Selective Synthesis and Luminescent Properties of Monazite- and Zircon-Type LaVO4:Ln (Ln ) Eu, Sm, and Dy) Nanocrystals Weiliu Fan,† Yuxiang Bu,‡ Xinyu Song,‡ Sixiu Sun,*,†,‡ and Xian Zhao*,† State Key Laboratory of Crystal Materials, and Department of Chemistry, Shandong UniVersity, Jinan, 250100, People’s Republic of China
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 11 2361–2366
ReceiVed NoVember 14, 2006; ReVised Manuscript ReceiVed July 27, 2007
ABSTRACT: Pure monazite-type monoclinic (m-) and zircon-type tetragonal (t-) phased LaVO4:Ln (Ln ) Eu, Sm, and Dy) nanocrystals could be obtained by a facile hydrothermal method in a controllable way without the presence of catalysts or templates. High-resolution transmission electron micrographs of the as-obtained samples showed high crystallinity. It was found that tuning the pH of the growth solution was a crucial step for the selective control of the structure and morphology of the LaVO4:Ln nanocrystals. When the pH value was lower than 3.5, only irregular nanoparticles of m-LaVO4:Ln were obtained. Increasing the pH value can induce the polymorph transformation from monazite to zircon type. As the pH value ranged from 4.5 to 6.0, uniform t-LaVO4:Ln nanorods with a diameter of 15 nm and length of about 100 nm were obtained. When the pH value was further increased to higher than 6.0, the as-obtained t-LaVO4:Ln nanocrystals exhibited particle-like morphology, with an average diameter of about 20 nm. The possible mechanism responsible for the phase control and anisotropic morphology evolution of the LaVO4:Ln nanocrystals was discussed. Their luminescent properties were studied and compared. The results indicated that the challenging transformation from monazite to the metastable zircon structure for LaVO4:Ln resulted in a remarkable improvement of the luminescent properties. This improvement was rationally analyzed, and the impact of the structure and shape on luminescent properties was explored. It suggests that we could obtain the function-improved materials by tailoring the structure and shape of the lanthanide-doped LaVO4 nanostructures. Introduction Nanoscale luminescent materials are of increasing importance for established technologies (for example, displays, lamps, and X-ray detectors) and are essential for upcoming applications. New applications of nanoscale phosphors include electroluminescent devices, integrated optics, luminescent fillers in transparent matrices (for example, glass or plastics), as well as biomedical applications, such as fluorescence resonance energy transfer (FRET) assays, biolabeling, optical imaging, or phototherapy.1 Rare earth compounds were extensively applied in luminescence and display, such as lighting, field emission display (FED), cathode ray tubes (CRTs), and plasma display panels (PDPs).2 It is expected that nanosized rare earth compounds can increase luminescent quantum efficiency and display resolution. Despite the fact that rare earth activator-doped vanadates of Y, Gd, and Lu have attracted great interest in view of luminescent applications,3 LaVO4 is not regarded as a suitable host for rare earth activators because of its ordinary monazite crystal structure at ambient conditions. Lanthanide orthovanadates crystallize in two polymorphs, namely, monoclinic (m-) monazite type and tetragonal (t-) zircon type. Generally, the larger Ln3+ ion prefers the monazite type because of its higher oxygen coordination number of 9 as compared to 8 of the zircon type. For this reason, at ambient conditions, bulk LaVO4 crystallizes solely in the monazite type as the thermodynamically stable state and other lanthanide orthovanadates, including Sc and Y, crystallize in the zircon type.4 Determined by its structural characteristics, m-LaVO4 (C2h5, P2l/n) is neither a suitable host for luminescent activators nor a promising catalyst compared to other orthovanadates.3a,5 On the contrary, it should * To whom correspondence should be addressed. Telephone: 86-53188364879. Fax: 86-531-88564464. E-mail:
[email protected] and zhaoxian@ sdu.edu.cn. † State Key Laboratory of Crystal Materials. ‡ Department of Chemistry.
be noticed that t-LaVO4 (D4h19, I4l/amd) has a similar structure to that of YVO4, and it is expected to be a promising phosphor candidate, as revealed by preliminary research.6 The main challenge lies in the synthesis of zircon-type LaVO4, because it is metastable and cannot be obtained by conventional methods. As a result, the selective synthesis of m- and t-LaVO4 nanocrystals not only has great theoretical significance in studying the polymorph conversion/phase-transition processes and the structure-dependent properties but also is very important for their potential applications. In recent years, much attention has been paid to the synthetically controlling the phase and shape of nanomaterials because both of them play crucial roles in physical and chemical properties of nanocrystals.7–11 Now, a few papers have been published on the preparation and properties of undoped t-LaVO4 crystals,4,12–15 but far fewer papers have been published on the selective synthesis and properties of lanthanide-doped m- and t-LaVO4 nanocrystals.6 In their case, Na3VO4 was used as the reactant and organic molecule additives, such as ethylenediaminetetraacetic acid (EDTA), must be added to play an important role in the polymorph selection of the doped LaVO4 nanocrystal. Although good control over nanocrystal structure and shape can be realized in these syntheses, however, the introduction of organic additives to the reaction system means a much more complicated process and may bring about an increase of the impurity concentration in the final product. Therefore, it is required to develop a more facile and reproducible method for the synthesis of the doped t-LaVO4 nanocrystal that hopefully has high crystallinity. In this paper, we controlled synthesized lanthanide ions (Eu3+, Sm3+, and Dy3+) doped monazite- and zircon-type phased LaVO4 nanocrystals by a facile hydrothermal method without the presence of catalysts or templates. Tuning the pH of the growth solution was a crucial step for the control of the structure transformation and morphology evolution of the final products. The detailed systematic study showed that a special dissolution–recrystallization transformation mechanism as well as an
10.1021/cg060807o CCC: $37.00 2007 American Chemical Society Published on Web 10/17/2007
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Ostwald ripening process were responsible for the phase control and anisotropic morphology evolution of the LaVO4:Ln nanocrystals. The photoluminescence spectra of the as-obtained samples indicated that, because of the structural transformation, zircon-type t-LaVO4:Ln (Ln ) Eu, Sm, and Dy), quite different from the monazite-type one, may be a promising red phosphor candidate. Experimental Section Analytical-grade lanthanide nitrate and anhydrous sodium metavanadate (NaVO3) were purchased from the Shanghai Chemical Reagent Company and were all used without further purification. Synthesis. In a typical procedure, an aqueous solution of La(NO3)3 (7.6 mL, 0.4 M) and Eu(NO3)3 (0.4 mL, 0.4 M) was mixed under vigorous stirring, then 8 mL of NaVO3 (0.4 M) aqueous solution was added dropwise to the above mixture at room temperature under constant magnetic stirring, and a yellow colloidal precipitate was formed. After stirring for 10 min, some amount of 1 M NaOH aqueous solution was added dropwise with stirring to tune the final pH of the growth solution to 2.5, 3.5, 4.5, and 6.0, respectively, to investigate the pH effect on the structure and morphology of the final product. The suspension was stirred for another 10 min. After dilution with deionized water, the resulting yellow suspension was poured into a 50 mL Teflon-lined stainless autoclave, sealed, heated at 180 °C for 48 h in a digital-type temperature-controlled oven, and then allowed to cool to room temperature naturally. The products were recovered by filtration, washed with deionized water and absolute alcohol, and finally dried at 80 °C in air for further characterization. The synthetic process described above can also be used for preparing LaVO4 nanocrystals doped with lanthanide ions other than Eu3+, such as Sm3+ or Dy3+. Characterizations. The phase purity and crystal structure of the obtained samples were examined by X-ray diffraction (XRD) using a Japan Rigaku D/Max-γA rotation anode X-ray diffractometer equipped with graphite monochromatized Cu KR radiation (λ ) 1.541 78 Å), employing a scanning rate of 0.02° s-1 in the 2θ range from 10–75°. The operation voltage and current were maintained at 40 kV and 40 mA, respectively. The morphologies and micro- and nanostructures of the as-synthesized doped LnVO4 nanocrystals were characterized by transmission electron microscopy (TEM), carried out using JEOL JEM100 CXII at an accelerating voltage of 100 kV. Further structural characterization was performed on a Philips Tecnai F30 high-resolution field-emission transmission electron microscope (HRTEM) operating at 300 kV. The samples for these measurements were dispersed in absolute ethanol by being vibrated in the ultrasonic pool. Then, the solutions were dropped onto a copper grid coated with amorphous carbon films and dried in air before performance. Ultraviolet–visible (UV–vis) absorption spectra of the suspensions with the same concentration were measured with a Lambda 35 spectrometer (Perkin-Elmer). Fluorescence spectra were recorded on a Hitachi FLS-900 spectrophotometer equipped with a 150 W Xe-arc lamp at room temperature, and for a comparison of the different samples, the emission spectra were measured at a fixed bandpass of 0.2 nm with the same instrument parameter [1.0 nm for the excitation split, 1.0 nm for the emission split, and 700 V for the photomultiplier tube (PMT) voltage].
Results and Discussion A. Crystal Structure and Morphology of LaVO4:Eu Nanocrystals. During the experiment, the most interesting phenomenon is that the colors of the as-grown LaVO4:Eu products gradually change from yellowish green (2.5 e pH < 3.0) to yellow (3.0 e pH < 3.5), then pale yellow (3.5 e pH < 4.0), and finally white powders (final pH g 4.0). This difference of the optical behavior may be related to the variation of the structure and/or shape of the resulting nanocrystals.16 XRD, TEM, and HRTEM were employed to analyze the crystalline structure and morphology of the samples. Figure 1 shows the XRD patterns of the LaVO4:Eu nanocrystals prepared at different pH. It can be seen that the structure
Figure 1. XRD patterns of LaVO4:Eu nanocrystals obtained under different pH conditions. (a) pH 2.5, (b) pH 3.5 (peaks signed with “9” were indexed to the m-LaVO4 phase), (c) pH 4.5, and (d) pH 6.0. The Eu3+ doping concentration is kept as 5 mol %. The standard XRD patterns of JCPDS file number 50-0367 (monazite-type LaVO4) and JCPDS file number 32-0504 (zircon-type LaVO4) are also shown.
of the samples was different based on the solution pH value at the synthesis process. The XRD pattern of the sample obtained at pH 2.5 was shown in Figure 1a. All of the diffraction peaks are well-indexed to the monoclinic phase of LaVO4 [Joint Committee on Powder Diffraction Standards (JCPDS) 50-0367], and no traces of other phases are examined. It is understandable that the host LaVO4 crystallizes in its thermodynamic state, the monazite structure. In addition, we found that in our reaction system m-LaVO4:Eu nanocrstals with a pure monazite structure can only be obtained with less than 5 mol % Eu3+ ion doping. Further increasing the amount of doping leads to traces of the tetragonal phase, which indicates that some of the Eu3+ ions crystallize as EuVO4 separately, and they cannot enter the lattice of monazite LaVO4 by occupying the La3+ ion sites. With the increase of the pH, however, the formation of m-LaVO4:Eu was restrained and the traces of t-LaVO4:Eu appeared, as can be seen from Figure 1b. Pure t-LaVO4:Eu could be ultimately obtained at the final pH g 4.5. Parts c and d of Figure 1 give the XRD pattern of the product obtained at final pH 4.5 and 6.0, respectively, from which we can see that the monoclinic phase of LaVO4 disappeared, and all of the peaks fit well with the tetragonal structure of LaVO4 (JCPDS 32-0504): space group I41/amd, with cell constants a ) 7.439 Å and c ) 6.545 Å. The unit-cell volume of 362.2 Å3 corresponds well with the 5 mol % doping level of the smaller Eu3+ compared to that of La3+. This decrease in the unit-cell volume is a good indication that the doping ions are homogeneously distributed in the host nanoparticles.6a,17 As a confirmation of the XRD analysis, Figure 2 shows typical TEM images of the LaVO4:Eu nanocrystals corresponding to Figure 1. From the image in Figure 2a, it can be found that the obtained m-LaVO4:Eu at pH 2.5 exhibits the forms of small irregularly shaped nanoparticles, with an average diameter of 20 nm and weak aggregation. Increasing the pH leads to the shape of the as-obtained nanostructures evolved to 1D by experiencing a nanoparticle and nanorod coexistence period (Figure 2b). The pH-dependent experiments were taken subsequently by increasing the pH to 4.5. Figure 2c indicates that the as-obtained pure t-LaVO4:Eu almost exhibits a uniform rod shape, with an average diameter of about 15 nm and length of about 100 nm. When the final pH value was increased to higher
m- and t-LaVO4:Ln Nanocrystals
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along the c axis, that is, the [001] direction, as indicated with an arrow in Figure 3c. B. Formation Mechanism of the Selective Synthesis of the m- and t-LaVO4:Eu Nanocrystals. Crystallographic-phase transformation in solution usually operates through a dissolution–recrystallization process to minimize the surface energy of the system. For our method, the whole reaction can be given in eq 1 as follows: La(Eu)(NO3)3+NaVO3 + H2O h LaVO4:EuV + NaNO3 + 2HNO3
(1)
EDTA
La(Eu)(NO3)3 + Na3VO4 98 LaVO4:EuV + 3NaNO3 pH 10
(2)
Figure 2. Corresponding TEM images of LaVO4:Eu nanocrystals. (a) pH 2.5, (b) pH 3.5, (c) pH 4.5, and (d) pH 6.0.
Figure 3. HRTEM images and corresponding FFT patterns of the selective-synthesized m- and t-LaVO4:Eu nanocrystals. (a and b) m-LaVO4:Eu nanoparticles as-obtained at pH 2.5. (c and d) t-LaVO4: Eu nanorods as-obtained at pH 4.5.
than 6.0, the habit of the anisotropic growth is destroyed and no rod-like morphology can be detected. Figure 2d indicates the irregular particle-like morphology of the sample prepared at pH 6.0, with an average diameter of about 20 nm. A characteristic HRTEM micrograph of the Eu3+-doped m-LaVO4 nanocrystal (pH 2.5) is shown in Figure 3a, which showed that the inside of m-LaVO4:Eu nanoparticles were wellcrystallized, and the lattice image clearly reveals the (111) lattice planes of m-LaVO4:Eu, with a d spacing of about 0.360 nm. The electron diffraction patterns (Figure 3b) also showed that the Eu3+-doped m-LaVO4 nanocrystals were single-crystals. Figure 3c shows the HRTEM image of the as-prepared t-LaVO4: Eu nanorod (pH 4.5). It shows that the t-LaVO4:Eu nanorod has an obvious anisotropic growth habit, and the nanorod is structurally uniform, free from defects and dislocations. The lattice spacing of about 0.374 and 0.330 nm corresponds to the (200) and (002) planes of the tetragonal-phased LaVO4 structure, respectively. Figure 3d is the corresponding fast Fourier transform (FFT) pattern of the HRTEM image (Figure 3c), which can be indexed to the [010] zone of a tetragonal LaVO4. Further studies of the HRTEM image and FFT pattern demonstrate that the direction of the t-LaVO4:Eu nanorod growth is
In our case, the reaction route is different from other reports (as illustrated in eq 2)6 because NaVO3 instead of Na3VO4 is used as the V source. From eq 1, we can obtain that the formation of LaVO4:Eu is a process of releasing H+, with the byproduct being HNO3. It is a strong acid, which can dissolve the as-obtained vanadate nucleus and thus speed up the dissolution, renucleation, and crystallization process, as well as Ostwald ripening process18 through the back reaction shown in the above equation. Additionally, the highly acidic environment may influence the growth rates by protonation of particular surfaces of the nanocrystals. Therefore, the structure and shape of the product can be well-controlled only by tuning the pH valve of the growth solution but without the presence of any other organic additives. A possible mechanism may be defined as follows: The thermodynamic behavior of small particles differs from that of the bulk material by the free-energy term γA, the product of the surface (or interface) free energy and the surface (or interfacial) area.19–21 When the surfaces of polymorphs of the same materials possess different interfacial free energies, a change in the phase stability can occur in response to both the changes in the surface environment and the particle size. On the basis of the experimental results, the pH value of the growth solution was a crucial parameter in the control of the structure and morphology of LaVO4:Eu nanocrystals. The main effect of tuning the pH value is to modulate the thermodynamics/ kinetics of nucleation and growth of the nanocrystal by controlling experimentally the interfacial tension (surface free energy). According to the acid–base surface properties of metal oxides, increasing the pH of precipitation decreases the surface charge density by desorption of protons and consequently increases the interfacial free energies of the system. Thermodynamic colloidal unstability may thus be reached; that is, in the specific reaction circumstance, the primarily thermodynamic stable state (m-LaVO4:Eu) becomes thermodynamically unstable, resulting in a considerable promotion of the ripening processes. The particles of m-LaVO4:Eu spontaneously grow by an Ostwald-ripening process to reach a new thermodynamic equilibrium state, where the t-LaVO4:Eu structure is more thermodynamically stable than the m-LaVO4:Eu phase, henceforth, making the phase transformation. In addition, when NaOH (1 M) was added to the growth suspension, the H+ ions were partially neutralized, which makes the localized [H+] varied on the nanocrystal surface and results in the surface free energies of the various crystallographic planes differing significantly. Obviously, the growth rate of those facets possessing higher free energy would be relatively faster, which affords the possibility of breaking the natural growth habit of the crystal and creating additional growth anisotropy. As revealed in the experiments, t-LaVO4:Ln nanorods can be obtained by tuning the pH of the growth solution.
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Figure 5. Emission spectra of (a) m-LaVO4:Eu nanoparticles, (b) t-LaVO4:Eu nanorods, and (c) t-LaVO4:Eu irregular nanoparticles under the excitation of 280 nm at room temperature, and the Eu3+ doping concentration is kept as 5 mol %.
Figure 4. UV–vis absorption spectra of (a) m-LaVO4:Eu nanoparticles, (b) t-LaVO4:Eu nanorods, and (c) t-LaVO4:Eu irregular nanoparticles.
C. Optical Properties of the m- and t-LaVO4:Eu Nanocrystals. Figure 4 shows the UV–vis absorption spectra of the as-prepared LaVO4:Eu nanocrystals suspension with 4.0 × 10-4 mol/L (the suspension was dispersed with an ultrasonic bath before the nanocrystals were measured). As Figure 4 shows, the absorption peaks of m-LaVO4:Eu nanoparticles (prepared at pH 2.5), t-LaVO4:Eu nanorods (prepared at pH 4.5), and t-LaVO4:Eu irregular nanoparticles (prepared at pH 6.0) are at 273, 282, and 277 nm, respectively. The UV–vis absorption of the as-obtained LaVO4:Eu nanocrystals does not significantly differ from that (λmax ) 272 nm) of the reported YVO4:Eu nanoparticles (in sizes ranging from 10 to 30 nm),3b and the absorption peak is attributed to the charge transfer from the oxygen ligands to the central vanadium atom inside the VO43groups. There is such a trend that the aborption peak of t-LaVO4: Eu nanorods shifts to a lower energy. It may be explained by quantum confinement effects;22 i.e., as the size of the nanorods increases, the energy gap narrows, so that the absorption peak shifts to a lower energy. Photoluminescence (PL) spectra of the as-prepared samples were recorded by a fluorescence spectrometer at room temperature. Figure 5 shows PL emission spectra of the as-prepared LaVO4:Eu nanocrystals with a doping level of 5 mol % under the excitation of 280 nm. All of them were normalized to their maximum. The spectra consist of sharp lines ranging from 580 to 720 nm, which are associated with the transitions from the excited 5D0 level to 7FJ (J ) 1, 2, 3, and 4) levels of Eu3+ activators; the most intense emission is the 5D0 f 7F2 transition located in the range of 600–620 nm, corresponding to the red emission, in good accordance with the Judd–Ofelt theory.23 In addition, we think the Na atoms did not enter the host LaVO4 structure because, if the Na atoms entered, pure LaVO4:Eu could not be obtained; therefore, the emission of the mixture will be different from that of pure LaVO4:Eu reported in the literature.6b We can see from the emission spectra (Figure 5) that the spectral splitting of zircon- and monazite-type LaVO4:Eu are quite different because of the stark effect of different crystal
fields. For Eu3+ ion-doped phosphors, the structure of the host and the lattice site of Eu3+ ions are very important factors that affect emission efficiency and the transformation from monazite to the metastable zircon structure resulted in a remarkable improvement of the luminescent properties. The zircon-type LaVO4 with a tetragonal crystal structure offers a crystal site with a D4h19 space group, which has a very low inversion symmetry;6a as a result, electric dipole transitions are more allowed, which results in a higher intensity of the electric dipole transitions than those of the monazite one. Figure 5 clearly shows the dominating peak at 617 nm of the 5D0 f 7F2 electric dipole transition. In comparison to the emission intensity of t-LaVO4:Eu irregular nanoparticles (Figure 5c), the t-LaVO4:Eu nanorods are expected to give a higher signal (Figure 5b), which may be caused by the oriented growth of the nanorods. The dipole field is not only influenced by the typical dimensions and dielectric constants of the hosts but also their shape. We suggest that the shape anisotropy affects the ionic dipole field and, therefore, the emission intensity. Moreover, in comparison to zero-dimensional nanoparticles, the shape anisotropy of a one-dimensional structure provided a better model system to investigate the dependence of optical properties on size confinement and dimensionality. As for the sample of t-LaVO4:Eu nanorods, the peak at 615 nm is obviously stronger than the peak at 618 nm (Figure 5b), which is in contrast to that of the t-LaVO4:Eu irregular nanoparticles (Figure 5c), and this may be caused by the change of symmetry of the crystal fields around Eu3+ ions. In addition, we can also see that the intensity ratio of 5D0 f 7F2 to 5D0 f 7F1 varied depending upon the morphology of the powders. In parts b and c of Figure 5, the intensity ratios of 5D0 f 7F2 to 5D0 f 7F1 in t-LaVO4:Eu nanorods and the irregular nanoparticles were determined to be 5.5 and 9.5, respectively. The intensity of the transitions between different J levels depends upon the symmetry of the local environment of the Eu3+ activators.24 According to the selective rules, the 5D0 f 7F1 lines originate from the magnetic dipole transition, while the 5D0 f 7F2 lines originate from the electric dipole transition. In terms of the Judd–Ofelt theory,23 the magnetic dipole transition is permitted and the electric dipole transition is forbidden, but for some cases in which the local symmetry of the activators is without an inversion center, the parity forbiddance is partially permitted, such as Eu3+ ions occupying D2d sites in t-LaVO4:Eu nanocrystals. Subsequently, when Eu3+ ions occupy inversion center sites, the 5D0 f 7F1 transition should be relatively strong, while the 5D0 f 7F2
m- and t-LaVO4:Ln Nanocrystals
Figure 6. XRD patterns of (a) t-LaVO4:Sm nanocrystals and (b) t-LaVO4:Dy nanocrystals. Both samples are prepared via the facile hydrothermal method (180 °C for 48 h) at pH 4.5, with the lanthanideion-doping concentration being kept as 5 mol %.
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Figure 8. Emission spectra of the t-LaVO4:Sm nanorods under the excitation of 320 nm at room temperature, with the Sm3+-doping concentration being kept as 5 mol %.
Figure 7. TEM and HRTEM images and corresponding FFT pattern of (a–c) t-LaVO4:Sm nanorods and (d–f) t-LaVO4:Dy nanorods. Both samples are prepared via the facile hydrothermal method (180 °C for 48 h) at pH 4.5, with the lanthanide-ion-doping concentration being kept as 5 mol %.
transition should be relatively weak. The results stated already indicate that, in the t-LaVO4:Eu nanorods, more Eu3+ ions occupy inversion center sites in comparison to the irregular t-LaVO4:Eu nanoparticles. D. Synthesis and Characterization of Zircon-Type LaVO4:Sm and LaVO4:Dy Nanorods. The synthetic process described above may as well be used for the preparation of LaVO4 doped with lanthanide ions other than europium. We have also synthesized and characterized the samarium- and dysprosium-doped nanocrystalline t-LaVO4 nanorods. Doping levels of 5 mol % in both cases have been employed, respectively. XRD, TEM, and HRTEM were employed to analyze the crystalline structure and morphology of the samples. The luminescence performance was also characterized. Figure 6 gives the XRD pattern of the t-LaVO4:Sm and t-LaVO4:Dy nanocrystals prepared under pH 4.5 via hydrothermal treatment at 180 °C for 48 h. The pattern fits well with the tetragonal structure of LaVO4 (JCPDS 32-0504). TEM images of the as-prepared t-LaVO4:Sm and t-LaVO4:Dy nanocrystals are shown in Figure 7. We can see that the doped t-LaVO4 nanocrystals have rod-like morphology, with a uniform diameter of about 30 nm and a length of about 150 nm. The HRTEM images and corresponding FFT patterns indicate high crystallinity of Sm3+/Dy3+-doped t-LaVO4 nanorods. Further studies of the HRTEM image and FFT pattern demonstrate that the direction of the doped t-LaVO4 nanorod growth is along the c axis, i.e., the [001] direction.
Figure 9. Emission spectra of the t-LaVO4:Dy nanorods under the excitation of 320 nm at room temperature, with the Dy3+-doping concentration being kept as 5 mol %.
We show the emission spectra (λex ) 320 nm) of t-LaVO4: Sm nanorods with the Sm3+-doping concentration being kept as 5 mol % in Figure 8. The emission spectrum shows the typical emissions of the Sm3+ ion at 566, 604, ∼630–650, and ∼700–710 nm of the transitions from the 4G5/2 level to the 6H5/2, 6 H7/2, 6H9/2, and 6H11/2 levels, respectively. Figure 9 shows the emission spectra of t-LaVO4:Dy nanorods under the excitation of 320 nm at room temperature, with the Dy3+-doping concentration being kept as 5 mol %. The emission spectrum shows the typical emissions of the Dy3+ ion; i.e., the spectra consist of sharp lines ranging from 450 to 700 nm, which are associated with the transitions from the excited 4F9/2 level to 6HJ (J ) 15/2, 13/2, and 11/2) levels of Dy3+ activators, which further demonstrates that energy transfer from the zircon-type LaVO4 host to the doping Dy3+ ion is also possible. Conclusions In conclusion, LaVO4:Ln (Ln ) Eu, Sm, and Dy) nanocrystals with a controlled structure and varied shape can be synthesized by a facile hydrothermal method without the presence of catalysts or templates. In this case, NaVO3 is used as the vanadium source. Tuning the pH of the growth solution is the crucial step to speed and drive the structure transformation and
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shape evolution of the lanthanide-doped LaVO4 nanocrystals. In comparison to the current published literature, the synthetic process and control of structures (monazite and zircon type) are quite simple. The mechanism of this reaction route has been well-illustrated. The emission properties of lanthanide-doped LaVO4 nanocrystals are mainly determined by the crystal structure of the host material. The impact of the structure and shape on luminescent property improvement was rationally analyzed. On the basis of these results, the possibility to make nanocrystals of different crystal structures and varied morphologies is therefore very important for tuning the luminescent properties of these kinds of doped insulator materials. Acknowledgment. This work was financially supported by the Foundation for the Excellent Project of Postdoctoral Research of Shandong Province, China (200601008).
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