Facile Synthesis and Optical Property of Porous Tin Oxide and

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J. Phys. Chem. C 2008, 112, 19939–19944

19939

Facile Synthesis and Optical Property of Porous Tin Oxide and Europium-Doped Tin Oxide Nanorods through Thermal Decomposition of the Organotin Weiqiang Fan,†,‡ Shuyan Song,†,‡ Jing Feng,†,‡ Yongqian Lei,†,‡ Guoli Zheng,†,‡ and Hongjie Zhang*,† State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry and Chinese Academy of Sciences, Changchun 130022, PR China, and Graduate School of the Chinese Academy of Sciences, Beijing, PR China ReceiVed: September 11, 2008; ReVised Manuscript ReceiVed: October 26, 2008

Porous SnO2 and SnO2-Eu3+ nanorods have been facilely prepared using triphenyltin hydroxide microrods as precursors. The porous structure of SnO2 nanorods, which was aggregated by small SnO2 nanocrystallites, has been confirmed by TEM images and nitrogen adsorption-desorption isotherms. The optical property of the porous SnO2-Eu3+ nanorods was investigated by UV-vis absorption and photoluminescence spectra. Introduction Recently, nanoscale materials have attracted intensive attention because they possess numerous interesting and new phenomena, as compared with solid or bulk forms, because of larger surface areas and nanometer-sized structures.1 Therefore, the surface areas and morphology of the nanomaterials may greatly affect their properties and fields of application. As is well-known, porous materials usually have large surface areas,2 and one-dimensional (1D) nanomaterials with 1D structure may possess intriguing properties and unique applications.3 Therefore, synthesis of 1D nanomaterials with porous structure will endow the nanomaterials with both large surface areas and 1D structure, which will offer great opportunities to explore their unexpected properties. Several methods have been applied to the preparation of the porous materials, such as the use of templates,4 polymerization-induced phase separation,5 etching,6 and thermal decomposition.7 The method of thermal decomposition, which is quite facile and convenient as compared with others, is usually used in the porous materials’ preparation. The formation of a porous structure can be commonly attributed to the gas given off during the thermal decomposition process of the organic parts. For instance, Gao et al.8 have introduced alkaline-earth metal phenylphosphonate nanorods as precursors and facilely synthesized mesoporous nanorods of Ca2P2O7 possessing a large surface area and high thermal stability by thermal decomposition to remove the organic part of Ca(HO3PPh)2 nanorods. Porous MOx (M ) Ti, Sn, In, and Pb) nanowires were also easily obtained by Xia’s group.9 They transformed alkoxide into a chainlike glycolate complex, which subsequently crystallized into uniform nanowires as the precursor. The glycolate complex precursor was thermally decomposed to porous SnO2 nanowires. Tin oxide (SnO2), an important n-type semiconductor, has long been of scientific and technological interest as a sensor and optical material.10 Facile synthesis of 1D SnO2 porous nanomaterials may be a challenge to explore novel SnO2 * To whom correspondence should be addressed. Phone: +86-43185262127. Fax: +86-431-85698041. E-mail: [email protected]. † Changchun Institute of Applied Chemistry. ‡ Chinese Academy of Sciences, Changchun 130022, PR China, and Graduate School of the Chinese Academy of Sciences.

applications. Recently, Zhao et al.11 reported porous SnO2 nanostructures consisting of nanoplates through thermal decomposition of the mixed solution composed of dibutyltin dilaurate and acetic acid, and porous nanostructures formed by removing organic parts of organotin. Inspired by the works above, we reason that using organooxotin as a precursor may offer a novel method to synthesize 1D SnO2 porous nanomaterial, since some organooxotins may be apt to form a 1D morphology because of the anisotropy of the organooxotin cluster with chainlike crystal structure which is usually prepared through hydrolysis of organotin.12 Otherwise, SnO2 with a wide band gap (3.6 eV) is an attractive semiconductor host for rare earth (RE) ions because of their potential applications in optoelectronic technology.13 When compared with other optical materials, RE complexes have long been used as phosphors and laser materials because of their sharp, intensely luminescent f-f electronic transitions,14 but direct excitation of the parity-forbidden intra-fshell RE ions crystal-field transitions is inefficient, so trying to incorporate RE ions into a wide-band gap semiconductor lattice will make lanthanide ions sensitized efficiently by exciton recombination in the host.15 Furthermore, if the SnO2 nanomaterials as host can be synthesized in 1D morphology, the RE-ion-doped SnO2 nanocrystals may offer more widespread applications. In this paper, we introduce chlorotriphenyltin (Ph3SnCl) as the source of tin. Hydrolysis of Ph3SnCl has been found to give triphenyltin hydroxide (Ph3SnOH) microrods, and porous SnO2 nanorods can be easily obtained by calcining Ph3SnOH microrods in a furnace from 600 to 800 °C for 1 h under air. Porous SnO2 nanorods doped with Eu3+ ions were synthesized similarly by adding europium complexes with sodium citrate into the hydrolysis reactor of chlorotriphenyltin. The SEM and TEM images show that the porous structure of the SnO2 nanorods was formed through nanoparticle aggregation. The surface area and the porous structure of the SnO2 nanorods were characterized by nitrogen adsorption-desorption isotherms. Photoluminescence properties of porous SnO2-Eu3+ nanorods have also been studied. Porous SnO2-Eu3+ nanorods, when excited at the wavelength of the absorption of the SnO2 host, exhibited relative high 5D0 f 7F1 magnetic-dipolar transition.

10.1021/jp8081062 CCC: $40.75  2008 American Chemical Society Published on Web 11/18/2008

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SCHEME 1: The Proposed Mechanism for the Formation of Porous SnO2 Nanorods

Experimental Section All chemical reagents were of analytical grade and were used as received without further purification. Hydrolysis of Ph3SnCl. A 38.5 mg portion of Ph3SnCl and X mg (X ) 2, 4, 6, 8, and 10 respectively) of sodium hydroxide were added into a 50 mL alcohol aqueous solution (alcohol/ deionized water molar ratio ) 2:3), then the reaction mixture was heated at reflux for 2 h. A white deposition was obtained and collected by filtration when the reaction was cooled to and dried at room temperature; Ph3SnOH microrods were then obtained. Synthesis of SnO2 Nanorods. The Ph3SnOH microrods as precursors were heated at 600, 700, and 800 °C for 1 h under air. Synthesis of SnO2 Nanorods Doped with Eu3+ Ions. The process is similar to the synthesis of the SnO2 nanorods above (38.5 mg Ph3SnCl and 10 mg NaOH), but a EuCl3 · 6H2O complex with sodium citrate was added into the hydrolysis reactor of chlorotriphenyltin. The Eu3+ ions/Sn molar ratio was 8:100; the Eu3+ ions/sodium citrate molar ratio was 1:1.5. Characterization. X-ray diffraction patterns (XRD) were recorded with a Rigaku-D/max 2500 V X-ray diffractometer equipped with a Cu KR radiation source (λ ) 1.541 78 Å). The morphologies and structures of the products were characterized by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). FESEM analysis was conducted on a Philips XL-30 field-emission scanning electron microscope operated at 15 kV, whereas TEM observation was carried out on a JEOL-JEM-2010 at 200 kV. Room temperature photoluminescence (PL) spectra were recorded on a Fluorolog-3 with a 450-W xenon lamp as the excitation source at room temperature. The decay curves for the 5D0 f 7F1 (588 nm) emission of SnO2-Eu3+ were measured with a 320 nm UV laser beam, which was frequency doubled in a 640 nm dye laser from a NarrowScan dye laser (Radiant Dyes Laser Accessories GmbH) bumped by a PP-8010 YAG laser (Continuum). FT-IR spectra were measured within a 4000-400 cm-1 region on an American BIO-RAD Company model FTS135 infrared spectrophotometer using the KBr pellet technique. The nitrogen sorption measurement was performed on a Mircromeritics ASAP 2020 surface area and porosimeter analyzer. UV-vis absorption spectra were measured by a Shimadzu UV-3600 spectrophotometer. Results and Discussion The proposed mechanism for the formation of porous SnO2 nanorods is described in Scheme 1. Ph3SnOH can be easily obtained by hydrolysis of Ph3SnCl in aqueous ethanol.16 We hydrolyzed the Ph3SnCl in alkaline aqueous ethanol, and the products were proved to be Ph3SnOH according to the IR spectra. when compared with the IR spectrum of the Ph3SnCl (Figure 1a), Figure 1b shows two new peaks at 896 and 911 cm-1, which can be attributed to δO-H, and a sharp peak at 3616 cm-1 was from the νO-H of the Ph3SnOH.17 When the Ph3SnOH precursor was calcined

Figure 1. IR spectra for (a) Ph3SnCl, (b) hydrolysis products, and (c) calcined products at 600 °C.

Figure 2. XRD patterns of (a) simulated XRD powder pattern of Ph3SnOH, (b) Ph3SnOH precursors (2 mg NaOH), (c) Ph3SnOH precursors (4 mg NaOH), (d) Ph3SnOH precursors (8 mg NaOH), and (e) Ph3SnOH precursors (10 mg NaOH).

at 600 °C for 1 h in air, the products (Figure 1c) exhibited a strong vibration around 610 cm-1, which could be assigned to the ν(Sn-O-Sn) of SnO2.18 To go deep into the structure of the hydrolysis products, X-ray diffraction was studied. XRD patterns for the hydrolysis products of Ph3SnCl with different quantities of NaOH (Figure 2b-e) show that all diffraction peaks can be readily indexed to simulated XRD powder patterns of Ph3SnOH (Figure 2a) that have been reported,19 which could further prove that the hydrolysis product of Ph3SnCl was Ph3SnOH. According the previous reference,12b the Ph3SnOH molecules tended to form zigzag chains (Figure 3e) due to the lone pair electrons of the O atom coordinated with the Sn atom. The long chains as building blocks were thought to be apt to further self-assemble into a 1D morphology (microrods),9 which corresponds to the SEM images of Ph3SnOH.

Porous SnO2 and SnO2-Eu3+ Nanorods

Figure 3. SEM of (a) the Ph3SnOH microblocks (2 mg NaOH), (b) Ph3SnOH microrods (4 mg NaOH), (c) Ph3SnOH microrods (8 mg NaOH), (d) and Ph3SnOH microrods (10 mg NaOH). (e) Chain structure of Ph3SnOH.

We have studied the morphology of the Ph3SnOH precursor in relation to the quantity of NaOH, and no deposition was exhibited when the Ph3SnCl was hydrolyzed without NaOH. As we can see in the Figure 3a, microblocks with lower aspect ratios were obtained when 2 mg of NaOH was added, then microrods formed with the increase in the quantity of the NaOH. We also found that the size of the microrods altered little when the quantity of the NaOH was above 4 mg. Otherwise, the yield of the Ph3SnOH precursor increased with the quantity of the NaOH, which could be attributed to the sufficient hydrolysis of the Ph3SnCl with abundant NaOH. It is also the reason why we synthesized with 10 mg NaOH the Ph3SnOH precursor doped with europium. In Figure 3b-d, we can see that large numbers of microrods of Ph3SnOH were obtained, that the width of the microrods is about 4 µm, and that the length is several tens of micrometers. SEM images of the samples after being calcined at different temperatures are shown in Figure 4. A 1D morphology of the Ph3SnOH precursors was retained, and the size of the sample shrank to nanoscale due to elimination of the organic parts of the precursors. TEM images reveal that the SnO2 nanorods have a highly porous structure consisting of interconnected nanocrystallites, and we can see that some parts of the porous SnO2 nanorods broke down into small nanocrystallites when the calcination temperature was raised to 800 °C (Figure 4f), which may be attributed to the interconnection of nanocrystallites becoming weaker with the increase in the calcination temperatures. According to the SAED pattern, the porous SnO2 nanorods were polycrystalline and rutile-phase SnO2. Therefore, porous SnO2 nanorods can be obtained by calcination of organic precursors, and the porous structure was influenced by the calcination temperature, so Ph3SnOH as a precursor may offer

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Figure 4. SEM and TEM images of porous SnO2 nanorods varied with calcination temperatures: (a, b) 600 °C, (c, d) 700 °C, and (e, f) 800 °C.

Figure 5. XRD patterns of porous SnO2 nanorods calcined at different temperatures.

a potential to synthesize SnO2 nanomaterials with special porous structure at a certain calcination temperature. Porous SnO2 nanorods were also studied by XRD (Figure 5), All the diffraction peaks can be indexed to the tetragonal phase of SnO2 with calculated lattice parameters of a ) 4.745 Å and c ) 3.193 Å, which are well in agreement with the reported values (JCPDS no. 77-0450).20 As the patterns show, the intensity of the diffraction peaks increased gradually with an increase in the calcination temperature, which may indicate that the degree of crystallization was enhanced at high calcination temperatures. The porous structure of the SnO2 nanorods was further confirmed by nitrogen adsorption-desorption isotherms. As shown in Figure 6, the isotherms are of type IV,21 with a hysteresis loop range of 0.6-1.0 P/P0. The BET surface areas of the samples were 47 (600 °C), 24 (700 °C), and 27 m2/g (800 °C), respectively. The relatively large surface area can be

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Figure 6. N2 adsorption-desorption isotherm for porous SnO2 nanorods.

Figure 7. UV-vis absorption spectra of porous SnO2-Eu3+ nanorods calcined at different temperatures. The inset shows the corresponding plot of (Rhν)2 vs photon energy and the linear fit over Eg.

due to the porous structure of the SnO2 nanorods aggregated by small nanocrystallites. The pore size distribution curves (see Figure S1 of the Supporting Information) showed that the average diameters of the samples were ∼10 nm, and the sample had the smallest average diameter of ∼7 nm when calcined at 600 °C. According to the measurement, the sample calcined at 600 °C has a relatively high surface area and small pore size, which may result from the close porous structure consisting of small nanocrystallites, as shown in the SEM and TEM images of Figure 4. Porous SnO2 nanorods doped with Eu3+ ions have a similar morphology (see Figure S2 of the Supporting Information), and the calcination temperatures varied from 600 to 800 °C. XRD patterns of SnO2-Eu3+ nanorods (see Figure S3 of the Supporting Information) were identified with the patterns of the rutile-phase SnO2, and the peaks of the Eu2O3 were not present, since impurities present in small amounts are usually not discernible in XRD patterns. In this sense, PL spectroscopy data is much more conclusive for investigating whether the Eu3+ ions were doped into the host SnO2. As the UV-vis absorption spectra shown (Figure 7), the samples exhibited broad bands from 400 to 200 nm which belonged to the host absorption of SnO2, and the maximal absorption was achieved at ∼300 nm. The inset shows that the band gaps (Eg) of samples calcined at different temperatures were between 3.4 and 3.6 eV, which were close to the band gap of SnO2 (3.6 eV).

Figure 8. (a) Excitation (monitored at 588 nm) and (b) emission spectra of porous SnO2-Eu3+ nanorods excited at the wavelength of the absorption of SnO2 host, and (c) emission spectra of porous SnO2Eu3+ nanorods excited at 393 nm.

Photoluminescence properties of the porous SnO2-Eu3+ nanorods have been studied. Figure 8a shows the excitation spectrum monitored at 588 nm (the transition of 5D0 f 7F1) of the porous SnO2-Eu3+nanorods. The broad bands are centered at 325 nm (calcined at 600 °C), 302 nm (700 °C), and 309 nm (800 °C), and the relatively sharp peaks at 393 nm are assigned to the 7F0 f 5L6 transitions within the Eu3+ ions.22 Moreover, the broad and strong band from 250 to 370 nm in the excitation spectrum of the porous SnO2-Eu3+nanorods, which can correspond to the band gap energy of the SnO2-Eu3+ (Figure 7), indicates the energy transfer from the nanosized SnO2 crystal to the Eu3+ ions. The mechanism of the energy transfer can be commonly explained as previously reported:23 the electron-hole pairs, which generated by the energy absorbed in the nanocrystals, recombined and then transferred to the Eu3+ ions.

Porous SnO2 and SnO2-Eu3+ Nanorods When the porous SnO2-Eu3+ nanorods were excited at the wavelength of the absorption of SnO2 host (Figure 8a), PL spectra (Figure 8b) mainly showed the transitions of 588, 593, and 599 nm (5D0 f 7F1), and 612 nm (5D0 f 7F2) of the Eu3+ ions, and there were obvious broad peaks at ∼530 nm, which were attributed to the host of SnO2.24 As is well-known, the transition 5D0 f 7F1 belongs to a magnetic dipolar transition that is insensitive to the surrounding environment of the Eu3+ ions, whereas 5D0 f 7F2 is an electric dipolar transition, and the relative intensity ratio (R) of 5D0 f 7F2 to 5D0 f 7F1 can indicate how far the local environment of the Eu3+ ions is centrosymmetric. In Figure 8b, we can see that the intensity of the 5D0 f 7F1 transition becomes stronger and the 5D0 f 7F2 transition gradually disappears, in contrast to the calcination temperature rise from 600 to 800 °C, which resulted in a decrease in the R ratio. The intensity of the 5D0 f 7F1 transition was enhanced at high calcination temperature, which can be due to the hydroxyl quenching effects on the photoluminescence.25 Otherwise, the change in the R ratio may reflect the altering of the surrounding environment of the Eu3+ ions. As previously reported,26 Sn4+ ions in the stoichiometric SnO2 have D2h or C2h point symmetry, and the Eu3+ ions can occupy the sites of the Sn4+ ions without the Sn4+ site symmetry being significantly distorted, although the ionic radius of the Eu3+ ions is bigger than the Sn4+ ions. This can be the reason for the split into three components of the 5D0 f 7F1 transition. As we know, the 5D0 f 7F2 transition should be totally forbidden if the Eu3+ ions are right at the D2h or C2h point symmetry of the Sn4+, but there is still a weak 5D0 f 7F2 transition, which may be considered as evidence that the site symmetry of the Eu3+ ions deviated from D2h or C2h.27 Therefore, the decrease in the R ratio can be attributed to the Eu3+ ions gradually approaching D2h or C2h sites with the rise of the calcination temperatures. The intensity of the 7F0 f 5L6 transition decreases as the calcination temperature increases from 600 to 800 °C also because the transition is forbidden in a centrosymmetric environment.28 The sharp and three splitting peaks of the 5D0 f 7F1 transitions, when excited at the wavelength of the absorption of SnO2 host, can be attributed to Eu3+ ions incorporated into the matrix of the SnO2 host. However, the different PL spectra were obtained when 393 nm was chosen as the excitation wavelength. As seen in Figure 8c, the PL spectra of the same samples show relatively broad peaks, and the main peaks at 612 nm belong to the 5D0 f 7F2 transitions, which indicates that these PL spectra may arise from Eu3+ ions out of the matrix of the SnO2 host and located at the disordered sites.29 Moreover, the intensity of the 5D0 f 7F2 transition was decreased with the increase in the calcination temperatures, which indicated that the Eu3+ ions may transform to the matrix of the SnO2 at high calcination temperatures. Therefore, according to Figure 8b and c, we infer that the Eu3+ ions are mainly at two sites in the porous SnO2-Eu3+ nanorods: a portion of the Eu3+ ions are inside the matrix of the SnO2, and another may be at the surface of the nanomateials. The decay curves of the porous SnO2-Eu3+ nanorods were fitted by double-exponential functions. The lifetimes of SnO2-Eu3+ (600 °C) are 0.52 (26.48%) and 4.62 ms (73.52%). The lifetimes of SnO2-Eu3+ (700 °C) are 0.30 (17.47%) and 6.68 ms (82.53%). The lifetimes of SnO2-Eu3+ (800 °C) are 1.02 (8.21%) and 8.40 ms (91.79%). According to the PL spectra, it is interesting that the porous SnO2-Eu3+ nanorods can exhibit different photoluminescences just by varying the excitation wavelength, which is due to the two different sites of the Eu3+ ions in the nanomaterials.

J. Phys. Chem. C, Vol. 112, No. 50, 2008 19943 Moreover, the SnO2-Eu3+ nanorods with the porous and 1D morphology may have the potential to enlarge their applications as optical materials. Conclusions Porous SnO2 and SnO2-Eu3+ nanorods have been successfully prepared by calcining Ph3SnOH microrods in a furnace from 600 to 800 °C for 1 h under air. TEM images revealed that the SnO2 nanorods have a highly porous structure consisting of interconnected nanocrystallites, and the XRD patterns indicated that the samples belonged to the tetragonal phase of SnO2. On the basis of the UV-vis absorption and photoluminescence spectra, the porous SnO2-Eu3+ nanorods showed the transition 5 D0 f 7F1 of the Eu3+ ions when excited at the wavelength of the absorption of SnO2 host, and the transition 5D0 f 7F1 was split into three components. However, the 5D0 f 7F2 transitions became the main peaks when 393 nm was chosen as the excitation wavelength. According to the optical property, we consider that some of the Eu3+ ions have been doped into the host of SnO2 host and others may be at the surface of the nanomateials. The porous structure, 1D morphology, and the special optical property of our samples will allow potential to extend the applications of SnO2 to sensors and optical and electrical fields. Acknowledgment. The authors are grateful for financial aid from the National Natural Science Foundation of China (Grant Nos. 20631040 and 20771099) and the MOST of China (Grant Nos. 2006CB601103, 2006DFA42610). Supporting Information Available: Additional information as noted in text. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Wang, X. D.; Summers, C. J.; Wang, Z. L. Nano Lett. 2004, 4, 423. (b) Song, S. Y.; Ma, J. F.; Yang, J.; Cao, M. H.; Zhang, H. J.; Wang, H. S.; Yang, K. Y. Inorg. Chem. 2006, 45, 1201. (2) (a) Corma, A. Chem. ReV. 1997, 97, 2373. (b) Rowsell, J. L. C.; Yaghi, O. M. Angew. Chem., Int. Ed. 2005, 44, 4670. (c) Scott, B. J.; Wirnsberger, G.; Stucky, G. D. Chem. Mater. 2001, 13, 3140. (3) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, Y. Q. AdV. Mater. 2003, 15, 353. (4) (a) Brian, T. H.; Blanford, C. F.; Stein, A. Science 1998, 281, 538. (b) Yang, P. D.; Deng, T.; Zhao, D. Y.; Feng, P. Y.; Pine, D.; Chmelka, B. F.; Whitesides, G. M.; Stucky, G. D. Science 1998, 282, 2244. (5) Nakanishi, K.; Tanaka, N. Acc. Chem. Res. 2007, 40, 863. (6) (a) Koker, L.; Wellner, A.; Sherratt, P. A. J.; Neuendorf, R.; Kolasinski, K. W. J. Phys. Chem. B 2002, 106, 4424. (b) Bisi, O.; Ossicini, S.; Pavesi, L. Surf. Sci. Rep. 2000, 38, 1. (7) (a) Yang, A.; Tao, X. M.; Pang, G. K. H.; Siu, K. G. G. J. Am. Ceram. Soc. 2008, 91, 257. (b) Song, S. Y.; Ma, J. F.; Yang, J.; Cao, M. H.; Li, K. C. Inorg. Chem. 2005, 44, 2140. (8) Gao, L. L.; Song, S. Y.; Ma, J. F.; Yang, J. Cryst. Growth Des. 2007, 7, 895. (9) (a) Wang, Y. L.; Jiang, X. C.; Xia, Y. N. J. Am. Chem. Soc. 2003, 125, 16176. (b) Jiang, X. C.; Wang, Y. L.; Herricks, T.; Xia, Y. N. J. Mater. Chem. 2004, 14, 695. (10) (a) Chowdhuri, A.; Gupta, V.; Sreenivas, K.; Kumar, R.; Mozumdar, S.; Patanjali, P. K. Appl. Phys. Lett. 2004, 84, 1180. (b) He, J. H.; Wu, T. H.; Hsin, C. L.; Li, K. M.; Chen, L. J.; Chueh, Y. L.; Chou, L. J.; Wang, Z. L. Small 2006, 2, 116. (11) Zhao, Q. R.; Zhang, Z. G.; Dong, T.; Xie, Y. J. Phys. Chem. B 2006, 110, 15152. (12) (a) Zheng, G. L.; Ma, J. F.; Yang, J.; Li, Y. Y.; Hao, X. R. Chem.sEur. J. 2004, 10, 3761. (b) Glidewell, C.; Liles, D. C. Acta Crystallogr. 1978, B34, 129. (13) (a) del-Castillo, J.; Rodrı´guez, V. D.; Yanes, A. C.; Me´ndez-Ramos, J. J. Nanopart. Res. 2008, 10, 499. (b) Morais, E. A.; Scalvi, L. V. A.; Tabata, A.; De Oliveira, J. B. B.; Ribeiro, S. J. L. J. Mater. Sci. 2008, 43, 345.

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