- Email: [email protected]
Synthesis of graphite polyhedral crystals using a combustion flame method
Carbon 43 (2005) 692–697 www.elsevier.com/locate/carbon
Synthesis of graphite polyhedral crystals using a combustion flame method H. Okuno a
a,*
, A. Palnichenko a, J.-F. Despres b, J.-P. Issi
a,c
, J.-C. Charlier
a,c
Unite´ de Physico-Chimie et de Physique des Mate´riaux (PCPM), Universite´ Catholique de Louvain, Place Croix du Sud 1, B-1348 Louvain-la-Neuve, Belgium b IMRA Europe, BP 213, 06904 Sophia Antipolis, France c Research Center for Micro- and Nano-materials and Electronic Devices (CERMIN), Universite´ Catholique de Louvain, B-1348 Louvain-la-Neuve, Belgium Received 28 January 2004; accepted 18 October 2004 Available online 21 December 2004
Abstract The combustion flame method is a high temperature chemical deposition process which has been extensively used in diamond film synthesis. In the present article, this technique is proposed for the growth of micro-sized graphitic polyhedral crystals. These microstructures possess either a rod-like or a pin-like structure, and exhibit complex axial symmetry including sometimes helicity. Their size can reach up to 3 lm in diameter and 15 lm in length. Raman spectroscopy, scanning (SEM) and high-resolution transmission electron microscopies (HR-TEM) have been used to characterize these carbon micro-rods which present a high crystalline perfection at the nanoscale. Ó 2004 Elsevier Ltd. All rights reserved.
1. Introduction The complex deposition and the highly versatile growth of graphitic carbon allotropic forms have come up with a large variety of low-dimensional structures, including fullerenes, nanotubes, whiskers and fibers. Conventional graphite is a hexagonal plate-like crystal with a very weak bonding between the graphene layers [1]. Graphite whiskers represent unusual forms of carbon based on the distortion of graphene sheets which could be roll into a scroll [2]. Carbon nanotubes are coaxial structures consisting of a various number of graphitic cylinders [3,4]. Recently carbon micro-trees [5] and graphite polyhedral crystals [6] have also been reported in the literature. Here, we report on the spectacular growth of microsized polyhedral graphitic crystals synthesized by the
*
Corresponding author. Tel.: +32 10 47 35 68; fax: +32 10 47 34 52. E-mail address: [email protected] (H. Okuno).
0008-6223/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.10.033
combustion flame technique [7]. Although such a technique has been widely used to produce diamond films [8], other carbon microstructures such as diamond-like carbon (DLC) [9] and gypsum flower-like carbon (GF) [10,11] have also been synthesized. Diamond films are composed of only sp3 bonded carbon, while DLC and GF also contain sp2 bonded carbon [9] in a various amounts. As such a growth technique involves complicated chemical and physical processes far from the thermodynamic equilibrium, the experimental conditions have been optimized for the growth of polyhedral carbon micro-crystals, which are pure graphitic structures (sp2 bonded carbon). The thermal conditions and the source gases composition are the key parameters for the production of these interesting carbon structures. 2. Experiment All the experiments are carried out using an oxyacetylene torch. The oxygen and acetylene source gases
H. Okuno et al. / Carbon 43 (2005) 692–697
are controlled by a standard mass flow meter system. Molybdenum plates are used as deposition substrate, mounted on a temperature controlled water-cooled cupper sample holder. The O2/C2H2 volume gas ratio is maintained typically at 0.9, with a C2H2 gas flow of 2.0 l/min. No catalyst is used. During the deposition time, the substrate is maintained in the acetylene vertical inner flame zone (acetylene feather), 1 mm below the center cone. The deposition time is fixed to 3 min. The substrate temperature ranges from 1000 to 1300 °C as determined by an infrared pyrometer. Field-emission scanning electron microscopy (SEM), transmission electron microscopy (TEM) at high resolution (HR-TEM) and Raman spectroscopy are used to characterize the samples.
693
Fig. 1. Scanning electron micrograph (SEM) of polyhedral crystals as synthesized by the oxy-acetylenic flame combustion technique.
3. Results Carbon micro-sized crystalline structures are produced in large amounts on the substrate (Fig. 1). These crystals are synthesized within peculiar and limited experimental conditions: the O2/C2H2 gas ratio being around 0.90 and the substrate temperature ranging from 1230 to 1280 °C. The crystal size can reach up to 3 lm in diameter and 15 lm in length. The carbon micro-crystals are called polyhedral crystals because of their complex polyhedral shapes with facets along their sides; the number of facets being frequently eight (Fig. 2a). Such polygonization of the crystal sides is assumed to be due to higher surface energy minimization when compared with the cylindrical shape. This phenomena has already been observed and reported in the literature for thermally annealed at 2000 °C [12–14]. At the end of the graphitic micro-crystals, various tips have been observed (Fig. 2). For example, a hexagonal symmetry is observed at the apex of the tip of a rodlike structure, illustrated in Fig. 2a. Fig. 2c shows a crystal exhibiting a pin-like apex at the end. In this case, the polyhedral
shape is twisted along the rod axis and the diameter of crystal becomes smaller towards the tips, leading to a quite sharp apex (Fig. 2c and d). Carbon nanotubes are also frequently observed protruding at the tip of these crystals, especially for the rod-like structure (Fig. 2b). The sizes of these carbon microstructures are large enough to allow selective micro-Raman analysis from the side face to the tip (Fig. 3). The Raman spectra were taken by focusing the laser beam with the excitation wavelength of 632.8 nm on an area of about 1 lm2. The Raman spectrum associated to the crystal faces is quite analogous to pure graphite, with a narrow G band (1580 cm1), nearly no D band (1350 cm1) and the second-order G 0 band peak (2700 cm1). The Raman spectrum obtained at the tip displays a D peak and an unusually strong second-order G 0 band peak that exceeds in intensity half of the G band of graphite. Such differences in the spectra could be attributed to the presence of topological defects, dangling bonds, and edge reconstruction at the end of the graphite rods.
Fig. 2. SEM images of representative polyhedral crystals with rod-like (a,b) and pin-like (c,d) morphologies, illustrating their polygonal shapes. The tip of these carbon microstructures is also highly symmetrical, exhibiting a crystalline apex as shown in the inset of (a).
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Fig. 3. First- and second-order Raman spectra obtained from the faceted part and the tip of a graphitic rod as illustrated in the inset using a SEM micrograph.
High-resolution TEM image of the side face of a rodlike crystal is presented in Fig. 4a. These (0 0 2) lattice fringes display the quite good ordering of the graphitic layers which are oriented parallel to the surface sides of the crystal. However, these planes need to reorganize at the rod end in order to form a facetted apex (see Fig. 2a). On the other hand, for the pin-like morphology, the graphitic layers are also parallel to the surface side but not to the crystal axis, where a reorganization of graphitic layers is observed at the crystal core (Fig. 4b). These two micrometer-sized graphite structures have already been reported in the literature [6,15]. However, to the best of our knowledge, it is the first time that both of them, the rod-like (Fig. 2a and d) and the pin-like (Fig. 2b and c) crystals, are observed within the same experiment. The main difference in their morphologies is the
orientation of the graphitic layers versus the core axis of the crystal. When the graphitic layers form a cylinder around the core axis, the diameter of the rod does not decrease significantly, leading to a rod-like structure. On the contrary, when the orientation of the graphitic planes is tilted, leading to a conical shape for the layers. The layers intersect at the crystal core and stop growing successively by reorganizing their free edges (Fig. 4b). Consequently, the corresponding structure displays a decrease of its diameter depending on the tilted angle of its composing graphitic layers versus its axis. These crystals also display various tips as previously illustrated in Fig. 2. The growth of the pin-like structure is supposed to stop when its diameter is inferior to a threshold value. On the other hand, the growth of rodlike crystals leads to the formation of various terminations as illustrated in Fig. 5a and b. HR-TEM reveals the orientations of the graphitic layers on these two terminations. The graphitic layers are frequently oriented parallel to the core axis up to the end of crystal (Fig. 5c). However, when the crystal ends, the graphitic layers do not stop abruptly. Nano-arches are observed at the edges of the graphitic planes (Fig. 5d). These nano-arches result in a swelling of the edge planes, producing a peculiar surface reconstruction in graphite [16]. Typical nano-arches are built by folding two graphene layers, but sometimes, the number of layers could increase to three or four. Less frequently, a single non-terminated graphene sheet is also observed enclosed between folded layers. Such kind of reconstruction does stabilize the structure by satisfying a large number of dangling bonds at the graphitic edge, and has also been reported for carbon filaments after thermal treatment at 2800 °C [17]. Other observations illustrate the presence of declinations within the graphitic layers (Fig. 5e). This could
Fig. 4. HR-TEM images of the graphitic layers of a rod-like (a) and a pin-like (b) micro-crystals. Both observations demonstrate the high crystallinity of these carbon structures and the orientation of the graphitic layers with the crystal axis.
H. Okuno et al. / Carbon 43 (2005) 692–697
695
Fig. 5. SEM images (a and b) of various tips of rod-like graphitic crystals. HR-TEM images characterizing the top of the crystal (c), folded carbon layers (d) and curved carbon layers (e). The arrows indicate the edge reconstruction in (d) and some declination due to topological defects in (e).
be due to the presence of topological defects such as pentagons (+60° declination) or heptagons (60° declination). Both the edge reconstruction (nano-arches) and the presence of topological defects located at the tip of these crystals could explain the D band in the corresponding Raman spectrum (Fig. 3). When an electro-deposited Ni–Co catalytic layer is introduced in the combustion flame technique, multiwall carbon nanotubes (MWNTs) are formed in the early stages of the deposition (5–10 s) [18]. After a deposition time of 1 min, the polyhedral crystals are synthe-
sized around these MWNTs (Fig. 6a). Several segments of crystal are grown simultaneously along the nanotube axis. Their selected-area diffraction pattern can be indexed by the graphite spacing as shown in Fig. 6b. This sharp symmetry profile exhibits isolated 100 and 101 spots, suggesting a highly oriented graphitic structure. All these micro-crystals possess an empty core (Fig. 6c), which can be evidenced as the MWNT which plays the role of support for the deposition. HR-TEM image (Fig. 6d) displays the 0 0 2 fringes of a MWNT pre-deposited and the polyhedral crystal. No
Fig. 6. SEM (a), electron diffraction pattern (b), TEM (c), and HR-TEM (d) images of a polyhedral crystal grown around a multi-wall nanotube.
696
H. Okuno et al. / Carbon 43 (2005) 692–697
discontinuity is observed in the stacking of the graphitic layers of the two structures which is probably due to the good graphitization imposed by the heat treatment.
4. Discussion In this work, graphitic structures with a peculiar unidimensional polyhedral shape and a quite high crystalline perfection are obtained as described above. Fig. 7a illustrates a possible growth model for the rod-like structure. At the early stages of the deposition, graphitic layers are formed on the substrate. The crystal growth may start from thermal fluctuations that would appear on these graphitic layers, leading to topological anomalies (Fig. 7a). The presence of a high thermal gradient in the process could promote the crystal growth in the perpendicular direction towards the flame source. The difference of temperature between the substrate and the flame source reaches about 1500 °C [19]. Although the substrate temperature is maintained around 1200 °C, the growth temperature is assumed to be higher. No obvious differences in crystal diameters are observed for different deposition times, suggesting that this diameter is fixed by the fluctuations in the graphitic layers. Only the length of the crystal varies with the deposition time, leading to an axial growth. In the solid state graphitization of pyrolytic carbon, crystallization starts in many places simultaneously to form randomly shaped and faceted particles consisting of inter-grown polycrystals [20]. As polyhedral crystals are progressively grown toward the flame at higher temperature, this technique might allow the synthesis of graphitic structure with crystalline perfection close to that of a single crystal. On the other hand, when MWNTs are preformed using catalysts, the crystal growth does not start on the substrate surface, but directly around the MWNTs (Fig. 7b). The nucleation appears simultaneously in several places due to the diffusion of the carbon species on the nanotube surface, and gives rise to several crystal segments (see Fig. 6a). In this case, the MWNT plays
the role of the deposition substrate, and is thus the core of the graphitic crystal. For longer deposition times, the crystal diameter is found to increase, leading to a possible control of the concentric growth for potential applications. Although the growth mechanism could be either concentric or axial, the same crystalline perfection is observed in both cases, suggesting that the high temperature conditions of the present technique are playing a key role in the graphitization process.
5. Conclusion Although the combustion flame technique varies from conventional CVD processes, where carbon structures grow with the help of a catalytic particle [21], the deposition of carbon under the extreme conditions of the oxy-acetylene torch is found to generate interesting structures with unusual morphologies. It is probably not surprising that carbon, with its ability to exist in so many allotropic forms, produces a large set of crystalline structures not observed for other materials. Nevertheless, both rod-like and pin-like microstructures represent a novel group of low-dimensional graphite crystals with an interesting variety of shapes. These novel structures are expected to have at least the mechanical properties of graphite whiskers (Young modulus of 800 GPa, and strength of 20 GPa2), and electronic properties similar to those of graphite. The polyhedral structure of the micro-rods could also provide a higher rigidity compared with cylindrical micro-fibers. When a MWNT is getting out of the structure, the micro-crystals could be used as probes for field emission sources. As their sizes are controlled by experimental conditions, growing larger and longer crystals could be challenging. Thus, this technique may provide a way to grow graphite single crystals to the size of a pencil, although preserving their crystalline perfection. If this can be carried out in practice, tailored microscopic carbon shapes can be produced with a degree of perfection never obtained before.
Fig. 7. Growth models for a polyhedral crystal grown (a) on a substrate, and (b) on the surface of a multi-wall nanotube.
H. Okuno et al. / Carbon 43 (2005) 692–697
Acknowledgments The authors acknowledge helpful discussions with Prof. P. Delhaes and are indebted to the Fonds pour la Recherche Fundamental Collective (FRFC, Belgium) for the transmission electron microscopy. J.-C.C. acknowledges the National Fund for Scientific Research [FNRS] of Belgium for financial support. This paper presents research results of the Belgian Program on Interuniversity Attraction Poles (PAI5/1/1) on Quantum Size Effects in Nano-structured Materials.
References [1] Bernal JD. The structure of graphite. Proc Roy Soc Lond Ser A 1924;106:749–73. [2] Bacon R. Growth, structure, and properties of graphite whiskers. J Appl Phys 1960;31:283–90. [3] Oberlin A, Endo M, Koyama T. Filamentous growth of carbon through benzene decomposition. J Cryst Growth 1976;32:335–49. [4] Iijima S. Helical microtubules of graphitic carbon. Nature 1991;354:56–8. [5] Ajayan PM, Nugent JM, Siegel RW, Wei B, Kowler-Redlich Ph. Growth of carbon micro-trees. Nature 2000;404:243. [6] Gogotsi Y, Libera JA, Kalashnikov N, Yoshimura M. Graphite polyhedral crystals. Science 2000;290:317–20. [7] Hirose H, Komaki K. Eur Pat Appl 1988;EP324538. [8] Hanssen LM, Snail KA, Carrington WA, Butler JE. Diamond and non-diamond carbon synthesis in an oxygen–acetylene flame. Thin Solid Films 1991;196:271–81.
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[9] Ravi KV. Combustion synthesis: is it the most flexible of the diamond synthesis processes? Diamond Relat Mater 1995;4:243–9. [10] Despres JF, Genseki A, Odawara O. Characterization of an unusual carbon entity: gypsum flower-like carbon. J Mater Chem 1997;7:1877–9. [11] Despres JF, Okuno H, Takahashi H, Odawara O. A phase transition between carbon allotropes synthesized by a combustion-flame method. Carbon 1997;35:1658–60. [12] Kmetko EA. The structure and graphitization of fine coke particles and of thermal carbon blacks. In: Proc Conf Carbon, 1956. p. 21–30. [13] Tsuzuku T. Graphitization stress in polycrystalline carbon as an origin of dislocations. J Phys Soc Jpn 1960;15:1373–9. [14] Akamatsu H, Kuroda H. On the sub-structure and the crystallite growth in carbon. In: Proc Conf Carbon, 1960. p. 355–69. [15] Zhang G, Jiang X, Wang E. Tubular graphite cones. Science 2003;300:472–4. [16] Ugarte D. Morphology and structure of graphite soot particles generated in arc-discharge C60 production. Chem Phys Lett 1992;198:596–602. [17] Murayama H, Maeda T. A novel form of filamentous graphite. Nature 1990;345:791–3. [18] Okuno H, Palnichenko A, Despres JF, Issi JP, Charlier JC. Catalytic synthesis of carbon nanotubes by oxy-acetylene combustion flame method, unpublished. [19] Matsui Y, Yabe H, Sugimoto T, Hirose Y. The structure of acetylene flames for diamond synthesis. Diamond Relat Mater 1991;1:19–24. [20] Dresselhaus MS, Dresselhaus G, Sugihara K, Spain IL, Goldberg HA. Graphite fibers and filaments. Springer series in materials science, vol. 5. Berlin: Springer-Verlag; 1988. [21] Endo M, Saito R, Dresselhaus MS, Dresselhaus G. In: Ebbesen TW, editor. Carbon nanotubes. Boca Raton: CRC Press; 1997. p. 35–111.
Synthesis of graphite polyhedral crystals using a combustion flame method H. Okuno a
a,*
, A. Palnichenko a, J.-F. Despres b, J.-P. Issi
a,c
, J.-C. Charlier
a,c
Unite´ de Physico-Chimie et de Physique des Mate´riaux (PCPM), Universite´ Catholique de Louvain, Place Croix du Sud 1, B-1348 Louvain-la-Neuve, Belgium b IMRA Europe, BP 213, 06904 Sophia Antipolis, France c Research Center for Micro- and Nano-materials and Electronic Devices (CERMIN), Universite´ Catholique de Louvain, B-1348 Louvain-la-Neuve, Belgium Received 28 January 2004; accepted 18 October 2004 Available online 21 December 2004
Abstract The combustion flame method is a high temperature chemical deposition process which has been extensively used in diamond film synthesis. In the present article, this technique is proposed for the growth of micro-sized graphitic polyhedral crystals. These microstructures possess either a rod-like or a pin-like structure, and exhibit complex axial symmetry including sometimes helicity. Their size can reach up to 3 lm in diameter and 15 lm in length. Raman spectroscopy, scanning (SEM) and high-resolution transmission electron microscopies (HR-TEM) have been used to characterize these carbon micro-rods which present a high crystalline perfection at the nanoscale. Ó 2004 Elsevier Ltd. All rights reserved.
1. Introduction The complex deposition and the highly versatile growth of graphitic carbon allotropic forms have come up with a large variety of low-dimensional structures, including fullerenes, nanotubes, whiskers and fibers. Conventional graphite is a hexagonal plate-like crystal with a very weak bonding between the graphene layers [1]. Graphite whiskers represent unusual forms of carbon based on the distortion of graphene sheets which could be roll into a scroll [2]. Carbon nanotubes are coaxial structures consisting of a various number of graphitic cylinders [3,4]. Recently carbon micro-trees [5] and graphite polyhedral crystals [6] have also been reported in the literature. Here, we report on the spectacular growth of microsized polyhedral graphitic crystals synthesized by the
*
Corresponding author. Tel.: +32 10 47 35 68; fax: +32 10 47 34 52. E-mail address: [email protected] (H. Okuno).
0008-6223/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.10.033
combustion flame technique [7]. Although such a technique has been widely used to produce diamond films [8], other carbon microstructures such as diamond-like carbon (DLC) [9] and gypsum flower-like carbon (GF) [10,11] have also been synthesized. Diamond films are composed of only sp3 bonded carbon, while DLC and GF also contain sp2 bonded carbon [9] in a various amounts. As such a growth technique involves complicated chemical and physical processes far from the thermodynamic equilibrium, the experimental conditions have been optimized for the growth of polyhedral carbon micro-crystals, which are pure graphitic structures (sp2 bonded carbon). The thermal conditions and the source gases composition are the key parameters for the production of these interesting carbon structures. 2. Experiment All the experiments are carried out using an oxyacetylene torch. The oxygen and acetylene source gases
H. Okuno et al. / Carbon 43 (2005) 692–697
are controlled by a standard mass flow meter system. Molybdenum plates are used as deposition substrate, mounted on a temperature controlled water-cooled cupper sample holder. The O2/C2H2 volume gas ratio is maintained typically at 0.9, with a C2H2 gas flow of 2.0 l/min. No catalyst is used. During the deposition time, the substrate is maintained in the acetylene vertical inner flame zone (acetylene feather), 1 mm below the center cone. The deposition time is fixed to 3 min. The substrate temperature ranges from 1000 to 1300 °C as determined by an infrared pyrometer. Field-emission scanning electron microscopy (SEM), transmission electron microscopy (TEM) at high resolution (HR-TEM) and Raman spectroscopy are used to characterize the samples.
693
Fig. 1. Scanning electron micrograph (SEM) of polyhedral crystals as synthesized by the oxy-acetylenic flame combustion technique.
3. Results Carbon micro-sized crystalline structures are produced in large amounts on the substrate (Fig. 1). These crystals are synthesized within peculiar and limited experimental conditions: the O2/C2H2 gas ratio being around 0.90 and the substrate temperature ranging from 1230 to 1280 °C. The crystal size can reach up to 3 lm in diameter and 15 lm in length. The carbon micro-crystals are called polyhedral crystals because of their complex polyhedral shapes with facets along their sides; the number of facets being frequently eight (Fig. 2a). Such polygonization of the crystal sides is assumed to be due to higher surface energy minimization when compared with the cylindrical shape. This phenomena has already been observed and reported in the literature for thermally annealed at 2000 °C [12–14]. At the end of the graphitic micro-crystals, various tips have been observed (Fig. 2). For example, a hexagonal symmetry is observed at the apex of the tip of a rodlike structure, illustrated in Fig. 2a. Fig. 2c shows a crystal exhibiting a pin-like apex at the end. In this case, the polyhedral
shape is twisted along the rod axis and the diameter of crystal becomes smaller towards the tips, leading to a quite sharp apex (Fig. 2c and d). Carbon nanotubes are also frequently observed protruding at the tip of these crystals, especially for the rod-like structure (Fig. 2b). The sizes of these carbon microstructures are large enough to allow selective micro-Raman analysis from the side face to the tip (Fig. 3). The Raman spectra were taken by focusing the laser beam with the excitation wavelength of 632.8 nm on an area of about 1 lm2. The Raman spectrum associated to the crystal faces is quite analogous to pure graphite, with a narrow G band (1580 cm1), nearly no D band (1350 cm1) and the second-order G 0 band peak (2700 cm1). The Raman spectrum obtained at the tip displays a D peak and an unusually strong second-order G 0 band peak that exceeds in intensity half of the G band of graphite. Such differences in the spectra could be attributed to the presence of topological defects, dangling bonds, and edge reconstruction at the end of the graphite rods.
Fig. 2. SEM images of representative polyhedral crystals with rod-like (a,b) and pin-like (c,d) morphologies, illustrating their polygonal shapes. The tip of these carbon microstructures is also highly symmetrical, exhibiting a crystalline apex as shown in the inset of (a).
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H. Okuno et al. / Carbon 43 (2005) 692–697
Fig. 3. First- and second-order Raman spectra obtained from the faceted part and the tip of a graphitic rod as illustrated in the inset using a SEM micrograph.
High-resolution TEM image of the side face of a rodlike crystal is presented in Fig. 4a. These (0 0 2) lattice fringes display the quite good ordering of the graphitic layers which are oriented parallel to the surface sides of the crystal. However, these planes need to reorganize at the rod end in order to form a facetted apex (see Fig. 2a). On the other hand, for the pin-like morphology, the graphitic layers are also parallel to the surface side but not to the crystal axis, where a reorganization of graphitic layers is observed at the crystal core (Fig. 4b). These two micrometer-sized graphite structures have already been reported in the literature [6,15]. However, to the best of our knowledge, it is the first time that both of them, the rod-like (Fig. 2a and d) and the pin-like (Fig. 2b and c) crystals, are observed within the same experiment. The main difference in their morphologies is the
orientation of the graphitic layers versus the core axis of the crystal. When the graphitic layers form a cylinder around the core axis, the diameter of the rod does not decrease significantly, leading to a rod-like structure. On the contrary, when the orientation of the graphitic planes is tilted, leading to a conical shape for the layers. The layers intersect at the crystal core and stop growing successively by reorganizing their free edges (Fig. 4b). Consequently, the corresponding structure displays a decrease of its diameter depending on the tilted angle of its composing graphitic layers versus its axis. These crystals also display various tips as previously illustrated in Fig. 2. The growth of the pin-like structure is supposed to stop when its diameter is inferior to a threshold value. On the other hand, the growth of rodlike crystals leads to the formation of various terminations as illustrated in Fig. 5a and b. HR-TEM reveals the orientations of the graphitic layers on these two terminations. The graphitic layers are frequently oriented parallel to the core axis up to the end of crystal (Fig. 5c). However, when the crystal ends, the graphitic layers do not stop abruptly. Nano-arches are observed at the edges of the graphitic planes (Fig. 5d). These nano-arches result in a swelling of the edge planes, producing a peculiar surface reconstruction in graphite [16]. Typical nano-arches are built by folding two graphene layers, but sometimes, the number of layers could increase to three or four. Less frequently, a single non-terminated graphene sheet is also observed enclosed between folded layers. Such kind of reconstruction does stabilize the structure by satisfying a large number of dangling bonds at the graphitic edge, and has also been reported for carbon filaments after thermal treatment at 2800 °C [17]. Other observations illustrate the presence of declinations within the graphitic layers (Fig. 5e). This could
Fig. 4. HR-TEM images of the graphitic layers of a rod-like (a) and a pin-like (b) micro-crystals. Both observations demonstrate the high crystallinity of these carbon structures and the orientation of the graphitic layers with the crystal axis.
H. Okuno et al. / Carbon 43 (2005) 692–697
695
Fig. 5. SEM images (a and b) of various tips of rod-like graphitic crystals. HR-TEM images characterizing the top of the crystal (c), folded carbon layers (d) and curved carbon layers (e). The arrows indicate the edge reconstruction in (d) and some declination due to topological defects in (e).
be due to the presence of topological defects such as pentagons (+60° declination) or heptagons (60° declination). Both the edge reconstruction (nano-arches) and the presence of topological defects located at the tip of these crystals could explain the D band in the corresponding Raman spectrum (Fig. 3). When an electro-deposited Ni–Co catalytic layer is introduced in the combustion flame technique, multiwall carbon nanotubes (MWNTs) are formed in the early stages of the deposition (5–10 s) [18]. After a deposition time of 1 min, the polyhedral crystals are synthe-
sized around these MWNTs (Fig. 6a). Several segments of crystal are grown simultaneously along the nanotube axis. Their selected-area diffraction pattern can be indexed by the graphite spacing as shown in Fig. 6b. This sharp symmetry profile exhibits isolated 100 and 101 spots, suggesting a highly oriented graphitic structure. All these micro-crystals possess an empty core (Fig. 6c), which can be evidenced as the MWNT which plays the role of support for the deposition. HR-TEM image (Fig. 6d) displays the 0 0 2 fringes of a MWNT pre-deposited and the polyhedral crystal. No
Fig. 6. SEM (a), electron diffraction pattern (b), TEM (c), and HR-TEM (d) images of a polyhedral crystal grown around a multi-wall nanotube.
696
H. Okuno et al. / Carbon 43 (2005) 692–697
discontinuity is observed in the stacking of the graphitic layers of the two structures which is probably due to the good graphitization imposed by the heat treatment.
4. Discussion In this work, graphitic structures with a peculiar unidimensional polyhedral shape and a quite high crystalline perfection are obtained as described above. Fig. 7a illustrates a possible growth model for the rod-like structure. At the early stages of the deposition, graphitic layers are formed on the substrate. The crystal growth may start from thermal fluctuations that would appear on these graphitic layers, leading to topological anomalies (Fig. 7a). The presence of a high thermal gradient in the process could promote the crystal growth in the perpendicular direction towards the flame source. The difference of temperature between the substrate and the flame source reaches about 1500 °C [19]. Although the substrate temperature is maintained around 1200 °C, the growth temperature is assumed to be higher. No obvious differences in crystal diameters are observed for different deposition times, suggesting that this diameter is fixed by the fluctuations in the graphitic layers. Only the length of the crystal varies with the deposition time, leading to an axial growth. In the solid state graphitization of pyrolytic carbon, crystallization starts in many places simultaneously to form randomly shaped and faceted particles consisting of inter-grown polycrystals [20]. As polyhedral crystals are progressively grown toward the flame at higher temperature, this technique might allow the synthesis of graphitic structure with crystalline perfection close to that of a single crystal. On the other hand, when MWNTs are preformed using catalysts, the crystal growth does not start on the substrate surface, but directly around the MWNTs (Fig. 7b). The nucleation appears simultaneously in several places due to the diffusion of the carbon species on the nanotube surface, and gives rise to several crystal segments (see Fig. 6a). In this case, the MWNT plays
the role of the deposition substrate, and is thus the core of the graphitic crystal. For longer deposition times, the crystal diameter is found to increase, leading to a possible control of the concentric growth for potential applications. Although the growth mechanism could be either concentric or axial, the same crystalline perfection is observed in both cases, suggesting that the high temperature conditions of the present technique are playing a key role in the graphitization process.
5. Conclusion Although the combustion flame technique varies from conventional CVD processes, where carbon structures grow with the help of a catalytic particle [21], the deposition of carbon under the extreme conditions of the oxy-acetylene torch is found to generate interesting structures with unusual morphologies. It is probably not surprising that carbon, with its ability to exist in so many allotropic forms, produces a large set of crystalline structures not observed for other materials. Nevertheless, both rod-like and pin-like microstructures represent a novel group of low-dimensional graphite crystals with an interesting variety of shapes. These novel structures are expected to have at least the mechanical properties of graphite whiskers (Young modulus of 800 GPa, and strength of 20 GPa2), and electronic properties similar to those of graphite. The polyhedral structure of the micro-rods could also provide a higher rigidity compared with cylindrical micro-fibers. When a MWNT is getting out of the structure, the micro-crystals could be used as probes for field emission sources. As their sizes are controlled by experimental conditions, growing larger and longer crystals could be challenging. Thus, this technique may provide a way to grow graphite single crystals to the size of a pencil, although preserving their crystalline perfection. If this can be carried out in practice, tailored microscopic carbon shapes can be produced with a degree of perfection never obtained before.
Fig. 7. Growth models for a polyhedral crystal grown (a) on a substrate, and (b) on the surface of a multi-wall nanotube.
H. Okuno et al. / Carbon 43 (2005) 692–697
Acknowledgments The authors acknowledge helpful discussions with Prof. P. Delhaes and are indebted to the Fonds pour la Recherche Fundamental Collective (FRFC, Belgium) for the transmission electron microscopy. J.-C.C. acknowledges the National Fund for Scientific Research [FNRS] of Belgium for financial support. This paper presents research results of the Belgian Program on Interuniversity Attraction Poles (PAI5/1/1) on Quantum Size Effects in Nano-structured Materials.
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