Fluffy carbon submicrospheres produced by a catalyzed solvent-thermal reaction

Letters to the Editor / Carbon 45 (2007) 1583–1595 [6] Harris PJF. Carbon nanotubes and related structures. New York: Cambridge University Press; 1999. [7] Coleman JN, Khan U, Blau WJ, Gun’ko YK. Small but strong: a review of the mechanical properties of carbon nanotube-polymer composites. Carbon 2006;44(9):1624–52. [8] Katz E, Willner I. Biomolecule-functionalized carbon nanotubes: applications in nanobioelectronics. ChemPhysChem 2004;5(8): 1085–1104.

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[9] Arunkumar AI, Srisailam S, Kumar TKS, Kathir KM, Chi YH, Wang HM, et al. Structure and stability of an acidic fibroblast growth factor from Notophthalmus viridescens. J Biol Chem 2002;277(48):46424–32. [10] Srisailam S, Kumar TKS, Rajalingam D, Kathir KM, Sheu HS, Jan FJ, et al. Amyloid-like fibril formation in an all beta-barrel protein – partially structured intermediate state(s) is a precursor for fibril formation. J Biol Chem 2003;278(20):17701–9.

Fluffy carbon submicrospheres produced by a catalyzed solvent-thermal reaction Cailiu Yin a, Qizhong Huang a,*, Yonggui Xie a, Xiufei Wang a, Zhiyong Xie a, Lianlong He b, Zhean Su a, Baorong Liu c b

a State Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083, China Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China c College of Physics Sciences and Engineering, Guangxi University, Nanning 530004, China

Received 7 July 2006; accepted 17 February 2007 Available online 27 February 2007

Discoveries of fullerenes and carbon nanotubes (CNTs) are the landmark of carbon materials science entering nano-era. Over the past ten years, carbon spheres with different structures are attracting extensive attention due to their excellent properties and potential applications as support of catalyst, electrode material of lithium-ion batteries, magnetic material, gas storage medium and structural reinforcement. However, compared to the studies of carbon nanotubes (CNTs) and fullerenes, investigations on carbon spheres are relatively limited and ever regarded as by-products or impurity and removed in early stage. Synthesis techniques of carbon spheres mainly are chemical vapor deposition [1], self-generated template approach [2], hydrothermal reaction [3], pyrolytic method [4,5] and arc discharge [6]. Serp et al. have classified the structures of carbon spheres according to their size [7]: carbon beads (one to several microns), carbon spheres (from 50 nm to 1 lm) and carbon onions (2–20 nm). In addition, according to Inagaki [8], carbon spheres can also be classified into three categories from their nanometer texture, concentric, radial and random arrangements of carbon layers. In order to improve the properties and expand the application field of carbon materials with new structures, new carbon precursors and techniques are developed. In previous studies, halogenated hydrocarbon or metal carbide as single carbon source has been applied to produce diamond [9], carbon nanotubes [10] and carbon spheres. Inspired *

Corresponding author. Tel.: +86 731 883 6078; fax: +86 731 883 6081. E-mail address: [email protected] (Q. Huang).

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from above two methods, Xie [11] proposed a method that both above carbon sources as reactants were used to synthesize diamond, carbon nanotubes and fullerenes though chemical reaction. Here, we synthesize fluffy carbon submicrospheres together with amorphous structure by using both calcium carbide and chloroform as carbon sources with ferrocene as catalyst at 350 C. The reaction formula could be proposed as 2CHCl3 ðlÞ þ 3CaC2 ðsÞ

FeðC5 H5 Þ2

! 8CðsÞ

þ 3CaCl2 ðsÞ þ H2 ðgÞ We call it the catalyzed solvent-thermal reaction. In this process, CHCl3 and CaC2 are oxidant and reducer, respectively. In this experiment, a 100 ml stainless autoclave was used as experimental set-up. The reaction temperature was recorded through a thermocouple installed in the autoclave. The air in autoclave was excluded with argon gas for several minutes. At first, 14.2 g of CaC2 powder (experimental reagent, >95%), 10 ml of CHCl3 (A.R grade purity, >99%) and 1.5 g of ferrocene powder (A.R grade purity, >99%) were put into the autoclave. Then the autoclave was sealed and heated to reaction temperature and kept at 350 C for 3 h. After reaction, the autoclave was cooled to room temperature naturally. A light and spongy dark product was collected and washed by dilute hydrochloric acid, absolute ethanol and distilled water for several times until the solution in neutrality. In the process of washing,

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Intensity (CPS)

12000

Table 1 Table composition of carbon spheres

26.26°

10000

Element

Carbon

Chlorine

Silicon

Oxygen

Sulfur

Calcium

8000

Content (wt.%)

95.27

0.17

0.34

3.79

0.10

0.33

6000 4000

44.38°

2000 0 20

40

60

80

2-theta (°)

Fig. 1. XRD pattern of the washed product.

ultrasonic dispersion was used in ethanol washing step. Finally, the purified product was filtrated and dried at 130 C for 8 h. The weight of the washed product is 0.52 g. X-ray diffraction (XRD), scanning electron microscope (SEM) with energy diffraction spectroscopy (EDS), transmission electron microscopy (TEM), high-resolution transmission electronic microscope (HRTEM), BET specific surface area and Raman spectroscopy were used to characterize the washed product. XRD pattern of the washed product is shown in Fig. 1. The two peaks of the XRD pattern at 26.26 and 44.38 can be indexed as graphitic (0 0 2) and (1 0 1) planes according to PDF#41-1487, respectively. The broad root of (0 0 2) diffraction peak presents that the amount of amorphous carbon is dominating. XRD pattern of graphitization degree (omitted) reveals that crystallization degree of this carbon product is very poor and only occupies 6.94%. The interlayer distance of (0 0 2) carbon plane measured by the XRD is 0.3391 nm. Fig. 2 shows the SEM image of the washed product. Most of the particles are spherical and adhere to each other like chains. The range of their diameters is between 50 and 250 nm and the most of carbon spheres have a size of 150 nm. The typical elemental analysis results of the carbon spheres using EDS are shown in Table 1. The EDS analysis reveals that the carbon content of carbon spheres is above 95 wt.%. The impurity elements in these carbonaceous

Fig. 2. SEM image of the washed product.

spheres are silicon, sulfur, calcium, chlorine, and oxygen. The silicon and sulfur elements are from raw CaC2, while chlorine and calcium elements are from the non-removed impurities in carbon aggregates or surface groups of carbon spheres, and oxygen comes from surface absorption. So, the global yield of carbon spheres occupies 8.3 wt.% (0.49 g) of the theoretical carbon yield (5.90 g) from CaC2 and CHCl3. According to TEM image of the sample shown in Fig. 3a at low magnification, the carbon spheres appear like carbon blacks and adhere together to chains. These chained spheres are still hard to be separated through supersonic dispersion. From TEM image of a typical carbon sphere at high magnification in Fig. 3b, it is very interesting that the whole sphere seems to be made of entangled filamentous carbon and looks fluffy. Due to their thickness, the centre of fluffy sphere is dark in the TEM image. Combining with the result of XRD pattern and the sizes of carbon spheres, we call it a fluffy carbon submicrosphere with amorphous structure. In order to better understand microstructure of the fluffy carbon spheres, HRTEM observations were carried out. HRTEM image of the fluffy sphere indicates that the filamentous carbon structure is composed of wavy graphitic layers, as shown in Fig. 3c. Long graphitic layer belts tangle together and constitute poor graphitic structure. The interlayer distance was measured to be in the range of 0.34–0.4 nm (as shown in Fig. 3d), far from 0.3354 nm of the ideal graphite crystal. A typical Raman spectrum of the fluffy carbon spheres is shown in Fig. 4, in which two broad peaks observed are 1324.34 cm1 (D-band) and 1584.50 cm1 (G-band), respectively. Compared with the Ref. [12], the broader peak reveals more amorphous characteristic, while the higher ID/IG (about 1.28) reveals the smaller graphitic crystallite size and the lower graphitization of the fluffy carbon spheres. The result is in agreement with XRD result and HRTEM observation. The BET surface area of carbon fluffy spheres is 48 m2/g. The possible growth process of the fluffy carbon spheres is briefly discussed. We carried out a series of experiments; the fluffy carbon spheres could be not gained in the same condition when two reagents among CaC2, CHCl3 and ferrocene were enclosed to the autoclave and heated. When the reaction began, the temperature of the autoclave chamber quickly exceeded 850 C in 1 min and was far higher than 450 C without catalyst. This temperature was far higher than the decomposition temperature of ferrocene [13], and ferrocene was reduced to atomic iron and cyclopentadienyl group [14]. Atomic irons agglomerated into iron clusters, and carbon clusters released grew on the iron

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Fig. 3. (a) Low magnification TEM image of the chained spheres; (b) high magnification TEM image of a typical carbon sphere; (c) HRTEM images of a typical carbon sphere and (d) its edge.

Intensity (a.u.)

1000

carbide and chloroform were the carbon sources and reactants, with ferrocene as the catalyst. The diameter of carbon spheres is in the range of 50–250 nm and the interlayer distance of (0 0 2) plane is from 0.34 to 0.4 nm. The global yield of fluffy carbon spheres is above 8.3 wt.%. Since, calcium carbide and chloroform are lowcost and easy to be obtained, this process is an economic method to produce fluffy carbon spheres.

1324.34

800

1584.50

600 400 200 0 800

Acknowledgements 1200

1600

2000

Wavelength (cm-1)

Fig. 4. Raman spectrum of the washed product.

clusters. As a result of the washing process, the most iron and impurities were removed, resulting in fluffy carbon spheres. However, catalysis of ferrocene in this process needs further investigation. Furthermore, no enough evidence is found to confirm cyclopentadienyl groups of ferrocene as carbon source to form carbon spheres. To sum up, fluffy carbon submicrospheres together with amorphous structure is synthesized by a catalyzed solventthermal reaction at 350 C in a sealed autoclave. Calcium

This work was supported by 973 Projects of China (Grant No. 2006CB600901). We thank Dr. Yong Du in Central South University has revised this manuscript in grammar and spelling. References [1] Qian HS, Han FM, Zhang B, Guo YC, Jun Y, BX P. Non-catalytic CVD preparation of carbon spheres with a specific size. Carbon 2004;42:761–6. [2] Hu G, Ma D, Cheng MJ, Liu L, Bao XH. Direct synthesis of uniform hollow carbon spheres by a self-assembly template approach. Chem Commun 2002;17:1948–9.

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[3] Wang ZF, Xiao PF, He NY. Synthesis and characteristics of carbon encapsulated magnetic nanoparticles produced by a hydrothermal reaction. Carbon 2006;44:3277–84. [4] Xu LQ, Zhang WQ, Yang Q, Ding YW, Yu WC, Qian YT. A novel route to hollow and solid carbon spheres. Carbon 2005;43:1090–2. [5] Shen JM, Li JY, Chen Q, Luo T, Yu WC, Qian YT. Synthesis of multi-shell carbon microspheres. Carbon 2006;44:190–3. [6] Sano N, Kikuchi T, Wang HL, Chhowalla M, Amaratunga GAJ. Carbon nanohorns hybridized with a metal-included nanocapsule. Carbon 2004;42:95–9. [7] Serp P, Feurer R, Kalck P, Kihn Y, Faria JL, Figueiredo JL. A chemical vapour deposition process for the production of carbon nanospheres. Carbon 2001;39:621–6. [8] Inagaki M. Discussion of the formation of nanometric texture in spherical carbon bodies. Carbon 1997;35:711–3.

[9] Li YD, Qian YT, Liao HW, Ding Y, Yang L, Xu CY, et al. A reduction–pyrolysis–catalysis of diamond. Science 1998;281:246–7. [10] Li YL, Yu YD, Liang Y. Novel method for synthesis of carbon nanotubes: low temperature solid pyrolysis. J Mater Res 1997;12(7): 1678–80. [11] Xie YG. A method to synthesize diamond, fullerences and carbon nanotubes with ionic carbon. CN Patent 03124799.7; 2003. [12] Ci LJ, Zhao ZG, Bai JB. Direct growth of carbon nanotubes on the surface of ceramic fibers. Carbon 2005;43:883–6. [13] Reshetenko TV, Avdeeva LB, Ismagilov ZR, Pushkarev VV, Cherepanova SV, Chuvilin AL, et al. Catalytic filamentous carbon structural and textural properties. Carbon 2003;41:1605–15. [14] Braun M, Hu¨ttinger KJ. Sintering of powders of polyaromatic mesophase to high-strength isotropic carbons: III. Powders based on an iron-catalyzed mesophase synthesis. Carbon 1996;34:1473–91.

Synthesis and morphology of carbon microcoils produced using methane as a carbon source Shaoming Yang *, M. Hasegawa, Xiuqin Chen, S. Motojima Department of Applied Chemistry, Faculty of Engineering, Gifu University, Gifu 501-1193, Japan Received 18 January 2007; accepted 7 February 2007 Available online 17 February 2007

We have found that regularly micro-coiled carbon fibers (CMCs) can be synthesized on a large scale with a high reproducibility by the catalytic pyrolysis of acetylene containing a small amount of a sulfur or phosphorus impurity, and have reported the preparation conditions, morphology, growth mechanism and some properties [1]. On the other hand, many carbon compounds other than acetylene [1], such as CO [2], propane [3], methane [4], ethylene, propylene, 1-butene, cis-2-butene, and 1,3-butadiene [5] can also be used as the carbon sources for growing carbon fibers. However, when using hydrocarbons other than acetylene, CMCs were rarely obtained under any reaction conditions. Metal-catalyzed hydrocarbons usually decompose to form a certain amount of acetylene, which is effectively available for growing carbon coils as we have already reported. Among the hydrocarbons, methane has a large cost advantage over acetylene and therefore its use as a carbon source for obtaining CMCs is of great interest. Although methane has been used to synthesize carbon nanotubes [6–8] and carbon nanofibers [9–11] by many researchers, there has not been any report on the preparation of CMCs using methane. In this study, we report our success in preparing CMCs by the Ni-catalyzed pyrolysis of methane, which was pre*

Corresponding author. Fax: +81 582933335. E-mail address: [email protected] (S. Yang).

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heated (decomposed) at high temperatures by a hot wire. Because the decomposition temperature of CH4 is 1200 C, which is much higher than that of C2H2, and this temperature is too high to generate the Ni catalytic anisotropy which is the driving force of coil formation, it is therefore impossible to form CMCs at this temperature [12]. Accordingly, the hot wire CVD process was developed to paralyze methane. The apparatus (Fig. 1) is composed of two reaction tubes, one for preheating, and the other for the CMC growth by chemical vapor deposition (CVD). A horizontal quartz reaction tube (the upper one, for preheating, 23 mm i.d. and 200 mm long) was fixed and contained a stainless steel rod around which a W filament was coiled, and this W filament acted as the hot wire to provide a high temperature of 1500 C for the methane pyrolysis. Another quartz reaction tube (the upper one, for CVD, 50 mm i.d. and 500 mm long) was heated by nichrome elements from the outside. Commercial Ni powder catalyst of several lm, size (Furuuchi Chem. Ltd. Co., Japan) was sprayed onto the graphite plate, which was placed in the central part of the tube. When the upper reaction tube reached 1500 C and the lower reaction tube reached 700–730 C, the source gases of N2, H2, H2S/H2, and CH4 with the flow rates of 150–200, 110–200, 30–60, 80–120 sccm, respectively, were introduced into the system. The reaction time was 30– 90 min. In this process, the source gases were pre-decom-