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Copyright 2010 Faculty of Science, Ubon Ratchathani University. All rights reserved.

Sci. J. UBU, Vol. 1, No. 2 (July December, 2010) 40-45 SCIENCE JOURNAL Ubon Ratchathani University http://scjubu.sci.ubu.ac.th

*Corresponding author. E-mail address: [email protected]

Research Article

Synthesis of Carbon Microspheres from Starch by Hydrothermal Process

S. Ratchahat1, N. Viriya-empikul2, K. Faungnawakij2, T. Charinpanitkul1, A. Soottitantawat1,*

1Center of Excellence in Particle Technology, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand.

2National Science and Technology Development Agency, 130 Thailand Science Park, Paholyothin Rd., Klong Luang, Pathumthani 12120, Thailand.

Received 10/03/10; Accepted 22/12/10

Abstract

This study showed a facile catalyst-free method to synthesize carbon microspheres (CMSs) via hydrothermal and carbonization process using various types of starch as starting materials. In hydrothermal process, starch was hydrolyzed, dehydrated, and polymerized to form carbon microspheres in water as a medium without involving any hazardous solvents. After hydrothermal process, the dried products were treated by heat in carbonization process under nitrogen atmosphere to develop their pore system of carbon microspheres. The two main types of starch, modified starch and native starch, were employed to address differences in particle size and morphology of resulting carbon microspheres. The SEM images clearly illustrated that the carbon microspheres have their perfect spherical morphology and smooth surface. The particle size distributions of the products with a size range of 0.4-4.0 m were determined by laser diffraction technique (Mastersizer). The particle size and particle size distribution of carbon microspheres strongly depended on types of starch. In other words, carbon microspheres from modified starch tended to smaller in particle size than carbon microspheres from native starch because of water-solubility of modified starch higher than native starch. After carbonization process, structural CMSs characterization performed by X-ray diffraction technique (XRD) indicated semi-hexagonal graphite structures which would be suitable for secondary lithium ion application. Elemental compositions of the carbonaceous products determined by energy dispersive X-ray spectroscopy (EDX), indicated that a main component was carbon being inert to many chemical reactions. Furthermore, these carbon materials have specific BET areas in the range of 400-500 m2/g which were formed during carbonization process. All N2 adsorption-desorption isotherms of these carbon materials have type I isotherm regarding to IUPAC classification that indicated micropore system.

Keywords: Carbon microspheres, Starch, Porous carbon, Hydrothermal process.

S. Ratchahat et al., Sci. J. UBU, Vol. 1, No. 2 (July December, 2010) 40-45


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1. Introduction

Since the significant finding of Buckminster-fullerene (C60) [1] and carbon nanotubes (CNTs) [2] considerable efforts have been made toward the synthesis of functional carbonaceous materials with diverse morpho-logies and structures, such as colloidal spheres [3], nanofibers [4], coin-like hollow carbons [5], macroflowers [6], and so on. Among the different morphologies of carbon-iceous materials, carbon microspheres (CMSs) have attracted widespread interest, owing to their potential properties in adsorb-ents [7], catalyst supports [8], and anode material for lithium ion batteries [9] and templates for fabricating core-shell or hollow structures [10]. The CMS particles have been synthesized by many techniques, such as pressure carbonization [11], chemical vapor deposition [12], mixed-valence oxide-catal-ytic carbonization [13], and reduction of carbides with metal catalysis [5]. Accord-ingly, various applications have been intens-ively developed [14]. Carbon microspheres (CMSs) with a perfect shape have the priority in catalyst support [15] and template utilization [16]. However, there are a few reports of CMSs with a uniform size and perfect spherical morphology from various types of starch. Consequently, in this study the synthesis of CMSs from various types of starch was investigated via hydrothermal process and following by carbonization pro-cess. The aim of this study is to investigate effects of types of starch on particle size and morphology of carbon microspheres. The two main types of starch, modified starch and native starch, were used to investigate differ-ences in particle size and morphology of the obtained carbon microspheres. After hydro-thermal process, the dried CMSs had been carbonized under nitrogen atmosphere. The carbonization process had highly developed microporosity of CMSs but had removed the reactive functional group (-OH,-COOH) on their surface [17]. The porous CMSs have highly microporosity and inert surface that can be used as adsorbents or gas storage materials. Without carbonization process, the CMSs have the reactive functional group on

CMSs surface which can be immobilize target reactive agents on the surface without further surface modification. The uniform CMSs can be determine particle morpho-logies and particle size distributions by scan-ning electron microscopy (SEM) and laser diffraction method (Mastersizer 2000), resp-ectively. The porous CMS particles were characterized by X-ray diffraction method (XRD) to reveal their crystalline properties. They were determined their specific surface area both before and after carbonization process to reveal the development of porous structure using adsorptiondesorption of nitrogen or the Brunauer, Emmett, Teller method (BET method). Moreover, elemental components of the porous CMS particles were determined by energy dispersive X-ray method (EDX) to demonstrate carbon content in their structure.

2. Materials and Methods

Materials. Two types of modified starch, Hi-CAP100 and CAPSUL, were obtained from National Starch and Chemical Ltd, (Bangplee, Thailand). These starches (HI-CAP100 and CAPSUL) were partially hydrolyzed of waxy maize starch and then derivatized to impart lipophilic properties with n-octenyl succinic anhydride. They can be immediately dissolved in water. The difference in structure of HICAP100 and CAPSUL, HICAP100 is a straight chain starch but CAPSUL is a branch chain starch. Therefore HICAP100 has water-solubility more than CAPSUL. Meanwhile, other native starch or crystallized starch (corn, tapioca, wheat, rice, and sticky rice starch) were obtained from a general commercial resource. High purity nitrogen gas (99.999%) was purchased from TIG. Synthesis of the CMSs. In the hydrothermal process, starch (5.0 g) was dispersed in 45.0 mL of de-mineralized water and magnetically stirred at 60C for 30 min. The mixture was filled in a 50 mL Teflon-lined stainless autoclave. Subsequently, the autoclave was put into an oven, which was heated at 180C



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for 12 h. The autoclave was cooled to room temperature naturally. Dark precipitates were collected and washed with de-mineralized water several times and dried in an oven at 100C for 24 h. The obtained powders were carbonized in a tube furnace under N2 atmosphere. The N2 flow rate, final temper-ature and heating rate of the furnace were 100 ml/min, 600C and 1C/min, respectively. Characterization. The particle size distribut-ions of synthesized carbon microspheres were determined by laser diffraction techn-ique (Mastersizer 2000: Malvern, United Kingdom). The samples were characterized by X-ray powder diffraction (XRD, SIEMENS D5000, Japan) using CuK radiation. The morphology observation of the samples was examined with scanning electron microscopy (SEM, JEOL: JSM-5410LV, Japan). The specific BET surface area was measured by N2 adsorption- desorption at -196C (BEL: BELSORP-mini, Japan). The carbonaceous products were confirmed by energy-dispersive X-ray spectra (EDX).

3. Results and Discussion.

Figure 1 shows the typical morphology of the synthesized CMSs from hydrothermal process. Perfect spherical shape and smooth surface can be observed in CMSs from all types of starch. These spherical particle formations are generally patterns in particle formation because they can keep the lowest their surface energy in spherical formation. When HI-CAP100 was used as a carbon precursor, the smallest size and highly uniform CMSs were obtained (Figure 1a, b) because of its high water-solubility. Therefore HI-CAP100 can be hydrolyzed immediately to form simultaneous nucleates of carbon microspheres. Nonetheless, the CMSs obtained from CAPSUL showed the big particle size more than carbon microspheres from HI-CAP100 because CAPSUL which is a branch modified starch can continuously be hydrolyzed to form a shell after nucleates forming. In other words,

shell growth formation plays an important role than nucleate formation. On the other hand, if native starches (unmodified starches) were used as starting material, the CMSs became larger and some of CMSs became aggregates with diameters ranging from 1.0 to 7.0 m (Figure 1f-h). In this work, it appears that HI-CAP100 could provide monodisperse CMSs when compared with other carbon sources. However, some of unmodified starches (corn and tapioca starch) can also result in the monodisperse CMSs (Figure 1d, e).

Based on our experimental results, CMS particle size and its distribution depended on types of starch (Figure 2). These particle size

Figure 1. SEM images of CMSs after carbonization: (a) HI-CAP100; (b) HI-CAP100 at 10,000 magnification; (c) CAPSUL; (d) corn; (e) tapioca; (f) sticky rice; (g) wheat; and (h) rice starch.



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2 (degree)

(002

)

(101

)

Inte

nsity

(a.u

.)

HI-CAP 100 Tapioca CAPSUL Sticky rice

distributions directly related to SEM observ-ations. The particle size distribution of CMSs from HICAP100 has the narrowest because HICAP100 has its high water-solubility. Contradictory, particle size distributions of carbon microspheres from native starch tended to broaden distributions that confirm-ed from SEM observations.

The energy-dispersive X-ray (EDX) analyses of carbon microspheres from various types of starch after carbonization process are shown in Table 1. These results showed that carbon is the main component of the CMSs in the range of 66-71wt%. However, the oxygen component in carbon microspheres might mainly come from the absorbed water molecules in pore structure [18].

In addition, the XRD patterns of some CMSs after carbonization are shown in Figure 3. There are the presences of two broad peaks at 2 = 24.8 and 43.5 which are reflections from the (002) plane and the (101) plane, respectively. The peaks can be indexed to a hexagonal graphite lattice. The broadening of the peaks suggests the presence of an amorphous carbon phase within the CMSs [19]. These semi-hexagonal graphite struct-ures took place during carbonization process at high temperature. In carbonization process, carbon atom in CMSs would be rearranged to form graphene sheets and partially collapsed to yield pore structures. This structure would be suitable for any substances storage which have their molecule sizes less than micropore (< 2 nm).

All N2 adsorption-desorption isotherms of CMSs after carbonization process were shown in Figure 4. These isotherms indicated that the carbonized CMSs exhibited type I adsorption isotherm due to its micropore structure regarding to IUPAC classification [20]. The BrunauerEmmettTeller (BET) surface areas of CMSs before and after carbonization were also summarized in Table 2. The CMSs surface areas were dramatically increased after carbonization process. The release of H, O and C during carbonization process increased large quantities of micro-pores throughout the bulk of the samples [7].

Figure 2. Particle size distribution of CMSs after carbonization which are ()HICAP-100, ()CAPSUL, ()Tapioca, ()Corn, ()Rice, ()Wheat, ()Sticky rice.

Table 1. The elemental components of CMSs from energy dispersive X-ray.

Elemental components Types of starches Carbon (%) Oxygen (%)

HICAP100 71.08 28.92 CAPSUL 68.15 31.85 Tapioca 69.11 30.89 Corn 66.67 33.33 Rice 68.44 31.56 Wheat 67.72 32.28 Sticky rice 69.01 30.99

Figure 3. X-ray diffraction patterns of CMSs after carbonization at 600 oC.



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4. Conclusions

High micropores and monodisperse CMSs were synthesized via a facile hydrothermal process without any catalysts. The low-cost starting materials and moderate reaction temperature provide an efficient way to fabricate solid CMSs. Furthermore, the particle size distribution of the CMSs strongly depended on types of starch. In other words, the smallest particle size of CMSs could be synthesized from HICAP100 because it can be hydrolyzed immediately to form nucleates of CMSs. On the other hand, broaden particle size distributions of CMSs from some types of native starch because they were continuously hydrolyzed during hydrothermal process to grow shell of CMSs. This mechanism came from their water-insolubility of native starch. Furthermore, high porosity of CMSs could be developed by treating the dried CMSs in carbonization process. The BET surface area of CMSs after carbonization process dramatically increased from 1-5 m2/g to 400-500 m2/g. In addition, the CMSs also increased their carbon contents by losing oxygen and hydrogen atoms during carbonization process. There-fore, the rearrangement of carbon atom in CMSs to form grapheme sheets took place.

Acknowledgements

This work was financially supported by the Centennial Fund of Chulalongkorn University for Center of Excellence in Particle Technology (CEPT) and Department of Chemical Engineering, Faculty of Engi-neering, Chulalongkorn University.

References

[1] Kroto, H.W., Heath, J.R., OBrien, S.C., Curl, R.F., & Smalley, R.E. (1985). C60: Buck-minsterfullerene. Nature, 318(6042), 162-3.

[2] Iijima, S. (1991). Helical microtubules of graphitic carbon. Nature, 354(6348), 56-8.

[3] Sun, X., & Li., Y. (2004). Colloidal carbon spheres and their core/shell structures with noble-metal nanoparticles. Angewandte Chemie-International Edition, 43(5), 597-601.

Figure 4. N2 adsorption-desorption isotherm of CMSs after carbonization which are ()HI-CAP100, ()CAPSUL, ()Tapioca, ()Corn, ()Rice, ()Wheat, ()Sticky rice.

Table 2. Specific BET surface area of CMSs.

Specific BET surface area, SBET [m2/g] Types of

starches Before carbonization

After carbonization

HI-CAP100

4.32 560

CAPSUL 3.41 530 Tapioca 3.23 546 Corn 3.57 520 Rice 2.89 457 Wheat 1.23 444 Sticky rice 3.12 415



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[4] Jang, J., & Bae, J. (2004). Fabrication of polymer nanofibers and carbon nanofibers by using a salt-assisted microemulsion polymer-ization. Angewandte Chemie-International Edition, 43(29), 3803-6.

[5] Yuan, D., Xu, C., Liu, Y., Tan, S., Wang, X., Wei, Z., & Shen, P.K. (2007). Synthesis of coin-like hollow carbon and performance as Pd catalyst support for methanol electro-oxidation. Electrochemistry Communicat-ions, 9(10), 2473-8.

[6] Xiao, Y., Liu, Y. , Cheng, L., Yuan, D., Zhang, J., Gu, Y., & Sun, G. (2006). Flower-like carbon materials prepared via a simple solvothermal route. Carbon, 44(8), 1589-91.

[7] Serp, P., Feurer, R., Kalck, P., Kihn, Y., Faria, J.L., & Figueiredo, J.L. (2001). Chemical vapour deposition process for the production of carbon nanospheres. Carbon, 39(4), 621-6.

[8] Auer, E., Freund, A. (1998). Carbons as supports for industrial precious metal catalysts. Applied Catalysis A: General, 173(2): 259-71.

[9] Vignal, V., Morawski, A.W., Konno, H., & Inagaki, M. (1999). Quantitative assessment of pores in oxidized carbon spheres using scanning tunneling microscopy. Journal of Materials Research, 14(3), 1102-12.

[10] Lou, Z., Chen, Q., Gao, J., & Zhang, Y. (2004). Preparation of carbon spheres consisting of amorphous carbon cores and graphene shells. Carbon, 42(1), 229-32.

[11] Inagaki, M., Vignal, V. (1999). Effects of carbonization atmosphere and subsequent oxidation on pore structure of carbon spheres observed by scanning tunneling microscopy. Journal of Materials Research, 14(7), 3152-7.

[12] Yang, R., Li, H., Qui, X., & Chen, L. (2006). A spontaneous combustion reaction for synthesizing Pt hollow capsules using colloidal carbon spheres as templates. Chemistry-A European Journal, 12(15), 4083-90.

[13] Mi, Y., Hu, W., Dan, Y., & Liu, Y. (2008). Synthesis of carbon micro-spheres by a glucose hydrothermal method. Materials Letters, 62(8-9), 1194-6.

[14] Yuan, D., Xu, C., Liu, Y., Tan, S., Wang, X., Wei, Z., & Shen, P.K. (2007). Synthesis of coin-like hollow carbon and performance as Pd catalyst support for methanol electro-oxidation. Electrochemistry Communicat-ions, 9(10), 2473-8.

[15] Tusi, M.M., Brandalise, M., Correa, O.V., Neto, A.O., Linardi, M., & Spinace, E.V. (2009). Preparation of PtRu/C electrocatal-ysts by hydrothermal carbonization process for m ethanol electro-oxidation. Portugaliae Electrochimica Acta, 27(3), 345-52.

[16] Sun, X., Li, Y. (2004). Colloidal carbon spheres and their core/shell structures with noble-metal. Nanoparticles. Angewandte Chemie-International Edition, 43(5), 597-601.

[17] Yang, R., & Qiu, X. (2005). Monodispersed hard carbon spherules as a catalyst support for the electrooxidation of methanol. Carbon, 43(1), 11-6.

[18] Mi, Y., Hu, W., Dan, Y., & Liu, Y. (2008). Synthesis of carbon microspheres by a glucose hydrothermal method. Materials Letters, 62(8-9), 1194-6.

[19] Zheng, M., Liu, Y., Xiao, Y., Zhu, Y., Guan, Q., Yuan, D., & Zhang, J. (2009). An easy catalyst-free hydrothermal method to prepare monodisperse carbon microspheres on a large scale. Journal of Physical Chemistry C, 113(19), 8455-9.

[20] Sing, K.S.W., & Gregg, S.J. (1982). Adsorption, surface area and porosity. (2), New York: Academic Press.

[21] Wang, Q., Li, H., Chen, L., & Huang, X. (2001). Monodispersed hard carbon sphe-rules with uniform nanopores. Carbon, 39(14), 2211-4.

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