- Email: [email protected]
Forming multiwalled carbon nanotubes by the thermal decomposition of Mo(CO)6
10 May 2002
Chemical Physics Letters 357 (2002) 267–271 www.elsevier.com/locate/cplett
Forming multiwalled carbon nanotubes by the thermal decomposition of MoðCOÞ6 Menachem Motiei a, Jose Calderon-Moreno b, Aharon Gedanken b
a,*
a Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel Materials and Structures Laboratory, Tokyo Institute of Technology, 4259, Nagatsuta, Midori-ku, 226-8503 Yokohama, Japan
Received 31 December 2001; in final form 19 March 2002
Abstract Multiwalled carbon nanotubes (MWCNTs) have been obtained by decomposing molybdenum hexacarbonyl under air or argon in a closed cell. In addition to the carbon nanotubes (CNTs), Mo2 C was obtained. A high percentage of CNT was obtained from only a few graphitic carbon particles. Our proposed mechanism suggests that CNT is formed via the Boudard reaction as well as by the reduction of CO by Mo. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Rakov [1] has recently reviewed the various methods by which single-walled and multiwalled carbon nanotubes (SWCNTs, MWCNTs) can be prepared. The methods can be divided into physical and chemical techniques. The general feature common to all the chemical methods is the use of a catalyst. Even in the case of the electric arc technique which is considered to be a physical method, the introduction of catalytic amounts of Fe, Co, or Ni or their mixtures appreciably influences the shape and yield of the carbon nanotubes (CNTs) [2,3]. A brief literature search points to nickel and cobalt, followed by iron as the most popular catalysts. Molybdenum is rarely employed as a catalyst but when it is used, its role is mostly as a
*
Corresponding author. Fax: +972-3-535-1250. E-mail address: [email protected] (A. Gedanken).
synergetic catalyst to one of the three magnetic transition metals, iron, cobalt, or nickel [4–10]. It is worth mentioning that one report has clearly found that when Mo is used as the sole catalyst for the disproportionation of CO (Boudard reaction), it is inactive [6]. This conclusion, however, does not contradict that of Dai and coworkers [11] who reported that a few nanometer sized particles of Mo catalyzed the disproportionation of CO. Later in the same article they clarify the nature of the catalyst claiming that ‘‘The Mo-containing compound is probably an oxide although detailed compositional analysis has yet to be carried out’’. On the other hand for the same reaction a synergetic affect is observed when Co and Mo are used as the catalysts. Unlike the transition metals (TMs), which have been widely used as catalysts for the fabrication of CNT, TM oxides are much less useful for this purpose. When metal oxides are used they are usually metal oxides, most frequently MgO, which
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 0 5 1 9 - 5
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M. Motiei et al. / Chemical Physics Letters 357 (2002) 267–271
is also involved [12,13] as the catalyst in the Boudard reaction. The role of MgO in the catalytic process is that of support for the metallic catalyst. We have recently obtained CNT and carbon nanoflasks from the dissociation of CoðCOÞ3 NO and FeðCOÞ5 in the presence of magnesium. The CNT are obtained through the reduction of CO by magnesium [14–16]. A different mechanism is offered for a similar reaction in which supercritical CO2 reacts with magnesium where the dominant step is the disproportionation of CO (Boudard reaction) [17]. It is worth mentioning that recently CNT were also prepared using non-catalytic reactions [18,19]. In the current study we have formed CNT from the reaction of MoðCOÞ6 in a closed cell at elevated temperatures in the presence of air or argon. Since in the current reaction, MoðCOÞ6 and air are the only reactants introduced in the reaction cell, it is worth remembering that the thermal decomposition of MoðCOÞ6 is known to yield molybdenum [20,21]. On the other hand when this decomposition is conducted under oxygen or air, molybdenum oxide is produced. Another important observation is that the sonochemical decomposition of MoðCOÞ6 under argon yields Mo2 C [22]. According to the proposed mechanism the molybdenum metal acts as the reducing agent.
2. Experimental CNTs samples were prepared by a catalytic reaction in which MoðCOÞ6 is the sole reactant in the reaction cell. The synthesis was carried out in a 4 ml closed vessel cell which was assembled from stainless steel Swagelok parts. A 1=200 union part is capped from both sides by a standard plug. For this synthesis, 528 mg of MoðCOÞ6 (0.002 mol) are introduced in the cell at room temperature under air ð1:6 104 mol air). The vessel was heated at 900 °C for 3 h. The reaction took place at the autogenic pressure of the precursors. When the reaction was complete the cell was cooled to room temperature and 430 mg of product was collected. Some 50 mg of material were found adhered to the Swagelok walls, and 48 mg were considered to be lost due to excess pressure. The product was
treated with 6 M HCl at 70 °C for 1 h, and was left in the HCl at room temperature overnight. It was then washed thoroughly with water and dried in vacuum. We could not observe a weight loss as the result of the acid treatment. The same reaction was also carried out with argon replacing the air. The pressure of the argon and oxygen was 1 atm. The X-ray diffraction patterns of the product were recorded using a Bruker AXS D8 Advance Powder X-ray Diffrac radiation). The tometer (using Cua k ¼ 1:5418 A transmission electron micrographs (TEMs) were obtained with a JEOL-JEM 100SX microscope, working at a 100 kV accelerating voltage. Samples for TEM were prepared by placing a drop of the sample suspension on a copper grid (400 mesh, electron microscopy sciences) coated with carbon film. The grid was then air-dried. A standard Renishaw Raman spectrometer was employed, using the 514.5 nm line of an Ar laser as the excitation source.
3. Results and discussion The XRD diffraction patterns of the as-prepared sample obtained in the air reaction are presented in Fig. 1. The diffraction patterns consists of eight distinct lines which are all assigned to Mo2 C based on an excellent fitting with literature data (JCPDS 35-0787). The diffraction peaks ob-
Fig. 1. X-ray diffraction of the as-prepared.
M. Motiei et al. / Chemical Physics Letters 357 (2002) 267–271
269
tained for the reaction in argon show the Mo2 C peaks and in addition the graphitic peak of carbon at 2h ¼ 26:38. TEM images of the as-prepared sample obtained in the air reaction are presented in Fig. 2. They show large irregularities in the shape of the CNT. Irregularity is detected in the widths of the tubes (ranging from 50 to 350 nm), their lengths (ranging from 0.5 to 25 l), and in their shape which vary from straight to highly curled CNT. The average diameter is 70–90 nm, and the average length is 10 lm, as calculated from measuring close to 100 tubes. It is also worth noting that almost no graphitic particles are observed in the TEM picture. The percentage of CNT in the TEM picture is very high. It is estimated that 90% of the carbon features observed on the grid are CNT. The amount of CNT obtained for this reaction is much larger than the amount obtained for CoðCOÞ3 NO or FeðCOÞ5 reacting with magnesium [14,16]. Another difference between the current reaction and those of [14,16] is that Mo carbonyl products open CNT, whereas Co and Fe carbonyls yield capped CNT. The CNT obtained for molybdenum were all empty. From our limited experience with we can conclude that the highest
yield of CNT is obtained in the case of the thermal decomposition of MoðCOÞ6 . The results of the thermal decomposition of MoðCOÞ6 in argon at the same temperature are similar to those of the air reaction. However the XRD diffraction, shows up a difference: the asprepared product displays a distinct graphitic peak at 26.38 in addition to the Mo2 C peaks. The TEM pictures are also crowded with CNT and perhaps with some more graphitic products (GPs) than in the air reaction. The Raman spectrum of the solid after HCl treatment is shown in Fig. 3. Only the D- and Gbands of graphitic carbon characteristic of MWCNTs are observed. Fig. 3 is a typical ‘‘averaged’’ Raman spectrum of the sample, obtained in the Raman macro-mode. It is averaged over areas of several mm2 . The ID =IG intensity ratio 0:12 indicates the high content of nanotubes in the treated sample and the presence of few lattice edges or defects in the multiwalls. Their crystallinity is comparable to than of MWNT obtained by arc synthesis [23]. Macro-Raman spectra displayed only the D and G bands of carbon, characteristic of carbon bonds. There is no evidence for the presence of Mo. The Raman spectra focused on the GP accompanying nanotubes (Fig. 4) were taken locally in the micro-Raman mode, over analyzed areas of 2 lm2 . The micro-Raman spectra showed bands at 940 and at 890 cm1 , which are assigned to Mo–O symmetric and asymmetric stretching in tetrahedrally coordinated Mo–O amorphous phase, respectively [24–27]. Some of the analyzed spots also revealed Raman bands at
Fig. 2. TEM image of carbon nanotubes.
Fig. 3. Raman image of carbon nanotubes.
270
M. Motiei et al. / Chemical Physics Letters 357 (2002) 267–271
Fig. 4. Raman image of carbon nanotubes.
820 and 990 cm1 , which we assigned to MoO3 nuclei scattered in the amorphous phase (Fig. 4A), while other areas did not reveal any Mo–O bonds (Fig. 4B). No evidence of molybdenum oxide crystals was detected. The Mo2 C, detected by XRD, has a metallic character and can not be detected by Raman spectroscopy.
4. Mechanism The two major products of the reaction are Mo2 C and carbon. According to our interpretation the first step in the proposed mechanism is the thermal decomposition of MoðCOÞ6 : MoðCOÞ6ðgÞ ! MoðsÞ þ 6COðgÞ
ð1Þ
The CNT is formed via the Boudard reaction, namely 3COðgÞ ! 1:5CðsÞ þ 1:5CO2ðgÞ
ð2Þ
The carbon monoxide reacts through two parallel reactions: in addition to reaction (2), it is proposed that Mo2 C is formed via the reaction between metallic Mo and CO: MoðsÞ þ 3COðgÞ ! 1=2Mo2 CðsÞ þ 3=2CO2ðgÞ þ CðsÞ ð3Þ It is possible that CNT are fabricated in reaction (3) as well as in the Boudard reaction. The loss of weight reported above is due to the escape of carbon dioxide from the reaction cell. Reaction (3)
is based on early reports of Lander and Germer [28], and Kuo and H€agg [29]. Lander and Germer observed the formation of Mo2 C while trying to plate metallic substrates, maintained at 300–475 °C, with molybdenum formed by the pyrolysis of MoðCOÞ6 in the presence of CO. Kuo and H€agg carburized molybdenum with CO, and obtained the b-phase of Mo2 C as well as the c-phase of MoC. A small amount of MoOx is also obtained in the air reaction. It is absent from the macro-Raman spectra and there is no evidence for its existence in the XRD either. It is detected only in the microRaman scan. It is formed according to our interpretation via the reaction of Mo and O2 . The small amount of MoOx is a result of the large ratio of Mo=O2 (60:1) or perhaps due to the reaction of MoOx with carbon yielding Mo2 C [30]. The EDAX results were taken separately for the CNT and for the GP. While a distinct Mo peak is detected for the GP, no sign of the presence of molybdenum atoms is found when the EDAX is measured for the CNT. This complicates considerably the mechanism of the creation of the CNT because it is not clear what role Mo plays in favoring the formation of CNT, if it is not found in their vicinity. Perhaps, the catalytic effect of the Mo is similar to that of the other metals but much stronger so that the other metals and small undetected amounts of a Mo species suffice to push the Boudard reaction forward. It is also difficult to determine whether the components of the stainless steel walls (Fe, Cr, etc.) catalyse this reaction since the absence of iron in the EDAX spectrum does not rule out its involvement.At least for one reaction [17] we have demonstrated that the components of the stainless steel walls do not play a catalytic role in the Boudard reaction.
5. Conclusions The thermal decomposition of MoðCOÞ6 in a closed cell produces a high yield of straight and coiled CNTs, with little amount of accompanying graphitic products. In contrast to the thermal decomposition of other carbonyls CoðCOÞ3 NO or FeðCOÞ5 , the nanotubes do not contain metal
M. Motiei et al. / Chemical Physics Letters 357 (2002) 267–271
fillings. The amount of CNT produced in the reaction is higher than in the decomposition of the Co and Fe carbonyls. The major byproduct is Mo2 C.
Acknowledgements A.G. thanks the support of the German Ministry of Science (BMBF) for support through the Deutsch-Israelische Projektpartnerschaft (DIP). We thank Prof. A. Gordon for editorial assistance.
References [1] E.G. Rakov, Russ. Chem. Rev. 69 (2000) 35. [2] S. Iijima, T. Ichichashi, Nature 358 (1992) 23. [3] D.S. Bethune, C.H. Kiang, M.S. de Vries, G. Gorman, R. Savoy, J. Vasquees, R. Beyers, Nature 363 (1993) 603. [4] O. Noury, T. Stockli, M. Croci, A.C.J.M. Bonard, Chem. Phys. Lett. 346 (2001) 349. [5] Y. Li, J. Liu, Y.Q. Wang, Z.L. Wang, Chem. Mater. 13 (2001) 1008. [6] W.E. Alvarez, B. Kitiyanan, A. Borgna, D.E. Resasco, Carbon 39 (2001) 547. [7] M. Su, B. Zheng, J. Liu, Chem. Phys. Lett. 322 (2000) 321. [8] B. Kitiyanan, W.E. Alvarez, J.H. Harwell, D.E. Resasco, Chem. Phys. Lett. 317 (2000) 497. [9] I. Willems, Z. Konya, J.F. Colomer, G. Van Tendeloo, N. Nagaraju, A. Fonseca, J.B. Nagy, Chem. Phys. Lett. 317 (2000) 71. [10] A.M. Cassell, J.A. Raymakers, J. Kong, H.J. Dai, J. Phys. Chem. B 103 (1999) 6484.
271
[11] H. Dai, A.G. Rinzler, P. Nikolaev, A. Thess, D.T. Colbert, R.E. Smalley, Chem. Phys. Lett. 260 (1996) 471. [12] P. Chen, H.-B. Zhang, G.-D. Lin, Q. Hong, K.-R. Tsai, Carbon 35 (1997) 1495. [13] A. Govindaraj, R. Sen, A.K. Santra, B.V. Nagaraju, Mater. Res. Bull. 33 (1998) 663. [14] S. Liu, X. Tang, L. Yin, Yu. Koltypin, A. Gedanken, J. Mater. Chem. 10 (2000) 1271. [15] S. Liu, J. Zhu, Y. Mestai, I. Felner, A. Gedanken, Chem. Mater. 12 (2000) 2205. [16] S. Liu, X. Tang, Y. Mastai, I. Felner, A. Gedanken, J. Mater. Chem. 10 (2000) 2502. [17] M. Motiei, Y. Rozenfeld Hacohen, J. Calderon-Moreno, A. Gedanken, J. Am. Chem. Soc. 123 (2001) 8624. [18] W. Marchet, A. Saveliev, L.A. Kennedy, A. Fridman, Chem. Phys. Lett. (2002). [19] J.M. Calderon-Moreno, M. Yoshimura, J. Am. Chem. Soc. 123 (2001) 741. [20] W.L. Loh, S. Jaenicke, G.K. Chuah, H.G. Ang, Talanta 45 (1998) 739. [21] S. Jaenicke, W.L. Loh, Catal. Today 49 (1999) 123. [22] T.H. Hyeon, M.M. Fang, K.S. Suslick, J. Am. Chem. Soc. 118 (1996) 5492. [23] M.S. Dresselhaus, G. Dresselhaus, M.A. Pimenta, P.C. Eklund, in: M.J. Pelletier (Ed.), Analytical Application of Raman Spectroscopy, Blackwell Science, Oxford, 1999, Chapter 9. [24] S. Yoshida, J. Phys. Chem. B 102 (1998) 2960. [25] S.R. Bare, Langmuir 14 (1998) 1500. [26] E. Haro-Poniatowski, C. Julien, J. Mater. Res. 13 (1998) 1033. [27] J.S. Cross, G.L. Schrader, Thin Solid Films 259 (1995) 5. [28] J.J. Lander, L.H. Germer, American Institute of Mining and Metallurgical Engineering Technology Publication No. 2259, 1947. [29] K. Kuo, G. H€agg, Nature 245 (1952). [30] S. Chaudhury, S.K. Mukerjee, V.N. Vaidya, V. Venugopal, J. Alloys Compd. 261 (1997) 105.
Chemical Physics Letters 357 (2002) 267–271 www.elsevier.com/locate/cplett
Forming multiwalled carbon nanotubes by the thermal decomposition of MoðCOÞ6 Menachem Motiei a, Jose Calderon-Moreno b, Aharon Gedanken b
a,*
a Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel Materials and Structures Laboratory, Tokyo Institute of Technology, 4259, Nagatsuta, Midori-ku, 226-8503 Yokohama, Japan
Received 31 December 2001; in final form 19 March 2002
Abstract Multiwalled carbon nanotubes (MWCNTs) have been obtained by decomposing molybdenum hexacarbonyl under air or argon in a closed cell. In addition to the carbon nanotubes (CNTs), Mo2 C was obtained. A high percentage of CNT was obtained from only a few graphitic carbon particles. Our proposed mechanism suggests that CNT is formed via the Boudard reaction as well as by the reduction of CO by Mo. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Rakov [1] has recently reviewed the various methods by which single-walled and multiwalled carbon nanotubes (SWCNTs, MWCNTs) can be prepared. The methods can be divided into physical and chemical techniques. The general feature common to all the chemical methods is the use of a catalyst. Even in the case of the electric arc technique which is considered to be a physical method, the introduction of catalytic amounts of Fe, Co, or Ni or their mixtures appreciably influences the shape and yield of the carbon nanotubes (CNTs) [2,3]. A brief literature search points to nickel and cobalt, followed by iron as the most popular catalysts. Molybdenum is rarely employed as a catalyst but when it is used, its role is mostly as a
*
Corresponding author. Fax: +972-3-535-1250. E-mail address: [email protected] (A. Gedanken).
synergetic catalyst to one of the three magnetic transition metals, iron, cobalt, or nickel [4–10]. It is worth mentioning that one report has clearly found that when Mo is used as the sole catalyst for the disproportionation of CO (Boudard reaction), it is inactive [6]. This conclusion, however, does not contradict that of Dai and coworkers [11] who reported that a few nanometer sized particles of Mo catalyzed the disproportionation of CO. Later in the same article they clarify the nature of the catalyst claiming that ‘‘The Mo-containing compound is probably an oxide although detailed compositional analysis has yet to be carried out’’. On the other hand for the same reaction a synergetic affect is observed when Co and Mo are used as the catalysts. Unlike the transition metals (TMs), which have been widely used as catalysts for the fabrication of CNT, TM oxides are much less useful for this purpose. When metal oxides are used they are usually metal oxides, most frequently MgO, which
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 0 5 1 9 - 5
268
M. Motiei et al. / Chemical Physics Letters 357 (2002) 267–271
is also involved [12,13] as the catalyst in the Boudard reaction. The role of MgO in the catalytic process is that of support for the metallic catalyst. We have recently obtained CNT and carbon nanoflasks from the dissociation of CoðCOÞ3 NO and FeðCOÞ5 in the presence of magnesium. The CNT are obtained through the reduction of CO by magnesium [14–16]. A different mechanism is offered for a similar reaction in which supercritical CO2 reacts with magnesium where the dominant step is the disproportionation of CO (Boudard reaction) [17]. It is worth mentioning that recently CNT were also prepared using non-catalytic reactions [18,19]. In the current study we have formed CNT from the reaction of MoðCOÞ6 in a closed cell at elevated temperatures in the presence of air or argon. Since in the current reaction, MoðCOÞ6 and air are the only reactants introduced in the reaction cell, it is worth remembering that the thermal decomposition of MoðCOÞ6 is known to yield molybdenum [20,21]. On the other hand when this decomposition is conducted under oxygen or air, molybdenum oxide is produced. Another important observation is that the sonochemical decomposition of MoðCOÞ6 under argon yields Mo2 C [22]. According to the proposed mechanism the molybdenum metal acts as the reducing agent.
2. Experimental CNTs samples were prepared by a catalytic reaction in which MoðCOÞ6 is the sole reactant in the reaction cell. The synthesis was carried out in a 4 ml closed vessel cell which was assembled from stainless steel Swagelok parts. A 1=200 union part is capped from both sides by a standard plug. For this synthesis, 528 mg of MoðCOÞ6 (0.002 mol) are introduced in the cell at room temperature under air ð1:6 104 mol air). The vessel was heated at 900 °C for 3 h. The reaction took place at the autogenic pressure of the precursors. When the reaction was complete the cell was cooled to room temperature and 430 mg of product was collected. Some 50 mg of material were found adhered to the Swagelok walls, and 48 mg were considered to be lost due to excess pressure. The product was
treated with 6 M HCl at 70 °C for 1 h, and was left in the HCl at room temperature overnight. It was then washed thoroughly with water and dried in vacuum. We could not observe a weight loss as the result of the acid treatment. The same reaction was also carried out with argon replacing the air. The pressure of the argon and oxygen was 1 atm. The X-ray diffraction patterns of the product were recorded using a Bruker AXS D8 Advance Powder X-ray Diffrac radiation). The tometer (using Cua k ¼ 1:5418 A transmission electron micrographs (TEMs) were obtained with a JEOL-JEM 100SX microscope, working at a 100 kV accelerating voltage. Samples for TEM were prepared by placing a drop of the sample suspension on a copper grid (400 mesh, electron microscopy sciences) coated with carbon film. The grid was then air-dried. A standard Renishaw Raman spectrometer was employed, using the 514.5 nm line of an Ar laser as the excitation source.
3. Results and discussion The XRD diffraction patterns of the as-prepared sample obtained in the air reaction are presented in Fig. 1. The diffraction patterns consists of eight distinct lines which are all assigned to Mo2 C based on an excellent fitting with literature data (JCPDS 35-0787). The diffraction peaks ob-
Fig. 1. X-ray diffraction of the as-prepared.
M. Motiei et al. / Chemical Physics Letters 357 (2002) 267–271
269
tained for the reaction in argon show the Mo2 C peaks and in addition the graphitic peak of carbon at 2h ¼ 26:38. TEM images of the as-prepared sample obtained in the air reaction are presented in Fig. 2. They show large irregularities in the shape of the CNT. Irregularity is detected in the widths of the tubes (ranging from 50 to 350 nm), their lengths (ranging from 0.5 to 25 l), and in their shape which vary from straight to highly curled CNT. The average diameter is 70–90 nm, and the average length is 10 lm, as calculated from measuring close to 100 tubes. It is also worth noting that almost no graphitic particles are observed in the TEM picture. The percentage of CNT in the TEM picture is very high. It is estimated that 90% of the carbon features observed on the grid are CNT. The amount of CNT obtained for this reaction is much larger than the amount obtained for CoðCOÞ3 NO or FeðCOÞ5 reacting with magnesium [14,16]. Another difference between the current reaction and those of [14,16] is that Mo carbonyl products open CNT, whereas Co and Fe carbonyls yield capped CNT. The CNT obtained for molybdenum were all empty. From our limited experience with we can conclude that the highest
yield of CNT is obtained in the case of the thermal decomposition of MoðCOÞ6 . The results of the thermal decomposition of MoðCOÞ6 in argon at the same temperature are similar to those of the air reaction. However the XRD diffraction, shows up a difference: the asprepared product displays a distinct graphitic peak at 26.38 in addition to the Mo2 C peaks. The TEM pictures are also crowded with CNT and perhaps with some more graphitic products (GPs) than in the air reaction. The Raman spectrum of the solid after HCl treatment is shown in Fig. 3. Only the D- and Gbands of graphitic carbon characteristic of MWCNTs are observed. Fig. 3 is a typical ‘‘averaged’’ Raman spectrum of the sample, obtained in the Raman macro-mode. It is averaged over areas of several mm2 . The ID =IG intensity ratio 0:12 indicates the high content of nanotubes in the treated sample and the presence of few lattice edges or defects in the multiwalls. Their crystallinity is comparable to than of MWNT obtained by arc synthesis [23]. Macro-Raman spectra displayed only the D and G bands of carbon, characteristic of carbon bonds. There is no evidence for the presence of Mo. The Raman spectra focused on the GP accompanying nanotubes (Fig. 4) were taken locally in the micro-Raman mode, over analyzed areas of 2 lm2 . The micro-Raman spectra showed bands at 940 and at 890 cm1 , which are assigned to Mo–O symmetric and asymmetric stretching in tetrahedrally coordinated Mo–O amorphous phase, respectively [24–27]. Some of the analyzed spots also revealed Raman bands at
Fig. 2. TEM image of carbon nanotubes.
Fig. 3. Raman image of carbon nanotubes.
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M. Motiei et al. / Chemical Physics Letters 357 (2002) 267–271
Fig. 4. Raman image of carbon nanotubes.
820 and 990 cm1 , which we assigned to MoO3 nuclei scattered in the amorphous phase (Fig. 4A), while other areas did not reveal any Mo–O bonds (Fig. 4B). No evidence of molybdenum oxide crystals was detected. The Mo2 C, detected by XRD, has a metallic character and can not be detected by Raman spectroscopy.
4. Mechanism The two major products of the reaction are Mo2 C and carbon. According to our interpretation the first step in the proposed mechanism is the thermal decomposition of MoðCOÞ6 : MoðCOÞ6ðgÞ ! MoðsÞ þ 6COðgÞ
ð1Þ
The CNT is formed via the Boudard reaction, namely 3COðgÞ ! 1:5CðsÞ þ 1:5CO2ðgÞ
ð2Þ
The carbon monoxide reacts through two parallel reactions: in addition to reaction (2), it is proposed that Mo2 C is formed via the reaction between metallic Mo and CO: MoðsÞ þ 3COðgÞ ! 1=2Mo2 CðsÞ þ 3=2CO2ðgÞ þ CðsÞ ð3Þ It is possible that CNT are fabricated in reaction (3) as well as in the Boudard reaction. The loss of weight reported above is due to the escape of carbon dioxide from the reaction cell. Reaction (3)
is based on early reports of Lander and Germer [28], and Kuo and H€agg [29]. Lander and Germer observed the formation of Mo2 C while trying to plate metallic substrates, maintained at 300–475 °C, with molybdenum formed by the pyrolysis of MoðCOÞ6 in the presence of CO. Kuo and H€agg carburized molybdenum with CO, and obtained the b-phase of Mo2 C as well as the c-phase of MoC. A small amount of MoOx is also obtained in the air reaction. It is absent from the macro-Raman spectra and there is no evidence for its existence in the XRD either. It is detected only in the microRaman scan. It is formed according to our interpretation via the reaction of Mo and O2 . The small amount of MoOx is a result of the large ratio of Mo=O2 (60:1) or perhaps due to the reaction of MoOx with carbon yielding Mo2 C [30]. The EDAX results were taken separately for the CNT and for the GP. While a distinct Mo peak is detected for the GP, no sign of the presence of molybdenum atoms is found when the EDAX is measured for the CNT. This complicates considerably the mechanism of the creation of the CNT because it is not clear what role Mo plays in favoring the formation of CNT, if it is not found in their vicinity. Perhaps, the catalytic effect of the Mo is similar to that of the other metals but much stronger so that the other metals and small undetected amounts of a Mo species suffice to push the Boudard reaction forward. It is also difficult to determine whether the components of the stainless steel walls (Fe, Cr, etc.) catalyse this reaction since the absence of iron in the EDAX spectrum does not rule out its involvement.At least for one reaction [17] we have demonstrated that the components of the stainless steel walls do not play a catalytic role in the Boudard reaction.
5. Conclusions The thermal decomposition of MoðCOÞ6 in a closed cell produces a high yield of straight and coiled CNTs, with little amount of accompanying graphitic products. In contrast to the thermal decomposition of other carbonyls CoðCOÞ3 NO or FeðCOÞ5 , the nanotubes do not contain metal
M. Motiei et al. / Chemical Physics Letters 357 (2002) 267–271
fillings. The amount of CNT produced in the reaction is higher than in the decomposition of the Co and Fe carbonyls. The major byproduct is Mo2 C.
Acknowledgements A.G. thanks the support of the German Ministry of Science (BMBF) for support through the Deutsch-Israelische Projektpartnerschaft (DIP). We thank Prof. A. Gordon for editorial assistance.
References [1] E.G. Rakov, Russ. Chem. Rev. 69 (2000) 35. [2] S. Iijima, T. Ichichashi, Nature 358 (1992) 23. [3] D.S. Bethune, C.H. Kiang, M.S. de Vries, G. Gorman, R. Savoy, J. Vasquees, R. Beyers, Nature 363 (1993) 603. [4] O. Noury, T. Stockli, M. Croci, A.C.J.M. Bonard, Chem. Phys. Lett. 346 (2001) 349. [5] Y. Li, J. Liu, Y.Q. Wang, Z.L. Wang, Chem. Mater. 13 (2001) 1008. [6] W.E. Alvarez, B. Kitiyanan, A. Borgna, D.E. Resasco, Carbon 39 (2001) 547. [7] M. Su, B. Zheng, J. Liu, Chem. Phys. Lett. 322 (2000) 321. [8] B. Kitiyanan, W.E. Alvarez, J.H. Harwell, D.E. Resasco, Chem. Phys. Lett. 317 (2000) 497. [9] I. Willems, Z. Konya, J.F. Colomer, G. Van Tendeloo, N. Nagaraju, A. Fonseca, J.B. Nagy, Chem. Phys. Lett. 317 (2000) 71. [10] A.M. Cassell, J.A. Raymakers, J. Kong, H.J. Dai, J. Phys. Chem. B 103 (1999) 6484.
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