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Hydrothermal Synthesis of Cerium Fluoride Hollow Nanostructures in a Controlled Growth Microenvironment Qiang Wu,* Ying Chen, Pei Xiao, Fan Zhang, Xizhang Wang, and Zheng Hu* Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, China ReceiVed: January 28, 2008; ReVised Manuscript ReceiVed: March 17, 2008
Hollow nanostructures have attracted increasing attention due to the unique properties and potential applications in many fields such as drug delivery, biomedical agents, energy storage, and reaction containers. In this paper, we report the preparation of cerium fluoride (CeF3) hollow nanostructures including nanocages, nanorings, nanococoons, and circular hollow disks through a facile hydrothermal process. The morphologies (hollow or solid) and cross sections (hexagonal or circular) of as-prepared nanostructures are dependent on the hydrothermal temperatures. All the precursors including KBrO3, Ce(IV) ions, H2SO4, and organic reducer (for example, citric acid or malonic acid), are necessary agents for the formation of CeF3 hollow nanostructures. On the basis of these experimental results, it is proposed that the growth microenvironment of CeF3 products is controlled by the Belousov-Zhabotinsky oscillating reaction, and the obtained hollow nanostructures could be regarded as the replicas of spatial patterns formed in the unstirred bromate-citric acid-Ce(IV)-H2SO4 oscillating system. The strong and uniform blue light emission of CeF3 hollow nanostructures implies their potential applications in light-emitting devices. 1. Introduction The past few decades have witnessed the discovery of various nanostructures with different sizes, shapes, and dimensions.1 These nanostructures show promising shape- or size-dependent properties and resulting potential applications due to their specific exposed surfaces, small granularity, and large surface areas.2 Among these nanostructures, hollow nanostructures such as nanocages, hollow spheres, and nanorings have attracted increasing attention because the hollow cavities inside may be extensively used in drug delivery, biomedical agents, energy storage, reaction containers, etc.3 Many synthetic approaches have been explored to prepare hollow nanostructures, most of which are based on the manipulation of crystal nucleation and growth by introducing templates or surfactants to confine the growth circumscription,4 or by using ultrasonic treatment5 or γ-ray or microwave irradiation6 to induce the hollow structures. As a classical Belousov-Zhabotinsky (BZ) oscillating reaction in the bromate-citric acid-Ce(IV)-H2SO4 system, the oxidation of citric acid (CA) by bromate ion in sulfuric acid solution is catalyzed by cerium ions, and the valences of cerium ions are periodically changed between Ce(III) and Ce(IV).7 In the unstirred reactor, this BZ system could generate a variety of spatial patterns,8 in which the cerium ions with different valence were orderly distributed to form concentric rings, spirals, etc. The spatial patterns might be “frozen in” to produce materials with specific structures as predicted by Epstein et al. in the BZ-aerosol OT system.9 If fluorine ions are introduced into this BZ system, Ce(III) ions could be “frozen in” to form insoluble cerium fluoride (CeF3) species through the reaction between Ce(III) and F-, while the Ce(IV)-distributed zone is vacant. Since Ce(III) ions are orderly distributed as annular spatial patterns, the growth microenvironment of CeF3 crystals * Corresponding author. E-mail:
[email protected]; zhenghu@ nju.edu.cn.
is controlled by the BZ oscillation and CeF3 nanostructures with hollow morphologies are possibly generated via such a mechanism. As one of the rare-earth fluorides with a layered structure, nanosized CeF3 nanostructures have promising applications as inorganic scintillators, luminescent host materials,10 as well as solid lubricants.11 Great efforts have been devoted to the synthesis of CeF3 nanostructures such as nanoparticles,12 nanodisks13 and nanowires.14 However, it is still a big challenge to synthesize CeF3 hollow nanostructures, which may find attractive applications in drug delivery due to two combined features of CeF3 nanostructures, i.e., the hollow cavity (for drug loading) and the luminescent properties (for drug tracking). Herein, we develop a hydrothermal approach to prepare CeF3 hollow nanostructures such as nanocages, nanorings, circular hollow disks, and nanococoons under control of BZ oscillating reaction. This approach is free of templates or surfactants, and also avoids the assistance of inductive irradiation, and thus is facile to mass production with low cost. The excellent luminescence property of CeF3 hollow nanostructures shows their promising potential in light-emitting devices. 2. Experimental Section Preparation of CeF3 hollow nanostructures is performed through a facile hydrothermal process with use of Ce(IV) salts as the cerium source and NaF as the fluorine source under control of the bromate-CA-Ce(IV)-H2SO4 oscillating reaction system. The reaction is carried out in a closed system with use of the Teflon-lined autoclave (50 mL capacity) as the reactor. In a typical run, 1 mL of concentrated sulfuric acid was added into 29 mL of distilled water, and then KBrO3 (0.3 g, 0.06 mol L-1), CA monohydrate (0.4 g, 0.063 mol L-1), and NaF (0.2 g, 0.16 mol L-1) were added into this solution in turn under stirring. Finally, (NH4)4Ce(SO4)4 · 4H2O (0.4 g, 0.02 mol L-1) was added into this clear solution under vigorous stirring and
10.1021/jp800838y CCC: $40.75 2008 American Chemical Society Published on Web 06/11/2008
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TABLE 1: Influence of the Dosages of Raw Materials, Reaction Temperatures, and Times on the Product Morphologies no.
H2SO4 (mL)
KBrO3 (g)
CA (g)
Ce(IV)a (g)
T (°C)
time (h)
morphologies
1 2 3 4 5 6 7 8 9 10 11
1 1 1 1 1 1 1 1 0 1 1
0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0 0.3
0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0
0.4 0.4 0.4 0.4 0.4 0.4 0.4b 0.22c 0.4 0.4 0.4
160 130 100 160 160 160 160 160 160 160 160
24 24 24 1 3 5 24 24 24 24 24
nanocages, nanorings, nanococoons circular hollow disks circular or semicircular solid disks circular or semicircular disks solid disks, nanocages (∼10%) solid disks, nanocages (∼50%) nanocages, nanorings, nanococoons nanoplates, nanoparticles (∼30 nm) hexagonal nanoplates (∼200 nm) solid spheres, short nanorods irregular nanoparticles and sheets
a
(NH4)4Ce(SO4)4 · 4H2O. b (NH4)2Ce(NO3)6. c using Ce(III) salt of CeCl3 · 7H2O.
the solution gradually turned opaque with white suspended particles. After the solution was stirred for 1 min, the autoclave was sealed and heated at 160 °C for 24 h without stirring. After the autoclave was cooled to room temperature naturally, the resulting precipitates were washed with deionized water until the solution was chemically neutral, and then dried at 60 °C in air overnight. For comparison, the dosages of the raw materials, the reaction temperatures, and the reaction times were changed to investigate their influences on the morphologies of the resulting products. The detailed experimental parameters and product morphologies were summarized in Table 1. Products were characterized by X-ray diffraction (XRD; Philips X’pert Pro X-ray diffractometer), transmission electron microscopy (TEM; JEM-1005 microscope), high-resolution TEM (HRTEM; JEM-4000EX microscope), and scanning electron microscopy (SEM; JSM-6300 microscope). X-ray photoelectron spectroscopy (XPS; ESCALAB MK II spectrometer) was employed to analyze the sample, using a Mg KR X-ray source (hν ) 1253.6 eV). The pressure in the analysis chamber was maintained at 2.9 × 109 mbar or lower during the measurement. The binding energies were calibrated against a value of the C1s hydrocarbon component centered at 285 eV. The excitation and emission spectra of CeF3 nanostructures were measured on a RF-5301PC spectrometer (Bruker) equipped with a Xe lamp as the excitation source. As-synthesized CeF3 products were dispersed in ethanol by ultrasonic treatment for 10 min to form a suspension. The absorption spectrum of the suspension was measured by ultraviolet-visible spectroscopy (UV-vis; UV-2401PC spectrometer). The suspension was also loaded into the quartz colorimetric tubes and irradiated with a 254 nm ultraviolet (UV) lamp, to observe their luminescent properties. To observe the uniformity of light emission, the CeF3-dispersed suspension was dropped on a glass slide and the emission was studied by using fluorescence microscopy (Nikon, ECLIPSE, TE2000-U) with the UV light as the excited source. 3. Results and Discussion 3.1. Morphologies and Structure of CeF3 Hollow Nanostructures. The representative SEM image (Figure 1a) of the product prepared via the typical run (experiment 1 in Table 1) reveals the hexagonal disk-like geometry of the nanostructures with the thickness of about 70-150 nm. TEM results (Figure 1b-e) show that nanocages, double-walled and trinal-walled nanocages, nanorings, and nanococoons (hollow nanocages filled with a small core) are observed in the product. The hollow nanostructures with irregular morphologies, for example, the deformed hexagonal cages and spiral nanocages, also exist
Figure 1. (a) SEM image of the product obtained at 160 °C. (b-f) Typical TEM images of hollow nanostructures: (b) nanocages and nanorings, (c) double-walled nanocages, (d) trinal-walled nanocage, (e) nanococoons, and (f) spiral nanocage and deformed nanocage. Nanocages are the predominant product.
Figure 2. (a) TEM image of a CeF3 nanocage. The inset is the corresponding ED pattern. (b) HRTEM image of the CeF3 nanocage taken from the boxed area in panel a.
(Figure 1f). The diameter of these nanostructures is in the range of 250-1200 nm, and the thickness of the side wall is about 30-150 nm. It is seen that the top and bottom faces of the nanocages are rather thin and frangible to expose the inside cavities. Figure 2a shows the magnified TEM image of a CeF3 nanocage. The corresponding electron diffraction (ED) pattern indicates this nanocage is single-crystalline. HRTEM image
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Figure 3. XRD patterns of the products obtained at different hydrothermal temperatures: (a) 160, (b) 130, and (c) 100 °C.
(Figure 2b) taken from the boxed area displays the clear lattice fringes with the interplanar spacing of 0.254 nm, in agreement with the d112 value of hexagonal CeF3 crystals. XPS results reveal that the product mainly consists of Ce(III) and F elements. No Ce(IV) ion is detected as concluded by the binding energy and the shapes of the peaks from cerium species.15 A small amount of O element is also present, which may arise from surface oxidation of the CeF3 product during the drying process (Supporting Information, Figure S1). The XRD pattern of the product shown in Figure 3a indicates that the hollow nanostructures consist of pure CeF3 with the hexagonal phase (space group P6322, JCPDS card 72-1436), in agreement with the HRTEM and XPS results. The unproportionate intensities of (002) and (004) diffraction peaks show the preferential exposed surface of these nanocrystals. An important feature of these hollow nanostructures is that their cross sections from the top/bottom view possess 6-fold structures associated with the crystal symmetry, while they exhibit elliptic projections from the side view (Supporting Information, Figure S2). It is indicated that the hollow nanostructures are hexagonal disks with slightly bulgy top and bottom surfaces. XRD patterns indicate that the products prepared at lower temperatures (experiments 2 and 3, Table 1) remain the composition of hexagonal CeF3 (Figure 3b,c). The intensities of the diffraction peaks are accordant with those of the powder samples as exhibited in JCPDS card 72-1436, indicating that no preferential exposed surface is present in these products. SEM images (Figure 4a) show that the product prepared at 130 °C is composed of circular disks, with a diameter of about 150-1000 nm and a thickness of about 60-150 nm. The holes on some disks (marked with arrows) indicate that the disks are hollow inside. TEM images (Figure 4b,c) show that many circular hollow disks have small cores in the center to form cocoonlike nanostructures. The HRTEM image (Figure 4d) of a CeF3 circular hollow disk indicates that the distance between two neighboring fringes is about 0.416 nm, in good agreement with the double value of CeF3 d300 spacing. The corresponding ED pattern reveals its single-crystalline nature. The products obtained at 100 °C are oblate circular disks and semicircular disks with a diameter of about 600-1500 nm (Figure 4e). The corresponding TEM result displays the solid morphologies of this product (Figure 4f). Further experimental results indicate that quasihexagonal hollow nanostructures form at the hydrothermal temperature higher than 145 °C, and at the lower reaction temperature of 130 °C, the anisotropic feature of CeF3 crystals is not exhibited, giving rise to the formation of circular hollow disks. When the reaction temperature is further decreased to 100 °C, the hollow nanostructures could
Figure 4. (a) SEM image and (b) TEM image of the product obtained at 130 °C. (c) Magnified TEM image of CeF3 circular hollow disks. (d) HRTEM image of the CeF3 hollow disks. The inset is the corresponding ED pattern. (e) SEM image and (f) TEM image of the product obtained at 100 °C.
not be obtained. These results exhibit that reaction temperature has a great influence on the final products. 3.2. Effect of Hydrothermal Time. The time of the hydrothermal reaction also greatly influences the final products (experiments 4-6, Table 1). The precipitates produced after mixing the precursors at room temperature were collected without experiencing the hydrothermal process and TEM observation indicates that the precipitates are solid particles with the diameter on the micrometer scale (Figure 5a). Hydrothermal reaction at 160 °C for 1 h leads to the formation of smaller circular or semicircular solid disks (Figure 5b). Some hollow nanocages (about 10%) appeared accompanied by solid disks after 3 h of reaction (Figure 5c). When the hydrothermal reaction was conducted for 5 h, about half of products have the morphology of hollow cages instead of solid disks (Figure 5d). XRD patterns reveal that the precipitates have the composition of K7Ce6F31 (Supporting Information, Figure S3a) and most of the K7Ce6F31 particles were converted into CeF3 after 1 h of hydrothermal reaction, and pure CeF3 products were obtained when the hydrothermal treatment was longer than 3 h (Supporting Information, Figure S3). With the increase of the reaction time, the ratio of hollow/solid nanostructures also increased. Hydrothermal reaction for about 12 h leads to the almost pure hollow nanocages. 3.3. Role of the Precursors. To reveal the role of Ce(IV) ions, the hydrothermal process was repeated with (NH4)2Ce(NO3)6 or Ce(III) salts like CeCl3 · 7H2O, Ce2(SO4)3 · 8H2O, and Ce(NO3)3 · 6H2O as the cerium source while other conditions were kept the same. The product prepared with (NH4)2Ce(NO3)6 as the cerium source (experiment 7, Table 1) also consists of nanocages, hexagonal nanorings, and nanococoons (Figure 6a)
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Figure 5. TEM images of the products synthesized for different hydrothermal times: (a) the precipitates formed after mixing of the precursors at room temperature and (b-d) the products obtained after the hydrothermal reaction for 1, 3, and 5 h, respectively.
Figure 7. TEM images of the products prepared without a certain precursor: (a) without Ce(IV) salt and with CeCl3 · 7H2O as the cerium source; (b) without KBrO3; (c) without H2SO4; and (d) without CA.
Figure 6. (a) TEM image and (b) XRD pattern of the product with (NH4)2Ce(NO3)6 as the cerium source.
nm are obtained in the hydrothermal process without the role of H2SO4. If CA is not added into the precursors, the product is composed of irregular nanoparticles and sheets as displayed in Figure 7d. From the above results, it is indicated that no CeF3 hollow nanostructure could be obtained if one of the precursors is not used in the hydrothermal reactions, suggesting that all the precursors are required and contribute to the formation of CeF3 hollow nanostructures. 3.4. Effect of Organic Reducers. The nature of organic reducers has a large influence on the morphologies of the eventual products. In the typical synthesis, CA acts as the reducer to convert Ce(IV) ions into Ce(III) ions, and plays an important role for the formation of CeF3 hollow nanostructures (Figure 7d). To compare the role of various organic reagents, malonic acid, oxalic acid, and sucrose are used to replace CA while keeping the other conditions. Figure 8 shows the typical TEM images of these products. Nanocages and nanorings are also obtained when malonic acid was used as the reducing agents, very similar to the product in the case of using CA (Figure 8a). If oxalic acid or sucrose is used as reducer, the products consist of nanoplates and no hollow nanostructure is observed (Figure 8b,c). For the case of oxalic acid, the experimental phenomena show that the solution was reddishorange after the hydrothermal process, which indicates the oxalic acid has stronger reducing ability than CA and reduces the bromate ions to Br2 molecules under acidic solution. During this process, the conversion of Ce(IV) ions into Ce(III) ions is much influenced. For another case of sucrose, its reducing ability is very weak and the conversion of cerium ions is more difficult than the case of using CA reducer. In both cases of oxalic acid and sucrose, the obtained nanostructures are similar to those formed with CeCl3 as the cerium source (Figure 7a). This indicates that the microenvironment for the growth of hollow nanostructures is destroyed when unsuitable reducers are used; hence, no CeF3 nanostructure could be produced. On the basis of the above results, it is revealed that the choice of organic reducing agents is crucial to the synthesis of CeF3 hollow nanostructures. 3.5. Possible Formation Mechanism. From the previous experiments and results, it is proposed that the formation of CeF3 hollow nanostructures involves a dissolution-recrystallization process during which the BZ oscillating reaction was used to control the growth microenvironment of CeF3 products. As mentioned above, K7Ce6F31 particles are immediately
with the composition of CeF3 (Figure 6b), which is very similar to the case with (NH4)4Ce(SO4)4 as the cerium source. However, only plate-like nanoparticles with a diameter of about 30 nm are observed in the product (experiment 8, Table 1) with use of equimolar Ce(III) salts of CeCl3 · 7H2O as the substitute for the Ce(IV) precursor (Figure 7a). The products obtained in the cases with other Ce(III) salts have similar plate-like morphologies (Supporting Information, Figure S4). This indicates Ce(IV) ions are indispensable in the precursors and could not be replaced by Ce(III) ions because the alteration to cerium ions would change the growth mechanism and thus influence the morphologies of the resulting nanostructural products. In addition, the hydrothermal process was conducted with conditions similar to those of the typical run, except that one of the precursors KBrO3, H2SO4, or CA is absent (experiments 9-11, respectively, Table 1). It is found that large particles as well as short nanorods are observed when KBrO3 was absent in the precursors (Figure 7b). The TEM image shown in Figure 7c reveals that hexagonal nanoplates with a size of about 200
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Figure 8. TEM images of the products prepared with other organic reducers as the substitute for CA: (a) malonic acid, (b) oxalic acid, and (c) sucrose.
Figure 9. Schematic illustration of the formation mechanism of CeF3 hollow nanostructures.
produced when the precursors are mixed at room temperature, and few Ce(IV) ions exist in the solution because K+ and Fions are excessive in comparison with the concentration of Ce(IV) ions. At 160 °C, the dissociation of K7Ce6F31 species in the aqueous solution could produce a small quantity of Ce(IV) ions to act as the catalyst for the BZ oscillating reaction. During the BZ oscillating reaction in an unstirred system, Ce(IV) and Ce(III) ions are distributed to form spatial dissipative structures.8 Ce(III) ions are “frozen in” through the reaction between Ce(III) and F- to form insoluble CeF3 while Ce(IV) ions could not be “frozen in” because the reaction condition facilitates the formation of M7Ce6F31 (M ) K+, Na+, or NH4+). Due to the dissociation ability of M7Ce6F31 at this temperature, CeF3 hollow nanostructures are produced. With the consuming of Ce(III) ions, K7Ce6F31 species were further dissolved to produce Ce(IV) ions, which could be converted into Ce(III) again through the BZ oscillation. As this process proceeds, CeF3 hollow nanostructures gradually formed. The formation mechanism is schematically illustrated in Figure 9. The CeF3 nanostructures obtained via such a mechanism should match the shapes of the spatial patterns of BZ oscillating waves. In fact, the morphologies of the hollow nanostructures are consistent with almost all of the features of the chemical waves such as concentric rings, spirals, deformation, and so on (Figures 1 and 4). This is convincing evidence for supporting our mechanism. On the other hand, based on this mechanism, these hollow nanostructures could not be obtained if the BZ oscillation did not occur. When we did not feed one of the reagents (required for the BZ oscillating reactions) into the hydrothermal reaction, the obtained products indeed show that no CeF3 hollow nanostructure was obtained (Figure 7).
It is worth noting that, at 160 °C, CeF3 hollow nanostructures formed after short hydrothermal time (e.g., for 5 h) have circular morphology as shown in Figure 5d, while after longer hydrothermal treatment (e.g., for 24 h), hexagonal nanocages are the predominant product (Figure 1). The hexagonal geometry of nanostructures may result from the anisotropic growth of CeF3 crystals because the dissolution-recrystallization equilibrium on the CeF3 crystal surface would lead to the formation of 6-fold structures due to their hexagonal symmetry. At a temperature of 130 °C, the anisotropic growth of CeF3 crystals is not obviously contributing due to the low temperature. At 100 °C, the solubility of K7Ce6F31 species is too low at this temperature to produce enough Ce(IV) concentration for the inducement of BZ oscillation, hence, solid CeF3 nanostructures were obtained. 3.6. Absorption and Emission Properties. Figure 10a gives the UV-vis absorption (1), excitation (2), and emission (3) spectra of the CeF3 hollow nanostructures. Four well-resolved absorption peaks centered at 252, 236, 220, and 207 nm are observed, which is similar to the absorption of disk-like CeF3 nanocrystals.13 The absorption peaks would come from the Ce3+ ions due to the electronic transitions from the ground state of Ce3+ to the different 5d states split by the crystal fields. The emission spectrum (curve 3) of CeF3 hollow nanostructures shows a broad emission band in the range of 270-400 nm peaking at 308 nm. Monitored with the emission wavelength of 308 nm, the excitation spectrum (curve 2) is obtained, displaying a strong band very similar to UV-vis spectrum with a maximum peak at 250 nm. This corresponds to the transitions from the ground state 2F5/2 of Ce3+ to the different components of the excited Ce3+ 5d states split by the crystal field.12a,16
Cerium Fluoride Hollow Nanostructures
J. Phys. Chem. C, Vol. 112, No. 26, 2008 9609 Acknowledgment. This work was financially supported by the National Basic Research Program of China (2007CB936300), the NSF of China (20601013, 20525312, and 20471028), and the Foundation of Jiangsu Province (BK2005416). Supporting Information Available: XPS results of the CeF3 nanostructures; the side view and schematic illustration of CeF3 nanostructure; XRD patterns of the products obtained with different hydrothermal time; TEM images of the products obtained at different conditions; the characterization results of luminescent property. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 10. (a) UV-vis absorption (1), excitation (2), and emission (3) spectra of CeF3 hollow nanostructures. (b and c) Fluorescence image and the corresponding optical photograph of CeF3 hollow nanostructures. The scale bar is 20 µm.
Fluorescence microscopy is used to study the emission uniformity of these CeF3 hollow nanostructures. The CeF3 product was dispersed in ethanol by ultrasonic treatment to form a suspension and then dropped on a glass slide. This specimen was irradiated by UV light, and the corresponding fluorescence image (Figure 10b) indicates strong blue fluorescence emits from the nanostructures. In comparison with the optical photograph (Figure 10c), it is seen that almost every dot could emit blue light, indicating the uniform emission of CeF3 nanostructures. In addition, the CeF3-dispersed ethanol suspension also exhibits blue light emission under irradiation of a 254 nm UV lamp (inset of Figure 10b). The emission can be assigned to the 5d-4f transition of Ce3+ ions.12,13 The doping of other rare-earth elements (such as La3+, Er3+, and Eu3+) into the CeF3 hollow nanostructures could be easily performed and the optical investigations are under way. Considering the simplicity of this synthetic approach, hollow morphology of the product, and the feasibility of doping other rare-earth elements into this material, the CeF3 hollow nanostructures have encouraging application potential in light-emitting devices. 4. Conclusion CeF3 hollow nanostructures such as nanocages, nanorings, nanococoons, and circular hollow disks are hydrothermally synthesized by controlling the growth microenvironment with the BZ oscillating reaction. The morphologies (hollow or solid) and cross sections (hexagonal or circular) of as-prepared nanostructures are dependent on the hydrothermal temperatures. The BZ oscillating reaction is crucial to the formation of hollow nanostructures, which could be regarded as the replicas of spatial patterns formed in the unstirred bromate-CA-Ce(IV)-H2SO4 oscillating system. The obtained CeF3 hollow nanostructures show encouraging potential in light-emitting devices due to the strong and uniform blue light emission.
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