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.     

Growth of carbon nanotube forests on carbon fibers with an amorphous silicon interface

Aligned multi-walled carbon nanotube forests were grown by chemical vapour deposition on carbon fibers by the use of an amorphous Si interface. The Si layer creates a barrier, hindering the Fe catalyst diffusion into the carbon fibers. This method provides a way to tailor the thermal, electrical and mechanical properties of the fiber-resin interface of a polymer composite.     

木质素基碳材料催化剂的制备及应用研究进展

木质素具有三维网状苯环结构、来源丰富、含碳量高、官能团丰富可控等特点,是一种理想的碳材料前体。通过化学改性和微结构调控制备具有特殊功能的木质素基碳材料,其在能源催化转化、电化学储能和环境修复等领域应用广泛。本文介绍了木质素基碳材料催化剂的国内外最新研究进展,总结了木质素基碳材料催化剂的制备方法,重点综述了木质素基碳材料催化剂在氧化反应、氢解反应、酯化反应、水解反应、脱水反应、费托合成等热催化反应、电解水析氢和锌空气电池氧还原等电催化反应、有机污染物降解等光催化反应的研究进展,但如何构筑高效、稳定、廉价、可规模生产的木质素基碳材料催化剂仍然是一个具有挑战性的课题。文章总结：今后研究中应加强对木质素的基础化学结构和微结构调控、活性组分与木质素碳材料载体间的相互作用、木质素基碳材料催化剂在催化反应中的作用机理等的研究,更好地发挥其低成本、三维结构易成型和微结构可调控等优势,拓展木质素生物质资源的高值化利用领域。     

Synthesis and Characterization of Magnetic Metal-encapsulated Multi-walled Carbon Nanobeads

A novel, cost-effective, easy and single-step process for the synthesis of large quantities of magnetic metal-encapsulated multi-walled carbon nanobeads (MWNB) and multi-walled carbon nanotubes (MWNT) using catalytic chemical vapour deposition of methane over Mischmetal-based AB₃ alloy hydride catalyst is presented. The growth mechanism of metal-encapsulated MWNB and MWNT has been discussed based on the catalytically controlled root-growth mode. These carbon nanostructures have been characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM and HRTEM), energy dispersive analysis of X-ray (EDAX) and thermogravimetric analysis (TGA). Magnetic properties of metal-filled nanobeads have been studied using PAR vibrating sample magnetometer up to a magnetic field of 10 kOe, and the results have been compared with those of metal-filled MWNT.     

Carbon nanotube junctions obtained by pulsed liquid injection chemical vapour deposition

The effect of substrate surface roughness on the synthesis of carbon nanotube (CNT) junctions is studied. CNTs were obtained by a pulsed liquid injection chemical vapour deposition system (PLICVD) and grown on quartz substrates with different roughnesses. Nickel particles were used as catalyst and acetone as the carbon precursor. Results shown that CNTs growth depend strongly on the substrate irregularity. When roughness is present, the presence of CNT junctions are increased. On the quartz surface, without any modification of roughness, CNTs are not obtained. Thus, a growth mechanism for CNT junctions, based on the substrate roughness is suggested. This method represents an important alternative to produce CNTs for applying them in nanoelectronic devices.     

Direct growth of MWCNTs on 316 stainless steel by chemical vapor deposition: Effect of surface nano-features on CNT growth and structure

Multi-walled carbon nanotubes were directly grown by chemical vapor deposition on as-received or pretreated 316 SS without application of an external catalyst. A detailed study of the size distribution of surface features formed by different steps of the synthesis process showed that the heating cycle and any complementary pretreatment may produce significant changes of the surface topography, thus suggesting that the influence of any primary characteristics of the original surface, as well as those caused by a pretreatment, should be assessed in conjunction with the effects of heating. Average lateral size of nano-features less than 60 nm (after heating) were shown to favor mainly the carbon nanotube growth while a larger features size was associated predominantly to the carbon nanofiber synthesis. Scanning and transmission electron microscopy observations suggest two different mechanisms for nanotube/nanofiber growth: (1) base growth mode caused by nanosized hills on the surface catalyzing the nanotube/nanofiber synthesis, (2) tip growth mode requiring substrate surface break-up as a preliminary step to form catalytic particles, with similarities to the “metal dusting” mechanisms. While untreated steel showed the best results concerning carbon nanotube coverage and homogeneity, oxidized-reduced samples showed an almost exclusive growth of carbon nanofibers with a full coverage.     

Direct growth of vertically aligned carbon nanotubes on a planar carbon substrate by thermal chemical vapour deposition

Uniform, vertically aligned multiwalled carbon nanotube arrays (VACNTs) were grown on glassy carbon-like thin films by thermal chemical vapour deposition (CVD). Thin (5 nm) aluminum and iron catalyst layers were pre-deposited by evaporation on the carbon substrates and VACNTs were grown at 750 °C by water-assisted CVD using ethylene as the carbon source. The aluminum layer was shown to be essential for aligned nanotube growth. VACNT arrays adhered strongly to the carbon film with low contact resistance between the VACNTs and the substrate. The VACNT arrays grown directly on the planar conducting carbon substrate have attractive properties for use as electrodes. Excellent voltammetric characteristics are demonstrated after insulating the arrays with a dielectric material.     

Catalytic Routes Towards Single Wall Carbon Nanotubes

Single wall carbon nanotubes (SWCNT) have become a strategic material in the area of nanotechnologies nowadays, and catalytic chemical vapor deposition seems to be the most promising technique in view of an industrial‐scale production. However, the selective catalytic production of single wall carbon nanotubes is still a challenge, since catalytic systems performances both in terms of selectivity and activity are still relatively low. One of the main challenges for the catalytic growth of SWCNT is the control of the catalyst nanoparticles size distribution along the high temperatures required by the process. This article provides a comprehensive overview of the state of the art of the strategies that have been followed to selectively grow single wall carbon nanotubes. It focuses on catalysts preparation and activity/selectivity and on the growth mechanism of these nanostructures. Particular attention is given to the identification of the parameters that control the selectivity of the reaction, such as the choice of the metal/support couple, the particle’s sizes, and the chemical vapor deposition conditions.     

Catalytic Routes Towards Single Wall Carbon Nanotubes

Single wall carbon nanotubes (SWCNT) have become a strategic material in the area of nanotechnologies nowadays, and catalytic chemical vapor deposition seems to be the most promising technique in view of an industrial-scale production. However, the selective catalytic production of single wall carbon nanotubes is still a challenge, since catalytic systems performances both in terms of selectivity and activity are still relatively low. One of the main challenges for the catalytic growth of SWCNT is the control of the catalyst nanoparticles size distribution along the high temperatures required by the process. This article provides a comprehensive overview of the state of the art of the strategies that have been followed to selectively grow single wall carbon nanotubes. It focuses on catalysts preparation and activity/selectivity and on the growth mechanism of these nanostructures. Particular attention is given to the identification of the parameters that control the selectivity of the reaction, such as the choice of the metal/support couple, the particle's sizes, and the chemical vapor deposition conditions.     

Catalytic CVD synthesis of double and triple-walled carbon nanotubes by the control of the catalyst preparation

We report the influence of catalyst preparation conditions for the synthesis of carbon nanotubes (CNTs) by catalytic chemical vapour deposition (CCVD). Catalysts were prepared by the combustion route using either urea or citric acid as the fuel. We found that the milder combustion conditions obtained in the case of citric acid can either limit the formation of carbon nanofibres (defined as carbon structures not composed of perfectly co-axial walls or only partially tubular) or increase the selectivity of the CCVD synthesis towards CNTs with fewer walls, depending on the catalyst composition. It is thus for example possible in the same CCVD conditions to prepare (with a catalyst of identical chemical composition) either a sample containing more than 90% double- and triple-walled CNTs, or a sample containing almost 80% double-walled CNTs.     

Synthesis of multi-walled carbon nanotubes by catalytic chemical vapor deposition using Cr2 − xFexO3 as catalyst

Chromium oxide and iron oxide solid solution was used as a catalyst for multi-walled carbon nanotubes synthesis by the catalytic chemical vapor deposition technique. The catalyst was prepared by the solution combustion synthesis method. Natural gas (NG) was employed as a carbon source for the carbon nanotube growth instead of methane, which is typically used. The carbon nanotube synthesis was carried out under H₂/NG and Ar/NG atmospheres at 950 °C. The Cr_2 − xFe_xO₃ catalyst was capable to produce carbon nanotubes only in H₂/NG atmospheres. Partial elimination of the catalyst after the synthesis was possible using a concentrated solution of HNO₃.     

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.     

Synthesis of aligned carbon nanotubes

Vertically aligned carbon nanotubes (ACNTs) are bundles of carbon nanotubes oriented perpendicular to a substrate, and horizontally aligned CNTs are parallel to the substrate. Their dense and orderly arrangement, along with outstanding physical and chemical properties, enables ACNTs to be used in various fields. The methods of synthesising ACNTs can be classified into single-step and double-step techniques. Thermal pyrolysis and flame synthesis are the common single-step methods, and both are relatively simple. The double-step methods, including catalyst coating and chemical vapour deposition, provide more control over the catalyst morphology. This review explores different methods used for ACNT growth, the process parameters that determine the morphology of ACNTs and the applications of structured ACNTs.     

Growth of multiwalled carbon nanotubes during the initial stages of aerosol-assisted CCVD

We report a study of the initial stages of growth of aligned multiwalled carbon nanotubes (MWNT) synthesised by catalytic chemical vapour deposition (CCVD) of liquid aerosol obtained from toluene/ferrocene solution. A special experimental procedure has been developed to stop the process after short durations (30 s to 2 min). Two different pyrolysis temperatures are considered: 800 and 850 °C. Both scanning and transmission electron microscopy (SEM, TEM) coupled to energy-dispersive X-ray (EDX) analyses are used in order to determine the location of catalyst particles and to examine their chemical nature, morphology and size distribution when nanotubes start to grow. During the early stages (30 s), we observe the formation of a layer of catalyst particles on silicon substrates before the growth of nanotubes. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements indicate the occurrence of iron oxide (γ-Fe₂O₃ or Fe₃O₄). In addition, XPS analysis reveals the formation of graphite-like carbon, demonstrating that iron oxide particles catalyse the decomposition of toluene vapour. SEM and TEM observations show that these particles are most often located at the nanotube root, suggesting a base growth mechanism responsible for the formation of aligned nanotube when prolonging growth time (2 min).     

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.     

Tantalum based coated substrates for controlling the diameter of carbon nanotubes

The diameter of carbon nanotubes deposited on TaN coated silicon substrates by catalytic chemical vapor deposition strongly depends on the N/Ta ratio in the coating. The coating characteristics are tailored by controlling the deposition parameters in a plasma reactive sputtering process. Photoelectron emission spectroscopy and X-ray diffraction of the coatings show the formation of a composite material. The presence of multi-wall nanotubes is confirmed by transmission electron microscopy. The results show that the structure of the TaN coating modifies the catalyst (Fe) effect changing the kinetics of the nanotube growth.     

Towards type-selective carbon nanotube growth at low substrate temperature via photo-thermal chemical vapour deposition

Carbon nanotubes have been intensively researched for electronic applications, driven by their excellent electronic properties, where the goals are control and reproducibility of growth, semiconducting/metallic type selectivity and maintaining high quality of carbon nanotubes, in a process that is temperature-compatible with the electronics. Photo-thermal chemical vapour deposition can achieve these goals and, through a thorough investigation of the parameter space, we achieve very high nanotube-quality and growth rates, and produce a phase-diagram that reveals distinct regions for growing semiconducting and metallic single-walled nanotubes, as well as multi-walled. Correlation with the carbon-catalyst phase diagram allows for the development of a novel growth model. We propose that the temperature-gradient induces carbon diffusivity-gradient across the catalyst to yield the high growth rate. This is attributed to the increase of α-iron of catalyst. The growth control demonstrated here allows for integration of the nanotube growth process by photo-thermal deposition into mainstream electronics manufacture.     

A kinetic study of multi-walled carbon nanotube synthesis by catalytic chemical vapor deposition using a Fe-Co/Al2O3 catalyst

A kinetic study was performed to describe the initial specific rate of multi-walled carbon nanotube synthesis by catalytic chemical vapor deposition (CCVD) on a bimetallic cobalt-iron catalyst at high temperature using ethylene decomposition to solid carbon and gaseous hydrogen. The study uses a mass spectrometer that allows reaction rate to be inferred from the exhaust gas composition measurements. The aim is to obtain a better understanding of the elementary steps involved in the production of carbon nanotubes so as to derive phenomenological kinetic models in agreement with experimental data. The best models assume the elimination of the first hydrogen atom from adsorbed ethylene as rate determining step and involve a hydrogen adsorption weak enough to be neglected. It was proved that hydrogen partial pressure has no influence on initial reaction rate of carbon nanotube synthesis with the catalyst used for this study. Activation energy and ethylene adsorption enthalpy were found to be equal to around 130 and −130 kJ mol⁻¹, respectively.     

Resistance–temperature dependence in carbon nanotube fibres

The electrical transport of a carbon nanotube assembly is determined by its morphology and composition. These vary with the assembly production processes and post-process treatments applied. Here, we present the study of the electrical – structural dependence of wire like assemblies of carbon nanotubes i.e. carbon nanotube fibres produced via floating catalyst chemical vapour deposition processes. We propose that the analysis of resistance – temperature characteristics of the fibres provides vast amount of information for the assessment of the quality of the fibres and thus the efficacy of fibre production and post-production processes. To aid qualitative and quantitative analysis of the experimental results we propose a new universal model which allows the fitting of experimental data in the full range of temperatures and a straightforward comparison of the recorded characteristics.     

Magnetic response of CVD and PECVD iron filled multi-walled carbon nanotubes

Plasma enhanced chemical vapour deposition (PECVD) and injection chemical vapour deposition (CVD) methods have been used to produce superparamagnetic iron nanoparticles (NPs) encapsulated in carbon nanostructures (core@shell). Morphological and structural properties of the Fe-filled CNTs synthesized by PECVD and CVD were carried out using a scanning and transmission electron microscope (SEM, TEM), high resolution electron microscopy (HRTEM) and selected area electron diffraction (SAED). Magnetometry results and electron microscopy observations reveal magnetic responses with different characteristics associated to the iron particles depending on the deposition method. The magnetic properties of these samples have been described in terms of the carbon nanotube anisotropic structural effects. This magnetic behaviour has potential for biomedical applications.