Advances in Condensed Matter Physics

Volume 2013 (2013), Article ID 347406, 6 pages

http://dx.doi.org/10.1155/2013/347406

Research Article

Hydrothermal Synthesis and Tunable Multicolor Upconversion Emission of Cubic Phase Y₂O₃ Nanoparticles

Haibo Wang,1,2 Chao Qian,¹ Zhigao Yi,1,2 Ling Rao,1,2 Hongrong Liu,¹ and Songjun Zeng¹

¹College 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

²Faculty of Materials, Optoelectronics and Physics, Key Laboratory of Low-Dimensional Materials and Application Technology (Ministry of Education), Xiangtan University, Xiangtan 411105, China

Received 29 September 2013; Accepted 12 November 2013

Academic Editor: Jianhua Hao

Copyright © 2013 Haibo Wang 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.

Highly crystalline body-centered cubic structure Y₂O₃ with lanthanide (Ln) codopants (Ln = Yb³⁺/Er³⁺ and Yb³⁺/Ho³⁺) has been synthesized via a moderate hydrothermal method in combination with a subsequent calcination. The structure and morphology of Y(OH)₃ precursors and Y₂O₃ nanoparticles were characterized by X-ray diffraction and transmission electron microscopy. The results reveal that the Y₂O₃ nanoparticles possess cubic phase and form the quasispherical structure. The upconversion luminescence properties of Y₂O₃ nanoparticles doped with different Ln³⁺ (Yb³⁺/ Er³⁺ and Yb³⁺/ Ho³⁺) ions were well investigated under the 980 nm excitation. The results show that the Yb³⁺/Er³⁺ and Yb³⁺/Ho³⁺ codoped Y₂O₃ nanoparticles exhibit strong red and light yellow upconversion emissions, respectively. It is expected that these Y₂O₃ nanoparticles with tunable multicolor output and intense red upconversion emission may have potential application in color displays and biolabels.

1. Introduction

In the last few years, upconversion (UC) nanoparticles have attracted great attention in many research areas owing to their unique antistokes emission processes of converting a longer wavelength radiation to short wavelength emission [1–3]. Lanthanide-doped rare-earth oxides and related materials are common phosphors in optical display devices and fluorescent bioimaging applications [4–12].

Y₂O₃, as one of the most important rare-earth materials, was considered as an ideal candidate for biological applications, because of their higher mechanical, thermal, and chemical stability [13–15]. Rare-earth oxides were usually synthesized by thermal decomposition of their oxysalt precipitates such as hydroxide and oxalate at a certain temperature [16–19]. However, the thermal decomposition method requires high temperature and inert gases protection, resulting in complex experimental operations. Therefore, it is of significant importance to develop a simple method for the preparation of rare-earth oxide nanoparticles. Recently, Ln-doped Y₂O₃ shows great potential application in optical display and lighting [20]. So, further investigations of tunable multicolor UC emission Y₂O₃ nanoparticles via codoping Ln ions (Yb³⁺/Er³⁺ and Yb³⁺/Ho³⁺) are still of great interest. Up to now, there are many researches to regulate the multi-color output of UC nanomaterials via doping rare-earth ions [21–25]. For example, Garry’s group have reported that Yb³⁺, Er³⁺, Ho³⁺, and Tm³⁺ codoped Y₂O₃ nanocrystals can generate red, green, blue, and white light [25]. However, to the best of our knowledge, few studies have focused on the synthesis of spherical-like Y₂O₃ nanoparticles with tunable multicolor UC emissions by using facile hydrothermal method.

In this paper, different Ln³⁺ (Yb³⁺/Er³⁺ and Yb³⁺/Ho³⁺) ions codoped Y(OH)₃ nanosheets have been successfully synthesized based on a facile and mild hydrothermal method. And the cubic phase Y₂O₃ nanoparticles were calcinated from these Y(OH)₃ precursors. In addition, the UC luminescence properties of the Y₂O₃ doped with different Ln ions (Yb³⁺/Er³⁺ and Yb³⁺/Ho³⁺) were investigated under 980 nm excitation. The UC mechanism and CIE (Commission Internationale del’Eclairage 1931 chromaticity) chromaticity coordinates were studied in detail.

2. Experimental

Y(NO₃)₃·6H₂O (99.99%), Er(NO₃)₃·6H₂O (99.99%), Yb(NO₃)₃·5H₂O (99.99%), and Ho(NO₃)₃·5H₂O (99.99%) were purchased from Sinopharm Chemical Reagent Co., Ltd (China). All other chemical reagents are of analytical grade and used directly without further purification.

2.1. Synthesis of Different Ln³⁺ Codoped Y₂O₃ Nanoparticles

Y(OH)₃ nanosheets were synthesized by a facile hydrothermal method [26, 27]. For a typical protocol, 1 mmol (total amounts) of Ln (NO₃)₃ (Ln = Y, Yb, Er, and Ho) with designed molar ratio (Y : Yb : Er = 78 : 20 : 2 and Y : Yb : Ho = 78 : 20 : 2) was added into 20 mL deionized water. And then the obtained solution was rapidly adjusted to pH = 14 by adding 10 wt% NaOH solution. The obtained mixture was then transferred into a 50 mL stainless Teflon lined autoclave and reacted at 120°C for 12 h. After reaction, the system was naturally cooled to room temperature. The resulting products were collected, washed several times with de-ionized water to remove residual NaOH, and then dried at 80°C for 3 h. Y₂O₃ nanoparticles were synthesized by sintered Y(OH)₃ precursor at 500°C for 6 h.

2.2. Characterizations

X-ray powder diffraction (XRD) patterns were recorded by using a D/max 2500/PC system X-ray diffractometer at 40 kV and 250 mA using Cu Kα radiation (λ = 1.5406 Å). The morphology and size of the samples were characterized by transmission electron microscopy (TEM, JEOL-2100F) equipped with an Oxford’s energy dispersive X-ray spectroscopy (EDS). The UC emission spectra were recorded by a spectrophotometer (R 500) equipped with 980 nm laser diode as the excitation source. The digital photographs of the as-prepared samples were taken by a commercial digital camera (Canon 650D).

3. Results and Discussion

3.1. Crystal Phase and Morphology Analysis

The phase composition of the as-prepared Y₂O₃:Yb/Er was investigated by XRD analysis. As shown in Figure 1, the samples present characteristic diffraction peaks centered at 20.60° (211), 29.32° (222), 33.90° (400), 36.08° (411) 39.96° (332), 43.62° (134), 48.72° (440), 53.44° (611), 56.38° (541), 57.88° (622), 59.30° (136), 60.72° (444), 64.76° (127), 71.28° (800), and 72.52° (811), which is matched well with the standard body-centered cubic Y₂O₃ (JCPDS card no. 88-1040). In addition, no other impurity diffraction peaks are observed, indicating that pure cubic phase nanoparticles were synthesized with good crystallinity and a homogenous Y-Yb solid solution structure was formed.

Figure 1: Typical XRD patterns of the Y₂O₃:Yb/Er samples and the standard cubic phase Y₂O₃ (JCPDS: 88-1040).

To reveal the morphology and structure, the as-prepared samples were characterized by TEM (Figure 2). The as-prepared Y(OH)₃:Yb/Er by hydrothermal method at 120°C for 12 h consists of nanosheet and frizzy nanosheet (Figure 2(a)). Y₂O₃:Yb/Er calcinated from the precursor of Y(OH)₃:Yb/Er at 500°C for 6 h present agglutinate structure of sphere-like nanoparticles (Figures 2(b) and 2(c)). As shown in Figure 2(d), the obvious lattice fringes in the high resolution transmission electron microscopy (HRTEM) images confirm the high crystallinity of the as-prepared Y₂O₃:Yb/Er nanoparticles. The interplanar distances between the adjacent lattices were measured to 2.64 Å, matching well with the d₄₀₀ spacing of the cubic phase Y₂O₃ (JCPDS 88-1040). From the above results, we can see that rare-earth hydroxides are unstable compounds. When the temperature reached a certain value, the nanosheet and frizzy nanosheet hydroxides were burnt to stable sphere-like nanoparticles. During the TEM measurement, the elemental components of the Y₂O₃:Yb/Er nanoparticles were detected by EDS. The sphere-like nanoparticles are mainly composed of Y, Er, Yb, and O, indicating that the doped Yb³⁺ and Er³⁺ were successfully incorporated into the Y₂O₃ host (Figure 2(e)). It is noted that the signals of Cu are attributed to the TEM copper grid.

(a)

(b)

(c)

(d)

(e)

Figure 2: (a) TEM image of Y(OH)₃:Yb/Er precursor; (b) low-magnification and (c) high-magnification TEM images of Y₂O₃:Yb/Er; (d) the HRTEM image of Y₂O₃:Yb/Er; (e) EDS of the Y₂O₃:Yb/Er nanoparticles (mainly composed of Y, Er, Yb, and O elements).

3.2. Upconversion Luminescence Properties

It is well known that multicolor UC luminescence can be achieved by doping various sensitizers and active Ln ions. Here, we discuss the UC luminescence properties of Y₂O₃ nanoparticles through codoping Yb³⁺/Er³⁺ and Yb³⁺/Ho³⁺ ions. Under 980 nm excitation, the corresponding UC luminescence spectra and the proposed energy transfer mechanism are shown in Figure 3. As demonstrated in Figure 3(a), the UC spectra of Y₂O₃:Yb/Er nanoparticles show the characteristic Er³⁺ UC emissions including the green emission peaks (522 nm and 541 nm) and the red emission peak centered at 660 nm. More importantly, it can be seen that the Y₂O₃:Yb/Er nanoparticles possess significant stronger red luminescence at 660 nm, which is different from previously reported NaYF₄ host that is usually presenting green UC emission [28]. Figure 3(c) shows the energy level diagram of the possible energy transfer processes between Yb³⁺ and Er³⁺. With absorbing 980 nm photon, the Yb³⁺ is excited to the ²F5/2 energy level and then transfers its energy to the nearby Er³⁺ ion. Immediately following excited state absorption, transfer of energy from excited state Yb³⁺ resulted in populating the ⁴F7/2 state of Er³⁺. The excited state (⁴F7/2) electrons of Er³⁺ nonradiatively relax to the ²H11/2, ⁴S3/2, and ⁴F9/2 levels (Er³⁺). With high Yb³⁺-doped concentration, the green emissions are almost quenched and the intensity ratio of the red to the green emission is large, which results in strong red emission [29]. The dominant red UC emission is mainly attributed to the two energy-transfer processes ⁴S3/2 (Er³⁺) + ²F7/2 (Yb³⁺) → ⁴I13/2 (Er³⁺) + ²F5/2 (Yb³⁺) and ⁴I13/2 (Er³⁺) + ²F5/2 (Yb³⁺) → ⁴F9/2 (Er³⁺) + ²F7/2 (Yb³⁺) [25, 29, 30]. The three emission peaks around at 522 nm, 541 nm, and 660 nm can be assigned to the ²H11/2 → ⁴I15/2 (green), ⁴S3/2 → ⁴I15/2 (green), and ⁴F9/2 → ⁴I15/2 (red) transitions of Er³⁺, respectively. As demonstrated in Figure 3(b), the relative intensity of green (⁵S₂/⁵F₄ → ⁵I₈) to red (⁵F₅ → ⁵I₈) emission of the Ho³⁺ ion is stronger than the ratio of green to red emission of Er³⁺. The insets of Figures 3(a) and 3(b) show the corresponding digital photographs of the water solution of the Y₂O₃:Yb/Er and Y₂O₃:Yb/Ho, respectively. As demonstrated, the eye-visible red and yellow light can be readily observed.

(a)

(b)

(c)

Figure 3: Upconversion luminescence spectra of Y₂O₃ nanoparticles doped with (a) 20% Yb, 2% Er; (b) 20% Yb, 2% Ho. (c) Simplified energy-level diagram of Yb³⁺/Er³⁺ and Yb³⁺/Ho³⁺ doped Y₂O₃ samples. The insets of (a) and (b) are the corresponding digital photographs of the water solutions of these nanoparticles.

To further reveal the UC multicolor output, we have calculated the CIE chromaticity coordinates of Y₂O₃:Yb³⁺/Er³⁺ and Y₂O₃:Yb³⁺/Ho³⁺. As shown in Figure 4, the chromaticity coordinates for Y₂O₃:Yb³⁺/Er³⁺ (, ) (Figure 4, point a) and Y₂O₃:Yb³⁺/Ho³⁺ (, ) (Figure 4, point b) are fallen into the red and yellow region, respectively, which is consistent with the results of the UC spectra and the digital photographs. The CIE diagram indicates that the UC colors including strong red and yellow can be obtained via co-doping Yb³⁺/Er³⁺ and Yb³⁺/Ho³⁺ in Y₂O₃ host material, respectively.

Figure 4: The CIE chromaticity diagram of Y₂O₃ nanoparticles doped with (a) 20% Yb, 2%Er (0.6611, 0.3357); (b) 20% Yb, 2% Ho (0.3884, 0.6026).

4. Conclusions

In summary, we have successfully synthesized spherical-like cubic phase Y₂O₃ nanoparticles with tunable multicolor UC emissions by using facile hydrothermal method. On the basis of the analysis of the XRD and TEM, the as-prepared Y(OH)₃:Yb/Er nanocrystals are composed of nanosheet and frizzy nanosheet and then the Y₂O₃:Yb/Er nanoparticles with a quasispherical cubic phase structure were obtained by calcining precursor. Under the excitation of the 980 nm laser, multicolor visible emissions including strong red and yellow can be obtained via co-doping Yb³⁺/Er³⁺ and Yb³⁺/Ho³⁺ in Y₂O₃ host material, respectively. These UC nanoparticles with multicolors suggest that they have potential application in color displays and biolabels.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (nos. 51102202 and 91230116), Specialized research Fund for the Doctoral Program of Higher Education of China (no. 20114301120006), Hunan Provincial Natural Science Foundation of China (Nos. 12JJ4056 and 13JJ1017), the Scientific Foundation of Ministry of Education (212119), and Scientific Research Fund of Hunan Provincial Education Department (13B062).

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