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Metal catalyzed synthesis of carbon nanostructures in an opposed flow methane oxygen flame
Combustion and Flame 135 (2003) 27–33
Metal catalyzed synthesis of carbon nanostructures in an opposed flow methane oxygen flame Alexei V. Saveliev, Wilson Merchan-Merchan, Lawrence A. Kennedy* Department of Mechanical Engineering, University of Illinois at Chicago, Chicago, IL 60607-7022, USA Received 10 February 2003; received in revised form 19 May 2003; accepted 19 May 2003
Abstract Results of an experimental study on metal-catalyzed synthesis of carbon tubular nanostructures in opposed flow flame are reported. The catalytic support made of Ni-alloy was positioned at the fuel side of the opposed flow flame formed by fuel (96%CH4⫹4%C2H2) and oxidizer (50%O2⫹50%N2) streams. The electron microscopy studies reveal the presence of highly organized carbonaceous structures with the configurations showing strong dependence on the flame location. Several typical structures were detected. These include: multiwalled nanotubes (MWNT), MWNT bundles, irregular high-density carbon nanofibers, long (up to 0.2 mm) uniform-diameter (⬃100 nm) nanofibers, helical regularly coiled tubular nanofibers, and ribbon-like coiled nanofibers with rectangular cross section. Transmission electron microscopy (TEM) studies performed on long nanofibers revealed the presence of highly organized multiple (⬃100) graphene layers. These layers are parallel to the nanofiber axis resembling the structure of MWNT. The TEM studies of coiled nanofibers show internal tubular structure and the presence of regular carbon lattice. The well-aligned bundles of nanotubes were examined by TEM showing tight packing of MWNTs with varying inner and outer diameters. The diversity of formed nanomaterials is attributed to the strong variation of flame properties along the flame axis including temperature, hydrocarbon and radical pool. This provides strong selectivity for formation of different nanoforms even without adjustment of catalyst properties. © 2003 The Combustion Institute. All rights reserved. Keywords: Carbon nanotubes; Combustion synthesis; Nano processing
1. Introduction Numerous potential applications of novel carbon nanomaterials include nanoconductors and nanoswitches, nanomechanical devices, composites with unique mechanical and electromagnetic properties, field displays, and storage of hydrogen and natural gas [1,2]. This fosters developments of synthesis and purification methods. Among widely applied and well-characterized methods are arc discharges [3,4], pulsed laser vaporization [5], and chemical vapor * Corresponding author. Tel.: ⫹1-312-996-2400; fax: ⫹1-312-996-8664. E-mail address: [email protected] (L.A. Kennedy).
deposition [6]; most of these synthesis techniques require the introduction of catalyst in the form of gas particulates or as a solid support. The selection of a metallic catalyst, such as cobalt, iron, or nickel has been shown to influence growth and morphology of generated nanotubular structures. The application of flames as a relatively inexpensive, robust, pyrolysing carbon source for growing tubular nanomaterials was recently studied in a number of publications. Synthesis methods with introduction of catalyst as a particle in the gas phase and in the form of solid support are implemented using mainly co-flow diffusion flame configurations. The formation of filamentous carbon on a wire probe [7] or metal screen [8] inserted in the fuel side
0010-2180/03/$ – see front matter © 2003 The Combustion Institute. All rights reserved.
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of the diffusion flames were reported in very early flame studies. The formed filaments were hollow with metal granules at the tips. The reported deposit structures closely resembled structures of carbon filaments formed by CO and CH4 decomposition over metal catalysts [9,10]. From our previous knowledge, it is doubtful that the pioneers in the field were adequately equipped to characterize the nanostructures formed in the deposited material. It is very reasonable to assume that nanotubes and nanofibers, if formed, were overlooked in these early studies. Recently, Yuan et al. [11] studied flame-based growth of nanotubes on a Ni/Cr support in laminar co-flow methane-air diffusion flame. Formation of entangled multiwalled nanotubes (MWNTs) with a diameter range of 20 to 60 nm was reported. In related work, Yuan and co-workers [12] used a catalytic support in the form of stainless steel grid to produce MWNTs in ethylene-air diffusion flame. The electroplating of grid with cobalt resulted in synthesis of aligned MWNT growth. The metal catalyst dispersed on TiO2 substrate was used by Vander Wal [13] to generate MWNTs in ethylene/air and acetylene/air diffusion co-flow flames. It was demonstrated that the structure of the obtained nanotubes and nanofibers strongly depends on the catalytic particle shape and chemical composition. The importance of the catalytic substrate positioning in flame is emphasized by the co-flow studies [11–13]. However, the influence of this factor has not been adequately studied due to the small flame volumes and strong flame-probe interactions. Use of counter-flow flame configuration can provide better sampling possibilities and improve interpretation of results. Additionally, it was shown recently [14,15], that a counter-flow oxy-flame has a strong potential for nanotube growth producing high temperatures and high radical concentrations. In the present work, we report results on carbon nanotubular structures formed on a catalytic probe inserted in the fuel zone of an opposed flow oxygen enriched flame. Experiments performed at fixed flame parameters with a single catalytic material show the presence of several characteristic regions with essentially varying properties of the formed carbon nanomaterials. Initial characterization of the formed nanostructures, performed by the scanning electron microscope (SEM) studies of the probe surface, shows the presence of MWNTs, irregular nanofibers, heavy-wall constant-diameter regular nanofibers, and regularly coiled nanotubes. The highresolution TEM studies of these objects are performed by dispersing the deposited material on the TEMs grids.
Fig. 1. Schematic of the experimental setup.
2. Experimental Figure 1 shows the experimental setup used in the present study. A counter-flow burner forms two opposite streams of gases; the fuel (methane seeded with 4% of acetylene) is supplied from the top nozzle and the oxidizer (50% O2⫹50%N2) is introduced from the bottom nozzle. The top and bottom nozzles have inside diameters of 40 mm. The fuel and oxidizer flows impinge against each other to form a stable stagnation plane, with a diffusion flame established from the oxidizer side. Co-flowing nitrogen is introduced through a cylindrical annular duct around the outer edge of the oxidizer nozzle, extinguishing the flame near the outer jacket and preventing it from dissipating into the environment. A detailed description of the burner is given by Beltrame et al. [16]. Technical purity methane (98%, AGA Gas Inc.) was seeded with Atomic Absorption purity acetylene (99.8%, AGA Gas Inc.). The experiments were conducted with constant fuel and oxidizer velocities and a strain rate equal to 20s⫺1. The 40-mm long catalytic probe was introduced radially through the flame-protecting shield to the yellow soot-containing region of the flame. The central part of the probe (⬃25 mm) was used to study the structure of deposit materials. The 0.64-mm diameter probe was fabricated from Ni-based alloy with composition 73%Ni⫹17%Cu⫹10%Fe. The axial position of the probe was controlled by a positioning system. Reported experiments were conducted with the residence time of 10 min. The horizontal sampling geometry of the probe was selected due to the presence of strong axial temperature gradients. It was found that vertical positioning of the probe essentially alters temperature profile of the flame. The initial surface scans of the catalyst probe were performed by a scanning electron microscope (JEOL Inc., Model JSM-6320F) with a cold field emission source. The specimens for TEM examinations were prepared by ultrasonic dispersion of car-
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Fig. 2. SEM image showing the catalyst substrate covered with high-density layer of carbon nanofibers and carbon nanotubes.
bon deposits collected from specific probe location in acetone; a drop of the suspension was placed on the electron microscope grid. The detailed structural characteristics of the deposited material were obtained from high-resolution electron microscopy studies performed on a JEOL JEM-3010 electron microscope. The magnification range was 50 to 1,500,000 times. Images were collected on a Gatan digital imaging system and processed by Digital Micrograph software.
3. Results and discussion SEM images collected from the location ⬃8.5 mm from the fuel nozzle show abundance of tubular nanostructures grown on the catalytic substrate (Fig. 2). Most of the formed nanofibers have inhomogeneous shapes containing frequent bends, kinks, and curved segments. The observed diameters vary from 50 to 100 nm. Some of the filaments are straight and uniform indicating presence of regular internal graphitic structure. High densities of formed nanofibers completely cover the catalyst surface. For transmission electron microscope studies, the carbon deposits from this location were transferred to the electron microscope grids. The TEM images of characteristic nanostructures are shown in Figs. 3 and 4. Figure 3 shows carbon nanofibers of tubular structure with varying diameter and wall thickness. Straight sections of uniform diameter are clearly observed. Detailed TEM studies of the multiple tubular fibers reveal the presence of catalytic pear-like particles on the tip of the growing nanofibers, clearly indicating their catalyst-aided mechanism of formation. High-resolution TEM studies show the occur-
Fig. 3. TEM image of carbon nanofibers transferred to the microscope grid. Corresponding SEM image of carbon deposit is shown in Fig. 2.
rence of nested carbon layers. However, these layers are found to be partially irregular, wrinkled and/or forming an angle with the nanofiber axis. Bundle of carbon nanotubes (Fig. 4) is another typical configuration observed in TEM studies. The MWNTs and nanofibers with diameters from 10 to 70 nm form a well-oriented structure with dense tubular packing. Relative stability of the structure allows us to suggest that it forms by simultaneous parallel growth of tightly packed nanotubes. Formation of well-aligned structures of carbon nanotubes are reported by Yuan et al. [11] in co-flow flames. However, significant separation of the tubes was observed in Yuan’s experiments.
Fig. 4. TEM micrograph of well-aligned MWNT bundle.
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Fig. 5. SEM image of regularly coiled helical carbon nanotube.
It is generally accepted, as described by Baker [17], that the growth of carbon tubules and nanofibers occurs by the extrusion of carbon dissolved in a metallic catalyst particle and/or active catalytic site on the metal surface. The carbon constantly precipitates on one side of the particle resulting in a carbon oversaturation and diffusion through the particle. As a result a graphene sheets are deposited on the other side of the particle forming a tubule with a diameter close to the particle size. The distinction between “tip growth” or “base growth” mechanisms does not appear to be essential; the dominant mechanism is often defined by the transport of the carbon to the active catalytic site. The particle geometry and precipitation rate of the carbon on its surface are the main factors controlling the shape and growth rate of produced nanomaterials. However, it should be noted that even perfectly prepared catalytic particles are to be matched by temperature and chemical environment to produce desirable carbon nanostructures. In fact, a number of experimental works demonstrated that active particles can be produced by initial dissolving of the carbon in the surface layer of the metal catalyst and subsequent extraction of the particle with predetermined size and geometry. Similarly, particles are often change size and geometry during the growth adjusting to the process parameters. SEM images collected closer to the flame front depict structural changes of formed nanotubular material. The high concentration of helically coiled carbon nanofibers is characteristic for this region. Figure 5 shows a typical regularly coiled carbon nanofiber surrounded by the specimens of the similar structure. The observed diameters of the coiled fibers varied from 20 to 100 nm; length exceeded several microns. TEM images from transferred deposits show the tubular structures of the coils (Fig. 6). The high-reso-
Fig. 6. TEM micrograph of regularly coiled helical carbon nanotube similar to one depicted in Fig. 5.
lution TEM image (Fig. 7) also suggests the presence of ordered carbon layers. The ordered layers were observed at various locations along the tube length by selective focusing of the electron beam. The complete reconstruction of the carbon lattice is hindered by the essentially 3-D form of the studied object. Carbon micro-coils obtained by chemical vapor deposition, eg., by catalyzed pyrolysis of acetylene, were studied by several researches [18,19]. These unique morphology structures are expected to have numerous applications including microsensors, nanomechanical devices and advanced composite materials. The regularly shaped nanotubular coils are expected to provide excellent properties, combining those of carbon nanotubes and solid carbon coils
Fig. 7. High-resolution TEM image of the regularly coiled helical nanotube shown in Fig. 6. Selective focusing of electron microscope shows presence of highly ordered graphene layers.
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Fig. 8. SEM image of coiled carbon ribbon-like nanofiber. The nanofiber has rectangular cross section with height of 300 nm and thickness of 100 nm. The presence of coiled fibers and large carbon deposits can be observed in the background.
Fig. 9. SEM image of long (⬃0.2 mm) uniform-diameter nanofiber.
[20,21]. Ni-catalyzed acetylene pyrolysis was used by Wen and Shen [21] to grow nanotubular coils with the addition of PCl3 as a co-catalyst. The mechanism of nanotubular coil formation, proposed by the authors, is based on the variation of carbon deposition rates from adjacent planes of catalytic nanocrystal particles. More general consideration, given by Amelinckx et al. [20], qualitatively describes formation of spiral and helical nanotubes by variation of deposition rates and, hence, extrusion velocities along the contact curve between the active catalytic particle and the already formed tube. Detailed consideration shows that this can produce a spiral-shaped tube in case of a circular contact area and a helixshaped tube in case of an elliptical contact area. It is possible to suggest that sharp gradients of temperature and chemical species in the vicinity of the flame zone induce sensible variations of carbon deposition rates providing the condition for the growth of helical structures. Another distinctive coiled carbon structure revealed by SEM analysis is shown in Fig. 8. These structures are found to be present closer to the fuel nozzle (⬃8 mm from the fuel nozzle). The nanofiber has a distinctive ribbon-like appearance. The rectangular cross section has a height of 300 nm and a thickness of 100 nm. Large carbon deposits can be observed in the background. The image (Fig. 8) also depicts several similar structures appearing like unwound ribbon rolls, suggesting that formation of these structures occurs by circular growth of the carbon fiber. It was reported previously [22], that growth of this rarely observed ribbon-like filaments could be catalyzed by small iron-containing particles in CO containing atmosphere at 700°C. The numerical simulations performed with the model developed
by Beltrame et al. [16] shows that this flame location is indeed characterized by temperature close to 700°C and the presence of carbon monoxide. Previous studies revealed that stacking of carbon layers in similar objects is not only very ordered but also aligned perpendicular to the ribbon surface providing numerous edges of graphite layers useful for development of catalysts and adsorbents. Long (⬃0.2 mm) regular carbon nanofibers with diameters from 50 to 200 nm are observed in this low-temperature zone of the flame (Fig. 9). Relatively low concentration of other carbon deposits can be seen in the background. Higher magnification reveals smooth uniform fiber surfaces and practically constant diameter. The characteristic length of the fiber suggests relatively high growth rates. Highresolution TEM images of these objects (Fig. 10) depict a texture of well-oriented concentric graphitic cylinders with average interplanar spacing close to 0.34 nm. The small hollow core inside the fiber is surrounded by large numbers (⬃100) of axi-parallel layers. The somewhat similar structured carbon nanofibers, recently reported by Endo et al. [23], were obtained by the pyrolysis of benzene/ferrocene feedstock in chemical vapor deposition experiments. Albeit the characteristic diameters of the obtained nanofibers are relatively large, their internal structure is very similar to the structure of MWNTs. Even though all current experiments were conducted with fixed flame parameters and by applying only a single catalyst material, the synthesized forms of carbon nanomaterials change dramatically. It is observed that a change in flame position induces variation in macro-morphology and in the microstructure of the carbon nanomaterials formed. The modification of growth conditions is directly related
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nanofibers, ribbon-like coiled nanofibers with rectangular cross section, and, finally, long (⬃0.2 mm) uniform-diameter (⬃100 nm) tubular nanofibers with regular internal structure of carbon layers. Some of the above structures were synthesized only recently at carefully designed chemical vapor deposition experiments in other laboratories. The diversity of formed nanomaterials is attributed to the strong variation of flame properties along the flame axis including temperature, hydrocarbon and radical pool. This provides strong selectivity for formation of different nanoforms even if a single catalytic support is utilized. Acknowledgments
Fig. 10. High-resolution TEM image of the wall structure for the carbon fiber similar to one shown in Fig. 9. Regular structure and orientation of the carbon layers parallel to the fiber axis is observed.
to the variation of the flame environment pertinent to the specific flame location. Temperature, radical, and hydrocarbon concentrations are strong functions of axial position in the flame. Availability of specific hydrocarbons at given temperature conditions alters the growth mechanism leading to the selective production of various nanoforms. The variety of the observed nanoforms leads to the conclusion that flames are a very powerful and highly selective tool for generation of carbon structures with varying morphology and structure. The optimal conditions for production of distinctive nanomaterials of interest include optimization of both catalyst and flame parameters. However, obtained results suggest that even for given catalytic substrate the specific nanofibers can be effectively grown by tailoring the flame environment.
4. Conclusions Formation of carbon nanotubes and carbon nanofibers was studied experimentally using a Ni-based catalytic support positioned at the fuel side of opposed flow flame formed by streams of fuel (50%CH4⫹4%C2H2) and oxidizer (50%O2⫹50%N2). For constant flame parameters and the same catalytic material, a variety of carbon nanostructures were observed for variations of catalyst’s position in the flame. Observed carbon structures include: MWNTs and MWNT bundles, nanofibers with varying degree of crystallinity, helical regularly coiled tubular carbon
This work was partially supported by National Science Foundation grant CTS 0304528 and the Air Liquide Corp. under Unrestricted Laboratory Development Grant. The authors extend special thank to Dr. Alan Nicholls, Mr. John Roth, and Ms. Kristina Jarosius from the UIC Research Resource Center for day-to-day help in SEM and TEM studies, encouragement and helpful discussions. The authors are also grateful to Professor C. Megaridis and Professor I. Puri for valuable comments and helpful discussions. References [1] M.S. Dresselhaus, G. Dresselhaus, P.C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic, San Diego, 1996. [2] T.W. Ebbesen, Carbon Nanotubes: Preparation and Properties, Boca Raton, Fl, CRC Press, 1997, p. 296. [3] C.H. Kiang, W.A. Goddard, R. Beyers, J R. Salem, D.S. Bethune, J. Phys, . Chem. 98 (1994) 6612– 6618. [4] Z. Shi, Y. Lian, X. Zhou, Z. Gu, V. Zhang, S. Iijima, L. Zhou, K.T. Yue, S. Zhang, Carbon 37 (1999) 1449 – 1453. [5] T. Guo, P. Nikolaev, A. Thess, D.T. Colbert, R.E. Smalley, Chem. Phys. Lett. 243 (1995) 49 –54. [6] R. Andrews, D. Jacques, A.M. Rao, F. Derbyshire, D. Qian, X. Fan, E.C. Dickey, J. Chen, Chem. Phys. Lett. 303 (1999) 467– 474. [7] A.G. Gaydon, H.G. Wolfhard, Flames, their structure, radiation, Chapman and Hall, London, 1960, Chap. 8. [8] J.M. Singer, J. Grumer, Seventh Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1959, p. 559. [9] H.B. Palmer, C.F. Cullis. In: P.L. Walker, Jr. (editor), Chemistry and Physics of Carbon, Vol. 1, Marcel Dekker, New York, 1965, p. 265. [10] R.T. Baker, P.S. Harris. In: P.L. Walker, Jr, P.A. Thrower (editors), Chemistry and Physics of Carbon, Vol. 14, Marcel Dekker, New York, 1978, p. 83. [11] L. Yuan, K. Saito, C. Pan, F.A. Williams, A.S. Gordon, Chem. Phys. Lett. 340 (2001) 237–241.
A.V. Saveliev et al. / Combustion and Flame 135 (2003) 27–33 [12] L. Yuan, K. Saito, W. Hu, Z. Chen, Chem. Phys. Lett. 346 (2001) p. 23. [13] R.L. Vander, Wal, Chem. Phys. Lett. 324 (2000) 217– 223. [14] W. Merchan-Merchan, A.V. Saveliev, L.A. Kennedy, A.A. Fridman, Chem. Phys. Lett. 354 (2002) 20 –24. [15] W. Merchan-Merchan, A.V. Saveliev, L.A. Kennedy, Combust. Sci. Technol. (in press). [16] A. Beltrame, P. Porshnev, W. Merchan-Merchan, A. Saveliev, A. Fridman, L.A. Kennedy, O. Petrova, S. Zhdanok, F. Amoury, O. Charon, Combust. Flame 124 (2001) 295–310. [17] R.T.K. Baker, Carbon 27 (1989) 315.
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[18] S. Motojima, I. Hasegawa, S. Kagiya, K. Andoh, H. Iwanaga, Carbon 33 (1995) 1167–1173. [19] X. Chen, S. Motojima, J. Mater. Sci. 34 (1999) 5519 – 24. [20] S. Amelinckx, X.B. Zhang, D. Bernaerts, X.F. Zhang, V. Ivanov, J.B. Nagy, Science 265 (1994) 635. [21] Y. Wen, Z. Shen, Carbon 39 (2001) 2369 –2386. [22] H. Murayama, M.A. Maeda, Nature 345 (1990) 791– 793. [23] M. Endo, Y.A. Kim, T. Takeda, S.H. Hong, T. Matusita, T. Nayashi, M.S. Dresselhaus, Carbon 39 (2001) 2003–2010.
Metal catalyzed synthesis of carbon nanostructures in an opposed flow methane oxygen flame Alexei V. Saveliev, Wilson Merchan-Merchan, Lawrence A. Kennedy* Department of Mechanical Engineering, University of Illinois at Chicago, Chicago, IL 60607-7022, USA Received 10 February 2003; received in revised form 19 May 2003; accepted 19 May 2003
Abstract Results of an experimental study on metal-catalyzed synthesis of carbon tubular nanostructures in opposed flow flame are reported. The catalytic support made of Ni-alloy was positioned at the fuel side of the opposed flow flame formed by fuel (96%CH4⫹4%C2H2) and oxidizer (50%O2⫹50%N2) streams. The electron microscopy studies reveal the presence of highly organized carbonaceous structures with the configurations showing strong dependence on the flame location. Several typical structures were detected. These include: multiwalled nanotubes (MWNT), MWNT bundles, irregular high-density carbon nanofibers, long (up to 0.2 mm) uniform-diameter (⬃100 nm) nanofibers, helical regularly coiled tubular nanofibers, and ribbon-like coiled nanofibers with rectangular cross section. Transmission electron microscopy (TEM) studies performed on long nanofibers revealed the presence of highly organized multiple (⬃100) graphene layers. These layers are parallel to the nanofiber axis resembling the structure of MWNT. The TEM studies of coiled nanofibers show internal tubular structure and the presence of regular carbon lattice. The well-aligned bundles of nanotubes were examined by TEM showing tight packing of MWNTs with varying inner and outer diameters. The diversity of formed nanomaterials is attributed to the strong variation of flame properties along the flame axis including temperature, hydrocarbon and radical pool. This provides strong selectivity for formation of different nanoforms even without adjustment of catalyst properties. © 2003 The Combustion Institute. All rights reserved. Keywords: Carbon nanotubes; Combustion synthesis; Nano processing
1. Introduction Numerous potential applications of novel carbon nanomaterials include nanoconductors and nanoswitches, nanomechanical devices, composites with unique mechanical and electromagnetic properties, field displays, and storage of hydrogen and natural gas [1,2]. This fosters developments of synthesis and purification methods. Among widely applied and well-characterized methods are arc discharges [3,4], pulsed laser vaporization [5], and chemical vapor * Corresponding author. Tel.: ⫹1-312-996-2400; fax: ⫹1-312-996-8664. E-mail address: [email protected] (L.A. Kennedy).
deposition [6]; most of these synthesis techniques require the introduction of catalyst in the form of gas particulates or as a solid support. The selection of a metallic catalyst, such as cobalt, iron, or nickel has been shown to influence growth and morphology of generated nanotubular structures. The application of flames as a relatively inexpensive, robust, pyrolysing carbon source for growing tubular nanomaterials was recently studied in a number of publications. Synthesis methods with introduction of catalyst as a particle in the gas phase and in the form of solid support are implemented using mainly co-flow diffusion flame configurations. The formation of filamentous carbon on a wire probe [7] or metal screen [8] inserted in the fuel side
0010-2180/03/$ – see front matter © 2003 The Combustion Institute. All rights reserved.
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of the diffusion flames were reported in very early flame studies. The formed filaments were hollow with metal granules at the tips. The reported deposit structures closely resembled structures of carbon filaments formed by CO and CH4 decomposition over metal catalysts [9,10]. From our previous knowledge, it is doubtful that the pioneers in the field were adequately equipped to characterize the nanostructures formed in the deposited material. It is very reasonable to assume that nanotubes and nanofibers, if formed, were overlooked in these early studies. Recently, Yuan et al. [11] studied flame-based growth of nanotubes on a Ni/Cr support in laminar co-flow methane-air diffusion flame. Formation of entangled multiwalled nanotubes (MWNTs) with a diameter range of 20 to 60 nm was reported. In related work, Yuan and co-workers [12] used a catalytic support in the form of stainless steel grid to produce MWNTs in ethylene-air diffusion flame. The electroplating of grid with cobalt resulted in synthesis of aligned MWNT growth. The metal catalyst dispersed on TiO2 substrate was used by Vander Wal [13] to generate MWNTs in ethylene/air and acetylene/air diffusion co-flow flames. It was demonstrated that the structure of the obtained nanotubes and nanofibers strongly depends on the catalytic particle shape and chemical composition. The importance of the catalytic substrate positioning in flame is emphasized by the co-flow studies [11–13]. However, the influence of this factor has not been adequately studied due to the small flame volumes and strong flame-probe interactions. Use of counter-flow flame configuration can provide better sampling possibilities and improve interpretation of results. Additionally, it was shown recently [14,15], that a counter-flow oxy-flame has a strong potential for nanotube growth producing high temperatures and high radical concentrations. In the present work, we report results on carbon nanotubular structures formed on a catalytic probe inserted in the fuel zone of an opposed flow oxygen enriched flame. Experiments performed at fixed flame parameters with a single catalytic material show the presence of several characteristic regions with essentially varying properties of the formed carbon nanomaterials. Initial characterization of the formed nanostructures, performed by the scanning electron microscope (SEM) studies of the probe surface, shows the presence of MWNTs, irregular nanofibers, heavy-wall constant-diameter regular nanofibers, and regularly coiled nanotubes. The highresolution TEM studies of these objects are performed by dispersing the deposited material on the TEMs grids.
Fig. 1. Schematic of the experimental setup.
2. Experimental Figure 1 shows the experimental setup used in the present study. A counter-flow burner forms two opposite streams of gases; the fuel (methane seeded with 4% of acetylene) is supplied from the top nozzle and the oxidizer (50% O2⫹50%N2) is introduced from the bottom nozzle. The top and bottom nozzles have inside diameters of 40 mm. The fuel and oxidizer flows impinge against each other to form a stable stagnation plane, with a diffusion flame established from the oxidizer side. Co-flowing nitrogen is introduced through a cylindrical annular duct around the outer edge of the oxidizer nozzle, extinguishing the flame near the outer jacket and preventing it from dissipating into the environment. A detailed description of the burner is given by Beltrame et al. [16]. Technical purity methane (98%, AGA Gas Inc.) was seeded with Atomic Absorption purity acetylene (99.8%, AGA Gas Inc.). The experiments were conducted with constant fuel and oxidizer velocities and a strain rate equal to 20s⫺1. The 40-mm long catalytic probe was introduced radially through the flame-protecting shield to the yellow soot-containing region of the flame. The central part of the probe (⬃25 mm) was used to study the structure of deposit materials. The 0.64-mm diameter probe was fabricated from Ni-based alloy with composition 73%Ni⫹17%Cu⫹10%Fe. The axial position of the probe was controlled by a positioning system. Reported experiments were conducted with the residence time of 10 min. The horizontal sampling geometry of the probe was selected due to the presence of strong axial temperature gradients. It was found that vertical positioning of the probe essentially alters temperature profile of the flame. The initial surface scans of the catalyst probe were performed by a scanning electron microscope (JEOL Inc., Model JSM-6320F) with a cold field emission source. The specimens for TEM examinations were prepared by ultrasonic dispersion of car-
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Fig. 2. SEM image showing the catalyst substrate covered with high-density layer of carbon nanofibers and carbon nanotubes.
bon deposits collected from specific probe location in acetone; a drop of the suspension was placed on the electron microscope grid. The detailed structural characteristics of the deposited material were obtained from high-resolution electron microscopy studies performed on a JEOL JEM-3010 electron microscope. The magnification range was 50 to 1,500,000 times. Images were collected on a Gatan digital imaging system and processed by Digital Micrograph software.
3. Results and discussion SEM images collected from the location ⬃8.5 mm from the fuel nozzle show abundance of tubular nanostructures grown on the catalytic substrate (Fig. 2). Most of the formed nanofibers have inhomogeneous shapes containing frequent bends, kinks, and curved segments. The observed diameters vary from 50 to 100 nm. Some of the filaments are straight and uniform indicating presence of regular internal graphitic structure. High densities of formed nanofibers completely cover the catalyst surface. For transmission electron microscope studies, the carbon deposits from this location were transferred to the electron microscope grids. The TEM images of characteristic nanostructures are shown in Figs. 3 and 4. Figure 3 shows carbon nanofibers of tubular structure with varying diameter and wall thickness. Straight sections of uniform diameter are clearly observed. Detailed TEM studies of the multiple tubular fibers reveal the presence of catalytic pear-like particles on the tip of the growing nanofibers, clearly indicating their catalyst-aided mechanism of formation. High-resolution TEM studies show the occur-
Fig. 3. TEM image of carbon nanofibers transferred to the microscope grid. Corresponding SEM image of carbon deposit is shown in Fig. 2.
rence of nested carbon layers. However, these layers are found to be partially irregular, wrinkled and/or forming an angle with the nanofiber axis. Bundle of carbon nanotubes (Fig. 4) is another typical configuration observed in TEM studies. The MWNTs and nanofibers with diameters from 10 to 70 nm form a well-oriented structure with dense tubular packing. Relative stability of the structure allows us to suggest that it forms by simultaneous parallel growth of tightly packed nanotubes. Formation of well-aligned structures of carbon nanotubes are reported by Yuan et al. [11] in co-flow flames. However, significant separation of the tubes was observed in Yuan’s experiments.
Fig. 4. TEM micrograph of well-aligned MWNT bundle.
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Fig. 5. SEM image of regularly coiled helical carbon nanotube.
It is generally accepted, as described by Baker [17], that the growth of carbon tubules and nanofibers occurs by the extrusion of carbon dissolved in a metallic catalyst particle and/or active catalytic site on the metal surface. The carbon constantly precipitates on one side of the particle resulting in a carbon oversaturation and diffusion through the particle. As a result a graphene sheets are deposited on the other side of the particle forming a tubule with a diameter close to the particle size. The distinction between “tip growth” or “base growth” mechanisms does not appear to be essential; the dominant mechanism is often defined by the transport of the carbon to the active catalytic site. The particle geometry and precipitation rate of the carbon on its surface are the main factors controlling the shape and growth rate of produced nanomaterials. However, it should be noted that even perfectly prepared catalytic particles are to be matched by temperature and chemical environment to produce desirable carbon nanostructures. In fact, a number of experimental works demonstrated that active particles can be produced by initial dissolving of the carbon in the surface layer of the metal catalyst and subsequent extraction of the particle with predetermined size and geometry. Similarly, particles are often change size and geometry during the growth adjusting to the process parameters. SEM images collected closer to the flame front depict structural changes of formed nanotubular material. The high concentration of helically coiled carbon nanofibers is characteristic for this region. Figure 5 shows a typical regularly coiled carbon nanofiber surrounded by the specimens of the similar structure. The observed diameters of the coiled fibers varied from 20 to 100 nm; length exceeded several microns. TEM images from transferred deposits show the tubular structures of the coils (Fig. 6). The high-reso-
Fig. 6. TEM micrograph of regularly coiled helical carbon nanotube similar to one depicted in Fig. 5.
lution TEM image (Fig. 7) also suggests the presence of ordered carbon layers. The ordered layers were observed at various locations along the tube length by selective focusing of the electron beam. The complete reconstruction of the carbon lattice is hindered by the essentially 3-D form of the studied object. Carbon micro-coils obtained by chemical vapor deposition, eg., by catalyzed pyrolysis of acetylene, were studied by several researches [18,19]. These unique morphology structures are expected to have numerous applications including microsensors, nanomechanical devices and advanced composite materials. The regularly shaped nanotubular coils are expected to provide excellent properties, combining those of carbon nanotubes and solid carbon coils
Fig. 7. High-resolution TEM image of the regularly coiled helical nanotube shown in Fig. 6. Selective focusing of electron microscope shows presence of highly ordered graphene layers.
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Fig. 8. SEM image of coiled carbon ribbon-like nanofiber. The nanofiber has rectangular cross section with height of 300 nm and thickness of 100 nm. The presence of coiled fibers and large carbon deposits can be observed in the background.
Fig. 9. SEM image of long (⬃0.2 mm) uniform-diameter nanofiber.
[20,21]. Ni-catalyzed acetylene pyrolysis was used by Wen and Shen [21] to grow nanotubular coils with the addition of PCl3 as a co-catalyst. The mechanism of nanotubular coil formation, proposed by the authors, is based on the variation of carbon deposition rates from adjacent planes of catalytic nanocrystal particles. More general consideration, given by Amelinckx et al. [20], qualitatively describes formation of spiral and helical nanotubes by variation of deposition rates and, hence, extrusion velocities along the contact curve between the active catalytic particle and the already formed tube. Detailed consideration shows that this can produce a spiral-shaped tube in case of a circular contact area and a helixshaped tube in case of an elliptical contact area. It is possible to suggest that sharp gradients of temperature and chemical species in the vicinity of the flame zone induce sensible variations of carbon deposition rates providing the condition for the growth of helical structures. Another distinctive coiled carbon structure revealed by SEM analysis is shown in Fig. 8. These structures are found to be present closer to the fuel nozzle (⬃8 mm from the fuel nozzle). The nanofiber has a distinctive ribbon-like appearance. The rectangular cross section has a height of 300 nm and a thickness of 100 nm. Large carbon deposits can be observed in the background. The image (Fig. 8) also depicts several similar structures appearing like unwound ribbon rolls, suggesting that formation of these structures occurs by circular growth of the carbon fiber. It was reported previously [22], that growth of this rarely observed ribbon-like filaments could be catalyzed by small iron-containing particles in CO containing atmosphere at 700°C. The numerical simulations performed with the model developed
by Beltrame et al. [16] shows that this flame location is indeed characterized by temperature close to 700°C and the presence of carbon monoxide. Previous studies revealed that stacking of carbon layers in similar objects is not only very ordered but also aligned perpendicular to the ribbon surface providing numerous edges of graphite layers useful for development of catalysts and adsorbents. Long (⬃0.2 mm) regular carbon nanofibers with diameters from 50 to 200 nm are observed in this low-temperature zone of the flame (Fig. 9). Relatively low concentration of other carbon deposits can be seen in the background. Higher magnification reveals smooth uniform fiber surfaces and practically constant diameter. The characteristic length of the fiber suggests relatively high growth rates. Highresolution TEM images of these objects (Fig. 10) depict a texture of well-oriented concentric graphitic cylinders with average interplanar spacing close to 0.34 nm. The small hollow core inside the fiber is surrounded by large numbers (⬃100) of axi-parallel layers. The somewhat similar structured carbon nanofibers, recently reported by Endo et al. [23], were obtained by the pyrolysis of benzene/ferrocene feedstock in chemical vapor deposition experiments. Albeit the characteristic diameters of the obtained nanofibers are relatively large, their internal structure is very similar to the structure of MWNTs. Even though all current experiments were conducted with fixed flame parameters and by applying only a single catalyst material, the synthesized forms of carbon nanomaterials change dramatically. It is observed that a change in flame position induces variation in macro-morphology and in the microstructure of the carbon nanomaterials formed. The modification of growth conditions is directly related
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nanofibers, ribbon-like coiled nanofibers with rectangular cross section, and, finally, long (⬃0.2 mm) uniform-diameter (⬃100 nm) tubular nanofibers with regular internal structure of carbon layers. Some of the above structures were synthesized only recently at carefully designed chemical vapor deposition experiments in other laboratories. The diversity of formed nanomaterials is attributed to the strong variation of flame properties along the flame axis including temperature, hydrocarbon and radical pool. This provides strong selectivity for formation of different nanoforms even if a single catalytic support is utilized. Acknowledgments
Fig. 10. High-resolution TEM image of the wall structure for the carbon fiber similar to one shown in Fig. 9. Regular structure and orientation of the carbon layers parallel to the fiber axis is observed.
to the variation of the flame environment pertinent to the specific flame location. Temperature, radical, and hydrocarbon concentrations are strong functions of axial position in the flame. Availability of specific hydrocarbons at given temperature conditions alters the growth mechanism leading to the selective production of various nanoforms. The variety of the observed nanoforms leads to the conclusion that flames are a very powerful and highly selective tool for generation of carbon structures with varying morphology and structure. The optimal conditions for production of distinctive nanomaterials of interest include optimization of both catalyst and flame parameters. However, obtained results suggest that even for given catalytic substrate the specific nanofibers can be effectively grown by tailoring the flame environment.
4. Conclusions Formation of carbon nanotubes and carbon nanofibers was studied experimentally using a Ni-based catalytic support positioned at the fuel side of opposed flow flame formed by streams of fuel (50%CH4⫹4%C2H2) and oxidizer (50%O2⫹50%N2). For constant flame parameters and the same catalytic material, a variety of carbon nanostructures were observed for variations of catalyst’s position in the flame. Observed carbon structures include: MWNTs and MWNT bundles, nanofibers with varying degree of crystallinity, helical regularly coiled tubular carbon
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