Crystallinity Of Carbon Nanotubes Research Articles

Enhancing purity and crystallinity of carbon nanotubes by magnetically assisted arc discharge and thermal purification and their field emission characteristics

This study used a magnetically assisted arc discharge (AD) method and thermal purification to enhance the purity and crystallinity of multi-walled carbon nanotubes (MWCNTs). CNT synthesis in the presence of a magnetic field increased the densities of ionic carbon species and the arc plasma temperature, thereby improving CNT production. AD-MWCNTs produced at an optimal magnetic strength exhibited enhanced purity, crystallinity, and thermal stability. Thermal purification was also conducted to increase the purity of the AD-MWCNTs further. Purity was evaluated using Raman spectroscopy. AD-MWCNTs were homogenized to overcome limitations in light-penetration depths to ensure accurate evaluation. The calculated purities were consistent with the analytical results, confirming the reliability of Raman spectroscopy for purity assessment. The number of working CNT emitters on the film surface was directly related to the purity. Highly pure crystalline AD-MWCNTs exhibited high-performance field-emission properties, namely a high current density of 2.7 A/cm2 and long-term emission stability (12 h at a current density of 500 mA/cm2), with no performance reduction.

Gas flow–directed growth of aligned carbon nanotubes from nonmetallic seeds

Kite growth is a process that utilizes laminar gas flow in chemical vapor deposition to grow long, well-aligned carbon nanotubes (CNTs) for electronic application. This process uses metal nanoparticles (NPs) as catalytic seeds for CNT growth. However, these NPs remain as impurities in the grown CNT. In this study, nanodiamonds (NDs) with negligible catalytic activity were utilized as nonmetallic seeds instead of metal catalysts because they are stable at high temperatures and facilitate the growth of low-defect CNTs without residual metal impurities. Results demonstrate the successful growth of over 100-μm-long CNTs by carefully controlling the growth conditions. Importantly, we developed an analysis method that utilizes secondary electron (SE) yield to distinguish whether or not CNTs grown from metal impurities. The absence of metallic NPs at the CNT tips was revealed by the SE yield mapping, whereas the presence of some kind of NPs at the same locations was confirmed by atomic force microscopy (AFM). These results suggest that most of the aligned CNTs were grown from nonmetallic seeds, most likely ND-derived NPs, via the tip-growth mode. Structural characterizations revealed the high crystallinity of CNTs, with relatively small diameters. This study presents the first successful use of nonmetallic seeds for kite growth and provides a convincing alternative for starting materials to prepare long, aligned CNTs without metal impurities. The findings of this study pave the way for more convenient fabrication of aligned CNT-based devices, potentially simplifying the production process by avoiding the need for the removal of metal impurities.

Control of morphology and crystallinity of CNTs in flame synthesis with one-dimensional reaction zone.

The growth of carbon nanotubes (CNTs) in a flame requires conditions that are difficult to achieve in a highly heterogeneous environment. Therefore, the analysis of the properties of the reaction zone within the flame is critical for the optimal growth of CNTs. In the present study, a comprehensive comparison between the CNT synthesis using a methane diffusion flame and a premixed flame is conducted regarding the morphology and crystallinity of the as-grown nanotubes. The premixed burner configuration created a flame that is stabilized through axisymmetric stagnation flow through sintered metal with one-dimensional geometry, different from a conventional co-flow flame. The significant difference in temperature distribution between the two flames causes a difference in the characteristics of the growth products. In the diffusion flame, the growth is limited to specific regions at certain height-above-burner (HAB) values with a temperature range of 750 to 950 °C at varying radial locations. The identified growth regions at different HAB values showed similar temperature distributions that yield CNTs of similar characteristics. Interestingly, the growth of CNTs in the premixed flame is dictated by only the HAB because the temperature distribution is relatively uniform along the radial directions but significantly different in the vertical direction. 17.3% variation in temperature in the axial direction successfully led to 44% and 66% variation in CNT diameter and crystallinity, respectively. The morphology control capability demonstrated in the present study is important for CNT functionalization for energy storage, nanosensor, and nanocomposite applications, where diameter and crystallinity are influential properties that govern the overall performance of the components.

Temperature dependence of Raman shift in defective single-walled carbon nanotubes

Raman scatterings of both pristine and defective single-walled carbon nanotubes were measured. Defects on carbon nanotubes (CNTs) were induced by UV/O3 treatment, and the correlation between the temperature dependence of the Raman shift of the G-band and the crystallinity of CNTs was investigated. In the temperature range of 250–600 K, a gradual negative change in the slope was observed; the linear shift of the Raman G-band frequency with respect to temperature increased as the crystallinity deteriorated. This phenomenon is attributed to the increase in the fourth-order phonon–phonon scattering interaction resulting from the induced defects.

Influence of Carbon Nanotube Attributes on Carbon Nanotube/Cu Composite Electrical Performances

Carbon nanotube (CNT)/copper composites offer promise as lightweight temperature-stable electrical conductors for future electrical and electronic devices substituting copper. However, clarifying how constituent nanotube structures influence CNT/Cu electrical performances has remained a major research challenge. Here, we investigate the correlation between the CNT/Cu electrical performances and nanotube structure by preparing and characterizing composites containing nanotubes of different structural attributes. We prepared three types of composites—single-wall (SW)-CNT/Cu wires, SW-CNT/Cu pillars, and multi-wall (MW)-CNT/Cu wires. The composites were fabricated from the corresponding CNT templates by two-step Cu electrodeposition, which retains template nanotube attributes through the fabrication process. The nanotube characteristics (diameter, G/D, alignment, etc.) in each template as well as the internal structure and electrical performances of the corresponding composites were characterized. SW-CNT/Cu wires and pillars outperformed MW-CNT/Cu wires, showing ≈ 3× higher room-temperature four-probe conductivities (as high as 30–40% Cu-conductivity). SW-CNT/Cu also showed up to 4× lower temperature coefficients of resistances i.e., more temperature-stable conductivities than MW-CNT/Cu. Our results suggest that few-walled small-diameter nanotubes can contribute to superior temperature-stable CNT/Cu conductivities. Better CNT crystallinity (high G/D), fewer nanotube ends/junctions, and nanotube alignment may be additionally beneficial. We believe that these results contribute to strategies for improving CNT/Cu performances to enable the real-world application of these materials as Cu substitutes.

Effects of ultrasonication on the microstructures and mechanical properties of carbon nanotube films and their based composites

Carbon nanotube (CNT) film has been considered as a promising preform of CNTs for developing high performance composites that possess high content of CNTs. However, due to the intertube van der Waals interactions, CNTs in the films are often tightly bundled together, making the impregnation of resin materials to CNTs very challenging. In this work, an eco-friendly ultrasonic-assisted resin infiltration process has been developed, and its effects on CNT bundles diameter, CNT crystallinity, CNT film stretchability, and the mechanical properties of CNT films and their based composites have been systematically investigated. It has been found that CNT bundles became thinner and the film stretchability became higher upon the ultrasonication, and thus the performance of CNT film-based composites were improved. However, extensive sonication would severely destroy the crystal structures of CNTs and thus degrade the mechanical properties of CNT films and their based composites. The underlying mechanisms for the evolution of CNT networks during the ultrasonic treatment were also discussed.

Polypropylene waste-derived carbon nanotubes (CNTs) via single-stage CVD technique: Determination of crystallinity

Basic structural information and carbonic composition of carbon nanotubes (CNTs) are necessary in quantifying their purity and possible applications. Utilizing plastic-derived carbon nanotubes (CNTs) in different fields is sometimes limited due to the presence and effect of large amount of amorphous carbon and metals residues arising from the catalyst during their synthesis. These materials result in defects in CNTs which are detrimental to the mechanical strength and electrical conductivity of the obtained CNTs. As a follow-up on a recent study, evaluation of crystallinity of CNTs obtained from waste polypropylene (PP) using a single stage chemical vapour deposition (CVD) technique over NiMo/MgO catalysts, prepared by different methods (sol-gel and incipient wet impregnation methods), via X-ray diffraction (XRD) analysis is reported in this study. The obtained crystallinity was compared to that of commercially available CNTs. It was observed that CNTs obtained over sol-gel prepared catalyst displayed almost similar fingerprint crystallinity to that of the commercially obtained CNTs.

Oxygen evolution activity limits the nucleation and catalytic growth of carbon nanotubes from carbon dioxide electrolysis via molten carbonates

Controlled catalytic synthesis of technologically valuable carbon nanomaterials by the electrochemical capture and conversion of CO2 is bottlenecked by limited understanding of combined electrochemical and catalytic mechanisms at play during conversion. Here, our findings provide new insight that a key factor for the electrochemical two-electrode catalytic growth of carbon nanotubes (CNTs) on the cathode surface is the oxygen evolution activity (OEA) occurring at the anode. Poor OEA can inhibit catalytic CNT formation altogether, while increased anode OEA is correlated to changes in diameter and crystallinity of CNTs with otherwise identical cathode catalyst layers and synthesis conditions. Specifically, comparing Cu, Ni/Al2O3, and Pt anodes with Fe/stainless steel cathodes we observe over 6x smaller CNT diameter distribution and over 2x lower D/G ratio when using Pt anodes with the highest OEA. This is attributed to the role of transport between the cathode and anode during synthesis; in the case of poor anode OEA, a build-up of oxide species near the cathode surface influences the surface tension at the catalyst-carbonate interface to modify CNT diameter and properties. This mechanistic insight opens the door to routes to controllably produce high quality carbon nanomaterials from the electrochemical capture and conversion of carbon dioxide.

Effect Grinding of Graphite on Structural and Morphological Characteristics of Carbon Nanotubes Grown by Microwave Oven

The influence of graphite grinding time on the formation of carbon nanotubes (CNTs), is investigated. Graphite with different grinding time is used for the growth of CNTs by a cost-effective method using a microwave oven. The samples produced using the different grinding time contain nanotubes with an average diameter in the range 31–50 nm as observed by field emission scanning electron microscopy (FESEM). The lowest intensity ratio of D and G bands (ID/IG) and full width at half maximum of G as identified by Raman spectroscopy for grinding time 20 minute indicates the improved crystallinity of CNTs.

Cost-effective single-step carbon nanotube synthesis using microwave oven

This paper reports the characterization of carbon nanotubes (CNTs) synthesised using a conventional microwave oven method, offering several advantages including fast, simple, low cost, and solvent free growth process. The procedure involves flattening of graphite/ferrocene mixture catalyst inside the microwave oven under ambient conditions for a very short duration of 5 s, which inhibits the loss factor of graphite and ferrocene. The effect of graphite/ferrocene mixture ratio for the synthesis of CNTs is investigated by transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), x-ray diffraction (XRD), Raman spectroscopy and UV–NIR–Vis measurements. The samples produced using the different ratios contain nanotubes with an average diameter in the range 44–79 nm. The highest yield of CNTs is attained with graphite/ferrocene mixture ratio of 70:30. The lowest ID/IG ratio intensity as identified by Raman spectroscopy for 70:30 ratio indicates the improved crystallinity of CNTs. Due to the capillary effect of CNTs, Fe nanoparticles are found to be encapsulated inside the tubes at different positions along the tube length. The obtained results showed that the smaller the diameter of graphite and ferrocene favors the synthesis of graphene oxide upon microwave radiation.

Effects of defects on thermoelectric properties of carbon nanotubes

Carbon nanotubes (CNTs) have recently attracted attention as materials for flexible thermoelectric devices. To provide a theoretical guideline of how defects influence the thermoelectric performance of CNTs, we theoretically studied the effects of defects (vacancies and Stone-Wales defects) on their thermoelectric properties; thermal conductance, electrical conductance, and Seebeck coefficient. The results revealed that the defects most strongly suppress the electron conductance, and deteriorate the thermoelectric performance of a CNT. By plugging in the results and the intertube-junction properties into the network model, we further show that the defects with realistic concentrations can significantly degrade the thermoelectric performance of CNT-based networks. Our findings indicate the importance of the improvement of crystallinity of CNTs for improving CNT-based thermoelectrics.

Continuous synthesis of well-crystalline VACNTs using CVD method for engineering applications

The extraordinary properties exhibited by carbon nanotubes (CNTs) make them one of the most active research studies among the researchers. In this paper, the optimum growth conditions for the synthesis of highly crystalline CNTs in order to improve their physical properties were presented. Chemical vapour deposition (CVD) was used to synthesise well-crystalline CNTs with uniform orientation using camphor oil as carbon source and argon gas as a carrier gas in the presence of ferrocene as a metal catalyst. The effects of four critical parameters, namely gas-flow rate, temperature, reaction time and the ratio of catalyst to carbon source on the crystallinity of CNTs, were investigated. The FESEM and TEM images, supported by the plot of Raman spectra emphasised the morphological improvement of the grown CNTs. It was found that crystalline CNTs can be synthesised using CVD method via restriction of the effective parameters.

Preparation, characterization, and application of vertically aligned CNT sheets through template assisted pyrolysis of PBI‐Kapton

This article investigates the synthesis of vertically aligned carbon nanotube (CNT) sheets through the pyrolysis of polybenzimidazole (PBI)‐Kapton inside the pores of anodized aluminium oxide (AAO). The nanocomposites of CNT/Alumina obtained were characterized by several techniques. It was found that the resulting carbon structures presented higher crystallinity compared with graphite. All of the CNT sheets had high antibacterial activity that was proportional to degree of CNT crystallinity. Moreover, the desalination of salty water using CNT sheets was performed. The results demonstrated that the vertically aligned CNT sheets could be used as an effective adsorbent for salty water desalination due to their very high adsorption capacity without CNT leakage into water.

Densification effects of the carbon nanotube pillar array on field-emission properties

In this study, a simple densification method for carbon nanotube (CNT) pillars is proposed to achieve high-performance field emission characteristics and stable emission. Through capillary force during solution evaporation, the CNT density in each pillar can be increased by about six times without causing damage to the crystallinity of CNTs. The densified CNT pillars exhibit lower series resistance, sharper pillars, better contacts, higher thermal conductivity, and better mechanical stiffness than as-grown ones. Therefore, the threshold field of the field emitter with such CNT pillars of 50 µm height can be reduced to 1.98 V/µm, as compared with 2.2 V/µm for the undensified ones. Moreover, the fluctuation of field-emission current decreases from 15.5 to 9.4% after the stress tests at a field of 2 V/µm for 1800 s. These findings imply that the densified CNT pillars are promising for the field-emission applications.

Diameter-control in synthesis of carbon nanotubes inside porous stainless steel block and application to glucose fuel cell electrode

Carbon nanotubes (CNTs) can be synthesized on inner surface of a porous stainless steel block by feeding ethanol or methanol as carbon source. In comparison with a conventional CNT growth in the porous stainless steel block by feeding ethylene, the diameter of CNTs in the newly-developed conditions using the alcohols are significantly smaller. In these conditions, a radial breathing mode were observed in Raman spectra, indicating the presence of single-walled carbon nanotubes. When the porous stainless steel block including CNTs directly grown on its inner surface is used for an anode in a glucose fuel cell, its power generation using the new CNT growth condition with ethanol or methanol can be improved by approximately three times higher than using the conventional condition with ethylene. Compared with the two alcohols used here, the fuel cell using the anode with the CNTs synthesized from methanol can exhibit 10% higher power than that from ethanol. According to Raman shift analysis and TEM observation, the surface area and the crystallinity of CNTs seem to be the key factors to realize the high electric generation performance.

Field-Emission Characteristics of the Densified Carbon Nanotube Pillars Array

In this study, the densification effects of carbon nanotube (CNT) pillar array have been proved to significantly increase the field emission characteristics. Without damage to the crystallinity of CNTs, the CNT density of each densified pillar can be increased about 6.25 times as compared to the as-grown ones. Consequently, the densified CNT pillars exhibit higher electrical conductivity, better contacts, and more emission sites, which are favorable to the field emission properties. What's more, the optimum field emission characteristics occur at the condition of R/H = 2 with H = 30 μm for both of the as-grown and densified samples because of the minimum screening effect and the proper aspect ratio. Therefore, the field emitter with densified CNT pillars of 30-μm in height and 60-μm in interspacing possesses the lowest turn on and threshold fields of 0.73 and 1.29 V/μm, respectively. It implies that the densified CNT pillars are promising for the field-emission applications.

Excellent electrode material of carbon nanotube macro-fibers for electric arc generator

Macroscopic carbon nanotube (CNT) fibers approximately 1 mm in diameter and 6 m long were prepared from CNT socks. The as-prepared macro-fiber exhibited excellent flexibility and manipulability. Microscopic examination showed that the fibers mainly consisted of CNTs with a fine graphitic structure and a large diameter (approximately 4.6 nm to 6.3 nm). Electric arcs were generated when the macro-fiber came in contact with a metallic electrode at a voltage as low as 30 V. These arcs had high intensity, and the generated temperature was sufficiently high such that the metallic anode electrode (stannum or copper wire) easily melted. CNT crystallinity was only slightly damaged after the electric circuit was discharged. The high intensity of the generated electric arcs suggested the potential applications of the macro-fiber in welding, melting metals, or other related fields.

Preparation and growth mechanism of carbon nanotubes via catalytic pyrolysis of phenol resin

Using nickel nitrate as catalyst precursor, a cost effective method for large scale preparation of carbon nanotubes (CNTs) was investigated through catalytic pyrolysis of phenol resin in the temperature range of 600–1200°C under an Ar atmosphere. The morphology and structure of pyrolysed resin were characterised by X-ray diffraction, scanning electron microscope, transmission electron microscope and energy dispersive X-ray spectroscopy. The results show that the morphology and structure of CNTs depend on pyrolysis temperature, and the starting growth temperature of CNTs is ∼600°C. When the pyrolysis temperature is raised, the length and crystallinity of CNTs increase remarkably. High aspect ratio and well crystallised CNTs with average diameter of about 50–60 nm and micrometer scale length could be obtained at 1000°C. Scanning electron microscopy and transmission electron microscopy analysis reveals that the catalyst particle was located at the top of each CNT, which indicates the tip growth mechanism for CNTs. The growth process of CNTs will go through the following stages: decomposition of hydrocarbon components into the carbon atoms, dissolution, diffusion and segregation. The growth of CNTs agrees with vapour–solid (V-S) model.

The characteristics of carbon nanotubes grown at low temperature for electronic device application

For the application of carbon nanotubes (CNTs) in flexible electronic devices, the CNTs were grown on Corning 1737 glass substrate by microwave plasma enhanced chemical vapor deposition (MPECVD) method. To deposit the catalyst layer, TiN buffer layer of 200nm thickness and Ni catalyst layer of 60nm were first deposited on the glass by r.f. magnetron sputtering method. The CH4 and H2 gases are used as the synthesis gas of CNTs and the working pressure was about 2.13 kPa, and the substrate bias was about −200V. The growth time was from 2min to 5min and the microwave power was about 800W. The substrate temperature as the main parameter was changed from 400°C to 550°C. The structural properties of CNTs synthesized with the substrate temperature were investigated using Raman, field emission scanning electron microscopy, and transmission electron microscopy methods. The surface and electrical properties of CNTs grown by MPECVD method were studied by scanning probe microscopy and four-point probe methods. We obtained the multi-walled CNTs (MW-CNTs). Multi-walled CNTs were vertically grown on Ni/TiN/glass substrates below 500°C without any glass deformations. As the substrate temperature was increased, the crystallinity of CNTs was improved. Ni catalyst was found at the tip of CNT by the TEM observation and the grown CNTs were found to have a multi-walled with bamboo like structure.

Influence of Various Plasma Treatment on the Properties of Carbon Nanotubes for Composite Applications

In present work, the effects of hydrogen and oxygen plasma treatments on the structural properties of carbon nanotubes (CNTs) synthesized by catalytic CVD (Chemical Vapor Deposition) have been systematically investigated. Field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HRTEM), and Raman spectroscopy were used to characterize the microstructural changes of the CNTs. The oxygen plasma treatment resulted in that the nanoparticles were appeared at the surface of CNTs. At high r.f. power (300 Watt), the microstructure of CNT was changed from nanotube type to nano particles. Long plasma treatment time changed the CNT morphology dramatically. For hydrogen plasma, however, there was no change in microstructure of CNT From the Raman analysis, the crystallinity of CNT was deteriorated by the plasma treatment, regardless of plasma power, treatment time, and gas types. The CNTs treated in oxygen plasma for 90 min showed excellent dispersion properties in aqueous solution.