Synthesis and purification of bimetallic catalysed carbon nanotubes in a horizontal CVD reactor

Carbon nanotubes (CNTs) were synthesised by a conventional chemical vapour deposition (CVD) method using acetylene as carbon source and a bimetallic catalyst of Fe–Co supported on a CaCO₃ support. The CNTs were characterised by transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy (RS), energy dispersive X-ray spectroscopy (EDS) and thermogravimetric analysis (TGA). The TEM images show clustered CNTs and reveal the outer and inner diameters of these nanomaterials. The XRD analysis shows the characteristic broad peak of graphitised carbon; the RS indicates that these materials have a high degree of crystallinity while the TGA shows the high thermal stability of the materials. EDS analysis also indicates that the purification method employed was able to remove the impurities in the CNT samples.

Keywords: CNTs; CVD; bimetallic catalyst; graphitised carbon; purity

The overwhelming attention that carbon nanotubes (CNTs) are currently receiving from researchers can be attributed to their exceptional properties and wide range of their applications [[1]]. The synthesis of these materials has been increasing on a daily basis to meet their demand for many technological applications. A number of technological advancements ranging from agriculture, electronics, medicine, aerospace and power, to automobiles, can be traced to the application of CNTs [[4]].

Apart from the electric arc discharge [[11]] and laser ablation methods [13], chemical vapour deposition (CVD) is one method of synthesising CNTs that has received a lot of attention from many researchers in recent times. This is due to its flexibility, low cost of fabrication and the ability to produce CNTs in large quantities [14]. Many modifications of the conventional CVD method have been adopted to enhance the production rate and purity of CNTs. Gaseous organic compounds are commonly used as carbon sources for synthesising CNTs. These carbon sources as well as catalysts, employed in the synthesis of CNTs by CVD method have significant effect on the type and quality of CNTs produced [[15]]. The catalysts used to synthesise these materials usually contain transition metals which have the ability to decompose the carbon sources [[18]].

The as-synthesised CNTs obtained by the CVD method are usually associated with metallic particles, amorphous carbon and support impurities. The removal of these accompanying impurities in CNTs is a great challenge since high purity is a fundamental requirement for the optimal performance of CNTs in many applications [21],[22]. Therefore, several purification methods have been developed to separate and eliminate these impurities [[23]].

In this study, CNTs were synthesised by CVD method with C2H2 as carbon source, nitrogen as the carrier gas and a bimetallic (Fe–Co) catalyst supported on CaCO3. The nucleation and growth mechanism of CNTs using this catalyst are also discussed. The synthesised CNT samples were purified with concentrated HNO3 and characterised by transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), energy dispersive X-ray spectroscopy (EDS) and Raman spectroscopy (RS).

The catalyst used to grow CNTs was prepared using Fe and Co metals supported on CaCO3. A 10 wt% catalyst, which contained equal proportions by weight of iron and cobalt, was prepared by dissolving 2.47 g of Co(NO3)2 · 6H2O and 3.62 g of Fe(NO3)3 · 9H2O in about 50 cm3 distilled water. This solution was added to 10 g of CaCO3 and the mixture was left to age for 60 min under constant stirring. The resulting slurry was allowed to semi dry at room temperature, after which it was dried in an air oven at 120°C for 12 h, cooled to room temperature, ground and finally screened through a 150 µm sieve. The fine powder (the catalyst) was then calcined at 400°C for 16 h. The total metal loading on the support was 5 wt% of both iron and cobalt (10 wt% total metals).

The experimental setup using a horizontal CVD reactor apparatus is as shown in Figure 1. The fabrication of the CVD reactor apparatus has been described elsewhere [[29]] and is similar to conventional CVD reactor reported by many authors [5],[13],[21].

Graph: Figure 1. Schematic of the horizontal CVD reactor.

CNTs were synthesised by the decomposition of acetylene (Afrox, 99% purity) in the tubular quartz reactor that was placed horizontally in the furnace. The furnace was electronically controlled such that the heating rate, reaction temperature and gas flow rates could be accurately maintained as desired. The catalyst was loaded into a quartz boat (120 mm × 15 mm) at room temperature and the boat was placed in the centre of the quartz tube. The furnace was then heated at 10°C/min while N2 was flowing over the catalyst at 300 mL/min. Once the temperature had reached 700°C, the N2 flow rate was reduced to 240 mL/min and C2H2 was introduced at a constant flow rate of 90 mL/min. After 60 min of reaction time, the C2H2 flow was stopped and the furnace was left to cool to room temperature under a continuous flow of N2 (40 mL/min). The boat with its contents was removed from the reactor and weighed to quantify the amount of CNTs that had been synthesised. All reactions were carried out at atmospheric pressure in the absence of oxygen.

In this study, a liquid-phase oxidation purification method was adopted to eliminate catalyst, support and amorphous impurities. This method was adopted to eliminate the possibility of damage to the structure of these materials [32],[33]. Since the CNTs usually synthesised by CVD methods are accompanied by many other types of carbon particles, it was essential to use a strong oxidant to purify the raw CNTs. Therefore, the as-prepared CNTs were purified with a mixture of concentrated HNO3 and H2SO4 in the ratio of 3 : 1 by volume. The amount of acid used was determined from the stoichiometry of reaction between iron and these acids. The raw CNT samples were soaked in the mixture of these acids and vigorously stirred for 48 h at room temperature. They were subsequently washed repeatedly with distilled water until a pH of about 7 was achieved. They were dried in air at 120°C for 12 h. The treated CNT samples were ground into fine particles and characterised using the TEM, EDX, XRD and TGA.

The catalysts that were prepared in our laboratory contained Fe and Co (10 wt% in a 1 : 1 ratio) supported on CaCO3 substrate. The choice of CaCO3 as catalyst support was due to its ability to reduce the formation of carbonaceous particles on the surface of CNTs. This quality of CaCO3 as a catalyst support can be attributed to the non-porous nature of CaCO3, which suppresses the formation of amorphous carbon during the growth of the nanotubes and consequently promote the selective formation of CNTs. Mhlanga et al. [34] reported in their work that silica gel or zeolites are excellent catalyst support for CNT synthesis. But porous material supports such as zeolite are liable for the accumulation of huge amount of amorphous carbon during CNT synthesis. This leads to a multi-step purification process, which in turn leads to structural damage and introduction of some other impurities to the CNTs. The choice of CaCO3 as support is thus favoured in preference to porous material support. The optimisation of the CNTs production parameters in this reactor has been fully investigated and presented elsewhere [31].

A two-step cyclic mechanism, which is similar to the one proposed by Magrez et al. [35], can be used to describe the nucleation of CNTs when CaCO3 is employed as the catalyst support. This mechanism starts with a dynamic equilibrium reaction of CaCO3 at the reaction temperature to yield CaO and CO2 (1). This is followed by the reaction of acetylene and CO2 to yield CNTs and regenerate CO and/or hydrogen (2) and (3). These reactions take place at a triple-point junction (Fe–Co/CaCO3/C2H2) around the catalyst–support interface to enhance the conversion of acetylene to nanotubes which grow during the reaction time, with the pyrolysis temperature employed [35].

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The growth of the tubes is enhanced at the temperature employed (700°C) by the existence of this triple joint of (Fe–Co/CaCO3/C2H2), while the CO2 regeneration is possible by the water gas shift reaction (4) or by CO disproportionation (5).

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The CO2 is assumed to act as an etching agent that prevents catalyst poisoning. It is also assumed that CO2 limits acetylene polymerisation, which occurs by a homogeneous radical chain reaction to produce more stable oligomers along with heavy oils [35]. Thus the production of CNTs over CaCO3 is a 'clean process' and suitable for industrial applications.

Figure 2 shows the representatives TEM images of CNTs deposited on the surface of Fe–Co/CaCO3 catalyst. The CNTs are about several millimetres long and their typical inner and outer diameters are 9.2 and 17.9 nm, respectively, with wall thickness of ∼5 nm (Figure 2c). The TEM images reveal that the CNT samples are relatively free of amorphous carbon. EDS analysis confirmed the presence of small amounts of Fe and Co in the samples which are residual particles encapsulated in the matrices of the tubes during CNT growth at the reaction conditions specified earlier (Figure 3).

Graph: Figure 2. TEM images of as-synthesised (a and b) clustered CNT samples (c) CNTs showing the wall thickness and diameter of the tube (d) CNT samples showing the catalyst entrapped within the matrix of the tubes.

Graph: Figure 3. EDS spectrum of raw CNTs showing the presence of Fe and Co.

The XRD pattern of the CNT sample (Figure 4) shows the characteristic pattern of graphitised carbon with the graphitic line (002) at diffraction peak of 25.8° which corresponds to inter-planner spacing of about 0.343 nm. This is characteristically attributed to multi-walled CNTs [3].

Graph: Figure 4. XRD pattern of the CNT sample.

The effect of the mass of the catalyst on the yield of CNTs produced was studied under the same reaction conditions: temperature (700°C); flow rate of acetylene (90 mL/min) and nitrogen (240 mL/min); and reaction time (60 min); and the results obtained are presented in Figure 5. The result indicates that as the amount of catalyst is increased, the quantities of CNTs produced also increased. This can be attributed to the fact that an increment in the mass of the catalyst provides more reaction surface for the selectivity of the nanotube formation, while the low mass only contributes to high carbon deposition activity, which possibly results in the formation of amorphous carbon on the surface of the CNTs [36]. The growth mode of the bimetallic catalyst is based on a strong interaction between the metals and the support which produced a good yield of CNTs. This pattern of results also indicates that CaCO3 is a good support for Fe/Co bimetallic catalyst for CNTs production as elucidated earlier. The mass of the catalyst was restricted by the size of quartz boat (120 mm × 15 mm) used to synthesise these materials.

Graph: Figure 5. Effect of mass of catalyst on the quantity of as-produced CNTs.

While the increase in mass of the catalyst positively affects the quantities of CNTs produced (Figure 5), the reverse is the case on the purity of the synthesised CNTs. The higher mass of the catalyst implies higher concentration of Fe/Co, which resulted in the increase in quantities of Fe/Co particles deposited on the surface of the CNTs, as shown in Figure 6.

Graph: Figure 6. HMTEM images of as-produced CNTs at different values of mass of catalyst (a) mass of catalyst = 0.2 g (b) mass of catalyst = 0.4 g (c) mass of catalyst = 0.6 g; (d) mass of catalyst = 0.8 g; and (e)–(f) mass of catalyst = 0.9 g.

Figure 6(a) shows the high-magnification TEM (HMTEM) images of CNT produced when the mass of catalyst is 0.2 g. The image shows a well-graphitised wall structure of CNTs and a little quantity of catalyst particles without an indication of amorphous carbon coverage on the CNTs. The images presented in Figure 6(a)–(d) show that the catalyst particles on the surfaces of the CNTs increased with an increase in mass of the catalyst. It can also be observed from Figure 6(f) that the high-magnification image of the CNTs produced with 0.9 g catalyst are a mixture of baboons and cylindrical structures with one end corked with the catalyst while the other end is closed. It could be inferred from the results obtained on the effects of the mass of the catalyst on the quantities of CNTs produced that the mass of the catalyst positively affects the quantities of CNTs, but it adversely affects the purity of the synthesised CNTs. This is also true of the increase in the metal catalyst loading on the quality of the CNTs synthesised, as reported elsewhere [31].

The CNTs produced by CVD reactors usually contain various impurities which are mainly catalyst metal particles. Other impurities present include: catalyst support materials and non-CNT microstructures such as amorphous carbon and fullerenes that impede the utilisation of the unique properties of CNTs. All these impurities have to be removed before they can be suitable for any application. Also, the as-synthesised CNTs usually have both ends closed which may hinder their adsorption and capillarity properties. Therefore, the purification procedure should involve both opening of the tubes and the elimination of these impurities. Among the various purification methods reported in literature, acid treatment using a mixture of concentrated nitric acid and sulphuric acid in the ratio 1 : 3 was the most effective. The mixture has been reported to cause no damage to the CNT structure. The mixture has also been reported to promote formation of functional groups [29],[[37]]. In our studies using these CNTs, we observed that the functional groups provide good surfaces for anchoring Pt nanoparticles for fuel cell applications [40].

The comparative analyses based on the surface morphology of the as-synthesised and purified CNT samples are presented in Figure 7. It can be seen from the images that the as-synthesised CNT sample contained impurities such as amorphous carbon, iron and cobalt particles. The presence of amorphous carbon impurities can be attributed to low pyrolysis temperature (700°C) used in the production of the CNTs [41], while the metal impurities are obtained from the excess metal catalyst particles that could not react during the synthesis of CNTs. The tiny black particles shown within the matrices of these materials are identified to be iron and cobalt particles, as confirmed by the EDS analysis (Figure 7).

Graph: Figure 7. TEM micrographs of (a)–(b) as produced CNTs and (c)–(d) purified CNTs.

The bimetallic (Fe and Co) employed for the synthesis of CNTs was supported on CaCO3. This support decomposed at the pyrolysis temperature to form CaO and CO2 as shown in the Equation (6) below.

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Therefore, traces of calcium oxide are likely to be present within the as-synthesised CNT sample. During the acid treatment, this metal oxide was dissolved by the highly concentrated nitric acid to produce soluble calcium nitrate (7), which was thoroughly washed and filtered away during the purification process.

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Based on the stoichiometry of the reaction, it requires about 4 mL of acid to dissolve 1 g of CaO, and 8.4 cm3 of concentrated HNO3 is enough to remove the metal impurities from the raw CNTs but for effective purification, excess of this acid was used in addition with H2SO4 for complete removal of the CaO and other metal impurities present in the as-synthesised CNT sample. The affinity of the HNO3 acid towards the amorphous carbon and crystalline carbon depends strongly on its concentration. Hence, the high concentration of the acids employed in this study produced effective purification of the CNT samples [39],[42],[43].

The HMTEM images of the purified sample (Figure 7(c)–(d)) show that the purification steps adopted in this study did not damage the CNT structures nor generate additional contaminants on these materials. The number of catalyst particles, which are the dark spots shown in Figure 7(a)–(b), is reduced. This observation was also confirmed by EDS analysis shown in Figure 8. It can be observed from the EDS analysis of the purified CNTs that the characteristic peaks for Fe and Co have been removed, which confirmed the total removal of the metallic impurities from the CNT samples.

Graph: Figure 8. EDS analysis of the purified CNT sample.

The RS is an effective way to evaluate the quality of CNTs. The Raman spectra of the purified and unpurified CNT samples shown in Figure 9 indicate two main peaks at around 1350 and 1600 cm−1, which are designated as the D-band and G-band [[44]]. The D-band represents the disorder-induced mode, while the G-band can be attributed to a C–C stretching mode of well-graphitised CNTs [45],[47],[48]. The purified CNT sample shows a broader peak for both D and the G modes than the as-synthesised sample. This closely indicates that the purified CNT sample contains as low impurities as possible.

Graph: Figure 9. Raman spectra of purified and unpurified CNT samples.

The relative intensity ratio of the D and the G bands, , is known to depend on the structural characteristics of carbon and is used to measure the disorder in CNTs [47]. The ratio gives information about the perfection of the graphite layer structure and reflects the properties of the edge plane or boundary of the graphite crystal faces. As the ratio increases, the defect structure increases and the degree of graphitisation becomes less [49]. The ratios of 0.86 and 0.85 were obtained for unpurified and purified CNT samples, respectively, which show that the purification process improves the crystallisation of the CNTs.

The TGA examines the kinetics of a chemical process, the percentage weight and derivative weight change in the sample as a function of temperature [50]. Figure 10 shows the TGA curves of the purified and as-synthesised CNT samples which were carried out in an oxygen atmosphere to monitor the weight loss and thermal stability of the samples. These analyses were carried out on Perkin Elmer Pyris 1 TGA analyser, by heating about 15 mg of CNT sample under a continuous flow of air. The temperature was initially held at 25°C for 1 min before being heated to 900°C at the rate of 10°C/min.

Graph: Figure 10. TGA analysis of purified and unpurified CNT samples.

The TGA curve for the purified CNT sample shows a mass loss of 98% at T < 500°C which indicates that CNT is relatively pure. Since amorphous carbons were expected to have been oxidised during acid and thermal treatment, the 2% impurity is suspected to be due to residue metallic particles, which were possibly trapped within the crystalline tubes and could not be removed by acid treatment. On the other hand, the curve for the as-synthesised CNT sample shows a mass loss of 83% (starting at a much lower temperature of 450°C). This behaviour can be traced to the presence of impurities, particularly the presence of traces of the support in this sample [31],[51].

CNTs were synthesised in a horizontal CVD reactor using acetylene as carbon source and bimetallic catalyst of Fe–Co prepared on CaCO3 support. The TEM images reveal that these materials have lengths of several millimetres with traces of metal impurities. The quantity of the CNT synthesised increased with increase in mass of the catalyst used while the quality decrease with increase in the quantity of the catalyst employed. The XRD analysis shows the characteristic broad peak of graphitised carbon, and the RS indicates that the CNTs have high degree of crystallinity. The TGA shows that the CNT sample is thermally stable up to 500°C, and the EDS analysis confirms that the purification employed removed the metal impurities without causing any harm to the CNT structures.

The authors acknowledge the support of DST/NRF Centre of Excellence in Strong Materials University of the Witwatersrand, Johannesburg, South Africa and Professor Mike Witcomb during TEM analysis.