Rare Metal Materials and Engineering Volume 43, Issue 10, October 2014 Online English edition of the Chinese language journal

Cite this article as: Rare Metal Materials and Engineering, 2014, 43(10): 2316-2320.

Received date: October 14, 2013 Foundation item: National Natural Science Foundation of China (51071117); Major State Basic Research Development Program of China (“973” Program) (2012CB619401); the Fundamental Research Funds for Central Universities and the Science Corresponding author: Yang Sen, Ph. D., Professor, School of Science, Xi'an Jiaotong University, Xi'an 710049, P. R. China, Tel: 0086-29-82667872, E-mail: yang.sen@mail.xjtu.edu.cn

Copyright © 2014, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

ARTICLE

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Synthesis and Cathodoluminescent Properties of Y2O3: Eu3+ Phosphor with Micron-sized Controllable Particles Wang Jieqiong, Yang Sen, Cao Zijun, Zheng Aqun, Sun Zhanbo, Song Xiaoping MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter & State Key Laboratory for Mechanical Behaviour of

Materials, Xi'an Jiaotong University, Xi'an 710049, China

Abstract: The luminescent properties of phosphors are sensitive to their morphologies, particularly to the sizes of phosphor particles. The optimal morphology of the phosphor for the field emission display (FED) is the microsized sphere-like particles. However, less attention has been paid to the phosphors with micron-size controlled particles. In this paper, we reported an improved co-precipitation method to synthesize the red-emitting Y2O3:Eu3+ phosphor with the particle sizes controlled in several microns. The optimum phosphors were obtained at the calcination temperature of 1050 °C and pH=5.5, showing the sphere-like particles with the size distribution from 0.9 μm to 3.1 μm. Compared with the commercial phosphors, such optimum phosphors greatly enhance the cathodoluminescent properties and the corresponding luminescence intensity under the low excitation voltage of 1 kV is increased by 50%. Results indicate an effective way to develop the high-quality FED phosphors.

Key words: optical materials; luminescence; field emission; precipitation

A field emission display (FED) is generally considered as the most promising flat panel display due to that it combines the advantages of cathode ray tube (CRT) display and liquid crystal display (LCD), namely high contrast level and high compact package [1]. Such a FED shares the same display principle with CRT display, where the micro-tip electron guns are used as the cathodes to provide electrons striking a phosphor to produce a color image. Thus, the key technique for fabricating the FED is how to develop the high-quality phosphors to guarantee the high brightness under the low excitation voltage [2-5].

Extensive investigations have revealed that the luminescent properties of phosphors are sensitive to their morphologies, particularly to the sizes of phosphor particles [6-8]. The study of controllable particle size is currently focused on two aspects: one is the commercial phosphor with a relatively larger particle size distributing from a few microns to dozens of microns and the other is the nano-sized phosphor with the particle size distribution from a few nanometers to hundreds

of nanometers [9]. As the large particle may produce more excited electrons to scatter the excitated beam, the commercial phosphors cannot provide sufficient luminescence intensity to support the high brightness under a low voltage. On the other hand, the nano-sized phosphors usually cause the surface- damaged region (e.g. amorphous surface layer) in the particles and thus result in a detrimental effect on phosphor efficiency due to the size limit of electron penetration depth [10]. The both phosphors mentioned above are unsuitable for developing the FED phosphors. To our knowledge, the optimal morphology of FED phosphors is the sphere-like particles with a few microns[11]. So far, few attentions have been paid to the phosphors with controlled particles distributing in several microns scale.

In this paper, we reported an improved co-precipitation method, which can control the particle sizes within a few microns, to synthesize the red-emitting Y2O3:Eu3+ phosphors. The optimum phosphors demonstrate the sphere-like particles with the size distribution from 0.9 μm to 3.1 μm. Compared

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with the commercial phosphors, such optimum phosphors greatly enhance the cathodoluminescent properties.

1 Experiment

Y2O3 (99.99%), Eu2O3 (99.99%), nitric acid (analytical grade) and ammonium hydrogen carbonate (analytical grade) were used as the starting materials. According to the stoichiometric composition (Y0.94Eu0.06)2O3, the yttrium and europium oxides were dissolved into aqueous nitric acid to form 0.5 mol/L nitrate solution; the ammonium hydrogen carbonate was dissolved into the distilled water to form the precipitant solution. Subsequently, the nitrate solution and precipitant solution were simultaneously titrated into the surfactant solution (i.e., PEG-2000) at the temperature of 50 °C to engender the precursor precipitation. Thus, the pH value of precipitation process can be adjusted by controlling the titration amount of precipitant solution. Consequently, the size of phosphor particle can be controlled as a result of controllable pH value, and therefore we designated this pH value controlled process of simultaneous titration as the improved co-precipitation method. After roasting the precursor at high temperature, the phosphor powders were finally formed. As a comparison, the commercial Y2O3:Eu3+ phosphors were provided by the Grirem Advanced Materials Co., Ltd.

We used the Fourier transform infrared spectroscopy (FTIR) to distinguish the precursors precipitated at different pH values and the thermogravimetry (TG) to analyze the calcination process. The crystal structure was determined by the X-ray diffractometry (XRD). The morphology of phosphor was investigated by the scanning electron microscope (SEM, JEOL-7000F) and the particle size distribution was analyzed by the laser scattering particle size distribution analyzer (Horiba LA-950V2). The cathodoluminescent (CL) properties were measured by a fiber-optic coupled spectrometer (SL-300), where the phosphors were excited by the electron beam under the excitation voltage of 1~5 kV.

2 Results 2.1 Fourier transform infrared spectroscopy and X-ray

diffractometry Fig.1 shows the FTIR spectra of precursors precipitated

from the solutions with different pH values. It is clearly seen that there exists the CO3

2- in the precursors at both pH=5.5 and pH=7.5 due to the absorption peaks [12-14] in the wavenumber of 757, 858, 1517, and 1427 cm-1. This means that the precipitation process is followed by

2Y(NO3)3+6NH4HCO3+nH2O→ Y2(CO3)3·nH2O↓+6NH4NO3+CO2↑+3H2O (1)

For the sake of simplicity, the minor Eu3+ is not included in the above equation. Thus, the precursor of Y2(CO3)3 hydrate is formed at pH=5.5, as shown in Fig.1a. When the pH value increases to 7.5, the Y2(CO3)3 precipitation continues to react

Fig.1 FTIR spectra of the precursors precipitation prepared by the

improved co-precipitation method with the controllable pH value: (a) pH=5.5 and (b) pH=7.5

with the NH4HCO3 solution to form the precursor of (NH4)2CO3Y2(CO3)3 hydrate by the following chemical process:

Y2(CO3)3·nH2O+2NH4HCO3 → (NH4)2CO3Y2(CO3)3· mH2O↓+(n－m+1)H2O+CO2↑ (2)

Correspondingly, the very typical absorption peaks of NH4+

occur in the FTIR spectra at the wavenumber of 1337 cm-1 due to the asymmetrical stretching vibration mode, as shown in Fig.1b. Compared with the traditional co-precipitation method without controllable pH value, the improved co-precipitation method with controllable pH value produces different precursors. As will be seen later (Fig.3), such a controllable pH value results in the phosphors with controllable micron-sized particles.

Fig.2 shows XRD patterns of Y2O3:Eu3+ phosphors pre- pared at pH=5.5 and pH=7.5 during the calcination process, at different temperatures. It is seen that there are no obvious diffraction peaks for the precursors. This is due to the amorphous nature of chemical precipitation hydrate. With the increase of calcination temperature, such hydrates gradually dehydrate as observed by the TG measurement in the insert of Fig.2. Thus, the cubic Y2O3:Eu3+ phase gradually grows from the amorphous precursors, and consequently results in the enhancement of diffraction intensity with increasing of tempe- rature. When the temperature reaches 950 °C, the precursors are totally crystallized and all the diffraction peaks correspond to the cubic transitions of the Eu3+ structure of Y2O3:Eu3+ phase (JCPDS card 25-1011). It should be noted that there is no diffraction peak corresponding to other impurity phase. This means that the Eu3+ ions completely enter the Y2O3

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Fig.2 XRD patterns of Y2O3:Eu3+ phosphors during the calcination

process at different temperatures with different pH values: (a) pH=5.5 and (b) pH=7.5 (the inserts show the thermo- gravimetry analysis during the calcination process)

crystal to occupy the Y3+ sites in the phosphors, which is helpful to the enhancement of cathodoluminescent properties. 2.2 Morphology

Fig.3 shows the optimum morphologies of Y2O3:Eu3+

phosphors investigated by SEM. For pH=5.5, the optimum morphology demonstrates the small sphere-like particle with the size less than 2 μm, as shown in Fig.3a. For pH=7.5, the optimum morphology shows the average particle size of about 3 μm, as shown in Fig.3b. Clearly, both particle sizes are controlled within several microns. This is mainly attributed to the improved co-precipitation method with controllable pH value. As a comparison, the morphology of commercial phosphor is presented in Fig.3c. It is seen that such morphology shows relatively large particles with irregular shape and thus results in relatively low cathodoluminescent intensity, as will be seen in Fig.5.

To well reflect the particle size distribution, we used the laser scattering particle size distribution analyzer to measure the particle size distribution. Fig.4 shows the curves of particle size distribution of the Y2O3:Eu3+ phosphors. Clearly, the phosphors prepared at pH=5.5 show the narrowest particle size distribution of 0.9~3.1 μm; the commercial phosphors show the broadest particle size distribution of 5.4~13.6 μm; the phosphors prepared at pH=7.5 show the moderate particle

Fig.3 Optimum morphologies of Y2O3:Eu3+ phosphors prepared

under different conditions: (a) pH=5.5, (b) pH=7.5, (c) the corresponding commercial phosphor

Fig.4 Curves of the particle size distribution for Y2O3:Eu3+

phosphors size distribution of 2.0~5.0 μm. Therefore, the particle size of phosphors can be controlled by adjusting the pH value of titration process in the improved co-precipitation method. 2.3 Cathodoluminescent properties

Fig.5 illustrates the cathodoluminescent (CL) spectra of Y2O3:Eu3+ phosphors with the optimum morphologies prepared at pH=5.5 and pH=7.5, as well as the commercial phosphor under the excitation voltage of 1 kV. It is shown that there are five narrow emission bands, which are denoted as the 5D0→7FJ (J=0, 1, 2, 3, 4) [15-17]. The strongest transition of 5D0→7F2 occurs at approximately 610 nm, corresponding to the red light emission. As seen in Fig.5a, it is a typical red-emission spectrum and furthermore the CL intensity is very sensitive to the size of phosphor particles. The highest CL intensity of 5D0→7F2 transition under low excitation voltage of 1 kV occurs in the phosphors prepared at pH=5.5, which is about 1.5 times of that of commercial phosphor. Obviously, this is mainly due to its narrow particle size distribution within several microns. With increasing excitation, the CL intensities are further enhanced, as shown in Fig.5b.

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Fig.5 Luminescent properties of Y2O3:Eu3+ phosphors: (a) cathodo-

luminescent spectra of Y2O3:Eu3+ phosphors with the optimum morphologies prepared at pH=5.5 and pH=7.5, as well as the commercial phosphor under the excitation voltage of 1 kV; (b) accelerating voltage dependence of cathodolu- minescent intensity of Y2O3:Eu3+ phosphors

Thus, the Y2O3:Eu3+ phosphors synthesized from the improve co-precipitation method with controllable pH value are suitable to the application in the FED.

3 Discussion As shown above, the CL intensity is very sensitive to the

morphologies of phosphors and the Y2O3:Eu3+ phosphors with narrow small particle size distribution show much higher CL intensity than that of commercial phosphor. This is attributed to the deeper penetration depth L of the electrons into the phosphor’s body [18,19], which can be estimated by the following empirical formula:

L=250 (A/ρ)(E/Z1/2)n (3) where n=1.2/(1－0.29log10Z), A is the atomic or molecular mass of the material, ρ is the bulk density, Z is the atomic number or the number of electrons per molecule in the case of compounds, and E is the accelerating voltage (kV) [20]. For Y2O3:Eu3+ phosphor, the atomic mass A=226, the atomic number Z=102, and the bulk density ρ=5.01 g/cm3. Thus, the estimated electron penetration depth at 1 kV accelerating voltage is about 1.5 nm. This is an average electron

penetration depth in the Y2O3:Eu3+ phosphor and the activation volume of Eu3+ ion is related to the phosphor particle size. As the particle sizes of obtained Y2O3:Eu3+ phosphors are much smaller than that of commercial phosphors, the total surface areas of particles in the obtained phosphors are much larger than that in the commercial phosphors. Thus, the obtained phosphors exhibit much larger activation volume than the commercial phosphor. The larger the activation volume occupying the phosphor body, the more the plasma will be produced. Thus, the more Eu3+ ions are excited and result in the increase of CL intensity. This is why the CL intensity of the obtained Y2O3:Eu3+ phosphors are much higher than that of commercial phosphor as shown in Fig.5a. Also for the same reason, the CL intensities of the obtained phosphors increase with the increase voltage and bombardment current as shown in Fig.5b. Clearly, phosphors synthesized from improved co-precipitation method are promising for application in field emission display devices.

4 Conclusions 1) The red-emitting Y2O3:Eu3+ phosphors with micron-size

controllable particles can be synthesized by the improved co-precipitation method.

2) By adjusting the pH value of titration process and subsequent roasting at high temperature, the phosphors with sphere-like particles in the size distribution of several microns can be obtained. Compared with the commercial phosphor, their luminescence intensities under low excitation voltage of 1 kV increase by 50%. Obviously, the improved co-precipitation method is an effective way to develop the high-quality FED phosphors.

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