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Ethylene flame synthesis of well-aligned multi-walled carbon nanotubes
28 September 2001
Chemical Physics Letters 346 (2001) 23±28
www.elsevier.com/locate/cplett
Ethylene ¯ame synthesis of well-aligned multi-walled carbon nanotubes Liming Yuan a, Kozo Saito a,*, Wenchong Hu b, Zhi Chen c a
Department of Mechanical Engineering, University of Kentucky, 521 CRMS Building, Lexington, KY 40506-0108, USA b Department of Chemical and Material Engineering, University of Kentucky, Lexington, KY 40506-0108, USA c Department of Electric Engineering, University of Kentucky, Lexington, KY 40506-0108, USA Received 14 May 2001
Abstract A stainless steel grid baked by a propane±air premixed ¯ame had iron, chromium and nickel oxide deposits on the grid surface. With this grid, entangled and curved shape multi-walled carbon nanotubes (MWNTs) were harvested from an ethylene±air diusion ¯ame with yield rate of 3 mg/min. Nitrogen addition to the ¯ame was found to straighten the entangled tubes probably by lowering the ¯ame temperature. A cobalt-electrodeposited stainless steel grid was ®nally applied to the nitrogen-diluted ethylene diusion ¯ame; well-aligned and well-graphitized carbon nanotubes consisting of 20 nm diameter and 10 lm long element tubes were obtained. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction This is the continuation of our previous work on carbon nanotubes. Previously, we reported synthesis of multi-walled carbon nanotubes using methane diusion ¯ames and discussed the role of iron oxide particles as the catalyst for the nanotube growth. In that Letter our focus was basic understanding on the mechanism of carbon nanotube formation in the ¯ame. This Letter discusses how to eectively synthesize well-aligned and well-graphitized carbon nanotubes using ethylene diusion ¯ames. Carbon nanotubes are known to possess exceptional mechanical, electrical and thermal properties because of their high strength, low
*
Corresponding author. Fax: +1-859-257-3304. E-mail address: [email protected] (K. Saito).
density, and high electrical and thermal conductivity [1]. Our previous Letter [2] summarized the current progress on dierent synthesis methods of carbon nanotubes. Our focus in this Letter is to use diusion ¯ames for commercial synthesis of multi-walled carbon nanotubes. There is a need of study in this area, because ¯ame synthesis of fullerenes has been reported, but the ¯ame synthesis of CNTs has met with little success. Some carbon nanotubes were found in the low pressure premixed hydrocarbon ¯ames and diusion ¯ames [3±5]; however, the nanotube yield was too low to be found under SEM analysis. We present here a unique method to synthesize carbon nanotubes without directly seeding materials into the ¯ame. We used our previous experimental apparatus and established a steady and stable laminar ethylene± air co-¯ow diusion ¯ame [6]. The growth mechanism of carbon nanotubes and the eect of nitrogen addition to the fuel stream on the nanotube growth rate were studied.
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 9 5 9 - 9
24
L. Yuan et al. / Chemical Physics Letters 346 (2001) 23±28
2. Experimental A laminar ethylene±air co-¯ow diusion ¯ame was used to synthesize MWNTs. A schematic diagram of the experimental apparatus was shown in our previous Letter [2]. Ethylene (99.5% purity) was issued from a 1.1 cm diameter stainless steel tube, which was surrounded by a 5 cm diameter tube through which air ¯owed. With the average linear fuel ¯ow rate of 4.7 cm/s and the average linear air¯ow rate of 63 cm/s, a steady and stable laminar ¯ame with a visible ¯ame height of 33 mm was established on the burner port. All syntheses were performed at normal atmospheric pressure. A stainless steel grid (type 304) was used as the substrate for the nanotube growth. The material deposited on the wire was examined directly by SEM (Hitachi S900, 3 kV). The deposit material was dispersed in ethanol using a mild sonication. Drops of the dispersion were placed on the copper TEM grids and dried for TEM analysis (JEOL JEM-2000-FX, 200 kV). TEM±EDX (energy dispersive X-ray) analysis was performed using an Oxford INCA detector attached to the TEM. A high-resolution TEM (JEOL 2010F, 200 kV) was also employed to study the microstructure of carbon nanotubes. 3. Results and discussion When the stainless steel grid was inserted into the ¯ame 4 mm above the exit port without any pre-treatment, and kept there for 10 min, some black material deposited on the grid. There were no nanotubes found under SEM analysis; however some MWNTs were found under TEM analysis. Most of the deposit material was amorphous soot particles. This indicates that the metal oxide (iron oxide and nickel oxide) particles formed on the metal surface were not suitable as the catalyst particles for carbon nanotube growth. In our previous study, a Ni±Cr wire was placed in the laminar methane diusion ¯ame for 10 min; the formed nickel oxide particles were favorable for the carbon nanotube growth [2]. The formation of iron oxide was faster than that of nickel oxide; the morphology of iron oxide layer on the metal sur-
face was dierent from that of nickel oxide. When the Ni±Cr wire was placed in the ethylene ¯ame for 10 min, some carbon ®bers loaded with amorphous carbon particles were produced, indicating the oxidation process in the ethylene ¯ame to be somewhat dierent from the methane ¯ame. To synthesize a pure form of carbon nanotubes, we pre-oxidized the stainless steel grid using a lean premixed propane ¯ame for about 5 s. SEM and TEM analyses on the pre-oxidized grid showed that a layer of several dierent types of particles whose diameter ranged from 10 to 200 nm was formed. The TEM±EDX spectra identi®ed these particles to be iron, chromium, nickel, oxygen and carbon. We believe that most of the metal elements exist in metal oxide form, some as metal carbide. When the grid was placed in the ethylene ¯ame for 10 min, a layer of dark gray deposit material that was identi®ed by SEM as a bundle of entangled and curved MWNTs with their diameter of 10±60 nm and some carbon ®bers with a diameter larger than 100 nm (Fig. 1). The length of nanotubes is at least several lm and the growth rate of gray material was more than 3 mg/min. 3.1. The sampling time Several dierent sampling times ranging from 1 to 10 min were applied to see whether or not the morphology of carbon nanotube changes. With an increase of sampling time, the production rate of carbon ®bers increased, probably due to a continuous deposition of pyrolytic carbon onto the carbon nanotubes [7]. We found that the length of carbon nanotubes reached the maximum length 2± 5 lm within less than 1 min, while with an increase of the sampling time beyond 1 min, the thickness of nanotube walls increased but the length remained the same. We also found after the deposition of pyrolytic carbon on the nanotube surface, the nanotube grew mainly in thickness, but not length, and eventually became a solid nano®ber. With a 10 min sampling time, the nanotube diameter increased to 20±60 nm, but the length remained the same probably due to the formation of pyrolytic carbon on the nanotube surface, a different result from our previous methane ¯ame synthesis, where pyrolysis products were stable
L. Yuan et al. / Chemical Physics Letters 346 (2001) 23±28
25
Fig. 1. SEM (a) and TEM (b) images of carbon nanotubes synthesized from an ethylene diusion-¯ame using a stainless steel sampling grid coated with catalyst particles created by a premixed propane ¯ame (magni®cation is shown in each ®gure).
and the pyrolytic carbon concentration was much lower than ethylene ¯ames. 3.2. The eect of nitrogen addition Our ethylene ¯ame was diluted with nitrogen, C2 H4 =N2 0:25, to change the temperature of the ¯ame. The stainless steel grid was pre-oxidized with a premixed propane ¯ame, and was placed for 10 min in the ethylene ¯ame at the same position where we obtained the Fig. 1 result. Fig. 2 shows SEM and TEM images of the carbon nanotubes that contain few carbon ®bers. The ¯ame temperature at the sampling location in this diluted ethylene ¯ame is 1244 °C, while the corresponding temperature for an undiluted ethylene ¯ame is 1547 °C. Thus, the observed signi®cant reduction of carbon ®bers may be due to lower temperature and lower fuel concentration conditions. The lower temperature that can reduce the pyrolysis rate of hydrocarbon products in the ¯ame may have something to do with an increase of nanotube formation but may suppress the growth of carbon ®bers probably due to the
lower chemical reaction rate of the hydrocarbon pyrolysis reaction. The nitrogen dilution eect may also have something to do with the creation of the nearly same diameter nanotubes, probably by limiting the growth of the nanotube wall thickness due to the limited chemical species and the lower chemical reaction rate. 3.3. Cobalt-coated grid The oxidized stainless steel grid produced entangled and curved multi-walled carbon nanotubes. To obtain straight and uniform diameter nanotubes, cobalt particles were electrodeposited on to the stainless steel surface by a two-probe dc method in 5 wt% CoSO4 7H2 O and 2 wt% H3 BO3 solution for 5 min. This cobalt-coated grid was placed for 5 min at the same location as the Figs. 1 and 2 result. Fig. 3 presents SEM and TEM images of well aligned and nearly uniform diameter carbon nanotubes obtained by the cobalt-coated grid. Interestingly, some nanotubes bend down (Fig. 3), while others are straight (Fig. 4), although both nanotubes were produced by the same cobalt-
26
L. Yuan et al. / Chemical Physics Letters 346 (2001) 23±28
Fig. 2. SEM (a) and TEM (b) images of carbon nanotubes synthesized from a nitrogen-diluted ethylene diusion-¯ame using a stainless steel sampling grid coated with catalyst particles created by a premixed propane ¯ame (magni®cation is shown in each ®gure).
(a)
(b)
Fig. 3. SEM (a) and TEM (b) images of carbon nanotubes synthesized from an ethylene diusion-¯ame using a cobalt-coated stainless steel grid (magni®cation is shown in each ®gure).
coated grid at the same ¯ame location. We noticed spacing between each nanotube is dierent: nanotubes in Fig. 3 are more packed than in Fig. 4,
suggesting that the mechanical strength required for each nanotube to shoot up straight may be weakened by friction between nanotubes.
L. Yuan et al. / Chemical Physics Letters 346 (2001) 23±28
Fig. 4. Vertically oriented, well-aligned nanotubes synthesized by a cobalt-coated stainless steel grid.
3.4. The growth mechanism of MWNTs The growth mechanism was investigated by examining the SEM, TEM images and TEM± EDX spectra. In the SEM images, some particles were found attached to the one end of the nanotubes. These particles were not well separated, and were closely connected to each other. The TEM images con®rmed that very few particles were encapsulated inside the nanotubes. The nanotubes were probably grown from some smaller particles, and entangled with each other. The nanotubes seemed to grow mainly by a root growth that was described by Baker [8]. The TEM±EDS (energy dispersion spectrum) showed that these particles were mainly iron oxide, although some are nickel oxide, con®rming our examination on the oxidized metal surface before nanotube growth. These particles acted as catalyst particles for nanotube growth. The interaction between metal surface and metal oxide particles seems very weak. SEM images showed that some particle block was detached from the surface because of the nanotube growth. But the interaction between these particles may be strong, because even after mild sonication, some particles are still connected to each other on the TEM
27
images. Experimental results showed that the diameters of carbon nanotubes were well correlated with the diameters of catalyst particles [9]. The smaller catalyst particles generated smaller nanotubes. Because the metal oxide particles were strongly bonded to each other, no catalyst particles could be pushed up to encapsulate nanotubes. For the nanotube growth out of cobalt particles, TEM±EDX spectra con®rm the existence of cobalt and oxygen. Some of the cobalt may be oxidized in the ¯ame. Fig. 3a SEM image of carbon nanotube synthesized from cobalt particles shows no obvious particles or block of particles attached to the tubes that were observed in the propane-oxidized grid sampling. Fig. 3b TEM image also displays no catalyst particles encapsulated inside the nanotubes indicating a strong bond between the cobalt catalyst particles and the metal surface. Fig. 5 high-resolution TEM image shows that the nanotubes synthesized from our ethylene ¯ame have been well graphitized. The production rate of MWNT is very sensitive to the sampling location. With an increase of the sampling location above the current location, the concentration of pyrolyzed hydrocarbon products increased. Some of the pyrolyzed hydrocarbon products can be decomposed by catalyst particles, but the rest will produce amorphous carbons, a contaminant for catalyst particles.
Fig. 5. HRTEM image of a typical nanotube synthesized by a cobalt-coated stainless steel grid (scale bar: 5 nm).
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L. Yuan et al. / Chemical Physics Letters 346 (2001) 23±28
4. Conclusions With a stainless steel grid as the substrate and oxidized by a premixed propane ¯ame, entangled nanotubes with diameters ranging from 10 to 60 nm were formed from a laminar ethylene±air diffusion ¯ame. The yield rate of the nanotubes from our current laboratory apparatus was approximately 3 mg/min. Well-aligned and well-graphitized multi-walled carbon nanotubes consisting of nearly uniform diameter nanotubes were synthesized from a nitrogen diluted laminar ethylene±air diusion ¯ame with a cobalt-electrodeposited stainless steel grid. Acknowledgements This study was sponsored by National Science Foundation under the MRSEC program (DMR-
9809686). We would like to acknowledge the valuable comments from Professor F.A. Williams and Professor A.S. Gordon. We also acknowledge Bob Gregory for his editing work. References [1] T.W. Ebbesen, Carbon Nanotubes: Preparation and Properties, CRC Press, Boca Raton, FL, 1997. [2] L. Yuan, K. Saito, C. Pan, F.A. Williams, A.S. Gordon, Chem. Phys. Lett. 340 (2001) 237. [3] R.L. Vander Wal, T.M. Ticich, V.E. Curtis, Chem. Phys. Lett. 323 (2000) 217. [4] J.B. Howard, K. Das Chowdhury, J.B. Vander Sande, Nature 370 (1994) 603. [5] H. Richter et al., Carbon 34 (1996) 427. [6] K. Saito, A.S. Gordon, F.A. Williams, W.F. Stickle, Combust. Sci. Technol. 80 (1991) 103. [7] M. Endo et al., Carbon 33 (1995) 873. [8] R.T. Baker, Carbon 27 (1989) 315. [9] W.Z. Li et al., Chem. Phys. Lett. 335 (2001) 141.
Chemical Physics Letters 346 (2001) 23±28
www.elsevier.com/locate/cplett
Ethylene ¯ame synthesis of well-aligned multi-walled carbon nanotubes Liming Yuan a, Kozo Saito a,*, Wenchong Hu b, Zhi Chen c a
Department of Mechanical Engineering, University of Kentucky, 521 CRMS Building, Lexington, KY 40506-0108, USA b Department of Chemical and Material Engineering, University of Kentucky, Lexington, KY 40506-0108, USA c Department of Electric Engineering, University of Kentucky, Lexington, KY 40506-0108, USA Received 14 May 2001
Abstract A stainless steel grid baked by a propane±air premixed ¯ame had iron, chromium and nickel oxide deposits on the grid surface. With this grid, entangled and curved shape multi-walled carbon nanotubes (MWNTs) were harvested from an ethylene±air diusion ¯ame with yield rate of 3 mg/min. Nitrogen addition to the ¯ame was found to straighten the entangled tubes probably by lowering the ¯ame temperature. A cobalt-electrodeposited stainless steel grid was ®nally applied to the nitrogen-diluted ethylene diusion ¯ame; well-aligned and well-graphitized carbon nanotubes consisting of 20 nm diameter and 10 lm long element tubes were obtained. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction This is the continuation of our previous work on carbon nanotubes. Previously, we reported synthesis of multi-walled carbon nanotubes using methane diusion ¯ames and discussed the role of iron oxide particles as the catalyst for the nanotube growth. In that Letter our focus was basic understanding on the mechanism of carbon nanotube formation in the ¯ame. This Letter discusses how to eectively synthesize well-aligned and well-graphitized carbon nanotubes using ethylene diusion ¯ames. Carbon nanotubes are known to possess exceptional mechanical, electrical and thermal properties because of their high strength, low
*
Corresponding author. Fax: +1-859-257-3304. E-mail address: [email protected] (K. Saito).
density, and high electrical and thermal conductivity [1]. Our previous Letter [2] summarized the current progress on dierent synthesis methods of carbon nanotubes. Our focus in this Letter is to use diusion ¯ames for commercial synthesis of multi-walled carbon nanotubes. There is a need of study in this area, because ¯ame synthesis of fullerenes has been reported, but the ¯ame synthesis of CNTs has met with little success. Some carbon nanotubes were found in the low pressure premixed hydrocarbon ¯ames and diusion ¯ames [3±5]; however, the nanotube yield was too low to be found under SEM analysis. We present here a unique method to synthesize carbon nanotubes without directly seeding materials into the ¯ame. We used our previous experimental apparatus and established a steady and stable laminar ethylene± air co-¯ow diusion ¯ame [6]. The growth mechanism of carbon nanotubes and the eect of nitrogen addition to the fuel stream on the nanotube growth rate were studied.
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 9 5 9 - 9
24
L. Yuan et al. / Chemical Physics Letters 346 (2001) 23±28
2. Experimental A laminar ethylene±air co-¯ow diusion ¯ame was used to synthesize MWNTs. A schematic diagram of the experimental apparatus was shown in our previous Letter [2]. Ethylene (99.5% purity) was issued from a 1.1 cm diameter stainless steel tube, which was surrounded by a 5 cm diameter tube through which air ¯owed. With the average linear fuel ¯ow rate of 4.7 cm/s and the average linear air¯ow rate of 63 cm/s, a steady and stable laminar ¯ame with a visible ¯ame height of 33 mm was established on the burner port. All syntheses were performed at normal atmospheric pressure. A stainless steel grid (type 304) was used as the substrate for the nanotube growth. The material deposited on the wire was examined directly by SEM (Hitachi S900, 3 kV). The deposit material was dispersed in ethanol using a mild sonication. Drops of the dispersion were placed on the copper TEM grids and dried for TEM analysis (JEOL JEM-2000-FX, 200 kV). TEM±EDX (energy dispersive X-ray) analysis was performed using an Oxford INCA detector attached to the TEM. A high-resolution TEM (JEOL 2010F, 200 kV) was also employed to study the microstructure of carbon nanotubes. 3. Results and discussion When the stainless steel grid was inserted into the ¯ame 4 mm above the exit port without any pre-treatment, and kept there for 10 min, some black material deposited on the grid. There were no nanotubes found under SEM analysis; however some MWNTs were found under TEM analysis. Most of the deposit material was amorphous soot particles. This indicates that the metal oxide (iron oxide and nickel oxide) particles formed on the metal surface were not suitable as the catalyst particles for carbon nanotube growth. In our previous study, a Ni±Cr wire was placed in the laminar methane diusion ¯ame for 10 min; the formed nickel oxide particles were favorable for the carbon nanotube growth [2]. The formation of iron oxide was faster than that of nickel oxide; the morphology of iron oxide layer on the metal sur-
face was dierent from that of nickel oxide. When the Ni±Cr wire was placed in the ethylene ¯ame for 10 min, some carbon ®bers loaded with amorphous carbon particles were produced, indicating the oxidation process in the ethylene ¯ame to be somewhat dierent from the methane ¯ame. To synthesize a pure form of carbon nanotubes, we pre-oxidized the stainless steel grid using a lean premixed propane ¯ame for about 5 s. SEM and TEM analyses on the pre-oxidized grid showed that a layer of several dierent types of particles whose diameter ranged from 10 to 200 nm was formed. The TEM±EDX spectra identi®ed these particles to be iron, chromium, nickel, oxygen and carbon. We believe that most of the metal elements exist in metal oxide form, some as metal carbide. When the grid was placed in the ethylene ¯ame for 10 min, a layer of dark gray deposit material that was identi®ed by SEM as a bundle of entangled and curved MWNTs with their diameter of 10±60 nm and some carbon ®bers with a diameter larger than 100 nm (Fig. 1). The length of nanotubes is at least several lm and the growth rate of gray material was more than 3 mg/min. 3.1. The sampling time Several dierent sampling times ranging from 1 to 10 min were applied to see whether or not the morphology of carbon nanotube changes. With an increase of sampling time, the production rate of carbon ®bers increased, probably due to a continuous deposition of pyrolytic carbon onto the carbon nanotubes [7]. We found that the length of carbon nanotubes reached the maximum length 2± 5 lm within less than 1 min, while with an increase of the sampling time beyond 1 min, the thickness of nanotube walls increased but the length remained the same. We also found after the deposition of pyrolytic carbon on the nanotube surface, the nanotube grew mainly in thickness, but not length, and eventually became a solid nano®ber. With a 10 min sampling time, the nanotube diameter increased to 20±60 nm, but the length remained the same probably due to the formation of pyrolytic carbon on the nanotube surface, a different result from our previous methane ¯ame synthesis, where pyrolysis products were stable
L. Yuan et al. / Chemical Physics Letters 346 (2001) 23±28
25
Fig. 1. SEM (a) and TEM (b) images of carbon nanotubes synthesized from an ethylene diusion-¯ame using a stainless steel sampling grid coated with catalyst particles created by a premixed propane ¯ame (magni®cation is shown in each ®gure).
and the pyrolytic carbon concentration was much lower than ethylene ¯ames. 3.2. The eect of nitrogen addition Our ethylene ¯ame was diluted with nitrogen, C2 H4 =N2 0:25, to change the temperature of the ¯ame. The stainless steel grid was pre-oxidized with a premixed propane ¯ame, and was placed for 10 min in the ethylene ¯ame at the same position where we obtained the Fig. 1 result. Fig. 2 shows SEM and TEM images of the carbon nanotubes that contain few carbon ®bers. The ¯ame temperature at the sampling location in this diluted ethylene ¯ame is 1244 °C, while the corresponding temperature for an undiluted ethylene ¯ame is 1547 °C. Thus, the observed signi®cant reduction of carbon ®bers may be due to lower temperature and lower fuel concentration conditions. The lower temperature that can reduce the pyrolysis rate of hydrocarbon products in the ¯ame may have something to do with an increase of nanotube formation but may suppress the growth of carbon ®bers probably due to the
lower chemical reaction rate of the hydrocarbon pyrolysis reaction. The nitrogen dilution eect may also have something to do with the creation of the nearly same diameter nanotubes, probably by limiting the growth of the nanotube wall thickness due to the limited chemical species and the lower chemical reaction rate. 3.3. Cobalt-coated grid The oxidized stainless steel grid produced entangled and curved multi-walled carbon nanotubes. To obtain straight and uniform diameter nanotubes, cobalt particles were electrodeposited on to the stainless steel surface by a two-probe dc method in 5 wt% CoSO4 7H2 O and 2 wt% H3 BO3 solution for 5 min. This cobalt-coated grid was placed for 5 min at the same location as the Figs. 1 and 2 result. Fig. 3 presents SEM and TEM images of well aligned and nearly uniform diameter carbon nanotubes obtained by the cobalt-coated grid. Interestingly, some nanotubes bend down (Fig. 3), while others are straight (Fig. 4), although both nanotubes were produced by the same cobalt-
26
L. Yuan et al. / Chemical Physics Letters 346 (2001) 23±28
Fig. 2. SEM (a) and TEM (b) images of carbon nanotubes synthesized from a nitrogen-diluted ethylene diusion-¯ame using a stainless steel sampling grid coated with catalyst particles created by a premixed propane ¯ame (magni®cation is shown in each ®gure).
(a)
(b)
Fig. 3. SEM (a) and TEM (b) images of carbon nanotubes synthesized from an ethylene diusion-¯ame using a cobalt-coated stainless steel grid (magni®cation is shown in each ®gure).
coated grid at the same ¯ame location. We noticed spacing between each nanotube is dierent: nanotubes in Fig. 3 are more packed than in Fig. 4,
suggesting that the mechanical strength required for each nanotube to shoot up straight may be weakened by friction between nanotubes.
L. Yuan et al. / Chemical Physics Letters 346 (2001) 23±28
Fig. 4. Vertically oriented, well-aligned nanotubes synthesized by a cobalt-coated stainless steel grid.
3.4. The growth mechanism of MWNTs The growth mechanism was investigated by examining the SEM, TEM images and TEM± EDX spectra. In the SEM images, some particles were found attached to the one end of the nanotubes. These particles were not well separated, and were closely connected to each other. The TEM images con®rmed that very few particles were encapsulated inside the nanotubes. The nanotubes were probably grown from some smaller particles, and entangled with each other. The nanotubes seemed to grow mainly by a root growth that was described by Baker [8]. The TEM±EDS (energy dispersion spectrum) showed that these particles were mainly iron oxide, although some are nickel oxide, con®rming our examination on the oxidized metal surface before nanotube growth. These particles acted as catalyst particles for nanotube growth. The interaction between metal surface and metal oxide particles seems very weak. SEM images showed that some particle block was detached from the surface because of the nanotube growth. But the interaction between these particles may be strong, because even after mild sonication, some particles are still connected to each other on the TEM
27
images. Experimental results showed that the diameters of carbon nanotubes were well correlated with the diameters of catalyst particles [9]. The smaller catalyst particles generated smaller nanotubes. Because the metal oxide particles were strongly bonded to each other, no catalyst particles could be pushed up to encapsulate nanotubes. For the nanotube growth out of cobalt particles, TEM±EDX spectra con®rm the existence of cobalt and oxygen. Some of the cobalt may be oxidized in the ¯ame. Fig. 3a SEM image of carbon nanotube synthesized from cobalt particles shows no obvious particles or block of particles attached to the tubes that were observed in the propane-oxidized grid sampling. Fig. 3b TEM image also displays no catalyst particles encapsulated inside the nanotubes indicating a strong bond between the cobalt catalyst particles and the metal surface. Fig. 5 high-resolution TEM image shows that the nanotubes synthesized from our ethylene ¯ame have been well graphitized. The production rate of MWNT is very sensitive to the sampling location. With an increase of the sampling location above the current location, the concentration of pyrolyzed hydrocarbon products increased. Some of the pyrolyzed hydrocarbon products can be decomposed by catalyst particles, but the rest will produce amorphous carbons, a contaminant for catalyst particles.
Fig. 5. HRTEM image of a typical nanotube synthesized by a cobalt-coated stainless steel grid (scale bar: 5 nm).
28
L. Yuan et al. / Chemical Physics Letters 346 (2001) 23±28
4. Conclusions With a stainless steel grid as the substrate and oxidized by a premixed propane ¯ame, entangled nanotubes with diameters ranging from 10 to 60 nm were formed from a laminar ethylene±air diffusion ¯ame. The yield rate of the nanotubes from our current laboratory apparatus was approximately 3 mg/min. Well-aligned and well-graphitized multi-walled carbon nanotubes consisting of nearly uniform diameter nanotubes were synthesized from a nitrogen diluted laminar ethylene±air diusion ¯ame with a cobalt-electrodeposited stainless steel grid. Acknowledgements This study was sponsored by National Science Foundation under the MRSEC program (DMR-
9809686). We would like to acknowledge the valuable comments from Professor F.A. Williams and Professor A.S. Gordon. We also acknowledge Bob Gregory for his editing work. References [1] T.W. Ebbesen, Carbon Nanotubes: Preparation and Properties, CRC Press, Boca Raton, FL, 1997. [2] L. Yuan, K. Saito, C. Pan, F.A. Williams, A.S. Gordon, Chem. Phys. Lett. 340 (2001) 237. [3] R.L. Vander Wal, T.M. Ticich, V.E. Curtis, Chem. Phys. Lett. 323 (2000) 217. [4] J.B. Howard, K. Das Chowdhury, J.B. Vander Sande, Nature 370 (1994) 603. [5] H. Richter et al., Carbon 34 (1996) 427. [6] K. Saito, A.S. Gordon, F.A. Williams, W.F. Stickle, Combust. Sci. Technol. 80 (1991) 103. [7] M. Endo et al., Carbon 33 (1995) 873. [8] R.T. Baker, Carbon 27 (1989) 315. [9] W.Z. Li et al., Chem. Phys. Lett. 335 (2001) 141.