Synthesis and reaction kinetics for monodispersive Y2O3:Tb3+ spherical phosphor particles

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Synthesis and reaction kinetics for monodispersive

Y

2

O

3

:Tb

3+

spherical phosphor particles

J.M. Sung, S.E. Lin, W.C.J. Wei

∗

Department of Material Science and Engineering, National Taiwan University Taipei 106, Taiwan, ROC Received 15 June 2006; received in revised form 28 October 2006; accepted 3 November 2006

Available online 5 January 2007

Abstract

Monodispersive Y2O3:Tb3+spherical phosphor was synthesized by homogenous precipitation process. By using X-ray diffractometer,

thermo-gravimetric analysis (TGA), transmission and scanning electron microscopes, and inductively coupled plasma (ICP)-optical emission spectrometry (OES), the precipitation behaviors were analyzed. The results indicated that the kinetics of the hydrolysis reaction of Y3+_{and Tb}3+_{was a zero-order}

reaction, controlled by the decomposition of urea with an activation energy of 131 kJ/mol. Besides, the effect of urea concentration, reaction temperature, and aging time for the control of the precipitate size were also investigated. The dopant Tb3+_{in the solution did not significantly}

influence the precipitation of the yttrium component. After 800◦C calcination, the Y2O3:Tb3+powder showed nano-crystalline characteristics and

Keywords: Y2O3; Phosphor

1. Introduction

Phosphors play an important role in display applications. Among all phosphors, yttria (Y2O3) is a well known host

mate-rial which has been widely used in cathode radiation tube (CRT), field emission display (FED), and thin film electrolu-minescence (TFEL) devices.1 _{According to previous studies,}

yttria doped with specific rare earth elements or materials shows interesting photoluminescent (PL) and electroluminescent (EL) properties.2,3 In addition to the optical properties, yttria has a high melting point (2698 K) with excellent chemical stability and low volatility in vacuum.

Traditional processes for synthesis of phosphors are mainly divided into two methods: solid-state reaction and wet-chemical process. The former method is popular for commercial produc-tion of phosphors. However, solid-state reacproduc-tion may experience the contaminations form ball milling or non-well-mixed dopant (second phase). Irregular morphologies and wide size distribu-tion are non-favorable for the phosphor powders obtained by such solid-state reaction.4

∗_{Corresponding author. Tel.: +886 2 23632684; fax: +886 2 23634562.}

E-mail address:wjwei@ntu.edu.tw(W.C.J. Wei).

Instead of solid-state reaction, wet-chemical process is con-sidered better for its molecular level mixing. Molecular level mixing offers the possibilities for controlling the chemical com-position, obtaining better homogeneity, single phase, and higher surface area powder.4,5Generally, wet-chemical processes refer to homogeneous precipitation method, hydrothermal method, and sol–gel method. Irregular shape and agglomerated pow-der was obtained often from sol–gel method.4 Among all wet-chemical processes, homogenous precipitation has the advantage of generating fine particles with relatively narrow size distribution compared to others. And it is also considered the best process for the ability to control the morphology of the precipitates.

When a specific type of organic compound, e.g., urea,6slowly decomposes in the solution which contains metal cations, the generated anions will combine with metal cations and form the precipitation nuclei. By adjusting the concentration of reactants, pH value of the solution, reaction temperature, and reaction time, we can easily control the concentration of nuclei and the growth rate of the precipitates. That means we can control the mor-phology, size, and size distribution of the precipitates.7,8 For example, Boschini et al.9_{, prepared nano-sized barium zirconate}

powder by thermal decomposition of urea in an aqueous solu-tion containing Ba2+and Zr4+. Another case was reported by Lin

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different aging time, temperature and urea additions. The reaction kinetics of the hydrolysis reaction of Y3+ and Tb3+ cations at the temperature 65–85◦C is investigated in this work.

2. Experimental procedure

2.1. Materials

Yttrium chloride (YCl3, 99.9% pure, Beyoung Chemical

Corp., Ltd., China), terbium nitrate (Tb(NO3)3, 99.9% pure,

Alfa Aesar, A Johnson Matthey, USA), and urea ((NH2)2CO,

99.5% pure, Acros, Geel, USA) were used for the preparation of Y2O3:Tb spherical particles.

2.2. Sample preparation

The solution concentration of YCl3was kept at 0.009 mol/L,

Tb(NO3)3 at 1.7× 10−4mol/L, and the urea concentrations

adjusted from 0.25 to 1 mol/L. The formulation and exper-imental condition are shown in Table 1. First, the urea was dissolved in de-ionized water, and then added to the diluted YCl3and Tb(NO3)3solution. Second, the mixture was sealed

in a glass jug and aged at the specific temperature. Finally, after aging the mixture for a specific period, the precipitate in the solution was separated by centrifugation for 30 min at 5000 rpm and the supernatant solution was discarded. This centrifuging treatment was repeated at least three times. For each cycle, the centrifuged particles were re-dispersed in de-ionized water.

transmission electron microscopes (Field Emission SEM with EDS, LEO 1530, Cambridge, UK; TEM 100 CX II, JEOL Co., Tokyo, Japan). The crystalline phase was examined by X-Ray diffractometry (XRD, Philips PW1710, Philip Co., Eindhoven, The Netherlands). The particle size was statistically measured from the SEM micrographs based on at least 300 particles.

3. Results and discussion

3.1. Hydration reactions

According to Aiken et al.5, homogenous precipitation reaction mechanism for yttrium precipitate can be classified in three steps. The overall chemical reactions can be described as follows:

Y3++ (NH2)2CO + (3 + x)H2O

→ Y(OH)CO3·xH2O+ 2NH4++ H+ (1)

The above equation shows that one mole of urea would react with one mole of yttrium cations to form an equi-molar yttrium compound (Y(OH)CO3·1H2O). Accordingly, the pH value of

the solution will decrease due to the generation of protons as the reaction proceeds.

In fact, the variation of pH value versus aging time inFig. 1

can be divided into three stages. In the first stage (for sample Y-3 was 0–1 h), the pH value increased with aging time. This phenomenon was caused by the decomposition of urea and the release of OH−anions into the solution. In the second stage (for sample Y-3 was 1–5 h), the pH value tended to slightly decrease due to the hydrolysis reaction of Y+3, as shown in Eq.(1). The

Table 1

Formulation and experimental conditions used in this study

Sample [Y3+] (mol/L) [urea] (mol/L) [Tb3+] (mol/L) Reaction temperature (◦C) Longest aging time (h)

Y-1 9.0× 10−3 0.5 1.7× 10−4 65 24 Y-2 9.0× 10−3 1.0 1.7× 10−4 65 20 Y-3 9.0× 10−3 0.25 1.7× 10−4 75 7 Y-4a 9.0× 10−3 0.5 1.7× 10−4 75 7 Y-4b 9.0× 10−3 0.5 0 75 7 Y-5a 9.0× 10−3 1 1.7× 10−4 75 5 Y-5b 9.0× 10−3 1 0 75 5 Y-6 9.0× 10−3 0.25 1.7× 10−4 85 5 Y-7 9.0× 10−3 0.5 1.7× 10−4 85 4

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Fig. 1. pH value of the solutions in this study plotted against the aging time.

ICP-OES data revealed that the concentration of Y+3cations in the solution decreased (over-saturated) in the second stage and the solution became turbid. The nuclei of the precipitate started to grow in this stage. Finally, in the third stage (for sample Y-3 was longer than 5 h), the pH value increased again. This was because of the complete reaction of the cations in the solution to form the precipitate. However, the residual urea would keep on decomposing and release OH−, which resulted in increasing the pH value.

3.2. Precipitation kinetics

Fig. 2shows the ICP-OES results of the residual concentra-tion of Y3+and Tb3+cations in the solution plotted against the aging time. As for Y3+cations (Fig. 2(a)), in the case of sample Y-3, the [Y3+] is constant for aging in 2 h and almost used up when aging time is longer than 7 h. Moreover, it also provided the evidence that, after aging for longer than 7 h, the particle size would remain constant throughout the reaction, which we will discuss later. Similar situations were observed for other cases. [Y3+] is almost constant for a few hours of the reaction and then falls to 2± 1 ppm level when aged for 20, 5, 4, 4, and 3 h for Y-1, Y-4a, Y-5a, Y-5b, and Y-7 cases, respectively. It was noticed that, in the cases of Y-5a and Y-5b, the addition of Tb3+did not have any apparent influence on the precipitation of [Y3+].

The reaction of Tb3+cations in solution is similar to that of [Y3+], as shown inFig. 2(b). However, according to the results of ICP-OES, we found that the initial precipitation point for [Tb3+] was about half an hour earlier than for [Y3+]. This may be due to a lower solubility of Tb3+cations in the solution. Therefore, [Tb3+] reached the over-saturated point half an hour earlier than [Y3+] cations to precipitate. This phenomenon did not cause a serious problem for the “co-precipitation” of [Tb3+] with [Y3+] during the synthesis.

After fitting the ICP-OES results, as shown inFig. 2(a and b), a linear relationship between the concentration of the residual cations and the aging time of the solution could be obtained, in which the residual [Y3+_{] and [Tb}3+_{] were higher than 2}_{± 1 ppm.}

For a zero-order reaction, the slope of the linear fitting line

Fig. 2. Residual: (a) [Y3+_{] and (b) [Tb}3+_{] in the solutions for specified conditions}

of this study, plotted against the aging time.

represents the reaction rates of the precipitate.

R = −d[C]

dt = k[C]

0 ₍₂₎

where k is the reaction rate constant (ppm/h), R the reaction rate, and [C] is the concentration of cations ([Y3+] or [Tb3+] in this study) in solution.

From ICP-OES results, we can also find that as the reaction temperature changes from 65 to 85◦C, the hydrolysis reaction rate of Y3+and Tb3+increases with the increase of urea addition. The slope of the reaction rate versus urea addition is defined as the reaction rate constant k and shown inFig. 3. The values of reaction rate constants of 65, 75, and 85◦C were 38.8, 221.4, and 575.3, respectively. The difference of the three cases results from the different decomposition rates of urea at the specified temperatures.

The activation energy for the precipitation of yttrium com-pound can be considered as the energy required to overcome the energy barrier for the reaction. The reaction rate constant k(T) varies as a function of the reciprocal absolute temperature shows as below: k(T ) = k0exp −Q RgasT (3)

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Fig. 3. The relationship between the reaction rate (ppm/h) of Y3+cations and urea concentration.

where k0is the pre-exponential factor, and Rgasthe gas constant

(J (mol K)−1), T is in absolute or Kelvin units.

An Arrhenius plot of the reaction rate constant of 65, 75, and 85◦C is also shown in Fig. 4. Shaw and Bordeaux11 reported the decomposition behavior of urea in aqueous solution at 65–100◦C. Recently, Lin and Wei10 showed the reaction kinetics of [In3+] with the decomposition of urea. The activation energy reported by Shaw (Q1) and by

Lin (Q2) was about 128 and 133 kJ/mol, respectively. The

activation energy for the precipitation of Tb3+ doped yttrium compound, denoted as Q, in this study was about 137 kJ/mol. Our result is in a good agreement with the reports of the others, implying that the factor controlling the mechanism of reactions in this study is indeed the decomposition of urea.

Fig. 4. Arrhenius plot of the reaction rate constant vs. the reciprocal Kelvin temperature. The activation energy for the decomposition of urea reported in literatures10,11_{was about 128 and 133 kJ/mol, respectively. The activation energy}

(Q) for the precipitate of yttrium compound in this study is 137 kJ/mol.

Fig. 5. SEM micrographs of the sample Y-4a (a) as-dried, (b) after calcination at 800◦C for 2 h. Aging time in the cases was 7 h.

3.3. Microstructural observation

Fig. 5(a and b) show the morphologies of the yttrium com-pounds with Tb3+ (samples Y-4a). SEM micrographs show that the morphology (size and shape) of the precipitation in both cases are similar. Size measurement indicated the aver-age particle size would reduce from about 110 nm (as-dried) to 90 nm (as-calcined at 800◦C for 2 h) for the Y-4a sample. The as-prepared particles should be porous, but sintered to higher density by the calcination. Later, TEM results (Fig. 9) will be presented to reveal the interior of the calcined particles.

The effect of urea concentration versus the average particle size is shown inFig. 6(a). It can be seen that the greater the urea concentration, the smaller the mean particle size, the more the nucleus density is generated in the solution per unit time. The high urea concentration would eventually lead to a smaller particle size.

Fig. 6(b) shows the variation of mean particle size of sample Y-4a versus the aging time during reaction period. In this case, up to 3 h of aging time, no obvious precipitate could be observed. However, the particle size grew from about 75 to 110 nm as the aging time increased from 3 to 5 h. The mean particle size

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Fig. 6. Mean particle size plotted against (a) the urea concentration (after complete reaction of samples Y-3, Y-4a, and Y-5a), (b) the reaction time of Y4-a, (c) size variation [a3_–_a3

o] vs. normalized aging time [t–to] plotted in log–log scale, (d) mean particle size vs. the reaction temperature (after complete reaction samples of

Y-1, Y-4a, and Y-7). All of the samples reported here were measured in as-synthesized state.

remained constant when aged longer than 5 h. It resulted from the complete reaction of the [Y3+] reactants (Fig. 2). The precipitate was no longer growing as holding for more than 5 h.

Fig. 6(c) shows the analysis results of the growth behavior shorter than 6 h. As the initial mean particle size ao= 75 nm

is taken, the growth kinetics (a3–a3o) show a linear

relation-ship between the normalized aging time (t–to) in the log–log

plot. The kinetics of this case performed an Ostwald ripening phenomenon,12 of which the diffusion-limited growth of the precipitates in the system has the t1/3time dependence.

The effect of reaction temperature versus mean particle size is shown inFig. 6(d). As observed from the figure, the mean particle size is smaller at higher reaction temperature. A higher reaction temperature generates more nuclei at the beginning of the reaction.

Fig. 7 shows the X-ray diffraction results of completely reacted sample Y-4a as-dried and after calcination at 800◦C for 2 h. From the XRD spectra, the yttrium compound precipitate is amorphous before calcination, and it transformed to Y2O3

crystalline phase after calcination. We have tried to find if the spectrum shifts after doping with Tb3+in the sample. However, the size of the Y3+_{and Tb}3+_{is 0.900 and 0.923 ˚}_{A, respectively,}

when the coordination number is 6.13 The difference between those two ions is less than 3%. As a result, the peak shift of the X-ray spectrum is hardly observed.

EDS analysis on randomly selected particles of the above sample is shown inTable 2, which lists five measurements. The average content of Tb3+solid solution in Y2O3is 5.0 at.%, and

appear a concentration variation of 0.45 at.% in a 90%

confi-dence. The Tb3+dopant distributes in every synthesized particles with the concentration enough for PL emission.

TGA results (Fig. 8) show that the mass of sample Y-4a remains constant after calcining to the temperatures higher than 650◦C. It reveals the fact that the phase transformation from amorphous yttrium compound to yttria finishes at 650◦C. A

Fig. 7. XRD results of sample Y-4a (a) as-dried and (b) after calcination at 800◦C for 2 h. Aging time in this case was 7 h.

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the theoretical mass loss (39%) by Eq.(1). The discrepancy of mass loss from sample to sample may be due to the residual water containing in the samples. If the mass loss between 220 and 700◦C was measured, the loss percentage was 33%, which is close to the conversion loss (32%) of Y(OH)CO3to Y2O3.

Their DTA results are similar, basically showing one endother-mic peaks at 200–400◦C, one exothermic at 450–700◦C. The peaks represents the dehydration of the sample and the decom-position of Y-carbonate to Y2O3, respectively.

The variation of the precipitate morphology and size distri-bution of sample Y-4a before and after calcination (heating rate: 5◦C/min, at 800◦C for 2 h) are shown inFig. 9, which shows the TEM micrographs of the microstructure of the Y-4a parti-cles after calcination (heating rate: 5◦C/min, at 800◦C for 2 h). FromFig. 9(a), some nanopores present in the particles can be observed, as shown by the arrows. The pores can be the residue of incomplete sintering, or induced by trapped water, or resulted by in-appropriate calcination schedule for the transformation of the yttrium compound to oxide. Besides, thickness fringes on the particles were observed. Since the particle size was smaller than the spot size of the electron beam, we could only get the ring pattern of these particles. After indexing, the grains were in the crystalline yttria phase, as shown inFig. 9(b).

Furthermore, in order to confirm the crystallinity of the yttria particles, center dark field (CDF) mode was used in this analysis. FromFig. 9(c and d), which are the images of sample Y-4a in the BF or CDF mode, respectively, the yttria is polycrystalline in each individual particle.

Fig. 8. TGA and DSC analysis of sample Y-1 (aging for 24 h). The heating rate was 10◦C/min.

Fig. 9. (a) TEM BF micrograph (b) ring pattern of the particles of the Y-4a particles (heating rate: 5◦C/min, at 800◦C for 2 h). The arrows point out the micropores in the particles and the thickness fringes also could be found in some particles. The imaging of the Y-4a sample in (c) BF mode and (d) CDF mode, which was illuminated by (222) Y2O3diffraction intensity.

3.4. PL properties

Fig. 10shows the result of PL spectrum for the sample Y-4a. The Tb3+_{doped yttria emitted green light with a maximal}

intensity at 546 nm. The corresponding energy transition was from5D4to7F5, as shown inFig. 10(a). However, Y-4b, yttria

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Fig. 10. PL spectra of calcined sample: (a) Y-4a and (b) Y-4b at 800◦C for 2 h.

without doping, did not show any PL spectrum peak, as shown inFig. 10(b).

4. Conclusions

The results of this work can be summarized as follows: (1) Monodispersive Y2O3:Tb3+spherical phosphors were

syn-thesized by homogenous precipitation at 65–85◦C with the addition of urea. The precipitation kinetics of Y3+and Tb3+ cations in the solution were investigated. The results show a zero-order reaction which is controlled by the decompo-sition of the urea with an activation energy of 137 kJ/mol. (2) The size of the precipitates can be controlled by adjusting

the concentration of urea, reaction temperature, and aging time of the solution. Besides, the results reveal that the doped Tb3+in the solution has no obvious influence on the morphology of the yttrium-based precipitate.

(3) The particles grow by the kinetics of Ostwald ripening in the stage of synthesis.

(4) As for the results of TGA, DSC, and X-ray diffraction, the yttrium compound (Y(OH)CO3·1H2O) is an amorphous

phase which transforms to yttria (Y2O3) crystalline phase

after calcination at the temperatures higher than 650◦C.

SEM results show that the precipitate experiences shrinkage after 800◦C for 2 h of calcination. From TEM results, we have observed the existence of nanopores in the particles. (5) Y2O3:Tb3+ powder showed a strong green light (546 nm)

PL emission under 254 nm UV light excitation.

Acknowledgements

This work was financially supported by National Science Council, under grant NSC93-2120-M-002-011. The authors thank the editing by Dr. K. Borgohain and valuable suggestions given by Professor C.F. Lin.

References

1. Kitai, A. H., Oxide phosphor and dielectric thin films for electroluminescent devices. Thin Solid Films, 2003, 445, 367–376.

2. Minami, T., Oxide thin-film electroluminescent devices and materials. Solid-State Electron., 2003, 47, 2237–2243.

3. Wakefield, B. G., Holland, E. and Dobson, P. J., Luminescence properties of nanocrystalline Y2O3:Eu3+. Adv. Mater., 2001, 13, 20.

4. Wuo, B. C., Modiﬁcation and characterization of strontium orthosilicate and yttrium oxide phosphors, MS Thesis, National Taiwan University, 2005. 5. Aiken, B., Hsu, W. P. and Matijevic, E., Preparation and properties of

monodispersed colloidal particles of lanthanide compounds: III, yttrium(III) and mixed yttrium(III)/cerium(III) system. J. Am. Ceram. Soc., 1988, 71(10), 845.

6. Sun, Y., Qi, L., Lee, M., Lee, B. I., Samules, W. D. and Exarhos, G. J., Photoluminescent properties of YO3:Eu3+phosphors prepared via urea

pre-cipitation in non-aqueous solution. J. Lumin., 2004, 109, 85–91.

7. Martinez-Rubio, M. I., Ireland, T. G., Silver, J., Fern, G., Gibbons, C. and Vecht, A., Effect of EDTA on controlling nucleation and morphology in the synthesis of ultrafine Y2O3:Eu3+phosphors. Electrochem. Solid-State Lett.,

2000, 3(9), 446–449.

8. Matijevic, E., Preparation and properties of uniform size colloids. Chem. Mater., 1993, 5, 412–426.

9. Boschini, F., Robertz, B., Rulmont, A. and Cloots, R., Preparation of nano-sized barium zirconate powder by thermal decomposition of urea in an aqueous solution containing barium and zirconium and by calcination of the precipitation. J. Eur. Ceram. Soc., 2003, 23, 3035–3042.

10. Lin, S. E. and Wei, W. C. J., Synthesis and growth kinetics of monodis-persive indium hydrate particles. J. Am. Ceram. Soc., 2006, 89(2), 527– 533.

11. Shaw, W. H. R. and Bordeaux, J. J., The decomposition of urea in aqueous media. J. Chem. Soc., 1955, 77, 4729.

12. Chiang, Y. M., Birnie III, D. P. and Kingery, W. D., Physical Ceramics Principles for Ceramic Science and Engineering. John Wiley & Sons, Inc., 1997, pp. 388–391.