Advances in Condensed Matter Physics

Volume 2013 (2013), Article ID 509374, 5 pages

http://dx.doi.org/10.1155/2013/509374

Research Article

Intense Red Upconversion Emission and Shape Controlled Synthesis of Gd₂O₃:Yb/Er Nanocrystals

Zhigao Yi, Boyun Wen, Chao Qian, Haibo Wang, Ling Rao, Hongrong Liu, and Songjun Zeng

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 5 October 2013; Accepted 23 October 2013

Academic Editor: Jianhua Hao

Copyright © 2013 Zhigao Yi 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.

Yb/Er codoped Gd(OH)₃ was synthesized firstly via a simple hydrothermal treatment of the corresponding nitrate in the presence of alkali by tuning the pH values. The Gd₂O₃:Yb/Er nanocrystals were obtained via sintering the corresponding hydroxides precursors. The as-prepared samples were characterized by the typical X-ray diffraction, energy-dispersive X-ray spectroscopy, transmission electron microscopy (TEM), high-resolution TEM, and spectrophotometer. The results revealed that two shapes of the as-prepared Gd₂O₃:Yb/Er nanocrystals can be readily tuned from lemon-like particle to rod-like structure via tuning pH values from 7 to 14. Moreover, compared with the samples prepared at pH 7, the Gd₂O₃:Yb/Er nanocrystals prepared at pH 14 exhibit enhanced red upconversion emission and higher upconversion luminescence intensity under the excitation of 980 nm laser.

1. Introduction

Rare-earth (RE) doped upconversion (UC) nanocrystals have been comprehensively used in high-quality lighting devices, magnets, and other functional materials due to their optical, electronic, and chemical properties arising from the 4f shell of their ions [1–6]. These capabilities are highly sensitive to the bonding states of RE ions. RE doping induced photoluminescence property in fluorides, and oxides systems have been universally reported [7–11]. For instance, Yb/Tm codoped Y₂O₃ nanocrystals can achieve remarkable green UC luminescent emission [8]. In Yb/Er codoped Lu₂O₃ system, UC spectra revealed that simultaneous green and red emissions can be obtained [11]. However, the intense UC emission located in the red region by doping Yb/Er is an enormous challenge owing to the fact that Yb/Er codoped UC materials have a multipeak emissions nature. Recently, Li’s group have developed a simple and controlled synthetic method [3–5], in which the shape transformation, phase evolution, and UC emission tuning of a series of RE oxides nanocrystals can be reached by simply adjusting reaction parameters, such as pH value, reaction time, and temperature.

Among various developed RE-based oxides UC nanomaterials, gadolinium oxide (Gd₂O₃) has its unique UC luminescent and magnetic properties. Therefore, RE (Nd³⁺, Er³⁺, and Tm³⁺) doped Gd₂O₃ nanocrystals have been selected for optical and magnetic studies, because of their high chemical durability, photothermal and photochemical stabilities, low photon-energy, and unpaired electrons of the 4f shell [12–17]. Of course, there are also many investigations of the UC luminescence nanocrystals mainly focused on the RE doped Gd₂O₃ with phase/structure control and UC luminescence tuning [15, 18–20]. Even though UC emissions of RE doped Gd₂O₃ nanocrystals can be enhanced via changing reaction parameters, intense red UC emission is still a large challenge for the various energy levels of RE ions. In general, these nanocrystals exhibit multipeak UC emissions due to the fact that the doped lanthanide ions have abundant energy levels. However, there are still limited reports about doping Yb/Er to achieve intense red UC emission.

In this work, the precursor Gd(OH)₃ codoped with Yb³⁺ and Er³⁺ ions was synthesized at first via a simple hydrothermal reaction. After that, the nanoparticle and nanorod structures of Gd₂O₃:Yb/Er were obtained by calcination of the corresponding Gd(OH)₃ precursors prepared at different pH values of 7 and 14, respectively. The UC properties of the Gd₂O₃:Yb/Er samples with different shapes were studied. Interestingly, greatly enhanced red UC emission can be realized by simply tuning the pH value from 7 to 14, which will expand their application from optics to the biological field.

2. Experimental

2.1. Materials and Regents

RE(NO₃)₃ · 6H₂O (Gd³⁺/Yb³⁺/Er³⁺, 99.99%) was purchased from Sigma-Aldrich. KOH was purchased from Sinopharm Chemical Reagent Co., China. All of the reagents were used as received with no further purification.

2.2. Synthesis of Yb/Er Codoped Gd(OH)₃ and Gd₂O₃ Nanocrystals

Gd(OH)₃:Yb/Er nanocrystals were synthesized by using a simple hydrothermal treatment [5] of the corresponding nitrate in the presence of alkali. In a typical synthesis, 1 mmol of Gd(NO₃)₃ (0.5 M), Yb(NO₃)₃ (0.5 M), and Er(NO₃)₃ (0.5 M) with the designed molar ratio of 78 : 20 : 2 was added into 20 mL of deionized (DI) water. And then, aqueous KOH (10 wt%) solution was added dropwise into the mixture until the pH was adjusted to a designated value (7/14). After another stirring for 20 min, the formed colloidal mixture was transferred into a 50 mL Teflon-lined stainless steel autoclave and then sealed and maintained at 180°C for 12 h. After the reaction was completed, the reaction system was permitted to cool to room temperature naturally and the pure solid product Gd(OH)₃:Yb/Er was obtained by washing with deionized water several times and drying at 60°C for 24 h. In addition, the corresponding oxide nanocrystals (Gd₂O₃:Yb/Er) have been prepared by calcination of Gd(OH)₃:Yb/Er at 450°C for 6 h.

2.3. Characterizations

Typical X-ray diffraction (XRD) patterns were recorded by a D/max-A System X-ray diffractometer at 40 kV and 250 mA with Cu-K radiation (). The shape and structure of the as-prepared samples were characterized by transmission electron microscopy (TEM, JEOL-2100F) equipped with the energy-dispersive X-ray spectroscopy (EDS). The UC emissions were tested by a spectrophotometer (R500) equipped with a 980 nm laser diode.

3. Results and Discussion

Crystal phase of the as-prepared Gd₂O₃:Yb/Er samples was identified by powder XRD pattern. After continuous heating for 6 hours at 450°C, Gd(OH)₃:Yb/Er (pH = 7) nanocrystals have completely transformed to Gd₂O₃:Yb/Er and the corresponding XRD analysis is showed in Figure 1(a). In addition, the phase of Gd₂O₃:Yb/Er (pH = 7) has transformed into pure cubic phase and no other peaks have been detected. When the pH value is tuned up to 14, pure cubic phase of as-synthesized products can be obtained as well as presented in Figure 1(b). The diffraction peaks of Gd₂O₃:Yb/Er nanocrystals (pH = 7/14) are well coincident with the peaks of the standard cubic phase Gd₂O₃ (JCPDS number 12-0797, Figure 1(c)).

Figure 1: XRD pattern of (a) Gd₂O₃:Yb/Er (pH = 7) and (b) Gd₂O₃:Yb/Er (pH = 14) nanocrystals and (c) standard cubic phase Gd₂O₃.

To further investigate the morphology and structure of these nanocrystals, TEM, high-resolution TEM (HRTEM), and selected area electron diffraction (SAED) assays were carried out. As shown in Figure 2(a), the precursor (Gd(OH)₃:Yb/Er, pH = 7) presented lemon-like shape. Figure 2(c) shows a typical TEM image of Gd₂O₃:Yb/Er (pH = 7). As demonstrated, the samples presented lemon- and rod-like structures. The interplanar spacing in the HRTEM image (Figure 2(d)) of a single particle was measured to be 3.10 Å, corresponding to the 111 crystal plane of cubic phase Gd₂O₃, which is matched well with the XRD analysis. Figure 2(e) shows the typical SAED result, further validating the formation of body-centered cubic phase structure. To further study the influence on morphology by tuning pH value, the TEM image of Gd₂O₃:Yb/Er (pH = 14) is presented in Figure 2(f). As shown in Figure 2(f), compared with Gd₂O₃:Yb/Er (pH = 7), the shape was completely changed to pure rod-like morphology. Figure 2(h) shows the typical HRTEM image taken from an individual nanorod (Figure 2(g)). The interplanar spacing was measured to be 2.68 Å, which agrees well with the 400 crystal plane of cubic phase Gd₂O₃, indicating the formation of cubic phase rod-like structure. The SAED result of nanorods (Figure 2(i)) also presents the characterized polycrystalline diffraction rings of body-centered cubic phase, further demonstrating the formation of cubic phase rod-like structure. Therefore, tuning pH value from 7 to 14 can successfully achieve the morphology evolution of Yb/Er codoped Gd₂O₃ nanocrystals from lemon-like particle to rod. Figure 2(b) exhibits EDS analysis of the as-prepared Gd₂O₃:Yb/Er (pH = 14), indicating that Gd, O, Yb, and Er are the major elements in these nanorods.

Figure 2: (a) TEM image of Gd(OH)₃:Yb/Er (pH = 7). (b) EDS analysis of Gd₂O₃:Yb/Er nanorods shown in (f). (c) TEM image of Gd₂O₃:Yb/Er (pH = 7) and the corresponding (d) HRTEM and (e) SAED, respectively. (f) TEM image of Gd₂O₃:Yb/Er (pH = 14). (g) TEM of a single nanorod and (h) the corresponding HRTEM image. (i) The SAED characterization of nanorods shown in (f).

The UC luminescence property of the Gd₂O₃:Yb/Er nanocrystals was studied at room temperature. As shown in Figure 3, Gd₂O₃:Yb/Er (pH = 7) exhibits intense red and weak green emissions (green curve) under excitation by continuous-wave 980 nm laser diode. Moreover, the later CIE calculated chromaticity coordinates (Figure 6) reveal that yellow light can be obtained. However, via varying the pH value from 7 to 14, the red UC emission efficiency and emission intensity of Gd₂O₃:Yb/Er have been extremely enhanced (red curve). As presented in Figure 4, the two green light emissions (labeled by green arrows) are attributed to the electronic transitions ²H11/2 → ⁴I15/2 and ⁴S3/2 → ⁴I15/2, which correspond with the green emissions (Figure 3) centered in 525 and 538 nm, respectively. On the other hand, the red emission (labeled by the red arrow) is ascribed to the transition of ⁴F9/2 → ⁴I15/2, which is well coincident with the red emission centered at 662 nm. To further reveal the UC emission property of these Gd₂O₃:Yb/Er nanorods, power dependent UC spectra have been studied (Figure 5). With increasing excitation power, the as-synthesized samples exhibit the trend of enhancing UC luminescence, especially red emission, which can be observed obviously from the red arrow shown in the CIE diagram (Figure 6). It is noted that the dominant red UC emission with very weaker green UC emission in the Yb/Er doped system is mainly attributed to the fact that the intensity ratio of the red to green emissions increased rapidly and the green emissions almost quenched for high doped content of Yb³⁺ (20 mol%) [11, 21].

Figure 3: UC spectra of Gd₂O₃:Yb/Er nanocrystals with different pH values. The blue curve stands for pH = 7, and the red one corresponds with pH = 14.

Figure 4: Schematic energy-level diagram of Yb³⁺ and Er³⁺ ions.

Figure 5: Dependence of PL spectra on pumping power in Gd₂O₃:Yb/Er nanorods.

Figure 6: The CIE 1931 chromaticity diagram and calculated chromaticity coordinates. The red arrow indicates the changing tendency of chromaticity coordinate of Gd₂O₃:Yb/Er nanorods (pH = 14) by increasing the excitation power.

4. Conclusions

In conclusion, body-centered cubic phase Gd₂O₃:Yb/Er nanocrystals with different shapes were successfully synthesized by a hydrothermal method and combination of a subsequent calcination. By tuning the pH value of the reaction system, the shape of Gd₂O₃:Yb/Er nanocrystals can be successfully tuned from lemon-like shape to rod. More interestingly, when adjusting the pH value from 7 to 14, the red UC emission and whole UC emission intensity have been remarkably enhanced. The bright red UC emission of these nanorods may expand their application varying from optics to the biological field.

Acknowledgments

This paper was supported by the National Natural Science Foundation of China (no. 51102202), specialized research Fund for the Doctoral Program of Higher Education of China (no. 20114301120006), Hunan Provincial Natural Science Foundation of China (no. 12JJ4056), and the Scientific Research Fund of Hunan Provincial Education Department (13B062).

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