- Email: [email protected]
Efficient encapsulation of gaseous nitrogen inside carbon nanotubes with bamboo-like structure using aerosol thermolysis
Chemical Physics Letters 396 (2004) 167–173 www.elsevier.com/locate/cplett
Efficient encapsulation of gaseous nitrogen inside carbon nanotubes with bamboo-like structure using aerosol thermolysis M. Reyes-Reyes a,b, N. Grobert b,c, R. Kamalakaran c, T. Seeger c, D. Golberg d, M. Ru¨hle c, Y. Bando d, H. Terrones a,b, M. Terrones a,e,* a
e
Advanced Materials Department, IPICYT, Camino a la Presa San Jose´ 2055, Col. Lomas 4a. Seccio´n, San Luis Potosı´ 78216, Me´xico b Fullerene Science Center, CPES, University of Sussex, Brighton BN1 9QJ, United Kingdom c Max-Planck-Institut fu¨r Metallforschung, Heisenbergstraße 3, D-70569 Stuttgart, Germany d Advanced Materials Laboratory, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan International Center for Young Scientists (ICYS), National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan Received 1 July 2004; in final form 27 July 2004
Abstract High yields of dense, ÔcleanÕ and uniform arrays of well-aligned carbon nanotubes, with bamboo-like structure encapsulating gaseous nitrogen, were obtained by thermolyzing uniform aerosols of ferrocene/benzylamine solutions at 850 °C. Electron energy loss spectroscopy (EELS) studies reveal that up to 90% of these tubes contain molecular nitrogen in their cores. The materials were characterized by scanning electron microscopy, X-ray powder diffraction, high-resolution transmission electron microscopy, and EELS elemental mappings using an Omega Filter microscope. We envisage the material useful for storing large concentrations of relatively heavy gases such as nitrogen in confined volumes. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction The hollow core of carbon nanotubes is able to host a wide variety of materials including gases and solid compounds [1–4]. In this way, carbon nanotubes could be used as nanolaboratories in order to observe dynamic effects and transformations of confined nanosystems (e.g., clusters, nanowires, pressurized gas molecules). Moreover, this encapsulation results in an enhancement of the flexural strength within the nanotube structure caused by the presence of a solid or compressed gas inside the tube core. Allied to these changes, the electronic and magnetic properties of the tubes may be also altered depending of the encapsulated material.
*
Corresponding author. Fax: +444 834 2010. E-mail address: [email protected] (M. Terrones).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.07.125
For solids compounds, nanowires of metals, metal carbides and/or oxides have been introduced/grown successfully inside the hollow core of carbon nanotubes [2–7]. In this context, arrays of Fe-filled carbon nanotubes could be used in the fabrication of magnetic devices with high-density data storage capacity [6] or inside integrated circuits used for nanoelectronics and photonics [7,8]. In the past, it has also been possible to encapsulate gaseous Ar [9], H2 [10,11] and N2 [12,13] inside carbon nanotubes. This may result in important applications for the generation of efficient gas storage devices, able to host highly compressed gases. Unfortunately, the yields of gas encapsulation in the nanotubes were not high, and some of these synthesis processes involved various steps [9]. In this account, we report a technique that is able to generate aligned dense arrays of aligned carbon nanotubes with bamboo-like structure, mostly filled with gaseous nitrogen (e.g., 50–80% yield). The material was
168
M. Reyes-Reyes et al. / Chemical Physics Letters 396 (2004) 167–173
obtained by pyrolyzing uniform aerosols of ferrocene/ benzylamine solutions at 850 °C using either an argonor an ultrasonic-driven atomizer. The products were analysed using electron microscopy and elemental characterization techniques. Our observations suggest that the tube growth process takes place via a Ôroot growthÕ mechanism.
2. Experimental Solutions consisting of mixtures of Fe(C5H5)2 (ferrocene – 2.5% by weight – Aldrich) in PhCH2NH2 (benzylamine) were prepared ultrasonically for 15 min. This solution was then transferred to the reservoir of the atomizer. One type of the aerosols generators used in this work is similar to that reported by Mayne et al. [14] (same operating conditions; high argon flow of ca. 2000 sccm), with the difference that it uses benzylamine/ferrocene solutions instead of benzene/ferrocene. An alternative ultrasonic apparatus was also used in order to obtain a uniform aerosol [15,16]. Both set-ups are able to generate a homogeneous aerosol from solutions that are then thermolyzed in the presence of Ar (1–3 l/min). The pyrolysis temperature was set to 850 °C, and all experiments were carried out inside quartz tubes (ID 25 mm; length 700 mm) for 15 min. At this point, the aerosol generator operation was suspended and a low argon flow (300 sccm) was kept until the furnace reached room temperature. The soot deposited on the walls of the quartz tube was scratched and analyzed by scanning electron microscopy (SEM) using a JEOL-JSM 6300F operating at 2–5 kV. Samples for high-resolution transmission electron microscopy (HRTEM) studies were prepared by dispersing the powders (1 mg) in acetone (10 ml) using a sonication bath for 5 min, and one drop of the suspension was then allowed to evaporate onto a lacey-carbon grid. The grids were examined using a Hitachi TEM 7100 operated at 120 kV, and a Field Emission Jeol-JEM3000F operated at 300 kV equipped with a Gatan 766 Energy Loss spectrometer 2D-DigiPEELS. Electron Energy Loss (EELS). Elemental mappings using EELS were acquired using: (a) 300 kV field emission JEM3010F equipped with an Omega Energy Filter and (b) Zeiss 912 TEM microscope (operated at 120 kV) with an attached Omega Energy Filter.
3. Results and discussion The black powder products possessed a characteristic ÔstickyÕ appearance and consisted of almost pure and aligned nanotubes. SEM images of these materials, recovered from the walls of the quartz tube, clearly showed flakes of aligned nanotubes (Fig. 1a–d); the
Fig. 1. (a–d) SEM images at different magnifications of samples obtained by pyrolyzing solutions of 2.5% ferrocene in benzylamine at 850 °C in an Ar atmosphere. It is clear that the material is rather uniform, and consist of aligned nanotubular structures (5–35 nm in diameter); the presence of unwanted particles and amorphous carbon are notably absent; (e–f) TEM images of the aligned material showing the compartmentalized structure of the nanotubes (e), and the metal elongated particles present at the tips of all tubes (f); and (g) powder X-ray diffraction pattern of the synthesized material exhibiting reflections characteristic of graphite, a-Fe and Fe3C.
average length of these elongated structures is ca. 100 lm with uniform outer diameters (15–40 nm; Fig. 1c). Our samples were ÔcleanÕ and contained large amounts of nanotubes (yield >95% by area), the rest of the material consisted of thin films of agglomerated carbon and/ or encapsulated iron particles (Fig. 1a–f), deposited only on the exposed surface (away from the substrate) of these carpet-like flakes. The opposite surfaces, closer to the SiOx substrate (observed by SEM), only showed the presence of isolated nanotube tips (Fig. 1d). TEM images of these regions (Figs. 1f and 2a) confirmed that at one end, individual nanotube tips encapsulating metal particles are formed, whereas the other ends, agglomerated bulk metal surfaces coated by carbon occur (Fig. 1e–f). HRTEM images (Fig. 2b,c) and energy dispersive
M. Reyes-Reyes et al. / Chemical Physics Letters 396 (2004) 167–173
Fig. 2. HRTEM images of an individual CNx nanotube showing metallic Fe encapsulated at the tip and nanocompartments along the tube: (a) a darker and amorphous regions are clearly observed at the end of the tip due to the presence of FeOx (EELS mapping not shown here); (b) higher magnification of the square shown in (a) revealing the high crystallinity of the carbon layers and the presence of an amorphous material at the end of the particle possibly due to noncrystalline CNx species extruded from the particle after precipitation; (c) highers magnification of (b) displaying the metal planes corresponding to Fe; (d) higher magnification of the compartment closure, showing some corrugation of the inner shells and a ÔblurredÕ inner region in the cores, possibly due to the encapsulation of gaseous material.
X-ray (EDX) spectroscopy studies suggested that the elongated metal inside these nanotube tips consisted of a-Fe covered by graphene layers. X-ray powder diffraction (XRD) patterns of the products (Fig. 1g) clearly reveal graphite-like reflections with small shifts towards lower angles in the (0 0 l) planes. These changes are caused by the curvature and turbostratic nature of the concentric graphene cylinders that form the carbon nanotube (d-spacing 0.34 nm, which is higher than in perfect AB stacked graphite 0.335 nm). The pattern also indicates the presence of a-Fe phase. Additional peaks from Fe3C (called cementite) could also be identified. From d-spacing similarities observed in the HRTEM images (Fig. 2c), it is difficult
169
to clearly differentiate between the a-Fe and Fe3C phases [14]. However, it is very likely that a thin layer of cementite (only a few atomic planes) is formed at the tube-metal interface (see below). TEM studies revealed that all nanotubes exhibited a bamboo-like (compartmentalized) structure (Figs. 1e and 2a) associated with the presence of nitrogen incorporated in the hexagonal carbon lattice; similar structures have also been obtained by other groups [12,17–19]. Fig. 2a depicts an iron particle at a nanotube end; three layered-carbon compartments are clearly shown within this nanotube (ca. 20–30 nm long each). The metal particle diameter changes from 8 (bottom) to 14 nm (tip), and is ca. 36 nm long. Fig. 2b,c amplifies the metal particle and the nanotube interface showing a high degree of crystallinity of the graphene cylinders as well as metal particle; lattice spacing of metal is ca. 0.2 nm, and corresponds to either the (1 1 0) reflection from the a-Fe phase or to the (0 3 1) reflection from the Fe3C phase. EDX studies reveal that the metal particle contains a thin layer on top containing Fe, O and traces of Si (Fig. 2a; top). We noted that the lattice spacings in all of the metal structures encapsulated observed inside the nanotube (Figs. 2a and 3a,d) remained always constant (ca. 0.2 nm). The Fe fillings inside the nanotubes exhibited lengths <180 nm and diameters from 10 to 20 nm (Fig. 3b). If ferrocene concentrations are increased in the pyrolysis experiments, the metal filling can be enhanced; similar results have also been confirmed using benzene [16,20] and toluene [21]. HRTEM images of the inner nanocompartments (Fig. 2d) reveal the existence of Ôamorphous-likeÕ material, possibly caused by the formation of non-crystalline
Fig. 3. (a–b) HRTEM images of CNx nanotube filled with Fe metal. In (a) the tip of the CNx tube is shown with some amorphous-type material at the top (identified as FeOx; see top inset). Insets display the high degree of crystallinity of the graphene shells and the filling material (a-Fe). At the metal–carbon interface it is believed that a few atomic layers of Fe3C are formed. However, it is difficult to identify these phase using standard electron microscopy techniques. The image shown in (b) displays an Fe-filled CNx tube.
170
M. Reyes-Reyes et al. / Chemical Physics Letters 396 (2004) 167–173
CNx species formed during growth. The graphene layers that separate each nanocage (see arrows in Fig. 2a,d) present various types of curvature and distortions, which are possibly due to pentagonal carbon defects and/or nitrogenated sites. It is noteworthy that the number of graphene layers close to the particle end (ca. 16 layers or 8 concentric cylinders) is always smaller than those around the waist of the tube (ca. 22 layers or 11 concentric cylinders) in the compartments or nanocages regions. We believe that the excess of graphene sheets along the tube waist is caused by the conical shape of the metal particle, which is responsible of different rates of C precipitation that results in the formation of additional layers of carbon via a diffusion process originating at the particle tip. EELS studies on the nanotube material showed the presence of nitrogen gas inside the nanotube cores (extremely sharp edges centered at ca. 401 eV) [12,13], and CNx layers (0.02 < x < 0.07) forming the tube walls. Elemental EELS mappings using Omega filtered microscopy clearly showed the presence of high concentrations of gaseous N2 inside most of the tube cores [12,13] (Fig. 4). Note that not all the tubes contain gaseous nitrogen in their cores (indicated by the arrows in the Carbon map; Fig. 4c). In some cases, it is clear that open nanotubes from the tips (e.g., GRAY arrow on left-hand side of Fig. 4c) do not contain gaseous N2 in their interior but N in the form of CNx on the tube walls (see spiral tube in Fig. 4d). From the nitrogen
map (Fig. 4d), it is also clear that some nanotubes contain high concentration of molecules due to the high blue contrast on the mapping. To the best of our knowledge, such high yields of gas encapsulation have never been observed previously. Line concentration profiles for C and N of individual tubes reveal that N is indeed encountered in the core of the tubes (Fig. 4e–f). It is important to note that the walls of the tubes contain low concentrations of N (e.g., 2–3%), and exhibit C–N bonds (two and three coordinated N). It is also likely that some of the N atoms or N2 molecules could be physisorbed on either the outer or inner walls of the tubes. Further investigations are needed along this direction. However, most of the N atoms we observed in this study are mainly encapsulated inside the cores of the tubes (Fig. 4). For the growth of bamboo-like tubular structures, we suggest that two different velocities of C/N precipitation through the metal particle take place: (a) The first one is fast and is responsible for the precipitation C and N of the outer cylinders (fast extrusions that occur along thin metal particle segments); (b) the second is slower due to the long extrusion time of C and N species occurring in the interior of the conical metal particles (thick region). Since the outer layers are extruded quicker than the inner shells, discontinuities of the inner cylinders growth occur and the formation of pentagons is catalysed, thus closing locally the inside layers. In addition, if the outer cylinders grow faster the chances to encapsulate un-
Fig. 4. (a) TEM image and (c–e) Elemental mappings of Fe, C and N using EELS with an Omega filter attached to a electron microscope. The Nitrogen map shows the filling in the interior of the tubes. It is clear that some tubes do not exhibit N due to the lack of gaseous nitrogen in the cores of the tubes. Note that not all the tubes contain nitrogen gas (indicated by the arrows in the Carbon map). The high contrast observed in the N map (intense blue) for some of the nanotubes suggest that extremely high pressures of N2 can be built inside the CNx nanotubes. Note that an open nanotube (GRAY arrow on left-hand side of (c) does not contain gaseous N2 in the interior but N in the form of CNx on the tube walls (see spiral tube in (d). Fe mapping demonstrates that most of the metal is always concentrated closer to the nanotube tips; (e–f) Line concentration profiles of a N-filled carbon nanotube revealing that most of the N gas is encapsulated in the nanotube core, indicating that the tube is filled with N. The C profile reveals a double maximum caused by the inner core.
M. Reyes-Reyes et al. / Chemical Physics Letters 396 (2004) 167–173
reacted C/N species in the hollow core are higher, thus giving rise to the encapsulation of C and N2 species. Very interestingly, the nanotube tips showed amorphous material at the very end (upper inset in Figs. 3a and 5a) containing Fe and O and traces of Si; similar structures have been observed by Katayama et al. [22].
171
It is possible that this FeOx amorphous material was formed at the Fe–SiOx interface of the substrate, and was responsible for the C and N migration and tube growth. From our previous observations, we believe that these type of nanotubes grow from the base upwards
Fig. 5. (a–b) SEM images of CNx nanotubes grown on the SiOx substrate showing that isolated tube tips can be clearly identified closer to the silica substrate, whereas an amorphous-like carbon agglomeration occurs at the tip of the flake. TEM images of a nanotube carpet shows: (c) the tips of the CNx tubes and (d) dense agglomerated particles at the other end of the carpet showing a conglomeration of metal and carbon; (e) proposed mechanism for the growth of CNx nanotubes filled with gaseous nitrogen. (i) Formation of Fe clusters, N2 and H2 molecules, as well as CNx species in the gas phase by the decomposition of (Cp2Fe) and (PhCH2NH2). (ii) Growth of Fe clusters and fixation of the cluster to the SiOx via the formation of FeOx–SiOx amorphous regions. We believe that H2 molecules are quickly carried out of the reaction zone, and therefore do not play a key role in the growth of our tubular structures. Similarly some hydrocarbons (CxHy) produced inside the furnace will be expelled relatively fast. (iii) Exposed Fe nanoparticles start reacting exothermally with the CNx species. The Fe particle remains fixed at the bottom and the CNx species are extruded upwards, in the form of crystalline cylinders consisting of CNx, at the opposite side of the particle. (iv) Elongation of the Fe particle via extrusion forces caused by the CNx precipitation through the metal particle along the Fe (1 0 0) planes, thus forming a conical shape. (v) Crystalline CNx material is extruded faster on the outside of the particle, and slow precipitation of carbon occurs at the inner particle region. It is believed that N2 molecules are also rapidly extruded at the other end of the metal particle. The different precipitation rates of CNx generate the fast formation of the outer cylinders and the fast encapsulation of CNx and N2 species in the tube cores (compartments are created by the slow precipitation of CNx and lack of material to keep inner nanotube to grow; these instabilities cause sudden closure of the inner layers).
172
M. Reyes-Reyes et al. / Chemical Physics Letters 396 (2004) 167–173
(root growth mechanism) using these FeOx clusters as reacting sites that are able to precipitate concentric graphene cylinders at the opposite end. SEM images of nanotubes grown on SiO2 substrate (Fig. 5a) suggest that the Fe from the gas phase first fragments from the ferrocene molecules and subsequently condenses on the SiOx surface, forming a type of amorphous Fe–FeOx layer consisting of uniformly distributed particles deposited on the substrate. In Fig. 5a, it is possible to observe that isolated nanotube tips are always present closer to the SiOx substrate. Additional TEM studies showed a bundle of aligned nanotubes containing well defined elongated Fe particles at one end (Fig. 5d), whereas the opposite end presented agglomerated material consisting of amorphous Fe and carbon (Fig. 5c). In our experiments, we noted that the use of benzylamine is responsible for obtaining extremely pure and thin nanotube materials (almost completely free of amorphous carbon) when compared to those reports for benzene–ferrocene or toluene–ferrocene pyolysis using atomisation methods [14,20]. Based on all our observations, we propose the following growth scenario for producing high yields of aligned CNx compartmentalized nanotubes containing large concentrations of nitrogen gas in their cores: (a) Fine and uniform droplets of Ferrocene (Cp2 Fe) and benzylamine (PhCH2NH2) molecules fragment rapidly and create Fe clusters, N2 and H2 molecules, as well as CNx species in the gas phase (Fig. 5e-i). (b) As the Fe clusters grow in size, they start condensing on the SiOx substrate forming nanoparticles (5–10 nm in diameter), which start reacting with gaseous molecules and species present in the environment. We believe that H2 molecules are too volatile and most of them are quickly carried out of the reaction zone, and therefore do not play a key role in the growth of our tubular structures. Similarly some hydrocarbons (CxHy) produced inside the furnace will be expelled relatively fast. (c) The substrate surfaces of the Fe nanoparticles establish strong bonds with the silica substrate via the formation of FeOx–SiOx amorphous regions. These regions result in the fixation of more-or-less homogeneous Fe-containing nanoparticles on the quartz tube (Fig. 5e-ii). (d) The exposed Fe particles then start reacting exothermally with the CNx species. Since one end of the particles is well fixed to the substrate, the Fe particle remains fixed at the bottom and the CNx species are extruded upwards, in the form of crystalline cylinders consisting of CNx, at the opposite side of the particle (Fig. 5e-iii).
(e) The extrusion forces of CNx species and the fixation of one end of the metal to the substrate, result in the elongation of the Fe particle thus forming a conical shape (Fig. 5e-iii). (f) During precipitation across the conical metal Fe particle, CNx species diffuse along the metal (1 0 0) planes and crystalline material is extruded faster on the outside of the particle (fast deposition growth). During CNx precipitation, we believe that N2 molecules are also rapidly extruded at the other end of the metal particle (Fig. 5e-iv). Simultaneously, CNx units diffuse along the inner regions (denser) of the particle at a smaller rate (low CNx precipitation velocity). These different precipitation rates generate the fast formation of the outer cylinders. (g) Due to the slow precipitation of the inner cylinders, the fast growth of the outer shells, and the presence of CNx and N2 species. It is very likely that numerous gaseous molecules are trapped in the interior of the tube core and the graphene-like inner shells left behind recombine and achieve closure by the introduction of pentagonal rings and CNx defects (Fig. 5e-v). (h) The closure of these inner shells result in the compartmentalized structure of the CNx nanotube. The trapping of gases occurs due to the fast growth of the outer layers and slow precipitation of CNx and N2 material in the inner tube regions (Fig. 5e-v). Therefore, the elongated shape of the metal particle, the fast extrusion of N2 molecules from the Fe particles, and the extremely fast growth of the outer shells are able to generate these fascinating bamboo-like CNx nanotubes with high yields of encapsulated gaseous nitrogen. We should emphasize that the pyrolysis of the same (benzylamine/ferroceno) solution, injected into the furnace using a spraying jet [20] with different concentrations (0.2 g ferrocene in 2 ml benzylamine), always resulted in a: (a) poor encapsulation of gaseous N2 inside the CNx tube cores, and (b) less degree of alignment of the grown nanotubes, when compared to those obtained in the present work. Various processing parameters affect the characteristics of the final products. For example, the size of the catalytic particle is very important for the dimensions and morphology of the produced nanotube. In this context, Rao et al. [23] proposed that when the particle has dimensions between 10 and 50 nm, multi-walled nanotubes are formed. In our experiments, the size of the aerosol droplet, the liquid flow and the coalescence kinetics occurring with the components of the solution play a significant role in the alignment and crystallinity of the nanotube products during pyrolysis [14].
M. Reyes-Reyes et al. / Chemical Physics Letters 396 (2004) 167–173
4. Conclusions Pyrolysis of homogenous aerosols has been carried out in order to obtain high yields of well-aligned CNx nanotubes containing large concentrations of molecular N2 in their cores. All tubes exhibited a compartmentalized bamboo-like structure with Fe elongated particles at their tips. Crystalline sheets of graphene always surrounded the Fe-fillings inside the nanotubes, and it is possible that Fe3C at the metal–carbon interface is also formed. We envisage this technique as a promising way to synthesize relatively long, thin and ÔcleanÕ nanotubes in high yields. The proposed growth mechanism in these systems occurs via a Ôroot-growthÕ process in which the presence of N2 and CNx species in the gaseous phase is crucial. It is also clear that the material is able to encapsulate large number of gaseous molecules, thus opening new avenues in the storage of heavy gases at high pressures inside CNx nanotubes. Acknowledgements We are indebted to Martine Mayne for her support and encouragement to work on these production meth´ rez and Lisette ods. We are grateful to Daniel Ramı Noyola for technical assistance. We also thank CONACYT-Me´xico for a Ph.D. scholarship (M.R.R.), and Grants: W-8001-millennium initiative (H.T., M.T.), G25851-E (H.T., M.T.), 36365-E (H.T.), 37589-U (M.T.), 41464-Inter American Collaboration (M.T.).
References [1] M. Terrones, Ann. Rev. Mater. Res. 33 (2003) 419.
173
[2] F. Banhart, N. Grobert, M. Terrones, J.-C. Charlier, P.M. Ajayan, Int. J. Mod. Phys. B 15 (2001) 4037. [3] J. Sloan, A.I. Kirkland, J.L. Hutchison, M.L.H. Green, Chem. Commun. 13 (2002) 1319. [4] P.M. Ajayan, S. Iijima, Nature 361 (1993) 333. [5] F. Kassubek, C.A. Stafford, H. Grabert, Phys. Rev. B-Condens. Matter 59 (11) (1999) 7560. [6] N. Grobert, W.K. Hsu, Y.Q. Zhu, J.P. Hare, H.W. Kroto, D.R.M. Walton, M. Terrones, H. Terrones, P. Redlich, M. Ru¨hle, R. Escudero, F. Morales, Appl. Phys. Lett. 75 (1999) 3363. [7] C.A. Ross, Annu. Rev. Mater. Res. 31 (2001) 203. [8] C.M. Lieber, Sci. Am. 285 (2001) 58. [9] G.E. Gadd, M. Blackford, S. Moricca, N. Webb, P.J. Evans, A.M. Smith, G. Jacobsen, S. Leung, A. Day, Q. Hua, Science 277 (1997) 933. [10] A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Heben, Nature 386 (1997) 377. [11] C. Liu, Y.Y. Fan, M. Liu, H.T. Cong, H.M. Cheng, M.S. Dresselhaus, Science 286 (1999) 1127. [12] M. Terrones, R. Kamalakaran, T. Seeger, M. Ru¨hle, Chem. Commun. 12 (2000) 2335. [13] S. Trasobares, O. Ste´phan, C. Colliex, W.K. Hsu, H.W. Kroto, D.R.M. Walton, J. Chem. Phys. 116 (2002) 8966. [14] M. Mayne, N. Grobert, M. Terrones, R. Kamalakaran, M. Ru¨hle, H.W. Kroto, D.R.M. Walton, Chem. Phys. Lett. 338 (2001) 101. [15] M. Glerup, H. Kanzow, R. Almairac, M. Castignolles, P. Bernier, Chem. Phys. Lett. 377 (2003) 293. [16] M. Mayne, et al. (in preparation). [17] B.C. Satishkumar, A. Govindaraj, C.N.R. Rao, Chem. Phys. Lett. 307 (1999) 158. [18] M. Terrones, Ph. Redlich, N. Grobert, S. Trasobares, W.K. Hsu, H. Terrones, Y.Q. Zhu, J.P. Hare, A.K. Cheetham, M. Ru¨hle, H.W. Kroto, D.R.M. Walton, Adv. Mater. 11 (1999) 655. [19] M. Terrones, H. Terrones, N. Grobert, W.K. Hsu, Y.Q. Zhu, H.W. Kroto, D.R.M. Walton, Ph. Kohler-Redlich, M. Ru¨hle, J.P. Zhang, A.K. Cheetham, Appl. Phys. Lett. 75 (1999) 3932. [20] R. Kamalakaran, M. Terrones, T. Seeger, Ph. Kohler-Redlich, M. Ru¨hle, Y.A. Kim, T. Hayashi, M. Endo, Appl. Phys. Lett. 77 (2000) 3385. [21] Ch. Singh, M.S.P. Shaffer, A.H. Windle, Carbon 41 (2003) 359. [22] T. Katayama, H. Araki, K. Yoshino, J. Appl. Phys. 91 (10) (2002) 6675. [23] C.N.R. Rao, B.C. Satishkumar, A. Govindaraj, M. Nath, Chem. Phys. Chem. 2 (2001) 78.
Efficient encapsulation of gaseous nitrogen inside carbon nanotubes with bamboo-like structure using aerosol thermolysis M. Reyes-Reyes a,b, N. Grobert b,c, R. Kamalakaran c, T. Seeger c, D. Golberg d, M. Ru¨hle c, Y. Bando d, H. Terrones a,b, M. Terrones a,e,* a
e
Advanced Materials Department, IPICYT, Camino a la Presa San Jose´ 2055, Col. Lomas 4a. Seccio´n, San Luis Potosı´ 78216, Me´xico b Fullerene Science Center, CPES, University of Sussex, Brighton BN1 9QJ, United Kingdom c Max-Planck-Institut fu¨r Metallforschung, Heisenbergstraße 3, D-70569 Stuttgart, Germany d Advanced Materials Laboratory, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan International Center for Young Scientists (ICYS), National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan Received 1 July 2004; in final form 27 July 2004
Abstract High yields of dense, ÔcleanÕ and uniform arrays of well-aligned carbon nanotubes, with bamboo-like structure encapsulating gaseous nitrogen, were obtained by thermolyzing uniform aerosols of ferrocene/benzylamine solutions at 850 °C. Electron energy loss spectroscopy (EELS) studies reveal that up to 90% of these tubes contain molecular nitrogen in their cores. The materials were characterized by scanning electron microscopy, X-ray powder diffraction, high-resolution transmission electron microscopy, and EELS elemental mappings using an Omega Filter microscope. We envisage the material useful for storing large concentrations of relatively heavy gases such as nitrogen in confined volumes. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction The hollow core of carbon nanotubes is able to host a wide variety of materials including gases and solid compounds [1–4]. In this way, carbon nanotubes could be used as nanolaboratories in order to observe dynamic effects and transformations of confined nanosystems (e.g., clusters, nanowires, pressurized gas molecules). Moreover, this encapsulation results in an enhancement of the flexural strength within the nanotube structure caused by the presence of a solid or compressed gas inside the tube core. Allied to these changes, the electronic and magnetic properties of the tubes may be also altered depending of the encapsulated material.
*
Corresponding author. Fax: +444 834 2010. E-mail address: [email protected] (M. Terrones).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.07.125
For solids compounds, nanowires of metals, metal carbides and/or oxides have been introduced/grown successfully inside the hollow core of carbon nanotubes [2–7]. In this context, arrays of Fe-filled carbon nanotubes could be used in the fabrication of magnetic devices with high-density data storage capacity [6] or inside integrated circuits used for nanoelectronics and photonics [7,8]. In the past, it has also been possible to encapsulate gaseous Ar [9], H2 [10,11] and N2 [12,13] inside carbon nanotubes. This may result in important applications for the generation of efficient gas storage devices, able to host highly compressed gases. Unfortunately, the yields of gas encapsulation in the nanotubes were not high, and some of these synthesis processes involved various steps [9]. In this account, we report a technique that is able to generate aligned dense arrays of aligned carbon nanotubes with bamboo-like structure, mostly filled with gaseous nitrogen (e.g., 50–80% yield). The material was
168
M. Reyes-Reyes et al. / Chemical Physics Letters 396 (2004) 167–173
obtained by pyrolyzing uniform aerosols of ferrocene/ benzylamine solutions at 850 °C using either an argonor an ultrasonic-driven atomizer. The products were analysed using electron microscopy and elemental characterization techniques. Our observations suggest that the tube growth process takes place via a Ôroot growthÕ mechanism.
2. Experimental Solutions consisting of mixtures of Fe(C5H5)2 (ferrocene – 2.5% by weight – Aldrich) in PhCH2NH2 (benzylamine) were prepared ultrasonically for 15 min. This solution was then transferred to the reservoir of the atomizer. One type of the aerosols generators used in this work is similar to that reported by Mayne et al. [14] (same operating conditions; high argon flow of ca. 2000 sccm), with the difference that it uses benzylamine/ferrocene solutions instead of benzene/ferrocene. An alternative ultrasonic apparatus was also used in order to obtain a uniform aerosol [15,16]. Both set-ups are able to generate a homogeneous aerosol from solutions that are then thermolyzed in the presence of Ar (1–3 l/min). The pyrolysis temperature was set to 850 °C, and all experiments were carried out inside quartz tubes (ID 25 mm; length 700 mm) for 15 min. At this point, the aerosol generator operation was suspended and a low argon flow (300 sccm) was kept until the furnace reached room temperature. The soot deposited on the walls of the quartz tube was scratched and analyzed by scanning electron microscopy (SEM) using a JEOL-JSM 6300F operating at 2–5 kV. Samples for high-resolution transmission electron microscopy (HRTEM) studies were prepared by dispersing the powders (1 mg) in acetone (10 ml) using a sonication bath for 5 min, and one drop of the suspension was then allowed to evaporate onto a lacey-carbon grid. The grids were examined using a Hitachi TEM 7100 operated at 120 kV, and a Field Emission Jeol-JEM3000F operated at 300 kV equipped with a Gatan 766 Energy Loss spectrometer 2D-DigiPEELS. Electron Energy Loss (EELS). Elemental mappings using EELS were acquired using: (a) 300 kV field emission JEM3010F equipped with an Omega Energy Filter and (b) Zeiss 912 TEM microscope (operated at 120 kV) with an attached Omega Energy Filter.
3. Results and discussion The black powder products possessed a characteristic ÔstickyÕ appearance and consisted of almost pure and aligned nanotubes. SEM images of these materials, recovered from the walls of the quartz tube, clearly showed flakes of aligned nanotubes (Fig. 1a–d); the
Fig. 1. (a–d) SEM images at different magnifications of samples obtained by pyrolyzing solutions of 2.5% ferrocene in benzylamine at 850 °C in an Ar atmosphere. It is clear that the material is rather uniform, and consist of aligned nanotubular structures (5–35 nm in diameter); the presence of unwanted particles and amorphous carbon are notably absent; (e–f) TEM images of the aligned material showing the compartmentalized structure of the nanotubes (e), and the metal elongated particles present at the tips of all tubes (f); and (g) powder X-ray diffraction pattern of the synthesized material exhibiting reflections characteristic of graphite, a-Fe and Fe3C.
average length of these elongated structures is ca. 100 lm with uniform outer diameters (15–40 nm; Fig. 1c). Our samples were ÔcleanÕ and contained large amounts of nanotubes (yield >95% by area), the rest of the material consisted of thin films of agglomerated carbon and/ or encapsulated iron particles (Fig. 1a–f), deposited only on the exposed surface (away from the substrate) of these carpet-like flakes. The opposite surfaces, closer to the SiOx substrate (observed by SEM), only showed the presence of isolated nanotube tips (Fig. 1d). TEM images of these regions (Figs. 1f and 2a) confirmed that at one end, individual nanotube tips encapsulating metal particles are formed, whereas the other ends, agglomerated bulk metal surfaces coated by carbon occur (Fig. 1e–f). HRTEM images (Fig. 2b,c) and energy dispersive
M. Reyes-Reyes et al. / Chemical Physics Letters 396 (2004) 167–173
Fig. 2. HRTEM images of an individual CNx nanotube showing metallic Fe encapsulated at the tip and nanocompartments along the tube: (a) a darker and amorphous regions are clearly observed at the end of the tip due to the presence of FeOx (EELS mapping not shown here); (b) higher magnification of the square shown in (a) revealing the high crystallinity of the carbon layers and the presence of an amorphous material at the end of the particle possibly due to noncrystalline CNx species extruded from the particle after precipitation; (c) highers magnification of (b) displaying the metal planes corresponding to Fe; (d) higher magnification of the compartment closure, showing some corrugation of the inner shells and a ÔblurredÕ inner region in the cores, possibly due to the encapsulation of gaseous material.
X-ray (EDX) spectroscopy studies suggested that the elongated metal inside these nanotube tips consisted of a-Fe covered by graphene layers. X-ray powder diffraction (XRD) patterns of the products (Fig. 1g) clearly reveal graphite-like reflections with small shifts towards lower angles in the (0 0 l) planes. These changes are caused by the curvature and turbostratic nature of the concentric graphene cylinders that form the carbon nanotube (d-spacing 0.34 nm, which is higher than in perfect AB stacked graphite 0.335 nm). The pattern also indicates the presence of a-Fe phase. Additional peaks from Fe3C (called cementite) could also be identified. From d-spacing similarities observed in the HRTEM images (Fig. 2c), it is difficult
169
to clearly differentiate between the a-Fe and Fe3C phases [14]. However, it is very likely that a thin layer of cementite (only a few atomic planes) is formed at the tube-metal interface (see below). TEM studies revealed that all nanotubes exhibited a bamboo-like (compartmentalized) structure (Figs. 1e and 2a) associated with the presence of nitrogen incorporated in the hexagonal carbon lattice; similar structures have also been obtained by other groups [12,17–19]. Fig. 2a depicts an iron particle at a nanotube end; three layered-carbon compartments are clearly shown within this nanotube (ca. 20–30 nm long each). The metal particle diameter changes from 8 (bottom) to 14 nm (tip), and is ca. 36 nm long. Fig. 2b,c amplifies the metal particle and the nanotube interface showing a high degree of crystallinity of the graphene cylinders as well as metal particle; lattice spacing of metal is ca. 0.2 nm, and corresponds to either the (1 1 0) reflection from the a-Fe phase or to the (0 3 1) reflection from the Fe3C phase. EDX studies reveal that the metal particle contains a thin layer on top containing Fe, O and traces of Si (Fig. 2a; top). We noted that the lattice spacings in all of the metal structures encapsulated observed inside the nanotube (Figs. 2a and 3a,d) remained always constant (ca. 0.2 nm). The Fe fillings inside the nanotubes exhibited lengths <180 nm and diameters from 10 to 20 nm (Fig. 3b). If ferrocene concentrations are increased in the pyrolysis experiments, the metal filling can be enhanced; similar results have also been confirmed using benzene [16,20] and toluene [21]. HRTEM images of the inner nanocompartments (Fig. 2d) reveal the existence of Ôamorphous-likeÕ material, possibly caused by the formation of non-crystalline
Fig. 3. (a–b) HRTEM images of CNx nanotube filled with Fe metal. In (a) the tip of the CNx tube is shown with some amorphous-type material at the top (identified as FeOx; see top inset). Insets display the high degree of crystallinity of the graphene shells and the filling material (a-Fe). At the metal–carbon interface it is believed that a few atomic layers of Fe3C are formed. However, it is difficult to identify these phase using standard electron microscopy techniques. The image shown in (b) displays an Fe-filled CNx tube.
170
M. Reyes-Reyes et al. / Chemical Physics Letters 396 (2004) 167–173
CNx species formed during growth. The graphene layers that separate each nanocage (see arrows in Fig. 2a,d) present various types of curvature and distortions, which are possibly due to pentagonal carbon defects and/or nitrogenated sites. It is noteworthy that the number of graphene layers close to the particle end (ca. 16 layers or 8 concentric cylinders) is always smaller than those around the waist of the tube (ca. 22 layers or 11 concentric cylinders) in the compartments or nanocages regions. We believe that the excess of graphene sheets along the tube waist is caused by the conical shape of the metal particle, which is responsible of different rates of C precipitation that results in the formation of additional layers of carbon via a diffusion process originating at the particle tip. EELS studies on the nanotube material showed the presence of nitrogen gas inside the nanotube cores (extremely sharp edges centered at ca. 401 eV) [12,13], and CNx layers (0.02 < x < 0.07) forming the tube walls. Elemental EELS mappings using Omega filtered microscopy clearly showed the presence of high concentrations of gaseous N2 inside most of the tube cores [12,13] (Fig. 4). Note that not all the tubes contain gaseous nitrogen in their cores (indicated by the arrows in the Carbon map; Fig. 4c). In some cases, it is clear that open nanotubes from the tips (e.g., GRAY arrow on left-hand side of Fig. 4c) do not contain gaseous N2 in their interior but N in the form of CNx on the tube walls (see spiral tube in Fig. 4d). From the nitrogen
map (Fig. 4d), it is also clear that some nanotubes contain high concentration of molecules due to the high blue contrast on the mapping. To the best of our knowledge, such high yields of gas encapsulation have never been observed previously. Line concentration profiles for C and N of individual tubes reveal that N is indeed encountered in the core of the tubes (Fig. 4e–f). It is important to note that the walls of the tubes contain low concentrations of N (e.g., 2–3%), and exhibit C–N bonds (two and three coordinated N). It is also likely that some of the N atoms or N2 molecules could be physisorbed on either the outer or inner walls of the tubes. Further investigations are needed along this direction. However, most of the N atoms we observed in this study are mainly encapsulated inside the cores of the tubes (Fig. 4). For the growth of bamboo-like tubular structures, we suggest that two different velocities of C/N precipitation through the metal particle take place: (a) The first one is fast and is responsible for the precipitation C and N of the outer cylinders (fast extrusions that occur along thin metal particle segments); (b) the second is slower due to the long extrusion time of C and N species occurring in the interior of the conical metal particles (thick region). Since the outer layers are extruded quicker than the inner shells, discontinuities of the inner cylinders growth occur and the formation of pentagons is catalysed, thus closing locally the inside layers. In addition, if the outer cylinders grow faster the chances to encapsulate un-
Fig. 4. (a) TEM image and (c–e) Elemental mappings of Fe, C and N using EELS with an Omega filter attached to a electron microscope. The Nitrogen map shows the filling in the interior of the tubes. It is clear that some tubes do not exhibit N due to the lack of gaseous nitrogen in the cores of the tubes. Note that not all the tubes contain nitrogen gas (indicated by the arrows in the Carbon map). The high contrast observed in the N map (intense blue) for some of the nanotubes suggest that extremely high pressures of N2 can be built inside the CNx nanotubes. Note that an open nanotube (GRAY arrow on left-hand side of (c) does not contain gaseous N2 in the interior but N in the form of CNx on the tube walls (see spiral tube in (d). Fe mapping demonstrates that most of the metal is always concentrated closer to the nanotube tips; (e–f) Line concentration profiles of a N-filled carbon nanotube revealing that most of the N gas is encapsulated in the nanotube core, indicating that the tube is filled with N. The C profile reveals a double maximum caused by the inner core.
M. Reyes-Reyes et al. / Chemical Physics Letters 396 (2004) 167–173
reacted C/N species in the hollow core are higher, thus giving rise to the encapsulation of C and N2 species. Very interestingly, the nanotube tips showed amorphous material at the very end (upper inset in Figs. 3a and 5a) containing Fe and O and traces of Si; similar structures have been observed by Katayama et al. [22].
171
It is possible that this FeOx amorphous material was formed at the Fe–SiOx interface of the substrate, and was responsible for the C and N migration and tube growth. From our previous observations, we believe that these type of nanotubes grow from the base upwards
Fig. 5. (a–b) SEM images of CNx nanotubes grown on the SiOx substrate showing that isolated tube tips can be clearly identified closer to the silica substrate, whereas an amorphous-like carbon agglomeration occurs at the tip of the flake. TEM images of a nanotube carpet shows: (c) the tips of the CNx tubes and (d) dense agglomerated particles at the other end of the carpet showing a conglomeration of metal and carbon; (e) proposed mechanism for the growth of CNx nanotubes filled with gaseous nitrogen. (i) Formation of Fe clusters, N2 and H2 molecules, as well as CNx species in the gas phase by the decomposition of (Cp2Fe) and (PhCH2NH2). (ii) Growth of Fe clusters and fixation of the cluster to the SiOx via the formation of FeOx–SiOx amorphous regions. We believe that H2 molecules are quickly carried out of the reaction zone, and therefore do not play a key role in the growth of our tubular structures. Similarly some hydrocarbons (CxHy) produced inside the furnace will be expelled relatively fast. (iii) Exposed Fe nanoparticles start reacting exothermally with the CNx species. The Fe particle remains fixed at the bottom and the CNx species are extruded upwards, in the form of crystalline cylinders consisting of CNx, at the opposite side of the particle. (iv) Elongation of the Fe particle via extrusion forces caused by the CNx precipitation through the metal particle along the Fe (1 0 0) planes, thus forming a conical shape. (v) Crystalline CNx material is extruded faster on the outside of the particle, and slow precipitation of carbon occurs at the inner particle region. It is believed that N2 molecules are also rapidly extruded at the other end of the metal particle. The different precipitation rates of CNx generate the fast formation of the outer cylinders and the fast encapsulation of CNx and N2 species in the tube cores (compartments are created by the slow precipitation of CNx and lack of material to keep inner nanotube to grow; these instabilities cause sudden closure of the inner layers).
172
M. Reyes-Reyes et al. / Chemical Physics Letters 396 (2004) 167–173
(root growth mechanism) using these FeOx clusters as reacting sites that are able to precipitate concentric graphene cylinders at the opposite end. SEM images of nanotubes grown on SiO2 substrate (Fig. 5a) suggest that the Fe from the gas phase first fragments from the ferrocene molecules and subsequently condenses on the SiOx surface, forming a type of amorphous Fe–FeOx layer consisting of uniformly distributed particles deposited on the substrate. In Fig. 5a, it is possible to observe that isolated nanotube tips are always present closer to the SiOx substrate. Additional TEM studies showed a bundle of aligned nanotubes containing well defined elongated Fe particles at one end (Fig. 5d), whereas the opposite end presented agglomerated material consisting of amorphous Fe and carbon (Fig. 5c). In our experiments, we noted that the use of benzylamine is responsible for obtaining extremely pure and thin nanotube materials (almost completely free of amorphous carbon) when compared to those reports for benzene–ferrocene or toluene–ferrocene pyolysis using atomisation methods [14,20]. Based on all our observations, we propose the following growth scenario for producing high yields of aligned CNx compartmentalized nanotubes containing large concentrations of nitrogen gas in their cores: (a) Fine and uniform droplets of Ferrocene (Cp2 Fe) and benzylamine (PhCH2NH2) molecules fragment rapidly and create Fe clusters, N2 and H2 molecules, as well as CNx species in the gas phase (Fig. 5e-i). (b) As the Fe clusters grow in size, they start condensing on the SiOx substrate forming nanoparticles (5–10 nm in diameter), which start reacting with gaseous molecules and species present in the environment. We believe that H2 molecules are too volatile and most of them are quickly carried out of the reaction zone, and therefore do not play a key role in the growth of our tubular structures. Similarly some hydrocarbons (CxHy) produced inside the furnace will be expelled relatively fast. (c) The substrate surfaces of the Fe nanoparticles establish strong bonds with the silica substrate via the formation of FeOx–SiOx amorphous regions. These regions result in the fixation of more-or-less homogeneous Fe-containing nanoparticles on the quartz tube (Fig. 5e-ii). (d) The exposed Fe particles then start reacting exothermally with the CNx species. Since one end of the particles is well fixed to the substrate, the Fe particle remains fixed at the bottom and the CNx species are extruded upwards, in the form of crystalline cylinders consisting of CNx, at the opposite side of the particle (Fig. 5e-iii).
(e) The extrusion forces of CNx species and the fixation of one end of the metal to the substrate, result in the elongation of the Fe particle thus forming a conical shape (Fig. 5e-iii). (f) During precipitation across the conical metal Fe particle, CNx species diffuse along the metal (1 0 0) planes and crystalline material is extruded faster on the outside of the particle (fast deposition growth). During CNx precipitation, we believe that N2 molecules are also rapidly extruded at the other end of the metal particle (Fig. 5e-iv). Simultaneously, CNx units diffuse along the inner regions (denser) of the particle at a smaller rate (low CNx precipitation velocity). These different precipitation rates generate the fast formation of the outer cylinders. (g) Due to the slow precipitation of the inner cylinders, the fast growth of the outer shells, and the presence of CNx and N2 species. It is very likely that numerous gaseous molecules are trapped in the interior of the tube core and the graphene-like inner shells left behind recombine and achieve closure by the introduction of pentagonal rings and CNx defects (Fig. 5e-v). (h) The closure of these inner shells result in the compartmentalized structure of the CNx nanotube. The trapping of gases occurs due to the fast growth of the outer layers and slow precipitation of CNx and N2 material in the inner tube regions (Fig. 5e-v). Therefore, the elongated shape of the metal particle, the fast extrusion of N2 molecules from the Fe particles, and the extremely fast growth of the outer shells are able to generate these fascinating bamboo-like CNx nanotubes with high yields of encapsulated gaseous nitrogen. We should emphasize that the pyrolysis of the same (benzylamine/ferroceno) solution, injected into the furnace using a spraying jet [20] with different concentrations (0.2 g ferrocene in 2 ml benzylamine), always resulted in a: (a) poor encapsulation of gaseous N2 inside the CNx tube cores, and (b) less degree of alignment of the grown nanotubes, when compared to those obtained in the present work. Various processing parameters affect the characteristics of the final products. For example, the size of the catalytic particle is very important for the dimensions and morphology of the produced nanotube. In this context, Rao et al. [23] proposed that when the particle has dimensions between 10 and 50 nm, multi-walled nanotubes are formed. In our experiments, the size of the aerosol droplet, the liquid flow and the coalescence kinetics occurring with the components of the solution play a significant role in the alignment and crystallinity of the nanotube products during pyrolysis [14].
M. Reyes-Reyes et al. / Chemical Physics Letters 396 (2004) 167–173
4. Conclusions Pyrolysis of homogenous aerosols has been carried out in order to obtain high yields of well-aligned CNx nanotubes containing large concentrations of molecular N2 in their cores. All tubes exhibited a compartmentalized bamboo-like structure with Fe elongated particles at their tips. Crystalline sheets of graphene always surrounded the Fe-fillings inside the nanotubes, and it is possible that Fe3C at the metal–carbon interface is also formed. We envisage this technique as a promising way to synthesize relatively long, thin and ÔcleanÕ nanotubes in high yields. The proposed growth mechanism in these systems occurs via a Ôroot-growthÕ process in which the presence of N2 and CNx species in the gaseous phase is crucial. It is also clear that the material is able to encapsulate large number of gaseous molecules, thus opening new avenues in the storage of heavy gases at high pressures inside CNx nanotubes. Acknowledgements We are indebted to Martine Mayne for her support and encouragement to work on these production meth´ rez and Lisette ods. We are grateful to Daniel Ramı Noyola for technical assistance. We also thank CONACYT-Me´xico for a Ph.D. scholarship (M.R.R.), and Grants: W-8001-millennium initiative (H.T., M.T.), G25851-E (H.T., M.T.), 36365-E (H.T.), 37589-U (M.T.), 41464-Inter American Collaboration (M.T.).
References [1] M. Terrones, Ann. Rev. Mater. Res. 33 (2003) 419.
173
[2] F. Banhart, N. Grobert, M. Terrones, J.-C. Charlier, P.M. Ajayan, Int. J. Mod. Phys. B 15 (2001) 4037. [3] J. Sloan, A.I. Kirkland, J.L. Hutchison, M.L.H. Green, Chem. Commun. 13 (2002) 1319. [4] P.M. Ajayan, S. Iijima, Nature 361 (1993) 333. [5] F. Kassubek, C.A. Stafford, H. Grabert, Phys. Rev. B-Condens. Matter 59 (11) (1999) 7560. [6] N. Grobert, W.K. Hsu, Y.Q. Zhu, J.P. Hare, H.W. Kroto, D.R.M. Walton, M. Terrones, H. Terrones, P. Redlich, M. Ru¨hle, R. Escudero, F. Morales, Appl. Phys. Lett. 75 (1999) 3363. [7] C.A. Ross, Annu. Rev. Mater. Res. 31 (2001) 203. [8] C.M. Lieber, Sci. Am. 285 (2001) 58. [9] G.E. Gadd, M. Blackford, S. Moricca, N. Webb, P.J. Evans, A.M. Smith, G. Jacobsen, S. Leung, A. Day, Q. Hua, Science 277 (1997) 933. [10] A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Heben, Nature 386 (1997) 377. [11] C. Liu, Y.Y. Fan, M. Liu, H.T. Cong, H.M. Cheng, M.S. Dresselhaus, Science 286 (1999) 1127. [12] M. Terrones, R. Kamalakaran, T. Seeger, M. Ru¨hle, Chem. Commun. 12 (2000) 2335. [13] S. Trasobares, O. Ste´phan, C. Colliex, W.K. Hsu, H.W. Kroto, D.R.M. Walton, J. Chem. Phys. 116 (2002) 8966. [14] M. Mayne, N. Grobert, M. Terrones, R. Kamalakaran, M. Ru¨hle, H.W. Kroto, D.R.M. Walton, Chem. Phys. Lett. 338 (2001) 101. [15] M. Glerup, H. Kanzow, R. Almairac, M. Castignolles, P. Bernier, Chem. Phys. Lett. 377 (2003) 293. [16] M. Mayne, et al. (in preparation). [17] B.C. Satishkumar, A. Govindaraj, C.N.R. Rao, Chem. Phys. Lett. 307 (1999) 158. [18] M. Terrones, Ph. Redlich, N. Grobert, S. Trasobares, W.K. Hsu, H. Terrones, Y.Q. Zhu, J.P. Hare, A.K. Cheetham, M. Ru¨hle, H.W. Kroto, D.R.M. Walton, Adv. Mater. 11 (1999) 655. [19] M. Terrones, H. Terrones, N. Grobert, W.K. Hsu, Y.Q. Zhu, H.W. Kroto, D.R.M. Walton, Ph. Kohler-Redlich, M. Ru¨hle, J.P. Zhang, A.K. Cheetham, Appl. Phys. Lett. 75 (1999) 3932. [20] R. Kamalakaran, M. Terrones, T. Seeger, Ph. Kohler-Redlich, M. Ru¨hle, Y.A. Kim, T. Hayashi, M. Endo, Appl. Phys. Lett. 77 (2000) 3385. [21] Ch. Singh, M.S.P. Shaffer, A.H. Windle, Carbon 41 (2003) 359. [22] T. Katayama, H. Araki, K. Yoshino, J. Appl. Phys. 91 (10) (2002) 6675. [23] C.N.R. Rao, B.C. Satishkumar, A. Govindaraj, M. Nath, Chem. Phys. Chem. 2 (2001) 78.