摘要:
Paper Mesoporous silicates as nanoreactors for synthesis of carbon nanotubes Mo�nika Urba�n,a Zolta�n Ko�nya,a Do�ra Me�hn,a Ji Zhub and Imre Kiricsia aDepartment of Applied and Environmental Chemistry, University of Szeged, Rerrich Be�la te�r 1, Szeged H-6720, Hungary. E-mail: kiricsi@chem.u-szeged.hu; Fax: 36-62-544-619; Tel: 36-62-544-623 bDepartment of Chemistry, University of California at Berkeley, CA 94720, USA Received 17th April 2002, Accepted 27th August 2002 Published on the Web 19th September 2002 Multi-wall carbon nanotubes (MWNT) were synthesized by a novel method using mesoporous MCM-41 and MCM-48 silicates as nanoreactors in the absence of any metal traces. Transmission electron microscopy (TEM) studies revealed that the quality of carbon nanotubes obtained was very promising, the diameter distribution is very narrow.Introduction Carbon nanotubes are in the focus of material science since they (i) are rather a new member of carbon allotropes, and (ii) have peculiar chemical, mechanical and electrical properties: therefore they represent a separate research field.1 In this carbon allotrope, three of the four electrons of the carbon atoms are sp2 hybridized and covalently bonded with neighboring carbon atoms, the remaining electron is delocalized similarly to the graphite structure. A single-wall carbon nanotube can be regarded as a rolled single graphite sheet, while a multi-wall derivative can be visualized as several graphite cylinders concentrically within each other. Depending on their structures, carbon nanotubes can be conductive, semiconductive or isolating, which makes them the most promising materials for applications in nanoelectronics.2 Due to their remarkable high mechanical resistance they might be one of the best potential strengthening materials in polymer chemistry.The single-wall carbon nanotubes are also regarded as optimal material for hydrogen storage, which would bring closer the use of hydrogen as fuel. Several methods are used for production of carbon nanotubes. 3 Generally the most frequently applied techniques differ either in the carbon source or the method to generate high temperature at which the carbon precursor converts to carbon nanotubes. Graphite, mixtures of transition metal compound and carbon, or various hydrocarbons and hydrocarbon derivatives in the vapor phase have been used as carbon sources.The high temperature is produced either by electric discharge4 or laser ablation.5 For catalytic syntheses, however, lower temperature is required, which can be achieved in a conventional electrical furnace.6 Using various metals supported on zeolite or silica as catalysts, Ivanov,7 Li,8 Mukhopadhyay9 and Hernadi10 reported the production of carbon nanotubes by the catalytic chemical vapor deposition (CCVD) method. However, no general explanation has been published on the formation mechanism of carbon nanotubes in these processes yet. It has been suggested that the role of catalyst is to dissolve the carbon generated from the hydrocarbon on the metal particles and after being saturated under the given experimental conditions, release the carbon and direct the nanotube formation.11 Among the CCVD technologies, there are only some methods using no transition metal catalyst. Porous anodic alumina film was applied as both substrate and catalytic material for production of carbon nanotube by catalytic decomposition of acetylene around 900 K.12 Dai13 used Fe supported on Si (1 0 0) wafer in catalytic conversion of ethylene at 1000K to produce carbon nanotube arrays.In this experiment the porous support was obtained by electrochemical etching of n1-type Si (1 0 0) wafer. Recently, Chinese researchers reported the synthesis of carbon nanotubes in the pore system of AlPO-5 type zeolite.14 Authors simply carbonized the template molecules used for synthesis of the zeolite.The carbonization of tripropylamine started at 673 K and the carbon nanotube formation was observed at 773 K. They investigated the carbon nanotube–zeolite composite system, i.e. the zeolitic component was not separated from the carbon nanotubes. Very thin single-wall (0.4 nm diameter) carbon nanotubes were produced and showed superconductivity.15–18 Last year, Korean scientists claimed the production of carbon–silicate composite via carbonization of divinylbenzene in the pores of mesoporous silicate of MCM-48 type.19 After dissolving the silicate, a highly ordered mesoporous carbon sample was obtained. This carbon structure had enhanced mechanical stability and high hydrophobicity. Ryoo showed that the pores of mesoporous silicates could be used for synthesis of carbon nanostructures from polymers without any transition metal catalyst.20 Despite all these research efforts, there are still remaining questions.For example, can well-graphitized carbon nanotubes form from the template of mesoporous silica when no or a small portion of template molecules contain oxygen? Can we observe any significant influence of the structure and dimension of pores on the quality of carbon nanostructures formed? In this communication we are intended to add some more details to answering these questions. Experimental Sample preparation Two M41S types of mesoporous silicate, MCM-41 and MCM-48 were synthesized using cetyl-trimethyl-ammonium bromide (CTMABr, Aldrich product)21 and a mixture of CTMABr and Triton X-100 (nonionic, block-(ethylene oxide–propylene oxide)-copolymer type detergent, Aldrich product), respectively.In the initial stage of the synthesis micelles were formed in aqueous solution and then, agglomerate 138 PhysChemComm, 2002, 5(20), 138–141 DOI: 10.1039/b203767j This journal is # The Royal Society of Chemistry 2002into hexagonal (MCM-41) and cubic (MCM-48) structures, while they are covered by partially polymerized silicate ions followed by the condensation of silanol groups resulting in solid material with well ordered structure. This means that the interiors of the as-synthesized MCM-41 and MCM-48 silicates are filled with well ordered organic molecules. Samples for the carbonization of polydivinylstyrene were prepared from the as synthesized materials after burning the template off.The samples were heat treated in nitrogen flow while the temperature was increased using a slow heating rate. At 823 K the nitrogen stream was switched for oxygen and the burning was conducted for additional 5 h. After cooling the sample to ambient temperature polymerization of divinylbenzene was performed as had been described.19 After each chemical step of preparation the samples were weighted and the weight-losses were compared. The weight changes gave indication on the effectiveness of the given step. Characterisation The as-synthesized samples were characterized by several physical methods. XRD patterns were recorded on the as synthesized, template free, graphitized samples. TEM images were taken on the carbon nanotube samples using a Phillips CM10 type microscope.High-resolution images were taken with a Topcon 002B transmission electron microscope. BET specific surface were determined from nitrogen adsorption data measured at 77 K with a Quantachrome NOVA Automated Gas Sorption Instrument. TG-DTA measurements were carried out on a Derivatograph Q instrument in the temperature range of 300–1300 K. Results In the first series of experiments, we attempted to transform the structure directing organic molecules into carbon nanotubes; no additional carbon source was added. The ordered mesopores of MCM-41 and MCM-48 can be viewed as nanoreactors for the production of carbon nanotubes. The as-synthesized samples were heat-treated in nitrogen at 1073 K for 30 min. We established that this quite short time is sufficient to graphitize the organic material in the mesoporous structure of MCM-41 and MCM-48.Using shorter or longer heat treatment the formation of carbon nanotubes was observed as well, however, 30 minutes seemed to be optimal concerning both the quality and the quantity of the carbon nanotubes. The produced MWNT–silica composite samples had high surface area and rather narrow diameter distribution. Treas, the inner diameters determined from the nitrogen adsorption data are listed in Table 1. From XRD profiles the basal spacing, i.e. the approximate pore diameters were estimated (Table 1). After dissolving the silicate in hydrofluoric acid, a black product consisting of carbon nanotubes were obtained. Fig. 1 shows the carbon nanotubes form MCM-41 and MCM-48.The messages of these experiments are as following: (i) transition metals are not required as catalyst for generation of carbon nanotubes in the pore systems of mesoporous silicates MCM-41 and 48. (ii) The mixed template containing oxygenated component can be converted to carbon nanotubes. (iii) After dissolving the silicate component the carbon nanostructures generated can be investigated without any influence of the silica. In the second series of experiments, the template molecules were burned off from the as-synthesized mesoporous silicate, then the pores of MCM-41 and MCM-48 silicates were filled with divinylbenzene and the divinylbenzene was polymerized in the pores. After heat treatment for a short time (generally between 5–30 min) at 1073 K followed by dissolution of the silicate using the method mentioned above carbonaceous material was obtained.TEM investigation of these samples revealed the presence of carbon nanotubes among the various carbonaceous materials. However, these carbon nanotubes are less graphitized as TEM images in Fig. 2 show. This is also confirmed by HREM. Representative images are shown in Fig. 3. Raman studies to estimate the degree of crystallinity of the tubes are in progress. Furthermore, significant difference was found between the amounts of carbon nanotubes formed in the two samples. In MCM-41 much more nanotubes were generated compared to MCM-48. Presumably, the 2D structure of the MCM-41 nanoreactors is preferred in nanotube formation in contrast to the 3D structure of MCM-48.Summarizing the observations, carbon nanotubes were found in the carbon nanostructures formed from polyvinylbenzene in the interior of both mesoporous silicates of MCM-41 and MCM-48 types. Discussion For a general catalytic chemical vapor deposition method, say when acetylene as carbon source is fed into the reactor for some Table 1 Physical characteristics of mesoporous silicates as synthesized (AS), template free forms (BO) and after the graphitization (GP) procedure MCM-41 MCM-48 XRD basal spacing AS/nm 4.02 3.84 XRD basal spacing GP/nm 4.02 3.76 BJH pore diameter BO/nm 3.12 2.92 BJH pore diameter GP/nm 3.08 2.89 TG organic content AS (wt.%) 33 47 TG organic content GP (wt.%) 5 8 BET surface area BO/m2 g21 941 1074 BET surface area GP/m2 g21 847 1000 Fig. 1 Carbon nanotubes formed in the (a) MCM-41 and (b) MCM-48 nanoreactors.PhysChemComm, 2002, 5(20), 138–141 139time, and some sort of zeolite impregnated with cobalt salt as the catalyst precursor, the carbon yield may reach 100 wt.%. This is the consequence of the excess carbon fed. This means that the amount of carbon generated may reach the weight of catalyst or even exceed it. In our case the amount of carbon precursor was limited, it was less than 50 wt.% of the mesoporous material. Actually only a part of this carbon may be converted to nanotubes, the most part is cracked off from the structure. Due to the weighting of the samples some rough estimation of the effectively utilized carbon can be made. Using these calculations we estimated that a maximum of 10 wt.% of the original carbon content of the samples was converted to structured carbon including nanotubes.We should mention again that the target of this work was not the elaboration of a new method for large-scale production technology. We wanted to show that without any transition metal catalyst, carbon nanotubes can be generated in pores of mesoporous silicate from the template molecules for MCM-41 and 48, and/or polymers filled into the pore system of such silicates. The results obtained reveal an additional very important feature of carbon nanotube generation. That is the influence of oxygen. The synthesis of carbon nanotubes is generally carried out in the absence of molecular oxygen – a very nice example is the procedure published by Sui, similar to our experiment, is when carbon nanotubes are prepared by pyrolysis of propylene in electrochemically formed alumina channels.12,22 However, nanotube formation can take place from oxygen containing compounds, such as CO in hydrogen atmosphere or acetone for the CCDV methods.9,23 In our experiments with MCM-41 the template molecule did not contain oxygen atoms.Initially we thought that this was the reason of the successful nanotube formation. However, we used Triton X-100, a non-ionic detergent consisting of poly(ethylene oxide) units as secondary template in the synthesis of MCM-48. After graphitization good quality carbon nanotubes were detected by TEM. From this it follows that the presence of oxygen does not influence negatively the generation of carbon nanotubes in the pore structure of mesoporous silicate when its concentration is limited.The carbon nanotubes synthesized by the method described in this paper were generated inside the pore structure of mesoporous silicates. We don’t want to speculate in advance the mechanisms of their formation; studies to understand this phenomenon are in progress; however, some important points should be mentioned here. The first, no transition metal clusters are present in the pores, therefore the simple catalytic way of the formation does not seem probable. However there could be some traces of catalytic impurities (as contamination during the preparation process) within silicates, which could also catalyze the growth of carbon nanotubes. The mesoporous silicate pores have free Si–OH groups of slightly acidic character.The question is whether these groups contribute to the formation of nanotubes, or not. Furthermore, the diameter of the tubes is larger than the original pore diameter of the silicate. At which stage of synthesis increases the diameter? We observed a rather uniform distribution of the diameter of carbon nanotubes. This may be the consequence of the regular arrangement of template molecules, since for these silicates rod-like micelles are formed in the aqueous solution and these micelles assembled to form either hexagonal or cubic structures while they are covered by silica. Studies to answer these questions are in progress and will be reported in subsequent papers. In conclusion, we have reported in this paper the production of carbon nanotubes in the interior of MCM-41 and MCM-48 mesoporous silicates as nanoreactors.The generation of MWNT took place both from the structure directing organic material as carbon source and from the polymer incorporated into the pores of the silicates prior to the Fig. 2 Carbon nanotube formation from polymerized divinylbenzene in (a) MCM-41, and (b) MCM-48 nanoreactors. Fig. 3 HREM images of carbon nanotubes formed in MCM-41 from (a) original template molecules, and (b) polymerized divinylbenzene. 140 PhysChemComm, 2002, 5(20), 138–141carbonization. It was found that the quality of carbon nanotubes was superior in the MCM-41 material. Acknowledgements This work was supported by the National Science Foundation of Hungary (OTKA No.T025246). ZK acknowledges support from FKFP 0216/2001, OTKA F038249 and Bolyai fellowship. References 1 R. Saito, M. S. Dresselhaus and G. Dresselhaus, in Physical Properties of Carbon Nanotubes, World Scientific Publishing, 1998; P. J. F. Harris, in Carbon Nanotubes and Related Structures – New Materials for the Twenty-First Century, Cambridge University Press, 1999; Carbon nanotubes: Synthesis, Structure, properties and Applications, ed. M. S. Dresselhaus, G. Dresselhaus and P. Avouris, Springer-Verlag, 2001. 2 D. Xu, G. Guo, L. Gui, Y. Tang, Z. Shi, Z. Jin, Z. Gu, W. Liu, X. Li and G. Zhang, Appl. Phys. Lett., 1999, 75, 481. 3 P. M. Ajayan, Chem. Rev., 1999, 99, 1787; C. Journet and P. Bernier, Appl. Phys. A: Mater. Sci. Process., 1998, 67, 1. 4 S. Iijima and T.Ichihashi, Nature, 1993, 363, 603. 5 C. D. Scott, S. Arepalli, P. Nikolaev and R. E. Smally, Appl. Phys. A: Mater. Sci. Process., 2001, 72, 573; W. K. Maser, E. Munoz, A. M. Benito, M. T. Martinez, G. F. de la Fuenet, Y. Maniette, E. Anglaret and J. L. Sauvajol, Chem. Phys. Lett., 1998, 292, 587. 6 H. Dai, Top. Appl. Phys., 2001, 80, 29. 7 V. Ivanov, A. Fonseca, J. B. Nagy, A. Lukas, X. B. Zhang, X. F. Zhang, D. Bernaerts, G. Van Tendeloo, S. Amelinckx and J. Van Landuyt, Chem. Phys. Lett., 1994, 223, 329. 8 W. Z. Li, S. S. Xie, L. X. Qian, B. H. Chang, B. S. Zhou, W. Y. Zhou, R. A. Zhao and G. Wang, Science, 1993, 274, 1701. 9 K. Mukhopadhyay, A. Koshio, T. Sugai, N. Tanaka, H. Shindohara, Z. Konya and J. B. Nagy, Chem. Phys. Lett., 1999, 303, 117. 10 K. Hernadi, A.Fonseca, J. B. Nagy, A. Siska and I. Kiricsi, Appl. Catal. A: Gen., 2000, 199, 245. 11 Z. Ko� nya, J. Kiss, A. Oszko, A. Siska and I. Kiricsi, Phys. Chem. Chem. Phys., 2001, 3, 155; A. Kukovecz, Z. Ko�nya, N. Nagaraju, I. Willems, A. Tamasi, A. Fonseca, J. B. Nagy and I. Kiricsi, Phys. Chem. Chem. Phys., 2000, 2, 3071. 12 Y. C. Sui, D. R. Acosta, J. A. Gonzalez-Leon, A. Bermudez, J. Feuchtwanger, B. Z. Cui, J. O. Flores and J. M. Saniger J. Phys. Chem., 2001, 105, 1523; Y. C. Sui, B. Z. Cui, R. Guardian, D. R. Acosta, L. Martinez and R. Perez, Carbon 2002, 40, 1011. 13 S. Fan, W. Liang, H. Dang, N. Franklin, T. Tombler, M. Chapline and H. Dai, Physica E, 2000, 8, 179. 14 H. D. Sun, Z. K. Tang, J. Chen and G. Li, Appl. Phys. A: Mater. Sci. Process., 1999, 69, 381. 15 H. D. Sun, Z. K. Tang and J. N. Wang, J. Magn. Magn. Mater., 1999, 198–199, 255. 16 N. Wang, G. D. Li and Z. K. Tang, Chem. Phys. Lett., 2001 339, 47. 17 Z. K. Tang, L. Zhang, N. Wang, X. X. Zhang, G. H. Wen, G. D. Li, J. N. Wang, C. T. Chan and P. Sheng, Science, 2001 292, 2462. 18 A. Jorio, A. G. Souza Filho, G. Dresselhaus, M. S. Dresselhaus, A. Righi, F. M. Matinaga, M. S. S. Dantas, M. A. Pimenta, Z. M. Li, Z. K. Tang and R. Saito, Chem. Phys. Lett., 2002 351, 27. 19 S. B. Yoon, J. Y. Kim and J. S. Yu, Chem. Commun., 2001, 559. 20 R. Ryoo, S. H. Joo and S. Jun, J. Phys. Chem. B, 1999, 103, 7743; R. Ryoo, S. Jun, J. M. Kim and M. J. Kim, Chem. Commun., 1998, 2225; S. H. Joo, S. J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki and R. Ryoo, Nature, 2001, 412, 169. 21 J. S. Beck, C. Vartuli, G. J. Kennedy, C. T. Kresge, W. J. Roth and S. E. Schramm, Chem. Mater., 1994, 6, 1816; R. Ryoo, S. H. Joo and J. Kim, J. Phys. Chem. B, 1999, 103, 7435. 22 F. Beguin, K. Metenier, G. Pellen, S. Bonnamy and E. Frackowiak, Mol. Cryst. Liq. Cryst., 2000, 340, 547. 23 J. H. Hafner, M. J. Bronikowski, B. R. Azamian, P. Nikolaev, A. G. Rinzler, D. T. Colbert, K. A. Smith and R. E. Smalley, Chem. Phys. Lett., 1998, 296, 195; P. Pinheiro, M. C. Schouler, P. Gadelle, M. Mermoux and E. Dooryhee, Carbon, 2000 38, 1469. PhysChemComm, 2002, 5(20), 138–141
ISSN:1460-2733
DOI:10.1039/b203767j
出版商:RSC
年代:2002
数据来源: RSC