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 multiwall carbon nanotubes over cobalt catalysts

CH₄ pyrolysis over Co/Al₂O₃ (60 wt %) catalysts, prepared by different methods and doped with tervalent-metal oxides, was studied gravimetrically under isothermal conditions at 600–750°C and the atmospheric pressure. The process starts at the maximum rate and decays with time. The pyrolysis rate as a function of linear CH₄ rate peaks at 40 cm/min. With the provision of the maximum process rate, the activation energy of CH₄ pyrolysis over various catalysts falls within a range of 150–240 kJ/mol. For catalysts with identical compositions, the activity depends on their preparation. Transmission electron microscopy showed that the product was multiwall carbon nanotubes with diameters ranging from 6 to 30 nm. Mathematical equations were proposed to describe the process kinetics. A productivity of 12 g/h is achieved on a laboratory continuous reactor.     

Synthesis of branched carbon nanotubes using copper catalysts in a hydrogen-filled DC arc-discharger

Branched multi-walled carbon nanotubes (b-CNTs) were deposited in a collar around the cathode in a DC arc-discharger in the presence of hydrogen and copper catalysts. Irrespective of the gas pressure or oxidation state of the catalysts, common morphologies (compartmentalization/segmentation, branching, partial metal filling) were observed when raw samples from the collar were analyzed by TEM. EDX confirmed the presence of metallic copper in the tips, in the branches and in the partially filled b-CNTs. These features have led to the proposal of a common growth model, in which reactions between metallic copper nanoparticles and gaseous carbon species that were formed in hydrogen, were used to rationalize the various CNT structures synthesised.     

Synthesis of supported metal nanoparticle catalysts using ligand assisted methods

The synthesis and characterization methods of metal nanoparticles (NPs) have advanced greatly in the last few decades, allowing an increasing understanding of structure-property-performance relationships. However, the role played by the ligands used as stabilizers for metal NPs synthesis or for NPs immobilization on solid supports has been underestimated. Here, we highlight some recent progress in the preparation of supported metal NPs with the assistance of ligands in solution or grafted on solid supports, a modified deposition-reduction method, with special attention to the effects on NPs size, metal-support interactions and, more importantly, catalytic activities. After presenting the general strategies in metal NP synthesis assisted by ligands grafted on solid supports, we highlight some recent progress in the deposition of pre-formed colloidal NPs on functionalized solids. Another important aspect that will be reviewed is related to the separation and recovery of NPs. Finally, we will outline our personal understanding and perspectives on the use of supported metal NPs prepared through ligand-assisted methods.     

Synthesis of single-walled carbon nanotube/metal nanoparticle hybrid materials from potassium-filled nanotubes

The reducing property of potassium-filled single-walled carbon nanotubes (SWCNTs) was used to synthesize single-walled carbon nanotube/metal nanoparticle hybrid materials. Electron transfer from potassium to SWCNTs gives rise to a substantial enhancement of the reducing ability of the carbon nanotubes. Metal ions with redox potentials lower than that of pristine SWCNTs can be reduced by potassium-filled SWCNTs. SWCNTs decorated with copper and zinc nanoparticles were synthesized through redox reactions between potassium-filled SWCNTs and metal ions. These redox reactions cannot take place if the potassium-filled SWCNTs have been exposed to air, because of oxidation of the carbon nanotubes which is shown by a shift of the G band frequency in Raman spectra.     

Synthesis of carbon nanotubes on Ni/carbon-fiber catalysts under mild conditions

Methane, n-hexane, benzene, and cyclopentadiene were decomposed at a relatively mild temperature (773 K) over a Ni catalyst supported on either vapor grown carbon fibers (VGCF) or graphitized carbon fibers (GCF). Transmission electron microscopy showed that the morphology of the fibers changed according to hydrocarbon and particle size. Decomposition of methane and n-hexane produced fishbone-type fibers. The fibers from n-hexane sometimes showed intermittent hollow structures but the diameters of the fibers were widely distributed. Decomposition of benzene and cyclopentadiene mainly produced winding type carbon nanotubes of relatively uniform diameters (10-20 nm). Bidirectional fishbone-type fibers (fibers growing outward from a central catalyst particle) were also observed as a by-product. Small Ni particles (10-20 nm) with stretched tails were present on the tips of tubular fibers, some of which frequently changed growth direction. The varying tubular morphologies can be ascribed to liquid-like Ni particles resulting from the freezing point depression due to a fast dissolution of carbons during decomposition of benzene or cyclopentadiene. The formation of bidirectional fibers was also observed in the decomposition of n-hexane. Relatively large well-faceted Ni particles (diameter 50-110 nm) grew bidirectional fibers.     

Synthesis of multi-walled carbon nanotubes on ‘red mud’ catalysts

Red mud, a toxic waste product from bauxite processing, was used as a catalyst for the synthesis of multi-walled carbon nanotubes (MWCNTs) by fluidised bed chemical vapour deposition. The products were analysed using thermogravimetric analysis, Raman spectroscopy, and transmission electron microscopy. Using ethylene at 650 °C a MWCNT yield of 375% (with respect to Fe loading) was obtained. Carbon products were approximately 75% MWCNTs with an I_G/I_D ratio from Raman spectroscopy of 1.43. The production technique and reaction conditions used are conducive to large-scale CNT production, offering a potential value-added commercial use for red mud.     

Synthesis of single-walled carbon nanotubes using hemoglobin-based iron catalyst

Hemoglobin (Hb) was used as a catalyst for the growth of single-walled carbon nanotubes (SWCNTs). Hb was deposited onto a hydrophilic treated substrate by spin coating method. After oxidation at 800 °C, protein chains were decomposed and iron oxide nanoparticles remained with an average diameter of 2.29 nm. High quality SWCNTs were synthesized with an average diameter of 1.22 nm. The protein chains prevent iron atoms aggregation and so the size of the nanoparticles is smaller than that from ferritin-like proteins.     

Cleaner water using bimetallic nanoparticle catalysts

Groundwater contaminated by hazardous chlorinated compounds, especially chlorinated ethenes, continues to be a significant environmental problem in industrialized nations. The conventional treatment methods of activated carbon adsorption and air‐stripping successfully remove these compounds by way of transferring them from the water phase into the solid or gas phase. Catalysis is a promising approach to remove chlorinated compounds completely from the environment, by converting them into safer, non‐chlorinated compounds. Palladium‐based materials have been shown to be very effective as hydrodechlorination catalysts for the removal of chlorinated ethenes and other related compounds. However, relatively low catalytic activity and a propensity for deactivation are significant issues that prevent their widespread use in groundwater remediation. Palladium‐on‐gold bimetallic nanoparticles, in contrast, were recently discovered to exhibit superior catalyst activity and improved deactivation resistance. This new type of material is a significant next‐step in the development of a viable hydrodechlorination catalysis technology. Copyright © 2008 Society of Chemical Industry     

Synthesis of aligned carbon nanotubes

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

Synthesis of vertically aligned single-walled carbon nanotubes with metallic chirality through facet control of catalysts

For nanotube synthesis, iron platinum (FePt) alloy particles have been prepared on a single crystalline magnesium oxide (MgO) substrate by alternate sputter deposition of FePt and MgO. Partially facetted {1 1 1}-nano particles of FePt have been epitaxially formed on the substrate and periodically exposed on the surface. The particles of FePt were half-buried between deposited MgO showed superior thermal stability and microparticulations were also achieved by optimization of film layer thickness. By using the substrates for growth of carbon nanotubes, vertically aligned single-walled carbon nanotubes (forest) have been successfully grown on the substrate containing the faceted FePt nanoparticles. Raman spectra of the forest have revealed prominent features of metallic nanotubes in the radial breathing-mode region.     

Controlling the growth of single-walled carbon nanotubes on surfaces using metal and non-metal catalysts

Thanks to the development of controlled synthesis techniques, carbon nanotubes, a 20-year-old material, are doing better at finding practical applications. The history of carbon nanotube growth with controlled structure is reviewed. There have been two main categories of catalysts used for carbon nanotube growth, metal and non-metal. For the metal catalysts, the growth process and the mechanism involved have been adequately discussed, with a widely accepted vapor–liquid–solid growth mechanism. The strategies for preparing single-walled carbon nanotube samples with well-defined structures such as geometry, length and diameter, electronic property, and chirality have been well developed based on the proposed mechanism. However, a clear mechanism is still being explored for non-metal catalysts with a hypothesis of a vapor–solid growth mechanism. Accordingly, the controlled growth of carbon nanotubes with a non-metal catalyst is still in its infancy. This review highlights the structure-control growth approach for carbon nanotubes using both metal and non-metal catalysts, and tries to give a full understanding of the possible growth mechanisms.     

Sulphate-activated growth of bamboo-like carbon nanotubes over copper catalysts

A sulphate-activated mechanism is proposed to describe the growth of bamboo-like carbon nanotubes (CNTs) over copper catalysts using chemical vapour deposition with helium-diluted ethylene. Sulphate-assisted copper catalysts afford a high-yield growth of bamboo-like CNTs at a mild temperature, 800 °C; however, non-sulphate-assisted copper catalysts, e.g., copper acetate and copper nitrate prepared catalysts, were inert to CNT growth and only gave amorphous carbons (a-C) surrounding copper nanoparticles under the same conditions. Nevertheless, the addition of sulphate ions in the preparation step for the two inert catalysts can activate their abilities for CNT growth with remarkable yields. Furthermore, Raman spectra analysis demonstrates a linear dependence between the concentration of sulphate ions in copper catalysts and the ratio of CNT-a-C in the as-grown carbon soot. The sulphate-activated effect on CNT growth over copper catalysts could be related to a three-way interaction of sulphate ions, copper nanoparticles and support. In situ TEM images of an as-grown CNT irradiated by electron beams without the inlet of carbon sources reveal a new pathway of carbon diffusion through the bulk of copper nanoparticles and an enlarged inner-wall thickness of the on-site CNT. This carbon diffusion model over copper catalysts can provide new insights into the CNT growth mechanism over non-magnetic metal catalysts.     

Synthesis of Y-junction single-wall carbon nanotubes

Y-junction single-wall carbon nanotubes (SWNTs) are synthesized using controlled catalysts by chemical vapor deposition. Mo-doped Fe nanoparticles supported by aluminum oxide particles are used as catalysts for growing Y-junction single-wall carbon nanotubes. Distribution of Mo-doped Fe particles plays an important role in Y-junction formation. Transmission electron microscopy confirmed the formation of single-walled structures of Y-junctions with diameters of 2-5 nm. Radial breathing mode peaks in Raman spectra show that our sample has both metallic and semiconducting nanotubes, indicating the possible formation of Y-branching with different electrical properties. The different electrical properties of branch and stem can be utilized in nanoscale three terminal electronic devices. The growth mechanism of Y-junction SWNTs is proposed.     

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.     

Synthesis of NiO-embedded carbon nanotubes using corona discharge enhanced chemical vapor deposition

Large-scale synthesis of NiO-embedded carbon nanotubes (CNTs) has been achieved using a nanoporous anodic aluminum (AAO) membrane as a template, with the aid of CH₄/H₂ corona discharge enhanced chemical vapor deposition (CVD). NiO nanoparticles are first introduced into the nanopores of the alumina template through wet impregnation method. The loading of NiO nanoparticles into the CNTs and the synthesis of the CNTs were simultaneously performed in the corona discharge reactor. Transmission electron microscopy characterization showed that the NiO nanoparticles are encapsulated into the walls of the CNTs, but not present on the outer surfaces.     

Tuning of Fe catalysts for growth of spin-capable carbon nanotubes

Growing spin-capable multi-walled carbon nanotube (MWCNT) forests in a repeatable fashion will become possible through understanding the critical factors affecting the forest growth. Here we show that the spinning capability depends on the alignment of adjacent MWCNTs in the forest which in turn results from the synergistic combination of a high areal density of MWCNTs and short distance between the MWCNTs. This can be realized by starting with both the proper Fe nanoparticle size and density which strongly depend on the sheet resistance of the catalyst film. We prove that a simple measurement of the sheet resistance can allow one to reliably predict the growth of spin-capable forests. Further investigation into the properties of pulled MWCNTs sheets demonstrated the relationship between their electrical resistance and optical transmittance. Overlaying either 3, 5, or 10 sheets pulled out from a single forest produces much more repeatable characteristics.     

Purity-controllable growth of bamboo-like multi-walled carbon nanotubes over copper-based catalysts

The growth of bamboo-like multi-walled carbon nanotubes (CNTs) without the formation of amorphous carbons was performed using copper-based catalysts by catalytic chemical vapour deposition (CVD) with diluted ethylene at 700–900 °C. The as-grown CNT soot was characterised by transmission electron microscopy, thermogravimetric analysis and Raman spectroscopy. The weak metal–support interaction of a sulphate-assisted copper catalyst (CuSO₄/SiO₂) can provide high-purity growth with remarkable yields of CNTs (2.24–6.10 CNT/g Cu·h) at 850–900 °C. Additionally, hydrogen-assisted CVD can activate inert copper catalysts, e.g., Cu(NO₃)₂/SiO₂ or Cu(CH₃COO)_2/SiO₂, for the growth of CNTs.     

Complete elimination of metal catalysts from single wall carbon nanotubes

The complete removal of entrapped metallic impurities (i.e. Ni and Co) incorporated within single wall carbon nanotubes (SWNTs) has been a long-standing issue. A sonication-mediated treatment of as-obtained SWNT soot in a 1:1 mixture of aqueous hydrofluoric and nitric acids resulted in the complete elimination of these impurities as shown by energy dispersive X-ray analysis (EDAX). Contact angle measurements indicated that the wetting of SWNTs is enhanced in the presence of HF. The presence of HNO₃ and surfactant was found essential in removing the catalyst due to SWNT etching of end-caps/defects and providing better dispersion, respectively. Moreover, Raman spectroscopy indicated that the structural purity of the SWNTs is not compromised by the HF/HNO₃ purification treatment.