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Carbon nanotubes synthesis by catalytic decomposition of ethyne using Fe/Ni catalyst on aluminium oxide support.


Carbon nanotubes (CNTs), that are long thin carbon wire, about a nanometer across, but up to many thousands of times longer-possess exciting mechanical, optical and electrical properties that would that make them ideal for nanoscale materials [1]. As other useful properties are discovered, particularly strength, interest has grown in potential applications for nano-materials. CNTs could be used, for example, in nanometer-sized electronics or to strengthen polymer materials for use in air and spacecrafts [2-6]. Carbon nanotubes can be produced by arc discharge, laser-ablation or by plasma-enhanced chemical vapor deposition (PECVD) as well as by thermal chemical vapor deposition.

A technique for the synthesis of carbon nanotubes by catalytic decomposition of hydrocarbons was recently reported in which hydrocarbons are decomposed directly into hydrogen and carbon [7]. Catalytic chemical vapor deposition (CCVD), arc discharge and laser ablation remain the three major synthesis routes for CNT production [8-11]. The catalytic chemical vapor deposition (CCVD) technique can be performed in the absence of a substrate that is homogeneous with the catalyst is in the gas phase [12].

The role of the catalyst is crucial to obtain high activity and selectivity towards CNT formation. Cobalt and other ferrous metals and their alloys are the active metals usually employed in the constitution of the catalyst [4,13-16].

In this study, iron-nickel catalyst on aluminium oxide support, have been prepared as [3,17] catalysts for the synthesis of Multi-Walled carbon nanotubes (MWCNT) with ethyne (acetylene) as carbon source. The use of iron and nickel with a combination of other metals as catalysts in the presence of acetylene for the synthesis of MWCNT has been reported by several researchers [18-21]. In this work, we have prepared mixed Fe and Ni catalyst only from nitrate salts of the metals to synthesize MWCNT by the decomposition of ethyne.


Preparation of catalyst

The catalyst was prepared by co-precipitation method [4,22]. A 0.25 M aqueous solutions of Fe[(N[O.sub.3]).sub.3] x 9[H.sub.2]O and Ni[(N[O.sub.3]).sub.2] x 6[H.sub.2]O were prepared and mixed in a 1:1 ratio with [Al.sub.2][O.sub.3] powder.

The pH of the synthesis was kept at a constant value of pH 9 by drop wise addition of a 1 M sodium hydroxide solution from a burette. The molar ratios used were [Fe.sup.3+]/[Ni.sup.2+] = 1:1. The slurry obtained was dried in the oven for different periods of time (10, 30, 60 180 and 300 min) at 125[degrees]C. The precipitates were then washed and dried at 40oC in an open air oven. The catalyst samples were calcined at 600[degrees]C in air. The final temperature was maintained for two hours.

Preparation of carbon nanotubes

Experimental procedure employed in the synthesis has been reported [23]. The reaction to synthesize carbon nanotubes were conducted in a horizontal tubular furnace at atmospheric pressure with ethyne gas as the carbon source. Pyrolysis of the hydrocarbon source was at 700[degrees]C. The reactor consisted of a 40 mm o.d x 70 cm long quartz tube heated by an electrical tube furnace with a temperature controller. Nitrogen gas flowing at 40 ml [min.sup.-1] was passed through the reactor for approximately 70 min., and after stabilization for 10 min, the nitrogen gas flow was maintained at 240 mL [min.sup.-1]. Ethyne gas with a flow rate of 90 mL [min.sup.-1] was then passed through the reactor for 60 min. The catalyst placed on a quartz boat was placed in the centre of the furnace during the synthesis. The flow rate of the gases was controlled by a mass flow controller (MFC). At the completion of the reaction, the reactor was cooled to room temperature with nitrogen flowing at 40 mL [min.sup.-1] for 3 to 4 hrs. The samples obtained by this method were then characterized as described below.


The morphological features of MWCNT were analyzed by Raman spectroscopy, FESEM, HR-TEM, EDS and TGA. The Raman spectra were obtained by a Raman spectroscope, Jobin-Yvon HR800 UV-VIS-NIR Raman spectrometer equipped with an Olympus BX 40 attachment. The excitation wavelength was 514.5 nm with an energy setting of 1.2 mV from a coherent Innova model 308 argon-ion laser. The Raman spectra were collected by means of back scattering geometry with an acquisition time of 50 seconds.

The surface morphology and EDS measurements were recorded with a JEOL 7500F Field Emission scanning electron microscope. The HR-TEM images of the sample were obtained by a CM 200 electron microscope operated

at 100 kV. The thermal behavior of the carbon nanotube and the catalyst were investigated by TGA using a Q500 TGA instrument under an air environment. The prepared MWCNT samples were heated in platinum crucibles with oxygen and nitrogen gases at a flow rate of 40 and 60 mL/min respectively. The dynamic measurement was made between ambient and 1000[degrees]C with a ramp rate of 10[degrees]C/min to 900[degrees]C.

Results and discussion

The Raman spectrum of the prepared CNT is presented in figure 1. Raman spectroscopy is a powerful tool for the characterization of CNTs, and can reveal several properties of CNTs. The Raman spectra of the sample, figure 2, have two major peaks: the D and G bands are observed which indicate the presence of crystalline graphitic carbon in the CNTs. The D band at 1340 [cm.sup.-1], has been attributed to the presence of amorphous carbon [22,23] due to surface defects of carbon nanotubes and graphitic carbon while the G-band band at about 1582 [cm.sup.-1] is that of ordered carbon nanotubes. The overtones due to these peaks in the spectrum is observed at 2673 [cm.sup.-1]. The intensity ratio of these two bands ([I.sub.D]/[I.sub.G]) is used as a parameter to characterize the quality of CNTs samples, with a higher intensity ratio indicating a higher degree of disorder in the CNTs. The intensity ratio for the two peaks obtained is moderate, showing that a good percentage of CNT was formed in the synthesis using the catalyst Fe/Ni on aluminium support.

The FE-SEM image of the prepared CNT is presented in figure 2. This shows the formation of carbon nanotubes using the mixed catalyst Fe/Ni supported on aluminium oxide. The synthesized carbon nanotubes have diameters ranging from 30 to 100 nm. The soot contains bundles of CNTs as well as a considerable amount of amorphous carbon and embedded metal particles. The scanning electron microscope image of the as-prepared carbon nanotubes show several carbon nanotubes embedded with carbonaceous material as well as catalyst particles, figure 3(b). At a higher magnification, figure 3(b), several carbon nanotubes could be seen.

Energy dispersive spectroscopy (EDS), figure 3, was employed to identify the concentration of catalyst embedded in the CNT. The EDS spectra of the synthesized CNTs indicate that the CNTs contain carbon as well as catalyst metals. The carbon is derived from the CNTs while iron nickel and aluminium are derived from the mixed catalyst and support material.




The HR-TEM micrograph of the prepared CNTs from Fe/Ni catalyst by decomposition of ethyne (acetylene) is presented in figure 4. The TEM image at 20 and 200 nm magnifications is a typical high resolution TEM image of carbon nanotubes, figure 5(a) and 5(b). The image at these magnifications indicates that there are catalyst particles encapsulated with the grown CNT which show that the CNT grew from the catalyst particles. In Figure 5(c), at 5 nm magnification, we see the tube consisting of several graphitic layers, approximately 60 layers with a hollow in the middle. Literature reports that the use of iron and nickel as catalysts leads to the formation of carbon nanotubes with an outer layer of thick amorphous carbon [24-26]. The outer layer of the carbon nanotubes in this figure shows a thick layer of amorphous carbon. The TEM image indicates that the carbon nanotubes are multiwalled.

The TGA profile of the CNTs is presented in figure 5. TGA analysis was employed to examine the thermal stability of the prepared CNTs over Fe/Ni catalyst. The temperature at which the CNTs are oxidized is an index of its stability. Thermogravimetric (TGA) and the derivative thermogravimetric curve of the weight loss (DTG) are in most instances used to investigate the presence of carbon nanotubes. The interpretations of these curves are not straight forward due to the presence of catalyst particles during weight loss analysis. The TGA graph show the TGA and derivative thermogravimetric curve of the prepared CNTs. Weight loss started at 582[degrees]C to about 670[degrees]C for the sample. We have attributed this to the removal of physisorbed and chemisorbed water molecules. A sharp endotherm observed in the TGA analysis. Sharp endotherms indicate crystalline rearrangements, a fusion or a solid-solid transition of a pure material. It is interesting to note from the weight loss analysis that only 30% of the carbonaceous material was burnt. No other secondary combustion took place during the TGA analysis. The curve of the derivative weight loss analysis shows only a single sharp peak with a broad base at 632[degrees]C. This characterizes the prepared carbon nanotubes as MWCNTs. The absence of any other further weight losses in the TGA/DTG graph after this temperature indicate that the catalyst supports are thermally stable above this temperature.




In conclusion, carbon nanotubes have been synthesized successfully by the decomposition of ethyne over Fe/Ni/[Al.sub.2][O.sub.3] catalyst. Raman spectral analyses as well as SEM images indicate that CNTs were produced while the TEM analyses show multiple layers in the Carbon nanotube at 5 nm.


This work was supported by a research grant from the Faculty of Applied and Computer Science Research and Publications Committee of Vaal University of Technology, Vanderbijlpark.


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Ezekiel Dixon Dikio * and Nolukhanyo Bixa

Department of Chemistry, Vaal University of Technology, P. O. Box X021, Vanderbijlpark 1900, Republic of South Africa

* Corresponding Author E-mail:
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Author:Dikio, Ezekiel Dixon; Bixa, Nolukhanyo
Publication:International Journal of Applied Chemistry
Date:Jan 1, 2011
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