Spindle-like Lanthanide Orthovanadate Nanoparticles: Facile Synthesis by Ultrasonic Irradiation, Characterization, and Luminescent Properties Cuicui Yu,†,‡ Min Yu,†,‡ Chunxia Li,† Cuimiao Zhang,† Piaoping Yang,† and Jun Lin*,†
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 2 783–791
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China, and Department of Chemistry, Northeast Normal UniVersity, Changchun 130024, P. R. China ReceiVed March 27, 2008; ReVised Manuscript ReceiVed NoVember 5, 2008
ABSTRACT: A general and facile ultrasonic irradiation method has been established for the synthesis of the lanthanide orthovanadate LnVO4 (Ln ) La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) nanoparticles from an aqueous solution of Ln(NO3)3 and NH4VO3 without any surfactant or template. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and photoluminescence (PL) spectra as well as kinetic decays were employed to characterize the as-prepared products. Ultrasonic irradiation has a strong effect on the morphology of the LnVO4 nanoparticles. The SEM and TEM images illustrate that the as-formed LnVO4 particles have a spindle-like shape with an equatorial diameter of 30-70 nm and a length of 100-200 nm, which are the aggregates of even smaller nanoparticles of 10-20 nm. The results of XRD confirm the formation of a well-crystallized LnVO4 phase with a tetragonal zircon structure. Eu3+ and Dy3+ doped zircon type LnVO4 (Ln ) La, Gd, Lu) samples show the characteristic dominant emissions of Eu3+ at 613 nm (5D0-7F2) and Dy3+ at 572 nm (4F9/2-6H13/2), respectively, as a result of an energy transfer from the VO43- to Eu3+ (Dy3+). The formation mechanism of the spindle-like LnVO4 nanoparticles with ultrasonic irradiation is discussed in the context.
1. Introduction Nanostructures which have received good recognition for their novel size- and shape-dependent properties, as well as their unique applications that complement those of their bulk counterparts, have been extensively investigated for over a decade.1,2 It is well-known that reduction of the particle size in a crystalline system can result in remarkable modifications of properties, which are different from those of the bulk because of a surface-to-volume ratio and the quantum confinement effect of nanometer materials. The generation of such small structures is essential to the advancement of many areas of modern science and technology, and a number of physical- and chemical-based synthetic methodologies have been developed.3-5 Chemical synthesis is developing an alternative and intriguing strategy for generating nanostructures with respect to material diversity, cost, versatility, synthetic tenability, and potential for largevolume production. However, to fabricate diverse nanomaterials with well-controlled shape, size, phase purity, crystallinity, and chemical composition, and desired properties on a large scale has become one of the most challenging issues faced by synthetic inorganic chemists.6 Lanthanide orthovanadates are an important family of rare earth compounds due to their unusual magnetic characteristics and useful luminescent and electronic properties. 7 They also have wide potential applications as laser hosts,8 catalysts,7a polarizers,7b etc. Lanthanide orthovanadates crystallize in two polymorphs, that is, monoclinic (m-) monazite type and tetragonal (t-) zircon type. Generally, with increasing ionic radius, Ln3+ ions show a strong tendency toward monazite-structured orthovanadate due to its higher oxygen coordination number of 9 as compared with * Corresponding author. E-mail:
[email protected]. † Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. ‡ Northeast Normal University.
8 of the zircon one. For this reason LaVO4 exists in the monazite type as the thermodynamically stable state while the other orthovanadates normally exist in the zircon type.9 Furthermore, m-LaVO4 is neither a suitable host for luminescent activators nor a promising catalyst because of its ordinary monoclinic (m-) monazite structure compared with other orthovanadates. In contrast, t-LaVO4 is expected to possess superior properties and is expected to be a promising phosphor host candidate. 10 But the synthesis of t-LaVO4 is the main challenge so far since it is metastable and cannot be obtained by conventional methods. To date, many investigations have been dedicated to obtaining t-LaVO4, the metastable phased material. Among them, solution processes categorized in “soft chemistry” sometimes work well. 9a,11 Zhao et al. have generated zircon type LnVO4 (Ln ) La, Nd, Sm, Eu, Dy) nanorods with Ln(NO3)3 and NaVO3 as starting materials by hydrothermal and microemulsion-mediated hydrothermal processes.12 In particular, recently Li et al. reported the oleic acid assisted hydrothermal method for the synthesis of colloidal rare earth orthovanadate nanocrystals.13 Nevertheless, the above methods often suffer from the requirements of high temperature, special conditions, tedious procedures, and catalysts or templates. Therefore, an easy, efficient, and general method needs to be developed for fabricating a large number of nanostructured LnVO4 materials. The sonochemical process has been proven to be an effective technique to obtain novel materials and to prepare nanomaterials with unique morphology and properties. During the sonication process, propagation of pressure waves is intense enough to make the formation, growth and implosive collapse of bubbles in liquid medium.14 These bubbles generate a localized hotspot, which has the extreme temperatures (>5000 K), pressure (>20 MPa) and the high cooling rates (1010 K · S-1).15 These extreme conditions can drive a variety of chemical reactions such as oxidation, reduction, dissolution and decomposition. So far, the sonochemical
10.1021/cg8003189 CCC: $40.75 2009 American Chemical Society Published on Web 12/31/2008
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Yu et al. Table 1. Refined Lattice Constants, the Ionic Radii of Ln3+, the Particle Size (Length × Diameter) and the Average Crystallite Size (D) of LnVO4 Spindle-like Nanoparticles refined lattice constants samples (Ln3+) m-LaVO4 t-LaVO4 CeVO4 PrVO4 NdVO4 SmVO4 EuVO4 GdVO4 TbVO4 DyVO4 HoVO4 ErVO4 TmVO4 YbVO4 LuVO4
a (Å)
c (Å)
a ) 7.043, b ) 7.279, c ) 6.721 7.437 6.553 7.416 6.480 7.349 6.466 7.277 6.471 7.250 6.390 7.240 6.369 7.194 6.340 7.197 6.292 7.142 6.308 7.119 6.299 7.090 6.276 7.073 6.270 7.046 6.241 7.034 6.235
ionic radii of particle size D (nm) Ln 3+ (Å) L × d (nm2) from CN ) 8a from TEM XRD 1.160
12 × 12
13
1.160 1.143 1.126 1.109 1.079 1.066 1.053 1.040 1.027 1.015 1.004 0.994 0.985 0.977
139 × 31 126 × 43 117 × 33 116 × 40 113 × 36 101 × 42 159 × 60 152 × 59 181 × 57 196 × 66 199 × 69 179 × 52 299 × 69 182 × 66
18 15 14 14 18 16 15 20 15 16 21 18 17 16
a CN ) coordination number. Data adopted from Shannon, R. D. Acta Crystallogr. A 1976, 32, 751.
Figure 1. The XRD patterns of the as-prepared lanthanide vanadates m-LaVO4, t-LnVO4 (Ln ) La-Lu) nanoparticles as well as the standard data for m-LaVO4 (JCPDS No. 50-0367), t-LaVO4 (JCPDS No. 320504).
method has been used to prepare various nanostructured materials, including PbWO4 spindles,16 Au nanobelts,17 PbS hollow nanospheres,18 ZnO nanorods,19 CePO4/Tb/LaPO4 core/shell nanorods. 20 However, a systematic investigation of the preparation of lanthanide orthovanadate nanostructures via the sonochemical method has not been performed thus far. Accordingly in this paper, we report the synthesis of the zircon type LnVO4 (Ln ) La-Lu) nanoparticles with spindle-like morphologies via a facile and efficient ultrasonic irradiation route at ambient temperature and pressure without any surfactant or template. The structure, formation mechanism, and photoluminescence (PL) properties of these orthovanadate nanoparticles are investigated in detail.
2. Experimental Procedures 2.1. Synthesis. All the reagents were used of analytical grade, including Ln2O3 (Ln ) La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu), Pr6O11, and Tb4O7 (all 99.999%) as well as Ce(NO3)3 · 6H2O were purchased from Science and Technology Parent Company of Changchun Institute of Applied Chemistry (China) and NH4VO3 (Tianjin Chemical Reagent Company, China) ammonia (Beijing Chemical Reagent Company, China). Ln(NO3)3 hydrate was prepared by dissolving the corresponding lanthanide oxides in a diluted nitric acid solution (3.6 M), and the water in the solutions was evaporated by heating. In a typical synthesis of LnVO4, 0.3 g of NH4VO3 was dissolved in 20 mL of diluted HNO3 aqueous solution (3.6 M), and then it was dripped into 20 mL (0.1 M) Ln(NO3)3 aqueous solution under vigorous stirring, and then its pH value was adjusted to 8 with ammonia. Finally, the mixed solution was exposed to ultrasonic irradiation under ambient atmosphere for 1 h with a high-intensity ultrasonic probe (JCS-206 Jining Co. China, Ti-horn, 23 kHz) immersed directly in a reaction solution. During the sonication, the temperature of the suspension was increased to about 70 °C with the occurrence of precipitation. The precipitates were centrifuged and washed with distilled water and absolute ethanol for several times in sequence, and dried in a vacuum at 60 °C for 12 h. Luminescent LnVO4/5 mol % Eu3+ and LnVO4/5 mol % Dy3+ (Ln
) La, Gd, Lu) samples were prepared in the similar way as that for pure LnVO4. For comparison, LaVO4 sample was also synthesized via vigorous stirring at room temperature without ultrasonic irradiation. 2.2. Characterization. The XRD pattern was performed on a Rigaku-Dmax 2500 diffractometer at a scanning rate of 12°/min in the 2θ range from 10 to 80°, with graphite monochromatized Cu KR radiation (λ ) 0.15405 nm). FT-IR spectra were obtained using Perkin-Elmer 580B infrared spectrophotometer using the KBr pellet technique. SEM micrographs and energy-dispersive X-ray spectroscopy (EDS) were obtained using a field emission scanning electron microscope (FE-SEM, XL30, Philips). The particle morphology and size were studied with transmission electron microscopy (TEM). The TEM and selected area electron diffraction (SAED) images were recorded on a JEOL 2010 transmission electron microscope, using an accelerating voltage of 200 kV. High-resolution transmission electron microscopy (HRTEM) was performed using FEI Tecnai G2 S-Twin with a field emission gun operating at 200 kV. The samples for TEM observation were prepared by dispersing a drop of the dispersion onto a carbon coated copper grid. The excess liquid was wicked away with filter paper, and the grid was dried at 70 °C. The photoluminescence (PL) excitation and emission spectra were recorded with a Hitachi F-4500 fluorescence spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The luminescence decay curves were obtained from a Lecroy Wave Runner 6100 Digital Oscilloscope (1 GHz) using a 250 nm laser (pulse width ) 4 ns, gate ) 50 ns) as the excitation. All the measurements were performed at room temperature.
3. Results and Discussion 3.1. Formation, Structure, and Morphology. As shown in Figure 1, all the X-ray powder diffraction (XRD) patterns of the as-prepared products via ultrasonic irradiation can be indexed to pure tetragonal phase of zircon type orthovanadate (JCPDS 32-0504 for t-LaVO4 as a reference). The cell parameters of all the as-formed orthovanadates LnVO4 (Ln ) La-Lu) are listed in Table 1. It can be seen from Figure 1 that no other impurities can be detected in the synthesized products. Moreover, careful observation reveals a systematic shift to smaller d values in the positions of the diffraction peaks (Figure 1) and a decrease of cell parameters (Table 1) from La to Lu due to the contraction of the ionic radii of lanthanides of the zircon type LnVO4. Here it is of great interest and importance to note that metastable t-LaVO4 can be facilely prepared via an ultrasonic irradiation route directly without any additive or template. In contrast, LaVO4 sample
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Figure 3. The FT-IR spectrum of LuVO4 sample.
Figure 2. Simulated crystal structures of (A) m-LaVO4 and (B) t-LaVO4.
was also synthesized via vigorous stirring at room temperature without ultrasonic irradiation. It is noted that when the ultrasonic irradiation is not applied in the synthesis system, the diffraction patterns of the LaVO4 sample cannot be indexed to the tetragonal zircon type phase, but to poorly crystallized monazite-type monoclinic (m-) LaVO4 (JCPDS 50-0367, P21/n space group) with lattice constants (a ) 7.043 Å, b ) 7.279 Å, c ) 6.721 Å listed in Table 1) (bottom of Figure 1). Thus, the role of ultrasonic irradiation is evidently illustrated in the formation of t-LaVO4. The nanocrystallite size of the samples can be roughly estimated from the Scherrer equation, D ) 0.941λ/β cos θ, where D is the average grain size, λ is the X-ray wavelength (0.15405 nm), θ and β are the diffraction angle and full-width at halfmaximum (fwhm) of an observed peak, respectively. 21 The strongest peaks (012) at 2θ ) 30.249° of m-LaVO4 and (112) around 32.203° of t-LnVO4 were used to calculate their average crystallite size (D) and as examples, respectively. The results are listed in Table 1. The estimated average crystallite sizes (take LaVO4 for example) are 18 nm for the t-LaVO4 and 13 nm for the m-LaVO4, respectively. The crystal structures of the two competitive products, m-LaVO4 and t-LaVO4, are shown in Figure 2A, B, respectively. In m-LaVO4 with space group of P21/n (14), the La3+ ion prefers 9 oxygen coordination numbers because of its large radius, and each vanadium atom is coordinated with tetrahedron of oxygen atoms with V-O distances ranging from 1.6191 to 1.9827 Å and O-V-O angles of 92.55° to 123.31° (Figure 2A).22 This is why LaVO4 tends to crystallize in the m-phase normally. The t-LaVO4 crystallizes in the I41/amd (141) space group, in which the La3+ ion chooses 8 oxygen coordination numbers, and the VO43- group exhibits regular V-center tetrahedron with uniform bond-lengths and bond-angles (Figure 2B). The different crystal structures between m-LaVO4 and t-LaVO4 will result in absolutely different luminescence properties when doping optically active ions (such as Eu3+) in them. To confirm the formation of the LuVO4 crystal structure, Fourier transform infrared (FT-IR) spectroscopy was performed
on the as-prepared LuVO4 nanoparticles, as illustrated in the Figure 3. A strong absorption band at 813 cm-1 and a weak band at 452 cm-1 can be attributed to the adsorption of V-O (from the VO43- group) and Lu-O bond, respectively. 23 This implies that the crystalline LuVO4 phase has formed in the asprepared particles. The absorption band located at 3413 cm-1 and 1633 cm-1 can be ascribed to the O-H stretching and bending vibration of water. Because the samples were prepared in aqueous solution, the surface of particles can be covered inevitably with the absorbed water molecules. The morphology and microstructure details of the as-prepared LnVO4 nanoparticles were investigated with SEM, TEM, HRTEM, and SAED. Figure 4A-C shows the typical SEM images of the as-formed GdVO4, HoVO4, and LuVO4 samples prepared via the sonochemical method, respectively. The SEM images clearly show that these three samples are spindle-like aggregates which consist of many even smaller nanoparticles with the size of 10-20 nm, basically consistent with the results estimated from the Scherrer equation. Furthermore, there is a small quantity of spindles interconnecting at the center to form flowerlike structures (Figure 4C). Figure 4D is a representative EDS pattern for the LuVO4 nanospindles, which reveals the presence of Lu, V, and O elements, respectively. Note that the Si signal is from the silicon wafer substrate on which the sample is deposited and the Au peak is from the coating for the measurement. The structure information of the nanospindles was further provided by TEM and HRTEM techniques. The TEM micrographs for the as-prepared m-LaVO4 and t-LnVO4 (Ln ) La-Lu) samples are shown in Figure 5A-O, respectively. In Figure 5A for m-LaVO4, the sample consists of irregular nanoparticles with an average size around 12.2 nm (listed in Table 1). However, in Figure 5B-O for zircon type LnVO4, the samples synthesized under the ultrasonic irradiation have the spindle-like morphologies with an equatorial diameter of 30-70 nm and a length of 100-200 nm. (The exact sizes are listed in Table 1.) Moreover, the above text has mentioned that the estimated nanocrystallite size of the samples calculated from the XRD patterns (Figure 1) are 18 nm for the t-LaVO4 and 13 nm for the m-LaVO4 particles. So it is believed that the spindlelike particles are self-assembled from the small crystallites. The SAED pattern (Figure 5P) taken from GdVO4 shows a set of rings instead of spots, revealing that they are polycrystalline in nature. The similar results were also obtained for the other 13 zircon type LnVO4 samples. Two representative HRTEM images for the spindle-like PrVO4 and LuVO4 samples are shown in Figure 6, panels A and B, respectively. From the HRTEM images, we can see clearly the well-resolved lattice fringes, and the distance (0.46
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Figure 4. SEM images with different magnifications (A) GdVO4, (B) HoVO4, (C) LuVO4, and the EDS of (D) LuVO4 nanoparticles.
Figure 5. Typical TEM images of the lanthanide orthovanadates samples (A) m-LaVO4, (B) t-LaVO4, (C) CeVO4, (D) PrVO4, (E) NdVO4, (F) SmVO4, (G) EuVO4, (H) GdVO4, (I) TbVO4, (J) DyVO4, (K) HoVO4, (L) ErVO4, (M) TmVO4, (N) YbVO4, and (O) LuVO4 together with the SAED pattern of (P) GdVO4 nanoparticles.
nm for PrVO4 and 0.45 nm for LuVO4) between the adjacent lattice fringes just corresponds to the interplanar distance of (101) planes for PrVO4 and LuVO4, agreeing well with the
d (101) spacing of the literature values (0.4858 nm in JCPDS No. 17-0879 for PrVO4 and 0.4661 nm in 17-0880 for LuVO4).
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Crystal Growth & Design, Vol. 9, No. 2, 2009 787 Scheme 1. Schemical Illustration for the Formation Process of the Spindle-like LnVO4 Nanoparticles by the Sonochemical Processa
a
Figure 6. HRTEM images of (A) PrVO4 and (B) LuVO4 nanoparticles.
3.2. Possible Growth Mechanism of the Spindle-like LnVO4 Nanoparticles. The above experimental results show that the m-LaVO4 nanoparticles are obtained without ultrasonic irradiation. This means that the product prepared at lower temperature was in favor of the formation of the m-LaVO4 because the macroscopically monoclinic phase is more thermodynamically stable than the tetragonal phase. On the contrary, the ultrasonic irradiation plays an important role in the formation of zircon type LnVO4 nanoparticles with spindle-like morphology. The instantaneous high-temperature and high-pressure field developed during ultrasonic irradiation provides a favorable environment for the growth of the zircon type LnVO4, nanoparticles. In order to investigate the formation mechanism of LnVO4 spindle-like aggregates morphologies, we take LuVO4 as a representative example to conduct detailed time-dependent experiments under ultrasonic irradiation. Figure 7 shows the SEM images of the corresponding intermediates. At t ) 2 min, irregular particles can be clearly seen from Figure 7A. Moreover, the careful observation of the SEM image shows that these particles have a tendency to form a spindle-like shape. With the reaction time up to 5 min, the dominant morphology of the products is nearly more uniform, monodisperse and complete spindles with a larger size with respect to the product prepared at t ) 2 min (Figure 7B). Moreover, the higher-magnification SEM image shows that the spindles are composed of even nanoparticles that are self-assembled into ordered chains that are aligned approximately parallel to the spindle long axis, as
Here an orange dot represents a single LnVO4 nucleus.
shown in Figure 7C. After 15 min of growth, there is no obvious change in the shape and size of the product. Furthermore, small fraction of spindles automatically connect to each other by cross sections to form flowerlike aggregates, as presented in Figure 7D. Therefore, on the basis of the experimental results discussed above, we propose the possible growth mechanism for the formation of LnVO4 spindle-like morphologies. The whole evolution can be divided into three steps. First, under the highintensity ultrasonic irradiation, the direct mixing of the two solutions containing metal (e.g., Ln3+) and VO43- (VO3- + OH) VO43- + H+) produces a large number of LnVO4 nuclei rapidly in the supersaturated solution. Second, these nuclei begin to aggregate in a certain way, most probably by electrostatic gravitation and intermolecular force, and finally the spindlelike structures are formed with the proceeding of the reaction, which is similar to the formation process of YF3 nanospindles.24 Scheme 1 shows the schematic illustration for the possible formation process of the LnVO4 spindle-like structure. 3.3. Luminescent Properties. Despite the fact that vanadates of La, Gd, and Lu doped with rare earth activators have attracted great interest in view of luminescent applications,11a,25 m-LaVO4 is not regarded as a suitable host for rare earth activators due to its ordinary monazite structure. 11a,26 However, recent research demonstrates that zircon type LnVO4 is an efficient host lattice for various optically active rare-earth ions. 27 Here we chose Eu3+, Dy3+ as doping ions to investigate their luminescence properties. In this work, 5 mol % Eu3+ and Dy3+ doped m-LnVO4 and t-LnVO4 (Ln ) La, Gd, Lu) nanoparticles were
Figure 7. SEM images of the as-prepared products at different time intervals of (A) 2 min, (B, C) 5 min, and (D) 15 min under ultrasonic irradiation.
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Figure 8. PL excitation (a, c) and emission (b, d) spectra of the as-prepared LnVO4/Eu3+ and LnVO4/Dy3+ (Ln ) La, Gd, Lu).
Figure 9. The CIE chromaticity diagram for (a) LaVO4/Dy3+, (b) GdVO4/Dy3+, (c) LuVO4/Dy3+, (d) LaVO4/Eu3+, (e) GdVO4/Eu3+ and (f) LuVO4/Eu3+.
prepared. All the PL spectra were measured under the same conditions. Only very weak photoluminescence was observed in the Eu3+ and Dy3+ doped m-LaVO4, but strong emission was observed in Eu3+ and Dy3+ doped zircon type LnVO4, respectively. The structurally induced luminescence for zircon type LnVO4/Eu3+ and LnVO4/Dy3+ mainly arises from the change of the symmetry of host and lattice site of Eu3+ and Dy3+. 28 The Eu3+ ion mainly shows a red emission in LnVO4 (Ln ) La, Gd, Lu) host lattices upon UV excitation. Figure 8a, b shows the PL excitation and emission spectra of zircon type LnVO4/ Eu3+ (Ln ) La, Gd, Lu), respectively. The excitation spectra
of these three zircon type LnVO4/Eu3+ nanoparticles (Figure 8a) consist of the characteristic excitation lines (397 nm, 7F05 L6; 418 nm 7F0-5D3; 467 nm 7F0-5D2) of Eu3+ within its 4f6 configuration and strong broad bands with the maxima at 309 nm respectively due to the absorption of VO43- groups.29 The strong absorption bands of VO43- groups in LnVO4 are due to a charge transfer from the oxygen ligands to the central vanadium atom inside the VO43- ions. 29 From the viewpoint of molecular orbital theory, it corresponds to transitions from the 1A2 (1T1) ground-state to 1A1 (1E) and 1E (1T2) excited states of VO43- ion. 30 The presence of the VO43- absorption band in the excitation spectrum of Eu3+ indicates an energy transfer takes place from VO43- to Eu3+ in the zircon type LnVO4/Eu3+, as reported previously for YVO4/Eu3+. 29 The emission spectra (Figure 8b) of LnVO4/Eu3+ (Ln ) La, Gd, Lu) obtained by excitation into vanadate group at 309 nm consist of peaks of Eu3+ within the wavelength range from 300 to 750 nm, corresponding to the transitions from the excited levels of 5D07 FJ (J ) 0-4), arising from the f-f transition of Eu3+, namely, 5 D0-7F1 (593 nm), 5D0-7F2 (613 nm), 5D0-7F3 (650 nm), and 5D07 F4 (697 nm), respectively.29 It is worth noting that the emission spectrum is dominated by the forced electric dipole transition 5 D0-7F2 (613 nm) (Figure 8b). It is well-known that the symmetry of the crystal sites in which Eu3+ are located will determine the relative intensity of the 5D0 f 7F1 and 5D0 f 7F2 transitions. If Eu3+ is located at a low-symmetry local site (without an inversion center), 5D0-7F2 electric-dipole transition (613 nm) with ∆J ) 2 is often dominated in emission spectrum, otherwise (with inversion symmetry) the 5D0 f 7F1 with ∆J ) 1 magneticdipole transition is dominant. 31 In our case, the Ln3+ ion (including for the doped Eu3+ and Dy3+) is located at lowsymmetry local site (D2d) in the zircon type LnVO4, resulting in the hypersensitive transition (5D0-7F2 of Eu3+) being the most prominent group in the emission spectrum. On the other hand, only very weak emission from vanadate group is observed around 450 nm, indicating the energy transfer from the vanadate group to Eu3+ is quite efficient. 29 The Dy3+ ion shows a whitish yellow emission in LnVO4 (Ln ) La, Gd, Lu) host lattices upon UV excitation. The
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Figure 10. The luminescence decay curves for Eu3+ in (a) LaVO4/Eu3+, (b) GdVO4/Eu3+, (c) LuVO4/Eu3+ and Dy3+ in (d) LaVO4/Dy3+, (e) GdVO4/Dy3+, and (f) LuVO4/Dy3+.
excitation and emission spectra for zircon type LnVO4/Dy3+ nanoparticles are shown in Figure 8c, d, respectively. The excitation spectra (Figure 8c) consist of strong bands with a maximum at 307 nm and some weak lines in the longer wavelength region. The former is due to the VO43- group, which is similar to those in zircon type LnVO4/Eu3+ (Figure 8a). The excitation spectrum positions and intensities of these three samples are also similar to those of Eu3+ doped LnVO4. The weak lines in the longer wavelength region are due to the f-f transitions (356 nm: 6H15/2-6P7/2; 370 nm: 6H15/2-6P5/2; 391 nm: 6 H15/2-6M21/2; 455 nm: 6H15/2-6I15/2) of Dy3+ within its 4f9 configuration.32 Excitation into the vanadate group at 307 nm yields the characteristic emission of Dy3+ in the blue region 460-505 nm with a maximum at 493 nm and yellow region of 570-600 nm with maximum at 572 nm, accompanied by a strong background emission around 450 nm (Figure 8d). The former emission lines correspond to the transitions from 4F9/2 to 6H15/2, 6H13/2 of Dy3+, and the latter is due to the emission from VO43- groups of the host lattices, respectively. The emission spectrum is dominated by the yellow emission 4F9/26 H13/2, because the Dy3+ ions occupy sites without an inversion center (similar to Eu3+) in zircon type LnVO4 host. 33 However, an obvious emission from the vanadate group can also be detected (at 450 nm), indicating the energy transfer from the vanadate group to Dy3+ is not very complete.
The red and whitish yellow emissions for the nanophosphors can be further confirmed by the CIE (Commission Internationale de I’Eclairage 1931 chromaticity) coordinates for the emission spectra of LaVO4/Dy3+ (x ) 0.2832, y ) 0.3236), GdVO4/Dy3+ (x ) 0.2894, y ) 0.3319), LuVO4/Dy3+ (x ) 0.2950, y ) 0.3250), LaVO4/Eu3+ (x ) 0.5613, y ) 0.3490), GdVO4/Eu3+ (x ) 0.5184, y ) 0.3167), LuVO4/Eu3+ (x ) 0.5338, y ) 0.3211), as shown in points a, b, c (whitish yellow region) and d, e, f (red region) in Figure 9, respectively. In order to study the dependence of the emission intensity of Eu3+ and Dy3+ on the LnVO4 (Ln ) La, Gd, Lu) host lattices, the same amount of the samples were closely packed into the sample holders and measured under identical instrumental conditions. Here it is interesting to note that both the emission intensity of Eu3+ and Dy3+ changes with different vanadate host lattices in the same sequence, that is, I (LaVO4) < I (GdVO4) < I (LuVO4). Generally, the differences in the PL spectra can be caused by factors such as the extent of crystallinity, morphology, size distribution, homogeneity, and dimension of the luminescent material. 34 XRD patterns showed that the LnVO4 nanoparticles derived from the ultrasonic irradiation method are highly crystalline with the same crystal structure and similar crystal intensity. SEM and TEM revealed that the t-LnVO4 nanoparticles almost have the same morphology, but their sizes increase
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in the sequence of LaVO4 (139 nm × 31 nm) < GdVO4 (159 nm × 60 nm) < LuVO4 (182 nm × 66 nm) (Table 1). It is well-known that surface area of materials increases along with decrease in size. The large surface area introduces a large number of defects into the phosphor crystal. Defects have serious drawbacks in PL intensity for phosphors as they provide nonradiative recombination routes for electrons and holes. 35 The surface area/volume ratio and the number of defects of the LnVO4 nanoparticles will decrease as LaVO4 > GdVO4 > LuVO4. As a result, the emission intensity of Eu3+ and Dy3+ increases as I (LaVO4) < I (GdVO4) < I (LuVO4). Additionally, the different ionic radii of Ln3+ can also account for the different emission intensity of Eu3+ and Dy3+ in LnVO4 (Ln ) La, Gd, Lu) host lattices to some extent. The ionic radii for La3+, Gd3+, and Lu3+ are 1.160, 1.053, and 0.977 Å, respectively (Those of Eu3+, Dy3+ are 1.066, 1.027 Å, respectively). 36 After substitution for the Ln3+ in LnVO4, it is expected that the stiffness for the environment of Eu3+ and Dy3+ increases as LaVO4) < GdVO4) < LuVO4). The stiffer the host lattice, the higher the emission intensity of the activator ions. 37 Thus, it is understandable that both the emission intensity of Eu3+ and Dy3+ increases in the sequence of I (LaVO4) < I (GdVO4) < I (LuVO4) in the LnVO4 nanoparticles. We believe that both the particle size and the ionic radii of Ln3+ have contributions to this result. Figure 10a-f shows the luminescence decay curves for the luminescence of Eu3+ (614 nm, 5D0-7F2) and Dy3+ (572 nm, 4 F9/2-6H13/2) in LnVO4/Eu3+ and LnVO4/Dy3+, (Ln ) La, Gd, Lu) nanoparticles, respectively. In general, all these decay curves for the Eu3+ and Dy3+ in LnVO4 host can be well fitted into a single-exponential function as I ) I0 exp(-t/τ), in which τ is the decay lifetime. 29 The lifetimes of Eu3+ (5D0 excited state) in LnVO4 host lattices are determined to be 0.42 ms for LaVO4, 0.25 ms for GdVO4,and 0.35 ms for LuVO4, and those of Dy3+ (4F9/2 excited state) are 0.010 ms (LaVO4), 0.015 ms (GdVO4), and 0.022 ms (LuVO4), respectively. It should be mentioned that the lifetime values of Eu3+ and Dy3+ in the current LnVO4 nanoparticles are much shorter than those (0.5-1.6 ms) in nanocrystalline YVO4 films annealing at high temperatures (700 °C), 29 indicating that there might be some defects and waterrelated impurities (undetectable by XRD) as nonradiative centers in the nanoparticles derived from the sonochemical process without postannealing.
4. Conclusions In summary, we have demonstrated that a facile and effective sonochemical approach can prepare the zircon type LnVO4 (Ln ) La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) nanoparticles from simple inorganic reactants without any surfactant or template. The zircon type LnVO4 nanoparticles are spindle-like with an equatorial diameter of 30-70 nm and a length of 100-200 nm, which are composed of densely packed small nanoparticles. The shape formation of the final t-LnVO4 products are based on an aggregation mechanism of primary nanoparticles. Under the excitation of UV light, the m-LaVO4 did not show any photoluminescence, but the as-formed zircon type LnVO4/Eu3+ and LnVO4/Dy3+ exhibit red (613 nm) and yellow (572 nm) luminescence, respectively. Both the emission intensity of Eu3+ and Dy3+ changes with different vanadate host lattices in the same sequence, that is, I (LaVO4) < I (GdVO4) < I (LuVO4) due to the increase of particle size and the increase of the stiffness of the host lattices.
Yu et al.
Acknowledgment. This project is financially supported by National Basic Research Program of China (2007 CB935502), and the National Natural Science Foundation of China (NSFC 50702057, 50872131, 00610227).
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