Agglomerated CNTs synthesized in a fluidized bed reactor: Agglomerate structure and formation mechanism

Agglomerated carbon nanotubes (CNTs) were synthesized by catalytic pyrolysis of propylene on Fe/Mo/Al₂O₃ catalysts in a nano-agglomerate fluidized-bed reactor (NAFBR) of 196 mm I.D. The macroscopic properties and microstructure of the CNTs and their evolution were systematically characterized by high resolution transmission electron microscopy, energy dispersive X-ray spectroscopy and Raman spectroscopy. The CNTs from the NAFBR are sub-agglomerates entangled with each other. Their formation involves the initial fragmentation of the catalyst support, sub-agglomerate formation and expansion of the agglomerates due to CNT growth. When the structure of the catalysts is destroyed, the release of stress inside the catalyst particles will result in structural defects in the CNT shells. More perfect CNTs are obtained in fully developed agglomerates. A model is proposed to explain the process of agglomerate formation and based on its formation mechanism, an approach to control CNT quality in an NAFBR is proposed.     

Synthesis of narrow-diameter carbon nanotubes from acetylene decomposition over an iron-nickel catalyst supported on alumina

Carbon nanotubes (CNTs) were synthesized by the catalytic decomposition of acetylene over 40Fe:60Al₂O₃, 40Ni:60Al₂O₃ and 20Fe:20Ni:60Al₂O₃ catalysts. High density CNTs of 20 nm diameter were grown over the 20Fe:20Ni:60Al₂O₃ catalyst, whereas low growth density CNTs of 40 and 50 nm diameter were found over 40Fe:60Al₂O₃ and 40Ni:60Al₂O₃ catalysts. Smaller catalyst particles enabled the synthesis of highly dense, long and narrow-diameter CNTs. It was found that a homogeneous dispersion of the catalyst was an essential factor in achieving high growth density. The carbon yield and the quality of CNTs increased with increasing temperature. For the 20Fe:20Ni:60Al₂O₃ catalyst, the carbon yield reached 121% after 90 min at 700 °C. The CNTs were grown according to the tip growth mode. Based on reports regarding hydrocarbon adsorption and decomposition over different faces of Ni and Fe, the growth mechanism of CNTs over the 20Fe:20Ni:60Al₂O₃ catalyst are discussed.     

Effect of adding nickel to iron-alumina catalysts on the morphology of as-grown carbon nanotubes

The synthesis of carbon nanotubes (CNTs) from ethylene decomposition by Fe/Al₂O₃ and Fe/Ni/Al₂O₃ catalysts (Fe:Ni=10:1) is studied. A small amount of nickel introduced into the catalyst can significantly increase the yield of CNTs, but the nanotubes change from straight tubes with concentric parallel carbon sheets to helical tubes of the fish-bone type. Raman characterization of CNTs prepared at 823 and 1023 K and CNTs annealed at 2473 K shows that CNTs deposited on the Fe/Ni/Al₂O₃ catalyst have poor crystallinity, as compared with that on the Fe/Al₂O₃ catalyst. These differences are explained by a mechanism of formation of helical tubes of the fish bone type that takes into consideration the differences in the chemical nature of the catalyst with and without nickel.     

New continuous gas-phase synthesis of high purity carbon nanotubes by a thermal plasma jet

We have developed a new gas-phase synthesis technique to produce carbon nanotubes (CNTs) with a continuous process and at high temperature, by using a thermal plasma jet. A thermal plasma jet was generated by applying a direct current of 100-300 A, using Ar as the plasma gas with a flow rate of ∼6 ksccm. The temperature of the thermal plasma jet was very high (∼10⁴ K) and the velocity was very fast (∼100 m/s). Fe(CO)₅ and CO were used as a catalyst precursor and carbon source, respectively. The yield of CNTs was dramatically increased by attaching a helical extension reactor at the end of the plasma nozzle. High purity (∼80%) CNTs were produced with a continuous process by using a thermal plasma jet with helical extension reactor equipment. The number of CNT walls produced was critically affected by the hydrogen gas injected as an auxiliary plasma gas. Without hydrogen gas, single-walled carbon nanotubes whose diameter was about 1 nm were mostly produced while with hydrogen gas double-walled carbon nanotubes (about 4 nm in diameter) were predominantly produced, with small amount of 3- and 4-walled carbon nanotubes.     

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.     

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.     

Preparation of Catalyst Ni–Cu/CNTs by Chemical Reduction with Formaldehyde for Steam Reforming of Methanol

The novel catalyst Ni–Cu alloys supported on carbon nanotubes (CNTs) was prepared by reduction with formaldehyde and applied in steam reforming of methanol. With nitric acid and sulfuric acid to create defects on the surface of CNTs and using ethanol to improve the hydrophilicity of CNTs, the Ni–Cu alloys were anchored on the surface of CNTs by co-reduction of Ni- and Cu-precursors under the use of tetra-n-methylammonium hydroxide to reduce the aggregation of Ni–Cu particles. In contrast, Ni–Cu catalyst supported on activated carbon (Ni–Cu/C) was prepared as well, and the bimetal of Ni and Cu supported on CNTs (Ni/Cu/CNTs) was attained by successive reduction of first Cu- and then Ni-precursors. The catalysts were characterized with XRD, ED, FESEM, transmission electron microscopy, and Thermogravimetric analysis. The hydrogen yield in steam reforming of methanol was near 100% at 360 °C over 20 wt.% Ni₂₀–Cu₈₀/CNTs. The catalytic activity of Ni₂₀–Cu₈₀/CNTs is much higher than that of Ni₂₀–Cu₈₀/C and Ni₂₀/Cu₈₀/CNTs.     

Synthesis of carbon nanotubes over Fe catalyst on aluminium and suggested growth mechanism

Carbon nanotubes (CNTs) have been grown by the decomposition of C₂H₂ over a thin catalyst film in order to investigate the growth mechanism of CNTs by chemical vapour deposition (CVD). The catalyst was prepared from an iron nitrate precursor solution that was spin-coated on an aluminium substrate. The density (mg cm⁻²) and the length of the CNTs were greatly influenced by the precursor concentration, the time of deposition, the temperature and the ratio of C₂H₂:N₂. Scanning and transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction measurements have been carried out in order to investigate the behaviour of the catalyst before and during the growth process. The iron nitrate film formed an amorphous iron oxide layer that transformed to crystalline Fe₂O₃, which was reduced to Fe₃O₄ and FeO in contact with the C₂H₂: N₂ reaction atmosphere. The CNTs synthesis took place on small iron carbide (Fe₃C) particles that were formed from the FeO.     

Strong influence of buffer layer type on carbon nanotube characteristics

Carbon nanotubes (CNTs) have been grown by chemical vapor deposition in a vacuum chamber equipped with in situ photoelectron spectroscopy technique that allows for precise characterization of the chemical state of substrate and catalyst before the CNTs growth. The CNTs were grown onto Si wafers covered with thin buffer layers of Al, Al₂O₃, TiN, and TiO₂, using Fe as catalyst. Marked differences were observed both in growth rate and nanotube characteristics, as determined by SEM, TEM and micro-Raman spectroscopy, depending on the choice of buffer layer.     

A treatment method to give separated multi-walled carbon nanotubes with high purity, high crystallization and a large aspect ratio

CNT agglomerates, prepared by catalytic chemical vapor deposition in a nano-agglomerate fluidized-bed reactor are separated and dispersed. The effects of shearing, ball milling, and ultrasonic and chemical treatments on the dispersing of the carbon nanotubes were studied using SEM, TEM/HRTEM and a Malvern particle size analyser. The resulting microstructures of the agglomerates and the efficiency of the different dispersion methods are discussed. Representative results of annealed CNTs are highlighted. The as-prepared CNT product exists as loose multi-agglomerates, which can be separated by physical methods. Although a concentrated H₂SO₄/HNO₃ (v/v=3:1) treatment is efficient in severing entangled nanotubes to enable their dispersion as individuals, damage to the tube-wall layers is serious and unavoidable. A high temperature annealing (2000 °C, 5 h) before the acid treatment (140 °C, 0.5 h) is recommended and can give well separated nanotubes with a high aspect ratio and 99.9% purity. These highly dispersed CNTs contain few impurities and minimal defects in their tube-bodies and will be of use in further research and applications.     

Effects of the catalyst pretreatment on CO2 sensors made by carbon nanotubes

A novel CO₂ sensor was made by carbon nanotubes (CNTs). The CNTs were synthesized by catalystic thermal chemical vapour deposition at 700 °C. Prior to the synthesis, the Fe catalysts were pretreated by H₂ plasma for different times. Two terminal resistance of the as-grown CNTs mat was measured under different CO₂ concentrations. It was found that without the catalyst pretreatment, the sensitivity was about 4% when the CNTs mat was exposed to 800 mTorr CO₂ concentration. However, with various catalyst pretreatment times of 5, 10, 15 and 20 min, the sensitivity was 3.69%, 6.27%, 9.54%, and 12.1%, respectively. The Raman spectroscopy showed the I_D/I_G decreased from 0.668 to 0.539 as the catalyst pretreatment time increased. The XPS also showed the correlation of surface chemical components with the Raman spectroscopy. The Fe catalyst H₂ plasma pretreatment affected both the graphitization and surface binding sites of CNTs.     

Optimizing the synthesis of cobalt-based catalysts for the selective growth of multiwalled carbon nanotubes under industrially relevant conditions

An industrially applicable cobalt-based catalyst was optimized for the production of multiwalled carbon nanotubes (CNTs) from ethene in a hot-wall reactor. A series of highly active Co–Mn–Al–Mg spinel-type oxides with systematically varied Co:Mn ratios was synthesized by precipitation and calcined at different temperatures. The addition of Mn drastically enhanced the catalytic activity of the Co nanoparticles resulting in an extraordinarily high CNT yield of up to 249 g_CNT/g_cat. All quaternary catalysts possessed an excellent selectivity towards the growth of CNTs. The detailed characterization of the obtained CNTs by electron microscopy, Raman spectroscopy and thermogravimetry demonstrated that a higher Mn content results in a narrower CNT diameter distribution, while the morphology of the CNTs and their oxidation resistance remains rather similar. The temperature-programmed reduction of the calcined precursors as well as in situ X-ray absorption spectroscopy investigations during the growth revealed that the remarkable promoting effect of the Mn is due to the presence of monovalent Mn(II) oxide in the working catalyst, which enhances the catalytic activity of the metallic Co nanoparticles by strong metal-oxide interactions. The observed correlations between the added Mn promotor and the catalytic performance are of high relevance for the production of CNTs on an industrial scale.     

Formation of bamboo-shape carbon nanotubes by controlled rapid decomposition of picric acid

In the presence of catalysts, carbon nanotubes (CNTs) can efficiently grow in the environment generated by the rapid decomposition of normal explosives. Controlling the reaction parameters of a mixture of picric acid (PA) with cobalt acetate and paraffin can lead to a well-defined morphology of CNTs. The formation of bamboo-shape tubes is favorable at relatively high Co(AC)₂/PA and paraffin/PA ratios. It is found that the bamboo-shape tubes are different in morphology and structure and can be categorized roughly into two types, according to the participation of the catalyst nanoparticles. The formation of the two types is discussed.     

Nickel ions removal from water by two different morphologies of induced CNTs in mullite pore channels as adsorptive membrane

In this study, chemical vapor deposition (CVD) method (with two proposed synthesis processes) was used for inducting two different morphologies of CNTs in mullite pore channels as a novel adsorptive membrane for nickel ions (Ni²⁺) removal from water. Cyclohexanol and ferrocene were used as carbon source and catalyst, respectively. The first proposed synthesis process involves coevaporation and pyrolysis of a mixed solution composed of cyclohexanol and ferrocene in a neutral atmosphere and the second involves sublimation and decomposition of ferrocene in a reactor individually and subsequently introduction of cyclohexanol as vapor to the reactor by a carrier gas during the reaction. Effects of synthesis parameters such as reaction time, catalyst content and reactor pressure on growth process, and structure and properties of the induced CNTs in pore channels of the mullite substrate were also investigated. Finally the optimized CNTs growth conditions for achieving a uniform distribution of the CNTs in the mullite pore channels were reported. The CNTs–mullite composite membranes prepared under the optimum conditions were oxidized with nitric acid and then successfully used as adsorptive membranes for nickel ions removal from water. Moreover, Langmuir and Freundlich isotherm models were used to describe adsorption behavior of nickel ions by the prepared adsorptive membrane.     

Modelling the growth of carbon nanotubes produced by chemical vapor deposition

An improved model of C₂H₂ deposition for the growth of carbon nanotubes (CNT) in a horizontal tube reactor has been developed. This includes detailed gas-phase reactions of acetylene pyrolysis, and surface catalytic reactions for CNT growth. Based on this model, the mechanism of CNT growth has been studied by analyzing the change of the CNT growth rate for different growth conditions such as pressure, temperature, number of catalyst nanoparticles per unit area, and diameters of catalyst nanoparticles. The influence of gas-phase reactions and their products on CNT growth has also been evaluated. It is found that although C₂H₂ is the main contributor to the growth of CNTs, the contribution from the gas-phase products could not be ignored, especially at high temperature.     

Synthesis of carbon nanotubes from acetone

The process of synthesizing carbon nanotubes (CNTs) using the method of catalytic gas-phase pyrolysis has been studied using acetone as a source of carbon. CNTs with outer diameters of 8–10 nm were prepared. The highest yield of the CNTs with the best quality is achieved when (Co, Mo)/MgO-Al₂O₃ catalyst is used. When (Fe, Co, Mo)/Al₂O₃ is used, the yield and quality of CNTs are lower. For comparison, CNTs obtained on the same catalysts but with propylene as the source of carbon have been investigated. It has been shown that, in this case, the best yield is achieved if (Fe, Co, Mo)/Al₂O₃ catalyst is used. According to the thermogravimetric data, CNT prepared at optimal conditions from acetone have fewer structural defects than those prepared from polypropylene. The optimal temperature and concentration conditions of the CNT synthesis from acetone have been determined. Based on the kinetic data, it has been assumed that the growth of CNTs takes place due to the ketene formed under the thermal decomposition of acetone. The ecological aspects of the CNT preparation from hydrocarbons and acetone are considered.     

The role of water vapor in carbon nanotube formation via water-assisted chemical vapor deposition of methane

Carbon nanotubes (CNTs) were synthesized over a CoO–MoO/Al₂O₃ catalyst via decomposition of methane in a horizontal quartz tube reactor. The effect of water vapor on the catalytic activity and catalyst lifetime was investigated for the first time in this system. We found that the introduction of an appropriate amount of water vapor (133.3 ppm) into the reaction environment enhanced and sustained the catalytic activity. A continuous supply of a controlled amount of water vapor was found to be optimal for producing CNTs with high crystallinity. The interruption of water vapor supply provoked the formation of an inner cap structure.     

Effect of Fe catalyst thickness and C2H2/H2 flow rate ratio on the vertical alignment of carbon nanotubes grown by chemical vapour deposition

Carbon nanotubes (CNTs) were fabricated by Chemical Vapour Depositon using a C₂H₂/H₂ mixture. They were grown on Si/SiO₂ substrate with Fe film as catalyst, deposited using thermal evaporation technique. The aim of this work is to emphasize the role of the Fe catalyst thickness and the C₂H₂/H₂ flow rate ratio to grow vertically aligned CNTs by thermal CVD. In order to investigate these aspects, four Fe metal films with thickness of 2.5, 3.5, 7.5 and 16 nm were deposited on Si/SiO₂ substrate and CNTs were grown with different C₂H₂/H₂ flow rate ratios, from 5/95 to 30/70 by thermal CVD at 750 °C. Results showed that CNTs were not vertically aligned with 16 nm catalyst thickness for all flow rate ratios, while CNTs were always vertically aligned for iron thickness less than 3.5 nm and vertically aligned only for a C₂H₂/H₂ flow rate ratio greater than 20/70 for the 7.5 nm catalyst thickness. Morphology and structural information about CNTs and Fe metal clusters were provided by field emission gun-scanning electron microscopy (FEG-SEM), atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM). Our results also indicate that for each flow rate ratio exists a critical thickness of iron catalyst under which vertically aligned CNTs are obtained.     

Role of iron catalyst particles density in the growth of forest-like carbon nanotubes

Carbon nanotubes (CNTs) were fabricated by Chemical Vapour Depositon using a C₂H₂/H₂ mixture. They were grown on Si/SiO₂ substrate with Fe film as catalyst, deposited using thermal evaporation technique. The aim of this work is to emphasize the role of the Fe catalyst and the C₂H₂/H₂ flow rate ratio to grow vertically aligned CNTs. Fe metal samples with the deposition times ranging from 1 min to 16 min were deposited and CNTs were grown with different C₂H₂/H₂ flow rate ratio, from 5/95 to 30/70 by thermal CVD at 750 ^oC. Results show that CNTs were not vertically aligned with the longest catalyst deposition time for all flow rate ratios, while CNTs were always vertically aligned for deposition time less than 4 min and vertically aligned only for a C₂H₂ flow rate greater than 20% for the 7 min catalyst deposition time. Morphological and structural information about CNTs and Fe metal clusters were provided by field emission gun-scanning electron microscopy (FEG-SEM), atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM). An accurate balance between the Fe metal clusters density and the C₂H₂/H₂ flow rate ratio favours to achieve of a good vertical alignment     

Catalytic growth of carbon nanotubes through CHNO explosive detonation

Multi-walled carbon nanotubes (CNTs) have been efficiently synthesized by a self-heating detonation process, operated at low loading densities of picric acid (PA), which acts as the explosive to provide needed high temperatures and parts of carbon sources. Paraffin or benzene provides additional carbon source for tube assembling and hydrogen source to capture oxygen in PA to form H₂O and thus to survive some carbon species from oxidation. Cobalt nanoparticles, in situ formed from a detonation-assisted decomposition and reduction of cobalt acetate, show good catalytic activity for nanotube nucleation and growth and for disproportionation reaction of CO generated from the PA detonation. The nanotubes and catalyst particles are characterized by SEM, TEM, EDX, SAED, XRD, and Raman spectroscopy techniques. Some tubes are well crystallized but others have lots of structural defects, especially for the tubes with thin walls and bamboo-like shapes. The catalyst particles show conical shapes and exhibit a fcc crystalline structure of parent cobalt. These data also experimentally show that tube growth is at a very high rate and suggest that it is possible for a large-scale synthesis of CNTs under high-density and high-pressure conditions.