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[10] J. Kastner, T. Pichler, H. Kuzmany, S. Curran, V. Blaw,. Weldon, Chem. Phys. Lett. 221, 53(1994). [11] M. A. Pimenta, A. Marruci, S. A. Empedocles, M. G..
Journal of the Korean Physical Society, Vol. 52, No. 5, May 2008, pp. 13721377

Carbon-Nanotube Growth over Iron Nanoparticles Formed on CaCO3 Support by Using Hydrogen Reduction

Nguyen Duc Dung, Nguyen Van Chuc, Ngo Thi Thanh Tam, Nguyen Hong Quang, Phan Hong Khoi and Phan Ngoc Minh Institute of Materials Science, Vietnamese Academy of Science and Technology, 18 Hoang Quoc Viet Road, Cau Giay District, Hanoi, Vietnam

(Received 15 July 2007) Carbon nanotubes (CNTs) were grown by chemical vapor deposition on a mixture of iron salt and CaCO3 . Salt mixtures of Fe(NO3 )3 9H2 O/CaCO3 and FeCl3 6H2 O/CaCO3 with various Fe weight contents were used as catalysts for growing the CNTs. A scanning electron microscope study revealed that the CNTs were densely grown on the Fe(NO3 )3 9H2 O/CaCO3 catalyst containing 5 wt.% Fe. The e ect of growth temperature on the segregation of Fe nanoparticles formed by hydrogen reduction is discussed. The result shows that 800  C is the optimal temperature for the formation of Fe nanoparticles over which CNTs grow with the highest yield of 78.61 %. A raman spectroscope and a scanning transmission electron microscope were utilized to characterize the multiwall structure of the CNTs. The 92.16 % purity of the CNTs was determined by using thermal gravimetric analysis. PACS numbers: 81.05.Tp Keywords: CNTs, CVD, CaCO3 , FeCl3

I. INTRODUCTION

Carbon nanotubes (CNTs) have been recognized as one of the most important nanomaterials. The superior properties of CNTs, such as high strength and good electrical conductivity, are critical for many applications in electronics [1], composites [2], polymers [3], etc. In addition to basic research on CNTs, technological investigation for large-scale and low-cost production satisfying industrial demands have really been challenges until now. The most common and optimal method for large-scale CNTs production is catalytic chemical vapor deposition (CCVD). In the CCVD process, catalyst supports, such as MgO, Al2 O3 , SiO2 , CaCO3 , etc., are the essential ingredients due to their high surface area for CCVD reaction. The choice of CaCO3 as a catalyst support was rstly proposed by Couteau et al. and is reported in Ref. 4. The advantages of this catalyst support are that it is soluble in acids, has low cost and is available in the market. The puri cation is simple and harmless to CNTs structure; thus, it is promising for large-scale, low-cost production of CNTs. However, a CNT yield of 21.74 % is low (70 mg of CNTs synthesized from 322 mg of C) and the puri cation procedure used rather concentrated acid 30 % HNO3 . In this research, using CaCO3 as a catalyst support, we  E-mail:

[email protected]

investigated the e ects of CVD conditions (catalyst, temperature, gas composition) in order to get higher CNT yields. We developed a simple method for making a catalyst only by grinding CaCO3 and Fe salts, therefore, neglecting the impregnating and drying steps used in Ref. 4, which reduces the number of stages in CNTs synthesis. The addition of H2 gas in the CVD process is not only to form Fe nanoparticles but also to promote the growth of CNTs and to increase the CNT yield. Another role of CaCO3 as a factor contributing to the formation of Fe nanoparticles is discussed. For separating the CNTs from the support, we used dilute 15 % HCl acid in the puri cation process.

II. EXPERIMENTS

Two kinds of salts, Fe(NO3 )3 9H2 O and FeCl3 6H2 O, were used as precursors for the catalytic Fe nanoparticles. CaCO3 was mechanically mixed with each salt with various weight contents to get the examined catalysts. Approximately 3 g of each examined catalyst was uniformly dispersed in a stainless-steel boat and the boat was settled into the center part of a quartz reactor (with a diameter of 3.5 cm and a length of 120 cm) placed horizontally inside a tube furnace (20-cm-long heating zone). Nitrogen ow of 300 sccm (standard cubic centimeters per minute) was supplied in the whole process.

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Table 1. CVD products for all samples obtained by using CVD at 800  C. Fe salts

Fe content (%)

CVD Product

Carbon deposited, Hc ( %)

Fe(NO3 )3 9H2 O

1 3 5 7 10 15 20

CNTs low density CNTs high density CNTs highest density CNTs large size and low density, amorphous CNTs large size and low density, CFs, amorphous CNTs few, high dense CFs, amorphous No CNTs, CFs, belts, rods, amorphous

17 24 30 32 35 37 39

FeCl3 6H2 O

1 3 5 7 10 15 20

Rods Rods, belts Slabs, belts Slabs High density Slabs, amorphous Amorphous Amorphous

18 25 29 33 37 39 42

A hydrogen ow of 100 sccm was let into the reactor at 800  C. After 10 minutes for the hydrogen reducing reaction, the decomposition of a 50 sccm acetylene ow was carried out for 30 minutes. The reactor was then cooled down to room temperature in the N2 medium and the CVD product was removed. For the CNT puri cation, the CVD product was sonicated in dilute HCl acid (15 %) for 1 h at room temperature, then ltered to remove the support, washed with distilled water and dried at 130  C for 20 h. A scanning electron microscope (SEM, Hitachi S-4800) was used to observe the morphology of CNTs in the CVD products. Raman spectroscopy (Micro - Raman LABRAM - 1B Jobin - Yvon, 632.8 nm He-Ne laser excitation) and scanning transmission electron microscope (STEM, Hitachi S-4800) were utilized to structurally characterize the puri ed CNTs. Impurities of the puri ed CNTs were estimated by using a thermal gravimetry analysis (TGA, Shimadzu TGA-50H) and energy dispersive X-ray uorescence spectroscopy (EDS, EDS-XT-9901).

III. RESULTS AND DISCUSSION

The Fe weight contents of 1 %, 3 %, 5 %, 7 %, 10 %, 15 % and 20 % in Fe(NO3 )3 9H2 O/CaCO3 and in FeCl3 6H2 O/CaCO3 mixtures were examined by using the CVD process to quantify the relative eciency of catalysts in terms of the amount of deposited carbon and the density of CNTs. The carbon deposition eciency (%) was evaluated in each CVD process by using Hc

(%) = 100(mp

)

mr =mr ;

(1)

Fig. 1. SEM images of CVD products from a FeCl3 9H2 O/CaCO3 mixture containing Fe with weight contents of (a) 5 %, (b) 10 %, (c) 15 % and (d) 20 %.

where mr is the weight of the salt mixture before decomposing acetylene and mp is the weight of the CVD product. Table 1 shows the results of the CVD products for all samples in our experiments. Figure 1 and 2 show typical SEM images of the CVD products from the two salt mixtures with various Fe weight contents. For FeCl3 6H2 O, we could not synthesize the CNTs, but could synthesize other con gurations of carbon (rod, slab, belt, amorphous) (see Figure 1). This can be explained by FeCl3 existing in liquid (FeCl3 ) and vapor (FeCl2 ) phases at the CVD temperature (800  C). In the CVD conditions (N2 , H2 and C2 H2 ows), these two

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Fig. 2. SEM images of CVD products from a Fe(NO3 )3 9H2 O/CaCO3 mixture containing Fe with weight contents of (a) 5 %, (b) 10 %, (c) 15 % and (d) 20 %.

Fig. 3. SEM images of the Fe(NO3 )3 9H2 O/CaCO3 mixture containing 5 wt.% Fe (a) before and (b) after annealing in N2 /H2 at 800  C for 10 minutes and (c) after growing CNTs.

phases do not form segregated Fe nanoparticles that are necessary to synthesize CNTs [5]. This also agrees with the results reported in Ref. 6. For Fe(NO3 )3 9H2 O, we can see that the Fe(NO3 )3 9H2 O/CaCO3 mixture containing 5 wt.% Fe reveals the highest density of CNTs and a moderate carbon deposition eciency, which are the optimum for CNT growth. For the catalysts having more than 5 wt.% Fe, Fe nanoparticles are probably dicult to form due to the cohesive agglomeration. That is why we observed low density and large diameter CNTs. In contrast, a Fe weight content of less than 5 % (1 %, 3 %) is good for the creation of the nanoparticles necessary to synthesize CNTs, but not for the growth of dense CNTs. We conclude that di erent Fe salt radicals play di erent catalytic roles in growing CNTs when using CaCO3 as a catalyst support. Our investiga-

tion validated those results by experimentally comparing Fe(NO3 )3 9H2 O with FeCl3 6H2 O. The behavior of the Fe(NO3 )3 9H2 O/CaCO3 mixture containing 5 wt.% Fe during the CVD process was studied to demonstrate the formation of Fe nanoparticles over which CNTs grow well. Figure 3(a) is a SEM image of the Fe(NO3 )3 9H2 O/CaCO3 mixture containing 5 % wt Fe before annealing. The glue-like clusters on the cubic shapes seen in Figure 3(a) are melting Fe(NO3 )3 9H2 O due to grinding in air. Therefore, the precursor laying on the CaCO3 surface is randomly done by mechanically mixing. This is the basis for the formation of Fe nanoparticles on the surface of the support. Figure 3(b) is a SEM image of the Fe(NO3 )3 9H2 O/CaCO3 mixture containing 5 wt.% Fe after annealing in N2 at room temperature and in N2 /H2 ( ow rate was N2 : H2 = 300 : 100 sccm) for 10 minutes at 800  C. Figure 3(c) is a SEM image of the Fe(NO3 )3 9H2 O and CaCO3 mix-

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Table 2. Dependence of CNTs performance on temperature. Temperature ( C) 700 800 900

Diameters (nm) 20 { 100 15 { 90 10 { 80

Carbon deposited, Hc (%) 28,6 30 33

CNT yield, Yc (%) 61.92 78.61 47.22

Fig. 4. SEM images of (a) the CVD product and of (b) the puri ed CNTs. (c) STEM image of the puri ed CNTs.

ture containing 5 wt.% Fe after growing CNTs over Fe nanoparticles on the support (CaCO3 or CaO) ( ow rate was N2 : H2 : C2 H2 = 300 : 100 : 50 sccm). For the annealed Fe(NO3 )3 9H2 O/CaCO3 mixture containing 5 % wt Fe (see Figure 3(b)), the transformation of the gluelike clusters into the bright segregated Fe nanoparticles can be primarily derived from the gas release in terms of the following reactions: 4Fe(NO3 )3 ! 2Fe2 O3 + 12NO2 " +3O2 "; CaCO3 ! CaO + CO2 "; Fe2 O3 + 3H2 ! 2Fe + 3H2 O " :

(2) (3) (4)

When the gas releasing reaction in Eq. (2) happens, NO2 and O2 erupt to create segregated Fe2 O3 nanoparticles. The segregation of Fe2 O3 nanoparticles is then enhanced by rough supporting surfaces (see Figure 3(b)), which can be derived from the release of CO2 gas according to reaction (3). Lastly, the segregated Fe nanoparticles are formed by reaction (4). An ex-situ measurement proving the formation of the bright segregated Fe nanoparticles (see reaction (4) and Figure 3(b)) is SEM observations of the CNTs. Based on Figure 3(c) and according to the mechanism of CNTs growth [7], the bright spots inside the CNTs are Fe nanoparticles. The above results indicate the e ectiveness of our study. By using the CVD process at 800  C, we exploit the self-gas release of the CaCO3 support through its high ability to form segregated Fe nanoparticles. We nally achieved an optimal catalyst for CNTs growth and demonstrated the formation of Fe nanoparticles on the support by proposing a gas release from the ironnitrate salt and calcium carbonate. As a result, we use a Fe(NO3 )3 9H2 O/CaCO3 mixture containing 5 wt.% Fe

as the catalyst for growing CNTs. In order to get the most advantageous CVD conditions, the dependence of CNTs performance (diameter, carbon deposition according to Eq. (1), CNTs yield in terms of Eq. (5) de ned below) on temperature was surveyed. The CNT yield is de ned by: Yc

(%) = 100mf =ms ;

(5)

where mf is the weight of puri ed CNTs and ms is the weight of carbon in the C2 H2 ow, which can be calculated by using Flowrate(l=min)  Time(min) ms =  24(g=mol): 22:4(l=mol) Table 2 catalogues the performance of the puri ed CNTs grown at 700  C, 800  C and 900  C. As the growth temperature increases, the Fe catalyst surface becomes rougher. Thus, they have smaller active regions nucleating smaller nanotubes. The SEM counted diameters of the puri ed CNTs in Table 2 also show a decrease with increasing growth temperature. The slight raise in carbon deposition is due to an increase in atomic mobility induced at the higher temperature, leading the catalyst to become \liquid-like", as suggested by Teo et al. [8]. The signi cant reduction in CNT yield at 900  C is probably due to the high rate of formation of iron carbide, limiting CNT growth. At 700  C the dispersion of segregated Fe nanoparticles is not high due to the low thermodynamic activity of ambient gases and the small amount of CO2 gas from the CaCO3 support. This explain why the CNT yield in this case is lower than it is in the case at 800  C, which is high enough to form segregated Fe nanoparticles, as mentioned above. Therefore, the growth temperature of 800  C was chosen for getting CNTs with the highest yield.

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Fig. 5. Raman spectrum of the puri ed CNTs.

The morphology, the structure and the graphite crystallinity of puri ed CNTs were characterized by using SEM, STEM, Raman Spectroscopy and TGA. The SEM image in Figure 4(a) reveals an overall view of the CVD product from which we can see CNTs laying randomly on the catalyst support. After puri cation, we received the CNTs shown in Figure 4(b). It is clearly seen that long ne CNTs without the catalyst support were obtained. This means that the catalyst support was totally removed. The multi-wall structure of the CNTs was con rmed by the STEM image in Figure 4(c). As shown in Figure 5, the Raman spectrum of the puri ed CNTs shows a D-line peak at 1325 cm 1 and a G-line peak at 1580 cm 1 . The D peak originates from vibrations of carbon atoms with dangling bonds in the plane terminations of turbostratic and poorly ordered carbon. The G peak corresponds to the vibration of sp2 bonded carbon atoms in a 2D hexagonal lattice. The D'line peak at 1615 cm 1 and no second order scattering peak at 1740 cm 1 suggest that our CNTs are multiwalled [9{11]. Thermal gravimetric analysis (TGA) is often used to get information on the degree of crystallinity of CNTs. Figure 6 shows TGA and DrTGA curves of the puri ed CNTs. The weight is reduced near 500  C and is completely burn out at 663  C. We can see that the carbon content in the puri ed CNTs is 92.42 %. The remains (7.58 %) are attributed to the iron oxide, iron carbide and other impurities. EDS was also employed to analyze the impurities of the puri ed CNTs. The results of the EDS analysis indicate that the Fe content in puri ed CNTs is 4.02 wt.% while the Ca content is undetectable. The discrepancy between the EDS and the TGA results can be explained as a transformation of the Fe composition into other forms due to the oxidation of iron in air during the TGA process. In summary, by using CaCO3 -supported iron salts, we have succeeded in synthesizing MWCNTs. With

Journal of the Korean Physical Society, Vol. 52, No. 5, May 2008

Fig. 6. TGA and DrTGA curves of the puri ed CNTs.

the conditions of 5 wt.% Fe in a mixture of Fe(NO3 )3 9H2 O/CaCO3 at 800  C and a 50 sccm C2 H2

ow, we can produce 1.16 g of puri ed MWCNTs in 30 minutes from 3 grams of catalyst by using CVD.

IV. CONCLUSIONS

We have proven the e ectiveness of our technique by achieving a high MWCNT yield of 78.61 %. MWCNTs (15 { 90 nm in diameter) were grown well over hydrogen-reduction-formed Fe nanoparticles, which originated from a Fe(NO3 )3 9H2 O/CaCO3 mixture containing 5 wt.% Fe at 800  C. MWCNTs with approximately 93 % purity were achieved by using a 15 % HCl acid solution to remove the supports. These results suggest an e ective approach for mass production of CNTs at low cost. If equipment for continuously supplying the catalyst support and for collecting the CVD product, is developed the method will be productive. ACKNOWLEDGMENTS

The authors would like to thank Mr. Le Dinh Quang and Mr. Dao Duc Khang for the experimental setup. This work is supported by the National Nanotechnology and the Basic Research Programs of Vietnam. We are grateful for the support of the Vietnamese - Korean bilateral Science Collaboration Program. REFERENCES

[1] Yancheng and O. Zhou, C. R. Physicque 4, 1021 (2003). [2] R. Khare and S. Bose, Minerals and Materials Characterization and Engineering 1, 31 (2005).

Carbon-Nanotube Growth over Iron Nanoparticles   { Nguyen Duc Dung et [3] J. G. Smith, Jr., Donavon, M. Delozier, J. W. Connel and K. A. Watson, Polymer 45, 1623 (2004). [4] E. Couteau, K. Hernadi, J. W. Seo, L. Thien-Nga, M. R. Gaal and L. Forro, Chem. Phys. Lett. 378, 9 (2003). [5] C. J. Lee, D. W. Kim, T. J. Lee, Y. C. Choi, Y. S. Par, W. S. Kim, Y. H. Lee, W. B. Choi, N. S. Lee, J. M. Kim, Y. G. Choi and S. C Yu, Appl. Phys. Lett. 75, 1721 (1999). [6] N. H. Quang, N. D. Dung, N. T. T. Tam, L. D. Quang, D. D. Khang, P. N. Minh and P. H. Khoi, Proceeding of National Physics Conference (Hanoi, 2005), p. 1424. [7] M. S. Dresselhaus, G. Dresselhaus and P. Avouris, Carbon Nanotubes (Springer-Verlag, Berlin and Hidelberg,

al.

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2001), p. 33. [8] K. Teo, C. Singh, M. Chhowalla and W. Milne, Encyclopedia of Nanoscience and Nanotechnology (American Scienti c Publishers, 2004), p. 665. [9] P. C. Eklund, J. M. Holden and R. F. Jishi, Carbon 33, 959 (1995). [10] J. Kastner, T. Pichler, H. Kuzmany, S. Curran, V. Blaw, Weldon, Chem. Phys. Lett. 221, 53(1994). [11] M. A. Pimenta, A. Marruci, S. A. Empedocles, M. G. Bawendi, E. B. Halon, A. M. Rao, Phys. Rev. B 58R, 16016 (1998).