Investigating the mechanism of collective bidirectional growth of carbon nanofiber carpets on metallic substrates

Bidirectional-growth of carbon nanofibers is a rare phenomenon found on free-standing catalyst particles, in contrast to the most commonly studied tip- and base-growth mechanisms for carbon nanostructures synthesized through thermal chemical vapor deposition. We reveal the underlying mechanisms of collective bidirectional growth in Ni_xPd_1−x-catalyzed carbon nanofiber carpets grown on a palladium substrate with varying nickel film thickness by monitoring the fiber growth evolution. The results show that the collective bidirectional growth is promoted and controlled by the chemical and physical restructuring of the sub-surface portion of the metal stack which undergoes micro-fragmentation as a result of the incorporation, diffusion, and precipitation of carbon. Carbon nanofiber growth can be controlled by engineering the catalyst-underlayer materials properties such as grain size, chemical composition and alloying. Since the determining factor whether carbon nanofibers or nanotubes are obtained is a strong function of catalyst size, the understanding of this growth mechanism can be transferred to the field of carbon nanotube synthesis. By keeping the grain size small enough to ensure carbon nanotube instead of carbon nanofiber growth, achieving dense, vertically aligned carbon nanotube carpets on metallic substrates might be possible, which is a prerequisite for carbon nanotube integration in integrated circuits.     

The effect of experimental parameters on the synthesis of carbon nanotube/nanofiber supported platinum by polyol processing techniques

Developing corrosion resistant carbon nanotube (CNT) and carbon nanofiber (CNF) supported Pt catalysts with optimized particle size is important for proton exchange membrane fuel cells. We investigated the effects of deposition technique (conventional refluxing and microwave irradiation), water content, carbon support and metal loading on the average Pt particle size and electrochemically active surface area (ECSA). BET surface area measurements, Fourier transform infrared spectroscopy, X-ray diffraction, thermal gravimetric analysis, transmission electron microscopy and cyclic voltammetry were used to characterize all the CNT/CNF supported catalysts. Processing was accelerated via microwave irradiation without significantly affecting Pt particle size as compared to conventional refluxing especially for high surface area CNT supported low Pt loading (10 wt%) catalyst. Adjusting the water content during synthesis effectively controlled the Pt particle size and size distribution regardless of the heating method, carbon support and metal loading. The ECSA of the samples was found to be dependent on Pt particle size which further depends on the water content during synthesis, support surface area and Pt loading. Optimization of deposition conditions leads to higher ECSA than seen in a commercially available carbon black supported catalyst.     

Synergetic carbon nanotube growth

We report an effect in vertically-aligned carbon nanotube growth in which small catalyst features in proximity to large features show an enhanced growth rate. We apply this so-called “synergetic growth” effect in micrometer-scale patterns to produce vertical-like growth horizontally. This approach enables carbon nanotube integration into device applications requiring a fair degree of carbon nanotube alignment. The synergetic growth effect corroborates growth mechanisms describing carbon nanotube growth as a more complex process than elemental carbon supersaturating metal catalyst.     

Carbon nanotube growth on conductors: Influence of the support structure and catalyst thickness

We investigate the formation and stability of Fe nanoparticles on TiN and poly-crystalline PtSi films, and their ability to grow carbon nanotubes forests. Using different-microstructure films, coated with or without their native oxides, we show that, upon purely-thermal catalyst pretreatment, PtSi favours the formation of homogenously sized nanoparticles and forest growth, partly due to its low surface energy. TiN, in contrast, leads to much less controllable processes and only when coated with its native oxide, or with thick catalyst films, yields large diameter nanotube forests. The microstructure of the material can dramatically limit catalyst diffusion into the bulk of the support during nanotube growth. These results allow us to establish the general behaviour expected for nanotube growth on any conductive materials.     

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.     

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.     

Gaseous adsorption of carbon tetrachloride onto carbon nanofiber arrays prepared by template-assisted synthesis

Adsorption of carbon tetrachloride onto aligned carbon nanofiber (CNF) arrays, prepared by a template-assisted synthesis, in vapor phase is conducted in the present study. Porous structure analysis indicated that various pore size distributions of the CNF arrays are found to vary with their tubular sizes. The increasing tubular size is accompanied by a decreasing micropore fraction as well as a vapor-phase adsorption capacity. Freundlich and Dubinin–Radushkevich models were employed to analyze the equilibrium adsorption data. The surface accessibility of CNF arrays, i.e., adsorption capacity per surface area, was found to decrease with the pore size, according to these models. It is suggested on the basis of the present work that the micropore fraction of CNFs plays an important role in determining the adsorption coverage. Both the equilibrium constant and free energy for the vapor-phase adsorption increase with the micropore proportion, indicating that the micropores act as a high-energy site for adsorption of carbon tetrachloride.     

Chemical vapour deposition of pyrolytic carbon on carbon nanotubes: Part 3: Growth mechanisms

As a first step to identify the growth mechanism of various pyrolytic carbon deposit morphologies onto multiwall carbon nanotubes (MWNTs) presented in earlier papers, we determined their growth chronology by carrying-out synthesis experiments involving a large time range. We propose that the formation of any of the deposit morphologies is the consequence of the primary formation of hydrocarbon liquid droplets in the gas phase and their subsequent deposition onto the MWNTs. This makes the formation mechanisms of the various deposit morphologies depend on physical phenomena related to the wetting of nanotube surfaces by the droplets, where the [droplet diameter]/[nanotube diameter] ratio plays an important role. The droplets are the result of the recombination of species issued from the cracking of the gaseous precursor (methane), and their characteristics (number, size, and aromaticity) depend on experimental parameters such as temperature, time of flight, and gas phase composition. The results bring a new light to the currently admitted hypotheses for the mechanisms of pyrolytic carbon deposition, and revitalise the liquid droplet theory formerly proposed by Grisdale in the 1950s, at least in the range of conditions investigated.     

Current understanding of the growth of carbon nanotubes in catalytic chemical vapour deposition

Due to its higher degree of control and its scalability, catalytic chemical vapour deposition is now the prevailing synthesis method of carbon nanotubes. Catalytic chemical vapour deposition implies the catalytic conversion of a gaseous precursor into a solid material at the surface of reactive particles or of a continuous catalyst film acting as a template for the growing material. Significant progress has been made in the field of nanotube synthesis by this method although nanotube samples still generally suffer from a lack of structural control. This illustrates the fact that numerous aspects of the growth mechanism remain ill-understood. The first part of this review is dedicated to a summary of the general background useful for beginners in the field. This background relates to the carbon precursors, the catalyst nanoparticles, their interaction with carbonaceous compounds and their environment. The second part provides an updated review of the influence of the synthesis parameters on the features of nanotube samples: diameters, chirality, metal/semiconductor ratio, length, defect density and catalyst yield. The third part is devoted to important and still open questions, such as the mechanism of nanotube nucleation and the chiral selectivity, and to the hypotheses currently proposed to answer them.     

First-principles based kinetic modeling of effect of hydrogen on growth of carbon nanotubes

We investigate the influence of hydrogen on carbon nanotube (CNT) growth in thermal catalytic chemical vapor deposition. Kinetic calculations of gas-phase transformations of hydrocarbons show that hydrogen interacts with gaseous carbon precursors, resulting in modification of the carbon supply rate to the catalyst particle. A surface-kinetic model of CNT growth is developed to study adsorption and decomposition kinetics of precursors on Ni catalyst particles. The detailed surface kinetics of carbon precursors and transport of carbon atoms through the catalyst particle are described in the framework of the surface/bulk site formalism, with the parameters of the reactions determined on the basis of first-principles calculations for Ni (1 1 1) and (1 1 3) surfaces. Using this model, different regimes of CNT growth, with and without hydrogen in the system, are analyzed. Hydrogen is shown to enhance desorption of hydrocarbons, leading to a decrease of the surface coverage and effective carbon supply rate.     

A method for structural characterization of the range of cylindrical nanocarbons: Nanotubes to nanofibers

Given the wealth of carbon multi-walled carbon nanotubes (MWNTs) and nanofiber synthesis strategies and resulting forms, there is an increasing need to better classify these materials in terms of their nanostructure. Apart from distinguishing the different nanoforms, such classification may be particularly useful for relating MWNT or nanofiber performance within various applications to their nanostructure. Demonstrated here is the use of image analysis algorithms applied to high resolution transmission electron microscopy (HRTEM) images of MWNTs and nanofibers. The analysis of the HRTEM images allowed for four separate measurements to quantify the graphitic content of the nanotube and nanofiber samples. Each measurement was based upon the features of individual carbon layer plane segments, which appear as fringes in HRTEM images. These measures included fringe length, separation, tortuosity and orientation. Distributions in the form of histograms serve to quantify data contained in the HRTEM images as represented by these parameters. Such information can serve as a measure of the physical characteristics and resulting chemical and mechanical properties of the nanotubes, nanofibers and their utility in applications.     

Microwave-assisted poly(glycidyl methacrylate)-functionalized multiwall carbon nanotubes with a ‘tendrillar’ nanofibrous polyaniline wrapping and their interaction at bio-interface

Multiwall carbon nanotubes (MWCNTs) were activated by microwave irradiation and covalently functionalized with poly(glycidyl methacrylate) (PGMA) through free radical polymerization using ‘fishing process’ when the propagating polymer radicals were attached onto the graphitic surface of the nanotube. The PGMA-functionalized MWCNTs were then used as a precursor to non-covalently wrap polyaniline (PAni) nanofiber onto them. The functionalized nanotubes exhibited stable dispersion up to 180 days in tetrahydrofuran, dimethyl formamide and dimethyl sulfoxide. Fourier transform infrared analyses indicated the attachment of the epoxide and benzenoid–quinoid functional moieties onto the nanotube surface. The PGMA coating on the nanotube and surrounding PAni nanofiber on the MWCNT scaffold were confirmed by transmission electron microscopy. The Raman spectroscopy confirmed the phonon-assisted modification of the nanotube. The differential action of the pristine and functionalized MWCNTs against an opportunistic bacterium (Escherichia coli ) and its plasmid deoxyribonucleic acid was also investigated. Pristine nanotubes exhibited bacterial inhibitory action and no condensation with the pET-32α(+) plasmid. On the other hand, the anti-bacterial PAni nanofiber and functionalized nanotubes showed complex formation with the bacterial plasmid.     

Systematic investigation of the growth mechanisms for conventional,doped and bamboo-shaped nanotubes

Fundamental physico-chemical mechanisms underlying the synthesis of nanotubes wereinvestigated, including conventional, doped, and bamboo-shaped nanotubes. The mechanisms are examined from the viewpoint of the well-known base growth (root growth) and tip growth mechanisms. The analysis of the surface characteristics of nanoparticles is key to the present approach. Surface and interface melting, surface and bulk diffusion through nanoparticle, and the formation of a hill due to over-segregation of the source species to the nanoparticle peripheral surface have also been investigated. The study may have led to an understanding of the basics and the differences between the base growth and the tip growths of nanotubes, and also of the formation of nanotube diaphragms (caps), if any. The proposed mechanisms have been used to attempt to explain various prior observations on the conventional, doped, and bamboo-shaped nanotubes. Experimental results available in the literature have been extensively employed to justify the validity of the mechanisms, and to highlight the possible appeal of these mechanisms.     

Study of fire retardant behavior of carbon nanotube membranes and carbon nanofiber paper in carbon fiber reinforced epoxy composites

Single-walled carbon nanotube (SWCNT) and multi-walled carbon nanotube (MWCNT) membranes (buckypaper) and carbon nanofiber (CNF) paper were incorporated onto the surface of epoxy carbon fiber composites, as proposed fire shields. Their flammability behaviors were investigated by a cone calorimeter. SWCNT buckypaper and CNF paper did not show notable improvement on fire retardancy. However, MWCNT buckypaper acted as an effective flame-retardant shield, reducing the peak heat release rate by more than 60% and reducing smoke generation by 50% during combustion. The pore structures of buckypapers and CNF paper were characterized by scanning electron microscopy (SEM), mercury intrusion porosimetry, and N₂ adsorption isotherms. Gas permeability of buckypaper and carbon nanofiber paper was measured. The correlation between buckypaper and CNF paper properties and their fire retardancy was discussed.     

Effective catalyst on SiO2 in ethanol CVD for growth of single-walled carbon nanotubes

We have compared catalytic activity of Co and Fe in a growth process of single-walled carbon nanotube (SWNT) by chemical vapor deposition using ethanol as a carbon source and SiO₂ as a catalyst-supporting material. Changes of the catalyst precursors (Co- and Fe acetate) in the growth process were carefully observed at three different stages: (i) after oxidation in air at 400 °C but before heating to the growth temperature (800 °C), (ii) after heating to the growth temperature in flowing Ar and H₂ but before starting the nanotube growth and (iii) after the growth process is over. During the growth of SWNT, the Co catalyst took the form of β-Co, resulting in a high yield growth. On the contrary, the Fe catalyst formed a silicate, Fe₂SiO₄, showing a poor catalytic ability. Our result shows that chemical reactions between the catalyst precursors and their supporting materials sensitively affect the catalytic ability.     

Plasma-enabled,catalyst-free growth of carbon nanotubes on mechanically-written Si features with arbitrary shape

Simple, rapid, catalyst-free synthesis of complex patterns of long, vertically aligned multiwalled carbon nanotubes, strictly confined within mechanically-written features on a Si(1 0 0) surface is reported. It is shown that dense arrays of the nanotubes can nucleate and fully fill the features when the low-temperature microwave plasma is in a direct contact with the surface. This eliminates additional nanofabrication steps and inevitable contact losses in applications associated with carbon nanotube patterns.     

Evolution of carbon film structure during its catalyst-free growth in the plasma of direct current glow discharge

Catalyst-free growth of nanocrystalline carbon films on silicon substrates under direct current glow discharge in a mixture of hydrogen and methane was studied by scanning and transmission electron microscopy, Raman spectroscopy, as well as X-ray photoelectron and near edge X-ray absorption fine structure spectroscopy (BESSY II, Berlin). The in-time development of the film structure on a carbided silicon substrate includes the formation of diamond-like particles, ultra-thin graphite flakes parallel to the surface, carbon nanowalls nucleated on the stacked flakes and their growth accompanied by a permanent decrease of the structural defect density, and finally nanotube nucleation at the nanowall edges. Based on the observation of the carbon nanotube/nanowall linear size variation in time and using the calculated binding energies and the diffusion thresholds obtained from the literature, we propose that direct attachment of the CH₃ radicals to the carbon nanowall edge is the predominant mechanism and the rate-limiting step of its growth, whereas carbon nanotube growth is controlled by radicals diffusing along its outer surface.     

Influence of synthesis process on preparation and properties of Ni/CNT catalyst

The Ni catalyst was deposited on multi-walled carbon nanotubes using electroless-plating method. The experimental results show that not only the concentrations of the compositions such as NaOH, HCHO (formaldehyde solution) (38 wt.%) and C₄H₄O₆KNa·4H₂O (Rochelle salt crystal) in Ni bath affect the rate of Ni deposition greatly, but also the acid pretreatment of carbon nanotube precursors have a great influence on Ni content and Ni particle size deposited on the surface of the carbon nanotubes and hence the catalytic properties of Ni/CNT in the synthesis of new carbon nanotubes.     

Scaled-up self-assembly of carbon nanotubes inside long stainless steel tubing

The self-assembly of carbon nanotubes (CNTs) on the inside wall of a relatively long stainless steel tubing for applications such as separations and chromatography, is reported in this paper. The CNTs were deposited by the chemical vapor deposition (CVD) using ethylene as the carbon source and the iron nanostructures in the stainless steel as the catalyst. The coating consisted of a layer of CNTs aligned perpendicular to the circumference of the tubes, often with an overcoat of disordered carbonaceous material, which could be selectively oxidized by exposing the CNT layer below to pure O₂ at 375 °C. Variation in uniformity in terms of the thickness and morphology of the deposited film and surface coverage were studied along the length of a tube by scanning electron microscopy (SEM). The effects of process conditions, such as flow rate and deposition time on the coating thickness, were studied. The catalytic effect of the iron nanostructures depended on surface conditioning of the tubing. It was found that the pretreatment temperature influenced the quality of the nanotube coating. The morphology of the CNT deposit supported the base-growth scheme and VLS (vapor-liquid-solid) growth mechanisms of CNTs.     

Interplay of interfacial compounds,catalyst thickness and carbon precursor supply in the selectivity of single-walled carbon nanotube growth

This study is devoted to elucidate the interplay of catalyst thickness and growth conditions in the activation and selectivity of single-walled carbon nanotube growth using cobalt deposited on Si/SiO₂ as a model system. In situ Raman studies reveal that thin catalyst layers require a higher pressure of carbon precursor to initiate nanotube growth. However, if the catalysts are pre-reduced, all catalyst thicknesses display the same low threshold pressure and a higher yield of single-walled carbon nanotubes. To explain these results, catalysts formed from a gradient of cobalt thickness are studied. Surface analyses show that during the catalyst preparation, catalyst atoms at the interface with silica form small and hard-to-reduce silicate nanoparticles while the catalyst in excess leads to the formation of large oxide particles. Weakly-reducing conditions of pretreatment or synthesis are sufficient to reduce the large oxide particles and to lead to the growth of large-diameter multi-walled carbon nanostructures. However, highly-reducing conditions are required to reduce the small silicate domains into small cobalt particles able to grow single-walled carbon nanotubes. These results show that reaction of the catalyst with the support to form more refractory compounds greatly impact the nucleation yield and the growth selectivity of single-walled carbon nanotubes.