Vertically aligned carbon nanotubes synthesized by the thermal pyrolysis with an ultrasonic evaporator

Thermal pyrolysis process is a catalytic chemical vapor deposition (CVD) based method, involving pyrolysis of mixed solution composed of liquid carbon sources and catalysts. This paper is focused on the synthesis of carbon nanotubes (CNTs) by the thermal pyrolysis process with an ultrasonic evaporator that atomizes the mixed liquid solution. The merit of this approach is that the apparatus can produce aligned and clean CNTs which can be easily controlled in a cost-effective manner. CNT samples synthesized by introducing the atomized solution into the pyrolysis furnace were analyzed to understand and characterize the growth mechanism. HRTEM and high magnification SEM analysis indicate that the samples are multiwalled carbon nanotubes (MWNTs) grown through tip growth mechanism. The results were confirmed by Raman spectrum curve. In addition, the effects of experimental parameters on the structure of CNTs were investigated by SEM images and themogravimetric analysis. The results reveal that synthesis time and temperature have a key influence on the size of CNTs, and the concentration of catalyst determines the amount of the filling yield in the inner part of CNTs.     

Growth kinetics of MWCNTs synthesized by a continuous-feed CVD method

Unlike two-step chemical vapor deposition (CVD) methods using pre-deposited catalyst particles, in a continuous-feed CVD process, the liquid feed (consisting of catalytic precursor and hydrocarbon source) is continuously supplied into the reactor causing catalyst particle formation, nucleation of carbon nanotubes (CNTs) and CNT growth to occur simultaneously throughout the reaction period. In order to observe these processes, CVD experiments were conducted for different durations (30 s to 3 h) and the product multiwalled carbon nanotubes (MWCNTs) were characterized using scanning electron microscopy. It was found that the nanotubes did not grow in the vapor phase and that substrates played an important role in the growth by providing a place for them to anchor before growth took place. Based on transmission electron microscopy images, it has been suggested that MWCNTs grew by root-growth mechanism from the catalyst particles that were deposited on the substrate during the early stages. At long process times, continuously supplied feed gas produced additional catalyst particles which were deposited mostly on the growing nanotube mat. Due to weak catalyst-mat interaction, the additional nanotubes grew by tip growth. A comprehensive MWCNT growth model has been presented for the continuous-feed CVD.     

Catalyst‐Assisted Growth of Single‐Crystalline Hafnium Carbide Nanotubes by Chemical Vapor Deposition

Single‐crystalline hafnium carbide (HfC) nanotubes were synthesized by a one‐step catalyst‐assisted chemical vapor deposition (CVD) method. The typical nanotubes had uniform diameters of ~60 nm and wall thicknesses of ~15 nm and preferentially grew along [201]. From HRTEM/EELS analysis, the growth mechanism based on carbon nanotubes (CNT) tip growth and CNT‐templated reaction was proposed for explaining the formation of HfC nanotubes. According to the mechanism, CNTs were first formed by diffusion of C atoms on the surface of solid Ni catalyst particles. Then, gaseous Hf species reacted with C atoms from CNTs to form HfC nanotubes. During the entire growth process, Hf atoms did not participate in the catalytic reaction. Thus, this process was distinguished from the conventional vapor–liquid–solid process.     

Effect of Addition of Ni metal catalyst onto the Co and Fe supported catalysts for the formation of carbon nanotubes

Production of novel porous material is a major target in current material science research due to its wide applications. As carbon nanotube (CNTs) is a one dimensional hollow structure it is also one of the promising materials in applications ranging from electronics to hydrogen storage medium. Catalytic chemical vapor deposition (CCVD) is a method whereby CNTs can be produced in large amount. Thus, in this work, we have synthesized CNTs via pyrolysis of acetylene using various supported transition-metal catalysts in a fixed-bed reactor. Scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to investigate the CNTs structure. The structures of nanotubes formed by acetylene pyrolysis were dependent on the catalysts used. It was found that alumina supported Ni/Fe catalyst inhibited the formation of CNTs growth while alumina supported Ni/Co catalyst gave high density of CNTs. However, nanotubes grown over alumina supported Ni/Fe catalyst were less dense due to the deactivation of the catalyst at the early stage of the pyrolysis process.     

Effect of Addition of Ni metal catalyst onto the Co and Fe supported catalysts for the formation of carbon nanotubes

Production of novel porous material is a major target in current material science research due to its wide applications. As carbon nanotube (CNTs) is a one dimensional hollow structure it is also one of the promising materials in applications ranging from electronics to hydrogen storage medium. Catalytic chemical vapor deposition (CCVD) is a method whereby CNTs can be produced in large amount. Thus, in this work, we have synthesized CNTs via pyrolysis of acetylene using various supported transition-metal catalysts in a fixed-bed reactor. Scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to investigate the CNTs structure. The structures of nanotubes formed by acetylene pyrolysis were dependent on the catalysts used. It was found that alumina supported Ni/Fe catalyst inhibited the formation of CNTs growth while alumina supported Ni/Co catalyst gave high density of CNTs. However, nanotubes grown over alumina supported Ni/Fe catalyst were less dense due to the deactivation of the catalyst at the early stage of the pyrolysis process.     

Preparation of sea urchin-like carbons by growing one-dimensional nanocarbon on mesoporous carbons

One-dimensional nanocarbons, including carbon nanotubes and nanowires, were grown on catalyst-seeded mesoporous carbons using thermal chemical vapor deposition. The catalyst was applied to the mesoporous carbons by a dip coating process followed by a high temperature reduction. The growth of carbon nanotubes/nanowires then took place through the thermal decomposition of methane at temperatures of 800 °C or 900 °C. Carbon nanotubes/nanowires grown on mesoporous carbon particles provide interconnects among the particles. Intimate contacts between the CNTs/CNWs and the mesoporous carbons were also observed. Due to these interconnects and the intimate contacts, the electrical resistance mesoporous carbons having carbon nanotubes/nanowires is approximately 30% lower than that of mesoporous carbons.     

Growth of high-quality carbon nanotubes on free-standing diamond substrates

Carbon nanotubes (CNTs) were grown on diamond-coated Si substrates and free-standing diamond wafers to develop efficient thermal interface materials for thermal management applications. High-quality, translucent, free-standing diamond substrates were processed in a 5 kW microwave plasma chemical vapor deposition (CVD) system using CH₄ as precursor. Ni and Ni-9%W-1.5%Fe catalyst islands were deposited to nucleate CNTs directly onto the diamond substrates. Randomly-oriented multi-walled CNTs forming a mat of ∼5 μm thickness and consisting of ∼20 nm diameter tubes were observed to grow in a thermal CVD system using C₂H₂ as precursor. Transmission electron microscopy and Raman analyses confirmed the presence of high-quality CNTs on diamond showing a D/G peak ratio of 0.2-0.3 in Raman spectra.     

The synthesis and characterization of carbon nanotubes grown by chemical vapor deposition using a stainless steel catalyst

Multi wall carbon nanotubes (MWCNTs) were grown on a stainless steel (SS) sheet by chemical vapor deposition without the addition of external metal catalyst. We found that the key for highly efficient growth includes the nanoscale roughness of the SS surface, as shown by scanning tunneling microscopy, that acts as catalyst/template in the nanotube formation. Raman spectroscopy and electron microscopy were used to check the nature and quality of the synthesized nanotubes. We conclude that stainless steel favors a base-growth mechanism. Transmission electron energy loss spectroscopy performed on single metallic particles found inside the nanotubes clarified the atomic nature of the catalytic particles supplied by the steel. Only unoxidized iron was found and no traces of nickel and chromium were detected. In addition, the SS substrate has been used for a second growth process after carefully removing the synthesized CNTs, proving that a continuous production of CNTs from the same substrate is achievable.     

Study of carbon nanotubes synthesis by spray pyrolysis and model of growth

Multi-walled carbon nanotubes (MWCNTs) were grown inside of quartz tubing by spray pyrolysis of ferrocene/benzene under argon flow. Carbon nanotubes (CNTs) with length of 200 μm were produced with reaction time of 10 min. The diameter of CNTs was influenced by the size of droplets formed in the nebulizer and the length was greatly influenced by ferrocene concentration and argon gas flow. It was found that temperature is a critical variable to produce CNTs at the experimental conditions used in this work. It was also found that CNTs only grew if ferrocene is added to gas flow, even if CNTs are previously seeded and formed on substrate, benzene cannot produce the CNTs without ferrocene. A model of CNTs formation and growth is proposed for spray pyrolysis of ferrocene/benzene, this mechanism consist of the formation of carbon/Fe nanoparticles during pyrolysis in the gas phase, these nanoparticles reach the walls of substrate, and the nanoparticles attach to substrate surface or to the nanotubes. Under proper conditions the displacement of Fe inside the graphitic structure induces the alignment of carbon walls, straightening this way the nanotubes.     

Synthesis of carbon nanotubes by microwave plasma-enhanced hot filament chemical vapor deposition

A combined system of microwave plasma-enhanced chemical vapor deposition (MPECVD) and hot filament CVD (HFCVD) has been developed for the growth of various carbon nanotubes, where the source gas (methane) can be decomposed independently through microwave plasma and hot tungsten filament. It is found that microwave plasma provides more efficiently carbon sources for the growth of carbon nanotubes (CNTs). By the help of microwave plasma, long CNTs array with length of 0.3 mm and high-density single-walled carbon nanotubes (SWNTs) have been grown by thermal CVD and hot filament CVD, respectively. Raman spectra of the SWNTs reveal high crystalline as well as narrow diameter distribution.     

Characteristics of carbon nanotubes grown by mesh-inserted plasma-enhanced chemical vapor deposition

Plasma-enhanced chemical vapor deposition (CVD) has the advantages of low temperature and vertical growth in synthesizing carbon nanotubes (CNTs), but has generally produced stubby CNTs, probably due to an ion bombardment effect. To suppress the ion bombardment, a metal mesh with the same electrical potential as that of the cathode was placed just above the substrate on the cathode. The anode was electrically grounded while the cathode and the mesh were both negatively biased, causing no plasma to occur below the mesh. The substrate was therefore separated from the plasma by the mesh so that the ion bombardment was suppressed. CNTs were grown on a 2 nm-thick Invar catalyst with different DC plasma powers of 0–112 W at 500 °C, 3.3 torr for 10 min, using C₂H₂ (28 sccm) and NH₃ (172 sccm). Compared to CNTs grown with no mesh, these CNTs showed smaller diameters and greater lengths. As the plasma power decreased, the CNTs grown with mesh were thinner and longer and resembled those grown at a higher temperature by thermal CVD. Etching these CNTs by N₂ plasma reduced their population density and considerably improved their field emission characteristics.     

Growth of branch carbon nanotubes on carbon nanotubes as support

A novel method combining the dense fluidized bed and the floating catalytically chemical vapor deposition method (FCCVD) to prepare carbon nanotubes (CNTs) was proposed. Propylene was decomposed at 660 °C, using the CNTs as supports and the metal particles from the in situ pyrolysis of ferrocene as catalyst. The conversion of propylene in this process was closed to 100% under an optimum condition. By SEM and TEM observations, the growth of new generation of CNTs was proven. It was demonstrated that the short and thin CNT branches exist on the tips or sidewalls of CNTs. Based on the analysis of the formation of catalytic sites in the FCCVD in a fluidized bed, a physical model of the formation of branches was proposed.     

Growth of aligned millimeter-long carbon nanotube by chemical vapor deposition

Carbon nanotubes (CNTs) of millimeters in length have been grown by an atmospherical pressure thermal Chemical Vapor Deposition (CVD). The experimental parameters controlling the growth have been systematically studied. Growth mechanism is investigated by TEM and a pulsed growth technique. Growth kinetics is revealed by studying time dependence of CNT length. We discuss that our high CNT yield is achieved by a combination of intermediate growth rate and long catalyst lifetime.     

Dynamics of catalyst particle formation and multi-walled carbon nanotube growth in aerosol-assisted catalytic chemical vapor deposition

In aerosol-assisted catalytic chemical vapor deposition (CCVD), the catalyst and carbon precursors are introduced simultaneously in the reactor. Catalyst particles are formed in situ and aligned multi-walled CNTs grow at a high rate. To scale-up the process, it is crucial to understand the chemical transformation of the precursors along the thermal gradient of the reactor, and to correlate nanotube growth with catalyst nanoparticle formation. The products synthesized along a cylindrical CVD reactor from an aerosol composed of ferrocene and toluene, as catalyst and carbon precursor, respectively, were studied. The product surface density and iron content are determined as a function of the location and the iron vapor pressure in the reactor. Samples are analyzed by electron microscopy, X-ray diffraction and Raman spectroscopy. We show the strong influence of the thermal gradient on location and rate of formation of both iron particles and CNTs, and demonstrate that catalyst particles are formed by gas phase homogeneous nucleation with a size which correlates with iron vapor pressure. They are gradually deposited on the reactor walls where nanotubes grow with an efficiency which is varying linearly with catalyst particle density. CNT crystallinity appears very high for a large range of temperature and iron content.     

On the kinetics of carbon nanotube growth by thermal CVD method

The role of ammonia (NH₃) on obtaining good quality vertically aligned multi-walled carbon nanotubes (CNTs) in thermal chemical vapor deposition (CVD) method has been widely studied. It was generally agreed that NH₃ helps to maintain catalyst metal surface active by reacting with amorphous carbon. In this article, a systematic study in varying the temperature and mixing ratio of gases was conducted in order to clarify the role of NH₃ and revealed a criterion for optimized condition window in the growth processes. In addition, this study has also carried out a statistical analysis through intensive TEM observations on the tube diameters, bamboo spacing, and the formation rate of each diaphragm under various temperatures and carbon source/NH₃ ratios. While the formation of the separation diaphragms were indeed a result of bulk diffusion of carbon atoms from bottom of the Ni nanoparticle following thermal dehydrogenization to the top of the Ni nanoparticle, there were other carbon atoms diffusing presumably via surface diffusion to the CNT-metal interface and contributed to the growth of tube wall; in other words, the CNTs growth is simultaneous renucleation and growth processes, instead of a continuous renucleation and growth process. This kinetics-based mechanism in combination with the proposed role of NH₃ could not only successfully explain the effects of the process parameters including temperature and the mixing gas ratio, but also could be used for pursuing the goal of lower growth temperature for thermal CVD method which is very important for many applications of CNTs.     

Cold-gas chemical vapor deposition to identify the key precursor for rapidly growing vertically-aligned single-wall and few-wall carbon nanotubes from pyrolyzed ethanol

Vertically-aligned carbon nanotubes (VA-CNTs) were rapidly grown from ethanol and their chemistry has been studied using a “cold-gas” chemical vapor deposition (CVD) method. Ethanol vapor was preheated in a furnace, cooled down and then flowed over cobalt catalysts upon ribbon-shaped substrates at 800 °C, while keeping the gas unheated. CNTs were obtained from ethanol on a sub-micrometer scale without preheating, but on a millimeter scale with preheating at 1000 °C. Acetylene was predicted to be the direct precursor by gas chromatography and gas-phase kinetic simulation, and actually led to millimeter-tall VA-CNTs without preheating when fed with hydrogen and water. There was, however a difference in CNT structure, i.e. mainly few-wall tubes from pyrolyzed ethanol and mainly single-wall tubes for unheated acetylene, and the by-products from ethanol pyrolysis possibly caused this difference. The “cold-gas” CVD, in which the gas-phase and catalytic reactions are separately controlled, allowed us to further understand CNT growth.     

Nanostructured Hybrid Carbon Nanotube/UltraHigh‐Temperature Ceramic Heterostructures: Microstructure Evolution and Forming Mechanism

An original catalytic method has been proposed to synthesize carbon nanotubes (CNTs)–ZrB₂–ZrO₂ heterostructures using ZrB₂ polymeric precursor. The pyrolysized gases from the ZrB₂ polymeric precursor are identified to be the carbon sources for CNTs growth. A parametric study is conducted to control the CNTs growth by optimizing parameters such as synthesis temperature and catalyst content. Observations show that the in situ grown CNTs are homogeneously dispersed in the powders, and the structure and the amount of CNTs are significantly dependent on the synthesis parameters. There are two kinds of grown CNTs existed in the produced hybrid heterostructures: (i) the kinking structured CNTs that are disordered and incomplete graphitization; (ii) the improved and graphitized CNTs. The ZrB₂ polymeric precursor during thermal pyrolysis provides capable of supplying substantial carbon source for CNTs nucleation and growth by homogeneous vapor–liquid–solid reactions.     

Direct growth of carbon nanotubes on carbon fibers: Effect of the CVD parameters on the degradation of mechanical properties of carbon fibers

Grafting carbon nanotubes (CNTs) directly on carbon fibers represents a promising approach in order to strengthen the weak interface between carbon fibers and polymer matrix in carbon fiber reinforced polymer composites (CFRCs). We have carried out direct growth of CNTs on carbon fibers by using two different catalytic chemical vapor deposition (CVD) processes, namely the conventional CVD process based on catalytic thermal decomposition of ethylene and the oxidative dehydrogenation reaction between acetylene and carbon dioxide. The effect of various CVD growth parameters, such as temperature, catalyst composition and process gas mixture, was for the first time systematically studied for both processes and correlated with the mechanical properties of carbon fibers derived from single-fiber tensile tests. The growth temperature was found to be the most critical parameter in the presence of catalyst particles and reactive gasses for both processes. The oxidative dehydrogenation reaction enabled decreasing CNT growth temperature as low as 500 °C and succeeded to grow CNTs without degradation of carbon fiber's mechanical properties. The Weibull modulus even increased indicating partial healing of present defects during the CVD process. The new insights gained in this study open a way towards simple, highly reproducible and up-scalable process of grafting CNTs on carbon fibers without inducing any damages during the CVD process. This represents an important step towards CNT-reinforced CFRCs with higher damage resistance.     

Carbon nanotubes based on anodic aluminum oxide nano-template

The effect of reaction gas and catalyst on the growth of carbon nanotubes (CNTs) in the anodic aluminum oxide (AAO) nano-template was investigated. A mechanism of CNT growth was proposed, which involves the competitive catalytic carbon deposition between on the Co catalyst particles electrodeposited at the bottom of the pores and on the AAO template itself. Presence of H₂ in the reacting gas mixture significantly affected the morphology and the wall structure of synthesized CNTs: CNTs of high crystallinity grew out of pores with H₂ while no CNTs overgrew in the absence of H₂. CNT synthesis by CO disproportionation showed a lower growth rate and a higher degree of ordering than those grown by C₂H₂ pyrolysis. The unified mechanism of CNT growth on AAO template is also proposed.     

Carbon nanotubes field emission devices grown by thermal CVD with palladium as catalysts

The effects of palladium (Pd) catalyst film thickness and ammonia (NH₃) in thermal chemical vapor deposition (CVD) growth of carbon nanotubes (CNTs) are systematically compared per the resulting morphologies, Raman spectra and field emission characteristics. The CNT field emitters were tested under identical experimental configurations. Field emission characteristics were described with Fowler-Nordheim field emission theory. Experimental results demonstrate that thermally grown CVD CNTs configured as diode field emitters exhibit low turn-on fields and high emission current density. The work is extended to include the study of gated field emitters or field emission triode, important to achieving high-resolution, full gray-scale imaging for field emission, flat-panel displays. The gated device was fabricated utilizing single-mask, self-aligned gate electrode with conventional integrated-circuit (IC) fabrication process. The CNT-triode showed gate-controlled modulation of emission current where higher gate voltage gives rise to higher anode currents. The triode fabrication process using silicon-on-insulator (SOI) wafers is discussed.