Synthesis of Y₂O₃:Eu³⁺ phosphors by surface diffusion and their photoluminescence properties

ZHANG Ming(张 明), LI Xin-hai(李新海), WANG Zhi-xing(王志兴), HU Qi-yang(胡启阳), GUO Hua-jun(郭华军)

School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China

Received 21 October 2008; accepted 13 March 2009

Abstract: Y₂O₃:Eu³⁺ phosphors were synthesized by the surface diffusion method (SDM). X-ray diffractometry (XRD), scanning electron microscopy (SEM), and energy dispersive spectrometry (EDS) were used to characterize the structure, morphology and component of Y₂O₃:Eu³⁺ phosphors. The photoluminescent (PL) properties were also investigated. The results reveal that the PL intensity of Y₂O₃:Eu³⁺ phosphors prepared by the surface diffusion method (SDM) is much higher than that prepared by homogeneous co-precipitation. The luminescence efficiency of the sample (Y_0.997, Eu_0.003)₂O₃ prepared by the SDM is almost 1.9 times that by homogeneous co-precipitation. The concentration of Eu³⁺ in the phosphor Y₂O₃:Eu³⁺ prepared by the surface diffusion can be reduced greatly owing to the activator, Eu³⁺ ions, distributing mainly in the outer layer of the phosphors where the photon generation process occurs.

Key words: phosphor; rare earth; surface diffusion method; photoluminescence properties

1 Introduction

Rare earth luminescent materials have considerable application in devices involving fluorescent lighting devices and display devices, such as tricolor lamps, cathode ray tubes (CRTs), liquid crystal displays (LCDs), field emission displays (FEDs), and plasma display panels (PDPs)[1-3]. Among them, europium-doped yttrium oxide (Y₂O₃:Eu³⁺) fulfills all the requirements for a good red-emitting phosphor because of its excellent luminescence efficiency, color purity and stability. It can be easily excited by 254 nm from mercury radiation and its sharp emission is located at 613 nm and all other emission lines are weak[4].

The conventional preparation method of Y₂O₃:Eu³⁺ phosphor is based on solid state reaction, in which all materials are mixed by grinding mill and then undergo a high temperature reaction[5]. It has been reported that the optical properties of luminescent materials can be greatly affected by the particle size and morphology. This motivated the development of other alternative preparation techniques[6-10], such as combustion synthesis, hydrothermal process, microwave irradiation method, co-precipitation method, spray pyrolysis and sol-gel route. Recently, it has been studied that the penetration depth of electrons increases exponentially with increasing the accelerating voltage. When the accelerating voltage was less than 1 kV, the penetration depth was less than several nanometers so that most of electrons were consumed in the surface layer[11-12]. As a result, only a thin outer layer of the micron-sized commercial phosphors took part in the photon generation process. With nanoparticles, the penetration by low-voltage electrons was sufficient for efficient material utilization[13]. However, the activators in the center of the phosphor prepared by these methods do not take part in the photon generation process because the energy of 254 nm radiation is too low to reach several microns. At the same time, the rare earth oxides are very expensive, limiting their application to a great extent. It has been found that coating relatively inexpensive cores with Y₂O₃:Eu³⁺ shell will reduce the amount of expensive rare earth phosphors used[14-15]. However, there is a problem that the intensity of these core-shell particles is lower than that of bulk Y₂O₃:Eu³⁺.

Our strategy to solve these problems has focused on materials with higher luminescent intensity and lower cost. In this work, we developed a simple and economical way to embed the activator Eu³⁺ into the outer surface layer of the bulk Y₂O₃; and surface diffusion method (SDM), in order to save the cost of the phosphors, was used.

2 Experimental

2.1 Materials

Yttrium oxide (Y₂O₃, 99.99%) and europium oxide (Eu₂O₃, 99.999%) were obtained from Hunan Research Institute of Rare Earth (Changsha, China). NH₄HCO₃ and HNO₃ were of analytical grade and used without further purification.

2.2 Sample preparation

Y₂O₃:Eu phosphors were prepared by surface diffusion and homogeneous co-precipitation. Surface diffusion method was follows. Stoichiometric amounts of Eu₂O₃ was firstly dissolved in nitric acid and evaporated to dryness, then dissolved in distilled water to form 0.02 mol/L europium nitrate solution. NH₄HCO₃ was dissolved in distilled water to form 0.07 mol/L solution. Secondly, 0.02 mol Y₂O₃ was ultrasonically dispersed in 200 mL distilled water for 30 min, then Eu(NO₃)₃ and NH₄HCO₃ solutions were slowly dripped into a continuously agitated Y₂O₃ suspension solution, simultaneously. The temperature of the solution was stabilized at 50 ℃ in a water bath. Soon, the precipitation was achieved on the Y₂O₃ surface. The precursor precipitate was filtered, washed with distilled water several times and dried at 80 ℃ for 24 h. After that, the precursor was ground and heated in air at 1 200 ℃ for 3 h. The resulting calcined Y₂O₃:Eu³⁺ samples were designated as SDM-n, where n stands for the concentration of Eu³⁺. For example, SDM-3 means that the concentration of Eu³⁺ is 0.3% (molar fraction).

For comparison, Y₂O₃:Eu³⁺ was prepared by traditional homogeneous co-precipitation method. The samples were designated as HCP-n, where n stands for the concentration of Eu³⁺. For example, HCP-3 means that the concentration of Eu³⁺ is 0.3% (molar fraction).

2.3 Characterization

X-ray diffraction (XRD) measurements of powder samples were carried out on a Rigaku diffractometer equipped with Cu K_α at 40 kV and 300 mA. Scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) were used to characterize the morphology and the component of particles by a Hitachi S530 spectrometer. The excitation and emission spectra of the samples were taken on a Hitachi F-4500 fluorescence spectrophotometer using a Xe lamp as the excitation source. All spectra were measured at room temperature.

3 Results and discussion

3.1 Luminescence properties

The excitation and emission spectra of (Y_0.997, Eu_0.003)₂O₃, synthesized by homogeneous co- precipitation (HCP-3) and surface diffusion (SDM-3) are shown in Fig.1. The excitation spectrum (Fig.1(a)) consists of a broad band with the maximum at 242 nm due to the charge transfer (CT) band, which relates to the electronic transition from the 2p orbital of O^2- to the 4f orbital of Eu³⁺. In Fig.1(b), the emission spectra of the two samples are composed of ⁵D₀→⁷F_J(J=0, 1, 2, 3, 4) emission line of Eu³⁺, dominated by the hypersensitive red emission ⁵D₀→⁷F₂ (forced) electric dipole transition of Eu³⁺ at 609 nm. All the other magnetic dipole transitions originating from the ⁵D₀ state to ⁷D_J (J=0, 1, 3, 4) level could also be observed, as labeled in Fig.1. Fig.1 demonstrates that the luminescent intensities of the two samples change greatly. The luminescent intensity in the SDM-3 increases remarkably over that in the HCP-3, for either the CT or the f-f inner-shell excitations. The consistency of the phenomenon was verified with different sets of such phosphor prepared for different times. To have a quantitative idea of the enhancement, the integrated luminescence efficiency in the wave length region of 550-640 nm, in each case, was determined by calculating the area under the curve. The ratio of the integrated luminescence efficiency of the two samples (SDM-3 to HCP-3, 27947.5?14725.4=1.9) shows that the luminescence efficiency of the SDM-3 sample is almost 1.9 times that of the HCP-3 sample.

Fig.1 Photoluminescence excitation (a) and emission (b) spectra of SDM-3 and HCP-3

To investigate the PL behaviors of Y₂O₃:Eu³⁺, the electron penetration depth into the phosphors should be considered. According to Feldman’s equation, the penetration depth of an electron will increase exponentially with increasing excitation energies[16]. The penetration depth of an electron is given as follows:

                            (1)

where R is the penetration depth of an electron; M_A is molecular mass; ρ is bulk density; Z_m is mean atomic number; n=1.2/(1-0.29lgZ_m); and E₀ is excitation energy. JUNG et al[12] suggested that the penetration depth   of a electron in (Y, Zn)₂O₃:Eu³⁺ phosphor increases exponentially with increasing the accelerating voltage. For Y₂O₂S:Tb, Zn₂SiO₄:Mn and La₂O₂S:Tb phosphors, GUTALER[17] calculated the penetration depth to be  5 nm at a screen potential of 300 V.

According the following equation, the energy of a photon of 254 nm UV is 4.88 eV:

E=hc/λ                                      (2)

where h is Planck’s constant; c is the velocity of light. The penetration depth of the photon is within several nanometers. It can be deduced that only the Eu³⁺ activator ions in the outer layer of micro-sized Y₂O₃:Eu³⁺ take part in the photon generation process, and most of the Eu³⁺ ions in the inner layer do not. According to the phenomenon that the PL intensity of SDM-3 increases greatly over that of HCP-3, we can deduce that the concentration of the Eu³⁺ ions in the outer layer of SDM-3 is more than that of HCP-3 because the PL intensity greatly depends on the concentration of activator ions.

3.2 Electron microscopic studies

Fig.2 shows the scanning electron micrographs of the Y₂O₃ and SDM-3. Obviously, the surface of Y₂O₃ is smooth and clear. After being coated, the surface of SDM-3 is covered with a discontinuous layer.

Fig.2 SEM photographs of Y₂O₃ (a) and SDM-3 prepared by surface diffusion method (b)

X-ray energy dispersive spectrum performed on the particles in Fig.3 suggests the existence of Y and O, as well as Eu. The Eu element was not detected clearly due to its low concentration.

Fig.3 EDS spectrum of SDM-3 prepared by surface diffusion method

3.3 X-ray diffraction patterns

In order to assess the crystal lattice of each phosphor, XRD was used. Fig.4 and Table 1 show respectively the XRD patterns and the crystal cell parameters of the (Y_1-xEu_x)₂O₃ synthesized by homogeneous co-precipitation (HCP) and surface diffusion method (SDM). Both samples have pure cubic Y₂O₃ phase with high crystallinity.

Fig.4 XRD patterns of SDM-3 (a) and HCP-3 (b)

Table 1 Cell parameters of (Y_1-xEu_x)₂O₃

As shown in Table 1, with increase in the concentration of Eu³⁺ from 0 to 3%, the lattice constant of the phosphors increases. Since the ionic radius of europium ion (0.095 nm) is larger than that of yttrium (0.09 nm), the substitution of Eu in cation sites takes place. However, the lattice constant of the sample SDM-3 has a special phenomenon. It is somewhat smaller than that of HCP-3, which has the same initial concentration of Eu³⁺ ion, and is close to that of Y₂O₃, in which the concentration of Eu³⁺ ion is 0. This indicates that the concentration of Eu³⁺ ion in the inner parts of the particles is small.

4 Conclusions

1) Cubic crystalline Y₂O₃:Eu³⁺ was successfully prepared using a novel approach, surface diffusion method. The results of the XRD, SEM and EDS investigation show that Eu³⁺ ions exist in the outer layer of the phosphors.

2) Compared with the traditional homogeneous co-precipitation method, the phosphors prepared by surface diffusion method show excellent PL properties. In the wave length region of 550-640 nm, the luminescence efficiency of the SDM-3 sample is almost 1.9 times that of the HCP-3 sample.

3) The concentration of Eu³⁺ in the phosphor Y₂O₃:Eu³⁺ prepared by the surface diffusion method can be reduced greatly owing to the activator, Eu³⁺ ions, distributing mainly in the outer layer of the phosphors where the photon generation process occurs. It is an efficient way to save the cost of the phosphors while the luminescent properties are retained.

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Foundation item: Project(2007CB613607) supported by the National Basic Research Program of China

Corresponding author: LI Xin-hai; Tel: +86-731-88836633; E-mail: xhli@mail.csu.edu.cn

DOI: 10.1016/S1003-6326(09)60106-0

(Edited by YANG Hua)