J. Phys. Chem. C 2007, 111, 14067-14071
14067
Multilayered Nanocrystalline SnO2 Hollow Microspheres Synthesized by Chemically Induced Self-Assembly in the Hydrothermal Environment Han X. Yang, Jiang F. Qian, Zhong X. Chen, Xin P. Ai, and Yu L. Cao* Department of Chemistry, Wuhan UniVersity, Wuhan, 430072, China ReceiVed: May 30, 2007; In Final Form: August 1, 2007
Multilayered nanocrystalline SnO2 hollow microspheres (HS-SnO2) were synthesized by chemically induced self-assembly in the hydrothermal environment. First, multilayered spherical SnO2-C composite was produced through a condensation polymerization and carbonization of sucrose accompanied by hydrolysis of SnCl4 in the hydrothermal reaction. Then, the HS-SnO2were obtained by removal of carbon via calcination. The as-prepared HS-SnO2 exhibit good electrochemical performance as the anode material in lithium ion batteries. Since this synthetic route is simple, convenient, and “green”, it is possible to extend this synthetic method to preparation of a wide range of the multilayered hollow spheres of metal oxides for functional material applications.
Introduction
Experimental Section
Tin oxide with an n-type wide-band gap (Eg ) 3.6 eV) is a functional material of great interest, which has been widely applied in gas sensors,1-3 optoelectronic devices,4 catalysis,5 and electrochemical energy storage. Recently, it was shown that the performances of SnO2 in these applications intimately depend on its structural features, such as nanoparticles,6,7 nanorods/belts/arrays,8-10 nanotubes,11 hollow spheres,12,13 and mesoporous structures.14,15 Particularly, hollow structures have attracted considerable attention because of their improvable performance, such as large surface area, efficient catalytic activity, and structural stability. Several methods have recently been reported to prepare SnO2 hollow spheres, including Localized Ostwald ripening,16 differential diffusion (nanoscale kirkendall effect),17,18 the “postsynthesis” encapsulation method,19 removable templates,20-22 and sacrificial carbon core generated by hydrothermal treatment of aqueous solutions of glucose and polysaccharides.23-25 However, only single-shelled hollow spheres are obtained by the above-mentioned methods. Recently, Lou and co-workers26 have successfully prepared the double-shelled SnO2 hollow colloids by shell-by-shell hydrothermal deposition on silica nanotemplates. In the present contribution, we report a novel approach for preparation of multilayered hollow microspheres composed of nanocrystalline SnO2 by chemically induced self-assembly in the hydrothermal environment. To form the multilayered SnO2 structure, the aqueous solution of SnCl4 is directly added in the aqueous solution of sucrose and then hydrothermally treated at 190 °C. This hydrothermal reaction produces a spherical SnO2-C composite through a condensation polymerization and carbonization of sucrose and concurrent hydrolysis of SnCl4 and then the pure SnO2 hollow spheres are formed by removal of carbon via high-temperature calcinations. To evaluate the electrochemical characteristics of this material, we tested the multilayered SnO2 hollow spheres (HS-SnO2) as lithium storage anodes for secondary Li batteries.
The HS-SnO2were prepared by using a simple hydrothermal method in aqueous sucrose/SnCl4 solution via chemically induced self-assembly. The typical experimental procedure is first to dissolve 8.76 g of SnCl4‚5H2O (analytical purity, 99.0%) and 8.56 g of sucrose (analytical purity) in distilled water to obtain 50 mL of mixture. The mixture was then transferred into a 60 mL Teflon-lined stainless steel autoclave. Then the autoclave was sealed and maintained at 190 °C for 24 h. After the reaction was complete, the autoclave was cooled naturally to room temperature. The resulting black precipitate was filtered and washed with distilled water and ethanol, and finally dried in a vacuum oven at 80 °C for 6 h. The product as treated is called the tin oxide-carbon composites. Subsequently, the tin oxide-carbon composites were calcined in air at 600 °C for 3 h to burn off carbon, leaving white multilayered SnO2 hollow microspheres (HS-SnO2). The tin oxide-carbon composites and HS-SnO2 were characterized with use of a Scanning Electron Microscope (Sirion 2000, FEI), Transmission Electron Microscopy (JEM2010, 200kV), and an X-ray powder diffraction (Shimadzu XRD-6000 diffractometer with Cu KR). Nitrogen adsorptiondesorption isotherms were determined with an ASAP 2020 (Micromeritics Instruments). Surface-area determination and pore analysis were performed by using the BET method and the BJH method. Thermogravimetric (TG) measurement was conducted on a TGA Q500 thermogravimetric analyzer (TA Instruments) in air at a scan rate of 10 deg/min from room temperature to 800 °C. The electrochemical performance of the HS-SnO2 as anode material was carried out with use of a three-electrode cell of a sandwiched design. The working electrode with a geometrical area of 2 cm2 consisted of 80% SnO2 microspheres, 12% acetylene black, and 8% polytetrafluoroethylene (PTFE) by weight and prepared by mixing the HS-SnO2, acetylene black, and PTFE emulsion with isopropanol to form an electrode paste, then rolling the paste into an ca. 0.1 mm- thick film and finally pressing the electrode film onto a nickel net. Lithium foil served as the counter and reference electrodes. The electrolyte was 1 mol‚L-1 LiPF6 dissolved in a mixed solution of ethylene
* Address correspondence to this author. E-mail:
[email protected]. Phone: 086-27-68754526. Fax: 086-27-87884476.
10.1021/jp074159a CCC: $37.00 © 2007 American Chemical Society Published on Web 09/06/2007
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Figure 1. SEM images of (a) the tin oxide-carbon composites, (b) the HS-SnO2 after calcination, (c) the magnified image of a single SnO2 hollow sphere, and (d) the magnified image of the surface of the SnO2 sphere.
carbonate (EC)-dimethyl carbonate (DMC)-ethylene methyl carbonate (EMC) (1:1:1, by weight). All the cells were assembled in a drybox filled with argon gas. The galvanostatic charge-discharge test was conducted with a BTS-55 Neware Battery Testing System. Results and Discussion Figure 1 shows the SEM images of the composite SnO2/C particles synthesized hydrothermally before and after calcinations. As shown in Figure 1a,b, the particles exhibit a uniform and rather spherical shape with diameters in the range of 2-3 and 1-2 µm, respectively. The diameter of SnO2 hollow spheres (Figure 1b) is distinctly less than that of the composite SnO2/C spheres (Figure 1a) due to removal of carbon and polymeric molecules in the SnO2 spheres. There is an open hole in the surface of the SnO2 spheres because of inner gas evacuation during calcination (Figure 1c). Through the hole, the inner hollow structure and inside smaller SnO2 sphere can be clearly seen. Though additional images inside the SnO2 sphere cannot be observed, it is speculated that the interior of the second SnO2 sphere could also be a hollow structure. So the as-prepared SnO2 spheres should have a multilayered hollow structure. As can be seen in Figure 1c, the thickness of the outer SnO2 shell is about 100 nm. These sphere shells are composed of aggregated nanoparticles of SnO2 with diameters of ∼10 nm (Figure 1d). The XRD analysis of the as-prepared products indicates that all the diffraction peaks can be very well indexed to a rutile structure of SnO2 (space group P42/mnm (136), a ) b ) 4.73397 Å, c ) 3.18693 Å, Joint Committee on Powder Diffraction Standards (JCPDS) No. 41-1445) (Figure 2), indicating that the HS-SnO2 has a perfect rutile crystalline lattice. By using Scherer’s formula,27 the mean crystalline size of the SnO2 nanoparticles is calculated to be about 10 nm, agreeing with the observation from Figure 1d. To obtain insight into the morphology of the HS-SnO2, we used TEM to visualize the detailed structure of the as-prepared SnO2 spheres. As shown in Figure 3a, the micrograph shows that every sphere has a hollow structure with a smaller sphere inside, in agreement with the SEM pattern in Figure 1c. The TEM image of a single sphere (Figure 3b) shows that there is a distinguishable sphere structure inside the second sphere shell. As confirmed by the ringlike selected-area electron diffraction
Figure 2. XRD pattern of the HS-SnO2.
(SAED) pattern (Figure 3c), the multilayered hollow spheres are also composed of phase-pure rutile SnO2. The magnified image (Figure 3d) of the SnO2 spheres shows a rough surface aggregated with nanoparticles of ∼10 nm diameter, which is in agreement with the data calculated from the XRD pattern (Figure 2). Also, the walls of the hollow sphere are highly porous. To determine the porous structure of the shell wall, we used nitrogen adsorption and desorption isotherms for acquiring the pore parameters of the HS-SnO2. It is found that the SnO2 spheres have a BET surface area of 36.17 m2‚g-1 with an average Barretl-Joyner-Halenda (BJH) pore diameter of 42 nm and a pore volume of 0.197 cm2‚g-1. The adsorption isotherm for the HS-SnO2 appears to be a type IV curve with a type III hysteresis loop (Figure 4a), indicating the presence of mesopores in the size range 2-50 nm (Figure 4b). Figure 5 shows the thermogravimetric (TG) curve and differential thermogravimetric (DTG) curve of the SnO2/C composite. There are obviously three steps of weight loss occurring between 300 and 500 °C in the DTG curve, which are all attributed to the combustion of carbon in different layers of the SnO2/C composite. For comparison, we also measured the TG and DTG curves of the pure carbon spheres produced in the same experimental conditions except for the absence of SnCl4 and found that only one step of weight loss is observed
Multilayered Nanocrystalline SnO2 Hollow Microspheres
J. Phys. Chem. C, Vol. 111, No. 38, 2007 14069
Figure 3. (a) TEM image of the HS-SnO2, (b) the magnified image of a single HS-SnO2, (c) the SAED pattern, and (d) the magnified image of the edge of a HS-SnO2.
Figure 5. TG/DTG curve of the tin oxide-carbon composites.
Figure 4. (a) Nitrogen adsorption and desorption isotherms and (b) pore-size distribution curve of HS-SnO2.
about 500 °C. The three steps of weight loss observed from the DTG curve in Figure 5 may suggest that the carbons contained
in the composite SnO2/C are located in different sphere shells and therefore have different combustion kinetics. The reaction mechanism for the formation of the monolayered hollow sphere via the hydrothermal treatment of glucose/water solutions has been reported,23,24,28 in which the formation of the monolayered sphere shell was assumed to involve the formation of the carbon sphere by the polymerization and carbonization reaction in the first step and subsequently the adsorption of metal ions and their resulting nanoparticles near the hydrophilic shell of the carbon particles. According to all the above observations, the reaction mechanism of the HSSnO2 formation in our case is different from that of the formation of the single-walled metal oxide sphere previously reported under the hydrothermal condition. The formation of HS-SnO2 could proceed through a chemically induced selfassembly process as shown in Figure 6. During the first stage of the reaction, the primary carbon spheres are formed by the condensation polymerization of sucrose and subsequent carbonization of the so-formed polymer (step I, Figure 6). Since
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Figure 6. Schematic illustration of the formation processes of the HS-SnO2.
Figure 7. (a) The initial charge-discharge curves of the HS-SnO2 and (b) the cycling performance of the HS-SnO2.
the surface of the carbon spheres is attached to a layer of OH groups, the outer surface of the carbon spheres is hydrophilic. Thus, a dehydration reaction can take place between the surface OH groups and the tin oxide hydrate in solution, leading to formation of a layer of tin oxide hydrate on the outside wall of the carbon spheres (step III, Figure 6). Subsequently, tin oxide hydrate on the surface of the carbon spheres reacts with the OH groups of oligosaccharides formed via the sucrose polymerization. After carbonization of the oligosaccharides, the second carbon shell is formed outside the primary carbon sphere (step IV, Figure 6). So, the well-defined carbon layer and the tin oxide (or hydrate) layer are assembled one by one via the chemically induced dehydration reaction (step V, Figure 6). As a consequence, the multilayered (three-layered) sphere shells with
carbon and tin oxide layers are alternately formed (step VI, Figure 6). After final calcinations, the HS-SnO2 are obtained. Recently, tin oxide has been a research highlight as a nextgeneration of anodic material for lithium ion batteries,29∼31 because of its high theoretical specific capacity (∼790 mAh‚g-1). However, the main hindrance against the practical application of SnO2 is its poor cycleability due to the large volume change (∼300%) during the charging-discharging process, which leads to mechanical failure and the loss of electrical contact. One possible approach to improve SnO2 electrochemical performance is downsizing SnO2 particles to the nanoscale level so as to enhance the crystalline stability. Li and co-workers31 have prepared monodisperse SnO2 nanofibers using sol-gel template synthesis, which can deliver high capacities and extraordinary
Multilayered Nanocrystalline SnO2 Hollow Microspheres cycling capabilities. Hollow SnO2 spheres can provide sufficient space to buffer the volume change during the chargingdischarging process and therefore are suggested as a cycleable anodic material for lithium ion batteries.12,20 Figure 7a displays the initial charge-discharge curve of the HS-SnO2 at a constant current of 100 mA‚g-1 with a cutoff voltage window of 5 mV and 1.5 V. This material has a high initial discharge capacity of 764 mAh‚g-1, close to its theoretical specific capacity, indicating a full utilization of all the SnO2 crystallites. Figure 7b shows the cycling performance of the HS-SnO2. The discharge capacity of this material remains 520 mAh‚g-1 after 20 cycles, exhibiting a good capacity retention. Conclusions In summary, the HS-SnO2 were prepared from aqueous sucrose/SnCl4 solution under hydrothermal environment via chemically induced self-assembly. The as-prepared HS-SnO2 exhibit good electrochemical performance as an anode material in lithium ion batteries. Furthermore, since this synthetic route is simple, convenient, and “green”, it may be extended to preparation of the hollow microspheres of other metal oxides. Because these hollow spheres are composed of nanoparticles and have large surface area, stable structure, and a particular inner environment, such materials may find great applications in gas sensors, heterogeneous catalysis, optical devices, and microreactors. Acknowledgment. The authors acknowledge financial support from the 973 Program, China (Grant No. 2002CB211800). References and Notes (1) Wang, Y. L.; Jiang, X. C.; Xia, Y. N. J. Am. Chem. Soc. 2003, 125, 16176. (2) Shirahata, N.; Shin, W.; Murayama, N.; Hozumi, A.; Yokogawa, Y.; Kameyama, T.; Masuda, Y.; Koumoto, K. AdV. Funct. Mater. 2004, 14, 580. (3) Camagni, P.; G.; Faglia, P.; Galinetto, Perego, C.; Samoggia, G.; Sberveglieri, G. Sens. Actuators, B 1996, 31, 99.
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