Luminescent and Magnetic Properties of Fe3O4@SiO2@Y2O3:Eu3+

Article pubs.acs.org/JPCC

Luminescent and Magnetic Properties of Fe3O4@SiO2@Y2O3:Eu3+ Composites with Core−Shell Structure Lizhu Tong, Jianhui Shi, Deming Liu, Quanhong Li, Xiaozhen Ren, and Hua Yang* College of Chemistry, Jilin University, Changchun, 130012, China ABSTRACT: Multifunctional Fe3O4@SiO2@Y2O3:Eu3+ composites were prepared with a facile solvothermal method followed by a subsequent heat treatment. Their structure and magnetic and luminescent properties were analyzed and discussed. Y2O3:Eu3+ phosphor as a shell was coated on the surface of Fe3O4@SiO2 as the core to form Fe3O4@SiO2@ Y2O3:Eu3+ composites. The particle size of the composites is 262.36 nm which consists of the magnetic core with about 210 nm in diameter and a silica shell with an average thickness of about 20 nm. It exhibits ferromagnetic behavior with the special saturation magnetization Ms of 12.62 emu/g, the negligible coercivity Hc, and remanence Mr at room temperature and exhibits a strong red emission peak originating from the electric−dipole transition 5D0 → 7F2 of Eu3+ ions at 611 nm. The current and potential biomedical uses include biological imaging, cell tracking, and magnetic bioseparation.

1. INTRODUCTION Magnetic and fluorescent inorganic nanocomposites are particularly important due to their broad range of potential applications. Various nanocomposite materials play a number of important roles in modern science and technology.1 The multifunctional nanocomposite materials combine with magnetic and luminescent properties in one entity, in particular those with potential applications in biotechnology and nanomedicine.2−7 Meanwhile, magnetic iron oxide nanoparticles have also been proved promising in a wide rang of biomedical applications such as cell separation, magnetic resonance imaging (MRI), magnetically assisted drug delivery, and tissue repair.8−11 Magnetic and fluorescent nanocomposites include a variety of materials such as silicabased, dye-functionalized magnetic nanoparticles and quantum dots−magnetic nanoparticle composites. However, little attention has been paid to the combination of magnetite materials with luminescent materials due to the quenching effect by Fe3O4 magnetite during mixture.11,12 It has recently been realized that metallic nanoparticles can be greatly stabilized against coalescence by encapsulation within inorganic layers.13−16 Thus, the design of an inert layer of SiO2 that can effectively separate the lanthanidebased luminescent component from the magnetite is very important in protecting the luminescence from quenching by magnetite. So far, various approaches have been developed to synthesize magnetic and fluorescent silica nanocomposites.17 For example, Wang et al. reported a method to make various types of nanoparticles, including CdTe quantum dots, Au nanoparticles, and Fe3O4 nanoparticles, assemble on silica microspheres. The silica shell coated on the surface of the magnetic cores is an excellent biocompatible material with good stability, easy functionalization, and low cytotoxicity.18−22 The combination of magnetic and luminescent properties into a single micro- or nanocomposite system through the SiO2 matrix © 2012 American Chemical Society

will allow the development of a novel multifunctional biomedical platform for multimodal imaging and simultaneous diagnosis. The silica-coated magnetic nanoparticles not only offer improved stability but also help in binding the various biological ligands on the nanoparticle surface.23,24 The properties of the assembled nanoparticles were well retained in the nanocomposite assemblies, and the controllable integration of magnetic and fluorescent properties was achieved.25 Veronica and his co-workers described a synthesis of composite silica spheres with magnetic and luminescent functionalities by a modified Stöber method combined with a layer-bylayer assembly technique.26 Even now, progress in functionalization of silica composites is made continuously. A lot of great efforts have been made to prepare silica spheres with more function. The composites are applied in the potential biomedical uses, such as biological imaging, cell tracking, magnetic bioseparation, nanomedicine, and bio-chemo-sensor.27−30 In this paper, we report that Fe3O4 nanoparticles synthesized by the facile solvothermal method were encapsulated with SiO2 via the Stöber method and further coated by the deposition of Y2O3:Eu3+ to prepare Fe3O4@SiO2@Y2O3:Eu3+ composites. Then, the relationship between the magnetic and fluorescent properties of the composites is discussed.

2. EXPERIMENTAL SECTION 2.1. Materials. All chemicals are of analytical grade and are used as received without further purification. Ferrous chloride FeCl3·6H2O, HCl (analytical reagent, A. R., Beijing Fine Received: December 29, 2011 Revised: February 2, 2012 Published: February 13, 2012 7153

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Chemical Company, China), CH3COONa (A. R. Beijing Fine Chemical Company, China), tetraethyl orthosilicate (TEOS), ammonia aqueous (28 wt %) (Shanghai Chem. Reagent Co.), and Ln2O3 (Y2O3, Eu2O3 99.99%) are purchased from the Beijing Chemical Reagent Company, the ethylene glycol (EG), polyethylene glycol (FW10000), and ethanol were from A. R., Beijing Fine Chemical Company of China. 2.2. Synthesis of Fe3O4 Nanoparticles. Fe3O4 nanoparticles were prepared by the solvothermal method. FeCl3· 6H2O (1.35 g, 5 mmol) was dissolved in ethylene glycol (40 mL), followed by the addition of NaAc (3.6 g) and polyethylene glycol (1.0 g) into the system. The mixture was stirred vigorously for 30 min and then sealed in a teflon-lined stainlesssteel autoclave (50 mL capacity). The autoclave was heated at 200 °C for 8−12 h and cooled to room temperature to form the black products. The products were washed several times with ethanol and dried at 80 °C for 12 h to prepare Fe3O4 nanoparticles. 2.3. Synthesis of Fe3O4@SiO2 Nanoparticles. The Fe3O4@SiO2 nanoparticles were prepared by a Stöber sol−gel process. In a typical procedure, 0.10 g of Fe3O4 particles was dispersed in the mixture solution of 40 mL of ethanol and 10 mL of H2O. After adding 1.5 mL of NH4OH (28 wt %), the tetraethyl-orthosilicate (TEOS, 3 mL) was added to the mixture solution, respectively. The mixture solution was stirred at 40 °C for 6 h to obtain the particles, and then the particles were washed with ethanol and deionized water to prepare Fe3O4@ SiO2 nanoparticles. 2.4. Synthesis of Fe3O4@SiO2@Y2O3:Eu3+ Composites. In a typical synthesis, 0.97 mmol of Y2O3 and 0.03 mmol of Eu2O3 were dissolved in dilute HCl, resulting to form a colorless solution of YCl3 and EuCl3. Then, CH3COONa (2.0 g) and mixing solution (37 mL) of ethylene glycol (EG) and water (volume ratio of EG:H2O is 35:2) were added to the mixture solution of YCl3 and EuCl3. Fe3O4@SiO2 nanoparticles (0.077 g) were taken into the mixture solution, stirred for 3 h, charged into a Teflon-lined stainless autoclave, and heated at 180 °C for 8 h to obtain the precursors. The precursors were separated by filtration, washed with the ethanol and distilled water several times, dried at 50 °C for 12 h, and heated at 800 °C for 3 h to prepare Fe3O4@ SiO2@Y2O3:Eu3+ composites. Synthesis routes of luminomagnetic bifunctional composites with the core−shell structure are in Scheme 1.

Magnetic and luminescent properties of the composites were tested by the vibrating sample magnetometer (VSM), and a spectrophotometer (Hitachi F-4500 spectrouorimeter equipped with a 150 W xenon lamp as the excitation source) was used for the photoluminescent (PL) measurement.

3. RESULTS AND DISCUSSION The morphology and crystal structure of the composites were characterized by SEM and TEM images in Figure 1. Figure 1a

Figure 1. SEM images of Fe3O4 (a), Fe3O4@SiO2 (b), and TEM images (c and d) of Fe3O4@SiO2@Y2O3:Eu3+.

shows the SEM image of PEG-coated Fe3O4 nanoparticles. It can be seen that the shape of nanoparticles is almost spherical with an average particle size of about 210 nm. The surface of the Fe3O4 particles is not smooth because they were composed of many reunited nanoparticles. Figure 1b shows a SEM image of Fe3O4@SiO2 microspheres. The microspheres are smooth on the surface of spherical particles with the average particle size about 230 nm. That is to say, the smooth Fe3O4@SiO2 microspheres are composed of magnetic cores with an average diameter of about 210 nm and a silica shell with thickness of ca. 20 nm on average. The thickness of the silica shell can be tuned by simply varying the initial amount of TEOS. Furthermore, the TEM image of Figure 1(c and d) reveals a core−shell structure of the Fe3O4@SiO2@Y2O3:Eu3+particles, in which a dark core of magnetite with the diameter 230 nm and a gray porous silica shell with the thickness 32.36 nm can be clearly seen. Most of the samples still are spherical shape with an average particles size ca. 262.36 nm. It is shown that Fe3O4@SiO2@Y2O3:Eu3+ composites have the core−shell structure, and the Y2O3:Eu3+ particles as the shell are well coated on the surface of Fe3O4@ SiO2 particles as the core. Synthesis routes of luminomagnetic bifunctional composites with the core−shell structure are in Scheme 1. Figure 2 exhibits the XRD patterns of Fe3O4, Fe3O4@SiO2, Y2O3:Eu3+, Fe3O4@Y2O3:Eu3+, and Fe3O4@SiO2@Y2O3:Eu3+, respectively. As shown in Figure 2a, the positions of all diffraction peaks match well with the standard JCPDS 89-0688 of Fe3O4 powder, which indicates that the Fe3O4 particles are single phase and belong to the cubic system. For Fe3O4@SiO2, the broad band from 22° to 25.0° can be assigned to the amorphous SiO2 shell (JCPDS No. 29-0085). In the case of Fe3O4@SiO2@Y2O3:Eu3+, the obvious XRD diffraction peaks at 20.59°, 29.12°, 33.78°, 48.50°, and 57.63° corresponding to

Scheme 1. Illustration for the Formation Process of the Spherical Fe3O4@SiO2@Y2O3:Eu3+ Composites

2.5. Characterization. The purities of all the samples were checked by X-ray diffraction measurements (XRD) at room temperature using Cu Kα radiation (Kα = 1.54059 Å). The morphology and microscope structure were characterized by a scanning electronic microscope (SEM, Philps XL-30) and transmission electron microscopy (TEM, JEOL Jem-1200EX). 7154

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from the phosphor is partially covered and irregularly reflected by the silica coating. As a result of this, a detected quantity of light coming out of the phosphor is diminished, thereby giving a reduction in luminescent intensity. The silica matrix acts as a barrier to protect Y2O3:Eu3+ from the quenching effects of Fe3O4. Thus, the intensities of both excitation and emission spectra of Fe3O4@ SiO2@Y2O3:Eu3+ are decreased. The luminescent intensities of the Fe3O4@SiO2@Y2O3:Eu3+ composites are decreased significantly compared with the ones of pure Y2O3:Eu3+ sample. The magnetic Fe3O4 is caused by the decrease of absorption UV energy of Y2O3:Eu3+and is a strong quenching effect on the Y2O3:Eu3+ sample. Therefore, the decreased volume for the photon−solid interaction on the surface of Fe3O4@SiO2@Y2O3:Eu3+ composites is a main reason for the decreasing luminescent intensities. Figure 4 shows the excitation and emission spectra of Fe3O4@ SiO2@Y2O3:Eu3+ composites under magnetic field effects 0.025 T

Figure 2. XRD patterns of Fe3O4 (a), Fe3O4@SiO2 (b), Y2O3:Eu3+ (c), Fe3O4@Y2O3:Eu3+ (d), and Fe3O4@SiO2@Y2O3:Eu3+ (e).

(211), (222), (400), (440), and (622) can be indexed to the JCPDS card No. (JCPDS 88-1040) of Y2O3:Eu3+ crystals, so Y2O3:Eu3+ crystals are coated successfully on the surface of Fe3O4@SiO2. There are little differences of XRD diffraction peaks between Fe 3 O 4 @Y 2 O 3 :Eu 3+ and Fe 3 O 4 @SiO 2 @ Y2O3:Eu3+, demonstrating that the incorporation of SiO2 has nearly no effect on the crystalline properties of the Fe3O4 as the core and Y2O3:Eu3+ as the shell. Additionally, no additional peaks for other phases can be detected, indicating the formation of pure products during the annealing process. The excitation and emission spectra of the Fe3O4@Y2O3:Eu3+ and Fe3O4@SiO2@Y2O3:Eu3+ composites are shown in Figure 3.

Figure 4. Excitation (A) and emission (B) spectra of Fe3O4@SiO2@ Y2O3:Eu3+ under the external magnetic field 0.25 T for 0 h (a), 1 h (b), 4 h (c), 5 h (d), and 3 h (e).

for different time which is 0, 1, 3, 4, and 5 h, respectively. Under the external magnetic field, the luminescent intensities of the nanocomposite are discussed. As can be seen from the excitation and emission spectra, the luminescent intensities of the composites are gradually increased until 3 h and then gradually decreased. It is shown that this causes a strong quenching effect of the magnetic field on the luminescent intensities of Fe3O4@SiO2@Y2O3:Eu3+ composites. The inducement obtained by the arrangement order of magnetic domains is reduced after 3 h under an external magnetic field, which leads to a reduction of the luminescent intensities. When the external magnetic field takes effect on the Fe3O4@SiO2, the magnetic energy is partly transferred to the Fe3O4@SiO2, and the rest is released in the form of heat. This effect also represented that the valence electron energy of Fe3O4 magnetite is increased, and then the energy is transferred from the valence electron of magnetite to that of phosphors.32 The other reason may be that the electrons of Eu3+ in the d−f transition process have a directional rearrangement process under the effect of external magnetic field, which weakened the effects of electronic cross-relaxation. The whole mechanism process is shown in Scheme 2. Magnetic properties of the products were examined by a VSM at room temperature. Figure 5 shows the magnetic hysteresis loops of @SiO2@Y2O3:Eu3+, Fe3O4@Y2O3:Eu3+, Fe3O4@SiO2, and Fe3O4 which have special saturation magnetizations Ms of 6.07, 12.62, 52.73, and 69.27 emu/g, respectively. Their special saturation magnetizations Ms are decreased evidently when the Fe3O4 are coated with SiO2 and further Fe3O4@SiO2@ are coated with Y2O3:Eu3+. It is clearly seen that all the samples have stronger magnetism with negligible coercivity Hc and remanence Mr at room temperature. The special saturation magnetization values of Fe3O4@ SiO2@, Fe3O4@SiO2@Y2O3:Eu3+, and Fe3O4@Y2O3:Eu3+ are

Figure 3. Excitation (A) and emission (B) spectra of Fe3O4@ Y2O3:Eu3+ (a) and Fe3O4@SiO2@Y2O3:Eu3+ (b).

Their characteristic of the excitation and emission bands are similar to those observed in a previous study for Y2O3:Eu3+ crystals which suggests that the luminescent properties are maintained in the product. The excitation spectra are monitored at 610 nm as the emission wavelength, while the emission spectra are measured at 254 nm as the excitation wavelength. The excitation spectra (Figure 3A) consist of a broad band with a maximum at 254 nm and some sharp peaks in the bigger wavelength region. The former is due to the charge-transfer band (CTB) of Eu3+−O2−, and the latter is from the f−f transition lines of Eu3+ with weak intensity compared with the Eu3+−O2− CTB.31The emission spectra of the europium-doped yttrium shows the strong, red hypersensitive electronic transition of 5D0 → 7F2 of Eu3+ ions at 610 nm excited at 254 nm. The emission intensities are increased from Fe3O4@ Y2O3:Eu3+ to Fe3O4@SiO2@Y2O3:Eu3+ because the light emitted 7155

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magnetic field. Figure 6b shows the particles also emitted obvious red emission when it accumulates with (b) external magnetic field. To illustrate dispensability in aqueous medium and magnetic response to the external magnetic field, a simple experiment is performed. An optical photograph of the magnetic microspheres attracted by an external magnet is exhibited in Figure 6. When a magnet is placed close to the glass vial, the composites are attracted toward the magnet very quickly and accumulated to the side of the glass vial near the magnet within two minutes, and the solution became clear and transparent. After removing the external magnet and sonication, the magnetic microspheres can be rapidly redispersed again. This simple experiment confirms that the composites possesse good water dispersity and magnetic separation characteristics. Fe3O4@SiO2@Y2O3:Eu3+ composites show the same property under UV-light irradiation. It is shown that the nanoparticles exhibit good magnetic responsibility and redispersed properties, which is a potential application for targeting and separation.

Scheme 2. Mechanism of the Modification for the Luminescence Intensity

4. CONCLUSIONS In summary, we demonstrate a successful synthesis of multifunctional Fe3O4@SiO2@Y2O3:Eu3+ composites with welldefined core−shell nanostructures. The magnetic and luminescent properties of Fe3O4@SiO2@Y2O3:Eu3+ composites are better than that of Fe3O4@Y2O3:Eu3+ composites. The silica interface as the shell is very important to a sandwich structured material with magnetic and luminescent properties, and when the composites are under the external magnetic field 0.25T for different times, their luminescent intensities are gradually increased for 3 h and then gradually decreased. There are the featuring tunable luminescent properties, strong magnetic response, and superior stability of the composites. We believe similar approaches can be used for the development of certain fluorescent−magnetic composites with low toxicity. Additionally, the design concept for the multifunctional nanomaterials may open up new opportunities in bioanalytical and biomedical applications, such as chemical/biosensor, nanoelectronics, and so on.

Figure 5. Magnetic hysteresis loops of Fe3O4@SiO2@Y2O3:Eu3+ (a), Fe3O4@Y2O3:Eu3+ (b), Fe3O4@SiO2 (c), and Fe3O4 (d).

lower than the Fe3O4. Compared with the Fe3O4 particles, the special saturation magnetization of the Fe3O4@SiO2 microspheres obviously is decreased because the diamagnetic contribution of the thick silica shell resulted in a low mass fraction of the Fe3O4 magnetic substance. Nevertheless, there is only a small difference between the special saturation magnetization of Fe3O4@Y2O3:Eu3+ and Fe3O4@SiO2@ Y2O3:Eu3+. The reason is the thin-layer SiO2 shell can not cause a larger mass fraction change of the Fe3O4 magnetic substance from Fe 3 O 4 @Y 2 O 3 :Eu 3+ to Fe 3 O 4 @SiO 2 @ Y2O3:Eu3+. Though the special saturation magnetization of Fe3O4@SiO2@Y2O3:Eu3+ particles is less than that for Fe3O4@ Y2O3:Eu3+, it may be believed to possess enough strong magnetic attraction for effectively magnetic separation. Figure 6 shows the photographs of the composites dispersed in an aqueous medium under UV irradiation at 365 nm and

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China.

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REFERENCES

(1) Kouassi, G. K.; Irudayaraj, J. Anal. Chem. 2006, 78, 3234−3241. (2) Ito, A.; Shinkai, M.; Honda, H.; Kobayashi, T. J. Biosci. Bioeng. 2005, 100, 1−11. (3) Hu, S. H.; Tsai, C. H.; Liao, C. F.; Liu, D. M.; Chen, S. Y. Langmuir 2008, 24, 11811−11818. (4) Li, M. J.; Chen, Z.; Yam, V. W. W.; Zu, Y. ACS Nano 2008, 2, 905. (5) Zhang, M.; Shi, S.; Meng, J.; Wang, X.; Fan, H.; Zhu, Y.; Wang, X.; Qian, Y. J. Phys. Chem. C 2008, 112, 2825−2830. (6) Zhang, Y.; Pan, S.; Teng, X.; Luo, Y.; Li, G. J. Phys. Chem. C 2008, 112, 9623−9626.

Figure 6. Photographs of Fe3O4@SiO2@Y2O3:Eu3+ composites without (a) and with (b) external magnetic field in natural light (inset) and UV irradiation at 365 nm.

natural light, respectively. In Figure 6a, the red emission of the composites can be observed under UV light without an external 7156

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(7) 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, 12387−12391. (8) Buathong, S.; Ung, D.; Daou, J.; Ulhaq-Bouillet, C.; Pourroy, G.; Guillon, D.; Ivanova, L.; Bernhardt, I.; Bégin-Colin, S.; Donnio, B. J. Phys. Chem. C 2009, 113, 12201−12212. (9) Lien, Y. H.; Wu, T. M. J. Colloid Interface Sci. 2008, 326, 517− 521. (10) Guo, J.; Yang, W.; Wang, C.; He, J.; Chen, J. Chem. Mater. 2006, 18, 5554. (11) Levin, C. S.; Hofmann, C.; Ali, T. A.; Kelly, A. T.; Morosan, E.; Nordlander, P.; Whitmire, K. H.; Halas, N. J. ACS Nano 2009, 3, 1379−1388. (12) Zhang, L.; Liu, B.; Dong, S. J. Phys. Chem. B 2007, 111, 10448− 10452. (13) Zhang, Y X; Pan, S S; Teng, X M J. Phys. Chem. C 2008, 112, 9623−9626. (14) Xu, Z. C.; Hou, Y. L.; Sun, S. H. J. Am. Chem. Soc. 2007, 129, 8698−8699. (15) Lu, Z. Y.; Dai, J.; Song, X. N.; Wang, G.; Yang, W. S. Colloid Surf. A 2008, 317, 450−456. (16) Zhang, Z. C.; Zhang, L. M.; Chen, L.; Chen, L. G.; Wan, Q. H. Biotechnol. Prog. 2006, 22, 514−518. (17) Selvan, S. T.; Patra, P. K.; Ang, C. Y.; Ying, J. Y. Angew. Chem., Int. Ed. 2007, 46, 2448. (18) Fu, W. Y.; Yang, H. B.; Li, M. H.; Yang, N.; Zou, G. T. Mater. Lett. 2005, 59, 3530−3534. (19) Watson, S.; Beydoun, D.; Amal, R. J. Photochem. Photobiol. A: Chem. 2002, 148, 303−313. (20) Kurinobua, S.; Tsurusakib, K.; Natuic, Y.; Kimatac, M.; Hasegawa, M. J. Magn. Magn. Mater. 2007, 310, e1025−1027. (21) Kostedtiv, W. L.; Drwiega, J.; Mazyck, D. W.; Lee, S. W.; Sigmund, W.; Wu, C. Y.; Chadik, P Environ. Sci. Technol. 2005, 39, 8052−8056. (22) Caruso, F. Nanoengineering of particle surfaces. Adv. Mater. 2001, 13, 11−22. (23) Jayanthi, K.; Chawla, S.; Joshi, A. G.; Khan, Z. H.; Kotnala, R. K. J. Phys. Chem. C 2010, 114, 18429−18434. (24) Baby, T. T.; Ramaprabhu, S. Talanta 2010, 80, 2016−2022. (25) Zhang, L.; Dou, Y. H.; Gu, H. C. J. Colloid Interface Sci. 2006, 297, 660. (26) Kwon, K. W.; Shim, M. J. Am. Chem. Soc. 2005, 127, 10269− 10275. (27) Wang, Z.; Wu, L.; Chen, M.; Zhou, S. J. Am. Chem. Soc. 2009, 131, 11276−11277. (28) Yang, S.; Liu, H.; Zhang, Z. New J. Chem. 2009, 3, 620. (29) Zhang, Y.; Wang, Sh.N.; Ma, S.; Guan, J. J.; Li, D.; Zhang, X. D.; Zhang, Z. D. J. Biomed. Mater. Res. Part A 2008, 85A, 840. (30) Levy, L.; Sahoo, Y.; Kim, K.-S.; Bergey, E. J.; Prasad, P. N. Chem. Mater. 2002, 14, 3715−21. (31) Kang, T.; Sung, J.; Shim, W.; et al. J. Phys. Chem. C 2009, 113, 5352−5357. (32) Xu, Y.; Karmakar, A.; Wang, D.; et al. J. Phys. Chem. C 2010, 114, 5020−5026.

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