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N-SWCNTs production by aerosol-assisted CVD method
Chemical Physics Letters 538 (2012) 108–111
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
N-SWCNTs production by aerosol-assisted CVD method Antal A. Koós a,⇑, Frank Dillon a, Rebecca J. Nicholls a, Lyubov Bulusheva b, Nicole Grobert a a b
Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK Nikolaev Institute of Inorganic Chemistry, 3, Acad. Lavrentiev Ave., Novosibirsk 630090, Russia
a r t i c l e
i n f o
Article history: Received 14 February 2012 In final form 19 April 2012 Available online 27 April 2012
a b s t r a c t The insertion of dopant atoms into the carbon nanotube framework allows one to tailor their electronic properties, reactivity and structure. We investigated the effect of the reaction parameters on the yield and quality of N-doped single-walled carbon nanotubes (N-SWCNTs) produced via aerosol chemical vapour deposition. In this context, aerosols of ethanol/benzylamine mixtures in conjunction with ferrocene were pyrolysed at temperatures between 950 and 1100 °C. As a result, we were able to produce NSWCNTs with a nitrogen content of ca. 1 at.%, diameters ranging between 0.9 and 1.8 nm, and production rates of more than 10 mg/h. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental details
Carbon nanotubes (CNTs) have several important applications [1,2], but nanotubes with well defined properties at reasonable price are needed [3]. In particular single walled CNTs (SWCNTs) are either metallic or semiconducting [4], but the selective growth or selection of SWCNTs with these properties is challenging. Fortunately, it is possible to tailor the electronic structure of SWCNTs by substituting carbon atoms with other atomic species such as nitrogen or boron [5,6]. Moreover doping makes it possible to fine tune the wall reactivity and mechanical strength of SWCNTs [7,8], but it is important to be able to carefully control the insertion of dopants into nanotubes [9–12]. Due to its proximity to carbon in the periodic table nitrogen is the most common dopant of CNTs [13–19]. Herein we report the production of nitrogen-doped single walled carbon nanotubes (N-SWCNTs) using aerosol chemical vapour deposition (ACVD), a cheap and scalable experimental method. Also as the catalyst particles are produced in situ there is no need for catalyst preparation and the experiments can be run continuously [20–25]. Furthermore, N-SWCNTs can be produced without toxic gases such as CO or NH3. The production of N-SWCNTs has been previously reported [26–28], however only small quantities of N-SWCNTs were produced. In order to increase the yield and to understand the effect of experimental parameters on the nanotube properties we systematically investigated the effect of doping, temperature and presence of hydrogen on the quantity and quality of the nanotubes.
N-SWCNTs were synthesised using a piezo-driven aerosol generator equipped with a quartz nozzle (2 mm ID) and connected to a quartz tube (2.2 cm ID) which was placed in a 50 cm long horizontal electrical tube furnace [29]. The nozzle was used in order to increase the heating rate of the precursor and reduce the size of the catalyst particles. Instead of heating the precursor together with the carrier gas according the temperature profile of the furnace, the cold precursor was introduced into the hot gas present in the furnace in approximately 0.01 s. Ferrocene (1.25 wt.%) containing ethanol aerosols and aerosols of ethanol:benzylamine (9:1) mixtures were pyrolysed at 950, 975, 1000, and 1150 °C. The furnace was heated and cooled in the presence of an argon flow (100 sccm). When the reaction temperature was reached a mixture of argon:hydrogen (9:1, at 2500 sccm total flow rate) was introduced for the duration of the reaction (60 min). N-SWCNTs containing soot formed and deposited on the quartz tube surface after the furnace, where the temperature was below 400 °C. The quality and morphology of the N-SWCNTs was analysed by transmission electron (TEM) and scanning electron microscopy (SEM), while their overall crystallinity was investigated using Raman spectroscopy. Electron energy loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS) was employed in order to confirm the nitrogen content of the nanotubes. The decomposition of the precursor during the synthesis of the nanotubes was monitored in situ by mass spectroscopy to get an insight into the nanotube growth. 3. Results
⇑ Corresponding author. E-mail address: [email protected] (A.A. Koós). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.04.047
N-SWCNTs formed as bundles and their surface was covered by amorphous carbon. Representative TEM images of these bundles
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109
Figure 1. TEM images of N-SWCNTs bundles. The nanotubes are covered by amorphous carbon and catalyst particles are attached to them.
Figure 2. SEM images of as collected N-SWCNTs carpets. The nanotubes formed thick bundles. The arrow shows a catalyst particle.
are shown in Figure. 1. The samples also contained many large Fe catalyst particles which seem unable to form N-SWCNTs. Based on TEM images we estimated that the samples contained approximately 50 wt.% Fe particles. A considerable decrease in the average diameter of the catalyst particles was seen when the nozzle was connected to the aerosol generator, but unfortunately further reduction is needed. The samples collected after the furnace did not contain multi-walled carbon nanotubes. The purity of the samples was also investigated with SEM. Figure 2 shows bundles of nanotubes with catalyst particles attached to them. Changing experimental parameters did not have a significant influence on the diameter of the bundles or the purity of the samples. However the yield was very sensitive to the variation of individual parameters. For example, at 975 °C using the nozzle the production rate doubled from 2–3 mg/h to 6 mg/h and the presence of H2 further increased the production rate to 13 mg/h. Therefore the use of hydrogen in large scale production of N-SWCNTs is highly recommended, even if the use of flammable gases increases the cost. Compared with conventional SWCNTs, the presence of nitrogen decreased the yield by approximately 10%. Unfortunately, the increase of the nitrogen content in the precursor, i.e. decomposition of 3:1 ethanol/benzylamine mixtures, decreased the production rate to ten percent of that achieved growing conventional SWCNTs. Since the incorporation of N atoms in graphitic carbon network is energetically unfavourable, N saturates the growing edge of the nanotube and inhibits the incorporation of carbon atoms [30]. Therefore scaling up the production of
N-SWCNTs is challenging. The highest production rate, 15 mg/h, was observed at 1000 °C. The effect of the furnace temperature, doping and presence of hydrogen in the carrier gas on the quality and diameter distribution of the N-SWCNTs was compared using Raman spectroscopy [31,32]. Figure 3a shows the Raman spectra of SWCNTs measured with a 532 nm laser. The increase of the furnace temperature increased both the D/G and 2D/G intensity ratios in N-SWCNTs, but a decrease was observed in the 2D/G intensity ratio measured on samples grown at 975 °C. The best samples, with the lowest D/G intensity ratio i.e. lowest defect concentration, were made at 1000 °C. In order to understand the effect of N doping on the quality of the nanotubes we compared the Raman spectra of SWCNTs and N-SWCNTs. For SWCNTs produced at 950 and 1150 °C the ID/IG ratio were 0.07 and 0.09, respectively. But the ID/IG ratio increased to 0.15 and 0.19 for N-SWCNTs produced at the same temperatures. This shows that the incorporation of N increased significantly the number of defects. Interestingly, the presence of H2 increased both the D/G and 2D/G intensity ratios in SWCNTs, but decreased both the D/G and 2D/G intensity ratios in N-SWCNTs. The most frequent diameters, indicated using Raman spectroscopy, were between 0.9 and 1.8 nm. The comparison of RBM regions measured with green and red laser are presented on Figure 3b and c, respectively. The diameter of the N-SWCNTs decreased with the increase of the furnace temperature indicating that it is possible to tune the diameter of the N-SWCNTs. N-SWCNTs produced using identical conditions to SWCNTs had smaller diameters. The
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Figure 4. Representative EELS spectrum of N-SWCNTs, inset showing the N edge.
Figure 5. XPS of the N 1s peak for the N-SWCNTs sample. The peaks correspond to the pyridinic (397.6 eV) and sp2 (400.5 eV) bonding, respectively.
Figure 3. Raman spectra of SWCNTs and N-SWCNTs. (a) Comparison of D, G and 2D peaks. (b and c) RBM measured with 532 and 632.8 nm laser, respectively. (d) Effect of the N doping on the position of the 2D peaks, measured with 532 and 632.8 nm lasers. The data was normalised and shifted in order to aid the comparison. The legend for a, b and c was the same.
presence of H2 decreased the diameter of SWCNTs, but increased the diameter N-SWCNTs. Maciel et al. reported that doping changes the position of the 2D peak [33], and therefore it is a sensitive tool to confirm the incorporation of nitrogen in the nanotubes. In order to allow the detailed investigation of 2D peaks, Figure 3d compares the SWCNTs and N-SWCNTs samples produced at 950 °C measured with both green and red laser. The position of 2D peaks in N-SWCNTs samples is shifted to the left in both cases, which confirms the presence of nitrogen. EELS [34] and XPS were used to measure the nitrogen content of N-SWCNTs. A representative EELS spectrum and XPS of the N 1s peak are presented in Figures 4 and 5. Approximately 1 wt.% nitrogen was incorporated according to both EELS and XPS measurements. We identified two types of nitrogen from the deconvoluted N 1s XPS spectrum: pyridinic (397.6 eV) and sp2 (400.5 eV) [28,29,35]. The sp2/pyridinic nitrogen ratio was approximately 1. Since nitrogen content of 2 at.% was already reported for SWCNTs grown using acetonitrile and a different CVD system [28] we made efforts to increase the N content of N-SWCNTs, but higher nitrogen content of the precursor blocked the nanotube growth. Decreasing the furnace temperature increased the nitrogen content in N-MWNT produced with similar experimental setup [29], however it decreased the N-SWCNTs yield.
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Acknowledgements We are grateful to the European Union FP6 Project BNC nanotubes 033350, BegbrokeNano, the Royal Society (NG), the European Research Council (ERC-2009-StG-240 500) (NG), and Engineering and Physical Sciences Research Council (EP/H046550/1) (RJN) for financial support. References [1] [2] [3] [4] [5] [6]
Figure 6. Mass spectra of the gasses leaving the furnace.
The experiments were monitored using mass spectrometry. For comparison the mass spectra of the gasses leaving the furnace in different experiments are presented in Figure 6. The spectra showed that the precursors were cracked to small molecules in all experiments. The most frequent molecule leaving the furnace was methane, which may be also formed due to catalytic hydrogenation of carbon [36]. Large quantities of acetylene and water were also detected, especially for SWCNTs made at low temperature. The highest detected mass corresponded to benzene, and therefore all benzylamine from the precursor was decomposed. These results indicate that small gas molecules formed in the furnace played a more important role in the nanotube growth than the original precursors, therefore N-SWCNTs may be produced from other precursors, too.
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
4. Conclusions We were able to produce N-SWCNTs with a nitrogen content of ca. 1 at.%, diameters ranging between 0.9 and 1.8 nm, and production rates of more than 10 mg/h using aerosol CVD method. The highest yield was obtained at 1000 °C in 9:1 Ar:H2 atmosphere. From Raman spectroscopy we conclude that nitrogen increases the defect density and inhibits the growth of large diameter tubes. Additionally, both the furnace temperature and presence of hydrogen are able to tune the diameter distribution. Because the nanotubes were deposited after the furnace where the temperature was less than 400 °C, this experimental method allows the in situ deposition of nanotubes on substrates sensitive to high temperatures. Since the precursors were cracked to small molecules in all experiments, we can conclude that these small molecules played the most important role in the SWCNTs production.
[26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36]
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Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
N-SWCNTs production by aerosol-assisted CVD method Antal A. Koós a,⇑, Frank Dillon a, Rebecca J. Nicholls a, Lyubov Bulusheva b, Nicole Grobert a a b
Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK Nikolaev Institute of Inorganic Chemistry, 3, Acad. Lavrentiev Ave., Novosibirsk 630090, Russia
a r t i c l e
i n f o
Article history: Received 14 February 2012 In final form 19 April 2012 Available online 27 April 2012
a b s t r a c t The insertion of dopant atoms into the carbon nanotube framework allows one to tailor their electronic properties, reactivity and structure. We investigated the effect of the reaction parameters on the yield and quality of N-doped single-walled carbon nanotubes (N-SWCNTs) produced via aerosol chemical vapour deposition. In this context, aerosols of ethanol/benzylamine mixtures in conjunction with ferrocene were pyrolysed at temperatures between 950 and 1100 °C. As a result, we were able to produce NSWCNTs with a nitrogen content of ca. 1 at.%, diameters ranging between 0.9 and 1.8 nm, and production rates of more than 10 mg/h. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental details
Carbon nanotubes (CNTs) have several important applications [1,2], but nanotubes with well defined properties at reasonable price are needed [3]. In particular single walled CNTs (SWCNTs) are either metallic or semiconducting [4], but the selective growth or selection of SWCNTs with these properties is challenging. Fortunately, it is possible to tailor the electronic structure of SWCNTs by substituting carbon atoms with other atomic species such as nitrogen or boron [5,6]. Moreover doping makes it possible to fine tune the wall reactivity and mechanical strength of SWCNTs [7,8], but it is important to be able to carefully control the insertion of dopants into nanotubes [9–12]. Due to its proximity to carbon in the periodic table nitrogen is the most common dopant of CNTs [13–19]. Herein we report the production of nitrogen-doped single walled carbon nanotubes (N-SWCNTs) using aerosol chemical vapour deposition (ACVD), a cheap and scalable experimental method. Also as the catalyst particles are produced in situ there is no need for catalyst preparation and the experiments can be run continuously [20–25]. Furthermore, N-SWCNTs can be produced without toxic gases such as CO or NH3. The production of N-SWCNTs has been previously reported [26–28], however only small quantities of N-SWCNTs were produced. In order to increase the yield and to understand the effect of experimental parameters on the nanotube properties we systematically investigated the effect of doping, temperature and presence of hydrogen on the quantity and quality of the nanotubes.
N-SWCNTs were synthesised using a piezo-driven aerosol generator equipped with a quartz nozzle (2 mm ID) and connected to a quartz tube (2.2 cm ID) which was placed in a 50 cm long horizontal electrical tube furnace [29]. The nozzle was used in order to increase the heating rate of the precursor and reduce the size of the catalyst particles. Instead of heating the precursor together with the carrier gas according the temperature profile of the furnace, the cold precursor was introduced into the hot gas present in the furnace in approximately 0.01 s. Ferrocene (1.25 wt.%) containing ethanol aerosols and aerosols of ethanol:benzylamine (9:1) mixtures were pyrolysed at 950, 975, 1000, and 1150 °C. The furnace was heated and cooled in the presence of an argon flow (100 sccm). When the reaction temperature was reached a mixture of argon:hydrogen (9:1, at 2500 sccm total flow rate) was introduced for the duration of the reaction (60 min). N-SWCNTs containing soot formed and deposited on the quartz tube surface after the furnace, where the temperature was below 400 °C. The quality and morphology of the N-SWCNTs was analysed by transmission electron (TEM) and scanning electron microscopy (SEM), while their overall crystallinity was investigated using Raman spectroscopy. Electron energy loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS) was employed in order to confirm the nitrogen content of the nanotubes. The decomposition of the precursor during the synthesis of the nanotubes was monitored in situ by mass spectroscopy to get an insight into the nanotube growth. 3. Results
⇑ Corresponding author. E-mail address: [email protected] (A.A. Koós). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.04.047
N-SWCNTs formed as bundles and their surface was covered by amorphous carbon. Representative TEM images of these bundles
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109
Figure 1. TEM images of N-SWCNTs bundles. The nanotubes are covered by amorphous carbon and catalyst particles are attached to them.
Figure 2. SEM images of as collected N-SWCNTs carpets. The nanotubes formed thick bundles. The arrow shows a catalyst particle.
are shown in Figure. 1. The samples also contained many large Fe catalyst particles which seem unable to form N-SWCNTs. Based on TEM images we estimated that the samples contained approximately 50 wt.% Fe particles. A considerable decrease in the average diameter of the catalyst particles was seen when the nozzle was connected to the aerosol generator, but unfortunately further reduction is needed. The samples collected after the furnace did not contain multi-walled carbon nanotubes. The purity of the samples was also investigated with SEM. Figure 2 shows bundles of nanotubes with catalyst particles attached to them. Changing experimental parameters did not have a significant influence on the diameter of the bundles or the purity of the samples. However the yield was very sensitive to the variation of individual parameters. For example, at 975 °C using the nozzle the production rate doubled from 2–3 mg/h to 6 mg/h and the presence of H2 further increased the production rate to 13 mg/h. Therefore the use of hydrogen in large scale production of N-SWCNTs is highly recommended, even if the use of flammable gases increases the cost. Compared with conventional SWCNTs, the presence of nitrogen decreased the yield by approximately 10%. Unfortunately, the increase of the nitrogen content in the precursor, i.e. decomposition of 3:1 ethanol/benzylamine mixtures, decreased the production rate to ten percent of that achieved growing conventional SWCNTs. Since the incorporation of N atoms in graphitic carbon network is energetically unfavourable, N saturates the growing edge of the nanotube and inhibits the incorporation of carbon atoms [30]. Therefore scaling up the production of
N-SWCNTs is challenging. The highest production rate, 15 mg/h, was observed at 1000 °C. The effect of the furnace temperature, doping and presence of hydrogen in the carrier gas on the quality and diameter distribution of the N-SWCNTs was compared using Raman spectroscopy [31,32]. Figure 3a shows the Raman spectra of SWCNTs measured with a 532 nm laser. The increase of the furnace temperature increased both the D/G and 2D/G intensity ratios in N-SWCNTs, but a decrease was observed in the 2D/G intensity ratio measured on samples grown at 975 °C. The best samples, with the lowest D/G intensity ratio i.e. lowest defect concentration, were made at 1000 °C. In order to understand the effect of N doping on the quality of the nanotubes we compared the Raman spectra of SWCNTs and N-SWCNTs. For SWCNTs produced at 950 and 1150 °C the ID/IG ratio were 0.07 and 0.09, respectively. But the ID/IG ratio increased to 0.15 and 0.19 for N-SWCNTs produced at the same temperatures. This shows that the incorporation of N increased significantly the number of defects. Interestingly, the presence of H2 increased both the D/G and 2D/G intensity ratios in SWCNTs, but decreased both the D/G and 2D/G intensity ratios in N-SWCNTs. The most frequent diameters, indicated using Raman spectroscopy, were between 0.9 and 1.8 nm. The comparison of RBM regions measured with green and red laser are presented on Figure 3b and c, respectively. The diameter of the N-SWCNTs decreased with the increase of the furnace temperature indicating that it is possible to tune the diameter of the N-SWCNTs. N-SWCNTs produced using identical conditions to SWCNTs had smaller diameters. The
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Figure 4. Representative EELS spectrum of N-SWCNTs, inset showing the N edge.
Figure 5. XPS of the N 1s peak for the N-SWCNTs sample. The peaks correspond to the pyridinic (397.6 eV) and sp2 (400.5 eV) bonding, respectively.
Figure 3. Raman spectra of SWCNTs and N-SWCNTs. (a) Comparison of D, G and 2D peaks. (b and c) RBM measured with 532 and 632.8 nm laser, respectively. (d) Effect of the N doping on the position of the 2D peaks, measured with 532 and 632.8 nm lasers. The data was normalised and shifted in order to aid the comparison. The legend for a, b and c was the same.
presence of H2 decreased the diameter of SWCNTs, but increased the diameter N-SWCNTs. Maciel et al. reported that doping changes the position of the 2D peak [33], and therefore it is a sensitive tool to confirm the incorporation of nitrogen in the nanotubes. In order to allow the detailed investigation of 2D peaks, Figure 3d compares the SWCNTs and N-SWCNTs samples produced at 950 °C measured with both green and red laser. The position of 2D peaks in N-SWCNTs samples is shifted to the left in both cases, which confirms the presence of nitrogen. EELS [34] and XPS were used to measure the nitrogen content of N-SWCNTs. A representative EELS spectrum and XPS of the N 1s peak are presented in Figures 4 and 5. Approximately 1 wt.% nitrogen was incorporated according to both EELS and XPS measurements. We identified two types of nitrogen from the deconvoluted N 1s XPS spectrum: pyridinic (397.6 eV) and sp2 (400.5 eV) [28,29,35]. The sp2/pyridinic nitrogen ratio was approximately 1. Since nitrogen content of 2 at.% was already reported for SWCNTs grown using acetonitrile and a different CVD system [28] we made efforts to increase the N content of N-SWCNTs, but higher nitrogen content of the precursor blocked the nanotube growth. Decreasing the furnace temperature increased the nitrogen content in N-MWNT produced with similar experimental setup [29], however it decreased the N-SWCNTs yield.
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Acknowledgements We are grateful to the European Union FP6 Project BNC nanotubes 033350, BegbrokeNano, the Royal Society (NG), the European Research Council (ERC-2009-StG-240 500) (NG), and Engineering and Physical Sciences Research Council (EP/H046550/1) (RJN) for financial support. References [1] [2] [3] [4] [5] [6]
Figure 6. Mass spectra of the gasses leaving the furnace.
The experiments were monitored using mass spectrometry. For comparison the mass spectra of the gasses leaving the furnace in different experiments are presented in Figure 6. The spectra showed that the precursors were cracked to small molecules in all experiments. The most frequent molecule leaving the furnace was methane, which may be also formed due to catalytic hydrogenation of carbon [36]. Large quantities of acetylene and water were also detected, especially for SWCNTs made at low temperature. The highest detected mass corresponded to benzene, and therefore all benzylamine from the precursor was decomposed. These results indicate that small gas molecules formed in the furnace played a more important role in the nanotube growth than the original precursors, therefore N-SWCNTs may be produced from other precursors, too.
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
4. Conclusions We were able to produce N-SWCNTs with a nitrogen content of ca. 1 at.%, diameters ranging between 0.9 and 1.8 nm, and production rates of more than 10 mg/h using aerosol CVD method. The highest yield was obtained at 1000 °C in 9:1 Ar:H2 atmosphere. From Raman spectroscopy we conclude that nitrogen increases the defect density and inhibits the growth of large diameter tubes. Additionally, both the furnace temperature and presence of hydrogen are able to tune the diameter distribution. Because the nanotubes were deposited after the furnace where the temperature was less than 400 °C, this experimental method allows the in situ deposition of nanotubes on substrates sensitive to high temperatures. Since the precursors were cracked to small molecules in all experiments, we can conclude that these small molecules played the most important role in the SWCNTs production.
[26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36]
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