Template Synthesis and Luminescence Properties of CePO4:Tb

$40.75 2008 American Chemical Society. Published on Web 12/04/2008 ... Oxford INCA), transmission electron microscopy (TEM, JEOL. 6300, 100 kV), a...
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J. Phys. Chem. C 2008, 112, 20217–20221

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Template Synthesis and Luminescence Properties of CePO4:Tb Nanotubes Guozhu Chen, Sixiu Sun,* Wei Zhao, Shuling Xu, and Ting You Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong UniVersity, Jinan 250100, China ReceiVed: July 18, 2008; ReVised Manuscript ReceiVed: October 30, 2008

In this paper, we report the synthesis of CePO4:Tb nanotubes by a solvothermal method using Ce(OH)CO3: Tb nanorods as both the physical and chemical templates. The formation of CePO4:Tb nanotubes involved a process of a solid-liquid interface chemical reaction in ethanol-water mixed solvent and a subsequent removal of unreacted Ce(OH)CO3:Tb sacrificial templates by acid treatment. It was found that the volume ratio between ethanol and water has some effect on the morphology of the final products. TEM and SEM images showed that the as-prepared nanotubes consist of nanoparticles/nanorods that preserve the basic morphology of the initial Ce(OH)CO3:Tb precursors. Furthermore, XRD patterns demonstrated phase evolution during the growth process. The photoluminescence properties were studied systematically, indicating that the as-prepared samples can serve as redox switches based on the reversible switching of the Ce4+/Ce3+ redox couple, which may be used as a biological label and probe. Introduction Nanomaterials have received wide recognition for their novel properties, derived from their unique structural and quantum size confinement effects.1-7 Especially, nanotubes are subjects of particular interest owing to their unique atomic structures and great potential in electronic, optical, mechanical, and bioscience applications.8-11 Furthermore, selective doping of these nanotubes is a promising way for further development of new curved nanomaterials, which could potentially change the electrochemical and optical properties along with accommodating different kinds of guest species without altering the structure of the tube walls.12-14 For example, Mao et al. synthesized erbium-doped yttrium oxide nanotubes (Er3+:Y2O3 nanotubes) with outstanding room-temperature photoluminescence by a hydrothermal procedure followed by a dehydration process from Er3+:Y(OH)3 nanotubes.15 Photoluminescence properties of Eudoped ThO2 nanotubes showed that strong visible light emission occurred at low concentration doping, and the luminescent intensity decreased at high concentration doping.16 More importantly, the tube structure may not only act as a physical shield for the inserted biomolecules but may also provide advantages for biomolecule delivery.17-19 The tube may be applied in the detection and monitoring at the level of single molecules in solution, as well as in the real-time visualization of cellular events,20-22 provided that the nanotube has luminescence and can be switched on and off easily. Therefore, it is of particular interest to seek a facile and high yield synthetic method to generate nanotubes which can be developed as new redox switches. As a consequence of their unique electronic structures and the numerous well-defined transition modes involving the 4f shell of their ions, lanthanide-doped phosphates with outstanding physical and chemical properties constitute an important domain of the nanostructure families.23-26 For example, Shi et al. synthesized a one-dimensional CePO4:Tb@LaPO4 heterostructure with improved luminescence in the presence of Pluronic * Corresponding author. E-mail: [email protected], tel: 86-531-88365432; fax: 86-531-88564464.

P123 (EO20PO70EO20).27 LnPO4 single-crystalline nanorod-based hollow microspheres and core-shell microspheres were synthesized via a composite functionalized aggregate composed of P123 (EO20PO70EO20), tetraphosphoric acid (H6P4O13), and Ln3+.28 Recently, Li and Yam described a facile route to synthesize CePO4:Tb nanowires with the assistance of β-cyclodextrin and demonstrated that the as-prepared samples possess luminescence switching behavior.29 However, to the best of our knowledge, the fabrication of Tb-doped CePO4 nanotubes has not yet been reported. Recently, we successfully fabricated two kinds of CeO2 nanotubes with distinctive structures and morphologies by using Ce(OH)CO3 nanorods as precursors.30 One kind of CeO2 nanotube was obtained by dissolving the unreacted Ce(OH)CO3 templates after a long solid-liquid interface reaction between Ce(OH)CO3 and NaOH at room temperature. Enlightened by this, we designed a solid-liquid interface reaction between Ce(OH)CO3:Tb nanorods and H3PO4 in ethanol-water mixed solvent, using Tb-doped Ce(OH)CO3 nanorods as precursors. The CePO4:Tb nanotubes were obtained after removal of the unreacted Ce(OH)CO3:Tb. Compared with the reported CePO4: Tb nanostructures,26-29 the CePO4:Tb nanotubes were obtained through a facile process without any additional surfactants or polymer assistance. As-prepared nanotubes show green emission upon UV illumination due to the energy transfer from Ce3+ to Tb3+ and could act as a redox-luminescent switch based on the reversible switching of the Ce4+/Ce3+ redox couple. Experimental Section All reagents, CeCl3 · 7H2O, TbCl3 · 6H2O, KMnO4, and ascorbic acid, are of analytic purity, obtained from Shanghai Sinopharm Chemical Reagent Ltd. Co. of China without further purification. In a typical synthetic procedure for Ce(OH)CO3:Tb precursors, 3.6 mmol of CeCl3 · 7H2O, 0.4 mmol of TbCl3 · 6H2O, and 24 mmol of urea were dissolved in distilled water to make a total volume of 80 mL. The clear solution was charged into a 100 mL wide-mouthed jar which was closed and kept at 80 °C for 24 h under magnetic stirring. The solution was then air-

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Figure 1. (a) XRD patterns of undoped Ce(OH)CO3 nanorods and Ce(OH)CO3 nanorods doped with 10 mol % Tb ions, respectively. (b) EDS data for Tb-doped Ce(OH)CO3 precursors. (c, d) TEM and SEM images of the Ce(OH)CO3:Tb precursors. The lower inset in panel c corresponds to the SAED pattern and the inset in d is a magnified nanorod with a smooth surface.

cooled to room temperature. The obtained white powder samples were centrifuged, washed with distilled water, and dried at 60 °C. For synthesizing CePO4:Tb nanotubes, the Ce(OH)CO3:Tb precursors (0.1 g) were dispersed into 14 mL of ethanol. Upon addition of 2 mL of 0.25 M H3PO4, the mixture solution was stirred for 10 min, and then the mixture was transferred into a 20 mL stainless Teflon-lined autoclave and heated at 160 °C for 6 h. After the mixture was cooled to room temperature, the precipitation was washed with HNO3 (1 M), distilled water, and absolute ethanol sequentially, and then it was dried in a vacuum at 60 °C for 12 h. CePO4:Tb nanotubes with different contents of Tb were obtained by the same procedure only by changing Tb-doped content in the precursors. To perform the luminescence switching behavior, KMnO4 (0.05 M) and ascorbic acid (0.05 M) were used to oxidize and reduce Ce3+ in each cycle. The treated samples were washed by water and ethanol several times to eliminate impurities before testing. The samples were characterized by X-ray diffraction (XRD) on a Japan Rigaku D/Max-γA rotating anode X-ray diffractometer equipped with graphite-monochromatized Cu KR radiation (λ ) 1.54178 Å) at a scanning rate of 0.02° s-1 in the 2θ range from 10° to 80°. The morphology and structure of as-synthesized CePO4:Tb nanotubes were characterized by field emission scanning electron microscopy (FESEM, JEOL JSM-6700F, 3 kV) equipped with energy-dispersive X-ray spectroscopy (EDS, Oxford INCA), transmission electron microscopy (TEM, JEOL 6300, 100 kV), and high-resolution transmission electron microscopy (HRTEM, JEM-2100, 200 kV). XPS spectra were recorded on an ESCALAB 250 spectrometer with a standard Al KR source. The charging of the samples was corrected by referencing all of the energies to the C 1s peak energy set at 285.1 eV, arising from adventitious carbon. Inductively coupled plasma-atomic emission spectrometry (ICP-AES) (Model IRIS Advantage) was used to measure the composition of the products. The luminescence spectra of dilute solutions were recorded in cuvettes (1 cm path length) on a WGY-10 spectrometer equipped with a 150-W xenon lamp as the excitation source at room temperature.

Results and Discussion Figure 1a displays the XRD patterns of undoped Ce(OH)CO3 nanorods and Ce(OH)CO3 nanorods doped with 10 mol % Tb ions. Only orthorhombic structures of Ce(OH)CO3 are detected (JCPDS files, no. 41-0013). It should be pointed out that in these Tb-doped samples no additional phase is observed, indicating that terbium has been successfully doped in the Ce(OH)CO3 crystalline lattice. The strong and sharp diffraction peaks demonstrate that the products are well crystallized. The existence of Tb in the EDS spectrum (Figure 1b) also clearly indicates that the Tb element is doped in the precursors (the Al signal is from the aluminum foil). The TEM micrograph shown in Figure 1c reveals a typical morphology of the prepared precursors. It can be seen the rod-like structures have a diameter about 200 nm, with length typically larger than 1 µm. The electron diffraction (ED) pattern (inset in Figure 1c) taken from an individual rod structure is indexed to the (300), (111) diffraction planes, respectively, of the orthorhombic structure of Ce(OH)CO3. It indicates that the as-prepared precursors are singlecrystalline, with a preferential growth direction along the [111] direction. The SEM image shown in Figure 1d exhibits a panoramic view of the precursors, and the inset magnified image shows the smooth surface of the nanorod, which agrees with the results of TEM tests. After the solvothermal process along with an interface reaction, CePO4:Tb nanotubes are obtained after acid treatment. The XRD pattern shown in Figure 2a reveals a pure phase, and all the diffraction peaks are very consistent with the reported XRD profile of hexagonal CePO4 (JCPDS files, no. 04-0632). The EDS results (Figure 2b) clearly confirm the presence of Ce, Tb, P, and O in the CePO4:Tb sample. The compositional analysis by ICP-AES shows that the percent of Tb is 9.2 mol %, which approximates the calculated values of Tb (10 mol %) in the precursors. The morphology and size of the products are evaluated by SEM and TEM observations. The representative image shown in Figure 3a exhibits a tube-like morphology, implying that the newly formed nanotubes well resemble the shape and size of the Ce(OH)CO3:Tb nanorod precursors. The inset in Figure 3a shows the SAED patterns obtained by focusing an electron beam onto the nanotubes. From this pattern, the

CePO4:Tb Nanotubes

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Figure 4. TEM and SEM images of the products obtained when the volume ratio between ethanol and water is 1. (a) Panoramic TEM image. (b) An individual nanotube with small nanorods adhered to its surface. (c, d) HRTEM images of a small nanorod and interior space of the nanotubes in panel a. (e) A ruptured nanotube.

Figure 2. XRD pattern and EDS data of the as-prepared CePO4:Tb nanotubes.

Figure 3. TEM and SEM images of CePO4:Tb nanotubes. (a) Panoramic TEM image and corresponding SAED pattern. (b) An individual magnified nanotube. (c, d) HRTEM images from the shell and interior space of the nanotube in panel b. (e) A ruptured CePO4: Tb nanotube.

reflections of (100), (101), (200), and (102) planes are clearly seen, indicating that the products are crystalline CePO4:Tb. A magnified TEM image of an individual CePO4:Tb nanotube is shown in Figure 3b. We can clearly see that the nanotube surface is coarse, composed of many small nanoparticles. More detailed inspection with high-resolution TEM (HRTEM) imaging (Figure 3c and 3d) shows that the shell of the as-prepared nanotubes is constructed by small nanoparticles. The interlayer spacing is measured for 0.607, 0.613, and 0.460 nm, which could be indexed to (001), (101) crystal planes of hexagonal CePO4. These nanoparticles have a random orientation with an average size around 10 nm, which is consistent with the ring pattern in selected-area electron diffraction. Figure 3e is a SEM image of a ruptured nanotube, showing a hollow interior of a tubular structure with shell thickness less than 30 nm. Controlled experiments are carried out to investigate the influence of the solvent compositions on the morphology of CePO4:Tb nanotubes. The final products still hold tube structure when the volume ratio between ethanol and water is 1 and other experimental conditions are kept same. As shown in Figure 4a, the brightness contrast between shell and core is still observed, indicating their hollow structure. The magnified TEM and SEM images shown in Figure 4b and 4e clearly display that small

nanorods with diameter about 12 nm attach together on the surface of nanotubes. Close observation from the HRTEM image (Figure 4c, 4d) reveals that the individual CePO4:Tb nanorod is single-crystalline and grows along the [001] direction. As we know, hexagonal CePO4 consists of infinite linear chains extending along the c axis, and the activation energy for the c axis direction of growth of hexagonal CePO4 is lower than that of growth perpendicular to the c axis. Therefore, from both a structural point of view and a thermodynamic perspective, CePO4 nanowires/nanorods grow preferentially along the [001] direction.31 When the mixed solvent is replaced by water, randomly distributed nanorods instead of nanotubes are found (TEM image not shown). To further demonstrate the influence of solvent composition on the morphology of CePO4, we allowed the cerium salt to directly react with H3PO4 under a different volume ratio of mixed solvent. As shown in Figure S1, when a higher relative volume of ethanol is selected, shorter nanorods are obtained. This trend is quite consistent with those obtained in the preparation CeO2, because the ε decreases with increasing volume percentage of ethanol in the mixed solvent and can result in smaller particle size.32 In the formation process of the as-prepared CePO4:Tb nanotubes, Ce(OH)CO3:Tb precursors are employed as both the physical and chemical templates, which not only cast the morphology of the precursors but also afford a Ce3+ source to initiate an interface reaction with H3PO4. As H3PO4 is introduced into the system, the following reaction occurs:

Ce(OH)CO3Tb + H3PO4 a CePO4 + CO2v + 2H2O (1) The CePO4:Tb products are formed by a surface deposition with a subsequent crystal growth procedure, during which the PO43- is deposited onto the surface of the Ce(OH)CO3:Tb precursors and then reacts gradually with the inner core to generate CePO4:Tb. Figure 5 displays a series of evolutional XRD patterns of the samples investigated. The phases of CePO4: Tb and Ce(OH)CO3:Tb (Figure 5a) coexist (Figure 5b) after solvothermal reaction at 160 °C for 6 h because the amount of H3PO4 is deficient in the typical synthesis. Because the diffraction intensity of the Ce(OH)CO3:Tb is much higher than that of CePO4:Tb, we find the highest diffraction peak (signed as a bracket) of the CePO4:Tb only in Figure 5b. The unreacted Ce(OH)CO3:Tb cores washed away by diluted HNO3 will result in the formation of interior space and a pure phase of CePO4: Tb (Figure 5c). The corresponding schematic illustration of the CePO4:Tb growth process is also shown in Figure 5. Successful doping is also evident from the luminescence spectrum. The room-temperature PL excitation and emission

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Figure 5. XRD patterens and corresponding schematic illustration of the CePO4:Tb nanotubes at different volume ratios of ethanol to water (7:1 or 1:1). (a) The newly prepared Ce(OH)CO3:Tb. (b) The partially reacted Ce(OH)CO3:Tb after reaction with H3PO4 at 160 °C for 6 h. (c) CePO4:Tb nanotubes synthesized after washing sample b with diluted HNO3.

Figure 6. PL excitation (a) and emission (b) spectra of a dilute colloidal dispersion of CePO4:Tb3+ nanotubes in ethanol.

spectra for CePO4:Tb nanotubes are shown in Figure 6a and 6b, respectively. The excitation spectrum consists of a broad band from 225 to 325 nm with a maximum at 277 nm due to the 4f-5d electronic transitions for Ce3+. Upon excitation at 277 nm, the obtained PL spectrum exhibits four well-resolved emission peaks, respectively. As displayed in Figure 6, the characteristic emission bands of Tb3+ at 492, 547, 589, and 623 nm are attributed to the transitions from 5D4-7F6, 5D4-7F5, 5D47 F4, and 5D4-7F3, respectively.26-29 The broad peak between 300 and 400 nm is a result of the d-f transitions of Ce3+. The cerium luminescence fails to be completely quenched by terbium, owing to the high Ce3+ concentration in the nanotubes. This result indicates that an incomplete energy transfer from Ce3+ to Tb3+ occurs in the CePO4:Tb nanotubes.29 The emission intensity of the Tb3+-doped CePO4 nanotubes is shown in Figure S2 as a function of the Tb3+ concentration. The emission intensity reaches its maximum at 10 mol % Tb3+ ions, after which it decreases quickly because of the concentration quenching effect. The emission intensity of the 4 mol % Tb3+ is weaker than that of 10 mol % Tb3+, possibly because the former nanotubes with lower doping Tb3+ concentration have fewer energy storage centers than the latter ones. It is interesting that upon the addition of KMnO4 solution to the as-prepared colloidal dispersion of CePO4:Tb3+ nanotubes, the emission spectrum changes into a horizontal line approximately (data not shown), and subsequent reduction of Ce4+ by adding aqueous ascorbic acid solution to the oxidized solution induces an increase in the luminescence. XPS data are applied to study the oxidation/reduction processes

Figure 7. Room-temperature emission (λex ) 277 nm) spectra of (a) thenewlypreparedsamplesand(b)thesampleafteroneoxidation-reduction cycle. The inset shows the green (Tb3+) luminescence from the sample recorded with a digital camera.

of Ce3+ and Tb3+ (Figure S3). The weak peaks of 901.3 and 885.2 eV ascribed to Ce4+ appear after adding KMnO4 while the profiles of Tb3+ are identical in the oxidation-reduction circle. As shown in Figure 7, the PL spectra and the energytransfer-sensitized green luminescence of the newly prepared CePO4:Tb nanotubes (a) and the samples after one oxidationreduction cycle (b) are almost identical, which demonstrate that the as-prepared CePO4:Tb nanotubes can be reversibly switched off and on. The chemical equations involved in these processes are shown in eqs 2 and 3.

3Ce3+ + MnO4- + 2H2O ) 3Ce4+ + MnO2 + 4OH- (2) 2Ce4+ + C6H8O6 a 2Ce3+ + C6H6O6 + 2H+

(3)

The luminescence is quenched (off) when the system is in the oxidized form while it is restored (on) in the reduced form, suggesting that the luminescence can be switched off and on by the alternate addition of KMnO4 and ascorbic acid to the colloidal dispersion of CePO4:Tb nanotubes. Furthermore, no obvious morphological change is observed from TEM images after several oxidation-reduction-oxidation cycles (Figure S4), which demonstrates that the as-prepared nanotubes can be used repeatedly. Conclusion In summary, CePO4:Tb nanotubes are successfully fabricated by using Ce(OH)CO3:Tb nanorods as precursors. Combining

CePO4:Tb Nanotubes XRD and EDS measurements, we confirm the Tb3+ ion has been successfully doped into Ce(OH)CO3 precursors and the doping is unaffected by the solvothermal conversion from Ce(OH)CO3: Tb to CePO4:Tb. This methodology may be extended to the synthesis of other kinds of doped nanotubes by rationally choosing doped precursors. The as-prepared CePO4:Tb nanotubes display luminescence properties that result from the energy transfer from Ce3+ to Tb3+. After addition of KMnO4, the Ce3+ oxidation state can be changed into Ce4+ which no longer transfers energy to Tb3+, so the luminescence is quenched. The quenched luminescence is again aroused by the addition of aqueous ascorbic acid which serves as a reducing reagent for Ce4+. These phenomena demonstrate that the as-prepared CePO4:Tb nanotubes possess redox switch function and can be used as a basis to develop a new system that may be applied in biological label and probe fields. Acknowledgment. This work was supported by the National 973 (2005CB623601) Program Projects of China. Supporting Information Available: TEM images of the products obtained by employing CeCl3 and H3PO4 as reactants, PL spectra at different contents of Tb in the as-prepared CePO4: Tb nanotubes, XPS data for Ce 3d and Tb 4d, and TEM images of the prepared CePO4:Tb nanotubes after oxidation and reduction. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Song, R. Q.; Xu, A. W.; Yu, S.-H. J. Am. Chem. Soc. 2007, 129, 4152. (2) Tan, Y.; Xue, X.; Peng, Q.; Zhao, T.; Wang, T.; Li, Y. Nano Lett. 2007, 7, 3723. (3) Wu, C.; Yu, S.-H.; Chen, S. J. Mater. Chem. 2006, 16, 3326. (4) Li, C.; Yang, J.; Yang, P.; Lian, H.; Lin, J. Chem. Mater. 2008, 20, 4317. (5) Hu, M.-J.; Lu, Y.; Zhang, S.; Guo, S.-R.; Lin, B.; Zhang, M.; Yu, S.-H. J. Am. Chem. Soc. 2008, 130, 11606. (6) Li, C.; Liu, X.; Yang, P.; Zhang, C.; Lian, H.; Lin, J. J. Phys. Chem. C 2008, 112, 2904.

J. Phys. Chem. C, Vol. 112, No. 51, 2008 20221 (7) Zhao, W.; Gu, J.; Zhang, L.; Chen, H.; Shi, J. J. Am. Chem. Soc. 2005, 127, 8916. (8) Zhou, W.; Han, Z.; Wang, J.; Zhang, Y.; Jin, Z.; Sun, X.; Zhang, Y.; Yan, C.; Li, Y. Nano Lett. 2006, 6, 2987. (9) Yan, C.; Xue, D. AdV. Mater. 2008, 20, 1055. (10) Wu, X.; Xu, Z.; Zeng, X. C. Nano Lett. 2007, 7, 2987. (11) Liu, Y.; Gilmore, K. J.; Chen, J. Chem. Mater. 2007, 19, 2721. (12) Takashi, K.; Susumu, S. Phys. ReV. B 2008, 77, 165417. (13) Gautam, U. K.; Bando, Y.; Zhan, J.; Costa, P. M. F. J.; Fang, X.; Golberg, D. AdV. Mater. 2008, 20, 810. (14) Xu, W.; Song, J.; Sun, L.; Yang, J.; Hu, W.; Ji, Z.; Yu, S. H. Small 2008, 4, 888. (15) Mao, Y.; Huang, J.; Ostroumov, R.; Wang, K.; Chang, J. J. Phys. Chem. C 2008, 112, 2278. (16) Lin, Z. W.; Kuang, Q.; Lian, W.; Jiang, Z. Y.; Xie, Z. X.; Huang, R. B.; Zheng, L. S. J. Phys. Chem. B 2006, 110, 23007. (17) Chen, C. C.; Liu, Y. C.; Wu, C. H.; Yeh, C. C.; Su, M. T.; Wu, Y. C. AdV. Mater. 2005, 17, 404. (18) Pantarotto, D.; Briand, J. P.; Prato, M.; Bianco, A. Chem. Commun. 2004, 16. (19) Bai, X.; Song, H.; Yu, L.; Yang, L.; Liu, Z.; Pan, G.; Lu, S.; Ren, X.; Lei, Y.; Fan, L. J. Phys. Chem. B 2005, 109, 15236. (20) Andre´asson, J.; Kodis, G.; Terazono, Y.; Liddell, P.; Bandyopadhyay, S.; Mitchell, R.; Moore, T.; Moore, A.; Gust, D. J. Am. Chem. Soc. 2004, 126, 15926. (21) Yan, P.; Holman, M.; Robustelli, P.; Chowdhury, A.; Ishak, F. I.; Adams, D. M. J. Phys. Chem. B 2005, 109, 130. (22) Lee, S.; Muller, A. M.; Al-Kaysi, R.; Bardeen, C. J. Nano Lett. 2006, 6, 1420. (23) Cao, M.; Hu, C.; Wu, Q.; Guo, C.; Qi, Y.; Wang, E. Nanotechnology 2005, 16, 282. (24) Riwotzki, K.; Meyssamy, H.; Kornowski, A.; Haase, M. J. Phys. Chem. B 2000, 104, 2824. (25) Wang, X.; Gao, M. J. Mater. Chem. 2006, 16, 1360. (26) Bu, W.; Chen, H.; Hua, Z.; Liu, Z.; Huang, W.; Zhang, L.; Shi, J. Appl. Phys. Lett. 2004, 85, 4307. (27) Bu, W.; Hua, Z.; Chen, H.; Shi, J. J. Phys. Chem. B 2005, 109, 14461. (28) Guan, M.; Sun, J.; Han, M.; Xu, Z.; Tao, F.; Yin, G.; Wei, X.; Zhu, J.; Jiang, X. Nanotechnology 2007, 18, 415602. (29) Li, Q.; Yam, V.W.-W. Angew. Chem., Int. Ed. 2007, 46, 3486. (30) Chen, G.; Xu, C.; Song, X.; Zhao, W.; Ding, Y.; Sun, S. Inorg. Chem. 2008, 47, 723. (31) Fang, Y. P.; Xu, A. W.; Song, R. Q.; Zhang, H. X.; You, L. P.; Yu, J. C.; Liu, H. Q. J. Am. Chem. Soc. 2003, 125, 16025. (32) Chen, H.-I.; Chang, H.-Y. Colloids Surf., A 2004, 242, 61.

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