[6] Lin C, Ritter JA. Effect of synthesis pH on the structure of carbon xerogels. Carbon 1997;35:1271–8. [7] Pekala RW, Alviso CT, Kong FM, Hulsey SS. Aerogels derived from multifunctional organic monomers. J Non-Cryst Solids 1992;145:90–8. [8] Mujumdar AS. Handbook of industrial drying, vol. 1. 2nd ed. New York: Marcel Dekker; 1995. 742 p. [9] Pekala RW. Structure of organic aerogels. 1. Morphology and scaling. Macromolecules 1993;26:5487–93. [10] Lecloux AJ. Texture of catalysts. In: Anderson JR, Boudart M, editors. Catalysis: Science and Technology, vol. 2. Berlin: Springer; 1981. p. 171–230. [11] Pirard R, Alie ´ C, Pirard JP. Specific behaviour of sol–gel materials in mercury porosimetry: collapse and intrusion. In: Handbook of sol–gel technology. Kluwer; 2004. p. 211–33. [12] Horikawa T, Hayashi J, Muroyama K. Size control and charac- terization of spherical carbon aerogel particles from resorcinol– formaldehyde resin. Carbon 2004;42:169–75. Formation of amorphous carbon nanotubes on ordered mesoporous silica support An-Hui Lu a,b, * , Wolfgang Schmidt a , Simion-Daniel Tatar b , Bernd Spliethoff a , Ju ¨ rgen Popp c , Wolfgang Kiefer b , Ferdi Schu ¨th a a Max-Planck-Institut fu ¨ r Kohlenforschung, 45470 Mu ¨ lheim an der Ruhr, Germany b Institut fu ¨ r Physikalische Chemie, Universita ¨t Wu ¨ rzburg, Am Hubland, D-97074 Wu ¨ rzburg, Germany c Institut fu ¨ r Physikalische Chemie, Friedrich-Schiller-Universitaet Jena, D-07743, Germany Received 4 August 2004; accepted 16 February 2005 Available online 23 March 2005 Keywords: Carbon nanotubes; Chemical vapor deposition; Transmission electron microscopy In the past decade, chemical vapor deposition (CVD) has been extensively employed for the synthesis of car- bon nanotubes (CNTs) with graphitic single-, double-, or multi-walls [1–3], but much less explored for the syn- thesis of CNTs with amorphous tube wall [4,5]. Herein, we present a synthesis of amorphous CNTs by a CVD procedure using mesoporous silica SBA-15 as matrix and an Fe 2 O 3 catalyst. These CNTs exhibit very special geometry with the carbon atom layers packed vertical to the tube axis. The synthesis of mesoporous silica SBA-15 is de- scribed elsewhere [6]. To introduce the catalyst, 3.0 g of SBA-15 was impregnated with 0.95 g of Fe(acac) 3 dis- solved in 200 ml of dried toluene under stirring. Subse- quently, the powder was filtrated and washed with toluene to remove the catalyst from the outer surface of the silica particles. After drying at room temperature and calcination at 550 °C for 5 h, a yellowish powder was obtained, denoted as Cat-1 consisting of about 2 wt.% of Fe 2 O 3 in SBA-15 silica. Ideally, the pores of the SBA-15 silica can be re- garded as nanovoids, which restrict the catalyst particle sizes during the processing, resulting in a particle size reaching a maximum value close to the pore size of SBA-15. Thus, it is possible to prepare catalysts with well-controlled particle size. One can therefore achieve a growth control of the CNTs, since the diameter of the resulting CNTs is usually dominated by the particle size of the catalyst [7, and references therein]. TEM investigation of sample Cat-1 reveals that no visible iron oxide nanoparticles aggregate on the outer surfaces or within the pores of the SBA-15 silica parti- cles (Fig. 1a). Moreover, the XRD patterns of the sam- ple shows no clear reflections in the wide angle range, further confirming the iron particles to be present as nanoparticles being highly dispersed in the pores of the silica [8]. Using this material, the catalytic growth of CNTs on catalyst particles anchored on the outer sur- face of SBA-15 can essentially be avoided. To grow CNTs, hexane was used as carbon source, thiophene as a growth promoter, and a hydrogen and argon mix- ture with a volume ratio of 1:4 as carrier gas. The CVD process was carried out at 820 °C for 30 min. After the CVD process, an entirely black product was ob- tained (composite-1). Fig. 1b reveals that numerous CNTs formed around the SBA-15 matrix with the 0008-6223/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.02.024 * Corresponding author. Address: Max-Planck-Institut fu ¨r Kohlen- forschung, 45470 Mu ¨lheim an der Ruhr, Germany. Tel.: +49 208 306 2367; fax: +49 208 306 2995. E-mail address: lu@mpi-muelheim.mpg.de (A.-H. Lu). Letters to the Editor / Carbon 43 (2005) 1778–1814 1811