Materials Letters 62 (2008) 3322–3324

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Materials Letters

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Preparation of carbon sphere from corn starch by a simple method

Shuo Zhao ⁎, Chengyang Wang, Mingming Chen, Zhiqiang ShiSchool of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China

a r t i c l e i n f o

⁎ Corresponding author. Tel./fax: +86 22 27890481.E-mail address: sharon0252003@yahoo.com.cn (S. Z

0167-577X/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.matlet.2008.02.057

a b s t r a c t

Article history:Received 14 November 2007Accepted 21 February 2008Available online 29 February 2008

Carbon spheres with a regular and perfect shape was successfully synthesized from corn starch by a two stageprocess: oxidation and carbonization. In this process, oxidation treatment was the main step; the optimaloxidation temperature was 210 °C. The resulting products were characterized by using combined techniquesincluding X-ray powder diffractometer (XRD), scanning electron microscopy (SEM), and high-resolutiontransmission electron microscopy (HRTEM). The results showed that the products could keep the originalshape of corn starch perfectly, and the diameters of carbon spheres ranged from 5 to 25 μm. It showedcharacteristic structure of non-graphitizing carbons heated at high temperatures. The preparationmechanismhas also been discussed.

© 2008 Elsevier B.V. All rights reserved.

Keywords:Carbon sphereCorn starchHeat treatmentMicrostructure

1. Introduction

The unique subtlety in bonding configurations that results fromcarbons ability to employ almost any conceivable combination of sp,sp2 and sp3 hybridization in a wide range of materials results in animmensely diverse range of structures and morphologies withextensive technical applications [1]. Spherical carbon particles as anew material has recently attracted considerable attention andinterest from carbon scientists worldwide owing to their uniqueapplications, including high-density and high strength carbon artifacts[2,3], energy storage [4], separation technologies [5], lubricants, etc.Different methods had been developed for the synthesis of carbonsphere, e.g., hydrothermal synthesis method [6–8], chemical vapordeposition [9–12], and pressure carbonization. These techniques,however, have some common features. Firstly, the process conditionare complicate, such as, they all need certain pressure duringsynthesis; secondly, the raw materials they used are all toxic andnon-renewable chemicals, and in a fewcases the catalystmust be used.

Starch is green, inexpensive, renewable, and available in largeamounts from different sources and has provided a viable alternativeto replace synthetic polymer, decreasing in consequence the environ-mental pollution [13,14]. Besides, starch itself has certain particleshape. So corn starch worked as carbon precursor in this paper. Mono-dispersed carbon sphere, which kept the original shape of starch, wasprepared by a simple method.

2. Experimental

The reagent (corn starch) used was obtained from ShandongJincheng Co., LTD. In this study, the corn starch was oxidized in

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the oven at air atmosphere for 24 h. The oxidation temperaturekept at 210 °C during oxidation. Carbonization of the starch wascarried out in a tube furnace with programmble controls, such asramp rate, final temperature and dwell time. Nitrogen gas waspassed through the reactor right from the beginning of carboniza-tion. The furnace temperature was increased at a rate of 2 °C/minfrom room temperature to 600 °C and held at this temperaturefor 1 h. Further, the carbonized starch was heat-treated at2600 °C.

Structure characterization was performed by X-ray diffraction(XRD) on a D/Max2500 X-ray diffractometer using CuKα radiation(40 kV, 200 mA, λ=1.54051 Å). The morphologies of the sample werecharacterized with a scanning electron microscopy (SEM, XL30ESEM).The high-resolution transmission electron microscopy (HRTEM) wasperformed with a Philips Tecnai G2 F20.

3. Results and discussion

The SEM was used to examine the morphology of the as-prepared carbon sphere.The SEM image in Fig. 1a showed the typical morphology of the corn starch particles.The diameter of most particles was very different, ranging from 5 to 25 μm. Fig. 1cwas the image of the sample carbonized at 600 °C. It indicated that the products keptperfectly the original shape of corn starch, and the diameter of the products becamesmaller. After graphitization, most of the products still kept spherical morphology (inFig. 1d). The surface of the received carbon sphere was very smooth without cracks.

To investigate the effect of oxidation process on the preparation of carbonsphere, a parallel experiment was carried out. It was found that the oxidation processplayed a critical role in the process. When the corn starch was directly carbonized atthe temperature of 600 °C, starch granules would melt together during carboniza-tion; and the morphology of as-prepared products like sponge (see from Fig. 1b).

A possible explanation for the as-prepared carbon sphere keeping the originalshape of precursor was proposed. When the corn starch was heated in the air, a largenumber of hydroxyl groups containing in the starch molecular were oxidized.Intermolecular cross-linking and rearrangement reaction occurred synchronously,followed by the releasing of small gaseous molecules. This made the melting process ofcrystallites not happen during carbonization [15]. A detailed study of the processmechanism was underway.

Fig. 1. SEM images of (a) corn starch, (b) corn starch carbonized at 600 °C without oxidation, (c) corn starch carbonized at 600 °C with oxidation, (d) corn starch graphitized at 2600 °Cafter oxidation.

3323S. Zhao et al. / Materials Letters 62 (2008) 3322–3324

The XRD and TEM were also powerful techniques for characterizing the micro-structure of carbon materials. The X-ray diffraction pattern of the carbon spheregraphitized at 2600 °C was shown in Fig. 2a. The broad bottom of the (002) peak andthe asymmetry of the (100) peak all showed that the sample had characteristics ofhard carbon after graphitization. The XRD pattern of 002 planes was shown in the Fig.2b. It showed that the T-constituent (turbostratic) and G-constituent (graphitic) had

Fig. 2. (a) XRD pattern of the products graphitized at 2600 °C; (b) XRD pattern of 002planes.

peaks at ca. 25,881° and 26,380°, respectively, analogous to those reported in manypapers [16]. The A was an amorphous constituent characteristic to non-graphitizingcarbons. When the temperature reached 2600 °C, part of A-constituent converted intoG-constituent. That was the reason why the G-constituent appeared after graphitiza-tion. Here, it should be emphasized that the present T-constituent showed a farnarrower peak than that of conventional turbostratic carbons, in other words, the T-constituent had very thick crystallites, in spite of a turbostratic structure. The layerdistance of carbon (002) plane from the XRD data were 0.34397 nm and 0.33758 nm,respectively.

A typical TEM image of the as-prepared carbon sphere was shown in Fig. 3a. TheTEM image indicated the carbon sphere in the sample appeared to be of nearlyperfect shape with different diameters, corresponding with the original shape of cornstarch. Fig. 3b presented the high-rate image of the thin edge of the carbon sphereindicated in the Fig. 3a. A lot of seriously entangled carbon ribbons appeared. This wasa typical and characteristic structure of non-graphitizing carbons heated at hightemperatures, corresponding with the results of XRD very well.

4. Conclusions

In summary, the carbon sphere keeping the original shape of cornstarch had been successfully synthesized by a simple process. Thecarbon sphere had a regular and perfect shape. The oxidation processwas found to have crucial effects on the final product. Meanwhile, thepreparation mechanism was suggested as the cross-linked bondformed during oxidation, contributed greatly to reducing the plasticityof the starch. Compared with other methods, this approach wassimple and feasible, and the raw material was green, inexpensive,renewable, and available in large amounts from different sources.

Fig. 3. (a) TEM image of carbon sphere; (b) HRTEM image of the thin region shown inthe white-circle area in (a).

3324 S. Zhao et al. / Materials Letters 62 (2008) 3322–3324

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