THE INFLUENCE OF SYNTHESIS PARAMETERS ON THE PRODUCTION OF MULTIWALLED CARBON NANOTUBES BY THE FERROCENE CATALYZED PYROLYSIS OF TOLUENE

A high yield (∼32 wt.%) of multiwalled carbon nanotubes was obtained in an iron catalyzed reaction. This was achieved in the temperature range 800-1000°C under an atmosphere of H₂/Ar by an improved solution injection method in a horizontal reactor using toluene as carbon source and ferrocene as catalyst precursor. The pyrolysis temperature, ferrocene concentration, solution feeding rate and carrier gas flow rate all influenced the yield of carbon nanotubes and the thickness of the aligned carbon nanotube films. The carbon nanotubes was prepared in high purity using optimized pyrolysis conditions.     

THE INFLUENCE OF SYNTHESIS PARAMETERS ON THE PRODUCTION OF MULTIWALLED CARBON NANOTUBES BY THE FERROCENE CATALYZED PYROLYSIS OF TOLUENE

ABSTRACT

A high yield (～32?wt.%) of multiwalled carbon nanotubes was obtained in an iron catalyzed reaction. This was achieved in the temperature range 800–1000°C under an atmosphere of H₂/Ar by an improved solution injection method in a horizontal reactor using toluene as carbon source and ferrocene as catalyst precursor. The pyrolysis temperature, ferrocene concentration, solution feeding rate and carrier gas flow rate all influenced the yield of carbon nanotubes and the thickness of the aligned carbon nanotube films. The carbon nanotubes was prepared in high purity using optimized pyrolysis conditions.     

Field emission study of carbon nanotubes forest and array grown on Si using Fe as catalyst deposited by electro-chemical method

Vertically aligned carbon nanotubes are synthesized by Low Pressure Chemical Vapor Deposition (LPCVD) on Si substrate coated with Fe as a catalyst at a pressure of 20 Torr and at a growth temperature of 600 degrees C. The catalyst film is prepared by electro-chemical method which is very unique and a low cost method. Three precursor gases Acetylene (C2H2), Ammonia (NH3) and Hydrogen (H2) at the flow rate of 20 sccm, 100 sccm and 100 sccm respectively are allowed to flow through the Low Pressure Chemical Vapor Deposition reactor for 10 minutes. Scanning Electron Microscope (SEM) images show that synthesized CNTs are vertically aligned and uniformly distributed with a high density. Raman analysis shows G-band at 1574 cm(-1) and D-band at 1370 cm(-1). The G-band is higher than D-band, which indicates that CNTs are highly graphitized. The field emission measurement reveals good field emission properties of as-grown vertically aligned carbon nanotubes with turn-on field of 1.91 V/microm at the current density 10 mA/cm2. The field enhancement factor is calculated to be 7.82 x 10(3) for as-grown carbon nanotubes.     

Effect of ferrocene concentration on the synthesis of bamboo-shaped carbon-nitrogen nanotube bundles

We have investigated the effect of ferrocene concentration on the synthesis of carbon-nitrogen (C-N) nanotubes. The bamboo-shaped carbon-nitrogen nanotubes were synthesized by spray pyrolysis of Fe(C5H5)2 and CH3CN solution using argon as a carrier gas at the optimum temperature of approximately 900 degrees C. The effect of ferrocene concentration on the length and concentration of nitrogen in nanotubes was studied. Micro-structural features of the nanotubes were monitored employing scanning and transmission electron microscopic techniques. SEM studies reveal that with decreasing ferrocene concentration from 25 mg ml(-1) to 5 mg ml(-1), the length of the nanotubes vary from 80 microm to 430 microm. A feasible growth model has been described and discussed. X-ray photoelectron spectroscopic studies have confirmed the formation of nitrogen-doped carbon nanotubes. These studies reveal that the nitrogen concentration in the nanotubes decreases with the increase of ferrocene concentration. The present synthesis route also provides means of producing carbon nanotubes with different concentrations of nitrogen.     

The growth of aligned carbon nanotubes on quartz substrate by spray pyrolysis of hexane

Vertically aligned multiwall carbon nanotubes were grown by spray pyrolysis of hexane as the carbon source in the presence of ferrocene as catalyst precursor on a quartz substrate. In recent work we used optimal experimental parameters for the feeding method, reactor conditions, reaction temperature and time, concentration of catalyst and flow rate of feed and gas. The process parameters were chosen so as to obtain multiwall carbon nanotubes and aligned multiwall carbon nanotubes. The tubes are around 15-80?nm in diameter. The morphology and structure of the samples were characterized by x-ray diffraction, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy analyses.     

Synthesis and characterization of aligned Fe-filled carbon nanotubes on silicon substrates

We describe the preparation and the properties of Fe-filled multi-walled carbon nanotubes on Co-coated oxidized silicon substrates. The material was grown by pyrolysis of ferrocene, using a chemical vapor deposition process. Scanning and transmission electron microscopy studies indicate that the material consists of filled and aligned MWNTs. They have outer diameters of 40–100 nm and diameters of the metal core of 20–40 nm. Energy dispersive X-ray analysis of individual tubes reveals that their filling consists of pure Fe. Alternating gradient magnetometry investigations demonstrate the ferromagnetic behavior of the filled tubes. We observe unique magnetic properties differing from those of bulk Fe.     

Large scale synthesis of bundles of aligned carbon nanotubes using a natural precursor: turpentine oil

Bundles of aligned carbon nanotubes (ACNTs) have been synthesised by spray pyrolysis of turpentine oil (inexpensive precursor) and ferrocene mixture at 800°C. Turpentine oil (C₁₀H₁₆), a plant-based precursor was used as a source of carbon and argon as a carrier gas. The bundles of ACNTs have been grown directly inside the quartz tube. The as-grown ACNTs have been characterised through X-ray diffraction, Raman spectroscopy, scanning and transmission electron microscopic techniques. Scanning electron microscope images reveal that the bundles of ACNTs are densely packed and are of ～70–130?µm in length. High-resolution transmission electron microscopy and Raman spectroscopy observations indicate that as-grown multi-walled carbon nanotubes (CNTs) are well graphitised. These CNTs have been found to have outer diameters between ～15 and 40?nm. This technique suggests a low-cost route for the large-scale formation of ACNTs bundles.     

Preparation of high yield multi-walled carbon nanotubes by microwave plasma chemical vapor deposition at low temperature

Vertically-aligned carbon nanotubes(CNTs) with multi-walled structure were successfully grown on a Fe-deposited Si substrate at low temperature below 330°C by using the microwave plasma chemical vapor deposition of methane and carbon dioxide gas mixture. This is apparently different from the conventional reaction in gas mixtures of hydrogen and methane, hydrogen and acetylene, and hydrogen and benzene ... etc. High quality carbon nanotubes were grown at lower temperature with CO₂ and CH₄ gas mixture than those used by the previous. After deposition, the microstructure morphology of carbon nanotubes was observed with scanning electron microscope and high-resolution transmission electron microscope. The characteristics of carbon nanotubes were analyzed by laser Raman spectroscopy. The results showed the variation of the flow rate ratio of CH₄/CO₂ from 28.5 sccm/30 sccm to 30/30 sccm and the DC bias voltage from –150 V to –200 V, at 300 W microwave power, 1.3–2.0 kPa range of total gas pressure, and substrate temperatures between 300°C and 350°C. Vertically aligned carbon nanotubes with the diameter of about 15 nm and multi-walled structure were illustrated by SEM and HRTEM. However, the highest yield of carbon nanotubes of about 50% was obtained at low temperature below 330°C by MPCVD for the CH₄/CO₂ gas mixture with properly controlled parameters.     

Influence of the crystallinity of the iron catalysts on the formation of carbon nanotubes

In this work, carbon nanotubes and minor amount of Fe/C core-shell structure nanoparticles were simultaneously synthesized by catalytic pyrolysis of ferrocene. Through high-resolution TEM observation and ED characterization, the results showed that the well-crystallized iron nanoparticles could catalyze the formation of carbon nanotubes, while the amorphous iron nanoparticles could not catalyze the formation of carbon nanotubes but form the Fe/C core-shell nanoparticles.     

Novel vapor phase reactions for the synthesis and modification of carbon nanotubes and inorganic nanowires

Several vapor phase methods have been developed for the preparation and modification of carbon nanotubes and inorganic nanowires. Thus, nebulized spray pyrolysis has been employed for the synthesis of carbon nanotubes and metal nanowires. Multi-walled carbon nanotubes (MWNTs) with fairly uniform diameters and aligned nanotube bundles have been obtained by nebulized spray pyrolysis using solutions of organometallics such as ferrocene in hydrocarbon solvents. Single-crystalline nanowires of zinc, cadmium, cobalt, and lead are obtained by the decomposition of metal acetates. By reacting acid-treated carbon nanotubes with vapors of metal halides, followed by reaction with water and calcination chemically-bonded oxide layers can be obtained on the nanotubes. A similar procedure has been employed to prepare chemically-bonded oxide layers on Al2O3, ZnO, and silicon nanowires by the reaction of the metal halides with the surface hydroxyl groups present on these nanowire surfaces.     

Study on the controllable scale-up growth of vertically-aligned carbon nanotube arrays

Vertically aligned carbon nanotubes (VA-CNTs) with high purity have been grown on quartz substrate via the gas phase catalytic chemical vapour deposition (CVD) by using ferrocene as the catalyst source and camphor as the carbon source. The effects of catalyst concentration, flow rate and water assistance on the morphology and structure of VA-CNTs are investigated by SEM, TEM, Raman and XPS characterizations. Under the optimized CVD conditions with modest ferrocene concentration and flow rate, dense and well VA-CNT arrays have been obtained. The water concentration should be controlled to improve CNTs alignment and impurity without damaging the walls of CNTs.     

Synthesis of multi-walled and single-walled nanotubes, aligned-nanotube bundles and nanorods by employing organometallic precursors

 Pyrolysis of organometallic precursors such as metallocenes and iron pentacarbonyl as well as of their mixtures with hydrocarbons such as acetylene or benzene has been carried out under a variety of conditions to synthesize nanotubes. While the use of benzene as a hydrocarbon source generally yields multi-walled nanotubes, it has been possible to obtain single-walled nanotubes (∼1 nm diameter) by pyrolyzing a metallocene or a mixture of metallocenes along with acetylene under a high flow rate of Ar. These experiments show that the organometallic precursor produces small nanoparticles of ∼1 nm diameter which then catalyze the formation of the single-walled nanotubes. Copious quantities of aligned-nanotube bundles have been obtained by the pyrolysis of acetylene in the presence of high concentrations of ferrocene. Nanorods have been produced by the pyrolysis of ferrocene under vacuum. Single walled nanotubes can be filled or decorated by metals. Received: 31 July 1998 / Accepted: 28 August 1998     

Effect of mixture ratios and nitrogen carrier gas flow rates on the morphology of carbon nanotube structures grown by CVD

We report on the growth of carbon nanotubes (CNTs) by thermal Chemical Vapor Deposition (CVD) and investigate the effects of nitrogen carrier gas flow rates and mixture ratios on the morphology of CNTs on a silicon substrate by vaporizing the camphor/ferrocene mixture at 750 °C in a nitrogen atmosphere. Carbon layers obtained after each CVD growth run of 15 min are characterized by scanning electron microscopy (SEM) and Raman spectroscopy. Growth of CNTs is found to occur on silicon substrates. The SEM micrographs helped better understand the nanotube growth morphology while Raman Spectroscopy was used to detect the presence of nanotubes and also identify their nature vizely semiconducting or metallic, single-walled or multi-walled. Raman Spectra was also useful to estimate the quality of the samples as a ratio of nanotube to non-nanotube content. The length and diameters of the aligned CNTs were found to depend on the pyrolysis temperatures, mixture ratio, and the nitrogen carrier gas flow rates.     

Three-dimensional micropatterns of well-aligned carbon nanotubes produced by photolithography

Three-dimensional micropatterns of well-aligned carbon nanotubes were prepared on photolithographically prepatterned substrates by pyrolysis of iron(II) phthalocyanine (FePc) under an Ar/H2 atmosphere at 800-1100 degrees C. The photopatterning was achieved by photolithographic cross-linking of a chemically amplified photoresist layer spin-cast on a quartz plate or a silicon wafer, coupled with solution development. Owing to an appropriate surface characteristic, the patterned photoresist layer was found in this case to support aligned carbon nanotube growth by pyrolysis of FePc, as were the photoresist-free substrate surfaces. The difference in chemical nature between the surface areas covered and uncovered by the photoresist film, however, caused a region-specific growth of the nanotubes with different tubular lengths and packing densities, leading to the formation of three-dimensional aligned nanotube patterns suitable for various device applications.     

Fabrication of carbon nanotube thermal interface material on aluminum alloy substrates with low pressure CVD

High quality vertically aligned carbon nanotube (VACNT) arrays have been synthesized on bulk Al alloy (Al6063) substrates with an electron-beam (E-beam) evaporated Fe catalyst using low pressure chemical vapor deposition (LPCVD). The pretreatment process of the catalyst was shown to play a critical role. This was studied comprehensively and optimized to repeatedly grow high quality VACNT arrays within a wide range of thicknesses of catalyst layer (2-11 nm) and acetylene (C(2)H(2)) flow rates (100-300 sccm). The thermal performance of the resulting VACNT arrays was evaluated. The minimum interfacial thermal resistance of the Si/VACNT/Al interfaces achieved so far is only 4 mm(2) K W(-1), and the average value is 14.6 mm(2) K W(-1).     

The effect of substrate morphology on the diameter distribution of carbon nanotubes grown on silica and ceramic substrates

Large-scale well-aligned carbon nanotube film and carbon nanotube bundles have been fabricated on smooth silica and rough polycrystalline ceramic substrates by pyrolysis of ferrocene/melamine mixtures. The images of transmission electron microscopy (TEM) and scanning electron microscope (SEM) show that carbon nanotubes grown on the silica substrate have uniform outer diameters of about ∼ 25 nm and lengths of about 40 μm, while those on the ceramic substrate have outer diameters from 10 to 90 nm and lengths up to 100 μm. Electron energy-loss spectroscopy (EELS) spectra show that nanotubes grown on the two different substrates are pure carbon tubes. The effects of substrate micro-morphologies on the diameters of carbon nanotubes have been discussed.     

Temperature Influence on the Morphology and the Magnetic Properties of Vertically Aligned Fe-filled Carbon Nanotubes

Arrays of vertically aligned Fe-filled multi-wall carbon nanotubes (MWNTs) on oxidized silicon substrates were prepared by pyrolysis of ferrocene in a dual furnace system and characterized by electron microscopy and magnetometry measurement. The effect of the growth temperature on both the filled nanotube morphology and their magnetic behavior was studied. Increasing the growth temperature in the range of 845-1035°C the nanotube alignment becomes worse and the diameter of the encapsulated Fe nanowires increases from 10 to 40 nm. Both the coercivity and the remanence ratio of the arrays of Fe-filled MWNTs decrease with the increase of the growth temperature. Factors causing the observed magnetic behavior are discussed.     

An efficient fabrication of vertically aligned carbon nanotubes on flexible aluminum foils by catalyst-supported chemical vapor deposition

An efficient and versatile growth of thin-layer carbon nanotubes on a flexible aluminum foil (for kitchen use) by catalyst-supported chemical vapor deposition is reported. The aluminum foil used in the present experiment is commercially available for kitchen use. The electron-beam vapor deposition and dip-coating have been used for preparing catalysts on the aluminum foil. Vertically aligned thin-layer CNTs with typical diameters of 2.5-6.0?nm and lengths up to 90?μm are obtained when ethanol is used in combination with Fe and Co catalyst particles at a growth temperature of around 650?°C under an Ar/H(2) gas flow. Thermo-gravimetric analyses together with HR-TEM observations indicate that the purity of the CNTs synthesized by the current technique is very high.     

A one-step technique to prepare aligned arrays of carbon nanotubes

A simple effective pyrolysis technique has been developed to synthesize aligned arrays of multi-walled carbon nanotubes (MWCNTs) without using any carrier gas in a single-stage furnace at 700?°C. This technique eliminates nearly the entire complex and expensive machinery associated with other extensively used methods for preparation of CNTs such as chemical vapour deposition (CVD) and pyrolysis. Carbon source materials such as xylene, cyclohexane, camphor, hexane, toluene, pyridine and benzene have been pyrolyzed separately with the catalyst source material ferrocene to obtain aligned arrays of MWCNTs. The synthesized CNTs have been characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and Raman spectroscopy. In this technique, the need for the tedious and time-consuming preparation of metal catalysts and continuously fed carbon source material containing carrier gas can be avoided. This method is a single-step process where not many parameters are required to be monitored in order to prepare aligned MWCNTs. For the production of CNTs, the technique has great advantages such as low cost and easy operation.     

Relation between Growth Parameters and Morphology of Vertically Aligned Fe-filled Carbon Nanotubes

Fe-filled multi-wall carbon nanotubes (MWNTs) were produced by pyrolysis of ferrocene in a dual furnace system. They grew vertically aligned on oxidized silicon substrates placed inside the reaction zone of a chemical vapor deposition reactor. A variation of the growth parameters has been performed in order to evaluate the possibility to control the Fe-filled nanotube growth process and thereby the nanotube- and the filling length, diameter and yield, and also the nanotube alignment. Electron microscopy studies show nanotubes with quite different morphologies. The relation between the aligned Fe-filled MWNTs growth and the most important growth parameters is discussed.