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Sonochemical production of a carbon nanotube
Ultrasonics Sonochemistry 6 (1999) 185–187 www.elsevier.nl/locate/ultsonch
Sonochemical production of a carbon nanotube R. Katoh *, Y. Tasaka, E. Sekreta, M. Yumura, F. Ikazaki, Y. Kakudate, S. Fujiwara National Institute of Materials and Chemical Research (NIMC), 1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Received 15 March 1999; received in revised form 8 April 1999
Abstract Sonochemical production of a carbon nanotube has been studied. The carbon nanotube is produced by applying ultrasound to liquid chlorobenzene with ZnCl particles and to o-dichlorobenzene with ZnCl and Zn particles. It is considered that the 2 2 polymer and the disordered carbon, which are formed by cavitational collapse in homogeneous liquid, are annealed by the interparticle collision induced by the turbulent flow and shockwaves. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Carbon nanotube; Heterogeneous sonochemistry; Inter-particle collision; Ultrasound
1. Introduction Various sonochemical reactions have been reported [1]. Reaction processes of sonochemistry can be divided into two categories, i.e. homogeneous and heterogeneous processes [2]. The homogeneous process proceeds at the hot spot produced by the cavitational collapse of a bubble in homogeneous liquid. The temperature at the hot spot exceeds 5000 K [3], and subsequently, the temperature surrounding the hot spot decreases very rapidly. Accordingly, the primary reaction of the homogeneous process can be regarded as pyrolysis of molecules around the hot spot. This model is called the ‘hot spot model’ [4]. In contrast, the heterogeneous process proceeds at a liquid–solid interface. At the surface of a solid, a micro-jet can be formed by applying ultrasound, and then, erosion of the surface can occur. On the surface of a small solid particle (smaller than 200 mm in diameter), the micro-jet cannot be formed. Such particles are able to move quickly due to the turbulent flow and shockwaves created by homogeneous cavitation [2]. Under this condition, high velocity inter-particle collision can occur, and, as a result, the thermal decomposition of molecules takes place at the site of the interparticle collision. Recently, sonochemical processes have been applied to the production of new materials [4,5]. In particular, we have studied the sonochemical production of carbon * Corresponding author. Fax +81-298-54-4487 E-mail address: [email protected] (R. Katoh)
materials. We previously reported a possible new route for the production of C by using ultrasound irradiation 60 of liquid benzene [6 ]. We have also studied the production of polymers from benzene derivatives [7] and that of disordered carbon from a CCl /benzene mixture 4 [8]. In these previous studies, the reaction temperature at a hot spot was considered sufficiently high for the production of highly crystallized graphitic carbons, though only low crystallized solid carbons, such as polymer and disordered carbon, were produced. This may have been due to a very short reaction time at a hot spot. In the study of pyrolytic synthesis of carbon materials, thermal annealing of low crystallized carbons is frequently applied to produce highly crystallized graphitic carbon. Therefore, we examine herein thermal annealing of the low crystallized carbons, which were produced by the homogeneous process, through the heterogeneous sonochemical process. Specifically, we applied ultrasound to liquid benzene derivatives with small solid particles, and observed the production of the carbon nanotube [9].
2. Experimental Samples were irradiated with an ultrasonic homogenizer (SMT, UH-600) equipped with a titanium tip 20 mm in diameter. The ultrasonic irradiation was carried out in an open glass vessel with Ar gas bubbling through the liquid. Liquid benzene ( Wako, S-grade), chlorobenzene (Junsei, GR-grade) and o-dichloro-
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R. Katoh et al. / Ultrasonics Sonochemistry 6 (1999) 185–187
benzene ( Tokyo Kasei, GR-grade) were used as received. The volume of the liquid was 50 ml. The glass vessel was cooled using an ice bath. The homogenizer was operated at 600 W, 20 kHz, in a pulsed mode, with a frequency of 1 s and a duty cycle of 0.5. For the heterogeneous experiments, ultrasound irradiation of liquid sample was carried out with solid particles (ZnCl Ni, Zn, ZnO and NiCl ). The particles were 2 2 approximately 200 mm in diameter in ZnCl , and 5– 2 10 mm in Ni, Zn, ZnO and NiCl . 2 The solid products formed in the liquid sample were analyzed with a transmission electron microscope ( TEM: JEOL, JEM-2000FX, 200 kV ). The sample specimen for the TEM observation was prepared by dipping a sample grid into the irradiated solution.
3. Results and discussion After ultrasound irradiation of liquid chlorobenzene without solid particles (homogeneous process), the liquid sample turned dark yellow, due to the formation of polymer [7]. Fig. 1 shows a TEM image of solid products obtained from irradiated liquid chlorobenzene without solid particles. From an analysis of the pattern
Fig. 1. TEM image of the product after ultrasound irradiation of chlorobenzene without ZnCl . The crystallized part is indicated by the 2 arrow.
of electron diffraction, we confirmed that the product is mainly polymer (the black part), and a small portion is highly crystallized graphitic carbon (indicated by arrow in Fig. 1). In order to proceed further crystallization of the low crystallized carbon, ultrasonic irradiation of liquid chlorobenzene was carried out with ZnCl particles (a hetero2 geneous process combined with a homogeneous process). Fig. 2 shows a TEM image of solid products obtained from irradiated liquid chlorobenzene with ZnCl particles. The shape of the products differs visibly 2 from that of the products in the homogeneous experiment ( Fig. 1). In the TEM image, spiny products can be observed. The shape of the spiny products is similar to the carbon nanotube produced by the arc discharge method [9]. Also, the pattern of electron diffraction shows that the spiny product has a layered structure, and the spacing between layers agrees with that in the carbon nanotube. Therefore, we concluded that the spiny products are carbon nanotubes. These results indicate that the polymers, which are produced by the homogeneous process, are annealed by the heterogeneous process. Mixing solid particles with a liquid sample enables effective crystallization of the polymers and the disor-
Fig. 2. TEM image of the product after ultrasound irradiation of chlorobenzene with ZnCl . 2
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Fig. 4. TEM image of the product after ultrasound irradiation of dichlorobenzene with Ni.
Fig. 3. TEM image of the product after ultrasound irradiation of dichlorobenzene with ZnCl . 2
dered carbon. In the present study, we used relatively small particles (<200 mm in diameter), a size at which inter-particle collision frequently occurs and for which the micro-jet formation is not efficient. Under these conditions, a high temperature can be achieved at the site of particle collision. After the collision, polymers, which are produced by the homogeneous process, can be converted into carbon nanotube. The effect of the source material on the production of highly crystallized carbon products was examined. When ultrasound was irradiated to liquid benzene with ZnCl particles, only polymers were produced. This 2 indicates that many hydrogen atoms remain in the products. In contrast, for irradiation of o-dichlorobenzene with ZnCl particles, carbon nanotubes were 2 produced, as shown in Fig. 3. Our previous studies on the homogeneous sonochemical decomposition of liquid benzene and its derivatives show that the hydrogen content of the polymers decreases by introduction of halogen atoms, which is due to the efficient coupling of halogen atoms with hydrogen atoms [7,8]. Therefore, the formation of polymers by ultrasound irradiation of
liquid benzene with ZnCl powder can be attributed to 2 the high content of hydrogen atoms in the precursor polymers. We also examined the heterogeneous process using different particles ( ZnCl , Zn, Ni, ZnO, NiCl ). In the 2 case of irradiation of o-dichlorobenzene with ZnCl and 2 Zn particles, carbon nanotubes were produced, but only graphitic particles [10] were observed from the irradiation with Ni ( Fig. 4), NiCl , and ZnO particles. This 2 clearly shows that annealing of the product proceeds effectively by the heterogeneous process, though the morphology of the crystallized carbons is influenced by the nature of the particles. Possibly, factors such as the size of the particles, thermal conductivity, and catalytic activity for crystallization are responsible for the morphology of the respective products.
References [1] T.J. Mason, Advances in Sonochemistry, Vol. 3, JAI Press, London, 1993. [2] K.S. Suslick, S.J. Doktycz, E.B. Flint, Ultrasonics 28 (1990) 280. [3] E.B. Flint, K.S. Suslick, Science 253 (1991) 1397. [4] K.S. Suslick, Adv. Organometall. Chem. 25 (1986) 73. [5] D. Peter, J. Mater. Chem. 6 (1996) 1605. [6 ] R. Katoh, E. Yanase, H. Yokoi, S. Usuba, Y. Kakudate, S. Fujiwara, Ultrason. Sonochem. 5 (1998) 37. [7] R. Katoh, H. Yokoi, S. Usuba, Y. Kakudate, S. Fujiwara, Ultrason. Sonochem. 5 (1998) 69. [8] R. Katoh, H. Yokoi, S. Usuba, Y. Kakudate, S. Fujiwara, Nippon Kagaku Kaishi (1998) 530. [9] S. Iijima, Nature 354 (1991) 56. [10] Y. Saito, T. Yoshikawa, M. Inagaki, M. Tomita, T. Hayashi, Chem. Phys. Lett. 204 (1993) 277.
Sonochemical production of a carbon nanotube R. Katoh *, Y. Tasaka, E. Sekreta, M. Yumura, F. Ikazaki, Y. Kakudate, S. Fujiwara National Institute of Materials and Chemical Research (NIMC), 1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Received 15 March 1999; received in revised form 8 April 1999
Abstract Sonochemical production of a carbon nanotube has been studied. The carbon nanotube is produced by applying ultrasound to liquid chlorobenzene with ZnCl particles and to o-dichlorobenzene with ZnCl and Zn particles. It is considered that the 2 2 polymer and the disordered carbon, which are formed by cavitational collapse in homogeneous liquid, are annealed by the interparticle collision induced by the turbulent flow and shockwaves. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Carbon nanotube; Heterogeneous sonochemistry; Inter-particle collision; Ultrasound
1. Introduction Various sonochemical reactions have been reported [1]. Reaction processes of sonochemistry can be divided into two categories, i.e. homogeneous and heterogeneous processes [2]. The homogeneous process proceeds at the hot spot produced by the cavitational collapse of a bubble in homogeneous liquid. The temperature at the hot spot exceeds 5000 K [3], and subsequently, the temperature surrounding the hot spot decreases very rapidly. Accordingly, the primary reaction of the homogeneous process can be regarded as pyrolysis of molecules around the hot spot. This model is called the ‘hot spot model’ [4]. In contrast, the heterogeneous process proceeds at a liquid–solid interface. At the surface of a solid, a micro-jet can be formed by applying ultrasound, and then, erosion of the surface can occur. On the surface of a small solid particle (smaller than 200 mm in diameter), the micro-jet cannot be formed. Such particles are able to move quickly due to the turbulent flow and shockwaves created by homogeneous cavitation [2]. Under this condition, high velocity inter-particle collision can occur, and, as a result, the thermal decomposition of molecules takes place at the site of the interparticle collision. Recently, sonochemical processes have been applied to the production of new materials [4,5]. In particular, we have studied the sonochemical production of carbon * Corresponding author. Fax +81-298-54-4487 E-mail address: [email protected] (R. Katoh)
materials. We previously reported a possible new route for the production of C by using ultrasound irradiation 60 of liquid benzene [6 ]. We have also studied the production of polymers from benzene derivatives [7] and that of disordered carbon from a CCl /benzene mixture 4 [8]. In these previous studies, the reaction temperature at a hot spot was considered sufficiently high for the production of highly crystallized graphitic carbons, though only low crystallized solid carbons, such as polymer and disordered carbon, were produced. This may have been due to a very short reaction time at a hot spot. In the study of pyrolytic synthesis of carbon materials, thermal annealing of low crystallized carbons is frequently applied to produce highly crystallized graphitic carbon. Therefore, we examine herein thermal annealing of the low crystallized carbons, which were produced by the homogeneous process, through the heterogeneous sonochemical process. Specifically, we applied ultrasound to liquid benzene derivatives with small solid particles, and observed the production of the carbon nanotube [9].
2. Experimental Samples were irradiated with an ultrasonic homogenizer (SMT, UH-600) equipped with a titanium tip 20 mm in diameter. The ultrasonic irradiation was carried out in an open glass vessel with Ar gas bubbling through the liquid. Liquid benzene ( Wako, S-grade), chlorobenzene (Junsei, GR-grade) and o-dichloro-
1350-4177/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S1 3 5 0 -4 1 7 7 ( 9 9 ) 0 0 01 6 - 4
186
R. Katoh et al. / Ultrasonics Sonochemistry 6 (1999) 185–187
benzene ( Tokyo Kasei, GR-grade) were used as received. The volume of the liquid was 50 ml. The glass vessel was cooled using an ice bath. The homogenizer was operated at 600 W, 20 kHz, in a pulsed mode, with a frequency of 1 s and a duty cycle of 0.5. For the heterogeneous experiments, ultrasound irradiation of liquid sample was carried out with solid particles (ZnCl Ni, Zn, ZnO and NiCl ). The particles were 2 2 approximately 200 mm in diameter in ZnCl , and 5– 2 10 mm in Ni, Zn, ZnO and NiCl . 2 The solid products formed in the liquid sample were analyzed with a transmission electron microscope ( TEM: JEOL, JEM-2000FX, 200 kV ). The sample specimen for the TEM observation was prepared by dipping a sample grid into the irradiated solution.
3. Results and discussion After ultrasound irradiation of liquid chlorobenzene without solid particles (homogeneous process), the liquid sample turned dark yellow, due to the formation of polymer [7]. Fig. 1 shows a TEM image of solid products obtained from irradiated liquid chlorobenzene without solid particles. From an analysis of the pattern
Fig. 1. TEM image of the product after ultrasound irradiation of chlorobenzene without ZnCl . The crystallized part is indicated by the 2 arrow.
of electron diffraction, we confirmed that the product is mainly polymer (the black part), and a small portion is highly crystallized graphitic carbon (indicated by arrow in Fig. 1). In order to proceed further crystallization of the low crystallized carbon, ultrasonic irradiation of liquid chlorobenzene was carried out with ZnCl particles (a hetero2 geneous process combined with a homogeneous process). Fig. 2 shows a TEM image of solid products obtained from irradiated liquid chlorobenzene with ZnCl particles. The shape of the products differs visibly 2 from that of the products in the homogeneous experiment ( Fig. 1). In the TEM image, spiny products can be observed. The shape of the spiny products is similar to the carbon nanotube produced by the arc discharge method [9]. Also, the pattern of electron diffraction shows that the spiny product has a layered structure, and the spacing between layers agrees with that in the carbon nanotube. Therefore, we concluded that the spiny products are carbon nanotubes. These results indicate that the polymers, which are produced by the homogeneous process, are annealed by the heterogeneous process. Mixing solid particles with a liquid sample enables effective crystallization of the polymers and the disor-
Fig. 2. TEM image of the product after ultrasound irradiation of chlorobenzene with ZnCl . 2
R. Katoh et al. / Ultrasonics Sonochemistry 6 (1999) 185–187
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Fig. 4. TEM image of the product after ultrasound irradiation of dichlorobenzene with Ni.
Fig. 3. TEM image of the product after ultrasound irradiation of dichlorobenzene with ZnCl . 2
dered carbon. In the present study, we used relatively small particles (<200 mm in diameter), a size at which inter-particle collision frequently occurs and for which the micro-jet formation is not efficient. Under these conditions, a high temperature can be achieved at the site of particle collision. After the collision, polymers, which are produced by the homogeneous process, can be converted into carbon nanotube. The effect of the source material on the production of highly crystallized carbon products was examined. When ultrasound was irradiated to liquid benzene with ZnCl particles, only polymers were produced. This 2 indicates that many hydrogen atoms remain in the products. In contrast, for irradiation of o-dichlorobenzene with ZnCl particles, carbon nanotubes were 2 produced, as shown in Fig. 3. Our previous studies on the homogeneous sonochemical decomposition of liquid benzene and its derivatives show that the hydrogen content of the polymers decreases by introduction of halogen atoms, which is due to the efficient coupling of halogen atoms with hydrogen atoms [7,8]. Therefore, the formation of polymers by ultrasound irradiation of
liquid benzene with ZnCl powder can be attributed to 2 the high content of hydrogen atoms in the precursor polymers. We also examined the heterogeneous process using different particles ( ZnCl , Zn, Ni, ZnO, NiCl ). In the 2 case of irradiation of o-dichlorobenzene with ZnCl and 2 Zn particles, carbon nanotubes were produced, but only graphitic particles [10] were observed from the irradiation with Ni ( Fig. 4), NiCl , and ZnO particles. This 2 clearly shows that annealing of the product proceeds effectively by the heterogeneous process, though the morphology of the crystallized carbons is influenced by the nature of the particles. Possibly, factors such as the size of the particles, thermal conductivity, and catalytic activity for crystallization are responsible for the morphology of the respective products.
References [1] T.J. Mason, Advances in Sonochemistry, Vol. 3, JAI Press, London, 1993. [2] K.S. Suslick, S.J. Doktycz, E.B. Flint, Ultrasonics 28 (1990) 280. [3] E.B. Flint, K.S. Suslick, Science 253 (1991) 1397. [4] K.S. Suslick, Adv. Organometall. Chem. 25 (1986) 73. [5] D. Peter, J. Mater. Chem. 6 (1996) 1605. [6 ] R. Katoh, E. Yanase, H. Yokoi, S. Usuba, Y. Kakudate, S. Fujiwara, Ultrason. Sonochem. 5 (1998) 37. [7] R. Katoh, H. Yokoi, S. Usuba, Y. Kakudate, S. Fujiwara, Ultrason. Sonochem. 5 (1998) 69. [8] R. Katoh, H. Yokoi, S. Usuba, Y. Kakudate, S. Fujiwara, Nippon Kagaku Kaishi (1998) 530. [9] S. Iijima, Nature 354 (1991) 56. [10] Y. Saito, T. Yoshikawa, M. Inagaki, M. Tomita, T. Hayashi, Chem. Phys. Lett. 204 (1993) 277.