Trans. Nonferrous Met. Soc. China 22(2012) 2471-2476

Selective synthesis and shape-dependent photoluminescence properties of (Y_0.95Eu_0.05)₂O₃ submicron spheres and microplates

ZHU Qi, LI Ji-guang, XU Yong, LI Xiao-dong, SUN Xu-dong

Key Laboratory for Anisotropy and Texture of Materials of Ministry of Education, Northeastern University,Shenyang 110819, China

Received 9 July 2012; accepted 1 August 2012

Abstract: Red-emission (Y_0.95Eu_0.05)₂O₃ submicron spheres and microplates were selectively obtained via hydrothermal precursor synthesis (150 ℃, 12 h) followed by calcination at 1000 ℃. Characterizations of the products were carried out by combined means of XRD, FT-IR, FE-SEM and PL analysis. The precursors could be modulated from basic-carbonate submicron spheres to normal carbonate microplates by increasing the molar ratio of urea to Y+Eu from 10 to 40-100. The resultant oxides largely retain their respective precursor morphologies at 1000 ℃, but morphology confined crystal growth was observed for the microplates, yielding more enhanced exposure of the (400) facets. Both the (Y_0.95Eu_0.05)₂O₃ spheres and microplates exhibit nearly identical positions of the PL bands and similar asymmetry factors of luminescence [I(⁵D₀→⁷F₂)/I(⁵D₀→⁷F₁), ~11] under 250 nm excitation, but the microplates show a significantly strong red emission at ~613 nm (~1.33 times that of the spheres) owing to their larger particle size and denser packing of primary phosphor crystallites.

Key words: optical material; powder processing; morphology-dependent physical phenomena; rare earths; optical spectroscopy

1 Introduction

Eu³⁺ doped Y₂O₃ solid solution (Y₂O₃:Eu³⁺) exhibits a sharp red emission at ~610 nm arising from the ⁵D₀→⁷F₂ intra-4f electronic transitions of the Eu³⁺ ions, upon short UV excitation into the charge transfer (CT) band at ~250 nm or under electron beam irradiation. The luminescent performance of a phosphor powder is known to be influenced by several factors, such as size, shape, and crystallinity of the particles. Over the past decades, a few of techniques have been developed for Y₂O₃:Eu³⁺ synthesis owing to its technological importance, such as combustion [1], sol-gel [2], high temperature pyrolysis [3,4], and thermal plasma processing [5,6], and much effort has been paid to particle size, particle shape, and crystallinity control of the products. Recently, a variety of novel Y₂O₃:Eu³⁺ nanostructures have been successfully produced, including monospheres [7-9], nanotubes [10], nanowires [11], nanorods [12], nanobelts [13,14], nanosheets [15], and microflowers [16].

It should be noted that a tunable morphology synthesis by a simple technique is of great importance but frequently remains a challenge in the field of particle science and engineering. In the present work, we selectively synthesized submicron spheres and microplates of (Y_0.95Eu_0.05)₂O₃ phosphors by calcining their precursors autoclaved from mixed nitrate solutions containing urea. Based upon characterizations by combined techniques of XRD, FT-IR, FE-SEM, and PL analysis, the effects of processing parameter on precursor characteristics as well as morphology-dependent crystallization and luminescence behaviors of the oxides were discussed.

2 Experimental

2.1 Powder synthesis

The yttrium and europium sources for particle synthesis were Y₂O₃ (99.99% purity, Huizhou Ruier Rare-Chem. Hi-Tech. Co. Ltd., Huizhou, China) and Eu₂O₃ (99.99% purity, Conghua Jianfeng Rare Earth Co. Ltd., Conghua, China), respectively. The other reagents were of analytical grade and were purchased from Shenyang Chemical Reagent Factory (Shenyang, China). Stock solutions of M(NO₃)₃ (M=Y and Eu) were prepared by dissolving the above oxides with a proper amount of hot nitric acid. The luminescence quenching concentration of Eu³⁺ in Y₂O₃ is widely observed to be ~5% (mole fraction), and thus in this work the molar ratio of Y to Eu was kept a constant of 19:1 for each run of the experiment. In a typical synthesis, a proper amount of urea was dissolved in 60 mL of 1 mmol M(NO₃)₃ stock solution, and the resultant mixture, after being constantly stirred for 30 min, was then transferred into a Teflon lined stainless-steel autoclave of 100 mL capacity. The autoclave was tightly sealed and was then put in an electric oven preheated to 150 ℃. After 12 h of reaction, the autoclave was left to cool naturally to room temperature, and the hydrothermal product was collected via centrifugation. The wet precipitate was washed with distilled water three times to remove the byproducts, rinsed with absolute ethanol, and was finally dried in air at 100 ℃ for 2 h to yield a white precursor powder for further characterizations. The red phosphor Y₂O₃:Eu³⁺ was obtained by calcining the precursor in air at 1000 ℃ for 4 h with a heating rate of 10 ℃/min at the ramp stage. The molar ratio of urea to total cation (R, R= urea/(Y+Eu)) was widely varied to investigate its influence on powder properties.

2.2 Characterization

Phase identification was performed by X-ray diffractometry (XRD, Model PW3040/60, Philips, Eindhoven, The Netherlands) operating at 40 kV/40 mA using nickel filtered Cu K_α radiation and a scanning speed of 4.0 (°)/min. Fourier transform infrared spectroscopy (FTIR, Model Spectrum RXI, Perkin-Elmer, Shelton, CT, USA) of the precursors was performed by the standard KBr method. Morphologies of the products were observed via field emission scanning electron microscopy (FE-SEM，Model JSM-7001F, JEOL, Tokyo, Japan). Photoluminescence (PL) spectra of the resultant Y₂O₃:Eu³⁺ phosphor were measured using an LS-55 fluorescence spectrophotometer (Perkin-Elmer, Shelton, CT) at room temperature.

3 Results and discussion

3.1 Effects of molar ratio of urea to Y+Eu on phase and morphology of precursor

Particle morphology of the precursor was found to be closely related to the molar ratio of urea to Y+Eu. Figure 1 shows the FE-SEM morphologies for three typical samples. At a low R value of 10, the product consists of loosely-agglomerated uniform spheres with a diameter of ~120 nm (Fig. 1(a)), showing a morphology very similar to that of the lanthanide basic carbonates classically made via urea-based homogeneous precipitation from open reaction systems under magnetic stirring [7-9]. Though in the present work, reactions proceeded under a closed hydrothermal system and without stirring. MATIJEVI? and HSU [17] have also produced monospheres of Y(OH)CO₃ via reaction at 90 ℃ for 2 h in capped test tubes and under a relatively low molar ratio of urea to Y of ~33. At higher R values of 40, 60 and 100, the hydrothermal products are all microplates having sections of up to ~35 μm × 5 μm, as shown in Fig. 1(c), for the product obtained at R=60. At R=20, the product is a mixture of the submicron spheres and the microplates (Fig. 1(b)), indicating that they are on the way of morphology transformation.

Fig. 1 FE-SEM micrographs showing morphologies of three typical precursors: (a) Submicron spheres (R=10); (b) Mixtures of submicron spheres and microplates (R=20); (c) Microplates (R=60)

Figure 2 shows the XRD patterns for two representative samples of the submicron spheres (R=10) and the microplates (R=60). The submicron spheres are essentially amorphous, similar to Y(OH)CO₃ made by MATIJEVI? and HSU [17]. The microplates can be nearly indexed to the orthorhombic structured yttrium carbonate hydrate phase (Y₂(CO₃)₃·3H₂O, JCPDS file No. 25—1010), except for changed relative intensities of the diffraction peaks due to a textured distribution of the microplates on the glass holder for XRD analysis.

Fig. 2 XRD patterns for two typical precursors of submicron spheres (R=10) and microplates (R=60)

Chemical compositions of the above two samples can be further confirmed by FT-IR (Fig. 3). Indeed, the amorphous spheres exhibit a spectral characteristic of the lanthanide basic carbonate hydrate (Ln(OH)CO₃·H₂O) monospheres classically made via urea-based homogeneous precipitation [7,8]. The strong and broad absorption bands in the region of 3000-3500 cm^-1 (peaked at ~3315 cm^-1) and the shoulder near 1655 cm^-1 provide evidence for water of hydration in the structure or surface adsorbed water, and they are assignable to the O—H stretching vibrations (n₁ and n₃) and H—O—H bending mode (n₂), respectively [18,19]. The absorption band observed in the region of 3500- 3750 cm^-1 (centered at ~3536 cm^-1) is indicative of hydroxyl (OH^- groups [18,19]. The presence of carbonate ions in the molecular structure is confirmed by the appearance of absorption doublets in the region of 1350- 1600 cm^-1 (n₃ of CO₃^2-, peaked at ~1415 and 1527 cm^-1) and also by the occurrence of the absorption at ~2362 cm^-1 and the multiple absorptions ranging from 500- 1000 cm^-1 (n₂ and n₄ of CO₃^2-) [18,19]. The microplates similarly show the water of hydration band at ~3315 cm^-1 and the CO₃^2- bands but lack the absorption by OH^- at 3536 cm^-1, suggesting a hydrated normal carbonate for the microplates as revealed by XRD.

Fig. 3 FT-IR spectra for two typical precursors of submicron spheres (R=10) (a) and microplates (R=60) (b)

The chemical composition of a hydrothermal product is the result of competition among the anionic species in combining with cations, and the final composition would always have the lowest solubility to minimize the free energy of the system. Though the soluble products of the basic carbonate and normal carbonate are not available to us, there is no doubt to say that they are the most stable forms under their respective hydrothermal conditions. The hydrolysis of urea yields ammonia and carbonate ions, and thus a higher molar ratio of urea to Y+Eu would give a higher solution pH and more CO₃^2-. It is also known that CO₃^2- tends to stabilize in a more alkaline solution. These may account for the formation of the normal carbonate at the high molar ratio of urea to Y+Eu of R=40-100. At even higher R values, a yttrium ammine carbonate (NH₃·Y₂(CO₃)₃·3H₂O) phase might be obtained, as inferred from the work of MATIJEVI? and HSU [17].

3.2 Shape-dependent crystallization and luminescence of (Y_0.95Eu_0.05)₂O₃ submicron spheres and microplates

Figure 4 shows the typical FE-SEM images of the (Y_0.95Eu_0.05)₂O₃ calcined at 1000 ℃ for 4 h. Clearly, the resultant oxides largely retain the original morphologies of their respective precursors except for a slight shrinkage in particle size by annealing. The calcination products have displayed all the characteristic diffractions corresponding to the cubic structured Y₂O₃ (space group: I (206), JCPDS: 89—5591), as shown in Fig. 5. The morphology confined crystal growth was observed for the microplates. These two samples have their respective (400)/(222) intensity ratios of ~25.4% (submicron sphere) and 58.9% (microplate), against a value of 24.6% from the data file. The obvious change of the (400)/(222) intensity ratio for the microplates indicates that Y₂O₃:Eu³⁺ nucleates within the microplates along with the thermal decomposition of the carbonate precursor but the subsequent free growth of the oxide nuclei is significantly restricted by the two-dimensional plate-like morphology [16]. As the (222) and (400) planes have a dihedral angle of ~54.7? and the microplates tend to lie parallel to the surface of the glass holder during XRD analysis, the substantially higher (400)/(222) intensity ratio thus implies an enhanced exposure of the (400) facets on plate surfaces.

Fig. 4 FE-SEM micrographs showing morphologies of (Y_0.95Eu_0.05)₂O₃ oxides calcined from submicron spheres (R=10) (a) and microplates (R=60) (b) at 1000 ℃ for 4 h

Fig. 5 XRD patterns of (Y_0.95Eu_0.05)₂O₃ oxides calcined from submicron spheres (R=10) and microplates (R=60) at 1000 ℃ for 4 h

It is widely observed that Y₂O₃:Eu³⁺ phosphors show a typical photoluminescence excitation band at ~250 nm, and are ascribed to the electronic transition from the 2p orbital of O^2- to the 4f orbital of Eu³⁺ [7-16, 20, 21]. Under UV excitation at 250 nm, the Y₂O₃:Eu³⁺ spheres and plates calcined at 1000 ℃ for 4 h exhibit PL bands at essentially identical positions. As marked in  Fig. 6, all the emission bands observed at 583, 589-601, 613, and 652 nm can be assigned to the ⁵D₀→⁷F_J (J = 0, 1, 2, 3) transitions of Eu³⁺, respectively. The cubic Y₂O₃ lattice is known to have two different types of crystallographic positions for Eu³⁺ substitution: the 24d site with C₂ symmetry and the 8b site with S₆ inversion symmetry [22,23]. It is well known that the ⁵D₀→⁷F₂ electric dipole transition is hyper-sensitive to the local environment of Eu³⁺ ions [23-25]. The intensity ratio of ⁵D₀→⁷F₂ transition (monitored at 613 nm) to ⁵D₀→⁷F₁ transition (monitored at 595 nm), called asymmetry factor, has been widely used as a very sensitive and efficient probe to detect the local structure around Eu³⁺. In our present work, both the samples have similar asymmetry factors of ~11.0, indicating that the local symmetries of Eu³⁺ in Y₂O₃ do not vary significantly between the two morphologies.

Fig. 6 Emission spectra of (Y_0.95Eu_0.05)₂O₃ red phosphors calcined at 1000 ℃ for 4 h under 250 nm excitation

The strongest emission peak near 613 nm is due to the forced electric dipole ⁵D₀→⁷F₂ transition of Eu³⁺. Taking the 613 nm emission for comparison, the microplates show an intensity of ~1.33 times that of the submicron spheres. The microplates have a larger particle size and thus a denser packing of the primary phosphor crystallites compared with the submicron spheres, which allows a less scattering of the evolved light and hence a stronger luminescence.

4 Conclusions

1) Red-emission (Y_0.95Eu_0.05)₂O₃ submicron spheres and microplates were selectively processed via hydrothermal precursor synthesis (150 ℃, 12 h) followed by calcination at 1000 ℃, without using any surfactant, catalyst, or template.

2) The precursor would change from (Y_0.95Eu_0.05)- (OH)CO₃·H₂O monospheres to (Y_0.95Eu_0.05)₂(CO₃)₃·3H₂O microplates along with an increase in the molar ratio of urea to Y+Eu from 10 to 40-100.

3) Morphology confined crystal growth was observed for the microplates, inducing more (400) facets exposed on plate surfaces.

4) Under UV excitation at 250 nm, the (Y_0.95Eu_0.05)₂O₃ submicron spheres and microplates exhibit nearly identical positions of the PL bands and similar asymmetry factors (~11.0) of luminescence but the latter shows a significantly strong red emission at ~613 nm, which was ascribed to the lager particle size and denser packing of primary phosphor crystallites of the microplates.

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(Y_0.95Eu_0.05)₂O₃亚微米球、微米板片的选择合成及荧光性能的形貌依存性

朱  琦，李继光，胥 永，李晓东，孙旭东

东北大学 材料各向异性与织构教育部重点实验室，沈阳 110819

摘  要：采用先水热合成 (150 ℃, 12 h)、后煅烧(1000 ℃)来实现(Y_0.95Eu_0.05)₂O₃亚微米球形和微米板片红色荧光颗粒的形貌可控合成。通过XRD、FT-IR、FE-SEM和PL等检测手段对样品进行分析。结果表明：将尿素与Y+Eu的摩尔比由10增大至40~100，得到的前驱体由碱式碳酸盐亚微米球转变为碳酸盐微米片；经1000 ℃煅烧所得氧化物能够继承前驱体的形貌特征；板片二维形貌的限制内部晶粒自由生长，使更多的(400)晶面暴露在板片颗粒表面；在250 nm紫外光的激发下，荧光颗粒的荧光发射峰位及荧光不对称因子[I(⁵D₀→⁷F₂)/I(⁵D₀→⁷F₁), ~11]均与颗粒形貌的相关性不强，但荧光强度呈现明显的形貌依存性；微米板片颗粒的尺寸大，从而其比表面积小，因此具有更高的荧光强度(微米板片在~613 nm处的荧光强度为球形颗粒的~1.33倍)。

关键词：发光材料；粉体合成；物理性能的形貌依存性；稀土；光谱学

(Edited by LI Xiang-qun)

Foundation item: Projects (50172030, 50972025, 50990303, 51172038) supported by the National Natural Science Foundation of China; Project supported by the Liaoning BaiQianWan Talents Program, China; Projects (N110802001, N100702001) supported by the Fundamental Research Funds for the Central Universities, China; Project supported by the China Scholarship Council

Corresponding author: LI Ji-guang; Tel: +86-24-83687787; Fax: +86-24-23906316; E-mail: lijg@smm.neu.edu.cn

DOI: 10.1016/S1003-6326(11)61487-8