Materials Research Bulletin 41 (2006) 1571–1577 www.elsevier.com/locate/matresbu
Hydrothermal synthesis and photoluminescence of novel green-emitting phosphor Y1xBO3:xTb3+ Yu Hua Wang a,b,*, Chun Fang Wu a, Jia Chi Zhang a a
Department of Materials Science, School of Physical Science and Technology, Lanzhou University, Lanzhou, GanSu Province 730000, PR China b State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, GanSu Province 730000, PR China Received 8 March 2004; received in revised form 24 March 2005; accepted 26 May 2005 Available online 14 June 2006
Abstract Y1xBO3:xTb3+ phosphors were first synthesized by hydrothermal reaction, and the samples were characterized by X-ray powder diffractometry, infrared absorption, nuclear magnetic resonance, scanning electron microscopy and photoluminescence. The results show that single phase is obtained with Tb concentration up to 0.28 and all the samples exhibit flake-like morphology. The sample was determined to be vaterite-type orthoborate and the boron is both four-coordinated (chief) and three-coordinated (few). The Y1xBO3:xTb3+ phosphors showed intense green emission at 550 nm and the intensity of the emission increases with Tb3+ substitution up to 0.22 and then decreases for higher Tb3+ content. In the phosphors prepared by the hydrothermal method the concentration quenching is higher than in the phosphors prepared by solid-state reaction; the intensity of emission is stronger in the former than that of the latter. Y1xBO3:xTb3+ is a promising phosphor for plasma display panels and hydrargyrum-free lamps. # 2006 Elsevier Ltd. All rights reserved. Keyword: A. Optical materials; B. Chemical synthesis; D. Luminescence
1. Introduction The flat panel display (FPD) such as the liquid crystal display (LCD), the electroluminescence display (ELD) and the plasma display panel (PDP) have a competitive edge in the large screen display market compared with large screen cathode ray tubes (CRT) because of their small size. When the screen size is greater than 40 in. or more, PDP are highly accepted because they not only involve relatively simple fabrication processes compared to the other two flat panels, but also offer a wide screen area (in excess of 40 in.), a flat profile, a wide viewing angle, and other advantages [1,2]. In PDP, three inorganic luminescent materials excited with 147 nm emit red, green and blue light. As far as green phosphor is concerned, Zn2SiO4:Mn is currently used today, however the performance of the PDP is disturbed by the properties of this phosphor because of its long decay time which makes the picture delayed. Zn2SiO4:Mn has increased decay time that constitutionally comes from the spin-forbidden emission (4T1 ! 6A1) of the activator Mn2+ ion [3]. Looking for an efficient green vacuum ultraviolet (VUV) phosphor is an urgent need. It is well known that Tb3+ has a reasonable green emission resulted from the 5D4 ! 7F5 transition [4–6]. As a host lattice, the rare-earth orthoborate * Corresponding author. Tel.: +86 931 8912772; fax: +86 931 8913554. E-mail address:
[email protected] (Y.H. Wang). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.05.031
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exhibits high absorption in VUV region (100–200 nm) and is stable at PDP operation temperature, especially the commercially applied red phosphors also take (Gd, Y) BO3 as host matrix. It can be expected that Tb3+ activated orthoborate will be a better green VUV phosphor candidate for PDP applications. Furthermore, if all tricolor phosphors choose orthoborate as a matrix, the process of PDP manufacturing can be simplified. The YBO3 doped with europium phosphors are mainly prepared by solid-state reaction (SR), sol–gel process (SG) and ultrasonic spray pyrolysis [7,8]. Every method has its advantages and disadvantages. For example, the solid-state reaction is easily operated, but large particles and agglomerates were often achieved. The spray pyrolysis process can produce spherical particles, but this method needs special equipment that is not widely available. The hydrothermal method is a wet-chemical technique for directly forming complex oxide powders as well as controlling their microstructure. And, the experiment where YBO3 takes terbium as an activator synthesized by a hydrothermal reaction was rarely reported [9]. In this study, in order to identify the new green VUV phosphor for PDP and possible applications for Hg-free lamps, YBO3:Tb phosphors were synthesized by a hydrothermal reaction and their photoluminescence were studied under UV and VUV excitation. 2. Experimental procedures The starting materials were Y2O3 (99.99%), Tb4O7 (99.99%), H3BO3 (99.5%). Stoichiometric amounts of each reagent were dissolved in diluted nitric acid by heating. The solution was evaporated until dry. The mixture was then transferred into a Teflon-lined, stainless steel autoclave with a filling capacity of 40% distilled water. The hydrothermal reaction lasted 6 h at different temperatures. A series of samples were prepared by doping different Tb contents under the same conditions. For comparison, the same Tb content phosphors were also synthesized by solidstate reaction at 1100 8C for 2 h. The crystal structure of the samples was characterized by X-ray powder diffractometry (XRD; Model D/max-2400, Rigaku Co. Ltd., Tokyo, Japan) operating at 40 kV/60 mA, using monochromatized Cu Ka radiation. Scanning electron microscopy (SEM; Model JSM-5600LV, Japan Electron Optics Laboratory Co. Ltd., Tokyo, Japan) was used to observe particle morphology. Measurements of infrared absorption (IR; Model Nexus 670, Nicolet Instrument Corporation, Madison, WI, USA) and nuclear magnetic resonance (NMR; Model Varian Infinity Plus 400, Varian Inc., USA) were carried out to study the coordination of the boron in the YBO3. The photoluminescence (PL) of samples were measured by UV spectrometer (Model RF-540, Shimadzu Corporation, Kyoto, Japan). 3. Results and discussion Fig. 1 shows the XRD patterns of YBO3 synthesized by the hydrothermal method at different temperatures. The location of each peak in the three samples are almost the same and are attributed to hexagonal symmetry. The lattice parameters (a) and (c) of the sample prepared at 260 8C are 0.3774 and 0.8791 nm, respectively. The relative intensity of different peaks changes with synthesis temperature. For example, the intensity ratio of (0 0 2) to (1 0 0) and (0 0 4)
Fig. 1. The X-ray powder diffractometry patterns of YBO3 synthesized at different temperatures.
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Fig. 2. The X-ray powder diffractometry patterns of Y1xTbxBO3 synthesized by solid-state process with x = 0.12 (a) and hydrothermal process with x = 0.12 (b) and x = 0.22 (c).
to (1 0 2) increases with an increase in preparation temperature. The result is in accordance with Wang’s work [9]. It indicates the preferential nucleation and growth of crystals. The phosphors Y1xTbxBO3 (0.00 x 0.28) and (0.00 x 0.16) were prepared by hydrothermal reaction (HR) and solid-state reaction (SR), respectively. As an example the XRD profiles for Y1xTbxBO3 phases with x = 0.12, 0.22 synthesized by HR and x = 0.12 synthesized by (SR) for comparison are showed in Fig. 2. No obvious shifting of the diffraction peaks is observed as x increases for phosphor synthesized by HR. All the peaks are indexed to hexagonal symmetry. The lattice parameters (a) and (c) of the phosphor with x = 0.22 are 0.3788 and 0.8841 nm, respectively. It can also be seen from Fig. 2 that no peaks corresponding to any of the second phases are found. This suggests that a well-crystalline, pure YBO3 is obtained and the Tb3+ ion is successfully doped into the matrix. The relationship between cell volume and Tb content are shown in Fig. 3. The hexagonal cell volume increased with the increased quantity of Tb3+ ions. The reason for this trend is that ˚ ) [10] is larger than that of Y3+ (1.015 A ˚ ) [11]. the radius of Tb3+ (1.181 A Fig. 4 shows IR absorption spectrum of YBO3 synthesized by hydrothermal process at 260 8C for 6 h. The peaks of 3404 and 1637 cm1 are ascribed to the OH stretching vibration mode and the HOH bending vibration mode of the H2O molecule, respectively. The reason for the existence of water is that the sample absorbed moisture during the IR measurement. Trivalent rare-earth orthoborates are isostructural with one of the three forms of calcite, aragonite and vaterite. Both the calcite and the aragonite-type borates involve trigonal boron with a monomeric ion, and the vaterite-
Fig. 3. The relationship between cell volume and Tb content.
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Fig. 4. The infrared spectrum of YBO3 prepared by hydrothermal method.
type borates involve tetrahedral boron with a cyclic trimeric ion. So both the calcite and the aragonite-type borates have n3 (asymmetric stretching) near 1300 cm1, n2 (out-of-plane bending) near 740 cm1, and n4 (in-plane bending) in the region 670–600 cm1. The vibrational spectra of vaterite-type borates are found to be markedly different from those of calcite and aragonite-type borates. There is a group of bands in the region 800–1200 cm1 in vaterite-type borates. The IR spectrum of this work was in accordance with the references [12–14]. Accordingly, the intense absorption band extending from 800 to 1200 cm1 ascribe to a vaterite-type borate. 436 cm1 is the out-of-plane blending of the BO4 group. A small peak at 1383 cm1 is asymmetric stretching of BO3 group. However, the peak near 570 cm1 cannot be distinguished. It can be an in-plane blending of the BO4 group or the BO3 group. Therefore, the information from the IR spectrum indicates that YBO3 is vaterite-type borate containing both BO4 tetrahedrons and BO3 trigonal (lesser), every three BO4 tetrahedron polymerized into a B3O99 ring as shown in Fig. 5. To affirm the coordination of boron atoms in YBO3 concluded from IR absorption spectrum, the nuclear magnetic resonance measurement [11,15] was performed. Fig. 6 shows NMR signals of B11 nuclei in the YBO3 sample derived from a hydrothermal process at 260 8C with 6 h. There are three peaks which appeared around 0, 80, 75 ppm, respectively. The intense peak at 0 ppm corresponds to boron in four-fold coordination. The result concluded from the NMR signals confirm the result concluded from the IR absorption spectra. Fig. 7(a) is an SEM photograph for Y0.88Tb0.12BO3 synthesized by the hydrothermal process at 260 8C with 6 h. The phosphor particles are flake-like and prove the conception of preferential nucleation in the hydrothermal synthesis concluded from the XRD patterns. The size of the particle is 8 mm on average. The morphology of other Tb content phosphors in this work are also flake-like and the particle sizes are almost same with Y0.88Tb0.12BO3 phosphor. For comparison the morphology of the same constituent phosphor prepared by SR is also shown in Fig. 7(b). It can be seen that the phosphor particles (Fig. 7(b)) are agglomerated, irregular and the particle size is approximately 3 mm.
Fig. 5. The structure of B3O99.
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Fig. 6. The nuclear magnetic resonance spectrum of YBO3 derived from hydrothermal process.
Fig. 8 shows the excitation (a) and emission (b) spectra of Y0.78Tb0.22BO3 prepared by the hydrothermal process at 260 8C for 6 h and Y0.88Tb0.12BO3 prepared by solid-state reaction. In the case of Y0.78Tb0.22BO3 prepared by HR, upon emission at 550 nm, there are two main peaks at about 240 and 275 nm corresponding to the spin allowed and the spin forbidden 4f to 5d transitions of Tb3+ in the excitation spectrum, respectively. In the emission spectrum, the
Fig. 7. The scanning electron microscopy images of Y0.88Tb0.12BO3 prepared by (a) hydrothermal process at 260 8C with 6 h and (b) solid-state process.
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Fig. 8. (a) The excitation spectra of Y0.78Tb0.22BO3 prepared by hydrothermal process. (b) The emission spectra of Y0.78Tb0.22BO3 prepared by hydrothermal reaction and Y0.88Tb0.12BO3 prepared by solid-state reaction.
phosphor exhibits a characteristic Tb3+ emission. The main emission peaks located near 500, 550, 590 and 628 nm are transitions from 5D4 to 7F6, 7F5, 7F4, 7F3, respectively. The green emission of 550 nm is dominant. The results are similar to that of the phosphors prepared by SR, but the excitation and emission intensity of the phosphors prepared by HR is higher than that of the phosphor prepared by SR. Generally under UV irradiation, Tb3+ is excited to 5D3 and 5D4 state at low Tb concentrations. The two excited states transport to 7FJ (J = 1, 2, 3, 4, 5, 6, respectively) leading to blue and green light. With the Tb concentration increasing, the 5D3 emission decreases and 5D4 emission increases for the occurrence of the radiationless relaxation from 5D3 state to lower 5D4 level that is called cross-relaxation. In this work, under 290 nm excitation, a blue emission attributed to the 5D3 to 7FJ transition at about 485 nm is observed when Tb% is less than 6%. Moreover, when the Tb% increased to 6%, the blue emission vanished because of the cross-relaxation (the 5D3 to 5D4 transition). In other words, the 5D3/5D4 intensity ratio is much lower than the solid-state derived phosphor at high concentration. Fig. 9 is the change of intensity emitting at 550 nm under 254 nm excitation, with activator Tb concentration in Y1xTbxBO3 phosphor synthesized by HR and SR. For the phosphors prepared by HR, the emitting intensity increases with Tb substitution up to 0.22 and then decreases for additional concentrations of Tb. This phenomenon is due to concentration quenching effect, the distance between two Tb3+ ions decreases when Tb3+ concentration increases, so the energy transfer between each Tb3+ ion becomes easier, which leads the concentration quenching to occur. As for phosphors prepared by SR the concentration for maximal emission intensity is 0.12. In samples synthesized by solidstate reaction the Tb3+ are perhaps more clustered and this leads to the observation of lower concentration quenching. More details about the quenching mechanism is under investigation.
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Fig. 9. The intensity of the emission at 550 nm with different Tb concentration.
The photoluminescence of YBO3:Tb in VUV region is currently being studied. Furthermore, the energy level of Ce3+, Gd3+ and Tb3+ indicates the possible energy transfer from Ce3+, Gd3+ to Tb3+. In our next work, we plan to dope Ce3+ and (or) Gd3+ as the sensitizer into the phosphor to enhance green emission brightness. 4. Conclusions In order to find an efficient green VUV phosphor, a series of Y1xTbxBO3 (0.00 x 0.28) phosphors were successfully prepared by a hydrothermal soft chemical process and the photoluminescence were investigated. The vaterite Y1xTbxBO3 (0.00 x 0.28) phosphors with uniform size and plate morphology were prepared by a mild hydrothermal process, which is about 900 8C lower than the solid-state reaction. The optimum luminescence intensity of the hydrothermal phosphors is much higher than those prepared by the solid-state process under ultraviolet excitation. YBO3:Tb as-prepared by hydrothermal synthesis is a promising phosphor for applications in lamps and displays. Acknowledgements This work was supported by the National High Technology Research and Development Program of China (863 Program, 2003AA324020), the NSFC (50272026), the EYTP (Excellent Young Teachers Program of MOE, China) and the Natural Science Foundation of Gansu Province, China (ZS031-A25-033-C). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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