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Synthesis of N-doped SWNT using the arc-discharge procedure
Chemical Physics Letters 387 (2004) 193–197 www.elsevier.com/locate/cplett
Synthesis of N-doped SWNT using the arc-discharge procedure M. Glerup a
d
a,b,*
, J. Steinmetz a,b, D. Samaille a, O. Stephan c, S. Enouz d, A. Loiseau d, S. Roth b, P. Bernier a
GDPC (UMR5581), Universite Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex 5, France b Max-Planck-Institut f€ ur Festk€ orperforschung, Heisenbergstrasse 1, 70569 Stuttgart, Germany c LPS, Universite Paris-Sud, Bat. 510, 91405 Orsay Cedex, France LEM, ONERA-CNRS (UMR104), 29 av. de la Division Leclerc, BP72, 92322 Ch^atillon Cedex, France Received 6 November 2003; in final form 6 November 2003 Published online:
Abstract Single-walled nitrogen doped nanotubes were prepared using two novel approaches in an arc-discharge process. In both cases a nitrogen-rich precursor was introduced into the anode rods together with graphite and the catalysts. The nitrogen rich precursors were organic and inorganic, respectively. Both synthesis routes gave nanotubes with a nitrogen concentration of a maximum of 1%, determined using electron energy loss spectroscopy (EELS) coupled with transmission electron microscopy (TEM). All samples were characterized with scanning electron microscopy (SEM), TEM, and high-resolution transmission electron microscopy (HRTEM). Ó 2004 Published by Elsevier B.V.
1. Introduction Carbon nanotubes have attracted much attention since their discovery by Iijima [1]. They are expected to bring significant breakthroughs in the electronic and mechanical engineering of materials. Doping carbon nanotubes with other chemical elements could be a particular interesting way for tuning these properties [2,3]. The electronic properties of single-walled carbon nanotubes (SWNTs) vary between semiconducting and metallic, depending on their helicity [4]. Nitrogendoped nanotubes are predicted to be either metallic or small gap semiconductors depending on the relative positions of the nitrogen and carbon atoms [5,6]. SWNTs are known to have extraordinary mechanical properties with a YoungÕs modulus of 1.25 Tpa [7]. Based on theoretical calculations, canonical nanotube stoichiometries on CN and C3 N4 have been predicted stable [8]. C3 N4 , iso-electronic to Si3 N4 , has been
*
Corresponding author. Fax: +33-467522504. E-mail address: [email protected] (M. Glerup).
0009-2614/$ - see front matter Ó 2004 Published by Elsevier B.V. doi:10.1016/j.cplett.2004.02.005
predicted to have hardness comparable to that of diamond [9]. Stephan et al. [10] initiated the synthesis of doped multi-walled nanotubes (MWNT) in 1994; they produced boron–nitrogen doped multi-walled carbon nanotubes using the arc-discharge method. Since then, a lot of work has been carried out concerning the direct synthesis of nitrogen and/or boron doped MWNTs in arc-discharge experiments [11–13]. The synthesis of doped single-walled nanotubes in arc-discharge experiment is not easily achieved. A few reports deal with the synthesis of nitrogen-doped SWNTs; the tubes are made by evaporation of the graphite rod together with the catalyst in a nitrogen gas-containing atmosphere [14]. Otherwise, the formation of single-walled CBx and CNy using substitution reactions with single-walled carbon nanotubes (C-SWNT) as templates have been described up to now [15]. In this work, we have chosen to take new approaches for the synthesis of nitrogen and/or boron doped SWNTs in arc-discharge experiments. Instead of carrying out the experiment in a nitrogen rich atmosphere we have added nitrogen-rich organic- and inorganic precursors into the graphitic anode-rod.
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M. Glerup et al. / Chemical Physics Letters 387 (2004) 193–197
2. Experimental Three different processes have been worked out: Synthesis process 1: the purpose of this series of experiments was to synthesize nitrogen doped SWNTs. Graphite (1–2 lm, Aldrich) was mixed with melamine (C3 N6 H6 , 99%, Aldrich) and Ni/Y catalyst (Ni: 99.999%, 100 mesh, Aldrich; Y: 99.9%, 40 mesh, Aldrich; Ni ¼ Y ¼ 0.6 w/o) and packed into the drillings of the anode rods. Samples were produced with amounts of melamine corresponding to 1, 2, 4 and 8 at.% nitrogen (denoted sample 1, 2, 3 and 4). The experiments were carried out with the optimized conditions for the synthesis of pure SWNTs. In the experiments with melamine, the direct current of 95–100 A in the arcdischarge was in the same range as in standard experiments, however the voltage was slightly lower. In general, the electric discharge was less stable than in conventional experiments and the distance of the two electrodes had to be adjusted manually. The more precursor used, the more the arc discharge was difficult to stabilize. Once stabilized, the synthesis proceeded as in standard experiments. The collarets produced were smaller than the normal SWNT collarets. Apart from the size, all characteristics of normal collarets could be observed, such as the spongy, nanotube-rich parts. Synthesis process 2: These experiments were carried out to synthesize single-walled CBx Ny nanotubes. Boron nitride powder (99.8%, Matthey Reagents) together with catalyst and graphite was packed into the drillings of the anode rods. Samples were produced with amounts of boron nitride corresponding to 1/1, 2/2, and 4/4 at.% boron and nitrogen (denoted sample 5, 6, and 7). The synthesis conditions were similar to those described above. Again the stability of the arc decreased with an increasing concentration of precursor added. Synthesis process 3: A third series of experiments was carried out to produce boron-doped nanotubes. As above, the 2 or 4 at.% of boron powder (95–97%, Riedel de Ha€en) was mixed with the graphite and the catalysts and filled into the anode rod. This time, it was very difficult to stabilize the arc; the growth of the collarets was impaired resulting in a low yield of nanotubes. Scanning electron microscopy (SEM) pictures were taken for the evaluation of the homogeneity and purity of the samples. SEM pictures were recorded without prior preparation using a Jeol 6300 F at 15 kV (field emission gun). The nanotubes were dispersed in absolute ethanol before preparing the grids for transmission electron microscopy (TEM) imaging (Philips CM20, 200 kV) and for the electron energy loss microscopy (EELS) experiments. For studying the structure of the nanotubes high-resolution transmission electron microscopy (HRTEM) experiments were carried out (Jeol 4000FX, 400/200 kV). A dedicated STEM (VG HB501) with a field emission gun operated at 100 keV and fitted with a
Gatan 666 parallel-EELS spectrometer optically coupled to a CCD camera was used for quantifying the C, N, and B concentrations of the nanotubes bundles.
3. Results and discussion In Fig. 1 are shown representative SEM images from the collaret (Fig. 1a) and for the soot (Fig. 1b) of sample 4. The soot consisted of white–greyish crystalline layer which was identified by means of medium-infrared spectroscopy (MIR) to consist of pure melamine (not shown here), indicating that a part of the melamine evaporates before participating in the reaction. This might not be that surprising since melamine has an evaporation temperature which is several thousand degrees lower than those of graphite. Melamine boils at 350 °C under standard conditions and starts decomposing at >280 °C. Thus, it is necessary to use a much higher concentration of melamine in the anode compared to what will participate in the formation of the tubes. The SEM images are representative for all samples synthesized using melamine as the nitrogen precursor. In Fig. 2 is shown a representative HRTEM image of the same sample and the corresponding EEL spectrum is seen in Fig. 3. From the C-K edge it can be seen that the nanotubes consist of a typical graphitic network with the typical sp2 -type bonding, and that the tubes are very well graphitized [16,17]. The nitrogen content in this sample was quantified to be at around 1 at.%. Theoretical calculations have suggested the existence of three possible ways to incorporate nitrogen, denoted ÔpyrolicÕ, ÔpyridinicÕ, and ÔgraphiticÕ [18,19] after the electronic structure of the nitrogen. In order to optimize the detection of such a low concentration of N atoms in the ropes of SWNTs and to observe the fine structures on the N-K edge, collections of 50 averaged spectra were acquired by rastering a slightly defocused probe over 10 10 nm2 areas. A specific method consisting of the acquisition of shifted spectra on the detector for posterior realignment was used to smooth the detector gain variation. The resulting N-K fine structures are shown in the inset of Fig. 3. The displayed spectrum is the sum of a few hundreds of spectra. In addition to the triangular r band above 405 eV, which is characteristic of C–N materials, two weak features are observed at lower energy at 398.6 and 402 eV. They can be attributed to p -states associated with pyridine-like and graphite-like nitrogen [18,19]. This in principle gives direct information about the electronic structure of the nitrogen atoms and the neighboring carbon atoms. We note that the detection of either pyridinic or graphitic nitrogen in nitrogen doped MWNTs has been commonly reported in the literature. EEL spectra were also measured on the amorphous/graphitic carbon from the collaret of sample 4 (not shown), approximately the
M. Glerup et al. / Chemical Physics Letters 387 (2004) 193–197
195
Fig. 1. SEM images of sample 4: (a) nanotubes taken from the collaret (N ¼ 1 at.%); (b) the soot collected in the reactor. The crystallites observed on this image is un-reacted, re-crystallized melamine.
CK NK
σ*
Instensity (arb. units)
402 eV 398.6 eV
395
4004
05 (eV)
4104
15
NK
250
300
350
400
450
500
Energy Loss (eV)
Fig. 2. HRTEM image of sample 4 (CNx nanotubes, N ¼ 1 at.%).
same or a slightly higher concentration of nitrogen was seen to be incorporated. The HRTEM images of the samples indicate that nitrogen is built into the carbon lattice of individual nanotubes and not intercalated between the tubes in the bundles since tube–tube distance is the same as for undoped SWNTs. Also X-ray diffraction experiments (not shown here) confirmed this observation. The charac1 corresponding to the teristic peak at Q ¼ 0:44 A nanotube bundles was observed as for un-doped SWNTs. We now turn to the samples resulting from synthesis process 2. Fig. 4a is a typical example of the collarets observed in sample 7. All samples contain 1 at.% of nitrogen and no boron was observed in any of the EEL spectra (within our experimental conditions, we estimate the detection limits for boron to be of the order of 0.1
Fig. 3. EEL core electron K-shell spectra of CNx nanotube bundles (sample 4). The nanotubes are doped with around 1 at.% nitrogen. For the C-K edge well defined p and r fine structure features are observed which are evidences of sp2 -hybridisation in graphitic structures. The inset is a magnification of the N-K edge.
at.%). It is observed that using a precursor with a higher evaporation temperature less precursor is needed for the incorporation of 1 at.% nitrogen into the SWNTs. From the SEM images (Fig. 4b) and the TEM investigations it was observed that the tubes were very long compared to samples 1–4 and to undoped SWNTs. Moreover, the nanotubes were more difficult to disperse than nanotubes from standard production. In Fig. 5 are shown representative HRTEM images. In samples 5–7 we observed an unusual high amount of nanoparticles (Fig. 5b). The particles are very well-graphitized onions arranged in chains. The inner shells are almost rectangular and mostly hollow. We correlate the formation of this type of nano-structures to the presence of boron during the synthesis. This is in agreement with similar observations on B-doped laser ablation experiments
196
M. Glerup et al. / Chemical Physics Letters 387 (2004) 193–197
Fig. 4. Representative SEM images of sample 7: (a) the collaret: (b) a very long bundle of SWNTs (>6 lm).
Fig. 5. (a) HRTEM image of sample 7. (b) Chains of mostly hollow carbon onions which are found in samples 4–7.
where morphological changes such as the formation of large numbers of graphitic onions were observed in SWNTs samples for high boron contents [20]. The samples resulting from process 3 did not indicate any incorporation of boron into the nanotube framework. The phenomenon of increased length of samples 5–7 is likely to be attributed to the presence of boron during the synthesis as it has previously been observed for boron-doped multi-walled carbon nanotubes [21,22]. During the formation process of the nanotube, boron atoms are preferably to be found close to the tubeÕs open edge where they hamper the closing of the tubes. This effect of the boron is largest for zig-zag nanotubes which can thus grow to a greater length than tubes with other helicities. Detailed observations will have to be done to check if these peculiarities are also present in the SWNT synthesized in this study. Generally, it was observed that the nitrogen doped single-walled are less stable than pure carbon nanotubes; it is actually possible to cut the bundles while exposing
them to the electron beam of the TEM. Also, while carrying out the EELS experiments it was observed that the nanotubes were more fragile than normally. These increased radiation damage effects might be a consequence of the dopants within the curved honey-comp lattice. Nitrogen dopants can in principle be regarded as defects in the nanotubes and an increased amount of defects in the tubes obviously makes the tubes more fragile. The work presented here confirms that the incorporation of nitrogen and boron in nanotubes is not comparable. No boron was seen to be incorporated in the nanotubes, meaning that it is much more difficult than nitrogen to incorporate into the lattice of the nanotubes. This phenomenon is also known from the synthesis of doped multi-walled nanotubes. Nanotubes containing more than around 5% boron have never been reported in the literature whereas nanotubes with an average concentration on 20 at.% of nitrogen have been synthesized [23]. The theoretical work by Blase et al. [21] and Hernandez et al. [22] explains the role boron plays
M. Glerup et al. / Chemical Physics Letters 387 (2004) 193–197
in the growth of single-walled nanotubes and how it favours the growth of long zig-zag tubes. The increased length of those nanotubes might be a feature which can be used to separate them for the rest of the tubes. This work gives rise to some new intriguing questions: Is the incorporation of boron in single-walled nanotubes possible? Can the use of small amount of chemical elements like boron be a way to synthesize nanotubes with a unique helicity? In all samples, the maximum incorporation of nitrogen was seen to be around 1 at.%, even though much more precursor was added to the synthesis. Does this mean that 1 at.% of nitrogen is the physical maximum? More theoretical and experimental research should be dedicated to this subject.
4. Conclusion It was shown that it is possible to directly synthesize nitrogen-doped nanotubes in an arc-discharge experiment using nitrogen-rich organic and inorganic precursors. This opens new routes for the synthesis of doped SWNTs in high temperature type experiments like arcdischarge and laser-ablation. This approach is seen to be promising though further work has to be carried out for the optimization of this type of experiments. This work demonstrates the possibility of designing experiments for controlling the length and helicity of the singlewalled carbon nanotubes.
Acknowledgements M.G. acknowledges the EC-TMR network FUNCARS for financial support. O.S. acknowledges the EC-IHP network Ôfullerene-like materialsÕ for financial support. Dr. M. Holzinger (UMII) is acknowledged for valuable discussions and D. Maurin (UMII) for the infrared measurements.
197
References [1] S. Iijima, Nature 354 (1991) 56. [2] X. Blase, J.C. Charlier, A. De Vita, R. Car, Appl. Phys. A 68 (1999) 293. [3] E. Hernandez, C. Goze, P. Bernier, A. Rubio, Phys. Rev. Lett. 80 (1998) 4502. [4] A. Hassanien, M. Tokumoto, Y. Kumazawa, H. Kataura, Y. Maniwa, S. Suzuki, Y. Achida, Appl. Phys. Lett. 73 (1998) 3839. [5] Yuanhe Huang, Jipeng Gao, Ruozhuang Liu, Synth. Met. 113 (2000) 251. [6] Yoshiyuki Miyamoto, M.L. Cohen, S.G. Louie, Solid State Commun. 102 (1997) 605. [7] A. Krishnan, E. Dujardin, T.W. Ebbesen, P.N. Yanilos, M.M.J. Treacy, Phys. Rev. B 58 (1998) 14013. [8] Y. Miyamoto, M.L. Cohen, S.G. Louie, Solid State Commun. 102 (2002) 605. [9] Teter, Hemley, Science 271 (1996) 53. [10] O. Stephan, P.M. Ajayan, C. Colliex, Ph. Redlich, J.M. Lambert, P. Bernier, P. Lefin, Science 266 (1994) 1683. [11] Ph. Redlich, J. Loeffler, P.M. Ajayan, J. Bill, F. Aldinger, M. R€ uhle, Chem. Phys. Lett. 260 (1996) 465. [12] W.K. Hsu, S. Firth, Ph. Redlich, M. Terrones, H. Terrones, Y.Q. Zhu, N. Grobert, A. Schilder, R.J.H. Clark, H.W. Kroto, D.R.M. Walton, J. Mater. Chem. 10 (2000) 1425. [13] K. Suenaga, C. Colliex, N. Demoncy, A. Loiseau, H. Pascard, F. Willaime, Science 278 (1997) 653. [14] R. Droppa Jr., P. Hammer, A.C.M. Carvalho, M.C. dos Santos, F. Alvarez, J. Non-Cryst. Solids 299–302 (2002) 874. [15] D. Goldberg, Y. Bando, K. Kurashima, T. Sato, Diamonds Relat. Mater. 10 (2001) 63. [16] O. Stephan, E. Sandre, P.M. Ajayan, F. Cyrot-Lackman, C. Colliex, Phys. Rev. B 53 (1996) 13824. [17] K. Suenaga, E. Sandre, C. Colliex, C.J. Pickard, H. Kataura, S. Iijima, Phys. Rev. B 63 (2001) 165408. [18] J. Casanovas, J.M. Ricart, J. Rubio, F. Illas, J.M. JimenezMateos, J. Am. Chem. Soc. 188 (1996) 8071. [19] I. Shimoyama, G.H. Wu, T. Sekiguchi, Y. Baba, Phys. Rev. B 62 (2000) R6053. [20] P.L. Gai, O. Stephan, Kris McGuire, A.M. Rao, M.S. Dresselhaus, G. Dresselhaus, C. Colliex. [21] X. Blase, J.-C. Charlier, A. De Vita, R. Car, Ph. Redlich, M. Terrones, W.K. Hsu, H. Terrones, D.L. Carroll, P.M. Ajayan, Phys. Rev. Lett. 83 (1999) 5078. [22] E. Hernandez, P. Ordej on, I. Boustani, A. Rubio, J.A. Alonso, J. Chem. Phys. 113 (2000) 3814. [23] M. Glerup, M. Castignolles, M. Holzinger, G. Hug, A. Loiseau, P. Bernier, Chem. Commun. 20 (2003) 2542.
Synthesis of N-doped SWNT using the arc-discharge procedure M. Glerup a
d
a,b,*
, J. Steinmetz a,b, D. Samaille a, O. Stephan c, S. Enouz d, A. Loiseau d, S. Roth b, P. Bernier a
GDPC (UMR5581), Universite Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex 5, France b Max-Planck-Institut f€ ur Festk€ orperforschung, Heisenbergstrasse 1, 70569 Stuttgart, Germany c LPS, Universite Paris-Sud, Bat. 510, 91405 Orsay Cedex, France LEM, ONERA-CNRS (UMR104), 29 av. de la Division Leclerc, BP72, 92322 Ch^atillon Cedex, France Received 6 November 2003; in final form 6 November 2003 Published online:
Abstract Single-walled nitrogen doped nanotubes were prepared using two novel approaches in an arc-discharge process. In both cases a nitrogen-rich precursor was introduced into the anode rods together with graphite and the catalysts. The nitrogen rich precursors were organic and inorganic, respectively. Both synthesis routes gave nanotubes with a nitrogen concentration of a maximum of 1%, determined using electron energy loss spectroscopy (EELS) coupled with transmission electron microscopy (TEM). All samples were characterized with scanning electron microscopy (SEM), TEM, and high-resolution transmission electron microscopy (HRTEM). Ó 2004 Published by Elsevier B.V.
1. Introduction Carbon nanotubes have attracted much attention since their discovery by Iijima [1]. They are expected to bring significant breakthroughs in the electronic and mechanical engineering of materials. Doping carbon nanotubes with other chemical elements could be a particular interesting way for tuning these properties [2,3]. The electronic properties of single-walled carbon nanotubes (SWNTs) vary between semiconducting and metallic, depending on their helicity [4]. Nitrogendoped nanotubes are predicted to be either metallic or small gap semiconductors depending on the relative positions of the nitrogen and carbon atoms [5,6]. SWNTs are known to have extraordinary mechanical properties with a YoungÕs modulus of 1.25 Tpa [7]. Based on theoretical calculations, canonical nanotube stoichiometries on CN and C3 N4 have been predicted stable [8]. C3 N4 , iso-electronic to Si3 N4 , has been
*
Corresponding author. Fax: +33-467522504. E-mail address: [email protected] (M. Glerup).
0009-2614/$ - see front matter Ó 2004 Published by Elsevier B.V. doi:10.1016/j.cplett.2004.02.005
predicted to have hardness comparable to that of diamond [9]. Stephan et al. [10] initiated the synthesis of doped multi-walled nanotubes (MWNT) in 1994; they produced boron–nitrogen doped multi-walled carbon nanotubes using the arc-discharge method. Since then, a lot of work has been carried out concerning the direct synthesis of nitrogen and/or boron doped MWNTs in arc-discharge experiments [11–13]. The synthesis of doped single-walled nanotubes in arc-discharge experiment is not easily achieved. A few reports deal with the synthesis of nitrogen-doped SWNTs; the tubes are made by evaporation of the graphite rod together with the catalyst in a nitrogen gas-containing atmosphere [14]. Otherwise, the formation of single-walled CBx and CNy using substitution reactions with single-walled carbon nanotubes (C-SWNT) as templates have been described up to now [15]. In this work, we have chosen to take new approaches for the synthesis of nitrogen and/or boron doped SWNTs in arc-discharge experiments. Instead of carrying out the experiment in a nitrogen rich atmosphere we have added nitrogen-rich organic- and inorganic precursors into the graphitic anode-rod.
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M. Glerup et al. / Chemical Physics Letters 387 (2004) 193–197
2. Experimental Three different processes have been worked out: Synthesis process 1: the purpose of this series of experiments was to synthesize nitrogen doped SWNTs. Graphite (1–2 lm, Aldrich) was mixed with melamine (C3 N6 H6 , 99%, Aldrich) and Ni/Y catalyst (Ni: 99.999%, 100 mesh, Aldrich; Y: 99.9%, 40 mesh, Aldrich; Ni ¼ Y ¼ 0.6 w/o) and packed into the drillings of the anode rods. Samples were produced with amounts of melamine corresponding to 1, 2, 4 and 8 at.% nitrogen (denoted sample 1, 2, 3 and 4). The experiments were carried out with the optimized conditions for the synthesis of pure SWNTs. In the experiments with melamine, the direct current of 95–100 A in the arcdischarge was in the same range as in standard experiments, however the voltage was slightly lower. In general, the electric discharge was less stable than in conventional experiments and the distance of the two electrodes had to be adjusted manually. The more precursor used, the more the arc discharge was difficult to stabilize. Once stabilized, the synthesis proceeded as in standard experiments. The collarets produced were smaller than the normal SWNT collarets. Apart from the size, all characteristics of normal collarets could be observed, such as the spongy, nanotube-rich parts. Synthesis process 2: These experiments were carried out to synthesize single-walled CBx Ny nanotubes. Boron nitride powder (99.8%, Matthey Reagents) together with catalyst and graphite was packed into the drillings of the anode rods. Samples were produced with amounts of boron nitride corresponding to 1/1, 2/2, and 4/4 at.% boron and nitrogen (denoted sample 5, 6, and 7). The synthesis conditions were similar to those described above. Again the stability of the arc decreased with an increasing concentration of precursor added. Synthesis process 3: A third series of experiments was carried out to produce boron-doped nanotubes. As above, the 2 or 4 at.% of boron powder (95–97%, Riedel de Ha€en) was mixed with the graphite and the catalysts and filled into the anode rod. This time, it was very difficult to stabilize the arc; the growth of the collarets was impaired resulting in a low yield of nanotubes. Scanning electron microscopy (SEM) pictures were taken for the evaluation of the homogeneity and purity of the samples. SEM pictures were recorded without prior preparation using a Jeol 6300 F at 15 kV (field emission gun). The nanotubes were dispersed in absolute ethanol before preparing the grids for transmission electron microscopy (TEM) imaging (Philips CM20, 200 kV) and for the electron energy loss microscopy (EELS) experiments. For studying the structure of the nanotubes high-resolution transmission electron microscopy (HRTEM) experiments were carried out (Jeol 4000FX, 400/200 kV). A dedicated STEM (VG HB501) with a field emission gun operated at 100 keV and fitted with a
Gatan 666 parallel-EELS spectrometer optically coupled to a CCD camera was used for quantifying the C, N, and B concentrations of the nanotubes bundles.
3. Results and discussion In Fig. 1 are shown representative SEM images from the collaret (Fig. 1a) and for the soot (Fig. 1b) of sample 4. The soot consisted of white–greyish crystalline layer which was identified by means of medium-infrared spectroscopy (MIR) to consist of pure melamine (not shown here), indicating that a part of the melamine evaporates before participating in the reaction. This might not be that surprising since melamine has an evaporation temperature which is several thousand degrees lower than those of graphite. Melamine boils at 350 °C under standard conditions and starts decomposing at >280 °C. Thus, it is necessary to use a much higher concentration of melamine in the anode compared to what will participate in the formation of the tubes. The SEM images are representative for all samples synthesized using melamine as the nitrogen precursor. In Fig. 2 is shown a representative HRTEM image of the same sample and the corresponding EEL spectrum is seen in Fig. 3. From the C-K edge it can be seen that the nanotubes consist of a typical graphitic network with the typical sp2 -type bonding, and that the tubes are very well graphitized [16,17]. The nitrogen content in this sample was quantified to be at around 1 at.%. Theoretical calculations have suggested the existence of three possible ways to incorporate nitrogen, denoted ÔpyrolicÕ, ÔpyridinicÕ, and ÔgraphiticÕ [18,19] after the electronic structure of the nitrogen. In order to optimize the detection of such a low concentration of N atoms in the ropes of SWNTs and to observe the fine structures on the N-K edge, collections of 50 averaged spectra were acquired by rastering a slightly defocused probe over 10 10 nm2 areas. A specific method consisting of the acquisition of shifted spectra on the detector for posterior realignment was used to smooth the detector gain variation. The resulting N-K fine structures are shown in the inset of Fig. 3. The displayed spectrum is the sum of a few hundreds of spectra. In addition to the triangular r band above 405 eV, which is characteristic of C–N materials, two weak features are observed at lower energy at 398.6 and 402 eV. They can be attributed to p -states associated with pyridine-like and graphite-like nitrogen [18,19]. This in principle gives direct information about the electronic structure of the nitrogen atoms and the neighboring carbon atoms. We note that the detection of either pyridinic or graphitic nitrogen in nitrogen doped MWNTs has been commonly reported in the literature. EEL spectra were also measured on the amorphous/graphitic carbon from the collaret of sample 4 (not shown), approximately the
M. Glerup et al. / Chemical Physics Letters 387 (2004) 193–197
195
Fig. 1. SEM images of sample 4: (a) nanotubes taken from the collaret (N ¼ 1 at.%); (b) the soot collected in the reactor. The crystallites observed on this image is un-reacted, re-crystallized melamine.
CK NK
σ*
Instensity (arb. units)
402 eV 398.6 eV
395
4004
05 (eV)
4104
15
NK
250
300
350
400
450
500
Energy Loss (eV)
Fig. 2. HRTEM image of sample 4 (CNx nanotubes, N ¼ 1 at.%).
same or a slightly higher concentration of nitrogen was seen to be incorporated. The HRTEM images of the samples indicate that nitrogen is built into the carbon lattice of individual nanotubes and not intercalated between the tubes in the bundles since tube–tube distance is the same as for undoped SWNTs. Also X-ray diffraction experiments (not shown here) confirmed this observation. The charac1 corresponding to the teristic peak at Q ¼ 0:44 A nanotube bundles was observed as for un-doped SWNTs. We now turn to the samples resulting from synthesis process 2. Fig. 4a is a typical example of the collarets observed in sample 7. All samples contain 1 at.% of nitrogen and no boron was observed in any of the EEL spectra (within our experimental conditions, we estimate the detection limits for boron to be of the order of 0.1
Fig. 3. EEL core electron K-shell spectra of CNx nanotube bundles (sample 4). The nanotubes are doped with around 1 at.% nitrogen. For the C-K edge well defined p and r fine structure features are observed which are evidences of sp2 -hybridisation in graphitic structures. The inset is a magnification of the N-K edge.
at.%). It is observed that using a precursor with a higher evaporation temperature less precursor is needed for the incorporation of 1 at.% nitrogen into the SWNTs. From the SEM images (Fig. 4b) and the TEM investigations it was observed that the tubes were very long compared to samples 1–4 and to undoped SWNTs. Moreover, the nanotubes were more difficult to disperse than nanotubes from standard production. In Fig. 5 are shown representative HRTEM images. In samples 5–7 we observed an unusual high amount of nanoparticles (Fig. 5b). The particles are very well-graphitized onions arranged in chains. The inner shells are almost rectangular and mostly hollow. We correlate the formation of this type of nano-structures to the presence of boron during the synthesis. This is in agreement with similar observations on B-doped laser ablation experiments
196
M. Glerup et al. / Chemical Physics Letters 387 (2004) 193–197
Fig. 4. Representative SEM images of sample 7: (a) the collaret: (b) a very long bundle of SWNTs (>6 lm).
Fig. 5. (a) HRTEM image of sample 7. (b) Chains of mostly hollow carbon onions which are found in samples 4–7.
where morphological changes such as the formation of large numbers of graphitic onions were observed in SWNTs samples for high boron contents [20]. The samples resulting from process 3 did not indicate any incorporation of boron into the nanotube framework. The phenomenon of increased length of samples 5–7 is likely to be attributed to the presence of boron during the synthesis as it has previously been observed for boron-doped multi-walled carbon nanotubes [21,22]. During the formation process of the nanotube, boron atoms are preferably to be found close to the tubeÕs open edge where they hamper the closing of the tubes. This effect of the boron is largest for zig-zag nanotubes which can thus grow to a greater length than tubes with other helicities. Detailed observations will have to be done to check if these peculiarities are also present in the SWNT synthesized in this study. Generally, it was observed that the nitrogen doped single-walled are less stable than pure carbon nanotubes; it is actually possible to cut the bundles while exposing
them to the electron beam of the TEM. Also, while carrying out the EELS experiments it was observed that the nanotubes were more fragile than normally. These increased radiation damage effects might be a consequence of the dopants within the curved honey-comp lattice. Nitrogen dopants can in principle be regarded as defects in the nanotubes and an increased amount of defects in the tubes obviously makes the tubes more fragile. The work presented here confirms that the incorporation of nitrogen and boron in nanotubes is not comparable. No boron was seen to be incorporated in the nanotubes, meaning that it is much more difficult than nitrogen to incorporate into the lattice of the nanotubes. This phenomenon is also known from the synthesis of doped multi-walled nanotubes. Nanotubes containing more than around 5% boron have never been reported in the literature whereas nanotubes with an average concentration on 20 at.% of nitrogen have been synthesized [23]. The theoretical work by Blase et al. [21] and Hernandez et al. [22] explains the role boron plays
M. Glerup et al. / Chemical Physics Letters 387 (2004) 193–197
in the growth of single-walled nanotubes and how it favours the growth of long zig-zag tubes. The increased length of those nanotubes might be a feature which can be used to separate them for the rest of the tubes. This work gives rise to some new intriguing questions: Is the incorporation of boron in single-walled nanotubes possible? Can the use of small amount of chemical elements like boron be a way to synthesize nanotubes with a unique helicity? In all samples, the maximum incorporation of nitrogen was seen to be around 1 at.%, even though much more precursor was added to the synthesis. Does this mean that 1 at.% of nitrogen is the physical maximum? More theoretical and experimental research should be dedicated to this subject.
4. Conclusion It was shown that it is possible to directly synthesize nitrogen-doped nanotubes in an arc-discharge experiment using nitrogen-rich organic and inorganic precursors. This opens new routes for the synthesis of doped SWNTs in high temperature type experiments like arcdischarge and laser-ablation. This approach is seen to be promising though further work has to be carried out for the optimization of this type of experiments. This work demonstrates the possibility of designing experiments for controlling the length and helicity of the singlewalled carbon nanotubes.
Acknowledgements M.G. acknowledges the EC-TMR network FUNCARS for financial support. O.S. acknowledges the EC-IHP network Ôfullerene-like materialsÕ for financial support. Dr. M. Holzinger (UMII) is acknowledged for valuable discussions and D. Maurin (UMII) for the infrared measurements.
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