Carbon Nanotube Samples Research Articles

Quantitative monitoring of the removal of non-encapsulated material external to filled carbon nanotube samples

The endohedral functionalization of carbon nanotubes with both organic and inorganic materials allows the development of tailored functional hybrids whose properties benefit from the synergistic effects of the constituent compounds. Bulk filling of carbon nanotubes (CNTs) results in samples that contain a large amount of non-encapsulated material external to the CNTs. The presence of the external material is detrimental to the processing and application of the resulting hybrids. Here we introduce the use of UV-Vis spectroscopy to monitor the cleaning process, i.e. the elimination of non-encapsulated compounds. Chrome azurol S has been employed to assess the bulk removal of external samarium(iii) chloride from filled single-walled carbon nanotubes. Chrome azurol S is of interest since it can be used to quantify a large variety of materials in a fast, accurate and reliable manner. The parameters that control the cleaning process have been optimized, including the time, temperature, volume and sonication, to achieve a fast and complete removal of the external material.

Effect of strain rate on tensile properties of polyurethane/(multiwalled carbon nanotube) nanocomposite

In this research, polyurethane (PU)/(carbon nanotube) (CNT) samples, with two different contents of multiwalled carbon nanotubes (MWCNTs; i.e., 0.5 and 1.0 wt%) were fabricated through a solution casting method. To investigate the effect of strain rate on tensile properties, tensile tests were done on standard samples at constant temperature and different strain rates (2 × 10−5 to 2 × 10−2 s−1). Eyring's model was performed to clarify the role of both strain rate and CNTs content on activation volume and activation enthalpy of PU. To elucidate the role of strain rate and CNTs content on fracture behavior of PU, fracture surfaces of some samples were also investigated by scanning electron microscopy. The results of tensile tests show the intense effect of strain rate on tensile properties of PU/MWCNTs nanocomposites. Also, it was proved that the dependency of tensile properties of PU nanocomposites on strain rate decreases as CNTs content increases. The microscopic observations of the samples also demonstrate that increasing the strain rate changes the behavior of the fracture surface to less a ductile fracture, and increasing CNTs content causes much surface roughness. Finally, by investigation of the activation enthalpies, it is confirmed that much higher enthalpy is needed to fracture the samples with increased MWCNTs content, as the activation enthalpy changes from 45 for neat PU to 131 kJ/mol for PU/(1% MWCNTs) samples. J. VINYL ADDIT. TECHNOL., 22:356–361, 2016. © 2014 Society of Plastics Engineers

High precision fractionator for use with density gradient ultracentrifugation.

The recent application of density gradient ultracentrifugation (DGU) for structural sorting of single-walled carbon nanotube samples has created a need for highly selective extraction of closely spaced layers formed in the centrifuged tube. We describe a novel computer-controlled device designed for this purpose. Through the use of fine needles, systematic needle motions, and slow flow rates, multiple sample layers can be aspirated under program control with minimal cross contamination between layers. The fractionator's performance is illustrated with DGU-sorted samples of single-walled carbon nanotubes.

Comparison of the equivalent width, the autocorrelation width, and the variance as figures of merit for XPS narrow scans

X-ray photoelectron spectroscopy (XPS) is widely used in surface and materials laboratories around the world. It is a near surface technique, providing detailed chemical information about samples in the form of survey and narrow scans. To extract the maximum amount of information about materials it is often necessary to peak fit XPS narrow scans. And while indispensable to XPS data analysis, even experienced practitioners can struggle with their peak fitting. In our previous publication, we introduced the equivalent width (EWXPS) as both a possible machine automated method, one that requires less expert judgment for characterizing XPS narrow scans, and as an approach that may be well suited for the analysis of complex spectra. The EWXPS figure of merit was applied to four different data sets. However, as previously noted, other width functions are also regularly employed for analyzing functions. Here we evaluate two other width functions for XPS narrow scan analysis: the autocorrelation width (AWXPS) and the variance (σXPS2). These widths were applied to the same four sets of spectra studied before: (a) four C 1s narrow scans of ozone-treated carbon nanotubes (CNTs) (EWXPS: ∼2.11–2.16eV, AWXPS: ∼3.9–4.1eV, σXPS2: ∼5.0–5.2eV, and a modified form of σXPS2, denoted σXPS2*: ∼6.3–6.8eV), (b) silicon wafers with different oxide thicknesses (EWXPS: ∼1.5–2.9eV, AWXPS: ∼2.28–4.9, and σXPS2: ∼0.7–4.9eV), (iii) hydrogen-terminated silicon surfaces, before and after modification with pentyl groups, and after annealing of the pentyl-terminated surface (EWXPS: ∼0.7–1.0eV, AWXPS: ∼1.2–1.6eV, and σXPS2: ∼0.12–0.19eV), and (iv) C 1s narrow scans from five different nanodiamond samples, three of which showed charging (EWXPS: ∼2.6–4.8eV, AWXPS: ∼3.8–6.9eV, and σXPS2: ∼1.6–4.2eV). All three of the width functions showed similar trends, except in the case of the C 1s spectra of the CNT samples, which were the most complex spectra evaluated, where σXPS2 showed poor correlation with the corresponding O/C ratios. Accordingly, we favor EWXPS and AWXPS. EWXPS is advantageous because it is conceptually simple, giving the most intuitive results. AWXPS has the advantage of not requiring the user to specify the height of the function at its maximum, which will be affected by noise. Because these functions are based on different mathematical operations/algorithms, best practices may involve the calculation of both widths for a set of narrow scans. The standard deviation, σXPS, i.e., the square root of the variance, was also examined. As expected, it gave results similar to σXPS2.

Quasi-aligned carbon nanotubes synthesised from waste engine oil

Quasi-aligned carbon nanotubes (CNTs) were successfully synthesised for the first time using waste engine oil (WEO) as the carbon source via thermal chemical vapour deposition (TCVD). The high carbon content of WEO was believed to promote the growth of the quasi-aligned CNTs. The synthesis process was performed at precursor and synthesis temperatures of 500 and 750°C, respectively, with a ferrocene catalyst concentration of 17.99wt%. Typical characterisation methods were employed to examine the CNTs: electron microscopy, energy dispersive X-Ray, X-ray diffraction and micro-Raman spectroscopy. The ability of CNT samples to emit electrons was also investigated by field electron emission (FEE) analysis. Electron microscopy and micro-Raman analysis revealed a dense mixture of quasi-aligned single- and multi-walled CNTs with a moderate ID/IG ratio of 0.90. The overall diameters of the CNTs ranged from 18.0 to 29.8nm, with the diameters of the single-walled CNTs estimated to be between 0.6 and 1.1nm. The FEE results showed that the quasi-aligned CNTs synthesised from WEO exhibited reasonable turn-on and threshold fields of 4.1 and 7.2Vμm−1, which corresponded to current densities of 0.1 and 1.0μAcm−2, respectively. This study highlights WEO as a new, cheap, abundant and easily available carbon source for quasi-aligned CNTs production with a potential application in electron emission devices.

Electrochemical oxidation of aniline at mono and bimetallic electrocatalysts supported on carbon nanotubes

Abstract The electrochemical mineralization of aniline, a compound that causes serious environmental problems, was investigated at Pt, Pd and Ru–Cu catalysts supported on multiwalled carbon nanotubes (CNTs). The modified electrodes, based on commercial CNT samples, were prepared and dispersed onto Toray carbon (CT) using a Nafion/water solution. The electroreactivity of aniline at the electrocatalysts based on CNTs, and the kinetic parameters of the redox reactions, were determined using cyclic voltammetry. In order to determine the conditions that allow an efficient mineralization, the electrochemical oxidation of aniline was carried out in a membrane separated electrochemical cell. The reaction products were identified and quantified using chromatographic methods in order to evaluate the mineralization efficiency. The highest conversions were obtained with the Ru–Cu/CNT/CT modified electrode.

Development of efficient digestion procedures for quantitative determination of cobalt and molybdenum catalyst residues in carbon nanotubes

Whatever the method used for the synthesis of carbon nanotubes (CNTs), they always contain residual catalysts in variable amount. Many methods have been proposed in the literature to purify CNTs, but their efficiency strongly depends on the experimental conditions. Although the presence of residual catalysts in small amount is generally not a problem for many applications, this can become a critical issue when a high purity is required, typically for magnetic properties or for biomedical applications (because of the intrinsic toxicity of most catalysts). Quantification of the amount of residual catalysts is usually obtained by classical chemical analysis, which requires a preliminary digestion (complete mineralisation) of the CNT samples. In this work, we systematically compared 3 different digestion protocols and optimised one, reaching 100% dissolution within a very limited time (1h) together with the requirement of only a few milligrams of sample, and safe experimental conditions. This method can be easily transferred for use in research laboratories, making accessible the quantitative analysis of CNT samples, and has been validated following ISO/IEC 17025:2005 for linearity, specificity, intermediate precision, limits of detection and quantification.

Selective adsorption of metoprolol enantiomers using 2-hydroxypropyl-β-cyclodextrin cross-linked multiwalled carbon nanotube.

This study investigates the ability of functionalized multiwalled carbon nanotubes (MWCNTs) for enantio-separation of metoprolol chiral forms. 2Hydroxypropyl-β-cyclodextrin (2HP-β-CD) was applied as a chiral selector to functionalize carbon nanotubes (CNTs). The modified multiwalled CNT samples were characterized using scanning electron microscopy and Fourier transform infrared spectroscopy. The results of analyses showed that CNTs were successfully cross-linked with 2HP-β-CD. To evaluate the enantio-separation property of the products, the separation of metoprolol chiral forms on the initial and final products was examined. Further, UV-visible spectroscopy and polarimeter analyses were used for characterization. The results indicate that MWCNT does not have any intrinsic enantio-separation ability, although its selectivity for enantio-separation can be enhanced by cross-linking it to 2HP-β-CD. Moreover, the optimal mass of adsorbent as well as optimal mass of functional groups is estimated to achieve maximum enantio-separation efficiency. The results indicate that applying large amounts of 2HP-β-CD to CNTs functionalization decreases the cross-linking efficiency, which consequently reduces enantio-separation efficiency.

Electronic property modification of single-walled carbon nanotubes by encapsulation of sulfur-terminated graphene nanoribbons.

The use of carbon nanotubes (CNTs) as cylindrical reactor vessels has become a viable means for synthesizing graphene nanoribbons (GNRs). While previous studies demonstrated that the size and edge structure of the as-produced GNRs are strongly dependent on the diameter of the tubes and the nature of the precursor, the atomic interactions between GNRs and surrounding CNTs and their effect on the electronic properties of the overall system are not well understood. Here, it is shown that the functional terminations of the GNR edges can have a strong influence on the electronic structure of the system. Analysis of SWCNTs before and after the insertion of sulfur-terminated GNRs suggests a metallization of the majority of semiconducting SWCNTs. This is indicated by changes in the radial breathing modes and the D and G band Raman features, as well as UV-vis-NIR absorption spectra. The variation in resonance conditions of the nanotubes following GNR insertion make direct (n,m) assignment by Raman spectroscopy difficult. Thus, density functional theory calculations of representative GNR/SWCNT systems are performed. The results confirm significant changes in the band structure, including the development of a metallic state in the semiconducting SWCNTs due to sulfur/tube interactions. The GNR-induced metallization of semiconducting SWCNTs may offer a means of controlling the electronic properties of bulk CNT samples and eliminate the need for a physical separation of semiconducting and metallic tubes.

Simulation and experimental characterization of polymer/carbon nanotubes composites for strain sensor applications

In this paper, a numerical model is presented in order to analyze the electrical characteristics of polymer composites filled by carbon nanotubes (CNTs) subject to tensile stress and investigate the possible usage of such materials as innovative sensors for small values of strain. The simulated mechano-electrical response of the nanocomposite is obtained through a multi-step approach which, through different modeling stages, provides a simple and effective tool for material analysis and design. In particular, at first, the morphological structures of the composites are numerically simulated by adopting a previously presented model based on a Monte Carlo procedure in which uniform distributions of the CNTs, approximated as of solid cylinders and ensuring some physical constraints, are dispersed inside a cubic volume representing the polymer matrix. Second, a geometrical analysis allows to obtain the percolation paths detected in the simulated structures. Suitable electrical networks composed by resistors and capacitors associated to the complex charge transport and polarization mechanisms occurring in the percolation paths are then identified. Finally, the variations of these circuit parameters, which are differently affected by the mechanical stresses applied to the composites, are considered to analyze the electromechanical characteristics of the composites and hence their performances as stress sensors. The proposed approach is used to investigate the impact on the electro-mechanical response of some physical properties of the base materials, such as the type of carbon nanotube, the height of energy barrier of polymer resin, as well as characteristics of the composite, i.e., the volume fraction of the filler. The tunneling effect between neighboring nanotubes is found to play a dominant role in determining the composite sensitivity to mechanical stresses. The simulation results are also compared with the experimental data obtained by performing stress tests on samples of a multi walled CNT filled composite based on poly (ε-caprolactone), a polymer which is of interest for its biocompatibility. Model simulations and measured data show generally satisfactory agreement, confirming the effectiveness of the proposed approach to account for the impact of the interactions between CNTs and the insulating resin on the electromechanical response of the composite.

Iron oxide and oxygen plasma functionalized multi-walled carbon nanotubes for the discrimination of volatile organic compounds

Oxygen plasma functionalized multiwall carbon nanotubes and iron oxide decorated multiwall carbon nanotubes are studied for detecting volatile organic compounds (VOCs) at room temperature. Alumina substrates with interdigitated gold electrodes are employed as transducers and the air-brushing method is used to coat them with a thin film of the differently treated carbon nanotube samples. The electrical characterization of the sensors in the presence of benzene, toluene, acetone, methanol and ethanol vapors is carried out. The different treatments allow for tuning the response of carbon nanotubes and it is found that combining the responses of two sensors (i.e., one decorated with iron oxide and the other treated in an oxygen plasma) enables discriminating the aromatic hydrocarbons from the other VOCs tested. Additionally, discriminating benzene from toluene also appears to be possible. Sensors as the ones presented here, which are operated at room temperature, may led to a new generation of affordable, mass produced portable/wearable detectors with important applications in the selective detection of benzene.

Biological response to purification and acid functionalization of carbon nanotubes

Acid functionalization has been considered as an easy way to enhance the dispersion and biodegradation of carbon nanotubes (CNT). However, inconsistencies between toxicity studies of acid functionalized CNT remain unexplained. This could be due to a joint effect of the main physicochemical modifications resulting from an acid functionalization: addition of surface acid groups and purification from catalytic metallic impurities. In this study, the impact on CNT biotoxicity of these two physiochemical features was assessed separately. The in vitro biological response of RAW 264.7 macrophages was evaluated after exposure to 15–240 µg mL−1 of two types of multi-walled CNT. For each type of CNT (small: 20 nm diameter, and big: 90 nm diameter), three different surface chemical properties were studied (total of six CNT samples): pristine, acid functionalized and desorbed. Desorbed CNT were purified by the acid functionalization but presented a very low amount of surface acid groups due to a thermal treatment under vacuum. A Janus effect of acid functionalization with two opposite impacts is highlighted. The CNT purification decreased the overall toxicity, while the surface acid groups intensified it when present at a specific threshold. These acid groups especially amplified the pro-inflammatory response. The threshold mechanism which seemed to regulate the impact of acid groups should be further studied to determine its value and potential link to the other physicochemical state of the CNT. The results suggest that, for a safer-design approach, the benefit-risk balance of an acid functionalization has to be considered, depending on the CNT primary state of purification. Further research should be conducted in this direction.

Clinically relevant CNT dispersions with exceptionally high dielectric properties for microwave theranostic applications.

We present a formulation for achieving stable high-concentration (up to 20 mg/ml) aqueous dispersions of carbon nanotubes (CNTs) with exceptionally high microwave-frequency (0.5-6 GHz) dielectric properties. The formulation involves functionalizing CVD-synthesized CNTs via sonication in nitric and sulfuric acid. The overall chemical integrity of the CNTs is largely preserved, as demonstrated via physical and chemical characterizations, despite significant shortening and functionalization with oxygen-containing groups. This is attributed to the protected inner walls of double-walled CNTs in the samples. The resulting CNT dispersions show greatly enhanced dielectric properties compared to a CNT-free control. For example, at 3 GHz, the average relative permittivity and effective conductivity across several 20 mg/ml CNT samples were increased by ∼ 70% and ∼ 400%, respectively, compared to the control. These CNT dispersions exhibit the stability and extraordinary microwave properties desired in systemically administered theranostic agents for microwave diagnostic imaging and/or thermal therapy.

Effect of hybrid carbon nanotube/short glass fiber reinforcement on the properties of polypropylene composites

The hybrid reinforcement effect of surface treated or untreated carbon nanotube (CNT)/glass fiber (GF); on the morphology, mechanical and electrical properties of polypropylene matrix composites were investigated. Surface treatment of the CNTs was performed by using 3-Amino propyl tri ethoxy silane (APTES). Composites were prepared by means of extrusion and injection molding techniques. FT-IR analysis of the CNT samples revealed the successful surface modification of nanotubes after silane treatment. XRD results showed that chemically functionalized carbon nanotubes have the same cylinder wall and crystalline structure with untreated carbon nanotubes. Tensile and dynamic mechanical analysis (DMA) test results revealed that GF and nanotube reinforced hybrid composites exhibited better tensile strength and modulus values than only GF or CNT reinforced composites. According to all these results, it can be concluded that simultaneous usage of glass fiber and carbon nanotube in the composites increases the reinforcing ability of nanotubes in polymer composites.

Upgrading non-oxidized carbon nanotubes by thermally decomposed hydrazine

We found that the electrical properties of conductive thin films based on non-oxidized carbon nanotubes (CNTs) could be further improved when the CNTs consecutively underwent a mild hydrazine adsorption treatment and then a sufficiently effective thermal desorption treatment. We also found that, after several rounds of vapor-phase hydrazine treatments and baking treatments were applied to an inferior single-CNT field-effect transistor device, the device showed improvement in Ion/Ioff ratio and reduction in the extent of gate-sweeping hysteresis. Our experimental results indicate that, even though hydrazine is a well-known reducing agent, the characteristics of our hydrazine-exposed CNT samples subject to certain treatment conditions could become more graphenic than graphanic, suggesting that the improvement in the electrical and electronic properties of CNT samples could be related to the transient bonding and chemical scavenging of thermally decomposed hydrazine on the surface of CNTs.

Surface and electrosurface characterization of surface-oxidized multi-walled N-doped carbon nanotubes

In our experiments, N-doped multi-wall carbon nanotubes (CNTs) were produced by chemical vapor deposition (CVD method). The bamboo-shaped carbon nanotubes were treated with various oxidizing agents, namely H2O2, HNO3, HClO4 and a binary mixture of H2SO4/HNO3 acid. The purity of the CNTs was tested with thermo-gravimetric methods.TEM and FTIR microscopy was used to investigate the relative content of oxygen-containing functional groups formed on the CNT surfaces. For the characterization of the electric double layer (EDL) of the surface-functionalized carbon nanotubes, the zeta-potential of the CNTs samples was measured as a function of pH and electrolyte concentration (for 1–1, 21–12 and 31–13 electrolytes). It has been shown that the electrokinetic behavior of CNTs essentially resembles the behavior of lyophobic colloids: (a) the ζ (CKCl) curves reveal a maximum or plateau region; (b) the addition of CaCl2 leads to a sharp decrease in the ζ-potential; (c) the increase of trivalent counterions leads to/causes a charge reversal of the surface. A correlation between the sedimentation stability of CNTs suspensions and the zeta-potential of nanotubes has been found.

Removing Aggregates from Single-Walled Carbon Nanotube Samples by Magnetic Purification

Purification of raw single-walled carbon nanotube (SWCNT) material remains an important challenge in nanotube research and applications. We describe here a simple but effective purification method that uses permanent magnets to remove many nanotube aggregates, as well as residual metallic catalyst, from aqueous suspensions of surfactant-coated SWCNTs. Samples have been characterized by optical absorption, fluorescence, and Raman spectroscopies; atomic force microscopy and near-infrared fluorescence microscopy; and thermogravimetric analysis. It is found that magnetic purification reduces absorption backgrounds and increases average fluorescence efficiencies to levels comparable to those in ultracentrifuged samples. The ratio of individualized SWCNTs to aggregates in magnetically processed HiPco samples is estimated to be approximately 4:1. As compared to ultracentrifugation, magnetic processing promises major advantages in cost, simplicity, energy consumption, and scalability.

Determination of Nanoparticle Surface Coatings and Nanoparticle Purity Using Microscale Thermogravimetric Analysis

The use of nanoparticles in some applications (i.e., nanomedical, nanofiltration, or nanoelectronic) requires small samples with well-known purities and composition. In addition, when nanoparticles are introduced into complex environments (e.g., biological fluids), the particles may become coated with matter, such as proteins or lipid layers. Many of today's analytical techniques are not able to address small-scale samples of nanoparticles to determine purity and the presence of surface coatings. Through the use of an elevated-temperature quartz crystal microbalance (QCM) method we call microscale thermogravimetric analysis, or μ-TGA, the nanoparticle purity, as well as the presence of any surface coatings of nanomaterials, can be measured. Microscale thermogravimetric analysis is used to determine the presence and amount of surface-bound ligand coverage on gold nanoparticles and confirm the presence of a poly(ethylene glycol) coating on SiO2 nanoparticles. Results are compared to traditional analytical techniques to demonstrate reproducibility and validity of μ-TGA for determining the presence of nanoparticle surface coatings. Carbon nanotube samples are also analyzed and compared to conventional TGA. The results demonstrate μ-TGA is a valid method for quantitative determination of the coatings on nanoparticles, and in some cases, can provide purity and compositional data of the nanoparticles themselves.

Investigation on Mechanical Properties of Nano Ferrous Composite

Nowadays metal matrix composites (MMCs) find application in various fields. Metals like Aluminium, Magnesium and Nickel etc., have been mixed with Carbon Nanotubes (CNTs) as reinforcement to prepare MMCs. There exists a growing interest in CNTs reinforced metal matrix composites (MMCs) due to the fact that CNTs can serve as very good reinforcing agents for MMCs. Reinforcement of the matrix metal with CNTs leads to enhancement in mechanical properties without any considerable increase in weight of the material. In this study, 3 different samples of multi-walled carbon nanotubes (MWNTs) reinforced iron composites were fabricated by using powder metallurgy technique by mixing the powdered iron and MWNTs manually and ball milling process, compacting the specimen using a cylindrical die and then sintering it. The compaction die was designed and fabricated using die steel. The various testing like tensile, compressive, hardness and chemical were carried out and surface characteristics were analysed. The test results show that the mechanical properties were found to be improved significantly when compared to unreinforced iron.

Microwave to Terahertz: Characterization of Carbon-Based Nanomaterials

Microwave engineering has been an exciting forefront of modern technology and one of the major enablers of the fast-expanding information era. Since its formal establishment in the 19th century, this classic field has experienced many revolutions powered by discoveries of new materials and inventions of related devices. Rapid developments in nanotechnology in recent years have offered exciting possibilities for revolutionary discoveries in many branches of human endeavor. Nanomaterials and associated devices are being widely studied and developed for applications in electronics, optics, biology, energy, etc. Although it has been suggested that nanomaterials such as carbon nanotubes (CNTs) and graphene-based devices may work well in the microwave or even terahertz (THz) range [1], [2], most of the previous measurements were done at dc or lower frequency ( f <; 300 MHz) [3]-[5]. The characterization techniques of nanomaterials at microwave and THz frequency are important for both fundamental research and practical applications before proposed components such as antennas, interconnections, and circuit building blocks [6]-[8] can be realized. In this article, a review of characterization methods and associated challenges for various CNT and graphene samples from microwave to THz frequencies is presented, and pros and cons of the approaches are highlighted.