Large-diameter Nanotubes Research Articles

Diameter-selective non-covalent functionalization of carbon nanotubes with porphyrin monomers.

We report on the spontaneous non-covalent functionalization of carbon nanotubes with hydrophobic porphyrin molecules in micellar aqueous solution. By monitoring the species concentrations with optical spectroscopies, we can follow the kinetics of the reaction and study its thermodynamical equilibrium as a function of the reagent concentrations. We show that the reaction is well accounted for by a cooperative Hill equation, reaching a molecular coverage close to a compact monolayer for a porphyrin concentration larger than a diameter-specific threshold concentration. The equilibrium constant is measured for 16 nanotube chiral species. The Gibbs energy of the reaction (of the order of -40 kJ mol(-1)) and its evolution with the nanotube diameter is consistent with theoretical calculations of the binding energy. This thermodynamical study shows a strong preferential binding of TPP molecules to larger diameter nanotubes. This original curvature selectivity can be used to induce diameter selective species enrichment.

Optimizing the degree of carbon nanotube dispersion in a solvent for producing reinforced epoxy matrices

The homogeneous incorporation of a small concentration of carbon nanotubes (CNTs) in epoxy matrices can enhance the mechanical properties of the composites. However, due to the high van der Waals interaction energy between CNTs, it is difficult to achieve a homogeneous dispersion of them in the epoxy matrix. Both processing conditions of the nanotubes, as well as the addition of surfactants have the potential to improve the dispersion. In this paper, the effects of sonication energy, CNT geometry and dispersant type on the dispersion degree of CNTs and consequent reinforcement of the epoxy matrix are investigated. Multi-walled CNTs with variable diameter and length were dispersed in ethanol using different sonication energies, with the aid of different dispersants. The results show that an increase in sonication energy initially leads to more CNT dispersion, although after a certain value of sonication energy, the level of dispersion remains constant. Moreover, excessive sonication energy was found to shorten the length of CNTs and thus decrease their aspect ratio. Due to the lower interaction energy of CNTs with larger (40–60nm) diameter, compared to those with smaller diameter (10–20nm), the larger diameter nanotubes showed a great ability to be dispersed, while CNT length showed no significant effect on the degree of dispersion. For greater CNT dispersion additives were incorporated. Copolymers which were able to disperse the CNTs primarily by steric repulsion were found to be more influential than those which dispersed the CNTs mainly by electrostatic repulsion, such as oleic acid. Furthermore, it was also found that copolymers yielding greater surface charge on the CNT were able to disperse CNTs more than those copolymers leading to a lower surface charge. It was found that, the incorporation of 0.1wt.% pristine CNT using optimized influential parameters resulted in high levels of homogeneous dispersion in the epoxy matrix, with a modest improvement in mechanical properties of epoxy. This study provides further insight into the dispersion of CNTs in composite matrix and the degree to which a controlled dispersion can allow the CNTs to reach their potential as reinforcing nanofillers.

Structure-Dependent Thermal Defunctionalization of Single-Walled Carbon Nanotubes

Covalent sidewall functionalization of single-walled carbon nanotubes (SWCNTs) is an important tool for tailoring their properties for research purposes and applications. In this study, SWCNT samples were first functionalized by reductive alkylation using metallic lithium and 1-iodododecane in liquid ammonia. Samples of the alkyl-functionalized SWCNTs were then pyrolyzed under an inert atmosphere at selected temperatures between 100 and 500 °C to remove the addends. The extent of defunctionalization was assessed using a combination of thermogravimetric analysis, Raman measurements of the RBM, D, and G bands, absorption spectroscopy of the first- and second-order van Hove peaks, and near-IR fluorescence spectroscopy of (n,m)-specific emission bands. These measurements all indicate a substantial dependence of defunctionalization rate on nanotube diameter, with larger diameter nanotubes showing more facile loss of addends. The effective activation energy for defunctionalization is estimated to be a factor of ~1.44 greater for 0.76 nm diameter nanotubes as compared to those with 1.24 nm diameter. The experimental findings also reveal the quantitative variation with functionalization density of the Raman D/G intensity ratio and the relative near-IR fluorescence intensity. Pyrolyzed samples show spectroscopic properties that are equivalent to those of SWCNTs prior to functionalization. The strong structure-dependence of defunctionalization rate suggests a new approach for diameter-sorting of mixed SWCNT samples on relatively large scales without ultrasonic agitation.

Structure-Dependent Thermal Defunctionalization of Single-Walled Carbon Nanotubes.

Covalent sidewall functionalization of single-walled carbon nanotubes (SWCNTs) is an important tool for tailoring their properties for research purposes and applications. In this study, SWCNT samples were first functionalized by reductive alkylation using metallic lithium and 1-iodododecane in liquid ammonia. Samples of the alkyl-functionalized SWCNTs were then pyrolyzed under an inert atmosphere at selected temperatures between 100 and 500 °C to remove the addends. The extent of defunctionalization was assessed using a combination of thermogravimetric analysis, Raman measurements of the D, G, and radial breathing bands, absorption spectroscopy of the first- and second-order van Hove peaks, and near-IR fluorescence spectroscopy of (n,m)-specific emission bands. These measurements all indicate a substantial dependence of defunctionalization rate on nanotube diameter, with larger diameter nanotubes showing more facile loss of addends. The effective activation energy for defunctionalization is estimated to be a factor of ∼1.44 greater for 0.76 nm diameter nanotubes as compared to those with 1.24 nm diameter. The experimental findings also reveal the quantitative variation with functionalization density of the Raman D/G intensity ratio and the relative near-IR fluorescence intensity. Pyrolyzed samples show spectroscopic properties that are equivalent to those of SWCNTs prior to functionalization. The strong structure dependence of the defunctionalization rate suggests an approach for scalable diameter sorting of mixed SWCNT samples.

The Dimension of Titania Nanotubes Influences Implant Success for Osteoclastogenesis and Osteogenesis Patients

Implants that can inhibit osteoclastogenesis and enhance osteogenesis are desirable for osteoporosis patients. In this study, titania nanotube (Ti-NT) materials, having nanotube diameters of 30, 80, and 120 nm, were produced separately by anodization at 10, 40, and 60 V, respectively. The introduction of Ti-NTs to titanium substrates significantly reduced the formation and activity of osteoclasts on samples. With the enlargement of the nanotube diameter, the osteoclasts number, tartrate-resistant acid phosphatase staining and activity, and related gene expressions of osteoclasts were further reduced. Osteogenic ability was enhanced by increasing the nanotube diameter. Thus, larger-diameter nanotube implants, such as NT60, were better able to inhibit bone absorption and enhance bone formation to prevent implant loss and failure, especially for osteoporosis patients.

Selective interactions of a high-molecular-weight polycarbazole with different commercial nanotube samples

The ability of a poly(2,7-carbazole) to disperse three samples of commercially available single-walled carbon nanotubes (raw HiPCO, purified HiPCO, and CoMoCAT nanotubes) has been investigated. UV–vis-NIR absorption spectroscopy, photoluminescence mapping, Raman spectroscopy, and atomic force microscopy were used to characterize the dispersions obtained. It was found that the polycarbazole preferentially interacted with semiconducting nanotubes and was efficient at dispersing smaller diameter (under 1.1 nm) carbon nanotubes. Larger diameter nanotubes could be dispersed to form stable suspensions in THF; however, it appeared that there were some small bundles present. It was found that annealing the purified HiPCO nanotubes at high temperature under an inert atmosphere greatly enhanced the ability of the polycarbazole to disperse the nanotubes and remove metallic species. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 2738–2747

Surfactant concentration dependent spectral effects of oxygen and depletion interactions in sodium dodecyl sulfate dispersions of carbon nanotubes.

Quenching of optical absorbance spectra for carbon nanotubes (CNTs) dispersed in sodium dodecyl sulfate (SDS) has been observed to be more pronounced at higher concentrations of the surfactant. The protonation-based quenching behavior displays wavelength dependence, affecting larger diameter nanotube species preferentially. Although absorbance may be recovered by hydroxide addition, pH measurements suggest that hydrolysis of SDS does not play a major role in the short term quenching behavior at high SDS concentrations. The degree of quenching is observed to correlate well with an increase in attractive depletion as SDS concentration is increased, while the extent of depletion is found to depend heavily on the concentration of preparation in comparison to the final SDS concentration. Attractive depletion in SDS is also found to be preferential for CNTs of larger diameter. It is proposed that depletion enhances the quenching effect due to close association of CNT-SDS complexes providing higher SDS densities on the CNT surface, leading to further oxidation. In addition, the quenching behavior in SDS is found to strongly suppress the optical and Raman signal from metallic nanotube species even at high pH. Displacement of SDS by sodium deoxycholate as a secondary surfactant is able to reverse the effects of protonation of metallic species, whereas hydroxide addition is only partially effective.

Effects of B–N co-doping into the ultra-small diameter zigzag single-walled carbon nanotubes: A density functional theory study

The boron(B)–nitrogen(N) co-doping ultra-small diameter (5,0) and (4,0) single-walled carbon nanotubes (SWCNTs) have been investigated by using the first-principles pseudopotential plane wave method. The results show that B–N pairs are easy to dope into ultra-small diameter zigzag nanotubes, which have numbers of differences with other large diameter nanotubes, and the B–N co-doping configurations are the energetically stable structures. Furthermore, by introducing B–N pairs, the (5,0) and (4,0) SWCNTs both transform from metal to semiconductor. These ultra-small diameter nanotubes have potential to be applied in semiconductor integrated circuit of extremely small size.

Doping Effect on Magnetism and Transport Property of Heterojunction between Carbon and Boron Nitride Nanotubes

First-principles study on the electronic structures and electron-transport properties of one-dimensional heterostructured carbon/boron nitride nanotubes is carried out. It is found that p-type atom substitutional doping, in which, e.g., one N atom is replaced with one C atom at the zigzag carbon nanotube edge, takes an electron from the energetically flat states below the Fermi level, which results in the spin-polarized ground-state electronic structure. On the other hand, n-type doping leads to the nonmagnetic ground-state electronic structure because of the strong σ–π hybridization in the smaller-diameter nanotube while the spin polarization is realized in the larger-diameter nanotube. More interestingly, in contrast to undoped nanotubes, where the edge states do not conduct electrons owing to the symmetry mismatch with the conducting carbon nanotube states, the fluctuation of the symmetry due to the presence of the doped atom can realize the spin-dependent resonant tunneling through the edge states. Our results indicate that the substituted atom plays important roles for the transport through the edge states as well as the spin-polarized electronic structure.

Reinforcement of CVD grown multi-walled carbon nanotubes by high temperature annealing

We report on the increase of the Young's modulus (E) of chemical vapor deposition (CVD) grown multi-walled carbon nanotubes (MWNTs) upon high temperature heat treatment. The post heat-treatment at 2200–2800ºC in a controlled atmosphere results in a considerable improvement of the microstructure, chemical stability and electro-physical properties of the nanotubes. The Young's modulus of MWNTs of different diameters was measured by the deflection of a single tube suspended across the hole of silicon nitride membrane and loaded by an atomic force microscope tip. Contrary to previous reports, a strong increase of E was feasible due to the improved growth conditions of pristine carbon nanotubes and to the improved heat treatment conditions. However, the elastic modulus of CVD grown MWNTs still shows strong diameter dependence resulting from the remaining structural inhomogeneities in large diameter nanotubes.

Bioactivity and generation of anodized nanotubular TiO2 layer of Ti–6Al–4V alloy in glycerol solution

This study examined the growth behavior of anodic TiO2 nanotubular layer in glycerol solutions and the bioactivity of a TiO2 nanotubular layer that was modified by precalcification treatment. A glycerol solution containing 1wt.% NH4F and 20wt.% deionized water was used as the electrolyte. Anodization was carried out at 20V at a current density of 20mA/cm2 for 1h. The anodized specimens were precalcified by soaking in a Na2HPO4 solution at 80°C for 30min followed by soaking in a saturated Ca(OH)2 solution at 100°C for 30min and heat-treatment at 500°C for 2h. To evaluate the bioactivity of the precalcified TiO2 nanotube layer, hydroxyapatite precipitation was examined in simulated body fluid at pH7.4 and 36.5°C for 10days. During the electrochemical oxidation process, a thin strip-like net structure was formed at the initial stage, which then developed into a well-arranged and self-organized nanotubular TiO2 layer. The nanotubular TiO2 layer had a dense structure due to the interpositioning of small diameter nanotubes between the larger diameter nanotubes, and the diameter of the nanotubes increased in the direction towards the substrate. The diameter of the TiO2 nanotubes increased with increasing applied potential and water content in the glycerol electrolyte solution. The diameters of the nanotubes grown on the samples treated in a glycerol solution containing 20wt.% H2O and 1wt.% NH4F at 20V were 122.9±15.2nm and 88.5±9.5nm, and the length of nanotubes was 567.0±27.2nm. The bioactivity of the Ti–6Al–4V alloy implant was improved with the formation of a nanotubular TiO2 layer and precalcification treatment in a 0.5M Na2HPO4 and saturated Ca(OH)2 solution.

Bioinspired Graphene Nanogut

Low-dimensional nanomaterials are attractive for various applications, including damage repair, drug delivery, and bioimaging. The ability to control the morphology of nanomaterials is critical for manufacturing as well as for utilizing them as functional materials or devices. However, the manipulation of such materials remains challenging, and effective methods to control their morphology remain limited. Here, we propose to mimic a macroscopic biological system—the gut—as a means to control the nanoscale morphology by exploiting the concept of mismatch strain. We show that, by mimicking the development of the gut, one can obtain a controlled wavy shape of a combined carbon nanotube and graphene system. We show that the scaling laws that control the formation of the gut at the macroscale are suitable for ultrasmall-diameter carbon nanotubes with a diameter smaller than 7 Å but do not account for the morphology of systems with larger diameter nanotubes. We find that the deviation is caused by cross-sectional buckling of carbon nanotube, where this behavior relates to the different constitutive laws for carbon nanotube and graphene in contrast to the macroscale biological system. Our study illustrates the possibility of downscaling macroscale phenomena to the nanoscale using continuum mechanics theory, with wide-ranging applications in nanotechnology.

Mapping Charge Transport by Electroluminescence in Chirality-Selected Carbon Nanotube Networks

We demonstrate random network single-walled carbon nanotube (SWNT) field-effect transistors (FETs) in bottom contact/top gate geometry with only five different semiconducting nanotube species that were selected by dispersion with poly(9,9-dioctylfluorene) in toluene. These FETs are highly ambipolar with balanced hole and electron mobilities and emit near-infrared light with narrow peak widths (<40 meV) and good efficiency. We spatially resolve the electroluminescence from the channel region during a gate voltage sweep and can thus trace charge transport paths through the SWNT thin film. A shift of emission intensity to large diameter nanotubes and gate-voltage-dependent photoluminescence quenching of the different nanotube species indicates excitation transfer within the network and preferential charge accumulation on small band gap nanotubes. Apart from applications as near-infrared emitters with selectable emission wavelengths and narrow line widths, these devices will help to understand and model charge transport in realistic carbon nanotube networks.

Type- and Species-Selective Air Etching of Single-Walled Carbon Nanotubes Tracked with in Situ Raman Spectroscopy

The thermal oxidation of carbon nanotubes in air is investigated by in situ Raman spectroscopy. Etching rates are directly seen to be diameter, chirality, and type dependent. We directly track the evolution of bundled nanotube networks that undergo air etching from 300 to 600 °C. Some species are more robust than others. Changes to radial breathing mode (RBM) and G- peak structures suggest that metallic species etch away more rapidly, with smaller diameter semiconducting species etching more slowly and large diameter nanotubes, including semiconductors, etching last. The decay in integrated G and D band intensities is tracked and fit reasonably well with biexponential decay. The RBM evolution is better represented by a single exponential. All bands are fit to activation plots with RBMs showing significantly different rates.

Formation of Nitrogen-Doped Graphene Nanoribbons via Chemical Unzipping

In this work, we carried out chemical oxidation studies of nitrogen-doped multiwalled carbon nanotubes (CNx-MWCNTs) using potassium permanganate in order to obtain nitrogen-doped graphene nanoribbons. Reaction parameters such as oxidation reaction, reaction time, the oxidizer to nanotube mass ratio, and the temperature were varied, and their effect was carefully analyzed. The presence of nitrogen atoms makes CNx-MWCNTs more reactive toward oxidation when compared to undoped multiwalled carbon nanotubes (MWCNTs). High-resolution transmission electron microscopy studies indicate that the oxidation of the graphitic layers within CNx-MWCNTs results in the unzipping of large diameter nanotubes and the formation of a disordered oxidized carbon coating on small diameter nanotubes. The nitrogen content within unzipped CNx-MWCNTs decreased as a function of the oxidation time, temperature, and oxidizer concentration. By controlling the degree of oxidation, the N atomic % could be reduced from 1.56% in pristine CNx-MWCNTs down to 0.31 atom % in nitrogen-doped oxidized graphene nanoribbons. A comparative thermogravimetric analysis reveals a lower thermal stability of the (unzipped) oxidized CNx-MWCNTs when compared to MWCNT samples. The oxidized graphene nanoribbons were chemically and thermally reduced and yielded nitrogen-doped graphene nanoribbons (N-GNRs). The thermal reduction at relatively low temperature (300 °C) results in graphene nanoribbons with 0.37 atom % of nitrogen. This method represents a novel route to preparation of bulk quantities of nitrogen-doped unzipped carbon nanotubes, which is able to control the doping level in the resulting reduced GNR samples. Finally, the electrochemical properties of these materials were evaluated.

Structure and Electronic Transport in Graphene Wrinkles

Wrinkling is a ubiquitous phenomenon in two-dimensional membranes. In particular, in the large-scale growth of graphene on metallic substrates, high densities of wrinkles are commonly observed. Despite their prevalence and potential impact on large-scale graphene electronics, relatively little is known about their structural morphology and electronic properties. Surveying the graphene landscape using atomic force microscopy, we found that wrinkles reach a certain maximum height before folding over. Calculations of the energetics explain the morphological transition and indicate that the tall ripples are collapsed into narrow standing wrinkles by van der Waals forces, analogous to large-diameter nanotubes. Quantum transport calculations show that conductance through these "collapsed wrinkle" structures is limited mainly by a density-of-states bottleneck and by interlayer tunneling across the collapsed bilayer region. Also through systematic measurements across large numbers of devices with wide "folded wrinkles", we find a distinct anisotropy in their electrical resistivity, consistent with our transport simulations. These results highlight the coupling between morphology and electronic properties, which has important practical implications for large-scale high-speed graphene electronics.

E-beam fabrication of antibacterial silver nanoparticles on diameter-controlled TiO2 nanotubes for bio-implants

To reduce the risk of infection, many researchers have applied antibacterial silver to TiO2 implant surfaces. In this study, silver nanoparticles were applied to TiO2 nanotubes with e-beam evaporation. In advance, the diameters of the nanotubes were controlled by varying anodization. The objective of this study was to optimize the antibacterial effects of silver nanoparticles, but also maintain the nanotubular surface of biocompatible TiO2 nanotubes. The surfaces were characterized with field emission scanning electron microscopy (FE-SEM), energy dispersive spectroscopy (EDS), high resolution X-ray diffraction (HR-XRD), X-ray photoelectron spectroscopy (XPS) and scanning probe microscopy (SPM). Contact angle measurements and 3 times simulated body fluid immersion (SBF) tests were performed for indirect verification of osteogenic properties. Results showed that the silver nanoparticles were efficiently fabricated on the nanotubes. We found that the contact angle was influenced more by the degree of anatase phase in the TiO2 than by surface roughness and apatite forming ability was proportionally increased as hydrophilicity was increased. Silver ion release was measured by the inductively coupled plasma mass spectrometry (ICP-MS). All specimens showed more than 0.1ppb of silver concentration which was known to be the level for the antibacterial property. The antibacterial activity test against Staphylococcus aureus (S. aureus) showed that bacterial colonization was effectively inhibited in all experimental groups. Water soluble tetrazolium (WST) assays showed that silver nanoparticles fabricated on large-diameter nanotubes were not cytotoxic. However, smaller diameter of nanotubes showed mild cytotoxicity, due to the excess of silver nanoparticles aggregated on the top surface. Overall, we concluded that large-diameter TiO2 nanotubes had favorable osteogenic properties, were non-cytotoxic, maintained a nanoporous surface, and displayed antibacterial properties that were highly beneficial for bio-implants.

Quantum Transport and Current Distribution at Radio Frequency in Multiwall Carbon Nanotubes

Multiwall carbon nanotubes represent a low-dimensional material that could serve as building blocks for future carbon-based nanoelectronics. The understanding of the electromagnetic performances at radio frequency of these materials for use in nanointerconnects is strictly related to the analysis of their transport properties as function of the working conditions. In this paper, we present an explicit expression of the conducting channels as function of diameter, temperature, doping, and supply voltage for both metallic and semiconducting carbon nanotubes. The proposed formula is based on the Dirac cone approximation of the conducting band energy of graphene nearby the Fermi points, combined with the Landauer-Buttiker formalism. Simplified expressions are also obtained in case of large diameter nanotubes. We show that the conductance, kinetic inductance, and quantum capacitance of each carbon shell are strongly affected by those parameters, and, consequently, that the current distribution among the shells of a multiwall carbon nanotube at radio frequency could be optimized with the proper definition of the nanotube configuration versus the working conditions.

The Role of Weak Bonding in Determining the Structure of Thiophene Oligomers inside Carbon Nanotubes

The orientation preference of methyl-terminated thiophene oligomers inside an armchair nanotube was evaluated using density functional theory calculations with dispersion corrections (B97-D functional). According to the B97-D calculations, internal thiophene oligomers prefer to be located near the walls of the armchair nanotubes rather than at their center. The attractive π···π and CH···π interactions operating between the thiophene guest and nanotube host are sufficient within the nanotube to overcome the energy cost of deforming the thiophene oligomer from the more stable gas-phase conformation. As a result, thiophene oligomers inside the nanotube tend to form nearly planar structures, even though adjacent thiophene rings rotate around the connecting bond in the gas phase. These weak attractive interactions enhance the planarity of thiophene oligomers confined within larger-diameter nanotubes.

Chlorination of carbon nanotubes

We report ab initio density functional theory calculations for chlorinated single-wall carbon nanotubes and investigate the atomic structure, energetics, and electronic structure of the chlorinated nanotubes, as well as the energetics of the desorption reaction. We find that the Cl atoms should be adsorbed in pairs and thus focus on doubly chlorinated nanotubes. Using the terminology of arene substitution patterns, ortho and para configurations are the most stable. The physisorption is preferable to the chemisorption in large-diameter nanotubes. The impurity states appear near the Fermi level ${E}_{F}$ in the electronic structure and may alter the electronic properties considerably. The bonding character for adsorption outside the nanotube is mainly covalent, but inside it consists of physical bonding. The adsorption of several Cl atoms inside a carbon nanotube leads to the formation of a charged Cl chain. Our calculated desorption barrier of $\ensuremath{\leqslant}1.4$ eV per Cl atom pair indicates that the cleansing by chlorination is a less damaging alternative with removable residue.