Preparation of Short Carbon Nanotubes and Application as an Electrode Material in Li‐Ion Batteries

A new approach is developed for cutting conventional micrometer‐long entangled carbon nanotubes (CNTs) to short ca. 200 nm long segments with excellent dispersion. CNTs with different lengths are used as anode materials in Li‐ion batteries. The reversible capacity of the Li‐ion batteries is increased and the irreversible capacity is decreased upon shortening the length of the CNTs. The reason for this is that the insertion/extraction of Li ions is easier into/from short CNTs as compared to long CNTs because of the shortened length and the presence of lateral defects. Moreover, short CNTs have a lower electrical resistance and Warburg prefactor, resulting in better rate performance at high current densities. The present study suggests that short segments of CNTs obtained by cutting long CNTs may possess novel properties that may be useful for a wide variety of applications.     

Poly(ϵ‐caprolactone)‐Functionalized Carbon Nanotubes and Their Biodegradation Properties

Biodegradable poly(?‐caprolactone) (PCL) has been covalently grafted onto the surfaces of multiwalled carbon nanotubes (MWNTs) by the “grafting from” approach based on in‐situ ring‐opening polymerization of ?‐caprolactone. The grafted PCL content can be controlled easily by adjusting the feed ratio of monomer to MWNT‐supported macroinitiators (MWNT‐OH). The resulting products have been characterized with Fourier‐transform IR (FTIR), NMR, and Raman spectroscopies, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). After PCL was coated onto MWNT surfaces, core/shell structures with nanotubes as the “hard” core and the hairy polymer layer as the “soft” shell are formed, especially for MWNTs coated with a high density of polymer chains. Such a polymer shell promises good solubility/dispersibility of the MWNT–PCL nanohybrids in low‐boiling‐point organic solvents such as chloroform and tetrahydrofuran. Biodegradation experiments have shown that the PCL grafted onto MWNTs can be completely enzymatically degraded within 4 days in a phosphate buffer solution in the presence of pseudomonas (PS) lipase, and the carbon nanotubes retain their tubelike morphologies, as observed by SEM and TEM. The results present possible applications for these biocompatible PCL‐functionalized CNTs in bionanomaterials, biomedicine, and artificial bones.     

Single‐Walled Carbon Nanotube Dispersions in Poly(ethylene oxide)

Dispersions of single‐walled carbon nanotubes (SWNTs) in poly(ethylene oxide) (PEO) assisted by a lithium‐based anionic surfactant demonstrate an electrical percolation of 0.03 wt.‐% and a geometrical percolation, inferred from melt rheometry, of 0.09 wt.‐%. Both the melting temperature and the extent of crystallinity of the PEO crystals decrease with increasing SWNT loading. Raman spectroscopy of the nanocomposites indicates a down‐shift of the SWNT G‐modes and suggests that the nanotubes are subjected to tensile stress transfer from the polymer at room temperature.     

Reactive Spinning of Cyanate Ester Fibers Reinforced with Aligned Amino‐Functionalized Single Wall Carbon Nanotubes

We report a new approach of reactive spinning to fabricate thermosetting cyanate ester micro‐scale diameter fibers with aligned single walled carbon nanotubes (SWNTs). The composite fibers were produced by first dispersing the SWNTs (1 wt %) in cyanate ester (CE) via solvent blending, followed by pre‐polymerization, spinning and then multiple‐stage curing. The pre‐polymerization, spinning and post‐spinning cure temperatures were carefully controlled to achieve good spun crosslinked fibers. Both pristine and amino‐functionalized SWNTs were used for the reinforced fiber spinning. Amino‐functionalized SWNTs (f‐SWNTs) were prepared by reacting acid‐treated SWNTs with toluene 2,4‐diisocyanate and then ethylenediamine (EDA). FTIR, optical microscopy and scanning electron microscopy (SEM) showed that the amino‐functionalized SWNTs were covalently and uniformly dispersed into the cyanate ester matrix and aligned along the fiber axis. The alignment was further confirmed using polarized Raman spectroscopy. The composite fibers with aligned amino‐functionalized SWNTs possess improved tensile properties with respect to neat CE fibers, showing 85, 140, and 420% increase in tensile strength, elongation and stress‐strain curve area (i.e., toughness), respectively. NH₂‐functionalization of SWNTs improves their dispersibility, alignment and interfacial strength and hence tensile properties of composite spun fibers. Fiber spinning to align SWNTs using thermosetting resin is novel. Others have reported fiber spinning to align SWNTs in thermoplastics. However, thermosetting CE resins offer the advantages of low and controllable viscosity during spinning and reactivity with amino functional groups to enable f‐SWNT/CE covalent bonding.     

Sensing of Damage Mechanisms in Fiber‐Reinforced Composites under Cyclic Loading using Carbon Nanotubes

The expanded use of advanced fiber‐reinforced composites in structural applications has brought attention to the need to monitor the health of these structures. It has been established that adding carbon nanotubes to fiber‐reinforced composites is a promising way to detect the formation of microscale damage. Because carbon nanotubes are three orders of magnitude smaller than traditional advanced fibers, it is possible for nanotubes to form an electrically conductive network in the polymer matrix surrounding the fibers. In this work, multi‐walled carbon nanotubes are dispersed into epoxy and infused into a glass‐fiber preform to form a network of in situ sensors. The resistance of the cross‐ply composite is measured in real‐time during incremental cyclic tensile loading tests to evaluate the damage evolution and failure mechanisms in the composite. Edge replication is conducted to evaluate the crack density after each cycle, and optical microscopy is utilized to study the crack mode and growth. The evolution of damage can be clearly identified through the damaged resistance parameter. Through analyzing the damaged resistance response curves with measurements of transverse crack density and strain, the transition between different failure modes can be identified. It is demonstrated that the integration of an electrically conducting network of carbon nanotubes in a glass fiber composite adds unique damage‐sensing functionality that can be utilized to track the nature and extent of microstructural damage in fiber composites.     

Modifying the Electronic Character of Single‐Walled Carbon Nanotubes Through Anisotropic Polymer Interaction: A Raman Study

Using Raman spectroscopy, we demonstrate that the anisotropic interaction between single‐walled carbon nanotubes (SWNTs) and poly(methyl methacrylate) (PMMA) causes significant changes in the electronic properties of their composites. Two different procedures were used to prepare the composites: melt blending and in‐situ UV polymerization. Resonant Raman studies relate the electronic density of states (DOS) of the SWNTs to the corresponding vibration symmetry changes of both the PMMA and the SWNTs. Our results show that, in the melt‐blended sample, the SWNTs—originally semiconducting—became predominantly metallic. The changes in the electronic properties were also confirmed by dielectric constant measurements. We propose that the anisotropic interaction between PMMA and SWNTs in the melt‐blended composite is the dominant reason for the observed electronic character change.     

Studies of Surface‐Modified Gold Nanowires Inside Living Cells

The cytotoxicity of various surface‐functionalized gold nanowires with different aspect ratios is investigated by (3‐(4,5‐dimethylthiazol‐2‐yl)2,5‐diphenyltetrazolium bromide) (MTT) assays for two cell lines, fibroblast and HeLa. It is found that functionalized gold nanowires with a diameter of 200 nm and lengths up to a few micrometers can be readily internalized by both types of cells regardless of the type of surface functionalization. However, the cytotoxicity of the gold nanowires is observed to depend on their surface modification. Serum‐coated gold nanowires are the least toxic, whereas more than 50 % of the cells are damaged in the presence of mercapto‐acid‐modified gold nanowires even at very low concentrations (10³ nanowires mL^–1). Nanowires with different aspect ratios exhibit the same cytotoxicity within limits of experimental error. However, the uptake efficiency is found to be higher for shorter nanowires as compared to their longer counterparts. Therefore, we conclude that internalized nanowires with high aspect ratios are more toxic to cells than nanowires with low aspect ratios. Positively charged aminothiol‐modified gold nanowires are employed to deliver both plasmid DNA and probe molecules into cells without compromising the viability of the cells. The local environment of individual nanowires within the cells is studied by monitoring the fluorescence signal from probe molecules attached to the nanowires.     

Synthesis and Properties of a Water‐Soluble Single‐Walled Carbon Nanotube–Poly(m‐aminobenzene sulfonic acid) Graft Copolymer

Poly(m‐aminobenzene sulfonic acid) (PABS), was covalently bonded to single‐walled carbon nanotubes (SWNTs) to form a water‐soluble nanotube–polymer compound (SWNT–PABS). The conductivity of the SWNT–PABS graft copolymer was about 5.6 × 10^–3 S cm^–1, which is much higher than that of neat PABS (5.4 × 10^–7 S cm^–1). The mid‐IR spectrum confirmed the formation of an amide bond between the SWNTs and PABS. The ¹H NMR spectrum of SWNT–PABS showed the absence of free PABS, while the UV/VIS/NIR spectrum of SWNT–PABS showed the presence of the interband transitions of the semiconducting SWNTs and an absorption at 17 750 cm^–1 due to the PABS addend.     

Reinforcing Epoxy Polymer Composites Through Covalent Integration of Functionalized Nanotubes

Strong interfacial bonding and homogenous dispersion have been found to be necessary conditions to take full advantage of the extraordinary properties of nanotubes for reinforcement of composites. We have developed a fully integrated nanotube composite material through the use of functionalized single‐walled carbon nanotubes (SWNTs). The functionalization was performed via the reaction of terminal diamines with alkylcarboxyl groups attached to the SWNTs in the course of a dicarboxylic acid acyl peroxide treatment. Nanotube‐reinforced epoxy polymer composites were prepared by dissolving the functionalized SWNTs in organic solvent followed by mixing with epoxy resin and curing agent. In this hybrid material system, nanotubes are covalently integrated into the epoxy matrix and become part of the crosslinked structure rather than just a separate component. Results demonstrated dramatic enhancement in the mechanical properties of an epoxy polymer material, for example, 30–70 % increase in ultimate strength and modulus with the addition of only small quantities (1–4 wt.‐%) of functionalized SWNTs. The nanotube‐reinforced epoxy composites also exhibited an increased strain to failure, which suggests higher toughness.     

In‐Situ Vis–Near‐Infrared and Raman Spectroelectrochemistry of Double‐Walled Carbon Nanotubes

Double‐walled carbon nanotubes (DWCNTs) are studied using in‐situ visible–near‐infrared (vis‐NIR) and in‐situ Raman spectroelectrochemistry. Electrochemical vis‐NIR spectroscopy reveals a complex picture of DWCNTs due to the overlap of the features of the inner and outer tubes and possible optical transitions, which are not predicted by the simple tight‐binding model. The optical transitions are bleached upon electrochemical doping. This is qualitatively understood to be a consequence of the Fermi‐level shift by the applied potential relative to the van Hove singularity. In‐situ Raman spectra are quenched by the applied cathodic/anodic potentials due to the loss of resonance by electrochemical charging. The electrochemical tuning of Raman spectra proceeds distinctly for inner and outer tubes. While the bands of outer tubes rapidly follow the potential change, the features of inner tubes respond relatively slowly to electrochemical perturbations. The Raman D‐mode of DWCNTs was found to be bifurcated upon electrochemical charging, which is similar to the behavior of the tangential displacement mode. Ionic liquids are good electrolytes for the spectroelectrochemistry of DWCNTs, even at extreme applied potentials. They allow the deconvolution of the tangential modes of the inner and outer tubes at both cathodic and anodic doping.     

Efficient Synthesis of Carbon Nanotube–Nanoparticle Hybrids

A simple and versatile approach has been developed to synthesize different carbon nanotube (CNT)–nanoparticle hybrid materials. The strategy is based on the nondestructive (noncovalent) functionalization of pristine CNTs and the subsequent in situ synthesis of a variety of different nanoparticles, including metal, semiconductor, and insulator particles, on the modified CNTs. This strategy has been demonstrated here with Pt, CdS, and silica nanoparticles. It is believe that this technique will provide a simple and convenient route to efficiently assemble a wide variety of nanoscale particles/clusters on the surfaces of CNTs, and will enable the construction of nanoscale heterostructures with novel functionalities in nanotechnology.     

Design of Dispersants for the Dispersion of Carbon Nanotubes in an Organic Solvent

The effect of dispersant structures for dispersing single‐walled carbon nanotubes (SWCNTs) is investigated. The monomer 3‐hexylthiophene is used as the starting material for the development of a series of oligomers that are used to disperse SWCNTs in an organic solvent. The series is obtained by varying the number of head groups, the regioregularity of head groups, and the head‐to‐tail ratios of the hexyl group in the oligomers. The SWCNT solutions are characterized with UV‐vis–near‐IR spectroscopy and transmission electron microscopy. An increase in the number of head groups improves the dispersity of SWCNTs, and a regioregular oligomer plays an important role in dispersing SWCNTs. Furthermore, Raman spectroscopy and X‐ray photoelectron spectroscopy shows that the sulfur atom head groups enhance interactions between the thiophenes and the SWCNT walls. The analysis demonstrated that a well‐designed thiophene oligomer could afford well‐dispersed SWCNT solutions with long‐term dispersion stability, even with an extremely low dispersant concentration (weight ratio of CNTs/dispersant is one and the dispersant concentration is 0.1 g L^–1).     

Clay Assisted Dispersion of Carbon Nanotubes in Conductive Epoxy Nanocomposites

Clay was introduced into single‐walled carbon nanotube (SWNT)/epoxy composites to improve nanotube dispersion without harming electrical conductivity or mechanical performance. Unlike surfactant or polymer dispersants, clay is mechanically rigid and known to enhance the properties (e.g., modulus, gas barrier, and flame retardation) of polymer composites. Combining nanotubes and clay allows both electrical and mechanical behavior to be simultaneously enhanced. With just 0.05 wt % SWNT, electrical conductivity is increased by more than four orders of magnitude (from 10^–9 to 10^–5 S cm^–1) with the addition of 0.2 wt % clay. Furthermore, the percolation threshold of these nanocomposites is reduced from 0.05 wt % SWNT to 0.01 wt % with the addition of clay. SWNTs appear to have an affinity for clay that causes them to become more exfoliated and better networked in these composites. This clay‐nanotube synergy may make these composites better suited for a variety of packaging, sensing, and shielding applications.     

Mechanical Reinforcement of Polyethylene Using Polyethylene‐Grafted Multiwalled Carbon Nanotubes

The incorporation of carbon nanotubes to a polymer generally improves the stiffness and strength of the polymer, but the ductility and toughness of the polymer are compromised in most cases. Here we report the mechanical reinforcement of polyethylene (PE) using polyethylene‐grafted multiwalled carbon nanotubes (PE‐g‐MWNTs). The stiffness, strength, ductility and toughness of PE are all improved by the addition of PE‐g‐MWNTs. The grafting of PE onto MWNTs enables the well‐dispersion of nanotubes in the PE matrix and improves MWNT/PE interfacial adhesion. The grafting was achieved by a reactive blending process through melt blending of PE containing 0.85 wt % of maleic anhydride and amine‐functionalized MWNTs. The reaction between maleic anhydride and amine groups, as evidenced by X‐ray photoelectron spectroscopy and Raman spectroscopy, leads to the grafting of PE onto the nanotubes.     

UV‐Light‐Driven Immobilization of Surface‐Functionalized Oxide Nanocrystals onto Silicon

TiO₂ nanorods (NRs) and γ‐Fe₂O₃ nanocrystals (NCs) passivated with unsaturated long‐chain carboxylic acids, namely 10‐undecylenic acid (10UDA) and oleic acid (OLEA), are covalently anchored to Si(100) at room temperature by UV‐light‐driven reaction of hydrogenated silicon with the carbon–carbon double bond (–C?C–) moieties of the capping surfactants. The high reactivity of vinyl groups towards Si provides a general tool for attaching particles of both materials via Si–C bonds. Interestingly, TiO₂ NRs were efficiently attached to silicon even when capped by OLEA. This latter finding has been explained by a photocatalytic mechanism involving the primary role of hydroxyl radicals that can be generated upon bandgap TiO₂ photoexcitation with UV light. The increased oxide coverage achievable on Si opens access to further surface manipulation, as demonstrated by the possibility of depositing an additional film of Au nanoparticles onto TiO₂ via TiO₂‐catalyzed visible‐light‐driven reduction of aqueous AuCl₄^– ions. Extensive morphological and chemical characterization of the obtained NC‐functionalized Si substrates is provided to support the effectiveness of proposed photochemical approaches.     

Hierarchical Self‐Assembly of Peptide‐Coated Carbon Nanotubes

Numerous applications, from molecular electronics to super‐strong composites, have been suggested for carbon nanotubes. Despite this promise, difficulty in assembling raw carbon nanotubes into functional structures is a deterrent for applications. In contrast, biological materials have evolved to self‐assemble, and the lessons of their self‐assembly can be applied to synthetic materials such as carbon nanotubes. Here we show that single‐walled carbon nanotubes, coated with a designed amphiphilic peptide, can be assembled into ordered hierarchical structures. This novel methodology offers a new route for controlling the physical properties of nanotube systems at all length scales from the nano‐ to the macroscale. Moreover, this technique is not limited to assembling carbon nanotubes, and could be modified to serve as a general procedure for controllably assembling other nanostructures into functional materials.     

Preparation of Homogeneously Dispersed Multiwalled Carbon Nanotube/Polystyrene Nanocomposites via Melt Extrusion Using Trialkyl Imidazolium Compatibilizer

Well‐dispersed multiwalled carbon nanotube (MWNT)/polystyrene nanocomposites have been prepared via melt extrusion, using trialkylimidazolium tetrafluoroborate‐compatibilized MWNTs. Quantification of the improvement is realized via transmission electron microscopy and laser scanning confocal microscopy image analysis. Differential scanning calorimetry and Fourier‐transform infrared and X‐ray diffraction analysis show evidence for a π‐cation, nanotube–imidazolium interaction and the conversion from an interdigitated bilayer, for the imidazolium salt, to an ordered lamellar structure, for the imidazolium on the surface of the MWNTs.     

Catalytic Twist‐Spun Yarns of Nitrogen‐Doped Carbon Nanotubes

The treatment of free‐standing sheets of multiwalled carbon nanotubes (MWNTs) with a NH₃/He plasma results in self‐supporting sheets of aligned N‐doped MWNTs (CN_x). These CN_x sheets can be easily twist spun in the solid state to provide strong CN_x yarns that are knottable, weavable, and sewable. The CN_x yarns exhibit tunable catalytic activity for electrochemically driven oxygen reduction reactions (ORR), as well as specific capacitances (up to 39 F·g^?1) that are 2.6 times higher than for the parent MWNTs. Due to a high degree of nanotube alignment, the CN_x yarns exhibit specific strengths (451 ± 61 MPa·cm³·g^?1) that are three times larger than observed for hybrid CN_x/MWNT biscrolled yarns containing 70 wt.% CN_x in the form of a powder. This difference in mechanical strength arises from substantial differences in yarn morphology, revealed by electron microscopy imaging of yarn cross‐ sections, as well as the absence of a significant strength contribution from CN_x nanotubes in the biscrolled yarns. Finally, the chemical nature and abundance of the incorporated nitrogen within the CN_x nanotubes is studied as function of plasma exposure and annealing processes using X‐ray photoelectron spectroscopy and correlated with catalytic activity.     

Properties of Carbon Nanotube Fibers Spun from DNA‐Stabilized Dispersions

The fabrication of single‐walled carbon nanotube (CNT) fibers containing (salmon) DNA has been demonstrated. The DNA material has been found to be adequate for dispersing relatively large concentrations (up to 1 % by weight) of carbon nanotubes. These dispersions are better suited for fiber spinning than previously studied dispersions based on conventional surfactants, such as sodium dodecyl sulfate (SDS). The DNA‐containing fibers were less conductive than the fibers based on SDS, but they were significantly stronger. Considerably increased conductivity was obtained by thermally annealing the CNT/DNA fibers, a process accompanied by a loss in mechanical strength. Smaller improvements in conductivity could be introduced by annealing the carbon nanotubes before fiber production, with no alteration of the fiber mechanical properties. Those CNT/DNA fibers that were mechanically strong and conductive also exhibited good electrochemical behavior and useful capacitance values (up to 7.2 F g^–1).     

Correlations between Percolation Threshold,Dispersion State,and Aspect Ratio of Carbon Nanotubes

Critical factors that determine the percolation threshold of carbon nanotube (CNT)‐reinforced polymer nanocomposites are studied. An improved analytical model is developed based on an interparticle distance concept. Two dispersion parameters are introduced in the model to correctly reflect the different dispersion states of CNTs in the matrix—entangled bundles and well‐dispersed individual CNTs. CNT–epoxy nanocomposites with different dispersion states are fabricated from the same constituent materials by employing different processing conditions. The corresponding percolation thresholds of the nanocomposites vary over a wide range, from 0.1 to greater than 1.0 wt %, and these variations are explained in terms of dispersion parameters and aspect ratios of CNTs. Important factors that control the percolation threshold of nanocomposites are identified based on the comparison between modeling data and experimental results.