Syntheses, Characterizations, and Applications in Lithium Ion

Jul 17, 2009 - ... Guo , Liping Zhang , Xiaochun Wu , Lina Zhang , Zhihua Han , Guangyao Liu , Jianbo Li , Guanghui Rao , Hanfu Wang , and Weiguo Chu...
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Syntheses, Characterizations, and Applications in Lithium Ion Batteries of Hierarchical SnO Nanocrystals Jiajia Ning,†,‡ Tao Jiang,‡ Kangkang Men,† Quanqin Dai,† Dongmei Li,† Yingjin Wei,*,‡ Bingbing Liu,† Gang Chen,†,‡ Bo Zou,*,† and Guangtian Zou† State Key Laboratory of Superhard Materials and College of Materials Science & Engineering, Jilin UniVersity, Changchun 130012, China ReceiVed: April 21, 2009; ReVised Manuscript ReceiVed: June 26, 2009

Hierarchical SnO nanocrystals are synthesized by a reproducible and facile way via decomposition of an intermediate product tin oxide hydroxide, Sn6O4(OH)4. By changing the amount of injecting water, layerplate-like, nest-like, stepwise-bipyramid-like, and defective stepwise-bipyramid-like hierarchical SnO nanocrystals could be obtained. All of these hierarchical SnO nanostructures are constructed by smaller nanosheets. The driving force of aggregation is reducing the surface energy of nanocrystals. Water played a key role in the control morphologies of hierarchical SnO nanostructures. The water control decomposition (WCD) mechanism was proposed to explain the effect of water on the morphologies. On the basis of reaction kinetics, the different superfluous injected water after reaction would restrain the decomposition of Sn6O4(OH)4 to SnO nanosheets; a different amount of superfluous injected water would induce a different reaction rate. At different reaction rates, SnO nanosheets would have different sizes and different approaches to aggregation, and different hierarchical SnO nanocrystals appeared by injecting different amounts of water into the reaction. Typically, hierarchical SnO nanocrystals as an anode material for lithium ion batteries are studied. These SnO nanocrystals show good potential for lithium battery materials. Among these SnO nanostructures, the stepwise-bipyramid-like nanostructure shows the best properties. Introduction Developing a facile way to manipulate the nanoscale building blocks via self-assembly and higher-ordered organization approaches into complex hierarchical architectures is a great challenge to both material science and advanced nanodevices.1-4 Due to the size- and shape-dependent electronic and optical properties of nanostructures, much effort has been made to organize nanoparticles, nanorods, and nanowires into threedimensional (3-D) ordered hierarchical nanostructures.5-8 Lots of synthetic routes have been developed on the basis of selfassembly or deposition techniques, such as vapor-induced phase separation, micelle aggregation, nanosphere lithography, and microwave heating.9-13 The solution approach is an important one, which has been used to synthesize metal oxides, semiconductors, and noble metal nanocrystals with hierarchical structures.14-17 However, synthesis of inorganic nanocrystals with various hierarchical structures has been an open problem until now. Developing simple and facile solution routes to produce hierarchical nanostructures is very important to nanoscience and synthetic chemistry. SnO has attracted lots of attention because of its potential applications as an anode material for lithium ion batteries.18,19 SnO has a high theoretical specific capacity of 875 mAh/g, which is much higher than that of commonly used carbon anode materials. The morphology (such as size and shape) is important for the electrochemical performance of anode materials,20 so it is expected to obtain good electrochemical performance by preparation of hierarchical SnO nanocrystals. Recently, a few * Corresponding authors. E-mail: [email protected] (B.Z.); yjwei@ jlu.edu.cn (Y.W.). † State Key Laboratory of Superhard Materials. ‡ College of Materials Science & Engineering.

synthetic routes have been used to synthesize SnO nanocrystals. Wang and co-workers have synthesized SnO nanoribbons21,22 and diskettes23 by thermal evaporation; SnO dendrites could also be produced by a self-catalytic vapor-liquid-solid (VLS) process.24 By aqueous approaches, SnO with complex hierarchical structures and/or bigger size can be achieved, such as plates,25,26 meshes,27 truncated bipyramids, and stacked combs.28 SnO nanoflowers have been produced in nonaqueous solution by a free-cation-induced mechanism, and SnO nanoflowers showed excellent properties as an anode material for lithium ion batteries.29 In this paper, we developed a simple and reproducible route to synthesize novel SnO hierarchical nanostructures by direct decomposition of tin oxide hydroxide. By adjusting the amount of injecting water, layer-plate-like, nest-like, stepwise-bipyramid-like, and defective stepwise-bipyramid-like hierarchical SnO nanocrystals could be obtained. These hierarchical nanostructures were constructed by smaller single crystal nanosheets. Water played a key role in the change morphology of hierarchical SnO nanocrystals. Water control decomposition (WCD) was proposed to explain the effect of water on the morphology of SnO nanocrystals. The electrochemical properties of the hierarchical SnO nanostructures were studied. The stepwisebipyramid-like nanostructures had the best properties as an anode material for lithium ion batteries among the hierarchical SnO nanostructures. Experimental Section Chemicals. SnCl2 (98%) and 1-octadecene (ODE, 90%) were purchased from Aldrich. Oleylamine (OLA, g70%) was purchased from Fluka. Toluene was purchased from Beijing Chemical Company. All chemicals were used in the experiments without further purification.

10.1021/jp905668p CCC: $40.75  2009 American Chemical Society Published on Web 07/17/2009

Lithium Ion Batteries of Hierarchical SnO Nanocrystals Synthesis. SnCl2 (0.1517 g, 0.800 mmol), OLA (1.9224 g, 7.200 mmol), and ODE (0.9072 g, 3.600 mmol) were loaded into a three-neck flask in a glovebox. Then, the flask was sealed and taken out to connect to the Schlenk line. After a certain amount of H2O was injected, yellow turbidness was produced. When the mixture was heated to a certain temperature (90 °C), the stirred yellow turbidness quickly turned into a brown suspension. By injecting 20, 30, and 40 µL of water, layerplate-like, nest-like, and stepwise-bipyramid-like nanostructures could be obtained, respectively. When the amount of water was more than 40 µL, defective stepwise-bipyramid-like nanostructures would be produced. After 10 min, aliquots were taken from the flask and quenched by room-temperature toluene. The samples were centrifuged and dispersed in toluene for characterization. Characterization. The scanning electron microscopy (SEM) measurements were carried out with a scanning electron microscope (JEOL, JSM-6700F) operated at an acceleration voltage of 8 kV. Powder X-ray diffraction (XRD) was obtained on a Bruker D8 diffractometer operating at 40 kV and 40 mA, using a Cu KR target. Data were collected from 15 to 80° with a sampling interval of 0.02° per step and a counting rate of 0.2 s per step. Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) were obtained with a Hitachi H-8100IV transmission electron microscope using an acceleration voltage of 200 kV. High resolution transmission electron microscopy (HRTEM) images were measured via a JEM-2100 transmission electron microscope at 300 kV. As for the electrochemical measurement, a coin battery cell was used. A metallic lithium foil served as the anode electrode. The cathode electrode was composed of hierarchical SnO nanostructures (as active materials, 75 wt %), carbon black conductive additive (10 wt %), and poly-vinylidenefluoride binder (PVDF, 15 wt %). Each electrode was 8 mm × 8 mm in size and separated by two pieces of Celgard 2400 membranes. The electrolytesa 1 mol/L lithium hexafluorophosphate (LiPF6) solutionswas dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) (EC:DMC ) 1:1, by v/v ratio). The battery cell was assembled in an argon-filled glovebox with the H2O and O2 concentrations below 1 ppm. Galvanostatic chargedischarge cycling was performed on a Land automatic battery tester (Wuhan, China) at a constant current density of 100 mA/g over a voltage window of 0.01-2.0 V. Results and Discussion Morphologies of the as-prepared products were obtained by SEM. The product which has gotten by injecting 20 µL of water had a layer-plate-like structure, with 600-1000 nm in size and about 300 nm in thickness (Figure 1a and b). The layer-platelike nanostructure was built by nanosheets. When the water amount was increased to 30 µL, a nest-like nanostructure was obtained (Figure 1c and d). The nest-like nanostructure with a size of 600-800 nm was built by smaller 2D nanosheets, as shown in Figure 1d. When we injected 40 µL of water into the solution, stepwise-bipyramid-like nanostructures appeared. The bipyramid was ended by two smooth facets. This structure was formed by assembly of round 2D nanosheets step by step. The length of bipyramid-like nanocrystals was about 2 µm. We also characterized the products obtained by injecting more than 40 µL of water, such as 50, 70, and 100 µL. The morphologies of the as-prepared samples were similar to those shown in Figure 1e and f, stepwise-bipyramid-like, as shown in Figure 2. These structures had many defects compared with the stepwisebipyramid in Figure 1e, such as nonsymmetrical, built out of

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Figure 1. SEM images of SnO nanostructures: (a and b) layer-platelike nanostructures; (c and d) nest-like nanostructures; (e and f) stepwise-bipyramid-like nanostructures, according to 20, 30, and 40 µL of water, respectively.

order. Thus, these morphologies could be named as defective stepwise-bipyramid-like nanostructures. The chemical composition and crystal structure of asprepared samples were confirmed by powder X-ray diffraction, as shown in Figure 3. These well-resolved diffraction peaks in the range of 2θ ) 15-80° correspond to the (001), (101), (002), (200), (112), (211), (202), (220), and (301) crystal planes of tetragonal SnO (JCPDS no. 06-0395). From Figure 3a to 3f, the diffraction peaks became more intensive and narrower, indicating that the crystallinity of SnO nanostructures was improved by adding more water for material preparation. Nanosheets became thicker and bigger from layer-plate-like nanostructures to defective stepwisebipyramid-like nanostructures (Figures 1 and 2). From the SEM images, we can see that the nanosheet is the basic unit of all of the SnO nanostructures. Deep research on nanosheets is important for understanding the formation mechanism of hierarchical SnO nanostructures. Figure 4a gives the HRTEM image of the nanosheet. The distance of the crystallgraphic planes given by HRTEM is 0.27 nm, corresponding to the (110) lattice fringe of tetragonal SnO. Nanosheets are single crystals in nature, as shown in SAED. The structure of nanosheets given by SAED agrees well with the result of XRD. Before the SnO nanostructures were formed, as shown in the experiment, yellow turbidness was produced at the beginning.

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Figure 2. SEM images of defective stepwise-bipyramid-like SnO nanostructures. Parts a, b, and c correspond to 50, 70, and 100 µL, respectively, of injected water in the reaction.

the precursor of SnO nanocrystals. Thus, the reaction of producing SnO nanocrystals from Sn6O4(OH)4 can be described as follows:29

Figure 3. XRD patterns of hierarchical SnO nanocrystals: (a) layerplate-like nanostructures; (b) nest-like nanostructures; (c) stepwisebipyramid-like nanostructures; (d-f) defective stepwise-bipyramid-like nanostructures which correspond to 50, 70, and 100 µL of water.

Figure 4. HRTEM images of nanosheets (a) and SAED patterns of nanosheets (b).

Figure 5. SEM image of tin oxide hydroxide (a) and XRD image of tin oxide hydroxide (b).

Then, the yellow turbidness transformed to SnO nanostructures. We characterized this yellow turbidness by SEM and XRD to get its morphology and chemical composition. Figure 5a gave the SEM images of the yellow turbidness. All of these were octahedrons, and the size was about 150 nm. XRD in Figure 5b was corresponding to tin oxide hydroxide, Sn6O4(OH)4 (JCPDS no. 46-1486). The octahedral tin oxide hydroxide was

Sn2+ + H2O f Sn6O4(OH)4

(1)

Sn6O4(OH)4 f SnO + H2O

(2)

The crystal growth mechanism of hierarchical nanostructures is very complicated, and until now, this has been an open question. The selective polymer adsorption, Ostwald ripening, and oriented attachment, etc., were proposed to explain the process of forming hierarchical nanostructures.30-32 Generally, the driving force of aggregation of smaller units to hierarchical structures is reducing the high surface energy of nanocrystals. From the HRTEM images and SAED patterns of the SnO nanosheet, the (110) facets have higher energy than other facets and the growth along the 〈001〉 direction can release more energy (Gibbs-Thomson law). Thus, aggregation of nanosheets along the 〈001〉 direction by attaching (110) facets is a facile way to reduce system energy. By only tuning the amount of water in the reaction, these nanosheets would aggregate to various hierarchical SnO nanocrystals, such as layer-plate-like, nest-like, stepwisebipyramid-like, and defective stepwise-bipyramid-like. H2O played a key role in controlling the morphology of SnO nanocrystals. A water control decomposition mechanism (WCD) was proposed to explain the effect of H2O on the morphology of nanocrystals. We could understand this WCD mechanism based on the reaction kinetics from reactions 1 and 2. From reaction 1, we can calculate that the amount of water reaction with SnCl2 (0.800 mmol) completely was 19.2 µL in the experiment. The amount of water injected into the reaction was more than 19.2 µL, so the amount of H2O would be superfluous after reaction 1. Based reaction kinetics, because one of reaction product of reaction 2 was H2O, the superfluous water would restrain Sn6O4(OH)4 decompose to SnO and reduce the reaction rate of reaction 2. The more water would induce the lower reaction rate of reaction 2. SnO nanosheets would have more time to grow and aggregate by increasing injection water amount. The size change of nanosheets can be seen from SEM and XRD (Figure 1, 2 and 3). After reaction 2, the produced SnO nanosheets would instant aggregate to hierarchical structure. When 20 µL of water was injected into the reaction, most of water was consumed in reaction 1, so superfluous after reaction 1 was few. On the basis of reaction kinetics, this is an advantage to decomposition of Sn6O4(OH)4 in reaction 2 and reaction 2 would run quickly in this case. SnO nanosheets had not enough time to grow and aggregate tightly (Figure 1a and

Lithium Ion Batteries of Hierarchical SnO Nanocrystals b), which can be seen as whole nanoplates. However, when 40 µL of water was injected into the reaction, the superfluous water was much enough to reduce the rate of reaction 2 greatly. SnO would have enough time to nucleate and grow to bigger nanosheets with high crystallinity. Then, these bigger nanosheets could aggregate to a regular hierarchical structure along the 〈001〉 direction, a stepwise-bipyramid-like structure appeared, and the boundary of nanosheets can be seen clearly (Figure 1f). When the water amount injected into the reaction was further increased, the rate of reaction 2 was further reduced. SnO would grow to much bigger nanosheets, and the big size of the nanosheets would induce a bigger stereohindrance effect in the process of aggregation, which is to the disadvantage of aggregation to ordered structures. Thus, these products had many defects compared with the products obtained by injecting 40 µL of water into the reaction. The most interesting result was the nest-like structure, obtained by injecting 30 µL of water into the reaction. This can be seen as an intermediate state between layer-plate-like and stepwise-bipyramid-like structure. When 30 µL of water was injected into the reaction, only 19.2 µL of water was consumed in reaction 1. More than 10 µL of water was left in the solution, which would decrease the rate of reaction 2. Moreover, the amount of superfluous water was more than that in injecting 20 µL of water and less than that in injecting 40 µL of water, so the rate of reaction 2 would be between that in injecting 20 µL of water and 40 µL of water. SnO nanosheets would have a little time to grow and aggregate. Thus, SnO nanosheets would grow bigger than nanosheets in nanoplates and smaller than that in a stepwise-bipyramid-like structure, which can be seen in Figure 1. These bigger nanosheets would aggregate to reduce the surface energy. Therefore, this time is not enough for aggregation of nanosheets step by step to regular structure, such as layer-plate-like and stepwise-bipyramid-like structure. Nest-like structure SnO nanocrystals were produced, which was with a certain degree of regular. From Figure 1c and d, we can observe that nanosheets in nest-like structure have a preferential direction, which have a constant angle with (110) facets of nanosheets; this aggregation of nanosheets along a preferential direction can also reduce the surface energy of nanosheets. In recent years, metal oxides with nanostructures have attracted much attention for lithium ion battery materials,33-35 such as MnO, Fe3O4, and TiO2. All of them showed lots of advantages as nanoelectrodes for lithium batteries, for example, higher capacity and longer cycle life. SnO is also an excellent anode material for lithium batteries because Li and Sn can form reversible alloy.36 A few studies on SnO as an anode material for lithium batteries have been reported, which showed good potential applications for lithium battery materials.29,37,38 First, discharge-charge curves of the as-prepared SnO nanostructures are shown in Figure 6. Stepwise-bipyramid-like structures have the highest first discharge capacity of 535.3 mAh/g compared to that of layer-plate-like and nest-like structure, which are 460 and 342 mAh/g, respectively. Furthermore, the Coulombic efficiency of stepwise-bipyramid-like structure could achieve 55.1%. The irreversible discharge capacity in the first cycle is mainly attributed to formation of irreversible Li2O and volume expansion when SnO became LixSn (0 e x e 4.4) alloy in the electrochemical process.33-36 Figure 7 gives the cycling behavior of layer-plate-like, nest-like, and stepwise-bipyramid-like SnO nanocrystals. The stepwise-bipyramid-like SnO nanostructures show the best electrochemical properties; the discharge capacity reaches 82.1% after 10 cycles which is higher than that of the layer-plate-like and nest-like structures. These different results

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Figure 6. First discharge-charge curves of layer-plate-like nanostructures, nest-like nanostructures, and stepwise-bipyramid-like SnO nanostructures between 0.02 and 3.0 V vs Li/Li+ at a contstant current density of 100 mAh/g.

Figure 7. Cycling behavior for electrodes of layer-plate-like nanostructures, nest-like nanostructures, and stepwise-bipyramid-like SnO nanostructures.

in electrochemistry show that morphology and structure greatly affect the electrochemical performance of nanocrystals. Maybe the particular stepwise-layer structure in stepwise-bipyramidlike nanocrystals is more favorable for improving electrochemical property than others because these stepwise-bipyramid-like structures have shorter path lengths for Li+ transport and more sufficient contact with conductive carbon during preparation of the electrode. Otherwise, a solid electrolyte interface (SEI) layer would be formed on the surface of the anode material, and a too thick SEI layer would block the electron transfer which is harmful for the material to obtain a high reversible capacity.35,36 Thus, the SEI layer with different thickness for layer-plate-like, nest-like, and stepwise-bipyramid-like nanostructures would be the other reason for different Coulombic efficiencies. More detailed research on reasons for the different electrochemical performances of these SnO nanocrystals is undergoing in the laboratory. Conclusion SnO nanocrystals with hierarchical structure were synthesized by a simple and reproducible way. By turning the amount of injecting water, layer-plate-like, nest-like, stepwise-bipyramidlike, and defective stepwise-bipyramid-like hierarchical SnO nanocrystals were produced. These complex nanostructures were

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built by nanosheets. By aggregation of nanosheets along the higher energy direction, the energy of the system was reduced; this is the drive force of aggregation of nanosheets to hierarchical structure. The amount of water was very important to the morphology of SnO nanocrystals. The water control decomposition (WCD) mechanism was proposed to explain this phenomenon. When the injecting water was more than needed in reaction 1, the superfluous water appeared, which would induce the slower decomposition rate of Sn6O4(OH)4 to SnO. The different reaction rate would make different sizes of nanosheets and different ways to aggregate, and various hierarchical nanostructures appeared by tuning the amount of water in the reaction. These hierarchical SnO nanostructures showed good potential as anode materials for lithium ion batteries. Among these nanostructures, the stepwise-bipyramid-like SnO nanostructures gave the best properties for lithium ion batteries because their particular structure was favorable for electrochemical properties. Both the structure-dependent property and the solid electrolyte interface (SEI) layer would be the main reason for the different electrochemical performances of these hierarchical SnO nanostructures. Acknowledgment. This work was supported by NSFC (Nos. 20773043 and 10674053), PCSIPT (IRT0625), NCET-06-0313, RFDP (No. 20060183073), the National Basic Research Program of China (Nos. 2005CB724400 and 2007CB808000), the Cultivation Fund of the Key Scientific and Technical Innovation Project of MOE of China, and the Postgraduate Innovative Foundation Program of Jilin University (20081221). References and Notes (1) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418. (2) Garcia-Ruiz, J. M.; Welham, N. J. Science 2003, 302, 1194. (3) Fan, H.; Yang, K.; Boye, S. T. Science 2004, 304, 567. (4) Park, S.; Lim, J. H.; Chung, S. W.; Mirkin, C. A. Science 2004, 303, 348. (5) Yang, H. G.; Zeng, H. C. Angew. Chem., Int. Ed. 2004, 43, 5930. (6) Yuan, J. K.; Li, W. N.; Gomez, S.; Suib, S. L. J. Am. Chem. Soc. 2005, 127, 14184. (7) Li, W. N.; Zhang, L. C.; Sithambaram, S.; Yuan, J. K.; Shen, X. F.; Aindow, M.; Suib, S. L. J. Phys. Chem. C 2007, 111, 14694. (8) Zhou, X. F.; Hu, Z. L.; Fan, Y. Q.; Chen, S.; Ding, W. P.; Xu, N. P. J. Phys. Chem. C 2008, 112, 11722. (9) Erbil, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Science 2003, 299, 1377.

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