J. Phys. Chem. C 2009, 113, 10085–10089
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Controllable Route to Solid and Hollow Monodisperse Carbon Nanospheres Yan Li, Jiafu Chen, Qun Xu,* Linghao He, and Zhimin Chen College of Materials Science and Engineering, Zhengzhou UniVersity, Zhengzhou 450052, P.R. China ReceiVed: February 10, 2009; ReVised Manuscript ReceiVed: May 4, 2009
An effective mechanism to prepare monodisperse carbon nanospheres with a regular round-ball-like shape is introduced. The route involves three steps: first, to synthesize monodisperse cross-linked polystyrene beads (CLPBs) by soap-free emulsion polymerization; second, to increase the cross-linking degree of CLPBs via Friedel-Crafts alkylation as a post-cross-linking reaction; third, to carbonize the reaction product. Both solid and hollow carbon spheres can be produced by adjusting the post-cross-linking reaction time. Fourier transform infrared spectra demonstrated the occurrence of a Friedel-Crafts alkylation reaction, and X-ray diffraction results showed that the carbon nanospheres are amorphous in structure. Typically, nitrogen adsorption/desorption measurements indicate that solid and hollow carbon nanospheres have a Brunauer-Emmett-Teller surface area as high as 498 and 451 m2/g, respectively. So it indicates that this method is novel in the field of preparation of carbon microspheres. 1. Introduction Carbonaceous materials, including nanotubes,1-3 onions,4,5 nanorods,3 nanocoils,6 fullerenes,7 and microspheres,8-10 have attracted considerable attention because of their excellent properties and potential applications in more and more fields. Among those materials, carbon microspheres are expected to play a significant role in many areas of modern science and technology, and they may serve as reinforcement substances for rubber,11 adsorbents,12 supports of catalytic systems,13 media for energy storage,14 and so on. Therefore, effective preparation methods of carbon microspheres mean a great deal to both academic research and industrial application. Generally, carbon spheres have two kinds of morphologies, solid and hollow carbon spheres. These two types of curved graphitic network structures have their own unique fabrication methods. For solid carbon spheres, the present methods for its synthesis involve chemical vapor deposition,15-18 hydrothermal reaction,19 arc plasma technique,20,21 pyrolysis,22 and metathesis reaction.23 For example, Pang and his group readily fabricated a large scale of carbon microspheres by metathesis reaction at low temperature, 300 °C, and the starting materials comprised CaC2 powder, C2Cl4, and CCl4 in the absence of any catalyst.23 But the obtained products are apparent in polydispersity. For hollow carbon spheres, preparation methods including template,24-26 core-shell polymer particles,27 reduction,28 laser pyrolysis,29 nanocasting process with dual template,30-32 and shock compression33 have been developed. Using silica spheres with a hard core/mesoporous shell as the template, Yoon et al.25 fabricated monodisperse hollow carbon spheres with a diameter of ca. 330 nm and its specific surface area reached as high as 1230 m2/g partially for the quantities of micropores and mesopores in the shell. For most of the current methods, only one type of carbon sphere, i.e., either solid or hollow, could be fabricated exclusively. At present, some researchers are trying to prepare solid or hollow carbon spheres by one method, but the final product is a mixture of solid and hollow carbon spheres that cannot be * To whom correspondence should be addressed. E-mail: qunxu@ zzu.edu.cn. Tel.: +86 371 67767827. Fax: +86 371 67767827.
separated easily.34 Herein, we introduce a controllable route to the preparation of solid and hollow carbon spheres. In this work, CLPBs are synthesized via polymerization with divinylbenzene (DVB) as a cross-linking agent, and Friedel-Crafts alkylation with AlCl3 as the catalyst is employed to accomplish the postcross-linking of cross-linked polystyrene beads (CLPBs), and a series of reaction conditions have been tried to investigate their influence on the morphology of the subsequent carbon product obtained after the carbonization process. The carbonization process is performed at 700 °C with nitrogen gas as a protective atmosphere. Both solid and hollow carbon nanospheres can be achieved controllably by adjusting the reaction time of the post-cross-linking reaction in the range of 2-24 h at 60 °C, and the mechanism is proposed in this study. 2. Experimental Section 2.1. Materials. Analytical-grade styrene, which was supplied by Kemiou Chemical Reagents Researching Center in Tianjin City, was purified by distillation under reduced pressure in a nitrogen atmosphere; DVB, containing 55% of the para- and meta-divinyl isomers and 45% of para- and meta-ethylvinyl isomers, was purchased from Aldrich Chemical Co.; potassium persulfate (KPS) was purchased from Shanghai Aijian Reactant Factory, and it was used after being recrystallized; water was distilled and deionized by a Milli-Q water purification systems; tetrachloromethane (CCl4), analytical grade, was obtained from Luoyang Chemical Reagent Factory in Luoyang City; acetone, analytical grade, was provided by Tianjin Chemical Reagent Factory; anhydrous aluminum trichloride (AlCl3), analytical grade, was supplied by Tianjin Damao Chemical Reagent Factory; dilute hydrochloric acid (10 wt %) was prepared from concentrated hydrochloric acid (36-38 wt %), which was offered by Luoyang Haohua Chemical Reagent Comany, Ltd., in Luoyang City. 2.2. Preparation of CLPBs. Soap-free emulsion polymerization was carried out via a batch process in a 250 mL, fournecked, round-bottom flask, which was put in a thermostated water bath. Styrene (2.1 g) and 90 g of water were added to the flask, and then nitrogen gas was passed through the above mixture under vigorous stirring (200 rpm). Thirty minutes later,
10.1021/jp901237r CCC: $40.75 2009 American Chemical Society Published on Web 05/14/2009
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the temperature started to rise. When the temperature reached 70 °C, 10 mL of aqueous solution containing 0.12 g of KPS was added to the flask, and this moment was regarded as the start time of the whole reaction. Then the reaction was carried out at this constant temperature of 70 °C. When the reaction had proceeded for 3 h, 0.9 g of DVB was injected into the flask stepwise by a syringe. The polymerization was ceased after the process for 24 h. The CLPBs were separated from the aqueous solution by centrifugation and added to the flask with CCl4 for the post-cross-linking reaction. 2.3. Post-cross-linking of CLPBs. The post-cross-linking reaction was conducted in a 125 mL, three-necked, roundbottom flask equipped with a polytetrafluoroethylene-bladed paddle stirrer and a water-cooled reflux condenser. In a typical reaction, 30 mL of CCl4 and 1.80 g of AlCl3 were put in the flask under vigorous stirring (160 rpm) at room temperature. Twenty minutes later, 0.30 g of CLPBs was added into the above mixture. Another 20 min later, the flask was put in a water bath of 40 °C, and this moment was regarded as the start time of the reaction. After the reaction was performed for 16 h, the product was collected and purified by centrifugating and washing with 15 mL of acetone three times, and then it was washed with 10 mL of dilute hydrochloric acid three times. The obtained solid product was collected and dried at 45 °C overnight. 2.4. Carbonization of Post-cross-linked CLPBs. In a tube furnace, post-cross-linked CLPBs were carbonized at high temperatures. After the sample was put in the quartz tube, the temperature program was executed: the temperature was elevated homogeneously to 700 °C and then was held at 700 °C for 2 h under a nitrogen flow. The product was collected after it was cooled to room temperature. 2.5. Characterization. A FEI Quanta 200 scanning electron microscope (SEM) was used to observe the morphologies of CLPBs and carbon spheres, and carbon spheres were also investigated by a FEI Tecnai G2 20 transmission electron microscope (TEM), which worked with an accelerating voltage of 120 kV. The crystallinity of some products was investigated by wide-angle X-ray powder diffraction (XRD) (Rigaku D/MAX3B) using graphite monochromatized Cu KR radiation. Fourier transform infrared (FT-IR) spectra were measured by using a Bruker Optics TENSOR 27 FT-IR spectrophotometer in the wavenumber range from 4000 to 400 cm-1. Nitrogen adsorption/ desorption of isotherms was conducted on a Quantachrome (NOVA 1000) sorption analyzer at 77 K, and the specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method. Raman spectra were recorded on a Horiba Jobin Yvon LabRAM HR800 UV confocal laser micro-Raman spectrometer equipped with an air-cooled Ar+ laser working at 514 nm. 3. Results and Discussion 3.1. Preparation of CLPBs and Carbon Nanospheres. As synthesis of monodisperse polymer beads has been realized successfully nowadays, these polymer beads can be treated as an ideal carbon source for the preparation of carbon spheres. Here, it is noted that the polymer beads must possess a large enough molecular weight to not decompose under high temperature during the process of carbonization. Large molecular weight of the polymers may be realized by post-cross-linking reaction. Herein, CLPBs were synthesized by soap-free emulsion polymerization. To obtain monodisperse CLPBs with a high degree of cross-linking while avoiding secondary nucleation and flocculation, cross-linking agent (DVB) was added using a delayed addition method.35 The monodisperse CLPBs obtained are shown in Figure 1. Their diameters are around 400 nm. CLPBs serve as the precursors of carbon nanospheres, so their
Li et al.
Figure 1. SEM image of the obtained CLPBs.
SCHEME 1: Schematic Illustration of the Post-cross-linking Reaction
ideal shape provides a possibility for preparing carbon nanospheres with ideal morphology. According to our original idea, carbon spheres may be obtained by carbonizing the CLPBs; however, it failed because CLPBs were found to begin decomposing at 306 °C and completely decompose at 345 °C. It is believed that the main reason for that was that the average molecular weight of the CLPBs was not large enough, and CLPBs decomposed before they were carbonized. So we must attempt to increase the molecular weight of CLPBs, and here we choose FriedelCrafts alkylation as the post-cross-linking reaction to accomplish it. By this reaction, some aromatic rings from different polymer chains are linked together covalently, which can increase the molecular weight of the polymer greatly. The schematic illustration of the reaction is shown in Scheme 1. At 40 °C, the post-cross-linking reaction was performed for 16 h, and the product obtained is shown in Figure 2. The postcross-linked CLPBs present a monodisperse and round-balllike shape with an average diameter of ca. 413 nm. The occurrence of mild coalescence between spheres may be due to the following: in the Friedel-Crafts alkylation process, sometimes the distance of two CLPBs is quite close because of the high concentration of CLPBs, and it is possible that some aromatic rings from different polymer beads are linked together covalently. To demonstrate the occurrence of Friedel-Crafts alkylation, an FT-IR experiment was performed. FT-IR spectra of CLPBs and post-cross-linked CLPBs obtained under the post-
Solid and Hollow Monodisperse Carbon Nanospheres
Figure 2. SEM image of post-cross-linked CLPBs obtained under the conditions of 40 °C/16 h.
Figure 3. FT-IR spectra of (a) CLPBs and (b) post-cross-linked CLPBs obtained under the conditions of 40 °C/16 h.
Figure 4. SEM and TEM images of carbon nanospheres corresponding to different reaction temperatures of post-cross-linking for 16 h: (A) 40 °C, (B) 50 °C, and (C) 60 °C.
cross-linking conditions of 40 °C/16 h are shown in Figure 3. For the latter (curve b), the peaks at 1282 and 1182 cm-1 may be ascribed to C-Cl bending vibration; the peak at 1492 cm-1 should be ascribed to C-H stretching of the aromatic ring,36 and its relative intensity appears to be low when compared with that of CLPBs (curve a) because many H atoms of the aromatic ring in CLPBs were lost after the Friedel-Crafts alkylation reaction, and it just demonstrates the occurrence of FriedelCrafts alkylation; an obvious peak at 1663 cm-1 in curve b may be ascribed to stretching of the CdO bond coming from residual acetone used as washing agent. After the carbonization process, carbon nanospheres with ideal morphology were obtained successfully, which are shown in parts A and A′ of Figure 4. Compared with 413 nm of the cross-linked CLPB spheres, the average diameter of a carbon sphere was ca. 260 nm, suggesting that an obvious volume shrinkage occurred during the carbonization process. 3.2. Effect of Different Experimental Conditions on the Morphology of the Carbon Nanospheres. Considering that the temperature of the post-cross-linking reaction may affect
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Figure 5. XRD pattern of (a) CLPBs and carbon nanospheres corresponding to different reaction temperatures of the post-crosslinking reactions for 16 h: (b) 40 °C, (c) 50 °C, and (d) 60 °C.
Figure 6. Raman spectra of carbon nanospheres corresponding to different reaction temperatures of post-cross-linking reactions for 16 h: (a) 40 °C, (b) 50 °C, and (c) 60 °C.
the morphology of the subsequent carbon product, the temperature was changed from 40 to 50 °C and to 60 °C. Figure 4 presents those corresponding carbon products. From these SEM images, we can see that the carbon products almost present a monodisperse and regular round-ball-like shape, and they almost share the same dimension. From these TEM images, it can be judged that the carbon spheres are all solid ones. We can also find severe coalescence between spheres corresponding to 40 °C (Figure 4A,A′) and 50 °C (Figure 4B,B′), and the coalescence apparently decreased when the reaction temperature increased to 60 °C (Figure 4C,C′). So the temperature of the post-cross-linking reaction at 60 °C may be the optimum one. The XRD pattern of CLPBs and carbon nanospheres corresponding to post-cross-linking reactions with different temperatures are shown in Figure 5. For CLPBs (curve a), an obvious peak at 18.6° indicates there is a certain amount of crystal components. Curves b, c, and d are the patterns of carbon nanospheres corresponding to the post-cross-linking temperatures of 40, 50, and 60 °C, respectively. The peak at around 23.5° in the curves appears to be wide, and it indicates that the most of the carbon nanospheres are in the amorphous phase. The Raman spectra for the carbon nanospheres corresponding to the post-cross-linking reaction conditions of 40 °C/16 h, 50 °C/16 h, and 60 °C/16 h are shown in Figure 6. The D-band around 1350 cm-1 was associated with the vibrations of carbon atoms with dangling bonds for the in-plane terminations of disordered graphite. The G-band around 1580 cm-1 corresponded to an E2g mode of graphite and was related to the vibration of sp2-bonded carbon atoms in a two-dimensional hexagonal lattice, such as in a graphitic layer.37-39 The low ratio of ID/IG was characteristic of a graphite lattice with perfect twodimensional order in the graphitic layer.40 The intensity ratio of ID/IG corresponding to the post-cross-linking reaction temperature of 40, 50, and 60 °C was respectively calculated to be 0.87, 0.86, and 0.88, and they are not profoundly affected by the post-cross-linking reaction temperatures.
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Li et al. successively. This, in effect, supplies a new platform to prepare carbon nanospheres in a controlled manner. Further nitrogen sorption-desorption experimental results indicate that the BET surface area of the obtained carbon nanospheres reached 498 and 451 m2/g. This allows for the potential of hydrogen storage; this needs further study as does the characterization of the obtained carbon spheres for their total pore volumes, micropore volumes, pore size distribution, etc. Acknowledgment. We are grateful for the financial support from the National Natural Science Foundation of China (No. 20804040), the Prominent Research Talents in the University of Henan Province, the Prominent Youth Science Foundation of Henan Province (No. 0512001200), and the open Project of State Key Laboratory of Supramolecular Structure and Materials (SKLSSM200704). References and Notes
Figure 7. TEM images of carbon nanospheres corresponding to different reaction times of post-cross-linking reactions at 60 °C: (A) 2 h, (B) 8 h, (C) 16 h, and (D) 24 h.
Next, we consider whether reaction time of post-cross-linking has an effect on the morphology of the carbon products, so a series of formulas employing different reaction times at 60 °C were carried out. The corresponding carbon products, which were obtained after carbonization, are shown in Figure 7. Although they share similar size, the body structures of them are not identical. The carbon nanospheres corresponding to the reaction time of 2 h (Figure 7A) present a hollow core structure; when the reaction time is increased to 8 h (Figure 7B), both hollow and solid carbon spheres appear; when it is increased to 16 h (Figure 7C) or 24 h (Figure 7D), only solid spheres can be found. In the Friedel-Crafts alkylation, CCl4 reacts with AlCl3 and produces intermediate ions, which may react with the aromatic rings in the polymer chains. At the beginning, the reaction almost occurs on the surface region of CLPBs. Meanwhile, CLPBs are swollen by CCl4; i.e., CCl4 molecules are able to penetrate CLPBs, which may extend the reaction locus to the interior of CLPBs. If the reaction time is short, reaction mainly occurs on the surface region of polymer beads. Therefore, in the following carbonization process, the outer layers are carbonized while the core decomposes under high temperature, and hollow carbon spheres are formed. If the reaction time is long, CLPBs can be greatly swollen by CCl4, and Friedel-Crafts alkylation reaction may occur thoroughly in all of the CLPBs, so the polymer beads can turn into solid carbon spheres after carbonization. So with adjustment of the different periods of time for Friedel-Crafts alkylation, the carbon spheres with the various morphologies can be fabricated controllably. Typically, nitrogen adsorption/desorption measurements of the carbon nanospheres corresponding to 60 °C/16 h and 60 °C/2 h were carried out, and the BET surface area calculated by the analyzer reached 498 and 451 m2/g, respectively, which was prominent in the family of carbon microspheres.18,19,23,24 Characterization of the obtained carbon spheres such as total pore volume, micropore volume, and pore size distribution needs examination in our future research. 4. Conclusions In summary, monodisperse carbon nanospheres with a regular round-ball-like shape were fabricated via an effective route. For the first time, hollow and solid nanospheres can be fabricated
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