Synthesis and luminescent properties of Zn0.890Nb2O6:Eu0.053+, Bi0.0053+, M0.055+ (M = Li, Na, K) phosphors

Current Applied Physics 13 (2013) 331e335

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3þ þ Synthesis and luminescent properties of Zn0.890Nb2O6:Eu3þ 0.05, Bi0.005, M0.055 (M ¼ Li, Na, K) phosphors

Fuwang Mo a, Liya Zhou a, *, Qi Pang b, Yuwei Lan a, Zhijuan Liang a a b

School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China Department of Chemistry and Biology, Yulin Normal University, Yulin 537000, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 May 2012 Received in revised form 30 July 2012 Accepted 7 August 2012 Available online 28 August 2012

3þ 3þ 3þ 3þ þ Zn1exNb2O6:Eu3þ x , Zn0.95yNb2O6:Eu0:05 , Biy , and Zn0.890Nb2O6:Eu0:05 , Bi0:005 , M0:055 (M ¼ Li, Na, K) redemitting phosphors were synthesized via solegel method. X-ray power diffraction, scanning electron microscopy, photoluminescence excitation, and emission spectra (PL) were used to characterize the phosphors. The obtained Zn1xNb2O6:Eu3þ x phosphor showed a stronger excitation band near 400 nm. When Eu3þ and Bi3þ were incorporated into the ZnNb2O6 lattice, a broad band from 300 nm to 350 nm with the center at 329 nm appeared. Taking the ion size difference of Liþ (59 pm), Naþ (99 pm), Kþ (137 pm), Eu3þ (95 pm), and Zn2þ (60 pm), the emitting intensity of the phosphor increased observably by adding Liþ and Naþ as charge compensators. The PL intensity of the Kþ-doped in Zn0.945Nb2O6:Eu3þ 0:05 , 3þ 3þ Bi3þ 0:005 was slightly less than those of Zn0.945Nb2O6:Eu0:05 , Bi0:005 . The chromaticity coordinates of 3þ þ Zn0.890Nb2O6:Eu3þ 0:05 , Bi0:005 , Li0:055 (x ¼ 0.67, y ¼ 0.34) were close to the standard of National Television Standard Committee values (x ¼ 0.670, y ¼ 0.330). The fabricated light-emitting diode (LED) further 3þ þ confirmed that the Zn0.890Nb2O6:Eu3þ 0:05 , Bi0:005 , Li0:055 phosphors can efficiently absorb up to 400 nm irradiation and emit red light, and are potential candidates as red-emitting components for white LED. Ó 2012 Elsevier B.V. All rights reserved.

Keywords: Luminescence Phosphor Solegel method

1. Introduction White light-emitting diode (WLED) is mostly recognized as a fourth generation illuminate lamp after incandescent lamps, fluorescent lamps, high-temperature tungsten filament lamps, and vapor discharge lamps. Due to its advantages in terms of energy saving, environmental friendliness, and long-lasting performance, LED is now known as the new lamp-house of the 21 century [1e3]. At present, two dominant commercial methods are available to achieve WLEDs: blue LED chip þ yellow phosphor (or green/red phosphors) and near-UV chip þ red/green/blue phosphors [4,5]. However, there exist at least two drawbacks in the aforementioned combinations. First, the overall efficiency is decreased rapidly when the correlated color temperature of the device is low. Second, the output light is deficient in the red region of the visible light spectrum [6e8]. Therefore, new red phosphors that can be excited efficiently under the near-UV range of 400 nm with intense emission and appropriate CIE chromaticity coordinates are essential. Eu3þ ion is an ideal red phosphor activator due to its 4f6 electronic configuration [9e13]. The Bi3þ ion was chosen for its closedshell 6s2 configuration, which is known to strongly influence the

* Corresponding author. Tel./fax: þ86 771 3233718. E-mail address: [email protected] (L. Zhou). 1567-1739/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cap.2012.08.004

luminescence of d0-complexes, such as the niobate group. Bi3þ acts as a sensitizer of Eu3þ through energy transfer from Bi3þ to Eu3þ in systems such as Y2O3 co-doped with Eu, Bi [14], LnVO4: Eu, Bi (Ln ¼ Y, La, Gd) [15], and Al2O3:Eu3þ, Bi3þ [16]. Yu-Jen Hsiao et al. [17] studied the synthesis and luminescence of ZnNb2O6. Fabrication of ZnNb2O6 has been studied by Cheng-Hsing Hsu et al. [18]. The structure and luminescence of LnNbO4:Bi3þ (La ¼ La, Gd, and Y) and YNbO4:Bi3þ, Dy3þ were reported by Xiuzhen Xiao [19]. Yuanyuan Zhou [20] et al. studied the photoluminescence of pure and Dy-doped ZnNb2O6 nanoparticles. However, to the best of our knowledge, there has been 3þ 3þ no report so far on Zn1xNb2O6:Eu3þ x , Zn0.95yNb2O6:Eu0:05 , Biy and 3þ þ , Bi , M (M ¼ Li, Na, K) phosphors. In the Zn0.890Nb2O6:Eu3þ 0:005 0:05 0:055 3þ , Zn Nb O :Eu present paper, a series of Zn1xNb2O6:Eu3þ 0.95y 2 6 0:05 0:05 , 3þ 3þ þ and Zn Nb O :Eu , Bi , M (M ¼ Li, Na, K) phosBi3þ 0.890 2 6 y 0:005 0:05 0:005 phors were synthesized by solegel method, and their luminescent properties were investigated. 2. Experimental 2.1. Synthesis 3þ The red phosphors Zn1xNb2O6:Eu3þ x , Zn0.95yNb2O6:Eu0:05 , 3þ 3þ þ and Zn0.890Nb2O6:Eu0:05 , Bi0:005 , M0:055 (M ¼ Li, Na, K) redemitting phosphors were prepared by the solegel method using

Bi3þ y ,

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Eu2O3 (99.99%), Bi2O3 (99.99%), Nb2O5(99.99%), alkali metal carbonates (A.R.), and Zn(NO3)2$6H2O (A.R.) as raw materials. A stoichiometric amount of Eu2O3 (99.99%), Bi2O3 (99.99%) and carbonates were dissolved in dilute nitric acid. The solution was then heated. While heating, the proper amount of distilled water was added to adjust the pH value to about 2. The solution was brought to the desired volume by distilled water after cooling. Afterward, 0.05 mol/L Eu(NO3)3 solution, 0.02 mol/L Bi(NO3)3 solution and 0.05 mol/L MNO3 (M ¼ Li, Na, K) solution were prepared. A stoichiometric amount of Nb2O5 was dissolved in the right amount of hydrofluoric (HF) acid. The settled solution was prepared after heating in hot water bath for 8 h and was brought to volume by distilled water after cooling using HF to adjust the pH value of the solution to about 2. Thus, the 0.10 mol/L NbF5 solution was completed. A stoichiometric amount of Zn(NO3)2$6H2O was weighed and put in a 200 mL plastic beaker. A stoichiometric amount of NbF5, Eu(NO3)3, Bi(NO3)3 and MNO3 (M ¼ Li, Na, K) solution were added to the plastic beaker. A sufficient amount of citric acid was added to the former solution as a chelating agent to form another solution. Citric acid to the total metal ions in the molar ratio of 3:1 was used for this purpose, and the addition of ammonium hydroxide was continued until a pH of about 2 at room temperature was achieved. The solution was stirred for 3 h. After heating in a water bath at 80  C, a gel was formed after evaporating the water. After heating in the drying oven at the temperature of 170  C, the gel filled the beaker, producing a foamy brown precursor. The precursor was ground in an agate mortar and then put into a corundum crucible. The phosphors were obtained after calcination at 1100  C for 5 h in air.

3. Results and discussion 3.1. XRD characterization The precursor samples burnt at 170  C were calcined at 900, 1000, and 1100  C for 5 h, respectively, and their XRD patterns are shown in Fig. 1. From the XRD patterns, diffraction peaks of ZnNb2O6 appeared when the samples were calcined at 900  C. However, there were still some miscellaneous peaks, which indicate that the ZnNb2O6 phase began to form at 900  C. The XRD patterns of ZnNb2O6 calcined at 1000 and 1100  C matched well with the literature (PDF 37-1371). These diffraction peaks became sharper gradually with increasing calcination temperature. When the temperature was raised to 1100  C, the intensity of the XRD peaks was enhanced, the half-width of the peaks became smaller, and no new peaks appeared. The behavior of the diffraction peaks showed that the crystal degree increases and the particles become larger with increasing temperature. Hence, 1100  C was selected as the calcination temperature. 3.2. SEM characterization The SEM image of Zn0.95Nb2O6:Eu3þ 0:05 phosphor calcined at 1100  C is shown in Fig. 2. The particles exhibited a rather serious agglomerate phenomenon and flake-like shapes with diameter of about 2 mme3 mm because the particles were calcined at high temperature. 3.3. Luminescent properties of Zn1xNb2O6:Eu3þ x phosphors

2.2. Characterization Fig. 3 shows the excitation and emission spectra of the Zn0.80Nb2O6:Eu3þ 0:20 phosphor. The excitation spectrum consisted of a broad band ranging from 200 nm to 500 nm with a maximum value at 275 nm and some sharp lines at the longer wavelength region. The wide band at about 275 nm was attributed to the transition toward the charge transfer state due to EueO interaction. Apart from the charge transfer band, some sharp lines were also seen in the excitation spectrum of Eu3þ, which corresponded to the fef transitions, including 7F0 / 5H3(311 nm), 7F0 / 5D4(363 nm),

The structure characterization was performed by X-ray powder A Rigaku/Dmax-2500, Rigaku diffractometer (XRD, Cu Ka ¼ 1.5406 Corporation of Japan). Scanning electron microscopy (SEM) using Hitachi Model S-3400N was utilized to observe particle sizes and shapes. Excitation and emission spectra were measured with a Hitachi F-2500 fluorescence spectrophotometer using Xe lamp as the excitation source. All measurements were carried out at room temperature. 7000 6000 5000 4000 3000 2000 1000 0

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PDF#37-1371(ZnNb 2 O 6 )

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2 /degree Fig. 1. XRD patterns of ZnNb2O6 phosphor calcined at 900  C, 1000  C and 1100  C, respectively.

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ex = 395 nm 3+

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Wavelength/nm Fig. 4. Emission spectra for samples of Zn1xNb2O6:Eu3þ x . Inset: Emission spectra of 3þ doping ratios. Zn1xNb2O6:Eu3þ x phosphors with different Eu

Fig. 2. SEM image of ZnNb2O6 phosphor calcined at 1100  C.

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F0 / 5L7(384 nm), 7F0 / 5L6(395 nm, stronger), 7F0 / 5D3 (418 nm), 7F0 / 5D2(466 nm, strong), respectively. They all originated from transitions within Eu3þ 4f6 configuration [21]. The emission spectrum showed two strong sharp peaks at 595 nm and 614 nm corresponding to the magnetic dipole transition 5D0 / 7F1 and electric dipole transition 5D0 / 7F2 of Eu3þ emission, respectively. When Eu3þ ion is local in a low symmetry site, it displays 5 D0 / 7F2 electric dipole transition, which appears dominantly and is highly sensitive to its local environment. The transitions from 5 D0 / 7F1 located at 595 nm are relatively weak, and thus are advantageous to obtaining saturated CIE chromaticity. The chromaticity coordinates of the phosphor Zn0.95Nb2O6:Eu3þ 0:05 (lex ¼ 395 nm) are calculated to be x ¼ 0.66, y ¼ 0.34, which are close to the standard of NTSC (x ¼ 0.67, y ¼ 0.33). The effect of the Eu3þ content in the Zn1xNb2O6:Eu3þ x (x ¼ 0.05, 0.10, 0.15, 0.20, 0.25) phosphors on the relative PL intensity is shown in Fig. 4. All the shapes of the spectra were similar except for relative intensity. The PL intensity at 614 nm was enhanced with increasing doped-Eu3þ content and reached the highest value at x ¼ 0.20. By contrast, the PL intensity decreased when the value of x was over 0.20. This quenching process is often attributed to the concentration when it reaches a certain of Eu3þdoped in the Zn1xNb2O6:Eu3þ x

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3þ 3.4. Luminescent properties of Zn0.95yNb2O6:Eu3þ 0:05 , Biy phosphors

Fig. 5 shows the excitation and emission spectra of the 3þ (y ¼ 0.00, 0.005, 0.01, 0.03, and 0.05) Zn0.95yNb2O6:Eu3þ 0:05 , Biy phosphors. From Fig. 5, when Eu3þ and Bi3þ ions were co-doped into the ZnNb2O6 lattice, a broad band from 300 nm to 350 nm with the center at 329 nm appeared due to the Bi3þ / O2 and Eu3þ / O2 charge transfer transition [25]. Moreover, the intensity of the excitation peak at 329 nm was enhanced with the increasing codoped-Bi3þ content and reached the highest at y ¼ 0.005. The shapes of the emission spectra of all samples were very similar except for intensity, indicating that the introduction of Bi3þ ions did not change the sublattice structure around the luminescent center of the Eu3þ ions. The main emission line was the 5D0 / 7F2 transition of Eu3þ at 614 nm and 5D0 / 7F1 transition of Eu3þ at 595 nm. The emission wavelength was independent of the concentration of Bi3þ, but the luminescent intensity was greatly influenced by the concentration of Bi3þ. The relative intensity reached its maximum value when the Bi3þ concentration was 0.5 mol %, which suggests that the energy absorbed by Bi3þ was transferred to the Eu3þ levels nonradiatively [19,26e28]. Additionally, the emission intensity of 3þ Zn0.945Nb2O6:Eu3þ 0:05 , Bi0:005 at 614 nm was almost equal to that of 3þ Zn0.80Nb2O6:Eu0:20 at 614 nm. The chromaticity coordinates of the 3þ phosphor Zn0.945Nb2O6:Eu3þ 0:05 , Bi0:005 (lex ¼ 395 nm) were calculated to be x ¼ 0.66, y ¼ 0.34, which were close to the standard of NTSC (x ¼ 0.67, y ¼ 0.33). 3þ þ 3.5. Luminescent properties of Zn0.890Nb2O6:Eu3þ 0:05 , Bi0:005 , M 0:055 (M ¼ Li, Na, K) phosphors

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The effect of charging compensators Liþ, Naþ, and Kþ ions on the 3þ þ red emission of Zn0.890Nb2O6:Eu3þ 0:05 , Bi0:005 , M0:055 is shown in Fig. 6. All the shapes of the spectra were similar except for relative

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value, upon which the energy of the electron in the excited state will be transmitted to the quenching center more easily. Moreover, an excessive amount of Eu3þ doped in Zn1xNb2O6:Eu3þ x will break the crystallization of the matrix [22e24].

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Wavelength/nm Fig. 3. Excitation (lem ¼ 614 nm, left) and emission (lex ¼ 395 nm, right) spectra of Zn0.80Nb2O6:Eu3þ 0:20 phosphor.

3þ intensity. Compared with the Zn0.945Nb2O6:Eu3þ 0:05 , Bi0:005 phosphor, the emitting intensity of the phosphor increased observably by adding Liþ and Naþ as charge compensators, and the emission intensity was about 1.9 and 1.2 times higher than that of

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3þ Fig. 5. Excitation (lem ¼ 614 nm, A) and emission (lex ¼ 395 nm, B) spectra of Zn0.95yNb2O6:Eu3þ 0:05 , Biy (y ¼ 0.00, 0.005, 0.01, 0.03, 0.05).

3þ Zn0.945Nb2O6:Eu3þ 0:05 , Bi0:005 , respectively. By contrast, the PL 3þ intensity of the Kþ-doped in Zn0.945Nb2O6:Eu3þ 0:05 , Bi0:005 was

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3þ slightly less than those of Zn0.945Nb2O6:Eu3þ 0:05 , Eu0:05 . The differþ þ ence in the ionic radii of Li (59 pm), Na (99 pm), Kþ (137 pm), Eu3þ (95 pm), and Zn2þ (60 pm) will result in a somewhat diverse sublattice structure around the luminescent center ions [29,30]. The ionic radius of Liþ (59 pm) and Naþ (99 pm) were similar to that of Zn2þ (60 pm) and Eu3þ (95 pm) so that Liþ and Naþ ions can substitute Zn2þ and Eu3þ ions without disturbing the crystal lattice. Taking the ion size difference of Kþ and Zn2þ into consideration, the crystal structure may be distorted and the luminescence intensity may be decreased. The chromaticity coordinates of the phosphor

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3þ þ Zn0.890Nb2O6:Eu3þ 0:05 , Bi0:005 , Li0:055 (lex ¼ 395 nm) were calculated to be x ¼ 0.67, y ¼ 0.34, which were also close to the standard of NTSC (x ¼ 0.67, y ¼ 0.33).

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Wavelength/nm 3þ Fig. 6. Effect of Mþ (M ¼ Li, Na, K) on emission spectra of Zn0.890Nb2O6:Eu3þ 0:05 , Bi0:005 , phosphor. Mþ 0:055

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3þ þ 3.6. Fabrication of LED with the Zn0.890Nb2O6:Eu3þ 0:05 , Bi0:005 , Li0:055 phosphor 3þ A LED was fabricated by coating the Zn0.890Nb2O6:Eu3þ 0:05 , Bi0:005 , phosphor onto a 370 nm-emitting InGaN chip. The emission Liþ 0:055 spectra of the original 370 nm-emitting InGaN chip (a) and the red3þ þ emitting LED with Zn0.890Nb2O6:Eu3þ 0:05 , Bi0:005 , Li0:055 (b) under 20 mA forward bias are shown in Fig. 7. The band close to 370 nm was attributed to the emission of the InGaN chip and the shoulder peak located at about 400 nm is led by the absorption of the red phosphor [31]. The sharp peak at 616 nm was generated by the 3þ absorption of the coated phosphor of Zn0.890Nb2O6:Eu3þ 0:05 , Bi0:005 , 3þ þ 3þ phosphor. Zn Nb O :Eu , Bi , Li can absorb Liþ 0.890 2 6 0:055 0:005 0:055 0:05 close to 400 nm excitation energy efficiently, so it is considered a good candidate for the red component of WLED application.

D0- F2

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4. Conclusions 0.2 3þ 3þ and Zn0.890Nb2O6: Zn1xNb2O6:Eu3þ x , Zn0.95yNb2O6:Eu0:05 , Biy 3þ 3þ þ Eu0:05 , Bi0:005 , M0:055 (M ¼ Li, Na, K) phosphors were prepared by sole

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gel method and the structures and photoluminescence properties of the phosphors as-prepared were studied in detail. The emission spectra showed strong red emission at 614 nm corresponding to the 3þ 3þ D0 / 7F2 transition of Zn1xNb2O6:Eu3þ x , Zn0.95yNb2O6:Eu0:05 , Biy

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Fig. 7. Emission spectra of the original 370 nm-emitting InGaN chip (a) and the red3þ þ emitting LED with Zn0.890Nb2O6:Eu3þ 0:05 , Bi0:005 , Li0:055 (b) under 20 mA forward bias.

3þ þ and Zn0.890Nb2O6:Eu3þ 0:05 , Bi0:005 , M0:055 (M ¼ Li, Na, K) phosphors

F. Mo et al. / Current Applied Physics 13 (2013) 331e335

under near-UV excitation (395 nm). Within the co-activator Bi3þ, a broad band from 300 nm to 350 nm with the center at 329 nm appeared due to the Bi3þ / Nb5 and Bi3þ / O2 charge transfer transition in the 300 nme350 nm region. The PL intensities of the Liþ 3þ and Naþ doped in Zn0.945Nb2O6:Eu3þ 0:05 , Bi0:005 were higher than those 3þ þ of Zn0.945Nb2O6:Eu3þ 0:05 , Bi0:005 . The PL intensity of the K -doped in 3þ Zn0.945Nb2O6:Eu3þ 0:05 , Bi0:005

was slightly less than those of

Bi3þ 0:005 . The fabricated LED further 3þ þ Zn0.890Nb2O6:Eu3þ 0:05 , Bi0:005 , Li0:055 phosphors can

Zn0.945Nb2O6:Eu3þ 0:05 ,

confirmed

efficiently that absorb close to 400 nm irradiation and emit red light. Thus, this phosphor is a potential candidate as a red-emitting component for WLED. Acknowledgements This work was financially supported by grants from the National Natural Science Foundation of China (No 61066006; 61264003); Science Foundation of Guangxi Province (No. 2011GXNSFA018054). Appendix A. Supplementary material Supplementary material associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cap.2012. 08.004. References [1] S. Ye, F. Xiao, Y.X. Pan, Y.Y. Ma, Q.Y. Zhang, Mater. Sci. Eng. R 71 (2010) 1e34. [2] J.K. Sheu, S.J. Chang, C.H. Kuo, Y.K. Su, L.W. Wu, Y.C. Lin, W.C. Lai, J.M. Tsai, G.C. Chi, R.K. Wu, Ieee Photonic. Tech. L 15 (2003) 18e20.

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