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Optical properties and local structure of Eu3+-doped synthetic analogue of the microporous titanosilicate mineral sitinakite
ARTICLE IN PRESS
Journal of Luminescence 128 (2008) 1108–1112 www.elsevier.com/locate/jlumin
Optical properties and local structure of Eu3+-doped synthetic analogue of the microporous titanosilicate mineral sitinakite Stanislav Ferdova,b,1, Rute A. Sa´ Ferreirab, Zhi Lina, a
Department of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal b Department of Physics, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal
Received 19 July 2007; received in revised form 14 October 2007; accepted 12 November 2007 Available online 4 December 2007
Abstract The synthetic analogue of the microporous titanosilicate mineral sitinakite has been hydrothermally synthesized and used as a host in the preparation of a new photoluminescent material. The inclusion of Eu3+ in the pores of the sitinakite doubles the unit cell volume and changes the symmetry of the initial sodium phase. The Eu3+-doped material displays a stable room temperature emission ascribed to the Eu3+ intra-4f6 5D0-7F04 transitions, with a maximum external quantum yield of 6%. The observation of two components for the nondegenerated 5D0-7F0 transition, the local field splitting of the 5D0-7F12 transitions, and the 5D0 emission decay curves point out the presence of two optically active Eu3+ sites. Possible structural distribution of the detected Eu3+ cations is discussed. r 2007 Elsevier B.V. All rights reserved. Keywords: Photoluminescence; Microporous titanosilicate; Host
1. Introduction The recent history of the family of the microporous titanosilicates is dominated by investigations of their classical zeolitic properties of catalysis, ion exchange, adsorption and separation. However, not long time ago a new possible application of these materials as a host for optically active environments has been proposed [1]. Here we turn our attention to the microporous titanosilicate analogue of the mineral sitinakite [2,3]. Recently, the synthetic sitinakite (Na2Ti2O3SiO4 2H2O) has attracted considerable interest due to its selective ion exchange properties, which makes it a promising material for remediation of ground water and certain type of nuclear wastes [4,5]. The structure of sitinakite has a onedimensional channel system composed of TiO6 octahedra occurring in clusters of four sharing edges to form a cubelike unit. The clusters are connected along c-axis by Ti–O–Ti bonds, and along a- and b-axis by SiO4 tetrahedra Corresponding author. Tel.: +351 234401519; fax: +351 234370084.
E-mail address: [email protected] (Z. Lin). On leave from the Central Laboratory of Mineralogy and Crystallography, Bulgarian Academy of Sciences. 1
0022-2313/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2007.11.085
whose oxygen atoms form part of the cluster. There are two crystallographic positions for eight Na+ ions in the unit cell. One sodium site is within the framework bonded to four oxygen atoms of silicate and two water molecules. The other site is in the tunnels with pore openings approximately 3.5 A˚ wide [3]. In this report, we extend the classical zeolitic properties of sitinakite and use its open framework as host to embedding Eu3+, which results in the creation of a new optical material. The properties and local distribution of the Eu3+ local coordination sites within the framework and in the channels are studied on the basis of photoluminescence spectroscopic data. In this paper, we also report, for the first time, absolute emission quantum yields on microporous titanosilicates, enabling the quantification of the photoluminescence features of such materials class.
2. Experimental The synthetic sitinakite was prepared using the following molar composition: 1.6Na2O:SiO2:0.78TiO2:20H2O. Typically, sodium silicate solution (27 wt% SiO2, 8 wt% Na2O, Merck), water, NaOH and anatase were mixed and heated
ARTICLE IN PRESS in a Teflon-lined autoclave at 170 1C for 6–7 days. Autoclave was removed and quenched in cold water. The resulting crystals were filtered and washed at room temperature with distilled water, and dried at 90–100 1C. Eu3+-doped sitinakite powder was prepared via ion exchange as follows: 0.36 g of europium(III) chloride hexahydrate (99.9%, Aldrich) was dissolved in 100 ml distilled water. Sitinakite powder (0.5 g) was added to Eu solution and the suspension was stirred at ca. 50 1C for 24 h. The final product was filtered and washed with distilled water and dried at 50 1C overnight. The morphology and crystal size of the samples were examined using scanning electron microscope (SEM) Hitachi S-4100. Energy-dispersive X-ray spectrometry (EDS) analysis was carried out by Ro¨mteck EDS System attached to the SEM. Powder X-ray diffraction (XRD) patterns were collected between 5 and 501 2y (step size 0.051) on a Philips X’pert MPD diffractometer using CuKa radiation. The photoluminescence spectra (14–300 K) were recorded on a Fluorolog-3 Model FL3-2T with a double excitation spectrometer (Triax 320), fitted with a 1200 grooves/mm grating blazed at 300 nm with bandpass of 2.1 nm/mm, and a single emission spectrometer (Triax 320), fitted with a 1200 grooves/mm with bandpass 2.6 nm/ mm coupled to a R928 Hamamatsu photomultiplier, using the front face acquisition mode. The excitation source was a 450 W Xe arc lamp. The emission spectra were corrected for detection and optical spectral response of the spectrofluorimeter and the excitation spectra were corrected for the spectral distribution of the lamp intensity using a photodiode reference detector. The lifetime measurements (14–300 K) were acquired with the setup described for the luminescence spectra using a pulsed Xe–Hg lamp (6 ms pulse at half-width and 20–30 ms tail) with emission slits of 0.30 mm and a starting delay of 0.050 ms. The measurements at 14 K were performed using a He closed cycle cryostat. The absolute emission quantum yields were measured at room temperature using a Quantum Yield Measurement System C9920-02 from Hamamatsu with a 150 W xenon lamp coupled to a monochromator for wavelength discrimination, an integrating sphere as sample chamber and a multi-channel analyzer for signal detection. 3. Results and discussion Fig. 1 shows direct comparison between powder XRD patterns of the as-synthesized and Eu3+-doped sitinakite. The phase retains its structural integrity although the inclusion of Eu3+ has resulted in vastly decreased crystallinity. Using the FullProf program [6] the unit cell parameters of the as-synthesized sodium sitinakite were refined (P42/mcm, a ¼ b ¼ 7.806(3)A˚, c ¼ 11.959(1) A˚, V ¼ 728.76 A˚3), and are in a fair agreement with the previously reported ones (P42/mcm, a ¼ b ¼ 7.8082(2)A˚, c ¼ 11.9735(4) A˚, V ¼ 730.00 A˚3) [3]. However, the attempts for lattice refinement of the Eu3+-doped sitinakite
Intensity (a.u.)
S. Ferdov et al. / Journal of Luminescence 128 (2008) 1108–1112
1109
Eu3+- doped Sitinakite
Sitinakite
10
20
30 2 Cukα
40
50
Fig. 1. Powder XRD patterns of sitinakite and Eu-doped sitinakite.
starting from the structural data reported in the literature [3] have failed. This fact suggested structural changes caused by the inclusion of the Eu3+ cations. Subsequently, the powder pattern was successfully indexed by using TREOR90 [7] and the lattice parameters were refined in space group P42/mbc, which was chosen after careful examination of the systematic absences. The obtained unit cell parameters (P42/mbc, a ¼ b ¼ 10.970(6)A˚, c ¼ 11.766(8)A˚; V ¼ 1416.91 A˚3) show almost doubled unit cell volume when compared with that of the assynthesized sodium sitinakite. These lattice changes and the estimated higher symmetry space group in Eu3+-doped sitinakite suggested a different distribution of Eu3+ cations included in the pore system when compared with sodium sites in sitinakite. A similar effect was also observed in the hydrogen converted as well as in the K+, Cs+ and Sr2+ exchanged forms of titanium and titano-niobium sitinakite [8,9]. The Rietveld refinement suggested that after K exchange, one cation site is located at the center of cavity while the other site is near the framework [8]. The low crystallinity of the Eu3+-doped sitinakite did not allow a Rietveld refinement and the local structural investigation of the Eu3+ cations is based on a photoluminescence spectroscopic study. Synthetic sitinakite crystallizes as 100–200 nm wellshaped prismatic crystals, which were possible to observe by SEM (Fig. 2A). The habit of the crystals does not change during the ion exchange (Fig. 2B). The chemical analyses of Eu3+-doped sitinakite were performed by EDS and did not show the presence of any sodium, indicating its complete exchange by europium. The measured molar ratios gave: Ti/Si—1.3, Ti/Eu—2.5. Fig. 3 depicts the low-temperature excitation spectrum of the Eu3+-doped sitinakite sample monitored within the 5 D0-7F2 transition lines. The spectrum displays a large broad band with two components peaking at ca. 250 and 305 nm and a series of straight lines ascribed to intra-4f 6
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Fig. 2. SEM images of the as-synthesized (A) and Eu-doped (B) sitinakite.
Fig. 3. Photoluminescence excitation spectra for the emission wavelength 613.0 nm of the Eu-doped sitinakite sample obtained at room temperature (a) and 14 K (b).
transitions between the 7F0,1 levels of the ground multiplet and the 5D41-5L6 and 5G24 excited states. The broad bands at ca. 250 and 305 nm are probably related to the
spin-allowed inter-configurational 4f 6-4f 55d1 band of Eu3+ [10] and to O2-Eu3+ ligand-to-metal charge transfer (LMCT) bands [11], respectively. The higher relative intensity of the LMCT band than that of the Eu3+ lines indicates that the 5D0 level is mainly populated via the LMCT states at 14 K, rather than by direct intra-4f 6 excitation. On the other hand, at 300 K the Eu3+ ions are mainly populated via direct intra-4f 6 excitation while the LMCT band relative intensity decreases. The thermal quenching of the LMCT states is a well-known process, which reinforces the LMCT nature attribution [12]. Fig. 4 shows in detail the emission spectra of the Eu3+doped sitinakite sample at 14 K and room temperature. All the spectra display the typical Eu3+ orange-red intra-4f 6 lines ascribed to the 5D0-7F04 transitions. At 14 K, two lines are unequivocally discerned for the 5D0-7F0 transition (Fig. 4B), and the Stark field splitting of the 5 D0-7F1,2 transitions is 4 and 6 clearly expressed components, respectively (Fig. 4C and D), readily indicating the presence of at least two Eu3+ local distinct environments. To render easier the Eu3+ local coordination discussion, the higher (17 300 cm1) and the lower (17 275 cm1) energy 5D0-7F0 transition will be ascribed to the Eu3+ local environments labeled as Sites 1 and 2 (Fig. 4B). Increasing the temperature, changes are observed in the energy and number of emission components. In particular, a single line is observed for the non-degenerated 5 D0-7F0 transition (Fig. 4B), peaking at 17 300 cm1, and the Stark field splitting of the 5D0-7F1,2 transitions is reduced to 3 and 5 clearly expressed components (Fig. 4C and D), respectively, suggesting that the Eu3+ local environment Site 2 is thermally quenched. The 5D0 lifetime values were acquired at 14 K through the monitoring of the emission decay curves around the two 5D0-7F0 transitions. All the curves are well reproduced by a single exponential function, yielding the lifetime values of 0.50170.086 (Site 1) and 0.21570.013 ms (Site 2). The details are listed in Table 1. At room temperature the emission decay curve monitored within the Eu3+ 5 D0-7F0 transition (17 300 cm1, Site 1) is characterized by a time decay constant of 0.27870.001 ms. Fig. 5 exemplifies the 5D0 emission decay curves acquired at room temperature and the respective single exponential fit. In this paragraph we will discuss the structural characterization of the two Eu3+ local coordination sites. Considering that the substitution of Na+ by Eu3+ is not isovalent, one can expect a low occupation of the cation sites. This means that in the structure of Eu3+ exchanged sitinakite each Eu3+ ion, theoretically, should substitute three Na+ ions. Since the XRD data suggest that Eu3+ exchanged sitinakite has the same space group of the K+ exchanged sitinakite, the Eu3+ position should be very similar to that of the K+ ions. It is reasonable to assume that the coordination configuration of the Eu3+ cations is also similar to that of the K+ cations. In the K+ exchanged sitinakite one of the cations, K1, is only connected to eight oxygen atoms of the silicate group,
ARTICLE IN PRESS S. Ferdov et al. / Journal of Luminescence 128 (2008) 1108–1112 5D
7 0→ F2
1111
5D
site 1
0→
7F 0
b site 2
576
578
580 5D
582
7 0→ F1
b
D0→7F3
7 0→ F0
5D
5D
0→
7F 4
7 0→ F1
574
b
a 585
590
595 5D
5
5D
Intensity (arb. units)
a
600 7 0→ F2
b a
a 570
598
626
654
682 710 Wavelength (nm)
608 613 618 623 628
Fig. 4. Emission spectra (A) of the Eu-doped sitinakite sample obtained at room temperature (a) and 14 K (b) for the excitation wavelength 393.0 nm. (B), (C) – (D) show in detail the 5D0-7F02 transitions, respectively. Table 1 Energy (E00, cm1), and FWHM (FWHM00, cm1) of the 5D0-7F0 lines and 5D0 lifetime value (t, ms) for the Eu3+-doped sitinakite obtained at 14 1K FWHM00
t
17 293.970.1 17 277.470.2
13.170.3 53.870.3
0.50170.086 0.21570.013
Ln (Intensity)
Site 1 Site 2
E00
0.000
1.000
2.000 Time (ms)
3.000
4.000
Fig. 5. Room temperature 5D0 emission decay curves of the Eu-doped sitinakite sample monitored at 578.0 nm and excited at 393.0 nm. The solid line represents the best fit using a single exponential function (r40.9979).
being 3.84 A˚ away from water molecules. Another one, K2, is situated near the framework but away from the Si–Si axis along the c-direction, and bonded to four oxygen atoms of
framework and two water molecules (K2O distance: 2.66 and 2.98 A˚) [8]. The estimated structural similarity between the K+ and Eu3+ exchanged sitinakites and the observation of two distinct Eu3+ local environments suggest that to some extent the Eu3+ cations in the Eu3+-doped sitinakite inherit the positions of the K+ ions. However, with the present powder XRD data one cannot estimate the right crystallographic positions of the incorporated Eu atoms and possible surface-bonded Eu3+ atoms are not excluded. Based on the above considerations and on the photoluminescence results we may say that the shorter lifetime value found for Site 2, when compared to that measured for Site 1 and the higher thermal quenching of Site 2 emission intensity, is in good agreement with the presence of OH oscillators from water molecules in the Eu3+ surroundings, which is conformed with the coordination of K2. The luminescence features were also quantified through the measurement of the absolute emission quantum yield (f). Under direct intra-4f6 excitation (395 nm, 5L6 and 465 nm, 5D2) the f value is 6%, whereas under excitation via the LMCT states (305 nm) a smaller value was estimated, 1%. The lower quantum yield value estimated for LMCT excitation is consistent with the fact that LMCT states constitute an important channel for depopulation of the lanthanide excited states, leading to luminescence quenching [13]. To the best of our knowledge, this is the first work presenting quantum yield results on a Eu3+-doped titanosilicate. The obtained value is substantially smaller than those of lanthanide-based silicate inorganic phosphors, such as oxyapatites (20–30%), suggesting that the concentration quenching effects may act as luminescence non-radiative channels, as previously described [14].
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4. Conclusion In summary, a synthetic analogue of the mineral sitinakite is synthesized and doped with Eu3+. The inclusion of Eu3+ in the pore system causes structural changes detected by powder XRD and studied by the photoluminescence spectroscopy. It is estimated that Eu3+-doped sitinakite has doubled unit cell volume and higher symmetry when compared with the initial sodium phase. Photoluminescence study indicates the presence of two active local europium sites, which supports the powder XRD investigation. The Eu3+-doped synthetic sitinakite displays a stable room temperature emission in the red spectral region, with a maximum absolute emission quantum yield of 6%, being the first quantum yield report on Eu3+-doped titanosilicate. Acknowledgments The authors thank the financial support from FCT, POCI2010 and FEDER. S. Ferdov also thanks FCT (SFRH/BPD/23771/2005) for the grant. References [1] Z. Lin, J.P. Rainho, J. Domingues, L.D. Carlos, J. Rocha, Microporous Mesoporous Mater. 79 (2005) 13.
[2] E.V. Sokolova, R.K. Rastsvetaeva, V.I. Andrianov, Y.K. EngorovTismenko, Y.P. Men’Shikov, Dokl. Akad. Nauk SSSR 307 (1989) 114. [3] D.M. Poojary, R.A. Cahill, A. Clearfield, Chem. Mater. 6 (1994) 2364. [4] A. Tripathi, D.G. Medvedev, A. Clearfield, J. Solid State Chem. 178 (2005) 253. [5] R.G. Anthony, C.V. Philip, R.G. Dosch, Waste Manage. 13 (1993) 503. [6] J. Rodriguez-Carvajal, FULLPROF, Program for Rietveld refinement and pattern matching analysis, in: Abstracts of the Satellite Meeting on Powder Diffraction of the XVth Congress of the International Union of Crystallography, Toulouse, France, July 1990, p. 127. [7] P.E. Werner, L. Eriksson, M. Westdahl, J. Appl. Crystallogr. 18 (1985) 367. [8] D.M. Poojary, A.I. Bortun, L.N. Bortun, A. Clearfield, Inorg. Chem. 35 (1996) 6131. [9] A. Tripathi, D.G. Medvedev, N. Nyman, A. Clearfield, J. Solid State Chem. 175 (2003) 72. [10] D. Ananias, M. Kostova, F.A. Almeida Paz, A. Ferreira, L.D. Carlos, J. Klinowski, J. Rocha, J. Am. Chem. Soc. 126 (2004) 10410. [11] G. Blasse, A. Bril, J. Inorg. Nucl. Chem. 29 (1967) 2231. [12] G.F. de Sa´, O.L. Malta, C. de Mello Donega´, A.M. Simas, R.L. Longo, P.A. Santa-Cruz, E.F. da Silva Jr., Coord. Chem. Rev. 196 (2000) 165. [13] C.W. Struck, W.H. Fonger, Understanding Luminescence Spectra and Efficiency Using Wp and Relating Functions, Springer, Berlin, 1991 and references therein. [14] S. Ferdov, R.A. Sa´ Ferreira, Z. Lin, Chem. Mater. 18 (2006) 5958.
Journal of Luminescence 128 (2008) 1108–1112 www.elsevier.com/locate/jlumin
Optical properties and local structure of Eu3+-doped synthetic analogue of the microporous titanosilicate mineral sitinakite Stanislav Ferdova,b,1, Rute A. Sa´ Ferreirab, Zhi Lina, a
Department of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal b Department of Physics, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal
Received 19 July 2007; received in revised form 14 October 2007; accepted 12 November 2007 Available online 4 December 2007
Abstract The synthetic analogue of the microporous titanosilicate mineral sitinakite has been hydrothermally synthesized and used as a host in the preparation of a new photoluminescent material. The inclusion of Eu3+ in the pores of the sitinakite doubles the unit cell volume and changes the symmetry of the initial sodium phase. The Eu3+-doped material displays a stable room temperature emission ascribed to the Eu3+ intra-4f6 5D0-7F04 transitions, with a maximum external quantum yield of 6%. The observation of two components for the nondegenerated 5D0-7F0 transition, the local field splitting of the 5D0-7F12 transitions, and the 5D0 emission decay curves point out the presence of two optically active Eu3+ sites. Possible structural distribution of the detected Eu3+ cations is discussed. r 2007 Elsevier B.V. All rights reserved. Keywords: Photoluminescence; Microporous titanosilicate; Host
1. Introduction The recent history of the family of the microporous titanosilicates is dominated by investigations of their classical zeolitic properties of catalysis, ion exchange, adsorption and separation. However, not long time ago a new possible application of these materials as a host for optically active environments has been proposed [1]. Here we turn our attention to the microporous titanosilicate analogue of the mineral sitinakite [2,3]. Recently, the synthetic sitinakite (Na2Ti2O3SiO4 2H2O) has attracted considerable interest due to its selective ion exchange properties, which makes it a promising material for remediation of ground water and certain type of nuclear wastes [4,5]. The structure of sitinakite has a onedimensional channel system composed of TiO6 octahedra occurring in clusters of four sharing edges to form a cubelike unit. The clusters are connected along c-axis by Ti–O–Ti bonds, and along a- and b-axis by SiO4 tetrahedra Corresponding author. Tel.: +351 234401519; fax: +351 234370084.
E-mail address: [email protected] (Z. Lin). On leave from the Central Laboratory of Mineralogy and Crystallography, Bulgarian Academy of Sciences. 1
0022-2313/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2007.11.085
whose oxygen atoms form part of the cluster. There are two crystallographic positions for eight Na+ ions in the unit cell. One sodium site is within the framework bonded to four oxygen atoms of silicate and two water molecules. The other site is in the tunnels with pore openings approximately 3.5 A˚ wide [3]. In this report, we extend the classical zeolitic properties of sitinakite and use its open framework as host to embedding Eu3+, which results in the creation of a new optical material. The properties and local distribution of the Eu3+ local coordination sites within the framework and in the channels are studied on the basis of photoluminescence spectroscopic data. In this paper, we also report, for the first time, absolute emission quantum yields on microporous titanosilicates, enabling the quantification of the photoluminescence features of such materials class.
2. Experimental The synthetic sitinakite was prepared using the following molar composition: 1.6Na2O:SiO2:0.78TiO2:20H2O. Typically, sodium silicate solution (27 wt% SiO2, 8 wt% Na2O, Merck), water, NaOH and anatase were mixed and heated
ARTICLE IN PRESS in a Teflon-lined autoclave at 170 1C for 6–7 days. Autoclave was removed and quenched in cold water. The resulting crystals were filtered and washed at room temperature with distilled water, and dried at 90–100 1C. Eu3+-doped sitinakite powder was prepared via ion exchange as follows: 0.36 g of europium(III) chloride hexahydrate (99.9%, Aldrich) was dissolved in 100 ml distilled water. Sitinakite powder (0.5 g) was added to Eu solution and the suspension was stirred at ca. 50 1C for 24 h. The final product was filtered and washed with distilled water and dried at 50 1C overnight. The morphology and crystal size of the samples were examined using scanning electron microscope (SEM) Hitachi S-4100. Energy-dispersive X-ray spectrometry (EDS) analysis was carried out by Ro¨mteck EDS System attached to the SEM. Powder X-ray diffraction (XRD) patterns were collected between 5 and 501 2y (step size 0.051) on a Philips X’pert MPD diffractometer using CuKa radiation. The photoluminescence spectra (14–300 K) were recorded on a Fluorolog-3 Model FL3-2T with a double excitation spectrometer (Triax 320), fitted with a 1200 grooves/mm grating blazed at 300 nm with bandpass of 2.1 nm/mm, and a single emission spectrometer (Triax 320), fitted with a 1200 grooves/mm with bandpass 2.6 nm/ mm coupled to a R928 Hamamatsu photomultiplier, using the front face acquisition mode. The excitation source was a 450 W Xe arc lamp. The emission spectra were corrected for detection and optical spectral response of the spectrofluorimeter and the excitation spectra were corrected for the spectral distribution of the lamp intensity using a photodiode reference detector. The lifetime measurements (14–300 K) were acquired with the setup described for the luminescence spectra using a pulsed Xe–Hg lamp (6 ms pulse at half-width and 20–30 ms tail) with emission slits of 0.30 mm and a starting delay of 0.050 ms. The measurements at 14 K were performed using a He closed cycle cryostat. The absolute emission quantum yields were measured at room temperature using a Quantum Yield Measurement System C9920-02 from Hamamatsu with a 150 W xenon lamp coupled to a monochromator for wavelength discrimination, an integrating sphere as sample chamber and a multi-channel analyzer for signal detection. 3. Results and discussion Fig. 1 shows direct comparison between powder XRD patterns of the as-synthesized and Eu3+-doped sitinakite. The phase retains its structural integrity although the inclusion of Eu3+ has resulted in vastly decreased crystallinity. Using the FullProf program [6] the unit cell parameters of the as-synthesized sodium sitinakite were refined (P42/mcm, a ¼ b ¼ 7.806(3)A˚, c ¼ 11.959(1) A˚, V ¼ 728.76 A˚3), and are in a fair agreement with the previously reported ones (P42/mcm, a ¼ b ¼ 7.8082(2)A˚, c ¼ 11.9735(4) A˚, V ¼ 730.00 A˚3) [3]. However, the attempts for lattice refinement of the Eu3+-doped sitinakite
Intensity (a.u.)
S. Ferdov et al. / Journal of Luminescence 128 (2008) 1108–1112
1109
Eu3+- doped Sitinakite
Sitinakite
10
20
30 2 Cukα
40
50
Fig. 1. Powder XRD patterns of sitinakite and Eu-doped sitinakite.
starting from the structural data reported in the literature [3] have failed. This fact suggested structural changes caused by the inclusion of the Eu3+ cations. Subsequently, the powder pattern was successfully indexed by using TREOR90 [7] and the lattice parameters were refined in space group P42/mbc, which was chosen after careful examination of the systematic absences. The obtained unit cell parameters (P42/mbc, a ¼ b ¼ 10.970(6)A˚, c ¼ 11.766(8)A˚; V ¼ 1416.91 A˚3) show almost doubled unit cell volume when compared with that of the assynthesized sodium sitinakite. These lattice changes and the estimated higher symmetry space group in Eu3+-doped sitinakite suggested a different distribution of Eu3+ cations included in the pore system when compared with sodium sites in sitinakite. A similar effect was also observed in the hydrogen converted as well as in the K+, Cs+ and Sr2+ exchanged forms of titanium and titano-niobium sitinakite [8,9]. The Rietveld refinement suggested that after K exchange, one cation site is located at the center of cavity while the other site is near the framework [8]. The low crystallinity of the Eu3+-doped sitinakite did not allow a Rietveld refinement and the local structural investigation of the Eu3+ cations is based on a photoluminescence spectroscopic study. Synthetic sitinakite crystallizes as 100–200 nm wellshaped prismatic crystals, which were possible to observe by SEM (Fig. 2A). The habit of the crystals does not change during the ion exchange (Fig. 2B). The chemical analyses of Eu3+-doped sitinakite were performed by EDS and did not show the presence of any sodium, indicating its complete exchange by europium. The measured molar ratios gave: Ti/Si—1.3, Ti/Eu—2.5. Fig. 3 depicts the low-temperature excitation spectrum of the Eu3+-doped sitinakite sample monitored within the 5 D0-7F2 transition lines. The spectrum displays a large broad band with two components peaking at ca. 250 and 305 nm and a series of straight lines ascribed to intra-4f 6
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Fig. 2. SEM images of the as-synthesized (A) and Eu-doped (B) sitinakite.
Fig. 3. Photoluminescence excitation spectra for the emission wavelength 613.0 nm of the Eu-doped sitinakite sample obtained at room temperature (a) and 14 K (b).
transitions between the 7F0,1 levels of the ground multiplet and the 5D41-5L6 and 5G24 excited states. The broad bands at ca. 250 and 305 nm are probably related to the
spin-allowed inter-configurational 4f 6-4f 55d1 band of Eu3+ [10] and to O2-Eu3+ ligand-to-metal charge transfer (LMCT) bands [11], respectively. The higher relative intensity of the LMCT band than that of the Eu3+ lines indicates that the 5D0 level is mainly populated via the LMCT states at 14 K, rather than by direct intra-4f 6 excitation. On the other hand, at 300 K the Eu3+ ions are mainly populated via direct intra-4f 6 excitation while the LMCT band relative intensity decreases. The thermal quenching of the LMCT states is a well-known process, which reinforces the LMCT nature attribution [12]. Fig. 4 shows in detail the emission spectra of the Eu3+doped sitinakite sample at 14 K and room temperature. All the spectra display the typical Eu3+ orange-red intra-4f 6 lines ascribed to the 5D0-7F04 transitions. At 14 K, two lines are unequivocally discerned for the 5D0-7F0 transition (Fig. 4B), and the Stark field splitting of the 5 D0-7F1,2 transitions is 4 and 6 clearly expressed components, respectively (Fig. 4C and D), readily indicating the presence of at least two Eu3+ local distinct environments. To render easier the Eu3+ local coordination discussion, the higher (17 300 cm1) and the lower (17 275 cm1) energy 5D0-7F0 transition will be ascribed to the Eu3+ local environments labeled as Sites 1 and 2 (Fig. 4B). Increasing the temperature, changes are observed in the energy and number of emission components. In particular, a single line is observed for the non-degenerated 5 D0-7F0 transition (Fig. 4B), peaking at 17 300 cm1, and the Stark field splitting of the 5D0-7F1,2 transitions is reduced to 3 and 5 clearly expressed components (Fig. 4C and D), respectively, suggesting that the Eu3+ local environment Site 2 is thermally quenched. The 5D0 lifetime values were acquired at 14 K through the monitoring of the emission decay curves around the two 5D0-7F0 transitions. All the curves are well reproduced by a single exponential function, yielding the lifetime values of 0.50170.086 (Site 1) and 0.21570.013 ms (Site 2). The details are listed in Table 1. At room temperature the emission decay curve monitored within the Eu3+ 5 D0-7F0 transition (17 300 cm1, Site 1) is characterized by a time decay constant of 0.27870.001 ms. Fig. 5 exemplifies the 5D0 emission decay curves acquired at room temperature and the respective single exponential fit. In this paragraph we will discuss the structural characterization of the two Eu3+ local coordination sites. Considering that the substitution of Na+ by Eu3+ is not isovalent, one can expect a low occupation of the cation sites. This means that in the structure of Eu3+ exchanged sitinakite each Eu3+ ion, theoretically, should substitute three Na+ ions. Since the XRD data suggest that Eu3+ exchanged sitinakite has the same space group of the K+ exchanged sitinakite, the Eu3+ position should be very similar to that of the K+ ions. It is reasonable to assume that the coordination configuration of the Eu3+ cations is also similar to that of the K+ cations. In the K+ exchanged sitinakite one of the cations, K1, is only connected to eight oxygen atoms of the silicate group,
ARTICLE IN PRESS S. Ferdov et al. / Journal of Luminescence 128 (2008) 1108–1112 5D
7 0→ F2
1111
5D
site 1
0→
7F 0
b site 2
576
578
580 5D
582
7 0→ F1
b
D0→7F3
7 0→ F0
5D
5D
0→
7F 4
7 0→ F1
574
b
a 585
590
595 5D
5
5D
Intensity (arb. units)
a
600 7 0→ F2
b a
a 570
598
626
654
682 710 Wavelength (nm)
608 613 618 623 628
Fig. 4. Emission spectra (A) of the Eu-doped sitinakite sample obtained at room temperature (a) and 14 K (b) for the excitation wavelength 393.0 nm. (B), (C) – (D) show in detail the 5D0-7F02 transitions, respectively. Table 1 Energy (E00, cm1), and FWHM (FWHM00, cm1) of the 5D0-7F0 lines and 5D0 lifetime value (t, ms) for the Eu3+-doped sitinakite obtained at 14 1K FWHM00
t
17 293.970.1 17 277.470.2
13.170.3 53.870.3
0.50170.086 0.21570.013
Ln (Intensity)
Site 1 Site 2
E00
0.000
1.000
2.000 Time (ms)
3.000
4.000
Fig. 5. Room temperature 5D0 emission decay curves of the Eu-doped sitinakite sample monitored at 578.0 nm and excited at 393.0 nm. The solid line represents the best fit using a single exponential function (r40.9979).
being 3.84 A˚ away from water molecules. Another one, K2, is situated near the framework but away from the Si–Si axis along the c-direction, and bonded to four oxygen atoms of
framework and two water molecules (K2O distance: 2.66 and 2.98 A˚) [8]. The estimated structural similarity between the K+ and Eu3+ exchanged sitinakites and the observation of two distinct Eu3+ local environments suggest that to some extent the Eu3+ cations in the Eu3+-doped sitinakite inherit the positions of the K+ ions. However, with the present powder XRD data one cannot estimate the right crystallographic positions of the incorporated Eu atoms and possible surface-bonded Eu3+ atoms are not excluded. Based on the above considerations and on the photoluminescence results we may say that the shorter lifetime value found for Site 2, when compared to that measured for Site 1 and the higher thermal quenching of Site 2 emission intensity, is in good agreement with the presence of OH oscillators from water molecules in the Eu3+ surroundings, which is conformed with the coordination of K2. The luminescence features were also quantified through the measurement of the absolute emission quantum yield (f). Under direct intra-4f6 excitation (395 nm, 5L6 and 465 nm, 5D2) the f value is 6%, whereas under excitation via the LMCT states (305 nm) a smaller value was estimated, 1%. The lower quantum yield value estimated for LMCT excitation is consistent with the fact that LMCT states constitute an important channel for depopulation of the lanthanide excited states, leading to luminescence quenching [13]. To the best of our knowledge, this is the first work presenting quantum yield results on a Eu3+-doped titanosilicate. The obtained value is substantially smaller than those of lanthanide-based silicate inorganic phosphors, such as oxyapatites (20–30%), suggesting that the concentration quenching effects may act as luminescence non-radiative channels, as previously described [14].
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4. Conclusion In summary, a synthetic analogue of the mineral sitinakite is synthesized and doped with Eu3+. The inclusion of Eu3+ in the pore system causes structural changes detected by powder XRD and studied by the photoluminescence spectroscopy. It is estimated that Eu3+-doped sitinakite has doubled unit cell volume and higher symmetry when compared with the initial sodium phase. Photoluminescence study indicates the presence of two active local europium sites, which supports the powder XRD investigation. The Eu3+-doped synthetic sitinakite displays a stable room temperature emission in the red spectral region, with a maximum absolute emission quantum yield of 6%, being the first quantum yield report on Eu3+-doped titanosilicate. Acknowledgments The authors thank the financial support from FCT, POCI2010 and FEDER. S. Ferdov also thanks FCT (SFRH/BPD/23771/2005) for the grant. References [1] Z. Lin, J.P. Rainho, J. Domingues, L.D. Carlos, J. Rocha, Microporous Mesoporous Mater. 79 (2005) 13.
[2] E.V. Sokolova, R.K. Rastsvetaeva, V.I. Andrianov, Y.K. EngorovTismenko, Y.P. Men’Shikov, Dokl. Akad. Nauk SSSR 307 (1989) 114. [3] D.M. Poojary, R.A. Cahill, A. Clearfield, Chem. Mater. 6 (1994) 2364. [4] A. Tripathi, D.G. Medvedev, A. Clearfield, J. Solid State Chem. 178 (2005) 253. [5] R.G. Anthony, C.V. Philip, R.G. Dosch, Waste Manage. 13 (1993) 503. [6] J. Rodriguez-Carvajal, FULLPROF, Program for Rietveld refinement and pattern matching analysis, in: Abstracts of the Satellite Meeting on Powder Diffraction of the XVth Congress of the International Union of Crystallography, Toulouse, France, July 1990, p. 127. [7] P.E. Werner, L. Eriksson, M. Westdahl, J. Appl. Crystallogr. 18 (1985) 367. [8] D.M. Poojary, A.I. Bortun, L.N. Bortun, A. Clearfield, Inorg. Chem. 35 (1996) 6131. [9] A. Tripathi, D.G. Medvedev, N. Nyman, A. Clearfield, J. Solid State Chem. 175 (2003) 72. [10] D. Ananias, M. Kostova, F.A. Almeida Paz, A. Ferreira, L.D. Carlos, J. Klinowski, J. Rocha, J. Am. Chem. Soc. 126 (2004) 10410. [11] G. Blasse, A. Bril, J. Inorg. Nucl. Chem. 29 (1967) 2231. [12] G.F. de Sa´, O.L. Malta, C. de Mello Donega´, A.M. Simas, R.L. Longo, P.A. Santa-Cruz, E.F. da Silva Jr., Coord. Chem. Rev. 196 (2000) 165. [13] C.W. Struck, W.H. Fonger, Understanding Luminescence Spectra and Efficiency Using Wp and Relating Functions, Springer, Berlin, 1991 and references therein. [14] S. Ferdov, R.A. Sa´ Ferreira, Z. Lin, Chem. Mater. 18 (2006) 5958.