An Easy Catalyst-Free Hydrothermal Method to Prepare

Apr 21, 2009 - E-mail: [email protected]. ... The size of the carbon microspheres can be easily controlled by regulating the concentration ... and Their ...
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J. Phys. Chem. C 2009, 113, 8455–8459

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An Easy Catalyst-Free Hydrothermal Method to Prepare Monodisperse Carbon Microspheres on a Large Scale Mingtao Zheng, Yingliang Liu,* Yong Xiao, Yong Zhu, Qiu Guan, Dingsheng Yuan, and Jingxian Zhang Department of Chemistry and Institute of Nanochemistry, Jinan UniVersity, Guangzhou 510632, People’s Republic of China ReceiVed: December 23, 2008; ReVised Manuscript ReceiVed: March 12, 2009

We report an easy catalyst-free method to prepare carbon microspheres via a hydrothermal carbonization process using starch solution as starting materials. SEM and TEM images show that the products consist of a large scale of monodisperse carbon microspheres with a size of about 2 µm. The size of the carbon microspheres can be easily controlled by regulating the concentration of the starch solution and the reaction temperature. Furthermore, the surface of the spheres is functionalized with hydroxyl and carboxyl groups, which make further surface modification unnecessary and facilitate immobilization of noble metal nanoparticles and fabrication of core-shell materials or hollow structures. Through SEM observation, the mechanism of the formation of carbon microspheres under hydrothermal conditions was suggested to be a direct dehydration polymerization and self-assemble fusion process. The present work provides a convenient yet effective method for large-scale synthesis of monodisperse carbon microspheres with high purity. 1. Introduction 1

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Since the discovery of fullerenes and carbon nanotubes, significant efforts have been made toward the synthesis of functional carbon materials with diverse morphologies and structures, such as coin-like hollow carbons,3 macroflowers,4 colloidal spheres,5 nanofibers,6 and so on. Among the different forms of carbon materials, carbon microspheres (CMSs) have attracted considerable attention, owing to their potential importance in catalyst supports,7-9 adsorbents,10,11 anode material for lithium ion batteries,12-14 and templates for fabricating core-shell or hollow structures.15-17 Carbon microspheres have been prepared by various methods, such as chemical vapor deposition,18 pressure carbonization,19 mixed-valence oxide-catalytic carbonization,20 and reduction of carbides with metal catalysis.21 Saccharide materials, such as glucose and sucrose, were excellent carbon precursors for preparing CMSs. For instance, Sun et al. prepared colloidal carbon spheres from glucose via a hydrothermal method;5 Wang et al. reported a two-step route to synthesize hard carbon spheres by a hydrothermal method using sugar as precursor.22 Recently, Mi and co-workers developed a hydrothermal method to synthesize CMSs with the aqueous glucose solution as starting materials under moderate conditions.23 However, as a cheaper and more available polysaccharide, starch was rarely used as a carbon precursor to prepare CMSs because it is hard to dissolve in water to form a clear solution. Cui et al. have reported a facile strategy for the synthesis of carbon particles with different nano- and microstructures by using a hydrothermal reaction and starch or rice grains as carbon sources catalyzed by free iron ions or iron oxide nanoparticles,24 yet carbon microspheres of high purity were extremely difficult to acquire by traditional hydrothermal process. Although carbon spheres were obtained from corn starch by a two-stage process,25 uniform spheres were difficult to obtain even though graphitized at very high temperature (2600 * To whom correspondence should be addressed. Fax: +86 20 8522 1697. E-mail: [email protected].

°C), and the particle size, which ranged from 5 to 25 µm, could not be controlled. Herein, we develop an easy and cost-effective method to prepare monodisperse CMSs on a large scale via a hydrothermal carbonization process under moderate conditions (500-600 °C). In this work, commercial soluble starch was chosen as the carbon precursor. Remarkably, it has been found that the size of CMSs could be easily controlled by regulating the concentration of the starch solution and the reaction temperature. Furthermore, the main process in this method was carried out in aqueous solution without involving any organic solvents or catalysts. This catalyst-free synthesis strategy will promote a better understanding of CMSs growth, and moreover, the assynthesized CMSs may lead to many new potential applications. 2. Experimental Section 2.1. Synthesis of the Carbon Microspheres. Commercial soluble starch was purchased from the Guangzhou Chemical Reagent Factory and used as the starting material without further purification. In a typical procedure, starch (4 g) was dissolved in 40 mL of distilled water that was magnetically stirred for 2 h, and then the mixture was heated at 60 °C for 30 min to form a clear solution. The solution was then sealed in a 50 mL stainless-steel autoclave. Subsequently, the autoclave was put into an electronic furnace, which was then heated to 600 °C with a rapid heating rate of 10 deg min-1 and maintained at 600 °C for 12 h. After the autoclave was cooled to room temperature naturally, the dark precipitates were collected and washed with distilled water and ethanol four times and dried in a vacuum at 60 °C for 6 h. Soft, light, cotton-like products were obtained. The yield of the products is about 95% according to carbon weight percent and about 40% according to starch. 2.2. Characterization. The samples were characterized by X-ray powder diffraction (XRD) performed on a MSAL-XD2 X-ray diffractometer with Cu KR radiation (40 kV, 20 mA, λ ) 1.54051 Å). The morphology observation of the samples was examined with scanning electron microscopy (SEM, Philips

10.1021/jp811356a CCC: $40.75  2009 American Chemical Society Published on Web 04/21/2009

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Figure 1. SEM images of the as-obtained products showing the spherical morphology: (a) low-magnification image; (b) magnified image; (c) TEM image; and (d) XRD pattern of as-prepared products. Conditions: starch (4 g), 600 °C for 12 h.

XL-30) and transmission electron microscopy (TEM, Philips Tecnai-10). The Raman spectrum of as-prepared samples was recorded at room temperature on a Renishaw RM2000 Raman microspectrometer with the 514.5 nm line of an argon laser. The Fourier transform infrared spectroscopy (FTIR) spectrum was measured by an EQUINOX 55 (Bruker) spectrometer with the KBr pellet technique ranging from 500 to 4000 cm-1. 3. Results and Discussion 3.1. Morphology and Characterization of the Products. Figure 1a,b shows the general morphology of the CMSs prepared by hydrothermal carbonization of 4 g of starch at 600 °C for 12 h. The panoramic SEM image in Figure 1a reveals that the sample consists of a large amount of uniform microspheres with a diameter of about 2 µm, and the morphological yield of the microspheres is approximately 100%. Most particles are single spheres and agglomerations are invisible. Figure 1b (an enlarged section of Figure 1a) shows the perfect spherical morphology of the CMSs with smooth surface. The spheres are nearly monodisperse and the diameter of the spheres is about 1.8-2 µm. Figure 1c presents the TEM image of the as-obtained CMSs. It can be observed that the CMSs are solid and monodisperse. The diameter of the CMSs falls in the range estimated by the SEM images. It can also be found that along with the CMSs, there are a very few solid nanospheres with a size of about 200-350 nm. A typical XRD pattern of the as-synthesized CMSs is shown in Figure 1d. There is a broad peak at 25.7° that corresponds to the (002) plane of graphite. In addition, a small shoulder peak at 43.1°, which corresponds to the (101) plane of graphite, can be observed. The broadening of the two peaks suggests the possible presence of an amorphous carbon phase within the carbon spheres. No other impurity is observed in the XRD pattern. Figure 2 shows the micro-Raman spectrum of the CMSs synthesized at 600 °C for 12 h. In the Raman spectrum, two strong peaks at 1333 and 1592 cm-1 can be seen. The peak at 1592 cm-1 (G band) is attributed to an E2g mode of graphite and is related to the vibration of sp2-bonded carbon atoms in a 2D hexagonal lattice, such as in a graphite layer. The peak at 1333 cm-1 (D band) corresponds to vibrations of carbon atoms with dangling bonds in plane terminations of disordered

Figure 2. Raman spectrum of the as-prepared products. Conditions: starch (4 g), 600 °C for 12 h.

Figure 3. FTIR spectra of as-obtained solid CMSs. Conditions: starch (4 g), 600 °C for 12 h.

graphite.26,27 The D and G bands show an intensity ratio of ID/IG ) 0.89 for the CMSs. This intensity ratio indicates an amorphous carbon structure, with a high content of lattice edges or plane defects within the analyzed CMSs, which is consistent with the XRD results. Moreover, the D band has a much wider width in the present observation. The spectrum is more characteristic of disordered graphite owning to the lower carbonization temperature. The CMSs derived from soluble starch under hydrothermal conditions have abundant functional groups remaining on the surface of the spheres. The FTIR spectrum in Figure 3 was used to characterize these microspheres. It reveals the strong characteristic peak at 3436 cm-1 is attributed to the O-H bending vibration, which implies the existence of residual hydroxyl groups. The absorption peak at 1118 cm-1 corresponds to the C-OH stretching and OH bending vibrations, and the peaks at

Catalyst-Free Preparation of Carbon Microspheres

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Figure 4. SEM images of the samples synthesized from the starch solution with different concentrations: (a) 0.5, (b) 6, (c) 8, and (d) 12 g of starch dissolved in 40 mL of distilled water and reacted at 600 °C for 12 h.

Figure 5. SEM images of carbon spheres synthesized from (a, b) 4 g of starch at 500 °C for 12 h, (c) 4 g of starch at 450 °C for 6 h and (d) 4 g of starch at 700 °C for 12 h, and (e, f) 4 g of starch at 600 °C for 12 h with the heating rate of 2 (e) and 20 °C/min (f).

2921 and 2852 cm-1 originate from the C-H stretching vibration. The infrared spectrum peaks at 1580 and 1636 cm-1 due to the stretching vibration of carboxyl groups. These results indicate that there are a large number of residues including hydroxyl and carboxyl groups on the surface of the as-prepared CMSs due to an incomplete carbonization process,28 and they play important roles in the formation process of the spherical structures. Furthermore, the functional groups provide a potential avenue to load other functional groups, molecules, ions, and nanoparticles or fabricate other core-shell or hollow structures. 3.2. Effect of the Reaction Conditions on Morphology and Size of the Carbon Microspheres. In this study, the hydrothermal carbonization process of starch solution in various concentrations for a proper reaction time and temperature yields very interesting results. The concentration of the starch solution plays a crucial role in controlling the diameter and size distribution of the products. Figure 4a-d shows the SEM images

SCHEME 1: Schematic Representation of the Possible Formation Process of Carbon Microspheres

of the products obtained from 0.5, 6, 8, and 12 g of soluble starch dissolved in 40 mL of distilled water and reacted at 600 °C for 12 h in the autoclave, respectively. It can be observed that the average size of the spheres is about 1, 3, 5, and 7 µm, respectively. These results indicate that the size of the CMSs tends to increase with the increase of the concentration of the starch solution. In addition, it can be seen that the dispersibility of the CMSs reduces with the increase of the concentration of

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Figure 6. (a) SEM image of the sample obtained at 400 °C for 3 h; (b) a close-up of the granular surface for the sample in Figure 5c was observed by high-magnified SEM.

starch solution. When the solution turns dense enough, the carbon spheres with a diameter of about 5-8 µm tend to aggregate and then to form larger particles (Figure 4d). On the other hand, the size of the CMSs varied greatly as the hydrothermal temperature changes while keeping other conditions constant. Figure 5a-d shows the SEM images of the products obtained at different hydrothermal temperatures. As shown in Figure 5a,b, the average size of the CMSs ranges from 1.2 to 1.5 µm when the reaction temperature is kept at 500 °C, indicating that the size of the spheres decreased with the decrease of the reaction temperature. It can be observed that there are some incompletely developed microspheres (indicated with a white arrow in Figure 5b) probably due to lower hydrothermal temperature. It is worth noting that the size of the CMSs increases abnormally and presents a wide distributing character ranging from 4 to 10 µm when the temperature is about 450 °C (Figure 5c). As shown in Figure 5d, the CMSs with a diameter of 6-8 µm aggregate with each other when the temperature is up to 700 °C, which is possibly due to the reduction of the functional groups on the surface of CMSs. Moreover, the heating rate plays a key role on the size distribution of the spheres. The rapid heating rate (>10 deg/ min) is essential to obtain monodisperse CMSs. At a lower heating rate, larger carbon spheres are acquired and the size of the spheres presents a wide size distribution character. For example, the diameter of the obtained CMSs ranges from 2.5 to 9 µm when the heating rate is about 2 deg/min (Figure 5e). On the contrary, the monodisperse microspheres with a diameter of about 1.5-1.8 µm can be obtained when the heating rate is about 20 deg/min (Figure 5f). These results indicate that the rapid heating rate results in small microspheres and a narrow size distribution. It was also found that varying the reaction time between 6 and 12 h did not significantly affect the morphology of the CMSs. When the time is shorter than 2 h, the reaction becomes very incomplete and the carbon yield is very low due to the short reaction time. Consequently, the appropriate heating rate to obtain monodisperse CMSs in a suitable yield ranges from 10 to 20 deg/min and the reaction time is about 12 h. 3.3. Formation Mechanism of the Carbon Microspheres. On the basis of all the above observations, a simple possible mechanism was proposed addressing the formation of CMSs under hydrothermal condition and schematically shown in Scheme 1. At lower temperature (about 160-200 °C, step I), the starch starts to hydrolyze and produce glucose and levulose, followed by further dehydrate and then polymerize to form colloidal carbon spheres with a uniform size of about several hundred nanometers. This process is similar to the growth of colloidal carbon spheres.5 As the temperature increases (about 300-400 °C, step II), the moving speed of the colloidal spheres is accelerated, resulting in the self-assembly and aggregation

of colloidal spheres because of the surface tension,29 thus the spherical aggregates formed. In this step, the heating rate is an important factor to control the size of the aggregates and it is found that a rapid one leads to the small aggregates. At higher temperatures (above 450 °C, step III), the number of functional groups in the colloidal carbon spheres decreased accordingly, and the “fusion” of the colloidal spheres and surface smoothing of the spherical aggregates proceeds further. Finally the aggregates are “fused” into solid carbon microspheres. This possible formation mechanism could be substantiated by the SEM observation of the intermediate products. Figure 6a shows a typical SEM image of the sample synthesized at 400 °C for 3 h with a heating rate of 10 deg/min. It can be seen that the sample consists of some aggregates. The aggregates present a wide size distribution from 2.5 to 5 µm, and consist of a large number of little spherical particles with an average size of about 200 nm. As shown in Figure 6b (a magnified image of Figure 5c), the spheres display a granular surface when examined under higher magnification. Individual colloidal spheres with diameters of about 150-300 nm can be distinguished clearly, and these carbon spheres appear to be “fused” together. These results indicate that the carbon microspheres are formed via aggregation and fusion of a large number of colloidal spheres rather than continuous growth of carbon colloidal spheres. Of course, to control the size of the CMSs accurately and its distribution, considerable kinetic and thermodynamic principles behind the formation of the microspheres in the hydrothermal system should be further studied. The work is now in progress. 4. Conclusion Highly uniform CMSs with smooth surface were synthesized via an easy hydrothermal carbonization process without any catalysts. The high-yield, low-cost starting materials and moderate reaction temperature provide an efficient way to fabricate solid CMSs. Furthermore, the size of the CMSs could be easily controlled by regulating the concentration of the starch solution, the heating rate, and the reaction temperature. Cheap raw materials and a simple preparation process make this technique of industrial importance. Moreover, the surface of the CMSs synthesized from hydrothermal carbonization of soluble starch is functionalized with hydroxyl and carboxyl groups, which makes further surface modification unnecessary. Therefore, the CMSs may greatly widen their potential applications in biochemistry, drug delivery, and catalyst supports, and they can be used as templates for fabricating core-shell or hollow structures. Acknowledgment. This work was financially supported by the Natural Science Union Foundations of China and Guangdong

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