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Synthesis and luminescent properties of novel RENbO4:Ln3+ (RE = Y, Gd, Lu; Ln = Eu, Tb) micro-crystalline phosphors
Journal of Non-Crystalline Solids 351 (2005) 3634–3639 www.elsevier.com/locate/jnoncrysol
Synthesis and luminescent properties of novel RENbO4:Ln3+ (RE = Y, Gd, Lu; Ln = Eu, Tb) micro-crystalline phosphors Xiuzhen Xiao, Bing Yan
*
Department of Chemistry, Tongji University, Siping Road 1239, Shanghai 200092, China Received 9 May 2005; received in revised form 22 August 2005 Available online 26 October 2005
Abstract Using rare earth coordination polymers with aromatic carboxylic acids as the precursors of rare earth oxide components, with polyethylene glycol (PEG) as the dispersing media, micro-crystalline phosphors RENbO4:Ln3+ (RE = Y, Gd, Lu; Ln = Eu, Tb) have been synthesized by an in situ co-precipitation method. Both X-ray diffraction and scanning electron microscopy have shown that the resultant samples present are crystalline with Ôrice glue ballÕ micro-morphology and crystalline grain sizes in the range of 1–2 lm. The luminescent properties of these phosphors have been studied, which show that the best photoluminescent performance is achieved for GdNbO4:Tb3+ or Eu3+. This was because Gd3+ plays an important role to enhance the luminescence of Tb3+ or Eu3+ in an energy transfer process. In addition, the influence of the doping concentration on the fluorescence behaviors has been examined. With increase of the doping concentration from 1 mol% to 5 mol%, both the red emission intensity of Eu3+ and the green emission intensity of Tb3+ increase. Ó 2005 Elsevier B.V. All rights reserved. PACS: 42.70.a; 78.55.m; 78.20.e; 81.20.Fw
1. Introduction Inorganic compounds doped with rare earth ions (Ce3+, Pr , Sm3+, Eu3+, Tb3+, Dy3+, Er3+) are extensively applied in luminescent devices, such as fluorescent lamps, cathode ray tubes, and lasers, etc. [1]. In the field of inorganic complex oxysalts, vanadates, molybdates, tungstates and niobates are of great interests as better hosts for rare earth ions to fabricate luminescent materials. This is because they have broad and intense charge-transfer (C-T) absorption bands in the near UV and are therefore capable of efficiently transferring energy to the activators by a non-radiative mechanism [2–5]. While for rare earth niobates, the fluorescence behavior of LnNbO4:RE have rarely been reported except for the niobates crystal structures [6–10]. It is well-known that LnNbO4 have a similar 3+
*
Corresponding author. E-mail address: [email protected] (B. Yan).
0022-3093/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.09.018
structure (Ln = La–Lu; Y), which belongs to T-phase (T-scheelite, I41/a) at high-temperature and to monoclinically distorted M-phase (M-fergusonite, C2) at low temperature [11–13]. Besides this, LnNbO4 is a well-known self-activated compound, presenting a strong broad band centered at 417 nm under ultraviolet excitation, which provides an opportunity for developing new, efficient luminescent materials [14–20]. In the early past studies, it was reported by Ropp that when LnNbO4 was doped with rare earth ions, there existed some degree of coupling between NbO3 4 and activators and the luminescent properties of LnNbO4:RE mainly depended on the nature of the activators [21] (i.e., Tb3+ shows moderate coupling, whereas Eu3+ shows a strong coupling, so YNbO4 doped with Eu3+ is a brighter red-emitting luminescent material [22–24]). However, the syntheses mainly focus on the high-temperature solid-state reactions providing agglomerated powders, which can permit to easily change the structural characteristics of the powders.
X. Xiao, B. Yan / Journal of Non-Crystalline Solids 351 (2005) 3634–3639
2. Experimental section RE1xLnxNbO4 (Ln = Y, Gd, Lu; Ln = Tb3+, Eu3+; x = 0.01, 0.03, 0.05) phosphors were prepared by a modified in situ co-precipitation technology. The purity of the starting materials for rare earth oxides is 99.99%, and for Nb2O5 is 99.5%. Also, the purity of nitric acid is 65.0– 68.0%. Firstly, both Tb4O7 and Eu2O3 were dissolved, respectively, in nitric acid to form 0.02 mol L1 solutions. Y2O3, Gd2O3 and Lu2O3 were also dissolved in nitric acid to obtain 0.2 mol L1 solutions. Then salicylic acid [n(sal):n(RE3+) = 3.2] was dissolved in ethanol solution, whose pH value was adjusted to be about 7.0 by ammonia solution. Then RE(NO3)3 and Ln(NO3)3 were transferred to obtain the emulsion. After stirring for 2 h, proper amount PEG and urea were introduced to the mixture for dispersant and fuel, and ammonia for the pH value about 8.5. Nb2O5 powder (0.266 g) was mixed with the emulsion. Through the in situ chemical precipitation, multi-hybrid precursors were obtained and finally were calcinated in a resistance furnace for 6 h at 1200 °C, then light yellow solid powders were produced. The typical process scheme of the synthesis can be shown below: ð1 xÞREðNO3 Þ3 þ xLnðNO3 Þ3 þ 3:2HOHBA þ mNH3 H2 O ! ð1 xÞ½REðOHBAÞ3 n þ x½LnðOHBAÞ3 n þ nPEG þ 0:5Nb2 O5 þ nCOðNH2 Þ2 þ mNH3 H2 O ! RE1x Lnx NbO4 þ CO2 " þ H2 O " þ NH3 " þ NO2 "
The structure and the particle size were determined by Bruke D8-Advanced X-ray diffraction using CuKa radiation at voltage of 40 kV and a current of 40 Ma. The morphology of the samples was examined with a JSM-6360LV scanning electron microscope. Luminescence spectra were measured at room temperature with a Perkin–Elmer LS-55 model fluorophotometer (excitation slit width = 10 nm, emission slit width = 5 nm).
3. Results and discussion In yttrium niobate, the niobate atom can be considered tetrahedrally coordinated to the oxygen atoms, although in a highly distorted site [27]. Fig. 1(A)–(C) gave the XRD patterns of LnNbO4:Eu3+. It can be seen that the structures of LuNbO4:Eu3+ and GdNbO4:Eu3+ phosphors were similar to that of YNbO4:Eu3+. The polycrystalline powders were found to crystallize in the monoclinic structure and X-rays diffraction data of the monoclinic LnNbO4:Eu3+ was used to identify the crystalline M-fergusonite phase. From the full width at half maximum of the diffraction peak by the Sherrer equation, the average crystallite size of all LnNbO4 phosphors was estimated to be in the range of 1–2 lm. The SEM images for these niobate phosphors of the LnNbO4:Eu3+ are shown in Fig. 2(A)–(C) for LnNbO4: Eu3+. The typical crystalline grain of all products was estimated to be around 0.5–2.5 lm in dimension, which agreed with the data from the XRD estimation. Also, a novel micro-morphology was observed, just like a kind of traditional Chinese, so called Ôrice glue ballÕ. Crystalline powder with micrometer dimension and high strength would be very useful for applications to obtain high efficient phosphors because these micro-crystalline materials can result especially if they have high luminescent intensities [28]. The organic polymer, PEG was combined as a dispersing medium to form the polybasic hybrid precursor template with micrometer size. So the particle size and morphology of LnNbO4:Eu3+ can be controlled and determined by the hybrid precursors prepared by in situ co-precipitation method. Although there was some conglomeration among the crystalline grain because of the high temperature 1200 °C of thermal decomposition, this preparation method can be expected to be a candidate for the synthesis of other luminescent materials based on rare earth oxides.
Relative intensities
In this paper, we put forward a novel modified synthesis technology through in situ chemical co-precipitation reactions, and obtain RENbO4:Ln3+ (RE = Y, Gd, Lu; Ln = Eu, Tb) crystalline phosphors. We select rare earth coordination polymers with ortho-hydroxylbenzoic acids as the precursors of rare earth species for their infinite chain-like polymeric structure similar to organic polymer templates. The multi-component hybrid polymeric precursors were assembled with other functional components such as Nb2O5 [25,26]. Besides this, organic polymer, i.e., polyethyl glycohol (PEG) was used as dispersing medium. Urea was used to modify the decomposition behavior and ammonia was used to control the pH value. Through an in situ co-precipitation process, multi-component hybrid polymeric precursors were constructed to produce the LnNbO4:Eu3+/Tb3+ with different doping concentrations. The luminescent properties of the materials produced are discussed in detail.
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C B A 20
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2θ/degree Fig. 1. XRD pattern of Gd0.95Tb0.05NbO4 (A), Y0.95Tb0.05NbO4 (B) and Lu0.95Tb0.05NbO4 (C).
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Fig. 2. SEM microscopy of Y0.95Tb0.05NbO4 (A), Lu0.95Tb0.05NbO4 (B) and Gd0.95Tb0.05NbO4 (C).
The excitation spectra of the LnNbO4:Eu3+ phosphors are depicted in Fig. 3, showing that no effective absorption transition of Eu3+ 4f configuration appeared in the long wavelength ultraviolet region of the range 300–400 nm and the effective energy absorption mainly took place in the narrow ultraviolet region of 200–300 nm. The excitation spectra of all Eu3+:phosphors, taken at an emission wavelength of 613 nm, consist of three bands centered at 223 nm, 243 nm and 256 nm, respectively. The peaks at 243 nm and 256 nm were ascribed to the charge transfer
state resulting from the ligand O2-2p orbit to the empty states of 4f configuration (Eu–O) transition. The band around 223 nm was attributed to the host charge transfer transition from oxygen to metal (Nb–O), which was agreement seen in the undoped YNbO4 [21]. Besides this, the intensities of three bands increased in the sequence of YNbO4:Eu3+, LuNbO4:Eu3+, GdNbO4:Eu3+. Fig. 4(A)–(C) was the emission spectra of all phosphors with the different doping concentration of Eu3+ under the excitation wavelength at 243 nm. The emission spectra of all Eu3+:phosphors showed the similar features, and they have involved in the following emission lines: 5D0–7F0, 5 D0–7F1, 5D0–7F2, 5D0–7F3 and 5D0–7F4, which were situated at 586 nm, 592, 612 and 622, 654 and 704 nm, respectively. The transition of electric-dipole 5D0–7F2 has shown similar properties in all three phosphors: it was found to be split into two components; of these, the intensity of this transition was strongest and was increasing when the Eu3+ concentration increased from 1 mol% to 5 mol%. Furthermore, we compared the luminescent lifetimes for 5 D0 ! 7F2 transition of Eu3+ for selected YNbO4:Eu3+ samples, which are determined to be 42.2, 46.8 and 49.6 ls, respectively, for 1 mol%, 3 mol% and 5 mol% Eu3+, separately. This indicates the similar characteristic to the luminescent intensities. As shown in Fig. 5, among these three powders GdNbO4, LuNbO4 and YNbO4, the red1 emission appears more intense in the GdNbO4 than in other two rare earth orthoniobates, which was consistent with the excitation intensity of three powders. This can be interpreted by the intermediate role of Gd3+ in GdNbO4. It is well-known that the energy level difference of 6GJ and 6PJ of Gd3+ is close to that of 7F1 and 5D0 of Eu3+, Gd3+ in 6GJ state 1 For interpretation of color in Fig. 5, the reader is referred to the web version of this article.
X. Xiao, B. Yan / Journal of Non-Crystalline Solids 351 (2005) 3634–3639
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Fig. 4. Emission spectra of Y1xEuxNbO4 (A), Lu1xEuxNbO4 (B) and Gd1xEuxNbO4 (C).
can excite Eu3+ into 5D0 state by resonance energy transfer, which results in the energy transfer of Gd3+ to Eu3+. The energy transfer process in GdNbO4:Eu3+ or Tb3 may be described as follows: The energy from the excitation source excited NbO3þ 4 firstly, and then the energy was trapped by Gd3+ and migrated along them until it is trapped by Eu3+ or Tb3+ [29]. The result was that the characteristic luminescence took place. Besides this, the NbO3þ 4 can transfer the energy to activators directly. Therefore, the luminescent
intensity of Eu3+ or Tb3+ in GdNbO4 is stronger than that of Y and Lu analog. Fig. 6 has shown the representative emission spectrum for YNbO4:5 mol% Eu3+ under the excitation wavelength 256 nm. It exhibited a strong band around 417 nm originated from the charge transfer transition (Nb5+–O2) and the characteristic lines ascribed to f–f transition of Eu3+. Especially we can observe the emission transition from high-level 5D1,2,3 to 7FJ. The presence of emission from higher energy is attributed to the low energy vibration 3 of NbO3 4 groups. The multi-phonon relaxation by NbO4 is not able to bridge the gaps between the higher energy levels (5D1, 5D2, and 5D3) and the 5D0 level of Eu3+ completely, resulting in emissions from these higher levels [30]. The excitation spectra of Tb3+ activated LnNbO4 powders in Fig. 7, presented three strong bands centered at 223, 243, 256 nm and a week band in the region 300–400 nm. The host absorption band (HAB) can be seen around
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X. Xiao, B. Yan / Journal of Non-Crystalline Solids 351 (2005) 3634–3639
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223 nm, suggesting that there existed an energy transfer 3+ process between NbO3þ 4 and Tb . Also, the band at 243 nm would seem to be attributed to the 7D level of the 4f–5d transition of Tb3+ due to spin-allowed 4f8–4f75d1 transitions and the band peaked at 256 nm could be assigned to the 9D level of the 4f75d1 configuration. Besides this, there also appeared a weak band in the region 300– 400 nm resulting from the Tb3+ f–f transition. As shown in Fig. 8(A)–(C), the emission spectra of Tb3+doped YNbO4, GdNbO4 and LuNbO4 have the same features. The representative emission spectra of Tb3+-doped GdNbO4 phosphors presented the four characteristic lines at 487, 546, 588 and 621 nm, corresponding to the transition of 5D4–7F6, 5D4–7F5, 5D4–7F4 and 5D4–7F3, respectively. Besides this, the intensity of 5D4–7F5 transition was the strongest among these lines and increasing when the doping concentration increased from 1 mol% to 5 mol%. In Fig. 9, we can find that the phosphor GdNbO4 doped with Tb3+ can perform more intense green fluorescence than other two phosphors, indicating that Gd3+ in GdNbO4:Tb3+ phosphor possibly plays a role in the energy transfer process: Gd3+ can function as an intermediate to absorb the ultraviolet excitation energy from NbO3 4 and then transfer to enhance the emission of Tb3+. Moreover, the luminescent lifetimes for 5D4 ! 7F5 transition of Tb3+ were monitored for selected YNbO4:Tb3+ samples, which are determined to be 46.2 ls (1 mol% Tb3+), 48.9 ls (3 mol% Tb3+) and 52.4 ls (5 mol% Tb3+), respectively.
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4. Conclusions
Fig. 8. Emission spectra of Lu1xTbxNbO4 (A), Y1xTbxNbO4 (B) and Gd1xTbxNbO4 (C).
In conclusion, we reported a modified in situ chemical co-precipitation synthesis process and fabricated LnNbO4 doped with Tb3+/Eu3+ compounds employing hybrid precursors. These phosphors exhibited a crystalline morphology with an interesting Ôrice glue ballÕ micro-morphology. Among the three kinds of rare earth orthoniobates:
LuNbO4, YNbO4 and GdNbO4, the red or green emission intensity of Eu3+ or Tb3+ in GdNbO4 was stronger than that of Y and Lu analog, which proved that there exist energy transfer processes from Gd3+ (as intermediate) to the activator. Furthermore, increasing the doping concentration from 1 mol% to 5 mol%, the ratio of green
X. Xiao, B. Yan / Journal of Non-Crystalline Solids 351 (2005) 3634–3639 5
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emission intensity to red emission intensity was increasing too. It was worthy to point out that the emission transition from higher level D1, D2 and D3 of all Eu3+:phosphors was observed possibly because of the low probability of multiphonon relaxation between the rare earth ions Eu3+ in LnNbO4 hosts. Acknowledgments The work was supported by the Science Fund of Shanghai Excellent Youth Scientists and the National Natural Science Foundation of China (20301013). References [1] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin, 1994. [2] M.L. Pang, J. Lin, M. Yu, J. Solid State Chem. 177 (2004) 2237.
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[3] P.M. Guo, F. Zhao, G.B. Li, F.H. Liao, S.J. Tian, X.P. Jing, J. Lumin. 105 (2003) 61. [4] S. Neeraj, N. Kijima, A.K. Cheetham, Solid State Commun. 131 (2004) 65. [5] B. Li, Z.N. Gu, J.H. Lin, M.Z. Su, Acta Phys.-Chim. Sinica 15 (1999) 794. [6] G. Blasse, Struct. Bond. 42 (1980) 1. [7] L.H. Brixner, Inorg. Chim. Acta 140 (1987) 97. [8] A. Santoro, M. Marezio, R.S. Roth, D. Minor, J. Solid State Chem. 35 (1980) 167. [9] S. Tsunekawa, Sci. Rep. Res. Inst. Tohoku Univ. Ser. A 29 (1980) 1. [10] S. Tsukawa, H. Takei, Phys. Status Solidi, Sect. A: Appl. Res. 50 (1978) 695. [11] J.M. Jehng, I.E. Wachs, Chem. Mater. 3 (1991) 100. [12] H. Weitzel, H.Z. Schro¨cke, Kristallographie 152 (1980) 69. [13] O. Yamaguchi, K. Matsui, T. Kawabe, J. Am. Ceram. Soc. 68 (1985) C275. [14] S. Tsunekawa, K. Hara, R. Nishitani, Mater. Trans. 36 (1995) 9. [15] J. Li, C.M. Wayman, J. Am. Ceram. Soc. 80 (1997) 803. [16] S. Maschio, A. Bachiorrini, R. Demonte, J. Mater. Sci. 30 (1995) 5433. [17] S. Tsukawa, Sci. Rep. Res. Inst. Tohoku Univ. 29 (1981) 1. [18] G. Blasse, J. Solid State Chem. 7 (1973) 169. [19] L.H. Brixner, Mater. Chem. Phys. 16 (1987) 253. [20] L.H. Brixner, H.Y. Chen, J. Electrochem. Soc. 130 (1983) 2435. [21] R.C. Ropp, Luminescence and the Solid State, Elsevier Science, Amsterdam, 1991. [22] G. Blasse, K.C. Bleijenberg, R.C. Powell, Luminescence and Energy Transfer, Springer, Heidelberg, 1980. [23] H.S. Sang, Y.J. Duk, S.S. Kyung, J. Appl. Phys. 90 (2001) 5986. [24] A.M.G. Massabni, G.J.M. Montandon, M.A. Couto dos Santos, Mater. Res. 1 (1998) 1. [25] B. Yan, H.H. Huang, J. Mater. Sci. 39 (2004) 3529. [26] B. Yan, L. Zhou, J. Alloy. Compd. 372 (2004) 238. [27] P.S. Pizani, E.R. Leite, F.M. Pontes, E.C. Paris, J.H. Rangel, J.H. Lee, E. Longo, P. Delega, J.A. Varela, Appl. Phys. Lett. 77 (2000) 824. [28] G. Blasse, A. Bril, J. Electrochem. Soc. Solid State Sci. 115 (1968) 1067. [29] X.Y. Wu, H.P. You, H.T. Cui, X.Q. Zeng, G.Y. Hong, C.H. Kim, C.H. Pyum, B.Y. Yu, C.H. Park, Mater. Res. Bull. 37 (2002) 1531. [30] M. Yu, J. Lin, Z. Wang, J. Fu, S. Wang, H.J. Zhang, Y.C. Han, Chem. Mater. 14 (2002) 2228.
Synthesis and luminescent properties of novel RENbO4:Ln3+ (RE = Y, Gd, Lu; Ln = Eu, Tb) micro-crystalline phosphors Xiuzhen Xiao, Bing Yan
*
Department of Chemistry, Tongji University, Siping Road 1239, Shanghai 200092, China Received 9 May 2005; received in revised form 22 August 2005 Available online 26 October 2005
Abstract Using rare earth coordination polymers with aromatic carboxylic acids as the precursors of rare earth oxide components, with polyethylene glycol (PEG) as the dispersing media, micro-crystalline phosphors RENbO4:Ln3+ (RE = Y, Gd, Lu; Ln = Eu, Tb) have been synthesized by an in situ co-precipitation method. Both X-ray diffraction and scanning electron microscopy have shown that the resultant samples present are crystalline with Ôrice glue ballÕ micro-morphology and crystalline grain sizes in the range of 1–2 lm. The luminescent properties of these phosphors have been studied, which show that the best photoluminescent performance is achieved for GdNbO4:Tb3+ or Eu3+. This was because Gd3+ plays an important role to enhance the luminescence of Tb3+ or Eu3+ in an energy transfer process. In addition, the influence of the doping concentration on the fluorescence behaviors has been examined. With increase of the doping concentration from 1 mol% to 5 mol%, both the red emission intensity of Eu3+ and the green emission intensity of Tb3+ increase. Ó 2005 Elsevier B.V. All rights reserved. PACS: 42.70.a; 78.55.m; 78.20.e; 81.20.Fw
1. Introduction Inorganic compounds doped with rare earth ions (Ce3+, Pr , Sm3+, Eu3+, Tb3+, Dy3+, Er3+) are extensively applied in luminescent devices, such as fluorescent lamps, cathode ray tubes, and lasers, etc. [1]. In the field of inorganic complex oxysalts, vanadates, molybdates, tungstates and niobates are of great interests as better hosts for rare earth ions to fabricate luminescent materials. This is because they have broad and intense charge-transfer (C-T) absorption bands in the near UV and are therefore capable of efficiently transferring energy to the activators by a non-radiative mechanism [2–5]. While for rare earth niobates, the fluorescence behavior of LnNbO4:RE have rarely been reported except for the niobates crystal structures [6–10]. It is well-known that LnNbO4 have a similar 3+
*
Corresponding author. E-mail address: [email protected] (B. Yan).
0022-3093/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.09.018
structure (Ln = La–Lu; Y), which belongs to T-phase (T-scheelite, I41/a) at high-temperature and to monoclinically distorted M-phase (M-fergusonite, C2) at low temperature [11–13]. Besides this, LnNbO4 is a well-known self-activated compound, presenting a strong broad band centered at 417 nm under ultraviolet excitation, which provides an opportunity for developing new, efficient luminescent materials [14–20]. In the early past studies, it was reported by Ropp that when LnNbO4 was doped with rare earth ions, there existed some degree of coupling between NbO3 4 and activators and the luminescent properties of LnNbO4:RE mainly depended on the nature of the activators [21] (i.e., Tb3+ shows moderate coupling, whereas Eu3+ shows a strong coupling, so YNbO4 doped with Eu3+ is a brighter red-emitting luminescent material [22–24]). However, the syntheses mainly focus on the high-temperature solid-state reactions providing agglomerated powders, which can permit to easily change the structural characteristics of the powders.
X. Xiao, B. Yan / Journal of Non-Crystalline Solids 351 (2005) 3634–3639
2. Experimental section RE1xLnxNbO4 (Ln = Y, Gd, Lu; Ln = Tb3+, Eu3+; x = 0.01, 0.03, 0.05) phosphors were prepared by a modified in situ co-precipitation technology. The purity of the starting materials for rare earth oxides is 99.99%, and for Nb2O5 is 99.5%. Also, the purity of nitric acid is 65.0– 68.0%. Firstly, both Tb4O7 and Eu2O3 were dissolved, respectively, in nitric acid to form 0.02 mol L1 solutions. Y2O3, Gd2O3 and Lu2O3 were also dissolved in nitric acid to obtain 0.2 mol L1 solutions. Then salicylic acid [n(sal):n(RE3+) = 3.2] was dissolved in ethanol solution, whose pH value was adjusted to be about 7.0 by ammonia solution. Then RE(NO3)3 and Ln(NO3)3 were transferred to obtain the emulsion. After stirring for 2 h, proper amount PEG and urea were introduced to the mixture for dispersant and fuel, and ammonia for the pH value about 8.5. Nb2O5 powder (0.266 g) was mixed with the emulsion. Through the in situ chemical precipitation, multi-hybrid precursors were obtained and finally were calcinated in a resistance furnace for 6 h at 1200 °C, then light yellow solid powders were produced. The typical process scheme of the synthesis can be shown below: ð1 xÞREðNO3 Þ3 þ xLnðNO3 Þ3 þ 3:2HOHBA þ mNH3 H2 O ! ð1 xÞ½REðOHBAÞ3 n þ x½LnðOHBAÞ3 n þ nPEG þ 0:5Nb2 O5 þ nCOðNH2 Þ2 þ mNH3 H2 O ! RE1x Lnx NbO4 þ CO2 " þ H2 O " þ NH3 " þ NO2 "
The structure and the particle size were determined by Bruke D8-Advanced X-ray diffraction using CuKa radiation at voltage of 40 kV and a current of 40 Ma. The morphology of the samples was examined with a JSM-6360LV scanning electron microscope. Luminescence spectra were measured at room temperature with a Perkin–Elmer LS-55 model fluorophotometer (excitation slit width = 10 nm, emission slit width = 5 nm).
3. Results and discussion In yttrium niobate, the niobate atom can be considered tetrahedrally coordinated to the oxygen atoms, although in a highly distorted site [27]. Fig. 1(A)–(C) gave the XRD patterns of LnNbO4:Eu3+. It can be seen that the structures of LuNbO4:Eu3+ and GdNbO4:Eu3+ phosphors were similar to that of YNbO4:Eu3+. The polycrystalline powders were found to crystallize in the monoclinic structure and X-rays diffraction data of the monoclinic LnNbO4:Eu3+ was used to identify the crystalline M-fergusonite phase. From the full width at half maximum of the diffraction peak by the Sherrer equation, the average crystallite size of all LnNbO4 phosphors was estimated to be in the range of 1–2 lm. The SEM images for these niobate phosphors of the LnNbO4:Eu3+ are shown in Fig. 2(A)–(C) for LnNbO4: Eu3+. The typical crystalline grain of all products was estimated to be around 0.5–2.5 lm in dimension, which agreed with the data from the XRD estimation. Also, a novel micro-morphology was observed, just like a kind of traditional Chinese, so called Ôrice glue ballÕ. Crystalline powder with micrometer dimension and high strength would be very useful for applications to obtain high efficient phosphors because these micro-crystalline materials can result especially if they have high luminescent intensities [28]. The organic polymer, PEG was combined as a dispersing medium to form the polybasic hybrid precursor template with micrometer size. So the particle size and morphology of LnNbO4:Eu3+ can be controlled and determined by the hybrid precursors prepared by in situ co-precipitation method. Although there was some conglomeration among the crystalline grain because of the high temperature 1200 °C of thermal decomposition, this preparation method can be expected to be a candidate for the synthesis of other luminescent materials based on rare earth oxides.
Relative intensities
In this paper, we put forward a novel modified synthesis technology through in situ chemical co-precipitation reactions, and obtain RENbO4:Ln3+ (RE = Y, Gd, Lu; Ln = Eu, Tb) crystalline phosphors. We select rare earth coordination polymers with ortho-hydroxylbenzoic acids as the precursors of rare earth species for their infinite chain-like polymeric structure similar to organic polymer templates. The multi-component hybrid polymeric precursors were assembled with other functional components such as Nb2O5 [25,26]. Besides this, organic polymer, i.e., polyethyl glycohol (PEG) was used as dispersing medium. Urea was used to modify the decomposition behavior and ammonia was used to control the pH value. Through an in situ co-precipitation process, multi-component hybrid polymeric precursors were constructed to produce the LnNbO4:Eu3+/Tb3+ with different doping concentrations. The luminescent properties of the materials produced are discussed in detail.
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C B A 20
30
40
50
60
2θ/degree Fig. 1. XRD pattern of Gd0.95Tb0.05NbO4 (A), Y0.95Tb0.05NbO4 (B) and Lu0.95Tb0.05NbO4 (C).
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2-
HBT
CTS( O
3+
Eu )
Relative intensities/a.u.
C
B
A
220
240
260
280
Wavelength/nm Fig. 3. Excitation spectra of Y0.95Eu0.05NbO4 (A), Lu0.95Eu0.05NbO4 (B) and Gd0.95Eu0.05NbO4 (C).
Fig. 2. SEM microscopy of Y0.95Tb0.05NbO4 (A), Lu0.95Tb0.05NbO4 (B) and Gd0.95Tb0.05NbO4 (C).
The excitation spectra of the LnNbO4:Eu3+ phosphors are depicted in Fig. 3, showing that no effective absorption transition of Eu3+ 4f configuration appeared in the long wavelength ultraviolet region of the range 300–400 nm and the effective energy absorption mainly took place in the narrow ultraviolet region of 200–300 nm. The excitation spectra of all Eu3+:phosphors, taken at an emission wavelength of 613 nm, consist of three bands centered at 223 nm, 243 nm and 256 nm, respectively. The peaks at 243 nm and 256 nm were ascribed to the charge transfer
state resulting from the ligand O2-2p orbit to the empty states of 4f configuration (Eu–O) transition. The band around 223 nm was attributed to the host charge transfer transition from oxygen to metal (Nb–O), which was agreement seen in the undoped YNbO4 [21]. Besides this, the intensities of three bands increased in the sequence of YNbO4:Eu3+, LuNbO4:Eu3+, GdNbO4:Eu3+. Fig. 4(A)–(C) was the emission spectra of all phosphors with the different doping concentration of Eu3+ under the excitation wavelength at 243 nm. The emission spectra of all Eu3+:phosphors showed the similar features, and they have involved in the following emission lines: 5D0–7F0, 5 D0–7F1, 5D0–7F2, 5D0–7F3 and 5D0–7F4, which were situated at 586 nm, 592, 612 and 622, 654 and 704 nm, respectively. The transition of electric-dipole 5D0–7F2 has shown similar properties in all three phosphors: it was found to be split into two components; of these, the intensity of this transition was strongest and was increasing when the Eu3+ concentration increased from 1 mol% to 5 mol%. Furthermore, we compared the luminescent lifetimes for 5 D0 ! 7F2 transition of Eu3+ for selected YNbO4:Eu3+ samples, which are determined to be 42.2, 46.8 and 49.6 ls, respectively, for 1 mol%, 3 mol% and 5 mol% Eu3+, separately. This indicates the similar characteristic to the luminescent intensities. As shown in Fig. 5, among these three powders GdNbO4, LuNbO4 and YNbO4, the red1 emission appears more intense in the GdNbO4 than in other two rare earth orthoniobates, which was consistent with the excitation intensity of three powders. This can be interpreted by the intermediate role of Gd3+ in GdNbO4. It is well-known that the energy level difference of 6GJ and 6PJ of Gd3+ is close to that of 7F1 and 5D0 of Eu3+, Gd3+ in 6GJ state 1 For interpretation of color in Fig. 5, the reader is referred to the web version of this article.
X. Xiao, B. Yan / Journal of Non-Crystalline Solids 351 (2005) 3634–3639
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Fig. 6. Emission spectra of Y0.95Eu0.05NbO4.
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Fig. 5. Emission spectra of Y0.95Eu0.05NbO4 (A), Lu0.95Eu0.05NbO4 (B) and Gd0.95Eu0.05NbO4 (C).
5
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Fig. 4. Emission spectra of Y1xEuxNbO4 (A), Lu1xEuxNbO4 (B) and Gd1xEuxNbO4 (C).
can excite Eu3+ into 5D0 state by resonance energy transfer, which results in the energy transfer of Gd3+ to Eu3+. The energy transfer process in GdNbO4:Eu3+ or Tb3 may be described as follows: The energy from the excitation source excited NbO3þ 4 firstly, and then the energy was trapped by Gd3+ and migrated along them until it is trapped by Eu3+ or Tb3+ [29]. The result was that the characteristic luminescence took place. Besides this, the NbO3þ 4 can transfer the energy to activators directly. Therefore, the luminescent
intensity of Eu3+ or Tb3+ in GdNbO4 is stronger than that of Y and Lu analog. Fig. 6 has shown the representative emission spectrum for YNbO4:5 mol% Eu3+ under the excitation wavelength 256 nm. It exhibited a strong band around 417 nm originated from the charge transfer transition (Nb5+–O2) and the characteristic lines ascribed to f–f transition of Eu3+. Especially we can observe the emission transition from high-level 5D1,2,3 to 7FJ. The presence of emission from higher energy is attributed to the low energy vibration 3 of NbO3 4 groups. The multi-phonon relaxation by NbO4 is not able to bridge the gaps between the higher energy levels (5D1, 5D2, and 5D3) and the 5D0 level of Eu3+ completely, resulting in emissions from these higher levels [30]. The excitation spectra of Tb3+ activated LnNbO4 powders in Fig. 7, presented three strong bands centered at 223, 243, 256 nm and a week band in the region 300–400 nm. The host absorption band (HAB) can be seen around
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Wavelength/nm Fig. 7. Excitation spectra of Lu0.95Tb0.05NbO4 (A), Y0.95Tb0.05NbO4 (B) and Gd0.95Tb0.05NbO4 (C).
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223 nm, suggesting that there existed an energy transfer 3+ process between NbO3þ 4 and Tb . Also, the band at 243 nm would seem to be attributed to the 7D level of the 4f–5d transition of Tb3+ due to spin-allowed 4f8–4f75d1 transitions and the band peaked at 256 nm could be assigned to the 9D level of the 4f75d1 configuration. Besides this, there also appeared a weak band in the region 300– 400 nm resulting from the Tb3+ f–f transition. As shown in Fig. 8(A)–(C), the emission spectra of Tb3+doped YNbO4, GdNbO4 and LuNbO4 have the same features. The representative emission spectra of Tb3+-doped GdNbO4 phosphors presented the four characteristic lines at 487, 546, 588 and 621 nm, corresponding to the transition of 5D4–7F6, 5D4–7F5, 5D4–7F4 and 5D4–7F3, respectively. Besides this, the intensity of 5D4–7F5 transition was the strongest among these lines and increasing when the doping concentration increased from 1 mol% to 5 mol%. In Fig. 9, we can find that the phosphor GdNbO4 doped with Tb3+ can perform more intense green fluorescence than other two phosphors, indicating that Gd3+ in GdNbO4:Tb3+ phosphor possibly plays a role in the energy transfer process: Gd3+ can function as an intermediate to absorb the ultraviolet excitation energy from NbO3 4 and then transfer to enhance the emission of Tb3+. Moreover, the luminescent lifetimes for 5D4 ! 7F5 transition of Tb3+ were monitored for selected YNbO4:Tb3+ samples, which are determined to be 46.2 ls (1 mol% Tb3+), 48.9 ls (3 mol% Tb3+) and 52.4 ls (5 mol% Tb3+), respectively.
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4. Conclusions
Fig. 8. Emission spectra of Lu1xTbxNbO4 (A), Y1xTbxNbO4 (B) and Gd1xTbxNbO4 (C).
In conclusion, we reported a modified in situ chemical co-precipitation synthesis process and fabricated LnNbO4 doped with Tb3+/Eu3+ compounds employing hybrid precursors. These phosphors exhibited a crystalline morphology with an interesting Ôrice glue ballÕ micro-morphology. Among the three kinds of rare earth orthoniobates:
LuNbO4, YNbO4 and GdNbO4, the red or green emission intensity of Eu3+ or Tb3+ in GdNbO4 was stronger than that of Y and Lu analog, which proved that there exist energy transfer processes from Gd3+ (as intermediate) to the activator. Furthermore, increasing the doping concentration from 1 mol% to 5 mol%, the ratio of green
X. Xiao, B. Yan / Journal of Non-Crystalline Solids 351 (2005) 3634–3639 5
7
D4 F5 C
Relative intensities/a.u.
5
D4
7
F6
5
7
D4 F4
B
5
D4
7
F3
A
450
500
550
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650
Wavelength/nm Fig. 9. Emission spectra of Lu0.95Tb0.05NbO4 (A), Y0.95Tb0.05NbO4 (B) and Gd0.95Tb0.05NbO4 (C).
emission intensity to red emission intensity was increasing too. It was worthy to point out that the emission transition from higher level D1, D2 and D3 of all Eu3+:phosphors was observed possibly because of the low probability of multiphonon relaxation between the rare earth ions Eu3+ in LnNbO4 hosts. Acknowledgments The work was supported by the Science Fund of Shanghai Excellent Youth Scientists and the National Natural Science Foundation of China (20301013). References [1] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin, 1994. [2] M.L. Pang, J. Lin, M. Yu, J. Solid State Chem. 177 (2004) 2237.
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