Growth Of Carbon Nanotube Arrays Research Articles

Analytical modeling of nucleation and growth of graphene layers on CNT array and its application in field emission of electrons

Carbon Nanotube (CNT) arrays and graphene have undergone several investigations to achieve efficient field emission (FE) owing to CNT’s remarkable large aspect ratio and graphene’s exceptional FE stability. However, when dense CNT arrays and planar graphene layers were used as field emitters, their field enhancement factor reduced dramatically. Therefore, in this paper, we numerically analyze the growth of a dense CNT array with planar graphene layers (PGLs) on top, resulting in a CNT-PGL hybrid and the associated field enhancement factor. The growth of the CNT array is investigated using Plasma Enhanced Chemical Vapor Deposition (PECVD) chamber in C2H2/NH3 environment with variable C2H2 flow, Ni catalyst film thickness, and substrate temperature followed by PGL precipitation on its top at an optimized cooling rate and Ni film thickness. The analytical model developed accounts for the number density of ions and neutrals, various surface elementary processes on catalyst film, CNT array growth, and PGLs precipitation. According to our investigation, the average growth rate of CNTs increases and then decreases with increasing C2H2 flow rate and catalyst film thickness. CNTs grow at a faster rate when the substrate temperature increases. Furthermore, as the chamber temperature is lowered from 750 °C to 250 °C in N2 environment and Ni film thickness grows, the number of the graphene layers increases. The field enhancement factors for the CNT array and hybrid are then calculated based on the optimal parameter values. The average height of the nanotubes, their spacing from one another, and the penetration of the electric field due to graphene coverage are considered while computing the field enhancement factor. It has been found that adding planar graphene layers to densely packed CNTs can raise its field enhancement factor. The results obtained match the current experimental observations quite well.

Machine learning for the regulation strategy and mechanism of the integrated growth of carbon nanotube arrays

Carbon nanotube (CNT) arrays are an attractive material, and achieving their low-cost and controllable growth is a difficult task.

Symmetrical growth of carbon nanotube arrays on FeSiAl micro-flake for enhancement of lithium-ion battery capacity

The efficient and economic growth of high-purity carbon nanotube (CNT) arrays remains a great challenge due to the synthetic complexity and difficulty of high-precision control. Herein, a high-yield production method for direct symmetrical growth of CNT arrays using FeSiAl micro-flake catalyst was developed. The FeSiAl micro-flakes combined the role of the substrate, buffer layer, and catalyst, which led to ultra-high catalytic efficiency of 297% during catalytic chemical vapor deposition (CCVD). After carefully considering the reaction temperature and time, the growth kinetics and corresponding mechanism were investigated. As conductive additives, CNT arrays have excellent application potential in high energy density ternary cathode materials. The average specific discharge capacity was ∼160 mAh g−1 at 5 C, which represents a great improvement relative to commercially available conductive additive super p (SP). This FeSiAl micro-flake catalyst provides a novel methodology for the efficient and fast growth of CNT arrays on a large scale with potential applications in the energy storage and micro-nano device fields.

Vertically aligned carbon nanotube arrays growth modeling at different temperatures and pressures in reactor

A model for the growth of carbon nanotubes was developed and the calculations for this model were compared with experiment. Experimental growth of carbon nanotubes arrays was carried out by the CVD method in a stream of acetylene, ammonia and argon at temperatures from 550 to 950 °C. A nickel film of 4 nm thickness on the surface of titanium nitride was used as a catalyst. The model takes into account the pyrolysis of hydrocarbons on the surface of the catalyst nanoparticles, the formation of a barrier layer on its surface, which slows down and stops the growth of the nanotube array, the interaction of the substance of the buffer layer with carbon in the catalyst nanoparticle. When developing the model, the mechanisms of individual processes were considered and the temperature dependences of the kinetic coefficients, which describe the growth of nanotubes, were obtained. It is these dependences that show a good agreement between the calculations and the experiment.

Effect of Hydrogen Concentration on the Growth of Carbon Nanotube Arrays for Gecko-Inspired Adhesive Applications

Vertically-aligned carbon nanotubes (VACNTs) have extraordinary structural and mechanical properties, and have been considered as potential candidates for creating dry adhesives inspired by adhesive structures in nature. Catalytic chemical vapor deposition is widely used to grow VACNTs; however, the influential mechanism of VACNT preparation parameters (such as H2 concentration) on its adhesion property is not clear, making accurate control over the structure of VACNTs adhesive an ongoing challenge. In this article, we use electron beam-deposited SiO2/Al2O3 as a support layer, Fe as catalyst, and C2H4/H2 gas mixtures as a feed gas to prepare VACNTs, while varying the ratio of the reducing atmosphere (H2) from 0% to 35%. VACNTs synthesized at a 15% H2 concentration (5 mm × 5 mm in size) can support a maximal weight of 856 g, which indicates a macroscopic shear adhesive strength of 34 N/cm2. We propose a hydrogen-concentration-dependent model for the shear adhesive performance of VACNTs. By adjusting the amount of hydrogen present during the reaction, the morphology and quality of the prepared VACNTs can be precisely controlled, which significantly influences its shear adhesive performance. These results are advantageous for the application of carbon nanotubes as dry adhesives.

Growth of carbon nanotube arrays on various CtxMey alloy films by chemical vapour deposition method

Carbon nanotube (CNT) arrays were fabricated on Ct-Me-N-(O) alloys with content of Ct in the range of 6–40 at.% by chemical vapour deposition. The Ct was a catalytic metal from the group of the following elements: Ni, Co, Fe, Pd, while Me was a transition metal from the group of IV–VII of the periodic table (where Me=Ti,V, Cr, Zr, Nb, Mo, Ta, W, Re). Carbon nanotubes were found to grow efficiently on the alloy surface with its composition containing Ti, V, Cr, Zr, Hf, Nb or Ta. The growth of CNTs was not observed when the alloy contained W or Re. Additions of oxygen and nitrogen in the alloy facilitated the formation of oxynitrides and catalyst extrusion on the alloy surface. Replacement of the metals in alloy composition affected the diameter of the resulting CNTs. The obtained results showed that the alloy films of varying thickness (10–500nm) may be used for the CNTs growth. The resulting CNT material was highly homogenous and its synthesis reproducible.

Parametric Investigation of the Kinetics of Growth of Carbon-Nanotube Arrays on Iron Nanoparticles in the Process of Chemical Vapor Deposition of Hydrocarbons

On the basis of the kinetic model of synthesis of carbon nanotubes on iron nanoparticles in the process of chemical vapor deposition of hydrocarbons, the parametric dependences of characteristics of arrays of vertically oriented nanotubes on the temperature of their synthesis, the concentration of acetylene in a reactor, and the diameter of the catalyst nanoparticles were investigated. It is shown that the maximum on the temperature dependence of the rate of growth of carbon nanotubes, detected in experiments at a temperature of ~700oC is due to the competing processes of increasing the catalytic activity of iron nanoparticles and decreasing the acetylene concentration because of the signifi cant gas-phase decomposition of acetylene in the reactor before it enters the substrate with the catalyst. Our calculations have shown that the indicated maximum arises near the transition point separating the low-temperature region where multiwall nanotubes are predominantly synthesized from the higher-temperature region of generation of single-wall nanotubes in the process of chemical vapor deposition of hydrocarbons.

Influence of oxide buffer layers on the growth of carbon nanotube arrays on carbon substrates

Controllable growth of carbon nanotube (CNT) arrays on carbon substrates requires an oxide buffer layer material such as Al2O3 and SiO2. This research analyzes the influence of these buffer layers on a model carbon substrate composed of pyrolyzed carbon film on flat silicon. Electron microscopy was used to analyze nanocatalyst morphology, particle density, CNT nucleation rate, and CNT morphology for selected growth conditions. It was observed that nanotubes grown on Al2O3 buffer layers had the fastest CNT nucleation rates and more uniform tube diameters, with residual nanocatalyst particles having elongated shapes. In contrast, CNTs formed on SiO2 buffer layers nucleated slower and had more variation in tube diameters, while the nanocatalyst particles remained spherical. Comparisons among SiO2 buffer layers deposited using three different techniques—atomic layer deposition, microwave plasma enhanced chemical vapor deposition and thermal oxidation—indicated that surface roughness plays an important role. Silica layers with the highest roughness had the highest catalytic particle density and tallest carpet height for identical growth conditions. These results provide new insight into the importance of buffer layer chemistry and morphology on growth and controllability of CNT arrays.

Catalyst-free growth of carbon nanotube arrays directly on Inconel® substrates for electrochemical carbon-based electrodes

Vertically aligned carbon nanotubes grown directly onto a metallic substrate without external catalyst can be used as the active electrode material and the current collector, respectively.

Control synthesis and applications of carbon nanotube arrays

Aligned carbon nanotube (CNT) arrays are the important forms of CNT aggregates, which can be controllably synthesized by chemical vapor deposition (CVD) methods. And they are the bridges that bring the excellent properties of CNT from nano-world to macro-world. Here this review gives a brief overview of the control growth of CNT arrays including horizontally aligned CNT arrays and vertically aligned CNT arrays, and their applications in nano-electronics, high performance fibers, functional thin-film devices, energy storages and so on. Meanwhile, we also prospect the future development in basic research and applications of the aligned CNT arrays.

The shape of the orientation distribution of carbon nanotubes in aligned arrays

Recent simulations of vertically aligned carbon nanotube arrays have shown that the shape of the orientation distribution of nanotubes within the array has a drastic effect on the electrical properties of the array. Orienting of shape-anisotropic objects can be carried out in several different ways such as shearing, magnetically steering, or by vibrating the objects. Nevertheless, perfect orientation is difficult if not impossible to achieve. In the case of the growth of carbon nanotube arrays, self-confinement can occur affecting the resultant orientation distribution. Yet so far the shape of the orientation distribution has not been quantified in great detail and it has been mostly assumed to be Gaussian or Lorentzian. In the present work, multi-walled carbon nanotube arrays were grown via aerosol-assisted chemical vapour deposition with iron catalyst and investigated using small-angle X-ray scattering, a method perfectly suited to characterizing the orientation of carbon nanotubes. Using a microfocused X-ray beam of 24 μm x 17 μm in size at beamline P03 of the PETRA III synchrotron storage ring in Hamburg, we determined the orientation distribution of the vertically aligned carbon nanotubes along the film height. Remarkably, the packing density of the carbon nanotubes seems to correlate not only with the width of the distribution but also its shape. The shape of the orientation distribution was then compared to that from different oriented systems. These findings indicate that by using alignment methods that are based on steric interaction between particles, such as shearing or self-confinement during particle growth, the system will reach an alignment with an orientation distribution closer to the Laplace distribution than to the normal distribution. Such a finding has profound implications for simulation studies of mechanical, electrical and other properties of many hierarchical materials.

Thickness-, alignment- and defect-tunable growth of carbon nanotube arrays using designed mechanical loads

We demonstrate the thickness-, morphology-, and defect-tunable growth and simultaneous integration of aligned carbon nanotube (CNT) arrays using a novel microscale platform. This platform consists of a micromechanical spring of desired stiffness, which applies a precise vertical load to a vertically aligned CNT array during its growth by chemical vapor deposition (CVD). The micromechanical spring is strained by the extrusive growth force output from the aligned CNT array during its growth and, at the same time, exerts a mechanical restoring force against the buckling resistance of the CNTs. This application of a designed vertical load on the CNTs allows modulation of the thickness and degree of alignment of the CNT array, as well as the structural quality of the individual CNTs. Consequently, the electrical resistance between two opposing CNT arrays can be tuned by adjusting the vertical load. In addition, their sensing responsiveness toward chemical species can also be enhanced by applying larger vertical load on the CNTs. In contrast to conventional growth methods for producing aligned CNT arrays, our approach offers an efficient way for the growth engineering and on-chip integration of aligned CNT arrays in a single step of the CVD.

Pretreatment Control of Carbon Nanotube Array Growth for Gas Separation: Alignment and Growth Studied Using Microscopy and Small-Angle X-ray Scattering

Aligned multiwalled carbon nanotube (CNT) arrays were prepared using chemical vapor deposition of C2H4 on Fe catalyst at 750 °C. CNT array height and alignment depends strongly on the duration of H2 pretreatment, with optimal height and alignment achieved using 10-15 min pretreatment. Small-angle X-ray scattering (SAXS) was used to quantify the alignment, distribution, and size of the CNTs in arrays produced from varying pretreatment times and the results correlated with microscopy measurements. SAXS analysis revealed that the higher section of the CNT arrays exhibited better alignment than the lower section. Combining these insights with transmission electron microscopy measurements of the CNT defects within each array enable a mechanism for the CNT growth to be proposed, where the loss of alignment arises from deformation of the CNTs during their growth. Gas permeation test across densified CNT arrays indicated that the alignment of the CNT array plays an important role in the gas transport, and that the gas diffusion across the well-aligned CNT arrays was enhanced by a factor of ~45, which is much more than that across the poorly aligned CNT arrays, with an enhancement factor of ~8.

Role of Catalyst Oxidation State in the Growth of Vertically Aligned Carbon Nanotubes

The impact of gas-phase pretreatment of supported iron-oxide catalyst utilized in aligned carbon nanotube (CNT) growth is studied to understand the correlation between the catalyst oxidation state and the growth characteristics of the aligned CNT forests. By varying the pretreatment conditions from a reducing to an oxidizing environment, notable changes are observed in both the collective CNT array growth behavior and the individual CNT characteristics. Although the greatest catalytic activity was observed following a full reduction to the zerovalent (metallic) Fe catalyst, growth is also observed from a catalyst composed of both Fe2O3 and Fe3O4 particles. XPS core-level analysis, following pretreatment of the catalyst, emphasizes the critical nature of the combined catalyst–underlayer interaction to achieve optimal catalyst activity during growth and hence the most efficient catalyst reduction process. Additionally, CNT diameters during growth were strongly affected by the pretreatment process. Overall, this work gives a collective picture of how the catalyst oxidation state affects the CNT growth based on the catalyst pretreatment environment and the nature of the catalyst–underlayer interactions. Such concepts are critical for the rational design of alternative catalyst–underlayer systems for efficient CNT synthetic processes.

Template effect of hydrolysis of the catalyst precursor on growth of carbon nanotube arrays

Iron catalyst films for the growth of carbon nanotube (CNT) arrays are prepared using sol–gel technique during different hydrolytic periods. It is shown that the extent of hydrolysis of the catalyst precursor has strong impacts on the size and density of iron catalyst particles, which distributed on surface of the film. The iron catalysts formed big clusters in the early stage of the hydrolysis, whereas the particle size decreased dramatically to approximate 20nm when the hydrolytic duration is as long as 150h. The reaction between the hydrolytic product of ethyl orthosilicate and the iron oxide particles effectively influence the structure of catalysts during the process of annealing precursor films and reducing the iron oxide particles into iron catalysts. We believe that the hydrolytic product limits the mobility of the catalyst particles, preventing them from aggregating into big clusters by Ostwald ripening. This catalyst film may be utilized to create a template to control the length and quality of CNTs.

One second growth of carbon nanotube arrays on a glass substrate by pulsed-current heating

We report the very rapid growth of carbon nanotubes (CNTs) at high temperatures that can be tolerated by glass substrates. Glass substrates with metal microelectrodes and sputtered catalysts are heated by a pulsed current in a chemical vapour deposition gas environment for 0.5–1s to synthesize CNTs of several micrometres in height without damaging the glass substrate. CNTs with structures from single-walled to multi-walled and morphologies from entangled networks to vertically aligned forests are grown simply by changing the nominal thickness of the catalyst, and such CNTs grown selectively on the microelectrodes worked as field emitters for cathodoluminescence. Rapid, easy growth of patterned CNT arrays on glass substrates without using furnaces/heaters or vacuum pumps will be useful for various applications of CNTs.

Improvement of oil adsorption performance by a sponge-like natural vermiculite-carbon nanotube hybrid

Sponge-like exfoliated vermiculite (EV)/carbon nanotube (CNT) hybrids with different CNT content were prepared by intercalating aligned CNT arrays into natural vermiculite layers for oil adsorption. The intercalated growth of CNT arrays among EV layers of the EV/CNT hybrids was characterized by scanning electron microscopy and transmission electron microscopy. Thermogravimetric analysis under air atmosphere showed that CNT contents of the hybrids varied from 11.4% to 94.8% for different growth duration. Pore size distributions measured by an ex-situ Hg penetration method showed that large amount of pores can be formed in the hybrids after the intercalation of CNT arrays. N 2 adsorption results showed that the specific surface areas of all hybrids were significantly improved compared with original EV particles. Oil adsorption tests were conducted on a class of organic solvents and oils in baker and the recovery process was carried out in a syringe. It was found that the oil adsorption capacities of the as-obtained sponge-like EV/CNT hybrids were significantly improved compared with original EV particles due to the large quantity of pores produced by the intercalated growth of aligned CNT. The highest adsorption capacity of the EV/CNT hybrids was 26.7 g/g for diesel oil when the CNT content reached 91.0%, which also exhibited good recycling performance. Furthermore, the adsorption capacity for diesel oil can be further increased to 70.6 g/g by transforming the EV/CNT hybrids into fluffy EV/CNT cotton through high-speed shearing.

Patterning Nanoparticles by Microcontact Printing and Further Growth of One‐Dimensional Nanomaterials

AbstractA general method for patterning various functional inorganic nanomaterials on SiOx/Si substrates by using microcontact printing (μCP) was developed. The application of hydrophobic poly(dimethylsiloxane) (PDMS) stamps is essential for the reproducibility of this process. By tuning the concentration of the nanoparticle solutions, we can control the thickness of the nanoparticle layers on surfaces. The patterned ZnO nanoparticles on surfaces are used as seeds to grow ZnO nanorod arrays, and the patterned Fe3O4 nanoparticles or Fe/Mo nanoclusters are used as catalysts for the patterned growth of carbon nanotube arrays. This shows that μCP is a facile method to pattern inorganic nanoparticles relying on PDMS stamps of designed surface affinity. Patterned 1Dnanomaterial arrays can subsequently be obtained by use of solution path seeded growth or catalyzed chemical vapor deposition.

Coupled process of plastics pyrolysis and chemical vapor deposition for controllable synthesis of vertically aligned carbon nanotube arrays

Efficient conversion of waste plastics into advanced materials is of conspicuous environmental, social and economic benefits. A coupled process of plastic pyrolysis and chemical vapor deposition for vertically aligned carbon nanotube (CNT) array growth was proposed. Various kinds of plastics, such as polypropylene, polyethylene, and polyvinyl chloride, were used as carbon sources for the controllable growth of CNT arrays. The relationship between the length of CNT arrays and the growth time was investigated. It was found that the length of aligned CNTs increased with prolonged growth time. CNT arrays with a length of 500 μm were obtained for a 40-min growth and the average growth rate was estimated to be 12 μm/min. The diameter of CNTs in the arrays can be modulated by controlling the growth temperature and the feeding rate of ferrocene. In addition, substrates with larger specific surface area such as ceramic spheres, quartz fibers, and quartz particles, were adopted to support the growth of CNT arrays. Those results provide strong evidence for the feasibility of conversion from waste plastics into CNT arrays via this reported sustainable materials processing.

Multi-Directional Growth of Aligned Carbon Nanotubes Over Catalyst Film Prepared by Atomic Layer Deposition.

The structure of vertically aligned carbon nanotubes (CNTs) severely depends on the properties of pre-prepared catalyst films. Aiming for the preparation of precisely controlled catalyst film, atomic layer deposition (ALD) was employed to deposit uniform Fe2O3 film for the growth of CNT arrays on planar substrate surfaces as well as the curved ones. Iron acetylacetonate and ozone were introduced into the reactor alternately as precursors to realize the formation of catalyst films. By varying the deposition cycles, uniform and smooth Fe2O3 catalyst films with different thicknesses were obtained on Si/SiO2 substrate, which supported the growth of highly oriented few-walled CNT arrays. Utilizing the advantage of ALD process in coating non-planar surfaces, uniform catalyst films can also be successfully deposited onto quartz fibers. Aligned few-walled CNTs can be grafted on the quartz fibers, and they self-organized into a leaf-shaped structure due to the curved surface morphology. The growth of aligned CNTs on non-planar surfaces holds promise in constructing hierarchical CNT architectures in future.