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The effect of catalysis on the formation of one-dimensional carbon structured materials
Diamond and Related Materials 11 (2002) 1019–1025
The effect of catalysis on the formation of one-dimensional carbon structured materials W.J. Jong, S.H. Lai, K.H. Hong, H.N. Lin, H.C. Shih* Department of Materials Science and Engineering, National Tsing Hua University, 101, Sec. 2, Kuang Fu Road, Hsinchu 300, Taiwan, ROC
Abstract Carbon nanotubes (CNTs) have been synthesized by catalytic chemical vapor deposition (CCVD). Catalysts, e.g. Fe, Ni, Co, etc., are very important in the formation of CNTs. Other elements, such as Cu, although showing no effect on growth, have been used to enhance the catalytic effect of Ni by alloying it with Ni. In this study, one-dimensional carbon structured materials were synthesized by microwave plasma enhanced chemical vapor deposition (MPECVD) with a mixture of methane and hydrogen as precursors at a temperature of ;600 8C. Subsequently, the samples were taken into thermal treatment (under a mixing atmosphere of Ar and H2, up to 1000 8C) in order to improve the graphitization of CNTs. The result shows that the increase of Cu from 40 to 60 at.% in NiyCu alloys changes the morphology of carbon from tubules to filaments, and meanwhile the tangled conditions were lessened in this Ni–Cu alloy range. Aligned carbon nanotubes were effectively catalyzed at Cu-40 at.%, and interestingly enough the existence of carbon nanoclusters at Cu)60 at.% was observed. Images from high-resolution transmission electron microscopy (HRTEM) showed that the CNTs were multiwalled and graphitized structures were confirmed by Raman spectra. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: One-dimensional carbon structured materials; Microwave plasma enhanced chemical vapor deposition; Ni–Cu alloys; Catalytic processes
1. Introduction The carbon nanotube has attracted a great deal of attention due to its advantageous properties, such as high Young’s modulus w1x, effective capability for the storage of a large amount of hydrogen w2x, field emission characteristics w3x, and structural diversities that make it possible for band gap engineering w4x. These useful properties of carbon nanotubes (CNTs) make themselves good candidates for various application fields, for instance, as a transistor, battery, field emission display, nanoscale inter-connects, and so on. Synthesis of CNTs has been achieved by several methods such as arc discharge w5x, laser vaporization w6x, and chemical vapor deposition (CVD) w7x. Among these, the CVD method has several advantages to control the structures with various growth parameters w8x. *Corresponding author. Tel.: q886-3-571-5131 ext. 3845; fax: q 886-35-71-0290. E-mail address: [email protected] (H.C. Shih).
Transition metals such as iron, cobalt and nickel are very active catalysts for the formation of carbon nanotubes. Many researchers observed that single-wall carbon nanotubes (SWCNTs) could be produced by the CVD method with two kinds of metal mixture catalysts. Hafner et al. reported that both CO decomposition on Mo catalyst particles at 850 8C or the reaction of C2H4 with FeMo particles at 700 8C appeared to generate SWCNTs w9x. Kitiyanan et al. noticed that Mo could be used to modify the selectivity of Co catalysts by changing the composition w10x. Experimental results indicated that the composition of CoyMos1:2 produced most SWCNTs. However, other metals, e.g. copper, do not appear to break carbon–carbon bonds and catalyze the nanotubes growth, but the Cu–Ni catalyst used in the growth of filamentous carbons in the heat chemical vapor deposition (HCVD) method has been extensively investigated w11,12x. Nishiyama et al. noticed that hydrogen had an impact on the carbon deposition when the Cu–Ni catalyst was used w13x. Kim et al. studied the hydrocar-
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W.J. Jong et al. / Diamond and Related Materials 11 (2002) 1019–1025
bon co-product on different Cu–Ni catalysts when methane and hydrogen were used in solid carbon deposition w14x. Rodriguez et al. observed that changes in the surface composition of the Cu–Ni catalyst influence the activity of filament growth w15x. Although various studies have been focused on the type of catalysts, there are, to date, few pictures of the morphology of one-dimensional carbon structured materials varying as a function the catalytic efficiency of the alloy. The relation between the composition of the Cu– Ni catalyst and the morphology of tubulesyfilaments is far from complete. Thus, this study aims to grow onedimensional carbon structured materials on the Ni–Cu catalyst by the MPECVD system instead of the HCVD method. Since many semiconductor processes required process temperatures lower than 700 8C, the MPECVD system, which could be operated in the lower temperature range, e.g. F600 8C, can satisfy this requirement. The relation between the morphology of carbon tubulesy filaments and the component of Ni–Cu catalyst is discussed. This will provide the information on the quality of the one-dimensional carbon structured materials. 2. Experimental We prepared the catalytic solution by dissolving the relevant metal salts in ethanol at various ratios. Nickel acetate tetrahydrate and copper chloride were used as the sources for nickel and copper, respectively. The solution was mixed using a magnetic stirrer in a closed vessel for 3 h. The well-stirred solution was spin-coated on a silicon wafer at a spinning velocity of 3000 rev. miny1 for 20 s. This step was repeated six times in order to get a thicker catalytic film. The film was dried at 120 8C for 10 min and then heated at 600 8C for 30 min in an atmosphere furnace. At this temperature, the alloy was prepared by the inter-diffusion of nickel and copper atoms. Carbon filaments were synthesized in a microwave plasma enhanced chemical vapor deposition system (MPECVD), as shown in Fig. 1, in which a power of 1100 W was applied under a controlled pressure of 30 torr. Hydrogen was used as a reducing medium to remove traces of oxygen as an initial step, the flow rate of which was set at 100 sccm. CH4 at the rate of 0.5 sccm was introduced as the carbon source in the second step. Meanwhile, a dc bias voltage of y350 V was applied to the substrate. The samples were then heated to 800 8C and 1000 8C for 2 h in a vacuum furnace under a mixing atmosphere of Ar and H2. The morphology and structure of the carbon filaments as well as the catalyst was characterized by HRTEM (400 kV, JEOL Model JEM-4000EX) with a point-topoint resolution of 0.17 nm. Raman spectra of deposited
Fig. 1. Schematic illustration of MPECVD.
carbon filaments in this study were measured on a Raman spectroscope (Ranishaw Ramanscope Model 2001), and excitation was achieved by means of the 514.4-nm (2.41 eV) line of an argon ion (Arq) laser (Coherent Innovr Model 90). The instrument is equipped with an optical microscope to allow the laser beam to focus on the sample surface. Slit widths were set manually. With the spectral slit width carefully adjusted, the scattering signals were detected by the charged carbon detector. 3. Results and discussion 3.1. Morphology and catalysis The morphology of the one-dimensional carbon structure in connection with various Cu contents in Cu–Ni alloy is shown in Fig. 2. When pure nickel was used, straight tubules were catalyzed by decomposing methane (Fig. 2a). The structures of the carbon nanotubes with 20% Cu–Ni alloys and pure Ni are almost identical. However, it is noted that with the alloy of 40% Cu, the structure changes from tubule to filament, resulting in both an increase in the diameter and the disorder degree of carbon filament. When the alloy reaches 60% Cu, one-dimensional carbon structured materials appear to undergo a further partial and simultaneous change in shape, rotating on an axis perpendicular to the direction of filament growth, and thus forcing the filament to conform to a spiral form. The catalyst particles, which contain 80% Cu, tend to produce filaments in a spiral mode during the whole growth period. When pure copper particles are used as catalyst, no evidence for the formation of filamentous carbon is observed.
W.J. Jong et al. / Diamond and Related Materials 11 (2002) 1019–1025
1021
Fig. 2. The TEM image and its SAD of the one-dimensional carbon structured materials synthesized by varying the percentage of copper in CuyNi alloy: (a) 0%, (b) 20%, (c) 40%, (d) 60%, and (e) 80%.
3.2. HRTEM characterization The selected diffraction patterns of the one-dimensional carbon structured materials shown in Fig. 2 provide information on graphitization. Fig. 2a shows the diffraction pattern of a tube with Ni used as the catalyst. The (0002) reflection presents as continuous arcs, which are due to a little curve tube. The rings from inner to outer correspond to {1010} and {1120} reflections. Fig. 2b shows six discrete spots located on the inner circle and another six on the outer circle, which are due to the hexagonal symmetry of graphite. The orientation of this pattern depends on the orientation of graphite segments
with respect to the tube axis, which means that it depends on the chirality of the tube. In poorly formed fibers, such as turbostratic graphite, spacing due to reflection of these planes is not definite and forms weak intensity rings. The weak intensity rings may be caused by a little stacking array of the graphite layer in the filament or twisty structure, as shown in Fig. 2c–e. The angular range 2u of the arcs is related to the structural parameter of the helix w16x. If the axis of the helix is normal to the incident beam, u is equal to the largest angle enclosed by the axis of the helix and the axis of the hypothetical tangential segments. By measuring the arc angles of Fig. 2a,b, 2u are 438 and 358,
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W.J. Jong et al. / Diamond and Related Materials 11 (2002) 1019–1025
respectively. In other words, they are less helical structures. The bend between two segments may be caused by insertion of a heptagon–pentagon pair in the graphite tubes, the pentagon, causing positive curvature, at the outer rim and the heptagon, negative curvature, at the inner rim of the helix w16x. Table 1 shows that the (0002), {1010} and {1120} plane spacing of carbon nanotubes with Ni–Cu alloy catalyst. All of (0002) plan spacings approach to the theoretical value. The spacing of {1010} and {1120} planes are far from the theoretical value. This is apparently due to the curve of the graphite plane in carbon nanotubes. The TEM images and their SAD patterns for various Cu–Ni catalysts are shown in Fig. 3. Table 2 lists the plane spacings of the catalysts.
Table 1 The interplanar spacing of carbon nanotubes
3.3. Raman spectroscopy
Since the substrate was heated by plasma only, the cooler sides of the carbon-saturated CuNi particles readily condense carbon species through the precipitation from the bulk and thus create favorable sites at the interface of CuNiySi for the nucleation and growth of the one-dimensional carbon structured materials. The original interface is readily replaced by the interface of CuNiyC and CySi which means that the interfacial free energy of CuNiySi (gCuNiySi) is higher than that of CuNiyC (gCuNiyC) and CySi (gCySi), namely, gCuNiySi) gCuNiyCqgCySi. However, this relation does not hold for Cu approaching 100 at.%, where Cu particles are ineffective in the formation of one-dimensional carbon structured materials.
Raman spectroscopy has been used to study multiwall nanotubes w17x and single-wall nanotubes w18,19x. Fig. 4. shows Raman spectra of carbon nanotubes synthesized by MPECVD using 20% Cu–Ni alloy as catalyst. The first-order Raman spectra of all the samples present two sharp peaks approximately 1590 and 1335 cmy1, which indicate the typical characteristics of graphitic carbon nanostructures. The peak at 1590 cmy1 can be identified as the G peak of crystalline graphite arising from the zone-center E2g mode and the peak at 1335 cmy1 as the D peak assigned to an A1g zone-edge phonon induced by the disorder due to the finitecrystalline size w20,21x. The second-order Raman spectra are also shown in Fig. 4. As mentioned above, the peak at approximately 2689 cmy1 (D*) is the overtone of the D peak, and the peak at 2929 cmy1 is attributed to the combination of the D and G peaks. The origin of the D peak has been explained as disorder-induced features due to the finite particle size effect or lattice distortion w22,23x. Besides, the large amount of crystalline domains on the nanometer scale and the surfaces of the tubes must account for the enhancement of the D peak w24x. When the degree of graphitization increases, the ratio of the intensity of the disorder peak at 1335 cmy1 to the graphite carbon peak at 1590 cmy1 (ID yIG ratio) decreases. The relative intensity of the G peak increases with increasing the annealing temperature. In other words, this shows that the crystallinity of the CNTs becomes better at higher temperatures than that at lower temperatures. The better crystallinity of the CNTs is believed to arise from the presence of the catalyst, which may lower the activation energy of the carbon atoms forming the graphitic structure. No deposits were found if no catalyst was present. The results show that the most ordered structure of the tubule is found when the ratio of Cu to Ni equals to 4:1 (80% Cu–Ni). There is a tendency of the disorder degree to increase when the Cu concentration rises. The result seems consistent with the hypothesis that there is
Ni:Cu
5:0
4:1
3:2
2:3
1:4
Theoretical ˚ value (A)
(0002) {1010} {1120}
3.49 2.03 1.21
3.58 2.18 1.29
3.43 2.00 1.61
3.65 2.23 1.52
3.54 2.15 1.26
3.44 2.45 1.415
no carbon deposition using pure copper as the catalyst. It is worth noting that the tubule formed with pure nickel particles as catalyst is not the most ordered structure. 3.4. Interfacial free energy consideration
3.5. Atomistic arrangement of the catalysis surface The copper atoms would cluster rather than uniformly disperse on the surface in the Cu-rich alloy particles. Because the Cu atoms are not the catalytic sites, there is no carbon deposited in these areas on the surface w15x. These clustered areas would limit the reactive areas of adsorption and precipitation of carbon and lead to Table 2 The interplanar spacing of the Ni–Cu catalyst Ni:Cu
Calculated ˚ value (A)
Theoretical ˚ value (A)
5:0 (131) (220) (111)
1.06 1.24 2.03
1.0624 1.2458 2.0344
3:2 (131) (200)
1.07 1.78
1.0734 1.7800
2:3 (220)
1.27
1.2651
1:4 (220) (200) (111)
1.27 1.78 2.07
1.2716 1.7983 2.0764
W.J. Jong et al. / Diamond and Related Materials 11 (2002) 1019–1025
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Fig. 3. The TEM image and its SAD of catalyst: (a) 0% Cu, (b) 40% Cu, (c) 60% Cu, and (d) 80% Cu.
filaments instead of tubular structures. There may be structure defects in the filaments due to the discontinuous nucleated sites. The more surface Cu atoms increase, the more disorder of filaments is. These defects may lead to twist of filaments in order to decrease the number of defects. The twisted filaments would form more compact structure. Then, the following carbon could deposit on the compact surface and form the straight filaments, as shown in Fig. 2d. If there were too many
Cu atoms clustered sites, the straight part would not appear. 4. Conclusions One-dimensional carbon structured materials grown by MPECVD system at a temperature (;600 8C) present an interesting result of the evolution of tubulesy filaments with a diameter varying from 50 to 200 nm
W.J. Jong et al. / Diamond and Related Materials 11 (2002) 1019–1025
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Fig. 4. Raman spectra of the carbon nanotubes synthesized by MPECVD using Ni–20%Cu alloy as catalyst.
depending on the catalytic efficiency of the CuNi alloy. A series of catalytic alloy component variance suggests the influence of copper on the carbon morphology and structure is significant. The existence of isolated Cu sites may be the reason that leads the morphology of carbon deposition to change. The defects, which were created by these no catalytic sites, would twist the filaments and increase the disorder degree. However, twist of filaments could eliminate defects and form compact surface, leading to the straight structure. Raman spectrum studies show that heat treatment is helpful for graphitization of carbon nanostructure. Temperature is therefore important for the degree of the graphitization of the one-dimensional carbon structure materials; the G-peak intensity exceeds the D-peak intensity at 1000 8C. Acknowledgments The authors gratefully acknowledge the support of this work by the National Science Council of the Republic of China under the contract of NSC 89-2216E-007-082. References w1x B.I. Yakobson, C.J. Brabec, J. Bernholc, Phys. Rev. Lett. 76 (1996) 2511.
w2x A.C. Dillon, K.B. Jones, T.A. Bekkedahl, C.H. Klang, D.S. Bethune, M.J. Heben, Nature (Lond.) 386 (1997) 377. w3x K.A. Dean, B.R. Chalamala, J. Appl. Phys. 85 (1999) 3832. w4x J.W. Mintmire, B.I. Dunlap, C.T. White, Phys. Rev. Lett. 68 (1992) 631. w5x C. Journet, W.K. Maser, P. Bernier, et al., Nature (Lond.) 388 (1997) 756. w6x A. Thess, et al., Science 273 (1996) 483. w7x Z.P. Huang, J.W. Xu, Z.F. Ren, J.H. Wang, M.P. Siegal, P.N. Provencio, Appl. Phys. Lett. 73 (1998) 3845. w8x Y.C. Choi, Y.M. Shin, Y.H. Lee, et al., Appl. Phys. Lett. 76 (2000) 2367. w9x J.H. Hafner, M.J. Bronikowski, B.R. Azamian, et al., Chem. Phys. Lett. 296 (1998) 195. w10x B. Kitiyanan, W.E. Alvarez, J.H. Harwell, D.E. Resasco, Chem. Phys. Lett. 317 (2000) 497. w11x V.B. Fenelonov, A.Yu. Derevyankin, L.G. Okkel, et al., Carbon 35 (1997) 1129. w12x L.B. Avdeeva, O.V. Goncharova, D.I. Kochubey, et al., Appl. Catalyst A: Gen. 141 (1996) 117. w13x Y. Nishiyama, Y. Tamai, J. Catal. 45 (1976) 1. w14x M.S. Kim, N.M. Rodriguez, R.T.K. Baker, J. Catal. 131 (1991) 60. w15x N.M. Rodriguez, M.S. Kim, R.T.K. Baker, J. Catal. 140 (1993) 16. w16x S. Amelinckx, D. Bernaerts, G. Van Tendeloo, et al., The morphology, structure and texture of carbon nanotubes: an electron microscopy study, Proceedings of the International Winterschool on Electronic Properties of Novel Materials, 1995, p. 515. w17x J. Kastner, T. Pichler, H. Kuzmany, et al., Chem. Phys. Lett. 221 (1994) 53.
W.J. Jong et al. / Diamond and Related Materials 11 (2002) 1019–1025 w18x A.M. Rao, E. Richter, S. Bandow, et al., Science 275 (1997) 187. w19x A. Kasuya, M. Sugano, Y. Sasaki, et al., Phys. Rev. B 57 (1998) 4999. w20x F. Tuinstra, J.L. Koenig, J. Chem. Phys. 53 (1970) 1126. w21x R.J. Nemanich, S.A. Solin, Phys. Rev. B 20 (1979) 392.
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w22x G. Vitali, M. Rossi, M.L. Terranova, V. Sessa, J. Appl. Phys. 77 (1995) 4307. w23x D.G. McCulloch, S. Prawer, A. Hoffman, Phys. Rev. B 50 (1994) 5905. w24x W. Li, H. Zhang, C. Wang, et al., Appl. Phys. Lett. 70 (1997) 2684.
The effect of catalysis on the formation of one-dimensional carbon structured materials W.J. Jong, S.H. Lai, K.H. Hong, H.N. Lin, H.C. Shih* Department of Materials Science and Engineering, National Tsing Hua University, 101, Sec. 2, Kuang Fu Road, Hsinchu 300, Taiwan, ROC
Abstract Carbon nanotubes (CNTs) have been synthesized by catalytic chemical vapor deposition (CCVD). Catalysts, e.g. Fe, Ni, Co, etc., are very important in the formation of CNTs. Other elements, such as Cu, although showing no effect on growth, have been used to enhance the catalytic effect of Ni by alloying it with Ni. In this study, one-dimensional carbon structured materials were synthesized by microwave plasma enhanced chemical vapor deposition (MPECVD) with a mixture of methane and hydrogen as precursors at a temperature of ;600 8C. Subsequently, the samples were taken into thermal treatment (under a mixing atmosphere of Ar and H2, up to 1000 8C) in order to improve the graphitization of CNTs. The result shows that the increase of Cu from 40 to 60 at.% in NiyCu alloys changes the morphology of carbon from tubules to filaments, and meanwhile the tangled conditions were lessened in this Ni–Cu alloy range. Aligned carbon nanotubes were effectively catalyzed at Cu-40 at.%, and interestingly enough the existence of carbon nanoclusters at Cu)60 at.% was observed. Images from high-resolution transmission electron microscopy (HRTEM) showed that the CNTs were multiwalled and graphitized structures were confirmed by Raman spectra. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: One-dimensional carbon structured materials; Microwave plasma enhanced chemical vapor deposition; Ni–Cu alloys; Catalytic processes
1. Introduction The carbon nanotube has attracted a great deal of attention due to its advantageous properties, such as high Young’s modulus w1x, effective capability for the storage of a large amount of hydrogen w2x, field emission characteristics w3x, and structural diversities that make it possible for band gap engineering w4x. These useful properties of carbon nanotubes (CNTs) make themselves good candidates for various application fields, for instance, as a transistor, battery, field emission display, nanoscale inter-connects, and so on. Synthesis of CNTs has been achieved by several methods such as arc discharge w5x, laser vaporization w6x, and chemical vapor deposition (CVD) w7x. Among these, the CVD method has several advantages to control the structures with various growth parameters w8x. *Corresponding author. Tel.: q886-3-571-5131 ext. 3845; fax: q 886-35-71-0290. E-mail address: [email protected] (H.C. Shih).
Transition metals such as iron, cobalt and nickel are very active catalysts for the formation of carbon nanotubes. Many researchers observed that single-wall carbon nanotubes (SWCNTs) could be produced by the CVD method with two kinds of metal mixture catalysts. Hafner et al. reported that both CO decomposition on Mo catalyst particles at 850 8C or the reaction of C2H4 with FeMo particles at 700 8C appeared to generate SWCNTs w9x. Kitiyanan et al. noticed that Mo could be used to modify the selectivity of Co catalysts by changing the composition w10x. Experimental results indicated that the composition of CoyMos1:2 produced most SWCNTs. However, other metals, e.g. copper, do not appear to break carbon–carbon bonds and catalyze the nanotubes growth, but the Cu–Ni catalyst used in the growth of filamentous carbons in the heat chemical vapor deposition (HCVD) method has been extensively investigated w11,12x. Nishiyama et al. noticed that hydrogen had an impact on the carbon deposition when the Cu–Ni catalyst was used w13x. Kim et al. studied the hydrocar-
0925-9635/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 1 . 0 0 5 5 1 - 9
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W.J. Jong et al. / Diamond and Related Materials 11 (2002) 1019–1025
bon co-product on different Cu–Ni catalysts when methane and hydrogen were used in solid carbon deposition w14x. Rodriguez et al. observed that changes in the surface composition of the Cu–Ni catalyst influence the activity of filament growth w15x. Although various studies have been focused on the type of catalysts, there are, to date, few pictures of the morphology of one-dimensional carbon structured materials varying as a function the catalytic efficiency of the alloy. The relation between the composition of the Cu– Ni catalyst and the morphology of tubulesyfilaments is far from complete. Thus, this study aims to grow onedimensional carbon structured materials on the Ni–Cu catalyst by the MPECVD system instead of the HCVD method. Since many semiconductor processes required process temperatures lower than 700 8C, the MPECVD system, which could be operated in the lower temperature range, e.g. F600 8C, can satisfy this requirement. The relation between the morphology of carbon tubulesy filaments and the component of Ni–Cu catalyst is discussed. This will provide the information on the quality of the one-dimensional carbon structured materials. 2. Experimental We prepared the catalytic solution by dissolving the relevant metal salts in ethanol at various ratios. Nickel acetate tetrahydrate and copper chloride were used as the sources for nickel and copper, respectively. The solution was mixed using a magnetic stirrer in a closed vessel for 3 h. The well-stirred solution was spin-coated on a silicon wafer at a spinning velocity of 3000 rev. miny1 for 20 s. This step was repeated six times in order to get a thicker catalytic film. The film was dried at 120 8C for 10 min and then heated at 600 8C for 30 min in an atmosphere furnace. At this temperature, the alloy was prepared by the inter-diffusion of nickel and copper atoms. Carbon filaments were synthesized in a microwave plasma enhanced chemical vapor deposition system (MPECVD), as shown in Fig. 1, in which a power of 1100 W was applied under a controlled pressure of 30 torr. Hydrogen was used as a reducing medium to remove traces of oxygen as an initial step, the flow rate of which was set at 100 sccm. CH4 at the rate of 0.5 sccm was introduced as the carbon source in the second step. Meanwhile, a dc bias voltage of y350 V was applied to the substrate. The samples were then heated to 800 8C and 1000 8C for 2 h in a vacuum furnace under a mixing atmosphere of Ar and H2. The morphology and structure of the carbon filaments as well as the catalyst was characterized by HRTEM (400 kV, JEOL Model JEM-4000EX) with a point-topoint resolution of 0.17 nm. Raman spectra of deposited
Fig. 1. Schematic illustration of MPECVD.
carbon filaments in this study were measured on a Raman spectroscope (Ranishaw Ramanscope Model 2001), and excitation was achieved by means of the 514.4-nm (2.41 eV) line of an argon ion (Arq) laser (Coherent Innovr Model 90). The instrument is equipped with an optical microscope to allow the laser beam to focus on the sample surface. Slit widths were set manually. With the spectral slit width carefully adjusted, the scattering signals were detected by the charged carbon detector. 3. Results and discussion 3.1. Morphology and catalysis The morphology of the one-dimensional carbon structure in connection with various Cu contents in Cu–Ni alloy is shown in Fig. 2. When pure nickel was used, straight tubules were catalyzed by decomposing methane (Fig. 2a). The structures of the carbon nanotubes with 20% Cu–Ni alloys and pure Ni are almost identical. However, it is noted that with the alloy of 40% Cu, the structure changes from tubule to filament, resulting in both an increase in the diameter and the disorder degree of carbon filament. When the alloy reaches 60% Cu, one-dimensional carbon structured materials appear to undergo a further partial and simultaneous change in shape, rotating on an axis perpendicular to the direction of filament growth, and thus forcing the filament to conform to a spiral form. The catalyst particles, which contain 80% Cu, tend to produce filaments in a spiral mode during the whole growth period. When pure copper particles are used as catalyst, no evidence for the formation of filamentous carbon is observed.
W.J. Jong et al. / Diamond and Related Materials 11 (2002) 1019–1025
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Fig. 2. The TEM image and its SAD of the one-dimensional carbon structured materials synthesized by varying the percentage of copper in CuyNi alloy: (a) 0%, (b) 20%, (c) 40%, (d) 60%, and (e) 80%.
3.2. HRTEM characterization The selected diffraction patterns of the one-dimensional carbon structured materials shown in Fig. 2 provide information on graphitization. Fig. 2a shows the diffraction pattern of a tube with Ni used as the catalyst. The (0002) reflection presents as continuous arcs, which are due to a little curve tube. The rings from inner to outer correspond to {1010} and {1120} reflections. Fig. 2b shows six discrete spots located on the inner circle and another six on the outer circle, which are due to the hexagonal symmetry of graphite. The orientation of this pattern depends on the orientation of graphite segments
with respect to the tube axis, which means that it depends on the chirality of the tube. In poorly formed fibers, such as turbostratic graphite, spacing due to reflection of these planes is not definite and forms weak intensity rings. The weak intensity rings may be caused by a little stacking array of the graphite layer in the filament or twisty structure, as shown in Fig. 2c–e. The angular range 2u of the arcs is related to the structural parameter of the helix w16x. If the axis of the helix is normal to the incident beam, u is equal to the largest angle enclosed by the axis of the helix and the axis of the hypothetical tangential segments. By measuring the arc angles of Fig. 2a,b, 2u are 438 and 358,
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W.J. Jong et al. / Diamond and Related Materials 11 (2002) 1019–1025
respectively. In other words, they are less helical structures. The bend between two segments may be caused by insertion of a heptagon–pentagon pair in the graphite tubes, the pentagon, causing positive curvature, at the outer rim and the heptagon, negative curvature, at the inner rim of the helix w16x. Table 1 shows that the (0002), {1010} and {1120} plane spacing of carbon nanotubes with Ni–Cu alloy catalyst. All of (0002) plan spacings approach to the theoretical value. The spacing of {1010} and {1120} planes are far from the theoretical value. This is apparently due to the curve of the graphite plane in carbon nanotubes. The TEM images and their SAD patterns for various Cu–Ni catalysts are shown in Fig. 3. Table 2 lists the plane spacings of the catalysts.
Table 1 The interplanar spacing of carbon nanotubes
3.3. Raman spectroscopy
Since the substrate was heated by plasma only, the cooler sides of the carbon-saturated CuNi particles readily condense carbon species through the precipitation from the bulk and thus create favorable sites at the interface of CuNiySi for the nucleation and growth of the one-dimensional carbon structured materials. The original interface is readily replaced by the interface of CuNiyC and CySi which means that the interfacial free energy of CuNiySi (gCuNiySi) is higher than that of CuNiyC (gCuNiyC) and CySi (gCySi), namely, gCuNiySi) gCuNiyCqgCySi. However, this relation does not hold for Cu approaching 100 at.%, where Cu particles are ineffective in the formation of one-dimensional carbon structured materials.
Raman spectroscopy has been used to study multiwall nanotubes w17x and single-wall nanotubes w18,19x. Fig. 4. shows Raman spectra of carbon nanotubes synthesized by MPECVD using 20% Cu–Ni alloy as catalyst. The first-order Raman spectra of all the samples present two sharp peaks approximately 1590 and 1335 cmy1, which indicate the typical characteristics of graphitic carbon nanostructures. The peak at 1590 cmy1 can be identified as the G peak of crystalline graphite arising from the zone-center E2g mode and the peak at 1335 cmy1 as the D peak assigned to an A1g zone-edge phonon induced by the disorder due to the finitecrystalline size w20,21x. The second-order Raman spectra are also shown in Fig. 4. As mentioned above, the peak at approximately 2689 cmy1 (D*) is the overtone of the D peak, and the peak at 2929 cmy1 is attributed to the combination of the D and G peaks. The origin of the D peak has been explained as disorder-induced features due to the finite particle size effect or lattice distortion w22,23x. Besides, the large amount of crystalline domains on the nanometer scale and the surfaces of the tubes must account for the enhancement of the D peak w24x. When the degree of graphitization increases, the ratio of the intensity of the disorder peak at 1335 cmy1 to the graphite carbon peak at 1590 cmy1 (ID yIG ratio) decreases. The relative intensity of the G peak increases with increasing the annealing temperature. In other words, this shows that the crystallinity of the CNTs becomes better at higher temperatures than that at lower temperatures. The better crystallinity of the CNTs is believed to arise from the presence of the catalyst, which may lower the activation energy of the carbon atoms forming the graphitic structure. No deposits were found if no catalyst was present. The results show that the most ordered structure of the tubule is found when the ratio of Cu to Ni equals to 4:1 (80% Cu–Ni). There is a tendency of the disorder degree to increase when the Cu concentration rises. The result seems consistent with the hypothesis that there is
Ni:Cu
5:0
4:1
3:2
2:3
1:4
Theoretical ˚ value (A)
(0002) {1010} {1120}
3.49 2.03 1.21
3.58 2.18 1.29
3.43 2.00 1.61
3.65 2.23 1.52
3.54 2.15 1.26
3.44 2.45 1.415
no carbon deposition using pure copper as the catalyst. It is worth noting that the tubule formed with pure nickel particles as catalyst is not the most ordered structure. 3.4. Interfacial free energy consideration
3.5. Atomistic arrangement of the catalysis surface The copper atoms would cluster rather than uniformly disperse on the surface in the Cu-rich alloy particles. Because the Cu atoms are not the catalytic sites, there is no carbon deposited in these areas on the surface w15x. These clustered areas would limit the reactive areas of adsorption and precipitation of carbon and lead to Table 2 The interplanar spacing of the Ni–Cu catalyst Ni:Cu
Calculated ˚ value (A)
Theoretical ˚ value (A)
5:0 (131) (220) (111)
1.06 1.24 2.03
1.0624 1.2458 2.0344
3:2 (131) (200)
1.07 1.78
1.0734 1.7800
2:3 (220)
1.27
1.2651
1:4 (220) (200) (111)
1.27 1.78 2.07
1.2716 1.7983 2.0764
W.J. Jong et al. / Diamond and Related Materials 11 (2002) 1019–1025
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Fig. 3. The TEM image and its SAD of catalyst: (a) 0% Cu, (b) 40% Cu, (c) 60% Cu, and (d) 80% Cu.
filaments instead of tubular structures. There may be structure defects in the filaments due to the discontinuous nucleated sites. The more surface Cu atoms increase, the more disorder of filaments is. These defects may lead to twist of filaments in order to decrease the number of defects. The twisted filaments would form more compact structure. Then, the following carbon could deposit on the compact surface and form the straight filaments, as shown in Fig. 2d. If there were too many
Cu atoms clustered sites, the straight part would not appear. 4. Conclusions One-dimensional carbon structured materials grown by MPECVD system at a temperature (;600 8C) present an interesting result of the evolution of tubulesy filaments with a diameter varying from 50 to 200 nm
W.J. Jong et al. / Diamond and Related Materials 11 (2002) 1019–1025
1024
Fig. 4. Raman spectra of the carbon nanotubes synthesized by MPECVD using Ni–20%Cu alloy as catalyst.
depending on the catalytic efficiency of the CuNi alloy. A series of catalytic alloy component variance suggests the influence of copper on the carbon morphology and structure is significant. The existence of isolated Cu sites may be the reason that leads the morphology of carbon deposition to change. The defects, which were created by these no catalytic sites, would twist the filaments and increase the disorder degree. However, twist of filaments could eliminate defects and form compact surface, leading to the straight structure. Raman spectrum studies show that heat treatment is helpful for graphitization of carbon nanostructure. Temperature is therefore important for the degree of the graphitization of the one-dimensional carbon structure materials; the G-peak intensity exceeds the D-peak intensity at 1000 8C. Acknowledgments The authors gratefully acknowledge the support of this work by the National Science Council of the Republic of China under the contract of NSC 89-2216E-007-082. References w1x B.I. Yakobson, C.J. Brabec, J. Bernholc, Phys. Rev. Lett. 76 (1996) 2511.
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