Synthesis of carbon nanotubes over gold nanoparticle supported catalysts

The synthesis of carbon nanotubes (CNTs) through the catalytic decomposition of acetylene was carried out over gold nanoparticles supported on SiO₂-Al₂O₃. Monodispersed gold nanoparticles with 1.3-1.8 nm in diameter were prepared by the liquid-phase reduction method with dodecanethiol as protective agent. The carbon products formed after acetylene decomposition consist of multi-walled carbon nanotubes with layered graphene sheets, carbon nanofilaments (CNFs), and carbon nanoparticles encapsulating gold particles. The observed CNTs have outer diameters of 13-25 nm under 850 °C. The influence of several reaction parameters, such as kind of carriers, reaction temperature, gas flow rate, was investigated to search for optimum reaction conditions. The CNTs were observed at a relatively low temperature (550 °C). The silica-alumina carrier showed higher activity for the formation of CNTs than others used in the screening test. With increasing temperature, the CNTs showed cured structures having thick diameters and inside compartments. When Au content on the support was over 5 wt.%, the gold nanoparticles coagulated to form large ones >20 nm in diameter and became encapsulated with graphene layers after decomposition of acetylene.     

Synthesis of multi-walled carbon nanotubes over tungsten-doped cobalt-based catalyst derived from a layered double hydroxide precursor

Multi-walled carbon nanotubes (CNTs) were prepared over a series of W-doped Co-based catalysts derived from layered double hydroxide precursor by catalytic chemical vapor deposition (CCVD) of acetylene. The materials were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), nitrogen adsorption–desorption experiments and Raman spectroscopy. The effect of the proportion of W in the Co-based catalysts on the carbon yield, diameter uniformity and quality of CNTs was investigated. The results demonstrated that with the increasing W/Co molar ratio from 0 to 1.0, both the mean number of walls and the average diameter of CNT produced over catalysts increased from about 8 to 28 nm and from about 12.1 to 23.7 nm, respectively. A small amount of tungsten added to the catalyst with the W/Co molar ratio of 0.3 could facilitate the dispersion of catalytically active Co species on the surface of support, and thus uniform and high-quality CNTs with a remarkably high yield of ca. 1600% were obtained.     

Carbon Nanotube Growth on Calcium Carbonate Supported Molybdenum-Transition Bimetal Catalysts

A comparison of different catalyst systems (Fe–Mo, Co–Mo or Ni–Mo nanoparticles supported on calcium carbonate) has been performed in order to optimize the carbon nanotube (CNT) growth. The influences of the reaction temperature, metal loading and carbon source on the synthesis of CNTs were investigated. Dense CNT networks have been synthesized by thermal chemical vapor deposition (CVD) of acetylene at 720 °C using the Co–Mo/CaCO₃ catalyst. The dependence of the CNT growth on the most important parameters was discussed exemplarily on the Co catalyst system. Based on the experimental observations, a phenomenological growth model for CVD synthesis of CNTs was proposed. The synergy effect of Mo and active metals was also discussed.     

Anodic titanium oxide: A new template for the synthesis of larger diameter multi-walled carbon nanotubes

Carbon nanotubes (CNTs) with larger diameter were synthesized over anodic titanium oxide (ATO) template by CVD method using acetylene as carbon source. The porous titanium oxide was obtained by anodization of titanium metal in a mixture of 1 M H₂SO₄ + 0.5% HF electrolyte at a constant applied potential of 40 V. The XRD analysis of anodized titanium revealed that rutile and anatase forms of TiO₂ are formed due to anodization. Further, SEM analysis was used to follow the development of pores on titanium surface. The TEM analysis revealed that the formed CNTs are straight and hollow with uniform wall thickness as well as larger diameter (70–80 nm). HRTEM study showed that the formed CNTs are multi-walled and their wall thickness is around 2–3 nm. Further, the structural features of the formed CNTs were studied by XRD. Raman spectroscopy was used to study the degree of graphitization of CNTs. The Lewis acid sites of TiO₂ present in the internal surface of the pores play an important role in the catalytic decomposition of acetylene and hence the formation of CNTs. When increasing the carbon deposition time, the wall thickness of CNTs is not increased significantly, indicating that the decomposition of acetylene is due to Lewis acid sites of TiO₂ and not due to thermal decomposition. Further, the morphology of CNTs formed over ATO template was compared with that of CNTs formed on Co electrodeposited ATO. There is no significant difference in morphology as well as wall thickness was observed between the CNTs grown over ATO with and without Co catalyst. But, still further investigations are necessary to study the structural differences between the CNTs grown over ATO with and without Co catalyst.     

Synthesis of carbon nanotubes over nickel–iron catalysts supported on alumina under controlled conditions

Carbon nanotubes (CNTs) were synthesized by catalytic decomposition of acetylene over Fe, Ni and Fe–Ni catalysts supported on alumina. The growth of CNTs was carried out at various reaction conditions. The growth density and diameter of CNTs could be controlled by varying the catalyst composition and the growth parameters. The growth density of CNTs increased with increasing the activation time of catalysts in H₂ atmosphere and/or decreasing acetylene concentration. At 600°C, higher density of CNTs was observed at 60 min for higher Fe containing catalyst, whereas at 90 min for higher Ni containing catalyst. The growth density of CNTs highly increased with increasing reaction time from 30 to 60 min. For all the catalysts, the diameter of CNTs decreased with increasing growth time further mainly due to hydrogen etching. Bimetallic catalysts produced narrower diameter CNTs than single metal catalysts. The growth of CNTs followed the tip growth mode and the CNTs were multi-walled CNTs.     

Production of Carbon Nanotube by Ethylene Decomposition over Silica-Coated Metal Catalysts

Formation of carbon nanofibers (CNFs) and carbon nanotubes (CNTs) through the decomposition of ethylene at 973 K was achieved using various metal catalysts covered with silica layers. CNFs of various diameters were formed by ethylene decomposition over a Co metal catalyst supported on the outer surface of the silica. In contrast, silica-coated Co catalysts formed CNTs with uniform diameters by ethylene decomposition. Silica-coated Ni/SiO₂ and Pt/carbon black also formed CNTs with uniform diameters, while CNFs and CNTs with various diameters were formed over Ni/SiO₂ and Pt/carbon black without a silica coating. These results indicate that silica layers that envelop metal particles prevent sintering of the metal particles during ethylene decomposition. This results in the preferential formation of CNTs with a uniform diameter.     

Catalytic synthesis of carbon nanotubes using Ni- and Co-doped calcium tartrates

Calcium tartrate doped with Ni and/or Co has been used as a catalyst source in the chemical vapor deposition synthesis of carbon nanotubes (CNTs). Thermolysis of doped calcium tartrate in an inert atmosphere was shown to yield Ni, Co or Ni-Co nanoparticles ∼6 nm in diameter dispersed in a calcium oxide matrix. The CNT synthesis was carried out by ethanol vapor decomposition at 800 °C. The structure of the products was characterized by transmission electron microscopy and Raman spectroscopy. It was found that Ni nanoparticles embedded in CaO provide the narrowest diameter distribution of CNTs, while the bimetallic Ni-Co catalyst allows the formation of the thinnest CNTs with the outer diameter of ∼2 nm. This type of CNT is more likely to be responsible for the lowest value of the turn-on field (∼1.8 V/μm) for the emission current detected for the latter sample.     

Kinetic study of carbon nanotube synthesis over Mo/Co/MgO catalysts

The kinetics of carbon nanotube (CNT) synthesis by decomposition of CH₄ over Mo/Co/MgO and Co/MgO catalysts was studied to clarify the role of catalyst component. In the absence of the Mo component, Co/MgO catalysts are active in the synthesis of thick CNT (outer diameter of 7-27 nm) at lower reaction temperatures, 823-923 K, but no CNTs of thin outer diameter are produced. Co/MgO catalysts are significantly deactivated by carbon deposition at temperatures above 923 K. For Mo-including catalysts (Mo/Co/MgO), thin CNT (2-5 walls) formation starts at above 1000 K without deactivation. The significant effects of the addition of Mo are ascribed to the reduction in catalytic activity for dissociation of CH₄, as well as to the formation of Mo₂C during CNT synthesis at high temperatures. On both Co/MgO and Mo/Co/MgO catalysts, the rate of CNT synthesis is proportional to the CH₄ pressure, indicating that the dissociation of CH₄ is the rate-determining step for a catalyst working without deactivation. The deactivation of catalysts by carbon deposition takes place kinetically when the formation rate of the graphene network is smaller than the carbon deposition rate by decomposition of CH₄.     

Microwave-polyol synthesis of nanocrystalline ruthenium oxide nanoparticles on carbon nanotubes for electrochemical capacitors

The ruthenium oxide nanoparticles dispersed on multi-wall carbon nanotubes (CNTs) were successfully synthesized via microwave-polyol process combined with forced hydrolysis without additional thermal oxidation or electrochemical oxidation treatment. The HRTEM, Raman spectra and TGA curve indicate that CNTs were uniformly coated with crystalline and partially hydrous RuO₂·0.64H₂O nanoparticles of 2 nm diameter and the loading amount of ruthenium oxide in the composite could be controlled up to 70 wt.%. The specific capacitance was 450 Fg⁻¹ of ruthenium oxide/CNT composite electrode with 70 wt.% ruthenium oxide at the potential scan rate of 10 mV s⁻¹ and it decreased to 362 Fg⁻¹ by 18% at 500 mV s⁻¹. The specific capacitance of ruthenium oxide in the composite was 620 Fg⁻¹ of ruthenium oxide at 10 mV s⁻¹. The ruthenium oxide nanoparticles in ruthenium oxide/CNT nanocomposite electrode had a high ratio of outer charge to total charge of 0.81, which confirmed its high-rate capability of the composite through the preparation of the nano-sized ruthenium oxide particles on the external surface of CNTs.     

Tailoring the microstructure and mechanical properties of arrays of aligned multiwall carbon nanotubes by utilizing different hydrogen concentrations during synthesis

We synthesize vertically aligned arrays of carbon nanotubes (CNTs) in a chemical vapor deposition system with floating catalyst, using different concentrations of hydrogen in the gas feedstock. We report the effect of different hydrogen concentrations on the microstructure and mechanical properties of the resulting material. We show that a lower hydrogen concentration during synthesis results in the growth of stiffer CNT arrays with higher average bulk density. A lower hydrogen concentration also leads to the synthesis of CNT arrays that can reach higher peak stress at maximum compressive strain, and dissipate a larger amount of energy during compression. The individual CNTs in the arrays synthesized with a lower hydrogen concentration have, on average, larger outer diameters (associated with the growth of CNTs with a larger number of walls), but present a less uniform diameter distribution. The overall heights of the arrays and their strain recovery after compression have been found to be independent of the hydrogen concentration during growth.     

Characteristics of airborne fractal-like agglomerates of carbon nanotubes

Airborne carbon nanotubes (CNTs) pose a health threat at workplaces and play more and more important roles in toxicity studies, yet data and model on their detailed characteristics are lacking. Thus we investigated aerosolized multiwalled CNTs (MWCNTs) by tandem measurement of the mass and mobility size in order to characterize the MWCNT agglomerates. The results revealed a fractal-like relationship between the mass and mobility size in the range 50–500 nm indicating quite compact agglomerate structure, and an effective density in the range of 0.51–0.83 g/cm³. With the tube diameters and intrinsic density, the tube length in an agglomerate was determined reliably from the mass. We developed a model to compute the porosity and geometrical outer diameter of the agglomerates and derived the fractal relation between the mass and the outer diameter with a fractal dimension of 2.6, which agreed well with fractal dimensions of bulk CNT assemblies determined by other analytical methods. The effective density based on the outer diameter was in the range of 0.11–0.35 g/cm³ and decreased with the increasing agglomerate size. Electron macroscopic images of the aerosolized MWCNTs provided comparable data for the outer diameter and fractal dimension.     

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.     

Characterization of epitaxial GaAs MOS capacitors using atomic layer-deposited TiO2/Al2O3 gate stack: study of Ge auto-doping and p-type Zn doping

Few-wall carbon nanotubes were synthesized by methane/acetylene decomposition over bimetallic Fe-Mo catalyst with MgO (1:8:40) support at the temperature of 900°C. No calcinations and reduction pretreatments were applied to the catalytic powder. The transmission electron microscopy investigation showed that the synthesized carbon nanotubes [CNTs] have high purity and narrow diameter distribution. Raman spectrum showed that the ratio of G to D band line intensities of I _G/I _D is approximately 10, and the peaks in the low frequency range were attributed to the radial breathing mode corresponding to the nanotubes of small diameters. Thermogravimetric analysis data indicated no amorphous carbon phases. Experiments conducted at higher gas pressures showed the increase of CNT yield up to 83%. M?ssbauer spectroscopy, magnetization measurements, X-ray diffraction, high-resolution transmission electron microscopy, and electron diffraction were employed to evaluate the nature of catalyst particles.     

Fabrication and properties of CNT/Ni/Y/ZrB2 nanocomposites reinforced in situ

Carbon nanotubes (CNTs) were synthesized in situ by chemical vapor deposition of methane over nano‐ZrB₂ matrix using Ni/Y catalysts. Well‐grown CNTs with tangled and long bodies and mainly composed of well‐crystallized graphite were obtained when the Ni content reaches 10 wt%. The CNT/ZrB₂ nanocomposites obtained by spark plasma sintering at 1400°C exhibited full density and optimal mechanical properties. The flexural strength and fracture toughness of the nanocomposites were 1184 ± 52 MPa and 10.8 ± 0.3 MPa·m^1/2, respectively. Results indicated that the dispersion of CNTs in situ can improve composite performance, rendering the mechanical properties of the CNT/ZrB₂ nanocomposites synthesized in situ considerably superior to those of monolithic ZrB₂ nanoceramics and CNT/ZrB₂ nanocomposites synthesized using the traditional method. The toughening mechanisms included crack deflection, crack bridging, CNT debonding, pull‐out, and fracture.     

Effects of bimetallic catalyst composition and growth parameters on the growth density and diameter of carbon nanotubes

Multi-wall carbon nanotubes (MWNTs) were synthesized by catalytic decomposition of acetylene over Fe, Ni and Fe-Ni bimetallic catalysts supported on alumina under various controlled conditions. The growth density and diameter of CNTs were markedly dependent on the activation time of catalysts in H₂ atmosphere, reaction time, reaction temperature, flow rate of acetylene, and catalyst composition. Bimetallic catalysts were apt to produce narrower diameter of CNTs than single metal catalysts. For the growth of CNTs at 600 ‡C under 10/100 seem flow of C₂H₂/H₂ mixture, the narrowest diameter about 20 nm was observed at the reaction time of 1 h for 20Fe : 20Ni : 60Al₂O₃ catalyst, but at that of 1.5 h for 10Fe : 30Ni : 60Al₂O₃ catalyst. It was considered that the diameter and density of CNTs decreased with the increase of the growth time mainly due to hydrogen etching. The growth of CNTs followed the tip growth mode.     

Co-Mo catalyzed growth of multi-wall carbon nanotubes from CO decomposition

We investigated the growth of multi-wall carbon nanotubes (CNTs) catalyzed by SiO₂-supported Co-Mo bi-metallic catalyst in flowing CO at 700 °C. We found that both Co and Mo are present in catalytic particles at the tips of CNTs, but their compositions vary from one catalytic particle to another and significantly deviate from the initial mixing composition. The Co concentration and distribution in the catalytic particle of a CNT largely determines the length of the CNT. The CNT growth process is carbon adsorption on exposed area of a catalytic particle and subsequent precipitation at the CNT-catalyst interface or open CNT wall edges. The encapsulation of a catalytic particle was found to occur by the growth of the open-edged graphene walls around the particle. Two types of long CNTs were observed: one with their CNT walls ended at the CNT-particle interface, and the other with their CNT walls open to the environment. The former have diameters similar to their catalytic particle size while the latter have larger diameters.     

Growth of few-wall carbon nanotubes with narrow diameter distribution over Fe-Mo-MgO catalyst by methane/acetylene catalytic decomposition

Few-wall carbon nanotubes were synthesized by methane/acetylene decomposition over bimetallic Fe-Mo catalyst with MgO (1:8:40) support at the temperature of 900°C. No calcinations and reduction pretreatments were applied to the catalytic powder. The transmission electron microscopy investigation showed that the synthesized carbon nanotubes [CNTs] have high purity and narrow diameter distribution. Raman spectrum showed that the ratio of G to D band line intensities of I_G/I_D is approximately 10, and the peaks in the low frequency range were attributed to the radial breathing mode corresponding to the nanotubes of small diameters. Thermogravimetric analysis data indicated no amorphous carbon phases. Experiments conducted at higher gas pressures showed the increase of CNT yield up to 83%. Mössbauer spectroscopy, magnetization measurements, X-ray diffraction, high-resolution transmission electron microscopy, and electron diffraction were employed to evaluate the nature of catalyst particles.     

Vertically aligned carbon nanotube arrays grown on a lamellar catalyst by fluidized bed catalytic chemical vapor deposition

Large amount of vertically aligned carbon nanotube (CNT) arrays were grown among the layers of vermiculite in a fluidized bed reactor. The vermiculite, which was 100-300 μm in diameter and merely 50-100 μm thick, served as catalyst carrier. The Fe/Mo active phase was randomly distributed among the layers of vermiculite. The catalyst shows good fluidization characteristics, and can easily be fluidized in the reactor within a large range of gas velocities. When ethylene is used as carbon source, CNT arrays with a relatively uniform length and CNT diameter can be synthesized. The CNTs in the arrays are with an inner diameter of 3-6 nm, an outer diameter of 7-12 nm, and a length of up to several tens of micrometers. The as-grown CNTs possess good alignment and exhibit a purity of ca. 84%. Unlike CNT arrays grown on a plane or spherical substrate, the CNT arrays grown in the fluidized bed remain their particle morphologies with a size of 50-300 μm and the good fluidization characteristics were preserved accordingly.     

Relating carbon nanotube growth parameters to the size and composition of nanocatalysts

A gas-phase approach to studying carbon nanotube (CNT) nucleation and growth from nanoparticle catalysts has been developed. Dimensionally- and compositionally-tuned metal particles with mean diameters between 2 and 5 nm and standard deviations less than 15% are initially synthesized from metallocene vapors in an atmospheric-pressure microplasma. The nanocatalysts are continuously fed with acetylene and hydrogen into a flow furnace reactor to grow CNTs. Kinetic studies are performed by in situ aerosol size classification of the nanotubes to relate the CNT length and thus, the growth rate in our thermal process to the catalyst size and composition. We find that reducing the catalyst size results in an increase in the growth rate while varying the catalyst composition affects the growth rate, activation energy, and the onset temperature for CNT growth.     

Single stage production of carbon nanotubes using microwave technology

Carbon nanotubes (CNTs) were produced by gas phase single stage tubular microwave chemical vapor deposition (TM–CVD) using ferrocene as a catalyst and acetylene (C₂H₂) and hydrogen (H₂) as precursor gasses. The effect of the process parameters such as microwave power, radiation time, and gas ratio of C₂H₂/H₂ was investigated. The CNTs were characterized using scanning and transmission electron microscopy (TEM), and by thermogravimetric analysis (TGA). Results reveal that the optimized conditions for CNT production were 900 W reaction power, 35 min radiation time, and 0.6 gas ratio of C₂H₂/H₂. TEM analyses revealed that the uniformly dispersed vertical alignment of multiwall carbon nanotubes (MWCNTs) have diameters ranging from 16 to 23 nm. The TGA analysis showed that the purity of CNT produced was 98%.