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Photoluminescence of NiNb2O6 nanoparticles prepared by combustion method
Materials Science and Engineering B 140 (2007) 128–131
Short communication
Photoluminescence of NiNb2O6 nanoparticles prepared by combustion method Yuanyuan Zhou, Mengkai L¨u ∗ , Zifeng Qiu, Aiyu Zhang, Qian Ma, Haiping Zhang, Zhongsen Yang State Key Laboratory of Crystal Materials, Shandong University, Shanda Nan Road, Jinan 250100, PR China Received 14 January 2007; received in revised form 13 March 2007; accepted 7 April 2007
Abstract Pure and Dy3+ -doped NiNb2 O6 nanoparticles have been prepared by a sol–gel combustion method using citric acid as fuel and complexing agent and nitrates as oxidants at a relatively low temperature compared to solid-state reaction method. The photoluminescence properties of these NiNb2 O6 nanoparticles were studied in detail. For the pure NiNb2 O6 nanoparticles, a blue emission band centered at 445 nm is observed. For NiNb2 O6 :Dy3+ nanoparticles, no characteristic luminescence of Dy3+ can be observed while the host blue emission enhances. It is suggested that energy transfer from Dy3+ to niobate groups maybe occurs. © 2007 Published by Elsevier B.V. Keywords: Combustion; Luminescence; NiNb2 O6
1. Introduction Recently, the photoluminescence properties of niobate have been extensively studied. For example, luminescence properties of rare-earth-metal ion-doped KLaNb2 O7 with layered perovskite structures have been investigated by Akihiko Kudo [1]; Ba5 Nb4 O15 exhibits a yellow emission upon excitation with short wavelength ultraviolet radiation [2]; the luminescence of niobate and energy transfer process between niobate groups and doped ions have been observed in La2 O3 –Nb2 O5 –B2 O3 glasses [3–5]; lanthanum niobates is also an interesting host for rareearth ions to fabricate luminescent materials [6–8]. But to our best knowledge, there are few investigations on transition metal niobate, e.g. NiNb2 O6 . NiNb2 O6 compound has two different structures: columbite (orthorhombic)-type and trirutile (tetragonal)-type. NiNb2 O6 with columbite structure has a prospective potential for efficient H2 production from water photocatalytically under visible light irradiation [9]. NiNb2 O6 was also reported as a useful precursor for the preparation of lead nickel niobate (PNN), a typical relaxor ferroelectric material [10,11]. However,
∗
Corresponding author. E-mail address: [email protected] (M. L¨u).
0921-5107/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.mseb.2007.04.002
no study about NiNb2 O6 with trirutile structure has been reported. Traditional solid-state mixed oxide method has been used in the preparation of single-phase NiNb2 O6 powders. However, powders prepared by the mixed oxide route have spatial fluctuations in their compositions and calcination conditions used in the mixed oxide process should be optimized [12]. It is necessary to develop a time-saving method to prepare NiNb2 O6 with pure phase at low temperature, and combustion synthesis may be a good choice because it can guarantee high purity, compositionally uniform, single phase, and small and uniform particle size in one single step [13,14]. Therefore, the main purpose of this work is to explore a simple sol–gel combustion synthetic route for the preparation of single-phase NiNb2 O6 powders with trirutile structure. The luminescence properties of pure and Dy3+ -doped powders will be studied and discussed in detail. The calcination temperature in this work is much lower than that in solid-state reaction method. This clearly emphasizes the advantages of combustion technique. In addition, Dy3+ is a very important activator for the luminescent materials because of its characteristic emissions due to the blue transition (4 F9/2 → 6 H15/2 ) and yellow one (4 F9/2 → 6 H13/2 ), respectively [15,16]. It is interesting to study the energy transfer process between Dy3+ and niobate groups.
Y. Zhou et al. / Materials Science and Engineering B 140 (2007) 128–131
129
2. Experimental
3. Results and discussion
Pure and Dy3+ -doped NiNb2 O6 nanocrystals have been synthesized via a sol–gel combustion process. Nickel nitrate (Ni(NO3 )2 ·6H2 O), niobium oxide (Nb2 O5 ), dysprosium oxide (Dy2 O3 ) and ammonium nitrate (NH4 NO3 ) were used as starting materials. Citric acid was used as both complexing agent for gel process and fuel for the combustion. All the reagents were of analytical grade without further purification. In a typical synthesis, first, required quantity of Nb2 O5 was dissolved in HF acid (40%) after heating in a hot water bath. Then, ammonia solution was added to this NbF5 solution to obtain Nb2 O5 ·nH2 O. The Nb2 O5 ·nH2 O precipitate was filtered, washed and dissolved in citric acid aqueous solution under heating at 80 ◦ C. Then stoichiometric Ni(NO3 )2 ·6H2 O, excessive NH4 NO3 were added and mixed homogeneously. After the water evaporated, the solution was turned into a yellow gel with high viscosity. The gel was then introduced into a muffle furnace preheated to 700, 750 and 800 ◦ C, respectively. As the ignition occurred, the reaction went on vigorously for a few seconds. Then, yellow fluffy products were obtained after the combustion reaction. The synthesis procedure of NiNb2 O6 :Dy3+ was adopted as the above mentioned except that Dy(NO3 )3 was introduced. Dy3+ was added in the form of Dy(NO3 )3 solution by dissolving Dy2 O3 into diluted HNO3 . The molar ratio of Dy3+ varied from 2% to 5% in relation to NiNb2 O6 . The phase was characterized by X-ray diffraction (XRD) using a Germany Bruker axs D8-advanced X-ray diffractometer system with graphite monochromatized Cu K␣ irradiation ˚ The morphology and microstructure were (λ = 1.5418 A). characterized with a Japan JEM-100CXII transition electron microscope. Absorption spectra were recorded on a U-3500 spectrophotometer. Excitation and emission spectra were measured on an Edinburgh 920 fluorescence spectrophotometer. All the measurements were taken at room temperature.
The crystal structures of the NiNb2 O6 compounds were investigated using the powder X-ray diffraction method. Fig. 1 shows XRD patterns of pure NiNb2 O6 samples prepared at different temperatures. All peaks correspond to tetragonal phase of NiNb2 O6 , which is in agreement with the literature (JPCDS no. 76-2355). The major peaks can be well indexed as (1 1 0), (1 0 1), (2 0 0), (1 1 1), (2 1 1), (2 2 0), (3 1 0) and (3 0 1), respectively. The calculated lattice parameters for the sample prepared ˚ and c = 3.05 A, ˚ respectively. No Charat 750 ◦ C are a = 4.70 A acteristic peaks arising from possible impurities are detected. It
Fig. 1. XRD patterns of the pure NiNb2 O6 samples prepared at 700, 750 and 800 ◦ C, respectively.
Fig. 2. TEM images of the undoped NiNb2 O6 samples prepared at 700, 750 and 800 ◦ C, respectively.
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Y. Zhou et al. / Materials Science and Engineering B 140 (2007) 128–131
Fig. 3. Absorption spectrum (a) and excitation spectrum (λem = 440 nm) (b) of the pure sample prepared at 750 ◦ C. Emission spectra (λex = 380 nm) (c) of the pure NiNb2 O6 samples prepared at 700, 750 and 800 ◦ C, respectively.
can also be observed that the diffraction peaks become sharp with increasing the reaction temperature, indicating better crystallinity of the samples, and the particle sizes calculated from Scherrer’s formula [17] are 22, 32 and 41 nm, respectively, for samples prepared at 700, 750 and 800 ◦ C. The particle size and morphology of the NiNb2 O6 powders obtained at different temperatures were examined by transmission electron microscopy as shown in Fig. 2. For all samples, the particles are irregular in shape and agglomerated. The average particle sizes for samples prepared at 700, 750 and 800 ◦ C are around 20, 30 and 43 nm, respectively, which are consistent with the XRD results. In addition, the degree of agglomeration tends to decrease with increasing calcination temperature. Fig. 3(a) shows the typical absorption spectrum of the pure sample prepared at 750 ◦ C. The broad band from 250 to 400 nm is attributed to a charge transfer from the oxygen ligands to the central niobium atom inside the NbO6 group. Fig. 3(b) shows the excitation spectrum (λem = 440 nm) of the same sample analysed in Fig. 3(a). Two broad excitation bands centered at 340 and 378 nm, respectively, can be observed. Compared with the absorption spectrum and the excitation spectrum, the absorption
edge is at about 378 nm and the band gap energy is Eg = 3.28 eV. This value is different from that reported by Jinhua Yea et al. [9] Fig. 3(c) shows the luminescence spectra of pure NiNb2 O6 samples (λex = 380 nm) prepared at different temperatures. A strong blue emission band centered at 440 nm can be observed for samples prepared at 700 and 750 ◦ C. This emission can be ascribed to the distorted NbO6 groups [2–5]. In the trirutile NiNb2 O6 crystal structure, there are two kinds of octahedra, NbO6 and NiO6 in the structure. The NiO6 and NbO6 octahedra connect to each other by sharing edges to form a straight chain along [0 0 1] direction. Octahedra chains link to each other by sharing corner O atoms [9]. Isolated and edge- or face-shared MO6 (M = Nb, Ta) octahedral groups show efficient luminescence with a large Stokes shift while corner-sharing of MO6 groups leads to exciton delocalization, smaller Stokes shift, lower energy band, energy migration and consequent luminescence quenching [3–5]. So the edge-shared NbO6 chains show efficient blue emission. It is noted that the emission intensity is lower in samples synthesized at higher temperature, and for sample prepared at 800 ◦ C, this blue emission is quenched, indicating that the cal-
Y. Zhou et al. / Materials Science and Engineering B 140 (2007) 128–131
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Fig. 4(b) shows the absorption spectrum of the 4 mol% Dydoped sample. Compared with the absorption spectrum of the undoped NiNb2 O6 (Fig. 3(b)), the absorption increases by the Dy doping. So the emission increase may be attributed to the energy transfer from Dy3+ to NbO6 groups. When the doping concentration of Dy3+ excesses 4 mol%, the emission intensity decreases due to concentration quenching effect. 4. Conclusion In summary, NiNb2 O6 nanoparticles have been successfully prepared via a sol–gel combustion method at as low as 700 ◦ C. Pure NiNb2 O6 nanoparticles show a blue emission centered at 440 nm due to the distorted edge-shared NbO6 groups. For the NiNb2 O6 :Dy3+ nanoparticles, no characteristic luminescence of Dy3+ can be observed but the blue emission intensity of the host NiNb2 O6 increases until the doping concentration reaches 4 mol%. It is attributed to the energy transfer from Dy3+ to NbO6 groups. Acknowledgement This work is supported by the awarded funds of Excellent State Key Laboratory (No. 50323006). References
Fig. 4. Emission spectra (λex = 380 nm) (a) of NiNb2 O6 :Dy3+ nanoparticles prepared at 750 ◦ C with doping concentrations of 0, 2, 4 and 5 mol%, respectively, and absorption spectrum (b) of the 4 mol% Dy-doped sample.
cination temperature has a visible effect on the emission of NiNb2 O6 . Fig. 4(a) shows the corresponding emission spectra of NiNb2 O6 :Dy3+ . Upon excitation at 380 nm, the characteristic emissions of Dy3+ due to the transitions of blue transition (4 F9/2 → 6 H15/2 ) and yellow one (4 F9/2 → 6 H13/2 ), respectively, are too weak to be checked, but the strong and broad blue emission band corresponding to the host NiNb2 O6 remains. The influence of Dy3+ concentration on the luminescent properties of NiNb2 O6 :Dy3+ was also investigated. As given in Fig. 4(a), as the concentration of Dy3+ increases from 2 to 4 mol%, the emission intensity of NbO6 enhances. It can be seen that the optimum concentration of Dy3+ is 4 mol% and the emission intensity is nearly 3.25 times stronger than that of the pure NiNb2 O6 .
[1] Akihiko Kudo, Chem. Mater. 9 (1997) 664. [2] A.M. Srivastava, J.F. Ackerman, W.W. Beers, J. Solid State Chem. 134 (1997) 187. [3] A.C.V. de Ara´ujo, I.T. Weber, W.D. Fragoso, C. de Mello Doneg´a, J. Alloy. Compd. 275–277 (1998) 738. [4] W.D. Fragoso, Celso de Mello Doneg´a, R.L. Longo, J. Lumin. 105 (2003) 97. [5] W.D. Fragoso, Celso de Mello Doneg´a, R.L. Longo, J. Non-Cryst. Solids 351 (2005) 3121. [6] Xiuzhen Xiao, Bing Yan, J. Non-Cryst. Solids 351 (2005) 3634. [7] Xiuzhen Xiao, Bing Yan, J. Non-Cryst. Solids 352 (2006) 3047. [8] Xiuzhen Xiao, Bing Yan, J. Alloy. Compd. 421 (2006) 252. [9] Jinhua Yea, Zhigang Zou, Akiyuki Matsushita, Int. J. Hydrogen Energy 28 (2003) 651. [10] E.F. Alberta, A.S. Bhalla, Mater. Lett. 54 (2002) 47. [11] Chao Lei, Kepi Chen, Xiaowen Zhang, Jun Wang, Solid State Commun. 123 (2002) 445. [12] O. Khamman, R. Yimnirun, S. Ananta, Mater. Lett. 61 (2007) 639. [13] J. Mckittrick, L.E. Shea, C.F. Bacalski, E.J. Bosze, Displays 19 (1999) 169. [14] S. Biamino, C. Badini, J. Eur. Ceram. Soc. 24 (2004) 3021. [15] H. Choi, C.-H. Kim, C.-H. Pyun, S.-J. Kim, J. Lumin. 82 (1999) 25. [16] A.N. Belsky, N.M. Khaidukov, J.C. Krupa, V.N. Makhov, A. Philippov, J. Lumin. 94–95 (2001) 45. [17] B.D. Cullity, Elements of X-ray Diffraction, Addison–Wesley, Reading, 1978, p. 102.
Short communication
Photoluminescence of NiNb2O6 nanoparticles prepared by combustion method Yuanyuan Zhou, Mengkai L¨u ∗ , Zifeng Qiu, Aiyu Zhang, Qian Ma, Haiping Zhang, Zhongsen Yang State Key Laboratory of Crystal Materials, Shandong University, Shanda Nan Road, Jinan 250100, PR China Received 14 January 2007; received in revised form 13 March 2007; accepted 7 April 2007
Abstract Pure and Dy3+ -doped NiNb2 O6 nanoparticles have been prepared by a sol–gel combustion method using citric acid as fuel and complexing agent and nitrates as oxidants at a relatively low temperature compared to solid-state reaction method. The photoluminescence properties of these NiNb2 O6 nanoparticles were studied in detail. For the pure NiNb2 O6 nanoparticles, a blue emission band centered at 445 nm is observed. For NiNb2 O6 :Dy3+ nanoparticles, no characteristic luminescence of Dy3+ can be observed while the host blue emission enhances. It is suggested that energy transfer from Dy3+ to niobate groups maybe occurs. © 2007 Published by Elsevier B.V. Keywords: Combustion; Luminescence; NiNb2 O6
1. Introduction Recently, the photoluminescence properties of niobate have been extensively studied. For example, luminescence properties of rare-earth-metal ion-doped KLaNb2 O7 with layered perovskite structures have been investigated by Akihiko Kudo [1]; Ba5 Nb4 O15 exhibits a yellow emission upon excitation with short wavelength ultraviolet radiation [2]; the luminescence of niobate and energy transfer process between niobate groups and doped ions have been observed in La2 O3 –Nb2 O5 –B2 O3 glasses [3–5]; lanthanum niobates is also an interesting host for rareearth ions to fabricate luminescent materials [6–8]. But to our best knowledge, there are few investigations on transition metal niobate, e.g. NiNb2 O6 . NiNb2 O6 compound has two different structures: columbite (orthorhombic)-type and trirutile (tetragonal)-type. NiNb2 O6 with columbite structure has a prospective potential for efficient H2 production from water photocatalytically under visible light irradiation [9]. NiNb2 O6 was also reported as a useful precursor for the preparation of lead nickel niobate (PNN), a typical relaxor ferroelectric material [10,11]. However,
∗
Corresponding author. E-mail address: [email protected] (M. L¨u).
0921-5107/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.mseb.2007.04.002
no study about NiNb2 O6 with trirutile structure has been reported. Traditional solid-state mixed oxide method has been used in the preparation of single-phase NiNb2 O6 powders. However, powders prepared by the mixed oxide route have spatial fluctuations in their compositions and calcination conditions used in the mixed oxide process should be optimized [12]. It is necessary to develop a time-saving method to prepare NiNb2 O6 with pure phase at low temperature, and combustion synthesis may be a good choice because it can guarantee high purity, compositionally uniform, single phase, and small and uniform particle size in one single step [13,14]. Therefore, the main purpose of this work is to explore a simple sol–gel combustion synthetic route for the preparation of single-phase NiNb2 O6 powders with trirutile structure. The luminescence properties of pure and Dy3+ -doped powders will be studied and discussed in detail. The calcination temperature in this work is much lower than that in solid-state reaction method. This clearly emphasizes the advantages of combustion technique. In addition, Dy3+ is a very important activator for the luminescent materials because of its characteristic emissions due to the blue transition (4 F9/2 → 6 H15/2 ) and yellow one (4 F9/2 → 6 H13/2 ), respectively [15,16]. It is interesting to study the energy transfer process between Dy3+ and niobate groups.
Y. Zhou et al. / Materials Science and Engineering B 140 (2007) 128–131
129
2. Experimental
3. Results and discussion
Pure and Dy3+ -doped NiNb2 O6 nanocrystals have been synthesized via a sol–gel combustion process. Nickel nitrate (Ni(NO3 )2 ·6H2 O), niobium oxide (Nb2 O5 ), dysprosium oxide (Dy2 O3 ) and ammonium nitrate (NH4 NO3 ) were used as starting materials. Citric acid was used as both complexing agent for gel process and fuel for the combustion. All the reagents were of analytical grade without further purification. In a typical synthesis, first, required quantity of Nb2 O5 was dissolved in HF acid (40%) after heating in a hot water bath. Then, ammonia solution was added to this NbF5 solution to obtain Nb2 O5 ·nH2 O. The Nb2 O5 ·nH2 O precipitate was filtered, washed and dissolved in citric acid aqueous solution under heating at 80 ◦ C. Then stoichiometric Ni(NO3 )2 ·6H2 O, excessive NH4 NO3 were added and mixed homogeneously. After the water evaporated, the solution was turned into a yellow gel with high viscosity. The gel was then introduced into a muffle furnace preheated to 700, 750 and 800 ◦ C, respectively. As the ignition occurred, the reaction went on vigorously for a few seconds. Then, yellow fluffy products were obtained after the combustion reaction. The synthesis procedure of NiNb2 O6 :Dy3+ was adopted as the above mentioned except that Dy(NO3 )3 was introduced. Dy3+ was added in the form of Dy(NO3 )3 solution by dissolving Dy2 O3 into diluted HNO3 . The molar ratio of Dy3+ varied from 2% to 5% in relation to NiNb2 O6 . The phase was characterized by X-ray diffraction (XRD) using a Germany Bruker axs D8-advanced X-ray diffractometer system with graphite monochromatized Cu K␣ irradiation ˚ The morphology and microstructure were (λ = 1.5418 A). characterized with a Japan JEM-100CXII transition electron microscope. Absorption spectra were recorded on a U-3500 spectrophotometer. Excitation and emission spectra were measured on an Edinburgh 920 fluorescence spectrophotometer. All the measurements were taken at room temperature.
The crystal structures of the NiNb2 O6 compounds were investigated using the powder X-ray diffraction method. Fig. 1 shows XRD patterns of pure NiNb2 O6 samples prepared at different temperatures. All peaks correspond to tetragonal phase of NiNb2 O6 , which is in agreement with the literature (JPCDS no. 76-2355). The major peaks can be well indexed as (1 1 0), (1 0 1), (2 0 0), (1 1 1), (2 1 1), (2 2 0), (3 1 0) and (3 0 1), respectively. The calculated lattice parameters for the sample prepared ˚ and c = 3.05 A, ˚ respectively. No Charat 750 ◦ C are a = 4.70 A acteristic peaks arising from possible impurities are detected. It
Fig. 1. XRD patterns of the pure NiNb2 O6 samples prepared at 700, 750 and 800 ◦ C, respectively.
Fig. 2. TEM images of the undoped NiNb2 O6 samples prepared at 700, 750 and 800 ◦ C, respectively.
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Y. Zhou et al. / Materials Science and Engineering B 140 (2007) 128–131
Fig. 3. Absorption spectrum (a) and excitation spectrum (λem = 440 nm) (b) of the pure sample prepared at 750 ◦ C. Emission spectra (λex = 380 nm) (c) of the pure NiNb2 O6 samples prepared at 700, 750 and 800 ◦ C, respectively.
can also be observed that the diffraction peaks become sharp with increasing the reaction temperature, indicating better crystallinity of the samples, and the particle sizes calculated from Scherrer’s formula [17] are 22, 32 and 41 nm, respectively, for samples prepared at 700, 750 and 800 ◦ C. The particle size and morphology of the NiNb2 O6 powders obtained at different temperatures were examined by transmission electron microscopy as shown in Fig. 2. For all samples, the particles are irregular in shape and agglomerated. The average particle sizes for samples prepared at 700, 750 and 800 ◦ C are around 20, 30 and 43 nm, respectively, which are consistent with the XRD results. In addition, the degree of agglomeration tends to decrease with increasing calcination temperature. Fig. 3(a) shows the typical absorption spectrum of the pure sample prepared at 750 ◦ C. The broad band from 250 to 400 nm is attributed to a charge transfer from the oxygen ligands to the central niobium atom inside the NbO6 group. Fig. 3(b) shows the excitation spectrum (λem = 440 nm) of the same sample analysed in Fig. 3(a). Two broad excitation bands centered at 340 and 378 nm, respectively, can be observed. Compared with the absorption spectrum and the excitation spectrum, the absorption
edge is at about 378 nm and the band gap energy is Eg = 3.28 eV. This value is different from that reported by Jinhua Yea et al. [9] Fig. 3(c) shows the luminescence spectra of pure NiNb2 O6 samples (λex = 380 nm) prepared at different temperatures. A strong blue emission band centered at 440 nm can be observed for samples prepared at 700 and 750 ◦ C. This emission can be ascribed to the distorted NbO6 groups [2–5]. In the trirutile NiNb2 O6 crystal structure, there are two kinds of octahedra, NbO6 and NiO6 in the structure. The NiO6 and NbO6 octahedra connect to each other by sharing edges to form a straight chain along [0 0 1] direction. Octahedra chains link to each other by sharing corner O atoms [9]. Isolated and edge- or face-shared MO6 (M = Nb, Ta) octahedral groups show efficient luminescence with a large Stokes shift while corner-sharing of MO6 groups leads to exciton delocalization, smaller Stokes shift, lower energy band, energy migration and consequent luminescence quenching [3–5]. So the edge-shared NbO6 chains show efficient blue emission. It is noted that the emission intensity is lower in samples synthesized at higher temperature, and for sample prepared at 800 ◦ C, this blue emission is quenched, indicating that the cal-
Y. Zhou et al. / Materials Science and Engineering B 140 (2007) 128–131
131
Fig. 4(b) shows the absorption spectrum of the 4 mol% Dydoped sample. Compared with the absorption spectrum of the undoped NiNb2 O6 (Fig. 3(b)), the absorption increases by the Dy doping. So the emission increase may be attributed to the energy transfer from Dy3+ to NbO6 groups. When the doping concentration of Dy3+ excesses 4 mol%, the emission intensity decreases due to concentration quenching effect. 4. Conclusion In summary, NiNb2 O6 nanoparticles have been successfully prepared via a sol–gel combustion method at as low as 700 ◦ C. Pure NiNb2 O6 nanoparticles show a blue emission centered at 440 nm due to the distorted edge-shared NbO6 groups. For the NiNb2 O6 :Dy3+ nanoparticles, no characteristic luminescence of Dy3+ can be observed but the blue emission intensity of the host NiNb2 O6 increases until the doping concentration reaches 4 mol%. It is attributed to the energy transfer from Dy3+ to NbO6 groups. Acknowledgement This work is supported by the awarded funds of Excellent State Key Laboratory (No. 50323006). References
Fig. 4. Emission spectra (λex = 380 nm) (a) of NiNb2 O6 :Dy3+ nanoparticles prepared at 750 ◦ C with doping concentrations of 0, 2, 4 and 5 mol%, respectively, and absorption spectrum (b) of the 4 mol% Dy-doped sample.
cination temperature has a visible effect on the emission of NiNb2 O6 . Fig. 4(a) shows the corresponding emission spectra of NiNb2 O6 :Dy3+ . Upon excitation at 380 nm, the characteristic emissions of Dy3+ due to the transitions of blue transition (4 F9/2 → 6 H15/2 ) and yellow one (4 F9/2 → 6 H13/2 ), respectively, are too weak to be checked, but the strong and broad blue emission band corresponding to the host NiNb2 O6 remains. The influence of Dy3+ concentration on the luminescent properties of NiNb2 O6 :Dy3+ was also investigated. As given in Fig. 4(a), as the concentration of Dy3+ increases from 2 to 4 mol%, the emission intensity of NbO6 enhances. It can be seen that the optimum concentration of Dy3+ is 4 mol% and the emission intensity is nearly 3.25 times stronger than that of the pure NiNb2 O6 .
[1] Akihiko Kudo, Chem. Mater. 9 (1997) 664. [2] A.M. Srivastava, J.F. Ackerman, W.W. Beers, J. Solid State Chem. 134 (1997) 187. [3] A.C.V. de Ara´ujo, I.T. Weber, W.D. Fragoso, C. de Mello Doneg´a, J. Alloy. Compd. 275–277 (1998) 738. [4] W.D. Fragoso, Celso de Mello Doneg´a, R.L. Longo, J. Lumin. 105 (2003) 97. [5] W.D. Fragoso, Celso de Mello Doneg´a, R.L. Longo, J. Non-Cryst. Solids 351 (2005) 3121. [6] Xiuzhen Xiao, Bing Yan, J. Non-Cryst. Solids 351 (2005) 3634. [7] Xiuzhen Xiao, Bing Yan, J. Non-Cryst. Solids 352 (2006) 3047. [8] Xiuzhen Xiao, Bing Yan, J. Alloy. Compd. 421 (2006) 252. [9] Jinhua Yea, Zhigang Zou, Akiyuki Matsushita, Int. J. Hydrogen Energy 28 (2003) 651. [10] E.F. Alberta, A.S. Bhalla, Mater. Lett. 54 (2002) 47. [11] Chao Lei, Kepi Chen, Xiaowen Zhang, Jun Wang, Solid State Commun. 123 (2002) 445. [12] O. Khamman, R. Yimnirun, S. Ananta, Mater. Lett. 61 (2007) 639. [13] J. Mckittrick, L.E. Shea, C.F. Bacalski, E.J. Bosze, Displays 19 (1999) 169. [14] S. Biamino, C. Badini, J. Eur. Ceram. Soc. 24 (2004) 3021. [15] H. Choi, C.-H. Kim, C.-H. Pyun, S.-J. Kim, J. Lumin. 82 (1999) 25. [16] A.N. Belsky, N.M. Khaidukov, J.C. Krupa, V.N. Makhov, A. Philippov, J. Lumin. 94–95 (2001) 45. [17] B.D. Cullity, Elements of X-ray Diffraction, Addison–Wesley, Reading, 1978, p. 102.