Highly Enhanced Photoluminescence from YVO - American

May 27, 2009 - nanocrystals and the typical YVO4:Eu3+@YPO4 core/shell heteronanostructures (R ) 1:4, 1:1, and 2:1). Figure 2a shows that the YVO4:Eu3+...
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Highly Enhanced Photoluminescence from YVO4:Eu3+@YPO4 Core/Shell Heteronanostructures Hongliang Zhu* and Diantai Zuo Center of Materials Engineering, Zhejiang Sci-Tech UniVersity, Xiasha UniVersity Town, Hangzhou 310018, China, and State Key Lab of Silicon Materials, Zhejiang UniVersity, Hangzhou 310027, China ReceiVed: January 9, 2009; ReVised Manuscript ReceiVed: April 8, 2009

Novel YVO4:Eu3+@YPO4 core/shell heteronanostructures with enhanced photoluminescence (PL) are proposed in this paper. They were readily formed by hydrothermal epitaxial growth of YPO4 onto YVO4:Eu3+nanocrystals because YPO4 and YVO4 have the same crystal structure and similar lattice parameters. Characterizations by means of X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy prove that the as-synthesized products were not a mixture of YVO4:Eu3+ and YPO4 nanocrystals or YV1-xPxO4: Eu3+ (0 < x < 1) solid solutions, but YVO4:Eu3+@YPO4 core/shell heteronanostructures. The heteronanostructures exhibited much stronger PL than the YVO4:Eu3+ nanocrystals under the same conditions; the optimal heteronanostructures with a YPO4/YVO4:Eu3+ ratio of 1:6 yield a PL intensity 44% higher than that of the YVO4:Eu3+ nanocrystals. The YVO4:Eu3+@YPO4 heteronanostructures have some distinct advantages such as high degree of lattice matching between YVO4 and YPO4. Finally, we discuss the mechanism of their PL enhancement. 1. Introduction Europium ion-doped yttrium orthovanadate (YVO4:Eu3+) has been widely used as red phosphor in cathode ray tubes (CRTs), in high-pressure mercury lamps, and in color television due to its excellent luminescent properties.1-3 In recent years, YVO4: Eu3+ nanophosphors have received a great deal of research attention due to the potential applications in higher resolution displays, drug delivery systems, and biological fluorescence labeling.4-7 In addition, smaller-sized phosphor particles result in higher packing density and better paste rheology leading to lower loading in lamps.8 Unfortunately, compared to micrometersized materials, much higher specific surface area of nanomaterials inevitablely results in more serious surface recombination and higher surface defects density.9 Consequently, luminescent efficiency of nanophosphors is usually lower than that of their corresponding bulk powders, which is due to the nonradiative relaxation originating from surface recombination, surface defects, and surface electronic states in the nanophosphors.10-12 Therefore, it is extremely important to increase luminescent efficiency for nanophosphors. During the past decade, some effort has been made to increase luminescence properties of nanophosphors by suppressing the energy-loss processes of the energy transfer. Among these strategies, coating surface of nanophosphors with a suitable inorganic material to form core@shell heteronanostructures has been regarded as an effective one.11,13 Up to now, some heterostructures based on YVO4:Eu3+ phosphor such as YVO4: Eu3+/YBO3:Eu3+, Y2O3:Eu3+@SiO2@YVO4:Eu3+, SiO2@YVO4: Eu3+, Y(OH)3-Eu3+/YVO4-Eu3+ composite, and YV(0.7)P(0.3)O4: Eu3+,Bi3+@SiO2 have been proposed.14-18 In this paper, we present a new kind of heteronanostructure, the YVO4: Eu3+@YPO4 core/shell heteronanostructure, which yields a photoluminescence intensity 44% higher than that of the YVO4: Eu3+ nanocrystals under the same conditions. Compared to other * To whom correspondence should be addressed. Phone/fax: +86 571 86843266. E-mail: [email protected].

heterostructures of YVO4:Eu3+,14-18 the YVO4:Eu3+@YPO4 core/shell heteronanostructures have some distinct advantages. First, YPO4 can be more readily epitaxially grown onto the YVO4:Eu3+ nanocrystals to form the heteronanostructure because both YVO4 and YPO4 have the same crystal structure (tetragonal zircon-type structure) and similar unit cell parameters.7 Second, the surface recombination, surface defects density, and surface state density of YVO4:Eu3+ nanocrystals can be greatly decreased via formation of the YVO4: Eu3+@YPO4 core/shell heteronanostructures due to the high degree of lattice matching between YVO4 and YPO4.7,18 Third, the YVO4:Eu3+@YPO4 heteronanostructures may have higher stability than the YVO4:Eu3+ nanophosphor in high-pressure mercury lamps due to surface protection from the stable YPO4 coating.7 Therefore, the heteronanostructures proposed here can be potentially used as high-performance composite nanophosphors. 2. Experimental Section 2.1. Preparation of the Heteronanostructures. The starting reagents used in this work were analytical-grade Y(NO3)3 · 6H2O, Eu(NO3)3 · 6H2O, (NH4)2HPO4, and NaVO3 · 2H2O. All samples were prepared by the hydrothermal method under the same conditions. We fixed the Eu3+ molar concentration of the YVO4: Eu3+ cores at 5%. Different YPO4/YVO4:Eu3+ molar ratios (abbreviated as R in this paper) such as 1:8, 1:6, 1:4, 1:2, 1:1, and 2:1 were adopted, so a total of six heteronanostructure samples were prepared. All preparation conditions were similar, so we take the YVO4:Eu3+@YPO4 core/shell heteronanostructures with a YPO4/YVO4:Eu3+ ratio of 1:6 as an example to present the detailed preparation procedures. The procedures involve two steps: (1) hydrothermal synthesis of the YVO4:Eu3+ cores and (2) hydrothermal epitaxial growth of YPO4 onto the YVO4:Eu3+ core. In the first step, 30.4 mL of Y(NO3)3 solution (0.15 mol/L), 1.6 mL of Eu(NO3)3 solution (0.15 mol/L), and 0.758 g of NaVO3 · 2H2O were added to 130 mL of deionized water under vigorous magnetic stirring for 30 min. The pH value of the solution was adjusted to 9.5 with

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Photoluminescence from YVO4:Eu3+@YPO4 ammonia under stirring. Then, the above solution was transferred into a Teflon-lined stainless steel autoclave (capacity 200 mL) and sealed. The autoclave was heated at 200 °C for 16 h and cooled naturally to room temperature. Finally, the YVO4:Eu3+ product was collected by centrifugation. In the second step, the YVO4:Eu3+ product obtained in the first step, 5.3 mL of Y(NO3)3 solution (0.15 mol/L), and 5.3 mL of (NH4)2HPO4 (0.15 mol/ L) were added to 150 mL of deionized water under vigorous magnetic stirring for 30 min. The pH value was adjusted to 8 with ammonia under stirring. Once again, the solution was hydrothermally treated at 200 °C for 16 h. Finally, the YVO4: Eu3+@YPO4 core/shell heteronanostructures (R ) 1:6) were collected and dried at 80 °C for 20 h in air. For comparison, YVO4:Eu3+ (Eu % ) 5) and YPO4 nanocrystals were also prepared by the above-mentioned hydrothermal approach, using the same process parameters. In other words, the YVO4:Eu3+ nanocrystals used for comparison were the product hydrothermally prepared in the first step. 2.2. Characterization and Photoluminescence Property. Phase identification of the products was carried out by X-ray diffraction (XRD) with a Thermo ARL X’TRA X-ray diffractometer with Cu KR radiation (λ ) 1.54178 Å). Morphology observation was performed on a JEOL JEM 200 CX transmission electron microscope. Chemical composition was analyzed by an Oxford Instrument’s INCA energy dispersive spectrometer (EDS). X-ray photoelectron spectroscopy (XPS) was used to analyze the samples. XPS measurement was performed on a X-ray photoelectron spectrometer (Model Axis Ultra DLD, Kratos Corp., UK) with a standard Mg KR (1256.6 eV) X-ray source operated at 150 W. All binding energies were referenced to the C1s peak at 284.6 eV of the surface adventitious carbon. Photoluminescent (PL) excitation and emission spectra of all samples were obtained on a Hitachi fluorescence spectrophotometer (Model F-4600, Hitachi Corporation, Japan). To estimate the PL enhancement effect of the YVO4:Eu3+@YPO4 heteronanostructures, all PL excitation and emission spectra were measured in powder form with use of the same measurement parameters. 3. Results and Discussion To determine that the YVO4:Eu3+ and YPO4 coexisted in the heteronanostructures, the products were characterized by XRD, and their typical XRD patterns are shown in Figure 1. Figure 1a shows the XRD patterns of the core material obtained in the first step. All XRD peaks of Figure 1a are in good agreement with the values of the YVO4 (JCPDS no. 72-0274). Likewise, all peaks of Figure 1h can be indexed to the tetragonal YPO4 (JCPDS no. 11-0254). As shown in Figure 1, YVO4:Eu3+ and YPO4 have similar XRD patterns except for a slight peak shift. This is because they have the same crystal structure with similar lattice constants (a ) 7.1 Å and c ) 6.27 Å for YVO4; a ) 6.9 Å and c ) 6.0 Å for YPO4). Interestingly, the peaks of the YPO4 become more and more visible as the YPO4/YVO4:Eu3+ ratio increases. As shown in Figure 1f,g, the YVO4:Eu3+@YPO4 core/shell heteronanostructure (R )1:1 and 2:1) clearly shows two series of XRD patterns, namely, those of the YVO4 and YPO4. The inset of Figure 1 shows the amplified (200) peak of the heteronanostructures (R ) 2:1), in which the (200) peaks of the YVO4 and YPO4 are clearly presented. Therefore, the core material obtained in the first step is YVO4:Eu3+, and the heteronanostructures obtained by the two-step hydrothermal process are composed of YVO4:Eu3+ and YPO4. Figure 2 shows the TEM images of the YVO4:Eu3+, YPO4 nanocrystals and the typical YVO4:Eu3+@YPO4 core/shell

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Figure 1. XRD patterns of the YVO4:Eu3+@YPO4 core/shell heteronanostructures with different YPO4/YVO4:Eu3+ molar ratios.

heteronanostructures (R ) 1:4, 1:1, and 2:1). Figure 2a shows that the YVO4:Eu3+ nanocrystals formed in the first step are of cube-like nanomorphology with a side length of 20 nm. Otherwise, the YPO4 nanocrystals exhibit quite different morphology from that of the YVO4:Eu3+ nanocrystals (see Figure 2b). They are smooth-surfaced nanorods with a diameter of 20 nm. True shell thickness of the core/shell heteronanostructures is somewhat difficult to measure due to their similar lattice constants, so is the amount of material used to grow YPO4 nanocoating as the reference shell thickness. In other words, the YPO4/YVO4:Eu3+ ratio is used as the reference shell thickness in this paper. TEM was employed to observe the morphology of core/shell heteronanostructures. If no independent YPO4 nanorods are found in the YVO4:Eu3+@YPO4 core/ shell heteronanostructures, such approximations are feasible. Panels c-d of Figure 2 show TEM images of the typical YVO4: Eu3+@YPO4 core/shell heteronanostructures (R ) 1:4, 1:1, and 2:1). When the YPO4/YVO4:Eu3+ ratio was low (R ) 1:4), the heteronanostructures exhibited almost the same morphology as that of the YVO4:Eu3+ nanocrystals (see Figure 2a), except that their size was slightly larger. Furthermore, no smooth-surfaced YPO4 nanorods were observed in the heteronanostructures (R ) 1:4). Figure 2d reveals that the diameter of the YVO4: Eu3+@YPO4 heteronanostructures (R ) 1:1) is around 45 nm, much larger than that of the YVO4:Eu3+ nanocrystals (see Figure 2a). In addition, they exhibit somewhat shuttle-like rather than cube-like morphology. Smooth-surfaced YPO4 nanorods were also not detected in Figure 2d. Figure 2e shows a TEM image of the heteronanostructures (R ) 2:1), which is quite different from that shown in panels c and d of Figure 2. A few smoothsurfaced YPO4 nanorods can be observed in Figure 2e. When the YPO4/YVO4:Eu3+ ratio is less than 1:1, the precursors of the YPO4 was epitaxially grown onto the YVO4:Eu3+ core to form the heteronanostructures, rather than form YPO4 nanorods via homogeneous crystalline nucleation in the solution. When the YPO4/YVO4:Eu3+ ratio was large (e.g., R ) 2:1), some of the precursors have been changed into YPO4 nanorods via homogeneous crystalline nucleation. Therefore, approximation

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Figure 3. EDS spectrum of the YVO4:Eu3+@YPO4 core/shell heteronanostructures with the YPO4/YVO4:Eu3+ molar ratio of 1:2.

Figure 2. TEM images of the YVO4:Eu3+, YPO4 nanocrystals, and the YVO4:Eu3+@YPO4 core/shell heteronanostructures: (a) YVO4:Eu3+, (b) YPO4, (c) YVO4:Eu3+@YPO4 (R ) 1:4), (d) YVO4:Eu3+@YPO4 (R ) 1:1), (e) YVO4:Eu3+@YPO4 (R ) 2:1). R is the YPO4/YVO4: Eu3+ molar ratio.

of true shell thickness via the amount of material used to form YPO4 nanocoatings is effective when the YPO4/YVO4:Eu3+ ratio is low (R < 1:1). To verify the YPO4/YVO4:Eu3+ molar ratios of the heteronanostructures, the YVO4:Eu3+@YPO4 heteronanostructures (R ) 1:2) were selected as a sample to be measured by EDS (see

Figure 3). The P/V atomic ratio was obtained by using the Oxford Instruments INCA software package (INCA Energy 2000). As given in the inset of Figure 3, the obtained P/V atomic ratio is 1:1.97, which is consistent with the theoretical value (R ) 1:2). XPS is the most commonly used technique for investigating the elemental composition of surface layers 1-5 nm in depth. If the YVO4:Eu3+ cores are effectively coated with the shell material (YPO4), the XPS signal of the cores will be lower than that of the uncoated YVO4:Eu3+. Therefore, the XPS signal of vanadium can be used as a measure for evaluating the formation of the core/shell heteronanostructures. The samples with different YPO4/YVO4:Eu3+ molar ratios such as 0:1, 1:8, 1:6, 1:4, 1:2, and 1:1 have been analyzed by XPS under the same conditions, and their XPS spectra in the range of 510-550 eV are presented in Figure 4a. As shown in Figure 4a, the uncoated YVO4:Eu3+ nanocrystals (R ) 0:1) exhibit a strong peak at around 517.5 eV, which is assigned to the V 2p3/2 band of vanadium.19,20 Interestingly, the V 2p3/2 peak decreases dramatically as the YPO4/YVO4:Eu3+ ratio increases, meaning that the YPO4 nanocoatings become thicker and thicker as the ratio increases. Figure 4b shows a plot of the intensity of the V 2p3/2 peak versus the YPO4/YVO4:Eu3+ molar ratio. It can seen from Figure 4b that the intensities of V 2p3/2 peaks display an almost linear decrease with the YPO4/YVO4:Eu3+ ratio. The intensity of the V 2p3/2 peak of the heteronanostructures (R ) 1:1) is only 5% of that of the YVO4:Eu3+ nanocrystals. Therefore, XPS analysis further confirms the formation of YVO4:Eu3+@YPO4 core/shell heteronanostructures. Figure 5 shows PL excitation and emission spectra of the YVO4:Eu3+@YPO4 core/shell heteronanostructures and the YVO4:Eu3+ nanocrystals. All PL excitation and emission spectra were measured in powder form with the same measurement parameters, so their respective PL emission intensity can relatively represent their PL efficiency. In other words, the photoluminescence enhancement effect of the core/shell heteronanostructures can be evaluated by the PL emission intensity of the samples. As shown in the excitation spectra (see the left side of Figure 5), the heteronanostructures and nanocrystals exhibit a strong broad excitation band in the range of 200-360 nm with a maximum value at 319 nm, which are assigned to charge transfer bands of Eu-O and V-O, and VO43- absorption bands.21 Both the YVO4:Eu3+ nanocrystals and the heteronanostructures show similar excitation spectra, meaning the shell material has no significant effects on the charge transfer. The emission spectra in the range of 585-645 nm (see the right side of Figure 5) exhibit two well-known emission bands of YVO4:Eu3+, which are assigned to the magnetic-dipole transition 5 D0-7F1 of Eu3+ (596 nm) and the forced electric-dipole transition 5D0-7F2 of Eu3+ (619 nm), respectively.22 More

Photoluminescence from YVO4:Eu3+@YPO4

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Figure 6. Plot of PL intensity of the 5D0-7F2 emission at 619 nm versus the YPO4/YVO4:Eu3+ molar ratio.

Figure 4. (a) XPS spectra of the YVO4:Eu3+ and YVO4:Eu3+@YPO4 core/shell heteronanostructures with different YPO4/YVO4:Eu3+ molar ratios (R); (b) plot of the intensity of the V 2p peak versus the YPO4/ YVO4:Eu3+ molar ratio.

Figure 5. Photoluminescence excitation (left side) and emission spectra (right side) of the YVO4:Eu3+@YPO4 core/shell heteronanostructures. R is the YPO4/YVO4:Eu3+ molar ratio.

importantly, all heteronanostructures except for the heteronanostructures (R ) 2:1) exhibit much stronger photoluminescence than the YVO4:Eu3+ nanocrystals. The optimal YPO4/YVO4: Eu3+ ratio is 1:6, and the heteronanostructures (R ) 1:6) yield a PL intensity 44% higher than that of the YVO4:Eu3+ nanocrystals. In addition, YV1-xPxO4:Eu3+ (0 < x < 1) solid solutions such as YV0.7P0.3O4:Eu3+ exhibit slightly lower PL emission than YVO4:Eu3+ under the same conditions.7 PL emission of the mixture of YVO4:Eu3+ and YPO4 is also lower than that of YVO4:Eu3+, because YPO4 is nonluminescent

material. Therefore, the PL enhancement is not due to formation of the YV1-xPxO4:Eu3+ (0 < x < 1) solid solution or the mixture, but to formation of the YVO4:Eu3+@YPO4 core/shell heteronanostructures. The surface recombination, surface defects density, and surface state density of the YVO4:Eu3+ core were greatly decreased by the YPO4 nanocoatings. Consequently, the nonradiative decay channels were effectively decreased, and enhanced PL emission was obtained. Figure 6 shows the plot of PL intensity of the 5D0-7F2 emission at 619 nm versus YPO4/YVO4:Eu3+ molar ratio (R). The change of PL intensity with R is a parabola-like curve that reaches the peak at R ) 1:6. Therefore, R has a critical role in the PL enhancement effect of the core/shell heteronanostructures. In theory, the YPO4 nanocoatings have two different effects on the PL emission of the core/shell heteronanostructures, one is positive and the other is negative. On the one hand, the YPO4 nanocoatings decrease the surface recombination, surface defects density, and surface state density of the YVO4:Eu3+ core, thus they have a positive effect on the PL emission of the core/shell heteronanostructures. On the other hand, YPO4 is nonluminescent material, so the PL efficiency of the core/shell heteronanostructures will decrease with its percentage increasing. On the basis of the above discussion, the change of PL intensity with R can be explained as follow: (1) When the YPO4/YVO4:Eu3+ ratio is very low (R < 1:6), the PL intensity increases with R because the surface defects density and surface state density of YVO4:Eu3+ nanocrystals decrease with increasing YPO4 coating. (2) When R ) 1:6, the surface recombination, surface defects density, and surface state density have been decreased greatly and the percentage of YPO4 is relatively low, so the best PL enhancement effect was obtained. (3) When the YPO4/YVO4: Eu3+ ratio gets higher (R > 1:6), YPO4 nanocoatings become thicker and consequently become better at decreasing the surface recombination and surface defects, but the percentage of the nonluminescent shell material (YPO4) also gets higher. Consequently, the PL intensity decreases with R when R > 1:6. 4. Conclusions The YVO4:Eu3+@YPO4 core/shell heteronanostructures were successfully prepared by a facile two-step hydrothermal approach. XRD patterns, TEM images, and XPS spectra of the as-synthesized products reveal that the shell material (YPO4) has been epitaxially grown onto YVO4:Eu3+ nanocrystals to form the heteronanostructures. The heteronanostructures exhibited much stronger photoluminescence than the YVO4:Eu3+ nanocrystals under the same conditions. The YPO4/YVO4:Eu3+

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molar ratio has an important role in the formation of the heteronanostructures and their photoluminescence properties. The optimal YPO4/YVO4:Eu3+ molar ratio is 1:6, and the heteronanostructures (R ) 1:6) yield a photoluminescence intensity 44% higher than that of the YVO4:Eu3+ nanocrystals under the same conditions. Compared to the reported heterostructures, the YVO4:Eu3+@YPO4 heteronanostructures have some obvious advantages such as high degree of lattice matching between YVO4 and YPO4 because they both have the same crystal structure and similar unit cell parameters. Highly enhanced photoluminescence from the heteronanostructures is mainly due to decreasing the surface recombination, surface defects density, and surface state density of the YVO4:Eu3+ nanocrystals by the YPO4 coating. Acknowledgment. This work was supported by the Open Foundation Project of the State Key Lab of Silicon Materials (2009). The authors also thank the Doctoral Science Foundation of Zhejiang Sci-Tech University (no. 0803611-Y) fo financial support. References and Notes (1) Itoh, L. O. Chem. ReV. 2003, 103, 3835. (2) Ju¨stel, T.; Nikol, H.; Ronda, C. Angew. Chem., Int. Ed. 1998, 37, 3084. (3) Wu, C. C.; Chen, K. B.; Lee, C. S.; Chen, T. M.; Cheng, B. M. Chem. Mater. 2007, 19, 3278. (4) Li, G.; Chao, K.; Peng, H.; Chen, K. J. Phys. Chem. C 2008, 112, 6228.

Zhu and Zuo (5) Chander, H. Mater. Sci. Eng. R 2005, 49, 113. (6) Giaume, D.; Poggi, M.; Casanova, D.; Mialon, G.; Lahlil, K.; Alexandrou, A.; Gacoin, T.; Boilot, J. P. Langmuir 2008, 24, 11018. (7) Zhu, H.; Yang, H.; Jin, D.; Wang, Z.; Gu, X.; Yao, X.; Yao, K. J. Nanopart. Res. 2008, 10, 1149. (8) Rao, R. P. J. Lumin. 2005, 113, 271. (9) Jang, E.; Jun, S.; Chung, Y.; Pu, L. J. Phys. Chem. B 2004, 108, 4597. (10) Abrams, B. L.; Holloway, P. H. Chem. ReV. 2004, 104, 12–5783. (11) Bu, W.; Hua, Z.; Chen, H.; Shi, J. J. Phys. Chem. B 2005, 109, 14461. (12) Mai, H.; Zhang, Y.; Sun, L.; Yan, C. J. Phys. Chem. C 2007, 111, 13721. (13) Chin, P. T. K.; de M. Donega’, C.; Bavel, S. S. v.; Meskers, S. C. J.; Sommerdijk, N. A. J. M.; Janssen, R. A. J. J. Am. Chem. Soc. 2007, 129, 14880. (14) Pan, G. H.; Song, H. W.; Bai, X.; Liu, Z. X.; Yu, H. Q. Chem. Mater. 2006, 18, 4526. (15) Chang, M.; Tie, S. Nanotechnology 2008, 19, 075711. (16) Yu, M.; Lin, J.; Fang, J. Chem. Mater. 2005, 17, 1783. (17) Pan, G.; Song, H.; Bai, X.; Fan, L.; Yu, H.; Dai, Q.; Dong, B.; Qin, R.; Li, S.; Lu, S.; Ren, X.; Zhao, H. J. Phys. Chem. C 2007, 111, 12472. (18) Darbandi, M.; Hoheisel, W.; Nann, T. Nanotechnology 2006, 17, 4168. (19) Hoffmann, R. C.; Jeurgens, L. P. H.; Wildhack, S.; Bill, J.; Aldinger, F. Chem. Mater. 2004, 16, 4200. (20) Barreca, D.; Depero, L. E.; Noto, V. D.; Rizzi, G. A.; Sangaletti, L.; Tondello, E. Chem. Mater. 1999, 11, 255. (21) Li, Y.; Hong, G. J. Solid State Chem. 2005, 178, 645. (22) Zhu, H.; Yang, D.; Zhu, L.; Li, D.; Chen, P.; Yu, G. J. Am. Ceram. Soc. 2007, 90, 3095.

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