Advances in Condensed Matter Physics

Volume 2013 (2013), Article ID 519869, 5 pages

http://dx.doi.org/10.1155/2013/519869

Research Article

Synthesis and Downconversion Emission Property of : Nanosheets and Nanotubes

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

Received 11 October 2013; Accepted 22 November 2013

Academic Editor: Jianhua Hao

Copyright © 2013 Chao Qian et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Ytterbium oxide (Yb₂O₃) nanocrystals with different Eu³⁺ (1%, 2%, 5%, and 10%) doped concentrations were synthesized by a facile hydrothermal method, subsequently by calcination at 700°C. The crystal phase, size, and morphology of prepared samples were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that the as-prepared Yb₂O₃ nanocrystals with sheet- and tube-like shape have cubic phase structure. The Eu³⁺ doped Yb₂O₃ nanocrystals were revealed to have good down conversion (DC) property and intensity of the DC luminescence can be modified by Eu³⁺ contents. In our experiment the 1% Eu³⁺ doped Yb₂O₃ nanocrystals showed the strongest DC luminescence among the obtained Yb₂O₃ nanocrystals.

1. Introduction

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 [11]. 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 Gd₂O₃:Eu³⁺ nanotubes and provided experimental evidence of anomalous thermalization [14]. In addition, the synthesis process of RE oxide doped Ln ions is much more simple and controllable [17]. For example, the Tb₄O₇ and Y₂O₃ nanotubes with open ends were synthesized by a simple hydrothermal method without adding any template [18]. 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 Y₂O₃, Ce₂O₃ and Gd₂O₃ has been widely investigated and the important applications of these RE oxides have been reported in recent years [19–22]. Although Yb₂O₃ is also a very promising matrix material [23], there is still lack of research about effect of Eu³⁺ dopant on the DC luminescence of Yb₂O₃.

In this paper, we reported the controllable synthesis of Eu³⁺ doped Yb₂O₃ 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 Eu³⁺ content and it is observed in the emission spectra that 1% Eu³⁺ doped Yb₂O₃ nanocrystals emit the strongest DC light in our experiment.

2. Experimental

2.1. Materials

The following chemical reagents Yb₂O₃, Eu₂O₃, HNO₃ and NaOH were used in synthesis of Eu³⁺ doped Yb₂O₃. Among them, Yb₂O₃ and Eu₂O₃ with purity of 99.99% were purchased from Sinopharm Chemical Reagent Co. Ltd. The analytical grade HNO₃ and NaOH were used as received without further purification.

2.2. Synthesis of Eu³⁺ Doped Rare-Earth Hydroxide/Oxide Nanosheets and Nanotubes

In order to prepare Eu³⁺ doped Yb₂O₃, we have to first prepare Yb(NO₃)₃ (0.5 M) and Eu(NO₃)₃ (0.1 M) by dissolving the Yb₂O₃ and Eu₂O₃ in HNO₃. Then the precursor Yb(OH)₃ was prepared by mixing Yb(NO₃)₃ and Eu(NO₃)₃ with addition of NaOH for adjusting the pH value of the liquid mixture. Finally, the Eu³⁺ doped Yb₂O₃ nanosheets and nanotubes with Eu³⁺ concentrations of 1, 2, 5, and 10% were synthesized by calcination at 700°C. The experimental details are listed below.

1 mmol RE(NO₃)₃ with the designed concentration of Eu(NO₃)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°C for 12 h. The as-prepared Yb₂O₃:Eu³⁺ 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⁻, and Na⁺. Then the precursor Yb(OH)₃ precipitates were dried at 60°C for 6 h. Finally the different concentration Eu³⁺ doped Yb₂O₃ nanocrystals were obtained by calcination of these precursors at 700°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-γA) with Cu-Kα radiation (λ = 1.5406 Å), and the 2θ range was from 10° to 80°. 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 SENS-9000 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 Yb₂O₃ structure (JCPDS: 87-2374). No extra diffraction peaks corresponding to other impurity phase were observed in this XRD pattern, indicating the pure cubic phase Yb₂O₃ obtained at low temperature (180°C).

Figure 1: XRD pattern of Yb₂O₃:1% Eu³⁺ nanocrystals (top) and standard diffraction of cubic phase Yb₂O₃ (JCPDS: 87-2374, bottom for comparison).

In order to establish the correlation between the Eu³⁺ doped Yb(OH)₃ and Yb₂O₃, the size and morphology of the precursors Yb(OH)₃:1% Eu³⁺ and Yb₂O₃:1% Eu³⁺ were characterized by TEM, respectively. Figures 2(a) and 2(b) show the typical TEM images of Yb(OH)₃:1% Eu³⁺. As demonstrated, the precursors have two typical shapes: sheet and tube-like structure. After calcination, Yb₂O₃:1% Eu³⁺ 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 Yb₂O₃:1% Eu³⁺ 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 [24]. Figure 2(e) shows the EDS analysis of Yb₂O₃:Eu³⁺ nanosheets, in which the Yb, Eu, and O except Cu resulting from copper grid are the major elements in sheets. As a result, the Eu³⁺ is successfully incorporated into the Yb₂O₃. It should be noted that the compositions of the Yb₂O₃:Eu³⁺ nanotubes were also analyzed by EDS equipped with the TEM and the similar results with nanosheets were obtained (data not shown).

(a)

(b)

(c)

(d)

(e)

Figure 2: (a) TEM image of Yb(OH)₃:Eu³⁺ nanosheets; (b) highly magnified TEM image of individual Yb(OH)₃:Eu³⁺ nanosheets; (c) TEM image of Yb₂O₃:Eu³⁺ nanosheets and nanotubes; (d) highly magnified TEM image of individual Yb₂O₃:Eu³⁺ nanosheets and nanotubes; (e) EDS analysis of Yb₂O₃:Eu³⁺ nanosheets. The inset of Figure 2(d) shows the SAED pattern of Yb₂O₃ nanosheets.

Under UV light excitation, the DC luminescence properties of Yb₂O₃:Eu³⁺ have been investigated. The photoluminescence excitation and emission spectra were recorded and shown in Figure 3, where the excitation spectrum of Yb₂O₃:1% Eu³⁺ 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 to levels (, and = 1–4) [6, 25]. The strongest DC emission was centered at 593 nm (due to the electron transition from to levels), leading to yellow emission light.

Figure 3: (a) Photoluminescence excitation spectrum of Yb₂O₃: 1% Eu³⁺ nanocrystals by detecting the emission at 593 nm and (b) emission spectrum under the excitation at 395 nm.

Possible mechanical energy level diagram for Eu³⁺ doped Yb₂O₃ nanocrystals after pumping at 395 nm is shown in Figure 4 [6, 25]. Under the excitation at 395 nm, the Eu³⁺ ion can be excited from the ground state to the excited state. The excited state Eu³⁺ ions will decay nonradiatively to the ⁵D₂, ⁵D₁ and ⁵D₀ levels, resulting in the corresponding blue (⁵D₂ → ⁷F₀), green (⁵D₁ → ⁷F0,1,3), yellow (⁵D₀ → ⁷F₁) and red (⁵D₀ → ⁷F2,3,4) emissions, respectively.

Figure 4: Scheme of energy levels of Eu³⁺ in Yb₂O₃ and possible mechanical energy level diagram for Eu³⁺ after pumping excitation at 395 nm.

The influence of Eu³⁺ concentration on the DC luminescence was investigated by comparison of the emission spectra (Figure 5) of the synthesized Yb₂O₃:X% Eu () samples. It can be seen that the strongest DC luminescence was achieved in 1% Eu doped sample. With further increasing the Eu³⁺ content, the DC luminescent intensity was decreased, which was mainly ascribed to the concentration quenching effect [26].

Figure 5: Photoluminescence emission spectra of different concentrations of Eu³⁺ doped Yb₂O₃: (a) 1% Eu³⁺, (b) 2% Eu³⁺, (c) 5% Eu³⁺, and (d) 10% Eu³⁺ by excitation at 395 nm.

4. Conclusions

Yb₂O₃:Eu³⁺ nanosheets and nanotubes doped with different concentrations of Eu³⁺ were successfully synthesized by a facile controllable hydrothermal process followed by calcination at 700°C. The XRD confirms the synthesized Yb₂O₃:Eu³⁺ with cubic structure. The TEM images revealed Yb(OH)₃:Eu³⁺ with sheet- and tube-like morphology, respectively. The DC emissions of the synthesized Yb₂O₃:Eu³⁺ 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 Eu³⁺, while the strongest intensity yellow DC luminescence was obtained at 1% Eu³⁺.

Acknowledgments

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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