Highly Uniform Tm3+-Doped NaYbF4 Microtubes: Controlled

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Highly Uniform Tm3+-Doped NaYbF4 Microtubes: Controlled Synthesis and Intense Ultraviolet Photoluminescence Songjun Zeng, Guozhong Ren, Wen Li, Changfu Xu, and Qibin Yang* Institute of Modern Physics, Xiangtan UniVersity, Xiangtan, 411105, China, and Key Laboratory of Low-Dimensional Materials and Application Technology, Ministry of Education, Xiangtan, 411105, China ReceiVed: March 10, 2010; ReVised Manuscript ReceiVed: May 17, 2010

Here, we present a facile and controlled synthesis of high-quality and uniform hexagonal phase NaYbF4 microtubes by a rational hydrothermal method. In addition, highly uniform and monodisperse NaYbF4 nanospheres and nanocubes are synthesized by simply tuning the reaction parameters, including sodium fluoride content, reaction temperature and reaction time. Under excitation with a 980 nm laser diode, intense ultraviolet (UV) and blue upconversion emissions were achieved from a single dopant ion (Tm3+) in NaYbF4. The effect of the crystal phase on the upconversion emission properties is also investigated. The results demonstrate that upconversion emission intensities from hexagonal phase NaYbF4 microtubes are much more efficient than the nanocubes by 2 orders of magnitude. These microtubes with unique upconvertion emissions demonstrate that NaYbF4 is an excellent host material, as compared with some of its other fluoride and oxide counterparts, and that it may find applications in UV and visible solid-state lasers or the fiber lasers. 1. Introduction The discovery of carbon nanotubes has stimulated considerable studies on one-dimensional (1D) structures with hollow interiors.1 Recently, inorganic materials with hollow interiors have attracted much attention due to their potential applications in photonic devices.2 Many methods, such as arc discharge,3 laser ablation,4,5 and template-assisted synthesis,6,7 have been developed to fabricate hollow nano- and micro-structures of oxides,8 semiconductors,9 nitrides,10 sulfides,11 silicates,12 and even organics.13 However, only a limited number of reports discuss hollow fluoride materials.14,15 As conventional functional materials, rare-earth (RE) iondoped fluorides based on NaLnF4 (Ln ) rare-earth element) have been widely investigated in recent years due to their interesting upconversion emission properties and potential applications in wide ranges of solid-state lasers, fluorescence imaging, optical data storage, and three-dimensional flat-panel displays.16-20 It has been recognized that well-defined rare-earth ion-doped fluoride nano-/microcrystals with uniform size and shape are necessary for the expected electronic structure, surface energy, and chemical reactivity. In particular, hollow morphologies of these nano-/microcrystals are significantly important for their potential technological applications in microelectronic devices. To date, many studies have been dedicated to the syntheses of upconverted RE fluorides micro- and nanocrystals.21-24 Studies that focus on fabricating RE fluoride nanostructures with hollow interiors, however, remain few. Recently, Wu reported the controlled synthesis of the both hexagonal phase NaLnF4 microtubes and cubic phase NaYF4 nanospheres via a hydrothermal method using ethylenediamine tetraacetic acid as the chelating agent.25 As demonstrated by Wu’s investigation, most of the NaLnF4 microcrystals have irregular shapes. Only a few, such as NaYF4, have well-defined tubular structures, and even these retained groovelike ends. In addition, the optical properties * To whom correspondence should be addressed. Phone: 86-73158292113. Fax: 86-731-58292113. E-mail: [email protected].

of Ln3+ ions are mostly restricted to NaYF4. Only red, green, and blue upconversion emissions were achieved by codoping Yb3+/Ho3+, Yb3+/Er3+, and Yb3+/Tm3+ in the NaYF4 host material.26-28 However, UV emissions via up-conversion remain challenging because they require more than three incident photons, which are obviously less efficient compared to twophoton processes generally observed for green and red upconversion emissions.29 In this work, high-quality and uniform hexagonal phase NaYbF4 microtubes are successfully synthesized by a rational hydrothermal method using oleic acid as a stabilizing agent. In addition, high uniformity and monodispersity of cubic phase NaYbF4 nanocrystals are easily produced from this method by simply adjusting the reaction parameters. Novel upconversion properties, including bright ultraviolet (UV) and eye-visible blue emissions, of NaYbF4 microtubes singly doped with Tm3+ are achieved and studied in detail. 2. Experimental Section Synthesis of Tm3+-Doped NaYbF4 Microtubes and Nanocrystals. All the RE oxides used for synthesis were of 99.99% purity. Rare earth nitrate [RE(NO3)3, RE ) Yb, Tm] solutions (0.5 and 0.1 M) were prepared by dissolving the corresponding RE oxide (99.99%) in nitric acid at elevated temperature. Excess nitric acid was removed by evaporation. Other chemicals were of analytical grade and were used as received without further purification. All doping ratios of Tm3+ in our experimental are presented in molar equivalents. NaYbF4 microtubes were synthesized by the following hydrothermal method.30-32 In a typical synthesis, 8 mL of ethanol was added into 2 mL of aqueous solution containing 1.2 g of NaOH under agitation to form a homogeneous solution, and 20 mL of oleic acid was added to form a metal-oleic acid complex. Then 1 mmol (total amount) of RE (NO3)3 (RE ) Yb, Tm) and 8 mL of NaF (1.0 M) aqueous solution were added to the solution under vigorous stirring. The resulting mixture was vigorously stirred for another 30 min. It was then transferred

10.1021/jp102175q  2010 American Chemical Society Published on Web 05/27/2010

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Figure 1. Typical XRD pattern of the as-prepared NaYbF4/1.0%Tm3+ microtubes at 190 °C, 24 h.

into a 50 mL stainless Teflon-lined autoclave, sealed, and kept at 190 °C for 24 h. After reacting, the system was allowed to cool to room temperature naturally. Products were deposited at the bottom of the vessel. The prepared samples were separated by centrifugation and washed with ethanol and deionized water several times to remove oleic acid and other remnants and then dried in air at 60 °C for 6 h. The final products can be welldispersed in a nonpolar solvent, such as cyclohexane, and aggregated by adding polar solvent, such as ethanol. Cubic phase nanocrystals with spherelike and cubiclike morphologies were synthesized by the same method as the hexagonal phase microtubes by adjusting the sodium fluoride contents, hydrothermal treatment temperature, and reaction time. Characterization. Powder X-ray diffraction (XRD) of the as-prepared products was performed by using a D/max-γA System X-ray diffractometer at 40 kV and 40 mA with a Cu KR radiation. The morphologies of the samples were observed by using a Leo-Supra35 field emission scanning electron microscopy (FE-SEM). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained using a JEOL-2100 high-resolution transmission electron microscopy operated at 200 kV. Samples for TEM and HRTEM assays were dispersed in cyclohexane and ultrasonicated to produce a well-dispersed suspension. One drop of the suspension was placed on a copper grid covered with hollow carbon films. The upconversion emission spectra were recorded by a spectrophotometer (R500) under the excitation of an unfocused 980 nm laser diode (LD) with power density of 3 W/cm2. 3. Results and Discussion Microstructural Investigations. The phase compositions of the as-prepared products were examined by XRD. The typical XRD patterns shown in Figure 1 revealed that the as-prepared NaYbF4 samples have high crystallinity. All of the strong and sharp reflection peaks in the XRD pattern can be readily indexed as pure hexagonal phase NaYbF4 with lattice constants comparable to the values in the literature data (JCPDS No. 27-1427). No other impurity phase is observed. Further morphological analysis of the as-prepared NaYbF4 samples was performed by FE-SEM observations. Figure 2 shows the typical FE-SEM images of the NaYbF4 samples. A large quantity of hexagonal-prismatic tubular structures with high quality and uniform sizes can be clearly identified in the low magnification FE-SEM images (Figure 2a). In the high magnification FE-SEM image (Figure 2b), it is noted that all

Figure 2. FE-SEM images for the as-synthesized NaYbF4/1.0%Tm3+ sample: (a) low magnification and (b) high magnification.

of the tubular structures with open ends have well-defined crystallographic facets and possess a uniform aspect ratio with an average of 3. From the symmetry of the morphology, the presence of hexagonal-prismatic tubes implies that the preferred growth direction of a single tube is along the [001] direction, which is further proved by the HRTEM characterization. Insights into the microstructure of the samples can be obtained by further analysis of the TEM and HRTEM assays. Figure 3 provides more details on the inner structures of the hexagonal-prism microtubes. The diameter of the microtubes is around 800 nm, on the basis of low magnification images (Figure 3a), which is consistent with the SEM analysis. Furthermore, microtubes with open ends can also be clearly identified in Figure 3a and b. However, the hollow interior structure could not be obviously observed in the TEM images (Figure 3a and b) because the walls of the microtubes were too thick to be identified by image contrasting. The corresponding HRTEM image of an individual microtube shown in Figure 3c indicates that the as-prepared NaYbF4 products with a singlecrystal nature have a high crystallinity. The crystals are also free of defects and dislocations. The interplanar distances are 3.45 Å, corresponding to the (001) lattice plane of hexagonal phase NaYbF4. Moreover the preferred growth direction of the microtube is along the [001] direction on the basis of the HRTEM analysis. Crystal Phase and Shape Control. To reveal the crystal phase and shape control, highly uniform and monodispersed

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Zeng et al. TABLE 1: Summary of the Reaction Parameters, Crystal Phase, Morphologies of NaYbF4 Nano-/Micro-Crystals Yb(NO)3 (0.5 M) NaF (1.0 M) T (°C) t (h) crystal phase morphology 1 mmol 1 mmol 1 mmol

Figure 3. Typical TEM and HRTEM images of the as-prepared NaYbF4/1.0%Tm3+ sample (a) overview image, (b) single microtube image, and (c) typical HRTEM image taken from the area marked by the black rectangle in part b.

NaYbF4 nano-/microcrystals were synthesized under different reaction conditions (Table 1). Figure 4 shows typical TEM images of NaYbF4 nanocrystals with a variety of shapes fabricated under different experimental conditions. All the TEM images demonstrate that the as-prepared NaYbF4 nanocrystals are high uniform, well dispersed, and self-assembled into twodimensional ordered arrays. As illustrated in Figure 4a and b, both the sphere- and cubic-shaped structures of the nanocrystals with mean sizes of 10 and 30 nm can be obtained at 140 and 160 °C, respectively.

6 mL 6 mL 8 mL

140 160 190

12 12 24

cubic cubic hexagonal

nanospheres nanocubes microtubes

The inset images in Figure 4a and b show the corresponding select area electron diffraction (SAED) patterns of the nanocrystals. It shows spotty polycrystalline diffraction rings corresponding to the (111), (200), (220), and (311) lattice planes of the cubic phase NaYbF4 (space group: Fm3m). The HRTEM image shown in Figure 4c shows that the as-prepared single NaYbF4 nanocube has a single crystalline structure. The interplanar distances are 2.72 and 1.92 Å, which are also matched well with the (200) and (220) lattice planes of the cubic phase NaYbF4, respectively. Hexagonal phase microtubes can be formed after 24 h at 190 °C (Figure 3a), indicating that the reaction temperature is a key factor in controlling the crystal phase and shape. At low temperatures, NaYbF4 nanocrystals form cubic phase nanospheres (140 °C) and nanocubes (160 °C). In contrast, higher temperature and prolonged reaction time favor the formation of more stable hexagonal phase microtubes, which further confirms that temperature plays a key role in controlling the crystal phase and shape. Photoluminescence Properties. Photoluminescence (PL) studies of the NaYbF4/xTm3+ (x ) 0.5, 0.8, 1.0, 1.5%) microtubes at room temperature were performed under the excitation of an 980 nm excitation laser diode. The PL spectra of the as-prepared NaYbF4/xTm3+ (x ) 0.5, 0.8, 1.0, 1.5%) microtubes are shown in Figure 5. The intense UV, blue, and weak red emissions centered at 346, 360, 450, 477, 649, and 690 nm are observed, which are attributed to the 1I6 f 3F4, 1D2 f 3H6, 1D2 f 3F4, 1G4 f 3H6, 1G4 f 3F4, and 3F3 f 3H6 transitions, respectively. The left inset in Figure 5 shows a photography image of the 1 wt % cyclohexane colloid of the as-prepared NaYbF4/0.8% Tm3+ sample. From the image, the bright blue upconversion luminescence can be observed by the naked eye with an excitation power density of 3 W/cm2. Although the UV luminescence intensity of the NaYbF4/1.0% Tm3+-doped sample cannot be seen by the naked eye, it had an intensity equivalent to the blue luminescence of the NaYbF4/ 0.8% Tm3+ sample (the left inset of Figure 5). On the other hand, the influence of different Tm3+ concentrations on the upconversion luminescence can be obtained from the emission spectrum. As shown in Figure 5, the PL intensity of the NaYbF4/ 1.0% Tm3+ sample is the strongest of the four samples, with different Tm3+ contents. The right inset of Figure 5 presents the variation of emission intensities as a function of the Tm3+ content in the samples. It is clear from the inset that the intensities obviously change with the Tm3+ ion concentration. The UV and blue emission intensities remarkably increase with increasing of Tm3+ content from 0.5 to 1.0%. When the Tm3+ concentration reaches 1.5%, however, the intensities fall abruptly, indicating the occurrence of concentration quenching. The possible upconversion mechanisms for the Tm3+ singly doped NaYbF4 microtubes are discussed on the basis of the simplified energy level diagram presented in Figure 6 according to the energy matching conditions. Under a 980 nm excitation, the Yb3+ ions are excited from the 2F7/2 level to the 2F5/2 level then transfer their energies to the nearby Tm3+ ions, and then three successive energy transfers (ET) from Yb3+ to Tm3+ populate the 3H5, 3F2, and 1G4 levels of Tm3+.33 Consequently,

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Figure 5. Dependence of upconversion emission spectra on Tm3+ concentration of NaYbF4/x% Tm3+ samples. The inset shows the photograph of 1 wt % cyclohexane solution of as-prepared NaYbF4/ 0.8% Tm3+ sample (left) and the variation of the ultraviolet and blue upconversion luminescence on the Tm3+ concentration (right).

Figure 6. Schematic energy-level diagram of Yb3+ and Tm3+, and mechanism of upconversion emissions.

Figure 4. TEM and HRTEM images of the NaYbF4 samples prepared under different reaction parameters: (a) nanoshperes (140 °C, 12 h), (b) nanocubes (160 °C, 12 h), and (c) HRTEM image of a single nanocube. The insets of parts a and b show the corresponding SAED patterns.

the blue emission at 477 nm is generated by the 1G4 f 3H6 transition of the Tm3+ ions; the red emissions at 649 and 690 nm are due to the 1G4 f 3F4 and 3F3 f 3H6 transitions of Tm3+ ions, respectively. Due to the large energy mismatch about 3500 cm-1, the 1D2 level of Tm3+ cannot be populated by the fourth photon from Yb3+ via ET to the 1G4. Therefore, the cross

relaxation process of 3F2 + 3H4 f 3H6 + 1D2 between Tm3+ ions may alternatively play an important role in populating the 1 D2 level from which the UV band at 360 nm and the blue band at 450 nm arise.34,35 After the 1D2 level is populated, the 3P2 level can be excited by another ET process from Yb3+ to Tm3+. It then nonradiatively decays to the 1I6 level, which results in another UV band emission centered at 346 nm. The fact that the UV emissions require excitation by several incident nearinfrared photons (980 nm) accounts for the observation that hexagonal phase NaYbF4 is an excellent host for Ln3+ doping and, in particular, for Tm3+ions, as compared with other fluoride or oxide matrices. Influence of Crystal Phase on Upconversion Luminescence. To reveal the influence of the microstructure on the PL properties, we also synthesized cubic phase NaYbF4 nanocubes with the same dopant concentrations (corresponding to 1 mol % Tm3+). The upconversion luminescence spectra and corresponding TEM images of NaYbF4/1.0% Tm3+ microtubes and nanocubes are shown in Figure 7. The upconversion luminescence intensities of the NaYbF4/1.0% Tm3+ nanocubes were

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Zeng et al. Scientific Foundation of Education Department of Hunan Province (No. 08C885). References and Notes

Figure 7. Upconversion luminescence spectra and corresponding TEM images of NaYbF4/1.0% Tm3+ nanocubes (a) and microtubes (b).

magnified 20 times for the sake of comparison. On the basis of Figure 7, the upconversion luminescence intensities of the microtubes are 2 orders of magnitude stronger than those of the nanocubic particles. These intense UV emissions and significant improvements are mainly ascribed to the following two factors: First, the cubic phase is known to be about an order of magnitude less efficient than the corresponding hexagonal phase.36 Second, hexagonal phase microtubes are significantly larger than cubic phase nanocubes, with an average size of 30 nm. Thus, the fraction of Ln ions near the surface of the microtubes is much less than that of the nanocubes.37,38 These two factors make the up-conversion of the hexagonal phase microtubes much more efficient than that of the nanocubes. 4. Conclusion In conclusion, we first synthesized highly uniform and welldefined hexagonal-prismatic NaYbF4 microtubes with intense UV and eye-visible blue PL emissions by a facile and controlled hydrothermal approach using oleic acid as a stabilizing agent. The crystal phases and morphologies (i.e., hexagonal phase microtubes, cubic phase nanospheres and nanocubes) can be controllably synthesized by tuning the sodium fluoride contents, hydrothermal treatment temperatures, and reaction times. The results demonstrated that higher temperatures and prolonged reaction time facilitated the formation of more stable hexagonal phase structures. Compared with the NaYbF4 nanocubes, hexagonal phase microtubes exhibit a more efficient PL emission that is 2 orders of magnitude higher than that of the nanocubes. Furthermore, the strong UV emission of NaYbF4/1.0% Tm3+ showed an intensity that is comparable to the blue emission of NaYbF4/0.8% Tm3+, which was eye-visible. The intense UV and highly efficient PL emissions confirm that the hexagonal phase NaYbF4 is an excellent host for Ln3+ doping, as compared with other fluoride or oxide matrixes. It is therefore expected that these single-crystal and ideal uniform tubular structures with strong UV and blue emission can be used for applications in UV and visible solid-state lasers or the fiber lasers. Acknowledgment. This work is supported by the National Natural Scientific Foundation of China (No. 10874144) and the

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