This content has been downloaded from IOPscience. Please scroll down to see the full text. Download details: IP Address: 190.109.177.42 This content was downloaded on 11/10/2013 at 23:17 Please note that terms and conditions apply. Single-walled carbon nanotubes synthesized by chemical vapor deposition of C 2 H 2 over an Al 2 O 3 supported mixture of Fe, Mo, Co catalysts View the table of contents for this issue, or go to the journal homepage for more 2011 Adv. Nat. Sci: Nanosci. Nanotechnol. 2 035007 (http://iopscience.iop.org/2043-6262/2/3/035007) Home Search Collections Journals About Contact us My IOPscience
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IP Address: 190.109.177.42
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Single-walled carbon nanotubes synthesized by chemical vapor deposition of C2H2 over an
Al2O3 supported mixture of Fe, Mo, Co catalysts
View the table of contents for this issue, or go to the journal homepage for more
Single-walled carbon nanotubessynthesized by chemical vapor depositionof C2H2 over an Al2O3 supported mixtureof Fe, Mo, Co catalysts
Thi Thanh Cao1,2, Thi Thanh Tam Ngo1, Van Chuc Nguyen1,Xuan Tinh Than1, Ba Thang Nguyen1 and Ngoc Minh Phan1
1 Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc VietRoad, Cau Giay District, Hanoi, Vietnam2 Department of Physics, Hanoi National University of Education, 136 Xuan Thuy Road,Cau Giay District, Hanoi, Vietnam
Received 25 March 2011Accepted for publication 10 June 2011Published 7 July 2011Online at stacks.iop.org/ANSN/2/035007
AbstractSingle-walled carbon nanotubes (SWCNTs) have been successfully synthesized by chemicalvapor deposition (CVD) using acetylene (C2H2) gas as a carbon source and a mixture ofFe/Mo/Co on an Al2O3 support as a catalyst. The effects of the weight percentage (wt%) ofmetals in the Fe/Mo/Co/Al2O3 catalysts, growth time, gas flow rate and growth temperatureon SWCNT growth were studied in detail. The optimum growth conditions were found to be agrowth time of 60 min, a growth temperature of 750 ◦C, Ar/H2/C2H2 flow rates of420/100/14 sccm and a catalyst composition of Fe/Mo/Co/Al2O3 = 5/3/1/80 (wt%). Themorphologies and structures of the grown SWCNTs were characterized by scanning electronmicroscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopytechniques.
Keywords: single-walled carbon nanotubes (SWCNTs), chemical vapor deposition (CVD),catalyst,
Classification number: 5.14
1. Introduction
Much effort has been made to develop technical andviable methods for synthesizing carbon nanotubes (CNTs)that have exhibited a wealth of fascinating electrical,optical and mechanical properties with a broad rangeof applications [1]. Currently, there are three majormethods for synthesizing CNTs: arc discharge [2], laserablation [3] and chemical vapor deposition (CVD) [4, 5].Among these methods, CVD is the best to produce CNTson a large scale at low cost [6–15]. Bimetal supportedcatalysts, such as Ni–Fe/Al2O3 [8], Co–Mo/SiO2 [9, 11],Co–Mo/MgO [12, 13], Fe-Co/MgO [14, 15] andFe–Mo/Al2O3 [16] catalysts, were found to be effective
in synthesizing CNTs with remarkable yields. Among thebimetallic catalysts, Mo containing catalysts showed betterperformance for single-walled carbon nanotube (SWCNT)production. From the above referred works, it can be realizedthat the ratio of SWCNTs and multi-walled carbon nanotubes(MWCNTs) can be controlled by changing many factors,such as the support and catalyst preparation technique, thecomposition of the catalysts, the system of CVD operationconditions and carbon source gases in the CVD process.
In this work, we prepared a series of Fe–Mo–Co/Al2O3
catalysts with varying weight percentages of metals(wt%). The growth capability of the SWCNTs from thedecomposition of acetylene gas at around 750 ◦C underdifferent conditions was investigated.
Adv. Nat. Sci.: Nanosci. Nanotechnol. 2 (2011) 035007 Thi Thanh Cao et al
(a) (b) (c)
Figure 1. SEM images of the CNTs grown over a Fe/Mo/Co/Al2O3 = 5/1/1/80 (wt%) catalyst for 60 min at (a) 600 ◦C, (b) 750 ◦C and(c) 800 ◦C with flow rates of Ar : H2: C2H2 = 420 : 100 : 14 sccm.
(a) (b)
Figure 2. SEM images of the CNTs grown during (a) 30 min and (b) 60 min at 750 ◦C under Ar : H2: C2H2 (420 : 100 : 14 sccm) using acatalyst of Fe/Mo/Co/Al2O3 (5/1/1/80 (wt%)).
2. Experimental
2.1. Preparation of supported catalysts
Fe–Mo–Co/Al2O3 catalysts were prepared by a conventionalimpregnation method with a varying weight percentageof metals (wt%). Fe(NO3)3.9H2O, (CH3COO)2Co.4H2Oand (NH4)6Mo7O24.4H2O were used as the metal sources.Aluminum oxide powder (Al2O3) was used as the catalystsupport. In all cases, the metal salts were dissolved inethanol by stirring, then a salt solution was impregnatedwith the support powder Al2O3. The mixture was sonicatedand then dried at 100 ◦C for 12 h to evaporate the ethanol.Subsequently, the product was calcined in air at a temperatureof 400 ◦C for 30 min. The calcined catalyst was ground tosizes of 50–300 nm.
2.2. CVD process
Synthesis of the SWCNTs was carried out at atmosphericpressure via catalytic decomposition of C2H2. Approximately20 mg of a catalyst sample was uniformly dispersed on aquartz board, which was placed in the center region of ahorizontal quartz tubular reactor. The diameter of the quartztubular reactor was 20 mm. An argon flow of 420 sccm wassupplied in the whole CVD process. A hydrogen flow of100 sccm was introduced to deoxidize the catalyst. After10 min, acetylene (C2H2) flow with varying flow rates wasadded to the CVD process. The growth time was 30, 60 and90 min. After the CVD process, the furnace was cooled down
to room temperature in Ar gas ambient to prevent oxidation ofthe CNTs.
The structure and morphology of the synthesized CNTswere characterized by using SEM, TEM and Ramanspectroscopy. Raman scattering was used to characterize theCNTs with an excitation wavelength of 633 nm at roomtemperature.
3. Results and discussion
3.1. Effect of growth temperature
Figure 1 shows SEM images of the CNTs synthesized for60 min over the composition Fe/Mo/Co/Al2O3 = 5/1/1/80(wt%) with flow rates of Ar/H2/C2H2 = 420/100/14 sccmat temperatures of 600, 750 and 800 ◦C. It is clear thatthe density of the CNTs was increased with increasinggrowth temperature. The density of CNTs grown at 600 ◦C(figure 1(a)) was very low and lower than that grown at 750 ◦C(figure 1(b)) and 800 ◦C (figure 1(c)). This result suggests thatmore active nucleation sites for the growth of CNTs could beformed at higher temperatures, resulting in a higher densityof CNTs grown at 800 ◦C compared to those grown at 600and 750 ◦C. Another way, at low temperature 600 ◦C, theC2H2 gas is not entirely decomposed, so the carbon sourcefor the growth process of the CNTs was very low. Whenthe temperature was increased to 750 ◦C, C2H2 gas could bedecomposed completely. Thus, the density of the CNTs grownat 750 and 800 ◦C was greater than that grown at 600 ◦C.
It was found that there were a lot of amorphous carbonson the surface of the CNTs grown at 800 ◦C and on the
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Adv. Nat. Sci.: Nanosci. Nanotechnol. 2 (2011) 035007 Thi Thanh Cao et al
(a) (b) (c)
Figure 3. SEM images of the CNTs grown over Fe/Mo/Co/Al2O3 = 5/4/1/80 (wt%) catalyst for 60 min at 750 ◦C withAr : H2 = 420 : 100 sccm, and (a) 7 sccm, (b) 14 sccm and (c) 30 sccm C2H2 flow rates.
(a) (b)
(c) (d)
Figure 4. SEM images of the CNTs grown during 60 min at 750 ◦C using a mix of Ar/H2/C2H2 (420/100/14 sccm), and (a)Fe/Mo/Co/Al2O3 = 5/3/1/80 (catalyst A), (b) Fe/Mo/Co/Al2O3 = 5/4/1/80 (catalyst B), (c) Fe/Mo/Co/Al2O3 = 5/1/1/80(catalyst C), (d) Fe/Mo/Co/Al2O3 = 7/1.5/1/30 (catalyst D).
other hand, the diameters of the CNTs were larger comparedto those grown at 750 ◦C. These results can be explainedas follows. When the temperature is higher, the rate ofhydrocarbon gas decomposition increases, and the rate oftransportation and diffusion of carbon into the catalyst alsoincreases. The diameter of the CNTs is therefore larger andthe CNTs are made dirtier by amorphous carbons that coverthe surface of the CNTs.
3.2. Effect of the growth time
To study the influence of the growth time on the synthesisof the SWCNTs, we executed the CVD process for 30, 60and 90 min at the same temperature and gas flow (750 ◦C and420 : 100 : 14 sccm Ar : H2: C2H2 flow rates). Figure 2 showsthat when the growth time was longer, the density of the CNTswas more dense at the same temperature and gas flow. We cansee that with a growth time of 30 min, the density of the CNTs
was less than that for 60 min. And for 90 min, the surface ofthe CNTs showed a lot of amorphous carbon.
It seems that the growth speed of the CNTs depends onthe catalyst conditions. In the initial period, the catalyst ispure, CNTs can grow very fast, and the density of the CNTsis high. Gradually, the catalytic particles are polluted, thecatalytic activation decreases and so CNT growth slows down.The excess decomposited carbons are deposited on the surfaceof the CNTs to form amorphous carbon.
3.3. Effect of gas source flow rates
Figure 3 shows SEM images of the CNTs grown on thecatalyst Fe/Mo/Co/Al2O3 = 5/4/1/80 (wt%) for 60 min at750 ◦C with C2H2 gas flows of (a) 7 sccm, (b) 14 sccm and(c) 30 sccm, respectively.
The SEM results show that the CNT growth at a 14 sccmC2H2 flow rate has higher purity compared to other flow rates.
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Adv. Nat. Sci.: Nanosci. Nanotechnol. 2 (2011) 035007 Thi Thanh Cao et al
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Figure 5. Raman spectra of the CNTs grown over (a)Fe/Mo/Co/Al2O3 = 5/3/1/80, (b) Fe/Mo/Co/Al2O3 = 5/4/1/80and (c) Raman spectra of the CNTs grown over A, B, C and Dcatalysts for 60 min at 750 ◦C with flow rates ofAr : H2: C2H2 = 420 : 100 : 14 sccm.
3.4. Effect of catalyst weight percentage (wt%)
Recently, many papers have studied the influence of Mo onthe synthesis process of SWCNTs with a catalyst mixture ofFe, Co, Ni, etc. or other catalysts, such as Al2O3, MgO, etc.In this paper, we expose the results in investigating the roleof metallic weight percentage in mixed catalysts (Fe, Mo, Co)that are covered on Al2O3 particles. The influence of Mo onthe SWCNT synthesis process is also studied in detail.
Figure 4 shows SEM images of CNTs synthesizedover Fe/Mo/Co/Al2O3 catalysts with varying (wt%)
Table 1. Diameter values of SWCNTs synthesized over catalystswith varying wt% metal loaded in this work.
Catalysts with varying wt% ωRBM(cm−1) d (nm)metal loaded
metals, namely: A of Fe/Mo/Co/Al2O3 = 5/3/1/80, B ofFe/Mo/Co/Al2O3 = 5/4/1/80, C of Fe/Mo/Co/Al2O3 =
5/1/1/80 and D of Fe/Mo/Co/Al2O3 = 7/1.5/1/30. TheCVD process was carried out for 60 min at 750 ◦C with a flowof Ar/H2/C2H2 = 420/100/14 sccm mixture. It can be seenthat the CNT density of the D catalyst was highest and theCNT density of the B catalyst was lowest. The CNT densitiesof the A and C catalysts were not as high as that of the Dcatalyst, but the CNT average diameter of the catalysts wassmaller than that of the D catalyst.
In order to understand the structure of the grown CNTs,Raman spectroscopic measurements were executed at theexcitation wavelength of 633 nm. The Raman spectra of thegrown CNTs at different growing conditions are shown infigure 5. It can be seen that in all cases the Raman spectraof the CNTs have two peaks: one at 1326 cm−1 (D-line) andthe other at 1592 cm−1 (G-line). The G-peak originates mainlyfrom the graphite Raman—active in plane E2g vibrationmode. The D-peak is attributed to a disordered carbonaceouscomponent. The Raman spectra of the CNTs grown over theA, B and C catalysts showed some distinct features comparedwith those that appeared for the D catalyst. In these cases,besides the characteristic peaks of the CNTs at 1326 and1592 cm−1, the pronounced radial breathing mode (RBM)peaks appeared between 180 and 300 cm−1, and the intensityof the RBM peak of A catalyst was highest. This showsthat Mo plays an important role in the synthesis process ofSWCNTs [5, 9–12]. At the CVD temperature (600–900 ◦C),using gas sources (CO, C2H2, CH4), Mo does not dissolveC to create the MoC phase. Moreover, the melt temperatureof Mo is higher than that of Fe and Co. So, Mo is an agentto separate catalytic metal particles and avoid agglomerationof the catalyst. Thus, the diameter of the particles is verysmall and is advantageous for the growth of the SWCNTs.We demonstrated that the Mo component in the catalystmixture of Fe/Mo/Co/Al2O3 = 5/3/1/80 is the best for theSWCNT growth process. From the measured ωRBM valueand using the relationship for SWCNT diameter d(nm) =
248/ωRBM [17, 18], where ωRBM is the frequency of theRBM in cm−1, the grown SWCNTs with diameter distributionranging from 1.2 to 1.4 nm were evaluated as shownin table 1.
Figure 6 shows TEM images of the CNTs grown overa catalyst of Fe/Mo/Co/Al2O3 = 5/3/1/80. It is seen thatthe SWCNTs created by this catalyst were formed in bunchtype and their diameters were about 3–6 nm. There was
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Adv. Nat. Sci.: Nanosci. Nanotechnol. 2 (2011) 035007 Thi Thanh Cao et al
catalytic particle
catalytic particle
Figure 6. TEM images of the CNTs grown on catalysts of Fe/Mo/Co/Al2O3 = 5/3/1/80 (wt%) for 60 min at 750 ◦C with flow rates ofAr : H2: C2H2 = 420 : 100 : 14 sccm.
no amorphous carbon on the surface of the CNTs and thecatalytic particles appear at the top of the CNTs.
4. Conclusion
The catalyst compositions and CVD conditions for thesynthesis of single-walled carbon nanotubes from C2H2
decomposition have been systematically investigated in orderto maximize the selectivity towards SWCNTs by chemicalvapor deposition. Single-walled carbon nanotubes weresuccessfully synthesized over Fe/Mo/Co/Al2O3 = 5/3/1/80(wt%) catalysts for 60 min at 750 ◦C at flow rates ofAr : H2: C2H2 = 420 : 100 : 14 sccm. From the Raman andTEM studies, the average diameter of synthesized SWCNTswas determined to be from 1.2 to 5 nm.
Acknowledgments
We would like to thank the Research and Developmentof Technology program, Vietnam Academy of Science andTechnology (VAST) for support. Part of this work was donewith the help of the National Basic Research Fund (Nafosted,code: 103.03.47.09) and the fund of AOARD 104140 Project.
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