LaF3, CeF3, CeF3:Tb3+

Feb 2, 2008 - LaF3, CeF3, CeF3:Tb3+, and CeF3:Tb3+@LaF3 (core-shell) 2D nanoplates have been successfully synthesized by a facile and effective ...
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J. Phys. Chem. C 2008, 112, 2904-2910

LaF3, CeF3, CeF3:Tb3+, and CeF3:Tb3+@LaF3 (Core-Shell) Nanoplates: Hydrothermal Synthesis and Luminescence Properties Chunxia Li, Xiaoming Liu, Piaoping Yang, Cuimiao Zhang, Hongzhou Lian, and Jun Lin* State Key Laboratory of Application of Rare Earth Resources, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ReceiVed: October 12, 2007; In Final Form: December 1, 2007

LaF3, CeF3, CeF3:Tb3+, and CeF3:Tb3+@LaF3 (core-shell) 2D nanoplates have been successfully synthesized by a facile and effective hydrothermal process. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) spectra as well as kinetic decays were used to characterize the samples. The experimental results indicate that the organic additive, trisodium citrate (Cit3-), as a shape modifier has the dynamic effect by adjusting the growth rate of different crystal facets, resulting in forming the anisotropic geometries of the final products. The possible formation mechanisms for different products have been presented. The CeF3, CeF3:Tb3+, and CeF3:Tb3+@LaF3 (core/shell) nanoplates show characteristic emission of Ce3+ (5d-4f) and Tb3+ (f-f), respectively. By coating the LaF3 shell on the surface of the CeF3:Tb3+ core, the distance between the luminescent lanthanide ions is increased and the surface quenchers are decreased, thus reducing the nonradiative pathways. The luminescent intensity and lifetime of the CeF3:Tb3+@LaF3 core/ shell nanoplates are enhanced with respect to the bare CeF3:Tb3+ nanoplates.

1. Introduction The ability to control and manipulate the physical and chemical properties of materials as we desire is one of the challenging issues in chemistry and materials science. Currently, inorganic nanocrystals with controllable and uniform size and shape have stimulated great interest because the morphology, dimensionality, and size of materials are well-known to have great effect on their physical, chemical, magnetic, and catalytic properties as well as for their application in optoelectronic devices.1,2 Much effort has been devoted to the fabrication of nanocrystals with various shapes, including zero-dimension (0D) isotropic spheres and cubes;3 1D rods, wires, tubes, and belts;4 2D plate, disk, and sheet;5 and hierarchical architectures such as star,6 multipods,7 and dendrites.8 In particular, 2D shaped nanocrystals with large surface area and high aspect ratio (the edge length over the thickness) have received considerable attention, which have potential applications in information storage, whisper gallery mode (WGM) lasers, transducer, light emitter, catalyst, and sensor.9 Although various chemical methods have so far been developed to prepare nanostructured materials with different plate-like shapes, these methods mainly focus on the fabrication of metals,10 semiconductors,11 and metal oxides.12 The fabrication of rare earth (RE) fluorides with 2D plated shape has drawn little attention.13 In the past decades, rare earth (RE) fluorides have become of the focus of intensive research due to their promising applications in lighting and displays, biological labels, and optical amplifies.14 In comparison with oxygen-based systems, fluorides possess very low vibrational energies and therefore the quenching of the excited states of the rare earth ions will be minimal.15 Furthermore, they exhibit adequate thermal and * Author to whom any correspondence should be addressed. E-mail: [email protected].

environmental stability and therefore are considered as ideal host materials for luminescent lanthanide ions. Recently, rare earth fluoride (REF3) nanocrystals with diverse shapes have been successfully synthesized through the thermolysis of organometallic precursors in surfactant solutions.16 However, exploring a simple and low-cost approach for the fabrication of 2D rare earth fluorides with well-defined shapes and in high yield remains a challenge. Hydrothermal synthesis, which may provide a more promising technique than conventional methods in terms of cost and potential for large-scale production, is considered as one of the useful and powerful pathways to prepare novel nanostructures. In addition, due to nonradiative decay from defects on the surface of the nanocrystals, the luminescence efficiency of nanostructural materials is usually lower than that of the corresponding bulk materials. To reduce these defects, the growth of a crystalline shell of a suitable inorganic material around each nanocrystal to form the core/ shell structures has been regarded as an effective strategy to improve luminescent efficiency. Here we report the hydrothermal synthesis and optical properties of 2D plate shaped rare earth fluoride nanocrystals: LaF3, CeF3, CeF3:Tb3+, and CeF3: Tb3+@LaF3 (core-shell). Organic additive trisodium citrate (Cit3-) plays double roles as both a coordination agent and structure-directing agent. The morphologies, structure, formation mechanism, and luminescence properties of these nanoplates are investigated in detail. 2. Experimental Section Preparation. The rare earth oxides La2O3 (99.999%) and Tb4O7 (99.999%) as well as Ce(NO3)3‚6H2O were purchased from Science and Technology Parent Company of Changchun Institute of Applied Chemistry and other chemicals were purchased from Beijing Chemical Company. All chemicals are analytical grade reagents and are used directly without further

10.1021/jp709941p CCC: $40.75 © 2008 American Chemical Society Published on Web 02/02/2008

LaF3, CeF3, CeF3:Tb3+, and CeF3:Tb3+@LaF3 Nanoplates purification. Rare earth chloride stock solutions of 0.2 M were prepared by dissolving the corresponding metal oxide in hydrochloric acid at elevated temperature. LaF3, CeF3, and CeF3:Tb3+ Nanoplates. In a typical procedure for the preparation of LaF3 nanoplates, 10 mL of LaCl3 (0.2 M) was added into 20 mL of aqueous solution containing 2 mmol of trisodium citrate (labeled as Cit3-) to form the metal-Cit3- complex (the molar ratio of Cit3- to RE3+ is 1:1). After vigorous stirring for 30 min, 30 mL of aqueous solution containing 25 mmol of NaF was introduced into the above solution. Then the mixing solution was transferred into a Teflon bottle held in a stainless steel autoclave, sealed, and maintained at 180 °C for 24 h. As the autoclave was cooled to room temperature naturally, the precipitates were separated by centrifugation, washed with deionized water and ethanol in sequence, and then dried in air at 80 °C for 12 h. CeF3, CeF3: 5% Tb3+ (molar ratio) nanoplates were prepared in a similar way as that for the LaF3 sample. It is noted that half of the CeF3:Tb3+ precipitate obtained was used for further characterization and the remaining half of that was used for the preparation of CeF3:Tb3+@LaF3 core-shell heterostructures. Additionally, different molar ratios (0, 1:2, 2:1, 4:1, 180 °C, 24 h) of Cit3-: La3+ and hydrothermal treatment times (0.5 h, 4 h, 180 °C) were selected to investigate the effects of these factors on the morphological and structural properties of the samples. CeF3:Tb3+@LaF3 Nanoplates. The remaining half of the CeF3:Tb3+ precipitate was dispersed in 30 mL of distilled water by sonication treatment. Meanwhile, 5 mL of LaCl3 (0.2 M) was added into 10 mL of aqueous solution containing 1 mmol of trisodium citrate to form the metal-Cit3- complex, which was added into the resulting CeF3:Tb3+ colloidal solution under vigorous stirring for 30 min. After that, 15 mL of aqueous solution containing 12.5 mmol of NaF was introduced into the above solutions. The following procedures were the same as those for the synthesis of LaF3 nanoplates as stated above. Characterization. Powers X-ray diffraction (XRD) measurements were performed on a Rigaku-Dmax 2500 diffractometer at a scanning rate of 15 deg/min in the 2θ range from 10° to 80°, with graphite monochromatized Cu KR radiation (λ ) 0.15405 nm). SEM micrographs were obtained with use of a field emission scanning electron microscopy (FE-SEM, XL30, Philips). Low- to high-resolution transmission electron microscopy (TEM) was performed using FEI Tecnai G2 S-Twin with a field emission gun operating at 200 kV. Images were acquired digitally on a Gatan multiople CCD camera. The photoluminescence (PL) excitation and emission spectra were recorded with a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The PL lifetimes of the samples were measured with a Lecroy Wave Runner 6100 Digital Oscilloscope (1 GHz) using a 266 nm laser wavelength (pulse width ) 4 ns) from YAG:Nd as the excitation source. All the measurements were performed at room temperature. 3. Results and Discussion Structures. The phase structures of the as-prepared samples were investigated by XRD. Figure 1 shows the XRD pattern of as-prepared LaF3 sample. All of the diffraction peaks can be readily indexed to a pure hexagonal phase [space group: P3hc1(165)] with lattice constants of a ) 0.7163 nm and c ) 0.7336 nm, very close to the reported data (a ) 0.7187 nm and c ) 0.7350 nm) in literature (JCPDS: 32-0483). Figure 2 shows XRD patterns of CeF3 (a), CeF3:Tb3+ (b), and CeF3:Tb3+@LaF3 core/shell (c) samples. The results of the XRD indicate that these

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Figure 1. XRD pattern of LaF3 sample. The line spectrum corresponds to the literature data of bulk LaF3 (JCPDS No. 32-0483).

Figure 2. XRD patterns of CeF3 (a), CeF3:Tb3+ (b), and CeF3: Tb3+@LaF3 (c). The line spectrum corresponds to the literature data of bulk CeF3 (JCPDS No. 08-0045).

three samples are well crystallized, and the patterns are consistent with hexagonal phase structure known from the bulk CeF3 crystal (JCPDS: 08-0045) with space group of P63/mcm (193). It is noteworthy that for our CeF3:Tb3+@LaF3 core/shell system, no obvious shift is observed for the diffraction peaks relative to those of the CeF3:Tb3+ core (as shown in Figure 2), which is due to the similar lattice constants between LaF3 and CeF3.17 Additionally, no other peaks can be found in the pattern, revealing that there is no impurity in the products. Morphologies. The morphologies and structures of samples were investigated by the SEM and TEM observations. Figure 3A is a typical SEM image of LaF3 sample, from which one can see that the product is composed of nanoplates with regular hexagonal shapes. A vast majority of nanoplates lie parallel to the substrate and some of them stand vertically. Furthermore, as shown in Figure 3A, a high yield of this polyhedral form can be easily achieved via this facile method. The highmagnification SEM image (Figure 3B) indicates that the thickness of the nanoplates is about 33 nm, the average edge length is 100 nm, and the mean diameter is 155 nm. TEM images can provide further insight into the nanometer-scale details of the hexagonal plated shape. Figure 3C is a TEM image of the product. The regular hexagonal cross sections can be clearly observed. The selected area electron diffraction (SAED) patterns are consistent with a hexagonal phase structure of LaF3 with strong ring patterns due to (002), (110), (111), and (300) planes (Figure 3C, inset), demonstrating its polycrystalline nature. To investigate the crystalline orientation of individual

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Figure 5. SEM (A, B) and TEM (Inset B) images of CeF3:Tb3+@LaF3 core-shell nanoplates.

Figure 3. SEM (A, B), TEM (C), and HRTEM (D) images of LaF3 hexagonal nanoplates. The lower inset in Figure 3D shows a schematic diagram of a LaF3 hexagonal nanoplate.

Figure 6. XPS spectra of CeF3:Tb3+ (a) and CeF3:Tb3+@LaF3 core/ shell (b) nanoplates. Inset: The XPS spectra in the range 820-920 eV.

Figure 4. SEM (A, B), TEM (C), and HRTEM (D) images of CeF3 round nanoplates. The lower inset in Figure 4D shows a schematic diagram of a CeF3 round nanoplate.

nanoplates, HRTEM was used to measure the distance of the lattice planes. As revealed by the HRTEM image (Figure 3D), the examined region is perfectly free of dislocation and distortion; the space of the parallel fringes is 0.33 nm, which is consistent with the d-spacing value of (111) lattice planes of LaF3. Under similar reaction conditions as those for preparing LaF3 sample, we also obtain CeF3 nanoplates. SEM and TEM images of the as-prepared CeF3 sample are depicted in Figure 4. The low-magnification SEM image (Figure 4A) shows the general view, clearly indicating that the products consist of largescale, monodisperse, and round nanoplates with a mean diameter of 60 nm and thickness of 120 nm. More careful examination of the magnified SEM image (Figure 4B) shows that the top/ bottom surfaces of the nanoplates are very rough, implying that nanoplates are aggregates of even smaller nanoparticles. Figure 4C is a representative TEM image of the CeF3 nanoplates, either lying flat on the faces or standing on the edges. The SAED pattern shown in the inset of Figure 4C also consists of a hexagonal phase structure of CeF3 with strong ring patterns indexed to the (002), (110), (111), and (300) planes, respectively, demonstrating its polycrystalline nature as that of LaF3 nanoplates. The HRTEM image in Figure 4D reveals that the samples are highly crystallized with the interplanar spacing of 0.33 nm corresponding to the (111) crystal plane of CeF3. Figure 5 presents SEM images of CeF3:Tb3+@LaF3 core-shell structure. From the low-magnification SEM image (Figure 5A), we can

clearly observe that the products are also composed of nanoplates. A higher magnification SEM image (Figure 5B) further indicates that the nanoplates have hexagonal structure. Herein, it is important to note that separate cores of CeF3:Tb3+ nanoplates were not observed via SEM in all of our samples, which indicates that the LaF3 shell growth is uniform, completely coating the CeF3:Tb3+ cores. Furthermore, a small quantity of nanoplates exist with hollow inners, as marked with the black arrows. From the TEM image of CeF3:Tb3+@LaF3 (inset in Figure 5B), regular and hollow hexagonal plates can be seen clearly. In TEM images, contrast depends on the electron scattering power of the object forming the images. The electron scattering power in turn depends on the electron density inside the object. 2b Hence, direct evidence can be obtained from the image contrast for the core/shell structure with different lattice parameters. However, in our case, because the core and the shell of the CeF3:Tb3+@LaF3 core-shell nanoplates have similar electron density and lattice parameters, the shell and core cannot be clearly distinguished. Compared with the CeF3:Tb3+ nanoplates, the CeF3:Tb3+@LaF3 ones show a remarkable increase in mean diameter from 60 to 125 nm, along with a nearly unchanged thickness. XPS. To further confirm the growth of the LaF3 shells around the CeF3:Tb3+ core round nanoplates, the CeF3:Tb3+ and CeF3: Tb3+/LaF3 core/shell nanoplates are subjected to XPS analysis. XPS is a very useful analytical technique for investigating the elementary states on the surfaces. Figure 6 shows the XPS spectra of CeF3:Tb3+ (a) and CeF3:Tb3+/LaF3 core/shell (b) nanoplates. The energies at 883.4 and 903.5 eV in Figure 6a are attributed to Ce 3d5/2 and 3d3/2 peaks, respectively.17 When the shell of LaF3 is grown onto the core of CeF3:Tb3+ nanoplates to form the core/shell structure, the Ce 3d5/2 and 3d3/2 peaks drop strongly and the La peaks at 837.9 (3d5/2) and 855.5 eV (3d3/2) dominate in the spectrum,18 as shown in Figure 6b. The XPS spectra in the range 820-920 eV can clearly distinguish the variation of peak intensity for La and Ce elements (inset in Figure 6). This is consistent with the proposed structure where LaF3 is the shell that encapsulates the CeF3:Tb3+ core. The oxygen and carbon detected in the XPS measurement may come

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Figure 8. TEM images of LaF3 samples (1:1 molar ratio of Cit3-: RE3+, 180 °C) as a function of reaction time of (A) 0.5, (B) 4, (C) 24 h.

SCHEME 1: Possible Formation Mechanisms for the LaF3 Nanoplates (a) and CeF3:Tb3+@LaF3 Nanoplates (b) Figure 7. TEM image of LaF3 particles prepared without the addition of Cit3- (A) and SEM images of LaF3 particles synthesized in the presence of different molar ratios of Cit3-/La3+ of 1:2 (B), 2:1 (C), and 4:1 (D). These samples are treated hydrothermally for 24 h at 180 °C and the other conditions are similar to those synthesizing LaF3 nanoplates.

from the atmosphere. Furthermore, low amounts of oxygen and carbon are frequently observed in the XPS spectra. Role of Cit3- in the Morphology Formation of LaF3 and CeF3 Nanoplates. During the growth process of crystals, not only its intrinsic structures, but a series of external factors, such as reaction temperature, time, the precursor solution pH value, and organic additives drastically influence the crystallization process and shape evolution of particles in a solution-based system.19 Here we stress the important effect of organic additive (Cit3-) on the shape formation of products in our current synthesis. By taking the LaF3 nanoplates as a representative example, a series of parallel experiments were carried out with varied amounts of Cit3- with the other conditions unchanged to obtain molar ratios of Cit3-/La3+ of 0, 1:2, 2:1, and 4:1. Figure 7 shows the corresponding particle sizes and morphology variation of LaF3 products. Without the use of Cit3-, the TEM image of as-prepared LaF3 sample reveals that the product consists of much smaller nanoparticles (Figure 7A), most of which also have hexagonal shape accompanied by a small quantity of irregular shaped nanoparticles. The presence of hexagonal nanoparticles is determined by the intrinsic crystal structure of LaF3. Generally, for materials with an intrinsic hexagonal structure, the anisotropic growth along crystallographically reactive directions is available to form hexagonal shaped nanoplates, as observed in the formation of β-NaYF4: Yb,Er hexagonal nanoplates.20 However, in comparison with that acquired in the presence of Cit3-, the products are more irregular, less uniform, and much smaller in size. This indicates that Cit3- plays a crucial role in determining the shape formation of homogeneous 2D hexagonal nanoplates of LaF3 crystals. Compared to the LaF3 prepared with 1:1 Cit3-:La3+, the morphologies of the products obtained with 1:2, 2:1, and 4:1 Cit3-:La3+ remain basically unchanged, as shown in Figure 7, parts B, C, and D, respectively. The diameters of the corresponding products are 145, 143, and 156 nm, and the thicknesses are 25, 25, and 26 nm, respectively. This implies the variation of amount of Cit3- has no obvious effect on the shape and size of the products. The exact mechanism for the change in morphology of LaF3 grown with and without Cit3- might be explained in terms of the kinetics of the crystal growth process. Laudise et al.21 claimed that the growth of crystals is related to the relative growth rate of different crystal facets and the difference in the growth rates of various crystal facets results

in a different outlook of the crystallite. On the basis of the manipulation of the kinetics of particle growth, an organic additive is introduced to the reaction system that preferentially adsorbs to certain crystal facets of the growing particles, resulting in the difference of the growth rates of different crystal facets, subsequently modulating the morphologies of the products. In our case, the tiny LaF3 nuclei formed at the first stage are treated hydrothermally for 0.5 h to yield nearly round LaF3 nanoplates with an average diameter of only 52 nm (Figure 8A). Then organic additive Cit3- can absorbed specifically on the (0001) planes of these smaller LaF3 nanoplates, which further slows down the growth rate along the 〈0001〉 orientation, consequently inhibiting significantly the longitudinal growth along the [0001] orientation with a relative enhancement of the growth sideways in the form of larger 2D nanoplates with the average size of 110 nm with the reaction proceeding to 4 h (Figure 8B). Moreover, some regular hexagonal nanoplates begin to appear. As the reaction time extends to 24 h, the morphology of the product is very regular hexagonal nanoplates along with the continuous increase of mean average to 155 nm (Figure 8C). To sum up, Cit3- species may have double functions on the growth of the LaF3 nanostructures. First, as a strong ligand, it can form a stable complex with La3+ ions through coordination interaction, which slows down the nucleation and subsequent crystal growth of LaF3 particles. This can be confirmed by the white precipitates produced after directly mixing the aqueous solution containing La3+ and Cit3-. Second, Cit3- acts as a structure-directing reagent binding to the surface of crystals, which directly affects the growth of different crystal facets by adjusting the growth rate of different facets, resulting in the formation of the anisotropically larger 2D plated geometry for the LaF3 crystals. Thus Cit3- plays dual roles as chelating ligand and shape modifier. Scheme 1a shows the possible formation process for the LaF3 nanoplates. Shape Formation of CeF3:Tb3+@LaF3 Core/Shell Nanoplates. The formation and evolution process of CeF3:Tb3+@LaF3 core/shell nanoplates can be divided into three steps. First, the CeF3:Tb3+ nanoplates are dispersed in the aqueous solution by ultrasonic treatment. Then the La3+-Cit3- complex and F- are introduced into the above solution. Under hydrothermal conditions (high temperature and pressure), the chelating of the La3+-

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Figure 10. Decay curve of Ce3+ luminescence in CeF3 nanoplates. Figure 9. The excitation (a) and emission (b) spectra of CeF3 nanoplates.

Cit3- complex will be weakened, and La3+ will be released gradually. On the other hand, F- in the solution reacts with La3+ to generate LaF3 nuclei. Second, large amounts of the newly formed LaF3 nuclei will grow by a diffusive mechanism into the CeF3:Tb3+ nanoplates, which will aggregate together to form the LaF3 shell in a process dominated by irreversible capture of the single particles. In the last step, much larger particles adopt a 2D hexagonal structure and segregate from each other to form their final morphology. Note that several hollow nanoplates observed in the SEM image can further testify to the above reasoning (Figure 5B). In the presence of hollow nanoplates it is highly possible that the larger CeF3:Tb3+ cores were broken off from the LaF3 shell because of sonication treatment during the SEM sample preparation process. Scheme 1b shows the possible formation process for the CeF3: Tb3+@LaF3 nanoplates. Photoluminescence Properties. Under 254 nm UV lamp irradiation, CeF3 nanoplates exhibit purple-blue emission while CeF3:Tb3+ and CeF3:Tb3+@LaF3 ones show bright green emission. CeF3, which shows quite strong emission at room temperature, is a luminescent material with 100% activator concentration.13 Figure 9 gives the excitation (a) and emission (b) spectra of CeF3 nanoplates. The emission spectrum (Figure 9b) of CeF3 nanoplates includes a broad band ranging from 300 to 500 nm peaking at 363 nm, which can be attributed to the 5d-4f transition of Ce3+.22 Monitored with the emission wavelength of 363 nm, the obtained excitation spectrum (Figure 9a) consists of a broad band with a maximum at 259 nm, which corresponds to the 4f-5d absorption of Ce3+.18 Figure 10 shows the luminescence decay curve of Ce3+ in CeF3 nanoplates. This curve can be well-fit into a single-exponential function as I(t) ) I0 exp(-t/τ) (I0 is the initial emission intensity at t ) 0 and τ is the 1/e lifetime of the emission center). The lifetime of Ce3+ is determined to be 36.59 ns. The short lifetime of Ce3+ is due to the allowed character for its 5d-4f transition. The Tb3+ ion doped CeF3 nanoplates show a strong green emission under UV excitation. Figure 11 shows its excitation (a) and emission (b) spectra. The excitation spectrum (Figure 11a) of the CeF3:Tb3+ sample is dominated by the 4f-5d absorption of Ce3+, which is similar to that of CeF3 nanoplates (Figure 9a), whereas the emission spectrum (Figure 11b) is characteristic of the Tb3+ ion. This indicates that an energy transfer from Ce3+ to Tb3+ occurs in the CeF3:Tb3+ sample, as for the case of the powder bulk materials.23 The emission of

Figure 11. The excitation (a) and emission (b) spectra of CeF3:Tb3+ sample.

Tb3+ is due to transitions between the excited 5D4 state and the (J ) 6-3) ground states of Tb3+ ions. No emission from the higher 5D3 level is observed due to a cross-relaxation effect at the high Tb3+ concentration.24 Besides the typical emission of Tb3+ ions, the weak and broad emission band of Ce3+ at 330 nm can be clearly seen, which is not completely quenched as observed in other Ce3+ and Tb3+ co-doped systems.17,25 Figure 12 shows the emission spectra of CeF3:Tb3+ (black line) and CeF3:Tb3+@LaF3 core/shell (green line) samples. The two samples are identical in profile (both consisting of the characteristic emission of Tb3+ with the 5D4-7F5 transition at 542 nm being the most prominent group). However, when the CeF3:Tb3+ nanoparticles were coated with the LaF3 shells, the emission intensity is improved by 28% with respect to that of CeF3:Tb3+ core particles. This increase of emission intensity may be attributed to the fact that a significant amount of nonradiative centers existing on the surface of CeF3:Tb3+ nanoplates are eliminated by the shielding effect of the LaF3 shell.18 In this core/shell structure, the distance between the luminescent lanthanide ions is increased and the surface quenchers are decreased, thus reducing the nonradiative pathways and suppressing the energy quenching in the energytransfer process. In addition, the enhanced emission intensity may further result from the suppression of OH groups quenching by the LaF3 shell. Because the residual OH groups, an inherent result of hydrothermal process, can quench the excited state of 7F J

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Figure 12. Emission spectra of CeF3:Tb3+ (black line) and CeF3: Tb3+@LaF3 (green line) nanoplates.

the lanthanide ions by dipole-dipole interaction, the proximity of the OH groups to the lanthanide ions results in a much higher extent of quenching.26 Namely, the OH vibrations can quench surface luminescent centers of CeF3:Tb3+ nanoparticles. However, the energy-loss processes on the surface luminescent centers can be significantly reduced by coating the LaF3 shell that grows on the CeF3:Tb3+ core and provides a barrier for energy migration to the outer surface of the shell.27 The luminescence decay curve of Tb3+ in CeF3:Tb3+ and CeF3: Tb3+@LaF3 core/shell samples also can be fitted into a singleexponential function, and the lifetimes of Tb3+ are determined to be 9.16 and 9.35 ms, as shown in Figure 13, parts a and b, respectively. The increase of the luminescence lifetime for Tb3+ in CeF3:Tb3+@LaF3 core/shell nanoplates relative to that in CeF3:Tb3+ core nanoplates shows that quenching from outside the particles is strongly reduced after the growth of a shell around the core.28 This is additional evidence for the formation of core/shell structure in CeF3:Tb3+@LaF3. 4. Conclusions In summary, 2D plate shaped LaF3, CeF3, CeF3:Tb3+, and CeF3:Tb3+@LaF3 (core-shell) nanoplates have been fabricated via a facile and effective hydrothermal route. Cit3-, as a coordination agent and structure-directing agent, plays a critical role in determining the morphology formation of the final products. The CeF3, CeF3:Tb3+, and CeF3:Tb3+@LaF3 (core/ shell) nanoplates show characteristic emission of Ce3+ (5d4f) and Tb3+ (f-f), respectively. By coating the LaF3 shell on the surface of the CeF3:Tb3+ core, the distance between the luminescent lanthanide ions is increased and the surface quenchers are decreased, thus reducing the nonradiative pathways. Therefore, the luminescent intensity and lifetime of the CeF3:Tb3+@LaF3 core/shell nanoplates are greatly enhanced in comparison to the bare CeF3:Tb3+ nanoplates. This synthetic methodology appears to provide a gateway into other rare earth fluoride compounds. Acknowledgment. This project is financially supported by the foundation of “Bairen Jihua” of the Chinese Academy of Science, the MOST of China (2003CB314707 and 2007CB935502), and the National Natural Science Foundation of China (NSFC 50572103, 20431030, and 50702057).

Figure 13. Decay curves of Tb3+ luminescence in CeF3:Tb3+ (a) and CeF3:Tb3+@LaF3 (b) samples.

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