Materials Letters 70 (2012) 54–56

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Materials Letters

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Preparation of bowl-like and eggshell-like hollow carbon microspheres frompotato starch

Shuo Zhao a,⁎, Xiao-yuan Li a, Cheng-Yang Wang b, Ming-Ming Chen b

a Chemistry and Chemical Engineering, Chongqing University, Chongqing 400030, Chinab Key Laboratory for Green Chemical Technology of Ministry of education, Tianjin University, Tianjin 300072, China

⁎ Corresponding author at: School of ChemistryChongqing University, No. 174, Shazheng Road, Sh400030, China. Tel./fax: +86 23 65111179.

E-mail address: sharon0252003@yahoo.com.cn (S. Z

0167-577X/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.matlet.2011.11.090

a b s t r a c t

a r t i c l e i n f o

Article history:Received 8 September 2011Accepted 23 November 2011Available online 3 December 2011

Keywords:Potato starchZnCl2 activationCarbon materialsPreparation mechanismMicrostructure

Hollow carbon microspheres (HCS) were prepared from potato starch by ZnCl2 activation. The morphologyand surface areas of the HCS were studied by scanning electron microscopy and N2 adsorption method, re-spectively. The results show that the HCS with bowl-like and eggshell-like structures are microporous mate-rials. The preparation mechanism of the HCS was investigated by X-ray diffraction and thermogravimetricanalysis. The results indicate that basic zinc chloride formed during impregnation decomposes into ZnO,HCl and H2O. ZnO and HCl play an important role in the preparation of the HCS. Moreover, removal of ZnOgrains by HCl washing contributes to the total surface areas of the HCS.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

In the past few years, carbon materials with diversiform morphol-ogy have emerged, such as fullerenes [1], carbon nanotubes [2], gra-phenes [3], and hollow carbon spheres [4,5], which greatly enrichesthe family of carbon materials. Among these carbon materials, hollowcarbon spheres have received wide attention owing to their uniquestructure and potential applications [6–8].

Numerous efforts have been devoted to the exploration of synthe-sis methods for hollow carbon spheres [9,10]. Nevertheless, thesemethods require rigorous conditions and complex reaction proce-dures. Some potential applications of hollow carbon spheres are alsosignificantly limited by their nano-scale particle size [11]. Therefore,it is of great significance to explore efficient and simple routes to pre-pare micron-scale hollow carbon spheres.

In this study, a convenient method was reported to prepare hollowcarbon microspheres (HCS), in which potato starch was employed ascarbon source and ZnCl2 as activation agent. The products were charac-terized by scanning electron microscopy (SEM), X-ray diffraction(XRD), thermogravimetric (TG) analyzer and N2 adsorption method.The preparation mechanism of the HCS was proposed.

and Chemical engineering,aping Ba District, Chongqing

hao).

l rights reserved.

2. Experimental

Potato starch from Shandong Jincheng Co., Ltd was impregnatedwith 10 wt.% ZnCl2 solution for 1 h. The impregnated starch wasdried and then carbonized for 2 h at 300, 400, 500 and 600 °C, respec-tively. The activated samples were boiled in 0.1 mol/L HCl solution,and then washed using distilled water until the pH was 7. The prod-ucts were denoted as HCS513 (i.e., HCS: hollow carbon microspheres;51: impregnation ratio is 5:1; 3: activation temperature is 300 °C),HCS514, HCS515 and HCS516, respectively.

The morphology of the samples was characterized by Philips XL30SEM. XRDwas carried out on a Rigaku D/max 2500v/PC system. TA-50TG analyzer was used to study the pyrolysis of samples. N2 adsorptionisotherms were recorded with a Micrometrics ASAP 3000 adsorptionanalyzer.

3. Results and discussion

3.1. Results

SEM images of potato starch and impregnated starch are shown inFig. 1a and b, respectively. It can be seen that impregnated starchretains the original morphology of potato starch, and clastic materialsappear. From Fig. 1a and c, it can be seen that the sphericity ofHCS513 before HCl washing is better than that of potato starch. More-over, HCS513 is hollow (Fig. 1d). SEM images of HCS514, HCS515 andHCS516 before HCl washing show the same results. After HCl wash-ing, the structure of HCS513 and HCS514 is called as bowl-like

Fig. 1. SEM images of (a) potato starch, (b) impregnated starch, (c) HCS513 before HCl washing, (d) sample in (c) after crushing, (e) HCS513 and (f) HCS514 after HCl washing.

10 20 30 40 50 60 70 800

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300

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Inte

nsity

(a.

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2θ (°)

2θ (°)

2θ (°)

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nsity

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HCS513 HCS514 HCS515 HCS516

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0 15 30 45 60 75 900

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21.7°

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nsity

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before HCl washingafter HCl washing

6.7°

c

Fig. 2. XRD curves of (a) impregnated starch, (b) activated samples before HCl washingand (c) HCS515 before and after HCl washing.

55S. Zhao et al. / Materials Letters 70 (2012) 54–56

(Fig. 1e) and eggshell-like (Fig. 1f) structures, respectively. SEM im-ages of HCS515 and HCS516 after HCl washing are the same as thatof HCS514.

Fig. 2 shows the XRD curves of impregnated starch and activatedsamples, and the curves were analyzed using Jade 5.0 XRD software.The results indicate that in Fig. 2a characteristic peaks of potato starchare at 15.1° and 17.2°, and other peaks are the characteristic peaks ofbasic zinc chloride (Zn5(OH)8Cl2·H2O). Therefore, it can be inferredthat clastic materials in Fig. 1b are Zn5(OH)8Cl2·H2O.

The analytical results from Jade 5.0 XRD software also show peaksin XRD curves of all samples are characteristic peaks of ZnO (Fig. 2b).From Fig. 2b, it can also be seen that with the increasing of activationtemperature the intensity of characteristic peaks of ZnO gets stronger,indicating that the size of ZnO grain increases. After HCl washing, twodispersive peaks representing the structure of activated carbon ap-pear at 6.7 and 21.7° (Fig. 2c), which indicates that ZnO grains are re-moved completely.

During activation, NaOH solution was used for detecting exhaustgas, and phenolphthalein was as indicator. It is found that pinkNaOH solution begins to get light at 200 °C, and becomes colorlessat 230 °C. According to the equation for decomposition of otherbasic salts, it can be inferred that Zn5(OH)8Cl2·H2O decomposes intoZnO, HCl and H2O.

Fig. 3 shows TG (and derivative thermogravimetric (DTG)) curves ofpotato starch and impregnated starch. It can be seen that TG curveof impregnated starch is similar with that of starch impregnated by

0 150 300 450 600 750 9000

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ght (

%)

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impregnated starch potato starch

82

CTemperature ( )°

C°

C°

C°

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tial r

esid

ual w

eigh

t (%

/C

)°

Fig. 3. TG (and DTG) curves of potato starch and impregnated starch.

Table 1Effect of HCl washing on the surface areas of samples.

Samples SBETa (m2/g) SBET

b (m2/g) SBETc (m2/g)

HCS514 0.34 582.29 581.95HCS515 475.90 613.69 137.79HCS516 425.19 520.33 95.14

a SBET: surface areas of activated sample before HCl washing.b SBET: surface areas of activated sample after HCl washing.c SBET: incremental surface areas by HCl washing, i.e., SBETc =SBET

b −SBETa .

56 S. Zhao et al. / Materials Letters 70 (2012) 54–56

NH4Cl solution [12]. The starting temperature of thermal decompositionand the temperature of maximum weight loss for impregnated starchare much lower than those of potato starch.

Surface areas of activated samples before and after HCl washingare shown in Table 1. Surface areas of HCS513 cannot be received,which may be that the pore structure of HCS513 is too complicated.From Table 1, it can be seen that after HCl washing surface areas ofall samples increase indicating that removal of ZnO also contributesto the total surface areas. When activation temperature increases to600 from 400 °C, incremental surface areas decrease to 95.14 from581.95 m2/g.

3.2. Discussion

HCS with bowl-like and eggshell-like structures can be preparedon a large scale from potato starch by ZnCl2 activation. The methodis simple and low-cost, and potato starch can be obtained from vari-ous renewable plant sources. Moreover, the range of potato starchgranule size is 15–100 μm. The HCS with different particle size canbe prepared by controlling the granule size of potato starch used ascarbon resource.

Preparation mechanism of the HCS is discussed from two aspects.Firstly, catalytic dehydration of HCl from the decomposition ofZn5(OH)8Cl2·H2O is the reason that the HCS can retain the originmorphology of potato starch [13]. The evolution of lots of small mol-ecules leads to better sphericity of the HCS than potato starch. Sec-ondly, the structure of the HCS is related with the size of ZnO grainsformed during activation. When the size of ZnO grains is small, thecarbon layer around ZnO grains is thick, and collapses after HCl wash-ing. At high activation temperature, size of ZnO grains increases(Fig. 2b), the carbon layer around ZnO grains becomes thin, and iseasy to destroy during HCl washing. Therefore, HCS with bowl-likeand eggshell-like structures were prepared at different activationtemperatures.

From Table 1, it can be concluded that two factors contributeto the surface areas of the HCS: the amorphous structure of activatedsamples, and the removal of ZnO grains. It is because that rearrange-ment and condensation reactions are the main reactions when activa-tion temperature is higher than 400 °C (Fig. 3). Therefore, thestructure of activated samples gets more orderly from amorphousstructure with the increasing of activation temperature and ZnOgrains are coated more closely by the carbon layer, leading to thedecrease of the incremental surface areas after removal of ZnO.

4. Conclusions

Bowl-like and eggshell-like HCS prepared in this paper exhibit mi-croporous properties with surface areas of 520–614 m2/g. The prepa-ration mechanism of the HCS was investigated. The results indicatethat Zn5(OH)8Cl2·H2O formed during impregnation decomposesinto ZnO, HCl and H2O during activation. The catalytic dehydrationof HCl makes the HCS retain sphericity. After removal of ZnO grains,the carbon coating layer around ZnO grains collapses or is destroyed,leading to bowl-like and eggshell-like structures. Moreover, the re-moval of ZnO grains also contributes to the total surface areas.

Acknowledgment

This work is financially supported by the Research Fund for theDoctoral Program of Higher Education of China (20100191120046).

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