Synthesis of multi-walled carbon nanotubes by catalytic chemical vapor deposition using Cr2 − xFexO3 as catalyst

Diamond & Related Materials 15 (2006) 1708 – 1713 www.elsevier.com/locate/diamond

Synthesis of multi-walled carbon nanotubes by catalytic chemical vapor deposition using Cr2 − xFexO3 as catalyst M.D. Lima ⁎, R. Bonadiman, M.J. de Andrade, J. Toniolo, C.P. Bergmann Av. Osvaldo Aranha, 99, sala 705-c, CEP 90035-190, Porto Alegre, Rio Grande do Sul, Brazil Ceramic Materials Laboratory, LACER, Materials Department, Federal University of Rio Grande do Sul, Brazil Received 31 July 2005; received in revised form 28 January 2006; accepted 17 February 2006 Available online 2 May 2006

Abstract Chromium oxide and iron oxide solid solution was used as a catalyst for multi-walled carbon nanotubes synthesis by the catalytic chemical vapor deposition technique. The catalyst was prepared by the solution combustion synthesis method. Natural gas (NG) was employed as a carbon source for the carbon nanotube growth instead of methane, which is typically used. The carbon nanotube synthesis was carried out under H2/NG and Ar/NG atmospheres at 950 °C. The Cr2 − xFexO3 catalyst was capable to produce carbon nanotubes only in H2/NG atmospheres. Partial elimination of the catalyst after the synthesis was possible using a concentrated solution of HNO3. © 2006 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Cr2O3; Natural gas

1. Introduction Carbon nanotubes (CNTs) are a new class of materials discovered in 1991 by S. Iijima [1]. They have attracted much interest from the scientific community because of their extraordinary mechanical, electrical and thermal properties. Several different processes for their synthesis have already been developed. Among the main ones are laser ablation technique, graphite electrodes discharge and catalyzed chemical vapor deposition (CCVD). The last one has the highest potential for the mass production on CNTs. The synthesis of CNTs by CCVD is dependent on the catalyst metallic particle formation. The growth of the CNTs occurs by deposition of carbon onto the catalyst particle, followed by its saturation with carbon and the precipitation of the carbon in the form of CNTs [2]. Eventually, the effect of the catalyst particle will cease because of excessive carbon deposition. This fact usually limits the efficiency of the catalyst. Despite the fact that CNTs have been discovered more than 13 years ago, and the intense research that has been done, a lowcost and high selectivity method for their synthesis was not ⁎ Corresponding author. E-mail address: [email protected] (M.D. Lima). 0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2006.02.009

developed yet, especially for single-walled carbon nanotubes (SWCNT). However, using the CCVD technique, several authors successfully synthesized CNTs employing different catalysts under diverse atmospheres and temperatures [3–11] with good efficiency. The synthesis effectiveness is measured by the ratio between the produced carbon during the synthesis and the amount of catalyst used in the process. In the case of multi-walled carbon nanotubes (MWCNT), a significant progress has been made concerning the controlled and continuous growth processes. Charanjeet et al. [3] achieved aligned MWCNT yields of 1.6 mg/cm2 h by the injection of a solution of ferrocene and toluene in a CVD reactor. Li et al. [4], using a non-continuous method and a Ni–Mo–MgO catalyst, achieved a very high yield (3000% in 30 min) in the MWCNT production. Some authors used the solution combustion synthesis (SCS) as a technique for the preparation of catalysts for carbon nanotubes production [12–17]. Using this technique, it is possible to produce oxides or oxide mixtures in a very quick manner, via an exothermic combustion reaction. Inorganic salts (usually nitrates) of the metals that will form the oxide catalysts are mixed in an aqueous solution. This solution is heated up until the water almost completely evaporates; after that, the temperature of the remaining material rises and it ignites [18]. In

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Table 1 Cr2 − xFexO3 catalysts formulations Formulation

% of metallic iron after reduction

Ferric nitrate (g)

Iron (mol)

Ammonium dichromate

Chromium (mol)

Fe/Cr (M)

Glycine (g)

0 1 2 3 4

0 10 20 30 40

0 3.05 6.86 11.76 18.3

0 0.008 0.017 0.029 0.045

12.95 12.95 12.95 12.95 12.95

0.100 0.100 0.100 0.100 0.100

0.00 0.08 0.17 0.29 0.45

3.75 4.66 5.79 7.25 9.19

order to keep the metallic ions in the solution and the system homogeneity, an organic complexant is added to the system before the water evaporation [18]. This complexant also acts as a combustible during the ignition of the mixture, which is oxidized by the nitrates decomposition. After the ignition, the resultant material is a mixture of several oxidized metals. Usually the SCS process produces homogeneous oxide mixtures with a high surface area and a small crystal size. During the CNTs synthesis, the catalyst is exposed to a reductive atmosphere which promotes the reduction of the catalyst metallic cations to the metallic state, forming the catalyst particles. The diameter of the catalyst particles depends on the local concentration of the catalyst metallic cations [19]. In order to obtain catalyst particles with uniform diameters it is important that the metallic ions are homogeneously dispersed in the support oxide. It is possible to achieve complete homogeneous dispersions of several metallic cations using compounds that form solid solutions (SS). Usually, catalysts produced by SCS for CNTs synthesis are SS between the catalyst metal oxide and the support oxide. Systems like Fe2O3–Al2O3 [20], NiO–MgO and CoO–MgO [21] are examples of S.S. catalysts. This approach ensures the homogeneous dispersion of the metallic catalyst and makes possible the mixture of oxides in any proportion within the solubility range, but only a restricted number of oxides can be mixed. It is also possible to produce crystalline compounds like the spinel LaFeO3, where LaO2 acts as the support [22], and the lacunar spinel phase Mg0.8MyMz′Al2O4 (M,M′ = Fe, Ni or Co, y + z = 0.2) [23]. There are also some reports about multiphase systems like Fe2O3–MO3–MgO [19], which segregate the components in nanometric particles intimately mixed.

In this work, we report the use of Cr2 − xFexO3 (a solid solution of Cr2O3 and Fe2O3) as a catalyst, which has never been used before. Since these oxides are widely miscible [24], a SS and a good dispersion of iron ions over the catalyst can be achieved, avoiding its segregation and the formation of nonuniform diameter catalyst particles under the reductive atmosphere used for CNTs synthesis. Given that, and considering that the mixture of the oxides is based on the formation of a solid solution, it is possible to mix these oxides at any desired proportion within the solubility limits of the Cr2O3– Fe2O3 system. Through the SCS technique it is possible to obtain nanometric oxides from the catalysts with a surface area higher than 50 m2/g [25]. Another innovative aspect of this work was the use of natural gas to the synthesis of the nanotubes, which is an extremely cheap source of carbon. Because of its low-cost and great availability, the use of natural gas could be interesting to the low-cost, mass production of carbon nanotubes. However, because natural gas is composed by several organic compounds and contains a certain amount of impurities (N2, CO2, H2O), the adjusting of optimum process parameters demanded considerable efforts. 2. Experimental procedure 2.1. Catalysts preparation The Cr2 − xFexO3 mixture was prepared by SCS, using glycine (C2H5NO2) as the complexant agent, (NH4)2Cr2O7 as the chromium oxide precursor, and Fe(NO3)3·9H2O as the iron oxide precursor. Ammonium nitrate was employed as an

Fig. 1. CNT synthesis apparatus: A) peristaltic pump, used for the injection of liquid precursors; B) flow meters; C) liquid precursors evaporator; D) electric oven; E) substrate for nanotubes growth; F) thermocouple; G) mullite tube; and H) boron silicate view window.

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Table 2 CNT synthesis parameters Condition Synthesis Synthesis temperature (°C) time (min)

Ar NG (l/h) (l/h)

H2 (l/h)

Heating/ cooling gas

1 2

100 –

– 100

Ar Ar

950 950

30 30

20 20

extra oxidizer in order to increase the volume of gases generated in the reaction, aiding to the increase in the surface area and to the reduction of the size of the crystallites [25]. Four different formulations were prepared (Table 1). All of them were dissolved in 100 ml of water. An aliquot of 5 ml of HNO3 was added to facilitate the dissolution process. A sample of 10 ml was placed in a stainless steel cup to evaporate the solvent and to ignite the mixture. This process was carried out in a pre-heated oven at 400 °C. The produced powders were then heat treated at 900 °C for two hours, in order to promote its crystallization and a better catalyst homogenization. After the synthesis, the catalysts were macerated with water to produce a viscous mass. One drop of each catalyst was placed over an alumina substrate, in such a way that several formulations could be tested at a determined synthesis condition simultaneously.

Fig. 3. Catalyst prepared according to formulation 4 (Table 1) and exposed to condition 2 (Table 2).

2.3. Catalysts and CNT characterization

2.2. CNT synthesis

Both catalysts and synthesized nanotubes were firstly characterized by X-ray diffractometry, using a Philips diffractometer (X'Pert MPD model) operating with a copper anode at 40 kV and 40 mA, and equipped with a graphite monochromator. Electron microscopy was also used in the samples characterization. A scanning electron microscope (JEOL–JSM 580) and a 200 kV transmission electron microscope (JEOL–JEM2010) were used. Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were employed to determine the temperatures of air oxidation of the carbonaceous materials and the carbon yield. The carbon yield was determined according to the equation:   ðMI −MF Þ Carbon–yield ðwt:%Þ ¼  100 MF

The CNT synthesis was carried out in a mullite tubular reactor with 50 mm diameter and 500 mm length (Fig. 1). The oven is externally heated by electric resistances. Natural gas (91.8 CH4, 5.580 C2H6, 0.970 C3H8, 0.030 C4H10, 0.100 C5H12, 0.800 CO2, 1.420 N2, vol.%) was employed as a carbon source. The catalysts were placed under the oven internal thermocouple, as can be seen in Fig. 1. The synthesis was made at room pressure. The time to reach synthesis temperature from room temperature was 30 min. The heating was performed under an argon atmosphere. The CNT synthesis conditions are shown in Table 2. When the synthesis time was elapsed, the natural gas flow was interrupted and the carrying gas flux was raised to 400 l/h during 2 min, in order to quickly remove the NG from the oven. Samples were cooled down to room temperature inside the oven with argon flowing.

where MF is the total weight obtained after DTA/TGA analysis, MI is the weight of the catalyst and carbon after the CNT synthesis. A Harrop model ST-736 equipment was used for that purpose. A

a

b

10

Intensity

Intensity

Cr1.3Fe0.7O Fe2O3

30

50

70 90 2θ (degree)

10

30

50

70 90 2θ (degree)

Fig. 2. X-ray diffraction pattern before (a) and after calcination at 900 °C for two hours (b) of formulation 4.

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105 100 95

weight loss (%)

90 85 80 75

catalyst from formulation 4 CNT from formulation 1

70

CNT from formulation 0 CNT from formulation 2

65

CNT from formulation 4 CNT from formulation 3

60 250

350

450

550

650

750 850 Temperature (°C)

Fig. 4. TGA analysis of the catalysts before and after the CNTs synthesis in condition 2.

sample of carbonaceous material of 50 mg was placed in a platinum crucible, and the test was conducted at a heating rate of 5 °C/min up to 800 °C with an airflow of 20 l/h, approximately. Raman spectroscopy was also employed. The equipment used a 10 mW He–Ne laser with an excitation radiation of 632.8 nm. The spectra were recorded by a CCD detector cooled by liquid nitrogen. 3. Results and discussion 3.1. Cr2 − xFexO3 catalyst Fig. 2 shows the X-ray diffraction patterns of the catalysts prepared according to formulation 4 (Table 1). These patterns

were obtained immediately after combustion synthesis (Fig. 2a) and after heat treating at 900 °C for two hours (Fig. 2b). The combustion synthesis process of the catalyst produced an amorphous powder (Fig. 2a), and it is not possible to confirm the formation of a solid solution between Fe2O3 and Cr2O3, which is a necessary condition for good iron dispersion within the catalyst. The formation of a solid solution occurred after heat treating (Fig. 2b). The expected composition of the formed compound, based on the amounts of reagents used in the formulation, should be Cr1.1Fe0.9O3; however, the obtained compound was identified by X-ray diffraction as the phase Cr1.3Fe0.7O3, indicating a slight deficiency of iron. Some part of the iron was consumed in the formation of hematite (Fe2O3),

Fig. 5. Cr2 − xFexO3 catalysts after CNTs synthesis in condition 2. Catalysts prepared according to formulations 1 (A), 2 (B), 3 (C) and 4 (D).

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which was also detected in the X-ray diffractometry. The other formulations containing iron presented similar X-ray diffraction patterns.

Cr7C3 Graphite/Nanotubes

Only the iron containing catalysts (formulations 1, 2, 3 and 4) exposed to H2/NG (condition 2, Table 2) atmosphere led to the production of carbon nanotubes. Scanning electron microscopy analyses (Fig. 3) and Raman analysis (Fig. 6) indicated that these tubes are multi-walled ones, with diameters ranging from 5 to 40 nm (Fig. 3). When the Ar/NG (condition 1) atmosphere was used, the presence of carbon nanotubes and carbon fibres was not detected by electron microscopy and the Raman spectra (Fig. 6) showed a low G/D ratio, characteristic of disordered carbon or high defective carbon nanotubes. The increase of the iron amount in the catalyst caused a higher production of CNTs, as shown by the TGA analysis (Fig. 4). The carbon yield was slightly higher (34%) using the catalyst containing 40% iron (formulation 4). This yield was higher than that obtained by Coquay et al. [13], who used an Fe–MgO catalyst obtained by S.C.S and urea as combustible (yield of 20%), but lower than the yield obtained by Flahaut et al. [12], who used a MgO–CoO–MoO3 catalyst and citric acid as combustible (yield of 56%), and also lower than that obtained by Li et al. [4]. The TGA analysis of the catalyst before the CNT synthesis showed that a weight loss occurs in temperatures between 250 and 650 °C, probably caused by the burning of residual organic material and water loss. However, the catalyst is relatively stable in the CNT oxidation temperature; thus, the weight loss in this temperature range is mainly caused by the CNTs combustion. Apparently, the increase on the iron amount in the catalyst did not cause significant changes in nanotubes diameters, as can be seen in the SEM micrographies (Fig. 5). Fig. 6 shows the Raman spectra of the CNT produced. For the CNTs synthesized in H2/NG (condition 2, Table 2) the G/D ratio lies between 1.65 D

Fe3C

Intensity

3.2. CNT synthesis

10

Cr2O3

30

50

70

90 2θ (Grads)

Fig. 7. X-ray diffraction pattern of the Cr2 − xFexO3 catalyst after CNTs synthesis under a H2/NG atmosphere.

and 1.95, which is characteristic of MWCNT. The iron concentration did not significantly modify the G/D ratio as well. The better performance of Cr2 − xFexO3 catalysts under H2/ NG atmospheres is probably related to the dissolution of the catalyst metal oxide in another more stable oxide. Since the iron oxide is dispersed in a chromium oxide matrix, its reduction to metallic iron by the oven atmosphere becomes more difficult, and a pure hydrogen atmosphere would be necessary in order to completely reduce Fe2O3. This was observed for the CoO– MgO system by Pinheiro et al. [26]. Despite the fact that the synthesis atmospheres used in this article are more suitable for production of SWNT, only MWCNT was observed. This could be attributed to the catalysts characteristics. According to Dupuis [27], the CNT diameter is determined by the diameter of the catalyst metallic particles, and a direct relation between the diameter of the catalyst particle and the diameter of the SWCNT can be observed. The catalyst particle diameter is strongly affected by the support oxide, by the nature of the catalyst metal and its concentration [29,30]. Usually, for the SWCNT growth, the diameter of the catalyst particle should be between 0.7 and 3 nm [28]. Probably, the

G Formulation 4- Cond. 1

Graphite/Nanotubes Cr2O3 Intensity

Formulation 2- Cond. 2

Formulation 3- Cond. 2

Formulation 4- Cond. 2

10

30

50

70

90 2θ (Grads)

1000

1100

1200

1300

1400

1500

1600

1700

1800

Fig. 6. Raman spectra of the CNTs produced using formulations 2, 3, and 4 under condition 2 and CNTs produced using formulation 4 under condition 1.

Fig. 8. X-ray diffraction pattern of the Cr2 − xFexO3 after CNTs synthesis under a H2/NG atmosphere and treatment with nitric acid (65%) at room temperature during two hours.

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catalyst particles formed in the system Fe2O3–Cr2O3 are outside this range, being more appropriated for MWCNT growth. The X-ray diffraction analysis of the catalyst after the CNT synthesis (Fig. 7) shows that the chromium oxide was reduced and transformed into chromium carbide under a H2/NG atmosphere. Almost all Cr2O3 was reduced. According to Mitchell et al. [31] and Grabke [32], chromium carbides like Cr7C3 are more stable than iron and nickel carbides, concerning their decomposition into carbon and metal. Carbon nanotubes can be produced from the continuous formation and decomposition of iron and nickel carbides in the catalyst nanoparticles (Pérez-Cabero et al. [33]; Schaper et al. [34]); however, for particles containing chromium carbides, the reaction could not proceed, because of their higher stability. In fact, using the catalyst with only chromium oxide (formulation 0) no CNT was synthesized, indicating that the chromium oxide or the chromium carbide doesn't act as a catalyst and only as support in the CNTs synthesis. The removal of the catalyst oxide after the CNT synthesis is facilitated by the reduction of the chromium oxide to chromium carbide. Chromium oxide is extremely resistant to acid etching, even those highly oxidants such as HNO3 [35]. However, after treatment with concentrated HNO3 (65%) at room temperature during two hours, it was possible to remove a considerable fraction of the support. As can be seen in Fig. 8, the peak corresponding to the [100] plane of graphite is much higher when compared to that showed in Fig. 7. 4. Conclusions Multi-walled carbon nanotubes were synthesized employing a Cr2 − xFexO3 solid solution as a catalyst. It was possible to synthesize carbon nanotubes using natural gas as a carbon source. The catalyst performance was better under H2/NG than in Ar/NG atmospheres. The H2/NG atmosphere led to the transformation of chromium oxide into chromium carbide. In absence of iron no CNT was synthesized, indicating that the chromium oxide or the chromium carbide doesn't act as a catalyst and only as support in the CNT synthesis. The higher amount of CNTs was obtained with the use of a formulation containing 40 wt.% of iron. Using a strong oxidizing acid as concentrated HNO3 was possible to remove partially the chromium compounds after the CNTs synthesis. Acknowledgments Authors are grateful to the members of LACER (Laboratory of Ceramic Materials) for their contribution to the development of this study, and to the CME/UFRGS (Electron Microscopy Center of the Federal University of Rio Grande do Sul) staff, who made the SEM and TEM analyses possible.

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