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Investigation of site occupancy and photoluminescence of Ce3+ in cubic borate Ba3Y2(B2O5)3 and Ce3+ → Tb3+ energy transfer behavior
Optical Materials 91 (2019) 246–252
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Investigation of site occupancy and photoluminescence of Ce3+ in cubic borate Ba3Y2(B2O5)3 and Ce3+ → Tb3+ energy transfer behavior
T
Pianpian Wua, Xubo Tonga, Yang Xua, Jin Hana, Hyo Jin Seob, Xinmin Zhanga,∗ a Hunan Province Key Laboratory of Materials Surface & Interface Science and Technology, School of Materials Science and Engineering, Central South University of Forestry and Technology, Changsha 410004, China b Department of Physics and Interdisciplinary Program of Biomedical, Mechanical & Electrical Engineering, Pukyong National University, Busan 608-737, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Phosphors Photoluminescence Borate Site occupancy Energy transfer
Ce3+, Tb3+ and Ce3+-Tb3+ activated Ba3Y2(B2O5)3 phosphors were synthesized by a sol-gel pyrolysis method. The synthesized phosphors were investigated using X-ray diffraction (XRD) analysis, photoluminescence emission and excitation spectra and luminescence decay curves. In the Ce3+ activated Ba3Y2(B2O5)3 samples, two different Ce3+ centers (marked as Ce(1) and Ce(2)) could exist. The Ba3Y2(B2O5)3:Tb3+ phosphor shows some emission peaks at ∼350–650 nm among which the green emission peak at 540 nm is the strongest. For the Ba3Y2(B2O5)3:Ce3+, Tb3+ co-doped phosphor, the existence of energy transfer process from Ce3+ center to Tb3+ center is confirmed and the interaction mechanism between Ce3+ and Tb3+ in Ba3Y2(B2O5)3:Ce3+,Tb3+ system is dipole-dipole interaction based on Inokuti–Hirayama (IeH) model.
1. Introduction The increasing variety of borate materials were widely investigated due to their wide applications in nonlinear optics (NLO) [1], plasma display panel (PDP) display and white light-emitting-diodes (LEDs) [2–5]. Recently, the phase relations in the ternary BaOeY2O3eB2O3 system were systematically investigated by Li et al. [6,7]. Six main compounds Ba2Y5B5O17, Ba3YB9O18, BaYB9O16, Ba3YB3O9, Ba3Y2B4O12 and BaY3B3O10 were found in this ternary system. Among these barium and yttrium double borates, the compound Ba3YB9O18 is a good candidate for scintillation crystal [8]. The luminescent properties of Ce3+ ion in the Ba2Y5B5O17 lattice were investigated [9]. Incorporating the Ce3+ ion into the Y3+ site yields efficiently blue photoluminescence upon UV-light excitation with an external quantum yield of 70% and is stable as a function of temperature with a quenching temperature of about 400 K, which indicates that Ba2Y5B5O17:Ce3+ is a suitable materials for UV chip excited white LEDs. Borates have been paid more attention as host lattices for luminescent materials because of the large band-gap, easy synthesis, stability, and low cost. Borates are suitable host lattices for luminescent materials, especially for white LEDs applications [10–12]. Among the rare earth ions, Eu2+, Ce3+ and Tb3+ are the well known ions for luminescent materials [13–19]. Ce3+ and Tb3+ co-doped Ba2Lu5B5O17 phosphor was employed for fabricating a UV-based white LED. A high
∗
color-rendering index of 91.4 at a low correlated color temperature of 3809 K is obtained [2]. The cerium-substituted barium lutetium borate, Ba2Lu5B5O17:Ce3+ shows an efficient blue emission when excited by UV-light with a photoluminescent quantum yield near 90%, a fast luminescence decay time (< 40 ns), and a thermal quenching temperature of 452 K [20]. A study of the ternary BaOeY2O3eB2O3 system has achieved a number of interesting luminescence materials for white LEDs. However, these phosphors still have some shortcomings. For example, rather low thermal stability with a T50 ≈ 200 K for Ba3YB9O18:Ce3+ and significant broad emission band for Ba2Y5B5O17:Ce3+ [8,9]. These shortcomings limit some potential applications. In pursuit of highly efficient, narrow emitting phosphors, research on red-emitting materials has suggested that a highly symmetric rare-earth coordination environment is critical to constrain the emission full-width at half-maximum (FWHM). Recently, we also focused on the photoluminescence properties of rare earth ions and transition metal ions activated borates phosphors [21–24]. There are three reasons for us to adopt Ba3Y2(B2O5)3 as a host for luminescence materials: (i) Zhao and Duke et al. reported the crystal structure of compound Ba3Y2(B2O5)3 [25]. The crystal structure consists of isolated [B2O5]4- groups, irregular (Ba/ Y)O9 polyhedra and regular YO6 polyhedra. Ba3Y2(B2O5)3 crystallizes in the cubic space group Ia3¯. Only five percent of all borates in the International Crystal Structure Database crystallize in a cubic space
Corresponding author. E-mail address: [email protected] (X. Zhang).
https://doi.org/10.1016/j.optmat.2019.03.030 Received 22 February 2019; Received in revised form 18 March 2019; Accepted 19 March 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
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group; (ii) to achieve narrow-band emission a highly symmetric activator coordination environment is very important [26]. Compound Ba3Y2(B2O5)3 crystallizes in the cubic space group Ia3¯. Y atoms are octahedrally coordinated by six oxygen atoms, forming perfect YO6 octahedra with a high site symmetry (3¯). If Ln3+ ion is incorporated into this host lattice, it will substitute for Y3+ site based on their isovalency and similar ionic size. Therefore, the luminescent center having a high site symmetry could result in desired narrow-band emission; (iii) this host lattice is also likely to have a high efficiency luminescence materials due to dense connectivity in the crystal structure [25]. For the three reasons we were interested to know whether the present compound (Ba3Y2(B2O5)3) offer potential as host lattice for luminescent materials. In this paper, we prepared Ce3+, Tb3+ and Ce3+-Tb3+ activated Ba3Y2(B2O5)3 phosphors through a sol-gel pyrolysis route, and studied their photoluminescence properties in detail. 2. Experimental Fig. 1. XRD patterns of (a) Simulated XRD pattern of Ba3Y2(B2O5)3 derived from the CIF file is presented as reference, (b) Ba3Y2(B2O5)3:4 mol% Ce3+, (c) Ba3Y2(B2O5)3:4 mol% Tb3+, (d) Ba3Y2(B2O5)3:4 mol% Ce3+,4 mol% Tb3+.
2.1. Sample preparation All the samples were prepared through a sol-gel pyrolysis route. The starting materials were Ba(NO3)2 (A.R.), Ce(NO3)3·6H2O (A.R.), Y2O3 (99.99%), Tb4O7 (99.99%) and H3BO3 (A.R.). Firstly, stoichiometric amounts of each oxides and nitrates were dissolved in dilute nitric acid or water respectively, and the corresponding nitrate solutions were mixed thoroughly, then citric acid (ncitric acid/nmetal ion = 5:1) and ethanol (CH3CH2OH) were added to the mixture under stirring for 0.5 h. The desired amount of ammonia (NH4OH) was added into the precursor solution and the pH value was adjusted to about 7–8. White sol was obtained after stirring for 2 h. A semi-transparent viscous gel can be obtained by evaporating the solvent at 75 °C in a water bath for 24 h until a homogenous and translucent gel was obtained. The resulting gel was dried at 150 °C for 12 h. Secondly, the resulting dry gel was ground and pre-calcined at 800 °C for 80 min in a muffle furnace in the air atmosphere. Thirdly, the samples were fully ground and calcined at 1000 °C for 2 h in a weak reductive (CO) atmosphere in order to reduce Tb4+ ions and prepare the final samples.
(r = 1.01 Å when CN = 6) and Tb3+ (r = 0.923 Å when CN = 6) ions incorporate into Y3+ sites (r = 0.900 Å when CN = 6) based on their same valency and similar ionic size [28]. In order to further understand the phase purity of Ce3+-activated Ba3Y2(B2O5)3:4% mol sample, the Rietveld structural refinement was performed by using the GSAS program and the result is shown in Fig. 2. Ba3Y2(B2O5)3 was served as an initial structure model. The finally refined unit cell parameters and residual factors are summarized in Table 1. The Rietveld analysis results indicate that the weighted profile R-factor (Rwp) and the expected R factor (Rp) are 10.66% and 7.39% for Ba3Y2(B2O5)3:4% mol Ce3+, indicating a good refinement quality. The excitation and emission spectra of the Ba3Y2(B2O5)3:4 mol% Ce3+ phosphor are presented in Fig. 3. The emission spectra present a broad band extending from 360 to 650 nm peaking at about 440 nm under different UV-light excitation, which can be attributed to the 5d1 → 4f1 transition of Ce3+. It is found that the excitation spectra of Ba3Y2(B2O5)3:4 mol% Ce3+ phosphor are dependent on the monitoring wavelengths, and different characteristics are observed. The excitation spectra for the longer-wavelength emissions (group I, curves c, d, e, f, g, h, i, j and k) show a broad band absorption in the range of 300–425 nm;
2.2. Characterization Identification of crystalline phased was performed at a X-ray diffraction (XRD) analysis (XD-2 diffractometer, Beijing Puxi Company) with Cu Kα radiation (λ = 1.5406 Å). High quality XRD data for Rietveld refinement were collected on a Bruker D8 Advance powder diffractometer with Cu Kα radiation over a 2θ range from 5 to 120° with step width of 0.01°. The Rietveld structure refinements were performed using the general structure analysis system (GSAS) program [27]. The excitation and emission spectra at 10 K and room temperature as well as fluorescence decay curves were recorded on an Edinburgh FLS920/FLS1000 steady-state and transient-state fluorescence spectrometer, which is equipped with a thermoelectric cooled red sensitive photomultiplier tube in the time-correlated single-photon counting mode. A 450 W xenon lamp, a 60 W μF900 flash lamp, and a 150 W nF900 flash lamp were used as the excitation light source. The temperature of the sample was adjusted by using a temperature controller (Tianjin Orient KOJI, China). 3. Results and discussion XRD patterns of the as-prepared Ba3Y2(B2O5)3:4 mol% Ce3+, Ba3Y2(B2O5)3:4 mol% Tb3+ and Ba3Y2(B2O5)3:4 mol% Ce3+, 4 mol% Tb3+ phosphors are shown in Fig. 1. The simulated XRD pattern of Ba3Y2(B2O5)3 host lattice derived from the CIF file is also shown in Fig. 1 for comparison. All diffraction peaks of the samples are consistent with the calculated cubic phase of Ba3Y2(B2O5)3. In the Ba3Y2(B2O5)3:Ce3+/Tb3+ system, it is assumed that the Ce3+
Fig. 2. Observed (crosses) and calculated (red curve) patterns as well as Bragg peak positions (short vertical lines, data for 2θ range 5–120°) and the difference profile (blue bottom trace) for the Rietveld structure analysis of Ba3Y2(B2O5)3:4 mol% Ce3+. 247
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Table 1 Crystallographic data and Ba3Y2(B2O5)3:4 mol% Ce3+.
Rietveld
refinement
data
of
Chemical formula
Ba3Y2(B2O5)3:4 mol% Ce3+
Crystal system Space group 2θ-interval α, β, γ (°) a, b, c (Å) Z Volume (Å3) Rwp (%) Rp (%) χ2
Cubic Ia3¯ 5–120 90 14.25782 (5) 8 2898.409 (31) 10.66 7.39 3.701
Fig. 5. The excitation and emission spectra of Ba3Y2(B2O5)3:4 mol% Ce3+ phosphor at 10 K.
exponential equation (see detail in Fig. 4). The fitted lifetimes for different emission wavelengths are in the order of nanosecond and listed in Fig. 4. It can be seen clearly that two different lifetimes are obtained, i.e., 26.4 ns and ∼43.5 ns, which is consistent with the decay time of Ce3+ 5d1 → 4f1 transition. The single exponential form of the decay curves indicates that the doped Ce3+ ions occupy one crystallographic site [29–31]. For a given transition of Ce3+ ion, the decay time is proportional to the square of the emission wavelength: τ ∼ λ2 [32,33]. Therefore, we conclude that the decay time of the Ce(1)3+ emission is 26.4 ns and that of the Ce(2)3+ emission is ∼43.5 ns? Usually, the emission and excitation spectra at lower temperature are used to confirm the existence of different optically active ion sites in a specific host [34,35]. The emission and excitation spectra of Ba3Y2(B2O5)3:4 mol% Ce3+ sample at 10 K are shown in Fig. 5. The emission spectra do not show obvious difference compared to those measured at room temperature (the hollow peak at 450 nm is due to the influence of fluorescence spectrometer); whereas the excitation spectra present some difference. The 5d1 configuration is split obviously by the crystal field into 2 and 3 components for Ce(1) site and Ce(2) site, respectively. The band at 357 nm (28 000 cm−1) (curve e) can be assigned to the excitation of the lowest 5d state of Ce(1), and the band at 390 nm (25 640 cm−1) (curves a, b, c and d) can be assigned to the lowest 5d state of Ce(2). In order to determine the emission peak positions for Ce(1) and Ce (2), the emission spectra are deconvoluted by using Gaussian profile and the results are shown in Fig. 6. The emission spectrum at room temperature under 340 nm excitation (curve m in Fig. 3) was fitted with a sum of three Gaussian profiles and the results are shown in Fig. 6a. The three Gaussian bands are marked as I (∼427 nm, 23 419 cm−1), II (∼459 nm, 21 786 cm−1) and III (∼506 nm, 19 763 cm−1). The energy difference is 2023 cm−1 between bands II and III. This value is consistent with the energy difference between 2F5/2 and 2F7/2 levels of Ce3+ [36,37]. Therefore, we assign the band I to the emission of Ce(1) center and the bands II and III to the emissions of Ce(2) center. For the emission spectrum at room temperature under 400 nm excitation (curve q in Fig. 3) can be fitted into two peaks (band IV, 448 nm and band V, 486 nm, see detail in Fig. 6b). The energy difference of bands IV and V is smaller than the theoretical value (about 1750 cm−1). We assign the bands IV and V to the emissions of Ce(2) center. The emission spectrum at 10 K under 340 nm excitation (curve f in Fig. 5) was fitted with a sum of four Gaussian profiles and the results are close to that in Fig. 6a. Due to the excitation spectral overlap of the broad regions (300–400 nm), selective excitation of Ce(1) or Ce(2) is not possible. Thus we cannot obtain the pure emission band of Ce(1) and Ce(2).
Fig. 3. The excitation and emission spectra of Ba3Y2(B2O5)3:4 mol% Ce3+ phosphor at room temperature.
while that for the 385 nm emission (group II, curve a) exhibits a broad band absorption in the range of 300–375 nm. The excitation spectrum for the 420 nm emission (curve b) should result from the overlap of group I and group II. Thus, we surmise that the different absorption bands could be corresponded to 4f1 → 5d1 transitions of two different Ce3+ sites (i.e., Ce(1) and Ce(2)). In order to confirm our reasoning, the decay curves of the Ba3Y2(B2O5)3:4 mol% Ce3+ sample were measured and the results are shown in Fig. 4. All the decay curves can be well fitted by a single-
Fig. 4. Luminescence decay curves of the Ba3Y2(B2O5)3:4 mol% Ce3+ sample at 400, 440, 480, 500 and 520 nm. 248
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Fig. 6. Gaussian fit of the emission spectra of Ba3Y2(B2O5)3:4 mol% Ce3+ phosphor under 340 and 400 nm excitation.
Fig. 8. The excitation and emission spectra of the Ba3Y2(B2O5)3:4 mol% Tb3+ phosphor.
The crystal structure of Ba3Y2(B2O5)3 was reported by Zhao et al. [25]. Ba3Y2(B2O5)3 crystallizes in the cubic system with space group Ia3¯. The Ba atoms are coordinated by nine O atoms in irregular ployhedra, while the Y atoms present two different crystallographic environments. Both Y1 and Y2 atoms are octahedrally coordinated by six O atom. The bond distances of Y1eO and Y2eO are 2.270 and 2.250 Å, respectively. The Y1 and Y2 sites have nearly Oh site symmetry and similar coordination surrounding. Recently, Duke reported that in the crystal structure of Ba3Y2(B2O5)3 compound Ba atoms are located at Wyckoff position 24d and share a bicapped hexagonal bipyramid with a small percentage of yttrium (91% Ba2+/9% Y3+) [12], i.e., part of Y atoms are coordinated by nine O atoms. In this case, we call these Y atoms as Y1 atoms; and the Y atoms octahedrally coordinated by six O atom as Y2 and Y3 atoms. The bond distances of Y2eO and Y3eO are 2.251 and 2.230 Å, respectively. The crystal structure of Ba3Y2(B2O5)3 compound viewed along [001] and the coordination environments of Ba/Y1, Y2, Y3 and (B2O5) 4- are presented in Fig. 7. Based on the same charge and similar size between Ce3+ and Y3+ ions, the Ce3+ luminescent center should substitute for the Y sites. The excitation and emission spectra of Ce3+ activated phosphors are mainly attributed to 4f ↔ 5d transitions. The energy positions of 5d levels are affected mainly by the coordination environment of Ce3+ ion [29]. Generally, the effects can be estimated by two factors, i.e., centroid shift and crystal field splitting. According to the formula proposed by Dorenbos [38,39], the centroid shift and crystal field splitting of 5d orbit depend
mainly on the distance between the Ce3+ ion and O2− surrounded. The distance between the Ce3+ ion and O2− in Y1 site is longer than that in Y2 and Y3 sites. Therefore, the corresponding values of centroid shift and crystal field splitting in Y1 site is smaller than that in Y2 and Y3 sites. Thus, we can assign that the Ce(1) substitutes for Y1 site, while the Ce(2) substitutes for Y2 and Y3 sites. The full width at half maximum (FWHM) of the emission spectra in Ba3Y2(B2O5)3:4 mol% Ce3+ sample is ∼80 nm, which is narrower than most other Ce3+ activated phosphors, such as the well-known YAG:Ce3+ (100 nm) [40], CaAlSiN3:Ce3+ (115 nm) [41], and Ba2Lu5B5O17:Ce3+ (115 nm) [42]. When incorporated into a highly symmetric environment, the luminescent center is likely to result in a narrow emission band [9,26]. As discussed above, the Y2 and Y3 sites have a high site symmetry, therefore, the relative narrow emission band observed in Ba3Y2(B2O5)3:4 mol% Ce3+ sample should be attributed to the highly symmetric local environment Ce3+ activator experienced. The excitation and emission spectra of the Ba3Y2(B2O5)3:4 mol% Tb 3+ phosphor are presented in Fig. 8. The emission spectrum is carried out under 234-nm UV light excitation and the excitation spectrum is obtained by monitoring at 540 nm emission. The emission spectrum shows the 5D4 emission lines (5D4→7F6, 486 nm; 5D4→7F5, 540 nm; 5 D4→7F4, 580 nm and 5D4→7F3, 625 nm). Among them, the 5D4→7F5 transition located at 540 nm is the strongest one. The 5D3 emission lines are not observed due to cross relaxation process between 5D3-5D4 and
Fig. 7. Crystal structure of Ba3Y2(B2O5)3 compound viewed along [001] and the coordination environments of Y3+ and Ba2+ cations in Ba3Y2(B2O5)3. 249
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Fig. 10. The emission spectra of Ba3Y2(B2O5)3:4 mol% Ce3+, yTb3+ phosphor under 340 nm excitation. The inset shows the emission intensity dependence of Tb3+ doping concentration.
Fig. 9. The excitation and emission spectra of the Ba3Y2(B2O5)3:4 mol% Ce3+,4 mol% Tb3+ phosphor. The excitation and monitoring wavelengths are marked in the figure.
shown in Fig. 10. The emission intensity dependence of Tb3+ content is depicted in the inset of Fig. 10. The emission intensity of Ce3+ decreases dramatically and monotonically with the increase of Tb3+ content, while the intensity of Tb3+ emission increases and has a maximum intensity when x = 0.2, which further confirms the existence of energy transfer between Ce3+ and Tb3+. The optimum Tb3+ content for the Tb3+ emission is about 0.2. In order to further identity the energy transfer from Ce3+ and Tb3+ ions, the photoluminescence decay curves of the Ce3+ ions in Ba3Y2(B2O5)3:0.04Ce3+,yTb3+ were measured with an excitation at 340 nm and monitored at 440 nm, as shown in Fig. 11. The decay curves derivate from single exponential behavior gradually, and the derivation becomes more predominant with Tb3+ concentration increasing, which indicates that a non-radiative process takes place. The observed decay curves were fitted by the following double-exponential expression:
F0e F6 transitions. The excitation spectrum does not exhibit obvious absorption lines in the range of 300–400 nm due to parity- and spinforbidden f→f transitions of Tb3+ ions. The 4f75d1 excitation state of Tb3+ consists of high spin (9DJ configurations) and low spin (7DJ configurations) states. According to the selection rule, the f→d transitions of Tb3+ include spin-allowed (7F6→7DJ) and spin-forbidden (7F6→9DJ) transitions. P. Dorenbos summed up the f→d transition energies of Ln3+ in various host lattices and concluded that a similar crystal–field splitting is expected for all Ln3+ ions in the same host lattice [43]. As discussed above, the lowest f–d transition of Ce3+ in Ba3Y2(B2O5)3 occurs about 390 nm (25 640 cm−1). The d level of free Ce3+ situates at 49 340 cm−1 [44]. Then, the value of crystal–field splitting is 23 700 cm−1. The d level energy of free Tb3+ ion is at 62 500 cm−1 for the low spin state [44]. Therefore, the f–d spin-allowed transition position of Tb3+ in Ba3Y2(B2O5)3 should locate at about 38 800 cm−1 (258 nm). Thus, the intense excitation band at about 250 nm in Fig. 9 can be assigned to the spin-allowed f→d transition of Tb3+. This absorption band has a heavy overlap with the host absorption band. The spin-forbidden f→d transition in Tb3+ is expected to occur about 6000 cm−1 lower energy than the spin-allowed one, i.e., at about 32 800 cm−1 (305 nm). A weak band at about 271 nm is observed and it could be ascribed to the spin-forbidden transition. The excitation and emission spectra of the Ba3Y2(B2O5)3:4 mol% Ce3+,4 mol% Tb3+ phosphor are shown in Fig. 9. Under 340-nm UV excitation, the emission spectrum consists of a broad band peaking at 432 nm and some line emissions at 485, 540, 588 and 624 nm. These line emissions are undoubtedly attributed to the Tb3+ 5D4→7FJ transitions. The broad band peaking at 432 nm can be assigned to Ce3+ 5d1 → 4f1 transition. The excitation spectra exhibit difference when different emission wavelengths are monitored. The excitation spectrum of the Tb3+ luminescence (curve a, λem = 540 nm) exhibits a strong band centered at 365 nm and a band centered at 256 nm. The excitation bands covering the 250–300 nm spectral range correlate with absorption bands of Tb3+ singly doped Ba3Y2(B2O5)3 sample (see Fig. 8), whereas the excitation bands covering the 300–425 nm spectral range are not consistent with those of Tb3+ singly doped Ba3Y2(B2O5)3 sample. As discussed above, these absorption bands correspond to the 4f1 → 5d1 transition of Ce3+ ions, indicating that energy transfer from Ce3+ to Tb3+ takes place [45]. To further investigate the energy transfer from Ce3+ to Tb3+, a series of samples Ba3Y2(B2O5)3:0.04Ce3+, yTb3+ (y = 0.02, 0.04, 0.06, 0.08, 0.1, 0.2, 0.5 and 1.0) were synthesized. The emission spectra of Ba3Y2(B2O5)3:0.04Ce3+,yTb3+ samples with different Tb3+ content are 7
7
I (t ) =
∑ i = 1,2
t ai exp ⎛− ⎞ + b ⎝ τi ⎠ ⎜
⎟
(1)
The average decay time τave was then calculated from:
Fig. 11. Luminescence decay curves of the Ba3Y2(B2O5)3:4 mol% Ce3+, yTb3+ samples at 440 nm under 340 nm excitation. The inset shows the decay time dependence of Tb3+ doping concentration. 250
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τave =
∑i = 1,2 ai τi2 ∑i = 1,2 ai τi
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(2)
The calculated lifetimes are shown in the inset of Fig. 11. The decay lifetime for the Ce3+ ions is found to reduce from 42 ns to 24 ns with an increase in Tb3+ concentration. This is due to the increase in nonradiative resonance energy transfer to the higher excited states of Tb3+ ions. At the same time, this confirms the increase in the energy transfer efficiency from Ce3+ to Tb3+ with the increase in Tb3+ ions in the system. If the donors and acceptors in the host lattice are distributed uniformly, the energy migration can be neglected. To analyze the mechanism of energy transfer between Ce3+ and Tb3+ ions, we attempted to analyze the decay curves within the framework of Inokuti–Hirayama (IeH) model [46]. 3
⎡ I (t) t C 3 t S⎤ = exp ⎢−⎛ ⎞ − ⎛ A ⎞ Γ ⎛1 − ⎞ ⎛ ⎞ ⎥ I0 τ C S τ ⎝ ⎠ 0 0 ⎝ ⎠ ⎝ ⎠ ⎝ 0⎠ ⎥ ⎢ ⎣ ⎦ ⎜
⎟
⎜
⎟
⎜
⎟
(3)
where τ0 is the radiative lifetime of an isolated Ce ion; Γ(1–3/S) is the Euler's function; CA is acceptor ion content; C0 is critical content defined as 3/(4πR 03) ; S is the multipole interaction parameter. If the value of S is 6, Γ(1–3/S) = 1.77. The fitted curves using IeH model are shown in Fig. 11. It can be seen that the best agreement between experimental data and theoretical fits is obtained when setting the value of S is 6. Therefore, the dominant interaction mechanism between the donors and acceptors for Ba3Y2(B2O5)3:Ce3+,Tb3+ sample is dipole-dipole interaction. 3+
4. Conclusions We prepared Ce3+, Tb3+ and Ce3+-Tb3+ activated Ba3Y2(B2O5)3 phosphors successively through a sol-gel pyrolysis route. The Ba3Y2(B2O5)3:Ce3+ phosphor showed a broad band extending from 360 to 650 nm, originating from two different Ce3+ centers (Ce(1) and Ce (2)). The Ba3Y2(B2O5)3:Tb3+ phosphor showed a main emission peak at 540 nm due to the 5D4→7F5 transition of Tb3+. In the Ce3+, Tb3+ codoped Ba3Y2(B2O5)3 codoped sample, the energy transfer from Ce3+ center to Tb3+ center takes place. The dominant interaction mechanism for Ba3Y2(B2O5)3:Ce3+,Tb3+ is based on the dipole-dipole interaction. 5. Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The project was sponsored by the Technology Program of Environmental Protection Department of Hunan (No. 2013–312). References [1] Z. Zhang, Y. Wang, B. Zhang, Z. Yang, S. Pan, CaB5O7F3: a beryllium-free alkalineearth fluorooxoborate exhibiting excellent nonlinear optical performances, Inorg. Chem. 57 (2018) 4820–4823. [2] Y. Xiao, Z. Hao, L. Zhang, X. Zhang, G.-H. Pan, H. Wu, H. Wu, Y. Luo, J. Zhang, An efficient green phosphor of Ce3+ and Tb3+-codoped Ba2Lu5B5O17 and a model for elucidating the high thermal stability of the green emission, J. Mater. Chem. C 6 (2018) 5984–5991. [3] S. Verma, K. Verma, D. Kumar, B. Chaudhary, S. Som, V. Sharma, V. Kumar, H.C. Swart, Recent advances in rare earth doped alkali-alkaline earth borates for solid state lighting applications, Phys. B Condens. Matter 535 (2017) 106–113. [4] Y.F. Liu, J. Silver, R.J. Xie, J.H. Zhang, H.W. Xu, H.Z. Shao, J.J. Jiang, H. C Jiang, An excellent cyan-emitting orthosilicate phosphor for NUV-pumped white LED application, J. Mater. Chem. C 5 (2017) 12365–12377. [5] Y.F. Liu, J.X. Zhang, C.H. Zhang, J.T. Xu, G.Q. Liu, J.J. Jiang, H.C. Jiang, Ba9Lu2Si6O24:Ce3+: an efficient green phosphor with high thermal and radiation
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Investigation of site occupancy and photoluminescence of Ce3+ in cubic borate Ba3Y2(B2O5)3 and Ce3+ → Tb3+ energy transfer behavior
T
Pianpian Wua, Xubo Tonga, Yang Xua, Jin Hana, Hyo Jin Seob, Xinmin Zhanga,∗ a Hunan Province Key Laboratory of Materials Surface & Interface Science and Technology, School of Materials Science and Engineering, Central South University of Forestry and Technology, Changsha 410004, China b Department of Physics and Interdisciplinary Program of Biomedical, Mechanical & Electrical Engineering, Pukyong National University, Busan 608-737, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Phosphors Photoluminescence Borate Site occupancy Energy transfer
Ce3+, Tb3+ and Ce3+-Tb3+ activated Ba3Y2(B2O5)3 phosphors were synthesized by a sol-gel pyrolysis method. The synthesized phosphors were investigated using X-ray diffraction (XRD) analysis, photoluminescence emission and excitation spectra and luminescence decay curves. In the Ce3+ activated Ba3Y2(B2O5)3 samples, two different Ce3+ centers (marked as Ce(1) and Ce(2)) could exist. The Ba3Y2(B2O5)3:Tb3+ phosphor shows some emission peaks at ∼350–650 nm among which the green emission peak at 540 nm is the strongest. For the Ba3Y2(B2O5)3:Ce3+, Tb3+ co-doped phosphor, the existence of energy transfer process from Ce3+ center to Tb3+ center is confirmed and the interaction mechanism between Ce3+ and Tb3+ in Ba3Y2(B2O5)3:Ce3+,Tb3+ system is dipole-dipole interaction based on Inokuti–Hirayama (IeH) model.
1. Introduction The increasing variety of borate materials were widely investigated due to their wide applications in nonlinear optics (NLO) [1], plasma display panel (PDP) display and white light-emitting-diodes (LEDs) [2–5]. Recently, the phase relations in the ternary BaOeY2O3eB2O3 system were systematically investigated by Li et al. [6,7]. Six main compounds Ba2Y5B5O17, Ba3YB9O18, BaYB9O16, Ba3YB3O9, Ba3Y2B4O12 and BaY3B3O10 were found in this ternary system. Among these barium and yttrium double borates, the compound Ba3YB9O18 is a good candidate for scintillation crystal [8]. The luminescent properties of Ce3+ ion in the Ba2Y5B5O17 lattice were investigated [9]. Incorporating the Ce3+ ion into the Y3+ site yields efficiently blue photoluminescence upon UV-light excitation with an external quantum yield of 70% and is stable as a function of temperature with a quenching temperature of about 400 K, which indicates that Ba2Y5B5O17:Ce3+ is a suitable materials for UV chip excited white LEDs. Borates have been paid more attention as host lattices for luminescent materials because of the large band-gap, easy synthesis, stability, and low cost. Borates are suitable host lattices for luminescent materials, especially for white LEDs applications [10–12]. Among the rare earth ions, Eu2+, Ce3+ and Tb3+ are the well known ions for luminescent materials [13–19]. Ce3+ and Tb3+ co-doped Ba2Lu5B5O17 phosphor was employed for fabricating a UV-based white LED. A high
∗
color-rendering index of 91.4 at a low correlated color temperature of 3809 K is obtained [2]. The cerium-substituted barium lutetium borate, Ba2Lu5B5O17:Ce3+ shows an efficient blue emission when excited by UV-light with a photoluminescent quantum yield near 90%, a fast luminescence decay time (< 40 ns), and a thermal quenching temperature of 452 K [20]. A study of the ternary BaOeY2O3eB2O3 system has achieved a number of interesting luminescence materials for white LEDs. However, these phosphors still have some shortcomings. For example, rather low thermal stability with a T50 ≈ 200 K for Ba3YB9O18:Ce3+ and significant broad emission band for Ba2Y5B5O17:Ce3+ [8,9]. These shortcomings limit some potential applications. In pursuit of highly efficient, narrow emitting phosphors, research on red-emitting materials has suggested that a highly symmetric rare-earth coordination environment is critical to constrain the emission full-width at half-maximum (FWHM). Recently, we also focused on the photoluminescence properties of rare earth ions and transition metal ions activated borates phosphors [21–24]. There are three reasons for us to adopt Ba3Y2(B2O5)3 as a host for luminescence materials: (i) Zhao and Duke et al. reported the crystal structure of compound Ba3Y2(B2O5)3 [25]. The crystal structure consists of isolated [B2O5]4- groups, irregular (Ba/ Y)O9 polyhedra and regular YO6 polyhedra. Ba3Y2(B2O5)3 crystallizes in the cubic space group Ia3¯. Only five percent of all borates in the International Crystal Structure Database crystallize in a cubic space
Corresponding author. E-mail address: [email protected] (X. Zhang).
https://doi.org/10.1016/j.optmat.2019.03.030 Received 22 February 2019; Received in revised form 18 March 2019; Accepted 19 March 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
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group; (ii) to achieve narrow-band emission a highly symmetric activator coordination environment is very important [26]. Compound Ba3Y2(B2O5)3 crystallizes in the cubic space group Ia3¯. Y atoms are octahedrally coordinated by six oxygen atoms, forming perfect YO6 octahedra with a high site symmetry (3¯). If Ln3+ ion is incorporated into this host lattice, it will substitute for Y3+ site based on their isovalency and similar ionic size. Therefore, the luminescent center having a high site symmetry could result in desired narrow-band emission; (iii) this host lattice is also likely to have a high efficiency luminescence materials due to dense connectivity in the crystal structure [25]. For the three reasons we were interested to know whether the present compound (Ba3Y2(B2O5)3) offer potential as host lattice for luminescent materials. In this paper, we prepared Ce3+, Tb3+ and Ce3+-Tb3+ activated Ba3Y2(B2O5)3 phosphors through a sol-gel pyrolysis route, and studied their photoluminescence properties in detail. 2. Experimental Fig. 1. XRD patterns of (a) Simulated XRD pattern of Ba3Y2(B2O5)3 derived from the CIF file is presented as reference, (b) Ba3Y2(B2O5)3:4 mol% Ce3+, (c) Ba3Y2(B2O5)3:4 mol% Tb3+, (d) Ba3Y2(B2O5)3:4 mol% Ce3+,4 mol% Tb3+.
2.1. Sample preparation All the samples were prepared through a sol-gel pyrolysis route. The starting materials were Ba(NO3)2 (A.R.), Ce(NO3)3·6H2O (A.R.), Y2O3 (99.99%), Tb4O7 (99.99%) and H3BO3 (A.R.). Firstly, stoichiometric amounts of each oxides and nitrates were dissolved in dilute nitric acid or water respectively, and the corresponding nitrate solutions were mixed thoroughly, then citric acid (ncitric acid/nmetal ion = 5:1) and ethanol (CH3CH2OH) were added to the mixture under stirring for 0.5 h. The desired amount of ammonia (NH4OH) was added into the precursor solution and the pH value was adjusted to about 7–8. White sol was obtained after stirring for 2 h. A semi-transparent viscous gel can be obtained by evaporating the solvent at 75 °C in a water bath for 24 h until a homogenous and translucent gel was obtained. The resulting gel was dried at 150 °C for 12 h. Secondly, the resulting dry gel was ground and pre-calcined at 800 °C for 80 min in a muffle furnace in the air atmosphere. Thirdly, the samples were fully ground and calcined at 1000 °C for 2 h in a weak reductive (CO) atmosphere in order to reduce Tb4+ ions and prepare the final samples.
(r = 1.01 Å when CN = 6) and Tb3+ (r = 0.923 Å when CN = 6) ions incorporate into Y3+ sites (r = 0.900 Å when CN = 6) based on their same valency and similar ionic size [28]. In order to further understand the phase purity of Ce3+-activated Ba3Y2(B2O5)3:4% mol sample, the Rietveld structural refinement was performed by using the GSAS program and the result is shown in Fig. 2. Ba3Y2(B2O5)3 was served as an initial structure model. The finally refined unit cell parameters and residual factors are summarized in Table 1. The Rietveld analysis results indicate that the weighted profile R-factor (Rwp) and the expected R factor (Rp) are 10.66% and 7.39% for Ba3Y2(B2O5)3:4% mol Ce3+, indicating a good refinement quality. The excitation and emission spectra of the Ba3Y2(B2O5)3:4 mol% Ce3+ phosphor are presented in Fig. 3. The emission spectra present a broad band extending from 360 to 650 nm peaking at about 440 nm under different UV-light excitation, which can be attributed to the 5d1 → 4f1 transition of Ce3+. It is found that the excitation spectra of Ba3Y2(B2O5)3:4 mol% Ce3+ phosphor are dependent on the monitoring wavelengths, and different characteristics are observed. The excitation spectra for the longer-wavelength emissions (group I, curves c, d, e, f, g, h, i, j and k) show a broad band absorption in the range of 300–425 nm;
2.2. Characterization Identification of crystalline phased was performed at a X-ray diffraction (XRD) analysis (XD-2 diffractometer, Beijing Puxi Company) with Cu Kα radiation (λ = 1.5406 Å). High quality XRD data for Rietveld refinement were collected on a Bruker D8 Advance powder diffractometer with Cu Kα radiation over a 2θ range from 5 to 120° with step width of 0.01°. The Rietveld structure refinements were performed using the general structure analysis system (GSAS) program [27]. The excitation and emission spectra at 10 K and room temperature as well as fluorescence decay curves were recorded on an Edinburgh FLS920/FLS1000 steady-state and transient-state fluorescence spectrometer, which is equipped with a thermoelectric cooled red sensitive photomultiplier tube in the time-correlated single-photon counting mode. A 450 W xenon lamp, a 60 W μF900 flash lamp, and a 150 W nF900 flash lamp were used as the excitation light source. The temperature of the sample was adjusted by using a temperature controller (Tianjin Orient KOJI, China). 3. Results and discussion XRD patterns of the as-prepared Ba3Y2(B2O5)3:4 mol% Ce3+, Ba3Y2(B2O5)3:4 mol% Tb3+ and Ba3Y2(B2O5)3:4 mol% Ce3+, 4 mol% Tb3+ phosphors are shown in Fig. 1. The simulated XRD pattern of Ba3Y2(B2O5)3 host lattice derived from the CIF file is also shown in Fig. 1 for comparison. All diffraction peaks of the samples are consistent with the calculated cubic phase of Ba3Y2(B2O5)3. In the Ba3Y2(B2O5)3:Ce3+/Tb3+ system, it is assumed that the Ce3+
Fig. 2. Observed (crosses) and calculated (red curve) patterns as well as Bragg peak positions (short vertical lines, data for 2θ range 5–120°) and the difference profile (blue bottom trace) for the Rietveld structure analysis of Ba3Y2(B2O5)3:4 mol% Ce3+. 247
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Table 1 Crystallographic data and Ba3Y2(B2O5)3:4 mol% Ce3+.
Rietveld
refinement
data
of
Chemical formula
Ba3Y2(B2O5)3:4 mol% Ce3+
Crystal system Space group 2θ-interval α, β, γ (°) a, b, c (Å) Z Volume (Å3) Rwp (%) Rp (%) χ2
Cubic Ia3¯ 5–120 90 14.25782 (5) 8 2898.409 (31) 10.66 7.39 3.701
Fig. 5. The excitation and emission spectra of Ba3Y2(B2O5)3:4 mol% Ce3+ phosphor at 10 K.
exponential equation (see detail in Fig. 4). The fitted lifetimes for different emission wavelengths are in the order of nanosecond and listed in Fig. 4. It can be seen clearly that two different lifetimes are obtained, i.e., 26.4 ns and ∼43.5 ns, which is consistent with the decay time of Ce3+ 5d1 → 4f1 transition. The single exponential form of the decay curves indicates that the doped Ce3+ ions occupy one crystallographic site [29–31]. For a given transition of Ce3+ ion, the decay time is proportional to the square of the emission wavelength: τ ∼ λ2 [32,33]. Therefore, we conclude that the decay time of the Ce(1)3+ emission is 26.4 ns and that of the Ce(2)3+ emission is ∼43.5 ns? Usually, the emission and excitation spectra at lower temperature are used to confirm the existence of different optically active ion sites in a specific host [34,35]. The emission and excitation spectra of Ba3Y2(B2O5)3:4 mol% Ce3+ sample at 10 K are shown in Fig. 5. The emission spectra do not show obvious difference compared to those measured at room temperature (the hollow peak at 450 nm is due to the influence of fluorescence spectrometer); whereas the excitation spectra present some difference. The 5d1 configuration is split obviously by the crystal field into 2 and 3 components for Ce(1) site and Ce(2) site, respectively. The band at 357 nm (28 000 cm−1) (curve e) can be assigned to the excitation of the lowest 5d state of Ce(1), and the band at 390 nm (25 640 cm−1) (curves a, b, c and d) can be assigned to the lowest 5d state of Ce(2). In order to determine the emission peak positions for Ce(1) and Ce (2), the emission spectra are deconvoluted by using Gaussian profile and the results are shown in Fig. 6. The emission spectrum at room temperature under 340 nm excitation (curve m in Fig. 3) was fitted with a sum of three Gaussian profiles and the results are shown in Fig. 6a. The three Gaussian bands are marked as I (∼427 nm, 23 419 cm−1), II (∼459 nm, 21 786 cm−1) and III (∼506 nm, 19 763 cm−1). The energy difference is 2023 cm−1 between bands II and III. This value is consistent with the energy difference between 2F5/2 and 2F7/2 levels of Ce3+ [36,37]. Therefore, we assign the band I to the emission of Ce(1) center and the bands II and III to the emissions of Ce(2) center. For the emission spectrum at room temperature under 400 nm excitation (curve q in Fig. 3) can be fitted into two peaks (band IV, 448 nm and band V, 486 nm, see detail in Fig. 6b). The energy difference of bands IV and V is smaller than the theoretical value (about 1750 cm−1). We assign the bands IV and V to the emissions of Ce(2) center. The emission spectrum at 10 K under 340 nm excitation (curve f in Fig. 5) was fitted with a sum of four Gaussian profiles and the results are close to that in Fig. 6a. Due to the excitation spectral overlap of the broad regions (300–400 nm), selective excitation of Ce(1) or Ce(2) is not possible. Thus we cannot obtain the pure emission band of Ce(1) and Ce(2).
Fig. 3. The excitation and emission spectra of Ba3Y2(B2O5)3:4 mol% Ce3+ phosphor at room temperature.
while that for the 385 nm emission (group II, curve a) exhibits a broad band absorption in the range of 300–375 nm. The excitation spectrum for the 420 nm emission (curve b) should result from the overlap of group I and group II. Thus, we surmise that the different absorption bands could be corresponded to 4f1 → 5d1 transitions of two different Ce3+ sites (i.e., Ce(1) and Ce(2)). In order to confirm our reasoning, the decay curves of the Ba3Y2(B2O5)3:4 mol% Ce3+ sample were measured and the results are shown in Fig. 4. All the decay curves can be well fitted by a single-
Fig. 4. Luminescence decay curves of the Ba3Y2(B2O5)3:4 mol% Ce3+ sample at 400, 440, 480, 500 and 520 nm. 248
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Fig. 6. Gaussian fit of the emission spectra of Ba3Y2(B2O5)3:4 mol% Ce3+ phosphor under 340 and 400 nm excitation.
Fig. 8. The excitation and emission spectra of the Ba3Y2(B2O5)3:4 mol% Tb3+ phosphor.
The crystal structure of Ba3Y2(B2O5)3 was reported by Zhao et al. [25]. Ba3Y2(B2O5)3 crystallizes in the cubic system with space group Ia3¯. The Ba atoms are coordinated by nine O atoms in irregular ployhedra, while the Y atoms present two different crystallographic environments. Both Y1 and Y2 atoms are octahedrally coordinated by six O atom. The bond distances of Y1eO and Y2eO are 2.270 and 2.250 Å, respectively. The Y1 and Y2 sites have nearly Oh site symmetry and similar coordination surrounding. Recently, Duke reported that in the crystal structure of Ba3Y2(B2O5)3 compound Ba atoms are located at Wyckoff position 24d and share a bicapped hexagonal bipyramid with a small percentage of yttrium (91% Ba2+/9% Y3+) [12], i.e., part of Y atoms are coordinated by nine O atoms. In this case, we call these Y atoms as Y1 atoms; and the Y atoms octahedrally coordinated by six O atom as Y2 and Y3 atoms. The bond distances of Y2eO and Y3eO are 2.251 and 2.230 Å, respectively. The crystal structure of Ba3Y2(B2O5)3 compound viewed along [001] and the coordination environments of Ba/Y1, Y2, Y3 and (B2O5) 4- are presented in Fig. 7. Based on the same charge and similar size between Ce3+ and Y3+ ions, the Ce3+ luminescent center should substitute for the Y sites. The excitation and emission spectra of Ce3+ activated phosphors are mainly attributed to 4f ↔ 5d transitions. The energy positions of 5d levels are affected mainly by the coordination environment of Ce3+ ion [29]. Generally, the effects can be estimated by two factors, i.e., centroid shift and crystal field splitting. According to the formula proposed by Dorenbos [38,39], the centroid shift and crystal field splitting of 5d orbit depend
mainly on the distance between the Ce3+ ion and O2− surrounded. The distance between the Ce3+ ion and O2− in Y1 site is longer than that in Y2 and Y3 sites. Therefore, the corresponding values of centroid shift and crystal field splitting in Y1 site is smaller than that in Y2 and Y3 sites. Thus, we can assign that the Ce(1) substitutes for Y1 site, while the Ce(2) substitutes for Y2 and Y3 sites. The full width at half maximum (FWHM) of the emission spectra in Ba3Y2(B2O5)3:4 mol% Ce3+ sample is ∼80 nm, which is narrower than most other Ce3+ activated phosphors, such as the well-known YAG:Ce3+ (100 nm) [40], CaAlSiN3:Ce3+ (115 nm) [41], and Ba2Lu5B5O17:Ce3+ (115 nm) [42]. When incorporated into a highly symmetric environment, the luminescent center is likely to result in a narrow emission band [9,26]. As discussed above, the Y2 and Y3 sites have a high site symmetry, therefore, the relative narrow emission band observed in Ba3Y2(B2O5)3:4 mol% Ce3+ sample should be attributed to the highly symmetric local environment Ce3+ activator experienced. The excitation and emission spectra of the Ba3Y2(B2O5)3:4 mol% Tb 3+ phosphor are presented in Fig. 8. The emission spectrum is carried out under 234-nm UV light excitation and the excitation spectrum is obtained by monitoring at 540 nm emission. The emission spectrum shows the 5D4 emission lines (5D4→7F6, 486 nm; 5D4→7F5, 540 nm; 5 D4→7F4, 580 nm and 5D4→7F3, 625 nm). Among them, the 5D4→7F5 transition located at 540 nm is the strongest one. The 5D3 emission lines are not observed due to cross relaxation process between 5D3-5D4 and
Fig. 7. Crystal structure of Ba3Y2(B2O5)3 compound viewed along [001] and the coordination environments of Y3+ and Ba2+ cations in Ba3Y2(B2O5)3. 249
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Fig. 10. The emission spectra of Ba3Y2(B2O5)3:4 mol% Ce3+, yTb3+ phosphor under 340 nm excitation. The inset shows the emission intensity dependence of Tb3+ doping concentration.
Fig. 9. The excitation and emission spectra of the Ba3Y2(B2O5)3:4 mol% Ce3+,4 mol% Tb3+ phosphor. The excitation and monitoring wavelengths are marked in the figure.
shown in Fig. 10. The emission intensity dependence of Tb3+ content is depicted in the inset of Fig. 10. The emission intensity of Ce3+ decreases dramatically and monotonically with the increase of Tb3+ content, while the intensity of Tb3+ emission increases and has a maximum intensity when x = 0.2, which further confirms the existence of energy transfer between Ce3+ and Tb3+. The optimum Tb3+ content for the Tb3+ emission is about 0.2. In order to further identity the energy transfer from Ce3+ and Tb3+ ions, the photoluminescence decay curves of the Ce3+ ions in Ba3Y2(B2O5)3:0.04Ce3+,yTb3+ were measured with an excitation at 340 nm and monitored at 440 nm, as shown in Fig. 11. The decay curves derivate from single exponential behavior gradually, and the derivation becomes more predominant with Tb3+ concentration increasing, which indicates that a non-radiative process takes place. The observed decay curves were fitted by the following double-exponential expression:
F0e F6 transitions. The excitation spectrum does not exhibit obvious absorption lines in the range of 300–400 nm due to parity- and spinforbidden f→f transitions of Tb3+ ions. The 4f75d1 excitation state of Tb3+ consists of high spin (9DJ configurations) and low spin (7DJ configurations) states. According to the selection rule, the f→d transitions of Tb3+ include spin-allowed (7F6→7DJ) and spin-forbidden (7F6→9DJ) transitions. P. Dorenbos summed up the f→d transition energies of Ln3+ in various host lattices and concluded that a similar crystal–field splitting is expected for all Ln3+ ions in the same host lattice [43]. As discussed above, the lowest f–d transition of Ce3+ in Ba3Y2(B2O5)3 occurs about 390 nm (25 640 cm−1). The d level of free Ce3+ situates at 49 340 cm−1 [44]. Then, the value of crystal–field splitting is 23 700 cm−1. The d level energy of free Tb3+ ion is at 62 500 cm−1 for the low spin state [44]. Therefore, the f–d spin-allowed transition position of Tb3+ in Ba3Y2(B2O5)3 should locate at about 38 800 cm−1 (258 nm). Thus, the intense excitation band at about 250 nm in Fig. 9 can be assigned to the spin-allowed f→d transition of Tb3+. This absorption band has a heavy overlap with the host absorption band. The spin-forbidden f→d transition in Tb3+ is expected to occur about 6000 cm−1 lower energy than the spin-allowed one, i.e., at about 32 800 cm−1 (305 nm). A weak band at about 271 nm is observed and it could be ascribed to the spin-forbidden transition. The excitation and emission spectra of the Ba3Y2(B2O5)3:4 mol% Ce3+,4 mol% Tb3+ phosphor are shown in Fig. 9. Under 340-nm UV excitation, the emission spectrum consists of a broad band peaking at 432 nm and some line emissions at 485, 540, 588 and 624 nm. These line emissions are undoubtedly attributed to the Tb3+ 5D4→7FJ transitions. The broad band peaking at 432 nm can be assigned to Ce3+ 5d1 → 4f1 transition. The excitation spectra exhibit difference when different emission wavelengths are monitored. The excitation spectrum of the Tb3+ luminescence (curve a, λem = 540 nm) exhibits a strong band centered at 365 nm and a band centered at 256 nm. The excitation bands covering the 250–300 nm spectral range correlate with absorption bands of Tb3+ singly doped Ba3Y2(B2O5)3 sample (see Fig. 8), whereas the excitation bands covering the 300–425 nm spectral range are not consistent with those of Tb3+ singly doped Ba3Y2(B2O5)3 sample. As discussed above, these absorption bands correspond to the 4f1 → 5d1 transition of Ce3+ ions, indicating that energy transfer from Ce3+ to Tb3+ takes place [45]. To further investigate the energy transfer from Ce3+ to Tb3+, a series of samples Ba3Y2(B2O5)3:0.04Ce3+, yTb3+ (y = 0.02, 0.04, 0.06, 0.08, 0.1, 0.2, 0.5 and 1.0) were synthesized. The emission spectra of Ba3Y2(B2O5)3:0.04Ce3+,yTb3+ samples with different Tb3+ content are 7
7
I (t ) =
∑ i = 1,2
t ai exp ⎛− ⎞ + b ⎝ τi ⎠ ⎜
⎟
(1)
The average decay time τave was then calculated from:
Fig. 11. Luminescence decay curves of the Ba3Y2(B2O5)3:4 mol% Ce3+, yTb3+ samples at 440 nm under 340 nm excitation. The inset shows the decay time dependence of Tb3+ doping concentration. 250
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τave =
∑i = 1,2 ai τi2 ∑i = 1,2 ai τi
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(2)
The calculated lifetimes are shown in the inset of Fig. 11. The decay lifetime for the Ce3+ ions is found to reduce from 42 ns to 24 ns with an increase in Tb3+ concentration. This is due to the increase in nonradiative resonance energy transfer to the higher excited states of Tb3+ ions. At the same time, this confirms the increase in the energy transfer efficiency from Ce3+ to Tb3+ with the increase in Tb3+ ions in the system. If the donors and acceptors in the host lattice are distributed uniformly, the energy migration can be neglected. To analyze the mechanism of energy transfer between Ce3+ and Tb3+ ions, we attempted to analyze the decay curves within the framework of Inokuti–Hirayama (IeH) model [46]. 3
⎡ I (t) t C 3 t S⎤ = exp ⎢−⎛ ⎞ − ⎛ A ⎞ Γ ⎛1 − ⎞ ⎛ ⎞ ⎥ I0 τ C S τ ⎝ ⎠ 0 0 ⎝ ⎠ ⎝ ⎠ ⎝ 0⎠ ⎥ ⎢ ⎣ ⎦ ⎜
⎟
⎜
⎟
⎜
⎟
(3)
where τ0 is the radiative lifetime of an isolated Ce ion; Γ(1–3/S) is the Euler's function; CA is acceptor ion content; C0 is critical content defined as 3/(4πR 03) ; S is the multipole interaction parameter. If the value of S is 6, Γ(1–3/S) = 1.77. The fitted curves using IeH model are shown in Fig. 11. It can be seen that the best agreement between experimental data and theoretical fits is obtained when setting the value of S is 6. Therefore, the dominant interaction mechanism between the donors and acceptors for Ba3Y2(B2O5)3:Ce3+,Tb3+ sample is dipole-dipole interaction. 3+
4. Conclusions We prepared Ce3+, Tb3+ and Ce3+-Tb3+ activated Ba3Y2(B2O5)3 phosphors successively through a sol-gel pyrolysis route. The Ba3Y2(B2O5)3:Ce3+ phosphor showed a broad band extending from 360 to 650 nm, originating from two different Ce3+ centers (Ce(1) and Ce (2)). The Ba3Y2(B2O5)3:Tb3+ phosphor showed a main emission peak at 540 nm due to the 5D4→7F5 transition of Tb3+. In the Ce3+, Tb3+ codoped Ba3Y2(B2O5)3 codoped sample, the energy transfer from Ce3+ center to Tb3+ center takes place. The dominant interaction mechanism for Ba3Y2(B2O5)3:Ce3+,Tb3+ is based on the dipole-dipole interaction. 5. Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The project was sponsored by the Technology Program of Environmental Protection Department of Hunan (No. 2013–312). References [1] Z. Zhang, Y. Wang, B. Zhang, Z. Yang, S. Pan, CaB5O7F3: a beryllium-free alkalineearth fluorooxoborate exhibiting excellent nonlinear optical performances, Inorg. Chem. 57 (2018) 4820–4823. [2] Y. Xiao, Z. Hao, L. Zhang, X. Zhang, G.-H. Pan, H. Wu, H. Wu, Y. Luo, J. Zhang, An efficient green phosphor of Ce3+ and Tb3+-codoped Ba2Lu5B5O17 and a model for elucidating the high thermal stability of the green emission, J. Mater. Chem. C 6 (2018) 5984–5991. [3] S. Verma, K. Verma, D. Kumar, B. Chaudhary, S. Som, V. Sharma, V. Kumar, H.C. Swart, Recent advances in rare earth doped alkali-alkaline earth borates for solid state lighting applications, Phys. B Condens. Matter 535 (2017) 106–113. [4] Y.F. Liu, J. Silver, R.J. Xie, J.H. Zhang, H.W. Xu, H.Z. Shao, J.J. Jiang, H. C Jiang, An excellent cyan-emitting orthosilicate phosphor for NUV-pumped white LED application, J. Mater. Chem. C 5 (2017) 12365–12377. [5] Y.F. Liu, J.X. Zhang, C.H. Zhang, J.T. Xu, G.Q. Liu, J.J. Jiang, H.C. Jiang, Ba9Lu2Si6O24:Ce3+: an efficient green phosphor with high thermal and radiation
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