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J. Phys. Chem. C 2008, 112, 4836-4843
Facile Route for the Fabrication of Porous Hematite Nanoflowers: Its Synthesis, Growth Mechanism, Application in the Lithium Ion Battery, and Magnetic and Photocatalytic Properties Suyuan Zeng,†,‡ Kaibin Tang,*,†,‡ Tanwei Li,† Zhenhua Liang,†,‡ Dong Wang,†,‡ Yongkun Wang,†,‡ Yunxia Qi,†,‡ and Weiwei Zhou†,‡ Nanomaterial and Nanochemistry, Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei, Anhui 230026, P. R. China, and Department of Chemistry, UniVersity of Science and Technology of China, Hefei, Anhui 230026, P. R. China ReceiVed: August 28, 2007; In Final Form: January 2, 2008
In this work, a facile route using a simple solvothermal reaction and sequential calcinations to synthesize porous R-Fe2O3 flower-like nanostructures without employing templates or matrices for self-assembly is presented. The flower-like nanostructures are composed of nanosheets with a thickness of about 20 nm, which are characterized by field-emission scanning electron microscopy (FESEM). Influencing factors such as the dosage of reactants and the solvents are systematically investigated. A possible formation mechanism for the flower-like nanostructure is proposed. A BET test shows that the product is porous and has a large surface area. The electrochemical, magnetic, and photocatalytic properties of the as-obtained R-Fe2O3 3D nanostructure are systematically investigated. The result shows that these properties are greatly affected by the porous structure.
1. Introduction The fabrication of complex nanostructures with controlled morphology, orientation, and dimensionality have attracted significant attention over the past decade since such control is crucial for the determination of structure-property relationships in many processes, the development of new pathways for materials synthesis, and novel applications of nanostructured materials.1-4 Over the past few years, ordered nanostructures and assemblies using nanoparticles, nanorods, nanobelts, and nanosheets as building blocks have attracted great interest because of the demand for precise control of increasing structural complexity.5-11 The simplest synthetic route to obtain these nanostructures is probably self-assembly, in which ordered aggregates formed in a spontaneous process.12 As a recently developed concept, the self-assembly technique has been shown to be an efficient “bottom-up” route in fabricating functional materials with different patterns and morphologies.13-14 Many complicated hierarchical nanostructures, including inorganic15 and organic nanostructures,16 have been fabricated by this simple and spontaneous process. However, it is still a big challenge to develop simple and reliable synthetic methods for hierarchically self-assembled architectures with designed chemical components and controlled morphologies, which will strongly affect the properties of nanomaterials. Hematite (R-Fe2O3), based on hexagonal close packing of oxygen with iron in 2/3 of the octahedral vacancy, is traditionally used as catalyst, pigment, gas sensors, and electrode materials17-19 due to its low cost, high resistance to corrosion, and environmentally friendly properties. Because of its excellent properties, * Corresponding author. Fax: +86-551-360-1791. E-mail: kbtang@ ustc.edu.cn. † Nanomaterial and Nanochemistry, Hefei National Laboratory for Physical Sciences at Microscale. ‡ Department of Chemistry.
much attention has been directed to the controlled synthesis of hematite particles with various morphologies. Till now, hematite particles possessing various relatively simple shapes have been available: for example, spheres, cubes, ellipsoids, spindles, disks, hexagonal platelets, peanuts, tubes, rods, wires, and belts.20 However, the self-assembly of these low-dimensional building blocks into complex 3D ordered nanostructures is still considerably more difficult. And now, continuous efforts are being made on the preparation of hematite 3D nanostructures because of their promising application in various fields. By direct thermal oxidation of iron substrates under the flow of O2, Yang’s group has successfully prepared uniform vertically aligned arrays of R-Fe2O3 nanobelts and nanowires.21 Using an AAO (anodic aluminum oxide) membrane as the template, Xue and coworkers also succeeded in the preparation of R-Fe2O3 nanowire arrays.22 However, 3D nanoarchitectures prepared by the template method or the CVD method usually suffer from the disadvantages related to high cost and tedious procedures, which may prevent them from large-scale applications. Thus, one would prefer a more facile and economic method for the preparation of the 3D nanostructures. As a result, many solutionbased methods such as hydrothermal and solvothermal methods were brought into the synthesis of R-Fe2O3 3D nanostructures. By treating K3[Fe(CN)6] in aqueous solution at 140 °C, Hu’s group succeeded in the preparation of single-crystal dendritic micro-pines of R-Fe2O3.23 Fu’s group reported the templatefree synthesis of the urchin-like hematite nanostructures by oxidizing FeSO4 with NaClO3 in the solution.24 As an alternative to the hydrothermal method, the solvothermal method has also been employed in the preparation of R-Fe2O3 3D nanostructures. Among all the solvothermal methods for the preparation of R-Fe2O3 3D nanostructures, the most common one was the ethylene glycol-mediated method. Using a mixed solution of ethylene glycol and water as the reaction medium, Yang and
10.1021/jp0768773 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/08/2008
Fabrication of Porous Hematite Nanoflowers co-workers also succeeded in the synthesis of airplane-like R-Fe2O3 nanostructures.25a Wan’s group reported the preparation of R-Fe2O3 nanoflowers by using ethylene glycol as the solvent.25b However, it still remains a great challenge for the synthesis of 3D R-Fe2O3 nanoarchitectures through a more economic and environmentally friendly way, which will facilitate our understanding on the shape-dependent properties of the products. As a low-cost and green solvent, ethanol has been used in the preparation of various 3D nanostructures, such as ZnO,26a CdS,26b CdSe,26b PbS,26c and CuS;26d however, the synthesis of hematite 3D nanostructures by using ethanol as the solvent is seldom reported. Herein, we report the synthesis of the 3D flower-like R-Fe2O3 nanostructures via an ethanol-mediated selfassembly process. Iron chloride and ethanol are employed as the starting materials. By decomposing the as-synthesized iron oxide precursor at elevated temperature, porous flower-like R-Fe2O3 nanostructures could be obtained. In our early research, we found that the properties such as the electrochemical, magnetic, and photocatalytic properties of hematite will change greatly once the product is porous.27 For example, the Morin transition of the porous nanorods is suppressed due to the porous structure. The discharge capacity and cycling performance are greatly improved by the porous structure of the compound. Thus it would be of great interest to investigate the electrochemical, magnetic, and photocatalytic properties of the porous hematite nanoflowers, which will facilitate the understanding of the surface-tailored properties of porous hematite. To the best of our knowledge, this is the first time that the electrochemical, magnetic, and photocatalytic properties of such 3D porous hematite nanoflowers been investigated. 2. Experimental Section 2.1. Preparation of Flower-like R-Fe2O3. 2.1.1. Preparation of the Flower-like Precursor. All the reagents are of analytical grade and used without further purification. In a typical experiment, 2 mmol of iron chloride hexahydrate (FeCl3‚6H2O) was added into 40 mL of ethanol and stirred until totally dissolved. The solution was then transferred into a 50 mL Teflon-lined stainless steel autoclave, kept at 140 °C for 12 h, and then cooled naturally. The product was collected by centrifugation and washed with deionized water and absolute ethanol several times and dried in air at 60 °C for 12 h. 2.1.2. Preparation of the Flower-like R-Fe2O3. In the next step, the as-prepared precursor was annealed in air at 400 °C for 2 h at the heating rate of 1 °C/min. A red solid was obtained, which was collected for further analysis. 2.2. Sample Characterizations. All products were characterized by X-ray diffraction (XRD) on a Philips X’pert Pro SUPER rotation anode with Cu KR radiation (λ ) 1.541874 Å). A scan rate of 0.0167° s-1 was applied to record the pattern in the 2θ range of 10-70°. The size and the morphology of the product were observed on a SIRION FEI field emission scanning electron microscope (FESEM) equipped with a GENESIS4000 energy dispersive X-ray spectroscope. The transmission electron microscope (TEM) image was taken on a Hitachi H-800 transmission microscope, using an accelerating voltage of 200 kV. The selected area electron diffraction (SAED) patterns and high-resolution transmission electron microscopy (HRTEM) images were obtained on a JEOL-2010 TEM at an acceleration voltage of 200 kV. Thermal gravimetric analysis (TGA) of the as-synthesized flower-like nanostructures was carried out on a Mettler Toledo TGA/ SDTA 851e thermal analyzer at a heating rate of 10 °C/min from room temperature to 800 °C in the air.
J. Phys. Chem. C, Vol. 112, No. 13, 2008 4837 Fourier transform infrared (FTIR) spectrometry was performed on KBr disks of the powdered samples using a Thomas Nicolet 670 FTIR spectrometer. The Brunauer-Emmett-Teller (BET) tests were determined via a Micromeritics ASAP-2000 nitrogen adsorption apparatus. The magnetic measurements were recorded on a SQUID magnetometer, Quantum Design MPMS. 2.3. Characterization of the As-Obtained Nanostructure in a Lithium Ion Battery. The performance of the R-Fe2O3 as a cathode was evaluated using a Teflon cell with a lithium metal anode. The cathode was a mixture of R-Fe2O3/acetylene black/ poly(vinylidene fluoride) with a weight ratio of 80:10:10. The electrolyte was 1 M LiPF6 in a 1:1 mixture of ethylene carbonate/diethyl carbonate, and the separator was Celgard 2500. The cell was assembled in a glove box filled with highly pure argon gas (O2 and H2O levels naoparticles, which agrees with the sequence that the surface areas of the samples decreased. It is generally accepted that the catalytic process is mainly related to the adsorption and desorption on the surface of the catalyst. The high specific surface area of the nanocatalysts results in more unsaturated surface coordination sites exposed to the reactants. In addition, the interconnected hollow pores in the catalyst enable storage of more reactant molecules. Therefore, the enhancement of catalytic activity by high surface area is reasonable. Considering the photocatalytic performance of the flower-like product and the facile preparation method, it is believed that these porous R-Fe2O3 nanoflowers may have potential application in the field of photocatalysis. Acknowledgment. Financial support by the National Natural Science Foundation of China (No. 20621061), the 973 Projects of China, and the Program for New Century Excellent Talents in university (NCET) are gratefully acknowledged. Supporting Information Available: XRD pattern, FT-IR spectrum, TG curve, and EDS spectrum. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Hu, J. T.; Odom, T. W.; Lieber, C. M. J. Am. Chem. Soc. 1999, 32, 435. (2) Law, M.; Goldberg, J.; Yang, P. D. Annu. ReV. Mater. Res. 2004, 34, 83. (3) Yuan, J. K.; Li, W. N.; Gomez, S.; Suib, S. L. J. Am. Chem. Soc. 2005, 127, 14184. (4) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 8124. (5) Co¨lfen, H.; Antonietti, M. Angew. Chem., Int. Ed. 2005, 44, 5576. (6) Tang, Z. Y.; Kotov, N. A. AdV. Mater. 2005, 17, 951. (7) Zhang, G. Q.; Lu, X. L.; Wang, W.; Li, X. G. Chem. Mater. 2007, 19, 5207. (8) Dong, L. F.; Gushtyuk, T.; Jiao, J. J. Phys. Chem. B 2004, 108, 1617. (9) Leontidis, E.; Orphanou, M.; Kyprianidou-Leodidou, T.; Krumeich, F.; Caseri, W. Nano Lett. 2003, 3, 569. (10) Li, Y. B.; Bando, Y.; Golberg, D. Appl. Phys. Lett. 2003, 82, 1962.
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