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Carbon nanotubes production by catalytic pyrolysis of benzene
Carbon Vol. 36, No.
5-6, pp. 681-683, 1998 0 1998ElsevierScienceLtd
Printedin GreatBritain.Allrightsreserved 0008-6223/98 $19.00+ 0.00 PII: SOOO8-6223(98)00039-6
CARBON NANOTUBES PRODUCTION BY CATALYTIC PYROLYSIS OF BENZENE A. M. BENITO,~ Y. MANIETTE,~ E. MuGoza and M. T. MART~NEZ~ ‘Instituto de Carboquimica, CSIC, Poeta Luciano Gracia 5,50015 Zaragoza, Spain bServicios Cientifico-Tkcnicos, Universidad de Barcelona, Lluis Sol&i Sabaris l&3,08028 Barcelona, Spain (Received 30 October 1997; accepted in revised form 4 November 1997)
Abstract-Pyrolysis of benzene at ca 6OC~9OO”Cover Ni powder generated different types of carbon nanostructures possessing a wide range of morphologies. The effects of temperature and time on carbon nanotubes growth were evaluated. The deposited carbon yield was measured, and the quality of the nanotubes was analyzed by transmission electron microscopy (TEM). 0 1998 Elsevier Science Ltd. All rights reserved. _ _
Key Words-A.
Carbon nanotubes, B. pyrolysis.
1. INTRODUCTION
i.d., 60 cm length) located in the first furnace (kept at 200°C). Nickel (Aldrich, 99.7%, -3pm particle size) was placed in the second furnace. Argon (flow rate 0.5 l/min) was used as a carrier gas. As soon as benzene is pumped into the first furnace, it vaporizes, and the vapor is passed to the second furnace where the pyrolysis takes place. Here the temperature and reaction times are systematically varied between 650 and 9OO”C, and S-60 minutes, respectively. After the pyrolysis is finished, the benzene flow was stopped, and the temperature of the second furnace was increased up to 950-lOOO”C, allowing it to operate for 60 minutes longer under Ar flow exclusively in order to favor the annealing. Soot was scraped from the tube wall in the second furnace, and analyzed by TEM (Philips CM30 operating at 300 kV). The samples for TEM were sonicated in ethanol, and then deposited on a copper grid covered with a carbon holey film. Observations were made preferentially on particles above the holes, in order to more easily obtain high resolution images.
Catalytic production of carbon nanofibers has become an area of increasing interest in material research [l-4]. The presence of catalytic particles in hydrocarbon pyrolysis systems leads to the production of carbon nanofibers exhibiting a wide range of morphologies [S-7]. Then, the metal particle is a crucial requirement for nanofiber growth, and appears responsible for the carbon agglomeration and subsequent fiber axial growth. The catalytically produced filaments appear to have analogous structures to the carbon nanotubes obtained by arc discharge techniques using graphite electrodes [8], and therefore to possess similar properties [4]. The nanotubes obtained by pyrolysis usually present larger lengths (up to 60 pm) than those produced by the arc discharge. Also, they have thicker diameter, which is related to the size of the active metal particles. On the other hand, pyrolytic production of carbon nanofibers exhibits the advantages of better temperature control and lower price production. Nevertheless, the graphitic layers of such nanofibers contain many defects, and these nanofibers are covered with amorphous carbon, a by-product of the thermal decomposition of hydrocarbons [91. In this paper, we present a detailed description of the catalytic deposition of carbon over nickel particles. The influence of the reaction parameters, temperature and time, on carbon nanotube growth with different diameters, lengths and structures was studied.
3. RESULTS AND DISCUSSION In this study, metallic particles of nickel were used as catalyst for the pyrolysis of benzene. The choice of carrier gas appeared to be very important. Experiments conducted under nitrogen atmosphere did not result in carbon nanotubes production in our conditions. In all our experiments, different structural forms of carbon were observed: amorphous carbon layers, graphitic polyhedral nanoparticles (GPNs) with metal particles inside, and multi-walled nanotubes composed of crystallized graphite layers usually covered with amorphous carbon on the outer layers. Formation of tubes was observed in all our experiments with lengths up to 5 pm, a diameter distribution from 20 to 100nm. The average number of layers, as measured by X-ray diffraction, was typically
2. EXPERIMENTAL
Pyrolysis experiments were carried out in a twostage furnace system in a flow reactor at atmospheric (flow pressure. Liquid benzene was pumped 0.5 ml/min) into one end of the quartz tube (6 mm 681
682
A. M. BI~UI IO <‘Itrl.
about 65. The interlayer spacing was found to be 3.46 A, which corresponds to the (002) distance in turbostratic graphite. The nanotubes usually contained metal particles encapsulated at the tips of the tubes with sizes typically about 25 nm (Fig. 1). At higher magnilication, Fig. 2 shows a representative tube with a nickel crystal at its tip. It must be noticed that the nickel crystal seems to be oriented with respect to the carbon tube. The selected area electron diffraction (SAED) pattern of the same area (inset to Fig. 2) actually shows a perfect coincidence of the nickel ( 111) and graphite (002) reflections. The amount of condensed carbon varied with the flow rate of benzene, temperature, and reaction time. The benzene flow rate was chosen as 0.33 mol C,H, h ~’ with an argon flow rate of 30 I Ar h ‘.
.
200
.
4
.
I
-
150,
650
(Ll)
700
750
800
Temwrature
850
5oc
t C1
3.1 Temperature infuencc Using the rate flow indicated above. the temperature influence on the nanotubes formation was evaluated in the range from 650 to 900°C. The Fig. 3. EtTect of (a) temperature and (b) time in the perccntage of deposited carbon (IV,,,, - WC,,/ W,,, x 100).
Fig. I. TEM
image showing a general view of a sample obtained at 700°C and 5 minutes.
deposited carbon percentage varied with time and temperature. being a maximum in the interval 700~~850°C (Fig. 3). However, the high percentage of carbon deposition does not mean we have reached the optimal conditions for nanotube production. At low temperatures (650°C) the tubes were found to be shorter and more entangled, with more defects in the graphitic layers, and GPNs appeared in high percentages. As the temperature was raised, the length of the tubes increased and they appeared rather straighter than curved, exhibiting a better degree of graphitization (Fig. 4). 3.2 Time influence Soot production takes place even at the first stages of the pyrolysis, and increases with time (Fig. 3(b)). Here, even at the shortest reaction times (5 minutes) nanotubes were observed by TEM (Fig. 1). Along with the formation of nanotubes and GPNs, increasing the reaction time leads to an enhanced appearance of amorphous soot covering the graphitic nanostructures up to high degrees. Then, the aspect of the samples appears to be highly agglutinated. Therefore. increasing the reaction does not necessarily lead to a maximum of nanotubes production. It is necessary to optimize time and temperature to obtain the highest nanotubes production with good quality.
4. CONCLUSIONS Fig. 2. High magnification TEM image of a nanotube with nickel particle at its tip. Inset& -SAED pattern of the area at the nanotube tip.
The pyrolytic decomposition of benzene over nickel metallic particles leads to the nanotubes formation
Carbon nanotubes production by catalytic pyrolysis of benzene
683
(4
(b) Fig. 4. (a) HRTEM image showing the walls of a typical nanotube obtained at 900°C and 15 minutes; (b) the electron diffraction pattern of this nanotube. in our conditions. The nanotubes size varied from 20 to 100 nm in diameter and up to 5 pm in length, and the orientation of the metallic particles influences the orientation of the deposited carbon. Temperature and time parameters are very important to obtain high yields of nanotubes with good quality. An increase in time results in an enhanced production of amorphous carbon along with the formation of nanotubes, while raising the reaction temperature leads to long and straight tubes having a higher degree of graphitization. Controlling the thickening and length by controlling the reaction parameters may add a valuable new dimension to the use of pyrolytic carbon nanotubes.
Acknowledgements-The authors would like to thank the DGICYT for project funding under Contract No. PB94-00224, and Wolfgang Maser for helpful discussions.
REFERENCES I. Chambers, A., Rodriguez, N. M. and Baker, R. T. K., J. Mater. Res., 1996, 11, 430.
2. Ivanov, V., Nagy, J. B., Lambin, P., Lucas, A., Zhang, X. B., Zhang, X. F., Bemaerts, D., Van Tendeloo, G., Amelinckx, S. and Van Landuyt, J., Chem. Phys. Left., 1994, 223, 329.
3. Fonseca, A., Hemadi, K., Nagy, J. B., Bemaerts, D. and Lucas, A. A., J. Molec. Catal. A: Chemical, 1996, 107(1-3),
159.
4. Rodriguez, N. M., J. Mater. Res., 1993, 8, 3233. 5. Amelinckx, S., Zhang, X. B., Bemaerts, D., Zhang, X. F., Ivanov, V. and Nagy, J. B., Science, 1994, 265, 635. 6. Terrones, H., Terrones, M. and Hsu, W. K., Chem. Sot. Rev., 1995, 24, 341.
I. Endo, M., Takeuchi, K., Shiraishi, M. and Kroto, H. W., J. Phys. Chem. Solids, 1993, 54, 1841. 8. Endo, M., Takeuchi, K., Igarashi, S., Kobori, K., Shiraishi. M. and Kroto. H. K.. J. Phvs. Chem. Solids. 1993. 54(i2), 1841. ’ ’ 9 Jose-Yacaman, M., Miki-Yoshida, M., Rendon, L. and Santiesteban, J. G., Appl. Phys. Lett., 1993, 62, 657.
5-6, pp. 681-683, 1998 0 1998ElsevierScienceLtd
Printedin GreatBritain.Allrightsreserved 0008-6223/98 $19.00+ 0.00 PII: SOOO8-6223(98)00039-6
CARBON NANOTUBES PRODUCTION BY CATALYTIC PYROLYSIS OF BENZENE A. M. BENITO,~ Y. MANIETTE,~ E. MuGoza and M. T. MART~NEZ~ ‘Instituto de Carboquimica, CSIC, Poeta Luciano Gracia 5,50015 Zaragoza, Spain bServicios Cientifico-Tkcnicos, Universidad de Barcelona, Lluis Sol&i Sabaris l&3,08028 Barcelona, Spain (Received 30 October 1997; accepted in revised form 4 November 1997)
Abstract-Pyrolysis of benzene at ca 6OC~9OO”Cover Ni powder generated different types of carbon nanostructures possessing a wide range of morphologies. The effects of temperature and time on carbon nanotubes growth were evaluated. The deposited carbon yield was measured, and the quality of the nanotubes was analyzed by transmission electron microscopy (TEM). 0 1998 Elsevier Science Ltd. All rights reserved. _ _
Key Words-A.
Carbon nanotubes, B. pyrolysis.
1. INTRODUCTION
i.d., 60 cm length) located in the first furnace (kept at 200°C). Nickel (Aldrich, 99.7%, -3pm particle size) was placed in the second furnace. Argon (flow rate 0.5 l/min) was used as a carrier gas. As soon as benzene is pumped into the first furnace, it vaporizes, and the vapor is passed to the second furnace where the pyrolysis takes place. Here the temperature and reaction times are systematically varied between 650 and 9OO”C, and S-60 minutes, respectively. After the pyrolysis is finished, the benzene flow was stopped, and the temperature of the second furnace was increased up to 950-lOOO”C, allowing it to operate for 60 minutes longer under Ar flow exclusively in order to favor the annealing. Soot was scraped from the tube wall in the second furnace, and analyzed by TEM (Philips CM30 operating at 300 kV). The samples for TEM were sonicated in ethanol, and then deposited on a copper grid covered with a carbon holey film. Observations were made preferentially on particles above the holes, in order to more easily obtain high resolution images.
Catalytic production of carbon nanofibers has become an area of increasing interest in material research [l-4]. The presence of catalytic particles in hydrocarbon pyrolysis systems leads to the production of carbon nanofibers exhibiting a wide range of morphologies [S-7]. Then, the metal particle is a crucial requirement for nanofiber growth, and appears responsible for the carbon agglomeration and subsequent fiber axial growth. The catalytically produced filaments appear to have analogous structures to the carbon nanotubes obtained by arc discharge techniques using graphite electrodes [8], and therefore to possess similar properties [4]. The nanotubes obtained by pyrolysis usually present larger lengths (up to 60 pm) than those produced by the arc discharge. Also, they have thicker diameter, which is related to the size of the active metal particles. On the other hand, pyrolytic production of carbon nanofibers exhibits the advantages of better temperature control and lower price production. Nevertheless, the graphitic layers of such nanofibers contain many defects, and these nanofibers are covered with amorphous carbon, a by-product of the thermal decomposition of hydrocarbons [91. In this paper, we present a detailed description of the catalytic deposition of carbon over nickel particles. The influence of the reaction parameters, temperature and time, on carbon nanotube growth with different diameters, lengths and structures was studied.
3. RESULTS AND DISCUSSION In this study, metallic particles of nickel were used as catalyst for the pyrolysis of benzene. The choice of carrier gas appeared to be very important. Experiments conducted under nitrogen atmosphere did not result in carbon nanotubes production in our conditions. In all our experiments, different structural forms of carbon were observed: amorphous carbon layers, graphitic polyhedral nanoparticles (GPNs) with metal particles inside, and multi-walled nanotubes composed of crystallized graphite layers usually covered with amorphous carbon on the outer layers. Formation of tubes was observed in all our experiments with lengths up to 5 pm, a diameter distribution from 20 to 100nm. The average number of layers, as measured by X-ray diffraction, was typically
2. EXPERIMENTAL
Pyrolysis experiments were carried out in a twostage furnace system in a flow reactor at atmospheric (flow pressure. Liquid benzene was pumped 0.5 ml/min) into one end of the quartz tube (6 mm 681
682
A. M. BI~UI IO <‘Itrl.
about 65. The interlayer spacing was found to be 3.46 A, which corresponds to the (002) distance in turbostratic graphite. The nanotubes usually contained metal particles encapsulated at the tips of the tubes with sizes typically about 25 nm (Fig. 1). At higher magnilication, Fig. 2 shows a representative tube with a nickel crystal at its tip. It must be noticed that the nickel crystal seems to be oriented with respect to the carbon tube. The selected area electron diffraction (SAED) pattern of the same area (inset to Fig. 2) actually shows a perfect coincidence of the nickel ( 111) and graphite (002) reflections. The amount of condensed carbon varied with the flow rate of benzene, temperature, and reaction time. The benzene flow rate was chosen as 0.33 mol C,H, h ~’ with an argon flow rate of 30 I Ar h ‘.
.
200
.
4
.
I
-
150,
650
(Ll)
700
750
800
Temwrature
850
5oc
t C1
3.1 Temperature infuencc Using the rate flow indicated above. the temperature influence on the nanotubes formation was evaluated in the range from 650 to 900°C. The Fig. 3. EtTect of (a) temperature and (b) time in the perccntage of deposited carbon (IV,,,, - WC,,/ W,,, x 100).
Fig. I. TEM
image showing a general view of a sample obtained at 700°C and 5 minutes.
deposited carbon percentage varied with time and temperature. being a maximum in the interval 700~~850°C (Fig. 3). However, the high percentage of carbon deposition does not mean we have reached the optimal conditions for nanotube production. At low temperatures (650°C) the tubes were found to be shorter and more entangled, with more defects in the graphitic layers, and GPNs appeared in high percentages. As the temperature was raised, the length of the tubes increased and they appeared rather straighter than curved, exhibiting a better degree of graphitization (Fig. 4). 3.2 Time influence Soot production takes place even at the first stages of the pyrolysis, and increases with time (Fig. 3(b)). Here, even at the shortest reaction times (5 minutes) nanotubes were observed by TEM (Fig. 1). Along with the formation of nanotubes and GPNs, increasing the reaction time leads to an enhanced appearance of amorphous soot covering the graphitic nanostructures up to high degrees. Then, the aspect of the samples appears to be highly agglutinated. Therefore. increasing the reaction does not necessarily lead to a maximum of nanotubes production. It is necessary to optimize time and temperature to obtain the highest nanotubes production with good quality.
4. CONCLUSIONS Fig. 2. High magnification TEM image of a nanotube with nickel particle at its tip. Inset& -SAED pattern of the area at the nanotube tip.
The pyrolytic decomposition of benzene over nickel metallic particles leads to the nanotubes formation
Carbon nanotubes production by catalytic pyrolysis of benzene
683
(4
(b) Fig. 4. (a) HRTEM image showing the walls of a typical nanotube obtained at 900°C and 15 minutes; (b) the electron diffraction pattern of this nanotube. in our conditions. The nanotubes size varied from 20 to 100 nm in diameter and up to 5 pm in length, and the orientation of the metallic particles influences the orientation of the deposited carbon. Temperature and time parameters are very important to obtain high yields of nanotubes with good quality. An increase in time results in an enhanced production of amorphous carbon along with the formation of nanotubes, while raising the reaction temperature leads to long and straight tubes having a higher degree of graphitization. Controlling the thickening and length by controlling the reaction parameters may add a valuable new dimension to the use of pyrolytic carbon nanotubes.
Acknowledgements-The authors would like to thank the DGICYT for project funding under Contract No. PB94-00224, and Wolfgang Maser for helpful discussions.
REFERENCES I. Chambers, A., Rodriguez, N. M. and Baker, R. T. K., J. Mater. Res., 1996, 11, 430.
2. Ivanov, V., Nagy, J. B., Lambin, P., Lucas, A., Zhang, X. B., Zhang, X. F., Bemaerts, D., Van Tendeloo, G., Amelinckx, S. and Van Landuyt, J., Chem. Phys. Left., 1994, 223, 329.
3. Fonseca, A., Hemadi, K., Nagy, J. B., Bemaerts, D. and Lucas, A. A., J. Molec. Catal. A: Chemical, 1996, 107(1-3),
159.
4. Rodriguez, N. M., J. Mater. Res., 1993, 8, 3233. 5. Amelinckx, S., Zhang, X. B., Bemaerts, D., Zhang, X. F., Ivanov, V. and Nagy, J. B., Science, 1994, 265, 635. 6. Terrones, H., Terrones, M. and Hsu, W. K., Chem. Sot. Rev., 1995, 24, 341.
I. Endo, M., Takeuchi, K., Shiraishi, M. and Kroto, H. W., J. Phys. Chem. Solids, 1993, 54, 1841. 8. Endo, M., Takeuchi, K., Igarashi, S., Kobori, K., Shiraishi. M. and Kroto. H. K.. J. Phvs. Chem. Solids. 1993. 54(i2), 1841. ’ ’ 9 Jose-Yacaman, M., Miki-Yoshida, M., Rendon, L. and Santiesteban, J. G., Appl. Phys. Lett., 1993, 62, 657.