A novel synthetic route to Y2O3:Tb3+ phosphors by bicontinuous cubic phase process

Materials and Design 31 (2010) 1737–1741

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Short Communication

A novel synthetic route to Y2O3:Tb3+ phosphors by bicontinuous cubic phase process Wen-Chen Chien a,*, Yang-Yen Yu b, Chun-Chen Yang a a b

Department of Chemical Engineering, Mingchi University of Technology, 84 Gunjuan Road, Taishan, Taipei Hsien 243, Taiwan Department of Materials Engineering, Mingchi University of Technology, 84 Gunjuan Road, Taishan, Taipei Hsien 243, Taiwan

a r t i c l e

i n f o

Article history: Received 19 September 2008 Accepted 25 January 2009 Available online 3 August 2009

a b s t r a c t This study applies a novel approach to prepare the terbium-doped yttrium oxide phosphors (Y2O3:Tb3+) using the bicontinuous cubic phase (BCP) process. The experimental results show that the prepared precursor powder was amorphous yttrium hydroxide Y1xTbx(OH)3 with a spherical shape and primary size 30–50 nm. High crystallinity phosphors with body-centered cubic structures were obtained after heat treatment above 700 °C for 4 h. The primary size of the phosphors grew to 100–200 nm, and dense agglomerates with a size below 1 lm were formed during the calcination. The obtained Y2O3:Tb3+ phosphor had a strong green emitting at 542 nm. The optimum Tb3+ concentration was 1 mol% to obtain the highest PL intensity. This study indicates that the calcining temperature of 700 °C needed for high luminescence efficiency in this work is much lower than 1000 °C or above needed for the conventional solidstate method. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Field emission displays (FEDs) have been recognized as one of the most promising technologies for flat panel displays (FPDs). In terms of practical applications, FEDs operate at a lower voltage electron excitation and higher current density. Sulfide-based phosphors have been shown to degrade at high current density, which lowers the efficiency of phosphors and leads to tip damage, thus shortening the life of the device [1]. Therefore, much research has been conducted on oxide-based phosphors because of their high stability. Rare-earth-doped yttrium oxide (Y2O3) and yttrium–aluminum oxide (Y3Al5O12, YAG) are considered to be two of the best kinds of oxide-based phosphors for FPD practical application because of their excellent luminescent efficiency, color purity, and chemical and thermal stability [2,3]. Many different techniques regarding developing terbium-doped yttrium oxide and yttrium–aluminum oxide phosphors (Y2O3:Tb3+ and Y3Al5O12:Tb3+) with a spherical shape, good crystallinity, high chemical purity, homogeneity and high luminescence efficiency have been reported in the literature, including solid-state reaction [4], combustion [5,6], spray pyrolysis [7,8], co-precipitation [9], sol–gel [10], solvothermal [11], hydrothermal [12], and chemical vapor reaction [13]. In general, the high temperature solid-state reaction and combustion produce large aggregates that must be ground or milled to obtain a finer powder. The efficiency of the phosphors decreases greatly in this process and the morphology of the particle is changed. The obtained phosphors usually have low composition homogeneity. Spray pyrolysis can produce spher* Corresponding author. Tel.: +886 2 29089899x4628; fax: +886 2 29083072. E-mail address: [email protected] (W.-C. Chien). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.01.046

ical and fine particles, but particles are often hollow and porous due to the short residence time and rapid pyrolysis of the spray drops in the high-temperature furnace. For the co-precipitation and sol–gel methods, the prepared phosphors have high purity, homogeneity and strong luminescence; however, the particles size distribution is board and the particle shape is non-spherical. For the solvothermal method, the prepared powders usually consist of aggregations of irregular grains, and are not at all desirable for phosphor applications. Moreover, the use of organic solvents increases the cost of production and may even pollute the environment. Hydrothermal and chemical vapor reactions are effective methods to produce fine, well-crystallized phosphors, but these processes require complicated and expensive equipment due to the strict operating temperature and pressure required. Therefore, further investigations of terbium-doped yttrium oxide phosphors and improvements in terms of its properties are required. It is well-known that many surfactants can self-assemble into a mesoporous phase with long range three-dimensional periodicity called the bicontinuous cubic phase (BCP) [14,15]. Unlike inverse micelles, which form isolated water pools, aqueous pores in the BCP are interconnected with neighboring ones through small channels. Due to the bicontinuous nature of the cubic phase, ions can diffuse from pore to pore in the hydrophilic aqueous region without passing through the surfactant membrane barrier. The BCP process has been used to prepare metal and sulfides nanoparticles. The prepared particles, such as Pd [16], PbS [17] and Mn-doped ZnS [18], show a spherical shape and monodispersed size distribution. Previously, we have prepared Y2O3:Eu3+ phosphors using the BCP process [19]. However, to our best knowledge, the literature contains no mention of using the BCP method to synthesize Y2O3:Tb3+ phosphors.

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In this study, the BCP method was applied to prepare the greenemitting phosphor Y2O3:Tb3+. The prepared phosphors were characterized by different techniques, such as thermogravimetric analysis (TGA), fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectrometer (EDS) and photoluminescence (PL). It is hoped that the study can result in a more comprehensive understanding of the BCP system and of the preparation procedure for oxide-based phosphors using the BCP process. 2. Experimental Y(NO3)36H2O (99.9%), Tb(NO3)36H2O (99.9%), NaOH, sodium diethyl hexylsulphosuccinate (AOT, 99.9%), and methanol (HPLC grade) were purchased from Aldrich and used as received. The main procedures for preparing the Y2O3:Tb3+ phosphors were as follows. About 4 g mixture of 0.2 M Tb(NO3)36H2O and Y(NO3)36H2O with a desired molar ratio of [Tb3+] to [Y3+] was added to the 16 g AOT in a 50 ml centrifugal tube followed by centrifugation for 2 min at 4000 rpm. After 4 h, the mixture was stirred using a spatula until visibly uniform. After 48 h incubation at room temperature, the mixture became transparent. The presence of the cubic phase was confirmed by observing the mixture under a polarized optical microscope. BCP was optically isotropic because of its cubic symmetry, and therefore it appeared dark between crossed polarizers. On the other hand, the lamellar and hexagonal phases were highly birefringent [17]. Then, the viscous mixture in the cubic phase was put into a 5 ml syringe and extruded in the shape of the cylinder into the 0.4 M NaOH solution to cause a homogeneous precipitation inside the BCP structure. After 1 h, the cylinder became white and then the excess NaOH solution was discarded by decantation. The cylinder was washed with deionized water and then dissolved in methanol. Centrifugation for 10 min at 9000 rpm led to complete precipitation of the white precursor. Then, the precursor was purified by washing several times with water and methanol and recovered through centrifugation. The precursor was dried at room temperature and then calcined at 400–1000 °C for 4 h and ground by hand to get fine Y2O3:Tb3+ phosphor powders. Thermal analysis was examined under a nitrogen flow using a DuPont Model 951 thermogravimetric analysis at a heating rate of 20 °C/min. Fourier transform infrared spectra were obtained on a KBr pellet using a Jasco Model FTIR 410 spectrophotomer. X-ray diffraction patterns were obtained with a Mac science Model MXP-3TXJ-7266 X-ray diffractometer using CuKa radiation. The atomic ratio of Tb/Y, particle size and morphology of the prepared phosphors were measured using a Hitachi H-7100 transmission electron microscope and field emission scanning electron microscopy and energy dispersive spectrometer (LEO 1530 FE-SEM + EDS). The photoluminescence measurements were performed using a JASCO 6500 spectrophotometer excited at the wavelength 360 nm.

(d) 100

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0

0

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Temperature( C) Fig. 1. TGA curves of: (a) AOT/precursor, (b) clean precursor, (c) AOT/precursor calcined at 900 °C for 4 h, and (d) clean precursor calcined at 700 °C for 4 h.

there is still about 45 wt% residual AOT in the precursor powder if the washing step was only repeated three times. On the other hand, the weight loss of clean precursor from room temperature to 700 °C is about 26 wt% that is close to the theoretical weight loss 28 wt% caused by the conversion of hydroxide to oxide, i.e. 2Y1xTbx(OH)3H2O ? TbxY2O3. Therefore, as shown in curves (c) and (d) no weight loss can be found when the AOT/precursor and clean precursor are calcined at 900 °C and 700 °C, respectively. Fig. 2 illustrates the SEM photographs of the prepared precursor powders with and without AOT residue. It can be seen in Fig. 2a

3. Results and discussion Fig. 1 illustrates the TGA curves of (a) AOT/precursor, (b) clean precursor, (c) AOT/precursor calcined at 900 °C for 4 h, and (d) clean precursor calcined at 700 °C for 4 h. The precursor with residual AOT was obtained by incomplete washing steps. Curve (a) shows that there is a about 70 wt% weight loss from room temperature to 1000 °C. The weight loss may be caused by the loss of absorbed water, hydroxyl groups, and the residual AOT. The residual AOT can be removed completely if the calcining temperature is over 900 °C. Comparing the curve (a) with curve (b), it is found that

Fig. 2. SEM photographs of the prepared precursor powders with and without AOT residue: (a) AOT/precursor and (b) clean precursor.

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that the white AOT existed in the precursor powders. From Fig. 2b, it shows that the primary particles of the precursor powders have a near spherical shape with a size about 30–50 nm, and narrow size distribution. These precursor powders are amorphous, as measured by XRD. Coagulation between the primary particles occurred and formed small aggregates during the vacuum drying step at room temperature. Generally, the precursor gel should be washed six times or more to prevent AOT residue; however, the washing step causes a decrease in the amount of obtained precursor powders. This is one disadvantage of the BCP process as compared to other conventional methods for the preparation of Y2O3-based phosphors. The precursor powders were calcined at different temperatures ranging from 400 to 1000 °C for 4 h to obtain the Y2O3:Tb3+ phosphors. Fig. 3 illustrates the XRD patterns of the prepared phosphors at different calcining temperatures and Tb3+ doping concentrations. The cubic Y2O3 phase (h k l) planes are also indicated in Fig. 3. It shows that the refractive peaks of 2h were observed at 20.6, 29.3, 33.9, 36.0, 40.0, 43.6, 48.7, 53.3, 56.3, 57.8, 59.1, and 60.5. These peaks correspond to the (211), (222), (321), (411), (332), (431), (440), (611), (541), (622), (631), and (444) planes, which can be indexed to the pure body-centered cubic Y2O3 (JCPDS No. 41-1105). The Y2O3:Tb3+ phosphors had a good crystallinity after calcination at 700 °C or above. No evident difference was observed between the XRD patterns of the samples calcined at 700 and 1000 °C. This result is consistent with that obtained from TGA analysis, where the hydroxides transfer completely into oxides as the calcining temperature exceeds 700 °C. Moreover, the crystal size of Y2O3:Tb3+ phosphor is calculated from the halfwidth of the peaks of the XRD powder patterns by using Scherrer’s Kk ffi equation: D ¼ pffiffiffiffiffiffiffiffiffi ; where D is the crystal size; K is the Scher2 2

1739

half-maximum (FWHM); and b is the instrumental broadening, here b = 0.0032. The FWHM and crystal size of Tb-doped yttrium oxide powders obtained at 700 and 1000 °C were 0.21 and 89 nm and 0.19 and 183 nm, respectively. As shown in Fig. 3, the crystal phase and the full width of the FWHM of these phosphors obtained from different Tb3+ concentrations did not exhibit any obvious differences with an increase in Tb3+ concentrations. It is known that the Tb3+ ion radius (0.923 Å) is close to the Y3+ (0.9 Å) [11]. Therefore, the crystal phase and size do not change significantly with an increase in Tb3+ concentrations. Besides, no evident difference was observed between the XRD patterns of the sample prepared at 700 °C by BCP method and at 1200 °C by the conventional solidstate reaction. Fig. 4 illustrates the SEM micrographs of Y2O3:Tb3+ phosphors calcined at 700 °C for 4 h. Fig. 4a shows that the primary particle size increased from 30–50 nm for the hydroxide precursors to about 100–200 nm for the oxide phosphors. The size values obtained using SEM are very close to those estimated from the XRD analysis, in which the particle sizes calculated using Scherrer’s equation are about 89–183 nm. Each primary crystal of oxide phosphors showed a smooth surface and a good crystal habit, which indicates that the growth mechanism of crystal dominated the process of phase transition from yttrium hydroxide to yttrium oxide. The larger aggregates with a size of several tens of micrometers were obtained after the calcination. These larger aggregates had a loose structure and thus could be easily ground by hand into smaller and separable dense aggregates. These dense aggregates with a size below 1 lm as shown in Fig. 4(b) were formed by the sintering of primary particles during the calcination. Further, the

B b cos h

rer shape factor, here K = 0.9; k is the X-ray wavelength, here 0

k = 1.5405 A Å; h is the Bragg angle; B is the measured full width at

(222)

(440) (622)

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Fig. 3. XRD patterns of the prepared Y2O3:Tb3+ phosphors at different calcining temperatures and Tb3+ doping concentrations.

Fig. 4. SEM photographs of the Tb-doped Y2O3 phosphors calcined at 700 °C and measured at different magnifications, (a) 10000X and (b) 65000X.

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atomic ratio of Tb/Y in the prepared phosphors was determined by the EDS spectrum. The result indicates that the homogeneous precipitation occurred in the BCP structure because the experimental atomic ratio of Tb to Y was very close to the theoretical value, which we added into the AOT. Fig. 5 illustrates the FTIR spectra of (a) pure AOT, (b) as-prepared precursor, (c) phosphors calcined at 500 °C, (d) phosphors calcined at 700 °C, and (e) commercial Y2O3. Spectrum (a) shows that the absorption bands at 1050, 1738, 2868, 2947 cm1 were observed for pure AOT. The bands at 1050 cm1 and 1738 cm1 were assigned to the S@O stretching vibration of the sulfonate group and C@O stretching vibration of the carbonyl group. The bands at 2868 and 2947 cm1 were assigned to the CH2–CH2 methylene symmetric and antisymmetric stretching vibrations, respectively. These absorption bands of AOT can be used to find the residual AOT in precursor powders before calcination. The FTIR spectrum of as-prepared precursor with residual AOT is shown in spectrum (b). The residual AOT was probably due to the nature of the viscous AOT and the strong hydrogen bonding between AOT and yttrium hydroxide. On the other hand, the broad absorption band at 3450 cm1 was assigned to the –OH group of Y(OH)3 and H2O, as it confirmed that the precursor powders are yttrium hydroxide. In spectrum (c), the characteristic bands of AOT disappeared for the phosphors calcined at 500 °C for 4 h, which indicates the residual AOT was removed by thermal decomposition. Further, the broad peak at 3400 cm1 reduced greatly, indicating a partial transformation of yttrium hydroxide to yttrium oxide. However, the band at 565 cm1 assigned to the vibration of the Y–O bond [20,21] was weak. As the calcining temperature increased to 700 °C or above, the strong absorption peak of Y–O at 565 cm1 became present. This result indicates that the yttrium hydroxide transferred completely into the yttrium oxide at 700 °C or above. The result corresponds to that obtained from the TGA and XRD analysis. It is clear that the Y–O absorption band sharpens as the calcining temperature increases. As to the cause of the sharpening, the effects upon characteristics of FTIR spectra were essentially due to the polarization charge induced at the particle surface by the external electromagnetic field [22]. It is assumed that the particle size increases with increasing calcining temperature, which causes the surface effects decrease so that damping of surface mode absorption decreases and the Y–O absorption band sharpens. The bands near 1540, 1410 and 3400 cm1 as shown in spectrums and OH (d) and (e) were assigned to the vibration of the CO2 3 group. These weak absorption bands were probably due to the adsorbed molecular water from KBr pellet technique for preparation

sample, or the residual carbonate or absorption of H2O and CO2 from the ambient atmosphere. Fig. 6 illustrates the PL excitation and emission spectra of the prepared Y2O3:Tb3+ phosphors calcined at 700 °C for 4 h under different Tb3+ concentrations. The excitation spectrum was recorded at 542 nm and the emission spectra were excited at 360 nm. Fig. 6 shows that the excitation spectra are composed of two overlapping bands whose maximum positions at 274 and 302 nm, in agreement with previous reports [6,23]. The origin of these bands has been associated with either charge transfer absorption or Tb3+ intra-band (4f8?4f75d) excitation [6]. The emission spectra are complex, containing several groups of sharp lines associated with electronic transitions of the Tb3+ ion. It is known that Tb3+ have four main emission bands corresponding to the 5D4 ? 7FJ transitions where J = 3, 4, 5 and 6. The strongest transition occurred at approximately 542 nm, which is characteristic of the green fluorescence of Tb3+ [16]. The most dominant peak at 542 nm arose from the 5D4 ? 7F5 transition, while the other peaks at 487, 584, and 620 nm corresponded to the 5D4 ? 7F6, 5D4 ? 7F4, and 5D4 ? 7F3, respectively. The Tb3+ doping concentration varied from 1 mol% to 9 mol%. It was found that the PL emission intensity decreased with the Tb3+ concentration. The results show that the optimum doping concentration was 1 mol%. The decrease of PL emission intensity of phosphors for Tb3+ concentration greater than 1 mol% may have been caused by the concentration quenching phenomena, which is caused by the cross-relaxation between neighboring Tb3+ ions. On the other hand, the line positions and line widths in the emission spectra did not change as Tb3+ concentration changed. This suggests that the nature of the Tb3+ activation does not change with the concentration. As shown in Fig. 6, the intensity of emission of phosphors prepared by BCP method is stronger than that prepared by solid-state method. The result shows that the calcining temperature of 700 °C needed for high luminescence efficiency in this work is much lower than 1000 °C or above needed for the conventional method in the literature [4]. Fig. 7 illustrates the PL spectra of the prepared Y2O3:Tb3+ phosphors at different calcining temperature and the same Tb3+ concentration 1 mol%. The PL emission intensity increased greatly with the increase in the calcining temperature from 400 to 700 °C. However, there was only a slight emission intensity increase when the calcining temperature increased from 700 °C to 1000 °C. This result is consistent with that obtained from the XRD analysis. In the XRD analysis, the prepared Y2O3:Tb3+ phosphors show a good crystallinity as calcining temperature is above 700 °C. The results indicate that the order of the environment surrounding Tb3+ has a great influence on the PL emission intensity. Compare to the conventional solid-state

Transmittance (a.u.)

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Fig. 5. FTIR spectra of (a) pure AOT, (b) as-prepared precursor, (c) phosphors calcined at 500 °C, (d) phosphors calcined at 700 °C, and (e) commercial Y2O3.

300

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Fig. 6. PL excitation and emission spectra of the prepared Y2O3:Tb3+ phosphors at different Tb3+ doping concentrations.

W.-C. Chien et al. / Materials and Design 31 (2010) 1737–1741

References

2.0x10 7 o

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method, the phosphor precursors prepared from BCP method have smaller and more uniform particle size and Tb3+ ions distribution. Therefore, at the same calcining temperature and Tb3+ concentration, the prepared phosphors have better crystalline and Tb3+ distribution and thus higher emission efficiency. 4. Conclusions In summary, Tb3+-doped Y2O3 phosphors have been successfully prepared using the BCP process. It was found that the homogeneous reaction occurred in the BCP structure for the precipitation of precursors, TbxY1x(OH)3, which could transfer completely into phosphors, TbxY2O3, when the calcining temperature was at 700 °C or above. SEM showed that the primary particles of the precursor had a spherical shape with a size of about 30– 50 nm. After calcination, the primary size of the Y2O3:Tb3+ phosphors grew to 100–200 nm and dense aggregates with a size below 1 lm were formed. XRD and PL analyses showed that the prepared Y2O3:Tb3+ phosphors had a good crystallinity and thus a high PL emission intensity as the precursors were calcined at 700 °C or above. The optimum Tb3+ doping concentration was 1 mol%. The results also reveal that not only the doping concentration but also the order of the environment surrounding doping ions has a great influence on the PL emission intensity of Y2O3:Tb3+ phosphors. Acknowledgment The author gratefully acknowledges the financial support of the National Science Council of the Republic of China.

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