- Email: [email protected]
Acrylonitrile, an advantageous precursor to synthesize nitrogen doped carbon nanotubes
Journal of Physics and Chemistry of Solids 104 (2017) 52–55
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Acrylonitrile, an advantageous precursor to synthesize nitrogen doped carbon nanotubes
MARK
⁎
A. Aguilar-Elguézabal , M. Román-Aguirre, L. De la Torre, E.A. Zaragoza Centro de Investigación en Materiales Avanzados S.C., (CIMAV), Laboratorio Nacional de Nanotecnología, Miguel de Cervantes 120, Chihuahua, Chih. 31136, Mexico
A R T I C L E I N F O
A BS T RAC T
Keywords: Acrylonitrile Carbon nanotubes (CNTs) Aerosol assisted chemical vapor deposition (AACVD)
The nitrogen doped carbon nanotubes present specific characteristics that offer better performance than pure carbon nanotubes for application like biomedicine, hydrogen adsorption and electrocataytic devices. This work present a simple method to obtain well-aligned nitrogen doped multi wall carbon nanotubes, which present open channels with diameter around 50 nm. These carbon nanotubes are obtained using acrylonitrile as carbon and nitrogen source, which offers some advantages on the use of other precursors like ammonia, pyridine, benzylamine, acetonitrile or melamine.
1. Introduction The interest on the insertion of nitrogen on the walls of carbon nanotubes (CNT) is due to the modification on chemical, physical and electronic properties of CNT [1–3]. The nitrogen atom on graphitic walls of carbon nanotubes is incorporated in at least three ways to the graphitic structure, as pyridine and pyrrol (see Fig. 1), which gives a metallic character to structure, and the substitution of a carbon atom, i.e., graphitic nitrogen, which according to Ghosh et al. [4], increases the electronic density of states at the doping site. Diversity of precursors have been used to incorporate nitrogen on CNT walls, and according to Nxumalo et al. [5], who synthesized nitrogen doped CNT (N-CNT) using more than ten different nitrogen sources, concentration of nitrogen in precursor solution is more important than nitrogen source to determine length and diameter. However, in their work, CNT presented bamboo like structure, which limits the application of carbon nanotubes since internal channels are inaccessible and a very high disordered structure is obtained. Other authors like Silva et al. [6], using ammonia as nitrogen precursor, Kumar et al. [7], using sesame oil, Wu and Ceng [8] using Prussian blue, or Suslova et al. [3] using acetonitrile, also obtained the bamboo like structure. In this work the synthesis of carbon nanotubes using acrylonitrile (ACN) as precursor is reported. Acrylonitrile is a substance with boiling point of 350 K, which facilitates its nebulization as a part of the process of CNT synthesis. Ferrocene, which is the catalyst used for CNT synthesis is soluble at required concentration on this solvent. Molecular configuration of ACN consist of three atoms of carbon and
⁎
one of nitrogen, where π electrons are delocalized along the C=C−C≡N structure by conjugation between C=C and C≡N bonds (Parent et al. [9]). Electron lone pair from nitrogen facilitates the interaction with metallic particles, as the formed by ferrocene during CNT synthesis, promoting the formation of graphene structure on iron nanoparticles, which is the intermediate precursor for the formation of CNT. 2. Experimental 2.1. Carbon nanotubes synthesis Nitrogen doped MWCNT (N-CNT) were synthesized by a process of aerosol assisted chemical vapor deposition (Aguilar-Elguézabal et al. [10]) using a mixture of acrylonitrile and ferrocene (36.2 g of ferrocene per liter of acrylonitrile), which was used as catalyst. The mixture was nebulized and fed on a quartz tube using a flow of 4 L/m of argon as gas carrier. Temperature on quartz tube was maintained constant (1073 K) and 20 min were enough to obtain N-CNT with a length of 100 µm. As a reference, carbon nanotubes were synthesized under the same conditions, but using toluene/ferrocene as precursor. 2.2. Carbon nanotubes characterization CNT were characterized by scanning electron microscopy in field emission gun equipment JMS 7000 F from JEOL, operated at 2 kV with EDAX Genesis Energy Dispersive X-ray EDS. For CNT characterization by transmission electron microscopy (TEM), a small amount of CNT was dispersed in ethanol and sonicated at room temperature, and
Corresponding author. E-mail address: [email protected] (A. Aguilar-Elguézabal).
http://dx.doi.org/10.1016/j.jpcs.2016.12.023 Received 6 September 2016; Received in revised form 7 November 2016; Accepted 22 December 2016 Available online 23 December 2016 0022-3697/ © 2017 Elsevier Ltd. All rights reserved.
Journal of Physics and Chemistry of Solids 104 (2017) 52–55
A. Aguilar-Elguézabal et al.
Quantachrome Instruments. Samples were outgassed under vacuum during three hours at 523 K before analysis. The infrared analysis was made by transmission in a Spectrum Magna FTIR from Nicolet. Carbon nanotubes were dried at 373 K during three hours to eliminate humidity taken from air during manipulation. After drying, carbon nanotubes were mixed with KBr powder and pressed to obtain disks. CNT/KBr mixture was diluted with more KBr until a transparent disk was obtained. Raman analyses were made using a Micro-Raman HORIBA HR LabRAM, which is equipped with a He-Ne laser with a wavelength of 632.8 nm and a thermoelectric CCD detector. 3. Results and discussion Fig. 2 shows SEM image of a mat of N-CNT synthesized from acrylonitrile/ferrocene. The image shows N-CNT as growth on quartz substrate. As can be seen CNT presents a uniform diameter and most of nanotubes have thin walls and are open on extreme exposed. The appearance of N-CNT is to be rigid with low flexibility. Fig. 3 shows that the obtained N-CNT presents diameters from 85 to 100 nm, and also is possible to observe that the external surface of CNT has deposits of nanoparticles. According to EDS analysis, these particles mainly consist of carbon and iron. General EDS analysis made to N-CNT showed that content of nitrogen was from 1.5% to 7.5% weight, being from 4% to 5% the most frequent value obtained. Surface area of the as synthesized N-CNT obtained from ACN/ ferrocene was around 50 m2/g, whereas for CNT synthesized by the use of toluene/ferrocene the surface area was of 20–25 m2/g. Considering that CNT obtained from toluene have the similar morphology, i.e. diameter and length, then to explain the results, samples were studied with more detail by electron microscopy. Fig. 4a shows SEM image of typical N-CNT obtained by the use of ACN/ferrocene. As can be seen the thickness of wall for N-CNT synthesized with ACN/ferrocene is around 13 nm, which is near the tenth part of the external diameter. Otherwise, CNT obtained from toluene/ferrocene are shown in Fig. 4b, and as can be seen, the internal channel diameter is lower than the fourth part of external diameter and this channel is obstructed by iron that was taken part during graphenic layers nucleation, which later conformed the carbon nanotube walls. Fig. 5 shows TEM images taken along a single N-CNT, where the absence of catalyst obstructing the internal channel of N-CNT is evident. Thus, for N-CNT inner surface as well as external surface of N-CNT is available to the adsorption of N2 molecules used to determine the surface area of nanotubes. As was shown, for typical MWCNT obtained from toluene/ferrocene the concentric graphene tubes that conform the nanotubes almost fill the
Fig. 1. - Configurations of N-insertion on graphene layer: 1.- N atom graphitic, 2.- N atom pyridinic, 3.- N atom pyrrolic.
Fig. 2. CNT synthesized from acrylonitrile/ferrocene mixture. The image was taken with CNT attached to quarts substrate, as they were growth.
then a drop of this solution was placed on a copper grid. CNT that remained on the copper grid were analyzed in a high-resolution transmission electron microscope (HRTEM) JEOL-JEM 2200FS equipped with a spherical aberration corrector in the condenser lens and operated at 200 kV. Determination of surface area of CNT was made by nitrogen adsorption at 77 K using an Autosorb-1C from
Fig. 3. N-CNT synthesized from acrylonitrile/ferrocene mixture. Diameter of N-CNT is around 80 – 100 nm, and small particles of a mixture of iron and carbon are attached to external surface of CNT.
53
Journal of Physics and Chemistry of Solids 104 (2017) 52–55
A. Aguilar-Elguézabal et al.
Fig. 4. (a) SEM image of N-CNT obtained from acrylonitrile/ferrocene pyrolysis. As can be seen, the thickness of wall is around the tenth part of nanotube diameter. TEM image of CNT obtained from toluene/ferrocene, where the reduced internal channel diameter can be observed, and the presence of iron, which obstructs the diffusion of molecules through this channel.
Fig. 5. TEM image of a single N-CNT. The image shows that the internal channel of nanotube is free from deposits from one extreme to the other, which allows the diffusion of molecules along the N-CNT channel.
Fig. 7. Raman spectra of samples of CNT obtained from toluene (nitrogen free) and from acrylonitrile.
channel, and iron remains inside the channels, impeding the diffusion of N2. Thus for same external dimensions, the available surface for toluene /ferrocene CNT is around the half of the area that is observed for N-CNT, which corresponds only to the external surface of nano-
Fig. 6. FTIR spectra of samples of CNT obtained from toluene (nitrogen free) and from acrylonitrile. Samples were dried before analysis in order to eliminate moisture from samples.
54
Journal of Physics and Chemistry of Solids 104 (2017) 52–55
A. Aguilar-Elguézabal et al.
Acknowledgments
tubes. The IR spectroscopy analysis made to N-CNT clearly indicated the absence of –C=N symmetric stretching band around 2200 cm−1 (Parent et al. [9], Bulusheva et al. [11]), which indicates that nitrogen from ACN, incorporates to CNT as a part of the wall, since the bond C-C in the required structure -C-C˭N is highly unstable at temperature of NCNT conditions synthesis. Small differences on FTIR spectra between N-CNT and undoped CNT can be find out as can be seen at Fig. 6. The stretching vibration of N-H bond can be observed, as was mentioned at experimental part, CNT were dried to avoid the overlapping with the signal of O-H that commonly appears on FTIR characterization of pure CNT (Bahgat et al. [12]). The stretching vibration of C-N, which is commonly located from 1180 to 1360 cm−1 is noticeable for N-CNT spectrum (Fig. 6). For the signal of C=N conforming the aromatic ring, signals overlap with C=C of carbon rings that conform the graphenic structure. Fig. 7 shows Raman spectra of N-CNT and undoped CNT, as can be seen D, G and G′ signals are present on both samples, and ratio G/D is similar for N-CNT and undoped CNT, however G′ signal is quite different. As is well known G′ is a signature of sp2 carbon materials (Jorio et al. [13]). A probable explanation of the reduction of G′-band for N-CNT is the change of density states that surrounds N-doped sites, which reduce the overtone of D-band. Signals observed below 300 cm−1 are typical of RBM, which are characteristic of SWCNT, however considering that diameter of most carbon nanotubes on N-CNT is around 80 nm or higher, the presence of RBM concerns the existence of isolated SWCNT coexisting with N-CNT. Since iron was used as catalyst, the presence of α-Fe2O3 was considered, however the positions of bands differ from those reported for that compound [14].
We thank Wilber Antúnez and Pedro Pizá for their valuable contribution for characterization of materials of this research. References [1] E.N. Nxumalo, N.J. Coville, Nitrogen doped carbon nanotubes from organometallic compounds: a review, Materials 3 (2010) 2141–2171. [2] O.S.G.P. Soares, R.P. Rocha, A.G. Gonçalves, J.L. Figueiredo, J.J.M. Órfão, M.F.R. Pereira, Easy method to prepare N-doped carbon nanotubes by ball milling, Carbon 91 (2015) 114–121. [3] E.V. Suslova, K.I. Maslakov, S.V. Savilov, A.S. Ivanov, L. Lu, V.V. Lunin, Study of nitrogen-doped carbon materials by bomb calorimetry, Carbon 102 (2016) 506–512. [4] K. Ghosh, M. Kumar, T. Maruyama, Y. Ando, Tailoring the field emission property of nitrogen-doped carbon nanotubes by controlling the graphitic/pyridinic substitution, Carbon 48 (2010) 191–200. [5] E.N. Nxumalo, P.J. Letsoalo, L.M. Cele, N.J. Coville, The influence of nitrogen sources on nitrogen doped multi-walled carbon nanotubes, J. Organomet. Chem. 695 (2010) 2596–2902. [6] R.M. Silva, A.J.S. Fernandes, M.C. Ferro, N. Pinna, R.F. Silva, Vertically aligned Ndoped CNT growth using Taguchi experimental design, Appl. Surf. Sci. 344 (2015) 57–64. [7] R. Kumar, R.K. Singh, R.S. Tiwari, Growth analysis and high-yield synthesis of aligned-stacked branched nitrogen-doped carbon nanotubes using sesame oil as a natural botanical hydrocarbon precursor, Mater. Des. 94 (2016) 166–175. [8] M.-S. Wu, Z.-Z. Ceng, Bamboo-like nitrogen-doped carbon nanotubes formed by direct pyrolysis of Prussian blue analogue as a counter electrode material for dyesensitized solar cells, Electrochim. Acta 191 (2016) 895–901. [9] Ph Parent, C. Laffon, G. Tourillon, Adsorption of acrylonitrile on Pt(ll1) and Au(ll1) at 95 K in the monolayer and multilayer ranges studied by NEXAFS, UPS, and FTIR, J. Phys. Chem. 99 (1995) 5058–5066. [10] A. Aguilar-Elguézabal, W. Antúnez, G. Alonso, F. Paraguay Delgado, F. Espinosa, M. Miki-Yoshida, Study of carbon nanotubes synthesis by spray pyrolysis and model of growth, Diam. Rel. Mater. 15 (2006) 1329–1335. [11] L.G. Bulusheva, A.V. Okotrub, Yu.V. Fedoseeva, A.G. Kurenya, I.P. Asanov, O.Y. Vilkov, et al., Controlling pyridinic, pyrrolic, graphitic, and molecular nitrogen in multi-wall carbon nanotubes using precursors with different N/C ratios in aerosol assisted chemical vapor deposition, Phys. Chem. Chem. Phys. 17 (2015) 23741–23747. [12] M. Bahgat, A.A. Farghali, W.M.A. El Rouby, M.H. Khedr, Synthesis and modification of multi-walled carbon nano-tubes (MWCNT) for water treatment applications, J. Anal. Appl. Pyrol 92 (2011) 307–313. [13] A. Jorio, R. Saito, G. Dresselhaus, M.S. Dresselhaus, Raman Spectroscopy in Graphene Related Systems, John Wiley & Sons, 2011 Chapter 12, Dispersive G'band and Higher-Order Processes: the Double Resonance Process. [14] V.V. Bolotov, V.E. Kan, E.V. Knyazev, P.M. Korusenko, S.N. Nesov, Y.A. Sten`kin, et al., An observation of the radial breathing mode in the Raman spectra of CVDgrown multi-wall carbon nanotubes, New Carb. Mater. 30 (2015) 385–390.
4. Conclusions The synthesis of carbon nanotubes using acrylonitrile allows the production of MWCNT that are characterized by an open channel with internal diameter of at least 80 nm, being wall thickness lower than 15% of external diameter. The availability of internal surface is evident since surface area is double than the surface area of undoped CNT were the internal diameter is lower and obstructed by iron nanoparticles. The content of nitrogen is around 4–5% weight; the nitrogen atom is integrated in the graphitic walls of CNT as pyridinic, pyrrolic and substituting graphitic carbon atoms.
55
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Acrylonitrile, an advantageous precursor to synthesize nitrogen doped carbon nanotubes
MARK
⁎
A. Aguilar-Elguézabal , M. Román-Aguirre, L. De la Torre, E.A. Zaragoza Centro de Investigación en Materiales Avanzados S.C., (CIMAV), Laboratorio Nacional de Nanotecnología, Miguel de Cervantes 120, Chihuahua, Chih. 31136, Mexico
A R T I C L E I N F O
A BS T RAC T
Keywords: Acrylonitrile Carbon nanotubes (CNTs) Aerosol assisted chemical vapor deposition (AACVD)
The nitrogen doped carbon nanotubes present specific characteristics that offer better performance than pure carbon nanotubes for application like biomedicine, hydrogen adsorption and electrocataytic devices. This work present a simple method to obtain well-aligned nitrogen doped multi wall carbon nanotubes, which present open channels with diameter around 50 nm. These carbon nanotubes are obtained using acrylonitrile as carbon and nitrogen source, which offers some advantages on the use of other precursors like ammonia, pyridine, benzylamine, acetonitrile or melamine.
1. Introduction The interest on the insertion of nitrogen on the walls of carbon nanotubes (CNT) is due to the modification on chemical, physical and electronic properties of CNT [1–3]. The nitrogen atom on graphitic walls of carbon nanotubes is incorporated in at least three ways to the graphitic structure, as pyridine and pyrrol (see Fig. 1), which gives a metallic character to structure, and the substitution of a carbon atom, i.e., graphitic nitrogen, which according to Ghosh et al. [4], increases the electronic density of states at the doping site. Diversity of precursors have been used to incorporate nitrogen on CNT walls, and according to Nxumalo et al. [5], who synthesized nitrogen doped CNT (N-CNT) using more than ten different nitrogen sources, concentration of nitrogen in precursor solution is more important than nitrogen source to determine length and diameter. However, in their work, CNT presented bamboo like structure, which limits the application of carbon nanotubes since internal channels are inaccessible and a very high disordered structure is obtained. Other authors like Silva et al. [6], using ammonia as nitrogen precursor, Kumar et al. [7], using sesame oil, Wu and Ceng [8] using Prussian blue, or Suslova et al. [3] using acetonitrile, also obtained the bamboo like structure. In this work the synthesis of carbon nanotubes using acrylonitrile (ACN) as precursor is reported. Acrylonitrile is a substance with boiling point of 350 K, which facilitates its nebulization as a part of the process of CNT synthesis. Ferrocene, which is the catalyst used for CNT synthesis is soluble at required concentration on this solvent. Molecular configuration of ACN consist of three atoms of carbon and
⁎
one of nitrogen, where π electrons are delocalized along the C=C−C≡N structure by conjugation between C=C and C≡N bonds (Parent et al. [9]). Electron lone pair from nitrogen facilitates the interaction with metallic particles, as the formed by ferrocene during CNT synthesis, promoting the formation of graphene structure on iron nanoparticles, which is the intermediate precursor for the formation of CNT. 2. Experimental 2.1. Carbon nanotubes synthesis Nitrogen doped MWCNT (N-CNT) were synthesized by a process of aerosol assisted chemical vapor deposition (Aguilar-Elguézabal et al. [10]) using a mixture of acrylonitrile and ferrocene (36.2 g of ferrocene per liter of acrylonitrile), which was used as catalyst. The mixture was nebulized and fed on a quartz tube using a flow of 4 L/m of argon as gas carrier. Temperature on quartz tube was maintained constant (1073 K) and 20 min were enough to obtain N-CNT with a length of 100 µm. As a reference, carbon nanotubes were synthesized under the same conditions, but using toluene/ferrocene as precursor. 2.2. Carbon nanotubes characterization CNT were characterized by scanning electron microscopy in field emission gun equipment JMS 7000 F from JEOL, operated at 2 kV with EDAX Genesis Energy Dispersive X-ray EDS. For CNT characterization by transmission electron microscopy (TEM), a small amount of CNT was dispersed in ethanol and sonicated at room temperature, and
Corresponding author. E-mail address: [email protected] (A. Aguilar-Elguézabal).
http://dx.doi.org/10.1016/j.jpcs.2016.12.023 Received 6 September 2016; Received in revised form 7 November 2016; Accepted 22 December 2016 Available online 23 December 2016 0022-3697/ © 2017 Elsevier Ltd. All rights reserved.
Journal of Physics and Chemistry of Solids 104 (2017) 52–55
A. Aguilar-Elguézabal et al.
Quantachrome Instruments. Samples were outgassed under vacuum during three hours at 523 K before analysis. The infrared analysis was made by transmission in a Spectrum Magna FTIR from Nicolet. Carbon nanotubes were dried at 373 K during three hours to eliminate humidity taken from air during manipulation. After drying, carbon nanotubes were mixed with KBr powder and pressed to obtain disks. CNT/KBr mixture was diluted with more KBr until a transparent disk was obtained. Raman analyses were made using a Micro-Raman HORIBA HR LabRAM, which is equipped with a He-Ne laser with a wavelength of 632.8 nm and a thermoelectric CCD detector. 3. Results and discussion Fig. 2 shows SEM image of a mat of N-CNT synthesized from acrylonitrile/ferrocene. The image shows N-CNT as growth on quartz substrate. As can be seen CNT presents a uniform diameter and most of nanotubes have thin walls and are open on extreme exposed. The appearance of N-CNT is to be rigid with low flexibility. Fig. 3 shows that the obtained N-CNT presents diameters from 85 to 100 nm, and also is possible to observe that the external surface of CNT has deposits of nanoparticles. According to EDS analysis, these particles mainly consist of carbon and iron. General EDS analysis made to N-CNT showed that content of nitrogen was from 1.5% to 7.5% weight, being from 4% to 5% the most frequent value obtained. Surface area of the as synthesized N-CNT obtained from ACN/ ferrocene was around 50 m2/g, whereas for CNT synthesized by the use of toluene/ferrocene the surface area was of 20–25 m2/g. Considering that CNT obtained from toluene have the similar morphology, i.e. diameter and length, then to explain the results, samples were studied with more detail by electron microscopy. Fig. 4a shows SEM image of typical N-CNT obtained by the use of ACN/ferrocene. As can be seen the thickness of wall for N-CNT synthesized with ACN/ferrocene is around 13 nm, which is near the tenth part of the external diameter. Otherwise, CNT obtained from toluene/ferrocene are shown in Fig. 4b, and as can be seen, the internal channel diameter is lower than the fourth part of external diameter and this channel is obstructed by iron that was taken part during graphenic layers nucleation, which later conformed the carbon nanotube walls. Fig. 5 shows TEM images taken along a single N-CNT, where the absence of catalyst obstructing the internal channel of N-CNT is evident. Thus, for N-CNT inner surface as well as external surface of N-CNT is available to the adsorption of N2 molecules used to determine the surface area of nanotubes. As was shown, for typical MWCNT obtained from toluene/ferrocene the concentric graphene tubes that conform the nanotubes almost fill the
Fig. 1. - Configurations of N-insertion on graphene layer: 1.- N atom graphitic, 2.- N atom pyridinic, 3.- N atom pyrrolic.
Fig. 2. CNT synthesized from acrylonitrile/ferrocene mixture. The image was taken with CNT attached to quarts substrate, as they were growth.
then a drop of this solution was placed on a copper grid. CNT that remained on the copper grid were analyzed in a high-resolution transmission electron microscope (HRTEM) JEOL-JEM 2200FS equipped with a spherical aberration corrector in the condenser lens and operated at 200 kV. Determination of surface area of CNT was made by nitrogen adsorption at 77 K using an Autosorb-1C from
Fig. 3. N-CNT synthesized from acrylonitrile/ferrocene mixture. Diameter of N-CNT is around 80 – 100 nm, and small particles of a mixture of iron and carbon are attached to external surface of CNT.
53
Journal of Physics and Chemistry of Solids 104 (2017) 52–55
A. Aguilar-Elguézabal et al.
Fig. 4. (a) SEM image of N-CNT obtained from acrylonitrile/ferrocene pyrolysis. As can be seen, the thickness of wall is around the tenth part of nanotube diameter. TEM image of CNT obtained from toluene/ferrocene, where the reduced internal channel diameter can be observed, and the presence of iron, which obstructs the diffusion of molecules through this channel.
Fig. 5. TEM image of a single N-CNT. The image shows that the internal channel of nanotube is free from deposits from one extreme to the other, which allows the diffusion of molecules along the N-CNT channel.
Fig. 7. Raman spectra of samples of CNT obtained from toluene (nitrogen free) and from acrylonitrile.
channel, and iron remains inside the channels, impeding the diffusion of N2. Thus for same external dimensions, the available surface for toluene /ferrocene CNT is around the half of the area that is observed for N-CNT, which corresponds only to the external surface of nano-
Fig. 6. FTIR spectra of samples of CNT obtained from toluene (nitrogen free) and from acrylonitrile. Samples were dried before analysis in order to eliminate moisture from samples.
54
Journal of Physics and Chemistry of Solids 104 (2017) 52–55
A. Aguilar-Elguézabal et al.
Acknowledgments
tubes. The IR spectroscopy analysis made to N-CNT clearly indicated the absence of –C=N symmetric stretching band around 2200 cm−1 (Parent et al. [9], Bulusheva et al. [11]), which indicates that nitrogen from ACN, incorporates to CNT as a part of the wall, since the bond C-C in the required structure -C-C˭N is highly unstable at temperature of NCNT conditions synthesis. Small differences on FTIR spectra between N-CNT and undoped CNT can be find out as can be seen at Fig. 6. The stretching vibration of N-H bond can be observed, as was mentioned at experimental part, CNT were dried to avoid the overlapping with the signal of O-H that commonly appears on FTIR characterization of pure CNT (Bahgat et al. [12]). The stretching vibration of C-N, which is commonly located from 1180 to 1360 cm−1 is noticeable for N-CNT spectrum (Fig. 6). For the signal of C=N conforming the aromatic ring, signals overlap with C=C of carbon rings that conform the graphenic structure. Fig. 7 shows Raman spectra of N-CNT and undoped CNT, as can be seen D, G and G′ signals are present on both samples, and ratio G/D is similar for N-CNT and undoped CNT, however G′ signal is quite different. As is well known G′ is a signature of sp2 carbon materials (Jorio et al. [13]). A probable explanation of the reduction of G′-band for N-CNT is the change of density states that surrounds N-doped sites, which reduce the overtone of D-band. Signals observed below 300 cm−1 are typical of RBM, which are characteristic of SWCNT, however considering that diameter of most carbon nanotubes on N-CNT is around 80 nm or higher, the presence of RBM concerns the existence of isolated SWCNT coexisting with N-CNT. Since iron was used as catalyst, the presence of α-Fe2O3 was considered, however the positions of bands differ from those reported for that compound [14].
We thank Wilber Antúnez and Pedro Pizá for their valuable contribution for characterization of materials of this research. References [1] E.N. Nxumalo, N.J. Coville, Nitrogen doped carbon nanotubes from organometallic compounds: a review, Materials 3 (2010) 2141–2171. [2] O.S.G.P. Soares, R.P. Rocha, A.G. Gonçalves, J.L. Figueiredo, J.J.M. Órfão, M.F.R. Pereira, Easy method to prepare N-doped carbon nanotubes by ball milling, Carbon 91 (2015) 114–121. [3] E.V. Suslova, K.I. Maslakov, S.V. Savilov, A.S. Ivanov, L. Lu, V.V. Lunin, Study of nitrogen-doped carbon materials by bomb calorimetry, Carbon 102 (2016) 506–512. [4] K. Ghosh, M. Kumar, T. Maruyama, Y. Ando, Tailoring the field emission property of nitrogen-doped carbon nanotubes by controlling the graphitic/pyridinic substitution, Carbon 48 (2010) 191–200. [5] E.N. Nxumalo, P.J. Letsoalo, L.M. Cele, N.J. Coville, The influence of nitrogen sources on nitrogen doped multi-walled carbon nanotubes, J. Organomet. Chem. 695 (2010) 2596–2902. [6] R.M. Silva, A.J.S. Fernandes, M.C. Ferro, N. Pinna, R.F. Silva, Vertically aligned Ndoped CNT growth using Taguchi experimental design, Appl. Surf. Sci. 344 (2015) 57–64. [7] R. Kumar, R.K. Singh, R.S. Tiwari, Growth analysis and high-yield synthesis of aligned-stacked branched nitrogen-doped carbon nanotubes using sesame oil as a natural botanical hydrocarbon precursor, Mater. Des. 94 (2016) 166–175. [8] M.-S. Wu, Z.-Z. Ceng, Bamboo-like nitrogen-doped carbon nanotubes formed by direct pyrolysis of Prussian blue analogue as a counter electrode material for dyesensitized solar cells, Electrochim. Acta 191 (2016) 895–901. [9] Ph Parent, C. Laffon, G. Tourillon, Adsorption of acrylonitrile on Pt(ll1) and Au(ll1) at 95 K in the monolayer and multilayer ranges studied by NEXAFS, UPS, and FTIR, J. Phys. Chem. 99 (1995) 5058–5066. [10] A. Aguilar-Elguézabal, W. Antúnez, G. Alonso, F. Paraguay Delgado, F. Espinosa, M. Miki-Yoshida, Study of carbon nanotubes synthesis by spray pyrolysis and model of growth, Diam. Rel. Mater. 15 (2006) 1329–1335. [11] L.G. Bulusheva, A.V. Okotrub, Yu.V. Fedoseeva, A.G. Kurenya, I.P. Asanov, O.Y. Vilkov, et al., Controlling pyridinic, pyrrolic, graphitic, and molecular nitrogen in multi-wall carbon nanotubes using precursors with different N/C ratios in aerosol assisted chemical vapor deposition, Phys. Chem. Chem. Phys. 17 (2015) 23741–23747. [12] M. Bahgat, A.A. Farghali, W.M.A. El Rouby, M.H. Khedr, Synthesis and modification of multi-walled carbon nano-tubes (MWCNT) for water treatment applications, J. Anal. Appl. Pyrol 92 (2011) 307–313. [13] A. Jorio, R. Saito, G. Dresselhaus, M.S. Dresselhaus, Raman Spectroscopy in Graphene Related Systems, John Wiley & Sons, 2011 Chapter 12, Dispersive G'band and Higher-Order Processes: the Double Resonance Process. [14] V.V. Bolotov, V.E. Kan, E.V. Knyazev, P.M. Korusenko, S.N. Nesov, Y.A. Sten`kin, et al., An observation of the radial breathing mode in the Raman spectra of CVDgrown multi-wall carbon nanotubes, New Carb. Mater. 30 (2015) 385–390.
4. Conclusions The synthesis of carbon nanotubes using acrylonitrile allows the production of MWCNT that are characterized by an open channel with internal diameter of at least 80 nm, being wall thickness lower than 15% of external diameter. The availability of internal surface is evident since surface area is double than the surface area of undoped CNT were the internal diameter is lower and obstructed by iron nanoparticles. The content of nitrogen is around 4–5% weight; the nitrogen atom is integrated in the graphitic walls of CNT as pyridinic, pyrrolic and substituting graphitic carbon atoms.
55