Hematite Hollow Spheres with a Mesoporous Shell: Controlled

Jul 8, 2008 - Hematite Hollow Spheres with a Mesoporous Shell: Controlled Synthesis and Applications in Gas Sensor and Lithium Ion Batteries. Zhengcui...
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J. Phys. Chem. C 2008, 112, 11307–11313

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Hematite Hollow Spheres with a Mesoporous Shell: Controlled Synthesis and Applications in Gas Sensor and Lithium Ion Batteries Zhengcui Wu,†,‡ Kuai Yu,† Shudong Zhang,† and Yi Xie*,† Department of Nanomaterials and Nanochemistry, Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei 230026, People’s Republic of China, and Anhui Key Laboratory of Functional Molecular Solids, College of Chemistry and Materials Science, Anhui Normal UniVersity, Wuhu 241000, People’s Republic of China ReceiVed: April 24, 2008; ReVised Manuscript ReceiVed: May 22, 2008

Novel R-Fe2O3 hollow spheres with uniformly distributed mesoporosity on the shell were successfully synthesized on a large scale by a smart complex precursor method, in which the composite mesoporous hollow structures were generated by utilizing different removing modes of oxalate ligands in ferric potassium oxalate. The shell of the hollow spheres exhibited honeycomb-like mesoporous nanostructures composed of single-crystal iron oxide nanoparticles and the as-obtained R-Fe2O3 composite hollow structures exhibited high gas sensitivity toward formaldehyde and ethanol at room temperature as well as favorable lithium ion battery performance. Introduction During the past few decades, much effort has been focused on designing new methods for the preparation of nanomaterials with special size, morphology, and porosity not only for their fundamental scientific interest but also for many technological applications. Hematite (R-Fe2O3), the most stable iron oxide with n-type semiconducting properties under ambient conditions, has attracted a great deal of attention from researchers in various fields including catalysis,1–3 environment protection,4–10 sensors,11 magnetic storage media,12 clinical diagnosis and treatment,13 and lithium ion electrode materials.11a,b,14 In particular, iron oxides nanostructures with a hollow interior could find better properties and applications because of their conspicuous physicochemical properties that differ markedly from those of nonhollow materials. Many efforts have been devoted to the synthesis of Fe2O3 hollow structures through various methods,11a,b,14a,15 and the fabrication of nano- and microscale hollow spheres was a continuous focus among researchers.14b,16 Composite hollow structures such as hollow spheres with mesoporous shell are more attractive due to their outstanding properties of low density, high surface area, and well-defined mesoporous wall structures that bring high permeability for controlled mass transport, hence, they can be used as confined nanocatalysts, adsorbents, targeted drug and gene delivery materials, as well as biomolecule encapsulation materials.17 There are two kinds of mesoporous hollow spheres according to different mesoporous construction on the shell: one is the intercrystal mesoporosity where the pores are in fact formed from the interspaces of the constituent nanoparticles, another is intracrystal mesoporosity where the pores existed in the interior of the constituent nanoparticles. Most of the reported mesoporous hollow spheres belong to intercrystal mesoporosity, such as the very recently reported R-Fe2O3 porous nanopheres.11c For the intracrystal mesoporosity, to the best of our knowledge, only silica and silica-related composite hollow structures have * To whom correspondence should be addressed. Phone/fax: 86-5513603987. E-mail: [email protected]. † University of Science and Technology of China. ‡ Anhui Normal University.

been reported, which were achieved through various template routes, including hard18 and soft templates such as micelles or emulsion.17,19 The complicated additional templates can direct the growth of target products but may introduce heterogeneous impurities. Therefore, the development of a simple and effective method for creating intracrystal mesoporous hollow spheres to the designed materials such as hematite R-Fe2O3 without employing additional template or matrix is important to technology and remains an attractive, but elusive goal. In the various efforts devoted to the synthesis of hollow spheres structure, the in situ gas-bubble templates have been widely used as a simple and effective method to construct hollow sphere structures of various inorganic materials.20 Meanwhile, a simple calcination process has been applied to obtain the mesoporous materials from corresponding precursors by the release of small gas molecules at elevated temperature.11b,14a,15f,21 These works inspire our idea that the combination of the above two methods may be a simple but effective choice for the construction of composite hollow structure. Thus, the reactant that cannot only supply an in situ gas bubble template in solution but also produce mesoporosity by decomposition from sintering will be expected to construct a composite hollow structure. As is known, the coordination compounds may provide various possibilities in acquiring nanostructured units and their assembly due to their abundant ligands and different coordination modes. For example, the different removal modes of ligands under different conditions may produce different nanostructures. Therefore, the complex has been thought of as an effective precursor for the construction of nanostructured materials.22 Some ligands can produce gases both from hydrolysis and from sintering, providing the idea to construct composite mesoporous hollow structure. Here, we selected ferric potassium oxalate as a smart precursor, utilizing two different kinds of removing mode of oxalate to achieve R-Fe2O3 mesoporous hollow spheres: first, the partial removal of oxalate by solvothermal reaction in the mixed solvent of water and ethylene glycol provides an in situ bubble template to generate iron oxide precursor hollow spheres; second, the gradual removal of the residual oxalate by calcination of the iron oxide precursor generates mesoporous structure on the shell of the hollow spheres, forming R-Fe2O3

10.1021/jp803582d CCC: $40.75  2008 American Chemical Society Published on Web 07/08/2008

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Figure 1. (a) Panoramic SEM image of the as-synthesized iron oxide precursor hollow spheres. (b) SEM image of the hollow spheres with higher magnification. (c, d) TEM images of the as-synthesized iron oxide precursor hollow spheres with low and high magnification. (e) HRTEM image of the as-synthesized iron oxide precursor hollow spheres, which shows there is no intracrystal mesoporous structure in the product.

mesoporous hollow spheres. Thanks to their composite hollow nanostructures, the gas sensitivity and lithium battery performance of as-prepared R-Fe2O3 mesoporous hollow spheres were significantly improved, compared with those of nanoparticle aggregations. Experimental Section All reagents were analytically pure and used without further purification. Synthesis. In a typical synthesis, 0.4 mmol of ferric potassium oxalate was dissolved in 20 mL of distilled water in a Teflonlined autoclave, followed by the addition of 20 mL of ethylene glycol. The autoclave was sealed, heated at 150 °C for 48 h, and allowed to cool to room temperature naturally. The precipitate was collected by centrifugation and washed with distilled water and ethanol several times, then dried in a vacuum at 50 °C for 10 h. Afterward, the precursor was placed in a crucible and carefully heated from room temperature to 450 °C at a rate of 10 deg/min and then maintained at 450 °C for 3 h in air. Finally, the products were collected. Characterization. The structure of these obtained samples was characterized with the XRD pattern, which was recorded on a Rigaku Dmax diffraction system with use of a Cu KR source (λ ) 1.54187 Å). The scanning electron microscopy (SEM) images were taken with a JEOL JSM-6700F field emission scanning electron microscope (FESEM, 20 kV). Transmission electron microscopy (TEM) images and highresolution transmission electron microscopy (HRTEM) images were obtained with a Hitachi 800 system at 200 kV and a JEOL2010 system also at 200 kV, respectively. The Fourier transform infrared (FTIR) spectroscopic study was carried out with a MAGNA-IR 750 (Nicolet Instrument Co.) FTIR spectrometer. Brunauer-Emmett-Teller (BET) nitrogen adsorption-desorption

was measured by using a Micromeritics ASAP 2020 accelerated surface area and porosimetry system. Measurements on gas sensitivity were performed with a WS-30A system (Weisheng Instruments Co., Zhengzhou, China). Electrochemical Measurements. The electrochemical measurements were carried out by using the as-prepared model test cells. The cathode was a mixture of R-Fe2O3/acetylene black/ poly(vinylidene fluoride) (PVDF) with weight ratio of 70:20: 10. A 10-20 mg amount of the mixture was pressed onto a Ni grid. The electrodes were dried at 120-130 °C in a vacuum furnace for 24 h before use, and Li foil was used as an anode. The electrolyte was a 1 M LiPF6 in a 1:1 mixture of ethylene carbonate/diethyl carbonate, and the separator was Celgard 2320. The cell was assembled in a glovebox filled with highly pure argon gas (O2 and H2O levels below 5 ppm). The galvanostatic charge/discharge experiment was performed between 3.0 and 0.5 V at constant discharge rates of 60 mA/g. The Li-ion battery electrodes experiments were carried out with use of a Land battery system (CT2001A). Results and Discussion In this strategy, the removal of oxalate ligands in the ferric potassium oxalate experienced two processes: the partial removal of oxalate by solvothermal reaction in the mixed solvent of water and ethylene glycol that contributes to the construction of iron oxide precursor hollow spheres and the gradual removal of the residual oxalate by calcination of iron oxide precursor that contributes to the construction of mesoporosity on the shell of hollow spheres. The hydrolysate of ferric potassium oxalate in the first step of the solvothermal reaction was well characterized. Figure 1a displays a panoramic SEM image of the as-prepared iron oxide precursor, which shows the sample is large-scale

Hematite Hollow Spheres with a Mesoporous Shell

Figure 2. XRD pattern of the R-Fe2O3 product obtained after calcining iron oxide precursor.

uniform hollow microspheres with diameters of 160-280 nm. The hollow nature of the microspheres could be concluded from the broken spheres, which is further confirmed from the high magnification SEM image shown in Figure 1b. The corresponding TEM images of the microspheres shown in Figure 1c,d further clearly identify the hollow structure of the sample, a strong contrast between the dark edges and the pale center confirms that almost all the spherical particles have a hollow cavity inside. The detailed observation in Figure 1d reveals the fact that the thickness of a shell is about 60 nm. The HRTEM image in Figure 1e shows there is no intracrystal mesoporous structure in the hollow spheres. The X-ray diffraction (XRD) pattern (Figure S1 in the Supporting Information) shows the emergence of diffraction peaks of FeC2O4 · XH2O (JCPDS No. 23-0293). The Fourier transform infrared (FTIR) spectrum of the as-prepared iron oxide precursor sample displays a notable character of FeC2O4 · XH2O (Figure S2 in the Supporting Information). In the region from 2000 to 1000 cm-1 of the observed spectrum, two absorption peaks at 1317 and 1362 cm-1 may be associated with the symmetrical stretching vibration of carboxyl and one absorption peak at 1629 cm-1 could be attributed to the asymmetrical stretching vibration of carboxyl. In the range from 4000 to 2000 cm-1, the broad and strong absorption peak at 3350 cm-1 is assigned as the band-stretching mode of hydroxyl, suggesting the existence of water in the sample. Therefore, the FTIR spectrum further confirms the precursor primarily contains FeC2O4 · XH2O. The above results demonstrate the hollow sphere structure has been synthesized as expected and the existence of oxalate in the iron oxide precursor offers the opportunity to create the porosity on the shell of the hollow spheres. The following calcination process was designed to create the porosity on the shell of the hollow spheres. As expected, R-Fe2O3 hollow microspheres with mesoporous shell have been prepared via calcining iron oxide precursor hollow spheres. Figure 2 shows the XRD pattern of the as-obtained product after calcination and the diffraction peaks are in good agreement with the standard XRD pattern of the pure hexagonal phase of R-Fe2O3 (JCPDS No. 80-2377), indicating that the pure phase of R-Fe2O3 can be obtained by calcining iron oxide precursor. FESEM and TEM images of the R-Fe2O3 mesoporous hollow spheres are shown in Figure 3. The panoramic observation in Figure 3a indicates that the as-obtained R-Fe2O3 sample basically remains the spherical morphology with a smaller diameter about 190 nm compared with that of the precursor before calcination, indicating a little shrinkage of the product after calcination. As seen from the magnified SEM image in Figure 3b, some fissures are observed on the spheres and the broken spheres are fewer

J. Phys. Chem. C, Vol. 112, No. 30, 2008 11309 than those of the precursor (see black arrows for the broken spheres). Figure 3c shows the TEM image of the resulting product, which clearly presents the honeycomb-like mesoporous nanostructures on the shell of the hollow spheres and reveals the spheres are not very round; this also indicates the microsphere is composed of many individual nanoparticles, consistent with the FESEM results. The high-magnification TEM images in Figure 3d,e further confirm the mesoporous structures on the shell of the hollow spheres and reveal that the diameter of the mesoporosity is about 4-12 nm. The pale center also proves the hollow nature of the sphere. The high-resolution TEM image in Figure 3f shows the lattice image obtained at the edge of the particle, demonstrating clearly the lattice fringes of the walls around several small mesopores, and the continuous lattice fringes demonstrate the single-crystal nature of the particle, the lattice spacings of 0.370 and 0.443 nm correspond to (012) and (003) lattice planes, respectively. The corresponding fast Fourier transform (FFT) spot diagram (Figure 3f inset) further clearly verifies the single-crystal nature of the particle. The HRTEM result confirms the small mesopores do not arise from the agglomeration or aggregation of smaller crystallites. To understand the growth mechanism of iron oxide precursor hollow spheres accurately, it is necessary to investigate the morphology evolution of the intermediates involved in the formation. Two intermediates caught at 2 and 4 h showed that the products were almost all hollow spheres with diameters of about 130 and 150 nm (Figure S3 in the Supporting Information), demonstrating the morphology evolution of the products with reaction time experienced the hollow spheres ripening process. Here, the presence of ethylene glycol plays an important role in the hydrolysis process of ferric potassium oxalate, which controls the formation of iron oxide precursor hollow spheres. Our comparative experiments confirmed that only pure R-Fe2O3 product was obtained without ethylene glycol (Figure S4 in the Supporting Information). At an elevated temperature, [Fe(C2O4)3]3- ions were gradually decomposed and reduced by ethylene glycol from Fe(III) to Fe(II), forming FeC2O4 · XH2O nanoparticles which had a tendency to aggregate, while at the same time, a large amount of gas microbubbles of CO2 was generated in the reaction supplied as an in situ soft template for the aggregation centers (step a in Scheme 1). Here, the viscosity of ethylene glycol could interfacially stabilize and reinforce the microbubbles. Driven by the minimization of interfacial energy, small FeC2O4 · XH2O nanoparticles may aggregate around the gas-liquid interface and finally FeC2O4 · XH2O hollow spheres were formed (steps b-d in Scheme 1). The hollow spheres obtained were covered with adsorbed polyol molecules that passivated the surface23 and hence prevented the oxidation of Fe2+ into Fe3+ and further hydrolyzation of the iron oxide precursor. In contrast, pure R-Fe2O3 nanoparticle aggregations with diameters of about 110 nm were obtained without ethylene glycol (Figure S4 in the Supporting Information), which is partly due to the fast release of the bubble from the pure water solution that could not be effectively used as aggregation centers for the self-assembly of iron oxide nanocrystallites. Therefore, our experiments found fundamental evidence that the existence of ethylene glycol was critical for the formation of the phase and morphology of iron oxide precursor, although the detailed mechanism still needs more investigation. After calcination in air at 450 °C for 3 h, iron oxide precursor was completely converted into R-Fe2O3 and a large number of uniform distributed honeycomb-like mesoporosity spheres were left on the shell of the hollow spheres due to the gradual releases of CO2 and H2O molecules from

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Figure 3. FESEM images and TEM images of as-prepared R-Fe2O3 hollow spheres with mesoporous shell after calcining iron oxide precursor hollow spheres at 450 °C for 3 h in air. (a, b) SEM images with low and high magnifications. (c-e) TEM images with different magnifications. (f) High-resolution TEM image obtained at the edge of the particle. The inset is the corresponding fast Fourier transform (FFT) spot diagram.

SCHEME 1: Schematic Illustration of the Evolution of r-Fe2O3 Mesoporous Hollow Spheres

the iron oxide precursor hollow spheres (step e in Scheme 1). The formation process of R-Fe2O3 mesoporous hollow spheres can be illustrated as shown in Scheme 1. In this strategy, as designed, the production of composite hollow structures comes from the removal of ligands in two chemical processes. Therefore, the production of iron oxide precursor hollow spheres that contain oxalate ligand is crucial to the final generation of R-Fe2O3 mesoporous hollow spheres structure. Series of contraing experiments were done to research the key factors that affected the formation of in situ CO2 gas bubble template, which affected the size and morphology of the iron oxide precursor. The experimental results indicated that

the concentration of ferric potassium oxalate, the volume ratio of the solvent of water and ethylene glycol, and temperature all played an important role. For example, when the concentration of ferric potassium oxalate was decreased to 0.2 mmol while maintaining other parameters constant, the morphology of the iron oxide precursor was still hollow spheres but with a smaller diameter of about 110 nm, and after calcination, R-Fe2O3 mesoporous hollow spheres with a diameter of about 85 nm and a uniform distributed honeycomb-like mesoporosity on the shell were obtained (Figure 4), which illuminated the reduced concentration of ferric potassium oxalate making iron oxide precursor nanoparticles aggregated on the smaller CO2 mi-

Hematite Hollow Spheres with a Mesoporous Shell

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Figure 4. (a) TEM image of iron oxide precursor hollow spheres prepared with 0.2 mmol ferric potassium oxalate. (b, c) TEM images of the corresponding R-Fe2O3 mesoporous hollow spheres with different magnifications after calcination.

crobubbles and the limited concentration also resulted in the final smaller hollow spheres. When the volume ratio of water and ethylene glycol excess was 3 or less than 1/3 (with a total volume of 40 mL) while keeping the concentration of ferric potassium oxalate constant at 0.4 mmol and the reaction temperature at 150 °C, the product was mostly made up of solid particles, demonstrating the physicochemical property of solvent dependent on its component concentration that affected the formation of the CO2 gas bubble template. When the temperature was increased to 200 °C while keeping other parameters invariable, only solid spheres were obtained, indicating the reduced viscosity at higher temperature was not favorable for the stabilization of the gas bubble template. These results suggest that it is possible to control and tune the size and morphology of iron oxide precursor by controlling the kinetic parameters, such as the concentration and the temperature. Prompted by their mesoporous appearance, it should have high surface areas. The measurement shows that the BrunauerEmmett-Teller (BET) surface area of the sample is 41.1 m2/g, where the two kinds of porous architecture of the pore within the microspheres and mesoporosities on the shell of the hollow spheres both endow this novel R-Fe2O3 sample with a large surface-to-volume ratio. Thus, it is expected that the as-prepared R-Fe2O3 mesoporous hollow spheres should be applied for fabricating sensors since it has been demonstrated that nanomaterials with desirable morphology and small size in the sensing layer may help to improve their own sensitivity at room temperature. The sensitivities of the R-Fe2O3 mesoporous hollow sphere sample toward trace levels of ethanol (C2H5OH) and formaldehyde (HCHO) gases were investigated at room temperature in dry air. The gas sensitivity is defined as the resistance ratio Rair/Rgas, where Rair and Rgas are the electrical resistances for sensors in air and in gas, respectively. Figure 5 shows the room temperature gas-sensing characteristic of the as-synthesized R-Fe2O3 mesoporous hollow spheres compared with R-Fe2O3 nanoparticle aggregations prepared in pure water (with BET surface area 25.7 m2/g). Panels a and b of Figure 5 display the plots of sensitivity versus the gas-vapor concentration when R-Fe2O3 mesoporous hollow spheres and nanoparticle aggregations were exposed to ethanol (C2H5OH) and formaldehyde (HCHO) gases, respectively. The sensitivities of the samples increase with an increase of ethanol and formaldehyde gas concentration. However, the nanoparticle aggregations are less sensitive. Whether exposed to C2H5OH or HCHO gas, the sensitivity of R-Fe2O3 mesoporous hollow spheres is excess twice that of the R-Fe2O3 nanoparticle aggregations under the same gas concentration and the sensitivities of the mesoporous hollow spheres to the two kinds of gases are distinguishable

Figure 5. Room temperature sensitivity of the sensors made of asprepared R-Fe2O3 samples to (a) ethanol (C2H5OH) and (b) formaldehyde (HCHO).

even at the concentration of 10 ppm, indicating its potential application in combustible and noxious gases detection. It is found the on and off responses for the products could be repeated many times without observing major changes in the signal, illustrating the favorable reversibility. For R-Fe2O3-based sensors, the change in resistance is mainly caused by the adsorption/desorption phenomena and reactions of gas molecules on the surface of the sensing structure. The coverage of the chemisorbed oxygen ions such as O-, O2- on the surface of the R-Fe2O3 sensors, which came from the atmosphere and the surface lattice oxygen (O2-) of the R-Fe2O3 sensor, can trap electrons from the detected gas, form space-change region, and further release to the metal oxide, resulting in the oxidation of the detected gases. Once the oxidation reaction occurs, electrons will enter into the sensors, leading to an increased conductivity

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Figure 6. First discharge curves of R-Fe2O3 mesoporous hollow spheres sample and nanoparticle aggregations sample at a constant current density of 60 mA/g. The inset shows the corresponding cyclic performance of the cell prepared from R-Fe2O3 mesoporous hollow spheres.

of the oxides.24 In this work, the as-prepared R-Fe2O3 mesoporous hollow spheres possess two kinds of porous architecture: one within the microspheres and the other on the shell of the hollow spheres. The interconnected honeycomb-like pores form a network that provides more sufficient space and active sites enabling both the target species and the background gas to access all the surfaces of R-Fe2O3 mesoporous hollow spheres contained in the sensing unit, improving the efficiency of gas diffusion and the interaction between adsorbed oxygen ions species and detected gases. The high sensitivity and reversibility under ambient conditions can be attributed to the intrinsic mesoporosity and high surface-to-volume ratio associated with the mesoporous hollow spheres.25 It is found that the lithium intercalation performance is related to the intrinsic crystal structure, where the lithium ions can intercalate into the interlayer, the tunnels, and the holes in the crystal structure.26 As related to R-Fe2O3, holes existing in its surface crystal will allow foreign atoms or molecules such as lithium ions to be introduced, and the lithium intercalation performance will improve by increasing the surface area or the porosity of the hematite crystals. Therefore, the gain of hematite nanostructures with higher surface area and porosity structures was expected to have a high capability because the intercalation capacities and affinities for Li+ to the more exposed holes in the hematite surface could shorten the diffusion length of lithium ions.27 The electrochemical performance of the as-prepared R-Fe2O3 mesoporous hollow spheres sample and nanoparticle aggregations sample in the cell configuration of Li/R-Fe2O3 was evaluated. Figure 6 shows the comparison discharge curves for the samples on the first cycle with a cutoff voltage of 0.5 V at a current density of 60 mA/g. The discharge curve of mesoporous hollow spheres electrode displays a long plateau and exhibits a high discharge capacity of 1279 mAh/g, while the electrode of nanoparticle aggregations exhibits 834 mAh/g. According to the results presented above, it is evident that the superior electrochemical activity of R-Fe2O3 mesoporous hollow spheres is consistent with that of the higher surface areas and mesoporous nanostructures, which increase proportion of atoms on the surface. The present R-Fe2O3 mesoporous hollow spheres, as indicated by the cycling performance in the inset of Figure 6, exhibit a good cycle performance and the reversible discharge capacity of the 15th cycle remains as 830 mAh/g, which is about 65% of the initial value, proving a favorable discharge capacity as well as capacity retention.

In summary, a smart complex precursor method in which the different removing modes of oxalate ligands in the reactant of ferric potassium oxalate was utilized to fabricate R-Fe2O3 hollow spheres with uniformly distributed honeycomb-like mesoporosity on the shell in high yield. The composite hollow spheres were composed of single-crystal iron oxide nanoparticles, which was not reported before. The as-obtained R-Fe2O3 mesoporous hollow spheres exhibited better sensitivity in the gas sensor and enhanced electrochemical activity in lithium ion battery compared to that of R-Fe2O3 nanoparticle aggregations. The facile preparation method provides a successful example for an alternative preparation of composite hollow structure without employing additional template or matrix, which may be of much significance in the synthesis of other composite hollow structure materials. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (No. 20621061) and the State Key Project of Fundamental Research for Nanomaterials and Nanostructures (2005CB623601). Supporting Information Available: XRD pattern, FTIR spectrum, and TEM images. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Zhang, J. L.; Wang, Y.; Ji, H.; Wei, Y. G.; Wu, N. Z.; Zuo, B. J.; Wang, Q. L. J. Catal. 2005, 229, 114. (2) Shekhah, O.; Ranke, W.; Schule, A.; Kolios, G.; Schlogl, R. Angew. Chem., Int. Ed. 2003, 42, 5760. (3) Brown, A. S. C.; Hargreaves, J. S. J.; Rijniersce, B. Catal. Lett. 1998, 53, 7. (4) Li, P.; Miser, D. E.; Rabiei, S.; Yadav, R. T.; Hajaligol, M. R. Appl. Catal., B 2003, 43, 151. (5) Oliveira, L. C. A.; Petkowicz, D. I.; Smaniotto, A.; Pergher, S. B. C. Water Res. 2004, 38, 3699. (6) Lai, C. H.; Chen, C. Y. Chemosphere 2001, 44, 1177. (7) Onyango, M. S.; Kojima, Y.; Matsuda, H.; Ochieng, A. J. Chem. Eng. Jpn. 2003, 36, 1516. (8) Wu, R. C.; Qu, J. H.; Chen, Y. S. Water Res. 2005, 39, 630. (9) Wu, R. C.; Qu, H. H.; He, H.; Yu, Y. B. Appl. Catal., B 2004, 48, 49. (10) Herrera, F.; Lopez, A.; Mascolo, G.; Albers, E.; Kiwi, J. Appl. Catal., B 2001, 29, 147. (11) (a) Chen, J.; Xu, L. N.; Li, W. Y.; Gou, X. L. AdV. Mater. 2005, 17, 582. (b) Wu, C. Z.; Yin, P.; Zhu, X.; OuYang, C. Z.; Xie, Y. J. Phys. Chem. B 2006, 110, 17806. (c) Gou, X. L.; Wang, G. X.; Park, J.; Liu, H.; Yang, J. Nanotechnology 2008, 19, 125606. (12) Zeng, H.; Li, J.; Liu, J. P.; Wang, Z. L.; Sun, S. H. Nature 2002, 420, 95. (13) Jordan, A.; Scholz, R.; Maier-Hauff, K.; Johannsen, M.; Wust, P.; Nadobny, J.; Schirra, H.; Schmidt, H.; Deger, S.; Loening, S.; Lanksch, W.; Felix, R. J. Magn. Magn. Mater. 2001, 225, 118. (14) (a) Zeng, S. Y.; Tang, K. B.; Li, T. W.; Liang, Z. H.; Wang, D.; Wang, Y. K.; Qi, Y. X.; Zhou, W. W. J. Phys. Chem. C 2008, 112, 4836. (b) Zeng, S. Y.; Tang, K. B.; Li, T. W.; Liang, Z. H.; Wang, D.; Wang, Y. K.; Zhou, W. W. J. Phys. Chem. C 2007, 111, 10217. (15) (a) Yuan, R. S.; Fu, X. Z.; Wang, X. C.; Liu, P.; Wu, L.; Xu, Y. M.; Wang, X. X.; Wang., Z. Y. Chem. Mater. 2006, 18, 4700. (b) Xiong, Y. J.; Li, Z. Q.; Li, X. X.; Hu, B.; Xie., Y. Inorg. Chem. 2004, 43, 6540. (c) Gong, C. R.; Chen, D. R.; Jiao, X. L.; Wang, Q. L. J. Mater. Chem. 2002, 12, 1844. (d) Lu, J.; Chen, D. R.; Jiao, X. L. J. Colloid Interface Sci. 2006, 303, 437. (e) Jiao, F.; Harrison, A.; Jumas, J. C.; Chadwick, A. V.; Kockelmann, W.; Bruce, P. G. J. Am. Chem. Soc. 2006, 128, 5468. (f) Zhong, L. S.; Hu, J. S.; Liang, H. P.; Cao, A. M.; Song, W. G.; Wan, L. J. AdV. Mater. 2006, 18, 2426. (16) (a) Titirici, M. M.; Antonietti, M.; Thomas, A. Chem. Mater. 2006, 18, 3808. (b) Bang, J. H.; Suslick, K. S. J. Am. Chem. Soc. 2007, 129, 2242. (c) Hu, C. Q.; Gao, Z. H.; Yang, X. R. Chem. Lett. 2006, 35, 1288. (d) Li, L. L.; Chu, Y.; Liu, Y.; Dong., L. H. J. Phys. Chem. C 2007, 111, 2123. (17) Djojoputro, H.; Zhou, X. F.; Qiao, S. Z.; Wang, L. Z.; Yu, C. Z.; Lu, G. Q. J. Am. Chem. Soc. 2006, 128, 6320.

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