CHEMPHYSCHEM ARTICLES DOI: 10.1002/cphc.201300783
A Strategy to Synthesize Hollow Micro/Nanospheres with Tunable Shell Thickness Gongzheng Yang,[a] Hao Cui,[a, b] and Chengxin Wang*[a, b] A simple one-step direct templating method is developed to synthesize hollow carbon and sandwich-like ZnO/C/ZnO micro/ nanospheres. The type and shell thickness of the final products can be controlled by simply adjusting the reaction temperature. The removal of the templates can also be easily controlled during the synthesis. At a low temperature, the templates remain in the products to form hollow sandwich-like micro/nanospheres. As the reaction temperature rises, the templates are consumed, which results in the preparation of
hollow carbon micro/nanospheres. On the basis of a series of experiments, we propose a simple plausible mechanism to address the original strategy for synthesizing these hollow micro/ nanospheres. Furthermore, the sandwich-like ZnO/C/ZnO nanospheres can be used as the anode in lithium-ion batteries, exhibiting an extraordinary cyclability and a high coulombic efficiency. This approach can be extended to the synthesis of other hollow spheres. Further investigation is underway in our group.
1. Introduction In the past decade, hollow spheres with tunable sizes and shell structures have received considerable attention because of their many attractive applications in catalysis,[1] biomaterials,[2] and electrochemical energy storage.[3] Various types of hollow spheres, including silica-,[4] carbon-,[5] polymer-,[6, 5c] and metaloxide-based structures,[3a,b, 7] have been synthesized. Among the numerous methods, templating[8] (including hard, soft, and sacrificial templating synthesis) is possibly the most widely used procedure to fabricate hollow spheres. However, the applications of these approaches are limited because: 1) it is difficult to choose suitable templates to encapsulate within the spheres; 2) the templates need to be removed, which may lead to expensive and tedious synthetic procedures; and 3) it is difficult to prevent the destruction of hollow spheres during chemical etching. Therefore, many efforts have been directed to exploring new synthetic methods for the large-scale preparation of hollow spherical materials. Recently, Zhang and Wan[3c] reported a new approach to synthesize tin nanoparticles encapsulated in elastic hollow carbon spheres with uniform size. Their excellent structure made them a promising anode material for high-performance lithium-ion batteries. However, about four major assembly processes were employed
to obtain the final products. Therefore, one would prefer a one-step method for the tunable preparation of hollow structures with tailored sizes, while the removal of templates in the final products can also be easily controlled during the synthesis process. Herein, we report a simple one-pot synthesis procedure for hollow C, ZnO/C/ZnO micro/nanospheres, which overcomes the shortcomings mentioned above. The synthesis is performed in a conventional horizontal tube furnace using Zn powders as the precursors (see Figure 1). In this method, Zn
[a] Dr. G. Z. Yang, Dr. H. Cui, Prof. C. X. Wang State key laboratory of optoelectronic materials and technologies, School of Physics Science and Engineering, Sun Yat-sen (Zhongshan) University Guangzhou 510275 (People’s Republic of China) E-mail: [email protected]
does not only act as a mold for the hollow carbon spheres but also induces a sandwich-like ZnO/C/ZnO structure with tunable shell thickness by simply adjusting the reaction temperature. Furthermore, the sizes of the final products can be tailored by varying the sizes of the Zn powders.
[b] Dr. H. Cui, Prof. C. X. Wang The Key Laboratory of Low-carbon Chemistry & Energy Conservation of Guangdong Province Sun Yat-sen (Zhongshan) University Guangzhou 510275 (People’s Republic of China)
2. Results and Discussion
Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201300783.
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Figure 1. Schematic diagram of the setup for the synthesis of hollow spheres.
Figure 2 A shows a low-magnification scanning electron microscopy (SEM) image of the collected products, obtained by thermally evaporating C60 onto Zn at 1250 8C. It is apparent ChemPhysChem 2014, 15, 374 – 381
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broad diffraction peaks—similar to those observed for carbon microspheres prepared at 1250 8C—appeared, indicating that amorphous-carbon/crystalline-ZnO (a-C/ZnO) composites were synthesized at 450 and 800 8C. Noting the similar structures of the microspheres synthesized at 450 and 800 8C, we wish to know how the crystalline ZnO clusters and amorphous carbon distribute within these spheres. Herein, electron back-scattering diffraction (EBSD) is applied to analyze the micro-area chemical composition, and the corresponding results are shown in Figure 5. In the EBSD image, the signals of the heavier elements, or those of composites containing heavier elements, can be deFigure 2. SEM image (A), magnified SEM image (inset), TEM image (B), and XRD pattern (C) of hollow carbon microspheres prepared at 1250 8C. D) EDX results obtained for the spheres in (B). tected as much brighter spots. Therefore, the bright parts in Figure 5 reveal the distributions of ZnO because the carbon has a much smaller atomic that the products consists of hollow microspheres with a diamnumber. As can be seen from Figure 5 B, almost continuous eter of 1 ~ 12 mm. The proportional relationship between Zn ZnO clusters distribute on both sides of the spheres. Besides, particle size and diameter of the hollow structures is shown in many small gaps have also been observed. By contrast, in FigFigure S1 (Supporting Information, SI), from which one can see ure 5 D, spheres exhibit the thinner shells which are interthat the sizes of the hollow microspheres are universally larger spersed by many discontinuous ZnO clusters. Interestingly, than those of the precursors. X-ray powder diffraction (XRD) these clusters are fragmental, but the spheres are actually and transmission electron microscopy (TEM), combined with complete. In response to this, we believe that these clusters energy dispersive X-ray (EDX) spectroscopy, were applied to must attach onto a continuous core, namely, the continuous confirm the crystallographic structure and elemental composiamorphous carbon sphere. For the spheres obtained at tion of the studied structures. In Figure 2 C, only a broad dif1250 8C (Figure 5 F), no ZnO signals are detected, showing that fraction peak can be observed at 20 ~ 308, indicating that low pure carbon microspheres have been synthesized. crystalline or amorphous carbon is synthesized.[9] The strong To further investigate the microstructures of a-C/ZnO comcontrast (Figure 2 B) between the dark edge and the pale posites, TEM imaging analysis was employed. A typical TEM center evidences the hollow nature of the structures. Furtherimage of an individual microsphere obtained at 450 8C is more, the EDX spectrum (Figure 2 D) indicates that hollow shown in Figure 6 A. One can see that this sphere seems to carbon microspheres with high purity have been obtained. By consist of the accumulations of many ZnO nanoparticles, acnow, one question will be easily proposed: where is the Zn? companied by the emergence of small gaps among the ZnO Therefore, to check the role that Zn plays in the growth of clusters. The high-resolution TEM (HRTEM) image in Figure 6 B the carbon microspheres, several temperature-dependent exshows details of the interface between the gap and crystalline periments were performed, keeping all the other reaction paZnO clusters. Taking the XRD and EDX (Figure 6 C) results into rameters constant. Figure 3 A shows the top-view morphology account, the disordered microstructure of these gaps (marked of hollow spheres synthesized at 450 8C. The magnified images in red) can be attributed to amorphous carbon. Figure 6 D exin Figure 3 B, C exhibit the thick shells of the hollow microhibits two a-C/ZnO microspheres synthesized at 800 8C. The spheres. When the reaction temperature was increased to shells of both spheres are so thin that several “holes” (amor800 8C, hollow microspheres with thinner shells were observed phous carbon) appear in the field of vision, as shown in Fig(see Figure 3 D, E). XRD patterns of the products obtained from ure 6 E. The elemental distributions across these spheres were 450 to 1250 8C (in Figure 4) show that the structures and comcharacterized by EDX spectroscopy. In Figure 6 C, the high positions of the products are dependent on the synthetic conatomic ratio of Zn and the low atomic ratio of C indicate that ditions. Five characteristic peaks matched the ZnO phase the a-C/ZnO microspheres obtained at 450 8C are coated by (PDF No. 36-1451) at 2 q = 31.77, 34.42, 36.25, 47.54, and 56.608 when the reaction temperature was reduced. Eventually, two a thick ZnO shell. For the samples obtained at 800 8C, the C 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 5. Secondary electron (SE) and back-scattering electron (BSE) images of the spheres obtained at 450 8C (A, B), 800 8C (C, D), and 1250 8C (E, F), in which both the SE and BSE micrographs are taken in the same field.
Figure 3. A) SEM image of the products obtained at 450 8C. B, C) Magnified SEM images corresponding to (A). D) Product prepared at 800 8C. E) Corresponding high-magnification SEM image of (D).
Figure 4. XRD patterns of hollow spheres produced at various reaction temperatures.
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content is 3.16 %, implying that more carbon is exposed on the surface and most areas of the spheres are also covered with relative thinner ZnO shells. We tried hard to obtain representative TEM images, where one could clearly see the layers of ZnO, C, and ZnO, but all the efforts failed, which might be caused by the difficulty to detect the thin carbon shell. Therefore, we directly probed the inner parts of the broken spheres, and the corresponding results are shown in Figure S2. One can clearly see that many ZnO nanoparticles are anchored on the inner shell of the hollow spheres, confirming the layered structure of the hollow spheres. More convincing evidence for the structures of the samples can be provided by Raman spectroscopy. Figure 7 shows the Raman spectra of the carbon sources and products synthesized at 450, 800 and 1250 8C. The Raman spectrum of the carbon sources shows bands at 267, 492, 710, 775, 1421, 1462, and 1566 cm 1, which correspond to crystalline C60.[10] The Raman spectra of all the spheres show two broad peaks centered at about 1348 and 1580 cm 1, which are associated with the vibrations of carbon atoms with dangling bonds for the in-plane terminations of disordered graphite and the vibrations in all sp2-bonded carbon atoms in a 2D hexagonal lattice, respectively.[11] Both the intensities of the D-band and G-band increase upon rising the reaction temperature. Under these reaction conditions, the intensities of the D-band are always higher ChemPhysChem 2014, 15, 374 – 381
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Figure 6. A–C) TEM, high-resolution TEM (HRTEM), and EDX results for a sphere synthesized at 450 8C. The inset in (B) is the Fourier-transform pattern of crystalline ZnO. D–F) TEM, HRTEM, and EDX results for two spheres obtained at 800 8C.
than those of the G-band, further reflecting the relative disorder of the carbon. Furthermore, it is worth noting the spectra of samples synthesized at 450 and 800 8C. A characteristic peak corresponding to ZnO (at 438 cm 1, see inset of Figure 7) is observed for both samples, its intensity being much higher for the samples synthesized at 450 8C. Besides, for the samples obtained at 800 8C, four bands at 710, 775, 1462, and 1566 cm 1 were also detected, which can be indexed to crystalline C60, implying that a small quantity of weak-crystalline C60 still exists in these spheres. By contrast, the signals of C60 were not detected for the samples obtained at 450 8C, indicating that these spheres are coated by thick ZnO shells. Since the formation of hollow structures strongly depends on the temperature of the 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
reaction, a comprehensive thermogravimetric analysis (TGA) was performed to learn about the processes that occur during the thermal formation of the hollow capsules, as shown in Figure S3. The results demonstrate that the products contain 97.4 %, 93.4 %, and 0 % ZnO, corresponding to the products synthesized at 450, 800, and 1250 8C, respectively. Thus, several kinds of hollow microspheres, including a-C/ ZnO composites with controllable thickness shells and pure carbon microspheres, have been synthesized by simply controlling the reaction temperature. On the basis of all the above observations, we propose a simple plausible mechanism to address the original strategy to the synthesis of these hollow microspheres, as shown in Figure 8. In our experiments, fullerene ChemPhysChem 2014, 15, 374 – 381
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more ZnO and C will be consumed, which finally leads to the formation of hollow carbon microspheres. Enlightened by the strategy to synthesize hollow microspheres, we thought that we could prepare hollow nanospheres by using nanometer-sized Zn powders instead of micrometer-sized Zn. Unfortunately, the initial experiments were not successful. The primary factor is the different size of the Zn powders. At low temperatures, such as 419 8C, the nanometersized Zn powders easily aggregate to form irregular particles, but this does not happen while using the micrometer-size Zn powders. In the mechanism proposed above, we can see that C60 not only serves as the carbon source but also plays an important role as separator, preventing the aggregation of Zn powders. With regard to this, we changed the experimental conditions (mixing C60 with nanometer-sized Zn powders) and finally successfully obtained ZnO/C/ZnO, C nanospheres similar Figure 7. Raman spectra of the carbon precursor and the products syntheto the microspheres mentioned above; the corresponding resized at 450, 800 and 1250 8C. sults are shown in Figure 9. The structures and compositions of the products are dependent on the synthetic conditions. Because of the higher activity and smaller size, the synthesis temC60 powders were used as the carbon sources. Compared with peratures of the hollowed-out sandwich-like ZnO/C/ZnO and other carbon materials, C60 possesses a higher activity and hollow C nanospheres decreased to 600 and 800 8C, respectivelower sublimation temperature. When the system is heated up ly. Figure 10 D shows a TEM image of the carbon shell obtained to 350 8C, plenty of C60 molecules will sublimate and deposit after the sandwich-like ZnO/C/ZnO nanospheres were etched onto the Zn powders (Figure S4).[12] As the temperature rises to using an HCl solution. This result further confirms our assumpclose to 419 8C (the melting point of Zn), the surface of Zn will tions. melt and aggregate with C60 molecules to form a homogeneous C60 shell. Then, the vapor pressure of Zn increases inside, To verify the structure of the nanospheres obtained at 600 8C, we carried out EDX and elemental mapping characterifinally bursting out from the weak portion of C60 shell, which zations. Figure 10 B shows the EDX spectrum of the nanoresults in the formation of the hollow structures (Figure S5). In spheres (collected from point 1 in Figure 10 A). The peaks corspite of this, a small quantity of liquid-Zn still adheres to the responding to C, O, and Zn are obviously identified. The eleinside of the C60 sphere. Meanwhile, due to the gaseous Zn exmental mappings further depict the element distributions. Figisting in the system, the outside of the C60 sphere will be wrapure 10 C shows a normal TEM image of a nanosphere. A holped up by a thin Zn shell. Furthermore, the residual oxygen relowed-out sphere possessing discontinuous clusters and mains in our furnace owing to the low-vacuum conditions. several holes is clearly visible. It can be verified that the sphere Therefore, Zn can easily react with O2 to form ZnO, which subis entirely covered with carbon (see Figure 10 D) and intersequently anchors onto the spheres. Thus, unique sandwichspersed with ZnO nanoparticles (see Figure 10 E, F). According like ZnO/C(a-C and C60)/ZnO hollow microspheres can be synto these observations, we can assume that these nanospheres thesized. When the temperature increases to values close to are C/ZnO composites similar to the microspheres which are 800 8C, C/CO will react vigorously with ZnO, resulting in the resynthesized using micrometer-sized Zn powders at 800 8C. The duction of the ZnO shells and the formation of hollowed-out chemical compositions of the ZnO/C/ZnO nanospheres obmicrostructures. Upon increasing the reaction temperature, tained at 600 8C were determined by TGA and the corresponding results, shown in Figure 11, confirm that the products contain a small amount of carbon (~ 3.8 %). For the anode material used in lithium-ion batteries, ZnO has a very high theoretical capacity of 987.7 mA h g 1.[13] However, it has rarely been reported as anode material owing to its poor cyclability compared to other materials. The volume effect and low electronic conductivity during the charge–discharge Figure 8. Schematic representation of the formation of hollow sandwich-like ZnO/C/ZnO and carbon spheres. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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ieve a high performance, which will be discussed in detail in the following. Coin cells with a metallic lithium anode were assembled to investigate the electrochemical performance of the ZnO/C/ZnO nanospheres at room temperature. Figure 12 A shows the charge–discharge profiles between 0.05 and 2.5 V (vs. Li/Li + ) at a current rate of 1.0C (1C was defined as 3 Li + per hour; 987.7 mA h g 1). The first discharge and charge capacities are 1142 and 707 mA h g 1, respectively. Figure 12 B shows the specific retention of the ZnO/C/ZnO nanospheres as a function of the charge–discharge cycling number. Even after the 200th cycle, the composite electrode delivers reversible capacities as high as 381 mA h g 1. A coulomb efficiency close to 100 % gives Figure 9. SEM images of the nanospheres prepared at 450 8C (A), 600 8C (C), and 800 8C (B). D) SEM image of the carbon shells obtained after the ZnO/C/ZnO nanospheres in (C) were etched using a HCl solution. further evidence of the stability of our products. To the best of our knowledge, such a stable cyclability has not been reported before for ZnO-based anodes of any type (see Table 1). The outstanding cyclability of our samples can be attributed to their unique nanostructure. Firstly, the hollow nanostructure is favorable to the infiltration of electrolyte, which then offers an ample space for Li + diffusion. Then, the sphere structure is helpful for mitigating the volume change in the charge–discharge process, resulting in a better stability. Secondly, many ZnO nanoparticles are present at the surface of the spheres—thanks to the sandwich-like structure—enlarging the contact area between Figure 10. TEM (A) and EDX (B) results for ZnO/C/ZnO nanospheres obtained at 600 8C. D–F) Elemental maps of C, Zn, and O in a ZnO/C/ZnO nanosphere. ZnO and electrolyte. The interlayer carbon itself is an electronic conductor, which enhances the charge transfer/Li + transport. Based on these points, it process are serious problems leading to a poor cyclability.[14] seems clear that hollow nanospheres with sandwich-like strucOne possible strategy to mitigate the volume change is to tures can lead to large improvements in the electrochemical design hollow nanomaterials.[3a,d] On the other hand, some performance. high-electronic-conductivity materials, such as C,[15] CoO,[16] and NiO,[17] are employed to dope ZnO to increase the chargetransfer rate. Herein, considering the unique sandwich-like nanostructure, this ZnO/C/ZnO nanosphere is expected to ach 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 11. TG curve of the ZnO/C/ZnO nanospheres at 5 8C min 1 in air. The residual weight ratios correspond to the content of ZnO in the composites.
3. Conclusions In summary, we have developed a facile and tunable synthetic procedure for the preparation of micro-/nanosized hollow spheres, including carbon and sandwich-like ZnO/C/ZnO spheres with tailored shell thickness. When we employ the sandwich-like ZnO/C/ZnO nanospheres as a lithium-battery anode, they exhibit an extraordinary cyclability and a higher coulombic efficiency compared to previously reported values. On the basis of a series of experiments, we propose a simple plausible mechanism to address the original strategy for synthesizing these hollow microspheres. Enlightened by this strategy, we think that it can be extended to the synthesis of other hollow spheres. Further investigation is underway in our group.
Experimental Section Synthesis Hollow carbon and sandwich-like ZnO/C/ZnO micro-/nanospheres with tunable shell thickness were synthesized in a conventional horizontal tube furnace, which is similar to that formerly used for synthesizing other nanomaterials.[21] In detail, 20 mg commercially available C60 powders (Peking University, China, 98 %) and 80 mg micro-/nanometer-sized Zn powders (Shanghai jingchun Reagent
Figure 12. Voltage profiles (A) and cycling performance (B) of the hollow sandwich-like ZnO/C/ZnO nanospheres at 1 C.
Co. China, 98 %) were used as carbon and Zn sources, respectively. Firstly, we dispersed the Zn powders into the ethyl alcohol. After supersonic processing, the Zn powders were dropped evenly onto most areas of a ceramic wafer. The C60 powders were positioned at the end of the ceramic wafer, as shown in Figure 1. Then the system was heated at a rate of 5 8C per minute. The characteristics of the final products can be tailored by simply adjusting the reaction temperature.
Materials Characterization
The morphologies of the as-synthesized products were examined by SEM and TEM, combined with EDX and selected-area electron diffraction (SAED). The crystal structure was characterized by XRD with CuKa radiation. TGA was performed using a PerkinElmer Pyris Table 1. Comparison of the lithium-storage properties of ZnO-based anodes. 6TG analyzer (heating rate of 5 8C min 1) under static-air condiMaterial Lithium-storage properties tions. Current density Cycle number Discharge Capacity [mAh g 1] Dandelion-like ZnO nanorod arrays[18] ZnO/carbon nanotube composite[15a] Carbon/ZnO nanorod arrays[15b] Flower-like ZnO nanostructures-Au[19] Porous ZnO nanosheets[20] Flower-like ZnO-NiO-C films[17a] Flower-like ZnO-CoO-C nanowall arrays[16] Core–shell ZnO-Ni3ZnC0.7-C nanocomposite[17b] Hollow sandwich-like ZnO/C/ZnO nanospheres
0.2 C 0.01 C 0.75 C 0.12 C 1C 1C 1C 0.5 C 1C
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40 30 30 50 50 50 50 50 200
310 180 360 382 300 380 280 372 381
Electrochemical Characterization Electrochemical experiments were performed using CR2032 coin-type cells. The working electrodes were prepared by mixing the ZnO/C/ ZnO nanospheres, carbon black (Super-P-Li), and poly(vinyl difluorChemPhysChem 2014, 15, 374 – 381
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CHEMPHYSCHEM ARTICLES ide) (PVDF, Aldrich) at a weight ratio of 80:10:10, was pasted on pure Cu foil (Shenyang Kejing Co. China, 99.6 %). The cells were formed using a Li-metal negative electrode, an electrolyte consisting of 1 m LiPF6 in a 1:1 ethylene carbonate and diethyl carbonate mixture, a separator (Celgard, 2325, USA), and a working electrode. The discharge–charge measurements of the cells were carried out at room temperature using a multichannel battery tester (Shenzhen Newware Technology Limited Co. China). The capacity values were calculated based on the mass of the active ZnO/C/ZnO nanospheres.
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[8] [9] [10] [11] [12]
Acknowledgements This work was financially supported by the National Nature Science Foundation of China (51125008, 11274392).
[13] [14]
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Received: August 25, 2013 Revised: November 23, 2013 Published online on December 27, 2013
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A Strategy to Synthesize Hollow Micro/Nanospheres with Tunable Shell Thickness Gongzheng Yang,[a] Hao Cui,[a, b] and Chengxin Wang*[a, b] A simple one-step direct templating method is developed to synthesize hollow carbon and sandwich-like ZnO/C/ZnO micro/ nanospheres. The type and shell thickness of the final products can be controlled by simply adjusting the reaction temperature. The removal of the templates can also be easily controlled during the synthesis. At a low temperature, the templates remain in the products to form hollow sandwich-like micro/nanospheres. As the reaction temperature rises, the templates are consumed, which results in the preparation of
hollow carbon micro/nanospheres. On the basis of a series of experiments, we propose a simple plausible mechanism to address the original strategy for synthesizing these hollow micro/ nanospheres. Furthermore, the sandwich-like ZnO/C/ZnO nanospheres can be used as the anode in lithium-ion batteries, exhibiting an extraordinary cyclability and a high coulombic efficiency. This approach can be extended to the synthesis of other hollow spheres. Further investigation is underway in our group.
1. Introduction In the past decade, hollow spheres with tunable sizes and shell structures have received considerable attention because of their many attractive applications in catalysis,[1] biomaterials,[2] and electrochemical energy storage.[3] Various types of hollow spheres, including silica-,[4] carbon-,[5] polymer-,[6, 5c] and metaloxide-based structures,[3a,b, 7] have been synthesized. Among the numerous methods, templating[8] (including hard, soft, and sacrificial templating synthesis) is possibly the most widely used procedure to fabricate hollow spheres. However, the applications of these approaches are limited because: 1) it is difficult to choose suitable templates to encapsulate within the spheres; 2) the templates need to be removed, which may lead to expensive and tedious synthetic procedures; and 3) it is difficult to prevent the destruction of hollow spheres during chemical etching. Therefore, many efforts have been directed to exploring new synthetic methods for the large-scale preparation of hollow spherical materials. Recently, Zhang and Wan[3c] reported a new approach to synthesize tin nanoparticles encapsulated in elastic hollow carbon spheres with uniform size. Their excellent structure made them a promising anode material for high-performance lithium-ion batteries. However, about four major assembly processes were employed
to obtain the final products. Therefore, one would prefer a one-step method for the tunable preparation of hollow structures with tailored sizes, while the removal of templates in the final products can also be easily controlled during the synthesis process. Herein, we report a simple one-pot synthesis procedure for hollow C, ZnO/C/ZnO micro/nanospheres, which overcomes the shortcomings mentioned above. The synthesis is performed in a conventional horizontal tube furnace using Zn powders as the precursors (see Figure 1). In this method, Zn
[a] Dr. G. Z. Yang, Dr. H. Cui, Prof. C. X. Wang State key laboratory of optoelectronic materials and technologies, School of Physics Science and Engineering, Sun Yat-sen (Zhongshan) University Guangzhou 510275 (People’s Republic of China) E-mail: [email protected]
does not only act as a mold for the hollow carbon spheres but also induces a sandwich-like ZnO/C/ZnO structure with tunable shell thickness by simply adjusting the reaction temperature. Furthermore, the sizes of the final products can be tailored by varying the sizes of the Zn powders.
[b] Dr. H. Cui, Prof. C. X. Wang The Key Laboratory of Low-carbon Chemistry & Energy Conservation of Guangdong Province Sun Yat-sen (Zhongshan) University Guangzhou 510275 (People’s Republic of China)
2. Results and Discussion
Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201300783.
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Figure 1. Schematic diagram of the setup for the synthesis of hollow spheres.
Figure 2 A shows a low-magnification scanning electron microscopy (SEM) image of the collected products, obtained by thermally evaporating C60 onto Zn at 1250 8C. It is apparent ChemPhysChem 2014, 15, 374 – 381
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broad diffraction peaks—similar to those observed for carbon microspheres prepared at 1250 8C—appeared, indicating that amorphous-carbon/crystalline-ZnO (a-C/ZnO) composites were synthesized at 450 and 800 8C. Noting the similar structures of the microspheres synthesized at 450 and 800 8C, we wish to know how the crystalline ZnO clusters and amorphous carbon distribute within these spheres. Herein, electron back-scattering diffraction (EBSD) is applied to analyze the micro-area chemical composition, and the corresponding results are shown in Figure 5. In the EBSD image, the signals of the heavier elements, or those of composites containing heavier elements, can be deFigure 2. SEM image (A), magnified SEM image (inset), TEM image (B), and XRD pattern (C) of hollow carbon microspheres prepared at 1250 8C. D) EDX results obtained for the spheres in (B). tected as much brighter spots. Therefore, the bright parts in Figure 5 reveal the distributions of ZnO because the carbon has a much smaller atomic that the products consists of hollow microspheres with a diamnumber. As can be seen from Figure 5 B, almost continuous eter of 1 ~ 12 mm. The proportional relationship between Zn ZnO clusters distribute on both sides of the spheres. Besides, particle size and diameter of the hollow structures is shown in many small gaps have also been observed. By contrast, in FigFigure S1 (Supporting Information, SI), from which one can see ure 5 D, spheres exhibit the thinner shells which are interthat the sizes of the hollow microspheres are universally larger spersed by many discontinuous ZnO clusters. Interestingly, than those of the precursors. X-ray powder diffraction (XRD) these clusters are fragmental, but the spheres are actually and transmission electron microscopy (TEM), combined with complete. In response to this, we believe that these clusters energy dispersive X-ray (EDX) spectroscopy, were applied to must attach onto a continuous core, namely, the continuous confirm the crystallographic structure and elemental composiamorphous carbon sphere. For the spheres obtained at tion of the studied structures. In Figure 2 C, only a broad dif1250 8C (Figure 5 F), no ZnO signals are detected, showing that fraction peak can be observed at 20 ~ 308, indicating that low pure carbon microspheres have been synthesized. crystalline or amorphous carbon is synthesized.[9] The strong To further investigate the microstructures of a-C/ZnO comcontrast (Figure 2 B) between the dark edge and the pale posites, TEM imaging analysis was employed. A typical TEM center evidences the hollow nature of the structures. Furtherimage of an individual microsphere obtained at 450 8C is more, the EDX spectrum (Figure 2 D) indicates that hollow shown in Figure 6 A. One can see that this sphere seems to carbon microspheres with high purity have been obtained. By consist of the accumulations of many ZnO nanoparticles, acnow, one question will be easily proposed: where is the Zn? companied by the emergence of small gaps among the ZnO Therefore, to check the role that Zn plays in the growth of clusters. The high-resolution TEM (HRTEM) image in Figure 6 B the carbon microspheres, several temperature-dependent exshows details of the interface between the gap and crystalline periments were performed, keeping all the other reaction paZnO clusters. Taking the XRD and EDX (Figure 6 C) results into rameters constant. Figure 3 A shows the top-view morphology account, the disordered microstructure of these gaps (marked of hollow spheres synthesized at 450 8C. The magnified images in red) can be attributed to amorphous carbon. Figure 6 D exin Figure 3 B, C exhibit the thick shells of the hollow microhibits two a-C/ZnO microspheres synthesized at 800 8C. The spheres. When the reaction temperature was increased to shells of both spheres are so thin that several “holes” (amor800 8C, hollow microspheres with thinner shells were observed phous carbon) appear in the field of vision, as shown in Fig(see Figure 3 D, E). XRD patterns of the products obtained from ure 6 E. The elemental distributions across these spheres were 450 to 1250 8C (in Figure 4) show that the structures and comcharacterized by EDX spectroscopy. In Figure 6 C, the high positions of the products are dependent on the synthetic conatomic ratio of Zn and the low atomic ratio of C indicate that ditions. Five characteristic peaks matched the ZnO phase the a-C/ZnO microspheres obtained at 450 8C are coated by (PDF No. 36-1451) at 2 q = 31.77, 34.42, 36.25, 47.54, and 56.608 when the reaction temperature was reduced. Eventually, two a thick ZnO shell. For the samples obtained at 800 8C, the C 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 5. Secondary electron (SE) and back-scattering electron (BSE) images of the spheres obtained at 450 8C (A, B), 800 8C (C, D), and 1250 8C (E, F), in which both the SE and BSE micrographs are taken in the same field.
Figure 3. A) SEM image of the products obtained at 450 8C. B, C) Magnified SEM images corresponding to (A). D) Product prepared at 800 8C. E) Corresponding high-magnification SEM image of (D).
Figure 4. XRD patterns of hollow spheres produced at various reaction temperatures.
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content is 3.16 %, implying that more carbon is exposed on the surface and most areas of the spheres are also covered with relative thinner ZnO shells. We tried hard to obtain representative TEM images, where one could clearly see the layers of ZnO, C, and ZnO, but all the efforts failed, which might be caused by the difficulty to detect the thin carbon shell. Therefore, we directly probed the inner parts of the broken spheres, and the corresponding results are shown in Figure S2. One can clearly see that many ZnO nanoparticles are anchored on the inner shell of the hollow spheres, confirming the layered structure of the hollow spheres. More convincing evidence for the structures of the samples can be provided by Raman spectroscopy. Figure 7 shows the Raman spectra of the carbon sources and products synthesized at 450, 800 and 1250 8C. The Raman spectrum of the carbon sources shows bands at 267, 492, 710, 775, 1421, 1462, and 1566 cm 1, which correspond to crystalline C60.[10] The Raman spectra of all the spheres show two broad peaks centered at about 1348 and 1580 cm 1, which are associated with the vibrations of carbon atoms with dangling bonds for the in-plane terminations of disordered graphite and the vibrations in all sp2-bonded carbon atoms in a 2D hexagonal lattice, respectively.[11] Both the intensities of the D-band and G-band increase upon rising the reaction temperature. Under these reaction conditions, the intensities of the D-band are always higher ChemPhysChem 2014, 15, 374 – 381
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Figure 6. A–C) TEM, high-resolution TEM (HRTEM), and EDX results for a sphere synthesized at 450 8C. The inset in (B) is the Fourier-transform pattern of crystalline ZnO. D–F) TEM, HRTEM, and EDX results for two spheres obtained at 800 8C.
than those of the G-band, further reflecting the relative disorder of the carbon. Furthermore, it is worth noting the spectra of samples synthesized at 450 and 800 8C. A characteristic peak corresponding to ZnO (at 438 cm 1, see inset of Figure 7) is observed for both samples, its intensity being much higher for the samples synthesized at 450 8C. Besides, for the samples obtained at 800 8C, four bands at 710, 775, 1462, and 1566 cm 1 were also detected, which can be indexed to crystalline C60, implying that a small quantity of weak-crystalline C60 still exists in these spheres. By contrast, the signals of C60 were not detected for the samples obtained at 450 8C, indicating that these spheres are coated by thick ZnO shells. Since the formation of hollow structures strongly depends on the temperature of the 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
reaction, a comprehensive thermogravimetric analysis (TGA) was performed to learn about the processes that occur during the thermal formation of the hollow capsules, as shown in Figure S3. The results demonstrate that the products contain 97.4 %, 93.4 %, and 0 % ZnO, corresponding to the products synthesized at 450, 800, and 1250 8C, respectively. Thus, several kinds of hollow microspheres, including a-C/ ZnO composites with controllable thickness shells and pure carbon microspheres, have been synthesized by simply controlling the reaction temperature. On the basis of all the above observations, we propose a simple plausible mechanism to address the original strategy to the synthesis of these hollow microspheres, as shown in Figure 8. In our experiments, fullerene ChemPhysChem 2014, 15, 374 – 381
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more ZnO and C will be consumed, which finally leads to the formation of hollow carbon microspheres. Enlightened by the strategy to synthesize hollow microspheres, we thought that we could prepare hollow nanospheres by using nanometer-sized Zn powders instead of micrometer-sized Zn. Unfortunately, the initial experiments were not successful. The primary factor is the different size of the Zn powders. At low temperatures, such as 419 8C, the nanometersized Zn powders easily aggregate to form irregular particles, but this does not happen while using the micrometer-size Zn powders. In the mechanism proposed above, we can see that C60 not only serves as the carbon source but also plays an important role as separator, preventing the aggregation of Zn powders. With regard to this, we changed the experimental conditions (mixing C60 with nanometer-sized Zn powders) and finally successfully obtained ZnO/C/ZnO, C nanospheres similar Figure 7. Raman spectra of the carbon precursor and the products syntheto the microspheres mentioned above; the corresponding resized at 450, 800 and 1250 8C. sults are shown in Figure 9. The structures and compositions of the products are dependent on the synthetic conditions. Because of the higher activity and smaller size, the synthesis temC60 powders were used as the carbon sources. Compared with peratures of the hollowed-out sandwich-like ZnO/C/ZnO and other carbon materials, C60 possesses a higher activity and hollow C nanospheres decreased to 600 and 800 8C, respectivelower sublimation temperature. When the system is heated up ly. Figure 10 D shows a TEM image of the carbon shell obtained to 350 8C, plenty of C60 molecules will sublimate and deposit after the sandwich-like ZnO/C/ZnO nanospheres were etched onto the Zn powders (Figure S4).[12] As the temperature rises to using an HCl solution. This result further confirms our assumpclose to 419 8C (the melting point of Zn), the surface of Zn will tions. melt and aggregate with C60 molecules to form a homogeneous C60 shell. Then, the vapor pressure of Zn increases inside, To verify the structure of the nanospheres obtained at 600 8C, we carried out EDX and elemental mapping characterifinally bursting out from the weak portion of C60 shell, which zations. Figure 10 B shows the EDX spectrum of the nanoresults in the formation of the hollow structures (Figure S5). In spheres (collected from point 1 in Figure 10 A). The peaks corspite of this, a small quantity of liquid-Zn still adheres to the responding to C, O, and Zn are obviously identified. The eleinside of the C60 sphere. Meanwhile, due to the gaseous Zn exmental mappings further depict the element distributions. Figisting in the system, the outside of the C60 sphere will be wrapure 10 C shows a normal TEM image of a nanosphere. A holped up by a thin Zn shell. Furthermore, the residual oxygen relowed-out sphere possessing discontinuous clusters and mains in our furnace owing to the low-vacuum conditions. several holes is clearly visible. It can be verified that the sphere Therefore, Zn can easily react with O2 to form ZnO, which subis entirely covered with carbon (see Figure 10 D) and intersequently anchors onto the spheres. Thus, unique sandwichspersed with ZnO nanoparticles (see Figure 10 E, F). According like ZnO/C(a-C and C60)/ZnO hollow microspheres can be synto these observations, we can assume that these nanospheres thesized. When the temperature increases to values close to are C/ZnO composites similar to the microspheres which are 800 8C, C/CO will react vigorously with ZnO, resulting in the resynthesized using micrometer-sized Zn powders at 800 8C. The duction of the ZnO shells and the formation of hollowed-out chemical compositions of the ZnO/C/ZnO nanospheres obmicrostructures. Upon increasing the reaction temperature, tained at 600 8C were determined by TGA and the corresponding results, shown in Figure 11, confirm that the products contain a small amount of carbon (~ 3.8 %). For the anode material used in lithium-ion batteries, ZnO has a very high theoretical capacity of 987.7 mA h g 1.[13] However, it has rarely been reported as anode material owing to its poor cyclability compared to other materials. The volume effect and low electronic conductivity during the charge–discharge Figure 8. Schematic representation of the formation of hollow sandwich-like ZnO/C/ZnO and carbon spheres. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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ieve a high performance, which will be discussed in detail in the following. Coin cells with a metallic lithium anode were assembled to investigate the electrochemical performance of the ZnO/C/ZnO nanospheres at room temperature. Figure 12 A shows the charge–discharge profiles between 0.05 and 2.5 V (vs. Li/Li + ) at a current rate of 1.0C (1C was defined as 3 Li + per hour; 987.7 mA h g 1). The first discharge and charge capacities are 1142 and 707 mA h g 1, respectively. Figure 12 B shows the specific retention of the ZnO/C/ZnO nanospheres as a function of the charge–discharge cycling number. Even after the 200th cycle, the composite electrode delivers reversible capacities as high as 381 mA h g 1. A coulomb efficiency close to 100 % gives Figure 9. SEM images of the nanospheres prepared at 450 8C (A), 600 8C (C), and 800 8C (B). D) SEM image of the carbon shells obtained after the ZnO/C/ZnO nanospheres in (C) were etched using a HCl solution. further evidence of the stability of our products. To the best of our knowledge, such a stable cyclability has not been reported before for ZnO-based anodes of any type (see Table 1). The outstanding cyclability of our samples can be attributed to their unique nanostructure. Firstly, the hollow nanostructure is favorable to the infiltration of electrolyte, which then offers an ample space for Li + diffusion. Then, the sphere structure is helpful for mitigating the volume change in the charge–discharge process, resulting in a better stability. Secondly, many ZnO nanoparticles are present at the surface of the spheres—thanks to the sandwich-like structure—enlarging the contact area between Figure 10. TEM (A) and EDX (B) results for ZnO/C/ZnO nanospheres obtained at 600 8C. D–F) Elemental maps of C, Zn, and O in a ZnO/C/ZnO nanosphere. ZnO and electrolyte. The interlayer carbon itself is an electronic conductor, which enhances the charge transfer/Li + transport. Based on these points, it process are serious problems leading to a poor cyclability.[14] seems clear that hollow nanospheres with sandwich-like strucOne possible strategy to mitigate the volume change is to tures can lead to large improvements in the electrochemical design hollow nanomaterials.[3a,d] On the other hand, some performance. high-electronic-conductivity materials, such as C,[15] CoO,[16] and NiO,[17] are employed to dope ZnO to increase the chargetransfer rate. Herein, considering the unique sandwich-like nanostructure, this ZnO/C/ZnO nanosphere is expected to ach 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 11. TG curve of the ZnO/C/ZnO nanospheres at 5 8C min 1 in air. The residual weight ratios correspond to the content of ZnO in the composites.
3. Conclusions In summary, we have developed a facile and tunable synthetic procedure for the preparation of micro-/nanosized hollow spheres, including carbon and sandwich-like ZnO/C/ZnO spheres with tailored shell thickness. When we employ the sandwich-like ZnO/C/ZnO nanospheres as a lithium-battery anode, they exhibit an extraordinary cyclability and a higher coulombic efficiency compared to previously reported values. On the basis of a series of experiments, we propose a simple plausible mechanism to address the original strategy for synthesizing these hollow microspheres. Enlightened by this strategy, we think that it can be extended to the synthesis of other hollow spheres. Further investigation is underway in our group.
Experimental Section Synthesis Hollow carbon and sandwich-like ZnO/C/ZnO micro-/nanospheres with tunable shell thickness were synthesized in a conventional horizontal tube furnace, which is similar to that formerly used for synthesizing other nanomaterials.[21] In detail, 20 mg commercially available C60 powders (Peking University, China, 98 %) and 80 mg micro-/nanometer-sized Zn powders (Shanghai jingchun Reagent
Figure 12. Voltage profiles (A) and cycling performance (B) of the hollow sandwich-like ZnO/C/ZnO nanospheres at 1 C.
Co. China, 98 %) were used as carbon and Zn sources, respectively. Firstly, we dispersed the Zn powders into the ethyl alcohol. After supersonic processing, the Zn powders were dropped evenly onto most areas of a ceramic wafer. The C60 powders were positioned at the end of the ceramic wafer, as shown in Figure 1. Then the system was heated at a rate of 5 8C per minute. The characteristics of the final products can be tailored by simply adjusting the reaction temperature.
Materials Characterization
The morphologies of the as-synthesized products were examined by SEM and TEM, combined with EDX and selected-area electron diffraction (SAED). The crystal structure was characterized by XRD with CuKa radiation. TGA was performed using a PerkinElmer Pyris Table 1. Comparison of the lithium-storage properties of ZnO-based anodes. 6TG analyzer (heating rate of 5 8C min 1) under static-air condiMaterial Lithium-storage properties tions. Current density Cycle number Discharge Capacity [mAh g 1] Dandelion-like ZnO nanorod arrays[18] ZnO/carbon nanotube composite[15a] Carbon/ZnO nanorod arrays[15b] Flower-like ZnO nanostructures-Au[19] Porous ZnO nanosheets[20] Flower-like ZnO-NiO-C films[17a] Flower-like ZnO-CoO-C nanowall arrays[16] Core–shell ZnO-Ni3ZnC0.7-C nanocomposite[17b] Hollow sandwich-like ZnO/C/ZnO nanospheres
0.2 C 0.01 C 0.75 C 0.12 C 1C 1C 1C 0.5 C 1C
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40 30 30 50 50 50 50 50 200
310 180 360 382 300 380 280 372 381
Electrochemical Characterization Electrochemical experiments were performed using CR2032 coin-type cells. The working electrodes were prepared by mixing the ZnO/C/ ZnO nanospheres, carbon black (Super-P-Li), and poly(vinyl difluorChemPhysChem 2014, 15, 374 – 381
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CHEMPHYSCHEM ARTICLES ide) (PVDF, Aldrich) at a weight ratio of 80:10:10, was pasted on pure Cu foil (Shenyang Kejing Co. China, 99.6 %). The cells were formed using a Li-metal negative electrode, an electrolyte consisting of 1 m LiPF6 in a 1:1 ethylene carbonate and diethyl carbonate mixture, a separator (Celgard, 2325, USA), and a working electrode. The discharge–charge measurements of the cells were carried out at room temperature using a multichannel battery tester (Shenzhen Newware Technology Limited Co. China). The capacity values were calculated based on the mass of the active ZnO/C/ZnO nanospheres.
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[8] [9] [10] [11] [12]
Acknowledgements This work was financially supported by the National Nature Science Foundation of China (51125008, 11274392).
[13] [14]
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Received: August 25, 2013 Revised: November 23, 2013 Published online on December 27, 2013
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