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Facile Template-Free Synthesis and Characterization of Elliptic r-Fe2O3 Superstructures Zhenguo An,†,§ Jingjie Zhang,*,† Shunlong Pan,† and Feng Yu†,§ Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China, Graduate School of the Chinese Academy of Sciences, Beijing 100049, P. R. China ReceiVed: January 15, 2009; ReVised Manuscript ReceiVed: April 1, 2009
In this work, elliptic single-crystalline R-Fe2O3 superstructures are prepared successfully by a facile hydrothermal method independent of surfactants or templates. The as-obtained products were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). The elliptic superstructures are composed of nanorods with a mean diameter of ca. 40 nm and an average length of ca. 200 nm. By properly monitoring the component of the reaction system, the morphology of the R-Fe2O3 superstructures can also be tuned to be spherical (consist of nanosheets) and quasi spindle-like (consist of integrated irregular blocks). On the basis of a series of contrast experiments over time, the probable growth mechanism and fabrication process of the products were proposed. The magnetic and electrochemical properties of the as obtained R-Fe2O3 superstructures with different morphologies are investigated systematically. The results show that these properties are greatly influenced by the special structures of the products. This work provides an additional strategy to prepared self-assembled superstructures with tailored morphologies (both for the building units and the overall products) and properties. 1. Introduction Research on the design and synthesis of inorganic complex structures in the micro- and nanosize with controlled morphology, orientation, and dimensionality has attracted considerable attention because different morphologies and crystallinity are believed to be responsible for their intrinsic properties. Moreover, such control is also believed to be crucial for the determination of size/structure-dependent properties and the development of new pathways for materials synthesis.1-5 In particular, it has become a hot topic to assemble low dimensional nanosized materials into organized and designed structures through selfassembly, for the assemblies usually exhibit novel properties different from bulk or discrete counterparts and show potential applications in many areas.6-11 Self-assembly has been shown to be a facile and efficient “bottom-up” route to assembled structures and up to now, many different kinds of metal, metal oxide, sulfide, phosphate, and hydrate with various hierarchical and complicated structures have been successfully fabricated by this spontaneous process.12-16 However, currently it remains a big challenge to develop simple and easy-controlled routes for the fabrication of hierarchically self-assembled architectures with designed chemical components and controlled morphologies through multidimensional interconnections of low-dimensional building units. R-Fe2O3 (hematite), with a rhombohedrally centered hexagonal structure of corundum-type with a close-packed oxygen lattice in which two-thirds of the octahedral sites are occupied by Fe(III) ions, is the most stable iron oxide under ambient atmosphere. Because of its low cost, high resistance to corrosion, and environmentally friendly properties, this transition metal oxide has been traditionally used as catalysts, pigments, gas * To whom correspondence should be addressed. E-mail: jjzhang@ mail.ipc.ac.cn. Phone/Fax: +86 10 82543691. † Technical Institute of Physics and Chemistry, Chinese Academy of Sciences. § Graduate School of the Chinese Academy of Sciences.
sensors, and electrode materials.17-21 Over the past decades, various simple R-Fe2O3 structures, such as cubes, rods, needles, wires, tubes, belts, disks, flakes, hollow spheres, and so on, have been successfully prepared by a variety of methods.22-28 Only very recently, a number of research work have been reported concerning the preparation of self-assembled hematite hierarchical structures due to their promising applications in many areas.29-32 However, the controlled assembly of low-dimensional building units into complex 3D ordered structures is still considerably difficult. To meet the ever-increasing nanotechnological demand, the diversity of assembled hematite structures with desired geometry and morphology need to be greatly expanded. Herein, we have successfully synthesized monodisperse elliptic R-Fe2O3 superstructures assembled by hematite nanorods by a facile hydrothermal process without the use of any template or organic surfactant. Ferric chloride, an inexpensive and nontoxic reagent, is employed as iron source and high purity R-Fe2O3 structures can be obtained after the hydrothermal process without further thermal treatment. The overall morphology of the hematite superstructures can be tuned simply by varying the component of the reaction system. The structuredependent magnetic and electrochemical properties of the final products have been investigated in detail, and the formation mechanism of such novel superstructures is also discussed. 2. Experimental Section Synthesis. All reagents were of analytical grade and used without further purification. Because ferric chloride absorbs water vapor significantly, the thermogravimetric analysis (TGA) of the raw hydrous ferric chloride was carried out. The TG curve (see Figure S1 of the Supporting Information) shows that FeCl3 contributes to 39.2% of the weight in the raw hydrous ferric chloride. In a typical experiment, 6 mmol of ferric chloride was added into 30 mL of deionized water and stirred until totally dissolved. The volume of the reaction solution was set to 35
10.1021/jp9004168 CCC: $40.75 2009 American Chemical Society Published on Web 04/21/2009
Elliptic R-Fe2O3 Superstructures mL by adding deionized water, and the pH value was 1.88. Unless otherwise mentioned, no other reagents were added into the reaction solution. In the attempt with different solvent, a mixture of ethanol and deionized water (9:1, v/v) was used instead of deionized water as the solvent in the preparation of the reaction solution. The reaction solution was then transferred into a 50 mL Teflon-lined stainless steel autoclave and heated slowly to 200 °C and kept for 12 h. Afterward, the autoclave was cooled to room temperature naturally and the red products were isolated by centrifugation, rinsed several times with distilled water, and then vacuum-dried at 60 °C. Characterizations. X-ray diffraction (XRD) analysis was carried out on a Rigaku D/max2200PC diffractometer with Cu KR radiation (λ ) 1.5406 Å). The scanning electron microscopy (SEM) images and energy dispersive X-ray (EDX) spectra were obtained using a Hitachi S-4300 microscope and EMAX Horiba, respectively. X-ray photoelectron spectroscopy (XPS) data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Mg KR radiation. The binding energies were referenced to the C1s line at 284.8 eV from adventitious carbon. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images and the selected area electron diffraction (SAED) pattern were performed with a Philips TECNAI-20 transmission electron microscope. Thermogravimetric analysis (TGA) was carried out on a TG/ DTA6300 thermal analyzer with a heating rate of 5 °C · min-1 in flowing argon atmosphere. Magnetic measurements were carried out at room temperature using a vibrating sample magnetometer (VSM, Lakeshore 7307, USA) with a maximum magnetic field of 1T. Electrochemical Performance of the As-obtained r-Fe2O3 Superstructures. For fabrication of the working electrodes, the as-prepared powders were mixed with acetylene black and polyvinylidene fluoride (PVDF) in weight ratio of 80:10:10 in N-methyl-2 pyrrolidinon (NMP). The obtained slurry was coated onto Al foil and dried at 80 °C for 4 h. Then, the dried tape was punched into round plates with diameter of 10.0 mm as the cathode electrodes. The electrodes were dried again at 120 °C for 5 h in a vacuum prior to use. Finally, the prepared anodes and Celgard 2400 separator (diameter of 16.0 mm) were placed into an argon atmosphere filled glovebox (H2O and O2 < 1 ppm) and assembled into a coin cell (CR2032) with lithium cathode, electrolyte of 1 M LiPF6 in EC-DEC-DMC (1:1:1 vol %) and the other components of the coin-type cell. The cells were charged and discharged at the current density of 0.2 mA · cm-2 on a battery tester in the voltage of 0.5-3.0V at room temperature. 3. Results and Discussion Structure and Morphology. The crystallinity and the phase information for the as-obtained iron oxide product have been confirmed with the X-ray diffraction (XRD) method, as shown in Figure 1. All the reflections can be indexed to a pure rhombohedral phase of R-Fe2O3 (hematite) (JCPDS no. 330664). The narrow sharp peaks suggest that the R-Fe2O3 superstructures are crystalline. No characteristic peaks due to the impurities of FeOOH, Fe3O4, or γ-Fe2O3 can be detected. X-ray photoelectron spectroscopy (XPS) analysis of the asobtained sample has also been carried out to examine the oxidation state of Fe (Figure 2). Two distinct peaks at binding energies of ca. 710.8 eV and ca. 724.3 eV are observed in the high resolution spectrum of Fe2p. The two peaks can be indexed to Fe2p3/2 and Fe2p1/2, which is characteristic of Fe3+ in Fe2O3.31,33
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Figure 1. XRD pattern of the as-obtained products.
Figure 2. X-ray photoelectron spectra (XPS) of the as-prepared products. Inset: high-resolution XPS Fe2p spectrum.
Figure 3. SEM images of the as-obtained elliptic R-Fe2O3 superstructures at different magnifications. Inset in (b): EDX spectrum of the sample.
The scanning electron microscopy (SEM) images and energy dispersive X-ray (EDX) spectra provide further insight into the morphology, microstructure, and the chemical composition of the Fe2O3 sample. The typical SEM images at different magnifications of the products are shown Figure 3. Figure 3a is a panoramic image of the as-prepared sample without any dispersion treatment, indicating the high-yield growth and good uniformity of the R-Fe2O3 superstructures. The magnified image shown in Figure 3b clearly demonstrates the elliptic morphology of the sample with an average center diameter of ca. 2 µm and length of ca. 3 µm. Result of the energy dispersive X-ray (EDX) analysis confirms the presence of oxygen and iron (Figure 3b,
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Figure 5. SEM images at different magnifications of the spherical R-Fe2O3 superstructures obtained using a mixed solvent of ethanol and deionized water (a and b), and the quasi spindle R-Fe2O3 superstructures prepared in the presence of urea (0.1 M) with a reaction time of 8.0 h. Figure 4. (a) TEM image of the as-prepared product, (b) enlarged TEM image of the head part of one individual R-Fe2O3 superstructure and the corresponding SAED pattern (inset), (c) HRTEM image of A area (indicated by a square in panel b), (d) HRTEM image of B area (indicated by a square in panel b).
inset). The SEM image of a single R-Fe2O3 superstructure indicates the rod-assembled nature of the sample (Figure 3c). A close observation on the elliptic R-Fe2O3 superstructure reveals that the nanorods as the building units possess a mean diameter of ca. 40 nm and an average length of ca. 200 nm (Figure 3d). The morphology and microstructure details of the R-Fe2O3 superstructures have been further examined by transmission electron microscopy (TEM) accompanied by selected area electron diffraction (SEAD). From the TEM image of two R-Fe2O3 particles shown in Figure 4a, the elliptic morphology and rough surface can be seen clearly, which agrees with the SEM result. A closer TEM observation of the head part of a single particle further confirms the rod-assembled nature of the products (Figure 4b). Noticeably, the corresponding SEAD pattern taken from the individual particle clearly suggests a single-crystal nature of the R-Fe2O3 products (inset of Figure 4b). To further reveal the fine structure of the R-Fe2O3 superstructures, high-resolution TEM (HRTEM) analysis has also been carried out. Shown in parts c and d of Figure 4 are the corresponding HRTEM images of the different areas marked by squares (A and B). The clear lattice image demonstrates the high crystallinity and single-crystal feature of the R-Fe2O3 superstructures, which is in good agreement with the XRD and the SEAD results. The typical d-spacings of 0.25 nm are consistent with the d-values of (110), indicating the growth direction (also the long axes of the elliptic particles) is along [110]. Effects of Reaction Conditions. In the present system, we find that the component of the reaction system is undoubtedly vital in the nucleation and growth of the R-Fe2O3 crystallites in the synthesis process and further determines the morphology of the final products. In the attempt with a different solvent, a mixture of ethanol and deionized water (9:1, v/v) was used instead of deionized water as the solvent in the preparation of the reaction solution. The calculated volume ratio of ethanol to water in the final reaction system was 6.2:1. The SEM images
of the as-obtained products are shown in Figure 5a,b. From the panoramic image (Figure 5a), it can be seen that spherical R-Fe2O3 superstructures are obtained on a large scale. Different from the nearly monodisperse elliptic particles, the spherical superstructures possess a broader diameter distribution of about 0.8-3.0 µm. A close observation of an individual spherical superstructure reveals that these superstructures are composed of nanoplatelets of ca. 30 nm in thickness. Each platelet is curled with a smooth surface. These nanoplatelets assemble layer by layer to form the R-Fe2O3 superstructures. Although the exact roles of ethanol are not yet clear, we propose that ethanol may play an important role as both solvent and starting material to form iron alkoxide precursor.34 Therefore, the nucleation, growth, assembly, and the ripening process of the R-Fe2O3 crystals will follow a different way from those that happen in pure deionized water solvent. A detailed study on this issue is still underway to further understand the exact roles of ethanol in determining the growth process and the final structure of the R-Fe2O3 superstructures. In addition to the composition of the solvent, it is found that the use of urea will also have certain effects on the morphology of the R-Fe2O3 assemblies. Parts c and d of Figure 4 depict the SEM images of the sample prepared when 0.1 M of urea was added into the reaction solution (deionized water as the solvent, the corresponding pH value of the reaction system was 1.92). It can be seen that after hydrothermal reaction for 8.0 h, quasi spindle R-Fe2O3 superstructures are obtained in large scale in the presence of urea (Figure 5c) and further prolonged reaction time has little influence on the morphology of the products. The as-prepared spindle-like R-Fe2O3 superstructures have relatively smooth surfaces with an average center diameter of ca. 3.0 µm and length of ca. 4.0 µm (Figure 5d). Urea is commonly used as precipitator in many reactions,35-37and we believe the urea in our reaction solution will accelerate the nucleation and growth of the iron oxide crystals, thus kinetically controlling the aggregation and growth of the R-Fe2O3 superstructures into spindle-like structures with a much integrated nature. The R-Fe2O3 superstructures with elliptic, spherical, and spindlelike morphologies mentioned above are assigned as samples A, B and C, respectively, for further discussion. Formation Mechanism. To understand the growth process of the products, the morphology evolution of the elliptic R-Fe2O3
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Figure 6. Schematic illustration of the possible formation process of the R-Fe2O3 superstructures. (a-c) SEM observations of the products at different reaction stages: (a) 1.0 h, (b) 5.0 h, and (c) 10.0 h.
superstructures has been studied by SEM. The possible formation processes and the SEM images of the samples obtained with different reaction times are shown in Figure 6. Because the elliptic R-Fe2O3 superstructures are synthesized without the use of complex precursors, templates, or reagents that can induce the directed growth of microcrystals and the formation of complicated hierarchical structures, we propose that the formation of our elliptic superstructures follows a three-step growth model involving the produce of R-Fe2O3 nanoparticles, the oriented aggregation of the nanoparticles to form rod-like nanocrystals, and finally the formation of elliptic superstructures through oriented attachment of the nanorods and Ostwald ripening. As shown in Figure 6a, at the early stage of the hydrothermal reaction, small iron oxide nanocrystals of ca. 20 nm were generated through conventional nucleation and subsequent crystal growth (step A). Under the influence of the dipole interactions and the thermodynamic driving force, the neighboring nanoparticles tended to join with each other through oriented aggregation. Together with the subsequent production and deposition of iron oxide, rod-like nanocrystals were formed (step B). As the reaction went on, these nanorods gradually assembled into quasi urchin-like structures by oriented attachment, and elliptic superstructures are formed with the rotation of the adjacent nanorods to share the same crystallographic orientations and the following combination of these nanorods (step C). During this process, there is also a thermodynamic driving force for the aggregated growth of the building units for the surface energy is reduced when the interface is eliminated. It is well accepted that the formation of the assembled superstructures dependent on the inherent structural characteristics and the external factors. In the present work, the reaction time, the component of the solvent, and the utilization of precipitator have been proved to have significant influences on the morphology of the R-Fe2O3 crystals. Indeed, several factors, including crystal-face attraction, electrostatic and dipolar fields associated with the aggregate, vander Waals forces, hydrophobic interactions, and hydrogen bonds, may have various effects on the formation and assembly.29,38,39 The discussion above mainly concerns the evolution of the morphology over the reaction time; more in-depth investigations are still underway to further understand the detailed influences of other factors on the formation, assembly, and the ripening process of the R-Fe2O3 crystals. Magnetic Properties. To investigate the influence of microstructure on the magnetic properties, the magnetization measurements of the R-Fe2O3 superstructures with different morphologies have been carried out in an applied magnetic field sweeping from -1 T to 1 T. Figure 7 depicts the roomtemperature hysteresis loops of the three samples with elliptic (sample A), spherical (sample B), and spindle-like (sample C) morphologies, respectively. No saturation of the magnetization as a function of the field is observed up to the maximum applied magnetic field for all three samples, and the magnetization at the maximum applied magnetic field of 1 T (Mmax) are 0.2629, 0.2948, and 0.3599 emu/g for samples A, B and C, respectively.
Figure 7. Room temperature hysteresis loops for the as-prepared samples A, B, and C. Mmax presents the magnetization at the maximum applied magnetic field of 1 T.
Figure 8. First charge-discharge curves of R-Fe2O3 superstructures with different morphologies (sample A: solid line; sample B: dashed line; sample C: dash-dotted line) at a current density of 0.2 mA cm-2.
All the three samples possess wide open M-H loops with the remanent magnetization (Mr) and the coercivity (Hc) of 0.1390, 0.1666, and 0.1962 emu/g, respectively, and 4539.3, 4225.7, and 3248.2 Oe, respectively. The higher Mmax and Mr of sample C may be attributed to the much integrated nature and larger particle size of the R-Fe2O3 superstructures.40 However, the presence of hierarchical structure and the smaller particle size of samples A and B lead to the increased surface/volume ratio and enhanced shape anisotropy.35,41,42 Therefore, much higher coercivities of samples A and B compared with sample C are observed. Performance of r-Fe2O3 Superstructures in a Lithium Ion Battery. Figure 8 shows the initial charge-discharge curves of the as-obtained R-Fe2O3 superstructures with different morphologies cycled in the voltage of 0.5-3.0 V at the current density of 0.2 mA · cm-2. It can be seen that the curves of all the three samples exhibit a typical flat discharge plateau closed to 0.75 V with the initial discharge capacities of 951.6 mAh · g-1,
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996.0 mAh · g-1, and 840.1 mAh · g-1 for samples A, B, and C, repectively. The reaction involves in the lithium ion battery may be summarized as: R-Fe2O3 + 6Li+ + 0.5e- f Li2O + 2Fe (discharge),43-45 and the calculated theoretical capacity of R-Fe2O3 is 1007 mAh · g-1 when six lithium ions in R-Fe2O3 are electrochemically extracted and reversibly reinserted.46,47 The three samples confirm capabilities with 94.5%, 98.9%, and 83.4% of the theoretical capacity. Sample B possesses the largest capability, which may be attributed to its special structure. As mentioned above, the spherical R-Fe2O3 superstructures of sample B are composed of loosely contacted nanosheets and have a broader size distribution; this special structure favors both the diffusion of the lithium ion and the contact between the electrolyte and the iron oxide.43,46,48,49 Therefore, better electrochemical property is observed for sample B. Conclusions In summary, a facile hydrothermal route to elliptic singlecrystalline R-Fe2O3 superstructures without employing surfactants or templates is found. The results show that the elliptic superstructures are formed through a combined process of nucleation, orientated attachment, and ripening. It is found that the existence of ethanol or urea in the reaction system possesses significant influences on the assembly and growth process and further affect both the overall morphology of the superstructures and that of the building units. The magnetic hysteresis measurements and the electrochemical performance measurements demonstrate that the as-obtained R-Fe2O3 superstructures show structure-dependent magnetic and the electrochemical properties. This work provides an additional strategy to prepared selfassembled superstructures with tailored morphology and properties. We believe such a synthetic route holds the potential to be extended for the preparation of assembled superstructures of other metal oxides or hydroxides. Acknowledgment. This work was supported by the National Natural Science Foundation of China (project no. 10476031) and the State High Technology Development Program 863 (2006AA09Z209). Supporting Information Available: TG curve of the raw hydrous ferric chloride used as the raw material. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Sun, Y.; Xia, Y. Science 2002, 298, 2176. (2) Li, Z. Q.; Ding, Y.; Xiong, Y. J.; Yang, Q.; Xie, Y. Chem. Commun. 2005, 7, 918. (3) Hakamada, M.; Mabuchi, M. Nano Lett. 2006, 6, 882. (4) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 8124. (5) Fang, X. S.; Ye, C. H.; Xie, T.; Wang, Z. Y.; Zhao, J. W.; Zhang, L. D. Appl. Phys. Lett. 2006, 88, 013101. (6) Lionel, V.; Conny, S.; Butorin, S. M. AdV. Mater. 2005, 17, 2320. (7) Service, R. F. Science 2005, 309, 95. (8) Alejandro, W.; Onur, A.; Braun, P. V. J. Am. Chem. Soc. 2005, 127, 16356. (9) Colfen, H.; Antonietti, M. Angew. Chem., Int. Ed. 2005, 44, 5576.
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