J. Cent. South Univ. Technol. (2010) 17: 895-898

DOI: 10.1007/s11771-010-0573-6

Carbon spheres prepared via solvent-thermal reaction method and their microstructures after high temperature treatment 

YIN Cai-liu(尹彩流)^{1, 2}, WEN Guo-fu(文国富)¹, HUANG Qi-zhong(黄启忠)²,

WANG Xiu-fei(王秀飞)¹, HE Liang-ming(何良明)¹, LIU Bao-rong(刘宝容)³

1. College of Physics and Electronic Engineering, Guangxi University for Nationalities, Nanning 530006, China;

2. State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China;

3. College of Physical Science and Technology, Guangxi University, Nanning 530006, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2010

Key words:

carbon spheres; solvent-thermal method; microstructure; high temperature treatment；

1 Introduction

Mesocarbon microbead (MCMB) and C₆₀ have been studied for more than 20 years, and their synthesis techniques and applications gain big progress. Recently, with the extensive application fields, another spherical carbon with size from 1 nm to several micrometers has absorbed more and more attention all over the world. Among spherical carbon family, solid carbon spheres [1], hollow carbon spheres [2] and carbon capsules [3] are the most interesting structures due to their particular properties and potential applications as anode materials for Li-ion secondary battery [4], polymer based vapor sensors [5], material reinforcements [6], catalyst carrier [7] and lubricant materials [8].

The synthesis technique mainly includes arc discharge [9], chemical vapour deposition (CVD) [10], solvent-thermal/hydrothermal route [11-12], template method [13] and sol-gel process [14]. Arc discharge is the earliest method to produce carbon nanospheres. UGRATE [9] found that most of carbon particles were formed by curved graphitic sheets and were extremely favorable at high temperature. This method, however, was low yield and needed violent condition. CVD technique and sol-gel route may be the most hope to produce spherical carbon because they can directly synthesize carbon spheres with or without catalyst and have some advantages such as high yield and uniform size distribution. While solvent-thermal/hydrothermal route, as a new method to synthesize spherical carbon, has been used only in recent several years, therefore, process parameters and carbon precursors need to be optimized, and the formation mechanism of carbon spheres needs to be investigated. In a word, scientists have been struggling in carbon material field as spherical carbon possesses different structures and excellent performances.

In this work, a simple method to synthesize carbon spheres via solvent-treatment reaction was studied with calcium carbide and chloroform as reactants. The morphology and structure change of the carbon spheres before and after high-temperature treatment (HTT) at   2 300 ℃ were investigated.

2 Experimental

In a typical experiment, 14.2 g of CaC₂ powder (experimental reagent, ＞95%) and 10 mL of CHCl₃ (A.R. grade purity, ＞99%) were enclosed to a stainless autoclave with a volume of 100 mL (Automatic Control Reaction Co., Ltd of Weihai, China). The air in the autoclave was excluded with argon gas before the reactants were put into the autoclave. Then, the autoclave was sealed and heated to the reaction temperature of  350 ℃ which was kept for 3 h. When the autoclave was cooled to room temperature naturally, dark power was collected and washed by dilute hydrochloric acid, absolute ethanol and distilled water respectively for several times. In the end, the washed product was filtrated and dried at 150 ℃ for 5 h. For HTT, part of carbon product was heated to 2 300 ℃ for 1 h under argon gas atmosphere. X-ray diffractometer (XRD, Rigaku, D/MAX-2550, Japan, 18 kW), scanning electron microscope (SEM, JEOL JSM-5600LV, Japan) with energy diffraction spectroscope (EDS), transmission electron microscope (TEM, JEOL2010, Japan, 200 kV), and BET (QuantaChrome monosorb，America) specific surface area analyzer were used to characterize carbon products before and after HTT.

3 Results and discussion

XRD patterns of the washed products before and after HTT at 2 300 ℃ are shown in Fig.1. The two peaks of the XRD pattern at 26.36? and 42.96? in curve (a) can be indexed as graphitic (002) and (100) planes according to PDF#41-1487, respectively. The broad root of (002) diffraction peak indicates that the dominating carbon is amorphous and low graphitization. The narrow full width

at half maximum (FWHM) of (002) diffraction peak means the long ordering structure and the high graphitization degree of carbon materials [15], and the FWHM of curve (a) is much smaller than that of curve (b) indicating less average in-plane crystallite dimensions. The change of 26.36? to 26.42? also verifies a decreased interlayer spacing d₍₀₀₂₎. Furthermore, (100), (101), (004), (110) and (112) peaks of carbon product after HTT are easily distinguished in curve (b) but not observed in curve (b), further demonstrating the development of long range graphitic ordering [4]. The interlayer distances of (002) carbon plane of carbon product before and after HTT measured by the XRD are 0.337 8 and 0.337 1 nm, respectively.

Figs.2(a) and (b) show the morphologies and size

Fig.1 XRD patterns of carbon product before (a) and after (b) HTT at 2 300 ℃

Fig.2 SEM images and EDS patterns of carbon spheres: (a) SEM, before HTT; (b) SEM, after HTT at 2 300 ℃; (c) EDS, before HTT; (d) EDS, after HTT at 2 300 ℃

distribution of carbon spheres before and after HTT at  2 300 ℃, respectively. Before HTT, the majority of the products is spherical and has a wide diameter range between 50 and 300 nm. Carbon spheres tend to aggregate bigger spheres (arrow 1) or adhere to each other like chains (arrow 2) (see Fig.2(a)). After HTT, smooth surface of carbon spheres becomes polyhedral, and the diameter of the spheres decreases (approximately 50-250 nm). The inset at top left corner in Fig.2(b) (enlarged image of arrow 3) displays a typical polyhedron structure, which has been reported by JIN  et al [4]. EDS patterns shown in Figs.2(c) and (d) reveal that the carbon content of carbon spheres is above 91% and 95% (mass fraction), respectively, and the impurity elements before HTT are oxygen, chloride and calcium; after HTT, only oxygen exists, and the aim of spraying Pt is to increase the electrical conductivity of carbon spheres. Chloride and calcium may come from surface groups (Cl—C) and reaction resultant (CaCl₂). After HTT, oxygen is still detected because the bigger BET specific surface (98 and 74 m²/g before and after HTT, respectively) results in surface absorption of carbon spheres.

Figs.3(a) and (c) show TEM images of carbon spheres before and after HTT. It is obviously observed that carbon spheres are cauliflower-like before HTT, but spheres become hollow polyhedrons with shell thickness of 15-30 nm after HTT. From HRTEM images of the sphere edge, cauliflower-like spheres are composed of curled carbon sheets entangled and their interlayer distance of (002) planes was measured to be in the range of 0.34-0.40 nm (as shown Fig.3(b)), far from      0.335 4 nm of the ideal graphite crystal; the planes of polyhedron shell consist of parallel carbon sheets with d₍₀₀₂₎ of 0.336 8 nm. The result is in agreement with XRD result and HRTEM observation. Carbon polyhedron structure is similar to onion-like [16] or polyhedral nanoparticles [17] by arc discharge.

Up to now, the formation mechanism of spherical carbon and related structures is not well established. In the following, growth mechanism of carbon sphere by solvent-thermal reaction is simply discussed and the structure change after HTT is analyzed. In this experiment, the reaction temperature of CaC₂-CCl₄ system was about 190 ℃. When the reaction began, carbon neutrals(C and C₂) and ions (C⁺) carbon clusters were released, and then coagulated with each other to form small clusters. At last, coalescence between carbon clusters finally formed the carbon spheres, which was similar to the formation mechanism of onion-like carbon structures or liquid carbon drop [9, 16-17]. However, solvent-thermal reaction was a mild reaction, therefore,

Fig.3 TEM image of carbon sphere and its edge before ((a), (b)) and after ((c), (d)) HTT at 2 300 ℃

carbon spheres belonged to amorphous structure, far from ideal graphite before HTT.

High temperature annealing effects on carbon spheres [4] and carbon nanotubes [18] showed that high temperature was helpful for enhancing graphitization degree or decreasing the layer distance of carbon (002) plane. From the above TEM observation, so-produced cauliflower-like carbon (Fig.3(a)) transformed into hollow polyhedron (Fig.3(c)) through high temperature at 2 300 ℃. HRTEM image in Fig.3(b) reveals that the curled carbon sheets entangle like cotton structure with many spaces between cottonwoods. Through HTT, the disordered carbon sphere transforms highly ordered structure (Fig.3(d)) by the energy gain from van der Waals interaction between shells [19]. Furthermore, the branches of internal shell of hollow carbon further verify that the growth of graphite layers is supposed to begin on the surface and progress toward the center [16].

4 Conclusions

(1) Carbon spheres of size of 50-300 nm are synthesized via solvent-thermal reaction with calcium carbide and chloroform as reactants. The original spherical morphology of carbon is converted to hollow and polyhedral via HTT.

(2) These carbon spheres possess amorphous character and are composed of curled and entangled graphic sheets with a short range order, while carbon hollow polyhedrons consist of parallel graphic flakes with a long range order.

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(Edited by YANG You-ping)

Foundation item: Project(2006CB600901) supported by the National Basic Research Program of China; Project(0991015) supported by Guangxi Natural Science Foundation, China; Project(200808MS083) supported by Guangxi Education Department Foundation, China

Received date: 2009-12-08; Accepted date: 2010-04-16

Corresponding author: HUANG Qi-zhong, PhD, Professor; Tel: +86-731-88836078; E-mail: qzhuang2007@163.com

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