- Email: [email protected]
Effect of the experimental parameters on the structure of nitrogen-doped carbon nanotubes produced by aerosol chemical vapour deposition
CARBON
4 7 ( 2 0 0 9 ) 3 0 –3 7
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Effect of the experimental parameters on the structure of nitrogen-doped carbon nanotubes produced by aerosol chemical vapour deposition Antal A. Koo´s*, Michael Dowling, Kerstin Jurkschat, Alison Crossley, Nicole Grobert* Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK
A R T I C L E I N F O
A B S T R A C T
Article history:
We describe the systematic study of multi-walled carbon nanotubes with different nitrogen
Received 16 June 2008
doping produced by aerosol chemical vapor deposition. Benzylamine:toluene mixtures of
Accepted 19 August 2008
0:100, 5:95, 10:90, 25:75, 50:50, 75:25 and 100:0 were thermally decomposed at 800–900 C
Available online 28 August 2008
under argon at atmospheric pressure, whereby the nitrogen content of the bulk material was varied between 0 and 2.2 at%. We also show how the presence of nitrogen in the precursor changed the nanotube morphology, i.e. nitrogen decreased the number of kinks incorporated into the carbon nanotubes, decreased their length and diameter and increased the proportion of ‘bamboo’ shaped nanotubes. Furthermore, due to the nitrogen doping, the oxidation resistance of the nanotube material was decreased. With concentrations above 10% benzylamine the increase of the reaction temperature had no significant effect on the quality of the nanotubes, however, at higher temperatures the nitrogen content was decreased. We demonstrate the control over the nanotube geometry, the nitrogen content and oxidation resistance of the nanotubes, and show that these properties are interlinked. 2008 Elsevier Ltd. All rights reserved.
1.
Introduction
Over the last decade, carbon nanotubes (CNTs) [1] have been the subject of extensive research due to their outstanding mechanical and electronic properties [2,3]. It has been shown that CNTs can behave as metals or semiconductors depending on their diameter and chirality. Although CNTs can be produced using a number of methods, there is still no control over nanotube morphology and hence their electronic behaviour. However, theoretical [4] and experimental studies [5] revealed that it is possible to tune the electronic properties of the nanotubes by incorporating hetero atoms within the carbon lattice [6]. The most frequently used dopants are boron and nitrogen because their atom size is similar to that of carbon [7–18] and because they serve as p- or n-type dopants, respectively [6]. The successful incorporation of nitrogen [15]
or boron [7] atoms within the graphitic carbon cylinders strongly depends on the choice of precursor, catalyst, reaction temperature, reaction time, gas flow rate and pressure [19]. Furthermore, the doping of nanotubes with hetero atoms also changes the nanotube structure [20], chemical reactivity [21] and mechanical stability [22]. Therefore, the use of nanotubes for future technological applications requires a precisely controlled introduction of dopants. In recent years, different methods have been developed for the production of nitrogen-doped carbon nanotubes (CNx nanotubes) including the arc discharge [23], ion implantation [13,17] and various techniques based on chemical vapor deposition (CVD) [19]. However, practical applications of CNTs or CNx nanotubes necessitate fairly large amounts of material at commercially viable prices. Aerosol CVD appears to be the most suitable synthesis method for industrial scale production of CNTs.
* Corresponding authors: Fax: +44 1865 27833. E-mail addresses: [email protected], [email protected] (A.A. Koo´s), [email protected] (N. Grobert). 0008-6223/$ - see front matter 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2008.08.014
CARBON
4 7 ( 2 0 0 9 ) 3 0 –3 7
For example, it is suitable for the continuous injection of catalyst precursor and carbon feedstock, it requires no additional catalyst preparation step and the samples need minimal or no purification [24,25]. Using aerosol CVD it is also possible to produce CNx nanotubes without using flammable or corrosive gases such as ammonia. Although several articles have been published about the synthesis of CNx nanotubes, the comparison of the results obtained using different experimental setups is difficult, and the influence of one chosen parameter on the quality of nanotubes cannot be determined. The dependence of the quality of nanotubes on the synthesis parameters can be understood only when one parameter is changed at a time. This study was carried out in order to identify structural trends in CNx nanotubes produced via aerosol CVD as a function of the synthesis parameters used during growth. The morphological changes of the CNx nanotubes were analysed using the following characterisation techniques: scanning electron microscopy, transmission electron microscopy, electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA).
2.
31
the aerosol generator was switched on and the argon flow was increased to 2500 sccm for the duration of the experiment (10, 20, 30 or 120 min). Subsequently, the flow rate was reduced again to 100 sccm while the furnace was left to cool to room temperature. Carbon soot was mainly produced in the centre of the furnace and some of it was carried by the gas flow to the acetone trap located at the opposite end to the aerosol generator. The CNTs produced on the inner wall of the quartz tube were removed with a sharp metal tool, while the nanotubes grown on the quartz plate were investigated as grown. For the structural characterisation, the as-produced nanotube soot was mounted on SEM stubs using conductive carbon tabs. For the TEM and EELS investigations, the nanotubes were sonicated in acetone in an ultra-sonic bath for 10 min. A few drops of the suspension were deposited on a lacey carbon copper grid and the TEM studies were performed at 400 keV. Raw CNT soot was deposited onto 1 · 1 cm conductive carbon tabs for the XPS measurements and for the TGA studies samples were heated in air from 30 to 850 C at rate of 10 C/min. Approximately 1.5 mg nanotubes were used for each TGA measurement.
Experimental
CNx nanotubes were produced using an aerosol-based CVD system consisting of a piezo-driven aerosol generator (RBI Pyrosol 7901), a quartz tube (2.2 cm inner diameter) and a 50 cm long horizontal electrical furnace, equipped with a gas flow controller and a gas trap filled with acetone. Solutions of 5 wt% ferrocene (Fe(C5H5)2, Aldrich 98%) in mixtures of toluene (C6H5CH3, Fluka 99.7%) and benzylamine (C6H5CH2NH2, Fluka 99%) were prepared in an ultrasonic bath for 30 min. Solutions were prepared for a range of benzylamine:toluene ratios, 0:100, 5:95, 10:90, 25:75, 50:50, 75:25 and 100:0. For the experiments, ferrocene containing mixtures were fed into the piezo-driven aerosol generator and a quartz plate (1 · 5 · 50 mm) was placed inside the quartz tube at the centre of the furnace. The generator (located as near as possible to the furnace in order to avoid the coalescence of droplets) was then connected to the quartz tube located in the horizontal tube furnace. Once the system was set-up, the aerosol generator and the quartz tube were flushed with 100 sccm argon while the furnace was heated to temperatures of 800, 850 or 900 C in order to remove any unwanted oxygen from the reactor. When the desired temperature was reached
3.
Characterisation of CNx nanotubes
3.1.
Overall structural investigation by SEM
SEM studies revealed that the nanotubes grew perpendicular to the substrate, forming flakes of ‘parallel’ aligned nanotubes. The representative SEM images of the nanotubes produced from toluene and benzylamine at 800 C are shown in Fig. 1. The CNTs produced from toluene contained many kinks and were undulating (wavy). The number of kinks decreased as benzylamine was introduced into the precursor, and when more than 10% benzylamine was used the nanotubes were straight. We compared the length of the nanotubes grown on the quartz substrates placed in the reaction tube. The length of the CNTs produced from toluene at 800 C in the 10 min experiments was found to be of the order of 110 ± 11 lm. The length was reduced to one third (36 ± 4 lm) by the introduction of only 10% benzylamine. The CNx nanotubes synthesised from benzylamine were 17 ± 2 lm long, i.e. only 15% of the length of the CNTs synthesised from toluene (see
Fig. 1 – SEM images of the nanotubes made from toluene (a) and benzylamine (b). The nanotubes produced from toluene were undulating while the tubes made from benzylamine were straight.
32
CARBON
4 7 ( 2 0 0 9 ) 3 0 –3 7
Fig. 2 – The length of nanotubes grown in 10 min at 800 C as a function of the benzylamine concentration of the precursor. The introduction of 10% benzylamine in toluene decreased to one-third the length of the nanotubes.
Fig. 3 – The length of nanotubes produced at 800 C from benzylamine and toluene as a function of the growth time.
Fig. 2). Consequently, the presence of nitrogen slowed down the nanotube growth significantly, even at low benzylamine concentrations. According to theoretical calculations [26] growth is inhibited because the nitrogen saturates the tube edge at the growing end, meaning nitrogen atoms need to be removed and replaced by carbon, or defects need to be incorporated into the graphitic network [26]. The weight of the samples produced from precursors with different benzylamine concentration was similar (approximately 300 mg/ experiment), despite the change in the nanotube length. The length of the nanotubes synthesised at 800 C from toluene and benzylamine is displayed in Fig. 3 as a function of experiment duration. The length of the CNTs produced from toluene in 30 min was 455 ± 45 lm and in 120 min was 785 ± 78 lm. The length of CNx nanotubes produced from benzylamine in 30 min was 36 ± 4 lm and in 120 min was 110 ± 11 lm, the same as the CNTs synthesised from toluene in 10 min.
3.2.
Characterisation of internal structure by TEM
TEM investigations showed that the samples were ‘clean’, i.e. they contained less than 5 wt% amorphous carbon and almost no polyhedral carbon particles, but the nanotubes incorporated defects typical to CVD synthesis. The samples
also contained Fe catalyst particles encapsulated in the inner core or attached to the nanotube surface. Using TEM it was possible to compare the samples, but it was difficult to quantify the quality of the nanotubes. We defined ‘quality’ in terms of the average distance between kinks. Changes caused by the incorporation of nitrogen, for example, stacked cone structure or corrugated walls were not considered as defects. The change of the nanotube structure as a function of the benzylamine concentration is presented in Fig. 4 using representative TEM images of samples synthesised at 800 C. The nanotubes produced from toluene contained many kinks, but their number decreased as the benzylamine concentration increased. This observation was confirmed by SEM. It was found, that the presence of benzylamine in the precursor changed the inner structure of the nanotubes, too. The samples contained CNx nanotubes exhibiting stacked-cone structure and tubes consisting of corrugated walls, together with the more crystalline nanotubes. In the following, stackedcone and corrugated nanotubes will be called ‘bamboo’ nanotubes. The CNx samples produced from precursors containing 5% benzylamine contained approximately 20% ‘bamboo’ nanotubes, this increased to 40% ‘bamboo’ nanotubes at 10% benzylamine, and more than 80% ‘bamboo’ nanotubes when benzylamine was used as the precursor. Fig. 5a shows a crystalline nanotube produced from toluene only. A CNx nanotube made from benzylamine exhibiting the typical stacked-cone structure is shown in Fig. 5c. The walls of CNx nanotubes are corrugated for nanotubes with stacked-cone structure and nanotubes with walls parallel to axis. Corrugation of the walls already occurs in tubes made from precursors containing 5% benzylamine. The formation of CNx nanotubes with corrugated walls is caused by the presence of nitrogen in the graphitic network, which induces curvature of the graphitic layer [26,27]. The formation of CNx nanotubes with periodic compartment structures can be explained by the difference between the carbon surface and bulk diffusion in the catalyst. It is likely that the walls of the CNx nanotubes were formed from the side surface of the catalyst mainly via carbon surface diffusion, while the inner layers form from top surface of the catalyst via bulk diffusion [28,29]. The incorporation of nitrogen atoms promotes tube closure, thereby triggering the presence of ‘bamboo’ structures [26]. The increase of the benzylamine content also increased the ratio of ‘bamboo’ nanotubes in the samples produced at higher temperatures, 850 and 900 C. The proportion of ‘bamboo’ nanotubes produced from the same precursor at different temperatures was similar. The density of the kinks in the CNTs made from toluene increased significantly with the increase of the temperature (Fig. 6). The CNx samples made at 900 C from precursors containing 5% benzylamine incorporated more defects than those made at 800 C, but above 10% benzylamine increasing the temperature from 800 to 900 C had no major impact on the quality of the samples. The outer diameter distribution as a function of benzylamine concentration is compared on Fig. 7. Approximately 300 tube diameters per sample were measured on TEM images taken of samples synthesised at 800 C in 10 min.
CARBON
4 7 ( 2 0 0 9 ) 3 0 –3 7
33
Fig. 4 – TEM images of nanotubes made from 100:0 (a), 95:5 (b), 90:10 (c) and 0:100 (d) toluene:benzylamine mixtures. The increase of nitrogen content in the precursor decreased the number of kinks, decreased the nanotube diameter and increased the proportion of ‘bamboo’ nanotubes.
The average outer diameter of the nanotubes was reduced from 62 nm, measured for toluene, to 52 nm for 5% benzylamine, and to 30 nm for CNx nanotubes grown from benzylamine. About 5% benzylamine reduced the most frequent outer diameter from 55 nm for toluene to 36 nm, and it decreased to 25 nm for CNx nanotubes made from benzylamine. The change in the FWHM was significant, too. The FWHM of 66 nm calculated for CNTs made from toluene was reduced to 37 nm by 5% benzylamine and to 24 nm by 10% benzylamine. At higher benzylamine concentrations, the FWHM did not change substantially. Thus, the increase of benzylamine content in the precursor decreased the average outer diameter of the nanotubes. Compared with the CNTs synthesised from toluene, the addition of only 5% benzylamine considerably reduced the average outer diameter of the CNx nanotubes. On the other hand, the average inner diameter of the nanotubes was increased from 9 nm, measured for tubes made from toluene, to 13 nm for CNx nanotubes grown from benzylamine. Hence, the nanotubes made from toluene only exhibit approximately three times more walls than the CNx nanotubes made from benzylamine. The increase of the reaction time from 10 to 20 min significantly changed the diameter distribution of the CNTs made from toluene, i.e. the number of thin nanotubes (ca. 25 nm outer diameter) was higher. In comparison, longer reaction times had no notable effect on the diameter distribution of the CNx nanotubes made from benzylamine. We suppose that within the first few minutes of the experiments the surface of
the quartz tube became covered with nanotubes. Consequently, not all of the incoming catalyst material could reach the substrate, and this caused the nanotubes themselves to act as a substrate for nanotube growth [30]. These new catalyst particles grown on the nanotubes were smaller than those grown on the surface of the quartz tube, and this increased the number of thin tubes [31]. The nanotubes grown from the new catalyst particles were shorter and contained more defects than the primary tubes.
3.3.
Characterisation of the nitrogen content via XPS
The incorporation of nitrogen within CNx nanotubes was confirmed by XPS measurements. The sample produced at 800 C from precursor containing 5 vol% benzylamine contained 0.28 ± 0.09 at% nitrogen, the nitrogen content increased to 0.49 ± 0.16 at% when the benzylamine concentration was 10 vol% (Fig. 8). The sample made from 1:1 benzylamine: toluene mixture contained 0.9 ± 0.16 at% nitrogen, while the nitrogen content in sample produced from benzylamine increased to 2.2 ± 0.3 at%. In this case the N/C ratio in the sample produced from benzylamine was to about 15% of the N/C ratio in the feedstock. The increase of the temperature decreased significantly the nitrogen content of the sample. The nitrogen content of the samples produced at 900 C was approximately half of those produced at 800 C. We determined the type of N bonding from the chemical shifts of the N 1s line for the samples produced at 800 C
34
CARBON
4 7 ( 2 0 0 9 ) 3 0 –3 7
Fig. 5 – (a) HRTEM image of a MWNT produced from toluene only, (b) close-up of (a) revealing a fairly crystalline structure and individual kinks typical pure carbon MWNTs grown by CVD. (c) HRTEM image of a MWCNx synthesised using benzylamine only, (d) close-up of (c) showing the corrugated nature frequently observed in MWCNx.
using a 1:1 mixture of benzylamine:toluene and benzylamine only. We identified tree types of nitrogen from the deconvoluted N 1s XPS spectrum: pyridinic (398.3 eV), sp2 (400.7 eV) and gaseous (404.5 eV) [32]. The sp2/pyridinic nitrogen ratio increased from 0.9, measured on the 1:1 benzylamine:toluene sample, to 1.1, measured on the sample produced from benzylamine only. In parallel the ratio of gaseous nitrogen also increased, i.e. 18–44%.
3.4. Investigation of nitrogen content of individual CNx nanotubes using EELS In addition to XPS, the nitrogen content of individual CNx nanotubes was estimated using EELS. The CNx nanotubes made from benzylamine contained approximately 2.5 wt% nitrogen, this decreased to approximately 1 wt% in nanotubes produced from 1:1 benzylamine:toluene mixture. This result agrees with the findings of Reyes-Reyes and co-workers. They reported 2–3% of nitrogen content in the walls of the CNx nanotubes produced from benzylamine using similar experimental setup and parameters [28]. EELS was not sensitive enough to measure the nitrogen concentration of CNx nanotubes produced from precursors with lower benzylamine content. The nitrogen distribution in the CNx nanotubes synthesised from benzylamine was investigated using Energy Filtered TEM. This showed uniform nitrogen distribution and it was not possible to identify islands with higher or lower nitrogen content.
3.5.
Oxidation resistance measured via TGA
The oxidation resistance of the nanotubes was determined using TGA. Data from two TGA measurements for each investigated benzylamine concentration is displayed in Fig. 9, the measurements were made on samples produced at 800 and 900 C in 10 min. In order to help the comparison of 14 TGA curves, Fig. 9 shows the temperatures where 40%, 60% and 80% of the sample weight remained during the TGA measurements as a function of the benzylamine concentration of the precursor. The CNTs synthesised at 800 C from toluene oxidised between 490 and 720 C, while the CNx nanotubes synthesised at the same temperature from benzylamine burned between 430 and 570 C. The CNx samples also showed decreasing oxidation temperature with the increase of the benzylamine concentration for intermediate precursor mixtures. A linear dependence was observed excluding small deviations (errors <4%). Interestingly, the most inert samples produced at 800 C were made from precursors containing 5% benzylamine. The TGA measurements of samples produced at 900 C showed similar trends. As an exception to this rule, the samples synthesised from toluene were less inert that those made from precursors containing 10% benzylamine. This behaviour confirms the TEM observations where the quality of the nanotubes synthesised from toluene and 5:95 benzylamine:toluene mixture decreased with the increase of the temperature
CARBON
4 7 ( 2 0 0 9 ) 3 0 –3 7
35
Fig. 6 – TEM images of nanotubes made at 850 (a and b) and 900 C (c and d) from toluene and benzylamine, respectively. The increase of the temperature increased the density of the kinks in the CNTs made from toluene, but it had no major impact on the quality of the CNx nanotubes made from benzylamine.
Fig. 7 – Outer diameter distribution of the nanotubes made at different benzylamine concentrations. The introduction of 10% benzylamine in toluene decreased significantly the outer diameter of the nanotubes.
Fig. 8 – The nitrogen content of the samples determined from XPS measurements. The increase of benzylamine concentration in precursor increased the nitrogen content of the samples produced at the same synthesis temperature. The increase of the synthesis temperature from 800 to 900 C decreased the nitrogen content of the samples to half.
(Fig. 6), but above 10% benzylamine content the temperature had no major influence on the quality of the CNx nanotubes. Above 10% benzylamine concentration the increase of the production temperature from 800 to 900 C increased the oxidation temperature of the CNx nanotubes. However, according to XPS this temperature increase also decreased the nitrogen content. The oxidation temperature plotted as a function of nitrogen content shows linear dependence, dem-
onstrating that the oxidation temperature is strongly correlated to the nitrogen content regardless of synthesis temperature. This result confirms the changes in the structure of the nanotubes and the change of ‘bamboo’ nanotube ratio determined with TEM. The nitrogen incorporated into the CNx nanotubes changed their structure, increasing the density of ‘defects’ and the reactivity of the walls. The sharp decrease of the weight
36
CARBON
4 7 ( 2 0 0 9 ) 3 0 –3 7
For reference, the key findings of this investigation are summarised below: With increasing benzylamine concentration the following trends were identified. • • • •
Fig. 9 – Temperatures where 40%, 60% and 80% of the sample weight remained during TGA measurements as a function of the benzylamine concentration in the precursor. Black symbols represent the nanotubes made at 800 C, grey symbols represent the nanotubes made at 900 C. The increase of benzylamine concentration in precursor decreased the oxidation resistance of the samples produced at the same synthesis temperature. The increase of the synthesis temperature increased the oxidation resistance.
of the samples made from precursors with a high benzylamine concentration indicates that the CNx nanotubes react with their environment on their whole surface, not only at randomly distributed defects like the CNTs. The increased reactivity and uniform defect distribution may help the sidewall functionalisation of the CNx nanotubes and improve their interaction with the matrix in composites [33].
4.
Conclusions
Multi-walled carbon nanotubes with different concentrations of nitrogen doping were produced by aerosol CVD from toluene and benzylamine mixtures. According to TEM and SEM the increasing benzylamine content of the precursor decreased the number of kinks incorporated into nanotubes, decreased their length and diameter, and increased the proportion of ‘bamboo’ nanotubes into the sample. The influence of the increase of benzylamine content on the structure was not uniform. Only 10% benzylamine mixed into precursor changed significantly the geometry of the CNx nanotubes compared with the CNTs, but the geometry was not so sensitive on nitrogen content at higher benzylamine concentrations. The nitrogen content of the samples could be tuned; the highest nitrogen content was 2.2 ± 0.3 at%. The N/C ratio in the CNx nanotubes produced from benzylamine at 800 C was about 15% of the N/C ratio in the feedstock. Only 100 C increase of the synthesis temperature decreased the nitrogen content of the CNx nanotubes to half. TGA measurements showed that the presence of nitrogen decreased the oxidation resistance of the nanotube walls, which may help the sidewall functionalisation of CNx nanotubes and improve the matrix-nanotube bonds in composites. The oxidation temperature had linear dependence on the nitrogen content of the nanotubes. These results show that it was possible to control the geometry, oxidation resistance and nitrogen content of the nanotubes, but these properties were not independent of each other.
• • • • • •
Increasing nitrogen-doping, up to 2.2 at%. Decreasing growth rate. Decreasing mean outer-diameter, from approximately 60 to 30 nm. Increasing proportion of bamboo and corrugated structures, from <10% to >85%. Gradual loss of crystallinity. Decreasing number of kinks. Reduced oxidation resistance. Decreasing nitrogen-doping; by a factor of half when temperature is increased from 800 to 900 C. The sp2/pyridinic nitrogen ratio and the ratio of gaseous nitrogen increased. Using these results it is possible to produce nanotubes with desired properties.
Acknowledgements This work has been supported in part by the European Commission under the 6 Framework Programme (STREP project BNC Tubes, contract number NMP4-CT-2006-03350). The support from Royal Society, WomenInNano, BegbrokeNano and the GDR-I nanotubes is acknowledged.
R E F E R E N C E S
[1] Iijima S. Helical microtubules of graphitic carbon. Nature 1991;354(6348):56–8. [2] Dresselhaus MS, Dresselhaus G, Saito R. Physics of carbon nanotubes. Carbon 1995;33(7):883–91. [3] Grobert N. Carbon nanotubes – becoming clean. Mater Today 2007;10(1–2):28–35. [4] Latil S, Roche S, Mayou D, Charlier J. Mesoscopic transport in chemically doped carbon nanotubes. Phys Rev Lett 2004;92(25):256805. [5] Terrones M, Ajayan P, Banhart F, Blase X, Carroll D, Charlier J, et al. N-doping and coalescence of carbon nanotubes: synthesis and electronic properties. Appl Phys A-Mater 2002;74(3):355–61. [6] Terrones M, Jorio A, Endo M, Rao A, Kim Y, Hayashi T, et al. New direction in nanotube science. Mater Today 2004;7:30–45. [7] Stephan O, Ajayan P, Colliex C, Redlich P, Lambert J, Bernier P, et al. Doping graphitic and carbon nanotube structures with boron and nitrogen. Science 1994;266(5191):1683–5. [8] Terrones M, Grobert N, Terrones H. Synthetic routes to nanoscale BxCyNz architectures. Carbon 2002;40(10):1665–84. [9] Endo M, Muramatsu H, Hayashi T, Kim Y, Van Lier G, Charlier J, et al. Atomic nanotube welders: boron interstitials triggering connections in double-walled carbon nanotubes. Nano Lett 2005;5(6):1099–105. [10] Mcguire K, Gothard N, Gai P, Dresselhaus M, Sumanasekera G, Rao A. Synthesis and Raman characterization of borondoped single-walled carbon nanotubes. Carbon 2005;43(2):219–27.
CARBON
4 7 ( 2 0 0 9 ) 3 0 –3 7
[11] Maultzsch J, Reich S, Thomsen C, Webster S, Czerw R, Carroll D, et al. Raman characterization of boron-doped multiwalled carbon nanotubes. Appl Phys Lett 2002;81(14): 2647–9. [12] Moradian R, Azadi S. Boron and nitrogen-doped single-walled carbon nanotube. Physica E 2006;35(1):157–60. [13] Kotakoski J, Krasheninnikov A, Ma Y, Foster A, Nordlund K, Nieminen R. B and N ion implantation into carbon nanotubes: insight from atomistic simulations. Phys Rev B 2005;71(20):205408. [14] Ewels C, Glerup M. Nitrogen doping in carbon nanotubes. J Nanosci Nanotechnol 2005;5(9):1345–63. [15] Kim S, Lee J, Na C, Park J, Seo K, Kim B. N-doped doublewalled carbon nanotubes synthesized by chemical vapor deposition. Chem Phys Lett 2005;413(4–6):300–5. [16] Terrones M, Terrones H, Grobert N, Hsu W, Zhu Y, Hare J, et al. Efficient route to large arrays of CNx nanofibers by pyrolysis of ferrocene/melamine mixtures. Appl Phys Lett 1999;75(25):3932–4. [17] Yamamoto K, Kamimura T, Matsumoto K. Nitrogen doping of single-walled carbon nanotube by using massseparated low-energy ion beams. Jpn J Appl Phys 2005;44(4A): 1611–4. [18] Villalpando-Paez F, Zamudio A, Elias A, Son H, Barros E, Chou S, et al. Synthesis and characterization of long strands of nitrogen-doped single-walled carbon nanotubes. Chem Phys Lett 2006;424(4–6):345–52. [19] Tang C, Bando Y, Golberg D, Xu F. Structure and nitrogen incorporation of carbon nanotubes synthesized by catalytic pyrolysis of dimethylformamide. Carbon 2004;42(12–13): 2625–33. [20] Nemes-Incze P, Daroczi N, Sarkozi Z, Koos A, Kertesz K, Tiprigan O, et al. Synthesis of bamboo-structured multiwalled carbon nanotubes by spray pyrolysis method, using a mixture of benzene and pyridine. J Optoelectron Adv Mater 2007;9(5):1525–9. [21] Nevidomskyy A, Csanyi G, Payne M. Chemically active substitutional nitrogen impurity in carbon nanotubes. Phys Rev Lett 2003;91(10):105502. [22] Hernandez E, Goze C, Bernier P, Rubio A. Elastic properties of single-wall nanotubes. Appl Phys A-Mater 1999;68(3): 287–92.
37
[23] Glerup M, Steinmetz J, Samaille D, Stephan O, Enouz S, Loiseau A, et al. Synthesis of N-doped SWNT using the arcdischarge procedure. Chem Phys Lett 2004;387(1–3):193–7. [24] Horvath Z, Kertesz K, Petho L, Koos A, Tapaszto L, Vertesy Z, et al. Inexpensive, upscalable nanotube growth methods. Curr Appl Phys 2006;6(2):135–40. [25] Grobert N. Nanotubes – grow or go? Mater Today 2006;9(10):64. [26] Sumpter B, Meunier V, Romo-Herrera J, Cruz-Silva E, Cullen D, Terrones H, et al. Nitrogen-mediated carbon nanotube growth: diameter reduction, metallicity, bundle dispersability, and bamboo-like structure formation. ACS Nano 2007;1(4):369–75. [27] Mandumpal J, Gemming S, Seifert G. Curvature effects of nitrogen on graphitic sheets: structures and energetics. Chem Phys Lett 2007;447(1–3):115–20. [28] Reyes-Reyes M, Grobert N, Kamalakaran R, Seeger T, Golberg D, Ruhle M, et al. Efficient encapsulation of gaseous nitrogen inside carbon nanotubes with bamboolike structure using aerosol thermolysis. Chem Phys Lett 2004;396(1–3):167–73. [29] Wang Y, Tang G, Koeck F, Brown B, Garguilo J, Nemanich R. Experimental studies of the formation process and morphologies of carbon nanotubes with bamboo mode structures. Diam Relat Mater 2004;13(4–8):1287–91. [30] Singh C, Shaffer M, Windle A. Production of controlled architectures of aligned carbon nanotubes by an injection chemical vapour deposition method. Carbon 2003;41(2):359–68. [31] Tapaszto L, Kertesz K, Vertesy Z, Horvath Z, Koos A, Osvath Z, et al. Diameter and morphology dependence on experimental conditions of carbon nanotube arrays grown by spray pyrolysis. Carbon 2005;43(5):970–7. [32] Ayala P, Gruneis A, Gemming T, Grimm D, Kramberger C, Rummeli MH, et al. Tailoring N-doped single and double wall carbon nanotubes from a nondiluted carbon/nitrogen feedstock. J Phys Chem C 2007;111:2879–84. [33] Fragneaud B, Masenelli-Varlot K, Gonzalez-Montiel A, Terrones M, Cavaille J. Efficient coating of N-doped carbon nanotubes with polystyrene using atomic transfer radical polymerization. Chem Phys Lett 2006;419(4–6): 567–73.
4 7 ( 2 0 0 9 ) 3 0 –3 7
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Effect of the experimental parameters on the structure of nitrogen-doped carbon nanotubes produced by aerosol chemical vapour deposition Antal A. Koo´s*, Michael Dowling, Kerstin Jurkschat, Alison Crossley, Nicole Grobert* Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK
A R T I C L E I N F O
A B S T R A C T
Article history:
We describe the systematic study of multi-walled carbon nanotubes with different nitrogen
Received 16 June 2008
doping produced by aerosol chemical vapor deposition. Benzylamine:toluene mixtures of
Accepted 19 August 2008
0:100, 5:95, 10:90, 25:75, 50:50, 75:25 and 100:0 were thermally decomposed at 800–900 C
Available online 28 August 2008
under argon at atmospheric pressure, whereby the nitrogen content of the bulk material was varied between 0 and 2.2 at%. We also show how the presence of nitrogen in the precursor changed the nanotube morphology, i.e. nitrogen decreased the number of kinks incorporated into the carbon nanotubes, decreased their length and diameter and increased the proportion of ‘bamboo’ shaped nanotubes. Furthermore, due to the nitrogen doping, the oxidation resistance of the nanotube material was decreased. With concentrations above 10% benzylamine the increase of the reaction temperature had no significant effect on the quality of the nanotubes, however, at higher temperatures the nitrogen content was decreased. We demonstrate the control over the nanotube geometry, the nitrogen content and oxidation resistance of the nanotubes, and show that these properties are interlinked. 2008 Elsevier Ltd. All rights reserved.
1.
Introduction
Over the last decade, carbon nanotubes (CNTs) [1] have been the subject of extensive research due to their outstanding mechanical and electronic properties [2,3]. It has been shown that CNTs can behave as metals or semiconductors depending on their diameter and chirality. Although CNTs can be produced using a number of methods, there is still no control over nanotube morphology and hence their electronic behaviour. However, theoretical [4] and experimental studies [5] revealed that it is possible to tune the electronic properties of the nanotubes by incorporating hetero atoms within the carbon lattice [6]. The most frequently used dopants are boron and nitrogen because their atom size is similar to that of carbon [7–18] and because they serve as p- or n-type dopants, respectively [6]. The successful incorporation of nitrogen [15]
or boron [7] atoms within the graphitic carbon cylinders strongly depends on the choice of precursor, catalyst, reaction temperature, reaction time, gas flow rate and pressure [19]. Furthermore, the doping of nanotubes with hetero atoms also changes the nanotube structure [20], chemical reactivity [21] and mechanical stability [22]. Therefore, the use of nanotubes for future technological applications requires a precisely controlled introduction of dopants. In recent years, different methods have been developed for the production of nitrogen-doped carbon nanotubes (CNx nanotubes) including the arc discharge [23], ion implantation [13,17] and various techniques based on chemical vapor deposition (CVD) [19]. However, practical applications of CNTs or CNx nanotubes necessitate fairly large amounts of material at commercially viable prices. Aerosol CVD appears to be the most suitable synthesis method for industrial scale production of CNTs.
* Corresponding authors: Fax: +44 1865 27833. E-mail addresses: [email protected], [email protected] (A.A. Koo´s), [email protected] (N. Grobert). 0008-6223/$ - see front matter 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2008.08.014
CARBON
4 7 ( 2 0 0 9 ) 3 0 –3 7
For example, it is suitable for the continuous injection of catalyst precursor and carbon feedstock, it requires no additional catalyst preparation step and the samples need minimal or no purification [24,25]. Using aerosol CVD it is also possible to produce CNx nanotubes without using flammable or corrosive gases such as ammonia. Although several articles have been published about the synthesis of CNx nanotubes, the comparison of the results obtained using different experimental setups is difficult, and the influence of one chosen parameter on the quality of nanotubes cannot be determined. The dependence of the quality of nanotubes on the synthesis parameters can be understood only when one parameter is changed at a time. This study was carried out in order to identify structural trends in CNx nanotubes produced via aerosol CVD as a function of the synthesis parameters used during growth. The morphological changes of the CNx nanotubes were analysed using the following characterisation techniques: scanning electron microscopy, transmission electron microscopy, electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA).
2.
31
the aerosol generator was switched on and the argon flow was increased to 2500 sccm for the duration of the experiment (10, 20, 30 or 120 min). Subsequently, the flow rate was reduced again to 100 sccm while the furnace was left to cool to room temperature. Carbon soot was mainly produced in the centre of the furnace and some of it was carried by the gas flow to the acetone trap located at the opposite end to the aerosol generator. The CNTs produced on the inner wall of the quartz tube were removed with a sharp metal tool, while the nanotubes grown on the quartz plate were investigated as grown. For the structural characterisation, the as-produced nanotube soot was mounted on SEM stubs using conductive carbon tabs. For the TEM and EELS investigations, the nanotubes were sonicated in acetone in an ultra-sonic bath for 10 min. A few drops of the suspension were deposited on a lacey carbon copper grid and the TEM studies were performed at 400 keV. Raw CNT soot was deposited onto 1 · 1 cm conductive carbon tabs for the XPS measurements and for the TGA studies samples were heated in air from 30 to 850 C at rate of 10 C/min. Approximately 1.5 mg nanotubes were used for each TGA measurement.
Experimental
CNx nanotubes were produced using an aerosol-based CVD system consisting of a piezo-driven aerosol generator (RBI Pyrosol 7901), a quartz tube (2.2 cm inner diameter) and a 50 cm long horizontal electrical furnace, equipped with a gas flow controller and a gas trap filled with acetone. Solutions of 5 wt% ferrocene (Fe(C5H5)2, Aldrich 98%) in mixtures of toluene (C6H5CH3, Fluka 99.7%) and benzylamine (C6H5CH2NH2, Fluka 99%) were prepared in an ultrasonic bath for 30 min. Solutions were prepared for a range of benzylamine:toluene ratios, 0:100, 5:95, 10:90, 25:75, 50:50, 75:25 and 100:0. For the experiments, ferrocene containing mixtures were fed into the piezo-driven aerosol generator and a quartz plate (1 · 5 · 50 mm) was placed inside the quartz tube at the centre of the furnace. The generator (located as near as possible to the furnace in order to avoid the coalescence of droplets) was then connected to the quartz tube located in the horizontal tube furnace. Once the system was set-up, the aerosol generator and the quartz tube were flushed with 100 sccm argon while the furnace was heated to temperatures of 800, 850 or 900 C in order to remove any unwanted oxygen from the reactor. When the desired temperature was reached
3.
Characterisation of CNx nanotubes
3.1.
Overall structural investigation by SEM
SEM studies revealed that the nanotubes grew perpendicular to the substrate, forming flakes of ‘parallel’ aligned nanotubes. The representative SEM images of the nanotubes produced from toluene and benzylamine at 800 C are shown in Fig. 1. The CNTs produced from toluene contained many kinks and were undulating (wavy). The number of kinks decreased as benzylamine was introduced into the precursor, and when more than 10% benzylamine was used the nanotubes were straight. We compared the length of the nanotubes grown on the quartz substrates placed in the reaction tube. The length of the CNTs produced from toluene at 800 C in the 10 min experiments was found to be of the order of 110 ± 11 lm. The length was reduced to one third (36 ± 4 lm) by the introduction of only 10% benzylamine. The CNx nanotubes synthesised from benzylamine were 17 ± 2 lm long, i.e. only 15% of the length of the CNTs synthesised from toluene (see
Fig. 1 – SEM images of the nanotubes made from toluene (a) and benzylamine (b). The nanotubes produced from toluene were undulating while the tubes made from benzylamine were straight.
32
CARBON
4 7 ( 2 0 0 9 ) 3 0 –3 7
Fig. 2 – The length of nanotubes grown in 10 min at 800 C as a function of the benzylamine concentration of the precursor. The introduction of 10% benzylamine in toluene decreased to one-third the length of the nanotubes.
Fig. 3 – The length of nanotubes produced at 800 C from benzylamine and toluene as a function of the growth time.
Fig. 2). Consequently, the presence of nitrogen slowed down the nanotube growth significantly, even at low benzylamine concentrations. According to theoretical calculations [26] growth is inhibited because the nitrogen saturates the tube edge at the growing end, meaning nitrogen atoms need to be removed and replaced by carbon, or defects need to be incorporated into the graphitic network [26]. The weight of the samples produced from precursors with different benzylamine concentration was similar (approximately 300 mg/ experiment), despite the change in the nanotube length. The length of the nanotubes synthesised at 800 C from toluene and benzylamine is displayed in Fig. 3 as a function of experiment duration. The length of the CNTs produced from toluene in 30 min was 455 ± 45 lm and in 120 min was 785 ± 78 lm. The length of CNx nanotubes produced from benzylamine in 30 min was 36 ± 4 lm and in 120 min was 110 ± 11 lm, the same as the CNTs synthesised from toluene in 10 min.
3.2.
Characterisation of internal structure by TEM
TEM investigations showed that the samples were ‘clean’, i.e. they contained less than 5 wt% amorphous carbon and almost no polyhedral carbon particles, but the nanotubes incorporated defects typical to CVD synthesis. The samples
also contained Fe catalyst particles encapsulated in the inner core or attached to the nanotube surface. Using TEM it was possible to compare the samples, but it was difficult to quantify the quality of the nanotubes. We defined ‘quality’ in terms of the average distance between kinks. Changes caused by the incorporation of nitrogen, for example, stacked cone structure or corrugated walls were not considered as defects. The change of the nanotube structure as a function of the benzylamine concentration is presented in Fig. 4 using representative TEM images of samples synthesised at 800 C. The nanotubes produced from toluene contained many kinks, but their number decreased as the benzylamine concentration increased. This observation was confirmed by SEM. It was found, that the presence of benzylamine in the precursor changed the inner structure of the nanotubes, too. The samples contained CNx nanotubes exhibiting stacked-cone structure and tubes consisting of corrugated walls, together with the more crystalline nanotubes. In the following, stackedcone and corrugated nanotubes will be called ‘bamboo’ nanotubes. The CNx samples produced from precursors containing 5% benzylamine contained approximately 20% ‘bamboo’ nanotubes, this increased to 40% ‘bamboo’ nanotubes at 10% benzylamine, and more than 80% ‘bamboo’ nanotubes when benzylamine was used as the precursor. Fig. 5a shows a crystalline nanotube produced from toluene only. A CNx nanotube made from benzylamine exhibiting the typical stacked-cone structure is shown in Fig. 5c. The walls of CNx nanotubes are corrugated for nanotubes with stacked-cone structure and nanotubes with walls parallel to axis. Corrugation of the walls already occurs in tubes made from precursors containing 5% benzylamine. The formation of CNx nanotubes with corrugated walls is caused by the presence of nitrogen in the graphitic network, which induces curvature of the graphitic layer [26,27]. The formation of CNx nanotubes with periodic compartment structures can be explained by the difference between the carbon surface and bulk diffusion in the catalyst. It is likely that the walls of the CNx nanotubes were formed from the side surface of the catalyst mainly via carbon surface diffusion, while the inner layers form from top surface of the catalyst via bulk diffusion [28,29]. The incorporation of nitrogen atoms promotes tube closure, thereby triggering the presence of ‘bamboo’ structures [26]. The increase of the benzylamine content also increased the ratio of ‘bamboo’ nanotubes in the samples produced at higher temperatures, 850 and 900 C. The proportion of ‘bamboo’ nanotubes produced from the same precursor at different temperatures was similar. The density of the kinks in the CNTs made from toluene increased significantly with the increase of the temperature (Fig. 6). The CNx samples made at 900 C from precursors containing 5% benzylamine incorporated more defects than those made at 800 C, but above 10% benzylamine increasing the temperature from 800 to 900 C had no major impact on the quality of the samples. The outer diameter distribution as a function of benzylamine concentration is compared on Fig. 7. Approximately 300 tube diameters per sample were measured on TEM images taken of samples synthesised at 800 C in 10 min.
CARBON
4 7 ( 2 0 0 9 ) 3 0 –3 7
33
Fig. 4 – TEM images of nanotubes made from 100:0 (a), 95:5 (b), 90:10 (c) and 0:100 (d) toluene:benzylamine mixtures. The increase of nitrogen content in the precursor decreased the number of kinks, decreased the nanotube diameter and increased the proportion of ‘bamboo’ nanotubes.
The average outer diameter of the nanotubes was reduced from 62 nm, measured for toluene, to 52 nm for 5% benzylamine, and to 30 nm for CNx nanotubes grown from benzylamine. About 5% benzylamine reduced the most frequent outer diameter from 55 nm for toluene to 36 nm, and it decreased to 25 nm for CNx nanotubes made from benzylamine. The change in the FWHM was significant, too. The FWHM of 66 nm calculated for CNTs made from toluene was reduced to 37 nm by 5% benzylamine and to 24 nm by 10% benzylamine. At higher benzylamine concentrations, the FWHM did not change substantially. Thus, the increase of benzylamine content in the precursor decreased the average outer diameter of the nanotubes. Compared with the CNTs synthesised from toluene, the addition of only 5% benzylamine considerably reduced the average outer diameter of the CNx nanotubes. On the other hand, the average inner diameter of the nanotubes was increased from 9 nm, measured for tubes made from toluene, to 13 nm for CNx nanotubes grown from benzylamine. Hence, the nanotubes made from toluene only exhibit approximately three times more walls than the CNx nanotubes made from benzylamine. The increase of the reaction time from 10 to 20 min significantly changed the diameter distribution of the CNTs made from toluene, i.e. the number of thin nanotubes (ca. 25 nm outer diameter) was higher. In comparison, longer reaction times had no notable effect on the diameter distribution of the CNx nanotubes made from benzylamine. We suppose that within the first few minutes of the experiments the surface of
the quartz tube became covered with nanotubes. Consequently, not all of the incoming catalyst material could reach the substrate, and this caused the nanotubes themselves to act as a substrate for nanotube growth [30]. These new catalyst particles grown on the nanotubes were smaller than those grown on the surface of the quartz tube, and this increased the number of thin tubes [31]. The nanotubes grown from the new catalyst particles were shorter and contained more defects than the primary tubes.
3.3.
Characterisation of the nitrogen content via XPS
The incorporation of nitrogen within CNx nanotubes was confirmed by XPS measurements. The sample produced at 800 C from precursor containing 5 vol% benzylamine contained 0.28 ± 0.09 at% nitrogen, the nitrogen content increased to 0.49 ± 0.16 at% when the benzylamine concentration was 10 vol% (Fig. 8). The sample made from 1:1 benzylamine: toluene mixture contained 0.9 ± 0.16 at% nitrogen, while the nitrogen content in sample produced from benzylamine increased to 2.2 ± 0.3 at%. In this case the N/C ratio in the sample produced from benzylamine was to about 15% of the N/C ratio in the feedstock. The increase of the temperature decreased significantly the nitrogen content of the sample. The nitrogen content of the samples produced at 900 C was approximately half of those produced at 800 C. We determined the type of N bonding from the chemical shifts of the N 1s line for the samples produced at 800 C
34
CARBON
4 7 ( 2 0 0 9 ) 3 0 –3 7
Fig. 5 – (a) HRTEM image of a MWNT produced from toluene only, (b) close-up of (a) revealing a fairly crystalline structure and individual kinks typical pure carbon MWNTs grown by CVD. (c) HRTEM image of a MWCNx synthesised using benzylamine only, (d) close-up of (c) showing the corrugated nature frequently observed in MWCNx.
using a 1:1 mixture of benzylamine:toluene and benzylamine only. We identified tree types of nitrogen from the deconvoluted N 1s XPS spectrum: pyridinic (398.3 eV), sp2 (400.7 eV) and gaseous (404.5 eV) [32]. The sp2/pyridinic nitrogen ratio increased from 0.9, measured on the 1:1 benzylamine:toluene sample, to 1.1, measured on the sample produced from benzylamine only. In parallel the ratio of gaseous nitrogen also increased, i.e. 18–44%.
3.4. Investigation of nitrogen content of individual CNx nanotubes using EELS In addition to XPS, the nitrogen content of individual CNx nanotubes was estimated using EELS. The CNx nanotubes made from benzylamine contained approximately 2.5 wt% nitrogen, this decreased to approximately 1 wt% in nanotubes produced from 1:1 benzylamine:toluene mixture. This result agrees with the findings of Reyes-Reyes and co-workers. They reported 2–3% of nitrogen content in the walls of the CNx nanotubes produced from benzylamine using similar experimental setup and parameters [28]. EELS was not sensitive enough to measure the nitrogen concentration of CNx nanotubes produced from precursors with lower benzylamine content. The nitrogen distribution in the CNx nanotubes synthesised from benzylamine was investigated using Energy Filtered TEM. This showed uniform nitrogen distribution and it was not possible to identify islands with higher or lower nitrogen content.
3.5.
Oxidation resistance measured via TGA
The oxidation resistance of the nanotubes was determined using TGA. Data from two TGA measurements for each investigated benzylamine concentration is displayed in Fig. 9, the measurements were made on samples produced at 800 and 900 C in 10 min. In order to help the comparison of 14 TGA curves, Fig. 9 shows the temperatures where 40%, 60% and 80% of the sample weight remained during the TGA measurements as a function of the benzylamine concentration of the precursor. The CNTs synthesised at 800 C from toluene oxidised between 490 and 720 C, while the CNx nanotubes synthesised at the same temperature from benzylamine burned between 430 and 570 C. The CNx samples also showed decreasing oxidation temperature with the increase of the benzylamine concentration for intermediate precursor mixtures. A linear dependence was observed excluding small deviations (errors <4%). Interestingly, the most inert samples produced at 800 C were made from precursors containing 5% benzylamine. The TGA measurements of samples produced at 900 C showed similar trends. As an exception to this rule, the samples synthesised from toluene were less inert that those made from precursors containing 10% benzylamine. This behaviour confirms the TEM observations where the quality of the nanotubes synthesised from toluene and 5:95 benzylamine:toluene mixture decreased with the increase of the temperature
CARBON
4 7 ( 2 0 0 9 ) 3 0 –3 7
35
Fig. 6 – TEM images of nanotubes made at 850 (a and b) and 900 C (c and d) from toluene and benzylamine, respectively. The increase of the temperature increased the density of the kinks in the CNTs made from toluene, but it had no major impact on the quality of the CNx nanotubes made from benzylamine.
Fig. 7 – Outer diameter distribution of the nanotubes made at different benzylamine concentrations. The introduction of 10% benzylamine in toluene decreased significantly the outer diameter of the nanotubes.
Fig. 8 – The nitrogen content of the samples determined from XPS measurements. The increase of benzylamine concentration in precursor increased the nitrogen content of the samples produced at the same synthesis temperature. The increase of the synthesis temperature from 800 to 900 C decreased the nitrogen content of the samples to half.
(Fig. 6), but above 10% benzylamine content the temperature had no major influence on the quality of the CNx nanotubes. Above 10% benzylamine concentration the increase of the production temperature from 800 to 900 C increased the oxidation temperature of the CNx nanotubes. However, according to XPS this temperature increase also decreased the nitrogen content. The oxidation temperature plotted as a function of nitrogen content shows linear dependence, dem-
onstrating that the oxidation temperature is strongly correlated to the nitrogen content regardless of synthesis temperature. This result confirms the changes in the structure of the nanotubes and the change of ‘bamboo’ nanotube ratio determined with TEM. The nitrogen incorporated into the CNx nanotubes changed their structure, increasing the density of ‘defects’ and the reactivity of the walls. The sharp decrease of the weight
36
CARBON
4 7 ( 2 0 0 9 ) 3 0 –3 7
For reference, the key findings of this investigation are summarised below: With increasing benzylamine concentration the following trends were identified. • • • •
Fig. 9 – Temperatures where 40%, 60% and 80% of the sample weight remained during TGA measurements as a function of the benzylamine concentration in the precursor. Black symbols represent the nanotubes made at 800 C, grey symbols represent the nanotubes made at 900 C. The increase of benzylamine concentration in precursor decreased the oxidation resistance of the samples produced at the same synthesis temperature. The increase of the synthesis temperature increased the oxidation resistance.
of the samples made from precursors with a high benzylamine concentration indicates that the CNx nanotubes react with their environment on their whole surface, not only at randomly distributed defects like the CNTs. The increased reactivity and uniform defect distribution may help the sidewall functionalisation of the CNx nanotubes and improve their interaction with the matrix in composites [33].
4.
Conclusions
Multi-walled carbon nanotubes with different concentrations of nitrogen doping were produced by aerosol CVD from toluene and benzylamine mixtures. According to TEM and SEM the increasing benzylamine content of the precursor decreased the number of kinks incorporated into nanotubes, decreased their length and diameter, and increased the proportion of ‘bamboo’ nanotubes into the sample. The influence of the increase of benzylamine content on the structure was not uniform. Only 10% benzylamine mixed into precursor changed significantly the geometry of the CNx nanotubes compared with the CNTs, but the geometry was not so sensitive on nitrogen content at higher benzylamine concentrations. The nitrogen content of the samples could be tuned; the highest nitrogen content was 2.2 ± 0.3 at%. The N/C ratio in the CNx nanotubes produced from benzylamine at 800 C was about 15% of the N/C ratio in the feedstock. Only 100 C increase of the synthesis temperature decreased the nitrogen content of the CNx nanotubes to half. TGA measurements showed that the presence of nitrogen decreased the oxidation resistance of the nanotube walls, which may help the sidewall functionalisation of CNx nanotubes and improve the matrix-nanotube bonds in composites. The oxidation temperature had linear dependence on the nitrogen content of the nanotubes. These results show that it was possible to control the geometry, oxidation resistance and nitrogen content of the nanotubes, but these properties were not independent of each other.
• • • • • •
Increasing nitrogen-doping, up to 2.2 at%. Decreasing growth rate. Decreasing mean outer-diameter, from approximately 60 to 30 nm. Increasing proportion of bamboo and corrugated structures, from <10% to >85%. Gradual loss of crystallinity. Decreasing number of kinks. Reduced oxidation resistance. Decreasing nitrogen-doping; by a factor of half when temperature is increased from 800 to 900 C. The sp2/pyridinic nitrogen ratio and the ratio of gaseous nitrogen increased. Using these results it is possible to produce nanotubes with desired properties.
Acknowledgements This work has been supported in part by the European Commission under the 6 Framework Programme (STREP project BNC Tubes, contract number NMP4-CT-2006-03350). The support from Royal Society, WomenInNano, BegbrokeNano and the GDR-I nanotubes is acknowledged.
R E F E R E N C E S
[1] Iijima S. Helical microtubules of graphitic carbon. Nature 1991;354(6348):56–8. [2] Dresselhaus MS, Dresselhaus G, Saito R. Physics of carbon nanotubes. Carbon 1995;33(7):883–91. [3] Grobert N. Carbon nanotubes – becoming clean. Mater Today 2007;10(1–2):28–35. [4] Latil S, Roche S, Mayou D, Charlier J. Mesoscopic transport in chemically doped carbon nanotubes. Phys Rev Lett 2004;92(25):256805. [5] Terrones M, Ajayan P, Banhart F, Blase X, Carroll D, Charlier J, et al. N-doping and coalescence of carbon nanotubes: synthesis and electronic properties. Appl Phys A-Mater 2002;74(3):355–61. [6] Terrones M, Jorio A, Endo M, Rao A, Kim Y, Hayashi T, et al. New direction in nanotube science. Mater Today 2004;7:30–45. [7] Stephan O, Ajayan P, Colliex C, Redlich P, Lambert J, Bernier P, et al. Doping graphitic and carbon nanotube structures with boron and nitrogen. Science 1994;266(5191):1683–5. [8] Terrones M, Grobert N, Terrones H. Synthetic routes to nanoscale BxCyNz architectures. Carbon 2002;40(10):1665–84. [9] Endo M, Muramatsu H, Hayashi T, Kim Y, Van Lier G, Charlier J, et al. Atomic nanotube welders: boron interstitials triggering connections in double-walled carbon nanotubes. Nano Lett 2005;5(6):1099–105. [10] Mcguire K, Gothard N, Gai P, Dresselhaus M, Sumanasekera G, Rao A. Synthesis and Raman characterization of borondoped single-walled carbon nanotubes. Carbon 2005;43(2):219–27.
CARBON
4 7 ( 2 0 0 9 ) 3 0 –3 7
[11] Maultzsch J, Reich S, Thomsen C, Webster S, Czerw R, Carroll D, et al. Raman characterization of boron-doped multiwalled carbon nanotubes. Appl Phys Lett 2002;81(14): 2647–9. [12] Moradian R, Azadi S. Boron and nitrogen-doped single-walled carbon nanotube. Physica E 2006;35(1):157–60. [13] Kotakoski J, Krasheninnikov A, Ma Y, Foster A, Nordlund K, Nieminen R. B and N ion implantation into carbon nanotubes: insight from atomistic simulations. Phys Rev B 2005;71(20):205408. [14] Ewels C, Glerup M. Nitrogen doping in carbon nanotubes. J Nanosci Nanotechnol 2005;5(9):1345–63. [15] Kim S, Lee J, Na C, Park J, Seo K, Kim B. N-doped doublewalled carbon nanotubes synthesized by chemical vapor deposition. Chem Phys Lett 2005;413(4–6):300–5. [16] Terrones M, Terrones H, Grobert N, Hsu W, Zhu Y, Hare J, et al. Efficient route to large arrays of CNx nanofibers by pyrolysis of ferrocene/melamine mixtures. Appl Phys Lett 1999;75(25):3932–4. [17] Yamamoto K, Kamimura T, Matsumoto K. Nitrogen doping of single-walled carbon nanotube by using massseparated low-energy ion beams. Jpn J Appl Phys 2005;44(4A): 1611–4. [18] Villalpando-Paez F, Zamudio A, Elias A, Son H, Barros E, Chou S, et al. Synthesis and characterization of long strands of nitrogen-doped single-walled carbon nanotubes. Chem Phys Lett 2006;424(4–6):345–52. [19] Tang C, Bando Y, Golberg D, Xu F. Structure and nitrogen incorporation of carbon nanotubes synthesized by catalytic pyrolysis of dimethylformamide. Carbon 2004;42(12–13): 2625–33. [20] Nemes-Incze P, Daroczi N, Sarkozi Z, Koos A, Kertesz K, Tiprigan O, et al. Synthesis of bamboo-structured multiwalled carbon nanotubes by spray pyrolysis method, using a mixture of benzene and pyridine. J Optoelectron Adv Mater 2007;9(5):1525–9. [21] Nevidomskyy A, Csanyi G, Payne M. Chemically active substitutional nitrogen impurity in carbon nanotubes. Phys Rev Lett 2003;91(10):105502. [22] Hernandez E, Goze C, Bernier P, Rubio A. Elastic properties of single-wall nanotubes. Appl Phys A-Mater 1999;68(3): 287–92.
37
[23] Glerup M, Steinmetz J, Samaille D, Stephan O, Enouz S, Loiseau A, et al. Synthesis of N-doped SWNT using the arcdischarge procedure. Chem Phys Lett 2004;387(1–3):193–7. [24] Horvath Z, Kertesz K, Petho L, Koos A, Tapaszto L, Vertesy Z, et al. Inexpensive, upscalable nanotube growth methods. Curr Appl Phys 2006;6(2):135–40. [25] Grobert N. Nanotubes – grow or go? Mater Today 2006;9(10):64. [26] Sumpter B, Meunier V, Romo-Herrera J, Cruz-Silva E, Cullen D, Terrones H, et al. Nitrogen-mediated carbon nanotube growth: diameter reduction, metallicity, bundle dispersability, and bamboo-like structure formation. ACS Nano 2007;1(4):369–75. [27] Mandumpal J, Gemming S, Seifert G. Curvature effects of nitrogen on graphitic sheets: structures and energetics. Chem Phys Lett 2007;447(1–3):115–20. [28] Reyes-Reyes M, Grobert N, Kamalakaran R, Seeger T, Golberg D, Ruhle M, et al. Efficient encapsulation of gaseous nitrogen inside carbon nanotubes with bamboolike structure using aerosol thermolysis. Chem Phys Lett 2004;396(1–3):167–73. [29] Wang Y, Tang G, Koeck F, Brown B, Garguilo J, Nemanich R. Experimental studies of the formation process and morphologies of carbon nanotubes with bamboo mode structures. Diam Relat Mater 2004;13(4–8):1287–91. [30] Singh C, Shaffer M, Windle A. Production of controlled architectures of aligned carbon nanotubes by an injection chemical vapour deposition method. Carbon 2003;41(2):359–68. [31] Tapaszto L, Kertesz K, Vertesy Z, Horvath Z, Koos A, Osvath Z, et al. Diameter and morphology dependence on experimental conditions of carbon nanotube arrays grown by spray pyrolysis. Carbon 2005;43(5):970–7. [32] Ayala P, Gruneis A, Gemming T, Grimm D, Kramberger C, Rummeli MH, et al. Tailoring N-doped single and double wall carbon nanotubes from a nondiluted carbon/nitrogen feedstock. J Phys Chem C 2007;111:2879–84. [33] Fragneaud B, Masenelli-Varlot K, Gonzalez-Montiel A, Terrones M, Cavaille J. Efficient coating of N-doped carbon nanotubes with polystyrene using atomic transfer radical polymerization. Chem Phys Lett 2006;419(4–6): 567–73.