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Synthesis of nanocomposites based on nanotubes and silicates
Applied Surface Science 258 (2012) 2540–2543
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis of nanocomposites based on nanotubes and silicates ˇ J. Breza a,∗ , K. Pastorková b , M. Kadleˇcíková a , K. Jesenák b , M. Caploviˇ cová b , M. Kolmaˇcka a , F. Laziˇst’an a a b
Slovak University of Technology, Ilkoviˇcova 3, 812 19 Bratislava, Slovakia Comenius University, Mlynská dolina CH-2, 842 15 Bratislava, Slovakia
a r t i c l e
i n f o
Article history: Received 31 May 2011 Received in revised form 18 October 2011 Accepted 18 October 2011 Available online 21 October 2011 Keywords: Carbon nanotube Aluminosilicate Zeolite Montmorillonite
a b s t r a c t In situ synthesis of nanocomposites based on carbon nanotubes and zeolite/montmorillonite was carried out in a hot filament CVD reactor where the precursors (methane and hydrogen) are activated by carbonized tungsten filaments heated up to 2200 ◦ C. In nanocomposites formed both on zeolite and montmorillonite we observed cross-linking of the catalytic particles by nanotubes and creation of carbon nanotube bridges and three-dimensional networks. The length of nanotube bridges was in a range from several nm to nearly 10 m. A high density of carbon nanotubes was observed in the whole volume of zeolite. The high catalytic efficiency of zeolite is most likely caused by its structure that allows anchoring of Fe3+ catalytic particles in the pores and prevents their migration from the sample. At the ends of the nanotubes grown on zeolite we observed particles of the catalyst. In montmorillonite, the particles catalyzing the growth of carbon nanotubes may be present not only on the external surface but also in the interlayer voids of the mineral. Its catalytic efficiency is enhanced as proved by the higher amount of CNTs and their bundles. In the course of CNTs synthesis probably also clumps of Fe3+ catalytic particles arise, which may be the reason for formation of bundles of nanotubes. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Electrical, thermal, optical and mechanical properties of carbon nanotubes (CNTs) have been in the focus of interest since their discovery at the beginning of the nineties [1]. Nowadays, nanocomposites with CNTs constitute one of the most rapidly developing areas of research of filamentous nanomaterials. Potential applications of nanocomposites based on CNTs and silicates include nanoporous filters, heat insulators, protecting materials for low-molecule gases, selective adsorbents or lubricants and light materials applicable for various purposes. The future prospects appear to be the so-called green nanocomposites, silicate nanocomposites [2] and biologically degradable carbon polymers in which both the catalytic and ion-exchange functions of the silicate matrix can be utilized [3]. Highly prospective is creation of cross-linked CNTs + silicate materials or nanocomposites on the basis of CNT grids and of a polymer [4]. The matrix of the nanocomposite consists of silicates – zeolite and montmorillonite. Whereas zeolite has a spatial threedimensional structure, see Fig. 1, montmorillonite has a layered structure as shown in Fig. 2. For synthesis, these substances provide both their outer surface and the unique types of internal structure. Zeolite and
∗ Corresponding author. Tel.: +421 2 60291328; fax: +421 2 65423480. E-mail address: [email protected] (J. Breza). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.10.089
montmorillonite are aluminosilicates, compounds containing SiO4 4− and AlO4 5− tetrahedrons. The ion exchange reaction takes place in such a way that cations present in the voids of the skeleton may be replaced by other cations, which is achieved by immersing zeolite into a solution containing respective cations, in our case Fe3+ . The layered structure of montmorillonite consists of one octahedral and two tetrahedral sheets. Since the atom of Al has an oxidation degree of III and replaces an atom of Si with oxidation degree IV, the aluminosilicate layers of montmorillonite obtain positive charge which is compensated by cations contained in the interlayer space. For montmorillonite it is typical that the whole charge is located on the octahedral mesh. Montmorillonite has the ability to increase its volume several times by absorption of water. The existence of negative charge on the sheets of montmorillonite has a marked consequence, namely that in water sediments the colloid particles of montmorillonite behave as anions with ion exchange properties. Hence, in a water solution of ferrous salts montmorillonite may be enhanced by ferrous ions and in this way, similarly like in zeolite, one may get a matrix containing Fe3+ (but also Fe2+ ) catalyst. In the case of synthesis of carbon nanotubes, particles of the catalysts represent reaction centres with a high probability of the growth of CNTs. The essence of the catalytic action of the catalyst is a change of sp3 hybridized carbon in the precursor (in our case in methane) into sp2 hybridized carbon in the structure of the CNT. The growth mechanisms of CNTs on nanoparticles of the catalyst are still not quite explained. The view of the mechanism of
J. Breza et al. / Applied Surface Science 258 (2012) 2540–2543
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concentrations: 0.4 M, 0.04 M and 0.004 M/100 g of silicate. The suspensions were stirred for 12 h and afterwards deposited on a polished Si wafer and allowed to dry quickly under an infrared lamp. In situ synthesis of carbon nanotubes and nanocomposites was carried out in a hot filament (HF) CVD reactor, where the precursors are activated by five carbonized tungsten filaments heated up to 2200 ◦ C. The working atmosphere was a mixture of methane and hydrogen. The pressure and temperature during deposition were 3000 Pa and approx. 600 ◦ C, respectively, and the synthesis time was 30 min. 3. Results
Fig. 1. The structure of zeolite consists of SiO4 4− and AlO4 5− tetrahedrons interconnected via oxygen atoms. The structure does not contain mutual connections of two AlO4 5− tetrahedrons [5].
CNT synthesis supported by the catalyst is as follows. At elevated temperature, carbon from the molecules of the activated gas is dissolved in the metallic catalyst, whereas hydrogen is released and part of the carbon atoms diffuse into the catalyst. Since the solubility of carbon in transition metals, such as iron, is finite, after a certain time saturation is reached and the surplus of carbon begins crystallizing in the form of nanotubes. During the growth of CNTs the catalyst is in a metastable state. An important role in the growth belongs also to interaction of the substrate (matrix) with the particles of the catalyst. In this respect, two growth modes are distinguished: the base or root growth, and the tip growth. More details on the growth mechanisms and on their predominance can be found in [7–9]. 2. Experimental materials and conditions The current experience and previous works of the authors [10,11] show that synthesis of CNTs can be performed on silicates with both layered and spatial structures. Herein, objective is to synthesize nanocomposites based on CNTs and zeolite as well as on CNTs and montmorillonite, and to study their properties. Metal infiltrated samples were prepared by immersing respective silicates into an aqueous solution of Fe(NO3 )3 ·9H2 O with different
Nanocomposites based on carbon nanotubes and silicates were obtained on both of the prepared substrates. Both the base and root growth modes were observed on in situ created nanocomposites. In the base growth, the catalyst is fixed at the surface of the substrate whereas in the tip growth mode the growing nanotubes lift the catalyst from the substrate. In case the forces between the catalyst and substrate are strong, the base growth takes place. The carbon saturated particle of the metal stays bonded to the substrate and excessive carbon is expelled into the space. If the interaction is weak, the metallic particle is lifted by the crystallizing carbon into the space, thus the growing nanotube pushes it in front of itself. The carbon nanotubes in both types of nanocomposites (based on zeolite and montmorillonite) are mutually interlaced, the pores and interlayer spaces are grown through by nanotubes. In characterizing the nanocomposites we focused on their morphology, on the type of CNTs and their alignment, on the presence of defects in their structure and of other carbon phases and on the homogeneity of the deposit. The phase of carbon on the silicate surface and the growth of CNTs inside the matrix were examined by scanning and transmission electron microscopies (SEM, TEM) and Raman spectroscopy. SEM measurements showed images of carbon nanotube bridges and networks in nanocomposites. Fig. 3 shows carbon nanotube bridges grown on Fe-zeolite. SEM micrographs confirmed the high catalytic efficiency of iron pre-treated zeolite. A high density of CNTs was observed in the whole volume of zeolite. The high catalytic efficiency is most likely caused by the structure of zeolite that allows anchoring of Fe3+ catalytic particles in the pores and prevents their migration from the sample. Fig. 4 is a TEM image of multi-walled carbon nanotubes grown on zeolite. At the ends of the nanotubes one can see particles of the catalyst. In both types of nanocomposites (on zeolite and montmorillonite) we observed also cross-linking of the catalytic particles by the nanotubes. Figs. 5 and 6 are SEM images of the bundles of CNTs
Fig. 2. The structure of montmorillonite [6].
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Fig. 3. SEM image of carbon nanotube bridges grown on Fe-zeolite pre-treated with Fe(NO3 )3 ·9H2 O. The green horizontal bar at the bottom denotes the size of 10 m. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Fig. 5. SEM image of the bundles of CNT grown on Fe-montmorillonite pre-treated with Fe(NO3 )3 ·9H2 O.
grown on Fe-montmorillonite pre-treated with Fe(NO3 )3 ·9H2 O. Since in the case of montmorillonite the particles catalyzing the growth of CNTs may be present not only on the external surface but also in the interlayer voids of the mineral, its catalytic efficiency is enhanced as proved by the higher amount of CNTs and their bundles. In the course of CNTs synthesis probably also clumps of Fe3+ catalytic particles arise, which may be the reason for formation of bundles of nanotubes. Fig. 7 shows creation of amorphous carbon on the surface of CNTs. The presence of the amorphous phase was proved by Raman spectroscopy. Amorphous carbon is an undesirable phase on CNTs, on the other hand it proves the adsorption potential of the surface of CNTs, see Fig. 8.
Fig. 6. Detail SEM image of the bundles of carbon nanotubes.
Fig. 4. TEM image of CNTs grown on Fe-zeolite. At the ends of multi-walled carbon nanotubes one can see particles of the catalyst. The particles of the catalyst are enclosed within the nanotube. We assume that on enclosing the catalyst the growth terminates. The white vertical bar on the right margin denotes the size of 200 nm.
Fig. 7. SEM image of carbon nanotube bridges on montmorillonite pre-treated by Fe(NO3 )3 ·9H2 O. Amorphous carbon is growing on the bridges and on the nanotubes grown in the matrix.
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Acknowledgements This work has been supported by Grant VEGA 1/0746/09 and Grants UK/273/2009 and UK/534/2010, and conducted in the Centre of Excellence CENAMOST. References
Fig. 8. TEM image visualizing a nude nanotube grown by the tip mode. The image illustrates the adsorption efficiency of carbon nanotubes. The thickness of coverage by amorphous carbon is larger than the nanotube itself.
4. Conclusion Nanocomposites based on CNTs and aluminosilicates were produced in situ by HF CVD. Zeolite and montmorillonite were used as substrates for immobilizing the catalytic particles of iron ions. The deposited CNTs formed a three-dimensional grid. Nanotube bridges with a length of several nm to nearly 10 m were created on both types of substrates. TEM and Raman measurements showed the presence of amorphous carbon on the surface of multi-walled nanotubes. Both base and tip growth modes were observed. Formation of carbon nanotube bridges and grids in the material can provide a promising way to produce new filamentous nanocomposites with improved mechanical and absorption properties.
[1] S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56–58. [2] S.S. Ray, M. Bousmina, Biodegradable polymers and their layered silicate nano composites: in greening the 21st century materials world, Progr. Mater. Sci. 50 (2005) 962–1079. [3] E.A. Stefanescu, C. Daranga, C. Stefanescu, Insight into the broad field of polymer nanocomposites: from carbon nanotubes to clay nanoplatelets, via metal nanoparticles, Materials 2 (2009) 2095–2153. [4] J.Y. Kim, Carbon nanotube-reinforced thermotropic liquid crystal polymer nanocomposites, Materials 2 (2009) 1955–1974. [5] http://www.isgtw.org/images/zeolite 1 large.jpg (accessed 30.05.2011). [6] S. Andrejkoviˇcová, PhD Thesis, Slovak Academy of Sciences, Bratislava, Slovakia, 2008. [7] C. Bower, O. Zhoub, W. Zhu, D.J. Werder, S. Jin, Nucleation and growth of carbon nanotubes by microwave plasma chemical vapor deposition, Appl. Phys. Lett. 77 (2000) 2767–2769. [8] S. Huang, M. Woodson, R. Smalley, J. Liu, Growth mechanism of oriented long single walled carbon nanotubes using fast-heating chemical vapor deposition process, Nano Lett. 4 (2004) 1025–1028. [9] A. Gohier, C.P. Ewels, T.M. Minea, M.A. Djouadi, Carbon nanotube growth mechanism switches from tip- to base-growth with decreasing catalyst particle size, Carbon 46 (2008) 1331–1338. [10] M. Kadleˇcíková, J. Breza, K. Jesenák, K. Pastorková, V. Luptáková, M. Kolmaˇcka, A. Vojaˇcková, M. Michalka, I. Vávra, Z. Kriˇzanová, The growth of carbon nanotubes on montmorillonite and zeolite (clinoptilolite), Appl. Surf. Sci. 254 (2008) 5073–5079. [11] K. Pastorková, F. Laziˇst’an, M. Kadleˇcíková, J. Breza, K. Jesenák, M. Kolmaˇcka, M. Michalka, Connections and bridges of carbon nanotubes grown on aluminosilicates (namely: montmorillonite and zeolite–clinoptilolite), in: Proc. of the 2nd NANOCON International Conference, Olomouc, Czech Republic, 2010, pp. 385–387.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis of nanocomposites based on nanotubes and silicates ˇ J. Breza a,∗ , K. Pastorková b , M. Kadleˇcíková a , K. Jesenák b , M. Caploviˇ cová b , M. Kolmaˇcka a , F. Laziˇst’an a a b
Slovak University of Technology, Ilkoviˇcova 3, 812 19 Bratislava, Slovakia Comenius University, Mlynská dolina CH-2, 842 15 Bratislava, Slovakia
a r t i c l e
i n f o
Article history: Received 31 May 2011 Received in revised form 18 October 2011 Accepted 18 October 2011 Available online 21 October 2011 Keywords: Carbon nanotube Aluminosilicate Zeolite Montmorillonite
a b s t r a c t In situ synthesis of nanocomposites based on carbon nanotubes and zeolite/montmorillonite was carried out in a hot filament CVD reactor where the precursors (methane and hydrogen) are activated by carbonized tungsten filaments heated up to 2200 ◦ C. In nanocomposites formed both on zeolite and montmorillonite we observed cross-linking of the catalytic particles by nanotubes and creation of carbon nanotube bridges and three-dimensional networks. The length of nanotube bridges was in a range from several nm to nearly 10 m. A high density of carbon nanotubes was observed in the whole volume of zeolite. The high catalytic efficiency of zeolite is most likely caused by its structure that allows anchoring of Fe3+ catalytic particles in the pores and prevents their migration from the sample. At the ends of the nanotubes grown on zeolite we observed particles of the catalyst. In montmorillonite, the particles catalyzing the growth of carbon nanotubes may be present not only on the external surface but also in the interlayer voids of the mineral. Its catalytic efficiency is enhanced as proved by the higher amount of CNTs and their bundles. In the course of CNTs synthesis probably also clumps of Fe3+ catalytic particles arise, which may be the reason for formation of bundles of nanotubes. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Electrical, thermal, optical and mechanical properties of carbon nanotubes (CNTs) have been in the focus of interest since their discovery at the beginning of the nineties [1]. Nowadays, nanocomposites with CNTs constitute one of the most rapidly developing areas of research of filamentous nanomaterials. Potential applications of nanocomposites based on CNTs and silicates include nanoporous filters, heat insulators, protecting materials for low-molecule gases, selective adsorbents or lubricants and light materials applicable for various purposes. The future prospects appear to be the so-called green nanocomposites, silicate nanocomposites [2] and biologically degradable carbon polymers in which both the catalytic and ion-exchange functions of the silicate matrix can be utilized [3]. Highly prospective is creation of cross-linked CNTs + silicate materials or nanocomposites on the basis of CNT grids and of a polymer [4]. The matrix of the nanocomposite consists of silicates – zeolite and montmorillonite. Whereas zeolite has a spatial threedimensional structure, see Fig. 1, montmorillonite has a layered structure as shown in Fig. 2. For synthesis, these substances provide both their outer surface and the unique types of internal structure. Zeolite and
∗ Corresponding author. Tel.: +421 2 60291328; fax: +421 2 65423480. E-mail address: [email protected] (J. Breza). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.10.089
montmorillonite are aluminosilicates, compounds containing SiO4 4− and AlO4 5− tetrahedrons. The ion exchange reaction takes place in such a way that cations present in the voids of the skeleton may be replaced by other cations, which is achieved by immersing zeolite into a solution containing respective cations, in our case Fe3+ . The layered structure of montmorillonite consists of one octahedral and two tetrahedral sheets. Since the atom of Al has an oxidation degree of III and replaces an atom of Si with oxidation degree IV, the aluminosilicate layers of montmorillonite obtain positive charge which is compensated by cations contained in the interlayer space. For montmorillonite it is typical that the whole charge is located on the octahedral mesh. Montmorillonite has the ability to increase its volume several times by absorption of water. The existence of negative charge on the sheets of montmorillonite has a marked consequence, namely that in water sediments the colloid particles of montmorillonite behave as anions with ion exchange properties. Hence, in a water solution of ferrous salts montmorillonite may be enhanced by ferrous ions and in this way, similarly like in zeolite, one may get a matrix containing Fe3+ (but also Fe2+ ) catalyst. In the case of synthesis of carbon nanotubes, particles of the catalysts represent reaction centres with a high probability of the growth of CNTs. The essence of the catalytic action of the catalyst is a change of sp3 hybridized carbon in the precursor (in our case in methane) into sp2 hybridized carbon in the structure of the CNT. The growth mechanisms of CNTs on nanoparticles of the catalyst are still not quite explained. The view of the mechanism of
J. Breza et al. / Applied Surface Science 258 (2012) 2540–2543
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concentrations: 0.4 M, 0.04 M and 0.004 M/100 g of silicate. The suspensions were stirred for 12 h and afterwards deposited on a polished Si wafer and allowed to dry quickly under an infrared lamp. In situ synthesis of carbon nanotubes and nanocomposites was carried out in a hot filament (HF) CVD reactor, where the precursors are activated by five carbonized tungsten filaments heated up to 2200 ◦ C. The working atmosphere was a mixture of methane and hydrogen. The pressure and temperature during deposition were 3000 Pa and approx. 600 ◦ C, respectively, and the synthesis time was 30 min. 3. Results
Fig. 1. The structure of zeolite consists of SiO4 4− and AlO4 5− tetrahedrons interconnected via oxygen atoms. The structure does not contain mutual connections of two AlO4 5− tetrahedrons [5].
CNT synthesis supported by the catalyst is as follows. At elevated temperature, carbon from the molecules of the activated gas is dissolved in the metallic catalyst, whereas hydrogen is released and part of the carbon atoms diffuse into the catalyst. Since the solubility of carbon in transition metals, such as iron, is finite, after a certain time saturation is reached and the surplus of carbon begins crystallizing in the form of nanotubes. During the growth of CNTs the catalyst is in a metastable state. An important role in the growth belongs also to interaction of the substrate (matrix) with the particles of the catalyst. In this respect, two growth modes are distinguished: the base or root growth, and the tip growth. More details on the growth mechanisms and on their predominance can be found in [7–9]. 2. Experimental materials and conditions The current experience and previous works of the authors [10,11] show that synthesis of CNTs can be performed on silicates with both layered and spatial structures. Herein, objective is to synthesize nanocomposites based on CNTs and zeolite as well as on CNTs and montmorillonite, and to study their properties. Metal infiltrated samples were prepared by immersing respective silicates into an aqueous solution of Fe(NO3 )3 ·9H2 O with different
Nanocomposites based on carbon nanotubes and silicates were obtained on both of the prepared substrates. Both the base and root growth modes were observed on in situ created nanocomposites. In the base growth, the catalyst is fixed at the surface of the substrate whereas in the tip growth mode the growing nanotubes lift the catalyst from the substrate. In case the forces between the catalyst and substrate are strong, the base growth takes place. The carbon saturated particle of the metal stays bonded to the substrate and excessive carbon is expelled into the space. If the interaction is weak, the metallic particle is lifted by the crystallizing carbon into the space, thus the growing nanotube pushes it in front of itself. The carbon nanotubes in both types of nanocomposites (based on zeolite and montmorillonite) are mutually interlaced, the pores and interlayer spaces are grown through by nanotubes. In characterizing the nanocomposites we focused on their morphology, on the type of CNTs and their alignment, on the presence of defects in their structure and of other carbon phases and on the homogeneity of the deposit. The phase of carbon on the silicate surface and the growth of CNTs inside the matrix were examined by scanning and transmission electron microscopies (SEM, TEM) and Raman spectroscopy. SEM measurements showed images of carbon nanotube bridges and networks in nanocomposites. Fig. 3 shows carbon nanotube bridges grown on Fe-zeolite. SEM micrographs confirmed the high catalytic efficiency of iron pre-treated zeolite. A high density of CNTs was observed in the whole volume of zeolite. The high catalytic efficiency is most likely caused by the structure of zeolite that allows anchoring of Fe3+ catalytic particles in the pores and prevents their migration from the sample. Fig. 4 is a TEM image of multi-walled carbon nanotubes grown on zeolite. At the ends of the nanotubes one can see particles of the catalyst. In both types of nanocomposites (on zeolite and montmorillonite) we observed also cross-linking of the catalytic particles by the nanotubes. Figs. 5 and 6 are SEM images of the bundles of CNTs
Fig. 2. The structure of montmorillonite [6].
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Fig. 3. SEM image of carbon nanotube bridges grown on Fe-zeolite pre-treated with Fe(NO3 )3 ·9H2 O. The green horizontal bar at the bottom denotes the size of 10 m. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Fig. 5. SEM image of the bundles of CNT grown on Fe-montmorillonite pre-treated with Fe(NO3 )3 ·9H2 O.
grown on Fe-montmorillonite pre-treated with Fe(NO3 )3 ·9H2 O. Since in the case of montmorillonite the particles catalyzing the growth of CNTs may be present not only on the external surface but also in the interlayer voids of the mineral, its catalytic efficiency is enhanced as proved by the higher amount of CNTs and their bundles. In the course of CNTs synthesis probably also clumps of Fe3+ catalytic particles arise, which may be the reason for formation of bundles of nanotubes. Fig. 7 shows creation of amorphous carbon on the surface of CNTs. The presence of the amorphous phase was proved by Raman spectroscopy. Amorphous carbon is an undesirable phase on CNTs, on the other hand it proves the adsorption potential of the surface of CNTs, see Fig. 8.
Fig. 6. Detail SEM image of the bundles of carbon nanotubes.
Fig. 4. TEM image of CNTs grown on Fe-zeolite. At the ends of multi-walled carbon nanotubes one can see particles of the catalyst. The particles of the catalyst are enclosed within the nanotube. We assume that on enclosing the catalyst the growth terminates. The white vertical bar on the right margin denotes the size of 200 nm.
Fig. 7. SEM image of carbon nanotube bridges on montmorillonite pre-treated by Fe(NO3 )3 ·9H2 O. Amorphous carbon is growing on the bridges and on the nanotubes grown in the matrix.
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Acknowledgements This work has been supported by Grant VEGA 1/0746/09 and Grants UK/273/2009 and UK/534/2010, and conducted in the Centre of Excellence CENAMOST. References
Fig. 8. TEM image visualizing a nude nanotube grown by the tip mode. The image illustrates the adsorption efficiency of carbon nanotubes. The thickness of coverage by amorphous carbon is larger than the nanotube itself.
4. Conclusion Nanocomposites based on CNTs and aluminosilicates were produced in situ by HF CVD. Zeolite and montmorillonite were used as substrates for immobilizing the catalytic particles of iron ions. The deposited CNTs formed a three-dimensional grid. Nanotube bridges with a length of several nm to nearly 10 m were created on both types of substrates. TEM and Raman measurements showed the presence of amorphous carbon on the surface of multi-walled nanotubes. Both base and tip growth modes were observed. Formation of carbon nanotube bridges and grids in the material can provide a promising way to produce new filamentous nanocomposites with improved mechanical and absorption properties.
[1] S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56–58. [2] S.S. Ray, M. Bousmina, Biodegradable polymers and their layered silicate nano composites: in greening the 21st century materials world, Progr. Mater. Sci. 50 (2005) 962–1079. [3] E.A. Stefanescu, C. Daranga, C. Stefanescu, Insight into the broad field of polymer nanocomposites: from carbon nanotubes to clay nanoplatelets, via metal nanoparticles, Materials 2 (2009) 2095–2153. [4] J.Y. Kim, Carbon nanotube-reinforced thermotropic liquid crystal polymer nanocomposites, Materials 2 (2009) 1955–1974. [5] http://www.isgtw.org/images/zeolite 1 large.jpg (accessed 30.05.2011). [6] S. Andrejkoviˇcová, PhD Thesis, Slovak Academy of Sciences, Bratislava, Slovakia, 2008. [7] C. Bower, O. Zhoub, W. Zhu, D.J. Werder, S. Jin, Nucleation and growth of carbon nanotubes by microwave plasma chemical vapor deposition, Appl. Phys. Lett. 77 (2000) 2767–2769. [8] S. Huang, M. Woodson, R. Smalley, J. Liu, Growth mechanism of oriented long single walled carbon nanotubes using fast-heating chemical vapor deposition process, Nano Lett. 4 (2004) 1025–1028. [9] A. Gohier, C.P. Ewels, T.M. Minea, M.A. Djouadi, Carbon nanotube growth mechanism switches from tip- to base-growth with decreasing catalyst particle size, Carbon 46 (2008) 1331–1338. [10] M. Kadleˇcíková, J. Breza, K. Jesenák, K. Pastorková, V. Luptáková, M. Kolmaˇcka, A. Vojaˇcková, M. Michalka, I. Vávra, Z. Kriˇzanová, The growth of carbon nanotubes on montmorillonite and zeolite (clinoptilolite), Appl. Surf. Sci. 254 (2008) 5073–5079. [11] K. Pastorková, F. Laziˇst’an, M. Kadleˇcíková, J. Breza, K. Jesenák, M. Kolmaˇcka, M. Michalka, Connections and bridges of carbon nanotubes grown on aluminosilicates (namely: montmorillonite and zeolite–clinoptilolite), in: Proc. of the 2nd NANOCON International Conference, Olomouc, Czech Republic, 2010, pp. 385–387.