs on water and ion transfer, is still unclear. Here, we report ab initio analysis of the energies involved in transfer of water and K+ and Cl- ions from solution to empty and water-filled 0.68 nm CNTs for different dielectric constants (ϵ) of the surrounding matrix. The transfer energies computed for 1 ≤ ϵ < ∞ permit a transparent breakdown of the transfer energy to three main contributions: binding to CNT, intra-CNT hydration, and dielectric polarization of the matrix. The latter scales inversely with ϵ and is of the order 102/ϵ kJ/mol for both ions, which may change ion transfer from favorable to unfavorable, depending on ion, ϵ, and CNT diameter. This may have broad implications for designing and tuning selectivity of nanochannel-based devices.

Interaction between chloride ions mediated by carbon nanotubes: a chemical attraction

The interaction between two Cl− ions separated by the wall of a narrow carbon nanotube has been investigated by density functional theory (DFT) and by DFT-based tight binding (DFTB+). The direct Coulomb interaction between the ions is screened by the nanotube, no matter if the latter is conducting or semiconducting. The presence of the ions induces changes in the electronic density of states of the nanotube, which results in an effective attraction between the ions of the order of 0.2–0.3 eV. The interaction of the outside ions with the tube has a covalent component, when the two ions are near there is even a direct chemical attraction between the ions. In contrast to the effective attraction between two Li+ ions reported before (Juarez et al., Phys Chem Chem Phys 22:10,603, 2020), the effect cannot be explained in terms of physical concepts alone. DFTB+ performs well when compared with DFT, and lends itself to fast calculations for large systems.