Narrow Carbon 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.

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.

Ab Initio Molecular Dynamics Simulation of Water Transport through Short Carbon Nanotubes.

Water transport through short single-walled (6, 6) carbon nanotubes (CNTs) was investigated with ab initio molecular dynamics (AIMD) simulation at different temperatures. The water molecules under extreme confinement present a one-dimensional jagged pattern owing to hydrogen bonding, with the near-perfect alignment of the dipole orientations. CNTs ending with dangling bonds can promote water dissociation near the entrance and the occurrence of dipole flipping along the water wire at high temperatures, accompanied by the formation of D defects and L defects in the hydrogen-bond network. In contrast, dissociation of water molecules rarely takes place if the dangling bonds at the ends of the CNTs are terminated with H atoms. Angular jumps of water molecules are commonplace inside the narrow CNTs, implying a low-energy barrier for hydrogen-bond exchange among water molecules in narrow CNTs. The simulation results demonstrate the high activity of dangling bonds at the ends of short CNTs, accompanying passivation processes and their profound impact on water structure and transport, which is important for diverse technological applications.

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.

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

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.

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 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.

Selecting Carbon Nanotubes with Diameters of Less than 1 nm by Laser Pulses: An Ab Initio Exploration.

Despite the thermal instability of carbon nanotubes (CNTs) with diameters of less than 1 nm, first-principles simulations indicate the possibility of selecting such narrow CNTs using laser pulses. The simulations suggested the possibility of selecting CNTs narrower than 1 nm under pulsed laser irradiation with a full width at half-maximum of 10 fs, a wavelength of 800 nm, and a maximum field intensity ranging from 4.5 to 5 V/Å when the polarization vector was set perpendicular to the CNT axis. This result was common to both zigzag and armchair CNTs, suggesting that the preferential survival of narrow CNTs is independent of the chirality. The mechanisms underlying the preferential survival of narrower CNTs are discussed from analogous simulations of graphene nanoribbons under various polarization directions of the laser field, and the possibility of selecting CNTs with subnanometer diameters is evaluated on the basis of the simulation results.

Reverse Translocation of Nucleotides through a Carbon Nanotube.

We report molecular dynamics (MD) simulations of the reverse translocation of single nucleotides through a narrow carbon nanotube (CNT), with a diameter of 1.36 nm, immersed in a 1 M KCl electrolyte solution under an applied electric field along the tube axis. We observe ion selectivity by the narrow CNT, which leads to a high flow of K+ ions, in contrast to a negligible and opposing current of Cl- ions. The K+ ions, driven by the electric field, force a negatively charged single nucleotide into the narrow CNT where it is trapped by the incoming K+ ions and water molecules, and the nucleotide is driven in the same direction as the K+ ions. This illustrates a novel mechanism of nucleotide reverse translocation that is controlled by ion selectivity. An increase in the CNT diameter to 2.71 nm or an increase in nucleotide chain length both lead to translocation in the normal direction of the applied field. The reverse translocation rate of single nucleotides is correlated to the ionic current of K+ ions in the narrow tube, unlike translocation in the normal direction in the wider tube.

Enhanced wettability of long narrow carbon nanotubes in a double-walled hetero-structure: unraveling the effects of a boron nitride nanotube as the exterior.

Studying the structure and dynamics of nano-confined water inside carbon nanotubes has consistently attracted the wide-spread interest of researchers. In the present work, molecular dynamics simulations indicated internal nonwetting behavior for the central region of the long and narrow single-wall carbon nanotube (5,5) (SWNT) and showed that continuous single-file water molecules are not formed through it. Unlike the SWNT, by adding boron nitride nanotubes (6,6) as an outer wall to the SWNT, a continuously long single-file water chain is formed through the double-walled carbon and boron nitride hetero-nanotube (DWHNT) and thorough internal wetting of the DWHNT is observed. The position and the number of water molecules, electrostatic potential heatmap of the nanotube's wall, free energy profile of nano-confined water, and number of hydrogen bonds between them confirmed the aforementioned results and complete internal wetting of the DWHNT. After using the boron nitride nanotube (6,6) as the outer wall, an homogeneous electrostatic potential distribution in the DWHNT and increase in the hydrophilic characteristics of the nano-channel wall are observed, bringing about gradual trapping of more water molecules through it. Finally, water molecules occupied the central region of the DWHNT and a thorough single-file water chain is formed inside the nano-channel. Water dipole orientation inside the DWHNT and their radial distribution function asserted the occurrence of the liquid-solid quasi-phase transition of single-file water molecules confined inside the long and narrow carbon nanotube (5,5) under ambient conditions.

Entropy deepens loading chemical potentials of small alcohols by narrow carbon nanotubes

Small alcohol confinement within narrow carbon nanotubes has been extensively and systematically studied via rigorous free-energy calculations. Employing molecular dynamics simulations, thermodynamic integration and thermodynamic cycling, the loading process of methanol and ethanol from aqueous solution into (6,6), (7,7) and (8,8) single-walled carbon nanotubes was computed and decomposed into its entropic and energetic terms. For all tubes and alcohols, loading is favoured from infinite dilution in water; for the same alcohol, wider tubes allow for the formation of a collective dipole which is cooperative in terms of electrostatics and reduce the rotational freedom of the loaded particles; narrow tubes only permit the formation of dipole-dipole dimers instead, with a (rotational) entropic gain that compensates for the loss of long-range dipole-dipole interactions. The latter renders deeper loading chemical potentials for narrower tubes when partitioning small alcohols from aqueous solution and it is a clear example of an entropy-energy compensation phenomenon.

Anomalous changes of intermolecular distance in aqueous electrolytes in narrow pores of carbon nanotubes

Structures of NaCl and LiCl aqueous solutions in pores of carbon nanotubes with diameter 1 and 2 nm were evaluated by using X-ray diffraction and molecular dynamics simulations. Water intermolecular distances in carbon nanotubes with 1 and 2 nm pore diameters were more elongated and shortened than bulk water, respectively. Those were results of weakened and strengthened hydrogen bonds. The structures of aqueous solutions in carbon nanotubes were considerably different from water despite tiny amounts of ions in the pores. The nearest neighbour distances in aqueous solutions were rarely changed from that in water system in 1 nm carbon nanotubes pore, while those were longer than water in 2 nm carbon nanotubes pore. On the other hand, the second nearest neighbour distances in aqueous solutions in 1 and 2 nm pore diameter carbon nanotubes were both decreased from those in water in carbon nanotubes. Significant hydration formation and cleavage of hydrogen bonds were thus observed in 2 nm carbon nanotubes pore, because the elongated nearest neighbour and shortened second nearest neighbour distances were observed. Anomalous feature of the unchanged nearest neighbour and shortened second neighbour distances in 1 nm carbon nanotubes pore is the result that water intermolecular distance was much longer than bulk and hydrogen bonds were thus severely separated. Ions might play dominant role to connect hydrogen bonds and of course form hydration shell. Those anomalous structure changes from water to aqueous solution were observed only in extremely narrow carbon nanotubes.