Nanorods Derived from Hydroxides

Oct 7, 2009 - Chinese Academy of Sciences, Changchun 130022, and Graduate University of ... attention has been paid to the synthesis of rare earth flu...
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DOI: 10.1021/cg900604j

Rare Earth Fluorides Nanowires/Nanorods Derived from Hydroxides: Hydrothermal Synthesis and Luminescence Properties

2009, Vol. 9 4752–4758

Zhenhe Xu, Chunxia Li,* Piaoping Yang, Cuimiao Zhang, Shanshan Huang, and Jun Lin* State Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, and Graduate University of the Chinese Academy of Sciences, Beijing 100049, P. R. China Received June 4, 2009; Revised Manuscript Received September 22, 2009

ABSTRACT: In this paper, we reported the synthesis of nearly monodisperse and well-defined one-dimensional (1D) rare earth fluoride (β-NaREF4) (RE = Y, Sm, Eu, Gd, Tb, Dy, and Ho) nanowires/nanorods by in situ acid corrosion and an ion exchange approach using RE(OH)3 as precursors via a facile hydrothermal route. X-ray diffraction (XRD), energy-dispersive X-ray (EDX) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) spectroscopy were used to characterize the samples. The results show that the as-prepared rare earth fluoride (β-NaREF4) nanowires/nanorods preserve the basic morphology of the initial RE(OH)3 precursors. The possible formation mechanism for β-NaREF4 nanowires/nanorods is presented in detail. Additionally, we investigated the PL properties of Eu3þ and Tb3þ doped down-conversion (DC) as well as Yb3þ/Er3þ and Yb3þ/ Tm3þ codoped up-conversion (UC) luminescence properties in β-NaYF4 host lattices.

1. Introduction Generally, the properties of nanomaterials strongly depend on the chemical composition, crystal structure, size, shape, and dimensionality. Controlled synthesis of nanocrystals with specific structures and research on their structural-based properties are important subjects in nanoscience.1 Among the various nanostructures, one-dimensional (1D) nanostructures with their inherent anisotropy are the smallest dimension structures, including nanorods, nanowires, nanotubes, and nanoprisms, which have attracted extensive synthetic interest over the past years due to their numerous potential applications in the fabrication of electronic, optical, optoelectronic, and magnetic devices.2 More applications and new functional materials might emerge if shape-controlled nanocrystals could be achieved with high complexity.3 Recently, much research attention has been paid to the synthesis of rare earth fluoride NaREF4 (RE = rare earth), because they normally possess a high refractive index and low phonon energy,4 which is a requirement to minimize nonradiative loss and maximize the radiative emission. Furthermore, they exhibit adequate thermal and environmental stability and therefore are regarded as excellent host lattices for down-conversion (DC) and upconversion (UC) luminescence of lanthanide ions.5 If rare earth fluoride NaREF4 were fabricated in the form of a 1D nanostructure, they would be expected to be highly functionalized materials, acting as electrically, magnetically, or optically functional host materials as well.6 Thus, in the past decades, much effort has been devoted to the controlled synthesis of NaREF4 nano-/microcrystals with a wealth of shapes and sizes by various strategies.5,7 In a recent review, Liu et al. have described systematically the recent development of synthetic strategies, surface modification, multicolor emission tuning, and biological applications of lanthanide-doped upconversion nanomaterials including rare earth fluorides nanocrystals.8 However, there are only several *Author to whom any correspondence should be addressed. E-mail: [email protected] (C.L.); [email protected] (J.L.). pubs.acs.org/crystal

Published on Web 10/07/2009

reports on the synthesis of 1D hexagonal (β)-NaREF4 nanowires or nanorods. Zeng et al. reported that uneven β-NaYF4: Er, Yb nanorods could be synthesized by a solvothermal route with the aid of cetyltrimethylammonuim bromide.9 Liang et al. have successfully obtained 1D branched β-NaYF4:Er, Yb nanocrystals by a mixed solvothermal route.10 A general liquid-solid solution (LSS) methodology provided by Li’s group was used to prepare uniform 1D β-NaYF4:Er, Yb nanorods, the success of which depends upon the effective complexation of linoleate on the surfaces of nanocrystals in a water-ethanol mixed-solution system.11 Despite these endeavors, it is still significant and urgent to develop more facile, efficient, and low-cost techniques to fabricate large-scale and well-crystallized 1D NaREF4. Recently, an anion exchange method has been used to prepare rare earth compounds such as REBO3, REVO4, and RE2O2S from the corresponding rare earth hydroxides RE(OH)3 precursors.12 However, to the best of our knowledge, only a few researchers have reported the synthesis of 1D β-NaREF4 nanostructures especially nanowires/nanorods through this route. Only in a recent study, Zhao et al. demonstrated the production of β-NaREF4 nanotubes through this in situ anion exchange reaction by using RE(OH)3 as a parent.13 Herein, in this paper, we try to synthesize 1D β-NaREF4 nanowires/nanorods using a similar method. First, we prepare 1D rare-earth hydroxides nanowires/nanorods through a large-scale and facile solution-based hydrothermal process without using any catalyst. Then, the 1D lanthanide hydroxides nanowires/nanorods are used as both the physical and chemical templates for the preparation of NaREF4 nanowires/nanorods. By using this synthetic pathway, we can manipulate the products to gain the expected 1D β-NaREF4 (RE = Y, Sm, Eu, Gd, Tb, Dy, and Ho) nanowires/nanorods. In addition, the DC emission for 5 mol % Eu3þ and 5 mol % Tb3þ doped β-NaYF4 and UC emission for 20 mol % Yb3þ/2 mol % Er3þ, and 20 mol % Yb3þ/2 mol % Tm3þ codoped β-NaYF4 samples have been thoroughly investigated. r 2009 American Chemical Society

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Figure 1. XRD patterns of Y(OH)3 (A) and β-NaYF4 (B) products. The standard data for Y(OH)3 (JCPDS card 83-2042) and β-NaYF4 (JCPDS card 16-0334) are also presented in the figure for comparison.

2. Experimental Section 2.1. Materials. The rare earth oxides RE2O3 (99.99%) (RE = Y, Sm, Eu, Gd, Dy, and Ho) and Tb4O7 (99.99%) were purchased from Science and Technology Parent Company of Changchun Institute of Applied Chemistry, and other chemicals were purchased from Beijing Fine Chemical Company. All chemicals were analytical grade reagents and used as purchased without further purification. 2.2. Preparation. Hydrothermal Synthesis of RE(OH)3 Nanowires/Nanorods. In a typical synthesis for the Y(OH)3 nanowires, 1 mmol of Y2O3 was dissolved in dilute HCl solution under heating with agitation, resulting in the formation of a colorless solution of YCl3. After evaporation followed by drying at 100 °C in ambient atmosphere, a powder of YCl3 was obtained. After evaporation, 30 mL of deionized water was added to form a clear aqueous solution. Then 10% NaOH solution was introduced dropwise to the vigorously stirred solution until pH = 14. After additional agitation for 1 h, the as-obtained white colloidal precipitate was transferred to a 50 mL autoclave, sealed, and heated at 180 °C for 12 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 dried in air at 80 °C for 12 h. Other rare earth hydroxide RE(OH)3 (RE = Sm, Eu, Gd, Tb, Dy, and Ho) nanowires/nanorods were hydrothermally prepared with in a manner similar to that for Y(OH)3 samples as stated above. Hydrothermal Synthesis of Y(OH)3:RE3þ Nanowires. The rareearth doped Y(OH)3: RE3þ (RE = Eu, Tb, Yb/Er, and Yb/Tm) nanowires were hydrothermally prepared with in a manner similar to that for Y(OH)3 samples as stated above, by using Eu2O3, Tb4O7, Yb2O3, Tm2O3, and Er2O3 together with Y2O3 as the starting materials. Hydrothermal Synthesis of β-NaREF4 Nanowires/Nanorods. In a typical procedure for the preparation of β-NaYF4 nanowires, 1 mmol of NaF and 0.7 mL of HF (40%) were dissolved in 35 mL of deionized water with stirring, then 0.32 g of Y(OH)3 nanowires/ nanorods prepared as above was added into the mixture solution. After additional agitation for 1 h, the as-obtained white colloidal precipitate was transferred to a 50 mL autoclave, sealed, and heated at 180 °C for 12 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 dried in air at 80 °C for 12 h. Other rare earth fluoride β-NaREF4 (RE = Sm, Eu, Gd, Tb, Dy, and Ho) nanowires/nanorods were hydrothermally prepared in a manner similar to that for Y(OH)3 samples as stated above. Hydrothermal Synthesis of β-NaYF4:RE3þ Nanowires. The rareearth doped β-NaYF4:RE3þ (RE = Eu, Tb, Yb/Er, and Yb/Tm) nanowires were hydrothermally prepared in a manner similar to that for β-NaYF4 nanowires except that the corresponding Y(OH)3: RE3þ as the starting materials. 2.3. Characterization. Powder X-ray diffraction (XRD) measurements were performed on a Rigaku-Dmax 2500 diffractometer with

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Figure 2. (A) Lower magnification, (B) higher magnification, (C) TEM, and (D) HRTEM images of the as-prepared Y(OH)3 nanowires. The inset of (A) is the EDX spectrum for the Y(OH)3 sample. Cu Ka radiation (λ = 0.15405 nm). The morphology and structure of the samples were inspected using a field emission scanning electron microscopy (FE-SEM, XL 30, Philips) equipped with energy-dispersive X-ray (EDX) spectrometer and a transmission electron microscope. Low- and high-resolution transmission electron microscopy (TEM) was performed by using an FEI Tecnai G2 S-Twin instrument 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 of the as-obtained β-NaYF4: RE3þ (RE=Eu and Tb) powders were recorded with a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The UC emission spectra of the ethanol solution of β-NaYF4:RE3þ (RE = Yb/Er and Yb/Tm) nanowires were obtained using a 980 nm laser from an OPO (optical parametric oscillator, Continuum Sunlite, USA) as the excitation source and detected by a R955 (HAMAMATSU) from 400 to 900 nm. All the measurements were performed at room temperature.

3. Results and Discussion 3.1. Formation and Morphology. 1D lanthanide hydroxides RE(OH)3 can be prepared on a large scale through a simple solution-based hydrothermal process.12c We take Y(OH)3-NaYF4 as a typical example to explain this transformation process. The composition and phase purity of the products were first examined by X-ray diffraction (XRD). Figure 1A shows the XRD pattern of the as-formed Y(OH)3 products through the hydrothermal process. All diffraction peaks for as-formed samples can be readily indexed to pure hexagonal phase [space group: P63/m] according to the Joint Committee on Powder Diffraction Standards (JCPDS) file no. 83-2042. Additionally, no other peaks can be found in the XRD patterns, revealing that the products are basically pure phase. The morphological and microstructural details of the as-prepared Y(OH)3 products were studied by scanning electron microscopy (SEM), TEM, and high-resolution transmission electron microscopy (HRTEM) techniques. Figure 2A,B shows the low- and high-magnification SEM images of the as-prepared Y(OH)3 samples. From Figure 2A, it can be clearly seen that the Y(OH)3 samples are composed of a large scale of nanowires. It can be calculated that the diameters and lengths of these nanowires are about 50-150 nm and 1-2 μm, respectively. More careful examination of the high-magnification SEM image (Figure 2B) shows that nanorods are very smooth and straight. The chemical composition of the Y(OH)3 nanowires was further investigated with EDX, which indicates that the samples are

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Figure 3. (A) Lower magnification, (B) higher magnification, (C) TEM, and (D) HRTEM images of the as-prepared β-NaYF4 nanowires. The inset of (A) is the EDX spectrum for the β-NaYF4 sample.

made of Y and O and the molar ratio of Y to O is approximately equal to 1:3 except the Si and Au peaks from measurement (inset of Figure 2A). To further study the fine structure of the above nanowires, TEM was performed. A representative TEM image for Y(OH)3 nanowires is shown in Figure 2C, clearly showing that the products are entirely composed of nanowires with diameters of about 50-150 nm and lengths of about 1-2 μm, consistent with the values shown in the SEM image (Figure 2B). The typical HRTEM image (Figure 2D) of Y(OH)3 nanowires clearly shows lattice fringes with interplanar spacing of 0.542 nm that corresponds to the (100) plane of the Y(OH)3 phase. The rare earth fluoride β-NaYF4 sample can be obtained after in situ acid corrosion and ion exchange hydrothermal process by reaction of Y(OH)3 nanowires as precursors with HF and NaF under hydrothermal conditions at 180 °C for 12 h. The XRD pattern shown in Figure 1B reveals the pure phase, and all diffraction peaks can be indexed easily as the hexagonal (β-) phase of NaYF4 [space group: P63/m], in good agreement with the values in the standard cards 16-0334 for β-NaYF4. No impurity peaks are observed, indicating that all Y(OH)3 transform completely into β-NaYF4. The morphology and microstructure details of the as-prepared β-NaYF4 nanowires were investigated with SEM, TEM, and HRTEM. Figure 3A,B shows the SEM images of as-prepared β-NaYF4 nanowires. It can be seen that the β-NaYF4 nanowires inherited their parents’ morphology. The EDX spectrum for the β-NaYF4 samples (inset of Figure 3A) shows the presence of all the necessary elements. The TEM micrograph for the as-prepared β-NaYF4 samples shows the obvious nanowires morphology (Figure 3C). Figure 3D is the HRTEM image for β-NaYF4, which reveals that the lattice plane of the nanowires afforded is highly crystallized, and the interplanar spacing of 0.515 nm that is coincident with (100) plane of β-NaYF4 phase. These results further confirm the presence of highly crystalline β-NaYF4 nanowires after in situ acid corrosion and an ion exchange approach, agreeing well with the XRD results.

Lanthanide atoms have close and gradually changed ionic radii, and as a result, other rare-earth fluorides β-NaREF4 (RE = Sm, Eu, Gd, Tb, Dy, and Ho) have also been synthesized under similar reaction conditions. Table 1 summarizes crystal structures, morphologies, and sizes of the rare-earth hydroxides RE(OH)3 and the corresponding β-NaREF4. We first prepared rare-earth hydroxides RE(OH)3 under a hydrothermal process. Similar to the case of Y(OH)3, the as-obtained other six RE(OH)3 samples all exhibit the peaks of pure crystalline hexagonal phase [space group: P63/m], as shown in Figure S1A (Supporting Information). Furthermore, the EDX spectra for these samples (Figure S2, Supporting Information) show the presence of all the necessary elements, confirming the formation of stoichiometric RE(OH)3. The products all take the shape of nanowires/nanorods with a diameter of 10-200 nm and a length of 0.1-2 μm, as shown in Figure S3 (Supporting Information). Then β-NaREF4 can be obtained after a hydrothermal in situ acid corrosion and ion exchange process by the reaction of RE(OH)3 nanowires/nanorods precursors with HF and NaF. The XRD patterns and EDX spectra for these samples show clearly that the products have a hexagonal phase of β-NaREF4 [space group: P63/m], as shown in Figures S1B and S4 (Supporting Information), indicating that the RE(OH)3 nanowires/nanorods parents have converted completely into β-NaREF4. More importantly, the products retain the shapes of nanowires/nanorods by this in situ replica process (Figure S5 of Supporting Information). Thus, it is reasonable to believe that the present synthetic conditions may be preferable for achieving β-NaREF4, indicating that this method is facile and effective to obtain crystalline pure products of β-NaREF4. 3.2. Formation Mechanism. To understand the formation mechanism of these kinds of β-NaREF4 nanowires/nanorods, the in situ acid corrosion and ion exchange processes to synthesize β-NaYF4 nanowires were investigated in detail at different reaction times. The XRD patterns and the corresponding SEM images of the intermediates obtained at

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Table 1. Optimal Experimental Conditions and Morphologies of Rare-Earth Hydroxides RE(OH)3 Nanowires/Nanorods and Rare-Earth Fluorides β-NaREF4 Nanowires/Nanorods RE(OH)3 or β-NaREF4

crystal phase

temperature [°C]

time [h]

morphology

diameter (nm)

length (μm)

Y(OH)3 Sm(OH)3 Eu(OH)3 Gd(OH)3 Tb(OH)3 Dy(OH)3 Ho(OH)3

hexagonal hexagonal hexagonal hexagonal hexagonal hexagonal hexagonal

180 180 180 180 180 180 180

12 12 12 12 12 12 12

nanowire nanowire nanorod nanorod nanorod nanorod nanowire

50-150 10-20 10-20 10-50 20-30 50-150 100-200

1-2 0.5-1 0.1-0.2 0.1-0.2 0.1-0.2 0.5-1 1-2

β-NaYF4 β-NaSmF4 β-NaEuF4 β-NaGdF4 β-NaTbF4 β-NaDyF4 β-NaHoF4

hexagonal hexagonal hexagonal hexagonal hexagonal hexagonal hexagonal

180 180 180 180 180 180 180

12 12 12 12 12 12 12

nanowire nanowire nanorod nanorod nanorod nanorod nanowire

50-150 10-50 10-20 10-50 20-40 50-150 100-200

1-2 0.5-1 0.1-0.2 0.1-0.2 0.1-0.2 0.5-1 1-2

Figure 4. XRD patterns of evolution for β-NaYF4 with different reaction times.

Figure 5. SEM images of evolution for β-NaYF4 samples at different reaction times: (A) 2 h, (B) 5 h, (C) 8 h, (D) 12 h.

different reaction time intervals are shown in Figures 4 and 5, respectively. They reveal that the intermediates display distinctively different XRD patterns at different reaction periods but their morphologies have no obvious change. After the Y(OH)3 precursors are reacted with HF and NaF for 2 h through the hydrothermal process, the Y(OH)3 and β-NaYF4 phases coexist (Figure 4B). A corresponding typical SEM image reveals that the product is composed of a large scale of nanowires, as shown in Figure 5A. As the reaction proceeds, the hexagonal phase of Y(OH)3 gradually reduces and the hexagonal phase of β-NaYF4 gradually grows (Figure 4C,D). These intermediate products also consist of nanowires, as presented in Figure 5, panels B and C, respectively. Finally, when the reaction time is prolonged to 12 h, hexagonal Y(OH)3 nanowires could completely transform into hexagonal β-NaYF4 nanowires (Figure 4E). The corresponding shape is fairly uniform hexagonal nanowires (Figure 5D). It is shown that temperature plays a critical role in controlling the crystal phase of the final products. When the other reaction conditions remain unchanged, the XRD patterns of the as-prepared NaYF4 products at different reaction temperatures are shown in Figure S6 (Supporting Information). At the lower reaction temperatures of 100 and 120 °C, the diffraction peaks of the samples can be indexed as a mixture of the β-NaYF4 (JCPDS No. 16-0334) (Figure S6A, Supporting Information) and Y(OH)3 (JCPDS No. 83-2042) (Figure S6F, Supporting Information), as presented in Figure S6, panels E and D, respectively. However, at higher reaction temperatures of

140 and 160 °C, the Y(OH)3-phase disappears completely and only the β-NaYF4-phase exists (Figure S6C,D, Supporting Information). From the above analysis, we can conclude that the reaction temperature is an important factor for this conversion. When the reaction is carried out using only HF under otherwise equal reaction conditions, XRD result (Figure S7, Supporting Information) shows that the pure YF3 (JCPDS No. 74-0911) can be obtained. When the reaction is carried out in the absence of HF, but only NaF, the product is an intermixture of Y(OH)3 (JCPDS No. 83-2042) and a small quantity of Y(OH)1.63F1.37 (JCPDS No. 80-2007) instead of β-NaYF4 (Figure S8, Supporting Information). On the basis of the above results, we believe that in the formation process of the as-prepared β-NaYF4 nanowires/ nanorods, Y(OH)3 precursors are employed as both the physical and chemical templates, which not only cast the morphology of the precursors but also afford a Y3þ source. The thermodynamic driving force should be a key to the formation of β-NaYF4 through the hydrothermal in situ acid corrosion and ion exchange approach.13 When HF and NaF react with Y(OH)3 nanowires/nanorods, HF first corrodes Y(OH)3 nanowires/nanorods. Then F- ions can substitute OH- ions to form a YF3 phase. However, in the hexagonal NaYF4 crystals, Naþ ions are randomly dispersed in different sites, so they can be easily trapped into the crystals to form a more stable hexagonal NaYF4 phase.13 During the process of phase transformation, because the hexagonal Y(OH)3 and β-NaYF4 have a similar crystal structure (Figure 6), the positions and arrangements of Y atoms

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Figure 6. Crystal structure of hexagonal-phased Y(OH)3 and hexagonal-phased β-NaYF4.

Figure 8. Room temperature PL excitation (λem = 615 nm) (A) and emission (λex = 398 nm) (B) spectra of 5 mol % Eu3þ-doped β-NaYF4 product. Figure 7. Luminescence photographs of β-NaYF4: 5 mol % Eu3þ (A) and β-NaYF4: 5 mol % Tb3þ (B) under the excitation of a 365 nm UV lamp in a dark room, and total upconversion luminescence photographs of β-NaYF4: 20 mol % Yb3þ, 2 mol % Er3þ (C), β-NaYF4: 20 mol % Yb3þ, 2 mol % Tm3þ (D) dispersed in ethanol solution under irradiation with a 980 nm laser.

remain unchanged. Thus, this in situ replica process insures the complete retention of morphology of the final product. 3.3. Photoluminescence Properties. It is well-known that the hexagonal β-NaREF4 is a much better host lattice for the luminescence of various optically active lanthanide ions, and different doping modes may lead to quite different emission behaviors, which are appealing to applications such as biological labeling and optics.14 Here, we mainly focus on the luminescence properties of Eu3þ, Tb3þ, Yb3þ/Er3þ, Yb3þ/Tm3þ in the asformed β-NaYF4 lattices, in an effort to reveal that our current method is an efficient process for the preparation of this kind of fluoride phosphor. It is noted that the doping with rare earth elements alters neither the crystal structure nor the morphology of the host materials, as shown in Figures S9-S12 (Supporting Information). Figure 7 shows the luminescence photographs of β-NaYF4:5 mol % Eu3þ (A, red), β-NaYF4: 5 mol % Tb3þ (B, green) powders under the excitation of a 365 nm UV lamp in a dark room, and total up-conversion luminescence photographs of β-NaYF4: 20 mol % Yb3þ, 2 mol % Er3þ (C, green), β-NaYF4: 20 mol % Yb3þ, 2% mol Tm3þ (D, blue) dispersed in ethanol solution under 980 nm IR laser excitation. A. β-NaYF4: 5 mol % Eu3þ. The excitation and emission spectra for 5 mol % Eu3þ doped β-NaYF4 are shown in Figure 8. The excitation spectrum (Figure 8A) consists of the characteristic excitation lines of Eu3þ within its 4f6 configuration from 200 to 570 nm. In general, most of the excitation lines can be clearly assigned (321 nm, 7F0 f 5H6; 365 nm, 7F0 f 5D4; 384 nm, 7F0 f 5G2; 397 nm, 7F0 f 5L6, strongest; 419 nm, 7 F0 f 5D3; 468 nm, 7F0 f 5D2; 528, 537 nm, 7F0,1 f 5D1) except for those weak ones at 254, 271, 289, and 301 nm (which have

little contribution to the excitation of Eu3þ and are of minor significance).15 Different from the excitation spectra for Eu3þ in rare earth hydroxides, in which a charge-transfer band (CTB) of Eu3þ-O2- is frequently observed between 200 and 300 nm. The CTB of Eu3þ-F- (generally located below 200 nm) is not present in this region because of the much greater energy needed to remove an electron from F- than from O2-.15 The fact that the excitation spectrum of the Eu3þ doped sample shows no CT band further demonstrates that no hydroxides Y(OH)3 are left after conversion into β-NaYF4. Excitation into the strongest 7F0 f 5L6 transition of Eu3þ at 397 nm yields the emission spectrum of the sample, which consists of all of the emission lines associated with the Eu3þ transitions from the excited 5D0,1,2 levels to the 7FJ level, that is, 465 nm, 5D2 f 7F0; 488 nm, 5D2 f 7F2; 510 nm, 5D2 f 7F3; 535 nm, 5D1 f7F1; 555 nm, 5D1 f 7F2; 591 nm, 5D0 f 7F1; 615 nm, 5D0 f 7F2; 649 nm, 5D0 f 7F3; 694 nm, 5D0 f 7F4,16 as shown in Figure 8B. There is no notable shift in the positions of the emission peaks compared to other Eu3þ doped systems because the 4f energy levels of Eu3þ are hardly affected by the crystal field because of the shielding effect of 5s25p6 electrons. Finally, from the emission spectrum of β-NaYF4: 5 mol % Eu3þ, it can be seen clearly that the 5D0 f 7F1 and 5D0 f 7F2 emissions have comparable intensity, indicating that the Eu3þ ions occupy the 1a and 1f sites (C3h symmetry with an inversion center) and 2h sites (Cs symmetry without inversion center) simultaneously in β-NaYF4 host lattices.17 B. β-NaYF4: 5 mol % Tb3þ. The β-NaYF4: 5 mol % Tb3þ sample emits bright-green light under UV excitation. Figure 9 shows the excitation and emission spectra of the sample, respectively. The excitation spectrum (Figure 9A) is composed of the characteristic f-f transition lines within the Tb3þ 4f8 configuration. Basically, the main excitation lines can be assigned as the transitions from the 7F6 ground state to the different excited states of Tb3þ, that is, 288 nm (5I6), 307 nm (5H6), 322 nm (5D0), 345 nm (5G2), 356 nm (5D2),

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Figure 9. Room temperature PL excitation (λem = 543 nm) (A) and emission (λex = 365 nm) (B) spectra of 5 mol % Tb3þ-doped β-NaYF4 product.

Figure 10. NIR to visible UC emission spectra of (A) hexagonal β-NaYF4: 20 mol % Yb3þ, 2 mol % Er3þ and (B) β-NaYF4: 20 mol % Yb3þ, 2 mol % Tm3þ products under 980 nm laser excitation.

372 nm (5G6), and 382 nm (5D3).18 Upon excitation into the 7F6 f 5D2 transition at 356 nm, the obtained emission spectrum exhibits four obvious lines centered at 489, 543, 584, and 619 nm, originating from the transitions from the 5 D4 excited state to the 7FJ (J = 6, 5, 4, 3) ground states of the Tb3þ ions, respectively (Figure 9B), with the 5D4 f 7F5 transition at 543 nm (green) being the most intense group. No emission spectral region from the high-energy 5D3 is observed. This is typical for luminescent materials with a high concentration of Tb3þ ions, because cross-relaxation produces an increase in the population of the 5D4 states at the expense of the 5D3 state.19 C. β-NaYF4: 20 mol % Yb3þ/2 mol % Er3þ and β-NaYF4: 20 mol % Yb3þ/ 2 mol % Tm3þ. Figure 10 shows the PL spectra of binary dopant systems (Yb3þ/Er3þ, Yb3þ/Tm3þ) nanocrystals under a 980 nm NIR laser excitation. The UC emission spectrum of β-NaYF4: 20 mol % Yb3þ/2 mol % Er3þ nanocrystals is shown in Figure 10A, in which the peaks centered at 409, 521, 539, and 654 nm can be assigned to 2H9/2 f 4I15/2, 2H11/2 f 4I15/2, 4S3/2 f 4I15/2, and 4F9/2 f 4 I15/2 transitions, respectively, of Er3þ.19 In Figure 10B for β-NaYF4: 20 mol % Yb3þ/2 mol % Tm3þ nanocrystals, the five emission bands centered at 450, 476, 646, and 698 nm correspond, respectively, to the 1D2 f 3F4, 1G4 f 3H6, 1G4 f 3 F4, and 3F3 f 3H6 transitions of Tm3þ.20 The strong NIR emission at 800 nm is attributed to the 3H4 f 3H6 transition. The mechanisms responsible for the UC luminescence are shown in Figure S13, Supporting Information.

β-NaREF4 samples inherit their parent’s morphology during this process of phase transformation. The possible formation mechanism for β-NaREF4 nanowires/nanorods has been presented in detail. Under longer UV excitation, the as-prepared Eu3þ- and Tb3þ-doped β-NaYF4 samples emit red and green light (DC luminescence), respectively. In contrast, under 980 nm NIR laser excitation, the Yb3þ/ Er3þ- and Yb3þ/Tm3þ-codoped β-NaYF4 samples exhibit strong green and blue UC luminescence, respectively. These results not only enrich the contents of lanthanide fluoride chemistry but also provide fundamental insight into the crystal growth and formation mechanism of nano-/microscale materials.

4. Conclusions In conclusion, we have developed a facile and effective in situ acid corrosion and ion exchange route to fabricate a series of rare earth fluoride β-NaREF4 nanowires/ nanorods by using RE(OH)3 (RE = Y, Sm, Eu, Gd, Tb, Dy, and Ho) nanowires/nanorods as precursors. The final

Acknowledgment. This project is financially supported by National Basic Research Program of China (2007CB935502, 2010CB327704), and the National Natural Science Foundation of China (NSFC 50702057, 50872131, 20901074). Supporting Information Available: XRD patterns of the asprepared other (A) RE(OH)3 and (B) β-NaREF4 (RE = Sm, Eu, Gd, Tb, Dy, and Ho) products (Figure S1), the energy-dispersive X-ray (EDX) spectroscopic analysis (Figure S2) and morphologies (Figure S3) of as-prepared RE(OH)3, EDX spectroscopic analysis (Figure S4) and morphologies (Figure S5) of asprepared β-NaREF4 products, XRD patterns of the as-prepared NaYF4 products at different reaction temperatures (Figure S6), XRD patterns of the as-prepared products only using HF (Figure S7) or NaF (Figure S8) under otherwise equal reaction conditions for preparing NaYF4, XRD patterns of the as-prepared Y(OH)3:RE3þ (RE = Eu, Tb, Yb/Er, Yb/Tm) (Figure S9), SEM, TEM, and HRTEM images of Y(OH)3:Yb3þ/Er3þ nanowires (Figure S10), XRD patterns of the as-prepared β-NaYF4: RE3þ (RE = Eu, Tb, Yb/Er, Yb/Tm) (Figure S11), SEM, TEM, and HRTEM images of β-NaYF4:Yb3þ/Er3þ nanowires (Figure S12), and the mechanisms responsible for the UC fluorescence of β-NaYF4:Yb3þ/Er3þ(Tm3þ) (Figure S13). This material is available free of charge via the Internet at http://pubs.acs.org.

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