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Effect of NH3 gas ratio on the formation of nitrogen-doped carbon nanotubes using thermal chemical vapor deposition
Materials Chemistry and Physics 183 (2016) 315e319
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Effect of NH3 gas ratio on the formation of nitrogen-doped carbon nanotubes using thermal chemical vapor deposition Chang-duk Kim, Hyeong-Rag Lee, Hong Tak Kim* Department of Physics, Kyungpook National University, Daegu 702-701, South Korea
h i g h l i g h t s The The The The
proportion of NH3 played a crucial role to control bonding types between N and CNTs. quaternary bonding between N and CNTs occurred above specific proportions of NH3. increase of NH3 flow rate reduces the defects and disorders in the CNTs. wall number and diameter of CNTs seldom changed with varying NH3 proportion.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 July 2015 Received in revised form 28 July 2016 Accepted 14 August 2016 Available online 15 August 2016
Nitrogen-doped carbon nanotubes (N-doped CNTs) were grown on Fe-coated Si substrates through thermal chemical vapor deposition using C2H2, H2, Ar, and NH3 gases. The effects of the flow rate of NH3 gas during the growth of N-doped CNTs were investigated. As the proportion of NH3 gas was increased from 0 to 30%, the growth rate exponentially decreased from 50 nm/s to 5 nm/s, while the diameter (10 e17 nm) and wall numbers (~7 layers) of the N-doped CNTs seldom changed. However, with the increased proportion of NH3, the nitrogen concentration in N-doped CNTs gradually increased. From XPS and Raman results, we observed that pyrrolic and pyridine bonding was dominant at low proportions of NH3, causing an increase in the number of defects and disorders in the CNTs. However, the quaternary bonding appeared at the NH3 proportions of 30%, while the concentration of pyrrolic and pyridine bonding saturated. Consequently, N-corporation inside the carbon honeycomb structure occurred above a specific proportion of NH3. This indicates that a high proportion of NH3 during the CNT growth is necessary to acquire the quaternary bonding between CNTs and nitrogen. © 2016 Elsevier B.V. All rights reserved.
Keywords: Nitrogen-doped Carbon nanotubes Thermal chemical vapor deposition NH3 gas
1. Introduction Carbon nanotubes (CNTs) are carbon derivatives with a cylindrical structure and possess specific optical, electrical, thermal, and mechanical properties [1]. Owing to their characteristics, CNTs have been found useful for a variety of applications: an electron emitter, a super-capacitor, nanosensors, battery anode materials, and membranes [2,3]. Recently, nitrogen-doped (N-doped) CNTs have been considered as a promising fuel cell electrode due to their high electrocatalytic activity during oxygen reduction [4]. Moreover, theoretical studies show that substantial doping of CNTs can induce semiconducting properties upon changing the band structure [5]. Thus, N-doped CNTs are of special interest, for both fundamental
* Corresponding author. Tel.: þ82 53 950 5316; fax: þ82 53 950 1739. E-mail address: [email protected] (H.T. Kim). http://dx.doi.org/10.1016/j.matchemphys.2016.08.033 0254-0584/© 2016 Elsevier B.V. All rights reserved.
and application studies. Usually, N-doped CNTs are prepared by chemical vapor deposition (CVD) including plasma and thermal techniques. The processes are carried out in an environment of nitrogen-rich gases [6,7]. During the CVD process for CNT growth, N atoms can be substantially doped in the CNTs. However, this process requires a high temperature and this implies that it is difficult to acquire highly doped CNTs using other synthetic techniques. In addition, the growth mode and the structure of CNTs are influenced strongly by variation in the concentration of nitrogen-rich gases [8]. Thus, understanding the N-doping process during the growth of CNTs, including the doping concentration, is necessary to control the properties of CNTs. In this study, we reported on a practical approach to synthesize highly aligned N-doped CNTs with a hollow structure. Usually, Ndoped CNTs show a bamboo-like structure and the compartment length in the CNTs are closely related to the concentration of
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C.-d. Kim et al. / Materials Chemistry and Physics 183 (2016) 315e319
nitrogen [9e11]. The hollow structural N-doped CNTs were grown on thin Fe catalytic film using a low pressure chemical vapor deposition (LP-CVD) technique, and the properties of nitrogen incorporation in N-doped CNTs were investigated according to the flow rate of NH3 gas. In addition, the experimental conditions for the structural change from the hollow to bamboo-like structure were studied via the analysis of literature data. 2. Materials and methods The N-doped CNTs were grown on a Fe-coated Si substrate by TCVD and the effects of the proportion of NH3 during the growth of CNTs were investigated. The Fe catalytic films with a thickness of 1 nm were formed on the Si substrate (2 cm 2 cm) using the ebeam deposition technique. The prepared substrate was placed at the center of a quartz tube furnace and the pressure of the furnace was reduced down to 4 104 Pa. The substrate was heated up to 950 C and was pretreated with a mixture of Ar (flow rate: 500 sccm) and H2 (flow rate: 500 sccm) gas for 5 s. Mixtures of C2H2 (flow rate: 500 sccm) and NH3 gas were then introduced into the furnace after the pretreatment of the Fe layer, and N-doped CNTs were grown at different proportions of NH3 gas varying from 0% to 30%. The process pressure was monitored using a capacitive manometer (Baratron, Type-626) and was ranged from 2.67 kPa to 3.33 kPa according to the proportion of NH3 gas. After subjected to the CNT growth process at 950 C for 10 min, the sample was moved to a cold zone in the furnace and rapidly cooled down to room temperature in an Ar atmosphere (pressure: 2 102 Pa). The length and surface morphology of N-doped CNTs were confirmed using scanning electron microscopy (SEM, Hitachi S-4500) and transmission electron microscopy (TEM, FEI Tecnai G2 F20) images. The vibrational analysis of N-doped CNTs was performed using microRaman system spectroscopy (TII Nanofinder 30), and X-ray photoemission spectroscopy (XPS, VG Microtech MT 500/1) was used to quantitatively assess the compositional and structural properties of the N-doped CNTs. 3. Results and discussion Fig. 1(a) shows the cross-sectional SEM images of the N-doped CNTs grown at different proportions of NH3. The growth direction of N-doped CNTs was vertically aligned for all conditions and the length of the CNTs decreased exponentially with increasing proportion of NH3. This means that the growth of CNTs, due to the super-saturation of carbon molecules on Fe surface, is very
Fig. 1. (a) Growth rate (inset: SEM images) and (b) TEM images of N-doped CNTs grown on Fe-coated Si substrate according to the proportion of NH3 gas.
sensitive to the proportion of NH3. The most-general growth mechanism of CNTs can be described as follows [1,9]: Hydrocarbon vapors are decomposed into carbon and hydrogen by contact with a catalytic metal. The decomposed carbons dissolve in the catalytic metal, and acquire a tubular shape on reaching the limit of carbon solubility. The difference in the growth of N-doped CNTs is additional formation of N-containing elements in catalytic material. In the case of Fe catalytic material, C and N elements are soluble in the Fe material according to the phase diagram [9,12] and the Fe metal reacts C and N elements, resulting in metal formations with carbide interfaces and FexN phases [9,13]. This implies that the existence of N-containing elements in the Fe catalytic metal strongly affects the solubility, surface (or subsurface) diffusion, and extrusion rate of carbon element. Therefore, the concentration of NH3 gas plays an important role in growth rate of N-dope CNTs, and it is conjectured the adsorbed N elements restrains carbon reactions in the Fe catalytic metal [14]. Fig. 1(b) shows the TEM images of N-doped CNTs grown at different proportions of NH3 (0, 10, 20, and 30%). The diameter and wall numbers of N-doped CNTs seldom changed with increasing proportion of NH3. The diameters of the grown CNTs ranged from 10 to 17 nm and the number of walls was on an average 7 layers, for all samples. The formation mechanism of as-grown CNTs was followed by the root-base model and the bamboo-like structure of grown CNTs was not observed for all samples. Usually, N-doped CNTs prefer the growth of bamboo-like structure, and the increase of N concentration in N-doped CNTs is accompanied with the decrease of the compartment length [9e11]. However, our results show that the presence of NH3 gas, at least in our reaction system, does not play an important role in bamboo-shape formations and it is conjectured that other parameters exist to obstruct the growth of the bamboo-like structure. Table 1 summarizes literature data for the transition from the hollow to bamboo structural N-doped CNTs according to different process parameters [15e17]. The analysis of literature data indicates that a low growth pressure, a small catalyst size, and a high growth temperature can induce the hollow structural N-doped CNTs. These results corresponded to the growth condition of N-doped CNTs in this study and it was conjectured that a low pressure (2.7e3.3 kPa), a thin catalyst thickness (1 nm) and a high temperature (950 C) led to build the hollow structural Ndoped CNTs. Thus, the change in proportion of NH3 hardly affects the structural properties of the CNTs but mainly influences other properties, including the growth rate of the CNTs. Fig. 2(a) shows the C 1s spectra of the N-doped CNTs according to the proportion of NH3. The background of all spectra was subtracted using Shirley's method [18]. The main peak, observed at approximately 284.5 eV was attributed to graphite-like C]C bonding, indicating that most of the C atoms were arranged in a conjugated honeycomb lattice [19]. The peak centered at approximately 285.8 eV is attributed to CeO bonding, which originated from the adsorption of oxygen components in atmosphere, which is confirmed by analyzing O 1s XPS peak (not shown here). The peaks centered at 285.2 eV originated from CeN and CeO bonding, respectively [20]. The CeN bonding appeared when NH3 gas was introduced to the reactor and this peak gradually increased with the rise in the proportion of NH3 in the reactor. Thus, the change in the proportion of NH3 strongly affects the N-doping concentration of the CNTs, while the structural properties of the CNTs seldom change. Fig. 2(b) shows the N 1s spectra of the N-doped CNTs synthesized at different proportions of NH3. The N 1s peaks exhibited an asymmetric shape for the N-doped CNTs, while the peaks were absent for pristine CNTs. The nitrogen concentration gradually increased with increasing proportion of NH3, as shown in Fig. 3(a). This means that there are various bonds between C and N in the N-doped CNTs and there is some variation in the
C.-d. Kim et al. / Materials Chemistry and Physics 183 (2016) 315e319
317
Table 1 Properties of N-doped CNTs on Fe catalyst according to process parameters. Variable
Method
Input Gas (sccm)
Catalyst thickness (nm)
Growth temp. ( C)
Growth pressure (kPa)
Structure
NH3 Flow Rate
CVD
1
950
2.67e3.33
Hollow
Catalyst Thickness
Plasma-CVD
C2H2: 500, NH3: 0e600 H2: 500, Ar: 500 C2H2: 18, NH3: 70
0.3e20
850
2.67
Growth Temp.
CVD
e
600e1050
101
Growth Pressure
CVD
C2H2: 10, NH3: 40 N2: 100 C2H2: 20, NH3: 80
e
700
0.08e101
2 nm Y: Hollow 5 nm [: Bamboo 700 C Y: Bamboo 700 C [: Hollow 6.67 kPa Y: Hollow 6.67 kPa [: Bamboo
Ref.
[15] [16] [17]
Important experimental parameters are in bold.
Fig. 2. XPS spectra of N-doped CNTs at different proportions of NH3 gas: (a) XPS spectra of C 1s peak and (b) N 1s peak according to the proportion of NH3.
Fig. 3. The change in concentration of nitrogen according to the proportion of NH3 gas: (a) [N]/[N þ C] and (b) the concentration of pyrrolic, pyridine, and quaternary bonding at different proportions of NH3 (inset: the schematic illustration of various CeN bondings).
concentration of each bond upon changing the proportion of NH3. Usually, the CeN bonding in CNTs can be classified as a pyridine, pyrrolic, and quaternary (or graphitic) bonding, as shown in Fig. 3(b) [20]. The pyridine and pyrrolic peaks centered at around
399 eV and 400 eV were only observed up to the NH3 proportion of 20% and then the quaternary bonding at about 401 eV, including two bonding, occurred at a NH3 proportion of 30%. The concentration of N elements in the N-doped CNTs linearly increased up to
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C.-d. Kim et al. / Materials Chemistry and Physics 183 (2016) 315e319
the NH3 proportion of 20% and the increase in rate decreased at higher NH3 proportion, as shown in Fig. 3(a). The concentration of pyrrolic bonding in the N-doped CNTs increased at the NH3 proportion of 10% and saturated at higher proportions of NH3. Also, the concentration of pyridine bonding in the N-doped CNTs increased gradually up to the NH3 proportion of 20%, and saturated at higher proportions of NH3. However, the quaternary bonding in the Ndoped CNTs appeared at the NH3 proportion of 30%, while the concentration of pyridine and pyrrolic bonding saturated. The pyrrolic and pyridine bonding are located at edge defect sites of pentagon and hexagon structure of N-doped CNTs, respectively, whereas the quaternary bonding is positioned inside hexagon structure of the CNTs [9,10,20], as shown in Fig. 3(b). Thus, it was concluded that N-incorporations inside the carbon honeycomb structure occurred after finishing reactions between the defect sites in the carbon network and the N-elements. This means that quaternary bonding in the N-doped CNTs could be acquired above a specific proportion of NH3. Fig. 4 shows Raman spectra of the N-doped CNTs at different proportions of NH3. In the spectra, three distinct peaks were observed at approximately 1350 cm1 (D-band), 1580 cm1 (Gband), 1615 cm1 (D’-band) and 2700 cm1 (2D-band). The G-band is known to originate from the C]C chain of all sp2 sites, which means that well-graphitized CNTs were formed. The D-band is related to the vibrations of chain clusters and breathing mode of sp2 sites in six-fold rings, indicating atomic displacement and disorder due to lattice defects in the CNTs. D’-band is related to the disorder of lattice defects [21] and 2D-band results from the second order Raman scattering and its position has almost double the wavenumber of the D-band [20e23]. Thus, the ratio of D band to G band (ID/IG) can be an indicator for the degree of defects in the CNTs. As
shown in Fig. 4, the ID/IG increased suddenly at the NH3 proportion of 10% and this was attributed to the increase of defects due to nitrogen incorporation in the N-doped CNTs. However, the ID/IG decreased at higher NH3 proportion, in spite of the increase of N concentration in the N-doped CNTs. This implied that the decrease of lattice defects in un-doped region of CNTs led to decrease the ratio of ID/IG, while the nitrogen-induced defects in the N-CNTs increased at higher NH3 proportion. The NH3 gas during CNT growth prevented from the formation of amorphous carbon, resulting in higher crystalline CNTs [24,25]. In addition, the increase of decomposed hydrogen due to the increase of NH3 flow rate could cause the removal of amorphous carbon phase on catalyst and the wall of CNTs, and this led to improve the quality of crystalline CNTs [26e28]. 4. Conclusions The N-doped CNTs were grown on Fe-coated Si substrates by a thermal chemical vapor deposition method using C2H2, H2, Ar, and NH3 gases and the effects of the proportion of NH3 gas during the growth of the CNTs were investigated. The growth rate of the CNTs exponentially decreased from 49.6 to 3.7 nm/s with the increase in the proportion of NH3, while the diameter and wall numbers of the N-doped CNTs seldom changed. The diameter of the CNTs ranged from 10 to 17 nm and the wall number was, on an average, 7 layers. However, the nitrogen concentration in N-doped CNT gradually increased with the rise in the proportion of NH3. The pyrrolic and pyridine bonding were dominant in the N-doped CNTs, which caused the increase in the number of defects and disorders in the CNTs. On the other hand, the concentration of pyridine and pyrrolic bonding saturated at NH3 proportion of 30%, while the quaternary bonding appeared. In addition, the ID/IG also decreased at higher proportion of NH3, indicating the reduction of defects and disorders in the N-doped CNTs. This implied that the increase of NH3 proportion caused the decrease of lattice defects in the region of undoped CNTs. Furthermore, N-doping inside carbon honeycomb structure occurred above specific proportions of NH3, implying that the high proportion of NH3 during the CNT growth is necessary to obtain the quaternary bonding between CNTs and nitrogen. Acknowledgment This work was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (No. 20133030011330). References
Fig. 4. Raman spectra of N-doped CNTs grown on Fe-coated Si substrates at different proportions of NH3 gas (inset: the variation of ID/IG according to the proportion of NH3).
[1] M. Kumar, Y. Ando, J. Nanosci. Nanotechnol. 10 (2010) 3739e3758. [2] M.F.L.D. Volder, S.H. Tawfick, R.H. Baughman, A.J. Hart, Science 339 (2013) 535e539. [3] S. Park, M. Vosguerichian, Z. Bao, Nanoscale 5 (2013) 727e752. [4] R. Rossetti, A. Nakahara, L.E. Brus, J. Chem. Phys. 79 (1983) 1086e1088. [5] D.V. Talapin, A.L. Rogach, A. Kornowski, M. Haase, H. Weller, Nano Lett. 1 (2001) 207e211. [6] J. Campos-Delgado, I.O. Maciel, D.A. Cullen, D.J. Smith, A. Jorio, M.A. Pimenta, H. Terrones, M. Terrones, ACS Nano 4 (2010) 1696e1702. [7] F. Villalpando-Paez, A. Zamudio, A.L. Elias, H. Son, E.B. Barros, S.G. Chou, Y.A. Kim, H. Muramatsu, T. Hayashi, J. Kong, H. Terrones, G. Dresselhaus, M. Endo, M. Terrones, M.S. Dresselhaus, Chem. Phys. Lett. 424 (2006) 345e352. [8] E.N. Nxumalo, N.J. Coville, Materials 3 (2010) 2141e2171. [9] O.Y. Podyacheva, Z.R. Ismagilov, Catal. Today 249 (2015) 12e22. €gl, Carbon 42 (2004) 3199e3207. [10] Y. Lu, Z. Zhu, D. Su, D. Wang, Z. Liu, R. Schlo [11] T. Sharifi, F. Nitze, H.R. Barzegar, C.W. Tai, M. Mazurkiewicz, A. Malolepszy, L. Stobinski, T. Wågberg, Carbon 50 (2012) 3535e3541. [12] K. Frisk, Calphad 11 (1987) 127e134.
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[21] M.S. Dresselhaus, A. Jorio, M. Hofmann, G. Dresselhaus, R. Saito, Nano Lett. 10 (2010) 751e758. [22] S. Mao, H. Pu, J. Chen, RSC Adv. 2 (2012) 2643e2662. [23] D. Chen, H. Feng, J. Li, Chem. Rev. 112 (2012) 6027e6053. [24] A.R. Biris, Z. Li, E. Dervishi, D. Lupu, Y. Xu, V. Saini, F. Watanabe, A.S. Biris, Phys. Lett. A 372 (2008) 3051e3057. [25] X.K. Wang, X.W. Lin, V.P. Dravid, J.B. Ketterson, R.P.H. Chang, Appl. Phys. Lett. 66 (1995), 2430e2342. [26] S.C. Chang, T.S. Li, T.C. Lin, Mater. Lett. 62 (2008) 1893e1895. [27] C.P. Juan, C.C. Tsai, K.H. Chen, L.C. Chen, H.C. Cheng, Jpn. J. Appl. Phys. 44 (2005) 8231e8236. [28] T. Cheng, Mater. Chem. Phys. 136 (2012) 140e145.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Effect of NH3 gas ratio on the formation of nitrogen-doped carbon nanotubes using thermal chemical vapor deposition Chang-duk Kim, Hyeong-Rag Lee, Hong Tak Kim* Department of Physics, Kyungpook National University, Daegu 702-701, South Korea
h i g h l i g h t s The The The The
proportion of NH3 played a crucial role to control bonding types between N and CNTs. quaternary bonding between N and CNTs occurred above specific proportions of NH3. increase of NH3 flow rate reduces the defects and disorders in the CNTs. wall number and diameter of CNTs seldom changed with varying NH3 proportion.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 July 2015 Received in revised form 28 July 2016 Accepted 14 August 2016 Available online 15 August 2016
Nitrogen-doped carbon nanotubes (N-doped CNTs) were grown on Fe-coated Si substrates through thermal chemical vapor deposition using C2H2, H2, Ar, and NH3 gases. The effects of the flow rate of NH3 gas during the growth of N-doped CNTs were investigated. As the proportion of NH3 gas was increased from 0 to 30%, the growth rate exponentially decreased from 50 nm/s to 5 nm/s, while the diameter (10 e17 nm) and wall numbers (~7 layers) of the N-doped CNTs seldom changed. However, with the increased proportion of NH3, the nitrogen concentration in N-doped CNTs gradually increased. From XPS and Raman results, we observed that pyrrolic and pyridine bonding was dominant at low proportions of NH3, causing an increase in the number of defects and disorders in the CNTs. However, the quaternary bonding appeared at the NH3 proportions of 30%, while the concentration of pyrrolic and pyridine bonding saturated. Consequently, N-corporation inside the carbon honeycomb structure occurred above a specific proportion of NH3. This indicates that a high proportion of NH3 during the CNT growth is necessary to acquire the quaternary bonding between CNTs and nitrogen. © 2016 Elsevier B.V. All rights reserved.
Keywords: Nitrogen-doped Carbon nanotubes Thermal chemical vapor deposition NH3 gas
1. Introduction Carbon nanotubes (CNTs) are carbon derivatives with a cylindrical structure and possess specific optical, electrical, thermal, and mechanical properties [1]. Owing to their characteristics, CNTs have been found useful for a variety of applications: an electron emitter, a super-capacitor, nanosensors, battery anode materials, and membranes [2,3]. Recently, nitrogen-doped (N-doped) CNTs have been considered as a promising fuel cell electrode due to their high electrocatalytic activity during oxygen reduction [4]. Moreover, theoretical studies show that substantial doping of CNTs can induce semiconducting properties upon changing the band structure [5]. Thus, N-doped CNTs are of special interest, for both fundamental
* Corresponding author. Tel.: þ82 53 950 5316; fax: þ82 53 950 1739. E-mail address: [email protected] (H.T. Kim). http://dx.doi.org/10.1016/j.matchemphys.2016.08.033 0254-0584/© 2016 Elsevier B.V. All rights reserved.
and application studies. Usually, N-doped CNTs are prepared by chemical vapor deposition (CVD) including plasma and thermal techniques. The processes are carried out in an environment of nitrogen-rich gases [6,7]. During the CVD process for CNT growth, N atoms can be substantially doped in the CNTs. However, this process requires a high temperature and this implies that it is difficult to acquire highly doped CNTs using other synthetic techniques. In addition, the growth mode and the structure of CNTs are influenced strongly by variation in the concentration of nitrogen-rich gases [8]. Thus, understanding the N-doping process during the growth of CNTs, including the doping concentration, is necessary to control the properties of CNTs. In this study, we reported on a practical approach to synthesize highly aligned N-doped CNTs with a hollow structure. Usually, Ndoped CNTs show a bamboo-like structure and the compartment length in the CNTs are closely related to the concentration of
316
C.-d. Kim et al. / Materials Chemistry and Physics 183 (2016) 315e319
nitrogen [9e11]. The hollow structural N-doped CNTs were grown on thin Fe catalytic film using a low pressure chemical vapor deposition (LP-CVD) technique, and the properties of nitrogen incorporation in N-doped CNTs were investigated according to the flow rate of NH3 gas. In addition, the experimental conditions for the structural change from the hollow to bamboo-like structure were studied via the analysis of literature data. 2. Materials and methods The N-doped CNTs were grown on a Fe-coated Si substrate by TCVD and the effects of the proportion of NH3 during the growth of CNTs were investigated. The Fe catalytic films with a thickness of 1 nm were formed on the Si substrate (2 cm 2 cm) using the ebeam deposition technique. The prepared substrate was placed at the center of a quartz tube furnace and the pressure of the furnace was reduced down to 4 104 Pa. The substrate was heated up to 950 C and was pretreated with a mixture of Ar (flow rate: 500 sccm) and H2 (flow rate: 500 sccm) gas for 5 s. Mixtures of C2H2 (flow rate: 500 sccm) and NH3 gas were then introduced into the furnace after the pretreatment of the Fe layer, and N-doped CNTs were grown at different proportions of NH3 gas varying from 0% to 30%. The process pressure was monitored using a capacitive manometer (Baratron, Type-626) and was ranged from 2.67 kPa to 3.33 kPa according to the proportion of NH3 gas. After subjected to the CNT growth process at 950 C for 10 min, the sample was moved to a cold zone in the furnace and rapidly cooled down to room temperature in an Ar atmosphere (pressure: 2 102 Pa). The length and surface morphology of N-doped CNTs were confirmed using scanning electron microscopy (SEM, Hitachi S-4500) and transmission electron microscopy (TEM, FEI Tecnai G2 F20) images. The vibrational analysis of N-doped CNTs was performed using microRaman system spectroscopy (TII Nanofinder 30), and X-ray photoemission spectroscopy (XPS, VG Microtech MT 500/1) was used to quantitatively assess the compositional and structural properties of the N-doped CNTs. 3. Results and discussion Fig. 1(a) shows the cross-sectional SEM images of the N-doped CNTs grown at different proportions of NH3. The growth direction of N-doped CNTs was vertically aligned for all conditions and the length of the CNTs decreased exponentially with increasing proportion of NH3. This means that the growth of CNTs, due to the super-saturation of carbon molecules on Fe surface, is very
Fig. 1. (a) Growth rate (inset: SEM images) and (b) TEM images of N-doped CNTs grown on Fe-coated Si substrate according to the proportion of NH3 gas.
sensitive to the proportion of NH3. The most-general growth mechanism of CNTs can be described as follows [1,9]: Hydrocarbon vapors are decomposed into carbon and hydrogen by contact with a catalytic metal. The decomposed carbons dissolve in the catalytic metal, and acquire a tubular shape on reaching the limit of carbon solubility. The difference in the growth of N-doped CNTs is additional formation of N-containing elements in catalytic material. In the case of Fe catalytic material, C and N elements are soluble in the Fe material according to the phase diagram [9,12] and the Fe metal reacts C and N elements, resulting in metal formations with carbide interfaces and FexN phases [9,13]. This implies that the existence of N-containing elements in the Fe catalytic metal strongly affects the solubility, surface (or subsurface) diffusion, and extrusion rate of carbon element. Therefore, the concentration of NH3 gas plays an important role in growth rate of N-dope CNTs, and it is conjectured the adsorbed N elements restrains carbon reactions in the Fe catalytic metal [14]. Fig. 1(b) shows the TEM images of N-doped CNTs grown at different proportions of NH3 (0, 10, 20, and 30%). The diameter and wall numbers of N-doped CNTs seldom changed with increasing proportion of NH3. The diameters of the grown CNTs ranged from 10 to 17 nm and the number of walls was on an average 7 layers, for all samples. The formation mechanism of as-grown CNTs was followed by the root-base model and the bamboo-like structure of grown CNTs was not observed for all samples. Usually, N-doped CNTs prefer the growth of bamboo-like structure, and the increase of N concentration in N-doped CNTs is accompanied with the decrease of the compartment length [9e11]. However, our results show that the presence of NH3 gas, at least in our reaction system, does not play an important role in bamboo-shape formations and it is conjectured that other parameters exist to obstruct the growth of the bamboo-like structure. Table 1 summarizes literature data for the transition from the hollow to bamboo structural N-doped CNTs according to different process parameters [15e17]. The analysis of literature data indicates that a low growth pressure, a small catalyst size, and a high growth temperature can induce the hollow structural N-doped CNTs. These results corresponded to the growth condition of N-doped CNTs in this study and it was conjectured that a low pressure (2.7e3.3 kPa), a thin catalyst thickness (1 nm) and a high temperature (950 C) led to build the hollow structural Ndoped CNTs. Thus, the change in proportion of NH3 hardly affects the structural properties of the CNTs but mainly influences other properties, including the growth rate of the CNTs. Fig. 2(a) shows the C 1s spectra of the N-doped CNTs according to the proportion of NH3. The background of all spectra was subtracted using Shirley's method [18]. The main peak, observed at approximately 284.5 eV was attributed to graphite-like C]C bonding, indicating that most of the C atoms were arranged in a conjugated honeycomb lattice [19]. The peak centered at approximately 285.8 eV is attributed to CeO bonding, which originated from the adsorption of oxygen components in atmosphere, which is confirmed by analyzing O 1s XPS peak (not shown here). The peaks centered at 285.2 eV originated from CeN and CeO bonding, respectively [20]. The CeN bonding appeared when NH3 gas was introduced to the reactor and this peak gradually increased with the rise in the proportion of NH3 in the reactor. Thus, the change in the proportion of NH3 strongly affects the N-doping concentration of the CNTs, while the structural properties of the CNTs seldom change. Fig. 2(b) shows the N 1s spectra of the N-doped CNTs synthesized at different proportions of NH3. The N 1s peaks exhibited an asymmetric shape for the N-doped CNTs, while the peaks were absent for pristine CNTs. The nitrogen concentration gradually increased with increasing proportion of NH3, as shown in Fig. 3(a). This means that there are various bonds between C and N in the N-doped CNTs and there is some variation in the
C.-d. Kim et al. / Materials Chemistry and Physics 183 (2016) 315e319
317
Table 1 Properties of N-doped CNTs on Fe catalyst according to process parameters. Variable
Method
Input Gas (sccm)
Catalyst thickness (nm)
Growth temp. ( C)
Growth pressure (kPa)
Structure
NH3 Flow Rate
CVD
1
950
2.67e3.33
Hollow
Catalyst Thickness
Plasma-CVD
C2H2: 500, NH3: 0e600 H2: 500, Ar: 500 C2H2: 18, NH3: 70
0.3e20
850
2.67
Growth Temp.
CVD
e
600e1050
101
Growth Pressure
CVD
C2H2: 10, NH3: 40 N2: 100 C2H2: 20, NH3: 80
e
700
0.08e101
2 nm Y: Hollow 5 nm [: Bamboo 700 C Y: Bamboo 700 C [: Hollow 6.67 kPa Y: Hollow 6.67 kPa [: Bamboo
Ref.
[15] [16] [17]
Important experimental parameters are in bold.
Fig. 2. XPS spectra of N-doped CNTs at different proportions of NH3 gas: (a) XPS spectra of C 1s peak and (b) N 1s peak according to the proportion of NH3.
Fig. 3. The change in concentration of nitrogen according to the proportion of NH3 gas: (a) [N]/[N þ C] and (b) the concentration of pyrrolic, pyridine, and quaternary bonding at different proportions of NH3 (inset: the schematic illustration of various CeN bondings).
concentration of each bond upon changing the proportion of NH3. Usually, the CeN bonding in CNTs can be classified as a pyridine, pyrrolic, and quaternary (or graphitic) bonding, as shown in Fig. 3(b) [20]. The pyridine and pyrrolic peaks centered at around
399 eV and 400 eV were only observed up to the NH3 proportion of 20% and then the quaternary bonding at about 401 eV, including two bonding, occurred at a NH3 proportion of 30%. The concentration of N elements in the N-doped CNTs linearly increased up to
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the NH3 proportion of 20% and the increase in rate decreased at higher NH3 proportion, as shown in Fig. 3(a). The concentration of pyrrolic bonding in the N-doped CNTs increased at the NH3 proportion of 10% and saturated at higher proportions of NH3. Also, the concentration of pyridine bonding in the N-doped CNTs increased gradually up to the NH3 proportion of 20%, and saturated at higher proportions of NH3. However, the quaternary bonding in the Ndoped CNTs appeared at the NH3 proportion of 30%, while the concentration of pyridine and pyrrolic bonding saturated. The pyrrolic and pyridine bonding are located at edge defect sites of pentagon and hexagon structure of N-doped CNTs, respectively, whereas the quaternary bonding is positioned inside hexagon structure of the CNTs [9,10,20], as shown in Fig. 3(b). Thus, it was concluded that N-incorporations inside the carbon honeycomb structure occurred after finishing reactions between the defect sites in the carbon network and the N-elements. This means that quaternary bonding in the N-doped CNTs could be acquired above a specific proportion of NH3. Fig. 4 shows Raman spectra of the N-doped CNTs at different proportions of NH3. In the spectra, three distinct peaks were observed at approximately 1350 cm1 (D-band), 1580 cm1 (Gband), 1615 cm1 (D’-band) and 2700 cm1 (2D-band). The G-band is known to originate from the C]C chain of all sp2 sites, which means that well-graphitized CNTs were formed. The D-band is related to the vibrations of chain clusters and breathing mode of sp2 sites in six-fold rings, indicating atomic displacement and disorder due to lattice defects in the CNTs. D’-band is related to the disorder of lattice defects [21] and 2D-band results from the second order Raman scattering and its position has almost double the wavenumber of the D-band [20e23]. Thus, the ratio of D band to G band (ID/IG) can be an indicator for the degree of defects in the CNTs. As
shown in Fig. 4, the ID/IG increased suddenly at the NH3 proportion of 10% and this was attributed to the increase of defects due to nitrogen incorporation in the N-doped CNTs. However, the ID/IG decreased at higher NH3 proportion, in spite of the increase of N concentration in the N-doped CNTs. This implied that the decrease of lattice defects in un-doped region of CNTs led to decrease the ratio of ID/IG, while the nitrogen-induced defects in the N-CNTs increased at higher NH3 proportion. The NH3 gas during CNT growth prevented from the formation of amorphous carbon, resulting in higher crystalline CNTs [24,25]. In addition, the increase of decomposed hydrogen due to the increase of NH3 flow rate could cause the removal of amorphous carbon phase on catalyst and the wall of CNTs, and this led to improve the quality of crystalline CNTs [26e28]. 4. Conclusions The N-doped CNTs were grown on Fe-coated Si substrates by a thermal chemical vapor deposition method using C2H2, H2, Ar, and NH3 gases and the effects of the proportion of NH3 gas during the growth of the CNTs were investigated. The growth rate of the CNTs exponentially decreased from 49.6 to 3.7 nm/s with the increase in the proportion of NH3, while the diameter and wall numbers of the N-doped CNTs seldom changed. The diameter of the CNTs ranged from 10 to 17 nm and the wall number was, on an average, 7 layers. However, the nitrogen concentration in N-doped CNT gradually increased with the rise in the proportion of NH3. The pyrrolic and pyridine bonding were dominant in the N-doped CNTs, which caused the increase in the number of defects and disorders in the CNTs. On the other hand, the concentration of pyridine and pyrrolic bonding saturated at NH3 proportion of 30%, while the quaternary bonding appeared. In addition, the ID/IG also decreased at higher proportion of NH3, indicating the reduction of defects and disorders in the N-doped CNTs. This implied that the increase of NH3 proportion caused the decrease of lattice defects in the region of undoped CNTs. Furthermore, N-doping inside carbon honeycomb structure occurred above specific proportions of NH3, implying that the high proportion of NH3 during the CNT growth is necessary to obtain the quaternary bonding between CNTs and nitrogen. Acknowledgment This work was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (No. 20133030011330). References
Fig. 4. Raman spectra of N-doped CNTs grown on Fe-coated Si substrates at different proportions of NH3 gas (inset: the variation of ID/IG according to the proportion of NH3).
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