Controlled Growth of Carbon Spheres Through the Mg-Reduction Route
© to the authors 2009
Received: 9 April 2009
Accepted: 3 September 2009
Published: 19 September 2009
Hollow spheres, hollow capsules and solid spheres of carbon were selectively synthesized by Mg-reduction of hexachlorobutadiene at appropriate reaction conditions. X-ray powder diffraction and Raman spectra reveal that the as-prepared materials have a well-ordered structure. A possible formation mechanism has been proposed.
The emergence of various carbon structures, such as fullerences, carbon nanotubes and closed spherical carbon shells, has triggered intense interest in this versatile material [1–3]. Various efforts have been made to synthesize different carbon structures and morphologies due to their wide range of applications in semiconductor device, gas storage, nanotweezers and electronics [4–8].
Among carbon structured materials, carbon solid and hollow spheres represent a special class of materials that exhibit unique properties such as low weight, thermal insulation and high compressive strength. Because of these excellent properties, carbon spheres can be applied to many industrial fields including gas/energy storage adsorbent, support of catalytic systems, electrode material of lithium–ion batteries, encapsulation of products for the controlled release of drugs or cosmetics. [9–11]. Up to now, various approaches have been carried out to prepare hollow and/or solid carbon spheres. For example, Wang and Yin produced graphitic carbon calabashes and solid spheres via a mixed-valent oxide-catalytic carbonization (MVOCC) process at 900–1,050 °C . Kroto et al. reported synthesis of carbon spheres on the large scale by the direct pyrolysis of hydrocarbons . Recently, direct chemical route has been introduced to synthesize carbon materials. Hu et al. synthesized hollow carbon spheres with a self-assembly approach by using hexachlorobenzene and Na as the reactants, the by-product NaCl generated during the reaction had to be removed by annealing the product above 1400 °C . A mild reduction reaction of Na2CO3, Mg and CCl4 at 450 °C  or the reduction of hexachlorobutadiene by NaN3 at 400 °C  has been reported to produce hollow carbon spheres successfully.
These earlier mentioned methods are usually involved with complicated processes or hazardous experimental conditions. Controlled preparation of nanostructures with desired shapes plays a key role in both nanomaterials science and technology. The carbon materials are known to have a different way of aggregating during reactive processes, which leads to the formation of various textures. By modification of reaction conditions and design of appropriate reaction route, it may be possible to obtain desired morphology of carbon materials. Herein, we report a convenient chemical route to shape-selectively synthesis of carbon hollow spheres, hollow capsules and solid spheres at different temperatures. These carbon materials were prepared by reduction of hexachlorobutadiene with metallic Mg powder as the reductant. The reaction system was conducted in an autoclave without the use of any catalyst. It is found that the shape of the carbon products and the reaction of carbon products can be controlled easily. A possible formation mechanism of the as-prepared carbon products has been proposed based on the experimental results.
In a typical procedure, an appropriate amount of anhydrous hexachlorobutadiene (0.01 mol) and Mg (0.03 mol) were put into a glass-lined stainless steel autoclave of 50 ml capacity. The glass liner can protect the inner wall of steel autoclave from being etched by the reaction. The autoclave was sealed and maintained for 5 h at 400, 480 or 600 °C, then cooled to room temperature. After pressure relief, the autoclave was opened and product was collected. The product was washed with absolute ethanol, dilute hydrochloric acid and distilled water to remove MgCl2 and other impurities. After drying in vacuum at 60 °C for 4 h, the final black powder product was obtained.
The morphology of the as-prepared samples was observed from transmission electron microscopy (TEM) images taken with a Hitachi H-800 transmission electron microscope. The high-resolution transmission electron microscopy (HRTEM) images were taken with a JEOL-2010 transmission electron microscope. Raman spectra were measured on a LABRAM-HR Raman spectrophotometer. The 5145 Å laser was used as an excitation light source. X-ray powder diffraction (XRD) pattern was carried out on a Rigaku Dmax-γA X-ray diffractometer with Cu Κα radiation (wavelength λ = 1.54178 Å).
Results and Discussion
We have demonstrated a convenient chemical route to synthesize carbon hollow spheres, hollow capsules and solid spheres by Mg-reduction of hexachlorobutadiene. The morphology of the product was found to be strongly dependent on the reaction temperature. This method provides a controllable and convenient approach for the preparation of desired carbon materials without a sophisticated technique. This approach could be further extended as a possible route to construct other desired carbon structures.
The financial support of this work by the National Natural Science Foundation of China (Grant No. 20771096) and the 973 Project of China (no. 2005CB623601) is gratefully acknowledged.
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