J. Phys. Chem. C 2009, 113, 20173–20177
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Synthesis and Luminescence Properties of Sheaflike TbPO4 Hierarchical Architectures with Different phase Structures Mei Yang,†,‡ Hongpeng You,*,† Yanhua Song,†,‡ Yeju Huang,†,‡ Guang Jia,†,‡ Kai Liu,†,‡ Yuhua Zheng,†,‡ Lihui Zhang,†,‡ and Hongjie Zhang*,† State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ReceiVed: May 11, 2009; ReVised Manuscript ReceiVed: October 14, 2009
Sheaflike terbium phosphate hydrate hierarchical architectures composed of filamentary nanorods have been fabricated by a hydrothermal method. The X-ray diffraction patterns and thermogravimetric/differential thermal analysis investigations reveal that the obtained terbium phosphate hydrate has a structural formula of TbPO4 · H2O, which can be readily indexed to the hexagonal phase GdPO4 · nH2O in JCPDS file 39-0232. The evolution of the morphology of the products has been investigated in detail. It is found that the addition of CTAB and Na2H2L (disodium ethylenediamine tetraacetate) plays an important role in controlling the final morphology of the products. A possible formation mechanism of the sheaflike architectures was proposed according to the experimental results and analysis. In addition, the phase structure of the product changes to monoclinic phase when it is annealed at 750 °C for 2 h in N2-H2 atmosphere. Tetragonal phase TbPO4 can be obtained when annealed temperature increases to 1150 °C. It was observed that some changes take place on the morphology and optical properties of the products annealed at different temperatures. 1. Introduction In the past decades, low-dimensional nanosized lanthanide compounds and lanthanide-ion-doped materials have drawn much attention because of their potential applications in the fields of light phosphor powder, advanced flat panel displays, biological and chemical labels, and so on.1-7 As an important sort of lanthanide inorganic salts, rare earth phosphates have attracted great interest due to their novel properties and applications such as tricolor fluorescent lamps, cathode ray tubes, and LCD displays.8,9 For the lanthanide phosphate nanomaterials, apart from zero-dimensional (0D) nanoparticles,10,11 different kinds of morphologies of nanostructures have been obtained by a variety of methods in the past few years, such as one-dimensional (1D),12,13 core/shell,14 and polygonal structures.15 In recent materials chemistry, large scale self-assembly of nanostructured building components with specific morphology and novel properties has been one topic of intensive investigation. Their size and shape as well as crystal structure play crucial roles in determining the chemical, optical, and electrical properties of these nanomaterials.16,17 To date, remarkable progress has been made in the self-assembly of highly organized building blocks of metals,18 semiconductors,19,20 copolymers,21 and biomaterials22 based on different driving mechanisms. However, the reports on the self-assembly of lanthanide phosphate compounds composed of nanomaterials are limited, although many different kinds of low dimensional lanthanide phosphate nanomaterials have been reported. In this paper, we provide a new example for the controlled growth of TbPO4 · H2O nanobundles and self-assembled sheaflike * To whom correspondence should be addressed. E-mail: (H.Y.)
[email protected]; (H.Z.)
[email protected]. † Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. ‡ Graduate University of the Chinese Academy of Sciences.
hierarchical architectures by a simple hydrothermal treatment. The influence of the reaction conditions on the morphology and the properties of the products were investigated, and a possible formation mechanism of the sheaflike TbPO4 · H2O architectures was proposed. Furthermore, the changes of the morphologies, phase structures, and the luminescent properties of the products obtained at different annealing temperatures were also studied. 2. Experimental Section 2.1. Synthesis of TbPO4 · H2O. Analytical grade disodium ethylenediamine tetraacetate [EDTA-2Na or Na2H2L, where L4-) (CH2COO)2N(CH2)2N(CH2COO)24-], cetaltrimethylammonium bromide (CTAB), and (NH4)2HPO4 · 12H2O were purchased from Beijing Chemical Corporation and used as received without further purification. Tb(NO3)3 · 6H2O was obtained by dissolving Tb4O7 (99.99%) (Wuxi Yiteng RareEarth Limited Corporation, China) in HNO3 solution under heating with agitation, and subsequently evaporated until desired products were obtained. In a typical synthesis, 0.9 g Tb(NO3)3 · 6H2O and 0.75 g Na2H2L were first dissolved in 10 mL distilled water. Then 20 mL ethanol-water solution containing 1.1 g CTAB and 0.4 g (NH4)2HPO4 · 12H2O were added into the above solution. The pH value of the solution was adjusted by HNO3. The resulting solution was transferred into a Teflon-lined autoclave (volume, 50 mL). Absolute ethanol or water was added until about 80% of the autoclave capacity was filled. Subsequently, the autoclave was sealed and maintained at 90 °C for 1-10 h followed by cooling to room temperature naturally. Finally, the precipitates were collected and washed with deionized water and ethanol several times. The final products were dried at 60 °C for 12 h in air. The proportions of the ethanol to water (v/v) for the obtained products were 1/35, 6/30, 11/25, and 16/20, respectively, and
10.1021/jp908697t CCC: $40.75 2009 American Chemical Society Published on Web 11/02/2009
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Figure 1. XRD patterns of S1-S4 products obtained at 90 °C for 10 h.
the corresponding products were labeled as S1, S2, S3, and S4, respectively. 2.2. Characterization. X-ray diffraction (XRD) pattern was measured by a Rigaku-D X-ray powder Diffractometer with Cu Ka radiation (λ ) 1.5418 Å). TG measurements were operated on Pyris Diamond thermogravimetric/differential thermal analysis (TG/DTA) with the heating rate of 10 °C min-1 from 40 to 600 °C in a N2 atmosphere to prevent the oxidation of the Tb3+ ions. Fourier transform infrared spectroscopy (FT-IR) spectra were measured with BRUKER Vertex 70 FTIR with the KBr pellet technique. The morphology of the products was inspected using a field emission scanning electron microscope (FESEM) (HITACHI S-4800). TEM images were collected on a JEOL JEM 2010 transmission electron microscope (TEM) operating at an accelerating voltage of 200 kV. Excitation and emission spectra were recorded with a HITACHI F-4500 fluorescence spectrophotometer equipped with a 150 W xenon lamp as the excitation source. All the measurements were performed at room temperature. 3. Results and Discussion 3.1. Structure and Morphology of the Products. Figure 1 shows the XRD patterns of S1-S4. Compared with all the standard XRD patterns of terbium phosphate or hydration terbium phosphate in JCPDS files, no one can agree with the obtained XRD patterns. Although there are some similar reports on the synthesis of the hexagonal TbPO4 · H2O with the phase structure indexed to PDF card No. 20-1244, one can clearly notice that the peaks at 2θ ) 40, 45, 50.5, 56, 58, and 61° appeared in the XRD patterns of the products do not exist in the PDF card No. 20-1244. With careful comparison, it can be found that the peak positions of the XRD patterns agree well with those of the hexagonal phase GdPO4 · nH2O in JCPDS file 39-0232. No additional peaks of other phases have been found, indicating that they are isostructural with the hexagonal phase GdPO4 · nH2O. It can be assumed that they should have a structural formula of TbPO4 · nH2O. The produce of this phase structure may be due to the similar ion radii and electrons structure of the outer shell of the Gd3+ and Tb3+ ions, which make the corresponding phosphate form isostructural hexagonal phase. In order to investigate the existence of the hydration of the obtained products, TG/DTA measurements were performed. The TG curve of S3 indicates that the weight loss occurs in two stages (Figure 2). The first one below 130 °C corresponds to the release of water molecules adsorbed on the product, with an weak endothermic peak centered at 100 °C detected in the
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Figure 2. TG and DTA curves of S3 sample obtained at 90 °C for 10 h.
Figure 3. XRD patterns of the products annealed at different temperature for 2 h.
DTA curve (A). The second one is in the range of 130-650 °C, corresponding to the progressive dehydration of the hexagonal phase. Two endothermic peaks at about 180 and 300 °C (B and C) can be found in the corresponding DTA curve. The hydration ratio of the product is roughly deduced from the weight loss during the second step, which indicates that the obtained phosphate hydrate should be TbPO4 · H2O. Furthermore, different temperature values in the range of 600-900 °C are given for the phase structure transition of the products. The exothermic peaks centered at 600 and 780 °C in DTA curve are due to the phase transition of the product (D and E). All speculates are confirmed by the corresponding XRD patterns of the samples annealed at different temperature. Figure 3 gives the XRD patterns of the product annealed at different temperature for 2 h in N2-H2 atmosphere to prevent the oxidation of the Tb3+ ions. It can be found that some changes take place on the phase structure of the products with increasing of the annealing temperature. When the temperature is lower than 650 °C, the phase structure of the product remains to be hexagonal phase and the intensity of the peaks of the XRD patterns increase with the temperature. When the temperature increases to 650 °C, the peaks of the monoclinic phase (PDF card No. 45-0040) appear, though their intensity is weak. With increasing the annealing temperature continually to 700 °C, the intensity of the peaks of the monoclinic phase increases, while the intensity of peaks of the hexagonal phase decreases. When the temperature is increased to 750 °C, the phase structure of the product completely changes into the monoclinic phase. With further increasing the annealing temperature, the peaks of the tetragonal phase (PDF card No. 75-0616) appear. When the temperature is increased to 1150 °C, tetragonal phase can be obtained.
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Figure 4. SEM images of S1-S4 products (a, S1; b, S2; c, S3; d, S4).
Figure 5. TEM image of one-half-sheave of S3 sample and the corresponding SAED pattern.
Figure 4 exhibits the representative SEM images of S1-S4. It can be found that the products is of a bundle of filamentary crystals banded in the middle, and fanned out from the middle to two ends to form sheaflike morphology with a length of 15 µm (Figure 4a). As the content of ethanol was increased to 6 mL, the size of the sheaves and the number of the composed filamentary nanorods increased dramatically (Figure 4b). When the content of ethanol was increased continually, more complex morphologies comprised of four or more half-sheaves emanating from the common core were obtained (Figure 4c,d). The TEM image of one-half-sheave of S3 and the corresponding selected area electron diffraction (SAED) pattern (Figure 5) confirm the results observed in the SEM images. It can be sure that the product is composed of filamentary nanorods extending from the same center, and the diameter of each nanorod is several dozens nanometers. The SAED pattern of the product is consistent with a hexagonal phase structure of TbPO4 · H2O with ring patterns due to (110), (111), (201), and (301) planes, which is in good agreement with the XRD pattern. Increasing annealing temperature also has influences on the morphology of the product. Figure 6 shows the SEM images of the product annealed at different temperature. It can be found that when the annealing temperature is lower than 700 °C, the sheaflike architectures composed of filamentary nanorods remain unchanged, though the size of the whole architectures shrinks a little (Figure 6a-e). As the annealing temperature is increased to 750 °C, the components of the sheaflike architectures change to nanoflakes with thickness of each about 20 nm (Figure 6f). With further increasing the temperature to 800 °C, the components of the architectures change to larger flakes (Figure 6g). When the annealing temperature is achieved to 1150 °C, they melt into a whole, and no space remains between flakes (Figure 6h). To investigate the possible growth mechanism, the influences of reaction time was further investigated. Supporting Information, Figure S1 shows the SEM images of the products obtained
Figure 6. SEM images of the products annealed at different temperature for 2 h (a, 90 °C; b, 250 °C; c, 450 °C; d, 650 °C; e, 700 °C; f, 750 °C (inset is the partial enlarged drawing); g, 800 °C; h, 1150 °C).
Figure 7. Schematic illustration of the formation and shape evolution of half sheaflike structure of TbPO4 · H2O
with different reaction time and the same other reaction conditions of S3. When the reaction time is 1 h, only transparent solution was obtained. With prolonging the reaction time to 1.5 h, lots of small bundle structures composed of filamentary nanorods were obtained (Supporting Information, Figure S1a). As the reaction time was increased to 2 h, the size of each bundle grew larger and sheaflike structure appeared (Supporting Information, Figure S1b). In the subsequent reaction process, the number of the filamentary nanorods in each sheaf structure increased greatly and the morphology became more complex (Supporting Information, Figure S1c,d). On the basis of the above time experimental results and analysis, the products may grow according to the crystal splitting theory proposed by Tang and Alivisatos to form the final sheaflike structure.23 At the beginning of the experiment, TbPO4 · H2O nuclei were obtained in the solution, which grew the building blocks for the final structure. In the following growth process, the nuclei grew into nanorods under the effects of the crystal structure and external powers. At the same time, branches were developed on the nanorods stems, leading to the splitting. With the increase of the reaction time, the branches further split into next generation of branches. The possible process of the growth of sheaflike structure is shown in Figure 7. The addition of Na2H2L facilitates the growth of large sheaflike rods with little splitting, which only take place on the
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Figure 8. FT-IR spectrum of S3.
two ends of the rods, and their splitting degree is small (Supporting Information, Figure S2a). This result should be due to the special structure and properties of Na2H2L. It is wellknown that L4- can provide chelating ability to form Tb3+complexes. As a result, the Tb3+ ions in the solution were decreased. When (NH4)2HPO4 was added into the solution, an ion-exchange reaction between PO43- and L4- ions took place and TbPO4 · H2O nuclei were formed under hydrothermal conditions. Moreover, the potential inducing action of Na2H2L also plays a role in controlling the formation of the final structure of the product by acting on the nuclei and nanorods to form the tighter textures of the nanorod-aggregated sheaves.24 The addition of CTAB facilitates the splitting growth of the nanorods with legible edge of each nanorod (Supporting Information, Figure S2b), but the diameter of the composed nanorods is larger than that of S3. This result should be attributed to the special structure of CTAB, which is an ionic compound and can ionize completely in water. The obtained ions are positively charged tetrahedron with long hydrophobic tail, while PO43- ions are negatively charged tetrahedron. Therefore, the structure complementarity between CTA+ and PO43- ions makes CTAB serve as a growth rate controller and an agglomeration inhibitor.25 When an amount of CTAB is added into the reaction solution, the CTA+ ions adsorb on the surface of the formed nuclei, which facilitates the 1D growth of the nuclei. Under the cooperation of the Na2H2L and CTAB, TbPO4 · H2O was formed with the sheaflike structures consisted of nanowires (Supporting Information, Figure S2c). The FTIR spectrum of S3 is shown in Figure 8. The band centered at 1052 cm-1 is ascribed to the asymmetry stretching vibration of the PO43- groups, and the bands centered at 609 and 536 cm-1 are attributed to the O-P-O bending vibrations. The broad vibration bands centered at 1627 and 3474 cm-1 are assigned to the O-H stretching vibration, which may be due to the water adsorbed on surface, the combined water of the samples and the -OH in the H4L.26 The bands centered at 1430 and 1317 cm-1 are attributed to the COO- vibrations, while the band centered at 805 cm-1 may be attributed to the vibrations of -(CH2)n-. These characterized absorptions indicate that there are still little amounts of the reagents of Na2H2L and CTAB remained on the surface of the final products, which further confirm their participation in the formation of the final morphology of the products. It should be noted that the addition of the ethanol also has an effect on the final morphology of the product by adjusting the stickiness and saturated vapor pressure of the solution under thermal conditions, which further affect the homogenization of the reactants in reaction medium, the amount of the individual nuclei formed, and the amalgamation as well as the direction
Figure 9. Excitation (a) and emission (b) spectra of the different phase structures of TbPO4.
preference of growing nuclei.27 As described in the above, when the content of the ethanol increased, the stickiness of the solution decreased dramatically, which can create more chances for the Tb3+ ions and PO43- ions to collide with each other and form nuclei. Under the cooperation of CTAB and Na2H2L, the following crystal splitting process was accelerated, which further induces the formation of final 3D hierarchical architecture. 3.2. Luminescent Properties. Figure 9 and Supporting Information, Figure S3 show the excitation and emission spectra of S3 annealed at different temperatures for 2 h in a N2-H2 atmosphere. By monitoring the 5D4-7F5 (543 nm) emission, the peaks in the excitation spectra ranging from 200 to 240 nm are due to the spin allowed 4f-5d transition of the Tb3+ ions. The bands from 250 to 280 nm are assigned to the spin-forbidden transition of the Tb3+ ions. The remaining peaks are assigned to the intra4f8 transitions between the 7F6 and 5F5,4, 5H7-4, 5D1,0, 5 L10-7, 5G6-2, and 5D2-4 levels. Excitation into the 4f-5d bands at 260 nm yields the characteristic green emission. These emission bands centered at 489, 543, 585, and 619 nm are due to the 5D4-7FJ (J ) 6, 5, 4, 3) transitions of the Tb3+ ions, respectively. The emissions from the 5D3 to the 7FJ levels are quenched by the crossrelaxation process described in the following formula
Tb3+(5D3) + Tb3+(7F6) f Tb3+(5D4) + Tb3+(7F0) This cross-relaxation process between 5D3-5D4 and 7F0-7F6 produces the rapid population of the 5D4 level at the expense of the 5D3 level, resulting in a strong green emission from the 5 D4 to the 7FJ levels. The emission spectra indicate that the intensity of the products increases first and then decreases with the enhancement of the temperature below 650 °C (Supporting Information, Figure S3b). This result may be due the remains of CTAB and Na2H2L in
Sheaflike TbPO4 Hierarchical Architectures the products, which carbonize at high temperature in N2-H2 atmosphere. The color of the products grows dark with the increase of the temperature, further supporting that the carbonization leads to luminescence quenching of the products. The excitation spectra (Figure 9a) of the TbPO4 with different phase structures exhibit that the intensity and the shape of the bands are different with the change of the phase structure. Compared with hexagonal TbPO4, the intensity of the peaks assigned to the intra4f8 transitions increased greatly in monoclinic TbPO4, and the band due to the spin allowed 4f-5d transition of the Tb3+ ions splits into two bands. For the tetragonal TbPO4, the intensity of the bands assigned to the spinforbidden transition of the Tb3+ ions increases dramatically compared with the other peaks or bands. The emission spectra (Figure 9b) reveal that the change of the phase structure has a little effect on the location of their luminescent lines. This situation is associated with the change of the 4f and 5d levels of the Tb3+ ions in the host lattices. It is well-known that the 4f-5d dipolar electric transitions are allowed by the Laporte parity selection rules and consequently they lead to intense absorption bands whose structure depends on the magnitude of the crystal field splitting, the symmetry of the occupied site, and the interaction between the 5d-electron and those remaining on f-orbitals. The 4f-5d bands are broad owing to their vibration property because of the large 5d-orbitial radial extension and a resulting strong coupling between the 4f-5d electronic transitions and the normal vibration modes of the host lattice. Therefore, the variation of the 4f-5d absorption bands reflects the different environments of the Tb3+ ion in different phase structures. In addition, the crystal field perturbation for 5dorbitial is much stronger than that for 4f-elctrons. So the luminescent properties of the Tb3+ ions exhibit a little change. 4. Conclusion In summary, we obtained a new phosphate hydrate TbPO4 · H2O with sheaflike architectures composed of numerous nanorods using hydrothermal method. The XRD results confirm that TbPO4 · H2O is isostructural with the hexagonal phase GdPO4 · nH2O. The morphology and the phase structure of hexagonal TbPO4 can be maintained well when the annealing temperature is below 650 °C. Monoclinic and tetragonal TbPO4 with different morphologies can be obtained when the products is annealed at 750 and 1150 °C. The investigation of synthesis parameters indicates that Na2H2L and CTAB as well as ethanol had great influences on the morphology and properties of the products. The experimental results and analysis reveal that the formation of the sheaflike TbPO4 · H2O architectures follow the crystal splitting theory. As the products are annealed at different temperature below 650 °C, the emission spectra exhibit that the intensity first increases with the temperature and then decreases due to the carbonization of the remains of CTAB and Na2H2L in the products, which has quenching effect on the luminescence. The excitation spectra of the products with different phase structures exhibit different properties, indicating that the Tb3+ ions have different environments. Emission spectra show a little change, due to a little effect of the f-f transitions of the different environments. The obtained sheaflike architectures may have potential applications in the fields of optoelectronic devices.
J. Phys. Chem. C, Vol. 113, No. 47, 2009 20177 Moreover, our method provides a possibility to synthesize other materials with architectures. Acknowledgment. This project is financially supported by the National Basic Research Program of China (973 Program, Grants 2007CB935502 and 2006CB601103) and the National Natural Science Foundation of China (Grant 20771098). Supporting Information Available: SEM images of the products obtained with different reaction time and the same other reaction conditions of S3 (Figure S1), SEM images of the products obtained with different reaction reagents and the same other reaction conditions of S3 (Figure S2), and excitation and emission spectra of S3 annealed at different temperatures for 2 h (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Li, L.; Liang, H. B.; Tian, Z. F.; Lin, H. H.; Su, Q.; Zhang, G. B. J. Phys. Chem. C 2008, 112, 13763. (2) Bu, W. B.; Hua, Z. L.; Chen, H. R. J. Phys. Chem. B 2005, 109, 14461. (3) Dai, Q. L.; Song, H. W.; Wang, M. Y.; Bai, X.; Dong, B.; Qin, R. F.; Qu, X. S.; Zhang, H. J. Phys. Chem. C 2008, 112, 19399. (4) Yu, L. X.; Song, H. W.; Liu, Z. X. J. Phys. Chem. B 2005, 109, 11450. (5) Tian, Z. F.; Liang, H. B.; Han, B.; Su, Q.; Tao, Y.; Zhang, G. B.; Fu, Y. B. J. Phys. Chem. C 2008, 112, 12524. (6) Jia, G.; Yang, M.; Song, Y. H.; You, H. P.; Zhang, H. J. Cryst. Grow. Des. 2009, 9, 301. (7) Riwotzki, K.; Meyssamy, H.; Kornowski, A. J. Phys. Chem. B 2000, 104, 2824. (8) Stouwdam, J. W.; van Veggel, F. C. J. M. Langmuir 2004, 20, 11763. (9) Stouwdam, J. W.; Hebbink, G. A.; Huskens, J.; Stouwdam, J. W.; Hebbink, G. A.; Huskens, J.; van Veggel, F. C. J. M. Chem. Mater. 2003, 15, 4604. (10) Schuetz, P.; Caruso, F. Chem. Mater. 2002, 14, 4509. (11) Xing, Y.; Li, M.; Davis, S. A.; Mann, S. J. Phys. Chem. B 2006, 110, 1111. (12) Yu, L. X.; Song, H. W.; Liu, Z. X.; Yang, L. M.; Lu, S. Z.; Zheng, Z. H. J. Phys. Chem. B 2005, 109, 11450. (13) Buissette, V.; Moreau, M.; Gacoin, T.; Boilot, J. P. AdV. Funct. Mater. 2006, 16, 351. (14) Li, L.; Jiang, W. G.; Pan, H. H.; Xu, X. R.; Tang, Y. X.; Ming, J. Z.; Xu, Z. D.; Tang, R. K. J. Phys. Chem. C 2007, 111, 4111. (15) Mai, H. X.; Zhang, Y. W.; Sun, L. D.; Yan, C. H. Chem. Mater. 2007, 19, 4514. (16) Zhu, L. P.; Xiao, H. M.; Zhang, W. D.; Yang, Y.; Fu, S. Y. Cryst. Grow. Des. 2008, 8, 1113. (17) Jung, S. H.; Oh, E.; Lee, K. H.; Yang, Y.; Park, C. G.; Park, W.; Jeong, S. H. Cryst. Grow. Des. 2008, 8, 265. (18) Kaltenpoth, G.; Himmelhaus, M.; Slansky, L.; Caruso, F.; Grunze, M. AdV. Mater. 2003, 15, 1113. (19) Yada, M.; Taniguchi, C.; Torikai, T.; Watari, T.; Furuta, S.; Katsuki, H. AdV. Mater. 2004, 16, 1448. (20) Fan, H. Y.; Yang, K.; Boye, D. M.; Sigmon, T.; Malloy, K. J.; Xu, H.; Lo´pez, G. P.; Brinker, C. J. Science 2004, 304, 567. (21) Ikkala, O.; ten Brinke, G. Science 2002, 295, 2407. (22) Shenton, W. S.; Pum, D.; Sleytr, U. B.; Mann, S. Nature 1997, 389, 585. (23) Tang, J.; Alivisatos, A. P. Nano Lett. 2006, 6, 2701. (24) Ma, L.; Chen, W. X.; Zheng, Y. F.; Xu, Z. D. Mater. Res. Bull. 2008, 43, 2840. (25) Yoshida, T.; Tochimoto, M.; Schlettwein, D.; Wohrle, D.; Sugiura, T.; Minoura, H. Chem. Mater. 1999, 11, 2657. (26) Li, L.; Jiang, W. G.; Pan, H. H.; Xu, X. R.; Tang, Y. X.; Ming, J. Z.; Xu, Z. D.; Tang, R. K. J. Phys. Chem. C 2007, 111, 4111. (27) Zhang, J.; Sun, L. D.; Yin, J. L.; Su, H. L.; Liao, C. S.; Yan, C. H. Chem. Mater. 2002, 14, 4172.
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