Open-ended Carbon Nanotubes Research Articles

Plasmon-Enhanced High-Order Harmonic Generation of Open-Ended Finite-Sized Carbon Nanotubes with Vacancy Defects

In this study, the plasmon-enhanced high-order harmonic generation (HHG) of H-terminated finite-sized armchair single-walled carbon nanotubes (SWCNTs) near Ag nanoparticles is investigated systematically. Multiscale methods that combine the real-time time-dependent Hartree–Fock (TDHF) approach at the semi-empirical intermediate neglected differential overlap (INDOS) Hamiltonian level for molecular electronic dynamics with the finite-difference time-domain (FDTD) and solving Maxwell’s equations are used. It is found that for intact CNTs, HHG is significantly enhanced due to plasmon resonance. However, the nonlinear optical properties are saturated when the tube length increases enough in the inhomogeneous near-field. For long CNTs, the large gradient of a near-field is unfavorable for the nonlinear excitation of electrons. But defects can further change the properties of the spectra. The HHG of hybrid systems can be enhanced very clearly by introducing vacancy defects in CNTs. This enhancement is affected by the energy and intensity of the incident light, the near-field gradient, and the number and location of defects.

Creating covalent bonds between Cu and C at the interface of metal/open-ended carbon nanotubes.

The unique electrical properties of carbon nanotubes (CNTs) are highly desired in many technological applications. Unfortunately, in practice, the electrical conductivity of most CNTs and their assemblies has fallen short of expectations. One reason for this poor performance is that electrical resistance develops at the interface between carbon nanomaterials and metal surfaces when traditional metal-metal type contacts are employed. Here, a method for overcoming this resistance using covalent bond formation between open-ended CNTs and Cu surfaces is investigated experimentally and supported by theoretical calculations. The open-ended CNTs are vertically oriented compared to the substrate and have carboxylic functional groups that react with aminophenyl groups (linkers) grafted on metal surfaces. The covalent bond formation, crosslinking carboxylic and amine, via amide bond formation occurs at 120 °C. The covalent bonding nature of the aminophenyl linker is demonstrated theoretically using (100), (110), and (111) Cu surfaces, and bridge-like bond formation between carbon and two adjacent Cu atoms is revealed. The electrical conductivity calculated for a single intramolecular-type junction supports covalent bond formation between Cu and CNTs. Experimentally, the robustness of the covalent bonding between vertically oriented CNTs is tested by exposing CNTs on Cu to sonication, which reveals that CNTs remain fixed to the Cu supports. Since bonding CNTs to metals was performed at low temperatures, the reported method of covalent bond formation is expected to facilitate the application of CNTs in multiple fields, including electronics.

Clar Covers and Zhang-Zhang Polynomials of Zigzag and Armchair Carbon Nanotubes

We derive here compact formulas for the Zhang-Zhang (ZZ) polynomials of two classes of finite open-ended carbon nanotubes: zigzag nanotubes (n, 0) of length d and armchair nanotubes (n, n) of length d. For zigzag nanotubes, the underlying Clar cover theory is trivial; in contrast, for armchair nanotubes, the Clar theory is complex and abundant in results. The ZZ polynomial formulas have been obtained using the interface theory of benzenoids and the transfer matrix methodology.

Carbon Nanotubes from Synthesis to Picomolar Detection Electrochemical Sensors

Electrochemistry at open ends and sidewalls of carbon nanotubes (CNTs) has been under debate, with opposing viewpoints as to which sites are more electrochemically active. A particular challenge in this field has been the ability to conduct electrochemical studies selectively at the open-ends of CNTs, without measuring contributions from the sidewalls. This talk will discuss the synthesis and assembly of CNTs into electrochemical sensor where open-ended CNTs were employed for electrochemical measurements. The assembly employs drawable CNTs that minimize sample handling and contamination, in the attempt to preserve the pristine nature of CNTs. Highly densified multiwalled carbon nanotube (HD-CNT) fibers were embedded within a polymer matrix protecting the sidewalls and limiting the reactions to the tips of the CNTs. Cyclic voltammetry was employed to examine the electrochemical properties of open-ended CNTs using a conventional bulk electrochemical cell and scanning electrochemical cell microscopy (SECCM). This assemblies have shown to detect extremely low concentrations of Pb2+ in water, neurotransmitters, NADH, furosemide (diuretic drug), and the electrodes were tested with multiple electrochemical techniques.

Chemical Bond Formation between Vertically Aligned Carbon Nanotubes and Metal Substrates at Low Temperatures

The exceptional physical properties of carbon nanotubes (CNTs) have the potential to transform materials science and various industrial applications. However, to exploit their unique properties in carbon-based electronics, CNTs regularly need to be chemically interfaced with metals. Although CNTs can be directly synthesized on metal substrates, this process typically requires temperatures above 350 °C, which is not compatible for many applications. Additionally, the CNTs employed here were highly densified, making them suitable as interconnecting materials for electronic applications. This paper reports a method for the chemical bonding of vertically aligned CNTs onto metal substrates that avoids the need for high temperatures and can be performed at temperatures as low as 80 °C. Open-ended CNTs were directly bonded onto Cu and Pt substrates that had been functionalized using diazonium radical reactive species, thus allowing bond formation with the open-ended CNTs. Careful control during grafting of the organic species onto the metal substrates resulted in functional group uniformity, as demonstrated by FT-IR analysis. Scanning electron microscopy images confirmed the formation of direct connections between the vertically aligned CNTs and the metal substrates. Furthermore, electrochemical characterization and application as a sensor revealed the nature of the bonding between the CNTs and the metal substrates.

Metal-free in situ carbon-nanotube-modified mesoporous graphitic carbon nitride nanocomposite with enhanced visible light photocatalytic performance

We synthesized a metal-free photocatalyst comprising mesoporous carbon nitride (mpg-C3N4) and carbon nanotubes (CNT), via a facile method involving polymerization of melamine as the precursor in the presence of CNT in refluxing methanol. Under visible light, the CNT/mpg-C3N4 composite resulted in a greater than 8.6 × faster rhodamine B photodecomposition rate compared with bulk graphitic C3N4, and 1.6 × faster compared with pure mpg-C3N4. We attribute the promoted photocatalytic performance of the composite to the synergistic effects of open-ended CNT and mpg-C3N4. The highly interconnected CNT in mpg-C3N4 effectively enhances the transfer of photo-generated electrons, harvests more incident visible light, and provides specific channels for oxygen adsorption and photoreactions. We propose a possible reaction mechanism and report a scavenger experiment that found that superoxide radical (·O2−) is mainly responsible for dye photodegradation. The CNT/mpg-C3N4 retained good stability after several cycles. This work supplies a promising method of exploiting a stable, metal-free, efficient, and visible light-driven semiconductor to enhance solar energy use for environmental remediation.

Plasmon-enhanced high order harmonic generation of open-ended finite-sized carbon nanotubes: The effects of incident field's intensity and frequency and the interference between the incident and scattered fields.

The nonlinear optical properties of hybrid systems composed of a silver nanosphere and an open-ended finite-sized armchair single-walled carbon nanotube (SWCNT) are systematically investigated by the hybrid time-dependent Hartree-Fock (TDHF)/finite difference time domain (FDTD) approach, which combines the real-time TDHF approach for the molecular electronic dynamics with the classical computational electrodynamics approach, the FDTD, for solving Maxwell's equations. The high order harmonic generation (HHG) spectra of SWCNTs are studied as a function of the intensity (I0) and frequency (ω0) of the incident field, and SWCNTs length as well. It is found that the near field generated by a Ag nanoparticle has an overall enhancement to the molecular HHG in all the energy range, and it extends the HHG spectra to high energy. The inhomogeneity of the near field results in the appearance of even-order harmonics, and their corresponding spectral intensities are sensitive to ω0, therefore the near field's gradient. When ω0 is far away from the frequency of plasmon resonance of the silver nanosphere (ωc), the interference between the incident and scattering light beams extends the spectral range and makes the HHG spectra more sensitive to I0, while at ω0 = ωc, the impact of the interference on the spectra is negligible.

Ab-initio investigation of lithium adsorption on short carbon nanotubes considering effects of the tube length

Carbon nanotubes (CNTs) are a candidate electrode material for lithium-ion batteries (LIBs). Tailoring CNTs to shorter tubes with open ends can enhance the lithium capacity. Recently, the carbon nanobelt (CNB) has been successfully synthesized. This work concentrates on ab initio modelling of lithium adsorption on short open-ended CNTs with different lengths based on density functional theory (DFT). Li atoms adsorbed on CNB (one-segment (6,6) CNT) and short (6,6) CNTs with 2–12 segments as well as periodical CNTs were modelled. The most stable Li-CNT configurations were obtained after the total system energies were minimized. Lithium adsorption energies of short CNTs were found to have a regular oscillatory dependence on the number of segments along tube axis. Energetics for lithium moving across the tube wall and along the tube axis were also calculated for open-ended CNTs. Ab initio molecular dynamics (ABMD) was performed to simulate the lithium diffusion behaviors in short CNTs. Among all short CNTs, the CNT with 2 segments was the most promising electrode material for LIBs and its lithium capacity was finally predicted.

Investigation of the interaction between carbon nanotube tip and silicon sample through molecular dynamic simulation

Atomic Force Microscopy (AFM) has become a standard tool in experimental surface metrology, revealing atomic scale features of the surfaces. Carbon nanotube represents ideal structure for AFM tips because of their tiny diameter, high aspect ratio and high strength. This article employs molecular dynamics (MD) simulation approach to investigate the atomic-scale interaction between the AFM tips represented by carbon nanotube and sample. The interaction force between two different types of single walled carbon nanotube (SWCNT) tips including open ended carbon nanotube and capped carbon nanotube while the deformation mechanism during the measurement was discussed in detail. Firstly, the interaction force between SWCNT tips and sample in contact mode was simulated by using hybrid potential. The stiffness of the two different types of single walled carbon nanotubes were verified. The simulation results indicate that capped SWCNT tip is more prone to tilt than open ended SWCNT tip, but the ability to resist deformation is stronger. Then the dynamic model of tip-sample system in tapping mode was established. The relationship between the tip of load and the distance in the process of approaching to the sample was obtained by curve fitting. The numerical solution was achieved by solving the dynamic equation and frequency spectrum was obtained with the method of Fourier transforms.

The pervaporative membrane with vertically aligned carbon nanotube nanochannel for enhancing butanol recovery

Vertically aligned (VA) and open-ended carbon nanotube (CNT) arrays filled polydimethylsiloxane (PDMS) composite membrane was fabricated, characterized, and evaluated for pervaporative recovery of butanol from acetone-butanol-ethanol (ABE) fermentation broth. The VA CNT (open-ended)/PDMS membrane displayed various advantages over homogeneous PDMS membrane, since the VA CNT (open-ended)/PDMS membrane demonstrated a simultaneous improved butanol flux and separation factor, which was the result of higher butanol solubility and diffusivity of this membrane. Both butanol flux and separation factor showed a sharp enhancement with increasing feed temperature (30–80 °C), and with increasing feed butanol concentration (5–25 g/L). The maximum total flux and butanol separation factor of the VA CNT (open-ended)/PDMS membrane reached 1093.7 g/m2 h and 37.6 at 80 °C, respectively. The composite membrane showed a stable performance for long-time continuous operation with fermentation broth for biobutanol production. Overall, the VA CNT (open-ended)/PDMS membrane showed higher butanol flux and selectivity than homogeneous PDMS membrane, and has great potentials for butanol separation from ABE fermentation.

Diabatic model for electrochemical hydrogen evolution based on constrained DFT configuration interaction.

The accuracy of density functional theory (DFT) based kinetic models for electrocatalysis is diminished by spurious electron delocalization effects, which manifest as uncertainties in the predicted values of reaction and activation energies. In this work, we present a constrained DFT (CDFT) approach to alleviate overdelocalization effects in the Volmer-Heyrovsky mechanism of the hydrogen evolution reaction (HER). This method is applied a posteriori to configurations sampled along a reaction path to correct their relative stabilities. Concretely, the first step of this approach involves describing the reaction in terms of a set of diabatic states that are constructed by imposing suitable density constraints on the system. Refined reaction energy profiles are then recovered by performing a configuration interaction (CDFT-CI) calculation within the basis spanned by the diabatic states. After a careful validation of the proposed method, we examined HER catalysis on open-ended carbon nanotubes and discovered that CDFT-CI increased activation energies and decreased reaction energies relative to DFT predictions. We believe that a similar approach could also be adopted to treat overdelocalization effects in other electrocatalytic proton-coupled electron transfer reactions, e.g., in the oxygen reduction reaction.

Meniscus Motion and Void Generation Inside Carbon Nanotubes

The hollow inside of a carbon nanotube (CNT) has great potential not only for flow rate enhancement of nanocapillary, but also for a material container which can be applied for drug delivery and nanoparticle infusion. However, these applications focus on after liquid infusion into CNTs, whereas the understanding of the filling process is still limited. We conducted capillary filling experiments using individual open-ended CNTs, which were stuck into an ionic liquid and visualized by scanning transmission electron microscopy. The results showed that the meniscus stopped inside the CNT, which is not predicted by the Lucas–Washburn equation. To explain this discrepancy, the intermolecular force between the liquid and CNT inner wall was proposed to provide an additional friction force. In addition, voids were observed in the liquid inside the CNT. The generation mechanism of voids was proposed to be induced by the instability of the thin liquid layer along the CNT inner surface caused by the advance of the th...

Tuned Fabrication of the Aligned and Opened CNT Membrane with Exceptionally High Permeability and Selectivity for Bioalcohol Recovery.

Synthetic membranes usually suffer from a ubiquitous trade-off between permeability and selectivity. Carbon nanotube (CNT)-based hybrid materials have shown attractive properties in high-performance membrane preparation; however, the aggregation of random CNTs in polymer remains a great challenge. Herein, the aligned and open-ended CNT/(polydimethylsiloxane) PDMS membranes are controllably fabricated to form a hamburger-like structure that possesses nanochannels (∼10 nm) in the intermediate layer as well as angstrom cavities in the embedded PDMS. These aligned CNT membranes surpass the filling content limitation of the nonaligned CNT/PDMS membrane (37.4 wt % versus ∼10 wt %), leading to excellent mechanical properties and a multiplying performance increase of mass flux and selectivity for the separation of alcohols. The membranes break the permeability-selectivity trade-off with both parameters remarkably increasing (maximum 9 times) for bioalcohol separation. The established pervaporative-ultrafiltration mechanism indicates that the penetrant molecules preferentially pass through CNT internal nanochannels with increasing membrane permeability, thereby paving a way to nanoscale design of highly efficient channeled membranes for separation application.

Three-dimensional nitrogen-doped carbon nanotubes/carbon nanofragments complexes for efficient metal-free electrocatalyst towards oxygen reduction reaction

Heteroatom-doped carbon materials as one of the most promising oxygen reduction reaction (ORR) catalysts have attracted much attention. Rational design and exploration of suitable heteroatom-doped carbon materials greatly affects their ORR performance. Herein, we successfully prepared nitrogen-doped carbon nanotubes/carbon nanofragments (NCNT/CNF) complexes by a pyrolysis process using oxidized open-ended carbon nanotubes (OCNT)/oxidized carbon nanofragments (OCNF) hybrids as carbon precursors. The effect of carbon precursors on the synthesis of the corresponding nitrogen-doped carbon products was systematically investigated. The result showed the OCNT retained good conductivity, while the OCNF offered adequate structure defects for efficient post-doping. Benefiting from the co-merits of sole constitute, the obtained NCNT/CNF1-15 (1–15 refers to the mass ratio) complexes possessed a typical three-dimensional architecture and much increased specific surface area, which facilitated reactant/electrolyte infiltration and ion/electron transfer. More importantly, they built the most optimized balance on ORR catalytic sites and conductivity. Thus, the NCNT/CNF1-15 complexes showed much enhanced ORR performance. Clearly, our work provides a good guidance on the design of advanced heteroatom-doped carbon-based ORR catalysts.

Exploration of gas molecules transport in composite poly (4-methyl-2-pentyne) and carbon nanotubes employing molecular dynamics simulation

In order to design optimal membrane composites it is important to have an in-depth understanding of penetrants transport in such membranes. In that spirit, molecular dynamics was employed to study the transport of gas molecules in a composite membrane consisting of poly (4-methyl-2-pentyne) (PMP) and close-ended carbon nanotubes (PMPCNTC) or open-ended carbon nanotubes (PMPCNTO). The logarithm plot of mean-squared displacement (MSD) versus time for the transport of methane in PMPCNTC and PMPCNTO was drawn. It is observed that there are three regimes: the ballistic regime, the subdiffusive regime and the Fickian diffusive regime. The ballistic regime in PMPCNTC is shorter than it is in PMPCNTO, because the average cavity size of PMPCNTC is smaller than that of PMPCNTO and the penetrants have more space to move freely in PMPCNTO than they do in PMPCNTC before they hit any matrix units. Then the diffusivities of penetrants were achieved according to the plot of MSD versus time for the transport of penetrants in the Fickian diffusive regime. The solubility coefficients and the permeability of gas molecules in the composites were also calculated. In PMPCNTC, the 25 locations that contribute most to solubilities are outside the nanotubes while in PMPCNTO, they are inside the nanotubes. In PMPCNTO it is the nanotube that solvates the majority of the gas molecules. The huge channels inside the nanotube cause the high solubility of these penetrants. Finally, the selectivity of n-C4H10 over CH4 in PMPCNTC is observed to be higher than it is in both PMPCNTO and pure PMP. The research results may be employed in membrane design.

Quantum chemical molecular dynamics simulation of carbon nanotube–graphene fusion

Here we report a quantum mechanical molecular dynamics (QM/MD) study of a fusion process of an open-ended carbon nanotube on a graphene hole, which results in the formation of a so-called pillared graphene structure – a three-dimensional nanomaterial consisting entirely of sp2-carbons. The self-consistent-charge density-functional tight-binding potential was adopted in this study. Two different sizes of graphene holes with 12 or 24 central carbon atoms removed from a graphene flake, and a (6,6) carbon nanotube with a compatible diameter were adopted. Formations of 6–7–6/5–8–5 defect structures were found on the fusion border between tube and graphene hole. The 6–7–6 structure was found to bear less curvature-induced strain energy and therefore to be more stable and much easier to form than the 5–8–5 structure.

Nano-Photoelectrochemical Cell Arrays with Spatially Isolated Oxidation and Reduction Channels.

Photoelectrochemical conversion of solar energy is explored for many diverse applications but suffers from poor efficiencies due to limited solar absorption, inadequate charge carrier separation, redox half-reactions occurring in close proximity, and/or long ion diffusion lengths. We have taken a drastically different approach to the design of photoelectrochemical cells (PECs) to spatially isolate reaction sites at the nanoscale to different materials and flow channels, suppressing carrier recombination and back-reaction of intermediates while shortening ion diffusion paths and, importantly, avoiding mixed product generation. We developed massively parallel nano-PECs composed of an array of open-ended carbon nanotubes (CNTs) with photoanodic reactions occurring on the outer walls, uniformly coated with titanium dioxide (TiO2), and photocathodic reactions occurring on the inner walls, decorated with platinum (Pt). We verified the redox reaction isolation by demonstrating selective photodeposition of manganese oxide on the outside and silver on the inside of the TiO2/CNT/Pt nanotubes. Further, the nano-PECs exhibit improved solar absorption and efficient charge transfer of photogenerated carriers to their respective redox sites, leading to a 1.8% photon-to-current conversion efficiency (a current density of 4.2 mA/cm2) under white-light irradiation. The design principles demonstrated can be readily adapted to myriads of photocatalysts for cost-effective solar utilization.

Chemical modification of pristine carbon nanotubes and their exploitation as the carbon hosts for lithium-sulfur batteries

In this work, we prepared nitrogen-doped open-ended carbon nanotubes (N-O-E-CNTs) through controllable chemical cutting followed by a facile hydrothermal process. Scanning electron microscopy, X-ray diffraction, Raman and some other techniques were used to characterize the as-prepared N-O-E-CNTs, which possessed enhanced specific surface area and pore volume, and excellent conductivity. The N-O-E-CNTs were investigated as carbon host for Li–S batteries by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance, and the electrochemical testing showed that the N-O-E-CNTs/S cathode presented much better cyclability and rate capability than the carbon nanotubes (CNTs)/S and open-ended carbon nanotubes (O-E-CNTs)/S cathodes. The main reasons for the improved performance were explained as follows. On one hand, the controllable chemical cutting for CNTs can effectively increase the specific surface area and pore volume while retaining good conductivity, and the formed open-ended structure is beneficial to trapping sulfur species in the internal nanotube cavity. On the other hand, the N doping for the N-O-E-CNTs can not only recover the conductivity, but also enable the N-O-E-CNTs to possess more superior compatibility and adsorbability for sulfur species, resulting in high utilization of active material and effective alleviation of polysulfides shuttling.

WIMP detection and slow ion dynamics in carbon nanotube arrays.

Large arrays of aligned carbon nanotubes (CNTs), open at one end, could be used as target material for the directional detection of weakly interacting dark matter particles (WIMPs). As a result of a WIMP elastic scattering on a CNT, a carbon ion might be injected in the body of the array and propagate through multiple collisions within the lattice. The ion may eventually emerge from the surface with open end CNTs, provided that its longitudinal momentum is large enough to compensate energy losses and its transverse momentum approaches the channeling conditions in a single CNT. Therefore, the angle formed between the WIMP wind apparent orientation and the direction of parallel carbon nanotube axes must be properly chosen. We focus on very low ion recoil kinetic energies, related to low mass WIMPs (approx 11 GeV) where most of the existing experiments have low sensitivity. Relying on some exact results on two-dimensional lattices of circular obstacles, we study the low energy ion motion in the transverse plane with respect to CNT directions. New constraints are obtained on how to devise the CNT arrays to maximize the target channeling efficiency.

Highly selective electrodeposition of sub-10 nm crystalline noble metallic nanorods inside vertically aligned multiwall carbon nanotubes

In this paper crystalline noble metallic nanorods including Au and Ag with sub-10 nm diameter, are encapsulated within prealigned and open-ended multiwall carbon nanotubes (MWCNTs) through an electrodeposition method. As the external surface of CNTs has been insulated by the epoxy the CNT channel becomes the only path for the mass transport as well as the nanoreactor for the metal deposition. Highly crystallized Au and Ag2O nanorods parallel to the radial direction of CNTs are confirmed by high-resolution transmission electron microscopy, energy dispersive x-ray spectroscopy and x-ray powder diffraction spectroscopy. The Ag2O nanorods are formed by air oxidation on the Ag metals and show a single crystalline structure with (111) planes. The Au nanorods exhibit a complex crystalline structure including twin-crystal and lattice dislocation with (111) and (200) planes. These crystalline noble metallic nanostructures may have important applications for nanocatalysts for fuel cells as well as nanoelectronic and nanophotonic devices. This method is deemed to benefit the precise deposition of other crystalline nanostructures inside CNTs with a small diameter.