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 Zhu b and Imre Kiricsi a a Department 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 b Department 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 pro- perties: therefore they represent a separate research field. 1 In this carbon allotrope, three of the four electrons of the carbon atoms are sp 2 hybridized and covalently bonded with neighboring carbon atoms, the remaining electron is deloca- lized 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. Depend- ing 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 nano- tubes. 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 deriva- tives in the vapor phase have been used as carbon sources. The high temperature is produced either by electric discharge 4 or laser ablation. 5 For catalytic syntheses, however, lower temperature is required, which can be achieved in a conven- tional electrical furnace. 6 Using various metals supported on zeolite or silica as cata- lysts, Ivanov, 7 Li, 8 Mukhopadhyay 9 and Hernadi 10 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 sug- gested 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 Dai 13 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 electrochemi- cal 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 tripropyl- amine started at 673 K and the carbon nanotube forma- tion was observed at 773 K. They investigated the carbon nanotube–zeolite composite system, i.e. the zeolitic com- ponent 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 2002