Synthesis and downconversion emission property of [Yb.sub.2][O.sub.3]:[Eu.sup.3+] nanosheets and nanotubes.
Recently, rare earth (RE) doped luminescence materials have attracted considerable attention owing to their excellent applications in optics, biological labeling and imaging, new light source, catalyst and so on, owing to their unique properties such as narrow band of spectrum, monochromatism and bright of emission light, much stronger light absorb, good thermal and chemical stabilities, and low biotoxicity [1-8]. Much more investigations have focused on fluorides because of their unique advantages such as low phonon energy and high upconversion (UC) and DC luminescence [9, 10]. For example, dual-modal (UC/DC) luminescence has been successfully realized through lanthanide (Ln) ions doped NaGdF4 core/shell nanocrystals by Chen's group . In addition to fluorides, RE doped oxides with much better high temperature thermal stability have also attracted much interests and they are applied in thin films, fiber laser, capacitor and precision optical glass, and so forth [12-16]. Chen's group have investigated the phonon confinement effects on the luminescence dynamics in [Gd.sub.2][O.sub.3]:[Eu.sup.3+] nanotubes and provided experimental evidence of anomalous thermalization . In addition, the synthesis process of RE oxide doped Ln ions is much more simple and controllable . For example, the [Tb.sub.4][O.sub.7] and [Y.sub.2][O.sub.3] nanotubes with open ends were synthesized by a simple hydrothermal method without adding any template . Via one-step hydrothermal method, the crystal phase, shape, and size controlling of these oxides can be easily obtained by changing the reaction parameters such as pH value of the solution, reaction temperature/time, and. As a result, DC luminescence of various RE oxides like Ln (Eu, Tb, Dy) doped [Y.sub.2][O.sub.3], [Ce.sub.2][O.sub.3] and [Gd.sub.2][O.sub.3] has been widely investigated and the important applications of these RE oxides have been reported in recent years [19-22]. Although [Yb.sub.2][O.sub.3] is also a very promising matrix material , there is still lack of research about effect of [Eu.sup.3+] dopant on the DC luminescence of [Yb.sub.2][O.sub.3].
In this paper, we reported the controllable synthesis of [Eu.sup.3+] doped [Yb.sub.2][O.sub.3] nanosheets and nanotubes with cubic structure by a general and facile hydrothermal method with combination of calcination. The yellow DC luminescence was displayed under ultraviolet (UV) excitation at 395 nm. Furthermore, intensity of DC luminescence can be modified by varying [Eu.sup.3+] content and it is observed in the emission spectra that 1% [Eu.sup.3+] doped [Yb.sub.2][O.sub.3] nanocrystals emit the strongest DC light in our experiment.
2.1. Materials. The following chemical reagents [Yb.sub.2][O.sub.3], [Eu.sub.2][O.sub.3], HN[O.sub.3] and NaOH were used in synthesis of [Eu.sup.3+] doped [Yb.sub.2][O.sub.3]. Among them, [Yb.sub.2][O.sub.3] and [Eu.sub.2][O.sub.3] with purity of 99.99% were purchased from Sinopharm Chemical Reagent Co. Ltd. The analytical grade HN[O.sub.3] and NaOH were used as received without further purification.
2.2. Synthesis of [Eu.sup.3+] Doped Rare-Earth Hydroxide/Oxide Nanosheets and Nanotubes. In order to prepare [Eu.sup.3+] doped [Yb.sub.2][O.sub.3], we have to first prepare Yb[(N[O.sub.3]).sub.3] (0.5 M) and Eu[(N[O.sub.3]).sub.3] (0.1 M) by dissolving the [Yb.sub.2][O.sub.3] and [Eu.sub.2][O.sub.3] in HN[O.sub.3]. Then the precursorYb(OH)3 was prepared by mixing Yb[(N[O.sub.3]).sub.3] and Eu(N[O.sub.3])3 with addition of NaOH for adjusting the pH value of the liquid mixture. Finally, the [Eu.sup.3+] doped [Yb.sub.2][O.sub.3] nanosheets and nanotubes with [Eu.sup.3+] concentrations of 1, 2, 5, and 10% were synthesized by calcination at 700[degrees]C. The experimental details are listed below.
1mmol RE[(N[O.sub.3]).sub.3] with the designed concentration of Eu[(N[O.sub.3]).sub.3] of 1, 2, 5, and 10% was, respectively, added into 30 mL distilled water and pH value of the liquid mixture was then tuned to 14 by adding NaOH solution. The above mixture was stirred by magnetic stirring apparatus for 15 minutes at room temperature and colloidal precipitation appeared in the solution. Then the mixture was transferred into 50 mL stainless Teflon lined autoclave and heated at 180[degrees]C for 12 h. The as-prepared [Yb.sub.2][O.sub.3]:[Eu.sup.3+] microcrystals were precipitated at the bottom of the vessel. The precipitates were taken out by pouring out the upper liquid and then washed with deionized water three times to remove impurities such as OH-, N[O.sub.3]and [Na.sup.+]. Then the precursor Yb[(OH).sub.3] precipitates were dried at 60[degrees]C for 6 h. Finally the different concentration [Eu.sup.3+] doped [Yb.sub.2][O.sub.3] nanocrystals were obtained by calcination of these precursors at 700[degrees]C for 3 h.
2.3. Characterizations. The crystal phase of the products was identified by XRD using an X-ray diffractometer (model: D/max-[gamma]A) with Cu-Ka radiation ([lambda] = 1.5406 [Angstrom]), and the 20 range was from 10[degrees] to 80[degrees]. The microstructure of the products was characterized by TEM (JEOL-2100F) together with energy-dispersive X-ray spectroscopy (EDS). The DC emission and excitation spectra were recorded by a SENS9000 spectrophotometer equipped with xenon lamp that was used as the UV light source. All of the above tests were performed at room temperature.
3. Results and Discussion
XRD pattern of the as-prepared products was shown in Figure 1, in which all of the diffraction peaks are matched well with the standard cubic phase [Yb.sub.2][O.sub.3] structure (JCPDS: 87-2374). No extra diffraction peaks corresponding to other impurity phase were observed in this XRD pattern, indicating the pure cubic phase [Yb.sub.2][O.sub.3] obtained at low temperature (180[degrees]C).
In order to establish the correlation between the [Eu.sup.3+] doped Yb[(OH).sub.3] and [Yb.sub.2][O.sub.3], the size and morphology of the precursors Yb[(OH).sub.3]:1% [Eu.sup.3+] and [Yb.sub.2][O.sub.3]:1% [Eu.sup.3+] were characterized by TEM, respectively. Figures 2(a) and 2(b) show the typical TEM images of Yb[(OH).sub.3]:1% [Eu.sup.3+]. As demonstrated, the precursors have two typical shapes: sheet and tube-like structure. After calcination, [Yb.sub.2][O.sub.3]:1% [Eu.sup.3+] presents the similar morphologies (Figures 2(c) and 2(d)) as the precursor hydroxides. The inset image in Figure 2(d) shows the selected area electron diffraction (SAED) pattern of a single [Yb.sub.2][O.sub.3]:1% [Eu.sup.3+] nanosheet, which reveal the single-crystalline nature of the nanosheet and can be readily indexed to cubic phase structure. In addition, as shown in Figures 2(a) and 2(b), some sheets were curled into tube-like shape, indicating the formation mechanism of tube structure with open ends coincident with self-rolled model, which is similar to Zhang's report . Figure 2(e) shows the EDS analysis of [Yb.sub.2][O.sub.3]:[Eu.sup.3+] nanosheets, in which the Yb, Eu, and O except Cu resulting from copper grid are the major elements in sheets. As a result, the [Eu.sup.3+] is successfully incorporated into the [Yb.sub.2][O.sub.3]. It should be noted that the compositions of the [Yb.sub.2][O.sub.3]:[Eu.sup.3+] nanotubes were also analyzed by EDS equipped with the TEM and the similar results with nanosheets were obtained (data not shown).
Under UV light excitation, the DC luminescence properties of [Yb.sub.2][O.sub.3]:[Eu.sup.3+] have been investigated. The photoluminescence excitation and emission spectra were recorded and shown in Figure 3, where the excitation spectrum of [Yb.sub.2][O.sub.3]:1% [Eu.sup.3+] nanocrystals was obtained by monitoring at 593 nm. Several wavelengths of UV light can excite emission with wavelength of 593 nm, but the strongest emission happened at the 395 nm UV light. Therefore, the 395 nm UV light is the best excitation wavelength. Meanwhile, the emission spectrum (Figure 3(b)) was measured under the excitation at 395 nm and eight emission peaks recorded at 470, 525, 530, 574, 593 610, 651, and 660 nm, respectively, were produced by the electron transition from [sup.5][D.sub.j] to [sup.7][F.sub.j'] levels (J = 0, 1, 2, and J' = 1-4) [6,25]. The strongest DC emission was centered at 593nm (due to the electron transition from [sup.5][D.sub.0] to [sup.7][F.sub.3] levels), leading to yellow emission light.
Possible mechanical energy level diagram for [Eu.sup.3+] doped [Yb.sub.2][O.sub.3] nanocrystals after pumping at 395 nm is shown in Figure 4 [6, 25]. Under the excitation at 395 nm, the [Eu.sup.3+] ion can be excited from the ground state to the excited state. The excited state [Eu.sup.3+] ions will decay nonradiatively to the [sup.5][D.sub.2], [sup.5][D.sub.1] and [sup.5][D.sub.0] levels, resulting in the corresponding blue ([sup.5][D.sub.2] [right arrow] [sup.7][F.sub.0]), green ([sup.5][D.sub.1] [right arrow] [sup.7][F.sub.0,1,3]), yellow ([sup.5][D.sub.0] [right arrow] [sup.7][F.sub.1]) and red ([sup.5][D.sub.0] [right arrow] [sup.7][F.sub.2,3,4]) emissions, respectively
The influence of [Eu.sup.3+] concentration on the DC luminescence was investigated by comparison of the emission spectra (Figure 5) of the synthesized [Yb.sub.2][O.sub.3]:X% Eu (X = 1, 2, 5, 10) samples. It can be seen that the strongest DC luminescence was achieved in 1% Eu doped sample. With further increasing the [Eu.sup.3+] content, the DC luminescent intensity was decreased, which was mainly ascribed to the concentration quenching effect .
[Yb.sub.2][O.sub.3]:[Eu.sup.3+] nanosheets and nanotubes doped with different concentrations of [Eu.sup.3+] were successfully synthesized by a facile controllable hydrothermal process followed by calcination at 700[degrees]C. The XRD confirms the synthesized [Yb.sub.2][O.sub.3]:[Eu.sup.3+] with cubic structure. The TEM images revealed Yb[(OH).sub.3] :[Eu.sup.3+] with sheet- and tube-like morphology, respectively. The DC emissions of the synthesized [Yb.sub.2][O.sub.3]:[Eu.sup.3+] centered at 470, 525, 530, 574, 593, 610, 651 and 660 nm were observed under the strongest excitation at 395 nm. The intensity of DC emission can be tuned by adjusting the concentration of [Eu.sup.3+], while the strongest intensity yellow DC luminescence was obtained at 1% [Eu.sup.3+].
This work was supported by the National Natural Science Foundation of China (no. 51102202), the New Century Excellent Talents in University (NCET-13-0787), the Specialized Research Fund for the Doctoral Program of Higher Education of China (no. 20114301120006) and Hunan Provincial Natural Science Foundation of China (nos. 12JJ4056 and 13JJ1017), the Scientific Foundation of Ministry of Education (212119) and the Scientific Research Fund of Hunan Provincial Education Department (13B062 and YB2012B027).
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Chao Qian, Tianmei Zeng, and Hongrong Liu
College of Physics and Information Science and Key Laboratory of Low-Dimensional Quantum Structures and Quantum Control of the Ministry of Education, Hunan Normal University, Changsha, Hunan 410081, China
Correspondence should be addressed to Hongrong Liu; firstname.lastname@example.org
Received 11 October 2013; Accepted 22 November 2013
Academic Editor: Jianhua Hao
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|Title Annotation:||Research Article|
|Author:||Qian, Chao; Zeng, Tianmei; Liu, Hongrong|
|Publication:||Advances in Condensed Matter Physics|
|Date:||Jan 1, 2014|
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