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Effects of copper ions on the near-infrared luminescence in Bi doped silicate glass via copper for sodium ion exchange
Journal of Non-Crystalline Solids 421 (2015) 30–34
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Effects of copper ions on the near-infrared luminescence in Bi doped silicate glass via copper for sodium ion exchange Xijia He a,b, Xuhui Xu a, Dacheng Zhou a, Cuiqiong Yan b, Jianbei Qiu a,⁎ a b
College of Materials Science and Engineering, Kunming University of Science and Technology, Xuefu RD, Kunming 650093, Yunnan, China College of Physics and Electronic Engineering, Qujing Normal University, Sanjiang RD, Qujing 655011, Yunnan, China
a r t i c l e
i n f o
Article history: Received 12 March 2015 Received in revised form 9 April 2015 Accepted 11 April 2015 Available online xxxx Keywords: Bismuth; Near infrared luminescence; Copper for sodium ion exchange
a b s t r a c t The copper (Cu) atoms were incorporated into the bismuth (Bi) doped silicate glasses by the copper for sodium (Cu–Na) ion exchange method. After the ion exchange process, the Cu ions were introduced into the glass matrix with different copper valence states, namely, Cu0, Cu+, and Cu2+. Bi3+ ions could be reduced to lower valence states of Bi containing Bi near infrared (NIR) active centers with the incorporation of the Cu ions, resulting in the enhancement of the Bi NIR luminescence intensity. During the heat treatment process, some lower valence states of Bi containing Bi NIR active centers could be oxidized, resulting in the attenuation of the NIR luminescence of Bi. Our research may extend the understanding of chemical environment modulation in Bi valence states and offer a valuable way to enhance the NIR luminescence intensity of Bi in glasses. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Due to the rapid development of telecommunication technology, which demands for optical amplifier with a wide and flat gain spectrum in the telecommunication window, bismuth (Bi) doped glasses with broad near infrared luminescence (NIR) gain in the telecommunication window have been widely studied over the past decade [1–8]. Though the origin of the Bi NIR active center(s) is still controversial [6,9–13], which impedes the application of the Bi doped materials, much of the current work put weight on Bi with low valence state(s) as the NIR active center(s), such as Bi+ [13–15]. Usually, Bi NIR active centers co-exist with other valence states of Bi ions. There is no effective way to prepare glasses doped with single Bi NIR active center [16–18]. It is reasonable to believe that the valence state distribution profiles of Bi in materials depend on their chemical environments [13,17,19,20], so the NIR emission profile may be tuned by modulating the chemical environment of the glass matrices. Ion exchange is a method to modify the local environment of the glass [21–26]. However, there is sparse investigation about the effect of ion exchange on the NIR luminescence properties of Bi doped glasses. In this work, the chemical environment of Bi was adjusted by the copper for sodium (Cu–Na) ion exchange method in the Bi doped silicate glass. After the ion exchange process, the Cu ions were introduced into the glass matrix with different copper valence states, namely, Cu0 , Cu +, and Cu 2+. The Bi NIR emission was adjusted by the Cu
⁎ Corresponding author. E-mail address: [email protected] (J. Qiu).
http://dx.doi.org/10.1016/j.jnoncrysol.2015.04.024 0022-3093/© 2015 Elsevier B.V. All rights reserved.
ion exchange process and the subsequent heat treatment process. The possible mechanisms were discussed in detail. 2. Experiments Glass sample with the composition (in mol%) of 60SiO2·10Al2O3· 20CaO·10Na2O·1Bi2O3 was prepared by the melt-quenching method. High grade reagents silicon dioxide (SiO2, 4 N, Aladdin Industrial Corporation), aluminum oxide (Al2O3, 4 N, Aladdin Industrial Corporation), calcium carbonate (CaCO3, 4 N, Sinopharm Chemical Reagent Co., Ltd.), sodium carbonate (Na2CO3, analytical reagent, Aladdin Industrial Corporation), and bismuth oxide (Bi2O3, 4 N, Xinju Mineral Resource Development Co. Ltd.) were selected as raw materials. The 50 g batches were mixed homogeneously in an agate mortar. Then they were melted in a corundum crucible at 1550 °C for 1 h in air. The melt was cast onto a stainless steel plate and heat treated at 520 °C for 8 h in air. Finally, the obtained glass was cut and polished into the same size. The Cu–Na ion exchanged glass samples were obtained by immersing the glasses in different molten baths of CuSO2/Na2SO2, with a copper sulfate molar concentration of 40%, 50% and 60%, and the corresponding samples were hereafter indicated as BiCu40, BiCu50 and BiCu60, at 550 °C for 1.5 h in air. The reference sample (indicated as Bi) did not undergo the Cu–Na ion exchange process. The mixed salts were contained in a silica crucible hold in a vertical furnace, in which the temperature was controlled to within ±1 °C. After inter-diffusion, samples were removed from the molten bath and washed with distilled water and alcohol to remove copper sulfate adhering to their surface. Then a part of the samples was further heat-treated at a temperature of 500 °C for 4 h in air, and the corresponding samples were hereafter indicated as BiCu40HT, BiCu50HT
X. He et al. / Journal of Non-Crystalline Solids 421 (2015) 30–34
31
and BiCu60HT. The base glass (indicated as Base) was prepared with the composition (in mol%) of 60SiO2·10Al2O3·20CaO·10Na2O, and the Cu–Na ion-exchanged and heat-treated glass samples were hereafter indicated as Cu50, and Cu50HT. Absorption spectra were recorded using a HITACHI U-4100 type spectrophotometer. The visible spectra were collected using a HITACHI F-7000 fluorescence spectrophotometer. The X-ray photoelectron spectroscopy (XPS) spectra were recorded on a PHI 5000 VersaProbe spectrometer, and the binding energies (BE) were referenced to the adventitious C1s line at 284.8 eV. The NIR luminescence spectra were obtained by a Zolix SBP300 spectrofluorometer with an InGaAs detector excited with an 808 nm laser diode. All measurements were carried out at room temperature.
3. Results
Fig. 2. Transmission electron microscope (TEM) image of glass BiCu50.
3.1. Ion-exchanged samples The XPS spectra give direct information on the charge state of atoms in materials. Unluckily, due to the fact that the content of the incorporated Cu ions is very little, the XPS spectra of Cu can hardly be distinguished from the background signal in our samples. However, the trace of Cu ions can be observed in the absorption spectra. The absorption spectra of the ion-exchanged samples are shown in Fig. 1. The absorption intensity increases with the introduction of the Cu ions in glass. The inset is the difference of the absorption of the sample BiCu50 from that of the sample Bi. The absorption at the ultraviolet (UV) region may arise from the d–s transition of Cu+ ions [27,28]. The absorption peak at ~570 nm, corresponding to the surface plasmon resonance (SPR) of Cu NPs, and the transmission electron microscope (TEM) image of glass BiCu50 (Fig. 2) indicate that there are Cu NPs formed in the ion exchange process [29,30]. The faint absorption in the range of 650–1000 nm can be attributed to the d–d transition of Cu2+ [31,32]. So we can conclude that the Cu ions are introduced into the glass matrix with different copper valence states, namely, Cu0, Cu+, and Cu2+. Usually, the characteristic emission peaks of Bi3+ occur in the UV region, blue or green wavelength region with variation of host materials [33–37]. Fig. 3 shows that the UV–visible luminescence intensity attenuates with the Cu concentration increase. The attenuation may be caused by the energy transfer from Bi3+ to Cu ions and/or the decreasing amount of Bi3+. As can be seen in Fig. 3, the characteristic visible emission of Cu+ ions is enhanced in the Cu/Bi co-doped sample
Fig. 1. Absorption spectra of samples Bi, BiCu40, BiCu50 and BiCu60. The inset is the difference of the absorption of the sample BiCu50 from that of the sample Bi.
BiCu50, so the energy transfer from Bi3+ to Cu ions is an attenuate factor. However, the possible reduction of Bi3+ cannot be ruled out directly. In order to reveal the variation of valence states, we measured the XPS spectra of Bi ions in samples Bi and BiCu50 (Fig. 4). The XPS spectrum of Bi in sample BiCu50 shifts to lower binding energy compared with that of Bi in the sample Bi [38,39], which indicates that the introduction of Cu ions into the glass matrix could reduce the higher valence states of Bi ions to lower valence species in glasses. That is the Cu–Na ion exchange could modulate the valence states of Bi in the glasses. Fig. 5 shows the NIR luminescence spectra of samples Bi, BiCu40, BiCu50 and BiCu60 excited at 808 nm. Excitingly, with the increase of the Cu concentration, the NIR luminescence intensity of Bi is enhanced.
3.2. Heat-treated samples Heat treatment was applied on a group of the ion-exchanged samples. The absorption spectra of samples Bi, BiCu50 and BiCu50HT are shown in Fig. 6. The inset is the difference of the absorption of the sample BiCu50HT from that of the sample BiCu50. The obvious dip at the situation of the SPR band indicates the dissolution of the Cu NPs during the heat treatment process. And the TEM images in Figs. 2 and 7 confirm this speculation. On the other hand, as can be seen in Fig. 8, the characteristic emission of Cu+ is enhanced in the single Cu ion exchanged
Fig. 3. Visible photoluminescence spectra of samples Base, Bi, Cu50, BiCu40, BiCu50 and BiCu60 excited at 310 nm. The inset is the normalized luminescence spectra of samples Bi, BiCu40, BiCu50 and BiCu60 excited at 310 nm.
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Fig. 4. X-ray photoelectron spectroscopy spectra of samples Bi, BiCu50 and BiCu50HT.
samples, which means that the content of Cu+ increases after heat treatment. In copper containing glasses, the metallic state Cu0 leads to a red coloration, Cu2+ causes blue to cyan, whereas Cu+ is colorless [40]. In our experiments, the colorless base glass shows light blue coloration after the ion exchange process, and turns into colorless after the heat treatment. So the disproportionation reaction as below should occur during the heat treatment process in these glasses: 2þ
Cu
0
þ
þ Cu →2Cu :
ð1Þ
The UV–visible luminescence intensity is strengthened by the heat treatment (Fig. 8), while the NIR luminescence of Bi is tampered by it (Fig. 9). The Bi XPS spectrum of sample BiCu50HT shifts to higher binding energy compared with that of the sample Bi (Fig. 4), which indicates that the heat treatment could oxidize the lower valence states of Bi ions to higher valence state species in these glasses. That is the heat treatment could modulate the valence states of Bi, too. 4. Discussion 4.1. Ion-exchanged samples The enhanced NIR emission in these samples may be discussed combing with the local field enhancement (LFE) effect of copper
Fig. 5. Near-infrared photoluminescence spectra of samples Bi, BiCu40, BiCu50 and BiCu60 excited at 808 nm.
Fig. 6. Absorption spectra of samples Bi, BiCu50 and BiCu50HT. The inset is the difference of the absorption of the samples BiCu50HT from that of the sample BiCu50.
nanoparticles (NPs), energy transfer from Cu ions, namely Cu0, Cu+, and Cu2+, to the NIR active centers, and the increased content of NIR active centers. Research results have shown the general features of the Cu–Na ion exchange as below [22,23,41]: (1) copper penetrates into glass mainly as Cu+ ions, replacing Na+ ions of the matrix; (2) only a trace of divalent Cu2+ ions can be incorporated into the glass matrix; (3) part of copper ions can be converted to metal Cu0; and (4) the Cu–Na ion exchange leads to a local structural rearrangement of the glass matrix. So we can discuss as follows. Firstly, since the characteristic SPR band is at about 500–650 nm, neither the exciting light nor the NIR emission band is overlapped by it, improved excitation and emission efficiency induced by SPR probably cannot occur in our experiment. Secondly, due to large mismatch between the SPR band and the NIR emission band, the energy transfer from Cu NPs to the NIR active centers can be ignored. Thirdly, the characteristic emission peak of Cu+ occurs at ~510 nm (see Fig. 4) [42,43], indicating that the possibility of energy transfer towards Bi NIR active centers should be rare. Fourthly, the characteristic photoluminescence of Cu2+ was not observed in these samples, and the characteristic absorption of Cu2+ was faint, so the energy transfer from Cu2+ to Bi NIR active centers should not be the key factor for the enhancement. Fifthly, the Cu–Na ion exchange could modulate the valence states of Bi in these glasses. Therefore, the enhanced NIR emission intensity of Bi may originate from the increased content of Bi NIR emission centers.
Fig. 7. TEM image of glass BiCu50HT.
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Fig. 8. a Visible photoluminescence spectra of samples Base, Bi, Cu50, Cu50HT, Bi, BiCu40, BiCu40HT, BiCu50, BiCu50HT, BiCu60 and BiCu60HT excited at 310 nm. b Intensity variations of Bi visible emission in ion exchanged and heat treated samples for different copper concentrations. The lines are guide for eye.
In the Cu–Na ion exchange process, Cu atoms replace the Na+ ions, which modify the local environment around the Bi ions. As previously reported, in tetrasilicate glass, Na K-edge extended X-ray absorption fine structure (EXAFS) analysis revealed that Na ions took a large coordination number (CN) of 4.3 [44]. However, Cu K-edge EXAFS analysis showed that Cu+ ion had a small CN of 2–3 [23]. It means that the Cu incorporation always causes local structural relaxations. Due to a large decrease of CN by the replacement of Cu for Na+, a large relaxation of local structure would be induced at the non-bridging oxygen (NBO), which means that the network gives new negative sites [45, 46]. The negative sites can prove electron to higher valence state of Bi/Cu ions. As the standard reduction potential (E°) of Cu2+/Cu+ (0.15 eV) is lower than that of Bi3+/Bix+, such as E°(Bi3+/Bi+) = 0.2 eV [47], the incorporation of Cu ions will be beneficial to reduce the Bi3+ to lower valence states of Bi containing Bi NIR active centers at the initial stage of the ion exchange process. Although, with the increase of ion exchange time, much lower valence states of Bi form in glass, more complicated redox equilibrium will happen, it is reasonable
to deduce that the NIR emission of Bi is enhanced by the increased content of Bi NIR emission centers. 4.2. Heat-treated samples The heat treatment could modulate the valence states of Bi, so the weakened NIR emission may originate from the reduction of the NIR active centers. In the heat treatment process, Cu atoms convert to Cu+ by redox reaction, which further modifies the local environment around the Bi ions. The Cu2+ ions always substitute the Na+ during the ion exchange process, so the formation of Cu+ consumes the NBO sites, which means that some lower valence states of ions can be oxidized. As the standard reduction potential (E°) of Cu+/Cu0 (0.52 eV) is higher than that of Bi3+/ Bix+, such as E° (Bi3+/Bi+) = 0.2 eV, E° (Bi3+/Bi0) = 0.31 eV [47], some lower valence states of Bi containing Bi NIR active centers will have the priority to be oxidized. Finally, the tampered NIR emission is caused by the decreased content of NIR active centers.
Fig. 9. a Near-infrared photoluminescence spectra of samples Bi, BiCu40, BiCu40HT, BiCu50, BiCu50HT, BiCu60 and BiCu60HT excited at 808 nm. b Intensity variations of Bi near infrared emission in ion exchanged and heat treated samples for different copper concentrations. The lines are guide for eye.
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5. Conclusions The Bi doped silicate glasses were prepared, and the adjustment of the NIR luminescent properties of Bi induced by Cu–Na ion exchange and subsequent heat treatment was investigated. We demonstrated that Bi NIR active centers can be modulated by the Cu ion exchange process and the subsequent heat treatment. During the Cu+ for Na+ ion exchange process, Cu atoms are incorporated into the samples. Due to the higher standard reduction potential, some Bi3+ ions can be transformed into lower valence states of Bi containing NIR active centers, which was confirmed by the enhanced NIR luminescence intensity and the shift of the XPS spectra of Bi. In the heat treatment procedure, owing to the lower standard reduction potential, some lower valence states of Bi containing NIR active centers turn into Bi3+. Finally, the NIR emission of Bi attenuates. According to the above discussion, the adjustment of the Bi NIR emission and the modulation in Bi valence states are more likely to originate from the variation of chemical environment leading by the Cu ion exchange and the subsequent heat treatment process. Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 51272097, 61265004 and 61307111), and the Nature and Science Fund from Yunnan Province Ministry of Education (No. 2011C13211708). References [1] Y. Fujimoto, M. Nakatsuka, Jpn. J. Appl. Phys. 40 (2001) L279–L281. [2] V.G. Plotnichenko, D.V. Philippovskiy, V.O. Sokolov, M.V. Sukhanov, A.P. Velmuzhov, M.F. Churbanov, E.M. Dianov, Opt. Mater. Express 4 (2014) 366–374. [3] A.N. Romanov, A.A. Veber, Z.T. Fattakhova, D.N. Vtyurina, M.S. Kouznetsov, K.S. Zaramenskikh, I.S. Lisitsky, V.N. Korchak, V.B. Tsvetkov, V.B. Sulimov, J. Lumin. 149 (2014) 292–296. [4] A.A. Veber, A.N. Romanov, O.V. Usovich, Z.T. Fattakhova, E.V. Haula, V.N. Korchak, L.A. Trusov, P.E. Kazin, V.B. Sulimov, V.B. Tsvetkov, J. Lumin. 151 (2014) 247–255. [5] B. Xu, S. Zhou, D. Tan, Z. Hong, J. Hao, J. Qiu, J. Appl. Phys. 113 (2013) 083503. [6] V.O. Sokolov, V.G. Plotnichenko, E.M. Dianov, Opt. Mater. Express 3 (2013) 1059–1074. [7] T.M. Hau, R. Wang, D. Zhou, X. Yue, Z. Song, Z. Yang, Y. Yang, X. He, J. Qiu, J. Lumin. 132 (2012) 1353–1356. [8] M. Peng, J. Qiu, D. Chen, X. Meng, L. Yang, X. Jiang, C. Zhu, Opt. Express 13 (2005) 1628–1634. [9] A.N. Romanov, Z.T. Fattakhova, A.A. Veber, O.V. Usovich, P.E. Haula, V.N. Korchak, V.B. Tsvetkov, L.A. Trusov, P.E. Kazin, V.B. Sulimov, Opt. Express 20 (2012) 7212–7220. [10] A.N. Romanov, Z.T. Fattakhova, D.M. Zhigunov, V.N. Korchak, V.B. Sulimov, Opt. Mater. 33 (2011) 631–634. [11] M. Peng, G. Dong, L. Wondraczek, L. Zhang, N. Zhang, J. Qiu, J. Non-Cryst. Solids 357 (2011) 2241–2245.
[12] M. Peng, C. Zollfrank, L. Wondraczek, J. Phys. Condens. Matter 21 (2009) 285106. [13] R. Ren, G. Dong, S. Xu, R. Bao, J. Qiu, J. Phys. Chem. A 112 (2008) 3036–3039. [14] A.N. Romanov, A.A. Veber, Z.T. Fattakhova, O.V. Usovich, E.V. Haula, Lev.A. Trusov, P.E. Kazin, V.N. Korchak, V.B. Tsvetkov, V.B. Sulimov, J. Lumin. 134 (2013) 180–183. [15] X. Meng, J. Qiu, M. Peng, D. Chen, Q. Zhao, X. Jiang, C. Zhu, Opt. Express 13 (2005) 1628–1634. [16] Z. Bai, M. Fujii, T. Hasegawa, S. Kitano, K. Imakita, M. Mizuhata, S. Hayashi, Opt. Mater. 34 (2012) 821–825. [17] B.I. Denker, B.I. Galagan, I.L. Shulman, S.E. Sverchkov, E.M. Dianov, Appl. Phys. B 103 (2011) 681–685. [18] S. Khonthon, S. Morimoto, Y. Arai, Y. Ohishi, Opt. Mater. 31 (2009) 1262–1268. [19] S.P. Singh, B. Karmakar, Opt. Mater. 33 (2011) 1760–1765. [20] T. Murata, T. Mouri, J. Non-Cryst. Solids 353 (2007) 2403–2407. [21] C. Maurizio, F. D'Acapito, C. Sada, E. Cattaruzza, F. Gonella, G. Battaglin, Mater. Sci. Eng. B 149 (2008) 171–176. [22] F. Gonella, A. Quaranta, E. Cattaruzza, S. Padovani, C. Sada, F. D_Acapito, C. Maurizio, Comp. Mater. Sci. 33 (2005) 31–36. [23] F. D'Acapito, S. Colonna, S. Mobilio, F. Gonella, E. Cattaruzza, P. Mazzoldi, Appl. Phys. Lett. 71 (1997) 2611–2613. [24] A. Quaranta, A. Rahman, G. Mariotto, C. Maurizio, E. Trave, F. Gonella, E. Cattaruzza, E. Gibaudo, J.E. Broquin, J. Phys. Chem. C 116 (2012) 3757–3764. [25] O. Veron, J.P. Blondeau, D. De Sousa Meneses, C.A. Vignolle, Plasmonics 6 (2011) 137–148. [26] X. He, X. Xu, Y. Shi, J. Qiu, J. Non-Cryst. Solids 409 (2015) 178–182. [27] J. Yang, H. Guo, Y. Wei, H.M. Noh, J.H. Jeong, Opt. Mater. Express 4 (2014) 315–320. [28] H. Guo, R.F. Wei, X.Y. Liu, Opt. Lett. 37 (2012) 1670–1672. [29] B. Kumar, G. Kaur, P. Singh, S.B. Rai, Appl. Phys. B 110 (2013) 345–351. [30] K.P. Rice, E.J. Walker Jr., M.P. Stoykovich, A.E. Saunders, J. Phys. Chem. C 115 (2011) 1793–1799. [31] S.P. Singh, R.P.S. Chakradhar, J.L. Rao, B. Karmakar, J. Magn. Magn. Mater. 346 (2013) 21–25. [32] B. Karthikeyan, Spectrochim. Acta A 66 (2007) 860–862. [33] S. Lai, Z. Yang, R. Wang, H. Wu, J. Liao, J. Qiu, Z. Song, Y. Yang, D. Zhou, J. Mater. Sci. 48 (2013) 8566–8570. [34] Y. Zorenko, M. Pashkovsky, A. Voloshinovskii, B. Kuklinski, M. Grinberg, J. Lumin. 116 (2006) 43–51. [35] W. Dong, C. Zhu, J. Phys. Chem. Solids 64 (2003) 265–271. [36] S.H. Shin, D.Y. Jeon, K.S. Suh, J. Appl. Phys. 90 (2001) 5986–5990. [37] G. Blasse, A. Bril, J. Chem. Phys. 48 (1968) 217–222. [38] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg, Handbook of X-ray Photoelectron Spectroscopy, 1979. 191. [39] P. Anil Kumar Reddy, B. Srinivas, K. Pruthu, V. Durgakumari, M. Subrahmanyam, Mater. Res. Bull. 46 (2011) 1766–1771. [40] J. Kaufmann, C. Rüssel, J. Non-Cryst. Solids 356 (2010) 1615–1619. [41] F. Gonella, F. Caccavale, L.D. Bogomolova, V.A. Jachkin, Appl. Phys. A 68 (1999) 539–546. [42] K. Annapurna, Anil Kumar, R.N. Dwivedi, N. Sooraj Hussain, S. Buddhudu, Mater. Lett. 45 (2000) 23–26. [43] K. Fukumi, A. Chayahara, K. Ohora, N. Kitamura, Y. Horino, K. Fujii, M. Makihara, J. Hayakaya, N. Ohno, Nucl. Instrum. Meth. B 149 (1999) 77–80. [44] S.N. Houde-Walter, J.M. Inman, A.J. Dent, G.N. Greaves, J. Phys. Chem. 97 (1993) 9330–9336. [45] F. Catan, D. De Sousa Meneses, J.P. Blondeau, L. Allam, J. Non-Cryst. Solids 354 (2008) 1026–1031. [46] S.E. Paje, M.A. Garcia, J. Llopis, M.A. Villegas, J. Non-Cryst. Solids 318 (2003) 239–247. [47] D.R. Lide, CRC Handbook of Chemistry and Physics, Electrochemical Series2004. 8–24.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Effects of copper ions on the near-infrared luminescence in Bi doped silicate glass via copper for sodium ion exchange Xijia He a,b, Xuhui Xu a, Dacheng Zhou a, Cuiqiong Yan b, Jianbei Qiu a,⁎ a b
College of Materials Science and Engineering, Kunming University of Science and Technology, Xuefu RD, Kunming 650093, Yunnan, China College of Physics and Electronic Engineering, Qujing Normal University, Sanjiang RD, Qujing 655011, Yunnan, China
a r t i c l e
i n f o
Article history: Received 12 March 2015 Received in revised form 9 April 2015 Accepted 11 April 2015 Available online xxxx Keywords: Bismuth; Near infrared luminescence; Copper for sodium ion exchange
a b s t r a c t The copper (Cu) atoms were incorporated into the bismuth (Bi) doped silicate glasses by the copper for sodium (Cu–Na) ion exchange method. After the ion exchange process, the Cu ions were introduced into the glass matrix with different copper valence states, namely, Cu0, Cu+, and Cu2+. Bi3+ ions could be reduced to lower valence states of Bi containing Bi near infrared (NIR) active centers with the incorporation of the Cu ions, resulting in the enhancement of the Bi NIR luminescence intensity. During the heat treatment process, some lower valence states of Bi containing Bi NIR active centers could be oxidized, resulting in the attenuation of the NIR luminescence of Bi. Our research may extend the understanding of chemical environment modulation in Bi valence states and offer a valuable way to enhance the NIR luminescence intensity of Bi in glasses. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Due to the rapid development of telecommunication technology, which demands for optical amplifier with a wide and flat gain spectrum in the telecommunication window, bismuth (Bi) doped glasses with broad near infrared luminescence (NIR) gain in the telecommunication window have been widely studied over the past decade [1–8]. Though the origin of the Bi NIR active center(s) is still controversial [6,9–13], which impedes the application of the Bi doped materials, much of the current work put weight on Bi with low valence state(s) as the NIR active center(s), such as Bi+ [13–15]. Usually, Bi NIR active centers co-exist with other valence states of Bi ions. There is no effective way to prepare glasses doped with single Bi NIR active center [16–18]. It is reasonable to believe that the valence state distribution profiles of Bi in materials depend on their chemical environments [13,17,19,20], so the NIR emission profile may be tuned by modulating the chemical environment of the glass matrices. Ion exchange is a method to modify the local environment of the glass [21–26]. However, there is sparse investigation about the effect of ion exchange on the NIR luminescence properties of Bi doped glasses. In this work, the chemical environment of Bi was adjusted by the copper for sodium (Cu–Na) ion exchange method in the Bi doped silicate glass. After the ion exchange process, the Cu ions were introduced into the glass matrix with different copper valence states, namely, Cu0 , Cu +, and Cu 2+. The Bi NIR emission was adjusted by the Cu
⁎ Corresponding author. E-mail address: [email protected] (J. Qiu).
http://dx.doi.org/10.1016/j.jnoncrysol.2015.04.024 0022-3093/© 2015 Elsevier B.V. All rights reserved.
ion exchange process and the subsequent heat treatment process. The possible mechanisms were discussed in detail. 2. Experiments Glass sample with the composition (in mol%) of 60SiO2·10Al2O3· 20CaO·10Na2O·1Bi2O3 was prepared by the melt-quenching method. High grade reagents silicon dioxide (SiO2, 4 N, Aladdin Industrial Corporation), aluminum oxide (Al2O3, 4 N, Aladdin Industrial Corporation), calcium carbonate (CaCO3, 4 N, Sinopharm Chemical Reagent Co., Ltd.), sodium carbonate (Na2CO3, analytical reagent, Aladdin Industrial Corporation), and bismuth oxide (Bi2O3, 4 N, Xinju Mineral Resource Development Co. Ltd.) were selected as raw materials. The 50 g batches were mixed homogeneously in an agate mortar. Then they were melted in a corundum crucible at 1550 °C for 1 h in air. The melt was cast onto a stainless steel plate and heat treated at 520 °C for 8 h in air. Finally, the obtained glass was cut and polished into the same size. The Cu–Na ion exchanged glass samples were obtained by immersing the glasses in different molten baths of CuSO2/Na2SO2, with a copper sulfate molar concentration of 40%, 50% and 60%, and the corresponding samples were hereafter indicated as BiCu40, BiCu50 and BiCu60, at 550 °C for 1.5 h in air. The reference sample (indicated as Bi) did not undergo the Cu–Na ion exchange process. The mixed salts were contained in a silica crucible hold in a vertical furnace, in which the temperature was controlled to within ±1 °C. After inter-diffusion, samples were removed from the molten bath and washed with distilled water and alcohol to remove copper sulfate adhering to their surface. Then a part of the samples was further heat-treated at a temperature of 500 °C for 4 h in air, and the corresponding samples were hereafter indicated as BiCu40HT, BiCu50HT
X. He et al. / Journal of Non-Crystalline Solids 421 (2015) 30–34
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and BiCu60HT. The base glass (indicated as Base) was prepared with the composition (in mol%) of 60SiO2·10Al2O3·20CaO·10Na2O, and the Cu–Na ion-exchanged and heat-treated glass samples were hereafter indicated as Cu50, and Cu50HT. Absorption spectra were recorded using a HITACHI U-4100 type spectrophotometer. The visible spectra were collected using a HITACHI F-7000 fluorescence spectrophotometer. The X-ray photoelectron spectroscopy (XPS) spectra were recorded on a PHI 5000 VersaProbe spectrometer, and the binding energies (BE) were referenced to the adventitious C1s line at 284.8 eV. The NIR luminescence spectra were obtained by a Zolix SBP300 spectrofluorometer with an InGaAs detector excited with an 808 nm laser diode. All measurements were carried out at room temperature.
3. Results
Fig. 2. Transmission electron microscope (TEM) image of glass BiCu50.
3.1. Ion-exchanged samples The XPS spectra give direct information on the charge state of atoms in materials. Unluckily, due to the fact that the content of the incorporated Cu ions is very little, the XPS spectra of Cu can hardly be distinguished from the background signal in our samples. However, the trace of Cu ions can be observed in the absorption spectra. The absorption spectra of the ion-exchanged samples are shown in Fig. 1. The absorption intensity increases with the introduction of the Cu ions in glass. The inset is the difference of the absorption of the sample BiCu50 from that of the sample Bi. The absorption at the ultraviolet (UV) region may arise from the d–s transition of Cu+ ions [27,28]. The absorption peak at ~570 nm, corresponding to the surface plasmon resonance (SPR) of Cu NPs, and the transmission electron microscope (TEM) image of glass BiCu50 (Fig. 2) indicate that there are Cu NPs formed in the ion exchange process [29,30]. The faint absorption in the range of 650–1000 nm can be attributed to the d–d transition of Cu2+ [31,32]. So we can conclude that the Cu ions are introduced into the glass matrix with different copper valence states, namely, Cu0, Cu+, and Cu2+. Usually, the characteristic emission peaks of Bi3+ occur in the UV region, blue or green wavelength region with variation of host materials [33–37]. Fig. 3 shows that the UV–visible luminescence intensity attenuates with the Cu concentration increase. The attenuation may be caused by the energy transfer from Bi3+ to Cu ions and/or the decreasing amount of Bi3+. As can be seen in Fig. 3, the characteristic visible emission of Cu+ ions is enhanced in the Cu/Bi co-doped sample
Fig. 1. Absorption spectra of samples Bi, BiCu40, BiCu50 and BiCu60. The inset is the difference of the absorption of the sample BiCu50 from that of the sample Bi.
BiCu50, so the energy transfer from Bi3+ to Cu ions is an attenuate factor. However, the possible reduction of Bi3+ cannot be ruled out directly. In order to reveal the variation of valence states, we measured the XPS spectra of Bi ions in samples Bi and BiCu50 (Fig. 4). The XPS spectrum of Bi in sample BiCu50 shifts to lower binding energy compared with that of Bi in the sample Bi [38,39], which indicates that the introduction of Cu ions into the glass matrix could reduce the higher valence states of Bi ions to lower valence species in glasses. That is the Cu–Na ion exchange could modulate the valence states of Bi in the glasses. Fig. 5 shows the NIR luminescence spectra of samples Bi, BiCu40, BiCu50 and BiCu60 excited at 808 nm. Excitingly, with the increase of the Cu concentration, the NIR luminescence intensity of Bi is enhanced.
3.2. Heat-treated samples Heat treatment was applied on a group of the ion-exchanged samples. The absorption spectra of samples Bi, BiCu50 and BiCu50HT are shown in Fig. 6. The inset is the difference of the absorption of the sample BiCu50HT from that of the sample BiCu50. The obvious dip at the situation of the SPR band indicates the dissolution of the Cu NPs during the heat treatment process. And the TEM images in Figs. 2 and 7 confirm this speculation. On the other hand, as can be seen in Fig. 8, the characteristic emission of Cu+ is enhanced in the single Cu ion exchanged
Fig. 3. Visible photoluminescence spectra of samples Base, Bi, Cu50, BiCu40, BiCu50 and BiCu60 excited at 310 nm. The inset is the normalized luminescence spectra of samples Bi, BiCu40, BiCu50 and BiCu60 excited at 310 nm.
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Fig. 4. X-ray photoelectron spectroscopy spectra of samples Bi, BiCu50 and BiCu50HT.
samples, which means that the content of Cu+ increases after heat treatment. In copper containing glasses, the metallic state Cu0 leads to a red coloration, Cu2+ causes blue to cyan, whereas Cu+ is colorless [40]. In our experiments, the colorless base glass shows light blue coloration after the ion exchange process, and turns into colorless after the heat treatment. So the disproportionation reaction as below should occur during the heat treatment process in these glasses: 2þ
Cu
0
þ
þ Cu →2Cu :
ð1Þ
The UV–visible luminescence intensity is strengthened by the heat treatment (Fig. 8), while the NIR luminescence of Bi is tampered by it (Fig. 9). The Bi XPS spectrum of sample BiCu50HT shifts to higher binding energy compared with that of the sample Bi (Fig. 4), which indicates that the heat treatment could oxidize the lower valence states of Bi ions to higher valence state species in these glasses. That is the heat treatment could modulate the valence states of Bi, too. 4. Discussion 4.1. Ion-exchanged samples The enhanced NIR emission in these samples may be discussed combing with the local field enhancement (LFE) effect of copper
Fig. 5. Near-infrared photoluminescence spectra of samples Bi, BiCu40, BiCu50 and BiCu60 excited at 808 nm.
Fig. 6. Absorption spectra of samples Bi, BiCu50 and BiCu50HT. The inset is the difference of the absorption of the samples BiCu50HT from that of the sample BiCu50.
nanoparticles (NPs), energy transfer from Cu ions, namely Cu0, Cu+, and Cu2+, to the NIR active centers, and the increased content of NIR active centers. Research results have shown the general features of the Cu–Na ion exchange as below [22,23,41]: (1) copper penetrates into glass mainly as Cu+ ions, replacing Na+ ions of the matrix; (2) only a trace of divalent Cu2+ ions can be incorporated into the glass matrix; (3) part of copper ions can be converted to metal Cu0; and (4) the Cu–Na ion exchange leads to a local structural rearrangement of the glass matrix. So we can discuss as follows. Firstly, since the characteristic SPR band is at about 500–650 nm, neither the exciting light nor the NIR emission band is overlapped by it, improved excitation and emission efficiency induced by SPR probably cannot occur in our experiment. Secondly, due to large mismatch between the SPR band and the NIR emission band, the energy transfer from Cu NPs to the NIR active centers can be ignored. Thirdly, the characteristic emission peak of Cu+ occurs at ~510 nm (see Fig. 4) [42,43], indicating that the possibility of energy transfer towards Bi NIR active centers should be rare. Fourthly, the characteristic photoluminescence of Cu2+ was not observed in these samples, and the characteristic absorption of Cu2+ was faint, so the energy transfer from Cu2+ to Bi NIR active centers should not be the key factor for the enhancement. Fifthly, the Cu–Na ion exchange could modulate the valence states of Bi in these glasses. Therefore, the enhanced NIR emission intensity of Bi may originate from the increased content of Bi NIR emission centers.
Fig. 7. TEM image of glass BiCu50HT.
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Fig. 8. a Visible photoluminescence spectra of samples Base, Bi, Cu50, Cu50HT, Bi, BiCu40, BiCu40HT, BiCu50, BiCu50HT, BiCu60 and BiCu60HT excited at 310 nm. b Intensity variations of Bi visible emission in ion exchanged and heat treated samples for different copper concentrations. The lines are guide for eye.
In the Cu–Na ion exchange process, Cu atoms replace the Na+ ions, which modify the local environment around the Bi ions. As previously reported, in tetrasilicate glass, Na K-edge extended X-ray absorption fine structure (EXAFS) analysis revealed that Na ions took a large coordination number (CN) of 4.3 [44]. However, Cu K-edge EXAFS analysis showed that Cu+ ion had a small CN of 2–3 [23]. It means that the Cu incorporation always causes local structural relaxations. Due to a large decrease of CN by the replacement of Cu for Na+, a large relaxation of local structure would be induced at the non-bridging oxygen (NBO), which means that the network gives new negative sites [45, 46]. The negative sites can prove electron to higher valence state of Bi/Cu ions. As the standard reduction potential (E°) of Cu2+/Cu+ (0.15 eV) is lower than that of Bi3+/Bix+, such as E°(Bi3+/Bi+) = 0.2 eV [47], the incorporation of Cu ions will be beneficial to reduce the Bi3+ to lower valence states of Bi containing Bi NIR active centers at the initial stage of the ion exchange process. Although, with the increase of ion exchange time, much lower valence states of Bi form in glass, more complicated redox equilibrium will happen, it is reasonable
to deduce that the NIR emission of Bi is enhanced by the increased content of Bi NIR emission centers. 4.2. Heat-treated samples The heat treatment could modulate the valence states of Bi, so the weakened NIR emission may originate from the reduction of the NIR active centers. In the heat treatment process, Cu atoms convert to Cu+ by redox reaction, which further modifies the local environment around the Bi ions. The Cu2+ ions always substitute the Na+ during the ion exchange process, so the formation of Cu+ consumes the NBO sites, which means that some lower valence states of ions can be oxidized. As the standard reduction potential (E°) of Cu+/Cu0 (0.52 eV) is higher than that of Bi3+/ Bix+, such as E° (Bi3+/Bi+) = 0.2 eV, E° (Bi3+/Bi0) = 0.31 eV [47], some lower valence states of Bi containing Bi NIR active centers will have the priority to be oxidized. Finally, the tampered NIR emission is caused by the decreased content of NIR active centers.
Fig. 9. a Near-infrared photoluminescence spectra of samples Bi, BiCu40, BiCu40HT, BiCu50, BiCu50HT, BiCu60 and BiCu60HT excited at 808 nm. b Intensity variations of Bi near infrared emission in ion exchanged and heat treated samples for different copper concentrations. The lines are guide for eye.
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5. Conclusions The Bi doped silicate glasses were prepared, and the adjustment of the NIR luminescent properties of Bi induced by Cu–Na ion exchange and subsequent heat treatment was investigated. We demonstrated that Bi NIR active centers can be modulated by the Cu ion exchange process and the subsequent heat treatment. During the Cu+ for Na+ ion exchange process, Cu atoms are incorporated into the samples. Due to the higher standard reduction potential, some Bi3+ ions can be transformed into lower valence states of Bi containing NIR active centers, which was confirmed by the enhanced NIR luminescence intensity and the shift of the XPS spectra of Bi. In the heat treatment procedure, owing to the lower standard reduction potential, some lower valence states of Bi containing NIR active centers turn into Bi3+. Finally, the NIR emission of Bi attenuates. According to the above discussion, the adjustment of the Bi NIR emission and the modulation in Bi valence states are more likely to originate from the variation of chemical environment leading by the Cu ion exchange and the subsequent heat treatment process. Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 51272097, 61265004 and 61307111), and the Nature and Science Fund from Yunnan Province Ministry of Education (No. 2011C13211708). References [1] Y. Fujimoto, M. Nakatsuka, Jpn. J. Appl. Phys. 40 (2001) L279–L281. [2] V.G. Plotnichenko, D.V. Philippovskiy, V.O. Sokolov, M.V. Sukhanov, A.P. Velmuzhov, M.F. Churbanov, E.M. Dianov, Opt. Mater. Express 4 (2014) 366–374. [3] A.N. Romanov, A.A. Veber, Z.T. Fattakhova, D.N. Vtyurina, M.S. Kouznetsov, K.S. Zaramenskikh, I.S. Lisitsky, V.N. Korchak, V.B. Tsvetkov, V.B. Sulimov, J. Lumin. 149 (2014) 292–296. [4] A.A. Veber, A.N. Romanov, O.V. Usovich, Z.T. Fattakhova, E.V. Haula, V.N. Korchak, L.A. Trusov, P.E. Kazin, V.B. Sulimov, V.B. Tsvetkov, J. Lumin. 151 (2014) 247–255. [5] B. Xu, S. Zhou, D. Tan, Z. Hong, J. Hao, J. Qiu, J. Appl. Phys. 113 (2013) 083503. [6] V.O. Sokolov, V.G. Plotnichenko, E.M. Dianov, Opt. Mater. Express 3 (2013) 1059–1074. [7] T.M. Hau, R. Wang, D. Zhou, X. Yue, Z. Song, Z. Yang, Y. Yang, X. He, J. Qiu, J. Lumin. 132 (2012) 1353–1356. [8] M. Peng, J. Qiu, D. Chen, X. Meng, L. Yang, X. Jiang, C. Zhu, Opt. Express 13 (2005) 1628–1634. [9] A.N. Romanov, Z.T. Fattakhova, A.A. Veber, O.V. Usovich, P.E. Haula, V.N. Korchak, V.B. Tsvetkov, L.A. Trusov, P.E. Kazin, V.B. Sulimov, Opt. Express 20 (2012) 7212–7220. [10] A.N. Romanov, Z.T. Fattakhova, D.M. Zhigunov, V.N. Korchak, V.B. Sulimov, Opt. Mater. 33 (2011) 631–634. [11] M. Peng, G. Dong, L. Wondraczek, L. Zhang, N. Zhang, J. Qiu, J. Non-Cryst. Solids 357 (2011) 2241–2245.
[12] M. Peng, C. Zollfrank, L. Wondraczek, J. Phys. Condens. Matter 21 (2009) 285106. [13] R. Ren, G. Dong, S. Xu, R. Bao, J. Qiu, J. Phys. Chem. A 112 (2008) 3036–3039. [14] A.N. Romanov, A.A. Veber, Z.T. Fattakhova, O.V. Usovich, E.V. Haula, Lev.A. Trusov, P.E. Kazin, V.N. Korchak, V.B. Tsvetkov, V.B. Sulimov, J. Lumin. 134 (2013) 180–183. [15] X. Meng, J. Qiu, M. Peng, D. Chen, Q. Zhao, X. Jiang, C. Zhu, Opt. Express 13 (2005) 1628–1634. [16] Z. Bai, M. Fujii, T. Hasegawa, S. Kitano, K. Imakita, M. Mizuhata, S. Hayashi, Opt. Mater. 34 (2012) 821–825. [17] B.I. Denker, B.I. Galagan, I.L. Shulman, S.E. Sverchkov, E.M. Dianov, Appl. Phys. B 103 (2011) 681–685. [18] S. Khonthon, S. Morimoto, Y. Arai, Y. Ohishi, Opt. Mater. 31 (2009) 1262–1268. [19] S.P. Singh, B. Karmakar, Opt. Mater. 33 (2011) 1760–1765. [20] T. Murata, T. Mouri, J. Non-Cryst. Solids 353 (2007) 2403–2407. [21] C. Maurizio, F. D'Acapito, C. Sada, E. Cattaruzza, F. Gonella, G. Battaglin, Mater. Sci. Eng. B 149 (2008) 171–176. [22] F. Gonella, A. Quaranta, E. Cattaruzza, S. Padovani, C. Sada, F. D_Acapito, C. Maurizio, Comp. Mater. Sci. 33 (2005) 31–36. [23] F. D'Acapito, S. Colonna, S. Mobilio, F. Gonella, E. Cattaruzza, P. Mazzoldi, Appl. Phys. Lett. 71 (1997) 2611–2613. [24] A. Quaranta, A. Rahman, G. Mariotto, C. Maurizio, E. Trave, F. Gonella, E. Cattaruzza, E. Gibaudo, J.E. Broquin, J. Phys. Chem. C 116 (2012) 3757–3764. [25] O. Veron, J.P. Blondeau, D. De Sousa Meneses, C.A. Vignolle, Plasmonics 6 (2011) 137–148. [26] X. He, X. Xu, Y. Shi, J. Qiu, J. Non-Cryst. Solids 409 (2015) 178–182. [27] J. Yang, H. Guo, Y. Wei, H.M. Noh, J.H. Jeong, Opt. Mater. Express 4 (2014) 315–320. [28] H. Guo, R.F. Wei, X.Y. Liu, Opt. Lett. 37 (2012) 1670–1672. [29] B. Kumar, G. Kaur, P. Singh, S.B. Rai, Appl. Phys. B 110 (2013) 345–351. [30] K.P. Rice, E.J. Walker Jr., M.P. Stoykovich, A.E. Saunders, J. Phys. Chem. C 115 (2011) 1793–1799. [31] S.P. Singh, R.P.S. Chakradhar, J.L. Rao, B. Karmakar, J. Magn. Magn. Mater. 346 (2013) 21–25. [32] B. Karthikeyan, Spectrochim. Acta A 66 (2007) 860–862. [33] S. Lai, Z. Yang, R. Wang, H. Wu, J. Liao, J. Qiu, Z. Song, Y. Yang, D. Zhou, J. Mater. Sci. 48 (2013) 8566–8570. [34] Y. Zorenko, M. Pashkovsky, A. Voloshinovskii, B. Kuklinski, M. Grinberg, J. Lumin. 116 (2006) 43–51. [35] W. Dong, C. Zhu, J. Phys. Chem. Solids 64 (2003) 265–271. [36] S.H. Shin, D.Y. Jeon, K.S. Suh, J. Appl. Phys. 90 (2001) 5986–5990. [37] G. Blasse, A. Bril, J. Chem. Phys. 48 (1968) 217–222. [38] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg, Handbook of X-ray Photoelectron Spectroscopy, 1979. 191. [39] P. Anil Kumar Reddy, B. Srinivas, K. Pruthu, V. Durgakumari, M. Subrahmanyam, Mater. Res. Bull. 46 (2011) 1766–1771. [40] J. Kaufmann, C. Rüssel, J. Non-Cryst. Solids 356 (2010) 1615–1619. [41] F. Gonella, F. Caccavale, L.D. Bogomolova, V.A. Jachkin, Appl. Phys. A 68 (1999) 539–546. [42] K. Annapurna, Anil Kumar, R.N. Dwivedi, N. Sooraj Hussain, S. Buddhudu, Mater. Lett. 45 (2000) 23–26. [43] K. Fukumi, A. Chayahara, K. Ohora, N. Kitamura, Y. Horino, K. Fujii, M. Makihara, J. Hayakaya, N. Ohno, Nucl. Instrum. Meth. B 149 (1999) 77–80. [44] S.N. Houde-Walter, J.M. Inman, A.J. Dent, G.N. Greaves, J. Phys. Chem. 97 (1993) 9330–9336. [45] F. Catan, D. De Sousa Meneses, J.P. Blondeau, L. Allam, J. Non-Cryst. Solids 354 (2008) 1026–1031. [46] S.E. Paje, M.A. Garcia, J. Llopis, M.A. Villegas, J. Non-Cryst. Solids 318 (2003) 239–247. [47] D.R. Lide, CRC Handbook of Chemistry and Physics, Electrochemical Series2004. 8–24.