Gas adsorption studies of CO2 and N2 in spatially aligned double-walled carbon nanotube arrays

Gas adsorption studies (CO₂ and N₂) over a wide pressure range on vertically, highly aligned dense double-walled carbon nanotube arrays of high purity and high specific surface area are reported. At high pressures, the adsorption capacity of these materials was found to be comparable to those of metal organic frameworks and mesoporous molecular sieves. These highly aligned CNT arrays were chemically modified by treating with oxygen plasma and structurally modified by decreasing the diameter of individual carbon nanotubes. Oxygen plasma treatment led to grafting of a large number of C–O functional groups onto the CNT surface, which further increased the gas adsorption capacity. It was found that gas adsorption is dependent on tube diameter and increases with decrease of the individual CNT diameter in the CNT bundles. As results of our studies we have found that at lower pressure regimes, plasma functionalized carbon nanotubes exhibit better adsorption characteristics whereas at higher pressures, lower diameter carbon nanotube structures exhibited better gas adsorption characteristics.     

Oriented association of multiwall carbon nanotubes upon efficient epitaxial organization of polyfluorene

Upon the epitaxial crystallization of poly(9,9-di-n-octyl-2,7-fluorene) from prior smectic organization, originally dispersed carbon nanotubes were found to be segregated into noncrystalline regions in between stacked crystalline lamellae. Arrays of oriented nanotube bundles subsequently resulted upon the establishment of regular lamellar stacking. Dependent on the domain sizes of resultant composite crystals, these intercalated nanotube bundles continuously extended for several micrometers in a slightly wavy manner. When the stacking of crystalline lamellae of polymer matrix is less efficient, the separation between neighboring bundles become wider and less uniform. Upon the increase of the extent of undercooling, the lamellar stacking effectively arrested the migration of carbon nanotubes, and the decline of both bundles diameter and waviness was observed. Accordingly the stacking of crystalline lamellae emerges to serve as an adjustable template for organizing carbon nanotubes within thin film of conjugated polymers. The dispersion of carbon nanotubes within smectic phase modifies the ultraviolet absorption spectrum and also the packing symmetry of later developed crystal structure. The inevitably involved irregularity of backbone distortion manifests the prevalence of the favored attractive interactions between nanotubes and fluorene backbones, which is viewed to render the self-aggregation of nanotubes at domain boundaries less favorable.     

TEM image simulation study of small carbon nanotubes and carbon nanowire

Recent findings of extremely small diameter carbon nanotube and nanowire in the core of a multi-walled carbon nanotube (MWCNT) have attracted interests from broad range of researchers. Direct observation of carbon nanotube is usually done using a transmission electron microscope (TEM). When nanotubes become smaller, it becomes harder to correctly understand the TEM images, not only because of the weak scattering, but also due to the artifact that starts to appear because of the interference effect and the inappropriate defocus condition.In this study, we have shown that the artifact such as ghost fringes due to inappropriate defocus conditions of the TEM appear in the core of an MWCNT, and can be misinterpreted as either carbon nanowire or small carbon nanotube. It is also shown that, in the TEM image, it is hard to distinguish a single-walled nanotube bundle from a double-walled carbon nanotube bundle. Finally, we propose that the cross-sectional observation is necessary for the correct characterization of single- and double-walled carbon nanotube bundles.     

Selective Deposition and Alignment of Single-Walled Carbon Nanotubes Assisted by Dielectrophoresis: From Thin Films to Individual Nanotubes

Dielectrophoresis has been used in the controlled deposition of single-walled carbon nanotubes (SWNTs) with the focus on the alignment of nanotube thin films and their applications in the last decade. In this paper, we extend the research from the selective deposition of SWNT thin films to the alignment of small nanotube bundles and individual nanotubes. Electrodes with “teeth”-like patterns are fabricated to study the influence of the electrode width on the deposition and alignment of SWNTs. The entire fabrication process is compatible with optical lithography-based techniques. Therefore, the fabrication cost is low, and the resulting devices are inexpensive. A series of SWNT solutions is prepared with concentrations ranging from 0.0125 to 0.2 mg/ml. The alignment of SWNT thin films, small bundles, and individual nanotubes is achieved under the optimized experimental conditions. The electrical properties of these samples are characterized; the linear current–voltage plots prove that the aligned SWNTs are mainly metallic nanotubes. The microscopy inspection of the samples demonstrates that the alignment of small nanotube bundles and individual nanotubes can only be achieved using narrow electrodes and low-concentration solutions. Our investigation shows that it is possible to deposit a controlled amount of SWNTs in desirable locations using dielectrophoresis.     

Mechanical modelling of carbon nanomaterials from nanotubes to buckypaper

A combination of molecular structural mechanics and finite element method is used to mechanically model graphene, single-walled and multi-walled carbon nanotubes, nanotube bundles, buckypaper, and buckypaper composites. The mechanical model developed is used to determine the elastic properties of these nanostructures including elastic and shear moduli. In each step, different parameters are investigated including length and orientation of graphene sheets, chirality, diameter, number of walls and length of nanotubes, number of nanotubes in bundles, porosity of buckypaper and alignment of bundles in a buckypaper composite. The results are in good agreement with both the experimental results and those reported by other researchers either experimentally or theoretically.     

Modelling gas-phase synthesis of single-walled carbon nanotubes on iron catalyst particles

A simple model for the gas-phase synthesis of carbon nanotubes on iron catalyst particles has been developed. It includes a growth model for the catalyst particles and describes nanotube growth processes through carbon monoxide disproportionation and hydrogenation. Models for particle-particle interactions and sintering are also included. When carbon arrives at a catalyst particle it can either dissolve in the particle until a saturation limit is reached, or form a graphene layer on the particle, or go on to form a nanotube. Two models for incipient nanotube growth are considered. The first allows nanotubes to form once a catalyst particle reaches the saturation condition. The second only allows nanotubes to form on the collision of two saturated particles. The particle system is solved using a multivariate stochastic solver coupled to the gas-phase iron chemistry using an operator splitting algorithm. Comparison with experimental data gives a good prediction of the nanotube length, and reasonable values of catalyst particle diameter and nanotube diameter. A parametric study is presented in which the carbon monoxide reaction rate constants are varied, as is the fraction of carbon allowed to form nanotubes relative to surface layers. The assumptions of the coagulation and sintering models are also discussed.     

Liquid crystal behavior of single wall carbon nanotubes

Single wall carbon nanotubes are dispersed in water with the water-soluble polymer polyvinylpyrrolidone and the surfactant sodium dodecylbenzene sulfonate, and then deposited by evaporative deposition onto degeneratively-doped silicon wafer substrates. These deposits were examined by scanning electron microscopy, which revealed highly-ordered arrays of large single wall carbon nanotube bundles. Various solution concentrations were prepared and deposition conditions were varied to determine their affect on the single wall carbon nanotube arrays. These observations were related to existing lyotropic liquid crystal theory and theories explaining the behavior of carbon nanotubes in solution, which allowed for further development and interpretation of the phase diagram which describes the behavior of single wall carbon nanotubes in lyotropic liquid crystal systems, and how competing liquid crystal systems in the same solution directly affect the ordering of the single wall carbon nanotube arrays.     

High pressure-high temperature synthesis of diamond from single-wall pristine and iodine doped carbon nanotube bundles

High pressure and high temperature experiments were performed on single-wall carbon nanotube bundles up to 14.5 GPa and 1800 K. Depending on the thermodynamic conditions, we have observed three different behaviors: at ambient temperature and for pressure lower than 24 GPa, minor structural changes are observed. Depending on the loss of hydrostatic conditions or on the combined application of pressure and temperature, partial or total graphitization is observed. For pressures of 14.5 GPa and temperatures of 1800 K the nanotubes are irreversibly transformed into cubic diamond, showing that it is possible to synthesize under high pressure and high temperature pure sp³ carbon structures from single-wall carbon nanotubes. In the case of iodine intercalated nanotubes, the same conditions of 14.5 GPa and 1800 K lead also to the transformation into diamond. No evidence of incorporation of iodine in the sp³ carbon structure was found. On the basis of our results, we discuss possibilities for new carbon-carbon composite engineering from single-wall carbon nanotube bundles.     

End cap nucleation of carbon nanotubes

An easily applied graphical approach for facilitating precise tailoring during computational construction of model uncapped or capped single-wall nanotubes or fullerenes is delineated and utilized in this paper. The main enabling concepts are the commonly suggested growth mechanisms for single- and multi-walled carbon nanotubes mediated by end cap structures. The construction protocol described herein can be used to rapidly create any type of armchair, zigzag, chiral or nonchiral defect-free nanotube. Any desired, feasible combination of length and diameter, along with specific placement of hexagonal and pentagonal rings or end caps, can also be controlled. A classification system for end caps is also developed in order to help simplify choosing or describing the particular carbon system under investigation. The suggested methodology is used to systematically model heats of formation of a variety of carbon nanotubes and related fullerenes using AM1 semiempirical calculations. The main factors affecting the calculated physical properties, other than size, are the structures of the various base and terminating end caps. Finally, we comment on the possible relationship of the construction methodology to mechanisms for carbon nanotube nucleation.     

Synthesis of close-packed multi-walled carbon nanotube bundles using Mo as catalyst

A simple mixture of porous magnesium oxide and commercial molybdenum oxide shows high efficiency for the synthesis of carbon nanotubes through the catalytic decomposition of methane at 900 °C. Field emission scanning electron microscopy (FE-SEM), Raman spectroscopy, and transmission electron microscopy (TEM) were used to characterize the products. The results indicate that close-packed multi-walled carbon nanotube (MWCNT) bundles were synthesized and the carbon nanotubes restricted within the bundles were about 5-7 nm in diameter. A growth mechanism for the bundles was suggested based on the FE-SEM images of bundles produced using different reaction times, and the X-ray diffractions of the raw products grown at the initial stage. Raman spectroscopy and FE-SEM results on the bundles grown using different methane flow rates confirmed the growth mechanism of close-packed MWCNT bundles.     

Controlling the growth of single-walled carbon nanotubes on surfaces using metal and non-metal catalysts

Thanks to the development of controlled synthesis techniques, carbon nanotubes, a 20-year-old material, are doing better at finding practical applications. The history of carbon nanotube growth with controlled structure is reviewed. There have been two main categories of catalysts used for carbon nanotube growth, metal and non-metal. For the metal catalysts, the growth process and the mechanism involved have been adequately discussed, with a widely accepted vapor–liquid–solid growth mechanism. The strategies for preparing single-walled carbon nanotube samples with well-defined structures such as geometry, length and diameter, electronic property, and chirality have been well developed based on the proposed mechanism. However, a clear mechanism is still being explored for non-metal catalysts with a hypothesis of a vapor–solid growth mechanism. Accordingly, the controlled growth of carbon nanotubes with a non-metal catalyst is still in its infancy. This review highlights the structure-control growth approach for carbon nanotubes using both metal and non-metal catalysts, and tries to give a full understanding of the possible growth mechanisms.     

Structure modification of single-wall carbon nanotubes

Various purification processes were applied to single-wall carbon nanotubes synthesized by metal catalyzed laser ablation. Structure modifications introduced by these processes were investigated by high-resolution transmission electron microscopy and Raman spectroscopy. An apparent structure modification after purification was the increase of bundle size although breaking of nanotubes and a change of nanotube diameter distribution were also observed. More vigorous attacking of single-wall carbon nanotube structure was identified by a strong mixed-acid treatment.     

Electrical conductivity of pure carbon nanotube yarns

The porosity of multi-walled carbon nanotube yarns can be varied over a wide range by adjusting the yarn construction, resulting in a dramatic change in yarn electrical conductivity. When the yarn electrical conductivity is converted into specific conductivity, its value remains approximately constant irrespective of the changes in yarn construction and porosity. The process of carbon nanotube yarn production involves two key steps, the formation of a network of carbon nanotube bundles spliced together by the entanglement of individual nanotubes and the compaction of the network into a cylindrical yarn. The splices formed from entangled individual nanotubes play a much greater role in electrical conduction than the cross-over contact formed between CNT bundles by compaction during spinning.     

Synthesis of carbon nanotubes with and without catalyst particles

The initial development of carbon nanotube synthesis revolved heavily around the use of 3d valence transition metals such as Fe, Ni, and Co. More recently, noble metals (e.g. Au) and poor metals (e.g. In, Pb) have been shown to also yield carbon nanotubes. In addition, various ceramics and semiconductors can serve as catalytic particles suitable for tube formation and in some cases hybrid metal/metal oxide systems are possible. All-carbon systems for carbon nanotube growth without any catalytic particles have also been demonstrated. These different growth systems are briefly examined in this article and serve to highlight the breadth of avenues available for carbon nanotube synthesis.     

Quantum Capacitance Extraction for Carbon Nanotube Interconnects

Electrical transport in metallic carbon nanotubes, especially the ones with diameters of the order of a few nanometers can be best described using the Tomanaga Luttinger liquid (TL) model. Recently, the TL model has been used to create a convenient transmission line like phenomenological model for carbon nanotubes. In this paper, we have characterized metallic nanotubes based on that model, quantifying the quantum capacitances of individual metallic single walled carbon nanotubes and crystalline bundles of single walled tubes of different diameters. Our calculations show that the quantum capacitances for both individual tubes and the bundles show a weak dependence on the diameters of their constituent tubes. The nanotube bundles exhibit a significantly large quantum capacitance due to enhancement of density of states at the Fermi level.     

Lithium storage performance of carbon nanotubes prepared from polyaniline for lithium-ion batteries

Carbon nanotubes with large surface area and surface nitrogen and oxygen functional groups are prepared by carbonizing and activating of polyaniline nanotubes, which is synthesized by polymerization of aniline with the self-assembly method in aqueous media. The physicochemical properties of the carbon nanotubes are characterized by scanning electron microscope, transmission electron microscopy, X-ray diffraction, Brunauer–Emmett–Teller, elemental analyses and X-ray photoelectron spectroscopy measurements. The surface area and pore diameter are 618.9 m² g⁻¹ and 3.10 nm. The electrochemical properties of the carbon nanotubes as anode materials in lithium ion batteries are evaluated. At a current density of 100 mA g⁻¹, the activated carbon nanotube shows an enormously first discharge capacity of about 1370 mAh g⁻¹ and a charge capacity of 907 mAh g⁻¹. After 20 cycling tests, the activated carbon nanotube retains a reversible capacity of 728 mAh g⁻¹. These indicate it may be a promising candidate for an anode material for lithium secondary batteries.     

A method for wet spinning of alginate fibers with a high concentration of single-walled carbon nanotubes

A method is described for the wet spinning of alginate fibers with a loading of single-walled carbon nanotubes as high as 23 wt%. Electrostatic assembling of polyelectrolytes and nanotubes coated with sodium dodecyl sulfate is exploited by using calcium as a cross-linking agent. The Young’s modulus of these fibers depends non-monotonically on nanotube concentration which is explained using Halpin-Tsai and Voigt models. Scanning electron microscope micrographs and resistivity analysis of the fibers suggest that the nanotube-alginate system undergoes a morphological transition from a composite structure of discrete nanotube bundles embedded in an alginate matrix to a complex continuous structure consisting of a nanotube network interwoven into a macromolecular network of alginate. The nanotube-alginate fibers have unprecedented high flexibility and a very high electrical conductivity similar to semimetals (between germanium and carbon).     

Mass production of high-quality multi-walled carbon nanotube bundles on a Ni/Mo/MgO catalyst

A new catalyst (Ni/Mo/MgO) is reported, with which one can synthesize multi-walled carbon nanotube (MWNT) bundles with a yield of more than 45 times the amount of the pristine catalyst, using a methane-hydrogen mixture as precursor. Powder X-ray diffraction, Raman spectroscopy and thermal gravimetric analysis show that the purity of the as-prepared MWNTs is over 97%. The diameter of the carbon nanotubes is 9-20 nm, measured by high-resolution electron microscopy on 421 individual MWNTs. The high purity of the as-prepared MWNTs allows us to omit the usual complex purification process required for carbon nanotubes synthesized by chemical vapor deposition. Because of its durable high activity, the Ni/Mo/MgO catalyst in its pristine state is ideal for mass production of high-quality MWNTs. The synergism of nickel and molybdenum is considered the main reason for the high yield of carbon nanotubes.     

Texture development in Fe-doped alumina ceramics via templated grain growth and their application to carbon nanotube growth

Fe-doped alumina (Fe-Al₂O₃) materials with a controlled microstructure could be designed for some special uses such as a substrate for carbon nanotube growth. In this study, Fe-doped Al₂O₃ ceramics with varying degrees of texture were prepared via Templated Grain Growth method and utilized for carbon nanotube synthesis by Catalytic Chemical Vapor Deposition in order to investigate how α-Al₂O₃ crystal orientation affects carbon nanotube growth in polycrystalline ceramics. The degree of texture increased with the Fe content in the presence of liquid phase. Three kinds of carbon filaments (few-wall carbon nanotubes bundles, individual multi-wall nanotubes and carbon nanofibres) were observed over Fe-doped Al₂O₃ ceramics with varying degrees of texture depending on the surface roughness, crystallographic orientation and the size of the catalyst nanoparticles. While well-textured substrates with a rough surface led to a small amount of randomly oriented carbon nanotube bundles, perpendicularly oriented individual multi-wall nanotubes were obtained over relatively smooth single crystal α-Al₂O₃ platelet surfaces (basal planes) which remained in the matrix without growing.