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Langmuir 2009, 25, 1818-1821
Facile Synthesis of Tin Oxide Nanoflowers: A Potential High-Capacity Lithium-Ion-Storage Material Jiajia Ning,†,‡,§ Quanqin Dai,†,‡,⊥ Tao Jiang,§ Kangkang Men,‡ Donghua Liu,‡ Ningru Xiao,‡ Chenyuan Li,| Dongmei Li,‡ Bingbing Liu,‡ Bo Zou,*,‡ Guangtian Zou,‡ and William W. Yu*,⊥ State Key Laboratory of Superhard Materials, Department of Materials Science and Engineering, and Department of Physics, Jilin UniVersity, Changchun 130012, People’s Republic of China, and Department of Chemistry and Biochemistry, Worcester Polytechnic Institute, Worcester, Massachusetts 01609 ReceiVed NoVember 12, 2008. ReVised Manuscript ReceiVed December 3, 2008 A facile and reproducible approach was reported to synthesize nanoparticle-attached SnO nanoflowers via decomposition of an intermediate product Sn6O4(OH)4. Sn6O4(OH)4 formed after introducing water into the traditional nonaqueous reaction, and then decomposed to SnO nanoflowers with the help of free metal cations, such as Sn2+, Fe2+, and Mn2+. This free cation-induced formation process was found independent of the nature of the surface ligand. It was demonstrated further that the as-prepared SnO nanoflowers could be utilized as good anode materials for lithium ion rechargeable batteries with a high capacity of around 800 mA h g-1, close to the theoretical value (875 mA h g-1).
Introduction Particular attention has been paid to the development of metal oxide nanocrystals with defined sizes and shapes in recent years.1-10 During this development, the unique optical, electronic, and magnetic properties of metal oxide nanocrystals have been utilized for the practical applications, such as transparent electrodes,11 catalysis,12 and gas sensors.13 Such applications mainly focus on metal oxide nanocrystals with zero- or onedimensional (0D or 1D) structures, because of the well-developed ability to control their size and size distribution. Comparatively less is known about three-dimensional (3D) metal oxide nanocrystals, accompanied by the synthetic complexity and difficulty. As one of these complicated 3D structures, metal oxide nanoflowers have recently attracted great interest for their unique technical potential that is inaccessible with the 0D or 1D * Corresponding author. E-mail:
[email protected] (B.Z.); wyu@ wpi.edu (W.W.Y.). † These authors contributed equally to this work. ‡ State Key Laboratory of Superhard Materials, Jilin University. § Department of Materials Science and Engineering, Jilin University. | Department of Physics, Jilin University. ⊥ Worcester Polytechnic Institute. (1) Hyeon, T.; Lee, S. S.; Park, J.; Chung, Y.; Na, H. B. J. Am. Chem. Soc. 2001, 123, 12798–12801. (2) Sun, S.; Zeng, H. J. Am. Chem. Soc. 2002, 124, 8204–8205. (3) Jana, N. R.; Chen, Y.; Peng, X. G. Chem. Mater. 2004, 16, 3931–3935. (4) Yu, W. W.; Falkner, J. C.; Yavuz, C. T.; Colvin, V. L. Chem. Commun. 2004, 2306–2307. (5) Narayanaswamy, A.; Xu, U. F.; Pradhan, N.; Kim, M.; Peng, X. G. J. Am. Chem. Soc. 2006, 128, 10310–10319. (6) Narayanaswamy, A.; Xu, U. F.; Pradhan, N.; Kim, M.; Peng, X. G. Angew. Chem., Int. Ed. 2006, 45, 5361–5364. (7) Niederberger, M.; Garnweitner, G. Chem.-Eur. J. 2006, 12, 7282–7302. (8) Jun, Y.; Choi, J.; Cheon, J. Angew. Chem., Int. Ed. 2006, 45, 3414–3439. (9) Chen, Y. F.; Johnson, E.; Peng, X. G. J. Am. Chem. Soc. 2007, 129, 10937– 10947. (10) Zhou, H. P.; Zhang, Y. W.; Mai, H. X.; Sun, X.; Liu, Q.; Song, W. G.; Yan, C. H. Chem.-Eur. J. 2008, 14, 3380–3390. (11) Presley, R. E.; Munsee, C. L.; Park, C. H.; Hong, D.; Wager, J. F.; Keszler, D. A. J. Phys. D: Appl. Phys. 2004, 37, 2810–2813. (12) Fierro, J. L. G. Metal Oxides: Chemistry and Applications; CRC Press: Boca Raton, FL, 2006. (13) Pinna, N.; Neri, G.; Antonietti, M.; Niederberger, M. Angew. Chem., Int. Ed. 2004, 43, 4345–4349.
counterpart.14-19 For instance, metal oxide nanoflowers were reported to be ideal structures for fabrications of high-rate electrochemical capacity in energy storage applications.19 Facile preparation of the complex 3D nanoflowers, however, still retains a great challenge in the field of materials science.20 Recently, Peng and co-workers reported the formation of In2O3, MnO, CoO, and ZnSe nanoflowers via limited ligand protection (LLP).5,6 When ligands were not enough to protect all of the nanoparticles, the individual nanoparticle became unstable and aggregated into 3D nanoflowers. Yan and co-workers subsequently synthesized CeO2 nanoflowers using a mechanism similar to that of Peng et al.10 Additionally, we observed a different formation process of ZnSe nanoflowers, which was based on the transformation from mononuclear complexes to polynuclear ones.21 These formation mechanisms, however, could not be utilized to explain the formation of SnO nanoflowers presented here. As one of the most important metal oxides, SnO is known as an excellent anode material for lithium ion rechargeable batteries.22,23 Its theoretical capacity was estimated to be as high as 875 mA h g-1, much higher than those of graphite (372 mA h g-1) and SnO2 (783 mA h g-1).22 Although nanostructured SnO crystals are difficult to synthesize due to their easy transformation into SnO2 by oxidization, some successful approaches have been reported for the preparation of SnO crystals (14) Chen, A. C.; Peng, X. S.; Koczkur, K.; Miller, B. Chem. Commun. 2004, 1964–1965. (15) Sun, X. H.; Lam, S.; Heigl, F.; Jrgensen, A.; Wong, N. B. J. Phys. Chem. B 2005, 109, 3120–3125. (16) Fang, X. S.; Ye, C. H.; Xie, T.; Wang, Z. Y.; Zhao, J. W.; Zhang, L. D. Appl. Phys. Lett. 2006, 88, 013101. (17) Du, J.; Yang, M.; Cha, S. N.; Rhen, D.; Kang, M.; Kang, D. J. Cryst. Growth Des. 2008, 8, 2312–2317. (18) Wei, Y.; Klajn, R.; Pinchuk, A. O.; Grzybowski, B. A. Small 2008, 4, 1635–1639. (19) Zhang, H.; Cao, G. P.; Wang, Z. Y.; Yang, Y. S.; Shi, Z. J.; Gu, Z. N. Nano Lett. 2008, 8, 2664–2668. (20) Wang, Z. L. AdV. Mater. 2003, 15, 432–436. (21) Dai, Q. Q.; Xiao, N. R.; Ning, J. J.; Li, C. Y.; Li, D. M.; Zou, B.; Yu, W. W.; Kan, S. H; Chen, H. Y.; Liu, B. B.; Zou, G. T. J. Phys. Chem. C 2008, 112, 7567–7571. (22) Idota, Y.; Kubota, T.; Matsufuji, A.; Maekawa, Y.; Miyasaka, T. Science 1997, 276, 1395–139. (23) Aurbach, D.; Nimberger, A.; Markovsky, B.; Sominski, E.; Gedanken, A. Chem. Mater. 2002, 14, 4155–4163.
10.1021/la8037473 CCC: $40.75 2009 American Chemical Society Published on Web 12/24/2008
Facile Synthesis of Tin Oxide Nanoflowers
with specific morphologies, such as diskettes,24 ribbons,25,26 belts,27 plates,28 meshes,29 and bipyramids.30 These structures, however, are of large size and/or are polycrystalline. In this work, we report a facile and reproducible approach for the preparation of nanoparticle-attached SnO nanoflowers via free cation-induced decomposition of an intermediate product tin oxide hydroxide (Sn6O4(OH)4). This free cation inducement, demonstrated by Sn2+, Fe2+, and Mn2+, was found independent of the nature of the surface ligand. The as-synthesized SnO nanoflowers were further observed to be good anodes for high-capacity lithium ion rechargeable batteries.
Experimental Section Chemicals. SnCl2 (98%), FeCl2 (99.9%), MnCl2 (99+%), and 1-octadecene (ODE, 90%) were purchased from Aldrich. Oleylamine (OLA, g70%), nonylamine (NLA, g97.0%), dodecylamine (DDA, g99.5%), and hexadecylamine (HDA, g99.0%) were purchased from Fluka. Toluene was purchased from Beijing Chemical Co. All chemicals were used in the experiments without further purification. Experiment 1 (Synthesis of SnO Nanoflowers). Typically, SnCl2 (0.1517 g, 0.800 mmol), OLA (1.9224 g, 7.200 mmol), and ODE (0.9072 g) were loaded into a three-neck flask in a glovebox. Next, the sealed flask was taken out and connected to the Schlenk line with nitrogen flow. After a certain amount (5-19 µL) of H2O was injected into the flask, yellow turbidness could be observed at room temperature. This turbidness characterized below was found to be Sn6O4(OH)4. When heated to a certain temperature (90-110 °C), the stirred Sn6O4(OH)4 mixture quickly turned slurry-like, meaning that SnO nanoflowers formed. At different reaction moments, aliquots were taken from the flask and were immediately quenched by equal volume of room-temperature toluene. The aliquots were centrifuged and then redispersed in toluene for characterization. In the above reaction, OLA was the ligand. Additionally, reactions with NLA, DDA, or HDA as the ligand were carried out as well, where SnO nanoflowers could be observed. Experiment 2 (Effect of Free Sn2+, Fe2+, or Mn2+ Cations on Decomposing Sn6O4(OH)4 to SnO). (A) 0.0625 mmol of Sn6O4(OH)4 was mixed with OLA (1.9224 g, 7.200 mmol) and ODE (0.9072 g), where no additional metal cations were added. This stirred mixture was gradually heated to 200 °C under nitrogen flow, but no slurry-like SnO nanoflowers appeared. (B) The only difference between this experiment and experiment 2A was that additional Sn2+ cations (0.425 mmol of SnCl2) were added into the mixture of Sn6O4(OH)4, OLA, and ODE. When the total mixture stirred with a magnetic stirrer was heated to 90 °C, slurry-like SnO nanoflowers were observed. (C) Instead of Sn2+ cations, Fe2+ cations (0.425 mmol of FeCl2) were added into the mixture of Sn6O4(OH)4, OLA, and ODE with the same amounts as in experiment 2A. When the temperature of the total stirred mixture reached 120 °C, slurry-like SnO nanocubes appeared. (D) In this experiment, 0.425 mmol of MnCl2 was used to replace the Sn2+ cations in experiment 2B. Slurry-like SnO nanocubes formed at 120 °C. Characterization. Powder X-ray diffraction (XRD) measurements were performed on a Bruker D8 diffractometer with a Cu KR target, operating at 40 kV and 40 mA. XRD data were collected from 15° to 80° with a sampling interval of 0.02° per step and a counting rate of 0.5 s per step. Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) images were obtained (24) Dai, Z. R.; Pan, Z. W.; Wang, Z. L. J. Am. Chem. Soc. 2002, 124, 8673– 8680. (25) Wang, Z. L.; Pan, Z. W. AdV. Mater. 2002, 14, 1029–1032. (26) Dai, Z. R.; Gole, J. L.; Stout, J. D.; Wang, Z. L. J. Phys. Chem. B 2002, 106, 1274–1279. (27) Orlandi, M. O.; Leite, E. R. J. Phys. Chem. B 2006, 110, 6621–6625. (28) Uchiyama, H.; Ohgi, H.; Imai, H. Cryst. Growth Des. 2006, 6, 2186– 2190. (29) Uchiyama, H.; Imai, H. Cryst. Growth Des. 2007, 7, 841–843. (30) Uchiyama, H.; Imai, H. Langmuir 2008, 24, 9038–9042.
Langmuir, Vol. 25, No. 3, 2009 1819 using a Hitachi H-8100IV transmission electron microscope at 200 kV. High-resolution transition 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. While a metallic lithium foil served as the anode electrode, the cathode electrode was composed of the as-prepared SnO nanoflowers (as active materials, about 4 mg, 75 wt %), carbon black conductive additives (10 wt %), and poly vinylidenefluoride binders (PVDF, 15 wt %). Each electrode was 8 mm × 8 mm in size and separated by two pieces of Celgard 2400 membranes. The electrolyte, a 1 mol/L lithium hexafluorophosphate (LiPF6) solution, was 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, where the H2O and O2 concentrations were below 1 ppm. The charge-discharge cycleability was measured on a Land automatic battery tester (Wuhan, China) at a constant current of 0.1 C over a voltage window of 0.01-2.0 V.
Results and Discussion On the basis of the amount of H2O added in the reactions, we named the as-prepared SnO nanoflowers as nanoflower-9, nanoflower-13, and nanoflower-17, meaning that 9, 13, and 17 µL of H2O were injected during the synthesis, respectively. The shape and size of all of these nanoflowers were characterized by TEM and HRTEM. The average diameters of nanoflower-9, nanoflower-13, and nanoflower-17 are 50, 70, and 110 nm, respectively (Figure 1), indicating that the overall size of these nanoflowers is extremely sensitive to the H2O amount. More H2O added in the reaction can generate larger SnO nanoflowers. As shown in Figure 1d-f, each individual nanoflower consists of many closely attached nanoparticles. This close attachment can be clearly observed in HRTEM images, where both nanoparticles with different orientations and boundaries among nanoparticles are visible in an entire nanoflower (Figure S1). Part of an individual nanoflower is illustrated in Figure 2a. The measured spacing of the crystallographic planes is 0.27 nm (Figure 2b), corresponding to the (110) lattice fringe of the tetragonal SnO. Figure 2c gives a SAED image of the as-synthesized SnO nanoflowers with highly crystalline structures. Five diffraction rings are shown, corresponding to (110), (200), (211), (113), and (301) planes of tetragonal SnO. The structure given by SAED is consistent with the XRD results. As shown in Figure 3, the well-resolved peaks in the XRD pattern correspond to (101), (110), (002), (200), (112), (211), and (202) crystal planes of the tetragonal SnO (JCPDS no. 06-0395). These peaks are relatively broad, indicative of small crystalline particles in the as-prepared SnO nanoflowers. From nanoflower-9 to nanoflower-17, the XRD patterns become more intensive and narrower, which shows that the crystallinity of SnO can be improved by adding more water in the reaction. The crystalline sizes of nanoflower-9, nanoflower13, and nanoflower-17, calculated by the Debye-Scherrer equation, are 8.7, 16.1, and 18.2 nm, respectively. These calculated sizes are consistent with the ones obtained from the TEM results (Figure 1d-f). As discussed above, the final slurry-like products in the reaction flask were confirmed to be SnO nanoflowers, which consist of many attached crystalline SnO nanoparticles. Before obtaining the slurry-like SnO nanoflowers, we observed yellow turbidness formed in the typical experiment. This yellow turbidness was detected to be tetragonal tin oxide hydroxide, Sn6O4(OH)4, as shown in Figure 4a (JCPDS no. 46-1486). The pure Sn6O4(OH)4 could be obtained before it transformed into slurry-like SnO nanoflowers. During the transformation, the coexistence of Sn6O4(OH)4 and SnO was observed (Figure 4b). Pure SnO products were obtained only after the yellow turbidness completely disappeared (Figure 4c). All of these clearly showed
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Figure 1. TEM images of nanoflower-9 (a and d), nanoflower-13 (b and e), and nanoflower-17 (c and f).
Figure 4. XRD patterns of Sn6O4(OH)4 (a), the transition stage from Sn6O4(OH)4 to SnO (b), and SnO nanoflowers (c). Figure 2. HRTEM (a and b) and SAED (c) of SnO nanoflowers.
Figure 3. XRD patterns of nanoflower-9 (a), nanoflower-13 (b), and nanoflower-17 (c).
that the SnO nanoflowers were derived from the decomposition of Sn6O4(OH)4. We further found that Sn6O4(OH)4 could not decompose to form nanoflower-shaped SnO products if no free metal cations existed in the reaction flask (Figures 5a and S2), where 0.0625 mmol of Sn6O4(OH)4 was mixed with OLA (1.9224 g, 7.200 mmol) and ODE (0.9072 g). Although this mixture was heated to 200 °C, no slurry-like SnO nanoflowers were observed. If free
Sn2+ cations derived from SnCl2 (0.425 mmol) were added into the above mixture of Sn6O4(OH)4, OLA, and ODE, SnO nanoflowers could be achieved by decomposition of Sn6O4(OH)4 (Figure 5b). This free cation-induced decomposition could be used to explain the formation of SnO nanoflowers in the typical experiment with 0.800 mmol of SnCl2 as the starting Sn2+ material. 0.800 mmol of Sn2+ could stoichiometrically react with 19.2 µL of H2O to form Sn6O4(OH)4. Therefore, when less than 19.2 µL of H2O was added into the reaction, there remained free Sn2+ cations after the formation of Sn6O4(OH)4. These unconverted Sn2+ cations induced the decomposition of Sn6O4(OH)4 to form SnO nanoflowers, such as nanoflower-9. In addition to the Sn2+ cation, other free cations (such as Fe2+ and Mn2+) could also cause the intermediate Sn6O4(OH)4 to decompose and form nanostructured SnO. As shown in Figure 5c and d, SnO nanocubes were produced with the existence of free Fe2+ or Mn2+ cations. Water is also important to formation of SnO nanoflowers, which provided oxygen to SnO by reacting with SnCl2 to produce Sn6O4(OH)4. Water did not take part in the decomposition of Sn6O4(OH)4, as it has been used up in the formation of Sn6O4(OH)4. Instead of OLA, other ligands (NLA, DDA, and HDA) with free Sn2+ cations available in the reactions were used to test both the decomposition of the intermediate Sn6O4(OH)4 and the formation of SnO nanoflowers (Figure S3). We also studied the role of ligand. Experiments showed that the tin oxide
Facile Synthesis of Tin Oxide Nanoflowers
Langmuir, Vol. 25, No. 3, 2009 1821
Figure 6. Charge-discharge capacity of SnO nanoflowers as anode materials for lithium ion rechargeable batteries.
Figure 5. TEM images of the products obtained in experiment 2A with no free metal cations available (a), experiment 2B with free Sn2+ cations available (b), experiment 2C with free Fe2+ cations available (c), and experiment 2D with free Mn2+ cations available (d). Scheme 1. Free Cation-Induced Formation of SnO Nanocrystals, Mn+ ) Sn2+, Fe2+, or Mn2+
nanoflowers for their applications in lithium ion rechargeable batteries. The preliminary results showed that our as-prepared SnO nanoflowers exhibited great potential as anode materials for lithium ion rechargeable batteries (Figure 6). Upon the charge-discharge cycleability, the initial charge capacity reached about 1350 mA h g-1 and then quickly decayed to ∼900 mA h g-1. The highest discharge capacity was about 800 mA h g-1, close to the reported theoretical value (875 mA h g-1).22 This high capacity might be attributed to the stable flower-like SnO nanostructure with high surface area.19 After 20 cycles, the capacity could retain 450 mA h g-1.
Conclusion
hydroxide would not be produced without ligand. These results showed that the free cation inducement is independent of the ligands and plays a key role in the formation of SnO nanocrystals by decomposition of Sn6O4(OH)4 (Scheme 1), and also aggregation of nanoparticles to nanoflowers to minimize the surface energy of nanocrystals is favorable in thermodynamics.5,6,8,10 Because free cations (Fe2+, Mn2+) were not found in the products, we think the free cations might be catalyst to the decomposition of tin oxide hydroxide. However, the detailed process of producing SnO nanocrystals with free cations available is not clear. This formation mechanism is obviously different from either the previous limited ligand protection mechanism5,6 or the polynuclear mechanism21 of nanoflowers. More detailed research on the new mechanism is undergoing in the laboratory. Complex 3D nanostructures of highly crystalline metal oxide semiconductors have been demonstrated to greatly affect their properties and performance.31,32 Therefore, it is important to morphologically control the preparation of crystalline SnO (31) Kuang, Q.; Jiang, Z. Y.; Xie, Z. X.; Lin, S. C.; Lin, Z. W.; Xie, S. Y.; Huang, R. B.; Zheng, L. S. J. Am. Chem. Soc. 2005, 127, 11777–11784. (32) Ding, Y. S.; Shen, X. F.; Gomez, S.; Luo, H.; Aindow, M.; Suib, S. L. AdV. Funct. Mater. 2006, 16, 549–555.
A simple route has been introduced to synthesize nanoparticleattached SnO nanoflowers via free cation-induced decomposition of the intermediate Sn6O4(OH)4. The free cation-induced formation of SnO nanoflowers was demonstrated by Sn2+, Fe2+, and Mn2+ cations and appeared independent of the nature of the surface ligand. Further experimental results showed that the assynthesized SnO nanoflowers could be potentially applied as good anodes for high-capacity lithium ion rechargeable batteries. Acknowledgment. This work was supported by NSFC (nos. 20773043 and 10674053), PCSIPT (IRT0625), NCET-06-0313, RFDP (no. 20060183073), the Key Research Program of the Education Ministry of China (no. 03057), the National Basic ResearchProgramofChina(nos.2005CB724400,2007CB808000), the Cultivation Fund of the Key Scientific and Technical Innovation Project of MOE of China, the Postgraduate Innovative Foundation Program of Jilin University, and the Worcester Polytechnic Institute. Supporting Information Available: HRTEM images of an entire SnO nanoflower; XRD patterns of Sn6O4(OH)4 and products of decomposing Sn6O4(OH)4 only in OLA and ODE; and TEM images of SnO nanocrystals obtained with NLA, DDA, and HAD as ligand. This material is available free of charge via the Internet at http://pubs.acs.org. LA8037473
