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Synthesis, Multi-Nonlinear Dielectric Resonance, and Excellent Electromagnetic Absorption Characteristics of Fe3O4/ZnO Core/Shell Nanorods Yu-Jin Chen,*,† Fan Zhang,† Guo-gang Zhao,‡ Xiao-yong Fang,§ Hai-Bo Jin,| Peng Gao,*,# Chun-Ling Zhu,# Mao-Sheng Cao,| and G. Xiao† College of Science, Harbin Engineering UniVersity, Harbin 150001, China, Heilongjiang Institute of Science and Technology, Harbin 150001, China, School of Science, Yanshan UniVersity, Qinhuangdao 06604, China, and College of Material Science and Chemical Engineering, Harbin Engineering UniVersity, Harbin 150001, China ReceiVed: December 25, 2009; ReVised Manuscript ReceiVed: March 30, 2010
Fe3O4/ZnO core/shell nanorods are successfully fabricated by combing an inorganic-phase reaction with a hydrogen annealing process. The transmission electron microscopy analysis indicates that the diameter and the length of the core/shell nanorods are 25-80 and 0.35-1.2 µm, respectively. Electromagnetic properties of the core/shell nanorod-wax composites are investigated. The permittivity of the composites shows four dielectric resonant peaks in 2-18 GHz, which can be explained by the transmission line theory. The resonant behavior mainly results from interface polarization induced by the special core/shell structures, dipole polarization of both Fe3O4 and ZnO, and electron transfer between Fe2+ and Fe3+ ions in Fe3O4. The maximum reflection loss is about -30 dB at 10.4 GHz for the composites with a thickness of 1.5 mm, and the absorption bandwidth with the reflection loss below -20 dB is up to 11 GHz for an absorber with the thickness in 2-4 mm. Thus, our results demonstrate that the Fe3O4/ZnO core/shell nanorods are attractive candidates for a new kind of the electromagnetic wave absorptive materials. Introduction Fe3O4 has attracted extensive attention due to its metalinsulator transition around 120 K, half-metallic character, and strong spin polarization at room temperature. Moreover, its magnetic properties can be tuned by changing size, shape, and dimension. Therefore, Fe3O4 has wide applications in many areas such as gas sensors, optoelectronic and spintronic devices, and biomedicine.1-8 Up until now, zero-dimensional (0D) Fe3O4 nanostructures with various shapes and different sizes have been synthesized. The spherical Fe3O4 nanoparticles with the diameter varied from 3 to 20 nm have been synthesized by combining the organic-phase reaction with seed-mediated growth.9 Superstable Fe3O4 nanocrystals dispersible in both organic and aqueous solutions have been successfully fabricated by using organic ligands.10 Recently, Kovalenko, Yang, Liu, and Kim et al have reported on the synthesis of Fe3O4 nanocubes with different sizes.11 Sun et al. has developed an oxidation route to fabricate hollow Fe3O4 nanoparticles.12 Meanwhile, some complex structures of Fe3O4 such as superlattices have also obtained.13 However, there are few reports on the synthesis and electromagnetic (EM) absorption characteristics of one-dimensional (1D) Fe3O4 nanostructures because the synthesis of 1D Fe3O4 nanostructures requires relatively complex approaches such as the template-assistant technique or the magnetic field induced method.14 Recently, some hierarchical and core-shell structures with new or enhanced EM absorption properties have been reported.15 * To whom correspondence should be addressed. E-mail: chenyujin@ hrbeu.edu.cn (Y.-J.C.); penggao@ hrbeu.edu.cn (P.G.). † College of Science, Harbin Engineering University ‡ Heilongjiang Institute of Science and Technology § School of Science, Yanshan University | Beijing institute of Technology # College of Material Science and Chemical Engineering, Harbin Engineering University
For example, Ni/Ag core/shell nanoparticles fabricated by Lee et al. showed dual-frequency electromagnetic absorption compared to the naked Ni particles.15f Thus, the composites containing magnetic cores are very promising for EM wave absorption materials. However, the nanocomposites reported for attenuating EM wave are mainly based on 0D materials. 1D composites may have stronger EM absorption properties due to the larger anisotropy along the axis direction.15i Previously, we have reported synthesis and EM absorption properties of 1D Fe3O4/Fe/SiO2 and Fe3O4/SnO2 nanocomposites.15b,c Herein, we assemble ZnO on the surface of Fe3O4 and successfully fabricate Fe3O4/ZnO core/shell nanorods. The nanocomposites exhibit multi-nonlinear dielectric loss behavior and have excellent EM absorption properties due to the presence of ZnO nanoshells that have strong EM attenuation characteristics.10a,16 Experimental Section Fe3O4/ZnO core/shell nanorods were fabricated by combining an inorganic-phase reaction with a hydrogen annealing process. (1) Fe2O3/ZnO nanorods were first obtained. Typically, β-FeOOH nanorods17 (∼ 0.05 g) were dispersed in 15 mL of ethylenediamine (0.15 mol · L-1) aqueous solution by ultrasonication. A total of 15 mL of Zn (AC)2 (0.1 mol · L-1) aqueous solution were then added into the solution above with vigorous stirring. After being stirred for 15 min, the mixture was transferred into a Teflon-lined stainless steel autoclave with a capacity of 50 mL for hydrothermal treatment at 120 °C for 12 h. As the autoclave cooled to room temperature naturally, the precipitates were separated by centrifugation, washed with distilled water and absolute ethanol, dried in air, and annealed at 500 °C for 3 h. After the obtained powder was treated again by the similar process described above, except annealing the sample in air, Fe2O3/ZnO core/shell nanorods were fabricated. (2) The Fe3O4/ ZnO core/shell nanorods were produced by annealing Fe2O3/
10.1021/jp912178q 2010 American Chemical Society Published on Web 04/30/2010
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Figure 1. Characterization of Fe2O3/ZnO core/shell nanorods: (a) XRD patterns; (b) TEM image; (c and d) HRTEM image.
ZnO at 360 °C for 5 h under a mixture of Ar/H2 flow. After H2 deoxidation, the color of sample changed from red-brown to black. The composite samples used for EM absorption measurement were prepared by mixing the Fe3O4/ZnO core/shell nanorods with wax with 50 wt % of the core/shell nanorods. To overcome the aggregation of the nanorods, we first dispersed the nanorods into the acetone solution by ultrasonication, and then the mixture was added into the molten wax under ultrasonication and stirring. The acetone was removed by drying the mixture at 50 °C. The mixtures were then pressed into toroidal-shaped samples (φout: 7.00 mm; φin: 3.04 mm). The complex permittivity and permeability of the composites were measured by using the T/R coaxial line method. The measurement setup consisted of an HP 8722ES vector network analyzer with a synthesized sweep oscillator source and an S-parameter test set. Results and Discussion Figure 1a shows the XRD pattern of Fe2O3/ZnO core/shell nanorods. Compared with the data in JCPDs No. 39-1346, all peaks indicated by Miller indices in the pattern can be indexed to Fe2O3. The other peaks indicated by asterisks corresponding to wurtzite ZnO (JCPDs No. 36-1451) are also observed, suggesting that the composites were composed of Fe2O3 and
ZnO. Figure 1b is a typical transmission electron microscope (TEM) image of as-synthesized Fe2O3/ZnO core/shell nanorods. It is clearly seen that the diameter and the length of the core/ shell nanorods are 25-80 nm and 0.35-1.2 µm, respectively. Two significant differences are clearly discerned between the Fe2O3/ZnO core/shell nanorods and the naked Fe2O3 nanorods (their detail microstructures were shown in ref 18). The first difference is that each individual Fe2O3 nanorod has many porous structures along its axial direction; however, the porous feature becomes unclear after ZnO coated the surface of Fe2O3 nanorods. The second one is that the significant difference in the light contrast between the core and shell regions for the composites, whereas it is not observed for the naked Fe2O3 nanorods. The detail microstructure of the core/shell nanostructures were further characterized by high-resolution TEM (HRTEM). The measurement results for the different core-shell nanorods show that the thickness of ZnO shell is in range of 6-25 nm, as shown in Figure 1c,d. The clear interface is presented between Fe2O3 core and ZnO shell regions, indicating the heterojunction formed at the interface (the magnified HRTEM images also shown in Figure S1). The spaces are 0.489 and 0.248 nm in the core and shell regions, corresponding to (111) of Fe2O3 and (101) planes of ZnO, respectively. Those
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Figure 2. Characterization of Fe3O4/ZnO core/shell nanorods. (a) XRD patterns. (b) The Fe 2p core level XPS spectra of both core/shell nanorods: upper curve, Fe2O3/ZnO; lower curve, Fe3O4/ZnO. (c) TEM image; inset shows the magnified TEM image of an individual Fe3O4/ZnO core/shell nanorod (scale bar: 30 nm). (d) HRTEM image.
results reveal that Fe2O3/ZnO core/shell nanorods are successfully fabricated by the present method. After H2 deoxidation, the Fe2O3/ZnO composites can be transformed into Fe3O4/ZnO core/shell nanorods. Figure 2a is the XRD pattern of the final products. In this pattern, all peaks indicated by Miller indices come from the cubic crystal structure of Fe3O4 according to JPCDs No. 72-2303. However, because of the very similar patterns between Fe2O3 and Fe3O4, the XRD analyses cannot provide enough evidence to confirm that Fe2O3 has been transformed into Fe3O4 after the H2 deoxidation process. X-ray photoelectron spectroscopy (XPS) can be used to determine the difference in the oxidation state of the iron. Figure 2b shows Fe 2p core level XPS spectra of both samples. In the XPS spectra of the Fe2O3/ZnO core/shell nanorods, besides two main peaks at about 711 and 725 eV corresponding to Fe 2p3/2 and Fe 2p1/2, the shakeup satellite structures are also observed at the higher binding energy sides of the main peaks, indicated by arrows. Those satellite peaks are the fingerprints of the electronic structures of Fe3+ and indicate that Fe2+ is absent.19 If Fe2O3 is reduced into Fe3O4, the satellite structures in XPS become unresolved due to the presence of Fe2+ ion.19 The satellite structures are not presented in the XPS spectra of the Fe3O4/ZnO core/shell nanorods, as shown in Figure 2b. It demonstrates that Fe2O3 in the nanocomposites has been transformed into Fe3O4 after the H2 deoxidation process. In addition, the color of the final product was changed into black from red-yellow (the photograph of the core/shell nanorods shown in Figure S2), which also supports the result above. The diffraction peaks of ZnO, shown by the asterisks, are still presented in the XRD pattern. It indicates that ZnO in the nanocomposties is not converted to Zn during the H2 deoxidation process. The reason is that the standard molar reaction Gibbs free energy at 360 °C is much larger than zero for the chemical
reaction, ZnO(s) + H2(g) f Zn(s) + H2O(g). Figure 2c is a TEM image of the Fe3O4/ZnO core/shell nanorods. From this figure, it can be seen that the core/shell nanorods have similar morphology and size of Fe2O3/ZnO composites. For the core/ shell structures, most of Fe3O4 cores are uniformly coated with ZnO as shown in the inset in Figure 2c. However, for the Fe3O4 nanorods with small diameters, the ends are uncoated due to the larger stress. The very small number of particles in the product formed during the synthesis process of β-FeOOH nanorods.17 The HRTEM image of Fe3O4/ZnO core/shell nanorods is shown in Figure 2d (the magnified HRTEM images also shown in Figure S3). The spaces are 0.482 and 0.248 nm in the core and shell regions, corresponding to (111) of Fe3O4 and (101) planes of ZnO, respectively. ZnO is a typical dielectric loss material for microwave, whereas Fe3O4 has dual-loss characteristics. Moreover, the heterojunctions have been formed at the interface for Fe3O4/ ZnO core/shell nanorods synthesized in the work. The heterojunction has a very important effect on the electronic and magnetic properties of materials. Therefore, the Fe3O4/ZnO core/ shell nanorods may exhibit good EM absorption properties. Figure 3a shows the frequency dependence of the real part (ε′) and imaginary part (ε′′) of relative complex permittivity (εr ) ε′ - jε′′) for the wax composites containing 50 wt % of the Fe3O4/ZnO core/shell nanorods. The ε′ value is in the range of 10.20-16.79. The ε′′ value is in the range of 2.61-6.14, which is much higher than those of ZnO-coated Fe nanocapsules,20 carbon/ferrite nanocomposites,21 and ZnO nanowires.16 It suggests that Fe3O4/ZnO core/shell nanorods have strong dielectric loss against EM wave. In terms of the electromagnetic theory, such high dielectric loss results from the naturally physical properties of Fe3O4 and ZnO and the special core/shell structures. First, the dipoles are presented in both Fe3O4 and ZnO,
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Figure 3. (a) Complex permittivity and (b) dielectric tangent loss of the core/shell nanorod-wax composites. (c) Schematic diagram and (d) equivalent circuit model of Fe3O4/ZnO core/shell nanorods.
especially when the size is in nanoscale.15a,16 The number of surface atoms with unsaturated bonds would greatly increase as the size decrease, resulting in an increase of the dipoles. The dipole polarizations will contribute to the dielectric loss.15a,16 It is one reason why ZnO nanowires have stronger EM absorption properties than ZnO microparticles.16 Second, the dielectric loss is related to the natural structure of Fe3O4. Its chemical formula can be written as FeA3+[ Fe2+, Fe3+ ]BO42-, where Fe cations occupy interstices of a face-centered cubic closed packed frame of oxygen ions, and A and B denote the tetrahedral sites and octahedral sites, respectively. Electrons can transfer between Fe2+ and Fe3+ ions in B sites as the field (EM wave) was applied, and thus, electronic spin and charge polarization has a significant effect on the loss.15b,c Third, the interfaces at the core and the shell are clearly observed, as shown in Figure 2d. Therefore, the interfacial polarization and the associated relaxation contribute to the dielectric loss. Such improved dielectric loss induced by the interfacial polarization has been observed in the other core/shell system.15d-g For example, Ag-Ni core-shell materials showed additional absorption that was induced by the interfacial polarization in 2-18 GHz compared to the naked Ni nanoparticles.15f In addition, the defects will form at the interfaces due to the crystal mismatch between Fe3O4 and ZnO. They can serve as polarized centers, leading to an additional dielectric loss.15h On the basis of the analyses above, the higher dielectric loss is related to the natural properties of Fe3O4 and ZnO and the special core/shell structures. Interestingly, the dielectric loss exhibits a very complex nonlinear resonant behavior, and four resonant peaks appear at the ε′′ curve. To determine the locations of those peaks, we plot the dielectric tangent loss (tan δe ) ε′′/ε′) versus frequency curve, as shown in the inset of Figure 3b. From the plot, the resonant peaks are at 7.3, 10.6, 13.8, and 16.6 GHz, respectively. It is possible that there are one or two resonant peaks for a single material. However, four resonant peaks are found forFe3O4/ZnO core/shell nanorods. Consequently, the nonlinear resonant behavior results from the special structure of the core/ shell nanorods. According to the testing principle of EM parameter (εr and µr), Fe3O4/ZnO core/shell nanorods could be considered as a transmission line with total terminal reflection.22 The schematic diagram of an individual core/shell nanorod and the corresponding equivalent circuit model are shown in the inset of Figure 3c,d. In the model, R1, C1 and R3, C3 denote the resistance and capacitance of ZnO, R2, L2I denote the resistance and inductance of Fe3O4, and RL denotes resistance of metal substrate, whereas L1, L2, and L3 also denote the resistances of
Figure 4. (a) Complex permeability and (b) magnetic tangent loss of the core/shell nanorod-wax composites. (c) Schematic diagram for the transformation of the electric field energy into the magnetic energy. (d) Magnetization hysteresis loops of Fe3O4/ZnO core/shell nanorods at room temperature.
the corresponding materials, which can be considered as inductances under EM irradiation conditions. It is found that there are five tuning loops (L1R1C1, L2C1, L2IC3, L3R3C3, RLC3) in the equivalent model. Both L1R1C1 and L3R3C3 loops, related to ZnO, have the same resonant frequency. Thus, the core/shell nanorods should have four resonant frequencies according to the equivalent circuit model, just corresponding to the four resonant peaks in the dielectric loss curve. Figure 4a shows the complex permeability (µr ) µ′r - jµ′′r) measured for Fe3O4/ZnO core/shell nanorods. It reveals that the value of the real part (µ′) is in the range of 0.8-1.12. Meanwhile, the image part (µ′′) is less than 0.1 over 2-18 GHz. It indicates that the values of the magnetic loss are largely less than those of the dielectric loss for the core/shell structures. Therefore, the EM attenuation mechanism of Fe3O4/ZnO core/ shell nanorods is mainly dependent on the dielectric loss. It is different from other nanocomposites previously reported such as Fe/ZnO core/shell nanoparticles, Fe-encapsulated carbon nanotubes, and so on.15b,c,20,22 It should be noted that the µ′′ value is negative, especially in the high frequency region (9-18 GHz). Generally, for left-hand materials, the real parts of both permittivity and permeability are negative. However, only µ′′ value of the composites is negative. Therefore, the phenomenon should not be attributed to left-hand properties of the materials. Recently, Deng et al. found that multiwalled carbon nanotube/ wax composites showed negative permeability, and they explained the phenomenon as that the magnetic energy was radiated out from the composites.23 The negative value means that the magnetic field energy is not reduced but increased after the composites being irradiated by EM wave. According to the principle of conservation of energy, the increase of the magnetic field energy should be resulted from the reduction of the electric field energy. To clarify it, we plot the magnetic tangent loss (tan δe ) ε′′/ε′) versus frequency curve, as shown in the inset of Figure 4b. When the data in Figure 3b with those in Figure 4b are compared, it can be found that the tendency of the increase of the dielectric loss (corresponding to reduction of electric field energy) and the decrease of the magnetic loss (corresponding to increase of magnetic field energy) is just consistent in high frequency region (9-18 GHz). It reveals that the energy conversion occurs as the composites being irradiated by the high frequency EM wave. One possible transforming
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Figure 5. Reflection loss for the Fe3O4/ZnO core/shell nanorods with thickness 1-4 mm.
route is described as follows. The induced electric current will be produced during the polarization processes (including dipole and interfacial polarization, etc.) due to the semiconductor characteristics of ZnO, as schematically shown in Figure 4c. Then one part of the electric current is dissipated as the formation of the heat by the resistance of ZnO, whereas another part can produce a magnetic field. Due to the ferromagnetic properties of the composites shown in Figure 4d, the internal magnetic field will be further enhanced. Therefore, the magnetic field energy may increase through the process above. However, how the transformation between the electric field and magnetic field energies is relished is relatively complicated and, thus, it requires further theoretical analysis. Fe3O4/ZnO core/shell nanorods exhibit multi-nonlinear dielectric resonance and have relatively larger ε′′ values over 2-18 GHz range. Therefore, the core/shell nanocomposites should have excellent EM absorption properties. According to the transmit-line theory,24 the reflection loss (RL) can be calculated by the following equations:
Zin ) (µr /εr)1/2 tan h[j2πfd/c)(µrεr)1/2]
(1)
,
RL(dB) ) 20 log
|
Zin - 1 Zin + 1
|
(2)
where Zin is the input impedance of the absorber, µr and εr are, respectively, the relative complex permeability and permittivity, c is the velocity of electromagnetic waves in free space,f is the frequency of microwaves, andd is the thickness of the absorber. The reflection loss (RL) of the core/shell nanorods with the thickness of the absorber varied from 1 to 4 mm can be calculated as shown in Figure 5. It indicates that the maximum reflection loss reaches -30 dB at 10.4 GHz for the absorber with the thickness in only 1.5 mm and up to -35 dB at 14.6 GHz for the absorber with the thickness in 2 mm. Moreover, the absorption bandwidth with the reflection loss below -20 dB is up to 11 GHz (from 4.5 to 15.5 GHz) for the absorber with the thickness in 2-4 mm. Therefore, the Fe3O4/ZnO core/ shell nanorods show excellent EM attenuation properties, including lightweight and strong and wide band absorption. The EM absorption properties are better than those of ZnO nanowires reported in our previous literatures, Ni/C nanocapsules and CNT/ CoFe2O4 nanocomposites. Therefore, Fe3O4/ZnO core/shell
In summary, Fe3O4/ZnO core/shell nanorods were successfully fabricated. The core/shell nanorods exhibited multinonlinear resonant behavior. According to the equivalent circuit model and the electromagnetic theory, it is related to Fe3O4, ZnO, and special interface structures for the core/shell nanorods. The maximum reflection loss reaches -30 dB at 10.4 GHz for the absorber with the thickness in 1.5 mm, and the absorption bandwidth with the reflection loss below -20 dB is up to 11 GHz for the absorber with the thickness in 2-4 mm. Our results demonstrate that the Fe3O4/ZnO core/shell nanorods obtained in this work are attractive candidates for the new types of EM wave absorptive materials. Acknowledgment. The authors acknowledge the support from the National Natural Science Foundation of China (Grant Nos. 50772025, 50972015) and the Natural Science Foundation of Heilongjiang Province, China (Grant Nos. TE2005-26, F200828 and E200839). The project was also supported by the Ministry of Science and Technology of China (Grant No. 2008DFR20420) and also the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20070217002). Supporting Information Available: Magnified HRTEM images and the photograph of Fe2O3/ZnO and Fe3O4/ZnO core/ shell nanorods. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Tang, J. K.; Wang, K. Y.; Zhou, W. L. J. Appl. Phys. 2001, 89, 7690. (b) Gee, S. H.; Hong, Y. K.; Erickson, D. W.; Park, M. H.; Sur, J. C. J. Appl. Phys. 2003, 93, 7560. (2) Huh, Y. M.; Jun, Y. W.; Song, H. T.; Kim, S. J.; Choi, J. S.; Lee, J. H.; Yoon, S.; Kim, K. S.; Shin, J. S.; Suh, J. S.; Cheon, J. J. Am. Chem. Soc. 2005, 127, 12387. (3) Lu, Z. L.; Xu, M. X.; Zou, W. Q.; Wang, S.; Liu, X. C.; Lin, Y. B.; Xu, J. P.; Lu, Z. H.; Wang, J. F.; Lv, L. Y.; Zhang, F. M.; Du, Y. W. Appl. Phys. Lett. 2007, 91, 102508. (4) Venkatesan, M.; Nawka, S.; Pillai, S. C.; Coey, J. M. D. J. Appl. Phys. 2003, 93, 8023. (5) Goya, G. F.; Berquo´, T. S.; Fonseca, F. C.; Morales, M. P. J. Appl. Phys. 2003, 94, 3520. (6) Lee, H.; Lee, E.; Kim, D. K.; Jang, N. K.; Jeong, Y. Y.; Jon, S. J. Am. Chem. Soc. 2006, 128, 7383. (7) Hu, F. Q.; Wei, L.; Zhou, Z.; Ran, Y. L.; Li, Z.; Gao, M. Y. AdV. Mater. 2006, 18, 2553. (8) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. D. Nature 2005, 437, 121. (9) Sun, S. H.; Zeng, H. J. Am. Chem. Soc. 2002, 124, 8204. (10) Kim, M.; Chen, Y. F.; Liu, Y. C.; Peng, X. G. AdV. Mater. 2005, 7, 1429. (11) (a) Kovalenko, M. V.; Bodnarchuk, M. I.; Lechner, R. T.; Hesser, G.; Scha¨ffler, F.; Heis, W. J. Am. Chem. Soc. 2007, 129, 6352. (b) Kim, D.; Lee, N.; Park, M.; Kim, B. H.; An, K.; Hyeon, T. J. Am. Chem. Soc. 2009, 131, 454. (c) Liu, Z. W.; Zhang, D. H.; Han, S.; Li, C.; Lei, B.; Lu, W. G.; Fang, J. Y.; Zhou, C. W. J. Am. Chem. Soc. 2005, 127, 6. (d) Yang, H. T.; Ogawa, T.; Hasegawa, D.; Takahashi, M. J. Appl. Phys. 2009, 103, 07D526. (12) (a) Peng, S.; Sun, S. H. Angew. Chem., Int. Ed. 2006, 46, 4155. (b) Cheng, K.; Peng, S.; Xu, C. J.; Sun, S. H. J. Am. Chem. Soc. 2009, 131, 10637. (13) Song, Q.; Ding, Y.; Wang, Z. L.; Zhang, Z. J. J. Phys. Chem. B 2006, 110, 25547. (14) (a) Li, X. L.; Liu, J. F.; Li, Y. D. Mater. Chem. Phys. 2003, 80, 222. (b) Terrier, C.; Abid, M.; Arm, C.; Serrano-Guisan, S.; Gravier, L.; Ansermet, J.-Ph. J. Appl. Phys. 2005, 98, 086102. (c) Wang, J.; Chen, Q. W.; Zeng, C.; Hou, B. Y. AdV. Mater. 2004, 16, 137. (d) Liu, Z. Q.; Zhang, D. H.; Han, S.; Li, C.; Lei, B.; Lu, W. G.; Fang, J. Y.; Zhou, C. W. J. Am. Chem. Soc. 2005, 127, 6. (e) Chang, M. T.; Chou, L. J.; Hsieh, C. H.; Chueh, Y. L.; Wang, Z. L.; Murakami, Y.; Shindo, D.
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