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The effect of B2O3 on the luminescent properties of Eu ion-doped aluminoborosilicate glasses
Journal of Non-Crystalline Solids 357 (2011) 2328–2331
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
The effect of B2O3 on the luminescent properties of Eu ion-doped aluminoborosilicate glasses Huidan Zeng a, Yifan Yang a, Zhenyu Lin a, Xiaoluan Liang a, Shuanglong Yuan a, Guorong Chen a,⁎, Luyi Sun b,⁎ a b
Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China Department of Chemistry and Biochemistry & Materials Science and Engineering Program, Texas State University—San Marcos, San Macros, TX 78666, USA
a r t i c l e
i n f o
Article history: Received 13 July 2010 Received in revised form 27 October 2010 Available online 16 December 2010 Keywords: Eu ions; Structure of glasses; Aluminoborosilicate glasses; PL properties
a b s t r a c t A series of Eu ion-doped aluminoborosilicate glasses was prepared by melt-quenching method and characterized by spectroscopic techniques, including Fourier transform infrared (FTIR), absorption, photoluminescence (PL), and electron paramagnetic resonance (EPR). The FTIR spectra showed that various glass structures formed when Al2O3 was partially replaced by B2O3. The PL characterization revealed that the emission intensity of Eu2+ ions firstly increased and then decreased with an increasing amount of Al2O3 replaced by B2O3. Meanwhile, the emission intensity ratio of [Eu2+]/[Eu3+] also followed the same trend. The EPR spectra confirmed the concentration variation of Eu2+ ions in the glass samples, which agreed well with the PL results. The possible mechanism of the effect of the glass network structure on the reduction behavior of Eu ions is discussed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Blue fluorescent materials have become a group of important functional materials in high-tech applications, including X-ray intensifying screens, large screen LED displays, and X-ray imaging panels, etc. [1–3]. Blue emission region of Eu2+-doped materials has been studied extensively [4–8]. Since Eu2O3 is the most typical starting compound to prepare Eu2+ doped fluorescent materials, Eu3+ ions must be reduced to obtain Eu2+ during the synthesis. In general, the reduction of Eu3+ to Eu2+ requires a reducing agent, such as CO, H2, carbon, etc. However, these complex preparation technologies not only increase the cost, but also potentially generate safety issues. Thus, it is highly desirable to develop facile and effective approaches to synthesize Eu2+ activated fluorescent materials. Täle et al. studied the synthesis of the Eu2+ activated Ba3(PO4)2 fluorescent material by high-temperature solid state reaction in air in 1979 [9]. Their work led to a safer and simpler means to obtain Eu2+. Recently, it has been disclosed that the reduction of Eu3+ to Eu2+ could be realized in host material (e.g. silicates, phosphates and borates) in air [3,10,11]. Peng et al. proposed that the spontaneous reduction of Eu3+ to Eu2+ in air has four requirements: (1) no oxidizing agents exist in the host compounds; (2) trivalent dopant Eu3+ cation could replace the divalent cation in the hosts; (3) the substituted cation has a similar radius to the
⁎ Corresponding authors. E-mail addresses: [email protected] (G. Chen), [email protected] (L. Sun). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.11.093
divalent Eu2+ cation; and (4) the host compound has an appropriate structure, which is composed of tetrahedral anion groups (e.g. AlO4, BO4, and SiO4) [10]. So far, the reduction of Eu3+ to Eu2+ in borophosphate, borate, and boroaluminate glass systems in air has been reported. While the earlier work mainly focused on the effect of alkaline earth metal ions and the optical basicity of the host materials on the luminescent properties of Eu ions doped compounds [3,11,12], the effect of the glass network structure on the reduction of Eu3+ ions has been largely ignored. In the present work, based on a series of Eu-doped aluminoborosilicate glasses, we investigated the relationship between the luminescent properties of the synthesized glasses and their network structure, which can be tuned by replacing Al2O3 with B2O3. The mechanism of the effect of intrinsic glass network structure on the reduction of Eu ions was proposed and discussed. 2. Experimental Eu-doped aluminoborosilicate glasses were prepared by melt quenching technique. Analytical reagents CaCO3, H3BO3, Al2O3, SiO2 and high purity Eu2O3 were used as starting materials to prepare a series of Eu-doped aluminoborosilicate glasses, whose nominal chemical compositions are listed in Table 1. The weighted raw materials (ca. 20.0 g) were mixed thoroughly and then transferred to a platinum crucible to melt at 1500–1550 °C for 1.5 h in air. The glass melt was poured into a preheated stainless steel mold for quenching, and subsequently annealed near its glass temperature Tg. The resulting glasses were polished and cut into 10 mm × 10 mm × 1 mm plates for characterizations.
H. Zeng et al. / Journal of Non-Crystalline Solids 357 (2011) 2328–2331
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Table 1 Nominal chemical composition of the glass samples. Glass no.
SiO2
Al2O3
B2O3
CaO
Eu2O3
G0 G1 G2 G3 G4
50 50 50 50 50
25 22 20 17 15
0 3 5 8 10
25 25 25 25 25
0.25 0.25 0.25 0.25 0.25
Fourier transform infrared (FTIR) spectra were collected on a Nicolet 5700 spectrophotometers (Thermo Companies) in the range of 400–2000 cm− 1 at room temperature. Optical absorption spectrometry characterization was performed using a JASCO V-570 spectrophotometer. Photoluminescence (PL) spectra for various glass samples were recorded on a Fluorolog-3-P fluorescence spectrometer (Jobin Yvon, France) with a xenon lamp as the excitation source. Electron paramagnetic resonance (EPR) characterizations were carried out using an EMX-8/2.7 EPR spectrometer (Bruker) operating in the X-band frequency (9.861 GHz).
Fig. 2. FTIR spectra of the glass samples.
The optical absorption spectrum of sample G0 is shown in Fig. 1 The absorption cut-off edge of Eu2+ ions could be observed in the range of 250 to 400 nm [13]. In addition, the inset of Fig. 1 exhibits two absorption levels, which are attributed to the 7F0→5D2 and 7 F1→5D1 transitions of Eu3+ ions [14]. Therefore, the absorption spectrum confirms the coexistence of Eu2+ and Eu3+ in sample G0, which proves that the reduction of Eu3+ to Eu2+ did take place in the glass sample during the preparation in air. Fig. 2 shows FTIR spectra of samples G0 to G4 at room temperature. The spectra exhibit four main absorption bands, B–O stretching of [BO3] units in 1590–1360 cm− 1, Si–O stretching of units [SiO4] and B–O stretching of units [BO4] units in 1200–900 cm− 1, Al–O bending of [AlO6] units in 820–560 cm− 1, and Si–O–Si and Si–O–Al bending in 530–400 cm− 1 [15–17]. The increasing intensity of the characteristic peak of B–O stretching of [BO3] units at 1590–1360 cm− 1 shows a good agreement with the formulation that more Al2O3 was replaced by B2O3 from samples G0 to G4. The breadth of the absorption band in the range of 1200–906 cm− 1 gradually narrows from samples G0 to G4. The breadth of the absorption band is determined by the number of non-bridging oxygen in the tetrahedron [16]. Thus, the breadth narrowing implies an increasing number of non-bridging oxygen existing in the glass network structure.
Furthermore, the absorption at 704 cm− 1 is owing to the bending of Al–O in [AlO6]. The increasing intensity from samples G0 to G4 indicates that the concentration of [AlO6] units in glass network is increasing. Considering the fact that an increasing amount of Al2O3 was replaced by B2O3 from samples G0 to G4, the simultaneous increasing concentration of [AlO6] units indicates that B2O3 can help promote the conversion of [AlO4] to [AlO6] units. Thus, although the overall concentration of Al atoms reduced, the concentration of [AlO6] units still increased, and meanwhile resulting in the further reduction of [AlO4] unit concentration in the microstructure. The absorption band at around 906 cm− 1, which is attributed to the coexistence of [BO4] and [AlO4] [15], increases firstly and then decreases. When Al2O3 was replaced by B2O3 in the composition, the concentration of [AlO4] units in the glass network structure decreased, as discussed above, which should lead to the weakening of the vibration absorption intensity of [AlO4]. However, this vibration absorption intensity firstly exhibits an increasing tendency, which indicates that the concentration of [BO4] units must increase firstly in samples G0 to G2. However, when the concentration of B2O3 was further increased from samples G2 to G4, [BO3] units were preferably formed in the glass network structure over [BO4] unit. Thus, the vibration absorption intensity of the band started to decrease. Therefore, the concentration of [BO4] units firstly increased and then decreased in the glass network from samples G0 to G4. Fig. 3 shows the emission spectra of samples G0 to G4 with 361 nm excitation at room temperature, in which the broad emission peak in the range of 375 to 570 nm could be assigned to 4f65d→4f7 transition
Fig. 1. Absorption spectrum of sample G0; the inset highlights the absorption spectrum from Eu3+ ions.
Fig. 3. Emission spectra of glass samples excited by 361 nm UV light. The inset is the intensity ratio of emission peaks of (4f65d→4f7)/(5D0→7F2).
3. Results and discussion
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H. Zeng et al. / Journal of Non-Crystalline Solids 357 (2011) 2328–2331
of Eu2+, while the emission peaks at 590 and 611 nm could be assigned to 5D0→7FJ (J = 1, 2) transitions of Eu3+. The emission spectra show that both Eu2+ and Eu3+ co-exist in Eu-doped glass samples; the reduction from Eu3+ to Eu2+ was achieved during the preparation process in air environment. In Fig. 3, with B2O3 replacing Al2O3 in glass composition, the emission peak intensity of Eu2+ ions increases and then decreases. The inset of Fig. 3 shows the ratio of Eu2+ emission peaking at 429 nm and Eu3+ emission peaking at 611 nm, which also exhibits the same trend. The reduction of Eu3+ to Eu2+ in Eu-doped glass in air is mainly owing to two reasons: (1) the trivalent dopant Eu3+ cations replace the divalent cations in the host; and (2) the host compound has an appropriate structure, which is composed of tetrahedral anion groups (e.g. [AlO4], [BO4], and [SiO4]) [3,10,18]. It is well known that the glass network structure is a hard three-dimensional network composed of tetrahedral anion groups, which could effectively shield the attack of oxygen on Eu2+ ions in the air condition [3]. Therefore, the changes in the glass network structure may indirectly influence the reduction of Eu3+ to Eu2+. As discussed above, because of the replacement of Al2O3 by B2O3 in the host, the number of [BO4] structure units in samples G0, G1 and G2 increased gradually, and the number of [AlO4] structure units decreased. The strength of B–O band is higher than that of Al–O band and the radius of B3+ is smaller than that of Al3+. Thus, comparing with [AlO4] unit, [BO4] unit has a more compact structure. It is reasonable to hypothesize that in the rigid three-dimensional network, the more restrained [BO4] units are more effective than [AlO4] units to shield Eu2+ from accessing by air. Therefore, more Eu2+ cations exist in the glass network structure. As a result, luminescence enhancement from Eu2+ ions was observed in the emission spectra. On the other hand, the reduction of Eu3+ could also be explained by the optical basicity theory [19]. Following the optical basicity rules, Eu2+ has a tendency to exist in host glass with lower optical basicity [11]. According to Duffy empirical equation, the optical basicity value (Λ) of each glass sample was calculated based on their formulations and is listed in Table 2. The optical basicity of host glasses gradually decreased with more Al2O3 replaced by B2O3 in the samples from G0 to G2. As a result, higher concentrations of Eu2+ ions stably exist in the glass network structure. Therefore, the luminescence intensity from Eu2+ ions gradually increased with B2O3 replacing Al2O3 in the samples from G0 to G2. However, when the optical basicity continues to decrease in samples from G2 to G4, the luminescence intensity from Eu2+ ions gradually decreased. As discussed above, besides the optical basicity, the host structure may also play an important role in the reduction of Eu3+ ions [10,18]. As FTIR spectra indicated, a large amount of [BO3] structure units formed in the samples from G2 to G4, which is expected to lead to the collapse of the intrinsic closed hard threedimensional network structure. Thus, the Eu2+ ions in the plane structure, such as [BO3] structure, would be easily accessed by oxygen during the preparation condition, resulting in the partial oxidation of Eu2+ to Eu3+ ions. EPR analysis can provide insightful information about Eu2+ ions. Fig. 4 presents the EPR spectra of the samples with varying B2O3 concentrations. The electron spin S and nuclear spin I of Eu2+ are 7/2 and 5/2, respectively, and the resonance signal at g values of 6.0, 4.7 and 2.8 can be found in the samples [4,8]. As shown in Fig. 4, the EPR signal intensity of Eu2+ ions increases firstly and then decreases with more B2O3 replacing Al2O3 in the glass composition, which agrees well with PL results. The EPR data further support the
Fig. 4. EPR spectra of glass samples.
above discussion on the reduction of Eu3+ to Eu2+. In summary, with more B2O3 replacing Al2O3 in the glass composition, the ratio of emission peak intensity of Eu2+ and Eu3+ is firstly increased and then decreased. 4. Conclusions In the present work, Eu ion-doped aluminoborosilicate glasses were prepared by melt-quenching method in air atmosphere. Reduction from Eu3+ to Eu2+ was achieved in the air environment. With the replacement of Al2O3 by B2O3 in the glasses, [BO4] units with more compact structure than [AlO4] units formed in the glass network structure. The increasing concentration of [BO4] units enables more Eu2+ ions to exist stably in the closed three-dimensional network. On the other hand, the optical basicity in host glass gradually decreased with Al2O3 replaced by B2O3, which also explains the formation of Eu2+ ions. Thus an enhancement of luminescence intensity from Eu2+ ions was observed. However, with further replacement of Al2O3 by B2O3 in the composition, [BO3] units, instead of [BO4] units tended to form in the glass network. The excessive [BO3] units would break intrinsic closed rigid three-dimensional network in glass, resulting in the partial oxidation of Eu2+ to Eu3+. As a result, the emission intensity from Eu2+ ions was lowered. Correspondingly, the emission intensity ratio of Eu2+ to Eu3+ is firstly increased and then decreased with B 2O 3 replacing Al2O 3 in the glass composition. Acknowledgements The authors wish to thank Prof. D. Chen (Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science) for helpful discussions. The present work is financially supported by the National Natural Science Foundation of China (NSFC 50702021), the Marie Curie International Incoming Fellowship (PIIF-GA-2008-219582), and the Shanghai Leading Academic Discipline Project (project B502). References [1] [2] [3] [4] [5]
Table 2 A list of optical basicity calculated based on nominal chemical composition. Glass no.
G0
G1
G2
G3
G4
Optical basicity (Λ)
0.5900
0.5819
0.5765
0.5684
0.5630
[6] [7] [8] [9] [10]
M. Thoms, H. von Seggern, A. Winnacker, Phys. Rev. B 44 (1991) 9240. K. Takahashi, J. Lumin. 100 (2002) 307. C. Zhu, Y. Yang, X. Liang, S. Yuan, G. Chen, J. Am. Ceram. Soc. 90 (2007) 2984. Q. Zhang, Y. Qiao, B. Qian, G. Dong, J. Ruan, X. Liu, Q. Zhou, Q. Chen, J. Qiu, D. Chen, J. Lumin. 129 (2009) 1393. S. Liu, G. Zhao, W. Ruan, Z. Yao, T. Xie, J. Jin, H. Ying, J. Wang, G. Han, J. Am. Ceram. Soc. 91 (2008) 2740. Z. Lian, J. Wang, Y. Lv, S. Wang, Q. Su, J. Alloys Compd. 430 (2007) 257. M. Peng, Z. Pei, Guangyan Hong, Q. Su, J. Mater. Chem. 13 (2003) 1202. Z. Lin, H. Zeng, Y. Yang, X. Liang, G. Chen, L. Sun, J. Am. Ceram. Soc. 93 (2010) 3095. I. Täle, P. Kulis, V. Kronghauz, J. Lumin. 20 (1979) 343. M. Peng, Z. Pei, G. Hong, Q. Su, Chem. Phys. Lett. 371 (2003) 1.
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[16] M. Sroda, C. Paluszkiewicz, Vib. Spectrosc. 48 (2008) 246. [17] S. Agathopoulos, D.U. Tulyaganov, J.M.G. Ventura, S. Kannan, A. Saranti, M.A. Karakassides, J.M.F. Ferreira, J. NonCryst. Solids 352 (2006) 322. [18] Z. Pei, Q. Zeng, Q. Su, J. Phys. Chem. Solids 61 (2000) 9. [19] Q. Luo, X. Qiao, X. Fan, S. Liu, H. Yang, X. Zhang, J. NonCryst. Solids 354 (2008) 4691.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
The effect of B2O3 on the luminescent properties of Eu ion-doped aluminoborosilicate glasses Huidan Zeng a, Yifan Yang a, Zhenyu Lin a, Xiaoluan Liang a, Shuanglong Yuan a, Guorong Chen a,⁎, Luyi Sun b,⁎ a b
Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China Department of Chemistry and Biochemistry & Materials Science and Engineering Program, Texas State University—San Marcos, San Macros, TX 78666, USA
a r t i c l e
i n f o
Article history: Received 13 July 2010 Received in revised form 27 October 2010 Available online 16 December 2010 Keywords: Eu ions; Structure of glasses; Aluminoborosilicate glasses; PL properties
a b s t r a c t A series of Eu ion-doped aluminoborosilicate glasses was prepared by melt-quenching method and characterized by spectroscopic techniques, including Fourier transform infrared (FTIR), absorption, photoluminescence (PL), and electron paramagnetic resonance (EPR). The FTIR spectra showed that various glass structures formed when Al2O3 was partially replaced by B2O3. The PL characterization revealed that the emission intensity of Eu2+ ions firstly increased and then decreased with an increasing amount of Al2O3 replaced by B2O3. Meanwhile, the emission intensity ratio of [Eu2+]/[Eu3+] also followed the same trend. The EPR spectra confirmed the concentration variation of Eu2+ ions in the glass samples, which agreed well with the PL results. The possible mechanism of the effect of the glass network structure on the reduction behavior of Eu ions is discussed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Blue fluorescent materials have become a group of important functional materials in high-tech applications, including X-ray intensifying screens, large screen LED displays, and X-ray imaging panels, etc. [1–3]. Blue emission region of Eu2+-doped materials has been studied extensively [4–8]. Since Eu2O3 is the most typical starting compound to prepare Eu2+ doped fluorescent materials, Eu3+ ions must be reduced to obtain Eu2+ during the synthesis. In general, the reduction of Eu3+ to Eu2+ requires a reducing agent, such as CO, H2, carbon, etc. However, these complex preparation technologies not only increase the cost, but also potentially generate safety issues. Thus, it is highly desirable to develop facile and effective approaches to synthesize Eu2+ activated fluorescent materials. Täle et al. studied the synthesis of the Eu2+ activated Ba3(PO4)2 fluorescent material by high-temperature solid state reaction in air in 1979 [9]. Their work led to a safer and simpler means to obtain Eu2+. Recently, it has been disclosed that the reduction of Eu3+ to Eu2+ could be realized in host material (e.g. silicates, phosphates and borates) in air [3,10,11]. Peng et al. proposed that the spontaneous reduction of Eu3+ to Eu2+ in air has four requirements: (1) no oxidizing agents exist in the host compounds; (2) trivalent dopant Eu3+ cation could replace the divalent cation in the hosts; (3) the substituted cation has a similar radius to the
⁎ Corresponding authors. E-mail addresses: [email protected] (G. Chen), [email protected] (L. Sun). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.11.093
divalent Eu2+ cation; and (4) the host compound has an appropriate structure, which is composed of tetrahedral anion groups (e.g. AlO4, BO4, and SiO4) [10]. So far, the reduction of Eu3+ to Eu2+ in borophosphate, borate, and boroaluminate glass systems in air has been reported. While the earlier work mainly focused on the effect of alkaline earth metal ions and the optical basicity of the host materials on the luminescent properties of Eu ions doped compounds [3,11,12], the effect of the glass network structure on the reduction of Eu3+ ions has been largely ignored. In the present work, based on a series of Eu-doped aluminoborosilicate glasses, we investigated the relationship between the luminescent properties of the synthesized glasses and their network structure, which can be tuned by replacing Al2O3 with B2O3. The mechanism of the effect of intrinsic glass network structure on the reduction of Eu ions was proposed and discussed. 2. Experimental Eu-doped aluminoborosilicate glasses were prepared by melt quenching technique. Analytical reagents CaCO3, H3BO3, Al2O3, SiO2 and high purity Eu2O3 were used as starting materials to prepare a series of Eu-doped aluminoborosilicate glasses, whose nominal chemical compositions are listed in Table 1. The weighted raw materials (ca. 20.0 g) were mixed thoroughly and then transferred to a platinum crucible to melt at 1500–1550 °C for 1.5 h in air. The glass melt was poured into a preheated stainless steel mold for quenching, and subsequently annealed near its glass temperature Tg. The resulting glasses were polished and cut into 10 mm × 10 mm × 1 mm plates for characterizations.
H. Zeng et al. / Journal of Non-Crystalline Solids 357 (2011) 2328–2331
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Table 1 Nominal chemical composition of the glass samples. Glass no.
SiO2
Al2O3
B2O3
CaO
Eu2O3
G0 G1 G2 G3 G4
50 50 50 50 50
25 22 20 17 15
0 3 5 8 10
25 25 25 25 25
0.25 0.25 0.25 0.25 0.25
Fourier transform infrared (FTIR) spectra were collected on a Nicolet 5700 spectrophotometers (Thermo Companies) in the range of 400–2000 cm− 1 at room temperature. Optical absorption spectrometry characterization was performed using a JASCO V-570 spectrophotometer. Photoluminescence (PL) spectra for various glass samples were recorded on a Fluorolog-3-P fluorescence spectrometer (Jobin Yvon, France) with a xenon lamp as the excitation source. Electron paramagnetic resonance (EPR) characterizations were carried out using an EMX-8/2.7 EPR spectrometer (Bruker) operating in the X-band frequency (9.861 GHz).
Fig. 2. FTIR spectra of the glass samples.
The optical absorption spectrum of sample G0 is shown in Fig. 1 The absorption cut-off edge of Eu2+ ions could be observed in the range of 250 to 400 nm [13]. In addition, the inset of Fig. 1 exhibits two absorption levels, which are attributed to the 7F0→5D2 and 7 F1→5D1 transitions of Eu3+ ions [14]. Therefore, the absorption spectrum confirms the coexistence of Eu2+ and Eu3+ in sample G0, which proves that the reduction of Eu3+ to Eu2+ did take place in the glass sample during the preparation in air. Fig. 2 shows FTIR spectra of samples G0 to G4 at room temperature. The spectra exhibit four main absorption bands, B–O stretching of [BO3] units in 1590–1360 cm− 1, Si–O stretching of units [SiO4] and B–O stretching of units [BO4] units in 1200–900 cm− 1, Al–O bending of [AlO6] units in 820–560 cm− 1, and Si–O–Si and Si–O–Al bending in 530–400 cm− 1 [15–17]. The increasing intensity of the characteristic peak of B–O stretching of [BO3] units at 1590–1360 cm− 1 shows a good agreement with the formulation that more Al2O3 was replaced by B2O3 from samples G0 to G4. The breadth of the absorption band in the range of 1200–906 cm− 1 gradually narrows from samples G0 to G4. The breadth of the absorption band is determined by the number of non-bridging oxygen in the tetrahedron [16]. Thus, the breadth narrowing implies an increasing number of non-bridging oxygen existing in the glass network structure.
Furthermore, the absorption at 704 cm− 1 is owing to the bending of Al–O in [AlO6]. The increasing intensity from samples G0 to G4 indicates that the concentration of [AlO6] units in glass network is increasing. Considering the fact that an increasing amount of Al2O3 was replaced by B2O3 from samples G0 to G4, the simultaneous increasing concentration of [AlO6] units indicates that B2O3 can help promote the conversion of [AlO4] to [AlO6] units. Thus, although the overall concentration of Al atoms reduced, the concentration of [AlO6] units still increased, and meanwhile resulting in the further reduction of [AlO4] unit concentration in the microstructure. The absorption band at around 906 cm− 1, which is attributed to the coexistence of [BO4] and [AlO4] [15], increases firstly and then decreases. When Al2O3 was replaced by B2O3 in the composition, the concentration of [AlO4] units in the glass network structure decreased, as discussed above, which should lead to the weakening of the vibration absorption intensity of [AlO4]. However, this vibration absorption intensity firstly exhibits an increasing tendency, which indicates that the concentration of [BO4] units must increase firstly in samples G0 to G2. However, when the concentration of B2O3 was further increased from samples G2 to G4, [BO3] units were preferably formed in the glass network structure over [BO4] unit. Thus, the vibration absorption intensity of the band started to decrease. Therefore, the concentration of [BO4] units firstly increased and then decreased in the glass network from samples G0 to G4. Fig. 3 shows the emission spectra of samples G0 to G4 with 361 nm excitation at room temperature, in which the broad emission peak in the range of 375 to 570 nm could be assigned to 4f65d→4f7 transition
Fig. 1. Absorption spectrum of sample G0; the inset highlights the absorption spectrum from Eu3+ ions.
Fig. 3. Emission spectra of glass samples excited by 361 nm UV light. The inset is the intensity ratio of emission peaks of (4f65d→4f7)/(5D0→7F2).
3. Results and discussion
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of Eu2+, while the emission peaks at 590 and 611 nm could be assigned to 5D0→7FJ (J = 1, 2) transitions of Eu3+. The emission spectra show that both Eu2+ and Eu3+ co-exist in Eu-doped glass samples; the reduction from Eu3+ to Eu2+ was achieved during the preparation process in air environment. In Fig. 3, with B2O3 replacing Al2O3 in glass composition, the emission peak intensity of Eu2+ ions increases and then decreases. The inset of Fig. 3 shows the ratio of Eu2+ emission peaking at 429 nm and Eu3+ emission peaking at 611 nm, which also exhibits the same trend. The reduction of Eu3+ to Eu2+ in Eu-doped glass in air is mainly owing to two reasons: (1) the trivalent dopant Eu3+ cations replace the divalent cations in the host; and (2) the host compound has an appropriate structure, which is composed of tetrahedral anion groups (e.g. [AlO4], [BO4], and [SiO4]) [3,10,18]. It is well known that the glass network structure is a hard three-dimensional network composed of tetrahedral anion groups, which could effectively shield the attack of oxygen on Eu2+ ions in the air condition [3]. Therefore, the changes in the glass network structure may indirectly influence the reduction of Eu3+ to Eu2+. As discussed above, because of the replacement of Al2O3 by B2O3 in the host, the number of [BO4] structure units in samples G0, G1 and G2 increased gradually, and the number of [AlO4] structure units decreased. The strength of B–O band is higher than that of Al–O band and the radius of B3+ is smaller than that of Al3+. Thus, comparing with [AlO4] unit, [BO4] unit has a more compact structure. It is reasonable to hypothesize that in the rigid three-dimensional network, the more restrained [BO4] units are more effective than [AlO4] units to shield Eu2+ from accessing by air. Therefore, more Eu2+ cations exist in the glass network structure. As a result, luminescence enhancement from Eu2+ ions was observed in the emission spectra. On the other hand, the reduction of Eu3+ could also be explained by the optical basicity theory [19]. Following the optical basicity rules, Eu2+ has a tendency to exist in host glass with lower optical basicity [11]. According to Duffy empirical equation, the optical basicity value (Λ) of each glass sample was calculated based on their formulations and is listed in Table 2. The optical basicity of host glasses gradually decreased with more Al2O3 replaced by B2O3 in the samples from G0 to G2. As a result, higher concentrations of Eu2+ ions stably exist in the glass network structure. Therefore, the luminescence intensity from Eu2+ ions gradually increased with B2O3 replacing Al2O3 in the samples from G0 to G2. However, when the optical basicity continues to decrease in samples from G2 to G4, the luminescence intensity from Eu2+ ions gradually decreased. As discussed above, besides the optical basicity, the host structure may also play an important role in the reduction of Eu3+ ions [10,18]. As FTIR spectra indicated, a large amount of [BO3] structure units formed in the samples from G2 to G4, which is expected to lead to the collapse of the intrinsic closed hard threedimensional network structure. Thus, the Eu2+ ions in the plane structure, such as [BO3] structure, would be easily accessed by oxygen during the preparation condition, resulting in the partial oxidation of Eu2+ to Eu3+ ions. EPR analysis can provide insightful information about Eu2+ ions. Fig. 4 presents the EPR spectra of the samples with varying B2O3 concentrations. The electron spin S and nuclear spin I of Eu2+ are 7/2 and 5/2, respectively, and the resonance signal at g values of 6.0, 4.7 and 2.8 can be found in the samples [4,8]. As shown in Fig. 4, the EPR signal intensity of Eu2+ ions increases firstly and then decreases with more B2O3 replacing Al2O3 in the glass composition, which agrees well with PL results. The EPR data further support the
Fig. 4. EPR spectra of glass samples.
above discussion on the reduction of Eu3+ to Eu2+. In summary, with more B2O3 replacing Al2O3 in the glass composition, the ratio of emission peak intensity of Eu2+ and Eu3+ is firstly increased and then decreased. 4. Conclusions In the present work, Eu ion-doped aluminoborosilicate glasses were prepared by melt-quenching method in air atmosphere. Reduction from Eu3+ to Eu2+ was achieved in the air environment. With the replacement of Al2O3 by B2O3 in the glasses, [BO4] units with more compact structure than [AlO4] units formed in the glass network structure. The increasing concentration of [BO4] units enables more Eu2+ ions to exist stably in the closed three-dimensional network. On the other hand, the optical basicity in host glass gradually decreased with Al2O3 replaced by B2O3, which also explains the formation of Eu2+ ions. Thus an enhancement of luminescence intensity from Eu2+ ions was observed. However, with further replacement of Al2O3 by B2O3 in the composition, [BO3] units, instead of [BO4] units tended to form in the glass network. The excessive [BO3] units would break intrinsic closed rigid three-dimensional network in glass, resulting in the partial oxidation of Eu2+ to Eu3+. As a result, the emission intensity from Eu2+ ions was lowered. Correspondingly, the emission intensity ratio of Eu2+ to Eu3+ is firstly increased and then decreased with B 2O 3 replacing Al2O 3 in the glass composition. Acknowledgements The authors wish to thank Prof. D. Chen (Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science) for helpful discussions. The present work is financially supported by the National Natural Science Foundation of China (NSFC 50702021), the Marie Curie International Incoming Fellowship (PIIF-GA-2008-219582), and the Shanghai Leading Academic Discipline Project (project B502). References [1] [2] [3] [4] [5]
Table 2 A list of optical basicity calculated based on nominal chemical composition. Glass no.
G0
G1
G2
G3
G4
Optical basicity (Λ)
0.5900
0.5819
0.5765
0.5684
0.5630
[6] [7] [8] [9] [10]
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