C A R B O N 4 7 ( 2 0 0 8 ) 3 1 3 – 3 4 7 331

Mechanism for the preparation of carbon spheres frompotato starch treated by NH4Cl

Shuo Zhao, Cheng-Yang Wang*, Ming-Ming Chen, Jing-Hui Sun

Key Laboratory for Green Chemical Technology of State Education Ministry, School of Chemical Engineering and Technology, Tianjin University,

Tianjin 300072, PR China

A R T I C L E I N F O

Article history:

Received 31 July 2008

Accepted 24 September 2008

Available online 30 September 2008

A B S T R A C T

Carbon spheres (CSs) retaining the morphology of potato starch were prepared by a two-

step process: impregnation followed by carbonization. In this process, potato starch was

impregnated in NH4Cl solution for 1 h. The mechanism of preparation was proposed.

The presence of HCl catalyzes the dehydration of impregnated starch at lower temperature

than its melting point, which destroys the crystallites in original starch totally. This makes

the microcrystalline melting difficult during the following carbonization. Scanning electron

microscopy results show that small cavity exists in the center of sphere. The CSs shows

high graphitization degree after graphitizing by X-ray diffraction analysis.

� 2008 Elsevier Ltd. All rights reserved.

Carbon spheres (CSs) have become an interesting research

object for many researchers owing to its wide application,

which includes high-density carbon artifacts [1] and lubri-

cants [2], etc. Therefore, the preparation of CSs has been a

subject of considerable interests from both scientific and

practical point of view. Up to now, various methods have been

developed for the preparation of CSs, e.g., hydrothermal syn-

thesis method [3], and chemical vapor deposition [4].

However, most of CSs are produced from the coal tar or

petroleum pitch. In light of the problems of fossil fuel re-

source, the preparation of CSs from alternative precursors at-

tracts considerable interest from researchers. Starch is

natural polymer and can be obtained from various plant

sources. We report a kind of CSs prepared from potato starch,

and the mechanism of preparation was investigated. In this

process, potato starch was impregnated in NH4Cl solution

for 1 h. The impregnated starch was dried at 40 �C for 5 h

and then carbonized at 600 �C for 1 h under N2. The carbon-

ized starch was further heat-treated at 2600 �C. The samples

were characterized by Philips XL30 scanning electron micros-

copy (SEM). Fourier transform infrared (FTIR) spectroscopy

was carried out using a Nicolet Magna-IR 560 FTIR spectrom-

eter. Thermogravimetric (TG) analysis was run on a TA-50

instrument under N2. Differential scanning calorimetry

(DSC) was carried out using NETZSCH DSC204HP calorimeter.

X-ray diffraction (XRD) was performed on a D/Max2500 X-ray

diffractometer.

Fig. 1 shows SEM images of the potato starch and the car-

bonized products. In Fig. 1a, some potato starch granules are

spheres or ellipsoids, and others are irregular particle shape.

When potato starch is carbonized directly at 600 �C, foam

structure is observed (Fig. 1b). The foam structure indicates

that phase transition process happens in the carbonization.

This can be proved by the following analysis. However, the

carbonized impregnated starch with the decrease in size re-

tains the original shape of potato starch (Fig. 1c). The image

of profile of carbonized impregnated starch indicates a small

cavity exists in the center of the CSs (Fig. 1d).

Fig. 2 gives the FTIR spectra of potato starch, impregnated

starch and NH4Cl. Comparing the spectrum of the three sam-

ples, it can be observed that there are no other characteristic

bands in the spectrum of impregnated starch except that of

potato starch and NH4Cl. It indicates that interaction between

potato starch and NH4Cl is only physical adsorption after

impregnation.

To investigate the catalysis of NH4Cl in the carbonization

of impregnated starch, TG experiments were carried out. As

shown in Fig. 3, the weight loss of original and impregnated

starch both occurs at two stages. The weight loss at first stage

is caused by the loss of absorbed water, and the maximum

weight loss occurs at about 82 �C. At second stage, the weight

loss is caused by the thermal degradation. The starting degra-

dation temperature and the maximum weight loss tempera-

ture of impregnated starch at this stage are much lower

than that of original starch. There are two competitive path-

ways in the pyrolysis of potato starch. The first pathway in-

volves dehydration at low temperature, rearrangement,

formation of carbonyl groups, evolution of CO and CO2, and

0008-6223/$ - see front matter � 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.carbon.2008.09.046

* Corresponding author: Fax: +86 22 27890481.E-mail address: cywang@tju.edu.cn (C.-Y. Wang).

formation of carbonaceous residue. In the second pathway,

the thermal scission of glycosidic bonds between the gluco-

pyranose units of the starch produces many oxygenated com-

pounds at high temperature. The presence of HCl (the

decomposed product of NH4Cl) at about 181 �C catalyzes

dehydration of hydroxyl groups in starch chain; especially

the dehydration of highly active primary hydroxyl group [5].

In other word, the pyrolysis of impregnated starch follows

the first pathway due to the catalysis of HCl.

Fig. 4 shows the DSC curve of potato starch. It can be seen

that there are two obvious endothermic characteristic peaks.

The first peak at about 81 �C corresponds to the DTG peak at

about 82 �C. The second peak at about 280 �C is corresponding

to the melting of crystallites in starch [6]. From TG analysis,

the starting degradation temperature of potato starch is at

around 283 �C, which is almost the same as its melting point.

It indicates that phase transition process happens when pota-

to starch is directly carbonized. However, the maximum

weight loss temperature of impregnated starch is 207 �C from

TG analysis which is far lower than 280 �C. In other word, the

violent dehydration before its melting point destroys the crys-

tallites in potato starch, which makes the crystalline melting

difficult during the following carbonization. The exothermic

peak at 320 �C is corresponding to the thermal degradation

Fig. 1 – SEM images of (a) potato starch; (b) carbonized potato starch; (c) carbonized impregnated starch and (d) the profile of

carbonized impregnated starch.

500 1000 1500 2000 2500 3000 3500 4000

Wave Number (cm-1)

Tran

smitt

ance

(%)

Potato starch

Impregnated starch

NH4Cl

N-HNH4+

C-HC-OO-H

C-OO-H

Fig. 2 – FTIR spectroscopies of potato starch, impregnated

starch and NH4Cl.

0 200 400 600 800

0

20

40

60

80

100

0

-1

-2

-3

-4

Res

idua

l wei

ght (

%)

Temperature (°C)

Diff

eren

tial r

esid

ual w

eigh

t (%

/°C)

TG

DTG

Potato starch

Impregnated starchImpregnated starch

NH4Cl

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

starch and NH4Cl.

332 C A R B O N 4 7 ( 2 0 0 8 ) 3 1 3 – 3 4 7

as evidenced by the main peaks marked out in Fig. 5. It is be-

cause the pyrolysis of impregnated starch following the first

pathway will result in the formation of species containing

four-carbon atoms, which would easily condense to lamellar

structure of graphite [5].

In conclusion, CSs retaining the morphology of potato

starch were prepared by a two-step process: impregnation fol-

lowed by carbonization. The CSs with small cavity in the cen-

ter shows high graphitization degree after graphitizing. The

mechanism of preparation was proposed. The presence of

HCl during carbonization catalyzes the dehydration of starch

at lower temperature than its melting point, which destroys

the crystallites in original potato starch totally. This makes

the microcrystalline melting very difficult during the follow-

ing carbonization.

Acknowledgements

Thanks to the National Science Foundation of Tianjin City,

No. 07jcybjc02500 and the Programme of Introducing Talents

of Discipline to Universities, No. B06006.

R E F E R E N C E S

[1] Inagaki M, Tamai Y, Naka S, Kamiya K. Texture andgraphitization behaviour of fluid coke. Carbon1974;12(6):639–43.

[2] Wang Q, Cao FY, Chen QW, Chen CL. Preparation of carbonmicro-spheres by hydrothermal treatment of methylcellulosesol. Mater. Lett. 2005;59(28):3738–41.

[3] Wang Q, Li H, Chen LQ, Huang XJ. Monodispersed hard carbonspherules with uniform nanopores. Carbon 2001;39(14):2211–4.

[4] Tosheva L, Parmentier J, Valtchev V, Vix-Guterl C, Patarin J.Carbon spheres prepared from zeolite Beta beads. Carbon2005;43(12):2474–80.

[5] Li H, Yang YG, Wen YF, Liu L. A mechanism study onpreparation of rayon based carbon fibers with (NH4)2 SO4 /NH4

Cl/organosilicon composite catalyst system. Compos. Sci.Technol. 2007;67(13):2675–82.

[6] Zaidul ISM, Absar N, Kim SJ, Suzuki T, Karim AA, Yamauchi H,et al. DSC study of mixtures of wheat flour and potato, sweetpotato, cassava, and yam starches. J. Food Eng.2008;86(1):68–73.

100 200 300 400 500

-3

-2

-1

0

1D

SC (m

W/m

g)

320°C

280°C

81°C

Temperature (°C)

Fig. 4 – DSC curve of potato starch.

10 20 30 40 50 60 70 80 90

0

1000

2000

3000

4000

5000

6000

Inte

nsity

(a.u

)

2θ (degrees)

002

100 004 110 112101

Fig. 5 – XRD pattern of impregnated starch after

graphitization.

C A R B O N 4 7 ( 2 0 0 8 ) 3 1 3 – 3 4 7 333

of potato starch during carbonization, which corresponds

with the DTG peak at around 321 �C.

Fig. 5 shows the XRD pattern of the impregnated starch

after graphitization. The (002) peak at 2h = 26.4� corresponds

to the interlayer spacing (d0 0 2) of 0.3377 nm, which is very

close to that of nature graphite. The XRD pattern also indi-

cates it has good crystalline structure after graphitization,