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Growth of Multiwall Carbon Nanotubes at 300°C Using CO2 as a Weak Oxidant to (CH4+H2) Microwave Plasma
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 18 (2019) 1411–1415
www.materialstoday.com/proceedings
ICN3I-2017
Growth of Multiwall Carbon Nanotubes at 300°C Using CO2 as a Weak Oxidant to (CH4+H2) Microwave Plasma Ajay Roy and Debajyoti Das* Nano-Science Group, Energy Research Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata – 700 032, INDIA
Abstract Multiwall carbon nano-tubes (MWCNTs) were successfully grown at a low temperature ~300°C using a CH4–CO2–H2 gas mixture in microwave (MW) PECVD and characterized by Raman spectroscopy and different electron microscopy e.g., AFM, SEM and TEM. Physical properties of the CNTs were found to be highly dependent on the gas ratio between CO2 and CH4. CO2 as a weak oxidizer played important roles in promoting the tip growth process, selectively removing the amorphous carbon from the catalyst surface and maintaining the growth of CNTs at low temperatures. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Nanotechnology: Ideas, Innovations & Initiatives-2017 (ICN:3i2017). Keywords: Carbon Nanotube; 2.54 GHz Microwave Plasma; Raman Scattering; Electron Microscopy.
1. Introduction Carbon nanotubes have well defined cylindrical structure consisting of graphene layers which show high current stability and remarkable field emission properties [1]. Chemical stability, high aspect ratio, high surface area of carbon nanotubes are appropriate for electrochemical storage, biological probe, fuel cells and various optoelectronic applications [2]. After the discovery of carbon nanotube, several growth processes and technics are introduced
* Corresponding author. Tel.: +91 (33)24734971; fax: +91 (33)24732805. E-mail address (D.Das): [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Nanotechnology: Ideas, Innovations & Initiatives-2017 (ICN:3i2017).
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A. Roy and D. Das / Materials Today: Proceedings 18 (2019) 1411–1415
e.g., arc discharge, thermal evaporation, chemical vapor deposition, etc, for its low temperature growth with improved physical properties. Chemical vapor deposition is a preferable technique in view of its lower substrate temperature requirement and uniformly controllable growth characteristics [3]. Well-aligned CNTs are necessary for proper applications. Various noble metals e.g., Fe, Ni, Co etc. have been used as catalysts for growing CNTs from CH4, C2H2 precursor gases [4]. For necessary reactions to take place ~500°C temperature is generally required [5]. As the synthesis of the stable CNT structures requires comparatively high-density plasma, microwave PECVD (MW-PECVD) are frequently used [6,7]. The growth of CNTs is feasible by using proper precursor gases, proper catalyst and by applying other special technics. Carbon diffusion rate is higher for Fe catalyst which has stronger adhesion with the growing CNTs than other transition metals e.g., Ni, Co, etc., and hence is more efficient in forming high-curvature (low diameter) CNTs such as SWCNT [8]. Accordingly, Fe has been widely used as the noble catalyst, and CH4 as the source gas, in combination with H2 as the diluent gas. In the present study, in addition to using the regular combination of Fe as the catalyst and CH4 and H2 as the precursor and diluent gas, CO2 has been used as a weak oxidant to the growing network enabling low temperature deposition of CNTs [9]. Parametric optimization has been pursued using tubular MW-PECVD system for enabling CNTs to grow at temperatures below the conventional one with the help of CO2 as a weak oxidant. The idea is to make it compatible for directly producing on device structures by reducing the production cost and removing temperature induced detrimental effect on other components of the device during fabrication. 2. Experimental Microwave plasma enhanced CVD (MW-PECVD : 2.45 GHz, 2 kW) tubular system, with 0.45 m length and 80 mm diameter quartz tube, was used for the growth of carbon nanotube thin films. CNT films were grown by MWPECVD at 600 W power, 45 Torr of gas pressure and at 300°C temperature of the quartz plate used as the substrate [6,7]. During deposition gas flow rate of CH4 and H2 remained constant at 7 SCCM each and flow rate of CO2 has was varied from 0 to 8 SCCM. Quartz substrates were pre-coated by 5 nm thick Fe catalyst layer deposited by RF magnetron sputtering, operated at 180 W power in Ar atmosphere at 30 mTorr pressure. Thin Fe films coated on quartz plates were annealed at 700°C in vacuum to form Fe nano-particles as active catalysts for CNT growth [10]. Fe nanoparticles were confirmed by Atomic Force Microscopy and X-ray diffraction spectroscopy. The CNT films were characterized by Raman measurements done by Renishaw inVia micro Raman spectrometer using 514 nm Ar+ Laser as the excitation source and operating at ~3 mW/cm2 of laser power, and also by XRD studies. Films were further characterized by JEOL JSM-6700F field effect scanning electron microscope and JEOL-JSM2010 highresolution transmission electron microscope operating at 200 kV. For HR-TEM study samples were transferred onto the carbon coated copper microscope grids (Pacific Grid-Tech, USA) by drop-cast coating method. 3. Results and discussion In order to introduce Fe as an active catalyst for CNT grown Fe nanoparticles are produced by vacuum annealing of magnetron sputtering deposited thin Fe films at 700°C. Formation of the nanoparticles was confirmed by atomic force microscopy. Figure 1 shows the AFM micrograph of the Fe nanoparticles which are typically of size
Figure 1. AFM micrograph of Fe nanoparticles formed by vacuum annealing at 700°C.
A. Roy and D. Das / Materials Today: Proceedings 18 (2019) 1411–1415
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~20-25 nm. Raman spectroscopy was used for nondestructive characterization of multi-wall/single-wall carbon nanotubes in thin films. Figure 2(a) shows the Raman shift of a set of four CNT films prepared by varying the CO2 flow rate to 0, 1, 2 and 8 SCCM. Each Raman spectrum exhibits a typical graphite vibration mode G-band at 1590 cm-1 and a disordered carbon mode D-band at 1350 cm-1. The presence of 1590 cm-1 peak indicates that CNTs are formed during growth and the 1350 cm-1 peak is due to defects in the graphite sheet [6]. In addition, the G’ band appears at ~2700 cm-1. The intensity of this G’ peak depends strongly on the metallic behavior of the nanotube. Figure 2(b) shows the variations in the intensity ratio of G-peak to D-peak, ID/IG, which reduced gradually with increased CO2 flow rate, identifying spontaneous defect elimination. In addition, IG’/IG ratio demonstrated the dominance of CNT growth in the films by its increasing magnitude, during increase in CO2 flow rate. It has been identified that a favored CNT growth has been accomplished corresponding to minimum defect associated in sample prepared with catalyst Fe-nanoparticles and CO2 flow rate in the range ~4 – 8 SCCM.
(a)
D-peak
CO 2 flow rate 0 1 4 8
G-peak 0.8
sccm sccm sccm sccm
G '-peak
(b)
1.2
ID/IG & IG'/IG
Normalized intensity
1.2
0.9
0.6
0.4
ID / IG IG '/ IG
0.3
0.0
0 1200
1500
2400
2600
-1
Raman Shift (cm )
2800
2
4
6
8
CO 2 flow rate (SCCM)
Figure 2. (a) Raman spectra of carbon nanotube thin films at various CO2 flow rate. (b) Changes in the intensity ratio of D-peak to G-peak and G’-peak to G-peak in Raman spectra for different CO2 flow rate variation.
Figure 3. MWCNTs in scanning electron microscopy.
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Further characterization of the CNT samples was pursued by electron microscopy. The FESEM image of the sample prepared at CO2 flow rate of 4 SCCM, shown in Fig. 3 demonstrates a high yield of nanofibers grown on the substrate surface. The TEM micrograph in Fig. 4(a) gives a better identification of its tubular structure with internal hollow feature. In addition, CNTs were having Fe catalyst particles on each of their tip, demonstrating the tip growth process being the prevailing mechanism of growth. Very high resolution probe by HRTEM categorically identified aligned multilayers of the CNTs in Fig. 4(b). CNTs were having typically outer diameters ~20 nm and inner diameters ~7 nm. During the actual period of growth of the CNTs generally a high temperature is needed. In the current study, optimization for multi-wall CNT growth at low temperature (~300°C) was pursued, using tubular MW-PECVD system aided by CO2. In addition to the presence of H2 as a good etchant [11], the inclusion of CO2 in the range 4 to 8 SCCM was helpful in removing the amorphous carbon component and promoting the CNT growth even at a relatively low temperature [5]. Further, small amount of added CO2 as a weak oxidizer plays as instrumental in facilitating the Fe catalyst nano-particles to maintain an acute contact angle with the substrate and make the catalyst-substrate interaction weak and thereby, promotes the Tip-growth process as the prevailing mechanism [12].
Figure 4. (a) TEM micrograph of MWCNTs demonstrating the Tip growth process, (b) High resolution micrograph demonstrating the layered structure of the CNT.
MWCNT Fe NP
Intensity (a.u.)
<002> CNT <100> CNT <200> FeO
<024> Fe2O3 <220> FeO
20
30
40
50
60
70
2(Degree) Figure 5. X-ray diffraction spectra of Fe nanoparticles and MWCNTs grown via Tip-growth process in MWCVD.
A. Roy and D. Das / Materials Today: Proceedings 18 (2019) 1411–1415
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Formation of Fe nanoparticles and the growth of CNTs were further investigated by X-ray diffraction studies. Figure 5 demonstrates the clear existence of Fe nanoparticles with different orientations. Further, the appearance of CNTs with dominant <002> orientation along with <100> orientation in lower intensity was confirmed in XRD signals. 4. Conclusions Growth of MWCNTs was successfully achieved at temperatures as low as 300°C, using a CH4–CO2–H2 gas mixture in MW-PECVD. CO2, supplied into the CVD reactor in a controlled manner, acted as a weak oxidizer and selectively eliminated amorphous carbon without damaging the growing CNTs. Again a small amount of added CO2 was effective in reducing the size of the support catalyst, by which few layer CNT was grown at such low temperature. Tip-growth process had been the prevailing mechanism making the catalyst-substrate interaction weak, i.e, the Fe catalyst nano-particles succeeded in maintaining an acute contact angle with the substrate which was supposed to be persuaded by the presence of small amount of added CO2 as a weak oxidizer. Acknowledgments The work was done under financial support from the CSIR and the DST, Govt. of India. One of the authors (AR) acknowledges CSIR, GoI, for providing the Research Fellowship. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
J.W.G. Wildoer, L.C. Venema, A.G. Rinzler, R.E. Smalley, C. Dekker, Nature, 391 (1998) 59–62. M. Elrouby, J. Nano. Adv. Mat. 1 (2013) 23–38. C. Liu, H.M. Cheng, Mat. Today, 16 (2013) 19–28. H.U. Rashid, K. Yu, Rev. Adv. Mater. Sci. 40 (2015) 235–248. Q. Wen, W. Qian, Chem. Mater. 19 (2007)1226–1230. D. Das, A. Banerjee, Surf. & Coat. Technol. 272 (2015) 357–365. A. Banerjee, D. Das, Appl. Surf. Sci. 273 (2013) 806–815. R.N.A.R. Seman, M.A. Azam, M.A. Mohamed, Adv. Nat. Sci. Nanosci. Nanotechnol. 7 (2016) 045016. J. Huang, Q. Zhang, Nano Res. 2 (2009) 872–881. A. Roy, D. Das, AIP Conf. Proc. 1832 (2017) 80028-1–3. D. Das : In “Solid State Phenomena” (Scitec Publication, Switzerland) Vol. 44-46 (1995) pp.227–258. A. Roy, D. Das, Diam. Relat. Mater. 88 (2018) 204–214.
ScienceDirect Materials Today: Proceedings 18 (2019) 1411–1415
www.materialstoday.com/proceedings
ICN3I-2017
Growth of Multiwall Carbon Nanotubes at 300°C Using CO2 as a Weak Oxidant to (CH4+H2) Microwave Plasma Ajay Roy and Debajyoti Das* Nano-Science Group, Energy Research Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata – 700 032, INDIA
Abstract Multiwall carbon nano-tubes (MWCNTs) were successfully grown at a low temperature ~300°C using a CH4–CO2–H2 gas mixture in microwave (MW) PECVD and characterized by Raman spectroscopy and different electron microscopy e.g., AFM, SEM and TEM. Physical properties of the CNTs were found to be highly dependent on the gas ratio between CO2 and CH4. CO2 as a weak oxidizer played important roles in promoting the tip growth process, selectively removing the amorphous carbon from the catalyst surface and maintaining the growth of CNTs at low temperatures. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Nanotechnology: Ideas, Innovations & Initiatives-2017 (ICN:3i2017). Keywords: Carbon Nanotube; 2.54 GHz Microwave Plasma; Raman Scattering; Electron Microscopy.
1. Introduction Carbon nanotubes have well defined cylindrical structure consisting of graphene layers which show high current stability and remarkable field emission properties [1]. Chemical stability, high aspect ratio, high surface area of carbon nanotubes are appropriate for electrochemical storage, biological probe, fuel cells and various optoelectronic applications [2]. After the discovery of carbon nanotube, several growth processes and technics are introduced
* Corresponding author. Tel.: +91 (33)24734971; fax: +91 (33)24732805. E-mail address (D.Das): [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Nanotechnology: Ideas, Innovations & Initiatives-2017 (ICN:3i2017).
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A. Roy and D. Das / Materials Today: Proceedings 18 (2019) 1411–1415
e.g., arc discharge, thermal evaporation, chemical vapor deposition, etc, for its low temperature growth with improved physical properties. Chemical vapor deposition is a preferable technique in view of its lower substrate temperature requirement and uniformly controllable growth characteristics [3]. Well-aligned CNTs are necessary for proper applications. Various noble metals e.g., Fe, Ni, Co etc. have been used as catalysts for growing CNTs from CH4, C2H2 precursor gases [4]. For necessary reactions to take place ~500°C temperature is generally required [5]. As the synthesis of the stable CNT structures requires comparatively high-density plasma, microwave PECVD (MW-PECVD) are frequently used [6,7]. The growth of CNTs is feasible by using proper precursor gases, proper catalyst and by applying other special technics. Carbon diffusion rate is higher for Fe catalyst which has stronger adhesion with the growing CNTs than other transition metals e.g., Ni, Co, etc., and hence is more efficient in forming high-curvature (low diameter) CNTs such as SWCNT [8]. Accordingly, Fe has been widely used as the noble catalyst, and CH4 as the source gas, in combination with H2 as the diluent gas. In the present study, in addition to using the regular combination of Fe as the catalyst and CH4 and H2 as the precursor and diluent gas, CO2 has been used as a weak oxidant to the growing network enabling low temperature deposition of CNTs [9]. Parametric optimization has been pursued using tubular MW-PECVD system for enabling CNTs to grow at temperatures below the conventional one with the help of CO2 as a weak oxidant. The idea is to make it compatible for directly producing on device structures by reducing the production cost and removing temperature induced detrimental effect on other components of the device during fabrication. 2. Experimental Microwave plasma enhanced CVD (MW-PECVD : 2.45 GHz, 2 kW) tubular system, with 0.45 m length and 80 mm diameter quartz tube, was used for the growth of carbon nanotube thin films. CNT films were grown by MWPECVD at 600 W power, 45 Torr of gas pressure and at 300°C temperature of the quartz plate used as the substrate [6,7]. During deposition gas flow rate of CH4 and H2 remained constant at 7 SCCM each and flow rate of CO2 has was varied from 0 to 8 SCCM. Quartz substrates were pre-coated by 5 nm thick Fe catalyst layer deposited by RF magnetron sputtering, operated at 180 W power in Ar atmosphere at 30 mTorr pressure. Thin Fe films coated on quartz plates were annealed at 700°C in vacuum to form Fe nano-particles as active catalysts for CNT growth [10]. Fe nanoparticles were confirmed by Atomic Force Microscopy and X-ray diffraction spectroscopy. The CNT films were characterized by Raman measurements done by Renishaw inVia micro Raman spectrometer using 514 nm Ar+ Laser as the excitation source and operating at ~3 mW/cm2 of laser power, and also by XRD studies. Films were further characterized by JEOL JSM-6700F field effect scanning electron microscope and JEOL-JSM2010 highresolution transmission electron microscope operating at 200 kV. For HR-TEM study samples were transferred onto the carbon coated copper microscope grids (Pacific Grid-Tech, USA) by drop-cast coating method. 3. Results and discussion In order to introduce Fe as an active catalyst for CNT grown Fe nanoparticles are produced by vacuum annealing of magnetron sputtering deposited thin Fe films at 700°C. Formation of the nanoparticles was confirmed by atomic force microscopy. Figure 1 shows the AFM micrograph of the Fe nanoparticles which are typically of size
Figure 1. AFM micrograph of Fe nanoparticles formed by vacuum annealing at 700°C.
A. Roy and D. Das / Materials Today: Proceedings 18 (2019) 1411–1415
1413
~20-25 nm. Raman spectroscopy was used for nondestructive characterization of multi-wall/single-wall carbon nanotubes in thin films. Figure 2(a) shows the Raman shift of a set of four CNT films prepared by varying the CO2 flow rate to 0, 1, 2 and 8 SCCM. Each Raman spectrum exhibits a typical graphite vibration mode G-band at 1590 cm-1 and a disordered carbon mode D-band at 1350 cm-1. The presence of 1590 cm-1 peak indicates that CNTs are formed during growth and the 1350 cm-1 peak is due to defects in the graphite sheet [6]. In addition, the G’ band appears at ~2700 cm-1. The intensity of this G’ peak depends strongly on the metallic behavior of the nanotube. Figure 2(b) shows the variations in the intensity ratio of G-peak to D-peak, ID/IG, which reduced gradually with increased CO2 flow rate, identifying spontaneous defect elimination. In addition, IG’/IG ratio demonstrated the dominance of CNT growth in the films by its increasing magnitude, during increase in CO2 flow rate. It has been identified that a favored CNT growth has been accomplished corresponding to minimum defect associated in sample prepared with catalyst Fe-nanoparticles and CO2 flow rate in the range ~4 – 8 SCCM.
(a)
D-peak
CO 2 flow rate 0 1 4 8
G-peak 0.8
sccm sccm sccm sccm
G '-peak
(b)
1.2
ID/IG & IG'/IG
Normalized intensity
1.2
0.9
0.6
0.4
ID / IG IG '/ IG
0.3
0.0
0 1200
1500
2400
2600
-1
Raman Shift (cm )
2800
2
4
6
8
CO 2 flow rate (SCCM)
Figure 2. (a) Raman spectra of carbon nanotube thin films at various CO2 flow rate. (b) Changes in the intensity ratio of D-peak to G-peak and G’-peak to G-peak in Raman spectra for different CO2 flow rate variation.
Figure 3. MWCNTs in scanning electron microscopy.
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Further characterization of the CNT samples was pursued by electron microscopy. The FESEM image of the sample prepared at CO2 flow rate of 4 SCCM, shown in Fig. 3 demonstrates a high yield of nanofibers grown on the substrate surface. The TEM micrograph in Fig. 4(a) gives a better identification of its tubular structure with internal hollow feature. In addition, CNTs were having Fe catalyst particles on each of their tip, demonstrating the tip growth process being the prevailing mechanism of growth. Very high resolution probe by HRTEM categorically identified aligned multilayers of the CNTs in Fig. 4(b). CNTs were having typically outer diameters ~20 nm and inner diameters ~7 nm. During the actual period of growth of the CNTs generally a high temperature is needed. In the current study, optimization for multi-wall CNT growth at low temperature (~300°C) was pursued, using tubular MW-PECVD system aided by CO2. In addition to the presence of H2 as a good etchant [11], the inclusion of CO2 in the range 4 to 8 SCCM was helpful in removing the amorphous carbon component and promoting the CNT growth even at a relatively low temperature [5]. Further, small amount of added CO2 as a weak oxidizer plays as instrumental in facilitating the Fe catalyst nano-particles to maintain an acute contact angle with the substrate and make the catalyst-substrate interaction weak and thereby, promotes the Tip-growth process as the prevailing mechanism [12].
Figure 4. (a) TEM micrograph of MWCNTs demonstrating the Tip growth process, (b) High resolution micrograph demonstrating the layered structure of the CNT.
MWCNT Fe NP
Intensity (a.u.)
<002> CNT <100> CNT <200> FeO
<024> Fe2O3 <220> FeO
20
30
40
50
60
70
2(Degree) Figure 5. X-ray diffraction spectra of Fe nanoparticles and MWCNTs grown via Tip-growth process in MWCVD.
A. Roy and D. Das / Materials Today: Proceedings 18 (2019) 1411–1415
1415
Formation of Fe nanoparticles and the growth of CNTs were further investigated by X-ray diffraction studies. Figure 5 demonstrates the clear existence of Fe nanoparticles with different orientations. Further, the appearance of CNTs with dominant <002> orientation along with <100> orientation in lower intensity was confirmed in XRD signals. 4. Conclusions Growth of MWCNTs was successfully achieved at temperatures as low as 300°C, using a CH4–CO2–H2 gas mixture in MW-PECVD. CO2, supplied into the CVD reactor in a controlled manner, acted as a weak oxidizer and selectively eliminated amorphous carbon without damaging the growing CNTs. Again a small amount of added CO2 was effective in reducing the size of the support catalyst, by which few layer CNT was grown at such low temperature. Tip-growth process had been the prevailing mechanism making the catalyst-substrate interaction weak, i.e, the Fe catalyst nano-particles succeeded in maintaining an acute contact angle with the substrate which was supposed to be persuaded by the presence of small amount of added CO2 as a weak oxidizer. Acknowledgments The work was done under financial support from the CSIR and the DST, Govt. of India. One of the authors (AR) acknowledges CSIR, GoI, for providing the Research Fellowship. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
J.W.G. Wildoer, L.C. Venema, A.G. Rinzler, R.E. Smalley, C. Dekker, Nature, 391 (1998) 59–62. M. Elrouby, J. Nano. Adv. Mat. 1 (2013) 23–38. C. Liu, H.M. Cheng, Mat. Today, 16 (2013) 19–28. H.U. Rashid, K. Yu, Rev. Adv. Mater. Sci. 40 (2015) 235–248. Q. Wen, W. Qian, Chem. Mater. 19 (2007)1226–1230. D. Das, A. Banerjee, Surf. & Coat. Technol. 272 (2015) 357–365. A. Banerjee, D. Das, Appl. Surf. Sci. 273 (2013) 806–815. R.N.A.R. Seman, M.A. Azam, M.A. Mohamed, Adv. Nat. Sci. Nanosci. Nanotechnol. 7 (2016) 045016. J. Huang, Q. Zhang, Nano Res. 2 (2009) 872–881. A. Roy, D. Das, AIP Conf. Proc. 1832 (2017) 80028-1–3. D. Das : In “Solid State Phenomena” (Scitec Publication, Switzerland) Vol. 44-46 (1995) pp.227–258. A. Roy, D. Das, Diam. Relat. Mater. 88 (2018) 204–214.