Growth of few-wall carbon nanotubes with narrow diameter distribution over Fe-Mo-MgO catalyst by methane/acetylene catalytic decomposition
- Vladimir A Labunov†1Email author,
- Alexander S Basaev2,
- Boris G Shulitski1,
- Yuriy P Shaman†1, 2,
- Ivan Komissarov1,
- Alena L Prudnikava1Email author,
- Beng Kang Tay3 and
- Maziar Shakerzadeh3
© Labunov et al; licensee Springer. 2012
Received: 6 December 2011
Accepted: 2 February 2012
Published: 2 February 2012
Few-wall carbon nanotubes were synthesized by methane/acetylene decomposition over bimetallic Fe-Mo catalyst with MgO (1:8:40) support at the temperature of 900°C. No calcinations and reduction pretreatments were applied to the catalytic powder. The transmission electron microscopy investigation showed that the synthesized carbon nanotubes [CNTs] have high purity and narrow diameter distribution. Raman spectrum showed that the ratio of G to D band line intensities of IG/ID is approximately 10, and the peaks in the low frequency range were attributed to the radial breathing mode corresponding to the nanotubes of small diameters. Thermogravimetric analysis data indicated no amorphous carbon phases. Experiments conducted at higher gas pressures showed the increase of CNT yield up to 83%. Mössbauer spectroscopy, magnetization measurements, X-ray diffraction, high-resolution transmission electron microscopy, and electron diffraction were employed to evaluate the nature of catalyst particles.
Carbon nanotubes [CNTs] due to their incredible properties have attracted scientific and practical interest for almost 20 years . High electrical and thermal conductance, striking mechanical strength, and an especially unique chirality-dependence electronic structure make CNTs one of the most promising alternatives to replace some of today's materials used in microelectronics manufacturing. In particular, due to an enormous field enhancement factor and high conductivity, CNTs are perfect sources for electron emission . Tremendous numbers of attempts have been made to use carbon nanotube arrays as the field-emission cathodes [FECs] . Single-wall carbon nanotubes [SWNT] provide a large field enhancement factor, low threshold voltage, and high emission currents, but the substantial degradation of emission currents is a serious bottleneck for the application of SWNT-based FECs . In contrast, multi-wall carbon nanotubes [MWNTs] have high emission stability but have a small field enhancement factor. Few-wall carbon nanotubes [FWNTs] are considered as an ideal choice for this application . Despite the progress in producing high-impurity and high-yield few-wall CNTs , the cost and simplicity of CNT fabrication still remain as the major problems limiting their wide application. In general, the main methods of CNT synthesis are arc discharge, pulsed laser ablation, and chemical vapor deposition [CVD]. CVD method has been recognized as the most suitable for commercial production of CNTs. The use of large specific surface powder with catalytic sites distributed on it is the common approach for high-yield CNT processes. Fabrication of a catalyst for synthesis of CNTs with desired geometrical properties (number of walls) is the art of today's carbon nanotube technology.
So far, bimetallic catalysts are accepted as the most efficient for high-yield production of CNTs. Metal alloys such as Co-Ni , Fe-Co [8, 9], Fe-Mo [6, 10], and Co-Mo [11, 12] were utilized in catalysts for CNT growth. There are a number of works discussing the transition metals: molybdenum ratio and role of molybdenum in the efficient growth of CNTs with uniform diameter distribution [13–16]. A high amount of molybdenum in Fe-Mo alloy leads to the growth of larger-diameter CNTs, and vice versa, decreasing the Mo content in bimetallic alloy favors smaller-diameter CNT synthesis. This fact most likely relates to the catalytic particle size dependence on FexMoyOz phases from which these particles are released during the reduction process; in turn, oxide phase formation depends on calcination (annealing) processes .
Usually, hydrogen serves as a reduction agent for catalyst activation. Variation of hydrogen flow during the reduction step and hydrogen/hydrocarbon ratio during the synthesis affects the quality and purity of the CNT product . On the other hand, transition metal catalysts themselves are known as promoters of methane decomposition for high-rate hydrogen conversion . It allows us to avoid the use of any additional reducing agent for catalyst activation.
Simplification of the high-yield production method and reducing the cost of catalyst components are two crucial steps towards the commercialization of the FWNT product. In this paper, we describe the developed technological approach for a simple and efficient FWNT synthesis.
Finally, the catalyst was placed in a quartz boat and then directly loaded into the hot zone of a 15-mm tubular furnace at the temperature of 900°C while the furnace was purged with 100 cm3/min of Ar. When the temperature was stable, 40 cm3/min flow of methane was introduced. After 15 min, the CH4 flow was set to zero simultaneously introducing 10 cm3/min of acetylene flow. The CNT growth process was stopped by switching off the C2H2 flow, and the product was immediately removed from the hot zone and cooled down to the room temperature at 300 cm3/min of Ar flow which took about 5 min. The synthesis of CNTs at higher pressure was performed at the same conditions described above except that after 45 min of the process, the gas pressure was increased up to 2 bars and the process continued additionally for 20 min.
Various techniques were employed in order to characterize the prepared CNTs. Raman spectroscopy was performed using the Renishaw spectrometer with a laser wavelength of 514 nm (Renishaw, Wotton-under-Edge, UK). A JEOL 2010 transmission electron microscope (JEOL Ltd., Akishima, Tokyo, Japan) operated at 200 kV was used to study the microstructure. A DRON-3M X-ray diffractometer with CuK source was employed to collect X-ray diffraction [XRD] spectra (Bourevestnik Inc., St. Petersburg, Russia). The Mössbauer spectra were collected in transmission geometry at room temperature using the MS2000 spectrometer with 57Fe/Rh source (40 mCu) (Belarusian State University, Minsk, Belarus). Magnetization of synthesized CNTs was measured by a ponderomotive method . Calorimetric measurements were performed with a Mettler-Toledo instrument TGA/DSC-1/1600 HF (Mettler-Toledo, Inc., Greifensee, Switzerland) setup in dry air flow and at a heating rate of 1°C/min.
There are few peaks in the range below 300 cm-1 in the Raman spectrum. Commonly, peaks in this range are assigned to the radial breathing mode [RBM] of SWCNTs. Using the simple inverse relation v = 9 +235/d (where ν is the frequency in units of inverse centimeter, d is diameter of nanotubes in nanometers), we estimated the diameters of the nanotubes in the range of 0.9 to 1.7 nm; the larger diameters cannot be estimated from the spectrum because of the spectrometer frequency cutoff. Besides, these peaks in the range below 300 cm-1, apart from the RBM mode of SWNTs, can be attributed to the RBM mode of the inner tubes of FWNTs .
As we pointed in the introduction part, the simple and cheap technology for producing CNTs with desired geometry of carbon nanotubes opens the way to a wide range of applications of CNTs. We would like to emphasize that our technological approach does not contain any additional calcinations or reduction step after the catalyst preparation. Besides, catalyst preparation process is consuming a lot less time; the elimination of a calcination step prevents the large-size catalytic particle formation, thus limiting the diameter distribution of CNTs in the final product. Although iron nanoparticles are initially in the oxide state, the presence of methane partially reduces the oxides, which leads to the enhancement of the methane decomposition by the presence of metal particles. Bimetallic catalysts show better methane decomposition performance, and the decomposition rate depends on the Fe/Mo ratio . In our work the Fe/Mo ratio is optimized for few-wall nanotube synthesis.
Moreover, it was revealed that reaction with methane not only reduces the iron oxide to iron, but also turns some fraction of it into cementite , which is reported to be not catalytically active for methane/acetylene decomposition . From the magnetic measurement data and Mössbauer spectrum, we conclude that most of the nanoparticles are in Fe3C phase in the final product. Only the selected area electron diffraction data indicated the presence of some amount of γ-Fe phase. We do believe that some part of cementite observed by our ex-situ measurements was formed during the cooling process when carbon interacting with the γ-Fe particles transited to the cementite phase .
Growth of CNTs is considered as a multistep process involving formation of Fe3C from iron catalyst nanoparticles [26–28]. Formation of CNTs from cementite was demonstrated by in-situ TEM measurements . However, there are still debates whether Fe3C could promote formation and growth of very thin CNTs . In general, catalytic activity and carbon solubility of catalytic species depend on the temperature, diffusion, and particle size. Thus, catalyst species could be active or inactive depending on the carbon atom kinetics. Molybdenum, due to the higher carbon solubility as compared to iron , can serve as a regulator of carbon atom kinetics for neighboring γ-Fe and Fe3C catalyst species, preventing their poisoning at the conditions we perform our synthesis. The Fe2MoC phase observed using HRTEM in our experiment claimed to be inactive for CNT growth . That phase most probably arises from the FexMoyOz phase, and its formation is limited due to the absence of calcination. The low amount of Fe2MoC can be explained this way (Mössbauer and XRD spectra do not show any evidence of this phase).
Let us briefly discuss the impact of synthesis pressure on CNT yield. It was revealed that prolongation of reaction time in our experiments for more than 45 min did not increase significantly the mass of the CNT product. That fact usually contributed to the diffusion issue of carbon feedstock molecules through the mat of CNTs . However, it was found that increasing the pressure up to 2 bars in the furnace during CNT synthesis, as was described in the experimental part, leads to the CNT-yield increase approximately by 20 wt.% (see Figure 9, red curve). Thus, we attribute the longer catalytic activity and, as a consequence, the higher CNT yield at the realized synthesis conditions to the increasing hydrocarbon molecular diffusivity as the gas pressure increases.
The technological approach for the simple high-yield CVD growth of few-wall carbon nanotubes was developed. The Fe-Mo-MgO catalyst with 1:8:40 molar ratio was prepared by impregnation method from purum quality components without any calcinations and reduction steps. TEM, Raman, and thermogravimetrical studies showed that decomposition of methane/acetylene at 900°C over the catalyst leads to the formation of CNTs with diameters in the range of 2 to 10 nm. The phase of catalytic species in CNT was investigated by ex-situ Mössbauer, XRD, HRTEM, and electron diffraction techniques. The phase transitions of metallic components of binary catalyst and their role in the CNT growth process were discussed. It was demonstrated that slightly increasing the gas pressure during the CNT growth leads to the increase in the CNT yield.
chemical vapor deposition
differential scanning calorimetry
few-wall carbon nanotube
high-resolution transmission electron microscopy
multi-wall carbon nanotube
single-wall carbon nanotube
transmission electron microscopy
The authors would like to thank J. A. Fedotova for the Mössbauer spectroscopy measurements and data analysis and K.I. Yanushkevich for the X-ray diffraction and magnetization measurements.
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