Crystalline Carbon Nanotubes Research Articles

Montmorillonite–carbon nanotube nanofillers by acetylene decomposition using catalytic CVD

Montmorillonite–carbon nanotube nanofillers (a binary hybrid material) were successfully synthesised for polymer reinforcing applications. Montmorillonite (Mt) supported iron oxide (20 and 25wt.%) catalysts were prepared using the conventional wet impregnation method and calcined at 450°C. In situ reduction of the catalysts and synthesis of CNT was carried out in a horizontal furnace at 750°C using acetylene along with nitrogen and hydrogen. The influence of gas composition on the carbon nanotube growth characteristics of montmorillonite supported iron oxide catalysts was studied. Growth experiments with two different gas compositions containing 5 and 8vol.% of acetylene were used to evaluate the effect of the concentration of acetylene gas on the yield, morphology and purity of the carbon nanotubes over each catalyst. Highly selective, crystalline and defect free carbon nanotubes were obtained at low acetylene concentration in the total gas flow.

Thin Films of Liquid Crystalline Phthalocyanines and their its Composites with Single-Walled Carbon Nanotubes: Properties and Alignment

In this study, liquid crystalline copper phthalocyanine and single walled carbon nanotubes were dispersed in solution to prepare novel composites. Thin films of these composites prepared by solving of the composite in dimetylformamide and then by spin coating the solution onto the substrates were studied by the methods of polarizing microscopy, polarized Raman and optical absorption spectroscopies. It was shown that pure CuPcR4forms the films with a herringbone arrangement of phthalocyanine molecules within adjacent columns. An addition of carbon nanotubes (0.5-2 wt %) to CuPcR4leads to formation of the films with edge-on orientation of the molecules. The higher conductivity values were found for the films of phthalocyanine containing SWCNT. The lateral conductivity tends to decrease with the increase of SWCNT concentration from 0.5 to 2 wt %.

Carbon nanotubes rooted montmorillonite (CNT-MM) reinforced nanocomposite membrane for PEM fuel cells

Nafion based nanocomposite membranes containing montmorillonite-carbon nanotubes (a binary hybrid material) were produced to develop high performance polymer electrolyte fuel cells. Multi walled carbon nanotubes were grown over 20 and 25 wt% iron loaded montmorillonite catalysts by CVD using acetylene as the carbon precursor. Growth experiments were carried out at optimised conditions to obtain highly selective crystalline carbon nanotubes. X-ray diffraction spectra of the catalysts were recorded for the structural characterisation and definition of particle size. The carbon nanotubes obtained were examined by various physico chemical characterisation studies such as SEM, TEM, Raman spectroscopy and TG analyses to understand the morphology and crystallinity of the CNTs. The MM-CNT hybrid material with ID/IG ratio of Raman spectral band as 0.53 represents the high selectivity towards CNTs. Thus the hybrid material produced was considered as the best nanofiller to develop polymer nanocomposites. Nafion based nanocomposite membranes were prepared by adding MM-CNT as nanofiller by solution casting method. A better dispersion of MM-CNT into the Nafion matrix was observed and the addition of the MM-CNT improved the thermal stability of the Nafion membrane.

Thermal stability of styrene–(ethylene butylene)–styrene-based elastomer composites modified by liquid crystalline polymer, clay, and carbon nanotube

The thermal decomposition behaviors of styrene–(ethylene butylene)–styrene (SEBS) thermoplastic elastomer filled with liquid crystalline polymer (LCP), organomontmorillonite (OMMT), and carbon nanotube (CNT) as a heat stabilizing filler, were comparatively investigated using nonisothermal- and isothermal-thermogravimetric analyses in air. The isoconversional method was employed to evaluate the kinetic parameters (Ea, lnA, and n) under dynamic heating. For neat samples, OMMT and CNT exhibited their respective lowest and highest thermal stabilities as revealed from the lowest and the highest Tonset values, respectively. The decomposition rates of the composites containing OMMT at the temperature >250 °C were higher than those containing CNT and LCP, respectively, whereas the elastomer matrix degraded with the highest rate. The obtained TG profiles and calculated kinetic parameters indicated that the incorporation of LCP, OMMT, and CNT into elastomer matrix improved the thermal stability. Especially, the CNT- and OMMT-containing composites significantly improved the thermal stability compared with the neat matrix polymer. Simultaneously recorded DSC thermograms revealed that the degradation processes for the neat polymers and their composites were exothermic in air. From the simultaneously recorded DSC data, the enthalpy of thermal decomposition for each composite system was found to be lower than that of the neat matrix and mostly decreasing with increasing filler loading. The isothermal decomposition stabilities of the neat SEBS and its composites containing the different fillers were in agreement with those of the nonisothermal investigation.

Photoconductive Hybrid Films via Directional Self‐Assembly of C60 on Aligned Carbon Nanotubes

AbstractHybrid nanostructured materials can exhibit different properties than their constituent components, and can enable decoupled engineering of energy conversion and transport functions. Novel means of building hybrid assemblies of crystalline C60 and carbon nanotubes (CNTs) are presented, wherein aligned CNT films direct the crystallization and orientation of C60 rods from solution. In these hybrid films, the C60 rods are oriented parallel to the direction of the CNTs throughout the thickness of the film. High‐resolution imaging shows that the crystals incorporate CNTs during growth, yet grazing‐incidence X‐ray diffraction (GIXD) shows that the crystal structure of the C60 rods is not perturbed by the CNTs. Growth kinetics of the C60 rods are enhanced 8‐fold on CNTs compared to bare Si, emphasizing the importance of the aligned, porous morphology of the CNT films as well as the selective surface interactions between C60 and CNTs. Finally, it is shown how hybrid C60–CNT films can be integrated electrically and employed as UV detectors with a high photoconductive gain and a responsivity of 105 A W−1 at low biases (± 0.5 V). The finding that CNTs can induce rapid, directional crystallization of molecules from solution may have broader implications to the science and applications of crystal growth, such as for inorganic nanocrystals, proteins, and synthetic polymers.

Catalytic Consequence of the Interface Between Iron Catalysts and Foils in Synthesis of Aligned Nanocarbons on Foils

Growth of aligned carbon nanotubes on different substrates such as titanium, copper, stainless steel and graphite foils has been investigated by different characterization techniques such as SEM, TEM, contact angle measurements and Raman spectroscopy. The study aims at gaining a better understanding of the effects of the surface properties of the foils on the alignment, diameter distribution, structure and the yield of nanocarbons. The results revealed that the surface tension of the foils has a significant influence on the size of the Fe catalyst particles on the foil surfaces obtained by ferrocene decomposition. The size of the Fe particles directly determined the structure and alignment of the nanocarbons on the foils. Small sized (about 10 nm) and highly crystalline carbon nanotubes were produced on foils such as Al and Ti with the low surface tension, while it is hard to grow any nanocarbon on the Cu foils due to too high surface tension. Lowering the surface tension of the Cu foils by surface oxidation made it possible to grow carbon nanofibers.

Fabrication of Highly Crystalline NbO<SUB><I>x</I></SUB> Nanotube/Cup-Stacked CNT Nanocomposites

Carbon nanotubes (CNTs) are promising catalyst supports for fuel cell applications. Metal oxide/CNT nanocomposites are also being studied for dye-sensitized solar-cell, photocatalyst, and sensor applications. The fabrication of nanocomposites consisting of highly crystalline NbOx nanotubes and cup-stacked carbon nanotubes (CSCNTs) is reported herein. The CSCNTs were selected for the carbon materials because of their distinctive structure. The CSCNTs were photochemically treated with vacuum ultraviolet light, which increased the amount of oxygen-containing functional groups therein. NbOx nanotubes with no defects were successfully prepared with the chemical treatment of highly crystalline, layered, flux-grown K4Nb6O17 crystals. First, K4Nb6O17 crystals were grown from a KCl flux at a holding temperature of 800 degrees C. Next, NbOx nanosheets were prepared from the layered K4Nb6O17 crystals via a two-step exfoliation process, which consists of proton exchange in an acid solution and intercalation of the tetrabutylammonium ions. The NbOx nanosheets were rolled up into nanotubes with diameters of about 20 nm and lengths of 100-500 nm on the surfaces of the CSCNTs; thus, unique and complex NbOx/CSCNT nanocomposites were successfully fabricated.

Millimeter-Long Carbon Nanotubes: Outstanding Electron-Emitting Sources

We are reporting the fabrication of a very efficient electron source using millimeter-long and highly crystalline carbon nanotubes. These devices start to emit electrons at fields as low as 0.17 V/μm and reach threshold emission at 0.24 V/μm. In addition, these electron sources are very stable and can achieve a peak current density of 750 mA cm(-2) at only 0.45 V/μm. In order to demonstrate intense electron beam generation, these devices were used to produce visible light by cathodoluminescence. Finally, density functional theory calculations were used to rationalize the measured electronic field emission properties in open carbon nanotubes of different lengths. The modeling establishes a clear correlation between length and field enhancement factor.

Bulk Synthesis of Narrow Diameter and Highly Crystalline Triple‐Walled Carbon Nanotubes by Coalescing Fullerene Peapods

Narrow diameter and highly crystalline triple-walled carbon nanotubes (TWNTs) are prepared from high-temperature treatment of double-walled carbon nanotubes encapsulating fullerenes at high temperatures in the range of 1500–2400 °C. The TWNTs have high structural stability and unique physical properties that are observed in the Raman and fluorescent signals. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Bending Deformation Mechanism and Defective Properties of Liquid Crystalline Carbon Nanotubes in Evaporating Droplets

Here, we explore the liquid crystalline behavior of carbon nanotubes (CNTs) through two types of commonly physical parameters (the Frank elastic constants kii (i = 1, 2, 3) in the liquid crystal field that typically describe the nature of molecular orientation, and the Young's modulus E that characterizes the peculiar mechanical properties of CNTs). Droplets of aqueous multiwall carbon nanotube (MWNT) suspensions are evaporated on silica wafer at room temperature in air and the resultant deposits exhibit the hallmark property of liquid crystalline formation, birefringence. And scanning electron microscopic (SEM) characterization further presents the concentric bending arrangement of MWNTs along the circumference at the edge. Then in analogy with liquid crystals, based upon the restraint of circular boundary as in the model of droplets, the CNTs' Frank elasticity is introduced and expresses the relationship with E, showing the dependence of nanotube rigidity and dimensions. By comparing the values of plane parameters k11 and k33, defective properties about CNT-assemblies are provided, with Frank elastic constant anisotropy (k11 ≠ k33) contributing to their various distortion structures in the central core of the defects.

碳纳米管的液晶性取向研究<br>Study on Liquid Crystalline Orientation for Carbon Nanotubes

碳纳米管的液晶性研究是当今碳纳米管研究领域的全新、前沿课题，其液晶性即有序取向性。这种取向有序来自模型化碳纳米管的排斥体积效应，并外现于典型的光学双折射现象。碳纳米管的棒状各向异性深入人心，它是纳米碳管经自组织形成取向液晶性缺陷的诱因，是高度一致纳米管取向形成的源泉，若经掺杂、剪切、外场等的作用，将实现由缺陷液晶性向高度有序液晶性的期待转变。本文着重从取向形貌对液晶性碳纳米管的取向特征进行了简要综述性介绍，强调碳纳米管的本征特性。碳纳米管的大尺寸使之取向液晶性清晰可见，而其自身的高杨氏模量特性及其间强的吸引相互作用亦使之液晶性取向易于呈现展曲性、有规可循。 The research on liquid crystallinity of carbon nanotubes (CNTs) is the novel and frontier subject. The liquid crystallinity is their orientation characteristic, which is from the effects of excluded volumes for CNTs and performs the typically optical birefringence. CNTs’ rodlike anisotropy lies in promoting formations of various orienting liquid crystalline defects and the nanotube uniform alignments, and the liquid crystalline defects will be expected to transform into highly uniform orienting structures with actions of doping, shearing, external fields and so on. The paper mainly makes simple reviews about orienting characteristics for liquid crystalline CNTs by means of morphology characterization, emphasizing their inherent properties. Magni-tudes of nanotubes make them visible under certain characterization (i.e. scanning electron microscopic characterization), as well as both of their inherent high Young’s modulus and the strong attractive interaction between them make the liquid crystallinity uncommon but regular (apt to behave splaying).

Precursor gas chemistry determines the crystallinity of carbon nanotubes synthesized at low temperature

Despite significant progress in carbon nanotube (CNT) synthesis by thermal chemical vapor deposition (CVD), the factors determining the structure of the resulting carbon filaments and other graphitic nanocarbons are not well understood. Here, we demonstrate that gas chemistry influences the crystal structure of carbon filaments grown at low temperatures (500 °C). Using thermal CVD, we decoupled the thermal treatment of the gaseous precursors (C 2H 4/H 2/Ar) and the substrate-supported catalyst. Varying the preheating temperature of the feedstock gas, we observed a striking transition between amorphous carbon nanofibers (CNFs) and crystalline CNTs. These results were confirmed using both a hot-wall CVD system and a cold-wall CVD reactor. Analysis of the exhaust gases (by ex situ gas chromatography) showed increasing concentrations of specific volatile organic compounds (VOCs) and polycyclic aromatic hydrocarbons (PAHs) that correlated with the structural transition observed (characterized using high-resolution transmission electron microscopy). This suggests that the crystallinity of carbon filaments may be controlled by the presence of specific gas phase precursor molecules (e.g., VOCs and PAHs). Thus, direct delivery of these molecules in the CVD process may enable selective CNF or CNT formation at low substrate temperatures. The inherent scalability of this approach could impact many promising applications, especially in the electronics industry.

High-Performance Rubber Sealant for Preventing Water Leaks

Considerable amounts of water are being lost due to leaks. One of the main reasons for experiencing such large water leaks is the premature failure of rubber sealants in valves. Water leaks commonly originate from the presence of chlorine ions in tap water. Here, we demonstrate that the incorporation of surface-modified and highly crystalline carbon nanotubes allows the rubber sealants to maintain their sealing properties without experiencing any distinctive degradation over a wide range of temperature and environmental conditions over their expected lifetime. It is envisaged that environmentally friendly nanotechnology will save huge amounts of energy for purifying and transporting water around the whole world.

Dual formation of carpets of large carbon nanofibers and thin crystalline carbon nanotubes from the same catalyst-underlayer system

A dense, micron-tall layer of carbon nanofibers (CNFs) was grown above a layer of carbon nanotubes (CNTs) during the same synthesis using a thick cobalt catalyst (15 nm). The CNFs had large diameters (100 nm) and were amorphous while the CNTs had small diameter (10–20 nm) and were crystalline. Base growth mechanism was at play for both the nanofibers and the nanotubes. High-resolution transmission electron microscopy characterization suggested that the main mechanisms leading to the growth of the two structures were based on the dewetting of the catalyst layer and its subsequent alloying with the Ta underlayer. We can extend these principles to grow diverse carbon nanostructures during the same synthesis using appropriate multilayer thin films for different applications, especially for electrochemical cells and supercapacitors.

Interactions between metals and carbon nanotubes: at the interface between old and new materials

The article reviews the interaction between crystalline metals and carbon nanotubes in nanocomposite systems. Starting with an introduction to the chemical interaction between metal atoms and graphitic layers, an overview of the fields of nanotechnology is given where metal-carbon interaction comes into play. The interface between metals and carbon nanotubes is of interest in junctions between nanotubes and their periphery, for example in metallic contacts for electronic devices or in metal supports for carbon nanotube components. Furthermore, metals determine the catalytic growth of carbon nanotubes. The behaviour of individual metal atoms in or on carbon nanotubes is treated as well as the interaction between crystalline metals and nanotube surfaces. Emphasis is put on the common mechanisms of metal-carbon interaction that play a role in such different fields as the electrical transport through a metal-nanotube contact or the catalytic growth of nanotubes from metal particles.

Thermally controlled synthesis of single-wall carbon nanotubes with selective diameters

High quality single-wall carbon nanotubes were synthesized on a MgO supported Fe-Co catalyst system using an inductive radio frequency catalytic chemical vapor deposition method with methane utilized as the hydrocarbon source. The synthesis temperature was varied between 700–1000 °C, and major differences when it comes to the size controllability of the catalyst nano-particles and the nanotube morphology were noticed when the reaction temperature was set at 800 and at 1000 °C. The structural and morphological properties of the catalyst system were analyzed by microscopy, X-ray diffraction and surface area analysis. The electron microscopy analyses of the catalyst system showed that the Fe/Co active metal nano-clusters have a very narrow size distribution when the catalyst system is thermally treated at 800 °C. As a result, highly crystalline single-wall carbon nanotubes with a narrow diameter distribution were successfully synthesized. It was found that by increasing the reaction temperature from 800 to 1000 °C, the diameter distribution of Fe/Co nano-particles broadens and hence affects the nanotube diameters. At 1000 °C the nanotubes presented a wider diameter distribution when compared to the ones grown at lower temperatures, a fact correlated to the changes in the catalyst structural morphology. Furthermore, the nanotubes synthesized at different reaction temperatures were analyzed using several techniques such as electron microscopy, thermogravimetric analysis, Raman and UV-Vis-NIR spectroscopy.

Simulations on the elastic response of amorphous and nanocomposite carbon

Theoretical calculations of the elastic response of carbon composites and amorphous carbon are reported. The studied composites consist of crystalline nanoinclusions, either spherical diamonds or carbon nanotubes, embedded in amorphous carbon matrices. The elastic constants of the composites were calculated and found to systematically increase as the density increases. The elastic recovery under hydrostatic pressure for all structures was also investigated and was found to be significantly high for both nanocomposite and amorphous carbon, but decreases as the material becomes more dilute.

Enhanced thermal conductivity of carbon fiber/phenolic resin composites by the introduction of carbon nanotubes

The authors report a significant enhancement in the thermal conductivity of a conventional carbon fiber/phenolic resin composite system when adding highly crystalline multiwalled carbon nanotubes. They demonstrate that 7wt% of carbon nanotubes dispersed homogeneously in a phenolic resin acted as an effective thermal bridge between adjacent carbon fibers and resulted in an enhancement of the thermal conductivity (e.g., from 250to393W∕mK). These results indicate that highly crystalline carbon nanotubes can be used as a multifunctional filler to enhance simultaneously the mechanical and thermal properties of the carbon fiber/phenolic resin composites.

Synthesis of Crystalline Carbon Nanotube Arrays on Anodic Aluminum Oxide Using Catalyst Reduction with Low Pressure Thermal Chemical Vapor Deposition

Well-aligned crystalline carbon nanotubes (CNTs) were synthesized on anodic aluminum oxide (AAO) template using low-pressure thermal chemical vapor deposition (TCVD). The AAO templates were fabricated using an anodization process, and Fe as a catalyst was electrochemically deposited inside the bottom of the pores on the AAO template. For promotion of catalytic reaction of Fe, pore widening and catalyst reduction were performed. The enhancement of the catalytic reaction between C2H2 and the Fe catalyst particles results in CNTs with good crystallinity and a high packing density without the deposition of amorphous carbon on the pore surface of the AAO templates. The diameter and density of the CNTs were approximately 50 nm and 7×109 CNTs/cm2 (70% of pore density), respectively. When the threshold voltage of the electron emission was measured with a gap of 300 µm, it showed a very low turn-on field of 0.87 V/µm and the field enhancement factor was greater than 5600.

Nitrogen in highly crystalline carbon nanotubes

Multiwall carbon nanotubes (MWCNTs) with an unprecedented degree of internal order were synthesised by chemical vapour deposition (CVD) adding a nitrogen-containing compound to the hydrocarbon feedstock. Ferrocene was used as the metal catalyst precursor. The remarkable crystallinity of these nanotubes lies both in the isochirality and in the crystallographic register of their walls, as demonstrated by electron diffraction and high resolution electron microscopy experiments. High resolution transmission electron microscopy analysis shows that the walls of the nanotubes consist of truncated stacked cones, instead of perfect cylinders, with a range of apex angles that appears to be related to the nitrogen concentration in the synthesis process. The structure of armchair, zigzag and chiral nanotubes is modelled and discussed in terms of density of topological defects, providing an interesting comparison with our microscopy experiments. A growth mechanism based on the interplay of base- and tip-growth is proposed to account for our experimental observations.