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Green Fabrication of Hierarchical CuO Hollow Micro/Nanostructures and Enhanced Performance as Electrode Materials for Lithium-ion Batteries Shuyan Gao,*,† Shuxia Yang,† Jie Shu,‡ Shuxia Zhang,† Zhengdao Li,† and Kai Jiang† College of Chemistry and EnVironmental Science, Henan Normal UniVersity, Xinxiang 453007, P. R. China, and Laboratoire de Re´actiVite´ et Chimie des Solides, UniVersite´ de Picardie Jules Verne, CNRS-UMR 6007, 33 Rue Saint Leu, 80039 Amiens Cedex 9, France ReceiVed: September 26, 2008; ReVised Manuscript ReceiVed: October 12, 2008

Engineering hierarchical CuO hollow micro/nanostructures was realized by a tyrosine-assisted green strategy. The morphology, composition, and phase structure of as-prepared powders were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), which showed that the sample was assembled from CuO nanosheets with average diameter of ca. 250 nm, selfwrapping to form hollow interiors with an outer diameter of 1.5-3 µm. The improved electrochemical performance toward Li uptake-release verifies their potential application as anode materials in lithium-ion batteries, which attributes to a three-dimensional current collector network and a chemical/mechanical robustness buffer of the as-formed novel CuO structure. Introduction Over the past few decades, the synthesis of hierarchical micro/ nanostructures with well-defined morphologies has progressively attracted considerable attention from both fundamental research and technological aspects.1 The physicochemical nature of their organic and inorganic components and the synergy between these components endow them with higher functionality and performance.2 Thus, design and construction of hierarchical structures facilitate tuning materials properties through tailoring the kinds and accessibility of functional components, curvature of interfaces, and style and degree of the internal organizations. None of these parameters can be manipulated by traditional material engineering in a single length scale.3 As a result, an ever-increasing research interest has been generated in developing a variety of approaches to construct hierarchical structures for the development of new functional materials.3,4 Up to now, many methods have been used to prepare complex hierarchical micro/nanostructures, such as hydrothermal methods,5 thermal reduction and oxidation process,6 oriented aggregation or selfassembly of building blocks,7 and template-assisted synthesis.8 As an important p-type transition-metal-oxide semiconductor with a narrow band gap (Eg ) 1.2 eV), copper oxide (CuO) has been widely exploited for a versatile range of applications such as superconductor,9 gas sensing,10 heterogeneous catalysis,11 magnetic storage media,12 field-emission sources,13 solar cell devices,14 lithium ion electrode materials,15 construction of a variety of organic-inorganic nanocomposites with unique characteristics,16 and so on. Among all these potential applications, the use of CuO as electrode materials for next generation rechargeable lithium-ion batteries have been intensively studied because of their high theoretical capacity, high safety, environmental benignity, low cost, etc.15 One of the bottlenecks restricting it from application in lithium-ion batteries is the large volume variation during the lithium uptake/release process, * To whom correspondence should be addressed. E-mail: shuyangao@ henannu.edu.cn. Phone: +86-373-3326544. Fax: +86-373-3326544. † Henan Normal University. ‡ Universite´ de Picardie Jules Verne.

which leads to severe mechanical strains and very rapid capacity decay. In this context, a great effort has been made to use nanoarchitectured electrodes for overcoming such drawback and improving the electrochemical performance.17 Hitherto, various methods, such as hydrothermal method,18 sol-gel technique,19 gas-phase oxidation,6 microemulsion,20 thermal decomposition, and template techniques, have been developed and improved for the organization of CuO micro/nanostructures with different morphologies such as nanoparticles, nanoellipsoids, nanorods, nanoneedles, nanoribbons, nanoshuttles, nanoleaves, nanotubes, three dimensional (3D) peanutlike patterns,21 pricky/layered microspheres,14 dandelion-like CuO hollow microspheres,22 and flower-shaped structures.3,23 All these techniques have indeed been demonstrated as fabrication tools for formulation of CuO nanoparticles into the manipulated position with controlled structures. But the required toxic raw materials, the removal of the template and the contamination of the byproducts limit their exploitation at the application level. Therefore, it is required to develop a clean and friendly method to synthesize complex CuO nanostructures in large-quantity under mild conditions. Nature is adept at greenly producing remarkable structures with optimization in form and property for their designated function,24 which is a great inspiration for materials scientists and chemists in the imitation of the hierarchical structures. Thus, when considering synthetic routes toward the growth of inorganic nanocrystals and subsequent organization into hierarchical and functional systems, one approach that is proving successful in addressing these problems is the use of biomolecules as templates, scaffolds, or interconnects.25 As a result, researchers in the field of nanoparticle synthesis and hierarchical assembly have turned to nature. How to utilize biomolecules’ special structures and strong assembling functions to fabricate functional nanocrystals of desired shape and to construct complicated superstructures from a single functional structure is very important in all of biology, chemistry, and materials science, but this approach is still in its infancy. Herein, we demonstrate biomolecule-assisted green method for synthesizing hierarchical hollow CuO micro/nanostructures simply by using CuSO4, NH3 · H2O, and tyrosine instead of any

10.1021/jp808545r CCC: $40.75  2008 American Chemical Society Published on Web 11/12/2008

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toxic and dangerous reagents. The complex CuO architecture shows improved electrochemical performance toward Li uptakerelease as anode materials in lithium-ion batteries, which benefits from the dual functions, a 3D current collector network and the chemical/mechanical robustness buffer, of the as-formed hierarchical CuO hollow micro/nanostructures. Experimental Section All chemical reagents were of analytical grade and used without further purification. All water used in this investigation was deionized by a Nanopure filtration system to a resistivity of 18 MΩ · cm. The preparation of tyrosine assisted hierarchical CuO micro/nanostructures is quite straightforward. One mL of 0.027 g/mL tyrosine aqueous solution and 1 mL of 28 wt % commercial ammonia were added under constant stirring, respectively, to 30 mL of 25 mM aqueous solution of CuSO4. The mixture was then transferred into a stainless steel autoclave with a Teflon liner of 50 mL capacity, and heated in an oven at 130 °C for 4 h. After the autoclave was air-cooled to room temperature unaided, a large quantity of black precipitate and a colorless supernatant were obtained. The resulting black precipitate was filtered and washed with distilled water. The product was redispersed in distilled water for further characterizations with field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) techniques to obtain detailed information on the morphology, component, and crystalline structure. The samples for characterization were prepared by adding an aliquot of the suspension on an ITO glass slide or glass slide, and allowing the solvent to slowly evaporate at room temperature. The FESEM images were obtained on an XL30 ESEM FEG scanning electron microscopy operating at 20 kV. The XRD pattern was recorded in the 2θ range of 20-70° on a RagakuD/Max 2500 V/PC X-ray diffractometer using Cu kR1 radiation (λ ) 1.54056 Å) at 40 kV and 200 mA. XPS was collected on an ESCALab MKII X-ray photoelectron spectrometer, using nonmonochromatized Mg KR X-ray as excitation source. The electrode for electrochemical testing was prepared by pasting a slurry containing 80 wt % active material, 10 wt % carbon black and 10 wt % polyvinylidene fluoride dissolved in N-methylpyrolline onto a copper foil. The slurry was uniformly spread on copper foil by a doctor blade technique. After coating, the film was dried in a vacuum oven at 120 °C for 24 h, and pressed, then cut into sheets with an area of 0.64 cm2. Two-electrode batteries for cycles were constructed in an Ar-filled glovebox using a metal lithium foil as counter electrode, 1 M LiPF6 in a 1:1 v/v mixture of ethylene carbonate and dimethyl carbonate as electrolyte and Celgard2300 polypropylene as separator. The water content in the electrolyte was low than 1 ppm. The moisture content and oxygen level of the Ar atmosphere in the glovebox were controlled under 1 ppm. Charge-discharge cycling was tested on multichannel Land Battery Test System. Results and Discussions The phase purity of the as-prepared product was determined by XRD as shown in Figure 1. All the diffraction peaks can be readily indexed to the monoclinic symmetry of CuO (space group C2/c, a ) 4.684 Å, b ) 3.425 Å, c ) 5.129 Å, β) 99.47°, JCPDS file no. 05-0661). The broadening of all the recorded peaks in the XRD pattern indicates that the component crystallites are of nanoscale character. No other impurities were detected by XRD analysis, indicating the high phase purity of the as-prepared sample. The surface and composition of the product was examined by XPS and shown in Figure 2. From

Figure 1. Typical XRD pattern of the sample.

Figure 2. Survey spectrum (a) and typical XPS spectrum of Cu 2p3/2 (b) of the sample.

the survey spectrum (Figure 2a), the peaks of Cu and O can be clearly detected. Besides them, the peak of N1s is also evident. The existence of N1s gives a solid piece of proof that the participation of biomolecule, tyrosine, in the synthesis of the CuO sample. The Cu 2p3/2 fine spectrum (Figure 2b) reveals a main peak at 934.2 eV, which is accompanied by two satellite peaks at 941.6 and 944 eV, respectively. These features correspond to a Cu2+ state for Cu atoms, which are well consistent with those observed in CuO.3,15c,26 Low-magnification FESEM observations show that the panoramic morphology of the as-obtained CuO product is in large quality and mainly composed of uniform, spheric architectures ranging from 1.5 to 3 µm in diameter (Figure 3a). The clear view (Figure 3b and c) displays that the surface of the architecture is not smooth and consists of many nanosheets with average diameter of ca. 250 nm. The examination of a crashed microsphere (Figure 3d) vividly reveals that the structure of these spheric architectures is assembled from nanosheets, selfwrapping to form hollow interiors with 1.5-3 µm in outer diameter. It is worth noting that all the constituent nanosheets

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Figure 3. FESEM images of the sample: (a) low-magnification image, (b) a single microspehere, (c) the surface of the microsphere, (d) crashed CuO micro/nanostructure, and (e) after sonication treatment. (f) FESEM image of the product in the absence of tyrosine.

are aligned without any substrate support, and the driving force originates from the biomolecule.2a,3,27 In our case, tyrosine is dispersed within liquid medium mainly as a random coil sol bearing abundant negatively charged carboxylic groups. These random coils behave as organic matrixes, binding many copper cations. The introduction of ammonia changed the pH value and caused Cu2+ ions to hydrolyze to form intermediate, which was still attached to tyrosine. Upon hydrothermal treatment, the intermediate decomposed and CuO could be formed. The gradual attachment of reactive constituents onto the nucleation centers leads to formation of relatively flat plates. With an increased growth rate, a highly curved structure may form due to the lattice tension or surface interaction.2a,3,27 As the reactant is consumed, the subsequent growth of the crystals takes an energetically favorable self-assembly of tiny sheets and is confined by the curved structure. As a result, ordered nanosheet-

based mesoporous shells are constructed. After sonication treatment of the sample for 10 min, the morphology can be well kept (Figure 3e), which shows the highly mechanical stability. In the absence of the tyrosine, the dominant morphology is mat composed of interconnected nanosheets as shown in Figure 3f. All these observations verify the formation of mechanically stable and hierarchical CuO micro/nanostructures with the help of biomolecule, tyrosine. To demonstrate the potential application of the present hierarchical CuO micro/nanostructures with high electronic conductivity, we carried out a preliminary investigation into their electrochemical performance toward Li uptake-release (Figure 4). It was found that the first discharge/charge voltage profile displayed three pseudoplateaus (2.5-2.0, 1.35-1.25, and 1.0-0.02 V vs Li+/Li, respectively) for the Li reaction with CuO, corresponding to the multistep electrochemical Li reaction

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Conclusion

Figure 4. First discharge/charge voltage profile of the present hierarchical CuO micro/nanostructures at a rate of C/10. The inset is variation in charge capacity versus cycle number for the as-prepared sample at a rate of C/10.

process or additional sites for Li uptake, whereas there are no obvious potential plateaus for Li release from the crystal lattice of CuO.15b,28 It is a typical reaction of transitional metal oxide with Li. Below 0.7 V, the potential tends to decrease gradually as the discharge depth increases. The behavior is similar to that described in the literatures.15b,c,28 The first charge capacity of hierarchical CuO micro/nanostructures is 560 mA h g-1, which is larger than that of reported CuO nanomicrospheres.15c This result indicates that the construction of hierarchical micro/ nanostructures is an effective way to improve the electrochemical performance. The improvement may be attributed to the following two factors. On one hand, the 3D configuration of hierarchical hollow micro/nanostructures can be considered as a 3D current collector network, which provides negligible diffusion times (short diffusion length), and enhanced electronic conductivity, and hence is the key to the good power performance. On the other hand, in view of the large volume expansion of CuO during Li+ uptake (CuO converts to Cu and Li2O, about a 174% volume expansion), the 3D hierarchical hollow micro/nanostructures may also be considered as an elastic buffer to relieve the strain associated with the volume variations during Li uptake-release, suppress particle pulverization, maintain electronic contact and guarantees good power performance and capacity retention.16a Unfortunately, it is reported that the initial discharge capacity of CuO is approximately 1240 mA h g-1, higher than the theoretical one (670 mA h g-1) based on a maximum uptake of 2 Li per CuO15b,28 and that of the as-prepared hierarchical CuO hollow micro/nanostructures (560 mA h g-1). Usually, the first discharge capacity of CuO considerably exceeds the nominal capacity and is ascribed to the electrolyte being reduced to form a solid electrolyte interphase (SEI) layer and organic conductive polymer,29 the reduction of the adsorbed impurities on CuO surfaces, the initial formation of lithium oxide due to the presence of some residual OH- groups in the surface of active CuO, and possibly interfacial lithium storage.30 Either electrolyte reduction decomposition or impurities will consume additional energy irreversibly during the charge/discharge process. Therefore, the first charge capacity is merely 560 mA h g-1, corresponding to 1.67 Li release. Owing to the hierarchical micro/nanostructures, favorable cycle calendar life and reversible capacity can be achieved as shown in the inset of Figure 4. It is suggested that practical electrode has outstanding chemical/mechanical robustness. Therefore, the use of this low-cost and high-performance hierarchical CuO micro/nanostructures for lithium ion batteries is feasible and promising.

In summary, a facile green synthetic route is demonstrated to construct complex CuO hollow micro/nanostructures with the use of biomolecule, tyrosine. These structures consist of CuO nanosheets self-organized into hollow micrometer-sized monoliths with a hierarchical architecture. The hierarchically hollow structures enhance mass transport in macroscales, promote the accessibility of the nanomaterials, and greatly facilitate dispersion, transportation, separation, and recycling of nanomaterials. As a demonstration, the synthesized hierarchical CuO hollow micro/nanostructures are used as anode materials in lithium-ion batteries with a high discharge/charge capacity. We believe, therefore, that the present system would be readily applicable to the creation of other novel supermicro/nanostructures and their functionalized derivatives. Acknowledgment. The authors are grateful to the National Natural Science Foundation of China (No. 20571025), the Henan Provincial Natural Science Foundation of China (082300420180), and Natural Science Foundation of Education Department of Henan Province (2008A150014) for their financial support. References and Notes (1) (a) Soler-Illia, G. J. d. A.; Sanchez, C.; Lebeau, B.; Patarin, J. Chem. ReV. 2002, 102, 4093. (b) Soten, I.; Ozin, G. A. Curr. Opin. Colloid Interface Sci. 1999, 4, 325. (c) Yuan, Z. Y.; Ren, T. Z.; Azioune, A.; Pireaux, J. J.; Su, B. L. Chem. Mater. 2006, 18, 1753. (d) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418. (e) Li, Y.; Li, C.; Cho, S. O.; Duan, G.; Cai, W. Langmuir 2007, 23, 9802. (f) Li, Y.; Cai, W. P.; Duan, G. T. Chem. Mater. 2008, 20, 615. (g) Fang, X.; Bando, Y.; Ye, C.; Shen, G.; Gautam, U. K.; Tang, C.; Golberg, D. Chem. Commun. 2007, 4093. (2) (a) Cao, A. M.; Hu, J. S.; Liang, H. P.; Song, W. G.; Wan, L. J.; He, X. L.; Gao, X. G.; Xia, S. H. J. Phys. Chem. B 2006, 110, 15858. (b) Yang, P. D.; Deng, T.; Zhao, D. Y.; Feng, P. Y.; Pine, D.; Chmelka, B. F.; Whitesides, G. M.; Stucky, G. D. Science 1998, 282, 2244. (c) Jenekhe, S. A.; Chen, X. L. Science 1999, 283, 372. (d) Fang, X. S.; Bando, Y.; Gautam, U. K.; Ye, C. H.; Golberg, D. J. Mater. Chem. 2008, 18, 509. (3) Cao, A.; Monnell, J. D.; Matranga, C.; Wu, J.; Cao, L.; Gao, D. J. Phys. Chem. C 2007, 111, 18624. (4) (a) Klajn, R.; Bishop, K. J. M.; Fialkowski, M.; Paszewski, M.; Campbell, C. J.; Gray, T. P.; Grzybowski, B. A. Science 2007, 316, 261. (b) Zhong, L. S.; Hu, J. S.; Cao, A. M.; Liu, Q.; Song, W. G.; Wan, L. J. Chem. Mater. 2007, 19, 1648. (c) Breen, T. L.; Tien, J.; Oliver, S. R. J.; Hadzic, T.; Whitesides, G. M. Science 1999, 284, 948. (d) Anandan, S.; Wen, X. G.; Yang, S. H. Mater. Chem. Phys. 2005, 93, 35. (e) Cao, B.; Cai, W.; Sun, F.; Li, Y.; Lei, Y.; Zhang, L. Chem. Commun. 2004, 1604. (5) Zheng, Y. H.; Cheng, Y.; Wang, Y. S.; Zhou, L. H.; Bao, F.; Jia, C. J. Phys. Chem. B 2006, 110, 8284. (6) (a) Zhang, J. T.; Liu, J. F.; Peng, Q.; Wang, X.; Li, Y. D. Chem. Mater. 2006, 18, 867. (b) Fang, X. S.; Ye, C. H.; Zhang, L. D.; Wang, Y. H.; Wu, Y. C. AdV. Funct. Mater. 2005, 15, 63. (7) (a) Jiang, P.; Zhou, J. J.; Fang, H. F.; Wang, C. Y.; Wang, Z. L.; Xie, S. S. AdV. Funct. Mater. 2007, 17, 1303. (b) Yu, J. G.; Yu, J. C.; Zhang, L. Z.; Wang, X. C.; Wu, L. Chem. Commun. 2004, 2414. (8) (a) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111. (b) Li, L.; Fang, X. S.; Chew, H. G.; Zheng, F.; Liew, T. H.; Xu, X. J.; Zhang, Y. X.; Pan, S. S.; Li, G. H.; Zhang, L. D. AdV. Funct. Mater. 2008, 18, 1080. (c) Li, L.; Yang, Y. W.; Li, G. H.; Zhang, L. D. Small 2006, 2, 548. (d) Li, Y.; Lee, E. J.; Cai, W.; Kim, K. Y.; Cho, S. O. ACS Nano 2008, 2, 1108. (9) (a) Schon, J. H.; Dorget, M.; Beuran, F. C.; Zu, X. Z.; Arushanov, E.; Cavellin, C. D.; Lagues, M. Nature 2001, 414, 434. (b) Hodges, J. A.; Sidis, Y.; Bourges, P.; Mirebeau, I.; Hennion, M.; Chaud, X. Phys. ReV. B 2002, 66, 020501. (c) Zheng, X. G.; Xu, C. N.; Tomokiyo, Y.; Tanaka, E.; Yamada, H.; Soejima, Y. Phys. ReV. Lett. 2000, 85, 5170. (10) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Taracon, J. M. Nature 2000, 407, 496. (11) Reitz, J. B.; Solomon, E. I. J. Am. Chem. Soc. 1998, 120, 11467. (12) (a) Kumar, R. V.; Diamant, Y.; Gedanken, A. Chem. Mater. 2000, 12, 2301. (b) Ziolo, J.; Borsa, F.; Corti, M.; Rigamonti, A.; Parmigiani, F. J. Appl. Phys. 1990, 67, 5864. (13) Zhu, Y. W.; Yu, T.; Cheong, F. C.; Xu, X. J.; Lim, C. T.; Tan, V. B. C.; Thong, J. T. L.; Sow, C. H. Nanotechnology 2005, 16, 88. (14) (a) Xu, Y.; Chen, D.; Jiao, X. J. Phys. Chem. B 2005, 109, 13561. (b) Xu, J.; Xue, D. J. Phys. Chem. B 2005, 109, 17157.

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