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Hydrothermal synthesis and luminescent properties of YBO3:Tb3+ uniform ultrafine phosphor
ARTICLE IN PRESS
Journal of Crystal Growth 286 (2006) 487–493 www.elsevier.com/locate/jcrysgro
Hydrothermal synthesis and luminescent properties of YBO3:Tb3+ uniform ultrafine phosphor Zhihua Lia,b, Jinghui Zenga, Chen Chenc, Yadong Lia, a
Department of Chemistry, Tsinghua University, Beijing, 100084, China College of Chemistry, Shandong Normal University, Jinan, Shandong 250014, China c Department of Chemistry, Beijing Institute of Technology, Beijing, 100081, China
b
Received 24 June 2005; received in revised form 20 September 2005; accepted 11 October 2005 Available online 1 December 2005 Communicated by K.W. Benz
Abstract The pure hexagonal phase YBO3:Tb3+ phosphors with good crystallinity and uniform size were prepared by hydrothermal reaction (HR). The particle size and morphology can be well controlled by adjusting the concentration of ammonium acetate and varying the reaction temperature and time. The phosphor prepared by HR emits an intense green light at 543 nm, which is stronger than that from crystals synthesized by solid-state reaction (SR). The influence of Tb3+-doping concentration on the crystallization and luminescent properties were investigated. The results showed that the samples exhibited a higher quenching concentration of Tb3+ in comparison with those prepared by the SR. The phenomena were also discussed. r 2005 Elsevier B.V. All rights reserved. PACS: 61.82.Rx Keywords: A1. Hydrothermal synthesis; A1. Luminescence; B1. YBO3:Tb; B2. Green phosphor
1. Introduction In recent years, much attention has been paid to phosphors for vacuum ultraviolet (VUV) excitation due to the demands of plasma display panels (PDP). In PDP, red, green and blue luminescent phosphors are the key materials. At present, Zn2SiO4:Mn is commonly used as the green phosphor for PDP. Its green emission is generated by the d-orbital electrons of the Mn2+ ions. The luminescence is by transition of electrons from excited state 4T1 to ground state 6A1 of Mn2+ ions, a forbidden one according to spin selection rule [1]. Therefore, this phosphor has a relatively long decay time, which will influence its use for (PDP) [1]. So, searching green phosphors with improved qualities, for example, with short decay time, is necessary to improve the luminance properties of PDP. Corresponding author. Tel.: +86 10 62772350; fax: +86 10 62788765.
E-mail address: [email protected] (Yadong Li). 0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.10.085
Higher efficiency and stronger VUV absorption are two important aspects for ideal VUV phosphors. Rare earth orthoborates, REBO3 (RE ¼ lanthanide and yttrium), is a series of interesting materials due to their high VUV transparency and exceptional optical damage threshold [2]. The properties of rare earth borate such as VUV transparency and absorption, optical damage threshold depends strongly on the structure of the borate ions [3,4]. YBO3, which belongs to space group P63/m (hexagonal vaterite-type structure) shows a high VUV absorption, is a promising host material for VUV phosphors. The YBO3:Tb phosphor is a promising candidate for the green component of PDP. It has a strong absorption band in the VUV range, and its luminance under VUV excitation is as high as conventional commercial green phosphors. The luminance property of PDP phosphor materials is strongly affected by the particle size and morphology [5,6]. Surface perfect and spherical-shaped phosphor always has high packing density, good slurry property, and smoother light intensity distribution. It can enhance the luminescent
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intensity and optimize the optical and geometrical structure of phosphor layers [5,7]. Moreover, the ultrafine phosphor particles with very small size can potentially lead to higher screen resolution, lower screen loading, and a higher screen density. At present, the commercialized phosphor YBO3:Tb which is mainly prepared by the solid-state reaction (SR) technique always have irregular shapes and agglomerated particles. Very recently, Jung et al. reported a spray pyrolysis method to synthesize YBO3:Tb phosphor, the morphology and size of the obtained phosphor is slightly improved [8]. However, the particles prepared by spray pyrolysis have a fatal disadvantage of hollowness configuration, which causes reduction in brightness and reduced long-term stability of the phosphor particles. In addition, excess boron is used in order to obtain a stoichiometric composition during the preparation of YBO3:Tb3+ phosphor by SR and spray pyrolysis. The excess boron may be covered on the phosphors surface. Boron is very volatile and may be harmful to the PDP. So, the excess boron should be evaporated. The post heat treatment is also an essential step in order to enhance luminescent intensity before using in PDP. The post heat treatment not only increases the production cost, but also leads to particle agglomeration. Therefore, how to prepare the ultrafine phosphor particles with fine, small, less surface defect, and spherical shape is a challenge. It is well known that hydrothermal method is a promising synthesis route, which can be better controlled from the molecular precursor to the reaction parameters, such as the reaction time and temperature to give highly pure and homogeneous materials. The technique allows low reaction temperatures, and controlled size and morphology of the products [9–15]. In this paper, we presented a hydrothermal method at 200 1C for 5–20 h. Controlled synthesis of well-crystallized YBO3:Tb3+ phosphor with uniform size and morphology is presented using ammonium acetate (CH3COONH4) as the ligand and buffer medium. The results on luminescent properties were also reported and discussed. 2. Experiment The starting reagents were analytical purity Y2O3, Tb4O7, H3BO3 and CH3COONH4. A series of samples with Y and Tb in molar ratio (Y:Tb ¼ 9.5:0.5, 9.0:1.0, 8.5:1.5, 8.0:2.0, 7.5:2.5, 7.0:3.0) were weighed, placed into a beaker (50 ml), and dissolved in diluted nitric acid by heating. The solution was evaporated to dryness. Rare earth nitrates were received. The nitrides were dissolved in deionized water. After that, H3BO3 (100% excess) and CH3COONH4 were added to the beaker. The concentration of CH3COONH4 was controlled in the range of 0.5–1.5 mol/L while the rare earth concentration was kept at 0.1 mol/L. The initial pH value was about 7. After fully dissolving, the solution was transferred into a Teflon lined (40 ml) stainless autoclave. The fullness is about 50%. The autoclave was then placed in an electric oven, heated at
200 1C for 5–20 h. After the autoclave cooled to room temperature, the products were separated by centrifugation, washed with distilled water and ethanol several times, then dried in an oven at 60 1C for 10 h. For comparison the same Tb3+ content phosphors were also synthesized by normal solid-state reaction technique, with analytical purity Ln2O3 and H3BO3 in the molar ratio 1:2 (Ln ¼ Y, Tb, Y:Tb ¼ 9.5:0.5, 9.0:1.0, 8.5:1.5, 8.0:2.0). The mixture was heated at 500 1C for 10 h and then ground, mixed and sintered at 1100 1C for 3 h. The obtained samples were characterized by X-ray powder diffractometer (XRD) using a Brucker D8-advance ˚ X-ray Diffractometer with Cu Ka radiation (l ¼ 1:5418 A), the operation voltage and current were 40 kV and 40 mA, respectively. The 2y range used was from 10 to 701 in steps of 0.021 with a count time of 0.2 s. Fluorescent spectra were recorded with a Hitachi F-4500 Fluorescence Spectrophotometer (ex slit: 1 nm; em slit: 1 nm). Scan speed was 240 nm/min. PMT Voltage was 700 V. All samples were measured at the same condition. Scanning electron microscopy (JSM6301F) was used for the observation of particle morphology. 3. Results and discussion It is well known that rare earth orthoborate crystals generally have a tendency of forming a sheet-like morphology during its growth process [16]. The sheet-like particles have a disadvantage of luminescent efficiency [16]. So, the crystallization environment of YBO3 should be improved in order to avoid the appearance of sheet-like morphology. Controlled synthesis of particles with special morphology by adding mineralizer to change the crystallization environment has been reported before [15]. It is noted that the formation of REBO3 is a process releasing H+ (RE3++H3BO3 ¼ REBO3+3H+), so the reaction happens easily in alkalescence solution. Alkaline condition, however, means quick reaction rate and it is difficult to control synthesis of the size and morphology of products in this reaction system. Therefore, the moderate reaction system should be employed. In our work, we introduced NH3 H2O, NH4NO3, NH4Cl and CH3COONH4 as mineralizer into (Y1xTbx)NO3 and H3BO3 system to change the crystallization environment of YBO3:Tb3+ in order to control the morphology of products. The experiments data were summed up in Table 1. As can be seen in Table 1, the moderate reaction condition formed when quantitative CH3COONH4 was added to the reaction system. The coordinating functions of CH3COO with lanthanide ions have strong influence on the morphology of YBO3:Tb3+. Hereby, we adopted CH3COONH4 as buffer medium and capping reagent to grow YBO3:Tb3+ phosphor. The well-crystallized YBO3:Tb3+ phosphor with regular morphology has been synthesized. Here, Tb0.2Y0.8BO3 was an example to demonstrate the growth process of phosphor particles. Experiments show that reaction temperature, time and the concentration of CH3COONH4
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Table 1 Summarization of experiments data Sample
Mineralizer
Mineralizer concentration
pH (initial and final)
Morphology of products
1
H3BO3
Stoichiometric Excess 100%
6–7; 6–7 6–7; 6–7
No products Nano-sheet
2
NH3 H2O
8, 6–7; 10, 7–8
Small sheet Sheet
3
NH4NO3
0.5 mol/L 1 mol/L
6, 6
No products
4
NH4Cl
0.5 mol/L 1 mol/L
6, 6
Small sheet Sheet
5
CH3COONH4
0.5 mol/L 1 mol/L 1.5 mol/L
7, 7
Irregular particles Near spherical particles Uniform particles
Initial materials are H3BO3 (excess 100%), Y2O3, Tb4O7 (Y:Tb ¼ 9:1, molar ratio), reaction temperature: 200 1C, reaction time: 20 h, [Ln3+] ¼ 0.1 mol/L.
Fig. 1. SEM image of Y0.8Tb0.2BO3, [CH3COONH4]: 1.5 mol/L, reaction time: 10 h, reaction temperature: (a) 220 1C, (b) 200 1C. The scale bar in a and b are 6 mm and 2 mm, respectively.
are the primary factors that influence the size and morphology of the particles. During chemical reaction process, reaction temperature is an important factor, which accelerates reaction rate markedly. The great reaction rate always leads to the formation of large particles in crystallization process. Fig. 1 is the SEM image of Tb0.2Y0.8BO3 prepared by hydrothermal reaction (HR) at 200 1C and 220 1C with 1.5 mol/L for 10 h, respectively. The particle sizes become larger with increasing reaction temperature. Fig. 2 showed that the particle size and morphology varied with the changing concentration of CH3COONH4 at the same reaction temperature. The crystallization process and the rate of seed formation cannot be controlled when the concentration of CH3COONH4 is equal to 0.5 mol/L, which led to the formation of large particles and agglomerations of particles (Fig. 2a). Fig. 2b indicated that the coordinating effect of CH3COO with lanthanide ions increased gradually when the concentration of CH3COONH4 is equal to 1.0 mol/L. So, the growth habit of YBO3 had been restricted. As the concentration of
CH3COONH4 increased to 1.5 mol/L, the large number of CH3COO enhanced the coordinating effect of CH3COO with lanthanide ions, which reduced the growth rate of phosphor particles. As a result, the particles have good crystallized surfaces. The value, 1.5 mol/L, may be an optimal concentration of CH3COONH4. Fig. 3 exhibited the influence on particle sizes by reaction time. The diameter of particles increased with the extended reaction time when the concentration of CH3COONH4 is maintained at 1.5 mol/L, but its morphology had no change. YBO3:Tb3+ phosphor could be barely obtained if the reaction time was less than 1.5 h at 200 1C. Experiments also showed that no matter what the concentration of CH3COONH4 is, the irregular particles are always obtained when the initial pH48 or pHo6. A little product with irregular shape and coarse particle size can be obtained when the reaction time lasted for 24 h at 180 1C. To sum up, the reaction temperature and time were the important conditions of synthesis YBO3:Tb3+phosphor, and CH3COONH4 was the absolutely necessary factor of morphology controlling. The effect of CH3COONH4 may
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Fig. 2. The influence of the CH3COONa concentration on the morphology. Sample: Y0.8Tb0.2BO3, reaction temperature: 200 1C, reaction time: 10 h.
Fig. 3. The time dependence of morphology evolution. Sample: Y0.8Tb0.2BO3 [CH3COONH4] ¼ 1.5 mol/L, reaction temperature: 200 1C.
serve as both a ligand and the buffer reagent. The large numbers of NH+ in the solution has 4 and CH3COO restricted the sheet-like growth habit of rare earth orthoborate. The optimized phosphors with uniform morphology and narrow diameter distribution can be obtained when the concentration of CH3COONH4 is 1.5 mol/L. The particles size and morphology can be well controlled by adjusting the concentration of ammonium acetate, the reaction temperature and time.
It is well known that rare earth ions have similar radius, coordination structure and physical–chemical properties. When one rare earth ion is replaced by another rare earth ion, the crystal structure does not change dramatically. Fig. 4 shows the XRD patterns of YBO3:Tb3+ synthesized by HR and SR. As can be seen, the XRD pattern of YBO3:Tb3+ is very similar to that of YBO3. Compared to the XRD pattern (SR), the diffraction intensity of (1 0 2), (1 0 4) and (1 0 6) faces increases and the diffraction
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106
114
200
004
101
002 Intensity (CPS)
202
HR 110 104 112
100
102
Z. Li et al. / Journal of Crystal Growth 286 (2006) 487–493
SR
ICSD No. 84653
10
20
30
40 2 (θ)
50
60
70
Luminescence Intensity (a.u.)
Fig. 4. XRD pattern of YBO3:Tb.
SR
HR
5
10
15 20 Tb3+: mol%
25
30
Fig. 5. Quenching concentration of YBO3:Tb for SR and HR, respectively. The emission wavelength is 543 nm.
intensity of (0 0 2) and (0 0 4) faces reduce in the XRD pattern (HR) owing to the well-crystallized samples prepared by HR. YBO3:Tb3+ particles with a perfect surface indicates that the phosphor may have stronger luminescence. Fig. 5 shows the Tb3+ concentration quenching curve. The saturation concentration of Tb3+ in the samples
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prepared by SR and HR were about 10% and 20% (molar ratio Tb/Y), respectively. In comparison with the phosphors synthesized by the conventional solid-state reaction, the quenching concentration of Tb3+ was largely increased for the samples prepared by HR. Many similar results have been reported before [17,19]. It can be stated that the emission intensity is related to the activator concentration of Tb3+. When the Tb3+ concentration increases to a limited level, the energy transfer between the Tb3+ ions become predominant by the nonradiation transitions. So the emission intensity decreases sharply. However, when the particle size is as small as nanoscale diameters, the interface effect would lead to a reduced energy-transfer rate, so less energy transfer can migrate to quenching sites. In other words, the change in concentration quenching suggests that the close proximity of the surface does not introduce a large number of quenching surface defects [18–20]. Thus, the concentration quenching inhibition is the result of cutting off the energy-transfer between Tb3+ by the interface effect of the nanoscale material. It is known that the luminescence intensity of the phosphors gradually increases with the increasing Tb3+ concentration until it reaches quenching concentration. As the activator concentration of Tb3+ contributes to the emission intensity, the luminescent intensity increases with increased doping level of Tb3+ cations, which will benefit their practical uses. Fig. 6 shows the excitation and emission spectra of Y0.8Tb0.2 BO3 prepared by HR and Y0.9Tb0.1 BO3 prepared by SR. As can be seen there is one peak at about 238 nm corresponding to the f–d transition of Tb3+ in the excitation (the emission wavelength is 543 nm). In the emission spectra, the main emission peaks are as follows: 489 and 497 nm- 5D4 to 7F6, 543 and 552 nm-5D4 to 7 F5, 582 nm-5D4 to 7F4. The dominant peak is the green emission of 543 nm. The similar curve is obtained for the phosphor prepared by SR, but the excitation and emission intensity of phosphors prepared by SR is lower than that of the phosphor prepared by HR. By comparison the asprepared phosphors by HR, we found that the larger the particles, the higher luminescence intensity they emit. The large particles have better structural perfection and smoother surface in general, which corresponds to fewer surface defects and lower reflection when excitation light radiates on phosphors. So the large particles have relatively higher luminescent efficiency than that of small ones under the same excitation intensity. An important application for next-generation PDP is high-definition television (HDTV). The particle size and morphology have an important effect on PDP because the uniform small size and spherical-shaped phosphor lead to higher screen resolution and a higher screen density, which meets the requisition of HDTV very well. Herein, the presented synthesis method, hydrothermal reaction, may be an environment-benign method owing to the most starting materials employed are water and CH3COONH4 etc, and they can easily control the particle size, phase purity, and morphology in comparison with other synthesis
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Acknowledgement
(a)
This work was supported by NSFC (90406003, 20401010, 50372030, 20025102, 20131030), the Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China and the state key project of fundamental research for nanomaterials and nanostructures (2003CB716901). Reference
238 nm Luminescence Intensity (a.u.)
543nm
(b)
HR HR
SR
SR
200 250 300 350 400 450 500 550 600 650 700 Wavelength (nm)
Fig. 6. (a) Luminescent image of Y0.8Tb0.2BO3 prepared by HR (200 1C, 6 h). (b) Excitation and emission spectra of Y0.8Tb0.2BO3 prepared by HR (200 1C, 6 h) and Y0.9Tb0.1BO3 prepared by SR (1100 1C, 3 h).
methods. Moreover, the easy operation process and simple synthesis equipments are suitable for large-scale production.
4. Conclusions In summary, the green phosphor of YBO3:Tb with uniform size and high luminescence was prepared by hydrothermal method. By adjusting the concentration of CH3COONH4, reaction temperature and time, we successfully carried out the controlled synthesis of YBO3 phosphor on the size and morphology of the particles. The prepared phosphor had a uniform morphology with a controlled size from 500 nm to about 1 mm and higher luminescent intensity, which meet the requirement of PDP and high-definition television (HDTV). In a word, our work presented an efficient way to obtain the YBO3:Tb particles with excellent quality, which may be used as a green phosphor for the application in PDP.
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Journal of Crystal Growth 286 (2006) 487–493 www.elsevier.com/locate/jcrysgro
Hydrothermal synthesis and luminescent properties of YBO3:Tb3+ uniform ultrafine phosphor Zhihua Lia,b, Jinghui Zenga, Chen Chenc, Yadong Lia, a
Department of Chemistry, Tsinghua University, Beijing, 100084, China College of Chemistry, Shandong Normal University, Jinan, Shandong 250014, China c Department of Chemistry, Beijing Institute of Technology, Beijing, 100081, China
b
Received 24 June 2005; received in revised form 20 September 2005; accepted 11 October 2005 Available online 1 December 2005 Communicated by K.W. Benz
Abstract The pure hexagonal phase YBO3:Tb3+ phosphors with good crystallinity and uniform size were prepared by hydrothermal reaction (HR). The particle size and morphology can be well controlled by adjusting the concentration of ammonium acetate and varying the reaction temperature and time. The phosphor prepared by HR emits an intense green light at 543 nm, which is stronger than that from crystals synthesized by solid-state reaction (SR). The influence of Tb3+-doping concentration on the crystallization and luminescent properties were investigated. The results showed that the samples exhibited a higher quenching concentration of Tb3+ in comparison with those prepared by the SR. The phenomena were also discussed. r 2005 Elsevier B.V. All rights reserved. PACS: 61.82.Rx Keywords: A1. Hydrothermal synthesis; A1. Luminescence; B1. YBO3:Tb; B2. Green phosphor
1. Introduction In recent years, much attention has been paid to phosphors for vacuum ultraviolet (VUV) excitation due to the demands of plasma display panels (PDP). In PDP, red, green and blue luminescent phosphors are the key materials. At present, Zn2SiO4:Mn is commonly used as the green phosphor for PDP. Its green emission is generated by the d-orbital electrons of the Mn2+ ions. The luminescence is by transition of electrons from excited state 4T1 to ground state 6A1 of Mn2+ ions, a forbidden one according to spin selection rule [1]. Therefore, this phosphor has a relatively long decay time, which will influence its use for (PDP) [1]. So, searching green phosphors with improved qualities, for example, with short decay time, is necessary to improve the luminance properties of PDP. Corresponding author. Tel.: +86 10 62772350; fax: +86 10 62788765.
E-mail address: [email protected] (Yadong Li). 0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.10.085
Higher efficiency and stronger VUV absorption are two important aspects for ideal VUV phosphors. Rare earth orthoborates, REBO3 (RE ¼ lanthanide and yttrium), is a series of interesting materials due to their high VUV transparency and exceptional optical damage threshold [2]. The properties of rare earth borate such as VUV transparency and absorption, optical damage threshold depends strongly on the structure of the borate ions [3,4]. YBO3, which belongs to space group P63/m (hexagonal vaterite-type structure) shows a high VUV absorption, is a promising host material for VUV phosphors. The YBO3:Tb phosphor is a promising candidate for the green component of PDP. It has a strong absorption band in the VUV range, and its luminance under VUV excitation is as high as conventional commercial green phosphors. The luminance property of PDP phosphor materials is strongly affected by the particle size and morphology [5,6]. Surface perfect and spherical-shaped phosphor always has high packing density, good slurry property, and smoother light intensity distribution. It can enhance the luminescent
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intensity and optimize the optical and geometrical structure of phosphor layers [5,7]. Moreover, the ultrafine phosphor particles with very small size can potentially lead to higher screen resolution, lower screen loading, and a higher screen density. At present, the commercialized phosphor YBO3:Tb which is mainly prepared by the solid-state reaction (SR) technique always have irregular shapes and agglomerated particles. Very recently, Jung et al. reported a spray pyrolysis method to synthesize YBO3:Tb phosphor, the morphology and size of the obtained phosphor is slightly improved [8]. However, the particles prepared by spray pyrolysis have a fatal disadvantage of hollowness configuration, which causes reduction in brightness and reduced long-term stability of the phosphor particles. In addition, excess boron is used in order to obtain a stoichiometric composition during the preparation of YBO3:Tb3+ phosphor by SR and spray pyrolysis. The excess boron may be covered on the phosphors surface. Boron is very volatile and may be harmful to the PDP. So, the excess boron should be evaporated. The post heat treatment is also an essential step in order to enhance luminescent intensity before using in PDP. The post heat treatment not only increases the production cost, but also leads to particle agglomeration. Therefore, how to prepare the ultrafine phosphor particles with fine, small, less surface defect, and spherical shape is a challenge. It is well known that hydrothermal method is a promising synthesis route, which can be better controlled from the molecular precursor to the reaction parameters, such as the reaction time and temperature to give highly pure and homogeneous materials. The technique allows low reaction temperatures, and controlled size and morphology of the products [9–15]. In this paper, we presented a hydrothermal method at 200 1C for 5–20 h. Controlled synthesis of well-crystallized YBO3:Tb3+ phosphor with uniform size and morphology is presented using ammonium acetate (CH3COONH4) as the ligand and buffer medium. The results on luminescent properties were also reported and discussed. 2. Experiment The starting reagents were analytical purity Y2O3, Tb4O7, H3BO3 and CH3COONH4. A series of samples with Y and Tb in molar ratio (Y:Tb ¼ 9.5:0.5, 9.0:1.0, 8.5:1.5, 8.0:2.0, 7.5:2.5, 7.0:3.0) were weighed, placed into a beaker (50 ml), and dissolved in diluted nitric acid by heating. The solution was evaporated to dryness. Rare earth nitrates were received. The nitrides were dissolved in deionized water. After that, H3BO3 (100% excess) and CH3COONH4 were added to the beaker. The concentration of CH3COONH4 was controlled in the range of 0.5–1.5 mol/L while the rare earth concentration was kept at 0.1 mol/L. The initial pH value was about 7. After fully dissolving, the solution was transferred into a Teflon lined (40 ml) stainless autoclave. The fullness is about 50%. The autoclave was then placed in an electric oven, heated at
200 1C for 5–20 h. After the autoclave cooled to room temperature, the products were separated by centrifugation, washed with distilled water and ethanol several times, then dried in an oven at 60 1C for 10 h. For comparison the same Tb3+ content phosphors were also synthesized by normal solid-state reaction technique, with analytical purity Ln2O3 and H3BO3 in the molar ratio 1:2 (Ln ¼ Y, Tb, Y:Tb ¼ 9.5:0.5, 9.0:1.0, 8.5:1.5, 8.0:2.0). The mixture was heated at 500 1C for 10 h and then ground, mixed and sintered at 1100 1C for 3 h. The obtained samples were characterized by X-ray powder diffractometer (XRD) using a Brucker D8-advance ˚ X-ray Diffractometer with Cu Ka radiation (l ¼ 1:5418 A), the operation voltage and current were 40 kV and 40 mA, respectively. The 2y range used was from 10 to 701 in steps of 0.021 with a count time of 0.2 s. Fluorescent spectra were recorded with a Hitachi F-4500 Fluorescence Spectrophotometer (ex slit: 1 nm; em slit: 1 nm). Scan speed was 240 nm/min. PMT Voltage was 700 V. All samples were measured at the same condition. Scanning electron microscopy (JSM6301F) was used for the observation of particle morphology. 3. Results and discussion It is well known that rare earth orthoborate crystals generally have a tendency of forming a sheet-like morphology during its growth process [16]. The sheet-like particles have a disadvantage of luminescent efficiency [16]. So, the crystallization environment of YBO3 should be improved in order to avoid the appearance of sheet-like morphology. Controlled synthesis of particles with special morphology by adding mineralizer to change the crystallization environment has been reported before [15]. It is noted that the formation of REBO3 is a process releasing H+ (RE3++H3BO3 ¼ REBO3+3H+), so the reaction happens easily in alkalescence solution. Alkaline condition, however, means quick reaction rate and it is difficult to control synthesis of the size and morphology of products in this reaction system. Therefore, the moderate reaction system should be employed. In our work, we introduced NH3 H2O, NH4NO3, NH4Cl and CH3COONH4 as mineralizer into (Y1xTbx)NO3 and H3BO3 system to change the crystallization environment of YBO3:Tb3+ in order to control the morphology of products. The experiments data were summed up in Table 1. As can be seen in Table 1, the moderate reaction condition formed when quantitative CH3COONH4 was added to the reaction system. The coordinating functions of CH3COO with lanthanide ions have strong influence on the morphology of YBO3:Tb3+. Hereby, we adopted CH3COONH4 as buffer medium and capping reagent to grow YBO3:Tb3+ phosphor. The well-crystallized YBO3:Tb3+ phosphor with regular morphology has been synthesized. Here, Tb0.2Y0.8BO3 was an example to demonstrate the growth process of phosphor particles. Experiments show that reaction temperature, time and the concentration of CH3COONH4
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Table 1 Summarization of experiments data Sample
Mineralizer
Mineralizer concentration
pH (initial and final)
Morphology of products
1
H3BO3
Stoichiometric Excess 100%
6–7; 6–7 6–7; 6–7
No products Nano-sheet
2
NH3 H2O
8, 6–7; 10, 7–8
Small sheet Sheet
3
NH4NO3
0.5 mol/L 1 mol/L
6, 6
No products
4
NH4Cl
0.5 mol/L 1 mol/L
6, 6
Small sheet Sheet
5
CH3COONH4
0.5 mol/L 1 mol/L 1.5 mol/L
7, 7
Irregular particles Near spherical particles Uniform particles
Initial materials are H3BO3 (excess 100%), Y2O3, Tb4O7 (Y:Tb ¼ 9:1, molar ratio), reaction temperature: 200 1C, reaction time: 20 h, [Ln3+] ¼ 0.1 mol/L.
Fig. 1. SEM image of Y0.8Tb0.2BO3, [CH3COONH4]: 1.5 mol/L, reaction time: 10 h, reaction temperature: (a) 220 1C, (b) 200 1C. The scale bar in a and b are 6 mm and 2 mm, respectively.
are the primary factors that influence the size and morphology of the particles. During chemical reaction process, reaction temperature is an important factor, which accelerates reaction rate markedly. The great reaction rate always leads to the formation of large particles in crystallization process. Fig. 1 is the SEM image of Tb0.2Y0.8BO3 prepared by hydrothermal reaction (HR) at 200 1C and 220 1C with 1.5 mol/L for 10 h, respectively. The particle sizes become larger with increasing reaction temperature. Fig. 2 showed that the particle size and morphology varied with the changing concentration of CH3COONH4 at the same reaction temperature. The crystallization process and the rate of seed formation cannot be controlled when the concentration of CH3COONH4 is equal to 0.5 mol/L, which led to the formation of large particles and agglomerations of particles (Fig. 2a). Fig. 2b indicated that the coordinating effect of CH3COO with lanthanide ions increased gradually when the concentration of CH3COONH4 is equal to 1.0 mol/L. So, the growth habit of YBO3 had been restricted. As the concentration of
CH3COONH4 increased to 1.5 mol/L, the large number of CH3COO enhanced the coordinating effect of CH3COO with lanthanide ions, which reduced the growth rate of phosphor particles. As a result, the particles have good crystallized surfaces. The value, 1.5 mol/L, may be an optimal concentration of CH3COONH4. Fig. 3 exhibited the influence on particle sizes by reaction time. The diameter of particles increased with the extended reaction time when the concentration of CH3COONH4 is maintained at 1.5 mol/L, but its morphology had no change. YBO3:Tb3+ phosphor could be barely obtained if the reaction time was less than 1.5 h at 200 1C. Experiments also showed that no matter what the concentration of CH3COONH4 is, the irregular particles are always obtained when the initial pH48 or pHo6. A little product with irregular shape and coarse particle size can be obtained when the reaction time lasted for 24 h at 180 1C. To sum up, the reaction temperature and time were the important conditions of synthesis YBO3:Tb3+phosphor, and CH3COONH4 was the absolutely necessary factor of morphology controlling. The effect of CH3COONH4 may
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Fig. 2. The influence of the CH3COONa concentration on the morphology. Sample: Y0.8Tb0.2BO3, reaction temperature: 200 1C, reaction time: 10 h.
Fig. 3. The time dependence of morphology evolution. Sample: Y0.8Tb0.2BO3 [CH3COONH4] ¼ 1.5 mol/L, reaction temperature: 200 1C.
serve as both a ligand and the buffer reagent. The large numbers of NH+ in the solution has 4 and CH3COO restricted the sheet-like growth habit of rare earth orthoborate. The optimized phosphors with uniform morphology and narrow diameter distribution can be obtained when the concentration of CH3COONH4 is 1.5 mol/L. The particles size and morphology can be well controlled by adjusting the concentration of ammonium acetate, the reaction temperature and time.
It is well known that rare earth ions have similar radius, coordination structure and physical–chemical properties. When one rare earth ion is replaced by another rare earth ion, the crystal structure does not change dramatically. Fig. 4 shows the XRD patterns of YBO3:Tb3+ synthesized by HR and SR. As can be seen, the XRD pattern of YBO3:Tb3+ is very similar to that of YBO3. Compared to the XRD pattern (SR), the diffraction intensity of (1 0 2), (1 0 4) and (1 0 6) faces increases and the diffraction
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106
114
200
004
101
002 Intensity (CPS)
202
HR 110 104 112
100
102
Z. Li et al. / Journal of Crystal Growth 286 (2006) 487–493
SR
ICSD No. 84653
10
20
30
40 2 (θ)
50
60
70
Luminescence Intensity (a.u.)
Fig. 4. XRD pattern of YBO3:Tb.
SR
HR
5
10
15 20 Tb3+: mol%
25
30
Fig. 5. Quenching concentration of YBO3:Tb for SR and HR, respectively. The emission wavelength is 543 nm.
intensity of (0 0 2) and (0 0 4) faces reduce in the XRD pattern (HR) owing to the well-crystallized samples prepared by HR. YBO3:Tb3+ particles with a perfect surface indicates that the phosphor may have stronger luminescence. Fig. 5 shows the Tb3+ concentration quenching curve. The saturation concentration of Tb3+ in the samples
491
prepared by SR and HR were about 10% and 20% (molar ratio Tb/Y), respectively. In comparison with the phosphors synthesized by the conventional solid-state reaction, the quenching concentration of Tb3+ was largely increased for the samples prepared by HR. Many similar results have been reported before [17,19]. It can be stated that the emission intensity is related to the activator concentration of Tb3+. When the Tb3+ concentration increases to a limited level, the energy transfer between the Tb3+ ions become predominant by the nonradiation transitions. So the emission intensity decreases sharply. However, when the particle size is as small as nanoscale diameters, the interface effect would lead to a reduced energy-transfer rate, so less energy transfer can migrate to quenching sites. In other words, the change in concentration quenching suggests that the close proximity of the surface does not introduce a large number of quenching surface defects [18–20]. Thus, the concentration quenching inhibition is the result of cutting off the energy-transfer between Tb3+ by the interface effect of the nanoscale material. It is known that the luminescence intensity of the phosphors gradually increases with the increasing Tb3+ concentration until it reaches quenching concentration. As the activator concentration of Tb3+ contributes to the emission intensity, the luminescent intensity increases with increased doping level of Tb3+ cations, which will benefit their practical uses. Fig. 6 shows the excitation and emission spectra of Y0.8Tb0.2 BO3 prepared by HR and Y0.9Tb0.1 BO3 prepared by SR. As can be seen there is one peak at about 238 nm corresponding to the f–d transition of Tb3+ in the excitation (the emission wavelength is 543 nm). In the emission spectra, the main emission peaks are as follows: 489 and 497 nm- 5D4 to 7F6, 543 and 552 nm-5D4 to 7 F5, 582 nm-5D4 to 7F4. The dominant peak is the green emission of 543 nm. The similar curve is obtained for the phosphor prepared by SR, but the excitation and emission intensity of phosphors prepared by SR is lower than that of the phosphor prepared by HR. By comparison the asprepared phosphors by HR, we found that the larger the particles, the higher luminescence intensity they emit. The large particles have better structural perfection and smoother surface in general, which corresponds to fewer surface defects and lower reflection when excitation light radiates on phosphors. So the large particles have relatively higher luminescent efficiency than that of small ones under the same excitation intensity. An important application for next-generation PDP is high-definition television (HDTV). The particle size and morphology have an important effect on PDP because the uniform small size and spherical-shaped phosphor lead to higher screen resolution and a higher screen density, which meets the requisition of HDTV very well. Herein, the presented synthesis method, hydrothermal reaction, may be an environment-benign method owing to the most starting materials employed are water and CH3COONH4 etc, and they can easily control the particle size, phase purity, and morphology in comparison with other synthesis
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Acknowledgement
(a)
This work was supported by NSFC (90406003, 20401010, 50372030, 20025102, 20131030), the Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China and the state key project of fundamental research for nanomaterials and nanostructures (2003CB716901). Reference
238 nm Luminescence Intensity (a.u.)
543nm
(b)
HR HR
SR
SR
200 250 300 350 400 450 500 550 600 650 700 Wavelength (nm)
Fig. 6. (a) Luminescent image of Y0.8Tb0.2BO3 prepared by HR (200 1C, 6 h). (b) Excitation and emission spectra of Y0.8Tb0.2BO3 prepared by HR (200 1C, 6 h) and Y0.9Tb0.1BO3 prepared by SR (1100 1C, 3 h).
methods. Moreover, the easy operation process and simple synthesis equipments are suitable for large-scale production.
4. Conclusions In summary, the green phosphor of YBO3:Tb with uniform size and high luminescence was prepared by hydrothermal method. By adjusting the concentration of CH3COONH4, reaction temperature and time, we successfully carried out the controlled synthesis of YBO3 phosphor on the size and morphology of the particles. The prepared phosphor had a uniform morphology with a controlled size from 500 nm to about 1 mm and higher luminescent intensity, which meet the requirement of PDP and high-definition television (HDTV). In a word, our work presented an efficient way to obtain the YBO3:Tb particles with excellent quality, which may be used as a green phosphor for the application in PDP.
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